Local Charge Storage in Thin Silicon Oxide Films: Mechanisms and

Article pubs.acs.org/JPCC

Local Charge Storage in Thin Silicon Oxide Films: Mechanisms and Possible Applications Carsten Maedler† and Harald Graaf*,‡ Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany ABSTRACT: We report on charge storage in silicon oxide below the micrometer scale as needed for the controlled attachment of charged particles. Thin silicon oxide films were locally charged using conductive atomic force microscopy. The charge was investigated using Kelvin probe force microscopy. We found that charges are stored in different traps in the silicon oxide matrix. We also discovered two different decay pathways: one parallel and one perpendicular to the surface. The parallel path, which is mediated by water adsorbed on the surface, was suppressed by a hydrophobic monolayer. This led to a stabilization of the stored charges in the oxide, which enabled the targeted attachment of ionic species from solution. As proof of concept, we deposited dye molecules selectively to previously written charge patterns on silicon oxide for the first time.

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water.11,13 This indicates that the charges are located close to the surface. Ushihashi et al. suggest two different phases of charge dissipation: a fast one due to surface conduction and a slow one where charges dissipate through the oxide and recombine at the silicon layer.12 Since water was found to be a major cause for charge spreading, low-dielectric liquids were used for the directed self-assembly of molecules onto the charge centers.14 The most successful approach to deposit watersoluble proteins while sustaining the previously deposited charges proved to be a water-in-oil emulsion prepared by sonication.15 In our study we investigated the charging mechanism and dissipation of trap states in silicon oxide on a submicrometer scale using Kelvin probe force microscopy. The charging experiments revealed multiple trap states and the presence of two different paths of charge dissipation. We studied the behavior of charges with respect to water by conducting the experiments as a function of relative humidity. In addition, we showed that passivation of the oxide surface with octadecylsiloxane substantially increases the lifetime of the charges. We were able to deposit dye molecules to previously written charge patterns, which was confirmed by confocal microscopy.

INTRODUCTION Silicon oxide has been shown to exhibit excellent charge storing properties if prepared properly.1 According to Sze, stable traps in silicon oxide layers are associated with defects in the oxide, which can be charged by introducing holes or electrons.2 The nature of those traps has been related to the diffusion of water into the oxide which results in the formation of silanol groups.3,4 These groups can be deprotonized by water and can result in a stable charge by capturing an electron: ≡SiOH + H 2O ⇄ SiO− + H3O+

(1)

SiO− + H3O+ + e− ⇄ SiO− + H 2O + H•

(2)

However, a long-term charge stability can only be achieved if both intrinsic bulk conductivity and surface conductivity are negligible. In addition to the charge trapping quality of silanol groups, they are also reported to be the major cause for charge spreading. Stored charge is transported along the surface through either their mobile protons or adsorbed water.5 Two approaches to reduce the surface silanol concentration are annealing6 or passivation of the silicon oxide surface with hydrophobic molecules involving the removal of silanol groups.7 In order to study the behavior of charges trapped in a laterally localized manner and to allow for the directed selfassembly of molecules onto those charges as proposed by Wright and Chetwynd,8 charge writing can be performed using an AFM.9 The evolution of charges can subsequently be monitored by Kelvin probe force microscopy.10 Studies about the charge retention in silicon oxide layers have found a clear size dependence. Charges stored over an area on the order of 100 × 100 nm2 have a considerably smaller lifetime (500 min, ref 11) than square millimeter-sized samples (projected lifetime of 500 years).7 There is disagreement on whether the stable traps are at the silicon−silicon oxide interface11 or whether they are on the oxide surface and in the bulk.10,12 Most groups found that the charges dissipate when the sample is immersed in © 2013 American Chemical Society

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EXPERIMENTAL SECTION Silicon substrates ((100), n-doped, 10−20 Ω·cm) with a 70 nm thermal oxide layer were cleaned with acetone and ethanol and rinsed with ultrapure water. Then, the wafers were sonicated either in piranha solution (2:3 volume fraction hydrogen peroxide and sulfuric acid) to obtain a high silanol concentration or in dichloromethane (all purchased from Merck). In order to remove the surface silanol groups, a third batch of wafers was passivated with an octadecylsiloxane monolayer by immersion in a 0.5 mM solution of Received: August 30, 2012 Revised: February 12, 2013 Published: February 13, 2013 5358

