72x36 Poster Template

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Resolving Leakage Currents In Strained Si/SiGe
Quantum Dots Jonathan HoffmanUniversity of
Wisconsin Platteville
Quantum Computing
Quantum Dot Structure
Results Aluminum Oxide Insulation
Aluminum Oxide Verification

• Quantum Bits (Qubits)
• Quantum computers use quantum mechanics to store
  and manipulate information.
• Quantum bits are analogous to classical bits in
  modern computing.
• Unlike classical bits, quantum bits are not
  limited to either 0 or 1, but can exist as a
  linear combination of both.

• Ohmic Contacts
• These contacts connect directly to the 2DEG a
  voltage can be applied across two of them to form
  a source and drain for the dot.

• An x-ray spectrum analysis was preformed on the
  deposited aluminum oxide as well as a sample of
  aluminum oxide, in order to determine if oxygen
  disassociation occurred.
• An atomic analysis of the spectrum showed that
  both samples had the same aluminum to oxygen
  ratio, indicating no loss of oxygen had occurred.

• Initial fabrication was unsuccessful the thin
  layer of titanium under the Schottky gates did
  not stick to the aluminum oxide, causing the
  gates to come off with the photoresist during
  lift off.

• Quantum Point Contact (QPC)
• Two contacts to which a negative voltage is
  applied to form a wave guide in the underlying
  2DEG and define the quantum dot (Gates 1,2,4,6,8
  in Fig. 5)

Si
Al
O
Fig. 21 Original Fabrication Procedure

• Schottky Gates
• Surface leads having no conduction path to the
  2DEG, used to influence the 2DEG using field
  effects. (Gates 1-8)

• A successful procedure for creating the Schottky
  gates on the aluminum oxide was determined. The
  palladium of the gates was applied directly to
  the aluminum oxide without a titanium buffer. The
  substrate was also cleaned using the RIE and a
  resist removing recipe.

• Loss-DiVincenso proposal
• A qubit based on lateral quantum dots in a
  semiconductor.
• Information is stored as electron spin.
• Logic operations can be achieved through
  interactions between electron spins in
  neighboring dots.

Al
C
O

• Radio Frequency Single Electron Transistor
  (RFSET)
• The RFSET is used as a charge sensor to measure
  charge fluctuations in the quantum dot.

Fig. 10 Sample Aluminum Oxide Spectrum
Fig. 9 Deposited Aluminum Oxide Spectrum
Fig. 22 New Fabrication Procedure
Device Fabrication
Results Increased Etch Depth

• Mesa
• Photolithography
• Develops a pattern of resist which will not be
  etched(mesa).
• Reactive Ion Etch (RIE)
• Plasma etches away the substrate and 2DEG around
  the mesa.
• E-Beam Evaporator Insulator Deposition
• Deposits an oxide in the etched area to prevent
  leakage currents.
• Ohmic Contacts
• Photolithography
• Develops a pattern exposing only the mesa pads.
• Buffered Oxide Etch
• Removes any oxides on the surface to ensure good
  contact.
• Thermal Evaporator Metal Deposition
• Deposits metal onto mesa pads.
• Strip Heater Annealing
• Metal diffuses through the silicon making a
  contact to the 2DEG
• Schottky Gate Leads
• Photolithography
• Aligns and develops a lead pattern to approach
  and run onto the mesa.

Source
1
2
Drain
Fig. 1 Loss-DiVincenzo Proposal Schematic
RFSET
3

• Etch depth was increased from 100 to 170
  nanometers.
• No oxide insulator was used between the Schottky
  gates and the mesa
• 11 out of 11 gates tested showed no leakage
  current.

Strained Si/SiGe Quantum Dots
Drain
Fig. 11 Etched Mesa
Fig. 12 Mesa Cross Section

• Two-Dimensional Electron Gas (2DEG)
• A 2DEG is a gas of electrons free to move in two
  dimensions, but tightly confined in the third.
• To form a 2DEG on a silicon wafer a few
  nanometers of silicon is grown on top of a layer
  of silicon germanium. The different lattice
  constants of silicon and silicon germanium causes
  the silicon to strain. This strain provides the
  essential conduction band offsets relative to the
  surrounding silicon germanium to form a quantum
  well. A layer of n-doped silicon germanium is
  then grown on top of the silicon carrier
  electrons from this layer populate the quantum
  well, creating the Two-Dimensional Electron Gas.

8
4
7
6
5
Fig. 13 RIE Schematic - Uses an electromagnetic
field to generate chemically reactive plasma
which selectively reacts with and destroys the
wafer surface.
Fig. 4 Quantum Dot Device Mesa
Fig. 5 Double Quantum Dot Pattern
Fig. 3 Si/SiGe Quantum Dot Device
Leakage Currents

• When energized, a current occurs between the
  metallic Schottky gates and the 2DEG under the
  surface.
• Leakage current interferes with quantum dot
  formation and measurement.
• Possible solutions include applying an aluminum
  oxide barrier under the gates, or etching deeper
  around the device mesa.

Fig. 23 Mesa New Etch Depth, No Oxide
Conclusion

• Increasing the depth of the mesa etch appears to
  solve the problem of leakage currents. This is
  most likely because the original depth did not
  fully etch away the underlying 2DEG.
• A procedure for aluminum oxide deposition was
  developed and tested. After deposition, it was
  shown to have the correct thickness and atomic
  makeup. A procedure for Schottky gate deposition
  on aluminum oxide was also developed and tested
  successfully.
• With these procedures it will be possible to
  create reliable quantum dots on silicon germanium
  with no leakage currents.

Fig. 15 Cross Section of Ohmic Contact
Fig. 14 Mesa and Ohmics
Fig. 16 Thermal Evaporator Schematic Passes a
large current through the source to evaporate the
material which is deposited on a sample suspended
in a vacuum.

• Step-graded layers, increasing from 5 germanium
  to 30, are used in order to allow relaxation to
  minimize defects.

Fig. 6 Voltage vs. Current in 10 Different
Schottky Gates
Aluminum Oxide Deposition

• Aluminum Oxide was deposited onto the substrate
  using an electron beam evaporator (E-Beam).
• Oxide depth was measured using atomic force
  microscopy measured depth was within 2 of
  intended depth.

• Benefits of Strained Si/SiGe over GaAs
• Increased coherence time of the spin information.
• Compatibility with semiconductor industry.

30 nm Al2O3
Acknowledgements
30 nm Al2O3
100 nm Al2O3

• Alex Rimberg, Timothy Gilheart, Joel Stettenheim,
  Feng Pan, and Mustafa Bal for their guidance.
• Ming Yun Yuan for her help with the aluminum
  oxide spectrum analysis, device fabrication, and
  leakage current testing.
• This project was funded by the NSF Research
  Experience for
• Undergraduates program and the DoD ASSURE Program.

Fig. 18 Cross Section of Lead Mesa Interface
Fig. 17 Mesa, Ohmics, and Schottky Leads
Fig. 8 E-Beam Evaporator Schematic

• The E-Beam evaporator uses a magnetic field to
  bend a beam of electrons into a crucible,
  evaporating its contents.

Fig. 2 Si/SiGe 2DEG material cross section and
band diagram
Fig. 20 CAD Dot Pattern (color signifies dose)
Fig. 19 Double Dot Pattern Done with E-Beam
Lithography
Fig. 7 3D AFM Plot of Deposited Aluminum Oxide