Interfacial state and potential barrier height associated with grain boundaries in polycrystalline silicon

Interfacial state and potential barrier height associated with grain boundaries in polycrystalline silicon

Materials Science and Engineering A 462 (2007) 61–67 Interfacial state and potential barrier height associated with grain boundaries in polycrystalli...

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Materials Science and Engineering A 462 (2007) 61–67

Interfacial state and potential barrier height associated with grain boundaries in polycrystalline silicon Sadahiro Tsurekawa ∗ , Kota Kido, Tadao Watanabe Department of Nanomechanics, Graduate School of Engineering, Tohoku University, 980-8579 Sendai, Japan Received 30 August 2005; received in revised form 10 January 2006; accepted 4 February 2006

Abstract Importance of polycrystalline silicon has been recognized in the electronic device technology. The interfacial states in the band-gap and potential barrier associated with grain boundaries in polycrystalline silicon can exert their detrimental influence on electrical conductivity and then on device performance. However, all grain boundaries are not similarly potential sites for electrical activity because individual grain boundaries have their own character depending on the orientation relation between two adjoining grains. We apply the electron-beam-induced current technique and the Kelvin probe force microscopy to observe the carrier recombination intensity and the potential barrier height, respectively, at well-characterized grain boundaries in semiconductor-grade polycrystalline silicon. The results are compared with the previously observed ones in solar-grade silicon to examine the factors affecting electrical activity of grain boundaries. © 2006 Elsevier B.V. All rights reserved. Keywords: Grain boundary; Polycrystalline silicon; Electrical property; Potential barrier; EBIC; KFM

1. Introduction Recently we are facing world wide environmental and energy problems like greenhouse effect. Numerous efforts have been made so far to find ways for solving the problems since last several decades of 20th century. In particular, the technology of solar cells is a very attractive solution for both environmental and energy problems. Thus, there are strong demands to increase the efficiency of solar energy–electricity conversion. From the reason for production cost of solar cells, polycrystalline silicon is extensively used, and then the total generation power of polycrystalline solar cells is now approximately twice as high as that of single crystal solar cells [1]. However, grain boundaries can often have a detrimental effect on the performance of solar cells by reducing electrical conductivity. This is because potential barriers and defect states within the band-gap are both generated in accordance with grain boundaries. They act as barriers against carrier transportation and as preferential sites for electron–hole recombination. However, recent works on grain boundary electrical activity using the electron-beam induced current (EBIC) technique have revealed that grain boundaries

do not always act as effective recombination sites for carriers [2–8]. We have also studied grain boundary electrical activity in solar-grade polycrystalline silicon, and showed that most grain boundaries possess shallow states irrespective of grain boundary character but high- and random boundaries would be accompanied with not only shallow states but also deep states [9–12]. Although there is a long-standing controversy concerning the origin of the electrical activity [13], it is widely accepted that electrical activity associated with grain boundaries depends on grain boundary character. Nevertheless, the origin of electrical activity is still unclear whether it is intrinsic structural disorder like dangling bonds or whether it is extrinsic chemical effects resulting from impurity segregation to grain boundaries. In the present paper, we observe grain boundary recombination activity and barrier height in semiconductor-grade (sc-grade) polycrystalline silicon, and then the results are compared with the previous observations for solar-grade polycrystalline silicon to clear the factors affecting grain boundary electrical property. 2. Experimental procedures 2.1. Specimen preparation



Corresponding author. E-mail address: [email protected] (S. Tsurekawa).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.471

The material used in this study was B-doped CZ silicon (p-type) single crystal with a doping concentration of

