Characterization of MgZnO films grown by plasma enhanced metal-organic chemical vapor deposition

MgZnO (magnesium-zinc-oxide) films were grown on (11-20) sapphire substrates and Zn-polar ZnO substrates by plasma enhanced metal-organic chemical vapor deposition (PE-MOCVD) employing microwave-excited plasma. Structural, electrical and optical properties were investigated by X-ray diffraction, atomic force microscope, Hall, transmittance and photoluminescence measurement. The c-axis lattice constant decreases proportionally to an increase in the Mg content of Mg_xZn_1 − xO films. Therefore, this indicates that Mg atoms can be substituted in the Zn sites. Mg contents in films on ZnO substrates increase up to 0.11. In addition, Ga doped ZnO films were grown on (11-20) sapphire substrates. The resistivity of the films on (11-20) sapphire is controlled between 1.2 × 10^− 3 Ω cm to 1 Ω cm by changing the process conditions. The overall results indicate the promising potential of this PE-MOCVD method for related (Zn, Mg)O films formation because of the reactivity of the radicals, such as oxygen radicals (O?).     

Phase transitions of room temperature RF-sputtered ZnO/Mg0.4Zn0.6O multilayer thin films after thermal annealing

We report the thermal stability of room-temperature RF-sputtered Mg_0.4Zn_0.6O thin films and ZnO/Mg_0.4Zn_0.6O superlattices at 600 °C and 800 °C. The phase of room-temperature as-sputtered Mg_0.4Zn_0.6O is crystalline ZnO embedded in an amorphous or short-range-ordered hexagonal MgZnO matrix. Annealing at either 600 °C or 800 °C for 5 min transforms the matrix into a crystalline hexagonal wurtzite structure, leading to a decrease of the optical bandgap (E_g) of Mg_0.4Zn_0.6O. This also results in a slight change near the absorption edge of the superlattice transmission spectrum. The films precipitate cubic MgZnO after heating Mg_0.4Zn_0.6O at 800 °C for 5 min; by contrast, precipitations take at least 3 h if the samples are heated at 600 °C. Heating at 800 °C for more than 3 h significantly reduces the film thickness and E_g, attributed to the decomposition of superlattices and diffusion of magnesium into the substrate, respectively. On the other hand, annealing the ZnO/Mg_0.4Zn_0.6O superlattice at 600 °C for 12 h also produces an initial slight change in the optical transmission spectra, yet the spectra remain essentially unchanged for the remainder of the annealing process.     

Annealing effect on crystallization behavior of cubic MgxZn1−xO films on A-plane and M-plane sapphire

Cubic Mg_xZn_1−xO thin films with Mg composition around 70% were deposited on A-plane and M-plane sapphire substrates by rf-reactive magnetron sputtering. Measured structural and optical properties of these thin films indicated an optimal annealing temperature of 700 °C which produced high quality cubic MgZnO thin films on both substrates. Moreover, when the annealing temperature exceeded 750 °C, a much rougher surface resulted, and several large mosaic particles on the surface of the annealed films appeared. From EDX results, the Mg composition was lower than that found in other sections of the annealed films. We attributed this to thermally induced reconstruction of the crystallites. This phenomenon was more obvious for annealed MgZnO films on A-plane sapphire than that on M-plane sapphire. Thermal expansion mismatch with the substrate is the principal reason.     

Large-sized-mismatched group-V element doped ZnO films fabricated on silicon substrates by pulsed laser deposition

Pulsed laser deposition has been utilized to synthesize impurity-doped ZnO thin films on silicon substrate. Large-sized-mismatched group-V elements (A^V) including P, As, Sb and Bi were used as dopants. Hall effect measurements show that hole concentration in the order of 10¹⁶-10¹⁸ cm⁻³, resistivity in the range of 10-100 Ω cm, Hall mobility in the range of 10-100 cm²/Vs were obtained only for ZnO:As and ZnO:Bi thin films. X-ray diffraction measurements reveal that the films possess polycrystallinity or nanocrystallinity with ZnO (002) preferred orientation. Guided by X-ray photoemission spectroscopy analyses and theoretical calculations for large-sized-mismatched group-V dopant in ZnO, the A_Zn^V-2V_Zn complexes are believed to be the most possible acceptors in the p-type A^V-doped ZnO thin films.     

