N-type ZnO Thin Films Research Articles

Effects of the addition of graphene on the defect-related photoluminescent and electrical properties of n-type ZnO thin films

This study determines the effect of the addition of graphene on the photoluminescent and electrical properties of ZnO films that are prepared using the sol-gel method. ZnO and graphene-doped ZnO thin films that are annealed at 500 °C for 30 min in nitrogen exhibit n-type conductivity. The correlation between photoluminescence (PL) spectral features and defects of ZnO means that there are oxygen vacancies (VO), zinc vacancies (VZn) and oxygen interstitials (Oi) in a ZnO thin film and there are VO, VZn, Oi in a graphene-doped ZnO thin film. The PL result shows that the incorporation of graphene into ZnO leads to a decrease in the number of VO, VZn and Oi and a transition from VZn to the substitution of carbon for zinc (CZn). The decreased number of VO and Oi is attributed to the substitution of VO by Oi, so equal numbers of acceptors and donors are annihilated and there is no change in the net electron carrier density. The transition from VZn to CZn increases the electron carrier concentration, which is opposite to the electrical observation. The results from PL, X-ray diffraction and X-ray photoelectron spectroscopy measurements demonstrate that a CZn+2Oi complex is formed in a graphene-doped ZnO thin film. The decrease in the electron concentration in a graphene-doped ZnO thin film is associated with the formation of the acceptor-like CZn+2Oi complex.

All-Oxide p-n Junction Thermoelectric Generator Based on SnOx and ZnO Thin Films.

Achieving thermoelectric devices with high performance based on low-cost and nontoxic materials is extremely challenging. Moreover, as we move toward an Internet-of-Things society, a miniaturized local power source such as a thermoelectric generator (TEG) is desired to power increasing numbers of wireless sensors. Therefore, in this work, an all-oxide p-n junction TEG composed of low-cost, abundant, and nontoxic materials, such as n-type ZnO and p-type SnOx thin films, deposited on borosilicate glass substrate is proposed. A type II heterojunction between SnOx and ZnO films was predicted by density functional theory (DFT) calculations and confirmed experimentally by X-ray photoelectron spectroscopy (XPS). Moreover, scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS) show a sharp interface between the SnOx and ZnO layers, confirming the high quality of the p-n junction even after annealing at 523 K. ZnO and SnOx thin films exhibit Seebeck coefficients (α) of ∼121 and ∼258 μV/K, respectively, at 298 K, resulting in power factors (PF) of 180 μW/m K2 (for ZnO) and 37 μW/m K2 (for SnOx). Moreover, the thermal conductivities of ZnO and SnOx films are 8.7 and 1.24 W/m K, respectively, at 298 K, with no significant changes until 575 K. The four pairs all-oxide TEG generated a maximum power output (Pout) of 1.8 nW (≈126 μW/cm2) at a temperature difference of 160 K. The output voltage (Vout) and output current (Iout) at the maximum power output of the TEG are 124 mV and 0.0146 μA, respectively. This work paves the way for achieving a high-performance TEG device based on oxide thin films.

Piezotronic effect on Rashba spin–orbit coupling based on MAPbI3/ZnO heterostructures

Rashba spin–orbit coupling (SOC) is a core issue in semiconductor spintronics, which allows the manipulation of electron spin through an electric field rather than an external magnetic field, revealing a bright prospect for advanced electronic devices with ultra-high speed and integration. Conversely, the emerging piezotronic effect is the born characteristic for many semiconductors that have a non-central symmetric structure, such as ZnO and GaN. Here, we design three heterostructure devices, based on piezoelectric p-type (CH3NH3)PbI3 single crystals and n-type wurtzite-structured ZnO thin films, to theoretically study how the piezotronic effect can effectively work on the Rashba spin–orbit coupling. Benefiting from large piezoelectric charges at the interface when a vertical strain is applied, a high concentration of two-dimensional electron gas is induced in the plane of the heterostructure, which can tune the built-in electric field at the interface and further manipulate the Rashba SOC. With the increase in pressure, both the Rashba parameter and spin splitting are found to first vanish and then increase linearly for ZnO with doping densities of 1015 and 1016 cm−3. This work provides insight for manipulating electron spins via the introduction of piezocharges, showing great application potential of the piezotronic effect in tuning spintronic devices.

