摘要:

Electron spin resonance was used to characterize concentrations of thermal equilibrium defects from room temperature of 280 °C in a 60‐micron‐thick hydrogenated amorphous silicon (a‐Si:H) film. I found a defect formation energy of 0.35 eV in material with 1×1015cm−3spins at 190 °C; an assumed two‐level configuration coordinate diagram would then have a ‘‘source’’ density of 1019cm−3. Annealing of defects quenched in from 250 °C yields an activation energy of 2.1 eV. When annealings of defects quenched in from high and low temperature are comparedthe defects introduced at the higher temperature always anneal faster; The metastable states with higher formation energies have smaller annealing activation energies. Additionally, the time constant for generation of defects at 205 °C is 10 times longer than the corresponding annealing time constant, consistent with the very high formation barrier expected for this two‐level system. The stretch parameter for defect generation is much closer to unity than for defect annealing. Easy‐to‐anneal, light‐induced defects can be described as a very high‐temperature distribution similar to that which might be quenched in as a result of kT&bartil;0.5 eV.

ISSN:0094-243X    

DOI:10.1063/1.41018

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have characterized metastable states in n‐type doped a‐Si:H due either to fast (quench) cooling or by light‐soaking followed by a series of partial dark anneals. These studies have been carried out for samples with a large range of PH3doping levels using drive‐level capacitance profiling to deduce the number of occupied bandtail states (NBT) and transient photocapacitance spectroscopy to measure the density of deep defects (ND). By comparing the changes in these quantities we are able to distinguish among many of the metastable defect reactions that have been proposed. In particular we have found conclusive evidence that quench cooling, although increasing NBTtypically by a factor of 2, does not modify NDin any of our samples. Marked changes in NBTwith negligible changes in NDare sometimes also obtained for some of the partial anneal steps following light‐soaking. These results are discussed in terms of possible defect reactions which can activate a phosphorous dopant atom without simultaneous formation of silicon dangling bonds.

ISSN:0094-243X    

DOI:10.1063/1.41019

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We report on concomitant measurements of the metastable light‐induced excess conductivity &sgr;eand of the defect density Ndin two series of n‐type a‐Si:H films doped with 30 or 300 ppm B2H6, respectively, and various volume parts of PH3. The data for &sgr;eare analyzed in terms of the light‐induced shift of the Fermi level, &Dgr;Ef. This quantity is determined by the density Neof light‐induced centers providing the negative excess charge (E‐centers) and by the density of states at the Fermi level, g(Ef). The changes of Neand g(Ef) give rise to the complicated dependence of &Dgr;Eton the exposure time teand on the Fermi‐level position, Ec−Ef, in the films. We also present data on the formation and annealing kinetics of the E‐centers. Their possible nature is discussed in the light of recent ideas on the mechanism of metastable phenomena in a‐Si:H films.

ISSN:0094-243X    

DOI:10.1063/1.41020

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The temperature dependence of the saturated light‐induced defect density (Nsat) of a‐Si:H was measured while precisely controlling sample temperature during illumination. Nsatdecreases if the sample temperature is held above 75 °C to 100 °C, and increases below 50 °C. At intermediate temperature Nsatlevels off. The two thermally activated regions of Nsatsuggest that the activation energy of the annealing process has a distribution with two peaks. In experiments geared to raising the stability of a‐Si:H, we found that Nsatis reduced to the lowest value measured so far by thermal annealing at 190 °C for 3 to 19 hours prior to light‐soaking. This result suggests that the microscopic structure of a‐Si:H plays on important role in stability.

ISSN:0094-243X    

DOI:10.1063/1.41042

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The computer program AMPS has been used to explore experimental situations for testing the validity of bimolecular and single‐carrier driven models of metastable defect generation in a‐Si:H. In this study, AMPS was used to evaluate the effect of blue laser soaking on the experimentally‐measured quantum efficiency of a‐Si:H Schottky barrier structures. Results of these simulations indicate that neither a surface degradation nor a bulk degradation model alone can satisfactorily explain the experimentally observed degradation in quantum efficiency due to blue laser soaking. These results suggest that neither a bimolecular nor single‐carrier‐driven defect generation mechanism can clearly account for the degradation experimentally observed.

