Semiconductor Based Hydrogen Sensor and Detecting System

1. Semiconductor Based Hydrogen Sensor and Detecting System Reporter: Dr. Kun-Wei Lin

2. Outline Part 1 Introduction Part 2 Experimental Part 3 Gas Sensing Characteristics of the Different Structure – Based Sensors Part4 96、98、99 Projects some application

3. Part 1 Introduction

5. 摘錄自網路

6. Applications of Hydrogen Hydrogen Fuel Cell Hydrogen Cylinder Hydrogen Transportation Hydrogen Storage Hydrogen Applications Hydrogen Refueling Station Hydrogen fueled aircraft Hydrogen -Domestic Use Liquid hydrogen fueled aircraft Helios Prototype http://www.mae.ufl.edu/NasaHydrogenResearch/index.php?src=h2webcourse

7. Introduction Application of hydrogen sensor ＊ Industrial fabrication processes ＊ Medical installations ＊ Laboratories (especially for semiconductor fabrication) ＊ Hydrogen-fueled motor vehicles Ingemar Lundström • Since 1976, Transistors and Schottky diodes based on Metal(Pd)-Oxide-Semiconductor(Si) MOS devices were used as hydrogen sensors. • —Lundstrom’s Group (Linkoping University, Sweden)

8. Capacitor Schottky diode Metal Insulator Metal Semiconductor Semiconductor Field-effect transistor Metal Insulator D S Semiconductor Types of Hydrogen Sensors Different type of gas sensors ＊ MOS capacitors (capacitance change) ＊ MOS field effect transistors (threshold voltage shift) ＊ MOS Schottky barrier diodes (current change) ＊ MS Schottky barrier diodes (current change) Gain= 8

9. The advantages of our device compare with Si-based structure • ＊Short response time • ＊ Obvious current variation • ＊ Operation at room temperature • ＊ widespread operating temperature regime

10. Mechanism of Hydrogen-Sensing H2(g) O2(g) H2O (g) ΔHS OHa Ha Ha Ha OaOa Surface Pd or Pt catalytic metal Hb Pd ΔHb - - - Interface ΔHi + + + Oxide ΔHio Semiconductor H2(g) :molecular hydrogen Ha : adsorbed hydrogen atoms on the Pd or Pt surface Hb :hydrogen atoms in the Pd or Pt bulk H i: hydrogen atoms at the Pd/oxide interface

11. Mechanism of Hydrogen-Sensing H2(g) 2Ha 2Hb 2Hi O2 + 2Ha 2(OHa) OHa + Ha H2O • Under atmospheric conditions • The catalytic reactionkineticsschemeofhydrogen adsorption and desorption k2 k3 k1 r1 r2 r3 where k1, k2, k3, and r1, r2 and r3 are adsorption and desorption rate constants. • In presence of oxygen, the addition reaction ofhydrogen desorption

12. Mechanismof Hydrogen-Sensing • Under steady-state conditions, b induced by hydrogen adsorption can be assumed as where b,max is the maximum change in barrier height and i is the hydrogen coverage at the interface. where K is a temperature-dependent rate constant; PH2 andPO2 areH2 and O2 partial pressures, respectively. The reaction order 1 for temperatures above 75℃ and 0.5 for the lower temperatures 12

13. Mechanism of Hydrogen-Sensing TheLangmuir form can be expressed in terms of B and Bmax as • From the relation of saturation current and barrier height, the Langmuir can also be deduced as where I0g,max is the maximum saturation current at hydrogen-contained ambient.

14. Mechanism of Hydrogen-Sensing According to the van’t Hoff equation where H is the change of enthalpy, S the change of entropy, and R the gas constant. The change of barrier height b can be rewritten as:

15. The schematic setup of the hydrogen measurement system Stainless Steel Chamber Heating Tape Mass Flow Control Sample Valve Valve Manometer Heater Exhaust Test Line Flange Heater and Thermometer Semiconductor Parameter Analyzer Air H2/Air Mixture

16. Measurement system implementation 16

17. Measurement system implementation 17

18. Part 2Experimental

19. Fabrication of the Device • Thin films were grown by MOCVD on S.I. GaAs substrate. • Conventional photolithography and wet etching technique is used. • Thermal oxide was grown by furnace at 120oC for 60 minutes. • Metal pattern was made by the thermal evaporation method. • The dimension of device is 2.05x10-3 cm2. • Ohmic contact : AuGe • Schottky contact : Pd Pd Schottky Contact AuGe Ohmic Contact 300Å n+-GaAs 50Å Thermal Oxide 3000Å AlGaAs active layer(n=2x1017cm-3 ) 5000Å GaAs buffer layer S.I. GaAs substrate Pd Schottky Contact Ohmic Contact

20. Why We Choose AlGaAs and Pd? • AlxGa1-xAs is lattice matched to GaAs, and the mole fraction of Al can be changed from 0 to 1. • The energy bandgap of AlGaAs is larger than GaAs and InP. • In compared with InGaP/GaAs and InP-based material system, the thermal oxide is more easily grown on AlGaAs/GaAs. • AlGaAs-based hydrogen sensor is suitable for higher operation temperature than InP-based system. • Pd metal shows excellent selectivity to hydrogen gas than other metals.