Raman-based measurement of carrier concentration in n-type ZnO thin films under resonant conditions

For Raman-based carrier concentration determination in thin film semiconductors, above-band-gap excitation is necessary, as the interference from underlying substrate is eliminated by limited photon penetration depth. With resonant Raman spectroscopy, this study investigates the plasmon-phonon coupled modes in n-type ZnO films to non-destructively probe the carrier concentration. The observed coupled mode slightly redshifts from A1(LO) frequency and fails to show monotonic carrier concentration dependency, which differs from the behavior under non-resonant conditions. The correlation between point defects and abnormal behaviors is studied both experimentally and theoretically. The second order LO phonons (2LO) instead of 1LO ones are applied in carrier concentration determination process, in order to avoid the influence from point defects. Reasonable results, especially in high carrier concentration range, are extracted from Raman spectra with the help of Lindhard-Mermin framework.

Influence of yttrium doping on the structural, morphological and optical properties of nanostructured ZnO thin films grown by spray pyrolysis

This study reports on the deposition of highly transparent, n-type ZnO thin films on glass substrate at 450 °C using spray pyrolysis processing, with the simultaneous insertion of yttrium (Y) at different percentages (0, 2, 5, 7 at%) as a dopant. The effect of Y doping on the structure, morphology and optical properties of Y doped ZnO (ZnO:Y) was investigated for optoelectronic applications. The obtained thin films were characterized by means of X-ray diffraction, field-emission scanning electron microscopy (FESEM), UV–visible absorbance measurements, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. The as-prepared films exhibit well-defined hexagonal wurtzite structure grown along [002]. Field emission scanning electron microscope micrographs of the pure ZnO and ZnO:Y showed that the films acquired a dominance of hexagonal-like grains, the morphology was influenced by Y incorporation. All the films showed high transparency in the visible domain with an average transmittance of 83%. The band gap energy, Eg, increased from 3.12 eV to 3.18 eV by increasing the Y doping concentration up to 5 at% and then decreased to 3.15 eV for 7 at% Y content. The PL and CL measurements reveal a strong ultraviolet (UV) emission, suggesting that the as-prepared ZnO:Y thin films can potentially be used in optoelectronic devices.

Metal Oxide Thin-Film Heterojunctions for Photovoltaic Applications.

Silicon-based tandem solar cells incorporating low-cost, abundant, and non-toxic metal oxide materials can increase the conversion efficiency of silicon solar cells beyond their conventional limitations with obvious economic and environmental benefits. In this work, the electrical characteristics of a metal oxide thin-film heterojunction solar cell based on a cuprous oxide (Cu2O) absorber layer were investigated. Highly Al-doped n-type ZnO (AZO) and undoped p-type Cu2O thin films were prepared on quartz substrates by magnetron sputter deposition. The electrical and optical properties of these thin films were determined from Hall effect measurements and spectroscopic ellipsometry. After annealing the Cu2O film at 900 °C, the majority carrier (hole) mobility and the resistivity were measured at 50 cm2/V·s and 200 Ω·cm, respectively. Numerical modeling was carried out to investigate the effect of band alignment and interface defects on the electrical characteristics of the AZO/Cu2O heterojunction. The analysis suggests that the incorporation of a buffer layer can enhance the performance of the heterojunction solar cell as a result of reduced conduction band offset.

