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Nano and Giga Challenges in Electronics and Photonics P-25 Quantum-Dot –Array Based Terahertz Detectors Tempe, Arizona, March 12-16, 2007 N. A. Kabir1, J. Song1, A. Markelz2, T. Morimoto3, Y. Ujiie3 3 3 3 3 1 N. Yumoto , K. Sudou , N. Aoki , Y. Ochiai , & J. P. Bird 1: Department of Electrical Engineering, University at Buffalo, the State University of New York, Buffalo, NY 14260, USA 2: Department of Physics, University at Buffalo, the State University of New York, Buffalo, NY 14260, USA 3: Department of Electronics and Mechanical Engineering, Chiba University,1-33 Yayoi-cho, Inage-ku, Chiba 263 8522, Japan We examine the terahertz conductivity response of lithographically defined quantum dot arrays as a function of temperature, dot size and photo excitation. Quantum dot structures have been explored as possible compact sources and detectors of THz radiation. Here we consider a lithographically defined array with high uniformity and high duty cycle to increase FIR optical density. Specifically we examine the cross over from continuum to discrete response as the quantized energy level spacing becomes commensurate with the FIR. Interaction of THz photons with a 2DEG in a semiconductor provides the basis for a number of different THz-detection schemes. One of the simplest approaches makes use of free carrier absorption to heat the electron gas relative to the lattice to change the conductivity. But it requires low-temperature (4.2 K) operation and does not provide frequency sensitivity. One way to realize such sensitivity is to make use of an appropriate electronic transition between the discretely-quantized states of quantum dot or quantum well, however, this again imposes the restriction of lowtemperature operation. Samples are lithographically defined using GaAs/Al0.3Ga0.7As and InAs/AlSb high-mobility, modulation doped two dimensional electron gas (2DEG )wafers. Quantum-dot arrays are fabricated using e-beam lithographically and wet chemical etching. Measurements of the complex conductivity response at THz frequencies are made using terahertz time domain spectroscopy (THz TDS). An alternative approach that should overcome these issues, allowing for higher temperature and frequency-specific operation, uses THz radiation to excite collective plasma oscillations in a high-mobility 2DEG, most commonly achieved by forming a metal grating on the heterostructure surface. Abstract 1. Background 2. 1) By exploiting the advantages of epitaxial growth, distance to the 2DEG can be reduced to a value of ~10 nm, about an order of magnitude smaller than the grating-2DEG separation in conventional devices, without any significant degradation of the 2DEG mobility. Given the exponential decay of the scattered electromagnetic-wave amplitude with distance from the grating, this is potentially a large effect contactless measurement of frequency dependent conductivity 2) The QD-array is characterized by its own plasmon excitations (grating plasmons) and the frequency of these should also lie in the THz range, similar to the plasmon mode in the adjacent 2DEG. When the frequency of the incident radiation is close to that of the grating plasmon, resonant excitation of these plasmons should cause the amplitude of the scattered electromagnetic field in the grating layer to increase dramatically and even exceed that of the external field measuring the complex conductivity response and assuming a Drude model one can directly access the momentum relaxation rate Phase vs Frequency 1.1 Transmission 0.9 5K 10K 25K 50K 75K 100K 150K 200K 250K 300K 0.8 0.8 1.2 Frequency (THz) 1.6 2 0.4 0.8 1.2 Frequency (THz) 1.6 o 8.7 10 rad / s o f0 0.14THz 2 0.8 1.2 Frequency (THz) 1.6 AlGaAs: 30 nm (GaAs BIG QD sample shown here) GaAs : 800 nm GaAs 2DEG sample – m* = 0.067 m0 ns = 2.5 X 1015 m-2 (RT) 2DEG GaAs Substrate : 600 μm Array of QD Samples 6. Fermi-energy calculation Assuming parabolic potential 1 * 2W2 E F m o 2 4 8 EF o m*W 2 En(ky) 2 -0.35 EF -0.4 0.4 0.8 1.2 Frequency (THz) k F2 37 EF 5.73 X 10 XnS * 2m Here, nS 2.5 X 1015 m 2 Absorbance 2 1.6 2 Results – BIG (500nmX500nm) QD 8. ky EF 9meV Calculations 9. 