Growing Thicker, More Boron-rich BAlN
With a comparable bandgap to aluminium-rich AlGaN, BAlN is a very promising alternative for making deep UV optical devices. One of its great attributes is a smaller lattice constant that can be used to tune the strain and thus enable optical polarization engineering. The addition of boron also reduces the refractive index, making it possible to fabricate BAlN/AlGaN distributed Bragg reflectors with reflectivity close to unity.
Up until now, to utilisation of BAlN has been held back by the low boron content of the wurtzite BAlN and its thickness: boron content has been limited to no more than 10 percent by MOCVD, and thickness has been restricted to 10 nm or less. According to reports, the root causes for these drawbacks include premature reaction, phase separation, and short diffusion length of boron atoms. The growth efficiency is also compromised as a result the premature reaction.
Recently, however, there has been a breakthrough by a team from Georgia Tech, Arizona State and KAUST, that is led by Xiaohang Li, Fernando Ponce, Russell Dupuis. By optimising growth conditions, in terms of V/III ratio and temperature, and turning to a pulse growth mode, these researchers were able to produce a 100-nm thick single-phase wurtzite BAlN thin films with boron compositions up to 14.4 percent
Scrutinising the films with transmission electron microscopy confirmed the wurtzite crystal structure. However, determining the material composition for BAlN is not trivial. X-ray diffraction can produce large errors, due to the lack of lattice references of BAlN and existence of strain and defects.
To address this concern, the researchers utilized Rutherford backscattering spectrometry (RBS) which is a reliable and accurate method for the composition of common thin film. However, the RBS signal of boron atoms was weak, due to the small nucleus. Large errors result, so to ensure a reliable figure, composition of aluminium was determined first from the RBS measurement, and that of boron deduced by simple subtraction, since BAlN is ternary.
Another remarkable result of the study is the high boron incorporation efficiency and growth efficiency. In this study, the B/III ratios leading to boron compositions of 11 percent and 14.4 percent were 12 percent and 17 percent. This indicates that the boron atoms in the precursors were incorporated into the thin films as efficiently as the aluminium atoms. The growth efficiency, on the other hand, was determined to be 2,000 um/mol, implying that 31 percent of the injected group-III precursors formed the thin films. This growth efficiency is similar to the ones of recent reports of efficient AlN growth, showing the suppression of parasitic reactions.
Despite the progress, the team believes that more effort is needed to improve the BAlN material quality. One interesting phenomenon described in previous reports, and observed by the team, is that a decreased growth temperature facilitates the formation of wurtzite BAlN. However, a lower temperature is undesirable for the AlN phase, which is dominant in the BAlN alloys with relative low boron compositions. This is apparently counterintuitive as a shorter covalent bond tends to result in the need of higher growth temperature for the "˜conventional' III-nitride binary alloys, including InN, GaN, and AlN.
This collaborative work, which received financial support from the U.S. National Science Foundation, Georgia Research Alliance, and KAUST, is detailed in the paper: 100-nm thick single-phase wurtzite BAlN films with boron contents over 10 percent, from Physica Status Solidi B, 1600699 (2017).