In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Substrates for gallium nitride epitaxy

Power semiconductor devices are key components in power conversion systems. Silicon carbide (SiC) has received increasing attention as a wide-bandgap semiconductor suitable for high-voltage and low-loss power devices. Through recent progress in the crystal growth and process technology of SiC, the production of medium-voltage (600?1700 V) SiC Schottky barrier diodes (SBDs) and power metal?oxide?semiconductor field-effect transistors (MOSFETs) has started. However, basic understanding of the material properties, defect electronics, and the reliability of SiC devices is still poor. In this review paper, the features and present status of SiC power devices are briefly described. Then, several important aspects of the material science and device physics of SiC, such as impurity doping, extended and point defects, and the impact of such defects on device performance and reliability, are reviewed. Fundamental issues regarding SiC SBDs and power MOSFETs are also discussed.

https://iopscience.iop.org/article/10.7567/JJAP.54.040103/pdf

Material science and device physics in SiC technology for high-voltage power devices

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

https://iopscience.iop.org/article/10.1088/0268-1242/31/3/034001/pdf

Recent progress in Ga2O3 power devices

Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7–4.9 eV for a high breakdown field of 8 MV cm−1. Low cost, high volume production of large single-crystal β-Ga2O3 substrates can be realized by melt-growth methods commonly adopted in the industry. High-quality n-type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal-semiconductor field-effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single-crystal Ga2O3 substrates and MBE-grown epitaxial wafers. The MESFETs delivered excellent device performance including an off-state breakdown voltage (Vbr) of over 250 V, a low leakage current of only few μA mm−1, and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near-unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications.

Development of gallium oxide power devices

Models for the electron mobility in the three most important silicon carbide (SiC) polytypes, namely, 4H, 6H, and 3C SiC are developed. A large number of experimental mobility data and Monte Carlo (MC) results reported in the literature have been evaluated and serve as the basis for the model development. The proposed models describe the dependence of the electron mobility on doping concentration, temperature, and electric field. The low-field mobility in 4H SiC is much higher than in 6H and 3C in the doping range interesting for RF power transistors (10/sup 16/ cm/sup -3/ ...10/sup 18/ cm/sup -3/), whereas the saturation velocities in the three polytypes investigated are nearly the same (slightly above 2/spl times/10/sup 7/ cm/s at 300 K). The models developed can be easily incorporated into numerical device simulators.

Electron mobility models for 4H, 6H, and 3C SiC [MESFETs]

The status of SiC bulk growth from an industrial point of view

Silicon carbide technology has made tremendous strides in the last several years, with a variety of encouraging device and circuit demonstrations in addition to volume production of nitride-based blue LEDs being fabricated on SiC substrates. The commercial availability of relatively large, high quality wafers of the 6H and 4H polytypes of SiC for device development has facilitated these exciting breakthroughs in laboratories throughout the world. These have occurred in numerous application areas, including high power devices, short wavelength optoelectronic devices, and high power/high frequency devices. This paper will describe progress made in increasing the quality and size of SiC wafers, advances in SiC epitaxy and some of the resulting device demonstrations and commercialization by Cree Research.

Progress in SiC : from material growth to commercial device development

The growth of high-purity, semi-insulating (HPSI) 4H-SiC crystals has been achieved using the seeded-sublimation growth technique. These semi-insulating (SI) crystals (2-inch diameter) were produced without the intentional introduction of elemental deep-level dopants, such as vanadium, and wafers cut from these crystals possess room-temperature resistivities greater than 109 Ωcm. Based upon temperature-dependent resistivity measurements, the SI behavior is characterized by several activation energies ranging from 0.9-1.5 eV. Secondary ion mass spectroscopy (SIMS) and electron paramagnetic resonance (EPR) data suggest that the SI behavior originates from deep levels associated with intrinsic point defects. Typical micropipe densities for wafers were between 30 cm-2 and 150 cm-2. The room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/mK for 4H-SiC, making these wafers suitable for high-power microwave applications.

High-purity semi-insulating 4H-SiC grown by the seeded-sublimation method

The production of large diameter, high quality SiC substrates is essential to realize the full potential of this important semiconductor material. The current status of SiC bulk sublimation growth for the production of these substrates is reviewed from an industrial point of view. Specific efforts towards larger diameter high quality substrates have led to the production of 50 and 75 mm diameter 4H and 6H wafers and the demonstration of high quality 100-mm wafers. We present thermal conductivity data for material of different doping levels, relevant for device applications. In SiC, micropipes are the most harmful defects for SiC device production. By continuous optimization of the growth process we were able to steadily decrease the micropipe density over the past several years, down to densities as low as 1.1 cm −2 for an entire 50-mm wafer, indicating that micropipes may be totally eliminated in the next few years. In order to achieve this goal for SiC substrates of increasing diameter, a thorough understanding of the growth process is essential. We will summarize results of modeling the growth process and its experimental verification. The effect of micropipe densities and their characteristic lateral distribution in SiC wafers on achievable device yields will be discussed, using large area Schottky diodes as an example.

Progress in the industrial production of SiC substrates for semiconductor devices

High-purity, semi-insulating (HPSI) 4H-SiC crystals with diameters up to 75 mm have been grown by the seeded sublimation technique without the intentional introduction of elemental deep-level dopants, such as vanadium. Wafers cut from these crystals exhibit homogeneous activation energies near mid gap and thermally stable semi-insulating (SI) behavior (>109 ohm-cm) throughout device processing. Secondary ion mass spectroscopy, deep-level transient spectroscopy, optical admittance spectroscopy, and electron paramagnetic resonance data suggest that the SI behavior originates from several deep levels associated with intrinsic point defects. Micropipe densities in HPSI substrates have been demonstrated to be as low as 10 cm−2 in 2-in. substrates, and the room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/cm·K for 4H-SiC. Devices fabricated on these HPSI wafers do not exhibit any substrate related back-gating effects and have power densities as high as 5.2 W/mm with 63% power added efficiency.

St. G. Müller

Papers

The status of SiC bulk growth from an industrial point of view

Progress in SiC : from material growth to commercial device development

High-purity semi-insulating 4H-SiC grown by the seeded-sublimation method

Progress in the industrial production of SiC substrates for semiconductor devices

High-purity semi-insulating 4H-SiC for microwave device applications