ZnO Metal-Semiconductor-Metal UV Photodetectors on PPC Plastic with Various Metal Contacts

The unique and attractive properties of the II-VI compound semiconductors have triggered an enormous incentive among the scientists to explore the possibilities of using them in industrial applications. Zinc Oxide (ZnO) is one of the compound semiconductors of the II– VI family with a direct band gap of 3.37 eV at room temperature, and a large excitation binding energy (60 meV). ZnO is low cost and easy to grow. It is also sensitive to the UV region because of its ultra violet absorbance and has high photoconductivity (Young et al., 2006). These properties make ZnO a promising photonic material for several applications, such as transparent conducting electrodes, surface acoustic wave filters, gas sensors, light emitting diodes, laser diodes and ultraviolet detectors (Kumar et al., 2009; Lim et al., 2006; Janotti et al., 2009; Bang et al.,2003). ZnO is of value and importance due to wide chemistry, piezoelectricity and luminescence at high temperatures. In industry, it can be used in paints, cosmetics, plastics and rubber manufacturing, electronics, and pharmaceuticals [http://www.navbharat.co.in/Clients.htm].

for GaN it is difficult. Secondly, the ZnO has large exciton binding energy which means that it has a lower threshold power for lasing. Thirdly, it is one of the few oxides that show quantum confinement in an experimentally accessible range of the size of the particles.
The preparation of ZnO thin film on flexible substrates such as plastic has so far received much interest due to its wide variety of applications as in flexible sensors and curved detector arrays. Plastic substrates provide lighter, more resistant to damage, flexible, and durable devices (Nandy et al., 2010); these attributes make them suitable for portable devices such as smart cards, personal digital assistants, digital cameras, cell phones, remote control, and circuits camcorders (Brabec et al., 2001). The substrate materials, that are commonly used for the above applications, include polyethylene terephthalate (PET), polyolefin, polytetrafluoroethylene (Teflon), Polycarbonate (PC), polyarylate (PAR), polyestersulfone (PES), polyimide (PI), poly(ethylene naphthalate) (PEN), thermoplastic polymethyl methacrylate (Perpex, Plexiglas), polyethylene naphthalate, and cellulose triacetate (Ma et al., 2008).
Most of ZnO studies have focused on fabrication issues with little attention given to the structural and optical properties of these films on flexible substrates. Interfacial phenomena in ZnO/polymer heterostructures are expected to be very different from those observed for inorganic substrates, there is a need for more comprehensive investigations to assess the potential of ZnO as a material that can be used in flexible electronic applications. Deposition of ZnO on polymers may open a new opportunities for the creation of novel multifunctional polymer/semiconductor heterostructures.
ZnO thin films deposited on plastics had a number of advantages compared with those on inorganic substrates. From the literature, various types of organic substrates have been explored by researchers to deposited ZnO.
Many researchers (Ott & Chang, 1999;Tsai et al., 2006;Lu et al., 2007) have grown ZnO on PET plastic substrates using different techniques for exemple: Ott and Chang (Ott & Chang, 1999) reported that their samples showed highly transparent (T > 80%) and conductive (ρ ~ 10 -3 Ω cm) and the best film grown on PET had a resistivity of 1.4 10 -3 Ωcm. Banerjee et al  deposited ZnO thin films with two different thicknesses (~260 and ~470 nm) for 4 and 5 hours using DC sputtering technique. They found that X-Ray diffraction (XRD) pattern for 4 h deposited ZnO films showed a weak intensity as well as broad peak at (002). But the film deposited for 5 h confirmed the formation of crystalline ZnO and three other peaks originated from (100), (002) and (101) reflections of hexagonal ZnO. Their films showed almost 80% to more than 98% visible transmittance and the bandgap (E g ) values were 3.53 eV and 3.31 eV for the films with (~260 and ~470 nm) thicknesses. Room temperature conductivities of the films were found ranging around 0.05 to 0.25 S cm -1 and the maximum carrier concentrations around 2.810 16 and 3.110 20 cm -3 with a variation in the deposition time of 4 and 5 h respectively.
On the other hand, Tsai et al (Tsai et al., 2006) reported that their ZnO/PET films showed a strong (002) peak from XRD measurements and the scanning electron microscopy (SEM) morphology of the films showed that no crack or bend appeared on the films. The average transmittance in the visible spectrum was above 80% for both substrates and the value of E g was found equal to 3.3 eV. The lowest resistivity obtained was 4.010 -4 Ω cm. Lu et al (Lu et al., 2007) found that XRD results of their deposited thin films showed a strong peak at (002) orientation and the root mean square (rms) were in the range 2.63~11.1 nm determined by atomic force microscopy (AFM). The average transmittance of the film was obtained over 80% in the visible spectrum. The lowest resistivity obtained was 4.010 -3 Ωcm.
Whereas,  deposited ZnO thin film at the first time on Teflon substrate by the rf magnetron sputtering. They reported that the XRD pattern of the film showed a strong peak at 2= 34.283° which corresponding to the (002) peak with full-width at half-maximum (FWHM) of 0.724° and the film also showed other peaks such as (100), (101), (102), (103) and the crystallite size was equal to 10 nm. The SEM image of ZnO thin film showed that the ZnO film structure consists of some columnar structured grains.
On the other hand, Kim et al  deposited ZnO films on PC and PES substrates by using rf sputtering system. They studied the effect of sputtering power ranged from 100 to 200 W on the characteristics of the films. XRD patterns of their deposited films showed strong 2 peaks at 34.4 o and they found that the intensities of the ZnO (002) peak and the grain size increased with increasing the sputtering power. The transmittance of the films on both substrates was 80-90%. Also Liu et al  prepared Transparent conducting aluminum-doped zinc oxide (ZnO:Al) films on PC substrates by pulsed laser deposition technique at low substrate temperature (room-100 °C. they reported that their experiments were performed at various oxygen pressures (3 pa, 5 pa, and 7 Pa). they studied the influence of the process parameters on the deposited (ZnO:Al) films. X-ray diffraction for their prepared films showed polycrystalline ZnO:Al films having a preferred orientation with the c-axis perpendicular to the substrate were deposited with a strong single violet emission centering about 377-379 nm without any accompanying deep level emission. The average transmittances exceed 85% in the visible spectrum.
Other organic substrates such as polyimide (Paul et al., 2007 andCraciun et al., 1994), polyarylate (Ianno et al., 1992), PEN (Koch et al., 1985) have also been tried by other researchers to deposit ZnO thin film. These ZnO thin films have been used to fabricate thin film transistor (TFT). The fabricated ZnO TFT on various organic substrates showed very encouraging results.
In summary, ZnO thin film deposited on organic substrates by different techniques are in the polycrystalline form with (002) orientation as the dominant peak. The thin films grown on these substrates usually demonstrated good optical transmittance characteristic, i.e. above 80% and the bandgap could be varied from 3.31-3.53 eV. In addition, the carrier concentration is dependent on the growth condition. Overall the ZnO thin films quality on organic substrates is comparable to those in organic substrates.

