Use of a small-voltage electrical response to measure sodium motion in rf sputtered SiO2 films

Journal of Non-Crystalline Solids 33 (1979) 391-403 © North-Holland Publishing Company

USE OF A SMALL-VOLTAGE ELECTRICAL RESPONSE TO MEASURE SODIUM MOTION IN RF SPUTTERED SiO2 FILMS M. MEAUDRE, R. MEAUDRE and A. DEGUIN Laboratoire de Physique Electronique, Universit~ de Lyon 1, 69621 Villeurbanne, France

Received 5 January 1979 Revised manuscript received 12 March 1979

Low voltage transient and ac responses of rf sputtered SiO2 films are studied in the temperature range 473-593 K. Experimental results can be explained in terms of a model with trapped carriers and with only one type of recombining mobile charge carriers. Two distinct. phenomena clearly appear in transient current curves; the rapid one is attributed to the space charge effect, the other to generation recombination. Theoretical and experimental curves are in good agreement. The concentrations and mobilities are deducted of carriers identified as Na÷ ions. Concentrations values are compared (for glass and silica substrates) with ion microprobe analysis results.

1. Introduction There are a considerable number o f publications concerning thermally grown SiO: films [1] but fewer projects have been carried out on rf sputtered SiO2 films, and those done have generally studied the influence o f experimental conditions on the thickness, density, structure and composition o f films [ 2 - 1 7 ] . Within the scope o f electrical measurements on rf sputtered SiO2 films we can quote: the investigation o f dielectric behaviour in the frequency range 103-107 Hz and dc leakage currents by Franklin [18]; the study o f thermally stimulated currents by Hickmott [19] to measure additional Na motion and b y Hino [20] and Hino et al., [21] to determine the relaxation time o f dipoles; the use o f conventional C - V method by Kumar and Gregor [ 14] and Schreiber and Fr6schle [ 16] to obtain the ion concentration or interface state density; the measurements o f breakdown fields by Franklin [18] Davidse and Maissel [2] and Schreiber and Fr6schle [16]. None o f these projects gives information on the motion o f the intrinsic charge carrier existing in the films. Therefore it was o f interest to study polarization processes likely to clarify this point. This investigation o f dielectric behaviour was performed b y transient and alternating current measurements. Such an analysis has never before been made neither in rf sputtered SiO2 films nor in thermally grown SiO: films. 391

392

M. Meaudre et al. / Sodium motion in R F sputtered SiO 2 films

2. Samples The rf sputtering equipment used for the deposition of SiO2 films was a diode sputtering system [22]. Typical data for the experiments were given in a previous paper [23]. The films obtained were analyzed by different methods. Electron diffraction has shown their amorphous character. It is important to state the amorphous character, as the position of absorption bands in the IR spectra can be influenced by crystalline zones present in the fihn. We obtained the infrared spectrum represented in fig. 1. The Si-O stretching band is shifted to 9.43/~m; for analogous displacements of the S i - O stretching band,Pliskin et al. [24] have calculated that these films are only slightly oxygen deficient, having a formula of SiOx with 1.95 < x < 1.99. Stoichiometry of our SiO2 films was confirmed by d, p nuclear reactions. Electrical measurements were made on A u - S i O 2 - A u samples. Au was pre-evaporated on glass (type I sample) or silica (type II sample) substrates. After sputtering of SiO2 an Au film was deposited to establish a capacitor Au-SiO2-Au. The contact areas were 3.14 mm 2. SiO2 film thickness varied between 500 and 3000 A. Before electrical measurements, in order to eliminate moisture, which generally masks true conductivity and frequency spectral characteristics, the films had been heated at 200°C in the vacuum chamber with a pressure lower than 10 -s Torr for about five days. 9

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Fig. 1. Infrared spectra for a rf sputtered SiO2 film, about 3000 A thick.

400

393

M. Meaudre et al. /Sodium motion in RF sputtered SiO 2 films

3. Experimental 3.1. M e t h o d

Electrical measurements have been made in a vacuum lower than 5 X 10 -6 Torr. The transient current response due to a step voltage of 50 mV was measured with a Keithley model 610 C electrometer and recorded. Capacitances Cp and conductances G o were inferred from the following method: a low frequency sinusoidal voltage u(t) = Re[V exp(/cot)] was applied to the sample and the ac current i(t) passing through it was measured. One has: i(t) = Re[(Gp +/Cpco) V(cos cot +/sin co0]

= GpVcos cot - CpcoV sin cot. The resulting i(t) = f(o) curve is an ellipse, i(t) was measured in the frequency range 5 X 10-4-0.2 Hz with a Keithley model 610 C electrometer and i(t) =rio)

