ZnO as the Leading Materials in the World of Optoelectronics, Sensors and Microelectronics - Research Paper Example

ZnO has arisen as one of the leading materials in the world of optoelectronics, sensors and microelectronics. This new found interest in thematerial has spear-led research and development into novel methods to produce schottky and ohmic contacts. This paper highlights the conventional technology and the new techniques used to form schottky and ohmic contacts from ZnO films. This paper also analyzes the most viable metals and metallization techniques that have been researched. Both n-type and p-type ZnO schottky and ohmic contacts their behavior, characteristics, I-V measurements and fabrication techniques have been discussed. Furthermore, as ZnO films and crystals may be interfaced with a variety of metals to form schottky and ohmic contacts. ZnO contacts have the capability of replacing conventional or time tested materials that have been used including the AlInGaN system and so, ZnO contacts may pave way for more reliable and less costly appliances. Background of ZnO ZnO has a hexagonal or wurtzite crystal structure. The wurtzite structure is formed with the Zn atoms being tetrahedrally coordinated with four O atoms. This allows the Zn d shell electrons to hybridize with the p shell of O. ZnO has a density of 5.606 g/cm3, melting point of 1975°C and exciton binding energy of 60mEV. The compound is a direct band gap semi conductor that has Eg=3.2eV. ZnO can be molded to produce desired electrical properties by divalent subsititution on the cation site. ZnO supports both n-type and p-type doping. The presence of Zn interstitials, O vacancies and hydrogen .The intrinsic level defects that cause n-type doping are 0.01-0.05eV below the conduction band. The material possesses an intrinsic direct band gap, a strong exciton state and gap states due to the presence of point defects. A study of the optical properties of ZnO by use of photoluminesence, photoconductivity and absorption confirms the presence of point defects[10].. ZnO is a wide gap semi conductor. Wide gap semi conductors are known to be more susceptible to n type doping; such as n-type doping through of ZnO through addition of excess Zn, Aluminum or Gallium. In this instance ZnO can be easily doped to form n-type material rather than p-type material. The reason behind the difficulty in doping ZnO to form p-type material can be linked to a number of reasons. In some cases, the inate point defects within the material compensate for the additional impurity by forming deep level traps. In other instances strong lattice relaxations push the dopant energy level deeper into the gap. For the case of ZnO the introduction of p-type dopants cause deep acceptor levels. Take the case of using copper as a p-type dopant. Copper causes an acceptor level of energy 0.17eV below the conduction band. Traditionally, nitrogen has been used as a dopant on the O site of ZnO to produce p-type material. Studies have suggested that group-V elements promise better p-type doping [10]. The ZnO system has been widely used in a number of applications such as optoelectronics, chemical and gas sensors as well as microwave devices. ZnO has a number of properties and characteristics that allow it to compete with the conventional use of AlInGaN system. The ZnO system in many ways has been found to be better than the AlInGaN system [10]. Comparison of few characteristics between the two systems yields the following facts; the ZnO system has larger exciton binding energy, ZnO crystals can be obtained at lower temperatures and ZnO has a 22% Indium composition when lattice matched with InGaN. In terms of application the comparison results hint towards the fact that ZnO has the capability of taking over a number of applications of AlInGaN which include UV lasers, high-density data storage systems, solid state lighting, secure communications and biodetection. Semiconductor Contacts ZnO has risen as one of the potential materials that may change the outlook of bandgap optoelectronics and microelectronics. The material is also used to design two semiconductor contacts; the Schottky barrier and Ohmic contacts. Ohmic contacts are non-rectifying junctions where current-voltage relationship (I-V curve) remains linear and symmetric. If, however, the current-voltage characteristic is non-linear and asymmetric, then a rectifying junction is created which is called a Schottky barrier contact. Schottky barrier contacts manufactured from ZnO have wide and variable barrier heights. This characteristic of schottky