H. Okushi

High quality homoepitaxial CVD diamond for electronic devices

Abstract Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH4 concentration of CH4/H2 gas system less than 0.15% CH4/H2 ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm2 and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH3)3,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm2/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.

Device-grade homoepitaxial diamond film growth

We have successfully synthesized homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using an extremely low CH 4 /H 2 ratio of CH 4 /H 2 gas less than 0.15% CH 4 /H 2 ratio and Ib(0 0 1) substrates with low misorientation angle (θ off ) less than 1.5. It was found that surface morphologies of the films strongly depend on a growth condition of CH 4 /H 2 ratio and θ off of the substrate and was suggested that the hydrogen etching and the θ off played an important role for the epitaxial diamond film growth with an atomically flat surface. On the other hand, from the cathodoluminescence spectra and Schottky junction properties of these diamond films with atomically flat surface, it has been clarified that these films have actually a high potentiality for electronic devices.

Homoepitaxial diamond films grown by step-flow mode in various misorientation angles of diamond substrates

Abstract Surface morphologies of device-grade homoepitaxial diamond films grown by microwave plasma chemical vapor deposition (CVD) with CH 4 /H 2 ratios and misorientation angles of diamond (001) substrates are discussed. We produced a map of surface morphologies as a function of both the misorientation angles of the diamond substrates and CH 4 /H 2 ratios. This map revealed that homoepitaxy under CH 4 /H 2 ratios greater than 0.05% was in good agreement with results reported in the literature. This could be explained in terms of migration length and CH 4 /H 2 ratios. On the other hand, when the CH 4 /H 2 ratios were lower than 0.15%, we found that the misorientation angle does not affect the surface morphology of the homoepitaxial diamond films so much, and we obtained atomically flat surfaces over the whole substrate area. The numerical result may be consistent with the fact that the migration length of precursors increases by orders of magnitude in the presence of a large amount of atomic hydrogen.

A theoretical study of a sulfur impurity in diamond

Abstract An isolated substitutional sulfur in diamond is found to be a deep donor. Some complex sulfur defects with nitrogen, boron and a vacancy may have donor levels in the range of 0.4–0.5 eV. However, formation energies of the defects of our interest are all high. Thus, we expect that, although sulfur can be a relatively shallow donor in diamond, its doping efficiency is quite low.

Low-compensated boron-doped homoepitaxial diamond films

Abstract We have synthesized device grade B-doped homoepitaxial diamond films with low compensation using a microwave plasma chemical vapor deposition (CVD) system. This study was based on an approach in which the synthesis of device grade films was performed in a clean CVD system. Among the films studied here, Hall effect measurements showed that the highest values of Hall mobility were 1840xa0cm 2 /Vxa0s at 290xa0K. The donor (compensation) concentration N D and compensation ratio N D / N Acceptor of the film mentioned above were 4±1×10 14 xa0cm −3 and about 0.4%, respectively. Furthermore, as a result of applying the two-step growth method to B-doped films, we obtained B-doped films without unepitaxial crystallites and pyramidal hillocks. We made high-quality Schottky junctions between the metal (Al, Cr, Ni, Au, or Pt) and the present B-doped diamond films. The current–voltage characteristics of these junctions indicated high-rectification properties with ideality factor n ∼1.1 and very low leakage currents at reverse bias. In particular, the junction fabricated with Pt was operational with n ∼1.3 at high temperature of around 731xa0K.

Schottky junction properties of the high conductivity layer of diamond

Junction properties of Al/high-conductivity layer (HCL) near the surface of hydrogenated homoepitaxial diamond films were re-examined to evaluate the published models for the origin of HCL. As compared with the junction properties of metal/oxidized B-doped diamond films, it is confirmed that carrier transport in the Al/HCL junction can be well explained by the conventional Schottky barrier theory for metal/p-type semiconductors. This means that the carrier depletion region of HCL (i.e. the downward band bending for p-type HCL) actually exists in the vicinity of the Al/HCL junction at thermal equilibrium (zero bias) as well as at reverse bias conditions. To keep the downward band bending of HCL, ionized acceptors must exist in this region. We discuss a downward band-bending model based on the hydrogen-related shallow acceptors to explain electrochemical behaviors (adsorbate effect) of HCL as well as the present Schottky junction properties.

High quality homoepitaxial diamond thin film synthesis with high growth rate by a two-step growth method ☆

Abstract Homoepitaxial diamond films grown in the condition of CH 4 /H 2 ratio lower than 0.15% in a microwave-assisted plasma chemical vapor deposition system had excellent electrical and optical properties without any unepitaxial crystallites (UCs). Under such a low CH 4 concentration condition, however, the growth rate becomes too slow to obtain a useful thickness. In order to overcome this problem, we attempted a two-step growth method. In the first step the substrate surface was treated by homoepitaxial growth of diamond in the presence of 0.05% CH 4 in H 2 ; in the second step the CH 4 concentration was increased. By considering the origin of UCs with cross-sectional transmission electron microscope studies, it was found that this method is based on surface improvement of the initial substrate by means of ultra-low CH 4 concentration growth. This method was quite useful for obtaining high quality films, with high growth rate and reproducibility.

Misorientation angle dependence of surface morphology in homoepitaxial diamond film growth at a low CH4/H2 ratio

Abstract We studied the misorientation angle dependence of the surface morphology in homoepitaxial diamond film grown by the microwave plasma chemical vapor deposition (CVD) method using an extremely low CH4 concentration gas system (less than 0.15% CH4/H2 ratio). We found that, for a misorientation angle (θoff) of less than 1°, the surfaces of diamond films are atomically flat surface. Their mean roughness (Ra) determined by atomic force microscopy (AFM) is less than 0.04xa0nm. Ra subsequently increases as θoff increases above θoff>1.5°. This tendency is opposite to that of the conventional homoepitaxial growth using a high CH4/H2 ratio. We also found that the θoff dependence of Ra of surfaces formed by hydrogen (H2) plasma etching has the same tendency as that of the CVD growth at the low CH4/H2 ratio mentioned above. Specifically, Ra is less than 0.17xa0nm for θoff

Theoretical modeling of sulfur-hydrogen complexes in diamond

Abstract We present ab initio energetics of sulfur (S)–hydrogen (H) complex defects in diamond. If the defects are in partial equilibrium with the dopants adsorbed on the restored (100) surface, some S–H complexes may be incorporated with less energy costs than that required for single vacancy ( V ) formation. However, they do not exhibit shallow donor characters. Thus, it is not quite obvious whether the S atoms doped in diamond can work as real donors.

Cross-sectional TEM study of unepitaxial crystallites in a homoepitaxial diamond film

Abstract We investigated the structure of unepitaxial crystallites (UC; non-epitaxial crystallites) in homoepitaxial diamond films on Ib (001) diamond substrate grown by the chemical vapor deposition (CVD), employing field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). The UC was classified into two types depending on the orientation relationship to the homoepitaxial film; one rotates by 70.5° around the common 〈110〉 axis, corresponding to Σ3 coincidence site lattice (CSL) relation. The other type does not have any particular angular relationship. It was found that the growth of the former type is closely related to a lattice dislocation on the substrate surface as well as the homoepitaxial film. On the other hand, there was hardly any lattice dislocation observed at the bottom of the latter type. A nanometer-sized crystalline diamond particle was observed at the nucleation site of the latter one.