N-type ZnO Thin Films Research Articles

Fabrication and Energy Band Alignment of n-ZnO/p-CuI Heterojunction

N-ZnO/P-CuI heterojunctions are fabricated by growing undoped n-type ZnO thin films on p-type γ-CuI (111) single-crystal substrates using radio-frequency magnetron sputtering. The ZnO films are identified to be columnar structured with c-axis-preferred orientation by using X-ray diffraction and scanning electron microscope. Measurements of the energy band alignment of ZnO/CuI interface by using X-ray photoelectron spectroscopy result in a valence band offset of 1.74 eV and a conduction band offset of -1.37 eV, meaning a type-II band alignment at the interface. A typical diodelike behavior of the current-voltage curve indicates its possible applications in optoelectronics with further development.

Oxide Solar Cells Fabricated Using Zinc Oxide and Plasma-Oxidized Cuprous Oxide

Oxide heterojunction solar cells composed of an n-type Al-doped ZnO (AZO) thin film on the surfaces of p-type Cu2O films were fabricated. The Cu2O films of about 0.34 to 1.67 µm thickness were grown by partial oxidation of a Cu sheet using microwave plasma. The AZO film of 400 nm thickness was deposited by magnetron sputtering. Energy conversion efficiencies of 0.12 to 0.30% were obtained in AZO/Cu2O cells under AM1.5 solar illumination.

Effects of Substrate Temperature on the Electrical and the Optical Properties of N-Type ZnO/P-Type 4H-SiC

We investigated the effect of the substrate temperature on the electrical and the optical properties of ZnO/4H-SiC structures. The n-type ZnO layer was grown on p-type 4H-SiC substrate by pulsed laser deposition to form p-n hetero-junction diode structure. The n-type ZnO thin films were deposited by pulsed laser deposition at different temperatures of 200, 400, and 600 °C, respectively. It was shown from transmission line method (TLM) and auger electron spectroscopy (AES) data that the sheet resistance of ZnO on SiC was increases from ~760 Ω/square to ~4000 Ω/square as the deposition temperature increases and the oxygen outdiffusion decreases. The I-V characteristics with and without illumination has also been studied.

Analysis of the electrical characteristics of the Ag/ZnO Schottky barrier diodes on F-doped SnO2 glass substrates by pulsed laser deposition

High quality semiconductor layer of n-type ZnO thin film was fabricated on F-doped SnO2 glass substrates by pulsed laser deposition. The Schottky junction diodes with configuration of Ag/ZnO/FTO have been fabricated to study the devices electrical properties by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The results show that the devices have good rectifying behaviors with an ideality factor of 1.64 and a Schottky barrier height of 0.85eV based on the I-V characteristics. Also, Cheung's functions and Norde's method were used to evaluate the I-V characteristics and to obtain the series resistance of the Schottky contact. From the C-V characteristics, the capacitance was determined to increase with decrease of frequencies. C-V measurements have resulted in higher barrier heights than those obtained from I-V measurements. The discrepancy between Schottky barrier heights obtained from I-V and C-V measurements was also interpreted.

Influence of post-deposition annealing on structural, optical and electrical characteristics of NiO/ZnO thin film hetero-junction

ABSTRACTTransparent p-n hetero-junction diodes are fabricated using, p-type NiO and n-type ZnO thin films deposited onto a Pt/Ti/glass substrate utilizing RF sputtering technique. The prepared hetero-junctions are studied for the structural, electrical and optical properties and the effect of post-deposition annealing is investigated through I-V measurements and XRD analysis. The as deposited hetero-junction is found to be giving ohmic behaviour while with post-annealing treatment it result in rectification with a ratio of forward-to-reverse current as high as 15 in the range -1.0 to 1.0 V. Forward threshold and the reverse breakdown voltages are found to be about 0.5 and -2.7 V, respectively. The forward-bias I-V characteristics are dominated by the flow of space-charge-limited current with an optical transmission of above 50 % in the visible region important for the transparent electronic device fabrication.

