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We have grown InN films directly on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD), without the need for any buffer layers. We have found that the growth temperature and the laser intensity exert a large influence on the crystalline quality of the InN films. We have also found that the electrical properties and the crystalline quality are improved with increasing film thickness. FWHM values for InN 0002 and 112¯4 X-ray rocking curves for a 1.5-μm-thick InN film were as small as 0.14° and 0.26°, respectively. The electron mobility and carrier density in the film were 1300 cm 2 /V s and 3.0×10 18 cm −3 , respectively. The fact that PLD direct growth of InN on sapphire substrates leads to formation of epitaxial material is quite consistent with the theoretical prediction from first principles calculations and is probably due to the enhanced surface migration of the InN precursors.

Interfacial chemical states and Schottky barrier heights (SBHs) for indium tin oxide (ITO) electrodes on Mg:GaN films have been investigated by high-resolution hard X-ray photoemission spectroscopy (HX-PES), and have been correlated with electrical properties. HX-PES has revealed that the large downward band bending of 2.6 eV was drastically reduced by ITO deposition and annealing, resulting in low SBH of 0.2 eV which is well correlated with good ohmic contact. Improvements of electrical properties can be attributed to the combination of (1) the interfacial layer with large work function, (2) the ordered interfacial dipole layer and (3) activation by ITO catalytic effect.

We have grown MnAs films on GaAs(1 1 1)B substrates by high-temperature molecular beam epitaxy (MBE) and have investigated their crystal structures and magnetic properties. X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXD) results showed that two kinds of zb-type MnAs films with lattice constants of 5.73 and 5.96 A were grown. The zb-type MnAs films show higher Curie temperature than 350 K and larger magnetization than the reported zb-type MnAs. At the initial stage of MnAs growth, Ga 1− x Mn x As was grown and this Ga 1− x Mn x As layer might play an important role of a buffer layer to enable the zb-MnAs growth.

We have grown AlN films on single-crystalline Mo(110), (100), and (111) substrates using a low temperature pulsed laser deposition (PLD) growth technique and investigated their structural properties. Although c-axis oriented AlN films grow on Mo(100), the films contain 30° rotated domains due to the difference in the rotational symmetry between AlN(0001) and Mo(100). AlN films with only poor crystalline quality grow on Mo(111) substrates, probably due to the poor surface morphology and high reactivity of the substrates. On the other hand, single crystal AlN films grow epitaxially on Mo(110) substrates with an in-plane relationship of AlN[11-20] // Mo[001]. Reflection high-energy electron diffraction or electron backscattered diffraction analysis has revealed that neither in-plane 30° rotated domains nor cubic phase domains exist in the AlN films. X-ray reflectivity measurements have revealed that the heterointerface between AlN and Mo prepared by PLD at 450 °C is quite abrupt. These results indicate that PLD epitaxial growth of AlN on single crystal Mo substrates is quite promising for the fabrication of future high frequency filter devices.

We have determined the valence band discontinuities of InN/Mg:GaN and InN/Si:GaN heterostructures by hard x-ray photoemission spectroscopy. InN thin films were grown by pulsed laser deposition method and streaky high-energy electron diffraction pattern was observed, indicating that good crystalline InN films were grown. The value of valence band discontinuity for InN/Mg:GaN is as small as 0.3 eV, while that for InN/Si:GaN is 1.4 eV. The fact that the InN/Mg:GaN interface has the small energy barrier suggests that InN is very promising for the interlayer between transparent conductive layers and Mg:GaN in light emitting diode structures. [DOI: 10.1380/ejssnt.2008.254]

We have demonstrated the first epitaxial growth of AlN films on single-crystalline Ta substrates by the use of a low-temperature growth technique based on pulsed laser deposition (PLD). Although previous AlN films grown on Ta(100) and (111) substrates have exhibited quite poor crystallinity, an epitaxial AlN(0001) film with an in-plane epitaxial relationship of AlN[112¯0]//Ta[001] has been obtained on a Ta(110) substrate at a growth temperature of 450 °C. We found that the full-width at half-maximum values for the crystal orientation distribution in the tilt and twist directions of the AlN film were 0.37° and 0.41°, respectively. Grazing-incidence X-ray reflection (GIXR) and X-ray photoelectron spectroscopy (XPS) measurements have revealed that the AlN/Ta heterointerface is quite abrupt, and that its abruptness remains unchanged even after annealing at 1000 °C.

