We discuss the potential benefits of using compound semiconductors for the detection of X- and γ-ray radiation. While Si and Ge have become detection standards for energy dispersive spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Compound semiconductors encompass such a wide range of physical properties that it is technically feasible to engineer a material to any application. Wide band-gap compounds offer the ability to operate in a wide range of thermal and radiation environments, whilst still maintaining sub-keV spectral resolution at hard X-ray wavelengths. Narrow band-gap materials, on the other hand, offer the potential of exceeding the spectral resolution of both Si and Ge, by as much as a factor of 3. Assuming that the total system noise can be reduced to a level commensurate with Fano noise, spectroscopic detectors could work in the XUV, effectively bridging the gap between the ultraviolet and soft X-ray wavebands. Thus, in principle, compound semiconductor detectors can provide continuous spectroscopic coverage from the far infrared through to γ-ray wavelengths. However, while they are routinely used at infrared and optical wavelengths, in other bands, their development has been plagued by material and fabrication problems. This is particularly true at hard X- and γ-ray wavelengths, where only a few compounds (e.g., GaAs, CdZnTe and HgI 2 ) have evolved sufficiently to produce working detection systems. In this paper, we examine the current status of research in compound semiconductors and by a careful examination of material properties and future requirements, recommend a number of compounds for further development. In the longer term, when material problems are sufficiently under control, we believe the future lies in the development of heterostructures and inserted interface layers to overcome contacting problems and quantum heterostructures and superlattices to facilitate low-noise readout.

Compound Semiconductor Radiation Detectors

A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Room-temperature compound semiconductor radiation detectors

An important aspect of the large expansion in the development and production of solid-state devices has been the demand for more sophisticated techniques for determining the electrical properties of semiconductors, especially silicon and the III-V compounds. A very wide range of measurement techniques now exists and it is the purpose of this article to review those techniques which are in widespread use or which show promise for future application, and at a time for continuing innovation in this area, to indicate present trends and material problems which may arise in the near future.

https://iopscience.iop.org/article/10.1088/0034-4885/41/2/001/pdf

The electrical characterisation of semiconductors

Signal variance in gamma-ray detector materials is reviewed with an emphasis on intrinsic variance. Phenomenological models of electron cascades are examined and the Fano factor ( F ) is discussed in detail. In semiconductors, F is much smaller than unity and charge carrier production is nearly proportional to energy. Based on a fit to a number of semiconductors and insulators, a new relationship between the average energy for electron–hole pair production and band gap energy is proposed. In scintillators, the resolution is governed mainly by photoelectron statistics and proportionality of light yield with respect to energy.

Signal variance in gamma-ray detectors-A review

Compton scattered photons included within the photopeak pulse-height window decrease the image contrasts of lesions. Furthermore, in the absence of an effective compensation procedure, the accuracy of the SPECT measurement is reduced. In the following sections the effect of scatter on SPECT is reviewed, and the influence of scattered photons on the effective linear attenuation coefficient used with multiplicative attenuation compensation methods is described. Criteria for evaluating scatter compensation procedures are proposed, and approaches to reducing the effect of scatter on SPECT imaging are described.

Scatter Compensation Techniques for SPECT

It has been hoped that high-Z semiconductors would offer efficient ?-ray detection at or near ambient temperatures with energy resolution significantly better than NaI (T1) scintillators. For use at X-ray energies, this goal has been achieved with both HgI2, CdTe, and GaAs detectors. However, at higher energies (~660 keV) all current detectors have one or more significant deficiencies in terms of attainable volume, charge collection efficiency, and polarization effects. Starting with first principles, all potential compounds which can be formed by the binary combination of elements from the periodic chart were considered as possible detector materials. A rank-ordered listing of the most promising materials for further development is given as well as an assessment of the prospects for future success.

What Can Be Expected from High-Z Semiconductor Detectors?

HgI2 x-ray detectors up to 16 mm3 have been made from single crystals grown from the vapor phase. Crystal growth, detector fabrication, and transport properties are described. Resolutions of 850 eV FWHM at 5.9 keV and 4.3 keV FWHM at 122 keV have been measured at 25° C in air.

Recent Advances with HgI2 X-Ray Detectors

Theoretical energy band structure calculations have been utilized to investigate several high-Z materials for potential use as ambient temperature radiation detectors Using the pseudopotential technique, the band structure for HgI2 has been determined and the effective masses of the holes and electrons have been estimated Theoretical mobilities of the electrons and holes as a function of temperature have been computed for HgI2 and CdTe and are compared to experimental data

/pdf/theoretical-band-structure-analysis-on-possible-high-z-2ocoolmlti.pdf

Theoretical Band Structure Analysis on Possible High-Z Detector Materials

Considerable interest has been generated recently in the use of high-Z semiconductors for x-ray spectroscopy. To aid in this study, a Monte-Carlo computational model has been used to simulate x-ray spectral response in semiconductor detectors. The model employs one-dimensional charge collection in an arbitrary electric field profile and includes trapping and electronics system effects. Spectra are calculated for several materials, including HgI2 and CdTe, and are compared to experimental results.

/pdf/prognosis-for-high-z-semiconductor-detectors-1hcg4s2iyr.pdf

Prognosis for High-Z Semiconductor Detectors

A high purity germanium gamma-camera has been developed and is currently being evaluated. This camera incorporates unique performance parameters such as a 2 mm full-width spatial response function with rejection of multiple-scatter in the detector, a 2.2% FWHM energy resolution for 99mTc, a 180 nsec paralyzable dead-time, and a 2 mu sec non-paralyzable dead-time. Imaging studies demonstrate the superior capabilities of this instrument.

G. A. Armantrout

Papers

What Can Be Expected from High-Z Semiconductor Detectors?

Recent Advances with HgI2 X-Ray Detectors

Theoretical Band Structure Analysis on Possible High-Z Detector Materials

Prognosis for High-Z Semiconductor Detectors

Imaging characteristics of a small germanium camera.