Diode Junctions in Single ZnO Nanowires as Half-Wave Rectifiers

Sep 24, 2009 - Under sinusoidal ac voltage, the nanowires act as half-wave rectifiers. Operation of the rectifiers at high frequencies is restricted b...
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J. Phys. Chem. C 2009, 113, 18047–18052

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Diode Junctions in Single ZnO Nanowires as Half-Wave Rectifiers Kallol Mohanta and Amlan J. Pal* Indian Association for the CultiVation of Science, Department of Solid State Physics, JadaVpur, Kolkata 700032, India ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: September 1, 2009

We introduce dopants in a section of ZnO vertical nanowires. This forms a junction in the nanowires that exhibit diode nature in current-voltage characteristics. Under sinusoidal ac voltage, the nanowires act as half-wave rectifiers. Operation of the rectifiers at high frequencies is restricted by a phase lag between current and applied ac voltage. We vary the length of the vertical nanowire junctions and study its effect on rectification characteristics. We find that the phase lag of current is less in shorter nanowire diodes than that in the longer ones. The shorter diodes hence operate until higher frequencies as half-wave rectifiers. 1. Introduction Semiconducting nanostructures offer interesting electronic and optoelectronic properties, which depend on the size and shape of the materials. In addition, the type of the semiconductors and the doping levels regulate the functioning of many of the devices. In this direction, ZnO is an ideal material with a wide band gap (3.3 eV) and high exciton binding energy in designing such devices.1,2 One may form various nanostructures of ZnO ranging from zero-dimensional quantum dots3 to two-dimensional nanosheets.4 While pristine ZnO is a natural n-type semiconductor due to intrinsic defects, such as vacancies at the oxygen sites (VO) and Zn interstitials (Zni), its doping level can be ranged to both p-type (with Li, Sb, P, or As) and further n-type or n+-type with Al, Ga, or In dopants.5-11 Combination of different types of nanomaterials may yield interesting electronic properties. In general, the rectifying nature of current-voltage characteristics is observed in these systems.12-22 In such cases, apart from the depletion layer, interfaces between the two materials or with the metal contacts play a predominant role in determining the rectifying properties. The interface between two nanomaterials can somewhat be straightened by introducing a dopant in a section of a nanowire.23 Such nanowires with a p- and an n-section, that is, a pn-junction in a nanowire, have yielded current rectification resulting in nanowire diodes or nanodiodes.24-26 Responses of a diode under ac voltage can be of interest in designing a half-wave or a full-wave rectifier. From the frequency response, the rate-limiting process of a rectifier can be determined that needs to be understood for inclusion in a circuitry. In this article, we study ac response of rectifying junctions in ZnO nanowires; we vary the length of the nanowire junctions and determine its effect on the time response of the rectifiers. 2. Experimental Section Zinc acetate dehydrate, acetone (99.9%), hexamethylene tetramine, aluminum chloride dehydrate, and sodium hydroxide palette were the required reagents. All these reagents were purchased from E-Merck (India) and used without further purification. Deionized water used in the reaction was obtained * To whom correspondence should be addressed. E-mail: sspajp@ iacs.res.in.