An overview of photodetectors
Photodetectors are basically semiconductor devices that convert the incident optical signal into an electrical signal which is usually revealed as photocurrent. The photodetectors can detect the optical signals over a range of the electromagnetic spectrum that is usually predominantly defined based on the material properties. A detector is selected based on the requirements of a particular application. The general requirements include wavelength of light to be detected, sensitivity level needed, and the response speed. In general, photodetectors respond uniformly within a specific range of the electromagnetic spectrum. Consequently, the wavelength of light detected determines the selection of the photodetector material and the target application and defines the structure (Sze, 2002). This will be explained further in the following sections.
When the incident photons with energy higher than the bandgap energy of a semiconductor; some of them will be absorbed within the semiconductor layer. Such a successful absorption process results in the generation of a free electron-hole pair. The energy gained by the electron which is called the work function must be sufficient to make the electron cross the barrier height between the metal contact and the semiconductor with kinetic energy (E e ). The kinetic energy can be given as (Sze, 2002).
where c is the light velocity, m  is the metal work function and  is the incident wavelength.
Since the photoelectric effect is based on the photon energy hv, the wavelength of interest is related to energy transition ∆E in the device operation, with the following relationship: where ∆E is the transition of energy levels.
Because the photon energy hv > ∆E can also cause excitation, Eq. 2 is often the minimum wavelength limit for detection. The transition energy ∆E, in most cases, is the energy gap of the semiconductor. It depends on the type of photodetector, and it can be the barrier height as in a metal-semiconductor (MS) photodiode. Alternatively, the transition energy can be between an impurity level and the band edge as in an extrinsic photoconductor. The type of photodetector and the semiconductor material are normally chosen and optimized for the wavelength of interest. The absorption of light in a semiconductor is indicated by the absorption coefficient. The transition energy does not only determine whether light can be absorbed for photoexcitation, but it also indicates where light is absorbed. A high value of absorption coefficient indicates light is absorbed near the surface where light enters. Whilst, a low value means the absorption is low that light can penetrate deeper into the semiconductor (Sze & Kwok 2006).

