Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Polycrystalline films of tungsten-doped indium oxide prepared by d.c. magnetron sputtering

Electrical and optical properties of polycrystalline films of W-doped indium oxide (IWO) were investigated. These films were deposited on glass substrate at 300 °C by d.c. magnetron sputtering using ceramic targets. The W-doping in the sputter-deposited indium oxide film effectively increased the carrier density and the mobility and decreased the resistivity. A minimum resistivity of 1.8 × 10^− 4 Ω cm was obtained at 3.3 at.% W-doping using the In₂O₃ ceramic targets containing 7.0 wt.% WO₃. The 2.2 at.% W-doped films obtained from the targets containing 5.0 wt.% WO₃, showed the high Hall mobility of 73 cm² V^− 1 s^− 1 and relatively low carrier density of 2.9 × 10²⁰ cm^− 3. Such properties resulted in novel characteristics of both low resistivity (3.0 × 10^− 4 Ω cm) and high transmittance in the near-infrared region.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

Effect of base and oxygen partial pressures on the electrical and optical properties of indium molybdenum oxide thin films

Indium molybdenum oxide thin films were RF sputtered at room temperature on glass substrates with a reference base pressure of 7.5 × 10^− 4 Pa. The electrical and optical properties of the films were studied as a function of oxygen partial pressures (OPP) ranging from 1.5 × 10^− 3 Pa to 3.5 × 10^− 3 Pa. The obtained data show that the bulk resistivity of the films increased by about 4 orders of magnitude (from 7.9 × 10^− 3 to 7.6 × 10¹ Ω-cm) when the OPP increased from 1.5 × 10⁻³ to 3.5 × 10^− 3 Pa, and the carrier concentration decreased by about 4 orders (from 1.77 × 10²⁰ to 2.31 × 10¹⁶ cm^− 3). On the other hand, the average visible transmittance of 30.54% of the films (brown colour; OPP = 1.5 × 10^− 3 Pa) was increased with increasing OPP to a maximum of 80.47% (OPP = 3.5 × 10^− 3 Pa). The optical band gap calculated from the absorption edge of the transmittance spectra ranges from 3.77 to 3.88 eV. Further, the optical and electrical properties of the films differ from those deposited at similar conditions but with a base pressure lower than 7.5 × 10^− 4 Pa.     

Contribution to carrier densities of the oxygen vacancy in a low-resistivity tin-doped indium oxide film by the hot-cathode plasma sputtering method

In order to clarify the contribution to carrier density by oxygen vacancies in tin-doped indium oxide (ITO) films prepared on glass substrates by the hot-cathode plasma sputtering method, we have investigated the effect of annealing on the electrical properties of an ITO film with a resistivity of 1.0 × 10^− 4 Ω cm. A drastic decrease in carrier density from 2.0 × 10²¹ to 0.88 × 10²¹ cm^− 3 was found with gradual increase in the Hall mobility from 29 to 35 cm² V^− 1 s^− 1 for repeated annealing cycles, when the ITO film was exposed for one hour to 400 °C oxygen gas at atmospheric pressure. The results indicate that the contribution of oxygen vacancies to carrier density was ca. 1.12 × 10²¹ cm^− 3 for the ITO film with an overall carrier density of 2.0 × 10²¹ cm^− 3.     

Temperature dependence of Fluorine-doped tin oxide films produced by ultrasonic spray pyrolysis

Fluorine-doped tin oxide (FTO) films were prepared at different substrate temperatures by ultrasonic spray pyrolysis technique on glass substrates. Among F-doped tin oxide films, the lowest resistivitiy was found to be 6.2 × 10^− 4 Ω-cm for a doping percentage of 50 mol% of fluorine in 0.5 M solution, deposited at 400 °C. Hall coefficient analyses and secondary ion mass spectrometry (SIMS) measured the electron carrier concentration that varies from 3.52 × 10²⁰ cm^− 3 to 6.21 × 10²⁰ cm^− 3 with increasing fluorine content from 4.6 × 10²⁰ cm^− 3 to 7.2 × 10²⁰ cm^− 3 in FTO films deposited on various temperatures. Deposition temperature on FTO films has been optimized for achieving a minimum resistivity and maximum optical transmittance.     

