Investigation of p-type behavior in Ag-doped ZnO thin films by E-beam evaporation

Ag-doped ZnO (ZnO:Ag) thin films were grown on glass substrates by E-beam evaporation technique. The structural, electrical and optical properties of the films were investigated as a function of annealing temperature. The films were subjected to post annealing at different temperatures in the range of 350-650 °C in an air ambient. All the as grown and annealed films at temperature of 350 °C showed p-type conduction. The films lost p-type conduction after post annealing treatment temperature of above 350 °C, suggesting a narrow post annealing temperature window for the fabrication of p-type ZnO:Ag films. ZnO:Ag film annealed at 350 °C revealed lowest resistivity of 7.25 × 10⁻² Ω cm with hole concentration and mobility of 5.09 × 10¹⁹ cm⁻³ and 1.69 cm²/V s, respectively. Observation of a free-to-neutral-acceptor (e,A^o) and donor-acceptor-pair (DAP) emissions in the low temperature photoluminescence measurement confirms p-type conduction in the ZnO:Ag films.     

Preparation and characteristics of transparent p-type ZnO film by Al and N co-doping method

Al-N co-doped ZnO films were fabricated by gaseous ammonia annealing at various temperatures. The structure and the electrical properties of Al-N-doped ZnO films strongly depend on the annealing temperature. XRD and SEM analysis indicate that the ZnO films possess a good crystallinity with c-axis orientation, uniform thickness and dense surface. Optical transmission spectra show a high transmittance (∼85%) in the visible region. Hall measurement demonstrates that ZnO films have p-type conduction with high carrier concentration of 8.3 × 10¹⁸ cm⁻³ and low resistivity of 25.0 Ω cm when the annealing temperature is 700 °C. Also the growth process of Al-N co-doped at various temperatures is discussed in detail.     

Epitaxial growth of Sc-doped ZnO films on Si by sol-gel route

The epitaxial growth of doped ZnO films is of great technological importance. Present paper reports a detailed investigation of Sc-doped ZnO films grown on (1 0 0) silicon p-type substrates. The films were deposited by sol-gel technique using zinc acetate dihydrate as precursor, 2-methoxyethanol as solvent and monoethanolamine (MEA) as a stabilizer. Scandium was introduced as dopant in the solution by taking 0.5 wt%¹ of scandium nitrate hexahydrate. The effect of annealing on structural and photoluminescence properties of nano-textured Sc-doped films was investigated in the temperature range of 300-550 °C. Structural investigations were carried out using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction study revealed that highly c-axis oriented films with full-width half maximum of 0.21° are obtained at an annealing temperature of 400 °C. The SEM images of ZnO:Sc films have revealed that coalescence of ZnO grains occurs due to annealing. Ostwald ripening was found to be the dominant mass transport mechanism in the coalescence process. A surface roughness of 4.7 nm and packing density of 0.93 were observed for the films annealed at 400 °C. Room temperature photoluminescence (PL) measurements of ZnO:Sc films annealed at 400 °C showed ultraviolet peak at about (382 nm) with FWHM of 141 meV, which are comparable to those found in high-quality ZnO films. The films annealed below or above 400 °C exhibited green emission as well. The presence of green emission has been correlated with the structural changes due to annealing. Reflection high energy electron diffraction pattern confirmed the nearly epitaxial growth of the films.     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

Induction of p-type conduction in sputtered deposited Al-N codoped ZnO thin films

Al and N codoped ZnO thin films were grown on n-Si (100) substrate by sputtering technique. Hall effect measurements of as-grown films exhibited n-type conduction, however 500 °C Ar annealed codoped films showed p-type conductivity with a hole concentration of 9.9 × 10¹⁶ cm^− 3, resistivity of 15.95 Ω-cm and hole mobility of 3.95 cm²/Vs, respectively. Codoped ZnO thin films were found to be highly c-axis oriented with good crystal quality. A neutral acceptor-bound exciton and donor-acceptor-pair emissions that appeared at room temperature photoluminescence measurement verify p-type conduction in Al and N codoped ZnO film. The current-voltage characteristics of p-n heterojunction evidently showed a diode like rectifying behaviour.     