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Figure 1. Analysis of deposited dotlike positive and negative charges. (a) KPFM image of the previously trapped charges; the cross section is indicated with a black line. (b) The cross section graph is fitted by two Gaussian curves indicated by the red line; the two main values for further analysis are the fwhm and the peak values denoted as max bias.

change in surface potential due to the negative applied voltages is also negative, which indicates the transfer of real charge. A polarization of the oxide can be ruled out since any polarization charge would be opposite in sign to the tip voltage.17 Applied voltages up to −30 V do not alter the topography (see for example Figure 7). Also, there is no cross-talk between the topography and the potential signal. A tip voltage of −30 V results in a peak electric field in the oxide of about 4.3 MV/cm. This is below the onset of Fowler−Nordheim tunneling, which starts at about 6−7 MV/cm. Therefore, no electric current flows through the oxide.18 The charging mechanism was investigated by varying the applied tip voltage and measuring the amount of trapped charge. Treating the tip−sample−counter electrode system as a simple capacitor, one would expect that the amount of trapped charge is proportional to the applied voltage. This is not the case as can be seen in Figure 2, where the measured peak

octadecyltrichlorosilane (OTS, from ABCR) and 8 mM ultrapure water in toluene (from Merck).16 Subsequently, the water contact angle of the wafers was measured to give an indication of the surface silanol group concentration using an OCA 20 contact angle meter from DataPhysics Instruments. The contact angles were 35°, 70°, and 105° for samples treated with piranha, dichloromethane, and OTS, respectively. The AFM used for charge writing and detection is a Level AFM from Anfatec Instruments. For the deposition of charges onto the samples, a voltage was applied between a conductive AFM tip (Ti−Pt coated, Mikromasch) and a counter electrode beneath the sample while the tip was in contact with the sample surface. Unless otherwise stated, charges were deposited at several spots on the sample by engaging the tip, applying a voltage, retracting the tip, setting the tip voltage to zero and readjusting its lateral position, and then engaging the tip again. These steps were repeated several times. As a result, the charge pattern is dotlike with rotational symmetry around the axis perpendicular to the sample surface. Kelvin probe force microscopy measurements for the characterization of the deposited charges were performed in dual frequency mode, which allowed for the simultaneous imaging of both topography and potential difference. Noise levels were as low as 20 mV. After imaging the dotlike charge patterns (Figure 1a), the cross section through their center was plotted and fitted with a Gaussian curve (Figure 1b). As a measure for the width of the charge distribution and the amount of charges, the full width at half-maximum (fwhm) of the Gaussian peak and the peak height Vmax were used, respectively. A precise control of the relative humidity (RH) was obtained by feeding a mix of dry and wet nitrogen into an environmental chamber enclosing the AFM. It was measured with a humidity sensor (Sensirion, SHT75) placed right next to the sample. A water-in-oil emulsion was prepared for the directed attachment of dye molecules as follows. Perfluorodecahydronaphthalene (Alfa Aesar) was used to disperse rhodamine 6G (Radiant Dyes Laser) by adding 10 mM ultrapure water and sonication. Then, samples were immersed in that solution for 30 s. Afterward, the samples were characterized by confocal microscopy.

Figure 2. Dependence of the amount of trapped charge on the tip voltage. The sample was passivated with octadecylsiloxane, the RH was 30%, and the dwell time was 1 s.

potential of the stored charges is plotted against the voltage applied to the tip for a sample passivated with ODS. The amount of stored charges grows faster than the applied voltage, which means that the number of available trap states exceeds the number of charged traps. Even though most silanol groups are removed from the surface by passivation with ODS, there seem to be enough silanol groups and residual water in the oxide. It can also be seen that the voltage measured is significantly smaller than the applied voltage. This might be due to rapid charge dissipation during the first minute after charging and to the fact that a voltage was applied only for 1 s, which might not be enough time for a complete charge transfer. Another reason

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RESULTS AND DISCUSSION Charging Mechanism. In order to verify both the ability of trapping charges and the accuracy of their subsequent detection by KPFM, charge patterns were written in silicon oxide layers by applying a voltage to a conductive AFM tip. The observed 5359

