Growth and Characterization of GaN and AlGaN Thin Films and Heterostructures and the Associated Development and Evaluation of Ultraviolet Light Emitting Diodes

Abstract

AlGaN-based thin film heterostructures have been grown and fabricated into ultraviolet light emitting diodes with and without p-type and/or n-type AlGaN carrier-blocking layers at the top and the bottom of the quantum wells, respectively, and having the principal emission at 353 nm. The highest values of this peak intensity and light output power were measured in the devices containing p-type carrier-blocking layers. Growth of an n-type carrier-blocking layer had an adverse effect on these device characteristics. A broad peak centered at ~540nm exhibited yellow luminescence and was present in the spectra acquired from all the devices. This peak is attributed to absorption of the ultraviolet emission by and re-emission from the p-GaN and/or to the luminescence from the AlGaN within quantum wells by current injection.

Individual AlxGa1-xN films (x<0<1) have been grown on Si- and C-terminated 6H-SiC{0001} substrates and characterized for electron emission applications. The large range in the values of x was achieved by changing the fraction of Al in the gas phase from 0 to 0.45. The ionized donor concentration in the n-type, Si-doped AlxGa1-xN films decreased as the mole fraction of Al was increased due to the reduction in the donor energy level and compensation. The use of the SiH4 flow rate, which yields a Si concentration of ~1E19 cm-3 in GaN, established the upper limit of the mole fraction of Al wherein n-type doping could be achieved at ~0.61. The electron affinity of the Si-doped Al0.61Ga0.39N films was as low as 0.1 eV. Increasing the Si doping concentration in AlN films to as high as 1E21cm-3 caused slight degradation in crystal perfection. No difference was found in the Al core level binding energies between undoped and Si-doped AlN films. The results of XPS and UPS experiments showed that the work function of N-polar AlN films was 0.6 eV lower than that of Al-polar films.