Composite GaN–C–Ga (“GaCN”) Layers with Tunable Refractive Index

Abstract

This article describes novel composite thin films consisting of GaN, C, and Ga (termed “GaCN”, as an analogue to BCN and other carbonitrides) as a prospective material for future optical applications. This is due to their tunable refractive index that depends on the carbon content. The composites are prepared by introducing alternating pulses of trimethylgallium (TMG) and ammonia (NH3) on silicon substrates to mimic an atomic layer deposition process. Because the GaCN material is hardly reported to the best of our knowledge, a comprehensive characterization is performed to investigate into its chemical nature, primarily to determine whether or not it exists as a single-phase material. It is revealed that GaCN is a composite, consisting of phase-segregated, nanoscale clusters of wurtzitic GaN polycrystals, in addition to inclusions of carbon, nitrogen, and gallium, which are chemically bonded into several forms, but not belonging to the GaN crystals itself. By varying the deposition temperature between 400 and 600 °C and the NH3 partial pressure between 0.7 × 10–3 and 7.25 mbar, layers with a wide compositional range of Ga, C, and N are prepared. The role of carbon on the GaCN optical properties is significant: an increase of the refractive index from 2.19 at 1500 nm (for carbon-free polycrystalline GaN) to 2.46 (for GaCN) is achieved by merely 10 at. % of carbon addition. The presence of sp2-hybridized C=N clusters and carbon at the interface of the GaN polycrystals are proposed to determine their optical properties. Furthermore, the formation of the GaN polycrystals in the composite occurs through a TMG:NH3 surface-adduct assisted pathway, whereas the inclusions of carbon, nitrogen, and gallium are formed by the thermal decomposition of the chemisorbed TMG species.

Thermal Atomic Layer Deposition of Polycrystalline Gallium Nitride

“…Our recent preceding publication on GaN ALD using the same precursors revealed significant gallium and carbon incorporation into the layers from the dissociation of TMG at similar elevated temperatures; we presented a clear relationship between the carbon content and the deposition temperature and pressure. 53 Besides, in the two earlier publications, no demonstration of the existence of an ALD window was provided, as presented here for the first time.…”

“…39 On the contrary, ALD with TMG or TEG typically demands resorting to low temperatures (<400 °C) since these Ga precursors tend to dissociate at higher temperatures, resulting in significant Gaand C-contamination in the layers. 53 In order to carry out the ALD at sub-dissociation temperatures of the organometallics, researchers resort to additional means of activation, such as with plasma, 30,37,39,43,45 hot filament, 48 or electron beam, 47 to dissociate the N precursor into radicals (NH x , x = 0−2 and H). Such radical-enhanced ALD lowers the process temperature and enables an ALD window for GaN.…”

Surface ligand removal in atomic layer deposition of GaN using triethylgallium

“…1,[4][5][6] A problem with most reported ALD process for GaN is a high amount of carbon contamination, on the order of several atomic percent. 7 The carbon emanates from the TMG precursor, most likely due to insufficient removal of methyl groups from the surface due to the lower temperatures in ALD, 200-400°C, compared to CVD. One route to circumvent this problem is to use Ga precursors with Ga-N bonds instead of Ga-C bonds, such as Ga[N(CH 3 ) 2 ] 3 which has been shown as a promising alternative for ALD of GaN.…”

Surface Ligand Removal in Atomic Layer Deposition of GaN Using Triethylgallium

“…1, 4-6 A problem with most reported ALD process for GaN is a high amount of carbon contamination, on the order of several atomic percent. 7 The carbon emanates from the TMG precursor, most likely due to insufficient removal of methyl groups from the surface due to the lower temperatures in ALD, 200-400 °C, compared to CVD. One route to circumvent this problem is to use Ga precursors with Ga-N bonds instead of Ga-C bonds, such as Ga(N(CH3)2)3 which has been shown as a promising alternative for ALD of GaN.…”