Microstructural and optical properties of Ga-doped ZnO semiconductor thin films prepared by sol-gel process

Transparent thin films of Ga-doped ZnO (GZO), with Ga dopant levels that varied from 0 to 7 at.%, were deposited onto alkali-free glass substrates by a sol-gel process. Each spin-coated film was preheated at 300 °C for 10 min, and then annealed at 500 °C for 1 h under air ambiance. The effects of Ga dopant concentrations on crystallinity levels, microstructures, optical properties, and electrical resistivities of these ZnO thin films were systematically investigated. Photoluminescence spectra of GZO thin films were examined at room temperature. XRD results revealed that the undoped ZnO thin films exhibited a preferred orientation along the (002) plane and that the ZnO thin films doped with Ga showed degraded crystallinity. Experimental results also showed that Ga doping of ZnO thin films could markedly decrease surface roughness, improve transparency in the visible range, and produce finer microstructures than those of undoped ZnO thin films. The most promising films for transparent thin film transistor (TTFT) application produced in this study, were the 3 and 5 at.% Ga-doped ZnO thin films, both of which exhibited an average transmittance of 90.6% and an RMS roughness value of about 2.0 nm.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Effect of post-annealing on the optoelectronic properties of ZnO:Ga films prepared by pulsed direct current magnetron sputtering

In this study, highly conductive films of ZnO:Ga (GZO) were deposited by pulsed direct current magnetron sputtering to explore the effect of post-annealing on the structural, electrical and optical properties of the films. XRD patterns showed that after annealing, the intensity of c-axis preferentially oriented GZO (002) peak was apparently improved. GZO film annealing at 300 °C for 0.5 h exhibits lowest resistivity of 1.36 × 10^− 4 Ω cm. In addition, the film shows good optical transmittance of 88% with optical band gap, 3.82 eV. Carrier concentration and optical band gap both decreases with the annealing temperature. Besides, the near-infrared transmittance at 1400 nm is below 5%, while the reflectivity at 2400 nm is as high as 70%.     

Substrate temperature dependence of the properties of Ga-doped ZnO films deposited by DC reactive magnetron sputtering

Ga-doped zinc oxide (ZnO:Ga) transparent conductive films were deposited on glass substrates by DC reactive magnetron sputtering. The influence of substrate temperature on the structural, electrical, and optical properties of ZnO:Ga films was investigated. The X-ray diffraction (XRD) studies show that higher temperature helps to promote Ga substitution more easily. The film deposited at 350 °C has the optimal crystal quality. The morphology of the films is strongly related to the substrate temperature. The film deposited is dense and flat with a columnar structure in the cross-section morphology. The transmittance of the ZnO:Ga thin films is over 90%. The lowest resistivity of the ZnO:Ga film is 4.48×10⁻⁴ Ω cm, for a film which was deposited at the substrate temperature of 300 °C.     

Structural, electrical and optical properties of Ga-doped ZnO films on cyclo-olefin polymer substrates

Ga-doped ZnO (GZO) films with a thickness of 100 nm were prepared on cyclo-olefin polymer (COP) and glass substrates at various temperatures below 100 °C by ion plating with direct-current arc discharge. The dependences of the characteristics of GZO films on the substrate temperature Ts were investigated. All the polycrystalline GZO films, which exhibited a high average visible transmittance of greater than 86%, were crystallized with a wurtzite structure oriented along the c-axis. The lowest resistivities of the GZO films were 5.3 × 10^− 4 Ωcm on the glass substrate and 5.9 × 10^− 4 Ωcm on the COP substrate.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.     

Deposition of low-resistivity gallium-doped zinc oxide films by low-temperature radio-frequency magnetron sputtering

The transparent and conductive gallium-doped zinc oxide (GZO) film was deposited on 1737F Corning glass using the radio-frequency (RF) magnetron sputtering system with a GZO ceramic target. (The Ga₂O₃ contents are approximately 5 wt. %). In this study, the effect of the sputtering pressure on the structural, optical and electrical properties of GZO films upon the glass or polyester film (PET) substrate was investigated and discussed in detail. The GZO film was grown under a steady RF power of 400 W and a lower substrate temperature from room temperature up to 200 °C. The crystal structure and orientation of GZO thin films were examined by X-ray diffraction. All of the GZO films under various sputtering pressures had strong c-axis (002)-preferred orientation. Optical transparency was high (> 80%) over a wide spectral range from 380 nm to 900 nm. According to the experimental data, the resistivity of a single-layered GZO film was optimized at  8.3 × 10^− 4 Ω cm and significantly influenced by the sputtering pressure. In further research, the sandwich structure of the GZO film/Au metal/GZO film was demonstrated to improve the electrical properties of the single-layered GZO film. The resistivity of the sandwich-structured GZO film was around 2.8 × 10^− 4 Ω cm.     

