Effects of thermal annealing on in induc

SIMS profiles of a as-grown, b 800 1C-annealed and c 1000 1C-annealed InGaN epitaxial layers prepared in this study... 2b shows DCXRD spectra of as-grown, 800 1C-annealed and 1000 1C-ann

Materials Science in Semiconductor Processing 10 (2007) 112–116

Effects of thermal annealing on In-induced metastable

defects in InGaN films

H Hunga, K.T Lamb, S.J Changa, , H Kuanc, C.H Chend, U.H Liawe

e

Department of Avionics, China Institute of Technology, Hsinchu 312, Taiwan

Available online 2 July 2007

Abstract

We investigated the effects of thermal annealing on the properties of InGaN layers From secondary ion mass spectroscopy results, it was found that severe In desorption occurred after annealing Photoluminescence and X-ray diffraction results indicate that significant amounts of In vacancy-related defects exist in the annealing samples It was also found that persistent photoconductivity decay time constants were 211, 893 and 1040 s, while the decay exponents were 0.153, 0.120 and 0.213 for the as-grown, 800 1C-annealed and 1000 1C-annealed InGaN epitaxial layers, respectively

r2007 Elsevier Ltd All rights reserved

Keywords: InGaN; MOCVD; PPC; XRD; SIMS

1 Introduction

Wide band-gap group III-nitride semiconductors

have attracted much attention in recent years These

materials are potentially useful in various

optoelec-tronic and high-speed device applications, such as

blue/green/ultraviolet (UV) light-emitting diodes,

laser diodes and high-power electronic devices[1–4]

The InGaN/GaN heterostructure has been

recog-nized as the essential structure for most

nitride-based devices However, it is well known that the

miscibility of InN in GaN is low The lattice

mismatch between InN and GaN is also large

Thus, In-rich InGaN clusters are often formed in InGaN epitaxial layers Due to the compositionally unstable nature of the alloy [5–8], it is still difficult

to grow high-quality InGaN epitaxial layers In order to improve the quality of InGaN, we need to reduce dislocation density and understand the nature of defects in these films[9–10]

It has been shown that one can use persistent photoconductivity (PPC) and photoluminescence (PL) measurements to evaluate the quality of InGaN films [11–12] PPC measurements could provide us with useful information about the metastable proper-ties of deep-level defects since these defects could be externally excited by shining light onto the epitaxial films Defects that interact with light could thus produce photocurrent that could last for an obser-vably long time [13] It has been reported that PPC

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doi: 10.1016/j.mssp.2007.05.002

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effects in GaN-related alloys originate from random

local fluctuations of alloy composition[14,15]

How-ever, no report on the annealing effect of PPC can be

found in the literature to our knowledge For

nitride-based devices, thermal annealing is often performed

to activate Mg in p-GaN and/or for ohmic contact

alloying In this study, we annealed InGaN epitaxial

layers at various temperatures The effects of thermal

annealing on the InGaN films will be discussed PCC

measurements were then performed on these samples

Measured electron-capture energies in these annealed

InGaN films were also reported

2 Experiments

Samples used in this study were all grown on

(0 0 0 1) sapphire (Al2O3) substrates by metalorganic

chemical vapor deposition system Details of the

growth can be found elsewhere [15–18] Briefly,

trimethylgallium (TMGa), trimethylindium (TMIn)

and ammonia (NH3) were used as indium, gallium

and nitrogen sources, respectively Disilane (Si2H6)

