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Báo cáo hóa học: " Spin effects in InAs self-assembled quantum dots"

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dos Santos et al. Nanoscale Research Letters 2011, 6:115 http://www.nanoscalereslett.com/content/6/1/115 NANO EXPRESS Open Access Spin effects in InAs self-assembled quantum dots Ednilson C dos Santos1, Yara Galvão Gobato1*, Maria JSP Brasil2, David A Taylor3, Mohamed Henini3 Abstract We have studied the polarized resolved photoluminescence in an n-type resonant tunneling diode (RTD) of GaAs/ AlGaAs which incorporates a layer of InAs self-assembled quantum dots (QDs) in the center of a GaAs quantum well (QW). We have observed that the QD circular polarization degree depends on applied voltage and light intensity. Our results are explained in terms of the tunneling of minority carriers into the QW, carrier capture by InAs QDs and bias-controlled density of holes in the QW. Introduction Our samples were processed into circular mesa structures of 400 μm diameter. A ring-shaped electrical Resonant tunneling diodes (RTDs) are interesting devices for spintronics because the spin character of the contact was used on the top of the mesa for optical carriers can be voltage selected [1-4]. Furthermore, spin access and PL and transport measurements under light properties of semiconductor quantum dots (QDs) are excitation. Magneto-transport and polarized resolved PL also of high interest because electron spins can be used measurements were performed at 2 K under magnetic as a quantum bit [5] for quantum computing [6] and fields up to 15 T parallel to the tunnel current by using quantum communication [7]. In this paper, we have stu- an Oxford Magnet with optical window in the bottom. died spin polarization of carriers in resonant tunneling The measurements were performed by using a Prince- diodes with self-assembled InAs QD in the quantum ton InGaAs array diode system coupled with a single well region. The spin-dependent carrier transport along spectrometer. A linearly polarized line (514 nm) from an Ar+ laser was used for optical excitation. Therefore, the structure was investigated by measuring the left- and right-circularly polarized photoluminescence (PL) photogenerated carriers in the device do not present any preferential spin polarization degree. The right (s+) intensities from InAs QD and GaAs contact layers as a and left (s-) circularly polarized emissions were selected function of the applied voltage, laser intensity and mag- netic fields up to 15 T. We have observed that the QD with appropriate optics (quarter wave plate and polarization degree depends on bias and light intensity. polarizer). Our experimental results are explained by the tunneling Results and discussion of minority carriers into the quantum well (QW), carrier capture into the InAs QDs, carrier accumulation in the Figure 1 shows the schematic potential profile and car- QW region, and partial thermalization of minority rier dynamics in our device. Under applied bias voltage, carriers. electrons are injected from the GaAs emitter layer into Our devices were grown by molecular beam epitaxy on the QW region. Resonant tunneling condition is a n+ (001) GaAs substrate. The double-barrier structure obtained when the energy of carriers is equal to the consists of two 8.3-nm Al0.4 Ga0.6 As barriers and a 12- energy of confined states in the QW. Under laser excita- tion, photogenerated holes tunnel through the QW and nm GaAs QW. A layer of InAs dots was grown in the can be captured by the QDs and eventually recombine center of the well by depositing 2.3 monolayers of InAs. radiatively. Carrier capture into QDs occurs within typi- Undoped GaAs spacer layer of width 50 nm separate the Al0.4 Ga0.6 As barriers from 2 × 1017 cm-3 n-doped GaAs cal times of about 1 ps which is much shorter than the layers of width 50 nm. Finally, 3 × 10 18 cm -3 n-doped characteristic dwell times of electrons and holes that are tunneling resonantly into the QW. Due to this fast car- GaAs layers of width 0.3 nm were used to form contacts. rier capture process, the QD photoluminescence will be very sensitive to the resonant tunneling condition and * Correspondence: yara@df.ufscar.br 1 Physics Department, Federal University of São Carlos, São Carlos, Brazil consequently to the applied bias voltage. Full list of author information is available at the end of the article © 2011 dos Santos et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
