Study On Growth And Property Of N-polar GaN Material By MOCVD

Because of its advantages such as wide bandgap, direct bandgap, high electron velocity and high breakdown field, photoelectric devices and microwave devices based on GaN semiconductor materials have played essential roles in many fields such as illumination, display, communication and radars. Most of the conventional GaN devices are made by Ga-polar GaN materials at the present time, because Ga-polar GaN materials are easy to controlled grow by both MOCVD and MBE, with superior crystal qualities and surface topographies. However, in comparison to Ga-polar GaN materials, N-polar GaN materials have many merits such as more active surface character, reversed polarization direction, easily formed low-resistivity Ohmic contact, so N-polar GaN materials have the potential to produce photoelectric devices, sensors and microwave devices with better performance.This dissertation systematically studies the growth mechanism, defects controlling method and properties characterization of N-polar GaN materials on different substrates by MOCVD method. The main work and outcomes are as follows:1. N-polar GaN materials are obtained on sapphire substrates by MOCVD through the methods of substrate surface deeply nitridation and high Ⅴ/Ⅲ ratio high temperature AlN nucleation layer, which remarkably change the chemical potential near the substrates at the early stage of growth process, and the polarity of N-polar GaN materials is confirmed by chemmical etching. The AFM results show that the RMS roughness value is 3.31 nm. The XRD results show that the density of screw dislocation and edge dislocation are 2.3×109 cm-2 and 5.3×109 cm-2 in N-polar GaN materials, respectively.2. The growth of N-polar GaN materials on small angle vicinal sapphire substrates are studied, and a novel mechanism of dislocation annihilation is firstly discovered. Stacking faults in GaN on vicinal substrates block the dislocations from propagating upwards. Utilizing this mechanism, N-polar GaN materials with high crystal qualities, good surface topographies and excellent optical properties are acquired combining with optimized growth conditions. XRD results show that the FWHM of (002) rocking curves for GaN films decreases from 793 arcsec for GaN on (0001) substrates to 630 arcsec on vicinal substrates, and the FWHM of (102) rocking curves for GaN films decreases from 743 arcsec for GaN on (0001) substrates to 626 arcsec on vicinal substrates, respectively. The dislocation density decreases from 2.2×109 cm-2 for GaN on (0001) substrates to 1.2×109 cm-2 on vicinal substrates, respectively. Both results show superior quality of N-polar GaN surface on vicinal substrates. The TEM results show that the stacking faults which appear near the AIN nucleation layer in N-polar GaN films with vicinal substrates block the dislocations from propagating upwards, which is the main reason for the improvement of GaN film quality on vicinal substrates.3. The carbon impurity incorporation in N-polar GaN materials are studied in depth, and a chemical model of carbon impurity incorporation on N-polar GaN material surfaces is proposed. The existence of C-Ga bonds are confirmed by XPS measurement, which verified the correctness of the model. The model reveals the relevance between surface topographies of N-polar GaN materials and carbon impurity incorporation:the better the surface topographies are, the lower the carbon impurity concentration is, and the more the yellow-luminescence are suppressed. The PL spectrum measurement results of N-polar GaN materials with different surface topographies are compared, and it is found that the ratio of near-band-edge emission to yellow-luminescence, IBE/IYL is 0.65 for the GaN on (0001) substrate, due to poor surface topographies, while IBE/IYL is as high as 14.3 for the GaN on vicinal substrate, because of good surface topographies. This confirmed the correctness of carbon impurity incorporation model.4. A method to change the polarity of GaN material surfaces on SiC substrates are put forward by controlling the Ⅴ/Ⅲ ratio of high temperature AIN nucleation layer. Experiments verify that N-polar GaN materials are obtained when high temperature AIN nucleation layer are grown under very high Ⅴ/Ⅲ ratio, which efficiently controls the chemical potential of growth surface. Study shows that when the Ⅴ/Ⅲ ratio of HT-A1N nucleation layer increases from 901 to 27026, the RMS roughness of the sample surface changes from 0.69 nm for Ga-polar to 6.14 nm for N-polar, and the Peak-Valley undulation value changes from 6.4 nm for Ga-polar to 54 nm for N-polar, while the square resistance decreases from high resistance for Ga-polar to low resistance for N-polar, which indicates the remarkable differences between Ga-polar and N-polar GaN materials.5. The relationship between oxygen impurities concentration and growth temperature are investigated by experiments, and the results show that the lower the growth temperatue is, the higher the oxygen impurities incorporation probability is. The experiment shows that the oxygen impurities concentration changes from 3.04×1018 cm-3 at 1050℃ to 3.15×1019 cm-3 at 900℃. The analysis shows that oxygen impurities in N-polar GaN materials come from surface adsorption, rather than upward diffusion from epitaxial layer inside.6. The characteristic of oxygen impurities in Ga-polar and N-polar GaN materials are systematically studied by first-principles calculation. The result shows that the position of oxygen impurities incorporation changes with different GaN polarities, and the change rules of surface adsorption energy differ from different GaN polarities. The calculation results indicate that oxygen adatoms are preferred on fcc position at Ga-polar GaN surfaces, while hcp position becomes the energetically preferred site at N-polar GaN surfaces. With increasing oxygen coverage from 0.25 ML to 0.75 ML, the the oxygen adsorption energy increases from-4.35 eV to-2.86 eV on fcc position for Ga-polar GaN. while it decreases from-3.35 eV to-3.86 eV on hcp position for N-polar GaN. The differences in variation tendencies for the oxygen adsorption energy illustrate that N-polar GaN surface is preferred to adsorb more oxygen adatoms.