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Vertical GaN power switch technology is expected to be utilized in next-generation medium to high voltage power converters due to the low ON-resistance and high breakdown voltage enabled by the improved critical electric field and mobility compared to Si and SiC-based devices. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth, allowing recent realization of several fundamental vertical power devices, including diodes with edge termination, trench MOSFETs, and CAVETs. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow vertical GaN power devices to achieve their full potential, notably the realization of repeatable thick drift layers with low background doping. To enable this, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for MOCVD growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. We have characterized multiple substrates from over eight suppliers and the homoepitaxy grown on those substrates. All the substrates had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling to detection of different concentrations of grain boundaries, sample impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conducive regions. The substrates can be characterized in two different categories: those with uniform characteristics, including carrier concentration, and those without. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Photoluminescence of the homoepitaxial surface along with Raman spectroscopy show that non-uniformities in the substrate carrier concentration can continue into the epitaxy. Following optical characterization, vertical Schottky diodes were fabricated to quickly evaluate device performance. While most of the films showed the ability to withstand high electric fields, those with more uniform properties in the substrate also showed more uniform electrical properties. These results show that the bulk substrates enable the path to high voltage vertical devices, but also show the significant influence that substrates can play in device performance.

Activation of implanted donors into a highly-resistive, nitrogen-doped homoepitaxial β-Ga2O3 has been investigated. Nitrogen acceptors with the concentration of ∼1017 cm−3 were incorporated during epitaxial growth yielding low-doped (net donor concentration 14 cm−3) films subsequently implanted with Si, Ge, and Sn. Upon Ohmic contact formation to the implanted regions, sheet resistance values of 314, 926, and 1676 Ω/sq were measured at room temperature for the Si-, Ge-, and Sn-implanted samples, respectively. Room temperature Hall measurements resulted in sheet carrier concentrations and Hall mobilities of 2.13 × 1014 /93, 8.58 × 1013/78, and 5.87 × 1013/63 cm2/(V s), respectively, for these three donor species. Secondary ion mass spectroscopy showed a volumetric dopant concentration of approximately 2 × 1019 cm−3 for the three species, resulting in carrier activation efficiencies of 64.7%, 40.3%, and 28.2% for Si, Ge, and Sn, respectively. Temperature-dependent Hall effect measurements ranging from 15 to 300 K showed a nearly constant carrier concentration in the Si-implanted sample, suggesting the formation of an impurity band indicative of degenerate doping. With a bulk carrier concentration of 1.3 × 1019 cm−3 for the Si implanted sample, a room temperature mobility of 93 cm2/(V s) is among the highest reported in Ga2O3 with a similar carrier concentration. The unimplanted Ga2O3:N regions remained highly resistive after the surrounding areas received implant and activation anneal. These results open the pathway for fabricating Ga2O3 devices through the selective n-type doping in highly resistive epitaxial Ga2O3.

Low ON-resistance and high breakdown voltage, made possible by improved critical electric field and mobility, has promoted GaN from a potential to the most promising semiconductor material for use in the next-generation of medium- and high voltage-power converters. The commercial availability of large area substrates, produced by HVPE (Hydride Vapor Phase Epitaxial, a quasi-bulk deposition technique) and ammonothermal bulk growth method, provides wafers with reduce concentrations of extended defect, allows the deposition of electronic grade epitaxial films and the realization of high performance electronic devices. Despite that, many steps of substrate preparation, such as miss-cut orientation and surface finishing, must be improved and standardized to yield reproducible epitaxial film growth. To verify the importance of substrate characteristics on the intrinsic properties of homoepitaxial MOCVD films, we initially evaluated substrate provided by various commercial supplies. The substrate effects on epi morphology, uniformity, and impurity incorporation were substantiated by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. All evaluated substrates had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling the detection of different concentrations of grain boundaries, impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conductive regions. The substrates can be grouped in two different categories: those with uniform characteristics, including carrier concentration, and those without. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Photoluminescence of the homoepitaxial surface along with Raman spectroscopy show that non-uniformities in the substrate carrier concentration can continue into the epitaxy. If time permits, results from vertical Schottky diodes, fabricated to quickly evaluate device performance, will presented. While most of the films showed the ability to withstand high electric fields, more uniform electrical properties were observed for those grown on substrates having more uniform properties. These results show that improving bulk substrates is the path to high voltage vertical devices, and that such substrates have a significant influence on device performance. Acknowledgments: Work at NRL is supported by the Office of Naval Research

