Surface morphology of ZnO buffer layer and its effects on the growth of GaN films on Si substrates by magnetron sputtering

ZnO thin films were first prepared on Si(111) substrates using a radio frequency magnetron sputtering system. Then the as-grown ZnO films were annealed in oxygen ambient at temperatures of 700, 800, 900, and 1000°C , respectively. The morphologies of ZnO films were studied by an atom force microscope (AFM). Subsequently, GaN epilayers about 500 nm thick were deposited on the ZnO buffer layers. The GaN/ZnO films were annealed in NH₃ ambient at 900°C. The microstructure, morphology and optical properties of GaN films were studied by x-ray diffraction (XRD), AFM, scanning electron microscopy (SEM) and photoluminescence (PL). The results are shown, their properties having been investigated particularly as a function of the ZnO layers. For better growth of the GaN films, the optimal annealing temperature of the ZnO buffer layers was 900°C.     

XRD study on the effect of the deposition condition on pulsed laser deposition of ZnO films

Using a pulsed laser deposition (PLD) process on a ZnO target in an oxygen atmosphere, thin films of this material have been deposited on Si(111) substrates. An Nd: YAG pulsed laser with a wavelength of 1064 nm was used as the laser source. The influences of the deposition temperature, laser energy, annealing temperature and focus lens position on the crystallinity of ZnO films were analyzed by X-ray diffraction. The results show that the ZnO thin films obtained at the deposition temperature of 400°C and the laser energy of 250 mJ have the best crystalline quality in our experimental conditions. The ZnO thin films fabricated at substrate temperature 400°C were annealed at the temperatures from 400°C to 800°C in an atmosphere of N₂. The results show that crystalline quality has been improved by annealing, the optimum temperature being 600°C. The position of the focusing lens has a strong influence on pulsed laser deposition of the ZnO thin films and the optimum position is 59.5 cm from the target surface for optics with a focal length of 70 cm.      

Nitrogen doping in pulsed laser deposited ZnO thin films using dense plasma focus

Pulsed laser deposition synthesized ZnO thin films, grown at 400 °C substrate temperature in different oxygen gas pressures, were irradiated with 6 shots of pulsed nitrogen ions obtained from 2.94 kJ dense plasma focus to achieve the nitrogen doping in ZnO. Structural, compositional and optical properties of as-deposited and nitrogen ion irradiated ZnO thin films were investigated to confirm the successful doping of nitrogen in irradiated samples. Spectral changes have been seen in the nitrogen irradiated ZnO thin film samples from the low temperature PL measurements. Free electron to acceptor emissions can be observed from the irradiated samples, which hints towards the successful nitrogen doping in films. Compositional analysis by X-ray photoelectron spectroscopy and corresponding shifts in binding energy core peaks of oxygen and nitrogen confirmed the successful use of plasma focus device as a novel source for nitrogen ion doping in ZnO thin films.     

Properties of Eu-doped zinc oxide thin films grown on glass substrates by radio-frequency magnetron sputtering

Eu-doped ZnO (EZO) thin films were prepared on glass substrates at various growth temperatures by radio-frequency magnetron sputtering. The properties of deposited thin films showed a significant dependence on the growth temperature. The preferential growth orientation of all the thin films was occurred along the ZnO (002) plane. The maximum crystallite size and the minimum average transmittance in the wavelength range of 450–1100 nm were observed for the EZO thin film deposited at 25 °C. A red shift of the optical band gap was observed in the growth temperature range of 25–300 °C. The highest figure of merit, an index for evaluating the performance of transparent conducting thin films, was obtained at 200 °C of growth temperature. These results indicated that the high-quality EZO film was obtained at a growth temperature of 200 °C.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.     

p-type conduction in ZnO dual-acceptor-doped with nitrogen and phosphorus

A dual-acceptor doping method was proposed to produce p-type conduction in ZnO. Both nitrogen and phosphorus were used as the p-type doping agents. ZnO:(N, P) films were prepared by spray pyrolysis. The p-type conduction was achieved by thermal annealing at appropriate temperatures (500-700 ^°C) for 20 min in O₂ ambient. The lowest resistivity of , with a hole concentration and Hall mobility of 5.3×10¹⁷ cm⁻³ and 0.94 cm² V ⁻¹ s⁻¹, respectively, was obtained at an optimal annealing temperature of 600 ^°C. The p-type behavior was reproducible and stable. The introduction of nitrogen and phosphorus in ZnO were identified by secondary ion mass spectroscopy.     

