R.F. MAGNETRON SPUTTERING OF a-si : H

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1 R.F. MAGNETRON SPUTTERING OF a-si : H A. Mirza, A. Rhodes, J. Allison, M. Thompson To cite this version: A. Mirza, A. Rhodes, J. Allison, M. Thompson. R.F. MAGNETRON SPUTTERING OF a-si : H. Journal de Physique Colloques, 1981, 42 (C4), pp.c4-659-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1981 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL UE YHYSIQCE (:olloque C4, suppl6merct czu no70, 'lbme 42, cctohre 1981 page C4-659 R,F, MAGNETRON SPUTTERING OF a-si:h A.R. Mirza, A.J. ~hodes*, J. Allison and M.J. ~hom~son*. napartmen l. oj^ BZectronic Zn'ng.inczri?lg, the University, Sheffie Ld, U. K. Abstract. - R.F. sputtering of films in a planar magnetron configuration, which is hecoming a well-established technique for high-rate deposition, has the additional advantage when used for the deposition of a-si-h that the presence of a magnetic ficld at the target effective1.y confines the cncrgetic charged species emitted from the target, thus prcvcnting their bombardment of the substrate on which the films are hcing depositcd. Since bombardment by ions has been found to strongly influence the properties of a-si-h [I], magnetron sputtering is of considerable interest, particularly for multijunction electronic devices, such as a-si MOS transistors, since damage of a depositcd layer by charged particle bombardment during sputtering of subsequent layers is avoided. The optical and electrical properties of magnetron-sputtered a-si-h films have been measured and are compared with those of more conventionally produccd r.f. diode sputtered material. Magnetron films of a-si-h prepared at PH = 6x10-~ torr possess good photoconductivity and Schottky diodc performance. Hydrogen content and i.r. vibrational spectra measurements show distinct differences between the two processes. Optical absorption, luminescencc,photovoltaicand other properties of magnetron-sputtered a-si-11 are also presented. Introduction. - During the past several years there has been considerable work done on the growth of thin film a-si-m for solar cells by r.f. diode sputtering 127 as an alternative to the glow discharge decomposition of silane [3]. R.F. sputtering is an attractive process when scaling up becomes a consideration, the tendancy being that the larger the area the more uniform is the final film. Uniform a-si films over 300cm7 have been fabricated in our laboratories; the active film thickness is only zlum and there is direct control over the hydrogen content in the a-si; this allows properties such as absorption spectrum to be tailored to a particular requirement. On a continuous on-line system with several sputtering targets metal, insulator and a-si films can be sputtered sequentially without breaking the vacuum, for multilayer devices. Over the last decade the use of planar magnetron sputtering [4] has opened up new areas for the application of sputtering. Sensitive MOS devices which could not be manufactured by conventional sputtering can now be produced using magnetron sputtering. The low sputtering voltage, characteristic of this process, reduces the number of higher energy sputtered particles. These lower energies produce a reduction in substrate heating. Thus the lower energy of the particles and the confinement of the plasma at the target face plus a fast deposition rate result in minimal damage to sensitive substrates by bombardment. Magnetrons can also be used for bias sputtering. In this paper we report the first stage of an investigation into the use of planar magnetrons for the preparation of a-si-h films and devices and a direct comparison is made with conventional r.f. diode sputtering for similar deposition condi tions. '~epartment of Physics *currently on leave at Xerox Corp., Palo Alto Article published online by EDP Sciences and available at

