The evolution of the programmed defect-state distributions in intrinsic hydrogenated amorphous silicon (a-Si:H) due to light soaking was qualitatively determined from charge deep-level transient spectroscopy. The defect-state distribution in a-Si:H was programmed by applying a particular bias voltage on the metal-oxide-semiconductor structure while annealing the structure above the equilibration temperature. The programmed distributions simulate defect-state distributions in different parts of an actual a-Si:H solar cell, particularly in the intrinsic regions close to the p/i and i/n interfaces.

The defect-state distribution in the bulk of the intrinsic layer is characterized by comparable contributions from the positively charged defect states above midgap, Dh, neutral states, Dz, and negatively charged states below midgap, De. In the programmedp-type (n-type) defect-state distribution there is an excess of the Dh (De) states. Light exposure modifies the p-type distribution that evolves to a broad distribution of states with a maximum around midgap. This distribution is dominated by Dz states with substantial contributions from Dh and De states. In case of n-type distribution light soaking only slightly influences the distribution by removing a part of the Dh states and by a small increase of Dz and De states.

A two terminal image and color sensitive detector based on two stacked sensing/switching p-i-n a-SiC:H diodes is presented. The imaging is performed in a write-read simultaneous process: the write exposure, which converts the optical image into a localized packet of charges and the readout which performs the charge to current conversion by detecting the photocurrent generated by a light beam scanner. By sampling the absorption region at appropriated voltages it is possible to extract separately the RGB integrated information with a good rejection ratio. Readout of 1000 lines per second is achieved allowing continuous and fast color recognition and image detection. A trade-off between the sensing and the switching diodes thicknesses is established in order to enhance the green recognition. When it is used a thin a-SiC:H, optimized for red transmittance and blue collection, and a switching absorber thick enough to absorb most of the incoming green light,the detector behaves itself as a filter giving information about the wavelength and the position of the optical image.

For a long time the application of silicon technology for optoelectronics has been limited by the extremely poor generation of light by bulk silicon. However, the properties of Si were found to depend on its structure at a nanometric scale and bright photoluminescence from silicon nanocrystals (Si-nc) was discovered. In this work, the results obtained for the mechanical connecting of single Si-nc by two independent techniques are presented. First, the room temperature approach of connecting and introducing Si-nc embedded in colloidal suspensions by capillary forces within multi-walled carbon nanotubes (MWCNTs) and second, the direct growth of MWCNTs on single Si-nc coated with iron catalyst are described.

Laser-crystallized polycrystalline silicon-germanium (poly-SiGe) thin films on glass substrates were characterized with energy dispersive X-ray and Raman spectroscopy. In the course of the crystallization strong lateral segregation occurs for laser-crystallized poly-Si1-xGex with 0.33 < x < 0.7, causing the local Ge content to differ by as much as 40 % from the average value. The segregation manifests itself in the appearance of well-resolved peaks in the Raman phonon modes. This mode splitting in the Raman spectra is interpreted as the formation of well defined alloy phases with a miscibility gap in between.

Polycrystalline SiGe layers have been oxidized in either dry or wet atmospheres. The evolution of the growing oxides and the SiGe layer during the oxidation processes have been characterized using Raman spectroscopy, X-ray diffraction, Rutherford backscattering and Fourier transform infrared spectroscopy. Formation of Ge nanocrystals has been observed for both oxidation atmospheres. Violet luminescence emission has been observed and its relation to the oxidation processes has been studied. The luminescence is unambiguously related to the presence of these Ge nanocrystals. Since it does not exhibit quantum confinement, it must be related to defects linked to the nanocrystals.

In this work we study the optical and electrical behavior of ZnO:Ga, ITO and IZO films deposited on glass after sustaining different hydrogen plasma conditions and exposure times. This work was complemented by analyzing the surface morphology of the set of films, which allow us to determine the role of hydrogen plasma on the film's properties such as Hall mobility, free carrier concentration, sheet resistance, optical transmittance, figure of merit and state of the surface. Apart from that, the performances of solar cells using an intrinsic layer constituted by nanocrystalline silicon will be also presented. The data show that the electrical properties of solar cells were improved by using ZnO:Ga as front contact, allowing a high current density collection and single pin solar cells with efficiencies exceeding 11%.

