High frequency photovoltaic ISB detectors in the near- and mid-IR SPIE Photonics West January 22, 2008 Daniel Hofstetter University of Neuchatel

Collaborations Uni NE Fabrizio R. Giorgetta, Esther Baumann CEA Grenoble Edith Bellet-Amalric, Fabien Guillot, Sylvain Leconte, Eva Monroy EPFL S. Nicolay, E. Feltin, J.-F. Carlin, N. Grandjean

Outline Introduction Optical transitions Infrared detectors GaN-based detectors for the near-IR Piezo- and pyro-electricity Spectral characterization / device optimization Results on high frequency testing Conclusions / outlook

IB / ISB transitions Interband transitions (IB) Transition energy determined by bandgap Lifetime on the order of 1 ns Opposite band curvature Intersubband transitions (ISB) Transition energy determined by QW width Lifetime on the order of 1 ps Parallel band curvature

Different types of ISB detectors ISB detectors QWIPs / QDIPs (photoconductors) QCDs (photovoltaic) Detectors based on optical rectification (photovoltaic)

First look onto materials Cubic arsenides grown on InP Hexagonal nitrides grown on sapphire Bandgap determines available conduction band discontinuity Bandgap vs. lattice parameter plot

Interest for III-nitride ISB transitions Large conduction band discontinuity (+) Short upper state lifetime (+) Potential high speed telecom devices (+) Heavy electron effective mass (-) Thin quantum well layers (-) Quality of AlN barriers (-)

GaN-based near-IR detectors Pecularities of the material Optical rectification Spectral properties High frequency testing

Piezo-electric properties No strain: centers of pos / neg charges coincide (crystal has no inversion center ! ) Strain: centers of pos / neg charges separate Microscopic dipoles are formed Macroscopic polarization occurs (pyro or piezo)

Crystal structure of III-nitrides Zincblende structure Cubic materials GaAs InP Cubic GaN Wurtzite structure Hexagonal materials ZnO SiC GaN

Optical transitions in III-nitride QWs Internal fields distort band structure IB transitions in the visible/UV ISB transitions in the mid-IR

Mechanism of optical rectification Excitation of e - leads to charge displacement Separation of charges => polarization Polarization => electric field => voltage

ISB band structure simulation Polarization fields change shape of QWs / barriers Large e - effective mass of 0.2 m e Very small layer thicknesses Maximal sublevel separation of 1.3 eV Monolayer fluctuations have a big effect Computed ISB wavelength vs. well thickness

Excellent material quality Generic layer structure 100 nm AlN cap AlN/GaN:Si SL 500 nm AlN buffer Sapphire substrate Layer properties Good reproducibility RMS roughness 2.7 nm Layer thicknesses in SL: 1.5 nm X-ray diffraction scans / SIMS analysis TEM cross-sectionAFM surface scan

Generic sample preparation Polish back and 45 ° wedges Deposit and structure SiN x isolation layer Evaporate Ti/Au contacts Dimensions: 100 µm x 100  m Separation 1 mm Typical sample preparation for absorption or photovoltage measurements

FTIR characterization Internal white light source (broad band) External cooled InAs detector (sensitivity) Common mirror / detector mount (lateral beam displacement) Voltage / current amplifier (feedback into FTIR) Schematic view of the experimental setup

ISB photovoltage spectra Confinement shift well visible Triple peak of thickest sample Reached 1.4 microns Measured photovoltage response of 4 different samples Baumann et al, APL, 2005

Series with AlN barrier thickness Different AlN barrier thicknesses Observe PV increase for thicker barriers Clear proof of NO resonant tunneling Signal vs. barrier thickness Hofstetter et al, APL, 2007

Room temperature ISB detector MBE growth Doping density 1e20cm -3 Cap layer thickness 5nm See ISB absorption and photovoltaic detection up to 300 K ISB absorption (direct and electro-modulated) Photovoltage at 300 K and 1.4 µm Hofstetter et al, APL, 2006

Improved performance at 300 K ISB absorption in TM polarization Small contacts for HF experiments Large contacts for spectral response Photovoltage signal 200 K

Experimental set-up Use direct modulation of laser diode Have large distance between pulser and detector amplifiers See response on spectrum analyzer

Frequency response Photovoltaic detection scheme 3 dB 0.2 GHz Maximum frequency of 2.94 GHz Observe still 7 dB at this frequency Frequency response Hofstetter et al, APL, 2007

Conclusions Growth of excellent short period GaN / AlN superlattices Testing of GaN-based PV detector for near-IR Demonstration of room temperature operation up to µm

Acknowledgements Professorship Program SNF NCCR Quantum Photonics SNF EU Project NitWave (#04170) ArmaSuisse