1

Department of Electrical Engineering, University of South Carolina,asif@engr.sc.edu

Work supported by Army, Navy, DARPA and NASA

Outline1. Why III-Nitrides2. Material Requirements and Issues3. Substrate Technology4. Thick Film Deposition (HVPE)5. MOCVD Growth6. MEMOCVD-Digital Epitaxy7. Ternary and Quaternary Digital Epitaxy8. Lateral Epitaxial Overgrowth9. Devices and Conclusions

By : Prof. Asif Khan,

CVD Deposition of Group-III Nitride Materials

2

III-N Materials

InNGaNAlN ?

Breakdown Field approximately 5-10 times of GaAs

3

tle

Nichia GaN Blue LEDs

4

Lighting and Display Technologies

GaAs 1980s GaN 1990s

Past and the Present

Lighting

Displays

5

Next Generation Lighting Systems

• Low voltage (< 10 V), Digital Controls• High efficiency, Lifetime > 2 years

6

Purifiers$ 20 billion*

Air

Water

White Lighting $ 10 billion*

Outdoor

Indoor

Bio-Med Sensors$ 10 billion*

Fixed

Portable

ApplicationsDeep Ultraviolet Light Emitting Diodes

* Strategies unlimited , Compound Semiconductors

λ ~ 254 nm λ ~ 265 nm λ ~ 280 nm

7

Technology ApplicationsSilicon GaAs GaN

Power: < 0.1 W/mm 1 W/mm 5-10 W/mmOperation T: 80 C 125 C > 300 C

5 mm

Photonics

Electronics

III-N Materials

8

sapphire

n+-AlGaN barrier

AlN/AlGaN SL

Ti/Al/A

u AlGaN MQW

AlN Buffer

p-AlGaNp+-GaN

Ni/Au

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

0.3 µ m

3 µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G D

µ m

µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

0.3 µ m

3 µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G D

µ m

µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

0.3 µ m

3 µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G D

µ m

µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

0.3 µ m

3 µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G D

µ m

µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

0.3 µ m

3 µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G D

SiO2µ m

µ m

I - SiC

Pd/Ag/Au

GaN

AlN

Pd/Ag/AuAlGaN

S G DSiO2

m

µ m

I-SiC

Pd/Ag/Au

GaNAlN

Pd/Ag/Au

n-AlGaN

S G D

LED Microwave Transistors

III-N Device Epilayer Needs

•N- and p-doped Layers•Heterojunctions•Quantum Wells and Superlattices

LEDs and Transistors

Choice of Substrates

Substrate Lattice constant (Angstroms) at 300 K

Thermal Conductivity W/cm-K at 300 K

Thermal expansion coefficient (10-

6 1/K) at 300 K

Bandgap (eV)

GaN a = 3.188 c = 5.185

2.0 3.1 (ave. 300 to 3.5 800 K)

3.39

AlN a = 3.112 c = 4.982

3.2 (c-axis) 2.30 2.69

6.2

6H SiC a = 3.081 c = 15.117

4.9 (a-axis) 2.9 2.9

3.03

4H-SiC a = 3.080 c = 10.082

~3.7 ~2.8 ~2.8

3.26

Sapphire a = 4.765 c = 13.001

0.35 (c-axis) 5.9 6.3

9.9

Si a = 5.4301 1.56 2.57 1.1 GaAs a = 5.6533 0.54 5.8 1.42

-0.160.920.380.38e14, C/m2

850090013510003200µn, cm2/V-s

1.418.22.866.23.390.89EG, eV

5.654.773.083.113.19 3.54 a, A

GaAsAl2O36H-SiC

AlNGaNInN

• No lattice matched substrate• Large polarization effects

Nitride Materials and Possible Substrates

Pd/Ag/Au

Al2O3

n+-AlGaNn-AlGaNn-AlGaNp-AlGaNp-GaN

100 µm

t = 1-5 µm

100 µ

m

LED

Bulk growth of GaN: direct synthesis

Melting conditions of semiconductors (without dissociating)

