High-Voltage Vertical GaN p-n Diodes by Epitaxial Liftoff From Bulk GaN Substrates (original) (raw)
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Thin-film GaN Schottky diodes formed by epitaxial lift-off
Applied Physics Letters, 2017
The unique material properties of GaN and related III-N semiconductors, such as a high critical electrical field, large band gap, high saturation electron velocity, good electron mobility, and high thermal conductivity 1,2 , have made GaN and related materials one of the most promising material systems for high-performance optoelectronics as well as next-generation power electronics. Despite the inherent material advantages of GaN and numerous device demonstrations, the actual performance of many lateral GaN devices has fallen short of the ultimate performance expected from consideration of fundamental material parameters. These performance discrepancies, which include effects such as large ideality factors, higher-than-expected reverse saturation currents, and the inability to support avalanche currents in diodes 3 , 4 and the presence of surface-and/or buffer-related effects such as dynamic on-state resistance, current-collapse, and hysteresis in FETs, 5,6 have thus far limited the applications that can be addressed using GaN electronics. The use of lattice-mismatched non-native GaN substrates (driven by the high cost and limited availability of native GaN substrates) also results not only in large dislocation densities but also limited thermal conductance for through-substrate heat removal. 7,8 As is well known, power devices are typically thermally limited 9,10 so that the die size is set by power dissipation and thermal resistance considerations, rather than by current density limitations. Epitaxial lift-off (ELO) processing offers an alternative approach to address these issues.
Materials Issues for Vertical Gallium Nitride Power Devices
ECS Transactions, 2013
In this paper we are addressing some of the fundamental materials issues for the development of vertical GaN-based power devices. Major components of such device are the n+ GaN freestanding substrate on which a thick (~50 µm), low defect density and low carrier concentration (<10 16 cm -3 ) n-GaN drift region is grown homoepitaxially. We show that the hydride-vapor-phase-epitaxy (HVPE) is a method capable of producing economically free standing n+ GaN substrates as well as the required thick and low defect and carrier concentration n-GaN drift region. The formation of freestanding GaN substrates by a natural separation mechanism effectively eliminates the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. A number of GaN thick films were grown onto sapphire substrates by the Hydride Vapor Phase Epitaxy (HVPE) method with thickness varying from 150μm to 3.8mm using either a low-temperature GaN or an AlN buffer as the nucleation step. We have found that samples grown on a low temperature GaN buffer naturally delaminate from the sapphire substrate post-growth over the entire thickness range studied. However, the GaN films grown on AlN buffers did not delaminate. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney's equation and assuming that the GaN buffer undergoes decomposition at the growth temperature. The structure of these films was determined by x-ray diffraction and the dislocation density was measured to be as low as 5x10 6 cm -2 . The lowest carrier concentration in these heteroepitaxially grown films was found to be 10 17 cm -3 . Furthermore, we have identified the origin of this n-type auto-doping and proposed method to reduce the carrier concentration to values 10 16 cm -3 or lower.
Investigation of the GaN-on-GaAs interface for vertical power device applications
Journal of Applied Physics, 2014
GaN layers were grown onto (111) GaAs by molecular beam epitaxy. Minimal band offset between the conduction bands for GaN and GaAs materials has been suggested in the literature raising the possibility of using GaN-on-GaAs for vertical power device applications. I-V and C-V measurements of the GaN/GaAs heterostructures however yielded a rectifying junction, even when both sides of the junction were heavily doped with an n-type dopant. Transmission electron microscopy analysis further confirmed the challenge in creating a GaN/GaAs Ohmic interface by showing a large density of dislocations in the GaN layer and suggesting roughening of the GaN/GaAs interface due to etching of the GaAs by the nitrogen plasma, diffusion of nitrogen or melting of Ga into the GaAs substrate. V C 2014 AIP Publishing LLC. [http://dx.
