Electrical and optical properties of aluminum-doped amorphous silicon carbide films

288

Applied Surface Science 4 8 / 4 9 11991 ) 288-296 North-Holland

Electrical and optical properties of aluminum-doped amorphous silicon carbide films P.K. Banerjee, J.S. K i m and S.S. Mitra Thin Film Lahorator)'. Department of Electrical Engttteertng. Umt'er,~'it)" of Rhode Island, King.won. RI 02881. USA Received 17 August 1990: accepted for publication 1 November 1990

Aluminum-doped amorphous silicon carbide films were prepared by RF co-spunering of SiC and Al-doped SiC. AES and XPS v,'as used to determine the composition of the resultant films. XPS results showed that concentrations of aluminum around 1.0% were easily obtained. The electrical conductivity was measured for the temperature range of 150 to 400 K and subjected to analysis in terms of the Mott hopping conductivity model. The conductivity of AI-doped a-SiC films was about five orders of magnitude higher than that of undoped films. Incorporating hydrogen during the growth of AI-doped a-SiC films results in a decrease of electrical conductiv+ty by about three orders of magnitude from its value in unhydrogenated films. The optical gap derived from the Tauc plot, was 1.13 eV for an AI-doped unhydrogenated film and 1.55 eV, for an AI-doped hydrogenated film. The structural properties of these films were studied by IR spectroscopy.

!. Introduction

2. Experimental

Several methods for doping amorphous semiconductors such as glow discharge [1], sputtering [2], ion implantation [3], etc. have been developed. Doping by glow discharge of a-SiC:H films was first achieved by Tawada [1]. Banerjee et al. [2] succeeded in doping a-SiC:H by RF sputtering. Most doping techniques for amorphous semiconductors have involved the use of phosphine and diborane to produce n- and p-type materials, respectively. In this paper, we describe doping of amorphous silicon carbide films with aluminum by RF cosputtering of SiC and Al-doped SiC without using hydrogen. The effect of hydrogen on the electrical properties was also investigated. The composition of the resultant films was analyzed and electrical and vibrational characteristics of AI-doped a-SiC films were studied.

Aluminum-doped a-SiC films were deposited by alternate RF co-sputtering from a Sit)rC0.4 target and an Al-doped SiC Target (AI ~ 4.0%) using an MRC 8667 sequential multi-target sputtering system. Coming glass 7059 and 1 to 3 ~2 cm n-type Si wafers were used as substrates. Deposition was carried out at room temperature in a pure Ar (99.999%) atmosphere. SiC and AI-doped SiC were sputtered at RF powers of 300 and 150 W, respectively. The partial pressure of Ar was kept fixed at 10 mTorr for all depositions. The distance between the target and the substrate was 65 mm. After deposition, the composition of the resultant films were determined by XPS (PerkinElmer PHI 5500). Infrared transmission and reflection were measured by a double-beam PerkinElmer spectrophotometer (Model 983) for wavenumbers between 200 and 1600 cm -~. Electrical

0169-4332/91/$03.50
P.K. Banerjee et al. / Electrical and optical properties of A I-doped a-SiC

conductivity was determined by the conventional two-point probe method for the temperature range of 150 to 400 K. The optical gap was determined from reflectance and transmittance data generated by a Cary 17 spectrophotometer. The thickness, as measured on a Sloan Dektak 11, was in the range of 0.4 to 0.7 #m.

film is depicted in fig. 3. The AES data show that the film has a composition of Si - 6 5 % and C - 35% which is very close to the composition of the SiC target used. Since the amount of aluminum was too small to detect from AES analysis, XPS measurements were carried out with M g K a X-rays (1253.6 eV) as exciting radiation. Analysis of the XPS bond of the AI2p feature at a binding energy of 74.0 eV indicated a concentration of about 1.0% AI content in the a-SiC film probed. Fig. 4a shows details of the Si2p (940-110.0 eV) peak; fig. 4b of the C ls (277.0-292.0 eV) peak, and fig. 4c of the Al2p peak (70-76 eV). The XPS spectrum of carbon in these films (fig. 4b) was deconvoluted into Gaussian type peaks (fig. 4d) to determine whether any diamond-like bonds (whose presence would result in a bond at 287.2 eV) are present in the film. The absence of the band indicates that there is no diamond-like bonding in these films, i.e. virtually no carbon atoms surrounded by four carbon neighbors.

3. Results and discussion

3.1. Composition The XPS spectrum of hydrogenated Al-doped amorphous films on an n-type substrate is shown in fig. 1. In this film, SiC and Al-doped SiC were sputtered for a total of 120 min at a sputtering time ratio of 8 : 2 for the SiC and the AI-doped SiC target. The oxygen and nitrogen peaks are advice from atoms of these gases adsorbed on the surface. Fig. 2a shows the AES spectrum of the surface of a film sputtered from an Al-doped SiC target only. Fig, 2b is the AES spectrum of a hydrogenated film (see table 1) produced by cosputtering of the Si0.66C0~4 and the Al-doped SiC targets. Note that an aluminum peak appears only in the film sputtered from the Al-doped SiC target. An AES depth profile taken for the hydrogenated

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3.2. Electrical conductivity The electrical conductivity data for AI-doped a-SiC films are shown in fig. 5. The conductivity

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Table I Composition of the surface of an AI-doped a-SiC film as determined by XPS [Element

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of unhydrogenated films depend nearly exponentially on inverse temperature. At 300 K, the magnitude of conductivity of the unhydrogenated AIdoped SiC film was about five orders of magnitude higher than that of an undoped a-SiC:H film [21. The effect of hydrogen incorporation on the conductivity of an Al-doped a-SiC:H film is also shown in fig. 5. As reported by other authors [2,5]. the incorporation of hydrogen results in a decrease

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P.K. Banerjeeet aL / Electricaland opncalpropertie~of A I-doped a-StC of electrical conductivity, in our case by about three orders of magnitude from the conductivity of our unhydrogenated AI-doped a-SiC films [2,5]. In amorphous semiconductors, the conductivity above room temperature can be approximated by the expression: o = o exp(-A/kT).

