Imaging transport: optical measurements of diffusion and drift in semiconductor materials and devices
Haegel, Nancy M.
Luscombe, James H.
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Knowledge of transport parameters is important to the development of new optoelectronic materials and devices, such as ultraviolet (UV) semiconductor lasers and advanced solar cells. A series of experiments was performed to measure fundamental transport parameters in luminescent semiconductor materials. Using a technique that couples a scanning electron microscope (SEM) in spot mode with a charge coupled display (CCD) camera, it is possible to image the recombination of charge created at a point. The goal is to extract fundamental transport parameters, such as minority carrier diffusion length (Ldiffusion)) and drift length (Ldrift), with high spatial resolution. Direct transport imaging was used to study diffusion without bias and drift under a range of applied electric fields. The recombination distribution as a function of applied bias was imaged. For the unbiased measurements, the results showed that for bulk n-type GaAs the spotwidth was independent of probe current indicating the luminescence distribution is primarily a function of generation volume and not diffusion length. For thin layer samples that could be approximated as two dimensional (2D), it was found that the spotwidth changed as a function of probe current indicating the potential to extract diffusion length data. Results are compared to numerical modeling of charge transport and the feasibility and limitations of this method for contact-free measurements of lifetime ()Ì¥ and mobility (Ã¦) are assessed.
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