Journal of ELECTRONIC MATERIALS, Vol. 44, No. 1, 2015
DOI: 10.1007/s11664-014-3502-x 2014 The Minerals, Metals & Materials Society
Structural, Optical, and Electrical Characterization of Al/n-ZnO/p-Si/Al Heterostructures RAJENDER KUMAR1,2 and SUBHASH CHAND1,3 1.—Department of Physics, National Institute of Technology, Hamirpur 177005, Himachal Pradesh, India. 2.—e-mail:
[email protected]. 3.—e-mail:
[email protected]
For heterojunction fabrication, zinc oxide thin films were grown on p-Si by pulsed laser deposition. X-ray diffraction patterns were used to study the grain size and morphology of the films. The optical properties of the films were studied by UV–visible and photoluminescence spectroscopy. Experimental observations confirmed that the deposited films have potential for sharp emission in the visible region. High-purity (99.999%) vacuum evaporated aluminium metal was used to make contacts to the n-ZnO and p-Si. The current–voltage characteristics of the Al/n-ZnO/p-Si(100)/Al heterostructure measured over the temperature range 60–300 K were studied on the basis of the thermionic emission diffusion mechanism. The equivalent Schottky barrier height and the diode ideality factor were determined by fitting measured current–voltage data to the thermionic emission diffusion equation. It was observed that the barrier height decreased and the ideality factor increased with decreasing temperature, and that the activation energy plot was nonlinear at low temperature. These characteristics are attributed to the Gaussian distribution of barrier heights. The capacitance–voltage characteristics of the Al/n-ZnO/p-Si(100)/Al heterostructure diode were studied over a wide temperature range. The impurity concentration in deposited n-type ZnO films was estimated from measured capacitance–voltage data. Key words: Current–voltage characteristics, capacitance–voltage characteristics, pulsed laser deposition, optical characterization, Al/n-ZnO/p-Si/Al heterojunction, photoluminescence
INTRODUCTION Because of its attractive properties, zinc oxide (ZnO) has become a prominent material with a variety of potential applications, for example in optoelectronic and electronic devices.1 ZnO has recently been used in more advanced applications, for example ultraviolet (UV) light-emitting diodes (LEDs),2–4 spintronics,5 and solar cells.6–8 ZnO with different nanostructure morphology can easily be grown,9 making it attractive for use in nanoscale devices. ZnO thin films are produced by use of a variety of methods, for example sol–gel synthesis,10–13 radio-frequency and direct-current (DC) sputtering,14,15 chemical vapor deposition,16,17 (Received July 19, 2014; accepted October 24, 2014; published online November 12, 2014)
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spray pyrolysis,18 electron cyclotron resonance-assisted molecular-beam epitaxy,19 and pulsed laser deposition (PLD).18–20 Among these techniques, PLD is unique for growth of oxide materials because the oxygen plasma created by the laser is of very high energy and its density is easily controllable by changing the oxygen pressure. PLD enables production of high-quality ZnO films at lower temperatures than other methods because of the high energy of ablated particles in the laser-produced plasma plume.21,22 Several important conditions determine the morphology and microstructure of thin films deposited by PLD, for example: 1 the energy and surface dynamics of adsorbed atoms, determined mainly by the laser wavelength, fluence, and substrate temperature;
Structural, Optical, and Electrical Characterization of Al/n-ZnO/p-Si/Al Heterostructures
