Crystal Truncation Rods from Miscut Surfaces with Alternating Terminations

TL;DR

This paper develops a theoretical framework for analyzing crystal truncation rods (CTRs) from miscut layered crystal surfaces with alternating terminations, aiding understanding of surface structure and growth kinetics.

Contribution

It introduces a general theory for CTR intensity distributions on miscut surfaces with two terminations, including calculations for GaN (0001) surfaces.

Findings

Identifies CTR positions sensitive to termination coverage.

Shows how CTR profiles differ between exactly oriented and vicinal surfaces.

Provides insights into surface structure from X-ray scattering data.

Abstract

Miscut surfaces of layered crystals can exhibit a stair-like sequence of terraces having periodic variation in their atomic structure. For hexagonal close-packed and related crystal structures with an {\alpha}{\beta}{\alpha}{\beta} stacking sequence, there have been long-standing questions regarding how the differences in adatom attachment kinetics at the steps separating the terraces affect the fractional coverage of {\alpha} vs. {\beta} termination during crystal growth. To demonstrate how surface X-ray scattering can help address these questions, we develop a general theory for the intensity distributions along crystal truncation rods (CTRs) for miscut surfaces with a combination of two terminations. We consider half-unit-cell-height steps, and variation of the coverages of the terraces above each step. Example calculations are presented for the GaN (0001) surface with various reconstructions. These show which CTR positions are most sensitive to the fractional coverage of the two terminations. We compare the CTR profiles for exactly oriented surfaces to those for vicinal surfaces having a small miscut angle, and investigate the circumstances under which the CTR profile for an exactly oriented surface is equal to the sum of the intensities of the corresponding family of CTRs for a miscut surface.