Takahiro Maruyama

Layered compound substrates for GaN growth

Abstract A feasibility study on layered compound substrates such as MoS 2 and mica for GaN growth was carried out. GaN films were successfully grown on MoS 2 by plasma enhanced molecular beam epitaxy and the crystal quality of GaN on MoS 2 was compared with that on Al 2 O 3 . The optical and structural properties, surface flatness and residual stress in GaN films on MoS 2 were remarkably improved compared with that on Al 2 O 3 substrate. These results suggest that MoS 2 , which has no dangling bonds on the surface and a lattice mismatch of 0.9%, has high potentiality for use as a substrate for GaN growth. The crystal quality of GaN on a mica substrate which has a lattice mismatch of 60% was not so good. The importance of lattice matching even on using layered compound substrates is suggested.

Optical and structural properties of GaN films grown on c-plane Al2O3 by electron cyclotron resonance molecular beam epitaxy

Optical and structural properties of undoped GaN films grown on Al2O3 (0 0 0 1) substrates under various nitrogen pressure and plasma power by electron cyclotron resonance molecular beam epitaxy were investigated by photoluminescence (PL) spectroscopy and X-ray diffraction (XRD). It was found that the optimum nitrogen pressure determined by full width at half maximum of XRD peak was in just agreement with that by PL spectra, and the GaN film grown under the optimum nitrogen pressure contains high density of dislocations. These results suggest that the optical quality is sensitive to the stress in the film and that the relaxation of stress depends on V/III ratio. We also found that the effects of increasing nitrogen pressure are not equivalent to the effects of increasing plasma power though they have been regarded as having the same effects.

Extended X-ray absorption fine structure study of heavily Cl doped ZnSe

Abstract The structural characteristics of heavily Cl doped ZnSe grown by molecular beam epitaxy were investigated by means of extended X-ray absorption fine structure (EXAFS) measurements. Two kinds of bond length between Cl and the nearest Zn atoms were observed; one is 0.25 nm and the other is 0.28 nm. The former is nearly equal to the bond length of SeZn (0.245 nm) in ZnSe indicating that four-coordinated Cl incorporated into the Se lattice site. The latter seems to indicate a defect structure, and a Clzinc vacancy complex is proposed as the defect model.

Compensation centers in ZnSeTe

Extended x-ray absorption fine structure (EXAFS), Rutherford-backscattering ion channeling, and particle induced x-ray emission channeling (PIXE/C) measurements have been performed in order to investigate compensation centers in Cl doped ZnSeTe. The EXAFS results from Cl doped ZnSeTe suggest that almost all Cl atoms are incorporated into substitutional Se lattice sites, which seems to indicate that Cl atoms themselves are not responsible for the compensation centers. The PIXE/C angular profiles were measured across the 〈100〉, 〈110〉, and 〈111〉 axes for undoped ZnSeTe. Comparing the angular profiles for Zn Kα, Se Kα, and Te Lα x-ray yields, it was found that some portion of the Te atoms (∼1020 cm−3) are located at tetrahedral interstitial sites. From these results, the difficulty of realizing n-type ZnSeTe is considered to be due to the existence of the interstitial Te atoms which act as acceptors.

Configuration of Cl atoms in ZnSe and ZnTe

Abstract The atomic configuration and charge states of Cl in ZnSe and ZnTe were studied by extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge structure (XANES) analysis. By measuring Cl K-edge EXAFS, it was found that the Cl atoms in ZnSe are incorporated into four-coordinated Se lattice sites, however, in ZnTe, the Cl atoms are displaced from Te lattice sites, and probably move towards the [1 1 1] direction and form a one-coordination. The peak position of the Cl K-edge in Cl : ZnTe XANES spectra is just the same as that of Cl : ZnSe. Since the charge state of Cl in ZnSe can be regarded as neutral, the charge state of Cl in ZnTe may be also neutral. The carrier saturation of Cl: ZnTe may originate in a deepening of the donor level caused by a displacement of dopant atom.

