Epitaxial Er-doped GaAs and Al0.5Ga0.5As films, 1.6 µm thick, grown by MBE on (100) GaAs substrates at 560 °C, with Er concentrations in the range 9 x 1017 to 2 x 1020 cm−3 were studied with RBS/channeling and photoluminescence techniques. Angular scans in the <110> and <111> axial and (111) planar directions indicate that the Er atoms in GaAs are located on interstitial sites. In Al0.5Ga0.5As doped with 5 x 1019 Er cm−3, 70% of the Er atoms are on positions slightly displaced from the interstitial site, the rest presumably substitutional. In Al0.5Ga0.5As doped with 9 x 1017 Er cm−3, more than 88% of the Er atoms are on substitutional sites. Photoluminescence around 1.54 µm is observed at room-temperature in Er-doped Al0.5Ga0.5As. Both the low and highly Er-doped samples show similar luminescence intensities; the luminescence lifetimes are on the order of 1 ms. The Er-doped GaAs does not show any measurable signal at room-temperature. Correlation of the luminescence data to the Er lattice location suggests that only substitutional Er in AlGaAs is in the luminescent trivalent state.

Monoclinic b-Ga2O3 and rhombohedral a-Ga2O3 have recently emerged as promising materials for “ultrawide” bandgap electronics for next-generation high voltage lateral and vertical power switching devices, thanks to their large bandgaps (4.9eV and 5.3eV, respectively), high expected critical breakdown fields (8MV/cm and 10MV/cm, respectively), and their correspondingly large high voltage and high frequency Baliga figures of merit (FOMs). These large FOMs ultimately translate to higher efficiencies for high power (>1kW) switching devices, which will enable more compact, higher performing power electronics systems. In order to realize the high voltages required for the most demanding applications, further development of epitaxial growth techniques which can produce thick Ga2O3 films is required. Halide vapor phase epitaxy (HVPE) is a scalable and inexpensive growth technique which currently appears to be well positioned to address such a need. Kyma Technologies has developed a low-cost HVPE reactor for the growth of thick Ga2O3 films which boasts high growth rates and smoothness while simultaneously being able to be lightly and controllably doped with Si and free of carbon. The reactor has been configured to grow both b-Ga2O3 and a-Ga2O3, and in this talk we will outline our recent growth results from both. Effects of substrate preparation, epi-readiness, and growth conditions on epilayer quality as well as on the corresponding Schottky barrier diode device performance will be presented.

We report the optical, electrical, and structural properties of Si doped $\beta$-Ga$_2$O$_3$ films grown on (010)-oriented $\beta$-Ga$_2$O$_3$ substrate via HVPE. Our results show that, despite growth rates that are more than one order of magnitude faster than MOCVD, films with mobility values of up to 95 cm$^2$V$^{-1}$s$^{-1}$ at a carrier concentration of 1.3$\times$10$^{17}$ cm$^{-3}$ can be achieved using this technique, with all Si-doped samples showing n-type behavior with carrier concentrations in the range of 10$^{17}$ to 10$^{19}$ cm$^{-3}$. All samples showed similar room temperature photoluminescence, with only the samples with the lowest carrier concentration showing the presence of a blue luminescence, and the Raman spectra exhibiting only phonon modes that belong to $\beta$-Ga$_2$O$_3$, indicating that the Ga$_2$O$_3$ films are phase pure and of high crystal quality. We further evaluated the epitaxial quality of the films by carrying out grazing incidence X-ray scattering measurements, which allowed us to discriminate the bulk and film contributions. Finally, MOS capacitors were fabricated using ALD HfO$_2$ to perform C-V measurements. The carrier concentration and dielectric values extracted from the C-V characteristics are in good agreement with Hall probe measurements. These results indicate that HVPE has a strong potential to yield device-quality $\beta$-Ga$_2$O$_3$ films that can be utilized to develop vertical devices for high-power electronics applications.

Halide vapor phase epitaxy was used to grow homoepitaxial films of β-Ga2O3 on bulk (010) crystals and heteroepitaxial films of α-Ga2O3 on c-plane sapphire substrates. The β-Ga2O3 substrates were prepared prior to growth to remove sub-surface damage and to apply various miscuts to their surfaces. Structural and electrical properties were found to be most impacted by the crystallinity of the β-Ga2O3 substrate itself, while the surface morphology was found to be most impacted by the miscut of the substrate. The appropriate choice of growth conditions and the miscut appear to be critical to realizing smooth, thick (>20 µm) homoepitaxial films of β-Ga2O3. The α-Ga2O3 films were grown on commercially available c-plane sapphire substrates, and the film morphology was found to be strongly impacted by the surface finish of the sapphire substrates. The α-Ga2O3 films were found to be smooth and free of additional phases or crystal twinning when the sapphire was sufficiently polished prior to growth.

Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than the 3.4 eV of GaN, represent an exciting and challenging new area of research in semiconductor materials, physics, devices, and applications. Because many figures-of-merit for device performance scale nonlinearly with bandgap, these semiconductors have long been known to have compelling potential advantages over their narrower-bandgap cousins in high-power and RF electronics, as well as in deep-UV optoelectronics, quantum information, and extreme-environment applications. Only recently, however, have the UWBG semiconductor materials, such as high Al-content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near-term possibility. In this article, the materials, physics, device and application research opportunities and challenges for advancing their state of the art are surveyed.

In this work, we outline our progress toward understanding the promises and limitations of hydride vapor phase epitaxy (HVPE) of homoepitaxial Ga 2 O 3 films in terms of the surface preparation, film nucleation, achievable growth rates, and doping capabilities. We also present evidence that hydrogen acts as a shallow donor in Ga 2 O 3 and show that background hydrogen concentrations can give rise to controlled doping at the 1E16 level and below.

Fe doped GaN was studied by time-resolved photoluminescence (PL) spectroscopy. The shape of PL transients at different temperatures and excitation powers allowed discrimination between electron and hole capture to Fe3+ and Fe2+ centers, respectively. Analysis of the internal structure of Fe ions and intra-ion relaxation rates suggests that for high repetition rates of photoexciting laser pulses the electron and hole trapping takes place in the excited state rather than the ground state of Fe ions. Hence, the estimated electron and hole capture coefficients of 5.5 × 10−8 cm3/s and 1.8 × 10−8 cm3/s should be attributed to excited Fe3+ and Fe2+ states. The difference in electron capture rates determined for high (MHz) and low (Hz) (Fang et al., Appl. Phys. Lett. 107, 051901 (2015)) pulse repetition rates may be assigned to the different Fe states participating in the carrier capture. A weak temperature dependence of the electron trapping rate shows that the potential barrier for the multiphonon electron capture is small. A spectral feature observed at ∼420 nm is assigned to the radiative recombination of an electron in the ground Fe2+ state and a bound hole.

In this paper, we present an overview of the current state-of-the-art in two-dimensional materials beyond graphene, and summarize device performance reported to-date. There is promise for these layered materials to be the foundation of a new area in low power and high frequency electronics, with early reports indicating 10s of gigahertz (GHz) operation without significant optimization of parasitic resistances or capacitances. In addition, we discuss the synthesis of transition metal dichalcogenides and the integration of as-grown material into heterostructures and electronic devices. Finally, we discuss the impact of surface preparation on the integration of dielectrics with MoS2 required to achieve GHz performance.

In this paper, we present an overview of the currently employed techniques to synthesize two-dimensional materials, focusing on MoS2 and WSe2, and summarize the progress reported to-date. Here we discuss the importance of controlling reactor geometries to improve film uniformity and quality for MoS2 through a combination of modeling and experimental design. In addition, development of processes scalable to provide wafer scale uniformity is explored using synthesis of WSe2 via metal-organic chemical vapor deposition. Finally, we discuss the impact of each of these processes for TMD synthesis on epitaxial graphene.

The economic and energy efficiency promise of solid state lighting drive a continuous need to reduce the cost and improve the efficiency of visible LEDs. Current III-N LED manufacturing is dominated by non-native substrate approaches which result in strained epitaxial films, high dislocation density in the buffer and device active regions, and large epiwafer bow, which together present major limitations in terms of LED device yield, performance, and cost. Large area sapphire substrates (6", 8", and beyond) are becoming available and present cost reduction potential yet they are even more vulnerable to strain and bow related issues. We present a novel approach to realizing a zero-bow relatively-thick GaN on sapphire template that can be made at essentially any diameter and which bodes well for improved device yield, performance, and cost. The elimination of bow is achieved through a simple cost-effective stress balancing technique involving backside deposition. The result is a large area GaN template which is flat at all temperatures and which has 10x lower dislocation density than current GaN buffers. We report on the properties of these templates and provide device data for LED structures grown thereon.

In0.20Ga0.80N/GaN multiquantum wells (MQWs) grown on [0001]-oriented GaN substrates with and without an InGaN buffer layer were characterized using three-dimensional atom probe tomography. In all samples, the upper interfaces of the QWs were slightly more diffuse than the lower interfaces. The buffer layers did not affect the roughness of the interfaces within the quantum well structure, a result attributed to planarization of the surface of the first GaN barrier layer, which had an average root-mean-square roughness of 0.18 nm. The In and Ga distributions within the MQWs followed the expected distributions for a random alloy with no indications of In clustering. High resolution Rutherford backscattering characterizations showed the ability to resolve the MQWs, and the resulting compositions and widths corroborated those determined from the atom probe analyses.

A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.

We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5nm and 7.5nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0nm exhibit the highest Hall mobility, approximately 1700cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5nm, coincident with the onset of strain relaxation.

AlN/GaN heterostructures with 1700-cm2/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (LG) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. LG = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency fT of 165 GHz, a maximum frequency of oscillation fmax of 171 GHz, and intrinsic average electron velocity ve of 1.5 ×107 cm/s. The 40-GHz load-pull measurements of LG = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high ve, and high RF performance in AlN/GaN transistors.

