Photon and Ion-Beam Induced Processes in Materials Studied by Hyperfine Interactions

Laser nitriding and laser cementation are investigated by Mössbauer spectroscopy and complementary methods. It is demonstrated how the backscattering versions of Conversion Electron and Conversion X-ray Mössbauer spectroscopy can contribute to the investigation of surface processes, like the laser-induced formation of nitrides and carbides. Additionally, the formation of semiconducting iron disilicide can be achieved by ion-beam mixing and pulsed laser irradiation of Fe/Si bilayers. The results of both processes are compared.

     

Laser nitriding investigated with Mössbauer spectroscopy

The effect of the nitrogen take‐up upon irradiation of iron or steel with excimer laser pulses in air or in nitrogen atmosphere is well established. The resulting phase compositions and nitrogen depth profiles were measured by a combination of simultaneous Conversion Electron Mössbauer Spectroscopy (CEMS), Conversion X‐ray Mössbauer Spectroscopy (CXMS), and Resonant Nuclear Reaction Analysis (RNRA) as a function of the nitrogen gas pressure during irradiation. A maximum nitrogen content and a maximum fraction of the ?-nitride was found at 0.1 MPa. This result is in accordance with hardness measurements performed by the nanoindentation technique.     

Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

Co掺杂ZnO薄膜的结构和磁学性能

研究了用单束脉冲激光沉积法制备的Co掺杂ZnO薄膜的结构和磁学性能。XRD表征结果表明制备的薄膜是具有沿c轴择优取向的纤锌矿点阵结构。然而,进一步的高分辨电子显微镜结果显示整个样品上的晶体取向并不完全相同。很难说明形成了单晶。结果分析表明Co占据了部分Zn的格点,并对电子结构产生了影响。室温下观察到了磁滞回线,显示掺杂Co可以实现ZnO的磁性翻转,但磁性比较小。该薄膜与我们以前用双束脉冲激光沉积法制备的Co掺杂ZnO薄膜具有相似的性能,提示我们其内部的机制可能相似。     

制成完全取向的D相准晶薄膜

用磁控溅射和后期退火的方法，首次制成了完全取向的Ｄ相准晶薄膜。这种薄膜对准晶的基础研究和应用研究都有重要意义。     

Progress in Optical Waveguiding Thin Films

Overview of recent activities in the development of active and passive thin film waveguides, waveguide lasers and laser-deposited waveguiding films is given. Published results and parameters are summarized. The focus is on films fabricated by laser deposition.     

Imaging the Magnetic Structure of Thin Films and Nanoparticles

Hyperfine Interactions - Nuclear resonant scattering of synchrotron radiation is applied to investigate the magnetic spin structure of nanoscale systems. The outstanding brilliance of modern...     

Muon Spin Rotation and Relaxation Experiments on Thin Films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy SR, LE-SR). After a brief overview of the LE-SR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa₂Cu₃O₇ film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional SR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.     

Critical Dynamics in Thin Films

Critical dynamics in film geometry is analyzed within the field-theoretical approach. In particular we consider the case of purely relaxational dynamics (Model A) and Dirichlet boundary conditions, corresponding to the so-called ordinary surface universality class on both confining boundaries. The general scaling properties for the linear response and correlation functions and for dynamic Casimir forces are discussed. Within the Gaussian approximation we determine the analytic expressions for the associated universal scaling functions and study quantitatively in detail their qualitative features as well as their various limiting behaviors close to the bulk critical point. In addition we consider the effects of time-dependent fields on the fluctuation-induced dynamic Casimir force and determine analytically the corresponding universal scaling functions and their asymptotic behaviors for two specific instances of instantaneous perturbations. The universal aspects of nonlinear relaxation from an initially ordered state are also discussed emphasizing the different crossovers occurring during this evolution. The model considered is relevant to the critical dynamics of actual uniaxial ferromagnetic films with symmetry-preserving conditions at the confining surfaces and for Monte Carlo simulations of spin system with Glauber dynamics and free boundary conditions.     

Grain Growth in Polycrystalline Thin Films of Semiconductors

Grain growth in thin films is usually abnormal, leading not only to an increase in the average grain size, but also to an evolution in the shape of the grain size distribution and to an evolution in the distribution of grain orientations. The latter can be driven by surface, interface or strain energy minimization, depending on film and substrate properties and on deposition conditions, and can lead to different final textures depending on which energy dominates.In semiconductor films, as in other materials, grain growth stagnation coupled with texture-selective driving forces leads to secondary grain growth, the rate of which is higher in thinner films. Self ion-bombardment enhances the rate of pre-stagnation grain growth, and doping of Si with electron donor leads to enhanced pre-stagnation grain growth as well as surface-energy-driven secondary grain growth. The effects of ion-bombardment and dopants on grain growth in Si can be understood in terms of associated increases in point defect concentrations and the effects of point defects on grain boundary mobilities.     

