Porous silicon implanted with nitrogen by plasma immersion ion implantation

Porous silicon implanted with nitrogen by plasma immersion ion implantation

Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 224±228 www.elsevier.nl/locate/nimb Porous silicon implanted with nitrogen by p...

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Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 224±228

www.elsevier.nl/locate/nimb

Porous silicon implanted with nitrogen by plasma immersion ion implantation A.F. Beloto a

a,* ,

M. Ueda b, E. Abramof a, J.R. Senna a, N.F. Leite a, M.D. da Silva a, H. Reuther c

Laborat orio Associado de Materiais e Sensores (LAS), Instituto Nacional de Pesquisas Espaciais (INPE), S.J. Campos, S. Paulo, Brazil b Laborat orio Associado de Plasma (LAP), Instituto Nacional de Pesquisas Espaciais (INPE), S.J. Campos, S. Paulo, Brazil c Research Center Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany

Abstract Porous silicon (PS) samples were prepared on (1 0 0) monocrystalline silicon wafers and implanted with nitrogen by plasma immersion ion implantation (PIII). Characterization by Auger electron spectroscopy (AES) showed the presence of the implanted nitrogen and also SiO2 and Si3 N4 compounds on the sample surfaces. The e€ect of the implantation and consequently of the compounds was analyzed measuring the re¯ectance for wavelengths between 200 nm and 800 nm on the implanted samples. The results show a strong reduction of the re¯ectance in the ultraviolet region of the spectrum, in agreement with a tendency for the intensity of the peak of the ultraviolet excited photoluminescence to increase with the PIII treatment time. These characteristics can be explored and appropriately used in the fabrication of optoelectronics devices based on PS. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Rm; 85.40.Ry; 78.40.)q; 78.55.Mb Keywords: Porous silicon; Plasma immersion ion implantation; Re¯ectance; Photoluminescence

1. Introduction Since the visible luminescence from porous silicon (PS) was discovered [1], this material has been studied as an option to be used in devices that require a proper integration of optical and electronic properties on the same wafer because crystalline silicon, that has been largely used in microelec-

*

Corresponding author. Fax: +55-123-345-6717. E-mail address: [email protected] (A.F. Beloto).

tronics industry, has very weak photoluminescence on the visible and near infrared region of the spectrum. Therefore, considerable research has been developed to achieve ecient visible luminescence from PS [2]. This material can be obtained by electrochemical anodization in hydro¯uoric acid and the current density, acid concentration, doping type and resistivity of the Si determine the morphology of the porous layer. Generally, lightly doped p-type Si results in sponge-like pore morphology whereas n-type or heavily doped p-type Si results in columnar type structure [3].

0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 6 1 7 - 0

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To produce optoelectronics devices, besides a speci®c type of PS, some requirements like surface passivation and stable structure are necessary. On the other hand, processes usually used to obtain these improvements can damage the structures of the pores [4]. For example, PS can be incorporated into a multicrystalline Si solar cell structure as an antire¯ective coating whose porous layer, with refractive index lower than that of bulk material, increases the light absorption [5]. These solar cells generally use some kind of passivation in order to improve the silicon solar cell performance and depending on the process applied in this passivation both the solar cell and the PS antire¯ective coating can be damaged. Plasma immersion ion implantation (PIII) is a novel ion implantation technique developed recently for the improvement of the surface properties of materials including semiconductors, metals and dielectrics [6]. It is inherently a non-line-of sight ion implantation method, which allows threedimensional surface treatment of manufactured workpieces of large dimensions and complex shapes, at high speed, in batch processing mode and in a cost-e€ective manner [7]. In PIII, the ions of interest are extracted directly from the plasma in which the samples to be processed are immersed, without the need of acceleration grids. This can be used as an alternative method to perform modi®cation of the pore walls. In this work, we produced and analyzed samples of sponge-like PS implanted with nitrogen by PIII in order to obtain surface modi®cations necessary to apply them in some optoelectronics devices. 2. Experimental PS was produced by anodic etching using ptype, 1 X cm, (1 0 0) Si wafers with dimensions approximately 26  26 mm2 . The PS layer was grown within a 2 cm diameter circle in the center of the wafer using a solution of HF:H2 O:ethanol 1:2:1 in weight. The current density and etching time was 113 mA=cm2 and 1200 s, respectively. After anodization, the samples were rinsed with ethanol and dried with N2 gas. The samples were

