We describe simulation of photon-induced modulation to understand the interface effects on the optical response of a low doped GaAs layer. We also present measured photoreflectance lineshapes from low doped (1 to 3 X 1016 cm -3) GaAs layers with different interfaces. These layers are GaAs; with a semi-transparent gold overlayer, with a heavily doped underlayer and an air exposed single layer. A comparison between the simulation and the experimental lineshape indicates three possible photomodulation mechanisms, each with its own characteristic lineshape. This work shows that it is essential to use a multilayer model in simulation in order to accound for the slight gradient in optical response of successive nanometer scale sublayers within a single layer of the material. In this study the gradient is caused by changes in the electric field within the space-charge region, which affects the optical response of the sample. We found that the details of photoreflectance lineshape of each layer depends on the electrical and structural parameters of its immediate interfaces and the electrical characteristics of its neighboring layers.
We extend our previous measurements of the lineshape of GaAs at the E1 transition (2.9 eV). This study covers the combined effect of temperature and carrier concentration along with a discussion of the effect of the electric field intensity and the field inhomogeneity within a depth of 20 nm from the surface. A systematic study of changes in the lineshape of the above bandgap transition, E1 as a function of temperature (80 - 400 K) and carrier concentration (CC) (2 - 200 X 1016cm-3umps with different penetration depths, so that the modulation has a gradient with respect to the depth within the sample. The application of this technique is demonstrated for a two-dimensional electron gas (2DEG) of a modulation doped heterojunction in comparison with conventional PR. In one case the heterojunction of interest was buried under two highly doped GaAs and AlGaAs layers 40 nm thick. We show that this heterojunction is barely distinguishable in a PR measurement. Nevertheless, at room temperatutric field on the photoreflectance lineshape is discussed. The observed effect may be applied as an optical measurement of the electric field and the carrier concentration within a depth of about 17 nm from the surface/interface.
This paper describes the application of a novel approach in electric field modulation spectroscopy, the differential photoreflectance (DPR), to study buried GaAs/AlGaAs heterostructures. We show that in most complicated device structures. DPR can provide selective photoreflectance (PR) response from layers buried within thin multilayer structures. Such responses are often superimposed on one another in conventional PR measurements. DPR measurement is achieved through alternative modulations from two laser pumps with different penetration depths, so that the modulation has a gradient with respect to the depth within the sample. The application of this technique is demonstrated for a two-dimensional electron gas (2DEG) of a modulation doped heterojunction in comparison with conventional PR. In one case the heterojunction of interest was buried under two highly doped GaAs and AlGaAs layers 40 nm thick. We show that this heterojunction is barely distinguishable in a PR measurement. Nevertheless, at room temperature DPR shows distinct peaked signals that correspond to the previously reported PR from a two-dimensional electron gas.
We report photoreflectance studies ofMOCVD grown doped GaAs at the higher energy
transition E1( 2.9 eV). We are especially interested in the variation ofboth the energy position
and the broadening parameter F of the E1 transition with doping concentration. Above 1 x
10'8cin3 for Si:GaAs and ' 7 X 1018 for Zn:GaAs, we observe an increasing overlap of B1 and
E1 + Li structures. Evaluation of r based on curve fitting of the KramersKronig analysed
data shows a nearly linear relation between F and the logarithm of carrier concentration. This
observation has potential application in the determination of carrier concentration for heavily
doped films.
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