Bae et al.23 used PLD for growth of ZnO films on (100) p-type silicon substrates and (001) sapphire substrates and studied the effect of deposition conditions on film quality. The best films were obtained by use of a laser fluence of approximately 2.5 J cm2 and substrate temperatures Ts between 200 and 600C. The morphology of ZnO films deposited at different pressures (1 9 106, 1, 20, 200, and 400 mTorr) had 3D growth features, as was evident from well-faceted hexagons. Because use of heterostructures is advantageous for control of the electronic and optoelectronic properties of semiconductor devices,24 several significant studies, especially on the photodiode properties of n-ZnO/p-Si heterojunctions, have been conducted on the fabrication of ZnO-based heterojunctions from such materials as p-Si and p-AlGaN. ZnO/Si heterojunctions are of particular interest because of their greater cost effectiveness and flexibility in optoelectronic device fabrication.25 Farag et al.26 fabricated Al/ZnO/p-Si/Al heterojunctions and determined the barrier height to be 0.56 eV. Zhang et al.27 studied Ag/ZnO Schottky barrier diodes on F-doped SnO2 glass substrates and determined the barrier height to be 0.85 and 1.68 eV (100 kHz) by current–voltage (I–V) and capacitance–voltage (C–V) measurements, respectively. Keskenler et al.10 studied the Ag/ZnO/p-Si/Al Schottky diode and determined the barrier height to be 0.71 eV. In this work we fabricated Al/n-ZnO/p-Si/Al heterojunction diodes and studied their optical properties and temperature dependent I–V and C–V characteristics in the temperature range 60–300 K. EXPERIMENTAL Al/ZnO heterojunction diodes were fabricated on boron-doped p-type (1 X-cm resistivity) silicon wafers of (100) orientation. The p/p+ silicon wafers had an 8.9 lm epitaxial p layer over the heavily doped p+ region. Before deposition of the back ohmic contact and Schottky contact, silicon wafers were thoroughly cleaned and etched to remove native oxide from the surface. The wafers were first cleaned with organic solvents (trichloroethylene, acetone, and methanol in succession) then rinsed in deionized water of resistivity 18 MX cm, and etched in 40% HF solution for 1 min. After each cleaning step, the silicon wafers were rinsed thoroughly in deionized water of resistivity 18 MX for 1 min. After cleaning and etching, the wafers were loaded inside a 12¢¢ diameter Hind High Vacuum coating system Model 12A4D. Ohmic contacts were established on the back (i.e. the p+ side) of the wafers by depositing high-purity (99.999%) aluminium at a pressure of ˚ . Back 3 9 106 mbar; the thickness was 2000 A ohmic contacts were annealed at 300C for 1 h in a vacuum of 1 9 103 mbar.
Before deposition of ZnO on the p-side of the silicon wafers for heterojunction formation, the wafers were cleaned and etched in 40% HF solution for one minute to remove the native oxide layer formed on the silicon. The back ohmic contact metal was protected from the etching process by coating with a layer of protective wax (picein). The ZnO film was deposited on the p-side of the silicon, by PLD, to form a heterojunction diode. To form contacts, alu˚ ) was subsequently minium film (thickness 2000 A deposited on the ZnO, by use of a 12¢¢ diameter Hind High Vacuum coating system Model 12A4D, through 1 mm diameter holes in a metal mask, in a vacuum of 3 9 106 mbar. For electrical characterization, temperature dependent I–V measurements were performed by use of a Keithley model 2400 programmable source meter in the temperature range 80–300 K, controlled by use of a Lakeshore model 331 temperature controller and a closed-cycle helium refrigeration system. All I–V data were recorded by use of data-acquisition software, resident in a personal computer, via an IEEE-488 interface card. C–V measurements were performed by use of a Wayne Kerr model 6520A precision impedance analyzer. RESULTS AND DISCUSSION X-ray Diffraction Study XRD spectra of the ZnO films on the p-type Si substrate are shown in Fig. 1. Indexing of the diffraction peaks is summarized in Table I. The peaks indicate formation of PLD-grown ZnO films with a hexagonal wurtzite structure. Keskenler et al.10 and Periasamy et al.28 obtained almost similar results for ZnO thin films produced on p-type Si substrate by sol–gel spin coating and vacuum coating, respectively. Table I also shows the lattice parameters estimated from the peaks by use of the relationship: 12000
Si (400)
10000
Intensity (arb. unit)
2 the ambient atmosphere in the deposition chamber; and 3 the type of substrate.
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ZnO (002)
8000 6000 ZnO (103)
4000 2000
ZnO (004)
0
20
30
40
50
60
70
80
2θ (degree) Fig. 1. X-ray diffraction spectra of ZnO film deposited on p-type silicon.
