The annealing effect for structural, optical and electrical properties of dysprosium–manganese oxide films grown on Si substrate

Samples of (Dy–Mn) oxide thin films were prepared on quartz and Si(p) substrates for optical and electrical investigations. These samples were annealed at different temperatures and characterised by UV–VIS absorption spectroscopy, X-ray fluorescence (XRF) and X-ray diffraction (XRD). The XRF spectrum was used to determine the weight fraction ratio of Mn to Dy in the prepared samples. The XRD shows that Dy oxide and Mn oxide prevent each other to crystallise alone or making a solid solution even at 600 °C. However, compound of DyMnO₃ was formed through the solid-state reaction for T > 800 °C. The ac-conductance and capacitance were studied, as a function of frequency and gate voltage and the fixed and interface charge densities were determined. It was found that the “correlated barrier hopping” CBH model controls the frequency dependence of the conductivity, while the Kramers–Kronig (KK) relations explain the frequency dependence of the capacitance. The parameters of CBH model were determined and show that the ac-conduction in crystalline (Dy–Mn) oxide is realised by bipolaron mechanism, where the barrier height of hopping is equal to the bandgap determined the UV–VIS absorption spectroscopy.     

Annealing effect on the dc transport mechanism in dysprosium oxide films grown on Si substrates

Crystalline Dy oxide thin films prepared on Si(p) substrates were annealed in vacuum and air at different temperatures and investigated by x-ray diffraction (XRD). Their capacitance-gate voltage characteristics were used to determine the charge densities in the samples and the voltage drop across the oxide layer itself in terms of gate voltage. The surface charge density was device-grade, on the order of 10¹¹ cm⁻². The dc current-voltage characteristic measurements show that the main mechanism controlling the current flow is the Richardson-Schottky (RS) mechanism. Parameters of the RS model, such as field-lowering coefficients and dynamic relative permittivity, were determined. The leakage current density of the samples was studied as a function of temperature from 293 to 400 K, and activation energies were determined.     

dc-Conduction mechanism in lanthanum–manganese oxide films grown on p-Si substrate

Thin films of (La–Mn) double oxide were prepared on p-Si substrates for electrical investigations. The samples have been characterised by X-ray fluorescence (XRF) and X-ray diffraction (XRD) methods. The XRF spectrum was used to determine the weight fraction ratio of Mn to La in the prepared samples. The XRD study shows the formation of grains of LaMnO₃ compound through a solid-state reaction for annealing at 800 °C. Samples used to study the electrical characteristics of the prepared films were constructed in form of a metal–oxide–Si MOS structures. Those MOS structures were characterised by the measuring their capacitance as a function of gate voltage C(V_g) in order to determine the oxide charge density Q_ox, the surface density of states D_it at the oxide/Si interface, and to extract the oxide voltage in terms of gate voltage. The extracted dielectric constant of the double oxide film is lower than that of pure La₂O₃ film and larger than that of pure Mn₂O₃ film, but the formation of LaMnO₃ grains by a solid-state reaction at 800 °C increases the relative permittivity to 11.5. These experimental conclusions might be useful to be used in the field of Si-oxide alternative technique. The leakage dc current density vs. oxide field J(E_ox) relationship for crystalline films follow the mechanism of Richardson–Schottky (RS), from which the field-lowering coefficient and the dynamic relative permittivity were determined. Nevertheless, the leakage current density measured in a temperature range of (293–363 K) was not controlled by the RS mechanism. It was observed that the temperature dependence of the leakage current in crystalline (La–Mn) oxide insulating films has metallic-like temperature behaviour, which might be important in the technical applications.     

DC conduction mechanisms in thin polyimide films

The current-voltage characteristics of polyimide films from 4000 Å to 11,000 Å thick have been studied over a temperature range from 26 °C to 100°C. Below 60°C the conduction mechanism appears to be a combination of the Schottky and the Poole-Frenkel mechanisms. Between 60°C and 100°C, the conduction appears to be ionic and can be described by the rate theory. Ionic jump distances of between 252Å and 274Å have been estimated.     

