Structural study of low-temperature grown superlattices of GaAs with delta-layers of Sb and P
Introduction
Epitaxial GaAs layers grown at a low temperature (LT) by molecular beam epitaxy (MBE) possess unique properties. This material (LT-GaAs) is characterized by subpicosecond carrier lifetime, high resistivity, and high electric breakdown fields [1]. Since the LT MBE is compatible with the standard MBE technology, LT-GaAs is very attractive for the use in electronic devices of the gigahertz and terahertz frequency ranges [2], [3]. The material properties originate from excess As, which is captured into the film during LT MBE in a form of antisite defects AsGa [4]. During subsequent annealing, the excess As forms a system of nanoinclusions (quantum dots, QDs) incorporated into the crystalline GaAs matrix [5]. The spatial arrangement of the As QDs can be achieved by δ-doping of the LT-GaAs matrix with isovalent In and Sb impurities [6]. Such doping, forming precursors for precipitation, results in formation of two-dimensional (2D) layers and superlattices of As QDs [7], [8], [9], [10].
In order to reveal the physics and to develop the technology of formation of the controlled and ordered As QD system, the detailed information is required on the actual As excess, thicknesses of the δ-doped layers, spacers between them and transition layers. Of special interest are the phase transformations taking place in LT-GaAs during annealing and associated with migration and precipitation of the excess As. One of the most effective methods of obtaining such information is the high-resolution X-ray diffractometry. This method allows one to obtain the data on the lattice mismatch between the epitaxial layer and GaAs substrate associated both with the presence of excess As and with isovalent δ-doping [11], [12]. The analysis and quantitative simulation of the rocking curves reveals features of the atomic structure and its variation during annealing. The X-ray diffraction method becomes most efficient in the structures with a periodic system of the layers δ-doped with isovalent impurities, which cause interference effects on the rocking curves.
In this paper we employ the high-resolution X-ray diffractometry to investigate LT-GaAs layers with periodic systems of the layers δ-doped with Sb, P (δ-Sb and δ-P, respectively) and their combinations. The study is aimed at the structural parameters of the δ-doped layers and spacers between them, variations in these parameters during annealing and formation of As QDs.
Section snippets
Experimental
The LT-GaAs samples were grown in a “Stat” MBE system on semi-insulating GaAs substrates. The growth rate and temperature were 1 μm/h and 200 °C, respectively. The films were periodically δ-doped with Sb or P or with both Sb and P separated by 5-nm-thick spacer. The period of the δ-layer location was about 220 nm. There were 7 periods in each sample. More details on the growth procedure can be found elsewhere [13].
The samples were cut into four parts. One part was kept as-grown, whereas the others
Experimental results and discussion
The X-ray diffraction curves for the LT-GaAs samples with δ-Sb, δ-P and their combination δ-Sb+δ-P are shown in Fig. 1, Fig. 2. In all curves, an intense central peak is observed. The location of this peak does not vary during annealing and corresponds to the Bragg diffraction from the stoichiometric GaAs substrate. In addition to this peak, there are multiple interference patterns caused by the periodic δ-doping of the samples. The central peak of the interference pattern (LT-GaAs 0SL) shifts
Conclusions
The study of X-ray diffraction allowed us to obtain the parameters of the LT-GaAs superlattices, which were grown at low temperature by molecular beam epitaxy and contain a system of thin δ-Sb and δ-P layers. We determined the thicknesses of the δ-layers and spacers as well as the concentration of the excess As in the as-grown samples. The latter data were found to be consistent with our measurements of the AsGa-related optical absorption. The structural transformations taking place in the
Acknowledgments
This study was supported by the Russian Foundation for Basic Research and by the Russian Academy of Sciences. Structural characterization was made on the equipment of the Joint Research Center “Material science and characterization in high technology” (Ioffe Institute, St.Petersburg, Russia).
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