2015 Master Thesis

A study on interface control of metal and AlGaN/GaN

for resistive Ohmic contact

13M36070

Mari Okamoto

Interdisciplinary Graduate School of Science and Engineering

Tokyo Institute of Technology

Supervisor:

Assoc.Prof. Kuniyuki Kakushima

Prof. Hiroshi Iwai

2

Februrary, 2015 Master Thesis

Contents

Abstract.................................................................................3

Chapter 1 Introduction........................................................9

Chapter 2 Experiment........................................................23

Chapter 3 Material for inducing VN from AlGaN............39

Chapter 4 TiSi2 electrode....................................................49

Chapter 5 TiC electrode……..............................................62

Chapter 6 Conclusions........................................................75

3

February, 2015 Abstract of Master Thesis

A study on interface control of metal and AlGaN/GaN

for resistive Ohmic contact

Supervisor: Associate Prof. Hiroshi Iwai

Supervisor: Prof. Kuniyuki Kakushima

Tokyo Institute of Technology

Department of Electrical and Electronic Engineering

13M36070 Mari Okamoto

AlGaN/GaN high-electron-mobility transistors (HEMTs) are expected for future power devices with

high efficiency, owing to its high mobility and large breakdown field. One of the issues is high

Ohmic contact with Ti-Al based electrode. As the mechanism is based on through dislocations in

AlGaN layer, the contact resistance would be increase for future high quality substrate. In this thesis,

we propose TiSi2 and TiC electrode as novel contact material, which show dislocation independent

conduction. The conduction is modeled based on interface reaction upon annealing and band

bending properties evaluation.

For TiSi2 electrode, Ohmic contact is achieved in the temperature range of 950oC~1100oC. The

minimum contact resistance c is 4.3×10-6 cm2 with annealing at 1075oC for 1 min. The contact

resistance is a little higher than conventional electrode with Ti-Al based (10-6~10-7 cm2). HE-XPS

analysis indicates higher shift of Al 1s spectra, which implies active donors like nitrogen

vacancy in AlGaN layer. The concentration of nitrogen vacancy increases to about 1.4×1018cm-3

4

estimated from fitting data. Moreover, optimization of annealing time realizes more lower contact

resistance, which value is 1.7×10-7 cm2 with annealing at 1075oC for 5min.

For TiC electrode, Ohmic contact is achieved in the temperature range of only 1000oC~1025oC.

The minimum contact resistance c is 6.1×10-3 cm2 with annealing at 1025oC for 1min. However

the c is much higher than conventional electrode, c can be decreased by increasing ratio of Ti for

TiC electrode. The minimum contact resistance with 90% Ti ratio is 8.2×10-8 cm2. Although XPS

analysis was not confirmed, increase in concentration of VN could be assumed. The

reason for increasing VN is that contact resistance was reduced with higher annealing

temperature despite sheet resistance Rsh is almost the same.

As a common points of TiSi2 and TiC electrode, one is that both of TiSi2 and TiC electrode can be

Ohmic contact without depending on dislocation in AlGaN. The other is decrease in thickness of

AlGaN was confirmed with higher annealing temperature. Therefore, reduction of AlGaN thickness

is an important role of lowering contact resistance.

Considered above discussion, TiSi2 and TiC can be Ohmic contact materials, which realize

dislocation independent conduction and also forming nitrogen vacancy at the surface of AlGaN.

Moreover, thickness of AlGaN was decreased without degrading 2DEG density and lower c can be

obtained due to thinner AlGaN barrier. Compared with conventional electrode, annealing

temperature is high, but these electrodes can be useful Ohmic-contact-material for future

dislocation-free wafers

5

Chapter 1, Introduction

1-1 A Power semiconductor device and demand of it………………........9

1-2 New materials for power semiconductor devices………....................10

1-3 AlGaN/GaN high electron mobility transistors (HEMTs)……….......10

1-4 Problems to realize high performance AlGaN/GaN HEMTs………...11

1-5 Current research for ohmic contact for AlGaN/GaN HEMTs……….12

1-6 Purpose of this study………………………………………………....18

References………………………………………………………………..20

6

Chapter 2,Experiments

2-1 Fabrication process.............................................................................23

2-2 Experimental principle........................................................................24

2-2-1 SPM cleaning and HF treatment......................................................24

2-2-2 SiO2 passivation by TEOS...............................................................24

2-2-3 Reactive Ion Etching........................................................................25

2-2-4 Wet etching by BHF.........................................................................25

2-2-5 RF magnetron sputtering..................................................................25

2-2-6 PMA in N2 ambient...........................................................................26

2-2-7 Transmission Electron Microscopy (TEM)......................................26

2-2-8 X-ray photoelectron spectroscopy.....................................................27

2-2-9 Transmission Line Method (TLM) with Four-terminal method........28

2-2-10 Sheet Resistance..............................................................................32

2-3 Current-Voltage characteristics.............................................................32

2-3-1 Metal-Semiconductor Contacts.........................................................32

2-3-2 Metal-AlGaN-GaN Contacts.............................................................34

References...................................................................................................37

7

Chapter 3, Material for inducing nitrogen vacancy

3-1 Introduction.........................................................................................39

3-2 TEM analysis .....................................................................................39

3-2-1 EDX analysis of Si single layer.......................................................43

3-2-2 Band bending observation of AlGaN witih Carbon.........................45

3-3 Conclusion………………………………………...............................47

Chapter 4, TiSi2 electrode

4-1 Introduction.........................................................................................49

4-2 Electrical characteristic of TiSi2 electrode..........................................49

4-3 XPS analysis........................................................................................51

4-4 Dependence on Annealing time..........................................................56

4-5 TEM and EDX analysis......................................................................57

4-6 Conclusions..........................................................................................59

8

Chapter 5, TiC electrode

5-1 Introduction.......................................................................................62

5-2 Curent-Voltage characteristic ...........................................................62

5-3 TEM analysis....................................................................................63

5-4 Dependency of Ti ratio for TiC electrode.........................................65

5-5 TEM analysis for TiC electrode........................................................68

5-6 Conclusion.........................................................................................74

References

Chapter 6,Conclusion

Acknowledgement

9

Chapter1

Introduction

1-1 A Power semiconductor device and demand of it..................................9

1-2 New materials for power semiconductor devices.................................10

1-3 AlGaN/GaN high electron mobility transistors (HEMTs)....................10

1-4 Problems to realize high performance AlGaN/GaN HEMTs................11

1-5 Current research for ohmic contact for AlGaN/GaN HEMTs...............12

1-6 Purpose of this study.............................................................................18

References...................................................................................................20

10

1-1 A Power semiconductor device and the demand

Power semiconductor devices are used in power conversion system which refers to convert voltage,

current, frequency, coherent, number of phase and wave shape. Figure 1.1 shows relation of

frequency and switching capacity where power device is applied to electric home applications

(microwave, electric fan and laundry machine) and industrial fields (railroad and robot weld) a wide

range of fields. At current, Silicon material is dominant for power devices with conversion efficiency

of number of about 94%. The reason is that insulating films are formed easily and Si has

high-quality of crystal by current technologies. However, high efficiency in converting electrical

power is a need to improve in order to achieve further lower loss. Incidentally, the electrical power

loss is determined by conduction loss and switching loss. Silicon carbide (SiC) and gallium nitride

(GaN) as new materials are studied for using new power devices that are expected to lower

conduction loss.

Figure 1.1 Applications of discrete power semiconductors

11

1-2 New materials for power semiconductor devices

Currently Silicon has been a dominant material to make power semiconductor devices (i.e IGBT,

MOSFET…). The conversion efficiency is about 94%. [1.1] But Silicon power devices have arrived

the limitation of the material property in the reducing electric power loss, the break down voltage

and things. However, a better power efficiency and higher breakdown voltage are required in

commercial fields. Therefore, new materials (GaN, SiC…) that have better material properties attract

increasing attention. [1.2] Table 1.1 is a table that compares the material properties (Silicon) with

two new competing materials (GaN, 4H-SiC).