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.     

Novel Ga-doped ZnO nanocrystal ink: Synthesis and characterization

Ga-doped ZnO (GZO) nanocrystals were synthesized via the hot-injection method for the first time. The characterizations of its structure, composition, morphology, and absorption properties were conducted by using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. The results indicated that GZO nanocrystals were single phase polycrystalline within a range of 5―10 nm. Optical measurements illustrated that GZO nanocrystals have a tunable band gap from 3.35 to 3.81 eV, depending on the Ga doping level. GZO nanocrystals were dispersed in nonpolar solvents to form a nanocrystal ink which could remain stable after a month of storage. The GZO thin film was fabricated by spin coating the GZO nanocrystal ink and annealing in air. The electrical resistivity of the film was measured to be 7.5 × 10⁻² Ω cm. This method, which eliminated the requirement of high vacuum and high temperature, was a promising alternative for transparent conducting oxide (TCO) fabrication.     

Ion sputter etching of ZnO:Ga thin film surfaces

In this article the modification of surface morphology of ZnO:Ga (GZO) thin films by ion sputter etching is presented. GZO thin films were prepared at room temperature on Corning glass substrates by both normal and oblique angle RF diode sputtering from ZnO:2%Ga ceramic target in Ar gas. Ion sputter etching was performed by RF re-sputtering of GZO thin films on substrates. During RF sputter etching, Ar pressure of 1.3 Pa and RF power of 250 W were kept constant, only the time of sputter etching was changed. Ion sputter etching had remarkable influence on surface morphology of GZO thin films: increase of roughness R_q and the “homogenization” of film surfaces, i.e. skewness (R_sk) and spikiness (R_ku) parameters (R_sk ≈ 0/R_ku ≈ 3).Surface root-mean-square roughness (R_q) increased from 15.3 nm (after sputter deposition) to 29.1 nm (after ion sputter etching). For obliquely thin films increased from 16.5 nm (after sputter deposition) to 38.2 nm. Changes of these parameters R_q, R_sk, R_ku influenced optical properties of GZO films, increased Haze parameter up to values 7.7% and width of optical band gap 3.44 eV, respectively.     

Heat generation properties of Ga doped ZnO thin films prepared by rf-magnetron sputtering for transparent heaters

Ga doped ZnO (GZO) thin films were prepared by rf-magnetron sputtering on glass substrate for window heater applications. Electrical and optical properties of these films were analyzed in order to investigate on substrate temperature and rf power dependencies. High quality GZO films with a resistivity of 1.30 × 10^− 4 Ω cm and a transparency above 90% in the visible range were able to be formed. GZO films have been patterned on glass substrate as a line heater. This GZO line heater showed the rapid heat radiation property from room temperature to 90 °C for 22 s at the applied voltage of 42 V. These results could provide a possibility to use GZO as effective transparent heaters.     

Low-temperature processing of transparent conductive indium tin oxide nanocomposites using polyvinyl derivatives

We report on the influence of additives on the electrical, optical, morphological and mechanical properties of transparent conductive indium tin oxide (In₂O₃:Sn; ITO) nanoparticle films by the use of polymers as matrix material. Key issues to fabricate layers suitable for use in electronic device applications are presented. Polyvinyl derivatives polyvinyl acetate, polyvinyl alcohol (PVA) and polyvinyl butyral were applied and their suitability to form transparent conductive ITO nanocomposite coatings at a maximum process temperature of 130 °C was investigated. A low-temperature treatment with UV-light has been developed to provide the possibility of curing ITO thin films deposited on substrates which do not withstand high process temperatures. Compared to best pure ITO layers (0.2 Ω^− 1 cm^− 1), the ITO-PVA nanocomposite coatings show a conductance value of 4.1 Ω^− 1 cm^− 1 and 5.9 Ω^− 1 cm^− 1 after reducing in forming gas. Sheet resistance of ca. 1200 Ω/□ with coexistent transmittance of 85% at 550 nm for a layer thickness of about 1.45 μm was achieved. The conductance enhancement is a consequence of nanoparticulate ITO network densification due to the acting shrinkage forces caused by the polymer matrix during film drying and additionally UV-induced crosslinking of PVA.     