ZnO薄膜近带边紫外发光的研究

用ZnO陶瓷靶,采用脉冲激光沉积(PLD)技术在c-Al₂O₃衬底上制备了ZnO薄膜。通过不同温度下光致发光(PL)光谱的测量,对样品的紫外发光机理进行研究。 在较低温度(10 K)下的PL光谱中,观测到一个位于3.354 eV处的束缚激子(D⁰X)发射,随着温度的升高(~50 K),在D⁰X的高能侧观测到了自由激子的发射峰。在10 K温度下,3.309 eV处出现了一个较强的发光带A,此发光带强度随着温度升高先增大然后减小,并且一直延续到室温。重点讨论了此发光带的起源,并认为A带可归属于自由电子-受主之间的复合发射。     

PLD制备的Cu掺杂SnS薄膜的结构和光学特性

利用脉冲激光沉积(PLD)在玻璃衬底上制备了Cu掺杂SnS薄膜.靶材是由SnS和Cu₂S粉末混合压制而成(Cu和Sn的量比分别为0%、2.5%、5%、7.5%和10%).利用X射线衍射(XRD)、拉曼光谱仪(Raman)、原子力显微镜(AFM)、紫外-可见-近红外分光光度计(UV-Vis-NIR)、Keithley 4200-SCS半导体参数分析仪研究了Cu掺杂量对SnS薄膜的晶体结构、表面形貌、光学性质和电学性能的影响.结果表明:所制备的SnS薄膜样品沿(111)晶面择优取向生长, SnS :5%Cu薄膜的结晶质量最好且具有SnS特征拉曼峰.随着Cu掺杂量的增大, 平均颗粒尺寸逐渐增大.不同Cu掺杂量的薄膜在可见光范围内的吸收系数均为10⁵ cm^-1数 量级.SnS :5%Cu薄膜的禁带宽度E_g为2.23 eV, 光暗电导率比值为2.59.同时, 在玻璃衬底上制备了p-SnS :Cu/n-ZnS 异质结器件, 器件在暗态及光照的条件下均有良好的整流特性, 并具有较弱的光伏特性.     

退火对溶胶-凝胶法制备的ZnO薄膜结构和光致发光的影响

采用溶胶-凝胶法在n-Si(100)衬底上制备ZnO薄膜并从三个方面对其研究。X射线衍射结果表明,在含氧气氛中退火的ZnO薄膜为多晶六角纤锌矿结构,有明显的c轴择优结晶取向;退火时间的长短和温度的高低对结晶取向性和粒径均有较大影响,通过进一步的研究发现最佳处理温度在500℃左右。用扫描电子显微镜观察样品的表面和侧面形貌,晶体的生长比较均匀,粒径平均在70~160nm范围内,与XRD测量结果相一致。室温下ZnO胶体的光致发光谱表明,随着胶体老化时间的延长,胶体的紫外峰位发生了蓝移。室温下ZnO薄膜的光致发光谱表明,紫外部分的发光峰位在365,390nm,发光强度较强;在可见光区的发光强度相对较弱,但是还没有被氧完全抑制掉。     

生长温度对Ga掺杂ZnO薄膜光电性能的影响

在不同衬底温度(室温~750 ℃)条件下,采用脉冲激光沉积(PLD)方法在石英玻璃和单晶硅(111)衬底上制备了Ga掺杂ZnO(GZO)薄膜。结果显示:衬底温度的变化导致衬底表面吸附原子扩散速率和脱附速率的不同,从而导致合成薄膜结晶质量的差异,衬底温度450 ℃时制备的GZO薄膜具有最好的结晶特性;GZO薄膜中载流子浓度随衬底温度升高而单调减小的现象与GZO薄膜中的本征缺陷密切相关,晶界散射强度的变化导致迁移率出现先增大后减小的趋势,衬底温度450 ℃时制备的GZO薄膜具有最小的电阻率~0.02 Ω·cm;随着衬底温度的升高,薄膜载流子浓度的单调减小导致了薄膜光学带隙变窄,所有合成样品的平均可见光透过率均达到85%以上。采用PLD方法制备GZO薄膜,衬底温度的改变可以对薄膜的光电性能起到调制作用。     

