Nonpolar nitrides

Historical Survey of Nonpolar Nitride Growth Achievements 5... 

The specific features of the morphology of the nonpolar heteroepitaxial films and heterostructure are directly related to the specific microstructure, including both point and structural defects. They are also related to the electrical and emission properties of the nonpolar nitrides. These relations are discussed in greater detail in several chapters of the book. 

The lattice and thermal mismatch between the substrate and the film, characteristic for the heteroepitaxial growth of the nitrides on foreign substrates, results in the presence of strain. This strain has a major effect on all fundamental material properties and device-relevant characteristics, and has been a subject of intense investigation. The strain in the nonpolar nitrides is supposed to be anisotropic owing to the in-plane anisotropies in the lattice matching and the thermal expansion coefficients of both the substrates and the layers along the ]0001] and [1-100] directions. 

The optical properties of the nonpolar nitrides have been extensively studied [94-106] by photoluminescence (PL), cathodoluminescence (CL), and photoreflectivity (PR). All the studies consistently reported two important features of this type of material that differ significantly from the optical properties of the nitrides grown in the [0001] direction. The first difference is related to the specific microstructure of these materials and is characterized by additional emission bands attributed to structural defects such as SFs. The second difference is related to the anisotropic in-plane character of the optical properties, in agreement with the theoretical predictions. 

The anisotropic strain in the nonpolar nitride materials films heteroepitax-ially grown in nonpolar directions inevitably leads to more complex behavior of the optical phonons. The theory predicts that some of the phonons in the nitrides split under anisotropic strain. Recent reports on a-plane nonpolar GaN... 

The applicability of the nonpolar and semipolar nitride material for devices emitting up to red wavelengths remains to be established. The contribution of low-dimensional QD and NW structures to low threshold lasing in nonpolar nitride cavities and the development of electrically pumped nanostructure lasers are currently under discussion. 

In this book, the present stage of development of the nonpolar nitrides is reviewed in detail, with emphasis on the three main topics, namely, crystal growth challenges, key properties and prospects for devices. 

We aim to cover all nonpolar nitride materials, as well as all the growth techniques, presently under development. Several chapters are focused on the specific features of the growth approaches, as used for the different types of material. The bulk of the discussion in each chapter is related to the physical properties of the nitride material obtained by the different techniques. 

We feel that sharing the present knowledge on the key issues of nonpolar nitrides from successfully working experts in the field would be very useful for further developments in the field. 

We have obtained 4H-A1N on 4H-SiC (1120). Isopolytypic growth resulted in very small XRD linewidth, 90 arcsec, one of the lowest values ever reported for heteroepitaxial growth of nonpolar nitride layers. However, TEM studies showed that the 4H-A1N layers still contain high densities of SFs and partial TDs. Further reduction of the densities of these defects is desirable to develop 4H-A1N grown on 4H-SiC (1120) for device applications. 

Since then, much effort has been expended on proving the advantage of nonpolar/semipolar LEDs over c-plane LEDs. The use of nonpolar nitrides potentially solves the problem of the large piezoelectric field, because no piezoelectric field is expected in the nonpolar heterostructures. 

To apply nonpolar nitrides to devices such as high-electron-mobility transistors and UV emitters/detectors, AlGaN is an essential material. However, when AlGaN is grown on GaN, cracks are generated by tensile stress when the layer... 

MOVPE growth, electrical conductivity control, heterostructure, and LED performance of the nonpolar nitrides were overviewed. Taking into account... 

Nonpolar Nitride Heterostructures and Devices grown by MOCVD... 

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