Jun Gotoh

Spiral growth of InGaN/InGaN quantum wells due to Si doping in the barrier layers

We have examined Si-doping effects in InGaN/InGaN quantum-well (QW) structures, especially the influence of Si-doped InGaN barrier layers on the growth mechanism of QW structures, by atomic force microscopy (AFM) and by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. Our AFM observations revealed that Si-doped InGaN barriers strongly affect the growth mode of overlying InGaN QW layers. This effect leads to the formation of nanoscale islands (with a density of 108 cm−2) due to spiral growth of the QW layers. The spirally grown nanoscale islands significantly increase the PL intensity. Through spatially resolved CL observations, we found that the number of dot-like CL bright spots increased dramatically when the barrier layers were Si doped, and the increased density of the spots was in good agreement with the increased density of the nanoscale islands observed by AFM. By combining these results, we show that the spirally grown QW structures produced by Si doping of the barriers effectivel...

Crystallographic microstructure and electrical characteristics of Au/Pt/Ti/Ni ohmic contacts on p‐type (001) ZnTe layers

The crystallographic microstructure and electrical characteristics of Au/Pt/Ti/Ni ohmic contacts on p‐type (001) ZnTe layers are investigated as a function of annealing temperature, by using the transmission line model method, cross‐sectional transmission electron microscopy, and Auger electron spectroscopy. The specific contact resistance decreases when the annealing temperature is increased and reaches a minimum at 300 °C. A minimum value of 1.1×10−6 Ω cm2 is obtained for a hole concentration of 3×1019 cm−3. The epitaxial NiTe2 that formed at the metal/semiconductor interface due to annealing is considered to play an important role in lowering the contact resistance. The excess Zn atoms created by the reaction between Ni and ZnTe are found to diffuse upward and to segregate at the Pt/Ni interface. A contact stability test performed at 102 °C suggests that these ohmic contact structures are stable even under high‐current injection.

Electrical properties and microstructures of Au/Pt/Ti/Ni ohmic contacts to p‐type ZnTe

Electrical properties and microstructures of Au/Pt/Ti/Ni ohmic contacts to p‐type ZnTe were investigated using the transmission line model method and cross‐sectional transmission electron microscopy. The specific contact resistance decreases when the annealing temperature is increased and reaches a minimum at 300 °C. The formation of NiTe2 from the reaction between Ni and ZnTe plays an important role in lowering the contact resistance. A contact stability test performed at 102 °C suggests that these ohmic contacts are stable even under high‐current injection.

DISORDERING OF THE ZNCDSE SINGLE QUANTUM WELL STRUCTURE BY CD DIFFUSION

The effects of annealing on a ZnCdSe single quantum well (SQW) structure with ZnCdSSe/ZnSSe superlattice optical guiding layers are investigated. X‐ray diffraction and photoluminescence (PL) measurements showed disordering of a ZnCdSSe/ZnSSe superlattice after annealing at about 500 °C. The PL peak energy of the SQW shifted to the higher energy side, and the linewidth narrowed in the sample annealed at 300 °C. Cadmium diffusion was confirmed by secondary ion mass spectrometry. We found that the disordering of the ZnCdSSe/ZnSSe superlattice and the changes in the emissions from the SQW were due to the Cd diffusion.

Effect of N-doped ZnTe layers on ZnSeZnTe graded superlattices

Abstract The effect of the nitrogen concentration on the properties of ZnTe-based contact structures consisting of a p-ZnTe layer, a ZnSe ZnTe graded superlattice (SL), and a p-ZnSe layer is described. The degree of nitrogen-doping into the ZnSe ZnTe SL and into the ZnTe layer was changed by modulation doping. The ohmic I-V characteristics were observed in samples with an N-doped ZnSe undoped ZnTe SL and in samples with a ZnTe layer with a nitrogen concentration of 3 × 1018cm−3. When the nitrogen concentration in the ZnSe ZnTe SL region or the ZnTe layer was increased, nitrogen diffusion into the ZnSe layer was observed by SIMS and PL measurement. The ZnSe layer then became highly resistive, probably because deep donors were induced by the nitrogen diffusion.