We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 °C, and compared their structural properties. Although the film quality of GaN grown at 700 °C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 °C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 °C has a Ga-polarity.

In this presentation, we will demonstrate successful operation of nitride devices prepared on various low cost substrates by the use of a newly developed low temperature epitaxial growth technique named pulsed sputtering deposition (PSD). We have found that performances of GaN devices such as LEDs or HEMTs fabricated with PSD are as good as those fabricated with conventional MOCVD. We have also found that the use of PSD low temperature epitaxial growth technique allows us to utilize chemically vulnerable materials that have never been used as substrates for GaN based devices. These results indicate that the low growth temperature PSD epitaxial process is quite advantageous for fabrication of nitride devices on various flexible substrates.

Although pulsed laser deposition (PLD) has been tested at various research institutes since the invention of the laser, it has recently attracted considerable attention for the growth of high-quality high-temperature superconducting thin films. This physical vapor deposition technique has been used extensively to manufacture thin films from various materials such as oxides, nitrides, carbides, organic materials, and metals. This chapter briefly introduces the physics and chemistry of PLD, typical experimental setups, and application of this technique to various materials.

We have grown AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We found that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[ 1 1 2 ¯ 0 ]//Ru[ 1 1 2 ¯ 0 ]. Electron backscattering diffraction observations revealed that neither 30° rotational domains nor cubic phase domains were present in the AlN films and the full-width at half-maximum of the distribution in the AlN[0 0 0 1] crystalline orientation was 0.56°. Spectroscopic ellipsometry measurements showed that the AlN/Ru hetero-interface was quite abrupt, which is important for fabrication of high-frequency film bulk acoustic resonators.

The authors have investigated the effects of low-temperature-grown (LTG) GaN buffer layers on the pulsed-laser deposition of GaN films on LiNbO3. LiNbO3 substrates with atomically flat surfaces have been prepared by annealing in a box constructed from blocks of single crystal LiNbO3 wafers. When the GaN growth temperature is lowered from 700to580°C, the GaN∕LiNbO3 interfacial layer thickness is reduced. When these LTG-GaN layers, which have a thickness of 20nm, are employed as buffers for the subsequent high temperature growth of 200-nm-thick GaN layers, a decrease in root-mean-square surface roughness and a reduction in tilt and twist mosaicities are observed. This technique seems quite promising for achieving high quality GaN on LiNbO3 substrates, which is attractive for the fabrication of future optoelectronic integrated circuits.

We have investigated a surface treatment for ZrB2 substrates, and its application to the heteroepitaxial growth of GaN. ZrB2 substrates exhibit atomically-flat terraces with a step-height equal to the c-axis lattice constant after the wet etching in a 0.3% HF solution followed by annealing at 1000 °C under ultra-high vacuum. We have grown 20 nm thick GaN films at various growth temperatures on cleaned ZrB2 substrates using pulsed laser deposition, and have found that high quality GaN grows in the layer-by-layer mode at low substrate temperatures. On the other hand, GaN with poor crystalline quality grows in the three-dimensional growth mode at high growth temperatures. Grazing-incidence angle X-ray reflectivity measurements have revealed that the interfacial layer thickness decreases from 16.2 to 1.0 nm as the growth temperature is reduced from 700 °C to room temperature. These results indicate that a reduction in growth temperature leads to the suppression of interfacial reactions, and makes it possible to take advantage of the small lattice mismatch of 0.6% between GaN and ZrB2.

