Hydrogenated microcrystalline silicon (μc-Si:H) films are prepared by hot-wire assisted plasma enhanced chemical vapor deposition, which controls the hydrogen radical density by filament temperatures, Tf, without changing other conditions. The effect of hydrogen radical on the properties of incorporated hydrogen into μc-Si:H films is studied using infrared absorption and gas effusion spectroscopies. The hydrogen concentration decreases with increasing Tf. The crystalline volume fraction, Xc, increases with Tf and shows a peak at Tf of 1850 °C. Integrated intensities of the modes near 2000 and 2100 cm-1 decrease with increasing Tf. Integrated intensity of the mode near 880 cm-1 shows almost same tendency of Xc. The effect of hydrogen radical on the properties of incorporated hydrogen into μc-Si:H films is discussed.

We have performed computer calculations to explore effects of the p/i interface on the open-circuit voltage in a-Si:H based pin solar cells. The principal conclusions are that interface limitation can occur for values of VOC significantly below the built-in potential VBI of a cell, and that the effects can be understood in terms of thermionic emission of electrons from the intrinsic layer into the p-layer. We compare measurements of VOC and electroabsorption estimates of VBI with the model calculations. We conclude that p/i interface limitation is important for current a-Si:H based cells, and that the conduction band offset between the p and i layers is as important as the built-in potential for future improvements to VOC.

A-SiGe n-i-p solar cells with i-layer deposited via plasma enhanced chemical vapor deposition (PECVD) with a germane to disilane ratio of 0.72 and hydrogen dilution R=(H2 flow)/(GeH4+Si2H6 flow) values of 1.7, 10, 30, 50, 120, 180 and 240 were deposited on stainless steel substrates. This germane to disilane ratio is what we typically use for the i-layer in the bottom cell of our standard triple-junction solar cells. Solar cell current-voltage curves (J-V) and quantum efficiency (QE) were measured for these devices. Light soaking tests were performed for these devices under 1 sun light intensity at 50° C. While device with R=30 showed the highest initial efficiency, the device with R=120 exhibit higher stabilized efficiency after 1000 hours of light soaking.

Single-layer a-SiGe films (∼500 nm thick) were deposited under the same conditions as the i-layer of these devices on a variety of substrates including 7059 glass, crystalline silicon, and stainless steel for visible-IR transmission spectroscopy, FTIR, and hydrogen effusion studies. It is interesting to note 1) the H content in the film decreased with increasing R based on both the IR and H effusion measurements, and 2) while the H content changes significantly with different R, the change in Eg is relatively small. This is most likely due to a change in Ge content in the film for different R.

Using the modulated photocurrent method we studied the deep defect creation and annealing kinetics of amorphous silicon-germanium alloys with Ge fractions below 10at.%. The modulated photocurrent spectroscopy clearly discloses the existence of two distinct bands of majority carrier traps in these alloys. The bands were identified as neutral Si dangling bonds and neutral Ge dangling bonds. Our studies show clearly that the Si and Ge defects directly compete with each other during annealing, implying a global reconfiguration mechanism. The creation kinetics reveal the usual t1/3 illumination time dependence for the total deep defect density. However, the individual densities of Si and Ge defects have different time dependencies. The details of the creation and annealing kinetics of Ge and Si defects are used to test predictions of certain defect creation models.

Microcrystalline silicon is a two-phase material composed of grains of crystalline silicon embedded in an amorphous silicon tissue. The boundary regions between the crystalline grains and the amorphous matrix are treated similarly to a heterojunction interface.The band offset at the grain boundaries causes the appearance of local electric field peaks, variables in intensity and direction. We present results obtained with two dimensional simulations of a μc-Si:H p-i-n junction in short circuit condition. Charge transport mechanism is described by the internal electric configuration assumed by the junction in thermodynamic equilibrium and illuminated with monochromatic radiations. Different configurations of the band offset at the grain boundaries are also considered and related to the transport properties in microcrystalline silicon

