Crystal Orientation Dynamics of Collective Zn dots before Preferential Nucleation

Abstract

The island nucleation in the context of heterogeneous thin film growth is often complicated by the growth kinetics involved in the subsequent thermodynamics. We show how the evolution of sputtered Zn island nucleation on Si(111) by magnetron sputtering in a large area can be completely understood as a model system by combining reflective second harmonic generation (RSHG), a 2D pole figure with synchrotron X-ray diffraction. Zn dots are then oxidized on the surfaces when exposed to the atmosphere as Zn/ZnO dots. Derived from the RSHG patterns of Zn dots at different growth times, the Zn dots grow following a unique transition from kinetic to thermodynamic control. Under kinetic-favoring growth, tiny Zn dots prefer arranging themselves with a tilted c-axis to the Si(111) substrate toward any of the sixfold in-plane Si<110> directions. Upon growth, the Zn dots subsequently evolve themselves to a metastable state with a smaller tilting angle toward selective <110> directions. As the Zn dots grow over a critical size, they become most thermodynamically stable with the c-axis vertical to the Si(111) substrate. For a system with large lattice mismatch, small volume dots take kinetic pathways with insignificant deviations in energy barriers.

Thermal effect of Zn quantum dots grown on Si(111): competition between relaxation and reconstraint

“…The micrographs and size distributions of ZnO/Zn dots were analyzed using FESEM (ZEISS-SUPRA 55-41-30). The net dipole contribution from the ZnO/Zn dots was determined through RSHG experiments; the setup and operation method of these experiments are described in [16]. Synchrotron XRD and XANES were performed at the 17B1 and 20A1 stations of the National Synchrotron Radiation Research Center in Taiwan.…”

“…ZnO/Zn dots. Zn dots can grow coherently on the Si(111) surface because of the pattern matching (3 m) between the Zn and Si(111) surface within the threshold limit of complete relaxation [16]. The constrained Zn dots follow the 3 m Si(111) surface with a tilt angle to the c-axis toward one of the six in-planes of Si〈110〉 and contribute to the noncancelled 3 m symmetrical dipole to RSHG [16].…”

“…However, the critical thickness of the constrained metal dot due to lattice mismatch between the metal and the semiconductor surface would restrict the size of the constrained metal dots [15]; this restriction is unfavorable for the actual development and application of nanometal contact. The existence of metastable states in Zn dots coherently grown on a Si(111) system has been recently discovered [16]. The formation of the metastable state before relaxation results from some 3D potential barriers between Zn(111) and one of the sixfold in-plane Si〈110〉.…”

“…The formation of the metastable state before relaxation results from some 3D potential barriers between Zn(111) and one of the sixfold in-plane Si〈110〉. The existence of the metastable state in the interface of Zn dots and oxide-free Si(111) increases the size of the constrained Zn dot and suppresses the surface energy of Zn to dominate the total free energy directly [16][17][18]. These results facilitate the development of a constrained metal dot/semiconductor for applications as contacts in future nanodevices.…”

“…Nonetheless, large ZnO/Zn dots remarkably contribute to the intensity of synchrotron XRD patterns. RSHG and synchrotron XRD are used to evaluate the net symmetrical dipole from constrained ZnO/Zn dots and the peak ratio of Zn (111) and Zn(002) in the XRD patterns to distinguish between constrained and relaxed ZnO/ Zn dots [16,18]. The compensative optical method enables the systematic analysis of the further structural evolution and crystal orientation of ZnO/Zn dots.…”