Effect of high temperature, high pressure annealing on GaN drift layers for vertical power devices
Journal of Crystal Growth, 2018
In this work, we evaluate the effect of the novel symmetric multicycle rapid thermal annealing (SMRTA) process on both Mg ion implanted and non-implanted thick unintentionally doped GaN drift layers for vertical power devices. The typical p-type behavior and restoration of implant damage are observed in implanted samples, but on non-implanted samples a reduction in background carrier concentration and associated reduction in leakage current and increase in breakdown voltage is observed. This indicates that the capping/annealing process itself is not detrimental to the crystal, and the annihilation of native point defects in the process has beneficial effects for device structures. Keywords B2. Semiconducting III-V materials; B3. vertical power devices; A1. doping; A1. ion implantation Highlights Symmetric multicycle rapid thermal annealing process applied to non-implanted GaN drift layers Improved crystal quality/breakdown voltage, lower leakage/background doping with annealing Mg-implanted samples still have improved crystal quality but high sheet and contact resistance
Semi-insulating GaN for vertical structures: role of substrate selection and growth pressure
Materials Science in Semiconductor Processing, 2020
Samples comprising 1.3 μm-thick C-doped semi-insulating (SI) GaN layer sandwiched between two n-GaN layers were grown on sapphire or conductive GaN substrates by metal-organic chemical vapor phase epitaxy at varied reactor pressure between 100 and 20 mbar. Vertical cylindrical resistors with a radius of 50 μm were defined by ~ 1.6 μm-deep mesa etching down to the bottom Si-doped n-GaN layer. X-ray diffraction rocking curves revealed almost invariant crystallographic quality of homo-epitaxial structures grown on the GaN substrate, while dislocations in GaN on sapphire lead to curve broadening and pits formations. C concentration in SI GaN grown on the GaN substrate was found to increase from ~ 1 ✕ 10 17 cm-3 to ~ 6 ✕ 10 18 cm-3 as the growth pressure decreased from 100 mbar to 20 mbar. However, one order of magnitude lower C concentration and inhomogeneous distribution was found for the sapphire substrate. I-V characterization showed that the electrical strength of SI GaN could be as high as 2.8 MV/cm if grown at 20 mbar on the GaN substrate. In this case the nondestructive breakdown voltage exceeds 350 V with a positive T coefficient pointing on impact ionization. Sample grown on the GaN substrate at 20 mbar demonstrated ln(I) ~ V dependence which is typical for a barriercontrolled leakage. Extracted barrier height of 0.41 eV at the n-GaN/SI GaN interface was explained by C-related compensation effect and by a shifting of the Fermi level. On the other hand, apart from SI GaN grown on GaN substrate at 20 mbar, all other structures showed I ¼ f(V n) dependence indicating a space-charge-limited current conduction mechanism. It is suggested that the optimized SI GaN grown on GaN substrate can be considered as a current blocking layer or as a channel in robust geometry vertical transistors.
Applied Physics Express, 2018
This work demonstrates the first nonpolar vertical GaN-on-GaN p-n power diodes grown on m-plane free-standing substrates by metalorganic chemical vapor deposition (MOCVD). The SEM and HRXRD results showed the good crystal quality of the homoepitaxial nonpolar structure with low defect densities. The CL result confirmed the nonpolar p-GaN was of high quality with well activated Mg acceptors and considerably reduced deep-level (DL) states. At forward bias, the device showed good rectifying behaviors with a turn-on voltage of 4.0 V, an on-resistance of 2.3 mΩ•cm 2 , a high on/off ratio of ~ 10 10 , and a low leakage current of ~ 10 −7 A/cm 2. At reverse bias, the current leakage and breakdown were described by the trap-assisted space-charge-limited current (SCLC) conduction mechanism, where I was proportional to V 4.5. The critical electrical field was calculated to be 2.0 MV/cm without field plates or edge termination, which is the highest value reported on nonpolar power devices. The high performance m-plane p-n diodes can serve as key building blocks to further develop nonpolar GaN power electronics and polarizationengineering-related advanced power device structures for power conversion applications.
IEEE Electron Device Letters, 2014
There is a great interest in wide band-gap semiconductor devices for power electronics application. In this letter, vertical GaN p-n diodes fabricated on bulk GaN substrates are discussed. The device layers are grown by MOCVD on low defect density (10 4 cm −2) bulk GaN substrates. The measured devices show breakdown voltages of 3.7 kV with an area differential specific on-resistance (R sp) of 2.95 m-cm 2 .
Applied Physics Letters, 2000
In this letter, the lateral and vertical transport in lightly doped n Ϫ -GaN films, grown by plasma assisted molecular beam epitaxy, were investigated in order to explore the role of electron scattering by charged dislocations. Lateral transport constants were determined by Hall effect measurements on n Ϫ -GaN films. The doping concentration and mobility of the investigated films was 1 -2 ϫ10 17 cm Ϫ3 and 150-200 cm 2 /V s, respectively. Vertical transport was studied by etching mesa structures and forming Schottky barrier diodes. The diodes exhibit near ideal forward currentvoltage characteristics with reverse saturation current densities in the 1 -10ϫ10 Ϫ9 A cm Ϫ2 range. The doping concentrations as well as the barrier height of the diodes were determined from capacitance-voltage measurements to be 8 -9ϫ10 16 cm Ϫ3 and 0.95-1.0 V, respectively. The analysis of the reverse saturation current, using the diffusion theory, leads to vertical mobility values of 950 cm 2 /V s. The significant increase in mobility for vertical transport is attributed to reduction in scattering by charged dislocations. © 2000 American Institute of Physics.