Ondrej Sik

Contact quality analysis and noise sources determination of CdZnTe-based high-energy photon detectors

Experimental studies of the transport and noise characteristics of a cadmium-zinc-telluride (CdZnTe) crystal with symmetric gold contacts and a guard ring electrode have been carried out. The current–voltage (IV) characteristics and the noise spectral density were measured at room temperature in the dark. The sample with a disconnected guard ring electrode showed symmetric characteristics for both bias voltage polarities. The shape IV characteristics indicated the presence of carrier injection, leading to IV characteristics nonlinearity. The semiconductor surface has been identified as the main noise and leakage current source. After connecting the guard ring electrode, the leakage currents were suppressed by two orders and the noise spectral density decreased by five orders. In the case of the connected guard ring electrode, the asymmetry of IV characteristics was observed. The contact with worse rectifying properties had a higher contribution of noise to the detector system. Increasing bias voltage causes steeper detector additive noise growth when the guard ring electrode is disconnected.

Study of electric field distribution and low frequency noise of CdZnTe radiation detectors

Polarization phenomena in a metal-semiconductor-metal (M-S-M) structure of metallic Schottky contacts deposited on CdZnTe radiation detectors were studied. We evaluate the distribution of the electric field along the biased M-S-M structure by Pockels measurements. The results show that almost all the electric field is developed across the depletion layer of the reverse-biased contact. The noise measurements of the CdZnTe detectors studied show that the dominant noise is 1/fm noise. The 1/fm noise, with the parameter m close to one, is present at frequencies below 100 Hz and its bandwidth decreases in the course of the polarization process. At higher frequencies, we observed an increase of the m parameter to 2, which indicates a strengthened effect of the generation-recombination processes. In the frequency band of dominating 1/fm = 1 noise, the increase of magnitude of the noise spectral density was proportional to the power of 6, in relation to the current through the detector. This high value is explained as a result of a screening effect of the space charge buildup during the polarization.

Notice of Retraction Comparison of relaxation properties of n-type and p-type CdTe single crystals

The bulk resistance of several CdTe single crystals was measured during long time interval with an applied external voltage. The measurements showed that the sample bulk resistance decreases very slowly after an external electric field was applied. Slow relaxation processes were also observed at temperature changes. The analysis showed that the main difference in relaxation processes between n-type samples and p-type samples is that the bulk resistance of n-type samples changes mainly due to the electron concentration with the temperature changing and the main process in p-type samples with the temperature changing is the hole mobility changing.

Contacts charge transport and additional noise properties of semiconductor CdTe sensors

Contact quality analysis of Cadmium-Telluride detector has been conducted. IV characteristics at operating temperatures T = 305 K, 315 K, 325 K were measured. Results showed asymmetry of IV characteristics for negative and positive bias indicated by increased leakage current in case of negative biasing. Noise contributions of contacts were evaluated. Reverse biased contact in negative was found as dominant source of low frequency noise.

Transport characteristics of CdTe radiation detectors

Transport characteristics of CdTe radiation detectors have been studied. Our aim is study of the charge carrier concentration and their mobility changing in the bulk with temperature. We found that the conductivity of the sample and the bulk conductivity increases very slowly after an external electric field is applied. Each sample shows very high value of relaxation time (hundreds and thousands of seconds).

Low frequency noise and ions diffusion in the CdTe bulk single crystals

The effect of ions diffusion on time dependence of noise and polarization phenomena of CdTe bulk single crystals has been carried out. We evaluated the instability of the reverse current of the Schottky contact at the P-type CdTe and the Au interface by a model which considers the effects of deep acceptors. The results shows small detrapping time compared to the experimental measurements. We suggested the diffusion of ions in depletion region at the metal - semiconductor interface causes an additional reduction of the barrier and could be considered as another responsible mechanism for the polarization effect. The CdTe samples showed noise source with dominant 1/ ƒn noise at low frequency, while the parameter n increases with repeating the measurements, indicating that another noise source has to be taken in account. The barrier fluctuating due to the ion diffusion in the depletion region can to be the another noise source,