Table 1.1 Material properties of Silicon, GaN and 4H-SiC

>3000>25001412Melting point [oC]

3.5-5.01.31.5Thermal

cond.λ [W/cmK]

2.02.01.0Sat. valocity vsat

[107cm/s]

2.23.30.25Critical Field Ec

[MV/cm]

900-120020001300Electron Mobility μ n

[cm2/Vs]

3.23.41.12Bandgap Eg[eV]

4H-SiCGaNSi

>3000>25001412Melting point [oC]

3.5-5.01.31.5Thermal

cond.λ [W/cmK]

2.02.01.0Sat. valocity vsat

[107cm/s]

2.23.30.25Critical Field Ec

[MV/cm]

900-120020001300Electron Mobility μ n

[cm2/Vs]

3.23.41.12Bandgap Eg[eV]

4H-SiCGaNSi

1-3 AlGaN/GaN high electron mobility transistor

As described in Table 1.1, GaN has greater material properties than Silicon. For example, the

critical field (3.3MV/cm) is tenfold as much as Silicon (0.25MV/cm), the saturated velocity

(2.0x107cm/s) is twice as much as Silicon (1.0x107cm/s). In addition, GaN can be grown on Si(111)

now [1.3], so the production cost will be lowered in the future. Therefore, GaN is very suitable for

the use as an important material in microwave power devices. [1.4] Also, high electron mobility

12

(2000cm2/Vs) compared to Silicon (1300cm2/Vs) is realized by use of AlGaN/GaN HEMT lateral

structure described in Figure 1.2 without intentionally doping. A high density of two-dimensional

electron gas (2DEG) occurs at the AlGaN/GaN surface by piezoelectric and spontaneous

polarization effects, as shown in Figure 1.3 [1.5-6]. Figure 1.4 shows the crystal structure of Ga-face

GaN that causes the spontaneous polarization. The spontaneous and piezo polarization can cause

high electric fields (5MV/cm) at AlGaN/GaN surface (~2nm), and they lead high sheet charge

density (6×1012 to 2×1013cm-2) to the surface [1.5].

Figure 1.2 AlGaN/GaN MOS-HFET

AlGaN

Metal

GaN

2DEGEC

EF

EV

Figure 1.3 A Band structure of an interface between AlGaN and GaN

13

Ga

N[0

00

1]

Spo

nta

ne

ou

s p

ola

riza

tion

Figure 1.4 Schematic of the crystal structure of Ga-face GaN

1-4 Problems to realize high performance AlGaN/GaN HEMTs

AlGaN/GaN HEMTs are attractive as a new power device, but there are some problems to solve in

order to realize an outstanding performance [1.7]. There are some main topics to solve, for example,

one is a normally-on characterization of the AlGaN/GaN channel layer that cause the power

consumption to increase, and two is a lot of crystal defects in AlGaN/GaN substrates that cause

breakdown voltage to decrease etc. Also, in terms of power consumption, high ohmic contact

resistance (Rcon) cause high electrical power losses and self-heating to increase in regardless of low

channel resistance (Rch). If the specific contact resistance (cm2) decrease from 10-5 cm2 to 10-6

cm2, 60% of the electrical power loss in breakdown voltages under 1000V could be reduced [1.9].

14

Figure 1.5 Technology issues in GaN HFETs [1.7]

1-5 Current researches for Ohmic contact to AlGaN/GaN HEMTs

At present, most low resistance Ohmic contact has been realized by using Ti-Al-based stacked

metallization (Ti/Al/Ni/Au [1.9-10], Ti/Al/W [1.11], Ta/Al/Ta [1.12], Ta/Si/Ti/Al/Ni/Ta [1.10]) or

by Also, Ohmic contact has been available at low temperature (550oC~600oC) with recess etching

that means AlGaN barrier in contact areas down to the location of 2DEG in a GaN substrate [1.13].

Their stacked metallization have achieved typical contact resistance (Rc) values below 1 mm (the

Rc value of Ti/Al/Ni/Au is 0.15 mm [1.9] etc.), and their specific contact resistance (C) is 0.7 to 2

×10-6 cm2[1.9-10]. Figure 1.6 shows the interface between electrodes (Ti/Al/Mo/Au [1.14]) and

the AlGaN/GaN substrate. It is considered that TiN in AlGaN are formed as shown in Fig1.6

depending on the reaction between AlGaN and Ti. Especially, TiN is formed on crystal defects

described as white arrows (TiN islands) [1.14]. TiN is a metallic compound with electrical resistivity

as low as 13 and Au shell presented around the islands could all enhance conductivity.

Therefore, electron would be transported efficiently through TiN protrusions into 2DEG, which

enables to achieve Ohmic characteristic. However, there is still the issue that if film-forming

technology for AlGaN/GaN upgrades in the future, the crystal defects in AlGaN/GaN will be

15

decreased and there is the possibility of increasing contact resistance because of the dependency to

crystal defects.

On the other hand, there is another competing ways for low resistance Ohmic contact with insertion

of Si3N4 layer to Ti-Al based metal (Si3N4/Ti/Al/Mo/Au [1.15]). Ohmic contact has been available

with annealing at 800oC and contact resistance is 0.91 mm. Ohmic contact has been achieved

because of formed pyramid AlN at the metal/AlGaN interface after annealing as shown in Figure 1.8.

These AlN pyramid was resulted from reaction that the N of AlN was extracted from AlGaN layer

by Al. N extraction from AlGaN causes n-type AlGaN, which enable to increase tunneling

conduction through AlGaN layer as shown in Figure 1.9. However AlN pyramid wasn’t formed on

dislocation, tunneling current through AlN dots is locally electron conduction. From this result, there

is still an issue that local electron conduction might lead concentration of electric field. A problem of

concentration of heat generation occurs by increased current, which was resulted from electric field

concentration. As shown in Figure 1.10, Ti/Al/Ti/A electrodes of the device were burned out in

measured Current –Voltage characteristic at a large current. This is the main reason that Al existing

in the ohmic contact scheme does not react completely and is subject to melting at these high

annealing temperatures. Thus heat generation degrades reliability for AlGaN/GaN HEMT device.

16

Figure 1.6 (a) Z-contrast image of the contacts on AlGaN/GaN after annealing.The threading

dislocations are labeled by white arrows (b) HRTEM image showing portions of the TiN island, GaN,

and AlGaN. (c) HRTEM image of one interfacial TiN grain [1.14]

Figure 1.7 A schematic of the spike mechanism for carrier injection [1.14]

17

Figure 1.8 Bright-field TEM image at the interface metal/AlGaN annealed at 800oC.[1.15]

Figure 1.9 A schematic of the AlN pyramid mechanism for carrier [1.14]

Figure 1.10 Simplified model of tunneling conduction at an interface (Metal/AlGaN)

18

Figure 1.10 The top view of samples burned out at large current

annealed at 750oC for 30 sec.[1.16]

1-7 Purpose of this study

In this study, I focused on reducing the contact resistance to AlGaN/GaN HEMTs. Although

common approach is achieved by direct conduction with Ti-Al based metal or tunneling through

AlGaN barrier with insertion of Si or Si3N4. These approaches depends on local electron conduction

from metal to 2DEG. Therefore the contact resistance would be increase for future substrate with less

crystal defects and reliability would be lost because of electric field concentration.

Here, we propose Ti-Si mixed metal and Ti-C mixed metal as new material for Ohmic contact

substituting Aluminum. The motivation for selecting these materials has two reasons. The one is that

silicide and carbide have thermal stability in high annealing temperature. The other one is that these

materials can extract N atoms from AlGaN, which cause band bending of AlGaN and increase

tunneling probability. Electrical characteristic and physical analysis of these electrodes were

examined with focus on interface reaction between AlGaN and these materials.

