A quantum wire is fabricated on (001)-GaAs at the intersection of two
overgrown cleaves. The wire is contacted at each end to n+ GaAs layers via
two-dimensional (2D) leads. A sidegate controls the density of the wire
revealing conductance quantization. The step height is strongly reduced from
2e^2/h due to the 2D-lead series resistance. We characterize the 2D density and
mobility for both cleave facets with four-point measurements. The density on
the first facet is modulated by the substrate potential, depleting a 2um wide
strip that defines the wire length. Micro-photoluminescence shows an extra peak
consistent with 1D electron states at the corner.Comment: 4 pages, 4 figure

Bichler, M.

Grayson, M.

Krenner, H. J.

Roth, S. F.

Schuh, D.

English

arXiv

Krenner, Hubert J.

Schuh, Dieter

Bichler, Max

Online-Publikationserver Augsburg

Vertical quantum wire realized with double cleaved-edge overgrowth

S. F. Roth

H. J. Krenner

D. Schuh

M. Bichler

M. Grayson

Crossref

OPUS - Augsburg University Publication Server

OPUS Augsburg

Appl. Phys. Lett. 89, 032102 (2006); https://doi.org/10.1063/1.2222347 89, 032102© 2006 American Institute of Physics.Vertical quantum wire realized with doublecleaved-edge overgrowthCite as: Appl. Phys. Lett. 89, 032102 (2006); https://doi.org/10.1063/1.2222347Submitted: 18 April 2006 . Accepted: 13 June 2006 . Published Online: 17 July 2006S. F. Roth, H. J. Krenner, D. Schuh, M. Bichler, and M. GraysonARTICLES YOU MAY BE INTERESTED INSurface acoustic wave regulated single photon emission from a coupled quantum dot–nanocavity systemApplied Physics Letters 109, 033105 (2016); https://doi.org/10.1063/1.4959079Combined electrical transport and capacitance spectroscopy of a MoS2-LiNbO3 field effecttransistorApplied Physics Letters 110, 023505 (2017); https://doi.org/10.1063/1.4973862Ultrasonically assisted deposition of colloidal crystalsApplied Physics Letters 105, 031113 (2014); https://doi.org/10.1063/1.4891171Vertical quantum wire realized with double cleaved-edge overgrowthS. F. Roth,a H. J. Krenner, D. Schuh, M. Bichler, and M. GraysonWalter Schottky Institut, Technische Universität München, 85748 Garching, GermanyReceived 18 April 2006; accepted 13 June 2006; published online 17 July 2006A quantum wire is fabricated on 001 GaAs at the intersection of two overgrown cleaves. The wireis contacted at each end to n+-GaAs layers via two-dimensional 2D leads. A side gate controls thedensity of the wire revealing conductance quantization. The step height is strongly reduced from2e2 /h due to the 2D lead series resistance. We characterize the 2D density and mobility for bothcleave facets with four-point measurements. The density on the first facet is modulated by thesubstrate potential, depleting a 2 m wide strip that defines the wire length.Microphotoluminescence shows an extra peak consistent with one-dimensional electron states at thecorner. © 2006 American Institute of Physics. DOI: 10.1063/1.2222347An elegant technique for fabricating clean one-dimensional 1D quantum wires is cleaved-edge overgrowthCEO,1 where the 1D wire arises at the intersection of atwo-dimensional 2D system and the perpendicular cleaveplane. Ballistic transport,2,3 optical spectroscopy,4,5 andlasing6,7 in such single-cleave wires have been investigated.Another advantage of CEO is that electron systems grown onthe cleave facet can be modulated with atomic precision bythe potential in the substrate. 2D CEO transport structureshave been modulated with a superlattice potential,8 and withsingle9 and double tunnel barriers.10 The twin CEO capabili-ties of reducing the dimension and modulating the potentialcan be combined in double-cleave structures to create modu-lated 1D structures such as quantum dot molecules.11,12 Todate, however, no transport structures have been imple-mented by double cleave due to the difficulty of fabricatingelectrical contacts.This letter demonstrates a simple quantum wire in adouble-cleave geometry with electrical contacts for transportmeasurements Fig. 1. The wire develops at the intersectionof two cleave planes in the accumulation edge of a depleted2D electron system 2DES. Four-point measurements revealthe density and mobility of electrons in each overgrownfacet,13 and when a 2 m wide low density