Johnson Matthey Technol. Rev., 2020, 64, (2), 202
A Short Review on Properties and Applications of Zinc Oxide Based Thin Films and Devices
ZnO as a promising material for applications in electronics, optoelectronics, biomedical and sensors
Zinc oxide has emerged as an attractive material for various applications in electronics, optoelectronics, biomedical and sensing. The large excitonic binding energy of 60 meV at room temperature as compared to 25 meV of gallium nitride, an III-V compound makes ZnO an efficient light emitter in the ultraviolet (UV) spectral region and hence favourable for optoelectronic applications. The high conductivity and transparency of ZnO makes it important for applications like transparent conducting oxides (TCO) and thin-film transistors (TFT). In this paper, the optoelectronic, electronic and other properties that make ZnO attractive for a variety of applications are discussed. Various applications of ZnO thin film and its devices such as light-emitting diodes (LED), UV sensors, biosensors, photodetectors and TFT that have been described by various research groups are presented.
The dependence of daily life on the products of the semiconductor industry has resulted in enormous growth of this industry. Progress demands the development of smaller and smaller devices with higher speed, flexibility, better performance and lower cost. This demand has resulted in the development of new technologies and materials to meet the requirements of the growing semiconductor industry. Nanotechnology, in which the products contain very small particles and demonstrate special properties, is one of the most recent and active areas of research. In this regard, thin-film technology plays an important role that allows deposition of very thin layers (from a few nanometres down to the angstrom level) of semiconductor material on a supporting substrate. The resulting material exhibits novel mechanical, chemical, optical and electrical properties with the reduction in size to the nanometre scale, which is the result of surface and quantum confinement effects.
A thin film is defined as a very thin layer (10 nm to 1–2 μm) of material deposited on a supporting material (substrate) by the controlled condensation of vapours, ions or molecules by a physical or chemical process (1). This technology is known as thin-film technology. Thin films are deposited over a wide range of substrates (2–4). Thin films can be classified based on material into various categories: for example metallic, dielectric, organic or semiconductor films. The material can be in monocrystalline, polycrystalline or amorphous forms. The properties of thin films are completely different from their bulk form. Materials in bulk form have fixed properties whereas the properties of thin films and devices depend on the quality of the surface rather than the bulk (5). Also, the properties of thin films can be modulated by various techniques like doping, thickness variation or surface treatments. Multilayer thin films can exhibit completely unknown properties. Thin-film technology also makes efficient use of raw material.
The progressive development of thin-film technology has resulted in its extensive use in fields of optics, electronics, aircraft, defence, space science and other industries. The categories in which thin-film technology finds applications are mechanical, chemical, thermal, electrical, magnetic, electronic, chemical, optical and optoelectronic (2). The main applications of thin-film technology primarily include optical coatings and semiconductor thin film devices. Various applications of thin-film technology are listed in Table I. A thin film of materials can be deposited from the gas, vapour, liquid or solid phase. In Figure 1, various thin film deposition methods are classified and summarised (6).
With advances in nanotechnology and thin film deposition techniques, significant interest has been developed in recent years for the development of photovoltaic devices, batteries, sensors, information storage, lighting and large-area electronics. Various materials like silicon, GaN, gallium arsenide and oxide-based semiconductors (including ZnO) (7–16) have continued to receive considerable attention for fundamental as well as application-oriented research. However, research interest in ZnO is enormously growing because of its excellent optical, electrical, magnetic, piezoelectric, catalytic and gas-sensing properties that make it specifically attractive for nanoelectronic, optoelectronic, nanophotonic and piezoelectric devices (17, 18). Different nanostructures of ZnO including nanorods, nanowires, nanotubes and nanoribbons (19, 20) can be deposited on various substrates using conventional thin film deposition methods like radiofrequency (rf) sputtering, thermal evaporation and sol-gel (11). With the availability of large single-crystal ZnO, epitaxial films with very few defects can be obtained hence very high performance electronic and optoelectronic devices can be fabricated. The processing temperature of ZnO nanostructures is very low. Therefore, cheap substrates like glass and plastic can also be used for fabricating ZnO-based devices. Moreover, the electrical and optical properties of ZnO can be easily tuned by post-deposition treatments like annealing, surface treatments and doping with materials like aluminium, gallium, indium, tin and copper (21–25). It is an n-type transparent material with a direct bandgap of 3.37 eV with good electrical conductivity (26–28). Therefore, it can also be used for near-UV emission and detection, as a transparent conductor and as a channel material in TFT.
This paper presents the various important properties that make ZnO suitable for electronic and optoelectronic applications. Further, research into applications of ZnO thin films and its devices including LED, biosensors, UV sensors, photodetectors and TFT given by various research groups are presented.
