Dependence of resistivity on structure and composition of AZO films fabricated by ion beam co-sputtering deposition

The correlation between the resistivity and the structure/composition in the aluminum doped zinc oxide (AZO) films fabricated by the ion beam co-sputtering deposition at room temperature was investigated. The various compositions of AZO films were controlled by the sputtered area ratio of Al to Zn target. The structure, Al concentrations and resistivities of the as-deposited films were determined by X-ray diffractometer (XRD), energy dispersive spectrometer (EDS) and four-point probe station, respectively. The lowest resistivity of the deposited film was 5.66 × 10⁻⁴ Ω-cm at the 0.7 wt.% aluminum concentration. The most intense ZnO (0 0 2) diffraction peak, the largest grain size, the longest mean free path, and the highest free carrier concentration in the film result in the lowest resistivity of 5.66 × 10⁻⁴ Ω-cm at room temperature; simultaneously, the thermal stability of the resistivity of the AZO film as a function of the sample temperature was investigated. Below 200 °C the film's resistivity was almost kept at a fixed value and the lowest resistivity of 4.64 × 10⁻⁴ Ω-cm at 247 °C was observed.     

Effect of composition and deposition temperature on the characteristics of Ga doped ZnO thin films

ZnO films doped with Ga (GZO) of varying composition were prepared on Corning glass substrate by radio frequency magnetron sputtering at various deposition temperatures of room temperature, 150, 250 and 400 °C, and their temperature dependent photoelectric and structural properties were correlated with Ga composition. With increasing deposition temperature, the Ga content, at which the lowest electrical resistivity and the best crystallinity were observed, decreased. Films with optimal electrical resistivity of 2-3 × 10⁻⁴ Ω cm and with good crystallinity were obtained in the substrate temperature range from 150 to 250 °C, and the corresponding C_Ga/(C_Ga + C_Zn) atomic ratio was about 0.049. GZO films grown at room temperature had coarse columnar structure and low optical transmittance, while films deposited at 400 °C yielded the highest figure of merit (FOM) due to very low optical absorption despite rather moderate electrical resistivity slightly higher than 4 × 10⁻⁴ Ω cm. The optimum Ga content at which the maximum figure of merit was obtained decreased with increasing deposition temperature.     

Surface textured ZnO:Al thin films by pulsed DC magnetron sputtering for thin film solar cells applications

Transparent conducting thin films of ZnO:Al (Al-doped ZnO, AZO) were prepared via pulsed DC magnetron sputtering with good transparency and relatively lower resistivity. The AZO films with 800 nm in thickness were deposited on soda-lime glass substrates keeping at 473 K under 0.4 Pa working pressure, 150 W power, 100 μs duty time, 5 μs pulse reverse time, 10 kHz pulse frequency and 95% duty cycle. The as-deposited AZO thin films has resistivity of 6.39 × 10⁻⁴ Ω cm measured at room temperature with average visible optical transmittance, T_total of 81.9% under which the carrier concentration and mobility were 1.95 × 10²¹ cm⁻³ and 5.02 cm² V⁻¹ s⁻¹, respectively. The films were further etched in different aqueous solutions, 0.5% HCl, 5% oxalic acid, 33% KOH, to conform light scattering properties. The resultant films etched in 0.5% HCl solution for 30 s exhibited high T_total = 78.4% with haze value, H_T = 0.1 and good electrical properties, ρ = 8.5 × 10⁻⁴ Ω cm while those etched in 5% oxalic acid for 150 s had desirable H_T = 0.2 and relatively low electrical resistivity, ρ = 7.9 × 10⁻⁴ Ω cm. However, the visible transmittance, T_total was declined to 72.1%.     

Influence of substrate temperature and post-treatment on the properties of ZnO:Al thin films prepared by pulsed laser deposition

Highly transparent conductive Al₂O₃ doped zinc oxide (AZO) thin films have been deposited on the glass substrate by pulsed laser deposition technique. The effects of substrate temperature and post-deposition annealing treatment on structural, electrical and optical properties of AZO thin films were investigated. The experimental results show that the electrical resistivity of films deposited at 240 °C is 6.1 × 10⁻⁴ Ω cm, which can be further reduced to as low as 4.7 × 10⁻⁴ Ω cm by post-deposition annealing at 400 °C for 2 h in argon. The average transmission of AZO films in the visible range is 90%. The optical direct band gap of films was dependent on the substrate temperature and the annealing treatment in argon. The optical direct band gap value of AZO films increased with increasing annealing temperature.     

