http://www.imdc.unsw.edu.au/ http://www.engineering.unsw.edu.au/energy-engineering/ Active Electronic Impurity Doping of Silicon Nanovolumes: Failure and Alternatives Dirk König Integrated Material Design Centre (IMDC) and School of PV and Renewable Energy Engineering (SPREE), University of New South Wales (UNSW), Sydney/Australia Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 1
Definition of Scope This presentation is about inducing electronic p- or n-type behaviour into ultrasmall Si nanovolumes such as nanocrystals (NCs), fins (for ULSI-FETs), nano-wires and -wells. It is not about • co-doping to improve luminescence properties • plasmonics which requires (semi-)metallic properties • chemical activation of solid surfaces, e.g. for catalysis which use doping densities in the ≥ 1 atom-% range Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 2
Outline 1. Conventional Doping, Theory 1.1 Broader Perspective 1.2 Dopant Formation Energies 1.3 Dopant Ionization Energies 2. Conventional Doping, Experiment 3. Phosphorus (P) in SiO 2 /Si NC Systems: h-DFT, APT, XANES 4. Interface Impact of Dielectric – Alternative 1 5. Excess Si (Ge) as Donor in Adjacent Barriers – Alternative 2 6. Modulation-Doped SiO 2 ; Preliminary Results – Alternative 3 7. Conclusions Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 3
1. Conventional Doping, Theory 1.1 Broader Perspective • macroscopic scale : Dopant, foreign atom segregation at high temperatures, cm range → Si float zone refinement, 1450 °C • microscopic scale : Dopant segregation to grain boundaries, µm range → Si solid phase crystallization, 600 to 800 °C • segregation anneal of Si nanocrystals (Si-NCs) in Si-rich SiO 2 or Si 3 N 4 carried out at 1050 to 1200 °C … What prevents doping of Si-NCs? # doping of Si-NCs requires energy (mechanical stress & surface tension, electrostatic interaction); NCs build up counter-stress → self-purification [1 − 6] # DBs saturating dopants (being inactivated) delivers energy → DB passivation at Si NC interface (||) by fully saturated dopants # ionization energy (E ion ) of dopants >> kT for NCs showing quantum confinement (QC) Theory Experiment [1] PRB 75 , 235304 (2007) [3] PRB 72 , 113303 (2005) [5] PRL 100 , 026803 (2008) [2] PRL 100 , 179703 (2008) [4] Nano Lett. 8 , 596 (2008) [6] PRB 80 , 165326 (2009) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 4
1. Conventional Doping, Theory 1.2 Dopant Formation Energies • small NCs: much increased dopant formation energy E form for many materials • atom size difference stress E form , triggering self-purification • E form (bulk Si) ≈ 0.1 eV [1] = k B T at T ≈ 900 °C, Si 146 BH 100 E form (Si NC) = 6 to 14 × E form (bulk Si) • studies so far do not include anions of dielectric or DBs which are likely to getter dopants d QD = 17.8 Å [2] [3] r r [1] PRB 75 , 235304 (2007) [2] PRL 100 , 179703 (2008) [3] PRB 72 , 113303 (2005) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 5
1. Conventional Doping, Theory 1.3 Dopant Ionization Energies • small Si NCs experience quantum confinement (QC) for d NC ≤ 2 r exc ≈ 9 nm • dopant is point defect (analogy to H atom) – QC only for d << 9 nm E ion (N D ) with d NC for donor on lattice site tiny ionization probability ( Ρ ion ) of dopants in Si NCs with notable QC, cf. [1] Si bulk, 300 K: Ρ ion = exp(-E ion /kT) = 0.15 with E ion (P as N D ) = 0.049 eV, Ρ ion increases further for N D ≥ 10 17 cm -3 [1] D. König, Chapter 8, Nanotechnology for Photovoltaic Devices, J. Valenta & S. Mirabella (Eds.), Pan Stanford, 2015 Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 6
Outline 1. Conventional Doping, Theory 2. Conventional Doping, Experiment 2.1 Free-Standing NCs 2.2 ULSI MISFETs 2.3 Sample Preparation Issues 2.4 Characterisation Strategy for Active Dopants 3. Phosphorus (P) in SiO 2 /Si NC Systems: h-DFT, APT, XANES 4. Interface Impact of Dielectric – Alternative 1 5. Excess Si (Ge) as Donor in Adjacent Barriers – Alternative 2 6. Modulation-Doped SiO 2 ; Preliminary Results – Alternative 3 7. Conclusions Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 7