Hexagonal boron nitride (hBN) is a wide, indirect bandgap semiconductor that holds great promise for optoelectronic devices in the ultraviolet and mid-infrared spectral regimes. The efficiency of o...

Plasma-enhanced atomic layer deposition (PE-ALD) is a promising method to grow conformal, epitaxial films at low temperatures (

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.

An all-epitaxial approach was demonstrated to create coaxial plasmon laser structures composed of an aluminum plasmonic metal/SiNx dielectric/InGaN quantum well shell surrounding a p-GaN nanowire core. Strong UV luminescence was observed from as-grown vertically-aligned arrays as well as horizontally-aligned nanowires transferred to a transparent carrier wafer. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silver nanoparticles dispersed on the surface of an inverted GaN LED were found to plasmonically enhance the nearbandedge emission. The resonant surface plasmon coupling led to a significant enhancement in the exciton decay rate and the ensemble of nanoparticles provided a mechanism to scatter the coupled energy as free space radiation. The inverted LED structure employed a tunnel junction to avoid the standard thick p+ GaN current spreading contact layer. In contrast to a standard design, the top contact was a thin n++ AlGaN layer, which brought the quantum well into the fringing field of the silver nanoparticles. This proximity allowed the excitons induced within the quantum well to couple to the surface plasmons, which in turn led to the enhanced band edge emission from the LED.

Thin monoclinic Ga2O3 films were deposited on c-plane sapphire substrates by low pressure chemical vapor deposition. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen gas (O2) as precursors. The effect of oxygen volume percentage on the growth rate of thin films was observed at two growth temperatures. Within the investigated growth window, a maximum growth rate of ∼2.9 μm/h was obtained for an oxygen volume percentage of 4.8% with a growth temperature at 800 °C. The film growth rate decreased as growth temperature increased when other growth parameters were kept the same. X-ray diffraction indicates that all films have the β-Ga2O3 structure with (−201) orientation, and those deposited with higher oxygen partial pressure are thicker and have improved crystalline quality. Polarized micro-Raman scattering is consistent with small grains of (−201) β-Ga2O3 having random in-plane orientations. The large variation of the relative intensities of overlapping emission bands contributing to the broad luminescence emission extending between 1.5 and 4.5 eV (∼825 and 275 nm) suggest that deposition conditions strongly affect different defect concentrations. Films deposited at 800 °C with a higher oxygen partial pressure yielded higher resistance, which may result from the incorporation of gallium vacancies, identified as a compensating point defect affecting the electrical conductivity of bulk monoclinic Ga2O3.

β-Ga2O3 is a promising ultrawide bandgap semiconductor for next generation radio frequency electronics. However, its low thermal conductivity and inherent thermal resistance provide additional challenges in managing the thermal response of β-Ga2O3 electronics, limiting its power performance. In this paper, we report the heteroepitaxial growth of β-Ga2O3 films on high thermal conductivity 4H-SiC substrates by molecular beam epitaxy (MBE) at 650 °C. Optimized MBE growth conditions were first determined on sapphire substrates and then used to grow β-Ga2O3 on 4H-SiC. X-ray diffraction measurements showed single phase ( 2 ¯ 01 ) β-Ga2O3 on (0001) SiC substrates, which was also confirmed by TEM measurements. These thin films are electrically insulating with a ( 4 ¯ 02 ) peak rocking curve full-width-at-half-maximum of 694 arc sec and root mean square surface roughness of ∼2.5 nm. Broad emission bands observed in the luminescence spectra, acquired in the spectral region between near infrared and deep ultraviolet, have been attributed to donor-acceptor pair transitions possibly related to Ga vacancies and its complex with O vacancies. The thermal conductivity of an 81 nm thick Ga2O3 layer on 4H-SiC was determined to be 3.1 ± 0.5 W/m K, while the measured thermal boundary conductance (TBC) of the Ga2O3/SiC interface is 140 ± 60 MW/m2 K. This high TBC value enables the integration of thin β-Ga2O3 layers with high thermal conductivity substrates to meliorate thermal dissipation and improve device thermal management.

Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm−3 and 1019 cm−3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor–shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm−3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

Epitaxial growth of monoclinic β-Ga2O3 on a-plane and c-plane sapphire substrates by metalorganic vapor-phase epitaxy (MOVPE) is reported. Crystalline phase, growth rate (∼150 nm/h), and energy gap (∼4.7 eV) were determined by x-ray diffraction and optical reflectance measurements. Film density of ∼5.6 g/cm3 measured by x-ray reflectivity suggests the presence of vacancies, and the O-rich growth regime implies the presence of Ga vacancies in the films. O/Ga ratio of 1.13, as measured by XPS for Ga2O3 grown on c-plane Al2O3, suggests that, near the surface, the film is O-deficient. Atomic force microscopy revealed smoother, smaller grain size when films were grown on c-plane Al2O3. Raman spectroscopy suggested inclusions of α-Ga2O3, likely present at the sapphire interface due to growth on nonnative substrate. Samples of β-Ga2O3 were selectively implanted with Si in the source/drain regions and subsequently annealed at 1000°C for 10 min. Normally-off transistors (V T ≅ 4.7 V) with 20-nm-thick Al2O3 gate oxide were fabricated, and a maximum drain–source current of 4.8 nA was measured.

Single crystal (−201) β-Ga2O3 substrates doped with Si and Sn have been thermally annealed in N2 and O2 atmospheres. Structural and electrical properties evaluation was performed via a number of experimental methods in order to quantify the effects of the doping and annealing ambient on the properties of these samples. All samples annealed in O2 exhibited significantly lower carrier concentration, as determined by capacitance–voltage measurements. Schottky barrier diodes exhibited excellent rectification when the Ga2O3 was annealed in N2, and significantly lower forward current using O2-annealed Ga2O3 substrates. Deep level transient spectroscopy revealed four deep trap levels with activation energies in the range of 0.40–1.07 eV. Electron spin resonance showed a decrease in shallow donor concentration, and cathodoluminescence spectroscopy revealed nearly two orders of magnitude lower emission intensity in O2-annealed Ga2O3 samples. Raman spectroscopy revealed a carrier concentration dependent Raman mode around 254 cm−1 observed only when the final anneal of (−201) β-Ga2O3 was not done in O2. Secondary ion mass spectroscopy measurements revealed diffusion of unintentional Fe towards the surface of the (−201) Ga2O3 samples after annealing in O2. Depth resolved positron annihilation spectroscopy showed an increased density of vacancy defects in the bulk region of O2-annealed Ga2O3 substrates.

The increasing need for devices capable of performing under extreme condition of power and temperature, and opto-electronic devices operating in the UV-Vis spectral range has been the principal driving force for research interest in III-Nitrides semiconductors over the last three decades. Progress in bulk growth and film deposition techniques for homo- and hetero-epitaxial growth of III-Nitride semiconductors has led to successful fabrication and commercialization of an increasing number of devices that are revolutionizing both the optoelectronic and electronic device industries. Many relevant figures of merit related to electronic device performance scale superlinearly with the band gap; hence, ultra-wide band gap (UWBG) materials (Eg > 3.4 eV) have the potential for far superior performance than conventional WBG materials such as GaN and SiC. UWBG materials will be critical enablers of future electronic components and systems, and a fundamental understanding of their synthesis and properties is critical to exploiting their advantages. In particular, cubic boron nitride (c-BN), among the semiconductor materials in the UWBG group (c-BN, AlN, Ga2O3, and diamond), with a band gap of ~6 eV and thermal conductivity of 13 W/mK (second only to diamond, 20 W/mK) offers the best combination of breakdown voltage and specific on-resistance. Another advantage of c-BN over other UWBG semiconductors is that it can be doped with donors and acceptors, enabling the fabrication of bipolar devices. The development of high quality GaN substrates and device grade films will be reviewed and a brief overview of the intrinsic properties of the WBG and UWBG semiconductors will be presented.