In chapter 1, power semiconductor devices and demand of it, new materials for power

semiconductor devices, AlGaN/GaN High Electron Mobility Transistors, problems to realize high

performance AlGaN/GaN HEMTs, current researches for ohmic contacts to AlGaN/GaN HEMTs.

19

In chapter 2, fabrication processes for samples, principles for experiments, conduction mechanisms

between metal and semiconductor (n-type) or AlGaN/GaN substrate are explained.

In chapter 3, the motivation for selecting Si and C is described. Especially focused on interface

reaction between Si or C and AlGaN layer, physical analysis is mainly observed.

In chapter 4, Current-Voltage characterization was measured with annealing temperature and time in

the case of using TiSi2/TiN as electrodes to AlGaN/GaN substrate. EDX analysis and XPS analysis

at AlGaN surface was observed, which indicates that band diagram of AlGaN/GaN.

In chapter 5, Current-Voltage characterization in the case of using TiC/TIN as electrode to

AlGaN/GaN substrate was measured. Current-Voltage measurement with varying Ti ratio is

described as electrical properties. TEM image of TiC/TiN electrode were shown in described

physical analysis.

In chapter 6, this chapter summarizes this study.

Figure 1.12 shows the contents of this thesis, which consists of 6 parts.

Figure 1.12 Structure of this thesis

20

References

[1.1] “Substrates for GaN Gased Devices: Performance Comparisons and Market

Assessment” (2006)

[1.2] Umesh K.Mishra, Primit Parikh and Yi-Weng Wu, “AlGaN/GaN HEMTs-An Overview of

Device Operation and Applications”, PROCEEDINGS OF THE IEEE, Vol. 90, No 6, June 2002

[1.3] F.Semond, P. Lorenzini, N. Grandjean, and J. Massies, “High-electron-mobility AlGaN/GaN

heterostructures grown on Si(111) by molecular-beam epitaxy”, Applied Physics Letters, Vol 78, No

3, January 2001

[1.4] M. Asif Khan, J.N.Kuznia, D.T Olson, W.J. Schaff, J.W. Burm, and M.S. Shur, “Microwave

performance of a 0.25 m gate AlGaN/GaN heterostructure field effect transistor”, Applied Physics

Letters, Vol. 65, No 9,August 1994

[1.5] O.Ambacher, J. Smart, J.R. Shealy, N.G. Weimann, K. Chu, M. Murphy, R. Dimitrov, L.

Wittmer, M. Stutzmann, W. Rieger and J. Hilsenbeck, “Two-dimensional electron gases induced by

spontaneous and piezoelectric polarization charges in N-and Ga-face AlGaN/GaN heterostructures”,

Journal of Applied Physics, Vol.85, No 6, 15 March 1999

[1.6] J.P. Ibbetson, P.T. Fini, K.D. Ness, S.P. DenBaars, J.S. Speck, and U.K. Mishra, “Polarization

effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect

transistors”, Applied Physics Letters, Vol. 77, No 2, July 2000

[1.7] Ho-young Cha, “High Efficiency GaN Power Device Technologies”, Hongik University,

NanoKISS, 2013

[1.8] W.Saito, Ichiro Omuram Tsuneo Ogura, Hiromichi Ohashi, “Theoretical limit estimation of

lateral wide band-gap semiconductor power-switching device”, Solid-State Electronics 48

1555-1562, 2004

[1.9] A.N. Bright, P.J. Thomas, M. Weyland, D.M. Tricker, C.J. Humphreys, R. Davies,

“Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts

using transmission electron microscopy”, Journal of Applied Physics, Vol 89, No 6, March 2001

[1.10] Yang Li, Geok lng Ng, Subramaniam Arulkumaran, Chandra Mohan Manoj Kumar, Kian

Siong Ang, Mulagumoottil Jesudas Anand, Hong Wang, Rene Hofstetter, and Gang Ye,

“Low-Contact-Resistance Non-Gold Ta/Si/Ti/Al/Ni/Ta Ohmic Contacts on Undoped AlGaN/GaN

High-Electron-Mobility Transistors Grown on Silicon”, Applied Physics Express, Vol 6, 116501,

October 2013

[1.11] Marleen Van Hove, Sanae Boulay, Sandeep R. Bahl, Steve Stoffels, Xuanwu Kang, Dirk

Wellekens, Karen Greens, Annelies Delabie, and Stefaan Decoutere, “CMOS Process-Compatible

High-Power Low-Leakage AlGaN/GaN MISHEMT on Silicon”, IEEE Electron device letters, Vol

33, No 5, May 2012

[1.12] A. Malmros, H. Blanck, and N. Rorsman, “Electrical properties, microstructure, and thermal

21

stability of Ta-based ohmic contacts annealed at low temperature for GaN HEMTs”, Semiconductor

Science and Technology, Vol 26, 075006, March 2011

[1.13] A. Firrincieli, B. De Jaeger, S. You, D. Wellekens, M. Van Hove and S. Decoutere, “Au-Free

Low Temperature Ohmic Contacts for AlGaN/GaN Power Devices on 200mm Si Substrates”,

Extended Abstracts of the 2013 International Conference on SSDM, p914-915, 2013

[1.14] Liang Wang, Fitih M. Mohammed, and Ilesanmi Adesida, “Differences in the reaction

kinectics and contact formation mechanisms of annealed Ti/Al/Mo/Au Ohmic contacts on n-GaN

and AlGaN/GaN epilayers”, Journal of Applied Physics, Vol 101, 013702, 2006

[1.15] B.Van Daele and G.Van Tendeloo, “Mechanism for Ohmic contact formation on Si3N4

passivated AlGaN/GaN high-electro-mobility transistors”, Applied Physics Letters 89, 201908,

2006.

[1.16] Zhu Yanxu, Cao Weiwei, Fan Yuyu, Deng and Xu Chen, “Effects of rapid thermal annealing

on ohmic contact of AlGaN/GaN HEMTs”, Journal of Semiconducors, Vol 35, No.2, 2014

22

Chapter2

Experiments

2-1 Fabrication process.............................................................................23

2-2 Experimental principle........................................................................24

2-2-1 SPM cleaning and HF treatment......................................................24

2-2-2 SiO2 passivation by TEOS...............................................................24

2-2-3 Reactive Ion Etching........................................................................25

2-2-4 Wet etching by BHF.........................................................................25

2-2-5 RF magnetron sputtering..................................................................25

2-2-6 PMA in N2 ambient...........................................................................26

2-2-7 Transmission Electron Microscopy (TEM)......................................26

2-2-8 X-ray photoelectron spectroscopy.....................................................27

2-2-9 Transmission Line Method (TLM) with Four-terminal method........28

2-2-10 Sheet Resistance..............................................................................32

2-3 Current-Voltage characteristics.............................................................32

2-3-1 Metal-Semiconductor Contacts.........................................................32

2-3-2 Metal-AlGaN-GaN Contacts.............................................................34

References...................................................................................................37

23

2-1 Fabrication process

In this section, a fabrication process for this study is shown by Figure 2.1. Figure 2.2 shows the

sample fabrication.

Figure 2.1 Fabrication process outline

Figure 2.2 sample fabrication

24

2-2 Experimental principles

Below is the detail of experiments in the fabrication process.

2-2-1 SPM cleaning and HF treatment

In the purpose of SPM cleaning and HF treatment are cleaning substrate surface. SPM cleaning is

one of the effective cleaning methods to degrease organic substance. HF treatment is used to

degrease oxidation products. Al0.25Ga0.75N/GaN substrate surface is coated by organic resist film in

order to protect from particles. The sample is degrease using 1% HF treatment and Liquid solution

which is made from H2SO4 and H2O2 (H2SO4: H2O2= 4:1). In this study, SPM cleaning is used on

heating at 180oC for Al0.25Ga0.75N/GaN substrate surface.

