Optical and structural properties of MgZnO/ZnO hetero- and double heterostructures grown by pulsed laser deposition

MgZnO thin films, MgZnO/ZnO heterostructures (HS) and double heterostructures (DHS) have been prepared on a-plane sapphire substrates by means of pulsed laser deposition (PLD). A linear blueshift of the MgZnO emission with increasing Mg content is observed in photoluminescence spectroscopy (PL) at 2 K. Cathodoluminescence measurements verify the spatial homogeneity of the emission properties of the MgZnO films. The film roughness is evaluated from atomic force microscopy scans. In MgZnO/ZnO HS the ZnO grows on all appearing MgZnO facets. PL investigations of such PLD-grown heterostructures show the high optical quality of thin ZnO films (d≤100 nm) grown on MgZnO. Capping those structures with a thin MgZnO layer further improves their luminescence intensity and enhances the emission of free-exciton luminescence from the ZnO layers. MgZnO/ZnO/MgZnO DHS with nominal ZnO layer thicknesses of d_nom≤6 nm show a clear intensification of the ZnO PL. Temperature dependent PL and transmission measurements between 4.4 and 300 K prove the dominating emission to be due to the recombination of excitons localized in the ZnO. At 2 K, due to confinement effects, their emission energy is blueshifted up to 51 meV compared to free excitons in bulk ZnO. PACS 81.15.Fg; 78.66.Hf; 68.37.Ps     

Study of structural and optical properties of ZnO films grown by pulsed laser deposition

Wurtzite zinc oxides films (ZnO) were deposited on silicon (0 0 1) and corning glass substrates using the pulsed laser deposition technique. The laser fluence, target-substrate distance, substrate temperature of 300 °C were fixed while varying oxygen pressures from 2 to 500 Pa were used. It is observed that the structural properties of ZnO films depend strongly on the oxygen pressure and the substrate nature. The film crystallinity improves with decreasing oxygen pressure. At high oxygen pressure, the films are randomly oriented, whereas, at low oxygen pressures they are well oriented along [0 0 1] axis for Si substrates and along [1 0 3] axis for glass substrates. A honeycomb structure is obtained at low oxygen pressures, whereas microcrystalline structures were obtained at high oxygen pressures. The effect of oxygen pressure on film transparency, band gap E_g and Urbach energies was investigated.     

Optical and electrical properties of aluminum-doped ZnO thin films grown by pulsed laser deposition

Transparent aluminum-doped zinc oxide (AZO) thin films were deposited on quartz glass substrates by pulsed laser deposition (PLD) from ablating Zn-Al metallic targets. The structural, electrical and optical properties of these films were characterized as a function of Al concentration (0-8 wt.%) in the target. Films were deposited at a low substrate temperature of 150 °C under 11 Pa of oxygen pressure. It was observed that 2 wt.% of Al in the target (or 1.37 wt.% of Al doped in the AZO film) is the optimum concentration to achieve the minimum film resistivity and strong ultraviolet emission. The presence of Al in the ZnO film changes the carrier concentration and the intrinsic defects.     

Violet luminescence from ZnO nanorods grown by room temperature pulsed laser deposition

ZnO thin films were deposited at room temperature by pulsed laser deposition (PLD) varying the oxygen pressure. Morphological analysis using scanning electron microscope (SEM) and atomic force microscopy (AFM) demonstrated the formation of ZnO nanorods at a particular oxygen pressure. Room temperature violet luminescence was observed from these ZnO nanorods and temperature dependence of luminescence was studied. Influence of oxygen pressure on the growth of ZnO thin films by PLD was studied using the X-ray photoelectron spectroscopy of both post ablated targets and deposited films. The ZnO films were crystalline and the formation of crystalline phase is found to follow a pressure–temperature (P–T) scaling with increase of temperature.     

