1. 1 Viraj Jayaweera Department of Physics Astronomy

2. GSU 2 Outline  Introduction  Dye-Sensitized Near-Infrared Detectors (DSID)  1/f Noise in DSID  Split-off Band Near-Infrared Detectors  Interfacial Workfunction Internal Photoemission (IWIP) Far-Infrared Detectors  Future Studies

3. GSU 3 The Electromagnetic Spectrum http://www.nasa.gov/centers/langley/science VisibleMicro wave near-IRmid-IRFar-IR 0.8 – 5  m 5 - 30  m 30 - 300  m Wavelength

4. GSU 4 IR Wavelength Range Classification –1-3 μmShort Wavelength Infrared SWIR –3-5 μmMedium Wavelength Infrared MWIR –5-14 μmLong Wavelength Infrared LWIR –14-30 μmVery Long Wavelength Infrared VLWIR –30-100 μmFar Infrared FIR –100-1000 μmSubmillimeter SubMM

5. GSU 5 Applications Infrared image of Orion Human Suspect climbing over fence at 2:49 AM in total darkness

6. GSU 6 Applications Breast Cancer Blood Flow

7. GSU 7 Applications Electrical Hotspots Energy Conservation Bad Insulation spots Loose contacts

8. GSU 8 Different Types of Infrared Detectors Photon Detectors Photovoltaic Photo Conductive Thermal Detectors Bolometer Thermopile Pyroelectric Detectors IR Detectors

9. GSU 9 Dye-Sensitized Near-Infrared Detectors (DSID) http://shs.starkville.k12.ms.us/~kb1/Hort1wk4.htm n-TiO 2 nanoparticle Dye p-CuSCN V n-type Dyep-type

10. GSU 10 Dye-sensitized electron injection to a semiconductor Light induced charge carrier generation in a semiconductor Direct and Sensitized Photo-Injection VB CB VB CB SemiconductorDye

11. GSU 11 Structure of Dye-Sensitized IR Detector Dye Platinum or Gold layer p-CuSCN n-TiO 2 Transparent CTO Glass TiO 2 nanoparticles CTO

12. GSU 12 Energy Level Diagram Appl. Phys. Lett., Vol. 85, No. 23, (2004)

13. GSU 13 IR Absorbing Dyes Anionic Dyes (readily anchor to the TiO 2 surface) Cationic Dyes (Not directly ancoring to TiO2 surface) Anionic compounds used for cationic Dyes IR 783IR 792Mercurochrome (MC) IR 820IR 1040 Bromopyrogallol Red (BPR) The number indicates the peak absorption wavelength in nanometers

14. GSU 14 Spectral Responsivity Appl. Phys. Lett., Vol. 85, No. 23, (2004)

15. GSU 15 1/f-like Noise Characteristics Spectral power density of noise Where f is frequency, 0<α <2 α = 0 white noise α = 1 pink noise (strict 1 / f) α = 2 brown noise α = 1 α = 0

16. GSU 16 Sample Preparation for Noise Measurements Glass Substrate Conducting Tin Oxide TiO 2 R = 56 k 18 V

17. GSU 17 Noise Measurement Setup Vacuum N 2, H 2 O (g) Heater Sample Temp. Sensor R Low Noise Pre- Amplifier (SR560) FT Signal Analyzer (SR785) PC

18. GSU 18 Noise in TiO 2 Nanocrystalline Films Semicond. Sci. Technol. 20 (2005) L40-L42 Infrared Phys. Techn. (2006) In Press, Corrected Proof TiO 2 (N 2 ) TiO 2 (N 2 RH >40%) TiO 2 (N 2,I 2 vapor) Adsorbed molecular species such as H 2 O and I 2 can generate 1/f noise These molecular species can produce electron acceptor state on the TiO 2 surface. It is suggested that the trapping and detrappng of electrons at the surface states is the cause of noise. α = 1.37 α = 1.25

19. GSU 19 Noise in TiO 2 +Dye Nanocrystalline Films TiO 2 /N3 (N 2, RH <40%) TiO 2 /N3 (N 2, RH=70%) TiO 2 /BPR (N 2, RH=70%)) TiO 2 /BPR (N 2, RH <40%) Semicond. Sci. Technol. 20 (2005) L40-L42 TiO 2 (N 2 RH=70%) TiO 2 (N 2,I 2 ) The dye coated TiO 2 suppresses the 1/f noise Higher relative humidity can partly desorbs dye from TiO 2 surface allowing water adsorption.

