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.     

As-doped p-type ZnO films grown on SiO2/Si by radio frequency magnetron sputtering

p-Type ZnO:As films with a hole concentration of 10¹⁶-10¹⁷ cm⁻³ and a mobility of 1.32-6.08 cm²/V s have been deposited on SiO₂/Si substrates by magnetron sputtering. XRD, SEM, Hall measurements are used to investigate the structural and electrical properties of the films. A p-n homojunction comprising an undoped ZnO layer and a ZnO:As layer exhibits a typical rectifying behavior. Our study demonstrates a simple method to fabricate reproducible p-type ZnO film on the SiO₂/Si substrate for the development of ZnO-based optoelectronic devices on Si-based substrates.     

Structural and optical properties of ZnO thin films prepared by sol-gel method with different thickness

In this work, ZnO thin films with different thickness were prepared by sol-gel method on glass substrates and the structural and optical properties of these films were studied by X-ray diffractometer, atomic force microscope, UV-visible spectrophotometer, ellipsometer and fluorophotometer, respectively. The structural analyses show that all the samples have a wurtzite structure and are preferentially oriented along the c-axis perpendicular to the substrate surface. The growth process of highly c-axis oriented ZnO thin films derived from sol-gel method is a self-template process. With the increase of film thickness, the structural disorder decreases and the crystalline quality of the films is gradually improved. A transition of crystal growth mode from vertical growth to lateral growth is observed and the transition point is found between 270 and 360 nm thickness. The optical analyses show that with the increase of film thickness, both the refractive index and ultraviolet emission intensity are improved. However, the transmittance in the visible range is hardly influenced by the film thickness, and the averages are all above 80%.     

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.     

Structural, optical and electrical properties of Al-N codoped ZnO films by RF-assisted MOCVD method

N-doped ZnO films were produced using N₂ as N source by metal-organic chemical vapor deposition (MOCVD) system which has been improved with radio-frequency (RF)-assisted equipments. The data of secondary ion mass spectroscopy (SIMS) indicate that the concentration of N in N-doped ZnO films is around 5 × 10²⁰ cm⁻³, implying that sufficient incorporation of N into ZnO can be obtained by RF-assisted equipment. On this basis, the structural, optical and electrical properties of Al-N codoped ZnO films were studied. Then, the effect of RF power on crystal quality, surface morphologies, optical properties was analyzed using X-ray diffraction, atomic force microscopy and photo-luminescence methods. The results illustrate that the RF plasma is the key factor for the improvement of crystal quality. Then the observation of A⁰X recombination associated with N_O acceptor in low-temperature PL spectrum proved that some N atoms have occupied the positions of O atoms in ZnO films. Hall measurements shown that p-type ZnO film deposited on quartz glasses was obtained when RF power was 150 W for the Al-N codoped ZnO films, while the resistivity of N-doped ZnO films was rather high. Compared with the Al-doped ZnO film, the obviously increased resistivity of codoped films indicates that the formation of N_O acceptors compensate some donors in ZnO films effectively.     

Effect of aging time of ZnO sol on the structural and optical properties of ZnO thin films prepared by sol-gel method

In this work, ZnO thin films were prepared by sol-gel method and the effect of aging time of ZnO sol on the structural and optical properties of the films was studied. The structural characteristics of the samples were analyzed by an atomic force microscope and an X-ray diffractometer. The optical properties were studied by a UV-vis spectrophotometer and a fluorophotometer. The results show that the ZnO thin film prepared by the as-synthesized ZnO sol had relatively poor crystalline quality, low optical transmittance in the visible range and relatively weak ultraviolet emission performance. After the as-synthesized ZnO sol was aged for 24 h, the degree of the preferred crystal orientation along the c-axis of the ZnO thin film prepared by this aged sol was improved. At the same time, this film had a very smooth surface with uniform grains and both its visible range transmittance and ultraviolet emission intensity were obviously increased. These results suggest that appropriate aging of ZnO sol is very important for the improvement of structural and optical quality of ZnO thin films derived from sol-gel method.     

Optical properties of Al-doped ZnO thin films by ellipsometry

Al-doped ZnO thin films (AZO) were prepared on Si (1 0 0) substrates by using sub-molecule doping technique. The Al content was controlled by varying Al sputtering time. The as-prepared samples were annealed in vacuum chamber at 800 °C for 30 min. From the XRD observations, it is found that all films exhibit only the (0 0 2) peak, suggesting that they have c-axis preferred orientation. The average transmittance of the visible light is above 80%. Spectroscopic ellipsometry was used to extract the optical constants of the films. The absorption coefficient and the energy gap were then calculated. The results show that the absorption edge initially blue-shifts and then red-shifts with increase of Al content.     