Fabrication of low resistivity tin-doped indium oxide films with high electron carrier densities by a plasma sputtering method

Tin-doped indium oxide (ITO) films fabricated on glass substrates using a hot-cathode plasma sputtering method exhibited low resistivity of 9.7 × 10⁻⁵ Ω cm, which is due to a high carrier density of 2.1 × 10²¹ cm⁻³. The change in the number of carriers, N, as a function of film thickness d, strongly suggests that oxygen extraction in the initial stages of ITO film growth on the glass substrate surface, creates oxygen vacancies as an electron carrier source for improvement in the resistivity of the films.     

Structural and optoelectronic properties of Al-doped zinc oxide films deposited on flexible substrates by radio frequency magnetron sputtering

Al-doped zinc oxide (AZO) thin films were deposited onto flexible polyethylene terephthalate substrates, using the radio frequency (RF) magnetron sputtering process, with an AZO ceramic target (The Al₂O₃ content was about 2 wt.%). The effects of the argon sputtering pressure (in the range from 0.66 to 2.0 Pa), thickness of the Al buffer layer (thickness of 2, 5, and 10 nm) and annealing in a vacuum (6.6 × 10^− 4 Pa), for 30 min at 120 °C, on the morphology and optoelectronic performances of AZO films were investigated. The resistivity was 9.22 × 10^− 3 Ω cm, carrier concentration was 4.64 × 10²¹ cm^− 3, Hall mobility was 2.68 cm²/V s and visible range transmittance was about 80%, at an argon sputtering pressure of 2.0 Pa and an RF power of 100 W. Using an Al buffer decreases the resistivity and optical transmittance of the AZO films. The crystalline and microstructure characteristics of the AZO films are improved by annealing.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

Effects of target bias voltage on indium tin oxide films deposited by high target utilisation sputtering

Indium tin oxide (ITO) films were deposited by reactive High Target Utilisation Sputtering (HiTUS) onto glass and polyimide substrates. The ion plasma was generated by an RF power source while the target bias voltage was varied from 300 V to 500 V using a separate DC power supply. The deposition rate, at constant target power, increased with DC target voltage due to increased ion energy reaching 34 nm/min at 500 V. All the films were polycrystalline and showed strong (400) and (222) reflections with the relative strength of latter increasing with target bias voltage. The resistivity was lowest at 500 V with values of 1.8 × 10^− 4 Ω cm and 2.4 × 10^− 4 Ω cm on glass and polyimide, respectively but was still less than 5 × 10^− 4 Ω cm at 400 V. All films were highly transparent to visible light, (> 80%) but the NIR transmittance decreased with increasing target voltage due to higher free carrier absorption. Therefore, ITO films can be deposited onto semiconductor layers such as in solar cells, with minimal ion damage while maintaining low resistivity.     

Effects of annealing temperature and heat-treatment duration on electrical properties of sol-gel derived indium-tin-oxide thin films

Transparent indium tin oxide (ITO) thin films have been deposited by the dip-coating process on silica substrates using solutions of 2,4-pentanedione, ethanol, indium and tin salts. The films have been first dried in air at 260 °C for 10 min and then annealed in a reducing atmosphere at different temperatures for various durations. The resistivity of ITO layers was found to decrease with increasing the metal concentration of the starting solution or the annealing temperature. Hence, by adjusting both metal concentration in the coating solution and heat-treatment, resistivities lower than 5 × 10^− 3 Ω cm for an annealing temperature of 550 °C and lower than 2 × 10^− 2 Ω cm for an annealing temperature of 300 °C, were obtained. These results are correlated with the density and the size of ITO grains in the films.     

Deposition of Al-doped ZnO thin-films with radio frequency magnetron sputtering for a source/drain electrode for pentacene thin-film transistor

Al-doped ZnO thin-films were deposited with the radio frequency magnetron sputtering technique at various temperatures and sputtering powers for a source/drain electrode in the pentacene thin-film transistor. With the increase in the deposition temperature and the decrease in the radio frequency sputtering power, the crystallinity was increased and the surface roughness was decreased, which lead to the decrease in the electrical resistivity of the film. Al-doped ZnO film deposited at 200 °C and sputtering power of 50 W showed a low resistivity (9.73 × 10⁴ μΩcm), high crystallinity, low roughness and uniform surface morphology. The pentacene thin-film transistor fabricated with Al-doped ZnO film as a source/drain electrode showed a device performance, (mobility: 7.89 × 10^− 3 cm²/Vs and on/off ratio: ~ 5 × 10⁴) which is comparable with an indium tin oxide electrode grown at room temperature.     

