Ultra-wide Bandgap Materials Research Articles

Engel-Vosko GGA calculations of the structural, electronic and optical properties of LiYO2

Structural, electronic and optical properties of lithium yttrium oxide (LiYO2) are investigated using density functional theory (DFT). These calculations are based on full potential linearized augmented plane wave (FP-LAPW) method implemented by WIEN2k. The generalized gradient approximation (GGA) is used as an exchange correlation potential with Perdew-Burk-Ernzerhof (PBE) and Engel-Vosko (EV) as exchange correlation functional. The structural properties are calculated with PBE-GGA as it gives the equilibrium lattice constants very close to the experimental values. While, the band structure and optical properties are calculated with EV-GGA obtain much closer results to their experimental values. Our calculations confirm LiYO2 as large indirect band gap semiconductor having band gap of 5.23eV exhibiting the characteristics of ultrawide band gap materials showing the properties like higher critical breakdown field, higher temperature operation and higher radiation tolerance. In this article, we report the density of states (DOS) in terms of contribution from s, p, and d-states of the constituent atoms, the band structure, the electronic structure, and the frequency-dependent optical properties of LiYO2. The optical properties presented in this article reveal LiYO2 a suitable candidate for the field of optoelectronic and optical devices.

Epitaxial ScAlN grown by molecular beam epitaxy on GaN and SiC substrates

ScxAl1-xN is a promising ultra-wide bandgap material with a variety of potential applications in electronic, optoelectronic, and acoustoelectric devices related to its large piezoelectric and spontaneous polarization coefficients. We demonstrate growth of ScxAl1-xN on GaN and SiC substrates using plasma-assisted molecular beam epitaxy with x = 0.14–0.24. For metal-rich growth conditions, mixed cubic and wurtzite phases formed, while excellent film quality was demonstrated under N-rich growth conditions at temperatures between 520 and 730 °C. An rms roughness as low as 0.7 nm and 0002 rocking curve full-width at half maximum as low as 265 arc sec were measured for a Sc0.16Al0.84 N film on GaN. To further demonstrate the quality of the ScAlN material, a high-electron-mobility transistor heterostructure with a Sc0.14Al0.86 N barrier, GaN/AlN interlayers, and a GaN buffer was grown on SiC, which showed the presence of a two-dimensional electron gas with a sheet charge density of 3.4 × 1013 cm−2 and a Hall mobility of 910 cm2/V·s, resulting in a low sheet resistance of 213 Ω/◻.

Band Alignments of Atomic Layer Deposited ZrO2 and HfO2 High-k Dielectrics with (-201) β-Ga2O3

The energy band alignment between atomic layer deposited (ALD) high-k dielectrics of ZrO2 and HfO2 with n-type β-Ga2O3 was evaluated using x-Ray photoelectron spectroscopy. The valence band offset was found to be −0.3 ± 0.04 eV and −0.5 ± 0.04 eV for ZrO2and HfO2, respectively, which produced type II, staggered gap alignments for both dielectrics with conduction band offsets greater than 1 eV. Capacitance-voltage measurements were conducted on metal-oxide-semiconductor (MOS) diodes and used to extract the dielectric constants (ɛ) of both oxides. ZrO2films had a nearly ideal ɛ of 24.7 (ɛ ∼ 25), while HfO2 exhibited a significantly lower ɛ of 14 (ɛ ∼ 25–30). These results, combined with the small hysteresis (≤0.09 eV) in the capacitance-voltage behavior for both films, is indicative of the high quality of these ALD oxides and their interface with the β-Ga2O3 making them potential candidates as gate dielectrics in power devices based on this relatively new ultra-wide bandgap material.

Point defect reduction in wide bandgap semiconductors by defect quasi Fermi level control

A theoretical framework for a general approach to reduce point defect density in materials via control of defect quasi Fermi level (dQFL) is presented. The control of dQFL is achieved via excess minority carrier generation. General guidelines for controlling dQFL that lead to a significant reduction in compensating point defects in any doped material is proposed. The framework introduces and incorporates the effects of various factors that control the efficacy of the defect reduction process such as defect level, defect formation energy, bandgap, and excess minority carrier density. Modified formation energy diagrams are proposed, which illustrate the effect of the quasi Fermi level control on the defect formation energies. These formation energy diagrams provide powerful tools to determine the feasibility and requirements to produce the desired reduction in specified point defects. An experimental study of the effect of excess minority carriers on point defect incorporation in GaN and AlGaN shows an excellent quantitative agreement with the theoretical predictions. Illumination at energies larger than the bandgap is employed as a means to generate excess minority carriers. The case studies with CN in Si doped GaN, H and VN in Mg doped GaN and VM-2ON in Si doped Al0.65Ga0.35N revealed a significant reduction in impurities in agreement with the proposed theory. Since compensating point defects control the material performance (this is particularly challenging in wide and ultra wide bandgap materials), dQFL control is a highly promising technique with wide scope and may be utilized to improve the properties of various materials systems and performance of devices based upon them.

