Growth And Proper Semipolar GaN

The spontaneous and piezoelectric polarization fields present in nitride heterostructures, while typically being advantageous in electronic devices such as nitride-based field effect transistors, the strong electric field produced by spontaneous and strain-induced piezoelectric polarizations is in the c-axis oriented hexagonal GaN. These polarization fields are undesirable for light-emitting devices whose active regions consist of quantum wells, because the associated electric fields separate the electrons and holes at the opposite interface of the well and reduce the overlap of their wave functions. This result in a reduction of the recombination efficiency in light-emitting devices and a redshift of the emission wavelength.To eliminate the internal polarization fields, group III nitride layers have been recently grown on nonpolar planes, such as the a-plane or the m-plane. In this case there are no polarization fields perpendicular to the layer interfaces, because the polar c-axis lies within the growth plane. However, extended defect densities in non-polar a-plane (11-20) GaN are very high. In this paper, we study the nonpolar and semipolar GaN materials. The main innovative results are:1. Using the home-made MOCVD system, the high quality nonpolar a-plane GaN crystal film with on r-plane sapphire has been achieved. The gradient nuclear layer has been used to inprove the quality of a-plane GaN, The full-width at half-maximum (FWHM) for the (11-20) plane of the high-resolution X-ray diffraction (HRXRD) rocking curve is802arcsec. Further more, we show that the insertion of AlN/AlGaN SLs improves crystal quality, the FWHM of (11-20) plane is660arcsec, which is the the top level of international.2. Si-doped (11-20) a-plane GaN grown on (1-102) r-plane sapphire substrate was obtained byMOCVD. The crystal quality, morphology, optical and electrical properties of the Si-doped a-plane GaN films were investigated by HRXRD, AFM, PL spectrum and Hall measurement. The results showed that the morphology and the crystal quality slightly degraded with Si doping, the mobility increases from5.05cm2/V sto66.61cm2/V s, from the study of the PL, the SiGa is found to increase intensity of the yellow luminescence.3. We have investigated the electrical anisotropic of the a-plane GaN, the electrical anisotropic is associated with the anisotropic of the crystal quality, further more, the mobility is changed with the degree of the van der Pauw square diagonal to the c direction, which shows significant electrical anisotropic, the study of the AlGaN/GaN by C. Y. Chang further evidence of our argument.4. Morphology of nonpolar (11-20) a-plane GaN epilayers on r-plane (1-102) sapphire substrate grown by were investigated after KOH solution etching. Many micron-and nano-meter columns on the a-plane GaN surface were observed by scanning electron microscopy. An etching mechanism model is proposed to interpret the origin of the peculiar etching morphology. The basal stacking fault in the a-plane GaN plays a very important role in the etching process. Our model is proved by comparing the etching of Ga-face and N-face GaN.5. The TiN mask has been abtained by nitrided the Ti interlayer, the crystal quality of a-plane GaN grown based on TiN technique is significant improved, the TiN is very easy to nitrided, so the time of the nitrided is reduced. The FWHM of (11-20) plane is432arcsec along the c-axis, the FWHM is497arcsec, both of the FWHM is less than500arcsec, the result, the results is much better than the results of conventional ELOG technology and the SiN interlay er technology.6. Using the home-made MOCVD system, the high quality semipolar a-plane GaN crystal film with on m-plane sapphire has been achieved. The pure (11-22) plane GaN is achieved by the nitrided before the growth, further more, we use the graded superlattices for defect reduction in semipolar (11-22) GaN films, the FWHM of the (11-22) plane decreases from944arcsec to761arcsec.7. The model of the impurity incorporation is proposed. It should be noted that oxygen comes from the decomposing of the "downside" sapphire, and the carbon incorporation most likely stems from Ga precursor molecule decomposing, namely it is from the "upside". The protection effect of the Ga atom to the N atom determines the concentration of C and O. The polar and the bonding structure play an important role in the impurity incorporation of C and O. The model has a good solution to the problem of impurity incorporation of GaN.8. The yellow band mechanism of the GaN is studied from the perspective of polarity. Based on our proposed combination of C and O impurities in the model, the luminescence mechanism of polar, nonpolar and semipolar GaN is studied, we found the non-polar GaN with the strongest yellow luminescence, the polar GaN materials is the middle, the semi-polar GaN has the weakest, the effect of dislocation and the VGa-ON is excluded by the test of the HRXRD and the SIMS, the C element is the main reason of yellow luminescence.9. The GaN nanowire is grown on nonpolar a-plane GaN by MOCVD. The nanowire origin from the polycrystalline GaN on the TiN, the nanowire has a very high growth speed. Further more, the Ga-face and N-face nanowire are grown at the same time, the method has a good solution to the problem of slow growth rate of GaN nanowire.