dos Santos et al. Nanoscale Research Letters 2011, 6:115 Page 2 of 5 http://www.nanoscalereslett.com/content/6/1/115 Figure 1 Schematic potential profile and carrier dynamics in the RTD. conditions. We point out that even at zero bias, the Figure 2 shows the I(V) characteristic curves for sev- QDs states which have a lower energy than the GaAs eral laser intensities. In dark condition, we have contacts, should be filled with electrons from the con- observed only one electron resonant peak which was tact layers, resulting in a negative charge accumulation associated to the resonant tunneling through the second in the QW region. The potential profile of our structure confined level e2 in the QW. It was shown previously should then be changed with respect to a reference sam- [8] that even when QDs are formed, a wetting layer is ple without quantum dots [8,9]. In this case, an asym- still present and changes the position of the first QW metry in the impurity concentration of the contact confined level (e1) to a new position below the GaAs layers should result in a non-zero electric field at the conduction-band. Therefore, resonant tunneling through quantum well and, thus, in a non-zero current, at zero e1 states cannot be observed in the I(V) characteristics bias. We have indeed observed that the crossing of the I curve. Under light excitation, holes are photocreated in (V) curves under light excitation occurs at a voltage contact layer region and tunnel through the double bar- slightly larger than zero, which indicates that there is a rier structure. An additional resonant peak associated to small asymmetry in the impurity concentrations of the hh2 hole resonance [8] is observed in lower voltage doped contact-layers. The crossing voltage corresponds region under higher laser intensities. We have also to the flat band condition of the RTD structure with observed that the photocurrent rapidly increases at low QDs. voltages (0.2 V), saturates in the region of about 0.2 and Figure 3a shows a typical PL spectrum obtained under 0.4 V, and eventually follows the similar resonant vol- zero magnetic field (B = 0 T). The GaAs contact layers tage dependence as the current measured in dark
dos Santos et al. Nanoscale Research Letters 2011, 6:115 Page 3 of 5 http://www.nanoscalereslett.com/content/6/1/115 the QD luminescence is sensitive to the resonant tun- A neling of carriers through the QW levels. Figure 3b also shows the voltage dependence of PL intensity from GaAs contact layer emission. Remark that QD and con- 0.16 tact emission are in anti-phase with each other. The 0mW (a) B=0T 0.14 observed reduction of contact emission and increase of T=2K e2 QD emission in low bias can be explained by the reduc- 0.12 tion of holes recombining in GaAs contact layer due to Current (mA) 0.10 the efficient capture into the QDs [8,9]. Figure 4 shows typical polarized resolved PL spectra 0.08 from QDs under applied bias and magnetic field (15 T). 0.06 Under magnetic field, the confined levels splits into 0.04 spin-up and spin-down Zeeman states and the optical recombination can occurs with well defined selection 0.02 rules probing the spin polarization of carriers in the 0.00 structure [10,11]. We clearly observe that the relative 0.0 0.2 0.4 0.6 0.8 1.0 1.2 intensities from s + and s - QD emission bands vary Bias (V) with the applied bias voltage even though the spin-split- B ting of the QD PL emission is negligible and does not show any appreciable variation with the applied voltage. Therefore the observed spin splitting does not explain the voltage dependence of the QD polarization degree. 0mW (b) 2.0 In fact, the confined states of the QD should not follow 2mW 10mW a simple thermal equilibrium statistics, as the polariza- 20mW 40mW tion of the carriers on those states should also depend 80mW 1.5 on the polarization of the injected carriers, as we discuss Current (mA) B=0T below. T=2K 1.0 Figure 5a shows the voltage dependence of the inte- hh2 grated PL intensity of QD emission at 15T. We have e2 observed a good correlation between the I(V) curve and 0.5 integrated PL intensity for the QD emission for both circular s+ and s- polarizations. Figure 5b shows the bias voltage dependence of the circular polarization 0.0 degree for the QD emission under low and high laser 0.0 0.2 0.4 0.6 0.8 1.0 1.2 intensities at 15T. We have observed that the QD circu- Bias (V) lar polarization degree is always negative