Optically transparent ZnO-based n-i-p ultraviolet photodetectors

An optically transparent tin-doped indium oxide/ZnO/NiO n-i-p heterostructure photodiode was fabricated by ion beam assisted e-beam evaporation. The diode clearly demonstrates rectifying current-voltage (J-V) characteristics with a current rectification ratio up to 10⁴ at bias ± 2 V and a low reverse current of ∼ 100 nA/cm² at − 5 V. Analysis of J-V characteristics including time dependence of the dark current shows that the leakage current at low biases is attributed to thermal generation via defect states, and at high biases, field-enhanced carrier generation from the ZnO layer dominates. Spectral response and linearity measurements indicate that such a diode is particularly suitable for low level of ultraviolet detection.     

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.     

UV-detector based on pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO/n-ZnO

A transparent ultraviolet (UV)-detector was fabricated using a high-quality pn-heterojunction diode composed of transparent oxide semiconductors, p-type NiO and n-type ZnO, and its UV-response was measured at room temperature. Transparent tri-layered oxide films of ZnO/NiO/ITO were heteroepitaxially grown on an YSZ (1 1 1) substrate by a pulsed-laser-deposition combined with a solid-phase-epitaxy technique and they were processed to fabricate a p-NiO/n-ZnO diode. The diodes exhibited a clear rectifying I-V characteristic with an ideality factor of ∼2 and a forward threshold voltage of ∼1 V. Although the photo-responsivity was fairly weak at the zero bias voltage, it was enhanced up to ∼0.3 A W⁻¹ by applying a reverse bias of −6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices.     

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.     

1.31 μm GaAs-based heterojunction p-i-n photodetectors using InGaAsNSb as the intrinsic layer grown by molecular beam epitaxy

GaAs-based double-heterojunction p-i-n photodetectors using In_zGa_1−zAs_1−x−yN_xSb_y in the i-layer are fabricated for the first time using the solid source molecular beam epitaxy growth method. A peak responsivity of ∼ 0.29 A/W, corresponding to quantum efficiencies (QE) of 38% is attained from the best p-i-n device between 0.9 and 1.1 μm. The dark current is ∼ 70 nA at a reverse bias of 2 V and the cutoff wavelength reaches ∼ 1.4 μm. The surfactant effect generated by the presence of Sb in this material allows thick high quality dilute nitride material growth. A Sb-free p-i-n device consisting of InGaAsN/GaAs is also fabricated to compare the device performance with the InGaAsNSb/GaAs p-i-n devices.     

Ultraviolet light emitting diode with n-ZnO:Ga/i-ZnO/p-GaN:Mg heterojunction

n-ZnO:Ga/p-GaN:Mg heterojunction light emitting diodes (LEDs) were fabricated at different growth temperatures and carrier concentrations in the n-type region. The effects of growth temperature and carrier concentration on the electrical and emission properties were investigated. The I-V and EL results showed that the improved device performances such as lower turn-on voltage and true ultraviolet emission were achieved with the insertion of a thin intrinsic layer between n-ZnO:Ga and p-GaN:Mg. This observation was attributed to a lowering of energy barriers for the supply of electrons and holes into intrinsic ZnO and recombination in the intrinsic ZnO with the absence of deep-level emission.     

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.     

Hybrid p-type ZnO film and n-type ZnO nanorod p-n homo-junction for efficient photovoltaic applications

Simple hybrid p-n homo-junctions using p-type ZnO thin films and n-type nanorods grown on fluorine tin oxide (FTO) substrates for photovoltaic applications are described. The ZnO nanorods (1.5 μm) were synthesized via an aqueous solution method with zinc nitrate hexahydrate and hexamethylenetetramine on ZnO seed layers. The 10-nm-thick ZnO seed layers showed n-type conductivity on FTO substrates and were deposited with a sputtering-based method. After synthesizing ZnO nanorods, aluminum-nitride co-doped p-type ZnO films (200 nm) were efficiently grown using pre-activated nitrogen (N) plasma sources with an inductively-coupled dual-target co-sputtering system. The structural and electrical properties of hybrid p-n homo-junctions were investigated by scanning electron microscopy, transmittance spectrophotometry, and I-V measurements.     

