Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na,N)/n-ZnO:Eu homojunction

Sodium and nitrogen dual acceptor doped p-type ZnO (ZnO:(Na, N)) films have been prepared by spray pyrolysis technique at a substrate temperature of 623 K. The ZnO:(Na, N) films are grown at a fixed N doping concentration of 2 at.% and varying the nominal Na doping concentration from 0 to 8 at.%. The XRD results show that all the ZnO:(Na, N) films exhibited (0 0 2) preferential orientation. The EDX and elemental mapping analysis shows the presence and distribution of Zn, O, Na and N in the deposited films. The Hall measurement results demonstrate that the Na–N dual acceptor doped ZnO films show excellent p-type conduction. The p-type ZnO:(Na, N) films with comparatively low resistivity of 5.60 × 10⁻² Ω cm and relatively high carrier concentration of 3.15 × 10¹⁸ cm⁻³ are obtained at 6 at.%. ZnO based homojunction is fabricated by depositing n-type layer (Eu doped ZnO) grown over the p-type layer ZnO:(Na, N). The current–voltage (I–V) characteristics measured from the two-layer structure show typical rectifying characteristics of p-n junction with a low turn on voltage of about 1.69 V. The ZnO:(Na, N) films exhibit a high transmittance (about >90%) and the average reflectance is 8.9% in the visible region. PL measurement shows near-band-edge (NBE) emission and deep-level (DL) emission in the ZnO:(Na, N) thin films.     

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.     

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.     

Realization of phosphorus-doped p-type ZnO thin films via diffusion and thermal activation

ZnO thin films were initially deposited on a heavily phosphorus-doped Si (n⁺-Si) substrate by radio frequency magnetron sputtering. The transition from n-type ZnO to p-type one was realized by phosphorus diffusing from Si substrate to ZnO film and being thermally activated during post annealing. Crystal structures of the ZnO films were confirmed to be highly c-axis oriented wurtzite structure by X-ray diffraction experiment. Photoluminescence spectra of the ZnO films showed strong ultraviolet emissions originated from the recombination of the band-edge excitons. The composition of the films was measured by X-ray photoelectron spectroscopy, and a typical concentration of phosphorus was about 0.48% corresponding to the order of atomic density of 10¹⁹/cm³. The hole concentration of the film was + 1.28 × 10¹⁹/cm³ measured by Hall effect apparatus. Formation of the p-type ZnO films can be further confirmed by the rectifying I-V curves of p-ZnO/n⁺-Si heterojunctions.     

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.     

Nitrogen-doped p-type ZnO thin films and ZnO/ZnSe p-n heterojunctions grown on ZnSe substrate by radical beam gettering epitaxy

We investigate the p-type doping in ZnO prepared by the method of radical beam gettering epitaxy using NO gas as the oxygen source and nitrogen dopant. Secondary ion mass spectroscopy measurements demonstrate that N is incorporated into ZnO film in concentration of about 8 × 10¹⁸ cm^− 3. The hole concentration of the N-doped p-type ZnO films was between 1.4 × 10¹⁷ and 7.2 × 10¹⁷ cm^− 3, and the hole mobility was 0.9-1.2 cm²/Vs as demonstrated by Hall effect measurements. The emission peak of 3.312 eV is observed in the photoluminescence spectra at 4.2  of N-doped p-type ZnO films, probably neutral acceptor bound. The activation energy of the nitrogen acceptor was obtained by temperature-dependent Hall-effect measurement and equals about 145 meV. The p-n heterojunctions ZnO/ZnSe were grown on n-type ZnSe substrate and have a turn-on voltage of about 3.5 V.     

Investigation of the electrical parameters of Ag/p-TlGaSeS/C Schottky contacts

p-type TlGaSeS single crystal was used to fabricate a Schottky device. Silver and carbon metals were used as the Ohmic and Schottky contacts, respectively. The device which displayed wide RF band at 13.200 and narrow band at 62.517 kHz with Q value of 1.4 and of 6.3 × 10⁴, respectively, is characterized by means of current (I)–voltage (V), capacitance (C)–voltage characteristics as well as capacitance–frequency (f) characteristics. The device series resistance, ideality factor and barrier height are determined from the I–V curve as 35.8 MΩ, 1.2 and 0.74 eV, respectively. The apparent acceptor density and the build in voltage of the device increased with increasing ac signal frequency. The high Q value, observed at 62.517 kHz, indicated a much lower rate of energy loss relative to the stored energy of the device. The energy loss (Q⁻¹) is much less than 0.001% of the stored value. The device was tested and found to remain at the same mode of resonance for several hours. It never switched or ceased unless it was tuned off.     

