Influence of As-stabilized surface on the formation of InAs/GaAs quantum dots

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Influence of As-stabilized surface on the formation of In As Ga As quantum dots

Chi-Che Tseng, Shu-Ting Chou, Yi-Hao Chen, Tung-Hsun Chung, Shih-Yen Lin, and Meng-Chyi Wu

Citation: Journal of Vacuum Science & Technology B 26, 956 (2008); doi: 10.1116/1.2912083 View online: http://dx.doi.org/10.1116/1.2912083

View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/26/3?ver=pdfcov

Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Influence of As-stabilized surface on the formation of InAs/ GaAs

quantum dots

Chi-Che Tseng

Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 300, Taiwan

Shu-Ting Chou

Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan

Yi-Hao Chen and Tung-Hsun Chung

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

Shih-Yen Lina兲

Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan and Department of Photonics, National Chiao-Tung University, Hsinchu 300, Taiwan

Meng-Chyi Wu

Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 300, Taiwan and Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

共Received 10 December 2007; accepted 25 March 2008; published 2 May 2008兲

In this article, we report the growth of InAs/GaAs quantum dots 共QDs兲 grown under different As4-supply procedures. The growth of the investigated samples carried out by the three procedures

of As shutter always opened, As shutter initially opened, and As shutter initially closed. The samples grown by the former two approaches show a uniform QD distribution and the multiple-peak luminescence, which correspond to ground-state, first-excited-state, and second-excited-state luminescence, while that grown by the latter only shows large InAs islands. The results suggest that the As-stabilized condition at the initial stage of QD growth is very critical for the high-quality QD formation. © 2008 American Vacuum Society. 关DOI: 10.1116/1.2912083兴

I. INTRODUCTION

Quantum-dot 共QD兲 structures prepared by molecular beam epitaxy共MBE兲 have attracted much attention in recent years due to their unique optical and electrical characteristics.1–3 The self-assembled QD structures grown under the Stranski–Krastanov共SK兲 growth mode are of high uniformity and defect free. Therefore, the QD structures are very promising for the applications of laser diodes and infra-red photodetectors.4It has been reported elsewhere that the uniformity and optical properties of InAs/GaAs QDs pre-pared by using an As4flux would be greatly improved under

the As-deficient condition.5 However, the influence of As4

overpressure on the formation of InAs/GaAs QDs has not been clarified yet. In this report, InAs/GaAs QDs grown under different As4-supply procedures are investigated.

Simi-lar surface morphologies and photoluminescence共PL兲 spec-tra are observed for QDs with both the As shutter always opened and initially opened procedures, while no uniform QD formation is observed for the sample with the As shutter initially closed procedure. The results suggest that the As-stabilized condition of the GaAs substrate surface is neces-sary for high-quality QD formation.

II. EXPERIMENT

The InAs/GaAs QD samples investigated in this study were prepared by solid-source MBE under As4 background

pressure. The As4 beam equivalent pressure was kept at 1

⫻10−5 _{torr during growth. Three samples with different}

As4-supply procedures were grown on共100兲 semi-insulating

GaAs substrates. The InAs QDs are formed by SK mode at 510 ° C. The InAs growth rate was kept at 0.08 ML/s, which corresponds to a V/III ratio of 50. The epitaxial structures are shown in Table I. The sample grown with As shutter always opened is referred as sample A, while the samples grown with As shutter initially opened and initially closed proce-dures are referred as samples B and C, respectively. For PL measurements, a 532 nm green laser is used as the pumping laser. The laser light is directed on the samples placed in a cryostat such that PL spectra can be measured from 10 to 300 K by temperature controller. The PLE spectrum is measured by using Jobin Yvon’s NanoLog3 system coupled with tungsten-halogen lamp as the light source. The PL and PLE signals were focused to a spectrometer coupled with an InGaAs p-i-n detector. Atomic force microscopy 共AFM兲 Digital Instruments Dimension 3100 was used for surface morphology observations.

