Efficient Light Scattering in Mid-Infrared Detectors Arvind P. Ravikumar, Deborah Sivco, and Claire Gmachl Department of Electrical Engineering, Princeton University, Princeton NJ 8544 MIRTHE Summer Symposium June 15 16, 215
Selection Rules The Bane of Intersubband Detectors Intersubband transitions transitions within a quantum well Conduction band offset (ΔE c ) E c Intersubband Material B Interband Material A Material A Normal angle absorption inherently not possible. E v Z 5 8 K Z Photocurrent (pa) 4 3 2 1 45 9 135 18 (TM) (TE) (TM) Polarization Angle
Normal Incidence Absorption Grating Coupled Photonic Crystals Corrugated-QWIPs Other lesser-known techniques: o Hole intersubband devices (band-mixing) o Quantum dot based infrared intersubband detectors Disadvantages: o Wavelength dependent each technique designed for a specific detector o Complicated fabrication techniques
Conventional Coupling Techniques: Limits 45 degree incidence Best case scenario Brewster s angle incidence (~ 17 ) I r θ 1 I n 1 R p = n 1 CCC θ 2 n 2 CCC θ 1 n 1 CCC θ 2 + n 2 CCC θ 1 T p = 1 R p 2 485 µm I t θ 2 n 2 I t =.5 T p SSSθ 2 I I t =.5 T p SSS θ 1 n 2 I 31 µm At Brewster s incidence, T p = 1 and θ 1 = θ B, we get, I t ~.15 I For current measurement conditions (θ 1 = 45 ), we get I t ~.93 I Coupling can be improved with gratings for normal incidence!
Can Scattering Be Used For Absorption? Can we use the sloped side walls of a wet-etched mesa for light transmission? If so, how much light will be absorbed? Assumptions wet-etched ridges are in close proximity. We will consider the first (I t (1) ) and the second (It (2) ) transmission. I π 2 θ i I t (1) = I π 1 R p SSS θ n dθ I t (2) I t (1) π 2 I t (2) = I π R p 1 R p SSS θ n dθ
Normal Incidence Absorption With Spirals And Waves Modeling sidewall slope as a quadrant of an ellipse, we get a total transmission, I t ~.15 I. Increase the number of sidewalls for light absorption SPIRALS and WAVES Side-view A oo Figure of merit can be given as, FFF = I t A e and the electrical device area, respectively. where A oo and A e are the optical area
Scattering Assisted Absorption Devices ~.5 mm diameter spirals For comparison, use standard mesa devices 485 µm 31 µm
Quantum Cascade Detector QC Detectors are unipolar photovoltaic infrared detectors. Detection wavelength is decoupled from the conduction band offset..4 E c A 2 B 1 A 1 C 1 D 1 F 1 Energy (ev).3.2.1 6. µm 6.5 µm 1 period ~ 3 nm Quantum Cascade Detectors. -.1 -.2 4 5 6 7 8 9 1 Position (Angstroms)
QC Detector Characterization Standard device geometry measured at 45 degrees Dark current measurements device resistance, noise properties o Room temperature 5 Ω Current (A) 1E-1 1E-2 1E-3 1E-4 1E-5 1E-6 Standard Mesa 3 K 8 K Eact ~ 8 mev Photocurrent (a.u.) 7 6 5 4 3 2 45 8 K 1E-7 1E-8-3 -2-1 1 2 3 Voltage (V) 1 2 K 1 125 15 175 2 Wavenumber (cm -1 )
Normal Incidence Photoresponse Photovoltage signal at normal incidence comparable to 45 degree incidence devices Enhanced normal incidence absorption in wavy detectors 2 15 T = 86 K 16 V_bias = Spiral Backside polished Wavy 14 Normal 12 V sig (mv) 1 V sig (mv) 1 8 6 5 Spiral, normal Wavy, normal Mesa, 45 deg Mesa, normal -6-4 -2 2 4 6 V bias (mv) 4 2 8 1 12 14 16 18 2 22 24 26 28 Temperature (K)
Dark Current Characteristics Current (A) Similar activation energy, Eact ~ 8 mev identical quantum design Higher resistance observed for spiral and wavy detectors increased in-plane resistance Method to improve Detectivity (signal to noise) without affecting responsivity 1E+ 1E-2 1E-4 1E-6 1E-8 Spiral detectors 1E-1-6 -4-2 2 4 6 Voltage (V) Resistance at bias (R ) 1E+8 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E+1 Temperature (K) 43 2 1 Spiral detectors E act = 8 mev Resistance at bias (R ).4.6.8.1.12.14 1/Temperature (K -1 ) Temperature (K) 43 2 1 1E+5 Mesa devices 1E+4 1E+3 1E+2 1E+1 E act ~ 8 mev.4.6.8.1.12.14 1/Temperature (K -1 )
Normal Incidence Photocurrent Spectra 86 K, bias dependence Intensity (a.u.) 8 6 4 2 Spiral: N 86 K V.1 V.2 V.5 V Intensity (a.u.) 175 15 125 1 75 5 Wavy: N 86 K 1. V.5 V.2 V.1 V V 25 V, Temperature dependence Intensity (a.u.) 12 13 14 15 16 17 18 19 Wavenumber (cm -1 ) 6 4 2 Spiral: N V b = 18 K 16 K 14 K 12 K 1 K 86 K Intensity (a.u.) 15 125 1 12 13 14 15 16 17 18 19 Wavenumber (cm -1 ) 75 5 Wavy: N V b = 18 K 16 K 14 K 12 K 1 K 86 K 25 12 13 14 15 16 17 18 19 Wavenumber (cm -1 ) 12 13 14 15 16 17 18 19 Wavenumber (cm -1 )
Spectral Comparison Identical spectral widths Small shift in peak photocurrent signal o Measurement resolution o Change in resistance Intensity (a.u.) 7 6 5 4 3 2 1 Mesa: 45 deg V b = 18 K 16 K 14 K 12 K 1 K 86 K 12 13 14 15 16 17 18 19 Wavenumber (cm -1 ) Intensity (a.u., normalized) 1..8.6.4.2. 86 K 54 cm -1 wavy spiral mesa 45 λ p-45 = 1587 cm -1 W p-45 = 18 cm -1 λ p-sw = 1527 cm -1 W p-sw = 17 cm -1 12 13 14 15 16 17 18 19 Wavenumber (cm -1 )
Conclusions Simple method to achieve normal incidence absorption in intersubband infrared detectors Wavelength-independent Standard optical lithography techniques Strong normal-incidence absorption standard packaging Spiral and Wavy detectors performance comparable with standard 45 degree mesa Responsivity similar order of magnitude Device resistance Larger by 3 4 orders Detectivity higher by 1 2 orders of magnitude Future work Optimization of spiral and wavy structures for maximum absorption Theoretical limits to structural improvements