Available online at www.sciencedirect.com PhysicaE17(2003)161–163 www.elsevier.com/locate/physe A novel InAs quantum wire system C.H.Yanga;∗,M.J.Yangb,K.A.Chenga,J.C.Culbertsonb aDepartmentofElectricalandComputerEngineering,UniversityofMaryland,CollegePark,MD20742,USA bNavalResearchLaboratory,Washington,DC20375,USA Abstract Wereporthereanewtechniqueinpatterninghighqualitylow-dimensionalelectronsinsingleInAsquantumwells.Grown by molecular beam epitaxy, the single InAs quantum well is sandwiched between AlSb barriers and capped by a thin layer ofInAs.WhentheInAscaplayerispatternedbyelectronbeamlithographyandselectivelyremoved,electronsareinduced in the InAs quantum well below due to the di2erent surface Fermi level pinning voltages on the exposed AlSb layer from theInAscap.One-dimensionalquantumwirescanthusbeconvenientlyde5nedbylithographyandnm-shallowetching.We demonstrate that these one-dimensional electrons posses a long elastic mean free path (¿1:4(cid:1)m) and a long coherence length(¿3(cid:1)m)at2K. ?2002ElsevierScienceB.V.Allrightsreserved. PACS:73.23.Ad;73.40.Kp;05.60.Gg Keywords:Quantumwire;InAs;One-dimensionalelectrons;SurfaceFermilevelpinning 1. Introduction advantages. The samples used for demonstrating this new approach are single InAs/AlSb quantum Tostudyquantumwiresandquantumdots[1–3],it wells grown on GaAs by molecular beam epitaxy. isdesirabletohaveasamplesystemwhoseelectronic A model structure has a thick AlSb/GaSb bu2er, a propertiescanbewellcontrolled.Forexample,ifthe 17nm InAs quantum well, and a 25nm AlSb top quantumdotsaretobeusedinsingleelectrondevices, barrier.Thesamplesarecappedwiththreethinlayers, a low number of electrons and a large energy sepa- InAs/AlSb/InAs,3nm each. ration between neighboring states are of interest. In the case of quantum wires, a single one-dimensional channel would be ideal. The most popular approach 2. Experimentalscheme is using GaAs-based high electron mobility transis- tor structures with Schottky gates. Here we report an Thefabricationprocessonlyinvolveselectron-beam alternative nanofabrication scheme that has several lithography for device pattering and shallow wet chemical etching for pattern transfer [4]. We use a focused electron beam to directly write patterns ∗Corresponding author. Tel.: +1-301-405-3673; fax: +1-301- onto a positive tone electron beam resist PMMA. 314-9281. E-mailaddress:[email protected](C.H.Yang). The written region is later washed away in the 1386-9477/03/$-seefrontmatter?2002ElsevierScienceB.V.Allrightsreserved. doi:10.1016/S1386-9477(02)00731-2 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 2. REPORT TYPE 3. DATES COVERED 16 JUN 2003 N/A - 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER A Novel InAs Quantum Wire System 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION U.S. Army Research Office P.O. Box 12211 Research Triangle Park, NC REPORT NUMBER 27709-2211 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE UU 13 unclassified unclassified unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 162 C.H.Yangetal./PhysicaE17(2003)161–163 development process. The patterned PMMA is then used as the etch mask. We have developed highly selective etchants that can easily remove either InAs orAlSbandleavetheotheroneintact. ThesurfaceFermilevelpinningvoltageforInAsis ∼0:15eV above its conduction band minimum. For AlSb, it is roughly ∼0:8eV above its valence band maximum.Wetakeadvantageofsuchadrasticdi2er- enceinsurfaceFermilevelpinningpositionstode5ne theelectronsintheInAsquantumwell. As part of the fabrication scheme, the as-grown sampleisintentionallydesignedtobenon-conductive. This is accomplished by p-doping in the three cap layersalongwiththestructureparametersandarela- tivelylowFermilevelpinningatInAssurface.Based on a self-consistent band bending calculation, the re- sulting Fermi level lies ∼100meV below the lowest Fig.1.Atomicforcemicrographofaquantumpointcontactdevice. Theconductanceoftheconductingchannel(fromtoptobottom) InAs quantum well subband at 4K. Hall measure- is modulated by voltage bias applied to the two side gates. The ments on as-grown samples con5rm their insulating surfaceofthewafershowsatomicterracesthatareoriginatedfrom characteristic. thethreadingdislocations. After3nmshallowetching,highmobilityelectrons are induced, forming a replica of the etched pattern. WehaveusedmagnetotransportonHallbarfabricated 3. Nanofabricationresults usingphotolithographytocharacterizetheconcentra- tionandmobilityoftwo-dimensionalelectronsat4K. This