Dynamics of photoreflectance from undoped GaAs

Abstract

We have studied the time constants involved in photoreflectance from several GaAs surface-intrinsic-n+ structures. The rise and fall times were determined from digital oscilloscope traces. We find that they depend on the intensity and wavelength of the pump and probe beams. The observed photoreflectance feature does not always follow a single exponential decay. The dependence of rise and fall times on intensity and wavelength of pump and probe beams can be accounted for by a theory based on majority-carrier flow. The characteristic time obtained can be used to determine the potential barrier height.

Optical electromodulation of surface-intrinsic-doped structures

“…They are defined by the fitting of exponential forms of the actual signal. 8 Contrary to the linear case, the time scales s as well as the observed transient time constants s are sensitive on the intensity of both, the probe and pump beams. The complete solution to Eq.…”

“…On one hand, the capture and emission rates of traps do not depend on the intensity of the beams and, on the other hand, traps are always energetically favorable to capture carriers. 8,11 Instead, the mechanism that is proposed by Shen et al in Ref. 8 is the modulation of the electric field by the surface photovoltage that is driven by the majority carrier dynamics.…”

“…8 In Sec. III we discuss in detail some characteristic features of the time resolved PR signals generated by the majority carrier flow mechanism.…”

“…Indeed, the presence of noise makes the observed signal indistinguishable from a pure exponential form. 8 To compare Eqs. ͑6͒-͑9͒ to data, we define the effective transient time for a given signal v(t) as the time constant of an exponential function f (t) that minimizes the quadratic error…”

“…8 can be linearized for exceedingly small beam intensities, i.e., for J probe ϩJ pump ӶJ 0 . Using kT/e as the unit for voltage, we recall that the solution of the linearized version 8,15 …”