With the continued growth of photonics, silicon oxynitride (SiOxNy) is becoming a popular material for optoelectronic applications owing to its large tunable refractive index. However, with the increase in refractive index, these films tend to show poor optical transmission characteristics. In this research we have investigated the influence of growth conditions on the loss characteristics of PECVD SiOxNy films. The films are grown at 350 °C substrate temperature and 1 Torr pressure with silane (SiH4) and nitrous oxide (N2O) precursor gases. The precursor flow rate and power input to the system are varied as the two primary parameters. It is observed that films grown at 100 kHz plasma frequency proved to be more transmissive than films grown at 13.56 MHz plasma frequency. Elastic recoil detection analysis results showed the hydrogen content is less in the low frequency films than the high frequency films, which is believed to be the reason for the low loss behavior. The details of these analysis results are discussed below.

In order to investigate the possibility of nanosecond activation in the non-melting state, we adopted the method of double-pulsed green laser annealing (DPSS), controlling effectively the combined pulse width with two pulsed lasers (pulse duration: ˜100ns, frequency: 1kHz). We investigated the formation of ultra-shallow junctions (USJ) less than 10nm in spite of the deep penetration depth of the green wavelength in crystalline Si (˜1000nm). In order to limit the depth of B implant, a Ge pre-amorphization implant was performed at an energy of 3keV to a dose of 3E+14/cm2. After the pre-amorphization implant, a B implant was performed at 0.2kev and doses of 5E+14/cm2 and 1E+15/cm2. The implanted B dopants remain within the pre-amorphized Si layer. The double-pulsed laser irradiation was performed with a homogenized line beam of 0.1mm x 17mm, scanning a sample stage at a constant velocity of 10mm/s, that is, at an overlap ratio of 90%. The non-melting state was found to be in the pulse energy density range of E ≤ 780mJ/cm2 at a delay time of 300ns. Overcoming the issues of the short annealing time (˜<1μs) and the deep penetration depth (˜1000nm), we succeeded in the ultra-shallow junction formation beyond the 45nm CMOS node: maximum junction depth of 6nm, minimum sheet resistance of 0.65kohm/sq at a B dose of 1E+15/cm2, an abruptness of 1.4nm/dec.

This paper describes two novel optical diagnostics that were recently introduced to the field of Si-based thin films, in particular for probing defect states present in the bulk and at the surface of a-Si:H films. It is expected that these diagnostics, when applied in situ or real time during film growth, can provide new insights into the a-Si:H film properties as well as into the fundamental surface processes during growth. The first method is cavity ringdown spectroscopy (CRDS). From ex situ measurements on a-Si:H thin films, it is shown that this method is very powerful for measuring absolute defect-related absorptions at subgap energies without the need for a calibration procedure, even for films as thin as 4 nm. It is also shown that the method can be used for measuring rare-earth dopants – here Er3+ in silicon-rich oxide – to the extent that issues about absorption cross-sections can be resolved by using thin samples instead of waveguides. Furthermore, the in situ application of the method for thin films is discussed by presenting the evanescent-wave cavity ringdown (EW-CRDS) technique. The second method is spectroscopic second harmonic generation (SHG). It has been found that this non-linear optical technique yields a photon energy dependent signal for as-deposited a-Si:H films and that this signal has a contribution from a-Si:H surface states. From a comparison with c-Si surface science studies, the possible origin of the signal from surface Si dangling bonds and strained Si-Si bonds is discussed. The application of SHG during real-time film growth is also presented.

Silicon nitride (SiNx:H) layers were deposited using a high deposition rate Hot-Wire CVD technique (up to 5 nm/s) and their application as passivating antireflection coating on multicrystalline silicon solar cells was investigated. An important aspect is the hydrogen release and diffusion during a short annealing treatment at temperatures well above the deposition temperature. Such annealing treatments (firing) are used during contact formation of multicrystalline solar cells.

