Onset Of Microcrystallinity Research Articles

Better control over the onset of microcrystallinity in fast-growing silicon network

In view of obtaining a Si:H network at the onset of microcrystallinity at a high deposition rate, we have adopted an intelligent approach to find out a tricky plasma condition in radio frequency (rf) plasma-enhanced chemical vapordeposition that provides a better control on growth introducing retarded microcrystallization. The deposition parameter includes a combination of high electrical power applied to the (SiH4+H2)-plasma and high gas pressure in thereaction chamber. High rf power increases the number density of film-forming precursors as well as atomic H density in the plasma, which helps to increase thefilm deposition rate and to promote microcrystallinity, respectively. In addition,high pressure helps not only to increase the film-growth rate by producing a dense plasma but also retards the microcrystallization process by increasing significantlythe gas phase collision frequency and consequently reducing the effective reactivityof atomic H on the surface of a fast-growing Si:H network. A combination of high-power and high-pressure plasma conditions provides a reasonably wide rangeof H2 dilution to work with and better control in producing a Si:H network at theonset of microcrystallinity, while increasing the film-growth rate.

Micro-photoluminescence and micro-Raman studies near the amorphous-to-microcrystalline transition in Si:H

Micro-photoluminescence and micro-Raman studies have been performed in the silicon–hydrogen system, near the onset of microcrystallinity, during the transition from amorphous-like to microcrystalline-like phase. Amorphous-like Si:H films before the onset of microcrystallization, as determined by a micro-Raman probe, exhibit a microcrystalline-like photoluminescence (PL) band, which is a doublet within the 1.0–1.2 eV energy band. These two satellite components exhibit two different natures of energy shifts with variation of either excitation intensity or temperature; however, both attribute to the germinate recombination of carriers. The ultimate line shape is determined by the low-energy component, because of its faster quenching with temperature. Two different characteristic temperatures of exponential band-tail states are obtained, contributing tail widths of 22 and 17 meV at lower and higher temperature regimes, respectively, across 200 K, as calculated in terms of the carrier thermalization model. PL-spectroscopy may offer a new means of diagnostics for Si:H network before the onset of microcrystallinity.

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased, (c) the low-energy optical absorption at photon energy <1.4 eV increased one to two orders of magnitude, (d) the low-energy PL band at ∼1.0 eV emerged with a decrease of total PL intensity, and (e) the conductivity activation energy decreased. The aforementioned changes correlated well with the crystallinity of the material. We attribute the observations mainly to the formation of the c-Si gain boundaries during crystallization.

Microstructure of amorphous and microcrystalline Si and SiGe alloys using X-rays and neutrons

This review describes recent experimental applications of small-angle X-ray (SAXS) and neutron scattering, as well as X-ray diffraction (XRD), for the determination of the microstructure of relevant solar cell thin film materials based on hydrogenated amorphous Si and SiGe alloys. Due to the recent trend toward use of amorphous Si and SiGe materials prepared near the onset of microcrystallinity, this review also describes the use of XRD to detect and determine the role of medium-range order and partial microcrystallinity in solar cell behavior. Some recent results from SAXS and XRD applied to microcrystalline Si are also summarized.

Onset of microcrystallinity in silicon thin films

The onset of microcrystallinity in silicon thin films was realized via an amorphous-to-microcrystalline phase transition. Undoped films have been deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique from silane diluted with hydrogen. Substrate temperature was set as the variable parameter in the deposition. The films were characterized by Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The presence of both amorphous and crystalline phases is indicated in the Raman spectra and the signature of grain boundary regions is not prominent for the transition films. The dominance of monohydride bonding in the amorphous matrix is revealed by FTIR spectra. TEM confirms the presence of small grains (∼50 Å) embedded in the amorphous matrix for the films prepared at the transition region. At the onset of crystallinity, films have a higher order of dark conductivity than a typical amorphous film, but are still photosensitive. Better stability under illumination is observed compared to amorphous silicon.

Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy

Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.

