How to measure packaging-induced strain in high-brightness diode lasers? Jens W. Tomm Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie Berlin Max-Born-Str. 2 A, D-12489 Berlin, Germany Outline 1. Introduction 1.1 Experimental 1.2 Approach 2. Experimental methods and results 2.1 Micro-Photoluminescence (µpl) 2.2 Photocurrent spectroscopy (PCS) 2.3 Degree-of-polarization PL or R (DoP-PL) 3. Summary Analysis of the electronic bandstructure 1
1.1 Experimental: High-brightness diode laser arrays: cm-bars 1 cm Best values reached so far: cw-output from a single bar 928 W H.X. Li, et al. SPIE Proc. 64560C-1-9 (2007). conversion efficiencies > 73% http://www.jold.de/ nlight 76% (2007) 2
1.2 Approach Strain creation within the structure or device, e.g., during 1. crystal growth 2. processing Epitaxial layer Substrate FEM-simulation according to A. Borowiec et al. Appl. Phys. Lett. 83, 225 (2003). intrinsic or built-in strain extrinsic processing-induced strain 3
3. creation of native point defects {110} extrinsic strain caused by defect creation 4
3. creation of native point defects {110} 4. packaging solder {110} 1 cm cm-bar Substrate α GaAs = 6.9 10-6 K -1 Uniaxial pressure along a <110> direction Epitaxial layer sequence Heat sink soldering temperature > 157 C extrinsic strain caused by defect creation α Copper = 16.7 10-6 K -1 extrinsic packaging-induced strain Spectroscopic strain analysis by checking the electronic bandstructure 5
Outline 1. Approach and Definitions 2. Experimental methods and results 3.1 Micro-Photoluminescence (µpl) 3.2 Photocurrent spectroscopy (PCS) 3.3 Degree-of-polarization PL or R (DoP-PL) 3. Summary 6
Micro-PL: Methodology Thickness 100 µm Excitation Laser bar Active region Length of cavity 1.5 mm PL-signal (a.u.) Wavelength (a.u.) Length 1cm MOTORIZED PRECISION GONIOMETER 1000 spectra per bar PL-peak position is extracted by a polynomial fit. 7
Plot: Spectral position of the PL-peak position versus local position along a laser bar 850 λ Raw PL wavelength shift (nm) 849 848 847 846 845 844 P. Martin et al. Appl. Phys. Lett. 75, 2521 (1999). 843 0 2000 4000 6000 8000 10000 Laser beam position (µm) Stress assessment of a mounted bar by the total bent of the curve. 8
Photocurrent spectroscopy (PCS) Photoluminescence (PL) Absorption 1,0 30000 0,8 25000 classical optical spectroscopy PL signal (a.u.) 0,6 0,4 0,2 0,0 1,4 1,6 1,8 2,0 2,2 photon energy (ev) α (cm -1 ) 20000 15000 10000 5000 1,4 1,6 1,8 2,0 2,2 photon energy (ev) We can not place a detector behind the device. PL or EL emission Absorption ~ Photocurrent! Device analysis 9
Different heat sinks compressed solder 1 cm cm-bar Substrate α GaAs = 6.9 10-6 K -1 Epitaxial layer sequence solder 1 cm cm-bar Substrate tensed Epitaxial layer sequence Heat sink α Copper = 16.7 10-6 K -1 α Heat sink Diamond = 1.0 10-6 K -1 Energy (ev) 1.550 1.548 1.546 1.544 1.542 1.540 1hh-1e 2.0 mev 2.5 mev 1.538 0 2 4 6 8 10 local position (mm) 1.608 1.606 1.604 1.602 1.600 1.598 1lh-1e hh- by lh-ratio=0.32 Energy (ev) 0.73 6.3 mev 3.4 mev 1.596 0 2 4 6 8 10 local position (mm) k*p perturbation theory Compression 0.36 Tension 0.67 10
1. Deformation along <110> u/u -0.0015-0.0010-0.0005 0.0000 0.0005 0.0010 0.0015 0 2 4 6 8 10 local position (mm) Copper Diamond Estimate for maximum strain: Soldering temperature: T s =157 C Thermal expansion coefficients: α GaAs = 6.9 10-6 K -1 α Copper = 16.7 10-6 K -1 α Diamond = 1.0 10-6 K -1 x/x = (α Device - α heat sink ) (T s -T ambient ) Copper x/x = - 0.00134 Diamond x/x = 0.00077 2. At the edges, the devices are unstrained. 11
Degree-of-polarization PL or R (DoP-PL) Strain fields of the grooves very nicely visible Strain (%) -0.12-0.10-0.08-0.06-0.04-0.02 0.00 0 2 4 6 8 10 Local position (mm) Strain (%) -0.12-0.10-0.08-0.06-0.04-0.02 hh1-e1 lh1-e1 0.00 0 2 4 6 8 10 Local position (mm) Tomm et al. Appl. Phys. Lett. 88, 1335504, 1-3 (2006). Check by PCS 12
3. Summary: Spectroscopic strain measurement is an indirect method. It uses the energetic bandstructure of quasi-particles in semiconductors. Practically mostly the electronic bandstructure is investigated (exception: built-in strain) Each method provides useful but incomplete (!) information. If you want to know something special and know what you expect: Apply the right method. If you do not know what you expect: Make a concerted approach and try to understand the puzzle. You can t fully determine all tensor components by measuring scalars. ISBN: 0071460322 13