Nanoparticles of amorphous silicon were grown by plasma-enhanced CVD. Their high hydrogen content has a great influence on the oxidation kinetics. Oxidation experiments done in a thermo-balance show that during a heating ramp under oxygen the onset of oxidation shifts to higher temperatures when hydrogen content is reduced by annealing. Apparently this higher oxidation rate of the as-grown particles is due to the great density of dangling bonds that are left behind just after the hydrogen is desorpted. This fact is supported by analyzing the oxidation dynamics under isothermal conditions. At lower temperature, when oxidation takes place just after hydrogen desorption, the oxidation transient begins with a finite slope which indicates that oxidation is not diffusion controlled. On the other hand, at higher temperatures, activation energy of the parabolic rate constant indicates that the oxide layer formed is less protective than the oxide layer formed on crystalline silicon.

We investigated the temperature dependence of the production and annealing kinetics of the light induced defect states in a:Si:H by electron spin resonance (ESR). At low temperatures (T ∼ 25 K) the silicon dangling bond production is about half as efficient as it is at 300 K. Defects, which are created below about 100 K, almost entirely anneal at room temperature. A sample of a-Si:H, which is subjected to several photo-excitation and annealing cycles, shows a very slow increase of both the degraded and annealed defect densities. The difference in the spin densities between the annealed and degraded states decreases with an increasing number of degradation/annealing cycles.

ECR silane plasmas for the deposition of a-Si:H and μc-Si films were investigated by in-situ mass spectroscopy (MS) using a quadrupole residual gas analyzer. The results showed that the intensities of ionic and neutral species (H, H2, He, Ar, Si and SiHx) in the 2 % SiH4/He plasma are strongly dependent on the deposition conditions such as chamber pressure, input power and hydrogen dilution. In all cases, the prevalence of Si ions was observed over SiH, SiH2 and SiH3 ions, suggesting a high decomposition rate of the silane in the plasma. In particular, the population of atomic hydrogen in the plasma seems to play a key role in the properties of both a-Si:H and μc-Si films. For example, the increased intensity of atomic hydrogen, compared to that of molecular hydrogen, resulted in the better quality a-Si:H film, showing a higher photo and dark conductivity ratio of ~105. The intensity of the hydrogen species was especially sensitive to the chamber pressure. The correlation between MS spectra and film properties is presented.

Polycrystalline silicon thin films with a thickness of 0.5-2μm were grown on 25nm thick Ni prelayers by d.c. magnetron sputtering from a Si target. In contrast to the conventional sputtering, the use of a thin Ni film allows the production of high crystallinity silicon at a temperature of 475°C and higher. The Ni disilicide grains formed at the Ni - growing Si film interface provide sufficient sites for the epitaxial growth of Si. The Si films with resistivity of 102-103Ω-cm possess a carrier lifetime of up to 11μs and a diffusion length of up to 3.4 μm, which makes them applicable to various microelectronic devices. As an example, Schottky diodes fabricated on 0.5μm thick Si films exhibit a forward-to-reverse current ratio of 107. The technique is easily implemented on a variety of substrates.

Heterogeneous silicon thin films exhibited various microstructures were prepared by plasma enhanced(PE)-CVD from gaseous mixture of SiF4+H2 (SiH4) on glass substrate. Efficient parameters for controlling the microstructures were reviewed together with the way to grow crystalline seeds on glass by repeating the deposition and etching. Two step growth where polycrystalline silicon thin films were epitaxially grown on the seeds made on glass was proposed to accelerated the growth rate. The relationship between the microstructure and the carrier-transport properties are addressed, as well.

In this work we investigate the density of states at amorphous-crystalline silicon interface that play the key role in the heterostructure solar cell application. In particular we analyzed the defect density arising from plasma treatment of the crystalline surface. This process is useful to clean the crystalline surface, but greatly influenced the electrical properties of the device. We used low temperature (20K-300K) capacitance measurement performed in a wide range of frequency of signal probe (1Hz-10kHz). Differences in the capacitance profile between samples with various plasma dry treatments indicate different defect density profile at interface. With the aid of a finite difference model of the capacitance as a function of temperature and frequency we extract information from the measurements about the defect energy distribution at interface. As a result, the density and the nature of defects at interface will be correlated to the technological parameters as: wafer cleaning procedure, hydrogen plasma treatment, type and concentration of dopants at interface.