Electrical transport parameters of heavily-doped zinc oxide and zinc magnesium oxide single and multilayer films heteroepitaxially grown on oxide single crystals

Heavily doped epitaxial ZnO:Al and Zn_1−xMg_xO:Al films were grown by radio frequency magnetron sputtering onto single crystalline substrates (sapphire, MgO, silicon) and characterized by structural and electrical measurements. It is the aim of this investigation to better understand the carrier transport and the doping mechanisms in heavily doped transparent conducting oxide (TCO) films. It was found that the crystallographic film quality determines only partly the mobilities and the carrier concentrations: ZnO:Al films on a-plane (110) sapphire and on MgO (100) exhibit the highest mobilities. The oxygen partial pressure during the deposition from ceramic targets is more important influencing especially the carrier concentration N of the films. Though the films grew epitaxially grain boundaries are still existent, which reduce the mobility due to electrical grain boundary barriers for N < 3 · 10²⁰ cm^− 3. From annealing experiments the role of point defects and dislocations for the carrier transport could be estimated. For carrier concentrations above 3 · 10²⁰ cm^− 3 ionized impurity scattering limits the mobility, which is in agreement with our earlier review [K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097].     

Characterization of Zn1 − xMgxO transparent conducting thin films fabricated by multi-cathode RF-magnetron sputtering

Transparent conducting thin films of Al-doped and Ga-doped Zn_1 − xMg_xO with arbitrary Mg content x were deposited on glass substrates by simultaneous RF-magnetron sputtering of doped ZnO and MgO targets, and their fundamental properties were characterized. MgO phase separation in Zn_1 − xMg_xO films was not detected by X-ray diffraction. The Zn_1 − xMg_xO films show high optical transparency in the visible region. Although the carrier density of the Zn_1 − xMg_xO films decreased with increasing x, the Zn_1 − xMg_xO films showed good electrical conductivity; electrical resistivity as low as 8 × 10^− 4 Ω ·cm was achieved for the Zn_0.9Mg_0.1O:Ga thin film.     

Preparation and electrical characterization of Cd0.8Zn0.2Te/Si thin films

Thin films of Cd_0.8Zn_0.2Te/Si structures were prepared by vacuum evaporation technique. The electrical properties such as activation energy, barrier height, and transport mechanism along with the capacitance-voltage characteristics are analyzed. The zero field activation energy calculated from the saturation current density with the inverse absolute temperature is found to be 0.37 eV and the barrier height is 0.54 eV. As the applied bias voltage increases the activation energy decreases from 0.3 to 0.22 eV for the bias range of 0-2 V. From the observed current voltage characteristics it is found that the surface state density is high for the films deposited at room temperature. From the high-frequency (1 MHz) C-V measurement the built in voltage is found to be 0.15 V. The plot of 1/C² vs the applied bias voltage behaviour is linear, indicating the presence of abrupt junction. The acceptor concentration as obtained from the 1/C² vs bias voltage is 1.4×10¹⁶ cm⁻³.     

Structural and photoluminescence properties of aligned Sb-doped ZnO nanocolumns synthesized by the hydrothermal method

Aligned Sb-doped ZnO nanocolumns were synthesized by a simple hydrothermal method. Based on the analyses of the X-ray diffraction and photoluminescence result, it could be confirmed that the Sb has successfully doped in the ZnO crystal lattices to form an accepter energy level. At 85 K, the recombination of the acceptor-bound exciton was predominant in PL spectrum, which was attributed to the transition of the (Sb_Zn-2V_Zn) complex bound exciton. The acceptor binding energy had been calculated to be 123 meV.     