We present novel approaches for the development of Ga2O3 Schottky barrier and heterojunction diodes. Samples of β- Ga2O3 were first annealed in N2 and O2 to demonstrate the effect of annealing on the carrier concentration. Cathodoluminescence and electron spin resonance measurements were also performed. Schottky barrier diodes on asgrown and O2-annealed epitaxial Ga2O3 films were fabricated and breakdown voltages were compared. Lower reverse current and a breakdown voltage of about 857 V were measured on the O2-annealed device. Finally, we report preliminary results from the development of anisotype heterojunctions between n-type Ga2O3 with a sputtered NiO layer. Rectifying current-voltage characteristics were obtained when the NiO was deposited both at room temperature and at 450 °C.

Elimination of basal plane dislocations (BPDs) in epitaxial 4H-SiC is demonstrated via a novel pulsed annealing technique in a moderate N2overpressure of 0.55 MPa. BPD removal in 15 µm thick epitaxial 4H-SiC was confirmed using ultraviolet photoluminescence (UVPL) imaging before and after the annealing process. The samples were capped with a carbon cap, introduced into the annealing chamber, and brought up to a base temperature (TBASE) of around 1550 °C for the pulsed anneal. The multicycle rapid thermal anneal (MRTA) was then performed in the TBASE:TMAXrange, where TMAX= 1875 °C was the peak temperature reached by the annealing cycles. Post-anneal surface quality and carrier lifetime were characterized by atomic force microscopy and time-resolved photoluminescence decay.

Ammonothermal Gallium Nitride (GaN) substrates are the most promising substrates for homoepitaxial growth of GaN films having low dislocation density. Growth-induced structural inhomogeneities in these substrates were investigated by high-resolution X-ray topography (HR-XRT) and high-resolution X-ray diffraction (HR-XRD). A one-to-one correlation of defects was observed in photoluminescence imaging. From the HR-XRD intrinsic rocking curve widths were found to be 16 arcsecs indicating superior crystalline quality. The lattice constants were measured from symmetric and asymmetric reflections to be similar to bulk values indicting low sample strian. The true dislocation density, from the HR-XRT images, was observed to be of the order of 102 cm–2 in the samples, and the radius of curvature was greater than 600 m. Growth striations were observed in the a and m-pane samples and are attributed to inhomogeneity in impurity incorporation during growth. Photoluminescence imaging showed deep level luminescent center...

Optical, electrical, and mass spectrometry experiments were carried out on solution grown GaN crystals to detect and identify the impurities incorporated in faces of different polarity at different growth condition. Selective etching, photoluminescence and cathodoluminescence imaging indicate that Mg acceptors incorporate preferentially in GaN nitrogen polar face. Larger incorporation of Mg was observed on crystals grown at lower temperatures. Capacitance–voltage measurements verified p-type conductivity due to an excess of Mg acceptors on N-polar face of the GaN crystals. These results suggest that epitaxial layers deposited from liquid phase may be the best approach to produce p-type conducting layers with high hole concentration and low defect density.

