Polarization comparison of InAlGaAs/GaAs superlattice

photocathodes having low conduction band offset

K. Ioakeimidi, T. Maruyama, J.E. Clendenin, A. BrachmannStanford Linear Accelerator Center

Yu.A.Mamaev, L.G.Gerchikov, Yu.P.Yashin, D. Vasilyev

St. Petersburg State Polytechnic University V.M.Ustinov and A.E.Zhukov

Ioffe Physico-Technical Institute

R. Prepost University of Wisconsin

85-90%85-90%

Main depolarization mechanisms• Interband absorption

smearing due to bandedge fluctuations

• Hole scattering between HH and LH states causes a LH broadening

• Optical phonon scattering

• Non polarized electrons generated in the BBR

• Scattering of electrons in the BBR

Vacuum

electron6nm GaAs

100nm

Ec

HH1

Valence BandLH1

Ev

AlGaAs

Buffer

Conduction Band

BBR

HH- LH separation determined by well/barrier thickness and strainEC/EV determined by SL structure

Motivation for a flat Conduction Band structure• Observed emission spectrum

• Emitted electron polarization

dRYem

1

11

emS

S

SS

dPP

1

1

00 1

: optical absorptionR: reflection coefficient1: electron lifetime in BBRem: time of electron emission in vacuum P0: initial electron polarization upon excitation with circularly polarized lightS0: surface recombination velocityS : spin relaxation time in SLS1 : spin relaxation time in BBR

A. Subashiev et. al. SLAC-PUB 10901

Vacuumelectron6nm GaAs

100nm

AlGaAsBuffer Conduction Band

HH1

BBR

Valence BandLH1

Ev

Ec

Design of a flat conduction band structureStrained Barrier AlyInxGa1-x-yAs/GaAs SL on GaAs substrate

y: determines the bandgapx: lowers bandgap, controls EC and induces strain

EC(x,y)=QC1*Eg1(x)+QC2,def*(Eg2(y)+EC,def)Yu. A. Mamaev et al., PST2003

Unstrained wells and compressively strained barriers GaAs/AlyInxGa1-x-yAs SL

E vl2

E vh2

E c2

E v1

E c1

e1

lh1

hh1

Al0.21 In0.20 Ga 0.62 As

GaAs

8

6

Yu. A. Mamaev et al., Mainz 2004

dQW(Å) E hh2 E lh1 E hh1 E e1

10 -0.2054E+00 -0.1160E+00 -0.5120E-01 1.429 15 -0.1752E+00 -0.1041E+00 -0.4413E-01 1.429 20 -0.1542E+00 -0.9352E-01 -0.3794E-01 1.428 25 -0.1389E+00 -0.8409E-01 -0.3267E-01 1.428 40 -0.9484E-01 -0.6199E-01 -0.2145E-01 1.427

PhotocathodesSample # In % Al

%SL

Bandgap

eV

BBR doping cm-3

E(meV)

LH-

HH

EC

(meV)

meV)meV)

