Study on Raman line at 1080.2 cm in ZnO thin films prepared under high RF power

ZnO thin films were prepared on glass or on homo-buffer/glass by a RF magnetron sputtering method at RF power of 100-550 W. The structural and Raman characteristics of the films were analyzed by X-ray diffraction and Raman scattering. There appeared a sharp peak of 1080.2 cm⁻¹ near the A₁(2LO) mode (1156 cm⁻¹) of ZnO in the Raman spectra when the RF power was higher than 300 W. In this case, the (100) peak of ZnO film appeared obviously. It was speculated that the Raman mode at 1080.2 cm⁻¹ was induced by the ordered distribution of Zn_i defects in ZnO lattice.     

Effect of dopant concentration on the structural, electrical and optical properties of Mn-doped ZnO films

The Mn-doped ZnO (Zn_1 − xMn_xO) thin films with manganese compositions in the range of 0-8 at.% were deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn_1 − xMn_xO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as the divalent state of Mn²⁺ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and saturates with Mn doping.     

Growth ambient dependent electrical properties of lithium and nitrogen dual-doped ZnO films prepared by radio-frequency magnetron sputtering

Lithium (Li) and nitrogen (N) dual-doped ZnO films with wurtzite structure were prepared by radio-frequency magnetron sputtering ZnO target with Li₃N in growth ambient of pure Ar and the mixture of Ar and O₂, respectively, and then post annealing techniques. The film showed week p-type conductivity as the ambient was pure Ar, but stable p-type conductivity with a hole concentration of 3.46 × 10¹⁷ cm^− 3, Hall mobility of 5.27 cm²/Vs and resistivity of 3.43 Ω cm when the ambient is the mixture of Ar and O₂ with the molar ratio of 60:1. The stable p-type conductivity is due to substitution of Li for Zn (Li_Zn) and formation of complex of interstitial Li (Li_i) and substitutional N at O site, the former forms a Li_Zn acceptor, and the latter depresses compensation of Li_i donor for Li_Zn acceptor. The level of the Li_Zn acceptor is estimated to be 131.6 meV by using temperature-dependent photoluminescence spectrum measurement and Haynes rule. Mechanism about the effect of the ambient on the conductivity is discussed in the present work.     

Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects on the properties of deposited thin films by direct current magnetron sputtering

To obtain a suitable sputtering target for depositing transparent conducting Al-doped ZnO (AZO) films by using direct current (DC) magnetron sputtering, this study investigates the possibility of using atmosphere controlled sintering of Al₂O₃ mixed ZnO powders to prepare highly conductive ceramic AZO targets. Experimental results show that a gas mixture of Ar and CO could produce a sintered target with resistivity in the range of 2.23 × 10^− 4 Ω cm. The fairly low resistivity was mainly achieved by the formation of both aluminum substitution (Al_Zn) and oxygen vacancy (V_O), thus greatly increasing the carrier concentration. Compared to usual air sintered target, the thin film deposited by the Ar + CO sintered target exhibited lower film resistivity and more uniform spatial distribution of resistivity. A film resistivity as low as 6.8 × 10^− 4 Ω cm was obtained under the sputtering conditions of this study.     

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.     

RF diode reactive sputtering of n- and p-type zinc oxide thin films

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O₂ and of Ar/N₂. Sputtering in Ar+O₂ working gas (up to 75% of O₂) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (〈0 0 2〉 direction). These films had good piezoelectric properties but also high resistivity (ρ≈10³ Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N₂ and 〈1 0 0〉 orientation for higher N₂ content. The presence of nitrogen N_O at O-sites forming N_O-O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 Ω cm, a p-type carrier concentration of 3.6×10¹⁴ cm⁻³ and a Hall mobility of 22 cm² V⁻¹ s⁻¹ were obtained at 75% N₂.     

