Crystal quality

Under Httle or no illumination,/ must be minimized for optimum performance. The factor B is 1.0 for pure diffusion current and approaches 2.0 as depletion and surface-mode currents become important. Generally, high crystal quality for long minority carrier lifetime and low surface-state density reduce the dark current density which is the sum of the diffusion, depletion, tunneling, and surface currents. The ZM product is typically measured at zero bias and is expressed as RM. The ideal photodiode noise current can be expressed as follows ... 

Three common uses of RBS analysis exist quantitative depth profiling, areal concentration measurements (atoms/cm ), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profilii of polymers, high-T superconductors, optical coatings, and catalyst particles. ... 

F. Otalora, J. M. Garcia-Ruiz, A. Moreno. Protein crystal quality studies using rod-shaped crystals. J Cryst Growth 168 93>, 1996. 

The crystal quality of the InGaN QWs becomes poor mainly due to the lattice-constant mismatch and the difference of the thermal expansion coefficient between InN and GaN with increasing the In composition [4,5]. Therefore, in order to improve the external quantum efficiency (i/ext) of the InGaN-based LEDs and LDs, it is important to elucidate and optimize the effects of the various growth conditions for the InGaN active layer on the structural and optical properties. Recently, we reported a fabrication of efficient blue LEDs with InGaN/GaN triangular shaped QWs and obtained a substantial improvement of electrical and optical properties of the devices [6,7]. 

Undoped, Mn, and Pr-doped ZnS namopartides synthesized by wet chemiral mdhod were optically annealed in air or vacuum. PL emission inoeas with annulling time. This increase is attributed to the photo-oxidation, enhancanent in the crystal quality, and diffiision of the luminescent ions. PL intensity of nanoparticles annealed in air increased more significantly due to the photo-oxidation compared with the nanoparticles annealed in vacuum. Mn and Pr-codoped ZnS nanoparticles emitted white light due to the effects of dopants. The optical annealing enhanced the emission intensity. 

At present, defect-free silicon crystals have been achieved at only at diameters of 200 mm. Comparisons of crystal quality were made among three techniques a typical conventional Czrochralski crystal growth technique, a slow-cooled controlled reaction and the perfect silicon process. The quality levels achieved in D-defect levels of the material is... 

The crystal quality depends on (1) inclusions that determine the content of impurities, and (2) shape and size distribution to which filterability of crystals is related. Formation of detrimental inclusions is caused by incorporation of strange bodies (gas, liquid, or solid) into the crystal. Any dirt, rust, or other fines in the solution can result in incorporation of these materials into the crystals. The mother liquor can also adhere to the crystal surface or can be... 

Compound 15 has an ill-defined structure 70) due to poor crystal quality. It is, however, interesting to note that 15 became resolved (space group 7 2,2, Z = 8) in an attempted cocrystallization experiment in the presence of optically pure 1. The two molecules of 15 in the asymmetric unit may be seen as a sort of a self inclusion , where molecules are again linked into infinite H-bond chains analogous to 13. 

Considering the crystal imperfections that are typically found in all crystals, the crystal quality of organic pigments is a major concern. The external surface of any crystal exhibits a number of defects, which expose portions of the crystal surface to the surrounding molecules. Impurities and voids permeate the entire interior structure of the crystal. Stress, brought about by factors such as applied shear, may change the cell constants (distances between atoms, crystalline angles). It is also possible for the three dimensional order to be incomplete or limited to one or two dimensions only (dislocations, inclusions). 

X-ray diffraction of pigment powder lends itself to the determination of pigment crystallinity. It is thus possible not only to determine the chemical configuration of a crystalline compound, but also the lattice system of the crystal through the diffraction pattern, in other words, the crystal quality size of crystallites, structural defects (Fig. 18). 

In the early days of silicon device manufacturing the need for surfaces with a low defect density led to the development of CP solutions. Defect etchants were developed at the same time in order to study the crystal quality for different crystal growth processes. The improvement of the growth methods and the introduction of chemo-mechanical polishing methods led to defect-free single crystals with optically flat surfaces of superior electronic properties. This reduced the interest in CP and defect delineation. 

