We report on the growth and properties of microcrystalline Si:H and (Si,Ge):H solar cells on stainless steel substrates. The solar cells were grown using a remote, low pressure ECR plasma system. In order to crystallize (Si,Ge), much higher hydrogen dilution (∼40:1) had to be used compared to the case for mc-Si:H, where a dilution of 10:1 was adequate for crystallization. The solar cell structure was of the p+nn+ type, with light entering the p+ layer. It was found that it was advantageous to use a thin a-Si:H buffer layer at the back of the cells in order to reduce shunt density and improve the performance of the cells. A graded gap buffer layer was used at the p+n interface so as to improve the open-circuit voltage and fill factor. The open circuit voltage and fill factor decreased as the Ge content increased. Quantum efficiency measurements indicated that the device was indeed microcrystalline and followed the absorption characteristics of crystalline ( Si,Ge). As the Ge content increased, quantum efficiency in the infrared increased. X-ray measurements of films indicated grain sizes of ∼ 10nm. EDAX measurements were used to measure the Ge content in the films and devices. Capacitance measurements at low frequencies ( ~100 Hz and 1 kHz) indicated that the base layer was indeed behaving as a crystalline material, with classical C(V) curves. The defect density varied between 1x1016 to 2x1017/cm3, with higher defects indicated as the Ge concentration increased.

In this work we have investigated the low-energy photoluminescence (PL) band with a peak between 0.8 eV and 1.0 eV for microcrystalline silicon films (μc-Si:H) grown under various growth conditions. At least four subbands are observed, the peaks of which are located near 0.80 eV, 0.87 eV, 0.92 eV, and 0.97 eV, respectively. It is suggested that the low-energy PL band basically arises from a superposition of these subbands, whose intensities strongly depend on deposition conditions, and thus its peak is determined by the sum of these subband intensities. From the results, it is suggested that the subband centered at 0.92 eV originates from defect-related radiative recombination in the amorphous phase rather than radiative band tail-to-tail transitions in the grain boundaries.

This paper first briefly reviews a few of the early studies that established some of the salient features of light-induced degradation in a-Si,Ge:H. In particular, I discuss the fact that both Si and Ge metastable dangling bonds are involved. I then review some of the recent studies carried out by members of my laboratory concerning the details of degradation in the low Ge fraction alloys utilizing the modulated photocurrent method to monitor the individual changes in the Si and Ge deep defects. By relating the metastable creation and annealing behavior of these two types of defects, new insights into the fundamental properties of metastable defects have been obtained for amorphous silicon materials in general. I will conclude with a brief discussion of the microscopic mechanisms that may be responsible.

A Metal-to-Metal (M2M) antifuse is formed using hydrogenated amorphous silicon as a dielectric material between two refractory metal electrodes. The M2M antifuse is used as a programmable device in an FPGA, where it is placed between two interconnect metal layers of a Logic CMOS process. The M2M device may then be programmed to interconnect logic circuits. The resulting programmed link is an alloy of amorphous silicon and barrier metal that forms a low resistance path between the logic circuits.

A field-assisted germanium-induced crystallization of amorphous silicon on glass is reported at temperatures below 500°C. Silicon films with a thickness of 0.1um are covered with 500Å of germanium as the seed of crystallization. Applying an electric field enhances the growth from both cathode and anode sides. XRD, SEM and TEM analyses have been used to study the crystallinity of the samples which have been treated under different annealing conditions, all confirming the polycrystalline nature of the annealed silicon films. The value of the applied voltage plays a crucial role in the crystalline quality of Si layers. While samples treated without an external voltage are not polycrystalline, an electric voltage of 10 V applied for a 1cm separation between anode and cathode, seems suitable for achieving good poly-crystalline Si layers. The size of grains varies between 0.1 and 0.2μm, as observed using SEM.