Properties of cobalt-doped zinc oxide thin films grown by pulsed laser deposition on glass substrates

Undoped and cobalt-doped zinc oxide (CZO) polycrystalline piezoelectric thin films (Co: 3, 5 at.%) using a series of high quality ceramic targets have been deposited at 450 °C onto glass substrates using a pulsed laser deposition method. The used source was a KrF excimer laser (248 nm, 25 ns, 2 J∕cm²). X-ray diffraction patterns showed that the Co-doped ZnO films crystallize in a hexagonal wurtzite type structure with a strong (0 orientation, and the grain sizes calculated from these patterns decrease from 37 to 31 nm by increasing Co doping. The optical waveguiding properties of the films were characterized by using a prism-coupling method. The distinct M-lines of the guided transverse magnetic (TM) and transverse electric (TE) modes of the ZnO films waveguide have been observed. With the aim of study the optical properties of the ZnO films, an accurate refractive index and thickness measurement apparatus was set up, which is called M-lines device. An evaluation of experimental uncertainty and calculation of the precision of the refractive index and thickness were developed on ZnO films. The optical transmittance spectra showed a good transparency in the visible region. Calculated optical band gap varying from 3.23 to 3.37 eV when the content of Co doping increases from 0 to 5 at.%.     

Electrical conduction in anodic tungsten oxide thin films

Thin films of tungsten oxide were prepared by anodic oxidation of 99.999% pure tungsten foils using an aqueous solution containing 0.4 M KNO₃ and 0.04 M HNO₃ as the electrolyte. Different films show different colours. The current-voltage characteristics are of the log I- V^1/2 type. These characteristics can be interpreted in terms of the Poole—Frenkel type of conduction mechanism. The colours may be duo to the incorporation of H⁺ ions and the formation of HxWO₃ during the anodizing process.     

Evaluation of PZT thin films on Ag coated Si substrates

Fabrication characteristics of hybrid thin film components are investigated. Lead zirconate titanate (PZT) films, thickness 10 μm, are fabricated by using laser ablation on the Ag electrode (about 1 μm thick) which is deposited on 200 μm Si substrates by evaporation. Composition close to the target material is obtained in PZT films even in air and without substrate heating. Low surface energy in the Ag−Si system causes spheroidization of the Ag layer on the fresh Si substrate, but the surface can be modified by grinding and oxidization. Only some cavities exist at the interface. The interface between the Ag electrode and PZT layer is physically continuous, as revealed by electron microscopy. After annealing at 750°C for 2 h, the PZT layer consists of the rhombohedral perovskite phase with a fraction of the pyrochlore phase. Detrimental interdiffusion between Pb and Si occurs during annealing if the PZT thin film is directly on the Si substrate. This is retarded by the presence of the Ag layer.     

The properties of Si/Si1-xGex films grown on Si substrates by chemical vapor deposition

A growth parameter study was made to determine the proper of a SiGe superlattice-type configuration grown on Si substrates by chemical vapor deposition (CVD). The study included such variables as growth temperature, layer composition, layer thickness, total film thickness, doping concentrations, and film orientation. Si and SiGe layers were grown using SiH₄ as the Si source and GeH₄ as the Ge source. When intentional doping was desired, diluted diborane for p-type films and phosphine for n-type films were used. The study led to films grown at ∼1000°C with mobilities from ∼20 to 40 percent higher than that of epitaxial Si layers and ∼100 percent higher than that of epitaxial SiGe layers grown on (100) Si in the same deposition system for net carrier concentrations of ∼8x10¹⁵ cm^-3 to ∼2x10¹⁷ cm^-3. Enhanced mobilities were found in multilayer (100)-oriented Si/Si_1-xGe_x films for layer thicknesses ≥400A, for film thicknesses >2μm, and for layers with x = 0.15. No enhanced mobility was found for (111)-oriented films and for B-doped multilayered (100)-orlented films. Supported in part by NASA-Langley Research Center, Hampton, VA, Contract NAS1-16102 (R. Stermer & A. Fripp, Contr. Mon.)     

在Si(211)衬底上分子束外延CdTe的剪切应变

利用高分辨率X射线衍射技术对分子束外延CdTe（211）B/Si（211）材料的CdTe外延薄膜进行了倒易点二维扫描,并通过获得的对称衍射面和非对称衍射面的倒易空间图,对CdTe缓冲层的剪切应变状况进行了分析.研究发现,对于CdTe/Si结构,随着CdTe厚度的增加,[1-1-1]、[01-1]两个方向的剪切角γ[1-1-1]和γ[01-1]都有变小的趋势,且γ[1-1-1]的大小约为γ[01-1]的两倍;对于CdTe/ZnTe/Si结构,ZnTe缓冲层的引入可以有效地降低CdTe层的剪切应变.     