and that its Figure 2 Current-voltage characteristic curves. (a) in dark and (b) value depends on both the applied bias voltage and the for several laser intensities. light excitation intensity. In general, its modulus pre- sents a maximum value near the resonant tunneling condition for photo-generated holes. For the high laser intensity condition, the polarization of the QD PL band s how two emission bands: the free-exciton transition is nearly constant (~-25%), but it shows a clear bias vol- from the undoped space-layer and the recombination tage dependence for the low laser excitation intensity. In between photogenerated holes and donor electrons from this case, the QD polarization degree clearly becomes the n-doped GaAs layers. The QD emission is observed more negative around the hole resonance and at about 1.25 eV and show lower PL intensity. We do approaches zero at the electron resonance. Those results not observe any emission from wetting layer because can be correlated to the density of carriers along the carriers preferentially recombine in lower energy states RTD structure and the electron and hole g-factors at in QDs. We have also observed that the QD PL intensity the accumulation layer. We point out two basic infor- depends strongly on the applied voltage at the region of mation that are fundamental for this analysis. First, it is low bias. We have observed a clear correlation between expected that the g-factors of electrons and holes have the I(V) curve and QD PL intensity (Figure 3b). Under opposite signs for GaAs and second, the minority car- applied bias, tunneling carriers can be promptly cap- riers tend to define the effective polarization of an opti- tured by QDs and then recombine radiatively. As cal recombination. Under high laser excitation intensity, explained before, due to this fast carrier capture process,
dos Santos et al. Nanoscale Research Letters 2011, 6:115 Page 4 of 5 http://www.nanoscalereslett.com/content/6/1/115 A B HH2 FE 10mW (b) 12 0.4 (a) V=0.20V 10 0.3 PL intensity (a.u.) e2 Current (mA) I PL (u.a.) 8 0.2 6 + n GaAs 0.1 GaAs 4 InAs QD's InAs QD's I(V) 0.0 2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.2 1.3 1.4 1.5 1.6 Bias (V) Energy (eV) Figure 3 Typical PL spectrum obtained and voltage dependence of PL intensity. (a) Typical PL spectrum and (b) PL integrated intensity as a function of applied voltage at 2 K, for B = 0 T and 10-mW laser excitation. condition (e2 resonant peak). Therefore, the QD polari- the photocreated holes become the majority carrier for zation should be mainly defined by electrons at low vol- the whole bias voltage range of our measurements as tages and by holes at high voltages, which explains that demonstrated by the fact that the photocurrent due to the negative polarization of the QD emission observed photogenerated holes is markedly larger than the elec- at low voltages tend to reduce its modulus and become tronic current in dark. Therefore, the negative polariza- more positive at high voltages. tion of the QD emission should be mainly defined by Our results indicate that the final polarization from the polarization accumulated electrons for all bias vol- QD emission cannot be solely attributed to the spin- tages, which is consistent with the g-factor for electrons splitting of the QD states under magnetic field and it in GaAs. Under low excitation condition, the majority depends on the spin polarization of the injected carriers carrier should change from holes at low voltages close into the QW, which are determined by the g-factors and to the hole resonant condition (hh2 resonant peak), to the density of electrons and holes along the RTD struc- electrons at high voltages, close to the electron resonant ture in a complex way. In fact, a quantitative calculation of the circular polarization degree from the QD T=2K 0.4 Degree of Polarization (%) PL intensity (a.u.) B=15T (a) 0.3 Current (mA) PL intensity (a.u.) I(V) 0.2 E2 P=10 mW 1V 0.1 B=15 T hh2 T=2 K 0.0 -10 (b) 0.15V -20 10 mW -30 100 mW 0V -40 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Bias(V) 1.20 1.22 1.24 1.26 1.28 1.30 1.32 Figure 5 Polarization of the injected carriers. (a) Integrated PL Energy (eV) intensity of QD emission as a function of applied voltage at 15 T. Figure 4 PL spectra for different applied voltages at 15 T and (b) Circular polarization degree of QD emission for lower and higher 2 K. laser intensity as function of applied voltage at 15 T and 2 K.
