EBIC MEASUREMENTS OF A TRIPLE JUNCTION IN POLYCRYSTALLINE SILICON

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EBIC MEASUREMENTS OF A TRIPLE JUNCTION IN POLYCRYSTALLINE SILICON

A. Bary, G. Nouet

To cite this version:

A. Bary, G. Nouet. EBIC MEASUREMENTS OF A TRIPLE JUNCTION IN POLY- CRYSTALLINE SILICON. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-423-C1-428.

�10.1051/jphyscol:1990165�. �jpa-00230332�

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COLLOQUE DE PHYSIQUE

Colloque Cl, suppl6ment au n0l, Tome 51, janvier 1990

EBIC MEASUREMENTS OF A TRIPLE JUNCTION IN POLYCRYSTALLINE SILICON

A. BARY and G. NOUET

Laboratoire d l E t u d e s et de Recherches sur les Materiaux, CNRS URA 1317, Institut des Sciences d e la Matiere et du Rayonnement, Boulevard du Marechal Juin, F-14032 Caen Cedex, France

Resume

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Une jonction triple isolee dans le silicium polycris- tallin Polix-Photowatt a ete examinee par MET et EBIC. Cette jonction triple est constituee dtun sous-joint, d'un joint proche d'une orientation de coincidence et d'un joint general.

Ces trois defauts contiennent des precipites. Par mesure quan- titative du signal EBIC il est montre que la vitesse de recom- binaison au niveau de ces differentes interfaces est propor- tionnelle a la densite de precipites.

Abstract

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A triple junction cut from a polycrystalline silicon ingot has been analyzed by TEM and EBIC. This triple junction is made up of a low angle grain boundary, a near coincidence grain boundary and a general grain boundary. These three defects contain precipitates. It is shown that the recombination velocities measured from the EBIC signal are proportional to the precipitate density.

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INTRODUCTION

In polycrystalline silicon the electrical activity of the geometri- cal defects such as grain boundaries or sub-grain boundaries can be detected by Electron Beam Induced Current (EBIC) measurements. This technique has been widely applied to characterize the electrical behaviour of semiconductors because of the straightforward sample preparation. This sensitive method to determine the active defects requires a collection structure of the minority carriers. A p-n junction or a Schottky barrier at the surface provides this possi- bility of collect.

By observation in a Transmission Electron Microscope (TEM) of the thinned specimen, identification of the structural nature of zones showing EBIC contrasts can be obtained. This procedure enables the EBIC signal and the structural information to be correlated. With this method different types of dislocations have been investigated and so their electrical activity has been classified /l-2,'. More recently some attention has been paid to the effects of segregation and precipitation at the grain boundaries / 3 / . So, it has been shown that after thermal treatments E 2 5 silicon bicrystals exhibit an dotted EBIC contrast. Moreover, the distance between these black dots is of the same order of magnitude that the distance measured between the precipitates imaged by TEM / 4 / . On the same type of ticrystals, the evolution of EBIC contrast has been analyzed accor- ding to different thermal treatments and the nature of the precipitates has been specified /5-7/. Some unexpected EBIC behaviour of 29 7rain boundaries has also been explained by considering selective

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990165

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Cl-424 COLLOQUE DE PHYSIQUE

segregation phenomena /8/. In most cases only qualitative evaluation of the electrical activity of the grain boundaries has been carried out. Subsequently Donolato /9/ has proposed a modelling of the EBIC contrast in order to determine the local diffusion length and recombination velocity. In this paper we are dealing with the EBIC contrast analysis of three grain boundaries, one low angle and two high angle grain boundaries, intersecting at a triple junction.

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^EBIC^ANALYSIS

Among the different types of charge collection signal, EBIC measurements have shown an increasing development in the electrical charac- terization of polycrystalline silicon solar cells. In this technique, electron-hole pairs are generated by scanning the semiconductor surface with an electron beam. The incident electron beam has typi- cally tens of keV energy and consequently thousands of electron-hole pairs can be generated in silicon since tte formation energy of these pairs is about three times the gap energy (E 3.6 eV).

