Depth profiling of vacancy-type defects in homogenous and multilayered a-si films by positron beam Doppler broadening

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1 Journal of Non-Crystalline Solids Depth profiling of vacancy-type defects in homogenous and multilayered a-si films by positron beam Doppler broadening X. Zou a,), D.P. Webb a, Y.C. Chan a, Y.W. Lam a, Y.F. Hu b, M. Gong b, C.D. Beling b, S. Fung b a Department of Electronic Engineering, City UniÕersity of Hong Kong, Tat Chee AÕenue, Kowloon, Hong Kong, Peoples Republic of China b Department of Physics, UniÕersity of Hong Kong, Pokfulam Road, Hong Kong, Peoples Republic of China Abstract Positron annihilation measurements have been carried out on a-si:h thin films deposited by plasma enhanced chemical vapour deposition at a high rate and on pinrpin double-junction diodes prepared conventionally by means of the variable-energy positron beam Doppler-broadening technique. The depth profiles of microvoids in films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the interior. The depth profiles of microvoid-like defects in the a-si:h films were extracted by use of the VEPFIT programme. Variable-energy positron-annihilation spectroscopy results on the diode show that the interfaces are of good quality except for the Aurn interface, consistent with the diode characteristic. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Depth profile; a-si:h; Positron-annihilation spectroscopy 1. Introduction The origin of metastability in amorphous Si has not been unambiguously identified. Microvoids have been suggested as metastability centres in a-si:h, especially in a-si:h films deposited at the greatest rates. Therefore, attention should be focused on microvoid characterisation. Of most defect characterisation techniques, a newly emerging technique, variable-energy positron-annihilation spectroscopy Ž VEPAS., is sensitive to vacancy-type defects in thin films and has ) Corresponding author. Tel.: q ; fax: q ; eeycchan@cityu.edu.hk. been successfully used to reveal the presence of vacancy-like defects such as clusters or vacancy-imwx 1. Extant positron annihilation purity complexes studies on a-si:h have utilised lifetime spectroscopy measurements w2,3 x. With regard to depth-resolved studies employing variable-energy positron beams, there have been few reports of attempts to depth profile and measure the microvoids in a-si:h using VEPAS. In this paper, we report on a VEPAS investigation of a-si:h thin films deposited at a deposition rate ) 1.5 nmrs, and double-junction pin diode prepared at a conventional deposition rate of about 0.1 nmrs, producing information on the depth profile of open volume defects, i.e., microvoids r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S

2 106 ( ) X. Zou et al.rjournal of Non-Crystalline Solids Experimental details The samples in this work were 1 to 2 mm thick intrinsic a-si:h thin films grown by pure silane Ž SiH. 4 RF glow-discharge plasma enhanced chemical vapour deposition Ž PECVD. at a deposition rate of 1.5 nmrs on quartz substrate, and a double-junction a-si:h pinrpin diode deposited by conventional glow discharge and covered with a 400 nm thick Au electrode from which side the positrons were implanted. The configuration was Aurn q i p q rn q i p q ritorglass. For deposition of the homogenous films, the RF power was varied between 20 and 40 W, and the substrate temperature varied between 300 and 3308C. The ratio of photoconductivity and dark conductivity of the a-si:h films is about 5 orders. The performance of the diode was good, as shown by the properties in Fig. 1. The positron beam annihilation experiments were performed using a mono-energetic positron beam of intensity 5=10 4 e q s y1 over an energy range of 0.15 to 25 kev in steps of 250 ev, details of which have been reported elsewhere wx 4. The variation of beam energy allows control over the mean depth of positron implantation into the samples. In general, the mean penetration depth of the positrons implanted at energy E can be determined from the following power law wx 1 x0 sae n Ž 1. where the constant A in Eq. Ž. 1 has been found n empirically to be A;400rr ArkeV, r being the 3 sample density in grcm, x is in A, 0 E is in kev, and the power nf1.6 for most materials w5,6 x. Thus for a-si:h Ž r s 2.2 grcm 3. the maximum positron beam energy, 25 kev, employed in the present study corresponds to a mean positron stopping depth of about 3 mm. This energy range was chosen so that at a certain positron implantation energy all positrons essentially annihilate in the a-si:h film while at the highest energies almost full penetration into the quartz substrate could be achieved, thus allowing depth profiling across the entire a-si:h film. The 511 kev annihilation g radiation from the sample was detected with a Ge detector of resolution 1.4 kev at 514 kev and a digitally stabilised multichannel analyser system. A total of 1=10 6 counts were collected under the annihilation photopeak of the g-ray for each positron energy. The photopeak line shape of the g-ray was described using the conventional valence and core annihilation parameters, S and W, which are the ratios of counts in the central and wing portions of the annihilation photowx 1 peak to the total counts in the peak, respectively. 3. Results Fig. 1. I V characteristic of a-si:h pinrpin double-junction diode. Positrons are trapped at neutral and negative vacancy defects because the missing positive charge of the ion cores causes the positron to experience a potential well. With increasing number of microvoids, the valence annihilation parameter, S, increases and the core annihilation parameter, W, decreases. A trapped positron at a vacancy overlaps mainly with the core electrons of the nearestneighbour atoms. Hence the shape and magnitude of