Structural and electrical properties of nitrogen and aluminum codoped p-type ZnO films

To resolve the problem of p-type doping in ZnO, nitrogen and aluminum (N-Al) codoped ZnO films were prepared by the ultrasonic spray pyrolysis (USP) technique. The structural and electrical properties of N-Al codoped ZnO films were investigated. The results demonstrate that the undoped ZnO films exhibit the preferential orientation of (002) plane, while ZnO films show high orientation of (101) plane after codoping with N and Al. The N-Al codoped ZnO films under optimum conditions show p-type conduction, with a low resistivity of 1.7×10⁻²Ω cm, carrier concentration of 5.09×10¹⁸ cm⁻³ and high Hall mobility of 73.6 cm² V⁻¹ s⁻¹. A conversion from p-type conduction to n-type was observed during the increase of measurement temperature.     

Ag-N doped ZnO film and its p-n junction fabricated by ion beam assisted deposition

Ag-N doped ZnO film was synthesized by ion beam assisted deposition and its electrical properties and annealing property were investigated. The films remained p-type even after annealing at 400 °C in air for 10 min. While the annealing temperature went up to 500 °C, the conduction type of these films shifted from p-type to n-type. The p-type ZnO film revealed low resistivity (0.0016 Ω cm), low Hall mobility (0.65 cm² V⁻¹ s⁻¹) and high carrier concentration (5.8 × 10²⁰ cm⁻³). ZnO p-n homojunction consisting of a p-type layer (Ag-N doped ZnO film) and an n-type layer (In-doped ZnO film) had been fabricated by ion beam assisted deposition. With electrical measurement, its current-voltage curve had a typical rectifying characteristic with current rectification ratio of 25 at bias ±5 V and a reverse current of 0.01 mA at −5 V. The depletion width was estimated 3.8 nm by using p-n junction equation.     

Effects of growth temperature on Li-N dual-doped p-type ZnO thin films prepared by pulsed laser deposition

Li-N dual-doped p-type ZnO (ZnO:(Li,N)) thin films have been prepared by pulsed laser deposition. The introduction of Li and N was confirmed by secondary ion mass spectrometry measurements. The structural, electrical, and optical properties as a function of growth temperature were investigated in detail. The lowest room-temperature resistivity of 3.99 Ω cm was achieved at the optimal temperature of 450 °C, with a Hall mobility of 0.17 cm²/V s and hole concentration of 9.12 × 10¹⁸ cm⁻³. The ZnO:(Li,N) films exhibited good crystal quality with a complete c-axis orientation, a high transmittance (about 90%) in the visible region, and a predominant UV emission at room temperature. The two-layer-structure p-ZnO:(Li,N)/n-ZnO homojunctions were fabricated on a sapphire substrate. The current-voltage characteristics exhibited the rectifying behavior of a typical p-n junction.     

Fabrication and characterization of magnetron sputtered arsenic doped p-type ZnO epitaxial thin films

Arsenic doped p-type ZnO thin films were grown on sapphire substrate by magnetron sputtering. As grown films reveal p-type conduction confirmed by Hall-effect and photoluminescence measurements. The p-type film with a hole concentration of 2.16× 10¹⁷ cm⁻³, mobility of 1.30 cm²/V.s and resistivity of 22.29 Ω-m were obtained at substrate temperature of 700 °C. ZnO homojunction synthesized by in-situ deposition of As doped p-ZnO layer on Al doped n-ZnO layer showed p-n diode like characteristics. X-ray pole figure and Transmission Electron Microscope studies confirm epitaxial nature of the films. Photoluminescence results exhibit the peaks associated with donor acceptor pair emission.     

Post-annealing influence on properties of N-In codoped ZnO thin films prepared by ion beam enhanced deposition method

N-In codoped ZnO thin films were prepared by ion beam enhanced deposition method (IBED) and were annealed in nitrogen and oxygen ambient after deposition. The influence of post-annealing on structure, electrical and optical properties of thin films were investigated. As-deposited and all post-annealed samples showed preferential orientation along (0 0 2) plane. Electrical property studies indicated that the as-deposited ZnO film showed p-type with a sheet resistance of 67.5 kΩ. For ZnO films annealed in nitrogen with the annealing temperature increasing from 400 to 800 °C, the conduction type of the ZnO film changed from p-type to n-type. However, for samples annealed in oxygen the resistance increased sharply even at a low annealing temperature of 400 °C and the conduction type did not change. Room temperature PL spectra of samples annealed in N₂ and in O₂ showed UV peak located at 381 and 356 nm, respectively.     

