Proton irradiation-induced point defects acting as Shockley-Read-Hall (SRH) recombination centers in homoepitaxial GaN p-n junctions were characterized based on analyses of recombination current. Positron annihilation spectroscopy (PAS) data indicated that the vacancies in the GaN specimens comprised Ga vacancies and divacancies. The SRH lifetimes were decreased with the increase of the 4.2 MeV proton dose. For the same dose, the carrier concentrations and the SRH lifetimes for p-/n+ junctions were significantly reduced compared with those for p+/n- junction. The results suggest the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes. The authors thank Mr. Takahide Yagi and Mr. Joji Ito of SHIATEX Co., Ltd., for performing proton irradiation and irradiation simulations. The authors thank Dr. Akira Uedono of Tsukuba Materials Research Co., Ltd., who is also a professor at the University of Tsukuba, for assisting in the assessment of vacancy types using PAS.The authors thank C-TEFs at Nagoya University for fabricating the devices used in this work.
Selective doping technique by ion implantation was applied to power devices. A novel activation method for p-type doping named as UHPA (Ultra High-Pressure Annealing) was recently developed. Using this method, edge termination of FLR (Field Limiting Rings) structure by Mg ion implantation for a pn diode was formed, and the improvement of the breakdown voltage was confirmed. JBS (Junction Barrier Schottky) diodes were also fabricated, which showed high performance such as breakdown voltage of 675V, specific on-resistance of 0.67mΩ·cm2 and avalanche capability. The activation of Mg using UHPA has enabled p-type selective doping even in GaN power devices.
Financial supports: MEXT (JPJ005357 and JPJ009777)
The results of complimentary time-resolved photoluminescence and positron annihilation measurements on Mg-implanted GaN on GaN fabricated using various I/I sequences will be shown to identify the species and quantify the concentrations and minority carrier capture coefficients of major midgap recombination centers (MGRCs) created by the I/I processes. Because vacancy clusters comprised of Ga vacancies (VGa) and N vacancies (VN) such as (VGaVN)3 were assigned as major vacancy-type defects and the room-temperature photoluminescence lifetime for the NBE emission increased with decreasing their concentration, (VGaVN)3 are assigned as major nonradiative recombination centers with electron capture coefficient of 5×10-6 cm3s-1, which is an order of magnitude larger than the hole capture coefficient of VGaVN in n-GaN (6×10-7 cm3s-1).
Financial supports: CSTI-SIP, MEXT (JPJ005357, JPJ009777, JP16H06427, JP21H01826), PNCRD TECHMATSTRATEG-III/0003/2019-00 and PNSC 2018/29/B/ST5/00338.
Behaviors of vacancy-type defects in ion-implanted GaN were studied by means of positron annihilation. Si or Mg ions were implanted into GaN to obtain 300-nm-deep box profiles of the impurities. The ion-implanted samples were annealed up to 1480°C under a N2 pressure of 1 GPa (ultra-high-pressure annealing: UHPA). For as-implanted GaN, the major defect species was identified as Ga-vacancy-type defects such as a divacancy (VGaVN). After annealing above 1000°C, vacancy clusters, such as (VGaVN)3, were introduced, and they were found to be remained even after 1480°C annealing. For Mg-implanted GaN with [Mg]=1018 cm-3, no large change in the depth distribution of Mg was observed before and after annealing at 1400°C. For the sample with [Mg]=1019 cm-3, however, Mg diffused into the bulk, which was attributed to the over-doping of Mg and their vacancy-assisted diffusion. The Mg diffusion was suppressed by sequential N-implantation, which was attributed to the reaction between Mg and vacancies under a N-rich condition. Interactions between vacancies, Mg, and H during UHPA were also discussed.
Vacancies in Mg-implanted GaN were probed using positron annihilation technique. Mg was implanted into GaN with [Mg] = 1E19 /cm3. For an as-implanted sample, the major defect species was identified as Ga-vacancy related defects. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000–1480C without a protective capping layer. Comparing with the sample annealed with the capping layer, although no large difference in the defect spices was observed, their concentration was decreased by the cap-less annealing. The diffusion of Mg during annealing was influenced by the presence of residual vacancies. H was unintentionally incorporated into the sample during annealing, and its diffusion property were also affected by vacancies and Mg. A part of this work was supported by MEXT “Research and development of next-generation semiconductor to realize energy-saving society (JPJ005357)” and the Polish National Science Centre through project No 2018/29/B/ST5/00338.
Ion implantation (I/I) and annealing techniques to enable high carrier doping into selected regions are one of important research fields for realization of GaN power devices. However, particularly difficult research challenges are present in Mg-I/I into GaN as p-type doping technique. First problem is related to nitrogen vacancies (VN), crystal defects introduced by Mg-I/I . [1] Second issue is connected to degeneration of GaN surface by pyrolysis reaction during high-temperature annealing process. We examined Mg/N co-implantation into GaN as p-doping in order to compensate of VN defects. Research on ultra-high pressure thermal activation process to maintain equilibrium conditions at high temperature was conducted to avoid degradation of GaN surface.
We prepared Mg/N co-implanted GaN-on-GaN samples with 300-nm-deep Mg-box-profile of 1E19 cm-3 and with N-box-profile of various concentrations in the range 0 ~ 1E20 cm-3. The samples were annealed without a protection cap layer at temperature between 1300 °C and 1480 °C under 1 GPa in N2 atmosphere. Samples capped with sputter-AlN were treated by conventional lamp annealing at 1300 °C for a comparison purpose.