Amorphous silicon deposited at high growth rates near the onset of microcrystallinity

Intrinsic a-Si:H samples grown with and without hydrogen (H 2) dilution of silane at different growth rates are studied. We find that the dilution reduces the defect density, in particular at larger growth rates. The defect density is smallest for samples grown using H 2 dilution conditions at growth rates as great as 1 nm/s and the corresponding solar cells have the greatest fill factors. Using transient photocapacitance measurements, we find evidence for a low concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.

Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

Medium-Range Order in a-Si:H Below and Above the Onset of Microcrystallinity

AbstractMedium range order (MRO) and the formation of microcrystallites in a-Si:H prepared by plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD) have been probed by systematic x-ray diffraction studies with films as thin as those used in solar cells. Effects of substrate temperature, hydrogen dilution, film thickness, and type of substrate have been examined. High-hydrogen-diluted films of 0.5 μm thickness, using optimized deposition parameters for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous provided a thin (20 nm) n-layer of a-Si:H or μc-Si:H is first deposited on the SS. The latter predeposition does not prevent partially microcrystallinity if the films are grown thicker (1.5 to 2.5 μm) and this is consistent with a recently proposed phase diagram of thickness versus hydrogen dilution. Analysis of the first (lowest angle) scattering peak of the a-Si:H phase demonstrates that its width, directly related to MRO, is reduced by heavier hydrogen dilution in PECVD growth or by increased substrate temperature in HWCVD growth. The narrowest width of fully amorphous material correlates with better solar cell stability and this is not likely related to bonded hydrogen content since it is quite different in the optimized PECVD and HWCVD a-Si:H. A wide range of MRO apparently exists in the residual amorphous phase of the mixed a/μc material.

Charge Transport in the Transition From Hydrogenated Amorphous Silicon to Microcrystalline Silicon

AbstractThe electronic transport properties of a series of samples prepared by hot-wire chemical vapor deposition with a transition from a-Si:H to μc-Si:H were measured applying the photoconductive frequency mixing technique. We found both improved stability against light-soaking and different values for the photomixing electron lifetime and mobility close to the onset of microcrystallinity as compared to the amorphous state. In particular, the mobility-lifetime product of charge carriers in some of the μc-Si:H samples turns out to lie about two orders of magnitude higher than that of a-Si:H films. The mobility on the other hand, is shown rather to decrease in the transition to the μc-region. Additional measurements of the range and the depth of long range potential fluctuations yield a possible explanation for our results in that grain boundaries may serve as scattering centers and barriers against recombination.

A Spectroscopic Investigation of the Amorphous to Microcrystalline Transition in Silicon Prepared by Reactive Magnetron Sputtering

Reactive magnetron sputtering of a c-Si target in an atmosphere containing argon and a varying partial pressure of hydrogen has been used to prepared a series of samples which span the transition from amorphous to microcrystalline silicon. Samples prepared at a substrate temperature of 200 °C, power of 100 W, and Ar partial pressure of 2.10 mtorr can be divided according to the partial pressure of H2, PH2, used in the deposition. Samples deposited using PH2 ≤ 1.00 mtorr are amorphous, those deposited using P H2≥ 1.25 mtorr are microcrystalline. Raman scattering is used to confirm amorphousity/microcrystallinity. Two types of changes are observed using infrared, IR, spectroscopy. First, the IR absorption spectrum changes at the onset of microcrystallinity. The absorption centered at 630 cm-1 changes shape while the feature at 2000 cm-1 appears to shift upward to 2090 cm-1. Second, microcrystalline samples deposited using PH2 ≥ 1.50 mtorr show significant post-deposition contamination upon atmospheric exposure. This is most clearly noted by the strengthening, over time, of Si02 related absorption features which appear at 1050 and 1150 cm-1, and a weakening of a narrow component of the 630 cm-1 absorption. This narrow component is not observed in amorphous material. The strength of the O-related absorption suggests that the oxide is a bulk property. The porous, low density network which is necessary for this contamination to occur is confirmed using HRTEM.