Photocurrent proh poperties of IIIp if-VV semiconductoidor ... · di ddesignedg bbyy EBL andd...
1
Diane A Diane A P f A F Professor: Anna Fontc Professor: Anna Fontc Exchange Professor Exchange Professor h Photocurrent pro Photocurrent pro Photocurrent pro i d semiconducto semiconducto semiconducto Background and Motivations Background and Motivations Nanowires have amazing properties such as 1D-electron conduction and Nanowires have amazing properties such as 1D electron conduction and la ge s face to ol me atio These niq e cha acte istics lead to ne large surface-to- volume ratio. These unique characteristics lead to new Iti optical and electrical behaviors compared to bulk components. The III - V Intrin optical and electrical behaviors compared to bulk components. The III V semiconductors have a direct bandgap and high electron mobility which S semiconductors have a direct bandgap and high electron mobility which • Sam largely enhance their absorption coefficient and electron conductivity 100% largely enhance their absorption coefficient and electron conductivity compared to Silicon Nowadays there is a need to know about their 100% Diam compared to Silicon. Nowadays there is a need to know about their • Diam properties at room temperature such as free carrier mobility and lifetime. the I properties at room temperature such as free carrier mobility and lifetime. This project involved electrical characterization of GaAs nanowires through the I This project involved electrical characterization of GaAs nanowires through their photoconductive properties. Single nanowire-FETs (Field Effect Figure 5: their photoconductive properties. Single nanowire FETs (Field Effect Transistor) have been fabricated in order to obtain the photo response of nanowires Transistor) have been fabricated in order to obtain the photo-response of 1, device # the nanowires in function of the source-drain voltage and the excitation 17). The r the nanowires in function of the source drain voltage and the excitation wavelength 700nm an wavelength. measureme and d) P Th Voltage V S Theory Gallium Arsenide average di Theory Gallium Arsenide (sample 3 • The bands structure diagram describes wavelength The bands structure diagram describes th l t ll d d f bidd of the slop the electrons allowed and forbidden obtain an a energy levels in the periodic crystal E energy levels in the periodic crystal l tti E g lattice. GaAs has a direct bandgap E (300K) D i • GaAs has a direct bandgap, E g (300K) = Dopi 1.424eV Dopi 1.424eV • Dec Dec • Due to phonon-electron interactions and ene Due to phonon electron interactions and lattice deformation the band gap Figure 6: lattice deformation, the band gap Figure 6: doped G increases as the temperature decreases, doped G V =1V increases as the temperature decreases, E (0K) = 1 519eV V SD =1V . significan E g (0K) = 1.519eV significan the squa Figure 1: Band structure of GaAs the squa approxim Figure 1: Band structure of GaAs approxim Photoconductivity Photoconductivity is the phenomenon of Photoconductivity • Photoconductivity is the phenomenon of Pass carrier generation under a certain light Pass carrier generation under a certain light exposure Dec exposure • Dec den • The photocurrent in a nanowire is: den No • The photocurrent in a nanowire is: • No Figure 7 Figure 2: Photoconductivity in a nanowire e = electron charge F = applied electric field GaAs nan Figure 2: Photoconductivity in a nanowire μ = carrier mobility I 0 = Illumination intensity Δ i ti f i i l it Al 2 O 3 pas Δn = variation of carrier conc. v = carrier velocity b) Norma Devices presentation and fabrication Devices presentation and fabrication Tem Tem type di t [ ] average length [μm] type average diameter [nm] average length [μm] sample 1 intrinsic 64 11 - 3 types of nanowires have been a) sample 1 intrinsic 64 11 l 2 d d 115 19 analyzed (see Table) a) sample 2 p-doped 115 19 analyzed (see Table). sample 3 intrinsic 190 12 ) b) a) b) - The electrodes were The electrodes were d i d b EBL d designed by EBL and deposited by thermal deposited by thermal ti evaporation (materials: Ti / Au) (materials: Ti / Au) Figure 8: a) temperature b ) Fi 3 ) t d) Z f obtain an a (bl li ) d) c) Figure 3: a) to d) Zoom from a entire pattern to a single nanowire (blue line) a entire pattern to a single nanowire- FET : FET : a) SEM picture of a complete a) SEM picture of a complete pattern, the big squares being the contact electrodes; b) Optical microscope picture (x20) of the centered pattern; ) O ti l i it ( 50) c) Optical microscope picture (x50) of the nanowire FETs; We have of