Takehiro Maehama

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.

Adsorption and decomposition of methylsilanes on Si(100)

Abstract We have investigated in-situ the adsorption and thermal decomposition of methylsilanes, SiH x (CH 3 ) 4− x ( x =1–3), on Si(100)(2×1), using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry. IRAS spectra revealed that at initial stages of adsorption, monohydride (–SiH) and CH 3 -substituted hydride species (–SiH x (CH 3 ) 3− x ) are generated with monohydride species being dominant. We suggest that upon room temperature adsorption of methylsilanes, breaking of the SiH bonds of methylsilane is favored over that of the SiC bonds. It is found that the dissociative adsorption of SiH 3 (CH 3 ) exhibits the second-order kinetics. Due to thermal annealing, surface species –SiH x (CH 3 ) 3− x are thermally decomposed to generate surface SiH and SiC bonds, and subsequently H 2 desorption from the SiH bonds occurs.

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.

Structural Analysis of Porous Silicon Multilayer using X-Ray Diffraction

A porous silicon monolayer (PSL) and four types of porous silicon multilayers (PSMLs) with 2-, 3-, 7-, and 50-layers were studied using the X-ray double crystal method. PSMLs were formed by controlling the anodization current density. Only three distinct peaks were observed in each rocking curve for these PSMLs, which showed two kinds of porous silicon layers with good crystallinity. It was also shown that the PSL and PSMLs were slightly different in the state of lattice strain. To observe the structure and depth-distribution of lattice strain of PSMLs, a new X-ray cross-sectional topographic technique (XCST) has been proposed. Applying XCST to a three-layer porous silicon consisting of 5 µm, 10 µm, and 5 µm layer thicknesses resulted in a clear X-ray cross-sectional topograph. From the topograph and the corresponding rocking curves, the precise distribution of the lattice strain was revealed. The XCST is useful for evaluating not only the PSML, but also general crystal multilayer structures such as superlattice hetero structures.

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.

Surface Modification of CdTe Crystal by Plasma Treatment Using Various Gases

Plasma treatments using various gases have been applied to CdTe(111) surfaces. The induced surface layer composition and morphology were analyzed by X-ray photoelectron spectroscopy and field-emission scanning electron microscopy. He and Ar plasma treatments yielded an approximately stoichiometric surface and morphologies with a pyramidal structure. The surface with the H2 plasma treatment had ball-like particles and was Cd-rich. Furthermore, the surface with the O2 plasma treatment was almost oxidized. An Al/CdTe Schottky contact formed on the surface after the He, Ar, and H2 plasma treatments showed good rectification properties due to elimination of a Te-rich layer.

Homoepitaxial diamond synthesis by DC arc plasma jet chemical vapor deposition

Homoepitaxial diamond films have been synthesized on (100) substrates by DC arc plasma jet chemical vapor deposition. It was shown that fabrication of epitaxial film with a smooth surface is possible at an appropriate substrate temperature T s with methane and carbon dioxide concentrations of 1% each. At T s ≥930°C, pyramidal hillocks form on the surface of the film, and irregular grain particles appear. However, if the methane concentrations are raised, even when substrate temperature is high, epitaxial films with smooth surfaces are formed. Epitaxial diamond film with a smooth surface shows much stronger cathodoluminescence than the film with pyramidal hillocks. From the results of the X-ray double-crystal analysis, the crystallinity of the epitaxial diamond layer is higher than or equal to that of the substrate of high-pressure-synthesized diamond. These results indicate that synthesis of high quality diamond film at a high growth rate, by DC arc plasma jet CVD, is possible.