Effect of different annealing temperature on Sb-doped ZnO thin films prepared by pulsed laser deposition on sapphire substrates

Influence of annealing temperature on the properties of Sb-doped ZnO thin films were studied. Hall measurement results indicated that the Sb-doped ZnO annealed at 950 °C was p-type conductivity. X-ray diffraction (XRD) results indicated that the Sb-doped ZnO thin films prepared at the experiments are high c-axis oriented. It was worth noting that p-type sample had the worst crystallinity. The measurements of low-temperature photoluminescence (PL) spectra indicate that the sample annealed at the temperatures of 950 °C showed strong acceptor-bound exciton (A⁰X) emission, and confirmed that it is related to Sb-doping by comparing with the undoped ZnO low-temperature PL spectrum.     

Effects of crystalline quality on the ultraviolet emission and electrical properties of the ZnO films deposited by magnetron sputtering

The ZnO films were deposited on c-plane sapphire, Si (0 0 1) and MgAl₂O₄ (1 1 1) substrates in pure Ar ambient at different substrate temperatures ranging from 400 to 750 °C by radio frequency magnetron sputtering. X-ray diffraction, photoluminescence and Hall measurements were used to evaluate the growth temperature and the substrate effects on the properties of ZnO films. The results show that the crystalline quality of the ZnO films improves with increasing the temperature up to 600 °C, the crystallinity of the films is degraded as the growth temperature increasing further, and the ZnO film with the best crystalline quality is obtained on sapphire at 600 °C. The intensity of the photoluminescence and the electrical properties strongly depend on the crystalline quality of the ZnO films. The ZnO films with the better crystallinity have the stronger ultraviolet emission, the higher mobility and the lower residual carrier concentration. The effects of crystallinity on light emission and electrical properties, and the possible origin of the n-type conductivity of the undoped ZnO films are also discussed.     

Fabrication of high hole-carrier density p-type ZnO thin films by N-Al co-doping

In order to obtain p-type ZnO thin films, effect of atomic ratio of Zn:N:Al on the electronic and structural characteristic of ZnO thin films was investigated. Hall measurement indicated that with the increase of Al doping, conductive type of as-grown ZnO thin films changed from n-type to p-type and then to n-type again, reasons are discussed in details. Results of X-ray diffraction revealed that co-doped ZnO thin films have similar crystallization characteristic (0 0 2 preferential orientation) like that of un-doping. However, SEM measurement indicated that co-doped ZnO thin films have different surface morphology compared with un-doped ZnO thin films. p-type ZnO thin films with high hole concentration were obtained on glass (4.6 × 10¹⁸ cm⁻³) and n-type silicon (7.51 × 10¹⁹ cm⁻³), respectively.     

Rapid thermal annealing induced changes on the contact of Ni/Au to N-doped ZnO

N-doped p-type ZnO (p ∼ 10¹⁸cm^-3) was grown on sapphire(0 0 0 1) substrate by metal-organic chemical vapor deposition method. Ni/Au metal was evaporated on the ZnO film to form contacts. As-deposited contacts were rectifying while ohmic behavior was achieved after thermally annealing the contacts in nitrogen environment. Specific contact resistance was determined by circular transmission line method and a minimum specific contact resistance of 8 × 10⁻⁴ Ω cm² was obtained for the sample annealed at 650 °C for 30 s. However, Hall effect measurements indicate that, as the rapid thermal annealing temperature increased up to 550 °C or higher the samples’ conductive type have changed from p-type to n-type, which may be due to the instability nature of the present-day p-type N-doped ZnO or the dissociation of ZnO caused by annealing process in N₂ ambient. Evolution of the sample's electric characteristics and the increment of metal/semiconductor interface states induced by rapid thermal annealing process are supposed to be responsible for the improvement of electrical properties of Au/Ni/ZnO.     

Improvement of electrical and optical properties of Ga and N co-doped p-type ZnO thin films with thermal treatment

Ga and N co-doped p-type ZnO thin films were epitaxially grown on sapphire substrate using magnetron sputtering technique. The process of synthesized Ga and N co-doped ZnO films was performed in ambient gas of N₂O. Hall measurement shows a significant improvement of p-type characteristics with rapid thermal annealing (RTA) process in N₂ gas flow, where more N acceptors are activated. The film rapid thermal annealed at 900 °C in N₂ ambient revealed the highest carrier concentration of 9.36 × 10¹⁹ cm⁻³ and lowest resistivity of 1.39 × 10⁻¹ Ω cm. In room and low temperature photoluminescence measurements of the as grown and RTA treated film, donor acceptor pair emission and exciton bound to acceptor recombination at 3.25 and 3.357 eV, respectively, were observed.     