fabrication technique proves to be versatile The electron concentration becomes 4:9×1011 cm2 for de5ning narrow quantum wires. Fig. 1 shows an andthemobilityis2×105 cm2=V s.Theelasticmean atomic force micrograph of a quantum point contact. freepathisthuscalculatedtobe2:3(cid:1)m Other than the source and the drain, two side gates We have further etched o2 the 3nm AlSb are de5ned by the same means. A conductive gate near the surface. The exposed 3nm InAs devel- servesthepurposeofmodulatingtheelectronconcen- ops its surface Fermi level pinning position again tration in the channel nearby. The non-uniformity of at ∼ 0:15eV above the conduction band mini- the lithography width is within several nanometers. mum. The two-dimensional electron concentration The depth of the wet etching is veri5ed to be consis- in the InAs quantum well becomes nearly zero tentwiththetargetedthickness,3nm. at4:2K. Fig. 2 shows the micrograph of a 300nm di- To further test the validity of the design, we ameter quantum ring. Note that the surface of the have etched o2 the second 3nm InAs quantum 5nishedsampleshowsmanybumpyfeatures,approx- well. Indeed, the induced electron concentration be- imately 2–3nm in height and 20–30nm in diame- comes 1:1×1012 cm2 and the mobility is increased ter. We attribute those to be the residue of PMMA to 4:27 × 105 cm2=V s. The corresponding elastic molecules. when these residue molecules are fully mean free path is calculated to be 7:0(cid:1)m. The 5nd- dissolved by chemical solvents, atomic terraces are ing of increased electron concentration, compared to clearly revealed, as shown in Fig. 1. Single electron the case where only the 5rst 3nm InAs is etched, transistor structures are also fabricated by the same is consistent with the design. The higher concentra- technique,andoneexampleisdemonstratedinFig.3. tion is due to the thinner dielectric layer in between Thefabricationtechniqueisversatile,asanypattern the InAs quantum well and the surface. In addition, de5ned on the resist in principle can be transferred the accompanied higher mobility is due to enhanced totheelectronlayer.Basedonmagnetotransportdata screening. takenfromaseriesofHallbarswithdi2erentchannel C.H.Yangetal./PhysicaE17(2003)161–163 163 Insummary,wehavedevelopedanoveltechnique forthefabricationofone-dimensionalquantumwires. The new fabrication method utilize the di2erent sur- faceFermilevelpinningpositionsofInAsandAlSb. Oncethesurfacematerial,eitherInAsorAlSb,ismod- i5ed, the surface Fermi level and the band bending change accordingly. Although there is no metal gate de5ned, the up shift of the surface Fermi level pin- ning position in the patterned area makes the system behave as if there is a metal gate with a +0:5V bias readilyapplied. Theelectronsarecon5nedinarelativelysteeplat- eral potential. Because the induced electrons are ap- proximately 31nm buried below the surface, there is Fig.2.Atomicforcemicrographofa300-diameterringforprobing no excess impurity scattering by surface states. The the coherence of one-dimensional electrons. The vertical bar is 10nm=division. elastic mean free path of narrow wires is the same as that for two-dimensional electrons; it is not de- graded by boundary scatterings, as evidenced by the measured nearly 100% specularity. In particular, the fabricated 15(cid:1)m-long wires show long elastic mean freepath(¿1:4(cid:1)m)andlongphasecoherencelength (¿3(cid:1)m) at 2K. This long characteristic transport length scale in conjunction with the short depletion lengthattheedges(¡15nm foreachside),indicate thatitisfeasibletode5neacompactquantumcircuit withinthecoherencelength. Acknowledgements ThisworkhasbeensupportedinpartbyONR,LPS, NSA,andARDA. Fig. 3. Atomic force micrograph of a single electron transistor. Theverticalleadsaredesignedforthesourceandthedrain.The References dot in the middle is the island. The bias applied to the two side gates is to modulate the tunneling current passing through the quantumdotisland. [1]M. Reed (Ed.), Semiconductors and Semimetals, Vol. 35, NanostructuredSystem,AcademicPress,SanDiego,1992. [2]G.Timp(Ed.),Nanotechnology,AIPPress,NewYork,1998. widths ranging from 5(cid:1)m to 100nm, the boundary [3]C.W.J. Beenakker, H. van Houten, in: H. Ehrenreich, D. Turnbull(Eds.),SolidStatePhysics,Vol.44,AcademicPress, scatteringisnearly100%specular.Asaresult,quan- SanDiego,1991,p.1. tum wires maintain a long elastic mean free path and [4]M.J. Yang, C.H. Yang, K.A. Cheng, J.C. Culbertson, Appl. alongcoherencelength. Phys.Lett.80(2002)1201.