the nanowire-FETs; d) SEM picture of one nanowire-FET We have nanowires d) SEM picture of one nanowire-FET . nanowires the photo Characterization method the photo vacuum e Characterization method vacuum e Characterization method spectrum spectrum by Varsh Monochromator Sample with by Varsh Chopper Sample with nanowire-FETs devices applicatio applicatio nanowire-FET Chopper nanowire FET Figure 4: Photoconductivity setup. The monochromatic light was oriented perpendicular to the nanowire and the EPFL Wom electrodes separation was of 1μm. a) Schematic of the setup, b) Picture of the real setup in NTU. EPFL Wom Ansermet Ansermet b i M l (EPFL) uberta i Morral (EPFL) uberta i Morral (EPFL) : Qihua Xiong (NTU) : Qihua Xiong (NTU) i f operties of III V operties of III-V operties of III V i or nanowires or nanowires or nanowires Res lts and Disc ssion Results and Discussion Results and Discussion i i nsic nanowires b) Diameter = 190 nm ) Di t 64 b) Diameter = 190 nm a) Diameter = 64 nm t lli t t e crystalline structure: % Zinc-Blende % Zinc Blende mete dependence on meter dependence on IV curves IV curves a) and b) IV curves of GasAs intrinsic with average diameter of a) 64 nm (sample #3-4) and b)190 nm (sample 3, device #16- d) c) red curves correspond to an illumination at d) c) nd the dark curves correspond to the ent of the dark current (no illumination); c) Photoconductivity spectrums (source-drain V SD =1V) of GasAs intrinsic nanowires with diameter of 64nm (sample 1) and 190nm 3): c) Normalized gains scanned on hs between 700 and 950nm; d) Linear fitting pe of the square root of the band edge to approximation of the bandgap. i ff ing effects a) b) ing effects crease of the bandgap crease of the bandgap i d d i ergy in p-doped nanowires 6: Photoconductivity spectrum of Intrinsic a p 6: Photoconductivity spectrum of Intrinsic a p- GaAs nanowires done at voltage source drain GaAs nanowires done at voltage source-drain a) Normalized photoconductive gains of two a) Normalized photoconductive gains of two nt nanowires; b) Linear fitting of the slope of nt nanowires; b) Linear fitting of the slope of uare root of the band edge to obtain an uare root of the band edge to obtain an mation of the bandgap mation of the bandgap. sivation effects a) b) sivation effects a) b) c ease of the s face states crease of the surface states nsity in doped nanowires nsity in doped nanowires impact on bandgap energy impact on bandgap energy 7: Photoconductivity spectrum of p-doped nowires (device #19-20) with and without an assivating layer coating of 10 nm: a) Real and alized photoconductive gains. perature dependence Varshni Equation: perature dependence Varshni Equation: b) c) b) c) a) Real Photoconductive gain spectrums (source-drain Voltage V SD =1V) of GasAs intrinsic nanowires in vacuum at 4 different f ( ) b) f f h l f h f h h b d d es, from 290K (Room Temperature) to 210K; b) Linear fitting of the slope of the square root of the photocurrent band edges to f h b d ) d d f h b d d d h ll h h approximation of the bandgaps; c) Temperature dependence of the band gap, determined theoretically with Varshni Equation d i ll ( d li ) and experimentally (red line). C l i Conclusion Conclusion observed a decrease of the bandgap energy in p-doped nanowires compared to intrinsic observed a decrease of the bandgap energy in p doped nanowires compared to intrinsic s In the case of the doped nanowires surface passivation is proved to slightly decrease s. In the case of the doped nanowires, surface passivation is proved to slightly decrease oconductive gain but highly improve the quality of the signal. The measurements in oconductive gain but highly improve the quality of the signal. The measurements in environment showed to improve the gain by increasing the absorption edge of the environment showed to improve the gain by increasing the absorption edge of the . Lower temperature proved to increase the bandgap energy of the material as predicted . Lower temperature proved to increase the bandgap energy of the material as predicted hni equation These different observations are noteworthy for future optoelectronic hni equation. These different observations are noteworthy for future optoelectronic ons of Gallium Arsenide nanowires. ons of Gallium Arsenide nanowires. Acknowledgement Acknowledgement men In Science and Humanities (WISH) Foundation for the Career Community Investment men In Science and Humanities (WISH) Foundation for the Career Community Investment
Transcript of Photocurrent proh poperties of IIIp if-VV semiconductoidor ... · di ddesignedg bbyy EBL andd...