Optical and structural properties of sol-gel prepared MgZnO alloy thin films

Mg_xZn_1−xO (x = 0-0.5) alloy thin films were prepared by a sol-gel dip-coating method. Mg_0.1Zn_0.9O and Mg_0.5Zn_0.5O films prepared were annealed in the range of 400-900 °C to investigate their thermal stability and temperature-dependent optical properties. The Mg_0.1Zn_0.9O films were thermally stable in the investigated annealing temperature range and exhibited the maximum ultraviolet emission at 800 °C. The segregation of MgO occurred in the Mg_0.5Zn_0.5O films, and the near-band-edge ultraviolet emission of this alloy was enhanced with increasing annealing temperature. The Mg saturation content in the sol-gel prepared MgZnO alloys was found to be about 0.23 where the band gap extended to 3.48 eV.     

Influence of doping concentration on the properties of ZnO:Mn thin films by sol-gel method

Thin films of Mn-doped ZnO with different doping concentration (0.8, 1, 3, 5 at%) were prepared on Pt/Ti/SiO₂/Si substrates by using sol-gel method. The effects of the doping concentration on the structural properties, electrical characteristics and element binding energy in films were investigated. X-ray diffraction (XRD) results showed that the c-axis orientation of ZnO films was affected by Mn²⁺ content. Current-voltage (I-V) measurements indicated that resistivities of ZnO films were observably enhanced by dopant of Mn²⁺ and the resistivities value increased with a doping level up to 5 at% Mn. X-ray photoelectron spectroscopy (XPS) patterns suggested that the binding energies of O1s and ZnL₃M₄₅M₄₅ were affected by the content of Mn²⁺.     

Stability of ZnMgO oxide in a weak alkaline solution

Zinc oxide (ZnO) is a chemical compound of great interest used, for example, as photocatalyst in the purification of wastewater or polluted air. However, neither dissolution, nor photo-dissolution of ZnO is negligible: indeed, both processes reduce significantly the efficiency of photocatalysis and then lead to a secondary pollution by free Zn^2 +. In the present study, the stability of ZnMgO thin films in weak alkaline solution is investigated. We demonstrate that the replacement of Zn^2 + ion with Mg^2 + ion results in the production of a Zn_0.84Mg_0.16O solid solution, whose stability is higher than that of the ZnO sample. This alloy, thus, constitutes an alternative to the use of ZnO in photocatalysis applications. To gain more insights into the higher resistance of such alloys to the dissolution process, X-Ray photoelectron spectroscopy measurements were performed. They highlighted the role of OH group adsorption in the experimentally observed enhancement of ZnMgO stability.     

Fabrication of p-type ZnO nanowires based heterojunction diode

Vertically aligned p-type ZnO (Li–N co-doped) nanowires have been synthesized by hydrothermal method on n-type Si substrate. X-ray diffraction pattern indicated a strong peak from (0 0 0 2) planes of ZnO. The appearance of a strong peak at 437 cm⁻¹ in Raman spectra was attributed to E₂ mode of ZnO. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peaks at 490 cm⁻¹ for ZnO stretching mode. Compositional studies revealed the formation of Li–N co-doped ZnO, where Li was bonded with both O and N. The junction properties of p-type ZnO nanowires/n-Si heterojunction diodes were evaluated by measuring I–V and C–V characteristics. I–V characteristics exhibited the rectifying behavior of a typical p–n junction diode.     

High frequency characteristics of tin oxide thin films on Si

High frequency characteristics of tin oxide (SnO₂) thin films were studied. SnO₂ thin films have been successfully grown on n-type Si (111) substrates by using a spray deposition technique. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of the metal-oxide-semiconductor (Au/SnO₂/n-Si) Schottky diodes were investigated in the high frequency range from 300 kHz to 5 MHz. It has been shown that the interface state density, D_it, ranges from 2.44 × 10¹³ cm⁻² eV⁻¹ at 300 kHz to 0.57 × 10¹³ cm⁻² eV⁻¹ at 5 MHz and exponentially decreases with increasing frequency. The C-V and G/ω-V characteristics confirm that the interface state density and series resistance of the diode are important parameters that strongly influence the electrical parameters exhibited by the metal-oxide-semiconductor structure.     

