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.     

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.     

Fabrication and characterization of magnetron sputtered arsenic doped p-type ZnO epitaxial thin films

Arsenic doped p-type ZnO thin films were grown on sapphire substrate by magnetron sputtering. As grown films reveal p-type conduction confirmed by Hall-effect and photoluminescence measurements. The p-type film with a hole concentration of 2.16× 10¹⁷ cm⁻³, mobility of 1.30 cm²/V.s and resistivity of 22.29 Ω-m were obtained at substrate temperature of 700 °C. ZnO homojunction synthesized by in-situ deposition of As doped p-ZnO layer on Al doped n-ZnO layer showed p-n diode like characteristics. X-ray pole figure and Transmission Electron Microscope studies confirm epitaxial nature of the films. Photoluminescence results exhibit the peaks associated with donor acceptor pair emission.     

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.     

Induction of p-type conduction in sputtered deposited Al-N codoped ZnO thin films

Al and N codoped ZnO thin films were grown on n-Si (100) substrate by sputtering technique. Hall effect measurements of as-grown films exhibited n-type conduction, however 500 °C Ar annealed codoped films showed p-type conductivity with a hole concentration of 9.9 × 10¹⁶ cm^− 3, resistivity of 15.95 Ω-cm and hole mobility of 3.95 cm²/Vs, respectively. Codoped ZnO thin films were found to be highly c-axis oriented with good crystal quality. A neutral acceptor-bound exciton and donor-acceptor-pair emissions that appeared at room temperature photoluminescence measurement verify p-type conduction in Al and N codoped ZnO film. The current-voltage characteristics of p-n heterojunction evidently showed a diode like rectifying behaviour.     

Transparent conductive ZnO:Al films grown by atomic layer deposition for Si-wire-based solar cells

Structural, electrical, and optical properties of atomic layer-controlled Al-doped ZnO (ZnO:Al) films grown by atomic layer deposition (ALD) on glass substrates were characterized at various growth temperatures for use as transparent electrodes. The Al atomic content in ZnO:Al films increased due to the reduced ZnO film growth rate with increasing temperature. The preferred orientation of ZnO:Al films was changed, and the optimum condition for best crystallinity was identified by varying the growth temperature. Furthermore, the carrier concentration of free electron was increased by substituting the Zn sites with Al atoms in the crystal, resulting from monolayer growth based on alternate self-limiting surface chemical reactions. The electrical resistivity of ZnO:Al film grown by ALD at 225 °C reached the lowest value of 8.45 × 10⁻⁴ Ω cm, with a carrier mobility of 9.00 cm² V⁻¹ s⁻¹ and optical transmittance of ∼93%. This result demonstrates that ZnO:Al films grown by ALD possess excellent potential for applications in electronic devices and displays as transparent electrodes and surface passivation layers.     

Structural, optical and electrical characterization of highly conducting Mo-doped In2O3 thin films

Highly conducting and transparent thin films of molybdenum-doped indium oxide were deposited on quartz by pulsed laser deposition. The effect of growth temperature and oxygen partial pressure on the structural, optical and electrical properties was studied. We find that the film transparency depends on the growth temperature. The average transmittance of the films grown at different temperatures is in range of 48-87%. The X-ray diffraction results show that the films grown at low temperature are amorphous while the films grown at higher temperature are crystalline. Electrical properties are found to be sensitive to both the growth temperature and oxygen pressure. Resistivity of the films decreases from 1.3 × 10⁻³ Ω cm to 8.9 × 10⁻⁵ Ω cm while mobility increases from 9 cm²/V s to 138 cm²/V s as the growth temperature increases from room temperature to 700 °C. However, with increase in oxygen pressure, resistivity increases but the mobility decreases after attaining a maximum. The temperature-dependent resistivity measurements show transition form semiconductor to metallic behavior. The film grown at 500 °C under an oxygen pressure of 1.0 × 10⁻³ mbar is found to exhibit high mobility (250 cm²/V s), low resistivity (6.7 × 10⁻⁵ Ω cm), and relatively high transmittance (∼90%).     

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).     

