GaAs quantum dot structures

GaAs quantum dot structures

ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 2229–2233 www.elsevier.com/locate/jcrysgro Growth and properties of InAs/InxGa1xAs/GaAs quant...

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ARTICLE IN PRESS

Journal of Crystal Growth 310 (2008) 2229–2233 www.elsevier.com/locate/jcrysgro

Growth and properties of InAs/InxGa1xAs/GaAs quantum dot structures Eduard Huliciusa,, Jirˇ ı´ Oswalda, Jirˇ ı´ Pangra´ca, Jan Vyskocˇila,b, Alice Hospodkova´a, Karla Kuldova´a, Karel Melichara, Tomislav Sˇimecˇeka a

Institute of Physics, Academy of Sciences of the Czech Republic, v. v. i., Cukrovarnicka´ 10, 162 53, Prague 6, Czech Republic b CTU FEE, Department of Mechanics and Materials Science, Technicka´ 2, 166 27 Prague 6, Czech Republic Available online 21 November 2007

Abstract Single- and double-layer InAs/GaAs quantum dot structures with strain-reducing layers (SRLs) were prepared by metalorganic vaporphase epitaxy using the Stranski–Krastanow growth mode. Structures were studied in-situ by reflectance anisotropy spectroscopy (RAS), and ex-situ by photoluminescence (PL). These structures, with very intense room temperature PL at wavelengths from 1.25 to 1.55 mm according to growth and structure parameters, were grown along while monitored with RAS. Strong correlation between RAS signal and PL intensity was found. Dependence of PL emission maximum position on SRL composition and capping layer thickness is shown. r 2007 Elsevier B.V. All rights reserved. PACS: 73.21.La; 73.61.Ey; 68.65.Hb; 78.55.Cr; 78.67.Hc Keywords: A1. Nanostructures; A3. Metalorganic vapor-phase epitaxy; B2. Semiconducting III–V materials

1. Introduction

2. Experiment

One of the most attractive properties of the InAs/GaAs quantum dot (QD) structures is the efficient, GaAs-based technology with stable and only slightly temperaturedependent emission in the near-infrared region, especially at 1.3 and 1.55 mm, wavelengths important for optical communications [1]. A promising way to increase homogeneity and density of QDs and to extend the emission to longer wavelengths is to use the strain reduction effect of InxGa1xAs matrix and covering strain-reducing layers (SRLs) [2,3]. The question of size of InAs/GaAs QDs is very important but is not the only aspect how to obtain the required emission wavelength. There are still many open or not fully resolved problems, for instance the dependence of QD emission wavelength and intensity on In content in InGaAs-covering SRL as well as the dependence of the wavelength on the thickness of the GaAs capping layer.

Single and double InAs QD layer structures (see Fig. 1) were prepared by low-pressure metalorganic vapor-phase epitaxy in AIXTRON 200 apparatus on SI (1 0 0) GaAs substrates using the Stranski–Krastanow growth mode. Precursors used for the growth of these structures were TMIn, TMGa and AsH3. The structures were grown at 70 hPa total pressure; total flow rate through the reactor was 8 slpm and the growth temperature was 490 1C, and the growth process is described elsewhere [4]. Several types of QD structures were prepared: single QD layer structure with and without ternary InxGa1xAs matrix and covering SRL and double QD layer structure with and without SRL covering the upper QD layer. The composition x of the 5 nm thick InxGa1xAs covering SRLs was varied between 0 and 0.35. GaAs cap layer of different thickness (0–42.5 nm) was grown on top of the structures. The list of studied samples is shown in Table 1. Prepared QD structures were studied during the growth by reflectance anisotropy spectroscopy (RAS) and ex-situ

Corresponding author. Tel.: +420 220318576; fax: +420 233343184.

E-mail address: [email protected] (E. Hulicius). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.11.055

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Fig. 1. Schematic drawing of double and single QD layer structures, some details are shown in Table 1.

Table 1 Samples with one QD layer and with two vertically correlated QD layers were prepared to find optimal growth parameters in order to achieve strong photoluminescence (PL) signal Sample

Structure type

TMIn flow (%)

WT (s)

In content x in InxGa1xAs SRL

CL thickness (nm)

PL int. (arb.u.)