from Milli-Q Academic system (resistivity ) 18.2 MΩ · cm). The nanowires were grown on N-type As-doped silicon wafers (111) with a resistivity of 3-10 mΩ · cm. The substrates were cleaned in Piranha solution (3:1 mixture of H2SO4 acid and 30% H2O2) for 30 min followed by through washing in deionized water (Warning: Piranha solution reacts violently, even explosively, with organic materials. It should not be stored or combined with significant quantities of organic material.). They were finally air-dried for further use. Vertical nanowires were grown from ZnO crystalline seeds, which were formed on the substrates by thermal decomposition of zinc acetate at 300-350 °C for 30 min. Annealing at this temperature results a seed layer with its (100) plane parallel to the substrate; this has been evidenced from the X-ray diffraction patterns of the nanowires that grew on these seeds.23,25 Vertical nanowires of ZnO were grown by dipping the seed layer in a mixture of 10 mM zinc acetate and 10 mM hexamine in aqueous media at 80 °C. Reaction time spanned from 15 to 90 min to grow nanowires of different lengths. To form a rectifying or a diode junction in a single nanowire, we introduced dopants in a controlled way. Since the nanowires were intrinsically n-type in nature, we made them further n-doped (n+-type) by adding 0.5 mM aluminum chloride (AlCl3) in the dipping solution. (We avoided p-doping due to instability in conduction in p-type ZnO nanowires.27) That is, after the growth of intrinsically doped n-type ZnO nanowires on the seeds for a certain period of time, growth of n+-type nanowires was continued for the same period of time. This resulted in an nn+-junction in each of the vertical nanowires. The growth sequence was also reversed by growing the doped n+-type ZnO nanowire first, so that n+n-junctions could be formed in the nanowires. To vary the length of the rectifiers, duration of the growth for each (of n-type and n+type) nanowire section was varied from 15 to 90 min. A junction with growth time of 30 min for both n- and n+-type wires had a total length of 300 nm; the length became 750 nm when the growth time was 90 min for each section. Additionally, nanowires with only intrinsically doped (n-type) and Al-doped (n+-type) ZnO were also grown and characterized for comparison. The nanowires were washed thoroughly in deionized water before characterization. The Si substrates with the vertical nanowires were placed facing downward in a bell jar under vacuum (10-3 torr). The junctions as well as the individual components (n- and n+-type

10.1021/jp906161w CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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nanowires) were characterized. While the highly doped Si substrate was the base electrode, a Hg drop peeping out of a metal needle of a syringe, which was controlled and raised from outside, acted as the other electrode. Bias was applied to the Si electrode with respect to the Hg one with a sweep speed of 50 mV/s. For current-voltage (I-V) measurements under a dc mode, a Yokogawa voltage source model 7651 and a Keithley 486 picoammeter were used. Capacitance-voltage (C-V) measurements at fixed frequencies were carried out with a Solartron 1260 Impedance Analyzer. The amplitude of the ac test signal was 100 mV rms; measurements were carried out in a parallel mode configuration. Short-circuit and open-circuit corrections for the external leads were carried out with standard normalization procedure. For ac measurements, a HewlettPackard Universal Source model 3245A provided the ac voltage in the 50 Hz -1 MHz region. Real time response of current in the circuit was recorded by digitizing the voltage drop across a 25 kΩ resistor through a Hewlett-Packard 54600B doublechannel oscilloscope; applied sinusoidal voltage was digitized simultaneously through the other channel of the oscilloscope. All the instruments were controlled through a General Purpose Interface Bus (GPIB). 3. Results and Discussion The vertical nanowires were characterized following standard procedure as mentioned in an earlier publication.25 In brief, the X-ray diffraction (XRD) spectrum of ZnO nanowires matched the data sheet of JCPDF (pdf number 36-1451) showing growth of ZnO crystals. Scanning electron microscope (SEM) images showed directional growth of the nanowires evidencing formation of closely packed vertical nanowires. The cross section of the nanowires was hexagonal in shape with a diameter of about 70 nm. Energy dispersive X-ray (EDX) analyses of pristine and Al-doped ZnO nanowires, obtained from SEM, yielded their composition. From the EDX studies, the atomic impurity in the n+-type nanowires turned to be about 5%. Under dc bias, all the junctions formed in the nanowires act as rectifiers.25 Here, we present I-V characteristics from nn+junctions in Figure 1a. The characteristics are asymmetric in nature. In nn+-junctions, current flow is favored from the n- to n+-section of the nanowire. The characteristics of the components, that is, n-type or n+-type nanowires separately, are symmetric. Results from the components, as presented in Figures 1b and c, respectively, rule out effects of the contacts in the observed rectifying characteristics from nn+- and n+n-junctions. Rectification in an nn+-junction in ZnO nanowires can best be substantiated by characterizing reverse junctions, namely, n+nnanowire junctions. A comparison of I-V characteristics from nn+- and n+n-junctions, as presented in Figure 1a, shows opposing rectification in the two cases. The results show that irrespective of the directionality of the junctions (that is, nn+and n+n-cases), current flow is always favorable from the n- to n+-section of the nanowires. Measurements from four different points on the films are presented in each of the cases (nn+- and n+n-junctions and n- and n+-type nanowires) to show the reproducibility of the results. The rectification ratio, which ranges between 8.5 and 10 at 2.0 V in the two junctions (averaged from four measurements each), matches quite reasonably. The results hence demonstrate that the rectification in the junctions has not yielded due to contacts with the electrodes. The results further confirm that the observed rectification arises due to the junction in the nanowire itself. In both the cases, current flow is favorable from the n- to n+-segment of the junction.