Annealing effect on dielectric and leakage current characteristics of Mn-doped Ba0.6Sr0.4TiO3 thin films as gate insulators for low voltage ZnO thin film transistor

We report on the dielectric properties and leakage current characteristics of 3 mol% Mn-doped Ba_0.6Sr_0.4TiO₃ (BST) thin films post-annealed up to 600 °C following room temperature deposition. The suitability of 3 mol% Mn-doped BST films as gate insulators for low voltage ZnO thin film transistors (TFTs) is investigated. The dielectric constant of 3 mol% Mn-doped BST films increased from 24 at in-situ deposition up to 260 at an annealing temperature of 600 °C due to increased crystallinity and the formation of perovskite phase. The measured leakage current density of 3 mol% Mn-doped BST films remained on the order of 5 × 10^− 9 to 10^− 8 A/cm² without further reduction as the annealing temperature increased, thereby demonstrating significant improvement in the leakage current characteristics of in-situ grown Mn-doped BST films as compared to that (5 × 10^− 4 A/cm² at 5 V) of pure BST films. All room temperature processed ZnO-TFTs using a 3 mol% Mn-doped BST gate insulator exhibited a field effect mobility of 1.0 cm²/Vs and low voltage device performance of less than 7 V.     

Growth and surface characterization of magnetron sputtered zinc nitride thin films

Zinc nitride films were deposited on Si(100) substrates at room temperature using RF-magnetron sputtering in pure N₂ and in Ar + N₂ atmospheres. Two active phonon modes (270.81 and 569.80 cm^− 1) are observed in Raman spectra for films deposited in Ar + N₂ atmosphere. Atomic force microscopy showed that the average surface roughness of the films deposited in pure N₂ atmosphere (1.3-3.33 nm) was less than for those deposited in a mixed Ar + N₂ atmosphere (10.3-12.8 nm). Low temperature cathodoluminescence showed two emission bands centered at 2.05 eV and 3.32 eV for both types of films.     

No ferromagnetic properties in polycrystalline Al-doped Zn0.97Mn0.03O diluted magnetic semiconductor

Zn_0.96Mn_0.03Al_0.01O powders have been synthesized using the co-precipitation technique. X-ray diffraction patterns show the typical würtzite structure with no additional peaks for all samples. Raman spectroscopy revealed that Mn²⁺ ions were substituted for Zn²⁺ into the ZnO matrix and that Al and Mn ions activate an additional vibration mode at 670 cm^− 1 which is specific to interstitial defects. The resistance measurements give a confirmation of the presence of free carriers. Nevertheless, magnetization measurements revealed only a mixture of paramagnetic and antiferromagnetic behaviors for all samples with no sign of ferromagnetism.     

p-type doping of ZnO by means of high-density inductively coupled plasmas

A custom-designed inductively coupled plasma assisted radio-frequency magnetron sputtering deposition system has been used to fabricate N-doped p-type ZnO (ZnO:N) thin films on glass substrates from a sintered ZnO target in a reactive Ar + N₂ gas mixture. X-ray diffraction and scanning electron microscopy analyses show that the ZnO:N films feature a hexagonal crystal structure with a preferential (002) crystallographic orientation and grow as vertical columnar structures. Hall effect and X-ray photoelectron spectroscopy analyses show that N-doped ZnO thin films are p-type with a hole concentration of 3.32 × 10¹⁸ cm^− 3 and mobility of 1.31 cm² V^− 1 s^− 1. The current-voltage measurement of the two-layer structured ZnO p-n homojunction clearly reveals the rectifying ability of the p-n junction. The achievement of p-type ZnO:N thin films is attributed to the high dissociation ability of the high-density inductively coupled plasma source and effective plasma-surface interactions during the growth process.     

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].     

