Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth
© Schaadt et al; licensee Springer. 2011
Received: 13 September 2010
Accepted: 11 March 2011
Published: 11 March 2011
In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination.
Quantum dots (QDs) are promising candidates for quantum information devices such as quantum bits in quantum computers or quantum memories. Self-assembled QDs were investigated in this context during the past decade. They can produce single photons and can be coupled to microcavity resonators [1, 2]. However, for large-scale applications it is essential to transfer the aforementioned schemes to well-positioned QDs in order to obtain a defined device architecture. One approach to site-selective QD growth utilizes substrate pre-structuring [3, 4]. Small holes are created on the substrate surface in order to alter the surface chemical potential which leads to an increased growth rate at the hole sites. Thus, QDs preferentially nucleate at the defined locations.
Various tools such as electron beam lithography (EBL) or local oxidation are available to pre-structure substrates [5, 6]. In most cases the procedure of pre-structuring involves several process steps including different chemicals which influence the substrate surface. For subsequent QD growth, however, it is necessary to provide a clean surface in order to minimize defects and uncontrolled QD nucleation. It is assumed that defects originating from the regrowth interface degrade the optical quality of the QDs. Therefore, great care has to be taken for surface cleaning after pre-structuring.
In this study we investigate the origin and effect of possible surface contamination which occurs during surface pre-structuring.
The samples were grown by molecular beam epitaxy (MBE) and pre-structured using conventional EBL. A GaAs epitaxial layer is grown on epi-ready (100) GaAs wafers followed by surface pre-structuring. During EBL 50-70 nm large holes were defined in a poly(methyl methacrylate)/(methacrylic acid) co-polymer resist on the surface. The holes are arranged on a square grid. Several arrays with varying lattice constants were defined that way. After development the holes were etched down 30 nm by wet chemical etching (WCE) using H2SO4:H2O2:H2O with a low etch rate of 1 nm/s. The resist was removed and the samples were cleaned in a series of solvent baths and ultrasonic cleaning. An additional cleaning step was introduced later on, which uses ozone generated by ultraviolet light to remove residual organic contamination.
Before QD growth the samples were heated up to 130°C for 1 h in the load lock chamber of the MBE system in order to get rid of volatile surface contamination. The surface oxide was removed in situ by Ga-assisted deoxidation . A 16 nm GaAs buffer layer (BL) was then grown at 500°C followed by 1.7 ML of InAs. The growth rates for GaAs and InAs were determined as 0.3 and 0.07 ML/s, respectively.
The pre-structured samples as well as the uncapped QD samples were characterized by atomic force microscopy (AFM). Transmission electron microscopy (TEM) was used in order to investigate the regrowth interface of QDs capped with 80 nm of GaAs.
Results and discussion
In general, two factors can account for the occurrence of the described defect holes. First, incomplete removal of the native oxide could leave residual oxide compounds on the surface which affect the proper GaAs regrowth. Second, insufficient surface cleaning after the lithography process could cause local organic contamination of the sample which also impacts the GaAs growth.
Incomplete deoxidation is rather unlikely since the defect holes are not randomly distributed. Some local areas are found with a high defect density whereas other areas seem very clean. In addition, by controlling the surface evolution during deoxidation using RHEED, it is made sure that enough Ga is provided to completely remove the native oxide. Furthermore, similar samples prepared by conventional thermal deoxidation as well contained comparable defects. That is why we focused on possibility two by analyzing and optimizing the cleaning procedure.
In conclusion we have investigated pre-structured GaAs sample surfaces for subsequent site-selective InAs QD growth. We have demonstrated the effect of different cleaning steps after EBL and introduced a UV-ozone cleaning procedure to remove the remaining organic contamination prior to regrowth. Successful operation of this method has been confirmed.
atomic force microscopy
electron beam lithography
molecular beam epitaxy
reflection high energy electron diffraction
transmission electron microscopy
wet chemical etching.
The Karlsruhe researchers acknowledge financial support from the Deutsche Forschungs-gemeinschaft (DFG) and the State of Baden-Württemberg through the DFG-Center for Functional Nanostructures (CFN) within subproject A2.6. We thank our collaborators J. Henrdrickson, G. Khitrova, H. Gibbs from the University of Arizona in Tucson, S. Linden from the University of Bonn, M. Wegener from the Karlsruhe Institute of Technology (KIT) for sample preparation. Furthermore, we would like to thank Heinrich Reimer for his help with designing and building the UV-ozone cleaner.
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