Semiconductors for UV photodetection
UV detection has usually been applied to narrow bandgap semiconductor photodiodes, thermal detectors, photomultiplier tubes (PMT), or charge-coupled devices (CCD) because www.intechopen.com they exhibit high gain and low noise and they can be rather visible-blind. However, PMT is a fragile and bulky device which requires high power supplies. On the other hand, CCD detectors are slow and their response does not depend on the wavelength. Semiconductor photodetectors require only a mild bias, and can be benefited for their small size and light weight, and being insensitive to magnetic fields. Their low cost, good linearity and sensibility, and capability for high-speed operation make them excellent devices for UV detection (Monroy et al., 2003).
The main disadvantage of these narrow-bandgap semiconductor detectors is device aging due to the exposure to radiation that has much higher energy than the semiconductor bandgap. Moreover, passivation layers, typically SiO 2 , reduce the quantum efficiency in the deep-UV range, and are also degraded by UV illumination (Caria et al., 2001)[43]. Another disadvantage of these devices is their sensitivity to low energy radiation, eventually, filters are required to block out visible and infrared photons, resulting in a significant loss of the effective area of the device. Finally, for high-sensitivity applications, the detector active area must be cooled to reduce the dark current. The cooled detector behaves as a cold trap for contaminants which leads to a lower detectivity (Monroy et al., 2003). Such problems make the wide-bandgap semiconductors, such as gallium nitride and ZnO, attractive alternatives. Therefore, ZnO has been used in optoelectronic, high power and high frequency devices.

Wide-bandgap semiconductors for UV photodetection
Detection of ultraviolet (UV) radiation is increasingly becoming important in a number of areas, such as flame detection, water purification, furnace control, UV astronomy, UV radiation dosimetry (Lakhotia et al., 2010).
Even though the responsivity of Si-based optical photodetectors in the UV region is low, they are still being used for light detection (Lakhotia et al., 2010). This has promoted some researchers to use wide direct band gap materials to fabricate optoelectronic devices that are sensitive in the UV region. Hence, GaN-based UV photodetectors have already become commercially available (Hiramatsu et al., 2007). ZnSe-based UV photodetectors, which is another wide direct band gap material, have also been manufactured (Hanzaz et al., 2007). Fabrication and characterization of low-intensity ultraviolet metal-semiconductor-metal (MSM) photodetectors based on AlGaN have also been reported (Gökkavas et al., 2007). ZnO is another semiconductor of wide direct bandgap that is also sensitive in the UV region and is of low cost and easy to manufacture. Therefore, ZnO will be focused in this chapter.
The importance of semiconductor UV PD has expanded the semiconductor industry and emphasized the development of low-light-level imaging systems for military and civilian surveillance applications. These detectors should: 1. not be sensitive to light at visible wavelengths (commonly referred to as being solar blind), 2. have a large response at the wavelength to be detected and have high quantum efficiency, 3. have a small value for the additional noise introduced by the detector.
There are many different types of semiconductor ultraviolet photodetectors such as: photoconductive or photoconductors detectors, p-n junctions photodiode, and MSM (metalwww.intechopen.com semiconductor-metal) photodetector (Liu et al., 2010). In the field of optical devices, several trends are pushing research to use new materials. For example, the UV PD is successfully fabricated based on wide bandgap semiconductors (E g > 3.0 eV). Photon detectors may be further subdivided according to their physical effects that make the detector responsive.

Metal-Semiconductor-Metal (MSM) photodiodes
Metal-semiconductor-metal (MSM) is a type of PD used for UV detection. MSM PDs consist of two interdigitated Schottky contacts which are called fingers deposited on top of an active layer as shown in Figure 1. These devices have a fast response and simple structure compared to other photodetectors of the same active area because of the interdigitated structure which reduces carrier transit time through close spacing of the electrodes, while maintaining a large active area.
The MSM PD operates when the incident light is directed on the semiconductor material between the fingers, electrons will be generated in the conduction band, and thus creating holes in the valance band of the undoped region of the semiconductor. This results in creating a photocurrent by means of one of two processes with the operative process determined by the magnitude of the incident photon energy (hv) relative to the energy bandgap (E g ) of the semiconductor and the metal work function ( Bn ). If E g is greater than hv and hv is greater than  Bn , then photoelectric emission of electrons from the metal to the semiconductor occurs. Alternatively, for the second process, if hv is greater than E g , then photoconductive electron-hole pairs are produced in the semiconductor. The generated electrons and holes are separated by an electric field intrinsically formed between the fingers.

