Minoru Toguchi

Quantitative Analysis of Polarization Phenomena in CdTe Radiation Detectors

Polarization phenomena in a Schottky-type CdTe radiation detector were studied. We evaluated the distribution of electric field in a biased CdTe detector by measuring the progressive change of Schottky barrier lowering with time. The parameters of deep acceptors such as detrapping time, concentration, and the depth of the energy level were quantitatively evaluated. In the case of applying the conventional model of charge accumulation, the obtained result shows that the CdTe bulk is never undepleted. We modified the charge accumulation model by taking account of the occupation state of the deep acceptor level. When a modified model is applied, the time that the depletion width in the bulk begins to diminish closely fits the time that the photopeak position begins to shift in radiation measurements. In this paper, we present a distribution of electric field during biasing and a simple method for the evaluation of the parameters of deep acceptors in CdTe bulk.

Formation of Aluminum Schottky Contact on Plasma-Treated Cadmium Telluride Surface

We have studied the effect of plasma treatment on the rectification property and performance of the CdTe radiation detector with the Al Schottky electrode. The Te-rich layer on the CdTe surface etched with Br-methanol caused the degradation of the rectification property of the Al/CdTe Schottky contact. To remove the Te-rich layer, plasma treatment was carried out. The plasma treatment did not roughen the CdTe surface, and it removed the Te-rich layer. In terms of current–voltage characteristics of the Al/CdTe Schottky contact, the leakage current of the samples with plasma treatment was lower than that of the samples without plasma treatment. Moreover, in terms of detector performance, the samples with plasma treatment showed a higher energy resolution than those without plasma treatment. We achieved a high energy resolution of 1.6 keV FWHM at 59.5 keV using the plasma-treated Al/CdTe/Pt detector, which is comparable to the value obtained using a conventional Schottky-type In/CdTe/Pt detector.

Effect of He Plasma Treatment on the Rectification Properties of Al/CdTe Schottky Contacts

We investigated the effect of He plasma treatment on the surface composition of CdTe and the electrical properties of Al/CdTe Schottky contacts. The composition of the initial CdTe surface is Te-rich due to Br-methanol etching. When Al Schottky contacts are formed on the CdTe surfaces with the Te-rich layer, the barrier height is low and the rectification property is not good. In this paper, we propose the He plasma treatment method to remove the Te-rich layer. From X-ray photoelectron spectroscopy measurement, it was found that the plasma treatment can remove the Te-rich layer. The rectification property of the Al Schottky contacts on the plasma-treated surfaces is improved, and their barrier heights are estimated to be about 0.65 eV. In γ-ray spectrometry, a high-energy resolution of 1.6 keV full width at half maximum at 59.5 keV was obtained from the plasma-treated Al/CdTe/Pt detector. The results indicate that the plasma treatment of CdTe surfaces significantly improves the energy resolution of Schottky-type Al/CdTe/Pt radiation detectors.

Analysis of Polarization Phenomenon and Deep Acceptor in CdTe Radiation Detector

High energy-resolution CdTe radiation detectors with Schottky contact show instability with operating time under bias voltage, which is termed as polarization phenomenon. Time stability is one of the important factors for its practical applications. It is considered that this phenomenon is responsible for the deep acceptor level in CdTe bulk. However, the energy position and concentration of the deep acceptor level have been not estimated accurately. We have studied the polarization phenomenon and the parameters of deep acceptor in a Schottky-type CdTe radiation detector. In this paper, we propose a new method which can quantitatively evaluate the parameters of the deep acceptor such as concentration and energy level by measuring the temperature dependence of the current-voltage characteristics of the CdTe radiation detector.

Effect of CO2 addition on diamond growth by DC arc plasma jet chemical vapour deposition

The effect of adding carbon dioxide ( CO2) to the reactant gases was studied in a dc arc plasma jet chemical vapour deposition (CVD) for the synthesis of diamond films. The fabricated films were characterized by Raman scattering spectroscopy and scanning electron microscopy. Without CO2, Raman spectra of these films display only graphitic peaks. As CO2 is added, faceted diamond crystal is observed in micrographs from scanning electron microscopy and a sharp diamond peak in the Raman spectrum appears. At a CO2/ CH4 ratio of 2.0, diamond films exhibit a pure diamond Raman peak at 1333 cm-1 with small nondiamond carbon peaks. In addition, plasma diagnosis was performed using optical emission spectroscopy.

