GaN Single Crystal Substrates Research Articles

Electrical and Structural Analysis of β‐Ga2O3/GaN Wafer‐Bonded Heterojunctions with a ZnO Interlayer

AbstractWafer bonding of β‐Ga2O3 and N‐polar GaN single crystal substrates is demonstrated by adding ZnO as a “glue” interlayer. The wafers are fully bonded such that Newton rings are not observed. Temperature‐dependent current‐voltage (I–V) measurements are conducted on the as‐bonded Ga2O3/ZnO/N‐polar GaN test structure and after annealing at 600 °C and 1100 °C. The impact of post‐annealing temperature on the electrical and structural characteristics of the bonded samples is investigated. A consistently ohmic‐like characteristic is obtained by annealing the bonded wafers at 1100 °C in N2, which is in part due to crystallization of ZnO and diffusion of Ga into ZnO which makes it n‐type doped. The wafer bonding of β‐Ga2O3 and GaN achieved in this work is promising to combine the material merits of both GaN and Ga2O3 targeting breakthrough high‐frequency and high‐power device performances.

Atomic Layer Deposition of ZnO for Modulation of Electrical Properties in n-GaN Schottky Contacts

ZnO films (5 nm and 20 nm) have been grown on GaN single-crystal substrates by thermal atomic layer deposition (ALD) and the electrical properties of n-GaN Schottky contacts modified by such ultrathin ZnO films have been characterized. Compared with 5-nm-thick ZnO, 20-nm-thick ZnO exhibited a better rectifying nature. The average barrier height and ideality factor at room temperature were extracted to be 0.64 eV and 2.33 eV, and 1.01 eV and 1.16 eV, for 5-nm- and 20-nm-thick ZnO, respectively. These results indicate that both the barrier height and ideality factor were altered effectively by changing the ZnO thickness. The temperature-dependent reverse current–voltage (I–V) characteristics revealed that tunneling was dominant for the 5-nm-thick ZnO. A laterally inhomogeneous barrier was appropriate to explain the forward I–V characteristics for both samples. Based on the parallel conductance method and forward I–V data, a lower interface state density was observed for 20-nm-thick ZnO, implying improved interface quality. These results suggest that the electrical properties of n-GaN Schottky contacts could be easily modulated by changing the ZnO thickness.

01aA06 Naフラックス法によるGaN単結晶基板の評価(半導体バルク(2),第36回結晶成長国内会議)

01aA06 Naフラックス法によるGaN単結晶基板の評価(半導体バルク(2),第36回結晶成長国内会議)

Wafer-scale porous GaN single crystal substrates and their application in energy storage

Porous GaN has recently attracted much interest due to its high surface area, shift of bandgap and efficient luminescence. However, the porous GaN materials obtained so far are mainly fabricated via an electrochemical etching or a photoelectrochemical etching method. Here, we report the fabrication of wafer-scale (2 inch) porous GaN by a novel and simple high temperature annealing process. A model is proposed, based on scanning electron microscopy as well as the stabilities of the GaN crystallographic plane, to explain the formation mechanism of the porous GaN. Improvement of the crystal quality, enhancement of the optical quality and strain relaxation of porous GaN are confirmed by the HRXRD, PL and Raman spectroscopy results. A three-electrode system based on the porous GaN electrodes is assembled. The good capacitive behavior and superior electrochemical reversibility validate the concept of porous GaN-based supercapacitors and point to their potential in energy storage applications.

GaN Substrate Technologies for Optical Devices

Large GaN single-crystal substrates with low dislocation density are the key materials for the commercial production of GaN-based laser diodes. We developed a new method to reduce the dislocations, named dislocation elimination by the epitaxial-growth with inverse-pyramidal pits (DEEP). A thick GaN film is epitaxially grown on a GaAs substrate with hydride vapor-phase epitaxy and then is separated from the GaAs substrate. The thick GaN layer grows with numerous large inverse-pyramidal pits. As the growth proceeds, dislocations in the GaN film are concentrated to the center of the pit and a wide area with low dislocation density is formed within the pit except the center area. To control the dislocation artificially, the position of the pits is fixed at a predetermined position by means of the selective growth of different polarity GaN film. This process was named as advanced DEEP (A-DEEP). GaN substrates with the A-DEEP method satisfied all the requirements for the violet laser diodes. We continued to develop new GaN substrates such as large-diameter c-plane substrates as well as nonpolar/semipolar GaN substrates. In particular, we overcame the green gap problem and developed the world's first true green laser diodes by selecting an optimal crystal plane.

Blue lasers on high pressure grown GaN single crystal substrates

Blue lasers on high pressure grown GaN single crystal substrates

Growth of AlxGa1?xN and Mg-Doped GaN Epilayers on Ga- and N-Faces of Bulk GaN Single Crystal Substrates by Molecular-Beam Epitaxy

AlGaN and Mg-doped GaN epilayers were grown by radio-frequency molecular beam epitaxy on both polar faces of bulk GaN single crystals with single polar domain, which were prepared by the pressure-controlled solution growth method. AlGaN epilayers on Ga-faces of bulk GaN substrates exhibited higher Al composition and narrower full-width at half maximum of 2θ — ω X-ray diffraction than those values obtained for AlGaN epilayers on N-faces at the same growth condition. P-type conduction was only achieved in Mg-doped GaN on Ga-faces of bulk GaN substrates without any post-treatments. These results indicate that the growth on Ga-faces (or with Ga-polarity) is more favourable for device fabrication even using high-crystallinity bulk GaN substrates.

