Effect of electron and proton irradiation on characteristics of SiC surface-barrier detectors of nuclear radiation

Structures with a Schottky barrier based on CVD-grown 4H-SiC films were irradiated with 8 MeV protons and 900 keV electrons. The maximum fluences were 10¹⁴ and 3 × 10¹⁶ cm^?2, respectively. It was found that, in the case of electrons, the primarily introduced radiation defects are closely spaced Frenkel pairs. Changes in the electrical characteristics of the structures were compared. Capacitance methods and nuclear spectrometry were employed. The latter technique was used to determine the charge collection efficiency under pulsed ionization with α-particles. It was determined that, under proton irradiation, the charge collection efficiency steadily decreases as the fluence increases. For electrons, the efficiency remains unchanged in the fluence range (1–3) × 10¹⁶ cm^?2. However, a fluence of 3 × 10¹⁶ cm^?2 leads to a pronounced increase in the non-uniformity of charge transport conditions throughout the sample volume.     

Instability of characteristics of SiC detectors subjected to extreme fluence of nuclear particles

The operation of detectors irradiated with 8-MeV protons at a fluence of 3 × 10¹⁴ cm^?2 has been studied. The detectors were based on modern CVD-grown n-4H-SiC films with a concentration of uncompensated donors equal to ～2 × 10¹⁴ cm^?3 and a thickness of 55 μm. The high concentration of primary radiation defects (～2 × 10¹⁷ cm^?3) determined the deep compensation of the films. The basic characteristics of the detectors—pulse amplitude and resolution—exhibited temporal instability. This effect is due to prolonged capture of nonequilibrium carriers by radiation centers and the resulting appearance of a polarization voltage in the bulk of the detector. The kinetics of attainment of steady values by the quantities specified above was analyzed.     

On the problem of the radiation hardness of SiC nuclear radiation detectors at high working temperatures

Owing to the radiation-induced pronounced conductivity compensation in silicon carbide, carrier localization (trapping) prevails over recombination in capture of nonequilibrium carriers. This makes it possible, by raising the temperature, to reduce the time of carrier retention by a trapping center to values shorter than the duration of signal shaping by electronic circuits. For structural defects created by 6.5-MeV protons, the temperature excluding degradation of the detector signal via carrier localization is estimated. The values of the appearing generation current the noise of which can restrict the operation of a detector in the spectrometric mode are determined.     

6H-SiC epilayers as nuclear particle detectors

Schottky diodes based on the n-n ⁺ 6H-SiC epilayers grown by sublimation epitaxy and also the layers produced by CREE company (USA) were used as detectors of α-particles of spontaneous decay. Since the thickness of n-layers was smaller than the range of the particles, geometrical parameters of the experiment differed from conventional ones; in the latter case, a particle is brought to rest in the region of electric field in the detector. The calculated and experimental data were compared to study the special features of transport of nonequilibrium charge under the conditions of complete and partial depletion of the structure. It is shown that characteristics of the material that govern the transport of charge carriers can be deduced from the analysis of the behavior of the signal amplitude and the shape of the pulse-height spectrum in relation to bias voltage applied to the Schottky diode. It follows from the results that the present-day sublimation-grown SiC layers are suitable for use as the basis for fabrication of nuclear particle detectors.     

Characteristics of nuclear radiation detectors based on semi-insulating gallium arsenide

The characteristics of detectors based on bulk semi-insulating GaAs (SI-GaAs) have been studied by α particle detection and spectrometry. A distinctive feature of these detectors is the dependence of the width of the space charge region W on reverse bias voltage U. The rate of increase in W(U) is ∼1 μm/V, which permits formation of a sensitive region a few millimeters thick. The main obstacle to applying kilovolt-range bias voltages U is the reverse current noise. The characteristics of diode structures in which a rectifying barrier to SI-GaAs was formed by metal deposition (Schottky diodes) and by growing heterostructures with heavily doped AlGaAs or GaAsSb epitaxial layers were compared. Nonequilibrium carrier transport in epitaxial structures capable of sustaining bias voltages above 1 kV was investigated in both weak (below 1 kV/cm) and strong (10–30 kV/cm) electric fields. In both cases, the carrier lifetimes were found to be about a few nanoseconds. Such low values are due to the high concentration of trapping centers (EL2-type native defects), which limits the carrier transport. An analysis of the spectral line shape revealed that the lifetime is almost constant throughout the detector volume. The charge introduced by a particle was found to be enhanced in fields of ∼30 kV/cm. This effect can be qualitatively explained by focusing the electric field lines at the vertex of the α-particle track, which leads to an increase in the local field strength to ∼10⁻⁵ V/cm and impact ionization by nonequilibrium electrons. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 490–497. Original Russian Text Copyright ? 2004 by Verbitskaya, Eremin, Ivanov, Strokan, Vasil'ev, Gavrin, Veretenkin, Kozlova, Kulikov, Markov, Polyakov.     

