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Title: Mobility-lifetime products in hydrogenated amorphous silicon

Abstract

The most important parameters characterizing the photoelectronic quality of a semiconductor are its charge-carrier mobility lifetime, {mu}{tau}, products. The two common experimental methods used to determine these parameters in hydrogenated amorphous silicon, {ital a}-Si:H, are the steady-state photoconductivity measurement and the life-of-flight charge-collection measurement. The two methods yield quite different results. We show that the difference can be resolved by an understanding of the physics involved in each of the measurements. We show that the steady-state {mu}{tau} is expected to be up to three orders of magnitude larger than the time-of-flight {mu}{tau} in undoped {ital a}-Si:H. This prediction is in excellent agreement with the corresponding experimental results.

Authors:
 [1];  [2]
  1. (Solar Energy Research Institute, Golden, Colorado 80401 (US))
  2. (The Racah Institute of Physics, The Hebrew University, Jerusalem (Israel))
Publication Date:
OSTI Identifier:
6012395
DOE Contract Number:
AC02-83CH10093
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; (USA); Journal Volume: 58:5
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; SILANES; CARRIER LIFETIME; CARRIER MOBILITY; HYDROGENATION; PHOTOCONDUCTIVITY; CHEMICAL REACTIONS; ELECTRIC CONDUCTIVITY; ELECTRICAL PROPERTIES; HYDRIDES; HYDROGEN COMPOUNDS; LIFETIME; MOBILITY; ORGANIC COMPOUNDS; ORGANIC SILICON COMPOUNDS; PHYSICAL PROPERTIES; SILICON COMPOUNDS; 360603* - Materials- Properties

Citation Formats

Crandall, R.S., and Balberg, I. Mobility-lifetime products in hydrogenated amorphous silicon. United States: N. p., 1991. Web. doi:10.1063/1.104622.
Crandall, R.S., & Balberg, I. Mobility-lifetime products in hydrogenated amorphous silicon. United States. doi:10.1063/1.104622.
Crandall, R.S., and Balberg, I. 1991. "Mobility-lifetime products in hydrogenated amorphous silicon". United States. doi:10.1063/1.104622.
@article{osti_6012395,
title = {Mobility-lifetime products in hydrogenated amorphous silicon},
author = {Crandall, R.S. and Balberg, I.},
abstractNote = {The most important parameters characterizing the photoelectronic quality of a semiconductor are its charge-carrier mobility lifetime, {mu}{tau}, products. The two common experimental methods used to determine these parameters in hydrogenated amorphous silicon, {ital a}-Si:H, are the steady-state photoconductivity measurement and the life-of-flight charge-collection measurement. The two methods yield quite different results. We show that the difference can be resolved by an understanding of the physics involved in each of the measurements. We show that the steady-state {mu}{tau} is expected to be up to three orders of magnitude larger than the time-of-flight {mu}{tau} in undoped {ital a}-Si:H. This prediction is in excellent agreement with the corresponding experimental results.},
doi = {10.1063/1.104622},
journal = {Applied Physics Letters; (USA)},
number = ,
volume = 58:5,
place = {United States},
year = 1991,
month = 2
}
  • The results of optical-absorption measurements determined by photothermal deflection spectroscopy, primary photoconductivity, and secondary photoconductivity on undoped and phosphorus-doped hydrogenated amorphous silicon films (a-Si:H) are reported. A normalization procedure for obtaining photocurrent spectra at constant generation rate is demonstrated. From these measurements, the efficiency-mobility-lifetime product (eta..mu..tau) for electrons is found to be constant from 2.0 to approx.0.9 eV for both undoped and phosphorus-doped a-Si:H. For excitations less than approx.0.9 eV, there is evidence that the product eta..mu..tau for electrons drops rapidly; similarly, the eta..mu..tau for holes exhibits a rapid decrease in undoped material for photon energies less than approx.1.5 eV.more » A model is proposed to explain the results, resolving a discrepancy between secondary- and primary-photoconductivity measurements of the optical absorption.« less
  • We have applied the junction recovery technique to different configurations of hydrogenated amorphous silicon type diodes and show that the recovered charge consists predominantly of holes. The technique is used for the measurement of the mobility-lifetime product for recombination, which was found to be dependent upon the level and type of doping.
  • In this paper, we use a model of hydrogenated amorphous silicon generated from molecular dynamics with density functional theory calculations to examine how the atomic geometry and the optical and mobility gaps are influenced by mild hydrogen oversaturation. The optical and mobility gaps show a volcano curve as the hydrogen content varies from undersaturation to mild oversaturation, with largest gaps obtained at the saturation hydrogen concentration. At the same time, mid-gap states associated with dangling bonds and strained Si-Si bonds disappear at saturation but reappear at mild oversaturation, which is consistent with the evolution of optical gap. The distribution ofmore » Si-Si bond distances provides the key to the change in electronic properties. In the undersaturation regime, the new electronic states in the gap arise from the presence of dangling bonds and strained Si-Si bonds, which are longer than the equilibrium Si-Si distance. Increasing hydrogen concentration up to saturation reduces the strained bonds and removes dangling bonds. In the case of mild oversaturation, the mid-gap states arise exclusively from an increase in the density of strained Si-Si bonds. Analysis of our structure shows that the extra hydrogen atoms form a bridge between neighbouring silicon atoms, thus increasing the Si-Si distance and increasing disorder in the sample.« less
  • The calculation of the space-charge density under illumination from surface photovoltage measurements has been adapted to analyze light-soaking experiments in intrinsic hydrogenated amorphous silicon (a-Si:H). We find that the positive space-charge densities increase and the space-charge widths decrease with light exposure while very little change occurs in the hole diffusion length. These results indicate that changes in electric field distribution with light soaking are important causes of degradation of amorphous silicon solar cells, rather than changes which may ocur in the hole diffusion length. We also review and discuss results of others regarding application of the surface photovoltage (SPV) tomore » study light-induced phenomena, evidence of two different types of light-induced defects, and potential limitations of SPV in a-Si:H.« less
  • The mobility-lifetime products ( ..mu..tau) and interface property have been examined through the photovoltaic studies in actual hydrogenated amorphous silicon (a--Si:H) p--i--n junction solar cells. A small amount of boron atoms included in a--Si:H enhances the ..mu..tau products of both electrons and holes up to the order of 10/sup -7/ cm/sup 2//V, which corresponds to the carrier diffusion length in excess of 5000 A. The doped window layer possessing inferior photoelectric property works as the recombination region for photocarriers generated in the active i layer, and practically dominates the interface property together with the surface recombination velocity S/sub 0/ atmore » the electrode/doped layer interface. The S/sub 0/ at the SnO/sub 2//p a--Si:H interface is estimated to be about 3 x 10/sup 2/ cm/s with an assumption of the electron mobility at 0.1 cm/sup 2//Vs. Prolonged light exposure causes a reversible change of the ..mu..tau products in every layer composing the p--i--n junction. These experimental results are discussed in connection with photovoltaic performances.« less