Physics and Material Science of Semiconductor Nanostructures PHYS 570P Prof. Oana Malis Email: omalis@purdue.edu Course website: http://www.physics.purdue.edu/academic_programs/courses/phys570p/ 1
Course overview Review of semiconductor physics band structures for 3D, 2D, 1D, and 0D Growth and fabrication of semiconductor nanostructures Electrical transport in semiconductor nanostructures Optical properties of semiconductor nanostructures Nanophotonic devices (lasers, detectors, etc.)
Textbooks T. Ihn, Semiconductor Nanostructures: Quantum States and Electronic Transport, Oxford (2010). John H. Davies, The Physics of Low Dimensional Semiconductors: An Introduction, Cambridge (1998). S. L. Chuang, Physics of photonic devices, Wiley 2009.
Textbooks References for background on semiconductors P.Y. Yu and M. Cardona, Fundamentals of Semiconductors. M. Balkanski and R.F. Wallis, Semiconductor Physics and Applications. N.W. Ashcroft and N.D. Mermin, Solid State Physics. C. Kittel, Introduction to Solid State Physics. J. Singh, Electronic and Optoelectronic Properties of Semiconductor Structures, Cambridge Univ. Press, 2003 P. Harrison, Quantum Wells, Wires, Dots, 2nd Ed., Wiley, 2005.
Nanotechnology mesoscopic 100 m 10 m 1 m 100nm 10nm 1nm 0.1nm Length scales in semiconductors (SC s) Mean free path: Coherence length: F of bulk: 10 Lattice constant: 1 ~ 10 2 nm Effective Bohr Radius: (see Electronic transport in mesoscopic systems, S. Datta, Cambridge Univ. Press) 10 1 ~ 10 1 ~ 10 nm 0 nm E E F k F a 2 1 2 F
bulk Aspects of Nanostructures Nano-Technology Room temp. kt~25mev dn/de (density of states) E Semiconductor nano-technology, Material engineering, etc ~100meV (for GaAs) 10nm Nano-scale Fundamental Interest Band structure, Many-body physics, Quantum optics etc.. Quantum-dot lasers, Photodetectors, Single electron devices, Single photon devices, Quantum computing, etc. Advanced Applications
Nanotechnology: Size Effects and Applications Quantization of electronic band structure Electrical and optical properties are altered Increase in surface area to volume of materials Novel mechanical properties of nanomaterials Applications: Nanoelectronics more transistors to be packed on a single chip High electron mobility due to symmetric and uniform nanostructures Energy conversion High light conversion efficiency solar cells with a continuum bandgap Nano-optics New light source in the mid-infrared region, the finger print region of most gases Consumer goods high strength, lightweight textiles that contain Carbon Nanotubes 7
Nanostructures Quantum wells Nanowire Quantum dots Carbon Nanotubes (CNT) Buckyball 8
Semiconductor nanostructures Gate-defined dot 1 m~100nm Mesa-etched dot 1µm~100nm Self-Assembled Quantum Dots ----- ----- -+ ~20nm ~20nm Quantum ring Three-dimensional STM image of an uncovered InAs quantum dot grown on GaAs(001). J. Marquez, et al, Appl. Phys. Lett. 78 (2001) 2309.
Semiconductor nanostructures Colloidal nanocrystals ~ few nm
Carbon nanotubes: One dimensional system (Courtesy Cees Dekker, Delft Institute of Technology, the Netherlands.) This research was reported in the 7 May 1998 issue of Nature. Here are some real-world nanotube materials, produced by laser ablation of a graphite target containing metal catalyst additives. On top is an atomic force microscopy image of a chiral tube with a diameter of 1.3 nanometers (Technical University, Delft: www.pa.msu.edu/cmp/csc/nanotube.html).
Observation of Nanostructures Scanning Electron Microscope (SEM) 10-40kV Electron beam Resolution>10nm * * See, for instance, University Physics, by Harrison Benson, John Wiley & Sons, Inc.
Observation of Nanostructures Transmission Electron Microscope (TEM) Electron beam 50-100kV diffraction Resolution>0.5nm
Observation of Nanostructures Scannning Tunneling Microscope (STM)* * Nobel prize in 1986 I=const Resolution: 0.001nm (vertical) 0.1nm (horizontal) Three-dimensional STM image of an uncovered InAs quantum dot grown on GaAs (001). J. Marquez, et al, Appl. Phys. Lett. 78 (2001) 2309.
