Stable p-type conductivity and enhanced photoconductivity from nitrogen-doped annealed ZnO thin film

We report on the growth of p-type ZnO thin films with improved stability on various substrates and study the photoconductive property of the p-type ZnO films. The nitrogen doped ZnO (N:ZnO) thin films were grown on Si, quartz and alumina substrates by radio frequency magnetron sputtering followed by thermal annealing. Structural studies show that the N:ZnO films possess high crystallinity with c-axis orientation. The as-grown films possess higher lattice constants compared to the undoped films. Besides the high crystallinity, the Raman spectra show clear evidence of nitrogen incorporation in the doped ZnO lattice. A strong UV photoluminescence emission at ~ 380 nm is observed from all the N:ZnO thin films. Prior to post-deposition annealing, p-type conductivity was found to be unstable at room temperature. Post-growth annealing of N:ZnO film on Si substrate shows a relatively stable p-type ZnO with room temperature resistivity of 0.2 Ω cm, Hall mobility of 58 cm²/V s and hole concentration of 1.95 × 10¹⁷ cm^− 3. A homo-junction p-n diode fabricated on the annealed p-type ZnO layer showed rectification behavior in the current-voltage characteristics demonstrating the p-type conduction of the doped layer. Doped ZnO films (annealed) show more than two orders of magnitude enhancement in the photoconductivity as compared to that of the undoped film. The transient photoconductivity measurement with UV light illumination on the doped ZnO film shows a slow photoresponse with bi-exponential growth and bi-exponential decay behaviors. Mechanism of improved photoconductivity and slow photoresponse is discussed based on high mobility of carriers and photodesorption of oxygen molecules in the N:ZnO film, respectively.     

Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Realization of high mobility p-type co-doped ZnO: AlN film with a high density of nitrogen-radicals

High mobility p-type ZnO:AlN thin films have been efficiently realized by utilizing pre-activated nitrogen (N) plasma sources with an inductively coupled dual target co-sputtering system. High density of N-plasma-radicals was generated with an additional RF power applied through a ring-shaped quartz-tube located inside the chamber during co-sputtering process. The AlN codoped ZnO film shows excellent p-type behavior with a high mobility and a hole concentration of 154 cm^2°V^− 1s^− 1 and about 3 × 10^18°cm^− 3 at 600 °C, respectively. Electrical properties of p-n homo-junction devices based on p-type ZnO film are also discussed.     

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.     

Stable p-type ZnO films grown by atomic layer deposition on GaAs substrates and treated by post-deposition rapid thermal annealing

Long-term stable p-type ZnO films were grown by atomic layer deposition on semi-insulating GaAs substrates and followed by rapid thermal annealing (RTA) in oxygen ambient. Significant decrease in the electron concentration and increase in the hole concentration, together with the intensity enhancement of acceptor-related A^oX spectral peak and the shift of bound exciton peak from D^oX to A^oX in the low-temperature photoluminescence spectra, were observed as the RTA temperature increased. Conversion of conductivity from intrinsic n-type to extrinsic p-type ZnO occurred at the RTA temperature of 600 °C. The p-type ZnO film with a hole concentration as high as 3.44 × 10²⁰ cm^− 3 and long-term stability up to 180 days was obtained as the RTA treatment was carried out at 700 °C. The results were attributed to the diffusion of arsenic atoms from GaAs into ZnO as well as the activation of As-related acceptors by the post-RTA treatment.     

Effect of Ag doping on the performance of ZnO thin film transistor

Ag-doped zinc oxide (SZO) thin film transistors (TFTs) have been fabricated using a back-gate structure on thermally oxidized and heavily doped p-Si (100) substrate. The SZO thin films were deposited via pulsed laser deposition (PLD) from a 1, 3, and 5 wt.% Ag-doped ZnO (1SZO, 3SZO, and 5SZO, respectively) target using a KrF excimer laser (λ, 248 nm) at oxygen pressure of 350 mTorr. The deposition carried out at both room-temperature (RT) and 200 °C. The SZO thin films had polycrystalline phase with preferred growth direction of (002) as well as a wurtzite hexagonal structure. Compare with ZnO thin films, the SZO thin films were characterized by confirming the shift of (002) peak to investigate the substitution of Ag dopants for Zn sites. The as-grown SZO TFTs deposited at RT and 200 °C showed insulator characteristics. However the SZO TFTs annealed at 500 °C showed good n-type TFT performance because Ag was diffused from Zn lattice site and bound themselves at the high temperature, and it caused generation of electron carriers. The post-annealed 5SZO TFT deposited at 500 °C exhibited a threshold voltage (V_th) of 11.5 V, a subthreshold swing (SS) of 2.59 V/decade, an acceptable mobility (μ_SAT) of 0.874 cm²/V s, and on-to-off current ratios (I_on/off) of 1.44 × 10⁸.     

