Origin of Coulomb Blockade Oscillations in SingleElectron Transistors

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Origin of Coulomb Blockade Oscillations in Single-Electron Transistors Fabricated with Granulated Cr/Cr 2 O

Origin of Coulomb Blockade Oscillations in Single-Electron Transistors Fabricated with Granulated Cr/Cr 2 O 3 Resistive Microstrips Xiangning Luo, Alexei O. Orlov, and Gregory L. Snider University of Notre Dame, Dept. of Electrical Engineering, Notre Dame, IN 46556 UNIVERSITY OF NOTRE DAME

Outline Ø Purpose: to understand single-electron devices with resistive microstrips instead of tunnel junctions

Outline Ø Purpose: to understand single-electron devices with resistive microstrips instead of tunnel junctions Ø Can single-electron transistor be built using only resistors with no tunnel junctions? Ø SETs with metal islands and resistive microstrips are fabricated and tested. Coulomb blockade oscillations are observed, but what is the origin of these oscillations? Ø Possible mechanisms for Coulomb blockade oscillations are investigated and discussed UNIVERSITY OF NOTRE DAME

Fabrication of Cr. Ox SETs by Two Steps of E -beam Lithography and Deposition

Fabrication of Cr. Ox SETs by Two Steps of E -beam Lithography and Deposition Ø Why two steps? To eliminate junctions! Ø First layer e-beam lithography and metal deposition define the Au electrodes and island (2 nm Ti and 10 nm Au) Ø The Cr. Ox resistive microstrips connecting the island to the electrodes are formed in the second e-beam lithography and deposition step. Ø Cr (8 nm-10 nm or 40 nm) was evaporated in the oxygen ambient. By controlling the oxygen pressure and deposition rate, different values of sheet resistance of Cr. Ox film were achieved. UNIVERSITY OF NOTRE DAME

Different Contact Designs (Type #1) Gate Cr Au Si. O 2 Island Cr Source

Different Contact Designs (Type #1) Gate Cr Au Si. O 2 Island Cr Source Drain Source Island Cr Drain Cr Cr Type #1: large tabs (wider than 300 nm in two dimensions) on both ends cover all of the steps where the two layers of metal overlap. Au Cr UNIVERSITY OF NOTRE DAME

Measurements on type #1 (tabs everywhere) SETs Ø Over 95% devices showed conductance at

Measurements on type #1 (tabs everywhere) SETs Ø Over 95% devices showed conductance at room temperature. Ø The Cr. Ox films were very uniform and lasted for a long time when exposed to air. Ø In the low temperature measurement (300 m. K) Ø R<2 kΩ/□, weak temperature dependence Ø 2 kΩ /□<R<7 kΩ /□, significant nonlinearities and a temperature dependence characteristic of variable range hopping were observed; however, none of the devices exhibited Coulomb blockade oscillations. Ø R>7 kΩ /□, all of the devices were frozen out, showing no conductivity below 5 KΩ. UNIVERSITY OF NOTRE DAME

Different Contact Designs (Type #2) Gate Au Cr Gate Cr Au Si. O 2

Different Contact Designs (Type #2) Gate Au Cr Gate Cr Au Si. O 2 Island Cr Source Drain Source Island Cr Cr Drain Cr Type #2: large tabs only cover the steps of source and drain and no tabs appear on the island. UNIVERSITY OF NOTRE DAME

Measurements on type #2 SETs Coulomb blockade oscillations were only observed in devices with

Measurements on type #2 SETs Coulomb blockade oscillations were only observed in devices with NARROW LINES touching the island The yield vs. resistance of type #2 devices Resistance range R<100 k <R<200 k <R<1 M R>1 M Total number of devices 12 9 5 4 Number of devices showed CBO 0 5 3 3 Yield 0% 56% 60% 75% Ø Coulomb blockade oscillations were only observed when the resistance of devices was greater than 100 k Ω. Ø Devices with higher resistance were more likely to show Coulomb blockade oscillations UNIVERSITY OF NOTRE DAME

Low Temperature Measurements (Type#2) (a) (b) (a) I-V curves of an SET in open

Low Temperature Measurements (Type#2) (a) (b) (a) I-V curves of an SET in open state and blocked state. (b) I-Vg modulation curve of the same SET of (a) measured at 300 m. K showed deep modulation by the gate. UNIVERSITY OF NOTRE DAME

Low Temperature Measurements (Type#2) Charging diagram of an SET measured at 300 m. K

Low Temperature Measurements (Type#2) Charging diagram of an SET measured at 300 m. K showed a charging energy of ~ 0. 4 me. V. UNIVERSITY OF NOTRE DAME

