P. Boháček

HYDROGENATED AMORPHOUS SILICON CARBON NITRIDE FILMS PREPARED BY PECVD TECHNOLOGY: PROPERTIES

Hydrogenated amorphous silicon carbon nitride films were grown by plasma enhanced chemical vapor deposition (PECVD) technique. The flow rates of SiH4 , CH4 and NH3 gases were 6 sccm, 30 sccm and 8 sccm, respectively. The deposition temperatures were 350, 400 and 450 ◦C. The RBS and ERD results showed that the concentrations of Si, C, N and H are practically the same in the films deposited at substrate temperatures in the range 350-450 ◦C. In photoluminescence spectra we identified two peaks and assigned them to radiative transitions typical for amorphous materials, ie band to band and defect-related ones. The electrical characterization consists of I(V ) measurement in sandwich configuration for voltages up to 100 V. From electrical characterization, it was found that with increased deposition temperature the resistivity of the amorphous SiCN film was reduced.

Electrical properties of semi-insulating GaAs irradiated with neutrons

Conductivity, Hall mobility and magnetoresistance in undoped semi-insulating GaAs samples irradiated by reactor neutrons of various fluences ranging from 1×1013 to 3×1015 cm−2 were measured and analysed in the temperature range 300–420 K. The conductivity and the apparent Hall mobility decrease while the magnetoresistance increases with increasing neutron fluence. The ratio of electron to hole concentration (n/p), resulting from the analysis of the room temperature parameters using a mixed conductivity model, decreases with increasing neutron fluence and reaches value less than one at the highest fluences used. Activation energy ≈0.78 eV was found for dominant deep acceptor deduced from the temperature dependences of the free charge carriers concentration in samples with n/p<1. A role of the thermal neutrons shielding at the irradiation, using a Cd-plate, is discussed.

Semi-insulating GaAs detectors optimized for fast neutron detection

Semi-insulating (SI) GaAs detectors with a HDPE (High Density PolyEthylene) conversion layer were optimized for detection of fast neutrons (from 0.5 MeV to 12 MeV). Based on previous simulations of neutron transport in HDPE and SI GaAs carried out by the Monte Carlo radiation transport computer code MCNPX (Monte Carlo N-Particle eXtended, version 2.5.0) we used a SI GaAs wafer with a larger thickness of 270 μm. The area of a single AuZn Schottky contact was enlarged from 6.25 mm2 to 7.36 mm2. Thanks to the pixel structure of the new metallization the breakdown voltage increased from 60 V to 280 V as deduced from the measured I−V characteristics. Various thicknesses of HDPE layers in the range from 100 μm to 2000 μm were used with SI GaAs detectors for neutron conversion. The measured relative detection efficiency for fast neutrons using SI GaAs detectors with various HDPE thicknesses varied from 0.07 to 0.12% at lower applied voltages, with a maximum for a 500 μm thick conversion layer.

Detection of fast neutrons using detectors based on semi-insulating GaAs

Detectors with AuZn square Schottky contact of the area of 2.5 ? 2.5 mm2 were fabricated. On the back side, the whole area AuGeNi eutectic ohmic contact was evaporated. The thickness of the base material (semi-insulating GaAs) was 220 ?m. The connection of 4 detectors in parallel was tested to get the detection area of 25 mm2. The 239Pu-Be fast neutron source with energies between 0.5 and 12 MeV was used in experimental measurements. We have investigated the optimal thickness of HDPE (high-density polyethylene) conversion layer for fast neutron detection. The spectra of the neutrons were measured by detectors covered by HDPE converter of different thicknesses. The fast neutron detection efficiency proved experimentally was compared with results from simulations performed by MCNPX (Monte Carlo N-Particle eXtended) code.

GaAs detectors irradiated by electrons at different dose rates

The radiation hardness of Semi-Insulating (SI) GaAs detectors against high-energy electrons was investigated. The detectors were irradiated by 5 MeV electrons. The influence of two irradiation parameters, the total absorbed dose (up to 24 kGy) and the applied dose rate (20, 40 and 80 kGy/h), on their spectrometric properties was studied. An 241Am gamma-ray source was used to evaluate the spectrometric properties. The applied dose has negatively affected the detector CCE (Charge Collection Efficiency) and has influenced also the energy resolution. Nevertheless, a global increase of detection efficiency with the dose was observed. Three different dose rates used during irradiation did not affect the CCE, but in the range of doses from 4 to 16 kGy an influence of the applied dose rate upon two other parameters was observed. With higher dose rates, a steeper increase in the detection efficiency and significant worsening of energy resolution were achieved.