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stored and neglects detrapping. The amount of different trap states is also not necessarily limited to two. Two different charging regimes were confirmed when varying several parameters, like relative humidity, applied voltage, and surface chemistry. The time constants are in the range from 0.14 to 0.19 s for fast trapping and from 5 to 7 s for slow trapping (see Table 1). Therefore, a strong dependence of the

is the detection of the charges by KPFM. Barry et al. proposed that the measured peak potential is 2−4 times smaller than the actual peak potential.19 Additionally, there is an offset between positive and negative charges and an overall threshold. For the same applied voltage only differing in sign, more negative than positive charge is trapped. This relates to the contact potential difference between the tip and oxide which was found to be about 500−700 mV (see background in Figure 1). Therefore, an offset between the absolute voltages for injection of positive and negative charges should be double that amount, thus 1−1.4 V, if assuming similar mechanisms for trapping of positive and negative charges. This agrees with the measured values (Figure 2). The threshold suggests electrostatic screening of the applied voltage. A possible explanation is the polarization of water molecules in the water meniscus that builds up between an AFM tip and the sample surface while both are in contact. Additionally, the ODS acts as an insulating layer.20 However, threshold voltages were observed on bare silicon oxide as well. In order to gain more insight into the charging mechanism, we studied the influence of charging time on the trapping of charge. For that purpose, charge was deposited at different spots on the sample by applying a voltage for different amounts of time between the tip and the sample while both were in contact. The amount of stored charge increases with increasing charging time and saturates (see Figure 3).

Table 1. Fitting Parameters for the Charging Curves of ODS Functionalized and Dichloromethane Treated Silicon Oxide at Different Relative Humidity (RH) and Applied Voltages (AV) sample ODS passivated, AV −30 V, RH 30% cleaned with dichloromethane, AV −10 V, RH 45% cleaned with dichloromethane, AV −30 V, RH 45% cleaned with dichloromethane, AV −30 V, RH 25% cleaned with dichloromethane, AV +30 V, RH 24%

Since, in this study, the amount of trapped charges is not limited by the amount of available traps but rather by the applied voltage, it is possible to describe the charging mechanism with an exponential behavior. However, the measured data were better approximated by a biexponential curve, which indicates that charges are stored in more than one trap state. Therefore, we suggest the following equation for the charging mechanism, where Vmax, Vmax,1, and Vmax,2 are the total peak voltage and the peak voltages due to the different traps, respectively, t is the time, and τc are time constants. ⎛ ⎡ t ⎤ Vmax ,1(∞) Vmax(t ) = Vmax(∞)⎜⎜1 − exp⎢ − ⎥ ⎢⎣ τc,1 ⎥⎦ Vmax(∞) ⎝ ⎡ t ⎤⎞ exp⎢ − ⎥⎟⎟ ⎢⎣ τc,2 ⎥⎦⎠ Vmax(∞)

Vmax,1(∞)/ Vmax(∞)

τc,2 (s)

Vmax,2(∞)/ Vmax(∞)

0.19

0.71

7

0.29

0.17

0.25

5

0.75

0.19

0.43

6

0.57

0.14

0.47

6

0.53

0.18

0.36

7

0.64

time constants on applied voltage, relative humidity, and surface chemistry can be ruled out within the accuracy of the measurements. Control experiments showed that rinsing the sample with water leads to a loss of all the charges in accordance with Jacobs et al.13 and Enikov et al.11 This suggests that the trapped charges are located close to the air−oxide interface. The fast trapping mechanisms are probably related to silanol groups according to formulas 1 and 2. Since the time constant of the slower process is a factor of about 30−40 higher, a larger energy barrier for the charge is assumed. This indicates that a chemical reaction such as the formation of an OH− attached to a silicon atom or an H3O+ to an oxygen atom, respectively, might be the charge trapping mechanism of the slower process. Bakos et al. calculated the energy barrier for such a reaction to be about 1.5 eV.21 It has been reported by other groups that the dissipation of charge is strongly correlated with the presence of a water layer on the sample surface.11,13 In order to gain information about the influence of relative humidity on the charging mechanism, experiments were performed at different RH values. Relative humidity was first increased to about 65% and then adjusted to lower values by carefully introducing nitrogen in the enclosed AFM chamber. This procedure was used to exclude any hysteresis effects as reported by Weeks et al.22 Because of residual water in the oxide, no dependence of charge trapping on the presence of water was found. However, more charge could be stored on a smaller area with decreasing RH values. This is evident from the graph in Figure 4 where the peak bias divided by the fwhm squared is plotted against the relative humidity. The charge density is greatest for the samples passivated with ODS. This indicates that the charge already spreads during the charging mechanism and during the time between charging and imaging and that this spreading increases with the amount of water on the sample. Charge Dissipation. The dissipation of stored charge was investigated by deposition of charge and subsequent continuous imaging of the area. The cross section through the center of the distribution is fitted for different time points after charging, and the time evolution of the maximum bias and fwhm are plotted.