2. Relevant Semiconductor Materials for Optoelectronic Applications
Based on the bandgap, semiconductor materials can be divided into two categories: narrow bandgap and wide bandgap materials. They can be further classified as indirect bandgap and direct bandgap materials. Narrow bandgap materials with a direct bandgap are desired for optoelectronic devices in the visible/infrared (IR) region whereas materials with a wide and direct bandgap are desired for optoelectronic devices in the UV/blue region. It is well known that Si dominates the semiconductor industry due to its exceptional material properties and compatibility with conventional processing. However, the indirect bandgap of Si greatly limits its application in optoelectronic devices. Therefore, GaAs, a direct bandgap material (Eg = 1.43 eV) with very high electron mobility (>8500 cm2 V–1 s–1) and related III-V compounds like indium gallium arsenide and aluminium gallium arsenide are used for fabricating optoelectronic devices like LED, lasers and other very high-speed electronic devices (7, 12). GaAs and its related materials have many advantages and are suitable for very high-speed electronic devices and optoelectronic devices in the near-IR region. However, due to the narrow bandgap of GaAs, it does not possess the properties for optoelectronic devices in the UV/blue spectral range. Optoelectronic devices in the UV/blue spectral range are in great demand for commercial applications in astronomy, medical, healthcare, water treatment and the military. The development of blue LED has resulted in the development of low-power white LED that is replacing incandescent and fluorescent lighting. The blue LED has also resulted in the development of blue-ray discs for storing high-definition video. Therefore, wide bandgap semiconductors such as GaN and ZnO have received considerable attention. For semiconductor-based photonic devices such as UV/blue LED and laser diodes, wide bandgap group III-nitrides have been the focus of intensive research due to their specific properties (9). However, research interest in ZnO is growing because of its large excitonic binding energy of 60 meV at room temperature as compared to 25 meV of GaN, which makes ZnO an efficient light emitter in the UV spectral region. Also, the crystal growth technology and processing of GaN is complex as compared to that of ZnO thin films and crystals that make it more attractive for optoelectronic devices in the UV/blue spectral range (29–31).
3. Relevant Semiconductor Materials for Thin-Film Transistors
At present hydrogenated amorphous-Si (a-Si:H) and polycrystalline-Si (poly-Si) are commercially used for large area display TFT and high-speed, high-resolution displays respectively. However, a-Si TFT has a low field-effect mobility value that makes it unacceptable for high-resolution displays with faster switching speeds. The field-effect mobility of poly-Si TFT is very high but it requires a very high-temperature crystallisation and is a very time-consuming process. As a result, the cost and time of production both increase. The high processing temperature restricts the use of substrates like glass and plastic. Poly-Si TFT suffer from non-uniform electrical properties due to its polycrystalline nature that makes it unsuitable for large-area displays. Also, Si is sensitive to light because of its low bandgap, therefore its characteristics degrade on exposure to visible light. Hence, shielding is required that limits the resolution of the display (11). Considering all these limitations there is a constant search for new materials and ZnO seems promising. ZnO thin film has high field-effect mobility, is insensitive to visible light and has a low processing temperature (18). Further, the quality of its film and devices can be very easily enhanced by doping with materials like In, Ga or Al.
4. Properties of Zinc Oxide
Some of the physical properties of ZnO that make it attractive for electronic and optoelectronic devices are summarised in Table II and are discussed one by one in the following sections.
4.1 Crystal Structure and Lattice Constant
In the crystal lattice, zinc and oxygen are arranged in tetrahedral geometry with each Zn atom surrounded by four O atoms and vice versa. ZnO exists in three crystal structures i.e., wurtzite, zinc blende and rock salt. At ambient conditions, ZnO exists in wurtzite form (11). A stable zinc blende phase can be achieved by growing ZnO on a cubic substrate (32–34). The rock salt structure can be obtained by applying very high pressure to the wurtzite structure (35). For the wurtzite structure, the lattice parameters a and b are equal and in the range 3.2475–3.5201 Å and c is in the range 5.2042–5.2075 Å. The bond between Zn and O in the crystal lattice possesses very strong ionic character. Therefore, ZnO is classed as being between an ionic and covalent compound (11).
4.2 Electronic Band Structure
ZnO is a direct bandgap material. Figure 2 shows the band structure of ZnO. It can be observed that in the Brillouin zone at k = 0, the lowest of the conduction band and topmost of the valence band lies at the same point. The electron configuration of Zn is 1s2 2s2 2p6 3s2 3p6 and O is 1s2 2s2 3p4. In a ZnO crystal, the bottom of the conduction band is due to occupied 2p states of O2– and the top of the valence band is due to the empty 4s states of Zn2+. The valence band further splits into three subvalence bands under the influence of spin that can be seen in Figure 2 (36).
4.3 Defects in Zinc Oxide
ZnO exhibits n-type properties due to intrinsic defects. The defects arise because of deviation from stoichiometry. Major defects present in ZnO are oxygen vacancies (VO) and zinc interstitials (Zni). However, which one of the defects dominates is still unclear (20). Due to these major defects, ZnO exhibits n-type characteristics. Figure 3 shows the defects and energy levels associated with it. In the Figure 3, Zn and O stand for zinc and oxygen respectively and V, and i correspond to vacancy and interstitial site respectively. Zni and VO result in a donor level in the forbidden gap whereas Zn vacancies create an acceptor level. The VO creates deep level donor states while the shallow level donor states are due to Zni. The difficulty in achieving p-type conductivity is due to the compensation of acceptor atoms by deep level donors that are the result of VO (37). The luminescence in green, blue and violet light regions is also attributed to these defects (38). Figure 3 shows possible luminescence from ZnO due to the various defect levels.
