Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3,...
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![Page 1: Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer.](https://reader036.fdocuments.in/reader036/viewer/2022062718/56649e8e5503460f94b91351/html5/thumbnails/1.jpg)
Preparation of Clean III-V Semiconductor Surfaces for
NEA PhotocathodesYun Sun1, 2, Zhi Liu3, Francisco Machuca3, Piero Pianetta1
and William E. Spicer1
1 Stanford Synchrotron Radiation Lab, Stanford, CA
2 Department of Chemistry, Stanford University, CA
3 Center for Integrated Systems, Stanford University, CA
Work is funded by Intevac, and is carried out at Stanford Synchrotron Radiation Lab (Department of Energy, Office of Basic Energy Sciences)
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Importance of InP Cleaning
• InP(100) based Negative Electron Affinity (NEA) photocathode– Surface cleanness critical for the
performance– Chemical cleaning of InP(100) not well
understood
• Other applications– Critical for MBE and CVD growth, etc.
![Page 3: Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer.](https://reader036.fdocuments.in/reader036/viewer/2022062718/56649e8e5503460f94b91351/html5/thumbnails/3.jpg)
Analysis Technique:Photoelectron Spectroscopy using
Synchrotron Radiation
• A systematic study by SR-PES– hv: 60 - 600eV, P2p, In4d, F1s, VB
– High resolution --> resolve chemical shift
– Short escape depth(~5Å) --> high surface sensitivity
• A controlled etching environment– Ar purged glove bag around load lock
![Page 4: Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer.](https://reader036.fdocuments.in/reader036/viewer/2022062718/56649e8e5503460f94b91351/html5/thumbnails/4.jpg)
1. 2.
3.
4.
5.
6.
7.
1. Argon/Nitrogen purged glove box
2. Connected Loadlock
3. XYZ-theta Stage with integrated heater in UHV chamber
4. Beam line (Radiation)
5. Hemispherical Energy Analyzer, Energy Resolution, 0.2eV.
6. Leak Valve for Gases
7. Alkali Metal doser
4.
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Typical One Step Cleaning Methods
• Hydrogen Peroxide based solutions*:
– H2SO4 : H2O2 : H2O (4:1:100)
– H2SO4 : H2O2 : H2O (4:1:1)
– NH3 : H2O2 : H2O (10:2:100)
– etc.
• Other: Br-CH3OH, HF, etc.
*H2SO4: 98%, H2O2 30%, NH3 30%
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One Step Chemical Cleaning
CHEMICALETCHING
VACUUMANNEAL
INERT(Ar) ATMOSPHERE UHV
• Not effective for InP(100) !
H2SO4 : H2O2 : H2O (4:1:100)
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One Step Chemical Cleaning H2SO4 : H2O2 : H2O (4:1:100)
K. E. / eV
Inte
nsi
ty
Oxide~0.5ML
K. E. / eV
Oxide
Inte
nsi
ty52504846444236343230282624
Etched
Etch +360oC
Etched
Etch +360oC
P2phv = 165eV
In4dhv = 70eV
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One Step Chemical Cleaning
In oxide
Kinetic Energy / eV
10
8
6
4
2
0
x10
3
5250484644
In4d for bulk InP
Inte
nsit
y
In4dhv = 70eV
525048464442
In in InP
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One Step Chemical Cleaning Not Enough
• Surface is left with oxide, which can not be removed completely by vacuum heating
• Add another step to remove the oxide– 36% HCl : H2O (1:3)
– HF (1%)
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Two Step Chemical Cleaning
CHEMICALETCHING
VACUUMANNEAL
OXIDE REMOVAL
INERT(Ar) ATMOSPHERE UHV
H2SO4:H2O2:H2O4:1:100
HCl(36%): H2O 1:3HF 1%
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HCl Solution as Second Step
Surface is Hydrophobic
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HCl Clean
Inte
nsi
ty /
a.u
.
3634323028262422
K.E. / eV
P2phv = 165eV
r.t.
230oC
290oC
330oC
360oC
Inte
ns
ity
/ a
.u.
3634323028262422
Kinetic Energy / eV
P2phv = 165eV
"Elemental" P
Inte
ns
ity
/ a
.u.
3634323028262422
Kinetic Energy / eV
P2phv = 165eV
oxide gone
r.t.
