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In most recent times, semiconductors have become the basis for electronic applications. Because of this, scientists and engineers have studied in detail about the electrical, conduction, and structural properties of semiconductors in order to produce semiconductors that function effectively under various temperatures and pressures. The invention of devices such as integrated circuits has shown the ability of scientists to create new structures in the development of semiconductors. Continuous research should be done to produce higher quality devices. Factors such as the size, durability, reliability, lifespan, functionality, and cost of the device have to be considered.
II-VI semiconductors are vastly researched during this day and time. An important property of these semiconductors is their energy gaps, which can be varied over a wide range by altering the mole fraction.
ZnO based semiconductors usually become a point of interest since they offer a wide field of applications ranging from optical UV filters and devices like Light Emitting Diode's, laser diodes, ultraviolet Schottky barrier photo detectors, metal-semiconductor field effect transistors (MESFETs), high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) due to a band gap [1-3]. Due to the large energy band gap and high bond strength, the material responds well for high temperature applications, because its intrinsic properties are maintained at much higher temperatures. This suggests that ZnO-based power devices can operate with less cooling and fewer high cost-processing steps associated with complicated structures designed to maximize heat extraction.
ZnO and its properties
Most of the II-VI binary compound semiconductors crystallize either in cubic zinc blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corners of tetrahedron, and vice versa. This tetrahedral coordination is typical of sp3 covalent bonding. In addition, these materials also possess substantial ionic character. ZnO is one of the II-VI compound semiconductors whose ionicity resides at the borderline between covalent and ionic semiconductors.
ZnO is a promising material for the realization and future of nanotechnology. With its wide band-gap (3.37 eV), high excitonic binding energy, and high breakdown strength, ZnO can be utilized for electronic and photonic devices, as well as for high-frequency applications. The availability of a native substrate and the potential for room-temperature operations opens the door to ZnO applications including chemical sensors and subscale electronic circuits .
Figure 1.1: Wurtzite crystal structure of ZnO
It exhibits three crystal structures namely, wurtzite, zinc-blende and rocksalt. Thermodynamically, most stable phase of ZnO is wurtzite. While the zinc-blende ZnO structure can be stabilized only by growing on cubic substrates, the rocksalt (NaCl) structure can be obtained at relatively high pressures. The wurtzite structure has a hexagonal unit cell with two lattice parameters, a and c, with a=d and c= âˆš8/3d,
3d, where d is the interplanar spacing. And the ratio c/a= âˆš8/3 =1.633. A schematic diagram of the wurtzite ZnO structure is shown in Figure 1.1. Table 1.1 shows the basic physical parameters of ZnO 
Table 1.1 Properties of wurtzite ZnO.
Zinc Oxide (ZnO) nitrides material is a wide band gap semiconductor material with potential applications in optoelectronic as well as in electronic devices operating at high power and high temperature conditions. Metal/ZnO contacts, both ohmic and Schottky are important for these device applications.
One of the serious problems with these devices is a large voltage drop across the semiconductor metal interface at ohmic contacts, which leads to poor device performance and reliability . In order to avoid this problem, the development of low resistance ohmic contacts is essential. Another serious problem in Schottky contacts is the high reverse leakage currents . Schottky barrier diodes with a low forward voltage drop, a low reverse leakage current, and high breakdown voltage etc., are important in electronic industry. Thus, further detailed studies are necessary for better understanding of ohmic and Schottky contact behavior.
A large barrier height leads to small leakage current and high breakdown voltage, which could result in improved responsivity and photocurrent to dark current contrast ratio. To achieve a large Schottky barrier height on ZnO, one can choose metals with high work functions .
This work involves the fabrication, structural, and electrical characterization of Schottky contact based photodetectors based on ZnO and Ag contactes, and we investigated the effect of annealing on the behavior of the I -V characterization[9-17].
