Lithium molybdate (LMO) is a hygroscopic material which is congruently soluble in water and was one of the first materials used to demonstrate cold sintering (4, 13, 18). LMO is conventionally sintered at 540°C but via cold sintering it can be densified at 120°C with the addition of 2–10 wt% distilled water under applied pressure (12, 17, 18). The properties of cold sintered LMO are comparable with conventional samples but a slight increase in dielectric loss is thought to relate to residual hydroxyl groups at the grain boundaries (12).

Materials related to LMO such as sodium molybdate (NMO) can also be densified via cold sintering. Whilst NMO is not hygroscopic, it is highly soluble in water making it another ideal candidate for cold sintering. NMO is conventionally sintered at 610°C, whilst the material can be cold sintered at 150°C with the addition of 5–10 wt% water and the application of 200 MPa of pressure. Wang et al. achieved relative densities of 87% after conventional sintering but 96% after cold sintering. The dielectric properties of NMO are also comparable between conventional and cold sintered. An increase in permittivity (ɛ_r) is observed (conventional: 11.6, CSP: 12.7), due to increased density whilst residual hydroxyl groups increased dielectric loss (19, 20).

As LMO and NMO are readily densified via cold sintering, they have been used as a starting point to create many composites with other materials with more favourable properties, but which are harder to cold sinter such as barium hexaferrite, sodium bismuth molybdate and bismuth lithium vanadium molybdate (19, 21, 22).

Zinc oxide has also gained interest as a material that can be densified via cold sintering. ZnO is wide band-gap (3.4 eV) semiconductor traditionally used in electronics such as varistors and requires temperatures in excess of 1100°C to sinter (23, 24). The high temperatures for conventional sintering leads to grain coarsening and other effects deleterious to the electrical properties and therefore methods of reducing sintering temperature have been widely investigated. Several studies have successfully cold sintered ZnO at ≤300°C. Unlike molybdate compounds, ZnO has limited solubility in water and therefore an alternative transient solvent such as aqueous acetic acid or zinc acetate (Zn-Ac) is utilised to achieve sufficient dissolution to promote densification (25, 26).

Recent work at The University of Sheffield has investigated the effects of pressure and temperature on the cold sintering of ZnO. Samples of ZnO were produced at temperatures 125–300°C and pressures of 187–375 MPa with 25–30 wt% of 1 M acetic acid. Scanning electron microscopy (SEM) of cold sintered ZnO showed that temperature and pressure have a significant effect on grain growth and morphology, helping to corroborate previous work by Funahashi et al. and Kang et al. (25, 26).

Funahashi studied cold sintering of ZnO using a wide variety of pressures, temperatures and concentrations of acetic acid (0.1–17.5 M) and water for comparison. The presence of the acetic acid was critical to achieve high density, with 1.0 M the optimal concentration. Pressures of 387 MPa combined with 300°C were reported to produce the highest densities from the test pressures and temperatures. The lower pressure of 77 MPa did produce high densities however no neck-growth was observed (26), in agreement with work done in The University of Sheffield. Pellets pressed at 250 MPa at 300°C showed high density (>98% theoretical) and grain growth but no necking, Figure 3(c). When pressure was increased to 374 MPa necking is observed (Figure 3(d)) producing a sample with a microstructure similar to conventional sintering.

Fig. 3.

Kang et al. studied a large range of processing conditions, including the use of various solvent chemicals and the effect of pressure, temperature, pH and ion concentration. Initial findings agreed with Funahashi with pressure promoting necking and temperature having the largest effect on grain growth (25, 26). However, it was proposed that pressure has a threshold below which densities are significantly affected. Beyond this threshold, densification becomes pressure independent. Other chemicals explored by Kang et al. (other than acetic acid or Zn-Ac) include hydrochloric acid, sulfuric acid, zinc chloride and zinc sulfate but only 70–75% densities were achieved, as well as unwanted cement and hydroxide phases (25). Densification was independent of pH due mainly to the presence of Zn²⁺ and acetate ions. The exchange of Zn²⁺ ions through solution enabled by applied pressure is the largest contributor to densification (25). Secondary phases observed by Kang et al. were also present, contradicting earlier suggestions that samples were single phase (26). Raman analysis showed evidence of residual acetate or Zn-Ac (25) which was confirmed in our studies. Figure 4 shows a comparison of ZnO cold sintered at 125°C and 300°C compared to a conventionally sintered sample produced at The University of Sheffield. At 125°C and 300°C, three peaks are observed which are not present in the conventional sample. The peak at ~943 cm^–1 is typical of a C–C bond, ~1435 cm^–1 a C–O bond and finally at ~2930 cm^–1 a mode characteristic of a C–H and the peak ~650 cm^–1 is still under investigation. These organic peaks are weaker in the 300°C sample, suggesting that some of the acetate has been removed. Acetate decomposes at ~225°C consistent with this hypothesis.

