n-ZnO Films Research Articles

PFE: ZnO hybrid nanocomposites for OLED applications: Fabrication and photophysical properties

In this work, ZnO nanoparticles (n-ZnO) and poly (9, 9-dioctyl-fluorenyl-2, 7-yleneethynylene) (PFE): n-ZnO based thin films were spin-coated onto glass substrates. Structural, morphological and optical properties of ZnO, PFE and the PFE: n-ZnO hybrid films with different n-ZnO mass ratios were investigated. n-ZnO films obtained by sol–gel technique are polycrystalline with a hexagonal wurtzite structure. They are also homogenous with an average grain size of about 35nm. For polymer nanocomposite, the optical properties are closely related to the ZnO content in the mixture. Among the tested active layers, the best performance is observed for that containing 2wt% of ZnO nanoparticles.

Direct formation of InN-codoped p-ZnO/n-GaN heterojunction diode by solgel spin-coating scheme

In this work p-ZnO/n-GaN heterojunction diodes were directly formed on the Si substrate by a combination of cost-effective solgel spin-coating and thermal annealing treatment. Spin-coated n-ZnO films on InN/GaN/Si wafers were converted to p-type polarity after thermal treatment of proper annealing durations. X-ray diffraction (XRD) analysis reveals that InN-codoped ZnO films have grown as the standard hexagonal wurtzite structure with a preferential orientation in the (002) direction. The intensity of the (002) peak decreases for a further extended annealing duration, indicating the greater incorporation of dopants, also confirmed by x-ray photoelectron spectroscopy and low-temperature photoluminescence. Hall and resistivity measurements validate that our p-type ZnO film has a high carrier concentration of 3.73×10¹⁷ cm⁻³, a high mobility of 210 cm²/Vs, and a low resistivity of 0.079 Ωcm. As a result, the proposed p-ZnO/n-GaN heterojunction diode displays a well-behaving current rectification of a typical p-n junction, and the measured current versus voltage (I-V) characteristic is hence well described by the modified Shockley equation. The research on the fabrication of p-ZnO/n-GaN heterojunctions shown here generates useful advances in the production of cost-effective ZnO-based optoelectronic devices.

Photovoltaic and photoelectrical response of n-ZnO/p-Si heterostructures with ZnO films grown by an Atomic Layer Deposition method

Zinc oxide (ZnO) films were used in silicon-based photovoltaic (PV) structures both as an n-type partner to p-type Si and (on top) as a transparent electrode (as a so-called TCO film). Photovoltaic response of such PV structures depends on the electrical properties of n-ZnO films as well as p-Si (100) substrates. The ZnO-based TCO films were deposited by an Atomic Layer Deposition (ALD) method. The films show a high electron concentration (~1020cm−3) and a relatively high electron mobility (>10cm2/Vs). Considering Si, we used fairly cheap commercial substrates with 360µm thickness, i.e., not optimized for PV cells applications. The best results were obtained for Si substrates with a moderate hole concentration (~1015cm−3) and resistivity of 10Ωcm. The photovoltaic efficiency achieved is 4.4%, with the possibility of further improvement.

Fabrication and Characterization of n-ZnO/n-Si Heterojunction

The n-ZnO/n-Si heterojunction are fabricated by depositing ZnO films on n-Si (111) films substrate using magnetron sputtering Al doped ZnO ceramic target. The structures of n-ZnO films, analyzed by X-ray diffraction (XRD) spectroscopy, and the preferential orientation of the ZnO grains is observed along the (101) and (100) axis aligning with the growth direction. The photoelectric properties, charge carrier transport properties and conductive mechanism were studied by testing the I-V, C-V characteristics with illumination and without illumination. Current-voltage (I-V) measurements of n-ZnO/n-Si heterojunctions show good diode characteristics and photovoltaic effects with illumination. The forward conduction is respectively determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of n-ZnO/n-Si heterojunctions has been proposed to explain the transport mechanism. As the conduction band and valence band offset in the ZnO/n-Si heterojunction is too big, the current transport mechanism is dominated by the space-charge limited current (SCLC) conduction at the forward voltage exceed 0.8 V. The results suggest the existence of a large number of interface states in ZnO/n-Si heterojunction, and the interface states can be reduced and the photoelectric properties can be further improved. DOI: http://dx.doi.org/10.11591/telkomnika.v11i9.3268