Effect of sputtering power on the properties of Cd1 − xZnxTe films deposited by radio frequency magnetron sputtering

Cd_1 − xZn_xTe films were prepared by radio frequency (r.f.) magnetron sputtering from Cd_0.9Zn_0.1Te slices target with different sputtering power (60-120 W). The effects of sputtering power on the properties of Cd_1 − xZn_xTe films were studied using X-ray diffraction (XRD), energy dispersive X-ray (EDX), atomic force microscopy (AFM), ultraviolet spectrophotometer and Hall effect measurements. The composition of the deposited films was determined by EDX. The Cd content was found always to be higher than the Te content, regardless of sputtering power. This behavior may be explained by the preferential sputtering of cadmium atoms in the target. XRD studies suggest that ZnTe secondary phases were coexisted in Cd_1 − xZn_xTe films. The origin of the secondary phase is ascribed to the lowest sticking coefficient of Zn atom. AFM micrographs show that the grain size increases with the sputtering power. The optical transmission data indicate that shallow absorption edge occurs in the range of 750-850 nm, and the sputtering power does not have a clear effect on the optical absorption coefficient. In Hall Effect measurements, the sheet resistivities of the deposited films are 1.988 × 10⁸, 8.134 × 10⁷, 8.088 × 10⁷ and 3.069 × 10⁷ Ω/sq, respectively, which increase with the increasing of sputtering power.     

Structural properties of low-temperature grown ZnO thin films determined by X-ray diffraction and X-ray absorption spectroscopy

The epitaxial growth of ZnO thin films on Al₂O₃ (0001) substrates have been achieved at a low-substrate temperature of 150 °C using a dc reactive sputtering technique. The structures and crystallographic orientations of ZnO films varying thicknesses on sapphire (0001) were investigated using X-ray diffraction (XRD). We used angle-dependent X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to examine the variation of local structure. The XRD data showed that the crystallinity of the film is improved as the film thickness increases and the strain is fully released as the film thickness reached about 800 Å. The Zn K-edge XANES spectra of the ZnO films have a strong angle-dependent spectral feature resulting from the preferred c-axis orientation. The wurtzite structure of the ZnO films was explicitly shown by the XRD and EXAFS analysis. The carrier concentration, Hall mobility and resistivity of the 800 Å-thick ZnO film were 1.84 × 10¹⁹ cm^− 3, 24.62 cm²V^− 1s^− 1, and 1.38 × 10^− 2 Ω cm, respectively.     

Mn-doped ZnO transparent conducting films deposited by DC magnetron sputtering

Mn-doped zinc oxide (ZnO:Mn) thin films with low resistivity and relatively high transparency were firstly prepared on glass substrate by direct current (DC) magnetron sputtering at room temperature. Influence of film thickness on the properties of ZnO:Mn films was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. As the thickness increases from 144 to 479 nm, the crystallite size increases while the electrical resistivity decreases. However, as the thickness increases from 479 to 783 nm, the crystallite size decreases and the electrical resistivity increases. When film thickness is 479 nm, the deposited films have the lowest resistivity of 2.1 × 10^− 4 Ω cm and a relatively high transmittance of above 84% in the visible range.     

Raman analysis of nitrogen doped ZnO

The mechanism of nitrogen doping is essential for making p-type ZnO. This paper demonstrates that Raman characterization is a potentially powerful tool to study the mechanism of nitrogen doping. We have observed new Raman features near 280, 510, 570, 642, 773, 1360 and 1565 cm^− 1 shift in nitrogen doped ZnO (ZnO:N) thin films compared with undoped ZnO films. Peaks at 280, 510, 570, 642, and 773 cm^− 1 are attributed to the nitrogen related defect complex. The Raman peaks at 1360 cm^− 1 and 1565 cm^− 1 shift are assigned to D—(disordered) and G—(Graphitic) bands associated with the carbon-related defect complex, respectively. The intensity and the intensity ratio of peaks at 1360 cm^− 1 and 1565 cm^− 1 have been found to be sensitive parameters that reflect the conductivity type of ZnO:N. Explanations are presented which correlate the Raman features to the electric conductivity of the films. From this analysis, we found that at temperature lower than or at 400 °C, nitrogen incorporation will form the nitrogen or possible nitrogen carbon related defect complex. As the growth temperature increases to 500 °C, the features associated with nitrogen are difficult to distinguish and the features associated to carbon begin to emerge. This observation possibly indicates the decrease of the nitrogen content and the increase of the carbon content in the ZnO:N film. The increase of carbon content may affect the donor behavior of the film. This observation suggests that growth conditions should be controlled to avoid carbon into the film.     

