Paper EM+MS-ThM1
Hollow Cathode Plasma-Assisted Atomic Layer Deposition of Wurtzite InN and In_xGa_1-xN Thin Films with Low Impurity Content

Thursday, October 22, 2015, 8:00 am, Room 210E

Among the III-nitride compound semiconductor family, InN is known with its unique properties which are crucial for both electronic and optoelectronic applications such as narrow band gap, small effective mass, and high electron mobility. Since InN and its alloys are currently the backbone of LED industry for bandgap tuning and are mostly grown using high-temperature epitaxy, experimental efforts on enabling low-temperature growth are critical to widen its perspective for applications like flexible (opto)electronics. In addition, a growth method in which indium composition can be precisely controlled for In_xGa_1-xN alloys is highly imperative for band gap engineering.

In this work, we summarize our recent progress on the development of crystalline InN and In_xGa_1-xN thin films with low impurity content at a substrate temperature as low as 200 °C by hollow cathode plasma-assisted ALD (HCPA-ALD). Deposition of polycrystalline wurtzite InN thin films was achieved using trimethylindium (InMe₃) and N₂ plasma sources. Process parameters including InMe₃ pulse time, N₂ flow rate and duration, purge time, deposition temperature, and plasma power were investigated. Detailed structural and optical characterizations of InN and In_xGa_1-xN were performed. N₂ plasma exposure time had a profound effect on the impurity content of the InN films. After saturating the surface of substrate with InMe₃ molecules, the ligands of InMe₃ were removed completely only after sufficient exposure dose of N₂ plasma. Insufficient exposure times of N₂ plasma resulted in InN films with higher carbon impurity contents as determined from XPS measurements, which were arising from methyl ligands of InMe_3. After optimizing the precursor dosages,XPS survey scan obtained from the bulk part of the InN film showed that h-InN films were carbon and oxygen free. On the other hand, indium composition in different In_xGa_1-xN thin films was determined by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X- ray diffraction. GIXRD measurements revealed the hexagonal wurtzite crystalline structure of the grown InN and In_xGa_1-xN thin films. Refractive index of the InN film at 750 nm was estimated to be 2.67 while refractive indices of In_xGa_1-xN thin films increased from 2.28 to 2.42 at wavelength of 650 nm with increase in indium composition. Optical band edge studies of the In_xGa_1-xN films confirmed the successful tunability of the optical band-edge with alloy composition. Our results show that HCPA-ALD is an alternative technique to grow crystalline InN and In_xGa_1-xN films at low substrate temperatures.