An amorphous silicon photoconductor to detect wavelengths between 180 nm to 550 nm without scintillator is presented. The photoconductor is based on a coplanar configuration of the electrodes, similar to measurement structures to determine material characteristics of amorphous layers, e.g. for the Constant Photocurrent Method (CPM). After passing through a thin transparent passivation layer, the incident radiation is directly absorbed in the intrinsic a-Si:H material. The carrier collecting electrical field is applied perpendicular to the incoming light. Test structures have been fabricated with 80 nm thick sputtered chromium contacts on top of a 60 nm carbonized hydrogenated i-layer and a SixNx passivation layer with a thickness of about 36 nm. The spacing between the Schottky contacts is varied between 3 μm and 100 μm. They are deposited on top or below the a-SiC:H layer. First experiments with this simple coplanar design show that with an increasing voltage a shift towards UV wavelengths can be observed. The new UV detector is applicable in the field of TFA image sensors (Thin Film on ASIC) and in the new Lab-on-a-Chip concept presently under development at the institute for microsystem technologies.

We have observed the recovery of the performance of amorphous silicon (a-Si:H) based solar cell (especially the fill factor) at temperatures between 25°C and 170°C after ∼600 hours of light soaking under 1 sun illumination at ∼40°C. We find that there is some recovery of the fill factor of the cells even at temperatures below the temperature of light soaking. The recovery is significantly greater in cells of poor quality. There is also some evidence of enhancement of the recovery rate due to an external electric field. Above the light soaking temperature, a sort of thermally activated recovery of the fill factor is observed.

Microcrystalline silicon deposited at low temperatures (<150°C) is a candidate material for use as the channel layer in thin film transistors deposited on plastic substrates. This would enable driver electronics to be integrated onto cheap flexible AMLCD panel.

In this study microcrystalline silicon was deposited by Electron Cyclotron Resonance Plasma Enhanced chemical Vapour Deposition (ECR-PECVD) at a temperature of 80°C, compatible with most plastic such as PET and PEN. A source gas mixture of SiH4 and H2 was employed. The structural and optical properties of samples deposited under a range of deposition conditions were measured.

Properties of p-type μc-Si prepared by Cat-CVD (Catalytic Chemical Vapor Deposition), often called Hot-Wire CVD, are studied for possible application to window layer of a-Si solar cells. Electrical, structural and optical properties are investigated. It is concluded that Cat-CVD p-type μc-Si is a suitable material as a window layer for Cat-CVD a-Si solar cells.

High silane to hydrogen flow ratios and optimum wire temperatures are the key process parameters to achieve high growth rate poly-silicon films by hot wire chemical vapour deposition (HWCVD) using a four-wire hot-wire assembly. Four tungsten wires, 4 cm apart from each other, were used as catalytic filaments. Growth rates higher than 7 nm/s have been achieved at a substrate temperature of ∼510°C. The increase in deposition rate was accompanied by deterioration of two physical properties i.e., decrease in photoresponse and increase in oxygen incorporation in the film, which is attributed to high porosity in the material that is commonly observed in these high growth rate materials. The process conditions to incorporate a high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures, poly-Si films deposited at 1.3 nm/s showed a high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as the i-layer in an n-i-p solar cell on a stainless steel substrate, without back reflector, showed an efficiency of 4.4 % and a high open circuit voltage of 0.58 V, which is attributed to effective passivation of defects and dopants by incorporated hydrogen.

We report on two experimental studies carried out to reveal insight into the interaction of SiH3 radicals with the a-Si:H surface as assumed essential in the a-Si:H growth mechanism. The surface reaction probability β of SiH3 on the a-Si:H has been investigated by spectroscopic means as a function of the substrate temperature (50 - 450°C) using the time-resolved cavity ringdown technique. The silicon hydrides –SiHx on the a-Si:H surface during deposition have been studied by the combination of in situ attenuated total reflection infrared spectroscopy and argon ion-induced desorption of surface hydrogen. For SiH3 dominated plasma conditions, it is found that the surface reactivity of SiH3 is independent of the substrate temperature with β = 0.30±0.03 whereas the silicon hydride composition on the a-Si:H surface changes drastically for increasing substrate temperature (from –SiH3 to =SiH2 to ≡SiH). The implications of these observations for the a-Si:H growth mechanism are addressed.

