Fabricatin and Characterizatin f Inverted GaAs/AlGaAs Heterstructures with Embedded InAs Wetting Layer Master Defence University f Basel 13. Januar 2009
Quantum infrmatin prcessing Quantum phenmena becme visible at the nanmetre scale A new degree f freedm the spin fr future applicatins The tw level system f the spin as candidate fr a qubit Cmplete cntrl f the spin state needed fr quantum cmputatin Physical implementatin: Lcalizatin f single electrns Islatin frm the envirnment Addressing f single spins Quantum dt (QD) Slide 2f 38
Quantum infrmatin prcessing Electrn cnfinement Lateral QD Self assembled (InAs) QD Vertically with a 2DEG Cnfinement depends n the shape Laterally with electrstatic tp Different band structure f InAs t gates GaAs http://www.springerlink.cm/cntent/0nu64r3047736q5g/fulltext.pdf Slide 3f 38
Quantum infrmatin prcessing Advantages Advantages InAs dts Lateral QDs Optical cntrl/readut f electrn spin Electrnic manipulatin Fast manipulatin f spin states Tunnel barriers tuneable (e.g. duble Lng distance interactins dt frmatin) (easier fr single addressing) Cmbine spin qubits with ptical techniques Electrn spin t phtn crrelatin Quantum infrmatin can be transprted ver large distances Hybrid quantum cmputer Slide 4f 38
Outline Inv 2DEG withut InAs dts and wetting layer Hybrid wafer (InAs wetting layer) AFM analysis, dt gradient Hall bar fabricatin Measurements (QHE, SdH) InAs dt lcalizatin Cnclusin Outlk Slide 5f 38
Inverted 2DEG Cupling InAs dts t a 2DEG Integrating InAs dts in an inverted 2DEG (n strain field t 2DEG, clser t surface) Wafer design Thickness f different layers Characterizatin f 2DEG prir t dt integratin InAs dts 2DEG Si dping Test cmplete fabricatin prcedure n prttype wafers Back gate Slide 6f 38
Inverted 2DEG (prepints) Tw different wafers withut InAs dts were grwn 2DEG spacer: 10 nm 30 nm 2DEG at 224 nm 2DEG at 130 nm Slide 7f 38
Measurements Use Hall bar gemetry t define vltage prbes Setup 3 He/ 4 He dilutin refrigeratr with magnet Fur terminal cnfiguratin S D Measure lngitudinal and Hall resistance Sweep magnetic field fr density, mbility measurements um Gates Ohmics White area: 2DEG Slide 8f 38
Results 10 nm spacer (prepint 1) 30 nm spacer (prepint 2) Pt/Au/Ge/Pt/Au (annealed at 530 C) R Ohmics 40 kω μ = 9400 cm 2 /Vs n = 2.1 e11 cm 2 R Ohmics 5.8 kω* μ, n nt meas. Imprtant t imprve the hmic cntacts Pt/Au/Ge/Pt/Au (50/100/55/40/50) nm Measurements at 4.2 K (*)AlGaAs dide: 60 s, 60 mw Slide 9f 41
Results 10 nm spacer (prepint 1) 30 nm spacer (prepint 2) Ni/Au/Ge/Ni/ Au (annealed at 500 C) R Ohmics 55 kω μ = 6800 cm 2 /Vs R Ohmics 25 kω* μ = 300 000 cm 2 /Vs Cnclusin: n = 2.6 e11 cm 2 n = 4.7e11 cm 2 Prepint 1: lw mbility (spacer t small, impurity scattering) Prepint 2: blcking barrier t small (n 2DEG) With Pt: hmics cnduct better (25%) fr 10 nm spacer wafer nly With Ni: mre hmics are usable especially fr 30 nm spacer wafer R Ohmics 1 kω nt yet reached Pt/Au/Ge/Pt/Au (50/100/55/40/50) nm Measurements at 4.2 K (*)AlGaAs dide: 300 s, 60 mw (mre hmics wrk) Slide 10 f 41
Hybrid wafer (pint 17) New wafer material with InAs dts available Tw pieces frm wafer with dt gradient B InAs dts at 137 nm 2DEG at 167 nm A 9 mm Slide 11 f 38
Stranski Krastinw Grwth Grwing InAs dts with MBE techinques (dt gradient) Surce: http://www.princetn.edu/~pccm/utreach/reu2004/s.%20lyon%20short%20course.pdf Slide 12 f 38
Hybrid wafer Atmic frce micrscpy (AFM) as a pwerful tl t lcalize the randmly self assembled dts Prir t AFM analysis Remve Gallium n back side with HCl Prtect frnt side with ebeam r UV phtresist Remve resist (remaining resist culd cver InAs dts) Ebeam: UV fld expsure, lift ff UV phtresist: develper Slide 13 f 38
Hybrid wafer Lw density (B) High density (A) Specificatins Width 200 nm Length 400 nm Height 1.5 2.5 nm Slide 14 f 38
Hybrid wafer
Hybrid wafer Very high dt gradient n piece A But wrng directin Cleaved side B A 1 mm frm cleaved side ~ 1200 dts/10 um^2 1.5 mm frm cleaved side ~ 10 dts/10 um^2 1.3 mm frm cleaved side ~ 200 dts/10 um^2 1.7 mm frm cleaved side ~ 1 dt/10 um^2 Slide 16 f 38
Hybrid wafer Dt gradient n whle wafer B d = 16.0 mm d A d = 11.75 mm Slide 17 f 38
Hybrid wafer Density prfile d (mm) 50 B 16.0 11.75 A 8.0 7.3 0 Slide 18 f 38
