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GaN Schottky Barrier Photodetectors with a -Ga2O3 Cap Layer

Zhen-Da Huang1, Ricky Wenkuei Chuang1;2;3, Wen-Yin Weng1,

Shoou-Jinn Chang1;2, Chiu-Jung Chiu1, and San-Lein Wu4

1Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan2Advanced Optoelectronic Technology Center (AOTC) and Center for Micro/Nano Science and Technology,

National Cheng Kung University, Tainan 70101, Taiwan3National Nano Device Laboratories (NDL), Tainan 74147, Taiwan4Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 833, Taiwan

Received August 24, 2012; accepted September 25, 2012; published online October 18, 2012

GaN Schottky barrier UV photodetectors (PDs) with a-Ga2O3 cap layer realized by furnace oxidation of GaN epitaxial layer were fabricated and

characterized. With the cap layer inserted, it was found that the reverse leakage current could be reduced by more than 4 orders of magnitude and

the UV-to-visible rejection ratio increased by 21 times. When compared with the conventional GaN PDs, incorporating an additional -Ga2O3 cap

layer helps to reduce the noise level and at the same time achieve a larger detectivity. # 2012 The Japan Society of Applied Physics

In the last few years, various configurations of GaN-based

photodetectors (PDs) have been demonstrated.14) When

compared with other bipolar-based PDs, Schottky barrier

PDs are relatively easier to fabricate and their response speedis also faster.5) However, the leakage current in Schottky

barrier PDs is comparably higher for a given applied bias.

Due to the large mismatches in lattice constant and thermal

expansion coefficient between GaN and sapphire,6,7) the

threading dislocation (TD) density in the epitaxial layer is

inevitably high for GaN grown on sapphire. These TDs could

function as leakage current paths at the metal/GaN interface,

which are primarily responsible for the high leakage current

typically observed in GaN-based Schottky barrier PDs.8) The

leakage current could be reduced by inserting an insulator

between GaN and metal. It has been reported that SiO2,

Ta2O5, Ga2O3, and Al2O3 can serve as the cap insulator for

nitride-based metalinsulatorsemiconductor devices.912)

In this study, -Ga2O3 is used as an insulator layer due to

its comparably larger bandgap energy in a range of 4.8

4.9 eV and higher resistivity. Various methods had already

been proposed to grow-Ga2O3 thin films, including pulsed

laser deposition (PLD), atomic layer deposition (ALD), metal

organic chemical vapor deposition (MOCVD), solgel proc-

ess, sputtering, and photoelectrochemical oxidation.1318)

In this study, we propose instead to modify the growth of

-Ga2O3 thin film by oxidizing the GaN epitaxial layer in a

furnace filled with oxygen at high temperature to convert part

of the GaN into -Ga2O3. When compared with the other

methods as mentioned earlier for the direct growth of-Ga2O3thin film, in addition to its cost-effective benefit, our

proposed technique is also expected to entail considerable

ease in the growth of the -Ga2O3 film. The resultant film

grown can readily be adapted into the fabrication of GaN

Schottky barrier PDs as a cap layer. The evaluations and

subsequent discussion of the physical, electrical, and optical

properties of the PDs can now proceed once the device

fabrication is completed.

Samples used in this study were all grown on 2-in. (0001)

sapphire substrates by metal organic chemical vapor deposi-

tion (MOCVD). The structure consists of a 30-nm-thick low-

temperature GaN nucleation layer, a 2-m-thick Si-doped

GaN buffer layer, and a 700-nm-thick undoped GaN layer.

First, the GaN/sapphire template was dipped in a diluted

hydrochloric acid water solution (HCl:H2O) for 4min to

remove the native oxide. The sample was subsequently

oxidized in a quartz tube furnace purged with 50 sccm of O2gas at 1000 C for 12 min. It was found that the thickness ofthe oxidized layer was about 100 nm (hereafter referred to as

PD A). The crystal quality of the as-grown samples was then

evaluated using MAC MXP18 X-ray diffractometer (XRD).

To fabricate -Ga2O3/GaN Schottky-barrier PDs, an induc-

tively coupled plasma (ICP) etcher was used to define the

circular pattern of the device. Standard photolithography and

liftoff were then utilized to define the contact electrodes. The

Ti/Al (30/100 nm) outer ring surrounding the circular diode

pattern with an inner diameter of 500m was deposited

afterward. To serve as ohmic contact, the circular Schottky

barrier PDs were annealed at 600C for 5min. We then

deposited a Ni/Au (5/5 nm thick) Schottky contact insidethe ring opening to serve as the semi-transparent Schottky

barrier contact. The diameter of a semi-transparent Schottky

barrier electrode was designed to be 490m. Additional

Ni/Au (50/100 nm thick) was then deposited directly on top

of Ni/Au (5/5 nm) to serve as the Schottky electrode contact

pad. Figure 1 shows a schematic diagram of the fabricated

PDs. Samples without the -Ga2O3 cap layer were also

prepared for comparison (hereafter referred to as PD B).