Nitrogen doped p-type SnO thin films deposited via sputtering

p-Type SnO thin films were fabricated via reactive RF magnetron sputtering on borosilicate substrates with an Sn target and Ar/O₂/N₂ gas mixture. The undoped SnO thin film consisted of a polycrystalline SnO phase with a preferred (1 0 1) orientation; however, with nitrogen doping, the preferred orientation was suppressed and the grain size decreased. The electrical conductivity of the undoped SnO thin films demonstrated a relatively low p-type conductivity of 0.05 Ω⁻¹ cm⁻¹ and it was lowered slightly with nitrogen doping to 0.039 Ω⁻¹ cm⁻¹. The results of the X-ray photoelectron spectroscopy suggested that the nitrogen doping created donor defects in the SnO thin films causing lower electrical conductivity. Lastly, both the undoped and doped SnO thin films had poor optical transmittance in the visible range.     

Enhanced conductivity of pulsed laser deposited n-InGaZn6O9 films and its rectifying characteristics with p-SiC

Wide band gap InGaZn₆O₉ films of thickness ~ 350 nm were deposited on sapphire (0001) at room temperature by using the pulsed laser deposition technique. The transparent films showed the optical transmission of > 80% with the room temperature Hall mobility of ~ 10 cm²/V s and conductivity of 4 × 10² S/cm at a carrier density > 10²⁰ cm^− 3. The electrical properties as a function of deposition temperatures revealed that the conductivity and mobility almost retained up to the deposition temperature of 200 °C. The films annealed in different atmospheres suggested oxygen vacancy plays an important role in determining the electrical conductivity of the compound. Room temperature grown heterostructure of n-InGaZn₆O₉/p-SiC showed a good rectifying behavior with a leakage current density of less than 10^− 9 A/cm², current rectifying ratio of 10⁵ with a forward turn on voltage ~ 3 V, and a breakdown voltage greater than 32 V.     

Study of Cu diffusion behavior in low dielectric constant SiOC(-H) films deposited by plasma-enhanced chemical vapor deposition

Carbon doped silicon oxide (SiOCH) thin films deposited using plasma-enhanced chemical vapor deposition (PECVD) are commonly used in multilevel interconnect applications. To enhance the electrical performance, the deposited SiOC(-H) films were annealed in a vacuum at various temperatures ranging from 250 to 450 °C. A Cu electrode was then deposited using thermal evaporation. The drift rate of Cu⁺ ions in the SiOC(-H) films with the Cu/SiOC(-H)/p-Si(100)/Al metal-insulator-semiconductor (MIS) structures after annealing was evaluated by C-V measurements with a flatband shift caused by bias-temperature stress (BTS). The samples were stressed at different temperatures of 150 to 275 °C and electric fields up to 1.5 MV/cm to examine the penetration of Cu⁺ ions into the SiOC(-H) films. The Cu⁺ ion drift diffusion behavior was observed by high-resolution transmission electron microscopy and depth profile analysis of the Auger electron spectra. The drift diffusion experiments suggested that the Cu⁺ ion drift rate in the of SiOC(-H) films increased with increasing annealing temperature. A thermal stress and BTS were used to evaluate the impact of Cu penetration on the dielectric properties of the SiOC(-H) films.     

