n-ZnO nanorod homojunctions

Current Applied Physics 12 (2012) 1326e1333

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Fabrication and structural, electrical characterization of i-ZnO/n-ZnO nanorod homojunctions _ Polat a, Y. Atasoy a S. Yılmaz a, b, *, E. Bacaksız a, I. a b

Department of Physics, Faculty of Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey School of Physical Sciences and National Centre for Plasma Science and Technology, Dublin City University, Glasnevin, Dublin 9, Ireland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 January 2012 Received in revised form 7 March 2012 Accepted 15 March 2012 Available online 23 March 2012

Well-aligned ZnO nanorods were synthesized by a vapor phase transport method on ZnO buffer layer coated n-Si substrates. X-ray diffraction and scanning electron microscopy results showed that the deposited ZnO nanorods crystallize in the wurtzite structure and are highly textured with their c-axes normal to the substrate and show a clearly hexagonal morphology. A heavily compensated and intrinsic ZnO layer (i-ZnO) doped with both Mg and Na was deposited on the nominally undoped ZnO nanorods (which show a natural n-type behavior) to produce an i-ZnO/n-ZnO homojunction. The i-ZnO layer consisted of the grainy shape nano-crystallites with the wavy surface morphology. The currentevoltage (IeV) characteristics of these structures in the temperature range of 150e300 K have been analyzed in the framework of standard thermionic emission (TE) theory with the assumption of a Gaussian distribution of the barrier heights. The values of zero bias barrier height (Fb0) and ideality factor (n) were found to be strongly temperature dependent whereby n decreases while Fb0 increases with increasing temperature. The ln(I0/T2) vs q/kT plot shows a straight line behavior and the values of activation energy (Ea ¼ Fb0) and the Richardson constant (A*) determined from the intercept and slope of the plot were 0.926 eV and 2.61  108 A cm2 K2, respectively. This value of A* is much lower than the known value of 32 A cm2 K2 for ZnO. Thus, a modified lnðI0 =T 2 Þ  ðs20 q2 =2k2 T 2 Þ vs. q/kT plot based on a Gaussian distribution of barrier heights was used which yields a mean barrier height ðFb0 Þ and modified effective Richardson (A**) of 1.032 eV and 34.85 A cm2 K2, respectively. This value of A** is much closer to the theoretical value of 32 A cm2 K2 for ZnO. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: ien ZnO homojunction (NaeMg) co-doping SEM XRD Temperature dependence of IeV characteristics

1. Introduction The ever decreasing dimensions of electronic devices has become a strong driver for many commercial applications and the development of self-assembled micro- and nano-structured materials systems has therefore become an increasingly important area of applied research. In parallel, there is also significant academic interest in nano-systems, as their properties can be remarkably different from those of the bulk materials due to quantum-size and other effects. In recent years, much attention has been paid to the one-dimensional (1D) ZnO nano-structured materials such as nanorods, nanotubes and nanobelts owing to their unique and specific properties as well as their application in nanoscale electronic and photonic devices [1,2]. For example, the advantages of using ZnO over GaN include its large exciton binding * Corresponding author. Department of Physics, Faculty of Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey. Tel.: þ90 462 377 25 53; fax: þ90 462 325 31 95. E-mail addresses: [email protected], [email protected] (S. Yılmaz). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.03.021

energy (60 meV) and its consequent ability to provide a highly efficient optical emission at room temperature. Due to these features, ZnO is a promising candidate for applications such as light-emitting devices (LEDs) in the region from blue to ultraviolet [3e5], photodiodes [6], solar cells [7] and spintronics [8]. Several deposition methods could be used for the construction of ZnO nano-sized structures including thermal evaporation [9], chemical bath deposition [10], solegel deposition [11], spray pyrolysis [12] and vapor phase transport method [13]. Among these methods, VPT method is widely used because of relatively simple apparatus requirements and has been employed to fabricate various nanostructures of ZnO [14,15]. In order to use ZnO in technological applications an important issue is to manufacture electrical junctions so that the material’s device behavior and potential can be studied. However, ZnO is a naturally unipolar material with significant n-type conductivity (in the range of 1017 cm3 even in nominally undoped material) arising from donor centers, due either to native point defects or other common impurities (e.g. such as H) [16e18]. It has proven to be very difficult or even impossible, to obtain consistent, stable and reproducible

