Nanocrystalline silicon — from disordered insulator to dirty metal

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Journal of Non-Crystalline Solids 198-200 (1996) 899-902

Nanocrystalline silicon - - from disordered insulator to dirty metal Mikio Taguchi, Yasuo Tsutsumi *, Ravin N. Bhatt, Sigurd Wagner Department of Electrical Engineering Princeton Uniuersity, Princeton, NJ 08544, USA

Abstract Selected results of a study of the metal-insulator transition in n-type nanocrystalline Si:H,F down to T = 30 K are presented. The transition occurs around an electron density of 1 X 1019 cm 3. On the insulator side, hopping conduction in a parabolic density of states near the Fermi level dominates below room temperature. On the metallic side, the conductivity reflects electron-electron interactions, with a temperature dependence of T ~/2 at temperatures less than 270 K.

1. Introduction Few systematic investigations of the electronic transport of nanocrystalline silicon have been reported [ 1,2]. W e deposited nanocrystalline silicon (nc-Si:H,F) films from silicon tetrafluoride (SiF4) and hydrogen (H2) by plasma-enhanced chemical vapor deposition. With one source g a s / a r s e n i c dopant composition the entire conductivity range from disordered insulator to dirty metal can be covered. W e present selected results of our electrical and optical evaluation. Our principal observation is the m e t a l - i n s u l a t o r transition at an electron density near 1 X ]019 c m 3.

* Corresponding author. Department of Electrical Engineering, Akashi College of Technology, Nishioka, Uozumi-cho, Akashi, Hyogo 674, Japan. Tel: +81-78 946 6120; fax: +81-78 946 6138; e-mail: [email protected].

2. Experimental procedures The radio frequency (rf) (13.56 MHz) deposition system and typical procedures were described earlier [3]. W e employed gas flow rates of 2.7 to 4.0 sccm of SiF4 and 4.0 to 27 sccm of H 2, substrate temperatures ~ of 250 to 290°C, a deposition pressure of 0.75 Torr (100 Pa), and a power density of 300 m W cm -2. The film structure was characterized by Raman scattering spectroscopy and X-ray diffraction, and chemical composition by secondary ion mass spectrometry (SIMS). The crystallites range in size from 5 to 50 nm. The Roman spectra show that the crystalline volume fraction is > 65%, and the amorphous volume fraction is < 35% [4]. For the highest-conductivity sample, SIMS measured hydrogen and arsenic concentrations of 1.3 X 1021 cm -3 (--~ 2.5 at%) and 8.5 X 102o cm 3 ( ~ 1.7 at%), respectively. The dc and ac conductivity measurements used

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093 (96)00079-8

M. Taguchi et al. / Journal of Non-Crystalline Solids 198-200 (1996) 899-902

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Fig. l. Temperature dependence of the dc conductivity of all four nc-Si:H,F films.

co-planar gap (length 1 = 0.5 mm, width w = 7.5 ram) electrodes of evaporated aluminum. The dc conductivity was measured using the two-terminal method over the temperature range of 30 to 400 K. The ac conductivity was measured using LCR meters (HP 4274 A and 4275 A) at frequencies from 10 2 to 107 Hz at temperatures ranging from 30 to 300 K. The electron density was determined with the van der Pauw method. The temperature dependence of the electron density and mobility on the highest-conductivity sample was measured between 180 and 400 K on a Hall bar structure. The optical absorption spectra were estimated by combining the results of transmission and of photothermal deflection spectra (PDS).

3. Results Table 1 provides a summary of the deposition conditions and the room-temperature transport properties of the four samples on which we concentrate in this paper. When the dc conductivity ~ra~ is plot-

f (Hz) Fig. 2. Frequency dependence of the ac conductivity for two nc-Si:H,F films at several temperatures.

ted as a function of temperature (Fig. 1), the key difference between the samples appears. The two films with higher o-d0 are metallic; their trd~(0), obtained by extrapolating ~r~c to T = 0 K on a linear scale, is positive. The two other films have ~rdc(0) < 0 and are insulators. The electron density of the mostconducting film, No. 811, remains constant between 180 and 4 0 0 K a t n e = l × 1 0 z° cm -3. The ac conductivity %c of one metallic and one insulating film is plotted as a function of frequency in Fig. 2 for several temperatures. The low-temperature (30 K) o:ac of the highest-conductivity sample 811 is independent of frequency up to 107 Hz. The other samples behave similarly to sample 603. Their O'a~ is constant at low frequency f. As f is raised, o:a~ begins to rise at a frequency fhop, which depends on temperature. We define fhop, where ac conduction begins to dominate over dc conduction, as the frequency where ~rac is 10% higher than O'dc. Fig. 3 shows the relation between o-ac and fhop estimated in this way. Clearly, O-dc and fhop are proportional to each other. This proportionality implies that ac and

Table 1 Preparation conditions, Tauc optical gap, and transport properties at 300 K for four nc-Si:H,F films Sample

L (°C)