Electrical and optical properties of ZnO films grown by molecular beam epitaxy

Zinc oxide (ZnO) films have been grown on sapphire by molecular beam epitaxy (MBE), and it is found that the grain size of the ZnO films increased with increasing the growth temperature. Photoluminescence (PL) study shows that the intensity ratio of near-band-edge emission to deep-level-related emission (NBE/DL) of the ZnO is significantly enhanced with increasing the growth temperature, and the dependence of the carrier mobility on the growth temperature shows very similar trend, which implies that there is a community factor that determines the optical and electrical properties of ZnO, and this factor is suggested to be the grain boundary. The results obtained in this paper reveal that by reducing the grain boundaries, ZnO films with high optical and electrical properties may be acquired.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Formation of p-type ZnMgO thin films by In-N codoping method

In-N codoped ZnMgO films have been prepared on glass substrates by direct current reactive magnetron sputtering. The p-type conduction could be obtained in ZnMgO films by adjusting the N₂O partial pressures. The lowest resistivity was found to be 4.6 Ω cm for the p-type ZnMgO film deposited under an optimized N₂O partial pressure of 2.3 mTorr, with a Hall mobility of 1.4 cm²/V s and a hole concentration of 9.6 × 10¹⁷ cm⁻³ at room temperature. The films were of good crystal quality with a high c-axis orientation of wurtzite ZnO structure. The presence of In-N bonds was identified by X-ray photoelectron spectroscopy, which may enhance the nitrogen incorporation and respond for the realization of good p-type behavior in In-N codoped ZnMgO films. Furthermore, the ZnMgO-based p-n homojunction was fabricated by deposition of an In-doped n-type ZnMgO layer on an In-N codoped p-type ZnMgO layer. The p-n homostructural diode exhibits electrical rectification behavior of a typical p-n junction.     

Effects of the sputtering time of ZnO buffer layer on the quality of GaN thin films

ZnO thin films with different thickness (the sputtering time of ZnO buffer layers was 10 min, 15 min, 20 min, and 25 min, respectively) were first prepared on Si substrates using radio frequency magnetron sputtering system and then the samples were annealed at 900 °C in oxygen ambient. Subsequently, a GaN epilayer about 500 nm thick was deposited on ZnO buffer layer. The GaN/ZnO films were annealed in NH₃ ambient at 950 °C. X-ray diffraction (XRD), atom force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to analyze the structure, morphology, composition and optical properties of GaN films. The results show that their properties are investigated particularly as a function of the sputtering time of ZnO layers. For the better growth of GaN films, the optimal sputtering time is 15 min.     

Fabrication of Sb-doped p-type ZnO thin films by pulsed laser deposition

p-Type ZnO thin films have been realized via monodoping antimony (Sb) acceptor by using pulsed laser deposition. The obtained films with the best electrical properties show a hole concentration in the order of 10¹⁸ cm⁻³ and resistivity in the range of 2-4 Ω cm. X-ray diffraction measurements revealed that all the films possessed a good crystallinity with (0 0 2)-preferred orientation. Guided by X-ray photoemission spectroscopy analysis and a model for large-sized-mismatched group-V dopant in ZnO, an Sb_Zn-2V_Zn complex is believed to be the most possible acceptor in the Sb-doped p-type ZnO thin films.     

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.     

Effects of cooling rate and post-heat treatment on properties of ZnO thin films deposited by sol-gel method

Sol-gel spin-coated ZnO thin films are cooled with different rates after the pre-heat treatment. Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman, and photoluminescence (PL) were carried out to investigate the effects of the cooling rate during pre-heat treatment on structural and optical properties of the ZnO thin films. The ZnO thin films cooled slowly exhibit mountain chain structure while the ones cooled rapidly have smooth surface. The ZnO thin films cooled rapidly have higher c-axis orientation compared to the ones cooled slowly. The narrower and the higher near-band-edge emission (NBE) peaks are observed in the ZnO thin films cooled rapidly.     

The Al-doping and post-annealing treatment effects on the structural and optical properties of ZnO:Al thin films deposited on Si substrate

Al-doped ZnO (ZnO:Al) thin films with different Al contents were deposited on Si substrates using the radio frequency reactive magnetron sputtering technique. X-ray diffraction (XRD) measurements showed that the crystallinity of the films was promoted by appropriate Al content (0.75 wt.%). Then the ZnO:Al film with Al content of 0.75 wt.% was annealed in vacuum at different temperatures. XRD patterns revealed that the residual compressive stress decreased at higher annealing temperatures. While the surface roughness of the ZnO:Al film annealed at 300 °C became smoother, those of the ZnO:Al films annealed at 600 and 750 °C became rougher. The photoluminescence (PL) measurements at room temperature revealed a violet, two blue and a green emission. The origin of these emissions was discussed and the mechanism of violet and blue emission of ZnO:Al thin films were suggested. We concluded that the defect centers are mainly ascribed to antisite oxygen and interstitial Zn in annealed (in vacuum) ZnO:Al films.     