Figure 2 shows the measured XRD spectrum of the

furnace-oxidized sample. The sharp peaks in the spectrum

are identified as (104), (202), (111), (113), (213), (302), and

(217), which can be indexed as -Ga2O3 (JCPDS file 11-

0370). The presence of these peaks shows that the thin

oxidized -Ga2O3 film has lattice constants ofa5:80 A,b3:04 A, andc12:23 A. Figure 3 shows the room-tem-perature (RT) currentvoltage (IV) characteristics of the

Fig. 1. Schematic diagram of the fabricated PDs.

E-mail address: rwchuang@mail.ncku.edu.tw

Applied Physics Express 5 (2012) 116701

116701-1 # 2012 The Japan Society of Applied Physics

http://dx.doi.org/10.1143/APEX.5.116701

http://dx.doi.org/10.1143/APEX.5.116701http://dx.doi.org/10.1143/APEX.5.116701http://dx.doi.org/10.1143/APEX.5.116701

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two fabricated PDs evaluated in the dark. The inset depicts

the RTIVcharacteristic associated with n-type contact pad

measured in dark, showing that this n-type contact is indeed

ohmic. Notice also that the measured current is larger than

100 mA, which is beyond the detection limit when the

applied bias is over 3 V. Under reverse bias, the measured

dark current of PD A is significantly smaller than that of

PD B. With10V applied bias, it was found that themeasured reverse leakage currents of PD A and PD B were8:541012 and 2:13107 A, respectively. A significantreduction in dark current is benefited by a thicker and higher

potential barrier incurred by the semi-insulating -Ga2O3cap layer when compared with a conventional Schottky

barrier PD (PD B). Under a forward bias, the turn-on voltage

of PD A is larger than that of PD B. The higher turn-on

voltage of PD A is attributed to the higher series resistance

and potential barrier encountered by the carriers as result

of incorporating the semi-insulating -Ga2O3 cap layer.

Figure 4 show the spectral responses of PD A and PD B

using a 300 W Xe lamp which is spectrally dispersed by a

monochromator as the excitation source. During these meas-

urements, the monochromatic light which has been pre-

viously calibrated by a UV-enhanced Si diode along with

an optical power meter is subsequently modulated by a

mechanical chopper and collimated thereafter onto the front-

side (a side associated with Ni/Au metal) of the devicesusing an optical fiber. The photocurrent is then recorded by

a lock-in amplifier. Notice that the photoresponses of the

photodetectors are flat in the short-wavelength region, while

a sharp cutoff occurs at 360 nm, which are typically ob-

served from both GaN-based PDs. Additionally, notice that

from Fig. 4 a similarity in responsivity between PD A and

PD B at wavelengths longer than 400 nm and a lower dark

current for PD A are observed. One possible explanation for

these observations could be due to a fact that any wavelength

longer than 400 nm is not effective absorbed by GaN layer,

thereby rendering both PD A and PD B to manifest a similar

photoresponsive behavior at wavelengths longer than

400 nm. In the short-wavelength region, it is found that the

responsivity measured from PD A is larger than that meas-

ured from PD B. The larger responsivity is attributed to the

effective passivation of surface states by the semi-insulating

-Ga2O3 cap layer. Note that the width of the transition

region observed from PD A is comparably narrower than

that from PD B, which is due to a much larger bandgap

associated with -Ga2O3, causing the trap states in the

-Ga2O3 cap layer to be relatively far away separated from

the 360 nm spectral region. Consequently, a narrower transi-

tion region is achieved for PD A. With an incident light

wavelength of 360 nm and1V bias applied, the measuredresponsivities of PD A and PD B are 1:4710

2

and6:72104 A/W, respectively. With5V bias applied,the measured responsivities of PD A and PD B are both

elevated to 3 and 1:96101 A/W, respectively. It is clearthat the responsivity increases in response to an applied

bias as shown in the inset, which implicitly indicates the

existence of the photoconductive gain in both PDs.19) It

was shown previously that photoconductive gain could

be induced by surface states.20) In fact, it needs to be

emphasized at this point that some deep level defects are

existed within the GaN layer. When the wavelength of the

incident light is longer than 360 nm, the photocurrent is

inevitably produced, which is believably caused by electrons

jumping from a defect level into conduction band. Here, the

UV-to-visible rejection ratio is defined as the responsivity

measured at 360 nm divided by the same quantity measured

at 450 nm. With such a definition in place, it is found that the

30 40 50 60 70

(104)

-Ga2O3(JCPDS File No. 11-0370)

Intensity(arb.unit)

2(deg)

(202)

(111)

(113)

(213)

(217)

(302)

Fig. 2. XRD spectrum of the furnace-oxidized sample.