In this study, thermoelectric properties of bulk and epitaxy GaN with various doping concentration are investigated. Seebeck coefficients decreased with the increase of carrier concentration for both bulk and epitaxial GaN samples, and the Seebeck coefficients of epitaxial GaN samples are found to be larger than that of bulk GaN samples in the similar carrier density due to the higher dislocation scattering. For epitaxial samples, a high power factor of 4.72 × 10 -4 W/m-K 2 is observed. The power factors of the bulk GaN samples are in the range of from 0.315× 10 -4 W/m-K 2 to 0.354× 10 -4 W/m-K 2 due to the low Seebeck coefficients.

Switches are at the heart of all pulsed power and directed energy systems, which find utility in a number of applications. At present, those applications requiring the highest power levels tend to employ spark-gap switches, but these suffer from relatively high delay-times (~10-8 sec), significant jitter (variation in delay time), and large size. That said, optically-triggered GaN-based photoconductive semiconductor switches (PCSS) offer a suitably small form factor and are a cost-effective, versatile solution in which delay times and jitter can be extremely short. Furthermore, the optical control of the switch means that they are electrically isolated from the environment and from any other system circuitry, making them immune from electrical noise, eliminating the potential for inadvertent switch triggering. Our recent work shows great promise to extend high-voltage GaN-based extrinsic PCSS state-of-the-art performance in terms of subnanosecond response times, low on-resistance, high current carrying capacity and high blocking voltages. We discuss our recent results in this work.

Systematic site-specific comparisons of the densities of threading dislocations (TDs) and associated V-defects in six-period InxGa1−xN/GaN multi-quantum wells (MQWs) with In mole fractions of x=0.1 and 0.2 have been conducted using several electron microscopy techniques including electron channeling contrast imaging. The MQWs were grown on GaN epitaxial films deposited on either (0001) GaN substrates or on (0001)AlN/6H-SiC heterostructures. The densities of TDs and V-defects in the surfaces of the homoepitaxial GaN films and the In0.1Ga0.9N/GaN MQWs were both 5.5×106cm−2. By contrast, the V-defect densities in the In0.2Ga0.8N/GaN MQWs exceeded by two and three times the densities of TDs measured in the surfaces of the underlying GaN films grown on GaN and AlN/SiC substrates, respectively. Similar observations of V-defects not associated with threading dislocations have been previously reported to occur in InGaN MQWs grown on GaN template films on sapphire substrates and having either an initially high In mole fraction or grown at the end of a series of MQWs with an increasing In mole fraction. From a synthesis of the results of this previous work using GaN/sapphire substrates with the present results for bulk GaN substrates, it is hypothesized that increases in stress generated by the larger mismatches in lattice parameters between the GaN film and the In0.2Ga0.8N/GaN MQWs and the additional stresses generated between the GaN barriers and the wells within these MQWs generated new TDs within the MQWs, which then acted as nucleation centers for new V-defects.

To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps. (C) 2013 AIP Publishing LLC.

The authors have investigated the growth and structural and electrical properties of homoepitaxial GaN layers and GaN/AlGaN heterostructures grown on free-standing, hydride vapor phase epitaxy grown, N-polar GaN:Fe substrates by rf-plasma molecular beam epitaxy. Secondary-ion mass spectroscopic analysis of unintentionally doped and Be-doped N-polar GaN layers indicate that oxygen is the dominant impurity in all layers and is largely insensitive to growth temperature in the range investigated (675 °C < TS < 760 °C). Transmission electron microscopy (TEM) indicates that threading dislocations are generated at the regrowth interface in these samples; in contrast to homoepitaxial growth on Ga-polar GaN, and that the density of threading dislocations diminishes as the growth temperature increases. However, examination by TEM indicates that threading dislocations are not generated at the regrowth interface of samples subjected to pregrowth substrate surface cleaning by gallium deposition and desorption and subsequent growth of ultrathin (15 Å) initial AlN layers. N-polar GaN/AlGaN heterostructures grown on Be-doped homoepitaxial N-polar GaN buffers exhibit low buffer leakage and Hall mobilities up to 1680 cm2/Vs at sheet densities of 1.3 × 1013 cm−2. High electron mobility transistors have been fabricated on these structures; drain current densities over 700 mA/mm and breakdown voltages as high as 70 V have been measured.

We present a detailed study of the below band-gap optical absorption at room temperature in bulk semi-insulating GaN:Fe versus the Fe-doping. It was established that the 1.24 eV photoluminescence band at 300 K consists of only vibrational replicas of the Fe3+ 4T1(G)→6A1(S) internal transition. We also studied the below band-gap photoluminescence excitation of the 1.24 eV band. The identical exponential rise of the photoluminescence excitation and the optical absorption coefficient identify the Fe3+/2+ charge-transfer as the main contributor to the 300 K optical absorption in the range 400-500 nm. Practical implications of these results for Fe-doping determination are discussed.