On the origin of photocurrent oscillation at Si electrodes

159

J. ElectroanaI. Chem., 351(1993) 159-168 Elsevier Sequoia S.A., Lausanne

JEC 02548

On the origin of photocurrent

oscillation at Si electrodes

H.J. Lewerenz and M. Aggour Bereich Phok&emische Energieumwandlung, Hahn -iUeitner Institut, Postfach, loo0 Berlin 39 (Germany) (Received 7 August 1992; in revised form 9 October 1992)

Abstract

Photocurrent oscillktions at n-type 8100) were investigated in ammonium fluoride solutions for a variety of electrolyte compositions and pH values. The data show a linear correlation between the etch rate for anodic oxides and the oscillation frequency at fmed applied voltage. A model based on minority carrier collection at the boundary between pores and the silicon substrate is suggested. Because of the large diffusion length of holes in silicon, the strong photocurrent modulation observed can be explained by changes in a relatively small number of charge-collecting pores while a substantial overall oxide thickness is maintained. A transition to noisy behaviour is also observed when the solution composition is changed to give a higher etch rate. The charge collection model used here to interpret the photocurrent oscillation is also the basic functioning principle of silicon point contact solar cells.

INTRODUCTION

Oscillation of current or potential in connection with electrochemical processes has been known for more than a century [ll. A large variety of electrochemical systems can show oscillatory behaviour. The waveforms of the current oscillations at constant or variable external potential are often remarkably similar in different systems. The appearance of current oscillations in silicon in fluoride solutions at high potentials was mentioned early by Turner [2] and has recently been addressed by several authors [3-71. It has been suggested that the presence of such oscillations can be caused only by a non-linear correlation between the formation and dissolution of the oxide. As part of an intensive study of silicon in different ammonium fluoride media; we present here a description of the oscillatory behaviour of the anodic dissolution of silicon in such electrolytes and the conditions for the appearance of this phenomenon. A model of a mechahism involving oxidation and dissolution at the surface is proposed.

160 EXPERIMENTAL

Single crystals of n-type and p-type silicon with the (100) faces exposed were used. The n-Si was doped with phosphorus in the range l-3 R cm and the p-Si was doped with boron (1-22 fl cm). The electrochemical measurements were made using a platinum counter-electrode, a saturated calomel electrode (SCE) as the reference electrode and a potentiostat-galvanostat (Heka model 128). The electrolytes were solutions of ammonium fluoride in triple-distilled water and the pH was adjusted by addition of H,SO,. A halogen lamp (Osram, 35 W) was used as a light source; the light intensity incident on the electrode was measured using a thermopile (LMT). The silicon electrodes were covered with thermal oxide to a thickness of 70 A; ohmic back-contact was made using a Gain amalgam. Oxide removal was achieved by etching the SiO,/Si structure in 0.5 M NH,F (pH 4.6) at +0.5 V/SCE. The etching produces the well-known dark current transient with a pronounced maximum when the etching front reaches the interface. Photocurrent oscillations were investigated for positive potentials of 6 V/SCE for n- and p-Si at a light intensity of 70 mW cmd2 for n-Si. The comparably high light intensity was chosen to facilitate comparison of the behaviour of n- and p-type Si. RESULTS AND DISCUSSION

Figure 1 shows the photocurrent versus time behaviour of n-Si in 0.1 M NH,F at various pH values under potentiostatic control. As soon as the light was

Time I s

Fig. 1. Current oscillations on illuminated n-Si in 0.1 M NH,F for different pH values: light intensity, 70 mW cm-*; electrode potential, + 6 V/SCE.