Epitaxial growth of GaN films on MgAl2O4(1 1 1) substrates by pulsed laser deposition (PLD) at temperatures ranging from 973 K to room temperature (RT) has been carried out in this work. It is found that the thickness of the interfacial layer between GaN and MgAl2O4 decreases with a decrease of growth temperature and the interfacial reaction is completely suppressed in the case of RT growth. The epitaxial relationship between GaN and MgAl2O4 is GaN(0 0 0 1)//MgAl2O4(1 1 1) and GaN [1 1 0]//MgAl2O4[0 1] at all temperatures. The crystal quality of the as-grown GaN changes with the growth temperature and the RT-grown GaN shows the best quality with FWHM values of 0.21° for the tilt distribution and 0.37° for the twist distribution, respectively. Only the growth of GaN at RT can completely suppress the diffusion of Mg from the substrate to the GaN film. These results evidently demonstrate the advantages of RT growth of GaN on MgAl2O4 substrates by PLD.

We have succeeded in the epitaxial growth of AlN on Cu(1 1 1) substrates using pulsed laser deposition (PLD) for the first time. We achieved atomically flat surfaces of Cu(1 1 1) by annealing in an ultra high vacuum chamber. In situ X-ray photoelectron spectroscopy and reflection high-energy electron diffraction studies revealed that the native oxide layers on the surfaces had been successfully removed and that the Cu surfaces were reformed by the segregation of antimony atoms, incorporated in the substrates as impurities. We found that AlN (0 0 0 1) grows epitaxially on Cu(1 1 1) at a substrate temperature of as low as 500 °C with in-plane epitaxial relationships of AlN [ 11 2 ¯ 0 ] / / Cu [ 1 1 ¯ 0 ] . Electron backscattering diffraction measurements showed that neither 30° rotational domains nor cubic phase domains exist in the AlN films. Spectroscopic ellipsometry measurements revealed that no interfacial layers exist at the AlN/Cu hetero interfaces within the limits of detection. These results indicate that the PLD low-temperature growth technique permits the preparation of high-quality AlN expitaxial films on single-crystal metal substrates.

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe 2 O 4 (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe 2 O 4 , which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe 2 O 4 , which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe 2 O 4 (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.

We have developed field-effect amorphous silicon solar cells (FESC) with comb-shaped electrodes (CSE) made of transparent conductive oxide (TCO). The FESC is based on the concept of utilizing a special window layer, in which carriers are induced by the field-effect instead of heavy impurity doping, to form a pin (or nip) junction. Pulsed laser deposition was used to fabricate a TCO CSE with a ZnO/ITO/ZnO stacked-layer structure. The FESC exhibited enhanced current output on applying a bias voltage for the field-effect. The conversion efficiency of the field-effect operation was improved by 39% compared with a FESC with a metal CSE. The fill factor was improved markedly by applying a TCO to the CSE.

In order to understand the mechanism of ohmic-contact formation by the annealing of Ti electrodes on n-type GaN, we have investigated the changes in the energy-band structure of Ti/n-type GaN depending on annealing temperature using photoemission spectroscopy. Valence-band spectrum for an as-deposited sample was explained by the simple summation of Ti and GaN spectra. Spectral line shapes significantly changed by annealing at 500 and 700 °C, suggesting the formation of a TiN layer. The peak shifts of Ga 3d and N 1s core levels are interpreted as the energy-band bending and interfacial reaction with the formation of the TiN layer. It is revealed that the ohmic-contact formation by the annealing is attributed to the formation of GaN with nitrogen vacancies, which is consistent with the current–voltage characteristics.

In electronic device fabrication, we frequently encounter the situations to deposit thin films of semiconductors, ceramics, and metals sequentially. Since the appropriate methods for depositing these films are frequently different, it should be beneficial for the research of new devices to develop a system that enables the depositions of different films not only sequentially but also in parallel by the procedure as prescribed. In this paper, we report on a tandem system composed of a pulsed laser deposition (PLD) and two plasma chemical vapor deposition (PCVD) chambers and its application in the fabrication of a prototype structure directed towards the field effect a-Si:H solar cell. PLD and PCVD were used to deposit conductive and semiconductive/insulative film layers, respectively. An exchangeable masking scheme was designed and installed in the system so we could make several device structures in parallel and quickly optimize the device and process parameters by the combinatorial methodology.