-10 -8 -6 -4 -2 0 2 410

-13

10-11

10-9

10-7

10-5

10-3

PD_B

PD_A

Forwardcurrent(mA)R

eversecurrent(A)

Applied bias (V)

0

2

4

6

8

10

-4 -3 -2 -1 0 1 2 3 4

-100

-50

0

50

100

Current(m

A)

Applied bias (V)

Fig. 3. RT IVcharacteristics of the two fabricated PDs (PD A andPD B) measured in dark as a direct comparison. The inset shows the RT

IVcharacteristics associated with n-type contact pad measured in dark.

280 300 320 340 360 380 400 420 440 460

10-4

10-3

10-2

10-1

100

101

-1 -2 -3 -4 -510-4

10-3

10-2

10-1

100

101

Applied bias (V)

Responsivity(A/W)

PD_A

PD_B

Respo

nsivity(A/W)

Wavelength (nm)

PD_B:-1V

PD_B :-5V

PD_A :-1V

PD_A :-5V

Fig. 4. RT spectral responses measured from PD A with-Ga2O3 cap

layer and PD B without -Ga2O3 cap layer. The inset depict the curves of

the responsivity versus applied bias.

Z.-D. Huang et al.Appl. Phys. Express5 (2012) 116701

116701-2 # 2012 The Japan Society of Applied Physics

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UV-to-visible rejection ratios for both PD A and PD B

biased at5V are 437 and 21, respectively. In contrast toPD B, a larger UV-to-visible rejection ratio is observed from

PD A, which is attributed to the effective suppression of

dark leakage current as result of inserting a -Ga2O3 cap

layer. Figure 5 shows noise power spectra measured from

these two PDs in the frequency range varied from 1 Hz to

1 kHz. The solid lines shown in the figure refer to the

experimental data, while the dashed lines are the corre-

sponding 1=f fittings. Under the same applied bias and

frequency, it is found that the noise power of PD A is

significantly lower than that of PD B due to a lower dark

current measured from PD A with the -Ga2O3 cap layer

incorporated. For a given bandwidthB, the total square noise

current hini2 can be estimated by integrating the noisespectral density Snf over the frequency range,21)

hini2 Z B0

Snf dfZ 1

0

Snf dfZ B1

Snf df

S0lnB1; 1whereSnfin the bandwidth range of 0 to 1 Hz is assumedto be the same and equal to Snf at 1 Hz. From the datashown in Fig. 4 and eq. (1), the total square noise currents

hini2 can then be determined as 1:731028 and 4:631023 A2 for PD A and PD B, respectively. The total squarenoise currenthini2 measured from PD A is about 5 orders ofmagnitude smaller than that from PD B due to an effectivepassivation of the sample surface as result of incorporating

a semi-insulating -Ga2O3 cap layer. The noise equivalent

power (NEP) and the normalized detectivity (D) can thenbe evaluated using,

NEPffiffiffiffiffiffiffiffiffihini2

pR

; 2

DffiffiffiffiA

p ffiffiffiffiB

p

NEP ; 3

whereR is the responsivity of the PDs, A is the device size,

and B is the bandwidth. Knowing the device size, we can

thus calculate NEPs andD

for a given bandwidth of 1 kHz.

It is found that NEP calculated are 4:381015 and 3:471011 W for PD A and PD B, respectively, while thecorresponding D values are 1:011013 and 1:28109cmHz0:5 W1. These findings indicate that a lower noise level

and a larger detectivity can be realized by introducing an

additional -Ga2O3 cap layer. With the semi-insulating -

Ga2O3 cap layer inserted, a substantial reduction in the

reverse leakage current can be achieved as a result of intro-

ducing a relatively higher potential barrier, thereby bringing a

significant improvement to the PD performance accordingly.

In summary, GaN Schottky barrier UV PDs with a

-Ga2O3 cap layer grown by furnace oxidizing the GaNepitaxial layer were fabricated and characterized. By this

oxidation technique introduced to form a -Ga2O3 cap layer,

a significant reduction of the dark leakage current and an

enhancement of the UV-to-visible rejection ratio can be duly

achieved. The impacts of growing an additional cap layer

effectively demonstrate that the measured NEPs and D

values for PDs with and without the-Ga2O3 cap are 4:381015 and 3:471011 W and 1:011013 and 1:28109cmHz0:5 W1, respectively.