4.4 Optical Properties
For materials to be used in optical emitting devices, they should have direct bandgap and high exciton energy. ZnO is a direct and wide bandgap semiconductor with high refractive index (2.008). Its bandgap is around 3.4 eV at room temperature. It has an exciton binding energy around 60 meV as compared to 25 meV of GaN. Due to this, exciton recombination is possible at room temperature and above. Therefore, ZnO is a stable light emitter as compared to GaN. Because of the excitonic process, emission in the UV region (380 nm) is observed from ZnO. However, due to the intrinsic defects of lower energy states, emission of violet, blue and green light has also been observed (39–41). Therefore, ZnO is an efficient material for phosphor applications (42). Stimulated emission under optical pumping has also been observed from ZnO. This phenomenon may be due to excitonic-excitonic scattering or emission (43, 44). Electrically pumped lasing from ZnO nanowires has also been achieved by some research groups (45–47).
4.5 Electrical Properties
The conductivity of a thin film mainly depends on carrier concentration and mobility. The relation between conductivity, mobility and carrier concentration is given by Equation (i):
where n is the density of electron (hole) concentration in the conduction band (valence band), q is the charge on the electron (1.6 × 10–19) and μ is the mobility of charge carriers. ZnO exhibits n-type characteristics due to the intrinsic defects (VO and Zni). The carrier concentration and mobility highly depend on the level of defects. In 2011 Torricelli et al. (48) proposed a multi-trapping-and-release-transport mechanism for charge transport phenomena in disordered ZnO. According to this model, the conductivity can be explained as Equations (ii) and (iii):
where μb is band mobility at infinite temperature, Nb is total states per unit volume in the transport band, To is the characteristic temperature that accounts for the energetic disorder, Nt is the total number of trap states and nt is the charge-carrier concentration in the trap states in the disordered ZnO. The authors assumed that the charge carriers nt in the trap state are much greater than that of the carriers in the transport band n. Therefore, the total carrier concentration nT was approximated as nT = n + nt ~ nt. The defects and hence the carrier concentration and mobility in the ZnO highly depend on the deposition method and the growth conditions. The concentration and mobility of electrons in ZnO have been found in the range 1016~ 1017 cm−3 and 20~400 cm2 V–1 s–1 respectively (11, 25, 49–51).
4.6 Ohmic and Schottky Contact
For high performance electronic and optoelectronic devices, high-quality metallic contact on the ZnO thin film is very important. The electrical properties of semiconductor devices are greatly affected by the contact used. The metallic contact on ZnO can be Schottky barriers or ohmic depending on the difference between the work function of the metal and the electron affinity of ZnO. For a Schottky contact on ZnO thin film, metals with high work function are required. Platinum, palladium, tantalum and gold are high work function metals that are generally used for making Schottky contact with ZnO film. Pd (φm = 5.12 eV) and Au (φm = 5.1 eV) have been reported to form the most stable Schottky barrier contact on ZnO thin films (52, 53). An ohmic contact plays an important role in the performance of devices like solar cells, TFT, varistors and LED. A good ohmic contact on a semiconductor film is characterised by a linear current-voltage (I-V) relationship and negligible contact resistance. To create an ohmic contact on ZnO, the work function of the metal should be close to the electron affinity of ZnO (χ = 4.35 eV) (54). Al, In and titanium have work function values close to 4.28 eV, their resistivity is very low and the contact resistance formed between these metals and ZnO is also negligible. Therefore, these metals can be a good choice for making ohmic contact with ZnO films.
5. Applications of Zinc Oxide
5.1 Transparent Conducting Oxides
TCO are widely used as electrodes in optical and electronic devices like displays, solar cells, LED and organic light-emitting diodes (OLED) (55). At present indium tin oxide (ITO) is used as a TCO due to its excellent transparency and conductivity but its availability is limited and this makes it very costly (56). As a result, the cost of devices incorporating ITO as electrodes is very high. ZnO is widely available, cheap and also has very good transparency in the visible region and good conductivity. Therefore, it can be an alternative choice as TCO. Highly crystalline transparent ZnO film with good conductivity is easy to process at low temperatures making it compatible with plastic and glass substrates (55, 57). The electrical conductivity of ZnO is not equivalent to ITO but the conductivity of ZnO can be modified by doping it with elements like Al, In and Ga (58). Agura et al. (59) and Jun et al. (60) have reported the lowest resistivity of Al-doped and Ga-doped thin films respectively. The reported resistivity was 8.1 × 10–5 Ω cm for Al-doped ZnO (AZO) and 7.7 × 10–5 Ω cm for Ga-doped ZnO (GZO) thin film. The transparency of GZO and AZO was found to be greater than 90% equivalent to the transparency of ITO (21, 61, 62). Therefore, it can be concluded that ZnO can be a good choice for TCO.
