Journal of Crystal Growth 227–228 (2001) 1062–1068 Modification of emission wavelength of self-assembled In(Ga)As/GaAs quantum dots covered by InxGa1xAs(04x40.3) layer Zhichuan Niu*, Xiaodong Wang, Zhenhua Miao, Songlin Feng National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China Abstract Red shifts of emission wavelength of self-organized In(Ga)As/GaAs quantum dots (QDs) covered by 3 nm thick InxGa1xAs layer with three different In mole fractions (x ¼ 0:1, 0.2 and 0.3, respectively) have been observed. Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was reduced due to introducing the InxGa1xAs (x ¼ 0:1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1xAs (x50:1) layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The temperature dependent PL spectra further confirm that the InGaAs covering layer can effectively suppress the temperature sensitivity of PL emissions. 1.3 mm emission wavelength with a very narrow linewidth of 19.2 meV at room temperature has been obtained successfully from In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm In0.2Ga0.2As strain reducing layer. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 68.35.Bs; 78.66.Fd; 42.55.Px Keywords: A1. Crystal morphology; A1. Quantum dots; A3. Molecular beam epitaxy; B2. Semiconducting gallium arsenide; B2. Semiconducting indium gallium arsenide 1. Introduction The study of semiconductor quantum dots (QDs) has gained much interest for its fundamental physics as well as potential applications for devices [1–6]. Particularly, the Stranski–Krastanow (SK) growth of InxGa1xAs islands on GaAs *Corresponding author. Tel.: +86-10-82304268; fax: +8682305056. E-mail address: [email protected] (Z. Niu). substrates has been considered as a promising method for the growth of high quality QDs. Recently, the growth of InAs/GaAs QDs covered by an InxGa1xAs or AlAs overgrowth layer or in an InxGa1xAs quantum well has been carried out in order to improve structural qualities or modify energy band structures particularly for obtaining longer wavelengths such as 1.3 or 1.55 mm [7–10]. It has been shown that strain reducing in the overgrowth layer, suppression of In segregation and compositional mixing between the cover layers 0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 8 9 - 7 Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 1063 and InAs islands play key roles for extending the QDs emission wavelength to 1.3 mm. It is effective to modify energy band structures of self-assembled QDs by overgrowth layers with different lattice constants or compositions such as InxGa1xAs instead of GaAs matrix. At the early stage of exploring the feasibility of long wavelength QDs emission, it is very important to know clearly how the structural and optical properties of InAs QDs are influenced by the inserted InxGa1xAs or AlAs covering layers. In this paper, we have carried out a systematic study on the influence of InxGa1xAs (04x40.3) cap layer on the structural and optical properties of InAs QDs grown on GaAs (1 0 0) substrates. It was observed by the transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements that the surface stress of the InAs islands along growth direction was reduced by overgrowth of InxGa1xAs layer on top of the QDs. Photoluminescence (PL) measurements revealed that the red shift of PL peak energy of the QDs was mainly due to the strain reduction of the InAs QDs by introducing the InxGa1xAs layer. It was further shown by calculations that the red shifts of emission energy and variation of linewidth of the PL spectra of the QDs depend on the contents of In mole fraction x of the InxGa1xAs layer. The temperature characterzation of the QDs was improved by the overgrowth of InxGa1xAs layer which served as a layer for preserving the size uniformity of the InAs islands. To further tune the emission wavelength of the QDs to a longer range, we performed systematic studies on the MBE growth and PL measurements of In0.5Ga0.5As/ GaAs QDs which were covered with In0.2Ga0.8As strain reducing layers. An emission wavelength of 1.3 mm with a very narrow linewidth of 19.2 meV at room temperature has been obtained from the In0.5Ga0.5As/GaAs QDs structures. In0.5Ga0.5As/GaAs QDs. After standard chemical cleaning, the substrates were mounted on Mo holders with indium. The growth sequence for the first four samples were as follows: firstly, a GaAs buffer layer grown at 6008C, then 2 monolayer (ML) InAs were deposited at 4508C to form the QDs. After a 5 s interruption, a 3 nm InxGa1xAs overgrowth layer (with different In mole fractions x ¼ 0:0 (i.e., GaAs), 0.1, 0.2 and 0.3 for the four samples) was grown on the top of InAs QDs, followed by a 50 nm GaAs cap layer. The growth temperatures for the upper InxGa1xAs and GaAs cap layers were kept at 4508C to minimize intermixing of In and Ga atoms for preserving the initial shape of the InAs islands. For the second three In0.5Ga0.5As/GaAs samples, the epitaxial layers were just the same as the above samples except the 16 ML