Vertically aligned Mn-doped zinc oxide nanorods by hybrid wet chemical route

Mn-doped zinc oxide (Mn:ZnO) nanorods were synthesized by incorporating manganese in aligned ZnO nanorods. For this, Mn was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing. The nanorods were preferentially oriented in (0 0 2) direction as indicated by the XRD measurement. Optical band gap was seen to decrease with increasing amount of Mn incorporation. XPS studies indicated that incorporated Mn was in Mn²⁺ and Mn⁴⁺ states. Mn²⁺ atomic concentration was found to be larger than Mn⁴⁺ concentration in all the samples. The Raman spectra of the Mn:ZnO nanorods indicated the presence of the characteristic peak at ∼438 cm⁻¹ for high frequency branch of E₂ mode of ZnO. The PL peak at ∼376 nm (∼3.29 eV) was ascribed to the band edge luminescence while the peak at ∼394 nm (∼3.15 eV) was assigned to the donor bound exciton (D^oX) and free exciton transition related to Mn²⁺ states.     

Preparation of zinc tin oxide films by reactive magnetron sputtering of Zn on liquid Sn

Zn is sputter-deposited on melted Sn films by radio-frequency magnetron sputtering in oxidizing plasma. The samples present an absorption cut-off wavelength close to the one of ZnO, and an optical transparency higher than 50% in the visible range. Ex-situ thermal annealing improves visible transparency and produces a slight blue-shift in the optical bandgap. X-ray diffraction patterns show typical spectra due to polycrystalline ZnO with evidence of the presence of crystalline SnO, before annealing, and Zn₂SnO₄, after annealing. Rutherford Backscattering studies reveal the existence of a ZnO layer on top of an O-rich (Zn, Sn)O thin film. After optimal thermal treatment, electrical characterization exhibits carrier concentrations of 10¹⁶-10¹⁷ cm⁻³ and mobilities of 20-80 cm² V⁻¹ s⁻¹ for the resulting (Zn, Sn)O n-type films.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.     

Giant pyroelectric coefficient of Mn-doped PLT thin films epitaxially grown on (001) Pt/MgO

The c-axis-oriented epitaxial thin films of Mn-doped Pb_1−xLa_xTi_1−x/4O₃ (PLT) on (001) Pt/MgO substrates were prepared by rf-magnetron sputtering. To investigate the effect of the doped ion, 0-1.7 mol% MnO₂ added to the PLT target powder. The temperature dependence of the relative dielectric constant ?_r measurements and modified Curie-Weiss plots suggested that the increasing of diffuseness n was induced by high-La substitution and the diffuseness n of PLT thin films decreased by the addition of Mn, considerably. Inner stress and thermodynamic analysis were carried out and the results propose that the increasing of γ with Mn doping caused by increasing the misfit strain of the c-axis-oriented epitaxial PLT thin films and substrate. As a result, giant pyroelectric coefficient (γ = 15.8 × 10⁻⁸ C/cm² K) of Mn-doped epitaxial PLT thin film was achieved.     

Optimization of sputtering parameters for deposition of Al-doped ZnO films by rf magnetron sputtering in Ar + H2 ambient at room temperature

The effects of sputtering pressure and power on structural and optical-electrical properties of Al-doped ZnO films were systemically investigated at substrate temperature of room temperature and H₂/(Ar + H₂) flow ratio of 5%. The results show that carrier concentration and mobility of the films show nonmonotone change due to the evolution of microstructure and lattice defect of the films caused by introduction of H₂ with increasing sputtering pressure and power. The transmittance of the films is also found to be related to the introduction of H₂ in addition to usually considered surface roughness and crystallinity. Finally, optimized sputtering pressure and power are 0.8 Pa and 100 W, respectively, and obtained minimum resistivity and highest transmittance are 1.43 × 10^− 3 Ω·cm and 90.5%, respectively. In addition, it is found that E_g of the films is mainly controlled by the carrier concentration, but crystallite size and stress should also be considered for the films deposited at different powers.