21. H2 adsorb on Pd surface and dissociate into atoms - + Dipole Layer H atoms diffuse into Pd bulk - + H2 molecule - + Hydrogen Sensing Mechanism • Steps of H2 sensing mechanism : • H2 molecules adsorb on Pd surface and then dissociate to atoms. • H atoms diffuse into the bulk of Pd metal. • H atoms adsorb on Pd/oxide interface and form thin dipole layer. • The barrier height is reduced by the formation of thin dipole layer. Air Pd Metal Oxide AlGaAs Ec Fermi-Level Ev

22. Current-Voltage Characteristics • The Pd/oxide/AlGaAs MOS device shows excellent performance from room temperature to 160oC

23. Barrier Height at Room Temperature • In compared with InGaP-based device, the barrier height of AlGaAs-based device is larger. • InGaP • 0.92eV in air • 0.77eV in 1% H2/air • AlGaAs • 1.05eV in air • 0.84eV in 1% H2/air

24. Barrier Height Variation • Barrier height variation at room temperature. • The barrier height variation of the AlGaAs-based device is larger than the InGaP-based device from 15ppm to 1% of hydrogen gas concentration. • Barrier height variation of InGaP & AlGaAs are 0.14 and 0.21 eV, respectively.

25. IH2 - Iair Iair S = Saturation Sensitivity • Pd/oxide/AlGaAs MOS device shows very high saturation sensitivity, especially at room temperature. • Over 155 times of sensitivity can be observed in 1% H2/Air at room temperature.

26. Saturation Sensitivity at R.T. • The saturation sensitivity is decreased with increasing the applied voltage. • Generally, the saturated sensitivity is increased with increasing the hydrogen concentration. • The saturated sensitivity is almost unity when the applied voltage is over 0.8V.

27. Transient Response at 30oC • The applied voltage is 0.35V. • Even at room temperature, the studied device shows good transient response characteristics under extremely low hydrogen concentration of 15 ppm H2/Air. • The maximum current of the studied device varies from 1.5x10-8 to 7.7x10-7 A under the condition of Air and H2/Air, respectively.

28. Transient Response at 95oC & 160oC

29. Response of 1% Hydrogen • τa : adsorption time constant, • τb : adsorption time constant are defined as the times reach e-1 of the final steady-state current values.

30. Conclusion • At room temperature, the extremely hydrogen concentration of 15ppm can be easily detected. • The detected transient-state response characteristic of 15ppm H2/air at room temperature is first reported. • The reverse current exhibit a highly sensitivity linearity, the current change from 1x10-10A(air) to 1x10-8A(1%) at 95oC. • High sensitivity of 155 under 0.3V and 1% H2/air can be obtained at room temperature. • The studied device shows a promise for high sensitivity, low leakage current, wide temperature operation regime and fast response speed for hydrogen sensor application.

31. Comparative studies of hydrogen sensing performance of Pd/InGaP MOS and MS Schottky diodes

32. The X-ray energy dispersive spectrometer (EDS能量散射) analysis

33. Measured I-V characteristics of the studied Pd/InGaP MOS Schottky diode • Measured I-V characteristics of the studied Pd/InGaP MOS Schottky diode, at T=400K, under atmospheric condition with different hydrogen concentrations. • The inset of this figure shows the corresponding forward I-V characteristics of studied device at different temperature of 300, 400, 500, 550, and 600K, respectively.

34. Measured I-V characteristics of the studied Pd/InGaP MS Schottky diode(400K) • Measured I-V characteristics of the studied Pd/InGaP MS Schottky diode, at T=400K, under atmospheric condition with different hydrogen concentrations. • The current variation of MOS structure is lager than that of MS Schottky diode. This is attributed to the reduction of the leakage current resulting from the improved interface properties under the presence of interficial oxide layer.

35. Barrier height as a function of hydrogen concentration in air

36. as a function of From slopes and intercepts, the equilibrium constant K values are obtained as 3.01, 1.38, and 0.7 for the Pd-MOS Schottky diode at 350, 400, and 450K, respectively. The equilibrium constant K is decreased as the temperature is increased.

