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

我们已经对硅尖的场发射电流进行了时间分辨测量。超快光谱是由飞秒场发射相机进行的，该相机最初是为了观察金属尖端上的单吸附动力学而开发的。作为半导体表面动力学的初步观测，我们进行了皮秒时间分辨率的测量。尽管目前装置中的最大电流不足以研究硅表面上的单原子动力学，但人们可以获得关于原子集体运动和硅表面电子隧穿的信息。在我们的研究中获得的最大电流密度与固态隧道器件的最大电流密度相当。我们还基于Tersoff势的分子动力学(MD)模拟研究了过冷液态硅(Si)中的成核和晶化过程。由多达8192个Si原子组成的MD盒被加热以产生熔融的Si，然后在有或没有温度梯度的不同过冷条件下对熔化的Si进行淬火，并显示了相应的形核过程。确定了临界形核半径与过冷度ΔT的函数关系，发现它与ΔT成反比。在线性温度梯度下过冷的情况下，形核首先发生在低温区，然后晶化向高温区进行，(111)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.