Effects of hydrogen flux on the properties of Al-doped ZnO films sputtered in Ar + H2 ambient at low temperature

Al-doped ZnO (AZO) transparent conductive thin films were grown by magnetron sputtering with AZO (98 wt.% ZnO, 2 wt.% Al₂O₃) ceramic target in Ar + H₂ ambient at a relatively low temperature of 100 °C. To investigate the dependence of crystalline and properties of as-grown AZO films on the H₂-flux, X-ray diffraction (XRD), X-ray photoemission spectrometer (XPS), Hall and transmittance spectra measurements were employed to analyze the AZO samples deposited with different H₂-flux. The results indicate that H₂-flux has a considerable influence on the transparent conductive properties of AZO films. The resistivity of 4.15 × 10⁻⁴ Ω cm and the average transmittance of more than 94% in the visible range were obtained with the optimal H₂-flux of 1.0 sccm. Such a low temperature growing method present here may be especially useful for some low-melting point photoelectric devices and substrates.     

Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach

Zn-Sn-O (ZTO) films with continuous compositional gradient of Sn 16-89 at.% were prepared by co-sputtering of two targets of ZnO and SnO₂ in a combinatorial method. The resistivities of the ZTO films were severely dependent on oxygen content in sputtering gas and Zn/Sn ratio. Except for the films with Sn 16 at.%, all the as-prepared films were amorphous and maintaining the stable amorphous states up to the annealing temperature of 450 °C. Annealing at 650 °C resulted in crystallization for all the composition, in which ZnO, Zn₂SnO₄, ZnSnO₃, and SnO₂ peaks were appeared successively with increasing Sn content. Above Sn 54 at.%, the ZTO films were deduced to have a local structure mixed with ZnSnO₃ and SnO₂ phases which were more conductive and stable in thermal oxidation than ZnO and Zn₂SnO₄ phases. The lowest resistivity of 1.9 × 10⁻³ Ω cm was obtained for the films with Sn 89 at.% when annealed at 450 °C in a vacuum. The carrier concentrations of the amorphous ZTO films that contained Sn contents higher than 36 at.% and annealed at 450 °C in a vacuum were proportional to the Sn contents, while the Hall mobilities were insensitive to Sn contents and leveling in the range of 23-26 cm²/V s.     

The enhanced conductivity and stability of AZO thin films with a TiO2 buffer layer

Aluminum doped zinc oxide (AZO) films were substitutes of the SnO₂:F films on soda lime glass substrate in the amorphous thin-film solar cells due to good properties and low cost. In order to improve properties of AZO films, the TiO₂ buffer layer had been introduced. AZO films with and without TiO₂ buffer layer were deposited on soda lime glass substrates by r.f. magnetron sputtering. Subsequently, one group samples were annealed in vacuum (0.1 Pa) at 500 °C for 120 s using the RTA system, and the influence of TiO₂ thickness on the properties of AZO films had been discussed. The XRD measurement results showed that all the films had a preferentially oriented (0 0 2) peak, and the intensity of (0 0 2) peak had been enhanced for the AZO films with TiO₂ buffer layer. The resistivity of TiO₂ (3.0 nm)/AZO double-layer film is 4.76×10⁻⁴ Ω cm with the maximum figure merit of 1.92×10⁻² Ω⁻¹, and the resistivity has a remarkable 28.7% decrease comparing with that of the single AZO film. The carrier scattering mechanism of TiO₂ (3.0 nm)/AZO double-layer film had been described by Hall measurement in different temperatures. The average transmittance of all the films exceeded 92% in the visible spectrum. Another group samples were heat treated in the quartz tube in air atmosphere, and the effect of TiO₂ thickness on thermal stability of AZO films had been discussed.     

