The structural and optical properties of GaSb/InGaAs type-II quantum dots grown on InP (100) substrate
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We have investigated the structural and optical properties of type-II GaSb/InGaAs quantum dots [QDs] grown on InP (100) substrate by molecular beam epitaxy. Rectangular-shaped GaSb QDs were well developed and no nanodash-like structures which could be easily found in the InAs/InP QD system were formed. Low-temperature photoluminescence spectra show there are two peaks centered at 0.75eV and 0.76ev. The low-energy peak blueshifted with increasing excitation power is identified as the indirect transition from the InGaAs conduction band to the GaSb hole level (type-II), and the high-energy peak is identified as the direct transition (type-I) of GaSb QDs. This material system shows a promising application on quantum-dot infrared detectors and quantum-dot field-effect transistor.
KeywordsGaSb Excitation Power Direct Interband Transition Increase Excitation Power InGaAs Buffer Layer
Quantum-size nanostructure materials have always been the research focus [1, 2, 3, 4, 5]. In recent years, staggered lineup type-II quantum-size nanostructures are of great research interest due to their possible application in many novel devices [6, 7]. Notably, a large research effort has been focused on the type-II quantum-size nanostructures composed of III and V direct-bandgap semiconductor materials, such as GaSb/GaAs [8, 9, 10], InAlAs/InP , InP/InGaP [12, 13], InP/GaAs , GaAsSb/GaAs , and InAs/GaSb [16, 17]. The reason is that they offer comparatively large bandgap energies and provide a possibility of covering the whole middle and far-infared optical range for photoelectric devices. Among these material systems, GaSb/GaAs quantum dot [QD] is an outstanding representative since its giant valence band offset, characteristic to this system, may result in practical applications for light-emitting devices in the spectral range of 1 to approximately 1.5 μm, such as in ophthalmology, neurology, and endoscopy .
Here, we provided another type-II QD material system, GaSb/InGaAs/InP, as another promising building block for optoelectronic and microelectronic applications. Compared to GaSb/GaAs type-II QDs, the bandgap of this system can be adjusted by both the QD-relevant structural characteristics and the In component in a InGaAs matrix. InP substrate is employed instead of GaAs for two reasons: one is that the lattice of InP is matched with the InGaAs buffer layer which has higher electron mobility; the other is that it may make the absorption peak position easily red shifted to 1.3 to approximately 1.55 μm. All these features show a promising application on the quantum-dot infrared detectors [QDIP] and quantum-dot field-effect transistors [QD-FET].
In this work, we investigated the structural and optical properties of type-II GaSb/InGaAs QDs grown on InP (100) substrate using molecular beam epitaxy [MBE].
GaSb/InGaAs type-II QDs were grown on the (100) semi-insulation Fe-doped InP substrate by a V80 MBE system (VG Semicon, East Grinstead, Weat Sussex, UK). The growth mode followed is the Stranski-Krastanow [SK] mode. Firstly, the surface oxides of the InP substrate were desorbed at a substrate temperature of approximately 500°C. A 500-nm In0.53Ga0.47As buffer layer matched with the InP substrate was then deposited at a growth rate of 5,000Å/h. Four-monolayer [ML] GaSb QDs were deposited with a slow growth rate of 0.12 ML/s. There was a 3-min growth interruption before and after QD growth. Afterwards, a 30-nm In0.53Ga0.47As capping layer was grown at a rate of 5,000Å/h. A 50-nm In0.52Al0.48As barrier layer was grown at a rate of 5,300Å/h. Finally, GaSb QDs were grown for the surface morphology measurements. The growth temperature used for the whole growth process was approximately 480°C. In the growth process of the sample, the InGaAs capping layer was doped with Si.
The morphology measurements of the QDs were characterized by a atomic force microscopy [AFM] and a scanning transmission electron microscope [STEM]. The AFM measurements were conducted in a tapping mode in air, and the STEM measurements were obtained using a Tecnai F20 super-twin machine (FEI Co., Hillsboro, OR, USA). The photoluminescence [PL] measurements were performed for the optical properties of the sample at 20 K using the source of a 532-nm line, and the excitation power is changed from 3 mW to 30 mW.
Results and discussion
To explain the PL mechanisms of the type-II GaSb/InGaAs QD structures, the schematic band diagrams of GaSb/InGaAs heterostructures are provided in Figure 4b. The spatial separation of electrons and holes will produce an electric field near the type-II GaSb/InGaAs interface. This electric field could make the band bended and form approximately triangular wells adjacent to the heterojunction. With increasing excitation power, the accumulation of electrons and holes at the GaAs/InGaAs interface would steepen the wells. In this case, upraised energy levels in the approximately triangular wells of electrons and holes would cause the PL peak to blueshift.
As is well known, the GaSb QDs only confine the holes, while the electrons are confined in the InGaAs matrix in the type-II GaSb/InGaAs QD heterostructure. We can take advantage of these features to accomplish a charge-discharge process of QDs and then to modulate the electric property of a two-dimensional electron gas [2DEG] in QD-FET. In this kind of QD-FET structure, type-II GaSb/InGaAs QDs are embedded; even if the GaSb QDs directly contacts with the 2DEG, electrons will still be blocked by the GaSb barrier and will not enter the QDs. Therefore, this structure will prolong the lifetime of the holes. So, the QD-FET based on the above band structure can be used to improve the sensitivity of existing InAs/GaAs QD-FET. In addition, this material system can be fabricated on InP substrates. The higher electron mobility InGaAs light absorption layer with lattice matched to the InP substrate has a strong optical absorption in the range of 1.3 to approximately 1.55 μm which is the low-loss optical fiber window. All of these features will promote the application of QD-FET on quantum communications, night vision, and other fields. Besides, owing to the spatially separated electrons and hole characters of type-II QDs, the GaSb/InGaAs QD-based QDIP could have obviously better performance than the InAs/(In)GaAs QD-based QDIP.
We have investigated the structural and optical properties of self-organized type-II GaSb/InGaAs heterostructure QDs grown on InP (100) using MBE. Formation of type-II GaSb/InGaAs heterostructure QDs centered on the PL peak at 0.75eV at 20 K. This type-II luminescence originates from radiative recombination of spatially separated electrons and holes. The PL peak positions are in proportion to the third root of the excitation power, which is a direct evidence of type-II luminescence. This structure was proposed for many important applications such as tunable laser, quantum-dot infrared detectors, and QD-FET.
This work was supported by the 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 No. 2010-CB327501).
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