We have observed a significant light-induced increase in the open-circuit voltage (Voc) of mixed-phase hydrogenated silicon solar cells. In this study, we investigate the kinetics of the light-induced effects. The results show that the cells with different initial Voc have different kinetic behavior. For the cells with a low initial Voc (less than 0.8 V), the increase in Voc is slow and does not saturate for light-soaking time of up to 16 hours. For the cells with medium initial Voc (0.8 ∼ 0.95 V), the Voc increases rapidly and then saturates. Cells with high initial Voc (0.95 ∼ 0.98 V) show an initial increase in Voc, followed bya Voc decrease. All light-soaked cells exhibit a degradation in fill factor. The temperature dependence of the kinetics shows that light soaking at high temperatures causes Voc increase to saturate faster than at low temperatures. The observed results can be explained by our recently proposed two-diode equivalent-circuit model for mixed-phase solar cells.

A simple lateral grain growth of polysilicon employing single excimer laser irradiation is proposed. In order to increase the size of silicon grain and to control the location of the large lateral grain, the oxide trench is employed under the amorphous silicon film in the proposed method. The proposed oxide trench, which is shaped like a triangle or a polygon with an acute angle, induces temperature gradient on the molten silicon film during the solidification. It was verified by SEM that about 2 μm-long silicon grains are successfully achieved near the oxide trench edge and the locations of lateral grains are controlled by the angular points of the diagram.

The electrical characteristics of SiH4-based PECVD gate oxide have been investigated with respect to gate oxide integrity (GOI) and its reliability. It was found that the GOI of poly-Si TFT integrated on glass substrate strongly depended on the charge trapping and deep level interface states generation under Fowler-Nordheim stress (FNS). By applying elevated temperature postanneal without vacuum break after the gate oxide deposition, highly reliable gate oxide was obtained. Under FNS, ID-VG curve showed severe shift and degradation of subthreshold slope, which were reduced by adopting post-annealed gate oxide. Besides, the TFT with post-annealed gate oxide showed around 10 times higher charge to breakdown than that of as-deposited gate oxide. Charge to breakdown of MOS capacitors were also studied. By applying post-annealed gate oxide, charge to breakdown drastically improved, which could be explained by reduced charge trapping under FNS.

This paper summarizes our recent studies of hydrogenated microcrystalline silicon (μc-Si:H) solar cells as a potential substitute for hydrogenated silicon germanium alloy (a-SiGe:H) bottom cells in multi-junction structures. Conventional radio frequency (RF) glow discharge is used to deposit hydrogenated amorphous silicon (a-Si:H) and μc-Si:H at low rates (∼ 1 Å/s), searching for the highest efficiency. We have achieved an initial active-area efficiency of 13.0% and stable efficiency of 11.2% using an a-Si:H/μc-Si:H double-junction structure. Modified very high frequency (MVHF) glow discharge is used to deposit a-Si:H and μc-Si:H at high rates (∼ 3-10 Å/s) for comparison with our a-Si:H/a-SiGe:H/a-SiGe:H triple-junction production technology. The deposition time for the μc-Si:H intrinsic (i) layer in the bottom cell should be less than 30 minutes in order to be acceptable for mass production. To date, an initial active-area efficiency of 12.3% has been achieved with the bottom cell deposited in 50 minutes. By increasing the deposition rate and reducing the bottom cell thickness, we have achieved an initial active-area efficiency of 11.4% with the bottom cell i layer deposited in 30 minutes. The cell stabilized to 10.4% after prolonged light soaking. We will address issues related to μc-Si:H material, solar cell design, solar cell analysis, and stability.

Surface topography of a-Si:H thin films, deposited at 75°C by Plasma-enhanced Chemical Vapor Deposition (PECVD) has been examined using helium/silane feedstock mixtures under different substrate bias conditions. Notable differences in the surface roughness evolution are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Smooth and apparently featureless surfaces result from deposition on RF powered surfaces, upon which a self-bias induces high-energy ion bombardment. Rougher surfaces result from films deposited on electrically grounded surfaces. These anodic films show that after a transition period, surface roughness grows linearly with processing time, exhibiting mounded type growth as evidenced by 2-D power spectral density functions of surface height measurements. Linear growth in roughness has been predicted for shadow growth models assuming film precursor sticking coefficients of one and random angle approach of film precursor species. Growth of this nature has not been reported before in a-Si:H studies, which usually assume directional deposition conditions and sticking coefficients less than unity – occurring even at low processing temperatures.