Room-temperature deposition of transparent conducting Al-doped ZnO films by RF magnetron sputtering method

Transparent conductive Al-doped zinc oxide (AZO) films with highly (0 0 2)-preferred orientation were deposited on quartz substrates at room temperature by RF magnetron sputtering. Optimization of deposition parameters was based on RF power, Ar pressure in the vacuum chamber, and distance between the target and substrate. The structural, electrical, and optical properties of the AZO thin films were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The 250 nm thickness AZO films with an electrical resistivity as low as 4.62 × 10⁻⁴ Ω cm and an average optical transmission of 93.7% in the visible range were obtained at RF power of 300 W, Ar flow rate of 30 sccm, and target distance of 7 cm. The optical bandgap depends on the deposition condition, and was in the range of 3.75-3.86 eV. These results make the possibility for light emitting diodes (LEDs) and solar cells with AZO films as transparent electrodes, especially using lift-off process to achieve the transparent electrode pattern transfer.     

Effect of substrate temperature on the growth of ITO thin films

Indium tin oxide (ITO) thin films were deposited onto glass substrates by rf magnetron sputtering of ITO target and the influence of substrate temperature on the properties of the films were investigated. The structural characteristics showed a dependence on the oxygen partial pressure during sputtering. Oxygen deficient films showed (4 0 0) plane texturing while oxygen-incorporated films were preferentially oriented in the [1 1 1] direction. ITO films with low resistivity of 2.05 × 10⁻³ Ω cm were deposited at relatively low substrate temperature (150 °C) which shows highest figure of merit of 2.84 × 10⁻³ square/Ω⋅     

Effects of post-annealing on structural, optical and electrical properties of Al-doped ZnO thin films

Aluminum-doped zinc oxide (AZO) films were deposited at 400 °C by radio-frequency magnetron sputtering using a compound AZO target. The effects of annealing atmospheres as well as hydrogen annealing temperatures on the structural, optical and electrical properties of the AZO films were investigated. It was found that the electrical resistivity varied depending on the atmospheres while annealing in air, nitrogen and hydrogen at 300 °C, respectively. Comparing with that for the un-annealed films, the resistivity of the films annealed in hydrogen decreased from 9.8 × 10⁻⁴ Ω cm to 3.5 × 10⁻⁴ Ω cm, while that of the films annealed in air and nitrogen increased. The variations in electrical properties are ascribed to both the changes in the concentration of oxygen vacancies and adsorbed oxygen at the grain boundaries. These results were clarified by the comparatively XPS analyzing about the states of oxygen on the surface of the AZO films. There was great increase in electrical resistivity due to the damage of the surfaces, when AZO films were annealed in hydrogen with a temperature higher than 500 °C, but high average optical transmittance of 80-90% in the range of 390-1100 nm were still obtained.     

Boron doped ZnO thin films fabricated by RF-magnetron sputtering

By using the radio frequency-magnetron sputtering (RF-MS) method, both pure ZnO and boron doped ZnO (ZnO:B) thin films were deposited on glass substrates at ambient temperature and then annealed at 450 °C for 2 h in air. It is found that both ZnO and ZnO:B thin films have wurtzite structure of ZnO with (0 0 2) preferred orientation and high average optical transmission (≥80%). Compared with the resistivity of 6.3 × 10² Ω cm for ZnO film, both as-deposited and annealed ZnO:B films exhibit much lower resistivity of 9.2 × 10⁻³ Ω cm and 7.5 × 10⁻³ Ω cm, respectively, due to increase in the carrier concentration. Furthermore, the optical band gaps of 3.38 eV and 3.42 eV for as-deposited and annealed ZnO:B films are broader than that of 3.35 eV for ZnO film. The first-principles calculations show that in ZnO:B thin films not only the band gap becomes narrower but also the Fermi level shifts up into the conduction band with respect to the pure ZnO film. These are consistent with their lower resistivities and suggest that in the process of annealing some substituted B in the lattice change into interstitial B because of its smaller ion radius and this transformation widens the optical band gap of ZnO:B thin film.     

Influence of post-annealing on the properties of Sc-doped ZnO transparent conductive films deposited by radio-frequency sputtering

Sc-doped ZnO transparent conductive films are deposited on glass substrates by radio-frequency sputtering. The influence of post-annealing on the structural, morphologic, electrical, and optical properties of the films is investigated by energy dispersion X-ray spectroscopy, X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The experimental results show that these films are polycrystalline with a preferred [0 0 1] orientation. The lowest resistivity of 2.6 × 10⁻⁴ Ω cm is obtained from the film annealed at 500 °C. The average optical transmittance of the films is over 90%. These results suggest that Sc-doped ZnO is a good candidate for fabricating high performance transparent conductive films.     