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1 × 1015 cm−3 . The concentration of oxygen was 1.2 × 1018 cm−3 , and the concentration of impurities such as Al, Cr, Cu, Fe, Na or K did not exceed 0.1 ppm. The p-type silicon was cast from the melt and annealed for 3 h with a high-purity carbon crucible in a vacuum of 10−3 Pa to prepare polycrystalline sample. In addition, P-doped polycrystalline silicon (n-type) with a doping concentration of 1 × 1015 cm−3 was used. The samples suitable for EBIC and KFM measurements were cut using a low-speed diamond cutter, then mechanically polished with SiC waterproof papers (grade 320–1500), buff-polished with Al2 O3 paste (3–0.3 ␮m) and colloidal silica to a mirror surface. Thereafter, the specimens were etched with a mixture of HF:HNO3 :CH3 COOH in the ratio of 5:10:2 by volume for 10 s to reveal the grain boundary position. 2.2. Determination of grain boundary character The character of individual grain boundaries in polycrystalline silicon samples was determined by automated electron backscatter diffraction (EBSD) (orientation imaging microscopy (OIM)) in combination with a scanning electron microscope with field emission gun (FEG-SEM). The OIM was made with an HITACHI S-4200 FEG-SEM instrument at 20 kV accelerating voltage, 10 ␮A emission current, 15 mm working distance and 3.5–4.0 ␮m beam step size. Furthermore, a sectioning method was used to determine the orientation of grain boundary planes [14].

2.3. EBIC observations EBIC observations were conducted at temperatures ranging from 50 K to room temperature using a liquid helium cooling stage with a TOPCON DS-130 SEM operating at 20 kV and 2 nA. A computer aided EBIC system made it possible to quantitatively analyze the EBIC contrast [15]. The EBIC contrast was defined by C=

Ib − IGB , Ib

(1)

where Ib and IGB are EBIC current at background and at a grain boundary, respectively. Samples suitable for EBIC observations were chemically polished with CP4 (HF:HNO3 :CH3 COOH = 3:5:3 by volume) at 353 K for 30 s after determination of the grain boundary character by OIM. Thereafter, a Schottky contact of Al was produced on the sample surface by about 250 nm thick in an ultra high vacuum (less than 10−6 Pa). 2.4. KFM observations The potential barrier heights at grain boundaries were measured by Kelvin probe microscopy (KFM) with a SHIMADZU SPM-9500J3 atomic force microscope (AFM). Details of the KFM method were given elsewhere [16–19]. A Pt/Ir-coated cantilever with 1.8–1.9 N m−1 in the spring constant and 72–73 Hz

Fig. 1. EBIC micrographs for semiconductor-grade (sc-grade) p-type polycrystalline silicon, showing variation of electrical activity of grain boundaries of different character with temperature.

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in the resonance frequency was used for the measurements. The barrier height, VGB was evaluated by the difference between the contact potentials at the grain boundary and in the grain interior as follows [20]: VGB = V¯ GB − V¯ G ,

(2)

V¯ A + V¯ B V¯ G = , (3) 2 where V¯ GB and V¯ G are the contact potential at the grain boundary and the average of the contact potentials of two adjoining grains, respectively. The value of V¯ GB , V¯ A and V¯ B were obtained by averaging 512 data points measured at the grain boundary, grain A and grain B, respectively.

Fig. 3. Temperature-dependence of EBIC contrast of grain boundaries in solargrade polycrystalline silicon [10–12].

3. Results and discussion 3.1. Electrical activity of grain boundaries at different temperatures Fig. 1 shows EBIC micrographs for semiconductor-grade (scgrade) p-type polycrystalline silicon at different temperatures, exhibiting variation of electrical activity of grain boundaries with different characters. It is evident that EBIC contrast of grain boundaries strongly depends on their character. The random boundaries show strong contrast compared with 3 and 9 coincidence site lattice (CSL) boundaries at 50 K. The EBIC contrasts of grain boundaries decrease with increasing temperature, and then 3 and 9 boundaries both show very weak EBIC contrast at 300 K. On the other hand, random boundaries still have a strong EBIC contrast at 300 K, indicating that random boundaries are electrically active. Quantitatively analyzed EBIC contrasts of grain boundaries are shown in Fig. 2 as a function of temperature. Except of 9 case, the EBIC contrasts of grain boundaries monotonically increase with decreasing temperature irrespective of their character. It is found that EBIC contrast is approximately two to three times as high at the random boundary as at 3 and 9 CSL boundaries in the whole range of temperatures. The EBIC contrasts of these CSL boundaries are less than 5% at 300 K and 10% at 50 K. In addition, there is little difference among the ¯ EBIC contrasts of {1 1 1}3, {1 1 2}3 and (1 1 1)/(5 1¯ 1)9 boundaries.