contacts is associated with crystal and material preparation [10]. For instance, the barrier heights of silver diodes on ZnO range from 0 to 1.2eV; which depends on the crystal, the surface preparation and the environmental conditions in which the contact is formed [7]. ZnO schottky barrier contacts have a wide variety of applications. Currently, ZnO is used as channels between the source and drain where charge exchange takes place in field effect transistors. The ZnO alters band bending and depletion width. The controllability of band bending is important for thin films and nanoscale layers; this controllability paves way for high speed transistors [7]. Furthermore, the material is used as a transparent electrode and window layer in solar cells. The material is also used to improve open circuit voltage at compound semiconductor hetrojunctions. The Schottyky model states that when a metal is fused into contact with a semiconductor, the ohmic characteristic of the contact depends on two variables; the first variable is the work function of the metal and the second is the electron affinity of the semiconductor [7]. The metal-semiconductor junction is considered ohmic if the barrier at the contact is zero. There resistance across the contact is near zero, this allows smooth flow of electrons to and fro from the semiconductor. For n-type semiconductors the work function of the metal must bear or smaller than the electron affinity of the semiconductor, on the other hand, in the case of p-type semiconductors the work function of the metal is close to or greater than the sum of electron affinity and bandgap energy. The barrier height and metal work function is dependent on one another for ionic semiconductors. For covalent semiconductors the barrier height is independent of the metal work function. ZnO is capable of producing ohmic contacts that have low contact resistivity [7]. This can be achieved by decreasing the barrier height or by increasing the surface doping density. In other words, ZnO has the ability to produce better quality contacts. These ohmic contacts are used in a variety of applications that include field effect transistors, light emitting diodes, solar cells and sensors. ZnO is especially applicable in devices that utilize nanoscale measures [7]. Nanoscale devices require low contact resistance; ZnO is able to provide this low contact resistance at nanoscales. Ohmic Contacts to ZnO Ohmic contacts having low contact resistivity and made of III-V compound semiconductors have their current transport mechanism explained through the field emission mechanism [2]. Field emission mechanism is the preferred method of current transfer for ohmic contacts. It involves the carriers tunneling across the full barrier width. ZnO is distinct from other semiconductor, in terms, that it possess both ionic and covalent characteristics. As a result of these characteristics ZnO ohmic contacts having low resistivity may be formed by; reducing barrier height in order to permit thermionic emission and increasing ZnO surface doping density to allow effective field emission [2]. There are two types of ZnO ohmic contacts; the non alloyed ZnO ohmic contacts and the alloyed ZnO ohmic contacts [7]. The fundamental difference between the two lies with the nature of deposited metal. Non alloyed ohmic contacts utilize metals that have electron affinity close to that of ZnO; these metals include Al and Ti. Such contacts allow smooth interfaces due to limited interface reaction, and so are more commonly used for shallow junction devices. The other ZnO ohmic contact is the alloyed ohmic contact. Alloyed ohmic contacts’ involve deposition of several metals on the surface followed by annealing. Annealing results in increase in carrier concentration, which is particularly important for n-type ZnO ohmic contacts [7]. There are two kinds of n-type ZnO ohmic contacts. The first is non-alloyed ohmic contacts. As mentioned afore n-type ZnO ohmic contacts have ZnO interfaced with metals that have electron affinity close that of ZnO. Non-alloyed n-etype ZnO ohmic contacts are formed through a solid-phase reaction at the semiconductor interface. A reduction in barrier width is introduced through two techniques; either interface diffusion or in situ doping during the growth phase. Such contacts possess smooth surface and interface morphology as their design does not include a liquid phase reaction. Non alloyed n-type ohmic contacts offer higher contact resistance and preferably employed for devices requiring shallow junction. The most common metals used to