Self-catalyst synthesis of aligned ZnO nanorods by pulsed laser deposition

High-density well-aligned ZnO nanorods were successfully synthesized on ZnO-buffer-layer coated indium phosphide (InP) (100) substrates by a pulsed laser deposition (PLD) method. Scanning electron microscopy images show that the ZnO buffer layer formed uniform drip-like structure and ZnO nanorods were well-oriented perpendicular to the substrate surface. The sharp diffraction peak observed at 34.46° in X-ray diffraction scanning pattern suggests that the ZnO nanorods exhibit a (002)-preferred orientation. The PL spectra of ZnO samples shows a strong near band edge emission centered at about 380 nm and a weak deep level emission centered at around 495 nm, and it demonstrates that the ZnO nanorods produced in this work have high optical quality, which sheds light on further applications for nanodevices. Supported by the National Natural Science Foundation of China (Grant No. 50532080), the Science & Technology Foundation for Key Laboratory of Liaoning Province (Grant No. 20060131), and the Doctoral Project by China Ministry of Education (Grant No. 20070141038)     

Structural and morphological properties of thin ZnO films grown by pulsed laser deposition

The pulsed laser deposition technique was used to produce zinc oxide thin films onto silicon and Corning glass substrates. Homogeneous surfaces exhibiting quite small Root Mean Square (RMS) roughness, consisting of shaped grains were obtained, their grain diameters being 40-90 nm at room temperature and at 650 °C growth respectively. Films were polycrystalline, even for growth at room temperature, with preferential crystallite orientation the (0 0 2) basal plane of wurtzite ZnO. Temperature increase caused evolution from grain to grain agglomeration structures, improving crystallinity. Compressive to tensile stresses transition with temperature was found while the lattice constant decreased.     

Electroluminescence from a ZnO homojunction device grown by pulsed laser deposition

A ZnO homojunction light emitting device was grown on n+ GaAs substrate by pulsed laser deposition. As-doped ZnO film by diffusion of As from the substrate was used for the p-type side and Al-doped ZnO film for the n-type side of the device. A distinct electroluminescence emission consisting of a dominant emission peak at ∼2.5 eV and a weak shoulder centered at ∼3.0 eV was observed at room temperature. The I-V characteristic of the ZnO homojunction showed a good rectifying behavior with a turn-on voltage of ∼4.5 V and a reverse breakdown voltage of ∼9 V.     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

Optical and waveguiding properties of Nd:KGW films grown by pulsed laser deposition

 Nd: KGd(WO₄)₂ thin films were deposited by KrF laser ablation on MgO, YAP, YAG and Si substrates at temperatures up to 800 °C. Film crystallinity, morphology, stoichiometry (WDX, RBS and PIXE), excitation spectra, fluorescence, refractive index and waveguiding properties were studied in connection with deposition conditions. The best films were crystalline and exhibited losses of approximately 5 dB cm^-1 at a wavelength of 633 nm. Received: 8 January 2001 / Accepted: 7 November 2001 / Published online: 11 February 2002     

Structural and optical properties of nano-structured tungsten-doped ZnO thin films grown by pulsed laser deposition

Novel highly c-oriented tungsten-doped zinc oxide (WZO) thin films with 1 wt% were grown by pulsed laser deposition (PLD) technique on corning 1737F glass substrate. The effects of laser energy on the structural, morphological as well as optical transmission properties of the films were studied. The films were highly transparent with average transmittance exceeding 87% in the wavelength region lying between 400 and 2500 nm. X-ray diffraction analysis (XRD) results indicated that the WZO films had c-axis preferred orientation with wurtzite structure. Film thickness and the full width at half maximum (FWHM) of the (0 0 2) peaks of the films were found to be dependent on laser fluence. The composition determined through Rutherford backscattering spectroscopy (RBS) appeared to be independent of the laser fluence. By assuming a direct band gap transition, the band gap values of 3.36, 3.34 and 3.31 eV were obtained for corresponding laser fluence of 1, 1.7 and 2.7 J cm⁻², respectively. Compared with the reported undoped ZnO band gap value of 3.37 eV, it is conjectured that the observed low band gap values obtained in this study may be attributable to tungsten incorporation in the films as well as the increase in laser fluence. The high transparency makes the films useful as optical windows while the high band gap values support the idea that the films could be good candidates for optoelectronic applications.     

