GaSb quantum dot growth using InAs quantum dot stressors

GaSb quantum dot growth using InAs quantum dot stressors

Journal of Crystal Growth 248 (2003) 333–338 GaSb quantum dot growth using InAs quantum dot stressors a, L. Muller-Kirsch . *, N.N. Ledentsova, R. Se...

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Journal of Crystal Growth 248 (2003) 333–338

GaSb quantum dot growth using InAs quantum dot stressors a, L. Muller-Kirsch . *, N.N. Ledentsova, R. Sellina, U.W. Pohla, D. Bimberga, I. H.auslerb, H. Kirmseb, W. Neumannb a

Institut fur Technische Universitat . Festkorperphysik, . . Berlin, Hardenbergstr. 36, 10623 Berlin, Germany b Institut fur . Physik, Humboldt Universitat . zu Berlin, Invalidenstr. 110, Berlin 10115, Germany

Abstract Metalorganic vapor phase epitaxy of GaSb quantum dots (QDs) grown on top of a layer of InAs seed QDs shows a vertically aligned correlation if thin GaAs spacer layers deposited at low temperature are used. Introduction of an annealing step after spacer deposition strongly improves the crystalline quality of the spacer layer. Vertically anticorrelated ordering is found on annealed spacers in addition to aligned ordering. Good optical properties are achieved for stacked QD structures with an InGaAs QD layer on top of seeded GaSb QDs. The photoluminescence of these structures is red shifted by 75 meV with respect to the emission of a single InGaAs QD layer. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 68.55. a; 78.66.Fd Keywords: A3. Metalorganic chemical vapor deposition; A3. Quantum dots; B1. GaSb

1. Introduction Quantum dots (QDs) are presently attracting much interest because of their huge potential for novel GaAs-based photonic devices [1]. An active medium consisting of QDs in a GaAs matrix allows to extend the emission wavelength of lasers and amplifiers into the near infrared range not covered by structures of higher dimensionality [2]. While high power lasers based on metalorganic chemical vapor deposition (MOCVD) grown InGaAs quantum dots as active medium were demonstrated at 1160 nm emission wavelength [3], *Corresponding author. Tel.: +49-30-314-22061; fax: +4930-314-22569. E-mail address: [email protected] (L. Muller-Kirsch). .

the important wavelength of 1300 nm has not been achieved so far using MOCVD. Sb containing QDs may present an alternative route to achieve this or even longer wavelengths. GaSb QDs in a GaAs matrix have only been scarcely investigated due their type II band alignment which provides strong confinement only for holes [4]. On the other hand, the spatially indirect transition in type II heterostructures provides the potential of wavelength tuning for a suitable design. A combined InGaAs (type I)– GaAsSb (type II) quantum well (QW) bilayer showed a red shift of the emission wavelength and enhanced electron confinement as compared to single GaAsSb quantum wells [5,6]. In this system, the holes reside in the GaAsSb layers with a larger localization energy than in the InGaAs layer while the electrons are confined in the InGaAs layer.

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 8 9 5 - X


L. Muller-Kirsch et al. / Journal of Crystal Growth 248 (2003) 333–338 .

Since both layers are compressively strained, plastic relaxation can only be avoided for thin layers or low Sb and In composition, limiting the use of this approach. The investigation of elastically relaxed QDs instead of quantum wells is thus an interesting approach. Moreover, one QD layer may serve as a seed layer to control independently density and size of the subsequently grown QD layers [7–9]. A first structural study of such a hybride system using InP QDs on InAs QD stressors demonstrated density control and the possibility to reduce the size inhomogeneity of the seeded InP QDs [10]. In this work, we present the first study of the structural and electronic properties of combined GaSb and In(Ga)As QDs.

