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Single grain TFTs are fabricated at a maximum temperature of 100oC for macroelectronics on a plastic substrate, as Si channels are fabricated at 100oC by combination of excimer laser crystallization and sputtering. The gate oxide is formed at 80°C by inductively coupled plasma enhanced chemical vapor deposition. These TFTs have shown a smaller threshold swing of 0.49 V/dec. and a higher field-effect mobility of 290 cm2/V·s, which can be used to directly fabricate system circuits or a high quality display on a plastic substrate.
Location-controlled grains with a diameter of 4 μm are successfully prepared by excimer laser crystallization of sputtered α-Si with μ-Czochralski (grain filter) process at a maximum processing temperature of 100°C. By a pulsed DC magnetron sputtering, α-Si film is deposited firstly on non-structured oxidized wafers for a test. It is found that a-Si film is easily ablated even with a low laser fluence when it is deposited with a substrate bias. From a non-biased sputtered α-Si precursor, grains with 1.8 mm in diameter can be prepared with excimer laser crystallization at room temperature. α-Si is then sputtered on the SiO2 with narrow holes (grain filters) and crystallized at room temperature. The location-controlled grains can be successfully prepared in a large energy density window. These location-controlled grains with a low temperature process are promising for single-grain thin film transistors (TFTs) on plastic substrate for an application to system integration on flexible microelectronics.
Defect states were quantitatively evaluated in single-grain (SG) Si thin-film transistors (TFTs), prepared by micro-Czochralski (grain filter) process with excimer-laser crystallization, by means of isothermal charge deep-level transient spectroscopy with a high sensitive charge/voltage converter. Its sensitivity reaches 10-16 C and it operates in the range of 2 microseconds - 10 ms. Measurements were performed on the SG-Si TFTs with various energy densities of laser crystallization, various channel areas, and positions in the grain. Our results indicate a direct correlation of fabrication parameters, parameters of the TFT determined from its transfer characteristics, and parameters of defect states (energy position in the band gap, concentration) induced by coincidence site lattice boundaries inside the location-controlled grains and by defects in the grain filter.
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.
The effects of 3-MeV electron irradiation on a-Si:H have been studied using Time-Resolved Microwave Conductivity (TRMC). A Van der Graaff electron accelerator is used to generate the probe-beam pulses for the TRMC experiment as well as for the in-situ irradiation of the samples for the degradation of the material. Using several probe-beam pulse doses, TRMC transients were obtained on samples that have been subjected to various radiation fluences. These transients were later analyzed using a simple model based on the Shockley-Read-Hall capture and emission processes. Using these simulations we deduce a relationship between the radiation fluence and the defect density in the material.
Computer simulations of single-junction hydrogenated amorphous silicon (a-Si:H) solar cells with different thickness of the intrinsic layer were carried out in order to study the role of charge gap states in their light-induced degradation. It is demonstrated that it is the decrease of positively charged states above midgap, Dh, and the increase of neutral states around midgap,Dz, and negatively charged states below midgap, De in the intrinsic layer that result in a drop of performance of the solar cells due to light soaking. These changes in the gap states are in accordance with our recent experimental results from the charge deep-level transient spectroscopy on undoped a-Si:H. The experimentally observed changes in the dark and illuminated J-V curves and spectral response could not be simulated with the same set of input parameters by only increasing the defect-state density in the intrinsic layer.
Temperature dependant I-V characteristics were measured on single-crystalline Si (c-Si) TFTs fabricated inside a location-controlled grain by [.proportional]-Czochralski process using an excimer-laser. At ON-state, temperature the activation energy (Ea) of the drain current drops to a negative value. The field effect mobility ([.proportional]FE) also decreases with temperature with a power of -1.86, which indicates that, the carriers transport are governed by acoustic phonon scattering. At OFF state with a small gate bias, leakage current is dominated by thermal generation, however the Ea was 0.9eV, i.e., near the band gap value of Si. This suggests that the carrier generation centers are not located at the mid-gap states. These distinctive results from a typical poly-Si TFTs are systematically investigated for c-Si TFTs having ECR- PECVD and LPCVD SiO2 gate insulator.
The charge deep-level transient spectroscopy (Q-DLTS) experiments on undoped hydrogenated amorphous silicon (a-Si:H) demonstrate that during light soaking the states in the upper part of the gap disappear, while additional states around and below midgap are created. Since no direct correlation is observed in light-induced changes of the three groups of states that we identify from the Q-DLTS signal, we believe that we deal with three different types of defects. Positively charged states above midgap are related to a complex formed by a hydrogen molecule and a dangling bond. Negatively charged states below midgap are attributed to floating bonds. Various trends in the evolution of dark conductivity due to light soaking indicate that the kinetics of light-induced changes of the three gap-state components depend on their initial energy distributions and on the spectrum and intensity of light during exposure.
