GSMBE growth on V-groove patterned substrates for InP-based quantum wires

Abstract

<p>Quantum wire (QWR) lasers are of interest because of predicted improvement of performance compared to quantum well lasers. For fiber optical communication, lasers are required that operate at wavelengths of 1.3 μm or 1.55 μm. This requires lasers grown in the InP-based materials. In the GaAs-based system, it has been shown that good QWR lasers can be achieved through epitaxial growth onto GaAs substrates having etched V-grooves. However, to date, no InP-based QWRs have been successfully grown in V-grooves. This arises from two problems: (i) the tendency for InP to planarize during growth, in contrast to the resharpening effect of AlGaAs; (ii) the potential for lattice-mismatch between InGaAs(P) and InP, which could result in strain relaxation and defect formation. Therefore, the purpose of this thesis is to establish the conditions whereby InGaAs/InGaAsP QWRs can be achieved and to confirm and characterize the one-dimensional behaviour. V-grooves with both(111)A and(111)B sidewalls are used in this study. They are obtained using chemical etching. The etching process is explained and conditions necessary to achieve each type of V-groove are established. Gas source molecular beam epitaxy has been used to grow various epitaxial structures. For InP layers grown under different growth conditions, it is found that the V/III flux ratio significantly affects the shape of a V-groove bottom and the roughness of both the V-bottom and the sidewalls. With the growth conditions optimized, InP layers can be grown which retain the sharpness and the smoothness of the V-groove. The sharpness of the bottom of the groove is related to growth conditions that decrease the growth rate at the bottom by limiting the supply of the group V component. InGaAs/InP quantum well structures have been grown using the optimized conditions. For (111)A V-grooves, transmission electron microscopy shows that all epilayers are defect-free and that InGaAs/InP quantum wires are successfully obtained with well thickness variation as high as a factor of 6. Lateral subband separations are estimated by a simple one-dimensional parabolic potential model with the thickness determined by TEM. Photoluminescence emission from the InGaAs quantum wires is spatially resolved with a spatially selective etch technique. For (111)B V-grooves, defects such as dislocations are observed in the bottom. However, the growth of bulk InGaAsP and InGaAs/InGaAsP yields different results. No extended defects have been observed in the InGaAsP layer grown in either (111)A or (111)B V-grooves. This is because, at the groove bottom the layer is both P and In rich, as analyzed by using energy dispersive X-rays. This tends to reduce the strain compared to InGaAs deposition, where the absence of P leads to In-rich, strained material. The growth of an InGaAsP layer in a (111)A V-groove results in a flat and wide bottom, which excludes the possibility of forming InGaAs/InGaAsP quantum wires. However, the growth of an InGaAsP layer in (111)B V-grooves results in a sharper bottom, such that crescent-shaped InGaAs/InGaAsP structures are formed when the InGaAs layer is very thin. Quantum-wire behaviour has been confirmed through the observation of lateral subbands in the photoluminescence (PL) spectra. This is further supported by polarization measurements. Also, the subband separation observed in PL spectra is consistent with a calculated value using the one-dimensional parabolic potential model. The InGaAs/InGaAsP QWRs should be able to be incorporated into laser structures.</p>