利用脉冲激光沉积法制备高Mg掺杂的六方相MgZnO薄膜

选用Mg0.2Zn0.8O陶瓷靶,利用脉冲激光沉积(PLD)法,在单晶Si(100)和石英衬底上生长了一系列MgZnO薄膜(MZO)。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱(EDS)和紫外可见光透射光谱(UV-Vis)等实验手段,研究了在不同工作压强下生长的薄膜样品的晶体结构、微观形貌和光学性能的变化。结果表明:所有的薄膜样品都是单一的ZnO六方相,禁带宽度随生长压强的升高而增加,变化范围在3.83～4.05eV之间,最短吸收边接近300nm。     

退火处理对YMnO3薄膜的结构和荧光性能的影响

室温下,采用脉冲激光沉积方法在Si(100)衬底上制备了YMnO3薄膜,并对其进行了不同温度的退火处理。采用X射线衍射和荧光光谱分析方法对薄膜的结构和荧光特性进行了研究。结果表明:通过退火处理,可以得到正交相和六方相共存的多晶态YMnO3薄膜,并且随着退火温度的升高,两相的比例发生变化,由正交相为主转变为六方相为主。YMnO3薄膜样品的荧光发射峰集中在波长430～620nm范围内,可能是由Mn3 离子从5T2到5E之间的能级跃迁所引起的。其荧光强度随着退火温度的升高逐渐增强,但峰位基本保持不变,说明薄膜结构的改变对Mn3 离子的能级跃迁几率有明显的影响,对能级位置的影响不大;而且荧光光谱还显示在同一薄膜中各个荧光峰的相对强度随着退火温度的变化不大。     

Zinc Oxide Thin Films for Memristive Devices: A Review

Zinc Oxide (ZnO) thin films have been addressed as promising candidates for the fabrication of Resistive Random Access Memory devices, which are alternative to conventional charge-based flash memories. According to the filamentary conducting model and charge trapping/detrapping theory developed in the last decade, the memristive behavior of ZnO thin films is explained in terms of conducting filaments formed by metallic ions and/or oxygen vacancies, and their breaking through electrochemical redox reactions and/or recombination of oxygen vacancies/ions. A comparative review of the memristive properties of ZnO thin films grown by sputtering, atomic layer deposition (ALD), pulsed laser deposition (PLD), and sol-gel methods is here proposed. Sputtered ZnO thin films show promising resistive switching behaviors, showing high on/off ratios (10–10⁴), good endurance, and low operating voltages. ALD is also indicated to be useful for growing conformal ZnO layers with atomic thickness control, resulting in important resistive switching characteristics, such as relatively high on/off ratios and low operating voltages. High insulating epitaxial ZnO thin films can be obtained by PLD, showing reliable switching properties at low voltages and with good retention. On the contrary, the sol-gel approach generally results in ZnO thin films with poor resistive switching behaviors. Nevertheless, thin films derived from ZnO NPs show improved switching performances, with higher on/off ratios and lower operating voltages. Independently of the synthetic approach, doped ZnO thin films exhibit better resistive switching behaviors than pristine ones, coupling a strong increase of the on/off ratio with a more stable switching response.     

在聚合物表面刻写方格子的新方法及理论研究

为了提高制作偶氮苯微结构的实验技术,在偶氮苯聚合物的光致异构和光致取向特性的基础上,利用两激光束的新实验方案,运用比较简便的分步制作的方法,成功的在偶氮苯功能化聚合物薄膜表面“刻写”出二维的正方格子结构。提出了一个新的光异构取向场理论：在两束偏振方向互相垂直的偏振光照射下,偶氮苯聚合物表面光栅的形成是光异构取向场的干涉的结果,并对实验现象进行了解释。用计算机对实验结果进行了模拟,将模拟结果与实验结果进行了比较,结果表明计算机模拟将会更加有利于偶氮苯聚合物微结构的设计和制作。     

梯度磁场中自旋-轨道耦合旋转两分量玻色-爱因斯坦凝聚体的基态研究

利用准二维Gross-Pitaevskii方程,研究了在梯度磁场中具有自旋-轨道耦合的旋转两分量玻色-爱因斯坦凝聚体的基态结构.探索了自旋-轨道耦合作用和梯度磁场对基态的影响.结果发现,在梯度磁场下,随着自旋-轨道耦合强度增大,基态结构由skyrmion格子逐渐过渡为skyrmion列.对于弱自旋-轨道耦合和小旋转频率情况,增大磁场梯度强度可导致基态由平面波相转变为half-skyrmion;对于强自旋-轨道耦合和大旋转频率情况,梯度磁场可诱导hidden涡旋的产生.梯度磁场、自旋-轨道耦合和旋转作为体系的调控参数,可用于控制不同基态相间的转化.