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Crystal Orientation Dynamics of Collective Zn dots before Preferential Nucleation.

Liu CC, Huang JH, Ku CS, Chiu SJ, Ghatak J, Brahma S, Liu CW, Liu CP, Lo KY - Sci Rep (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan.

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.

No MeSH data available.


ss-RSHG parameters simulated from Eq. (3).(a) a1 and (b) a3 for different deposition times. The three insets in (a) depict the distribution of the c-axis orientation of Zn/ZnO dots.
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f3: ss-RSHG parameters simulated from Eq. (3).(a) a1 and (b) a3 for different deposition times. The three insets in (a) depict the distribution of the c-axis orientation of Zn/ZnO dots.

Mentions: The RSHG experimental setup is detailed in Ref. 31. The laser source was a pulsed Q-switched Nd-YAG laser with a wavelength of 1,064 nm, a pulse duration time of 6 ns, and a repetition rate of 10 Hz. The laser spot was not tightly focused to prevent damage to the sample. The detection area of each sample is about 10 mm2, which covers a large number of dots. The results of the optical ss-RSHG experiments reveal the anisotropic character and degree of coherence of these polar dots on a statistical basis. The ss-RSHG patterns of the Zn/ZnO dots grown on Si(111) are shown in Fig. 2 with the least squares curve fitting using Eq. (3) in red solid lines. The similar analyses for Zn/ZnO dots grown on Si(100) has no any symmetrical RSHG pattern and their RSHG signals are tiny (<0.1 same scale as Fig. 2). Since the surface symmetry of Si(100) is 4 mm that is not the same type as the surface symmetry of Zn, there is no any strain between Zn and Si(100), and hence no coherence between the structure of Zn and Si(100). The results reflect to the coherent growth between Zn and the substrate should be based on the same type of crystalline symmetry as least. The fitted parameters a1 and a3 are plotted as a function of growth time in Fig. 3. The value of a1 starts with zero until the growth time of 20 min, and then peaks at 25 min. Subsequently, a1 gradually returns to zero at 35 min growth time and remains zero thereafter. The results reveal an unusual growth phenomenon in which epitaxial dots tilt their crystallographic orientations during growth, implying a large lattice mismatch. To interpret the evolution of a1, the following scenarios are speculated (the associated possible crystal orientations are drawn in Fig. 3). (i) When the growth time is less than 20 min, the c-axes of the dots are tilted off-normal with the azimuth directions evenly distributed among in-plane Si<110>, rendering the a1 values cancelled out by each other. Besides, this sixfold tilting of the dots still makes a constructive contribution to a3, as shown in Fig. 3b. The a3 value reaches the maximum at 15 min of growth and starts to decrease thereafter, corresponding to when the dots increase to the critical volume and then start to lose their coherency for the happening of stress relaxation. (ii) When the growth time is between 20 and 35 min, a1 is non-zero but a3 keeps decreasing. When the dot volume increases, the lattice coherency keeps losing as the decreasing trend of a3. Correspondingly, the dots reorient themselves toward a more symmetric geometry to minimize the surface energy as normal to Si(111). During this process, the accumulative c-axes of the dots have asymmetrically projected orientations on some of the sixfold in-plane Si<110>, causing a nonzero resultant a1. The parameter a1 soon reaches the maximum of 0.4 for 25 min-grown Zn/ZnO dots, representing the stable kinetic configuration with χs ≠ 0 and ϕazi = 0° deduced from the ss-RSHG pattern showing periodic step-up petals (Fig. 2c). This a1 is not contributed from grain boundaries27 since ϕazi should be 30° in the grain boundary case. (iii) When the growth time is longer than 35 min, a1 = 0 and a3 keeps decreasing. This phenomenon implies that most dots have the c-axis perpendicular to Si(111) for the thermodynamic stable configuration. The RSHG parameters of a1, a3 and χs reflect the net dipole contribution from Zn/ZnO dots covered by irradiated area, which reveals the coherent growth and the evolution of crystal orientation of the collective Zn dot system during growth. These results would be a guide to do further analyses.