dos Santos et al. Nanoscale Research Letters 2011, 6:115 Page 5 of 5 http://www.nanoscalereslett.com/content/6/1/115 Bennett CH, DiVincenzo P: Quantum information and computation. Nature emission is a rather complex issue as it depends on var- 7. 2000, 404:247. ious parameters, including the g-factors of the different 8. Patane A, Levin A, Polimeni A, Eaves L, Main PC, Henini M, Hill G: Carrier layers, the resonant and non-resonant tunneling pro- thermalization within a disordered ensemble of self-assembled quantum dots. Phys Rev B 2000, 62:13595. cesses, the capture dynamics of the carriers by the QDs, 9. Vdovin EE, Levin A, Patanè A, Eaves L, Main PC, Khanin YN, Dubrovskii YV, the density of carriers along the structure and the Zee- Henini M, Hill G: Imaging the electron wave function in self-assembled man and Rashba effects. This suggestion is also sup- quantum dots. Science 2000, 290:122. ported by previous results obtained for p-i-n and n-type 10. Fiederling R, Keim M, Reuscher G, Ossau W, Schmidt G, Waag A, Molenkamp LW: Injection and detection of a spin-polarized current in a RTDs without QDs [3,4]. It was observed that the high light-emitting diode. Nature 1999, 402:787. QW polarization degree observed on those measure- 11. Ohno Y, Young DK, Beschoten B, Matsukura F, Ohno H, Awschalom DD: ments is mostly due to a highly spin polarized carriers Electrical spin injection in a ferromagnetic semiconductor heterostructure. Nature 1999, 402:790. from the two dimensional gas formed in the accumula- tion layer next to the emitter barrier. We also point out doi:10.1186/1556-276X-6-115 Cite this article as: dos Santos et al.: Spin effects in InAs self-assembled that the density of carriers along the RTD structure can quantum dots. Nanoscale Research Letters 2011 6:115. be voltage and light controlled, which can be used to vary the circular polarization degree from QDs emission. Conclusion In conclusion, we have observed that the QD circular polarization in an n-type RTD can be voltage and light controlled. A maximum value of spin polarization of about -37% was obtained for the hole resonant tunnel- ing condition and for low-laser intensities. We asso- ciated this effect to the voltage and light dependence of charge accumulation in the QW region. Author details 1 Physics Department, Federal University of São Carlos, São Carlos, Brazil 2 Physics Institute, UNICAMP, Campinas, Brazil 3School of Physics and Astronomy, Nottingham Nanotechnology and Nanoscience Centre, University of Nottingham, Nottingham, UK Authors’ contributions EdS carried out the PL and transport measurements, prepared figures and participated in the analyses of the data. YGG conceived of the study, analyzed the data and wrote this paper. MJSPB participated in the draft of the manuscript. MH has grown the sample and DAT has processed the sample. Competing interests The authors declare that they have no competing interests. Received: 13 August 2010 Accepted: 3 February 2011 Published: 3 February 2011 References 1. Slobodskyy A, Gould C, Slobodskyy T, Becker CR, Schmidt G, Molenkamp LW: Voltage-controlled spin selection in a magnetic resonant tunneling diode. Phys Rev Lett 2003, 90:246601. 2. de Carvalho HB, Galvão Gobato Y, Brasil MJSP, Lopez-Richard V, Submit your manuscript to a Marques GE, Camps I, Henini M, Eaves L, Hill G: Voltage-controlled hole spin injection in nonmagnetic GaAs/AlAs resonant tunneling structures. journal and beneﬁt from: Phys Rev B 2006, 73:155317. 3. de Carvalho HB, Brasil MJSP, Galvão Gobato Y, Marques GE, Galeti VA, 7 Convenient online submission Henini M, Hill G: Circular polarization from a nonmagnetic p-i-n resonant 7 Rigorous peer review tunneling diode. Appl Phys Lett 2007, 90:62120. 7 Immediate publication on acceptance 4. dos Santos LF, Galvão Gobato Y, Lopez-Richard V, Marques GE, Brasil MJSP, 7 Open access: articles freely available online Henini M, Airey RJ: Polarization resolved luminescence in asymmetric n- type GaAs/AlGaAsresonant tunneling diodes. Appl Phys Lett 2008, 7 High visibility within the ﬁeld 92:143505. 7 Retaining the copyright to your article 5. Loss D, DiVincenzo DP: Quantum computation with quantum dots. Phys Rev A 1998, 57:120. 6. Steane A: Quantum computing. Rep Prog Phys 1998, 61:117. Submit your next manuscript at 7 springeropen.com

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