Progress in Monolithic Photonic Integration Using Quantum Well Shape Modification Enhanced by Ion Implantation
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Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
QWs, as judged by PL intensity, loss, etc. and thus the advantages of QWs in device structures was lost. Ion Implantation Induced OW Shape Modification The fundamental issues in QW shape modification have always been the reliability, reproducibility, and simplicity of the spatially selective enhancement process. Early work, using impurity free QW shape modification in GaAs/AlGaAs QWs, (Ga vacancies formed in GaAs at elevated temperatures under a SiO 2 cap layer were believed to be the mechanism of enhanced interdiffusion) produced large bandgap shifts. 4 Unfortunately, the process, while effective, was not very reproducible. The difficulty appeared to be related to the variable porosity of the deposited Si0 2 cap layer and/or the semiconductor surface treatment. A simple technique for avoiding this problem is to produce vacancies using ion implantation. Everything is identical with the dielectric cap technique, except the technique for generating vacancies near the surface. Ion implantation has the virtues of being simple, of being insensitive to surface morphology or conditions, of being highly reproducible, and of being compatible with any material system. The intermixing effect is due to the thermal diffusion of vacancies generated in the surface region of the sample, not due to deposition of ions deep in the sicture in the middle of the QWs as is the case in other ion implantation intermixing techniques. FUNDAMENTAL PHYSICS OF QW SHAPE MODIFICATION In the following sections, factors which govern the magnitude of the QW shape modification process and the resultant fundamental bandgap shift are considered, with specific reference to the strain free GaAs/AlGaAs QW material system. Some examples of Si0 2 cap modified QWs will be used as the influence of vacancy generation and diffusion direction are exceptionally clear in this technique. The magnitude of the effect of QW shape modification on its optical and electrical properties are dependent on a number of parameters. These can be grouped into three areas: those parameters relating to the QW structure itself, those dealing with the details of the generation of the shape modification enhancing entities (e.g., vacancies), and those having to do with the activating process itself (i.e., the anneal process). Unless otherwise specified, all of the experimental results given below will be based on ion implantation QW shape modification. OW Structure Enhanced QW shape modification typically produces an increase (blue shift) of the bandgap energy of the QW material (Eg' > Eg) due to the movement of barrier atoms into the well material and a "rounding" of the as-grown, abrupt QW interfaces, which are typical of epitaxial growth (Figure 1). The maximum bandgap shift possible in a given QW material system is equal to the total "depth" of the well (i.e., the sum of the conduction and valence band QWs). However, there is a strong dependence of the magnitude of bandgap shift on the QW width (Figure 2). Note
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