Vertical GaN power switch technology is expected to be utilized in next-generation medium to high voltage power converters due to the low ON-resistance and high breakdown voltage enabled by the improved critical electric field and mobility compared to Si and SiC-based devices. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, the properties of nitrides are no longer dominated by defects introduced by heteroepitaxial growth, allowing recent realization of several fundamental vertical power devices, including diodes with edge termination, trench MOSFETs, and CAVETs. However, additional materials challenges are coming to the forefront that need to be understood and surmounted in order to allow vertical GaN power devices to achieve their full potential, notably the realization of repeatable thick drift layers with low background doping. To enable this, a deeper understanding of substrate preparation and the effects of the substrate and growth initiation on the characteristics of the epitaxial layers is required for MOCVD growth of homoepitaxial films. We investigate these effects on epi morphology, uniformity, and impurity incorporation by growing simultaneously on wafers from different vendors. The goal of this work is to detect and identify defects in GaN substrates with a series of quick, non-destructive, inexpensive techniques with capabilities of mapping whole wafers. Four different substrates - three grown by hydride vapor phase epitaxy and the fourth by ammonothermal technique were obtained from multiple vendors. All had nominally similar as-received specifications (resistivity, thickness, off-cut angle, bow, surface finish). The substrates were evaluated with a variety of techniques including Raman spectroscopy, photoluminescence, white light interferometry, and Nomarski imaging, enabling to detection of different concentrations of grain boundaries, sample impurities, point defects, v-shaped pits, polishing defects, crystal stress damage, and non-uniform insulating and conducive regions. Comparing these results to those from homoepitaxial growth on the same wafers, the effects are both subtle and overt. Macroscopic surface morphology, which has shown a direct correlation to leakage current, copies and exaggerates that of the underlying substrate. Donor incorporation in the epitaxy is relatively uniform, regardless of the substrate doping. However, homoepitaxy on substrates with inhomogeneous impurity incorporation did show some near band edge broadening by PL. Carbon incorporation varied with substrate offcut angle, which is supported by DFT calculations. All interfaces between epitaxy and substrate have a large Si peak, which requires further study. Following optical characterization, vertical Schottky diodes were fabricated to quickly evaluate device performance. While most of the films showed the ability to withstand high electric fields, those with more uniform properties in the substrate also showed more uniform electrical properties. These results show that the bulk substrates enable the path to high voltage vertical devices, but also show the significant influence that substrates can play in device performance.

Cubic boron arsenide (c-BAs) exhibits an ultrahigh thermal conductivity (κ) approaching 1300 Wm−1 K−1 at room temperature. However, c-BAs is believed to incorporate high concentrations of crystal imperfections that can both quench κ and act as sources of unintentional p-type conductivity. Although this behavior has been attributed to native defects, we show here, using optical and magnetic resonance spectroscopies together with first-principles calculations, that unintentional acceptor impurities such as silicon and/or carbon are more likely candidates for causing the observed conductivity. These results also clarify that the true low-temperature bandgap of c-BAs is 0.3 eV higher than the widely reported value of ∼1.5 eV. Low-temperature photoluminescence measurements of c-BAs crystals reveal impurity-related recombination processes (including donor-acceptor pair recombination), and electron paramagnetic resonance experiments show evidence for effective mass-like shallow acceptors. Our hybrid density functional calculations indicate that native defects are incapable of giving rise to such signals. Instead, we find that group-IV impurities readily incorporate on the As site and act as shallow acceptors. Such impurities can dominate the electrical properties of c-BAs, and their influence on phonon scattering must be considered when optimizing thermal conductivity.

4H-SiC samples irradiated with high energy protons were probed by low temperature photoluminescence (PL) and room temperature micro-Raman scattering spectroscopies. The quench of the near band-edge emission and the presence of a number of new sharp lines in the luminescence spectra of the proton-irradiated samples confirm the formation of various new defects. The changes of the line-shape and peak position of the longitudinal optical phonon-plasmon coupled (LOPC) mode in 4H-SiC are consistent with the decrease in the free carrier concentrations due to the introduction of carrier traps induced by the high energy proton irradiation. The estimated penetration depths for 6 and 8 MeV energy proton were 180 μm and 300 μm, respectively, which are in good agreement with the Monte Carlo numerical simulation results. At the 180 μm and 300 μm depths, the carrier concentrations were reduced by approximately 34% and 21%, respectively.

Gallium nitride’s (GaN) material properties of broadband transparency, high thermal conductivity, and wide-band gap make it a promising candidate for high-power frequency conversion devices. The strong internal polarization of GaN leads to large second-order nonlinearity, but conventional phase matching is prevented due to weak birefringence. To obtain efficient nonlinear optic frequency conversion, patterned inversion growth has been developed to induce quasiphase matching (QPM). We have fabricated and tested periodically oriented GaN (PO-GaN) devices to obtain QPM frequency conversion. This report discusses our recent measurements of second harmonic generation resonances for these devices.