360oC
P in InP
~0.42ML
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HCl Clean
Inte
ns
ity
/ a
.u.
525048464442
K.E. / eV
In4dhv = 70eV
r.t.
230oC
290oC
330oC
360oC
new component
Inte
ns
ity
525048464442Kinetic Energy (eV)
In4dhv = 70 eV
Surface Shift
Inte
nsit
y
525048464442Kinetic Energy (eV)
In4dhv = 70eV
r.t.
360oC
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One Step
Two Step
+330 oC
K.E. / eV K.E. / eV
One Step
Two Step
+330 oC< 0.1ML
C1s O1s
565452504846 70686664626058
C1s And O1s Spectra at different stagesIn
ten
sity
Inte
nsi
ty
hv = 340eV hv = 600eV
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HF Solution as Second Step
Surface is Hydrophilic
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HF CleanIn
tens
ity
36343230282624Kinetic Energy (eV)
r.t.
180oC
230oC
360oC
P2phv = 165eV
oxide
Inte
nsity
525048464442Kinetic Energy (eV)
In4dhv = 70eV
r.t.
120oC
180oC
230oC
360oC
In-F
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In4d Fit after HF CleanIn
ten
sit
y
525048464442Kinetic Energy (eV)
In4dhv = 70eV
In-F
Inte
ns
ity
525048464442Kinetic Energy (eV)
In4dhv = 70eV
In-F
Inte
nsit
y
525048464442Kinetic Energy (eV)
In4dhv = 70eV
In-F
Inte
ns
ity
525048464442Kinetic Energy (eV)
In4dhv = 70eV
In-O
SurfaceShift
r.t. 120oC.
180oC. 230oC.
In in InP
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HF CleanIn
ten
sity
158156154152150
Kinetic Energy ( eV)
F1shv = 800eV
r.t.
180oC
230oC
Inte
nsity
65605550Kinetic Energy (eV)
VBhv = 70eV
r.t.
120oC
180oC
230oC
360oC
F2p
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In-F and F Coverage
Temperature(oC)
In-F Coverage(monolayer)
F coverage(monolayer)
r.t. (~27oC) 0.44 0.48
120oC 0.38 --
180oC 0.30 0.31
230oC 0 0
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HCl Clean v.s. HF Clean
HCl Clean HF Clean
Surface Property Hydrophobic Hydrophilic
SurfaceTermination
“Elemental” P~0.42ML
F terminate In sites~0.48ML
Vacuum Anneal “Elemental” Premoval at 330oC
F removal at 230oC
Clean Resultafter Heating
Clean, no oxide Almost clean,~0.05ML oxide
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GaAs(100) Cleaning recipe
H2SO4:H2O2:H2O *
4:1:100
+
Vacuum annealing at 500C
*H2SO4 98%; H2O2 30%; NH3 30%
![Page 22: Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer.](https://reader036.fdocuments.in/reader036/viewer/2022062718/56649e8e5503460f94b91351/html5/thumbnails/22.jpg)
After chemical clean
Peak found on sample after chemical clean
Inte
nsi
ty [A
rbitr
ary
Un
its]
166164162160158156
Kinetic Energy
Ga(As) Bulk
Elemental As
Oxide
As 3d
Inte
nsi
ty [A
rbitr
ary
Un
its]
188186184182180178
X-Axis Value
Ga 3d
(Ga)As Bulk
GaOx
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After thermal annealing
Peak found on sample after annealing
Inte
nsi
ty [A
rbitr
ary
Un
its]
166164162160158156
Kinetic Energy
Ga(As) Bulk
As 3d
Inte
nsi
ty [A
rbitr
ary
Un
its]
188186184182180178
Kinetic Energy
(Ga)As Bulk
Ga 3d
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C 1s and O 1s
Inte
nsity
[Arb
itrar
y U
nits
]
7570656055
Kinetic Energy
O 1s
Sample as received
After chemical Clean
After annealing
Kinetic Energy
Inte
nsity
[Arb
itrar
y U
nits
]
6058565452504846
C 1s
After annealing
After chemical clean
Sample as received
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Result of chemical cleaning
GaAs(100) InP(100)
Step 1:H2SO4:H2O2:H2O4:1:100
> 2ML Elemental As < 0.2ML suboxide > 0.5ML C
~0.5ML oxide > 0.5ML C
Step2:HCl: H2O1:3
--------------------- ~0.4ML
Elemental P ~0.2ML C
Vacuum Anneal(30 min)
No oxide< 0.1ML C
No oxide< 0.1ML C
![Page 26: Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer.](https://reader036.fdocuments.in/reader036/viewer/2022062718/56649e8e5503460f94b91351/html5/thumbnails/26.jpg)
Chemical Cleaning (Ex situ)
• Optimize the sulfuric acid cleaning used for other III-V’s for GaN --> 4:1 H2SO4:H2O2 (~90oC)
Annealing Ambient (In situ)
• Testing the use of ammonia back pressure in comparison tovacuum annealing --> Anneal in vacuum better
Annealing Temperature (In situ)
• Find an effective temperature --> 700oC
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9.