ISSN:0094-243X    

DOI:10.1063/1.41043

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Arguments are presented that the performance of amorphous silicon solar cells is inadequately described by the density of deep defects in the material. In addition to the density of deep defects, carrier extraction (or fill factors) is limited by the carrier mobilities. Carrier mobilities are determined by the presence of charged defects, which will affect the bandmobility via potential‐fluctuations of the band edges. The primary degradation reaction is postulated to be the generation of additional charged defects which will affect the solar cell performance. Because temperature will establish an equilibrium between charged and neutral defects, the commonly observed increase of the spin densities or midgap defect density occurs. However, this increase is not the major cause for the inferior performance after light‐soaking. The model presented here is not a radical departure from the models used to decribe the electronic properties, but rather an attempt to provide one consistent description using well established concepts.

ISSN:0094-243X    

DOI:10.1063/1.41045

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A complete investigation of the kinetics for defect generation by Stutzmann, Jackson, and Tsai (SJT) necessitates numerical solution of the rate equations. The time where saturation of defects occurs, depends critically on temperature and activation energy for annealing. This saturation time can vary by orders of magnitude according to different experimental conditions. It is shown how high‐intensity light or 20 keV‐electron irradiation data can be explained in terms of the SJT model.

ISSN:0094-243X    

DOI:10.1063/1.41046

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Defect density as a function of light exposure time and the value of the saturation defect density in undoped hydrogenated amorphous silicon (a‐Si:H) have been measured. We varied conditions of temperature and light intensity to test the predictions of both a stretched exponential description of the kinetics and the steady‐state solution to the underlying rate equation. A stretched exponential fits the time dependence, with a stretching parameter that increases and a time constant that decreases with temperature over the range 270 to 360 K. The saturation density of light‐induced defects is both temperature and intensity dependent in the range 420–480 K for generation rates 6×1020to 2×1022cm−3sec−1. The observed decrease in saturation density with increasing temperature and increase in saturation density with increasing intensity are consistent with the model predictions.

ISSN:0094-243X    

DOI:10.1063/1.41021

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have used the photoluminescence (PL) generated in a thin‐film sample of a‐Si:H to probe low absorption levels by measuring the absorption of the PL as it travels down the length of the film in a waveguide mode. This technique, which we have called PL absorption spectroscopy of PLAS, allows the measurement of values of the absorption coefficient &agr; down to about 0.1 cm−1. Because this technique probes the top and bottom surfaces of the a‐Si:H sample, it is important to separate surface from bulk absorption mechanisms. An improved sample geometry has been employed to facilitate this separation. One sample consisted of an a‐Si1−xNix:H/a‐Si:H/ a‐Si1−xNx:H/NiCr layered structure where the silicon nitride layers served as the cladding layers for the waveguide. In a second sample the a‐Si:H layer was interrupted near the middle for two separate, thin (100 A˚) layers of a‐Si1−xNx:H in order to check for the importance of the absorption at the silicon/silicon nitride interfaces in these PLAS measurements. Changes in the below‐gap absorption on light soaking were examined using irradiation from an Ar+laser (5145 A˚, ∼200 mW/cm2for 5.5 hours at 300 K). The silicon/silicon nitride interface is responsible for an absorption which has a shoulder near 1.2 eV while the bulk a‐Si:H absorption exhibits no such shoulder. The metastable, optically‐induced increase in the below gap absorption appears to come entirely from the bulk of the a‐Si:H. These low temperature PLAS measurements are compared with those obtained at 300 K by photothermal deflection spectroscopy.

ISSN:0094-243X    

DOI:10.1063/1.41022

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We report experimental studies of metastability in the p‐layer of hydrogenated amorphous silicon p‐n junction devices. After reverse bias annealing, we observe a completely reversible metastable increase in the capacitance caused by hole emission from defects in the p‐layer. Hole recapture over an energy barrier during subsequent zero‐bias annealing restores the initial capacitance. The hole emission has a thermal activation energy of 1.3±0.2 eV and the hole capture has an activation energy of 0.89±0.1 eV. We discuss microscopic models of the metastable defect and relate our findings to other carrier‐induced metastability in hydrogenated amorphous silicon.

ISSN:0094-243X    

DOI:10.1063/1.41023

出版商:AIP    

年代:1991

数据来源: AIP