Amorphous Gallium Nitride by Low-Temperature MOCVD Growth for Back-End-Of-Line Transistors

GaN is a promising material for BEOL access transistors due to its high electron mobility, large bandgap (for low leakage), and stability in a hydrogen environment (to minimize V_t variability). However, GaN is normally grown at temperatures (~1000 °C) much higher than the 450 °C typically considered BEOL-compatible. Due to the lack of thermal energy, metal-organic chemical vapor deposition (MOCVD) precursor decomposition is less efficient and adatom diffusion is insufficient, preventing the growth of high-quality GaN using typical growth strategies under BEOL constraints.<br/><br/>Amorphous GaN has been proposed as a promising material for electronic applications [1]. Here, we demonstrate the growth of amorphous GaN at BEOL compatible temperatures ≤ 450 °C by MOCVD with electrical properties starting to approach those necessary for access transistors. Through a detailed study of the growth conditions such as the V/III ratio of the precursors and low-temperature BEOL-compatible anneals, we were able to obtain amorphous GaN (as confirmed by XRD) with a 3.2 eV bandgap as measured by UV-Vis. Room temperature mobilities of 10-15 cm²/V-s are demonstrated and initial transistors exhibit I_on/I_off ratios greater than 10⁷ with subthreshold swings ~110 mV/decade. Crystalline GaN, grown using conventional two-step growth methods, shows a steep drop-off in mobility from the 350 cm²/V-s when grown at 1000 °C down to 0 when grown at 450 °C. Our amorphous GaN grown at 450 °C has electrical properties that are comparable to crystalline GaN grown at ~750 °C. We will discuss the role of oxygen incorporation to suppress electron carrier concentrations and will provide detailed characterization of the films from XPS, Raman, TEM, XRD, EDX, and EXAFS.<br/><br/>This work was supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041. This work was also supported in part by the SRC through the Global Research Collaboration (GRC) program.<br/><br/><b>References</b><br/>[1] Stumm, P., and D. A. Drabold., <i><b>Physical Review Letters 79</b></i><b><i> </i></b>(4),(1997) 677