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Fermi Level Pinning of Si, Ge and GaAs - starts by asking tons of questions

Fermi Level Pinning of Si, Ge and GaAs - starts by asking tons of questions. Can Ef be bended?. No, Fermi level can not be bended. It can be pinned. Is Ef a bulk or a surface property?. Ef is a bulk property, not a surface property. What can influence the position and movement of Ef?.

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Fermi Level Pinning of Si, Ge and GaAs - starts by asking tons of questions

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  1. Fermi Level Pinning of Si, Ge and GaAs- starts by asking tons of questions Wei-E Wang

  2. Can Ef be bended? • No, Fermi level can not be bended. It can be pinned. Wei-E Wang

  3. Is Ef a bulk or a surface property? • Ef is a bulk property, not a surface property Wei-E Wang

  4. What can influence the position and movement of Ef? • Ef is simply a energy state which will give probability ½ for hole from valence band to conduction band • Ef vs CNL • Ef is influenced by the electron and hole concentration inside the bulk • Anything that perturbs e-h equilibrium will impact Ef position and movements • In Si crystal, defects are minimized in a way that the only thing that matters is dopant. • Therefore, Ef in Si is a sole function of dopant concentration • However, in III-V materials (or even Ge), this is not the case. These materials have intrinsic defects which perturb the e-h equilibrium Wei-E Wang

  5. Electrical bath of Si and III-V (Ge) Si Ef= f(dopants , intrinsic carriers) GaAs Ef= f(dopants, native defects, intrinsic carriers) Wei-E Wang

  6. Does MBE grown GaAs any different from bulk? • GaAs MBE is grown under high V/III ratio. • Proven thermodynamic equilibrium exists for all dopants (except Pb) Wei-E Wang

  7. Ef for Si vs III-V • Recalling again, Ef is a bulk property and it never bends • No need to worry about bulk when deals with Ef pinning in Si • Need to worry about bulk when deals with III-V and Ge • Ef can be pinned in III-V and Ge by two ways: by surface states or by bulk defects. • Ef for Si can only b pinned by surface defects (states) Wei-E Wang

  8. Is undoped GaAs an intrinsic semiconductor? • Probably not. There are many point defects in GaAs. The electroneutrality of GaAs at high temp is actually controlled by the charged native defects (VAs+). Wei-E Wang

  9. Why GaAs is harder to dope with n (donor)? • Some donors are black sheep and they like to gang up with a VGa- to form a complex (donor-VGa- ) which actually counter dope their own brothers. • e.g., TeAsVGa- andSiAsVGa- • Strong n+ annealing effect due to unsaturated VGa- . • At the crystal cools from growth temp to Rt, almost all the charged vacancies and interstitials are annihilated. Antisite GaAs appears to be very low. However, AsGa (EL2) ~ E16/cm3. This means that the surface density~ E10-11/cm2. Wei-E Wang

  10. Comparing Si, Ge and GaAs from Ef perspective • Si: Ef can only be pinned at surface (bulk is too good and can be easily passivated by hydrogen) • Ge: Ef can be pinned at surface, but weekly pinned in the bulk. Harder to be passivated by hydrogen though • GaAs: Ef can be pinned at both surface and the bulk. That’s the key reason why it’s so sticky (even at region of low Dit). It’s due to pinning inside the bulk. Ef can not move as freely as Ge and Si. Wei-E Wang

  11. About the GV model • Tau is function of x-section, temp and DE. • Capture X-section has a strong temp effect. In addition, more different type of defects which are not activated at low T will start to participate in the game at higher temp. • This means that the tau formula needs to be generalized Wei-E Wang

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