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Quasiregular quantum-dot-like structure formation with postgrowth thermal

annealing of InGaN

Õ

GaN quantum wells

Yen-Sheng Lin, Kung-Jen Ma, and Cheng Hsu

Department of Mechanical Engineering, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan, Taiwan, Republic of China

Yi-Yin Chung, Chih-Wen Liu, Shih-Wei Feng, Yung-Chen Cheng, C. C. Yang,a) and Ming-Hua Mao

Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering, and Graduate Institute of Electronics Engineering, National Taiwan University, I, Roosevelt Road, Section 4,

Taipei, Taiwan, Republic of China

Hui-Wen Chuang, Cheng-Ta Kuo, and Jian-Shihn Tsang

Advanced Epitaxy Technology Inc., Hsinchu Industrial Park, Taiwan, Republic of China Thomas E. Weirich

The Center of Electron Microscopy, Aachen University of Technology, Ahornstrasse 55, D-52074 Aachen, Germany

共Received 22 October 2001; accepted for publication 5 February 2002兲

Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content共19%兲 was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence共PL兲 measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1467983兴

Quantum-dot-like structures have been observed in InGaN/GaN quantum wells 共QWs兲. Such structures are formed because of the large lattice mismatch between InN and GaN.1–3 With high-resolution transmission electron mi-croscopy共HRTEM兲, randomly distributed clusters of indium aggregation and phase-separated InN were widely observed. The cluster structures form potential minimums共called local-ized states兲 for trapping carriers for efficient photon emission.4 –7 Such indium-aggregated distributions are basi-cally located in the quantum-well layers; however, they usu-ally diffuse into barriers, extensively under certain condi-tions. Typically, such out-diffusion becomes more extensive in a sample with a higher nominal indium content.3

It has been reported that postgrowth thermal annealing could alter the sizes and distributions of self-organized InAs quantum dots.8,9 In InGaN compounds, postgrowth thermal annealing processes for changing cluster structures and their photon emission properties were also reported.10–12In one of the previous results, a better-confined quantum-well struc-ture, i.e., weaker indium out-diffusion, after thermal anneal-ing was reported.12 The photoluminescence 共PL兲 intensity was increased with the stronger quantum-well confinement effect. Postgrowth thermal annealing can provide means for device manufacturers to tune the photon emission wave-length or to tailor the gain spectrum by changing the size/ composition of the quantum-dot-like structures. The infor-mation of thermal annealing is also helpful for crystal

growers to design growth procedures. In this letter, we report the formation of quasiregular quantum-dot-like structures from randomly distributed indium-aggregated clusters after postgrowth thermal annealing of an InGaN/GaN QW sample. It was found that with thermal annealing temperature at 900 °C, indium distribution was better confined in the quantum-well layers. Although the better confinement in a high-indium sample 共⬎45%兲 has been reported,12the obser-vation of quasiregular quantum-dot-like structures has never been reported. In particular, the indium content of our sample is much lower 共19%兲. Also, we found that such a trend of regular structure disappeared when the thermal annealing temperature was 950 °C. The measurements of optical prop-erties showed quite a consistent trend.

The sample used in this study was grown with a low-pressure metal–organic chemical-vapor deposition reactor. The InGaN/GaN QW sample consisted of ten periods of In-GaN wells with 3.5 nm thickness. The barrier was 10 nm GaN. In the sample, the QW layers were sandwiched be-tween a 1.5␮m GaN buffer layer on a共0001兲 sapphire sub-strate and a 50 nm GaN cap layer. The growth temperatures were 1010 and 720 °C for GaN and InGaN, respectively. The designated indium content of the sample was about 19%. As-grown samples were thermally annealed in a quartz tube furnace at different temperatures ranging from 800 to 950 °C in nitrogen ambient for 30 min. HRTEM and energy filter transmission electron microscopy 共EFTEM兲 were used to characterize material structures.

For HRTEM observation, cross-sectional samples were prepared in the conventional manner by grinding, dimpling,

a兲_{Electronic mail: ccy@cc.ee.ntu.edu.tw}

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 14 8 APRIL 2002

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and Ar⫹-ion milling with 6 kV, 1 mA, and an incident angle of 4°. Based on previous experiences, for samples containing indium, the ion-milling step needs to be carried out with the specimen holder at liquid-nitrogen temperature in order to minimize ion-beam damage. The HRTEM investigations were conducted with a 200 keV Philips CM 200 and a 300 keV JEM 3010 microscope. All the high-resolution micro-graphs were taken at Scherzer defocus and the sample was viewed along a 关11–20兴 zone axis. The 300 keV JEM 3010 microscope was equipped with a 2 k⫻2 k slow-scan charge-coupled-device camera and a Gatan imaging filter共GIF兲. Re-garding the EFTEM images, the drift between the two preedge and the postedge images was recorded by using the cross-correlation algorithm available in the Digital Micro-graph software of the GIF. In order to remove the back-ground contribution underneath the ionization edges and to obtain elemental maps, the jump-ratio method was used to avoid noise and artifacts. However, it cannot be quantified.