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Table I. Indexing of diffraction peaks of ZnO thin films deposited on Si by pulsed laser deposition Sample no.
Peak position (2h) degrees
Miller indices of plane and associated phase
Calculated lattice parameters ˚) (A
34.559 61.249 69.205 72.741
(002) of ZnO (103) of ZnO (400) of Silicon (004) of ZnO
c = 5.187 a = 3.605 a = 5.426 c = 5.199
1 2 3 4
100
UV–Visible Study
80 15
Eg = 3.43 eV
60
(α hυ) 2 (eVm-1)2
Transmittance %
6x10
40
15
5x10
15
4x10
15
3x10
15
2x10
15
20
1x10
3.0 3.1
0 300
400
500
3.2 3.3
3.4 3.5
hυ (eV)
600
700
3.6 3.7
800
Wavelength (nm) Fig. 2. Optical transmittance spectrum, as a function of wavelength, of a ZnO thin film on quartz. The inset shows a plot of (aht)2 as a function of photon energy (ht).
1 4 h2 þ hk þ k2 l2 ¼ þ 2 2 2 d 3 a c
(1)
where d is the interplanar spacing of the atomic planes and (hkl) are the Miller indices. These ZnO thin films have larger lattice constants a than the standard data of the Joint Committee on Powder ˚ Diffraction Standards (c = 5.20661 A and ˚ ).29 a = 3.24982 A The average crystallite size, D, of the ZnO thin films deposited on silicon was calculated from the full width at half maximum (FWHM) of the (002) peak by use of the Debye–Scherrer formula:30 D¼
0:94k ; b cos h
(2)
where b is the width, in radians, of the diffraction peak, measured at half of its maximum intensity, h is the Bragg angle, and k is the wavelength of the x-rays used. The average crystallite size of the ZnO films calculated from the (002) peak was 22.6 nm. Crystallite size for ZnO was reported to be 27.453 nm by Zahedi et al.31
Optical transmission spectra of ZnO thin films grown on quartz, while simultaneously growing films on silicon, by PLD were acquired by use of a Perkin–Elmer Lambda 750 UV–visible spectrophotometer. Figure 2 shows the transmission spectrum as a function of wavelength. The ZnO thin film has maximum transmittance of 91% in the visible region. In the ultraviolet region the transmission spectrum has sharp absorption edge characteristic of an optical band gap. The band gap energy Eg of the ZnO thin films was determined from the dependence of absorption coefficient on photon energy:32 1=2 aht ¼ A ht Eg (3) where A is a constant. A plot of (aht)2 against photon energy in the region of high optical absorption, i.e. the Tauc plot,32 is shown in the inset of Fig. 2. The energy band gap of ZnO thin films was calculated from the intercept obtained by extrapolating the linear part of (aht)2 versus (ht) plot to the energy axis. The band gap obtained for the sample was 3.43 eV. This value is higher than that reported by Keskenler et al.10, Marotti et al.33, and Srinivasan and Kumar,34 who found the optical band gap of ZnO film on p-type silicon prepared by sol–gel spin coating was 3.27 eV. Ratana et al.35 reported a higher optical band gap, 3.70 eV, for a ZnO thin film. Optical band gaps of ZnO comparable with our value have been reported for nanopowders and bulk ZnO.36,37 Photoluminescence Study Figure 3 shows the room-temperature PL spectrum of a ZnO thin film formed on a p-Si semiconductor. It contains three emission peaks. The peak in the UV region corresponds to band edge emission at approximately 376 nm corresponding to the energy of 3.30 eV. The spectrum contains a very high-intensity peak at 746 nm (red luminescence at 1.66 eV). This energy value of 1.66 eV is approximately half the bandgap energy of ZnO, which implies the presence of mid-gap deep-level defect states in the ZnO films which gives the red luminescence. Similar red luminescence has been reported for thin films of ZnO prepared by the sol–gel
Structural, Optical, and Electrical Characterization of Al/n-ZnO/p-Si/Al Heterostructures 3500 ZnO mid gap defect emission
3000
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
8. 160K
10
-7
10. 120K
2000 Si bandgap
1500
emission
10
-8
ZnO bandgap emission
0 300
1. 300K 2. 280K
600
750
900
1050
1200
1350
Wavelength (nm) Fig. 3. Room temperature photoluminescence spectrum of a ZnO film grown on p-type Silicon.
