Superlattice period

Fig. 12—Hardness of TiN/NbN superlattice after Shinn et al. [100], TiN/VN superlattices after Helmersson et al. [101], and TiN/Ti superlattice after Shih et al. [102] as functions of the superlattice period.

Stoichiometric modulations through the superlattice period all have substantial effects on these properties. 

The challenge is to form compounds with structures that we design, not Mother Nature. Superlattices are examples of nano-structured materials [1-3], where the unit cell is artificially manipulated in one dimension. By alternately depositing thin-films of two compounds, a material is created with a new unit cell, defined by the superlattice period. 

Determine the superlattice period from the spacing of adjacent satellites. 

In order to ascertain how such a 708 superlattice periodicity could arise, we have investigated a number of possible model systems by optical diffractometry. The method is illustrated in Fig. 11. Computer-simulated, graphical projections of the crystal structure are prepared onto 35mm photographic film as black and white transparency masks, which then serve as the object for optical diffraction. As can be seen, the technique faithfully yields the required rectangular (Fig. lid) and triangular (Fig. lie) diffraction patterns for simulated ZSM-5 and ZSM-11 ([010] projection), respectively. Further the 4o8 periodicity along the a axis for the particular , -sequence shown in Fig. 11c is clearly reflected in the appropriate transform (Fig. Ilf). 

Different surface-sensitive techniques respond differently to the various kinds of disorder. The measurement of LEED beams is unique in largely filtering out all defects that are unrelated to the superlattice periodicity that defines the beams. Other techniques (including diffuse LEED) generally include contributions from all defects, for instance from adsorbates located at undesired steps and crystallite boundaries. Then only a reduction of the defect concentration can remove defect contributions from the experimental data. Rotational disorder of adsorbed molecules does not matter for techniques which measure only bond lengths, and not bond orientations. Thus, NEXAFS is more sensitive to such disorder than SEXAFS. 

Doublets of folded longitudinal acoustic (LA) phonons due to the superlattice periodicity [143] can also be seen in the Raman spectra of the SLs (indicated by arrows in Fig. 21.2). The positions of the doubled peaks agree well with the first doublet frequencies calculated within the elastic continuum model [144]. The observation of the LA phonon folding suggests that these superlattices possess the requisite structural quality for acoustic Bragg mirrors and cavities to be used for potential coherent phonon generation applications [145-147]. 

In Fig. 2C we illustrate a schematic model of such a superpattern. Two graphite sheets that are rotated relative to each other are superimposed, which results in a giant honeycomb lattice. By choosing an angle of 9° for the misorientation, we obtain a superlattice period of 16 A. 

When nonlocality is not taken into account, the value of the x-component of dielectric displacement vector averaged over the superlattice period is (see also Section 8.1)... 

Then averaging over the superlattice period according to the rules... 

The angle brackets denote an arithmetic average over the superlattice period. It is clear that this average is equivalent to averaging of the fields over the thickness of the superlattice d L. 

One can experimentally derive the degree of charge transfer (Z) [73-75] between donor and acceptor from the location of the superlattice X-ray spots (diffuse or sharp). Let the superlattice periodicity due to 2kp or 4kp scattering be A., and let d be the regular (or high-temperature static) lattice spacing. Then it can be shown that ... 

Debye-Hfickel screening parameter observed, nuclear and magnetic widths of neutron diffraction lines superlattice period Bohr magneton effective de Geimes factor magnetic coherence length density of states... 

Semiconductors, Electrochemical Atomic Layer Deposition (E-ALD), Fig. 2 STM images (a) Au vapor deposited on glass (b) 30 superlattice periods. 

A hybrid approach to nanowire synthesis developed by Wu et al. [22] combines laser ablation with a flow reactor process, and has yielded superlattice nanowires of Si/SiGe. The role of the flow furnace is to produce Si nano wires, while the laser ablation process introduces Ge periodically into the vapor. The Ge is absorbed by the catalyst and incorporated into the forming nanowire. Through judicious control of the process, Wu s group has achieved superlattice periodicities of Si/ SiGe of approximately 250 nm. Moreover, the results of this study bode well for superlattice nanowire devices. 

Figure 8 Vickers hardness of nitride superlattice structures as a function of superlattice period X. (From Ref. 47, 48.)...

Wang, S., Mingo, N. (2009). Tailoring interface roughness and superlattice period length in electron-filtering thermoelectric materials. Physical Review B, 79, 115316. 

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