We obtained the dominant Green Luminescence (GL) emission at 2.3 eV and the recessive Donor Acceptor Pair (DAP) emission at 3.2 eV from a low-temperature cathodoluminescence (LT-CL) spectra in Mg-I/I samples without N co-implantation. This is in good agreement with results published before.[1][2] In addition to this, we found that the Mg/N co-implantation, for optimized N-ion dose, suppresses the GL intensity while maintaining the DAP intensity. Furthermore, in the ultra-high pressure thermal activation process, the DAP intensity markedly increases with anneal temperature and GL intensity suppression is also visible. These results strongly suggest that the origin of GL is related to VN, and the VN defect compensation occurs by N co-implantation. We also compare the AFM images of GaN surface roughness. The ultra-high pressure annealed surfaces were as smooth (RMS = 0.2~0.3 nm) as the as-implanted samples (0.3 nm), whereas surface treated with conventional lamp was rough (1.9 nm).
We can, therefore, conclude that Mg/N co-implantation and ultra-high pressure thermal activation process allows activation of the Mg acceptor and recovery of p-GaN crystal quality. Such treatment results in VN defect compensation and a smooth surface of the annealed sample.
This work was supported by MEXT “Program for research and development of next-generation semiconductor to realize energy-saving society.”
[1] A. Uedono et al., Phys. Status Solidi B 252, No. 12 (2015)
[2] K. Kojima et al., Appl. Phys. Express 10, 061002 (2017).
An electrostatic-actuated suspended bridge structure composed by single-crystalline silicon carbide (SiC) is fabricated.
The structure is entirely made of homoepitaxially grown single-crystalline 4H-SiC. Electrical isolation between the
suspended bridge and the base plate is established with a pnp junction formed by multiple ion implantation. The structure
is fabricated by a combination of reactive ion etching (RIE) and doping-selective photoelectrochemical (PEC) etching.
The suspended bridge is actuated by applying a voltage between the bridge and the base plate.
The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were investigated in a wavelength range
from the near band edge (392 nm for SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a
temperature range from room temperature to 512°C. Optical interference measurements with vertical incident
configuration were employed to precisely evaluate ordinary refractive indices. In visible region, the thermo-optic
coefficient of GaN has the largest value in these materials. Optical simulation of GaN-based tunable band-pass filter with
AlGaN/GaN distributed Bragg reflectors (DBRs) was also carried out by using the obtained thermo-optic coefficients. It
revealed that 9 nm red-shift can be obtained from room temperature to 500°C.
Microelectromechanical systems (MEMS) devices made of single crystalline silicon carbide (SiC) are attractive for applications in harsh environment, because SiC is chemically inert and semiconductor devices made of SiC can be operated at very high temperature. On the other hand, due to its chemical inertness, controllable etching of SiC has been difficult. Molten KOH etching has been widely used to detect crystalline defects in SiC as etch pit in crystal growth researchers. Some etch pits have hexagonal shape, indicating anisotropic etching nature. Therefore, molten KOH etching may have potential as a SiC MEMS fabrication process. In this study we have developed the anisotropic wet chemical etching of single crystalline hexagonal SiC using molten KOH for SiC bulk micromachining. 6H-SiC (0001)Si face and (000-1)C face substrates are used. Etching rates of (0001)Si and (000-1)C faces at 490 °C are evaluated to be 37 nm/min and 3.1 μm/min, respectively, indicating that the (0001)Si face is etched almost 100 times slower than the (000-1)C face is. Cross sectional analysis of etched structure of (000-1)C face revealed that inclined crystal plane was formed as a sidewall with some undercut. To assess in-plane etching anisotropy, ring shape mesa structures are formed on SiC (0001)Si face by RIE and then etched by molten KOH. Ring shape changed into hexagonal shape, clearly indicating etching rate along <11-20> direction is larger than <1-100> direction in (0001)Si face.
Electrically conductive zirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based nitride semiconductors. In this paper, important properties of ZrB2 as a substrate for nitrides, such as, thermal expansion coefficient, thermal conductivity, optical reflectivity and cleavage, are reviewed. Then, heteroepitaxial growth of GaN and AlN on the substrate by molecular-beam epitaxy (MBE) are discussed. Direct growth and two-step growth using low-temperature GaN nucleation layers as well as characterization of the surface condition of ZrB2 substrates by X-ray photoelectron spectroscopy (XPS) and the effect of surface treatment on grown layers are presented.
Gas-source molecular beam epitaxy (GSMBE) was applied for the growth of ZnMgSSe layers and quantum well (QW) structures. The source materials were elemental Zn and Se, as well as gas sources of bis- methylcyclopentadienyl-magnesium ((MeCp)2Mg) and H2S. Mg and S compositions were well controlled by the flow rate of (MeCp)2Mg and H2S, respectively. ZnSe/ZnMgSSe QWs with abrupt heterointerface have successfully been grown on [100]-oriented GaAs substrates under in-situ monitoring of specular beam intensity oscillation in reflection high energy electron diffraction (RHEED). Photoluminescence (PL) at 4.2 K revealed sharp and intense emission from single QWs, which is attributed to n equals 1 heavy-hole free exciton. The photopumped lasing of a double heterostructure was achieved at room temperature with low threshold excitation intensity (110 kW/cm2), suggesting formation of well-defined heterostructures and promising potential of GSMBE for device applications.
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