Objectives and Outline of the project
The objectives and outline of this project are written in the following steps:
1. To synthesis ZnO on Si by sputtering.
2. To fabricate Ag and Al Schottky contacts on ZnO.
2. To study the electric properties of these contacts.
3. To study the effect of temperature on the electric properties of contacts (I-V-T).
This project contains six chapters. The first chapter provides background information on the ZnO and its properties and applications. Theory and literature review on the metals semiconductors contacts, Schottky and ohmic is discussed in the second chapter of this project. Chapter three is about the methodology, experiment and characterization techniques. Results of the experiment is in chapter four, while discussion in chapter five. Finally, conclusions are collected in chapter six.
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THEORY AND LITERATUE REVIEW
ZnO is very important in industrial application and spatially in photoelectric application, such as photodetector. However, we cannot use ZnO or other semiconductors directly; we must use a contact between semiconductors and its applications. Metal/semiconductor contact is very important and easy to fabricate.
2.2 Metal-semiconductor contacts
The metal semiconductor contact or metal-semiconductor junction is undeniably an essential part of all semiconductor devices. In order to form a reliable and a quality device, an ideal or a high quality metal semiconductor junction must be formed according to the design requirements of the device. Moreover, many of the useful properties of a p-n junction can be achieved by simply forming an appropriate metal-semiconductor contact. Basically, metal-semiconductor contacts can be categorized into two types, that is the rectifying metal-semiconductor contact which is equivalent to a p-n junction diode and the non-rectifying (ohmic) metal-semiconductor contact. Two of these metal-semiconductor contacts play an important role in all the semiconductor devices because they are obviously attractive due to their fabrication simplicity, and are particularly useful when high-speed rectification is required.
Fig. 2.1 shows an n-type semiconductor with a metal work function Ñ„m is brought in contact with a semiconductor having a work function Ñ„s, charge transfer occurs until the Fermi levels align at equilibrium. In Fig. 2.1, it shows the case where Ñ„m > Ñ„s with the semiconductor Fermi level is initially higher than that of the metal before contact is made. To align the two Fermi levels, the electrostatic potential of the semiconductor must be raised, i.e. the electron energies must be lowered relative to that of the metal, thus resulting in the formation of a depletion region, W near the junction. The positive charge due to uncompensated donor ions within W matches the negative charge on the metal. The electric field and the bending of the bands within W are similar to that of a p-n junction.
The equilibrium contact potential V0, is the potential which prevents further net electron diffusion from the semiconductor conduction band into the metal while Î¦b is the potential barrier height, or better known as the Schottky barrier height, which act as a barrier for electron injection from the metal into the semiconductor conduction band is Ñ„m- Ï‡. Here Ï‡ is called the electron affinity, is measured from the vacuum level to the semiconductor conduction band edge. Just like in the p-n junction, the potential difference V0 can be decreased or increased by the application of either forward- or reversed-bias voltage.
When discussing about the transport mechanisms, which determine the conduction properties of Schottky barriers, it is known that there are various ways in which electron can be transported across a metal-semiconductor junction under forward bias.
Fig. 2.1: A Schottky barrier is formed by contacting an n-type semiconductor with a metal having a larger work function: a) band diagrams for the metal and the semiconductor before joining; b) equilibrium band diagram for the junction.
Fig. 2.2 shows the case for an n-type semiconductor under forward bias. The inverse processes occur under reverse bias. The mechanisms are:
emission of electrons from the semiconductor over the top of the barrier into the metal;
quantum-mechanical tunneling through the barrier;
recombination in the space-charge region;
recombination in the neutral region (hole injection).
Fig. 2.2 Transport processes in a forward-biased Schottky barrier.
It is possible to make practical Schottky barrier diodes in which process (a) is the most important and such diodes are generally referred to as 'nearly ideal'. Processes (b), (c), and (d) cause departures from this ideal behaviour.
On the other hand, the ohmic or non-rectifying metal-semiconductor contact is formed when the charge induced in the semiconductor in aligning the Fermi levels is provided by majority carriers (Fig. 2.3).
Fig. 2.3 (a) Ohmic metal-semiconductor for an n-type semiconductor (Ñ„m < Ñ„s), and (b) the equilibrium band diagram for the junction.
