The dark conductivity (σd) has been measured from 300 to 440K on undoped hydrogenated microcrystalline silicon (μc-Si:H) films having different thicknesses. The carrier transport is found to be thermally activated with single activation energy (Ea) in all the samples. The Ea increases as the film thickness decreases. At the same time logarithmic of dark conductivity prefactor (σo) is found to follow a linear relation with activation energy, known as the Meyer-Neldel rule (MNR). Results are explained in terms of increased degree of disorder in thinner samples. Thus change in Ea with the film thickness is directly related to the density of localized states at the Fermi level in grain boundary (GB). Therefore varying the film thickness and, hence, the exponential density of states induces a statistical shift of Fermi level which gives rise to the observed MNR.

Measurements of capacitance transients due to charge carrier emission from metastable defects in p-type hydrogenated amorphous silicon (a-Si:H) p/n junction structures, show that both holes and electrons can be metastably trapped in the p layer. At 350 K electrons and holes are emitted at the same rate defining the isokinetic temperature (Tiso) for the Meyer-Neldel rule. The enthalpy changes for hole and electron emission are 0.94 and 0.51 eV, respectively. The entropy changes for hole and electron emission are 31 and 16 Boltzmann constant, respectively. Charge emission rates are measured, for the first time, above and below Tiso. Below Tiso electrons are emitted faster than holes and above Tiso the reverse is true. These relative changes in emission rates are a direct consequence of the large entropy changes in the defect reactions. Tiso, within experimental error is the same as the H glass equilibration temperature (360 K) measured by others in p-type a-Si:H, suggesting that the defect reactions measured here are involved in H glass equilibration. Activation and deactivation of B atoms with accompanying charge emission likely govern both processes.

We measured the transient photocurrent decay from the steady state in microcrystalline silicon from plasma-enhanced chemical vapor deposition and hot-wire chemical vapour deposition. Samples exhibiting a range of photoconductive properties in terms of both the majority and minority carrier mobility-lifetime products or sub-gap absorption coefficients were studied. Measurements were made over a wide range of steady-state photogeneration rates for which we detail the variation with generation rate of the decay time. Samples with a short steady-state photocarrier lifetime show a long decay time. We relate the slow decay process to the much larger density of traps in the band gap in the poor-quality samples. Trapped carriers are released, undergo emission and trapping processes and eventually recombine in these samples, on a much longer time-scale than in the higher-quality samples so that the decay time cannot be taken as a fingerprint for photo-electronic quality. Analytical and numerical modeling indicate bimolecular recombination behavior during the decay. Results are in agreement with free-carrier interaction with exponentially distributed band-tail states.

We evaluated a-Si:H TFTs fabricated on polyimide foil under uniaxial compressive or tensile strain. The strain was induced by bending or stretching. All experiments confirmed that the on-current and hence the electron linear mobility depend on strain å as μ = μ0 (1 + 26·ε), where tensile strain has a positive sign. Upon the application of stress the mobility changes instantly and then remains unchanged in measurements up to 40 hours. In the majority of the TFTs the off-current and leakage current do not change. In tension, the TFTs fail mechanically at a strain of ∼ 3x10-2 but recover if the strain is released ‘immediately’.

In general terms this paper analyzes effectiveness of ZnO/p+/i a-Si:H/n+/Al solar cells made on commercial glass substrate with double-sided antireflection coatings. The primary topic, however, is the ZnO/p+/i a-Si:H junction based on layers obtained by the reactive magnetron sputtering (RMS) of Si:B, Si:P, Al, Zn:Al targets, where the “i” layer is a protocrystalline RFCVD layer. These single layers, as well as the respective structures, were studied using primarily Grazing Incidence X Ray Analyzes and AFM spectroscopy. In particular, the microcrystallization, surface roughness and chemical composition of the films were studied with respect to film thickness, RMS power and hydrogen dilution. The nature of the influence of ZnO/p+/i a-Si:H crystallization on pin a-Si:H solar cells performance is presented. Optical transmission and reflection measurements were used to measure the light flux reaching the “i” layer. We determined the experimentally observed angular distribution of scattered light in transmittance through glass/ZnO:Al/p+ silicon and double antireflection coatings on glass/ ZnO:Al/p+ silicon junctions. The optoelectronic properties of glass/ZnO/pin a-Si:H/Cr/Al cells are examined by systematic investigation of their I-V characteristics subsequent to steady-state conditions.

