Inadequacy of the Conventional View of Hydrogenated Amorphous Silicon
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INADEQUACY OF THE CONVENTIONAL VIEW OF HYDROGENATED AMORPHOUS SILICON
M. P. SHAW,', AND V. CANNELLA" M. SILVER, D. ADLER, Massachusetts Institute of *Center for Materials Science and Engineering, Technology, Cambridge, MA 02139 tDepartment of Electrical Engineering and Computer Science, Wayne State University, Detroit, MI 48202 ftOvonic Display Systems, Troy, MI 48084
ABSTRACT The conventional view of the electronic structure of hydrogenated amorphous silicon is: (1) the material is characterized by a mobility gap of about 1.8 eV, with exponential band tails due to disorder and deep defect states arising from silicon dangling bonds (T 3 centers); (2) substitutional doping occurs because of the formation of chargedimpurity/dangling-bond pairs, e.g. P + - T3 , at the substrate temperature; (3) the effective correlation of the T 3 center is about 0.4 eV; (4) T3° centers are the predominant recombination center; (5) the three intrinsic 0 ESR signals are due to electrons on T 3 centers, electrons in the conduction band tail, and holes in the valence band tail. It is the purpose of this paper to demonstrate that this model is in sharp disagreement with an array of basic experimental data, and much of the evidence presented in its favor is based on self-inconsistent logic. We conclude that it is very likely that large concentrations of charged intrinsic defect pairs are present in all hydrogenated amorphous silicon films. RELATIVELY NON-COROVERSIAL PROPERTIES OF ffYDROGEUATJI
AMORPHOUS SILICON
Although many of the properties of hydrogenated amorphous silicon (a-Si:H) are sample and even history dependent, several results appear to be universal. These include: (1) the material is a semiconductor with a mobility gap near 1.8 eV [11; (2) undoped samples are n-type, with the low-temperature Fermi energy, EF, typically about 0.6 eV below the conduction band mobility edge, Ec [2]; (3) doping with phosphorus increases EF monotonically with concentration, [P], until it saturates about 0.2 eV below Ec [2]; (4) doping with boron decreases EF monotonically with [B], saturating about 0.2 eV above the valence band mobility edge, Ev; the samples become p-type when EF drops below mid-gap [2]; (5) Arrhenius plots of the electrical conductivity as a function of temperature are relatively linear below 400 K [31; (6) the conduction and valence band tails are exponential, and of the form:
with Tc (7) appears appears samples
g(E) = go(E) exp [-(Ec - E)/kTc]
(1)
g(E) = gv(E) exp [-(E -
(2)
Ev)/kTv]
Z 300 K and Tv z 500 K [4]; three intrinsic ESR signals are observed one with g = 2.0055 that in undoped and lightly doped samples one with g = 2.004 that in n-type samples, and one with g = 2.01 that appears in p-type [5].
Mat. Res. Soc. Symp. Proc. Vol. 70. '1986 Materials Research Society
114
On less firm ground are the observations of localized peaks in the gap, arising from defect centers. The vast majority of reports are consistent with a peak about 0.9 eV below Ec in n-type samples [6]. In addition, both undoped and doped sampl
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