For example, for an n-type semiconductor with the case of Ñ„m < Ñ„s as shown in Fig. 2.3(a), the Fermi levels are aligned at equilibrium by transferring electrons from the metal to the semiconductor. This raises the semiconductor electron energies (lowers the electrostatic potential) relative to the metal at equilibrium Fig. 2.3(b). In this case, the barrier to electron flow between the metal and the semiconductor is small and easily overcome by a small voltage. No depletion region occurs in the semiconductor in these cases since the electrostatic potential difference required to align the Fermi levels at equilibrium calls for accumulation of majority carriers in the semiconductor.
For metal-semiconductor contacts, the ohmic contact is needed for connections to other devices or circuit elements because of its linear current-voltage (I-V) characteristics in both biasing directions (with minimal resistance and no tendency to rectify signals), the Schottky contact, besides functioning as a rectifying junction, poses an advantage in fast-switching application over the p-n junction counterpart. The Schottky barrier diode is a majority carrier device. This fact means that there is no diffusion capacitance associated with a forward-biased Schottky diode. The elimination of the diffusion capacitance makes the Schottky diode a higher frequency device than the p-n junction diode. Moreover, when switching a Schottky diode from forward to reverse bias, there is no minority carrier stored charge to remove, as is the case in the p-n junction diode. Since there is no minority carrier storage time, the Schottky diodes can be used in fast- switching applications. A typical switching time for a Schottky diode is in the picosecond range, while for a p-n junction it is normally in the nanosecond range.
2.3 Thermoionic emission model
The forward I-V characteristics was analyzed using standard thermionic emission relation for electron transport from a metal-semiconductor with low doping concentration and the equation is given by[1-7]
where Vd is the voltage across the diode, n the ideality factor, k is the Boltzman constant, and Is is the saturation current given by
where q is the electron charge, T the temperature, S the contact area, A** effective Richardson constant and ï¦b the Schottky barrier height. The value of ï¦b can be deduced directly from the I-V curves if the effective Richardson constant, A** is known. Eqs (1) can be finally rewritten as
Here, the plot of ln [Iexp (qV/kT)] vs. V will give a straight line with the slope = q/nkT and y-intercept at lnIs. And from this value of Is we can calculate the value of Schottky barrier height ï¦b from Eqs (2)
The theoretical value of A** is 32.4 A cm-2K-2 based on the effective mass of ZnO (m* = 0.27me) and from A** =4ï°e m* K2 /(h3) =120 (m*/me) .
Or in easy way at V > 3kT/q we can simplified the model to 
In addition, by take the logarithmic this equation become
We can easy to calculate Is from the y-intercept and n from the slope.
2.4 Series Resistance
At large currents there will a voltage drop across the series resistance. The large series resistance is attributed to the large spacing between the contacts. We can calculate the series resistance Rs from the Cheung function
a plot of H (I) vs. I will give a straight line with the y-axis intercept equal to nï¦b and the slope of this plot is the series resistance.
2.5 Ag and Al-ZnO contact
Because the work function of Ag larger than ZnO as we see in table 1.1, Ag used in contact with ZnO to form schottky contact, see figure 2.4
Schottky contacts are required for some UV detectors, transistor devices, and for material characteristics of ohmic and Schottky contacts to n-type ZnO have been reported in the literature using Ag [9-11] and Al as well as others, where the characteristics of the contacts were reported for a given metal and ZnO materials. Aluminum has typically resulted in ohmic behavior with specific constant resistivity.
Recently, Allen et al  measured the ideality factor n to be 1.1 and the SBH to be 0.77 eV for a Ag/ZnO SBD produced on the O-polar surface of ZnO. Also Sheng et al  declared that the ideality factor of Ag/ZnO varied from 1.37 at 265 K to 1.29 at 340 K.
Figure 2.4 Ag-ZnO schottky contact.