S. Yılmaz et al. / Current Applied Physics 12 (2012) 1326e1333

p-type conductivity in bulk, thin film and nano-structured ZnO, due, amongst other factors, to the low dopant solubility, selfcompensation effect of intrinsic defects [19,20] and the influence of surface effects such as surface conduction channels [21]. However, intrinsic (i), semi-insulating, compensated ZnO can be readily obtained by using group I dopants such as Li, Na and K as substitutional dopants for Zn, which act as deep acceptors [22e24]. Since intrinsic ZnO layer is highly resistive, it is widely used in both copper indium diselenide (CIS) and cadmium sulfide (CdS)-based solar cells as an intermediate layer to protect the cells against efficiency losses and therefore this layer helps to improve the cell performance [25,26]. Intrinsic ZnO also can be used in the form of peien structure for LEDs, where serves as an emissive layer [27]. To fabricate a ZnO homojunction-based device, Hwang et al. produced a nei ZnO homojunction on Al2O3 substrates by using an insulator ZnO layer (i-ZnO) and an n-ZnO layer grown by radio frequency magnetron sputtering. They expressed the structure as a ZnO-based light-emitting metaleinsulatoresemiconductor diode and investigated the currentevoltage (IeV) characteristics of the structure [28]. The fabrication and characterization of an nei ZnO homojunction diode therefore seems to be a useful first step in the absence of p-type material, in terms of studying the material behavior in devices. Analysis of the forward bias IeV characteristics of metalesemiconductor (MS) and pen homojunctions are often based on standard thermionic emission (TE) theory, especially at low temperatures. These analyses usually reveal an increase in the built-in barrier height (BH) voltage, Fb0, and a decrease in the ideality factor, n, with increases in temperature [29e45]. This behavior of Fb0 and n at low temperatures leads to non-linearity in plots of activation energy vs. q/kT and may be attributed to structural defects or dislocations at the semiconductor interface, in addition to roughness and non-uniformly at the interfaces and trapped interfacial charges [38e43]. Assuming a variation in BH with a Gaussian distribution (GD) is one of the most successful approaches, which satisfactorily explains many seemingly unusual electrical characteristics of diodes at low temperatures [29e44]. According to this model, there is a distribution of nanoscale interfacial regions with varying BHs and the total current is considered to be a sum of currents flowing through these individual regions in parallel. Schmitsdorf et al. [46] used Tung’s theoretical approch [47,48] and they found a linear correlation between the experimental zero bias BHs and ideality factors. The issues discussed above will be common to both pen and nei homojunctions and thus studies of nei homojunctions should provide useful initial data to understand such effects in the absence of ZnO pen homojunctions at present. Furthermore, there is much less research on the use of vapor phase transport method for the fabrication of one-dimensional ZnO homojunctions (pen or nei) in the literature and it is our intention to undertake a novel and necessary study of the use and potential of this growth method to grow ZnO-based devices. In this paper, we have studied the structural properties of (i) n-ZnO nanorods grown by VPT on ZnO buffer layer coated Si substrates and (ii) intrinsic (NaeMg doped) ZnO layer produced by solegel dip coating method on three steps n-ZnO deposits coated Si substrates. Additionally, nei ZnO homojunctions were fabricated and their currentevoltage (IeV) characteristics have been measured in the temperature range of 150e300 K and then analyzed using the standard theory outlined above with the assumption of a GD of the BH. 2. Experimental details Well-aligned ZnO nanorods were grown on n-Si substrates using a three step approach consisting of an initial seeding of

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substrates and followed by a chemical bath deposition (CBD) and finally vapor phase transport (VPT) nanorod growth. Full details of the growth method may be found in [49]. i-ZnO layer was deposited on these structures by solegel dip coating method. 2.1. Seed layer preparation n-Type Si (100) wafers (thickness ¼ 0.5 mm, resistivity 5e9 U cm) were cut into 1  1.5 cm2 pieces as substrates. These were cleaned ultrasonically in sequence with acetone, ethanol and then blown dry by a nitrogen flow. To grow the ZnO seed layer, zinc acetate dihydrate (Zn (OOCCH3)2$2H2O) was dissolved in absolute ethanol with a concentration of 5 mM and this solution was coated onto Si wafer slides by drop coating with a solution volume per substrate area of 3.5 mL/cm2. The substrates were rinsed with fresh ethanol after drop coating after 20 s, and then dried with a nitrogen flow. This procedure was repeated five times for each sample and then the samples were annealed at 350  C for 30 min in ambient air to produce the initial textured ZnO seed layer which acts a nucleation and alignment template for the ZnO nanorods to be subsequently grown by the CBD method, as described in [49,50]. 2.2. Chemical bath deposition (CBD) In the CBD process, 25 mM zinc nitrate hexahydrate (Zn(NO3)2$6H2O) was dissolved in deionized water (40 ml) and 25 mM hexamethylenetetramine (C6H12N4) was added to the solution to induce the formation of ZnO nanorods [51]. ZnO seeded substrates were immersed vertically into the aqueous solution and heated at a constant temperature of 80  C using water baths for 40 min, with stirring. After deposition, samples were taken from the solution, ultrasonically cleaned with deionized water for 5 min to remove white loosely adherent powder precipitates, rinsed with ethanol and finally dried with nitrogen. 2.3. Vapor phase transport deposition (VPT) For the VPT growth stage, 0.06 g of ZnO and graphite powders were thoroughly mixed together and placed in the middle of an alumina boat. Substrates which were covered ZnO buffer layers (seed layer plus CBD) were placed directly above the source material with the growth surface facing the powder. The boat was placed in the central part of a tube furnace. The growth was carried out at 925  C with a 90 sccm flow of argon for 1 h. The furnace was then cooled down to room temperature and the samples were taken out. 2.4. Preparation of i-type ZnO layer i-Type ZnO layer grown on the three steps n-ZnO deposits coated Si substrates was prepared by a solegel dip coating method. The initial solution is prepared using zinc acetate dihydrate Zn (OOCCH3)2$2H2O), MgCl2$H2O and NaCl in methanol (CH3OH) to form a mixed solution with a designed Na and Mg doping ion concentration. Diethanolamine (DEA) (HN(CH2eCH2OH)2), then, was added into the mixture as a stabilizer leading to a DEA/(Zn (OOCCH3)2$2H2O) molar ratio of 1:1. The mixture was stirred by a magnetic stirrer at 60  C for 1 h until a clear solution formed. Finally, the doping concentration of the mixed solution was controlled at w0.5 mol/L. The NaeMg doped ZnO samples with a constant Na/Zn molar ratio of 0.02 and Mg/Zn molar ratio of 0.06 were prepared [52]. The entire back side of the Si substrate slide was etched in a solution of H2SO4, H2O2 and 20% HF (hydrofluoric acid) and then was further etched in 6HNO3:1HF:35H2O, 20% HF. After each

