High-quality GaN Substrates Research Articles

(Invited) A Brief Overview and Future Outlook on the Progress of III-Nitride Semiconductors Research

The increasing need for devices capable of performing under extreme condition of power and temperature, and opto-electronic devices operating in the UV-Vis spectral range has been the principal driving force for research interest in III-Nitrides semiconductors over the last three decades. Progress in bulk growth and film deposition techniques for homo- and hetero-epitaxial growth of III-Nitride semiconductors has led to successful fabrication and commercialization of an increasing number of devices that are revolutionizing both the optoelectronic and electronic device industries.Many relevant figures of merit related to electronic device performance scale superlinearly with the band gap; hence, ultra-wide band gap (UWBG) materials (Eg > 3.4 eV) have the potential for far superior performance compared to conventional WBG materials such as GaN and SiC. UWBG materials will be critical enablers of future electronic components and systems, and a fundamental understanding of their synthesis and properties is critical to exploiting their advantages. In particular, cubic boron nitride (c-BN), among the semiconductor materials in the UWBG group (c-BN, AlN, Ga2O3, and diamond), with a band gap of ~6 eV and thermal conductivity of 13 W/mK (second only to diamond, 20 W/mK), offers the best combination of breakdown voltage and specific on-resistance. Another advantage of c-BN over other UWBG semiconductors is that it can be doped with donors and acceptors, enabling the fabrication of bipolar devices.The development of high quality GaN substrates and device grade films will be reviewed, and the challenges encountered in the realization of GaN devices will be compared to those facing the development of UWBG materials and devices. Finally, a brief overview of the intrinsic properties of the WBG and UWBG semiconductors will be presented.This work was supported by the Office of Naval Research.

(Invited) A Brief Overview and Future Outlook on the Progress of III-Nitride Semiconductors Research

The increasing need for devices capable of performing under extreme condition of power and temperature, and opto-electronic devices operating in the UV-Vis spectral range has been the principal driving force for research interest in III-Nitrides semiconductors over the last three decades. Progress in bulk growth and film deposition techniques for homo- and hetero-epitaxial growth of III-Nitride semiconductors has led to successful fabrication and commercialization of an increasing number of devices that are revolutionizing both the optoelectronic and electronic device industries.Many relevant figures of merit related to electronic device performance scale superlinearly with the band gap; hence, ultra-wide band gap (UWBG) materials (Eg > 3.4 eV) have the potential for far superior performance than conventional WBG materials such as GaN and SiC. UWBG materials will be critical enablers of future electronic components and systems, and a fundamental understanding of their synthesis and properties is critical to exploiting their advantages. In particular, cubic boron nitride (c-BN), among the semiconductor materials in the UWBG group (c-BN, AlN, Ga2O3, and diamond), with a band gap of ~6 eV and thermal conductivity of 13 W/mK (second only to diamond, 20 W/mK) offers the best combination of breakdown voltage and specific on-resistance. Another advantage of c-BN over other UWBG semiconductors is that it can be doped with donors and acceptors, enabling the fabrication of bipolar devices.The development of high quality GaN substrates and device grade films will be reviewed and a brief overview of the intrinsic properties of the WBG and UWBG semiconductors will be presented.

Breakdown phenomenon dependences on the number and positions of threading dislocations in vertical p-n junction GaN diodes

The correlation between current-voltage (I–V) characteristics and threading dislocations was evaluated using p-n junction diodes on a high-quality GaN substrate with an average threading dislocation density (TDD) ≤ 4 × 105 cm−2 using the newly developed maskless 3D (M-3D) method. For the forward I–V characteristics, it was found that the “on” resistance (R on) increased as the number of dislocations in the diode increased. This result indicates that reducing the number of dislocations is effective in improving the forward I–V characteristics. On the other hand, for the reverse I–V characteristics, it was found that there was no clear correlation between the breakdown voltage (V B) and the number of dislocations. Also, the p-n diode’s destruction point and dislocation position did not match in almost all of the measured p-n diodes. These results suggest that dislocation has very little effect on V B and the p-n diode destruction.