Table 2.1: Values of work functions and electronegativities for some common metals
(eV). (Rhoderick and Williams, 1988)
2.4 Theory of Characterization Techniques
2.4.1 Scanning Electron Microscopy (SEM)
SEM is an electron microscope which utilizes an electron beam to produce a magnified image of a sample. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography.
In SEM, electrons are thermionically emitted from an electron gun (tungsten filament) through a series of lenses to be focused and scanned across the sample. The beam, which typically has an energy ranging from a few hundred eV to 40 keV passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. Through continuous random scattering events that primary beam effectively fills a tear shaped interaction volume with a multitude of electronic excitations. Fig 2.4 shows the schematic of SEM.
Fig 2.4: Schematic of SEM.
2.4.2 X-Ray Diffraction (XRD)
XRD is a powerful, non-destructive technique to analyze crystalline structures with high accuracy. The interplanner spacing d values for a particular material and for a particular structure are unique. From this, information about the crystal characteristics such as structural properties (lattice parameters, strain, grain size, epitaxy, phase composition, preferred orientation order-disorder transformation, thermal expansion), atomic arrangement and thickness of the material can be obtained.
X-ray diffraction is based on constructive interference of monochromatic x-rays and a crystalline sample. X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law:
This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2Î¸ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacing allows identification of the mineral because each mineral has a set of unique d-spacing as mentioned above. This is achieved by comparison of d-spacing with standard reference patterns. Fig 2.5 shows the diffraction of X-rays by a crystal .Fig 2.6 shows the schematic for XRD .
Fig 2.5: Diffraction of X-rays by a crystal.
Fig 2.6: Schematic of XRD.
Annealing is a heat treatment that is given to a material to alter its properties such as strength and hardness. Annealing is used to induce ductility, relieve internal stresses, refine the structure and improve cold working properties. In the semiconductor industry, silicon wafers are annealed, so that dopant atoms, usually boron or phosphorus, can diffuse into substitution positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material .
Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility. Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered.
2.3 Energy Dispersive X-ray Analysis (EDX)
One of the most useful features of SEM analysis is Energy Dispersive X-ray Analysis (EDX). An accessory to an SEM, this analytical tool allows simultaneous non-destructive elemental analysis of a sample. The electron beam in an SEM has an energy typically between 5,000 and 20,000 electron volts (eV). The energy holding electrons in atoms (the binding energy) ranges from a few eV up to many kilovolts. Many of these atomic electrons from one of the innermost shells are dislodged as the incident electrons beam strikes the surface of a conducting sample, thus ionizing atoms of the sample, i.e. this leaves the atom in an excited state with a vacancy near the core of the atom. Relaxation to the original state follows by an electron from an outer shell of the atom that falls inward to fill the vacancy. The difference in energy between the two energy states is released in the form of an x-ray. Because the emitted x-ray has energy equal to the difference between two sharply defined levels characteristic of the atom, it is called a characteristic x-ray. Unlike the x-ray continuum, characteristic x-rays have discrete energies, which serve to clearly identify the atom type involved in the transition. They provide the fingerprint, or signature for identification of almost any element in the periodic table.
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This chapter will elaborate on fabrication processes undertaken to acquire micrometer (micron) sized contacts on ZnO by applying Al and Ag metallization as suggested in earlier chapters. The fabrication process is segregated into wafer cleaning, sputtering for ZnO, metal deposition, contact patterns definition and I-V measurements.
3.1 Cleaning wafer with RCA method
In this experiment n-type, Si (100) was selected as substrates to sputter ZnO thin films on its surface. Thus surface cleaning on the Si wafers were carried out before any fabrication process is initiated to remove any contamination on the wafers.
Currently steps using in Nor Lab cleaned Si wafers:
Cleaning Si in H2O/NH4OH/H2O2 in ratio 5:1:1 in glass cup at temperature 75oC for 10min.
Cleaning in HF: H2O in ratio 1:50 in plastic cub for 10 to 15 sec.
Finally in H2O/HCL/H2O2 in ratio 6:1:1 at temperature 75oC for 10min.
The sample was rinsed in deionized water after each step.