form non alloyed n-type ohmic contacts are In, Ti, Al, Al-Ti and Pt-Ga, The metals mentioned possess an electron affinity that is near to ZnO. The interface with each metal utilizes a different technique. The ZnO interface with each of these metals is different since each metal exhibits different characteristics when interfaced with the semiconductor [10] . The In-ZnO n-type ohmic contact has been experimentally formed by use of a KrF excimer laser [7]. The technique involves hydrothermally growing a n-ZnO substrate that has a resistivity of 2000 Ώ-cm. A Zn rich superficial layer on the ZnO substrate was produced through excimer laser irradiation. A metal layer of In was then introduced, thus forming a In-ZnO interface. This interface displayed n-type conduction and possessed ohmic characteristics. To incorporate low resistance into n-type ZnO ohmic contacts, another metallization scheme was introduced. This system involved a combination of Au and Ti that has Al/Pt layer sandwiched. This metal system was then used to form contacts on heavily doped n-ZnO, which resulted in the formation of contacts having a low specific contact resistance; in the range of 10-7 ~ 10-8 Ώ-cm2 . The preparation involved P-doped n-type ZnO films to be deposited on the substrate by using a pulse-laser deposition. The metals were deposited through e-beam evaporation. The experiment revealed specific contact resistances of 3.910x10-7 Ώ-cm2 with carrier concentration of 6.01019 cm3 at 30°C and a carrier concentration of 2.41018cm3 at 200°C. These results demonstrate that Ti/Al/Pt/Au are all viable metals for production of non alloyed n-type ZnO ohmic contacts. The drawbacks of these metals include low thermal stability and low adhesion between the metal and the semiconductor. The other n-type ohmic contacts are alloyed ohmic contacts. These contacts are formed by depositing the metal on the semiconductor surface and heating the interface above the eutectic temperature [10]. As the interface cools, the regrowth of the semiconductor surface takes place along with dopant incorporation. The field emission is enhanced as a result of reduced barrier width due to higher concentration of carriers. Alloyed ohmic contacts utilize three main metallization schemes; single layer metallization, double layer metallization and multi layer metallization [7]. The single layer metallization scheme uses an interface between Ru and Al doped ZnO. The double layer metallization scheme is different from the single layer metallization scheme in two ways. The double layer metallization scheme commonly uses Ti/Au, Al/Ti and Ta/Au. Secondly, the metal interfaces with ZnO on the top layer and on the bottom layer. The multi layer metallization scheme uses Ti/Al/Pt/Au, ITO/Ti/Au, Re/Ti/Au and TiB2/Pt/Au as metals. As the name suggests the multi layer scheme has at least three layers; the contact metal layer, a diffusion barrier and a metal covering layer. The addition of layers in alloyed n-type ZnO ohmic contacts induces greater thermal stability and consistent specific contact resistance over a range of temperatures [7]. The p-type ZnO ohmic contact requires metals that possess large work functions to be interfaced with ZnO. The fabrication process of p-type ZnO ohmic contacts requires the creation of lower barrier heights and an abundance of p-type carrier concentration at the ZnO surface [2]. The formation of p-type ZnO contacts involves the formation of Zn vacancies or O interstitials on the ZnO surface layer, that act as acceptors. As with n-type alloyed ZnO ohmic contacts, the p-type ZnO ohmic contact also showcases three metallization schemes; the single layer metallization scheme, the double layer metallization scheme and the multilayer metallization scheme. The single layer metallization scheme produced p-type ohmic contacts that had specific contact resistance of 3.15 x 10-3 Ώ-cm2. The p-type ZnO layer was grown in metalorganic molecular beam epitaxy (MOMBE) using N doping [10]. The resulting as-deposited samples produced schottky contacts with barrier heights of 2.35eV. Low specific contact resistance was introduced through rapid thermal annealing (RTA) at atmospheric pressure nitrogen at 520°C for 2 minutes [7]. The double metallization scheme involves the formation of metal bilayers with combination of different metal types such as Ni/Au, Ti/Au and In/Au. An experiment was designed to analyze which bilayer combination best would serve as a p-type ZnO ohmic contact. The contacts were grown using Hybrid Beam Deposition and had layer thicknesses of 30nm/100nm [10]. The results