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PHOTOCAPACITY STUDY OF GRAIN BOUNDARY RECOMBINATION IN SILICON
A. Broniatowski, D. Bernard
To cite this version:
A. Broniatowski, D. Bernard. PHOTOCAPACITY STUDY OF GRAIN BOUNDARY RECOM- BINATION IN SILICON. Journal de Physique Colloques, 1989, 50 (C6), pp.C6-155-C6-155.
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REVUE DE PHYSIQUE APPLIQU~E
Colloque C6, Supplement au n06, Tome 24, Juin 1989
PHOTOCAPACITY STUDY O F G R A I N BOUNDARY RECOMBINATION I N SILICON(')
A. BRONIATOWSKI and D. BERNARD
Groupe de Physique des Solides de llE.N.S., Universite de Paris V I I , 2, Place Jussieu, F-75251 ~ a r h Cedex 05, France
Key features of the grain boundary recombination in semiconductors derive from the limitation of the electron and hole flows to the interface, due to diffusion through the potential barrier of the boundary. There follows a feed- back effect in the process of recombination, as the electrostatic potentlal in the barrier is determined in turn by the charge state of the boundary traps. To our knowledge, no satisfactory account has been given so far of t h ~ s feedback effect on the kinetics of grain boundary recombination. To obtain a consistent picture of the electronic processes taking place at the interface, one has thus to write down a set of coupled e.quations describing (I) the diffusion .of fhe carriers through the potentlal barrler of the boundar ; ii the recomblnatlon of electrons and holes at the boundary traps (Shoc~le!-kead recombination statistics)i and (iii) the relationship between the variation of the electrosta- tic potentla1 In the barrier and the occupancy of the boundary traps (Poisson equation). These equations a!low one to determine the, magnitude of the boun- dary charge and the recombination current as a functlon of the denslty of In- jected carriers. Last, by computing the current density throu h the interface
B
for a small ac applied voltage, one obtains the expression or the complex impedance of the boundar that one needs to interpret the transport properties of a bicrystal u n k r light illumination.
in order to check the validity of this approach, we have carried out an experi- mental study of grain boundary recombination b the means of hotocapacity
Yr P
measurements on an n-doped silicon bicrystal. he experimenta setup cons- ists of an optical bench fitted with a YAG laser ( 1 . 0 6 ~ ) and a defocusing dev- ice to mon~tor the densjty of minority carriers injected in the specimen. A light chopper and a lock-ln amplifier have been used to detect the photocapa- c~tance signal at low in ection levels. The measurements have been made on a bicrystal, whose boun
d
ary states had been previously characterlsed by Deep Level Translent Spectroscopy (DLTS) [I], and known to. reduce to a single level of a known density and energy location. The only f~ttrng parameters left In the model are thus the majority (electron) and the minority (hole) carrier capture cross-sections. By adjustin the theoretical master-curve to the ex- perimental data for the varlat~on o f the photocapacity vs. the density of in- jected carriers, we find for the e ctro 'c capture cross-section of the boun- dary traps the value of ( 6 * 2 ) ~ 1 0 - ~ ~ cm2. The adjustment IS found practically insensitive to the value taken for the minority carrier capture cross-section:this can be readily explained in the framework of the model, as the majorlty carrier density at the jnterface is actually less than that of the minority carriers due to. the barr~er effect of the boundary. Majority carrier trappln
%
istherefore the l ~ m ~ t l n g process of recomblnat~on at the boundary, rather t an minorit carrier trapping as the case would be for recombination centres in the
huh.
Thjs study also provides a new method of determining the recombination vel- ocrty of the bound ry. The value deduced from the hotocapac~ty measure-
R
ments is ( 9 + l ) x l 0 $ m/s. It would be interesting to c eck t h ~ s value against that obtained by experiments of a different type (Electron or Light Beam Ind- uced Current measurements), on a slmllar specimen.
[ I ] A. Broniatowski, Phys. Rev. B
x,
5895 (1987).(' ) This study is part of D. Bernard's doctoral thesis in physics.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989615