SiO2 as buffer layer on Si :
To enhance the growth of ZnO on Si (100), and to reduce the mismatch between ZnO and Si the SiO2 was using as a buffer layer, By annealing the Si (100)sample in furnace at 1100oC for 3hr with O2 flow.
R F Sputtering for ZnO
Various deposition techniques such as reactive evaporation, solution growth, spray pyrolysis, metallo organic chemical vapour deposition, ion beam sputtering, dc/rf magnetron sputtering etc. were employed for the preparation of ZnO films. Among these methods, dc reactive magnetron sputtering received much attention because of sputtering from elemental target in the presence of reactive gas for preparation ofcompound films with high energy of sputtered species, low pressure operation and low substrate temperature rise, made it as an attractive technique to deposit films on different substrates. When compared to other physical deposition techniques, magnetron sputtered films have better adhesion and greater uniformity over large areas. The physical properties of ZnO films prepared by dc reactive magnetron sputtering mainly depend on the sputtering parameters such as substrate temperature, oxygen partial pressure and sputtering pressure apart from the target-substrate distance, sputtering power and deposition rate. In this work, we use n-type Si (100) as a substrate. See figures 3.1 and 3.2.
3.1 Diagram of a simplified DC magnetron sputter used for this study
3.2 Picture of the equipment in the lab
Check the Film in SEM and EDX
To check the morphology of the surface of ZnO thin film Scanning electron microscope SEM was used, and to show the ratio and purity of ZnO film EDX characterization was used.
X-ray diffraction of ZnO thin film
As we, see in chapter two x-ray is a good way to check the quality of ZnO thin film. In our work after spurting ZnO on Si(100), the film was characterized by x-ray diffraction.
To enhance the properties of thin film we annealed it at 900oC in furnace see figure 3.3.
Figure 3.3 Annealing furnace.
Evaporate Al and Ag on the portion of the surface of ZnO
First Al was evaporated by using thermal evaporating system and second Ag was evaporated on another portion of ZnO film, using these steps:
Source of Al and Ag (99.999% pure) in the form of granular (0.040" in diameter) was cut into small pieces and loaded onto a tungsten crucible in the vacuum chamber. 2 tungsten crucibles can be simultaneously loaded into the chamber but only 1 source on the crucible can be melted at a time.
The vacuum chamber was evacuated and the pressure inside the chamber was brought down to between 2.0-x10-5 to 3.4x10-5 torr before the source was heated.
The tungsten crucible was heated with 5.0A DC current for 1 minute. Al source was melted instantly at this current level.
After 1 minute, the shutter was opened to allow deposition of Al onto the ZnO film.
The shutter was opened to allow deposition of Al onto the ZnO film.
The shutter was opened for 5 minutes and the heating current was turned off immediately after that as well.
After depositing Al, the crucible containing silver is rotated and ready to be melted. Step 4 to 7 was repeated except the tungsten crucible was heated with 5.0 a DC current for 1 minute.
During the deposition pressure inside the chamber reached 8.0x10-5 torr.
Vacuum in the chamber was finally released and the wafer was taken out for the next processing step.
Figure 3.4 Diagram of the evaporator used for this study
Figure3.5 Picture of the equipment in the lab
Finally after evaporate Ag and Al, the film was annealed at 900oC, to get schottky contacts, we repeat the experiment in different way, but this was the good way to get Schottky.
3.8 I-V-T measurements with different temperature
The basic equipment required to perform device electrical tests is the parametric analyzer system shown in figure 3.6 with the capability of positioning needle like probes on the contact structures and applying the correct voltage, current and polarities to the device. All the devices measurements are made in basically the same way. A voltage is applied to the component contact probe and the resultant current flowing between the contacts is measured with ammeter. The results are then exhibited on the display screen or the monitor. In addition, we make all measurements at different temperature starting from room temperature (300K) up to (523K). We repeated this step more times to get a good schottky.
Figure 3.6 Parametric Analyzer (I-V System)
In this chapter, we will offer our data and results of characterization of ZnO thin film. The quality and the properties of ZnO thin film synthesis by dc-sputtering techniques will be shown in this chapter. In addition, the results of the contacts of Ag/ZnO/Al will be offered.
