Effect of self-assembled InAs islands on the interfacial roughness of optical-switched resonant tunneling diode
© Tian et al; licensee Springer. 2012
Received: 19 September 2011
Accepted: 14 February 2012
Published: 14 February 2012
Embedding a quantum dot [QD] layer between the double barriers of resonant tunneling diode [RTD] is proved to be an effective method to increase the sensitivity of QD-RTD single-photon detector. However, the interfacial flatness of this device would be worsened due to the introduction of quantum dots. In this paper, we demonstrate that the interfacial quality of this device can be optimized through increasing the growth temperature of AlAs up barrier. The glancing incidence X-ray reflectivity and the high-resolution transmission electron microscopy measurements show that the interfacial smoothness has been greatly improved, and the photo-luminescence test indicated that the InAs QDs were maintained at the same time. The smoother interface was attributed to the evaporation of segregated indium atoms at InGaAs surface layer.
73.40.GK, 73.23._b, 73.21.La, 74.62.Dh
KeywordsQuantum dots high-resolution TEM glancing incidence X-ray reflectivity interface flatness molecular beam epitaxy
Nevertheless, if the quantum dot layer was built in the double barriers of RTD, the interfacial flatness may be worsened . Because the interfacial flatness has a strong influence on the properties of RTD [7–9], the use of InAs QDs within double barriers may possibly deteriorate the performance of practical device. In this paper, it is shown that the use of QDs within AlAs barriers does deteriorate the interfacial quality, and then, this weakness was optimized through changing the growth condition. Detailed measurements show that the interfacial flatness has been greatly improved without dissolving the InAs QDs.
All samples were grown by V80H solid-source molecular beam epitaxy system on GaAs semi-insulating (100) substrates, the growth rates were determined by the reflection high-energy electron diffraction oscillation technique. In order to investigate the effect of quantum dot layer on interface quality of QD-RTD, we prepared two samples which were assigned as RTD-1 and RTD-2, respectively. The growth process of RTD-2 can be described as follows: first, a 500-nm thick GaAs buffer layer was grown at 580°C, a 3-nm AlAs down barrier layer [DBL] was deposited afterwards at 610°C, and then the substrate temperature was lowered to 500°C. Subsequently, a 1-nm In0.15Ga0.85As-strained layer, a 1.6 mono-layer [ML] of InAs QDs, and a 4-nm In0.15Ga0.85As capping layer were grown, respectively, with the rate of 0.022 ML/s for InAs and 1.3 nm/s for the In0.15Ga0.85As layer. A 2-min growth interruption was introduced before the InAs layer growth and another 20 s after the formation of QDs. Finally, a 3-nm AlAs was deposited as the up barrier layer [UBL]. The structure of sample RTD-1 was the same as RTD-2 except that it did not contain an InAs QDs layer.
The glancing incidence X-ray reflectivity [GIXRR] was operated on a Bede D1 high-resolution triple-axis diffractometer (Bede Scientific Incorporated, 14 Inverness Drive East, Englewood, CO, USA). The high-resolution transmission electron microscopy [HRTEM] observation was conducted on a JEOL 2010 system (EM Lab Services, Inc., KA, USA). The photo-luminescence [PL] measurements were performed at 77 kelvin [K] using the 532-nm line. The resulting luminescence signal was analyzed with a grating mono-chromator and detected by a photon counting system.
Results and discussion
Simulated result of RMS roughness of every layer of RTD-1, RTD-2, and RTD-3
InAs QDs layer
InGaAs capping layer
The penetration depth of X-ray is very low in the test of GIXRR, and the reflection light intensity will show a power exponent downward trend with the increase of penetration depth [10–12]. So, the GIXRR better reflects the quality of the interface close to the surface. Hence, it is believable that the decline of AlAs UBL/InGaAs interface flatness contributes to the deteriorative interfacial quality of RTD-2 more than AlAs DBL/InGaAs interface. Because the AlAs UBL/InGaAs hetero-junction was deposited after the growth of InAs QDs, it will definitely increase the difficulty to obtain a flat hetero-junction interface. Obviously, in order to put this type of QD-RTD into real use, the interfacial flatness must be improved.
To improve the interface quality, three methods may be feasible. The first one is depositing thicker InGaAs capping layer, but the increasing thickness of quantum well will introduce a serious degradation on resonant tunneling performance of RTD , at the same time, this change of material structure will increase the total strain accumulation of InAs/InGaAs system . The second one is to raise the growth temperature of InGaAs capping layer. Higher temperature is conducive to increase the atom migration ability and, thus, improve the interface flatness. However, for the sake of the weaker In-As chemical bond, this approach may lead to the deviation of indium component of InGaAs capping layer from the setting value and may even cause the InAs QDs to dissolve. The last one is to increase the growth temperature of depositing AlAs UBL. Because the InGaAs layer is strained, the system tends to reduce the strain energy through segregating indium atoms onto surface [15, 16]. This phenomenon will increase the roughness of InGaAs surface. Raising temperature after InGaAs growth as an annealing treatment can evaporate excess indium atoms at InGaAs layer surface and will result a better growth of front flatness [17, 18]. So, we adopted the last method to grow simple RTD-3. Its structure was exactly the same as RTD-2; the only difference was the improved growth temperature of the AlAs UBL from 500°C to 610°C with an interruption of 2 min.
Figure 2c shows the GIXRR curve of RTD-3. The appearance of multi-level satellite peaks indicates that the interface quality has been truly improved. The experimental data were simulated with RFS, and the RMS roughness of every layer was also listed in Table 1. According to the simulation of RTD-3, the RMS roughness for InAs QDs layer was 4.62 nm which was consistent with the average height of the Stranski-Krastanov growth mode QDs (4 to 7 nm) [19, 20], indicating that the simulating result was very close to real value; so, this simulation should be convincing. In Table 1, it is shown that the flatness of UBL has been greatly optimized via the improvement of growth condition. The RMS roughness of RTD-3 UBL decreased from 0.31 nm (of RTD-1) to 0.18 nm. In addition, it should also be noticed that UBL and DBL of RTD-3 have RMS roughness of 0.17 nm and 0.18 nm, respectively. Obviously, the interfacial flatness of UBL has been improved to be close to the level of DBL.
We have studied the interface quality of QD-RTD with a novel structure of InAs QDs incorporated in the double barriers of RTD. GIXRR was employed to test the roughness of InGaAs/AlAs hetero-junction interfaces. It is found that the interfacial flatness was positively deteriorated due to the deposition of InAs QDs layer. In order to optimized this defect, higher growth temperature was used in the growth of AlAs UBL. GIXRR measurement shows that the interfacial flatness of UBL has been improved to be close to the level of DBL, and subsequently, this result was verificated by HRTEM test. Meanwhile, PL measurement demonstrates that the InAs QDs were well maintained after the changing of growth condition. The improving quality of interface that could be ascribed to annealing treatment can evaporate excess indium atoms of InGaAs layer surface which resulted from indium segregation. This result could be used to further improve the performances of this potential structure of QD-RTD.
This work was supported by the National Natural Science Foundation of China (grant nos. 10874212 and 61106013), the National High Technology Research and Development Program of China (grant no. 2009AA033101), and the National Basic Research Program of China (grant nos. 2010CB327501 and 2011CB925604).
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