Raman and Photoluminescence Mapping of Lattice Matched InGaP/GaAs Heterostructures
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RAMAN AND PHOTOLUMINESCENCE MAPPING OF LATTICE MATCHED InGaP/GaAs HETEROSTRUCTURES G. Attolini, P.Fallini, F.Germini, C. Pelosi MASPEC-CNR Institute, Parco Area Delle Scienze, 37/A Fontanini, 43010 Parma, Italy O. Martínez, L.F.Sanz, M.A. González, J. Jiménez Física de la Materia Condensada, ETSII, 47011Valladolid, Spain ABSTRACT The influence of the substrate on composition and CuPt-type spontaneous order of MOVPE lattice matched InGaP/GaAs layers was studied. The study was carried out by microRaman and microphotoluminescence. The order was determined by the band gap, while the Raman parameters were also contributed by the surface topography that was also related to the type of substrate. The spontaneous order increases with Si- doping of the substrates. Doping the layers with Zn randomises the alloy. INTRODUCTION InGaP layers lattice matched to GaAs substrates have potential applications for electronic devices, e.g. high efficiency tandem solar cells, single heterojunction bipolar transistors, tunable laser diodes, etc. It has some advantages in relation to AlGaAs ternary alloys, which are the base of many devices. Lattice matched InGaP presents a direct band gap of 1.9 eV, which is approximately equal to the maximum direct band gap that can be obtained with AlGaAs (Ga molar fraction of 0.4) [1]. InGaP is insensitive to oxygen and humidity inside the reactor, which is a well known cause of Al instability in AlGaAs [2]. InGaP is affected by other problems, in particular the composition control and the existence of an spontaneous ordered phase. In1-xGaxP matches the GaAs lattice for x=0.516 at room temperature; however due to the large lattice and thermal mismatches between InP, GaP and GaAs the composition is critical to avoid residual strain in the layers [3]. The second problem of interest regards the spontaneous order. InGaP can appear under a CuPt-type ordered phase [4, 5]. In the ordered phase the cations are not randomly distributed, but seggregate spontaneously into alternate (111) planes giving a CuPt-type structure in the cation sublattice. This phase critically influences the optical and electrical properties of InGaP/GaAs layers. The most important is the shrinking of the band gap, which can be reduced by 100 meV for lattice matched composition [5, 6]. Also, the minority carrier lifetime and mobility are sensitive to cation order [6]. The spontaneous order is related to the growth method and the specific growth conditions, such as the substrate temperature and the V/III ratio. The nature of the susbstrate is shown here to influence the properties of the layers. We present herein a detailed study of the properties of InGaP layers grown by LP-MOVPE (Low pressure metal organic vapor phase epitaxy) on different substrates aiming to understand the influence of the substrate on the spontaneous order. The analysis is carried out by High Resolution X-Ray diffraction (HRXRD), Atomic Force Microscopy (AFM), micro-Raman (µ-R) and micro-Photoluminescence (µ-PL).
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EXPERIMENTAL AND SAMPLES Layers were grown
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