Ordering and Phase Separation in Movpe InGaP Alloys and Unicompositional Quantum Wells
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ABSTRACT The microstructures of In0. 5 Ga 0 .5 P alloys grown on (100) GaAs by MOVPE have been characterized with cross-section TEM and their optical emission examined with photoluminescence at low temperatures. All the alloys exhibit spinodal-like decomposition with compositional modulations along directions in the growth plane. Alloys grown at 775°C have the highest emission energy, 2.0 eV; growth at 675°C gave the lowest, 1.89 eV, due to strong CuPt-type ordering of In and Ga. The ordered domains are platelets 20 to 200 nm wide and 1020 nm thick, with antiphase boundaries 1-2 nm apart. We have also formed "unicompositional" quantum wells of thin (1.3-20 nm) ordered layers grown at 675°C between disordered barriers grown at 750°C. Ordering is found only in the active layer, with domains similar to those of thick layers. The emission energy increases by 90 meV as the well thickness is decreased from 10 to 1.3 nm, thus demonstrating quantum size effects solely through disorder-order phenomena. INTRODUCTION AND APPROACH Alloys of InGaP have the highest direct bandgap of ternary III-V systems, making them attractive candidates for optoelectronic devices operating in the visible range (green to red). Compositions of In0. 5 Ga0. 5 P can be grown by metal organic vapor phase epitaxy (MOVPE) with a lattice constant matching that of GaAs. The optical emission energy of these alloys varies with growth temperature [1] and is lowest near 675°C [2]. It is now well established that the reduced energy is due to CuPt-type ordering of In and Ga on ( 111 } planes, as confirmed with electron diffraction [3,4]. Our laboratory has previously examined the emission characteristics with photoluminescence (PL) as a function of growth temperature and substrate orientation [2]. In order to determine the microscopic origin of the PL variations and control them, we have characterized key alloys of that work with transmission electron microscopy (TEM) [4]. We find contrast modulations like those of spinodally decomposed materials [5] in all the alloys. Strong ordering is seen for growth at 675°C but none for 750°C, consistent with the PL energies [2]. We have also used the growth-temperature dependence to form "unicompositional" disorderorder-disorder (DOD) quantum wells by growing thin ordered layers at 675 0 C between disordered "barrier" layers grown at 750°C [6]. Dark-field TEM images demonstrate that the quantum well is indeed ordered and has a domain structure like that of thick layers. The PL energy of these structures increases with decreasing thickness of the ordered layer, consistent with quantum confinement of carriers in a low-energy well. The DOD quantum wells thus provide size quantization solely through order-disorder phenomena, and may give a unique perspective on optical transitions in nominally ordered material. More generally, our work shows that ordering can be an additional tool to tailor electronic properties of III-V alloys. The alloys were grown using trimethylindium, triethylgallium and phosphine in a lowpressure hori
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