was used as the n-type doping source We first

prepared a low-temperature 25-nm-thick GaN

nucleation layer at 500 1C We then raised the

reactor temperature to 1050 1C to grow a

4-mm-thick Si-doped GaN n-cladding layer Subsequently,

the temperature was ramped down to 760 1C to

grow an In0.25Ga0.75N layer with an electron

concentration of 1018cm3 The as-grown samples

were then furnace annealed in N2 ambient for

10 min at 800 or 1000 1C

Crystal qualities of these epitaxial layers were

then evaluated by room temperature (RT) PL and

double-crystal X-ray diffraction (DCXRD) A

Bio-Rad rpm 2000 system with a low 5 mW HeCd laser

operated at 325 nm was used for PL measurement,

and a Bede QC2A system was used for DCXRD

measurement Secondary ion mass spectroscopy

(SIMS) was also used to evaluate distribution

profiles of In, Ga and N atoms before and after

thermal annealing After these measurements, Ti/Al

contacts were deposited onto the sample surfaces to

serve as contact electrodes Photocurrent

measure-ments were subsequently performed using HeCd

laser as the excitation source at various

tempera-tures In order to measure transient responses, we

switched the HeCd laser ON and OFF during PPC

measurements We also applied a constant DC

voltage onto the samples and measured current

transients by an HP 4156 semiconductor parameter

analyzer

3 Results and discussion

Figs 1(a)–(c) show SIMS profiles of as-grown,

800 1C-annealed and 1000 1C-annealed InGaN

1000 800 600 400 200 0

Depth (nm)

10 0

10 1

102

10 3

104

105

106

10 7

1000 800 600 400 200 0

Depth (nm)

100

10 1

10 2

10 3

104

10 5

10 6

10 7

1000 800 600 400 200 0

Depth (nm)

100

10 1

102

10 3

104

10 5

106

10 7

As-grown

N Ga In

N Ga In

N Ga In Annealed at 800 °C

Annealed at 1000 °C

b

c

Fig 1 SIMS profiles of (a) as-grown, (b) 800 1C-annealed and (c)

1000 1C-annealed InGaN epitaxial layers prepared in this study.

epitaxial layers prepared in this study It can be seen

clearly that In atoms could only be observed near

the sample surfaces, which agrees well with our

initial design However, it was found that relative

intensity of In decreased after annealing,

particu-larly for the 1000 1C-annealed samples Such a result

suggests that In atoms were desorbed from the top

InGaN layers after annealing As a result, average

In composition in the InGaN epitaxial layers should

decrease Furthermore, defects related to In

vacan-cies should also be generated after annealing

Fig 2(a) shows measured PL profiles of the three

samples It was found that PL intensity decreased

while PL full-width at half-maximum increased

after annealing These results also suggest that the

quality of the samples was degraded after annealing

It was also found that PL peak position shifted to

the long-wavelength side This is probably due to

the fact that deep-level-related luminescence became

dominant for the annealed samples due to the

increased defect density It is well known that

defect-related yellow luminescence (YL) is often

observed from GaN epitaxial layers[11] Instead of

In clusters[16], we believe that the deep-level-related

broad luminescence peak at 585 nm observed from

the annealed samples is similar to the previously

reported YL Fig 2(b) shows DCXRD spectra of

as-grown, 800 1C-annealed and 1000 1C-annealed

InGaN epitaxial layers prepared in this study It

was found that we could clearly observe the

InGaN-related XRD peak from the as-grown sample

However, intensities of InGaN-related XRD peaks

seemed to decrease and eventually merged into

the GaN main peak for the annealed samples

This again can be attributed to In desorption during annealing

The inset of Fig 3 shows a typical transient response of our samples It can be seen that fall-time

is much longer than rise-time, which indicates that the PPC effect indeed exists in our samples.Fig 3

shows current transients measured at 80 K for the as-grown and annealed samples as we turned OFF the HeCd laser It was found that we could clearly observe PPC effect from all these three samples As

we turned off the excitation, it was found that we could fit the current transients by the following stretched-exponential function [17]:

IPPCðtÞ ¼ exp½ðt=tÞb ð0obo1Þ, (1)

3000 1500

0 -1500 -3000

Omega-2Theada (arcsec)

10 0

10 1

102

103

104 1.4 × 107

1.2 × 107

1.0 × 107

8.0 × 106

6.0 × 106

As-grown Annealed at 800

Annealed at 1000

500 525 550 575 600 625 650

Wavelength (nm)

as-grown 800 1000

Fig 2 Measured (a) PL profiles and (b) DCXRD spectra of the three samples.