3 ( ) X. Zou et al.rjournal of Non-Crystalline Solids the core electron momentum distribution can be used to identify the vacancy acting as a positron trap. Assuming a uniform defect profile in an a-si:h thin film, the positron trapping rate in the defects is related to the effective positron diffusion length by wx 1 mc s yl Ž 2. vac D q 2 L q,eff b where Dq is the positron diffusion coefficient, lb the free-positron annihilation rate, m the specific trapping rate by vacancies, Cvac the vacancy concen- tration, and Lq,eff the effective diffusion length of positron in the material. This is a useful equation for evaluating the density of defects in a-si:h, since Lq,eff can be experimentally determined. In this study, we assume that the annihilations occur either in the a-si:h layer, in the quartz substrate, at the surface of the a-si:h for the homogenous film or at the surface of the Au electrode for the diode, or as epithermal positrons. Each annihilation state has a distinct S-parameter. The total measured S-parameter is then given by Ss FS Ž 3. Ý i i i with Fi the annihilation fraction of positrons in the ith layer. Based upon Eq. Ž. 3, we used VEPFIT to perform the fitting procedure. Fig. 2 shows the positron line-shape parameter, S, measured vs. incident energy for a-si:h two films prepared under different process conditions. The same general trend is observed in both samples. The results clearly demonstrate that there is a depth variation in microvoid volume fraction. A deposition condition dependency can also be seen. The S parameters vary from about to at low positron implanting energy. This variation is attributed to positrons annihilating within the surface region. As the implanting energy increases, more positrons annihilate in the interior of a-si:h and this location results in an increase of the S-parameter to about 0.583, which may be identified as characteristic of the a-si:h. The S-parameter decreases with further increase in positron implanting energy as more positrons annihilate in the quartz substrate and thus finally approaches the magnitude characteristic of bulk quartz for all samples. It can be easily seen that for the a-si:h films deposited at the greatest rate with different RF power and substrate temperature, the microvoid profile appears the same in the interior, but obvious differences exist in the surface region for positron beam incident energies below 1 kev, corresponding to about 18 nm according to Eq. Ž. 1. It seems that larger RF power coupled with higher substrate temperature is of benefit in eliminating microvoids within Fig. 2. Doppler-broadening line-shape parameter S as a function of incident positron energy for high-rate deposited a-si:h thin film with different RF powers and substrate temperatures. The filled circle curve is for an RF power of 40 W and a substrate temperature of 3308C, and the open circles for an RF power of 20 W and a substrate temperature of 3008C. Solid and dash lines show VEPFIT fits to the data.

4 108 ( ) X. Zou et al.rjournal of Non-Crystalline Solids the surface region of the greatest rate deposited a-si:h. This elimination is mainly due to the relative concentrations of monosilicon radicals when the RF glow discharge is off. As is well known, the number of vacancies is closely correlated with the SiH 2 density in the plasma wx 7. At larger RF power, the amounts of radical SiH 2 will decrease, which is conducive to eliminating microvoids within the surface region of a:si:h, PECVD grown at the greater rates. 4. Discussion The density of microvoids in a-si:h can be found using Eq. Ž. 2. The effective diffusion length of positrons in the surface region and in the interior of a-si:h respectively were found to be, by VEPFIT, ; 19 nm and ; 11 nm for 40 W deposited and ;17 nm and ;11 nm for 20 W deposited a-si:h thin films. Adopting as the value for the specific positron trapping rate for vacancy defects to be that wx 14 y1 in crystalline Si 8, ms3=10 s, we derived defect concentrations from the S-parameter data of 2.1" 0.5 = 10 atomic fraction Ž at.fr.. in the sur- face region and 6.5"0.5 =10 at.fr. in the bulk for the 40 W, and 2.5"0.5 =10 at.fr. in the surface region and 6.2" 0.5 = 10 at.fr. in the bulk for the 20 W deposited sample. Again using c-si data the diffusion coefficient of positrons was taken as Ds2.2 cm 2 rs. The annihilation rate, corresponding to c-si divacancies, was taken as l b s 9 y = 10 s wx 9. The evaluated results clearly demonstrate the effect of RF power on the production of microvoids in the near surface region of an a-si:h thin film. Eq. Ž. 2 shows that the position-dependent diffusion length, LŽ x., is a primary source of information about the defect concentration profile, C Ž x. vac, inan a-si:h thin films. To fit the S parameter profiles, a 5-layer model, consisting of a surface layerža- Si:H. r3 interior layers Ž a-si:h. and a quartz substrate layer, was adopted. Fig. 3 presents the depth, L q,eff, and defect concentration profiles of a sample deposited at 20 W and 3008C. A variation at the interface between the surface layer Ž ; 13 nm. and one of the interior layers can be seen from the figure, and a lower defect concentration exists in the initially deposited material to a thickness of ; 1000 nm. The increase in microvoid concentration at the interface indicates that the structure of the a-si:h thin film takes a few atomic layers to stabilise as a Fig. 3. Extraction of the position-dependent effective diffusion length Lq,eff and the depth profile of defects concentration, versus positron penetration depth, in an a-si:h homogenous film Ž 20 W, 3008C.. Lines are drawn as guides for eyes.

5 ( ) X. Zou et al.rjournal of Non-Crystalline Solids Fig. 4. The lineshape parameter S as a function of incident positron energy for a-si:h double-junction diode. result of lattice mismatch, which is an implication of the interface effects possibly caused by the mismatch between the networks of the a-si:h thin film and the quartz substrate. VEPAS was also carried out on an a-si:h double-junction pinrpin diode. Fig. 4 shows the measured data and VEPFIT fit. The measured S parameter is about at smaller positron implanting energies. This magnitude is attributed to positrons annihilating on or close to a surface. As the implanting energy increases, more positrons annihilate in the Au overlayer and this annihilation results in a de- Fig. 5. The SrW parameter as a function of incident positron energy for a-si:h double-junction diode.

6 110 ( ) X. Zou et al.rjournal of Non-Crystalline Solids crease of the S-parameter to about The S- parameter increases with increasing positron implanting energy as more positrons annihilate in the interior of the diode. The S-parameter moves towards a valley corresponding to the p q rito interface, increases thereafter and finally approaches a magnitude corresponding to the ITOrglass interface. Thus the internal interfaces of the double-junction pinrpin diode do affect the S-parameter, indicating there is no large density of voids at the interfaces and hence that the interfaces are of good quality, in agreement with the conclusion drawn from the diode properties of Fig. 1. To confirm the results above, an SrW-E plot has been drawn in Fig. 5, The SrW ratio can be used to distinguish the surface annihilation mode from the interior annihilation mode more clearly than just S, because the surface and interface annihilation modes give a very small W. SrW is approximately constant, except at the Aurn q interface, in particular at the p q rito interface, again implying good internal interface quality. 5. Conclusions By means of the slow positron beam Dopplerbroadening technique, the depth profiles of microvoids in the films have been resolved. It was found that for the homogenous films there is a smaller void fraction in the surface layer, to a depth of approximately 18 nm, than there is in the interior. The depth profile of defect concentrations in a-si:h were extracted by VEPFIT fitting, which indicated an approximately uniform density of defects throughout the films, but with an increased concentration at both the near surface region and the substrate interface due to a network mismatch. VEPAS results on the pinrpin double-junction diodes indicate that the diode interfaces are of good quality, except for the Aurn interface, in agreement with conclusions drawn from the diode properties. Acknowledgements The authors would like to acknowledge the financial support from the Universities Grants Council of Hong Kong Government ŽCERG Project No References wx 1 P.J. Schultz, K.G. Lynn, Rev. Mod. Phys. 60 Ž wx 2 S. Dannefaer, D. Kerr, B.G. Hogg, J. Appl. Phys. 54 Ž wx 3 A.L. Jung, Y.H. Wang, G. Liu, J.J. Xiong, B.S. Cao, W.Z. Yu, D. Adler, J. Non-Cryst. Solids 74 Ž wx 4 C.D. Beling, S. Fung, H.M. Weng, C.V. Reddy, S.W. Fan, Y.Y. Shan, C.C. Ling, American Institute of Physics, Conference Proceedings Series 303 Ž p wx 5 A.P. Mills Jr., R. Wilson, Phys. Rev. A 26 Ž wx 6 A. Vehanen, K. Saarinen, P. Hautojarvi, H. Huomo, Phys. Rev. B 35 Ž wx 7 W. Luft, Y. Simon Tsuo, Hydrogenated Amorphous Silicon Alloy Deposition Processes, Marcel Dekker, New York, Ž p. 15. wx 8 S. Dannefaer, G.W. Dean, D.P. Kerr, B.G. Hogg, Phys. Rev. B 14 Ž wx 9 E. Soininen, J. Makinen, D. Beyer, P. Hautojarvi, Phys. Rev. B 46 Ž