In this work we study the possibility to use amorphous silicon nitride, grown by plasma, as an alternative way to realize buffer layer in a-Si:H/c-Si heterostructure. We experimented several growing condition for silicon nitride depending on deposition parameters, obtaining samples highly transparent and with optical gap varying in the range 2.4 – 5.2 eV. We found evidence that the gap of the material is principally due to the NH3/N2 ratio. The very low absorption obtainable on this material was successfully utilized to increase the short circuit current density of the device respect to the standard cell with intrinsic amorphous silicon buffer, particularly in the low wavelength region as confirmed by quantum yield measurements. We optimized the thickness of the SiNx buffer layer respect to the photovoltaic parameters of the solar cell. A 0.5 nm thick SiNx ensures good photogeneration in blue region of the visible spectrum and does not appreciably degrade the transport mechanism of the heterojunction.

It is shown that inert gas effusion employing implanted neon and argon atoms is a useful tool for microstructure characterization of a-Si based alloys. The method measures sensitively interconnected voids and gives information about sizes of microstructure. Limitations of the method are discussed. The results show network reconstruction effects in a-Si:O:H and a-Si:C:H alloys as a function of annealing.

Gas phase chemistry is believed to play an important role in hot-wire CVD of amorphous silicon, serving to convert the highly-reactive atomic Si produced at the wire into a less-reactive species by reaction with ambient SiH4. In this paper, we use quantum chemistry computations (B3LYP/cc-pvTZ) to examine the energetics and rates of possible gas-phase reactions between Si and SiH4. The results indicate that formation of disilyne (Si2H2) is energetically favorable. Unlike other products of this reaction, Si2H2 does not require collisional stabilization, and thus this species is the most likely candidate for a benevolent precursor that participates in the growth of high-quality Si films.

The concentration profiles of B, P, Zn, and O in the active intrinsic (i) νc-Si:H layer and across the interfaces in p-i-n and n-i-p structures have been measured with SIMS. For the ZnO/νc-p/νc-i sequences, an apparent B and O profile extends over several hundred nanometers into the i-layer, and high levels of Zn can be found well above the ZnO substrate layer. These profiles are not affected by annealing at the deposition temperature. Much lower impurity concentrations are measured for n-i-p deposition sequences, or when the p-layer or the i-layer is amorphous. Ruling out diffusion and intermixing processes, evidence for the presence of pinholes in the material is presented, which explain most of the experimental findings.

Thermal influence on film preparation in catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD, is investigated since heat radiation from catalyzer and thermal transport by introduced gases are one of the dominant factors to determine the film properties. It was found that catalyzer-surface area is a key parameter determining not only the deposition rate but also the thermal influence. “Catalytic platE” instead of the conventional wire was proposed in order to suppress the heat radiation with keeping the catalyzer-surface area. Cat-CVD method employing the catalytic plate is superior to conventional “hot-wirE” CVD method.

The topological local cluster (or Schläfli cluster) concept of Marians and Hobbs is used to detect topologically crystalline regions in models of disordered tetrahedral semiconductors. We present simple algorithms for detecting both Wells-type shortest circuits and O'Keeffe-type rings, which can be used to delineate alternative forms of the Schläfli cluster in models.

Hydrocarbon ions are implanted into silicon by pulse biasing Si to a high voltage of -45 kV in a methane plasma. The resulting SiCx:H films are examined with respect to their composition and chemical binding by RBS, NRA and IR spectroscopy. The process may yield all C/Si ratios, up to pure C films. The H depth profile is shown to be strongly governed by the C depth profile. Silicon carbide bonding as well as C–H bonds can be proven in the implanted region.