The contribution deals with electronic properties of thin oxide/amorphous hydrogenated silicon (a-Si:H) measured by capacitance-voltage (C-V) and charge version of deep level transient spectroscopy (Q-DLTS). The interest was focused on the studies of the interface properties of very thin dielectrics formed by dielectric barrier discharge (DBD) or natively on the a-Si:H layer. These properties were compared with those of oxide layers prepared by chemical oxidation in HNO3. The DBD was used for the preparation of a very thin SiO2 layer on a-Si:H for the first time to our knowledge. Preliminary electrical measurements confirmed that a very low interface states density was detected in the case of the native oxide/a-Si:H and DBD oxide/a-Si:H

Brunner, R.

Bucek, A.

Cernak, M.

Gleskova, H.

Kobayashi, H.

Kral, M.

Pincik, E.

Rusnak, J.

Zahoran, M.

English

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryKral, M. and Bucek, A. and Gleskova, H. and Cernak, M. and Kobayashi, H. and Rusnak, J.and Zahoran, M. and Brunner, R. and Pincik, E. (2005) Electronic properties of very thin nativeSiO2/a-Si:H interfaces and their comparison with those prepared by both dielectric barrier dischargeoxidation at atmospheric pressure and by chemical oxidation. Acta Physica Slovaca, 55 (4). pp.373-378. ISSN 0323-0465Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/acta physica slovaca vol. 55 No. 4, 373  378 August 2005ELECTRONIC PROPERTIES OF VERY THIN NATIVE SiO2/a-Si:H INTERFACESAND THEIR COMPARISON WITH THOSE PREPARED BY BOTH DIELECTRICBARRIER DISCHARGE OXIDATION AT ATMOSPHERIC PRESSURE AND BYCHEMICAL OXIDATION1M. Kr·al’2,a, A. Buceka, H. Gleskov·ab, M. Cern·aka, H. Kobayashic, J. Rusn·akd,M. Zahorana, R. Brunnerd , E. Pinc·kdaDepartment of Experimental Physics, Faculty of Mathematics, Physics and Informaticsomenius University, Mlynska´ dolina F2, SK-842 48 Bratislava, SlovakiabDepartment of Electrical Engineering, Princeton University, Princeton, NJ 08544, USAcInstitute of Scientific and Industrial Research, and CREST Japan Science and TechnologyOrganization, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapandInstitute of Physics, Du´bravska cesta 9, SK-845 11 Bratislava, SlovakiaReceived 11 February 2005, in final form 10 May 2005, accepted 12 May 2005The contribution deals with electronic properties of thin oxide/amorphous hydrogenated sil-icon (a-Si:H) measured by capacitance-voltage (C-V) and charge version of deep level tran-sient spectroscopy (Q-DLTS). The interest was focused on the studies of the interface prop-erties of very thin dielectrics formed by dielectric barrier discharge (DBD) or natively on thea-Si:H layer. These properties were compared with those of oxide layers prepared by chemi-cal oxidation in HNO3. The DBD was used for the preparation of a very thin SiO2 layer ona-Si:H for the first time to our knowledge. Preliminary electrical measurements confirmedthat a very low interface states density was detected in the case of the native oxide/a-Si:H andDBD oxide/a-Si:H.PACS: 52.77.Dq, 61.43.Dq, 73.20.-r1 IntroductionSilicon is widely used in microelectronics as a substrate (c-Si) and silicon oxides as insulationlayers. Because of a fast and cheep production, silicon in amorphous state (a-Si) is used in hy-drogenised form (a-Si:H) for production of solar cells and thin film transistors (TFT). The a-Si:Hcan be deposited on various substrates mainly by Plasma Enhanced Chemical Vapor Deposition(PECVD). Silicon oxide can be produced by several techniques, such as oxygen plasma im-mersion ion implantation [1], chemical oxidation by HNO3 [2], monoenergetical Ar+ ion beamtreatment followed by an exposure to oxygen [3], etc. Another possibility is to expose a prepared1Presented at SSSI-IV (Solid State Surfaces and Interfaces IV) Conference, Smolenice, Slovakia, 8–11 Nov. 2004.2E-mail address: martin.kral@gmail.com0323-0465/05 c© Institute of Physics, SAS, Bratislava, Slovakia 373374 M. Kra´l’ et al.a-Si:H layer to air and the thin native SiO2 is consequently formed on the surface. Additionaltechnique is based on dielectric barrier discharge (DBD) [4].To improve electrical parameters of solar cells and TFT, the SiO2/a-Si:H structures are in-tensely studied at present time. This paper is devoted to investigation of some parameters ofSiO2/a-Si:H structures (interface states density, oxide layer thickness, form of built-in charge).SiO2 layers were prepared by the chemical oxidation, native oxidation and DBD oxidation. DBDwas used for SiO2 layer preparation on a-Si:H for the first time and it was compared with chem-ical and native oxides. The properties of interfaces and oxide layers were measured by the C-Vand Q-DLTS techniques.2 ExperimentAll the oxide layers were prepared on