Giant temperature coefficient of resistance in Co-doped ZnO thin films

A novel high-performance thermistor material based on Co-doped ZnO thin films is presented. The films were deposited by the pulsed laser deposition technique on Si (111) single-crystal substrates. The structural and electronic transport properties were correlated as a function of parameters such as substrate temperature and Co-doped content for Zn_1?x Co _x O (x=0.005,0.05,0.10 and 0.15) to prepare these films. The Zn_1?x Co _x O films were deposited at various substrate temperatures between 20 and 280 °C. A value of 20 %/K for the negative temperature coefficient of resistance (TCR) with a wide range near room temperature was obtained. It was found that both TCR vs. temperature behavior and TCR value were strongly affected by cobalt doping level and substrate temperature. In addition, a maximal TCR value of over 20 %?K^?1 having a resistivity value of 3.6 Ω?cm was observed in a Zn_0.9Co_0.1O film near 260 °C, which was deposited at 120 °C and shown to be amorphous by X-ray diffraction. The result proved that the optimal Co concentration could help us to achieve giant TCR in Co-doped ZnO films. Meanwhile, the resistivities of the films ranged from 0.4 to 270 Ω?cm. A Co-doped ZnO/Si film is a strong candidate of thermometric materials for non-cooling and high-performance bolometric applications.     

Nickel-induced microwheel-like surface morphological evolution of ZnO thin films by spray pyrolysis

Nickel?Czinc oxide (Ni?CZnO) thin films were deposited onto glass and tin-doped indium oxide-coated glass substrates by using a pneumatic spray pyrolysis technique at 450?°C from aqueous solutions of zinc acetate and nickel acetate precursors. The effect of nickel doping on structural, morphological and optical properties of the ZnO thin films has been studied. The X-ray diffraction patterns confirmed the polycrystalline nature of the films having hexagonal crystal structure. Ni?CZnO films with appropriate nickel doping revealed the occurrence of novel wheel-like surface morphology. The absorption edge of the Ni?CZnO films showed a red shift, meaning that the optical band gap energy decreases as the nickel doping concentration increases. A growth model is developed and proposed for the novel wheel-like morphology. All the thin films exhibited room-temperature photoluminescence. Pure ZnO and Ni?CZnO thin films were tested for their photoelectrochemical performance in 0.5 M Na₂SO₄ electrolyte solution. The values of fill factor and open circuit voltage were improved for the Ni?CZnO thin films.     

Characteristics of ZnO epilayer on the post-annealed buffer layer on GaN/sapphire substrate by pulsed laser deposition

We employed epi-GaN substrates for ZnO film growth, and studied the deposition and post-annealing effects. ZnO films were grown by pulsed laser deposition (PLD) method. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were investigated and the post-annealed ZnO films grown at T _g =700^oC have very smooth surfaces and the rms with roughness is about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epilayer and GaN/sapphire substrates. It is confirmed by AFM that growth temperature of 700^oC helps the films grow in step-flow growth mode. It is observed by cathode luminescence spectrum that the ZnO film grown at 700^oC has very low visible luminescence, indicating the decrease of the deep level defects. It is also revealed by Hall measurements that carrier concentration is decreased by increasing the growth temperatures. It is suggested that low temperature buffer layer growth and post-annealing technique can be used to fabricate ZnO hetero-epitaxy.     

Photoluminescence of ZnO and Mn-Doped ZnO Polycrystalline Films Prepared by Plasma Enhanced Chemical Vapour Deposition

ZnO and Mn-doped ZnO polycrystalline films are prepared by plasma enhanced chemical vapour deposition at low temperature （220℃）, and room-temperature photoluminescence of the films is systematically investigated. Analysis from x-ray diffraction reveals that a11 the prepared films exhibit the wurtzite structure of ZnO, and Mndoping does not induce the second phase in the films. X-ray photoelectron spectroscopy confirms the existence of Mn^2＋ ions in the films rather than metalic Mn or Mn^4＋ ions. The emission efficiency of the ZnO film is found to be dependent strongly on the post-treatment and to degrade with increasing temperature either in air or in nitrogen ambient. However, the enhancement of near band edge （NBE） emission is observed after hydrogenation in ammonia plasma, companied with more defect-related emission. Furthermore, the position of NBE shifts towards to high-energy legion with increasing Mn-doped concentration due to Mn incorporation into ZnO lattice.     