If an internal electric field is applied to the specimen, the carriers generated within it are swept out in opposite directions so that steady state is established. Charge collection can occur at barriers such as p-n junctions or Schottky barriers.

Inhomogeneities or localized defects are revealed in EBIC mode as dark regions arising from recombination of generated carriers on the traps. From the experimental profile perpendicular to the grain boundary the recombination velocity and diffusion length can be cal- culated. Donolato /9/ has shown that the suitable parameters are the area and variance of the EBIC profile. Charge collection efficiency,

q (E), as a function of the beam incident energy can be used to determine the diffusion length at the vicinity of the defect /10/.

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EXPERIMENTAL

The specimen was cut from a polycrystalline silicon ingot (Polix, Photowatt). The directional solidification leads to a columnar structure with grains larger than one centimeter. This specimen has been analysed in EBIC mode after POC13 diffusion so that a n+p junction parallel to the surface is made at 0.4 pm deep. The accelera- ting voltage is 30 kV and the depth distribution of pairs is maximum at 7 pm. The doping rate is approximatly 1 0 ' ~ c m - ~ and the low injec- tion condition is valid if the incident beam intensity is lower than 10 nA.

After EBIC analysis, the specimen was thinned by ion-milling ( ~ r + , 6 kV) for observation in TEM. The orientation relationships between adjacent grains were performed from the diffraction patterns.

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RESULTS AND DISCUSSION

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^EBIC^azalysis

The EBIC image of Fig. la shows the three different EBIC contrasts : Fig. 2 gives the three profiles used to calculate the corresponding

diffusion lengths and recombination velocities (Table 1).

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Fig. la : TEM image of the triple junction

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^b^{E B I C}contrasts of three different grain boundaries (A . : coincidence graing boundary deviating from 1 2 1 ; B : general grain boundary ; C : 13' low angle grain boundary).

Table 1 : Diffusion length and recombination velocity of the three interfaces.

We csn already notice that the diffusion lengths measured in the same grain by using the two limiting grain boundaries are different whereas the recombination velocity values are rather equal. Since this last parameter is an intrinsic characteristic of the grain boundary, this result makes comparable the values associated with each interface. So it is clear that the minority carrier recombination is not similar for each interface.

C 3 15.5

6.3 Grain boundary

Grain

Diffusion length L (pm) Recombination

velocity V, X 10~cm.s"

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TEM analysis

This triple junction is consisting of a grain boundary deviating from the E21 coincidence orientation (A interface), of a general grain boundary (B interface) and of a low angle grain boundary (C 'nterface) (Fig. lb). The deviation from the Z21 coincidence orien-

A 1 10 24.2

A 2 9.5 29.5

C 2 12.5

7.5 B

1 15 11.3

B 3 12 10.1

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COLLOQUE DE PHYSIQUE

E B l C V, = 30

k v ;

Ib =0.4nA

1'

104% ;11.3-.8 lW4Vs. l 0 1 a-

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P i g . ² : EBIC p r o f i l e s of t h e t h r e e i n t e r f a c e s A , B and C.

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F i g . 3 : TEM image showing the different densities of precipitates in the three boundary planes.

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Cl-428 COLLOQUE DE PHYSIQUE

tation is compensated by a secondary dislocation network which can be seen in Fig. 3a. The rotation angle of the low angle grain boundary is about 13".

In fact, the common feature of these three interfaces is the decora- tion of the grain boundary planes with some precipitates (Fig. 3).

Although the nature of these precipitates has not been identified, they can be considered as the origin of the EBIC contrast. An evaluation of the precipitate density for each interface shows a good correlation with the recombination velocity (table 2).

Table 2 : Recombination velocity of the three grain boundaries as a function of the precipitate density.

Grain boundary Precipitate density

pm- l

Recombination velocity V, X 10~crn.s-'

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CONCLUSION

EBIC and TEM analyses of a triple junction have shown that the recombination velocity of minority carriers at grain boundaries is proportional to the density of precipitates decorating these three interfaces.

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