Acknowledgments This work was financially supported in part by the

National Science Council (NSC) of Taiwan under Contract Number NSC 98-

2221-E-006-015-MY3. Furthermore, the authors would like to acknowledge the

use of shared facilities funded by the Ministry of Education in Taiwan under the

Program of Top 100 Universities Advancement. The additional financial support

provided by the Bureau of Energy, Ministry of Economic Affairs of Taiwan, is

greatly appreciated. Finally, the authors also wish to thank the LED Lighting and

Research Center and Ocean Energy Research Center of NCKU for assistance in

relevant devices analyses.

1) E. Monroy, E. Munoz, F. J. Sanchez, F. Calle, E. Calleja, B. Beaumout, P.

Gibart, J. A. Munoz, and F. Cusso: Semicond. Sci. Technol. 13 (1998)

1042.

2) G. Parish, S. Keller, P. Kozodoy, J. A. Ibbetson, H. Marchand, P. T. Fini,

S. B. Fleischer, S. P. DenBaars, and U. K. Mishra: Appl. Phys. Lett. 75

(1999) 247.

3) Q. Chen, J. W. Yang, A. Osinsky, S. Gangopadhyay, B. Lim, M. Z. Anwar,M. Asif Khan, D. Kuksenkov, and H. Temkin:Appl. Phys. Lett. 70 (1997)

2277.

4) Z. M. Zhao, R. L. Jiang, P. Chen, D. J. Xi, Z. Y. Luo, R. Zhang, B. Shen,

Z. Z. Chen, and Y. D. Zheng: Appl. Phys. Lett. 77 (2000) 444.

5) M. Razeghi and A. Rogalski: J. Appl. Phys. 79 (1996) 7433.

6) T. Hino, S. Tomiya, T. Miyajima, K. Yanashima, S. Hashimoto, and M.

Ikeda: Appl. Phys. Lett. 76 (2000) 3421.

7) Y. Xin, E. M. James, I. Arslan, S. Sivananthan, N. D. Browning, S. J.

Pennycook, F. Omnes, B. Beaumont, J.-P. Faurie, and P. Gibart: Appl.

Phys. Lett. 76 (2000) 466.

8) S. J. Chang, S. M. Wang, P. C. Chang, C. H. Kuo, S. J. Young, T. P. Chen,

S. L. Wu, and B. R. Huang: IEEE Sens. J. 10 (2010) 1609.

9) H. C. Casey, G. G. Fountain, R. G. Alley, B. P. Keller, and S. P. DenBaars:

Appl. Phys. Lett. 68 (1996) 1850.

10) L. W. Tu, W. C. Kuo, K. H. Lee, P. H. Tsao, C. M. Lai, A. K. Chu, and

J. K. Sheu: Appl. Phys. Lett. 77 (2000) 3788.11) C. T. Lee, H. W. Chen, and H. Y. Lee:J. Appl. Phys. Lett. 82 (2003) 4304.

12) C. Wang, N. Maeda, M. Hiroki, T. Kobayashi, and T. Enoki:Jpn. J. Appl.

Phys. 44 (2005) 7889.

13) K. Matsuzaki, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M.

Hirano, and H. Hosono: Thin Solid Films 496 (2006) 37.

14) C. L. Dezelah, J. Niinisto, K. Arstila, L. Niinisto, and C. H. Winter:Chem.

Mater. 18 (2006) 471.

15) H. W. Kim and N. H. Kim:Mater. Sci. Eng. B 110 (2004) 34.

16) Y. X. Li, A. Trinchi, W. Wlodarski, K. Galatsis, and K. Kalantar-zadeh:

Sens. Actuators B 93 (2003) 431.

17) C. T. Lee, H. W. Chen, F. T. Hwang, and H. Y. Lee:J. Electron. Mater. 34

(2005) 282.

18) A. C. Lang, M. Fleischer, and H. Meixner:Sens. Actuators B 66 (2000) 80.

19) E. Munoz, E. Monroy, J. A. Garrido, I. Izpura, F. J. Sanchez, M. A.

Sanchez-Garca, E. Calleja, B. Beaumont, and P. Gibart:Appl. Phys. Lett.

71 (1997) 870.

20) J. C. Carrano, T. Li, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C.

Campbell: J. Appl. Phys. 83 (1998) 6148.

21) S. Wang, T. Li, J. M. Reifsnider, B. Yang, C. Collins, Jr., A. L. Holmes,

and J. C. Campbell: IEEE J. Quantum Electron. 36 (2000) 1262.

1 10 100 1000

10-31

10-29

10-27

10-25

10-23

1/f fitting

1/f fitting

Noisepowerdensity(A2/Hz)

Frequency (Hz)

PD_A

PD_B

Biased at -5 V

Fig. 5. Noise power spectra measured associated with the two fabricated

PDs (PD A and PD B).

Z.-D. Huang et al.Appl. Phys. Express5 (2012) 116701

116701-3 # 2012 The Japan Society of Applied Physics

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