The development and characterization of nitride QW structures grown by atomic layer epitaxy (ALEp) for device applications are discussed. We have grown epitaxial thin films (4-10nm) covering the full range of binary and ternary III-N compositions by ALEp. In this work, ALEp-grown QW structures are presented. Optical characteristics are discussed. Characterization of layer interfaces and composition are critical to the development of this growth technique for quantum-based devices. Structures to study this by atom probe tomography have been created. By understanding the structure of crystalline ALEp films with nanometer-scale thickness, the unique properties of these materials can be advanced for quantum-scale applications.

Periodically oriented GaN (PO-GaN) devices have been fabricated and tested in order to obtain quasi-phase matched frequency conversion. This report discusses recent measurements of second harmonic generation resonances and characterization of the PO-GaN devices.

Boule of GaN crystal was grown with 50 mm diameter and 3 mm thickness by hydride vapor phase epitaxy and cut using a wire saw to produce freestanding wafers. These were, mechanically polished with diamond slurry and followed with chemical mechanical polishing for final surface preparation. Surface morphology was examined by non-contact mode atomic force microscopy before and after thermal annealing process performed at 700, 800, 900, and 1000 °C for 1 h, in air. Wafers with optimum surface quality submitted to thermal annealing treatment at 900 °C were characterized by reduced scratched density and residual stress, and surface roughness of 0.096 nm. Surface quality improvement was confirmed by relatively large recovery of the room temperature near band edge luminescence intensity.

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

No reliable results were reported up-to-date on electrical activation of Mg implanted GaN without co-doping with other ions. The main reason of the poor ion-implanted activation in GaN is lack of the adequate GaN annealing technique. We have developed a new approach, Multicycle Rapid Thermal Annealing to overcome this limitation and enable longer annealing times at high temperature. We have applied this new technique to Mg-implanted GaN, and demonstrated p-type conductivity.

The morphology, structural and optical properties of void-assisted freestanding HVPE-AlN films were investigated by a combination of non-destructive microscopic and spectroscopic techniques. The freestanding approximately 80 μm thick clear film has a wurtzite crystalline structure with remarkable properties around the central film region. The E 2 (high)-phonon frequency coincides with reported stress-free film phonon frequency. The low temperature luminescence study of the growth and interface sides of the film is consistent with the incorporation of a high concentration of oxygen impurities. These results are promising as the growth method amenable to the production of freestanding stress-free large area substrates for epitaxial growth.

The structural and optical properties of self-nucleated crystals grown by a near atmospheric pressure solution growth method are presented. High-resolution room temperature Raman scattering studies demonstrate that stress-free crystals with low free-electron background have been produced. Low and room temperature photoluminescence experiments confirm the presence of shallow donors and an unknown shallow acceptor. Large relative intensity variations of the emission bands assigned to recombination process involving donors and acceptor, resulting from significant changes in the incorporation and/or activation of defect associated with each recombination channel, reflect major changes in the intrinsic material properties.

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm 3 , or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm 3 ) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c -axis, with approximate g -values, g ∥ =1.951 and g ⊥ =1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.

Cubic boron arsenide (c-BAs) has been predicted to exhibit thermal conductivity values around 1400 W/m-K at room temperature by first-principles calculations and, thus, shows promise to be one of the solutions for the thermal management challenge faced by the semiconductor industry. However, the size of single crystals of c-BAs in reported growth efforts is still less than 1 mm. In addition, the measured thermal conductivity is well below the predicted value due to the presence of defects in the grown crystal. Here we report the growth of c-BAs single crystals up to a few mm by the chemical vapor transport technique based on a TeI4 transport agent under gas pressures up to 8 atm. Raman spectroscopy revealed a sharp P1 phonon mode, suggesting good crystalline quality. High-angle annular dark-field scanning transmission electron microscopy revealed the AsB (As-atom on B) and BAs (B-atom on As) antisite pairs. A bulk thermal conductivity value of 133 ± 17 W/m-K at 300 K was obtained using steady-state comparative measurements. Although the bulk thermal conductivity is still one order of magnitude lower than the calculation, the ability to increase the size of the crystal is an important step toward the synthesis of high quality bulk c-BAs crystals.