of the experiment indicated that only the Ni/Au metal bilayer was able to form an ohmic contact. Recent research on p-type ZnO ohmic contacts focuses on lowering the specific contact resistance. It has been found that specific contact resistance could be decreased by increasing the RTA and by the inclusion of interfacial solid solutions between the top metal and intermediate metal, and the intermediate metal to p-type ZnO surface [7]. The multilayer metallization scheme uses Pt, Ni or Au metals with p-type ZnO for formation of p-type ohmic contacts. In an experiment, Sb doped ZnO films were grown on n-type Si with Au/Ni contacts. The contact exhibited ohmic behavior with specific contact resistance of 3.0x10-4Ώ-cm2, after annealed at 800°C for 60 seconds [2]. Though the acceptor levels created by Zn vacancies were found to be deep, shallower acceptor levels could be created with addition of activated Sb atoms [10]. In another experiment, to confirm the presence of an ohmic contact between the Pt electrode and ZnO film through the work function control of InSb doped ZnO, a schottky diode was fabricated. The particular schottky diode created possessed n-type property. Three separate schottky diodes were fabricated in accordance to the time given for the InSb thin layer to deposit. In this case, the three diodes corresponded to deposition times of 3, 4 and 5 seconds (I1, I2 and I3) at a constant deposition rate of 5nm/s [4]. The performance of the schottky diodes were analyzed by subjecting them to I-V measurements. The results revealed I3 has a high leakeage current under reverse bias condition while schottky diode I1 and I2 exhibit rectifying behavior [4]. The leakage current values at -2.0V were 1.52x10-5 A and 1.23x10-5 A for schottky diodes I1 and I2 [4]. These values indicate that an InSb layer may be capable of producing ohmic contacts. On the other hand the schottky diode I3 was incapable of producing an ohmic contact, since the the InSb layer acted more like a energy barrier than a surface modification [2]. The final results of this experiment indicated that a InSb-ZnO n-type ohmic contact could be successfully fabricated if the thickness of the InSb layer was kept below 20nm [4]. Schottkey contacts to ZnO In recent years researchers prefer Zn over Ga due to the following advantages. First, it has a large excitation binding energy that allows the emission of light at room temperature. Second, it forms nano structures very easily that conduct easy charge transfer and light emission. Third, it is abundantly available and fourth it is biocompatible [10]. It is more difficult to understand the basic concept behind Schottkey formation on ZnO owing to the surface defects such as oxygen vacancies and zinc interstitials that lead to high donor concentration on the surface. Additionally, the range of barrier heights on ZnO for a given metal is wide and varies greatly. For instance, Au contact on ZnO can yield barrier heights ranging from 0 to 1.2 eV. Similarly, the contacts formed by metals such as Pt and Ta result in barrier heights that vary on large scale. The practical situation where the n-type barrier heights for contact with high work function metal are lower than expected range, poses another problem. Thus using this latter piece of information, it can be inferred that the Schottkey barrier heights are related to work function of different materials [8]. One published work suggests that the high density of defects on the surface of ZnO might lead to a reduction in the work function of metal and hence it is imperative to study the metal/ZnO interface for adjusting the Schottkey properties. According to another published work, the ohmic contact due to a metal and semiconductor interaction depends on the work function of the metal as well as on the electron affinity of the semiconductor. The contact is established such as to allow free movement of carrier which ultimately leads to a development of minimum resistance across the contact. This concept of minimal resistance helps to comprehend that for an n-type semiconductor the work function of the metal in contact, must be very near or smaller than electron affinity of the semiconductor. Conversely, for p-type semiconductor, the work function of the metal in contact must be near to or larger than the sum of electron affinity and band gap energy. However, another experimentally based research suggests that the former mentioned theory discussing the strong relationship between the barrier height and metal work function is only applicable as far as ionic semiconductors are