David J. Smith

We report on the fabrication and temperature-dependent characterization of metalorganic chemical vapor deposition-grown quasi-vertical AlN Schottky barrier diodes (SBDs) on bulk AlN substrates. The SBDs exhibited high current densities exceeding 2 kA/cm2 at 10 V, with a turn-on voltage of ∼3.0 V. Cross-sectional TEM analysis revealed a thin (∼5 nm) AlNxOy interfacial layer formed between the AlN and the Ni contact. Stable rectifying operation was maintained up to 300 °C, and the temperature dependence of the Schottky barrier height and ideality factor was characterized. Capacitance voltage measurements showed strong temperature dependence, resulting in an apparent increase in the net donor concentration from ∼5 × 1017 cm−3 at 300 K to ∼1 × 1018 cm−3 at 373 K. Temperature-dependent reverse-bias characteristics were consistent with Poole–Frenkel emission, with a trap energy of ∼0.34 eV.

We report plasma-assisted molecular beam epitaxial growth of AlBN thin films on a nitrided c-plane Al2O3 substrate. The AlBN film epitaxially grows in rotational alignment with an out-of-plane/in-plane directions of AlBN [0001¯]/[101¯0]‖AlN nucleation layer [0001¯]/[101¯0]‖Al2O3[0001]/[112¯0]. The B composition of the AlBN layer is varied from 0% to 15% by varying the growth temperature, exploiting the reaction rate-controlled growth mechanism. X-ray diffraction and high-resolution transmission electron microscopy are used to determine the structural properties as a function of boron composition. A monotonic decrease in the c-lattice constant and a non-monotonic change in the a-lattice constant are observed with an increase in the B content in AlBN films grown on nitrided sapphire. While the control AlN film showed a bandgap of 6.1 eV, the AlBN films with ∼15% boron showed a bandgap of 5.9 eV. The AlBN films with 15% B exhibit a fivefold increase in the nonlinear second-harmonic generation intensity compared to AlN. AlBN films exhibit higher thermal conductivity than AlScN films with comparable alloy compositions, and at equal or smaller thicknesses. The findings indicate several opportunities for AlBN films in applications of deep UV optoelectronics, nonlinear photonics, and high-power electronics devices, especially in high-voltage and high-temperature environments.

Cubic BN (c-BN) is attractive for high-power and extreme-environment electronics. However, thin-film synthesis is challenging due to its propensity to form mixed-phase microstructures of sp2-and sp3-bonded polytypes. We combine time-domain thermoreflectance and time-resolved magneto-optic Kerr effect measurements to determine the effect phase composition, bond disorder, and interfacial structure have on thermal transport of BN thin-films grown on boron-doped diamond by plasma-enhanced chemical vapor deposition. High-resolution transmission electron microscopy reveals BN film grown on (001) diamond is turbostratic (t-BN), while the film grown on (111) diamond is c-BN. At 300 K, the t-BN film has a thermal conductivity of 17 ± 2.5 W m−1 K−1, while the c-BN film has a graded conductivity that varies between 5 and 20 W m−1 K−1 as a function of depth. The c-BN film’s thermal conductivity is ≈ 40× lower than that of bulk c-BN, indicating that defects limit phonon mean-free-paths to ∼10 nm. The c-BN/diamond interface conductance is ≈300 +/− 140/100 MW m−2 K−1, about threefold higher than t-BN/diamond. We credit this factor of three difference to interfacial bonding caused in the sp2 vs sp3 bonded polytypes.