Energy of PL max. (eV)

A B C D E

2  QD 2  QD 2  QD The same as C, RAS at 2.65 eV 8%InGaAs matrix layer +1  QD +23%InGaAs SRL, color plot 1  QD 1  QD with 5 nm InxGa1xAs SRL 1  QD with 5 nm InxGa1xAs SRL 1  QD with 5 nm InxGa1xAs SRL 1  QD with 5 nm InxGa1xAs SRL 1  QD with 5 nm InxGa1xAs SRL

86 98 100 100 100

30 30 15 15 15

– – – – 0.23

42.5 42.5 42.5 42.5 42.5

1300 27,000 160,000 68,000 14,000

0.962 0.981 0.984 0.997 0.938

100 100 100 100 100 100

15 15 15 15 15 15

– 0.23 0.29 0.35 0.35 0.35

42.5 42.5 42.5 42.5 5 0

28,500 17,000 7500 3500 8000 30,000

1.000 0.943 0.937 0.934 0.838 0.900

F G H I J K

The thickness of the GaAs separation layer between the two QD layers is 30 nm, capping layer (CL) varied from 0 to 42.5 nm, growth time of the wetting layer was 20 s, waiting time (WT) was 15 and 30 s.

by photoluminescence (PL). The growth interruption waiting time, after 20 s long InAs wetting layer deposition, was also monitored using RAS. RAS Laytec EpiRAS 200TT equipment was used for insitu monitoring of the growth process using color plot and time-resolved modes. The principle of this method was described in Ref. [5], where the RAS spectra of thin InAs layers grown step by step on GaAs are presented. The RAS signal in the wavelength range from 2 to 4.5 eV during growth of the whole structure can be seen on colorplot (Fig. 2a). This mode can be used for the determination of the most suitable energy for time-resolved measurements. However, transient mode of RAS (signal measured at one reflected photon energy) has better time resolution (Fig. 2b). We have used energy 2.65 eV, which is sensitive to the GaAs (and InGaAs) layer growth and is also suitable for the determination of the

growth rate from the monolayer (ML) oscillations, and 4.2 eV energy, which is sensitive to the QD formation (described in detail in Fig. 3a). Although at the energy 2.65 eV the changes of RAS signal intensity are much higher during InAs deposition (see Fig. 2b), at 4.2 eV there is an important feature of RAS spectra during InAs deposition: the signal increases first, then reaches its maximum near 1 ML of InAs thickness [5], then it starts to decrease again. This feature enables us to monitor and control the wetting layer deposition and QD formation. PL was excited by a semiconductor laser with emission wavelength 670 nm, detected by a Ge detector using standard lock-in technique, and analyzed by SDL 1 monochromator. All measurements reported here were performed at room temperature with the excitation light power density of 10 W/cm2.

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Fig. 3. (a) RAS time-resolved signal at 4.2 eV of the QD structure growth, samples A (green line), B (blue) with waiting times (WT) 30 s and C (red, WT ¼ 15 s) and (b) relevant RT photoluminescence measurements.

Fig. 2. (a) RAS color plot of sample E and (b) time-resolved measurement of sample C (4.2 eV) and sample D (2.65 eV) with temperature profile (red line) measured during QD structure growth.

3. Results and discussion The RAS signal at 4.2 eV appeared to be very sensitive to QD formation and enabled us to monitor QD growth process in-situ. Fig. 3a shows RAS time-resolved signal at 4.2 eV of the QD structures grown under different conditions (see Table 1, samples A, B, and C). During the growth of InAs wetting layers the RAS signal increases (up to the deposition of 1 ML of InAs [5]), followed by its decrease when the InAs layer is more than 1 ML thick. For the first QD layer growth no significant change of RAS signal appears during the waiting time for measured double QD layer samples. This can be caused by surfactant behavior of In atoms on the sample surface or by the strong In memory effect—sticking and releasing TMIn on

or from the susceptor and reactor walls. For the second QD layer significant increase of RAS signal was observed during waiting time for samples B and C (see blue and red curves of the second QD layer growth in Fig. 3a). This is a sign of QD formation. At very low InAs growth rates the RAS signal increase could be observed even when the second QD layer is growing. We suppose that this increase is caused by the decrease of the original InAs wetting layer thickness to 1 ML, when QDs are formed and the maximal signal is restored during this process. The RAS signal during the QD growth for samples with very low PL intensity has no significant change of intensity during the waiting time after the second QD layer growth (see the green curve in Fig. 3a). The dosage of InAs (the H2 flow through TMIn bubbler, the growth time and precursor temperature were held constant) in these cases is probably too low for QD formation. The importance of the second (upper) QD layer in our double-stacked QD structures is accentuated by the fact that the second QD layer is responsible for the PL of the