Equilibrium curve for GaN

Bulk crystal of GaN, grown at 10 – 20 Kbar, and 1400 – 1600 °C without seed, along the 10-10 direction). Squares grids have 1 mm sides

2Ga (melt) + N2 = 2GaN

AlN sublimes dissociatively at the hotter source and condenses reversibly at the colder seed

Sublimation Growth of AlN

15 mm Diameter AlN Boule

Courtesy of Crystal IS

Bulk AlN PVT

Bulk Crystal Growth Facilities

Crystal Growth

GRINDING

CUTTING

(2" dia. ingot )

SiC CRYSTALS

(0001)

(1120)

Halide Vapor Phase Epitaxy (HVPE)

Epitaxial Nitride Films by HVPE

! Gallium transport by halide (chloride) formation

2HCl(g) + 2Ga(l) → 2GaCl(g) +H2(g) T = 800 oC

! Reaction with chloride to form the nitride

GaCl(g) + NH3(g) → GaN(s) + HCl(g)+ H2(g); T = 1030 oC

• Growth rate is determined by HCl flux

• High growth rates are possible due to low probability of gas phase nucleation

• Growth rates can exceed 100 mm/min

HVPE GaN

B. Monemar, J Crystal Growth, Vol. 208, p. 18, 2000

Defect Reduction with Thickness

Defects in HVPE GaN Films

Cross section of pit with crack Cross section of crack

Surface with small pits Featureless surface

Free Standing GaN Wafer by HVPE

R. Vaudo, ATMI

Tom Kuech

Growth of AlN & AlGaN by HVPE

MOCVD growth system

NH3

bis-Mg

Various problems associated with mismatches

1.Lattice (a-lattice constant) mismatch

2.Vertical (c-lattice constant mismatch)

3. Coefficient of thermal expansion mismatch

4. Low thermal conductivity

5. Different chemical composition of the epitaxial film

6. Polar surface

Substrate Property Consequence

1. All problems typically associated with high dislocation density

2. Anti-phase boundaries, inversion domain boundaries

3. Thermally induced stress, cracks in epitaxial films

4. Poor heat conduction; unsuitability for high power devices

5. Contamination, interface states, poor wetting of surface during growth

6. Mixed polarity; inversion domains

MOCVD III-N growth issues

Strain/thermal mismatch

Lattice mismatched Substrates

Growth Temperature compatibility

• InN 600 C• GaN 1000 C• AlN 1150 C

SLs buffered

SLs Strain-management for crack-free AlGaN growth

Conventional-MOCVD

MOs

NH3

Surface roughening

Pre-reaction!!!

gas-phase reaction and low surface migration

Growth front

Growth steps

III-N Conventional MOCVD growth

ProblemNative substrates

availablity

Heteroepitaxy

Technology in infancy

Thermal mismatch

resulted cracking

Lattice mismatch induced huge surface

energy difference results in random island

growth

Analogy tomercury on glass

Low T amorphous

AlN/GaN/InNbuffer developed

t

T or flow

T

NH3

MOs

Annealing/nitridation

LT-buffer

growth

Well-established procedure for III-nitrides on sapphire

GaN on Sapphire substrate

! The lattice mismatch with GaN is 13.9%

! The steps for GaN growth includes: (a) Nitridation and (b) low temperature buffer layer (usually AlN) growth

! Growth on c-plane of sapphire gives c-plane GaN, while growth on r-plane gives a-plane GaN

! Energy gap of sapphire is > 8eV so light extraction possible from substrate side for LEDs

Growth steps of GaN on sapphire

Microstructure of GaN on Sapphire

Standard MOCVD vs MEMOCVD

0 10 20 30 40 50 60 70 80 90 100

MOCVD precursor scheme

on

onMOs

NH3

Time0 10 20 30 40 50 60 70 80 90 100

on

off

high

lowNH

3

MOs

MEMOCVD precursor scheme

time

* Gas-phase prereaction

* Low surface mobility* Precursors overlap adjustable * Minimize prereaction and enhance

migration* AlInGaN digital alloys and SLS

Custom MEMOCVD system for III-Nitrides

MOCVD system contains:Vacuum system

Gas delivery system

Control systemHeating system

MOs

NH3

MEMOCVD of III-N Materials

Minimized pre-reaction

Enhanced migration

allowing V/III separation hence reducinggas-phase reaction & enhancing surface migration

