Fabrication of high hole-carrier density p-type ZnO thin films by N-Al co-doping

In order to obtain p-type ZnO thin films, effect of atomic ratio of Zn:N:Al on the electronic and structural characteristic of ZnO thin films was investigated. Hall measurement indicated that with the increase of Al doping, conductive type of as-grown ZnO thin films changed from n-type to p-type and then to n-type again, reasons are discussed in details. Results of X-ray diffraction revealed that co-doped ZnO thin films have similar crystallization characteristic (0 0 2 preferential orientation) like that of un-doping. However, SEM measurement indicated that co-doped ZnO thin films have different surface morphology compared with un-doped ZnO thin films. p-type ZnO thin films with high hole concentration were obtained on glass (4.6 × 10¹⁸ cm⁻³) and n-type silicon (7.51 × 10¹⁹ cm⁻³), respectively.     

High-conductive nanocrystalline silicon with phosphorous and boron doping

Intrinsic, P- and B-doped hydrogenated amorphous silicon thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited samples were thermally annealed at the temperature of 800 °C to obtain the doped nanocrystalline silicon (nc-Si) films. The microstructures, optical and electronic properties have been evaluated for the undoped and doped nanocrystalline films. X-ray photoelectron spectroscopy (XPS) measurements demonstrated the presence of the substitutional boron and phosphorous in the doped films. It was found that thermal annealing can efficiently activate the dopants in films accompanying with formation of nc-Si grains. Based on the temperature-dependent conductivity measurements, it was shown that the activation of dopant by annealing increased the room temperature dark conductivity from 3.4 × 10⁻⁴ S cm⁻¹ to 5.3 S cm⁻¹ for the P-doped films and from 1.28 × 10⁻³ S cm⁻¹ to 130 S cm⁻¹ for the B-doped films. Meanwhile, the corresponding value of conductivity activation energies was decreased from 0.29 eV to 0.03 eV for the P-doped films and from 0.3 eV to 5.6 × 10⁻⁵ eV for the B-doped films, which indicated the doped nc-Si films with high conductivity can be achieved with the present approach.     

Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

The crystalline structure,conductivity and optical properties of Co-doped ZnO thin films

Transparent conductive Co-doped ZnO thin films were deposited by ultrasonic spray technique. Conditions of preparation have been optimized to get good quality. A set of cobalt (Co)-doped ZnO (between 0 and 3 wt%) thin films were grown on glass substrate at 350 °C. The thin films were annealed at 500 °C for improvement of the physical properties. Nanocrystalline films with hexagonal wurtzite structure and a strong (0 0 2) preferred orientation were obtained. The maximum value of grain size G = 63.99 nm is attained with undoped ZnO film. The optical transmissions spectra showed that both the undoped and doped ZnO films have transparency within the visible wavelength region. The band gap energy decreased after doping from 3.367 to 3.319 eV when Co concentration increased from 0 to 2 wt% with slight increase of electrical conductivity of the films from 7.71 to 8.33 (Ω cm)⁻¹. The best estimated structure, optical and electrical results are achieved in Co-doped ZnO film with 2 wt%.     

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.     

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.     

p-Type conduction in phosphorus-doped ZnO thin films by MOCVD and thermal activation of the dopant

Phosphorus-doped p-type ZnO thin films have been realized by metalorganic chemical vapor deposition (MOCVD). The conduction type of ZnO films is greatly dependent on the growth temperature. ZnO films have the lowest resistivity of 11.3 Ωcm and the highest hole concentration of 8.84 × 10¹⁸ cm⁻³ at 420 °C. When the growth temperature is higher than 440 °C, p-type ZnO films cannot be achieved. All the films exhibited p-type conduction after annealing, and the electrical properties were improved comparing with the as-grown samples. Secondary ion mass spectroscopy (SIMS) test proved that phosphorus (P) has been incorporated into ZnO.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Structural, electrical and optical properties of Gd doped and undoped ZnO:Al (ZAO) thin films prepared by RF magnetron sputtering

The influence of the gadolinium doping on the structural features and opto-electrical properties of ZnO:Al (ZAO) films deposited by radio frequency (RF) magnetron sputtering method onto glass substrates was investigated. X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [0 0 2] direction. The Gd doped ZAO film with a thickness of 140 nm showed a high visible region transmittance of 90%. The optical band gap was found to be 3.38 eV for pure ZnO film and 3.58 eV for ZAO films while a drop in optical band gap of ZAO film was observed by Gd doping. The lowest resistivities of 8.4 × 10⁻³ and 10.6 × 10⁻³ Ω cm were observed for Gd doped and undoped ZAO films, respectively, which were deposited at room temperature and annealed at 150 °C.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.     

