Preparation and study of high Quality hydrogenated silicon Films from Amorphous to Microcry stalline Transition Range for Photovoltaic Applications

The films are prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD) and Hot Wire Chemical Vapor Deposition (HWCVD) methods by varying different deposition parameters and extensively studied for structural, optical and electrical properties. The films are found to be quite homogeneous despite the use of argon dilution. Raman studies show that the films range from purely amorphous to a maximum of 65 % crystalline in nature and possess good short- and medium-range order (SRO and MRO).

The stability of the films is attributed to improved order and hydrogen in the monohydride mode. The films show the transition from amorphous to nanocrystalline when the Ts is increased from 150 to 200oC. In Chapter 8, studies are reported on the films prepared from pure silane by varying the silane flow rate (SFR).

The hydrogen content of these films varies with very high monohydride fraction (70-92%) and the films have good SRO and MRO. In the last chapter, a comparative report and a summary of all the films in this thesis is mentioned.

Introduction 1

Growth model for amorphous and microcrystalline silicon

Different methods of deposition of a-Si:H/ µc-Si:H

Current issues regarding a-Si:H/ µc-Si:H photovoltaics

Motivation for present work

Experimental Techniques 29

Film Preparation

Radio Frequency Plasma Enhanced Chemical Vapour Deposition
Hot Wire Chemical Vapour Deposition (HWCVD) method

The system consists of a high-vacuum chamber generally connected to a containment load chamber, a gas supply system, and a discharge source. These gases dissociate into various film-forming radicals by inelastic collisions with high-energy electrons (about a few tens of electron volts) present in the plasma, followed by further gas-phase reactions. The radicals thus formed undergo secondary reactions in the gas phase, mostly with basic SiH4 and H2, as indicated in Figure 2.3, which form a steady state.

The radicals formed in secondary gas-phase reactions are then deposited on the substrate, yielding the film. In this method, the source gases are broken down into film-forming radicals by means of a filament heated to high temperatures, generally in the range of 1600-2200 oC [2, 10-12]. The radicals thus formed by catalytic cracking can further undergo chain gas phase reactions and be modified before being deposited on the substrate [13-15].

Like PECVD, the HWCVD system also consists of a high vacuum chamber equipped with a gas inlet system and a substrate heater. In contrast to the PECVD technique, where SiH3 is the main film-forming radical, here Si and H are the primary radicals released from the filament surface [10, 20].

Characterization techniques

Structural Characterizations

X-ray diffraction
Scanning electron microscopy
Raman scattering spectroscopy
Transmission electron microscopy
Atomic force microscopy

Optical Characterizations

UV-Vis-NIR spectroscopy
Fourier transform infrared spectroscopy
Photoluminescence spectroscopy

Electrical Characterizations

Temperature dependent dark and photoconductivity
Stability studies
Diffusion Length measurements by SSPG method

The expansion of the first peak, i.e. due to (111) plane, is estimated by deconvoluting the peak into three components, sharp peak due to the nanocrystalline silicon and a broad peak originating from the amorphous tissue in the film, both centered at 2θ value near 28o and a third broad peak centered at 23o due to the glass substrate. This instrument is also equipped with an energy study of the surface morphology of the films. As the bond angle deviation increases, the material becomes more disordered with a broadening of the amorphous TO peak.

In 2.4 In the case of nanocrystallites embedded amorphous films, the crystalline peak shifts from 520 cm-1 towards lower value due to the size limitation of the crystallites, smaller the crystallite size, larger is the shift. This accelerating voltage of 200 KeV is used to study the microstructure of the film. The measurements are performed on the films used to obtain the film surface morphology.

For each scan size, a statistical average of the roughness from different scanned areas is estimated. The thickness and the optical constant of the films are estimated from the interference edges in the transmission spectrum according to Swanepoel [32]. The thus estimated thickness value is used to calculate the deposition rate (rd) of the films, while the band gap is calculated from the plot of α νh vs.

The sample holder is placed, the photoconductivity of the films is measured by the two-probe method. The sample holder is placed inside a vacuum chamber which provides a high vacuum of the order of 10-5 mbar. The conductivity of films on Corning and ITO coated glass are measured in coplanar and sandwich geometries, respectively.

In the case of coplanar geometry, if l is the length of the electrodes, d is the distance between them, t is the thickness of the film, V is the applied voltage and I is the measured current, then the conductivity is given by the following relation ,. Light soaking is performed for 12-14 hours by the same lamp used for photoconductor measurements through a water filter to avoid heating of the film. Due to the formation of low and high resistance regions, the photocurrent perpendicular to the lattice edges is less than is the case with uniform illumination.

