Nonpolar III-Nitrides Optoelectronic Devices by Plasma-Assisted Molecular Beam Epitaxy

Abstract

III-Nitrides are excellent semiconductors very much suitable for modern electronic and optoelectronic applications specifically in the fields of solid-state lighting, solar cells, high power/frequency electronics and photodetectors. Most of the devices of these wurtzite crystals are grown along [0002] polar direction, which are strongly affected by spontaneous and piezoelectric polarizations at the interfaces. This not only decreases the efficiency of recombination but also changes the metal-semiconductor junction properties, thus adversely affecting the photodetection properties. On the other hand, nonpolar orientations such as [10-10] m-plane and [11-20] a-plane do not have such polarization along the growth directions. This intrinsic property makes nonpolar III-nitride materials more suitable for optoelectronic devices although growth of high-quality nonpolar material has been a challenge because of anisotropic lattice plane. The thesis work can be divided as follows. Basic working principles of molecular beam epitaxy system and different characterization tools employed in the present work are explained. Three different growth approaches were employed for the growth of nonpolar a-plane GaN on r-plane Al2O3 and their crystalline and UV photodetection properties were compared. These results show that new growth methods with reduced and efficient set of growth steps can be used, considering the UV detection properties of a-plane GaN. A close analysis on nonpolar wurtzite crystal reveals that it basically has three azimuth directions [0002], [10-12] and [10-10] for in-plane transport. Hall-bar and UV-photodetectors were aligned along these azimuth directions and it was found that mobility, transit times and responsivity are azimuth dependent and they were found to be fastest and highest along [0002] respectively. Five different samples of a-plane were grown by PAMBE and the transport was limited towards [0002] azimuth direction. Spectral response was carried out along [0002] azimuth direction with variation of the bias voltage (0-5V). It was found that with increase in the bias voltage responsivity also increases. Highest responsivity estimated for these devices was 400 A/W, which is the highest responsivity reported for epitaxial GaN thin film to the best of our knowledge. Finally, self-powered temporal response for UV-A (364 nm) was carried out on these devices and very stable response with very fast transit times was observed. The last part of this work deals with the growth and characterization of nanostructures on r-plane Al2O3. The I-V characteristics using MSM electrode of Au meatal on these samples reveal that the density of nanostructures controls the Schottky barrier and leads to, Schottky to Ohmic transition with increase in the nanorods density.