Guiding principles for obtaining stabilized amorphous silicon at larger growth rates
Introduction
Understanding complicated reactions occurring in silane glow-discharge plasmas and on the film-growing surface are essential to improve the properties of resulting hydrogenated amorphous silicon films (a-Si:H). Deterioration of film properties especially after light soaking (photo-induced degradation) occurs in the resulting a-Si:H when films are deposited from a silane glow-discharge plasma at greater growth rates when keeping the substrate temperature constant [1].
In this report, we have studied the formation kinetics of higher order silane-related reactive species (HSRS). These are recognized as the responsible reactive species during film growth, which cause the photo-induced metastability in the resulting film through changes in the network structure [1]. The incorporation of dihydride-mode bonding configuration in a film is the cause of instability [1]. We propose a guiding principle to reduce the contribution of HSRS during film growth and we demonstrate an improvement of film properties especially after light soaking using Schottky diode photocurrent as a function of voltage as an evaluation technique.
Section snippets
Structural properties of a-Si:H deposited at a high growth rate
It is a well known experimental fact that opto-electronic properties of a-Si:H deteriorate after light soaking when films are deposited at larger growth rates [1]. The fill factor (FF) of photocurrent as a function of voltage properties in a-Si:H Schottky diodes decreases with increasing deposition rate of a-Si:H prepared under conventional plasma conditions in plasma-enhanced chemical-vapor deposition (PECVD) [1]. We have reported that FF of Schottky diodes after light soaking has a
Experimental results
Conventional capacitively coupled PECVD system is used in this study [1]. Fig. 1 shows the electron density and the electron temperature measured by a conventional Langmuir probe method as a function of the excitation frequency in an Ar plasma, where an electrode distance of 40 mm, a working pressure of 50 mTorr, a power density of 0.13 W/cm2 are used. Fig. 2 shows the dc self-bias voltage in the SiH4 plasma (an electrode distance of 40 mm, a working pressure of 20 mTorr, a power density of
Summary
We have studied the effect of the plasma-excitation frequency and the H2-dilution ratio during film growth on the opto-electronic properties especially after light soaking of the resulting films through the control of the HSRS contribution ratio. The guiding principles are reduction of the electron temperature in the plasma by increasing the excitation frequency, which suppresses the generation rate of SiH2 and proper H2-dilution ratio, which scavenges SiH2 during the HSRS formation process.
Acknowledgements
We express our thanks to Professor Kawai and Dr Shindo of Kyushu University for helpful assistance in Langmuir probe measurement.
References (5)
- et al.
Thin Solid Films
(1999) - et al.
Thin Solid Films
(1999)