Mitsuyuki Yamanaka

Electrical properties of Si nanocrystals embedded in an ultrathin oxide

We investigate Si nanocrystals fabricated by the rapid thermal oxidation (RTO) of an ultrathin chemical vapour deposition (CVD) amorphous Si (a-Si:H) film. It is found from the transmission electron microscope (TEM) observation that the ultrathin RTO film contains Si nanocrystals of around or less than 5 nm in size. The dynamic electrical conduction measurement of the RTO diode structure including the Si nanocrystals reveals novel features such as the N-shaped tunnel current versus gate voltage characteristics and the hysteresis. It is also found that the gate voltages at the first and second current rise are fixed and the current reduction in the fixed time interval is observed at the constant gate voltage. These findings can be explained by the fixed-amount electron charging effect at the Si nanocrystals and the consequent screening effect on the tunnel current flowing through the diode structure.

Deep defect states in hydrogenated amorphous silicon studied by a constant photocurrent method

Experimental studies by means of a constant photocurrent method (CPM) have been carried out on the deep defect states in undoped hydrogenated amorphous silicon (a‐Si:H). Assuming Gaussian energy distributions, two types of defect states (ST1 and ST2) have been found from careful analysis of CPM spectra; one of these states (ST1) is a neutral Si dangling‐bond (Si D0) state, and the other (ST2) is a negatively charged dangling‐bond‐like defect state located in the lower gap. ST2 lies deeper in energy by 0.1 eV, and has a narrower full width at half‐maximum (FWHM) when compared with ST1. Possible candidates for ST2 have been discussed by referring to the previous models of defects in a‐Si:H. The FWHM of ST1 (the Si D0 state) does not always depend on the overall structural disorder estimated from Raman spectra. The energy position of the Si D0 state measured from the valence‐band edge is almost independent of the optical band gap. Long exposure to light increases the density of ST1 (the Si D0 state) and decr...

Effects of Optical Confinement in Textured Antireflection Coating using ZnO Films for Solar Cells

The effect of optical confinement in textured antireflection coating (AR coating) was investigated. Textured ZnO films prepared by metalorganic chemical vapor deposition were applied to solar cells as AR coating. The reflectance of the cells decreased as the grain size increased with film thickness, especially at long wavelength. This reduction in the reflectance caused an increase in short-circuit current and spectral response of the cells at near-infrared. The effect of optical confinement in the cell by this film was shown as far as long-wavelength range was concerned.

Properties of hydrogenated amorphous silicon prepared by alternatively repeating chemical‐vapor deposition from disilane and hydrogen plasma treatment

Experimental studies have been carried out to characterize hydrogenated amorphous silicon prepared by alternatively repeating chemical‐vapor deposition (CVD) from disilane and hydrogen plasma (HP) treatment (referred to as HP treated CVD a‐Si:H). It has been found that hydrogen plasma treatment induces structural relaxation of Si network in addition to the passivation of Si dangling bonds. Two types of defect states with different Gaussian energy distribution exist in this material, and this type of distribution has been found to be common in undoped a‐Si:H prepared by a variety of methods. High film quality, i.e., a low defect density (<1016 cm−3) and a fairly long ambipolar diffusion length (0.12 μm), and significant reduction of light‐induced changes have been simultaneously achieved in HP treated CVD a‐Si:H. Steady state transport of electrons and holes under illumination are controlled by the states other than neutral Si dangling bond state (D0). The behavior of these states with long exposure to lig...

A new characterization parameter for hydrogenated amorphous silicon: B (the square of the gradient of the (αh/ω)1/2 versus h/ω plot)

We have found that B (the square of the gradient of (αh/ω)1/2 versus h/ω plot) is dependent on the preparation conditions of a‐Si:H films and has a distinct correlation with photoconductivity. B includes information on the tail states and the band edge of extended states of a‐Si:H. B is proposed as one of important characterization parameters for a‐Si:H as well as photoconductivity.

