Abstract
Carrier diffusion is of paramount importance in many semiconductor devices, such as solar cells, photodetectors, and power electronics. Structural defects prevent such devices from reaching their full performance potential. Although a large carrier diffusion length indicates high material quality, it also implies increased carrier depletion by an individual extended defect (for instance, a dislocation) and obscures the spatial resolution of neighboring defects using optical techniques. For commonly utilized photoluminescence (PL) imaging, the spatial resolution is dictated by the diffusion length rather than by the laser spot size, no matter the spot is at or below the diffraction limit. Here, we show how Raman imaging of the LO phonon-plasmon-coupled mode can be used to recover the intrinsic spatial resolution of the optical system, and we demonstrate the effectiveness of the technique by imaging defects in GaAs with diffraction-limited optics, achieving a 10-fold improvement in resolution. Furthermore, by combining Raman and PL imaging, we can independently and simultaneously determine the spatial dependence of the electron density, hole density, radiative recombination rate, and non-radiative recombination rate near a dislocation-like defect, which has not been possible using other techniques.
Highlights
While both point defects (PDs) and extended defects (EDs) may yield qualitatively similar effects[1,2,3], for example, depletion of carriers that are supposed to generate radiative recombination or carry electrical current, they often play competitive roles in affecting the device performance[4,5]
Note that the effective defect impact range already appears to be significantly smaller than that given by the diffusion length (DL) because of the improved spatial resolution of the raster scan mode compared to the uniform illumination mode[16]
The PL intensity reaches the background value within approximately 10 μm, which is roughly equivalent to half of the DL, in the L/L mode, but the Raman intensity reaches the background level within approximately 2 μm, which is approximately 1/10 of the DL
Summary
While both point defects (PDs) and extended defects (EDs) may yield qualitatively similar effects[1,2,3], for example, depletion of carriers that are supposed to generate radiative recombination or carry electrical current, they often play competitive roles in affecting the device performance[4,5]. PDs suppress carrier diffusion and may diminish the impact of EDs. It is relatively easy to saturate PDs in a moderately highquality material with a high carrier density, but an ED tends to introduce a very high density of defect states that are practically impossible to saturate by increasing the carrier injection level. Before being saturated by increasing illumination power, a dislocation can mutate into a defect network that is more detrimental than the original form[5]. It is important to distinguish and investigate EDs individually and, to identify their atomistic structures
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