161

0

50

100

150

2@3

250

303

Timels

Fig. 2. Current oscillations on illuminated n-Si in 0.1 M, 0.2 M and 0.4 M NH,F intensity, 70 mW cm-*; electrode potential, + 6 V/SCE.

at pH 4.5: light

switched on, the current rose to a large value (not displayed here), then fell within 2 s to a low value of 80 PA cm-* and subsequently began to oscillate until it gradually reached a stationary current. The initial peak is related to photocurrent multiplication on the oxide-free silicon surface as long as surface atoms can interact directly with fluoride ions from the solution [8-111. The height of this peak increases rapidly with increasing fluoride concentration, illumination intensity and applied voltage. The effect can be attributed to the concordant injection of charge carriers from surface complexes, formed under illumination by minority carriers, into the conduction band. In a similar experiment [12] a distinct variation of the ellipsometric parameter A, which can be attributed to thickness changes, has been observed during photocurrent oscillations. The photocurrent is modulated strongly although the underlying oxide, which is obviously unaffected, is about 80 w thick. The oscillations and their duration depend on the type and. doping level of the silicon substrate, the fluoride concentration, the pH, the illumination and the applied bias. As shown in Fig. 1, the oscillation frequency increased as the solution pH was decreased. The frequency also increases when the fluoride concentration is increased (Fig. 2) at a constant pH of 4.5. In more concentrated ammonium fluoride (above 0.4 M) the oscillations disappear with n-Si and we observe that the platinum electrode releases bubbles of gas. For example, the duration of the oscillations in 0.1 M NH,F (pH 4) is longer by a factor of 60 than that in a solution with pH 2.5 (Figs. 1 and 3). The same difference is observed between 0.4 M and 0.1 M NH,F, both at pH 4 (Fig. 2). Therefore the stationary limit is shown only for

0

50

150

XYI

Time/s

Fig. 3. Current oscillations on p-Si in 0.1 M NH,F + 6 V/SCE.

at different pH values: electrode

potential,

the more strongly damped oscillations. The height of the stationary current increases rapidly with increasing fluoride concentration, but the influence of the pH also is large. The maximum stationary current is observed around pH 2.5 in dilute NH,F ( < 0.5 Ml. The behaviour of p-type specimens is similar. The current on the oxide-free specimen is initially controlled by the applied bias. At higher bias, oscillations are also generated on p-type samples, as can. be seen in Fig. 3 for 0.1 M NH,F at 6 V/SCE and different pH values. Here, too, the oscillation frequency increases for lower values of the pH. In contrast with n-Si, the oscillations on p-Si at higher NH,F concentrations (O-4-0.6 M) occurred without releasing gas bubbles at the counter-electrode. This may be due to the reduced number of holes at the n-Si interface because of limited illwnination intensity. To investigate the parameters leading to the current oscillations in more detail, the correlation with the ~m~sition of the NH,F electrolyte is analysed. NH,F is completely dissociated in pure H,O according to NH,F NHZ+F(1) The addition of a small amount of H,SO, to the NH,F solution produces three fluoride species: FL, HF; and HF. Th e relative concentrations of these species are given by K,[HF] = [H+][F-] K, = 1.3 x 10-3 (2) I(, = 0.104 K,[HF;I = EWEF-1 (3) where K, and K, are the dissociation constants. The values given are those most frequently cited in the literature 1131.

163

0.12

I

I

I O.lM

I

I

I

NH,F -_

-

PH Fig. 4. Mole fraction of fluoride ions F-, HF; and HF calcukted for 0.1 M NH,F using the constants from refs. 14 and 15.

Since it is well known that HF and HF; are the only hydrogen complexes present in dilute solution [14], the total concentration of fluorine-containing species in NH,F is given by [F] = [F-] + [HF] + 2[HF;] (4) Substituting eqns. (2) and (3) into eqn. (4) gives 2K,tHF12/&[H+] + (1 +WtH+l)Wl = PI (5) Figure 4 shows the variation of the concentrations [F-l, [I-IF;] and [HFI with pH for a 0.1 M solution. The curve can be divided into three regions. In strongly acidic solution, about pH 0 to pH 2, HF is the dominating species. In the range pH 2 to pH 4, HF; begins to increase and reaches a maximtmr at pH 3.2 at the i;oint where [HF] = [F-l and decreases for smaller pH. Above this pH range Fbecomes a significant species and increases monotonically up to pH 5, where it reaches a plateau. In earlier work on etching silicon oxides in fluoridic solution, the etch rate of SiO, has been related to the concentration of HF; and HF, with the former acting four to five times faster than the latter 1151. The overall reaction for the dissolution of SiO, in HF-containing solution is SiO, + 6HF H,SiF, + 2H20 (6) and the attack of HF; on SiO, films can be described by SiO, + 3HF; SiFi- + OH-+ H,O (7)

164

0.2

0 0

1

2

4

3

5

6

1

PH

Fig. 5. pH dependence of the etch rates for SiOz at different NH,F concentrations.

