We have developed an efficient scheme for simulating silicon nanowires with crystalline cores and amorphous sheaths, using molecular dynamics. By starting with a crystalline nanowire and performing high temperature anneal an amorphous outer sheath can be grown with controlled thickness on the nanowire. Simulations for [001] nanowires with diameters of 12 nm find low energy facets between the amorphous and crystalline layers. Simulations for [110] nanowires find weak faceting and an inhomogeneous amorphous-crystalline boundary.

We have recently reported that hydrogenated nanocrystalline silicon (nc-Si:H) solar cells exhibit metastability behavior that is significantly different from hydrogenated amorphous silicon (a-Si:H) solar cells. First, we studied the light-induced change of nc-Si:H cells under different light spectra, and observed that no light-induced degradation occurs when the photon energy is lower than the bandgap of a-Si:H, while light-induced degradation indeed occurs when the photon energy is higher than the bandgap of a-Si:H. Based on this observation, we conclude that the light-induced defect generation occurs mainly in the amorphous and/or grain boundary regions. Secondly, we found that forward-bias current injection in the dark does not cause any degradation in the performance of nc-Si:H cells, in contrast to observations in a-Si:H cells. This phenomenon can be explained by assuming transport percolation through crystallites, where excess carrier recombination does not cause degradation. Third, we found that a reverse electrical bias does not reduce, but rather enhances the light-induced degradation in the nc-Si:H cell performance. This enhancement increases with the magnitude of the reverse bias. We carried out a systematic study including measurements of quantum efficiency losses and fill factors under different color filters. In view of the heterogeneity of the material structure, we proposed a “back-to-back” diode model which explains most of the experimental results. Finally, we present results of the improvement of stability of nc-Si:H cells made using hydrogen dilution profiling. We find that the nc-Si:H cells with an optimized hydrogen dilution profile show very little light-induced degradation though these cells have a high amorphous volume fraction. This indicates that the amorphous volume fraction is not the only factor determining the degradation. Other factors, such as the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries, apparently play important roles in the overall stability of nc-Si:H cells.

The main objective of this work is to investigate the possibility of combining pulsed laser deposition (PLD) and pulsed laser annealing to realize p-type Si1-xGex thin films suitable for post-processing MEMS on top of standard pre-fabricated driving electronics. The main advantage of this approach is that the substrate is kept at a CMOS backend compatible temperature throughout the deposition and thus the MEMS integration process will have no thermal impact on the underlying electronics. In addition, it is demonstrated that PLD Si1-xGex has good adhesion to SiO2 and accordingly there is no need for a silicon nucleation which is the case for LPCVD and PECVD. Furthermore, this technique is much more economical than CVD as it does don't imply using expensive gas precursors such as germane and silane.

The microstructure of a-Si:N:H films, which are applied as antireflection coating and for defect passivation in multicrystalline silicon (mc-Si) solar cells, was studied by gas effusion experiments. The results show for as-deposited material of low substrate temperatures (TS = 200 – 300°C) a predominant diffusion of molecular hydrogen for temperatures up to 800°C in agreement with the presence of interconnected openings (voids). At higher substrate temperatures, the material has a more compact structure and atomic hydrogen is the dominant diffusing species in the accessible temperature range. Annealing effects were also studied. The results are consistent with the concept that atomic hydrogen released from the a-Si:N:H coating serves for defect passivation in μc-Si solar cells.

The creation of metastable defects in the bulk of the intrinsic layers of a-Si:H p-i-n solar cells by recombination of photo-generated carriers as well as those injected under far forward bias has been investigated. The evolutions in the defects, created with volume-absorbed red light at open circuit voltage and constant far forward bias currents, were characterized with the Shockley-Reed-Hall recombination obtained from the dark currents under low forward bias voltages. This allowed the kinetics to be studied at different temperatures in the absence of isothermal annealing. It is found that the form of the kinetics obtained with the two methods are the same and a detailed study was carried out with currents from 1 to 100mA/cm2 with carrier injection during which the electron and hole concentrations remain essentially constant. This kinetics is similar to that reported for thin film results exhibiting stretched exponential behavior with a t1/3 relation over a limited regime before the onset of saturation. By separating the contributions of the metastable defects from the intrinsic defects, their evolution is found to have a t1/2 dependence over the entire range prior to the onset of saturation. Since these kinetics also exhibit an (intensity)2t dependence they point to a rate equation for the creation of metastable defects, Nms, dNms / dt ~ (intensity)2 / Nms, which is suggestive of new creation mechanisms for SWE defects.