Optical and structural parameters of the ZnO thin film grown by pulsed filtered cathodic vacuum arc deposition

Transparent conductive ZnO film was deposited on glass substrate by pulsed filtered cathodic vacuum arc deposition (PFCVAD). Optical parameters such as absorption coefficient α, the refractive index n, energy band gap E_g and dielectric constants have been determined using different methods. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. The spectra of the dielectric coefficient were used to calculate the energy band gap and the value was 3.24 eV. The experimental energy band gap was found to be 3.22 eV for 357 nm thick ZnO thin film. The envelope method was also used to calculate the refractive index and the data were consistent with K-K relation results. The structure of the film was analyzed with an x-ray diffractometer and the film was polycrystalline in nature with preferred (002) orientation.     

Growth of ZnO nanoparticles and nanorods with ultrafast pulsed laser deposition

In this work, we study the application of ultrafast pulsed laser deposition (PLD) in ZnO nanomaterial synthesis, including nanoparticles and nanorods. PLD using long pulse (nanosecond) lasers has been widely used as a method for growing prototype materials. The recently-emerged ultrafast PLD is expected to be able to overcome the problem of large liquid droplet formation. Using near infrared and femtosecond laser pulses in ablation, we first characterize the ablation plume using a Langmuir probe and plasma optical emission spectroscopy. We then examine the structural properties of the nanoparticles generated during low-fluence ablation. Finally, we demonstrate that using nanoparticle aggregates as templates, assisted by plume-excited nitrogen radicals at a high fluence, high quality ZnO nanorods can be grown free of metal catalysts.     

Optical and structural properties of pulsed laser ablation deposited ZnO thin film

A limited number of reports exists in the literature concerning the systematic study of the structural and optical properties of ZnO thin films, produced by pulsed laser ablation, in correlation with the deposition parameters adopted. In this paper we present a characterization of a sample prepared by this technique and studied by photoelectron spectroscopy and X-ray diffraction. The dielectric function of both target and films has been deduced by reflection electron energy loss spectroscopy.     

Alumina capped ZnO quantum dots multilayer grown by pulsed laser deposition

To extend the applicability of ZnO, with the bulk band gap of about 3.3 eV, into deep UV region, we have grown a multilayer of alumina capped ZnO quantum dots of mean in-plane sizes in the range of ∼1.8-3.6 nm at room temperature using alternate Pulsed Laser Deposition. Size dependent blue shift of the band gap of these dots up to ∼4.5 eV is observed in the optical absorbance spectra. The observed blue shift can be understood using the effective mass approximation in weak and strong confinement regimes.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.     

Optical investigation of A-plane ZnO/ZnMgO multiple quantum wells grown by pulsed laser deposition

In this study, we present the optical characteristics of A-plane ZnO/ZnMgO multiple quantum wells (MQWs) with different well widths grown on R-plane sapphire substrates by pulsed laser deposition (PLD). The energy gaps of ZnO and ZnMgO have been observed by photoluminescence (PL) and absorption spectra. The electrons confined in the ZnO wells transit from the electron ground sub-band to the heavy-hole ground sub-band (noted as 11H) located at 3.40 and 3.57 eV for the ZnO/ZnMgO MQWs samples with well widths of 5.6 and 1.2 nm, respectively. The strong anisotropic polarization characteristic has been studied by polarization-dependent PL measurements. For comparison, we also calculated the transition energies of different well thicknesses varying from 1 to 6 nm. The theoretical results match quite well with the experimental values and revealing the suitable conduction band offset Q_c=0.6. The temperature dependence of PL spectra is being investigated, in the temperature range between 10 and 300 K.     