Growth front

Growth steps

MEMOCVD AlN/ALGaN SLs-complex

g=0002

AlGaN

AlN

AlN

AlGaN

Deep UV LEDs (250-280 nm)X-ray spectra of MEMOCVD AlN/AlGaN SL buffer

25 30 35 40 45

Sapp

hire

(006

)su

b-SL

+1

sub-

SL-2

sub-

SL-1 su

b-SL

0

2θθθθ-ωωωω ( o )

Inte

nsity

(a.u

)

Inte

nsity

(a.u

)

2θθθθ-ωωωω ( o )

35 36 37

HQAlN

SL+2

SL0SL-1

SL-2

SL-3

SL+1

N-AlGaN on MEMOCVD AlN+SLs buffer:Al0.66Ga0.34N for sub-260nm LEDs

RMS=7A with atomic steps

SapphireAlN

AlN

AlGaN

N-AlGaNHigh etch pits density: 1010 cm-2

MOCVD vs MEMOCVD AlN

TMA

NH3

TMA

t

Conventional MOCVD

on

on

t

NH3

PALEon

offonoff

PALE AlNMOCVD AlN

MEMOCVD AlInGaN digital alloys

0 10 20 30 40 50 60 70

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

TMG

TMI

NH3

TMA

Time (second)

The number of repeats of Al, In, and Ga pulses in the unit cell are 2, 2, and 1, respectively. Pulse length is kept as 6 seconds.

A typical EDAX spectrum for our AlInGaN samples

EDAX In fraction as a function of m, In pulses within one growth unit cell for (3,m,1)

MEMOCVD AlInGaN digital alloys Composition control

MEMOCVD AlInGaN Digital Alloy PL

260 280 300 320 340 360 3800.0

0.5

1.0

1.5

2.0

2.5

(2,2+,1)150

(1,3+,1)150

(3,1,1)150

GaN

PL

inte

nsity

(a.u

.)

Wavelength (nm)

MEMOCVD AlInGaN MQWs 4x(2,2++,1)30 /(1,3++,1)10

GaN

Al2O3

AlInGaN(2,2++,1)AlInGaN(1,3++,1)AlInGaN(2,2++,1)AlInGaN(1,3++,1)AlInGaN(2,2++,1)AlInGaN(1,3++,1)AlInGaN(2,2++,1)AlInGaN(1,3++,1)AlInGaN(2,2++,1)

Quaternary Digital SuperlatticesXRD Spectra

The majority of dislocations in GaN result from the coalescence of misoriented islands

• Dislocations can interact and be annihilated

Stimulated Emission at 258 nm in AlN/AlGaN Quantum Wells Grown on Bulk AlN Substrates

PLOG GaN(IĪ0I)

(IĪ00)(II20)

(1101)(1120)

MOCVD GaN

Figure 2(a)

PLOG GaN

SiO2

(1120)

(1100)

10µm

Completely coalesced PLOG GaN

Pulsed Lateral Overgrowth (PLOG)Different Pulse time for NH3 on and Off

SiC

SiO2

Window region Mask region

2 µm

Low dislocation density

High dislocation density

Pulsed Lateral Overgrowth (PLOG)TEM X-section Image

Complete coalescence of Complete coalescence of GaN GaN by by lateral overgrowth methodlateral overgrowth method

Plane view Cross sectional view

Coalesced film

Surface roughness of PLOG Surface roughness of PLOG GaNGaN

RMS roughness

PLOG GaN = 7-10 Å

* No step termination observed

* Reduction of screw component threading dislocation

05

100150

200

60nm

µm

Device Design

SiO2 mask

Edge Emitting UV LEDs

a-GaN (ELOG)or AlN or c-GaN

R-sapphire / bulk AlN /HVPE GaN

n+(a-GaN) / AlN / GaN

n-AlGaNMQW

p-AlGaNp+-GaN

Non Polar III-N Device Development

R-Sapphire

a-GaN template

a-GaN ELOG

1 st SiO2 mask

2 nd SiO2 mask SALE a-GaN

Approach 2: Selective Area Lateral Epitaxy (SALE)