We have fabricated pixel circuits consisting of two bottom-gate staggered source-drain amorphous silicon thin-film transistors (a-Si:H TFTs) on flexible stainless steel foils. Stainless steel is attractive because it allows for high processing temperatures of >300°C and is a perfect barrier to oxygen and moisture. Our steel foils were 75m thick, with a peak-to-peak surface roughness of >1.2m. This rough, as-rolled, conductive surface needed a thick planarization and passivation (electrical isolation) layer. The surface was planarized with 1.6m of spin-on-glass, which reduced the roughness to ~0.3m peak-to-peak. A passivation layer of 0.6m of SiNx deposited by plasma-enhanced chemical vapor deposition was used to reduce leakage currents and capacitive coupling to the substrate. The 92m × 369m voltage-programmed pixel circuits employ a switching (Sw) TFT (W/L=50/5m), a driver (Dr) TFT (W/L=200/5m), and a 2pF storage capacitor between the gate and source of the Dr TFT. With a supply voltage of VDD=20V and a drive bias of 20V the circuits deliver 26A of current. The vertical stripe pixels were integrated into 48 × (4) × 48 arrays and passivated with SiNx. Anode metal of Al-1% Si was also deposited, preparing the displays for subsequent OLED fabrication. Pixel circuits with this performance can drive top-emitting organic light emitting diodes (OLEDs) and therefore can be used in backplanes for flexible, high-resolution, active-matrix OLED displays.

An asymmetric dual gate poly-Si thin film transistors (TFTs), which is consist a long-gate TFT and a short-gate TFT, were fabricated in order to suppress the kink current and increase the reliability. The long-gate TFT operates in a linear regime and limits the total current flow by its current operation region. The asymmetric dual-gate does not exhibit from the kink current in a high drain bias due to the distribution of lateral electric field. The asymmetric dual-gate structure improves kink-free characteristics compared with conventional single and dual-gate TFTs. The hot-carrier stress reliability is successfully improved due to kink current suppression.

The crystal-field potential at the Er3+ ion surrounded by six oxygen ions is expanded in terms of polynomials. After converting it into equivalent angular momentum operators, the Stark-splitting of the 4I15/2 ground state of the Er3+ ion is calculated. Influence of the change in the environment of the Er3+ ion on the shift of the energy levels is investigated and compared with the observed Er photoluminescence spectrum in a-Si:H. The scattering of the calculated energy levels by the structural fluctuation around the Er3+ ion is also compared with the linewidth of the component photoluminescence lines.

A novel T-shaped-gated (T-Gate) polycrystalline silicon thin-film transistor (poly-Si TFT) with vacuum gaps has been proposed and fabricated only with a simple process. The T-Gate structure is formed only by a selective undercut-etching technology of the Mo/Al bi-layers. Then, vacuum gaps are in-situ embedded in this T-Gate structure subsequent to capping the SiH4-based passivation oxide under the vacuum process chamber. Experimental results reveal that the proposed T-Gate poly-Si TFTs have excellent electrical performance, which has higher maximum on-off current ratio of 4.6 e107, and the lower off-state leakage current at VGS = -10 V and VDS = 5V of about 100 times less than that of the conventional one. It is attributed to the additional undoped offset region and the vacuum gap to reduce the maximum electric field at drain junction while ascribed to the sub-gate to maintain the on-current. Therefore, such a T-Gate poly-Si TFT is very suitable for the applications and manufacturing in active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diodes (AMOLEDs).

Systematic measurements of dark current density versus voltage (J-V) characteristics were carried out on a-Si:H, a-SiGe:H, and nc-Si:H solar cells at different temperatures from 23°C to 150°C. In a first order approximation, the dark J-V characteristics follow the standard diode characteristic formula for most high quality solar cells. The temperature dependence of the reverse saturation current can be used for deducing the intrinsic layer bandgap for the three types of solar cells. From a detailed analysis using the derivative of the measured dark J-V characteristics, we obtain different features in the plot of ideality factor versus bias voltage for the three types of solar cells. We also deduce the distribution of density of states using a recently proposed procedure [1]. In general, a peak near the middle of the bandgap appears for the three types of solar cells; below the middle of the bandgap, there is an exponential-like broad distribution of gap states. However, the calculated gap state distribution shows a temperature dependence, which may be caused by the simplification of the calculation procedure.