2-2-2 SiO2 passivation film by plasma-TEOS

Plasma chemical vapor deposition (CVD) is used Tetraethyl orthosilicate (TEOS) films formation

shown in Figure 2.3. Feature of CVD is formed using a rapid chemical reaction of TEOS at under

200oC temperature.

CH

H H

H

O

H

C CO

H H

H

H

H

C

C H

H

H

O

HC

H

Si

C H

O

H

H

HC

H

CH

H H

H

O

H

CCH

H H

H

O

H

C CO

H H

H

H

H

CCO

H H

H

H

H

C

C H

H

H

O

HC

H C H

H

H

O

HCC

H

SiSi

C H

O

H

H

HC

H C H

O

H

H

HCC

H

Figure 2.3 Tetlaehtyl orthosilicate

25

2-2-3 Reactive ion etching (RIE)

Reactive ion etching (RIE) is used to both AlGaN etching and metal contact patterning. Feature of

RIE etching can be formed uniformly.

2-2-4 Wet etching by BHF

BHF (Buffered HF) consists of NH4F and HF. SiO2 can be removed by using BHF. In order to open

contact holes, the chemical substance was used in this study.

2-2-5 RF magnetron sputtering

Radio frequency (RF) magnetron sputtering is used to deposit metal (Ti, Si, C, B, TiSi2, TiC, TiN).

Sputtering is one of the vacuum processes used to deposit thin films on substrates. A high voltage

across a low pressure gas is applied to become “plasma” which is consisted of electrons and Ar gas

ions in a high-energy state. After, the accelerated plasma ions strike the “target” which is sputtering

metal seed as shown in Figure 2.3. In this study, Ti, Si, C, B, TiSi2, TiC and TiN for contact

electrode were deposited by sputtering process.

Figure 2.4 Simplified schematic of RF magnetron sputtering

26

2-2-6 PMA in N2 ambient

Post metallization annealing (PMA) is needed to recover arising from processing defects. In this

study, different annealing conditions, annealing temperature and time are examined.

2-2-7 Transmission Electron Microscopy (TEM)

TEM is one of the electron microscopes. By irradiating electrons to the thin sample, some electrons

are scattered and others are transmitted. Because amount of transmitted electrons depends on the

structure or component of each portion, the specimen shape and surface structure in addition to

information of the internal material which is the degree of cohesion, crystalline patterns, presence of

lattice defect, and such as orientation directions of the crystal can be known by observing the

internal structure of the sample. Typically a TEM consists of three stage of lens as shown figure 2.3.

The stages are condenser lenses are responsible for primary beam formation, whilst the objective

lenses focus the beam that comes through the sample itself. The projection lenses are used to expand

the beam onto the fluorescent or other imaging device, such as film.

27

Figure 2.5 Illustration of TEM system

2-2-8 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is one of the most effective method of determining the

elements, which composing the sample. XPS spectra are obtained by irradiating a material with a

beam of X-rays while simultaneously measuring the kinetic energy (Ek) and number of electrons,

which escape from the material being analyzed. The relation of energies as follow:

EE bkhv (2.1)

Where h is energy of the x-ray, Ek is the kinetic energy of the emitted electron and Eb is binding

energy of emitted electron. Eb is determined by measuring Ek and his incident X-ray energy, which

is constant. Eb is observed to energy peak, which is determined by composition of sample. In this

28

study, the chemical composition of the sample was measured by hard XPS at Spring-8 BL46XU as

shown in Figure 2.4. The mean free path of excited electrons is about 10-nm thick.

Figure 2.6 Illustration of XPS system

2-2-9 Transmission Line Method (TLM) with Four Terminal Method

TLM is the one of measurement that derives a specific resistivity of electrodes. Figure 2.6 (a)

shows the electrodes from the top view, and (b) shows from the cross-section view. A Wide length of

the electrodes is Z (190 m), a gap between the electrodes is d (d1~6=50 m, 100 m, 150 m, 200

m, 250 m, 300 m) and A lateral wide length of the electrodes is L (80 m).

shcT RZ

dRR 2 （2.1）

29

where RT is the total resistance between electrodes, Rc is the contact resistance ,and Rsh is the sheet

resistance between electrodes.

Rc could be derived from the approximately line as shown in Figure 2.9.A specific contact resistance

(c) is

T

c

cZL

R (2.2)

where LT is an effective transfer length illustrated by Figure 2.7.

)2(2 T

shshc Ld

Z

RR

Z

dR

(2.3)

Figure 2.5 (a) A top view of electrodes for TLM (b) A cross-section figure of electrodes

30

Current

Electrode

Current

ElectrodeLT

Figure 2.6 Effective transfer lengths

LT LT

Electrode Electrode

Substrate

V

d

Figure 2.7 A voltage drop between electrodes

5μ m 10μ m 15μ m 20μ m

an approximately straight line

RT

2RC

-2LT d

measured data

Figure 2.8 A transfer length method test structure and a plot of total resistance as a function of

contact spacing

31

In this study, Current-Voltage characteristic was measured with TLM patterning by four-terminal

method as shown in Figure 2.9. Four terminal method is utilized in measuring low resistance

(beneath 1 ). However, two terminal method is also used for measuring resistance, resistance

measured by two terminal method consists of wiring resistance and contact resistance between

measuring instrument, wiring and sample (Figure). Therefore, measured value of two terminal

method is higher than that of four terminal method. In study, we measured low contact resistance

using transmission line method with four terminal method (Figure 2.9).

Figure 2.9: Four point terminal for measuring resistance with extra resistance

2-2-10 Sheet Resistance

Sheet resistivity of electrode was measured by four-terminal method with patterned thin-film metal

shown in Figure 2.10. Sheet resistance Rsh (/sq) is

L

WRRsh (2.4)

32

Where L (290 m) is a gap between electrode 2 and electrode 3 and width of metal path is W (25

m).

Figure 2.10 Sheet resistivity with four-terminal method

2-3 Current-Voltage characteristics

This section covers current-voltage characteristics between metal and semiconductor (n-type) or

AlGaN/GaN. The understanding of a conduction mechanism between metal and semiconductor is

important for reducing contact resistance.

2-3-1 Metal-Semiconductor Contacts

Figure 2.11 and Figure2.12 shows the flow of electron in an n-type Schottky diode that thermal

equilibrium and applied forward bias respectively. φBn is Schottky barrier height, Ef is Fermi level,

Ec is conduction band, φbi is built in potential respectively. Showed black dot of electron can

transport upper barrier height which direction is shown arrows. In Figure 2.11, there is thermal

equilibrium, there is an equal and opposite flow of electron. In Figure 2.12, caused Vapp is applied

voltage, Fermi level Ef is moved upper direction. Thermionic emission theory current density for

Schottky diode as follow:

]1))[exp(exp(* 2 kT

qV

kT

qTAJ

appBn (2.4)

Figure 2.11 shows a principal of electrons transport process.

33

EFEF

EC

Metal Semiconductor (n-type)

qφ Bnqφ bi

Figure 2.11 At thermal equilibrium, electrons transport processes between metal and semiconductor

(n-type)

EF

EF

EC

Metal Semiconductor (n-type)

qφ Bn

qφ bi-qVapp

qVapp

Figure 2.12 At forward bias, electrons transport processes between metal and semiconductor (n-type)

EFEF

EC

Metal

Semiconductor (n-type)

EV

Thermionic emission

Thermionic field emission

Field emission

Figure 2.13 A principal of electrons transport processes

34

2-3-2 Metal-AlGaN-GaN Contacts

A new thin surface barriers (TSB) model assuming the presence of extrinsic surface donors was

proposed, where the TFE /FE process through the TSB region determines current transport. In

metal-GaN contact,TFE /FE model cannot explain the measured reverse I-V date quantitatively.

Figure 2.16 shows TSB model and the band diagram.