AFM Images (a) (b) Au island Cr. Ox wire Large tab Gate Cr. Ox

AFM Images (a) (b) Au island Cr. Ox wire Large tab Gate Cr. Ox wire with tabs Au island (a) AFM image of a Cr. Ox wire deposited on the edge of Au island. (b) The AFM image of an abnormal SET revealed that only two edges were covered by large tabs in the sample with a pattern shift. UNIVERSITY OF NOTRE DAME

Step Edge Junctions or Resistive microstrips with “right” resistance and capacitance? (b) Au island

Step Edge Junctions or Resistive microstrips with “right” resistance and capacitance? (b) Au island Cr (a) Au island Cr Si. O 2 Au Cr (c) Cr Si. O 2 Au island R>RQ C « e 2/2 k. BT Top view (a), cross section (b) of step edge junction, the areas where step edge junctions formed are marked by circles, and cross section (c) showing resistive microstrips with “right” resistance and capacitance. UNIVERSITY OF NOTRE DAME

AFM Image Island Gate Au layer Cr. Ox Gate The abnormal devices which had

AFM Image Island Gate Au layer Cr. Ox Gate The abnormal devices which had a very rough surface of Cr. Ox films. UNIVERSITY OF NOTRE DAME

Multiple Frequencies in I-Vg Modulation Curves Multiple frequencies in I-Vg modulation curve of abnormal

Multiple Frequencies in I-Vg Modulation Curves Multiple frequencies in I-Vg modulation curve of abnormal devices with a very rough Cr. Ox surface. UNIVERSITY OF NOTRE DAME

SETs with Thicker Cr. Ox Wires (Type #2) Ø SETs with thicker (~ 40

SETs with Thicker Cr. Ox Wires (Type #2) Ø SETs with thicker (~ 40 nm) Cr. Ox wires were also fabricated using pattern design type #2 with different widths of island (80 nm and 500 nm). Ø The room temperature sheet resistance of the devices showing significant nonlinearity in I-V curves at 300 m. K is around 5 kΩ/□, which is about the same as our previous SETs with thinner (8 -10 nm) Cr. Ox wires. Ø Among those devices having significant nonlinearity, about 95% (21 out of 22) exhibited Coulomb blockade oscillations, which is much higher than that of SETs with thinner Cr. Ox wires. Ø Tunnel barriers other than step edge junction formed at the interface of Au island Cr. Ox providing small enough capacitance and resistance lager than RQ to fulfilled the two requirements of Coulomb blockade oscillations UNIVERSITY OF NOTRE DAME

Low Temperature Measurements SETs with Thicker Cr. Ox Wires (Type #2) (a) (b) (a)

Low Temperature Measurements SETs with Thicker Cr. Ox Wires (Type #2) (a) (b) (a) I-Vg modulation curves of an SET with 40 nm thick Cr. Ox strips showed deep modulation by the gate. (b) Charging diagram of the same SET of (a) measured at 12 m. K UNIVERSITY OF NOTRE DAME

Low Temperature Measurements SETs with Thicker Cr. Ox Wires (Type #2) Temperature dependence of

Low Temperature Measurements SETs with Thicker Cr. Ox Wires (Type #2) Temperature dependence of an SET with thicker Cr. Ox wires UNIVERSITY OF NOTRE DAME

Low Temperature Measurements of a Cr. Ox Wire Crossed Over Two Au wires Au

Low Temperature Measurements of a Cr. Ox Wire Crossed Over Two Au wires Au Cr Source Gate Drain (a) Schematic of the layer of a Cr. Ox wire crossed over two Au wires. (b) Coulomb blockade oscillations observed on this structure at 300 m. K. UNIVERSITY OF NOTRE DAME

SETs with Pt as the First Layer Ø SETs with Pt instead of Au

SETs with Pt as the First Layer Ø SETs with Pt instead of Au as the first layer and thicker (~ 40 nm) Cr. Ox as the second layer were also fabricated using pattern design type #3. Ø Most of the devices showed significant nonlinearity in I-V curves at 300 m. K. Ø None of these devices showed any gate dependence. Ø More experiments are needed. UNIVERSITY OF NOTRE DAME

Conclusions Ø Two basic requirements to observe single electron tunneling effects: Ø the total

Conclusions Ø Two basic requirements to observe single electron tunneling effects: Ø the total capacitance of the island, C, must be small enough that the charging energy EC = e 2/2 C >> k. BT. Ø the resistance of the tunnel barriers, RT > RQ = 25. 8 k to suppress quantum charge fluctuations. Ø Resistive microstrip itself does not provide localization of electrons in the island - first requirement may not be fulfilled. Ø Two possible explanations: Ø Step edge “break junctions” with low C are formed at the connecting interface between Cr. Ox wires and Au wires Ø Microstrips with small overlapping area and high resistance may satisfy both requirements UNIVERSITY OF NOTRE DAME