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cut into pieces of 13  13 mm2 , each one containing a polished Si region as well. The PIII treatments were carried out in a system [8] where the plasma is produced by a DC glow discharge source. The nitrogen glow discharge plasma was obtained typically at 7:6  10 4 mbar pressures, with plasma densities of about 2:5  1010 cm 3 and temperatures of 5 eV. The plasma potential reaches 350 V at the center of the chamber where the Si samples are placed for treatment. The high voltage pulser was run at the peak voltage of 12 kV, with pulse duration of about 100 ls (triangular shape) and at a repetition frequency of 670 Hz. The PS wafers and some polished Si wafers were PIII implanted with nitrogen during di€erent exposure times. Table 1 shows the relation between exposure times and implanted dose, estimated from the collected current and the sheath propagation model. To make easier the characterization of PS samples, they were loaded onto a 12 sample supporting holder made of stainless steel (SS) AISI304, together with some polished silicon samples (for other analyses). At the end of the PIII process, the PS samples had a region with PS ®lm and a region with polished Si, both implanted for the same implantation time. Surface analysis was carried out with a high resolution AES device made by FISONS Instruments Surface Science, model MICROLAB 310-F. The total re¯ectance was measured between 200 nm and 800 nm using a Spectrophotometer Hitachi U-3501/40001 equipped with an integrating sphere. The photoluminescence properties were analyzed using a Renishaw 2000 Raman System and using a system set up to measure the visible Table 1 Relation between time of PIII implantation and the estimated dose implanted Time of PIII implantation (min)

Estimated implanted dose (10 7 cm 2 )

3 5 10 15 30

1.2 2.0 4.0 6.0 12.0

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light (orange/red) emitted when the PS samples were excited with ultraviolet light.

3. Results and discussion Fig. 1 shows the pro®les of nitrogen and other impurities implanted during the PIII process carried out for 15 min. The measurements were done by AES technique in the implanted part of the polished Si region of the PS sample. A high concentration of implanted nitrogen (up to 60% atomic concentration) occurred at about 20 nm. The implanted nitrogen reaches a depth at least of 90 nm. The presence of oxygen and carbon species in signi®cant quantities at small depths was expected, since native silicon oxide on the samples was not removed just prior to the PIII processing, and also because of the vacuum system used, comprised of di€usion and mechanical pumps. Iron in almost negligible quantities was detected and it apparently comes from the support structure or from the walls of the chamber. In Fig. 2 we show the high resolution Auger electron spectra from the same sample. The pro®les of the three species (Si, SiO2 and Si3 N4 ) can be seen clearly from this spectra, for di€erent depths in the Si wafer. Each level corresponds to approximately 10 nm in depth. Therefore, in the ®rst layer (10 nm

Fig. 2. High energy resolution Auger electron spectra from the polished region of the PS wafer sample exposed to 15 min, 670 Hz PIII implantation for di€erent levels. Each level corresponds to a 10 nm step in depths. Peaks from left to right can be identi®ed as belonging to SiO2 , Si3 N4 and Si.

Fig. 3. Total re¯ectance measurements carried out in polished Si samples with and without PIII implantations for 0.5 min, 3 min, 10 min and 30 min.

Fig. 1. Nitrogen concentration depth pro®le in the polished part of the PS wafer sample exposed to 15 min, 670 Hz PIII implantation. Included are pro®les for the implanted impurities.

depth) we ®nd the SiO2 compound signature appearing at the energy of about 1609 eV. Between 15 nm and 30 nm, we can identify the Si3 N4 compound with its peak near 1612 eV. In the third layer it is mixed up with the silicon peak at 1616 eV. As Si3 N4 was detected by AES in the implanted samples, we decided to investigate the re¯ectance behavior of the PS samples as well as of the polished Si samples. Fig. 3 shows the re¯ectance measurements of the polished Si samples for un-

A.F. Beloto et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 224±228

implanted Si and for 0.5 min, 3 min, 15 min and 30 min PIII implanted Si. It can be seen clearly that the re¯ectance decreases with the increase of the PIII implantation time especially in the ultraviolet region and this re¯ectance decrease saturates after approximately 10 min. A similar behavior can be observed with the PS samples (Fig. 4) but in this case we can observe some discrepancy on the re¯ectance curves, that is, the decrease of the re¯ectance does not necessarily follow the increase of PIII implantation time for all wavelengths. However, this behavior can be understood in the light of our previously published results [8], where we discussed the sputtering of the Si surface caused by high plasma potential (350 V). In the present work, we measured the etching depth for 3 min, 15 min  140 A  and 340 A,  and 30 min obtaining 100 A, respectively. We can conclude that even though we have continuous Si3 N4 formation during PIII treatment, we have also an etching process that can keep the coating thickness constant and also change the samples' porosity and the porous layer depth with increasing time. Other works [9] have shown that the re¯ectance changes with the PS thickness. In polished Si the compromise between surface etching and Si3 N4 formation seems to have less in¯uence on the re¯ectance curves. In order to verify the e€ect of the re¯ectance reduction on the photoluminescence curves, the samples were excited with ultraviolet light with spectral intensity peaked at approximately 360 nm.

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Fig. 5. Photoluminescence measured in the visible region of the spectrum for PS sample implanted by PIII for 5 min, 10 min and 30 min. The peak of the wavelength used for excitation is 360 nm.

Generally the emission increased with the PIII time of implantation. Fig. 5 shows the results for the same samples analyzed in Fig. 4 and identical behavior was observed with the other samples. We can see that the results are not in complete agreement with the ones of Fig. 4 and this fact can be explained because of the in¯uence of sample inhomogeneity, size of the implanted PS region and the ultraviolet beam width. The photoluminescence was also measured with 514 nm excitation (Ar‡ laser) using a Raman spectroscopy. In this case, the samples showed a little redshift of the photoluminescence peak (approximately 40 nm) and an increase of the luminescence signal with increasing PIII exposure time. We believe that the re¯ectance changes observed in the 514 nm region are related to these changes in the intensity of the photoluminescence.

4. Conclusion

Fig. 4. Total re¯ectance measurements carried out in PS samples with and without PIII implantations for 5 min, 10 min and 30 min.

Porous silicon wafers implanted with nitrogen by PIII showed an e€ective reduction of the re¯ectance in the ultraviolet region with a corresponding increase of the photoluminescence in the visible region. High energy resolution AES measurements have shown that the nitrogen implantation causes SiO2 and Si3 N4 formation on the samples during the PIII process. We believe that the presence of these

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layers on the PS samples can explain the reduction of the re¯ectance in the ultraviolet region like the one shown in Figs. 3 and 4. On the other hand, there is a compromise between the PIII exposure time and the re¯ectance behavior because the nitrogen plasma causes a progressive sputtering of the PS surface, keeping constant the rate of Si3 N4 formation but changing the PS thickness and porosity. References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82 (1997) 909.

[3] R.T. Collins, P.M. Fauchet, M.A. Tischler, Phys. Today, January (1997) 24. [4] A. Manuaba, I. Pinter, E. Szilagyi, G. Battistig, C. Ortega, A. Grosman, G. Amsel, Mater. Sci. Forum 248±249 (1997) 233. [5] S. Strehlke, D. Sarti, A. Krotkus, K. Grigoras, C. LevyClement, Thin Solid Films 297 (1997) 291. [6] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [7] D.J. Rej. Handbook of Thin Film Process Technology, IOP, Bristol, 1996, pp. E2:3:1±E2:3:25. [8] M. Ueda, L.A. Berni, G.F. Gomes, A.F. Beloto, E. Abramof, H. Reuther, J. Appl. Phys. 86 (1999) 4821. [9] L. Schirone, G. Sotgiu, F.P. Califano, Thin Solid Films 297 (1997) 296.