In0.5Ga0.5As layers which were gorwn by cycled monolayer deposition methods. For each cycle of (InAs)n/(GaAs)n, n=1 ML, 1.5 ML and 2.0 ML for the three samples, respectively. In order to carry out AFM measurements, QD samples were also grown with the same growth sequence as above but without the 50 nm GaAs cap layer. The growth rates of GaAs and InAs were 1 mm/h and 0.11 mm/h, respectively. The Arsenic (As4) pressure was kept at 3 107 Torr during the growth. The TEM images were taken using a JEM 200 CX electron microscope. The TEM samples were mechanically polished to 70 mm. AFM measurements were performed ex situ using Nanoscope 3 from Digital Instrument with a silicon tip. For the PL measurements, a He–Ne laser (524.5 nm) was used as an excitation source. The excitation power was 1 mW and the sample temperature could be raised from 7 to 300 K. The signals from the samples were detected by a liquid nitrogen cooled Ge detector. 2. Experimental 3. Results and discussion Two series of samples were grown by VG V80 H MKII MBE system on semi-insulating (1 0 0) GaAs substrates. The first four samples were InAs/GaAs QDs. The second three samples were 3.1. TEM and AFM investigation of the samples Fig 1(a) and (b) show the cross-sectional dark field TEM pictures of the InAs QDs covered by 1064 Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 Fig. 1. Cross-sectional dark-field TEM image of 2 ML InAs islands covered by (a): GaAs cap layers and (b): 3 nm-thick In0.1Ga0.9As overgrowth and GaAs cap layers. GaAs or 3 nm In0.1Ga0.9As overgrowth layer, respectively. In Fig. 1(a) one can clearly identify the InAs QDs and the GaAs capping and buffer layers through the contrast of the images and the dark contrast expands obviously to the capping and buffer layers. In Fig. 1(b), however, the contrast of the image is weaker than that in Fig. 1(a). The dimensions of the QDs are estimated to be about 18 nm in base diameter and 3 nm in height. In-rich clusters are clearly seen but no misfit dislocations are observed. Fig. 2(a) and (b) show AFM images (scanning field 1 mm 1 mm) for both the samples with In mole fractions x ¼ 0:2 and 0.3, respectively. The morphology of the 2.0 ML InAs QDs covered by a 3 nm thick In0.2Ga0.8As overgrowth layer is quite flat with the waviness being less than 1 nm, as shown in Fig. 2(a). However, in the case of the QDs covered by the In0.3Ga0.7As overgrowth layer, the surface morphology shows larger islands (about 60 nm in bottom diameters) indicating the formation of the InxGa1xAs islands on the top of InAs QDs, as shown in Fig. 2(b). The weaker contrast in the TEM image of Fig. 1(b) directly evidences that the strain field in the QDs covered by the In0.1Ga0.9As layer along the growth direction is smaller than that of the QDs covered by GaAs layers, since the dark contrast in the TEM images around the islands has been proved to be due to the strain effect induced Fig. 2. Atomic force microscope top graphical images of InAs QDs covered by 3 nm-thick (a): In0.2Ga0.8As and (b): In0.3Ga0.7As cover layers. Scanning fixed: 1.0 mm 1.0 mm. in the capping and buffer layers [11]. We have known that [8] the stress component and consequent strain in the growth direction can be reduced by reducing the mismatch of the lattice constant between the layers; therefore, further reduction of the strain can be realized by increasing the In content in the InxGa1xAs cover layers. In our experiment, the In0.2Ga0.8As layer with less indium has large lattice mismatch to InAs dots and preferred to grow around the dots flattening the regions besides the dots, as shown in Fig. 2(a). However, if the In content x of the InxGa1xAs layer is above 0.3, the overgrowth InxGa1xAs layer will easily nucleate to bigger islands on the top of InAs dots, leading to unsymmetrical distribution of the stress on the top of InAs islands and inevitably resulting in size fluctuation Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 of the InAs QDs. These will be proved by the following PL measurements. 3.2. PL measurements of the samples The PL spectra taken at 10 K are shown in Fig. 3, in which the inset shows the spectra of PL integrated intensity versus full width at half maximum (FWHM) for all samples with different In contents of the InxGa1xAs layers. In the case of the InAs QDs covered by a GaAs layer (x=0.0), the PL peak energy is positioned at 0.97 mm (1.27 eV) with an FWHM of 62 meV. In all cases of the QDs covered by a 3-nm-thick InxGa1xAs (x50.1) strain-reducing layer, considerable red shifts of the PL peak energies with enhanced integrated intensity were observed. The minimum FWHM of 36 meV is obtained for the case of x=0.2, while the FWHM is 39 meV for the case of x=0.3. Generally, the red shifts of the PL peak of the InAs QDs covered by InxGa1xAs layers are mainly due to the following reasons [8]: (1) the strain in the growth direction is decreased by the overgrowth of InxGa1xAs layer. (2) InAs/GaAs intermixing during the growth can be effectively reduced. Here we separately