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Nanodevices and Nanostructures – Quantum Wires and Quantum Dots

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Nanodevices and Nanostructures – Quantum Wires and Quantum Dots 1 Nanodevices and Nanostructures – Quantum Wires and Quantum Dots Wan-Ching Hung n L = ! (2) Abstract— This article describes how quantum dots and 2 quantum wires are integrating to semiconductor nanostructures Where L = the full width of the well and the single electron effects such as quantum confinement, n = a quantum number with the value of tunnel effect, and Coulomb blockade. In the end, several any positive integer. fabrication processes are introduced. = de Broglie wavelength ! Index Terms—Quantum dots, Quantum wires n2h2 E (3) n = 2 8mL Where En = the totoal engergy level. I. INTRODUCTION n = a quantum number with the value of quantum dot typically contains between 1 to 200 atoms any positive integer. A in diameter and its length, width, and high are generally h = a modified Planck’s constant defined less than 100nm. The key point determines whether a m =the mass of the electron. semiconductor nanostructure is a quantum dot or not is the When the isolation layer in transistor shrinks to nanometer motion of electrons having 0 degree of freedom. This is in size, the electrons could tunnel through the potential barrier. retrained by Fermi wavelength (1). The tunneling effect would cause transistors fail but it can also 2! "F = be used to develop scanning tunneling microscope. The ability kF (1) Where !F = the Fermi wavelength; wavelength of carriers that dominate electrical transport. kF = the Fermi wave vector. The Quatum wires confine the motion of electrons or holes to one spatial direction Fig. 2. Tunneling effect II. SINGLE ELECTRON EFFECTS of a single electron passes through barrier is called tunneling effect (Figure 2). When nanodevices work in quantum state, there are 3 In order to let single electron tunnel through one atom to effects: quantum confinement, tunneling effect, and Coulomb another atom, the electron must overcome the Coulomb blockade that can be observed. Quantum confinement effect blockade energy. The equation (4) defines the Coulomb occurs when one or more of the dimensions of the blockade energy. nanomaterial are smaller than the Fermi wavelength, the E = e2 / 2C (4) boundary conditions of electrons and holes are not infinite and c ! restricted in one or more dimensions. The equation (2), (3) and Where Ec = the Coulomb blockade energy, which is the repelling energy of the previous electron to the next electron. e = the electron charge C! = the capacitance. The Coulomb blockade is the increased resistance at small bias voltage of an electronic device comprising at lease one Fig. 1. Quantum confinement low-capacitance tunnel junction. The tunnel junction capacitor is charged with one elementary charge by the tunneling Figure 1 are the quantum confinement of electrons. electron. If the capacitance is very small, the voltage buildup can be large enough to prevent another electron from tunneling. The electrical current is then suppressed at low bias 2 voltages and the resistance of the device is no longer constant. quantum well to create nano devices such as single electron The increase of the differential resistance around zero bias is transistor and single electron memory. There are four called the Coulomb blockade. fabrication processes are currently used. The self-assembly method and chemical colloidal method are bottom-up approach. III. QUANTUM DOTS AND QUANTUM WIRES APPLICATIONS The quantum dot applications in various fields include A. Self-assembly method blue-laser diodes, single electron transistor (Figure 3), There are two methods to accomplish the process. One of light-emitting devices, etc. The single electron transistor the methods is called dip coating. It dips the material in to (SET) [1] which is based on Coulomb blockade and tunneling solution and washes away the part that is unwanted to form a effect is a single electron device in which the addition or film. The other method uses molecular-beam epitaxy or subtract of a small numbers of electrons to/from an electrode chemical vapor deposition to effectively form the quantum dot can be controlled with one-electron precision using the charge arrays on specific material under the theory of lattice effect. These quantum dot applications have the advantages of mismatch. It is a combination of techniques, where particle small size, low power consumption, and high speed. arrangement is controlled by differences in reactivity – a characteristic determined by exposing particles and surfaces to an assortment of chemical treatments. Solar cell, light-emitting diodes, and capsule in drug delivery system are using this process to fabricate. (Figure 5) Fig. 3. Silicon based single electron transistor The Blue-laser diodes are a kind of quantum dot lasers which succeeds in minimizing temperature sensitive output Fig. 5. Self-assembly method fluctuations, something that not possible with previous semiconductor lasers. The blue-lasers diodes are made of GaN and used in optical data communications and optical networks. B. Chemical colloidal method The commonly seen commercial product of blue-laser diodes The process is easy and can be used in mass production to is used as light source of High Definition DVD. produce multilayered quantum dots. An example for chemical A quantum wire application is nanobarcodes [2] which is colloidal method is to grow CdSe with sizes between 2~6nm. used in medical field. Nanobarcodes (Figure 4) are made CdSe is a material used to made quantum dot light emitting different quantum wires of different metals that have different diodes. Figure 6 is the process flow of how to cap for CdSe. reflectivity. Barcode readout is accomplished by bright field Nanobarcodes is also fabricated by this process. reflectance imaging, typically using blue illumination to enhance contrast between Au and Ag stripes Fig. 4. Nanobarcodes Fig. 6. Synthesis of CdSe IV. FABRICATION METHODS There are 2 ways to realize nanodevices. One of them is based on the current integrated circuits to minimize the line C. Abbreviations and Acronyms width. It is called “top-down’ approach. The electronic Besides photolithography which is the most commercial devices only shrink in size and the basic structure of electronic form, a large number of promising of promising devices do not change. The other way is called “bottom-up” nanolithography methods are including electron bream approach. It is totally different from the structure of current lithography, ion beam lithography, nanoimpring lithography integrated circuit and it uses quantum dot, quantum wire, and [3], and dip pen nanolithography [4]. 3 [6] Nanotechnology Knowledge Working Group, Nanotechnology Handbook, Nikkei Business Publications, Japan, 2003. Fig. 7. Dip pen nanolithography Etching is the process of using liquid acids or gas to dissolve away or remove unwanted material such as semiconductor material. Dry etching and wet etching are two commonly known processes in semiconductor fabrication. D. Split-gate approach It uses additional voltage to create 2 dimensional confinements to control the shape and size of the quantum dot’s gate. It means metal gates with a sub-micron sized gap between them are deposited onto a semiconductor substrate. (Figure 5) Fig. 8. The concept of split-gate technique[5] The black regions correspond to metallic gates and in the figure 5 on the left the gates are grounded and there is no effect on the underlying two dimensional electrons. In the figure 5 on the right, a negative voltage is applied to the depleting electrons underneath them and leaving a narrow region of electrons in the gap between the gates. The split-gate approach offers a number of advantages compared to other techniques available for the fabrication of nanostructures. Electrical contact to the nanostructure of interest is easily achieved. It’s a combination of electron beam lithography, evaporation, lift off, and contact annealing. However, this method is suitable for research. REFERENCES [1] K. K. Likharvey, “Single-Electron Devices and Their Applications”, Proc. IEEE, vol. 87, no. 4, pp. 606-632. 1999. [2] http://research.chem.psu.edu/cdkgroup/sensors.htm [3] http://www.princeton.edu/~chouweb/newproject/page3.html [4] http://www.chem.northwestern.edu/~mkngrp/dpn.htm [5] http://www.eas.asu.edu/~bird/ .
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