Figure 1. (a) Current-voltage characteristics of an nn+- and an n+njunction. Characteristics from the components are shown in (b) and (c). Growth time of n- and n+-type nanowires was 30 min each in the junction and its components. Measurements from four different points on the films are presented with different colored lines in each of the cases. Inset of (a) represents the schematic diagram of the measurement.

Figure 2. Current-voltage characteristics of n+n-junctions of different lengths. Legends display duration of growth of n- and n+-sections of the junctions. Inset shows the characteristics of the diodes and its components as plots of I/V versus V (normalized to 1).

The rectifying nature of I-V characteristics in the nanowire junctions must have appeared due to formation of a depletion layer at the interface between n- and n+-segments. We aimed to change the total length of the junction and observe its effect on the rectification. We targeted to reduce the total length of the nanowire diodes well within the width of depletion layer. I-V characteristics of the nanowire junctions of different lengths are presented in Figure 2. Here, the duration of growth of nanowires that is directly related to the length of the junction is shown as legends. The corresponding rectification ratios (RR), measured at 2.0 V, are presented in Table 1. The results show that with a decrease in the length of nanowire diodes, the RR initially has remained unaltered and finally has decreased. A decrease in the RR in shorter nanowire junctions implies that

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TABLE 1: Rectification Ratio, Ideality Factor, and Series Resistance of Different Nanowire Diodes Grown for Different Lengths of Time growth time of n+- and n-section of n+n-nanowire junctions 15 20 30 50 90

and and and and and

15 20 30 50 90

min min min min min

rectification ratio (RR) at 2.0 V

ideality factor (n)

series resistance (RS) in MΩ

1.7 2.4 8.9 7.7 8.2

5.0 4.2 5.6 5.8 5.8

0.6 1.8 11.1 14.3 15.9

the length of the diode must have fallen within the width of the depletion layer. The transport mechanism in these nanowire diodes can be studied from the I-V characteristics. Talin and Le´onard have recently proposed a unique representation;28 in the inset of Figure 2, we have followed their representation to plot the characteristics as I/V versus V with the I/V value normalized to unity at 2.0 V. The linear fit with the same slope for all the nanowire junctions signifies a I ∝ V2 behavior;29,30 a spacecharge limited (SCL) current conduction is hence applicable to these nanowire diodes. In the figure, we have included results from the bulk wire or the n- and n+-sections. The plots fit to straight lines at the higher-voltage region. The deviation from the linear fit at low voltages may be due to Schottky barriers with the contacts. We also have tried to fit the I-V characteristics of the nanowire diodes with the general empirical equation

