Real-time metrology of self assembled quantum dots by reflection high energy electron diffraction

ABSTRACT Based on atomistic analysis and kinematic diffraction theory, it has been previously predicted that quantum dot heights can be extracted from RHEED intensity profiles along the chevron tails (Feltrin A. and Freundlich A , J. Cryst. Growth 301-302 , 38-41-2007). Here we report the existence of such periodic RHEED intensity fringes and demonstrate experimentally the possibility of monitoring real time the evolution of the average dot-size in the archetype InAs/GaAs system. The methodology when combined with RHEED information on the dot surface coverage and facet orientation is shown to provide a full metrology of self assembled quantum dots and could be valuable in assessing the QD growth kinetics and improving process reproducibility. Keywords: Quantum dots, RHEED electron diffraction, Molecular Beam Epitaxy, InAs, GaAs, Diffraction Modeling 1. INTRODUCTION The determination of quantitative properties of quantum dots, real time during growth has been a long standing problem in semiconductor research community. For the envisaged applications of quantum dots in optoelectronic devices [1] and electronic storage devices, it is critical to control and reproduce the quantitative properties of quantum dots during growth. So far only post growth techniques (STM, TEM, AFM and x-ray diffraction analysis) have been successfully employed in extracting the quantitative parameters of quantum dots [2  6]. In this paper, RHEED is explored as a technique that can be utilized in quantitative analysis of epitaxially grown InAs/GaAs Stranki-Krastanow quantum dots, during growth. RHEED is a standard non intrusive technique which is applied in situ to provide real time structure evolution during thin film growth in epitaxy. Our early analysis based on theoretical calculations using kinematic diffraction theory showed that quantum dot heights could be directly extracted from the periodicity of the predicted intensity fringes along the RHEED chevron tails [7,8]. In the present work we aim at exploiting these theories in determination of the size of epitaxialy grown InAs quantum dots on GaAs(001) during growth using a hybrid approach. We calculate RHEED intensity profiles along chevrons concomitantly to the experimental RHEED intensity profiles along chevrons to confirm consistency of our method. In particular, we want to clarify the role of size distribution of quantum dots for the RHEED intensity fringes along the chevrons. It has been acknowledged that the InAs/GaAs SKQDs are mainly bounded by shallow {137} family of facet planes [2,9]. At the same time our previous RHEED studies have shown that the included angle between chevrons is related to the facet orientation of the InAs /GaAs SKQDs. Particularly electrons diffracted from the {137} facet planes give rise to appearance of RHEED chevrons at azimuth with an included angle of 56 degrees.[8] The periodicity of the intensity fringes along a chevron streak extracted with respect to the azimutal angle axis in the RHEED image is inversely proportional to the vertical height of the QD.[8] With the knowledge of the facet orientation combined with the extracted dot height from the fringe periodicity leads to the most important information, which is the real time size of the dot. *