Fig. 4.

Gonzalez-Julian et al. densified ZnO via cold sintering and a combined cold sinter-FAST/SPS method, using Kelvin probe force microscopy (KPFM) on resulting samples, to better understand the mechanisms behind cold sintering (27). For both CSP and CSP‐SPS, powders were moistened with 1.6–3.2 wt% distilled water or a 0.5% Zn–Ac solution. Samples were then sintered according to parameters in Table II. The addition of Zn–Ac to the transient solvent was found to significantly reduce the onset temperature of densification from 90–130°C to ~25°C at all pressures. This demonstrates the crucial role of powder dissolution in densification via cold sintering.

KPFM was used to analyse the surface potentials of samples sintered via CSP-SPS at 150 MPa. The addition of water increased surface potential compared to the as-received powder, which implies an increase in defect concentration. A contrasting effect is seen with the addition of Ac-H₂O, where surface potentials are reduced indicating a lower defect concentration, which the authors attribute to the observed grain growth. The increase in surface potential indicates that the solvent phase not only promotes transport but also raises the sintering potential through the creation of activation energy lowering defects, as OH^– and H⁺ ions diffuse into the surface of the crystal structure (25–27).

From impedance spectroscopy, ZnO densified using Ac-H₂O had the highest conductivity with significantly lower total activation energy than other conditions. The bulk activation energy of ZnO was found to be significantly reduced by sintering with both water and acetate, whereas the grain boundary (E_a) was found to be increased with H₂O and decreased with Ac-H₂O. This lowering of activation energy is thought to be due to the manufacture of highly defective diffusion pathways, which helps to encourage sintering at low temperatures.

Overall Gonzalez-Julian et al. theorised the liquid phase has five main roles during the CSP in ZnO: (a) a better initial packing of the powder material due to interparticle friction; (b) dissolution of Zn²⁺ and O^2– ions from the powder surface; (c) formation of defects in ZnO crystals due to H⁺ and OH^– diffusion; (d) formations of highly defective diffusion pathways between grains and (e) elimination of carbonates. These effects are thought to be further enhanced by the presence of the acetate phase by improving dissolution. This paper indicates that the liquid phase has a more complex role than first suggested by initial studies; better understanding of defect chemistry effects is in understanding and improving the results achieved from the CSP.

To unite cold and flash sintering, Nie et al. studied the effect of an aqueous transient liquid phase on flash sintered ZnO. An electrode green body produced by uniaxial pressing was placed in a flash chamber and flowing wet argon + 5% hydrogen was introduced after 1 h. The conductivity of the hydrated pellet increased by a factor of four compared to the unhydrated form (3 × 10^–7 S cm^–1 to ~7 × 10^–3 S cm^–1). The presence of water was found to trigger flash sintering at room temperature due to the higher conductivity, producing relative densities of ~98%. The water was also proposed to assist with densification via mass transport due to partial dissolution of the substrate (28).

As already discussed, Li, NMO and ZnO are relatively easy to cold sinter and coarse powders can be densified with the addition of water or acetic acid as the transient solvent. To cold sinter a wider variety of materials, with a broader range of properties, several methods are employed, such as reducing the particle size to nanoscale, thereby increasing the reactivity of the powder and altering liquid additive to include more complex acids, alkalis or ionic solvents.

In cases where the powder dissolves incongruently, a method of hydrothermal assisted cold sintering is utilised through reactive intermediate phases. The liquid utilised in cold sintering of incongruent materials is often a solution containing a deep eutectic reaction precursor to form the desired products at temperatures below that of a solid‐state process (29–32).

When particles of BaTiO₃ are exposed to water, Ba ions leach from the surface, leaving a titanium‐rich layer (33). To cold sinter BaTiO₃, Guo et al. utilised nanoscale particles of BaTiO₃ and a barium hydroxide on titania suspension in deionised water. This prevents the dissolution of Ba during cold sintering and the Ba(OH)₂ and TiO₂ react to form BaTiO₃ during annealing at 700–900°C.