${\rm NH}_{3}$ Sensing by $p$-ZnO Thin Films

AlN and AlAs codoped <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -ZnO thin films are fabricated by RF magnetron sputtering for gas sensing applications. For comparison, Al monodoped <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO film is also grown. The conductivity of the films is confirmed by the Van der Pauw Hall effect measurement system. The structural and elemental properties of the films are studied by X-ray diffraction and time-of-flight secondary ion mass spectroscopy. Sensitivity measurements are performed with the fabricated <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO films for different concentrations of ammonia (200-1200 ppm) at different operating temperatures (room temperature: 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and 150 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C). It is found that both <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -ZnO films have higher sensitivity than that of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO film. Furthermore, both <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -ZnO films exhibit lower response and recovery times than <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO film.

Nitrogen doping of ZnTe for the preparation of ZnTe/ZnO light-emitting diode

Nitrogen-doped p-type zinc telluride (p-ZnTe) films are prepared by sputtering in a mixture of nitrogen/argon plasma. The effect of doping level N (ratio of N2 flow rate to that for the mixed gas) in the range 0–10 % on the films properties is investigated. Heterojunction diodes are prepared on stainless steel flexible substrate from the p-ZnTe (doping level, N = 5 %) and solution-grown n-ZnO films. The junction parameters and light-emission properties of diodes are investigated. Doping level beyond N = 1 %, changes the cubic crystal structure of ZnTe to hexagonal and reduces the size of crystallites. At the doping level N = 2–4 %, films with the highest hole density of 2.5 × 1018 cm−3 and lowest band gap energy of 1.4 eV are obtained. The diodes junction built-in potential and the donor density in n-ZnO films are found to be in the range 0.4–0.7 V and 1.5 × 1017–1.4 × 1018 cm−3, respectively. Diodes exhibit electroluminescence in the UV and visible regions due to the band edge and defect emissions in ZnO.

Synthesis and electrical properties of Pb(Zr0.52Ti0.48)O3 thick films embedded with ZnO nanoneedles prepared by the hybrid sol–gel method

Pb(Zr0.52Ti0.48)O3 thick films embedded with ZnO nanoneedles (PZT–ZnOn) were successfully prepared on Pt/Cr/SiO2/Si substrates by the hybrid sol–gel method via spin-coating ZnOn suspension and lead zirconate titanate (PZT) sol. To control the orientation of the films, a PbTiO3 (PT) layer was first deposited as a seed layer. Effects of annealing method and ZnOn contents on the corresponding orientation and crystallization of PZT–ZnOn films were investigated by XRD and SEM. The results show that all the PZT–ZnOn composite thick films have pure perovskite structure and high-quality film surface. The dielectric and ferroelectric properties of the PZT–ZnOn films are close to the PZT films, and have a little decrease with the increasing of the ZnOn contents.

UV-visible detector and LED based n-ZnO/p-Si heterojunction formed by electrodeposition

In this study, we have investigated the heterojunctions formed by n-ZnO thin films deposited on (100) p-Si:B using electrochemical deposition (ECD) technique. Structural, electrical and luminescence features of the thin films were respectively measured. Optimal sets of growth conditions seem to be the ones that are undergone for the samples D1 and D2. It was observed that n-ZnO thin films have dominantly preferred orientation of (002). It has been shown that the heterostructures exhibited reasonable rectifying behavior with turn-on voltage of about 1.2 V and ideality factor of 2.1. In case of illumination with 400 nm wavelength light, significant increase occurred especially in reverse bias current by a factor of 103 and 102 for the D1 and the D2, respectively. Bandgap of ZnO thin films has been determined to be 3.4 eV at the room temperature by using the band edge photoluminescence measurements. Finally, the room temperature electroluminescence (EL) results show that the heterostructures exhibits observable broad luminescence centered at the wavelengths of 390 and 510 nm for D1 and 470 nm for D2, respectively. Additionally, sharp lasing peaks are also observed in the EL spectra, probably due to the multiple scattering effects.