37. as a function of • The corresponding K values of the studied Pd-MS Schottky diode are 2.36, 2.11, and 1.85 at 350, 400, and 450K, respectively. • The equilibrium constant K is decreased as the temperature is increased. • The interface coverage i is decreased with elevating the temperature at the same hydrogen partial pressure. • The water production rate is increased with increasing the operating temperature.

38. lnK as a function of the reciprocal of temperature According to the van’t Hoff equation where H is the initial heat of hydrogen adsorption, S the change of entropy, and R the gas constant. • From slopes of this plot, the calculated Hvalues for Pd/InGaP MOS and MS Schottky diodes are 355 and 65.9 meV/atom, respectively.

39.  i/(1-i) as a function of The change of barrier height b can be rewritten as: • The calculated max values are 163, 103, 88.6, and 82 meV for Pd-MOS Schottky diode at 300, 350, 400, and 450K, respectively.

40. Transient response curves • Transient response curves upon the introduction and removal of 97, 537, and 9090ppm H2/air gases of the studied Pd/InGaP MOS Schottky diode at 400K. • With increasing the hydrogen concentration from 97 to 9090ppm H2/air, the response time constant of adsorption (a) for the studied MOS Schottky diode is decreased from 35 to 5.4 sec.

41. Transient response curves • Transient response curves upon the introduction and removal of 97, 537, and 9090ppm H2/air gases of the studied Pd/InGaP MS Schottky diode at 400K. • With increasing the hydrogen concentration from 97 to 9090ppm H2/air, the response time constant of adsorption (a) for the studied MS Schottky diode is decreased from 64 to 7.8 sec.

42. Transient response curves • The transient response curves of the studied MOS Schottky diode at 350 and 400 K vary gradually increase. • This implies that the coverage sites at the Pd metal and oxide interface are not all occupied and the water production rate is lower than adsorption rate. • At a higher temperature of 600K, the interface coverage sites are all occupied and the water production rate is larger than the adsorption rate.

43. Transient response curves • At low temperature of 350K, the unsaturated behaviors of transient response are found. • At 400 and 500K, due to the absence of interface coverage site in MS Schottky diode, the adsorption and absorption on the Pd surface are depend on the temperature and the Pd surface property.

44. Summary • The Pd/InGaP hydrogen sensors based on the MOS and MS Schottky diodes have been fabricated and studied. The studied devices exhibit significantly wide operating temperature regimes. • Even at 300K and low hydrogen concentration of 15ppm H2/air, the remarkable hydrogen detection can be observed. • Under the presence of oxide layer in device structure, the hydrogen detection sensitivity is improved. • From the van’t Hoff equation, heats of hydrogen adsorption are 355 and 65.9 meV/atom for studied MOS and MS-type devices, respectively. • These values confirm that hydrogen atoms populated at the interface between Pd metal and oxide layer causes the improved hydrogen detection characteristics of MOS type structure.

45. Comparative studies of hydrogen sensing performance of Pd- and Pt- InGaP MOS Schottky diodes

46. Current-voltage (I-V) characteristics of Pd-InGaP MOS Schottky diode hydrogen sensor • The forward currents of the studied Pd-InGaP MOS Schottky diode are substantially increased with increasing the hydrogen concentration and temperature. • The current variations of InGaP Schottky diode based on Pd metal are more sensitivite than those of Pt metal under low hydrogen concentration (< 937 ppm H2/air) and low operating temperature (T< 400 K) regimes.

47. Current-voltage (I-V) characteristics of Pt-InGaP MOS Schottky diode hydrogen sensor • The forward currents of the studied Pt-InGaP MOS Schottky diode are substantially increased with increasing the hydrogen concentration and temperature. • At high operating temperature, the Pt/InGaP sensor has better detecting properties. Particularly, at 600K, the current variations of Pt/InGaP Schottky diode are significantly higher than those of Pd/InGaP Schottky diode.

48. Current variation as a function of hydrogen concentration • Current variation as a function of hydrogen concentration for Pd-InGaP Pd-InGaP MOS Schottky diode hydrogen sensors at different temperature. • Upon exposing to low hydrogen concentration ambient, however, the Pd-InGaP Schottky exhibits better hydrogen detecting capability.

49. Current variation as a function of hydrogen concentration • Current variation as a function of hydrogen concentration for Pt-InGaP MOS Schottky diode hydrogen sensors at different temperature. • By comparing with the hydrogen sensing response from current variations, generally, the Pt/InGaP Schottky diode is more sensitive to hydrogen than the Pd-InGaP Schottky diode.

50. Barrier height as a function of hydrogen concentration • Barrier height as a function of hydrogen concentration for Pd-InGaP MOS Schottky diode hydrogen sensor at different temperature. • The barrier height variation is significant under low hydrogen concentration for Pd-InGaP MOS Schottky diode.