The composition,structure and optical properties of weakly magnetic Co-containing amorphous Si and Ge films

This study investigated the composition, structure and optical properties of amorphous SiCo and GeCo films. The samples were prepared by radio frequency sputtering. Films were deposited with Co atomic concentrations in the range of 1.7–10.3 at.%. After deposition, the films were submitted to thermal treatments up to 900 °C and investigated by energy dispersive X-ray spectrometry, X-ray diffraction, Raman scattering and optical transmission spectroscopy. Additionally, magnetic force microscopy measurements were performed at room temperature. For comparison purposes, Co-free samples were also prepared, annealed and characterized following a similar procedure. The experimental results indicated the following: (1) the Co atoms were effectively and homogeneously incorporated into the amorphous hosts; (2) the as-deposited films (either pure or containing Co) were essentially amorphous; (3) annealing the films at high temperatures induced crystallization; (4) after crystallization, non-magnetic CoSi₂ (silicide) and CoGe₂ (germanide) phases were identified in the Co-containing Si and Ge films, respectively; (5) the optical properties of the films were significantly affected by the insertion of Co and by the annealing temperature; and (6) the samples exhibited a reduced magnetic signal at room temperature. These experimental observations were systematically studied, which are presented and discussed in this report.     

Substrate biasing effects on the microstructure of magnetron-sputtered AlN films investigated by in situ reflectance interferometry

In situ reflectance interferometry (RI) at 400 nm wavelength was used to investigate the effect of the substrate negative bias on the microstructure of aluminium nitride (AlN) films deposited at room temperature on Si substrates by magnetron sputtering. Their surface reflectance recorded during film deposition promptly yields real-time information on the microstructures developed under oxygen contamination and bias change. Specifically, the refractive index n and the extinction coefficient k are deduced from reflectance using appropriate multilayer optical models and validated by spectroscopic ellipsometry. These optical constants correlate appreciably with the microstructure that evolves between columnar-crystallized and purely amorphous phases including in-between amorphous states containing dispersed nano-AlN grains. These microstructures were identified using ex situ energy dispersive X-ray spectroscopy, transmission electron microscopy and diffraction, X-ray diffraction and Auger electron spectroscopy. The simple and cost-effective in situ RI thus appears a powerful tool in controlling the microstructures of thin AlN films for desired applications.     

Sn掺杂Ge-Sb-Te相变薄膜的晶化特性

用磁控溅射法制备了掺杂Sn的Ge2Sb2Te5相变材料薄膜,研究了Sn含量对结晶性能的影响.薄膜的X射线衍射(XRD)表明,热处理使薄膜发生了从非晶态到晶态的相变,并出现Sn-Te相.通过示差扫描量热(DSC)实验测出在不同加热速率下非晶态薄膜粉末的结晶峰温度,计算了材料的结晶活化能.根据结晶动力学分析和结晶活化能数据,掺杂Sn后的Ge-Sb-Te具有更高的结晶速率,用于光存储时将具有更高的擦除速度.     

Microstructure and crystallization kinetics analysis of the (In15Sb85)(100-x)Zn(x) phase change recording thin films

The (In15Sb85)(100-x)Zn(x) films (x = 0 - 17.4) were deposited on nature oxidized Si wafer and glass substrate at room temperature by magnetron co-sputtering of Sb target and InZn composite target. The thermal property of the films was examined by a homemade reflectivity thermal analyzer. Microstructures of the films were analyzed by transmission electron microscope (TEM). We examined the effects of Zn addition on the thermal property, crystallization kinetics, and crystallization mechanism of the In15Sb85 recording film. As x = 0 - 17.4, thermal analysis shows that the (In15Sb85)(100-x)Zn(x) films have two phase transition temperature ranges which are 189 degrees C-215 degrees C and 300 degrees C-350 degrees C. It is found that the activation energy is increased with Zn content. This indicates that the thermal stability of amorphous state is improved by doping Zn. The optical contrasts of the films are all larger than 15%, as x = 0 - 6.2, indicating that the films have the potential in blue laser optical recording media application.     