The F- ion is excluded as the etching species of SiO, but can react with silicon as follows: Si + 2F-+ 2h+ SiF, (8) The rate of dissolution of SiO, films in fluoride solutions can be qualitatively described by R/A

s-l =a([HF]/mol

1-l) + b([HF;]/mol

1-i) + c

(9) with the following values for a, b and c at 25°C [El: c= -0.14 a = 2.5 b = 9.66 The constant c corresponds to the error in fitting the data; its value will be lower for a better fit. Figure 5 shows the etch rates R of SiO, calculated using eqn. (9) and the above values of Q and b with c = 0 (no negative etch rates) as a function of the pH for various NH,F concentrations. It shows a high etch rate in the strongly acidic range. At high NH,F concentrations, the maximum etch rate is found to be around pH 3.2 where [IIF;] and [HF] have their optimum values. Since eqn. (9) describes the dissolution of thermal oxides, the question arises as to whether the etch rate deduced can be correlated with experimental results on anodically formed oxides. It has been shown that, at low NH,F concentrations, normalized etch rates are almost identical for various types of silicon oxide [16]. This indicates that the ratio b/a in eqn. (9) is relatively independent of the oxide composition and morphology. Therefore we assume that the b/a ratio is not substantially altered for the anodic oxides and plot the etch rate according to eqn. (9) in Fig. 7 below. Only the relative etch rate can be given because of the uncertainty concerning the absolute etch rates of HF and I-IF;.

165 160 -

n-Si

140

-

-

-

0.05 M O.lM 0.2 M

120 100 En 60 40 20 n 2

2.5

Fig. 6. Dependence

3

3.5

4

4.5

5

5.5

PH of the period of oscillation on the calculated

rate.

The experimental data for the period of photocurrent oscillation at different concentrations are shown in Fig. 6 as a function of the pH; the curves show the polynomial fittings. It is clear that the slopes of the fitted curves decrease at higher concentrations. The variations of the etch rate R with pH (Fig. 5) and the oscillation period T with pH (Fig. 6) allow us to plot the oscillation period for different pH values and concentrations as a function of the calculated etch rate. The result is shown in Fig. 7 for n-Si. The fit of the data by a mean least-squares approximation yielded the

0

0.5

Relative etch rate Fig. 7. Dependence of the relative etch rate on the oscillation period for n-S at different NH,F concentrations and pH values: y = 3.4928x( - l.OlOS), R = 0.82761.

166

Electrolyte

4 4: .

Electrolyte

Electrolyte

A-v

F&. 8. Schematic illustration of oxidation and dissolution across point defects at the Si-SiO, interface.

functional dependence T =AR-’ where A = 3.49. Hence the etch rate is directly proportional to the oscillation frequency; f = T-l for a wide range of concentrations and pH values. The result suggests that the oscillation frequency is directly related to oxide removal. To aid our understanding of the origin of the current oscillations, we have constructed three Si-SiO, interface models (Fig. 8). Because of the very high positive potentials and illumination of the silicon, a positively charged layer forms at its surface. A thin layer containing a high concentration of negatively charged ions (OH-, HF;, F-) is formed in the electrolyte. The ions react with silicon and lead to oxide formation and silicon dissociation, and the following possible reactions are considered [S]: Si + 40H,

+ Ah+ -

Si(OH), + (4 - h)eI I----+

Si + 4HF + (4 - A)h+

d

(10)

SiF, + 4H++ heI I2

where OH,

SiO, + 2H,O

0 < 4)

