Influence of Ar/O2 ratio on the properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering

Ga-doped zinc oxide (ZnO:Ga) transparent conductive films with highly (002)-preferred orientation were deposited on glass substrates by DC reactive magnetron sputtering method in Ar + O₂ ambience with different Ar/O₂ ratios. The structural, electrical, and optical properties were investigated by X-ray diffraction, Hall measurement, and optical transmission spectroscopy. The resistivity and optical transmittance of the ZnO:Ga thin films are of the order of 10^− 4 Ω cm and over 85%, respectively. The lowest electrical resistivity of the film is found to be about 3.58 × 10^− 4 Ω cm. The influences of Ar/O₂ gas ratios on the resistivity, Hall mobility, and carrier concentration were analyzed.     

An approach to enhanced acceptor concentration in ZnO:N films

Owing to the low doping concentration of nitrogen and strong compensation of intrinsic donors, the attainment of highly conductive p-type ZnO films remains one of the largest challenges for the application of ZnO. An approach has been proposed to increase the doping concentration of nitrogen in ZnO by exposing the ZnO:N films in the ambient of nitrogen plasma periodically in this paper. Hall measurements and photoluminescence spectroscopy indicate that this approach is effective in improving the hole concentration in ZnO films. Under the optimized conditions, a p-type ZnO film with a hole concentration of 1.68 × 10¹⁸ cm⁻³ has been achieved.     

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.     

Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition

Nanocrystalline gallium doped zinc oxide (ZnO:Ga) thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). A statistical design of experiments (DOE) was employed to optimize electrical conductivity. A carrier concentration of 5.5×10²⁰/cm³ and a mobility of 15 cm²/V s yielding a resistivity of 7.5×10⁻⁴ Ω cm resulted from the conditions of high pressure, rf power, and electrode gap. X-ray diffraction showed that gallium doping had a profound impact on film orientation. Atomic force microscopy (AFM) revealed that the films were nanostructured, with an average grain size of 80 nm and a surface roughness of ∼2 nm. This unique morphology benefited optical transmission, but limited electrical performance. Average transmission across the visible spectrum was ∼93% as scattering losses were minimized. Temperature dependent Hall and optical transmission measurements demonstrated that structural defects and ionized impurities were equal contributors to electron scattering.     

Improvement in moisture durability of ZnO transparent conductive films with Ga heavy doping process

Moisture durability of ZnO transparent conductive films was achieved with Ga heavy doping by off-axis type rf magnetron sputtering. The resistivity of 10.9 at.% Ga-doped ZnO was 1.3 × 10⁻³ Ωcm and changed less than 5% of resistivity over a 9400-h reliability test at a temperature of 85 °C and humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicated that the c-axis was oriented in various directions as well as the perpendicular direction to the substrate surface. The heavily doped Ga disorders the crystal growth of ZnO films and forms a different crystal structure from conventional ZnO.     

Investigations on the structural, optical and electrical properties of Nb-doped SnO2 thin films

Niobium-doped tin oxide thin films were deposited on glass substrates by the chemical spray pyrolysis method at a substrate temperature of 400 °C. Effects of Nb doping on the structural, electrical and optical properties have been investigated as a function of niobium concentration (0–2 at.%) in the spray solution. X-ray diffraction patterns showed that the films are polycrystalline in nature and the preferred growth direction of the undoped film shifts to (200) for Nb-doped films. Atomic force microscopy study shows that the surface morphology of these films vary when doping concentration varies. The negative sign of Hall coefficient confirmed the n-type conductivity. Resistivity of ~4.3 × 10⁻³ Ω cm, carrier concentration of ~5 × 10¹⁹ cm⁻³, mobility of ~25 cm² V⁻¹ s⁻¹ and an average optical transmittance of ~70% in the visible region (500–800 nm) were obtained for the film doped with 0.5 at.% niobium.     

