Project âSviluppo di convertitori termofotovoltaici ad alta efficienza basati su nanostrutture di semiconduttoriâ. The authors would like to acknowledge Ms. I.
Mat. Res. Soc. Symp. Proc. Vol. 722 © 2002 Materials Research Society
Defect free InGaAs-based strain balanced MQW grown on virtual substrate by metallorganic chemical vapor deposition A. Passaseo, R. Cingolani, M. Mazzer1, M. Lomascolo1, S. Tundo1, L. Lazzarini2, L. Nasi2 G. Salviati2, K.W. Barnham3 National Nanotechnology Laboratory of INFM, Dept. Ing. Innovazione, University of Lecce, via Arnesano Lecce, ITALY 1 CNR-IME Institute, Via Arnesano, I-73100 Lecce, Italy 2 CNR-MASPEC Institute, Parco Area delle Scienze 37/A, I 43010 Fontanini- Parma, Italy 3 Quantum Photovoltaics Group, Imperial College of Science and Technology, London SW7 2AZ, UK
ABSTRACT In this work we describe a novel system for photovoltaic applications which combines InGaAs based strain-balanced multiple quantum wells (MQWs) with a "virtual substrate", designed to extend the absorption edge of the photovoltaic devices to about 1 eV. The virtual substrate is designed by properly choosing a sequence of InGaAs layers having different In content, in order to obtain the desired lattice parameter at the topmost layer and to confine at the deepest interfaces the misfit dislocations, well away from the QW active region. A series of InGaAs p-i-n junctions, containing a strain balanced MQW in the intrinsic region, were deposited by metallorganic chemical vapor deposition on different virtual substrates. In all the samples the virtual substrates were proved to be successful to grow zero net strain MQW and to confine defects at the buffer/substrate interface. Transmission electron microscopy observation shows that no defects propagate from the strain accommodating layers to the active region. The total density of threading dislocations reaching the surface was found to be less than 1*E5/cm2. The confined misfit dislocation network, however, results in marked cross-hatched morphology that was found to affect the lateral strain distribution in the whole structure. By optimizing the growth condition of the structures, the influence of the surface roughness induced by CH pattern is partially suppressed.
INTRODUCTION Strained InGaAs/GaAs quantum wells (QWs) are currently used for a large variety of micro-electronic and optoelectronic devices, including high efficiency photovoltaic cells. However, on such structures, the lattice mismatch places severe limits in extending the absorption edge to significant lower energies and on the number of QWs that can be accommodated before strain relaxation takes place. In order to overcome substrate lattice parameter limitations, strain balanced approach in GaAs based QW solar cell can be used, by compensating the compressive strain of the InGaAs QWs with appropriated tensile strain barriers which lead to an intrinsic region average lattice parameter equal to that of GaAs. With this method a moderate shift in the absorption edge (up to 960 nm), together with an higher density of active layer, have been obtained [1]. However, in order to move the spectral response to significantly higher wavelengths and to obtain a large number of QWs in the intrinsic region without plastic relaxation, is necessary to combine the strain balanced approach with the use of a proper virtual substrate designed to accommodate the higher lattice misfit while confining misfit dislocations (MDs) far from the QW region. K11.8.1
In this work we describe a GaAs-based heterostructure were InGaAs strain-balanced MQW, designed to extend the absorption edge of the photovoltaic devices to about 1 eV, is grown on a virtual substrate by metallorganic chemical vapor deposition (MOCVD). Transmission electron microscopy (TEM), cathodoluminescence (CL) and atomic force microscopy (AFM) were used to characterize the structures. In all the samples the virtual substrates were proved to be successful to grow zero net strain MQW and to confine defects at the buffer/substrate interface. CL measurement shows that the absorption wavelengths were efficiently shifted up to about 1300 nm.