Studies on highly resistive ZnO thin films grown by DC-discharge-assisted pulsed laser deposition

It was found that by changing the substrate temperature from room temperature to ～850 °C, ZnO thin films with widely varying resistivity values could be grown on sapphire substrates using DC-discharge-assisted pulsed laser deposition (PLD) in oxygen ambient. The resistivity of the film grown at room temperature was too high to measure using our existing setup. However, as the growth temperature was increased from 550 °C to 750 °C, the resistivity first decreased slowly from ～14.0 to 4.4 Ω?m and then dropped suddenly to get saturated at ～2.0×10^?3 Ω?m as the growth temperature was further increased. In contrast to these, when there was no DC-discharge, the variation of resistivity for ZnO thin films grown by PLD was marginal up to the substrate temperature of ～850 °C. The reason for these observations was found to be the combined effects of reduction in donor defect densities like oxygen vacancies and zinc interstitials, introduction of acceptor type defects like interstitial oxygen and zinc vacancies, and the resultant poor carrier mobility at lower growth temperatures. At higher growth temperatures (800 °C and above), the appearance of oxygen vacancies and increase in mobility due to better crystalline quality were found to be responsible for reducing the resistivity. The PL of these films had significant emission in the green and red regions of the spectrum due to the aforesaid defect related transitions. Such highly resistive and luminescent films might be suited for applications such as resistive RAM, UV-photo detector, TFT, piezoelectric, transparent phosphor, and broadband LED applications.     

Study of thermal stability of ZnO:B films grown by LPCVD technique

Zinc oxide thin films with different boron doping levels (ZnO:B) are prepared by low pressure chemical vapor deposition (LPCVD) technique. All films here exhibit a pyramid-like surface texture. Stability of the ZnO:B films is systematically investigated through a post heat treatment at ambient temperatures of 300 °C and 250 °C for different durations. It is found that total transmission (TT) of these films at near infrared (NIR) wavelength range increases with the enhanced thermal treating intensity, which could be attributed to decrease of free carrier concentration inside the films. Moreover, light absorption in NIR wavelength range decreases profoundly with the increasing carrier concentration after a post thermal treatment in particular for highly doped ZnO:B films. However, morphology of these ZnO:B films does not vary after the thermal treatment and thus the corresponding light scattering properties do not change as well. Therefore, the thermally treated ZnO:B films may lead to an increase in light-generated current and resulting a higher cell efficiency due to the enhancement of TT when they work as front contact in silicon thin film solar cells.     

Characterization of Cd1−xFexS diluted magnetic semiconductors grown at near phase conversion temperature

Fe-based cadmium sulfide alloy thin films have been grown on c-plane sapphire substrates by a low-pressure metalorganic chemical vapor deposition technique at different growth temperatures. From X-ray diffraction and absorption spectra of the samples, the evolutions with growth temperature show an inflexion at the growth temperature of 300 ^°C. This was attributed to the phase transformation from zinc-blende to wurtzite. With increasing growth temperature from 270 ^°C to 360 ^°C, Fe concentration in the films increases monotonously. The electronic states of Cd_1−xFe_xS were investigated by X-ray photoelectron spectroscopy. Magnetic measurement shows Van Vleck paramagnetism of the Cd_1−xFe_xS thin film in the temperature region below 7 K.     

Functionalized Pd/ZnO Nanowires for Nanosensors

A method for surface doping and functionalization of ZnO nanowires (NWs) with Pd (Pd/ZnO) in a one‐step process is presented. The main advantage of this method is to combine the simultaneous growth, surface doping, and functionalization of NWs by using electrochemical deposition (ECD) at relatively low temperatures (90 °C). Our approach essentially reduces the number of technological steps of nanomaterial synthesis and final nanodevices fabrication with enhanced performances. A series of nanosensor devices is fabricated based on single Pd/ZnO NWs with a radius of about 80 nm using a FIB/SEM system. The influence of Pd nominal composition in Pd/ZnO NW on the H₂ sensing response is studied in detail and a corresponding mechanism is proposed. The results demonstrate an ultra‐high response and selectivity of the synthesized nanosensors to hydrogen gas at room temperature. The optimal concentration of PdCl₂ in the electrolyte to achieve extremely sensitive nanodevices with a gas response (S_H2) ≈ 1.3 × 10⁴ (at 100 ppm H₂ concentration) and relatively high rapidity is 0.75 µM. Theoretical calculations on Pd/ZnO bulk and functionalized surface further validated the experimental hypothesis. Our results demonstrate the importance of noble metal presence on the surface due to doping and functionalization of nanostructures in the fabrication of highly‐sensitive and selective gas nanosensors operating at room temperature with reduced power consumption.     