Dependence of ZnTe‐based contact structure properties on nitrogen concentration

The effect of nitrogen concentration on the properties of ZnTe‐based contact structures, which consist of p‐ZnTe layers, ZnSe/ZnTe graded superlattices and p‐ZnSe layers is described. The nitrogen concentration of the ZnTe layer was varied by using a modulation doping method. Ohmic I–V characteristics were observed when the nitrogen concentration of the ZnTe layer was as low as 3×1018 cm−3. When the nitrogen concentration rose above 1×1019 cm−3, however, no current was observed in the I–V curves. Nitrogen diffusion from the ZnTe layer to the superlattice and the ZnSe layer was observed by SIMS analysis and PL measurement. The ZnSe layers became highly resistive, probably because deep donors were induced by the nitrogen diffusion.

Control of the II–VI/GaAs interface reaction using hydrogen radical and Zn/As fluxes

The lifetime of II–VI-based blue-green laser diodes on GaAs substrates is limited by rapid degradation in the active layers. This degradation has been observed as dark defects in the active layer during the laser operation, where defects occurred due to stacking faults that originated from the Ga2Se3 compounds at the ZnSe/GaAs interface. The reported value of the density of stacking faults of the II–VI lasers was in the order of 105 cm−2. To extend the lifetime, surface treatment of the GaAs substrate and control of the interface reaction are necessary. We investigated a new treatment technique using hydrogen-radical and Zn/As fluxes. We fabricated ZnSe-based double-hetero (DH) structures on a treated GaAs substrate and measured the density of dark defects in the light emitter area by electroluminescence microscopy. Chemical bonds at the interface were evaluated by x-ray photoelectron spectroscopy. A dark defect density of less than 105 cm−2 was obtained when the As-terminated GaAs surface was Zn treated....

GaN/W/W-oxide metal base transistor with very large current gain and power gain

We demonstrate a GaN/W/W-oxide metal base transistor (MBT) whose collector is formed by oxidizing the intrinsic W base. The thickness of the nonoxidized intrinsic base of the fabricated collector-up MBT on a sapphire substrate was estimated to be 2–3 nm. Although the MBT showed large leakage, subtraction of the leakage from collector current revealed that the transistor had a very large small-signal direct current (dc) current gain of 87 dB and a dc power gain of 50 dB. This indicates that the GaN-based MBT is a possible candidate for microwave and millimeterwave amplifiers as well as for high-speed integrated circuits used in optical fiber communication system.

High On/Off-Current Ratio in Bottom-Gated Microcrystalline-Silicon Thin-Film Transistors With Vertical-Offset Structure

The high on/off-current ratio of microcrystalline-silicon thin-film transistors (TFTs) with vertical offset was demonstrated. These TFTs have a bottom-gate structure and offset regions formed along the side surfaces of the thick interlayer films. Due to a decrease of maximum electric-field intensity and a narrow distribution of high electric field, the on/off current ratio of vertical-offset TFTs is about three orders of magnitude higher than that of conventional TFTs.

Blue-green light-emitting diodes with p-ZnSSe highly doped with nitrogen grown by metalorganic molecular beam epitaxy and molecular beam epitaxy

Abstract The p-type doping of ZnSSe grown by MOMBE (metalorganic molecular beam epitaxy) and MBE (molecular beam epitaxy) has been comparatively studied. The p-type doping by using ammonia as a dopant source in MOMBE resulted in p-ZnSe with hole concentration of up to 10 17 cm -3 . Perfect ohmic contact characteristics were achieved by MOMBE for Au/p-ZnSe:N with hole concentration of 5.5 × 10 17 cm -3 . On the other hand, p-type doping in MBE by using nitrogen plasma as a dopant resulted in p-ZnSe with net acceptor concentration of up to 7.6 × 10 17 cm -3 . I–V characteristics with the lowest Schottky barriers were obtained for Au/p-ZnSe with net acceptor concentration of 6.6 × 10 16 to 7.6 × 10 17 cm -3 for ZnSe:N and of 2.8 × 10 16 to 1.2 × 10 17 cm -3 for ZnS x Se 1− x :N ( x = 0.03−0.12). The characteristics of ZnSe light emitting diodes with p-ZnSe grown by using ammonia in MOMBE are described. The characteristics of a ZnCdSe/ZnSSe laser diode with p-ZnSSe grown by using nitrogen plasma in MBE are discussed.