Enos Gombia

Study of Surface Treatment Effects on the Metal-CdZnTe Interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the oxidized surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

Effects of the quantum dot ripening in high-coverage InAs∕GaAs nanostructures

We report a detailed study of InAs∕GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15nm and 2×1011cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25nm and densities ranging from 2×108to1.5×109cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the fo...

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

Deep levels in virtually unstrained InGaAs layers deposited on GaAs

The dislocation-related deep levels in InxGa1−xAs layers grown by molecular beam epitaxy on GaAs substrates have been investigated. Virtually unstrained InGaAs layers with mole fraction x of 0.10, 0.20, and 0.30 have been obtained by properly designing the In composition of linearly graded InxGa1−xAs buffers. Two electron traps, labeled as E2 and E3, whose activation energy scales well with the energy gap, have been found. Unlike E2, E3 shows: (i) a logarithmic dependence of the deep level transient spectroscopy amplitude on the filling pulse width and (ii) an increase of concentration as the buffer/InGaAs interface is approached. These findings, together with the observation that, in compressively strained In0.2Ga0.8As, the E3-related concentration is definitely higher than that of virtually unstrained In0.2Ga0.8As, indicate that this trap is likely originated by extended defects like threading dislocations.

Study of surface treatment effects on the metal-CdZnTe interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-Ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous etching solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the etched surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

Photoelectric properties of GaSb Schottky diodes

Electrical and photoelectrical properties of GaSb Schottky diodes obtained by evaporating gold metal dots on sulphur treated or chemically etched surfaces of Te-doped n-GaSb crystals (grown from melt by Czochralski method), with Hall carrier density in the range of 1.8–6.5×1017 cm−3, were studied. J/V characteristics with an ideality factor ranging between 1.17 and 1.22 were measured on Schottky diodes prepared on sulphur passivated surfaces. After image force effect correction, photoelectric determination of the barrier height (qΦb=0.598±0.006 eV) has been found to be independent of the surface treatment and, in the case of the sulphur-treated diodes, in good agreement with the value obtained through C−2 vs reverse bias measurements (qΦb=0.6±0.01 eV). Through spectral response analysis of Schottky diodes, an estimation of minority carrier diffusion length value is given. A major role of sulphur passivation on preparation of Schottky barrier with good and well reproducible electrical properties is confirmed.

Use of spatially dependent electron capture to profile deep‐level densities in Schottky barriers

A new procedure to profile deep‐level densities within the space‐charge region of Schottky barriers is presented. The method takes advantage of the spatial dependence of the time constant for the free‐electron capture by deep‐donor traps. The amplitude of the slow component of the capture capacitance transient following a negative reverse voltage pulse is simply related to the trap density at the point where the Fermi level crosses the trap level itself. The density profile of a given trap can be achieved by measuring the slow‐component transient signal as a function of the reverse voltage at a suitably chosen constant temperature. The estimated spatial resolution of the method was near 50 A in a practical case. Experimental density profiles for EL14, EL8, EL3, and the EC−ET =0.37 eV level in Cr/GaAs and Al/GaAs Schottky barriers are presented and discussed. The procedure is expected to be also applicable to the case of trap densities comparable with the shallow‐donor density without introducing large errors.

Boron oxide encapsulated vertical Bridgman grown CdZnTe crystals as X-ray detector material

CdZnTe crystals doped with indium were grown by the boron oxide vertical Bridgman technique, with a diameter of 2-inches. The crystals showed large single crystalline yield and low etch pit density. The background impurity content was dominated by boron in concentration lower than 1 ppm. High resistivity was obtained and a procedure for contact preparation was developed. The mobility-lifetime product of the material was determined by both X-ray irradiation and photocurrent spectroscopy. The X-ray detector prepared with this material showed good spectroscopic characteristics.

DX-CENTER-RELATED FEATURES BY CAPACITANCE MEASUREMENTS IN ALGAAS

The role of quasistatic C‐V measurements in investigating DX‐center‐related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron‐capture rate by the DX center, at low temperature, is responsible for a frozen‐step‐like density profile of positively charged DX centers near the metal‐semiconductor interface. This causes a knee‐shaped 1/C2‐vs‐V plot and gives rise to an apparent built‐in potential. The low‐temperature freezing in of the free‐electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low‐temperature C‐V measurements performed on the sample cooled at different cooling rates.

Hole diffusion length investigation by photon and electron excitation of GaAs Schottky barriers

A hole diffusion length in liquid‐encapsulated‐czochralski (LEC) GaAs monocrystals (Si‐doped, n≂1015–1016 cm−3) in the range (0.4–3.0) 10−4 cm, has been determined by photon and electron bombardment through semitransparent Au or Cr Schottky electrodes. A minority‐carrier lieftime in the range (0.3–9.7) 10−9 s was estimated. Schottky barrier diodes with abrupt junctions giving optimum electrical characteristics were prepared. The absorption length of the examined samples was evaluated by optical transmission measurements. The diffusion length investigation, which was carried out by the steady‐state surface photovoltage (SPV) and the scanning electron microscope electron‐beam‐induced current (SEM‐EBIC) techniques, has been related to the spectral quantum efficiency of Schottky diodes. SPV and EBIC techniques are emphasized as complementary methods for the investigation of bulk photoelectronic properties.