Comparative study of crystallization processes in Sb2Te3 films using laser and thermal annealing techniques

Adherent and pin-hole free amorphous Sb₂Te₃ thin films have been obtained by vacuum evaporation at substrate temperatures ≤25 °C. The films have been crystallized by thermal and laser annealing, and the crystallization processes monitored as a function of annealing temperature and laser scan speed. A comparative study of topography reveals disk-shaped crystallized areas in thermal crystallization and dendrite growth in the laser induced process. The crystallized films in both cases contain a single Sb₂Te₃ phase. Activation energy of 2 eV for crystallization, determined using differential scanning calorimetery indicates good room temperature stability of the amorphous states.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

InGaZnO thin films grown by pulsed laser deposition

We fabricated InGaZnO (IGZO) ceramic target (In: Ga: Zn = 1: 1: 4 in atomic ratio) using solid-state reaction at ambient atmosphere, and deposited IGZO thin films on quartz glass at room temperature under various oxygen partial pressures using the pulsed laser deposition method. Influence of oxygen pressure on crystal structure, surface morphology, optical and electrical properties were investigated. It was found that all the films deposited at room temperature exhibit amorphous structure. On the other hand, the physical properties of the films like transparency, electron mobility, and free-electron concentration were found to be correlated to the oxygen pressure during the deposition and in turn to the possible oxygen vacancies or metallic interstitials in the films. The analysis of X-ray photoelectron spectra (XPS) of the films indicated that there are no metallic 3d states of In, Ga and Zn, suggesting that oxygen vacancies could be main defects that affect physical properties of the films.     

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb₂Te₃ and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.     

Ge-Sb-Te-O相变薄膜的结晶动力学研究

用磁控溅射法制备了Ge-Sb-Te和Ge-Sb-Te-O相变材料薄膜,由热处理前后薄膜的X射线衍射(XRD)发现,热处理使薄膜发生了从非晶态到晶态的相变.通过非晶态薄膜粉末的示差扫描量热(DSC)实验测出不同加热速率条件下的结晶峰温度,并计算了材料的摩尔结晶活化能、原子激活能和频率因子.根据结晶动力学结晶活化能E判据得出结论为:与Ge-Sb-Te相比,掺杂氧后的Ge-Sb-Te更容易析晶,具有更快的结晶速率.     

GaGeTe films as phase-change optical recording media

Ga₂₀Ge₃₀Te₅₀ thin films deposited by vacuum evaporation on various substrates have been studied for their structural and optical properties. The as-deposited amorphous films were crystallized by thermally annealing them. The optical constants of the amorphous films indicate semiconducting behaviour (n> k). The optical bandgap (E_g) determined from Tauc's plot is 0.7 eV. The change in reflectance on crystallization has been utilized to obtain maximum optical contrast by optimising the thickness of the film.     

Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films

Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiC_x:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiC_x:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV-Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiC_x:H thin films.     

Properties of aluminum oxide thin films deposited by pulsed laser deposition and plasma enhanced chemical vapor deposition

The chemical, structural, mechanical and optical properties of thin aluminum oxide films deposited at room temperature (RT) and 800 °C on (100) Si and Si-SiO₂ substrates by pulsed laser deposition and plasma enhanced chemical vapor deposition are investigated and compared. All films are smooth and near stoichiometric aluminum oxide. RT films are amorphous, whereas γ type nano-crystallized structures are pointed out for films deposited at 800 °C. A dielectric constant of ∼ 9 is obtained for films deposited at room temperature and 11-13 for films deposited at 800 °C. Young modulus and hardness are in the range 116-254 GPa and 6.4-28.8 GPa respectively. In both cases, the results show that the deposited films have very interesting properties opening applications in mechanical, dielectric and optical fields.     

Thermal and optical properties of Ge5Bi18Se77 films

Amorphous films of Ge₅Bi₁₈Se₇₇ deposited by vacuum evaporation have been studied for their thermal and optical properties. Differential scanning calorimetry (DSC) has been used to perform the thermal analysis to estimate the activation energy for crystallization (E_c) and the order of crystallization (m) of this material. The high value of (E_c), 1.672 eV, indicates good stability of the amorphous phase. The optical constants of the as-deposited, amorphous and the thermally annealed crystalline films indicate semiconducting behavior and the band gap (E_g) determined from Tauc's plot are 0.92 eV and 0.8 eV for the amorphous and crystalline films, respectively. The value of the absorption coefficient () is of the order of 10⁴cm^-1 in the optical range for both amorphous and crystalline films. The studies on optical and thermal properties confirm the suitability of these firms in phase change optical recording.     

Aluminum-induced in situ crystallization of HWCVD a-Si:H films

Aluminum-induced in situ crystallization (AIC) of amorphous silicon films deposited by hot wire chemical vapor deposition (HWCVD) on glass is demonstrated. Aluminum was deposited at temperatures varying from room temperature to 300 °C on HWCVD a-Si:H films. The AIC was observed to take place in situ during the deposition of Al films, when the glass/a-Si:H temperature is kept 300 °C. A 20-nm Al film was effective in inducing crystallization of about 63% in the a-Si:H film. Thus, separate post-deposition annealing step can be avoided. For an Al film thickness comparable to the amorphous silicon film deposited at an optimum deposition rate, crystallization at temperature as low as 200 °C is observed. It was also observed that the growth pattern of c-Si in case of AIC without post-deposition annealing was identical to AIC with annealing step.     