Interfacial passivation of n-ZnO/p-Si heterojunction by CuI thin layer * * Project supported by the "333 High-Level Talents Training Project" in Jiangsu Province of China (No. BRA2016111), the Qing Lan Project of Jiangsu Higher Education, the Science and Technology Program of Changzhou (No. CE20175031), the Jiangsu Province Key R & D Projects (No. BE2016200), and the High-Tech Key Laboratory of

The ZnO/Si heterojunction diode can be integrated with the Si process, which has attracted great attention in recent years. However, the large number of interface states at the ZnO/ Si heterojunction interface could adversely affect its optoelectronic properties. Here, n-type ZnO thin film was deposited on p-Si substrate for formation of an n-ZnO/p-Si heterojunction substrate. To passivate the ZnO/Si interface, a thin CuI film interface passivation layer was inserted at the ZnO/p-Si heterojunction interface. Electrical characterization such as I–V and C–V characteristic curves confirmed the significant improvement of the heterojunction properties e.g. enhancement of forward current injection, reduction of reverse current and improvement of the rectification ratio. These results showed that the passivation of interface is critical for ZnO/Si heterojunctions.

Deposition and characterization of spin-coated n-type ZnO thin film for potential window layer of solar cell

Spin-coating technique has been utilized to deposit ZnO thin films on ITO substrates. The dependence of the structural, optical, and electrical properties of the prepared films on both the sol–pH values and annealing temperatures has been investigated. X-ray diffraction analysis has revealed that the optimal sol–pH value was found to be 8 at annealing temperature 450 °C. At these conditions, the optical transmission and reflection data have also shown a clear enhancement in both the morphology and the grain size of the films. In addition, the analysis of I–V and C–V measurements of the Schottky Au/ZnO diode has shown a high agreement with both the investigated structural and optical properties. Furthermore, a new empirical method based on the calculated electrical parameters in the context of both the thermionic emission and Mott–Schottky models has been proposed to evaluate the approximate value of the electron effective mass in the conduction band of the ZnO (m* = 0.224 mo).

Synthesis and characterization of PbS/ZnO thin film for photocatalytic hydrogen production

Abstract In this work, PbS/ZnO thin film was designed in order to assess the photocatalytic hydrogen production from the water splitting. A p-type PbS thin film was prepared by chemical bath deposition onto a glass substrate, followed by an n-type ZnO thin film deposition by RF magnetron sputtering. The structural and morphological characterization was carried out by grazing angle x-ray diffraction (GA-XRD), and atomic force microscopy (AFM), respectively, while the optical properties were studied using ultraviolet-visible spectroscopy (UV–Vis) and spectroscopic ellipsometry (SE). The photocatalytic activity was evaluated by quantifying the hydrogen production under UV light irradiation using gas chromatography. The results showed a higher photocatalytic hydrogen production of PbS/ZnO thin film (7.38 μmol cm −2 h −1 ) compared to PbS (3.35 μmol cm −2 h −1 ) and ZnO (2.45 μmol cm −2 h −1 ) thin films. Linear sweep voltammetry (LSV) test showed the synergistic effect between PbS and ZnO thin films for the generation and transport of the charge carriers.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu<sub>2</sub>O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited <br />Zn<sub>1-x</sub>Mg<sub>x</sub>O

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Investigation of nanostructured Pd–Ag/n-ZnO thin film based Schottky junction for methane sensing