Then a presence of TSB regions having a thickness D, where a thin surface barrier is formed in each

region by high density of unintentional donors, is assumed.

In each TSB region, it is assumed that TFE/FE emission through TSB regions is the main

mechanism of overall current transport [2.3].

In the case of AlGaN/GaN, Ti reacts with AlGaN and N vacancy occurs in AlGaN. The N vacancy

has become high density of unintentional surface donors in confined location. Due to a lot of

non-uniform dislocations in GaN substrate, there is high possibility that TSB region is distributed

locally as shown in Figure 2.15. In the area, surface donors (N vacancy) sharpen the schottky barrier

as shown in Figure 2.14, so similarly it might be assumed that TFE/FE emission through TSB

regions is the main mechanism of the current transport. In Figure 2.16, the electric potential at x =D,

which is the thickness of the thin surface barrier, is defined as D. The shape of the electrical

potential from x= 0 to x= D is a sharp parabola, whose minimum potential is defined as 0. Also V0

is defined as the bias voltage as 0=D, and V0 and 0 are given by the following equations:

n

S

B VDqN

V 2

0

02

(2.5)

where Vn is given by

D

C

nN

NkTV ln

(2.6)

For V<V0

35

n

D

S

DS

D

S

D VVDDVVqNN

NqN

2

22

0

0

0

0 ))(2

12

(2.7)

And for V>V0

nVV 0 (2.8)

The energy location of the Gaussian main peak qm for the TFE process is given by

)/(cosh

1

00

2

0

0

kTEB

m

(2.9)

where E00 is given by

0

00002

)( S

DD

m

NNEE

(2.10)

and the ideality factor for forward and reverse currents are

kT

E

kT

EnF

0000 coth (2.11)

1

000000 tanh

kT

E

kT

E

kT

EnR (2.12)

The average forward saturation current density is given by

]/)(exp[)/exp()/cosh(

)/tanh()(

00

00000kTnVqkTqV

kTEk

kTEEqTAJ FnBn

B

SF

(2.13)

)/)(exp()/(cosh

)(2

1

00

2

00kTnVq

kTqE

qVVq

k

EqTAJ FRB

BnRSR

for TFE (2.14)

)/)]/([sin()]/([

])(3/2exp[

00

2/1

0

2/1

0

2/1

000

2/3

00

EqqkTqqk

qEqETAJ

BBBB

BBSR

36

for FE (2.15)

In the current researches, these conduction paths are considered as a main mechanism when ohmic

contact between metal and AlGaN/GaN substrate is realized.

AlGaN

Metal

GaN

2DEGEC

EF

EVN vacancys

Tunneling conduction

Metal

AlGaN

GaN

EC

EF

EV

2DEG

Thin Surface Barrier

Compounds

generation

Figure 2.14 A Thin Surface Barrier Model (1) [2.3]

D

metal semiconductor

Thin surface barrier(TSB) region

Figure 2.14 TSB model (2) [2.4]

37

TSB regionqφB

qVnqV

qφD

qφ0

DEF

Em = qφm

x

Figure 2.15 The band diagram of TSB model [2.4]

References

[2.1] Dieter K. Schroder, “SEMICONDUCTOR MATERIAL AND DEVICE

CHARACTERIZATION”, IEEE PRESS,2006

[2.2] Yuan Taur and Tak H. Ning, “Fundamentals of Modern VLSI Devices”, Cambridge University

press, 1994

[2.3] Junji Kotani, Tamotsu Hashizume, and Hideki Hasegawa, “Analysis of Excess Leakage

Currents in Nitride-Based Schottky Diodes Based on Thin Surface Barrier Model”, Techical Report

of IEICE, ED 2004-42

[2.4] Hideki Hasegawa and Susumu Oyama, “Mechanism of anomalous current transport in n-type

GaN schottky contacts”, J.Vac.Sci. Technol. B 20(4), 2002

38

Chapter3

Material for inducing nitrogen vacancy

3-1 Introduction.........................................................................................39

3-2 TEM analysis ......................................................................................39

3-2-1 EDX analysis of Si single layer........................................................43

3-2-2 Band bending observation of AlGaN witih Carbon.........................45

3-3 Conclusion………………………………………...............................47

39

3-1 Introduction

The motivation for selecting Si and C has been proved in this chapter focused on interface reaction

of Si/C and AlGaN. Si and C have high melting point to endure being annealed at high temperature.

Moreover, alloy compounded with carbon could be treated at annealing in high temperature to

produce donor like N vacancies at an interface between metal and AlGaN/GaN substrates. In this

chapter, electrical properties and physical analysis was described.

3-2 TEM analysis

TEM analysis of interface between AlGaN and Si or C is shown in this section. Si and C (100 nm)

was deposited on AlGaN/GaN substrate by RF sputtering after chemical cleaning with SPM (H2O2

and H2SO4). As shown in Figure3.1, the image corresponding to the device structure was obtained

by TEM. AlGaN/GaN substrates contain a lot of crystal defects in Figure 3.1. They are considered as

a factor that decreases the breakdown voltage.

Figure 3.1 TEM image of structure (×30,000)

Figure 3.2~Figure 3.4 shows that cross-sectional TEM image of the sample with Si single layer

stacked on AlGaN/GaN substrate annealed 1075oC in N2 ambient for 1 min. Thin interface layer

40

between Silicon and AlGaN was observed in Figure 3.3. Detailed analysis of interface layer was

mentioned in subsection 3-2-1.

Figure 3.2 TEM image of Si layer on AlGaN (×200,000)

Figure 3.3 TEM image of Si layer on AlGaN (×500,000)

41

Figure 3.4 TEM image of Si layer on AlGaN (×2,000,000)

Figure 3.5~Figure 3.7 shows that cross-sectional TEM image of the sample with C single layer on

AlGaN/GaN substrate annealed 1075oC in N2 ambient for 1 min. Thickness of AlGaN layer was

decreased and interface between C and AlGaN has been rough rather than interface between Si and

AlGaN. Therefore, C atoms can more promote reaction between Carbon and AlGaN.

42

Figure 3.5 TEM image of C layer on AlGaN (×200,000)

Figure 3.6 TEM image of C layer on AlGaN (×500,000)

43

Figure 3.7 TEM image of C layer on AlGaN (×2,000,000)

3-2-1 EDX analysis of Si single layer

From cross sectional TEM image of Si/AlGaN in Figure 3.2 and Figure 3.3, thin interface layer was

confirmed on AlGaN/GaN substrate. In this subsection, Energy dispersive x-ray (EDX) analysis of

interface layer was described. Measurement point was shown in Figure 3.8 (Point 1 in Carbon layer,

Point 2 and Point3 was in interface layer and Point 4 was in Carbon layer). Figure 3.9 includes EDX

analysis of four points in C/AlGaN. N in Point 1 was hardly detected. That’s the negligible influence

of EELS’s noise signal. Si and O spectra were detected in Point 1 and Only N spectra was detected

in Point 4.

There is a little difference between point 2 and point3. Si, N and O spectra are detected in interface

layer, which proves that interface layer were composed Si-O-N mixed layer. Si-O-N layer was

formed by the reaction that Si induced nitrogen vacancy from AlGaN layer. From these result,

Silicon would be a material for generating nitrogen vacancy in AlGaN, nevertheless there is a

44

problem that Si is likely to reacts with Oxygen.

