The Ohmic Contacts To GaN HEMT Epilayers And Its Applications To Hall Measurements

Gallium nitride and the related III—V nitrides have been exploited for optoelectronic devices in the blue and ultraviolet wavelengths, and they have more recently been explored for high-temperature and high-power electronic devices. In developing such devices, thermally stable, low-resistance ohmic contacts with good surface morphologies are essential. But the wide bandgap and lager electron affinity of GaN compared to other III-V semiconductors generally result in the difficulties of preparing ohmic contact to GaN. Otherwise, Hall measurement plays an important part in testing the electrical parameters of the epilyers, but is restricted by the influence of the ohmic contact to GaN when testing GaN materials . In this thesis, systematical research on ohmic contacts to GaN materials and fabrications of the electrodes of Hall measurement were performed. Following are the main works:GaN HEMT epilayers were grown by Metal organic chemical vapor deposition (MOCVD). X-ray Diffraction (XRD) and photoluminescent (PL) were used for analysing the influences of the thicknesses of GaN low-temperature buffer layers on epilayers, it was showed that the suitable thickness of GaN low-temperature buffer layers was about 30nm.Ti/Al/Ni/Au were deposited by electron beam evaporation to make contact to GaN HEMT epilayers, after rapid thermal annealing(RTA) in N₂ for 30s at temperatures of 750℃, 780℃, 800℃, 820℃, 850℃, 880℃, 900℃ and 930℃, the special contact resistivities ρ_c of the samples were obtained from transmission line method (TLM). The results indicted that ρ_c was dependent on annealing temperatures. The values of ρ_c decreased as the annealing temperature increased at first, when the annealing temperature went up to 880℃, ρ_c decreased to 6.94x10^-7Ω·cm², and then, ρ_c turned to increase with the further increase in temperature. So, anneal at 880℃for 30s was thebetter annealing condition.Surface morphology of the samples anneled under different conditions were characterized by optical microscope and scanning electron microscope (SEM). The results showed that the surface was smooth before annealing;after annealing in the temperature from 780°C to 820 °C, the alloy was insufficient;when the temperature exceed 900 °C, the surface turned to be ragged;when the samples were annealed at the temperatures of 850°C or 880°C, the surface with sufficient alloying and clear edge profile were obtained.Ti/Al/Ni/Au were used for fabricating the electrodes of Hall measurement, after rapid thermal annealing at 880 °C for 30s, the contact between the electrodes and wafers turned to be ohmic contact. The results obtained from Hall measurement was compared with those obtained from C-V method. Atomic force microscopy (AFM) was used for observing the surface morphology of GaN, and it was showed that the surface morphology almost had no change before and after annealing. This indicted that anneal at 880°C for 30s did no harm to the samples. Therefore, the processes of preparing ohmic contacts could be used for fabricating Hall electrodes, and then Hall measurement could be performed to test GaN samples.