Low temperature synthesis of sol–gel derived Al-doped ZnO thin films with rapid thermal annealing process

A series of sol–gel derived Al-doped ZnO (AZO) thin films with rapid thermal annealing process at low temperature were studied to examine the influence of annealing temperature and the Al doping concentration on their microstructure, electrical and optical transport properties. Crystalline AZO thin films were obtained following an annealing process at temperatures between 400 and 600 °C for 10 min in argon gas ambient. AZO thin films with Al doping of 1 at% were oriented more preferentially along the (002) direction, and have larger grain size and lower electrical resistivity, while the highest average optical transmittances of 92% were observed in AZO films with Al doping of 2 at%. With the annealing temperature increasing from 400 to 600 °C, the grain size of AZO films increased, the optical transmittance became higher, and the electrical resistivity decreased to a lowest value of 1.2 × 10⁻⁴ Ω cm resulting from the increase of the carrier concentration and the mobility.     

Characterizations of gallium-doped ZnO films on glass substrate prepared by atmospheric pressure metal-organic chemical vapor deposition

Ga-doped zinc oxide (ZnO:Ga) films were grown on glass substrate by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as an n-type dopant gas. The structural, electrical and optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga-doping plays an important role in forming microstructures in ZnO films. A smooth surface with a predominant orientation of (101) was obtained for the ZnO:Ga film grown at a flow rate of TEG = 7.5 sccm. Moreover, a lowest resistivity of 3.6 × 10^− 4 Ω cm and a highest mobility of 30.4 cm² V^− 1 s^− 1 were presented by the same sample, as evaluated by Hall measurement. Otherwise, as the flow rate of TEG was increased, the average transmittance of ZnO:Ga films increased from 75% to more than 85% in the wavelength range of 400-800 nm, simultaneously with a blue-shift in the absorption edge. The results obtained suggest that low-resistivity and high-transparency ZnO films can be obtained by AP-MOCVD using Ga-doping sufficiently to make the films grow degenerate and effect the Burstein-Moss shift to raise the band-gap energy from 3.26 to 3.71 eV.     

Effect of substrate temperature on structural, optical and electrical properties of ZnO thin films deposited by pulsed laser deposition

ZnO thin films were grown by the pulse laser deposition (PLD) method using Si (100) substrates at various substrate temperatures. The influence of the substrate temperature on the structural, optical, and electrical properties of the ZnO thin films was investigated. All of the thin films showed c-axis growth perpendicular to the substrate surface. At a substrate temperature of 500 °C, the ZnO thin film showed the highest (002) peak with a full width at half maximum (FWHM) of 0.39°. The X-ray Photoelectron Spectroscopy (XPS) study showed that Zn was in excess irrespective of the substrate temperature and that the thin film had a nearly stoichiometrical composition at a substrate temperature of 500 °C. The photoluminescence (PL) investigation showed that the narrowest UV FWHM of 15.8 nm and the largest ratio of the UV peak to the deep-level peak of 32.9 were observed at 500 °C. Hall effect measurement systems provided information about the carrier concentration, mobility and resistivity. At a substrate temperature of 500 °C, the Hall mobility was the value of 37.4 cm²/Vs with carrier concentration of 1.36 × 10¹⁸ cm⁻³ and resistivity of 2.08 × 10⁻¹ Ω cm.     

Electrical and optical properties of transparent conducting tin doped ZnO thin films

Electrical, optical and structural properties of tin doped ZnO thin films were investigated for various tin (Sn) doping concentrations. Sol gel method was used to deposit the films on microscopic glass slides and silicon substrate. UV–Visible spectrometer analysis showed excellent optically transparent oscillating natures with transparency above 85% in the visible range. Band gap of 3.24 eV was deduced for Sn doping concentration of 4 at% using envelope method. Scanning electron microscopy (SEM) was employed to study the morphology of the films. Crystallinity of the film was investigated by X-Ray diffraction (XRD), which revealed polycrystalline nature with orientation towards c-axis. Resistivity of 3.11 Ω-cm with minimum stress value of 8.11 × 10⁻³ MPa was measured for Sn doping concentration of 4 at%.     