Influence of deposition parameters on preferred orientation of RF magnetron sputtered BST thin films

Ba_0.65Sr_0.35TiO₃ (BST) thin films have been deposited by radio frequency magnetron sputtering. The effects of the deposition parameters on the crystallization and microstructure of BST thin films were investigated by X-ray diffraction and field emission scanning electron microscopy, respectively. The crystallization behavior of these films was apparently affected by the substrate temperature, annealing temperature and sputtering pressure. The as-deposited thin films at room temperature were amorphous. However, the improved crystallization is observed for BST thin films deposited at higher temperature. As the annealing temperature increased, the dominant X-ray diffraction peaks became sharper and more intense. The dominant diffraction peaks increased with the sputtering pressures increasing as the films deposited at 0.37–1.2 Pa. With increasing the sputtering pressure up to 3.9 Pa, BST thin films had the (110) + (200) preferred orientation. Possible correlations of the crystallization with changes in the sputtering pressure were discussed. The SEM morphologies indicated the film was small grains, smooth, and the interface between the film and the substrate was sharp and clear.     

非化学计量比SbOx薄膜的结晶动力学研究

利用直流磁控反应溅射法在不同氧分压下制备了SbOx薄膜,对退火前后薄膜的X射线衍射(XRD)分析表明,退火后薄膜结构发生了从非晶态向晶态的转变.利用示差扫描量热法(DSC)测出不同升温速度条件下非晶态薄膜粉末的晶化峰温度,用Kissinger公式计算了材料的结晶活化能计算结果表明,随着溅射时氧分压的增加,薄膜的结晶活化能增加,而相应的非晶态与晶态之间的焓差则呈现出相反的变化趋势.     

ZnTe薄膜特性研究

用真空共蒸发法在室温下制备了ZnTe:Cu多晶薄膜.用XRD表征薄膜结构,刚沉积未掺Cu和适度掺Cu的薄膜为立方结构,高度(111)择优,重掺Cu的为立方和六方混合相.室温时薄膜的形貌和光能隙取决于掺Cu浓度和退火温度,并通过透射光谱的测量计算出光能隙.重掺Cu的薄膜具有反常电导温度关系.     

Study of surface morphology and optical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films with annealing

Nb₂O₅ films have been deposited on variety of substrates using the sol-gel dip coating technique. As-deposited films on all substrates are amorphous. Films were annealed under controlled ambience at 300, 400 and 600°C for 5 h. As-deposited films on glass substrate show uniform surface structure. The crystal structure and surface topography are found to depend strongly on the annealing temperature and nature of the substrates. The average grain size of 40 nm is observed in films annealed at 300°C. On annealing at 400°C increasing grain size and resulting fusing of them, enhanced surface roughness. Films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single crystal Si substrates crystallized into hexagonal phase after annealing at 600°C. The as-deposited films show very high transmittance (>90%) in the visible region. The optical band gap is observed to increase from 4.35 eV when the films are in amorphous state to 4.87 eV on crystallization.     

Crystallization kinetics of SbxSe100−x thin films

The crystallization kinetics in Sb_xSe_100−x films with 39≤x≤58is studied by monitoring the optical transmission of the films during both isothermal and constant rate heatings. The structure of the films upon crystallization and at certain intermediate stages is studied by electron microscopy techniques. The results are analyzed in the frame of the Johnson-Mehl-Avrami theory in order to determine the kinetic parameters (Avrami exponent, activation energy and frequency factor) in addition to the crystallization temperature. The results show that film crystallization is always preceeded by a relaxation process which modifies substantially the optical properties of the amorphous material. Amorphous films with compositions close to the stoichiometric compound (Sb₂Se₃) are found to show the highest activation energy for crystallization.     

Room temperature photoluminescence of amorphous GaAs

We report what is believed to be the first observation of room temperature photoluminescence of amorphous GaAs. By means of low-temperature pulsed-laser deposition amorphous GaAs thin films were deposited on glass and Si substrates. Almost identical room temperature emission spectra with a peak centered at 1.368 eV have been observed for both samples. The results stress the capability of laser deposition to form hetero-paired optical device structures for room temperature operations independent of substrate morphologies.