D.G Ebling

Comparison of CdTe, Cd0.9Zn0.1Te and CdTe0.9Se0.1 crystals: application for γ- and X-ray detectors

CdTe is one of the most encouraging semiconductor materials in the field of room temperature γ- and X-ray spectroscopy. To improve the detector properties, the ternary systems (Cd,Zn)Te and Cd(Te,Se), and CdTe were grown by vertical Bridgman technique. To achieve low noise detectors, the resistivity of all materials was increased by chlorine doping. The crystals were characterized by electrical (Hall measurements, photoinduced current transient spectroscopy) methods. The numbers of deep levels influencing the resistivity were reduced by introducing Se into the CdTe system. A common deep level close to the middle of the bandgap has been identified, responsible for the compensation effect in all three systems. In addition high resistivity and n-type conductivity were achieved in CdTeSe materials for the first time

Evaluation of active layer properties and charge collection efficiency of GaAs particle detectors

Abstract According to Ramos theorem the charge collection efficiency of a particle detector is mainly influenced by the field distribution between the contacts of a Schottky diode. In semi-insulating GaAs material a space charge layer is formed due to deep levels needed for the compensation of acceptors. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by different methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of ∼10 15 cm −3 at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of ∼16 16 cm −3 as measured by infrared absorption. The Schottky barrier leakage current is found to be responsible for the variation of the electrically active deep centers and it therefore influences the charge collection efficiency (c.c.e.). The c.c.e. turns out to be inversely proportional to the active concentration of deep centers. These results are supported by our modelling of the c.c.e.: Using the transport and the Poisson equation the electrical field distribution can be calculated through the coupling of the quasi-Fermi levels and the compensation mechanism. The model calculations of charge collection efficiencies for both alpha particles and protons are confirmed by the experimental results. The work is performed within the framework of the RD8 project.

Radiation detector properties of CdTe0.9Se0.1:Cl crystals grown under microgravity in a rotating magnetic field

Abstract The influence of growth conditions on radiation detector performance was studied by examining Cd(Te,Se):Cl crystals. They were grown by the travelling heater method under microgravity conditions during the PHOTON 8 mission. An additional forced convection was produced by a rotating magnetic field (400 Hz) of 2 mT. The influence on crystal quality was analyzed by several characterization methods such as time dependent charge measurement, photo-induced current transient spectroscopy and admittance spectroscopy. The charge collection efficiency of detectors was increased from 10% without magnetic field up to 66% by the influence of the rotating magnetic field. The improved detector performance is explained by the increased resistivity and a reduced number of deep levels.

Near band-edge transitions in AlN thin films grown on different substrates

Abstract The optical properties of epitaxial aluminum nitride (AlN) films grown on sapphire, SiC, and Si substrates have been investigated in the (2–6.3) eV photon energy range using cathodoluminescence (CL) at liquid nitrogen temperature. Besides a broad luminescence band at 3.2 eV the CL spectra shows multiple, relatively narrow excitonic features in the near band-edge region at approximately 6.1 eV. The nature of the excitonic transitions is discussed and an estimate of the band-gap energy is given.

Studies of the compensation mechanism in CdTe grown from the vapour phase

Abstract Semi-insulating titanium and vanadium doped cadmium telluride crystals were grown from the vapour phase. We show results of the electrical characterisation obtained by Hall measurements, photoinduced current transient spectroscopy (PICTS), admittance spectroscopy and time-of-flight measurements. The crystals with a resistivity of about 10 9 Ω · cm showed a deep level, which was identified for CdTe:V at −0.77 eV and a similar value at −0.66 eV for CdTe:Ti, both with reference to the conduction band. The μτ products are almost the same for both materials in the range of 10 −5 cm 2 /V. A compensation model, which was developed for semi-insulating GaAs, is adapted to semi-insulating vanadium- and titanium-doped cadmium telluride. We show that the data from the compensation model correspond to the measured properties.

Optical properties and ordering of AlxGa1-xN MBE-layers

To study the influence of the growth parameters on structural properties and crystal quality epitaxial layers of AlN and Al x Ga 1-x N (x = 0-1) were grown in an RF-plasma enhanced MBE-system on Si-terminated (00.1) SiC, (00.1) sapphire and (111) Si substrates by varying substrate temperature, growth rate and III/V ratio. The alloy composition of the Al x Ga 1-x N layers was studied by RBS-measurements and the results were compared with X-ray diffraction and with the optical properties obtained from cathodoluminescence (CL). The formation of an Al x Ga 1-x N superstructure is observed with ordered Al and Ga distribution. Increasing degree of chemical ordering is observed with inereasing growth temperature. The CL-spectra at 77 K show relatively sharp near-hand-edge transitions for pure AlN and Ga-rich compositions indicating high material quality. The near-band-edge transition follows the composition of the solid solution connected with a broadening of the peaks and evident optical bowing for samples grown at low growth rates (<0.35 μm/h). A decrease of the bowing parameter is observed for the entire range of alloy composition by increasing the growth rate.

Investigation of CdTe:Cl grown from the vapour phase under microgravity conditions with time dependent charge measurements and photoinduced current transient spectroscopy

Abstract CdTe: Cl crystals were grown from the vapour phase by the sublimation travelling heater method (STHM) in space on board of the unmanned EURECA I mission. They were characterized by time dependent charge measurement (TDCM) and photoinduced current transient spectroscopy (PICTS). Axial cuts of the space grown crystals investigated by TDCM reveal an axial symmetric resistivity distribution along the growth direction, which can be explained by segregation effects of chlorine.

Optical investigation of AlxGa1−xN epitaxial films grown on AlN buffer layers

Abstract In this study we investigate AlxGa1−xN/AlN heterostructure layers grown on c-plane sapphire by RF-plasma enhanced molecular beam epitaxy. The Al mole fraction was varied through the entire range of compositions (0≤x≤1), and was determined from Rutherford backscattering (RBS) analysis and X-ray diffraction (XRD). The thicknesses of the AlxGa1−xN layers ranged from 650 nm to ≈2 μm, and those of the AlN buffer layers from ≈100 nm to ≈400 nm. Reflection and cathodoluminescence measurements were carried out in the photon energy range between 3.0 and 6.2 eV at various cryogenic temperatures down to liquid helium temperature. The reflection measurements yield a bowing parameter b=(0.91±0.14) eV for the dependence of the band-gap energy on the alloy composition.

AlN on sapphire and on SiC : CL and Raman study

Aluminium nitride (AIN) layers were grown on sapphire (0 0 01) and on Si-terminated SiC (0 0 01) in a RF-plasma enhanced MBE system with integrated UHV-STM/AFM. The layers have been investigated by cathodoluminescence (CL) and Raman measurements. The CL measurements show defect related transitions in the region of 2-5 eV and resolved excitonic features in the near band-edge region followed by multiple A 1 (LO) replica. From both CL and Raman studies a superior quality of the AIN/SiC layers in comparison to the A1N/sapphire lavers is deduced.

Characterization of CdTe:Cl crystals grown under microgravity conditions by time dependent charge measurements (TDCM)

Abstract CdTe:Cl crystals were grown from the liquid and from the vapour phase under microgravity (μg) conditions on board the unmanned EURECA I mission. The resistivity distribution of the grown crystals was measured by time dependent charge measurement (TDCM). Photo induced current transient spectroscopy (PICTS) was used to investigate the deep level properties. The axial resistivity distributions of the crystals grown in space differ significantly from 1 g reference crystals. In the case of vapour growth, these differences can be explained by an additional laminar flow under 1 g conditions. Supercooling has to be considered in growth from a Te zone under μg.