strip in the firstcleave plane is depleted, the conductance shows quantiza-tion, indicating 1D transport at the corner. The plateau stepsare significantly reduced from G0=2e2 /h due to the 2D leadseries resistance. The quantum wire is also observed in pho-toluminescence as a second peak in the spectrum above theGaAs band gap energy.Figure 1a shows the design of the vertical quantumwire. The substrate and first CEO are shown, with the secondCEO taking place on the front facet. First, a 001 substrateis overgrown vertically with 1m n+ doped GaAs bottomcontact, 2 m undoped GaAs, 2m Al0.3Ga0.7As AlGaAs,another 2 m GaAs layer, and finally a 1m n+ top contact.This topmost layer is patterned with photolithography andpartly etched away, leaving four n+ finger contacts13 whichlead up to each cleave plane. Then the 110 facet is cleavedand overgrown, giving rise to the first 2DES 2DES I. Theovergrowth consists of an 8 nm wide GaAs quantum well, a50 nm AlGaAs spacer, Si  doping, 500 nm AlGaAs, and an+ side gate to control the 2D density. The 2 m wide Al-GaAs substrate barrier modulates the density of 2DES I, bothdue to quantum confinement8 and because charged defects inAlGaAs can reduce the density next to the barrier.14,15 Thereduced density region of 2DES I is notated as 2DES I*. Toaccumulate 1D states at the edge of 2DES I, the perpendicu-lar 11̄0 cleave facet is overgrown with a modulation dopedbarrier: 20 nm AlGaAs spacer, Si  doping, and 300 nmAlGaAs topped with a 20 nm GaAs cap. The accumulationaElectronic mail: roth@wsi.tum.deFIG. 1. a The double-cleave sample consists of a 001 grown substrateand a first 110 and second 11̄0 cleave, both overgrown with modulationdoping. The 2DES regions from the two cleaves are indicated with I andII, and the 2 m substrate barrier defines a reduced density strip in the firstcleave denoted as I*. The wire dotted line runs vertically at the corner of2DES I and II and is contacted to the n+ top and bottom contacts. Thesecond CEO is in the plane of the front facet. b The conductance ofsamples 1a and 2 exposes steps between VG=0 and 0.2 V, indicative of 1Dconductance quantization. The 2D lead resistance suppresses the step heightfrom 2e2 /h.APPLIED PHYSICS LETTERS 89, 032102 20060003-6951/2006/893/032102/3/$23.00 © 2006 American Institute of Physics89, 032102-1potential also induces a second 2DES 2DES II at the sec-ond cleave plane, split by the substrate AlGaAs barrier.Before measuring the wire, it is helpful to first charac-terize 2DES I, I*, and II with four-point magnetoresistancemeasurements using the n+ finger contacts. Standard lock-intechniques were used after illuminating the sample with a redlight-emitting diode LED. Figures 2a and 2b show theShubnikov de Haas SDH oscillations measured at 330 mKin a magnetic field perpendicular to the first and secondcleave facets, respectively.At a side gate bias VG=0.0 V inset of Fig. 2a onlyone density is apparent in the resistance oscillations on thefirst facet. The density and mobility nI=2.151011 cm−2 andI=2.6104 cm2/V s are presumed to correspond to 2DESI, since 2DES I* should be depleted. When nI is increasedwith side gate bias to VG=0.8 V Fig. 2a the zero fieldresistance drops more than three orders of magnitude andadditional minima appear. Both behaviors are evidence forthe onset of conduction in 2DES I*. The low resistance arisesbecause of a short to the bulk-doped bottom contact, and theadditional minima arise from the lower density SdH oscilla-tions in 2DES I*. The resistances Rtop-top between two fingercontacts and Rtop-bottom from a finger to the bottom contactFig. 3 inset confirm that two pinch-off conditions occur atVG=−0.1 and +0.3 V, attributed to the depletions of 2DES Iand 2DES I*, respectively. On the second cleave facet Fig.2b, the density and mobility of 2DES II are nII=4.31011 cm−2 and II=2.9104 cm2/V s, respectively.We can quantify the densities of 2DES I and I* at posi-tive gate voltage by plotting Landau fans of the Rxx minimaversus VG Fig. 3. The fit lines plot B=nIh /e black andB* =nI*h /e* gray, where B is the magnetic field of theth minima, and the densities nI and nI* vary linearly withgate voltage with a fixed density difference n=nI−nI*=0.921011 cm−2. The fits clearly identify all the observedminima. Just as in the inset of