Microcrystalline Si Thin Film Research Articles

Lateral matching of periodic front and back textures in thin film silicon solar cells

We systematically study the influence of the relative lateral position of the front and back periodic textures on the light absorption in microcrystalline Si thin film solar cells by finite difference time domain method. We show that there is an optimum lateral position match between the periodic front and back textures, which allows maximum light absorption to be obtained in the Si thin film solar cells. A relative lateral shift between the front and back periodic textures breaks the symmetry of the conformal cell structure, which can result in more wave modes in the Si thin film solar cells.

Microcrystalline Si thin films by re-crystallization of electrophoretic deposited Si nanoparticle films

ABSTRACTA new route to producing microcrystalline silicon (µc-Si) thin films by re-crystallizing Si nanoparticle films by flash lamp method is presented. High quality Si nanoparticle films with high uniformity and high particle packing density were obtained using a stable non-aqueous Si nanoparticle suspension and the electrophoretic deposition (EPD) method. Morphology and crystallinity of as-deposited and flash lamp re-crystallized Si nanoparticle films were studied.

The growth characteristics of microcrystalline Si thin film deposited by atmospheric pressure plasma-enhanced chemical vapor deposition

Microcrystalline silicon thin film was grown by atmospheric pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique with a cylindrical rotary electrode supplied with 150 MHz very-high-frequency power. The crystalline volume fraction could be controlled by changing the flow rate ratio of silane and hydrogen gas during AP-PECVD. We could also control it by regulating the substrate scanning speed. At low substrate scanning speed, the silicon film had a low crystalline volume faction and layer-by-layer structure with alternating layers of amorphous and microcrystalline Si. On the other hand, at high substrate scanning speed, silicon crystals of sizes 25 nm grew homogeneously throughout the whole film.

Electrical properties of boron- and phosphorous-doped microcrystalline silicon thin films prepared by magnetron sputtering of heavily doped silicon targets

The boron(B)- and phosphorous(P)-doped microcrystalline silicon (Si) thin films were prepared by magnetron sputtering of heavily B- and P-doped Si targets followed by rapid thermal annealing (RTA), their electrical properties were characterized by temperature-dependent Hall and resistivity measurements. It was observed that the dark conductivity and carrier concentration of the 260 nm B-doped Si films annealed at 1,100 °C in Ar were 3.4 S cm−1 and 1.6 × 1019 cm−3, respectively, which were about one order of magnitude higher than that of P-doped Si films. The activation energy of the B- and P-doped Si films were determined to be 0.23 eV and 0.79 eV, respectively. The dark conductivity of B- and P-doped Si films increased with the increase of film thickness, RTA temperature, and the incorporation of H2 in Ar during RTA. The present work provides an easy and non-toxic method for the preparation of doped microcrystalline Si thin films.

Amorphous/crystalline silicon heterojunction solar cells via remote inductively coupled plasma processing

Low-frequency inductively coupled plasma (ICP) has been widely used to deposit amorphous or microcrystalline Si thin films, but the intrinsic drawback namely ion bombardment effect limits its application in Si heterojunction solar cells. In this letter, we redesigned typical ICP and realized a remote plasma deposition with suppressed ion bombardment effect. This remote ICP system enables the synthesis of high quality amorphous Si layers with a compact network and a high hydrogen content (10.5%). By using this remote ICP system, we achieved amorphous/crystalline silicon heterojunction solar cells with an efficiency of 14.1% without any back surface field or textures.

Shape evolution of patterned amorphous and polycrystalline silicon microarray thin film electrodes caused by lithium insertion and extraction

Silicon is the most promising high capacity anode material to replace graphite for developing next generation high energy density Li-ion batteries. In this approach, patterned amorphous and microcrystalline Si thin film electrodes (a-Si and μc-Si) have been prepared by rf-sputtering and etched further by a reactive ion etching (RIE) system to form square-shape microcolumn electrodes with controllable size (5 × 5 μm width, 500 nm height, aspect ratio of width/height is 10:1) and array distance (5 μm). It has been found that the volume expansion and contraction of a-Si and μc-Si are anisotropic, about 180% along vertical direction and 40% along lateral direction. The total volume variation changes linearly with the increase of lithium insertion content up to ∼310% for a-Si and ∼300% for μc-Si. It occurs nearly reversibly. In addition, it is observed that the original square-shape Si column transforms into the dome-like appearance after lithium insertion and changes into bowl shape after lithium extraction gradually. Radial-like curved cracks are formed after 5−10 cycles and the neighboring Si columns tend to merge together when the distance of the columns is less than 1 μm.

Influence of ignition condition on the growth of silicon thin films using plasma enhanced chemical vapour deposition

The influences of the plasma ignition condition in plasma enhanced chemical vapour deposition (PECVD) on the interfaces and the microstructures of hydrogenated microcrystalline Si (μc-Si:H) thin films are investigated. The plasma ignition condition is modified by varying the ratio of SiH4 to H2 (RH). For plasma ignited with a constant gas ratio, the time-resolved optical emission spectroscopy presents a low value of the emission intensity ratio of Hα to SiH* (IHα/ISiH*) at the initial stage, which leads to a thick amorphous incubation layer. For the ignition condition with a profiling RH, the higher IHα/ISiH* values are realized. By optimizing the RH modulation, a uniform crystallinity along the growth direction and a denser μc-Si:H film can be obtained. However, an excessively high IHα/ISiH* may damage the interface properties, which is indicated by capacitance—voltage (C−V) measurements. Well controlling the ignition condition is critically important for the applications of Si thin films.

Rf excited optical emission spectrum of radicals generated during hot wire chemical vapour deposition for the preparation of microcrystalline silicon thin film

To study the radicals behavior in the hot wire chemical vapour deposition (HWCVD) process for the preparation of microcrystalline Si (μc-Si: H) thin film, a weak radio frequency (rf) power was introduced to excite the radicals generated in HWCVD chamber. The spectrum of rf-excited HWCVD (rf-HWCVD) was obtained by subtracting the emission of hot wires from the spectrum measured by OES. The influence of the rf power on the rf-HWCVD spectrum can be neglected as the rf power density was less than 0.1 W/cm2. Under the same deposition parameters,the emission spectra for rf-HWCVD and plasma enhanced CVD (PECVD) processes are different. Under the low deposition pressure (7.5 Pa), the intensities of SiH* and Hα vary with the hot wire temperature reversely, which is characteristic of HWCVD with high gas dissociation rate and high concentration of atomic H. The ratio of intensity of Hα to SiH* in the emission spectrum of rf-HWCVD varying with deposition pressure is consistent with the crystalline fraction of μc-Si: H film. The results indicate that the optical emission spectroscopy measurement is a suitable method for the investigation of the HWCVD process excited by a weak rf-power.

Improvement in the optical absorption of PECVD microcrystalline Si thin film through modification of the crystalline fraction through an annealing process

Intrinsic microcrystalline silicon (μc-Si) thin films with varying crystalline fractions ( X C) were deposited using plasma-enhanced chemical vapor deposition. Effort was made to modify the X C and optical properties by annealing at different temperatures, pressures, and ambient gases. After annealing, the amount of Si–H in the μc-Si thin film was decreased for an initial X C of 7%, while that of Si–H 2 mainly decreased to an initial X C of 60–70%. The X C of μc-Si with an initial X C of 7% increased by annealing. However, initial X C of 60% and 70% resulted in a decrease of X C upon annealing. It is suggested that an increase in crystal size of μc-Si films by annealing decreases X C, while a decrease in crystal size increases X C. Depending on the annealing conditions, the optical absorption property of the μc-Si thin film changed. When the initial X C was 7%, the absorption coefficient increased as X C increased to 16%, but slightly decreased with X C values from 51% to 70%. Therefore, we suggest that there is an optimum X C that exhibits the highest optical absorption.

Requirements for TCO Substrate in Si-based Thin Film Solar Cells -Toward Tandem

AbstractNewly developed SnO2:F films having “double–texture (W-texture)” were evaluated in terms of optical and electrical properties and compared with Asahi type-U substrate. “W-textured” transparent conductive oxide (TCO) film was composed of a combination of 300-500nm large hills and small pyramidal texture covering them. “W-textured” TCO could show a large haze value exceeding 80% even at 800nm. Microcrystalline Si (μc-Si:H) thin film solar cells were fabricated on these TCOs and characterized. The result showed that a quantum efficiency of μc-Si:H solar cells was improved with “W-textured” TCOs significantly in longer wavelength region.

