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.     

Effect of ZnO sacrificial layer thickness on the structure and optical properties of GaN nanoparticles prepared by RF magnetron sputtering

GaN nanoparticles were prepared on sapphire (0001) substrates with ZnO sacrificial layers by self assembly of Ga₂O₃ films in their reaction with NH₃. ZnO sacrificial layers with different thicknesses and Ga₂O₃ films were deposited on sapphire substrates in turn by a radio frequency (RF) magnetron sputtering system. Nitridation of the Ga₂O₃ films was then carried out in a quartz tube furnace. The effect of ZnO sacrificial layer thickness on the structure and optical properties of nanoparticles prepared by RF magnetron sputtering were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL). GaN nanoparticles with ZnO sacrificial layers of different thicknesses possess hexagonal wurtzite crystal structure and have a preferred orientation with c axis perpendicular to the sapphire substrates. XRD, SEM, and AFM results reveal that the better-crystallinity, uniform, and well-dispersed GaN nanoparticles (～30 nm) without agglomeration were obtained with a ZnO sacrificial layer 300-nm thick. The PL result reveals that the optical properties of the GaN nanoparticles are improved with a ZnO sacrificial layer 300-nm thick. Therefore, we suggest that a ZnO sacrificial layer 300-nm thick is the most suitable condition for obtaining better-quality GaN nanoparticles with good luminescence performance. Moreover, the mechanism of the formation of GaN nanoparticles with ZnO sacrificial layers is also discussed.     

ZnMgO生长中压强和衬底对薄膜性质的影响

利用金属有机源化学气相沉积(MOCVD)生长方法,在2.5 kPa和5 kPa生长压强下,分别以sapphire (Al2O3)和ZnO为衬底生长ZnMgO薄膜.研究分析了样品的晶体结构、表面形貌、光电学性质.结果表明,衬底和生长压强对ZnMgO薄膜的生长有重要影响.5 kPa高压生长和以ZnO为衬底均有利于ZnMgO...     

Effects of sapphire substrates surface treatment on the ZnO thin films grown by magnetron sputtering

The surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering were studied. The sapphire substrates properties have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Results show that the intensity of (0 0 2) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching was strongest, FWHM of (0 0 2) diffraction peak is the narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550  $^{\circ }\mathrm{C}$ ). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.     

Electronic and optical properties in ZnO:Ga thin films induced by substrate stress

The effects of biaxial stress in ZnO:Ga thin films on different substrates, e.g., sapphire(0001), quartz, Si(001), and glass have been investigated by X-ray diffraction, atomic force microscopy, and electrical transport and ellipsometric measurements. A strong dependence of orientation, crystallite size, transport, and electronic properties upon the substrate-induced stress has been found. The structural properties indicate that a tensile stress exists in epitaxial ZnO:Ga films grown on sapphire, Si, and quartz, while a compressive stress appears in films grown on glass. The resistivity of the films decreased with increasing biaxial stress, which is inversely proportional to the product of the carrier concentration and Hall mobility. The refractive index n was found to decrease with increasing biaxial stress, while the optical band gap E₀ increased with stress. These behaviors are attributed to lattice contraction and the increase in the carrier concentration that is induced by the stress. Our experimental data suggest that the mechanism of substrate-induced stress is important for understanding the properties of ZnO:Ga thin films and for the fabrication of devices which use these materials.     

Fabrication of ZnO thin films by femtosecond pulsed laser deposition

The growth of ZnO thin films on sapphire substrate using the femtosecond PLD technique is reported. The effect of substrate temperature and oxygen pressure on the structural properties of the films was studied. Highly c-axis oriented ZnO films can be grown on sapphire substrates under vacuum conditions using the femtosecond PLD process. There is an optimum substrate temperature for the pulsed laser deposition of ZnO film that enhances the thermodynamic stability and allows the formation of well-crystallized thin films. The crystal quality of the films can be further improved by increasing the deposition time and introducing oxygen during the pulsed laser deposition process.     

