GaAs self-assembled quantum dots

GaAs self-assembled quantum dots

Physica E 13 (2002) 224 – 228 www.elsevier.com/locate/physe Photoluminescence linewidth narrowing of InAs=GaAs self-assembled quantum dots S. Kiravi...

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Physica E 13 (2002) 224 – 228

www.elsevier.com/locate/physe

Photoluminescence linewidth narrowing of InAs=GaAs self-assembled quantum dots S. Kiravittaya ∗ , Y. Nakamura, O.G. Schmidt Max-Planck-Institut fur Festkorperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany

Abstract The e0ects of desorption and di0usion of indium adatoms on the photoluminescence (PL) from InAs self-assembled quantum dots (QDs) are investigated by introducing growth interruptions after QD formation. Large, low-density and small, high-density QDs were grown by molecular beam epitaxy using low (0:01 ML=s) and high (0:2 ML=s) growth rates, respectively. The PL from the QDs grown at 0:01 ML=s and with various growth interruption times exhibit decreasing linewidths from 40 to 32 meV with increasing growth interruption time up to 30 s. The narrowing of the PL linewidth results from improved size homogeneity due to desorption and di0usion of adatoms from small (¡ 30 nm) InAs clusters. The narrowing of the PL linewidth from the InAs dots is combined with low-temperature GaAs capping to obtain a linewidth of 26 meV. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 78.68.Hc; 68.65.Hb; 81.15.Hi Keywords: Photoluminescence; InAs; Self-assembled quantum dots; Molecular beam epitaxy

1. Introduction The zero-dimensional nature of quantum dots (QDs) is of great interest for high-performance optoelectronic devices such as semiconductor lasers [1–3]. Self-assembled Stranski–Krastanov (SK) growth is a promising technique to realize defect free, high-density QDs. Size @uctuations of QDs, formed via the SK growth mode, however, hinders the realization of superior laser performance. To minimize the e0ects of size @uctuation, the growth parameters have to be well understood and optimized [3–8]. Special growth techniques were proposed to minimize the size ∗

Corresponding author. Tel.: +49-711-689-1315; fax: +49-711689-1010. E-mail address: [email protected] (S. Kiravittaya).

distribution, for example: tuning of arsenic pressure to utilize the self-size-limiting e0ect [7], lowering of growth temperature when growing the capping layer to reduce intermixing e0ect [7], and strain reduction in QDs by growth of InGaAs capping layers [9]. In this paper, we have experimentally examined the e0ect of desorption and di0usion of indium adatoms via growth interruption (GI) of self-assembled InAs QDs grown by molecular beam epitaxy (MBE). Two types of QDs are studied by photoluminescence (PL) and atomic force microscopy (AFM): (1) large, low-density and (2) small, high-density QDs. PL linewidth narrowing is observed in the case of large, low-density QDs when growth interruptions ¡ 60 s are introduced. The growth interruption was combined with low-temperature growth of the GaAs capping layer to further reduce the PL linewidth.

1386-9477/02/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 5 2 5 - 2

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2. Experimental procedure The samples were grown by solid source MBE on ◦ semi-insulating GaAs (001) ± 0:1 . After desorbing ◦ the oxide at 670 C, a 400-nm GaAs bu0er, 20-nm Al0:4 Ga0:6 As cladding layer, followed by a 20-nm ◦ GaAs layer were deposited at 610 C. The growth rate (GR) of GaAs and Al0:4 Ga0:6 As were 0.60 monolayer (ML)=s and 1:0 ML=s, respectively. The beam equivalent pressure of As4 was 8×10−6 mbar. A 1:8 ML InAs self-assembled QD layer was deposited ◦ at 500 C, which was calibrated by the [email protected] high-energy electron di0raction (RHEED) transition from (2×4) to c(4×4) reconstruction of the GaAs surface. The InAs GR was previously calibrated by the QD formation time at low substrate temperature ◦ (450 C). InAs grown at 0:01 ML=s is referred to as low-GR sample and at 0:20 ML=s is referred to as high ◦ GR. An InAs desorption rate of 0:004 ML=s at 500 C was measured by comparing the dot formation time at di0erent growth temperatures. After the InAs was grown on the GaAs surface, the GI was introduced immediately followed by growth of a 100-nm-thick GaAs cap to cover the dot for PL measurement. A 20-nm Al0:4 Ga0:6 As, 20-nm-thick GaAs and an InAs QD growth sequence with the same GI time were then deposited for surface analysis. The sample was cooled down immediately to freeze the QDs on the surface for the characterization by ex situ AFM. Photoluminescence (PL) was measured at 300 K. PL was excited by a 488 nm Ar + laser at 5 and 50 mW to observe the spectrum from the QDs and the wetting layer (WL), respectively. The laser beam diameter was about 160 m. The signal was detected by an LN2 -cooled Ge photodetector using standard lock-in technique. 3. Results and discussion In Fig. 1, the 1×1 m2 AFM images of the QDs without and with 30 s GI grown at low GR (Fig. 1(a) and (b)) and high GR (Fig. 1(c) and (d)) are shown. The 500×500 nm2 area on the top-right corner of each image was adjusted to yield a high-contrast height scale. 1 In Fig. 1(a), besides large InAs QDs, 1

Note that the horizontal dark lines around the dot come from a high-contrast image processing error.