4.1 Scanning Electron Microscope (SEM) and EDX of ZnO thin film
The scanning electron micrograph (SEM) picture of the representative ZnO film deposited on Si substrate is shown in figure 4.1; we can see the surface morphologies of ZnO film and the grain size.
Figure 4.1 SEM picture of ZnO thin film growth by dc/ r f sputtering.
To check the existence of Zn and O ratio we characterized the film by using EDX techniques. The spectrum is shown in figure 4.2
Figure. 4.2 EDX of ZnO thin film growth by dc/ r f sputtering.
4.2 X-ray Diffraction Data of ZnO/Si thin film
The crystalline structures of sample (ZnO/Si) film was then examined by x-ray diffraction analysis (XRD), which is a good technique to check the quality of the crystalline structure. XRD data are shown in figure 4.3.
We can see two peaks located at 2ï± = 34.4o and 69.3o. The first peak represents the reflection of hexagonal ZnO (0002) and the second represents the substrate Si (100).
Figure 4.3 XRD of deposited ZnO film on Si (100) using dc sputtering.
4.3 I-V measurements of Ag/ZnO/Al contacts
To study the electric properties of this contact, I-V-T measurements were used. The forward and reverse of I-V-T characterization of Ag/ZnO/Al thin film are shown in figure 4.4 and figure 4.5. The measurements were taken at different temperatures started from room temperature up to 573K.
Figure 4.4 I-V of forward and reverse of Ag/ZnO/Al contacts at different temperature in logarithmic scale.
Figure 4.5 I-V of forward and reverse of Ag/ZnO/Al contacts at different temperature.
In addition, for more information we plot forwarded and reverse behavior separately at different temperatures in logarithmic scale as shown in figure 4.6 and 4.6.
Figure 4.6 Forward I-V of Ag/ZnO/Al contacts at different temperature.
Figure 4.7 Reverse I-V of Ag/ZnO/Al contacts at different temperature.
Figures from 4.8 to 4.12 show the I-V for Ag/ZnO/Al contacts at different temperature separately.
Figure 4.8 I-V of forward and reverse of Ag/ZnO/Al contacts at T= 300K
Figure 4.9 I-V of forward and reverse of Ag/ZnO/Al contacts at T= 323K
Figure 4.10 I-V of forward and reverse of Ag/ZnO/Al contacts at T= 373K
Figure 4.11 I-V of forward and reverse of Ag/ZnO/Al contacts at T= 423K.
Figure 4.12 I-V of forward and reverse of Ag/ZnO/Al contacts at T= 523K.
4.4 Barrier heights and the ideality factor calculations
From the thermoionic emission model equations 2.1, 2.5 and 2.6 a plot of ln(I) vs. V can be shown. The plots give straight lines with a slope = q/nkT and y-intercept at lnIs. From the value of Is we can calculate the value of Schottky barrier height ï¦b from Eqs (2.4). We repeat the plot for all measurements at different temperature, (see figures 4.13 - 4.18). The values of Schottky barrier height ï¦b, saturation currents and ideality factor n were calculated and shown in table.1.
Figure 4.13 plot of ln( I)vs. Voltage for Ag/ZnO/Al contacts at temperature T=300K
4.14 plot of ln( I)vs. Voltage for Ag/ZnO/Al contacts at temperature T=323K
Figure 4.15 plot of ln (I) vs. Voltage for Ag/ZnO/Al contacts at temperature T=373K
Figure 4.16 plot of ln (I) vs. Voltage for Ag/ZnO/Al contacts at temperature T=423K
Figure 4.17 plot of ln (I) vs. Voltage for Ag/ZnO/Al contacts at temperature T=473K
Figure 4.18 plot of ln (I) vs. Voltage for Ag/ZnO/Al contacts at temperature T=523K.
