Epitaxial Deposition of Low-Defect Aluminum Nitride and Aluminum Gallium Nitride Films

The bjective of my research was to develop low-defect AlN and AlGaN templates to enable pseudo-homoepitaxial deposition of UV-LEDs. Two approaches have been used to achieve this objective. Firstly, hydride vapor phase epitaxy (HVPE) process was used to prepare thick AlN films with lower defect density. Interactions of dislocations in thicker films result in their annihilation. Secondly, since thick films grown on sapphire tend to crack beyond a critical thickness (3-5 mum), epitaxial lateral overgrowth (ELOG) approach was employed to eliminate cracking and to further reduce the defect density. The growth technique was switched from HVPE to Metalorganic chemical vapor deposition (MOCVD) due to much improved material quality with the later method.;An HVPE growth system was first designed and constructed from ground up [1]. It is a vertical system with a quartz chamber and a resistively heated furnace. AlCl3 and NH3 were used as the precursors. AlCl3 was generated by passing HCl gas (diluted with H2) through Al metal source. A linear relationship between growth rate and HCl flow rate indicated that the growth rate is limited by mass transportation. Growth parameters including temperature, chamber pressure and V/III ratio were optimized to improve the film quality. Thick films of AlN with thicknesses exceeding 25 mum were grown with growth rates as high as 20 mum/hr [2]. AFM study revealed that surface roughness of HVPE grown AlN films strongly depends on the growth rate. The lowest RMS roughness for HVPE grown film was 1.9 nm. These films had typical (002) full-width at half maximum (FWHM) values ranging from 24 -- 400 arcsec, depending on the growth rate of the respective films. The crystalline quality of the films was also found to be deteriorating as the growth rate increased. It is inferred that the growth mode changes from two dimensional to three dimensional at higher growth rates due to reduced adatom migration length. PL spectrum exhibited near-band-edge (NBE) emission line along with broader deep level-related bands, presumably due to donor-acceptor and band-acceptor transitions. Etch-pit density measurement revealed threading dislocation density of 1x109 cm-2, which is at least 5 times smaller than conventionally grown MOCVD films.;AlN and AlGaN films grown on sapphire substrate tend to crack and even peel off beyond a critical thickness of 2 -- 5 mum. To prevent thick films from cracking, ELOG technique was utilized. This technique has been successfully employed to deposit several hundred micrometers thick crack-free GaN films, which demonstrate 2-3 orders of magnitude reduction in TDD. Crack-free thick layers of AlN and AlGaN were deposited using ELOG technique. MOCVD was used as the growth method due to improved material quality. Trimethylaluminum (TMAl), trimethylgallium (TMGa) and ammonia (NH3) were used as the precursors. The growth was performed under low pressure with H2 as the carrier gas. First about 2 mum thick AlGaN layers were deposited on c-plane sapphire substrates. Such thin AlGaN layers, grown without any dislocation reduction technique, usually show TDD in the mid 1010 cm-2 range. Linear trench patterns along [11¯00] AlGaN were fabricated in the AlGaN layers using conventional photolithography and RIE (reactive ion etching). The patterns included 2 mum wide mesas with 5 mum wide trenches. The trenched templates were reloaded into the MOCVD chamber for overgrowth of AlN and high Al-content AlGaN. The overgrowth was carried out at a temperature of 1200 °C with the growth rate of 1 to 2 mum/hr. Migration enhanced MOCVD (MEMOCVDRTM) [3] was combined with ELOG approach to develop a novel Migration Enhanced Lateral Epitaxial Overgrowth (MELEO) technique [4]. As part of this novel technique, precursor pulses were varied to control the ratio of lateral/vertical growth rate. Coalescence related low-angle grain boundaries and edge dislocations were reduced by promoting coherent coalescence using MELEO technique. Fully coalesced, AlN and AlGaN films with thicknesses as high as 30 mum were deposited. SEM was used to characterize the geometry of the coalescence region. HRXRD was used to evaluate the crystalline quality of the thick films X-ray rocking curves for AlN (0002) and (101¯2) diffractions had full-width at half maximum (FWHM) values of 150 and 291 arcsec, respectively. AFM images illustrate much improved surface morphology with long parallel atomic steps. TEM study shows bending and interaction of dislocations. (Abstract shortened by UMI.).