Low-temperature growth of gallium oxide thin films by plasma-enhanced atomic layer deposition

Abstract

Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.

Recent Progress of Deep Ultraviolet Photodetectors using Amorphous Gallium Oxide Thin Films

“…The use of oxygen plasma further lowers down deposition temperature by increasing the chemical reaction rate while decreasing the interlayer diffusion rate, thereby effectively broadening the ALD temperature window. [ 89 ] Several groups have reported their results using TMGa and oxygen plasma for Ga 2 O 3 deposition with a temperature window between 50 and 400 °C and a growth rate ranging from 0.53 to 0.7 Å per cycle. [ 86,89,95,98 ]…”

“…[ 89 ] Several groups have reported their results using TMGa and oxygen plasma for Ga 2 O 3 deposition with a temperature window between 50 and 400 °C and a growth rate ranging from 0.53 to 0.7 Å per cycle. [ 86,89,95,98 ]…”

“…[ 97 ] Most of these precursors could only be used at a high temperature (>300 °C) typically resulting in undesired impurities within the films such as carbon and hydrogen, whose concentrations strongly depend on the growth temperature. [ 89 ] As TMGa has a high vapor pressure and is highly volatile, it could be used as a gallium source at lower temperatures. However, as previously reported, TMGa with H 2 O 2 or H 2 O could not grow gallium oxide via ALD, even at a higher deposition temperature of 350 °C because the surface became poisoned by surface methyl species after only a few cycles and these methyl species could not be removed by H 2 O.…”

“…[87] To date, different Ga and oxygen precursors have been reported for the deposition of Ga 2 O 3 thin films in ALD processes, as shown in Table 3. The ever-reported Ga precursors include trimethylgallium (TMGa), [14,86,89,90,93,95,98,99] triethylgallium (TEGa), [92] tris (2,2,6,6-tetramethyl-3,5-heptanedionato) gallium(III) (Ga(TMHD) 3 ), [96] and gallium tri-isopropoxide (GTIP). [97] Most of these precursors could only be used at a high temperature (>300 C) typically resulting in undesired impurities within the films such as carbon and hydrogen, whose concentrations strongly depend on the growth temperature.…”

“…For example, in the study by Comstock and Elam, [99] alternating exposures to TMGa and ozone yielded a growth rate of 0.52 Å/cycle for deposition temperatures between 200 and 375 C. The use of oxygen plasma further lowers down deposition temperature by increasing the chemical reaction rate while decreasing the interlayer diffusion rate, thereby effectively broadening the ALD temperature window. [89] Several groups have reported their results using TMGa and oxygen plasma for Ga 2 O 3 deposition with a temperature window between 50 and 400 C and a growth rate ranging from 0.53 to 0.7 Å per cycle. [86,89,95,98] Except using TMGa as Ga precursor, some other metal organics containing Ga element have also been developed to synthesize Ga 2 O 3 thin films.…”