Photosensitivity of ZnO/Cu2O thin film heterojunction

ZnO/Cu2O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure and optical properties of n-ZnO and i-ZnO films deposited on glass substrates were characterized by X-ray diffraction (XRD) and spectrophotometer, respectively. The current–voltage (I–V) characteristics of two ZnO/Cu2O n–i–p heterojunctions were investigated, which n-ZnO layer thicknesses were 60nm and 180nm. Typical diode rectification behavior was observed in both heterojunctions. The heterojunctions exhibit photosensitivity under the irradiation with a halogen lamp. The dark current and photocurrent are higher in the heterojunction with thinner n-ZnO layer. The photosensitivity (135) of the heterojunction with 180-nm n-ZnO layer is higher than that (79) of 60-nm n-ZnO layer. An energy band diagram was proposed to analyze I–V characteristics of the heterojunction.

Ultraviolet electroluminescence from n-ZnO/i-MgO/p+-GaN heterojunction light-emitting diodes fabricated by RF-magnetron sputtering

Based on the easily controllable radio frequency magnetron sputtering, n-ZnO and i-MgO thin films were fabricated on p+-GaN substrate to construct heterojunctional light-emitting diodes for ultraviolet emission from the near band edge exciton recombination of ZnO. Effects of the insulator MgO layer on the electroluminescent performance of the n-ZnO/i-MgO/p+-GaN light-emitting diodes have been investigated. It was found that the light-emitting diode presented stronger near band-edge emission with blue shift emission peak under the lower working current when i-MgO layer was inserted. The fabrication process, characteristics and the mechanism were discussed in detail.

Studying trivalent/bivalent metal ion doped TiO 2 as p-TiO 2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO 2 thin films (M x Ti 1− x O 2, M = Cr 3+, Fe 3+, Ni 2+, Co 2+, Mn 2+ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti 4+ oxidation state of the Ti2p band in the doped p-TiO 2. The homogenous M x Ti 1− x O 2 was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current ( I)–voltage ( V) characteristics for the Ag/n-ZnO/M x Ti 1− x O 2/ITO/glass assembly showed rectifying behavior with small turn-on voltages ( V 0) < 1 V. The ideality factor ( η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.

Novel fabrication method of ZnO films utilizing solid-phase crystallized seed layers

ABSTRACTA novel fabrication method of ZnO films utilizing solid-phase crystallized seed layers has been developed. In this method, solid phase crystallization (SPC) is performed by annealing amorphous ZnON films, which are prepared by sputtering of ZnO targets in Ar/N2 mixed gases, in an oxidization atmosphere. The grain size of ZnO films deposited on the seed layers is significant larger than that of ZnO films directly deposited on glass substrates, which is considered to be due to the low grain density of seed layers. By utilizing this technique, the resistivity of ZnO:Al (AZO) films is decreased from 20 × 10-4 Ωcm to 5 × 10-4 Ωcm at the film thickness of 30nm. Furthermore, we observed that SPC seed layers are in-plane aligned when Al2O3 substrates are used, which suggests that the fabrication method proposed here is also promising for synthesizing epitaxial ZnO films.

Annealing effect on the optical response and interdiffusion of n-ZnO/p-Si (111) heterojunction grown by atomic layer deposition

Optical and structural properties of n-ZnO films grown on a p-Si (111) substrate by atomic layer deposition were observed using in situ synchrotron x-ray diffraction during annealing. The photoluminescence showed a complicated photon response with increasing annealing temperature. In situ x-ray diffraction indicated the growth of grains for an annealing temperature from 500 to 800 °C with the orientation altering from polycrystalline to preferential (200). Measurements with a time-of-flight secondary-ion mass spectrometer indicated that the outgassing of hydrogen atoms and ZnO/Si interdiffusion behavior were correlated with the intensity and position of emissions in photoluminescence spectra.

THE PERSPECTIVES OF FABRICATION FOR ULTRAVIOLET PHOTORESISTORS BASED ON ZNO FILMS

Undoped and nitrogen doped n-ZnO films were deposited on Si and SiNx/Si substrates by magnetron sputtering. The methods of thermal vacuum deposition and standard LIFT-OFF lithography were used for fabrication ohmic In and Al contacts to ZnO. The influence of crystal perfection for undoped ZnO films and magnitude of interelectrode spacing of contacts on photosensitivity and response speed of the photoresistors on their basis were investigated. For photoresistors based on undoped films the photocurrent-to-dark current ratio equal to 24 at response speed about ten minutes were achieved. The photoresistors based on nitrogen doped films showed the photosensitivity with photocurrent-to-dark current ratio is equal to 250 at λ = 390 nm with time constant of photoresponse about 10 μs.