technique, using zinc acetate dihydrate as precursor, then spin coating.38 However the intensity of emission by their samples was very low compared with the very strong intensity for our samples. This red emission at approximately 746 nm is believed to arise as a result of oxygen and zinc anti-sites.39 Thus the room temperature PL measurements imply the band gap is 3.30 eV, which is slightly lower than that reported in the literature (3.37 eV). Current–Voltage Characteristics The ln(I)–V characteristics of Al/ZnO/p-Si/Al heterojunction diodes in the temperature range 60–300 K are shown in Fig. 4. Near room temperature, a linear portion is observable only over a low bias range, probably because of the low barrier height of the heterojunction. At lower temperatures these plots are clearly linear over several orders of magnitude of current. Further, they progressively become straight over a wide bias range with decreasing temperature. The increase of the straight-line portion of the ln(I)–V curve and their gradual shift toward higher bias with decreasing temperature are in agreement with thermionic emission diffusion Eq. (4): qðV IRS Þ qðV IRS Þ 1 exp I ¼ IS exp gkT kT (4) with IS ¼ Ad A T 2 exp
q/b0 kT
(5)
where Is is the saturation current at zero bias, Ad is the diode area, A** is the effective Richardson constant, T is the temperature in Kelvin, k is the Boltzmann constant, q is the electronic charge, /b0
13
6. 200K 7. 180K 9. 140K 11. 100K 12. 80K 13. 60K
-1.0
450
4. 240K 5. 220K
1000 500
1
3. 260K
Current (A)
Intensity (a. u.)
2500
197
-0.5
0.0
0.5
1.0
Voltage (V) Fig. 4. Current–voltage characteristics of the Al/n-ZnO/p-Si/Al heterojunction diode at different temperatures.
is the zero-bias barrier height, g is the ideality factor, and Rs the diode series resistance. It is customary to extract the saturation current (Is) from the straight line portion of the ln(I)–V plots, by extrapolation to zero-bias, and to determine the ideality factor from the slope of the straight line portion. Also, Rs data are found from the best fit of experimental I–V data to Eq. (4) by the least-squares fitting method. Alternatively, computer software is used to fit experimental I–V data to Eq. (4) by iteration, taking Is , g and Rs as adjustable. Once Is is known, the barrier height /b0 can easily be determined from Eq. (5) at any temperature for a given diode area Ad and Richardson constant A** (3.2 9 105 Am2K2 for n-ZnO and p-Si).10,21–27 Zero-Bias Barrier Height and Ideality Factor Zero-bias barrier height data obtained by fitting experimental I–V data to Eq. (4) are shown in Fig. 5. The zero-bias barrier height /b0 decreases rapidly from 0.44 eV at 300 K to 0.22 eV at 60 K. Keskenler et al.10 reported the barrier height of Ag/ ZnO/p-Si/Al to be 0.71 eV. Variation of the ideality factor with temperature is shown in Fig. 6. The extracted ideality factor is slightly high at room temperature because the ln(I)– V plots have a short linear region. At temperatures below 160 K, however, the linear portion increases and the derived ideality factor is also low. Below this temperature the ideality factor initially increases marginally with decreasing temperature; it then decreases substantially below 120 K, and has a value of 2.99 at 60 K. The barrier height can also be determined in yet another way from the activation energy plot. For this, Eq. 5 can be expressed as: IS q/ 1 (6) ln 2 ¼ lnðAd A Þ b0 T k T
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Kumar and Chand 0.45 -20
Activation Energy = 0.39 eV 4 A** = 3.79 × 10 Am-2 K -2
0.42
ln I s /T 2 (AK-2)
Barrier height (eV)
-25
0.39 0.36 0.33 0.30 0.27
-30 -35 -40 -45
0.24
-50 4
0.21 50
100
150
200
250
300
Temperature (K) Fig. 5. Barrier height as a function of temperature for the Al/n-ZnO/ p-Si/Al heterojunction diode.