Semiconductor detector based on 4H-SiC and analysis of its active region thickness

The 4H polytype silicon carbide is a promising material for radiation-resistant sensors of ionizing particles. The wide band gap of 3.26 eV offers operation at increased temperatures up to several hundred degrees of Celsius. In this work we focused on the analysis of active region thickness of detectors based on 4H-SiC. The detectors investigated are fabricated from a 105 ?m epitaxial layer grown on 350 ?m 4H-SiC substrate. The circular Schottky contacts with diameter of 1.4 mm using an Au/Ni double layer were evaporated onto both sides of the detector material. Three methods for determination of the active thickness were used. The capacitance-voltage measurements allowed us to estimate free carrier concentration profile. The second method was based on detection of ?-particles generated by an 241Am source. Using SRIM calculations and known Bragg curve absorbed energy in detector volume was estimated. The last method consists of measuring detection efficiency of ?-rays at reverse bias voltages up to 500 V.

Detector of fast neutrons based on silicon carbide epitaxial layers

High quality liquid phase epitaxial layer of 4H-SiC with a thickness of 105 μm was used for fabrication of detector with the circular Schottky contact formed by using Au/Ni double layer contact metallization. The detector structure was characterized by current-voltage measurements showing low reverse current density at room temperature. Following the detection of fast neutrons was studied using the 239Pu-Be source with the mean neutron energy of about 4 MeV. Detected spectrum revealed silicon and carbon-recoil ion continuum. The simulation of detection efficiency vs. thickness of the detector active layer shows linear dependence.

Correlation of crystal defects and galvanomagnetic parameters of semi-insulating InP with performance of radiation detectors fabricated from characterised materials

Semi-insulating (SI) InP substrates from four producers have been studied by galvanomagnetic methods, X-ray diffraction, laser scattering tomography, and scanning electron beam induced current (EBIC) techniques. The detection performances of radiation detectors fabricated from the materials were tested using 59.5 and 122 keV gamma ray sources. The highest apparent Hall mobility has been observed in a substrate with low Fe content, produced by LEC method and a post-growth wafer annealing process. Detectors based on this material exhibited the best overall detection performance. SI InP material grown by VGF method showed the lowest dislocation and precipitate densities and also acceptable detection performance. Room temperature (RT) operation of SI InP detectors is demonstrated.

The semi-insulating GaAs-based particle detector at IEE SAS: first imaging results ☆

Abstract A particle detector made of semi-insulating GaAs is used for preliminary study of X-ray imaging. A standard technology developed in our laboratory is applied to prepare this detector. The detector works at room temperature in the photon counting mode. Results show that the quality of images taken at chosen exposure range is limited only by the photonic noise from used X-ray source 241 Am.

Simulation of the reverse I–V characteristics of the Schottky barrier radiation detector structures prepared on semi-insulating GaAs

The reverse current–voltage (I–V) characteristics of the Schottky barrier radiation detector structures prepared on semi-insulating gallium arsenide (SI GaAs) have been simulated using a modified thermionic field emission (TFE) model. In order to explain the charge–current transport, the effect of tunnelling and thermionic field emission together with the Schottky barrier lowering were considered taking into account the voltage drop on the quasi-neutral bulk region. The modified TFE model describes reverse I–V characteristics in the temperature range between 300 K and 360 K. An observed agreement between measured and simulated characteristics enabled us to determine the Schottky barrier height and specific resistance of the SI GaAs base from the theoretical simulation. The results of calculated resistivities are compared with resistivities determined by the van der Pauw method. The value of the Schottky barrier height is in good agreement with the previously published data. A much higher value of the calculated ohmic current compared to the current corresponding to the diode structure at a low bias/current region is revealed. A detected difference increases with increasing temperature. This fact led us to conclude that the current in the linear part of the I–V characteristics in the low bias region does not correspond to the ohmic transport, being linear; that is, in contradiction with the more generally considered view.