Study On The Rule Of Structural And Bonding Evolutionduring Amorphization For Several Semiconductors:Insights From First-principles Calculations

Semiconductor is the “blood” in electronic industry. Amorphous semiconductors exhibits may unique properties because they have a structure with short range order but long range disorder. Therefore, amorphous semiconductors have been widely used in the field of electronic science such as information storage and energy conversion. It will beneficial for improving performance of amorphous semiconductor devices, if we explore the atomic structure and electronic structure of amorphous semiconductors. Generally, we can obtain amorphous semiconductor by traditional melt-quench method or solid state phase transition induced by laser electronic excitation. In this paper, we will try to analysis the bonding structure and amorphization process of amorphous chalcogenides, such as AlSb, GeSbTe, and graphite, by first-principles calculations. By the way, we also study the defect behavior of Hydrogen in ZnS semiconductor. Details are as follows:1. Some chalcogenides have the ability of rapid and reversible transition between their crystalline and amorphous phase under laser or electrical pulse. This is the basic rules for phase change memory to storage information. These chalogenides are also known as phase change material. Generally, the phase-change materials have three or more kinds of elements. A new kind of phase change material, AlSb, is made up of two metal elements. In spite of this, AlSb shows good stability of amorphous phase. Meanwhile, it is reported that Al keeps in amorphous state during phase switching. Here, by molecular dynamic simulation, we obtain the atomic structure of amorphous AlSb. In order to find the underlying causes of its good amorphous phase stability, we analysis the bond angle and coordination number distribution, electron localization function and charge density difference of amorphous AlSb. We find that both Al and Sb hold the sp3 bonding structure, which build up the basic atomic network in amorphous AlSb. Besides, Al tends to accumulate and form three fold rings which show metallic characteristic. As such, amorphous AlSb can be divided into Al-rich part and Sb-rich part. The Sb-rich part could hold the ability of fast phase transition. While, the Al-rich part shows large difference in structure between crystallize and amorphous AlSb. This is reason why AlSb exhibits good stability of amorphous phase.2. By electronic excitation effect caused by laser, amorphous semiconductor, such as Ge2Sb2Te5(GST), has been reported to be obtained by a kind of solid-state phase transition. Based on this, we further explore the effect of electronic excitation on amorphous GST. By monitoring the evolution of the amount of four fold rings, we demonstrate that amorphous GST can still hold amorphous states instead of transfer into crystalline state under electronic excitation. The amorphous GST will be more stable if lager amount of electronic excitation degree is applied.3. Laser electronic excitation can not only lead to amorphization in materials but also significantly change its bonding characteristics. Here, we study the evolution of layer-style graphite on the surface of diamond under electronic excitation. We find that first of all the excitation can make the graphite disorder. On the other hand, a part of original bonds with sp2 bonding characteristic can change to the ones with sp3 bonding. If we can align the honeycomb motif in graphite with the six-fold ring structure in(111) plane of diamond, the excitation could have a possibility of transferring sp2 graphite to sp3 diamond.4. Defect is a kind of disorder in perfect crystal. In this thesis, we study the origin of hydrogen defects, their microscopic picture, and their transition path in cubic ZnS semiconductor. We find that the Zn-S bond center site is the most favorite site for H atom, that indicate other element impurities and carrier can have significant impacts on such kind of H defect. In comparison, the interstitial site of hydrogen has a higher energy and can be not so stable. On the other hand, by first-principles molecular dynamics, we find that the interstitial hydrogen can be easily moved even in room temperature, which indicates the high-temperature annealing may be a possible route to remove hydrogen in ZnS.