Low-dimensional physics in semiconductor nano-structures, such as quantum wells, quantum wires and quantum dots (QDs) have attracted much interest in recent years. Among these nano-structures, the two-dimensional (2-D) wells and barriers have been grown since 1970s. The defect-free zero-dimensional QDs have also been achieved since the early 1990s using the self-assembly growth technique. Many interesting properties and phenomena of these QDs have been studied. In contrast, the study on the growth and characterization of quantum antidots (QADs) is seriously lacking. Recently, many interesting phenomena related to antidots have been observed. However, in these studies, almost all the antidots were fabricated using external processing techniques, such as e-beam lithography, atomic force microscopy related methods and focused ion beam. The defect-free, self-assembled growth of antidots was rarely studied.
In the growth of self-assembled InAs/GaAs QDs in Stranski–Krastanow mode, the surface energy minimization dominates the surface morphology of InAs layer. When the deposited amount of InAs increases to the so-called transition thickness (about 1.5ML), the 2-D to 3-D transition happens due to the strain-induced increment of surface energy. The surface energy depends on many factors, such as lattice mismatch between GaAs and InAs, substrate temperature, growth rate, III/V ratio, etc. To obtain a quantum-size 3-D island, it is necessary to have a large enough lattice mismatch between two materials. In InAs/GaAs system, the lattice mismatch is about +7%. For InAs QDs, the strain in the dots is compressive. For GaAs/InAs anti-dots, the strain becomes tensile in the dots. The 2-D to 3-D transition for the anti-dot formation naturally also depends on the amount of lattice mismatch, material elastic constant and other material parameters. These factors determine the transition thickness under definite growth conditions. In this work, we studied experimentally the GaAs antidots growth in InAs matrix on (100) InAs substrate. Under proper growth conditions, 3-D island formation has been observed clearly by both atomic force microscope (AFM) and Transmission electron microscope (TEM) methods. The transition thickness and antidots sizes are discussed in detail.
2. Experimental result and discussion
The samples were grown on the (100) InAs substrate by a solid-source Varian Gen II molecular beam epitaxy (MBE) system equipped with an arsenic cracker cell. After native oxide desorption at 510°C, a 0.5μm InAs buffer layer was deposited before the GaAs growth. With about 40nm InAs spacer, 1.5, 1.75, 2, 2.25, and 2.5ML GaAs were deposited sequentially. After the last GaAs layer was deposited, the sample was cooled down under arsenic flux. Migration-enhanced epitaxy (MEE) method was used for each GaAs layer growth. That is, after each 0.25ML GaAs deposition, we introduced growth interruption for 10s. In this 10s period, the arsenic shutter was kept open for the first 5s, and then closed for the next 5s. The growth temperature and the growth rate for GaAs were 500°C and 0.1μm/hr, respectively. The III/V beam equivalent pressure ratio of In (Ga) was 25(10).
Fig. 1 shows an AFM image of the grown sample. The measurement was performed in the tapping mode by a DI-5000 AFM system. From the image, a clear 3-D, dot-like morphology is observed. From the surface profile analysis of the AFM, the shape of the islands is almost isotropic, with about 15–35nm in base diameter and about 2–4nm in height. The density of the GaAs antidots was about 3–4×1010cm-2 averaged over several observed 1×1μm2 images.
Fig. 1. AFM image of 2.5ML GaAs on InAs.
Fig. 2 and Fig. 3 show the TEM images for the sample. In Fig. 2, there are 5 layers of GaAs with different thickness as stated above. From the figure, we can see that for less than or equal to 2.25ML GaAs deposition, there is no 3-D island formation. For the fourth layer (with 2.25ML GaAs) from the bottom, strain fields in some isolated spots were observed. In the layer with 2.5ML GaAs deposition, clear QADs were observed. The high-resolution TEM image for one of the GaAs antidots formed on the sample surface is shown in Fig. 3. From the figure, the exact size of the GaAs antidot could be obtained. The base diameter and height are about 20 and 2.5nm, respectively. It is consistent with the AFM observation.
Fig. 2. TEM image of five layers GaAs with different thickness, 1.5, 1.75, 2.0, 2.25 and 2.5ML from bottom to top.
Fig. 3. The magnified TEM image of a GaAs antidot on the surface.
From our study, we can conclude that the transition thickness of 2-D to 3-D morphology transition for GaAs antodots on InAs is between 2.25 and 2.5ML, which is considerably larger than 1.5ML, the transition thickness for InAs QD formation on GaAs. However, the obtained transition thickness is also larger than the previous reported ones, which obtained from RHEED pattern or in situ STM measurement . To certify there is no re-evaporation of Ga atoms during the growth due to the absence of arsenic with our MEE method, we have grown another uncapped sample with 2.25ML GaAs under the same growth conditions. From the measured AFM image of this sample, as shown in Fig. 4, there is no 3-D island formed with 2.25ML GaAs. Hence, unambiguously, the 2D–3D transition thickness of GaAs is larger than 2.25ML.
Fig. 4. AFM image of 2.25ML GaAs on InAs without migration-enhanced interruption.
In summary, we have grown GaAs antidots in InAs matrix successfully. The quantum-sized 3-D islands were observed clearly in both AFM and TEM measurements. From these observations, the transition thickness is determined to be between 2.25 and 2.25ML. For 2.5ML GaAs deposition, the grown antidots have a size about 15–35nm in base diameter and about 2–4nm in height with a density about 3–4×1010cm-2.
Source:Physica E: Low-dimensional Systems and Nanostructures
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