The effects of silicon doping on the structural and electrical properties of III-nitrides grown by MBE
Date of Issue2017-02-01
School of Electrical and Electronic Engineering
Group III-nitrides such as GaN, AlN and AlGaN alloys are gaining a lot of interest in the field of optoelectronics due to their ability to emit light from the infrared (Eg, InN =0.7 eV) to the deep ultra-violet (Eg,AlN = 6.2 eV) region of the spectrum. AlN and GaN are the most preferred materials for UV LEDs and laser diodes, because of their wide band gap, high thermal conductivity and high breakdown voltage, but suffer from difficulties in controlling electrical conduction. Due to their wide band gaps, AlN and GaN have low intrinsic carrier concentration, which makes it difficult for them to conduct at room temperature. Thus, doping is introduced by incorporating dopants such as silicon for n-type conductivity and magnesium for p-type conductivity. This dissertation involves the structural and electrical characterization of GaN and AlN samples doped with silicon. The growth of GaN and AlN samples is carried out using Plasma Assisted Molecular Beam Epitaxy (PA-MBE) method. The substrate used for the growth of AlN is sapphire because it is cost-effective and available in many sizes, especially 2-inch. Silicon (111) is used for the growth of GaN due to its low cost, which reduces the manufacturing cost of III-nitride based devices. Structural characterization was carried out on the grown samples using optical microscopy and atomic force microscopy (AFM). Electrical characterization was performed using Hall-effect measurements. AlN layers of 250 nm thickness were grown on 2-inch sapphire substrates. Al droplets were observed on the surface of the undoped samples grown with high Al flux. Al flux was optimized in the next few growths in order to prevent the formation of Al droplets on the surface of the samples. The RMS roughness decreased with the increase in silicon doping of the AlN samples. Thus, the incorporation of silicon led to the improvement in the structural quality of the AlN samples. The electrical properties such as resistivity, electron mobility and electron carrier concentration were measured using Buffer amplifier at room temperature. The undoped samples could not be measured due to their highly resistive nature. The highest electron carrier concentration obtained is 7.024x1014 cm-3, very low mobility of 5.52 cm2/V-s and resistivity of 1611 Ω-cm for the sample having a silicon concentration of 1.5x1020 cm-3.With the increase in electron carrier concentration, electron mobility decreased. Hence, the lowest mobility was obtained for the highest doped sample. Post-deposition annealing degraded the crystal quality and the electrical properties of the annealed sample could not be measured. During the growth of GaN samples on Si (111) substrates, a 100 nm thick AlN layer was grown, before growing GaN to reduce threading dislocations due to the large lattice mismatch between GaN and Si. Followed by the growth of AlN layer, 500 nm thick GaN layer was grown. The GaN samples exhibited rough surface morphology with the increase in silicon doping. Another possible reason for rough surface morphology of heavily doped GaN films is silicon cell radiation heating. An n-type conductivity is achieved with a high electron concentration of 1.93x1020 cm-3, carrier mobility of 68.2 cm2/V-s declining with increase in silicon doping and low resistivity of 0.00047 Ω-cm.
DRNTU::Engineering::Electrical and electronic engineering