Transparent conductive ZnO:Al films grown by atomic layer deposition for Si-wire-based solar cells

Structural, electrical, and optical properties of atomic layer-controlled Al-doped ZnO (ZnO:Al) films grown by atomic layer deposition (ALD) on glass substrates were characterized at various growth temperatures for use as transparent electrodes. The Al atomic content in ZnO:Al films increased due to the reduced ZnO film growth rate with increasing temperature. The preferred orientation of ZnO:Al films was changed, and the optimum condition for best crystallinity was identified by varying the growth temperature. Furthermore, the carrier concentration of free electron was increased by substituting the Zn sites with Al atoms in the crystal, resulting from monolayer growth based on alternate self-limiting surface chemical reactions. The electrical resistivity of ZnO:Al film grown by ALD at 225 °C reached the lowest value of 8.45 × 10⁻⁴ Ω cm, with a carrier mobility of 9.00 cm² V⁻¹ s⁻¹ and optical transmittance of ∼93%. This result demonstrates that ZnO:Al films grown by ALD possess excellent potential for applications in electronic devices and displays as transparent electrodes and surface passivation layers.     

Influence of deposition temperature on the crystallinity of Al-doped ZnO thin films at glass substrates prepared by RF magnetron sputtering method

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering method under different substrate temperatures. The microstructural, electrical and optical properties of AZO films were investigated in a wide temperature range from room temperature up to 350 °C by X-ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), Hall measurement, and UV–visible meter. The nature of AZO films is polycrystalline thin films with hexagonal wurtzite structure and a preferred orientation along c-axis. The crystallinity and surface morphologies of the films are strongly dependent on the growth temperature, which in turn exerts a great effect on microstructural, electrical and optical properties of the AZO films. The atomic arrangement of AZO film having an wurtzite structure was indeed identified by the HRTEM as well as the Selected Area Electron Diffraction (SAED). The defect density of AZO film was investigated by HRTEM. The film deposited at 100 °C exhibited the relatively well crystallinity and the lowest resistivity of 3.6 × 10⁻⁴ Ω cm. The average transmission of AZO films in the visible range is all over 85%. More importantly, the low-resistance and high-transmittance AZO film was also prepared at a low temperature of 100 °C.     

Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition

Transparent conducting Al-doped ZnO (AZO) thin films have been deposited by sol-gel route. Starting from an aqueous solution of zinc acetate by adding aluminum chloride as dopant, a c-axis oriented polycrystalline ZnO thin film 100 nm in thickness could be spin-coated on glass substrates via a two-step annealing process under reducing atmosphere. The effects of thermal annealing and dopant concentration on the structural, electrical and optical properties of AZO thin films were investigated. The post-treated AZO films exhibited a homogenous dense microstructure with grain sizes less than 10 nm as characterized by SEM photographs. The annealing atmosphere has prominent impact on the crystallinity of the films which will in turn influence the electrical conductivity. By varying the doping concentrations, the optical and electrical properties could be further adjusted. An optimal doping concentration of Al/Zn = 2.25 at.% was obtained with minimum resistivity of 9.90 × 10⁻³ Ω-cm whereas the carrier concentration and mobility was 1.25 × 10²⁰ cm⁻³ and 5.04 cm² V⁻¹ s⁻¹, respectively. In this case, the optical transmittance in the visible region is over 90%.     

Effects of substrate temperature and Zn addition on the properties of Al-doped ZnO films prepared by magnetron sputtering

Al-doped ZnO (AZO) films prepared at different substrate temperature and AZO films with intentional Zn addition (ZAZO) during deposition at elevated substrate temperature were fabricated by radio frequency magnetron sputtering on glass substrate, and the resulting structural, electrical, optical properties together with the etching characteristics and annealing behavior were comparatively examined. AZO films deposited at 150 °C showed the optimum electrical properties and the largest grain size. XPS analysis revealed that AZO films deposited at elevated temperature of 450 °C contained large amount of Al content due to Zn deficiency, and that intentional Zn addition during deposition could compensate the deficiency of Zn to some extent. It was shown that the electrical, optical and structural properties of ZAZO films were almost comparable to those of AZO film deposited at 150 °C, and that ZAZO films had much smaller etching rate together with better stability in severe annealing conditions than AZO films due possibly to formation of dense structure.     