H,SiF,

(11)

is the adsorbed hydroxyl ion, and h+ and e- represent a hole and an

167

electron respectively. The participation of holes and electrons in the reaction depends on’ the silicon carrier type. The current reaches a maximum and decreases with oxide growth. Because of the difference in molecular volume between SiO, and silicon, there is a need for a free volume at the oxidizing interface. This free-volume requirement is the origin of point defects in the oxidizing film, leading to pores in the structure [17]. As the formation of the oxide is completed, positive charge accumulates at the silicon surface and an inversion layer develops in n-Si (accumulation layer in p-Si) leading to a potential drop across the Hehnholtz layer resulting in passivation. In fluoride solution, however, silicon oxide is not stable and will be dissolved by HF and HF; ions, especially across pores and point defects at the Si-SiO, interface, with an increased etch rate (Fig. 8). The holes have to migrate through the sample following trajectories due to diffusion and drift as shown in Fig. 8(a) to reach the uncovered part of the silicon surface and contribute to the current. All holes present in the half-sphere with the radius L, CL, is the diffusion length of the holes) can reach the surface. The silicon surface then depletes and forms an intrinsic region. The oxidation is then rapid; therefore the current increases and reaches a maximum. Because of the increased oxidation of pores, the current decreases, leading to blocking of carriers [18]. Since the diffusion lengths are large in single-crystal silicon, a small number of uncovered surfaces, separated by about 50-100 pm, suffices to collect the holes resulting in high quantum efficiencies for n-Si (QY > 0.8). However, other point defects and pores will be attacked by the etchant (Fig. 8(b)), leading to the extension of existing pores and the formation of new pores where the positive charges are then again collected efficiently (Fig. 8(c)). The model suggested here resembles the charge-collection principle in point contact silicon concentrator solar cells which is also based on the spatial distribution of charge-collecting centres spaced such that the minority-carrier diffusion length allows for high quantum efficiency 1191. The oscillations of current at constant external potential are dependent on the number of point defects and pores, their radii and separation at the surface of the semiconductor, and the etch rate. The number of point defects and pores on SiO, increases with the acidity and concentration of the fluoride solution and also with time. In an acidic solution the number of point defects and pores is higher and their attack by NH,F is so strong that the current oscillates rapidly. At pH values below 2.5, the oscillation disappears and the observed stationary current remains relatively high. With respect to less acidic solution, the growth of SiO, occurs at a small number of point defects. The decreased etch rate (Fig. 5) enables us to predict a large oscillation period for the current. At pH values above 5, the oscillation disappears because the etch rate is too small to compete with oxidation, and the stationary current remains relatively small (passivation). At pH 4.5, a transition from sustained oscillation toward decaying oscillations is observed (compare Fig. 21, leading to noisy behaviour (full line in Fig. 2) for t > 150 s. Since the proposed model incorporates competition between oxidation at pores and etching of the oxide which is synchronized by the electrical field, sustained oscillations as

168

well as noisy behaviour and damping can be described qualitatively. Noise is expected to occur for large numbers of pores with similar but different lengths so that the etching process results in statistical current signals. The correlation time then depends on the density and length of the point defects and/or pores. The stationary current also depends on the reaction mechanism of eqns. (10) and (11) and then on the dissolution rate R, which explains the variation of the stationary current and the maximum observed around pH 2.5 in Fig. 5. The synchronization of the oscillation is believed to originate from the external electrical field applied between sample and solution [20], thus defining the supply of holes to the surface and creating the same potential at spatially separate interfaces at which bare or less oxidized silicon is exposed. The proposed model correlates very well with recent in-situ ellipsometric data [21] and ex-situ X-ray photoelectron spectroscopy analyses of immersed samples during current oscillations [22]. In both these independent investigations it was found that the current oscillation is related to a thickness change of about 10% in an already existing oxidic film of thickness 35 A. This would be well explained by the fluctuating pore model since the overall thickness change is believed to be small. ACKNOWLEDGEMENT

This work has been supported by the BMFT (grant 0328926A). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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