M.J. Williams

Fabrication and performance of thin film transistors, TFTs, incorporating doped μc-Si source and drain contacts, and boron-compensated μc-Si channel layers

PH 3 doped n + μc-Si, and lightly B 2 H 6 doped intrinsic μc-Si, iμc-Si, thin films have been integrated into bottom-gate TFTs. The use of n + μc-Si as a source/drain contact material in a-Si:H TFTs reduces the threshold voltage compared to n + a-Si:H contacts. The use of iμc-Si as the TFT channel material, combined with a post-deposition, back-channel exposure to atomic-H yielded low-temperature processed TFTs with effective channel mobilities of -6.5 cm 2 /V-s

Post‐deposition relaxation of electronic defects in hydrogenated amorphous silicon

Electronically active defects in hydrogenated amorphous silicon thin films, deposited by the conventional glow discharge process in the temperature range between about 225 and 325 °C with ∼10–15 at. % hydrogen, undergo a thermally activated relaxation during film deposition. We determine the kinetics of this relaxation process in films with similar hydrogen concentrations deposited by reactive magnetron sputtering at a substrate temperature of ∼40 °C, and annealed at temperatures greater than 150 °C. We present a quantitative relationship between the relaxation time, and the deposition and/or annealing conditions required to produce low defect density material.

Reduction of defects by high temperature annealing (150°C–240°C) in hydrogenated amorphous silicon films deposited at room temperature

Abstract We have deposited undoped a-Si:H films with hydrogen content from 0% to 19% with the substrate temperature near room temperature (Ts=40°C). We find an “intrinsic” defect density of 1018cm−3 in the as-deposited films with [H]=12%. Annealing the films at temperatures >150°C reduces the defect density and increased the photoresponse to levels the same as those in films deposited at Ts > 200°C.

Integrated processing of amorphous and microcrystalline Si thin film transistors by plasma-assisted chemical-vapor deposition

Abstract Thin film transistors (TFTs) have been fabricated in an ultra-high vacuum compatible integrated processing system with on-line surface analysis diagnostics - Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). This paper deals with TFTs that include dual-layer oxide-nitride dielectrics, and either hydrogenated amorphous and/or microcrystalline Si thin films for the channel, and source and drain regions. The emphasis is on the integrated processing of bottom-gate device structures, and on the way the electrical performance of the TFTs are correlated with the properties of the dielectric and semiconducting films, their included internal interfaces and the external exposed surfaces.

Hydrogenated amorphous silicon-nitrogen alloys, a-Si,N:H: A candidate alloy for the wide band gap photo-active material in tandem photovoltaic (PV) devices

Abstract a-Si,N:H alloys were prepared by remote plasma-enhanced chemical-vapor deposition (PECVD), and evaluated for applications as wide band gap, photo-active materials for PV devices. These alloys have been deposited using SiH 4 as the silicon-atom source gas, and either N 2 or NH 3 as the nitrogen-atom source gas to yield films with E 04 band gaps up to ∼2.2 eV. We have characterized the microstructure, and have studied selected optical and electrical, and transport properties, comparing films prepared from N 2 and NH 3 N-atom source gases.

Deposition and Characterization of Near “Intrinsic” μc-Si Films Deposited by Remote Plasma-Enhanced Chemical-Vapor Deposition - RPECVD

Undoped films of μc-Si deposited by RPECVD are n-type with a room temperature dark conductivity of ∼6×10 -4 S/cm and an activation energy of ∼0.3 eV. This is due to native donor-like defects . We report on the conductivity and photoconductivity of boron-doped μc-Si, with emphasis on low doping levels that are designed to compensate exactly these native donor-like defects. We describe the dark conductivity and the photoconductivity as functions of dark conductivity activation energy and the average boron concentration, and present a model for the photoconductivity based on band off sets between the crystalline and amorphous regions of the μc-Si.

Photoconductivity and optical stability of intrinsic μc-Si films formed by remote plasma-enhanced chemical-vapor deposition, remote pecvd

μc-Si deposited by RPECVD is n-type with a room temperature dark conductivity of ∼6×10 −4 S/cm and an activation energy of ∼0.3 eV, due to native donor-like defects . We report conductivity and photoconductivity in B-doped μc-Si, emphasizing doping levels that are used to exactly compensate the native defects, and produce a high resistivity, highly photoconductive form of μc-Si.

Hydrogenated Amorphous Silicon-Nitrogen, a-Si,N:H ALLOYS: An Alternative to A-SI,C:H for the Wide Band Gap Photo-Active Material in Tandem PV Cells

We have investigated a-Si,N:H alloys as an alternative wide band-gap, photo-active material. The entire alloy range between a-Si:H and a-Si 3 N 4 :H can be formed by a remote plasma-enhanced chemical-vapor deposition (PECVD) process. Other studies have demonstrated that a-Si,N:H alloys could be doped to form window materials for p-i-n devices. This paper focuses on alloy materials with E 04 bandgaps to about 2.2 eV. We have prepared these a-Si,N:H alloys, characterized their microstructure, and studied their photoconductivity, sensitivity to light-soaking and transport properties. For example, with increased alloying we show that i) the white-light photoconductivity and ii) the kinetics and magnitude of the decay of photoconducitivity under intense illumination (the Staebler-Wronski effect), are about the same as for PV-grade a-Si:H.

Nitrogen: Not a Dopant in Crystalline Si (C-Si), But an N-Type Dopant in A-Si:H, Why?

We have incorporated N-atoms into hydrogenated amorphous silicon in the Si-rich alloy regime to ∼12 at.% N, and have observed a transition from n-type doping to alloying as the concentration of N-atoms is increased above about 5 at.%. By analogy with the local bonding arrangements of P-donors in n-doped a-Si:H, we attribute the doping to four-fold coordinated N-atoms with second neighbor H-atoms as in N + -Si-H linkages. The occurrence of these arrangements is supported by (i) IR studies which indicate a non-statistical association of N and H-atoms bonded to the same Si-atom, and (ii) a chemical bonding model in which the large effective electronegativies of four-fold coordinated N + atoms and neutral O-atoms promote similar bonding properties with respect to their nearest-neighbor arrangements with Si and H atoms such as N + (O) -Si-H linkages

The role of dangling bond states in the picosecond recovery of photoinduced absorption in a-Si:H

We have investigated the influence of deep defect states on the picosecond decay of photoinduced absorption in a-Si:H using femtosecond laser-pulses. For device-quality a-Si:H material with a low density of deep defect states, N d ≃10 16 cm -3 , the recovery of photoinduced absorption is controlled by an intrinsic bimolecular recombination process