The origin of electrical property deterioration with increasing Mg concentration in ZnMgO:Ga

Transparent conductive Ga-doped Zn_1 − xMg_xO (ZnMgO:Ga) films were epitaxially grown via Pulsed Laser Deposition on sapphire by optimizing the substrate temperature and other parameters of deposition. Zn_0.68Mg_0.31Ga_0.01O/sapphire films deposited at 400 °C have a Hall mobility (μ) of 9.2 ± 0.5 cm² V^− 1 s^− 1 and a free electron density (n) of 1.79 × 10²⁰ ± 0.06 × 10²⁰ cm^− 3, yielding an electrical conductivity (σ) = 262 ± 22 S/cm. Zn_0.90Mg_0.09Ga_0.01O/sapphire films, deposited under the same growth conditions, have similar crystalline quality, but significantly better electrical properties (σ = 1450 ± 10 S/cm, μ = 24.5 ± 2.5 cm² V^− 1 s^− 1, n = 3.81 × 10²⁰ ± 0.20 × 10²⁰ cm^− 3). This comparison provides evidence of electrical property deterioration in doped ZnMgO bulk material with increasing Mg content, independent of crystalline quality. Electrical properties of ZnMgO:Ga are further deteriorated by the decrease of the crystalline quality. Polycrystalline Zn_0.90Mg_0.09Ga_0.01O/a-SiO₂ samples deposited under identical conditions on amorphous silica substrates had both inferior crystal quality and inferior transport properties (μ = 2.5 ± 0.2 cm² V^− 1 s^− 1, n = 2.04 × 10²⁰ ± 0.20 × 10²⁰ cm^− 3, σ = 80 ± 8 S/cm) compared to their epitaxial counterparts. Overall, the results of this study indicate that both bulk material properties and crystalline quality influence the electrical properties of single-phase ZnMgO:Ga thin films.     

Fabrication of Al and large radii mismatch As codoped p-ZnO thin film and homojunction

We report Al³⁺ and large radii mismatched As⁵⁺ codoped p-ZnO thin films by As back diffusion from GaAs substrate and sputtering Al₂O₃ (1, 2 and 4 at%) mixed ZnO target. Hall effect measurements showed that the hole concentration increased upon codoping (As⁵⁺ and Al³⁺) compared to the monodoped (As⁵⁺) film. Also, it showed that 1 at% Al doped ZnO:As has low resistivity with high hole concentration due to best codoping. The crystallinity of the films has been studied by X-ray diffraction. The p-type formation mechanism has been investigated by X-ray photoelectron spectroscopy and low temperature photoluminescence analysis. It implies that As⁵⁺ substitutes on Zn²⁺ instead on O^2 − site that leads to the formation of (As_Zn-2V_Zn) complex which gives rise to p-conductivity. Further, the fabricated p-n homojunction using best codoped p-ZnO film shows typical rectifying characteristics of a diode.     

Analysis of surface states and series resistance in Au/n-Si Schottky diodes with insulator layer using current-voltage and admittance-voltage characteristics

In order to good interpret the experimentally observed Au/n-Si (metal-semiconductor) Schottky diodes with thin insulator layer (18 Å) parameters such as the zero-bias barrier height (Φ_bo), ideality factor (n), series resistance (R_s) and surface states have been investigated using current-voltage (I-V), capacitance-frequency (C-f) and conductance-frequency (G-f) techniques. The forward and reverse bias I-V characteristics of Au/n-Si (MS) Schottky diode were measured at room temperature. In addition, C-f and G-f characteristics were measured in the frequency range of 1 kHz-1 MHz. The higher values of C and G at low frequencies were attributed to the insulator layer and surface states. Under intermediate forward bias, the semi-logarithmic Ln (I)-V plot shows a good linear region. From this region, the slope and the intercept of this plot on the current axis allow to determine the ideality factor (n), the zero-barrier height (Φ_bo) and the saturation current (I_S) evaluated to 2.878, 0.652 and 3.61 × 10⁻⁷ A, respectively. The diode shows non-ideal I-V behavior with ideality factor greater than unity. This behavior can be attributed to the interfacial insulator layer, the surface states, series resistance and the formation barrier inhomogeneity at metal-semiconductor interface. From the C-f and G-f characteristics, the energy distribution of surface states (N_ss) and their relaxation time (τ) have been determined in the energy range of (E_c − 0.493E_v)-(E_c − 0.610) eV taking into account the forward bias I-V data. The values of N_ss and τ change from 9.35 × 10¹³ eV⁻¹ cm⁻² to 2.73 × 10¹³ eV⁻¹ cm⁻² and 1.75 × 10⁻⁵ s to 4.50 × 10⁻⁴ s, respectively.     

