Boron-doped Microcrystalline Silicon Films Research Articles

Density of States in Thin Boron-Doped Microcrystalline Silicon Films Estimated from the Thermally Stimulated Conductivity Method

In this work, a series of boron-doped microcrystalline silicon samples [μc-Si:H(B)] were deposited by plasma-enhanced chemical vapor deposition, using silane (SiH4) diluted in hydrogen, and diborane (B2H6) as a dopant gas. The concentration of B2H6 in SiH4 was varied in the range of 0 100 ppm. The density of states was obtained from the thermally stimulated conductivity technique and compared with results obtained by the modulated photoconductivity methods. To explain the poor agreement between the density of states obtained from the thermally stimulated conductivity and the other methods, it is shown by means of numerical simulations that the density of states is very sensitive to experimental errors introduced in the calculation of the μnτn product (mobility of electron × lifetime of the electron). The thermally stimulated conductivity method is applied here for the rst time to calculate the density of defect states in the forbidden band of μc-Si:H samples.

Effects of deposition pressure and plasma power on the growth and properties of boron-doped microcrystalline silicon films

Using diborane as doping gas, p-doped μc-Si:H layers are deposited by using the plasma enhanced chemical vapour deposition (PECVD) technology. The effects of deposition pressure and plasma power on the growth and the properties of μc-Si:H layers are investigated. The results show that the deposition rate, the electrical and the structural properties are all strongly dependent on deposition pressure and plasma power. Boron-doped μc-Si:H films with a dark conductivity as high as 1.42 Ω−1·cm−1 and a crystallinity of above 50% are obtained. With this p-layer, μc-Si:H solar cells are fabricated. In addition, the mechanism for the effects of deposition pressure and plasma power on the growth and the properties of boron-doped μc-Si:H layers is discussed.

Effect of boron on the microstructure of hydrogenated microcrystalline silicon thin films

AbstractIn this work, a series of boron doped microcrystalline silicon films (μc‐Si:H (B)) were deposited by plasma‐enhanced chemical vapor deposition (PECVD), using silane (SiH4) diluted in hydrogen, and diborane (B2H6) as a dopant gas. The concentration of B2H6 was varied in the range of 0–100 ppm. The microstructure and morphology of samples were analyzed by atomic force microscopy (AFM), X‐ray diffraction (XRD), and Raman spectroscopy. A trend towards increasing crystalline volume fraction and grain size were observed as boron concentration in the samples increased; while the XRD spectra show that the peak intensity at 2θ ≈ 47° decreases and becomes gradually amorphous with the increasing degree of doping. The doped microcrystalline silicon films presented a crystallographic preferential orientation in the plane (220). Correlations between structural and electric properties were also studied. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of substrate temperature on the growth and properties of boron-doped microcrystalline silicon films

Highly conductive boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films are prepared by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at the substrate temperatures (TS) ranging from 90°C to 270°C. The effects of TS on the growth and properties of the films are investigated. Results indicate that the growth rate, the electrical (dark conductivity, carrier concentration and Hall mobility) and structural (crystallinity and grain size) properties are all strongly dependent on TS. As TS increases, it is observed that 1) the growth rate initially increases and then arrives at a maximum value of 13.3 nm/min at TS=210°C, 2) the crystalline volume fraction (Xc) and the grain size increase initially, then reach their maximum values at TS=140°C and finally decrease, 3) the dark conductivity (σd), carrier concentration and Hall mobility have a similar dependence on TS and arrive at their maximum values at TS=190°C. In addition, it is also observed that at a lower substrate temperature TS, a higher dopant concentration is required in order to obtain a maximum σd.

Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane (B2H6) as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120to420K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range (300–120K), variable range hopping (VRH) was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hopping parameters, are calculated. A correlation between the hopping parameters for both models is deduced. A numerical factor that improves the value of each parameter is calculated.

Aging effects in microcrystalline silicon films studied by transient photoconductivity

Measurements of dark conductivity, steady-state and transient photoconductivity in undoped and boron-doped microcrystalline silicon films exposed to room air are reported. Two aging processes are identified: (i) an increase in dark conductivity and mobility-lifetime product occurring over several days, that may be reversed by heating to 160 °C under vacuum, and subsequently re-cycled, (ii) an irreversible change in the density of states occurring over a period of several months. It is proposed that the reversible effect is associated with charge transport in a region of electron accumulation induced by weakly-adsorbed water, whereas the irreversible effect, identified from its transient photocurrent signature, is associated with a true change in the density of states within the transport path, possibly as a result of slow chemical reactions at silicon grain boundaries.

Low-temperature growth of thick intrinsic and ultrathin phosphorous or boron-doped microcrystalline silicon films: Optimum crystalline fractions for solar cell applications

The growth kinetics and optoelectronic properties of intrinsic and doped microcrystalline silicon (μc-Si:H) films deposited at low temperature have been studied combining in situ and ex situ techniques. High deposition rates and preferential crystallographic orientation for undoped films are obtained at high pressure. X-ray and Raman measurements indicate that for fixed plasma conditions the size of the crystallites decreases with the deposition temperature. Kinetic ellipsometry measurements performed during the growth of p-(μc-Si:H) on transparent conducting oxide substrates display a remarkable stability of zinc oxide, while tin oxide is reduced at 200°C but stable at 150°C. In situ ellipsometry, conductivity and Kelvin probe measurements show that there is an optimum crystalline fraction for both phosphorous- and boron-doped layers. Moreover, the incorporation of p-(μc-Si:H) layers produced at 150°C in μc-Si:H solar cells shows that the higher the crystalline fraction of the p-layer the better the performance of the solar cell. On the contrary, the optimum crystalline fraction of the p-layer is around 30% when hydrogenated amorphous silicon (a-Si:H) is used as the intrinsic layer of p–i–n solar cells. This is supported by in situ Kelvin probe measurements which show a saturation in the contact potential of the doped layers just above the percolation threshold. In situ Kelvin probe measurements also reveal that the screening length in μc-Si:H is much higher than in a-Si:H, in good agreement with the good collection of microcrystalline solar cells

Thin boron doped microcrystalline silicon films

We report a study of the electronic properties of thin (<40 nm) boron-doped microcrystalline silicon films. Our results show that the crystallinity of thin layers depends largely on the material of the substrates. Changes of the conductivity with the film thickness, deposition pressure and deposition temperature have been observed. A conical growth model is proposed, and the importance of the density of nucleation centers is discussed.

Properties and photovoltaic applications of microcrystalline silicon films prepared by rf reactive sputtering

Undoped, phosphorus-, and boron-doped microcrystalline silicon films were prepared by rf reactive sputtering, and their properties were investigated through structural, optical, and transport measurements. The merits of microcrystalline films for the p and n contacts in photovoltaic devices were demonstrated through the fabrication of single and tandem p-i-n solar-cell structures with best efficiencies between 5 and 6%.