Fabrication technology of a Si nanowire memory transistor using an inorganic electron beam resist process

We report on a novel fabrication technology of a Si nanowire memory transistor using an inorganic SiO2 electron beam (EB) resist process. The inorganic EB resist process technique was put to practical use in Si nanodevice fabrication for the first time. We have successfully demonstrated the 15-nm-wide and 20-nm-thick Si nanowire memory transistor with a Si nanodot less than 10 nm in diameter. In the fabricated Si nanowire nanodot memory transistor, we have observed a large electron memory effect, i.e., a threshold voltage shift ΔVth of 2.2 V at room temperature. It is experimentally shown that the inorganic EB resist process is promising for fabricating various Si nanodevices.

Relationship between carrier diffusion lengths and defect density in hydrogenated amorphous silicon

Experimental studies and numerical analysis have been carried out to clarify the relationship between carrier diffusion lengths and defect density in undoped a-Si:H. It has been confirmed that in device quality plasma-deposited a-Si:H, the diffusion lengths of both electrons and holes under steady-state illumination of the intensity equivalent to normal solar cell operating conditions are determined by the density of Si dangling-bond defects ranging between 3×1015 and 8×1016 cm−3. This rather trivial result, however, has not been obtained in previous studies in which the carrier transport data obtained by the steady-state photocarrier grating method were treated incorrectly. The ratio of the drift mobility of electrons to that of holes and the ratio of electron lifetime to the hole lifetime in a-Si:H under illumination have been determined and their implications discussed.

Characterization of hydrogenated amorphous silicon by capacitance-voltage and surface photovoltage measurements using liquid Schottky barriers

Experimental studies have been made on the electrical properties of hydrogenated amorphous silicon (a‐Si:H) using liquid Schottky barriers. We have found that the quasi‐static capacitance‐voltage (C‐V) method can be applied to the a‐Si:H/quinone‐hydroquinone (Q‐HQ) liquid Schottky junction. This method enables us to determine the net density of positive space charge due to ionized traps and impurities in a‐Si:H (Ne), the built‐in potential (Vb), and the width of the surface space‐charge layer (W), of this liquid junction. The barrier height of an undoped a‐Si:H/Q‐HQ junction has been estimated to be more than 1 eV from the value of Vb thus obtained. By C‐V and surface photovoltage (SPV) measurements on the same samples, we have studied the changes in the properties of a‐Si:H with doping and with prolonged illumination. It has been found that phosphorous (P) doping drastically decreases the hole diffusion length measured by the SPV method and increases the value of Ne. Slight boron (B) doping increases the...

Stress as a governing parameter to control the crystallization of amorphous silicon films by thermal annealing

We have fabricated nanocrystalline dots by thermal annealing (TA) of thermal chemical-vapor-deposited amorphous silicon (a-Si:H) films. In S. Hazra, I. Sakata, M. Yamanaka, and E. Suzuki, Appl. Phys. Lett. 80, 1159 (2002), we observed that ultrathin a-Si:H films (2–10 nm) are stressed because of the presence of deformed crystallites or paracrystallites. With the increase of thickness, volume fraction of paracrystallites decreased in the films and stress in the films gradually reduced. Therefore, by changing the thickness, we can control the stress in the a-Si:H films and thereby the dimensions (in the range of 5 to 10 nm) as well as volume fractions of nanocrystallites formed by TA. On the other hand, it has been found that relaxed a-Si:H networks form polycrystalline films by TA.

Very Low Temperature Epitaxial Growth of Silicon Films for Solar Cells

Low-temperature n-type silicon epitaxial growth has been studied for the application to n+/p homojunction solar cells. Epitaxial Si films can be grown at the substrate temperatures from room temperature (RT) to 200 °C by plasma-enhanced chemical vapor deposition. Two-step growth is effective for obtaining high-quality layers above 150 °C. In this growth, a very thin Si layer has been deposited on Si substrates at a low hydrogen-to-silane gas-flow ratio, and subsequently an epitaxial Si layer has been grown at a higher ratio. Smooth nucleation with reduced local stress has been enhanced in the initial stage of the epitaxial growth. An open-circuit voltage, Voc, of 0.610 V and a conversion efficiency, η, of 13.54% have been obtained for an n+/p cell, where the n+ layer was deposited at 200 °C. Film growth mode changes below 100 °C. The stress in the film during the epitaxial growth is relieved by introducing a small fluctuation of crystal orientation and the voids in the film at these temperatures. We have succeeded in fabricating a solar cell with a Voc of 0.608 V and an η of 13.52% having an n+ layer deposited at RT.