Pulsed laser deposition of Zr–N codoped <Emphasis Type="Italic">p</Emphasis>-type ZnO thin films

Present p-type ZnO films tend to exhibit high resistivity and low carrier concentration, and they revert to their natural n-type state within days after deposition. One approach to grow higher quality p-type ZnO is by codoping the ZnO during growth. This article describes recent results from the growth and characterization of Zr–N codoped p-type ZnO thin films by pulsed laser deposition (PLD) on (0001) sapphire substrates. For this work, both N-doped and Zr–N codoped p-type ZnO films were grown for comparison purposes at substrate temperatures ranging between 400 to 700 °C and N₂O background pressures between 10⁻⁵ to 10⁻² Torr. The carrier type and conduction were found to be very sensitive to substrate temperature and N₂O deposition pressure. P-type conduction was observed for films grown at pressures between 10⁻³ to 10⁻² Torr. The Zr–N codoped ZnO films grown at 550 °C in 1×10⁻³ Torr of N₂O show p-type conduction behavior with a very low resistivity of 0.89 Ω-cm, a carrier concentration of 5.0×10¹⁸ cm⁻³, and a Hall mobility of 1.4 cm² V⁻¹ s⁻¹. The structure, morphology and optical properties were also evaluated for both N-doped and Zr–N codoped ZnO films.     

Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target

Zinc oxide thin films have been obtained in O₂ ambient at a pressure of 1.3 Pa by pulsed laser deposition (PLD) using ZnO powder target and ceramic target. The effect of temperature on structural and optical properties of ZnO thin films was investigated systematically by XRD, SEM, FTIR and PL spectra. The results show that the best structural and optical properties can be achieved for ZnO thin film fabricated at 700 °C using powder target and at 400 °C using ceramic target, respectively. The PL spectrum reveals that the efficiency of UV emission of ZnO thin film fabricated by using powder target is low, and the defect emission of ZnO thin film derived from Zn_i and O_i is high.     

Influence of annealing temperature on optical properties of zinc oxide thin films analyzed by spectroscopic ellipsometry method

Zinc oxide (ZnO) thin films were sol–gel spin coated on glass substrates, annealed at various temperatures 300 °C, 400 °C and 500 °C and characterized by spectroscopic ellipsometry method. The optical properties of the films such as transmittance, refractive index, extinction coefficient, dielectric constant and optical band gap energy were determined from ellipsometric data recorded over the spectral range of 300–800 nm. The effect of annealing temperature in air on optical properties of the sol–gel derived ZnO thin films was studied. The transmission values of the annealed films were about 65% within the visible range. The optical band gap of the ZnO thin films were measured between 3.25 eV and 3.45 eV. Also the dispersion parameters such as single oscillator energy and dispersive energy were determined from the transmittance graph using the Wemple and DiDomenico model.     

Effect of Annealing Temperature on Structural and Optical Properties of N-Doped ZnO Films

Nitrogen-doped ZnO （ZnO：N） films are prepared by thermal oxidation of sputtered Zn3N2 layers on A1203 substrates. The correlation between the structural and optical properties of ZnO：N films and annealing temperatures is investigated. X-ray diffraction result demonstrates that the as-sputtered Zn3N2 films are transformed into ZnO：N films after annealing above 600℃. X-ray photoelectron spectroscopy reveals that nitrogen has two chemical states in the ZnO：N films： the No acceptor and the double donor （N2）o. Due to the No acceptor, the hole concentration in the film annealed at 700℃ is predicted to be highest, which is also confirmed by Hall effect measurement. In addition, the temperature dependent photoluminescence spectra allow to calculate the nitrogen acceptor binding energy.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.