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Selective area growth of cubic group III-Nitrides on nano-patterned 3C-SiC (001) substrates

Subject Area Experimental Condensed Matter Physics
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 418748882
 
Final Report Year 2023

Final Report Abstract

Within this project selective growth of cubic GaN epilayers on 3C-SiC (001) substrates covered with SiO2 is demonstrated. In comparison to the standard growth conditions at about 730°C the substrate has to be raised to about 930°C for selective growth. Despite of the increased substrate temperature phase pure cubic GaN could be grown with improved surface roughness. Due to the geometry of our MBE equipment and the arrangements of our effusion cells shadow effects occur during overgrowth of the mask structure. This effect limits the aspect ratio (ratio hole depth to hole width) to below 2. For efficient aspect ratio trapping (ART) a depth to width ratio of larger than 1 is necessary, wherefore the thickness of silicon dioxide mask has to selected according to the mask hole diameter. Silicon dioxide (SiO2) was used for the growth mask to avoid nucleation of GaN on the covered surface. Plasma deposited and thermal SiO2 was used and structured by nano-sphere-lithography (NSL), block-copolymer-lithography (BCP) and e-beam-lithography (EBL). NSL and BCP were utilized to achieve hole arrays with 130 nm and 17 nm in diameter. EBL was employed for patterning of U- and V-shaped grooves. Patterns were transferred into SiO2 by Reactive Ion Etching. 3C-SiC and SiO2 were etched by SF6 and CHF3/Ar plasmas, respectively. On all structures selective growth of cubic GaN was verified and selective nucleation at nano scales from 17 nm to 100 nm at <001> and <111> facets of the 3C-SiC was demonstrated. The hexagonal content of GaN was measured by transmission-electron-microscopy (TEM) and high-resolution X- ray diffraction (HRXRD). On checker board patterns with opening of 3 mm, hexagonal phase content below 1 % were achieved. On V-shaped grooves, formed by <111> facets along [110] in 3C-SiC, a hexagonal phase content of about 17,6% was estimated. For all structures TEM-images evidences phase pure nucleation. Investigations on V-shaped grooves also demonstrate the influence of antiphase domains (APD) and the influence of orthogonal arranged V-grooves on tilted substrates. The reason for the different growth on the orthogonal directions is probably the different polarity of the (111) facets. Using GaN samples with SiO2 structures made by block-copolymer lithography (17nm wide holes) first experiments also demonstrate coalescence with dominant cubic phase after selective growth within the 17 nm wide holes and overgrowth afterwards. After coalescence the lowest hexagonal content of 29% was measured with X-ray diffractometry (including edge regions with parasitic hexagonal inclusions). One critical point are the edges of the side walls of the SiO2 masks. It turns out that the roughness of the RIE etched sidewalls give raise to the formation of new stacking faults starting at the side walls, which prevents the implementation of ART filtering effect. An additional optimization of the mask structure and a wet chemical post-treatment should improve the roughness of the sidewalls and should enable aspect ratio trapping. In addition the epitaxial growth at the high growth temperatures necessary for selective growth have to be studied in more detail and have to be optimized for each used structure.

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