This paper presents the advantages and disadvantages of using triethylgallium (TEGa) and trimethylgallium (TMGa) for realization of high-purity, carbon-free, epitaxial Ga2O3 films. Critical process conditions and MOCVD reactor geometries on achieving high purity β-Ga2O3 films with high electron mobility and low background carrier concentration, including doping control in this range, will be discussed. This paper will also discuss the MOCVD growth of high Al composition (up to 30%) high quality strained β-(AlGa)2O3/Ga2O3 heterostructures. To obtain semi insulating films, we will discuss Ga2O3 doping with nitrogen, comparing various nitrogen precursors. We will present the limitations of MOCVD technique for achieving heavily doped (>1020 1/cm3), highly conductive β-Ga2O3. We will present a new in-situ etching process for Ga2O3 in MOCVD using Ga and Cl based chemistry.
MOCVD growth of high-quality β-(AlGa)2O3 on (010), (001), and (100) β-Ga2O3 substrates using nitrous oxide (N2O), TEGa, and TMAl as sources will be presented. Coherently strained β-(AlGa)2O3 layers with Al composition of ~40% and ~30% were realized on (100) and (010) substrates, which were coloaded during the growth. The films were smooth, but the layers grown on (100) substrates were smoother (~0.3 nm). The N2O can also dope the layers with nitrogen. β-(AlGa)2O3 films with [N] ranging from ~5x1017 to ~2×1019 cm-3 were achieved. The effects of substrate temperature and Al composition on N incorporation will be discussed.
β-Ga2O3 epitaxy from metal-organic chemical vapor deposition (MOCVD) has exhibited low background defects and high mobility which are promising for high-power devices. Vertical field-plate Schottky diodes have been fabricated using MOCVD β-Ga2O3 epitaxy that showed punch through breakdown with a specific on-resistance (Ron,sp) of 0.67 mΩ-cm2. This Ron,sp is among the lowest of comparable β-Ga2O3 drift layer thickness reports and can be contributed from the high-mobility MOCVD β-Ga2O3 epitaxy. We also demonstrated Pseudo vertical diodes fabricated on a thicker MOCVD β-Ga2O3 on Fe-doped substrate that showed higher voltage, better leakage current, and improved surface properties compared to the thinner films on Sn-doped substrate epitaxy.
Ga2O3 is the only ultra-wide bandgap semiconductor with melt-growth substrate technology similar to that of Si, heterostructure device technology similar to that of the III-Nitride family, and high growth rate (GR) epitaxial technologies such as MOCVD and HVPE to support the development of ultra-high-breakdown voltage devices competitive with SiC technology. We report a Ga2O3 transistor device based on a high-GR MOCVD technology (Agnitron Technology’s Agilis 100 reactor). We have demonstrated for the first time a β-Ga2O3 MOSFET grown by high-GR MOCVD resulting in significantly improved epilayer quality. The high GR demonstrated via this method paves the road for demonstration of high breakdown voltage devices on a thick Ga2O3 buffer layer.
Ga2O3 is the only ultra-wide bandgap semiconductor with melt-growth substrate technology similar to that of Si, heterostructure device technology similar to that of the III-Nitride family, and high growth rate (GR) epitaxial technologies such as MOCVD and HVPE to support the development of ultra-high-breakdown voltage devices competitive with SiC technology. We have demonstrated for the first time a β-Ga2O3 MOSFET grown by high-GR MOCVD (Agnitron Technology’s Agilis 100 reactor) with record high mobility of 170 cm2/Vs, despite increased carrier scattering rate in the doped channel, facilitated by a significant improvement in epilayer quality. The high GR demonstrated via this method paves the road for demonstration of high breakdown voltage devices on a thick Ga2O3 buffer layer. [1] M.J. Tadjer et al., J. Phys. D: Appl. Phys. 54 (2021) 034005.
