Resonant tunnelling via InAs self-organized quantum dot states

Resonant tunnelling via InAs self-organized quantum dot states

Microelectronic Engineering 43–44 (1998) 341–347 Resonant tunnelling via InAs self-organized quantum dot states a, a a a b ,1 Jiannong Wang *, Ruigan...

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Microelectronic Engineering 43–44 (1998) 341–347

Resonant tunnelling via InAs self-organized quantum dot states a, a a a b ,1 Jiannong Wang *, Ruigang Li , Yuqi Wang , Weikun Ge , David Z.-Y. Ting a

Physics Department, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China b Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

Abstract InAs quantum dots formed by submonolayer insertion of InAs into the GaAs quantum well of a GaAs /AlAs double barrier resonant tunnelling structure were studied. A series of sharp resonant tunnelling peaks in I–V characteristics of resonant tunnelling diodes with InAs insertions were observed. Temperature and magnetic field dependent I–V studies and theoretical modeling led us to conclude that these peaks are the result of resonant tunnelling through localized states associated with InAs quantum dots.  1998 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; Resonant tunnelling; InAs / GaAs; Self-organized growth

Semiconductor quantum dot structures represent the ultimate in manmade low-dimensional electron systems. Most extensively studied quantum dots are fabricated by sophisticated surface patterning and gate electrodes. Recently, strain induced self-organized growths of InAs Quantum Dots (QDs) on GaAs substrates has attracted much attention. It was shown that the growth of a highly lattice mismatched semiconductor layer onto a substrate could lead to the spontaneous formation of semiconductor islands with sizes in the quantum range. This has been exploited for the InAs / GaAs system ( | 75 lattice mismatch) to produce self-organized InAs QDs in GaAs matrix by InAs submonolayer insertion and by InAs coherent island formation during Molecular Beam Epitaxy (MBE) growth of InAs / GaAs heterostructures. In the former case, coherent InAs islands of monolayer height can assume a similar sizes, and form quantum dots in the GaAs matrix [1,2]. In the latter case, the initial stage of highly strained hetero-epitaxial growth of InAs on GaAs proceeds first by a two-dimensional growth and then, at above a critical thickness (between 1.5 and 3.0 monolayers), by the formation of single crystal dots on a residual 2D wetting layer [3]. Up to now, self-organized InAs QDs were extensively studied by Photoluminescence (PL) and PL excitation spectroscopies [1,2,4–7], capacitance spectroscopy, and far-infrared absorption spectroscopy [8,9]. However, little work has been reported on the study of quantum transport properties of InAs QDs. Very recently, a few groups have reported the resonant tunnelling study of InAs QDs [10,11]. In those studies, InAs QDs were formed by coherent island growth and embedded in AlAs *Corresponding author. 1 Also with Department of Applied Physics, California Institute of Technology, Pasadena, California. 0167-9317 / 98 / $19.00 Copyright  1998 Elsevier Science B.V. All rights reserved. PII: S0167-9317( 98 )00183-X

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barriers. A variety of features in current–voltage characteristics were observed. However, in this paper we report our study of InAs self-organized QDs which are formed by InAs submonolayer insertion incorporated into the GaAs QW of a GaAs /AlAs double barrier resonant tunnelling structure with an In 0.1 Ga 0.9 As emitter. We have observed a series of sharp peaks in the current–voltage characteristics of the structure. By carrying out temperature and magnetic field dependent experiments and theoretical modeling we conclude that these peaks are from quantized states associated with InAs QDs. The Resonant Tunnelling Diodes (RTDs) were fabricated from the GaAs /AlAs heterostructure as shown in Fig. 1. The GaAs /AlAs double barrier resonant tunnelling structure was grown by MBE on a [001] semiinsulating GaAs substrate. The structure consisted of a 3 nm GaAs Quantum Well (QW) sandwiched between two 4.5 nm AlAs barriers. In the middle of the GaAs QW, a one-eighth monolayer of InAs was embedded. A 20 nm undoped In 0.1 Ga 0.9 As emitter layer was incorporated between each AlAs barrier and the adjacent doped GaAs contact layers. The growth temperature was set to 5108C during the growths of In 0.1 Ga 0.9 As layers, AlAs barriers, the GaAs QW and the InAs submonolayer, and 5808C for the rest of the structure. An As 2 source was used, and two-second

Fig. 1. Layer structure of the MBE wafer.

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growth interruptions were introduced before and after the one-eighth monolayer InAs insertion. A control structure, without the submonolayer InAs insertion, was also grown at the same growth conditions. For comparison, two additional structures incorporating a standard 20 nm undoped GaAs

Fig. 2. Schematic conduction-band profile at zero bias for (a) a structure with undoped GaAs spacer layer adjacent to AIAs barrier; (b) a structure with undoped InGaAs layer adjacent to AlAs barrier.