Structural and opto-electric properties of Ga and F codoped ZnO thin films on PC substrates

Ga–F codoped ZnO (GFZO) thin films were firstly prepared on polycarbonate (PC) substrates by MF magnetron sputtering and the results were compared in detail with the Al doped ZnO (AZO) thin films. The influence of dopants on the structural and optoelectric properties of ZnO films was studied. X-ray diffraction and scanning electron microscopy results show that the cracks formed in both GFZO and AZO films due to the high residual stress, originating from the different thermal expansion coefficients between film/substrate. However, the GFZO films show better crystallinity and deliver larger grain size than AZO. A relative low resistivity of 1.4 × 10⁻³ Ω cm and a transmittance of 81% for the GFZO thin films are achieved, better than that of 2.3 × 10⁻² Ω cm and 75% for the AZO films. The results illustrate that the combined effects of Ga–F codoping and smaller crack density can optimise the opto-electric properties of ZnO based thin films.     

Microstructure and properties of Al-doped ZnO thin films by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing

A series of Al-doped ZnO (AZO) thin films deposited by nonreactive DC magnetron sputtering at room temperature following rapid thermal annealing was studied to examine the influence of these Al doping concentration, sputtering power and annealing temperature on their microstructure, electrical and optical transport properties. AZO thin films with Al dopant of 3 wt% were oriented more preferentially along the (002) direction, bigger grain size and lower electrical resistivity The resistivity of AZO films decreases with the increase of Al content from 1 to 3 wt%, sputtering power from 60 to 100 W and the annealing temperature from 50 to 250 °C. Sputtering power and annealing had some effect on the average transmittance of AZO thin films. For AZO thin films with Al doping level of 3 wt%, the lowest electrical resistivity of 5.3 × 10⁻⁴ Ω cm and the highest optical transmittance of 88.7% could gain when the sputtering power was 100 W and the annealing temperature was 200 °C or above.     

Physical properties of CdS:Ga thin films synthesized by spray pyrolysis technique

This paper reports the investigation of physical properties of CdS:Ga thin films grown for the first time by a simple spray pyrolysis method as a function of Ga-doping level from 0 to 8 at.%. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive photoelectron spectroscopy, transmittance, photoluminescence, Hall effect and resistivity measurements are utilized to search for the structural, morphological, chemical, optical and electrical properties of as-prepared samples. XRD data confirm the presence of hexagonal structure with a strong (101) preferred orientation. SEM results show that the surface morphology varies significantly via Ga-doping, particularly 6 at.% doping level. Optical transparency is improved by the lower Ga-doping (2 and 4 at.%) whereas higher doping concentration (6 and 8 at.%) causes a poor transmission in the visible region. With respect to CdS (2.42 eV), the calculated band gap values at first enhances for 2 at.% Ga-doping and reaches to 2.43 eV. But, further increase in Ga-doping amount leads to a drop in the band gap value (2.39 eV) for 8 at.% Ga-doping. Electrical analyses display that 2 at.% Ga-doped CdS thin films exhibit a maximum carrier density and a minimum resistivity that are related to the substitutional incorporation of Ga³⁺ ions at Cd²⁺ ions. However, higher doping of Ga atoms into CdS gives rise to a gradual diminish in the carrier concentration and a rise in the resistivity. Based on all the data, it should be concluded that 2 at.% Ga-doped CdS thin films exhibit the best optical and electrical properties that can be used in the optoelectronic applications.     

Effect of zinc addition and vacuum annealing time on the properties of spin-coated low-cost transparent conducting 1 at% Ga–ZnO thin films

Abstract

Pure and 1 at% gallium (Ga)-doped zinc oxide (ZnO) thin films have been prepared with a low-cost spin coating technique on quartz substrates and annealed at 500 °C in vacuum ～10^?3 mbar to create anion vacancies and generate charge carriers for photovoltaic application. Also, 0.5–1.5 at% extra zinc species were added in the precursor sol to investigate changes in film growth, morphology, optical absorption, electrical properties and photoluminescence. It is shown that 1 at% Ga–ZnO thin films with 0.5 at% extra zinc content after vacuum annealing for 60 min correspond to wurtzite-type hexagonal structure with (0001) preferred orientation, electrical resistivity of ～9 × 10^?3 Ω cm and optical transparency of ～65–90% in the visible range. Evidence has been advanced for the presence of defect levels within bandgap such as zinc vacancy (V_Zn), zinc interstitial (Zn_i), oxygen vacancy (V_o) and oxygen interstitial (O_i). Further, variation in ZnO optical bandgap occurring with Ga doping and insertion of additional zinc species has been explained by invoking two competing phenomena, namely bandgap widening and renormalization, usually observed in semiconductors with increasing carrier concentration.     

