Kazufumi Azuma

Optical emission spectroscopy study toward high rate growth of microcrystalline silicon

Abstract A systematic optical emission spectroscopy (OES) study was carried out to enhance the deposition rate of microcrystalline silicon (μc-Si:H) with conventional r.f. plasma-enhanced chemical vapor deposition (r.f. PECVD). Among the various plasma parameters, the combination of total pressure, r.f. power, electrode distance and cathode heating was effective to promote the deposition rate without deteriorating the film crystallinity. Strong correlations among the OES intensity, SiH, intensity ratio, I H α / I Si* , deposition rate and Raman intensity ratio, I μc-Si / I a-Si were confirmed in the case of r.f. SiH 4 and H 2 PECVD. A relatively high deposition rate was achieved of ∼5 A/s in the μc-Si:H film growth by optimizing the deposition parameters. The effects of higher pressure, higher r.f. power, inter electrode distance and cathode heating (SiH 4 gas heating) are demonstrated in the growth of μc-Si:H from strong H 2 -diluted SiH 4 by a conventional r.f. glow discharge.

Surface Wave Plasma Oxidation at Low Temperature under Rare Gas Dilution

Surface wave plasma oxidation was investigated under Kr gas dilution for the formation of the gate dielectric of poly-Si thin film transistors (TFTs). We clarified the plasma kinetics of plasma oxidation under rare gas dilution and the relationship between the oxidation rate and the plasma characteristics. It was also clarified that the oxygen atom in the surface wave plasma under Kr gas dilution is mainly generated by two processes: the electron impact dissociation of O2 gas and the quenching of Kr metastables through the reaction with O2 gas. The plasma kinetics under rare gas dilution for plasma oxidation can be explained as follows: The plasma highly diluted with rare gas generates high-density electrons and rare gas metastables, which efficiently generate high-density oxygen atoms from a small amount of O2 gas. Then the high-density oxygen atoms increase plasma oxidation rate. The structure and interface properties of the plasma-oxide films at 300°C are very similar to those of the thermal-oxide. At low temperatures, surface wave plasma oxidation is very effective especially under rare gas dilution.

Gap States in a-SiGe:H Examined by the Constant Photocurrent Method

Gap states in a-SiGe:H alloys were examined by the deconvolution analysis of subgap absorption spectra obtained by the constant photocurrent method. A broad distribution of defect states was found in a-SiGe:H alloys by the analysis postulating a Gaussian distribution of defect states. Analysis postulating two Gaussian peaks suggested that defects in a-SiGe:H can be divided into Si- related and Ge-related types. The energy positions of two defect peaks relative to the conduction band edge were almost constant irrespective of the optical gap of alloy films, and the position of one peak agreed with the defect peak in a-Si:H. The other peak which locates around 0.75 eV below the conduction band edge appeared when Ge was introduced into a-Si:H. The ratio of these two peaks exhibited a fairly good correlation to the Ge/Si ratio of alloys.

Chemical Vapor Deposition of a-SiGe:H Films Utilizing a Microwave-Excited Plasma

Hydrogenated amorphous silicon-germanium films were prepared by a chemical vapor deposition method utilizing a microwave-excited plasma produced at low gas pressures (~10-3 Torr). Deposition methods in which SiH4 and GeH4 were decomposed by a plasma stream of Ar or hydrogen or by direct excitation were examined at 200°C and at the pressures optimized for the preparation of hydrogenerated amorphous silicon films. Highly photoconductive narrow gap films (an optical gap of 1.5 eV) with photoconductivity higher than 10-5 S/cm and photosensitivity of about 104 were obtained by these methods, keeping a high deposition rate.

Microwave-Excited Plasma CVD of a-Si:H Films Utilizing a Hydrogen Plasma Stream or by Direct Excitation of Silane

The microwave-excited plasma CVD of a-Si:H films, in which SiH4 was decomposed by a plasma stream of hydrogen or by direct excitation at low gas pressures (10-4 to 10-3 Torr), was examined and compared with a previously studied Ar plasma stream method. It was clarified that an Ar plasma stream was not necessary to obtain highly photoconductive a-Si:H films. However, the deposition characteristics in these deposition methods were different from the Ar plasma stream method. On the other hand, the influence of the hydrogen plasma stream was not apparent when the SiH4 flow rate was high, suggesting an ionization cross-section dependence of the plasma discharge.