5506 17 18 1.449 1e19 52 19 15 20

5501 20 21 1.454 1e19 70 19 15 20

5503 23 25 1.469 1e19 68 10 15 20

5777 20 23 1.471 1e19 60 3 17 25

6329 20 22 1.463 7e18 61 11

6410 28 35 1.542 7e18 90 23

QW: 1.5nmQB: 4nmBBR: 6nmSL doping: 4e17cm-3

GaAsP/GaAs SL: 89meV LH-HH splitting EC=36meV, EV=19meV

650 700 750 800 850

10-2

10-1

100

QE SL-5506 27.09.2005 QE SL-5506 29.09.2005 Al

18In

17Ga

65As(4nm)-GaAs(1.5nm); ==15meV

Al18

In17

Ga65

As(4nm)-GaAs(1.5nm); ==15meV + BBR

QE,

%

wavelenght, nm

0

10

20

30

40

50

60

70

80

90

POL SL-5506 27.09.2005 POL SL-5506 29.09.2005 Al

18In

17Ga

65As(4nm)-GaAs(1.5nm); ==15meV; P*0.95

Al18

In17

Ga65

As(4nm)-GaAs(1.5nm); ==15meV + BBR

P, %

Green curve – without BBR absorption; Black curve – with BBR

#5506: In.17Al.18Ga.65As/GaAs SL

650 700 750 800 850

10-2

10-1

100

QE SL-5501 03.10.2005 QE SL-5501 04.10.2005 QE SL-5501 05.10.2005 QE SL-5501 06.10.2005 Al

21In

20Ga

59As(4nm)-GaAs(1.5nm); =15 meV =20meV

+BBR

QE,

%

wavelenght, nm

0

10

20

30

40

50

60

70

80

90

100

POL SL-5501 03.10.2005 POL SL-5501 04.10.2005 POL SL-5501 05.10.2005 POL SL-5501 06.10.2005 Al

21In

20Ga

59As(4nm)-GaAs(1.5nm); =15 meV =20meV; P*0.95

Al21

In20

Ga59

As(4nm)-GaAs(1.5nm); =15 meV =20meV; P*0.95+BBR

P, %

Green curve – without BBR absorption; Orange curve – with BBR

#5501: In.20Al.21Ga.59As/GaAs SL

650 700 750 800 850

10-3

10-2

10-1

100

QE SL-5503 21.09.2005 QE SL-5503 22.09.2005 QE SL-5503 23.09.2005 Al

25In

23Ga

52As(4nm)-GaAs(1.5nm); ==30meV

Al23

In23

Ga54

As(4nm)-GaAs(1.5nm); ==30meV Al

25In

23Ga

52As(4nm)-GaAs(1.5nm); ==30meV; + BBR

QE

, %

wavelenght, nm

0

10

20

30

40

50

60

70

80

90

POL SL-5503 21.09.2005 POL SL-5503 22.09.2005 POL SL-5503 23.09.2005 Al

25In

23Ga

52As(4nm)-GaAs(1.5nm); ==30meV; P*0.90

Al23

In23

Ga54

As(4nm)-GaAs(1.5nm); ==30meV; P*0.90 Al

25In

23Ga

52As(4nm)-GaAs(1.5nm); ==30meV; P*0.95 + BBR

P, %

Green curve – In0.23Al0.25Ga0.54As – 1.5 nm GaAs without BBR absorption

Blue curve – , In0.23Al0.23Ga0.54As – 1.5 nm GaAs without BBR absorption; Black curve – In0.23Al0.23Ga0.54As – 1.5 nm GaAs with BBR

#5503: In.23Al.25Ga.52As/GaAs SL

1.4 1.5 1.6 1.7 1.8 1.9

20

40

60

80

100

Pol

ariz

atio

n, %

Photon energy, eV

Experiment SL 5-777 Theory In

20Al

23GaAs(40)/GaAs(15)*0.95, =25, =17meV

Theory In20

Al23

GaAs(36)/GaAs(15)*0.95, =25, =17meV

#5777: In.20Al.23Ga.59As/GaAs SL

92%

Simulations vs data analysis• Data indicates a blue shift for the polarization peak for

samples 5501, 5503, 5506

• In theory longer wavelengths give higher polarization because they obey better the selection rules

• In practice longer wavelengths photogenerate electrons primarily in the BBR where there is no polarization selectivity and also

• the electrons photogenerated in the SL structure in this case thermalize more in the BBR and they have lower tunneling probabilities due to tighter confinement and due to the CB peak at the interface between the SL and the BBR.

#6329: In.20Al.22Ga.58As/GaAs SL

85

80

75

70

65

60Po

lari

zati

on (

%)

860850840830820

Wavelength (nm)

St. P 520 C 540 C 540 C

St. Petersburg 78%CTS 76 ~ 77%

850800750700650

Wavelength (nm)

80

60

40

20

Pola

riza

tion

(%

)

St. P CTS

#6410: In.28Al.35Ga.37As/GaAs SL

80

60

40

20

Pola

riza

tion

(%

)

800780760740720700680660

Wavelength (nm)

St. P CTS

85

80

75

70

65

60

Pola

riza

tion

(%

)

800795790785780

Wavelength (nm)

St. P CTS 1st activation CTS 2nd activation

St. Petersburg 74%CTS 77%

(004) X-ray

Indium fraction (assume 100% strain): 5-777 19.6% 5501 20.7%

Superlattice thickness: 5-777 5.01 nm 5501 5.10 nm

5-777 is better

(004) simulation

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

(deg.)

0

+1

SimulationI n0.20Al0.21Ga0.59As (4 nm) - GaAs (1.5 nm)Assume 100% strain

Summary of Polarization Results

Sample # In % Al% Polarization %

5506 17 18 82-85

5501 20 21 84-90

5503 23 25 75-82

5777 20 23 92

6329 20 22 76-78

6410 28 35 75-82

How can we explain 90%?• Polarization calibration seems OK

within 2-3%.• 5-777 had an As-cap, allowing a

low temp heat-cleaning.• When 5-777 was heated to >540 C,

polarization dropped to 84%, which is consistent with SVT-5501.

• Surface effect supported by the temperature of the heat cleaning dependence of the results

• X ray shows decreased barrier size that would imply higher strain

• Strained barrier structures do not preserve strain in the BBR; we think that 5777 might have reduced BBR thickness.

Conclusions• The flat CB structures show promising polarization

results with a record polarization of 92%.

• Further analysis needs to be done in order to understand depolarization effects primarily in the BBR.

• The flat CB photocathodes show sensitivity to heat cleaning temperature. The heat cleaning effect needs to be explored further with SIMS analysis.