Figure 3.8 Schematic illustration of sample and measurement point

Figure 3.9 EDX analysis

45

3-2-2 Band bending observation of AlGaN witih Carbon

The reaction of C with AlGaN layer was characterized by hard x-ray photoelectron spectroscopy

(HXPES) with an x-ray energy of 7940 eV. For this characterizations, a carbon layer of 10 nm in

thickness was used to efficiently collect photoelectrons under the layer. As the inelastic mean free

path for photoelectrons Al 1s is 12 nm [3.1], all the photoelectrons arising from AlGaN layer can be

collected. Figure 3.10 shows Al 1s spectrum before and after annealing in 1075 oC in nitrogen

ambient for 1 min. The binding energy was calibrated by each Al 1s spectrum obtained from each

sample. A positive shift in energy at the peak intensity from 1561.1 to 1561.6 eV is observed with

the annealed sample. Moreover, a spectrum broadening from 1.16 to 1.26 eV in full width at half

maximum (FWHM) can be confirmed. These two facts strongly suggests that the AlGaN layer is

further bent down in additional to the effect of spontaneous and piezoelectric polarization. From

deconvolution analysis, the spectrum change can be interpreted as nitrogen vacancy creation to form

n+-AlGaN at the surface of AlGaN, which is in good agreement with thermodynamic prediction of

the reaction of C and AlGaN. We should note that downward bending of AlGaN layer is

advantageous as it increases the density of 2DEG. From above results, the energy band diagram of C

on AlGaN/GaN structure before and after annealing can be drawn as Figure 3.11.

46

Figure 3.10 Al 1s spectra of AlGaN layer before and after annealing at 1075oC with C.

Figure 3.11 Band diagram of AlGaN before and after annealing

47

3-4 Conclusion

It has been suggested that Si and C can be electrode for inducing nitrogen vacancy with annealing at

1075oC for 1 min. These material can be candidate for Ohmic contact material being substituted for

Aluminum. Moreover, interface layer and interface roughness between AlGaN and Si (or C) has

been uniform rather than Ti-Al based metal. These results indicates that Si and C are Ohmic material

for forming nitrogen vacancy like donors in high thermal treatment without depending on

dislocations.

Reference

[3.1] S. Tanuma, C.J.Powell and D.R. Penn, “Calculations of electron inelastic mean free paths .IX.

Data for 41 elemental solids over the 50eV to 30 keV range”, Willey Online Library, 2010.

48

Chapter4

TiSi2 electrode

4-1 Introduction.........................................................................................49

4-2 Electrical characteristic of TiSi2 electrode..........................................49

4-3 XPS analysis........................................................................................51

4-4 Dependence on Annealing time..........................................................56

4-5 TEM and EDX analysis......................................................................57

4-6 Conclusions..........................................................................................59

49

4-1 Introduction

Silicon was deposited with Titanium and TiSi2 was selected as a contact electrode. Current-Voltage

characteristics and physical analysis have been measured by TLM patterning in this chapter. TiN (50

nm) layer was deposited as a cap layer to prevent electrode from oxidation.

4-2 Electrical characteristic of TiSi2 electrode

Fig 4.1 shows Current-Voltage characteristics across a 300-μm gap. The electrode area is 80 m×

190 m and thickness of TiSi2 film is 20 nm. As shown in Figure 4.1, almost total current increase as

the annealing temperature increases from asdepo to 1100oC. Ohmic contact to AlGaN/GaN substrate

has been realized as the annealing temperature is 950oC~1100oC. Maximum current flew in annealed

at 1075oC. Compared with conventional Ohmic contacts (700~950oC), the process window is narrow

and the annealing temperature is high.

Figure 4.1 Current-Voltage characteristic of TiSi2/TiN electrode with different annealing temperature

50

Figure 4.2 shows contact resistance of some samples with different annealing temperature. The

minimum contact resistance is 0.4 mmwith 1075 oC annealing for 1min. Contact resistance is 0.4

mm. Sheet resistance of 2DEG is almost the same regardless of annealing temperature, as shown

in Figure 4.3. Therefore, specific contact resistance c is 4.9×10-6cm2. Therefore, contact

resistance for TiSi2 is a little higher than conventional Ti/Al/Mo/Au electrode.

Figure 4.2 Current-Voltage characteristic of TiSi2/TiN electrode with different annealing temperature

51

Figure 4.3 2DEG sheet resistance with different annealing temperature

4-3 XPS analysis

The reaction of TiSi2 with AlGaN layer was characterized by hard x-ray photoelectron spectroscopy

(HXPES) with an x-ray energy of 7940 eV. TiSi2 layer of 10 nm in thickness was deposited on the

AlGaN/GaN substrate with 1075oC annealing and without annealing. As the inelastic mean free path

for photoelectrons Al 1s is 12 nm, all the photoelectrons arising from AlGaN layer can be collected.

Assuming a potential profile of ψ(z), where z is the distance from the surface of the film or bulk, the

total photoelectrons at a binding energy (BE), J(E), from a core level can be written as

dzzEEIeEJ

z

0

0

sin sin)(

where I(E−E0) is a typical spectrum with a peak energy of E0, , , and E0 are the inelastic mean free

path (IMFP), take-off angle measured from the sample surface and BE of electrically neutral region,

respectively. In the case of germanium substrate, potential profiles of depletion condition are

approximated as

52

2

21)(

z

qNZ

sAlGaN

bs

where ψs, εAlGaN, q, and Nb are the surface potential at z = 0, the permittivity of AlGaN, the

electronic charge, and the concentration of either donor or acceptor, respectively [4.1]. The core

energy level also bends along with the potential profile at the surface. Therefore, the measured

spectrum consist of photoelectrons with different peak energies as shown in Figure 4.4

Deconvolution of the spectrum enables to extract the bending profile in the substrate as well as the

surface potential.

Figure 4.5 and Figure 4.7 show the fitting Al 1s spectrum of TiSi2/AlGaN with and without

annealing at 1075oC respectively. Figure 4.6 and Figure 4.8 show deconvolution of AlGaN substrate

with and without annealing at 1075oC respectively. The BE of the peak intensity along with the

depth shifts to higher energy direction owing to the bend down, as expected by 0.27 eV. A clear shift

toward higher binding energy indicates downward band bending of the substrate to increase the

surface potential, which implies active donors like nitrogen vacancy in AlGaN layer. Active donor

concentration donor depth estimated from fitting data are 1.4×1018 cm-3, 4.7 nm respectively. The

band diagram of TiSi2/AlGaN/GaN was described in Figure 4.9 from these analysis.

53

Figure 4.4 Band bending of AlGaN surface result in a spectra broad by deconvolutionof the spectrum,

the bending profile as well as the surface potential can be derived.[4.1]

Figure 4.5 Experiment and fitting data of Al 1s spectrum (asdepo)

54

Figure 4.6 The fitting result of the Al 1s spectrum (asdepo)[4.2]

Figure 4.7 The fitting result of the Al 1s spectrum (asdepo)

55

Figure 4.8 The fitting result of the Al 1s spectrum (asdepo)

Figure 4.9 The fitting result of the Al 1s spectrum (asdepo)

56

4-4 Dependence on Annealing time

Contact resistance was examined with different annealing time from 1min to 50min at 1075oC, at

which the minimum contact resistance (0.4 mm) was achieved for 1min. However, contact

resistance became lower with longer annealing time from 1min to 5min, contact resistance became

higher from 10min to 60min, as shown in Figure 4.10. The minimum contact resistance is 0.53 mm

for 5min. Considering that both of metal sheet resistance and 2DEG sheet resistance is almost the

same from 10 min to 50 min, there would be incomprehensible problem that contact resistance

increased with longer annealing time (Figure 4.11). This problem was described in this subsection

4-5.

Figure 4.10 Contact resistance and metal sheet resistance with annealing time at 1075oC

57

Figure 4.11 2DEG sheet resistance with annealing time

4-5 TEM and EDX analysis

TEM image at the interface between TiSi2 and AlGaN substrate was shown in Figure 4.12 and

Figure 4.13. The sample of TEM image was annealed at 1075oC for 50 min for N2 ambient. One can

observe a flat surface of TiSi2 and a formation of uniform interface layer at TiSi2 and AlGaN layer.

Composition of interface layer consist of Ti, Si, Al, O and N. Interface layer including O atoms

would be reason that contact resistance with longer annealing time was increased.