concerned. It states that for non-ionic or covalent semiconductors the barrier height and work function of metal are independent of each other. The reason for which is that the intrinsic or extrinsic interface states in the band gap reduce the Fermi energy level to a narrow energy range. Since the ZnO semiconductor is on the border that distinguishes ionic semiconductors and non-ionic semiconductors, the formation of ohmic contacts with minimal resistance with this particular semiconductor involves two ways. The first being the simple reduction of barrier height and the second is the increment in the doping density on the surface of this semiconductor which would lead to a reduction in barrier width allowing free movement of the carriers [10]. The requirement of the high quality Schottkey contacts with ZnO is significant while constructing optical and electronic devices. According to a research study, the metals used for Schottkey contact development on the ZnO semiconductor are majorly Pt, Pd and Au only, due to the major problem of low affinity of oxygen. To improve the quality of Schottkey contact several methods have been employed so far. One of the methods is to treat the surface of the semiconductor with metals such as Hydrogen Peroxide, ozone, sulfide and oxygen plasma which results in a better quality of rectifying contact. The technique of improving the surface morphology along with others such as removal of carbon and hydroxyl contaminants and reduction in surface conductivity deems better results as far as improving contact quality is concerned. Yet, still the concepts of these techniques are not very well understood [3]. Another research study emphasizes that the properties of the semiconductor junction depend mostly on the metal used for contact, the surface of the semiconductor and the treatment of this surface with a particular chemical. It suggests that the most widely used metals for contact are Pt, Pd, Au and Ag. According to this publication, the Schottkey contact quality o the surface of ZnO wafers was improved after treating the surface with irradiation of oxygen or oxygen/helium plasma, sulfide treatment, UV ozone cleaning, boiling hydrogen peroxide treatment and surface etching with chemicals such as nitric acid, hydrochloric acid and phosphoric acid. It further evaluates that the boiling treatment with hydrogen peroxide led to a decrease in the leakage current [1]. One publication emphasizes on the need of the Schottkey contacts on the n-type semiconductor to increase the performance of the semiconductor device. This study explains that the electrical performance of the diodes with Schottkey contacts increases when the semiconductor surface is treated with nitric acid. Furthermore, it also discusses that most researchers believe that owing to issues of tunneling, interface states’ effects along with the effect of deep recombination centers, the ideality factor of these semiconductor diodes with Schottkey contacts increase greatly from 1. Additionally, it states that the rectifying property of the ZnO semiconductor with Pt or W contact is improved when the semiconductor surface is treated with the UV-Ozone cleaning before either of the metal is deposited. The direct deposition of Pt metal on ZnO semiconductor surface requires the prior treatment of the surface of this semiconductor with UV-Ozone cleaning whereas the direct deposition of W contacts don’t require this surface treatment. In both cases of direct deposition, the rectifying quality is improved. The study also suggests that the Pt or W contacts’ quality is improved by removing the carbon contamination from the surface of ZnO. The studies on p-type semiconductor and its association with metals to form Schottkey contacts are limited due ease in availability of the p-type material. However, one fact has been established that to increase the contact quality, the barrier height and thermal stability parameters of the contact has to be improvised [7]. Since most research has been conducted on the n-type ZnO semiconductor, the electrical properties of this particular type with different metals have been studied mostly as compared to the p-type semiconductor. The carrier density for an n-type ZnO with Au or Ag contacts has been found to be 1017 cm-3 and the barrier heights for Au contact with ZnO are 0.65 eV [2]. For Au metal contact, the I-V characteristics suggest that the ideality factor is close to 2. This ideality factor is associated with the tunneling issue [2]. In addition, due to dependence of saturation currents on temperature the activation