Gad Bahir – Technion Nanotechnology Workshop Quantum Dots Infrared Photodetectors (QDIPs) Gad Bahir Collaboration: E. Finkman, (Technion) D. Ritter (Technion) S. Schacham (Ariel) P. Petroff (USCB USA) F. Julien (CNRS France) M. Gendry (Lyon France) Graduate students T. Raz M. Girzel N. Shual InAs InAlAs

Gad Bahir – Technion Nanotechnology Workshop Outline  Self assembled quantum dots  Infrared photodetectors from bandgap engineering to “ artificial atoms ” QWIPs vs QDIPs

Gad Bahir – Technion Nanotechnology Workshop What are quantum dots? A medium whose dimensions are of the order of the electron’s de Broglie wavelength  3D confinement LxLx LyLy LzLz L x, L y, L z  deBroglie Density of States

Gad Bahir – Technion Nanotechnology Workshop Self-Assembled Growth of Quasi-zero Dimensional Systems Frank-van der Merwe: 2d layer by layer Stranski-Krastanow: initial 2D growth leads to 3D island growth Vollmer-Weber: 3D island growth Increasing Strain AFM images of Surface InAs QDs  GaAs/InAs (UCSB-Technion)  InP/InAlAs/InAs (France-Technion)  SiGe/Si (France-Technion)  InP/InGaP/InAs (Technion) Wetting layer

Gad Bahir – Technion Nanotechnology Workshop MOMBE Growth of InAs/InP Quantum Dots 1.76 ML1.83 ML1.97 ML2.17 ML2.38 ML QD Density vs. InAs Nominal Thickness AFM image of single dot Tal Raz et al., PRB 2003 submitted

Gad Bahir – Technion Nanotechnology Workshop MOMBE Growth of InAs/InP Quantum Rings Tal Raz et al., APL 2003

Gad Bahir – Technion Nanotechnology Workshop QDs Structures Intra-band transition Inter-band transition Self organized islands are Formed after a few Monolayers of layer by Layer growth. Typical Dimensions: nm lateral size 5-8 nm vertical heights Barrier Wetting layer Substrate QD QW

Gad Bahir – Technion Nanotechnology Workshop QDs properties  The presence of a discrete energy spectrum distinguishes quantum dots from all other solid state systems and caused them to be called “ artificial atoms ”  The atom like properties make QDs a good venue for studying the physics of confined carriers and also could lead to novel device applications in the field of quantum computing, optics and optoelectronics.  These “ artificial atoms ” can, in turn, be positioned and assembled into complexes that serve as a new material. Single dot exciton spectra Gammon Science 1996

Gad Bahir – Technion Nanotechnology Workshop MWIR and LWIR Applications Thermal imaging, night vision, reconnaissance Chemical spectroscopy Optical remote sensing Atmospheric applications Medical diagnostics Vegetation recognition Fire fighting, Crime Prevention, Forensics Space-based Remote Sensing, Astronomy

Gad Bahir – Technion Nanotechnology Workshop Quantum Well IR photodetector – QWIP the Bandgap Engineering concept e - barrier well barrier well Band to band Intra-band Man made IR detector in wide band-gap semiconductor

Gad Bahir – Technion Nanotechnology Workshop QWIP structure Top view

Gad Bahir – Technion Nanotechnology Workshop QWIPs do not work with normal incidence light Complicated coupling technique

Gad Bahir – Technion Nanotechnology Workshop Noise mechanisms in QWIPs E vs. K || (c) (a) (b) Conduction band of multi-quantum wells structure (a)Tunneling (b)Field induced tunneling (c)Thermionics emission Recombination time ~1 p sec

Gad Bahir – Technion Nanotechnology Workshop From bandgap engineering to “artificial atoms” QWIP vs. QDIP QWIP Limitations:  Polarization Selection Rule – QE immediately limited to 50%  Short lifetime of photoexcited electrons – carriers relax back to the ground state before they can escape from the quantum well (~10 ps) QDIP (expected) Advantages:  3D Confinement - intrinsically sensitive to normal incidence photoexcitation  Much longer relaxation (~100 ps) / capture times ( “ phonon bottleneck ” ) - leads to increased gain and thus, higher responsivity and detectivity

Gad Bahir – Technion Nanotechnology Workshop Device structure and image AFM Image Device structure [1-10] [110] 2 electrons per dot

Gad Bahir – Technion Nanotechnology Workshop Front illumination QDIP photoconductive spectra as function of bias Finkman et al., PRB 2001

Gad Bahir – Technion Nanotechnology Workshop Polarization dependence [1-10] [110] Polarization dependence of 100 mV peak Dot shape and orientation [1-10] Front illumination pc signal Dual band detector with polarization selectivity Bound to continuum Bound to bound + tunneling Bahir SPIE 4820 (2002) [1 1 0]

Gad Bahir – Technion Nanotechnology Workshop I-V as function of temperature (dark current full line, background radiation 300K dashed line)

Gad Bahir – Technion Nanotechnology Workshop PC Spectra for various temperatures S. Schacham et al., PRB 2003

Gad Bahir – Technion Nanotechnology Workshop QDIP advantages over QWIP 1.Normal incidence absorption Normal incidence (without grating) was indeed observed 2.Phonon bottleneck Absence of phonon bottleneck in most experimental results. There is no advantage to QDIP over QWIP ? The QD does not “ work ” as an artificial atom and we have to consider strong interaction between carriers and lattice vibrations.

Gad Bahir – Technion Nanotechnology Workshop Compatition between tunneling and decay Following bound to bound excitation, electrons can either tunnel out and become free carriers or decay back. As the temperature is raised, the decay rate of the 100 meV signal increases due to increased LA phonon concentration while tunneling is independent of temp. Polaron formalism for coupling strength between electron and phonon. Bound to continuum 250 meV peak Bound to bound + tunneling

Gad Bahir – Technion Nanotechnology Workshop Model fit to temperature dependence S. Schacham et al., PRB 2003 The decrease of signal with temperature is associated with reduced polaron life time due to increased LA phonon population with temperature 100 meV

Gad Bahir – Technion Nanotechnology Workshop Conclusion  Unlike bulk material or quantum wells, the relaxation in QDs is not due to emission of one LO phonon, but is a results of multiphonon process.  There is no need for the two electron states to differ exactly by one LO phonon energy, i.e. no phonon bottleneck.  The atom-quantum dot analogy should not be carried too far: unlike electron in an isolated atom, carriers in semiconductor quantum dot, which contain a few thousands of atoms in a nearly defect free 3D crystal lattice interact strongly with lattice vibrations and in a unique way which should be studied.