1.4 n e n e q 2 p S * . S* . m ( qt ) m L r 0 • No illumination effects observed 1.2 Frequency (THz) 7.88 109 p 2 L for L=500nm 1 GaAs 2DEG 0.9 0.8 0.8 0.6 Fmin1_GaAs_BIG Fmin2_GaAs_BIG Fmin_InAs Fmin_GaAs_SML p 7.88 10 rad / s 12 0.4 p fp 1.25THz 2 1 10 100 Temperature (K) • At low T: true “dot” response for L=200nm p 12.44 10 rad / s 12 p fp 1.98THz 2 Calculations InAs 2DEG 1000 • True “dot” response is seen at low T and washes out at above ~100K due to electron-phonon scattering effect 0.7 • At High T (above ~100K): response determined by electron-phonon scattering – as Ns changes drastically for both samples 11. • Inflection points from the phase data shows absorbance due to plasmon oscillation in the dots – visible only in GaAs BIG (~500nmX500nm) QD sample 2nd harmonic 8.6 for W=200nm o 2.17 1012 rad / s f 0 o 0.35THz 2 n-AlGaAs: 40 nm 2 for W=500nm 11 GaAs BIG QD = 500nmX500nm GaAs SML QD = 200nmX200nm InAs QD = 200nmX200nm Occurence of minima 2 o 5K 10K 25K 50K 75K 100K 150K 200K 250K 300K 0.4 2 Oscillator and plasmon frequency calculation 8 EF m*W 2 0.9 0.7 Results – SMALL (200nmX200nm) QD 7. 1 0.8 -0.4 0.4 2nd harmonic -0.35 0.7 Drawbacks of existing detectors 3. 5K 10K 25K 50K 75K 100K 150K 200K 250K 300K -0.3 Phase Transmission 1 5) Distance to 2DEG can not be reduced below a few hundred nm, as it deteriorates the 2D electron mobility and the frequency resolution of the plasmon system restricting the coupling efficiency of the metal grating and detection sensitivity -0.25 1.2 5K 10K 25K 50K 75K 100K 150K 200K 250K 300K -0.3 4) Fourier harmonics of the electromagnetic field are attenuated by the time they reach the 2DEG Phase vs Frequency Transmission vs Frequency -0.25 1.1 3) Amplitude of the induced electric field penetrating to 2DEG cannot exceed the externally-incident radiation due to the total electric field inside the metal grating fingers being equal to zero at all points due to screening Phase 1.2 15-DOT CRYSTAL GaAs CAP: 5 nm Schematic of THz TDS 5. Ns-Hall (Xe16 /m2) Transmission vs Frequency 2) Electromagnetic wave that is scattered by the grating gate corresponds to an evanescent mode whose amplitude decays exponentially with increasing distance from the gate with degrading sensitivity coherent detection Motivation 4. 1) Metal gate induces a periodic modulation of the normally-incident external THz radiation, which lies in the submillimeter range and is much larger than the grating period ultra-fast optical technique that allows us to measure the dielectric response in the range 5 – 85 cm-1 In order to overcome the problems noted above, we propose to investigate the characteristics of a new class of plasmonic THz detectors in which the metal grating gate is replaced with nanostructured arrays of quantum dots with the following advantages: 10. The critical issue in plasmon-based detection schemes concerns an efficient 9-DOT CRYSTAL coupling of the external electromagnetic radiation and the 2D plasmons which suffers due to the following reasons: 8.4 0.6 8.2 0.5 • No proof of carrier transitions between different energy sub-bands but there is definite presence of 2nd order harmonic oscillations 8 7.8 0.4 7.6 1 10 100 • GaAs SMALL QD response seems to indicate that it is always depleted 0.3 0.2 1 10 Temperature (K) 100 Plasmonic detection • InAs QD shows a weaker confinement with a constant Ns 12. Conclusion NANOELECTRONIC MATERIALS & DEVICES RESEARCH GROUP (NoMaD) Department of Electrical Engineering, University at Buffalo