Fig. 2. EBIC contrast of grain boundaries with different character as a function of temperature for sc-grade polycrystalline silicon. Legend: () {1 1 2}3, () {1 1 1}3, () 9, and (䊉) random boundaries.

Fig. 3 presents the temperature-dependence of EBIC contrasts of grain boundaries in solar-grade polycrystalline silicon reported so far [10–12]. It is found that EBIC contrasts of these boundaries in solar-grade silicon are considerably stronger than those in sc-grade silicon. In addition, they exhibit a peak around 100 K irrespective of their characters, whereas those in sc-grade silicon monotonically increase with decreasing temperature as shown in Fig. 2. The peak temperature is found to be higher by approximately 30 K for the random boundary than for low-angle and CSL boundaries. On the basis of the Shockley–Read–Hall statistic [21,22], the temperature dependence of recombination intensity of minority carriers was predicted for different levels of the defect states in the band-gap by Kusanagi et al. [23]. They showed that the recombination intensity increases with decreasing temperature and has a peak at a certain temperature. Of particular importance is the finding that the peak temperature shifts to higher temperature region according as the energy level of defect states approaches the mid-gap energy (Fig. 4 [23]). Judging from the different features of temperature-dependences of EBIC contrasts of grain boundaries in solar-grade silicon, we can see that the grain boundary in solar-grade silicon would be accompanied with a shallow defect state in the range of energy level from Ev +60 meV to Ev +100 meV, where Ev is the energy level of the valence band. Probably, the energy of defect states is slightly deeper at random boundary than at coincidence boundaries. Since the sc-grade silicon is likely to have

Fig. 4. Temperature-dependence of recombination intensity of minority carriers for different levels of the defect states in band-gap [23].

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a peak in EBIC contrast at a temperature less than 50 K, we can consider that impurity contamination probably shifts the defect states associated with grain boundaries towards deeper energies. Furthermore, impurity contamination will also increase the density of grain-boundary defect states because the EBIC contrasts of grain boundaries are much higher in solar-grade silicon than in sc-grade silicon. In addition, one can find that difference between the EBIC contrasts of CSL boundaries and random boundaries becomes more pronounced as impurity concentration decreases. The behavior dependent on impurity concentration is similar to that observed in the studies of intergranular fracture [24,25] and grain boundary energy [26]. 3.2. Influence of the deviation angle from exact coincidence relation on electrical activity of grain boundaries If the misorientation angle deviates from the exact coincidence relation, a regular array of DSC dislocations would be introduced to accommodate the deviation, following the equation D = b/θ, where D is the spacing of DSC dislocations, b the magnitude of Burger’s vector for DSC dislocations and θ is the deviation angle from the exact coincidence relation. These DSC dislocations would be an intrinsic origin of electrical activity of grain boundaries. Fig. 5 shows the change in EBIC contrast of

Fig. 5. Change in EBIC contrast of grain boundaries in sc-grade silicon as a function of the deviation angle from the exact coincidence relation. Legend: () 3, () 9, () 5 and () 27 boundaries.

grain boundaries in sc-grade silicon according to the deviation from the exact coincidence relation. It was found that EBIC contrast of grain boundaries increases with increasing the deviation angle, namely with increasing the density of DSC dislocations. Because the nature of temperature-dependence of EBIC contrast does not change according to the deviation angle, DSC dislocations will be accompanied with shallow states, and then they are the origin of electrical activity. Thus, an increase of the density of DSC dislocations involves an increase of EBIC contrast.

Fig. 6. Observations of grain-boundary potential barriers in B-doped p-type (a and b) and p-doped n-type (c and d) polycrystalline silicon by KFM.