(Invited) Ultra-Wide-Bandgap AlGaN Power Diodes and Transistors

As the wide-bandgap materials SiC and GaN mature and become commercially available, “Ultra” Wide-Bandgap (UWBG) semiconductors with bandgaps exceeding 3.4 eV are gaining interest as the next-generation materials that will enable dramatic leaps in power electronics performance. This is because the Figure-of-Merit (FOM) comparing breakdown voltage (VB) and area-normalized “specific” on-resistance (Ron,sp), defined as VB 2/Ron,sp, scales as the cube of the critical electric field for vertical devices and the square of the critical electric field for lateral devices. The critical electric field for avalanche breakdown in turn scales approximately as the 5/2 power of the bandgap, implying a very strong dependence of FOM on bandgap. Representative UWBG materials include diamond, Ga2O3, and Al x Ga1-x N. The AlGaN system is particularly attractive due to the availability of heterostructures as well as the ability to utilize polarization doping; both of these properties mitigate the issue of energetically deep impurity dopants and consequent incomplete ionization which universally affects UWBG semiconductors. Thus, our group’s focus has been on the development of AlGaN for the fabrication of next-generation power devices with both vertical and lateral architectures. Quasi-vertical AlGaN PiN diodes were grown on thick (1.3 mm) sapphire substrates using Metal Organic Vapor Phase Epitaxy (MOVPE). The initial epitaxial layers consist of an AlN layer followed by an unintentionally-doped AlGaN layer to facilitate the transition from the substrate to the active device layers. Dislocation density is 1-2×109 cm-2; additional reduction in dislocation density is possible using patterned AlGaN and overgrowth techniques. The transition layer is followed by a heavily-doped n-type AlGaN:Si contact layer and the thick lightly-doped n-type AlGaN:Si drift layer. The drift layer is critical since it supports the high voltage in the reverse-bias blocking mode and is also responsible for the on-state resistance in the forward-bias conducting mode. This is followed by a p-type AlGaN:Mg anode layer and finally graded to a very thin GaN:Mg p+ contact layer. Ion implantation into the p-layers is utilized to form a Junction Termination Extension (JTE) to prevent premature breakdown. A mesa etch is performed down to the n-contact layer, and Ohmic contacts (TiAlMoAu) are deposited; this is necessary since the substrate is insulating and, due to the lateral current transport in this layer, the device configuration is termed “quasi-vertical” (the conduction through the pn junction and drift layer is in the vertical direction). The top Ohmic contact is PdAu, the surface is passivated with silicon nitride, and a shallow trench isolates the JTE from the mesa sidewalls. For a device utilizing Al0.3Ga0.7N for the active layers, and with a 4.3 um thick drift region doped in the mid-1016 cm-3 range, VB exceeded 1.5 kV (Figure 1a); combined with the measured Ron,sp of 16 mOhm·cm2, a FOM of approximately 150 MW/cm2 is achieved. To our knowledge this is the first time a vertical kV-class AlGaN PiN diode has been demonstrated, and the FOM is the highest reported for an AlGaN diode. Further, the current density under forward bias exceeds 3.5 kA/cm2, and the critical electric field for the Al0.3Ga0.7N is estimated to be 6.0 MV/cm. Lateral High Electron Mobility Transistors (HEMTs) were also grown and fabricated in Al-rich AlGaN alloys. The epitaxial layers are likewise grown on sapphire substrates by MOVPE. Following the growth of an AlN nucleation layer, a 400 nm thick Al0.85Ga0.15N buffer/channel layer is grown, which is capped by a 48 nm thick AlN barrier layer. All layers are unintentionally doped. A combination of mercury-probe capacitance-voltage and contactless resistance measurements indicate a sheet resistance of approximately 4200 Ohm/square and a channel mobility of 250 cm2/V·s, and the pinch-off voltage is approximately -4.0 V. Circular HEMTs were fabricated using a six-mask photolithographic process. The key step in the process is the fabrication of the source and drain Ohmic contacts, for which n+ GaN:Si is re-grown in trenches etched through the AlN barrier, and a TiAlNiAu metal stack is afterwards deposited. The Schottky gate is NiAu and the surface is passivated with silicon nitride; no field plates are utilized. Drain current vs. drain voltage curves are shown in Figure 1b and clear gate control is evident. However, the current density is much lower than expected based on the measured sheet resistance and known device geometry, indicating that further optimization of the Ohmic contacts is necessary (the contact resistivity is estimated to be 0.1 Ohm·cm2). Despite this, the devices show excellent performance with Ion/Ioff > 107 and VB > 800 V, and to our knowledge are the highest-bandgap transistors ever reported. Figure 1