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small InAs clusters (¡ 30 nm in width and 2–3 nm in height) marked by an arrow are distinctly observed on the @at WL surface area. The clusters completely disappear when a 30 s GI was introduced (Fig. 1(b)). Fig. 1(c) and (d) show that the density of high-GR QDs decreases when a 30 s GI is introduced and that the dots slightly increase in size. The PL spectra of 1:8 ML InAs QDs at low and high GRs with 0, 30, 60, and 120 s GIs are shown in Fig. 2. The PL peak energies of the ground state, the Krst excited state and WL from the low-GR QDs are resolved. In the case of the high-GR QDs, the peaks are not clearly separable due to the greater size inhomogeniety of these dots. With low GR, the dot size uniformity is improved by using 30 s GI due to the preferential di0usion of indium adatoms from small InAs clusters to the larger InAs QDs [10]. Since the indium adatoms on the clusters are likely to possess a higher energy [10] than those on the QDs, these atoms will di0use and desorb at the Krst stage (0 –30 s) after the QD formation. The indium adatoms prefer to cooperate with the smaller QDs due to a lower energetic barrier at the QD edges [10]. Through this process, the size distribution of QDs can be improved. However, for longer GIs the indium desorption acts to decrease the dot size homogeneity and consequently the PL line width increases with increased GI (longer than 30 s) as shown in Fig. 2(a). The situation is entirely di0erent for the high-GR QDs. In this case, the desorption and di0usion of indium adatoms from the clusters cannot improve the size distribution when the GI is applied (see Fig. 1(c) and (d)). The AFM result is [email protected] in the PL spectra of the high-GR QDs in Fig. 2(b), where the linewidth broadens with longer GI. This linewidth broadening as a function of GI has also been reported in Ref. [5]. Fig. 3 shows the dependence of the PL peak energy on the GI time. The PL blueshift of low-GR QDs with more than 45 s GI is attributed to a height reduction of the QDs due to the indium desorption. From the AFM results of these QDs with long GI (45 –120 s), it is seen that the density of the low-GR QDs do not change but the height is decreased. The PL spectra of the high-GR QDs, in contrast, show a clear redshift, which we assign to an increasing diameter and height of the existing QDs (measured by AFM). We attribute the increase in the QD size to the

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Fig. 1. 1×1 m2 AFM images of 1:8 ML InAs QDs grown at low GR (0:010 ML=s) with (a) 0 s GI, and (b) 30 s GI and InAs QDs at high GR (0:20 ML=s) with (c) 0 s GI, and (d) 30 s GI. The inset of Fig. 1(a) and (b) are high-contrast images.

300 K 1.8 ML InAs low growth rate

300 K 1.8 ML InAs high growth rate

WL

Intensity (a.u.)

56 meV

38 meV

87 meV

120s GI

69 meV

60s GI

32 meV

59 meV

30s GI

as grown

1.0 1.1 1.2 1.3 1.4 1.5 Energy (eV)

60s GI

30s GI

51 meV

38 meV

(a)

120s GI x5

as grown

1.0 1.1 1.2 1.3 1.4 1.5 (b)

Energy (eV)

Fig. 2. PL spectra of 1:8 ML InAs QDs with various GI time at (a) low GR and (b) high GR.

S. Kiravittaya et al. / Physica E 13 (2002) 224 – 228

Low growth rate WL peak

90

1.36 1.34 1.14 1.12

80 High growth rate Ground state Low growth rate st

1.10

1 excited state

1.08 1.06 1.04 1.02

Low growth rate Ground state

0 30 60 90 120 150 180 210 240

GI time (s) Fig. 3. PL peak energy dependence of 1:8 ML InAs QDs on various GI time at low GR (square) and high GR (circle).