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HRTEM observations have revealed that dangling bonds at {1 1 1}3 and {1 1 2}3 boundaries in silicon and germanium can be reconstructed [27–29]. Kohyama et al., who used a semi-empirical tight-binding method, have reported that no deep states or band tails are observed in the band-gap on the reconstructed {1 1 2}3 boundary [30]. Accordingly, electrical activity observed at these boundaries would be attributed to the DSC dislocations on the boundaries. 3.3. Measurement of grain boundary potential barrier height by KFM Fig. 6 [20] shows examples of observations of potential barriers at grain boundaries in p-type (a and b) and n-type (c and d) sc-grade silicon. As for the contact potential, the higher is the contact potential, the brighter is the KFM contrast. It is found for p-type silicon that the contact potential is higher at grain boundaries than in the grain interior. This is because downwards band bending occurs in the vicinity of the grain boundaries in p-type silicon. On the other hand, the contact potential is lower at grain boundaries than in the grain interior for n-type silicon. Of particular importance is the finding that the KFM contrast depends on the grain-boundary character. Random boundaries (R) display greater contrast than a 9 coincidence boundary. Fig. 6(a) also demonstrates that a {1 1 1}3 (coherent twin) boundary shows weak KFM contrast, while a {1 1 2}3 (incoherent twin) boundary appears as a brighter line in the image. Thus, it is evident that the grain-boundary potential height depends on the inclination of the grain-boundary plane as well as on the grain-boundary character. The potential barrier heights at individual grain boundaries in p-type silicon were quantitatively evaluated on the basis of Eqs. (2) and (3), and are plotted as a function of misorientation angle in Fig. 7. The measured grain-boundary potential heights are lower than 100 meV, in good agreement with the values reported by Nabhan et al. [31] and with the EBIC observations mentioned in Section 3.1. Here, it should be noted that the potential barrier height varies with boundary misorientation

Fig. 7. The relationship between grain-boundary potential barrier height and misorientation. Legend: () 3, () 9, () 27 and (䊉) random boundaries. The dashed lines in the figure show the average values of the barrier heights of random and low-energy special boundaries. The Miller indices beside the data points show the orientation of grain boundary planes with respect to the lattice.

angle. The low- coincidence boundaries characterized as 38.9◦ (9) and 60.0◦ ({1 1 1}3) show lower potential barrier ranging from 10 to 50 meV, while most random boundaries have higher potential barriers ranging from 60 to 80 meV. The average value of the potential barrier heights for random boundaries is almost twice as high as that for low- coincidence boundaries. The dependence of the barrier height on the grain-boundary character would arise from different levels of defect states in the band-gap. As seen in Fig. 7, {1 1 2}3 boundaries have potential barrier heights similar to random boundaries. Although the reason is not yet clear, the electrical activity may be affected by the junction between {1 1 1}3 coherent and {1 1 2}3 incoherent twin boundaries because the observed {1 1 2}3 boundaries were

Fig. 8. EBIC micrographs for sc-grade silicon showing that segments of grain boundaries near triple junction and short segment of facetted grain boundaries both are at origin of strong electrical activity, particularly at room temperature.

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Fig. 9. The potential barrier heights measured near a triple junction (TJ) and at a short segment (S) of facetted grain boundaries. The Miller indices in the figure show the orientation of grain-boundary planes with respect to the lattice. Legend: () 3, () 9, () 27 and (䊉) random boundaries.

