Takao Inokuma

Properties of “Stoichiometric" Silicon Oxynitride Films

The properties of amorphous SiO x N v films prepared by rf glow discharge of a SiH 4 -O 2 -NH 3 mixture at 300°C are investigated as functions of the O content, x, and the N content, y. A relationship of 2x + 2.8y = 4 was found between x and y. This result suggests that the densities of homobonds, such as Si-Si, O-O and N-N, and of Si-H and O-H bonds are sufficiently lower than those of Si-O and Si-N bonds. The absence of splitting of stretching absorption bands arising from Si-O and Si-N bonds suggests that the film is a homogeneous alloy. The properties of the dangling bonds and their origins are discussed.

Inversion channel diamond metal-oxide-semiconductor field-effect transistor with normally off characteristics.

We fabricated inversion channel diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with normally off characteristics. At present, Si MOSFETs and insulated gate bipolar transistors (IGBTs) with inversion channels are widely used because of their high controllability of electric power and high tolerance. Although a diamond semiconductor is considered to be a material with a strong potential for application in next-generation power devices, diamond MOSFETs with an inversion channel have not yet been reported. We precisely controlled the MOS interface for diamond by wet annealing and fabricated p-channel and planar-type MOSFETs with phosphorus-doped n-type body on diamond (111) substrate. The gate oxide of Al2O3 was deposited onto the n-type diamond body by atomic layer deposition at 300 °C. The drain current was controlled by the negative gate voltage, indicating that an inversion channel with a p-type character was formed at a high-quality n-type diamond body/Al2O3 interface. The maximum drain current density and the field-effect mobility of a diamond MOSFET with a gate electrode length of 5 μm were 1.6 mA/mm and 8.0 cm2/Vs, respectively, at room temperature.

Effects of the Addition of SiF4 to the SiH4 Feed Gas for Depositing Polycrystalline Silicon Films at Low Temperature

Polycrystalline silicon films with a strong (110) texture were prepared on a corning 7059 glass substrate at 400° C by a plasma-enhanced chemical vapor deposition method using a mixture of gases with different SiF4 flow rates ( [SiF4] = 0–0.5 sccm) under a fixed SiH4 flow rate ( [SiH4] = 1 or 0.15 sccm). The effects of the addition of SiF4 to SiH4 on the structural properties of the films were studied by Raman scattering, X-ray diffraction (XRD), atomic force microscopy and stress measurements. For [SiH4] = 1 sccm, the crystallinity and the (110) XRD grain size monotonically increased with increasing [SiF4], and their respective maxima reach 90% and 900 A. In contrast, for [SiH4] = 0.15 sccm, both the crystallinity and the grain size decreased with [ SiF4]. Mechanisms causing the change in crystallinity are discussed, and it was suggested that an improvement in the crystallinity, due to the addition of SiF4, is likely to be caused by the effect of a change in the surface morphology of the substrates along with the effect of in situ chemical cleaning. On the other hand, both the effect of in situ chemical etching on the growing surface and the effect of a change in the capability for surface migration of adsorbates may provide a minor contribution in determining the crystallinity.

Effects of Addition of SiF4 During Growth of Nanocrystalline Silicon Films Deposited at 100?C by Plasma-Enhanced Chemical Vapor Deposition

Structural properties of nanocrystalline silicon (nc-Si) films, deposited at 100°C using SiF4/SiH4/H2 by plasma-enhanced chemical vapor deposition, were investigated by changing the SiF4 flow rate, [SiF4]. At a certain low [SiF4] value (=[SiF4]S), both the crystallinity and the grain size had minimum values. The Raman peak shifts corresponded well with a change in stress, and films with [SiF4]S were suggested to be free from random stress in the local Si–Si networks. The photoluminescence spectra had the highest intensity and the highest peak energy at [SiF4]S. It was proposed that nc-Si films with high [SiF4] have microvoid-like grain boundaries with high densities of Si–F and SiH2 bonds, exhibiting an increase in crystallinity and susceptibility to O contamination after deposition. These results were interpreted in terms of the change in the etch rates by H and F radicals, depending on deposition temperature.

Initial Growth of Polycrystalline Silicon Films on Substrates Subjected to Different Plasma Treatments

Undoped 0.2-µm-thick polycrystalline Si (poly-Si) films were deposited on fused quartz substrates by plasma-enhanced chemical vapor deposition from a SiH4–H2 mixture. All films were prepared under the same deposition conditions, just after the substrates were exposed for 1 min to CF4–He plasma excited with various rf powers. Poly-Si films with improved crystallinity and large grains were obtained when the films were deposited on substrates with the proper degree of surface roughness of uniform size and shape. These films were also found to have lower values of stress and higher values of g, as compared with those of poly-Si films on substrates with a flat surface or an extremely rough surface. The X-ray diffraction (XRD) spectra exhibited only the texture, and the intensity was proportional to the third power of the average grain size estimated from the width values of the XRD spectra. These results suggested that the growth of grains is three-dimensionally controlled and depends on the surface morphology of the substrates, while the concentration of grains per unit area is roughly independent of the morphology of the substrates.