We have grown thin GaN epitaxial films on Si(1 1 1) and sapphire (0 0 0 1) with AlN buffer layers using pulsed laser deposition and investigated the distortion of the films using synchrotron radiation grazing incidence-angle X-ray diffraction for the first time. We have found that the strains in the AlN films were almost fully relaxed at the growth temperature (800 °C) by the introduction of the misfit dislocations. However, we have also found that the introduction of misfit dislocations at the interfaces between the AlN buffer layers and the GaN films is not rigorous enough to release the stain fully at the growth temperature.

We have grown InN films on c-plane sapphire substrates by pulsed laser deposition (PLD) with a radio frequency nitrogen radical source for the first time and investigated the effect of the substrate surface nitridation on the structural and electrical properties of InN films with reflection high energy electron diffraction (RHEED), atomic force microscope, the Hall effect measurements and high-resolution X-ray diffraction (HRXRD). RHEED and HRXRD characterizations revealed that high-quality InN grows epitaxially on sapphire by PLD and its epitaxial relationship is InN (0 0 0 1)∣∣sapphire (0 0 0 1) and InN [2 -1 -1 0]∣∣sapphire [1 0 -1 0]. The InN crystalline quality and the electron mobility are improved by the substrate nitridation process. The area of the pits at the InN surface is reduced by the substrate nitridation process probably due to the reduction in the interface energy between InN and the substrate. The full width at half maximum of the -1 -1 2 4 X-ray rocking curve for InN grown by the present technique without using any buffer layers was as small as 34.8 arcmin. These results indicate that the present technique is promising for the growth of the high-quality InN films.

We have grown GaAs epitaxial films on MnZn-ferrite substrates using MnAs buffer layers and investigated their heterointerfaces with glazing incidence-angle X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the heterointerfaces for this structure are quite abrupt and the roughness at the GaAs/MnAs and MnAs/MnZn-ferrite interfaces are 1.1 and 0.2 nm, respectively. We also found that the diffusion of atoms through the GaAs/MnAs interface into the GaAs film is negligible. These results indicate that the MnAs buffer layer for the GaAs/ferrite structure is chemically stable and promising for the application to the future magnetic electronics.

We have grown GaN films on LiGaO2 (LGO) (0 0 1) substrates by pulsed laser deposition (PLD) for the first time and investigated their structural properties using generalized grazing incidence-angle X-ray diffraction. By the use of PLD, we can grow high quality hexagonal GaN on LGO epitaxially without the use of any buffer layers, which is important to take advantage of the nearly lattice matched substrate. It has been shown that thin GaN films grown on LGO substrates are compressed in the lateral directions and that the lattice constant of GaN in the LGO [1 2 0] direction is slightly larger than that in the LGO [0 1 0] direction. This anisotropy is probably related to the crystal orientation dependence of the thermal contraction rate for LGO during the cooling down from the growth temperature.

We propose a novel solar cell structure based on the concept of forming p or n window layers by the field effect instead of impurity doping, and we verify its performance experimentally. The device, a field-effect amorphous silicon solar cell (FESC), was designed with the aid of a device simulator and fabricated by a plasma chemical vapour deposition system. We have verified that the output current of the FESC was amplified by the field-effect bias application to the gate electrode. The fundamental properties of this new type of amorphous silicon solar cell are demonstrated for the first time.

Effect of the AlN buffer layer on the characteristics of the GaAsy(MnZn)Fe O structures has been investigated with atomic 24 force microscopy, high resolution X-ray diffraction (HRXRD), and reflection high-energy electron diffraction (RHEED). We have found that the use of the AlN buffer layer improves crystalline quality of GaAs thin films. HRXRD and RHEED observations have revealed that the epitaxial relationship for this structure is GaAs ( 111 )yyAlNw 0001 xyy(MnZn)Fe O w 111 x and GaAs 24 w1 y 10 xyyAlNw 10 y 10 xyy(MnZn)Fe O w1 y 10 x. We have also found that the surface morphology is remarkably improved by 24 the use of AlN buffer layer. 2003 Elsevier Science B.V. All rights reserved.