EXPERIMENTAL DETAILS The p-i-n samples studied in this work consist of 500 nm p-doped layer, 10 period of InGaAs strain balanced QWs as intrinsic region and 1600 nm n-doped layer. The structures were deposited on fully relaxed virtual substrates having the equilibrium lattice parameters equivalent to that of an InxGa(1-x)As alloy with x varying from x = 0.15 and x = 0.35. The virtual substrate consist in a sequence of InGaAs layers with thickness and compositions chosen to drive the relaxation to occur without exceeding at any interface the dislocation density generated in the case of a single InGaAs layer having the designed equilibrium lattice parameter. The empirical model used to design the virtual substrate will be reported elsewhere. The growth was performed in a horizontal LP-MOCVD system (AIXTRON 200 AIX), equipped with a rotating substrate holder, at 20 mbar and with a growth temperature of 650 °C. TMGa, TMIn-solution and pure AsH3 were used as source materials and palladium purified H2 with a flow rate of 7 slm was used as a carrier gas. The TMIn and TMGa partial pressures were properly adjusted in each layer in order to obtain the designed In composition at the fixed growth rate of 4 µm/h. Only the InGaAs wells were grown at the growth rate of 2 µm/h. Growth was interrupted for 40 s between subsequent layer for surface and flows stabilization. The nominal In content in the layers and the growth rate were verified on thick samples by scanning electron microscopy (SEM) and X-ray measurement. Transmission Electron Microscopy (TEM) was performed on a Jeol 2000FX microscope operated at 200 kV. Observations were made on (100) plan view and (-110) cross section specimens with g=220 and g=200 which are known to be highly sensitive to strain field and chemical composition respectively. The CL investigation were performed in plan view on a Cambridge Stereoscan 350 SEM equiped with an Oxford Instrument mono-CL system. A Digital Nanoscope IIIA microscope was used in contact mode for Atomic Force Microscopy (AFM) investigations. RESULTS AND DISCUSSION Figures 1 show the cross sectional TEM (g=220) micrographs of samples grown on virtual substrates with equilibrium lattice parameter (ELP) corresponding to the In content x = 0.15 (a) and x = 0.35 (b). On both samples, MDs were found to be efficiently confined in the inner InGaAs layers forming the virtual substrates, and, apart from the surface undulations caused by the non-homogeneous strain field induced by the dislocations, no defects have been observed to propagate from the strain-accommodating layers to the active region. Large area plan-view TEM investigation carried out on the sample of fig. 1(a), proved that the threading
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Figure 1. DF 220 TEM cross sectional image of samples with equilibrium lattice parameter corresponding to the In content x = 0.15 (a) and x = 0.35 (b). The presence of the straininduced contrast pattern is clearly visible. dislocations reaching the free surface are less than 105 cm-2 and that the MQW region is not affected by the presence of MDs. This is confirmed by X-Ray reciprocal lattice maps (not reported here) were is found that, according to the buffer design, the top of the structure is almost completely relaxed. The InGaAs cladding layers and the QW structure appear tilted with respect to the substrate, leading to the conclusion that the misfit dislocations lie almost completely at the substrate-buffer interface [2] The underlying misfit dislocation network, however, results in marked cross-hatched (CH) morphology with a periodicity of about 1 µm, as evidenced by AFM measurements, that affect the lateral strain distribution in the whole structure. Actually, the CH morphological features are spatially correlated to the presence of regular bright and dark columnar regions that invert contrast across the middle of each ridge of the CH surface morphology; this straininduced contrast modulation has the same periodicity of the CH pattern. Superimposed to the described strain modulation, a wavy fine contrast modulation, confined only in the MQW region, is found in the samples shown in figures 1. This correspond to thickness fluctuations of the MQW layers that are localised only in the CH valley, as demonstrate by figure 2, were MQW grown in correspondence of the CH valley are evidenced.
