Hiroki Tokunaga

AN ANALYSIS OF TEMPERATURE DEPENDENT PHOTOLUMINESCENCE LINE SHAPES IN INGAN

Photoluminescence (PL) line shapes in InGaN multiple quantum well structures have been studied experimentally and theoretically between 10 and 300 K. The higher temperature PL spectra can be fitted quantitatively with a thermalized carrier distribution and a broadened joint-density-of-states. The low temperature PL line shapes suggest that carriers are not thermalized, as a result of localization by band-gap fluctuations. We deduce a localization energy of ∼7 meV as compared with an activation energy of ∼63 meV from thermal quenching of the PL intensity. We thus conclude that this activation energy and the band-gap fluctuation most likely have different origins.

Uniform Growth of AlGaN/GaN High Electron Mobility Transistors on 200 mm Silicon (111) Substrate

Crack-free AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on a 200 mm Si substrate by metal–organic chemical vapor deposition (MOCVD) is presented. As grown epitaxial layers show good surface uniformity throughout the wafer. The AlGaN/GaN HEMT with the gate length of 1.5 µm exhibits a high drain current density of 856 mA/mm and a transconductance of 153 mS/mm. The 3.8-µm-thick device demonstrates a high breakdown voltage of 1.1 kV and a low specific on-resistance of 2.3 mΩ cm2 for the gate–drain spacing of 20 µm. The figure of merit of our device is calculated as 5.3×108 V2 Ω-1 cm-2.

Quantum chemical mechanism in parasitic reaction of AlGaN alloys formation

The mechanism of parasitic reactions among trimethylaluminum (TMA), trimethylgallium (TMG), and NH3 in atmospheric pressure (AP) MOVPE for growth of AlGaN is theoretically studied using the quantum chemical method. The calculations show that metal–nitrogen chain growth reaction easily proceeds through the successive reactions of ‘complex formation with NH3’ and ‘CH4 elimination by the bimolecular mechanism’. Additionally, a parasitic reaction in APMOVPE using other raw material is also investigated. The calculated result shows that small change of raw material raises activation energy of parasitic reaction, and, thus, the parasitic reaction is suppressed. This result suggests a way to improve APMOVPE by a suitable choice of substituent.

Highly uniform growth in a low-pressure MOVPE multiple wafer system

Abstract A novel horizontal metal organic vapor phase epitaxy (MOVPE) system, which is capable of handling six 3 inch wafers or eighteen 2 inch wafers mounted on a 10 inch diameter susceptor, has been developed for the growth of III–V compound semiconductors. The characteristic features in this system are “triple flow channel” gas injection and “face-down” wafer setting configuration. The inlet for the source gas flow is divided into three zones (upper, middle and lower flows for hydrides, organometals and hydrogen, respectively) to control the concentration boundary layer and the growth area. The wafers are placed inversely to prevent thermal convection and particles on the growing surface. The independent controlled three-part heating system is also adopted to achieve a uniform temperature distribution over an 8 inch growing surface. The thickness and the doping of GaAs, Al 0.3 Ga 0.7 As, In 0.48 Ga 0.52 P and In 0.2 Ga 0.8 As grown by this system are uniform within ± 2% over all 3 inch wafers.

An analysis of temperature dependent piezoelectric Franz–Keldysh effect in AlGaN

Strong Franz–Keldysh oscillations near the band gap of AlGaN are observed in the contactless electroreflectance (CER) studies of a GaN/InGaN/AlGaN multilayer structure. The line shape analysis of the CER spectra at different temperatures provides an accurate determination of the AlGaN band gap energies and the built-in electric fields. Using the existing data of the thermal expansion coefficients of GaN and sapphire, and the piezoelectric constants of AlGaN, the temperature dependence of the electric field is estimated and is in good agreement with the experimental results between 15 and 300 K. We attribute such electric field to the piezoelectric strain effect.

