Electrical transport and charge detection in nanoscale phosphorus-in-silicon islands Fay Hudson, Andrew Ferguson, Victor Chan, Changyi Yang, David Jamieson, Andrew Dzurak, Bob Clark 24 th January 2006

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

Previous work on buried islands with leads Victors device comprises: - Diffused ohmic contacts - Implanted phosphorus leads - Implanted phosphorus island - Barrier control gates (B 1 and B 2 ) - Control gates (C 1 and C 2 ) 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 ~ nm V.C. Chan et. al., condmat/ 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

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, B 1 and B 2 V.C. Chan et. al., condmat/ G sd (10 -2 e 2 /h) Vsd = 0  V Vsd = 350  V

Scaling-down: nanoscale island with leads (no SET) Smaller devices: ~ few hundred P atoms (c.f. Victor’s ~ 10 4 ) Island implant aperture ~ 30 nm diameter Areal implant doses~ 8.5 × cm -2 (~ 10 x bulk MIT for P in Si)  600 P atoms per island ~ 4.2 × cm -2 (~ 5 x bulk MIT for P in Si)  300 P atoms per island Diffused ohmic contacts Buried leads Buried island Barrier gate (V b ) (too small for separate barrier gates) Control gate (V g ) d = 20, 40, 60, 80 nm (Victor’s devices ~ 80nm) 30 nm island 65 nm gaps Test implant mask 50 nm island nm gaps P implanted through mask

Fabrication (no SETs) 1  m20  m Bond pads (Optical lithography) Diffused ohmic Contact (Optical) Thin oxide region (Optical) Ion implanted source-drain leads and island (EBL) V b – barrier gate V g – control gate (EBL) Ti/Pt Alignment marks (EBL)

Ion implanted source-drain and island (pre-anneal) 50 nm island nm gaps 50 nm island nm gaps 50 nm island nm gaps no island 120 nm gap SEM images of device: post- implant, pre-anneal Dark areas show location of implanted 31 P + and damage in Si Can check fabrication and record individual dimensions 50 nm island nm gaps

Looking for gate dependent events Coulomb blockade seen in bias spectroscopy DC measurements (50 nm island, 35 nm gaps) 1  m VbVb VgVg SD Device turns on ~ 1.5V (looks like a MOSFET)

DC measurements (50 nm island, 35 nm gaps) 3 mV V sd (mV) 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 3 mV V g = 175 mV F.E. Hudson et. al., to appear in Microelectronic Engineering 2006

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 6 mV

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

Previous work on double island devices Demonstrated control over interdot coupling Voltage on middle gate, V M, 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) Previous work by Victor Chan: V.C. Chan et. al., paper in preparation V M = 0.81 V V M = 1.0 V

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

Si:P double island – device 1 (50 nm, 80 nm gaps) with SET 500 nm VLVL VRVR SET SET gate n, m n+1, m n+1, m+1 n, m+1 SET aligned centrally between dots Device 1 – 600 ions per island nm

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

Coupling between dots is much smaller than coupling of dots to gates (by x) 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 Si:P double island – device 1 (50 nm, 80 nm gaps) with SET 600 ions per island

Future work rf in dc offset 0.4 pF 100 pF SET 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 1) Combined SET and interdot coupling control 100 pF