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)

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.

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

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.

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%

One Step Chemical Cleaning

CHEMICALETCHING

VACUUMANNEAL

INERT(Ar) ATMOSPHERE UHV

• Not effective for InP(100) !

H2SO4 : H2O2 : H2O (4:1:100)

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

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

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%)

Two Step Chemical Cleaning

CHEMICALETCHING

VACUUMANNEAL

OXIDE REMOVAL

INERT(Ar) ATMOSPHERE UHV

H2SO4:H2O2:H2O4:1:100

HCl(36%): H2O 1:3HF 1%

HCl Solution as Second Step

Surface is Hydrophobic

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

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

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

HF Solution as Second Step

Surface is Hydrophilic

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

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

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

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

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

GaAs(100) Cleaning recipe

H2SO4:H2O2:H2O *

4:1:100

+

Vacuum annealing at 500C

*H2SO4 98%; H2O2 30%; NH3 30%

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

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

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

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

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

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

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

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.

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.

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: fmachuca@stanford.edu

• Electron Gun Test Column• Current density and brightness measurement• Energy spread measurement

VB after HCl Cleanof InP(100)

Inte

ns

ity

/ a

.u.

686664626058565452

K.E. / eV

VBhv = 70eV

r.t.

230oC

290oC

330oC

360oC

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

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

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+

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

GaAs(100) Valence Band

Inte

nsi

ty [A

rbitr

ary

Un

its]

8075706560

Kinetic Energy

VB

After annealing

After chemical clean