Tomoyoshi Mishima

Ultrahigh relaxation oscillation frequency (up to 30 GHz) of highly p‐doped GaAs/GaAlAs multiple quantum well lasers

We have demonstrated a relaxation oscillation frequency (fr) of up to 30 GHz in highly p‐doped (Be=1×1019 cm−3) GaAs/GaAlAs multiple quantum well (MQW) lasers grown by molecular beam epitaxy. This is the highest fr achieved at room temperature for III‐V semiconductor lasers. The high fr is attained by the large differential gain in the p‐doped MQW structure having a low threshold electron density resulted from the excess hole density in addition to the quantum size effect.

Vertical GaN p-n Junction Diodes With High Breakdown Voltages Over 4 kV

Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages (VB) of 4.7 kV combined with low specific differential ON-resistance (RON) of 1.7 mΩcm2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low RON. A Baligas figure of merit (VB2/RON) was 13 GW/cm2. These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.

Over 3.0

This letter describes a new two-step electrode process on p-GaN and characteristics of GaN p-n junction diodes on free-standing GaN substrates with low specific ON-resistance R_οn and high breakdown voltage V_B. We develop a two-step process for anode electrodes in order to avoid plasma damage to the p⁺-GaN contact layer during the sputtering process. The specific ON-resistance is further improved due to a new low-damage process. The breakdown voltage of the diodes with the field-plate (FP) structure is over 1100 V, and the leakage current was low, i.e., in the range of 10^-9 A. The specific ON-resistance of the diodes of 50 μm in diameter with the FP structure was 0.4 mΩ · cm². Baligas figure of merit (V_B²/R_on) of 3.0 GW/cm² is obtained. These are the best values ever reported among those achieved by GaN p-n junction diodes on free-standing GaN substrates.

\hbox{GW/cm}^{2}

Detailed experimental results on p-type modulation-doped multi-quantum well (MD-MQW) lasers are discussed. The GaAs/GaAlAs MD-MQW laser was grown by low temperature (600°C) molecular beam epitaxy using Be as the p-type dopant. The realization of the p-type MD-MQW structure was fully confirmed by high resolution secondary ion mass spectroscopy, transmission electron microscopy, and photoluminescence measurement. An ultrahigh relaxation oscillation frequency of up to 30 GHz was realized, which is 2.5 times that of undoped MQW lasers and also 5 times that of double heterostructure lasers. In addition, the spectral chirping width of p-type MD-MQW lasers was experimentally confirmed to be 1/5 that of undoped MQW lasers. These improvements result from the enhancement of the differential gain due to the modulation doping effect in addition to the quantum size effect. These results are in good agreement with the theoretical analyis.

Figure-of-Merit GaN p-n Junction Diodes on Free-Standing GaN Substrates

The fundamental material parameters associated with GaN, which are important for the design of devices such as light-emitting diodes and laser diodes, were investigated using large high-quality GaN single crystals fabricated through hydride vapor phase epitaxy using the void-assisted separation method. The thermal-expansion coefficients (298–573K) along the C[0001], A[112¯0], and M[101¯0] axes (αC, αA, and αM) were measured. Thermal expansion in each direction, approximately proportional to the temperature, was observed throughout the measured temperature range. Although the thermal-expansion coefficients in the high-temperature range, i.e., αC(573K)=7.2±0.02×10−6∕K, αA(573K)=5.7±0.2×10−6∕K, and αM(573K)=5.8±0.2×10−6∕K,were relatively close to the reported values, the thermal-expansion coefficients along the C axis in the low-temperature range, i.e., αC(298K)=5.3±0.02×10−6∕K, was significantly larger than the reported values. Thermal conductivities parallel and perpendicular to the C axis were almost the ...