A series of SiNx:H layers with Si/N ratio in the range of 0.7-1.45 was first characterized using optical R/T measurements, Fourier Transformed Infrared Spectroscopy (FTIR) and Elastic Recoil Detection (ERD) to determine optical constants n and k, Si-H, N-H and Si-N bond densities, Si-H peak position and H content. To get more insight in the passivation process during firing, which takes place at roughly 800 °C, samples with a N/Si ratio of 1.2 were annealed at this temperature for different times (15 s – 10 min). The total bonded H content, measured with FTIR, using conventional proportionality constants as proposed by Lanford and Rand [1], seems to show an increase within the first 60 s. At longer annealing times, the bonded H content decreases, as expected. The behavior of the H content under annealing measured by FTIR and ERD can be explained by using different matrix elements for calculating the H content from the FTIR data. The matrix element for the N-H stretching mode (3343 cm-1) was determined to be (4.7±0.2)x1020 cm-2 while the Si-H stretching (2193 cm-1) was found to be (0.55±0.04)x1020 cm-2. Such high matrix elements for the N-H stretching mode have not been reported earlier. The value of the Si-H stretching mode matrix element is within the reported values for different back bonding scenarios, but low for a peak position of 2193 cm-1. The ratio between the matrix elements of 8.5 differs strongly from the ratio of 2 observed by Lanford [1] or Bohne [2].

Results are presented on the defect state distributions in intrinsic a-Si:H layers with and without hydrogen dilution in p-i-n solar cells obtained directly from the analysis of dark forwardbias current-voltage (JD-V) characteristics. It is shown that there are distinct differences in the distributions of both native and light induced defect states between the two types of i-layers. Computer simulations using these distributions are presented which show excellent agreement with the experimental results not only for the JD-V but more importantly for the bias dependent differential diode quality factor n(V) characteristics. Results are also presented on the nature of the gap states and their evolution with light induced degradation as well as their effects on the performance and stability of high quality a-Si:H solar cells.

The growth of hydrogenated silicon (Si:H) and silicon-germanium alloys (Si1-xGex:H) by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) substrates has been studied by real time spectroscopic ellipsometry (RTSE). The motivation is to develop deposition phase diagrams that can provide greater insight into the optimization of amorphous Si1-xGex:H (a-Si1-xGex:H) for multijunction photovoltaics. In initial studies, the phase diagram for bottom cell a-Si1-xGex:H (Eg ˜ 1.4 eV) is found to exhibit fundamental similarities to that for Si:H when both materials are prepared under standard PECVD conditions that optimize pure a-Si:H. These similarities suggest directions for optimizing a-Si1-xGex:H by identifying conditions under which a smooth, stable surface is obtained to the largest possible bulk layer thickness. In phase diagram development for PECVD Si1-xGex:H on c-Si, it has been found that the highest surface stability and smoothest surfaces are obtained using cathodic deposition (self bias: ˜-20 V) with a H2-dilution level just below that of the amorphous-to-(mixed-phase microcrystalline) [→(a+μc)] transition for a thick layer. Due to the promising nature of these results, full phase diagrams are compared for cathodic and anodic Si1-xGex:H as well as for cathodic and anodic Si:H, all on c-Si substrates. The cathodic phase diagram for Si1-xGex:H reveals a narrow range of significant improvement in surface structural evolution near the →(a+μc) transition, and for a-Si:H reveals an extension of the ultrasmooth protocrystalline regime to a much wider range of thickness.

Of late, high index Si surfaces like, (5 5 12) are being explored for the formation of 1D nanostructures in the form of nanowires or chains. The surface topography of Si (5 5 12) presents an unique template for the growth of 1D nanostructures. In this paper we report the adsorption-desorption studies of Sb onto (2×1) reconstructed surface of Si (5 5 12). Motivated by our earlier studies of adsorption/desorption of Sb on low index surfaces (001) and (111), studying the interaction on high index surface like (5 5 12) is found to be interesting. The experiments have been performed in UHV with in-situ growth and probed by using AES, LEED and EELS techniques. The uptake curve shows initially a simultaneous multilayer growth of Sb up to 4 ML and then at 20 ML the entire Si surface is covered by Sb. The LEED patterns show a disorder during growth at RT above 1 ML. EELS studies at 250 eV, primary beam energy show that as the adsorption proceeds the features of bulk Sb start appearing above 4ML coverage. However, the bulk Sb features are not complete until a high enough coverage of Sb (˜20ML) which suggests the existence of the primary rows up to a higher coverage. Annealing studies have also been performed on the RT adsorbed system, to study the residual thermal desorption characteristics. Annealing the system to about 7900C results in Si (337) facets at a low coverage of 0.2 ML. The anisotropy in the LEED spots suggests an ordered Sb adsorption along (1 10) and a local disorder in the (665 ) direction. Thus we provide evidence for the formation of a zig-zag 1D-nanowire of Sb grown on the (5 5 12). The new phase is also accompanied by forming a dangling bond state at 3 eV in EELS.