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etching step, the area was rinsed thoroughly with methanol. High purity Au with a thickness of about 200 nm (as determined by a PHE 102 spectroscopic ellipsometer) was then thermally evaporated (at a pressure of w105 Torr) onto the slide’s back side. In order to obtain a low resistivity back side Ohmic contact, Au/n-Si structure was annealed at 450  C for 5 min in N2 ambient [53]. For the front side contacts, circular Au dots of 1 mm in diameter and 200 nm thickness were deposited on the upper surface of Au/n-Si/ n-ZnO/i-ZnO through a metal shadow mask. 2.5. Characterization X-ray diffraction (XRD) data were taken using a Bruker AXS D8 advance texture diffractometer with CuKa radiation over the range 2q ¼ 20e60 with a step of 0.01 at room temperature. The surface morphology was studied with Zeiss EVOLS 15 scanning electron microscopy (SEM), an accelerating voltage of 20 kV was used in this study. The resistivity and carrier concentration of the i-ZnO layer were determined by four probe method and Hall Effect measurements, respectively, at room temperature. Temperature dependent IeV measurements were carried out in a He cryostat in the temperature range 150e300 K, in 25 K steps. 3. Results and discussion 3.1. Structural properties The XRD pattern of ZnO nanorods grown on a ZnO buffer layer coated Si substrate is shown in Fig. 1. The peak positions are in good agreement with those expected from the ZnO wurtzite (hexagonal) structure in the standard data (JCPDS, 36-1451). As seen from this figure, the sample has a strong (002) preferred orientation and but also shows weaker diffraction peaks belonging to (100) and (110) planes. In addition, the smaller diffraction peaks located at 31.09 and 32.99 can be indexed to the (200) and (013) planes of the Zn2SiO4, respectively, in accordance with JCPDS card no, 24-1469, suggesting that the reactions between the Si, SiO2 and ZnO may take place the during both the heat treatment of ZnO seed layer and vapor phase transport process performed at high grown temperature [54,55]. The lattice parameters a and c were calculated from the positions of the XRD peaks of the hexagonal phase as 0.325 nm and 0.519 nm, respectively, in reasonable agreement with the known values of bulk ZnO (JCPDS, 36-1451).

XRD pattern of i-type ZnO layer grown on the three steps n-ZnO deposits coated Si substrate is given in Fig. 2. Compared to the XRD pattern of the n-type ZnO nanorod (Fig. 1), it was found that the intensity of the peak (002) decreased enormously. The peak due to (013) plane of the Zn2SiO4 disappeared and extra characteristic (101) peak of ZnO appeared, suggesting some change in the crystal alignment. The calculated lattice parameters are slightly lower than that of n-type ZnO nanorods due to the smaller ionic radii of both Na and Mg ions compared to Zn ions. The SEM micrograph showing the cross-sectional view of the ZnO nanorods grown on a ZnO buffer layer coated Si substrate is given Fig. 3(a). The micrograph shows a typical image of ZnO vertical nanorod arrays grown by VPT method on a thin ZnO buffer layer (seed layer þ CBD) coated Si substrate. It can be seen from the figure that the ZnO nanorods grow predominantly vertical to the Si substrate, consistent with the XRD data and almost all nanorods have the same length of w4 mm and diameters in the range 150e200 nm. A top view image of i-type ZnO layer deposited on three steps n-type ZnO deposits (seed layer þ CBD þ VPT) coated Si substrate is shown in Fig. 3(b). As can be seen from the figure, a wavy surface morphology with porous structure was formed over a large area. The magnified image of the top view is presented in the inset of Fig. 3(b), illustrating that the grainy shape structure with the average diameter of w200 nm is successfully obtained. Fig. 3(c) displays the cross-section image of i-ZnO layer grown on three steps n-ZnO deposits coated Si substrate. As clearly seen, i-ZnO has accumulated on upper surface of the ZnO nanorods with a thickness of w1.5 mm. The electrical properties of the n-ZnO nanorods are also of relevance to analyses of the present type. Firstly we note that the present samples are in the form of nanorods, and thus do not form a continuous conducting film (noting also that the underlying buffer layer is quite thin and with a small grain size and thus will not show significant conductivity). This morphology means that measuring the conducting properties of individual nanorods is quite challenging. However, these morphologies are reported to display background n-type conductivity in the nominally undoped condition due to intrinsic defects and impurities consistent with overwhelming majority of literature reports for all nominally undoped ZnO materials which are known from the literature to have carrier densities and mobilities of the order of 1017 cm3 and 10 cm2 V1 s1, respectively [56,57]. We therefore consider that our nominally undoped samples, of similar crystalline quality and

600

300

200 * (110)

*(101)

400

300

200

100

0

*(110)

+ (013)

Intensity (counts)

400

100

* ZnO + Zn2SiO4

500

*(002)

4

+(200)

2

+ (200) * (100)

Intensity (counts)

500

600

* (002)

* ZnO + Zn SiO

0

20

25

30

35

40

45

50

55

60

2θ (degree) Fig. 1. XRD pattern n-type ZnO nanorods grown on a ZnO buffer layer coated Si substrate.