5.2 Gas Sensors
Gas sensors have many important applications like environmental pollution control, fire detection, as an alcohol breath analyser, industrial process controller or for detection of harmful gas leaks in mines and other industries (63). Semiconducting oxide-based gas sensors are easy to fabricate, have low cost and their surfaces have good sensitivity to the adsorbed gases (64). For good sensitivity, the film surface should have high grain density with a porous surface (65). ZnO being physically and chemically stable can be a good choice for thin film gas sensors. Doping ZnO with suitable elements in appropriate amounts increases the surface density of grains and porosity thereby improving the sensing selectivity and response time of the film (66). The sensitivity further improves at high temperature. The conductivity of ZnO thin film surfaces will increase or decrease depending upon the nature of reaction (oxidation or reduction) of the adsorbed oxygen on the surface of the ZnO thin film and the gas under test (65). There are numerous reports of ZnO thin film gas sensors for detecting species such as ammonia, ammonium, nitrogen dioxide, water, ozone, carbon monoxide, hydrogen, hydrogen sulfide and ethanol for various applications (17). Chou et al. (63) reported Al-doped ZnO thin film by rf sputtering method with interdigitated Pt electrodes that can be used as a breath analyser for sensing ethanol. Kim et al. (67) reported a Sn-doped ZnO thin film gas sensor for NO2 detection with improved selectivity. Other reported works include Pd-doped ZnO gas sensors for H2 detection by Al-zaidi et al. (68) and a ZnO thin film gas sensor by rf sputtering for H2, NO2 and hydrocarbon detection by Sadek et al. (69). Balakrishnan et al. (70) reported the detection of NH3 gas by a p-type ZnO thin film. The p-type thin film was obtained by co-doping with aluminium nitride and aluminium arsenide and then depositing with rf sputtering method.
5.3 Light-Emitting Diodes
The large bandgap of ZnO and high exciton energy makes it an ideal material for blue and UV LED. ZnO is widely available and cheap, so it has an advantage over GaN from the cost point of view. The limiting factor in realising ZnO based LED was the lack of stable and reproducible p-type ZnO. The alternative approach was that n-type ZnO thin film was grown on other p-type materials like Si, GaN, zinc telluride, copper(I) oxide and GaAs (11, 27). Various ZnO based heterojunction LED in the UV and visible ranges (red, blue, green or white) have been reported. Rogers et al. (71) and Alivov et al. (72) have reported n-ZnO/p-GaN and n-ZnO and p-AlGaN LED in the UV range of 375 nm and 385 nm by pulsed laser deposition (PLD) and chemical vapour deposition (CVD) processes, respectively. Yang et al. (73) and Alivov et al. (74) have reported n-ZnO/p-GaN and n-ZnO/p-GaN/Al2O3 blue LED by metalorganic chemical vapour deposition (MOCVD) and CVD processes, respectively. An n-ZnO/n-MgZnO/n-CdZnO/p-MgZnO LED emitting red light has been reported by Ohashi et al. (75) by a MOCVD process. Chichibu et al. (76) fabricated greenish-white LED using helicon-wave-excited plasma-sputtering from p-type copper gallium sulfide heterojunction diodes using n-type ZnO as an electron injector. They also reported IR-LED (780 nm) by using a p-CuGaS2/n-ZnO-Al structure fabricated by the helicon-wave-excited plasma-sputtering method (77). Earlier it was difficult to achieve p-type doping in ZnO but, at present, several researchers have reported p-type ZnO and homojunction LED based on it. Wang et al. (78) reported p-ZnMgO/ZnO/n-ZnMgO p-n junction LED. Tsukazaki et al. (79) represented a p-i-n homojunction structure on a (0 0 0 1) ScAlMgO4 substrate. The p-type conductivity was achieved by doping ZnO with nitrogen. Ryu et al. (80) fabricated arsenic-doped p-type ZnO and demonstrated (Zn, Be)ZnO/n-ZnO-based LED. Lim et al. (81) fabricated p-ZnO/n-ZnO/sapphire LED by rf sputtering method.
For short-wavelength semiconductor laser diodes, wide bandgap materials are ideal (82). At present blue and UV lasers are based on GaN materials (83). Because of the large exciton binding energy of 60 meV as compared to 25 meV of GaN, ZnO could be a promising material for UV and blue laser applications. The lasing phenomenon in ZnO occurs due to exciton-exciton scattering. Various researchers have observed stimulated emission from ZnO (84–87). Stimulated emission from the surface and edges of a ZnO thin film is observed under optical pumping. ZnO has high excitonic energy hence lasing is observed under moderate pumping. Therefore, ZnO-based lasers have a low threshold value (11). Ozgur et al. (83) in 2004 reported low threshold exciton-exciton scattering-induced stimulated emission in rf-sputtered ZnO thin films. Random stimulated emission from a ZnO polycrystalline thin film was observed by Cao et al. (88). Gadallah et al. (89) in 2013 reported surface and edge emission under optical pumping from a ZnO thin film grown on a sapphire substrate by PLD with the highest gain and lowest loss (at that period). Waveguide assisted random lasing was also observed from an epitaxial ZnO thin film (90). Although there are various reports on lasing through ZnO, there are no reports on ZnO-based laser diode. The limitation in fabricating ZnO-based laser diodes was that a stable p-type ZnO thin film was not realisable. But now with the various reports on p-type ZnO (78–81), it is expected that a ZnO-based laser diode will be available soon.
