Study On The Growth Of N-polar Ⅲ-nitrides Films By MOCVD And Light Emitting Diodes

Ⅲ-nitrides with outstanding optical and electrical properties have been widely used in light-emitting diodes (LEDs), laser diodes, photodetectors and high-power transistors. Metal-polar nitrides generally show good crystalline quality. In contrast, the growth of high quality N-polar nitrides is still a challenge. Therefore, most of Ⅲ-nitrides devices are realized using Metal-polar materials. The understanding of N-polar nitrides is not enough in-depth comparing with metal-polar Ⅲ-nitrides. However, N-polar nitrides with inversed internal electrical field have advantages in many applications of devices, such as enhancement mode and highly scaled transistors, Photon detectors, solar cells, Zener tunneling diodes. N-polar structure is beneficial to improve the properties of LEDs and laser diodes. N-polar nitrides have attracted researchers’great interests. Our research has been focused on the growth of N-polar GaN, InGaN and InN films, the detailed research contents are as follows:We have studied the effects of the offcut angle of sapphire substrate, the thickness of nucleation layer, growth temperature and Ⅴ/Ⅲ ratio of GaN on the properties of N-polar GaN. The results indicate that the crystalline quality, optical property and surface morphology of N-polar GaN are greatly improved as the offcut angle of sapphire substrate increases from 0.3° to 0.8 °. Crystal misorientation can promote step flow growth as it leads to the formation of a high density of surface steps with short terrace length between steps, increasing the probability for Ga-adatoms to reach step and kink positions. The optimal thickness of nucleation layer is 20 nm. As the growth temperature of GaN template increases from 1000 to 1080℃, the yellow luminescence of N-polar GaN decreases remarkably. That implies a high growth temperature can improve the optical property of N-polar GaN. The screw type and edge type dislocations densities, root mean square (RMS) roughness value, electron concentration and mobility of the optimal N-polar GaN are 4.70×107 cm-2,3.13 108 cm-2,0.331 nm,2.35×1017/cm3 and 509 cm2/V·s, respectively. These properties are similar to the properties of Ga-polar GaN.N-polar InGaN films are grown on N-polar GaN template. We have studied the effects of growth temperature, pressure, V/III source ratio and trimethylindium (TMIn) input flow on the properties of N-polar InGaN films. The results indicate that low growth temperature and large TMIn input flow are more effective ways to increase In content in the films. However, the increased In contents deteriorate the crystalline quality of InGaN films. A high growth temperature is beneficial to improve the crystalline quality and optical properties of InGaN films. N-polar InGaN films grown at temperatures higher than 760℃ show sharp near band edge luminescence peaks, whereas the films grown at temperatures lower than 740℃ show no prominent peak. The equilibrium partial pressure of In is much higher than that of Ga, the high equilibrium partial pressure of In means that a considerable part of the total input In species is in the gas phase and does not contribute to the incorporation process, therefore, the total group III input flow rates have a larger impact on the amounts of In atoms to incorporate into InGaN films. The RMS roughness value of InGaN films decreases as growth temperature or pressure increases. The relatively high growth pressure can improve the crystalline quality and increase the In content of InGaN films simultaneously.We have fabricated violet-blue and blue-green emitting N-polar LEDs. High order satellite peaks of the samples appear in XRD spectrum, that indicate the N-polar LEDs have high crystalline quality. The RMS roughness values of violet-blue and blue-green emitting N-polar LEDs are 1.45 nm and 1.75 nm, respectively. At a injection current of 200 mA, the peak wavelengths of violet-blue and blue-green emitting N-polar LEDs are 411 nm and 483 nm, respectively.N-polar InN films are grown on N-polar GaN template. We have studied the effects of growth temperature, growth mode, ammonia and TMIn input flows on the properties of N-polar InN films. The results indicate that N-polar InN films have flat surfaces and In-polar InN films show faceted surfaces. Besides, In-polar InN domains exist in N-polar InN films. We suggest that the In-polar InN domains originate from the boundaries of N-polar GaN domains. The density of In-polar InN domains increases as the growth temperature or TMIn flow rate increases, and decreases as NH3 flow rate increases. These boundaries on GaN surface might block the migration of In-adatoms, and the blocked In-adatoms formed In-polar nuclei. The mobility of In-adatoms varies under different growth conditions, therefore, the In-adatoms with different mobilities have different possibilities to migrate to the boundaries of N-polar GaN domains and form In-polar InN nuclei. The etching rate of N-polar InN is higher than that of In-polar InN. The XRD results show that the diffraction peaks of N-polar InN films and In-polar InN domains are located at 31.3°and 32.1°, respectively. The SEM and XRD pole figure measurements confirm the existence of zinc blende InN (ZB-InN) in N-polar InN films. Besides, the tilted In-polar wurtzite InN (WZ-InN) domains are grown on the In-polar side wall of the ZB-InN. The full width at half maximum of (0002) X-ray rocking curve of the optimal N-polar InN is 1.35°.