Large-grained poly-silicon thin films by aluminium-induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si (μc-Si:H) thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) on SiO 2-coated Si wafers has been studied. The starting structure was substrate/μc-Si:H/Al. Annealing this structure at a temperature of 520 °C resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. Grain sizes exceeding ∼60 μm have been achieved in films displaying a preferential (1 0 0) orientation. The length of time the samples are kept under ambient conditions before annealing plays a key role in controlling grain size and orientation. It is likely that this time delay influences the formation of the interface between the Si and Al layers and, hence, the crystallisation process. These poly-Si layers exhibit an average surface roughness ( R a) generally in the range ∼7–12 nm.

Poly-silicon thin films by aluminium induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR–PECVD) on glass and SiO 2 coated Si wafers has been studied. The starting structure was substrate/μc-Si/Al. Annealing this structure in the temperature range 370–520 °C, immediately following deposition of the Al layer, resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. The top poly-Si layer exhibited grain sizes generally in the range ∼2–6 μm, although larger grains were also sparsely present. The films did not exhibit any appreciable degree of preferred orientation. The surface roughness was relatively high with a R a value of ∼20 nm.

The compact microcrystalline Si thin film with structure uniformity in the growth direction by hydrogen dilution profile

The hydrogen dilution profiling (HDP) technique has been developed to improve the quality and the crystalline uniformity in the growth direction of μc-Si:H thin films prepared by hot-wire chemical-vapor deposition. The high H dilution in the initial growth stage reduces the amorphous transition layer from 30–50 to less than 10 nm. The uniformity of crystalline content Xc in the growth direction was much improved by the proper design of hydrogen dilution profiling which effectively controls the nonuniform transition region of Xc from 300 to less than 30 nm. Furthermore, the HDP approach restrains the formation of microvoids in μc-Si:H thin films with a high Xc and enhances the compactness of the film. As a result the stability of μc-Si:H thin films by HDP against the oxygen diffusion, as well as the electrical property, is much improved.

High Density Plasma Processing of Microcrystalline Si Thin Films

AbstractIn the present work, we report on the fabrication of high quality microcrystalline Si thin films by high-density PECVD technique. The typical deposition rate of the HD-PECVD μ-Si thin films was greater than 350 Å/min in the H2/SiH4 ratio range of 20-100. For a 150-nm-thick film deposited at a H2/SiH4 ratio of 20, the typical microcrystalline volume fraction and the average crystallite size corresponding to <111> orientation were 75% and 160 Å, respectively. The observed growth and properties of the μ-Si thin films show the potential of the high-density PECVD technique for the low temperature processing of high quality films with superior control of bulk and interfacial characteristics.

Barrier-controlled carrier transport in microcrystalline semiconducting materials: Description within a unified model

A recently developed model that unifies the ballistic and diffusive transport mechanisms is applied to the carrier transport across potential barriers at grain boundaries in microcrystalline semiconducting materials. In the unified model, the conductance depends on the detailed structure of the band edge profile and in a nonlinear way on the carrier mean free path. Equilibrium band edge profiles are calculated within the trapping model for samples made up of a linear chain of identical grains. Quantum corrections allowing for tunneling are included in the calculation of electron mobilities. The dependence of the mobilities on carrier mean free path, grain length, number of grains, and temperature is examined, and appreciable departures from the results of the thermionic-field-emission model are found. Specifically, the unified model is applied in an analysis of Hall mobility data for n-type μc-Si thin films in the range of thermally activated transport. Owing mainly to the effect of tunneling, potential barrier heights derived from the data are substantially larger than the activation energies of the Hall mobilities. The specific features of the unified model, however, cannot be resolved within the rather large uncertainties of the analysis.