Correlation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (Eg) results in a higher Voc but a lower Jsc. However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in Voc. For solar cells with good interface characteristics, a linear relation FF = βVoc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V-1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H2 dilution to no H2 dilution, we find that a 58 mV enhancement in Voc with H2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher Voc. We did not include this effect in the analysis due to lack of available data for correlation between H2 dilution and band tail narrowing.

We investigate low-temperature epitaxial growth of thin silicon films on Si [100] substrates and polycrystalline template layers formed by selective nucleation and solid phase epitaxy (SNSPE). We have grown 300 nm thick epitaxial layers at 300°C on silicon [100] substrates using a high H2:SiH4 ratio of 70:1. Transmission electron microscopy confirms that the films are epitaxial with a periodic array of stacking faults and are highly twinned after approximately 240 nm of growth. Evidence is also presented for epitaxial growth on polycrystalline SNSPE templates under the same growth conditions.

This paper presents a fast and accurate method for extraction of the above-threshold physical parameters (such as threshold voltage, power parameter, effective mobility, and contact resistance) from measurement data in the linear and saturation regions of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) fabricated on glass and plastic substrates. The method of extraction is different from techniques that are currently used by virtue of the departure from the square law dependence of the current-voltage characteristics. In addition, a broader range of process-induced variation in material properties is expected, which is accentuated by the effects of different substrates, leading to wide-ranging device parameters. In particular, non-ideal parameters such as contact resistance may vary by orders of magnitude due to process variations, thus strongly influencing the extracted values of TFT parameters if its effect is not considered in the extraction method. In this paper, the effect of contact resistance and other non-ideal parameters is systematically identified and eliminated using TFTs with different channel length. The extracted values for TFTs on glass and plastic substrates clearly highlights the differences in material properties stemming from the different process conditions and substrate properties, and provide insight that is invaluable for subsequent device/process optimization.

High-performance microcrystalline and amorphous silicon solar cells are the key elements for a successful combination to form the “micromorph” tandem cell [1,2]. A microcrystalline silicon (μc-Si:H) solar cell in the n-i-p configuration was fabricated by the VHF PE-CVD deposition process. The cell has a conversion efficiency exceeding 9% (VOC=520 mV, FF=73%, JSC=24.2 mA/cm2). This result was achieved by a successful combination of the following elements: first a fine-tuning of the silane concentration (SC) in hydrogen feedstock gas used for deposition of the intrinsic <i> absorber layer, second, the incorporation of an optimised back-reflecting substrate into the cell; and, third, the ideal combination of each of these key-components.

Compared to earlier results with n-i-p-type μc-Si:H solar cells, a substantial increase in VOC was now obtained, while maintaining reasonable JSC-values. Earlier investigations on the role of the i-layer material had revealed a trade-off between cells with high JSC but low VOC or cells of low JSC and high VOC. In the present contribution the authors now show the successful combination of a cell with an acceptable VOC and good JSC generation in the long-wavelength region (above 700 nm). This is mainly because of suitable light-diffusing back-reflectors which perform well with respect to both, optical and electrical aspects.

Excess leakage currents under reverse bias (known as shunting) and spontaneous reductions of this excess leakage under increased reverse bias (known as curing) were investigated in hydrogenated amorphous silicon (a-Si:H) based single junction p-i-n type diodes. An increase in the frequency of shunting was observed when the front contacts were switched from tin oxide to zinc oxide, most likely due to defects in the previously deposited zinc oxide coated glass was observed. Storage in the dark and light soaking up to 100 hours were both observed to independently increase the leakage current in previously leaking diodes. Models for the distribution of shunt-causing defects within a given cell area were considered. Comparing the measured frequency of shunting using cells of varying area (1 to 16 mm2) to the models' predictions indicate a distribution of point defects separated by relatively large average distances that are slightly larger for tin oxide (5-6 mm) than for zinc oxide (4 mm).