Effects of growth variables on the properties of single crystalline ZnO thin film grown by inductively coupled plasma metal organic chemical vapor deposition

Single crystalline undoped and Ga-doped n-type zinc oxide (ZnO) films were grown on sapphire (Al₂O₃) substrates by inductively coupled plasma (ICP) metal organic chemical vapor deposition. Effects of growth variables on the structural, optical, and electrical properties of ZnO films have been studied in detail. Single crystal films with flat and smooth surfaces were reproducibly obtained, with application of sample bias and O₂ ICP. The best film properties were obtained at the growth condition of 650 °C, 400 W ICP power, − 94 V bias voltage, O/Zn (VI/II) ratio of 75. Single crystalline Ga doped n-ZnO films were also obtained, with free carrier concentration of about 1.5 × 10¹⁹/cm³ at 1 at.% Ga concentration.     

Fabrication of n-Zn1−xGaxO/p-(ZnO)1−x(GaP)x thin films and homojunction

Ga doped ZnO (GZO) and GaP codoped ZnO (GPZO) thin films of different concentrations (1–4 mol%) have been grown on sapphire substrates by RF sputtering for the fabrication of ZnO homojunction. The grown films have been characterized by X-ray diffraction (XRD), photoluminescence (PL), Hall measurement, energy dispersive spectroscopy (EDS), time-of-flight secondary ion mass spectrometer (ToF-SIMS), UV–Vis–NIR spectroscopy and atomic force microscopy (AFM). Unlike in conventional codoping, here we directly doped (codoped) GaP into ZnO to realize p-ZnO. The Hall measurements indicate that 2 and 4% GPZO films exhibit p-conductivity due to the sufficient amount of phosphorous incorporation while all the monodoped GZO films showed n-conductivity as expected. Among the p-ZnO films, 2% GPZO film shows low resistivity (2.17 Ωcm) and high hole concentration (1.8 × 10¹⁸ cm^?3) by optimum incorporation of phosphorous due to best codoping. Similarly, among the n-type films, 2% GZO shows low resistivity (1.32 Ωcm) and high electron concentration (2.02 × 10¹⁹ cm^?3) by optimum amount of Ga incorporation. The blue shift and red shift in NBE emission observed from PL acknowledged the formation of n- and p-conduction in monodoped and codoped films, respectively. The neutral acceptor bound exciton recombination (A⁰X) observed by low temperature PL for 2% GPZO confirms the p-conductivity. Further, the high concentration of P atoms than Ga observed from ToF-SIMS (2% GPZO) also supports the p-conductivity of the films. The fabricated p–n junction with best codoped p-(ZnO)_0.98(GaP)_0.02 and best monodoped n-Zn_0.98Ga_0.02O films showed typical rectification behavior of a diode. The diode parameters have also been estimated for the fabricated homojunction.     

