F. Dubecký

GaAs detectors irradiated by low doses of electrons

Semi-insulating (SI) GaAs detectors were irradiated by 5 MeV electrons up to a dose of 69 kGy, in order to test their radiation hardness. The electric and spectrometric stability of detectors was examined as a function of the absorbed dose. Investigated detectors showed a very good detector radiation resistance within a dose up to 40 kGy followed by deterioration of some spectrometric and electric properties. However, the reverse current and the detector charge collection efficiency showed minimum changes with the overall applied doses. The obtained results will be used as a preliminary study for further radiation-hardness investigations of GaAs detectors against high energy electrons. This will complete our previous studies of GaAs detector radiation hardness against fast neutrons and γ-rays.

Detection properties of LEC SI GaAs radiation detectors with the symmetrical contact configuration

Abstract I–V characteristics and detection properties (CCE, FWHM) of the semi-insulating GaAs detectors (base length 200 μm) with the Au-Au and W-W Schottky barriers and N + -N + ohmic contacts, measured at room temperature, are presented for α , β and heavier particles (up to Z ⋍ 10). The best results (average values of CCE and FWHM were 87% and 5%, respectively, for 5.48 MeV α particles from 241 Am) give detectors with the Au-Au contact configuration. Detectors with the N + contacts have worse parameters, about 30% for both, the CCE and FWHM, respectively. Nevertheless, such detectors operate at a low bias voltage, under 20 V.

High resolution alpha particle detectors based on 4H-SiC epitaxial layer

We fabricated and characterized 4H-SiC Schottky diodes as a spectrometric detector of alpha particles. A thin blocking contact of Ni/Au (15 nm) was used to minimize the influence on alpha particles energy. Current-voltage characteristics of the detector were measured and a low current density below 0.3 nAcm−2 was observed at room temperature. 239Pu241Am244Cm was used as a source of alpha particles within the energy range between 5.1 MeV and 5.8 MeV for detector testing. The charge collection efficiency close to 100 % at reverse bias exceeding 50 V was determined. The best spectrometric performance shows a pulse height spectrum at a reverse bias of 200 V giving an energy resolution of 0.25 % in the full width and half maximum for 5.486 MeV of 241Am.

Characterization of epitaxial 4H-SiC for photon detectors

High purity epitaxial 4H-SiC became a serious candidate for the fabrication of spectrometric radiation detectors with a high resistance to neutrons and gamma rays damage and suitable for applications in hot plasma diagnostics. The present work reports on i) the characterization of high purity epitaxial 4H-SiC grown by liquid phase epitaxy on SiC substrates and ii) the performances of metal/4H-SiC detectors fabricated on the same material. X-ray diffraction and topography as well as I-V, C-V and DLTS measurements are used for the evaluation of the material properties and device characteristics. The UV and X-ray detection abilities are evaluated by photocurrent measurements in the 3-6 eV region and pulse-height spectra measurements of the 241Am, respectively. Preliminary results of the detector hardness to fast neutron and gamma ray radiations are also reported.

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.

GaAs detectors for hard X-ray astronomy

Particle detectors fabricated from bulk semi-insulating (SI) GaAs of 4 different suppliers are investigated. I-V, C-V characteristics, material transport parameters as well as inhomogeneities are studied and correlated with detector performances. It was observed that the charge collection efficiency and the energy resolution of detectors tested by 5.48 MeV α-particles and 122 keV X-rays is strongly affected by physical properties of the base material. It is concluded that detectors fabricated from the graded bulk SI GaAs material are suitable for application in hard X-ray astronomy.

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.

The work functions of Au/Mg decorated Au(100), Mg(001), and AuMg alloy surfaces: A theoretical study

A plane-wave density functional theory is used to predict the work functions of Au/Mg decorated Au(100), Mg(001), and stochiometric AuMg alloy surfaces. We find, that irrespective of the details, all Au/Mg systems containing Mg on the surface reveal the Mg-dominated work functions, i.e., significantly shifted toward the work function of clean Mg(001) surface. The reported analyses suggest, that this general trend stems from a strong charge transfer from Mg to Au and consequent enhancement of a surface dipole. The calculated properties of the AuMg alloy well agree to the experiment. The reported results may readily find applications in Au/Mg/AuMg surface physics and technology of metal/semiconductor contacts.

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.