More SVT results

80

70

60

50

40

30

20

Pola

riza

tion

(%

)

850800750700650

Wavelength (nm)

0.001

0.01

0.1

QE (%

)

80

70

60

50

40

30

20

10

Pola

riza

tion

(%

)

850800750700650

Wavelength (nm)

2

3

4

5

678

0.1

2

3

4

5

678

1

QE (%

)

SVT-5503 SVT-5506

SVT-5501 (5-777 duplicate)

2

3

4

5

678

0.1

2

3

4

5

678

1

2

QE (%

)850800750700650

Wavelength (nm)

80

60

40

20

Pola

riza

tion

(%

)

95

90

85

80

75

70

65

60

Pola

riza

tion

(%

)

850845840835830825820

Wavelength (nm)

1st activation 500 C 24 hr af ter 1st activation 2nd activation 500 C 3rd activation 540 C

Peak polarization ~84%

Dear Takashi,

We have analyzed your recent data on SL-5501-5506 and agree with your conclusion that the polarization losses could be due to the surface effect. To confirm this assumption we compare your experimental data (points) with our calculations (curves - see figures below).

First we calculate the initial polarization of photoelectrons in the working layer using your data of layer composition and thickness.

Then we reduce the resulting polarization of emitted electrons by 5% due to the polarization losses in the surface region (. “Energy resolved spin-polarized electron photoemission from strained GaAs/GaAsP heterostructure”, Yu.A.Mamaev, A.V.Subashiev, Yu.P.Yashin, H.-J.Drouhin and G.Lampel, Solid State Comm., Vol. 114, No 7, 2000, pp 401-405.).

We have reasonable agreement between the theory and experiment for the overall behavior of polarization spectra for SL 5501 and 5506. For SL 5503 the agreement can be achieved if we assume that the actual Al concentration in barrier layer is 2% lower, i.e. SL 5503 4 nm In0.23Al0.23Ga0.56As – 1.5 nm GaAs.

However there is a systematical discrepancy between the theory and experiment in the region of polarization maximum, namely experimental peak is blue shifted and smaller in amplitude. In our calculations the position of the polarization maximum is close to the photoabsorption edge. The QY spectrum behavior near the edge evidences that the position of photoabsorption edge is calculated correctly. Thus the observed blue shift of polarization maximum and large polarization losses might be caused by an additional physical reason.

We assume that this effect can arise from the photoabsorption in the surface (BBR) layer (see our last joint paperA. V. Subashiev, L. G. Gerchikov, Yu. A. Mamaev, Yu. P. Yashin, J. S. Roberts, D.-A. Luh, T. Maruyama, and J. E. Clendenin “Strain-Compensated AlInGaAs-GaAsP Superlattices for Highly-Polarized Electron Emission”, Appl. Phys. Lett. 86 (2005) 171911). In figures below we show the polarization spectra accounting for the BBR contribution to electron emission. Figures demonstrate that this contribution cuts the red side of the polarization maximum resulting in its sizable blue shift and decreasing of amplitude.

The similar effect we also observe in SL 6-329 and 6-410. However it seems that in our best sample, SL 5-777, there is no visible BBR contribution. In the last figure we show the polarization spectrum of SL 5-777 together with our calculations. We use the latest Ioffe PTI data on its composition. According to this data SL 5-777 is 3.6 nm In0.20Al0.23Ga0.57As – 1.5 nm GaAs. The barrier thickness we reduce by 0.4nm according to X-ray results, though this change does not almost affect the polarization spectrum. Figure show that our calculations without BBR contribution (initial polarization – 5% surface losses) reproduce well the experimental spectrum. Unfortunately we do not understand at the moment why some samples are good and some samples with close or even the same design are bad.

Vacuum

electron6nm GaAs

100nm

Ec

HH1

Valence Band

LH1

E v

AlGaAs

Buffer Conduction BandBBR

InAlGaAs-GaAs superlattice

• St. Petersburg group observed 90% polarization from InAlGaAs-GaAs superlattice (5-777).

• SVT grew three wafers– 5506: 4 nm In0.17Al0.18Ga0.59As – 1.5 nm GaAs 18 periods– 5501: 4 nm In0.20Al0.21Ga0.59As – 1.5 nm GaAs 18 periods (duplicate of 5-777)– 5503: 4 nm In0.23Al0.25Ga0.59As – 1.5 nm GaAs 18 periods

• Peak polarization of SVT-5501 was 84%.• Three samples from St. Petersburg

– 5-777 Could not activate to high enough QE. No measurements.– 6-329 4 nm In0.2Al0.22Ga0.58As – 1.5 nm GaAs 18.5 periods

– 6-410 3.07 nm In0.28Al0.354Ga0.366As – 0.56 nm GaAs 15 periods

• X-ray diffraction of Ioffe 5-777 and SVT-5501