Single Electron Charging in Optically Active Nanowire Quantum Dots

Abstract

We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot.KEYWORDS Nanowires, optically-active quantum dots, single electron charging, opto-electronics S ingle, optically active quantum dots are widely investigated due to the ability to combine both single electron charging 1,2 and single 3 or entangled 4 photonemission, which are all key requirements for quantum information processing applications. 5 Nanowire quantum dots (NW-QDs) offer additional functionalities over selfassembled quantum dots since they are embedded in a onedimensional system instead of a three-dimensional host matrix. Therefore, the single electron (hole) transport channel is naturally aligned to the optically active quantum dot in the nanowire, advantageous for combining both quantum optics 6 and transport. [7][8][9] In addition, due to the small radial dimensions of the nanowires, electrostatic gate geometries are highly versatile 7,8 and axial heterostructure design is not limited by strain. As an example, the combination of Si sections, which are free of nuclear spins, with optically addressable electronic levels in III-V materials is promising for extending electron spin storage times. Prior to the work presented here, we have shown that a single InAsP quantum dot grown in an InP nanowire geometry is optically active, exhibits narrow emission lines, spin polarization memory effects, 10 and can be embedded in a LED device geometry. 11 Furthermore, it is predicted that NW-QDs are ideal sources of entangled photons due to the nanowire symmetry. 12 Recently, an electron spin-to-charge conversion read-out scheme has been proposed, 13 which is compatible with controlled storage of carriers up to microseconds. 14 Such storage times are promising since single spins in selfassembled quantum dots (SA-QDs) have been initialized, coherently manipulated, and read-out within picosecond time scales. 15,16 The proposed spin read-out scheme 13 highly depends on the overall tunnel coupling between the SA-QD energy levels and the continuum, determined by the quantum dot-to-contact spatial separation, which is fixed during growth. 17 In this letter, we present electrical control and optical read-out of the number of electrons residing in a single InAs 0.25 P 0.75 quantum dot embedded in an InP nanowire. We first identify the neutral exciton by photocurrent spectroscopy. Second, we demonstrate that the electron number can be controlled by an electric field applied along the nanowire growth direction or by an electrostatic bac...

Population dynamics and dephasing of excitons and electron-hole pairs in polytype wurtzite/zinc-blende InP nanowires

“…This flexibility in NW design and growth has enabled novel device applications including NW lasers 10,22,23 , light emitting and detection devices 2,[24][25][26] , ultrahigh density transistors 27 single-electron charging devices 28,29 as well as single photon emitters 15,30,31 and detectors 32 .…”

Single quantum dot nanowire photodetectors

“…2 In addition, NW devices have shown excellent light detection properties. [3][4][5][6] Recently, quantum dots ͑QDs͒ embedded in NW devices have shown both single electron control [7][8][9][10] and single photon emission, 11 which are important in quantum information applications. The unique combination of an on-chip light emitter and detector is useful for near field optical circuits using plasmon waveguides.…”

“…17 The InP NWs studied here are unintentionally doped and have typical lengths of 4 m and tapered diameters ranging from 20 to 60 nm. 10 The NWs were grown by metal-organic vapor phase epitaxy, using 20 nm gold colloids as catalysts for vapor liquid solid nucleation. 18 The InAs 0.25 P 0.75 QD is ͑5 Ϯ 2͒ nm high with ͑33Ϯ 1͒ nm diameter and is positioned in the middle of the NW.…”

“…The processing of these NW-QD devices has been described in detail elsewhere. 10 In Fig. 1, we present the photoresponse and carrier dynamics of a single NW photodetector at low temperature ͑T =10 K͒.…”

“…The dark current ͑bot-tom trace͒ is lower than 0.5 pA up to V sd = 6 V. Schottky contacts and the low carrier concentration in the NW produce a high signal to noise ratio ͑ϳ800͒ in the photodetection and allow for the application of an electric field along the NW axis. 10 Since the laser is aligned to the QD, the asymmetry in the I-V indicates that the QD is positioned closer to the drain contact. 20 In Fig.…”