Boron-doped Amorphous Silicon Films Research Articles

Femtosecond Laser Fabrication of Anisotropic Structures in Phosphorus- and Boron-Doped Amorphous Silicon Films.

Femtosecond laser-modified amorphous silicon (a-Si) films with optical and electrical anisotropy have perspective polarization-sensitive applications in optics, photovoltaics, and sensors. We demonstrate the formation of one-dimensional femtosecond laser-induced periodic surface structures (LIPSS) on the surface of phosphorus- (n-a-Si) and boron-doped (p-a-Si) amorphous silicon films. The LIPSS are orthogonal to the laser polarization, and their period decreases from 1.1 ± 0.1 µm to 0.84 ± 0.07 µm for p-a-Si and from 1.06 ± 0.03 to 0.98 ± 0.01 for n-a-Si when the number of laser pulses per unit area increases from 30 to 120. Raman spectra analysis indicates nonuniform nanocrystallization of the irradiated films, with the nanocrystalline Si phase volume fraction decreasing with depth from ~80 to ~40% for p-a-Si and from ~20 to ~10% for n-a-Si. LIPSS' depolarizing effect, excessive ablation of the film between LIPSS ridges, as well as anisotropic crystalline phase distribution within the film lead to the emergence of conductivity anisotropy of up to 1 order for irradiated films. Current-voltage characteristic nonlinearity observed for modified p-a-Si samples may be associated with the presence of both the crystalline and amorphous phases, resulting in the formation of potential barriers for the in-plane carrier transport and Schottky barriers at the electric contacts.

Spectroscopic ellipsometry investigation to study the microstructure evolution in boron-doped amorphous silicon films as a result of hydrogen dilution

Spectroscopic ellipsometry (SE) is a sophisticated technique to find the optical constants, bandgap and microstructure of thin layer. SE is used to study the microstructure evolution in boron-doped amorphous silicon films for different hydrogen flow rates (HFR). Spectral dependance of the real and imaginary parts of pseudo-dielectric constant is obtained at a fix angle of incidence (70°). Tauc–Lorentz (T–L) optical model is used to estimate the thickness, bandgap, optical constant and thickness of the top rough layer of the films, whereas Bruggeman effective medium approximation (BEMA) is applied to find the volume fractions of amorphous, crystalline and void phases. A shift in peak position from 3.65 to 4.1 eV in dielectric constant is observed as the hydrogen flow rate is increased from 30 to 70 SCCM. This is accompanied by the emergence of a peak near 3.4 eV, which belongs to the direct bandgap of c-Si. These observations suggest an improvement in microstructure of the films deposited at higher HFR. It is also supported by the observation that films deposited at higher HFR have higher magnitude of amplitude parameter and less broadening. Fitting of experimental data using BEMA also suggests that crystalline fraction increases and amorphous fraction decreases at higher HFR. The bandgap and thickness of top rough layer estimated from SE data are matched well with those obtained using transmission data and atomic force microscopy.

Characterization of Boron Doped Amorphous Silicon Films by Multiple Internal Reflection Infrared Spectroscopy

ABSTRACTIn this study, we employed Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) to characterize chemical bonding structures of boron doped hydrogenated amorphous silicon (a-Si:H(B)). This technique has been shown to provide over a hundred fold increase of detection sensitivity when compared with conventional FTIR. Our MIR-IR analyses reveal an interesting counter-balance relationship between boron-doping and hydrogen-dilution growth parameters in PECVD-grown a-Si:H. Specifically, an increase in the hydrogen dilution ratio (H2/SiH4) was found to cause the increase in the Si-H bonding and a decrease in the B-H and SiH2 bonding, as evidenced by the changes in corresponding IR absorption peaks. In addition, although a higher boron dopant gas concentration was seen to increase the BH and SiH2 bonding, it also resulted in the decrease of the most stable SiH bonding configuration. The new chemical bonding information of a-Si:H thin film was correlated with the various boron doping mechanisms proposed by theoretical calculations.

Low temperature back-surface-field contacts deposited by hot-wire CVD for heterojunction solar cells

The growing interest in using thinner wafers (< 200 μm) requires the development of low temperature passivation strategies for the back contact of heterojunction solar cells. In this work, we investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by Hot-Wire CVD. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. The microstructure of the deposited thin films has been comprehensively studied by Spectroscopic Ellipsometry in the UV–visible range. The effective recombination velocity at the back surface has been measured by the Quasi-Steady-State Photoconductance technique. Complete double-side heterojunction solar cells (1 cm 2) have been fabricated and characterized by External Quantum Efficiency and current–voltage measurements. Total-area conversion efficiencies up to 14.5% were achieved in a fully low temperature process (< 200 °C).