Preparation of wide gap Cu(In,Ga)S2 films on ZnO coated substrates

We have prepared Cu(In,Ga)S₂ films at growth temperatures from 300 °C to 580 °C with a homogeneous gallium depth distribution (estimated band gap 1.67 eV) onto soda lime glass (SLG) substrates with one of three different kinds of back contact: Mo(1000 nm), ZnO(500 nm), and Mo(30 nm)/ZnO(500 nm), respectively. We have also investigated the depth profiles of Zn and Na (diffused from SLG) in Cu(In,Ga)S₂ films by secondary ion mass spectroscopy (SIMS). The efficiency of solar cells on Mo increases with increasing growth temperature. It is higher on Mo/ZnO than on ZnO, and increases from 350 °C to 450 °C, then decreases above 450 °C. It was observed by SIMS that the amount of Zn in Cu(In,Ga)S₂ on Mo/ZnO is lower than it is on ZnO up to 450 °C, and a large amount of Zn diffuses into absorbers over 450 °C, which contributes to decreasing efficiency. The amount of Na in the back contact increases with growth temperature. The depth distribution of Na in Cu(In,Ga)S₂ films on Mo is almost constant in the order of 10¹⁷-10¹⁸ cm^− 3, on ZnO and Mo/ZnO the Na concentration increases towards the surface and is in the range of 10¹⁵-10¹⁷ cm^− 3.     

Structure and optical properties of ZnO:V thin films with different doping concentrations

A series of ZnO thin films doped with various vanadium concentrations were prepared on glass substrates by direct current reactive magnetron sputtering. The results of the X-ray diffraction (XRD) show that the films with doping concentration less than 10 at.% have a wurtzite structure and grow mainly along the c-axis orientation. The residual stress, estimated by fitting the XRD diffraction peaks, increases with the doping concentration and the grain size also has been calculated from the XRD results, decreases with increasing the doping concentration. The surface morphology of the ZnO:V thin films was examined by SEM. The optical constants (refractive index and extinction coefficient) and the film thickness have been obtained by fitting the transmittance. The optical band gap changed from 3.12 eV to 3.60 eV as doping concentration increased from 1.8 at.% to 13 at.% mol. All the results have been discussed in relation with doping concentration.     

Cu(In,Ga)Se2 films prepared by sputtering with a chalcopyrite Cu(In,Ga)Se2 quaternary alloy and In targets

This study reports the successful preparation of Cu(In,Ga)Se₂ (CIGS) thin film solar cells by magnetron sputtering with a chalcopyrite CIGS quaternary alloy target. Bi-layer Mo films were deposited onto soda lime glass. A CIGS quaternary alloy target was used in combination with a stack indium target for compensating the loss of indium during annealing process. A one-stage annealing process was performed to form CIGS chalcopyrite phase. Experimental results show that the optimal adhesion strength, residual stress, and resistivity were obtained at a thickness ratio of 67% of bi-layer Mo films and a working pressure of 0.13 Pa. The CIGS precursor was layered through selenization at 798 K for 20 min. The stoichiometry ratios of the CIGS film were Cu/(In + Ga) = 0.91 and Ga/(In + Ga) = 0.23, which approached the device-quality stoichiometry ratio (Cu/(In + Ga) <0.95, and Ga/(In + Ga) <0.3). The resistivity of the sample was 11.8 Ωcm, with a carrier concentration of 3.6 × 10¹⁷ cm⁻³ and mobility of 1.45 cm²V⁻¹s⁻¹. The resulting film exhibited p-type conductivity.     

Co-electrodeposition and Characterization of Cu (In, Ga)Se2 thin films

Cu (In, Ga)Se₂(CIGS) thin films were electrodeposited on Mo-coated soda lime glass substrate by the electrodeposition technique. The chemical bath for co-electrodeposition was prepared from copper chloride, indium chloride, gallium chloride and selenous acid. The effect of different chemical bath concentration of the CIGS films on the microstructure and electric properties has been investigated. The microstructure and morphology of the selenized CIGS thin films were investigated by X-ray diffraction and scanning electron microscopy. The composition of the selenized CIGS thin films were characterized by energy dispersive spectroscopy. Hall coefficient, Conductivity and Mobility of the selenized CIGS thin films were measured by ACCENT HL5500 Hall System. The results indicate CIGS thin films deviate little from the ideal stoichiometric one and single chalcopyrite structure. At room temperature, electrical conductivity, Hall mobility and charge-carrier concentration of the films vary from 49.63 to 64.56 ( cm)^‒1,271 to 386 cm²V^‒1 s^‒1, 8.026 × 10¹⁷ to 1.4 87×10¹⁸ cm^‒3, respectively, and are dependent on the composition of the films.