Step 1. a-plane GaN pillar on R-plane Sapphire

SEM image of fully coalesced

SALE a-plane GaN layer

Non Polar III-N Device DevelopmentSelective Area Lateral Epitaxy (SALE)

Step 2. a-plane GaN pillar after SiO2 deposition

10 µµµµm

a-plane GaN Template, ELOG, SALE

Template:RMS ~ 50 A

ELOG:RMS ~ 4.5 A

SALE:RMS ~ 4.5 A

RMS surface Roughness

X-Ray Rocking Curve Comparison a-plane GaN

a-plane GaN Template, ELOG, SALE

362 nm Peak Emission LED over r-sapphire

Edge Emitting Non Polar UV LEDs

300 350 400 450 500 550 600

100 mA

50mA

20mA

Inte

nsity

, a.u

.

Wavelength, nm

0 2 4 6 8 100

10

20

30

40

50

Rd = 65 ΩΩΩΩ

Cur

rent

, mA

Voltage, V

C. Chen et. al. Jpn. J. Appl. Phys., 42, Part 2, No. 9A/B, pp. L1039-L1040 (2003).

SALE a-plane GaN cavity

]0211[

]1001[

[0001]

N2-laser beam path

Lasing emission

364 366 368 370 372 374 376 378 380

0

200

400

600

800

1000 T = 300 K

270 kW/cm2

330 kW/cm2

400 kW/cm2

500 kW/cm2

Inte

nsity

, a.u

.

Wavelength, nm

Edge Emitting Non Polar UV Laser

Deep UV LEDs (250-280 nm)I-V and Spectral Emission

200 300 400 500

100 µµµµm x 100 µµµµm

Inte

nsity

, a.u

.

Wavelength, nm

0 2 4 6 8 100

10

20

30

40

50

255nm90ΩΩΩΩ

265nm40ΩΩΩΩ

280nm 30ΩΩΩΩ

Cur

rent

, mA

Voltage, V

Deep UV LEDs (250-280 nm)Pulsed powers

280 nmEQE 0.355%WPE 0.122%

265 nmEQE 0.217%WPE 0.107%

0 200 400 600 800 1000 1200 1400 16000

5

10

15

20

25

280 nm267 nm

Northwestern

265 nm

280 nm

500ns, 0.5%

2 x 2 array of 100 µµµµm x 100 µµµµm

Po

wer

, mW

Current, mA

Dr. Hongmei WangDr. E. KuokstisDr P. MaruskaDr. Mikhail Gaevski

Dr. Grigory SiminDr. Alex KoudymovDr. Max Shatalov

RESEARCH

TEAM

Dr. Jinwei YangDr. Wenzhong SunDr. Changqing ChenDr. C. Kim

Dr. Vinod AdivarahanDr. Ashay ChitnisMs. Irina MokinaDr. Y. LeeDr. H. Cho

StudentsMs. Husna FatimaMs. Gong ZhengMr. Ping HuangMr. M. IslamMr. Salih SaygiMr. Wenhua GuMr. J. WangMr. J. LiMr. Shiva RaiMr. Shuai WuMr. A. SattuMr. Z. YangMr. N. Tiperneni

Professor Asif KhanPML Integrated AlGaN Research Team

SupportMr. Ahmad HedariMr. Bin ZhangMr. D. JohnsonMs. Quinhua ZhangMs. T. OsborneMs. Pat Dedman

Matl. Growth

Matl. Test

Device Process

Device Test

Scientist

PML

Photonics Microelectronics Lab 20,000 sq. ft. class 100 clean rooms

Materials

Matl. Testing

Lithography Device Process

Device Package

Electrical TestOptical Test