Effects of oxygen concentration on the properties of sputtered SnO2:Sb films deposited at low temperature

Antimony doped tin oxide (SnO₂:Sb) films have been prepared by d.c. magnetron sputtering and the properties of the films depend on deposition conditions, such as O₂ gas ratio, were investigated. The gas composition was found to affect the properties of the films. With the incorporation additional oxygen, the electrical and optical properties of films significantly improved. The minimum value of resistivity of the films was 4.9 × 10^− 3 Ω cm at the oxygen concentration of 30% and the optical transmittance was over 80%.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Investigation of defects with electron spin resonance

Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σ_D = 10^− 6-10^− 1 S/cm) and shows a narrow central ESR line (g ≈ 2.003, peak-to-peak linewidth ?H_pp ≈ 4 G) with two pairs of satellites and a spin density N_S = 10¹⁹ cm^− 3. Al-doping results in the compensation of dark conductivity to as low as σ_D = 10^− 11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity (I_C^IR), ESR line shifts to g ≈ 2.01 and becomes as broad as ?H_pp ≈ 30 G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5 × 10¹⁷ cm^− 3 before it increases again for the highly doped material.     

AlN codoping and fabrication of ZnO homojunction by RF sputtering

The ZnO homojunction fabricated from undoped and 1 mol% AlN doped (codoped) ZnO targets by RF magnetron sputtering has been reported. The grown films on Si (100) substrate have been characterized by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and Hall measurements. The increase of d-space value (compared with unstressed bulk) found from XRD for AlN codoped ZnO film supports the formation of p-ZnO due to the N incorporation. The presence of N in the film has been confirmed by EDS and XPS analysis. Further, the p-conductivity in AlN codoped ZnO has been evidenced by low temperature PL (donor-acceptor-pair emission) and room temperature PL (red shift in near-band-edge emission). Hall measurement shows that 1 mol% AlN codoped ZnO has the hole concentration of 3.772 × 10¹⁹ cm⁻³. The fabricated homojunction with 1% AlN doped ZnO (p-type) and undoped ZnO (n-type) exhibits a typical rectification behavior with high breakdown voltage, and rectification ratio, 13.4. The junction parameters such as ideality factor, barrier height and series resistance have also been calculated for the fabricated p-n junction. The energy band diagram has been proposed for the fabricated homojunction.     

Synthesis of ZnTe nanowires onto TiO2 nanotubular arrays by pulse-reverse electrodeposition

Growth of ZnTe nanowires using a pulse-reverse electrodeposition technique from a non-aqueous solution is reported. ZnTe nanowires were grown on to an ordered nanotubular TiO₂ template in a propylene carbonate solution at 130 °C inside a controlled atmosphere glove box. The pulse-reverse electro deposition process consisted of a cathodic pulse at − 0.62 V and an anodic pulse at 0.75 V Vs Zn²⁺/Zn. Stoichiometry growth of crystalline ZnTe nanowires was observed in the as-deposited condition. The anodic pulse cycle of the pulse-reverse electrodeposition process presumably introduced zinc vacancies as deep level acceptors at an energy level of E_v + 0.47 eV. The resultant ZnTe nanowires showed p-type semiconductivity with a resistivity of 7.8 × 10⁴ Ω cm and a charge carrier density of 1.67 × 10¹⁴ cm^− 3. Annihilation of the defects occurred upon thermal annealing that resulted in marginal decrease in the defect density.     

Enhanced ferroelectric properties of Mg doped (Ba,Sr)TiO3 thick films grown on (001) SrTiO3 substrates

Highly (001)-oriented 1 mol% Mg doped (Ba_0.67,Sr_0.33)TiO₃ (BST) films with a thickness of 1.25 μm were grown on (110) SrRuO₃/(001) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction measurements reveal that the BST thick films have very high crystalline quality, and have a distorted lattice with a large tetragonality a/c = 1.012. The BST thick films have a remanent polarization (P_r) value as large as 10.1 μC/cm² and a coercive electric field (E_c) value of 65.0 kV/cm. The films possess dielectric constant and loss values of ε_r = 385.36 and tgδ = 0.038 at 1 kHz and room temperature. The leakage currents of the films are on the order of 10^− 5 A/cm² at ± 150 kV/cm. The mechanism for enhancing electric properties of the Mg doped BST films was also discussed.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Growth and properties of transparent p-NiO/n-ITO (In2O3:Sn) p-n junction thin film diode