undoped a-Si:H layers ∼ 0.5 µm thick prepared by PECVD technique at 13.56 MHz from a mixture of SiH4 (10 sccm) and H2 (90 sccm) gases bypressure 120 Pa, power 0.03 W cm−2, sample temperature 160 ◦C and duration of depositionwas 20 min.Samples were exposed to ambient air after the deposition of a-Si:H layer and by this way atroom temperature the native oxide layer of 1-2 nm thickness have grown. Abbreviation of thesamples is “am3”.Other method for oxide preparation that was used is chemical oxidation. This method of lowtemperature oxidation by use of nitric acid, i.e. nitric acid oxidation of silicon (NAOS), has beendeveloped in Osaka University [5, 6]. On the top of the a-Si:H surface, an oxide layer of 5-8nm thickness was prepared in 68 % water solution of HNO3 acid at boiling temperature (121◦C)during 30 minutes. Abbreviation of the samples is “naos”.Additional type of oxide layer was prepared by DBD. Dielectric barrier discharges are char-acterized by discharge gaps and the presence of one or more insulating layers in between thegaps. Typical operation conditions are pressures between 10 and 1000 kPa and frequencies inthe range of 50 Hz to 100 kHz. Three basic configurations to produce DBDs can be distin-guished, the volume discharge, the surface discharge and the coplanar discharge arrangement.The volume discharge arrangement is most common. It consists in general of a gas gap betweenparallel electrode plates with one or both electrodes covered by a dielectric layer. The ignitionvoltage is mainly determined by the size of the gap, the kind of gas and its pressure.Surface discharge arrangement consists of one or more long high voltage electrodes on thesurface of a dielectric and an extended counter electrode on its reverse side. Pure surface dis-charges appear on the surface of the dielectric. The discharge ignites at Paschen minimum con-ditions. Its area on the surface is proportional to the amplitude of the applied voltage.The coplanar discharge arrangement on the other hand consists of pair(s) of extended elec-trodes that are embedded in a dielectric close its surface. Most often the electrode separationis in the order of 100 µm. The discharge occurs on the surface of the dielectric in between thelocation of the electrodes.In our experiments with utilization of DBD for a-Si:H surface exposure the coplanar dis-charge arangement has been used with following parameters:Frequency: 14 kHz; voltage: 7 kV, distance of electrodes: 0.5 mm; width of electrodes: 1.65mm; material of dielectrics: Al2O3 based.Electronic properties of very thin native SiO2/a-Si:H interfaces... 375Fig. 1. C-V record of structure native oxide/a-Si:H/c-Si measured with various bias voltages by380 K and 430 K.Fig. 2. C-V record of structure DBD oxide/a-Si:H/c-Si. Plasma exposure time: 30 s.Pressure 133 kPa of O2 at flow regime was used during the experiment. Distance of samplefrom the surface of dielectric was 0.25 mm. Time of exposure was 30 s (bd1) and 60 s (bd2),respectively. The samples were exposed at room temperature. Abbreviation of the samples was“bd1”, “bd2” according to time of exposure (see FigsG. 2, 3, 5, 6).The samples were treated using a atmospheric plasma source which is a modification of thesurface-dielectric-barier-discharge plasma source described in details in reference [7].3 MeasurementsThere was a C-V measurement at ≈ 100 kHz and Charge version of Deep Level Transient Spec-troscopy (Q-DLTS) using a spectrometer developed at the Institute of Physics of SAS [3,8] usedto characterize the prepared samples. Both measurements were realized with the same measure-ment equipment in the same cryostat. Q-DLTS was measured by the voltage steps of 1V, 4V, 6V,(Fig. 4, 5 and 6) and by 0.2V in the case of the structures with chemical oxides (Fig. 7 and 8).4 ResultsMain results concern the behavior of the Q-DLTS spectra and the C-V characteristics.i. Accumulation capacitance was not reached under given conditions for the samples withnative oxides (Fig. 1, the corresponding capacitance is very low) whereas for DBD oxides(see Fig. 2, 3), the accumulation was observed. From the hysteresis on Fig. 2 and 3, it canbe deduced the presence of a mobile charge in oxide layer which is not observed for thenative oxide. The results indicate that the DBD and chemical oxidations influence also thebulk of the a-Si:H layer, not only the oxide/a-Si:H interface.376 M. Kra´l’ et al.Fig. 3. C-V record of structure DBD oxide/a-Si:H/c-Si by various bias voltages. Exposure time:60 s.Fig. 4. Q-DLTS spectra of the structure nativeoxide/a-Si:H/c-Si annealed under various condi-tions according picture label.Fig. 5. Q-DLTS spectra of the structure with DBDoxide/a-Si:H/c-Si annealed 20 min by 425K andmeasured by various conditions. Preparation timeof the oxide: 60 s.Fig. 6. Q-DLTS spectra of the structure with DBDoxide/a-Si:H/c-Si annealed 20 min by 425K andmeasured by various conditions. Preparation timeof the oxide: 30 s.ii. A “standard” behaviour of deep level spectra (according Powel and Dean model [9, 10])was measured on the samples with the chemical oxide layer (see Fig. 7 and 8). However,only two distinct dangling bond components (at lower temperatures) could be identified,namely Dh at 320 K and Dz at 390 K at the rate window of 100 s−1, similarly as in [3].The structures had an increased conductivity at higher temperatures.Electronic properties of very thin native SiO2/a-Si:H interfaces... 