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

p-Type ZnO thin films have been realized via doping Li as acceptor by using pulsed laser deposition. In our experiment, Li₂CO₃ was used as Li precursor, and the growth temperature was varied from 400 to 600 °C in pure O₂ ambient. The Li-doped ZnO film prepared at 450 °C possessed the lowest resistivity of 34 Ω cm with a Hall mobility of 0.134 cm² V⁻¹ s⁻¹ and hole concentration of 1.37 × 10¹⁸ cm⁻³. X-ray diffraction (XRD) measurements showed that the Li-doped ZnO films grown at different substrate temperatures were of completely (0 0 2)-preferred orientation.     

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.     

Influence of the deposition pressure on the properties of transparent conducting zirconium-doped zinc oxide films prepared by RF magnetron sputtering

Transparent and conducting zirconium-doped zinc oxide films with high transparency and relatively low resistivity have been successfully prepared by RF magnetron sputtering at room temperature. The deposition pressure was varied from 0.6 to 2.5 Pa. A transformation from a relatively compact structure to individual grains was observed with the increase of deposition pressure. As the deposition pressure increases, the resistivity increases sharply due to both, the decrease of hall mobility and carrier concentration. The lowest resistivity achieved was 2.07 × 10⁻³ Ω cm at a deposition pressure of 0.6 Pa with a hall mobility of 16 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.95 × 10²⁰ cm⁻³. The films are polycrystalline with a hexagonal structure and a preferred orientation along the c-axis. All the films present a high transmittance of above 90% in the visible range. The optical band gap decreases from 3.35 to 3.20 eV as the deposition pressure increases from 0.6 to 2.5 Pa.     

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.     

Effects of growth temperature on Li-N dual-doped p-type ZnO thin films prepared by pulsed laser deposition

Li-N dual-doped p-type ZnO (ZnO:(Li,N)) thin films have been prepared by pulsed laser deposition. The introduction of Li and N was confirmed by secondary ion mass spectrometry measurements. The structural, electrical, and optical properties as a function of growth temperature were investigated in detail. The lowest room-temperature resistivity of 3.99 Ω cm was achieved at the optimal temperature of 450 °C, with a Hall mobility of 0.17 cm²/V s and hole concentration of 9.12 × 10¹⁸ cm⁻³. The ZnO:(Li,N) films exhibited good crystal quality with a complete c-axis orientation, a high transmittance (about 90%) in the visible region, and a predominant UV emission at room temperature. The two-layer-structure p-ZnO:(Li,N)/n-ZnO homojunctions were fabricated on a sapphire substrate. The current-voltage characteristics exhibited the rectifying behavior of a typical p-n junction.     

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.     

Influence of oxygen partial pressure on optoelectrical properties of aluminum-doped CdO thin films

Highly conducting and transparent aluminum-doped CdO thin films were deposited on quartz by ablating the sintered target of CdO containing 2 wt% of aluminum with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). The effect of oxygen partial pressure on structural, electrical, and optical properties was studied. It is observed that the (2 0 0) plane is highly preferred for the films grown under high oxygen pressure. The conductivity, carrier concentration and mobility of the films decrease with increase in the oxygen pressure after attaining maximum. Low resistivity (2.27 × 10⁻⁵ Ω cm), and high mobility (79 cm² V⁻¹ s⁻¹) is observed for the film grown under oxygen pressure of 1.0 × 10⁻³ mbar. The optical band gap is found varying between 2.68 and 2.90 eV for various oxygen pressure.     

Ag-N doped ZnO film and its p-n junction fabricated by ion beam assisted deposition

Ag-N doped ZnO film was synthesized by ion beam assisted deposition and its electrical properties and annealing property were investigated. The films remained p-type even after annealing at 400 °C in air for 10 min. While the annealing temperature went up to 500 °C, the conduction type of these films shifted from p-type to n-type. The p-type ZnO film revealed low resistivity (0.0016 Ω cm), low Hall mobility (0.65 cm² V⁻¹ s⁻¹) and high carrier concentration (5.8 × 10²⁰ cm⁻³). ZnO p-n homojunction consisting of a p-type layer (Ag-N doped ZnO film) and an n-type layer (In-doped ZnO film) had been fabricated by ion beam assisted deposition. With electrical measurement, its current-voltage curve had a typical rectifying characteristic with current rectification ratio of 25 at bias ±5 V and a reverse current of 0.01 mA at −5 V. The depletion width was estimated 3.8 nm by using p-n junction equation.     