Moreover, thickness of AlGaN layer would be confirmed to be decreased from 25 nm to 15.3 nm.

58

Figure 4.12 Cross sectional TEM image of TiSi2/TiN electrode at 1075 oC annealing (×550,000)

Figure 4.13 Cross sectional TEM image of TiSi2/TiN electrode at 1075 oC annealing (×2,550,000)

59

Figure 4.14 (a) Cross sectional TEM image of TiSi2/TiN

(b),(c) EDX spectra of TiSi2 and AlGaN at dislocation (*1) and AlGaN (*2).

Figure 4.14(b), (c) show EDX analysis of point 1 and point 2 which are indicated Figure 4.14(a). As

there is little difference between both spectra and also no intrusion of Ti and Si atoms has been

observed. Therefore, TiSi2 electrode could be no-independent-dilocation contact for AlGaN/GaN

structure.

4-6 Conclusion

Ohmic contact has been achieved by using TiSi2 electrode. From XPS analysis, the reaction of TiSi2

and AlGaN with high thermal treatment causes VN in AlGaN layer. Donor concentration is enough

high for obtaining Ohmic characteristic. Cross-sectional TEM image of TiSi2/AlGaN shows that

interface layer is formed uniform rather than conventional Ti-Al based electrode. Therefore, TiSi2 is

a candidate for contact material for AlGaN/GaN structures independent of dislocation density of

epi-wafers.

60

Reference

[4.1] A.Y.C. Yu, “Electron tunneling contact resistance of metal-silicon contact barriers”,Solid-State

Electronics,13, p.230-p.247, 1970.

[4.2] K. Kakushima, K. Okamoto, K.Tachi, K. Tachi, S.Sato, T. Kawanago. K. Tsutsui, N.Sugii, P.

Ahmet, T. Hattori and H. Iwai, “Observation of band bending of metal/high-k Si capacitor with high

energy x-ray photoemission spectroscopy and its application to interface dipole measurement”

Journal Applied Physics, 104, 104908, 2008.

61

Chapter5

TiC electrode

5-1 Introduction........................................................................................62

5-2 Curent-Voltage characteristic ...........................................................62

5-3 TEM analysis.....................................................................................63

5-4 Dependency of Ti ratio for TiC electrode..........................................65

5-5 TEM analysis for TiC electrode.........................................................68

5-6 Conclusion..........................................................................................74

References

62

5-1 Introduction

Carbon was deposited with Titanium and TiC was selected as a contact electrode. Current-Voltage

characteristics and physical analysis have been measured by TLM patterning in this chapter. As a

reference electrode, Ti/TiN (20/50 nm) and C/TiN (20/50 nm) were deposited on AlGaN/GaN

substrate. TiN (50 nm) layer was deposited as a cap layer to prevent electrode from oxidation.

5-2 Current-Voltage characteristics

TiC/TiN (20/50 nm) is stacked as electrodes by RF sputtering, and Current-Voltage characteristics

have been measured by TLM patterning with different annealing temperatures in N2 ambient for

1min. Figure 5.1 shows Current-Voltage characteristics across a 300-μm gap. The electrode area is

190m×80m. As shown in Fig 3.1, Total current increase as the annealing temperature increases

until 1025oC. Ohmic contact to AlGaN/GaN substrate has been realized as the annealing temperature

is between 950oC and 1025oC. However, Ohmic contact was lost as the annealing temperature is

1050oC. It means that the contact between electrodes and substrates became schottky barrier junction.

Compared with conventional ohmic contacts (700~950oC), the process window is so narrow and the

annealing temperature is high. The lowest contact resistance is 18 mm and specific contact

resistance is 6.1×10-3 cm2 with the sample annealed at 1050 oC for 1 min. The value of contact

resistance is much higher than conventional Ti/Al/Mo/TiN electrode.

63

Figure 5.1 Current-Voltage characteristics across a 300-μm gap (TiC/TiN)

5-3 TEM analysis of TiC electrode

Figure 5.2 shows cross-sectional TEM image of TiC/TiN electrode that was stacked by RF

sputtering and annealed at 1025oC for 1min. Thickness of AlGaN layer was decreased from 30 nm to

14 nm with high annealing temperature. As shown in Figure 5.3, interface layer was formed, which

is independent of dislocation. There is a problem that annealing temperature for achieving Ohmic

contact is so high.

64

Figure 5.2 TEM image of TiC/TiN annealed at 1025oC (×500,000)

Figure 5.3 Enlarged TEM image of TiC/TiN annealed at 1025oC (×500,000)

65

5-4 Dependency of Ti ratio for TiC electrode

Dependency of contact resistance has been measured by depositing different Ti/C ratio. Figure 5.4

shows structures of electrodes deposited by RF sputtering. Six kinds of TiC electrode were prepared

with various Ti ratio, which are 50 %, 75 %, 83.4 %, 87.5 %, 90 % and 95 %. The thickness of Ti is

0.8 nm with 50 % Ti ratio: type(a), 2.4 nm with 75 % Ti ratio: type(b), 4.0 nm with 83.4 % Ti ratio:

type(c), 5.6 nm with 87.5 % Ti ratio: type(d), 7.2 nm with 90% Ti ratio: type(e) and 15.2 nm with

95% Ti ratio: type(f). In spite of each Ti ratio for TiC electrode, constant thickness of C is 0.45 nm.

Then Current-Voltage characteristics are measured after annealed at different temperatures (800o

C~1050oC) in 1min by using TLM patterning.

Figure 5.4 Structure of electrode with different Ti ratio

66

Figure 5.5 Contact resistance as a function of annealing temperature with different Ti ratio (a) ~ (f)

Figure 5.6 Contact resistance as a function of annealing temperature

with different Ti ratio (a), (b), (e), (f)

67

Figure 5.7 Contact resistance as a function of Ti ratio at1075oC

Figure 5.5 and Figure 5.6 shows relationship between contact resistance and different annealing

temperature. Ti/Al/Mo/TiN (15/60/35/50 nm) electrode was prepared as a reference contact

electrode. There is a tendency that contact resistance and annealing temperature for Ohmic

characteristic is decreased with higher Ti ratio for TiC electrode. Annealing temperature for Ohmic

contact is 1000oC for 50 % Ti ratio, 900oC for 75 and 83.4% Ti ratio, 850oC for 87.5% and 90%,

825oC for 95% Ti ratio.

There has also tendency that lowest contact resistances with each Ti ratio are indicated in annealed at

1025oC. Therefore, Figure 5.7 shows contact resistance with each Ti ratio annealed at 1025oC for

1min. The minimum contact resistance for TiC electrode with 90 % Ti ratio is 0.03 mm and

specific contact resistance is 2.1×10-7 cm2 with 1025oC, which is less than that of conventional

Ti/Al/Mo/TiN electrode (Rc 0.26 mm, 6.14×10-6 cm2).

In terms of contact resistance, TiC electrode with higher Ti ratio can be competitive Ohmic contact,

because the contact resistance of common electrode is about 10-6~10-7cm2. Though, the annealing

temperature for Ohmic contact is a little higher than common electrode (800oC~).

68

5-5 TEM analysis for TiC with 90% Ti ratio

Here, TiC means that TiC electrode with 90% Ti ratio. Figure 5.7 shows the cross-sectional TEM

image of TiC without annealing and Figure 5.8 shows the magnified TEM image of the sample at

crystal defect. In the following figures likewise, Figure 5.9 and Figure with 825 oC annealing, 5.10

and Figure 5.11 with 850 oC annealing and Figure 5.12 Figure 5.13 Figure 5.14 with 1025oC

annealing. As shown in magnified TEM images with all annealing temperature ranges, interface

between TiC and AlGaN is almost flat. TiC with 90% Ti ratio can be independent-of-dislocation

electrode for AlGaN/GaN structure, as well as TiC with Ti 50 % ratio.