We have grown AlN films on (Mn,Zn)Fe 2 O 4 by pulsed laser deposition and investigated the growth temperature dependence of their structural properties using grazing-incidence X-ray reflection (GIXR), atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED) and X-ray diffraction. We have confirmed that high-quality AlN films can be grown in the substrate temperature range from room temperature to 800 °C. We have also found that the films grown at low temperatures have greater strain and smoother surfaces due to the reduced motion of atoms during growth. GIXR measurements have revealed that the abruptness of the heterointerface between the AlN films and the substrates is improved by a reduction in the growth temperature.

We have investigated low-temperature epitaxial growths of AlN on nearly lattice-matched MnO(1 1 1) with pulsed laser deposition (PLD). Coaxial impact collision ion scattering spectroscopy (CAICISS) analysis has revealed that the surface of MnO(1 1 1) just before the growth consists of both the Mn-plane and the O-plane with a coverage ratio of 1:1. We have confirmed that the epitaxial growth temperature for AlN on MnO can be reduced down to 400 °C by the use of PLD. The roughness of the AlN/MnO interface decreased from 1.3 to 2.1 nm by the reduction of the growth temperature, which is probably due to the suppression of the intermixing reactions at the interface. On the contrary, the surface morphology of the AlN films was degraded by the reduction of the growth temperature probably due to the stress buildup caused by the suppression of the interface reactions.

The generalized grazing-incidence-angle x-ray scattering technique has been developed for analysis of near-surface structures, namely, the anisotropy of the structure in the directions perpendicular and parallel to its surface, and its change within the depth. In this method, a specimen is irradiated by an x-ray beam with a grazing-incidence angle as small as the critical angle of the surface, and both in- and out-of-plane scattering are analyzed simultaneously. The technique has been applied to investigate the structures of InAs quantum dots grown on Si with a diameter of 37 nm and a surface coverage of 11%. It has been found that the InAs dots were dilated by 1.7% along the direction perpendicular to the surface and compressed by 1.4% in the lateral direction. The compression in the lateral direction shows a maximum value at the interface, and decreases as the distance from the interface increases. The existence of 1.7% compressive strain means that most of the 11.7% lattice mismatch between the InAs do...

Local structures around Tb ions implanted in SiO 2 on Si, which exhibited strong green luminescence (∼540 nm) at room temperature, were investigated by using fluorescence X-ray absorption fines structure (XAFS) analysis at Tb L III -edge. The X-ray absorption near-edge structure spectra showed that Tb ions implanted in SiO 2 were incorporated as Tb 3+ . The extended X-ray absorption fines structure analysis revealed that two types of local structures coexisted, in which two and six oxygen atoms were coordinated around Tb ions, respectively. Amount of Tb–6O structure in the annealed sample increased compared with that in the as-implanted one. Photoluminescence measurement showed enhancement of luminescence intensity by annealing. Therefore, it is considered that formation of Tb–6O structure is related to the enhancement of luminescence intensity.

We have grown group III nitrides on SrTiO3 (STO) substrates using pulsed laser deposition (PLD) and investigated their structural properties by high resolution X-ray diffraction (HRXRD) and reflection high energy electron diffraction (RHEED). We have found that AlN and GaN grow epitaxially on STO (111) substrates. The epitaxial relationship between the nitrides and SrTiO3 substrates is nitrides(0001) ∥ SrTiO3(111) and nitrides[10—10] ∥ SrTiO3[11—2]. The in-plane alignment is rotated by 30° along the c-axis from that expected by the notion of lattice matching. It has also been found that AlN grown on SrTiO3 is dilated by 0.4% in the [0001] direction due to the lattice mismatch and the smaller thermal contraction rate.

We have investigated the effect of AlN buffer layers on characteristics of GaN films grown on nearly lattice matched MnO (111) substrates by pulsed laser deposition (PLD). We have found that the polarity of the GaN films grown on the MnO substrates changes from the N-polarity to the Ga-polarity with the use of AlN buffer layers. Atomic force microscopy (AFM) observations and photoluminescence (PL) measurements have revealed that the surface morphology and the optical properties of the GaN films on MnO can be improved by the AlN buffer layers.