ZnO as a UV detector
As mentioned before, ZnO is one of the most prominent semiconductors in the metal-oxide family because of its excellent properties which attract many researcher groups to use this material in UV detection applications. Below, a briefly discussion of ZnO as a UV detector will present and some earlier studies about the fabrication of ZnO as UV detector will also be provided.
ZnO was one of the first semiconductors to be prepared in a rather pure form after silicon and germanium. It was extensively characterized as early as in the 1950`s and 1960`s due to its promising properties (Plaza et al., 2008). Wide band gap semiconductors have so far gained much attention for the last decade because they can be used in optoelectronic devices in the short wavelength and also in the UV region of the electromagnetic spectrum. These   . Table 1 below shows a summary of some of the important properties of these wide band gap semiconductors. Initially, ZnSe and GaN based technologies made significant progress in the blue and UV light emitting diode and injection laser. No doubt, GaN is considered to be the best candidate for the fabrication of optoelectronic devices. However, ZnO has great advantages for light emitting diodes (LEDs) and laser diodes (LDs) over the currently used semiconductors. Recently, it has been suggested that ZnO is promising for various technological applications, especially for optoelectronic short wavelength light emitting devices due to its wide and direct band (Nause & Nemeth, 2005).

Wide band gap semiconductor
From the literature ZnO UV PDs have been widely investigated by many researchers on different substrates through different methods. Table 2 compiles and summarizes the structural of ZnO UV PDS reported by other researchers.

Schottky barrier height calculation of MSM PD
A MSM PD is a unipolar device with two back-to-back Schottky junctions formed on the same semiconductor surface. Under the application of bias voltage, one of the diodes becomes reverse biased, forming a depletion region that tends to sweep out photocarriers. The other diode becomes forward biased, allowing the collected photocurrent to flow out just as an ohmic contact. Under sufficiently high bias voltage, the depletion region extends and touches the small space-charge region under the forward biased electrode. Figure 2 shows the energy-band diagram of the MSM PD in the biased state. The vertical displacement of the electrode metals indicates the bias voltage applied to the device resulting in the forward biased condition of the left-hand metal-semiconductor interface and the reverse bias of the right-hand interface. Upon biasing, the semiconductor between the electrodes becomes fully depleted of free carriers. The reversed biased interface prevents the current from flowing through the device when there is no optical signal. The depleted regions between the electrodes are the photodetector active regions. As mentioned before, a photon with energy greater than the band gap of the semiconductor will be absorbed by an electron; this electron will get excited to the conduction band as shown by the process labeled as (1) in Figure 2. The photogenerated electron and hole are swept by the high applied fields to the positive and negative electrodes resulting in an electronic output signal (Haas, 1997).
One can assume that other current transport processes also contribute to the movement of electrons within the depletion region and across the barrier in Schottky contacts to determine the barrier height. The equations usually used to determine the barrier height in a Schottky diode. Assuming pure thermionic emission and V3KT, the general I-V equations usually used to determine the barrier height in a Schottky diode are represented by (Daraee et al., 2008) where I 0 is the saturation current, n is the ideal factor, K is the Boltzmann's constant, T is the absolute temperature,  B is the barrier height, A is the area of the Schottky and A * is the effective Richardson coefficient. Eq. 4 indicates the dependence of the barrier height on the saturation current (I o ). The theoretical value of A * can be calculated using Eq. (5) below.
where h is Planck's constant and m *  0.27m o is the effective electron mass for n-type ZnO so that A *  32 A/cm 2 K 2 (Liang et al 2001). (1) the photogeneration of signal charges and (2) the thermally-generated carriers overcoming barrier heights adding to device dark current MSM-PD performance is critically dependent on the quality of the Schottky contacts. Therefore, it is necessary to measure the Schottky barrier height of the actual contact which is a constituent part of the MSM PD under investigation. As mentioned before, a MSM-PD essentially consists of two Schottky contacts connected back-to-back. When a bias is applied within the MSM; this will put one Schottky barrier in forward direction (anode) and the other is reverse direction (cathode).
Following the analyses of Sze (Sze et al., 1971) high, and the hole is a minor carrier. Hence the hole current can be neglected, assuming that the dark current can mostly consist of the electron current (Jun at al., 2003). The dark current of the MSM PD is approximately as shown below (Yam & Hassan, 2008) Thus, Equation (6) can be re-written follows:

Photodetectors characteristics
There are many characteristics that describe the performance of a PD. These performance characteristics indicate how a detector responds. The response of the detector should be great at the wavelength to be detected. Whilst, the additional noise created by the detector should be small. The response speed should be high so that the variations in the input optical signal can also be detected. The performance characteristics of the photodetectors are summarized in the following subsections.