Aluminum-seeded growth of microcrystalline Si thin film onto SnO2 substrate

To improve the crystallinity of thin boron-doped microcrystalline Si (p-μc-Si:H) films at an initial growth stage, we tried an Al-seeding method, which is to deposit an ultrathin (∼50 Å) Al layer onto a transparent conducting oxide before preparing the p-μc-Si:H film by using a mercury-sensitized photochemical vapor deposition method. It was found that the Al layer serves as seeds to facilitate the p-μc-Si:H with high crystallinity at an initial growth stage and the Al seeds stimulate the nucleation of Si crystallites. It was also found that the absorption loss in the ultrathin (∼50 Å) Al-seed layer is negligible in the visible wavelength region due to its natural oxidation. When used in superstrate-type p-i-n amorphous silicon solar cells, even the Al-seeded p-μc-Si:H film having a several hundred angstrom thickness acts as an excellent p layer compared to the conventional p-μc-Si:H.

PLASMA PROCESSES IN PV SOLAR CELLS (a-Si:H)

The state of the art of amorphous silicon thin films deposited in PECVD processes for solar cells applications is reviewed . Precursors such as silane, tri and tetra chloro-silane as monomers in Ar, H2 and Ar+H2 gas mixtures or pure silane plasmas were mainly used in the deposition of amorphous or micro-crystalline Si thin films. Reactions kinetics in the plasma bulk and in the plasma surface (solid) interactions as measured by mass spectrometry in real time, on line, show the governing species the particulate contamination's and the rate determining steps in the deposition process. The degradation of a-Si:H solar cells at intense illumination i.,e.,at elevated temperatures and the inhibition of the degradation processes by the addition of Ar to the silane plasmas will be discussed.

Microanalytical Characterization of Structure and Defects for the Development of Low Temperature Silicon Epitaxial Growth

Abstract The nano-scale dimensions of next generation VLSI and ULSI devices will drive the development of a variety of new processing requirements. Currently device processing conditions from substrate cleaning to thin film deposition require temperatures in the range of 600°C to 1200°C. In order to realize a Si device circuit architecture which integrates Si/Ge structures or the needed super abrupt junctions of buried channel CMOS, low temperature processes must replace those in current production lines. For these processes to be successfully developed and implemented, proper characterization techniques must be used. In the case of epitaxy, cross-sectional TEM is the tool of choice. We will discuss the prominent role that TEM has played in the development of a new Si epitaxy technology. Recently, at the National Renewable Energy Laboratory (NREL), we have shown low temperature, 195°C to 400°C, Si epitaxy via hot-wire chemical vapor deposition- HWCVD. In the past HWCVD has been used to produce amorphous, micro-crystalline, and polycrystalline Si thin films.

Low temperature Si crystal growth by alternating deposition and hydrogen etching sequences and its application to the p-layer of a-Si:H solar cells

Abstract Low temperature silicon crystal growth by alternating deposition and hydrogen etching sequences has been studied. The large area remote ECR H 2 plasma was used as an atomic hydrogen source. Epitaxial, polycrystalline, and microcrystalline Si thin films have been obtained below 400°C corresponding to various deposition conditions and kinds of substrates. Epitaxial films with electron mobilities exceeding 800 cm 2 /Vs were obtained on Si(100) substrates. By applying this method, very thin p-type microcrystalline Si films have been prepared on SnO 2 substrates below 200°C. The cross-sectional TEM micrographs showed that the longitudinal crystal size of microcrystalline on SnO 2 substrates was about 100–200 A. The a-Si:H solar cells with p-type microcrystalline Si had a higher open-circuit voltage than that of conventional cells by as much as 0.02–0.04 eV.

Integrated processing of amorphous and microcrystalline Si thin film transistors by plasma-assisted chemical-vapor deposition

Thin film transistors (TFTs) have been fabricated in an ultra-high vacuum compatible integrated processing system with on-line surface analysis diagnostics - Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). This paper deals with TFTs that include dual-layer oxide-nitride dielectrics, and either hydrogenated amorphous and/or microcrystalline Si thin films for the channel, and source and drain regions. The emphasis is on the integrated processing of bottom-gate device structures, and on the way the electrical performance of the TFTs are correlated with the properties of the dielectric and semiconducting films, their included internal interfaces and the external exposed surfaces.