This work refers to a study performed on polymorphous silicon (pm-Si:H) at excitation frequencies of 13.56 and 27.12 MHz in a large area PECVD reactor. The plasma was characterised by impedance probe measurements, aiming to identify the plasma conditions that lead to produce pm-Si:H films. The films produced were characterised by spectroscopic ellipsometry, infrared and Raman spectroscopy and hydrogen exodiffusion experiments, which are techniques that permit the structural characterisation of the pm-Si films and to study the possible differences between the films deposited at 13.56 and 27.12 MHz. Conductivity measurements were also performed to determine the transport properties of the films produced. The set of data obtained show that the 27.12 MHz pm-Si:H can be grown at higher rates with less hydrogen dilution and power density, being the resulting films denser, chemically more stable and with improved performances than the pm-Si:H films grown at 13.56 MHz.

The open-circuit voltage (Voc) of mixed-phase hydrogenated silicon solar cells has been found to increase after light soaking. In this study, we use micro-Raman to investigate the heterogeneous structure of solar cells in the amorphous-to-nanocrystalline transition region. For a cell with Voc = 0.981 V, Raman spectra show a typical broad Gaussian lineshape around 480 cm-1, a signature of typical amorphous material. A cell with Voc = 0.674 V displays a sharp Lorentzian peak around 516 cm-1, indicative of nanocrystallinity. A cell with Voc = 0.767 V was systematically scanned for 20 different positions in 500 μm increments. Most spectra show a typical Gaussian lineshape around 480 cm-1, several spectra reveal a hint of a nanocrystalline shoulder around 512 cm-1, and one spectrum exhibits a distinct nanocrystalline peak. We conclude that the nanocrystallite distribution in the mixed-phase material is very non-uniform even within a mm dot. This result provides direct evidence supporting a recently proposed two-diode equivalent-circuit model to explain the light-induced effect.

High-rate (> 1 nm/s) and low-temperature (50– 400°C) deposition of silicon nitride (a-SiNx:H) films has been investigated by the expanding thermal plasma (ETP) technique using SiH4 as Si-containing and N2 or NH3 as N-containing precursor gases. The structural, optical and electrical properties of the a-SiNx:H films have been studied by elastic recoil detection, spectroscopic ellipsometry, infrared spectroscopy, dark conductivity measurements and atomic force microscopy. The film properties of the ETP deposited a-SiNx:H films in this low-temperature range are discussed in terms of deposition rate, atomic composition, UV-VIS optical and IR vibrational properties, conductivity, and surface topography of the films.

A study was carried out on hydrogenated amorphous silicon (a-Si:H) n-i-p (substrate) solar cell structures with p-a-SiC:H and highly diluted p-Si:H layers grown with different dilution ratios R=[H2]/[SiH4]. The contributions of the recombination at the p/i interfaces to the forward bias dark current characteristics were identified and quantified for the different cell structures. In both cell structures the role of the p/i interfaces was identified and it is found that the lowest p/i interface recombination is obtained with protocrystalline p-Si:H layers having no microcrystalline component. The results with p-Si:H layers are attributed not only to their properties but also to the subsurface modification of the intrinsic layer. Evidence is also presented that points to the beneficial effects of the high hydrogen dilution and power used in the deposition of these p-layers in creating the p/i interface regions. The limitations on 1 sun open circuit voltage (VOC) imposed by the p/i recombination present in all the cell structures is consistent with the mechanisms proposed by Deng et al.[1]. The results presented here also point to why the 1 sun VOC in protocrystalline p-Si:H solar cells is higher than that in p-a-SiC:H cells.

Alpha quartz can be transformed to a metamict phase in a physical reaction such as irradiation with fast ions, neutrons and electrons. As truly inspired by this fact, we design a simple experiment to produce disordered silica and at the same time watch the transformation using a transmission electron microscope. This work simply presents electron-damaged silica in dark field at Bragg reflection. Since the transformation is not complete, disordered silica (at the sample edge) and retained quartz can be compared in each image. Not surprisingly, more speckles are found in the quartz than in the disordered structure. But it is too early to say no medium-range ordering in amorphous silica because the experiment only shows one Bragg diffraction. Systematic work to complete and measure the disordering again is being pursued.

