Brent P. Nelson

Deposition of device quality, low H content amorphous silicon

Device‐quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device‐quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.

Microvoids in amorphous Si1−xCx:H alloys studied by small‐angle x‐ray scattering

The microstructure of hydrogenated amorphous silicon‐carbon alloys has been analyzed by small‐angle x‐ray scattering, infrared absorption, and density measurements. Decreasing density with C incorporation is due to microvoids about 0.6 nm in average radius, which are either approximately spherical in shape or randomly oriented nonspheres. The microvoid number density increases from about 5×1019/cm3 for a‐Si:H to about 4×1020/cm3 for a‐Si0.7 C0.3 :H. The CH3 species probably causes the enhanced microvoid formation in these alloys. A large fraction of the microvoid surfaces is not hydrogenated.

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased...

Structural properties of hot wire a-Si:H films deposited at rates in excess of 100 Å/s

The structure of a-Si:H, deposited at rates in excess of 100 A/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 A/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 A/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in th...

Deposition of device quality, low H content a-Si:H by the hot wire technique

We report measurements of the Urbach edge and density of gap states on a series of hydrogenated amorphous silicon (a-Si:H) films deposited by hot-wire-assisted chemical vapor deposition (HW). We compare the properties of these films to those of a series of a-Si:H films deposited by the traditional radio frequency (rf) glow discharge (GD) technique, where we varied the substrate temperature to change the bonded H content (CH). We show for the first time that, as CH is decreased below the value traditionally associated with device quality GD a-Si:H (∼10 at.%), the electronic properties of the GD films deteriorate in the traditional manner while those for the HW samples remain device quality. Properties of these low CH HW samples will be presented and compared to those of GD films containing comparable CH. Because several indications exist that the structure of the HW films is different than that of the GD films, Raman and Small Angle X-Ray Scattering (SAXS) measurements are presented to illustrate structural differences.

High-deposition rate a-Si:H n–i–p solar cells grown by HWCVD

Abstract We grow hydrogenated amorphous silicon (a-Si:H) solar cells in a device structure denoted as SS/n–i–p/ITO. We grow all the a-Si:H layers by hot-wire chemical vapor deposition (HWCVD) and the indium-tin-oxide (ITO) by reactive evaporation. We are able to grow HWCVD i-layer materials that maintain an AM1.5 photoconductivity-to-dark-conductivity ratio of 105 at deposition rates up to 130 A/s. We have put these high-deposition rate i-layer materials into SS/n–i–p/ITO devices and light-soaked them for ≥1000 h under AM1.5 conditions. We obtain stabilized solar cell efficiencies of 5.5% at 18 A/s, 4.8% at 35 A/s, 4.1% at 83 A/s and 3.8% at 127 A/s.

Saturated defect densities of hydrogenated amorphous silicon grown by hot-wire chemical vapor deposition at rates up to 150 Å/s

Hydrogenated amorphous-silicon (a-Si:H) is grown by hot-wire chemical vapor deposition (HWCVD) at deposition rates (Rd) exceeding 140 A/s (∼0.8 μm/min). These high rates are achieved by using multiple filaments and deposition conditions different than those used to produce our standard 20 A/s material. With proper deposition parameter optimization, an AM1.5 photo-to-dark-conductivity ratio of 105 is maintained at an Rd up to 130 A/s, beyond which it decreases. In addition, the first saturated defect densities of high Rd a-Si:H films are presented. These saturated defected densities are similar to those of the best HWCVD films deposited at 5–8 A/s, and are invariant with Rd up to 130 A/s.

Amorphous silicon films and solar cells deposited by HWCVD at ultra-high deposition rates

Abstract The deposition conditions for hydrogenated amorphous silicon, deposited by hot wire chemical vapor deposition, are linked to the film structure as we increase deposition rates (Rd) to >100 A/s. At low Rd ( 100 A/s), optimum films are deposited under silane depletion conditions as high as 75–80%, and all structural properties except for the SAXS results once again indicate a compact material. We relate changes in the film electronic structure (Urbach edge) with increasing Rd to the increase in the SAXS signals, and note the invariance of the saturated defect density versus Rd, discussing reasons why these microvoids do not play a role in the Staebler–Wronski effect for these films. Finally, we present device results over the whole range of Rd that we have studied and suggest why, at high Rd, device quality films can be deposited at such high silane depletions.

Effects of dilution ratio and seed layer on the crystallinity of microcrystalline silicon thin films deposited by hot-wire chemical vapor deposition

Abstract We deposited microcrystalline silicon (μc-Si) by hot-wire chemical vapor deposition (HWCVD) at different thickness and dilution ratio, with and without seed layer. As the dilution ratio increased, we observed an increase in the amount of microcrystalline phase in the film, a change in the structure of the grains and a loss of the (220) preferential orientation. The films deposited over a seed layer had a larger fraction of crystalline phase than films deposited with the same parameters but without a seed layer. For high dilution ratios (R=100), most of the film grows epitaxially at the interface with the Si substrate, but a microcrystalline film slowly replaces the single-crystal phase. For low dilution ratios (R=14), the film starts growing mostly amorphously, but the amount of crystalline phase increases with thickness.

The observation of microvoids in device quality hydrogenated amorphous silicon

Abstract The size, shape, and number density of microvoids in device quality glow discharge deposited hydrogenated a-Si have been obtained by small angle x-ray scattering (SAXS). By combining the SAXS results with infrared measurements, we deduce that the interior surfaces of these microvoids contain ∼4–9 bonded H atoms. We suggest that these H atoms are the clustered H atoms previously detected by multiple quantum NMR.