Responsivity (R)
Responsivity is defined as the ratio of the photocurrent output (in amperes) to the incident optical power (in watts). It is expressed as the absolute responsivity in amps per watt (A/W). Therefore, responsivity is to measure the effectiveness of the detector for converting www.intechopen.com the electromagnetic radiation to the electrical current. Responsivity depends on wavelength, bias voltage, and temperature. Reflection and absorption characteristics of the detector's material change with wavelength and hence the responsivity also changes with wavelength (Sze & Kwok 2006).
The responsivity in terms of the photo current can be written as: where ph I is the photo current (A), inc P is the incident optical power (W),  is the external quantum efficiency of the PD.
The responsivity in terms of wavelength can also be written as: where is the incident wavelength.
Responsivity is an important parameter that is usually specified by the manufacturer. Through responsivity, the manufacturer can determine how much detector's output is required for a specific application.

Quantum efficiency
Quantum efficiency of a photodetector (QE) ( %) is defined as the ratio of the electron generation rate to the photon incidence rate. QE is related to the photodetector responsivity by the following equation (

Characteristics of ZnO thin films deposited on PPC plastic
From the literature, the deposition of ZnO thin film on organic substrate such as PPC has not been reported by other groups. Therefore, PPC shows great potential as a substrate for ZnO thin film. The PPC has been chosen for its excellent properties such as low cost, high dielectric strength and high surface resistivity; and it is considered as one of the important polymers for biomedical and engineering application . It also exhibits high transparency and superior mechanical strength (Song et al., 2009). These attractive properties make the PPC a good substrate for a variety of microelectronic applications. Figure 4 shows the chemical structure of PPC plastic (Lu et al., 2005). This section briefly presents the structural, optical and electrical properties of ZnO deposited on PPC substrate.

The structural properties
ZnO thin film had been deposited on PPC plastic substrate with thickness of the 1µm. In order to study the structure of the film, XRD diffraction pattern of the deposited ZnO thin film on PPC plastic substrate was measured as shown in Figure 5. The XRD pattern shows that the film is ZnO as compared to the International Center for Diffraction Data (ICDD) library, and it is oriented in c-axis, which is the preferred orientation axis for such a material. The grain size also calculated from the FWHM of the XRD spectrum based on Scherrer formula as in Eq. 12 (Tan et al., 2005) was found to be about 26.8 nm.

2(deg.)
where B is the full width at half maximum[(FWHM) in radians] intensity of XRD,  is the Xray wavelength (Cu Kα =0.154 nm),  is the Bragg diffraction angle, and K is a correction factor which is taken as 0.9 (Tan et al., 2005). Thus we can conclude that the film deposited on PPC plastic is nanostructured crystal. This finding is in agreement with the result reported by Myoung et al (Myoung et al., 2002).
The surface morphology of the film was studied using SEM as shown in Figure 6. The figure shows that the ZnO has smooth surface morphology and relatively smaller particles, which are well connected to each other; it strongly adheres to the substrate and has tightly bounded particles. Inset in Figure 6 shows the SEM image reported by Ergin et al (Ergin et al., 2009). A close visual inspection reveals that the prepared sample in this work has similar surface morphology as reported by Ergin. These good surface properties have strong effect on the optical properties such as transmittance and absorbance of the UV light when this material is used as UV detector (Jandow et al., 2010b).

Fig. 6. SEM image of ZnO film on PPC substrate
The elemental analysis of the sample was investigated by EDX as shown in Figure 7.The EDX result shows that Zn, O and C elements were present in the sample. Zn and O elements came from ZnO film, on the other hand, C element was not anticipated in the film; obviously, the presence of C was due to the PPC substrate. Similarly, Ergin et al (Ergin et al., 2009) who studied the properties of ZnO deposited on glass substrate pointed out that Si and Ca elements were not expected to be in ZnO film and these two elements could have come from the glass substrates. The inset in Figure 7 shows the EDX image reported by them. The film surface was examined by AFM as shown in Figure 8. A typical AFM image shows that ZnO thin film consists of some columnar structure grains with root mean square (rms) equal to 10 nm; this nanostructure of the film surface may be useful in the absorption of the UV light when this material is used as a UV detector (Jandow et al 2010c).