energy observed in practical is much lower than that predicted on the basis of barrier height. Lower reverse currents were observed for diodes that were prepared on undoped and etch-free surfaces of ZnO semiconductor as compared to the etched surfaces. The ideality factor and the C-V characteristics, however, remain the same for both types. The ZnO with Ag metal contact has similar electrical properties as that of Au Schottkey contact. I-V characteristics, C-V characteristics, low reverse current and thermal stability remain the same for both Au and Ag types [2]. However, it should be noted that due to less thermal stability of both types, the diodes based on these contacts have very limited applications in semiconductor devices. According to research studies, a single-crystalline ZnO can be grown in bulk quantities via the three following techniques: Hypodermal growth (HT), growth form melt at high temperatures (MT) and Chemical Vapour Transport (CVT) [9]. Amongst the three HT is beneficial in producing ZnO with lesser defects and high degree of crystallanity owing to low growth temperature and maintenance of thermal equilibrium at growth. It was observed that when Schottkey contacts were fabricated for single crystal ZnO, they had a large value for ideality factor and low rectification ratio ( ratio of the value of current I at a large voltage V divided by the corresponding value of current I at the negative voltage –V) [9]. It was further reported that Schottkey contacts can be developed using the interface of Pd metal with single-crystal ZnO semiconductor which would yield an ideality factor which is approximately unity. Using the similar concept, Au and Ag Schottkey diodes can be designed with single-crystal ZnO as the substrate. Studies have shown that when the surface of the single-crystal ZnO is treated with ozone or hydrogen peroxide the rectification ratio and ideality factors improve for Schottkey diodes that are designed using the HT or CVT method [9]. Additionally, the hydrogen plasma treatment improves the quality of Schottkey diodes that have single-crystal ZnO designed via HT method. However, the same technique has alternate effects on the Schottkey diodes with single-crystal ZnO substrate designed using the CVT or MT methods [9]. Conclusion The development and fabrication of ZnO contacts has refined over the years. The material characteristics and conduction mechanism has been further understood as well. This new found understanding has opened the doors for better refinement and fabrication ability of ZnO contacts. The role of surface adsorbates, impurities, and defects on ZnO contacts has been better understood, due to the availability of high crystal quality and clean surface preparation. The contacts possess low specific resistance, good interface and surface morphology. Furthermore, the material exhibits exceptional characteristics at a nano level. For this reason, ZnO has inarguably established itself as one of the leading materials in the fabrication of nano-devices. Works Cited [1]Bhattacharya, A., Gupta, R. K., Kahol, P. K., & Ghosha, K. (2010). Electrical properties of rectifying contacts on selectively carrier controlled. JOURNAL OF APPLIED PHYSICS . [2]Ipa, K., Thalera, G., Yanga, H., Hana, S. Y., Lia, Y., & Nortona, D. (2006). Contacts to ZnO. Journal of Crystal Growth , 287, 149-156. [3]Jandow, N. N., Ibrahim, K., & Hassan, H. A. (2010). I-V Characteristic for ZnO MSM Photodetector. American Institute of Physics, (pp. 424-427). [4]Jee, S. H., Kakati, N., Lee, S. H., Yoon, H. H., & Yoon, Y. S. (2011). Ohmic contact between ZnO and Pt by InSb layer in a ZnO Schottky diode. APPLIED PHYSICS LETTERS , 98. [5]Kim, Y., & Yun2, H. (2011). Effect of Hydrogen Peroxide Treatment on the. American Institute of Physics, (pp. 953-954). [6]Kolkovsky, V., Scheffler, L., Hieckmann, E., Lavrov, E. V., & Weber, J. (2011). Schottky contacts on differently grown n-type ZnO single crystals. APPLIED PHYSICS LETTERS . [7]Leonard J. Brillson, Y. L. (2011). ZnO Schottky barriers and Ohmic contacts. JOURNAL OF APPLIED PHYSICS , 121301-33. [8]Nagata, T., Volk, J., Haemori, M., Yamashita, Y., Yoshikawa, H., & Hayakawa, R. (2010). Schottky barrier height behavior of Pt–Ru alloy contacts on single-crystal. JOURNAL OF APPLIED PHYSICS . [9]Nakamuraa, A., & Temmyo, J. (2011). Schottky contact on ZnO nano-columnar film with H2O2 treatment. JOURNAL OF APPLIED PHYSICS . [10]Pearton, S. J., Norton, D. P., Ip, K., Heo, Y. W., & Steiner, T. (2004). Recent advances in processing of ZnO. 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