Bottom-gate amorphous silicon thin-film transistors were fabricated on a polyethylene terephthalate substrate. The maximum processing temperature was 100°C. The transistor characteristics are comparable, although still inferior, to those of standard amorphous silicon transistors fabricated on glass substrates. To obtain these characteristics, an extended anneal the processing temperature was required. The devices were fabricated using separately optimized low-temperature active layer, contact layer and gate dielectric layer. To achieve good electronic properties for these layers, hydrogen dilution was required.

We proposed a new carbon source, 1,3-disilabutane (H3Si-CH2-SiH2-CH3:1,3-DSB), to grow hydrogenated amorphous silicon carbide (a-SiC:H) films by mercury-sensitized photochemical vapor deposition (photo-CVD). We described preliminary results of undoped and p-type a-SiC films deposited using 1,3-DSB. It was found that the optical energy gap of the films was changed even at very small 1,3-DSB/silane ratios of few percents. P-type doping was carried out by using diborane and we obtained the films with a darkconductivity of 1.3x10-4 S/cm at the optical bandgap of 2.1 eV. In addition, we applied this material for a p-layer of a p-i-n type a-Si based solar cell and we have achieved relatively high conversion efficiency of 9.55%.

A Pecvd silicon nitride film, Damascene Nitride™, is deposited in a PECVD chamber with a hollow cathode faceplate using silane and ammonia as precursor gases. Various techniques (FTIR, RBS-HFS, SIMS, TDS and BTS) were used to characterize the structure, composition, density and wet etch rate of the film. FTIR analysis indicates that Damascene Nitride is very similar to a high density plasma (HDP) nitride film. HFS analysis shows the film's hydrogen content to be 13%,∼6% less than other PECVD nitride films, leading to a 20% improvement in etch selectivity to FSG. The film wet etch rate is 2 times slower than that of other PECVD nitrides, and the dielectric constant k was measured to be 6.8, which is lower compared to other PECVD nitrides and HDP CVD nitrides where k∼ 7.0 and 7.5, respectively. SIMS analysis shows that Cu diffusion is <250Å in the nitride, and low leakage current (10-10 A) is confirmed through BTS testing. The higher density of Damascene Nitride leads to higher etch selectivity and better Cu barrier properties, allowing a thinner nitride film to be used. Thinner nitride layers, in addition to the lower k of Damascene Nitride, leads to a 5-6% reduction in RC delay when Damascene Nitride is used with low k dielectric materials.

A cascaded arc expanding thermal plasma is used to deposit intrinsic hydrogenated amorphous silicon at growth rates between 0.2 and 3 nm/s. Incorporation into a single junction p-i-n solar cell resulted in an initial efficiency of 6.7%, whereas all the optical and initial electrical properties of the individual layers are comparable with RF-PECVD deposited films. In this cell the intrinsic layer was deposited at 0.85 nm/s and at a deposition temperature of 250°C, which is the temperature limit for growing the p-i-n sequence. The cell efficiency is limited by the fill factor and using a buffer layer at the p-i interface deposited with RF-PECVD at low growth rate can increase this. The increase in fill factor is a result of a lower initial defect density near the p-i interface then obtained with the expanding thermal plasma, resulting in better charge carrier collection. To use larger growth rates, while maintaining the material properties, higher deposition temperatures are required. Higher deposition temperatures result in a smaller optical bandgap for the intrinsic layer and deterioration of the p-type layer, resulting in a lower opencircuit voltage. First results on applying a buffer layer will also be presented.