Impact of threading dislocations in GaN p–n diodes on forward I–V characteristics

The correlation between on-resistance (Ron) and the number of dislocations under the anode electrode was evaluated using p–n junction diodes on a high-quality GaN substrate with an average threading dislocation density of ≤4 × 105 cm−2 by a newly developed maskless 3D method. A positive correlation was observed between Ron and the number of dislocations. The microdiodes with an anode electrode size of diameter 6 μm of having one dislocation and no dislocation under the anode electrode were evaluated to estimate the current reduction by one dislocation, which was derived to be about 0.37 mA at a forward voltage of 5 V. The reduction of the forward current seemed to be caused by non-radiative recombination centers around dislocation lines which weakened the photon recycling effects and conductivity modulation. On the other hand, there was no clear correlation between the breakdown voltage and the number of dislocations.

(Invited) A Brief Overview and Future Outlook on the Progress of III-Nitride Semiconductors Research

The increasing need for devices capable of performing under extreme condition of power and temperature, and opto-electronic devices operating in the UV-Vis spectral range has been the principal driving force for research interest in III-Nitrides semiconductors over the last three decades. Progress in bulk growth and film deposition techniques for homo- and hetero-epitaxial growth of III-Nitride semiconductors has led to successful fabrication and commercialization of an increasing number of devices that are revolutionizing both the optoelectronic and electronic device industries.Many relevant figures of merit related to electronic device performance scale superlinearly with the band gap; hence, ultra-wide band gap (UWBG) materials (Eg > 3.4 eV) have the potential for far superior performance than conventional WBG materials such as GaN and SiC. UWBG materials will be critical enablers of future electronic components and systems, and a fundamental understanding of their synthesis and properties is critical to exploiting their advantages. In particular, cubic boron nitride (c-BN), among the semiconductor materials in the UWBG group (c-BN, AlN, Ga2O3, and diamond), with a band gap of ~6 eV and thermal conductivity of 13 W/mK (second only to diamond, 20 W/mK) offers the best combination of breakdown voltage and specific on-resistance. Another advantage of c-BN over other UWBG semiconductors is that it can be doped with donors and acceptors, enabling the fabrication of bipolar devices.The development of high quality GaN substrates and device grade films will be reviewed and a brief overview of the intrinsic properties of the WBG and UWBG semiconductors will be presented.

Strain Recovery and Defect Characterization in Mg‐Implanted Homoepitaxial GaN on High‐Quality GaN Substrates

The evolution of defects due to high‐pressure annealing of magnesium ion‐implanted epitaxial GaN grown on high‐quality GaN substrates is investigated. Changes in the implant‐induced strain are quantified as a function of annealing temperature and time. After annealing at 1300 °C for 10 min, the implant‐induced strain is fully relieved and accompanied by the presence of extended defects such as basal plane stacking faults and prismatic loops. Approximately one‐third of the original implant‐induced strain remains after annealing at 700 °C, and 5% of the original strain remains at 1000 °C for 100 min. In all cases, nearly all of the recovered strain occurs within first few minutes of annealing. A prominent increase in the asymmetric (104) triple axis X‐ray rocking curve full width at 0.01 maximum (FW0.01M) is observed after annealing at 1300 °C for 10 min. After annealing at 1300 °C for 100 min, a subsequent decrease in FW0.01M is correlated with a reduction of the extended defect density from 4 × 108 to 3 × 107 cm−2, determined through transmission electron microscope (TEM) measurements. Further reduction in the density of the extended defects by optimizing annealing temperature and time is expected to improve the performance of GaN‐based vertical power devices.

Wafer-scale analysis of GaN substrate wafer by imaging cathodoluminescence

With the progress in large-size high-quality GaN substrates, wafer-scale analysis is needed to evaluate homogeneity and defect distribution. We demonstrate the mapping of a 2 inch GaN substrate wafer based on the imaging cathodoluminescence technique. Macro pit defects with sizes varying from microns to millimeters are visualized and classified into three types according to their optical and structural properties. The formation mechanisms of the different types of pit defects are discussed with consideration of the facet growth involved in substrate growth.

Long range, non-destructive characterization of GaN substrates for power devices

It has been well established that GaN grown on native substrates yields higher quality films compared to those grown on non-native substrates, such as SiC or Si. However, homoepitaxy is hindered by the lack of inexpensive high-quality GaN substrates. The importance of substrate quality, in particular defects and impurities, arises from its impact on device performance. Many of the imperfections in GaN substrates are inhomogeneous, thus are often not detected when short-range techniques are used.Therefore, methods for evaluating substrate quality over long ranges need to be developed. This research reports the study of these defects on four different GaN wafers from four different vendors using quick, long-range, non-destructive techniques that can be performed in a few hours. All samples are expected to be n+ and should theoretically be suited for use in GaN homoepitaxial vertical power devices, though had different properties that could impact device performance. Through evaluation by Raman spectroscopy, photoluminescence, white light optical profilometry, and Nomarski imaging concentrations of grain boundaries, impurity incorporations, point defects, v-shaped cone defects, polishing defects, crystal stress damage, and insulating and conductive regions were identified across the various GaN substrates studied.