Effect of organic-buffer-layer on electrical property and environmental reliability of Ga-doped ZnO films prepared by RF plasma assisted DC magnetron sputtering on plastic substrate

Ga-doped ZnO (GZO) transparent conductive films have been prepared by RF plasma assisted DC magnetron sputtering under a reductive atmosphere on organic-buffer-layer (OBL) coated polyethylene telephthalate (PET) substrates without intentionally heating substrates. Electrical and optical properties, crystallinity, and environmental reliability of the GZO films have been investigated. The distributional characteristic of resistivity is observed in the GZO film deposited on the OBL-coated PET substrates. The high resistivity at facing the erosion area in the source target is reduced by providing the RF plasma and H₂ gas near the substrate, resulting in a uniform distribution of the sheet resistance. It has been also found that the increase of resistivity by an accelerated aging test performed under a storage condition at 60 °C and at a relative humidity of 95% is suppressed by employing the OBL. The OBL suppresses the formation of cracks, which are induced by the aging test. These facts are thought to contribute to a high environmental reliability of GZO films on PET substrates. Values of resistivity, Hall mobility and carrier concentration are obtained: 5.0-20 × 10⁻³ Ω cm, 4.0 cm²/Vs, and 3.8 × 10²⁰ cm⁻³, respectively. An average transmittance of the GZO film including OBL and PET substrate is 78% in a visible region. The OBL enables to realize the practical use of GZO films on PET sheets.     

Effects of substrate on the structural, electrical and optical properties of Al-doped ZnO films prepared by radio frequency magnetron sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on substrates including alkali-free glass, quartz glass, Si, and SiO₂ buffer layer on alkali-free glass by using radio frequency magnetron sputtering. The effects of different substrates on the structural, electrical and optical properties of the AZO films were investigated. It was found that the crystal structures were remarkably influenced by the type of the substrates due to their different thermal expansion coefficients, lattice mismatch and flatness. The AZO film (100 nm in thickness) deposited on the quartz glass exhibited the best crystallinity, followed sequentially by those deposited on the Si, the SiO₂ buffer layer, and the alkali-free glass. The film deposited on the quartz glass showed the lowest resistivity of 5.14 × 10^− 4 Ω cm among all the films, a carrier concentration of 1.97 × 10²¹ cm^− 3 and a Hall mobility of 6.14 cm²/v·s. The average transmittance of this film was above 90% in the visible light spectrum range. Investigation into the thickness-dependence of the AZO films revealed that the crystallinity was improved with increasing thickness and decreasing surface roughness, accompanied with a decrease in the film resistivity.     

Optical and electrical properties of ZnO films prepared by URT-IP method

Optical and electrical properties were studied on thin polycrystalline ZnO films (200-nm thick) deposited on glass substrates at 200 °C by a DC-arc ion plating method (URamoto-Tanaka-type ion plating method). Effects of the oxygen flow rate (OFR) on film properties were examined. The resistivity of undoped films changed from 4.2×10⁻³ to 9.6×10⁻¹ Ω cm, corresponding to the carrier concentration of 1.0×10²⁰-1.2×10¹⁸ cm⁻³, depending on the increase in OFR from 0 to 40 sccm. The Hall mobility tends to be the maximum value of 28 cm² (V s)⁻¹ at OFR of 10 sccm. Photoluminescence (PL) spectra exhibited a dominant near-band-edge (NBE) emission together with weak PL bands at 2.2 and 3.2 eV. Intensity of NBE was maximum at OFR of 10 sccm. Intensity of the PL band at 2.2 eV increased with increase in OFR. As a result of Ga-doping, the resistivity decreased and the carrier concentration increased by one order of magnitude. The optical transmittance was more than 90% in 400-1200 nm. The ZnO:Ga (3 and 4 wt.% Ga-doped) thin films with the lowest resistivity of 2.6×10⁻⁴ Ω cm, the highest mobility of 25 cm² (V s)⁻¹, and the highest PL intensity were obtained at OFR of 10 sccm. Further increase of OFR led to the decrease in both mobility and PL intensity.     