Diane ADiane AP f A FProfessor: Anna FontcProfessor: Anna Fontc
Exchange ProfessorExchange Professorg
hPhotocurrent proPhotocurrent proPhotocurrent propi dsemiconductosemiconductosemiconducto
Background and MotivationsBackground and MotivationsgNanowires have amazing properties such as 1D-electron conduction andNanowires have amazing properties such as 1D electron conduction andla ge s face to ol me atio These niq e cha acte istics lead to nelarge surface-to-volume ratio. These unique characteristics lead to new
I t ioptical and electrical behaviors compared to bulk components. The III-V Intrinoptical and electrical behaviors compared to bulk components. The III Vsemiconductors have a direct bandgap and high electron mobility which Ssemiconductors have a direct bandgap and high electron mobility which • Samlargely enhance their absorption coefficient and electron conductivity 100%largely enhance their absorption coefficient and electron conductivitycompared to Silicon Nowadays there is a need to know about their
100%Diamcompared to Silicon. Nowadays there is a need to know about their • Diam
properties at room temperature such as free carrier mobility and lifetime. the Iproperties at room temperature such as free carrier mobility and lifetime.This project involved electrical characterization of GaAs nanowires through
the IThis project involved electrical characterization of GaAs nanowires throughtheir photoconductive properties. Single nanowire-FETs (Field Effect Figure 5:their photoconductive properties. Single nanowire FETs (Field EffectTransistor) have been fabricated in order to obtain the photo response of nanowiresTransistor) have been fabricated in order to obtain the photo-response of 1, device #the nanowires in function of the source-drain voltage and the excitation 17). The rthe nanowires in function of the source drain voltage and the excitationwavelength 700nm anwavelength. measureme
and d) P
ThVoltage VS
TheoryGallium Arsenide average diTheoryGallium Arsenide (sample 3
• The bands structure diagram describes wavelengthThe bands structure diagram describesth l t ll d d f bidd
of the slopthe electrons allowed and forbidden obtain an a
energy levels in the periodic crystalE
energy levels in the periodic crystall ttiEg lattice.
GaAs has a direct bandgap E (300K) D i• GaAs has a direct bandgap, Eg (300K) = Dopig1.424eV
Dopi1.424eV
• DecDec• Due to phonon-electron interactions and eneDue to phonon electron interactions and
lattice deformation the band gap Figure 6:lattice deformation, the band gap Figure 6:doped Gincreases as the temperature decreases, doped GV =1Vincreases as the temperature decreases,
E (0K) = 1 519eVVSD=1V.significanEg (0K) = 1.519eV significanthe squag
Figure 1: Band structure of GaAsthe squaapproximFigure 1: Band structure of GaAs approxim
PhotoconductivityPhotoconductivity is the phenomenon of
Photoconductivity• Photoconductivity is the phenomenon of
Passcarrier generation under a certain light Passcarrier generation under a certain lightexposure
Decexposure
• Decden• The photocurrent in a nanowire is: denNo
• The photocurrent in a nanowire is:• No
Figure 7Figure 2: Photoconductivity in a nanowiree = electron charge F = applied electric field GaAs nanFigure 2: Photoconductivity in a nanowire
µ = carrier mobility I0 = Illumination intensityΔ i ti f i i l it
Al2O3 pasΔn = variation of carrier conc. v = carrier velocity b) Norma
Devices presentation and fabricationDevices presentation and fabrication Temp Temtype di t [ ] average length [µm]type average diameter [nm] average length [µm]
sample 1 intrinsic 64 11 - 3 types of nanowires have been a)sample 1 intrinsic 64 11
l 2 d d 115 19
ypanalyzed (see Table)
a)sample 2 p-doped 115 19 analyzed (see Table).sample 3 intrinsic 190 12
) b)a) b) - The electrodes wereThe electrodes wered i d b EBL ddesigned by EBL andg ydeposited by thermaldeposited by thermal
tievaporationp(materials: Ti / Au)(materials: Ti / Au)
Figure 8: a)temperatureb
) Fi 3 ) t d) Z fobtain an a(bl li )d) c) Figure 3: a) to d) Zoom from a
entire pattern to a single nanowire(blue line) a)
entire pattern to a single nanowire-FET :FET :
a) SEM picture of a completea) SEM picture of a completepattern, the big squares beingp , g q gthe contact electrodes;
b) Optical microscope picture (x20)of the centered pattern;
) O ti l i i t ( 50)c) Optical microscope picture (x50)of the nanowire FETs; We haveof the nanowire-FETs;
d) SEM picture of one nanowire-FET We havenanowires
d) SEM picture of one nanowire-FET.
nanowiresthe photo
Characterization methodthe photovacuum eCharacterization method vacuum eCharacterization methodspectrumspectrumby VarshMonochromator Sample with by VarshChopper
o oc o ato Sample withnanowire-FETs devices
applicatioapplicatio
nanowire-FETChopper
nanowire FET
Figure 4: Photoconductivity setup. The monochromatic light was oriented perpendicular to the nanowire and the EPFL Womg y p g p pelectrodes separation was of 1µm. a) Schematic of the setup, b) Picture of the real setup in NTU.