100 80 60 40 20 0

Time (sec) 0.2

0.4 0.6 0.8 1.0

as-grown 800 1000

20 15 10 5 0 Time (sec)

Photocurrent (a.u.) light on

light off

Fig 3 Current transients measured at 80 K for the as-grown and annealed samples as we turned OFF the HeCd laser The inset shows typical transient response of our samples.

where IPPC(0) is the initial photocurrent, t is the

PPC decay time constant and b is the decay

exponent For comparison, we normalized the

current byIPPC(t) ¼ [I(t)Id]/[I(0)Id][18] In other

words, the current was normalized to unity at the

time when illumination was switched OFF Here,

I(t) is the current measured at time t, I(0) is the

current immediately taken after the termination of

the excitation source and Id is the initial dark

current It has been shown previously that PPC

effects of p-type GaN and InGaN/GaN quantum

wells were related to DX-like deep-level defects and

In-induced potential fluctuation [7,8] Since In

atoms were desorbed after annealing, we observed

significant PPC effect from the annealed samples

due to severe In fluctuation and the large number of

defect-induced deep-level centers

Using the normalized current transients shown in

Fig 3, we can calculate the PPC decay time constant

tand decay exponent b As listed inTable 1, it was

found that the PPC decay time constants were 211,

893 and 1040 s, while the decay exponents were 0.153,

0.120 and 0.213 for the as-grown, 800 1C-annealed

and 1000 1C-annealed InGaN epitaxial layers,

respec-tively Compared with the as-grown sample, it was

found that PPC decay time constant t was larger

while decay exponent b was smaller for the 800

1C-annealed sample These values result in slower decay

for the 800 1C-annealed sample In contrast, it was

found that PPC decay time constant t and decay

exponent b of the 1000 1C-annealed sample were both

larger than those observed from the as-grown sample

These values result in faster decay for the 1000

1C-annealed sample The exact reasons for these

observations were not clear yet It is possible that In

vacancy-related deep-level centers could result in

slower decay for the 800 1C-annealed sample On

the other hand, the average In composition in the

InGaN layer was much smaller for the 1000

1C-annealed sample It is possible that the number of

non-radiative recombination centers became larger due to severe In desorption Thus, we observed a much larger decay exponent b and a faster decay for the 1000 1C-annealed sample To clarify these points,

we need to perform experiments such as deep-level transient spectroscopy Such experiments are under way and the results will be reported separately

Table 1

PPC decay curve parameters for as-grown and annealed InGaN

epitaxial layers

time constant

t (s)

Decay exponent b

Electron-capture barrier DE

(MeV)

Annealed at 800 1C 893 0.120 743

Annealed at 1000 1C 1040 0.213 247

14 12 10 8 6 4 2 5 6

1000/T (K-1)

14 12 10 8 6 4 2

1000/T (K-1) 1000/T (K-1)

7 8

∆ E = 247meV

∆ E = 98meV

∆ E = 743meV

6 8 10 12 14

4.5 5.0 5.5 6.0

As-grown

Annealed at 800°C

Annealed at 1000°C

16

c b

Fig 4 Arrhenius plots of t for the (a) as-grown, (b) 800 1C-annealed and (c) 1000 1C-1C-annealed InGaN epitaxial layers.