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emitter instead of In 0.1 Ga 0.9 As emitter were grown with and without the InAs submonolayer insertion. RTDs with active area of a few square micrometers were fabricated by the method discussed in [12]. Low temperature (10 K) current–voltage (I–V ) characteristics of RTDs fabricated from all four structures were studied. No resonance features were observed in I–V curves obtained from RTDs of the structure with standard GaAs emitter and without InAs submonolayer insertion. As applied bias increases from zero to around 2 to 3 V no current was measured through RTDs. Above that, monotonic and rapid increase of the current was observed. This can be understood as follows. The energy level of a two-dimensional (2D) quasi-bound state in a 3 nm GaAs QW is well above the GaAs conduction band edge. By applying a bias high enough to align the 2D energy level in the emitter to that in the GaAs QW, we put the structure into a regime where Fowler–Nordheim tunnelling currents are dominant, and thus masking out resonant tunnelling currents. For RTDs of the structure with standard GaAs emitter and with InAs submonolayer insertion, we also did not observe any features in I–V curves. The explanation for this result is as shown schematically in Fig. 2(a). The energy levels associated with the InAs submonolayer insertion are very close to the GaAs band edge. For an applied bias sufficiently high to allow accumulation of 2D electrons in the undoped emitter layer, the InAs levels would already have been biased to below the 2D energy level in the emitter, so that no currents tunnelling through them would be detected. It is these studies of the standard set of GaAs /AlAs double barrier resonant tunnelling structures which led us to believe that a modified structure is necessary for revealing resonant tunnelling through energy level associated with InAs submonolayer insertion. Our modified structure uses an undoped In 0.1 Ga 0.9 As layer instead of standard undoped GaAs spacer layer as the emitter. As shown in Fig. 2(b), the introduction of an In 0.1 Ga 0.9 As layer before the

Fig. 3. Experimental I–V curve measured at 10 K far a RTD with 1 / 8 monolayer InAs insertion and InGaAs emitter.

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AlAs barrier significantly lowers the emitter energy level so that quantized states close to the GaAs band edge can be probed. Indeed, I–V characteristics of the modified double barrier resonant tunnelling structure with InAs submonolayer insertion showed a series of sharp resonances. A typical I–V curve from RTDs with 1 / 8 monolayer InAs insertion measured at 10 K is shown in Fig. 3. Negative bias represents that electrons flow from substrate side. Three sharp peaks at 0.89 V, 0.934 V and 0.972 V were observed together with a few weaker features. In contrast, we observed no resonance features in I–V curves of a control RTDs grown without the InAs submonolayer insertion. This result indicates that the peaks in Fig. 3 are originated from energy levels associated with InAs sub–monolayer insertion. We performed simple 3D quantum transport modeling of the structures using the planar supercell stack method [13]. In our calculation, the active region of the structure is taken as a stack of layers along the growth direction. Each layer in cross-section contains a periodic array of rectangular planar supercells. Within each cell the potential assumes lateral variations as determined by device geometry. We start out with a GaAs /AlAs double barrier structure with 4-monolayer barriers and a 11monolayer QW. The well-center monolayer is then replaced by a 50%–50% random alloy of InAs and GaAs. The transmission coefficient spectra for several random alloy configurations corresponding to different average InAs island sizes, along with that for a structure where the random potential for the

Fig. 4. Calculated transmission coefficient for a RTD with 1 / 2 monolayer InAs insertion at the QW center with different structure configurations of InAs. The shaded curve is for a reference structure with an averaged, effective InGaAs alloy potential; the dashed line, dash-dotted line, and solid line curves are the result with random InAs island configurations of average island sizes of l 5 0.56 nm, 11.8 nm, and 21.0 nm respectively.

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center layer is replaced by an averaged effective In 0.5 Ga 0.5 As potential using the Virtual Crystal Approximation (VCA), are shown in Fig. 4. Our calculation shows that the transmission property of these structures depends heavily on the configuration of the 50% coverage of InAs. We can see that while the VCA structure and the structure with small island size each show a single transmission resonance, as the InAs island size increases many additional features appear in the transmission coefficient curves, with resonance positions shifting closer to the GaAs band edge. This is in qualitatively agreement with what we have observed experimentally, and we therefore conclude that the sharp peaks observed in the experiment may come from the InAs islands of different sizes. In our experiments the details of the peaks observed in I–V curve varies from one RTD to another. Some RTDs did not show any resonance features in I–V curves at all. This is probably due to uneven distribution of InAs islands, as there was only about an average of 12% InAs coverage in the structures studied, and each of our RTDs has an active area of only a few square micrometers. Temperature dependent experiments show that the peaks persist up to 50 K, in contrast to features associated with impurities. Furthermore, applied in-plane magnetic field (up to 12 T), i.e. perpendicular to current direction, had little effect on the resonance features observed in I–V curves, and thereby strongly suggesting the zero-dimensional nature of the states responsible for these features. In conclusion, we have studied I–V characteristics of a GaAs /AlAs double barrier resonant tunnelling structure with InAs submonolayer insertion and InGaAs emitter. We observed a series of sharp peaks in the I–V curves of the structure. We believe that these sharp peaks are the results of resonant tunnelling through the localized states associated with individual InAs islands or quantum dots.

Acknowledgements J. Wang would like to thank the Croucher Foundation for its financial support of the study. J. Wang, R.Li and Y. Wang would like to thank Research Grant Council, Hong Kong for its financial support. D. Ting acknowledges support from the Taiwan National Science Council through Contract NSCB62112-M-007-001.

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