Saburo Shimizu

Essential Change in Crystal Qualities of GaN Films by Controlling Lattice Polarity in Molecular Beam Epitaxy

GaN heteroepitaxial growth on sapphire (0001) substrates was carried out by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). A Ga-polarity growth was achieved by using an AlN high-temperature buffer layer. The epilayer polarity was characterized directly by coaxial impact collision ion scattering spectra (CAICISS). It was found that the properties of the GaN films showing Ga-face polarity, including their structural and electrical properties, were dramatically improved compared to those of films with N-face polarity. This important conclusion is considered to be a breakthrough in the realization of high-quality III-nitride films by MBE for device applications.

Characterization of Polarity of Wurtzite GaN Film Grown by Molecular Beam Epitaxy Using NH3

Effects of the initial nitridation of a sapphire(0001) substrate by NH3 on the polarity of GaN{0001} film have been investigated by coaxial impact collision ion scattering spectroscopy. The polarity of ammonia-molecular beam epitaxial (MBE) film grown on the substrate nitrided using NH3 is assigned as (0001). The effect of the initial nitridation of the substrate by NH3 is found to be contrary to that by nitrogen plasma, where the GaN film grown on the nitrided substrate shows the polarity of (0001). The polarity of GaN film grown by rf plasma-assisted MBE on the substrate which is nitrided using NH3 is also (0001). These findings suggest the possibility of polarity control of the grown GaN film by choosing the N source for initial nitridation of the substrate.

A Molecular and Ion-Beam Epitaxy System for the Growth of III-V Compound Semiconductors Using a Mass-Separated, Low-Energy Group-V Ion Beam

A new technique for growing III-V compound semiconductor films has been developed, in which the sticking coefficient of the group V element can be increased by implanting mass-separated, high-purity group-V ions at low energy. The newly-constructed growth system consists of an ordinary MBE system and a UHV low-energy ion-implantation system. The deceleration characteristics of the group V (arsenic and phosphorus) ion beams obtained with this low-energy ion implantation system are described. Single-crystal GaAs films are obtained with this growth method between 220°C and 550°C even with a flux ratio of unity by implanting As+ ions at 100 and 200 eV. The PL spectra at 4.2 K of the non-doped films grown at 450°C or higher show well-defined features due to bound exciton and donor-acceptor pairs. High-quality InP layers are also obtained between 210°C and 420°C even with a flux ratio of unity by implanting P+ ions at 100 and 200 eV. These results indicate that the controllability of the group V element in MBE can be greatly improved by implanting mass-separated, high-purity group-V ions at low energy.

Computer Simulation for Analysis of Lattice Polarity of Wurtzite GaN{0001} Film by Coaxial Impact Collision Ion Scattering Spectroscopy

Signal intensities of coaxial impact collision ion scattering spectra have been computed for c-axis-oriented GaN films at various incident angles in order to analyse the lattice polarity based on the three-dimensional two-atom triple-scattering model. It was found that Ga and N signal intensities show specific incident angle dependences on (0001) and (000) surfaces. The physical image of each characteristic feature in the spectra was also clarified.

Terminating Structure of Plasma-Assisted Molecular Beam Epitaxial GaN{0001} Film Surface Identified by Coaxial Impact Collision Ion Scattering Spectroscopy

Terminating structures of the GaN{0001} films grown on nitrided sapphire(0001) substrates by plasma-assisted molecular beam epitaxy (MBE) have been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). The analyses of incidence angle dependences of time of flight (TOF) spectra have shown that the surfaces of GaN films grown under both N-rich and Ga-rich conditions are (0001) N-planes terminated with Ga atoms. This implies that (0001), N-terminated surfaces of GaN films grown under these conditions are unstable and a Ga-rich condition is required to avoid the N-deficiency in the grown GaN film.

Characterization of Polarity of Plasma-Assisted Molecular Beam Epitaxial GaN{0001} Film Using Coaxial Impact Collision Ion Scattering Spectroscopy

Effects of the initial nitridation of a sapphire(0001) substrate and the thickness of a low-temperature GaN buffer layer on the polarity of GaN{0001} films grown by rf plasma-assisted molecular beam epitaxy (MBE) have been investigated by coaxial impact collision ion scattering spectroscopy (CAICISS). The volume ratios of each domain in grown GaN{0001} films, that is, (0001) and (000), were evaluated by making comparisons between the experimental results of the incident angle dependences of Ga and N signal intensities and simulated ones. It was determined that the initial nitridation of a sapphire(0001) substrate increases the volume ratio of the (000) domain in the grown film and, on the contrary, a thick low-temperature GaN buffer layer increases that of the (0001) domain.

A high-current low-energy multi-ion beam deposition system

A high-current, low-energy multi-ion beam deposition system has been developed aiming at the fabrication of new materials. This system consists of two ion sources, a dual-sector-type mass analyzer and a decelerating system. Several ion species are extracted successively from the two ion sources by switching the mass analyzer selection. Artificially structured materials, especially having a layered structure, can be grown by the fine control of the growth process of each layer. The developed ion beam deposition system, including the design concept, is described in detail. The deceleration characteristics of this system using Ar+ ion are also shown.

Molecular Beam Epitaxy of InP Using Low Energy P+ Ion Beam

A new film growth technique of III-V compound semiconductors is developed where the sticking coefficient of the group V element can be increased by implanting mass separated, high purity group V ion at low energy. Using this growth technique, the epitaxial growth of InP on (100)InP substrate is studied. Single crystal films are grown between 210°C and 420°C at the P+ ion energy of 200 eV and with the flux intensities of In molecular beam and P+ ion beam of 1.5×1013 cm-2s-1 and 3.0×1013 cm-2s-1, respectively. The films grown between 300°C and 420°C show sharp 2×4 structure in the RHEED pattern as in the case of MBE, and also show well defined features due to bound excitons and donor-acceptor pairs in the photoluminescence spectra at 4.2 K.

Epitaxial Growth of α-Fe Film on Si(111) Substrate by Low-Energy Direct Ion Beam Deposition

Fe film growth on Si(111) substrate was carried out by low-energy direct ion beam deposition. Fe films of about 1000 A thick were grown at Fe+ ion energies of 10 eV, 20 eV, 50 eV and 100 eV without removing the native oxide layer on the Si substrate. Single-crystal α-Fe(111) films were obtained at 20 eV, 50 eV and 100 eV at room temperature, while (110)-preferred-oriented α-Fe film was obtained at 10 eV. Cross-sectional high-resolution transmission electron microscopy (TEM) studies show that the removal of the native oxide layers on the Si substrates by Fe+ ion irradiation results in the epitaxial ordering of the α-Fe films when the films are grown at 20 eV, 50 eV and 100 eV.

Low-Temperature Silicon Epitaxial Growth by Photochemical Vapor Deposition Using Vacuum Ultraviolet Light

Homoepitaxial Si growth by photochemical vapor deposition (photo-CVD) of Si2H6 using vacuum ultraviolet (VUV) light from a microwave-excited D2 lamp has been investigated. Epitaxial Si films can be obtained by this method at a growth temperature of 650°C, which is much lower than that used in conventional thermal CVD. A surface cleaning method of Si substrates by VUV light irradiation has been proposed. It was found from AES and RHEED studies that light irradiation prior to film growth is effective for the elimination of oxide and carbon contaminants on the substrate. Crystalline quality of grown films and dissociation mechanism of Si2H6 by VUV light are also described.