Thick microcrystalline silicon (mc-Si:H) films were exposed to atomic hydrogen plasma at substrate temperature of 220°C after deposition. The microstructure of μc-Si:H films after exposure was characterized using Raman back scattering spectroscopy and transmission electron microscopy (TEM). Raman spectra reveal that the intensity near 520 cm−1 significantly increases after hydrogen exposure, indicating an increase of crystallinity in the films. TEM micrographs of μc-Si:H films exposed to atomic hydrogen also show an increase in the size of grains and a growth of crystalline grains ranging from surface to bulk. These results suggest that crystalline grain formation in μc-Si:H films is likely to be caused by chemical annealing.

The spatial distribution of the SiH3 radicals between the electrodes of a hydrogen diluted silane VHF plasma under thin film hydrogenated microcrystalline silicon (μc-Si:H) growth conditions has been measured using the time resolved cavity ringdown (τ-CRD) absorption spectroscopy technique. The μc-Si:H growth rate is estimated from the measured spatial SiH3 profiles using a simple model based upon diffusion controlled flux of SiH3 radicals to the electrode surface, where the SiH3 can react with the film surface. The calculated value of μc-Si:H growth rate roughly agrees with the value of the experimentally determined growth rate. This agreement implies that the SiH3 radical is the main growth contributor to the μc-Si:H growth. Furthermore, the τ-CRD reveals the growth kinetics of the clusters in the plasma by light scattering at these clusters on time scales of 1 s after the plasma ignition.

Thin-beam directional X'tallization (TDX) is a low-temperature-substrate compatible crystallization method that can form directionally solidified poly-si films directly on glass substrates. Without using a mask, extremely long uniformly-spaced poly-Si grains can be formed by TDX. In this paper, we have investigated the influence of laser energy and scan pitch on the properties of the resulting poly-Si films. Grain size and surface morphology of TDX-processed poly-Si films were observed by SEM and AFM. The AFM results demonstrate that, due to the lateral growth phenomenon, TDX poly-Si films are much smoother than films produced by the current industry standard method, Excimer Laser Annealing (ELA). The breakdown of the TDX method is also discussed in this paper. When the laser scan pitch is larger than the lateral growth length that is characteristic of a given poly-Si film and beam property combination, a discontinuous poly-Si film is formed with vertical protrusions and grain boundaries occurring perpendicular to the scan direction. Poly-Si films with vertical protrusions and grain boundaries perpendicular to the direction of current flow will degrade the resulting thin-film transistor (TFT) performance.

In this paper, we wish to report our preliminary experimental results from the photoluminescence (PL) studies in a Mn-ion implanted silicon-rich oxide (SRO) thin film. At 4 K, a broad PL peak, centered at ~ 1.2 eV, was observed. It is blue-shifted from the Si substrate peak at ~ 1.1 eV. The temperature (T) dependence of PL was carried out at zero magnetic (B) field and B = 0.5 Tesla, respectively, and showed quantitatively different behaviors. At B = 0, the PL intensity increases very slowly at low temperatures and reaches a maximal value at ~ 40 K. It then decreases as T is further increased. At B = 0.5 Tesla, the peak temperature (Tpeak), whether the intensity is maximal, moves to ~ 80-100 K, and the decreasing rate beyond Tpeak is much smaller than that at B = 0. We speculate that these two different behaviors might reveal, possibly, a ferromagnetic ordering in Mn-ion doped silicon nanocrystals.

We investigated the structural quality of polysilicon films fabricated by the aluminium induced crystallization (AIC) of amorphous silicon on alumina substrates. We analyzed the overall crystallographic quality of the poly-Si films in terms of grain size distribution and grain orientation versus crystallization temperature. For these studies, we used extensively the orientation imaging micrograph (OIM) technique, a very powerful tool that allows elucidating the inner-grain structure, the grain boundaries, the grain orientation. From our analysis, we may conclude that the polysilicon films formed by AIC on alumina substrates have the following features: (i) for all investigated temperatures, most of the silicon grains have a deviation angle from (100) crystallographic orientation between 5 and 25°; (ii) increasing the annealing temperature tends to decrease the (100) preferred orientation; (iii) the angular boundary distribution revealed that the main defects are those that have been observed inside isolated dentrites, namely low angle boundaries (<2°) and coincident site lattice boundaries such as Σ3, Σ9 and Σ27.

