研究目的
Investigating the properties of composite GaN-C-Ga (‘GaCN’) layers with tuneable refractive index for future optical applications.
研究成果
The study concludes that GaCN is a composite material with phase-segregated, nano-scale clusters of wurtzitic GaN polycrystals and inclusions of carbon, nitrogen, and gallium. The refractive index of GaCN layers can be tuned by varying the carbon content, making them suitable for future optical applications. The formation of GaN polycrystals in the composite occurs through a TMG:NH3 surface-adduct assisted pathway, while the inclusions of carbon, nitrogen, and gallium are formed by the thermal decomposition of the chemisorbed TMG species.
研究不足
The study is limited by the thermodynamically unstable nature of gallium carbide, which may affect the stability and performance of GaCN layers under certain conditions. Additionally, the exact mechanisms of carbon incorporation and its effects on the optical properties require further investigation.
1：Experimental Design and Method Selection:
The GaCN layers were deposited on 4-inch Si wafers from alternating pulses of trimethylgallium (TMG) and ammonia (NH3), to mimic an ALD process. An inert (argon) gas purge was applied between consecutive precursor doses. The depositions were performed between 400 – 600 oC, at a range of reactor pressures between
2：01 and 10 mbar, corresponding to NH3 partial pressures between 7x10-3 and 25 mbar, respectively. A polycrystalline AlN layer (25 nm thick) was first in-situ deposited on Si by ALD, as a buffer layer. The GaCN layer growth was monitored in-situ by a spectroscopic ellipsometer. Sample Selection and Data Sources:
The layer thickness and morphology were studied by a Merlin scanning electron microscope (SEM) from Zeiss equipped with an Energy Selective Backscattered (ESB) detector, and an Icon atomic force microscope (AFM) from Bruker. The phase segregation study was carried out by a Fourier transform infrared (FTIR) spectrometer from Thermo Scientific, a Quatera SXM X-ray photoelectron spectrometer (XPS) from PHI, a X’pert Powder X-ray diffractometer (XRD) from Malvern Panalytical and a CM300ST-FEG transmission electron microscope (TEM) from Philips.
3：List of Experimental Equipment and Materials:
The layer growth was in-situ monitored, determining the thickness and the optical constants (i.e., refractive index and extinction coefficient) by spectroscopic ellipsometry (SE), using a Woollam M-2000 ellipsometer and J.A. Woollam CompleteEASE software.
4：Experimental Procedures and Operational Workflow:
The pulsed deposition approach was undertaken to minimize gas-phase reactions between the precursors and grow the layer merely through surface reactions.
5：Data Analysis Methods:
The optical constants were determined by multi-sample analysis approach using a Kramers-Kronig consistent B-spline parameterization-based optical model, while also accounting for the surface roughness of the layers.
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