Hydrogenated polymorphous silicon (Pm-Si:H) being an admixture of amorphous and ordered phase silicon shows improved optical and electrical properties due to the presence of nanocrystallites. In order to compare the dynamic and steady state electrical properties in a-Si:H and pm-Si:H, bottom gate Thin Film Transistors (TFT) of these materials were fabricated with SiO2 as the insulating layer. The active materials were deposited using plasma-enhanced chemical vapor deposition (PECVD) by varying pressure, temperature and hydrogen dilution. Transfer characteristics of TFTs made using pm-Si:H show lower leakage current, higher on-current and sharper volt per decade change as compared to similar TFTs made from a-Si:H. Density of states in pm-Si:H as calculated from field effect conductance using incremental method is observed to be an order of magnitude lower than in a-Si:H based devices. To compare dynamic characteristics, we studied the switch-on transient characteristics of polymorphous and amorphous silicon TFTs by pulsing the gate to different voltages in the temperature range of 150-300K. The switch-on transients are trap limited with overall better switching characteristics for pm-Si:H samples. An initial rising transient in case of pm-Si:H is activated with an effective energy of 0.3 eV. The origins of transients are interpreted in terms of trap limited carrier dynamics and charge redistribution within the distribution of localized states.

Reduction of cluster amount in silane discharges is the key to decreasing microstructure parameter Rα of a-Si:H films deposited with the discharges. The cluster amount is found to be reduced more than one order of magnitude using 60 MHz discharges instead of 28 MHz ones or using H2 dilution of an H2/SiH4 ratio of 5. The cluster-suppressed plasma CVD using 60 MHz discharges realizes deposition of a-Si:H films of Rα~ 0 at a fairly high rate of 0.55 nm/s. Moreover, a downstream cluster collection method of high sensitivity has been developed for detecting a small amount of clusters formed under deposition conditions of Rα < 0.01.

We present hole drift-mobility measurements on hydrogenated amorphous silicon from several laboratories. These temperature-dependent measurements show significant variations of the hole mobility for the differing samples. Under standard conditions (displacement/field ratio of 2×10-9 cm2/V), hole mobilities reach values as large as 0.01 cm2/Vs at room-temperature; these values are improved about tenfold over drift-mobilities of materials made a decade or so ago. The improvement is due partly to narrowing of the exponential bandtail of the valence band, but there is presently little other insight into how deposition procedures affect the hole drift-mobility.

We compare the deposition rate, hydrogen incorporation and optoelectronic properties of hydrogenated polymorphous silicon films produced either by the decomposition of silane-hydrogen or of silane-helium mixtures. Our results clearly show that He dilution allows to drastically reduce the RF power needed to achieve the same deposition rate as in the case of H2 dilution. Infrared spectroscopy and hydrogen effusion experiments show clear differences in the hydrogen bonding and content in both series of films. Interestingly, both He and hydrogen dilution result in films with improved transport properties, in particular the hole diffusion length, with respect to standard amorphous silicon. These results indicate that He dilution is a good alternative to H2 dilution to prepare intrinsic layers for solar cells.

Nanocrystalline germanium thin-films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen were doped with phosphorus and boron. We report some electrical transport and structural properties of Ge films as a function of dopant species and doping levels. The dark conductivities of the phosphorus- and boron-doped films are approximately three to four orders of magnitude higher than the intrinsic nanocrystalline germanium. In the solid phase, phosphorus comprised about 1 atomic percent in the Ge bulk over the range of source gas ratios used, and the conductivity remained fairly constant, indicating saturated conditions. Boron comprised about 10 atomic percent in Ge at the highest dark conductivity, while increased doping turned the films amorphous. To test the doped layers for device applications, an all-nanocrystalline germanium p-i-n diode was constructed and showed rectification when measured in the dark at room temperature.

A hollow electrode enhanced RF glow plasma excitation technique has been newly developed. In this technique, the reactor is divided into a capacitively-coupled RF glow discharge space and a processing space by the counter electrode, which includes a hollow structure and is placed between a RF electrode and the substrate. After introducing hydrogen gas into the chamber and applying RF power to the electrode, high intensity plasma emission is observed near and inside the hollow structure attached to the counter electrode. By using hollow RF electrode excitation in addition to the hollow counter electrode technique, it is found that plasma emission is further enhanced. The application of these discharge types for semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition (PECVD) of hydrogenated microcrystalline silicon thin films. High crystallinity, photo-sensitivity and a maximum deposition rate of 4.9nm/s can all be achieved at a plasma excitation frequency of 13.56MHz and a temperature of 300°C. Properties of these plasmas are investigated by observing the plasma emission pattern and optical emission spectrum analysis. It is found that, using additional hollow RF electrode discharge, faster processing of device grade hydrogenated microcrystalline silicon thin films can be achieved under lower RF power compared to hollow counter electrode technique alone.