We have achieved an independently-confirmed 17.8% conversion efficiency in a 1-cm2, p-type, float-zone silicon (FZ-Si) based heterojunction solar cell. Both the front emitter and back contact are hydrogenated amorphous silicon (a-Si:H) deposited by hot-wire chemical vapor deposition (HWCVD). This is the highest reported efficiency for a HWCVD silicon heterojunction (SHJ) solar cell. Two main improvements lead to our most recent increases in efficiency: 1) the use of textured Si wafers, and 2) the application of a-Si:H heterojunctions on both sides of the cell. Despite the use of textured c-Si to increase the short-circuit current, we were able to maintain the same 0.65 V open-circuit voltage as on flat c-Si. This is achieved by coating a-Si:H conformally on the c-Si surfaces, including covering the tips of the anisotropically-etched pyramids. A brief atomic H treatment before emitter deposition is not necessary on the textured wafers, though it was helpful in the flat wafers. It is essential to high efficiency SHJ solar cells that the emitter grows abruptly as amorphous silicon, instead of as microcrystalline or epitaxial Si. The contact on each side of the cell comprises a thin (< 5 nm) low substrate temperature (~100°C) intrinsic a-Si:H layer, followed by a doped layer. Our intrinsic layers are deposited at 0.3-1.2 nm/s. The doped emitter and back-contact layers were deposited at a higher temperature (>200°C) and grown from PH3/SiH4/H2 and B2H6/SiH4/H2 doping gas mixtures, respectively. This combination of low (intrinsic) and high (doped layer) growth temperatures was optimized by lifetime and surface recombination velocity measurements. Our rapid efficiency advance suggests that HWCVD may have advantages over plasma-enhanced (PE) CVD in fabrication of high-efficiency heterojunction c-Si cells; there is no need for process optimization to avoid plasma damage to the delicate, high-quality, Si wafers.

The application of amorphous silicon as a thermo-optically active material in MEMS-based, tunable optical filters is presented. The thin film interference filter employs a solid-state silicon cavity and silicon-based distributed Bragg reflectors (DBR). Tuning is achieved by varying the the resonator's refractive index and thus its optical thickness through thermal modulation. The filter is configured as a membrane and operates up to a temperature of 450 °C. Such a filter is essential for monitoring and reconfiguration of optical data communication networks and is also applicable for chemical and gas sensing.

A novel method of locating Si thin-film grains by combining metal nano-imprint and excimer laser annealing (ELA) is demonstrated. Metal nano-imprint at the a-Si film surface is used for the purpose of creating Si crystal nuclei which act as the seed for the subsequent crystallization by using ELA. The annealing to form nuclei at imprinted sites was carried out at temperatures below 450oC. ELA using XeCl laser of the sample with capping SiOx film resulted in the formation of over 2.0 μm-sized Si crystal grains at controlled position. Electron back-scattering pattern (EBSP) analysis showed that about 75% of the boundaries inside the location-controlled grains were the coincidence site lattice boundaries.

The deposition process of silicon nitride (SiNx) by hot-wire chemical vapor deposition (HW CVD) is investigated by exploring the effects of process pressure and gas-flow ratio on the composition of the deposited SiNx films. Furthermore, experiments with D2 and deuterated silane were performed to gain further insight in the deposition reactions taking place. It appeared that the N/Si ratio in the layers determines the structural properties of the deposited films and since the volume concentration of Si-atoms in the deposited films is constant with N/Si ratio, the structure of the films are largely determined by the quantity of incorporated nitrogen. Because the decomposition rate of the ammonia source gas is much smaller than that of silane, the properties of the SiNx layers are largely determined by the ability to decompose the ammonia and to incorporate nitrogen into the growing material. It appeared that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high value of 7 nm/s for dense transparent SiNx films, much faster than conventional deposition techniques for SiNx can offer. Despite this high deposition rate good control over the composition is achieved by varying the flow ratio of the source gasses. Depositions performed with deuterated silane as a source gas reveal that almost all hydrogen in N-rich films originates from ammonia, probably caused by SiNx matrix formation by cross linking reactions

A unique nanowire growth was accomplished at 575 oC by the metal-induced growth (MIG). That is a spontaneous reaction between metal and Si. The deposited metal worked as a catalyst layer to grow nanowire in the solid-state. Various metals (Ni, Co, and Pd) were used in MIG nanowire fabrication and Ni-induced case was successful in demonstrating that metal species should be a dominant mover in nanowire growth. Transmission electron microscopy investigation was performed to observe the nanowire growth direction. The Ni to Si composition was studied by energy dispersive spectroscopy showing the Ni diffusion inside the nanowire as well as the Ni silicide layer. The practical application MIG nanowire was proven by fabricating nanoscale contacts.