Strontium titanate is conventionally sintered at over 1400°C (34). Boston et al. developed a method of cold sintering for SrTiO₃ which utilised reactive intermediate phases (29). Nanoscale SrTiO₃ and TiO₂ powders were mixed with 0.2 ml of a 1.5 M strontium chloride aqueous solution with 1.5 M equivalent of anatase nanoparticles. The mixture was pestle and mortared to produce a free-flowing powder which was then pressed at 750 MPa for 10 min at room temperature before increasing to 180°C for 60 min. After cold sintering, a 4 h heat treatment at 950°C was utilised to promote microreactions between SrCl₂ and TiO₂ intermediate phases, forming SrTiO₃. Electrical testing of cold sintered SrTiO₃ showed similar trends to conventionally sintered materials, however the relative permittivity values exhibited frequency dependence. Particle size in the conventionally sintered samples is shown to affect the permittivity and loss (Table III).

Cold sintering is an exciting area for development in ceramic science. There are however a number of challenges which will need to be overcome to improve the commercial prospects of this new technology.

Most developments in cold sintering so far have been on an empirical, material-specific, ‘trial and error’ basis. A greater understanding of the mechanisms and how they relate to processing parameters will allow a wider range of materials to be densified via cold sintering. There are numerous processing parameters which can be altered to tailor the densification of material during cold sintering, including composition of transient liquid phase, volume of transient liquid required, pressure, temperature and powder particle size.

The transient phase should allow for the congruent dissolution of the solid phase or react to form a desirable composition upon heating during sintering or subsequent heat treatment. Therefore, it is important to understand the dissolution behaviour of the ceramic within the temperature range of sintering. The amount of liquid used during cold sintering is mostly quoted in weight percent of the solid phase. This does not consider the effect of surface area, far greater for nano- as opposed to micropowders. The purpose of pressure during cold sintering is the rearrangement of particles, but it also plays a more complex role in dissolution, grain growth and activation of reactions due to inhomogeneous pressure distributions.

The temperatures used during cold sintering are largely dependent on the evaporation point of the solvent. Grain growth has also been observed in some materials cold sintered significantly above the solvent evaporation temperature but below conventional sintering temperatures, this could be used to achieve specific grain sizes and structures.

In some cases, secondary phases can form during cold sintering, due to reactions between the solid phase and transient solvent or residual solvent after sintering completion. Kang et al. used a number of acids in the cold sintering of ZnO to observe their effectiveness as solvent phases. For a ZnCl₂ solution, significant amounts of zinc oxychloride phases were observed, with similar results produced when sulfate and nitrate based solvents were used. While the use of such strong solvents is an extreme example, it demonstrates the importance of correct solvent choice for the sintering process to prevent secondary phase formation. Even for more successful solvent phases used to densify ZnO, such as acetic acid and Zn-Ac, small amounts of residual acetate phases have been detected and affect properties.

When BaTiO₃ interacts with distilled water, Ba is leached from the material leading to an amorphous Ti-rich surface layer. This preferential leaching is overcome using a solution containing high concentrations of Ba and Ti ions but amorphous material forms which requires a further post CSP crystallisation step at high temperatures.

While the amount of energy required to densify material via cold sintering has shown to be significantly reduced, the energy of nanopowder production has not been routinely considered when evaluating the total energy consumed. To produce nanopowders significant amounts of energy or complex chemical reactions are often required, transferring the energy consumption and environmental costs to a different stage of the manufacturing process.

Microwave (MW) dielectric materials show strong interactions with electromagnetic waves, making them extremely important in modern communications as resonators, filters and substrates (22, 35). The three selective parameters of MW dielectric ceramics are high quality factor (Qf), near-zero temperature coefficient of resonant frequency (TCF) and high ɛ_r (19, 22, 35).

With fifth generation (5G) network technologies beginning to be utilised and installed in numerous countries, the material challenge is to develop systems of very high resonant frequency and low latency. Whilst fourth generation (4G) systems operate in the 2–8 GHz range, the operating range of 5G systems will eventually be up to 30 GHz. For these 5G systems, the dielectric loss of polymeric substrates used in 4G is too high and other substrates must be investigated (36–39).

Cold sintering has shown promise within this area at The University of Sheffield with the densification of several known MW ceramics achieved at low temperatures. However, none of the early materials such as LMO and NMO exhibited near zero TCF. Consequently, Wang et al. (17, 19) has developed several, two component temperature stable MW ceramic composites via CSP.