III. RESULTS AND DISSCUSSION

Figure 1 shows the 1⫻1␮m2_{AFM images of the three}

samples. As shown in the figure, samples A and B show a uniform QD distribution with a QD density of ⬃2.8 ⫻1010_cm−2_{, a mean dot diameter of 54 nm, and a mean}

height of 6 nm, while sample C only exhibits large InAs islands. Although the total As4supplies for samples B and C

are the same, it is shown from the AFM images that the

a兲_{Electronic mail: shihyen@gate.sinica.edu.tw}

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initial surface conditions are critical for the InAs QD forma-tion. For sample B, the As are sufficient for the initial InAs epitaxial growth with the As shutter initially opened proce-dure. Therefore, similar surface morphology with As shutter always opened procedure, sample A, is obtained for sample B. For the case of sample C with In shutter initially closed procudure, the In coverage is merely 0.43 ML within the 5 s As shutter closed period. Although the period of time is too short for clear reflection high energy electron diffraction pat-tern observation, it is reasonable to assume that the back-ground As₄ pressure is not sufficient to prevent metal-rich surface formation. In this case, possible In droplet formation may be obtained at the initial growth stage of sample C.6The following In adatoms would tend to accumulate nearby the In droplets, which leads to large InAs island formation for sample C.

Figure 2 shows the 10 K PL spectra of samples A, B, and C. Samples A and B exhibit the multiple-peak luminescence, which correspond to ground-state, first-excited state, and second-excited-state luminescence. The similar PL perfor-mance from samples A and B shows that both the samples have similar crystal quality. This also reveals that as long as the initial InAs film is grown under As-stabilized condition, the close/open procedure of the As shutter does not play an important role of the QD formation. For sample C without the QD formation, the higher-energy peaks in the PL spec-trum should be attributed to the InGaAs clusters resulting from In/Ga interdiffusion at the initial stage. The PL results are consistent with the observation of AFM images for the three samples.

To further investigate the optical characteristics of the QD samples, Fig. 3共a兲 shows the normalized PL spectra at 10 K of sample A measured for different laser excitation powers. At an excitation power of 34 mW, there are three peaks de-noted as E₀ 共1.12 eV兲, E₁共1.19 eV兲, and E₂共1.25 eV兲, cor-responding to the ground-state, first-excite-state, and second-excited-state luminescence, respectively. The phenomena are attributed to the state-filling effect.7 Due to the longer elec-tron lifetime of excited states of the QD structures; the pho-toexcited electrons would fully occupy the states with lower energies. Therefore, luminescence from the higher-energy state would be observed from the PL spectra. By reducing the laser excitation power to 15 mW, the E2peak disappears

and the intensity ratio of peak E1to peak E0also decreases,

as shown in Fig. 3共a兲. The results are attributed to the gen-eration rate of photoexcited electrons at the first excited state to be lower than their recombination rate. Therefore, the first excited states are empty for the sample under such a mea-surement condition. The multiple-peak luminescence of sample A observed under such a low excitation power exhib-ited its high crystal quality. To verify the attribute of the transition mechanisms discussed above, the 10 K photolumi-nescence excitation共PLE兲 spectrum of sample A with detec-tion energy at E0 共1.12 eV兲 is shown in Fig. 3共b兲. As shown

in the figure, the peaks located at 1.19 and 1.25 eV are ob-served from the PLE spectrum, which are consistent with the TABLEI. Wafer structures and As shutter close/open procedures of samples

A, B, and C.

FIG. 1. 1⫻1␮m2_{AFM images for the three samples A, B, and C.}

FIG. 2. 10 K PL spectra of samples A, B, and C.

957 Tseng et al.: Influence of As-stabilized surface 957

JVST B - Microelectronics and Nanometer Structures

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PL peaks denoted as E1and E2. The identical peak positions

obtained from two different measurement methods have con-firmed the attribute of transition mechanisms discussed above. Also shown in the PLE spectrum are two additional

peaks located at 1.47 and 1.51 eV, which result from the InAs wetting layer and GaAs bandedge luminescence, respectively.8

IV. CONCLUSIONS

We have demonstrated the growth of InAs/GaAs QDs under different As4-supply procedures. Similar surface

mor-phologies and PL spectra are observed for the quantum dots grown with both the As shutter always opened and initially opened procedures, while no uniform QD formation are ob-served with the As shutter initially closed procedure. The phenomenon is attributed to the In droplet formation at the GaAs surface under the As-deficient condition. In this case, the In adatoms would tend to accumulate nearby the In drop-lets such that large InAs islands would be observed. The results suggest that the As-stabilized condition at the initial stage of QD growth is very critical for the high-quality QD formation. Identical peak positions observed from the multi-peak PL spectrum and PLE spectrum have revealed the higher-order confinement states of the QD structures.

ACKNOWLEDGMENTS

This work was supported in part by the National Science Council, Taiwan, under Grant No. NSC 96-2221-E-001-030.

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