In this work, we report on the design and fabrication of p-NiO/Ga2O3 trench junction barrier schottky diodes (JBSDs) integrated with space-modulated junction termination extension (SM-JTE) and compare the performance with planar Ni/Ga2O3 schottky barrier diodes (SBDs) and p-NiO/Ga2O3 heterojunction diodes (HJDs). The JBSDs achieved breakdown voltages exceeding 1.8 kV along with low leakage currents (<10−2 A/cm2), while displaying a low turn on voltage (VON) of ∼1 V, which is similar to that of planar Ni/Ga2O3 SBDs. The fabricated devices showed excellent forward characteristics with low differential on-resistance (Ron,sp) ranging from 4 to 10.5 mΩ cm2, for fin width between 0.6 and 1.25 μm. The best performing device with fin width of 0.85 μm showed a unipolar figure of merit of ∼0.7 GW/cm2. This work showcases the benefits of trench JBS design along with SM-JTE edge-termination for efficient high-performance kilovolt-class β-Ga2O3 diodes.

Devices based on ultrawide bandgap heterostructures are attracting increased attention for applications that require high fields or high-temperature environments. In this article, we demonstrate an ultrawide bandgap BN/AlN metal-insulator-semiconductor Schottky diode on single-crystal AlN substrates, where the AlN epilayers were grown via metalorganic chemical vapor deposition, and the BN layer was deposited using microwave plasma chemical vapor deposition. The BN/AlN heterostructure was characterized by x-ray photoelectron spectroscopy, transmission electron microscopy, and electron-energy-loss spectroscopy. Capacitance-frequency measurements indicated a low interface state density of 0.67–3.55 × 1011 eV−1 cm−2 at the BN/AlN interface. At forward bias, the device showed good rectifying behavior. At reverse bias, 2D variable range hopping and trap-assisted tunneling were found to be the dominant mechanisms. This work provides valuable guidance for developing ultrawide bandgap nitride heterostructures.

Diamond electronics has attracted attention for high power and high frequency device applications. Cubic boron nitride (c-BN) may be considered as a suitable dielectric layer for electron channel diamond metal–insulator–semiconductor field effect transistors (MISFETs) provided that its valence band edge can be positioned above that of diamond. This study reports experimental measurement of the valence band offset (VBO) between c-BN and nitrogen-plasma terminated boron-doped diamond (111). Nitrogen plasma processing was used to produce C–N bonding at the diamond surface. Electron cyclotron resonance plasma enhanced chemical vapor deposition was then used to deposit epitaxial c-BN films on the N-terminated diamond substrate, as confirmed by cross-sectional high-resolution electron microscopy. X-ray and ultraviolet photoemission spectroscopies indicated that the valence band maximum of c-BN is positioned 0.4 eV above that of diamond resulting in a type II staggered band alignment. This result is consistent with theoretical predictions of the VBO between the two materials in the (111) surface orientation, indicating that c-BN with C–N interface bonding can be used as a dielectric layer for electron channel diamond (111) MISFET devices.

Epitaxial β-Ga ₂ O ₃ films grown on (001) diamond substrates with a β-(Al/Ga) ₂ O ₃ buffer layer exhibit strong texture with multiple rotational domain variants sharing a common growth axis. This texture is attributed to two interrelated structural and geometrical factors: (1) pseudo-symmetry about the growth direction due to the high symmetry of the oxygen sublattice which results in close interplanar spacings of different lattice planes that determine the in-plane lattice mismatch, and hence two different crystallographic relationships and types of domain variants; (2) the lack of higher-order symmetry in the C 2/ m monoclinic structure of β-Ga ₂ O ₃ and the higher symmetry of the diamond substrate that leads to various subvariants. The microstructure of the films consists predominantly of domain clusters, with domains rotated by either ∼60° or 120° (±10°) relative to each other about the growth axis ∼[ 1 02]. Some domain boundaries (DBs), visible near edge-on in cross-sectional and plan-view projections, exhibit a high degree of coherency. These highly coherent DBs with small lattice rotation and/or distortion near the DB are observed where small in-plane and off-plane DB lattice mismatch is expected. Larger lattice mismatch between domains is accommodated by relatively large lattice rotation and/or distortion near the DB, as well as changes in DB structure and shape. Understanding the origin of texture and the characteristics of common planar defects in β-Ga ₂ O ₃ will offer insights into their impact on thermal and electrical transport properties and enable effective microstructural optimization for successful integration into future power electronics.

