Optical properties of ZnO nanowire arrays electrodeposited on n- and p-type Si(1 1 1): Effects of thermal annealing

Electrodeposition is a low temperature and low cost growth method of high quality nanostructured active materials for optoelectronic devices. We report the electrochemical preparation of ZnO nanorod/nanowire arrays on n-Si(1 1 1) and p-Si(1 1 1). The effects of thermal annealing and type of substrates on the optical properties of ZnO nanowires electroplated on silicon (1 1 1) substrate are reported. We fabricated ZnO nanowires/p-Si structure that exhibits a strong UV photoluminescence emission and a negligible visible emission. This UV photoluminescence emission proves to be strongly influenced by the thermal annealing at 150-800 °C. Photo-detectors have been fabricated based on the ZnO nanowires/p-Si heterojunction.     

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.     

Transparent, amorphous and organics-free ZnO thin films produced by chemical solution deposition at 150 °C

We have studied the low-temperature processing of ZnO by chemical solution deposition. A transparent, stable precursor solution prepared from zinc acetate dihydrate dissolved in 2-methoxyethanol was spin-coated on SiO_x/Si, soda-lime glass and polymer substrates and heated at 150 °C. Selected thin films deposited on SiO_x/Si were additionally heated at 450 °C.Microstructural and chemical analyses showed that the thin films heated at 150 °C in air were amorphous, contained no organic residues and had a root mean square roughness of 0.7 nm. The films deposited on SiO_x/Si and heated at 450 °C were crystallised and consisted of randomly oriented grains with a diameter of about 20 nm. All thin films were transparent, exhibiting a transmission of over 80% in the visible range. The resistivity of the 120-nm thick ZnO films processed at 150 °C was 57 MΩ cm and upon heating at 450 °C it decreased to 1.9 kΩ cm.     

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.     

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.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.     

Low-temperature atomic layer deposition of ZnO thin films: Control of crystallinity and orientation

Low-temperature atomic layer deposition (ALD) processes are intensely looked for to extend the usability of the technique to applications where sensitive substrates such as polymers or biological materials need to be coated by high-quality thin films. A preferred film orientation, on the other hand, is often required to enhance the desired film properties. Here we demonstrate that smooth, crystalline ZnO thin films can be deposited from diethylzinc and water by ALD even at room temperature. The depositions were carried out on Si(100) substrates in the temperature range from 23 to 140 °C. Highly c-axis-oriented films were realized at temperatures below ~ 80 °C. The film crystallinity could be further enhanced by post-deposition annealing under O₂ or N₂ atmosphere at 400-600 °C while keeping the original film orientation intact.     

Structure and optical properties of Zr1−xSixN thin films on sapphire

A series of zirconium silicon nitride (Zr_1−xSi_xN) thin films were grown on r-plane sapphire substrates using reactive RF magnetron co-sputtering of Zr and Si targets in a N₂/Ar plasma. X-ray diffraction pole figure analysis, X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), optical microscopy, and optical absorption spectroscopy were used to characterize the film stoichiometries and structures after growth at 200 °C and post-deposition annealing up to 1000 °C in ultra-high vacuum. The atomically clean r-plane sapphire substrates induce high quality (100) heteroepitaxy of ZrN films rather than the (111) orientation observed on steel and silicon substrates, but the addition of Si yields amorphous films at the 200 °C growth temperature. After the annealing treatment, films with Si content x < 0.15 have compressive stress and crystallize into a polycrystalline structure with (100) fiber texture. For x > 0.15, the films are amorphous and remain so even after ultra-high vacuum annealing at 1000 °C. XPS spectra indicate that the bonding changes from covalent to more ionic in character as Si―N bonds form instead of Zr―N bonds. X-ray reflectivity, atomic force microscopy (AFM) and optical microscopy data reveal that after post-deposition annealing the 100 nm thick films have an average roughness < 2 nm, except for Si content near x = 0.15 corresponding to where the film becomes amorphous rather than being polycrystalline. At this stoichiometry, evidence was found for regions of film delamination and hillock formation, which is presumably driven by strain at the interface between the film and sapphire substrate. UV-visible absorption spectra also were found to depend on the film stoichiometry. For the amorphous Si-rich films (x > 0.15), the optical band gap increases with Si content, whereas for Zr-rich films (x < 0.15), there is no band gap and the films are highly conductive.     

Phase transitions of room temperature RF-sputtered ZnO/Mg0.4Zn0.6O multilayer thin films after thermal annealing

We report the thermal stability of room-temperature RF-sputtered Mg_0.4Zn_0.6O thin films and ZnO/Mg_0.4Zn_0.6O superlattices at 600 °C and 800 °C. The phase of room-temperature as-sputtered Mg_0.4Zn_0.6O is crystalline ZnO embedded in an amorphous or short-range-ordered hexagonal MgZnO matrix. Annealing at either 600 °C or 800 °C for 5 min transforms the matrix into a crystalline hexagonal wurtzite structure, leading to a decrease of the optical bandgap (E_g) of Mg_0.4Zn_0.6O. This also results in a slight change near the absorption edge of the superlattice transmission spectrum. The films precipitate cubic MgZnO after heating Mg_0.4Zn_0.6O at 800 °C for 5 min; by contrast, precipitations take at least 3 h if the samples are heated at 600 °C. Heating at 800 °C for more than 3 h significantly reduces the film thickness and E_g, attributed to the decomposition of superlattices and diffusion of magnesium into the substrate, respectively. On the other hand, annealing the ZnO/Mg_0.4Zn_0.6O superlattice at 600 °C for 12 h also produces an initial slight change in the optical transmission spectra, yet the spectra remain essentially unchanged for the remainder of the annealing process.     