We have grown c-BAs single crystals up to 1000 μm size by the chemical vapor transport (CVT) technique using combined As and I2 transport agents with the As:I ratio of 1:3 under gas pressures of up to 35 atm. Raman spectroscopy revealed a very sharp (∼2.4 cm−1) P1 phonon mode and an interesting splitting behavior of P1 from detailed polarization studies. Electron paramagnetic resonance (EPR) experiments revealed no evidence for EPR active growth-related defects under the experimental resolution. Finally, a moderate thermal conductivity value of ∼132 W/m-K was obtained using a transient thermal grating technique. These results suggest that although the high As gas vapor pressure environment in CVT growth can increase the transport rate of c-BAs significantly, it may not be efficient in reducing the defects and enhancing the thermal conductivity in c-BAs significantly.

Monoclinic β-Ga2O3 has attracted significant attention as a high power electronics material based on its ultra-wide energy gap (4.9 eV), high theoretical critical field (8 MV/cm), as well as the commercially availability of inexpensive substrates grown from the melt. In order to achieve breakdown field consistent with the theoretical expectations, however, very high quality epitaxial films with very low doping and defect density will be required. Wong et al. have reported a β-Ga2O3 MOSFET with carrier concentration below 4x1014 cm-3, which is the lowest reported value for epitaxial β-Ga2O3 to date [1]. This talk will discuss the structural and electronic properties for thick homoepitaxial β-Ga2O3 with doping as low as 8x1012 cm-3, corresponding to breakdown voltage in excess of 2.38 kV (3.18 MV/cm), measured without field termination for vertical measurements or edge effects correction in the case of lateral devices. Our results show that current β-Ga2O3 halide vapor phase epitaxial technology can produce epilayers with breakdown field approaching the theoretical value of GaN. In addition, we will discuss the annealing of bulk and epitaxial β-Ga2O3 films in O2 atmosphere as a potential pathway to reduce carrier concentration and possibly improve the breakdown voltage of Ga2O3 Schottky diodes. We show that annealing Ga2O3 in O2 will reduce ND-NA by about an order of magnitude, similar to earlier observations by Kuramata et al. [2]. Furthermore, we will present our approach for overcoming the polaron-induced suppression of room-temperature p-type conductivity in Ga2O3, causing holes to preferentially self-trap instead of being available as free carriers [3]. We show that natural p-type semiconducting oxides such as NiO can be integrated with β-Ga2O3 to form a rectifying heterojunction as a fundamental building block for any viable Ga2O3 power device. Finally, we will discuss the low thermal conductivity of Ga2O3 and present our preliminary thermal management simulations and experimental results. [1] A. Kuramata, et al., Jpn. Jour. Appl. Phys. 55, 1202A2 (2016). [2] M.H. Wong, et al., Appl. Phys. Express 10, 041101 (2017). [3] J.B. Varley, et al., Phys. Rev. B 85, 081109(R) (2012).

The effect of various types of in-grown stacking faults and threading screw/edge type dislocations on carrier lifetime and diffusion lengths in 4H-SiC epitaxial films was investigated through cathodoluminescence decays and charge collection efficiencies of electron beam induced current signals at specific defects sites. Most stacking faults yielded ~40% reduction in the carrier lifetime. Moreover, drastic lifetime reductions were observed in regions containing surface triangular defects and bulk 3C polytype inclusions. Dislocations of both types serve as efficient recombination centers, though stronger reduction in diffusion lengths was observed in the vicinity of screw type dislocations.