The optical properties
The PL of the prepared ZnO thin film on PPC plastic was measured at room temperature. The result is given in Figure 9. Two luminescence peaks can be found in the figure. The first luminescence peak is the UV emission of ZnO thin films at 379.5 nm and as mentioned before it corresponds to the near band edge emission (NBE) due to the electronic transition from the near conduction band to the valence band as reported by Young et al and Gao and Li (Young et al., 2006;Gao &Li 2004).
The other luminescence peak is the blue-green emission ranged from 452.0 to 510.0 nm as shown in the inset in Figure 9, which is due to the defect related to deep level emission (Tneh et al., 2010). This result is in agreement with Wei et al. and Wu et al (Wei et al., 2007;Wu et al., 2007), which is attributed to the transition of electron from defect level of Zn interstitial atoms to top level of the valence band. In addition, the high UV to the visible emission ratio indicates a good crystal quality of the film which means a low density of surface defects (Jandow et al., 2010b).
The transmission and absorption spectra of ZnO film are shown in Figure 10. It can be seen that the transmission values of the film are low at short wavelengths (≤ 380nm) and high at long wavelengths. Therefore, the film behaved as an opaque material because of its high absorbing properties at short wavelengths as shown in the same figure and as a transparent material at long wavelengths.
This situation is related to the energy of the incident light; when energies of photons are smaller than the bandgap of ZnO film, they are insufficient to excite electrons from the valence band to the conduction band. However, ZnO has oxygen vacancies and interstitial Zn atoms, which act as donor impurities (Jandow et al., 2010d).
These impurities may be ionized by these low energies, so the film has low absorbance and high transmittance values at long wavelengths. The transmittance values increased higher www.intechopen.com than 80% in the visible region remarkably as the wavelength increased and this range refers to the fundamental absorption region (Shan et al., 2005). This indicates that the film could be used as transparent windows for UV light or as electrodes in a metal-semiconductor-metal MSM PDs (Jandow et al., 2010e). The dependence of (αh) 2 against the photon energy (h) is plotted as in Figure 11. By extrapolating the linear part of the plot to (αh) 2 = 0 the value of the energy gap was found to be about 3.33 eV.
h ( Fig. 11. The bandgap derivation for ZnO thin film on PPC substrate from the dependences of (αh) 2 on h Similar results were reported by Banerjee et al  who deposited ZnO thin film on PET plastic substrate and also by many other authors who prepared their ZnO thin films on glass and silicon (Gümüş et al., 2006;Khoury et al., 2010;Kang et al., 2007;Lai et al., 2008).

The electrical properties
The Hall measurements show that the film is n-type with resistivity,  of about 1.39 10 -1 Ωcm and mobility, μ, of 26 cm²/V-s. The carrier concentration, n, was measured and it was found to be 1.72 ×10 18 cm -3 .

Substrate ZnO Electrical properties
Ref.

Resistivity, 
(Ω-cm ) From the literature, the electrical properties of ZnO thin film deposited on organic substrates have rarely been reported. Table 3 summarizes some of the reported electrical properties. From the table, it was found that the resistivity value for our sample is about two orders of magnitudes higher than typical reported values (Ott et al., 1999;Tsai et al., 2006) and the mobility, μ, is 1.3 times higher than the results in . On the other hand, the carrier concentration, n, of our sample is about two orders of magnitudes higher as compared to the reported value in Ref .