Structure, morphology and electrical characterizations of direct current sputtered ZnO thin films

ZnO thin films were deposited on glass substrates by direct current (DC) sputtering technique at room temperature (RT) to 400 °C with a 99.999% pure ZnO target. Then the samples deposited at RT were annealed in air from the RT to 400 °C. The effects of substrate temperature (T_s) and annealing treatment (T_a) on the crystallization behavior and the morphology have been studied by X-ray diffraction and atomic force microscopy. We also compared the structural properties of samples deposited at 400 °C on glass to those deposited on Pt/silicon substrate. The resistivity, surface roughness and size of the grains have also been studied and correlated to the thickness of ZnO films deposited on Pt/Si substrates. The experimental results reveal that the substrate has a major influence on the structural and morphological properties. For the films deposited on glass, below 400 °C, T_s and T_a have a similar influence on the structure of the films. Moreover, the ZnO samples deposited at RT and annealed in air have poor electrical properties.     

Preparation, dielectric and ferroelectric properties of Pb5Ge3O11 and Pb5Ge2.85Si0.15O11 thin films fabricated by sol-gel process

Lead germanate-silicate (Pb₅Ge_2.85Si_0.15O₁₁) ferroelectric thin films were successfully fabricated on Pt/Ti/SiO₂/(100)Si substrates by the sol-gel process. The thin films were fabricated by multi-coating at preheating temperatures of 350 and 450 °C. After annealing the thin films at 600 °C, the films exhibited c-axis preferred orientation. The degree of c-axis preferred orientation of the thin films preheated at 350 °C was higher than that of films preheated at 450 °C. Grain growth was influenced by the annealing time. The thin films exhibited a well-saturated ferroelectric P-E hysteresis loop when preheated at 350 °C and annealed at 600 °C for 1.5 h. The values of the remanent polarization (Pr) and the coercive field (Ec) were approximately 2.1 μC/cm² and 100 kV/cm, respectively.     

The effect of high temperature annealing on Schottky diode characteristics of Au/n-Si contacts

The current-voltage and capacitance-voltage characteristics of Au/n-Si/Al Schottky barrier diode were measured in the temperature range of 100-800 °C. Au/n-Si/Al Schottky barrier diode annealed at temperatures from 100 °C to 400 °C for 5 min and from 500 °C to 800 °C for 7 min in N₂ atmosphere. The electronic parameters such as barrier height and ideality factor (n) of the device were determined using Cheung's method. To determine whether or not a Schottky diode is ideal it can be used the ideality factor (n) found from its forward current-voltage (I-V) characteristics. It has been found that the value of Φ_b (0.82 or 0.83 eV) remains constant up to 500 °C and 0.80 and 0.79 eV in 600, 750 °C respectively in the forward I-V mode. An ideality factor value of 1.04 was obtained for as-deposited sample. The ideality factor n varied from 1.04 to 2.30. The experimental results have shown that the ideality factor (n) values increases with increasing annealing temperature up to 750 °C. This has been explained in terms of the presence of different metallic-like phases produced by chemical reactions between the Au and Si substrate because of the annealing process. The Φ_b (C-V) values obtained from the reverse-bias C⁻²-V curves of the as-deposited and annealed diode are in the range 0.99-1.12 eV. The difference between Φ_b (C-V) and the Φ_b (I-V) is in close agreement with values reported in literature. Besides Fermi energy level and carrier concentration determined by using thermionic emission (TE) mechanism show strong temperature dependence. It has been seen current-voltage characteristics of the diode show an ideal behavior.     

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.     

Effects of annealing temperature on microstructure and electrical properties of Mn-Co-Ni-O thin films

Mn_1.85Co_0.3Ni_0.85O₄ (MCN) thin films were prepared on Al₂O₃ substrates by chemical solution deposition method at different annealing temperature (650, 700, 750 and 800 °C). Effects of annealing temperature on microstructure and electrical properties of MCN thin films were investigated. The MCN thin film annealed at 750 °C is of good crystallization and compact surface. It shows lower resistance (4.8 MΩ) and higher sensitivity (3720.6 K) than those of other prepared films. It also has small aging coefficient (3.7%) after aging at 150 °C for 360 h. The advantages of good properties make MCN thin film very promising for integrated devices.     

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.     

Fabrication of p-type ZnO thin films via magnetron sputtering and phosphorus diffusion

ZnO thin films were deposited on heavily phosphorus-doped (n⁺-Si) substrates by radio frequency magnetron sputtering. The films were changed from n-type to p-type by phosphorus diffusion from the n⁺-Si substrates to the ZnO films and being activated thermally during deposition. n-Type ZnO (n-ZnO) films were also deposited onto the p-type ZnO (p-ZnO) films to form n-ZnO/p-ZnO/n⁺-Si multilayer structures. The cross section of the multilayer structure was examined by scanning electron microscopy. Crystal structures of the p-ZnO films were studied by X-ray diffraction and were confirmed to be highly c-axis oriented primarily perpendicular to the substrate. Photoluminescence spectra of the p-ZnO films showed that band-edge UV emission predominated. The hole concentration of the p-ZnO films was between +1.78×10¹⁸ cm⁻³ and +1.34×10¹⁹ cm⁻³, and the hole mobility was 13.1-6.08 cm²/V s measured by Hall effect experiment. The formation of p-ZnO films was confirmed by the rectifying characteristics of the p-ZnO/n⁺-Si heterojunctions and the n-ZnO/p-ZnO homojunction on the multilayer structure as well as by the experimental results of Hall effect.     

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.     

Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature

Bi_3.25Eu_0.75Ti₃O₁₂ (BET) thin films were deposited on Pt/Ti/SiO₂/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600-800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10^− 6 C/cm² under 300 kV/cm), remnant polarization (65.7 × 10^− 6 C/cm² under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d₃₃ (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices.     

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.