6
8
10
1000/T ( K
-1
12
14
16
18
)
Fig. 7. Conventional activation energy (lnðIs =T 2 Þ as a function of 1000/T) plot for the Al/n-ZnO/p-Si/Al heterojunction diode.
first sight indicative of deviation from the pure thermionic emission–diffusion mechanism and warrant further investigation.
3.0
Barrier Height Inhomogeneities Ideality Factor
2.5
2.0
1.5
1.0 50
100
150
200
250
300
Temperature (K)
The significant decrease of the zero-bias barrier height and increase of the ideality factor, especially at low temperatures, are possibly caused by barrier height (BH) inhomogeneities resulting from variation in the thickness and composition of the ZnO films. To describe the BH inhomogeneities with a Gaussian distribution function, Eqs. 4 and 5 get modified such that /ap appears in place of /b0 .40 The /ap is termed as apparent zero-bias barrier height and is given by:
Fig. 6. Ideality factor as a function of temperature for the Al/n-ZnO/ p-Si/Al heterojunction diode.
2 qr /ap ¼ / b0 2kT
Therefore, a plot of ln(Is/T2) as a function of 1/T should yield a straight line, the slope of which determines the zero-bias barrier height /b0 and with the intercept at the ordinate giving the Richardson constant for known diode area Ad. This method gives a single value of the barrier height from the entire I–V data over the entire temperature range. Figure 7 shows the plot of ln(Is/T2) as a function of 1000/T obtained from the experimental I–V data at different temperatures. The plot shown in Fig. 7 is not linear; it deviates from linearity at low temperature but can be fitted to a straight line in the high temperature region. The activation energy obtained from the linear portion at high temperature is 0.39 eV, and the Richardson constant A** obtained from the intercept of the straight portion with the ordinate is equal to 3.79 9 104 Am2K2, which is much lower than the known value (3.2 9 105 Am2K2). The decrease in barrier height, increase in ideality factor with decreasing temperature, and nonlinear behavior of activation at low temperatures are at
stands for mean zero-bias barrier height where / b0 and r is the standard deviation of Gaussian distribution of barrier heights. To see the occurrence of Gaussian distribution of barrier heights for Al/nZnO/p-Si/Al the zero-bias barrier height is plotted as function of 1/T, as shown in Fig. 8. Clearly the data are a good fit to a straight line, which is indicative of Gaussian distribution of the barrier height. The slope of this may be used to derive the mean of the distribution r, and the intercept on the ordinate yields the value of the zero bias mean barrier height . / b0 The intercept of the best fit yields a mean barrier height of 0.51 eV. From the slope of the best fit straight line the value of standard deviation of the distribution is evaluated as 0.054 V. Thus, it is obvious that the decrease of the zero-bias BH is caused by the Gaussian distribution of barrier heights. Substituting the value of /ap in Eq. 6 gives the modified activation plot: IS q2 r2 q/ (8) ¼ lnðAd A Þ b0 : ln 2 2 2 T 2K T kT
(7)
Structural, Optical, and Electrical Characterization of Al/n-ZnO/p-Si/Al Heterostructures -15
0.45
Intercept = 0.51 eV
0.42
ln (Is/T 2)- 2q 2 /2K2T2 (AK-2)
Slope = -17.2 meVK
0.39
Barrier height (eV)
199
0.36 0.33 0.30 0.27 0.24
-30
-45
-60
-75
-90
Un-modified plot Modified plot
0.21 3
6
9
12
15
18
-105 3
-1 1000/T (K )
Figure 9 shows the modified activation energy plot and the original activation energy plot. Clearly the modified activation energy plot is linear over the entire temperature range of measurement from 300 K down up to 60 K. The Richardson’s plot obtained from the modified activation energy plots is found to be 2.35 9 105 Am2K2, in close agreement with the known value of 3.2 9 105 Am2 K2, and the barrier height obtained is 0.505 eV.
