ABSTRACT
A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (µτ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Green's function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity. The approach enables measurement of spatial variations in the µτ product with a high degree of spatial resolution.
ABSTRACT
The response of extrinsic photoconductors to a step change in incident photon flux has long been known to exhibit a sharp transient feature, particularly at higher signal levels, known as the hook effect. We demonstrate experimentally and theoretically that the hook effect can be due to reduced illumination adjacent to the injecting contact. This nonuniformity can be produced by the transverse illumination of the detector that is common for far-infrared Ge:Ga devices. The hook effect has been demonstrated to be either present or absent in the same Ge:Ga photoconductor, at comparable signal size, depending on the nature of the contact illumination. Numerical finite-difference calculations of the transient response support this explanation and produce features that replicate the experimental results.
ABSTRACT
A numerical model for the transient response of extrinsic photoconductors is applied to the behavior of Ge:Ga and GaAs:Te detectors. Photoconductors display a two-component response to changes in illumination. The characteristic time and magnitude for the slow component have been studied as a function of background flux, applied field, temperature, device length, and signal size. For large-signal applications, the background flux affects the transient response even when the signal is orders of magnitude greater than the background. Experimental results are presented to support key predictions of the modeling. Because the ratio of fast to slow components is independent of both background and signal size, we propose the operation of detectors in such a way that final signal levels are derived from the fast component.
ABSTRACT
Doped germanium photoconductors are the most sensitive detectors for astronomy in the wavelength range 40-240 µm. Under the extremely low background conditions encountered in cooled satellite instruments, these devices exhibit a number of transient effects, such as slow relaxation after a step change in illumination or bias, and spontaneous spiking at high signal levels. Such behavior can degrade the excellent instantaneous sensitivity of these detectors and create calibration uncertainties. These effects have been observed in the Ge:Be photoconductors and the stressed and unstressed Ge:Ga photoconductors in the Long Wavelength Spectrometer, one of the instruments on the Infrared Space Observatory. A systematic investigation of the transient response of the Long Wavelength Spectrometer detectors to a step change in illumination as a function of operating temperature, bias electric field, and illumination step size has been carried out to determine operating conditions that minimize the effects of this behavior. The transient effects appear to be due primarily to carrier sweep out, but they are not fully explained by existing models for transient response.
