Growth and Characterization of High-Temperature, High-Quality, Nitrogen-Polar InAlN Films Using Plasma Assisted Molecular Beam Epitaxy

Commercially available GaN-based electronics and optoelectronics devices are fabricated mainly on Ga-polar GaN substrates. However, recent results from the University of California Santa Barbara have shown superior performance of N-polar GaN high electron mobility transistors (HEMTs) in W-band compared with the Ga-polar GaN HEMTs. Meanwhile, research and progress on Nitrogen-polar films have been limited due to the fact that it is very hard to grow films of high quality, unlike the metal-polar case.<br/>Although AlGaN has been commonly used as the barrier in GaN HEMTs, InAlN could be potentially a great candidate to enable high charge in HEMTs with scaled channels. In particular, In_0.18Al_0.82N which is lattice-matched to GaN, has a wide bandgap, large spontaneous polarization, and high refractive index contrast with GaN. Therefore, it is attractive as the barrier material in (HEMTs) and UV light emitting diodes (LEDs).<br/>Regardless, the growth of InAlN is challenging due to a large difference in the thermal stability of InN and AlN. Therefore, InAlN has been traditionally grown at temperatures much lower than optimized growth temperature for GaN using both metal-organic chemical vapor deposition (MOCVD) and MBE [1-3]. Lower growth temperature leads to higher incorporation of impurities such as oxygen and carbon. Additionally, the lower growth temperature of InAlN would require several growth interruptions to change the substrate temperature for the growth of other (Al,Ga)N layers in the epi-structure. In this work, we are going to present the first demonstration of high-temperature growth of InAlN on N-polar GaN substrate by plasma assisted molecular epitaxy (PAMBE).<br/> <br/>A GenXplor (MBE) system was used for the epitaxial growth of samples in this work. First, a 200 nm-thick GaN layer was grown at 750 °C to ensure a smooth surface morphology. The growth was then interrupted to change the substrate temperature for the growth of InAlN film. A number of InAlN samples were grown at different temperatures ranging from 600 °C to 800 °C. By tuning the Al and In fluxes, InAlN films with InN mole fraction ranging from 8% to 37% was achieved. The surface morphology of samples was characterized using atomic force microscopy (AFM). The alloy composition was extracted using high-resolution X-Ray diffraction (HRXRD). In0.18Al0.82N film with smooth surface morphology and high structural quality was achieved at 750 °C (the optimum temperature for growth of N-polar GaN).<br/> <br/>1- Appl. Phys. Lett. <b>104</b>, 072107 (2014).<br/>2- AIP Advances <b>6</b>, 035211 (2016);<br/>3- <i>Mater Renew Sustain Energy</i> <b>2, </b>10 (2013).