Nanoscale Phosphorus-in-Silicon Islands: Electrical Transport & Charge Detection

1. Electrical transport and charge detection in nanoscale phosphorus-in-silicon islandsFay Hudson, Andrew Ferguson, Victor Chan, Changyi Yang, David Jamieson, Andrew Dzurak, Bob Clark24th January 2006

2. Overview • Introduction to previous work by PhD student, Victor Chan • Large buried P islands with buried leads • Motivation for this work • Smaller islands, approaching few hundred atoms per island • Fabrication – e-beam lithography, phosphorus implantation through PMMA mask, activation of phosphorus, surface gates and SETs • Single island measurements – direct transport • Double island measurements – with SET charge detection • Future work and conclusions

3. Previous work on buried islands with leads • Experiments by Victor Chan • To learn about the phosphorus-in-silicon system • Also, to further confirm that fabrication works – P ion implantation through a mask, P ion activation • Victors device comprises: • Diffused ohmic contacts • Implanted phosphorus leads • Implanted phosphorus island • Barrier control gates (B1 and B2) • Control gates (C1 and C2) • Phosphorus implanted at 14keV to a depth of ~ 20 nm below the Si surface • Island contains a few tens of 1,000’s of phosphorus atoms, 500 nm x 100 nm • Gap between leads and island ~ 80-100 nm V.C. Chan et. al., condmat/0510373

4. Vsd = 0 mV Gsd (10-2 e2/h) Gsd (10-2 e2/h) Vsd = 350 mV Previous work on buried islands with leads • Experiments by Victor Chan • Coulomb blockade was observed (data left) • Tunnel barriers could be independently controlled via surface gates, B1 and B2 V.C. Chan et. al., condmat/0510373

5. P implanted through mask Test implant mask 50 nm island35-45 nm gaps 30 nm island65 nm gaps Scaling-down: nanoscale island with leads (no SET) Smaller devices: ~ few hundred P atoms (c.f. Victor’s ~ 104) Island implant aperture ~ 30 nm diameter Areal implant doses ~ 8.5 × 1013 cm-2 (~ 10 x bulk MIT for P in Si) 600 P atoms per island ~ 4.2 × 1013 cm-2 (~ 5 x bulk MIT for P in Si) 300 P atoms per island • Diffused ohmic contacts • Buried leads • Buried island • Barrier gate (Vb) (too small for separate barrier gates) • Control gate (Vg) d = 20, 40, 60, 80 nm (Victor’s devices ~ 80nm)

6. Fabrication (no SETs) Ti/Pt Alignment marks(EBL) 20 mm 1 mm Thin oxideregion(Optical) Bond pads(Optical lithography) Ion implantedsource-drain leadsand island(EBL) Diffused ohmic Contact(Optical) Vg – controlgate (EBL) Vb – barriergate

7. 50 nm island17-20 nm gaps 50 nm island35-45 nm gaps 50 nm island50-60 nm gaps 50 nm island65-75 nm gaps no island120 nm gap Ion implanted source-drain and island (pre-anneal) • SEM images of device: post-implant, pre-anneal • Dark areas show location of implanted 31P+ and damage in Si • Can check fabrication and record individual dimensions

8. Vb 1 mm S D Vg DC measurements (50 nm island, 35 nm gaps) Coulomb blockade seenin bias spectroscopy Looking for gate dependent events Device turns on ~ 1.5V (looks like a MOSFET)

9. 3 mV 3 mV Vsd (mV) Vg = 175 mV 3 mV DC measurements (50 nm island, 35 nm gaps) • Coulomb blockade peaks have period ~ 3mV • Also found at a higher barrier voltage range • Indicates a structure with constant capacitance – charging to a regular potential F.E. Hudson et. al., to appear in Microelectronic Engineering 2006

10. 6 mV Devices with different gaps • 20 nm device – shows weaker gate dependent events with similar period (6 mV) • All devices with ~20 nm gaps are conducting at zero barrier-gate voltages • All devices with >40 nm gaps are not conducting at zero barrier-gate voltage • Small change in gap size  large changes in device characteristics • Barrier control is important to adjust for fabrication variations

11. Previous work on double island devices with SETs Previous work by Victor Chan: Large (500nm) double-dot structure with two rf-SETs: van der Wiel et. al., Rev. Mod. Phys. 75 1 (2003) Double-dot hexagon cell structure was observed V.C. Chan et. al., paper in preparation

12. VM = 0.81 V VM = 1.0 V Previous work on double island devices Previous work by Victor Chan: • Demonstrated control over interdot coupling • Voltage on middle gate, VM, is increased • Interdot coupling is increased – seen is a separation of the triple points • Eventually, the two dots merge and exhibit single island characteristics (parallel lines) V.C. Chan et. al., paper in preparation

13. Implant test mask:50 nm islands 60 nm gaps Implant test mask:50 nm islands80 nm gaps Double island devices – fabrication • Make smaller islands, with few hundred electrons in each • Two surface control gates and use just one SET (no room for two) 50 nm 60 nm 50 nm 80 nm

14. n m Si:P double island – device 1 (50 nm, 80 nm gaps) with SET Device 1 – 600 ions per island SETgate SET n+1, m+1 n, m+1 n, m n+1, m VL VR 500 nm SET aligned centrally between dots

15. n, m+1 n+1, m+1 n, m n+1, m Si:P double island – device 2 (50 nm, 60 nm gaps) Device 2 – 300 ions per island SET aligned slightly closer to right dot (m)

16. Si:P double island – device 1 (50 nm, 80 nm gaps) with SET 600 ions per island • Coupling between dots is much smaller than coupling of dots to gates (by 50-100x) • Need control over tunnel coupling between dots • Difficult to fit tunnel barrier gates, plus SET, plus control gates… • Would be interesting to see the effect of tunnel coupling control - could float an rf-SET relative to buried structure to control tunnel barriers

17. Future work 1) Combined SET and interdot coupling control rf in 100 pF dc offset SET 100 pF 0.4 pF SET control gate • Add capacitor to one side of the SET – rf sees ground but dc floating • rf-SET is still operational and also a DC bias can be applied which acts as the tunnel barrier control gate • Increase the SET antenna size to cover barriers • Lose ability to bias the SET, but can operate normal (B ~ 0.5T) - no need for bias 2) Is it possible to make these devices smaller – towards observing quantum states? – with current fabrication, N ~ 50