Epitaxial ZnO films grown on ZnO-buffered c-plane sapphire substrates by hydrothermal method

ZnO films are hydrothermally grown on ZnO-buffered c-plane sapphire substrates at a low temperature of 70 °C. A radio-frequency (RF) reactive magnetron sputtering has been used to grow the ZnO buffer layers. X-ray diffraction, scanning electron microscopy, and room temperature photoluminescence are carried out to characterize the structure, morphology and optical property of the films. It is found that the films are stress-free. The epitaxial relationship between the ZnO film and the c-plane sapphire substrate is found to be ZnO (0 0 0 1)||Al₂O₃ (0 0 0 1) in the surface normal and in plane. Sapphire treatment, as such acid etching, nitridation, and oxidation are found to influence the nucleation of the film growth, and the buffer layers determine the crystalline quality of the ZnO films. The maximum PL quantum efficiency of ZnO films grown with hydrothermal method is found to be about 80% of single-crystal ZnO.     

Effects of sapphire substrate annealing on ZnO epitaxial films grown by MOCVD

The annealing effects of sapphire substrate on the quality of epitaxial ZnO films grown by metalorganic chemical vapor deposition (MOCVD) were studied. The atomic steps formed on (0 0 0 1) sapphire (α-Al₂O₃) substrate surface by annealing at high temperature was analyzed by atomic force microscopy (AFM). The annealing effects of sapphire substrate on the ZnO films were examined by X-ray diffraction (XRD), AFM and photoluminescence (PL) measurements. Experimental results indicate that the film quality is strongly affected by annealing treatment of the sapphire substrate surface. The optimum annealing temperature of sapphire substrates is given.     

Photoluminescence of ZnO films prepared by r.f. sputtering on different substrates

Polycrystalline ZnO films with good orientation were deposited on sapphire, quartz, Si and 7059 glass substrates by r.f. magnetron sputtering. A strong UV photoluminescence (PL) peak (located at 356 nm) and a weak blue emission peak (located at 446 nm) were observed at room temperature (RT) for the films deposited on sapphire, quartz and Si substrates when excited with 270 nm light. For the films prepared on Corning 7059 glass, only a strong 446 nm blue emission peak was found, and the PL intensity decreased with increasing oxygen pressure during films deposition. The intensity of the UV emission increased 7 and 14 times, respectively, for the films on sapphire and quartz substrates after high temperature annealing in vacuum. The UV emission originates from the inter-band transition of electrons and the blue emission is due to transition of electrons from the shallow donor level of the oxygen vacancies to the valence band.     

Surface luminescence in ZnO nanoparticles

Spectral and time-resolved photoluminescence (TRPL) measurements were performed on ZnO nanoparticles of different sizes (17-300 nm). Under a low photon energy excitation of 2.33 eV, the time-integrated PL spectra (TIPL) clearly exhibit broad emission in the range of 1.2-2.3 eV. Upon increase of the particle size, a red-shift in the PL peak position was observed. Gaussian analysis indicates that this red-shift corresponds to the increased relative magnitude of the Gaussian combination in the low energy region. In addition, TRPL demonstrates a clear relationship between the particle diameters and the PL decay times. The shortening of the PL lifetime could be explained by a surface states model.     

Epitaxial ZnO thin films grown by pulsed electron beam deposition

In this work, the pulsed electron beam deposition method (PED) is evaluated by studying the properties of ZnO thin films grown on c-cut sapphire substrates. The film composition, structure and surface morphology were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Optical absorption, resistivity and Hall effect measurements were performed in order to obtain the optical and electronic properties of the ZnO films. By a fine tuning of the deposition conditions, smooth, dense, stoichiometric and textured hexagonal ZnO films were epitaxially grown on (0001) sapphire at 700 °C with a 30° rotation of the ZnO basal plane with respect to the sapphire substrate. The average transmittance of the films reaches 90% in the visible range with an optical band gap of 3.28 eV. Electrical characterization reveals a high density of charge carrier of 3.4 × 10¹⁹ cm^?3 along with a mobility of 11.53 cm²/Vs. The electrical and optical properties are discussed and compared to ZnO thin films prepared by the similar and most well-known pulsed laser deposition method.     

Influence of annealing atmosphere on ZnO thin films grown by MOCVD

ZnO films were deposited on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Annealing treatments for as-deposited samples were performed in different atmosphere under various pressures in the same chamber just after growth. The effect of annealing atmosphere on the electrical, structural, and optical properties of the deposited films has been investigated by means of X-ray diffraction (XRD), atomic force microscope (AFM), Hall effect, and optical absorption measurements. The results indicated that the electrical and structural properties of the films were highly influenced by annealing atmosphere, which was more pronounced for the films annealed in oxygen ambient. The most significant improvements for structural and electrical properties were obtained for the film annealed in oxygen under the pressure of 60 Pa. Under the optimum annealing condition, the lowest resistivity of 0.28 Ω cm and the highest mobility of 19.6 cm² v⁻¹ s⁻¹ were obtained. Meanwhile, the absorbance spectra turned steeper and the optical band gap red shifted back to the single-crystal value.     