Figures 1共a兲–1共d兲 show the HRTEM bright-field images of the as-grown 共a兲, 800 °C-annealed 共b兲, 900 °C-annealed

共c兲, and 950 °C-annealed 共d兲 samples. The contrast variations

in the pictures represent the fluctuations of indium composi-tion. The diffusive InGaN/GaN QW interfaces can be clearly seen in the as-grown sample关Fig. 1共a兲兴. The average size of indium-rich clusters is larger than 10 nm. The clusters are irregularly dispersed and extended into the GaN barriers. In-creasing the annealing temperature leads to a better confine-ment of indium-rich clusters near InGaN QWs, as shown in Figs. 1共b兲 and 1共c兲. The average size of the indium-rich clus-ters becomes smaller after the annealing treatment. The size homogeneity of quantum-dot-like structures was also im-proved. Interestingly, one can observe that very-fine indium-rich quantum-dot-like structures 共2–5 nm兲 were regularly dispersed inside the InGaN QWs after the annealing treat-ment at 900 °C关Fig. 1共c兲兴. However, further increase of an-nealing temperature to 950 °C leads to a highly irregular structure, as shown in Fig. 1共d兲. EFTEM scanning was per-formed for the samples. Indium composition profiles across and along the QWs were obtained. Line-scan results of in-dium composition 共%兲 across QWs in the as-grown sample

共a兲, and samples annealed at 900 °C 共b兲 and 950 °C 共c兲, are

shown in Fig. 2. Compared to the as-grown sample, the

sample annealed at 900 °C manifests better confinement of indium. However, after annealing at 950 °C indium becomes dispersive again. Figure 3 shows EFTEM line-scan results of indium composition共%兲 along a QW in the as-grown sample

共a兲, and samples annealed at 850 °C 共b兲, 900 °C 共c兲, and

950 °C共d兲. Annealing did result in more regular structures of clusters. Quasiregularly arrayed quantum-dot-like structures with nearly the same indium concentration at the cores of the quantum-dot-like clusters can be observed in the sample with 900 °C annealing. The irregular distribution of compositional fluctuation can again be observed after postgrowth annealing at 950 °C. In the InGaN/GaN QW sample, thermal energy at 900 °C is sufficient to drive the system to a state of lower potential energy, i.e., a quasiregular structure. The relation-ship between the period of the indium-rich clusters and the width and nominal indium content of QWs is worth investi-gating further. When the annealing temperature reached 950 °C, the higher temperature caused coarsening of indium-rich clusters with the sizes exceeding the QW width.8Such a process destroyed the QW structure, leading to the results shown in Figs. 1共d兲, 2共c兲, and 3共d兲.

Figures 4 and 5 show the PL characteristics of these samples. Because of the oscillatory variations in PL spectra due to the Fabry–Pe´rot effect, normal PL peak position and full width at half maximum could not be accurately cali-brated. PL peak positions were then obtained through Gauss-ian fitting, and the root-mean-square 共rms兲 spectral widths were calculated. Figure 4 shows the blueshifts of PL peak positions after thermal annealing. The usually observed S-shape variation of PL peak position is unclear in Fig. 4.

FIG. 1. HRTEM bright-field images of the as-grown共a兲, 800 °C-annealed

共b兲, 900 °C-annealed 共c兲, and 950 °C-annealed 共d兲 samples.

FIG. 2. EFTEM line-scan results of indium composition共%兲 across QWs in the as-grown sample共a兲, and samples annealed at 900 °C 共b兲 and 950 °C 共c兲.

Typically, annealing at higher temperature results in a larger blueshift. However, the result of 950 °C annealing seems to have different behaviors between the low- and high-temperature ranges. Figure 5 shows the PL rms width varia-tions of various samples. The as-grown sample shows an irregular variation in the high-temperature range. Typically, thermal annealing leads to more regular indium cluster struc-tures. Therefore, the PL spectral width is expected to be

smaller, as confirmed with the behaviors of the annealed samples in Fig. 5. The larger rms spectral width of the 950 °C annealing sample, compared with the other three an-nealed samples, is consistent with the irregular structure shown in Figs. 1共d兲, 2共c兲, and 3共d兲.

In summary, we have conducted postgrowth thermal an-nealing of an InGaN/GaN QW sample with a medium level of nominal indium content. From material analyses, we found that thermal annealing at 900 °C led to quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was further raised to 950 °C.

This research was supported by the National Science Council, The Republic of China, under Grant Nos. NSC 2218-E-002-094, NSC 2218-E-002-095, and NSC 89-2215-E-002-051. This research was also supported by the Chung Shan Institute of Science and Technology, Taiwan, R.O.C.

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FIG. 4. Fitted PL peak positions as functions of temperature of various samples.

FIG. 5. PL root-mean-square spectral widths as functions of temperature of various samples.

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