Influence of sputter-etching of substrate on the microstructural and optical properties of ZnO films deposited by RF magnetron sputtering

ZnO films were prepared using radio frequency magnetron sputtering on Si(1 1 1) substrates that were sputter-etched for different times ranging from 10 to 30 min. As the sputter-etching time of the substrate increases, both the size of ZnO grains and the root-mean-square (RMS) roughness decrease while the thickness of the ZnO films shows no obvious change. Meanwhile, the crystallinity and c-axis orientation are improved by increasing the sputter-etching time of the substrate. The major peaks at 99 and 438 cm⁻¹ are observed in Raman spectra of all prepared films and are identified as E₂(low) and E₂(high) modes, respectively. The Raman peak at 583 cm⁻¹ appears only in the films whose substrates were sputter-etched for 20 min and is assigned to E₁(LO) mode. Typical ZnO infrared vibration peak located at 410 cm⁻¹ is found in all FTIR spectra and is attributed to E₁(TO) phonon mode. The shoulder at about 382 cm⁻¹ appearing in the films whose substrates were sputter-etched for shorter time (10-20 min) originates from A₁(TO) phonon mode. The results of photoluminescence (PL) spectra reveal that the optical band gap (Eg) of the ZnO films increases from 3.10 eV to 3.23 eV with the increase of the sputter-etching time of the substrate.     

Growth of transparent conducting nano-structured In doped ZnO thin films by pulsed DC magnetron sputtering

Transparent conducting nano-structured In doped zinc oxide (IZO) thin films are deposited on corning 7059 glass substrates by bipolar pulsed DC magnetron sputtering with variation of pulsed frequency and substrate temperature. Highly c-axis oriented IZO thin films were grown in perpendicular to the substrate on the 30 kHz and 500 °C. The IZO films exhibited surface roughness of 3.6 nm similar to the commercial ITO and n-type semiconducting properties with electrical resistivity (carrier mobility) of about 5 × 10⁻³ Ω cm (14 cm²/V s). The optical characterization showed high transmittance of over 85% in the UV-vis region and exhibited the absorption edge of near 350 nm. In micro-Raman spectra, the origin of two additional modes is attributed to the host lattice defect due to the addition of In dopant. These results suggest that the IZO film can possibly be applied to make transparent conducting electrodes for flat panel displays.     

Correlation between electrical and optical properties of Cr:ZnO thin films grown by pulsed laser deposition

Highly transparent and conducting Chromium doped ZnO (Cr:ZnO) thin films with preferential c-axis orientation were grown on (0 0 0 1) sapphire substrates using buffer assisted pulsed laser deposition. The resistivity of Cr:ZnO thin films was found to decrease to a minimum value of ∼1.13×10⁻³Ω cm with the increasing Cr concentration up to ∼1.9 at.% and then increase with further increase of Cr concentration. On the contrary, the band gap and carrier concentration of Cr:ZnO thin films increased up to ∼3.37 eV and ∼2×10²⁰ cm⁻³, respectively, with the increase of Cr concentration up to ∼1.9 at.%, then decreased with further increase of Cr concentration. The increase of carrier concentration and conductivity with Cr doping at low Cr concentrations (<1.9 at.%) could be attributed to the presence of Cr in +3 valence state in ZnO thus acting as donor while decrease of carrier concentration beyond ∼1.9 at.% of Cr concentration could be attributed to the charge compensating effect due to the presence of acceptor like point defects such as oxygen interstitials. This was experimentally confirmed using x-ray photoelectron spectroscopy. The observed variation in the band gap of Cr:ZnO thin films with increasing Cr doping was attributed to the competing effects of the high free carrier concentration induced Burstein-Moss blue shift and band gap narrowing.     

Determination of electrical types in the P-doped ZnO thin films by the control of ambient gas flow

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5 × 10¹⁷ cm⁻³ and 2.5 cm²/V s, respectively. X-ray diffraction showed that the ZnO (0 0 0 2) peak shifted to lower angle due to the positioning of P³⁻ ions with a larger ionic radius in the O²⁻ sites. This indicates that a p-type mechanism was due to the substitutional P_O. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.