Electrical characteristics of Au/n-GaAs Schottky barrier diodes with and without SiO2 insulator layer at room temperature

In this study, our main goal is fabricated with and without insulator layer Au/n-GaAs Schottky barrier diodes (SBDs) to explain whether or not the insulator layer is effective on some electric parameters such as Φ_B, n, N_ss, and R_s. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-semiconductor (Au/n-GaAs) are investigated and compared with metal-insulator-semiconductor (Au/SiO₂/n-GaAs) Schottky diodes. From the room temperature I-V characteristics of these devices, the main electrical parameters such as, ideality factor (n) and zero bias barrier height (Φ_bo) values of 1.25 and 0.73 eV for Au/n-GaAs, and 1.51 and 0.75 eV for Au/SiO₂/n-GaAs, were obtained. The interface distribution profile (N_ss) as a function of (E_c − E_ss) was extracted from the forward-bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_e) and series resistance (R_s) for the Schottky diodes. The N_ss values obtained taking into account the series resistance values are lower than those obtained without considering the series resistance. The diodes show non-ideal I-V behavior with ideality factor greater than unity. This behavior is attributed to the interfacial insulator layer and the interface states. The I-V characteristics confirmed that the distribution of N_ss, R_s, and interfacial insulator layer are important parameters that influence the electrical characteristics of metal-semiconductor and metal-insulator-semiconductor Schottky diodes.     

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 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.     

Fabrication of p-type ZnO nanowires based heterojunction diode

Vertically aligned p-type ZnO (Li–N co-doped) nanowires have been synthesized by hydrothermal method on n-type Si substrate. X-ray diffraction pattern indicated a strong peak from (0 0 0 2) planes of ZnO. The appearance of a strong peak at 437 cm⁻¹ in Raman spectra was attributed to E₂ mode of ZnO. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peaks at 490 cm⁻¹ for ZnO stretching mode. Compositional studies revealed the formation of Li–N co-doped ZnO, where Li was bonded with both O and N. The junction properties of p-type ZnO nanowires/n-Si heterojunction diodes were evaluated by measuring I–V and C–V characteristics. I–V characteristics exhibited the rectifying behavior of a typical p–n junction diode.     

Effect of thickness of ZnO active layer on ZnO-TFT's characteristics

We have investigated the electrical characteristics of ZnO thin film transistors with respect to the thickness of ZnO active layers. The ZnO layers with the thickness of 30 nm to 150 nm were deposited on bottom gate patterned Si substrate by RF sputtering at room temperature. The low-temperature oxide served as gate dielectric. As ZnO channel layer got thicker, the leakage current at V_DS = 30 V and V_G = 0 V greatly increased from 10^− 10 A to 10^− 6 A, while the threshold voltage decreased from 15 V to 10 V. On the other hand, the field effect mobility got around 0.15 cm²/V s except for the 30-nm-thick channel. Overall, the 55-nm-thick ZnO channel layer showed the best performance.     

High frequency characteristics of tin oxide thin films on Si

High frequency characteristics of tin oxide (SnO₂) thin films were studied. SnO₂ thin films have been successfully grown on n-type Si (111) substrates by using a spray deposition technique. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of the metal-oxide-semiconductor (Au/SnO₂/n-Si) Schottky diodes were investigated in the high frequency range from 300 kHz to 5 MHz. It has been shown that the interface state density, D_it, ranges from 2.44 × 10¹³ cm⁻² eV⁻¹ at 300 kHz to 0.57 × 10¹³ cm⁻² eV⁻¹ at 5 MHz and exponentially decreases with increasing frequency. The C-V and G/ω-V characteristics confirm that the interface state density and series resistance of the diode are important parameters that strongly influence the electrical parameters exhibited by the metal-oxide-semiconductor structure.     

Analysis of surface states and series resistance in Au/n-Si Schottky diodes with insulator layer using current-voltage and admittance-voltage characteristics