3. Structural correlation of the GaSb and InAs quantum dots 3.1. Samples without annealing of the GaAs spacer Nucleation control of QDs by stressor QDs is known to induce a vertical ordering of the quantum dots [4–6]. Cross-sectional TEM images of 2.5 ML thick GaSb QD layers, separated by GaAs spacer layers of 3.5 and 7.5 nm thickness from the InAs seed QDs are shown in Fig. 1. The GaAs spacer layers have not been annealed in these samples. In both samples, the GaSb QDs are vertically aligned to the InAs seed QDs. Since both QD layers are compressively strained, the

2. Experimental setup The samples were grown in a low-pressure MOCVD reactor on semi-insulating GaAs (0 0 1) substrates using TEGa, TMIn, TESb and AsH3 for the QD layers and TMGa, TMAl and AsH3 for the buffer and cap layers. The buffer layer consists of 100 nm GaAs and 100 nm Al0.33Ga0.77As grown at 7001C, followed by 100 nm GaAs grown at 6001C. Subsequently, the temperature was reduced to the growth temperature of the InAs QDs. Deposition of 2.5 ML InAs was followed by the growth of a low-temperature GaAs spacer layer and deposition of the GaSb layer. Samples were prepared with and without an annealing step after epitaxy of the GaAs spacer. Before GaSb deposition, the temperature was reduced to 4701C and AsH3 was switched off for 5 s to remove residual As from the reactor. The GaSb layers were deposited at a V/III ratio of 6 in the gas phase and a growth rate of B0.15 monolayer/s (ML/s), followed by a growth interruption of 5 s without Sb flux. After the subsequent deposition of a 7 nm thick part of the GaAs cap, the growth temperature was ramped up to 6001C for growth of the remaining cap layer consisting of 40 nm GaAs, 20 nm Al0.33Ga0.77As and 5 nm GaAs. The samples were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and by photoluminescence (PL) using the 514 nm Ar+ line for excitation and a Ge pin diode for detection.

Fig. 1. Dark field cross-sectional TEM images using the chemically sensitive (0 0 2) reflection of GaSb QDs seeded by InAs QD layers with a separating low-temperature GaAs spacer layer of (a) 3.5 nm and (b) 7.5 nm thickness. (c) Dark field image of GaSb QDs with InAs seed QDs using the strain sensitive (2 2 0) reflection. Before deposition of 2.2 ML GaSb the GaAs spacer was annealed as described in the text.

L. Muller-Kirsch et al. / Journal of Crystal Growth 248 (2003) 333–338 .

variation of the surface strain above the InAs QDs favors nucleation of the GaSb QDs on the top of the InAs seed QDs, in analogy to the previously studied growth of seeded InAs QDs [6]. The surface of the 3.5 nm thick GaAs spacer is corrugated (Fig. 1a). In contrast, the surface of the 7.5 nm thick spacer is flat (Fig. 1b), resulting in an approximately constant width of the GaAs barrier between the InAs and GaSb QDs. Each InAs QD induces a nucleation of a GaSb QD on top in both cases. Consequently, the density of GaSb QDs is defined by the density of the InAs seed QDs of 1  1011 cm 2 as deduced from plan view TEM images (not shown here). Without InAs QD seeding a GaSb QD density of 3  1010 cm 2 was observed for identical growth conditions of the GaSb layer [4]. This demonstrates the successful nucleation control of GaSb QDs and the ability to take advantage of the achievable high density of InAs/GaAs QDs. 3.2. Samples with annealed GaAs spacers The InAs and GaSb QDs studied above are expected to be electronically coupled due to the thin spacers used. However, PL spectra revealed degraded optical properties of these hybride structures. The PL intensity is reduced by two orders of magnitude compared to samples containing only InAs QDs. Additionally, a broad luminescence background appears at lower energies. The structure is assumed to induce a spacial separation of electrons and holes, reducing strongly the electron–hole overlap necessary for light emission. Defects related to the low-temperature growth of the GaAs barrier may hence reduce efficiently the integral luminescence intensity. In order to increase the optical yield, an annealing step was introduced to improve the crystalline quality of the spacer layer. In InGaAs QD stacks such a procedure was found to enhance the luminescence intensity as well as the performance of laser devices [3]. Here, quantum wells were used to optimize the annealing step. An InAs QW was capped with 3 nm GaAs which was deposited at the same low temperature and subsequently annealed for 12 min at 6001C before growing a GaSb QW. This procedure enhances the