Semiconductor Quantum Wells (QWs) A narrow gap semiconductor is sandwiched between layers of a wide band gap semiconductor Quantum confinement takes place when the well thickness is comparable to the de Broglie wavelength of the particle Electron movement is confined in the quantum well growth direction Examples: GaAs/AlAs, InGaAs/AlInAs. 15
Growth of Quantum Wells Molecular Beam Epitaxy (MBE) slow deposition rate, typically less than 1000 nm per minute films to grow epitaxially evaporated atoms do not interact with each other or any other vacuum chamber gases until they reach the wafer 16
Semiconductor growth: Molecular Beam Epitaxy Prof. Manfra s GaN and GaAs MBE machines at Purdue Device fabrication at the Birck Nanotechnology Center
Application of QWs Diode Laser Light Electrode Light Electrode p-algaas GaAs n-algaas Conduction band Band gap +V Valence band Disadvantages: Emission wavelength depends on material Very difficult to generate more than one color per laser Difficult to generate long wavelength, i.e., colors in the mid- to farinfrared region 18
Quantum Laser Generation Quantum Effect Conventional laser High band gap material Low band gap material Electron High band gap material Conduction band Photon Quantum Laser Injector Active region Injector Conduction band Sub-band Band gap Valence band Hole Thickness is bigger than wavelength Advantages: Band gap Sub-band Wavelength depends on layer thickness (flexible design); Use well-mastered materials for long wavelength Layer thickness Thickness must be smaller than wavelength Make it possible to generate multiple colors in same laser Sub-band 19
Quantum Cascade Laser (QCL) Generation ħω ħω ħω Cross Section of a QCL: Note that the layer thickness is smaller than the wavelength One layer Electric field Cascade effects One electron emits N photons to generate high output power Typically 20-50 stages make up a single quantum cascade laser 10 m Dime coin Quantum cascade laser 20
Light Emission in Mid-Infrared Region Sun Part of the Spectrum Wave Length (µm) 0.3 0.4 UV 0.5 VIS 0.6 0.7 1.5 NIR 2.0 3.0 10.0 20.0 30.0 MIR (3~30 µm) 40.0 (FIR) 50.0 The wave length of most of quantum cascades laser lies in the mid-infrared region (3~30 µm) Most gases with strong light absorption in mid- Infrared region: CO, NH 3,, NO, SO 2,, etc. 21
Gas Detection Using Quantum Cascade Lasers Trace gas detection principle Gas mixture Laser diode driver Temperature monitor Quantum Cascade Laser (QCL) Mid-Infrared detector Pulse Generator Quantum cascade laser generator Room Air Trigger Data Acquisition To Vacuum PA detector PC Signal Trace gas detection system (Kosterev et al. 2002) Trace gas detection functional diagram 22
Results For Gas Detection 0.6 Wavelength ( m) Laser frequency and absorption percentage CO concentration, ppm 0.5 0.4 0.3 0.2 0.1 11:20 13:20 15:20 17:20 19:20 21:20 Time (hh:mm) Detected CO concentration at different time From Gmachl, et al. 2003 23
Multi-Wavelength Emission by Quantum Cascade Lasers Wave function Well material Ga x In 1-x As Barrier material Al x In 1-x As Fundamental wavelength: 9.2 µm SHG wavelength: 4.6 µm 24
Quantum Well Infrared Photodetector (QWIP) Physical model for QWIP Packaged QWIP chip 25
Quantum Dot Solar Cells Au grid bar 200 nm n + GaAs 0.5 µm intrinsic region 30 nm n GaInP 100 nm n GaAs UD GaAsP InGaAs QDs UD GaAsP InGaAs QDs UD GaAsP InGaAs QDs UD GaAsP 100 nm p GaAs p + GaAs (311)B substrate Au contact Lattice parameter GaAsP/InGaAs QD layers CB IB VB h(ν 1 + ν 2 ) or hν 3 E CI E IV Absorption is increased due an intermediate band created by quantum dots QD Solar Cell design 26
Possible Applications Single electron transistor, quantum computation, QW, QD and QC lasers Terahertz radiation Quantum dot infrared photodetectors, QDIPs Optical memories Single-Photon sources Bio-sensing
Issues and Challenges in Semiconductor Nanostructures 1. Nanostructured Materials by design The ability to measure, control and restructure matter at the nanoscale in order to change those properties and functions 2. Nano-manufacturing Assembling nanoscale devices in high rate processes that are reliable and environmentally friendly 3. Toxicity 4. Stability 28
The American Physical Society (APS) No matter what their research area Every Physics Graduate Student, + every undergrad who wants to go to graduate school should join the APS!! The first year's membership is FREE to students & the following student years are highly discounted!
The Materials Research Society (MRS) Graduate students working in Solid State, Condensed Matter, or Materials Physics should also consider joining the MRS!! The MRS is another large professional organization, but it has a very interdisciplinary membership. This reflects the fact that people with many different backgrounds are doing various kinds of materials research. For example, it has members with backgrounds in Physics, in Chemistry, & in various types of Engineering.