AlN/diamond heterojunction field effect transistors with p-channel and normally-on depletion mode are developed, and the device performance is improved by reducing the contact resistances at source and drain contacts reported previously. The heterojunction structure is constructed from a c-axis-oriented AlN epilayer grown on oxygen-terminated (111) diamond substrate by metal-organic vapor phase epitaxy at temperatures as high as 1250 °C. Thermal treatment in the mixed hydrogen (H2) and ammonia atmosphere just before AlN growth improves the AlN adhesion to diamond surface. In addition, this treatment simultaneously produces a much larger surface hole-carrier density than that obtained by conventional H2-plasma treatment. X-ray photoelectron spectroscopy reveals the existence of carbon-nitrogen bonds at the diamond surface, and these may be responsible for such a large hole density. These results are promising in relation to new opportunities for developing diamond-based power electronic devices.

We investigate the microstructures of domain boundaries in an AlN layer grown on a (001) diamond substrate by metal-organic vapor phase epitaxy. The AlN layer has a two-domain structure with crystal orientation along either <112̄0> AlN [named by AlNI domain] or <101̄0> AlN [named by AlNII domain] parallel to [110] direction of diamond. The AlNI and AlNII domains are not atomically bonded at two-domain boundary from initial to final step of growth, while an edge-type dislocation is generated at single-domain boundary (SDB). In addition, an inversion AlNI domain [named by AlNI⁎] is randomly-ordered at the initial stage of the coalescence between the AlNI domains. The AlNI⁎ is easily terminated with increasing the thickness of AlNI domain. The inversion domain boundary changes to the edge-type dislocation at the SDB with further growth, which reduces the defect density in the AlNI domains.

In order to understand a substrate effect on photoresponse properties of diamond-based Schottky-barrier photodiode (SPD), we utilize a heavily boron-doped p+-diamond substrate for a thin epilayer growth and the SPD fabrication. Vertical- and planar-types SPD's show excellent rectifying property with ideality factor close to unity and on-resistance as small as 1 Ω·cm2. Both the SPD's are operable in photovoltaic and reverse bias modes with a quantum efficiency of 2.2 ± 0.5% and does not provide a photoconductivity gain and a persistent photocurrent before and after annealing at 500 °C for 120 min. By comparing the photoresponse properties of SPD's on the p+-diamond substrate with on a Ib-diamond substrate, we conclude that a homojunction between p-diamond epilayer and Ib-diamond substrate governs a photo-carriers generation and a transport process, which produces an excellent spectral response property of SPD on the Ib-diamond substrate.

We investigate electrical and optical properties of a vertical-type Schottky-barrier photodiode (SPD) using a boron (B)-doped p-diamond epilayer grown on a heavily B-doped p+-diamond (100) substrate with B concentration ([B]) of 1 × 1020 cm− 3 by microwave plasma chemical-vapor deposition. Surface morphology and [B] in the epilayer are strongly affected by the growth temperature (Tg). Smooth surface and low [B] of (3 ± 2) × 1015 cm− 3 with an abrupt [B] profile at epilayer interface are reproducibly obtained for Tg lower than 900 °C. The vertical-type SPD with a semitransparent WC Schottky contact is fabricated on an oxidized surface of the p-diamond epilayer. The SPDs with an ideality factor lower than 1.1 and a reverse leakage current less than 10− 14 A are reproducibly obtained. The SPD is operable at zero or reverse bias mode with a fast response speed less than 1 s. The external quantum efficiency for 220-nm light illumination is measured to be almost constant value of 3.5 ± 0.5% with increasing the reverse bias voltage from 0 to 5 V before and after annealing at 400 °C for 10 min. It is found that the vertical-type SPD using p+-diamond substrate does not provide the photoconductivity gain property.

Wurtzite AlN layers are grown on a (0 0 1) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using atomic force microscopy, X-ray diffractometry, and transmission electron microscopy. At the initial stage of AlN growth, AlN crystalline particles with various orientations are randomly nucleated on the (0 0 1) diamond surface. At the second step of growth, AlN grains predominantly oriented along the c-axis grow, incorporate the randomly oriented AlN grains, and then grow further. At the final step of growth, a continuous AlN layer with a c-axis-oriented two-domain structure is obtained on the (0 0 1) diamond substrate. Microstructural analysis reveals that either the 〈112¯0〉 AlN domain or 〈101¯0〉 AlN domain is aligned on the [1 1 0] diamond direction. The growth mechanism governs by the higher growth rate of the AlN grains along the [0 0 0 1] direction than along other directions at high growth temperatures up to 1270 °C.

c-Axis-oriented wurtzite (0 0 0 1) AlN layers are grown on a (1 1 1) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using X-ray diffractometry and transmission electron microscopy. At the initial stage of AlN growth, AlN particles with a two-domain structure are nucleated on the (1 1 1) diamond surface. The orientation is either 〈1 1 2¯ 0〉AlN ∥ [1 1¯ 0] diamond [named the AlNI domain] or 〈1 0 1¯ 0〉AlN ∥ [1 1¯ 0] diamond with a rotation angle of 30° from AlNI around the c-axis [named the AlNII domain]. The initial AlN particles are predominantly composed of AlNII. Subsequently, the AlNII grains incorporate the quite-low-density AlNI grains upon further growth. Finally, a continuous AlN layer dominated by the AlNII domain is obtained on the diamond. The growth dynamics is significantly different from that of the AlN layer grown on (1 1 1) Si. The growth mechanism is understood by considering the periodic atomic arrangement of AlN and (1 1 1) diamond. We conclude that AlNII on (1 1 1) diamond has a geometrically appropriate epitaxial relationship that reduces the stress energy of the AlN lattice.