Conductive GaN Substrate Research Articles

Vertical GaN Transistor with Semi‐Insulating Channel

Herein, vertical GaN transistors with a semi‐insulating (SI) 1.3 μm thick channel layer and C doping of 1 × 1017 cm−3 are studied. Structures are grown using a metal–organic chemical vapor deposition on conductive GaN substrates. SI GaN is sandwiched between 2.5 μm thick n‐GaN drift layer (Si doping of ≈ 1 × 1017 cm−3) and a top n‐GaN contact layer. A circular mesa region with a diameter of 180 μm is patterned using a deep dry etching. The gate contact formed on the mesa sidewall is insulated from the vertical channel using a 20 nm thick Al2O3 grown by an atomic layer deposition. Despite a robust layout, transistors transfer characteristics indicate normally off behavior if extracted from the linearly scaled current–voltage characteristics and an open channel drain current of 30 mA at the gate bias of 4 V. Achieved on/off ratio is 107 at −2 V subthreshold gate bias when the full channel depletion is reached. And, 200 ns long gate pulse characteristics show only a marginal trapping even though no post‐metallization annealing is performed. By comparing experimental results with modeling, mobility of free electrons in the channel is found to be about 45 cm2 V−1 s−1.

Cobalt Phosphate-Modified (GaN)1–x(ZnO)x/GaN Branched Nanowire Array Photoanodes for Enhanced Photoelectrochemical Performance

Photoelectrochemical water splitting based on suitable catalysts has attracted wide attention as a promising strategy to utilize solar energy to produce clean and renewable hydrogen fuel. Herein, we reported cobalt phosphate-modified (GaN)1–x(ZnO)x/GaN nanowire arrays on a high-temperature conductive GaN substrate toward enhanced photoelectrochemical water splitting by a facile two-step Au-assisted chemical vapor deposition method. The highly conductive Si-doped GaN substrate is designed to serve as a current collector and epitaxial substrate to grow GaN nanowires at high temperature. Meanwhile, high density of branched (GaN)1–x(ZnO)x nanowires with a tunable band gap, strong visible-light absorption, and high catalytic activity is deposited on the surface of GaN nanowires to act as active components to harvest light toward the oxygen evolution reaction. Such multi-junction heterostructures effectively enhance wide spectral utilization and light absorption and simultaneously accelerate the separation of electrons and holes and the transfer of photogenerated electrons from (GaN)1–x(ZnO)x nanowires to the GaN substrate collector and Pt electrode. The photocurrent density of the (GaN)1–x(ZnO)x/GaN nanowire array photoanode can reach 37.5 μA cm–2 at 1.23 V vs reversible hydrogen electrode and could be further enhanced to 186 μA cm–2 by modifying the cobalt phosphate cocatalyst, showing promising potential in clean hydrogen production.

Semi-insulating GaN for vertical structures: role of substrate selection and growth pressure

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 ✕ 1017 cm-3 to ~ 6 ✕ 1018 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 non-destructive break-down 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 barrier-controlled 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 = f(Vn) 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.

Theoretical considerations on efficiency degradation due to thermal effect in a planar GaN-based LED with a GaN substrate

In this paper, using a fully-coupled, three-dimensional electro-thermal device simulator, we study the mechanism of efficiency degradation at high current operation in planar GaN-based light emitting diodes (LED). In particular, the improvement of the efficiency degradation using thicker conductive GaN substrates has been demonstrated. First, it is found that local Joule heating inside thin conductive GaN substrates degrades internal quantum efficiency (IQE) and increases the series resistance. Then, we introduced thicker conductive GaN substrates and simulated distributions of the current density and temperature inside the substrate. It is found that the maximum current density inside the GaN substrate decreases by about six times for a 100-µm-thick substrate compared to that for a 5-µm-thick substrate. Therefore, the maximum junction temperature decreases, and then IQE and the driving voltage are improved. The present study proves that thick GaN substrates are effective to improve the properties of planar LEDs at high current operation.

