Article pubs.acs.org/crystal
Epitaxial Growth of Cubic and Hexagonal InN Thin Films via PlasmaAssisted Atomic Layer Epitaxy Neeraj Nepal,*,† Nadeemullah A. Mahadik,† Luke O. Nyakiti,† Syed B. Qadri,‡ Michael J. Mehl,‡ Jennifer K. Hite,† and Charles R Eddy, Jr.† †
Electronics Science & Technology Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, United States ‡ Material Sciences and Technology Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, United States ABSTRACT: InN thin films possessing either a novel cubic or a hexagonal phase were grown by plasma-assisted atomic layer epitaxy on an a-plane sapphire, Si(111), and GaN/sapphire templates, simultaneously. Two ALE growth temperature windows were found between 175−185 °C and 220− 260 °C, in which the growth process is self-limiting. In the lower temperature ALE window, InN on an a-plane sapphire crystallized in a face-centered cubic lattice with a NaCl type structure, which has never been previously reported. InN grown on other substrates formed the more common hexagonal phase. In the higher temperature ALE window, the InN films grown on all substrates were of hexagonal phase. The NaCl phase and the epitaxial nature of the InN thin films on the a-plane sapphire grown at 183 °C are confirmed independently by X-ray diffraction, transmission electron microscopy, and numerical simulations. These results are very promising and demonstrate the tremendous potential for the PA-ALE in the growth of crystalline III-N materials with novel phases unachievable by other deposition techniques.
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INTRODUCTION mong III-nitride semiconductors, indium nitride (InN) has the smallest direct band gap, largest electron saturation velocity and mobility, and smallest electron effective mass.1 It has long been an attractive semiconductor material for application in optical, electrical, and optoelectronic device technologies such as solar cells and high electron mobility and high-frequency devices. The most thermodynamically stable phase of the InN is a wurtize structure (a hexagonal phase); however, the growth of zincblende (a cubic phase) InN on InAs/GaAs has been demonstrated at 450 °C by plasmaassisted molecular beam epitaxy (MBE) .2 Cubic InN has a smaller band gap and superior electronic properties as its lattice is isotropic and possesses lower phonon scattering .3,4 Significant progress has been made in the growth of hexagonal InN by various growth methods such as MBE,5 metal organic chemical vapor deposition (MOCVD),6 and high-pressure chemical vapor deposition (HPCVD).7 Other methods such as radio frequency sputtering and pulsed laser deposition (PLD) have also been used to grow InN films.8,9 Karam et al. reported atomic layer epitaxy (ALE) of InxGa1−xN alloys up to x = 27% at temperatures between 650−750 °C, using a modified-ALE process.10 There are reports on atomic layer epitaxy/deposition of GaN, and AlN,11−13 but the epitaxial layer growth by ALE is challenging. To our knowledge, there are no previous reports on epitaxial growth of InN by true-ALE at low temperatures (
