Ultra-fast Time-resolved Field-emission-current Measurements of Surface Adsobates on Si-tip

我们已经对硅尖端的场发射电流进行了时间分辨测量。利用飞秒场发射相机进行了超快速光谱分析，该相机最初是为了观察金属尖端上的单一吸附质动力学而开发的。作为对半导体表面动力学的初步观察，我们进行了皮秒时间分辨率的测量。尽管目前装置的最大电流不足以研究硅表面上的单原子动力学，但人们可以从硅表面获得原子集体运动和电子隧穿的信息。在我们的研究中获得的最大电流密度可与固态隧道装置所达到的电流密度相媲美。我们还研究了过冷液态硅（Si）中基于分子动力学（MD）模拟的原子成核和结晶过程。将8192个Si原子组成的MD细胞加热生成熔融Si，然后在有温度梯度或无温度梯度的各种过冷条件下对熔融Si进行淬火，并观察相应的成核过程。临界成核半径与过冷量ΔT成反比，与ΔT成反比。在线性温度梯度下过冷时，首先在低温区成核，然后在高温区结晶，（111）表面大部分平行于温度梯度。

We have carried out time-resolved measurements of field-emission currents from silicon tips. The ultra-fast spectroscopy has been carried out by a femtosecond field emission camera originally developed to observe single adsorbate dynamics on metal tips. As a preliminary observation of semiconductor surface dynamics, we have performed measurements in picosecond time resolution. Even though the maximum current in the present set-up is not enough to study the single atomic dynamics on silicon surfaces, one can gain information on collective motions of atoms and on electron tunneling from silicon surfaces. The maximum current density achieved in our study is comparable to those reached by solid state tunneling devices.We have also investigated atomistic processes of nucleation and crystallization in supercooled liquid silicon (Si) based on molecular-dynamics (MD) simulations using the Tersoff potential. MD cells composed of up to 8192 Si atoms were heated to produce melted Si, and then melted Si was quenched under various supercooled conditions with or without a temperature gradient and the corresponding nucleation processes were visualized. The critical nucleation radius was determined as a function of the amount of supercooling ΔT and it was found to be inversely proportional to ΔT. It was also found, in the case of supercooling under a linear temperature gradient, that nucleation first occurred at the lower temperature region and then crystallization proceeded toward the high temperature region with the (111) surface mostly parallel to the temperature gradient.