MOVCD Epitaxial Growth Of Low Dislocation GaN And Low Temperature AlN

With the development of ?-nitride epitaxy technology and advances in device fabrication processes,various types of optoelectronic devices based on GaN have developed rapidly.To meet the requirements of high-performance high-power GaN-based optoelectronic devices and power electronic devices,it is imperative and challenging to continually prepare a high-quality GaN with a lower TDD.The as-grown GaN on sapphire heteroepitaxial substrates has high threading dislocation density and the methods for reducing dislocations mainly include lateral epitaxy overgrowth and patterned sapphire substrates.However,these methods are complicated in process and high in production cost,and in-situ grown amorphous SiN_x can also block the threading dislocations in GaN materials.Based on this,the dissertation proposes the defect preferential passivation method to reduce the threading dislocation density of heteroepitaxial GaN on sapphire substrates.On the other hand,low-temperature aluminum nitride thin films are commonly used in transistors as an insulating layer and an interface layer,which are often prepared by ALD.But the ALD method has a slow growth rate and a serious waste of raw materials.Previously,our laboratory carried out a half-faced gas-injection modification of the MOCVD equipment,which can theoretically achieve an intake mode similar to ALD method.On this basis,the dissertation uses MOCVD half-faced gas-injection mode to grow low temperature AlN film and studies its growth characteristics and surface morphology,crystallinity and optical properties of AlN film.The main contents are as follows:?1?we proposed a defect preferential passivation method in order to reduce the threading dislocation density of heteroepitaxial GaN on sapphire substrates.First,a GaN seed layer is epitaxially grown on a sapphire substrate and the etch pits are generated on the GaN seed layer using the wet etching technique.The in situ SiN_x layer is then deposited on etch pits,followed by a regrown GaN layer using the MOCVD equipment.The dislocation density of the second layer of GaN is greatly reduced by the amorphous SiN_x preferentially filling the dislocation pits to block the upward propagation of the threading dislocations.In this method,the conditions of wet etching and the growth conditions of the SiN_x interlayer are the key to reducing dislocations.We systematically studied the effects of SiN_x growth time and wet etching dislocation pit size and depth on reducing GaN dislocations,and discussed the mechanism of this method to reduce dislocations:the prevention effect by SiN_x covering etch pits.Finally,by optimizing the growth conditions of the second layer of GaN,the threading dislocation density of the GaN crystal is further reduced.The threading dislocation density of the GaN epitaxial wafer prepared by this method is reduced by an order of magnitude compared to the conventional two-step method.?2?We used a modified MOCVD equipment to grow a low-temperature AlN film on a heterogeneous substrate using the half-faced gas-injection mode,and the thickness of the AlN film was investigated as a function of growth temperature,TMAl and NH₃flux,and different substrates.The results show that the growth rate of the AlN film does not increase with the increase of TMAl and NH₃ at 330-370?,which has a self-limiting growth effect.The growth of the AlN film on the patterned sapphire substrate indicates that this method also has good conformality.Further,we studied the material density and surface morphology of AlN films as a function of temperature.As the growth temperature increases,the average grain size of the AlN film increases and the surface roughness increases due to the AlN growth mode changes to island growth as the growth temperature increases.The material density also increases with increasing growth temperature,probably due to the higher growth temperature leading to a more complete reaction of TMAl and NH3.Finally,we studied the crystallinity and optical properties of AlN films.At a growth temperature of 350-400?,the grown AlN film is wurtzite-type polycrystal and preferentially grows the?002?plane.And the optical band gap of the AlN film increases with increasing growth temperature,which may be caused by the difference in material density and thickness of the AlN film grown at different temperatures.