A polariton-based light emitter is a extraordinary concept as an alternative to a light-emitting diode (LED) or laser diode. The physics of a polariton laser is fundamentally different from the spontaneous emission process of an LED or the inversion and stimulated emission process of a laser diode. The rapid decay and emission from this polariton–exciton state bypasses the normal irreversible spontaneous emission and associated non-radiative decay mechanisms. An AlGaN/AlN nucleation bilayer was employed on r -plane sapphire to deposit non-polar GaN quantum wells embedded in an AlGaN-based cavity surrounded by top and bottom distributed Bragg reflectors (DBRs). The reflectance data show that the exciton and photon states can be tuned (by changing the angle of the sample) to the same energy. The characteristic strong coupling was observed in the reflectance data where the states of the exciton and photon do not overlap; rather they split into an upper polariton state and a lower polariton state. The photoluminescence (PL) showed a strong emission at a low stimulation level at a similar energy and angle.

Although β-Ga2O3 thin films and nanowires (NWs) show promise as very stable and reliable active components for high temperature gas sensors, their use at room temperatures is limited due to poor electrical conductivity. To address this problem, we grew Sn-doped β-Ga2O3 nanowires by the vapor−liquid−solid (VLS) approach. Sn-doped β-Ga2O3 NWs with diameters of 100−250 nm retained the monoclinic β-Ga2O3 structure, though photoluminescence (PL) emission was red-shifted by up to 50 nm relative to the deep defect band typically observed for pure β-Ga2O3 NWs. When higher amounts of Sn were introduced, individual Ga2O3−SnO2 heterostructures (HS) self-assembled, to form three distinctive parts: monocrystalline Sn-doped β-Ga2O3, poorly crystalline Sn-doped β-Ga2O3, and polycrystalline Ga-doped SnO2, thus realizing a p-n junction within a single HS. Factors responsible for the self-assembly of Ga2O3−SnO2 HS are the different vapor pressures of Sn and Ga and different growth kinetics of Ga2O3 and SnO2. Inhomogeneity ...

Cathodoluminescence real-color imaging and spectroscopy were employed to study the properties of Ga(2)O(3) nanowires grown with different Sn/Ga ratios. The structures grown under Sn-rich conditions show large spectral emission variation, ranging from blue to red, with a green transition zone. Spectral emission changes correlate with changes in the chemical composition and structure found by energy dispersive spectroscopy and electron diffraction. A sharp transition from green to red emission correlates with a phase transition of beta-Ga(2)O(3) to polycrystalline SnO(2). The origin of the green emission band is discussed based on ab initio calculation results.

Photo- and cathodo-luminescence measurements of a variable-diameter ensemble of GaN nanowires revealed a diameter-dependent, spectral emission distribution between 350 nm and 850 nm. Spectral analysis indicated that wires with a diameter less than 400 nm were dominated by a yellow luminescence with a weaker near UV/violet emission also present. Examination of this ensemble showed that there was a general trend in the ratio of near-UV-to-yellow emission intensities with increasing nanowire diameter. Additionally, a broad green emission appears in the nanowires with a diameter above approximately 200 nm. A calculation based on the nanoheteroepitaxy model indicates that this diameter represents a transitional thickness where strain is relieved by defect formation mechanisms with a characteristic green emission.

A novel erbium chloride seeded growth process was developed for the growth and fabrication of erbium doped GaN nanocrystals. This erbium chloride seeded technique simplifies the delivery of erbium into a metal organic chemical vapor deposition system. Additionally, this selective growth of the GaN nanocrystal only occurred in the presence of the ErCl 3 seed. Strong green emission, distinctive of the Er 3+ ion, was observed in the GaN nanocrystals.

The behavior of the D1 center in semi-insulating 4H-SiC substrates revealed by low-temperature photoluminescence was investigated after post-growth high temperature anneals between 1400 and 2400oC. The influence of different post-anneal cooling rates was also studied. The optical signature of D1 was observed up to 2400oC with intensity maxima at 1700 and 2200oC. We propose that the peak at 1700°C can be related to the formation and subsequent dissociation of SiC native defects. It was found that changes in the post-annealing cooling rate drastically influence the behavior of the D1 center and the concentrations of the VC, VSi, VC-VSi and VC-CSi lattice defects.