A biosensor is a transducer that detects a biological response and converts it into an equivalent electrical signal. Biosensors have many important applications especially in the healthcare and food-processing industries. They are used for chemical and biological analysis. They are also used for clinical analysis and environmental monitoring. Materials to be used for biosensors should be biocompatible and non-toxic so that the biological activity of the element to be recognised is retained. It should also present a high surface area to the element to be detected for better sensitivity. The large surface-to-volume ratio and electron and phonon confinement of nanomaterials make them favourable for biosensors. ZnO due to its biocompatibility, non-toxicity and antibacterial properties is a good choice for biosensors. ZnO has a high isoelectric point (9.5) therefore elements with a low isoelectric point can also be immobilised on it through electrostatic interaction. ZnO nanostructures as biosensors can be used to detect species like hydrogen peroxide, urea, protein, glucose, human immunoglobulin G (IgG), DNA (phosphinothricin acetyltransferase (PAT) gene), phenol, catechol or cholesterol (26, 91).
A photodetector is a device that senses electromagnetic waves. It converts the optical signal into an equivalent electrical signal. If a light wave with energy greater than or equal to the bandgap of the semiconductor falls on it, then electron-hole pairs are generated. The charge pairs drift towards the anode and cathode respectively under the influence of the appropriate electric field resulting in the generation of current. This current is called photocurrent, and it is proportional to the intensity of light falling on the semiconductor. Photodetectors can be classified as photoconductors and photodiodes. Photodiodes are further classified as metal-semiconductor-metal (MSM) photodiodes, Schottky photodiodes, p-n homojunction photodiodes and p-n heterojunction photodiodes. In a photoconductor, the conductivity of the semiconductor changes under the influence of light. If light with energy greater than or equal to the bandgap of the semiconductor falls on it, an electron in the valence band absorbs energy and jumps to the conduction band. The concentration of electrons in the conduction band increases hence the conductivity also increases. Thus, the current in the external circuit under biased conditions increases proportionally to the photocurrent. A photodiode works in reverse bias mode. In a diode, a depletion region is formed at the junction of p and n regions and the width of this region increases on increasing the reverse bias voltage. In this region, there are no free charge carriers but because of thermal energy, very few electron-hole pairs may be generated. Under the influence of an electric field in the depletion region, the electrons and holes drift towards the n-region and the p-region respectively. This current has very small magnitude and is known as leakage current or dark current (current in the absence of visible light). This current depends on the ambient temperature, reverse bias voltage and external series resistance. If the diode is exposed to a light wave of appropriate wavelength, more electron and hole pairs generate, and more current flows in the external circuit. That current is the sum of dark current and photocurrent. The photocurrent is proportional to the intensity of light falling on the diode.
For efficient detection of light, the photodetector should have some desirable features. It should be sensitive in the required spectral region with high responsivity, high quantum efficiency, fast response time and small noise equivalent power (NEP). It should have low noise current in the undesired spectral range (92). ZnO is mainly used for detecting UV rays. Mollow (93) in 1940 was the first to observe the UV photoresponse of ZnO thin films. The 3.4 eV bandgap of ZnO makes it very sensitive to UV rays compared to visible and IR rays. The bandgap can, however, be tuned by doping with materials like In, Al or magnesium to make a detector for a specific wavelength. UV sensors have a wide range of applications. They are used in space applications for communication and in the military for missile warning and guiding systems. They can be used for environmental monitoring as an ozone layer monitor and for commercial purposes as a fire detector (92, 94). Materials to be used for space and military applications should be thermally, mechanically and chemically stable and should have high radiation resistance. ZnO is an ideal material with all these properties along with high gain and high photoresponse. ZnO is almost transparent in IR and in the visible region hence ZnO-based UV detectors exhibit less dark current and better sensitivity to UV rays as compared to Si-based UV detectors. Figures 4(a)–4(c) show some important structures of ZnO-based UV detectors.
Figure 4(a) shows the structure of a ZnO photoconductor. It is very simple to fabricate. The ohmic contacts are patterned over a ZnO thin film layer. For ohmic contacts, Al, Ti and ITO can be used (95–97). A ZnO based photoconductor exhibits high internal gain. The disadvantage is that it exhibits a very high dark current. The responsivity and linear dynamic range are also low (92).