Low-temperature growth of polycrystalline Silicon thin films has been investigated to fabricate thin film transistors by a new thermal CVD with the reactive source gases, Si2H6 and F2, resulting in the film growth at a low-temperature less than 500°C. In order to establish the optimal condition, gas pressure, total gas flow rate of Si2H6+F2 and He as a carrier gas, and residence time, τ, were tuned. Deposition rates and film crystallinity were influenced by the gas flow rations. The growth rate was 3.2-4.2[nm/min] and film uniformity was within ±6.5% over 4cm2 area. High crystallinity films showed a sharp peak at 520[cm-1] in Raman spectra whose full width at half maximum was 6-8[cm-1]. The high crystallinity even at the early stage of film growth was confirmed by transmission electron microscopy. The conductivity and activation energy is on the order of 10-5-10-6 [S/cm] and 0.53[eV], respectively, after hydrogenation. We fabricated poly-Si bottom-gate TFT that have field effect mobility as high as 32.3cm2/Vs and on/off current ratio of 104.

This paper presents the fabrication and characterization of MEMS structures on glass substrates using thin film silicon technology and surface micromachining. The technology developed to process bridge and cantilever structures as well as the electromechanical characterization of these structures is discussed. This technology can enable the expansion of MEMS to applications requiring large area and/or flexible substrates. The main results for the characterization of the movement of the structures are as follows: (1) in the quasi-DC regime and at low applied voltages, the response is linear with the applied dc voltage. Using an electromechanical model which takes into account the constituent materials and geometry of the bilayer, it is possible to extract the deflection of the structures. This estimate suggests that it is possible to control the actuation of these structures to deflections on the sub-nanometric scale; (2) resonance frequencies of up to 20 MHz have been measured on hydrogenated amorphous silicon (a-Si:H) bridge structures with quality factors (Q) of 70-100 in air. The frequency depends inversely on the square of the structure length, as predicted by the mechanical model; and (3) using an integrated permanent magnet/magnetic sensor system, it is possible to measure the structure movement on-chip and to obtain an absolute calibration of the deflection of the structures.

In this work silicon nitride films are formed as substrate temperatures 250°C by combination of catalytic-nitridation and catalytic-vapor deposition method in a catalytic chemical vapor deposition system. It is found that inserting nitridation layer about 2 nm-thick before growing the silicon nitride films, injection-type hysteresis of capacitance-voltage curve is drastically reduced from 1.4 V to 0.05 V for 40 nm-thick SiNx.

Polycrystalline silicon (polysilicon) is a key component of integrated circuit technology, micromechanical systems and displays. High-mobility polysilicon thin-film transistors are replacing amorphous-silicon transistors in advanced active-matrix displays, and are also vital to the emerging fields of microsensors and microactuators [1]. In this study, the deposition processes (LPCVD) of polysilicon thin films were carefully studied and the basic kinetics were analyzed. The films were patterned into submicron islands and the relation of the deposition temperature versus the crystallinity of polysilicon islands was observed. A “threshold” deposition temperature for polysilicon crystals larger than 0.35 μm was found. The relationships of polysilicon crystal sizes vs. annealing temperature and vs. annealing duration were also systematically studied. Polysilicon islands of ∼ 0.1 μm lateral dimensions were successfully fabricated by ebeam lithography and several types of annealings were performed to increase micro-crystal size, with the goal of creating single-crystal high-mobility islands. Theoretically, transistors built from such islands should match the performance of state-of-the-art bulk CMOS devices. The crystal size was measured using TEM and AFM analysis. Moreover, EXAFS (Extended X-ray Absorption Fine Structure) measurements at SSRL (Stanford Synchrotron Radiation Lab) were employed to study the crystallization processes of polysilicon films and the correlation between the absorption curves, the thin-film electrical properties and the polysilicon grain sizes.

We study the H NMR line shapes of a sample of a-Si:H under several conditions: 1) as grown, 2) light-soaked for 600 hours, and 3) light-soaked followed by annealing at different temperatures. At T = 7 K, the NMR line shape of the sample after light soaking exhibits an additional doublet compared to that of the sample as-grown. This doublet is an indication of a closely separated hydrogen pair. The distance between the two hydrogen atoms is estimated to be about (2.3 ± 0.2) Å. The concentration of these hydrogen sites is estimated to be between 1017 and 1018 cm-3 consistent with ESR measurements of the defect density after light soaking. This doublet disappears after the sample is annealed at 200°C for 4 hours.

