Scott Morrison

Low dark current and blue enhanced a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode for imaging applications

This paper presents an a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode with low dark current and enhanced short wavelength responsivity suitable for low-level light detection applications. Junction properties and carrier transport are investigated in terms of current-voltage characteristics, photocurrent transient measurements, and spectral photoresponse. It is demonstrated that introduction of a thin (∼40A) undoped a-SiC:H buffer (δi) at the p-i interface significantly reduces the reverse dark current and recombination losses at this interface. A dark current density of ∼10pA∕cm2 at reverse bias of 1V is achieved for the n-i-δi-p structure, in which the p-type a-SiC:H window layer and the undoped δi buffer layer have a band gap of 2eV.This paper presents an a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode with low dark current and enhanced short wavelength responsivity suitable for low-level light detection applications. Junction properties and carrier transport are investigated in terms of current-voltage characteristics, photocurrent transient measurements, and spectral photoresponse. It is demonstrated that introduction of a thin (∼40A) undoped a-SiC:H buffer (δi) at the p-i interface significantly reduces the reverse dark current and recombination losses at this interface. A dark current density of ∼10pA∕cm2 at reverse bias of 1V is achieved for the n-i-δi-p structure, in which the p-type a-SiC:H window layer and the undoped δi buffer layer have a band gap of 2eV.

High Deposition Rate Amorphous Silicon Solar Cells and Thin Film Transistors Using the Pulsed Plasma Pecvd Technique

The pulsed plasma deposition can increase the deposition rate of amorphous silicon (a-Si) without an increase in the particulate count in the plasma which is an important factor determining the yield of commercial products such as active matrix displays and solar cells. In this paper, we report the deposition of a-Si at rates of up to 15 A/sec a using a modulation frequency in the range of 1-100kHz and the impact it has on solar cell conversion efficiency and thin film transistor performance.

Amorphous and Microcrystalline Silicon Solar Cells Grown by Pulsed PECVD Technique

The Pulsed PECVD technique involves modulating the standard 13.56 MHz RF plasma, in the kHz range. This allows an increase in the electron density during the ‘ON’ cycle, while in the ‘OFF’ cycle neutralizing the ions responsible for dust formation in the plasma. In this work, we report the increase of i-layer growth rate and silane gas utilization rate (GUR) for amorphous Si p-i-n solar cells grown in a large area (30 cm × 40 cm) single chamber deposition system. The i-layer growth rate of 5.4 A/sec with a GUR of >15% has been achieved, which shows a device efficiency of 8.3% (almost same as of our conventional PECVD grown a-Si:H solar cell with ilayer growth rate of ∼1 A/sec). We also deposited microcrystalline Si p-i-n devices using the Pulsed PECVD technique. The crystallite orientation of the films changes from a random to a (220) orientation near the microcrystalline-to-amorphous transition. The effects of crystallite orientation, grain boundaries and ion bombardment during growth on the solar cell performances are investigated. An efficiency of 4.8% for single junction μc-Si:H p-i-n device has been achieved for the i-layer thickness of 0.9 μm.

Deposition of amorphous and microcrystalline silicon using a graphite filament in the hot wire chemical vapor deposition technique

The use of a graphite filament in the “hot wire” chemical vapor deposition technique is demonstrated to produce “state-of-the-art” intrinsic and doped (p- and n-) amorphous silicon (a-Si:H) material and microcrystalline silicon (μc-Si) materials. Preliminary p-i-n type solar cells have led to a conversion efficiency of >8.5%. The filament is found to be rugged and remains intact even after deposition of ∼500 μm in thickness. This is in contrast to the use of conventional filament materials, such as W or Ta, whose longevity is limited to less than a few microns of deposition. Unlike the case of a Ta filament, the deposition rate remains constant with the use of a graphite filament.

Deposition of microcrystalline silicon solar cells via the pulsed PECVD technique

Abstract In this paper, we report the deposition of microcrystalline Si materials and microcrystalline n–i–p and p–i–n devices via the Pulsed PECVD technique. The crystallite orientation of the films changes from a random orientation to (2 2 0) orientation near the microcrystalline-to-amorphous transition. The observed change in orientation ( 2 2 0 vs. 1 1 1 ) is correlated with the solar cell performance, with the best efficiency seen for (2 2 0) oriented i-layers. The role of ion bombardment and grain boundary interfaces on the VOC of these devices is also investigated.