integral luminescence yield of the closely stacked QWs dramatically, demonstrating the impact of the improved GaAs spacer. The effect of annealing on the surface morphology of such GaAs spacers used in hybride QD samples is seen in AFM images. In the left and the middle part of Fig. 2 the surface of the lowtemperature GaAs layer is shown before and after the annealing step, respectively. For the as-grown surface a high density of monolayer steps is found. The annealing step leads to a reorganization of the growth surface to a morphology similar to that of a step flow mode observed at higher growth temperatures. A significant part of the luminescence yield improvement is attributed to a reduction of the defect density. The right part of Fig. 2 shows an AFM image after the deposition of 2 ML GaSb on the annealed 3 nm thick GaAs spacer. Large holes, 3 nm deep and B200 nm wide, show that InAs evaporated during the annealing step at places where the QDs were not sufficiently covered. The GaSb layer deposited afterwards leads to the formation of B10 nm high and

Fig. 2. AFM images of InAs quantum wells capped with 3 nm low-temperature GaAs (left) without and (middle) with an annealing step, respectively. (right) InAs QDs capped with 3 nm GaAs and 2 ML GaSb using an annealing step after the GaAs deposition.

L. Muller-Kirsch et al. / Journal of Crystal Growth 248 (2003) 333–338 .

4. Optical properties of stacked GaSb and InAs quantum dots 4.1. Properties of GaSb dots on InAs seed dots In Fig. 3a, the room temperature luminescence of a sample with annealed 7.5 nm thick GaAs spacer is compared to that of a reference sample containing only the InAs QD seed layer. The InAs QD luminescence at 1.16 eV is reduced in the hybride sample by two orders of magnitudes and a broad band labeled A centered at 0.98 eV appears. This luminescence is supposed to originate from hybride QD structures. With increasing excitation density (not shown here) band A shows a blue shift and a saturation faster than the luminescence of InAs WL

T = 300K

PL-Intensity (arb. units)

B100 nm wide clusters, which are so large that they exceed the critical thickness for plastic relaxation. Only some small objects are found that may act as QDs. This study shows that the thickness of the GaAs spacer is a very critical parameter when the important annealing step is applied. The annealing step also effects the structural correlation of the stacked QDs. A cross-sectional TEM image of a sample with an annealed 7.5 nm thick GaAs spacer is shown in Fig. 1c. A 2.2 ML thick GaSb layer was deposited, closer to the critical thickness for QD formation (about 2 ML) as compared to the samples described above. Different kinds of vertical correlation appear in this sample. In the left part of the image, two QDs are vertically aligned. In contrast, the InAs QD next to it induces two small anticorrelated GaSb QDs with an angle of about 381 relative to the center of the InAs QD. In addition, some InAs QDs do not lead to a nucleation of a QD in the subsequent GaSb layer. This effect can be explained by the reduced deposition thickness of GaSb in this sample. Anticorrelated growth of the GaSb QDs was confirmed on several TEM images of three different samples grown under similar conditions. Furthermore, AFM images of these samples show pairs of GaSb QDs with a separation also observed in TEM images. Anticorrelated ordering was previously reported for the PbSe/PbEuTe material system [11] and for CdZnSe submonolayer islands in ZnSe [12]. For the IV–VI material system, the strong anisotropy of the strain field in the crystal leads to trigonal QD lattices in subsequently stacked QD layers. An anisotropy of the strain tensor also exists in GaAs (0 0 1) though being much weaker than in rock salt materials [13]. Consequently, the strain-driven anticorrelated ordering is particularly found in samples with a smooth spacer surface morphology, i.e. samples with annealed spacers. It should be noted that the reduced GaSb deposition thickness in the sample presented in Fig. 2c favors the observation of small QDs. Depositing more material leads to larger sizes of the QDs which at last merge into one single QD on the top of a buried QD.