Hall bar fabricatin Test whle prcess with piece B (n dts, nly wetting layer) Optical image Define 2DEG: Mesa UV phtlithgraphy H 2 SO 4 :H 2 O 2 etching Slide 19 f 38
Hall bar fabricatin Ohmics UV phtlithgraphy Evapratin f (Au/Ge) 2 /Pt (120/60/120/60/85) nm Annealing f ally at different temperatures (60 s) Wire bnding t chip carrier Optical image Slide 20 f 38
Ohmics n hybrid wafer Measure resistance f hmics at 4.2 K T annealed fr 60 s vs. R Ohmics 440 C: nt cnducting 460 C: 13 kω 480 C: 9 kω 500 C: 15 kω Pt/Au/Ge/Pt/Au (530 C): R Ohmics 40 kω (Au/Ge) 2 /Pt (480 C): R Ohmics 9 kω Still nt R Ohmics 1 kω:what else can we d? Slide 21 f 38
Density and mbility R xx and R xy as functin f B perp and V back gate Density n and mbility μ at lw B fields (Drude mdel) n s = μ = 1, erh 1 ensρ xx, R ρ H xx = = dr db R xx xy ( B = 0) W L 20 mk Slide 22 f 38
Measurements: hybrid wafer Pint 17 with wetting layer (n InAs dts) Density and mbility at 20 mk n = 1.1 e11 cm 2 (R H = 5800 Ohm/T) μ = 1.4 e6 cm 2 /Vs (ρ xx = 40 Ohm) Illuminatin with AlGaAs dide (λ = 875 nm), 10 mw, 60 s n LED = 2.3 e11 cm 2 (R H = 2700 Ohm/T) μ LED = 1.6 e6 cm 2 /Vs (ρ xx = 17 Ohm) Slide 23 f 38
Measurements: hybrid wafer Quantum Hall effect at different T Plateaus if number f delcalized states is cnstant Slide 24 f 38
Measurements: hybrid wafer Shubnikv de Haas (SdH) scillatins and spin splitting Temperature dependence Only ne subband ccupied Spin splitting (Zeeman energy is higher than the Landau level bradening) Slide 25 f 38
Measurements: hybrid wafer SdH scillatins R xx = 0 in the regin f quantized Hall resistance, n scattering Number f Landau levels (filling factr) f the transversal resistance Slide 26 f 38
Measurements: hybrid wafer Electrn density tuneable with back gate n 1 er s =, RH = H dr db R H and ρ xx as a functin f V BG xy ns( V μ( V BG BG ) = ) = 1, erh 1, ensρxx R H = R xy ρxx = R xx ( V B BG ) ( B = cnst, V BG W ) L n 1.1 t 1.6 e11 cm 2 μ 1.4 t 1.9 e6 cm 2 /Vs Data incnsistent fr VBG 0 Slide 27 f 38
Measurements: hybrid wafer Ohmic cntacts limit back gate measurements Depletin Extraplatin f density data Cex = dq du = e dn du = 4.8 10 5 C 2 m V = 4.8 10 5 F m 2 C 1 5 F εε 0 = 4.38 10, 2 = 2.63 μm, ε = 13 2 DEG d m BG = d BG Estimate depletin vltage fr back and tp gate V BG, dep = 3.6 V VTG, dep = 230 mv Slide 28 f 38
Measurements: hybrid wafer Use anther quantum effect t cnfirm density data Frequency f SdH scillatins 2e SdH scillatins are peridic in 1/B with spacing Δ( 1/ B) = hns -1 15 2 With Δ(1/ B) = 0.394 T ns = 1.2 10 m, frm QHE ns = 1.1 10 15 m 2 Slide 29 f 38
Measurements: hybrid wafer Effective mass * ΔR ( B, T ) D( m, T ), xx * with D( m, T ) = fr ln(sinh( χ)) χ χ, sinh( χ) χ = * 2πk m T B heb Δ ln R xx ( B, T ) B T = C 1 + C 2 * m T Slide 30 f 38
Measurements: hybrid wafer Further analysis Transprt and quantum life time t determine main scattering mechanism (relevant if InAs dts present) Electrn g factr determinatin Slide 31 f 38
InAs dt lcalizatin AFM analysis t determine dt density Marker grid with ebeam lithgraphy Slide 32 f 38
InAs dt lcalizatin Phtluminescence analysis (Parisa Fallahi) Exact lcalizatin f dts Slide 33 f 38
Address single InAs dt Next step Quantum pint cntact with gate Slide 34 f 38
New mask design Thicker vltage prbes t 2DEG Zig zag hmics Crystal axis dependence, edge effects (larger cntact area)? R Ohmics 1 kω Slide 35 f 38
Cnclusin Surface analysis by AFM Imaging f self assembled InAs dts embedded in an inv 2DEG structure at 137 nm Lcalizatin f InAs dts t marker grid Quantum Hall effect and SdH scillatins at lw magnetic fields belw 1 K Analysis f inv 2DEG with InAs wetting layer High mbility Tuneable electrn density and mbility Resistance f hmic cntacts 1 kω with new mask design Slide 36 f 38
Outlk Change electrn density with tp gate Ohmics nt influenced Density and mbility measurements with wafer cntaining InAs dts Address single dt Charge sensing with QPC, tunnel rate measurements lateral QD cupling, etc. Slide 37 f 38
Acknwledgement C wrkers Zürich: Antni Badlat Parisa Fallahi Atac Imamglu Basel: Dminik Zumbühl Flrian Dettwiler Charulata Barge & rest f the grup Vreni Thmmen Slide 38 f 38