Kinetic Monte Carlo study of step asymmetry and stable step orientations on GaAs(001)

Abstract

Step morphology in the homoepitaxial growth of a GaAs(001)-␤2(2ϫ4) surface is studied by kinetic Monte Carlo simulations. The growth simulations revealed that one of the two types of B steps we examined grows by keeping the mean step direction parallel to ͓110͔ with punctuated protrusions of As dimer rows along the ͓1 10͔ direction, whereas the other type tends to have a step orientation parallel to one of the two orthogonal ͗010͘ directions. These results can be examined by observing vicinal surfaces of GaAs͑001͒ with scanning tunneling microscopy.The combined use of epitaxial growth and reflection highenergy electron diffraction ͑RHEED͒ is a popular technique to fabricate heterostructures 1 and to integrate optoelectronic devices. To accomplish these tasks, it is necessary to control growth with the accuracy of an atomic layer thickness so as to realize the interface abruptness. Accordingly, ab initio density-functional calculations have been used to explore the atomic and electronic structures of various surface reconstructions with the aim at revealing their roles in growth. The prototypical example of this kind of study is the finding of the bistability of Si͑001͒ between an S A step and an S B step, as first pointed out by Chadi with tight-binding total-energy calculations, 2 and later confirmed by scanning tunneling microscope ͑STM͒ observations. 3 Here, we will point out that a similar bistability also exists on a GaAs(001)-␤2(2ϫ4) reconstructed surface, i.e., on the best-studied III-V compound semiconductor surface. 4,5 Since many optoelectronic devices are made of III-V semiconductors, this phenomenon may be used to control growth and, hence, to improve the quality and performance of such devices. However, the best method to study this is obviously not to perform ab initio density-functional energyminimization calculations as it has the limitations on the system size and the time scales to simulate growth phenomena. Instead, it is practically more suitable to carry out kinetic Monte Carlo ͑kMC͒ simulations 6 to fulfill this task.The surface reconstruction of our concern is the ␤2(2 ϫ4) structure, which appears during molecular-beam epitaxial growth at TӍ580°C when the arsenic pressure is kept sufficiently high. 7 For convenience, the ␤2(2ϫ4) reconstruction is depicted in Fig. 1, where the dark and bright disks denote As and Ga atoms, respectively, and their radii decrease according to their depths. The shaded areas in Figs. 1͑a͒ and 1͑b͒ indicate the sites to which an additional As dimer may stick and initiate the growth of this structure in the ͓110͔ direction and the ͓1 10͔ direction, respectively.To investigate the evolution of the surface morphology on GaAs͑001͒, we have carried out kMC simulations based on the two-species growth model, in which both Ga atoms and As 2 molecules are deposited simultaneously onto a ␤2(2 ϫ4)-reconstructed surface at Tϭ580°C. 8,9 The interaction diagrams of the As part used in the model are seen in Figs. 2͑a͒-2͑e͒. 10 Among them, the interactions depicted in Figs. 2͑d͒ ...

High‐Voltage Spinel Cathode Materials: Navigating the Structural Evolution for Lithium‐Ion Batteries

“…It is reasonable to anticipate the electrochemical‐driven local structural degradation accumulates over prolonged cycles. Itoh et al., [ 116 ] analyzed the fine structure of 1st and 28th cycled d ‐LNMO electrodes based on Rietveld refinement of the SXRD data. It revealed a decrease of Ni content in d ‐LNMO together with an increase of up to 0.6 wt% for the RSP impurity in the cycled ACM.…”