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Here e is the electronic charge, a is the hopping distance, vph is a photon frequency obtained from the Debye temperature (%h = 2.7 × 103/S for SIC), k is the Boltzmann constant, a is the inverse rate of the fall-off of the wave function of a localized state, ~, is a dimensionless constant and has the value 18.1 according to Ambegaokar et al. [7] and N ( E v ) is the density of states at the Fermi level. In fig. 6, we plot o T ~/2 versus T '/4 for Ohop (determined as described above) for a film containing hydrogen and a film without nitrogen, l"he straight lines in this semilog plot suggest that eq. (2) is applicable, i.e. the conductivity depends on hopping. We obtained the values of a from ooT w2 as described in ref. [8]:

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P.K. Baneqee et at / Electricaland optwal properties of AI-doped a-SiC

The average hopping distance R and the average hopping energy W are given by R = [~ral,'TN( E v)] ,/4 cm

We used these equations to obtain Mott's parameters for the hydrogenated film whose experimental data is plotted in fig. 5. The results are given in table 2. F r o m this table, it is seen that for this sample the parameters seem to be reasonably within Mott's requirements of the localized-state model, i.e. a R >> 1, W>> kT. Somewhat larger values of a and N ( E r) probably result from uncertainties involved in estimating oo and To. When the same analysis was applied to an unhydro-

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3.3. Optical properties The quadratic dependence of the absorption coefficient on the photon energy is shown in fig. 7.

The optical gap was determined by extrapolating the (ahz,) 1/2 versus hv straight lines to zero absorption. Its value is about 1.19 eV for the unhydrogenated film, which is almost the same as the value reported for undoped a-SiC carbide films [2]. The optical gap of the hydrogenated film was slightly larger and its value was around 1.55 eV, i.e., a blue shift by about 13.36 ¢V. Fig. 8 shows the infrared transmission spectra of an AI-doped a-SiC film and of the same film

P.K. Banerjee et a L / Electrical :rod optwal properttes of A I-doped a-SiC

294

annealed at 300 ° C for 0.5 h. The Si-C stretching band around 800 cm-~ [9,10] is obvious in both spectra. Annealing the film at 300°C causes the intensity of the band around 615 cm -t to disappear and the intensity of the Si-C stretching band around 800 cm-~ to increase. This suggests that the band at 615 cm-~ is associated with bonding between AI and either Si or C. Silicon-silicon bonds at 520 cm-~ [5] are also evident in the spectrum. The peak at 1115 cm -~ is attributed to either the oxygen incorporated into the Al-doped target o~ oxygen adsorbed on the surface.

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P.K. Banerjee et al. /Electricai and optical properties of A I-doped a-SiC Table 2 Mott parameters for an Al-doped hydrogenated a-SiC film Thickness (A)

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c a n b e fitted b y a single e x p o n e n t i a l o v e r a w i d e r a n g e o f t e m p e r a t u r e s . I n h y d r o g e n a t e d films, t w o e x p o n e n t i a l s a r e r e q u i r e d to fit t h e d a t a w h i c h w a s s u b j e c t e d to a n a l y s i s in t e r m s o f t h e M o l t m o d e l Meaningful Molt parameters were obtained from that analysis. The thermoelectric power measured at r o o m t e m p e r a t u r e w a s p o s i t i v e in all o f t h e films. A l u m i n u m d o p i n g h a s little effect o n t h e o p t i c a l g a p w h o s e v a l u e in a n u n h y d r o g e n a t e d f i l m w a s a r o u n d 1.19 eV. I n c o r p o r a t i o n o f h y d r o gen into the AI-doped a-SiC films increased the o p t i c a l g a p to a b o u t 1.55 eV.

Acknowledgements

W e w o u l d like to t h a n k t h e R h o d e I s l a n d C e n t e r for Thin Film and Interface Research for partial

support of this work, especially for surface characterization of our samples using their PerkinElmer PHI 5500 surface analyzer.

References

[1] Y. Tawada, in: Amorphous Semiconductor Technologies and Devices, Vol. 6, Ed. Y. Hamakawa (Ohmsha, Tokyo, 1983). [2] P.K. Banerjee, J.M.T. Pereira and S.S. Mitra, J. Non-Cryst. Solids 87 (1986) 1. [3] G.W. Anderson, J.E. Davey, J. Comas, N.S. Saks and W.H. Luckey. J. Appl. Phys. 45 (1974) 4528. [4] A. Tabata, S. Fujii, T. Mizutani and M. Icda. J. Phys. D (Appl. Phys.) 23 (1990) 316. [5] R. Dutta, P.K. Banerjee and S.S. Mitra, Solid State Commun. 42 (1982) 219. [6] N.F. Mort, J. Non-Cryst. Solids B 8-10 (1971) l.

296

P.K. Banerjee et al. / Electrwal and optical properties of A I-doped a-SiC

[7] V. Ambegaokar. ill. Halperin and J.S. Langer. Phys. Rev. 34 (19711 2612. (8] K. Nair and S.S. Mitra. J. Non-Cryst. Solids 24 (1977) 1. [91 P.K. Banerjee. Properties of Amorphous Silicon (INSPEC. The Institute of Electrical Engineers. London, 1989).

[10] A. Asano, T. Ichimura and H. Sakai. J. Appl. Phys. 65 (1989) 2439.