20

25

30

35

40

45

50

55

60

2θ (degree) Fig. 2. XRD pattern of i-type ZnO layer grown on the three steps deposits coated Si substrate.

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Fig. 3. (a) A cross-sectional SEM image of the ZnO nanorods grown on a ZnO buffer layer coated Si substrate, (b) a top view image of i-type ZnO layer deposited on three steps n-type ZnO deposits coated Si substrate (Inset shows in the magnified image of i-ZnO layer), (c) the cross-section image of i-ZnO layer grown on three steps n-ZnO deposits coated Si substrate.

grown at similar temperatures, likely display similar electrical properties. On the other hand, the resistivity and carrier concentration of the i-ZnO layers are found to be 5.76  105 U cm and 1.34  1013 cm3. The resistivity value of i-ZnO samples is relatively high when compared with the reported value (57 U cm) for p-type ZnO in the literature [52]. The reasons for this change in resistivity may be due to the larger density of extrinsic traps at the grain boundaries due to oxygen chemisorptions as well as the differences in surface morphologies [58]. Au/n-Si/n-ZnO/Au and Au/i-ZnO/Au contacts were measured at room temperature and observed that these junctions exhibited ohmic behavior as seen in Fig. 4. Thus we can conclude that the observed rectifying effect originates purely from the n-ZnO/i-ZnO diode.

For a diode under a significant forward bias V (V > 3kT/q), and assuming the existence of a series resistance (Rs), the relation between I and V can be expressed as follows:

 
 qðV  IRs Þ 1 I ¼ I0 exp nkT

  I0 qFb0 ¼ A T 2 exp  A kT

(2)

where A is the effective diode area, Fb0 the zero bias barrier height and A* is the theoretical Richardson constant of 32 A cm2 K2 for ZnO. The experimental values of n and Fb0 are determined from slopes and intercepts of the forward bias ln I vs. V plot at each temperature, respectively. The values of n can be obtained from the slope of the linear regions of the forward bias ln I vs. V plots and can be expressed as:

Au/i-ZnO/Au Au/n-Si/n-ZnO/Au

1e-5

(1)

where I0 is the reverse saturation current, q is electronic charge, V is applied voltage, Rs is series resistance, IRs is the voltage across on the Rs, n is the ideality factor, k is Boltzmann constant and T is the absolute temperature in Kelvin [59]. The saturation current density J0 is defined by

J0 ¼

2e-5

Current (A)

3.2. Forward bias IeV characteristics as a function of temperature

0

n ¼

-1e-5

q dðV  IRs Þ kT dðlnðIÞÞ

(3)

The zero bias barrier height in Eq. (2) is obtained from the intercept of this plot and can be expressed as:

-2e-5 -2

-1

0

1

Voltage (V) Fig. 4. IeV characteristics of Au/n-Si/n-ZnO/Au and Au/i-ZnO/Au junctions.

2 qfb0 ¼ kTln



AA T 2 I0

 (4)

S. Yılmaz et al. / Current Applied Physics 12 (2012) 1326e1333

A schematic view of the ZnO homojunction structure can be seen in Fig. 5(a). Fig. 5(b) shows a set of experimental semilogarithmic forward bias IeV characteristics (using a semilogarithmic graph) of the i-ZnO/n-ZnO homojunction measured at different temperatures ranging from 150 K to 300 K. The IeV characteristics were shown in the inset in Fig. 5(b) and it may be seen that the n-ZnO/i-ZnO homojunction clearly displays rectifying behavior with a turn on voltage of w1.5 V at 300 K. This value is in agreement with previous p-ZnO/n-ZnO film homojunction diode which was reported that the turn on voltage appears in the range of 1e3 V [60,61]. Our value is also much lower than those of other wide band gap materials such as GaN and ZnSe, regarding larger band gap energy of ZnO, which could be attributed to the high defect concentration in the interface [62]. It is well-known that a low value of the turn on voltage is critically important in device applications. Generally, it is known that the ideality factor n increases while the zero bias barrier height Fb0 decreases with decreasing temperature [29e37]. The experimental ideality factor (n) and the apparent ideality factor (nap) (denoted by closed circles and open squares, respectively) are shown in Fig. 6. The apparent ideality factor values were obtained using the method indicated by Eq. (3). These data display the general trend of increasing at lower temperatures. The ideality factors shown in Fig. 6 are in all cases much higher than that those expected for ideal pen junctions n ¼ 1e2. However, other authors have reported ideality factor values exceeding our values (5.4e6.6) such as 3e25 for p-ZnO:As/ n-ZnO [63] and 10e20 for n-ZnO/p-(Zn,Mg)O:P diodes [64]. These increased values of the measured ideality factor in homojunctions may be due to: (1) current leakage effects originating from the surface imperfections at the junction interface such as Zn2SiO4 layer observed in our XRD patterns in both Figs. 1and 2 [55,65] and (2) carrier recombination in the space-charge region, such as deep level assisted tunneling and/or parasitic rectifying junctions within the device [66]. Fig. 7 shows the zero bias barrier height Fb0 and the apparent barrier height Fap (denoted by closed circles and open squares, respectively) as a function of temperature for the forward biased junction. It is seen that the zero bias barrier height increases from 0.43 to 0.75 eV with increasing temperature. The increase in the barrier height is probably due to the increase in the depletion layer

a

Ohmic contact Au

7.0 Au/n-Si/n-ZnO/i-ZnO 6.8 Experimental Apparent

6.6 6.4 Ideality Factor

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6.2 6.0 5.8 5.6 5.4 5.2 100

150

200

250

300

350

Temperature (K) Fig. 6. Temperature dependence of the ideality factor (n). The open squares show the estimated value of the ideality factor using Eq. (8).