The observation of PPC effect implies that there is

an insufficient amount of energy for carriers to

overcome a capture barrier DE created by localized

defects, preventing recapture of electrons by

deep-level-related non-radiative recombination centers We thus

performed current transient measurements and

calcu-lated PPC decay time constant t and decay exponent b

at various temperatures For most III–V and II–IV

semiconductor materials, it has been shown that the

temperature-dependent PPC decay time constant t

could be fitted well by t ¼ t0exp[DE/kT], where t0is

the high-temperature limit of the time constant while

DE is the capture barrier[19–22].Figs 4(a)–(c) show

the Arrhenius plots of t for the as-grown, 800 1

C-annealed and 1000 1C-annealed InGaN epitaxial

layers, respectively From these figures, we can

determine the electron-capture energy, DE (i.e., the

energy barrier for electrons to relax to ground state), of

the samples As listed inTable 1, it was found that the

electron-capture energies DE were 98, 743 and

247 MeV for the as-grown, 800 1C-annealed and

1000 1C-annealed InGaN epitaxial layers, respectively

Fig 5shows the configuration-coordinate diagram of

the fabricated samples[23] The vertical axis ofFig 5

symbolizes the electronics and the strain energy of the

defect center while the horizontal axis symbolizes the

position of the defect and its neighboring atoms After

annealing, we believe that the vacancies left by the

vanished In atoms will change the configuration of the

system Deep-level defect centers also shift toward the

associated shallow states after annealing Thus, we also

observed a change in capture energy, DE, after

annealing due to the change of system configuration

and the change of electron transition between its

ground state and metastable state

4 Conclusions

In summary, the effects of thermal annealing on the properties of InGaN layers were investigated From SIMS results, it was found that severe In desorption occurred after annealing PL and XRD results indicate that significant amounts of In vacancy-related defects exist in the annealing samples It was also found that the PPC decay time constants were 211, 893 and

1040 s, while the decay exponents were 0.153, 0.120 and 0.213 for the as-grown, 800 1C-annealed and

1000 1C-annealed InGaN epitaxial layers, respectively Reference

[1] Nakamura S, Senoh M, Iwasa N, Nagahama S Appl Phys Lett 1995;67:1868.

[2] Nakamura S, Fasol G The blue laser diode Berlin, Germany: Springer, 1997.

[3] Wu LW, Chang SJ, Wen TC, Su YK, Chen JF, Lai WC,

et al IEEE J Quantum Electron 2002;38:446.

[4] Nakamura S, Senoh M, Nagahama SI, Iwasa N, Yamada T, Matsushita T, et al Jpn J Appl Phys Lett 1998;37:L309 [5] Dang XZ, Wang CD, Yu ET, Boutros KS, Redwing JM Appl Phys Lett 2001;72:2745.

[6] Chen CH, Hang DR, Chen WH, Chen YF Appl Phys Lett 2003;82:1884.

[7] Johnson C, Lin JY, Jiang HX, Khan MA, Sun CJ Appl Phys Lett 1996;68:1808.

[8] Yang HC, Lin TY, Chen YF Appl Phys Lett 2001;78:338 [9] Zeng KC, Lin JY, Jiang HX, Salavador A, Popovici G, Tang H, et al Appl Phys Lett 1997;71:1368.

[10] Deguchi T, Shikanai A, Torii K, Sota T, Chichibu S, Nakamura S Appl Phys Lett 1998;72:3329.

[11] Reddy CV, Balakrishnan K, Okumura H, Yoshida S Appl Phys Lett 1998;73:244.

[12] Grandjean N, Massies J, Leroux M Appl Phys Lett 1991;68:43.

[13] Li JZ, Lin JY, Jiang HX, Geisz JF, Kurtz SR Appl Phys Lett 1999;75:1899.

[14] Chang MCY, Tsang KO, Li EH, DenBaars SP MRS Internet J Nitride Semicond Res 1999;4:6d.

[15] Romano LT, McCluskey MD, Krusor BS, Bour DP, Chua

C, Brennan S, et al J Cryst Growth 1998;189:38.

[16] Feng SW, Lin EC, Tang TY, Cheng YC, Wang HC, Yang

CC Appl Phys Lett 2003;83:3906.

[17] Li JZ, Lin JY, Jiang HX J Appl Phys 1997;82:1227 [18] Lee YC, Shu GW, Chou WC, Shen JL, Uen WY Solid State Commun 2002;123:421.

[19] Chen YF, Huang SF, Chen WS Phys Rev B 1991;44:12478 [20] Lin JY, Dissanayake A, Brown G, Jiang HX Phys Rev B 1990;42:5855.

[21] Lin JY, Dissanayake A, Jiang HX Solid State Commun 1993;87:787.

[22] Dissanayake A, Huang SX, Jiang HX, Lin JY Phys Rev

B 1991;44:13343.

[23] Redfield D, Bube RH Photoinduced defects in semiconduc-tors Cambridge: Cambridge University Press; 2006.

Configuration Coordinate Q

D-800 °C

D-1000 °C D-As grown

∆ E

Ec

Ev

Fig 5 Configuration-coordinate diagram for defect centers in

InGaN after annealing at different temperature.