Fig. 3, the pinch-off of 2DESI occurs at −0.4 V relative to the pinch off of 2DES I*,though differing cooldowns and illumination conditions shiftthe absolute gate voltage. At gate voltages where 2DES I* isdepleted, transport in the 1D accumulation edge of 2DES I*can be investigated.Conductance steps as a function of side gate bias in Fig.1b are the principal evidence of 1D conduction in thisstructure. An ac voltage of 100 V is applied between a topfinger contact and the bottom contact, and the current is mea-sured with lock-in techniques at 17 Hz. In Fig. 1b the con-ductance at 4.2 K of sample 1a after illumination is zero at agate voltage of VG=0 V, but rises to a plateau value of about0.3G0 between VG=0.03 V and VG=0.07 V. A second pla-teau appears at 0.6G0 between VG=0.07 V and VG=0.09 V,indicating 1D transport. Sample 2 behaves similarly, with theconductance onset shifted to a slightly higher gate voltageand lower plateau values. The plateaus in both samples arestrongly suppressed from G0, due to a high 2D lead resis-tance as well as possible 2D-1D scattering at the accumula-tion edge.2,16,17Evidence of a quantum wire is also observed opticallyvia microphotoluminescence -PL. The sample is cooleddown to 15 K in a flow cryostat, and a HeNe laser =632.8 nm is focused to a diameter of 1 m on the 11̄0facet the front facet of Fig. 1a. Figure 4 shows PL spectrarecorded for low excitation densities P0=200 W/cm2. Asthe spot is positioned exactly above 2DES I* position A inFig. 1a, a large peak appears in the PL signal at a detectionenergy h=1562 meV and a smaller side peak at 1576 meV.When moving the laser spot along the 001 direction to restabove 2DES I, both peaks disappear position B in Fig. 1a,because here no confinement for holes exists and only PLfrom the GaAs bulk is detected at the band gap energy of1514 meV.We calculate the interband absorption spectrum of an8 nm GaAs quantum well embedded in an AlGaAs matrix.The lowest electron to heavy-hole transition was found to beat Ee−hh=1574 meV, as indicated by the arrow in Fig. 4a.Accordingly, we attribute the high energy peak to a transitionbetween 2D electron and 2D hole states. We expect to findthe 1D electron states shifted to lower energy, as shown byFIG. 2. Shubnikov–de Haas oscillations of a 2DES I for VG=0.8 V andVG=0.0 V inset and b 2DES II. A side gate tunes the density of 2DES I.Extra minima at VG=0.8 V are attributed to a second density in region2DES I*. Filling factors are labeled  for density nI of 2DES I and * fordensity nI* of 2DES I*. Symbols indicate minima in the data and arrowsindicate the theoretical position of minima as described in the text.FIG. 3. Inset The gate voltage dependent resistances Rtop-top and Rtop-bottomdemonstrate the pinch off of the low density central 2DES I* and of the highdensity 2DES I with decreasing VG. The positions of the minima are plottedvs VG and fit to two Landau fans. A density difference of n=nI−nI*=0.921011 cm−2 is able to explain all the observed minima.032102-2 Roth et al. Appl. Phys. Lett. 89, 032102 2006the level diagram in Fig. 4a. Consequently, we assign thelow energy peak to a transition between bound 1D electronand a continuum of 2D heavy-hole states.5 No holes are con-fined to 1D in our structure. The energy difference of14 meV between the two peaks is the binding energy ofthe quantum wire and is in good agreement with Hartreecalculations.A final test for the existence of a quantum wire is exci-tation power dependent and polarization resolved spectros-copy. The results are presented in Figs. 4b and 4c, respec-tively. For increasing excitation powers up to 100P0 bothpeaks gain intensity and the 1D peak shows a clear saturationbehavior in contrast to the 2D signal. This observation can beexplained with a consecutive filling of the 1D and 2D levels.Due to the higher density of states, the 2D peak gains inten-sity superlinearly, the 1D peak sublinearly. For the sum ofboth signals an exponent of 0.97±0.05 was found, demon-strating the single exciton nature and the decay to a commonhole level. In Fig. 4c we plot the degree of polarizationI / I+ I of the 1D and 2D signals. For =0° perpen-dicular to the quantum well growth direction of 2DES I, wefind a maximum degree of polarization of 80% for both sig-nals. Since the PL