width originating from the accumulation of minority charge carriers at the interface at elevated temperatures. The interface barrier inhomogeneity is also sometimes is responsible for an apparent increase in the barrier height, implying that the IeV characteristics do not necessarily represent the true mean barrier height at a particular temperature [66]. To determine the zero bias barrier height in another way, Eq. (2) can be rewritten as:

  I qFb0 ¼ lnAA  ln 02 kT T

(5)

The Richardson constant is usually determined from the intercept of ln (I0/T2) vs. q/nkT graph [45]. The experimental data, shown in Fig. 8, fit to a straight line over the entire temperature range. The activation energy (Ea ¼ Fb0) is usually determined from the slope of the ln (I0/T2) vs. q/nkT plot: its value for our structure is Ea ¼ 0.926 eV while the intercept yields a Richardson constant (A*) of 2.61  108 A cm2 K2 which is much lower than the known theoretical value of 32 A cm2 K2 for ZnO although a similar value of 8.60  109 A cm2 K2 was reported by Mtangi et al. [67]. This

b 1.0e-3 Au/n-Si/n-ZnO/i-ZnO

Au/n-Si/n-ZnO/i-ZnO

0.8

1.0e-4

Φbo Φap

n-type VPT ZnO n-type CBD ZnO Seed layer ZnO

0.7

1.0e-6

T=300 K 4e-5

1.0e-7

3e-5 2e-5

1.0e-8

n-Si

300 K 275 K 250 K 225 K 200 K 175 K 150 K

0 -2 -1 0

1

2

0.4

-1e-5

1.0e-10 0.0

0.5

1.0

0.6

0.5

1e-5

1.0e-9

Ohmic contact Au

Φbo and φap (eV)

i-type ZnO

Current (A)

1.0e-5

1.5

2.0

2.5

Voltage (V)

150

200

250

300

Temperature (K) Fig. 5. (a) A view of schematic layering in the ZnO homojunction diode and the contacts for IeV data. (b) Semilog IeV characteristics of the Au/n-Si/n-ZnO/i-ZnO homojunction at various temperatures with the IeV inset showing the turn on voltage.

Fig. 7. Variation of the zero bias barrier height Fb0 and the apparent barrier height Fap of ZnO homojunction with temperature.

S. Yılmaz et al. / Current Applied Physics 12 (2012) 1326e1333

Au/n-Si/n-ZnO/i-ZnO

Ln (I0/T2) (A.K-2)

-28

-30

-32

-34

y = (-0.926)x - 22.59 φb0 = 0.926 eV A* = 2.61x10-8A.cm-2K-2

-36 6

8

10

12

14

q/nkT (eV-1) Fig. 8. Richardson plots of ln(J0/T2) versus q/nkT or q/kT.

discrepancy between the experimental and theoretical values of A* has already been reported by many researchers in recent years [35,36,38]. Such fluctuations in A* can be attributed to the spatially inhomogeneous BH and potential fluctuations at the interface which consist of low and high barrier areas. As a consequence, the current transport paths through the diode flow preferentially through the lower barrier regions. As was explained by Horvath [31], the value of A* obtained from temperature dependence studies of the forward bias IeV characteristics may be affected by lateral inhomogeneity in the interface. Therefore, the lower values of experimentally determined A* indicate that the effective active area is in fact much smaller than the physical diode rectifier contact area due to the presence surface defects or dislocations in the material, interface roughness and processing-induced contaminations in environment of the surfaces [30,35]. With increased temperature a much greater fraction of the area of the interface region with larger BH values can be accessed more current flows through the entire interface region. We will apply the method of inhomogeneous-barrier analysis to determine the modified Richardson constant of our ZnO homojunctions.

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and the standard deviation at zero bias, respectively. Fb0 , ss0 and the voltage coefficients r2, r3 obey Eqs. (6) and (7), respectively. Using this approach, and based on Eqs. (6) and (7), a plot of zero bias barrier height Fb0 versus q/2 kT should show a straight line, allowing Fb0 and s2s0 to be determined from the intercept and slope, respectively. This plot is shown in Fig. 9 (denoted by closed circles). A least-square linear fitting of these data yields values of 1.041 eV and 0.1 eV for Fb0 and ss0, respectively. The barrier height of 1.041 eV is in close agreement with the one reported by Gopalakrishnan et al. although our value ss0 is lower than that reported by those authors implying a more homogeneous interfacial barrier height distribution [68]. The n11 versus q/2 kT plot is also shown in Fig. 9 (denoted by open squares). According to Eq. (7), this plot should be a straight line whose intercept and slope allow the voltage coefficients r2 and r3 from the intercept and slope, respectively. The values of r2 ¼ 0.783 and r3 ¼ 0.0017 V were obtained from this plot. Since r3 is negative it is clear from Eq. (7) that it is responsible for the increase in nap with decreasing temperature. Since r2 is also negative, we conclude that both the barrier height and standard deviation decrease as the bias voltage is increased. These results reveal that increased bias voltage effectively homogenizes the barrier height distribution and reduces fluctuation in this quantity, i.e., the higher the bias, the narrower the barrier height distribution. This can be explained by the fact that image charge effects shift the effective barrier maximum deeper into the semiconductor when the bias voltage increases, thus effectively homogenizing the barrier height distribution [69]. The Richardson plot is now modified by combining Eqs. (2) and (6) to give:

  I ln 02  T

s20 q2 2k2 T 2

! ¼ lnAA 

qFb0 kT

(8)

This modified lnðI0 =T 2 Þ  ðs20 q2 =2k2 T 2 Þ versus q/kT plot should also be a straight line with the slope and intercept directly yielding the mean barrier height Fb0 and the modified Richardson constant A**, respectively (Fig. 10). It can be seen that the modified Richardson plot has quite a good linearity over the entire temperature range and these data yield a mean barrier height and modified Richardson constant from least-squares fitting of Fb0 ¼ 1.032 eV and A** ¼ 34.85 A cm2 K2, respectively. This value

3.3. Inhomogeneous-barrier analysis and modified Richardson plots

Fap ¼ Fb0 

qs2s0 2kT

(6)

Au/n-Si/n-ZnO/i-ZnO n-1-1

0.8

Φap

-0.82

-0.84

-1

0.6

(n -1)

0.7

0.5 -0.86

and

  1 qr    1 ¼ r1 T ¼ r2  3 2kT nap T

-0.80

0.9

φap(eV)

The IeV characteristics shown earlier show that there is an increase in Fb0 and a decrease in the ideality factor with increasing temperature possibly caused by barrier height inhomogeneities resulting from the variation in thickness and composition of the interface layer, and non-uniformity of interfacial charges. In some studies this has been successfully analyzed on the basis of a TE mechanism with a GD of barrier heights. The GD of the apparent barrier height and the apparent ideality factor with temperature are characterized by the following relations [29e37]:

0.4

(7)

r1, r2 and r3 are the voltage coefficients which may depend on T, and which quantify the voltage deformation of the barrier height distribution, while Fb0 and ss0 represent the mean barrier height

0.3 15

φap = (- 0.016)(q/2kT) - 1.041 -1 (n -1) = (- 0.0017)(q/2kT) - 0.7833

20

25

30

35

-0.88 40

q/ 2kT(eV-1) Fig. 9. Zero bias apparent barrier height Fap and n1  1 versus q/2 kT plots.

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References

-40

2 2

2 2

Ln (I0/T2) - (σ q /2k T ) (A.K-2)

Au/n-Si/n-ZnO/i-ZnO

-60

-80

-100 30

y = (-1.032)x + 0.0902 φb0 = 1.032 eV A** = 34.85 A.cm-2K-2 40

50

60

70

80

q/kT(eV-1) Fig. 10. Modified Richardson plots of lnðI0 =T 2 Þ  ðs20 q2 =2k2 T 2 Þ versus q/kT.

of Fb0 ¼ 1.032 eV is very close to the value of Fb0 ¼ 1.041 eV obtained from the plot of Fap versus q/2 kT shown in Fig. 9, while the modified Richardson constant A** ¼ 34.85 A cm2 K2 is slightly higher than the theoretical value of A* ¼ 32 A cm2 K2, however we also note that modified Richardson constant values of 27 and 167 A cm2 K2 are reported in the literature and are in good order of magnitude agreement with our data [67,70]. 4. Conclusions In this study, well-aligned n-type ZnO nanorods on ZnO buffer layer coated Si substrates were obtained by VPT growth. SEM and XRD measurements reveal that these nanorods crystallize in the hexagonal crystalline structure with a preferential orientation of the nanorod long axis (which is parallel to the crystal c-axis) perpendicular to the substrate. We subsequently fabricated Na- and Mg-doped i-ZnO by a solegel method and formed Au/Si/n-ZnO/iZnO junctions and electrical measurements confirmed that these structures displayed rectifying IeV characteristics. The IeV characteristics of these structures in the temperature range of 150e300 K have been analyzed using standard TE theory assuming a GD of the BH. The non-ideality of the forward bias IeV characteristics in the ZnO homojunction diode was attributed to inhomogeneity in the barrier height as well as the effects of series resistance. The value of Fb0 increases and the ideality factor n decreases with increasing temperature. In addition, the ln (I0/T2) vs q/nkT plot shows an almost linear behavior and values for (Ea ¼ Fb0) and A* of 0.926 eV and 2.61  108 A cm2 K2, respectively, are obtained. A modified lnðI0 =T 2 Þ  ðs20 q2 =2k2 T 2 Þ vs q/kT plot based on a GD of BHs yielded values for Fb0 and A** of 1.032 eV and 34.85 A cm2 K2, respectively. The value of A** is close to the theoretical value of 32 A cm2 K2 for ZnO. As a result, we have shown that the temperature dependent IeV characteristics of the Au/Si/n-ZnO/i-ZnO homojunctions can be successfully explained on the basis of the TE mechanism with a GD of interfacial BHs. Acknowledgment The corresponding author (SY) gratefully acknowledges the support of the Council of Turkish Higher Education in the form of a fellowship to support extended visits to foreign institutions. This work was supported by the research fund of Karadeniz Technical University, Trabzon, Turkey, under contract no. 2010.111.001.3.