45x103
40
35
30
25
20
Nor
mal
ized
Inte
nsity
[cou
nts/
sec]
60565248
Kinetic Energy (eV)
Carbon 1shv=340eV
After H2SO4
/H2O2
700C Vacuum Anneal
No Clean
6x103
5
4
3N
orm
aliz
ed In
tens
ity [c
ount
s/se
c]
7570656055
Kinetic Energy (eV)
Oxygen 1shv=600eV
After H2SO4
/H2O2
700C VacuumAnneal
No Clean
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10.
35x103
30
25
20
15
10
5
Nor
mal
ized
Inte
nsity
[cou
nts/
sec]
188186184182180178
Kinetic Energy (eV)
Gallium 3dhv=200eV
After H2SO4
/H2O2
700C Vacuum Anneal
No Clean
25x103
20
15
10N
orm
aliz
ed In
tens
ity [c
ount
s/se
c]
64605652
Kinetic Energy (eV)
Nitrogen 1shv=460eV
AfterH2SO4
/H2O2
700CVacuumAnneal
No Clean
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H2SO4/H2O2
H
C
H H
C
H H H
CGaN GaN
O
C
O O
C
O O O
C
Anneal
GaN
• Carbons 1s spectroscopy shows a conversion of hydrocarbons to more volatile oxides of carbon.
• Carbon contamination reduced to 1% and Oxygen 8% of a monolayer.
12.
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I. A sulfuric acid/hydrogen peroxide treatment followed by a vacuum anneal at 700C reduces carbon and oxygen concentrations to a few percent of a monolayer.
II. An ammonia annealing ambient is worse than a vacuum ambient for the thermal desorption of carbon and oxygen at temperatures at or below 740C.
III. The chemical state is predominantly a volatile oxide of carbon.
IV. We have a reproducible and clean GaN surface for our photocathode research achieved by a non-destructive sulfuric acid cleaning technique.
13.
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More on GaN
• Quantum yield• 20-30 % by Cs• 40-50 % by Cs+O
• Works in Transmission Mode with Sapphire/AlN/GaN
• Looking for collaboration: [email protected]
• Electron Gun Test Column• Current density and brightness measurement• Energy spread measurement
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VB after HCl Cleanof InP(100)
Inte
ns
ity
/ a
.u.
686664626058565452
K.E. / eV
VBhv = 70eV
r.t.
230oC
290oC
330oC
360oC
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Valence Band after Two Step cleaning and Annealing for InP(100)
Inte
ns
ity
686664626058565452 Kinetic Energy / eV
VBhv = 70eV
O2p
One Step
Two Step
Two Step
+330oC
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Surface Etching(2)
H2O2
In POO
OHO
3/2
InP
H+
In3+
POOH
OHOH
SO42-H+ H+ H+H2O2
Grow oxide:
InP + H2O2 + H+ In(HPO4)3/2
Etch oxide:In(HPO4)3/2 + H+ In3+ + H3PO4
In POO
OHO
3/2
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Oxide Removal
In POO
OHO
3/2
InP
H+
In3+
POOH
OHOH
SO42-
H+
Oxide Removal:In(HPO4)3/2 + H+ In3+ + H3PO4
H+
H+
InP
P
Elemental P Generation:H3PO4 + InP + H+ In3+ + P
In3+
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Hydrophilic Surface after HF Clean
In In In In In In In
F F F F F F F
H H H H O O O OH H H H
HydrogenBonds
InP
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GaAs(100) Valence Band
Inte
nsi
ty [A
rbitr
ary
Un
its]
8075706560
Kinetic Energy
VB
After annealing
After chemical clean