Successors to sapphire in the GaN market

2002 has seen a resurgence of interest in the development of GaN single crystal substrates. The number of companies in the field has grown and they are at last receiving major funding to help them overcome the next hurdles. The impetus behind this lies in the shortcomings of the sapphire-based technologies currently in use. There is still a question mark against the commercial viability of pure GaN wafers when and if they will ever match the market need for low-price materials.

Dynamics of excitonic recombination and interactions in homoepitaxial GaN

The dynamics of free and bound excitons and their interactions have been analyzed from the results of time-resolved photoluminescence (PL) in GaN layers fabricated by metalorganic chemical vapor deposition on GaN single-crystal substrates. The PL transients of an acceptor-bound exciton ${(A}^{0}X)$ a donor-bound exciton ${(D}^{0}X),$ and free excitons A and B ${(FX}_{A}$ and ${\mathrm{FX}}_{B})$ at 3.467 eV, 3.472 eV, 3.478 eV, and 3.483 eV, respectively, have been measured. At low excitation density (100 ${\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ the PL decay times in GaN range from 30 ps for ${\mathrm{FX}}_{B}$ up to 1000 ps for ${A}^{0}X.$ The dependence of time constants versus excitation and temperature has been analyzed. Below 50 K, the most important decay process of the $\mathrm{FX}$ is trapping on donors. At higher temperatures, delocalization of excitons is observed, and free-exciton emission becomes relatively strong. The delocalization of excitons influences the decay process. Consequently, decay times initially increase, then decrease with temperature. They are longest at about $T=70 \mathrm{K}.$ The time constants of the ${\mathrm{FX}}_{A}$ and ${D}^{0}X$ are of the order of a few hundred ps. The exciton temperature ${T}_{X}$ has been determined from the slope of the blue wing of the $\mathrm{FX}$ peak. Initially, the excitons are hot (due to high photon energy); then they cool due to interactions with phonons on a time scale of about 80 ps. In the same time range, a ${D}^{0}X\ensuremath{-}\mathrm{FX}$ interaction leading to a broadening of the ${D}^{0}X$ peak is observed.

Photoluminescence study of homoepitaxial N-polar GaN grown on differently misoriented single crystal substrates

We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal (0 0 0 1 ̄ ) GaN single crystal substrates. Off-angles of 2° and 4° towards the [1 1 2 ̄ 0] direction as well as 4° in the [1 0 1 ̄ 0] direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the [1 1 2 ̄ 0] direction results in an N-polar epilayer of the best optical quality.

GaN/AlGaN quantum wells for UVemission: heteroepitaxy versus homoepitaxy

GaN/Al0.1Ga0.9N quantum wells (QWs)are grown by molecular beamepitaxy on (0001) sapphire and (0001) GaN single-crystalsubstrates. Their optical properties are investigated bytemperature-dependent photoluminescence (PL). At roomtemperature, the integrated PL intensity of the homoepitaxialQW is 20 times higher than that of the heteroepitaxially grown QW.In the latter case, the PL intensity rapidly decreases even inthe low-temperature range (10-100 K). This is ascribed to thenon-radiative recombination of excitons on threadingdislocations. In contrast, the PL intensity quenching of thehomoepitaxial QW is purely governed by carrier thermal escape.These results demonstrate that GaN bulk substrates offer newopportunities for UV optoelectronics.

Light emitters fabricated on bulk GaN substrates. Challenges and achievements.

AbstractWe used high-pressure grown GaN single crystal substrates to fabricate dislocation free optoelectronic devices like light emitting diodes and laser diodes structures. The latter ones demonstrated laser action under optical pumping condition with the threshold of about 200 kW/cm2 at room temperature. In the present paper we would focus on the specific aspects of the homoepitaxial growth by MOVPE method including epi-ready substrate preparation and surface polarity choice. We believe that our results demonstrate clearly the feasibility of device fabrication based on high-pressure grown GaN bulk crystals.

Growth of GaN single crystal substrates

Self-separating single crystal GaN substrates were grown by hydride-metalorganic vapor phase epitaxy (H-MOVPE) on closely lattice-matched LiGaO 2 substrates. A critical process for obtaining high quality GaN films on LiGaO 2 was the initial surface nitridation of the oxide substrate with NH 3. This nitriding process and the cooling schedule after growth were critical to achieving film-substrate self-separation. The as-grown single crystal GaN substrate exhibited a flat surface without any mechanical or chemical treatment. No cracks or residual strain were observed. Different characterization techniques were used to assess the quality of the substrates and films quality. The surface morphology was examined by AFM and the structural quality was analyzed by XRD; the chemical composition was investigated by AES, ESCA, SNMS and SIMS were used to determine composition profiles near the surface. Micro Raman spectroscopy was applied for film and substrate characterization.