This paper compares a-Si:H p-i-n diodes having a spatially uniform distribution of defect states with diodes in which the defect distribution is non-uniform, i.e. equilibrated according to the Defect-Pool model. Diodes with a uniform defect distribution exhibit a clear dependence of the current-voltage characteristics on the width of the intrinsic region, whereas in equilibrated diodes, this dependence is absent. This difference is explained by comparing the space-charge distribution and the recombination profile of the intrinsic region in both types of diodes.
The deposition processes and the properties of a-SiC:H and a-SiGe:H films in 55 kHz glow discharge were investigated. The analysis of deposition rate and RBS measurements showed that the chemical reactions between SiHn spices and CH4 control the incorporation of C in a-SiC:H films. High deposition rates of a-SiC:H and a-SiGe:H films fabricated by 55 kHz PECVD is caused by the increase of radical fluxes to the growth surface. The specific features of a-SiC:H and a-SiGe:H microstructure were revealed by IR and AFM analysis. In a-SiC:H films the islands of low size were distinguished on the surfaces of large islands. The large variation of the total hydrogen content in a-SiGe:H did not affect the optical bandgap, while the hydrogen related microstructure controlled the electronic properties such as dark conductivity, 11p.r product, defect density and Urbach slope.
The results of optoelectronic properties and SW effect measurements of 55 kHz a-SiC:H and a-SiGe:H films demonstrated the increased stability in comparison with a-Si:H.
The effect of interface roughness on the optical properties of amorphous silicon (a-Si:H) solar cells was investigated using rms roughness measurements and computer modeling. We deposited four single junction a-Si:H solar cells on Asahi U type substrate each with a different intrinsic layer thickness. The roughness of the substrate and of the subsequent interfaces of the cells was measured by Atomic Force Microscopy. The relations between the computer input parameters, which describe the diffuse part of reflected and transmitted light at a rough interface, and the rms roughness of the interfaces in a-Si:H solar cell are presented. After obtaining a good matching between the simulated and measured external quantum efficiencies (QE) of the four cells, we investigated the effect of interface roughness on the absorption in all individual layers of the a-Si:H solar cell using the determined scattering parameters.
The rms roughness of the Asahi U-type substrate surface is determined to be 40 nm. Deposition of a 9 nm thick p-type a-SiC:H layer on the substrate has not changed the surface roughness. Due to a high refractive index of n-type a-Si:H the back contact interface acts as a nearly perfect diffuser for the reflected light when the rms roughness of the interface is higher than 25 nm. The rms roughness of the back interface of the four cells is found to be dependent on the intrinsic layer thickness and is larger than 25 nm.
A calibration procedure for determining the model input parameters of standard a-Si:H layers, which comprise a single junction a-Si:H solar cell, is presented. The calibration procedure consists of: i) deposition of the separate layers, ii) measurement of the material properties, iii) fitting the model parameters to match the measured properties, iv) simulation of test devices and comparison with experimental results. The inverse modeling procedure was used to extract values of the most influential model parameters by fitting the simulated material properties to the measured ones. In case of doped layers the extracted values of the characteristic energies of exponentially decaying tail states are much higher than the values reported in literature. Using the extracted values of model parameters a good agreement between the measured and calculated characteristics of a reference solar cell was reached. The presented procedure could not solve directly an important issue concerning a value of the mobility gap in a-Si:H alloys.
While a-Si:H tandem solar cells deposited on textured TCO substrates have already shown high efficiency and good stability , the light absorption profile in such cells is still one of the important quantities to know. An accurate absorption profile is required when we do solar cell analysis, design, and optimization, or when we do computer simulations.
With the computer program GENPRO2D  which was developed in our laboratory, and was based on the incoherent Multi-rough-interfaces (scattering) Model, we are able to calculate the light absorption profile in a tandem a-Si:H solar cell for different light spectra and different light incident angles, provided that the scattering of light at every interface in the cell is known. In this paper, we will present our calculated results with comparison to the measured results of a real tandem cell.
The optimization of the back contact reflectivity for thin film a-Si:H solar cells has been performed. The results of optical calculations show that a-Si:H/TCO/Metal interfaces with a proper TCO thickness reflect much more than their a-Si:H/Metal counterparts. We compared solar cells which were deposited on a flat substrate with different back contacts. The back contacts consisted of a metal layer (aluminum, silver/aluminum) or combined TCO/metal layers (TCO/Al, TCO/Ag/Al). The same was done with solar cells which were deposited on a textured substrate. The solar cells with a TCO/metal back contact showed not only a significantly increased short-circuit current density but also an increase in the spectral response. The cells with TCO/Ag/Al back contact showed the best result.
We present the results of a series of numerical study of a-Si:H solar cells on different V- grooved substrates. The light absorption profiles were calculated by taking into account the V-groove inner reflection and the reflection at the glass/air interface. These profiles were used for modeling the performance of solar cells with the 1-D Amorphous Semiconductor Analysis (ASA) computer package, which has been developed in our laboratory. The J-V characteristics under AMI.5 and the external quantum efficiency axe simulated for V- grooves with different tilt-angles (0 degree for flat substrate). Results show that effective light trapping occurs for V-grooves with tilt-angle larger than 21 degrees.
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