Electrical properties of alumina films grown on Si at low temperature using catalytic CVD

The electrical properties of alumina films formed at substrate temperatures as low as 27 °C using tri-methyl aluminum (TMA) and molecular oxygen (O₂) by catalytic chemical vapor deposition (Cat-CVD) have been investigated by capacitance-voltage (C-V), current-voltage (I-V) measurements and X-ray photoelectron spectroscopy (XPS). Substrate temperature dependence of dielectric constant and leakage current of the films has been explained on the basis of deficiency in oxygen. Interface trapping density of the order of 10⁹ ev^− 1cm^− 2 has been obtained. Angle resolved XPS measurements have revealed that the direct bonding of alumina and Si was realized with very small interface trapping density.     

Study on the optical properties of Zn1−xMgxS (0 ≤ x ≤ 0.55) quantum dots prepared by precipitation method

Zn_1−xMg_xS (0 ≤ x ≤ 0.55) quantum dots (QDs) were successfully synthesized by precipitation method. The crystal structures, microstructures, and optical properties of the Zn_1−xMg_xS QDs were investigated using X-ray diffraction, scanning electron microscopy, and ultraviolet-visible and photoluminescence (PL) spectroscopy. The Zn_1−xMg_xS QDs were found to have a cubic crystal structure and an average crystallite size of 6.40-7.96 nm. It has been shown that an increase in doping Mg²⁺ concentration in Zn_1−xMg_xS QDs led to a gradual widening of the band gap and a weakening in the PL intensity of the Zn_1−xMg_xS QDs.     

Facile synthesis of highly branched jacks-like ZnO nanorods and their applications in dye-sensitized solar cells

Highly branched, jacks-like ZnO nanorods architecture were explored as a photoanode in dye-sensitized solar cells, and their photovoltaic performance was compared with that of branch-free ZnO nanorods photoanodes. The highly branched network and large pores of the jacks-like ZnO nanorods electrodes enhances the charge transport, and electrolyte penetration. Thus, the jacks-like ZnO nanorods DSSCs render a higher conversion efficiency of η = 1.82% (V_oc = 0.59 V, J_sc = 5.52 mA cm⁻²) than that of the branch-free ZnO nanorods electrodes (η = 1.08%, V_oc = 0.49 V, J_sc = 4.02 mA cm⁻²). The incident photon-to-current conversion efficiency measurements reveal that the jacks-like ZnO nanorods DSSCs exhibit higher internal quantum efficiency (∼59.1%) than do the branch-free ZnO nanorods DSSC (∼52.5%). The charge transfer resistances at the ZnO/dye/electrolyte interfaces investigated using electrochemical impedance spectroscopy showed that the jacks-like ZnO nanorods DSSC had high charge transfer resistance and a slightly longer electron lifetime, thus improving the solar-cell performance.     

Carrier transport mechanisms and photovoltaic characteristics of p-H2Pc/n-Si heterojunctions

Heterojunction cells of p-H₂Pc/n-Si were fabricated by vacuum deposition of p-H₂Pc thin films onto n-Si single crystals. Measurements of the current-voltage (I-V) and the capacitance—voltage (C-V) characteristics have been evaluated to identify the mechanisms of barrier formation and, consequently, current flow. The forward current involves tunneling and could be explained by a multi-step tunneling recombination model due to a high density of interface defects. The C-V characteristics indicate an abrupt heterojunction model. The devices exhibit strong photovoltaic characteristics with an open-circuit voltage of 0.34 V, a short-circuit current density of 17.5 mA/cm² and a power conversion efficiency of 1.5%. These parameters have been estimated at room temperature and under constant illumination of 150 mW/cm².