富矽氮化物中矽量子點晶體生長之電腦模擬

Abstract

我們計畫利用最近發展的self-consistent-charge density-functional tight-binding (SCC-DFTB)方法進行富矽氮化物中矽量子點晶體生長之電腦模擬，此法應用DFTB算出原子作用力再用分子動力學進行動態模擬，DFTB是密度泛涵理論的近似法，可以得到接近密度泛涵理論的結果而在運算上快很多，首先，我們用亂數產生Si 及N在空間中的分布，再優化到某一個溫度下的穩定幾何結構，此結構將近似低溫PECVD製程的初始結構，晶粒析出的現象則預期在中、高溫的回火中產生。 我們的主要目標為：(1)找出成核機制，(2)估算適當的回火溫度，(3)估算適當的矽氮比例，(4)估算快速成核的初始結構，這些資訊將助富矽氮化物中矽量子點晶體生長實驗找到有效的實驗參數。

We are proposing to use the recently developed self-consistent-charge density-functional tight-binding (SCC-DFTB) method to perform simulations of silicon quantum dots nucleation process in silicon-rich amorphous silicon nitride samples. The imulation will be performed using molecular dynamics with atomic forces determined from quantum DFTB calculations. The forces will be calculated at every molecular dynamics frame. Such an approach is only possible owing to the exceptional computational efficiency of the DFTB method that can be achieved owing to careful and well-tested approximations introduced in the usual density functional formalism. Therefore, we can expect an almost DFT-like accuracy of our simulations at drastically reduced computational costs. The simulation will use initial silicon-rich silicon nitride structures generated by random distribution of silicon and nitrogen atoms and pre-optimized to ensure geometrical stability of the samples at the simulation temperature. By choosing such a theoretical framework we expect to reproduce the experimental conditions that have been recently used in the low-temperature PECVD processes. The nucleation phenomena are anticipated to be observed in the simulated annealing process of the initial samples at moderate and high temperatures (500° C to 900° C).Our main objectives are: (i) establish the mechanism of nucleation, (ii) determine the optimal annealing temperature, (iii) find the optimum silicon-to-nitrogen ratio in the initial samples, and (iv) elucidate the local geometrical molecular structure in the initial silicon-rich silicon nitride samples that leads to the most rapid nucleation grain formation. The obtained theoretical values will be used to propose an efficient experimental setup that can yield the silicon quantum dots with good effectiveness and to assist the experimental synthesis process.