Hassan Elhadidy
Hassan Elhadidy
Project: DETNOISE - Noise in semiconductor detectors of X-ray and gamma ray radiation
Person in Charge: prof. Ing. RNDr. Josef Šikula, DrSc.
Host institution: Department of Physics, The Faculty of Electrical Engineering and Communication, Brno University of Technology
Country of Origin: Egypt
Country of scientific activity: Egypt, Czech
Project duration: 30 months
Scientific panel: Physics
Abstract:
Detector technology based on semiconductor materials which can operate at room temperature is a strategically important area of interest in the field of international research and industrial applications for radiation sensing. In order to achieve a significant improvement, the next generation of sensor devices will be based on the II-VI group of semiconductor materials, which provide direct conversion with high efficiency. The high performance of CdTe-based sensors relies upon their efficiently collecting free carriers generated by irradiation; this ability, in turn, critically depends upon the high resistivity of this material and its low carrier trapping. One of the key device properties required for an excellent spectroscopic semiconductor detector is the high signal-noise ratio. So far, most effort concentrated to maximize the signal, while much less attention was paid to noise properties of detector material and contacts. Methodology consists in experimental study of fluctuation phenomena caused by charge carrier transport and quantum transitions related to electron interaction with photons and phonons, and stochastic processes with long relaxation constant due to ion diffusion. Measurable quantities are noise voltage or current and their spectral density dependence on electric field intensity, temperature and intensity of light illumination. For electrical characterization and determination of localized energy levels in CdTe crystals the galvanomagnetic measurements, photoluminescence, photoconductivity and thermoelectric effect spectroscopy will be used. The effort will concentrate to determine, evaluate and suppress noise sources originating in the material (point defects, inclusions and precipitates, dislocation clusters), in the metal-semiconductor interface area and in the volume of the sensor influenced by band bending due to different work functions of metal and CdTe (CdZnTe).
The ongoing project summary:
Detector technology based on semiconductor materials is a strategically important area of interest in the field of international research and industrial applications for radiation sensing. In order to achieve a significant improvement, the next generation of sensor devices will be based on the II-VI group of semiconductor materials, which provide direct conversion with high efficiency. The high performance of CdTe-based sensors relies upon their efficiently collecting free carriers generated by irradiation; this ability, in turn, critically depends upon the high resistivity of this material and its low carrier trapping. One of the key device properties required for an excellent spectroscopic semiconductor detector is the high signal-noise ratio. So far, most effort concentrated to maximize the signal, while much less attention was paid to noise properties of detector material and contacts. Methodology consists in experimental study of fluctuation phenomena caused by charge carrier transport and quantum transitions related to electron interaction with photons and phonons, and stochastic processes with long relaxation constant due to ion diffusion.
The main goals of the project are:
- Fundamental characterization of point and volume defects in CdTe and CdZnTe samples by a complex of experimental techniques. Search for principal sources of noise signal in semiinsulating Au/CdTe/Au system.
- Evaluation of influence of annealing on defect structure and noise spectra. Development of noise model. Study of influence of contact metal on noise spectra.
- Summary of noise spectroscopy and defect studies. Proposal of optimized process of detector fabrication including choice of contact metal.
In the first year of the project, the research was mainly based on evaluation of influence of each part of CdTe detector system on its overall electrical properties. The parts of detector are:
· Detector surface. New samples were made at our partner institution- the Department of Physics, Charles University, Prague. For these samples, the IV characteristics were measured. In order to isolate influence of surface current, two detectors were made. These detectors are made from the same crystal with the same geometrical dimensions. One sample was equipped with an extra guard ring electrode that grounds surface currents. Experimentally was found that the benefit of surface current to total current is approx. 40 procent.
· Semiconductor-metal interface area. Transport characteristics measurements were conducted. In order to suppress effects of semiconductor bulk, measurements were carried out by the four-point method. The goal of these measurements was to evaluate linearity of metal-semiconductor junction.
· Detector bulk. To characterize homogeneity of analysed samles bulk, which acts as semiconductor ionisation chamber, the study by optoelectrical methods was carried out at the Dept. of Physics, Charles University. The Pockels effect measurements were carried out. This method can determine electric field distribution among semiconductor bulk. Impurities and inhomogenities cause local decrease of electric field intensity in regions of defects.
The next part of research was focused on nature of macroscopic electric quantities during relaxation process of detector. The long-lime resistance change measurements of detector, fed by constant voltage, were conducted. Simultaneously, the progress of fow-frequency noise signal spectrum of detector output was observed. Measurements showed that the electrical quantities relaxation time of detector lasts in order of 104 seconds. After applying constant voltage on sample, current started to increase monotonously. So, the detector resistivity decreases during relaxation process. From low-frequency noise spectra analysis was found that in time region > 600 seconds after detector biasing, the noise spectral density magnitude becomes constant, even though the process of relaxation had not ended and we still observed sample current increase. From this, we found out that in late relaxation, detectors additional noise is not proportional to current.
Conclusions of recent work
IV characteristics analysis of detector, equipped with guard ring electrode showed that signifficant part of unwanted leakage current is caused by surface current. In this analysis, we find absolutely neccessary to investigace on influence of guard ring electrode on electrical field shape. Guard ring can negatively affect efficiency of charge carrier’s collection. The charge carriers are generated by interaction of incoming ionizing radiation with detector bulk. The effect of electric field modulation can lead to deteriorated spectral resolution of detector. Furthermore, is essential to deeply study possibility od leakage current reduction by detector surface technological modification, which would brink higher surface resistivity and, hand in hand, act as protection layer against corrosive influence of environment.
From the Pockels effect measurements we found out that the entire electric field intensity drop takes place at metal-semiconductor interface of reversively biased contacts. So, we assume fluctuation of depleted barrier length as a primal cause of long relaxation time of detectors. In this area, a higher concentration of deep level energy traps occurs. These energy traps are result of imperfect contact manufacturing technology.
The noise specral analysis is easy to implement tool to evalouate manufacturing quality of contacts.By comparing additional noise (as an indicator of deffective charge carrier transport), generatet by reverse biased contact, we estimated contacts quality of analysed samples.
Our recent findings are important for qualitative improvement of nowadays commercially manufactured detectors. The results will help EU region companies (such as Eurorad, Strassburg) to get technological lead in field of semiconductor spectroscopic dadiation detectors.