A. Ruzin

Study of the charge collection efficiency of CdZnTe radiation detectors

The charge collection efficiency of CdZnTe radiation detectors with two different configurations: aSchottky diode detector and aresistive detector are compared. The average charge collection efficiencies for three different directions of irradiation (negative electrode, positive electrode and perpendicular to the electric field) are calculated. The mobility-lifetime product of the CdZnTe substrates is evaluated from the dependence of the measured spectra upon detector bias voltage. The measurement of the average charge collection efficiency is based on monitoring the shift of the peak channel with bias voltage in an experimental setup which is well calibrated. Two types of radiation are used:gamma photons from several radioactive sources andalpha particles from an241Am source. The models for the evaluation of mobility-lifetime product from the measured data for the two types of detector configurations as well as for the two types of radiation sources, are compared and discussed. The CdZnTe (Zn = 10%) substrates under study are obtained commercially and are grown by the high pressure Bridgeman method. The mobility-lifetime products and specific resistivity of the two types of detectors are evaluated and compared. A lower resistivity material has a narrower depletion region and behaves like a thinner detector thus exhibiting better collection efficiencies. Therefore, medium resistivity material which is completely inadequate for resistive detectors can still yield high performance Schottky detectors. The preferred direction of irradiation, i.e. from the negative electrode, is possible only in the case of n-type material which is reverse biased by negative voltages applied to the Schottky gate. The mobility-lifetime products that are derived on both the resistive detector (with specific resistivity of ≈1.1010 ω.cm) and the Schottky diode (with specific resistivity of ≈1.106 Ω.cm) are μnτn ≈-4.10−4 cm2V−1 and μpτp≅ 8.10−5 cm2V−1.

Comparison of radiation damage in silicon induced by proton and neutron irradiation

The subject of radiation damage to Si detectors induced by 24-GeV/c protons and nuclear reactor neutrons has been studied. Detectors fabricated on single-crystal silicon enriched with various impurities have been tested. Significant differences in electrically active defects have been found between the various types of material. The results of the study suggest for the first time that the widely used nonionizing energy loss (NIEL) factors are insufficient for normalization of the electrically active damage in case of oxygen- and carbon-enriched silicon detectors. It has been found that a deliberate introduction of impurities into the semiconductor can affect the radiation hardness of silicon detectors.

Studies of radiation hardness of oxygen enriched silicon detectors

Abstract Detectors for high-energy particles sustain a substantial amount of structural defects induced by the particles during the operation period. Some of the defects have been found to be electrically active, degrading the detectors performance. Understanding the mechanisms of the electrical activities and learning to suppress their influence are essential if long “lifetime” detectors are required. This work reports about radiation hardness of silicon P-I-N devices fabricated from oxygen enriched high resistivity material. The high and nearly uniform concentration of oxygen in Float Zone silicon has been achieved by diffusion of oxygen from SiO 2 layers.

Methodology for evaluation of mobility-lifetime product by spectroscopy measurements in CdZnTe spectrometers

A novel methodology is presented for extraction of the semiconductor electron and hole mobility-lifetime products by x- and γ-ray spectroscopy analysis. The methodology is based on the analysis of spectroscopy results, namely the maximal and the average charge collection efficiencies as discussed below, for different photon energies at various bias voltages. The methodology enables the evaluation of both electron and hole mobility-lifetime products without the need to use α particle measurements. The evaluation is carried out by a single parameter fitting of the models analyzed in this study. Mobility-lifetime products for CdZnTe substrates grown by Bridgman and high pressure Bridgman (HPB) methods are reported and compared. Typical values of μe⋅τe=2⋅10−3 and μh⋅τh=1⋅10−5 cm2/V are extracted for HPB grown CdZnTe with 10% Zn concentration. Values of μe⋅τe=9⋅10−4 and μh⋅τh=1⋅10−7 cm2/V are obtained for CdZnTe with 10% Zn concentration grown by the Bridgman method.

Monolithic coupling of a SU8 waveguide to a silicon photodiode

We present quantitative results of light coupling from SU8 waveguides into silicon p-n photodiodes in monolithically integrated structures. Multimode, 12 μm thick, and 20 μm wide SU8 waveguides were fabricated to overlap 40×180 μm2 photodiodes, with three different waveguide-photodiode overlap lengths. The attenuation due to leaky-mode coupling in the overlap area was then calculated from photocurrent measurements. The overlap attenuation ranged from a minimum of 2.2 dB per mm overlap length to a maximum of about 3 dB/mm, comparing favorably with reported nonpolymeric waveguide-Si photodiode attenuations.