20. GSU 20 Dye-Sensitized IR Detector Noise Power spectral density of the dark current noise of the hetrojunction n-TiO 2 /MC-IR792/p-CuSCN n-TiO2/MC-IR792/p-CuSCN

21. GSU 21 Advantages and Disadvantages AdvantagesDisadvantages 1.Cheep 2.Low noise 3.Fully Solid State 4.Detection Wavelength Range Can be change using suitable Dye 5.Detection limits can be extend using Suitable Pair of Dyes. 6.Readily applicable to large area detectors 1.Response Time is slow 2.Long term stability is low 3.Experiment is more important to find a suitable Dyes. (Lower prediction capability)

22. GSU 22 HIWIP (Homojunction Interfacial Workfunction Internal Photoemission Detectors) Absorption is due to free carriers Barrier formed by Homojunction (p-type) Δ comes from doping APL 78, 2241 (2001) APL 82, 139 (2003) Barrier Undoped GaAs Emitter p + GaAs Δ h+h+ hνhν Δ biased zero bias p + GaAs Undoped GaAs

23. GSU 23 HEIWIP (HEterojunction Interfacial Workfunction Internal Photoemission Detectors) Absorption is due to free carriers Interface is sharp (no space charge) Barrier formed by Heterojunction (p-type) Δ comes from Al fraction (x) and doping APL 78, 2241 (2001) APL 82, 139 (2003) Δ h+h+ hνhν Δ biased zero bias p + GaAs Al x Ga 1-x As Barrier Al x Ga 1-x As Emitter p + GaAs

24. GSU 24 Spin Split-off Transition Based IR Detectors

25. GSU 25 Detector Structure (HE0204) After processing Substrate ~1000 A Metal p GaAs AlGaAs p GaAs AlGaAs p GaAs ++ + + + n Periods Top Contact Barrier 1250 Å Emitter 188 Å

26. GSU 26 Split-off Mechanism IR Photon excites holes from the light/heavy hole bands to the split- off band (Solid Arrow) Excited holes may escape in split-off band or, May scatter into the light/heavy hole bands and then escape (Dashed Arrow) E k Heavy Hole Band Split-off Band EfEf Δ L/H Δ SO Light Hole Band Conduction Band

27. GSU 27 E k Light Hole Band Split-off Band EFEF Δ L/H escape Free Carrier Absorption Light/Heavy Hole Band Split-off Band Δ SO Response Mechanism I Heavy Hole Band The photoexcitation process consists of the standard free carrier absorption.

28. GSU 28 E k Heavy Hole Band Split-off Band EFEF Δ L/H Split-off Absorption Light/Heavy Hole Band Split-off Band scattering Δ SO Light Hole Band direct photoabsorption to the split-off band, followed by a scattering to the light/heavy hole band. Response Mechanism II

29. GSU 29 E k Heavy Hole Band Split-off Band EfEf Δ L/H escape Split-off Absorption Light/Heavy Hole Band Split-off Band Δ SO Light Hole Band Single indirect photoabsorption into the split-off band. Response Mechanism III

30. GSU 30 E k Heavy Hole Band Split-off Band EfEf Δ L/H escape Split-off Absorption Light/Heavy Hole Band Split-off Band scattering Δ SO Light Hole Band Response Mechanism IV indirect photoabsorption, followed by a scattering event to the light or heavy hole band.

31. GSU 31 Split-off Absorption

32. GSU 32 Quantum Efficiency of Split-off Detector 2.55.07.510.012.515.0 0.00 0.01 0.02 Split-off Response Free Carrier Response Quantum Efficiency Wavelength (µm) Sample 1332 T = 50K

33. GSU 33 Split-off Detector Response Threshold for mechanism (III) Threshold for mechanism (II / IV)

34. GSU 34 Advantages Increased operating Temperature Use of the split-off band provides increased absorption at short wavelengths Increased escape due to high carrier energies Increased gain due to impact ionization from high energy carriers h+h+ i p+p+ i h+h+ i p+p+ i h+h+ i p+p+ i Δ Δ Δ E SO Dark Current ~e -Δ/kT Δ Δ