Structure and Magnetic Properties of Mn-Implanted ZnO Films

Unintentionally doped n-ZnO(0002) films prepared by pulse laser deposition were implanted with various doses of Mn+. Photoluminescence measurement revealed that the incorporation of Mn in the films shifts the near band edge position towards the higher energy side. Magnetism measurements of the Mn-implanted samples showed clear magnetic properties. Magnetization–field hysteresis loops were observed at 300 K and the temperature-dependent magnetization showed magnetic behavior up to 350 K, which is in agreement with the theoretical prediction.

Microstructural investigation of Ti/Au ohmic contacts on Ga doped single crystalline n-ZnO films

Characteristics of Ti/Au ohmic contacts on Ga doped n-type ZnO films were investigated in detail, before and after the thermal annealing. It was observed that a minimum specific contact resistance was obtained after annealing the samples at 200–300 °C, and the contact properties deteriorated by annealing at higher temperatures. Observation using the transmission electron microscopy and the X-ray diffractometer showed no phase reactions at the film interfaces after the annealing at 200 °C. It was observed in the case of 500 °C annealed samples that Ti-Zn oxide formed within the Ti layer and substantial out-diffusion of Zn and O occurred from ZnO to Ti. It was proposed that the Zn/O out-diffusion and the Ti-Zn oxide formation are responsible for the deterioration of contact properties after the high temperature annealing. Formation of Ti–Au compounds was not observed in any of the annealed samples.

Mechanism Investigation of p-i-n ZnO-Based Light-Emitting Diodes

Using a cosputtering technique to deposit P-ZnO : AlN film and using a vapor cooling condensation system to deposit n-ZnO : In and i-ZnO films on sapphire substrates, thin-film-type ZnO-based light-emitting diodes (LEDs) were fabricated. A Nd : YAG laser with a wavelength of 413 nm is utilized to identify the defect-related emissions of p-ZnO, i-ZnO, and n-ZnO films. The characteristics of i-ZnO layer of ultraviolet (UV) emissions were analyzed using temperature-dependent photoluminescence. The mechanism of the UV electroluminescence emission peak at 3.20 eV observed from the p-i-n ZnO-based LEDs were attributed to the low deep-level defects and the radiative recombination occurred in the i-ZnO layer.

Thin-film polycrystalline n-ZnO/p-CuO heterojunction

Results of X-ray diffraction and spectral-optical studies of n-ZnO and p-CuO films deposited by gas-discharge sputtering with subsequent annealing are presented. It is shown that, despite the difference in the crystal systems, the polycrystallinity of n-ZnO and p-CuO films enables fabrication of a heterojunction from this pair of materials.

Optical and structural characteristics of Ga-doped ZnO films

The n-ZnO films doped with Ga to the content 2.5 at % are produced by pulse laser deposition onto the (0001) oriented single crystal sapphire substrates. The transmittance spectra of the ZnO films in the range from 200 to 3200 nm are studied in relation to the Ga dopant content. It is established that an increase in the Ga content shifts the fundamental absorption edge to the blue region, but reduces the transparency of the ZnO films in the infrared spectral region. The dependence of the band gap on the level of doping with Ga is determined. The photoluminescence spectra of the ZnO films doped to different levels are recorded. It is established that the PL intensity and peak position vary unsteadily with the level of doping. X-ray diffraction studies of the structure of the films are carried out. It is found that the crystallographic parameters (the lattice constant c) of the ZnO film depend on the Ga dopant content and the conditions of deposition of the films.

MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode

The growth, fabrication, and subsequent electroluminescence (EL) characterization of an n-ZnO/p-GaN heterojunction light-emitting diode prepared on c-Al 2O 3 substrate are presented. The diode-like I– V characteristics and room temperature EL spectrum with an intense broadband emission in the yellow-green spectral region has been observed with forward bias applied. Photoluminescence (PL) and Raman spectra of the n-ZnO and p-GaN films were also measured. By comparing PL and EL spectra, it was concluded that the deep-level defect-related emission mainly originated from the GaN epitaxial layer.