We have grown AlN on (Mn,Zn)Fe2O4 substrates by pulsed laser deposition (PLD) and investigated their structural properties using high resolution X-ray diffraction (HRXRD), reflection high energy electron diffraction (RHEED), and atomic force microscopy (AFM). We have observed the transition of the RHEED pattern from sharp streaks into clear spots at the early stage of the film growth, which indicates that the growth mode of AlN changed from the two-dimensional mode to the three-dimensional mode due to the stress buildup. RHEED and XRD observations have revealed that hexagonal AlN (0 0 0 1) grows on (Mn,Zn)Fe2O4 (1 1 1) with the in-plane epitaxial relationship of [1 1 −2 0]AlN//[0 1 −1](Mn,Zn)Fe2O4. The lattice mismatch for this alignment is calculated to be 6%. The FWHM value of the AlN (0 0 0 2) X-ray rocking curve is as low as 77 arcsec, which indicates that the density of the threading screw dislocations in the AlN film is quite low.

GaN films were grown on (La0.29,Sr0.71)(Al0.65,Ta0.35)O3 (LSAT) (111) substrates, the lattice constant of which matched the 3×3 structure of GaN (0001) and the thermal expansion coefficient of which was close to that of GaN, by atmospheric metalorganic chemical vapor deposition. It was found that the surface of LSAT having a perovskite crystal structure was damaged in ambient of NH3 and TMG gas. However, the epitaxial growth of GaN film on the LSAT substrate was achieved by using an AlN blocking layer to prevent the damage by these gases. The crystallographic orientation was evaluated from a phi-scan of 4-cycle X-ray diffraction to be GaN[1100]∥LSAT[110] rotating in plane by 30° against the expected orientation (GaN[2110]∥LSAT[110]). The 30° rotation would be caused by the bond configuration of the surface of the LSAT substrate. The interface structure at the substrate and the threading dislocation in the films were also investigated using a cross-sectional transmission electron microscope.

The activation of metallorganic chemical vapor deposition-grown Mg-doped GaN by N 2 annealing with thin Co and Pt films has been investigated. The Hall effect measurements revealed that both the Co and Pt films enhance activation of Mg acceptors as catalysts at low temperatures. A maximum hole concentration of p-type GaN was achieved at annealing temperature of 600 °C for the samples activated with both the Co and Pt films. It was also revealed that the activation of the acceptors is strongly affected by the thickness of the Co film.

We have grown group III nitride epitaxial films on various substrates by pulsed laser deposition and investigated structural properties of the surfaces and the hetero-interfaces using grazing incidence angle X-ray reflectivity (GIXR) and atomic force microscopy (AFM). It has been found that hetero-interfaces between PLD AlN and conventional substrates such as Al 2 O 3 and Si are quite abrupt (about 0.5 nm) probably due to a less reactive growth ambience. However, we observed a thin interfacial layer (less than 10 nm) at the hetero-interface between AlN and (Mn,Zn)Fe 2 O 4 . The surface roughness of AlN is mainly determined by the extent of the lattice mismatch. It has been also found that the roughness at the hetero-interface between GaN and AlN formed by PLD coincides with the surface roughness of the AlN layer.

Electrical characteristics of Mg-doped p-GaN activated with Ni catalytic layers have been investigated by means of temperature-dependent Hall effect measurements. It has been revealed that the Ni layer on GaN enhances activation of the acceptor in the whole annealing temperature range. This enhancement is remarkable especially at temperatures below 500 °C and can be attributed to the catalytic effect for the hydrogen desorption. We have found that the donor concentrations in the samples activated with the Ni catalysts are also higher than those activated without Ni. This is probably due to the catalytic effect of Ni for the nitrogen desorption. In addition, it has been found that the ionization energies of the Mg acceptors in GaN activated with the Ni layer are decreased from 170 to 118 meV as the acceptor concentration increases.

