Semipolar surfaces

Here we present a very brief survey of the most important steps in the growth development of Group Ill-nitride materials and devices with nonpolar and semipolar surfaces. We follow a chronological order, and try to establish the connections between the steps and their driving forces. We do not intend to... 

Figure 1.3 The number of publications per year focused on nitrides with nonpolar and semipolar surfaces versus time, showing the strong increase in research activity in this field...

One of the most important aspects of the films and heterostructures with nonpolar and semipolar surfaces is related to the polarization dependence of their optical properties. The polarization anisotropy has been studied both theoretically and experimentally in nonpolar GaN [87, 88], as well as in InN [105]. The optical polarization anisotropy in wurtzite nitrides originates from their valence band structure, which can be significantly modified by the anisotropic in-plane strain in the films. 

The future development of the nitride materials and devices with nonpolar and semipolar surfaces inevitably goes in several directions, namely, further optimizations of the material quality, better understanding of the effects, phenomena, and mechanisms typical for this material system, further optimizations of the device performance, and expansion of the device capabilities. 

The thorough understanding of the specific properties of this type of material is of critical importance toward disclosing their full potential. As already mentioned, there are strong research efforts worldwide to investigate and develop high-quality nitride materials with nonpolar and semipolar surfaces. 

An alternative approach for producing templates with nonpolar or semipolar surfaces is growth of GaN boules in the conventional [0001] direction up to a significant thickness by HVPE that could then be sliced into bars or vrafers with desired orientations. This type of GaN material was characterized by different techniques such as X-ray diffraction, TEM, and CL, and confirmed to be of high structural quality with TD density in the range of 10 cm [103,104]. 

Still insufficient compared to the classical III-V semiconductors. This applies even more to semipolar and nonpolar surfaces of these materials, which have started to gain more attention recently. One of the reasons is that nitride layers grown on nonpolar and semipolar surfaces are less influenced by the quantum-confined Stark effect, which hmits the performance of optoelectronic devices. 

The preparation and analytical results of semipolar organoboron compounds containing Cl or P were reported and the surface activity and fireproofing mechanism of these substances were described [283]. 

A stable floe may also be produced by dispersing insoluble particles in a turbid or hazy vehicle consisting of finely dispersed or emulsified semipolar, liquid droplets, which cause the droplets to be adsorbed on the surface of the insoluble drug particles, resulting in a stable floe. Turbid aqueous vehicles have been prepared by the interaction of non-ionic surfactants and preservatives. The concentration of surfactant and preservative required for haze formation may be reduced by the addition of small amounts of sorbitol to the vehicle. 

The solubility of semipolar and nonpolar solutes in water has been related to the term molecular surface area of the solute and some interfacial tension term. This model was later analyzed by various investigators in much greater detail. °... 

The monitoring of compounds with dedicated analytical methods is often insufficient to assess and maintain the quality of surface waters. Many unknown compounds can be present, some of them are harmful for the environment and also for humans. For the identification of semipolar and polar target compounds present in water, LC database fingerprints in combination with LC-MS/MS identification or LC database toxic fingerprint in combination with LC-MS/MS identification can improve the efficiency on relevant compounds. 

One of the most useful application of surfactants in pharmaceuticals is solubilization of poorly water soluble drugs by surfactant micelles. Solubilization is the preparation of a thermodynamically stable isotropic solution of a substance (normally insoluble or sparingly soluble in a given solvent) by incorporation of an additional amphiphilic component(s). It is the incorporation of the compound, referred to as sol-ubilizate or substrate, within a micellar or reverse micellar system. The compound can be incorporated into various locations within the micelle depending on its structure and polarity. For nonpolar solubilizates, the preferred location is the hydrocarbon core of the micelle. For a polar substrate, the preferred location is in the PEO core of the micelle or simply adsorbed at its surface. For a semipolar compound, the molecule can be incorporated between the surfactant molecules in the micelle, either in short or deep penetration. 

GaN-O (1110). We also studied the semipolar GaN (10-11) surfaces in the presence of mygen, in which we allowed both variable number of surface atoms and variable cell size (restricted to a 2 x 2 or... 

Many chemists and engineers believe that, in order to have a good protective film, one must have a third component in addition to the polar heads attached to the surface and the coat of hydrocarbon tails. The third component is a film of oil attracted to the hydrocarbon tails. This theory gains support from the fact that some of the semipolar film formers are not effective in 100% aqueous systems, where there is no oil available to form a film over the hydrocarbon tails. ... 

Optical properties of GaN plates with a semipolar (1122) surface sliced from thick HVPE layers as well as of homoepitaxial layers grown by MOCVD on such semipolar substrates have also been reported [106,107]. As in the case of the nonpolar bars, low-temperature PL spectra exhibit narrow DBE lines and well-resolved Xa and Xb EEs. However, the polarized PL spectra measured in backscattering geometry show some peculiarities, the most prominent of... 

In our study, semipolar 1122 planes played a key role. Figure 14.2 illustrates the crystal structure of GaN containing a (1122) plane. The (1122) and (H22) surfaces possess different bond configurations and, thus, have different chemical natures. However, the influence of the bond configuration on, for example, the growth characteristics and optical properties has yet to be clarified. In terms of the polarization anisotropy in the optical properties, the key in-plane directions are the [1100] direction, which is perpendicular to the [0001] direction, and the [H23] direction, which is parallel to the projection of the [0001]-axis on the (1122) plane and perpendicular to... 

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