Characterization of high-performance polycrystalline silicon complementary metal-oxide-semiconductor circuits

Polycrystalline silicon (poly-Si) complementary metal–oxide–semiconductor (CMOS) circuits have been fabricated by using an advanced excimer-laser annealing method and a plasma-oxidation method. The 1-µm-long thin-film transistors (TFTs) were fabricated on arrays of laterally grown long and narrow grains, so that the majority of carriers were free from scattering at grain boundaries during propagation through the channel. The propagation delay time measured by a 21-stage ring oscillator was 175 ps and a power-delay product of 9×10-13 J/gate was obtained at a supply voltage of 3.3 V. The obtained propagation delay time was almost the same as those of bulk Si devices having the same gate length. Furthermore, we expect that 1-µm-long CMOS TFT circuits on glass will have a performance superior to that of 1-µm-long bulk Si devices when the short channel effect and threshold voltage fluctuation are controlled well.

X-ray photoelectron spectroscopy study on SiO2/Si interface structures formed by three kinds of atomic oxygen at 300 °C

Using the high-brilliant synchrotron radiation at SPring-8 we have studied the SiO2/Si interface structures, the interface state densities, and the uniformities of ∼1-nm-thick oxide films formed by three kinds of atomic oxygen at 300 °C by measuring Si 2p photoelectron spectra at the photon energy of 1050 eV and the energy loss spectra of O 1s photoelectrons at the photon energy of 714 eV. Among silicon oxide films studied here the abrupt compositional transition at SiO2/Si interface, the smallest deviation in interface state density, the interface state density comparable to that for thermal oxide formed in dry oxygen at 950 °C, and the highest uniformity was obtained with oxide film formed in krypton-mixed oxygen (Kr:O2=97:3) plasma.

Chemical Vapor Deposition of a-Si:H Films Utilizing a Microwave Excited Ar Plasma Stream

A Microwave-excited Ar-plasma stream produced at low gas pressures (10-4 to 10-2 Torr) was applied to a preparation of hydrogenated amorphous silicon films made from SiH4. The electric and optical properties of the films were investigated as functions of the deposition conditions. The film properties greatly depended upon the substrate temperature and the deposition pressure and slightly upon the microwave power. The conductivity of the films prepared at 200°C and at pressures ranging between 0.6 and 0.8 mTorr were about 5×10-10 Scm-1 in the dark and 4×10-4 Scm-1 under AM 1.5 illumination. The films prepared at the lower pressures were porous and contain a large amount of gases; the films prepared at higher pressures were deteriorated in the same way as those prepared by an RF glow discharge from Ar-diluted SiH4.

Characterization of Novel Polycrystalline Silicon Thin-Film Transistors with Long and Narrow Grains

Excellent characteristics of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with long and narrow grains aligned one-dimension have been experimentally clarified for the first time. The field effect mobility and on-off transition slope of n-channel and p-channel devices were as high as 685 cm2 V-1 s-1 and 190 mV/decade and 145 cm2 V-1 s-1 and 104 mV/decade, respectively. Fluctuations of characteristics were considerably reduced by widening the channel, and uniform characteristics were observed when there were approximately twenty long grains within the channel. These results were obtained when the TFT channel was formed within a region free from grain boundaries formed by head-on collision of laterally growing grains and seeds used to initiate lateral grain growth. Material properties are discussed from the viewpoint of device characteristics.

Medium-Range Order of Amorphous Silicon Germanium Alloys: Small-Angle X-Ray Scattering Study

A medium-range order of hydrogenated amorphous silicon germanium (a-Si1-xGex:H) alloys is investigated using small-angle X-ray scattering. It is found that the scattering intensities I(h) of a-Si:H and a-Ge:H decrease with scattering vector h as represented by I(h)~h-3. However, in the case of a-Si1-xGex:H alloys, the scattering intensity decreases with h as represented by I(h)~h-P (P<3). This shows that the alloys take on a mass fractal structure. At around x=0.5, the fractal dimension tends to approach the value of 2.