Epitaxially grown ZnO thin films on 6H-SiC(0 0 0 1) substrates prepared by spin coating-pyrolysis

Epitaxially grown ZnO thin film on 6H-SiC(0 0 0 1) substrate was prepared by using a spin coating-pyrolysis with a zinc naphthenate precursor. As-deposited film was pyrolyzed at 500 °C for 10 min in air and finally annealed at 800 °C for 30 min in air. In-plane alignment of the film was investigated by X-ray pole-figure analysis. Field emission-scanning electron microscope, scanning probe microscope, and He-Cd laser (325 nm) was used to analyze the surface morphology, the surface roughness and photoluminescence of the films. In the photoluminescence spectra, near-band-edge emission with a broad deep-level emission was observed. The position of the near-band-edge peak was around 3.27 eV.     

Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering

Pure and Cu-doped ZnO (ZnO:Cu) thin films were deposited on glass substrates using radio frequency (RF) reactive magnetron sputtering. The effect of substrate temperature on the crystallization behavior and optical properties of the ZnO:Cu films have been studied. The crystal structures, surface morphology and optical properties of the films were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and a fluorescence spectrophotometer, respectively. The results indicated that ZnO films showed a stronger preferred orientation toward the c-axis and a more uniform grain size after Cu-doping. As for ZnO:Cu films, the full width at half maxima (FWHM) of (0 0 2) diffraction peaks decreased first and then increased, reaching a minimum of about 0.42° at 350 °C and the compressive stress of ZnO:Cu decreased gradually with the increase of substrate temperature. The photoluminescence (PL) spectra measured at room temperature revealed two blue and two green emissions. Intense blue-green luminescence was obtained from the sample deposited at higher substrate temperature. Finally, we discussed the influence of annealing temperature on the structural and optical properties of ZnO:Cu films. The quality of ZnO:Cu film was markedly improved and the intensity of blue peak (∼485 nm) and green peak (∼527 nm) increased noticeably after annealing. The origin of these emissions was discussed.     

Influence of substrate temperature on the optical and piezoelectric properties of ZnO thin films deposited by rf magnetron sputtering

In this study, ZnO thin films were fabricated using the rf magnetron sputtering method and their piezoelectrical and optical characteristics were investigated for various substrate temperatures. The ZnO thin film has the largest crystallization orientation for the (0 0 2) peak and the smallest FWHM value of 0.56° at a substrate temperature of 200 °C. The surface morphology shows a relatively dense surface structure at 200 °C compared to the other substrate temperatures. The surface roughness shows the smallest of 1.6 nm at a substrate temperature of 200 °C. The piezoelectric constant of the ZnO thin film measured using the pneumatic loading method (PLM) has a maximum value of 11.9 pC/N at a substrate temperature of 200 °C. The transmittance of the ZnO thin film measured using spectrophotometry with various substrate temperatures ranged from 75 to 93% in the visible light region. By fitting the refractive index from the transmittance to the Sellmeir dispersion relation, we can predict the refractive index of the ZnO thin film according to the wavelength. In the visible light range, the refraction index of the ZnO thin film deposited at a substrate temperature of 200 °C is the range of 1.88-2.08.     

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.     

The electrical, optical and magnetic properties of Si-doped ZnO films

In this paper, the influences of Si-doping on electrical, optical and magnetic properties of ZnO films have been systematically investigated. It is found that the resistivity of the films decreases from 3.0 × 10³ to 6.2 × 10^-2 Ωcm with Si-doping due to the increase of carrier concentration. The bandgap of ZnO films increases from 3.28 to 3.52 eV with increasing of Si concentration, which is found to be due to the collective effects of bandgap narrowing and Burstein-Moss effect induced by high carrier concentration. With increase of Si concentration, the near band edge (NBE) emission decreases due to the deterioration of crystal quality, while the yellow emission enhances due to the increase of extrinsic impurity or defects. The additional Si-doping has a profound influence on the enhancement of magnetic property and the maximum magnetic moment of 2.6 μ_B/Si is obtained. The ferromagnetic ordering is seen to be correlated with carrier concentration and structural defects.