1 / 25

Engineering of Disorder in MBE grown Ultra-High Mobility 2D Electron System

Engineering of Disorder in MBE grown Ultra-High Mobility 2D Electron System. Vladimir Umansky Braun Center for Submicron Research Weizmann Institute of Science, Rehovot, Israel. Collaborators: Moty Heiblum & group (Braun Center for Submicron Research)

taffy
Télécharger la présentation

Engineering of Disorder in MBE grown Ultra-High Mobility 2D Electron System

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Engineering of Disorder in MBE grown Ultra-High Mobility 2D Electron System Vladimir Umansky Braun Center for Submicron Research Weizmann Institute of Science, Rehovot, Israel Collaborators: Moty Heiblum & group(Braun Center for Submicron Research) Jurgen Smet & group (Max-Planck-Institut für Festkörperforschung, Stuttgart)

  2. Preface: 2DEG and Mesoscopic Physics Mobility: ~25,000 cm2/V∙s

  3. Electron mobility progress

  4. Outlook • 2D Electron Gas - basics • DX centers – why we are lucky to have them? • How to observe 5/2 quasiparticles ? • New ideas for band gap engineering • Ultra – High Mobility. Is it enough ? • How to control disorder ? • Conclusions

  5. 2DEG in AlGaAs/GaAs 2DEG in AlGaAs/GaAs -scattering Illumination Background Impurities Remote Ionized Impurities GaAs AlGaAs(x~0.3) T<1K Doping Spacer (d) BG RI 2DEG ΔEc E0 EF 2DEG Total Depth (D)

  6. DX centers Gates Delta or uniformdoping 30-40% AlGaAs spacer Shallow donor 2DEG Pure GaAs DX center The “standard” 2DEG structure: In the dark: Pros: Frozen charge (in the dark) allows gating Cons: Low doping efficiency (in the dark) → high RI scattering After Illumination in the dark: Pros: Almost double density after illumination → high mobility. Cons: Parallel conduction/gate instability.

  7. Applications 5/2 Gateable 2DEG: QDs, QPC, Spin-pump, Quantum shot noise, etc… Deep structures Measurements after illumination Shallow structures Measurements in the dark

  8. 5/2 in the “standard” 2DEG Data from ~1998 “Standard” Al0.36Ga0.64As/GaAs 2DEG Mobility: ~14 ×106 cm2/V∙s Density: 2.2 ×1011 cm-3 Measurements: After illumination Ungateable 5/2

  9. How to Achieve Ultra-High Mobility ? Background Impurity Scattering MBE system design Raw materials (i.e. Gallium (7N) → 2÷5×1015 cm-3 ) (*) Optimal growth conditions (rate, temperature, III/V ratio, etc…) Optimal 2DEG structure design Optimal growth sequence design (*) background impurity density ~ 1×1014 cm-3 limits mobility by ~1÷2 ×106 cm2/V∙sec

  10. Double – Side Doping ns* d d EF E0 2DEG Total Depth (D) W For the same spacer width: Concern: Interface scattering in QW → Inverse interface Used first by L. Pfeiffer to produce samples with > 30 ×106 cm2/Vsec

  11. Doping in Short Period Super-Lattice 6ML AlAs Γ X ~250 meV 9ML GaAs Higher transfer efficiency Higher mobility due to better screening by X electrons No parallel conductance due to ~3 times shorter Bohr radius Short Period Super-Lattice - SPSL

  12. Results on Electron Mobility e e RIBER MBE32 machine Uniform Doping in Al0.35Ga0.65As EF 2DEG SPSL d-doping ~36x106cm2/V·s 2DEG in QW EF SPSL d-doping

  13. Is Mobility a Relevant Parameter for FQHE ?

  14. BG scattering vs RI scattering uniform doping EF 2DEG SPSL d-doping EF EF Spacer 80 nm BG limited mobility ~ 16 ×106 cm2/V∙s For spacer > 80 nm contribution of RI scattering < 13÷15 %

  15. Mobility, Disorder & FQHE BG BG BG RI Disorder In high mobility 2DEG the main scattering mechanism – BG scattering BG impurities ~1013 cm-3 in 30 nm QW→ average distance ~2 mm RI disorder potential characteristic length → spacer → ~80÷100 nm FQHE is governed by RI induced disorder

  16. How to control the RI disorder? Over-doping: Freeze-out temperature: (Efros A.L. 1988) Introduce Spatial Correlations between Ionized Donors !!! g ~ 20

  17. Over-doping & FQHE e e SPSL d-doping EF 2DEG Uniform Doping in Al0.35Ga0.65As Concern: Over-doping leads to “Parallel” conductance Minimal Doping ~2×1011 cm-2 Average distance between donors ~200 Ǻ Bohr Radius for X-electron 20÷30 Ǻ → over-doping of ~ 2÷5 times looks feasible

  18. Application for 5/2 EF SPSL d-doping

  19. Measurements of ¼ electrons charge

  20. There’s no such thing as a free lunch Double side doped 2DEG n~(3.0÷3.3)×1011 cm-2, m~(29÷33)×106 cm2/V∙s g ≈ 2 g ≈ 2.5 g ≈ 2.3 5/2

  21. Phase transition in Donor layer (s) B g ≈ 2 +1 +2 0 0 g ≈ 2.3 g~2.3 g~1.1

  22. Phase Transition in Disordered 2DES QPC

  23. Ideal 2D system for mesoscopic device Ultra-high purity 2DEG Spatially correlated 2D electron system However, frozen at low T

  24. Engineering of Disorder: Doping Schemes Shallow donor DX center • Using another AlAs-GaAs SPSL for doping • Using multiple doping layers in SPSL • Using “shallow” DX centers in AlGaAs

  25. Conclusions • High mobility (low total scattering rate) is just a precondition to obtain very low disordered 2D systems. • FQHE is governed by RI induced disorder • Spatial Correlations of Remote Ionized Donors are necessary to obtain perfect 5/2 FQHE

More Related