[1] X.W. Sun, J.Z. Huang, J.X. Wang, Z. Xu, Nano Lett. 8 (2008) 1219. [2] X.D. Wang, J. Zhou, C.S. Lao, J.H. Song, N.S. Xu, Z.L. Wang, Adv. Mater. 19 (2007) 1627. [3] P. Klason, M.M. Rahman, Q.H. Hu, O. Nur, R. Turan, M. Willander, Microelectron J. 40 (2009) 706. [4] T.M. Borseth, B.G. Svensson, A.Y. Kuznetsov, Appl. Phys. Lett. 89 (2006) 262112. [5] S. Chu, J.H. Lim, L.J. Mandalapu, Z. Yang, L. Li, J.L. Liu, Appl. Phys. Lett. 92 (2008) 152103. [6] P. Uthirakumar, J.H. Kang, B.D. Ryu, H.G. Kim, H.K. Kim, C.-H. Hong, Mater. Sci. Eng. B 166 (2010) 230. [7] R.S. Mane, W.-J. Lee, C.D. Lokhande, B.W. Cho, S.-H. Han, Curr. Appl. Phys. 8 (2008) 549. [8] S. Yılmaz, E. McGlynn, E. Bacaksız, J. Cullen, R.K. Chellappan, Chem. Phys. Lett. 525 (2012) 72. [9] H. Tang, L. Zhu, H. He, Z. Ye, Y. Zhang, M. Zhi, Z. Yang, B. Zho, T. Li, J. Phys. D: Appl. Phys. 39 (2006) 2696. [10] K. Jacobs, D. Balitsky, P. Armand, P. Papet, Solid State Sci. 12 (2010) 333. [11] S.E. Ahn, J.S. Lee, H. Kim, S. Kim, B.H. Kang, K.H. Kim, G.T. Kim, Appl. Phys. Lett. 84 (2004) 5022. [12] S. Yılmaz, M. Parlak, S¸. Özcan, M. Altunbas¸, E. McGlynn, E. Bacaksız, Appl. Surf. Sci. 257 (2011) 9293. [13] S. Yılmaz, E. McGlynn, E. Bacaksız, S¸. Özcan, D. Byrne, M.O. Henry, R.K. Chellappan, J. Appl. Phys. 111 (2012) 013903. [14] C.X. Xu, X.W. Sun, B.J. Chen, Appl. Phys. Lett. 84 (2004) 1540. [15] M. Biswas, E. McGlynn, M.O. Henry, M. McCann, A. Rafferty, J. Appl. Phys. 105 (2009) 094306. [16] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [17] R. Schifano, E.V. Monakhov, J.S. Christensen, A. Yu. Kuznetsov, B.G. Svensson, Phys. Status Sol. A 205 (2008) 1998. [18] Y.R. Park, J. Kim, Y.S. Kim, Appl. Surf. Sci. 255 (2009) 9010. [19] Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87. [20] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205. [21] L.T. Tsai, S.P. Chiu, J.G. Lu, J.J. Lin, Nanotechnology 21 (2010) 145202. [22] B. Xiao, Z. Ye, Y. Zhang, Y. Zeng, L. Zhu, B. Zhao, Appl. Surf. Sci. 253 (2006) 895. [23] W. Liu, F. Xiu, K. Sun, Y.H. Xie, K.L. Wang, Y. Wang, J. Zou, Z. Yang, J. Liu, J. Am. Chem. Soc. 132 (2010) 2498. [24] M.K. Gupta, N. Sinha, B.K. Singh, B. Kumar, Mater. Lett. 64 (2010) 1825. [25] J.C. Lee, K.H. Kang, S.K. Kim, K.H. Yoon, I.J. Park, J. Song, Sol. Energy Mater. Sol. Cells 64 (2000) 185. [26] J. Perrenoud, L. Kranz, S. Buecheler, F. Pianezzi, A.N. Tiwari, Thin Solid Films 519 (2011) 7444. [27] W.S. Han, Y.Y. Kim, B.H. Kong, H.K. Cho, Thin Solid Films 517 (2009) 5106. [28] D.-K. Hwang, M.-S. Oh, J.-H. Lim, Y.-S. Choi, S.-J. Park, Appl. Phys. Lett. 91 (2007) 121113. [29] Y.P. Song, R.L. Meirhaeghe, W.H. Laflere, F. Cardon, Solid-State Electron. 29 (1986) 633. [30] J.H. Werner, H.H. Güttler, J. Appl. Phys. 69 (1991) 1522. [31] Zs.J. Horvarth, Solid-State Electron. 39 (1996) 176. [32] S. Chand, J. Kumar, Semicond. Sci. Technol. 10 (1995) 1680. [33] R.T. Tung, Phys. Rev. B 45 (1992) 13509. [34] M.K. Hudait, S.B. Krupanidhi, Mat. Sci. Eng. B 87 (2001) 141. [35] S¸. Karatas¸, S¸. Altındal, A. Türüt, A. Özmen, Appl. Surf. Sci. 217 (2003) 250. [36] G.P. Ru, R.L.V. Meirhaeghe, S. Forment, Y.L. Jiang, X.P. Qu, S. Zhu, B.Z. Li, Solid State Electron. 49 (2005) 606. [37] W. Yang, J. Marino, A. Monson, C. Wolden, Semicond. Sci. Tech. 21 (2006) 1573. [38] O. Pakma, N. Serin, T. Serin, S¸. Altındal, J. Appl. Phys. 104 (2008) 14501. [39] K. Ejderha, N. Yıldırım, B. Abay, A. Turut, J. Alloy. Compd. 484 (2009) 870. lu, S¸. Altındal, J. Alloy. Compd. 484 (2009) 405. [40] A. Tatarog [41] M. Soylu, B. Abay, Y. Onganer, J. Alloy. Compd. 509 (2011) 5105. [42] B. Abay, J. Alloy. Compd. 506 (2010) 51. [43] M.E. Aydın, J. Appl. Phys. 102 (12007) 043701. [44] S. Chand, J. Kumar, Semicond. Sci. Tech. 11 (1996) 1203. [45] A. Gümüs¸, A. Türüt, N. Yalçın, J. Appl. Phys. 91 (2002) 245. [46] R.F. Schmitsdorf, T.U. Kampen, W. Mönch, Surf. Sci. 324 (1995) 249. [47] R.T. Tung, J. Appl. Phys. 85 (1999) 1935. [48] J.P. Sulvian, R.T. Tung, M.R. Pinto, W.R. Graham, J. Appl. Phys. 70 (1991) 7403. [49] D. Byrne, E. McGlynn, K. Kumar, M. Biswas, M.O. Henry, G. Hughes, Cryst. Growth Des. 10 (2010) 2400. [50] L.E. Greene, M. Law, D.H. Tan, M. Montano, J. Goldberger, G. Somorjai, P. Yang, Nano Lett. 5 (2005) 1231. [51] L.L. Yang, Q.X. Zhao, M. Willander, J. Alloy. Compd. 469 (2009) 623. [52] Z.Q. Ma, W.G. Zhao, Y. Wang, Thin Solid Films 515 (2007) 8611. [53] I. Dökme, S. Altındal, T. Tunç, I. Uslu, Microelectron. Reliab. 50 (2010) 39. [54] J. Zhou, J. Liu, X. Wang, J. Song, R. Tummala, N.S. Xu, Z.L. Wang, Small 3 (2007) 622. [55] D. Byrne, R.F. Allah, T. Ben, D.G. Robledo, B. Twamley, M.O. Henry, E. McGlynn, Cryst. Growth Des. 11 (2011) 5378. [56] J. Goldberger, D.J. Sirbuly, M. Law, P. Yang, J. Phys. Chem. B 109 (2005) 9. [57] H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, W. Cai, Adv. Funct. Mater. 20 (2010) 561. [58] J. Sun, T. Yang, G. Du, H. Liang, J. Bian, L. Hu, Appl. Surf. Sci. 253 (2006) 2066. [59] S.S. Lin, Z.Z. Yel, J.G. Lu, H.P. He, L.X. Chen, X.Q. Gu, J.Y. Huang, L.P. Zhu, B.H. Zhao, J. Phys. D: Appl. Phys. 41 (2008) 155114.