Diamond is an attractive substrate choice for the growth of gallium oxide for high-power device applications due to its high thermal conductivity. However, the cubic crystal structure of diamond is very different from that of monoclinic β-Ga2O3. Adding to the complexity, there is a large surface energy difference between the two materials, and a high-temperature oxide environment is required for Ga2O3 growth. Here, we demonstrate a viable pathway that enables thick adherent Ga2O3 layers to be grown epitaxially on diamond (001) substrates, using metal–organic chemical vapor deposition. This growth is achieved using an intermediary (Al0.06Ga0.94)2O3 (AGO6) buffer layer. Omission of the strain management buffer results in Ga2O3 films that exfoliate into membranes for thicknesses greater than about 500 nm. Implementation of the buffer layer allows for the alleviation of the excess strain energy developed within the grown film and assists in improving film quality, increasing grain size, reducing dislocations, and providing epitaxially textured thin films relative to layers without the buffer. Adherent 750-nm-thick β-Ga2O3 epitaxial layers have been achieved using the AGO6 buffer, with a full-width at half-maximum of 1.36° and a root-mean-square roughness of 6.24 nm. The thermal conductivity of the β-Ga2O3 thin films with and without the AGO6 buffer was measured to be 6.0–6.5 W/mK. Thus, integration of the monoclinic gallium oxide with the diamond cubic substrate using the intermediary buffer layer improves the crystal quality but does not impact the thermal conductivity, suggesting the future feasibility of using diamond substrates for a wide range of Ga2O3 device applications.

Epitaxial films of cubic boron nitride (c-BN) have been grown on single-crystal boron-doped diamond substrates by electron cyclotron resonance plasma-enhanced chemical vapor deposition using gas mixtures of Ar–He–N2–BF3–H2. The resulting c-BN films have been characterized using in situ x-ray photoelectron spectroscopy to establish the growth surface bonding (i.e., sp3 or sp2). The interface and film crystal structure were characterized with high resolution electron microscopy and electron-energy-loss spectroscopy. This study considers three stages of the growth process: in situ surface preparation, initial nucleation and growth of c-BN, and growth of the epitaxial c-BN layer. Prior studies from our group have established that hydrogen gas phase concentration affects fluorine-induced etching and c-BN nucleation. The results of this study establish that by optimizing the surface chemistry for all three stages of the growth process, it is possible to achieve an adherent, oriented epitaxial c-BN layer, a workable growth rate (∼50 nm/hr), cubic phase BN throughout, and negligible sp2 bonding except at the interface.

Cross-sectional transmission electron microscopy has been used to characterize the morphological features of thin boron nitride films grown on single-crystal boron-doped diamond substrates (lattice mismatch of 1.36%) using electron cyclotron resonance-plasma enhanced chemical vapor deposition. The effect of gas precursor concentration, growth temperature, and substrate cleaning method on determining the BN phase (either cubic or turbostratic), the defect density, and the orientation relationship between c-BN domains and the diamond substrate were investigated. A nucleation step involving a hydrogen-limited gas mixture promoted etching of sp2-bonded BN phases and increased the fraction of cubic phase present in the films. A growth temperature of 820 °C resulted in larger BN grains and reduced defect densities particularly in regions away from the BN/diamond interface. Substrate cleaning with hydrogen plasma was found to be associated with twin-related BN growth rather than a simple epitaxial relationship. High-resolution electron micrographs showed complex contrast features caused by the presence of a high density of twin domains and stacking faults near the BN/diamond heterointerface for samples with predominant cubic phase. Electron-energy-loss spectroscopy was used to differentiate between regions of sp2 and sp3 bonding, and showed distinct transitions to the latter for the growth of cubic materials. Growth experiments at even higher temperatures and optimal substrate cleaning methods are needed for improved defect mitigation in BN films.