5.6.2 Metal-Semiconductor-Metal Photodiode
Figure 4(b) shows the structure of a ZnO based MSM photodetector. The Schottky metal contacts are patterned in an interdigitated form on ZnO thin film. The Schottky contact should have a large barrier height and should form a stable contact with ZnO. The larger the barrier height, the lower will be the leakage current and better will be the photocurrent to dark current contrast ratio. However, at the same time, quantum efficiency and responsivity will decrease (92, 97, 98). The main advantage of a MSM photodiode is that there is very low capacitance between the Schottky contact and the thin film. Therefore, it has high speed. High work function metals like Pd, Au, Pt, nickel, chromium, ruthenium or silver are preferred for making interdigitated Schottky contact on ZnO thin films (27, 28, 92). The two interdigitated contacts are similar but creating dissimilarities in the two contacts may result in a self-powered device. Chen et al. (99) reported a self-powered ZnO MSM photodetector with Au contact. One interdigitated contact had narrow Au fingers whereas others had wide Au fingers. The observed responsivity was very high. The responsivity was reported to be highest, at 20 nA W–1, when the asymmetric ratio was 20:1. Very low-cost ZnO based MSM detectors have also been reported using graphite electrodes and paper substrate (100, 101). Gimenez et al. (100) in 2011 and Hasan et al. (101) in 2012 reported ZnO nanocrystals based MSM photodetectors with interdigitated graphite contacts drawn by pencil on paper. The interdigitated pattern was drawn on paper by using an appropriate pencil and ZnO nanocrystals were grown by a solution-based technique and then transferred to the paper. This MSM detector was very easy to fabricate and very cheap with performance comparable to a MSM photodetector with metal contacts.
5.6.3 Schottky Photodiode
Figure 4(c) shows the structure of a ZnO based Schottky diode. It has well-patterned Schottky and ohmic contacts. It has many advantages over photoconductor and MSM photodiodes including low dark current, high contrast ratio, high speed and high quantum efficiency. As discussed earlier various high work function metals like Pt, Ni, Cr, Ru, Ag and Pd can be used for making Schottky contacts on ZnO. In 1986 the first ZnO Schottky photodiode was reported by Fabricius et al. (102). Au and manganese were used to form Schottky and ohmic contacts respectively. The observed efficiency was not good. After that various efforts were made to improve efficiency. Most of them used Pd, Au and Pt due to the stability of the Schottky contact with ZnO films and Al as the ohmic contact. Recently Tang et al. (103) fabricated a graphene nanodots array (GNDA) with ZnO nanofilm spin-coated on it for UV photodetection. They found a two-fold increase in external quantum efficiency (9.32%) and responsivity (22.55 mA W−1) of ZnO/GNDA for 20 nm and 30 nm sizes. As the size of GNDA increased to 45 nm, the performance was comparatively poor. Su et al. (104) fabricated a high performance and self-powered beryllium zinc oxide based dual-colour UV photodetector through a one-step electron beam evaporation of an asymmetric Ti/Au pair. The device exhibits ultrafast response speed, with a rise time of ~35 μs and a decay time of ~880 μs and also two cut-off response wavelengths at ~275 nm and ~360 nm under zero bias, which correspond to the UVA and UVC regions. Very high-performance UV detectors have so far been reported by groups like Somvanshi et al., Ali et al. and Oh et al. (52, 105–108).
5.6.4 p-n Heterojunction Photodiode
ZnO based heterojunction photodiodes can be fabricated by depositing a ZnO thin film on other p-type films or substrates like GaN, Si, silicon carbide, nickel(II) oxide, ZnTe and Cu2O (11, 92, 109). Generally, the p-Si substrate is used because of its low cost, easy availability and compatibility with Si-based complementary metal-oxide-semiconductor (CMOS) technology. By using a Si substrate, it is possible to integrate ZnO-based devices with Si-based CMOS technology (52). The problem with the n-ZnO/p-Si UV detector is that ZnO is transparent to visible light whereas Si exhibits photocurrent in the visible region so it cannot be used in the presence of visible light. This problem can be solved by either insertion of an insulator layer between ZnO and Si (110) or coating the surface with nanoparticles (111). Zhang et al. (110) reported a n-ZnO/insulator-MgO/p-Si visible-blind UV photodetector. A visible-blind n-ZnO/p-Si UV detector was also obtained by Chen et al. (111) by coating the surface of ZnO with silica nanoparticles. Hu et al. (112) reported a high-performance UV photodetector (nearly 104 at zero set bias under 370 nm (~0.85 mW cm−2)) with high signal-to-noise ratio, high speed, high selectivity and high detectivity. Ouyang et al. (113) reported a heterojunction photodetector in which a CdMoO4–ZnO composite film was prepared by spin-coating CdMoO4 microplates on ZnO film. The responsivity was 18-fold higher and the decay time was half compared to ZnO film by optimising the amount of CdMoO4 microplates. Further, the photocurrent was two-fold higher if Au nanoparticles are deposited to the CdMoO4–ZnO composite film. Zhao et al. (114) fabricated a highly crystallised, self-powered solar-blind (200–280 nm) ZnO–Ga2O3 core-shell heterostructure using a one-step CVD method. The device exhibited a sharp cut-off wavelength at 266 nm, fast response speed and decay time and showed an ultrahigh responsivity (9.7 mA W−1) at 251 nm with a high UV:visible rejection ratio (R251 nm:R400 nm) of 6.9 × 102 under zero bias. The device was highly suitable in practical self-powered solar-blind detection.
