SUPERLATTICE
PHOTOCATHODES:
An Overview
Tarun Desikan
PPRC, Stanford University
[email protected]
OUTLINE

Spin polarized electrons quick study


Semiconductor polarized electron sources
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Method and results
Superlattice characterization
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The need for strain
Simulation of superlattice band structures
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Uses and requirements
X-ray diffraction and photoluminescence
Results
USES OF POLARIZED ELECTRONS

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High-energy physics
Surface analysis and imaging

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“Quantum” applications

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SPLEEM and SPSPM
Computing, cryptography
Single-electron devices
Spintronics
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Spin-polarized currents
Enhanced performance
REQUIREMENTS IN HEP

High polarization

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High charge

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1013 electrons per train
High speeds


Increases effective luminosity of Collider
>90%
Picosecond micro-bunches
Semiconductor photocathodes
SEMICONDUCTOR SOURCE
CB
E
k
mj = +1/2
+
Eg
+3/2
HH



SO
LH
-1/2
S1/2
-
+1/2
-1/2
+1/2
-1/2
-3/2
P3/2
P1/2
Photo-excitation by polarized laser beam
HH -> CB populates one spin state
LH -> CB populates the other spin state
Maximum polarization = 50%
NEGATIVE ELECTRON AFFINITY SURFACE
GaAs
Polarized electrons
Cs2O
Tunneling current
CB
VB
Circularly polarized laser photons


Polarized e- tunnel through to NEA material
and escape
Atomically clean surface at UHV
STRAINED PHOTOCATHODES
CB
E
mj = +1/2
Eg
k
+
+3/2
HH



SO
LH
+1/2
+1/2
-1/2
S1/2
-1/2
-1/2
-3/2
P3/2
P1/2
HH and LH no longer degenerate at k=0
HH -> CB populates one spin state,
LH -> CB does not occur
Maximum polarization = 100%
SAMPLE STRUCTURE
1000 A
25mm
25mm
Active Region
GaAs0.64P0.36
Buffer
GaAs(1-x)Px Graded
Layer
GaAs Substrate
SUPERLATTICE PHOTOCATHODES


Critical thickness (~100 A) limits the size of
strained active region
Practical limit is ~1000 A


Active region partially relaxes
Multiple quantum wells

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Strained material sandwiched between unstrained
layers
Strained region thickness < critical thickness
Band engineering
SUPERLATTICE BAND CALCULATIONS

k•p transfer matrix method

1
Chuang (UIUC), David Miller, Jim Harris (Stanford)
2
3
4
N+1
 AN  2 
 A1 

B   T B
 1
 N 2 
T  D1 .P2 D2 .P3 D3 ...PN 1 DN 1
N+2
SUPERLATTICE BAND CALCULATIONS

Must account for CB, HH, LH and SO

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CB decoupled
HH, LH and SO interact
Matrix solution to Schrödinger's equation
8x8 Hamiltonian
Strain effects incorporated into Hamiltonian
Boundary conditions
Reach MATLAB noise floor
SINGLE QUANTUM WELL SIMULATION
MULTIPLE QUANTUM WELL SIMULATION
SIMULATION RESULTS

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Must use consistent
parameters
Easy wrap-around
scripts
Spot trends
Compare with
experiments?
E
Lw
Effective
Band Gap
HH–LH Splitting
X-RAY DIFFRACTION


High-resolution XRD to analyze crystal
Study layer attributes

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Thickness
Composition
Strain
Tilt
Vendor specifications
XRD THEORY
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
Bragg’s Law:
n* = 2*d*sin()
All lattice planes
d


contribute to
Bragg diffraction
(004), (224), (113) planes commonly used
Every layer contributes a Rocking Curve peak
Repeating series of thin layers causes
additional peaks
ROCKING CURVES
RECIPROCAL SPACE MAP
cps

2
RECIPROCAL SPACE MAP
cps

2
SAMPLE STRUCTURE
1000 A
25mm
25mm
Strained GaAs
GaAs0.64P0.36
Buffer
GaAs(1-x)Px Graded
Layer
GaAs Substrate
OTHER CHARACTERIZATION TOOLS

Photoluminescence

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Band structure analysis
Check simulation predictions
SIMS
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Doping profile
Destructive
STRAINED SUPERLATTICE SVT-3682
1000 A
25mm
25mm
Active Region
GaAs0.64P0.36
Buffer
GaAs(1-x)Px Graded
Layer
GaAsP
30 A
Strained GaAs
30 A
GaAsP
Strained GaAs
GaAsP
Strained GaAs
GaAs Substrate
BAND STRUCTURE SIMULATION
BAND STRUCTURE
CB1
1.65 eV
HH1
0.86 eV
GaAsP

GaAs
GaAsP
LH1
GaAs
GaAsP
Photoluminescence confirms the simulation
prediction
LAYER THICKNESSES

(004) scan [above] as well as (224)
SVT-3682 ANALYSIS
Active Region
GaAs0.64P0.36
Buffer
GaAs(1-x)Px Graded
Layer
GaAs Substrate



aGaAs
6
aGaAs
P
0.64 0.3
6
aGaAs
aGaAs
Ideal
Actual
Well Width = Barrier Width = 32 A
Phosphorus fraction in GaAsP = 0.36
Strained GaAs does not relax significantly
P
0.64 0.3
SVT-3682 PERFORMANCE
Quantum
Efficiency
Polarization
SVT-3682 PERFORMANCE

Peak polarization of ~86%

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High QE

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A record at SLAC
> 0.2 % is great
No charge limit
A great photocathode!
Repeatable?
CONCLUSIONS
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High performance superlattice photocathodes
fabricated using GaAs/GaAsP
Further improvement by optimizing
parameters
Need to test validity of band structure
simulations
Extend simulation model to calculate
polarization