We have investigated the effect of an MnAs buffer layer for the epitaxial growth of GaAs on MnZn ferrite substrates using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM), and glazing incidence-angle X-ray diffraction (GIXD). The RHEED and XRD observation have shown that the crystal quality of the GaAs layer is dramatically improved by inserting the MnAs buffer layer in spite of its large lattice mismatch. AFM observation has revealed that the use of the MnAs buffer also leads to the improvement in surface morphology. The epitaxial relationship for GaAs/MnAs/ferrite structure turned out to be [1 1 1]GaAs//[0 0 0 1]MnAs//[1 1 1]ferrite and [1 1 2 ] GaAs// [1 1 0 0] MnAs// [1 1 2 ] ferrite. These improvements can be attributed to the efficient relaxation of the lattice strain due to the softness of the MnAs metal bonding.

We have grown group III nitride epitaxial films on Mn–Zn ferrite substrates by laser MBE and investigated the surface distortion of the film using generalized grazing incidence-angle X-ray diffraction (G-GIXD). In-plane and out-of-plane diffraction spots were observed and the lattice constant of the surface region was calculated from its position. It has been found that AlN and GaN films directly grown on Mn–Zn ferrite are dilated in the perpendicular direction to the surface and compressed in the lateral direction. On the other hand, GaN grown on Mn–Zn ferrite using an AlN buffer layer is compressed in the perpendicular direction to the surface in spite of its relaxation in the lateral direction. This result indicates that the simple Hooke’s law of the bulk crystal does not hold at surfaces of GaN. It can be concluded that G-GIXD is powerful technique to analyze the lattice distortion in the surface-active region of the semiconductor devices.

We have characterized GaN films and their interfaces with sapphire (0001) substrates prepared by pulsed laser deposition (PLD) using coaxial impact collision ion scattering spectroscopy (CAICISS). Grazing incidence X-ray reflection (GIXR) measurements have revealed the absence of the interfacial layer. The abruptness of the heterointerface between the films and the substrate is estimated to be 0.7 nm. CAICISS analysis has revealed that the GaN films without and with an AlN buffer layer have the polarity of (0001) (N face; -c) and (0001) (Ga face; + c), respectively, which indicates that the insertion of AlN causes a change in the crystal polarity.

We have grown GaN on (110) NdGaO 3 (NGO) substrates by pulsed laser deposition (PLD) and characterized their structural properties using reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), grazing incidence-angle X-ray diffraction and reflectivity (GIXD and GIXR). RHEED and GIXD measurements have revealed that the single crystal hexagonal (1120) GaN grows on NGO (110) and its in-plane epitaxial relationship is GaN [0001]//NGO [111]. The lattice mismatches for this alignment in NGO [111] and NGO [111] direction are calculated to be 0.97 and −4.86%, respectively. It is found that the GaN film on NGO (110) is almost fully relaxed in the (1120) plane. GIXR measurements revealed that the hetero-interface between GaN and the NGO substrate was steep and its interface roughness was as low as 0.5 nm.

We have investigated the hetero-interface between molecular beam epitaxy grown MnAs thin films and Mn–Zn ferrite substrates using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). The RHEED observations have shown that the mismatch between MnAs and Mn–Zn ferrite is fully relaxed at the film thickness of

Catalytic effect of Ni for activation of metalorganic chemical vapor deposition-grown Mg-doped GaN in N2 and N2O has been investigated. P-type GaN has been obtained at an annealing temperature as low as 200°C using the Ni catalytic film in N2O as well as in N2. Moreover, the hole concentrations for the samples obtained by annealing in N2O were in good agreement with those obtained in N2 in the whole annealing temperature range. These results indicate that the Ni film effectively acts as a catalyst to enhance hydrogen desorption at the surface of GaN, which results in the reduction of hydrogen concentration in GaN at low annealing temperatures. It has also been suggested that the catalytic activity of Ni to enhance hydrogen desorption is much stronger than that of N2O. Therefore, an introduction of oxygen in the annealing atmosphere is not necessary for effective removal of hydrogen under the existence of the Ni film.