di0usion of the atoms from less stable QDs to more stable QDs. This also corresponds to a density reduction of the high-GR QDs when the GI time is increased. Fig. 2 shows that the e0ects of the GI on PL spectra of the high-density, high-GR QDs are evidently di0erent from low-density, low-GR QDs. The former shows a linewidth narrowing and a peak blueshift, the latter shows a linewidth broadening and continuous redshifting of the PL peak. Fig. 4 shows the relation between the full-width at half-maximum (FWHM) of the QD PL emission spectrum and the GI time. Whereas the FWHM of the high-GR QDs becomes increasingly broad, the low-GR QDs exhibit a distinct minimum at around 30 s GI. To our knowledge, this is the Krst time that the introduction of a GI causes a narrowing of the PL linewidth originating from an InAs QD ensemble. The inset of Fig. 4 shows the PL spectrum from 30 s GI, low-GR QDs where the GaAs capping layer ◦ was grown at lower growth temperature (470 C). The PL linewidth of this sample (26 meV) is considerably narrower than that of Fig. 2 (32 meV), which we attribute to reduced intermixing of indium and gallium atoms at lower growth temperature. It is also narrower than the 28 meV-linewidth of low-temperature capped low-GR QDs without the GI (not shown).

High growth rate Ground state Low growth rate Ground state

70 60

Energy (eV)

0.9

50

1.1

1.3

300 K 5 mW Low Temperature GaAs Overgrowth 26 meV

40 30 0

Intensity (a.u.)

300 K

FWHM (meV)

PL Peak Energy (eV)

1.38

227

30 60 90 120 150 180 210 240

GI time (s) Fig. 4. FWHM dependence of 1:8 ML InAs QDs on various GI times for low GR (square) and high GR (circle) QDs. The inset shows the room-temperature PL spectrum for 30 s GI, with a low-temperature GaAs capping procedure. The FWHM is 26 meV.

4. Conclusion In conclusion, we investigated the e0ects of a GI on two di0erent kinds of InAs QD ensembles (low-GR (large and low-density) QDs and high-GR (small and high-density) QDs). We observe a PL linewidth narrowing of the low-GR QDs after a 30 s GI, which corresponds to a size distribution improvement of the QDs. This improvement does not occur in the case of the high-GR QDs, which is [email protected] in an FWHM broadening. A PL blueshift and redshift of low-GR and high-GR QDs, respectively, can be explained by the indium adatoms di0usion and desorption from the QDs after disappearance of small InAs clusters. Moreover, the size homogeneity enhancement can be combined with a low-temperature capping procedure, which reduces the PL line width to 26 meV. Acknowledgements This research work was Knancially supported by the “Bundesministerium fMur Bildung, Wissenschaft, Forschung und Technologie” within the III=V quantum structure Project No. (01BM906=4) and Thailand

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research fund (TRF) through DAAD-Royal Golden Jubilee Scholarships. We would like to thank C. MMuller for assistance on the AFM measurements. References [1] D.L. Hu0aker, G. Park, Z. Zou, O.B. Shchekin, D.G. Deppe, IEEE J. Sel. Top. Quant. 6 (2000) 452. [2] D. Bimberg, M. Grundmann, F. Heinrichsdor0, N.N. Ledentsov, V.M. Ustinov, A.E. Zhukov, A.R. Kovsh, M.V. Maximov, Y.M. Shernyakov, B.V. Volovik, A.F. Tsatsul’nikov, P.S. Kop’ev, Zh.I. Alferov, Thin Solid Films 367 (2000) 235. [3] Y. Nakata, K. Mukai, M. Sugawara, K. Ohtsubo, H. Ishikawa, N. Yokoyama, J. Crystal Growth 208 (2000) 93.

[4] P.B. Joyce, T.J. Krzyzewski, G.R. Bell, T.S. Jones, S. Malik, D. Childs, R. Murray, Phys. Rev. B 62 (2000) 10 891. [5] Z.M. Wang, S.L. Feng, X.P. Yang, Z.D. Lu, Z.Y. Xu, H.Z. Zheng, F.L. Wang, P.D. Han, X.F. Duan, J. Crystal Growth 192 (1998) 97. [6] T.R. Ramachandran, A. Madhukar, I. Mukhametzhanov, R. Heitz, A. Kalburge, Q. Xie, P. Chen, J. Vac. Sci. B 16 (1998) 1330. [7] K. Yamaguchi, K. Yujobo, T. Kaizu, Jpn. J. Appl. Phys. 39 (2000) L1245. [8] R. Murray, D. Childs, S. Malik, P. Siverns, C. Roberts, J.-M. Hartmann, P. Stavrinou, Jpn. J. Appl. Phys. 38 (1999) 528. [9] K. Nishi, H. Saito, S. Sugou, J.-S. Lee, Appl. Phys. Lett. 74 (1999) 1111. [10] W. Seifert, J. Johansson, N. Carlsson, A. Gustafsson, J.-O. Malm, J. Crystal Growth 197 (1999) 19.