Dielectric behavior of spin-deposited nanocrystalline nickel-zinc ferrite thin films processed by citrate-route

Nanocrystalline nickel-zinc ferrite thin films with the general formula Ni_1−xZn_xFe₂O₄, where x=0.0, 0.2, 0.4 and 0.6 were fabricated via a chemical route known as the citrate precursor route. These films were spin-deposited on indium-tin oxide coated glass, fused quartz and amorphous Si-wafer substrates, and annealed at various temperatures up to 650 °C. The films annealed below 400 °C were found to be X-ray amorphous, while the films annealed at and above 400 °C were polycrystalline exhibiting a single-phase spinel structure. The average grain size of the films evaluated by transmission electron microscopy, is found to be in the range 4-8.5 nm. The room temperature DC resistivity of the films is in the range 10³-10⁷ Ω m. Dielectric constant and dielectric loss were measured in the frequency range 100 Hz-1 MHz. Dielectric constant of the films is found to lie between 25 and 44, while the loss factor is if the order of 10⁻². The higher values of the dielectric constant for films having higher zinc concentration are attributable to the enhanced hopping between Fe²⁺ and Fe³⁺ ions in these samples. The M-H hysteresis measurement of the nickel ferrite thin films annealed at 650 °C showed narrow hysteresis loop—a characteristic of soft ferromagnetic material.     

High work function of Al-doped zinc-oxide thin films as transparent conductive anodes in organic light-emitting devices

Deposition of Al-doped ZnO (AZO) films with various film thicknesses on glass substrates was performed to investigate the feasibility of using AZO films as anode electrodes in organic light-emitting devices (OLEDs). The electrical resistivity of the AZO films with a 180-nm thickness was 4.085 × 10⁻² Ω cm, and the average optical transmittance in the visible range was 80.2%. The surface work function for the AZO films, determined from the secondary electron emission coefficients obtained with a focused ion beam, was as high as 4.62 eV. These results indicate that AZO films grown on glass substrates hold promise for potential applications as anode electrodes in high-efficiency OLEDs.     

Structural, optical and electrical properties of Al-N codoped ZnO films by RF-assisted MOCVD method

N-doped ZnO films were produced using N₂ as N source by metal-organic chemical vapor deposition (MOCVD) system which has been improved with radio-frequency (RF)-assisted equipments. The data of secondary ion mass spectroscopy (SIMS) indicate that the concentration of N in N-doped ZnO films is around 5 × 10²⁰ cm⁻³, implying that sufficient incorporation of N into ZnO can be obtained by RF-assisted equipment. On this basis, the structural, optical and electrical properties of Al-N codoped ZnO films were studied. Then, the effect of RF power on crystal quality, surface morphologies, optical properties was analyzed using X-ray diffraction, atomic force microscopy and photo-luminescence methods. The results illustrate that the RF plasma is the key factor for the improvement of crystal quality. Then the observation of A⁰X recombination associated with N_O acceptor in low-temperature PL spectrum proved that some N atoms have occupied the positions of O atoms in ZnO films. Hall measurements shown that p-type ZnO film deposited on quartz glasses was obtained when RF power was 150 W for the Al-N codoped ZnO films, while the resistivity of N-doped ZnO films was rather high. Compared with the Al-doped ZnO film, the obviously increased resistivity of codoped films indicates that the formation of N_O acceptors compensate some donors in ZnO films effectively.     

Thermal stability of Ti/Al/Pt/Au and Ti/Au Ohmic contacts on n-type ZnCdO

The specific contact resistivity and chemical intermixing of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn_0.05Cd_0.95O layers grown on ZnO buffer layers on GaN/sapphire templates is reported as a function of annealing temperature in the range 200-600 °C. A minimum contact resistivity of 2.3 × 10⁻⁴ Ω cm² was obtained at 500 °C for Ti/Al/Pt/Au and 1.6 × 10⁻⁴ Ω cm² was obtained at 450 °C for Ti/Al. These values also correspond to the minima in transfer resistance for the contacts. The Ti/Al/Pt/Au contacts show far smoother morphologies after annealing even at 600 °C, whereas the Ti/Au contacts show a reacted appearance after 350 °C anneals. In the former case, Pt and Al outdiffusion is significant at 450 °C, whereas in the latter case the onset of Ti and Zn outdiffusion is evident at the same temperature. The improvement in contact resistance with annealing is suggested to occur through formation of TiO_x phases that induce oxygen vacancies in the ZnCdO.