The use of SiC triode structures as detectors of nuclear particles

The possibility of the internal amplification of the signal by a factor of approximately 100 for SiC-based detectors of short-range ions was demonstrated. The detectors were fabricated from the growth of p-epilayers on 6H-SiC n ⁺-substrates. The film thickness was approximately 10 μm, and the doping level was 2.8×10¹⁵ cm⁻³. The Schottky barriers were formed on the films by magnetron sputtering of Ni. The structure of the detectors was n-p-n ⁺, and their parameters were investigated in the floating-base mode. The α particles from ²⁴⁴Cm with an energy of 5.8 MeV were used, and the increase in the signal (E) with a rise in applied voltage (U) was investigated. The structures irradiated are equivalent to the phototriodes. The superlinear rise of E was observed with a considerable (by a factor of tens) amplification of the nonequilibrium charge introduced by the α particle. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 3, 2002, pp. 375–378. Original Russian Text Copyright ? 2002 by Strokan, Ivanov, Savkina, Davydov, Bogdanova, Lebedev.     

Carrier transport in a SiC detector subjected to extreme radiation doses

The efficiency of charge transfer in SiC detectors irradiated with 8 MeV protons at a dose of 10¹⁴ cm^?2 has been studied. The number of defects originally created in this irradiation mode is equivalent to that of disruptions produced in the lattice of SiC detectors used in experiments on the modernized SLHC collider (CERN). Methods of nuclear spectrometry were employed, with the detectors tested with 5.4 MeV alpha particles. Taking into account the deep compensation of SiC in the course of irradiation, the use of structures in the unconventional forward-biased mode is suggested. In this mode, the electric field strength is distributed across the detector thickness more uniformly. An illustrative model of carrier transport is suggested for processing of experimental data.     

Effect of low-temperature annealing on characteristics of SiC detectors with radiation-induced defects

p ⁺-n-n ⁺ detector structures based on CVD films with an uncompensated donor concentration of 2 × 10¹⁴ cm^?3 have been studied. The p ⁺-region was created by implantation of Al ions. The detectors were preliminarily irradiated with 8-MeV protons at a fluence of 3 × 10¹⁴ cm^?2 and then annealed at 600°C for 1 h. In measurements performed in the temperature range 20–150°C, the forward-and reverse-bias modes were compared. It is shown that the annealing leads to a higher collection efficiency of carriers generated by nuclear radiation and to a decrease in the amount of charge accumulated by traps in the course of testing. Despite the positive effect of the annealing, a considerable amount of radiation defects remain, which is manifested, in particular, in the kinetics of the forward current.     

Effect of nuclear radiation on Selfoc microlenses

Exposure of quarter pitch SLS and SLW Selfoc microlenses to moderate radiation doses ~50 Gy has been found to induce optical attenuations of as much as 1 dB at 0.85 ?m. Although significant recovery is noted in the SLW lens over 24 h following the irradiation, little decrease has been measured in the SLS lens. Spectral measurements of the induced absorption have revealed a monotonic decrease with increasing wavelength (0.6?1.6 ?m).     

不同形状防辐射屏自发辐射对探测器的影响

为了研究不同形状的防辐射屏自发辐射对探测器的影响并找出影响最小的防辐射屏形状, 采用理论计算对抛物面、双曲面、半椭球面(含半球面)、台柱面(含圆柱面和圆椎面)的防辐射屏内屏自发辐射至探测器的功率进行了系统分析。结果表明, 各种形状防辐射屏自发辐射至探测器的功率随防辐射屏高度与底面半径比值的增大而先减小再增大, 其中最大值为与探测器尺寸相关的常数, 最小值出现在防辐射屏高度与底面半径接近或相等时; 当防辐射屏为半椭球面且高度和底面半径相等(即半球面)时, 自发辐射至探测器的功率小于其它任何形状的防辐射屏, 且与防辐射屏尺寸及顶部开小孔与否无关。本研究能为防辐射屏结构设计提供有价值的参考。     

气溶胶粒子特性和垂直分布对辐射的影响

数值模拟了在给定条件下气溶胶粒子群平均有效半径和折射率虚部不同时大气层顶的反射强度和到达地面的透射强度,以及不同气溶胶垂直分布对各高度层的反射、透射强度和辐照度的影响。结果表明,当大气气溶胶光学厚度相同时,气溶胶垂直分布对15 km以下的反射和透射辐射影响较大;气溶胶粒子群平均有效半径和折射率虚部越小,大气层顶的反射强度和到达地面的透射强度越大。因此,对于准确地计算大气辐射不仅需要考虑气溶胶总光学厚度,还需考虑气溶胶粒子群的平均有效半径、复折射率和气溶胶垂直分布;计算中若只使用气溶胶模型中的经验值会带来较大误差。     