Vertical β-Ga2O3 Schottky diodes from metal-organic chemical vapor deposition (MOCVD) epitaxy are reported for high-power devices. The field plate Schottky barrier diode (SBD) showed a differential specific on-resistance (Ron,sp) of 0.67 mΩ-cm2 and an average breakdown electric field of 2.28 MV/cm. To the best of our knowledge, this Ron,sp is the lowest among the available vertical β-Ga2O3 SBD reports, and contributed from the high-mobility MOCVD β-Ga2O3 epitaxy. Moreover, the average electric field of 2.28 MV/cm is higher compared to most of the vertical β-Ga2O3 punch-through SBDs. These results suggest that the high-quality MOCVD β-Ga2O3 can be promising for high-power devices.
High quality germanium doped β-Ga2O3 epitaxial film was grown by PMBE technique and fabricated into a vertical type Schottky photodiode with a Pt/nGa2O3/n+Ga2O3(010) structure. The photodiode exhibited excellent rectifying characteristics with a turn on voltage ~ 1V and near zero bias leakage current ~ 100 fA. The photoresponse measurement showed a true solar blind sensitivity with cutoff wavelength ~260 nm and an out of band rejection ratio of ~104. A maximum responsivity of 0.09 A/W at 230 nm was measured at zero bias, corresponding to an external quantum efficiency of ~52 %. The time response of the photovoltaic diode is in the millisecond range and has no long-time decay component which is very common in the MSM photoconductive wide bandgap devices. The photodiode performance remains stable up to 300°C, suggesting its potential use for high temperature applications.
MgZnO is an attractive semiconductor alloy for UV optoelectronic and electronic devices. Due to recent progress and availability of high quality Ga2O3 substrates and its high solar-blind bandgap of ~4.9 eV, it is desirable to investigate its application for solar-blind applications as a potential substrate alternative to sapphire for MgZnO. MgZnO alloys have been grown using plasma-assisted molecular beam epitaxy on Sn doped n-type (010) β-Ga2O3 substrates. It was found MgZnO growth with a MgO buffer layer has a rocksalt lattice structure. In-situ RHEED observations show that the sample grown with a MgO buffer shows two-dimensional growth and a surface roughness with root-mean-square (RMS) below 2 nm. On the other hand, MgZnO grown without a MgO buffer has a mixed phase of rocksalt and wurtzite lattice structures. Additionally, as the initial step for the fabrication of tunable wavelength solar-blind photodetectors, Schottky barrier photodetectors have been fabricated, demonstrating zero (0 V) bias responsivity of 0.1 μA/W (rocksalt MgZnO), 0.7 μA/W (mixed phase MgZnO) and 1.3 μA/W (mixed phase MgZnO) at 230 nm, 310 nm and 335 nm, respectively.
We report on high quality, wurtzite MgxZn1-xO (MgZnO) epitaxial films grown via the PMOCVD method with a record high Mg content up to 51 %. A series of MgZnO films with various Mg content were grown on ZnO (~30 nm)/Al2O3(0001) and ZnO (~30 nm)/AlN (~25 nm)/Al2O3(0001) substrates. The band gap for the films estimated using UV-visible transmission spectroscopy ranges from 3.24 - 4.50 eV, corresponding to the fraction of Mg between x=0.0 to x=0.51, as determined by Rutherford backscattering spectroscopy (RBS). The cathodoluminescence (CL) measurement showed a blue shift in the spectral peak position of MgZnO, indicating an increase in Mg content. No multi-absorption edges and CL band splitting were observed, suggesting the absence of phase segregation in the as grown films. The phase purity and crystal structure of the films were further confirmed by XRD. The absence of phase separation is attributed to the fast periodic transition steps in the PMOCVD, creating a non-equilibrium system where radicals that are formed will have insufficient time to reach their energy minimum. AFM analysis of the films had decreasing surface roughness with increasing Mg content. MSM photodetector was fabricated from the films to characterize the spectral response. The devices exhibit peak response ranging between 276 - 383 nm, covering a large portion of the solar blind spectral window. Moreover, the Schottky barrier was enhanced by treating the MgZnO surface with H2O2, reducing the device’s dark current.