ZnO thin films on Si(1 1 1) grown by pulsed laser deposition from metallic Zn target

ZnO thin films with highly c-axis orientation have been fabricated on p-type Si(1 1 1) substrates at 400 °C by pulsed laser deposition (PLD) from a metallic Zn target with oxygen pressures between 0.1 and 0.7 mbar. Experimental results indicate that the films deposited at 0.3 and 0.5 mbar have better crystalline and optical quality and flatter surfaces than the films prepared at other pressures. The full width at half maximum (FWHM) of (0 0 0 2) diffraction peak decreases remarkably from 0.46 to 0.19° with increasing annealing temperature for the film prepared at 0.3 mbar. In photoluminescence (PL) spectra at room temperature, the annealed film at 700 °C exhibits a smaller ultraviolet (UV) peak FWHM of 108 meV than the as-grown film (119 meV). However, an enhanced deep-level emission is observed. Possible origins to above results are discussed.     

Influence of substrate temperature on electrical and optical properties of p-type semitransparent conductive nickel oxide thin films deposited by radio frequency sputtering

Nickel oxide thin films were deposited on fused silica and Si(1 0 0) substrates at different substrate temperatures ranging from room temperature to 400 °C using radio frequency reactive magnetron sputtering from a Ni metal target in a mixture of O₂ and Ar. With the increase of substrate temperature, nickel oxide films deposited on the Si substrates exhibit transition from amorphous to poly-crystalline structures with different preferred orientations of NiO(2 0 0) and (1 1 1). The films deposited at higher temperature exhibit higher Ni²⁺/Ni³⁺ ratio. With substrate temperature increasing from room temperature to 400 °C, the electrical resistivities of nickel oxide films increase from (2.8 ± 0.1) × 10⁻² to (8.7 ± 0.1) Ω cm, and the optical band-gap energies increase from 3.65 to 3.88 eV. A p-nickel oxide/n-zinc oxide heterojunction was fabricated to confirm the p-type conduction of nickel oxide thin film, which exhibited a steadily rectifying behavior.     

Preparation of CuInGeSe4 thin films by selenization method using the Cu-In-Ge evaporated layer precursors

CuInGeSe₄ quaternary compounds are known to have a chalcopyrite-like structure and have band gaps of about 1.3 eV, suitable for optimum conversion efficiency for solar cells. We have prepared the CuInGeSe₄ thin films by the selenization method using the Cu-In-Ge evaporated layer precursors. The analyses of X-ray diffraction show that the single phase of CuInGeSe₄ is obtained by the selenization of precursors at 450-500 °C. The SEM observation of film surface shows that the grain sizes are in the order of 1-2 μm. The band gaps of selenized films close to 1.6 eV are wider than that of bulk crystals (about 1.3 eV). These films have p-type conduction and higher electrical resistivities than more 10⁵ Ω cm at room temperature.     

The effect of post-annealing on surface acoustic wave devices based on ZnO thin films prepared by magnetron sputtering

Zinc oxide (ZnO) thin films were deposited on unheated silicon substrates via radio frequency (RF) magnetron sputtering, and the post-deposition annealing of the ZnO thin films was performed at 400 °C, 600 °C, 800 °C, and 1000 °C. The characteristics of the thin films were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The films were then used to fabricate surface acoustic wave (SAW) resonators. The effects of post-annealing on the SAW devices are discussed in this work. Resulting in the 600 °C is determined as optimal annealing temperature for SAW devices. At 400 °C, the microvoids exit between the grains yield large root mean square (RMS) surface roughness and higher insertion losses in SAW devices. The highest RMS surface roughness, crack and residual stress cause a reduction of surface velocity (about 40 m/s) and increase dramatically insertion loss at 1000 °C. The SAW devices response becomes very weak at this temperature, the electromechanical coupling coefficient (k²) of ZnO film decrease from 3.8% at 600 °C to 1.49% at 1000 °C.     

Influence of growth temperature of TiO2 buffer on structure and PL properties of ZnO films

A series of ZnO films with TiO₂ buffer on Si (1 0 0) substrates were prepared by DC reactive sputtering. Growth temperature of TiO₂ buffer changed from 100 °C to 400 °C, and the influence on the crystal structures and optical properties of ZnO films have been investigated. The XRD results show that the ZnO films with TiO₂ buffer have a hexagonal wurtzite structure with random orientation, and with the increase of growth temperature of TiO₂ buffer, the residual stresses were released gradually. Specially, the UV emission enhanced distinctly and FWHMs (full width half maximum) decreased linearly with the increasing TiO₂ growth temperature. The results all come from the improvement of crystal quality of ZnO films.