Study of depth-dependent radiation-induced defects using coherent acoustic phonon spectroscopy
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Study of depth-dependent radiation-induced defects using coherent acoustic phonon spectroscopy A. Steigerwald1, J. Gregory1, K. Varga1, A.B. Hmelo1, X. Liu2, J. K. Furdyna2 L. C. Feldman1,3, N. Tolk1 1 Department of Physics and Astronomy, Vanderbilt University Nashville, TN, 37235 2 Department of Physics, University of Notre Dame, Notre Dame, Indiana, 46556 3 Institute of Advanced Materials, Devices, and Nanotechnology, Rutgers University, New Brunswick, NJ, 08901 ABSTRACT Here we study the effect of radiation-induced point defect distributions on the optical reflectivity signal in GaAs using coherent acoustic phonon spectroscopy. We demonstrate that the presence of point defects significantly modifies the optical response, allowing estimation of the depth-dependent defect distribution in a nondestructive and noninvasive manner. We show that the observed changes are dependent on defect-induced changes to the electronic structure, namely defect-induced band tailing of the direct 1.43eV band edge. This provides a method for subsurface investigations on the complex interaction between different defects species and optoelectronic structure. INTRODUCTION Identifying and characterizing the presence of point defects and their effect on material properties is often a difficult task. Such defects may arise during materials growth processes, through diffusive processes during operation at elevated temperatures, or in applications where materials are exposed to significant levels of radiation. Many useful techniques to study defect properties fail to capture depth-dependent information as they either average over an entire sample (linear spectroscopy) or lack spatial dimension (electrical measurements). Techniques that do attempt depth-dependent measurements can be difficult, require beamlines (channeling), have inherent depth limits (positron annihilation), and can be destructive in nature (crosssectional imaging). Here we show that the ultrafast time-resolved pump-probe technique known as coherent acoustic phonon spectroscopy (CAP, alternatively known as picosecond ultrasonics) is able to detect subsurface point defect concentrations in a nondestructive manner. In these experiments an intense optical pump pulse is absorbed at the sample surface. The subsequent thermal expansion launches an elastic wave of longitudinal acoustic phonons that travel into the bulk at the acoustic speed of sound. A second, weaker probe pulse then reflects off both the sample surface and the optical discontinuity created by the elastic wave from the bulk interior (Figure 1(a)). The two components combine at the detector to create a long-lasting oscillatory response whose properties (period, amplitude, attenuation, etc.) at any time delay (Δt) depend on the local optoelectronic properties at the location of the strain wave. Since the strain wave travels at the acoustic speed of sound (Vs) the time delay can be correlated with depth by multiplying
z=(Δt)(vs). Many excellent descriptions of the strain wave generation and the probe detection techniq
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