Effect of SiO2 capping layer(C/L) on recrystallization of amorphous Si (a-Si) film was investigated. When a thick C/L over 500 Å was deposited on an a-Si film before crystallization, fine p-Si grains less than 100nm were obtained at full range of energy density window. However, when a thin C/L below 200 Å was used, Si-melt spouted out through C/L at over critical energy density. When Si-melt started spouting, abrupt change of grain size also occurred. These large grains could be explained by a non-uniformity of heat flow caused by Si-melt spouting. With this polycrystalline Si (p-Si) material having appreciable grain size protected by C/L, fabrication of low-cost Low Temperature Poly Si (LTPS) without additional cleaning of p-Si surface could be successfully developed.

A study has been carried out on the forward bias dark current and the short circuit current -open circuit voltage characteristics of a-Si:H p-i-n solar cells over wide range of illumination intensities. Results are presented with superposition of these characteristics over extended current voltage regimes. This and the observed separation between these characteristics are consistent with the arguments presented based on first principle arguments. The conclusions drawn about the role of photo-generated carrier lifetimes, the densities of defects and the potential barriers in the i-layers adjacent to the n and p contacts are confirmed by numerical simulations. The key role of these potential barriers to the split in the characteristics offer new insight into both why the lack of superposition has been observed and the erroneous conclusions drawn about carrier transport for a-Si:H solar cells in the dark and under illumination.

In recent work, the electrical properties of metal-induced-grown (MIG) Si thin films were studied by using current-voltage (I-V) data from a metal/Si Schottky contact. It was found that controlling the doping level of the films and annealing in forming gas (15% H2 and 85% N2) can improve the quality of the nc-Si films. From SIMS analysis on the nc-Si film deposited from a highly doped target, the nc-Si can duplicate the doping level of the sputtering target. Study of p-type doped nc-Si films showed that the fabrication of Schottky diodes on nc-Si films made from an extremely high-doped target (∼1020 cm-3) or low-doped target (∼1015cm-3) was not successful. For highly doped p-type films, tunneling causes Ohmic conduction instead of rectifying conduction. For the nc-Si film deposited from a low-doped p-type target, the film shows conversion to n-type characteristics when measured by a hot probe. This might be due to defects or oxygen in the film. N-type films at the middle doping level (∼1017cm-3) gave good Schottky diodes after annealing the film in forming gas at 700°C. The Schottky diodes fabricated by high work-function metal (Au) gave the rectifying ratio of ~∼103. Several techniques, e.g. slow/fast two-step sputtering at low working pressure and surface polishing, were used to improve the photo response of Schottky photodiodes. The open-circuit voltage (Voc) of 0.164V and short-circuit current density (Jsc) of 2.5 mA/cm2 were achieved under 100mW/cm2 illumination.

Amorphous (a-Si:H) and microcrystalline (μc-Si:H) hydrogenated silicon films were obtained using DC saddle field glow discharge. The structure of the films was determined by Raman spectroscopy, SEM and TEM. The optoelectronic characteristics of both a-Si:H and μcSi:H were investigated using FTIR, UV/VIS spectroscopy, dark electrical conductivity (σd) and photoconductivity (σph) measurements. Boron and phosphorous doping of a-Si:H and μc-Si:H films was also investigated. The results show that both a-Si:H and μc-Si:H undoped films are highly resistive (σd=10-8-10-10 Ω-1cm-1). The doping efficiency of μc-Si:H films is much higher than a-Si:H films. The Tauc gap for a-Si:H was in the range 1.8-1.9 eV and for μc-Si:H films it was in the range 1.9-2.5 eV. The photoconductivity measurements of undoped films indicate a higher photosensitivity of a-Si:H films (σph/σd=104)than that of μc-Si:H films (σph/σd=10-100).