Control of diameter of ZnO nanorods grown by chemical vapor deposition with laser ablation of ZnO

We made a study of controlling diameters of well-aligned ZnO nanorods grown by low-pressure thermal chemical vapor deposition combined with laser ablation of a sintered ZnO target, which was developed by us. Until now, it has been impossible to control diameters of ZnO nanorods, while the growth orientation was maintained well-aligned. In this study we developed a multi-step growth method to fabricate well-aligned nanorods whose diameters could be controlled. Metal Zn vapor and O₂ are used as precursors to grow ZnO nanorods. N₂ is used as a carrier gas for the precursors. A substrate is an n-Si (111) wafer. A sintered ZnO target is placed near the substrate and ablated by a Nd–YAG pulsed laser during ZnO nanorod growth. The growth temperature is 530 C and the pressure is 66.5 Pa. A vertical growth orientation of ZnO nanorods to the substrate is realized in the first-step growth although the diameter cannot be controlled in this step. When an O₂ flow rate is 1.5 sccm, well-aligned nanorods with 100 nm diameter are grown. Next, the second-step nanorods are grown on only the flat tip of the first-step nanorods. The diameters of the second-step nanorods can be controlled by adjusting the O₂ flow rate, and the growth direction is kept the same as that of the first-step nanorods. When the O₂ flow rate in second-step growth is smaller than 0.6 sccm, the diameter of the second-step nanorods is 30–50 nm. When the O₂ flow rate is between 0.75 and 3.0 sccm, the diameter is almost same as that of the first-step nanorods. When the O₂ flow rate is larger than 4.5 sccm, the diameter is increased with increasing O₂ flow rate. Further, the third-step ZnO nanorods with gradually increased diameters can be grown on the second-step nanorods with 1.5 sccm O₂ flow rate and without laser ablation.     

Correlation between electrical and optical properties of Cr:ZnO thin films grown by pulsed laser deposition

Highly transparent and conducting Chromium doped ZnO (Cr:ZnO) thin films with preferential c-axis orientation were grown on (0 0 0 1) sapphire substrates using buffer assisted pulsed laser deposition. The resistivity of Cr:ZnO thin films was found to decrease to a minimum value of ∼1.13×10⁻³Ω cm with the increasing Cr concentration up to ∼1.9 at.% and then increase with further increase of Cr concentration. On the contrary, the band gap and carrier concentration of Cr:ZnO thin films increased up to ∼3.37 eV and ∼2×10²⁰ cm⁻³, respectively, with the increase of Cr concentration up to ∼1.9 at.%, then decreased with further increase of Cr concentration. The increase of carrier concentration and conductivity with Cr doping at low Cr concentrations (<1.9 at.%) could be attributed to the presence of Cr in +3 valence state in ZnO thus acting as donor while decrease of carrier concentration beyond ∼1.9 at.% of Cr concentration could be attributed to the charge compensating effect due to the presence of acceptor like point defects such as oxygen interstitials. This was experimentally confirmed using x-ray photoelectron spectroscopy. The observed variation in the band gap of Cr:ZnO thin films with increasing Cr doping was attributed to the competing effects of the high free carrier concentration induced Burstein-Moss blue shift and band gap narrowing.     

Optical, structural and optoelectronic properties of pulsed laser deposition PbS thin film

Lead sulfide (PbS) nano-structured film has been grown on quartz substrates by the PLD technique. The deposited films were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Formation of cubic phase of PbS nanocrystals is proven. The absorption and emission spectra were measured for different thicknesses of the films. I–V characteristics and photoconductivity of the deposited film were also measured. The results indicate an efficient performance of the deposited films as an optical detector.     

Effects of deposition temperature on the structural and morphological properties of thin ZnO films fabricated by pulsed laser deposition

ZnO thin films were grown on Si(1 0 0) substrates using pulsed laser deposition in O₂ gas ambient (10 Pa) and at different substrate temperatures (25, 150, 300 and 400 °C). The influence of the substrate temperature on the structural and morphological properties of the films was investigated using XRD, AFM and SEM. At substrate temperature of T=150 °C, a good quality ZnO film was fabricated that exhibits an average grain size of 15.1 nm with an average RMS roughness of 3.4 nm. The refractive index and the thickness of the thin films determined by the ellipsometry data are also presented and discussed.