Undoped nanocrystalline n-type ZnO thin film was deposited by chemical deposition technique on a thermally oxidized p-Si (~5 Ω cm resistivity and orientation) substrate. Formation of stable zinc oxide thin film was confirmed by two-dimensional X-Ray Diffraction (XRD) and EDX analysis. The average crystallite size of the ZnO sample was evaluated as ~50 nm. The surface was characterized by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) that confirm the formation of nanocrystalline (grain size ~50 nm) ZnO thin film with surface roughness of ~100 nm. Good conversion of precursor into ZnO thin film in the chemical deposition method was evident by Fourier Transform Infrared Spectroscopy (FTIR). A small peak at 479 cm−1was observed in the FTIR spectrum confirming the formation of quartzite structure of the ZnO. The band gap (~3.44 eV) of the material was calculated from the optical absorption spectroscopy. To prepare Pd–Ag/n-ZnO Schottky junction, Pd–Ag contacts were taken by electron beam evaporation method. I–V characteristics of the junction were studied at different temperatures in inert and reducing ambient (N2 and N2 + CH4) with turn on voltage of around 0.2 V. The parameters like ideality factor (η), saturation current (I 0), series resistance (Rs), and barrier height (Φ BO) of the junction were calculated in the temperature range 50–200 °C in N2 as well as in 1 % CH4 + N2 ambient. It was observed that the ideality factor decreases in the temperature range 50–200 °C (η = 12.34 at 50 °C and η = 1.52 at 200 °C) in N2 ambient and η = 1.18 in N2 +CH4 ambient at 200 °C. Schottky Barrier Height (Φ BO) of the Pd–Ag/n-ZnO junction was found to increase with temperature. A close observation of Pd–Ag/n-ZnO junction in the presence of methane was performed to appreciate its application as methane sensor. The sensing mechanism was illustrated by a simplified energy band diagram.

Fabrication and characterization of light-sensing device based on transparent ZnO thin film prepared by sol-gel

In this work, sol-gel method is used for obtaining the ZnO films. The device is constructed from aluminum and n-type ZnO thin film deposited on top of an n type-silicon substrate. Although the structure is formed from the same type semiconductor materials, it exhibits a rectifying behavior. While the similar studies are found in the open literature for ZnO and Si single crystal, we focus on photoelectrical characteristics of n-ZnO/n-Si isotype device. It is seen that reverse bias I–V characteristics are sensitive to light. It has been seen that the interface capacitance for the device is increased with the illumination effect. It is noted that the illumination dependence of interface capacitance can be used to determine the photo-induced interface states.

Investigation of a Reliable Ohmic Contact to n-Type ZnO Thin Films Prepared by Sol–Gel Method

In this letter, we investigated Ti (20 nm)/Al (30 nm)/Au (60 nm) metallic contacts to Al-doped ZnO thin films prepared by the sol–gel method. In order to measure specific contact resistance, circular transfer length method structures were patterned on the samples by a lithography process. The samples were annealed at the different temperatures and electrical characterization ( $I$ – $V$ ) was performed. The annealing process was optimized to produce ohmic contacts with the lowest specific contact resistance. The results showed that the sample annealed at 350 °C had the lowest specific contact resistance of $\sim 3.7\times 10^{-7}~\Omega \cdot \textrm {cm}^{2}$ . Surface and cross section energy dispersive X-ray analysis was used to determine the distribution of different elements during the annealing process. The results indicated effective inter-diffusion of Al and Ti as well as out-diffusion of oxygen, resulting in an increase in the n-type carrier concentration close to the surface of the ZnO. Scanning electron microscopy and atomic force microscopy analysis were used to determine the effect of annealing on the surface morphology.

Distributed Bragg reflector assisted low-threshold ZnO nanowire random laser diode

An electrically pumped nitrogen doped p-type ZnO nanowires/undoped n-type ZnO thin film homojunction random laser with a 10-period SiO2/SiNx distributed Bragg reflector is demonstrated. The formation of p-n homojunction is confirmed by the current-voltage and photocurrent characteristics. The random lasing behaviors with a low threshold of around 3 mA are observed. The output power is measured to be 220 nW at a drive current of 16 mA.

Inverse spin Hall effect induced by spin pumping into semiconducting ZnO

The inverse spin Hall effect (ISHE) of n-type semiconductor ZnO thin films with weak spin-orbit coupling has been observed by utilizing the spin pumping method. In the ferromagnetic resonance condition, the spin pumping driven by the dynamical exchange interaction of a permalloy film injects a pure spin current into the adjacent ZnO layer. This spin current gives rise to a DC voltage through the ISHE in the ZnO layer, and the DC voltage is proportional to the microwave excitation power. The effect is sizeable even when the spin backflow is considered.