A set of 8 rf deposited a-Si:H thin films of various thickness (4-1031nm) have been used to explore the applicability of two optical techniques, thin film cavity ringdown spectroscopy (tfCRDS) and second harmonic generation (SHG), for the measurement of small defect-related absorptions. In this paper we will give a first overview of the different aspects of these techniques, which are novel in the field of amorphous silicon materials. It is shown that tf-CRDS is capable of measuring defect-related absorptions (associated with dangling bonds) as small as 10-7 for a single measurement, without the need for elaborate calibration procedures. The results are compared with photothermal deflection spectroscopy (PDS) for a broad spectral range (0.7 – 1.7 eV) and show good agreement. Furthermore the existence of a defect-rich surface layer with a defect density of 1.1×1012 cm-2 has been proven. The absorption spectrum of a 4 nm thin film has revealed a different spectral signature than a bulk dominated (1031 nm) film. The SHG experiments on a-Si:H films have shown that the second harmonic signal arises from the surface states and polarization dependent studies have revealed that the surface states probed have an ∞m-symmetry. From this it can be deduced that the absorbing surface states are isotropically distributed. A spectral scan suggests that the second harmonic signal, whose origin has not been unrevealed yet, has a resonance at an incident photon energy of 1.22 eV.

The effects of NH3 and H2 plasma passivation on the characteristics of poly-Si thin-film transistors with source/drain extensions induced by a bottom sub-gate were studied. Our results show that significant improvements in device performance can be obtained by both passivation methods. Moreover, NH3-plasma-treatment appears to be more effective in reducing the off-state leakage, subthreshold swing, compared to H2 plasma passivation. NH3 plasma treatment is also found to be more effective in reducing the anomalous subthrehold hump phenomenon observed in non-plasma-treated short-channel devices. Detailed analysis suggests that all these improvements can be explained by the more effective passivation of the traps distributed in both the front and back sides of the channel by NH3 plasma treatment.

The electronic properties of amorphous silicon films prepared by the expanding thermal plasma technique have been studied using steady-state and transient photoconductivity measurements. It is found that films deposited at a substrate temperature of 400°C have a conduction band tail slope of 29 meV, deep defect density of order 3×1016 cm-3, an Urbach tail slope of 65 meV, defect absorption of 5-10 cm-1, and a mobility-lifetime product of 1.3×10-7 cm2 V-1. Aslight increase in defect density and reduction in mobility-lifetime product is observed on moderate light-soaking. The overall optoelectronic quality is somewhat poorer than commercial PECVD material, but there is scope for improvement as deposition conditions are further optimised.

We report an ITO/a-SiNx:H/a-Si:H MIS photodetector with improved performance in terms of its dark current, stability, and spectral response in the blue region. The a-Si:H and a-SiNx:H thin film layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) on a glass substrate with patterned Mo back contact. The ITO was polycrystalline with a wide band gap (>3.75 eV) and was deposited at room temperature by magnetron sputtering. SIMS (Secondary Ion Mass Spectrometer) measurements show that an ultra thin a-SiNx:H film (a few nm) can effectively block the diffusion of oxygen from the ITO to the a-Si:H. In addition, the insulator layer provides a barrier for electrons, which serves to reduce the dark current. This is in contrast to the ITO/a-Si:H Schottky photodiode whose electrical and optical performance is impaired by the large defect density at the interface due to impurity diffusion from the ITO layer. At a reverse bias of 1 V, the dark current density of the MIS photodetector is as low as 4 nA/cm2. Photoresponse measurements show a dramatically enhanced sensitivity in the UV/blue spectral region. A high quantum efficiency (∼80%) is achieved at a wavelength of 440 nm, which can be attributed to reduction of both optical and recombination loses by virtue of the highly transparent polycrystalline ITO and the low defect density at the a-SiNx:H/a-Si:H interface.