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396

M. Meaudre et al. / Sodium motion in RF sputtered SiO 2 films

curves were plotted using a Hewlett Packard model 7000 A / X - Y recorder. A Tektronix model 547 oscilloscope was used in the frequency range 0 . 2 - 2 0 Hz. For ~otl = 2k~r ,

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3.2. Transient current

The time response of the current due to a step voltage was studied in the temperature range 473-593 K. The current-time curves obtained for samples P17C~ (type I sample) and P2oC 6 (type II sample) are shown in figs. 2 and 3. Depolarization currents were also studied. Polarization and depolarization currents followed Ohm's law closely for applied electric fields up to 10 6 V m -I. The principle of superposition was obeyed. 3.3. L o w frequency behaviour

Dielectric behaviour of sample P2oC6 (type II sample) was investigated from 5 × 10-4-20 Hz in the temperature range 473-530 K. Figs. 4(a) and 4(b) show a dielectric dispersion at low frequencies, this dispersion is characterized by large variations in capacitance. Alternating currents were not measured in sample P~7C1 but the dielectric behaviour of analog type I films was previously given [23]. The static capacitances of type I samples was found to be larger than those of type II samples.

4. Results

The large experimental values of static capacitance and the fact that they do not depend practically on electrode separation L (for type I film having L = 600 A, 1200 A, 2500 A, we measured respectively 1.8/aF, 2/~F, 1.6/IF) lead us to exclude a dipolar model or the Friauf or Macdonald explanation [25,26]. Jaffe and Lemay [27] have given an expression for the transient current in the case of non-recombining positive and negative carriers with equal mobilities and the same boundary conditions. They assumed a uniform electric field so that Poisson's equation is not obeyed and the voltage is at least as large as 8KT/e. The theoretical time constant ratios were not observed experimentally. In the same case of non-recombining carders with equal mobilities Macdonald [28] gave an expression for the current which only applies to interfaces, and the corresponding (non-blocking situation) static capacitance is proportional to L. Fatuzzo and Coppo [29] have taken into account

M. Meaudre et al. / Sodium motion in RF sputtered Si02 films

397

the space charge layers spontaneously formed at the crystal surfaces; they assumed completely blocking electrodes and equal carrier mobilities. The transient current is governed by only one voltage dependent time constant; this fact and the complete blocking of carriers were not observed by us. Fonash et al., [30-31] have investigated the case of Schottky barriers but the relation between initial currents and time constants were not satisfied, furthermore an electronic conduction seemed unlikely in our low voltage low frequency situation. Our model [32] taking into account generation recombination at all the states of the gap can give theoretical dielectric dispersion curves similar to experimental ones but leading to a high and unacceptable density of states. Finally when only one type of carriers can recombine at a single level we have shown that the transient response is made up of the sum of two exponential decays [33-35]; when certain experimental curves are fitted to this model it is impossible to satisfy the theoretical relations existing between the different coefficients. For all the previous reasons we studied a slightly more sophisticated model [36]. We considered a sample with concentration M d of immobile neutral centres before dissociation. After dissociation n(x, t) carriers of mobility g and M a trapped carriers appear, nd(x, t) centres are not dissociated. The number of carriers produced per second at position x is proportional to the product of na(x, t) and a generation coefficient k, the number of recombinations per second is ~n(x, t) [Ma - na(x, t)]. When a low step voltage V is applied at time t = 0, the transient current for electrodes of unit area may be stated in the following form:

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398

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When using a least squares computer program our experimental results were successfully fitted with eq. (1). Second order terms such as 13(Ma + no)~no/P2o are not taken into account during the fitting of transient current curves, nevertheless they were given in eq. (1) because they lead to first order terms in the expression of the complex dielectric constant. Theoretical curves and experimental points are shown in figs. 2 and 3; a very good agreement is obtained and this was for transient current durations as long as eight hours. Values of parameters are listed in table I, they have also been used in eqs. (2) and (3) to calculate the theoretical Cp and Gp curves of fig. 4. One can note from the Gp curves that for frequencies larger than 0.1 Hz, another phenomenon appears, a work in progress shows that a hopping process is very likely.