5.6.5 p-n Homojunction Diode
There are very few reports on ZnO-based p-n homojunction UV detectors due to difficulty in achieving p-type ZnO thin films as discussed earlier. But a few groups like Liu et al. (115), Moon et al. (116) and Chiu et al. (117) have succeeded in growing stable p-type ZnO thin films and have reported ZnO based p-n homojunction UV detectors. ZnO was doped mainly with As, nitrogen or antimony to achieve p-type conductivity.
5.7 Thin-Film Transistor
The TFT was first patented (in 1952) as a solid-state amplifier (118). It is a three-terminal (source, drain and gate) device similar to the metal-oxide-semiconductor field-effect transistor (MOSFET) with the same working principle. It has a substrate (for providing mechanical support to the structure), a dielectric layer, an active channel layer and source/drain and gate contacts. Charge carriers are injected through the source electrode at one end and collected at the drain electrode at another end. The gate electrode is to control the flow of charge between the source and drain terminal. The dielectric layer between the gate electrode and the channel layer is to prevent the flow of charge carriers between them. The difference between MOSFET and TFT is that the channel in the TFT is formed by the accumulation of charges and in the MOSFET the channel is formed by inversion. Figures 5(a)–5(c) show the working principle of n-channel TFT and Figures 5(d)–5(e) represent the output characteristics of n-channel TFT (119, 120).
For n-channel TFT, a positive bias is applied between the drain to source and gate to source contact. The source contact is biased at 0 V. Figures 5(d)–5(e) show the graphs ID vs. VG and ID vs. VD respectively for an n-channel TFT. In Figure 5(e), Region 1 is known as the linear region. In this region VD<<VG and the drain current ID is given by Equation (iv):
where W is the width, L is the length of the channel, μFE is the mobility of electrons, Vth is the turn-on voltage, Ci is the capacitance of gate insulator per unit area, VGS is gate-to-source voltage and VDS is drain-to-source voltage. Since VD<<VG the drain current can be approximated by Equation (iv). It can be observed that in the linear region ID varies linearly with VDS, Equation (v):
Region 2 is known as the saturation region. In this region (VG–Vth) >> VD and the drain current is given by Equation (v). It can be seen that the value of drain current in this region is constant and does not vary with VDS, Equation (vi):
Based on the position of the gate terminal and source/drain electrodes there can be four possible TFT structures. Figures 6(a)–6(d) show the possible structures of TFT: (a) staggered bottom-gate, (b) co-planar bottom-gate, (c) staggered top-gate and (d) co-planar top-gate (115–117, 121–123). In the coplanar structure, the source/drain contacts and the gate contact are placed on the same side of the semiconductor/oxide interface. In staggered structures, the gate electrode is placed on one side, and the source/drain contact is placed on the other side of the semiconductor/oxide interface. The bottom gate structure is easy to fabricate, but the disadvantage is that the channel layer is exposed directly to the atmosphere. Therefore, the performance of the bottom gate TFT is easily affected by the presence of light, gases and humidity. Passivation of the channel layer is required to prevent exposure. The passivated bottom gate TFT is more stable, reliable and gives a much better performance as compared to an unpassivated one (124). The top gate structure has the advantage that the active layer is covered by the gate oxide and the gate contact, but an extra masking step is needed to fabricate it. The first TFT was based on cadmium selenide material. In 1962 Weimer et al. (125) reported the first TFT in which he used CdSe as an active channel material. In 1973, Brody et al. (126) demonstrated the use of TFT in the LCD. He used a matrix of 120 × 120 CdSe TFT for switching of pixels. But the very high cost and issues like reliability, stability and the invention of low power CMOS technology limited the research interest in TFT at that time. In 1979 le Comber et al. (127) reported the a-Si:H TFT. The channel layer was deposited using plasma-enhanced chemical vapour deposition (PECVD) and doping of hydrogen was done by a glow discharge technique. After that, it took ten years for TFT LCD to become attractive in the commercial market thereby increasing the research interest in the field of TFT. At present, active-matrix liquid-crystal display (AMLCD) technology is based on a-Si:H. But it has many disadvantages. The field-effect mobility of a-Si is very low (~1 cm2 V–1 s–1). This makes it unsuitable for ultra-high-definition displays where very high switching rates are required. Poly-Si has a very high field-effect mobility (>50 cm2 V–1 s–1) but the problem is that it requires a very high processing temperature (>500°C) and the crystallisation process is very time-consuming. The high temperature makes it incompatible with cheap glass and plastic substrates and hence the cost of a poly-Si TFT display is very high. Due to its polycrystalline nature, it exhibits different characteristics across the film area. Therefore, poly-Si TFT is unsuitable for large-area displays. The common problem with Si-based TFT is that they are sensitive to visible light, so a shield in the form of an array is required that blocks the backlight, therefore the resolution of the display is degraded. These limiting factors turned attention toward other materials especially to wide bandgap materials due to their insensitivity to visible light (11).
