Toshio Nakashita

Defect compensation in doped CVD amorphous silicon

Abstract Structural defects in chemically vapor-deposited amorphous silicon have been shown to be passivated by phosphorus doping. The ESR centers due to defects are eliminated in the doping range 4×10 −4 3 )/N(SiH 4 ) −3 . This defect compensation has been directly confirmed as a marked reduction in the gap state density above midgap. A tentative defect model for explaining the defect compensation is discussed. Effects of H + ion implantation in CVD a-Si are also understood by the defect model.

Surface States in Tunnelable MOS Structures

The bias dependence of the tunneling current, low frequency capacitance and differential conductance of MOS structures with very thin SiO2 films on p-type Si substrates have been investigated over the temperature range from 151 K to 324 K. The surface state density of about ~8×1011 states/cm2 is obtained for p–Si(100) orientation and of a little higher value for p–Si(111), but the energy distribution of surface states near the valuence band depends strongly on the crystallographic orientation. The surface state capture cross-sections for p–Si(111) and p–Si (100) MOS are 2.0×10-16 cm2 and 3.1×10-16 cm2, respectively. Experimental evidence of the tunneling current component from metal to surface states is seen in DC conductance and surface state density measurements.

Gap States and ESR of Boron-Doped CVD Amorphous Silicon

The density distribution of the gap states in boron-doped CVD amorphous silicon has been determined by the field-effect technique. The measured density of the midgap states as high as 2×1018 cm-3eV-1 for undoped a-Si is appreciably reduced by doping with boron atoms. Electronic states arising from unpaired spins are found at energies near Ev+0.30 eV, creating a bump in the gap states. The experimental results are discussed in conjunction with a divacancy model.

Localized States in Amorphous and Polycrystallized Si

Optical and electrical properties of electron-beam deposited amorphous Si films have been measured in a wide range of annealing temperatures to investigate the nature of localized states. Polycrystallization of Si films was found to be accompanied by an abrupt decrease in the refractive index and film thickness. The localized state density near the Fermi level is obtained to be ~1020 cm-3eV-1 for as-deposited samples from capacitance measurements of a-Si/SiO2/p+Si tunnel junctions. Field effect results showed that this density is reduced to ~1018 cm-3eV-1 by annealing at 650°C and subsequently to ~1017 cm-3eV-1 after polycrystallization. On the other hand, the density at ~0.21 eV above the Fermi level remains almost unchanged around 5×1019 cm-3eV-1 irrespective of annealing and polycrystallization, suggesting the presence of a common structural defect in amorphous and polycrystalline Si.

Localized-State-Density Distribution in Post-Hydrogenated CVD Amorphous Silicon

Chemically-vapor-deposited amorphous Si films containing no bonded hydrogen were hydrogenated by hydrogen plasma annealing. The remarkable reduction of the localized state density in the mobility gap produced by the hydrogenation was confirmed quantitatively by the field-effect technique. The localized state density near the center of the mobility gap was reduced to less than 1×1018cm-3eV-1, and the widths of the tail states below the conduction band and above the valence band were also decreased by the hydrogenation. An energy band model involving an electron trapping level ~0.12 eV below the conduction band and hole trapping levels ~0.35 and ~0.20 eV above the valence band is proposed, and the ~1.25 eV- and ~1.10 eV-luminescence peaks are explained in terms of the radiative transitions between these trapping states.

Defect States and Electronic Properties of Post-Hydrogenated CVD Amorphous Silicon

Chemically vapor-deposited a-Si films containing no detectable bonded hydrogen were annealed in a hydrogen plasma. Bonded hydrogen to a density much more than the density of ESR centers in as-grown a-Si was found to be present in the hydrogenated films, with a depth profile described by a complementary error function. The effective spin-elimination-depth is estimated to be 0.5 µm, in which the spin density is reduced to 7×1016 cm-3. Photoconductivity and photoluminescence are closely correlated with the in-depth profile of hydrogens.

Structural changes of amorphous Ge1−xSnx alloy films by annealing

Microcrystalline (µc-) grains of Ge1-ySny (0.1<y<0.4) were precipitated by thermal treatments of amorphous films of a Ge1-xSnx(x<0.4) alloy deposited by co-sputtering. At higher temperatures grains of β-Sn came out, co-existing with those of µc-Ge1-ySny. Mossbauer spectroscopy was used to characterize states of Sn in a Ge-Sn alloy film. Optical properties, such as the real part e1 of the complex dielectric constant for Ge0.65Sn0.35, also changed as the structure change, especially at a photon energy of 1.6~1.8 eV, where e1 took a maximum. It was suggested that an amorphous Ge-Sn alloy might be a good material for archival-type optical storage.

Schottky Barrier Solar Cells of Weakly Hydrogenated CVD Amorphous Silicon

Electronic properties of CVD a-Si were remarkably improved by hydrogen plasma annealing. As a result, the Schottky-barrier solar cells without an antireflection coating have provided a conversion efficiency of 2.7% at 100 mW/cm2, and no Staebler-Wronski effect has been observed in the hydrogenated CVD a-Si cell. It is also found that the fill factor is dependent on incident light intensity, because of changes in its series and parallel resistances by light illumination.

Defect Compensation by Bonded Hydrogen in Undoped a-Ge:H Films with Mono- and Dihydride Bonding

Hydrogenated amorphous Ge(a-Ge:H) thin films are prepared by the capacitive-coupled plasma chemical vapor deposition method using GeH4 as a reactive gas. Two kinds of films, one which involves only monohydride bonding and the other, which involves mainly dihydride bonding, are deposited in different deposition conditions. Although no impurity atoms, such as P or B atoms, are doped, both n- and p-type films are formed, depending on the deposition conditions. Infrared absorption spectra, electronic properties including photo- and dark conductivities, and optical properties of a-Ge:H have been studied. The effect of mono- and dihydride bonding on defect compensation is confirmed from the measurements, and it is concluded that the monohydride-bonding hydrogen atoms terminate the gap states mainly below midgap, and that the dihydride-bonding atoms chiefly decrease the number of gap states above midgap.

Electronic properties of post-hydrogenated lightly-boron-doped CVD amorphous silicon

Lightly-boron-doped CVD amorphous Si with a doping ratio between 3.3×10-7 and 2.5×10-5 was hydrogenated by hydrogen plasma annealing, and the electronic properties including photoconductivity and photoluminescence were investigated. At a doping ratio of 4×10-6 amorphous Si with the intrinsic property is produced, of which the photoconductivity at AM1 (100 mW/cm2) is 3×10-5 Ω-1cm-1, the localized state density in the mobility gap is lowest, and the photoluminescence intensity is highest. It is suggested that some boron atoms become inactive, forming B-H bonds with penetrating hydrogen atoms at doping ratios above 4×10-6.