Analysis of Layer Structure Variation of Periodic Porous Silicon Multilayer

Structures of periodic porous silicon multilayer, which were formed by the current density modulation method, were investigated by cross-sectional scanning electron microscopy (SEM) observation. When the layers were formed with a current density of 20 mA/cm2 or less the thickness of the layers was constant regardless of the stack position of the layer. When the layers were formed with a current density of 50 mA/cm2 or more, the thickness of the layers decreased and the porosity of the layers increased as the stack position of the layer became deep. The layer peeled off when the porosity of the layer increased up to aproximately 100%. The thickness variation of the layers and the peeling were prevented by raising the temperature of the electrolyte.

Preparation of Amorphous Hydrogenated Carbon Films by RF Sputtering at a Low-Hydrogen-Flow-Rate Region for Hydrogen-Reactive Substrates

Amorphous hydrogenated carbon (a-C:H) films have been deposited by RF magnetron sputtering system in H2/He plasma. Hydrogen incorporated into the a-C:H film plays an important role in the release of internal stress in the film. On the other hand, hydrogen plasma has deoxidizing, etching and reduction effects on a substrate. Some oxides and compound semiconductors such as CdTe substrate receive serious damage during the deposition process of a-C:H film. Therefore, the amount of hydrogen radicals in sputtering plasma should be as small as possible for suppressing the damage. In this study, we investigate the effects of hydrogen incorporated to an a-C:H film which are deposited at a wide range of relative hydrogen gas flow rate (RH) from 0.1% to 50%. It has been clarified that the a-C:H films deposited in low RH have high quality and that the films can be grown on a CdTe substrate without etching by hydrogen ions and radicals at RH=0.2% and the nearest-neighbor region.

Hydrogen storage phenomenon in amorphous phase of hydrogenated carbon nitride

A hydrogen storage characteristic of the amorphous phase of hydrogenated carbon nitride (a-CNx:H) deposited using a gas mixture of CH4 and N2 by a chemical-vapor-deposition method enhanced by electron-cyclotron-resonance plasma was evaluated under high-pressure hydrogen atmosphere. The hydrogen content in the sample at 300 K and 77 K was directly measured using a volumetric analysis established for evaluation of hydrogen absorption of metal alloys. The contents of stored hydrogen in the sample under 12 MPa were determined to be 1.1 wt.% and 2.2 wt.% at 300 K and 77 K, respectively. The contents of stored hydrogen in the sample were larger than those of a-CNx:H deposited from the dissociated reaction of CH3CN and BrCN. The results of the storage behavior of hydrogen and infrared absorption suggest that the mechanism of hydrogen storage might be explained as chemical and physical absorption.

Raman analysis of trans-polyacetylene chains in hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited using a RF magnetron sputtering system in H2/He plasma. The bonding hydrogen concentration (nH), optical gap, hardness and Raman spectrum of these a-C:H films were measured. Two significant peaks centered at ~1150 and ~1450 cm-1 were observed in the Raman spectra of the a-C:H films with a high nH. The two Raman peaks are attributed to the vibration modes of trans-polyacetylene (trans-PA), and the ~1150 cm-1 peak is used to analyze the trans-PA chaining configuration. As a result, it is found that as the nH increases, the relative intensity of this peak increases and its peak position downshifts. These results indicate that the trans-PA chain content and its conjugate length increase with nH. In this paper, we show that the chaining configuration of the a-C:H films can be analyzed using the ~1150 cm-1 peak in the Raman spectrum.

Improvement in γ-ray Detection Quality of Al/CdTe/Pt Schottky-Type Radiation Detector by Sulfur Treatment

We have studied the effect of sulfur treatment on the performance of a CdTe radiation detector with an Al Schottky electrode. After sulfur treatment, the leakage current was 1.51 ×10-9 A at a bias voltage of 250 V, whereas that of a sample without sulfur treatment was 7.90 ×10-9 A. Moreover, samples with sulfur treatment were capable of measuring the γ-radiation from 241Am for 150 min, while samples without sulfur treatment were incapable of measuring the γ-radiation after 30 min. The improvement in the polarization of the CdTe radiation detector was achieved by the sulfur treatment of the CdTe surface.