Properties of MOCVD GaN/AlGaN heterostructures grown on polar and non‐polar bulk GaN substrates

AbstractIn this work we present structural, optical and electrical properties of GaN epilayers and AlGaN/GaN MOCVD heterostructures obtained on polar and non‐polar 10x10 mm2, 1 and 1.5 inch bulk ammonothermal GaN substrates. The results were obtained based not only on a single‐point measurement but also on mapping across the sample area. In contactless electroreflectance (CER) spectra CER resonances related to optical transitions between excited states were clearly observed for GaN/AlGaN QWs grown on c ‐plane GaN substrates, whereas such features were not noted in the case of GaN/AlGaN QWs grown on m ‐plane GaN substrates. This experimental result clearly shows that the polarization‐related electric field leads to the quantum confinement of some extra states in the polar QW system. To check the advantage of homo‐epitaxy using ammonothermal GaN, high electron mobility transistors (HEMTs), and light emitting diodes (LEDs) structures were grown on (0001) Ga‐polarity, semi‐insulating and conductive GaN substrates respectively (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anomalous photoresponse of GaN x-ray Schottky detectors

GaN based materials are believed to be very stable materials, in particular, under irradiation by high energy photons such as x rays. We have studied x-ray detectors based on GaN Schottky diodes. Vertical Schottky diodes were fabricated based on a 20μm thick undoped GaN layer grown on a conductive GaN substrate. Their photoresponse to near UV light and to x rays was measured. While the response to near UV light was fast and linear as expected, anomalous behaviors were observed under x-ray illumination. The photocurrent increases as the third power of the incident x-ray flux. The photocurrent transient when the x rays is turned on are long and nonexponential (S shape) and strongly differs from the off transient which is fast and exponential. Also, a very strong quenching of the x-ray photoresponse is observed when the detector is simultaneously illuminated with visible light. All of these anomalous behaviors are explained in the frame of a complete model involving traps and tunnel currents. A reasonable quantitative agreement between the model and the experimental data is obtained.

Schottky rectifiers fabricated on bulk GaN substrate analyzed by electron-beam induced current technique

We report the fabrication of specially-designed Schottky rectifiers with thin Schottky metal on bulk GaN substrate through a homoepitaxial process. The Schottky rectifiers show consistent easy turn-on behaviors but with a wide distribution of reverse leakage current levels. A turn-on voltage as low as 0.9 V with a low on-resistance of 2.56 mΩ cm 2 was obtained. The easy turn-on properties can be attributed to the low series resistance achieved by fabricating vertical geometry devices on highly conductive GaN substrate. Electron-beam induced current analysis indicates that although the dislocation density in the homo-epilayer is generally low and on the order of ∼1 × 10 7 cm −2, mesoscopic defects such as grain boundaries and “dislocation walls” are present in the epilayer and serve as strong recombination centers. By comparing the reverse leakage current levels and the EBIC images of selected devices, we found that the “dislocation walls” observed are most likely to be the major source of high reverse leakage current. This study not only shows the promise of making high-performance GaN power devices based on homoepitaxial growth approach, but also indicates the importance of successful substrate polishing to the development of bulk-GaN-based devices.

Low leakage Schottky rectifiers fabricated on homoepitaxial GaN

The development of a vertical GaN Schottky rectifier on freestanding bulk GaN substrate by employing homoepitaxial growth technique is reported. The n−∕n+ Schottky diode structure was grown on conductive GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the homoepitaxial layer determined by cathodoluminescence mapping technique is ∼6×106cm−2 with surface roughness less than 1nm. The fabricated Schottky rectifiers with dielectric overlap edge termination show a low reverse leakage current of <3×10−8A at −100V, which can be explained by the low dislocation density in the epilayer and high Schottky barrier height (1.37eV) of the Pt∕GaN metal-semiconductor contact. Due to the reduced series resistance, the bulk GaN Schottky rectifiers also exhibit a low turn-on voltage of ∼1.3V and an on-resistance of ∼5.3mΩcm2.