Figure 3. Dependence of the trapped charge on the time for which a voltage was applied on ODS-covered silicon oxide. The applied voltage was −30 V and the RH 30%. A biexponential fit approximates the data points better than a monoexponential fit.

−

τc,1 (s)

Vmax ,2(∞)

(3)

This equation is only a simplification because it assumes that the peak voltage is proportional to the amount of charges 5360

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Table 2. Time Constants of the Exponential Decay of the Peak Potential of Trapped Charges in Different Samplesa sample ODS passivated, RH 30% cleaned with dichloromethane, RH 24% cleaned with dichloromethane, RH 45% a

Figure 4. Trapped charge density versus relative humidity for silicon oxide cleaned with dichloromethane. The dwell time was 0.01 s; the applied voltages are −20 V (squares) and +20 V (triangles). The lines are guides for the eyes.

charge

τD,1 (min)

τD,2 (min)

positive negative positive

1370 ± 30 1600 ± 40 184 ± 2

2360 ± 230

negative positive

210 ± 30 7.6 ± 0.3

2000 ± 500 162 ± 8

negative

12.5 ± 0.7

160 ± 10

The indicated errors are statistical errors due to fitting.

follows that the fast dissipation is due to water-assisted surface conduction as proposed by various groups.10,11,13 This is further confirmed by comparing fwhm curves for the different RH values. The fwhm increases by a factor of 3 during the first 400 min at 45% RH but only by a factor of 1.5 at 24% RH. Charge dissipated even more rapidly on samples cleaned with piranha solution (high density of silanol groups). Only very few data points could be observed due to the imaging time. Charge could only be detected until about 30 min after charging. The concentration of silanol groups and the amount of adsorbed water are also a likely reason for faster charge dissipation in experiments performed by other groups.11,12 In order to decrease surface conduction, a monolayer of ODS was chemisorbed on the silicon oxide as hydrophobic coating. The corresponding decay curve (Figure 6) shows no

First, the dissipation of stored charge on silicon oxide cleaned with dichloromethane and charged at RH 24% was studied. The plots of the maximum bias and the fwhm versus time after charging are shown in Figure 5. It is apparent that the

Figure 5. Decay curve of the maximum bias and the fwhm (inset) for silicon oxide cleaned with dichloromethane taken at RH 24%. The dwell time was 0.01 s; the applied voltages are −20 V (squares) and +20 V (triangles). The lines are biexponential fits.

maximum bias decreases and the fwhm increases with time. However, positive charges could still be detected more than 20 h after charging. Fitting the values in Figure 5 with an exponential decay as proposed by Crisci et al.23 yields a poor approximation of the curves. Looking more closely at the fwhm, two sections of the curve become visible. The first section is characterized by an increase in fwhm, whereas the fwhm remains almost constant for the second section. The kink in the curve occurs just before 400 min. A similar bend is seen in the decay of the maximum bias, also at about 400 min. Therefore, a two-phase decay is proposed. The fast decrease of maximum bias in the first section is due to surface conduction of traps with a low energy barrier, and the second section is dominated by dissipation of charges trapped in bulk states close to the oxide−air interface through the oxide. The suggested decay is thus biexponential. The obtained values for different samples are summarized in Table 2. Unlike the charging mechanism, which did not depend on the relative humidity, such a dependence seems to play an important role in the charge decay. Decay curves obtained at a relative humidity of 45% exhibited a more rapid dissipation of charge at the initial stage (not shown here). Since a higher RH causes an increase in the water layer on top of the sample, it

Figure 6. Decay curve of the maximum bias and the fwhm (inset) for silicon oxide passivated with ODS charged at RH 30% with −10 V (squares) and +15 V (triangles). The lines are monoexponential fits.

bend like those for the other samples did. Instead, the curve was approximated with a monoexponential decay with time constants of 1400 and 1600 min for positive and negative charges, respectively. This indicates that the rapid dissipation of charge due to surface conduction is inhibited, which can also be seen in the fwhm versus time after charging plot (inset of Figure 6). Here, the increase of the fwhm is smaller than for samples cleaned with dichloromethane (Figure 5). The passivation of electrets with hydrophobic layers is a known method for reducing dissipation of stored charges.7,11,12 However, the stability of charges obtained here is higher than the stability achieved by other groups in comparable nanoscale experiments with a coating of HMDS. This is because water contact angles for monolayers of HMDS (70°−73° in the experiments performed by Uchihashi et al.12) on silicon oxide are smaller than for ODS (105° in this work), which means that more water is present on monolayers of HMDS. Furthermore, 5361