Ti/Au 금속과 n-type ZnO 박막의 Ohmic 접합 연구

The Ohmic contact of Ti/Au metals on n-type ZnO thin film deposited on c-plane sapphire substrates by pulsed laser deposition was investigated by TLM (transfer length method) patterns. The Ti/Au metal films with thickness of 35 nm and 90 nm were deposited by electron-beam evaporator and thermal evaporator, respectively. By using the photo-lithography method, the TLM patterns with gaps were formed. To improve the electrical properties as well as to decrease an interface states and stress between metal and semiconductor, the post-annelaing process was done in oxygen ambient by rapid thermal annealing system at temperature of for 1 min. In this study, it appeared that the minimum specific contact resistivity shows about in annealed sample, which may be originated from formation of oxygen vacancies of ZnO during an oxidation of Ti metal at the interface of Ohmic contacts.

Fabrication and characterization of heterojunction diodes composed of La 0.5Sr 0.5CoO 3 and Al-doped ZnO Thin Films

Oxide heterojunction diodes with La 0.5Sr 0.5CoO 3 (LSCO) and n-type Al-doped ZnO (AZO) thin films on Si substrate were fabricated by radio frequency (r. f.) magnetron sputtering technology. The AZO thin film was first deposited on the Si substrate and annealed in an argon atmosphere. The heterostructure was obtained by stacking the LSCO thin film on the AZO thin film without annealing. The thicknesses of the AZO and LSCO thin films were about 300 nm and 200 nm, respectively. The LSCO/AZO diode exhibited an asymmetric current–voltage relationship with a reverse shape of a good rectifying property. The build-in voltage of the diode was about 1.5 V and was found to be independent of the annealing temperature of the AZO film. The diode with an AZO thin film annealed at 500 °C exhibited the maximum forward-to-reverse current ratio of about 0.25 mA/V under an applied voltage of − 8 V to + 8 V.

ELECTRICAL AND OPTICAL PROPERTIES OFZnOTHIN FILMS PREPARED BY R.F. MAGNETRON SPUTTERING

High transparency and conductivity of transparent conducting oxide thin film are very important for improving the efficiency of solar cells. ZnO thin film is a better candidate for transparent conductive layer of solar cell. N-type ZnO thin films were prepared by radio-frequency magnetron sputtering on glass substrates. ZnO thin films underwent annealing treatment after deposition. The influence of the sputtering power on the surface morphology, the electrical and optical properties were studied by AFM, XRD, UV2450 and HMS-3000. The experimental results indicate that the crystal quality of ZnO thin film is improved and all films show higher c-axis orientation with increasing sputtering power from 50 to 125 W. The average transparency of ZnO thin films is higher than 90% in the range of 400–900 nm between the sputtering power of 50–100 W. After the rapid thermal annealing at 550°C for 300 s under N2ambient, the minimum resistivity reach to 10-2Ω⋅ cm .

Optically Transparent of N-ZnO/p-NiO Heterojunction for Ultraviolet Photodetector Application

Transparent p–n heterojunction diodes consisting of n-type ZnO and p-type NiO thin films were prepared on glass substrates by r.f. magnetron sputtering. The structural and optical properties of the n-ZnO/p-NiO heterojunction were characterized by X-ray diffraction (XRD), UV–visible spectroscopy, Hall measurement, and I-V photocurrent measurements. The XRD shows that ZnO films are highly crystalline in nature with preferred orientation along the (0 0 2) orientation. The optical transmittances of ZnO and NiO films are 87% and 80%, respectively. The current–voltage curve of the heterojunction demonstrates obvious rectifying diode behavior in a dark environment. The lowest of leakage current is 7.73x10−8 A/cm2 for n-ZnO/p-NiO heterojunction diode. Upon UV irradiation, it was found that the detector current was increased by more than one order of magnitude. It was also found that the corresponding time constant for turn-on transient was τon = 27.9 ms while that for turn-off transient was τoff= 62.8 ms.

