mance (Section 5.3) and describe means by which these defect bands can be experimentally ... Figure 5.2 shows three atomic models for the disloca- tion core ...
The Electronic Structure ot
Grain Boundaries in Polycrystalline Semiconductor Thin Films LEWIS M.
FRAAS and
Chevron Research Company Richmond, California
5.1 5.2
5.3 5.4 5.5 5.6
KENNETH ZANIO Hughes Research l-aboratories Malibu, California
Introduction Internal Boundaries, Electronic Structure, and Electronic Transport l. Electronic Structure of a Dislocation 2, Electronic Structure at a Free Surface 3. Electronic Structure of a Grain Boundary Grain Boundary Effects in Polycrystalline Thin-Film Devices Measurement of Grain Boundary Structures Grain Boundary Passivation Conclusions References
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5.1
INTRODUCTION
The revolution in solid-state electronics began with a theory of a perfect periodic lattice with donor or acceptor impurities. This theory allowed the development of bulk-effect devices such as junction diodes and bipolar transistors. Next, extension of the theory of the solid state to surfaces paralleled the development of surface-effect devices such as fieldeffect transistors (FETs) and charge-coupled devices (CCDs). In these two areas, semiconductor process development led to successful commercial devices because the solid-state theory provided predictability. Polycrystalline semiconductor thin-fitm development, on the other hand, has proceeded by correlating process variables with device variables 153
P2LYCRrsTALLtNE AND nuoabuous THIN FILMS AND DEVICES
Copyright o 1980 by Academic Press, Inc. AIl rights of reproduction in any form reserved ISBN 0-12-403880-8
LEWIS M. FRAAS AND KENNETH ZANIO
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withour the aid of a theory allowing predictability. Success has been limited at best. In this paper, we take the point of view that solicl-state theory must be extendecl from the bulk periodic lattice and the external interfaces to describe the one- and two-dimensional defect arrays within polycrystalline thin films (i.e., dislocations, stacking faults, and grain bounclaries). Since these defect arrays can, and generally do, have associated electronic banding states within the bulk material energy gap, it is desirable to charactertze these states (i.e., describe their origins, inrpr,rrity interactions, energy-level positions, state densities, and effects on clevice performance). However, very little rvork has been done along these lines. But work can begin in the sense that the experimental and theoretical techniques are available. An integrated point of view is required. In this paper, we attempt to provide the required integreition b;' first clescribing the origins of the one- and two-dimensional defect bands (Section 5.2). Then, we relate the defect bands to thin-fiim device performance (Section 5.3) and describe means by which these defect bands can be experimentally measured (Section 5.4). Finally, we suggest ways ti"rat the ciefect bands rnight be modified by technology to lead to grain bounciary and dislocation passivation (Section 5.5)"
5.2
INTERNAL BOUNDARIES, ELECTRONIC STRUCTURE, AND ELECTRONIC TRANSPORT
structure of dislocations and free surfaces has developeci from experimentation over the past few years. This section begins with a review of this work. Then the theory of dislc>cation states arid surface states is generalized to describe the electronic structure of grain boundaries. This is followed by a description of electronic conduction perpendicular and parallel to grain boundaries. A later section discttsses electronic transport across a p-n junction in the region where e grain boundary intersects this junction.
A picture of the electronic
1.
Electronic Structure of a Dislocation
A dislocation is a one-dimensional periodic defect array. Dislocations can be seen experimentally in many ways. Figure 5.1 presents two microscopic photographs of dislocations. Figure 5.1a shows a cathodoluminescence [] image of dislocations in GaAs. The black lines represer-rt the dislocations. The one-dimensional periodicity is evident, as the dislocations run as far as 50 pm. Figure 5.1b is a transmission electron microsct-rpe photograph of the core region [2] of a 60o dislocation in germanium as vierved along the drslocation axis. The fringe separation in this photograph
5.
Electronic Structttre
oJ"
Grain Brtundories
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(a) Cathodoluminescence image of Te-doped GaAs. A number of dislocations are visible as black lines and dots where the recombination of electron-hole pairs induced by electron bombar