S. Yılmaz et al. / Current Applied Physics 12 (2012) 1326e1333 [60] Y. Yang, X.W. Sun, B.K. Tay, G.F. You, S.T. Tan, K.L. Teo, Appl. Phys. Lett. 93 (2008) 253107. [61] A. Kumar, M. Kumar, B.P. Singh, Appl. Surf. Sci. 256 (2010) 7200. [62] W.Z. Xu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, B.H. Zhao, L. Jiang, J.G. Lu, H.P. He, Appl. Phys. Lett. 88 (2006) 173506. [63] Y.R. Ryu, T.S. Lee, J.H. Leem, H.W. White, Appl. Phys. Lett. 83 (2003) 4032. [64] Y.W. Heo, Y.W. Kwon, Y. Li, S.J. Pearton, D.P. Nortona, Appl. Phys. Lett. 84 (2004) 3474. [65] J.L. Pau, J. Piqueras, D.J. Rogers, F.H. Teherani, K. Minder, R. McClintock, M. Razeghi, J. Appl. Phys. 107 (2010) 033719.

1333

[66] Y.W. Song, K. Kim, J.P. Ahn, G.E. Jang, S.Y. Lee, Nanotechnology 20 (2009) 275606. [67] W. Mtangi, F.D. Auret, C. Nyamhere, P.J. Janse van Rensburg, M. Diale, A. Chawanda, Physica B: Condensed Matter 404 (2009) 1092. [68] N. Gopalakrishnan, L. Balakrishnan, V.S. Pavai, J. Elanchezhiyan, T. Balasubramanian, Curr. Appl. Phys. 11 (2011) 834. [69] S. Zhu, C. Detavernier, R.L.V. Meirhaeghe, F. Cardon, G.P. Ru, X.P. Qu, B.Z. Li, Solid-State Electron. 44 (2000) 1807. [70] K. Sarpatwari, O.O. Awadelkarim, M.W. Allen, S.M. Durbin, S.E. Mohney, Appl. Phys. Lett. 94 (2009) 242110.