In summary, it is important to determine crystal quality, unit cell dimensions of the crystal (a larger crystal absorbs X rays more strongly, 0.3-0.5 mm is considered the optimal size), the crystal s space group, and how many protein molecules are in the unit cell and in one asymmetric unit. Actually, the great majority of crystals useable for X-ray crystallography are not ideal but contain lattice defects. This is true for protein crystals, which are also weak scatterers since the great majority of the component atoms are light atoms, C, N, and O. 

There are few strictly comparable structurally characterized alkynyl/diynyl/poly-ynyl systems, which make detailed comparisons between the various unsaturated ligands difficult. For diynyl and poly-ynyl complexes, this is no doubt due to the poor packing qualities of the rod-like mono-metallic systems and resulting poor crystal quality. Structural features of poly-ynyl complexes are summarized in Table II. 

Saridakis, E. and Chayen, N. E. (2000). Improving protein crystal quality by decoupling nucleation and growth in vapor diffusion. Protein Sci. 9, 755-757. 

A fully automated protein crystallography beamline at a third-generation synchrotron source can screen several hundred crystals daily. Automatic evaluation of the diffraction images to ascertain crystal quality is, therefore, a critical step for high-throughput data acquisition. Evaluation of each image requires software that mimics the traditional visual assessment of crystal quality. 

Automatic diffraction analysis is available through the Diffraction Image Screening Tool and Library (DISTL) (Zhang et al., 2006). DISTL incorporates automatic indexing and identification of extraneous features such as ice rings, and provides an estimate of the resolution of the diffraction data. Neural networks have also been tested for the evaluation of crystal quality (Berntson et al., 2003). 

Initial screens can be distinguished between methods that are used to determine what factors are most important, and follow-up screens that allow optimization and improvement of crystal quality (Table 14.1). In experimental design, this is known as the Box-Wilson strategy (Box et al., 1978). The first group of screens is generally based on a so-called factorial plan which determines the polynomial coefficients of a function with k variables (factors) fitted to the response surface. It can be shown that the number of necessary experiments n increases with 2 if all interactions are taken into account. Instead of running an unrealistic, large number of initial experiments, the full factorial matrix can... 

It should be noted that the theory described above is strictly vahd only close to Tc for an ideal crystal of infinite size, with translational invariance over the whole volume. Real crystals can only approach this behaviour to a certain extent. Flere the crystal quality plays an essential role. Furthermore, the coupling of the order parameter to the macroscopic strain often leads to a positive feedback, which makes the transition discontinuous. In fact, from NMR investigations there is not a single example of a second order phase transition known where the soft mode really has reached zero frequency at Tc. The reason for this might also be technical It is extremely difficult to achieve a zero temperature gradient throughout the sample, especially close to a phase transition where the transition enthalpy requires a heat flow that can only occur when the temperature gradient is different from zero. 

Since we had to irradiate crystals within their growth solution in their original orientation, and at the same time to avoid other crystals in close proximity, we were strongly dependent not only on the intensity and the stability, but also on the size and shape of the incident beam. As a consequence, improvements in the quality of the data collection facilities were needed, alongside our efforts to improve the crystal quality. 

As with neutron powder methods, both monochromatic and time-of-flight (Laue) techniques have been used for single-crystal neutron studies. Such studies are much less numerous, however, as a result of the difficulty of compressing suitably sized (>1 mm ) single-crystal sample to the required pressure while maintaining the crystal quality. The piston cylinder cell design of Me Whan and others [128] has been used at the ILL reactor source for a number of neutron structural studies to... 

Figure 4.18 (a) RBS and (b) AFM data of 4H-SiC implanted with Al at 700°C. Note the crystal quality is degraded due to implant damage, as expected. RBS and AFM data are courtesy of, respectively. Dr. Tami Isaac-Smith from the group under Professor John Williams at Auburn University and Dr. A. ]. Hsieh of the Army Research Laboratory. 

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