Effects of oxygen pressure on n-ZnO/p-Si heterojunctions fabricated using pulsed laser deposition

ZnO films were prepared on p-Si substrates using pulsed laser deposition (PLD) and n-ZnO/p-Si heterojunctions were fabricated at different oxygen partial pressures. The effects of oxygen pressures on crystallinity and surface morphology of ZnO films and the I-V characteristics of n-ZnO/p-Si heterojunctions were studied. It was found that the films grown in the oxygen pressure range from 10⁻⁵-10⁻² Torr were all c-axis oriented. The surface morphologies were strongly dependent on the oxygen partial pressure. The current-voltage (I-V) characteristics of the heterojunctions could be classified into two categories depending on the oxygen pressure. At low pressure (10⁻⁵-10⁻⁴ Torr), the I-V curves were similar to those of common p-n junctions. As the oxygen pressure increased to 10⁻³ Torr, the I-V curves changed markedly. Based on the I-V characteristics, an energy band diagram of n-ZnO/p-Si was proposed.     

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.     

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.     

Raman scattering studies of Mn-doped ZnO thin films deposited under pure Ar or Ar + N2 sputtering atmosphere

This paper investigates the anomalous and specific Raman modes present in Mn-doped ZnO thin films deposited using the magnetron co-sputtering method. To trace these peaks, we prepared Mn-doped ZnO films with different Mn concentrations by altering the sputtering power of the Mn target in a pure Ar or Ar + N₂ sputtering atmosphere. A broad band observed in the Raman spectra of heavily Mn-doped ZnO films ranges from 500 to 590 cm^− 1. This band involves the enhanced A₁ longitudinal mode and activated silent modes of ZnO, as well as a characteristic mode of Mn₂O₃. Four anomalous Raman peaks at approximately 276, 510, 645 and 585 cm^− 1 are present in pure and Mn-doped ZnO films deposited under the Ar + N₂ sputtering atmosphere. The peaks at 276 cm^− 1 and 510 cm^− 1 may originate from the complex defects of Zn_i-N_O and Zn_i-O_i, respectively, while the peak at approximately 645 cm^− 1 could be due to a complex defect of Zn_i coupled with both the N and Mn dopants. The results of this study suggest classifying the origins of anomalous and specific Raman peaks in Mn-doped ZnO films into three major types: structural disorder and morphological changes caused by the Mn dopant, Mn-related oxides and intrinsic host-lattice defects coupled with/without the N dopant.     

Ni-doped vertically aligned zinc oxide nanorods prepared by hybrid wet chemical route

Ni-doped zinc oxide (Ni:ZnO) nanorods were synthesized by incorporating nickel in vertically aligned ZnO nanorods. Ni was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing for dispersing Ni in ZnO nanorods. The optical band gap decreased with increasing amount of Ni incorporation. The origin of the photoluminescence peak at ∼ 400 nm was related to the defect levels introduced due to substitution of Ni²⁺ in the Zn²⁺ site with annealing. The Raman spectra indicated the presence of the characteristic peak at ∼ 436 cm^− 1 which was identified as high frequency branch of E₂ mode of ZnO. The Fourier Transformed Infrared spectra indicated the existence of the distinct characteristic absorption peak at 481 cm^− 1 for ZnO stretching modes. Current-voltage characteristics indicated that the current changed linearly with voltage for both the doped and undoped samples.     

Study on the Hall-effect and photoluminescence of N-doped p-type ZnO thin films

N-doped, p-type ZnO thin films have been grown by plasma-assisted metal-organic chemical vapor deposition method. The results under optimized growth conditions included a resistivity of 1.72 Ω cm, a Hall mobility of 1.59 cm²/V s, and a hole concentration of 2.29 × 10¹⁸ cm^− 3, and were consistently reproducible. A N-related free-to-neutral-acceptor emission and an associated phonon replica were evident in room temperature photoluminescence spectra, from which the N acceptor energy level in ZnO was estimated to be 180 meV above the valence band maximum.     