69

Figure 5.7 TEM image of with90 % Ti ratio without annealing (×200,000)

Figure 5.8 Magnified TEM image of TiC electrode at defect site without annealing (×2,000,000)

70

Figure 5.9 TEM image of TiC electrode with 825oC annealing (×200,000)

Figure 5.10 Magnified TEM image of TiC electrode at defect with 825oC annealing (×200,000)

71

Figure 5.11 TEM image of TiC electrode at defect with 850oC annealing (×200,000)

Figure 5.12 Magnified TEM image of TiC electrode at defect with 850 oC annealing (×2,000,000)

72

Figure 5.13 TEM image of TiC electrode at defect with 1025 oC annealing (×200,000)

Figure 5.14 TEM image of TiC electrode at defect with 1025 oC annealing (×2,000,000)

73

Figure 5.15 Comparison of AlGaN thickness of TiC electrode

Figure 5.15 shows the cross-sectional TEM image of TiC electrode with different annealing

compared with AlGaN thickness. Decrease in AlGaN thickness was observed with higher annealing

temperature. However there is little different thickness between 825oC and 850oC annealing, Ohmic

characteristic hasn’t been achieved at below 825oC. From this comparison between 825oC and 850oC,

there is no relationship between AlGaN thickness and Ohmic characteristic.

Then interface layer, whose thickness is 29 nm, was confirmed at 1025oC and it should be noted that

contact resistance with 1025oC annealing is lower than any other annealing temperature and also Ti

ratio for TiC electrode. This means that existence of interface layer is related to lowering contact

resistance. However, the detail analysis of interface layer was not measured, the correlation cannot

be detected.

74

5-6 Conclusion

Tic electrode can be independent-of-dislocation Ohmic contact in AlGaN layer from the TEM

image. Higher Ti ratio for TiC electrode has reduced contact resistance and annealing temperature

for obtain Ohmic characteristic. The minimum contact resistance is 0.17 mm, specific contact

resistance is 2.1×10-7cm2 for TiC with 90% Ti ratio in 1025oC annealing. Therefore especially

focused on TiC with 87.5% Ti ratio, cross sectional TEM image of electrode was observed. Then,

AlGaN thickness wasn’t related with Ohmic characteristic, compared with AlGaN thickness with

each annealing temperature. There is no visible changes between 825oC and 850oC, so nitrogen

vacancy at 850oCis higher than it at 825oC. This assumption suggested that Ohmic contatact can be

achieved by using TiC electrode annealed at 850oC because of enough for nitrogen vacancy with

850oC annealing.

75

Chapter6

Conclusions

76

(1) Introduction

TiC and TiSi2 was used as an Ohmic contact material and examined by electrical analysis and also

physical analysis in this study.

(2) Thermodynamic prediction

Ohmic contact has been realized by using TiSi2 electrode and TiC electrode. As TiSi2 electrode,

annealing temperature for achieving Ohmic contact is 950oC~1100oC. The minimum contact

resistance is 0.4 mm with 1075oC annealing for 1min. In the case of TiC electrode (including all

ratio of Ti), annealing temperature for Ohmic contact is 825oC~1050oC. Contact resistance

decreased with higher proportion of Ti and the minimum contact resistance is 0.17 mm at 1025oC

annealing for 1min.

Here, thermodynamic prediction was indicated below an equation of the reaction between AlGaN

and TiSi2 or TiC electrode. These are exoergic reaction equations as follows.

molkJGaAlNSiTiNTiSiNGaAl /6.84

3

4

1

11

2

11

3

11

343275.025.0 (6.1)

molkJGaAlCNTiNTiCNGaAl /7.1902

3

2

12 75.025.0 (6.2)

From the result of these equations and XPS analysis additionally, both TiSi2 and TiC electrode can

be Ohmic contact material, which extract N atoms from AlGaN.

(3) Schematic illustration

As shown in schematic illustration of TiSi2 electrode, thickness of AlGaN was reduced to 15.3 nm

and donor concentration was 1.4×1019cm-3 to 4.7 nm from the surface AlGaN. However contact

resistance is competitive with conventional electrode (10-6~10-7cm2), annealing temperature for

Ohmic characteristic is much higher than conventional electrode. Therefore annealing temperature

77

should be lower.

Interface layer between TiSi2 and AlGaN was confirmed at 1075oC for 1min. The composition of

interface layer is Si-O-N. This means that N atoms were extracted by TiSi2 electrode with high

temperature annealing. Additionally, thickness of AlGaN is also observed. From these results,

conditions for obtaining Ohmic characteristic would be Si-O-N interface layer or recess thickness of

AlGaN.

Figure 6.1Illustration of TiSi2 electrode

Ohmic

Interface layer

TiC

1025oC

TiCTiC TiC

asdepo825oC 850oC 1025oC

TiC (Ti :87.5%)

TiC (Ti :50%)

Interface leyaer

TiC

Figure 6.2 Illustration of TiC electrode with different Ti ratio and annealing temperature

78

As shown in schematic illustration of TiC electrode, AlGaN thickness was also decreased with

higher annealing temperature. However there is little different thickness of AlGaN between 825oC

and 850oC, Ohmic contact has been achieved at only 850.oC. Considering this result, this means that

achieving Ohmic characteristic is not related with AlGaN thickness. Therefore concentration of

nitrogen vacancy with 825oC annealing might be higher than that with 850oC annealing.

Compared with the sample with 850oC and 1025 oC, less AlGaN layer and interface layer above TiC

electrode was confirmed. Contact resistance with 1025oC annealing is 0.17 mm and is lower than

that with 850oC annealing. This means that thickness of AlGaN needs to be decreased to lower

contact resistance. On the other hand, little difference between TiC with 50% Ti ratio and 90% Ti

ratio was observed, which means that thickness of AlGaN and interface layer are almost the same.

But contact resistance with 1025oC annealing for TiC with 90% Ti ratio is lower than it for 50 % Ti

ratio. Therefore, concentration of nitrogen vacancy for TiC with 90% Ti ratio is higher than it for

TiC with 50% Ti ratio. Increasing nitrogen vacancy is also necessary for lowering contact resistance.

In conclusions, TiSi2/TiN and TiC/TiN electrodes can be Ohmic contact for AlGaN/GaN structures.

However, the minimum contact resistance of both TiSi2 and TiC electrode competed with common

electrode’s minimum contact resistance, annealing temperature is higher. As a TiC electrode,

annealing temperature to be Ohmic characteristic is lower with higher Ti ratio, but minimum contact

resistance is made at 1025oC. So there is a problem that annealing temperature is much higher than

common electrode, which should be solved. But TiSi2 and TiC electrode can be

independent-of-dislocation contact, so TiSi2 and TiC electrode can be Ohmic contact electrode for

the future substrate with less dislocation.

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Acknowledgements

I would like to express my appreciation for my supervisor Assoc.Prof. Kuniyuki Kakushima and

Prof. Hiroshi Iwai because they gave me his continuous encouragement and helpful advice for my

study.

Also I am deeply grateful to Prof. Yoshinori. Kataoka, Prof. Akira. Nishiyama, Prof.

Nobuyuki.Sugii, Prof. Hitoshi. Wakabayashi, Prof. Kazuo. Tsutsui, and Prof. Kenji. Natori for

advices and great help.

This study was supported by Toshiba corp., and I would like to express my gratitude to Dr. Wataru.

Saito, who is a researcher of Toshiba Corp. Semiconductor Company, for constructive advices at

regular meetings.

I thank to my colleagues of Kakushiam/Iwai Lab. for supporting my study.

Without their guidance and persistent help this thesis would not have been possible.

I would like to appreciate the support of secretaries, Ms. Nishizawa and Ms. Matsumoto.

Besides, I want to thank my parents, grandfather, grandmother, and my sister for their tremendous

supporting.

Mari Okamoto

Feburary, 2015