Semipolar nitrides

Despite the intensive growth optimizations and significant progress of the crystal quality of the nonpolar and semipolar nitrides, materials, and devices, there is still considerable scope for development. All the material properties appear more complicated and impose more challenges on measurements and analyses. As it will be described in the following, in order to find a way to control and improve the material characteristics, several phenomena have to be evaluated and better understood, as whereas their nature and driving forces have to be clarified. 

Nonpolar and Semipolar Nitride-based Devices Today 23... 

The nonpolar and semipolar nitride devices also have the potential to use the polarization light emission. This property is especially beneficial for backlighting liquid crystal displays (LCD) and projectors. The polarized light sources do not require polarizing filters and thus the screens can be made thinner, lighter, and more energy efficient. 

The optoelectronic devices based on nonpolar and semipolar nitrides will be further developed aiming higher efficiency at different emission wavelengths. 

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 2000, Waltereit, Brandt, and coworkers in Klaus Ploog s group at the Paul Drude Institute in Berlin reported the first planar nonpolar (1100) m-plane films on (100) oriented y-LiAl02 substrates [16]. In their breakthrough paper in Nature, Waltereit demonstrated the absence of internal electric fields in m-plane GaN quantum wells with AlGaN barriers. Waltereit s work motivated a worldwide research effort in nonpolar and semipolar nitrides. 

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. 

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. 

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. 

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