9
12
1000/T ( K
-1
15
)
5x10
17
4x10
17
1. 300K 2. 280K 3. 260K 4. 240K 5. 220K 6. 200K 7. 180K 8. 160K 9. 140K 10. 120K 11. 100K 12. 80K 13. 60K
13
17
3x10
17
2x10
1
17
1x10
Capacitance–Voltage Characteristics The capacitance of Al/n-ZnO/p-Si/Al heterojunction diode was measured as a function of reverse bias at different temperatures. The C–V characteristics are described by conventional heterojunction theory:41 1 2 e1 ND þ e2 NA ¼ ðVbi V Þ (9) C2 A2 qe1 e2 NA ND where A is the diode area, q is electronic charge, ND and NA are donor density in n-ZnO and acceptor density in p-Si, respectively, e1 and e2 are the dielectric constants of n-ZnO and p-Si, respectively, Vbi is the built-in voltage of the diode, and V is the externally applied reverse bias. Figure 10 shows plots of C2 as a function of V for the Al/n-ZnO/p-Si/ Al heterojunction diode at different temperatures and at a frequency 500 kHz. The built-in voltage, Vbi, can be determined from the intercept of 1/C2 with the voltage axis. The barrier height can be estimated from the built in voltage Vbi by use of the relationship:41 kT kT NV þ ln /b ¼ Vbi þ (10) q q NA where k is the Boltzmann constant, T is temperature, and NV is the effective density of states in the valance band, and is given by:42,43
18
Fig. 9. Modified activation energy plot corresponding to the standard deviation r = 0.054 V.
C-2 ( F -2)
Fig. 8. The barrier height /ap obtained from I–V measurements as a function of inverse temperature.
6
1
2
3
4
5
6
Reverse bias (V) Fig. 10. The C12 versusV characteristics of the Al/n-ZnO/p-Si/Al heterojunction diode at a frequency of 500 kHz and different temperatures.
3 2p mh kT 2 NV ¼ 2 h2
(11)
Here, mh is the effective mass of holes and h is Plank’s constant. The barrier heights derived from the capacitance– voltage data for the Al/n-ZnO/p-Si/Al heterojunction diode are shown in Fig. 11, with the barrier height derived from the I–V data. Clearly the barrier height derived from the C–V data is larger than the barrier height derived from the I–V data. As is evident from Eq. 10 the equivalent barrier height is slightly higher than the built-in voltage of the Al/nZnO/p-Si/Al heterojunction diode. The p-type silicon on which the ZnO is deposited has a resistivity of 1 X cm, which corresponds to an acceptor density NA equal to 1.3 9 1016 cc1. From the slope of C2 versus V plots and the acceptor concentration NA in the p-silicon the donor density in n-ZnO is calculated to be 1.41 9 1017 cc1. In
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500KHz
I-V derived BH
Barrier height (eV)
0.90
C-V derived BH
0.75
0.60
0.45
0.30
0.15 50
100
150
200
250
300
Temperature (K) Fig. 11. Barrier height derived from I–V and C–V measurement for the heterojunction diode at 500 kHz frequency.
