a-Si∶H薄膜的再结晶技术发展概述

论述了非晶硅薄膜的主要再结晶技术,包括传统的炉子退火、金属诱导晶化、微波诱导晶化、快速热退火和激光晶化。着重指出了各种晶化技术已取得的研究成果、优缺点、有待进一步研究的内容及其在多晶硅薄膜太阳电池工业生产中的应用前景。     

Glow discharge deposited a-Si:H,Al thin films

The modification of the electrical and optical properties of a-Si : H by the incorporation of aluminium has been studied. The a-Si : H,Al films were obtained by means of dc glow discharge decomposition of gas silane and simultaneous evaporation of alumunium using the same discharge conditions and varying only the evaporation rate of Al. The study of the films properties includes V-NIR transmission spectroscopy, temperature dependence of dark conductivity and steady state photoconductivity. From these measurements an important variation of both optical gap and dark conductivity has been observed as the aluminium content in the films increases. Electrical conduction probably takes place, in the analyzed temperature range (300–450 K), through extended states of the valence band. It has also been observed that the normalized photoconductivity of the slightly Al doped films is three or four orders of magnitude lower than that of undoped a-Si : H film.     

Electrical properties of a-Si:H thin films as a function of bonding configuration

Hydrogenated amorphous silicon (a-Si:H) thin films were fabricated by Radio Frequency (RF) magnetron sputtering. For solar-cell applications, a-Si:H layers are required to show low dark conductivity and high photoconductivity and, thus, high photosensitivity. Hydrogen flow ratio and working pressure were mainly adjusted to control bonding configurations and hydrogen concentration in the films. At a high working pressure of 12 mTorr, all of the prepared amorphous and microcrystalline silicon films showed a dominant IR absorption peak at 2100 cm⁻¹, which indicates a Si-H₂ stretching mode, grain boundaries and microvoids. When the working pressure was decreased to as low as 3 mTorr with a hydrogen flow ratio of 0.1, the bonding configuration of the films was mainly Si-H as determined by the dominant IR absorption peak at 2000 cm^−1, and the photosensitivity of the films was maximized to 760.     

IR, ESCA and Auger analysis of thermally hydrogenated a-Si:H thin films

Hydrogenated amorphous silicon thin films have been prepared by thermal evaporation. IR investigations showed the existence of all expected Si---H. The electron spectroscopy for chemical analysis exhibited the existence of oxygen and carbon atoms on the silicon surface which led to a shift in the Si2p core level. Auger spectroscopy also exhibited a peak shift in the kinetic energy. Both shifts are interpreted on the bases of the environmental change in the amorphous structure.     

Electrical conductivity in a-Si:H:Cl films

The temperature dependence of the electrical conductivity was investigated in chlorinated and hydrogenated amorphous silicon films (a-Si:H:Cl) deposited by glow discharge in the SiCl₄SiH₄ mixtures. The influence of chlorine on the different transport processes is determined according to the investigated temperature range. On the base of the experimental results and the speculative considerations regarding the film structure the inference is drawn that chlorination of hydrogenated amorphous silicon causes a decrease in the gap state density and an increase in the structural disorder. It is supposed that a more definite improvement of the electronical properties of glow discharge amorphous silicon could be expected for the relatively low chlorine concentration of about (1.0–2.0)×10²¹ at/cm³.     

a-Si:H thin film microstructure influenced by HOMOCVD-process parameters

Hydrogenated amorphous silicon films prepared by homogeneous chemical vapor deposition (HOMOCVD) process were studied with respect to their microstructure by field-assisted ion exchange (FAIE) techniques. Results concerning the influence of HOMOCVD process parameters such as temperature, gas pressure, flow and substrate temperature are presented. Two series of deposition experiments were carried out: at constant gas phase conditions and at the variable ones. The obtained results showed that the amount of micropores was not determined by the growth rate. This inference is in contrast with the results obtained for plasma CVD a-Si:H. On the basis of the presented results it is possible to choose the deposition conditions, which yield a-Si:H films with the lowest micropore density.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Characterization of a-Si:H films via analysis of multiple photocurrent spectra

The spectral response of photoconductive materials applicable to solar cells and photosensors, is commonly characterized using different experimental approaches, such as photocurrent spectra normalized to incident light intensity, photoconductivity spectra measured at constant photon flux, the constant photocurrent method (CPM) [3–5], and the dependence of the photocurrent on the photon flux.In this work, a procedure is proposed, which allows the complementary results of the above mentioned approaches to be gained by analysing several photocurrent spectra, obtained by routine measurements at different illumination levels (Multiple Spectra). This procedure is used for investigation of two a-Si:H samples. The spectra, corresponding to a constant photon flux and a constant photocurrent, as well as the dependencies of the photocurrent on the photon flux at various wavelengths I−Φ spectral map) are derived from the Multiple Spectra and are subsequently used to obtain the following characteristics of the investigated samples: the subbandgap optical absorption spectra; the bulk density of “defect” states; the Urbach tail width; and the power index of the I−Φ)_λ dependencies for the bulk and for the interface regions.     

Influence of laser scribing in the electrical properties of a-Si:H thin film photovoltaic modules

The third (P3) laser patterning step of thin film photovoltaic devices is studied experimentally using a diode pumped solid state laser with 532 nm wavelength and a delay generator. The effect on the electrical characteristics of the devices due to the patterning process is investigated by performing scribes on single, thin-film solar cells. As it is shown, in this type of experiments the inertia in the motion systems or in the scanner controlling the direction of the laser beam plays a critical role in the results. By controlling externally the output of the laser beam it is possible to overcome the inertia and investigate the real effect of the P3 laser scribing on the device electrical characteristics. When the laser scribing conditions are optimized and the inertia in the system is taken care of, the P3 process has very little effect on the device electrical characteristics. Translated to modules this means that by optimizing the P3 process, the decrease in the efficiency found when up-scaling from single cells to modules can be minimized (as far as the P3 process is concerned) to that coming from the removed area.     

The effect of annealing on the optical and electrical properties of a-Si:H sputtered films

In this work we study the influence of an annealing treatment at high temperatures on the optical and electrical properties of a-Si:H sputtered films. The results show important changes of the different parameters determined after annealing. A possible model of the silicon matrix evolution during annealing is proposed, and the dependence of the energy gap on disorder and hydrogen concentration is discussed.     

Effect of halogen additives on the stability of a-Si:H films deposited at a high-growth rate

We deposited a-Si : H,F films at a high-growth rate (15 Å/s) using a SiH₄ and SiF₄ gas mixture to examine the effect of halogen additives on the film stability against light exposure. Fluorinated a-Si : H films show a high conductivity over 5×10⁻⁵ S/cm and the Schottky cells made with fluorinated films exhibit an improved fill factor after light-soaking. SIMS measurements show an increased oxygen incorporation into the film at a SiF₄ flow of 5 sccm or larger, while virtually no increase is seen when a small SiF₄ flow rate of 1 sccm is used. This is presumably an indication that a small amount of SiF₄ can actually help improve the stability of a-Si : H films against light exposure.