Fabrication of an efficient GLAD-assisted p-NiO nanorod/n-ZnO thin film heterojunction UV photodiode

The glancing angle deposition (GLAD) configuration in RF magnetron sputtering has been utilized to deposit p-type NiO nanorods, which are integrated with n-type ZnO thin film to realize a high performance p–n heterojunction diode for the efficient detection of UV photoradiation. The fabricated nano p-NiO/n-ZnO heterojunction shows excellent rectifying characteristics (rectification ratio ∼ 4 × 103) with a very low reverse saturation current (∼10−10 A). The heterojunction photodiode exhibits a high current gain (3.71 × 103), high photoresponsivity (3.24 × 105 A W−1), with a relatively fast response (∼200 ms) and recovery speed (∼25 ms) towards a very low UV intensity of 0.62 μW cm−2 (λ = 300 nm). Recombination tunneling and space-charge limited current are the probable conduction mechanisms at low and high applied voltages respectively for the fabricated heterojunction diode. The observed high response speed and excellent responsivity of the diode towards deep-UV (λ = 300 nm) radiation might lead to potential military applications such as missile tracking, in addition to optical communications.

Electrical Characterization of n-ZnO/p-Si Heterojunction Prepared by Spray Pyrolysis Technique

The study reports the experimental and the electrical junction properties analysis of current–voltage characteristics of n-ZnO/p-Si heterostructures. Wide band gap semiconducting layer of n-type ZnO thin film was fabricated on p-type Si wafer with spray pyrolysis technique at 550C° to form n-ZnO/ p-Si heterojunctions. The current-voltage characteristic of the n-Zn0/ p-Si heterojunction device has been measured at room temperature in the dark and under illumination (lamp/160 W). The characteristic parameters of the structure such as barrier height, ideality factor and series resistance were determined from the current-voltage measurement.

Investigation of VO–Zni native donor complex in MBE grown bulk ZnO

In this paper, we have experimentally investigated the theoretical predictions of VO–Zni to be a native donor in ZnO. Intrinsically zinc-rich n-type ZnO thin films having ND ∼ 6.23 × 1018 cm−3 grown by molecular beam epitaxy on Si (0 0 1) substrate were annealed in oxygen environment at 500–800 °C, keeping a step of 100 °C for 1 h, each. Room temperature Hall measurements demonstrated that free donor (VO–Zni) concentration decreased exponentially and Arrhenius plot yielded activation energy to be 1.2 ± 0.01 eV. This value is in agreement with theoretically reported activation energy of VO–Zni donor complex in ZnO. We argue; this observation can be explained by two-step process: (i) incoming oxygen fills VO of VO–Zni complex leaving behind Zni; (ii) Zni releases its energy and moves to a lower energy state with respect to the conduction band minima and/or occupies an inactive location. Consequently, Zni–VO complex loses its donor role in the lattice. Our experimental data supported theoretical predictions of VO–Zni to be a native donor. Results from photoluminescence spectroscopy carried out on Zn-rich ZnO additionally justify the existence of VO–Zni complex.

The Electrical Characteristics of ZnO :Ga/p-Si Junction Diode

ABSTRACTThe junction characteristics between ZnO:Ga (GZO) film and p-Si substrate are discussed in the research. For the transparent semiconductor ZnO, the element Ga is chosen to be the dopant source to produce a high quality n-type ZnO thin film. The ZnO:Ga (GZO) film shows a average transmittance is 84.7% (above 400 nm), a bandgap energy of 3.37 eV, a carrier concentration of 7.29×1013 cm−3and a resistivity of 118 Ω-cm. For the GZO/p-Si junction, it shows a junction barrier height of 0.54 eV with an ideality factor of 1.24. The capacitance-voltage measurement shows that it has a uniform reverse bias depletion layer. The Cheung function is also brought to discussion the diode characteristics.