PLD法生长Al2O3基ZnO薄膜的特性

在不同衬底温度下,用脉冲激光沉积法(PLD),在Al2O3(0001)平面上生长了ZnO薄膜。研究了衬底温度对其结晶质量、电学性质以及发光性质的影响。结果显示:XRD在2θ为34°处出现了唯一的ZnO(0002)衍射峰;ZnO薄膜的电阻率随衬底温度的升高而增大;在衬底温度为500℃时,出现了位于410nm附近的特殊的光致发光(PL)峰。     

硅(001)衬底上生长的ZnO薄膜的AFM研究

对采用电子束反应蒸镀方法在低温下在硅（001）衬底上外延生长的ZnO薄膜的表面构像进行了原子力显微镜（AFM）观察，分析研究不同的衬底温度对薄膜表面形貌及结构特性的影响。在250℃衬底温度下获得的ZnO薄膜，膜表面平整，结构致密，表面平均不平整度小于3nm，说明在该衬底温度下获得的ZnO薄膜是高透明度、高质量、高度取向的单晶薄膜。     

Properties of CdTe nanocrystalline thin films grown on different substrates by low temperature sputtering

CdTe nanocrystalline thin films have been prepared on glass, Si and Al2O3 substrates by radio-frequency magnetron sputtering at liquid nitrogen temperature. The crystal structure and morphology of the films were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The XRD examinations revealed that CdTe films on glass and Si had a better crystal quality and higher preferential orientation along the (111) plane than the Al2O3. FESEM observations revealed a continuous and dense morphology of CdTe films on glass and Si substrates. Optical properties of nanocrystalline CdTe films deposited on glass substrates for different deposited times were studied.     

不同衬底上纳米晶CdTe薄膜的低温制备及光性能研究

CdTe nanocrystalline thin films have been prepared on glass, Si and Al2O3 substrates by radio-frequency magnetron sputtering at liquid nitrogen temperature. The crystal structure and morphology of the films were charac-terized by X-ray diffraction （XRD） and field-emission scanning electron microscopy （FESEM）. The XRD examinations revealed that CdTe films on glass and Si had a better crystal quality and higher preferential orientation along the （111） plane than the Al2O3. FESEM observations revealed a continuous and dense morphology of CdTe films on glass and Si substrates. Optical properties of nanocrystalline CdTe films deposited on glass substrates for different deposited times were studied.     

Optical and microstructural characterization of the effects of rapid thermal annealing of CdTe thin films grown on Si (100) substrates

The effects of rapid thermal annealing (RTA) on CdTe/Si (100) heterostructures have been studied in order to improve the structural quality of CdTe epilayers. Samples of CdTe (111) polycrystalline thin films grown by vapor phase epitaxy (VPE) on Si (100) substrates have been investigated. The strained structures were rapidly thermally annealed at 400°C, 450°C, 500°C, 550°C, and 600°C for 10 sec. The microstructural properties of the CdTe films were characterized by carrying out scanning electron microscopy (SEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). We have shown that the structural quality of the CdTe epilayers improves significantly with increasing annealing temperature. The optimum annealing temperature resulting in the highest film quality has been found to be 500°C. Additionally, we have shown that the surface nucleation characterized by the island size distribution can be correlated with the crystalline quality of the film.     

Crystal perfection of GaP films grown on Si substrates by solid-source MBE with atomic hydrogen

GaP films have been grown by migration-enhanced epitaxy on Si (001) substrates tilted by 6° about the 〈011〉 axis. High-energy electron diffraction, transmission electron microscopy, and X-ray diffraction analysis were used to demonstrate that introduction of atomic hydrogen in the course of epitaxy markedly improves the overall structural quality of GaP films. Up to thicknesses of about 0.1 μm, the full width at half-maximum of the (004) reflection in the X-ray diffraction pattern of these films almost coincides with the theoretical value for defect-free films, which indicates that their state is nearly pseudomorphic.     