Statistical models for charge collection efficiency and variance in semiconductor spectrometers

Charge collection efficiency and the variance in the collected charge of semiconductor spectrometers are modeled. The model is based on a statistical approach and the extended Ramo theorem. The model yields an expression for variance in charge collection efficiency as a function of photon energy, bias voltage, and semiconductor parameters. These calculations as a function of absorption depth are particularly important in semiconductors with high atomic numbers, such as CdZnTe, since in these materials a uniform absorption cannot be assumed for a wide range of energies. Three different spectrometer configurations were considered: resistive, partially depleted Schottky barrier, and fully depleted Schottky barrier. An analytical model for the resistive configuration is presented and the results are compared to numerically obtained results of the Schottky configuration.

Passivation and surface leakage in CdZnTe spectrometers

The subject of surface leakage and passivation in spectrometers fabricated in high pressure Bridgman CdZnTe crystals with evaporated gold contacts has been examined. The surface leakage has been found to be the dominant mechanism of dark current leakage in metal–semiconductor–metal detectors with noninjecting contacts. Several passivation techniques have been examined with various degrees of success in terms of reducing the mean value and the power spectral density of dark noise characteristics. An insulating polymer was applied on the bare surface reducing the total leakage current by the factor of 6–7, while favorably affecting the dark noise spectral density by decreasing the corner frequency of the low frequency noise. However, a considerable surface leakage has been observed even in these apparently passivated spectrometers. A three terminal device with iso-potential high voltage contacts has been fabricated and tested, exhibiting a reduction of the total dark current leakage by approximately two ord...

Radiation hardness of silicon detectors manufactured on epitaxial material and FZ bulk enriched with oxygen, carbon, tin and platinum

Abstract Recent results on the radiation hardness of silicon detectors fabricated on epitaxial and float zone bulk silicon enriched by various impurities, such as carbon, oxygen, tin and platinum are reported. A new methodology of measurements of electrical properties of the devices has been utilized in the experiment. It has been shown that in the case of irradiation by protons, oxygen enriched silicon has better radiation hardness than standard float zone silicon. The carbon enriched silicon detectors, on the other hand, exhibited significantly inferior radiation hardness compared to standard detectors. This study shows for the first time, a violation of the widely used narmalization technique of the various particle irradiations by NIEL coefficients. The study has been carried out in the framework of the RD48 (ROSE) collaboration, which studies the radiation hardening of silicon detectors.

A model for the growth of CdTe by metal organic chemical vapor deposition

A kinetic model for the metalorganic chemical vapor deposition (MOCVD) growth of CdTe over a wide temperature range is presented. The model yields the growth rate as a function of the gas-phase concentrations of the constituents. The model is corroborated with experimental results obtained by the MOCVD growth of CdTe at 380° C. The major features of the model are the observed two-step surface-controlled pyrolysis and surface saturation, leading initially to a growth rate that increases with the square root of the concentrations of the reacting species and subsequently to a decrease of the growth rate as the concentrations increase. At even higher concentrations, an additional increase of growth rate is observed and modeled.

Silicon wafer oxygenation from SiO2 layers for radiation hard detectors

Abstract Radiation produces lattice damage in silicon by displacing the atoms from their original positions and thereby generating the corresponding defects. As a result, new states are created in the semiconductorforbidden band gap, negatively affecting the electrical performance of the devices. Endurance to radiation can be improved by having a high oxygen concentration in the silicon. For detector fabrication, high resistivity silicon is also needed, thus float zone wafers are preferred; however, this kind of material exhibits a low oxygen concentration. Although different ways to incorporate oxygen in float zone silicon have been proposed, all of them imply modifications during the ingot growth. Thermal diffusion from SiO 2 layers on polished wafers is an interesting alternative to improve their oxygen content. Different thermal processes aimed at obtaining oxygen enriched silicon for the fabrication of radiation hard detectors have been tested. Attention has also been paid to carbon introduction during processing since, high concentrations of this element has been proved deleterious.