Thermal Atomic Layer Deposition of Polycrystalline Gallium Nitride

Abstract

We report the successful preparation of polycrystalline gallium nitride (poly-GaN) layers by thermal atomic layer deposition (ALD) at low temperatures (375−425°C) from trimethylgallium (TMG) and ammonia (NH 3 ) precursors. The growth per cycle (GPC) is found to be strongly dependent on the NH 3 pulse duration and the NH 3 partial pressure. The pressure dependence makes the ALD atypical. We propose that the ALD involves (i) the reversible formation of the hithertounreported TMG:NH 3 surface adduct, resulting from NH 3 physisorbing on a TMG surface site and (ii) the irreversible conversion of neighboring surface adducts to Ga−NH 2 −Ga linkages. The pressure dependence arises from the presumed reversible nature of the adduct formation on the surface, equivalent to the known reversible nature of its formation in the gas phase in metal organic chemical vapor deposition reactions. Using in situ spectroscopic ellipsometry (SE), the GPC monitored as a function of several ALD parameters is as high as 0.1 nm/cycle at 60 s NH 3 pulse and 1.3 mbar NH 3 partial pressure. The changes in the growth pattern (as monitored by SE) caused by changes in the ALD parameters support the proposed growth model. Ex situ characterization reveals that the layer is carbon-free, has a polycystalline wurtzitic structure, and shows a decent conformaility over Si trenches. Tuning the ALD recipe allows us to vary the layer composition from Ga-rich to stoichiometric GaN. The Ga richness is attributed to the simultaneous TMG dissociation at the deposition temperatures. This work is the first full-scale report on low temperature thermal ALD of poly-GaN from industrial precursors, occurring via a novel chemical pathway and not requiring any radical assistance (such as plasma) as used before.

Epitaxial GaN using Ga(NMe2)3 and NH3 plasma by atomic layer deposition

“…Additionally, thermal ALD with TMG and NH 3 requires deposition temperatures of 400 1C, which is relatively high for an ALD process. 13 Similar issues has been reported for ALD of GaN using TEG, were impurities of C and O was the primary issue. 14,15 ALD routes using GaCl and GaCl 3 together with NH 3 deposited at high temperature (4400 1C) have also been investigated, but render films with Cl impurities.…”

“…This is in accordance with previous XPS measurements on GaN thin films. 11,13,[35][36][37] After sputter cleaning the film surface, XPS measurements gave an initial overall composition of 25.0 at% Ga, 74.5 at% N, 0.5 at% O with no detectable C. It should be noted that the N 1s peak overlaps with Ga Auger peaks, which results in a broad N 1s peak and an overestimate of the N content. To obtain the Ga/N ratio in the film without Ga Auger interference, the Auger peaks were subtracted from the contribution to the N content.…”

“…Deposition of GaN by ALD has been achieved using TMG or triethylgallium (TEG) with N 2 /H 2 plasma, 11 NH 3 plasma 11,12 or thermally with NH 3 . 13 However, these processes suffer non-stoichiometric Ga/N ratios and high carbon and oxygen impurities. Additionally, thermal ALD with TMG and NH 3 requires deposition temperatures of 400 1C, which is relatively high for an ALD process.…”

Epitaxial GaN using Ga(NMe2)3 and NH3 Plasma by Atomic Layer Deposition

“…Additionally, thermal ALD with TMG and NH3 requires deposition temperatures of 400 C, which is relatively high for an ALD process. 13 Similar issues has been reported for ALD of GaN using TEG, were impurities of C and O was the primary issue. 14,15 .…”

“…To meet these challenges, the low temperature time-resolved CVD route known as atomic layer deposition (ALD) has been studied for GaN. Deposition of GaN by ALD has been achieved using TMG or triethylgallium (TEG) with N2/H2 plasma 11 , NH3 plasma 11,12 or thermally with NH3 13 . However, these processes suffer non-stoichiometric Ga/N ratios and high carbon and oxygen impurities.…”