In order to good interpret the experimentally observed Au/n-Si (metal-semiconductor) Schottky diodes with thin insulator layer (18 Å) parameters such as the zero-bias barrier height (Φ_bo), ideality factor (n), series resistance (R_s) and surface states have been investigated using current-voltage (I-V), capacitance-frequency (C-f) and conductance-frequency (G-f) techniques. The forward and reverse bias I-V characteristics of Au/n-Si (MS) Schottky diode were measured at room temperature. In addition, C-f and G-f characteristics were measured in the frequency range of 1 kHz-1 MHz. The higher values of C and G at low frequencies were attributed to the insulator layer and surface states. Under intermediate forward bias, the semi-logarithmic Ln (I)-V plot shows a good linear region. From this region, the slope and the intercept of this plot on the current axis allow to determine the ideality factor (n), the zero-barrier height (Φ_bo) and the saturation current (I_S) evaluated to 2.878, 0.652 and 3.61 × 10⁻⁷ A, respectively. The diode shows non-ideal I-V behavior with ideality factor greater than unity. This behavior can be attributed to the interfacial insulator layer, the surface states, series resistance and the formation barrier inhomogeneity at metal-semiconductor interface. From the C-f and G-f characteristics, the energy distribution of surface states (N_ss) and their relaxation time (τ) have been determined in the energy range of (E_c − 0.493E_v)-(E_c − 0.610) eV taking into account the forward bias I-V data. The values of N_ss and τ change from 9.35 × 10¹³ eV⁻¹ cm⁻² to 2.73 × 10¹³ eV⁻¹ cm⁻² and 1.75 × 10⁻⁵ s to 4.50 × 10⁻⁴ s, respectively.     

Nanostructured solar cell by spray pyrolysis: Effect of titania barrier layer on the cell performance

ZnO nanostructured solar cells with CuInS₂ absorber layer were prepared by chemical spray method. In order to increase chemical stability of ZnO nanorods against dissolution in the next steps of the cell preparation, and reduce the electrical shorts between the front and back contacts, an amorphous TiO₂ layer was deposited on ZnO nanorods by ALD or sol-gel spray technique. The thicknesses of the layer (≤ 5 nm by spray and ≤ 1 nm by ALD), which did not impede the collection of carriers, were determined. TiO₂ thicknesses above these optimal values led to s-shaped I-V curves, causing the decrease in solar cell efficiency from 2.2 to 0.7% due to the formation of an additional junction blocking charge carrier transport in the device under forward bias. Nanostructured cells suffered from somewhat higher interface recombination but showed still two times higher current densities (~ 10 mA/cm²) than the planar devices did.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Development of NiO-based thin film structures as efficient H2 gas sensors operating at room temperatures

P-type NiO thin films have been developed on high resistivity Si and SiO₂ substrates by a pulsed laser deposition technique using an ArF^? 193 nm excimer laser at deposition temperature of 300 °C and in 40 Pa partial oxygen pressure. Structures based on such NiO films as host material in the form of Au-NiO Schottky diodes have been subsequently developed under vacuum. In a different procedure, an n-SnO₂ layer has been deposited by a CVD technique on a NiO film to produce a p/n heterojunction. The sensing properties of all above structures have been tested upon exposure to a H₂ flow in air ambient gas at various operating temperature ranging from 30 to 180 °C. For the NiO films, the optimum temperature was about 150 °C exhibiting a sensitivity of 94%. After surface sensitization of NiO by Au the NiO films showed an H₂ response at operating temperature of 30 °C. The sensitivity of p-NiO/n-SnO₂ heterojunction devices was extracted from I-V measurements in air and under H₂ flow mixed in air. In this case a dramatic increase of the sensitivity was achieved at operating temperature of 30 °C for a forward bias of 0,2 V.     

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112?0) and ZnO (0001?) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance-voltage (C-V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C-V measurements the sheet carrier density in a Zn_0.8 Mg_0.2O/ZnO/Zn_0.8 Mg_0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10¹⁰ cm^− 2. For the films deposited on sapphire 2D growth is observed in the Burton-Cabrera-Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.     

Electrical and photocurrent characteristics of Au/PVA (Co-doped)/n-Si photoconductive diodes

In this work, we investigate the some main electrical and photocurrent properties of the Au/PVA(Co-doped)/n-Si diodes by using current–voltage (I–V) measurements at dark and various illumination intensity. Two types of diodes with and without polyvinyl alcohol (PVA) (Co-doped) polymeric interfacial layer were fabricated and measured at room temperature. Results show that the polymeric interfacial layers and series resistance (R_s) strongly affect the main electrical parameters of these structures. Also, metal/polymer/semiconductor (MPS) diode with PVA (Co-doped) interfacial organic layer is very sensitive to the light such that the current in reverse bias region increase by 10³–10⁴ times with the increasing illumination intensity. The open circuit voltage V_oc and short-circuit current I_sc values of this MPS diode under 100 mW/cm² illumination intensity were found as 0.28 V and 19.3 μA, respectively.