Epitaxial necking in GaAs grown on pre-pattemed Si substrates

We report a new method for the natural reduction of threading dislocations in GaAs on Si by growing on patterned Si substrates. We also explore other effects of patterning on dislocation formation during growth: stress relief near the mesa edges at high aspect ratios, and limited dislocation nucleation and propagation. Prior to growth, the Si substrates were processed to produce a plurality of mesas varying in width (5-170 μm) and geometry (circular, rectangular, and square mesas). After growth of the GaAs, the material was characterized with cathodoluminescence (CL) and secondary electron microscopy. For a GaAs growth temperature of 570° C and a thickness of 10 μm, the GaAs grown on the 40μ-wide Si mesas show a factor of 1.6 increase in luminescence intensity over the luminescence intensity from the unpatterned control area. Also, the emission wavelength from the smaller mesas is shifted to shorter wavelengths as compared to GaAs/GaAs and the unpatterned control area. The emission wavelength and CL intensity varies across the mesas; for 40 μm wide mesas, the emission wavelength is fairly constant across the mesa and the CL intensity decreases near the edges, whereas for larger mesas the emission wavelength decreases and the CL intensity increases at the mesa edges. For the 40 μm wide mesas, the integrated CL intensity is equal to that of a control GaAs/GaAs grown with the same doping level. No cracks were observed in the GaAs grown on the Si mesas, even though the thickness of the GaAs was 10 μ,m.     

Study of the effects of rapid thermal annealing in generation–recombination noise in MBE grown GaN thin films

Low-frequency excess noise was measured from GaN thin films deposited by plasma assisted molecular beam epitaxy (MBE). The noise power spectra is dominated by 1/f noise at low frequency and by Lorentzians at frequencies beyond 3 kHz. The temperature dependencies of the Lorentzians were examined from room temperature to about 90 K. From the Arrhenius plot of the time constants, the thermal activation energy of the fluctuation time constant was found to be around 30 meV. From the temperature dependencies of voltage noise power spectra, we estimated the magnitudes of the capture and emission activation energies. Based on the results, we have formulated a model, which stipulates that the generation–recombination (G–R) noise arises from the capture and emission of carriers by localized states in the bulk of the film. The process leads to fluctuations in the carrier mobility due to the modulation of the Coulombic scattering rate. We next conducted a systematic investigation on the effects of rapid thermal annealing on G–R noise in GaN thin films. Experimental results showed that annealing at 900°C resulted in the minimum FWHM in the rocking curve. Furthermore, we observed a substantial reduction in the noise level, indicating that rapid thermal annealing can be used as an effective means for noise reduction in GaN based devices.     

Antiphase boundary of GaAs films grown on Si(001) substrates by molecular beam epitaxy

Single and double domain GaAs film growth on double domain Si(001) substrates by molecular beam epitaxy were observed. The domain structure of the film did not succeed to the domain structure of the substrate surface but was decided by a direction of slight misorientation of the substrate. To explain this, it has been proposed that the antiphase boundary (APB) of the film is dominantly non-stoichiometric,i.e., the APB is composed of the same polar {11n} or {nn1} (n = 1, 2, ….) planes of the adjacent domains. Growth simulation based on this model about the APB has explained well the experimental results that a double domain film can grow on a Si(001) surface which is exactly oriented or is misoriented towards a 〈100〉 direction.     

Sputter-deposition of NiMnSb magnetic thin films from a composite target onto Si substrates

We have deposited ferromagnetic NiMnSb thin films by sputtering from a single composite target onto Si wafer substrates. Similarly to earlier results using glass substrates, we find that a combination of low radio frequency power, low argon pressure, and moderate substrate temperature is successful at directly obtaining stochiometric, single-phase polycrystalline films with the bulk C1_b crystal structure. The use of Si substrates, however, is compatible with standard electronic processing and integration into electronic device structures. The similarity of the films to bulk NiMnSb suggests that the predicted half-metallic (100% spin-polarized) electronic properties of NiMnSb can be reproduced in a magnetically active thin-film device structure.     

Interface properties of ZnO nanotips grown on Si substrates

ZnO nanotips have been grown on Si (100) using metalorganic chemical vapor deposition (MOCVD). The growth temperature is optimized for good crystallinity, morphology, and optical properties. ZnO nanotips exhibit a strong near band edge emission of ∼376 nm at room temperature with negligible green band emission. Pregrowth substrate treatment using diluted hydrofluoric acid (HF) and minimized oxygen exposure before the initial growth significantly reduces the interfacial SiO₂ thickness, while maintaining good morphology. An n-ZnO nanotips/p-Si diode is fabricated and its I–V characteristic is measured. The threshold voltage of the diode is found to be below 2.0 V with small reverse leakage current. The ZnO/p-Si diodes provide the possibility of integrating the ZnO nanotips with Si-based electronic devices.