DEEP LEVEL TRANSIENT SPECTROSCOPY OF MO GAAS SCHOTTKY BARRIERS PREPARED BY DC SPUTTERING

DC sputtering of molybdenum allows the realization of near ideal Schottky barriers provided suitable deposition conditions are utilized. DLTS investigation shows that this kind of deposition process does not introduce damage defects if the sputtering voltage is below 2.5 kV, in our experimental conditions. The electrical characteristics get worse and a reduction in EL2 signal is evident for diodes prepared at the highest sputtering voltage. The reduction in EL2 signal is not an apparent effect related to the lowering of the barrier but it is strictly correlated to the damage due to the metal deposition process. A model, based on the outdiffusion of interstitial and antisite As, is proposed to interpret the experimental data

TI/GAAS SCHOTTKY BARRIERS PREPARED BY ION-BEAM SPUTTERING

Schottky barrier diodes of Ti on n-GaAs wafers were made by means of an ion beam sputtering (IBS) system under various sputtering conditions. We have investigated Schottky diode parameters by I-V and C-V measurements, and deep level centers by deep level transient spectroscopy (DLTS). The electrical characteristics are typical of good quality Ti/GaAs Schottky diodes for all samples, although a slight deterioration of the electrical performances has been observed for samples prepared at higher beam voltage. DLTS investigation shows the presence of damage defects in concentration lower than 1 x 10(14) cm-3 for all samples with the exception of those prepared at higher beam voltage. Such low concentration of defects introduced by the sputtering deposition shows that IBS can be a very useful technique to prepare Schottky barriers on GaAs

CHARACTERIZATION OF A DEFECT LAYER AT A SCHOTTKY-BARRIER INTERFACE BY CURRENT AND CAPACITANCE MEASUREMENTS

The electrical properties of a Ti/GaAs Schottky diode were investigated using electrical measurements under forward bias. The current, the low and high frequency capacitance and DLTS measurements evidence and describe in a consistent way the existence of a thin layer of defects induced by the metal deposition process near the interface. The quasi-saturation of the current and the excess low frequency capacitance are analysed by means of a generalization of a dipole layer model. The high density of states extending for several hundreds of Angstrom in the semiconductor is also evidenced by high frequency capacitance measurements. DLTS measurements using positive pulses confirm the presence of the traps and, together with the temperature analysis of the cut-off frequency, allow to study the trap emission process over a wide range of temperatures

X-RAY-IMAGING USING A PIXEL GAAS DETECTOR

The electrical characteristics, charge collection efficiency and energy resolution of a GaAs detector made out of LEC material have been studied. A matrix of 36 square pixels was deposited on a 70 mu m thick crystal; each pixel had an area of 200 X 200 mu m(2) with 20 mu m spacing between adjacent pixels. This detector showed a charge collection efficiency uniformity among the pixels better than 88% when exposed to 60 keV photons, which ensures that a comfortable common threshold setting can be adopted without affecting the uniformity of the measurement. It was successful in recording (60 keV photon energy) the image of a wolframium slab, 100 mu m thick. RI Cola, Adriano/G-2379-201

GaAs devices with vertical and planar structures for optically activated high-voltage switching

A class of semi-insulating gallium arsenide diodes has been manifactured and tested, with a Schottky contact and a special ohmic contact on the back, which can be operated in reverse bias well above 5 V/mu m were leakage currents are in the 1-10 mu A range (for 3 mm diodes). Preliminary measurements show their feasibility as PhotoConductive Semiconductor Switches (PCSS). At fixed high-voltage bias, the current drawn by this device can be increased at least by two orders of magnitude, by illuminating the diode with low-intensity near-infrared light. The resulting change in resistance can be used in order to switch off (DC mode) the high voltage either in a series or a parallel load configuration. Measurements are reported for 3 mm diameter GaAs diodes, 0.2 mm thick, as for: current-voltage characteristics, current increase upon illumination with a 880 nm LED, linearity of photocurrent vs. illumination level. In addition to these diodes with vertical structure, special GaAs switches with a planar structure have been developed and tested at 670 nm with a focused laser diode beam; they are made of a 0.2 mm substrate and a series of two Schottky contacts facing each other on the same side of the substrate. The planar structures enable multiple independent switches on the same substrate, each one being able to withstand a voltage bias as high as 700 V with typical leakage currents below 20 nA, and in addition, they may provide a photocurrent/dark current gain as high as 10(6). Both types of devices can be activated either locally or remotely, e.g. with illumination via single or multiple optical fibers, thus providing a special type of PCSS for optically controlled high-voltage DC switching. We plan to use devices of this type in high-energy physics experiments, specifically, in order to control remotely the high-voltage bias of microstrip gas chambers. (C) 1998 Elsevier Science B.V. All rights reserved. RI Cola, Adriano/G-2379-201

Irradiation of optically activated SI-GaAs high-voltage switches with low and high energy protons

Semi-Insulating Gallium Arsenide (SI-GaAs) devices have been tested for radiation hardness with 3-4 MeV or 24 GeV proton beams. These devices can be operated in de mode as optically activated electrical switches up to 1 kV. Both single switches (vertical Schottky diodes) and multiple (8) switches (planar devices) have been studied, by analyzing their current-voltage (I-V) reverse characteristics in the dark and under red light illumination, both before and after irradiation. We propose to use them in the system of high-voltage (-600 V) switches for the microstrip gas chambers for the CMS experiment at CERN. Low energy protons (3-4 MeV) were used in order to produce a surface damage below the Schottky contact: their fluence (up to 2.6x10(15)) p/cm(2)) gives a high-dose irradiation. The high energy proton irradiation (energy: 24 GeV, fluence: 1.1x10(14) p/cm(2)) reproduced a ten years long proton exposure of the devices in CMS experiment conditions. For low energy irradiation, limited changes of the I-V curves in the dark have been observed, with at most a fourfold increase of the leakage current: after exposure, however, the breakdown voltage decreases significantly. For high energy irradiation, we observed - for the vertical Schottky diodes biased at -600 V - an increase of the leakage current and a reduction of the photocurrent after irradiation, with respect to pre-irradiation conditions. For these diodes, the reduction of the photocurrent/dark current ratio was 25:1. At the same proton energy, an analogous behaviour was shown by the planar devices, but after irradiation the current gain may reduce over three orders of magnitude

Electrical and optical properties of ITO and ITO/Cr-doped ITO films

In this paper we report on the effects of the insertion of Cr atoms on the electrical and optical properties of indium tin oxide (ITO) films to be used as electrodes in spinpolarized light-emitting devices. ITO films and ITO(80 nm)/Cr-doped ITO(20 nm) bilayers and Cr-doped ITO films with a thickness of 20 nm were grown by pulsed ArF excimer laser deposition. The optical, structural, morphological wand electrical properties of ITO films and ITO/Cr-doped structures were characterized by UV-Visible transmission and reflection spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM) and Hall-effect analysis. For the different investigations, the samples were deposited on different substrates like silica and carbon coated Cu grids. ITO films with a thickness of 100 nm, a resistivity as low as similar to 4 x 10(-4) Omega cm, an energy gap of similar to 4.3 eV and an atomic scale roughness were deposited at room temperature without any post-deposition process. The insertion of Cr into the ITO matrix in the upper 20 nm of the ITO matrix induced variations in the physical properties of the structure like an increase of average roughness (similar to 0.4-0.5 nm) and resistivity (up to similar to 8x10(-4) Omega cm). These variations were correlated to the microstructure of the Cr-doped ITO films with particular attention to the upper 20 nm