2. Conventional Doping, Experiment 2.1 Free Standing NCs • conductivity, 270 K [1] : 1 / 3 / 500 for N D = 0 / 1.6 × 10 19 / 1.5 × 10 20 doping fails; 1.6 × 10 19 cm -3 one donor every 4 nm [1] Pearson, Bardeen [2] : Semiconductor – metal transition at N D ≈ 1.5 × 10 20 cm -3 ( ≈ 0.3 atom-%) • [P] EPR s.c. = P concentration at the NC after processing, incl. inactive donors • [P] EPR = P concentration which shows EPR signal of unpaired e – (built-in donor) as required condition for active doping; ≤ 10 -4 [P] nom [1] PRL 100 , 026803 (2008) [2] Phys. Rev. 75 , 865 (1949) [3] [3] PRB 80 , 165326 (2009) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 8
2. Conventional Doping, Experiment 2.2 ULSI MISFETs ultra-high density doping in Drain/Source (D/S) areas: dopant activation problem • dopant out-diffusion of 5 nm (flash anneal [FLA]) to 15 nm (spike RTA) [1] MISFET channel length diminished, or channel even shorted (spike RTA) [2] • dopant clustering and inactivation (spike RTA) [2] Entropy of lattice modification limits dopant formation: Profiles diffuse out ULSI: How can we introduce n- and p-type behaviour into ultrasmall Si nanovolumes without dopant clustering and out-diffusion ? [2], plasma doping + spike RTA [2], plasma doping + spike RTA [1] JAP 104 , 093709 (2008) [2] Nanotechnology 24 , 275705 (2013) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 9
2. Conventional Doping, Experiment 2.3 Sample Preparation Issues: Excess Si • Too much excess Si, forming a-Si/SiNC networks behaving like a-Si • In such networks, conventional doping works to some extent but # No control over Si NC properties (energy gap, charge storage, PL response) # low carrier mobility ( ULSI), high recombination rate ( Solar Cells) • TEM images only show NCs with right (low index) orientation to image plane • Full account of Si content requires ∈ -specific imaging methods: energy-filtered (EF) TEM, electron energy loss spectroscopy (EELS) or APT kMC = kinetic Monte-Carlo SIM Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 10
2. Conventional Doping, Experiment 2.3 Sample Preparation Issues: Excess Si • Si NC/SiO 2 SL, CV data presented as evidence for Si NC donor (P) doping [1,2] : # requires continuous SCR continuous Si NC / a-Si network # Oxide capacity C Ox with frequency due to low majority carrier mobility and defect density typical for a-Si; “higher Si% P doped sample” = ??? # separate Si NCs in SiO 2 behave very different [3] • similar issues with # SiO 0.66 /SiO 2 vertical superlattice (SL) and CV for NC doping with Boron [2] # SiO 0.7 NC SL solar 25 Si QD SL periods cell [4] : [1] E gap = 1.8 eV; [1] E gap (a-Si:H) = 1.6 … 1.7 eV ∆ C Ox [1] Phys. Stat. Sol. B 248 , 472 (2011) [2] JAP 118 , 154305 (2015) [3] JAP 113 , 133703 (2013) [4] APL 95 , 153506 (2009) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 11
2. Conventional Doping, Experiment 2.3 Sample Preparation Issues: Excess Dopant Densities Too much P, resulting in SiP x O y ternary oxide Pearson, Bardeen: Si semiconductor-metal transition at ≈ 0.3 at-% dopant concentration [1] • P concentration in most publications range from ≈ 1 [2] via ≈ 9 [3] to ≈ 12 [4] atom-%; • data from [4] below, using SiO 0.9 as pre-cursor for Si NCs in SiO 2 ; rectification ratio (current densities at +/- 10 V) ≈ 9 to 35 • What do Photolumines- cence (PL) data descibe? • How to obtain clear evi- dence for active dopants (yielding charge carriers)? [1] Pearson, Bardeen, Phys. Rev. 75 , 865 (1949) [2] APL 87 , 211919 (2005) [3] APL 92 , 123102 (2008) [4] APL 102 , 013116 (2013) Dirk König slide / 39 IMDC and SPREE, Eng. Faculty, UNSW 19 Nov 2015 12
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