Basic ammonothermal growth of Gallium Nitride – State of the art, challenges, perspectives

Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5 × 104 cm−2. Crystals with different types of conductivity, n-type with free electron concentration up to 1019 cm−3, p-type with free hole concentration of 1016 cm−3, and semi-insulating with resistivity exceeding 1011 Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown.

Characterization of Homoepitaxial GaN Films on Commercial GaN Substrates

The realization of next-generation medium to high power vertical GaN devices relies heavily on the epitaxial growth of GaN on commercially available, high-quality GaN substrates. Recent advances in commercially available GaN substrates have enabled the design and fabrication of vertical GaN devices that demonstrate promise and feasibility, but are limited by materials challenges of homoepitaxial GaN growth. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates include defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. In this presentation, a systematic study of the morphological, electrical, and optical properties of GaN films on GaN substrates will be presented to identify substrate and epitaxial film metrics that administer high mobility, low leakage current, low on-resistance, and high breakdown voltage.

Growth of High-Quality GaN Template from Nanometer-Size Lattice Channels by Hydride Vapor Phase Epitaxy

GaN are an attractive material in power devices for energy-saving measures in consumer products, automobiles, and industrial machines. To realize such GaN devices, however, high-quality GaN substrates are indispensable. In this paper, we describe the growth of high-crystalline-quality GaN template by HVPE with the nano-FIELO technique, where GaN growth starts by forming facet structures from nanometer-size channels which are opened on SiO2 layer deposited on GaN/sapphire substrate. The lattice pattern consisting of 500 nm × 500 nm square SiO2 masks surrounded by 80-nm-wide channels is used in this study. A nanoimprinting technique is applied followed by dry etching to fabricate lattice channels. From cross-sectional TEM observation, it is shown that the dislocation density is significantly reduced by the lattice pattern. Uniform GaN growth over 2-inch-diameter wafer is realized.

K16200 窒化物半導体デバイスの今後の展望(【K16200】情報・知能・精密機器部門企画,基調講演)

Market trend of GaN-based white LEDs and RF HFET is surveyed. Key points for the future white light source for general lighting and high power electron devices are discussed. Advantage of using high-quality GaN substrates grown by Na flux method on the performance of high-current density operation LEDs and also high power electron devices will be shown. Also, merit of using high-density nitrogen radical source on the growth of thick InGaN by plasma-assisted (PA-) molecular beam epitaxy (MBE) is presented.

Improvement of the droop efficiency in InGaN‐based light‐emitting diodes by growing on GaN substrate

AbstractThe high quality GaN substrates were fabricated by hydride vapor phase epitaxy (HVPE). The threading dislocation densities (TDDs) in the InGaN/GaN multiple‐quantum‐well light‐emitting diodes (LEDs) on high quality GaN substrate was substantially reduced, leading to a significant reduction in an improved droop effect at high current density. Compared to the same devices grown on Al2O3, these LEDs showed the enhancement in output power and reduction in droop. In this study, we demonstrate the optical and electrical properties of LEDs grown on Al2O3 and GaN substrate. The LEDs on GaN substrate outperformed the LEDs on Al2O3 at forward voltage, output power, and external quantum efficiency (EQE) for 1.5%, 40%, and 15% enhanced, respectively (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous emission from GaN/AlGaN terahertz quantum cascade laser grown on GaN substrate

AbstractThe radiation properties of a GaN/AlGaN‐based terahertz‐quantum cascade laser (THz‐QCL) device fabricated on a high quality GaN substrate were investigated in this work. The radiant intensity from a THz‐QCL fabricated on a GaN substrate showed a value (∼20 pW) about 10 times higher than that from a THz‐QCL grown on a metal organic chemical vapor deposition (MOCVD)‐GaN template that we had used before. Furthermore, we observed for the first time spontaneous THz emission with a peak at 1.37 THz in the case of a THz‐QCL grown on a GaN substrate. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vapor-phase epitaxy of high-crystallinity GaN films using Ga 2O vapor and NH 3

