Ricarda M. Kemper

Anti-phase domains in cubic GaN

The existence of anti-phase domains in cubic GaN grown on 3C-SiC/Si (001) substrates by plasma-assisted molecular beam epitaxy is reported. The influence of the 3C-SiC/Si (001) substrate morphology is studied with emphasis on the anti-phase domains (APDs). The GaN nucleation is governed by the APDs of the substrate, resulting in equal plane orientation and the same anti-phase boundaries. The presence of the APDs is independent of the GaN layer thickness. Atomic force microscopy surface analysis indicates lateral growth anisotropy of GaN facets in dependence of the APD orientation. This anisotropy can be linked to Ga and N face types of the {111} planes, similar to observations of anisotropic growth in 3C-SiC. In contrast to 3C-SiC, however, a difference in GaN phase composition for the two types of APDs can be measured by electron backscatter diffraction, μ-Raman and cathodoluminescence spectroscopy.

Cubic GaN on Nanopatterned 3C-SiC/Si (001) Substrates

In this chapter we demonstrate the growth and characterization of nonpolar relaxed cubic GaN by plasma-assisted molecular beam epitaxy on prepatterned 3C-SiC/Si (001) substrates. Nanopatterning of 3C-SiC/Si (001) was achieved by two different fabrication techniques: nanosphere lithography (NSL) to generate large-area pattern, and conventional electron beam lithography (EBL) for tailoring particular surface morphologies. Both methods were followed by a lift-off and a reactive ion etching (RIE) process. We analyze the influence of the substrate on the GaN growth and show that it is possible to grow single phase and defect-reduced cubic GaN crystals on 3C-SiC nanostructures. Furthermore cubic GaN/AlN multiquantum wells were grown on 3C-SiC nanostructures, which is a further step toward nanoscaled device applications.

ECR-Ectching of Submicron and Nanometer Sized 3C-SiC(100) Mesa Structures

Anisotropic etching processes for mesa structure formation using fluorinated plasma atmospheres in an electron cyclotron resonance (ECR) plasma etcher were studied on Novasic substrates with 10 µm thick 3C-SiC(100) grown on Si(100). To achieve reasonable etching rates, a special gas inlet system suitable for injecting SF6 into the high density downstream Ar ECR plasma was designed. The influence of the etching mask material on the sidewall morphology was investigated. Masking materials with small grain sizes are preferable to achieve a desired shape. The evolution of the mesa form was investigated in dependence on the gas composition, the applied bias, the pressure and the composition of the gas atmosphere. The achieved sidewall slope was 84.5 deg. The aspect ratios of the fabricated structures in the developed residue free ECR plasma etching process were between 5 and 10. Mesa structures aligned to [100] and [110] directions were fabricated.

Schottky and Ohmic Contacts on Non-Polar Cubic GaN Epilayers

In this work we focus on the fabrication of ohmic contacts and of Schottky barrier devices (SBD) on non-polar cubic GaN epilayers grown by molecular beam epitaxy (MBE). A Ti/Al/Ni/Au metallization was used for ohmic contacts and the contact resistance was measured by transmission line measurements (TLM). Ni, Pd, Ag and NiSi Schottky barrier devices 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN epilayers. The current-voltage (I-V) and the capacity-voltage (C-V) characteristics were studied at room temperature in detail. A clear rectifying behavior was measured in all SBDs. In the Ni and Ag SBDs an abnormal large leakage current under reverse bias was observed. Isochronal thermal annealing of these Ni and Ag based SBDs at 200°C in air improved the reverse characteristics by up to three orders of magnitude. This is in contrast to the Pd contacts, where the as grown contact showed already good performance and thermal annealing had nearly no influence on the I-V characteristics. For all SBDs the magnitude of the reverse current is generally larger than that expected due to thermionic emission and an exponential increase of the reverse current is observed with increasing reverse voltage. In-depth analysis of the I-V characteristic showed that a thin surface barrier is formed at the metal semiconductor interface and that crystal defects like dislocations may be the reasons for the discrepancy between experimental data and thermionic emission theory.

Hydrogen Effects in ECR-Etching of 3C-SiC(100) Mesa Structures

An anisotropic etching process for mesa structures using fluorinated plasma with hydrogen addition was developed in an electon cyclotron resonance setup. The evolution of the mesa morphology was studied in dependence on the gas composition, the applied bias and the pressure. The achieved side wall slope approached 90° with a negligible trenching. The aspect ratios of the fabricated structure in the developed residue free ECR plasma etching process were between 5 and 20.