Effects of ionizing radiation on glow discharge detectors

Glow discharge detectors in the abnormal glow mode can be used as sensitive detectors of microwave and millimeter wave radiation even in the presence of very intense γ ionizing radiation fields where semiconductor devices cannot operate. In the subnormal glow mode, glow discharge detectors give promise of being able to monitor ionizing radiation fields so intense that the usual electronic detectors saturate.     

Development of nuclear radiation detectors with energy resolution capability based on CdTe-n/sup +/-GaAs heterojunction diodes

CdTe-n/sup +/-GaAs heterojunction diodes for room-temperature nuclear radiation detectors have been developed and demonstrated. The heterojunction diode was fabricated by growing a thin n-type CdTe buffer layer followed by the undoped p-like CdTe layer of about a 100 /spl mu/m thickness on the n/sup +/-GaAs substrates using metal-organic vapor phase epitaxy. The diode detectors exhibited good rectification property and had the reverse leakage currents of a few /spl mu/A/cm/sup 2/ at 40 V bias. The detector clearly demonstrated its energy resolution capability by resolving the 59.54 keV gamma peak from the /sup 241/Am radioisotope during the radiation detection test.     

Nonpolarizing radiation detectors based on wide-gap semiconductor crystals

Wide gap, insulating semiconductor crystals offer great promise in photoelectric devices, especially as detectors of electromagnetic and nuclear radiation, but are not widely used because they become polarized during operation. During operation of detectors using these crystals with a high concentration of deep impurity levels, electrical charges build up and produce a change, over time, in the electric field within the crystal and in the magnitude of the detector photoresponse. Since it is impossible to avoid impurity centers in these crystals at this time, we propose new approaches to creating radiation detectors-dosimeters which do not become polarized over time, but rely on productive use of the polarization charges that accumulate in them. Fiz. Tekh. Poluprovodn. 33, 1475–1478 (December 1999)     

Radiation hardness of SiC ion detectors under relativistic protons

Schottky diodes based on 6H-SiC epitaxial films exposed to 1000 MeV protons at a dose of 3×10¹⁴ cm^?2 have been studied by precision alpha spectrometry. Parameters of deep levels introduced by protons were determined by deep-level transient spectroscopy. The number of vacancies generated in proton tracks was found using TRIM software. The width of the space charge region and the hole diffusion length before and after irradiation were obtained by processing the alpha-spectrometry and capacitance measurements. Minor variations in the charge transport properties of epitaxial 6H-SiC detectors were observed.     

Spectrometric properties of SiC detectors based on ion-implanted p +-n junctions

Results of spectrometric studies of nuclear radiation detectors based on p ⁺-n junctions formed in 4H-SiC films are presented for the first time. The junctions were fabricated by ion implantation of aluminum into 26-μm-thick CVD-grown epitaxial 4H-SiC layers with an uncompensated donor concentration of (3–5) × 10¹⁵ cm^?3. The detector characteristics were measured in testing with natural-decay alpha particles with energies of 3.35 and 5.4 MeV. The collection efficiency of charge generated by 3.35 MeV alpha particles was as high as 100% at an energy resolution of ? 2%.     

制备参数和退火对激光诱导化学汽相沉积合成纳米硅的粒径和红外光谱的影响

用激光诱导化学汽相沉积（LICVD）法制备纳米硅，发现：激光强度存在低限阈值，SiH4的流速存在着高限阈值，二者正相关，以维持SiH4裂解所需的高温。为了使纳米硅粒小而均匀，应加大激光强度，并相应加快SiH4的流速，以提高纳米硅粒的成核率，减少每一个纳米硅核所吸收的硅原子数，并缩短每一个纳米硅核的生长期。纳米硅制取后退火脱H，纳米硅的红外吸收光谱发生变化：4条特征吸收带的位置、强度和形状各有改变。这是因为纳米硅的表面积很大，表面氧化使组态改变。为了减轻这样的氧化，纳米硅应在Ar气氛中而不是在空气中退火，并且开始退火的温度低于300℃。     

Excitonic detectors of infrared and submillimeter radiation

Journal of Infrared, Millimeter, and Terahertz Waves - The theory of new type detectors based on the “quenching” of secondary emission in direct-gap semiconductors (lines of Raman light...     

Study of contacts to CdZnTe radiation detectors

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.