100 nm Figure 2. DF 200 TEM cross sectional image showing the MQW thickness modulation at the CH valleys of the sample with ELP of x=0.15 shows in figure 1(a). K11.8.3
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Figure 3. AFM image of sample with ELP of x=0.15 showing macro steps of the CH pattern (a). The image shows the same geometrical distribution of the wavy strain modulation observed in TEM plan view (b). This morphological instability have been already observed in strain balanced structures and explained by compositional fluctuations [3]. However, in our samples, step bunching phenomena is more likely to be responsible for MQW lateral thickness modulation. Actually, CH consists of nearly flat topmost surface separated by V-shaped valley (as shown by the AFM of figures 3(a)), where an higher density of step is present; the geometrical distribution of the valley is the same of the fine strain modulation observed in TEM plan view (figure 3(b)). The ribbon-like morphology is then developed by the preferential incorporation of the growing species at these steps. Moreover, in correspondence of each valley, an In-rich thin vertical region is presents, due to the preferential In incorporation on the small (100) surface that lies at the bottom of the V-shaped valley, like already found in InGaAs quantum wire samples [4]. The described morphological instability affects more severely samples grown on virtual substrates having higher equilibrium lattice parameter, as evidenced by the sample shown in figure 1(b), grown with an equilibrium lattice parameter corresponding to the In content x = 0.35. In this sample the MDs are still efficiently confined at the buffer-substrate interface, demonstrating that the virtual substrate is working properly, but the intensity of the strain induced contrast is significantly stronger respect to the sample with ELP of x=0.15 of figure 1(a). More important, new dislocations are created at the topmost MQW layers grown in correspondence of the CH grooves and propagate up to the sample surface. The stronger CH-induced lateral strain distribution found on this sample results in a larger thickness fluctuations of the MQW layers whose thickness locally overcome the critical layer thickness, inducing the generation of new dislocation. Hovewer, in all the samples the approach to combine strain balanced structures with the use of a proper virtual substrate is effective to shifts the sample emission wavelengths significantly above with respect the GaAs. Cathodoluminescence measurements carried out at 77 K on the samples shown in figure 1 are shown in figures 4. The sample grown with a lower equilibrium lattice parameter exhibit a fairly sharp peak around 1025 nm with higher efficiency with respect the cladding layer emission, reflecting the better structural quality obtained for the MQW structure. On the contrary, the MQW emission of the sample grown with the highest equilibrium lattice parameter, centered at the designed wavelength of 1300 nm, result to be broadened by the larger MQW thickness modulation and by dislocations.
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Figure 4. CL emission of the sample grown with equilibrium lattice parameter corresponding to the In content x = 0.15 (a) and x = 0.35 (b). In both samples the spectra were collected at 77 K and beam accelerating voltage of 20 keV
The structural and morphological phenomena described so far are generated mainly by the non planar surface resulting after the growth of the virtual substrate. In order to reduce, or even suppress, the influence of the CH morphology, we have grown a sample having the same structure and equilibrium lattice parameter of the sample shown in figure 1(a), where growth conditions, that should lead to surface planarization, were chosen for the growth of the InGaAs cladding layer. The results obtained with the different groth condition are shown in figures 5. As evident, the desired results was partially obtained. The strain-induced contrast modulation induced by the CH pattern starts to be visible only in the upper region of the cladding layer and results to be significantly reduced. The same happens for the In-rich thin vertical regions, indicating that the influence of the CH morphology was partially suppressed on this sample. As a consequence, the growth of the MQW undergoes a reduced morphological instability, with the larger part of the MQW left nearly unaffected by thickness modulations with respect to the sample with ELP x=0.15 of figure 1(a).
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Figure 5. Cross sectional TEM micrograph of the whole structure (a) and of the MQW region (b) of the sample
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CONCLUSIONS Strain balanced InGaAs/InGaAs MQWs were combined for the first time with the use of virtual substrates obtaining a significant shift of the absorption edge with respect to GaAs. The strain model used for the design of the structures has been proved to be successful to grow zero net strain MQW on completely relaxed virtual substrates and to confine defects at the buffer/substrate interface far from the active region. The confined MDs network, however, results in a marked CH morphology that induce a lateral strain modulation in the whole structure. Step-bunching at the cross-hatched valleys leads to lateral thickness modulations in the strain balanced MQW region that, for structures grown on virtual substrate having equilibrium lattice parameter corresponding to the In content of x = 0.35, induce generation of MDs at the topmost MQW layers. By optimizing the growth condition of the structures, the influence of the surface roughness induced by CH was partially suppressed. ACKNOWLEDGMENTS This work was supported by the ASI. Project “Sviluppo di convertitori termofotovoltaici ad alta efficienza basati su nanostrutture di semiconduttori”. The authors would like to acknowledge Ms. I. Tarantini, for her valuable technical assistance. REFERENCES 1. Ekins-Daukes N J, Barnham K W J, Connolly J P, Roberts J S, Clark J C, Hill G and Mazzer M Appl. Phys. Lett. 75, 4195 (1999) 2. L. Lazzarini, L. Nasi, C. Ferrari, M. Mazzer, A. Passaseo, K. Barnham Proceeding of Msm XII Oxford, 2001 in press 3. M. Mitsuhara, M. Ogasawara, H. Sugiura. J..of Cr. Growth 210, 463 (2000) 4. A.Passaseo, M.Longo, R.Rinaldi, R.Cingolani, M.Catalano, A.Taurino, and L.Vasanelli, J.Cr. Growth, 197, 777 (1999)
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