I ) I0[exp(eV/nkBT) - 1] where I0 is the reverse saturation current and kB, T, and n are the Boltzmann’s constant, temperature, and diode ideality factor, respectively. Here n determines the transport mechanism applicable in the rectifying diode; n ) 1 corresponds to thermionic emission, while n > 2 leads to electron-hole recombination. To account for the voltage drop across the series resistance (RS) of the nanowire diodes, one generally replaces V in the above equation by (V - IRS). It has been shown that while the slope of (dV)/(d ln(I)) versus I plot yields series resistance, its intercept with the y-axis returns the ideality factor of the diode.31 The values of RS and n for the nanowire diodes of different lengths have been quoted in Table 1. The results show that with a change in the length, n ranged between 5.0 and 5.8. This shows that for all the nanowire diodes, recombination current predominates. The dominance of recombination current could be due to the high temperature and low voltage range of our measurements, at which concentration of minority carriers is relatively large in these nanowire junctions. The series resistance of the nanowire diodes expectedly increases with their length. To determine doping concentration of intrinsic ZnO nanowires, we have recorded C-V characteristics at different fixed frequencies. In Figure 3, we show such a plot for all the nanowire diodes at 100 kHz, at which the contribution of diffusion capacitance becomes negligible, since the carriers at the interface cannot follow the fast ac signal. The plots of the square of inverse of junction capacitance versus reverse bias voltage (Figure 3) fit to straight lines for the nanowire junctions of different lengths (except the longest nanowires). From the slope of the plots that do not vary with the length of the nanowire junctions, we could determine carrier concentration

Figure 3. 1/C2 versus voltage characteristics at 100 kHz of n+njunctions of different lengths. Legends display duration of growth of n- and n+-sections of the junctions. Broken lines fit to the experimental points in the respective cases. Experimental points for the 50 and 90 min cases were divided by 5 to fit in the same graph.

of less-doped or intrinsic segment. Its value, 1.2 × 1024 m-3, matched reasonably well with the carrier concentration of pristine ZnO quantum structures.32 The intercept of the plots, on the other hand, has resulting contact potential. The value of the potential, which ranged between 0.4 and 1.5 V, decreased with an increase in the length of the nanowire rectifier. This decrease could be due to an increase in the voltage drop along the series resistor of the rectifiers. In the following, we will show how the nanowire junctions can act as rectifiers when an ac bias, instead of a dc one, is applied. We have recorded current response of the junctions under ac voltage of varied frequencies. In Figure 4, we present sinusoidal voltage and its corresponding current from nn+- and n+n-junctions. Frequency of the applied voltage was 100 Hz. Both the nanowire junctions clearly act as half-wave rectifiers. Directions of rectification in the two diodes are opposite to each other. In an nn+-nanowire diode, current response in the positivebias section of sinusoidal ac voltage is higher as compared to that in the negative-bias one. In the n+n-case, it is the opposite. This shows that under ac voltage also, current flow in all the nanowire junctions is preferred in the n- to n+-direction. Control experiments with the components, that is, with nand n+-type nanowires, were also carried out under ac bias of different frequencies. Results for the 100 Hz case are presented in Figure 5. In both the nanowires, current is symmetric in the two bias sections of sinusoidal voltage. This is in clear contrast to the results from the junctions shown in Figure 4a, b. Magnitude of current in the n+-type case is slightly higher than that in n-type nanowires. This is due to higher conductivity in the latter system, which has a higher doping level and correspondingly a higher carrier concentration. The control experiments with the components show that the half-wave rectifying characteristics in nn+- and n+n-junctions, as presented in Figures 4a and b, respectively, did occur due to the junction in the nanowires and not due to interfaces with the metal electrodes. Responses at higher frequencies show a phase lag between voltage and current. Figures 6a and b show results from nn+and n+n-nanowire diodes operated under a high-frequency ac voltage. The direction of half-wave rectification in one nanowire diode is opposite of that in the other. In both the nanowires, current response lags the sinusoidal voltage. Since the phase lag limits operation of a rectifier at high frequencies, we have characterized the nanowire diodes under different frequencies.

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Figure 4. Applied sinusoidal voltage (broken line) and its corresponding current response (continuous line) from nn+- and n+n-nanowire junctions. Legends specify duration of growth of nanowire junctions. Frequency of the applied voltage was 100 Hz.

Figure 5. Applied sinusoidal voltage (broken line) and its corresponding current response (continuous line) from n- and n+-type nanowire. Legends specify the frequency of applied voltage. Growth time of n- and n+-type nanowires was 30 min each in the nanowires.