The deposition of micro-crystalline silicon by means of PECVD with a new linear microwave plasma source is investigated. This plasma source has successfully been introduced in the large scale production of multi-crystalline Si solar cells for the deposition of passivating silicon nitride layers. Advantages of this linear plasma source are the high deposition rates and the large area (up to 80 cm width, no length limitations) on which a homogeneous deposition can be achieved. Since this source has not been applied for deposition of micro-crystalline silicon before, we explored a large parameter space (substrate temperature, pressure, MW-power, gas flow rates), in order to find optimum growth conditions. It is observed that with this microwave source it is possible to grow micro-crystalline layers at significantly higher silane/hydrogen ratios and higher deposition rates than for conventional RF PECVD. In this paper, structural properties of the silicon layers, as investigated by Raman and FTIR spectroscopy, XRD and SEM measurements are discussed.

Microcrystalline silicon (μìc-Si:H) solar cells were prepared in a wide range of deposition parameters using high pressure 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD). Focus was on the influence of deposition pressure, electrode distance and the application of a pulsed plasma on high rate deposition of solar cells. At electrode distances between 5 and 20 mm solar cells with efficiencies >8 % were prepared. A medium electrode distance of 10 mm yielded best device performance. Pulsed plasma deposition leads to good results at medium deposition rates of ∼5 Å/s, for higher rates a strong decrease of efficiency was observed. The highest efficiencies in a small area reactor were 8.9 % for CW and 8.4 % for pulsed plasma. We also succeeded in preparing μc-Si:H and a-Si:H/μc-Si:H solar cells in a 30x30 cm2 reactor with efficiencies of 9 % and 12.5 %, respectively.

The influence of the preparation conditions in hot wire chemical vapour deposition (HWCVD) on the electronic properties of microcrystalline silicon is investigated in view of application of the material in thin film solar cells. Poor grain boundary passivation, as a result of hydrogen etching at strong hydrogen dilution of the process gas or thermal desorption of hydrogen at high deposition temperatures, is considered a main obstacle for material optimisation. We conclude that optimum μc-Si:H solar cell material, both from HW-CVD and from plasma enhanced CVD, is not necessarily obtained with largest grain sizes and apparent highest crystalline content, but rather by a material prepared under conditions which yield a compact morphology with an effective grain boundary passivation.

We have studied the structural evolution in amorphous and microcrystalline silicon deposited from silane-argon mixture by radio frequency plasma enhanced chemical vapour deposition (PECVD) method. Sharp increase in small angle x-ray scattering (SAXS) intensity, in accordance with tilt measurements, indicates columnar morphology in the sample deposited in the amorphous-microcrystalline transition region. The variation of SAXS measured heterogeneity and a gradual shift of Si-H stretching vibrational frequency at 2000 cm-1 towards higher wave number with increase of power density indicate structural modifications in the films. Observation of sharp increase in the ratio of the intensity of Ar* to SiH* in the transition region may explain the surface modification assisted by Ar* and hence the structural changes in the material.

Electron spin resonance accompanied by conductivity measurements in n-type microcrystalline silicon with different doping concentrations and different structure compositions has been applied for the study of the density of gap states and the influence of these states on charge carrier density. We studied doping concentrations close to the defect density where the doping induced Fermi level (EF) shift is determined by compensation of gap states. We found a correlation between the EF shift, the intrinsic defect density and structural changes.

The effect of thin seed layers on the subsequent growth of thick, high growth rate bulk μc-Si is investigated by XRD, SEM and cross sectional TEM. All layers were deposited by hotwire chemical vapor deposition (HWCVD). When the seed layer as observed by XRD is highly crystalline, made by using high H2 dilution (H2:SiH4 100:1), the amorphous incubation layer typical of μc-Si growth is virtually eliminated. Furthermore, with this seed layer, bulk layer deposition conditions that would otherwise produce highly amorphous material enable a composite film with significant crystallinity. When the seed layer is predominantly amorphous, made using a much lower H2 dilution (10:1), there is evidence that very small crystallites, undetected by XRD, still facilitate immediate nucleation and enable the formation of larger grains in the subsequent bulk layer. In concurrence with other HWCVD results, lowering the filament temperature results in significant improvements in film compactness, photoresponse and grain size, while maintaining significant crystallinity. Such films have been incorporated into high deposition rate solar cells.