Nanostars in Highly Si-Doped GaN

Abstract

Understanding the relation between surface morphology during epitaxy of GaN:Si and its electrical properties is important from both the fundamental and application perspectives. This work evidences the formation of nanostars in highly doped GaN:Si layers with doping level ranging from 5 × 1019 to 1 × 1020 cm–3 grown by plasma-assisted molecular beam epitaxy (PAMBE). Nanostars are 50-nm-wide platelets arranged in six-fold symmetry around the [0001] axis and have different electrical properties from the surrounding layer. Nanostars are formed in highly doped GaN:Si layers due to the enhanced growth rate along the a-direction ⟨112̅0⟩. Then, the hexagonal-shaped growth spirals, typically observed in GaN grown on GaN/sapphire templates, develop distinct arms that extend in the a-direction ⟨112̅0⟩. The nanostar surface morphology is reflected in the inhomogeneity of electrical properties at the nanoscale as evidenced in this work. Complementary techniques such as electrochemical etching (ECE), atomic force microscopy (AFM), and scanning spreading resistance microscopy (SSRM) are used to link the morphology and conductivity variations across the surface. Additionally, transmission electron microscopy (TEM) studies with high spatial resolution composition mapping by energy-dispersive X-ray spectroscopy (EDX) confirmed about 10% lower incorporation of Si in the hillock arms than in the layer. However, the lower Si content in the nanostars cannot solely be responsible for the fact that they are not etched in ECE. The compensation mechanism in the nanostars observed in GaN:Si is discussed to be an additional contribution to the local decrease in conductivity at the nanoscale.

Diffusion-Dependent Pattern Formation on Crystal Surfaces

“…Adoption of such growth conditions generally leads to the formation of fragmented islands on the surface of the crystal. In Figure , we show the results obtained after 10 5 time steps, what at room temperature and assuming typical attempt frequency values 10 13 and typical diffusion barrier 0.5 eV translates into time of 2 or 3 s. The shape of obtained islands are dendrite-like and in some sense resembles the experimentally observed structures in highly Si-doped GaN layers . We observed that for higher values of diffusional rate islands grow larger with a more regular and dense structure.…”

“…In Figure 2, we show the results obtained after 10 5 time steps, what at room temperature and assuming typical attempt frequency values 10 13 and typical diffusion barrier 0.5 eV translates into time of 2 or 3 s. The shape of obtained islands are dendrite-like and in some sense resembles the experimentally observed structures in highly Si-doped GaN layers. 27 We observed that for higher values of diffusional rate islands grow larger with a more regular and dense structure. To be sure that the structure with lower n DS is not an early stage of structure with higher one, we performed calculations for longer time steps.…”