4.5 Series Resistance Calculations
At large currents through the junction there will be a voltage drop across the series resistance. The large series resistance is attributed to the large spacing between the contacts. We calculated the series resistance Rs from the Cheung function equation (2.7) and from equation (2.8)
Using the n value determined from the slope of Equ.2.6 , a plot of H (I) vs. I will give a straight line with the y-axis intercept equal to nï¦b and the slope of this plot is the series resistance. All calculations of Rs are in table 4.1.
Table 4.1. Schottky barrier height, Saturation currents, ideality factor and series resistance at different temperature.
This chapter will offer a broader view in understanding and interpreting the physical structures and the electrical properties of the contacts based on the characterization data. The effect of temperature on I-V measurements of Ag and Al on ZnO film. Barrier height of the contact and ideality factor will be calculated.
5.1 ZnO thin Film Quality
From SEM picture figure 4.1, we see homogenous distributions of ZnO grains on the surface of Si (100), and from EDX spectrum figure 4.2, we can see only Zn and Oxygen (O), which indicate that the ZnO thin film was a high quality.
XRD spectrum of ZnO thin film has two peaks at 2ï±= 34.4o and 69.3o for reflection from the plane h-ZnO (0004) and Si(100) respectively. This ideated that only a single crystal of ZnO on Si substrate. The average size of ZnO crystals has been calculated from Scherrer formula where D is the average size of the crystal, k is a constant equals 0.9, ï¬ is the incident x-ray wave and ï¢ is given by ï¢= ï°/2 [FHWM * ï°/180]. The value of D was 15.1nm.
5.2 Effect of Temperature on Schottky barrier height and ideality factor
The contact properties obtained from the I-V-T characteristics (forward and reverse) of Ag/ZnO/Al as function of temperature are shown in Fig.4.1 and 4.2. It is observed that the characteristics of Al/Ag/ZnO Schottky contacts are uniform over different contacts. The forward I-V-T characteristics was obey the standard thermionic emission relation equation 2.1 and curves obtained for the I-V-T measurements indicate a very strong temperature dependence of the Ag/ZnO/Al Schottky diodes, and thermionic emission becomes the dominant process.
The barrier height and the ideality factor are plotted as a function of temperature in Fig. 5.1 and fig. 5.2. The plot shows that the ideality factor exhibits a decreasing with increasing temperature, while the barrier height increase with increasing temperature which indicate that the thermionic emission was dominant [1,2]. In addition, we can see the diode was stable with temperature increasing.
In Fig 5.3 the plot between saturation current and temperature show that as temperature increase the saturation current increase which indicate that the conductivity increase with temperature, and this meaning that this device better on high temperature than low.
Figure 5.1. Barrier height as a function of temperature in the temperature range (300-523 K) for the Ag/ ZnO/Al Schottky diode after annealing at 900C.
Figure 5.2. Ideality factor as a function of temperature in the temperature range (300-523 K) for the Ag/ ZnO/Al Schottky diode after annealing at 900C
From figure 5.3, we can see the inverse relation between barrier height and ideality factor. This indicates that as barrier height increase the contacts between metal and semiconductor enhancing.
Figure 5.3 Plotting of Barrier height vs. Ideality factor of Ag/ZnO/Al at different temperatures.
4.2 Series Resistance
Figure 5.4 shows the relation between series resistance and temperature for Ag/ZnO/Al contacts. We can see the decrease of resistance as temperature increase witch indicates that this diode operate at high temperature than at low temperature. Figure 5.5 shows the relation between saturation current and temperatures; we can see the saturation current was increase as temperature increase, which indicates the resistance was decreased. Figure 5.6 shows the relation between saturation current and series resistance.
Figure 5.4. Series resistance as a function of temperature in the temperature range (300-523 K) for the Ag/ ZnO/Al Schottky diode after annealing at 900C.
Figure.5.5 Saturation Current as a function of temperature in the temperature range (300-523 K) for the Ag/ ZnO/Al Schottky diode after annealing at 900C.
Figure 5.6. Saturation current vs. series resistance for the Ag/ ZnO/Al Schottky diode after annealing at 900C.