Structural and electrical characterization of oxidated, nitridated and oxi-nitridated (100) GaAs surfaces

Oxidation by the UV & ozone process, nitridation by the nitrogen helicon-wave-excited plasma process, and the combination of these processes are applied to (100) GaAs wafers. An atomic force microscope, X-ray photoelectron spectroscopy, a transmission electron microscope, photoluminescence and electrical characteristics (current–voltage and capacitance–voltage) were used to analyze the influences of these processes on the structure and composition of the surfaces and the interfaces. Metal–insulator–semiconductor (MIS) diodes and Schottky diodes were fabricated in order to investigate the electrical influences of these processes. The oxidation slightly disorders GaAs surfaces. Nitridation of a bare surface creates about a 2-nm-thick strongly disordered layer, which strongly deteriorates the electrical and photoluminescence characteristics. Nitridation of oxidated wafers (oxi-nitridation) forms firm amorphous GaON layers, which contain GaN, with very flat and sharp GaON/GaAs interfaces, where crystal disorder is hardly observed. It improves the current–voltage (I–V) and capacitance–voltage (C–V) characteristics and the photoluminescence intensity. Results of the structural and the electrical characterizations qualitatively coincide well with each other.

Studies of Effects of Adsorption of Silicon or Germanium on the Electronic States of (100) GaAs Surfaces

The electronic states of both bare Ga-terminated and As-terminated (100)-(1×1) GaAs surfaces and the same surfaces with adsorbed Si or Ge were studied by molecular orbital calculations using small cluster models and the discrete variational Xα method. The most stable adsorption sites for Si and Ge are bridge sites on the (100)-GaAs surface, regardless of whether they are Ga- or As-terminated. When Si or Ge atoms are adsorbed, the density of the surface states in the forbidden band of the Ga or As surface layers markedly decreases. This suggests that both the Ga-terminated and As-terminated surfaces have a clear band gap when either Si or Ge are adsorbed on the surfaces. These adsorbates have states in the lower half of the band gap or in the valence band.

Two Levels of Ni/n-GaAs Schottky Barrier Heights Formed on a Wafer by Controlling pH of Pretreatment Chemicals: Effect of Oxygen Adsorption

Schottky barrier heights of Ni/n-GaAs junctions were controlled by changing the pH of pretreatment chemicals. An effective barrier height of 0.8 eV was obtained by treatment with dilute HCl liquid (pH = l), and 0.6 eV by treatment with dilute NH4OH liquid (pH = 13). Surface analysis by an x-ray photoelectron spectroscopy indicated the existence of about twice the density of oxygen at the surface of the HCl-treated wafer as compared with that pretreated by the NH4OH liquid. The former shows nearly linear C-2–V characteristics, while the latter shows larger but less frequency dependent capacitances. Two levels of Ni/n-GaAs Schottky barriers were formed on a wafer by successive pretreatments and selective Ni evaporations.

Effects of Nitrogen Addition to Fluorinated Silicon Dioxide Films

Amorphous fluorinated silicon dioxide (a-SiO2:F) films doped with nitrogen were deposited by changing the ammonia flow rate using plasma-enhanced chemical vapor deposition from SiH4–O2–CF4–NH3 mixtures. The effects of nitrogen addition to the films on both the dielectric constant (es) determined from the capacitance vs. voltage characteristics and the bonding properties examined by infrared absorption measurements, were investigated. These results were also discussed in terms of a change in the partial charge on the constituent Si, O, F and N atoms caused by adding F and N atoms to SiO2 films. When a-SiO2 films are doped by 2-3 at.% with both fluorine and nitrogen under high rf power and high deposition temperature (Td) conditions, films with low es(≈3.2) and high water resistivity were obtained. It was suggested that Si–F bonds, which act to decrease es value, are stabilized by forming Si–N bonds near the Si–F bonds and by removing weaker Si–F bonds under high rf power and high Td.

TEMPERATURE EFFECTS ON THE STRUCTURE OF POLYCRYSTALLINE SILICON FILMS BY GLOW-DISCHARGE DECOMPOSITION USING SIH4/SIF4

Polycrystalline silicon (poly-Si) films were prepared on glass substrates by the plasma- enhanced chemical vapor deposition method using SiH4/SiF4 mixtures as a function of deposition temperature, Td, from 150 to 400°C, and the structural properties were investigated. In addition, the effects of addition of H2 to the SiH4/SiF4 gas were also examined. The crystallinity and grain size of Si films with added were found to have maximum values at around Td = 250–300°C. However, poly-Si films without H2 addition contain numerous microvoids, and exhibit easy O contamination, and their crystallinity monotonically increased with Td. The change in the SiH and SiH2 bond density with H2 addition was consistently interpreted in terms of this model. As a result, H2 addition at low Td was suggested to suppress O contamination and improve the crystalline quality. By contrast, H2 addition at high Td is likely to deteriorate crystalline quality. The results were discussed in terms of fluorine and hydrogen chemistry.