Quantum Well Intermixing for Optoelectronic Applications
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Mat. Res. Soc. Symp. Proc. Vol. 484 ©1998 Materials Research Society
device fabrication. Other techniques could be classified into two categories, one is impurity induced intermixing and another is impurity free intermixing. Impurity induced intermixing has been extensively studied [23-29] and main impurities used for these studies are Zn and Si. However, impurity induced interdiffusion usually introduces substantial concentration of impurities in the active regions which is undesirable particularly for active devices such as lasers. Impurity free interdiffusion [30, 31] is achieved by using cap layers of Si0 2. Annealing of quantum wells with cap layers such as SiO 2 leads to creation of Gallium vacancies which enhance the atomic interdiffusion. In the GaAs/AlGaAs system, SiO 2 and Si3N4/SrF 2 are commonly used to enhance and suppress interdiffusion, respectively. However, Si0 2 reacts with Al when in direct contact with A1GaAs, thus generates Si which acts as an impurity source. Reproducibility has been found to be one of the main problems as interdiffusion has been
found to be sensitive to the deposition conditions of SiO 2. Laser assisted interdiffused InGaAs/InGaAsP lasers have been fabricated and has been mainly used for this material system [18]. Ion implantation is widely used in microelectronics industry, but found limited use in optoelectronic device fabrication. This is mainly due to creation of defects during ion bombardment and difficulties associated with removal of defects during post-implant thermal processing. Despite these limitations, ion implantation has been used for creation of electrical and optical isolation [32] as well as fabrication of intermixed lasers [33]. Recently, we have also developed a novel intermixing technique, namely anodic oxide induced intermixing [34, 35]. This is based on creation of pulsed anodic oxides on GaAs substrates and subsequent thermal processing of the quantum well structures with anodic oxide cap layers. We have applied this technique for intermixing of quantum well structures based on GaAs/AIGaAs system and also to increase confinement on quantum wire structures grown on patterned substrates [36]. In this paper, we present results of systematic studies of variation of various implantation, anodic oxidation and annealing parameters on the intermixing of GaAs/AlGaAs quantum well structures. This material system is chosen for its simplicity due to close lattice matching which will avoid further complications of strain induced effects. EXPERIMENTAL Four quantum wells of nominal thicknesses 1.4 (QW#1), 2.3 (QW#2), 4.0 (QW#3) and 8.5 (QW#4)nm, were grown by low pressure metal organic chemical vapour deposition (MOCVD) on a semi-insulating (100) GaAs substrate for ion implantation study and on p+-GaAs for anodic oxide induced intermixing study. GaAs quantum wells are sandwiched between A10 .54 Ga0 .46 As barriers of 50 nm thick, with narrowest wells closer to the surface. All layers were undoped and grown at 750TC. The structure was then terminated by a GaAs
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