In this study, vapor-phase epitaxy (VPE) of GaN oriented-film was performed using Ga 2O vapor as the Ga source. Ga 2O vapor was obtained by reducing Ga 2O 3 powder with H 2 gas at 1000 °C. The Ga 2O vapor was then reacted with NH 3 on a seed substrate at 1100–1150 °C. A high quality GaN substrate (1 mm thick, with full widths at half maximum of GaN (0 0 0 2) X-ray rocking curve of 107–110 arcsec) prepared by the Na-flux method were used as the seed substrate. After 30 min of growth, a 3-μm flat GaN (0 0 0 1) epitaxial layer was grown on the seed substrate. X-ray diffraction (XRD) measurements showed that the FWHM of the GaN epitaxial layer was 74–111 arcsec, showing high crystallinity. Secondary ion mass spectrometry (SIMS) analysis showed that the oxygen concentration in the epitaxial layer was 1.5×10 18 atoms/cm 3. Although an oxide was used as the raw material, oxygen concentration close to those in GaN crystal grown by the hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE) were achieved. We concluded that the VPE method using Ga 2O vapor has potential as a simple vapor-phase-growing technique for high-quality GaN films.

Freestanding high quality GaN substrate by associated GaN nanorods self-separated hydride vapor-phase epitaxy

This work proposes a method for fabricating 2 in. freestanding GaN substrates of high crystallographic quality and low residual strain. Arrays of GaN nanorods with sidewalls coated with silicon dioxide (SiO2) were randomly arranged on the sapphire substrate as a growth template for subsequent hydride vapor-phase epitaxy (HVPE). The passivation of the sidewalls coated with SiO2 prevents the coalescence of GaN grains in spaces between the rods, causing them to grow preferentially on the top of individual rods. The proposed method significantly improves GaN crystal quality and results in self-separation from the underlying host sapphire substrate due to the relaxation of thermal strains in the HVPE cooling-down process.

GaN Bulk Substrates for GaN Based LEDs and LDs

In the present work, we discuss the importance of bulk GaN substrates for GaN based LEDs and LDs. As crystal growth methods for growing bulk GaN, we study the hydride vapor phase epitaxial (HVPE) method and the pressure-controlled solution growth (PC-SG) method. The recent results obtained by these two methods are presented. The maximum size of the grown single crystals was 3 × 4 cm2 for the HVPE method and 12 mm diameter for the PC-SG method. Both methods can provide high quality GaN substrates, with a low dislocation density of the level of less than 106 cm–2.

Structural characterization of Al 1− xIn xN lattice-matched to GaN

Ternary Al 1− x In x N films of various compositions were grown by atmospheric pressure metalorganic vapor-phase epitaxy. The films were structurally characterized by X-ray diffraction and atomic force microscopy. Although the crystallinity of Al 1− x In x N grown directly on a sapphire substrate was quite low, in the case of Al 1− x In x N grown on sapphire using a low-temperature-deposited AlN buffer layer, the crystallinity was a little higher. Moreover, when Al 1− x In x N was grown on a high-quality GaN layer which was grown on sapphire with a low-temperature-deposited AlN buffer layer, the crystallinity became much better. In addition, in this case, Al 0.83In 0.17N, which is lattice-matched to GaN, had the highest crystallinity among Al 1− x In x N samples of various compositions. It was found that a high-quality GaN substrate and lattice matching are essential for the growth of high-quality Al 1− x In x N.

Growth of a GaN layer on a glass substrate by metal organic chemical vapor deposition

This paper reports for the first time the successful growth of a single GaN layer on an amorphous glass substrate using metal organic chemical vapor deposition (MOCVD) technique. The layers were characterized by room temperature photoluminescence and X-ray diffraction measurements. In addition, the lateral overgrowth of GaN on this substrate was also tried, which could provide an effective method to obtain high-quality GaN substrates in the near future. In comparison to the overgrowth on sapphire substrate, the lateral overgrowth on our substrate is independent of the mask stripe direction, which could indicate a different growth mechanism.

Precise control of pn-junction profiles for GaN-based LD structures using GaN substrates with low dislocation densities

Doping profiles of pn-junctions for GaN-based laser diode structures were studied using SIMS and EBIC measurements. Mg impurity, a p-type dopant, was found to diffuse heavily into the n-type region of the order of 1017cm−3 when a sample is grown on a sapphire substrate. This unwanted diffusion appears closely related to the presence of threading dislocations, and can be eliminated by using high-quality GaN substrates for epitaxial growth.