Effect of an organic compound (Methyl Red) interfacial layer on the calculation of characteristic parameters of an Al/Methyl Red/p-Si sandwich Schottky barrier diode

An Al/Methyl Red/p-Si sandwich Schottky barrier diode (SBD) has been fabricated by adding a solution of the organic compound Methyl Red in chloroform onto a p-Si substrate, and then evaporating the solvent. Current-voltage (I-V) measurements of the Al/Methyl Red/p-Si sandwich SBD have been carried out at room temperature and in the dark. The Al/Methyl Red/p-Si sandwich SBD demonstrated rectifying behavior. Barrier height (BH) and ideality factor values of 0.855 eV and 1.19, respectively, for this device have been determined from the forward-bias I-V characteristics. The Al/Methyl Red/p-Si sandwich SBD showed non-ideal I-V behavior with the value of ideality factor greater than unity. The energy distribution of the interface state density determined from I-V characteristics increases exponentially with bias from 3.68 × 10¹² cm^− 2 eV^− 1 at (0.81 − E_v) eV to 9.99 × 10¹³ cm^− 2 eV^− 1 at (0.69 − E_v) eV.     

Effects of growth ambient on electrical properties of Al-N co-doped p-type ZnO films

p-Type zinc oxide thin films with c-axis orientation were prepared in N₂O-O₂ atmosphere by an Al-N co-doping method using reactive magnetron sputtering. Secondary ion mass spectroscopy (SIMS) measurements indicate that as-grown ZnO films were co-doped with Al and N. Hall effect measurements show a dependence of types of conduction, carrier concentration and mobility of as-grown ZnO films on N₂O partial pressure ratios. p-Type ZnO thin films deposited in a N₂O partial ratio of 10% show the highest hole concentration of 1.1×10¹⁷ cm⁻³, the lowest resistivity of about 100 Ω cm, and a low mobility of 0.3 cm² V⁻¹ s⁻¹.     

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.     

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.     

Low temperature solution synthesis and characterization of ZnO nano-flowers

Synthesis of flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods was achieved by the solution process using zinc acetate dihydrate and sodium hydroxide at very low temperature of 90 °C in 30 min. The individual nanorods are of hexagonal shape with sharp tip, and base diameter of about 300-350 nm. Detailed structural characterizations demonstrate that the synthesized products are single crystalline with the wurtzite hexagonal phase, grown along the [0 0 0 1] direction. The IR spectrum shows the standard peak of zinc oxide at 523 cm⁻¹. Raman scattering exhibits a sharp and strong E₂ mode at 437 cm⁻¹ which further confirms the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement shows the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p_3/2) and 1044.3 eV (Zn 2p_1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrate a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm, suggests that the flower-shaped ZnO nanostructures have good optical properties with very less structural defects.     

Microstructural and electrochemical properties of vertically aligned few layered graphene (FLG) nanoflakes and their application in methanol oxidation

Vertically aligned few layered graphene (FLG) nanoflakes were synthesised on silicon substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Transmission electron microscopy (TEM) shows that the structures have highly graphitized terminal planes of 1–3 layers of graphene. Raman spectroscopy revealed a narrow G band with a FWHM of ∼23 cm⁻¹ accompanied by a strong G′ (2D) band, with a FWHM of ∼43 cm⁻¹ and an I_G/I_G′ ratio of 1, which are all the characteristics of highly crystallized few layered graphene. The FLG electrodes demonstrate fast electron transfer (ET) kinetics for Fe(CN)₆^3−/4− redox system with an electron transfer rate, ΔE_p, of 60 mV. Platinum (Pt) nanoparticles of ∼6 nm diameter were deposited on as grown FLGs using magnetron DC sputtering for methanol oxidation studies. When used as electrodes for methanol oxidation, a mass specific peak current density of ∼62 mA mg⁻¹ cm⁻² of Pt is obtained with a high resistance to carbon monoxide (CO) poisoning as evident by a high value of 2.2 for the ratio of forward to backward anodic peak currents (I_f/I_b).     

Room temperature electroluminescence from the n-ZnO/p-GaN heterojunction device grown by MOCVD

The heterojunction light-emitting diode with n-ZnO/p-GaN structure was grown on (0 0 0 1) sapphire substrate by metalorganic chemical vapor deposition (MOCVD) technique. The heterojunction structure was consisted of an Mg-doped p-type GaN layer with a hole concentration of ∼10¹⁷ cm⁻³ and a unintentionally doped n-type ZnO layer with an electron concentration of ∼10¹⁸ cm⁻³. A distinct blue-violet electroluminescence with a dominant emission peak centered at ∼415 nm was observed at room temperature from the heterojunction structure under forward bias conditions. The origins of the electroluminescence (EL) emissions are discussed in comparison with the photoluminescence spectra, and it was supposed to be attributed to a radiative recombination in both n-ZnO and p-GaN sides.