Carrier Decay and Diffusion Dynamics in Single-Crystalline CdTe as Seen via Microphotoluminescence

Abstract

The ability to spatially resolve the degree to which extended defects impact carrier diffusion lengths and lifetimes is important for determining upper limits for defect densities in semiconductor devices. We show that a new spatially and temporally resolved photoluminescence (PL) imaging technique can be used to accurately extract carrier lifetime values in the immediate vicinity of dark-line defects in CdTe=MgCdTe double heterostructures. A series of PL images captured during the decay process show that extended defects with a density of 1.4 × 10 5 cm −2 deplete photogenerated charge carriers from the surrounding semiconductor material on a nanosecond time scale. The technique makes it possible to elucidate the interplay between nonradiative carrier recombination and carrier diffusion and reveals that they both combine to degrade the PL intensity over a fractional area that is much larger than the physical size of the defects. Carrier lifetimes are correctly determined from numerical simulations of the decay behavior by taking these two effects into account. Our study demonstrates that it is crucial to measure and account for the influence of local defects in the measurement of carrier lifetime and diffusion, which are key transport parameters for the design and modeling of advanced solar-cell and light-emitting devices.

Overcoming diffusion-related limitations in semiconductor defect imaging with phonon-plasmon-coupled mode Raman scattering

“…Fig. 3e plots the (normalized) PL transition rate W r ( r ) = I PL ( r )/ n ( r ), which turns out to be strongly spatially dependent, rather than constant as one might expect 14 . This result reveals very important information about the defect that is not readily available from PL mapping alone in either CW or time-resolved mode.…”

“…Examples of radiative defects that facilitate this approach include nitrogen vacancy centers in diamond 11 and nitrogen pairs in GaAs 12 . However, for non-radiative defects such as dislocations in GaP 4 and dislocations and grain boundaries in CdTe 6 , 13 , 14 , the common approach is to image the band edge PL/EL to reveal the location where the luminescence signal is weakened by defect-induced carrier depletion. In this situation, the spatial resolution is drastically degraded relative to the capability of the optical system when the carrier diffusion length (DL) is greater than the optically defined spatial resolution.…”

“…Though the spatial dependence of the PL intensity near an ED can be measured by PL imaging 5 , 14 , 16 , it is not possible to independently analyze the spatial dependence of the carrier density n ( r ), radiative recombination rate W r ( r ), and non-radiative recombination W nr ( r ) because the measured I PL ( r ) = n ( r ) W r ( r ) is a product of two quantities, and W nr ( r ) is implicitly involved. Even if spatial and time-resolved imaging are performed simultaneously, one still cannot separate W r ( r ) and W nr ( r ) because the local PL decay time τ ( r ) is given by τ ( r ) −1 = W r ( r ) + W nr ( r ).…”

“…Even if spatial and time-resolved imaging are performed simultaneously, one still cannot separate W r ( r ) and W nr ( r ) because the local PL decay time τ ( r ) is given by τ ( r ) −1 = W r ( r ) + W nr ( r ). Since W r ( r ) is not experimentally accessible, it is typically assumed to be constant throughout the material 14 . However, since LOPP Raman imaging provides a straightforward method to obtain the spatial variation n ( r ) near the defect, combined with PL mapping, we are able to obtain spatial profiles for both the radiative and non-radiative recombination rates W r ( r ) and W nr ( r ) near a defect.…”

Optical Imaging of Diffusive Electrons with Different Kinetic Energies in a Semiconductor

“…Because electrons with different kinetic energies emit photons at different wavelengths, the spatial variation of the PL signal for different wavelengths had to be measured and compared to test the hypothesis. In previous studies, both electron and laser beams have been used to generate a local electron population, but the spatial profiles of light emission were typically measured either by including the whole emission spectrum (non-wavelength selective) [5] or by selecting a single narrow band near the bandgap [6,8,9,10]. Thus, the potential wavelength dependence had not been explicitly examined.…”