consider how the two factors modify the energy band gaps of the InAs QDs. Firstly, we consider the effect of strain reduction in the overgrowth layer which can modify the energy gap of the QDs. We could assume simplistically that the QDs are spherical in shape and covered by InxGa1xAs or GaAs. In the case of QDs covered by In0.2Ga0.8As layer, the strain component and pressure within the QDs can be expressed as [12] P ; ery ¼ eyf ¼ efr ¼ 0; err ¼ ð1Þ 3l þ 2m P¼ 4ð3l þ 2mÞða 1Þ ; 4a þ 8 ð2Þ where l and m are Lame’s constants, and a (=1.0565) is the ratio of the lattice constants of InAs and In0.2Ga0.8As. So the changes of conduction- and valence-band edges are dEc;v ¼ ac;v ð3err Þ ¼ 0:055ac;v ðeVÞ; ð3Þ where ac;v in Eq. (3) are the deformation potentials of InAs. Using ac=5.08 eV, av=1.00 eV [13], so a=(acav)=6.08 eV, we obtain an energy-gap change of dE ¼ dEc dEv ¼ 0:337 eV: ð4Þ Similarly, the energy change of the band gap of the QDs covered by a GaAs layer can be calculated (by the same equations as above) as follows: dE 0 ¼ 0:425 eV: Fig. 3. PL spectra of InAs QDs covered by 3 nm-thick InxGa1xAs (x=0, 0.1, 0.2, and 0.3) strain-reducing layer at 10 K with an excitation intensity of 1 mW. In the inset, the linewidth of the PL spectra versus In contents of the InxGa1xAs layer is shown. 1065 ð5Þ So the value of the red shift of the emission energy deduced from Eqs. (4) and (5) is 88 meV. Secondly, we consider the reduction effect of InAs/GaAs intermixing due to the In segregation during the growth which also leads to some red shifts of the PL emission energy. In fact, the compositional mixing of the InAs/GaAs interfaces not only degrades the size uniformity of the islands but also modifies the energy band structures of the QDs. It has been estimated theoretically that the red shifts of the emission energy of the QDs caused by the In segregation are approximately 20– 30 meV [14]. Based on the calculations discussed above, one can estimate approximately all PL energy 1066 Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 shifts for the samples with different In mole fractions of InxGa1xAs cover layers as shown in Fig. 4, in which the experimental results are presented for comparison, revealing the fact that the main contribution to the red shift of PL peak energy is due to the effect of the InxGa1xAs layers. The narrowing of the PL linewidth of the QDs covered by InxGa1xAs layer is possibly due to the InxGa1xAs overgrowth layers which suppress the composition mixing or In segregation effectively during the growth of covering layers preserving the size fluctuations of the InAs/GaAs islands [8]. The initial shape of the InAs islands covered by InxGa1xAs layer can be preserved at a relatively earlier stage during the overgrowth of InxGa1xAs layer than during the overgrowth of GaAs capping layer. The In segregation during the growth of InxGa1xAs covering layer on the top of the InAs islands will be suppressed, because the In elemental source is supplied during the growth of InxGa1xAs covering layer. In our experiment, for the case of InxGa1xAs cover layer with the In mole fraction x=0.2, the size uniformity of the QDs was improved as shown in the AFM images of Fig. 2(a) and the PL spectra of Fig. 3. While for the case of the InxGa1xAs covering layer having higher In contents x50.3, the bigger In0.3Ga0.7As islands were nucleated on the top of the InAs islands as shown in AFM pictures of Fig. 2(b). The nucleation of In0.3Ga0.7As islands might create unsymmetrical distribution of stress on the top of InAs islands, leading to an increment of the FWHM from 36 emV for the case of x=0.2– 39 emV for the case of x=0.3. It will be further discussed in the following that the InxGa1xAs cover layer also plays a key role for the improvement of temperature dependent characterization of the InAs QDs. The temperature dependent spectra of the PL peak energy for the four samples are compared as shown in Fig. 5. The red shifts of the PL peak energy correspond to the In mole fraction x of the InxGa1xAs cover layers. The red shift of the temperature dependent PL signals for the cases of x=0.1, 0.2 and 0.3 is smaller than that of x=0.0, revealing less dependency or sensitivity of PL emission to the variation of temperatures. In contrast, there is no difference in the red shifts of the PL peak between the cases of x=0.2 and 0.3. It can accordingly be concluded that the red shift of the temperature dependent PL peak energy is effectively preserved mainly due to the overgrowth of InxGa1xAs layer leading to improvements of the size uniformity and relaxation of the surface strain of the QDs, and partly due to the bandgap narrowing effect at higher temperature. It has been well-recognized [15] that the self-organized InAs QDs system can be regarded as a coupled system in which the wave functions of Fig. 4. Calculated (dashed line) and experimental (solid line) data of the PL peak energy shifts vs In content of the InxGa1xAs (04x40.3) cover layers. Fig. 5. Temperature dependent spectra of the red shifts of the PL peak energy of the QDs corresponding to different In mole fractions from 0.0 to 0.3. Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 carriers in an individual QD penetrate into adjacent dots. The coupling and relaxation effects of the carriers will be enhanced with temperature increase due to increased electron–photon interaction in the QDs. As a result, photogenerated carriers transfer and relax into energetically lowlying states, giving rise to the fast red shift of the exciton energy. For the samples of InAs QDs covered by InxGa1xAs (0.3>x50.1), the size uniformity of the QDs was improved by the overgrowth of InxGa1xAs layer, resulting in better uniformity of energy states. Therefore, the shifts of the PL peak energy caused by the temperature increase will be smaller due to better uniformity of the energy states of the InAs QDs covered by InxGa1xAs layers. It is reasonable that the suppression of intermixing or In segregation for the case of x=0.3 should be stronger than that for the case of x=0.2. But the size distribution of the QDs is obviously worse due to the nucleation of InxGa1xAs on the top of InAs islands as evidenced by the AFM images and PL measurements. These two contrary aspects lead to the same energy shifts for x=0.2 and 0.3 cases as shown in our spectra of the PL peak energies versus temperatures. In precise treatment for the explanation, the temperature dependence of the bandgap narrowing effect should be taken into account. It has been shown [14] that the nonlinear decrease of the sublevel separation of the QDs with InGaAs overgrowth layer is certainly due to the strain lattice distortion in the embedded dot structures, while the QDs without overgrowth layer change linearly during the increase of temperature. For complete understanding of the strain field dominating effects on the temperature dependence of the PL peak of the InAs QDs covered by the InGaAs layers, we need further investigation for the relationship between sublevel and strain field distribution or lattice distortion around the dots. In order to further increase the red shifts of the PL emission wavelength, the In0.5Ga0.5As/GaAs QDs have been grown. From the above discussion, we know that within three kinds of strain reducing layers with different In mole fractions, the In0.2Ga0.2As layer is the best one to modify the structural and optical properties of the InGaAs 1067 QDs. So the In0.2Ga0.2As cover layer was used for the In0.5Ga0.5As/GaAs QDs samples. Their optical properties were studied by PL measurements. As shown in Fig. 6, 1.3 mm ranged emission wavelengths at room temperature are obtained from all three samples. Particularly, the linewidth of the PL spectra of the sample n=1.0 is 19.2 meV which is one of the best results, to our knowledge, so far. The inset of Fig. 5 shows PL spectra dependency on laser excitation power intensity taken at 10 K from the sample of n=1.0, showing clearly the excited states at higher excitation power. Those PL results indicate that the three In0.5Ga0.5As/GaAs quantum dots have very good optical properties with a useful emission wavelength of 1.3 mm. Although the deposition cycle (n=1.0, 1.5, 2.0) for the growth of In0.5Ga0.5As/ GaAs QDs is different, which might cause different strain states to remain in the InGaAs/GaAs Fig. 6. Room temperature PL spectra for three In0.5Ga0.5As/ GaAs QDs samples grown via cycled (InAs)n/(GaAs)n (n=1.0, 1.5, 2.0, respectively) monolayer deposition, in which the narrowest FWHM of 19.2 meV is shown for the sample n=1.0. In the inset, the excitation power dependent PL spectra are shown for the sample n=1.0, the excited energy states are clearly resolved at higher excitation power intensity. 1068 Z. Niu et al. / Journal of Crystal Growth 227–228 (2001) 1062–1068 interfaces and lead to variation of energy band structure of the QDs [16], the similar emission wavelength between the three samples reveals that the In0.2Ga0.2As covering layer modifies the stress in the interfaces of the InGaAs and GaAs layers leading to better uniformity in strain distribution. So this strain reducing effect induced by InGaAs overgrowth layer on top of InGaAs/GaAs QDs structure is confirmed again as a very important factor for reproducibility of growth of QDs with good optical quality. obtained successfully, showing good reproducibility in the growth of such QD structures. 4. Summary References The influence of InxGa1xAs covering layer on the structural and optical properties of InAs/GaAs self-organized QDs grown by molecular beam epitaxy has been investigated by using TEM, AFM and PL measurements. The TEM images demonstrate that the stress in the InAs dots along the growth direction can be reduced by introducing an InxGa1xAs (0.14x40.3) layer between the InAs islands and GaAs cap layers. The AFM pictures confirm that the surface of InAs islands was flattened during the growth of 3-nm-thick InxGa1xAs (x=0.2) cover layer on the InAs islands. 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