Na_0.5Bi_0.5MoO₄-Li₂MoO₄ (NBMO-LMO) composite samples were produced by mixing NBMO and LMO powders with 5–10 wt% of deionised water and pressing pellets 30 min at 150°C and 200 MPa. Sintered pellets were dried for 24 h at 120°C to remove any residual moisture (22). The NBMO‐LMO ceramic composites in this study showed no chemical reaction between the phases during cold sintering and near zero TCF was achieved at ~20% LMO with ɛ_r = 17 and Qf = 8000 GHz (22) (Figure 5).

Fig. 5.

(Bi_0.95Li_0.05)(V_0.9Mo_0.1)O₄-Na₂Mo₂O₇ (BLVMO‐NMO) composites were also sintered by combining the mixtures with 5–10 wt% of deionised water and hot pressing for 30 min at 150°C and 200 MPa. A post sintering drying step of 120°C for 24 h was performed to remove residual moisture (19). Electrical and MW analysis of the BLVMO-NMO showed similar trends to the NBMO-LMO and near zero TCF was obtained at ~20% NMO with ɛ_r ~40 and Qf = 4000, Figure 6.

Fig. 6.

Although the Qf values of these composites do not compete with conventionally sintered ceramics for resonator applications, their properties, ease of integration and low energy consumption show promise for a wide range of novel devices.

Graded-index (GRIN) lenses (19, 43) are able to convert a point electromagnetic source to a planar wave and vice versa. They are normally used in optics but CSP can be used to fabricate devices from ceramics in MW applications. A MW GRIN lens (19, 22) consists of concentric rings of material, with decreasing ɛ_r towards the outer edge of the structure, preferably reaching values close to ɛ_r = 1. Dimensions and ɛ_r of the layers are tailored to ensure they have the same focal point (O) to convert the incident spherical wave to a plane wave; a schematic GRIN lens design is shown in Figure 7 and a simulation (CST Microwave Studio, Dassault Systèmes, France) of a working GRIN lens is shown in Figure 8.

Fig. 7.

Fig. 8.

The simulated lens was illuminated with an open-ended K_a-band waveguide (7.112 mm × 3.556 mm). Across the whole frequency range the boresight directivity was increased from 26 GHz to 40 GHz, the relative increase compared to the case with no lens was between 4.6 dB and 8.5 dB. The simulated BLVMO-NMO and NBMO-LMO lenses exhibited an aperture efficiency ~70% at 26 GHz and ~78% at 34 GHz respectively (19, 22).

Due to the ability to control lateral dimensions during cold sintering, it is possible to co‐sinter multiple layers of ceramics without the detrimental effects of differential shrinkage and divergent thermal expansion coefficients. Wang et al. co‐sintered three layers of ceramic in the LMO‐NBMO system (LMO, NMBO-10 wt% LMO and NMBO-50 wt% LMO) to create a macroscopic ceramic-ceramic composite (22). This demonstrated the ability to utilise cold sintering to produce graded dielectrics and thus illustrated proof of concept for the fabrication of a MW GRIN lens. Simulations were performed to understand the potential efficiency of lenses composed of six concentric rings of radially reducing ɛ_r, illuminated with a K_α-band waveguide between 26–40 GHz. Peak aperture efficiencies were 78% at 34 GHz for the NBMO-LMO lens and 70% at 26 GHz for the BLVMO-NMO lens. Demonstrating high conversion rates between input and output of the lens.

Microstrip patch antennae (MPA) are a low-profile form of antenna that can be integrated effectively where space and weight restrictions apply and maybe printed onto polymer-based printed circuit boards (PCBs) for mobile device applications (44, 45).

At The University of Sheffield, Wang et al. made use of cold sintering to produce an MPA from a calcium titanite-potassium molybdate (CTO-KMO) composite. Composites were produced at 150°C under a uniaxial pressure of 200 MPa for 30 min, achieving high density for all compositions. From energy-dispersive X-ray spectroscopy (EDS) mapping, the authors showed that two chemically discrete regions are present in the composites after cold sintering indicating that no reaction occurs between the two phases during sintering (45).

For 5G antenna substrates, materials should have low ɛ_r (<15), a near-zero TCF and high-quality factor (45, 46). In previous studies, the dielectric properties of composites approximately follow known mixing laws and can therefore be tailored to suit specific applications. A CTO-KMO composite produced with 92 wt% KMO had TCF ~–4 ppm °C^–1, ɛ_r = 8.5 and Qf ~11,000 GHz. This composition was then used to create a cold sintered MPA which operates at 2.51 GHz with a 62% radiation efficiency (Figure 9). The combination of high antenna performance and low temperature densification demonstrate the potential for the direct fabrication of antenna substrates onto PCBs (45).