the calculation the permittivity (e1) of n-ZnO is taken to be 8.5.44 The donor density of n-ZnO obtained in our work is comparable with that reported for n-ZnO films grown on p-Si films by magnetron sputtering at different substrate temperatures, which was found to vary from 8.62 9 1016 cc1 to 1.03 9 1019 cc1.41 Thus, the observed results are quite consistent with reported values. As expected, the barrier height values derived from the C–V measurement are higher than those obtained from the I–V measurements. This discrepancy can be explained by the different nature of the C–V and I–V measurement techniques. The barrier heights deduced from two techniques are not always the same. If the barriers are uniform and ideal, the two measurements yield the same value; otherwise, they yield different values. The different behavior of Schottky barrier height obtained from the two techniques can be explained on the basis of a distribution of Schottky barrier height because of inhomogeneities at the metal–semiconductor interface.45,46 In addition, the C–V technique averages over the whole area and measure the barrier height of a Schottky diode. In contrast, the barrier height from the I–V method includes any barrier-lowering effect because of the interfacial insulator layer or the interface states and an effective barrier height is measured. Other than this, determination of the Schottky barrier height from I–V characteristics is only reliable if one can be confident that the current is determined by TE theory. For this to be so, the forward portion of the characteristic must be a good straight line with a low value of the ideality factor.43,47 CONCLUSIONS Al/n-ZnO/p-Si/Al heterojunction diodes were fabricated by use of the pulsed laser deposition technique. The ZnO film has a hexagonal wurtzite
structure with a strongly preferred (002) direction perpendicular to the substrate. In addition, lowintensity peaks corresponding to the (103) and (004) planes are also observed in the XRD pattern. The lattice constants c and a of ZnO films deposited on ˚ , respectively. silicon are found to be 5.19 and 3.60 A The band gap of ZnO thin films deposited by pulsed laser deposition was found to be 3.43 eV from UV– visible spectra. Photoluminescence spectra show ZnO band gap emission at 3.30 eV. The ZnO films also emit strong red luminescence at 746 nm; this arises as a result of mid gap defect states. The forward I–V characteristics of the Al/n-ZnO/ p-Si/Al heterojunction diode were interpreted on the basis of a thermionic emission–diffusion mechanism to find the equivalent barrier height. Whereas the zero-bias barrier height /b0 decreases with decreasing temperature the ideality factor increases, the changes being quite substantial at very low temperatures. The zero-bias barrier height of Al/ n-ZnO/p-Si/Al heterojunction diodes was found, from the activation energy fit at high temperatures, to be 0.505 eV, and the plot of ln(Is/T2) as a function of 1000/T deviates from linearity at low temperatures. The concept of barrier height inhomogeneities was used to explain the decrease in barrier height, increase of ideality factor, and non-linear activation plot at low temperatures. The inhomogeneities are described by a Gaussian distribution of barrier heights with mean barrier height 0.51 eV and standard deviation 0.054 V. The capacitance–voltage characteristics of the Al/n-ZnO/p-Si/Al heterojunction diode at 500 kHz frequency were measured in the temperature range 60–300 K. The built-in voltage and the barrier height of the Al/n-ZnO/p-Si/Al heterojunction diode were also obtained from the C–V measurements. The ionized donor concentration in n-ZnO was estimated to be 5.32 9 1017cc1. REFERENCES 1. C.W. Litton, T.C. Collins, D.C. Reynolds, P. Capper, S. Kasap, and A. Willoughby, Zinc Oxide Materials for Electronic and Optoelectronic Device Applications (New York: Wiley, 2011). 2. C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, and D. Wang, Nano Lett. 7, 1003 (2007). 3. T. Zhai, L. Li, X. Wang, X. Fang, Y. Bando, and D. Golberg, Adv. Funct. Mater. 20, 4233 (2010). 4. Y.S. Choi, J.W. Kang, D.K. Hwang, and S.J. Park, IEEE Trans. on Electron Devices 57, 26 (2010). 5. S.J. Pearton, D.P. Norton, Y.W. Heo, L.C. Tien, M.P. Ivill, and Y. Li, J. Electron. Mater. 35, 862 (2006). 6. M. Law, L.E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt, and P. Yang, J. Phys. Chem. B 110, 22652 (2006). 7. J. Qiu, X. Li, F. Zhuge, X. Gan, X. Gao, W. He, S.J. Park, H.K. Kim, and Y.H. Hwang, Nanotechnology 21, 195602 (2010). 8. K.S. Yeong, K.H. Maung, and J.T.L. Thong, Nanotechnology 18, 185608 (2007). 9. Z. Wang, Nanowires and Nanobelts: Materials, Properties and Devices: Volume 2: Nanowires and Nanobelts of Functional Materials, (New York: Springer, 2005).
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