Fabrication and properties of p-type K doped Zn 1− xMg xO thin film

A series of K doped Zn 1− x Mg x O thin films have been prepared by pulsed laser deposition (PLD). Hall-effect measurements indicate that the films exhibit stable p-type behavior with duration of at least six months. The band gap of the K doped Zn 1− x Mg x O films undergoes a blueshift due to the Mg incorporation. However, photoluminescence (PL) results reveal that the crystallinity decreased with the increasing of Mg content. The fabricated K doped p-type Zn 0.95Mg 0.05O thin film exhibits good electrical properties, with resistivity of 15.21 Ω cm and hole concentration of 5.54 × 10 18 cm −3. Furthermore, a simple ZnO-based p–n heterojunction was prepared by deposition of a K-doped p-type Zn 0.95Mg 0.05O layer on Ga-doped n-type ZnO thin film with low resistivity. The p–n diode heterostructure exhibits typical rectification behavior of p–n junctions.

Fabrication of Transparent p-Type CuxZnyS Thin Films by the Electrochemical Deposition Method

CuxZnyS thin films were deposited on indium–tin oxide-coated glass substrates by the electrochemical deposition (ECD) method using aqueous solutions containing CuSO4, ZnSO4, and Na2S2O3. The film deposited under optimum conditions exhibited a high optical transmission, and its energy band gap was about 3.2 eV. It was confirmed that CuxZnyS showed p-type conduction and photosensitivity. To fabricate a ZnO/CuxZnyS heterojunction, an n-type ZnO thin film was deposited on CuxZnyS by ECD. In a current–voltage measurement, the heterojunction showed rectification properties.

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500°C are measured to be about 3.6×10−4Ω cm, 23.8cm2/Vs and 7.1×1020cm3, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~0.9nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85V at 1kHz. Low turn-on voltage of about 1.3V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.

Two-Dye-Molecule-Stacked Structures on ZnO Films Formed by Liquid-Phase Molecular Layer Deposition (MLD) for Waveguide-Type Photo-Voltaic Devices

We propose a waveguide-type two-dye-sensitized photo-voltaic device with flat thin film semiconductors. P-type dye molecule (rose bengal (RB)) and n-type dye molecule (crystal violet (CV)) were successively deposited on n-type ZnO thin films by the liquid-phase molecular layer deposition (MLD) to form a n/p/n structure of ZnO/p-type dye/n-type dye. The photocurrent spectra were widened in the n/p/n structure, where, electrons excited in the n-type dye molecules and the p-type dye molecules are injected into ZnO with large efficiency. The guided light configuration achieved sufficient light absorption in the dye molecules to enhance the photocurrents by about one-order of magnitude.

ZnO-based metal-semiconductor field-effect transistors on glass substrates

We investigate the influence of quartz glass and borosilicate glass substrates on the electrical properties of ZnO-based metal-semiconductor field-effect transistors (MESFETs). The n-type ZnO thin-film channels were grown by pulsed-laser deposition and MESFETs were processed by reactive dc sputtering of AgxO-Schottky gate contacts. All devices are in the normally-off state. They exhibit very low off-currents in the range of 10−13A and on/off ratios of maximum 6 decades. The channel mobilities are highest for ZnO on quartz with 1.3cm2/Vs. The glass substrates introduce a compensating effect on the conduction of the ZnO channel resulting in higher on/off-voltages and lower on-current.

Structural and electrical properties of metal contacts on n-type ZnO thin film deposited by vacuum coating technique

The authors report on fabrication and characterization of Al and Pt metal contacts on ZnO thin films grown on ITO coated glass substrates using thermal evaporation technique. The structural and surface properties of ZnO thin film were studied by using x-ray diffraction and atomic force microscopy techniques. Atomic force micrographs revealed that ZnO microparticles have perfect pyramidal shape with small surface roughness (average rms value of 3nm). The current voltage characteristics of Pt∕ZnO and Al∕ZnO contacts were studied by scanning tunneling microscopy. The Pt contact on ZnO thin films behaves as a rectifying contact with a barrier height of 0.72eV, while Al contact on ZnO thin film turns out to be Ohmic in nature. The band gap of ZnO thin film was estimated to be 3.10eV from absorption spectroscopic measurements.