is detected perpendicular to the quantumwell QW growth direction heavy-hole and light-hole statescan be distinguished. In contrast to light holes, heavy-holetransitions are polarized perpendicular to the QW growth di-rection due to the missing z component of the central cellpart of the wave function. The strong in-plane polarization ofthe 1D and 2D signals is a clear indication for a recombina-tion of conduction band electrons with a heavy-hole state,ruling out a multiple peak structure from the light-hole en-ergy spectrum. All four optical findings, the limited spatialarea, the peak emission energy, the power dependent levelfilling, and the heavy-hole polarization selection rules, pro-vide evidence for the formation of a bound 1D level forelectrons below the 2D continuum.In conclusion, we have fabricated a vertical quantumwire with twofold application of CEO. In conductance mea-surements, steps characteristic of 1D transport are observed.In optical investigations a peak in the PL spectrum on thelow energy side of the quantum well signal is confirmed toarise from bound wire states with power and polarizationdependent measurements. The vertical orientation of the wiremakes it possible to fabricate samples with a 1D potentialmodulated by atomically precise barriers.This work is supported by the Bundesministerium forBildung und Forschung BmBF through Project No.01BM469 and Deutsche Forschungsgemeinschaft Ab 35/4and 35/5. The authors thank Jonathan J. Finley for assistancewith PL measurements and Gerhard Abstreiter for discus-sions and continued support.1L. Pfeiffer, K. West, H. Stormer, J. Eisenstein, K. Baldwin, and D.Gershoni, Appl. Phys. Lett. 56, 1697 1997.2A. Yacoby, H. L. Stormer, N. S. Wingreen, L. N. Pfeiffer, K. W. Baldwin,and K. W. West, Phys. Rev. Lett. 77, 4612 1996.3R. de Picciotto, H. L. Stormer, L. N. Pfeiffer, K. W. Baldwin, and K. W.West, Nature London 411, 51 2001.4J. Hasen, L. N. Pfeiffer, A. Pinczuk, S. He, K. West, and B. S. Dennis,Nature London 390, 54 1997.5A. R. Goni, L. N. Pfeiffer, K. W. West, A. Pinczuk, H. U. Baranger, andH. L. Stormer, Appl. Phys. Lett. 61, 1956 1992.6W. Wegscheider, L. N. Pfeiffer, M. M. Dignam, A. Pinczuk, K. W. West,S. L. McCall, and R. Hull, Phys. Rev. Lett. 71, 4071 1993.7Y. Hayamizu, M. Yoshita, S. Watanabe, H. Akiyama, L. N. Pfeiffer, and K.W. West, Appl. Phys. Lett. 81, 4937 2002.8R. A. Deutschmann, W. Wegscheider, M. Rother, M. Bichler, G.Abstreiter, C. Albrecht, and J. H. Smet, Phys. Rev. Lett. 86, 1857 2001.9W. Kang, H. Stormer, L. Pfeiffer, K. Baldwin, and K. West, NatureLondon 403, 59 2000.10F. Ertl, S. Roth, D. Schuh, M. Bichler, and G. Abstreiter, Physica E Am-sterdam 22, 292 2004.11W. Wegscheider, G. Schedelbeck, G. Abstreiter, M. Rother, and M.Bichler, Phys. Rev. Lett. 79, 1917 1997.12G. Schedelbeck, W. Wegscheider, M. Bichler, and G. Abstreiter, Science279, 1792 1997.13M. Grayson, S. F. Roth, Y. Xiang, F. Fischer, D. Schuh, and M. Bichler,Appl. Phys. Lett. 87, 212113 2005.14E. P. D. Poortere, Y. P. Shkolnikov, and M. Shayegan, Phys. Rev. B 67,153303 2003.15S. F. Roth, M. Grayson, M. Bichler, D. Schuh, and G. Abstreiter, AIPConf. Proc. 772, 915 2005.16R. de Picciotto, H. L. Stormer, A. Yacoby, L. N. Pfeiffer, K. W. Baldwin,and K. W. West, Phys. Rev. Lett. 85, 1730 2000.17Sample names: 03-04-05.1e 1a, 03-04-05.1c 1b, and 04-05-06.3c 2.FIG. 4. a PL of a double-cleave sample measured on the square well Aand the triangular well B see Fig. 1. Two peaks appear at A and areattributed to a transition from 1D and 2D electron states to 2D heavy-holestates. The peaks disappear if the laser spot is moved to B. The arrowindicates the calculated transition energy of the ground state. b At lowlaser powers only the 1D peak is visible. The 2D peak appears as the poweris increased and electrons fill up higher states. When the lowest energy levelis filled, the intensity of the 1D peak saturates. c Both peaks are stronglypolarized as expected for transitions to heavy-hole states. The polarizationangle =0° is perpendicular to the quantum well growth direction.032102-3 Roth et al. Appl. Phys. Lett. 89, 032102 2006

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Vertical quantum wire realized with double cleaved-edge overgrowth

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