V. P. LaBella, H. Y. Yang, D. W. Bullock, and P. M. Thibado
Department of Physics, The University of Arkansas, Fayetteville, Arkansas 72701
P. Kratzer and M. Scheffler
The structure of the GaAs(001)-(2x4) reconstructed surface has been resolved by scanning tunneling microscopy (STM) and first principles electronic structure calculations. A combined effort of STM imaging at the University of Arkansas (USA) and Theoretical calculations done at the Fritz-Harber Institute in Berlin Germany has resolve the 10+ year long controversy of the atomic structure of this very important surface.
The picture above shows four different structural models of this surface that have been debated over the past 10 to 15 years. All the models satisfy the electron counting model. Theoretical calucaltions have predicted that the ß2 is the lowest energy structure. The ß2 (c) is the structure that was resolved in the STM image below.
The figure above shows the STM data (a) which resolves the atomic positions of the ß2 structural model on a well-ordered GaAs(001)-(2x4) surface. The image in (b) is a surface of constant local density of states (LDOS) extracted from a first-principles density functional theoretical calculation (FP-DFT) within the local density approximation (LDA) of the ß2 structural model. The agreement between the theory and the data further supports the ß2 model. (c) shows the ball-and-stick structural model to scale with the theory and data. (b) shows a plot of the height across the dimer rows from the STM image in (a). The height difference is about 0.3 nm or one monolayer of GaAs, which further supports the ß2 model.
The figure above shows sections of two STM images taken from the same region but imaged at -3.0 V (a) and -2.1 V (b). Below, (c)-(d) the LDA theory taken and two different biases to simulate the STM biases shown above. At lower biases the theoretical images reveal that the trenches widen (i.e., the wave functions from the top-layer As-dimers retract). This is the novel mechanism (sample sharpening) that allows the STM tip to image the structure in the trench.