3 C4-660 JOURNAL DE PHYSIQUE Experimental Conditions. - The a-si-h films have been prepared in a Nordiko r.f. sputtering sys tem with a 0.5m diameter stainless steel bell-jar containing two 20cm.D. targets and a 15cm.D. magnetron target. The maximum transverse component of magnetic field in front of the target is loog (0.OlT) and the target to substrate spacing is 7cm. The system is pumped down to less than torr before deposition, by a turbomolecular pump. The substrate temperature is held at 240 C and the total argon and hydrogen sputtering pressure is 5x10-~ torr. The r.f. power to the magnetron target is maintained at loow producing a target d.c. bias of -400V compared with -850V for diode sputtering at similar powers. The hydrogen partial pressure (PH) has been varied from 2x10-' torr to 8x10-' torr to determine whether the optimum PH (6x10-' torr) for r.f. diode sputtering remains the same when using the magnetron target. The a-si-h films, all approximately l ~ m thick, are deposited onto a matrix of glass (Corning 7059),crystalline silicon and quartz substrates, each 2.5 x 2.5cm. Results and Discussion. - Using the r.f. magnetron rather than the conventional sputtering target causes an appreciable increase in deposition rate, from 0,2~m/hr to 0.7vrnlhr. The photoconductivity of the samples, given by the ~ U T product, prepared with PH from 2x10-' torr to 8x10-' torr, varies between lo-' and 10-'O~m-~ and the peak in photoconductivity shifts to higher energies with increasing hydrogen partial pressure. This compares well with the photoconductivity of diode sputtered a-si-h 151. An estimate of the optical band gap has been made for the magnetron samples. Figure 1 shows the extrapolation of the optical absorption curves to give the optical band gap as the intercept on the energy axis. Above ZXIO-' torr the optical band gap remains at approximately 1.78eV, which is similar to the optical band gap obtained for films prepared by r.f. diode sputtering. The I-R vibrational spectra however do show a distinct difference between the two processes. Figure 2 shows the ratio of the 2000cm-' to the total stretching band absorption (2000cm-I and 2100cm-') as a function of ph, as well as showing the total hydrogen absorption. For r.f. diode material the total absorption is much higher, around 12.4%, but the fraction of singly bonded hydrogen is lower at Data on the density of states in the pseudo-gap, derived from low-frequency capacitance measurements, in fig. 3, show a minimum in defect density for PH = 6x10-' torr. This minimum is in fact lower than that produced by conventional diode sputtering. Some Pt/a-Si-H Schottky barrier devices (not solar cells) have been fabricated and their illuminated J-V characteristics measured under AM1 illumination. Figure 4 shows the results of these tests carried out at room temperature. The best characteristic is produced at PH = 6x10-' torr which gives open circuit voltages (>700mV) well above those for r.f. diode sputtered devices. The fill factor is poor in these devices owing to bulk and contact resistances. Luminescence data also support the trends seen in the illuminated characteristics of these devices. For Pll = 2xl0-' torr samples, the luminescence output at 290K contains a distinct 0.9eV defect related band. However, as PH increases, the spectra becomes broader, due to the emergence of the band-edge luminescence at 1.2eV, indicating an improved efficiency. At the optimum PH = 6-8x10-' torr the 1.2eV band is dominant, entirely masking the defect luminescence. The dark J-V for the Pt-Schottky diode prepared at PH = 6xl0-~ torr is shown in figure 5. This shows ideal characteristics with good saturation, high rectification ratio and an ideality factor n = 1.45 indicating diffusion limited rather than recombination limited current. For this sample, figure 6 shows a plot of n conductivity versus 1/T to obtain its dark activation energy for nichrome ohmic contacts. A single activation energy of 0.82eV is exhibited throughout the temperature range indicated and no localised state conduction is seen at low temperatures. These results are also typical for optimised r.f. diode sputtered films. Conclusions. - Despite an appreciable increase in deposition rate, no degradation in film quality has been seen when r.f. magnetron sputtering is used instead of r.f. diode sputtering, under otherwise similar preparation conditions. In fact some

4 improvements are made, e.g. an apparent increase in the single bonded H, a reduction in thc density of states in the pseudo-gap and an increase in V~C. Where there are differences in properties of the two processes these seem to indicate that the magnetron process is not yet fully optimised. For instance, changes in the deposition rate will influence the arrival rate of sputtered atoms on the substrate and this together with PI^ will influence the formation of SiH in the gas and on the substrate For this reason we are still actively developing this process and in particular are investigating magnetron bias-sputtering. It is felt that magnetron sputtering will certainly be of considerable use for the subsequent sputtering of multijunction devices. The authors wish tu acknowledge the invaluable help provided by Mr. H.L. Fernandez-Canque who carricd out the C-V measurements and Mr. D.P. Turner for many useful discussions, for which they are most grateful. References 1. Allison, J., Thompson, M.J., Turner, I).P., and Thomas, I.P., 1980, Proc. Photo. Solar Energy Conf., Cannes, France, Thompson, M.J., Allison, J., Al-Kaisi, M.M., and Thomas, I.P., Proc. of the Tnt. Conf. on Photovoltaic Solar Energy, Lux. 1977, Gibson, R.A., Le Comber, P.G., and Spear, W.E., Solid-State and Electron Devices, June 1978, Vo1.2, Waits, R.K., J. Vac. Sci and Techn. 15(2) Mar/Apr.1979, 179, 5. Turner, D.P., Thomas, I.P., Allison, J., Thompson, M.J. et al. Proc. American Inst. Phys., to be published. Fig.1. Optical absorption for Fig.2. Fraction of singly bonded hydrogen varying hydrogen partial and hydrogen content for varying pressures, PH = (a) 2, hydrogen partial pressures. (b) 4, (c) 6, (d) 8 x tvrr.

5 JOURNAL DE PHYSIQUE Fig.4. J-V characteristics of Pt-Schottky 17 L---J barrier diodes under AM1 illumin ation, a-d as Fig.1. E,-E (ev) Fig.3. Density of states in the pseudo-gap, a-d as Fig. 1, (e) PH =6 x torr, r. f. diode sputtered. VOLTS Fig.5. Dark J-V characteristics for a-si-h/pt Schottky diode, ph = h x torr. Fig.6. Dark conductivity v reciprocal torr. temperature, PH = 6 x lo-'