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

2. 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

3. 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

4. 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

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

6. 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

7. 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 (-)

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

9. 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)

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

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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

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

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

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

23. 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 2.94 GHz @ 1.55 µm

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