Scanning probe microscopy (SPM) with a conducting tip has been used to electrically probe silicon nanocrystals (NCs) on an insulating substrate. NC samples were produced by aerosol techniques followed by a sharpening oxidation. The size of NCs is in the range of 10-50nm and deposited on a silicon substrate with a density of around 1011/cm2. Using a conducting tip, the charge was injected directly into the NCs, and the bias dependent images due to the trapped charges in the NCs were monitored. Charging effects affected by the size of NCs and injection direction were also estimated from the apparent height differences of the NCs with respect to the applied bias.

In 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.

Major development challenges in application of hydrogenated amorphous silicon (a-Si:H) technology to large area digital X-ray imaging and technological attributes such as low temperature deposition and high uniformity over large area evolve around dynamic imaging modalities such as fluoroscopy, which demands both high speed readout and signal amplification capabilities in addition to long term device stability. This work reports on initial results of a variety of TFT active pixel sensor (APS) structures in a-Si:H technology, each demonstrating unique capabilities such as enhancements in signal gain, TFT threshold voltage immunity, and real-time high speed readout.

1H NMR studies of hydrogenated amorphous silicon (a-Si:H) with ˜1017 cm-3 defects grown by PECVD with a rate of 5 Å/s show the existence of a hydrogen doublet for both asgrown and light-soaked samples. We observe the doublet over the temperature range from 5 to 20 K in a sample where no light soaking has occurred. The doublet line shapes display no narrowing over this temperature range. Vibrational modes characteristic of SiH2 wagging and scissor modes are seen from infrared spectroscopy. These results suggest that the doublet is due to SiH2 that occurs at a density of approximately 1 at. % in this sample. From line shape analysis, we estimate a lower limit of 1.8 ˜ for the hydrogen-to-hydrogen separation.

We apply the publicly available device modeling tool SC-Simul for simulating experiments with user-defined heterojunction diodes to discuss the role of the electric field in solar cells. For amorphous silicon/crystalline silicon heterodiodes, the role of interface defects, an amorphous silicon buffer layer and low-cost crystalline silicon is studied by simulation of current-voltage characteristics and photoluminescence. Photoluminescence is sensitive to the minority carrier density in the volume of the device and can be used to monitor minority carrier properties in these diodes.

Microcrystalline silicon with properties relevant to highly efficient solar cells can be suc-cessfully prepared on glass for material characterization if a thin intrinsic ‘seed layer’ coating of the substrate is used. This is demonstrated by a detailed structure analysis on the base of Raman spectroscopy and photothermal deflection spectroscopy. The coating turns out to be crucial (1) for achieving a crystalline content as high as that of solar cell absorber material, (2) for creating a homogeneous structure in growth direction, and (3) for extending the range of deposition pa-rameters which lead to films with high crystallinity towards the regime of amorphous growth. Regarding electrical transport, ‘seed layer’ assisted growth results in a structure dependence of the dark conductivity which is very similar to that of material grown on bare glass. Regarding optical absorption spectra, residual interference fringes, which indicate structure non-uniformities, are clearly suppressed by means of ‘seed layers’. It is concluded that appropriate ‘seed layers’ play an important role for a comprehensive characterization and development of microcrystalline silicon layers for thin film devices.

Localized synthesis of 3-8 nm Si nanocrystals (nc-Si) in PECVD-grown Si-rich SiO2 (SRSO) film is demonstrated using CO2 laser annealing at an intensity below the ablation-threshold (6.0 kW/cm2). At an optimized surface temperature of 1285°C, the precipitated nc-Si in CO2-laser-annealed SRSO film results in near-infrared photoluminescence (PL) at 806 nm, whereas the ablation damage induced at higher laser intensities as well as temperatures results in blue PL at 410 nm related to structural defects. The refractive index of the laser-annealed SRSO at 633 nm increases from 1.57 to 2.31 as the laser intensity increases from 1.5 to 6.0 kW/cm2. Transmission electron microscopy analysis reveals that the average size and volume density of Si nanocrystals embedded in the SRSO film are about 6 nm and 4.5×1016 cm-3, respectively. The CO2 laser annealing with controlled intensity and spot size can potentially accomplish in-situ, localized annealing of the SRSO film without causing irreversible damage to nearby electronics.