There are various reports on ZnO-based TFT dated from 1968. The insensitivity to visible light, low processing temperature, deposition of a highly crystalline thin film over a large area by conventional processes like sputtering and devices with very high field-effect mobility in the range of 0.2 cm2 V–1 s–1 to 40 cm2 V–1 s–1 have made ZnO a very attractive channel material for TFT (128, 129). The first ZnO TFT was reported by Boesen et al. in 1968 (130). Numerous ZnO TFT have since been reported with very high mobility as compared to a-Si TFT. Due to the transparent nature of ZnO in the visible region, it is possible to realise ZnO based fully transparent TFT. In 2003 Hoffman et al., Carcia et al. and Masuda et al. (131–133) reported fully transparent ZnO TFT. In 2008 Hirao et al. (134) demonstrated a 1.46 inch LCD with 61,600 pixels driven by bottom gate ZnO TFT arrays.
The main performance parameters for TFT are turn-on voltage, drain current on-to-off ratio (Ion:Ioff) and channel mobility. Turn-on-voltage is the minimum gate voltage required to turn on the TFT. The lower the turn-on voltage, the lower the requirement of biasing voltages leading to lower power consumption. TFT with high mobility and a high Ion:Ioff ratio can work at a higher frequency and are suitable for high-resolution displays. ZnO TFT with field effect mobility up to 50 cm2 V–1 s–1 and Ion:Ioff ratio greater than 105 can be obtained. The performance of the ZnO TFT can be further improved by various techniques like the use of high-k dielectric, doping of ZnO and post-deposition treatments.
Memristor is of interest to many research groups as it finds important application in fields like non-volatile memory, neural networks optoelectronics, radiation sensors and neuromorphic systems. There are some reports on ZnO based memristor devices. Patil et al. (135), Fauzi et al. (136), Barnes et al. (137), Santos et al. (138) and Le et al. (139) have reported ZnO based memristor with low power and fast switching activity.
ZnO has emerged as an important semiconductor material because of its excellent electrical, optical, piezoelectric and gas sensing properties. Hence, it can be used for near-UV emission or detection and as a transparent electrode. It has a large excitonic binding energy of 60 meV at room temperature as compared to 25 meV of GaN, an III-V compound having limited prospects. This makes ZnO an efficient light emitter in the UV spectral region and comparably favourable for optoelectronic applications. The high conductivity and transparency of ZnO are important for applications like transparent conducting oxides and TFT. ZnO is fast emerging as a future material for the fabrication of low cost, high performance electronic and optoelectronic devices including transparent conductive films, solar cells, LED and TFT. However, there are certain challenges and limitations. First is the realisation of stable p-type ZnO. It is very difficult to achieve p-type conductivity hence the fabrication of ZnO-based p-n junction devices and CMOS is not currently viable. The most important factor is the stability of electrical characteristics in the presence of oxygen. ZnO reacts with the oxygen in the environment. Due to this, the conductivity varies and the electrical properties change over time. The variation of the electrical properties makes ZnO-based devices unstable. The characteristics of devices based on silicon technology are highly reproducible and stable under varying ambient conditions, and the devices are highly reliable. For commercialisation of ZnO based devices, it is very important to resolve these issues.
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aluminium gallium arsenide
aluminium gallium nitride
active-matrix liquid-crystal display
hydrogenated amorphous Si
aluminium-doped zinc oxide
beryllium zinc oxide
charge coupled device
cadmium molybdenum oxide
cadmium zinc oxide
copper gallium sulfide
chemical vapour deposition
graphene nanodots array
gallium-doped zinc oxide
indium gallium arsenide
indium tin oxide
light amplification by stimulated emission of radiation
magnesium zinc oxide
metalorganic chemical vapour deposition
metal-oxide-semiconductor field-effect transistor
noise equivalent power
organic light-emitting diode
plasma-enhanced chemical vapour deposition
pulsed laser deposition
scandium aluminium magnesium oxide
superconducting quantum interference devices
transparent conducting oxides
very large scale integrated circuits
zinc magnesium oxide
Sumit Vyas received his BE degree in Electronics and Communication Engineering from Shri Vaishnav Institute of Technology and Science, India and an MTech degree in Microelectronics and VLSI Design from Motilal Nehru National Institute of Technology Allahabad, India, in 2010 and 2012 respectively. He completed his PhD degree in 2016 from Motilal Nehru National Institute of Technology Allahabad. Currently he is working as an Assistant Professor in Thapar Institute of Technology, India. His current research interests include fabrication, characterisation and modelling of semiconducting oxide thin film based electronic and optoelectronic devices. He has published more than 17 papers in various international journals and conference proceedings.