Fig. 9.

Multilayer ceramic capacitors (MLCCs) consist of alternate layers of ceramic and metallic electrode and over three trillion are produced every year (20). MLCCs are conventionally sintered at high temperatures which presents several challenges, not least the electrode melting point and chemical compatibility with the ceramic.

Capacitors are used in a wide variety of applications and conditions and they are characterised by the temperature dependency of their properties. C0G (or NP0) Class 1 dielectric materials do not show a significant variation in capacitance over a wide range of temperatures. Materials with positive and negative temperature coefficients of capacitance (TCC) can be mixed to create a temperature stable ceramic composite. Figure 10 compares the TCC of several common Class 2 and C0G capacitors.

The TCC of BLVMO and NMO ceramics are approximately +81 ppm °C^–1 and –99 ppm °C^–1 respectively. The combination of these materials creates temperature stable composites <10 ppm °C^–1 with low dielectric loss (tan δ ≈ 0.001) and ɛ_r = 40 (19, 20). Using BLVMO-xNMO (x = 0.2) composites, researchers at The University of Sheffield have demonstrated the use of cold sintering to produce multilayer ceramic capacitors with comparable properties to conventional calcium zirconate C0G MLCCs manufactured at 1100°C (20). Laminated stacks were made from tape cast BLVMO-NMO with screen printed silver electrodes. After binder burnout, at 180°C for 3 h, stacks were exposed to water vapour in a sealed beaker at 80°C. The moistened stacks were then cold sintered at 150°C under 100 MPa of pressure for 30 min. The SEM image of the cross-section of the cold sintered MLCC in Figure 11 shows the dielectric layers are well densified, well laminated and unwarped. The Ag electrodes also appear well defined indicating no reaction at the metal-ceramic interface (20). The room temperature ɛ_r and loss at 1 MHz were found to be 39 and 0.01 respectively and the TCC was within 0.013% up to 150°C.

Fig. 10.

Fig. 11.

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).

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 O^2– 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).

Fig. 2.

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 (V_O) and zinc interstitials (Zn_i). 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. Zn_i and V_O result in a donor level in the forbidden gap whereas Zn vacancies create an acceptor level. The V_O creates deep level donor states while the shallow level donor states are due to Zn_i. The difficulty in achieving p-type conductivity is due to the compensation of acceptor atoms by deep level donors that are the result of V_O (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.

Fig. 3.

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).

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):

(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 (V_O and Zn_i). 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):

(ii)

(iii)

where μ_b is band mobility at infinite temperature, N_b is total states per unit volume in the transport band, T_o is the characteristic temperature that accounts for the energetic disorder, N_t is the total number of trap states and n_t is the charge-carrier concentration in the trap states in the disordered ZnO. The authors assumed that the charge carriers n_t in the trap state are much greater than that of the carriers in the transport band n. Therefore, the total carrier concentration n_T was approximated as n_T = n + n_t ~ n_t. 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).

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 (φ_m = 5.12 eV) and Au (φ_m = 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.

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.

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.

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.

Fig. 4.

5.6.1 Photoconductor

5.6.2 Metal-Semiconductor-Metal Photodiode

5.6.3 Schottky Photodiode

5.6.4 p-n Heterojunction Photodiode

5.6.5 p-n Homojunction Diode

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).

Fig. 5.

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 I_D vs. V_G and I_D vs. V_D respectively for an n-channel TFT. In Figure 5(e), Region 1 is known as the linear region. In this region V_D<<V_G and the drain current I_D is given by Equation (iv):

(iv)

where W is the width, L is the length of the channel, μ_FE is the mobility of electrons, V_th is the turn-on voltage, C_i is the capacitance of gate insulator per unit area, V_GS is gate-to-source voltage and V_DS is drain-to-source voltage. Since V_D<<V_G the drain current can be approximated by Equation (iv). It can be observed that in the linear region I_D varies linearly with V_DS, Equation (v):

(v)

Region 2 is known as the saturation region. In this region (V_G–V_th) >> V_D 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 V_DS, Equation (vi):

(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).

Fig. 6.

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 (I_on:I_off) 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 I_on:I_off 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 I_on:I_off 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.