SiFa:H 2 (sccm)

Eg (eV)

O-ac (S cm -t)

ne (cm-3)

/~e (cm2 V 1 s-I)

811 610 809 603

290 250 290 250

2.7:27 2.5:25 4.0:20 4.0:20

1.80 1.90 1.67 1.91

30.9 5.60 1.16 0.657

1.0 1.7 2.6 1.4

1.93 2.06 2.78 29.3

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M. Taguchi et al. / Journal of Non-Co,stalline Solids 198-200 (1996) 899 902 101

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dc conduction are closely correlated and arise from the same mechanism. The conductivity data suggest that conduction proceeds at the Fermi level, which lies in extended states for samples 811 and 610, and in localized states for samples 809 and 603. This conclusion is supported by the optical absorption spectra a ( h v ) of Fig. 4, where we also plot the spectra for crystalline Si and a typical a-Si:H film. At low photon energies samples 811 and 603 (and the other two nc-Si:H,F films) absorb strongly, due to high defect densities and free carrier absorption. The pronounced rise of a of sample 811 toward low h ~, suggests free carrier absorption.

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where the value of x depends on the dimensionality and the nature of the hopping process. Variable-range hopping would make x = 1 / 4 for a constant density of states [8], or x = 1 / 2 for a parabolic density of states with a soft gap, (i.e. N ( E ) ~ ( E EF)2). Such a soft gap is consistent with ac conductivity measurements in doped crystalline silicon [9]. To decide between these alternatives, we log-log plot the temperature-dependent component of Crdc, A~r(T) = O-d~(T) -- ~rac(0), for the two metallic films (Fig. 5). Film 811 exhibits a clear T ~/2 dependence at T < 270 K. Therefore, electron-electron interaction dominates the inelastic scattering in this sample. -

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902

M. Taguchi et al. / Journal of Non-Co'stalline Solids 198-200 (1996) 899-902

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The electrical transport properties and the optical absorption spectra of highly conducting nc-Si:H,F films were studied. We observed the metal-insulator transition at an electron density of ~ 1 × 1019 cm -3. Overall, the film properties are similar to those of heavily doped crystalline silicon near the m e t a l - i n sulator transition, except for the lower electron mobility.

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T-v2 (K-l/2) Fig. 6. O'dc of insulatingfilms as a functionof T- l/2. Sample 610, however, deviates from the T 1/2 behavior below 80 K. This film exhibits ~ c ( T ) similar to film 603 (Fig. 2), so that it must lie close to the metal-insulator transition. On the insulating side, the hopping conductivity ~rdc with variable activation energy is described by Eq. (2). Through detailed data analysis we have obtained the value of the exponent x, which is ~ 0.5 at 30 to 150 K. Fig. 6 shows log O-dc as a function of T-~/2 in this T-range for the two insulating films. The T - i/2 dependence predicted by Eq. (2) is obeyed well, and suggests hopping in a parabolic density of states. The high-frequency ~,c is known to follow an approximate power law of the form ~ (x w '~, with an exponent s less or equal to unity [9,10]. While analysis of the frequency-dependent part of ~ c , A ~rac = ~r~ - ~rdo, above fhop yields values for s of 1 / 3 to 1 / 2 , the accessible range of w may be too small for a reliable determination of s. The temperature dependence of OLc and o-ac are similar, suggesting that the ac transport proceeds via localized states near the mobility edge.

Acknowledgements This work is supported in part by the Electric Power Research Institute and by the New Energy and Industrial Technology Development Organization ( N E D O / M I T I ) and ARPA.

References [1] S. Usui and M. Kikuchi, J. Non-Cryst. Solids 34 (1979) 1. [2] G. Willeke, W.E. Spear, D.I. Jones and P.G. Le Comber, Philos. Mag. B46 (1982) 177. [3] J. Kolodzey, S. Aljishi, R. Schwarz, D. Slobodin and S. Wagner, J. Vac. Sci. Technol. A4 (1986) 2499. [4] M. Taguchi and S. Wagner, Mater. Res. Soc. Symp. Proc. 358 (1994) 739. [5] B.L. Altshuler and A.G. Aronov, Solid State Commun. 30 (1979) 115. [6] N.F. Mott, Metal-Insulator Transitions,2nd Ed. (Taylor and Francis, London, 1990) p. 42. [7] R.F. Milligan,T.F. Rosenbaum,R.N. Bhatt and G.A. Thomas, in: Electron-ElectronInteractionsin Disordered Systems, ed. A.L. Efros and M. Pollak (North-Holland,Amsterdam, 1985) p. 231. [8] N.F. Mott, J. Non-Cryst. Solids 1 (1968) 1. [9] M.A. Paalanen, T.F. Rosenbaum, G.A. Thomas and R.N. Bhatt, Phys. Rev. Lett. 51 (1983) 1896. [10] A.R. Long, in: Hopping Transport in Solids, ed. M. Pollak and B.I. Shklovskii (North-Holland, Amsterdam, 1991) p. 207.