Transient response in doped germanium photoconductors under very low background operation

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.     

Determination of the distribution of air and water in porous media by electrical impedance tomography and magneto-electrical imaging

Monitoring the distribution of water content is essential for understanding hydrological processes in the lithosphere and the pedosphere. The movement of water in unsaturated rock formations and in the vadose zone is influenced by different processes (mainly infiltration, evaporation, percolation and capillary flow) which may be rate determining depending on the actual conditions. The interdependence of these processes also strongly influences the transport and distribution of solutes in the pore space. In order to gain a better understanding of the movement and distribution of water in unsaturated media, systematic investigations with non-invasive or minimal invasive methods appear to be most suitable. Studies on the distribution of electrical conductivity can improve risk analysis concerning waste disposals in general and nuclear waste repositories in particular. Induced polarization and magnetic flux density determined with two highly sensitive accessories yield additional information and may allow for better discrimination of coupled flow processes. Electrical impedance tomography (EIT) with 20 current injection and 48 voltage electrodes was used here to monitor the evaporation of tap water from a container filled with sand under laboratory conditions at 20 °C. The results are compared with data obtained by determining spectral induced polarization (SIP) of sand during desaturation in a multi-step outflow equipment. Infiltration processes and evaporation from sand saturated with 0.01 M CaCl₂ were determined by magneto-electrical resistivity imaging technique (MERIT). The results were obtained from a long-term experiment under controlled conditions.     

Preparation, Processing, and Characterization of Nano-Crystalline BaTiO3 Powders and Ceramics Derived from Microemulsion-Mediated Synthesis

Monodisperse and nanocrystalline BaTiO₃ powders of different stoichiometries were synthesized through the hydrolytic decomposition of mixed metal alkoxide solutions with water-in-oil microemulsions as reaction media. The method allows a tailor-made preparation of powder particles as small as 10 nm in average. Characterization with respect to purity, morphology, and behavior upon annealing was conducted in order to determine appropriate processing conditions for the preparation of pure, dense nanocrystalline ceramics. As a result, ceramics with a mean grain size as small as 35 nm were achieved. Raman spectroscopy revealed for the first time local tetragonality in pure nanocrystalline bulk BaTiO₃-ceramics with such a fine microstructure.     

Origin of the hook effect in extrinsic photoconductors

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.     

Transient behavior of infrared photoconductors: application of a numerical model

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.     

Electron beam induced current imaging of near-contact regions insemi-insulating GaAs

Electron beam induced current (EBIC) in a scanning electron microscope has been used to image the internal electric field regions near implanted contacts on semi-insulating GaAs. Planar n⁺-i-p⁺ structures were fabricated with intercontact distances ranging from 5 to 100 μm. In cases where the diffusion length is short compared to the lengths of interest, the current collected is determined primarily by the local electric field profile. With no externally applied bias, we observe large current collection regions adjacent to the n⁺ contact, extending ~10-20 μm into the bulk material. Two-dimensional (2-D) imaging indicates that the regions are highly nonuniform. For small intercontact distances, the contact-related fields, which are produced by the diffusion and trapping of carriers from the contacts, can dominate the entire region. Changes in EBIC signal with the application of forward or reverse bias are used to monitor the interaction of the zero bias field and the applied field. This approach provides a good estimate of the field distributions in trap-dominated, high resistivity materials like semi-insulating GaAs, with a spatial resolution generally not obtained with other field imaging techniques     

Performance and materials aspects of Ge:Be photoconductors

Ge:Be photoconductors have been developed for low photon background applications in the 30–50 μm wavelength region. These detectors provide higher responsivity and lower noise equivalent power (NEP) than the Ge:Ga detectors currently operating in this wavelength range. Berylliumdoped single crystals were grown by the Czochralski method from a carbon susceptor under a vacuum of ～ 10^?6 torr. We report an optimum detective quantum efficiency of 46% at a background flux of 1.5×10⁸ photons/second (7×10^?13 W). Ge:Be detector performance is strongly influenced by the absolute concentrations and the concentration ratio of residual shallow donors and shallow acceptors.     

Three-dimensional transport imaging for the spatially resolved determination of carrier diffusion length in bulk materials

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.