Magnetic and optical properties of epitaxial n-type Cu-doped ZnO thin films deposited on sapphire substrates

In this paper, we report on pulsed laser deposition of n-type Cu-doped ZnO thin films on c-plane sapphire substrates at 700°C. XRD and HRTEM were employed to study the epitaxial growth relationship between the Zn1−xCuxO film and sapphire substrate. Absorption measurements showed excitonic nature of the thin films and a decrease in the bandgap energy with increased Cu concentration was observed. Such as-deposited films showed room temperature ferromagnetism with Curie temperature (Tc) at around 320 K. The moment per Cu atom decreases as the Cu concentration increases. The largest magnetic moment about 0.81μB/Cu atom was observed for Zn0.95Cu0.05O thin films. The presence of any magnetic second phase was ruled out and the ferromagnetism was attributed to Cu ions substituted directly into the ZnO lattice.

Transparent ZnO Thin-Film Transistors on Glass and Plastic Substrates Using Post-Sputtering Oxygen Passivation

High-current level ZnO thin-film transistors (TFTs) on transparent substrates are demonstrated using low-temperature RF sputtering. Oxygen passivation induced fill of oxygen vacancies within the sputtered n-type ZnO thin films on glass substrates is investigated to manipulate the performance of top-gate ZnO TFTs. The surface oxygen passivation effectively enlarges grain size of the ZnO on glass substrates from 7 nm to 20 nm and increases the oxygen composition ratio from 30% to 35%, which essentially yields a TFT with its significantly increase of drain-source current and Ion/Ioff ratio, as compared with a typical ZnO based TFT. The optimum duration of oxygen passivation in this study yields a device with a drain-source current level 0.87 mA under a bias condition VGS = 5 V and VDS = 15 V, Ion/Ioff ratio 1.4 times 106. We further demonstrate high-performance top-gate ZnO TFTs by applying similar low-temperature process on a flexible polymer substrate. The device shows an IDS 26 muA under a bias condition VGS = 15 V and VDS = 25 V with gate size W/L = 600 mum/300 mum. The average optical transmission of the entire flexible TFT structure in the visible spectrum range is about 82% while the transmission at 550 nm is 88%.

Electrical characterization of Au/n-ZnO Schottky contacts on n-Si

Wide band gap semiconducting layer of n-type ZnO thin film was fabricated on n-type Si substrate with electrochemical deposition technique and the current–voltage ( I– V) and the capacitance–voltage/frequency ( C– V/ f) characteristics of the structure have been measured at room temperature. The characteristic parameters of the structure such as barrier height, ideality factor and series resistance were determined from the current–voltage measurements. Also, Cheung functions and Norde method were used to plot the I– V characteristics and, to extract the characteristic parameters of the Schottky contact. It was seen that trap-filled space charge-limited current (SCLC) is the dominant transport mechanism at large forward bias. The capacitance measurements showed that the values of capacitance were almost independent of frequency up to a certain value of frequency whereas at high frequencies the capacitance decreased quickly. The higher values of capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the ZnO that can follow the alternating current (ac) signal.

Positive Hall coefficients obtained from contact misplacement on evident n-type ZnO films and crystals

We report on the effect of sample non-uniformity on the results of Hall-effect measurements. False positive Hall coefficients were obtained from an evidently n-type ZnO single crystal, although four electrodes with low contact resistance were made and the Van der Pauw parameter for this electrode configuration was close to 1.00. Further position-sensitive characterization revealed that the false positive Hall coefficient was due to non-uniform electrical properties of the sample. To demonstrate a false positive sign of the Hall coefficient due to sample non-uniformity, we devised a model structure made from evident n-type ZnO thin film and successfully reproduced a false positive Hall coefficient from n-type ZnO.

Sb-doped p-ZnO∕Ga-doped n-ZnO homojunction ultraviolet light emitting diodes

ZnO p-n homojunction light emitting diodes were fabricated based on p-type Sb-doped ZnO∕n-type Ga-doped ZnO thin films. Low resistivity Au∕NiO and Au∕Ti contacts were formed on top of p-type and n-type ZnO layers, respectively. Au∕NiO contacts on p-type ZnO exhibited a low specific resistivity of 7.4×10−4Ωcm2. The light emitting diodes yielded strong near-band-edge emissions in temperature-dependent and injection current-dependent electroluminescence measurements.