The recent development of contrast echography has made renal enhancement possible through an intravenous injection of microbubble-based contrast. In animal models, tissue perfusion can be quantified using contrast echography by measurement of the rate at which microbubbles replenish tissue after their ultrasound-induced destruction. Our purpose in this study was to evaluate renal blood flow with contrast echography in humans. To increase the sensitivity for microbubbles, we used a combination of power Doppler harmonic and intermittent imaging. The pulsing interval (PI) was changed from 10 cardiac cycles to 1 cardiac cycle during an intravenous infusion of the contrast agent, and alterations in the intensity of the renal cortex were represented as a decline ratio (DR). In 24 patients with various renal diseases, we were able to observe all 48 kidneys with adequate enhancement of the renal cortex. At PI of 10 cardiac cycles, the enhancement was homogeneous and strong, while, obviously, changing PI from 10 to 1 cardiac cycles caused a decline of enhancement. An excellent correlation was found between DR using contrast echography and renal plasma flow determined by clearance and radionuclide measurements. An excellent correlation was found between the DR values determined by contrast echography and the renal plasma flow values determined using clearance and radionuclide measurements. These results suggest that DR may be useful for evaluation of both total and split renal blood flow. Thus the contrast echographic method presented here could succeed in assessing renal cortical blood flow less invasively than conventional methods in humans.

Structural properties of InAs quantum dots grown on Si (1 0 0) substrate have been investigated with G-GIXD reciprocal space mapping. Although both the volume (37 nm in diameter) and the surface coverage (about 11%) of the InAs dots are small, strong diffraction spots have been observed. To investigate the origin of the peak broadening, we compared the experimental data with structural factors calculated with FEM. We have found that the size of diffraction spots from the InAs dots is determined not only by their size and within-a-dot strain distribution, but also by the dot-to-dot strain distribution.

Highly conductive Ge-doped GaN epitaxial layers were grown by low-temperature pulsed sputtering, and their fundamental structural and electrical properties were investigated. The room-temperature (RT) electron concentration was increased to 5.1 × 1020 cm−3 by the Ge doping, and the atomically flat stepped and terraced surface and the crystalline quality of the layers were maintained. Consequently, the RT resistivity was reduced to 0.20 mΩcm, which is comparable to that for typical transparent conductive oxides such as indium tin oxide. The contact resistance of Ge-doped GaN with a Ti/Al/Ti/Au metal stack prepared without annealing was as low as 0.087 Ωmm. Furthermore, the selective formation of a Ge-doped region using an SiO2 mask was demonstrated. The results clearly indicate the strong potential of pulsed sputtering Ge-doped GaN growth for forming low-parasitic-resistance contact layers of various electrical and optical devices.

We grew thick InN films on yttria-stabilized zirconia (YSZ) substrates by high-growth-rate pulsed sputtering deposition. X-ray diffraction analysis revealed that the in-plane orientation distribution of the InN (0001) films on YSZ decreased as the film thickness increased. The edge dislocation density of the 9.0-µm-thick InN film grown at a rate of 6 µm h−1 is calculated to be 1.7 × 109 cm−2, and the atomically flat surface exhibited a step-and-terrace structure. In addition, the crystallinity of semipolar InN is improved by increasing the film thickness. These results indicate that high-growth-rate pulsed sputtering deposition can be used to prepare InN substrates for the fabrication of high-speed electron devices and InGaN-based, long-wavelength light-emitting devices.

We demonstrated the crystal growth of wafer-scale hexagonal boron nitride (hBN) films on AlN template substrates by pulsed sputtering deposition using Fe catalysts. It was found that hBN films with high crystalline quality were formed at the hetero-interfaces between the Fe catalytic layers and the AlN templates. The full width at half maximum value of the E2g Raman spectrum for the hBN film was as small as 5.1 cm−1. The hBN film showed a highly c-axis-oriented structure and a strong near-band-edge deep ultraviolet emission at room temperature. The present results indicate that the sputter synthesis of hBN films via interface segregation opens a new pathway for the wafer-scale production of high-quality hBN films, and we envisage its potential applications in the fabrication of prospective hBN-based optoelectronic devices.