Characteristics of ZnO:Al thin films co-doped with hydrogen and fluorine

Fluorine and hydrogen co-doped ZnO:Al (AZO) films were prepared by radio frequency (rf) magnetron sputtering of ZnO targets containing 1 wt.% Al₂O₃ on Corning glass at substrate temperature of 150 °C with Ar/CF₄/H₂ gas mixtures, and the structural, electrical and optical properties of the as-deposited and the vacuum-annealed films were investigated. In as-deposited state, films with fairly low resistivity of 3.9-4 × 10⁻⁴ Ω cm and very low absorption coefficient below 900 cm⁻¹ when averaged in 400-800 nm could be fabricated. After vacuum-heating at 300 °C, the minimum resistivity of 2.9 × 10⁻⁴ Ω cm combined with low absorption loss in visible region, which enabled the figure of merit to uplift as high as 4 Ω⁻¹, could be obtained for vacuum-annealed film. It was shown that, unlike hydrogenated ZnO films which resulted in degradation upon heating in vacuum at moderately high temperature, films with fluorine addition could yield improved electrical properties mostly due to enhanced Hall mobility while preserving carrier concentration level. Furthermore, stability in oxidizing environment could be improved by fluorine addition, which was ascribed to the filling effect of dangling bonds at the grain boundaries. These results showed that co-doping of hydrogen and fluorine into AZO films with low Al concentration could be remarkably compatible with thin film solar cell applications.     

Effect of indium doping in CdO thin films prepared by spray pyrolysis technique

Preparation of transparent and conducting indium doped CdO thin films by spray pyrolysis on glass substrate is reported for various concentration of indium (2-8 wt%) in the spray solution. The electrical, optical and structural properties of indium doped CdO films were investigated using different techniques such as Hall measurement, optical transmission, X-ray diffraction and scanning electron microscope. X-ray analysis shows that the undoped CdO films are preferentially orientated along (2 0 0) crystallographic direction. Increase of indium doping concentration increases the films packing density and reorient the crystallites along (1 1 1) plane. A minimum resistivity of 4.843×10⁻⁴ Ω cm and carrier concentration of 3.73×10²⁰ cm⁻³ with high transmittance in the range 300-1100 nm were achieved for 6 wt% indium doping. The band gap value increases with doping concentration and reaches a maximum of 2.72 eV for 6 wt% indium doping from 2.36 eV of that of undoped film. The minimum resistivity achieved in the present study is found to be the lowest among the reported values for In-doped CdO films prepared by spray pyrolysis method.     

Electrical and optical properties of ZnO films prepared by sputtering of ZnO targets containing AlN

ZnO films prepared from the ZnO target containing 2% AlN are transparent irrespective of radio frequency (RF) power. The obtained ZnO films have the carrier density of 3.8 × 10²⁰ cm⁻³ or less and the low mobility of 5.3-7.8 cm²/(V s). In the case of 5% AlN target, ZnO films prepared at 40, 60 and 80 W are transparent, whereas ZnO films prepared at 100 and 120 W are colored. As RF power increases from 40 to 120 W, the carrier density increases straightforwardly up to 5.5 × 10²⁰ cm⁻³ at 100 W and is oppositely reduced to 3.2 × 10²⁰ cm⁻³ at 120 W. In the case of 10% AlN target, ZnO films prepared at 60 W or more are colored, and have the carrier density of 4 × 10²⁰ cm⁻³ or less. The N-concentration in these colored films is estimated to be 1% or less. The Al-concentration in the ZnO films prepared from the 5 and 10% AlN targets is higher than 2%. The carrier density of the ZnO films containing Al and N atoms is nearly equal to that of ZnO films doped with Al atoms alone. There is no evidence in supporting the enhancement of the carrier density via the formation of N-Al_xZn_4−x clusters (4 ≥ x ≥ 2).     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

Boron doped ZnO thin films fabricated by RF-magnetron sputtering

By using the radio frequency-magnetron sputtering (RF-MS) method, both pure ZnO and boron doped ZnO (ZnO:B) thin films were deposited on glass substrates at ambient temperature and then annealed at 450 °C for 2 h in air. It is found that both ZnO and ZnO:B thin films have wurtzite structure of ZnO with (0 0 2) preferred orientation and high average optical transmission (≥80%). Compared with the resistivity of 6.3 × 10² Ω cm for ZnO film, both as-deposited and annealed ZnO:B films exhibit much lower resistivity of 9.2 × 10⁻³ Ω cm and 7.5 × 10⁻³ Ω cm, respectively, due to increase in the carrier concentration. Furthermore, the optical band gaps of 3.38 eV and 3.42 eV for as-deposited and annealed ZnO:B films are broader than that of 3.35 eV for ZnO film. The first-principles calculations show that in ZnO:B thin films not only the band gap becomes narrower but also the Fermi level shifts up into the conduction band with respect to the pure ZnO film. These are consistent with their lower resistivities and suggest that in the process of annealing some substituted B in the lattice change into interstitial B because of its smaller ion radius and this transformation widens the optical band gap of ZnO:B thin film.     

Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition

Transparent conducting Al-doped ZnO (AZO) thin films have been deposited by sol-gel route. Starting from an aqueous solution of zinc acetate by adding aluminum chloride as dopant, a c-axis oriented polycrystalline ZnO thin film 100 nm in thickness could be spin-coated on glass substrates via a two-step annealing process under reducing atmosphere. The effects of thermal annealing and dopant concentration on the structural, electrical and optical properties of AZO thin films were investigated. The post-treated AZO films exhibited a homogenous dense microstructure with grain sizes less than 10 nm as characterized by SEM photographs. The annealing atmosphere has prominent impact on the crystallinity of the films which will in turn influence the electrical conductivity. By varying the doping concentrations, the optical and electrical properties could be further adjusted. An optimal doping concentration of Al/Zn = 2.25 at.% was obtained with minimum resistivity of 9.90 × 10⁻³ Ω-cm whereas the carrier concentration and mobility was 1.25 × 10²⁰ cm⁻³ and 5.04 cm² V⁻¹ s⁻¹, respectively. In this case, the optical transmittance in the visible region is over 90%.     

Oxygen influence on sputtered high rate ZnO:Al films from dual rotatable ceramic targets

In this study, the influence of oxygen on high rate (up to 110 nm m/min) sputtered aluminum doped zinc oxide films (ZnO:Al) was systematically investigated. Different oxygen gas flows from 0 sccm to 8 sccm were inputted into the chamber during the preparation of ZnO:Al films from dual rotatable ceramic targets under high discharge power (14 kW). The resistivity increases from 4.2 × 10⁻⁴ Ω cm to 4.3 × 10⁻² Ω cm with the rising oxygen gas flow. While both the carrier concentration and mobility drop by one order of magnitude from 3.4 × 10²⁰ cm⁻³ to 2.5 × 10¹⁹ cm⁻³ and from 43.5 cm²/V s to 5.6 cm²/V s, respectively. The as-grown ZnO:Al films and after-etched ZnO:Al films after a chemical wet etching step in diluted HCl solution (0.5%) exhibit different surface structures. All films show high light transmission and low light absorption but different light scattering properties (diffusion and haze) because of different surface structures. Moreover, ZnO:Al films display different optical bandgaps between 3.51 eV and 3.27 eV, which are corresponding to different carrier concentrations. The variation of mobility and morphology is related with chemisorption of oxygen in the grain boundaries as well as high energetic oxygen ions bombardment.     

Research on the properties of ZnO thin films deposited by using filtered cathodic arc plasma technique on glass substrate under different flow rate of O2

ZnO thin film has been deposited on the glass substrate at a temperature of 200 °C using the filtered cathodic arc plasma (FCAP) technique with the oxygen flow rate of 1.0, 3.0, 5.0, 7.0, 9.0 and 10.0 sccm. The deposition processes are only held in pure oxygen atmosphere. The as-grown films exhibit a polycrystalline hexagonal wurtzite structure. With the oxygen flow rate increase, the crystallinity of the samples first increases and then decreases as measured by X-ray diffractometry (XRD). And the tensile stress exists in all the as-grown thin films. The small grain with a mean diameter of 13 nm is observed by the field emission scanning electron microscopy (FESEM). The electrical resistivity values of the thin films are very low ranging from 5.42 × 10⁻³ Ω cm to 4.0 × 10⁻² Ω cm. According to the result from room temperature photoluminescence spectra measurement, the luminescent bands also depend on the oxygen supply.     

Improvement of electrical and optical properties of Ga and N co-doped p-type ZnO thin films with thermal treatment

Ga and N co-doped p-type ZnO thin films were epitaxially grown on sapphire substrate using magnetron sputtering technique. The process of synthesized Ga and N co-doped ZnO films was performed in ambient gas of N₂O. Hall measurement shows a significant improvement of p-type characteristics with rapid thermal annealing (RTA) process in N₂ gas flow, where more N acceptors are activated. The film rapid thermal annealed at 900 °C in N₂ ambient revealed the highest carrier concentration of 9.36 × 10¹⁹ cm⁻³ and lowest resistivity of 1.39 × 10⁻¹ Ω cm. In room and low temperature photoluminescence measurements of the as grown and RTA treated film, donor acceptor pair emission and exciton bound to acceptor recombination at 3.25 and 3.357 eV, respectively, were observed.