Effect of substrate and annealing on the structural and optical properties of ZnO:Al films

ZnO:Al thin films with c-axis preferred orientation were deposited on glass and Si substrates using RF magnetron sputtering technique. The effect of substrate on the structural and optical properties of ZnO:Al films were investigated. The results showed a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. However, the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (0 0 2) peaks decreased evidently and the grain size increased. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (∼445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.     

Effect of electrically degenerated layer on the carrier transport property of ZnO epitaxial thin films

ZnO films were prepared on (1 1 1) YSZ and (0 0 0 1) sapphire by pulsed laser deposition method. Effect of lattice mismatch on the carrier transport properties of ZnO epitaxial thin films was investigated. The carrier mobility of the ZnO films on YSZ was larger than that of ZnO/sapphire due to smaller lattice mismatch when the thickness was below 150 nm. The effect of electrically degenerated layer on the carrier transport property increased with decreasing the film thickness of ZnO film. The carrier density and electron mobility of 20 nm-thick-ZnO film on either substrate were regardless of the temperature. We concluded that the dominant carrier scattering mechanism in ZnO ultra thin films is double Schottky barriers at the grain boundary and that their height depends on the carrier concentration.     

Room-temperature ferromagnetic Mn-alloyed ZnO films obtained by pulsed laser deposition

Ferromagnetic, semi-insulating Mn-alloyed ZnO films with a Curie temperature above 375 K have been grown by pulsed laser deposition on c-plane sapphire substrates. Antiferromagnetic coupling is revealed by temperature-dependent magnetization measurements. The antiferromagnetic coupling would be compatible with the observed weak ferromagnetism by assuming that the magnetic moments order antiferromagnetically but nonparallel (canted). We find a clear correlation between coercivity and mosaicity of the ferromagnetic Mn-alloyed ZnO films and explain it on the basis of a coercivity mechanism known from soft magnetic materials.     

Annealing effects of sapphire substrate on properties of ZnO films grown by magnetron sputtering

The annealing effects of sapphire substrates on the quality of epitaxial ZnO films grown by dc reactive magnetron sputtering were studied. The atomic steps formed on (0001) sapphire (α-Al₂O₃) substrates surface by annealing at high temperature were analyzed by atomic force microscopy. Their influence on the growth of ZnO films was examined by X-ray diffraction and photoluminescence measurements. Experimental results indicate that the film quality is strongly affected by annealing treatment of the sapphire substrate surface. The optimum annealing temperature of sapphire substrates for ZnO grown by magnetron sputtering is 1400 °C for 1 h in air. PACS 81.40.Ef; 68.55.Jk; 81.05.Dz; 81.15.Cd     

Low-temperature MOVPE growth of ZnO thin films by using a buffer layer

ZnO thin films have been grown on a-plane (1,1,−2,0) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at low substrate temperature of 350 °C. It is showed that the crystal and electrical quality of the thin films was improved by using a ZnO buffer layer. The photoluminescence (PL) measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.     

ZnO Nanostructures Grown on A1N/Sapphire Substrates by MOCVD

ZnO nanorods and nanotubes are successful synthesized on A1N/sapphire substrates by metal-organic chemical vapour deposition （MOGVD）. The different morphology and structure properties of ZnO nanorods and nanotubes are found to be affected by the A1N under-layer. The photoluminescence spectra show the optical properties of the ZnO nanorods and nanotubes, in which a blueshift of UV emission is observed and is attributed to the surface effect.[第一段]     

Realization of controllable etching for ZnO film by NH4Cl aqueous solution and its influence on optical and electrical properties

ZnO films were deposited on c-plane Al₂O₃ substrates by pulsed laser deposition. The etching treatments for as-grown ZnO films were performed in NH₄Cl aqueous solution as a function of NH₄Cl concentration and etching time. It was found that NH₄Cl solution is an appropriate candidate for ZnO wet etching because of its controllable and moderate etching rate. The influence of etching treatment on the morphology, optical and electrical properties of the ZnO films has been investigated systematically by means of X-ray diffraction, atomic force microscope, photoluminescence and Hall effect. The results indicated that the surface morphology and optical properties of the films were highly influenced by etching treatment.