The characteristics of ZnO UV PDs prepared on PPC
ZnO UV PDs with different metal contacts i.e. Pd, Ni and Pt have been fabricated on PPC plastic substrates. Figure 12 shows the fabricated ZnO UV PD with Pd contacts. The UV detector fabricated on PPC is very flexible and low cost. On the other hand, when the sample was illuminated with UV wavelength of 385 nm with power of 58.4 μW; the photocurrent (I ph ) (at same voltage) biased at 0.8V was 5.27, 7.44 and 8.80 µA, and n was found to be 1.21, 1.46 and 1.50, respectively for the fabricated ZnO PD with Pd, Ni and Pt contact electrodes. The results obtained for this study can be explained as follows: when light impinges onto the MSM UV detector, high-energy photons will be absorbed by the ZnO thin film, and with an appropriate bias, photon-generated carriers will drift toward the contact electrodes and a photocurrent will be observed (Young et al., 2006).
Further application of reverse bias acts to increase the electric field magnitude within the depletion region and to reduce the barrier height (Sze & Kwok 2006). When the energy of the incident photon is higher than the bandgap of ZnO, electron-hole pairs are generated inside ZnO thin film by light absorption. At the same time, the electron-hole pairs are separated by the electric field inside the depletion region of the ZnO thin film to generate the photocurrent.
In this study, the application of a bias on each of the Pd, Ni or Pt metallic fingers will create an electric field within the underlying ZnO thin film that sweeps the photo generated carriers out of the depletion region. The speed and the collection efficiency of the device vary, depending upon the magnitude of the applied bias, the finger separation and the average depth at which the photo generated carriers are produced (Jandow et al., 2010b). Comparing among the three PDs, PD with Pd has the lowest dark current, this follows by Ni and Pt. the dark current for PT and Ni contacts are about 4.3 and 3.9 times of Pd. The difference of the dark current for PT and Ni contacts is relatively insignificant. This could be anticipated from the SBHs, as shown in Table 4. The difference of SBH value for these contacts is very small i.e. 5 meV.

Schottky barrier height calculation
As mentioned earlier, one of the most interesting properties of a metal-semiconductor (MS) interface is its Schottky barrier height (SBH), which is a measure of the mismatch of the energy levels for majority carriers across the MS interface. The SBH controls the electronic transport across MS interfaces and is, of vital importance to the successful operation of any semiconductor device (Tung et al., 2001). SBH plays an important role to modulate the dark and photo currents.
In particular, the forward-bias portion of the I-V characteristics has often been used to deduce the magnitude of SBH (Cho et al., 2000). The transport of carriers across a MS interface is very sensitively dependent on the magnitude of the energy barrier, SBH.
SBHfor the three PDs could be determined by using Eqs. (3)(4)(5)(6)(7)(8) Figure 14. I 0 was derived from the intercept with y-axis and by substituting this I 0 value in Eq. 4,  B was found to be 0.738 eV. Similarly for Ni/ZnO and Pd/ZnO, the  B determined from the plot under dark and illuminated conditions are summarized in Table 4.
Many metals such as Ag, Au, Pd and Pt have been used as Schottky contacts for ZnO, and resulted in SBH of between 0.6-0.8 eV .  From Figure 13 it can be found that the ZnO MSM UV PD with Pt electrode has the highest light current. That is because it has the lowest Schottky barrier height at the Pt/ZnO interface. While still the light current for the PD with Pd contact is the lowest for the same reason.
One can find that the difference among the electrical characteristics of the three PDs may mean that their barriers are different. Since the barrier height of the PD with Pd contact is higher than the barrier height for the PDs with Ni and Pt contacts, the integrated number of carriers above the barrier height for the PD with Pd contact will be less than the others, which caused the total current over the barrier to be lower. Eventually the lowest dark current was obtained with the Pd contact compares with the PDs with Ni and Pt contacts.
From Table 4, it was found that the calculated SBH value for the PD with Pd and Ni contacts is higher than that with Pt contact although the work function of Pt is the highest. This indicated that the barrier height is independent of the work function and many research groups Wright et al., 2007;Ip et al., 2005;Liu et al 2004;Dong and Brillson, 2008;Kahng, 1063;Vanlaar and Scheer, 1965;Andrews and Phillips, 1975;Mead and Spitzer, 1963;Yıldırım et al., 2010;Roccaforte et al., 2010;Rabadanov et al., 1982;Mead, 1966; have reported that the barrier heights did not correlate with the metal work functions, suggesting many possible influences such as surface states, surface morphology, and surface contamination play important roles in the electrical properties of the contacts.
The increase of current when PD is illuminated by an UV source can be explained by work function of metal and semiconductor in the energy band diagram Rabadanov et al., 1982;. In this case the metal and semiconductor are taken as Pd and ZnO.
ZnO which has a work function of  ZnO = 4.1 eV is also known to be a natural n-type semiconductor due to the oxygen vacancy which acts as trap center. The Pd work function ( Pd = 5.6 eV) is higher than that of ZnO. When a contact is formed electrons flow from ZnO to Pd metal until the Fermi levels align resulting in band bending as shown in Figure  15, where E 0 , E C , E FS , E V and are the vacuum level, the conduction band, the Fermi level, the valence band and the work function of the semiconductor (S) which in our case is ZnO, respectively, and E FM is the Fermi level of the metal contact (M) which is Pd as shown in the figure, respectively. In ZnO, the trapping mechanism administrates the photoconduction. The oxygen molecules in the ZnO thin film surface capture electrons from the semiconductor. Under UV illumination, the photon energy releases the trapped electrons and also generates photo-induced electrons, causing an enhancement of the current.
The results obtained in this work are in agreement with the results reported by Young et al (Young et al., 2006) who fabricated ZnO MSM PD with Ag, Pd and Ni contact electrodes, and the barrier height for Ag/ZnO, Pd/ZnO and Ni/ZnO interfaces were 0.736, 0.701 and 0.613 eV, respectively although the work function for Ag =4.74, Pd =5.60 and Ni =5.42 eV. This result showed that although the work function of Ag is the lowest but the barrier height for Ag/ZnO was the highest, which indicated that the barrier height independent on the work function. Furthermore, Polyakov et al (Polyakov et al., 2003) results also showed that Schottky barrier heights were equal to 0.65-0.70 eV from capacitance-voltage measurements by using Au and Ag Schottky contacts on the bulk n-ZnO crystals; Au/ZnO SBH was lower than Ag/ZnO even though the work function of Au 5.47 is higher than the Ag. As well as, Neville and Mead (Neville & Mead 1970) results showed the barrier energy for Au is 0.66 eV and for Pd is 0.60 eV dependent on the forward current-voltage characteristics and as mentioned earlier the work function of Pd is higher than Au. The independence of SBH on the metal work function has been explained by many researchers Rabadanov et al., 1981;Mead, 1966;, they pointed out SBH could be affected by the interface structure and the associated interface states Figure 16 shows the responsivity as a function of the incident wavelength for the three ZnO MSM UV PDs with Pd, Ni and Pt contact electrodes. At 0.8 V and by applying Eq. 9, it was found that the maximum responsivity for the three PDs were found to be 0.08, 0.09 and 0.11 A/W, which corresponds to quantum efficiency ( %) of 27.1 % 31.7 % and 37.7% respectively which are shown in Figure 17. It is observed that the PD with Pt contacts has higher responsivity and quantum efficiency comparing to the PDs with Pd and Ni contacts due to the lowest barrier height and highest photocurrent. Table 5 summarizes the responsivity and quantum efficiency for Pd, Ni and Pt contacts, respectively.