377Fig. 7. Q-DLTS spectra of the structure with chem-ically formed oxide/a-Si:H/c-Si.Fig. 8. Q-DLTS spectra of structure with chemi-cally formed oxide/a-Si:H/c-Si. Bias annealing at420 K.iii. Interface properties investigated by Q-DLTS indicate that the native oxide layer/a-Si:H andDBD oxide/a-Si:H are without remarkable signal coming of the deep states (Fig. 4, 5 and6) comparing to the MIS structures with the chemical oxide. The absence of the signal iscaused probably by the preparation properties.iv. It is possible to redistribute such a type of deep local states as it can be seen in the case ofchemical oxides by a “bias annealing” (see Fig. 8) at –1V (+0.5V), 380K (425K) for 10min. It was impossible with the samples with the native oxide and DBD oxide layers (seeFigs. 4, 5 and 6).5 ConclusionThe interest was focused on the experimental research of the interface properties of very thindielectrics formed natively on the a-Si:H surface after the PECVD of 1 µm thick a-Si:H layergrown at 250 ◦C on a crystalline Si substrate, DBD oxide/a-Si:H and chemical oxide/a-Si:Hstructures.The deep level spectra have been analyzed and compared with previous results published andinterpreted on the basis of the Powell-Dean theoretical model.We compare the results obtained on the very thin SiO2/a-Si:H structure prepared by the oxi-dation using DBD at atmospheric pressure in oxygen with the chemical and native oxides. TheDBD technique was used for a very thin SiO2 layer formation on a-Si:H for the first time to ourknowledge,.The preliminary electrical measurements confirmed that a very low density of interface stateswas detected in the cases of the native oxide/a-Si:H and DBD oxide/a-Si:H. Relying on Q-DLTSwe suppose that a a-Si:H layer was partly transformed into a Si micro-crystalline-like layer during378 M. Kra´l’ et al.the preparation of a very thin oxide layer and consequently the “standard” distribution of the a-Si:H deep states was not measured. On the other hand, the Q-DLTS spectra do not contain anysignal confirming existence of deep states in the band gap of the semiconductor in 100 – 350 Kregion. Other parameters concerning the electrical quality of the dielectric layers also indicatethat such a type of the SiO2 layers prepared by the DBD technique can be applied in sub-microntechnology, e.g. for production the MOS based devices.Acknowledgement: This work was partly supported by the Slovak Grant Agency (VEGA)projects no. 2/4105/05 and no. 1/1011/04.References[1] K. Volz, W. Ensinger: Surf. Coat. Technol. 156 (2002) 237[2] Asuha, T. Kobayashi, O. Maida, M. Inoue, M. Takahashi, Y. Todokoro, H. Kobayashi: Appl. Phys.Lett. 81 (2002) 3410[3] E. Pincˇı´k, H. Kobayashi, M. Takahashi, N. Fujiwara, R. Brunner, H. Gleskova´, M. Jergel, J. Mu¨llerova´,M. Kucˇera, C. Falcony, L. Ortega, J. Rusna´k, M. Mikula, M. Zahoran, R. Jura´ni, M. Kra´l’: Appl. Surf.Sci. 235 (2004) 351[4] J.J. Yu, I.W. Boyd: Thin Solid Films 453-454 (2004) 63[5] Asuha, M. Takahashi, H. Kobayashi: Surf. Sci. 547 (2003) 275[6] H. Kobayashi, Asuha, M. Takahashi, H. Iwasa: J. Appl. Phys. 94 (2003) 7328[7] M. Sˇimor, J. Ra´helˇ, M. Cˇerna´k, Y. Imahori, M. Sˇtefecˇka, M. Kando: Surf. Coat. Tech. 172 (2003) 1-6[8] J. Rusna´k: in: Proc. of the International Conference SNST. (Eds. S. Jurecˇka, J. Mu¨llerova´) Podbanske´,Slovakia, February 29 – March 3, 2004, p. 132, ISBN 80-89186-03-3[9] M. J. Powell, S. C. Dean: Phys. Rev. B 48 (1993) 10815[10] M. J. Powell, S. C. Dean: Phys. Rev. B 53 (1996) 10121

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Thin Solid Films 453-454

Electronic properties of very thin native SiO2/a-Si:H interfaces and their comparison with those prepared by both dielectric barrier discharge oxidation at atmospheric pressure and by chemical oxidation

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Electronic properties of very thin native SiO2/a-Si:H interfaces and their comparison with those prepared by both dielectric barrier discharge oxidation at atmospheric pressure and by chemical oxidation

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