fairly short compared with {1 1 1}3 boundaries. The influence of triple junction and boundary irregularity on grain boundary electrical activity will be described in the following section. 3.4. Influence of triple junction on grain boundary electrical activity It has been reported that grain boundaries often possess an unstable structure due to the restriction of atomic relaxation from triple junctions and grain-boundary irregularities [32]. These sites could exert an influence on electrical activity of grain boundaries. Buis et al. [3] reported that a high density of microfacetting with a scale of approximately 50 nm can be sites for electrical activity, as revealed by the EBIC technique. We have also observed that triple junctions and corners of facetted boundaries exhibit strong EBIC contrast [9,12], as shown in Fig. 8 [12]. Furthermore, extremely high potential barriers, 250–500 meV, were often observed at the segment of grain boundaries near triple junctions (TJ), while most barrier heights at grain boundaries are lower than 100 meV. It was also found that facetted 3 boundaries (particularly a short incoherent grain boundary) possess extremely high potential barriers as well. These values obtained from the grain boundaries near triple junction (TJ) and from a short boundary (S) are plotted in Fig. 9. 4. Conclusions We have observed carrier recombination intensities and potential barriers associated with grain boundaries in sc-grade polycrystalline silicon by EBIC and KFM, respectively, and discussed the results in terms of the grain-boundary character. The chief results obtained are as follows:

(i) Grain boundaries in solar-grade silicon and sc-grade silicon both generate shallow states in the band-gap irrespective of their characters. The energy of defect states associated with grain boundaries are not higher than Ev +100 meV for both solar-grade and sc-grade silicon. The defect states are slightly deeper at random boundary than at CSL boundaries. (ii) Impurity contamination not only shifts the defect states associated with grain boundaries towards deeper energies but also increases the density of their states. (iii) The difference between EBIC contrasts of CSL boundaries and random boundaries becomes more significant when the concentration of impurities decreases. (iv) The EBIC contrast of grain boundaries increases with increasing the deviation from the exact coincidence relation. (v) The potential barrier heights of grain boundaries in scgrade polycrystalline silicon vary in the range 10–100 meV according to the grain boundary character. Random boundaries possess barrier heights almost twice as high as coincidence boundaries. The potential barrier height was found to depend on the grain boundary inclination as well as on its character. (vi) Both grain boundary segments near triple junctions and short segments associated with facetted boundaries exhibit extremely high potential heights ranging from 250 to 500 meV. Acknowledgements The authors would like to express their thanks to Prof. T. Sekiguchi and Dr. J. Chen (National Institute for Materials Science, Japan) for their help with EBIC observations. References [1] PV News, April (2006). [2] C.R.M. Grovenor, J. Phys. C: Solid State Phys. 18 (1985) 4079. [3] A. Buis, Y.-S. Oei, F.W.C. Schapink, Trans. Jpn. Inst. Met. Suppl. 27 (1986) 221. [4] G. Poullain, A. Bary, B. Mercey, P. Lay, J.-L. Chermant, G. Nouet, Trans. Jpn. Inst. Met. Suppl. 27 (1986) 1069. [5] M. Aucouturier, A. Broniatowski, A. Chari, J.L. Maurice, Springer Proc. Phys. 35 (1989) 64. [6] N. Tabet, C. Monty, Phil. Magn. B57 (1988) 763. [7] R. Rizk, G. Nouet, Interf. Sci. 4 (1997) 303. [8] J. Chen, T. Sekiguchi, D. Yang, F. Yin, K. Kido, S. Tsurekawa, J. Appl. Phys. 96 (2004) 5490. [9] Z.J. Wang, S. Tsurekawa, K. Ikeda, T. Sekiguchi, T. Watanabe, Interf. Sci. 7 (1999) 197. [10] S. Hamada, K. Kawahara, S. Tsurekawa, T. Watanabe, T. Sekiguchi, Mater. Res. Soc. Symp. Proc. 586 (2000) 163. [11] S. Tsurekawa, S. Hamada, M. Shibata, K. Kawahara, T. Watanabe, T. Sekiguchi, Ann. Chim. Sci. Mater. 27 (Suppl. 1) (2002) S255. [12] S. Tsurekawa, K. Kido, S. Hamada, T. Watanabe, T. Sekiguchi, Z. Metallkd. 96 (2005) 197. [13] J.L. Maurice, Rev. Phys. Appl. 22 (1987) 613. [14] V. Randle, The Measurement of Grain Boundary Geometry, Institute of Physics Publishing, Bristol and Philadelphia, 1993, p. 86. [15] T. Sekiguchi, K. Sumino, Rev. Sci. Instrum. 66 (1995) 4277.

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