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it is known that water penetrates more easily through a layer of HMDS than a layer of ODS due to the longer alkyl chains of ODS.24 After increasing the RH to 60%, a fast initial decay is observed even on the ODS-coated sample accompanied by a broadening of the charge distribution. The rapid decrease in peak potential is attributed to water-assisted surface conduction like it occurs on the uncoated samples. This means that between RH 30% and RH 60% a water layer builds up on the hydrophobic surface (water contact angle 105°) which enables dissipation of the charges on the surface. Nevertheless, the modification of the oxide surface by the hydrophobic monolayer stabilizes the stored charges, which enables a charge storage with high density. To demonstrate this, we correlate the applied voltage with a gray scale of a picture for each pixel of 150 × 150 nm2 (see Figure 7). The information can be stored over several hours without a significant loss of information.

Figure 8. Directed assembly of dye molecules onto charge pattern. (a) Charge pattern on silicon oxide coated with ODS. Voltage pulses of −40 V were applied for 0.1 s at RH 35%. (b) Confocal microscopy image of rhodamine 6G molecules selectively attached to charge centers trapped in silicon oxide coated with ODS. CUT is an acronym for Chemnitz University of Technology.

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CONCLUSION In conclusion, we performed the first systematic study on charge storage in silicon oxide on a submicrometer scale using Kelvin probe force microscopy and attached dye molecules selectively to previously written charge patterns. We found at least two states at the electret−air interface that act as traps for the charge carriers introduced by an electric field applied at a conducting atomic force microscope tip. The traps are likely protonated or deprotonated silanol groups, and OH− groups bound to Si atoms or H3O+ bound to oxygen atoms, respectively. Investigations of the charge decay suggest two different pathways. One path along the electret surface due to conduction mediated by silanol groups can be selectively closed by the introduction of a self-assembled monolayer yielding a hydrophobic surface and an effective termination of the silanol groups. The slower pathway can be attributed to dissipation of charges through the oxide. This method of studying charged states in silicon oxide is complementary to existing studies and should be built upon. Measurements in vacuum (as this can lead to narrower and more stable structures27,28) and eliminating residual water from the oxide are two examples of further improvements that would reveal more information about the nature of trap states. We were able to increase the lifetime of stored charges considerably and successfully attached dye molecules selectively to previously written charge patterns.

Figure 7. Charge pattern created on silicon oxide coated with an ODS monolayer by applying different negative voltages to an AFM tip in contact mode on a 128 × 128 grid. The applied voltage ranges from −30 to 0 V according to 256 different shades of gray of the original image. The pulse length at every grid point was 10 ms. Images of (a) the topography, (b) the electric potential of the written pattern, and (c) the original picture of Mount Hood.

Directed Attachment. The achieved lifetime and the density of stored charges on silicon oxide layers facilitate the directed self-assembly of nanoparticles and biomolecules in a controllable fashion. For precise control of successful attachment, cationic dye molecules were selectively deposited onto charge patterns and subsequently observed by confocal microscopy.25 In order to attach rhodamine 6G molecules to the samples, a charge pattern was written as shown in Figure 8a. Since rhodamine 6G molecules have the tendency to adsorb on the silicon oxide surface, no selective attachment could be achieved on plain silicon oxide. However, rhodamine 6G does not attach to the hydrophobic monolayer of ODS which enables the selective attachment to charge centers. Rhodamine 6G molecules were selectively attached to previously trapped charges in silicon oxide coated with an ODS monolayer from water-in-oil emulsions. The resulting confocal microscopy image is shown in Figure 8b. It reveals a correlation between the surface potential image and the confocal microscopy image, which means that the dye molecules primarily attach to the negative charge centers. Thus, we showed the selective attachment of rhodamine 6G molecules to previously deposited charge in silicon oxide coated with a monolayer of ODS for the subsequent characterization with confocal microscopy for the first time. To improve the homogeneity of the coverage, the water droplet size in the emulsion should be decreased. This could be achieved using microfluidic systems.26

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AUTHOR INFORMATION

Corresponding Author

*Phone ++49-371-531-34807; Fax ++49-371-531-834807; email [email protected]. Present Addresses †

Boston University, Boston, MA. University of Kassel, Germany.

‡

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Prof. C. von Borczyskowski for discussions of the results and thank the DFG (GR 2659/4) for financial support.

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REFERENCES

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