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double-layer QD structures because QDs in the upper layer are usually bigger and thus their electron energy levels are lower than in the smaller size first layer QDs. Due to overlapping of electron wave functions in the QD columns with thin separation layer between QD layers (our case), the radiative recombination originates from QDs in the upper layer [2,6]. The influence of waiting time on the RAS signal and consequently on PL is crucial, see Fig. 3a and b. An example for two waiting times is presented. Sample B with the longer waiting time exhibits six times lower PL intensity than sample C, because the size of QDs continue to increase at the expense of their density during the longer waiting time in the case of sample B. Sample A, which does not show RAS signal intensity increase during waiting time, has more than two orders of magnitude lower PL than sample C. This effect was observed for all similar RAS behavior cases during the growth of our samples. The reason is probably that QDs were not created at all or at sufficient densities under these growth conditions, and thus the structure surface stayed smooth. The similar effect can also be caused by small changes of wetting layer thickness during the QD creation. Influence of matrix and covering InxGa1xAs SRL on PL was also studied. Single QD structures with room temperature PL at wavelengths from 1.25 to 1.55 mm were prepared. The use of In0.08Ga0.92As 5-nm-thick matrix layer before InAs QD growth increases the QD density from 2  109 to 7  109 cm2 [4,7]. The composition of the InxGa1xAs SRL and the thickness of GaAs cap layer influence the PL spectrum. The content of In in the InxGa1xAs SRL was changed from 0% to 35%. For high concentration of In the

Fig. 5. Comparison of normalized room temperature PL spectra of single QD layer samples I, J and K with 5 nm In0.35Ga0.65As SRL and with different thickness of GaAs capping layer. The dependence of the position of PL maxima on the thickness of capping layer is shown in the inset.

decrease of PL intensity was observed (see Fig. 4). Saturation of the red shift of PL emission position is visible (see the inset in Fig. 4). Surprisingly, the dependence of the position of PL emission maximum on the thickness of GaAs capping layer was observed up to 42 nm of its thickness. An example of this behavior of our single QD layer structures with 5 nm In0.35Ga0.65As SRL is shown in Fig. 5. This dependence cannot be explained only by the increase of strain in the structure with increasing thickness of GaAs capping layer, but probably there are some morphological changes of QDs during the capping layer growth.

4. Conclusions

Fig. 4. Comparison of room temperature PL spectra of single QD layer samples F, G, H and I with 5 nm InxGa1xAs SRL. The dependence of the position of PL maxima on the In content is shown in the inset.

The thickness of GaAs capping layer considerably influences the PL spectra of QD structures. Decrease of emitted maximum wavelength with increasing capping layer thickness was observed up to the 42-nm-thick GaAs capping layer. Increase of In content in InxGa1xAs SRL above a certain value is useless from the point of view of the PL red shift, because its saturation was observed with increasing content of In in InxGa1xAs SRL above 23% and was accompanied by substantial decrease of PL intensity. Using of ternary matrix SRL below QD layer increases the density of QDs. RAS in the time-resolved mode measurement at 4.2 eV is very suitable for in-situ monitoring of QD creation and can be used for the estimation of PL intensity of the prepared QD structures.

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Acknowledgments This work was supported by GAAV Grant nos. A100100719, B101630601, GACR Grant nos. 202/06/ 0718, 202/05/0242, and project AV0Z 10100521. References [1] D. Bimberg, J. Phys. D: Appl. Phys. 38 (2005) 2055. [2] A. Hospodkova´, E. Hulicius, J. Oswald, J. Pangra´c, T. Mates, K. Kuldova´, K. Melichar, T. Sˇimecˇek, J. Crystal Growth 298 (2007) 582.

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