Electron-phonon interaction in boron-doped silicon nanocrystals: Effect of Fano interference on the Raman spectrum

Raman spectroscopy is employed for studying silicon nanocrystal arrays in boron-doped amorphous silicon films. The nanocrystals were formed in the initial amorphous films by the pulsed impact of an excimer laser. The electron-phonon interaction effects are observed experimentally in the heterostructure formed by a silicon nanocrystal and an amorphous matrix. These effects can be described in the framework of the familiar Fano interference model.

Precipitation of Boron in Highly Boron-Doped Silicon

The clustering of boron in highly boron-doped silicon and its influence on electrical deactivation are reported. Highly boron-doped crystalline silicon was fabricated as a starting material by solid phase epitaxy of boron-doped amorphous silicon films. Boron can be supersaturated in the crystallized samples annealed at a low temperature of about 600°C. A lot of precipitates, containing clustered boron, were observed in the samples annealed at high temperatures of about 1000°C. The chemical states and the atomic configuration of boron in samples annealed at various temperatures corresponded to the electrical deactivation of boron.

Thermoelectric power of amorphous silicon under illumination

Abstract Both the photoconductivity σ and the photothermoelectric power S have been measured on undoped, phosphorus-doped and boron-doped amorphous silicon films prepared by r.f. decomposition of the appropriate silane gas mixture. A significant feature of the work was the preparation of suitable contacts to the amorphous film. Considerable care was required to reduce photovoltaic effects to a sufficiently low level before measurements could be made. Measurements of σ and S are presented in the temperature range 150–500 K at a number of illumination intensity levels. Room-temperature measurements are also given as a function of incident photon energy. For each sample the Q function, defined as Q=In σ+|eS|K, is independent of illumination intensity and varies linearly with reciprocal temperature. It is concluded that photoconduction between 150 and 500 K occurs through extended states, in the conduction band for the undoped and n-type samples and in the valence band for the p-type sample. The temperature v...

Intrinsic stress of phosphorus- and boron-doped amorphous silicon films

Abstract The relationship between the intrinsic stress and hydrogen-bonding configuration for plasma-produced hydrogenated amorphous silicon (a-Si: H) films doped with phosphorus and boron has been studied in the gas-phase doping ratios from 0 to 5 × 10−3 prepared with different substrate temperatures. We found the same strong correlation between the sign and strength of intrinsic stress σi and the infrared stretching-mode absorption ratio I(SiH2)/I(SiH) (= R) as in a previous study on undoped films. The compressive stress rapidly increases as R decreases below 1·2, while the tensile stress increases and finally saturates as R increases above 1·2; consequently, zero stress is reached at R ≊ 1.2. This stress change was found not to be associated with the dopants and the doping levels but with the plasma conditions. The experimental σi-R curve has been interpreted by a phenomenological theory based on a simplified two-phase structural model.

Density of midgap states and Urbach edge in chemically vapor deposited hydrogenated amorphous silicon films

Intrinsic or lightly boron-doped amorphous silicon films were produced by chemical vapor deposition (CVD) from disilane at temperatures from 380 to 460 °C and at growth rates from 0.1 to 40 Å/s. The density of states (DOS) near the middle of the mobility gap was determined by space-charge-limited conduction on n-i-n structures and by steady-state capacitance-temperature spectroscopy on p-i-n structures. Very close agreement was found between the two techniques for intrinsic layers deposited under similar conditions. The DOS distribution is rather flat from 0.60 to 0.75 eV from the conduction-band edge and has a range of 1–6×1017 cm−3 eV−1 for intrinsic films, with the minimum occurring for depositions at 440 °C independent of growth rate or contaminants in the disilane. Diborane levels from 9 to 18 ppm in the gas phase reduces the DOS at 440 °C to 3–6×1016 cm−3 eV−1. The exponential absorption below the band edge in the range 1.4–1.6 eV was determined from primary photocurrent spectra of p-i-n structures. The Urbach edge parameter E0 ranged from 0.048 to 0.056 eV with the minimum also occurring for depositions at 440 °C, independent of growth rate and boron content. Comparison to glow-discharge a-Si:H films indicates that CVD a-Si:H films have a similar amount of structural disorder, determined from E0, but an inherently larger DOS deep in the gap. A model is proposed in which boron reduces the high midgap dangling bond density caused by insufficient weakly bonded hydrogen in the film as a consequence of the high deposition temperatures. The model is consistent with other measurements, such as hole collection width.