5. Discussion Electronic conduction may be due to the motion of free carriers or alternatively to the motion of quasi-localized carriers. Mobility values o f free carriers [ 3 7 - 3 9 ] are considerably larger than the ones given in table 1. Mobility values o f carriers hopping between localized sites can be very low and only the smallness of the activation energy remains a plausible argument for determining the nature of the carriers. As a general rule, activation energies in excess o f 0.6 eV indicate an ionic transport [40], as activation energies deducted from figs. 5(a) and 5(b) are 1 eV we conclude electronic transport is not responsible for the observed effects. Furthermore, ionic conduction is characterized by low mobilities, indeed, in fused silica Lee [41] reported a mobility of 2.8 × 10 -12 m 2 V -I s -1 for H ÷ at 140°C, Owen and Douglas [42] reported one of 2.8 × 10 -18 m s V -1 s -1 for Na ÷ at 140°C; in thermally grown SiO2 films Hofstein [43] measured a mobility of 1.4 × 10 -13 m 2 V -1 s -1 for H ÷ at 140°C, Sugano et al., [44] reported one of 6 × 10 -16 m s V -1 s -1 for Na ÷ at 200°C and Snow et al., [45] used a mobility value found by Owen and Douglas to explain their results. Though slightly lower, our mobility values may be compared with the previous

400

M. Meaudre et al.

/ Sodium

motion in R F sputtered SiO 2 films

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(b) Fig. 5. Plots of carrier mobility as a function of reciprocal temperature. (a) Type I film (P17C1 sample), (b) type II film (P2oC6 sample).

ones for Na ÷. Activation energy f o r Na ÷ mobility in SiO2 was found 0 . 9 - 1 . 4 eV [ 4 4 - 4 5 ] . According to Kriegler and Bartnikas [46] values of about 1 eV, which is in good agreement with ours, would really characterize the mobility, whilst higher values would characterize trapping effects at the interfaces. So observed conduction mechanisms seem t o be due to Na ions. Analysis of films by an ion probe microanalyzer confirmed the presence of sodium. Profiles of Na concentration are given in figs. 6(a) and 6(b) for type I and type II films. Na concentrations deducted from electrical measurements are higher than Na concentrations measured by analysis, but both methods give the same variation of content between films o f type I and type II. Consequently polarization effects are undoubtedly due to Na ion motion. In the case of ionic conduction, according to some authors, [ 4 7 - 5 0 ] , the Ein-

401

M. Meaudre et al. / Sodium motion in R F sputtered SiO 2 films

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stein relation may be replaced by I~/D = e/fKT where f is a correlation factor smaller than unity; this leads to smaller concentration values ( × f ) a n d thus to a better agreement with microprobe analysis results. One has now to justify the set of equations [36] leading to eqs. (1), (21) and (3). In SiO2 films, some S i - O - S i linkages are broken, resulting in oxygens attached to only one silicon, or non-bridging oxygens. According to Charles [51 ] there are positions of stability for Na ÷ ions about non-bridging oxygens separated from each other by energy barriers. We then consider that the M a immobile neutral centres existing per unit volume before dissociation, consist of non-bridging oxygen, that is a negatively charged oxygen, with an Na ÷ ion in the neighbourhood [52]. After dissociation n(x, t) positive carriers of mobility/a (Na ÷ ions) and M a trapped carriers appear, ha(X, t) centres are not dissociated. Charge motion is due to the applied electric field and diffusion process. Na ÷ ions are partially blocked at the electrodes. Eqs. (1), (2) and (3) have been obtained in the case of negative mobile and trapped carriers but it is easily shown that changing the sign of these carriers does not change anything in the final result. The presence of trapped ions has often been observed [53-56]. It must be underlined that our/3no values are of the same order of magnitude as the k's derived from Hickmott [54] in the case of rf sputtered SiO2 films for 1-1.65 eV trap depth.

402

M. Meaudre et al. / Sodium motion in R F sputtered SiO 2 films

6. Conclusion Low voltage electrical behaviour of rf sputtered SiO2 films has been studied by t r a n s i e n t and alternating Current measurements. The interpretation o f experimental results has necessitated the change o f a previously published model by adding trapped carriers which do not take part in conduction. Good fitting o f experimental curves has allowed us to obtain the concentration and mobility o f carriers considered as Na ÷ ions. When glass substrates with a high Na content are used, SiO2 films are polluted by sodium contained in substrates. When silicon dioxide substrates are used, very clean films are obtained since the Na content was o f the same order as that estimated by Ridley [57] for the purest thermally grown SiO2 films. Differences between the Na concentrations obtained by microprobe analysis and deduced from electrical measurements probably arise from our model which is too simple. Different boundary conditions leading to smaller concentrations could be envisaged [58,59], unfortunately the corresponding expressions for the transient current are not yet available. Furthermore it is very likely that the main discrepancy arises from non-uniform ionic distributions such as those appearing in figs. 6(a) and 6(b) and not taken into account by the theory.

Acknowledgements The authors wish to thank Mrs A. Joly and Mr R. Chifflet for computer analysis, and Dr B. Blanchard for ion microprobe analysis.

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M. Meaudre et al. /Sodium motion in R F sputtered SiO 2 films

[14] [15] [16] [17] ] 18]

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