Structural Characterization of SiF4, SiH4 and H2 Hot-Wire-Grown Microcrystalline Silicon Thin Films with Large Grains

In this paper, we present a comprehensive study of microcrystalline silicon thin film samples deposited by a novel growth process intended to maximize their grain size and crystal volume fraction. Using Atomic Force Microscopy, Raman spectroscopy, and x ray diffraction the structural properties of these samples were characterized qualitatively and quantitatively. Samples were grown using a Hot-Wire Chemical Vapor Deposition process with or without a post-growth hot-wire annealing treatment. During Hot-Wire Chemical Vapor Deposition, SiF4 is used along with SiH4 and H2 to grow the thin films. After growth, some samples received an annealing treatment with only SiF4 and H2 present. These samples were compared to each other in order to determine the deposition conditions that maximize grain size. Large microcrystalline grains were found to be aggregates of much smaller crystallites whose size is nearly independent of deposition type and post-annealing treatment. Thin films deposited using the deposition process with SiF4 partial flow rate of 2 sccm and post-growth annealing treatment had the largest aggregate grains ~ .5 µm and relatively high crystal volume fraction.

Reduction of dark current under reverse bias in a-Si:H p-i-n photodetectors

This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm 2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

Deposition of microcrystalline silicon films and solar cells via the pulsed PECVD technique

The pulsed plasma CVD technique has been shown to increase the deposition rate without an increase in the particulate count in the plasma which is an important factor in determining the yield of commercial products such as solar cell modules. The technique is also more easily scaled to larger areas than the VHF-PECVD technique. In this paper, we report on the deposition of microcrystalline silicon (/spl mu/c-Si) films over large area substrates (30 cm /spl times/ 40 cm) as well on the optimization of /spl mu/c-Si solar cell devices. The effects of nucleation and substrate pre-treatment on the p/i interface are discussed.

Deposition of amorphous silicon solar cells via the pulsed PECVD technique

The pulsed plasma CVD technique has been shown to increase the deposition rate without an increase in the particulate count in the plasma, which is an important factor in determining the yield of commercial products such as solar cell modules. In this paper, the authors report on the use of this technique in a small area deposition system, and show deposition rates of a-Si:H of up to 15 Angstroms/sec can be achieved using a modulation frequency in the range of 1-100 kHz. Simple solar cells of the p/i/n configuration, deposited using this technique, have shown initial efficiencies of 9% with intrinsic-layer deposition rates of up to 7 A/sec. The application of this technique to a large-area 30 cm/spl times/40 cm system is also discussed. In particular, they report a deposition rate of rate(>25%), and the performance of small area (0.25 cm/sup 2/) devices using this approach.

DEPOSITION OF THIN FILM SILICON USING THE PULSED PECVD AND HWCVD TECHNIQUES

Use of a pulsed plasma-enhanced-chemical-vapor-deposition technique for “state of the art” a-Si:H materials and solar cells at a deposition rate of up to 15 Å/sec using a modulation frequency in the range of 1–100 kHz is reported. The approach has also been developed to deposit materials and devices onto large-area, 30 cm×40 cm, substrates with thickness uniformity of <5%, and a large gas utilization rate of >25%. In a newly developed “Hot Wire” chemical vapor deposition method, a graphite filament material, has so far shown no appreciable degradation after deposition of 500 μm of amorphous silicon. It is reported that this technique can produce “state-of-the-art” a-Si:H and that a solar cell of p/i/n configuration exhibited an initial efficiency approaching 9%. The use of microcrystalline silicon (μc-Si) materials to produce low cost stable solar cells is gaining considerable attention. It is demonstrated that both of these techniques can produce thin film μc-Si, dependent on process conditions, with 111 and/or 220 orientations and with a grain size of approximately 500 Å. Inclusion of these types of materials into a solar cell configuration is discussed.