Low resistivity transparent conducting CdO thin films deposited by DC reactive magnetron sputtering at room temperature

Transparent conducting cadmium oxide (CdO) films were deposited on PET (polyethylene terephthalate) substrate by DC reactive magnetron sputtering at room temperature. All the films deposited at room temperature were polycrystalline in rock-salt structure. Dependences of the physical properties of the CdO films on the oxygen partial pressure were systematically studied. The films deposited at low oxygen flow rate were (200) oriented, while the films deposited at an oxygen flow rate greater than 20 sccm were (111) oriented. The average grain size of the CdO films decreased as the oxygen flow rate increases as determined by XRD and SEM. The Hall effect measurement showed that CdO films have high concentration, low resistivity, and high mobility. Both the mobility and the concentration of the carrier decreased with the increase of the oxygen flow rate. A minimum sheet resistance of 36.1 Ω/□, or a lowest resistivity of 5.44 × 10^− 4 Ω cm (6.21 × 10²⁰/cm³, μ = 19.2 cm²/Vs) was obtained for films deposited at an oxygen flow rate of 10 sccm.     

Improvements of spatial resistivity distribution in transparent conducting Al-doped ZnO thin films deposited by DC magnetron sputtering

The relationship between two techniques developed for improving the resistivity distribution on the substrate surface in transparent conducting Al-doped ZnO (AZO) thin films prepared at a temperature of 200 °C by dc magnetron sputtering depositions (dc-MSD) using various sintered AZO targets has been investigated. One improvement method superimposes an rf component onto the dc-MSD (rf + dc-MSD). The other improvement method uses conventional dc-MSD with a low resistivity AZO target prepared under optimized conditions. An improvement of resistivity distribution resulted from a decrease in the resistivity of targets used in the preparation of AZO thin films by dc-MSD either with or without superimposing rf power. However, the resistivity distribution of AZO thin films resulting from depositions using rf-superimposed dc-MSD with lower-resistivity targets was not significantly improved over that of AZO thin films prepared by conventional dc-MSD using targets with the same low resistivities. The use of rf superimposition only resulted in improved resistivity distribution in thin films when the AZO targets had a resistivity higher than around 1 × 10^− 3 Ω cm. It should be noted that sintered AZO targets optimized for the preparation of AZO thin films with lower resistivity as well as more uniform resistivity distribution on the substrate surface tended to exhibit a lower resistivity.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.     

Electrical, optical, and structural properties of InZnSnO electrode films grown by unbalanced radio frequency magnetron sputtering

We have investigated the electrical, optical, structural, and annealing properties of indium zinc tin oxide (IZTO) films prepared by an unbalanced radio frequency (RF) magnetron sputtering at room temperature, in a pure Ar ambient environment. It was found that the electrical and optical properties of unbalanced RF sputter grown IZTO films at room temperature were influenced by RF power and working pressure. At optimized growth condition, we could obtain the IZTO film with the low resistivity of 3.77 × 10^− 4 Ω cm, high transparency of ~ 87% and figure of merit value of 21.2 × 10^− 3Ω^− 1, without the post annealing process, even though it was completely an amorphous structure due to low substrate temperature. In addition, the field emission scanning electron microscope analysis results showed that all IZTO films are amorphous structures with very smooth surfaces regardless of the RF power and working pressure. However, the rapid thermal annealing process above the temperature of 400 °C lead to an abrupt increase in resistivity and sheet resistance due to the transition of film structure from amorphous to crystalline, which was confirmed by X-ray diffraction examination.     

ITO thin films deposited by advanced pulsed laser deposition

Indium tin oxide thin films were deposited by computer assisted advanced PLD method in order to obtain transparent, conductive and homogeneous films on a large area. The films were deposited on glass substrates. We studied the influence of the temperature (room temperature (RT)-180 °C), pressure (1-6 × 10^− 2 Torr), laser fluence (1-4 J/cm²) and wavelength (266-355 nm) on the film properties. The deposition rate, roughness, film structure, optical transmission, electrical conductivity measurements were done. We deposited uniform ITO thin films (thickness 100-600 nm, roughness 5-10 nm) between RT and 180 °C on a large area (5 × 5 cm²). The films have electrical resistivity of 8 × 10^− 4 Ω cm at RT, 5 × 10^− 4 Ω cm at 180 °C and an optical transmission in the visible range, around 89%.