Modulation-Doped Multi-Quantum Well (MD-MQW) Lasers. II. Experiment

In this letter, we describe the characteristics of Gallium Nitride (GaN) p–n junction diodes fabricated on free-standing GaN substrates with low specific on-resistance Ron and high breakdown voltage VB. The breakdown voltage of the diodes with the field-plate (FP) structure was over 3 kV, and the leakage current was low, i.e., in the range of 10-4 A/cm2. The specific on-resistance of the diodes of 60 µm diameter with the FP structure was 0.9 mΩcm2. Baligas figure of merit (VB2/Ron) of 10 GW/cm2 is obtained. Although a certain number of dislocations were included in the device, these excellent results indicated a definite availability of this material system for power-device applications.

Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

We investigated, by atomic force microscopy (AFM), the effect of the V/III ratio on the surface morphologies of GaN epilayers during the early stage of high-temperature (HT) GaN growth. The epilayers were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We observed that initial HT-GaN islands in the early stage of HT-GaN growth were larger when a lower V/III ratio was used during the low-temperature (LT) GaN-buffer-layer growth or during the early stage of HT-GaN growth (the HT-GaN-buffer growth). These larger islands were attributed to a reduced nuclei density when a lower V/III ratio was used during the LT-GaN-buffer-layer growth, or to an increased growth aspect ratio (the ratio of vertical and lateral growth rates) when a lower V/III ratio was used in the early stage of HT-GaN growth. The larger islands led to fewer defects when they coalesced, which demonstrated that the quality of GaN epilayers grown on sapphire substrates by LP-MOCVD can be improved by using a low V/III ratio during the LT or HT GaN-buffer-layer growth.

High-Breakdown-Voltage and Low-Specific-on-Resistance GaN p–n Junction Diodes on Free-Standing GaN Substrates Fabricated Through Low-Damage Field-Plate Process

Abstract We have fabricated high quality In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As high electron mobility transistors (HEMTs) with thin buffer layers lattice-mismatched on GaAs substrates by molecular beam epitaxy (MBE). Our step-graded In y Al 1− y As buffer layers efficiently terminate lattice-misfit dislocations at each step interface according to cross-sectional transmission electron microscopy (TEM) and plan-view TEM observations. The buffer layers facilitate high mobility (approximate values of 10,000 cm 2 / V · s at room temperatures), even though the total buffer thickness is smaller than 600 nm. The mobility is comparable to that for HEMTs grown on InP substrates and better than that reported for HEMTs grown on GaAs with much thicker buffers. For a device with a 0.5 μm long, 20 μm wide gate and a 600 nm buffer layer, the peak extrinsic G m is 750 mS/mm, which is higher than that for previously reported HEMTs with the same gate length, including devices on InP substrates. In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As single quantum wells (SQWs) grown on the step-graded In y Al 1− y As buffer layers show intense, sharp photoluminescence spectra, comparable to those of SQWs grown lattice-matched on InP. These results show that the tin step-graded buffer enabled growth of high-quality In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As heterostructures on GaAs substrates.

Control of Initial Nucleation by Reducing the V/III Ratio during the Early Stages of GaN Growth

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...

High Gm In0.5Al0.5As/In0.5Ga0.5As high electron mobility transistors grown lattice-mismatched on GaAs substrates

We have developed GaAs heterojunction bipolar transistors (HBTs) with low turn-on voltage by using pseudomorphic GaAsSb as the base layer. The turn-on voltage of GaAs/GaAs0.91Sb0.09 HBT is 0.10 V lower than that of InGaP/GaAs HBT. The lower turn-on voltage is attributed to the smaller band gap of the GaAsSb base layer, indicating that GaAsSb is useful material for reducing turn-on voltage of GaAs HBTs. The current gain of 20 is obtained for GaAs/GaAs0.91Sb0.09 HBT, which is larger than those of previously reported GaAs/GaAsSb HBTs owing to the pseudomorphic, fully strained GaAsSb with no misfit dislocations. The knee voltage of 0.47 V is attained at the collector current density of 5×104 A/cm2. These results indicate that GaAs/GaAsSb HBTs have a great potential for reducing operating voltage and power dissipation.