35. GSU 35 Different material will cover different split-off ranges Antimonides – 1-2 µm Arsinides – 3-5 µm Phosphides – 8-15 µm Nitrides – 40-60 µm

36. GSU 36 GaSb HIWIP THz Detector 0.05 μm 5×10 18 cm -3 p ++ GaSb Substrate 2×10 18 cm -3 p + emitter Undoped-GaSb barrier 2×10 18 cm -3 p + emitter 5 × 10 18 cm -3 p ++ 2 μm 0.1 μm Metal contact Δ GaSb p + GaSb Energy EFEF EFEF Top Contact Bottom Contact ΔEVΔEV

37. GSU 37 GaSb HIWIP THz Detector IV APPLIED PHYSICS LETTERS 90, 111109 2007

38. GSU 38 GaSb HIWIP THz Detector Response 3.7 V 3.4 V 3.0 V 2.0 V 1.0 V T = 4.9 K (a)(b) 3.0 V 2.0 V 1.0 V 97 μm 157 Frequency (THz) 10151.5 Frequency (THz) 4 2

39. GSU 39 Future Studies Design optimized split-off band detector operating near room temperature for 3 -5 μm range. Use different material system to cover different wavelength range e.g. Nitrides – 40-60 µm Phosphides – 8-15 µm InGaSb/GaSb HEIWIP design for THz detection (a) Design single layer HEIWIP detector as first step (b) Improve performance using multi layer and resonant cavity structures. (c) Use surface plasmon to enhance detector performance. (using metal grid pattern on top detector)

40. GSU 40 Future Studies GaSb In 0.02 Ga 0.98 Sb 0.2 μm 2.1 μm 0.7 μm In 0.02 Ga 0.98 Sb GaSb Substrate GaSb In 0.02 Ga 0.98 Sb GaSb 0.2 μm 2.1 μm 0.7 μm GaSb Substrate

41. GSU 41 Acknowledgement Committee Members Dr. Unil Perera Dr. Kirthi Tennakone Dr. Douglas Gies Dr. Xiaochun He Dr. Vadym M. Apelcov Committee Members Dr. Unil Perera Dr. Kirthi Tennakone Dr. Douglas Gies Dr. Xiaochun He Dr. Vadym M. Apelcov Lab Members Dr. Steve Matsick Dr. Mohammad Rinzan Mr. Aruna Weerasekara Mr. Gamini Ariyawansa Mr. Ranga Jayasinghe Lab Members Dr. Steve Matsick Dr. Mohammad Rinzan Mr. Aruna Weerasekara Mr. Gamini Ariyawansa Mr. Ranga Jayasinghe

42. GSU 42

43. GSU 43 Extra stuff

44. GSU 44 n-typeDyep-typen-type Dyep-type

45. GSU 45 HIWIP HIWIP (Homojunction Interfacial Workfunction Internal Photoemission Detector) Barrier formed by Homojunction (n-type) (Δ comes from doping) n + doped GaAs GaAs Δ zero bias e-e- in+n+ ECnECn EFEF hνhν Δ biased JAP 77, 915 (1995)

46. GSU 46 E k Heavy Hole Band Split-off Band Light Hole Band Conduction Band E k Heavy Hole Band Split-off Band Light Hole Band Conduction Band Intrinsic (InSb, HgCdTe)Quantum Well Detector Mechanisms E SO

47. GSU 47 In this Presentation… Wavelength (μm)

48. GSU 48 Modal Results

49. GSU 49 Split-off Mechanism IR Photon excites holes from the light/heavy hole bands to the split-off band (Solid Arrow) Excited holes may escape in split- off band or, May scatter into the light/heavy hole bands and then escape (Dashed Arrow) E k Heavy Hole Band Split-off Band EfEf Δ L/H Δ SO Light Hole Band Conduction Band Transition is entirely in hole bands Carrier energies are continuous not quantized Split-off response is inherently broadband

50. GSU 50 GaSb Absorption

51. GSU 51

52. GSU 52 Dye-Sensitized IR Detector Noise Power spectral density of the dark current noise of the hetrojunction n- TiO 2 /MC-IR792/p-CuSCN

53. GSU 53 IR Absorbing Dyes Anionic Dyes (readily anchor to the TiO 2 surface) Cationic DyesAnionic compounds used for cationic Dyes IR 783 IR 792 Mercurochrome IR 820 Bromopyrogallol Red