EPFL Womp µ ) p, ) p
AnsermetAnsermetb i M l (EPFL)uberta i Morral (EPFL)uberta i Morral (EPFL)
: Qihua Xiong (NTU): Qihua Xiong (NTU)g ( )
i foperties of III Voperties of III-Voperties of III V pior nanowiresor nanowiresor nanowires
Res lts and Disc ssionResults and DiscussionResults and Discussion
i insic nanowires b) Diameter = 190 nm) Di t 64 b) Diameter = 190 nma) Diameter = 64 nmt lli t te crystalline structure:
% Zinc-Blende% Zinc Blendemete dependence onmeter dependence onIV curvesIV curves
a) and b) IV curves of GasAs intrinsicwith average diameter of a) 64 nm (sample
#3-4) and b)190 nm (sample 3, device #16-d)c)red curves correspond to an illumination at d)c)
nd the dark curves correspond to theent of the dark current (no illumination); c)Photoconductivity spectrums (source-drain
VSD=1V) of GasAs intrinsic nanowires withdiameter of 64nm (sample 1) and 190nm3): c) Normalized gains scanned onhs between 700 and 950nm; d) Linear fittingpe of the square root of the band edge toapproximation of the bandgap.
i ffing effects a) b)ing effects ) )
crease of the bandgapcrease of the bandgapi d d iergy in p-doped nanowires
6: Photoconductivity spectrum of Intrinsic a p6: Photoconductivity spectrum of Intrinsic a p-GaAs nanowires done at voltage source drainGaAs nanowires done at voltage source-drain
a) Normalized photoconductive gains of twoa) Normalized photoconductive gains of twont nanowires; b) Linear fitting of the slope ofnt nanowires; b) Linear fitting of the slope of
uare root of the band edge to obtain anuare root of the band edge to obtain anmation of the bandgapmation of the bandgap.
sivation effectsa) b)
sivation effectsa) b)
c ease of the s face statescrease of the surface statesnsity in doped nanowiresnsity in doped nanowiresimpact on bandgap energyimpact on bandgap energy
7: Photoconductivity spectrum of p-dopednowires (device #19-20) with and without an
assivating layer coating of 10 nm: a) Real andalized photoconductive gains.
perature dependence Varshni Equation:perature dependence Varshni Equation:
b) c)b) c)
a) Real Photoconductive gain spectrums (source-drain Voltage VSD=1V) of GasAs intrinsic nanowires in vacuum at 4 differentf ( ) b) f f h l f h f h h b d des, from 290K (Room Temperature) to 210K; b) Linear fitting of the slope of the square root of the photocurrent band edges to
f h b d ) d d f h b d d d h ll h happroximation of the bandgaps; c) Temperature dependence of the band gap, determined theoretically with Varshni Equationd i ll ( d li )and experimentally (red line).
C l iConclusionConclusionobserved a decrease of the bandgap energy in p-doped nanowires compared to intrinsicobserved a decrease of the bandgap energy in p doped nanowires compared to intrinsics In the case of the doped nanowires surface passivation is proved to slightly decreases. In the case of the doped nanowires, surface passivation is proved to slightly decreaseoconductive gain but highly improve the quality of the signal. The measurements inoconductive gain but highly improve the quality of the signal. The measurements inenvironment showed to improve the gain by increasing the absorption edge of theenvironment showed to improve the gain by increasing the absorption edge of the. Lower temperature proved to increase the bandgap energy of the material as predicted. Lower temperature proved to increase the bandgap energy of the material as predicted
hni equation These different observations are noteworthy for future optoelectronichni equation. These different observations are noteworthy for future optoelectronicons of Gallium Arsenide nanowires.ons of Gallium Arsenide nanowires.
AcknowledgementAcknowledgement
men In Science and Humanities (WISH) Foundation for the Career Community Investmentmen In Science and Humanities (WISH) Foundation for the Career Community Investment