Crystal Orientation Dynamics of Collective Zn dots before Preferential Nucleation.

Liu CC, Huang JH, Ku CS, Chiu SJ, Ghatak J, Brahma S, Liu CW, Liu CP, Lo KY - Sci Rep (2015)

ss-RSHG parameters simulated from Eq. (3).(a) a1 and (b) a3 for different deposition times. The three insets in (a) depict the distribution of the c-axis orientation of Zn/ZnO dots.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4515603&req=5

f3: ss-RSHG parameters simulated from Eq. (3).(a) a1 and (b) a3 for different deposition times. The three insets in (a) depict the distribution of the c-axis orientation of Zn/ZnO dots.
Mentions: The RSHG experimental setup is detailed in Ref. 31. The laser source was a pulsed Q-switched Nd-YAG laser with a wavelength of 1,064 nm, a pulse duration time of 6 ns, and a repetition rate of 10 Hz. The laser spot was not tightly focused to prevent damage to the sample. The detection area of each sample is about 10 mm2, which covers a large number of dots. The results of the optical ss-RSHG experiments reveal the anisotropic character and degree of coherence of these polar dots on a statistical basis. The ss-RSHG patterns of the Zn/ZnO dots grown on Si(111) are shown in Fig. 2 with the least squares curve fitting using Eq. (3) in red solid lines. The similar analyses for Zn/ZnO dots grown on Si(100) has no any symmetrical RSHG pattern and their RSHG signals are tiny (<0.1 same scale as Fig. 2). Since the surface symmetry of Si(100) is 4 mm that is not the same type as the surface symmetry of Zn, there is no any strain between Zn and Si(100), and hence no coherence between the structure of Zn and Si(100). The results reflect to the coherent growth between Zn and the substrate should be based on the same type of crystalline symmetry as least. The fitted parameters a1 and a3 are plotted as a function of growth time in Fig. 3. The value of a1 starts with zero until the growth time of 20 min, and then peaks at 25 min. Subsequently, a1 gradually returns to zero at 35 min growth time and remains zero thereafter. The results reveal an unusual growth phenomenon in which epitaxial dots tilt their crystallographic orientations during growth, implying a large lattice mismatch. To interpret the evolution of a1, the following scenarios are speculated (the associated possible crystal orientations are drawn in Fig. 3). (i) When the growth time is less than 20 min, the c-axes of the dots are tilted off-normal with the azimuth directions evenly distributed among in-plane Si<110>, rendering the a1 values cancelled out by each other. Besides, this sixfold tilting of the dots still makes a constructive contribution to a3, as shown in Fig. 3b. The a3 value reaches the maximum at 15 min of growth and starts to decrease thereafter, corresponding to when the dots increase to the critical volume and then start to lose their coherency for the happening of stress relaxation. (ii) When the growth time is between 20 and 35 min, a1 is non-zero but a3 keeps decreasing. When the dot volume increases, the lattice coherency keeps losing as the decreasing trend of a3. Correspondingly, the dots reorient themselves toward a more symmetric geometry to minimize the surface energy as normal to Si(111). During this process, the accumulative c-axes of the dots have asymmetrically projected orientations on some of the sixfold in-plane Si<110>, causing a nonzero resultant a1. The parameter a1 soon reaches the maximum of 0.4 for 25 min-grown Zn/ZnO dots, representing the stable kinetic configuration with χs ≠ 0 and ϕazi = 0° deduced from the ss-RSHG pattern showing periodic step-up petals (Fig. 2c). This a1 is not contributed from grain boundaries27 since ϕazi should be 30° in the grain boundary case. (iii) When the growth time is longer than 35 min, a1 = 0 and a3 keeps decreasing. This phenomenon implies that most dots have the c-axis perpendicular to Si(111) for the thermodynamic stable configuration. The RSHG parameters of a1, a3 and χs reflect the net dipole contribution from Zn/ZnO dots covered by irradiated area, which reveals the coherent growth and the evolution of crystal orientation of the collective Zn dot system during growth. These results would be a guide to do further analyses.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan.

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.

No MeSH data available.