Figure 6. Applied sinusoidal voltage (broken line) and its corresponding current response (continuous line) from nn+- and n+n-nanowire junctions. Legends specify duration of growth of nanowire junctions. Frequency of the applied voltage was 100 kHz.

Figure 7 sums up the frequency response of the results from a diode of length 750 nm. Responses from a shorter diode (15 and 15 min) are also presented in the figure. For all the diodes, neither forward-bias or reverse-bias current nor their ratio, which is defined as the rectification ratio, has depended strongly on the frequency in the frequency range of our measurement.

Invariance of current or the ratio until higher frequencies implies that mechanism of current conduction is faster than the time scale of a half-cycle of a sinusoidal wave. This is, however, expected in a rectifying junction. The little decrease in current and ratio could be due to RC time constant of the circuit. When the characteristics of a diode are compared with another, it is

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J. Phys. Chem. C, Vol. 113, No. 42, 2009 18051 lag and vice versa. Since the phase lag would probably depend on other factors apart from the length of the junction, it was not possible to quantify the phase lag with the length of the nanowire junctions. As such, the shorter nanowire diodes, which give rise to a lower rectification than a longer one, can be operated until a higher frequency; a longer diode having a higher rectification ratio as its output has a truncated range of operation in the high-frequency range. The results also show that finally the phase lag limits operation of nanowire-based half-wave rectifiers at higher frequencies. 4. Conclusions

Figure 7. Frequency response of amplitude of forward-bias current and rectification ratio for two nanowire diodes of different frequencies. While current is shown with circles, squares represent rectification ratio. Open and filled symbols represent nanowire diodes with n- and n+ZnO section grown for 15 min each and 90 min each, respectively. Duration of growth of nanowire junctions is also specified in the legends.

In conclusion, we have introduced dopants in a section of ZnO vertical nanowires to form junctions. Nanowire junctions of different lengths have been characterized under dc bias and also under ac bias of 50 Hz-1 MHz frequency. The junction in the nanowire has resulted rectifying I-V characteristics under dc bias; ideality factors calculated from the I-V characteristics have shown that recombination current has predominated in all the nanowire diodes. Under sinusoidal voltage, the nanowire rectifiers have yielded half-wave rectification. Frequency response of the rectifiers has resulted in a phase lag in current with respect to the applied voltage. With an increase in frequency, the phase lag of current has increased limiting operation of the half-wave nanowire rectifiers. In junctions of shorter lengths, the phase lag has been less. The results have showed that the shorter ZnO nanowire rectifiers can be operated until higher frequencies for half-wave rectification. Acknowledgment. K.M. acknowledges a CSIR Junior Research Fellowship No. 9/80(491)/2005-EMR-I, Roll No. 509342. The Department of Science & Technology, Government of India financially supported the work through Ramanna Fellowship SR/S2/RFCMP-02/2005. References and Notes

Figure 8. Phase lag of current with respect to sinusoidal voltage as a function of frequency of the ac voltage. Plots for nanowire diodes of different lengths are shown in the figure. Legends display duration of growth of n- and n+-sections of the junctions.

found that, at any frequency, rectification ratio of a longer nanowire diode is higher than that of a shorter one. It may be recalled that under dc bias too, rectification ratio was higher in the longer nanowire diodes (Table 1). In other words, the nature of current rectification under dc bias has been translated to the ac mode without any hindrance. The role of RC time constant has been manifested as phase lag between applied voltage and its corresponding current responses. Figures 6a and b have shown that at higher frequencies, the current starts to lag the applied voltage for both nn+- and n+n-junctions. In Figure 8, phase lag of current with respect to the applied bias has been plotted as a function of frequency of ac voltage. This has been done for all the nanowire rectifiers of different lengths. The figure shows that the phase lag increases with an increase in the frequency. At any frequency, the phase lag is higher in a longer nanowire rectifier. Figure 8, in conjunction with Figure 6, reveals that the decrease in current with frequency has a one-to-one correspondence with the phase lag. The correspondence holds true even when results from all the nanowire diodes of different lengths are considered. That is, at any frequency the shorter diodes yield lower phase

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