Uniform growth of compound-semiconductor film over 10 inch circle by controlling entrance effects by diffusion mixing between 3 layered flows

Abstract Entrance effects at platen edge of a wafer holder bring about a predominantly high growth rate in a horizontal reactor of high flow rate of metal organic vapor phase epitaxy (MOVPE) of compound-semiconductors. To solve this problem, we have developed a horizontal reactor with 3 layered flow injections. In this reactor, metal-alkyl source gases diffuse through a nearby flow layer while flowing down stream. By adjusting the thickness and flow rate of the diffusion layer, we can control an amount of metal-alkyl molecules that reach the substrate surface at the entrance region. This method also brings about a freedom of choice in a growth conditions that sometimes contradict with each other such as doping, composition and thickness. As a results, ±1% thickness uniformity of GaAs, AlGaAs and InGaP, and ±1% doping uniformity of Si as well as ±2% doping uniformity of Zn have been obtained.

Epitaxial Growth and Properties of Mg-Doped Gan Film Produced by Atmospheric Mocvd System With Three Layered Lammar Flow Gas Injection

Mg-doped GaN films with a variety of Mg concentrations were grown on sapphire (0001) by horizontal atmospheric metalorganic chemical vapor deposition (MOCVD) system with three layered laminar flow gas injection in an attempt to study the Mg doping effects on film quality. The increase of Mg concentration induced an increase of x-ray rocking curve full width at half maximum (FWHM) and degradation of surface morphology. Secondary ion mass spectroscopy (SIMS) analysis shows increase of Si and O, associated with Mg-doping concentration. Si and O concentrations of Mg-doped film are up to 5×10 16 cm −3 and 5×10 17 cm −3 at Mg concentration of 4.5×10 19 cm −3 , respectively. Strong 380nm emission and weak 430nm emission were observed by photoluminescence (PL) measurement at room temperature for as-grown Mg-doped GaN films which shows p-type conductivity after thermal annealing. While, in highliy Mg-doped GaN films which do not show the p-type conduction after thermal annealing, 430nm and/or 450nm emission were dominating. The highest room temperature free hole concentration achieved was p=2.5× 10 18 cm −3 with mobility μ p =l.9cm 2 /V s.

Opportunities and challenges in GaN metal organic chemical vapor deposition for electron devices

The current situation and next challenge in GaN metal organic chemical vapor deposition (MOCVD) for electron devices of both GaN on Si and GaN on GaN are presented. We have examined the possibility of increasing the growth rate of GaN on 200-mm-diameter Si by using a multiwafer production MOCVD machine, in which the vapor phase parasitic reaction is well controlled. The impact of a high-growth-rate strained-layer-superlattice (SLS) buffer layer is presented in terms of material properties. An SLS growth rate of as high as 3.46 µm/h, which was 73% higher than the current optimum, was demonstrated. As a result, comparable material properties were obtained. Next, a typical result of GaN doped with Si of 1 × 1016 cm−3 grown at the growth rate of 3.7 µm/h is shown. For high-voltage application, we need a thick high-purity GaN drift layer with a low carbon concentration, of less than 1016 cm−3. It is shown that achieving a high growth rate by precise control of the vapor phase reaction is still challenge in GaN MOCVD.

MOCVD growth of nitride semiconductors

Abstract In this chapter, specific characteristics of nitride semiconductor metal organic chemical vapor deposition are described about the following topics: growth mechanisms, Mg doping, visible multi quantum wellMQW growth, and ultraviolet materials growth by using high flow speed reactor. The importance of controlling vapor phase reaction is highlighted in view of growing low carbon materials for better electrical and optical properties as well as growing high aluminum content AlGaN alloys.

Criteria for versatile GaN MOVPE tool: high growth rate GaN by atmospheric pressure growth

Growth rate has a direct impact on the productivity of nitride LED production. Atmospheric pressure growth of GaN with a growth rate as high as 10 μm/h and also Al0.1Ga0.9N growth of 1 μm/h by using 4 inch by 11 production scale MOVPE are described. XRD of (002) and (102) direction was 200 arcsec and 250 arcsec, respectively. Impact of the growth rate on productivity is discussed.