We have grown “all oxide” transparent p-n junction thin film nanostructure device by using chemical solution deposition and E-beam evaporation onto SiO₂ substrate. Combined grazing incidence X-ray diffraction and atomic force microscopy confirm phase pure, mono-disperse 30 nm NiO and 2 at. wt.% Sn doped In₂O₃ (ITO) nanocrystallites. Better than 70% optical transparency, at a wavelength of 600 nm, is achieved across 160 nm thick p-n junction. The optical band gap across the junction was found to decrease as compared to the intrinsic ITO and NiO. The current-voltage (I-V) characteristics show rectifying nature with dynamic transfer resistance ratio of the order of 10³ in the forward bias condition. Very small reverse leakage current with appreciable breakdown was observed under the reverse bias condition. The observed optical and electrical properties of oxide transparent diode are attributed to the heteroepitaxial nature and carrier diffusion at the junction interface.     

Effect of TiOx seed layer on the texture and electric properties in La and Ca modified PbTiO3 thin films

Lanthanum-and calcium-modified PbTiO₃ (PLCT) ferroelectric thin films were successfully prepared on Pt(111)/Ti/SiO₂/Si substrates by pulsed laser deposition. Influence of TiO_x seed layer on texture and electric properties of PLCT films was investigated. It is found the PLCT films without seed layer exhibited highly (100)-textured, while using about 9 nm TiO_x as seed layer lead to highly (301)-textured. The PLCT film with TiO_x seed layer possess higher remnant polarization (Pr = 26 µC/cm²), better pyroelectric coefficient and figure of merit at room temperature (p = 370 µC/m²k, F_d = 190 × 10^− 5 Pa^− 1/2) than that of film without seed layer. The mechanism of the enhanced electric properties was also discussed.     

Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide

Highly doped indium-tin oxide films exhibit resistivities ρ as low as  1.2 × 10^− 4 Ω cm, while for ZnO films resistivities in the range of 2 to 4 × 10^− 4 Ω cm are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10²⁰ cm^− 3. By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 × 10²⁰ cm^− 3, independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10¹² and 3 × 10¹³ cm^− 2 were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10¹⁸ cm^− 3. Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.     

Electrical characterisation of p-doped distributed Bragg reflectors in electrically pumped GaInNAs VCSOAs for 1.3 μm operation

The high resistivity that is encountered in p-type DBRs is an important problem in vertical cavity surface emitting lasers and optical amplifiers (VCSELs and VCSOAs). This is because the formation of potential barriers at the interfaces between layers of high and low refractive index inhibits the carrier flow, thus increasing the DBR series resistance. In this work, the electrical characteristics of two p-type doped DBR structures grown on undoped and p-type doped GaAs substrates have been investigated. The DBRs are designed for VCSOAs operating at 1.3 μm and consist of 14-periods of alternating GaAs and Al_0.9Ga_0.1As in the first sample and 14-periods of GaAs and Al_0.3Ga_0.7As/Al_0.9Ga_0.1As in the second one. For the longitudinal transport sample, Hall mobility and sheet carrier density were measured in the temperature range from 77 to 300 K. In the vertical transport sample, current–voltage (I–V) measurements across the DBR layers were carried out at different temperatures in the range between 15 and 300 K. We achieved resistivity reduction in our samples by using an interface composition grading technique aimed at improving the VCSOA characteristics.     

P-type transparent Ti-V oxides semiconductor thin film as a prospective material for transparent electronics

In this paper electrical and optical properties of mixed titanium and vanadium oxides (Ti-V oxides) thin films have been outlined. Thin films were deposited by sputtering of mosaic Ti-V target in reactive oxygen plasma using high energy magnetron sputtering process. From elemental analysis results, 19 at.% of vanadium was incorporated into thin films and X-ray diffraction investigations displayed their amorphous behavior. However, images obtained by the use of an atomic force microscope displayed a densely packed nanocrystalline structure. It has been found that V addition considerably improves the electrical conduction of prepared Ti-V oxide thin films as compared to undoped TiO₂ and results in p-type electrical conduction. Resistivity of Ti-V oxides thin films was found at the order of 10⁵ Ω cm. Optical measurements have shown the average transmission coefficient of about 73% in the visible spectral range and that the position of fundamental absorption edge has been shifted by 40 nm towards the longer wavelength as compared to the undoped TiO₂. The results testified that the prepared Ti-V oxides thin films might be considered as a p-type transparent oxide semiconductors for future application in transparent electronics.