Distribution of chalcogen atoms in ZnSSe and ZnMgSSe: an EXAFS study

Abstract Extended X-ray absorption fine structure (EXAFS) measurements were performed to investigate the local bonding structure in ZnS x Se 1 − x ( x = 0.06) and Zn 1 − x Mg x S y Se 1 − y ( x = 0.18, y = 0.2) with zincblende structure. It is found that the first nearest neighbor SZn distance and the second nearest neighbor SS distance remain close to those in ZnS with zincblende structure. This implies a deviation from Vegards law in these compounds. The second nearest neighbor structure around sulphur shows that the ratio of the number of sulphur atoms to that of selenium is proportional to the respective molar ratio both in ZnSSe and in ZnMgSSe. These results indicate that there is no clustering in ZnSSe and that chalcogen atoms are also distributed with negligible clustering in ZnMgSSe.

Effect of surface oxygen vacancies on electronic states of reduced SrTiO3(110) surface

Abstract The electronic structure of reduced SrTiO3(110) surface prepared by annealing in ultrahigh vacuum (UHV), has been studied using photoemission spectroscopy (PES) and scanning tunneling microscopy/spectroscopy (STM/STS). In the PES spectra of reduced SrTiO3(110) surface after annealing in UHV at 800°C, a clear metallic state with a sharp Fermi cut-off and a broad state centered at ∼1.1eV below the Fermi level are observed in the band gap region. A Ti3p->3d resonance enhancement is occurred for the metallic state, but not for the ∼1.1eV state. On the other hand, after the sample was annealed in UHV at 1000°C, it has been shown that the metallic state becomes drastically weak, and that only the ∼1.1eV state is seen in the band gap region. Thereafter, the ∼1.1eV state exhibits a clear Ti3p->3d resonance enhancement. The difference in the spectral feature of these band gap states is directly related to the surface geometric structural change.

Extended X-ray absorption fine structure study of defects in Cl doped ZnSe

Abstract Local structures around the dopant in heavily Cl doped ZnSe films were investigated by extended X-ray absorption structure (EXAFS) analysis. Measurements were performed for a carrier saturated sample (with N d − N a = 3 × 10 19 cm −3 ) without and with post-growth annealing treatment. In addition to the bond identified as the normally substituted Cl atoms at Se sites, the bond with the length of 0.28 nm was observed for the non-annealed sample. In the annealed sample, only a bond of 0.29 nm was observed. We consider the 0.28–0.29 nm-bond corresponds to the ClZn vacancy complex, which is formed by heavy doping or thermal treatment.

Cathodoluminescence of GaN films grown under Ga and N rich conditions by radio-frequency-molecular beam epitaxy

Cathodoluminescence (CL) topographs were taken at room temperature at 364 nm on undoped GaN films grown by plasma-enhanced molecular beam epitaxy at various V/III ratios. The CL topographs were compared with scanning electron microscope (SEM) images. The CL topograph was found to be significantly influenced by the V/III ratio during growth. For GaN films grown at low V/III ratio, spatially nonuniform luminescence intensity was observed. This nonuniformity in luminescence may be due to the local variation of stoichiometry of GaN induced by Ga microsegregation. At nitrogen-rich condition, almost flat surfaces in SEM image were observed and the uniform intensity of luminescence was obtained by CL topograph. Thus, the nitrogen-rich condition is favourable to get structually and optically uniform films.

Defect Structures Determined by EXAFS

Extended X-ray absorption fine structure (EXAFS) measurements were performed to study the atomic structure in undoped ZnSSe, ZnMgSSe, and Cl-doped ZnSe, ZnTe and ZnSeTe. From the measurements of the S K-edge EXAFS, it was found that the S atoms are distributed randomly in ZnSSe and ZnMgSSe. By measuring the Cl K-edge EXAFS for doped samples, it was found that the Cl atoms in ZnSe are incorporated into the fourfold coordinated Se lattice site, however, in ZnTe, the Cl atoms are displaced from the Te lattice site. The cause of the difficulty to get n-type ZnTe may be the displacement of Cl atoms. In ZnSeTe whose Te content is 0.15, the Cl atoms seem to be incorporated into anion lattice sites forming fourfold coordination. The compensation center in ZnSeTe seems to be different from that in ZnTe.