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500 °C are measured to be about 3.6 × 10⁻⁴ Ω cm, 23.8 cm²/Vs and 7.1 × 10²⁰ cm³, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~ 0.9 nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85 V at 1 kHz. Low turn-on voltage of about 1.3 V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.     

Characteristics of transparent ZnO based thin film transistors with amorphous HfO2 gate insulators and Ga doped ZnO electrodes

Coplanar type transparent thin film transistors (TFTs) have been fabricated on the glass substrates. The devices consist of intrinsic ZnO, Ga doped ZnO (GZO), and amorphous HfO₂ for the semiconductor active channel layer, electrode, and gate insulator, respectively. GZO and HfO₂ layers were prepared by using a pulsed laser deposition (PLD) and intrinsic ZnO layers were fabricated by using an rf-magnetron sputtering. The transparent TFT exhibits n-channel, enhancement mode behavior. The field effect mobility, threshold voltage, and a drain current on-to-off ratio were measured to be 14.7 cm²/Vs, 2 V, and 10⁵, respectively. High optical transmittance (> 85%) in visible region makes ZnO TFTs attractive for transparent electronics.     

Effect of Thickness on the Properties of Ga-doped Nano-ZnO Thin Films Prepared by RF Magnetron Sputtering

Nano transparent conductive oxide (TCO) Ga-doped ZnO (GZO) thin films with thickness from 260 nm to 620 nm were prepared on glass substrates by RF magnetron sputtering from a powder target with 3 at.% Ga₂O₃. The substrate temperature was kept at 300 °C. The effect of thickness on the structural, electrical, and optical properties of GZO thin films was investigated. It shows that the nano-GZO films are dense and flat, and have polycrystalline structure with preferentially in the (002) orientation. With the increase of thickness, the crystallinity and the grain sizes of the films are improved, meanwhile the carrier concentration increases and the lowest resistivity of 3.685×10⁻³ Ω cm occurs in the 620 nm thick GZO film. The average optical transmittance of all the films is over 80% in the visible range. Decreasing the thickness, the optical transmission of the films increase, and the absorption edge shifts to shorter wavelength, which means the optical band gap is broadened.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Near infrared transparent conducting cadmium oxide deposited by MOCVD

Polycrystalline cadmium oxide has been deposited on to glass substrates for use as transparent conducting oxides for thin film photovoltaics. The films were deposited by metal organic chemical vapour deposition (MOCVD). The oxygen precursor was tertiary butanol, which was chosen to avoid the pre-reaction often seen for more reactive oxygen sources when combined with the cadmium source, dimethylcadmium. This combination of precursors promoted a surface reaction to yield high quality CdO films with large grain size. The cadmium oxide, deposited at 280 °C, yielded a minimum resistivity of 3.9 × 10^− 4 Ω cm whilst maintaining an average transmittance between 450 and 2500 nm of 80%. This exceptional near-IR transmittance coupled with good electrical conduction is well suited to match the infrared requirements of multi-junction photovoltaics designed to capture greater proportions of the solar spectrum.     

Effect of acetic acid on ZnO:In transparent conductive oxide prepared by ultrasonic spray pyrolysis

Undoped and indium doped zinc oxide (ZnO) transparent conductive oxide were prepared by a low-cost Ultrasonic Spray Pyrolysis. The influence of acetic acid on properties of the ZnO thin films was investigated. The complex formed by [CH₃COO⁻] and [Zn²⁺] in precursor solution was better for the growth of ZnO film. The acetic acid added in precursor solution can supply [CH₃COO⁻] for both [Zn²⁺] and [In³⁺] to form complexes. That made the [Zn²⁺] and [In³⁺] have similar statement, which can promote the indium doping in the ZnO films. The surface morphology, structural and electrical properties of the ZnO thin films were influenced by the acetic acid adding. The total transmittance of the ZnO thin films is above 80% in the wide wavelength region from 400 nm to 2000 nm.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.