Responsivity and quantum efficiency
As shown in the figure, the PD responsivity was nearly constant in the UV region 320-360 nm; it started to increase from 365 nm, of which the responsivity peaked at 385 nm and began to decrease whenever became close to the visible region. This can be explained as follows; when ZnO detector is irradiated by UV light with energy higher than the bandgap (3.37 eV for ZnO), electron-hole pairs will be generated, as a result these excess charge carriers contribute to photo current and result in the response to the UV light. The cut-off at wavelength of 385 nm is nearly close to the ZnO energy bandgap of 3.37 eV; the responsivity decreased at the shorter wavelength range due to the decrease of the penetrating depth of the light, resulting in an increase of the surface recombination Jandow et al., 2010a;Yan et al., 2004).

Conclusion
In summary, an overview of the characteristics of the deposited ZnO thin film on various organic substrates such as polyethylene terephthalate (PET), polyolefin, polytetrafluoroethylene (Teflon) and Polycarbonate (PC) and their potential applications in various areas has been presented. A review of semiconductor PDs, types of PDs as well as their characteristics has been demonstrated. Apart from that, the properties of the ZnO thin films deposited on PPC plastic substrate have been expressed. ZnO UV detectors prepared on PPC substrate with different electrodes i.e. Pd, Ni and Pt have been fabricated and investigated. The results showed that the deposited ZnO thin film had good structural and optical properties. In addition ZnO UV PDs fabricated on PPC with different metal contacts showed that Pt/ZnO MSM UV PD has the highest quantum efficiency.

Acknowledgments
This work was conducted under the grant no. (100/PFIZIK/8/4010) support from Universiti Sains Malaysia is gratefully acknowledged.
Zhang, B., Wakatsuki, K., Binh, N., Usami, N., & Segawa, Y. (2004). Effects of growth temperature on the characteristics of ZnO epitaxial films deposited by metalorganic chemical vapor deposition. Thin Solid Films, Vol.449, pp. 12-19 Zhang, J., Kang, J., Hu, P., & Meng, Q. (2007) In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies.