Microcrystalline Si Films Research Articles

Mechanism of Hydrogenated Microcrystalline Si Film Deposition by Magnetron Sputtering Employing a Si Target and H2/Ar Gas Mixture

The mechanism of hydrogenated microcrystalline silicon (µc-Si:H) film deposition by magnetron sputtering employing a Si target and H2/Ar gas mixture has been investigated by measuring Si and H atom densities in the gas phase by laser-induced fluorescence spectroscopy. The crystalline volume fraction of the film correlated positively with H atom density. The variation in Si atom density indicated the increase in sputtering yield from the Si target in the H2/Ar discharge. The surface of the Si target immersed in the H2/Ar discharge was hydrogenated. Therefore, it is reasonable to expect the production of SiHx molecules (typically SiH4) from the hydrogenated Si target via reactive ion etching. Since SiHx molecules produced from the target may function as a deposition precursor, the mechanism of µc-Si:H film deposition is considered to be similar to that of plasma-enhanced chemical vapor deposition (PECVD) employing a SiH4/H2 gas mixture. The advantage of magnetron sputtering deposition over PECVD is the production of SiHx molecules without using toxic, explosive SiH4.

TCOs for nip thin film silicon solar cells

AbstractSubstrate configuration allows for the deposition of thin film silicon (Si) solar cells on non‐transparent substrates such as plastic sheets or metallic foils. In this work, we develop processes compatible with low Tg plastics. The amorphous Si (a‐Si:H) and microcrystalline Si (µc‐Si:H) films are deposited by plasma enhanced chemical vapour deposition, at very high excitation frequencies (VHF‐PECVD). We investigate the optical behaviour of single and triple junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of low pressure CVD (LP‐CVD) ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 µm thick LP‐CVD ZnO. We observe that, for a‐Si:H, the cell performance depends critically on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a‐Si:H, a thick LP‐CVD ZnO front contact provides more light trapping on the 2D periodic substrate. Then, we investigate the influence of the thick and thin TCOs in conjunction with thick absorbers (µc‐Si:H). Because of the different nature of the optical systems (thick against thin absorber layer), the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP‐CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. Copyright © 2008 John Wiley & Sons, Ltd.

Microcrystalline/Amorphous Thin Si Films Deposition by a Newly Developed Dual Injection System Employing Hydrogen Plasma and Silicon Radicals at Low Temperature (300 °C) Chemical Vapor Deposition Process

Low temperature (300 °C) deposition of thin microcrystalline/amorphous silicon films by using newly developed single shower, dual injection, microwave excited high density plasma (1011 cm-3) system is described. This is an original setup based on an experimental modification of a dual shower system that was reported by us earlier. For the first time, for this system, experimental results are presented correlating between the deposition and plasma parameters with the resulting Si films atomic structure. The results consistent interdependence enables to pre-determine, in a wide range, by design, the growth of microcrystalline or amorphous thin Si films, by setting the proper deposition conditions for each film type.

Melting and Solidification of Microcrystalline Si Films Induced by Semiconductor Diode Laser Irradiation

Rapid thermal annealing of microcrystalline Si (µc-Si) films induced by cw semiconductor diode laser (SDL) irradiation has been investigated. Owing to the higher absorption coefficient of µc-Si than that of amorphous Si (a-Si), 1.2-µm-thick µc-Si films are melted and recrystallized within 3 ms, whereas no phase transformation of a-Si films is observed under the same annealing condition. The annealed Si films show a high crystalline volume fraction of 97% and [111] preferential orientation. Characteristic triangle surface structures aligned to the laser scanning direction, which suggests that the lateral solidification from molten Si is observed.

Role of microstructure in electronic transport behavior of highly crystallized undoped microcrystalline Si Films

Significant correlation is established between the observed electrical properties (room temperature value of dark conductivity and its activation energy) of the plasma deposited highly crystallized undoped hydrogenated microcrystalline silicon (μc-Si:H) films with their microstrucural properties (various aspects like fractional composition of constituent grains, morphology, and crystalline orientation). The conductivity shows three distinct trends with increasing film thickness irrespective of the different deposition parameters or conditions, and these three zones are applicable to all the samples, indicating that all the samples fundamentally belong to three different microstructural classes (with distinct microstructural attributes like thickness and features of grains and conglomerates) corroborative with these three zones of electrical behavior.

Boron Doping Effects in Microcrystalline Silicon

AbstractConditions leading to high conductivities (up to 300 S/cm) in chlorosilane-based boron-doped microcrystalline Si:Cl:H films are investigated. It is found that the high conductivity originates primarily from the growth of highly crystalline material with a high concentration of boron. Furthermore, these films grow with relatively low chlorine and hydrogen concentrations of a few percent and, according to effusion measurements of hydrogen and implanted helium, in a relatively compact structure. At a boron doping level of 1%, admixture of 10% silane to the tetrachlorosilane results in the growth of amorphous material of low conductivity while for admixture of up to 90% of silicontetrafluoride, microcrystalline Si films with high conductivities can be grown.

Intrinsic crystalline-to-amorphous transition above 400°C in plasma-deposited Si thin films

There has been long debate concerning why microcrystalline Si formation is disrupted at a deposition temperature above around 400°C: is it due to the loss of surface hydrogen coverage or to the incorporation of oxygen impurities? The authors reduced the atmospheric concentration of such impurities as O, C, and N in this work by using an ultraclean plasma deposition apparatus and demonstrate that microcrystalline Si films showed improved crystallinity at a deposition temperature of 350°C, resulting in a mobility of 5. As the deposition temperature was raised to 450°C, however, the Raman spectrum shows an amorphous structure of the film under such clean conditions. According to secondary ion mass spectrometry, the O, C, and N concentrations in this film were as low as 1017, 1016, and 1017cm−3, respectively, demonstrating that the crystalline-to-amorphous transition around 400°C should be intrinsic under the present conditions and may be correlated with thermal hydrogen desorption on a film-growing surface.

High quality microcrystalline Si films by hydrogen dilution profile

Novel hydrogen dilution profiling (HDP) technique was developed to improve the uniformity in the growth direction of μc-Si:H thin films prepared by hot wire chemical vapor deposition (HWCVD). It was found that the high H dilution ratio reduces the incubation layer from 30 nm to less than 10 nm. A proper design of hydrogen dilution profiling improves the uniformity of crystalline content, X c, in the growth direction and restrains the formation of micro-voids as well. As a result the compactness of μc-Si:H films with a high crystalline content is enhanced and the stability of μc-Si:H thin film against the oxygen diffusion is much improved. Meanwhile the HDP μc-Si:H films exhibit the low defect states. The high nucleation density from high hydrogen dilution at early stage is a critical parameter to improve the quality of μc-Si:H films.

Study of anomalous behavior of steady state photoconductivity in highly crystallized undoped microcrystalline Si films

The photoconductivity (σph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) films was measured as a function of light illumination over a wide temperature range (∼15–325K). A thermal quenching behavior in σph was observed at ∼240K. The photoconductivity exponent (γ) was found to be sublinear with γ as low as 0.13. A density of states (DOS) profile having a steep conduction band tail, and valence band tail with two distinct distributions was found to be necessary to understand the electronic transport behavior in the inherently heterogeneous μc-Si:H films.

Fabrications of Si Thin-Film Solar Cells by Hot-Wire Chemical Vapor Deposition and Laser Doping Techniques

In this paper, we report a novel low-temperature process for fabricating a Si thin-film solar cell on a glass substrate. The cell structure was composed of glass/Al/p–i–n Si/Ag (grid), where the Si intrinsic layer was deposited by hot-wire chemical vapor deposition. All the doped Si layers were produced using a postgrowth laser-doping process. The hot-wire-deposited amorphous, microcrystalline and polycrystalline Si films showed significant differences in band gap and structural properties as determined by Raman spectroscopy, spectral optical transmission measurements, and transmission electron microscopy. The corresponding crystalline volume fractions were 93, 73, and 12%, respectively. It was found that the best solar cells were fabricated with a Si intrinsic layer deposited at the transition from microcrystalline to polycrystalline regimes. A preliminary efficiency of 1.9% was obtained for an n–i–p structured solar cell on an untextured glass substrate.

R&D activities of silicon‐based thin‐film solar cells in China

AbstractThe status and progress of R&D activities of silicon‐based thin‐film solar cells in China are described briefly in this paper, including amorphous Si solar cells and microcrystalline Si film solar cells based on PECVD technology and polycrystalline film solar cells based on RTCVD technology. Especially, the microcrystalline thin‐film solar cells and the tandem solar cells of amorphous Si with microcrystalline Si have made great progress. The polycrystalline film solar cells have made remarkable achievements as well. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and electrical studies of plasma-deposited polycrystalline silicon thin-films for photovoltaic application

Plasma-deposited polycrystalline Si films [or microcrystalline Si (μc-Si) films] produced by plasma enhanced chemical vapor deposition (PECVD) have attracted considerable attention as novel, low-cost and stable materials for the photovoltaic i-layer in p–i–n junction thin-film solar cells. The μc-Si films prepared under various deposition conditions show widely various microstructures, from a crystalline–amorphous mixed state to an almost perfect crystalline state, with different crystallographic orientations. These structural changes directly influence the carrier transport properties that play a dominant role in determining photovoltaic performance. Furthermore, obtaining a uniform built-in electric field throughout the i-layer is a crucial issue for achieving excellent photovoltaic performance. To obtain a uniform electric field, the following terms should be required for the i-layer: ‘truly’ intrinsic characteristic (or not n-type characteristics) as well as structural uniformity in the growth direction without an incubation layer. Here, structural properties of μc-Si for achieving truly intrinsic characteristics are reviewed with an emphasis on collations with the crystalline volume function and the degree of (2 2 0) crystallographic preferential orientation in the crystalline phase. In addition, we reviewed a growth mechanism for the μc-Si film that is actually used in the photovoltaic i-layer in highly efficient solar cells: hybrid-phase growth consisting of conventional vapor-phase growth at the surface and the solid-phase crystallization that simultaneously occurs in the film. That growth is very effective in producing structural uniformity along the growth direction and for formation of crystallites directly on the underlying doped layer.

Recombination traffic in highly crystallized undoped microcrystalline Si films studied by steady state photoconductivity

The dependences of photoconductivity ( σ ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films on the temperature and the intensity of light were measured in a temperature range 15–325 K. The variation in light intensity exponent ( γ) is found to be 0.5 ≤ γ ≤ 0.9 for thin samples, and 0.15 ≤ γ ≤ 0.8 in a thick fully crystallized sample. Based on our simulation and experimental results, we have been able to construct density of state map for both the samples of different microstructure. In particular, we found that the fully crystallized thick sample has a very steep conduction band tail compared to thin sample. The valence band tails (VBT) are classified into two parts: one with sharper tail near the edge originating from grain boundary defects and other one with less steep tail associated with the defects in columnar boundary regions. Capture cross section for the deeper tail is smaller than the shallower one. The shallower VBT states extend up to more depth in thin sample. Our simulation results for thick sample show the transformation of recombination traffic taking place from shallower states to deeper tail states in the temperature region where γ < 0.5. Therefore, these deeper tail in thick sample works as a “safe hole traps”.

Gas-jet electron beam plasma chemical vapor deposition method for solar cell application

Abstract A novel gas-jet electron beam plasma chemical vapor deposition method for high-rate deposition of silicon films is presented. The method is based on the activation of initial gas molecules by electron beam and fast convective transfer of the radicals to a substrate by means of a supersonic free jet. We report on the results of applying the method for growth of amorphous, microcrystalline, homoepitaxial silicon films and transparent conducting oxides (ZnO:Al). Our method demonstrates the high deposition rates (up to 5 nm/s) of microcrystalline Si films at low temperatures on large area substrates (area up to 15×15 cm 2 ).

Charge Assisted Deposition of Polycrystalline Silicon Thin Films by Cesium Sputter Ion Beam Deposition

Polycrystalline Si thin films were deposited on a Si (100) substrate using negative Si ion beams and electrons generated from a highly doped Si target by the bombardment of highly energetic cesium ions. The negative Si ion energy was fixed at 100 eV and the target current was manipulated from 100 to 200 µA. The microstructure of the deposited films was analyzed from diffraction pattern and transmission electron micrographs. From the results of Transmission electron Microscopy (TEM) data, a polycrystalline Si film was deposited at a target current of 200 µA while amorphous film or a microcrystalline Si film was obtained at a target current of 100 µA, result in charged defects generated by ion beams and sufficient ion fluxes make such a structural difference.

Oxygen impurity doping into ultrapure hydrogenated microcrystalline Si films

We have systematically performed oxygen impurity doping into ultrapure hydrogenated microcrystalline Si (μc-Si:H) films grown at ∼200°C where the lowest spin densities are obtained. Two threshold oxygen concentrations [O] were observed: no crystallinity deterioration below [O]≈1×1020cm−3 and rapid carrier density increase above [O]≈1×1018cm−3. In these cases, electron carrier density roughly obeys a 1.4th power law with respect to [O] in the range between these two thresholds, while crystallinity remains almost unchanged. An implication of these results is discussed that includes plausible microscopic structures of oxygen donors and their locations.

Increase of temperature and crystallinity during electrical switching in microcrystalline silicon

ABSTRACTWe investigate electrical stressing and switching in hydrogenated microcrystalline silicon (mc-Si:H) by thermal, and optical and electrical measurements of Cr/mc-Si:H/metal thin-film structures. Boron-doped microcrystalline Si films of 30-50 nm thick are deposited by hot-wire chemical vapor deposition (HWCVD) on Cr-coated glass at 160°C and contacted with Ag or Al. Switching in devices of size 5 to 30 mm is stimulated by a current-ramp from 10 nA to 50 mA. We find that the voltage across the mc-Si:H devices initially increases logarithmically with current, then saturates at 2∼3 V, and finally drops to a low value of 1 to 1.5 V. This drop indicates a permanent decrease of device resistance to below 1 kW. During current stressing, the surface temperature increases with the bias current, and the surface reflectivity changes. After switching, a small increase in crystalline fraction can be observed by micro-Raman scattering measurements. The observations suggest electrothermal processes which cause changes in microstructure of the mc-Si bulk during current stress.

Iron disilicide formed in a-Si〈Fe〉 thin films by magnetron co-sputtering

The formation of β-FeSi2 precipitates was observed for the first time in microcrystalline Si films. The Fe-doped amorphous Si films (a-Si:Fe) were obtained using magnetron sputtering. Subsequent short-time thermal treatment led to the transition of amorphous Si to microcrystalline Si and to the formation of β-FeSi2 precipitates. The synthesized samples emitted at a wavelength of λ≈1.54μm at 100K.

Highly Conductive Boron Doped Microcrystalline Si Films Deposited by Hot Wire Cell Method and its Application to Solar Cells

Boron doped microcrystalline silicon (p-µc-Si) films were successfully deposited by the Hot Wire Cell method using a gas mixture of pure silane (SiH4) and diborane (0.5% B2H6 in H2). The influence of various deposition parameters on the structural and electrical properties of the films was investigated to obtain highly conductive p-µc-Si films. A high dark conductivity (σd) of 84.6 S/cm and a low activation energy of 0.02 eV were achieved for 550-nm-thick films. For thin films with a thickness of 15 nm, a σd of 4×10-2 S/cm was obtained. The thin p-µc-Si was incorporated into p–i–n amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) solar cells, in which intrinsic layers were deposited by photo chemical vapor deposition (photo-CVD). The initial conversion efficiencies of 9.04% and 6.17% were obtained for a-Si and µc-Si solar cells, respectively.

The formation of β-FeSi2 precipitates in microcrystalline Si

The formation of β-FeSi2 precipitates was observed for the first time in microcrystalline Si films. The Fe-doped amorphous Si films (a-Si:Fe) were obtained using magnetron sputtering. Subsequent short-time thermal treatment led to the transition of amorphous Si to microcrystalline Si and to the formation of β-FeSi2 precipitates. The samples synthesized emitted at the wavelength λ≈1.54 μm at 100 K.