GaAs InAs QDs


1.0 (a)




Energy (eV)

T = 300K PL-Intensity (arb. units)


InGaAs QDs


InGaAs/GaSb QDs 0.9






Energy (eV)

Fig. 3. (a) PL spectra of GaSb QDs with an InAs stressor layer and of a single InAs QD layer (gray line). (b) PL spectrum of 2 ML GaAs and InGaAs QDs deposited subsequently to the GaSb QDs. For comparison the luminescence of a single InGaAs QD layer is included (gray line).

L. Muller-Kirsch et al. / Journal of Crystal Growth 248 (2003) 333–338 .

the InAs QDs that dominates the spectrum at high excitation densities. The electronic properties are determined by the spatial separation of electrons and holes in the InAs and GaSb layers, respectively. From the respective confinement of the electrons and holes, a lowering of the transition energy to about 1.0 eV as well as a reduction of the oscillator strength are expected. The latter might explain the pronounced nonlinearity of the 1.0 eV PL peak intensity.


decrease of the luminescence intensity is attributed to strain-induced defects, as the strain in the structure is strongly increased by the additional QD sheet. On the other hand, the moderate reduction of the PL intensity at high excitation density suggests still large overlap of the electron and hole wave functions. This is a promising issue; e.g. the gain in a QD-based optical device is much more critical than for higher-dimensional nanostructures due to the reduced volume fill factor of the active material.

4.2. Inverted structure with InGaAs dots on seeded GaSb dots 5. Conclusions The structural studies in Section 3 showed that the GaAs spacer thickness is constricted by the required annealing step. For spacers thinner than B7 nm the evaporation of InAs destroys the smooth surface morphology. Therefore, for further optimization the sequence of InAs and GaSb QDs was extended with 2 ML GaAs on top of the GaSb QDs, followed by an InGaAs layer forming a third QD layer. In this structure, the sequence of GaSb and In(Ga)As QDs is inverted in the upper layers with respect to the samples studied above. For the growth of the InGaAs QD layer the growth temperature was increased by 201C after deposition of the 2 ML GaAs. The GaAs layer serves as a protection during the temperature ramp and prevents an Sb termination of the surface before InGaAs deposition. 2.4 ML InGaAs with a nominal In content of 0.65 was deposited at B4901C. The luminescence spectrum of such an inverted structure is shown in Fig. 3b for an excitation density of 5 W/cm2. In addition, the spectrum of a sample containing only an InGaAs QD layer is included as a reference. The inverted structure with three QD layers shows a QD luminescence, which is red shifted by 75 meV with respect to the reference sample. In addition, the intensity of the ground state QD luminescence is decreased by a factor of 8 at 5 W/cm2 and by factor of 3 at 5 kW/cm2 excitation density (not shown). The shift of the luminescence is assigned to the closely stacked GaSb and InGaAs QDs, demonstrating the possibility to lower the emission energy as compared to single InGaAs QD structures. The

Vertically aligned and anticorrelated growth of GaSb QDs was observed using InAs seed QD layers. The kind of correlation depends sensitively on growth conditions, namely annealing of the separating GaAs spacer layer and the thickness of the GaSb layer. The anticorrelated nucleation of the GaSb QDs is attributed to the anisotropy of strain in the GaAs spacer. Hybride QD structures with inverted sequence were fabricated depositing an InGaAs QD layer closely above the GaSb QDs. These structures exhibit a red shift of the PL by 75 meV as compared to single InGaAs QD layers. The structures demonstrate the potential of extension of the wavelength range of MOCVD grown QD-based light emitting devices to the IR.

Acknowledgements This work was supported by SFB296 and IB of BMBF (Rus01/228). The authors like to thank Dr. V. Shchukin for helpful discussions, L.M.-K. is particularly indebted to Bookham Technologies for financial support.

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