This paper reports temperature-dependent DC and small-signal RF characteristics of a 0.4-mm-radius sapphire-based GaN p-i-n diode between -60°C and 175°C. Deep levels approximately 1 eV below the conduction band were observed in both persistent
photo-conductance and photo-capacitance measurements. Self-heating effects were also observed and modeled with the measured thermal resistance and time constant. Based on these characteristics, an equivalent-circuit model was constructed, which accurately predicted the temperature-dependent DC and RF characteristics of the diode.
Effects of electron irradiation on GaN and AlxGa1-xN doped with acceptor-forming species (Mg, C, Fe, and Mn) were
studied by cathodoluminescence and electron beam induced current techniques. Low energy electron beam irradiation was shown to induce a systematic decay of the cathodoluminescence intensity, which is accompanied by increased electronic carrier diffusion length, indicating the increase of carrier lifetime. Temperature-dependent
cathodoluminescence measurements allowed to estimate the activation energy for irradiation-induced effects, which was found to be comparable to the ionization energy of the dominant acceptor species. These observations are consistent with trapping of non- equilibrium electrons on deep, non-ionized acceptor levels. In (Al) GaN:Mg and GaN:C electrons are trapped by the ground state of the neutral acceptor atom, while in TM-doped compounds, electron irradiation induced processes appear to involve a more energetically accessible excited states of the acceptors.
We report on a technique for optimizing transport properties in p- and n-type AlGaN/GaN and GaN/InGaN superlattices. As we show highly conductive heterostructures can be obtained by inserting a graded doped layer, which reduces the barrier height while maintaining high sheet carrier density. For optimized p-type AlGaN/GaN SL, an eight fold reduction of the barrier height and a 1.5 times increase in sheet hole density is obtained compared to typical SL. The optimized structure yields 13 orders of magnitude improvement in vertical conductivity (σV) compared to typical SL, and 35 times improvement in lateral conductivity (σL) compared to bulk p-GaN. For optimized p-type GaN/InGaN SL, an improvement of more than 10 orders of magnitude in σV compared to typical SL is obtained with σL better than that of bulk p-InGaN. We also investigate n-type SLs as current spreading layers. A significant improvement in current distribution is obtained for the optimized SLs.
This paper reviews of some of the recent progress made in the development of high quality MgZnO and ZnCdO layers grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys for this entire range of compositions was confirmed by X-ray diffraction. Compositional analysis was performed using SIMS and RBS. Strong optical emission in the 380 nm to 574 nm spectral range was achieved at RT from CdxZn1-xO with various compositions, demonstrating a great potential for use in LEDs. Compositional fluctuations in a Cd0.16Zn0.84O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ~0.5 μm diameter were seen on the intensity maps. Dependence of the fundamental optical band gap on the composition of CdxZn1-xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd0.16Zn0.84O film. High conductivity and optical transparency of the CdZnO films with high Cd-mole fraction is attractive for making high performance electrodes. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells (MQW).
In the extensive research dedicated recently to metal- organic chemical vapor deposition (MOCVD)-grown high- efficiency GaN LED device design, a significant effort has been made to increase the conductivity of p-GaN layers, while n-GaN layers received relatively little attention. We demonstrated, both experimentally and theoretically, that the resistivity of n-GaN layers has a profound effect on blue InGaN LED performance. Optimization of n-GaN epitaxial layers allows the achievement of device series resistances below 15 Ohms and forward voltages as low as 2.9 Volts at 20 mA. We have also shown that contactless measurements of sheet resistivity of the entire LED epitaxial structure closely correlate with the ohmic resistance of the GaN layer measured in the fabricated devices. This provides an excellent non-destructive characterization tool for n-GaN optimization. Insufficient n-GaN conductivity is shown to trigger a distinct degradation mechanism by initiating current crowding in a localized device area. InGaN LED lamps with optimized n-GaN layers had a high external quantum efficiency and a good long-term reliability.
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