Fabrication of Experimental Set ups 47

HWCVD system fabrication

Conductivity measurement set up

Steady State Photo Carrier Grating (SSPG) set up

Argon dilution of PECVD films 59

Experimental Details

Results and Discussions

X-ray diffraction studies
Thickness, optical constants and optical band gap calculation
Photoluminescence studies
Temperature dependent conductivity and stability studies
Diffusion length measurements by SSPG method

Conclusion

Hydrogen dilution of HWCVD films 79

UV-Vis-NIR transmission spectroscopy
Infra Red transmission spectroscopy
Photoluminescence spectroscopy
Carrier Diffusion length by SSPG method

Conclusion

Infrared absorption studies

Conclusion

Hydrogen diluted HWCVD films: Effect of Process

Infrared absorption spectroscopy

Conclusion

HWCVD films: Effect of Silane Flow Rate 155

Conclusion

HWCVD films: Effect of Filament Temperature 177

Experimental Details

Well-cleaned Corning 1737, ITO-coated glass, c-Si wafers, and carbon-coated Cu grids are used. The deposition is carried out by keeping the substrate temperature (Ts) and silane flow rate (SFR) fixed at 250oC and 3SCCM respectively for all films while the filament temperature (TF) is varied in the range of 1600-1900oC. During deposition, a constant process pressure (PP) of 0.05 mbar is maintained by throttling the chamber outlet.

The distance between the filament and the substrate is fixed at 6 cm, like all other HWCVD films described in this thesis. This distance is chosen so that the heating of the substrate due to the filament is negligible. The films are structurally characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and atomic force microscopy (AFM).

Transmission electron microscopy is performed on the films deposited on Cu lattices using JEOL TEM. Raman studies are performed with a Witec Alpha Raman spectrometer with an excitation wavelength of 514.1 nm in the scan range of 100-1000 cm-1. Atomic force microscopy is performed in acoustic mode with silicon cantilevers with a force constant of 2.5 N/m and a resonant frequency of 80 KHz using Picoplus Molecular Imaging.

For optical studies, UV-Vis-NIR transmission spectra are recorded in the range of 3000-350 nm using the Shimadzu UV 3101 PC spectrometer, while IR transmission spectra are recorded in the range of 400-4000 cm-1. The transport studies are carried out by temperature-dependent dark and photoconductor measurements and stability studies using light. A white light source of 100 Watts is used for photoconductor measurements and stability studies using light.

More details about the experimental procedure and estimation of various parameters are explained in Chapter 2.

Results and Discussions

This reveals the formation of nanocrystallites in the amorphous matrix for films deposited at high TF. It is observed that low TF films contain some dark areas randomly dispersed in a homogeneous light matrix. In the high TF films, white thread-like structures surrounding the brighter regions with a diameter of about 20 n are additionally observed.

The dark areas in the high TF films show the silicon lattice spacing nanocrystallites under high resolution (Figure 9.6), which is not observed in the low TF films (amorphous films). Figures 9.7a-9.7d show the surface morphology of the films reported in this chapter as obtained from AFM studies. It is observed that the films have a very small rms roughness (rrms) of approx. 3-4.5 nm.

The thickness, optical constants and deposition rate (rd) of the films as estimated from the interference fringes in the UV-Vis-NIR transmission spectra are listed in Table 9.4. However, there are reports of high band gaps in the films produced by HWCVD [11–12] and also by PECVD methods [13]. Polyhydride bonding (SiH2)n in the films is almost absent as can be seen by the absence of any absorption in the 750-850 cm-1 range.

The films are oxygen-free, except for the film deposited at TF 1900 oC, where a small dip in the transmission spectrum near 1050 cm-1 is observed, corresponding to the SiO bond. The nanocrystalline nature of the films at high TF may also be responsible for the low CH of the films. Annealing at 200 oC for 2 hours increases the dark conductivity for all films and the photoconductivity of films with embedded nanocrystallites.

Among the films described in this chapter, the film deposited at 1700 oC has the highest photosensitivity. Films with low TF are found to be amorphous, while films deposited at higher TF contain nanocrystallites embedded in an amorphous matrix. The stability of the films is attributed to the improved order, monohydride bonding and also the low hydrogen content of the films.

Conclusion

Conclusion and Scope for future work 199

Conclusion

Scope for future work

Argon dilution of PECVD films