Growth and Properties of the Dilute Bismide Semiconductor GaAs1-xBix

Introduction•

Bismuth as a surfactant in semiconductor epitaxy•

Electronic structure of bismide alloys•

Transport properties•

Unsolved problems•

Outline of Presentation

T. Tiedje University of British Columbia

Introduction

Bi

Sb

As

P

N -

+

Group VBi is heaviest non-Radioactive element•

Non-toxic•

Strong tendency to surface segregate in MBE•

Large spin-orbit coupling•

Devices

Semiconductor alloys with temperature •insensitive bandgap (Oe, Kyoto)

Low threshold, low power HBT (Hase, Sony •US6936871, 2006)

Local strain compensator for nitrogen, solar •cells (Mascarenhas, NREL)

Bandgap bowing with Bi:88 meV/%Bi

Francoeur et al, APL 82 (2003)

Lattice constant 6.32 Å

GaBi

GaN

Giant Bandgap Bowing in Both Nitrides and Bismides

Tixier et al. APL 82, 2245 (2003)

C onductio nB and

Va lenceB an d

E n e rg y

E g

N N 2

H H

N 2s

S O

LH

C B C B

G a N xAs 1-x G a A s 1-xB ix

S O

LH

B i c lus te r ?

B i 6p

H H

E g

∆ o

States of Impurity Elements Resonant with Band Edges

N 2s orbital resonant with the bottom of the conduction band and Bi 6p resonant with the top of the valence band

Band Alignment

Drawn to scale for 3% Bi, N•

Favourable band alignment for •GaAs/GaAsBi HBT

Nitride Bismide

Giant Bandgap Bowing Effect

Bismide Nitride

0 5%[Bi]

Linear interpolation

In bismides, 85 meV/% bandgap change much bigger than in •InGaAs which is 11 meV/%

>100 meV/% in dilute nitride, opposite sign!•E. Nodwell et al Phys Rev. B 155210 (2004)

Bismide-Nitride Bandgap Map

Bandgap control with low N content

Example, 3.5% Bi + 2% N gives 1.55 µm bandgap, lattice matched to GaAs

Tixier et al. APL, 86, 112113 (2005)

VG-V80H solid source MBE deposition system•

Helical RF plasma source for nitrogen•

Conventional Knudsen effusion cell for Bi•

Bi BEP up to 10-5 Torr, 450-750oC•

incorporation < 2 x 1017 cm-3

Substrate temperatures 400oC - 600oC•

V/III flux ratio between 1 and 8•

GaAs Substrate

GaNAs Film ~225 nm

Surfactant MBE Growth of Dilute Nitrides

Surfactant Effect on Surface Morphology

No Bi FluxV/III ratio =1, rms ~ 1.2 nm

Vertical scale: 10 nm

High Bi Flux (~10-5 torr)V/III ratio =1, rms ~ 0.1 nm

Vertical scale: 0.8 nm

2 x 2 µm AFM images GaN0.004As0.996

Step flow growth at 460C, no Bi incorporation in SIMS•

Ideal surfactant?• Tixier et al., J. Cryst. Growth 251 (2003), 449

220

200

180

160

140

120

100

Spe

cula

r RH

EE

D In

tens

ity (a

rb. u

nits

)

140120100806040200time (s)

Substrate Temperature (°C)440 452 475 505 560

Measurement of Bi Coverage with RHEED

Desorption

Open Bi Close Bi

Change to 3x1 reconstruction with Bi flux•Specular intensity increases with Bi coverage•

Bi Flux = 1x10-6 torr

1.0

0.8

0.6

0.4

0.2

0.0

Bi C

over

age

(ML)

3.0x10-62.01.00.0Bi Flux (Torr)

modified langmuir modelRHEED data

Pressure Dependence of Bi Coverage

U = 1 eVzε = 0.2 eV

1.0

0.8

0.6

0.4

0.2

0.0

Bi C

over

age

(ml)

700650600550500450Bismuth cell temperature (°C)

Open BiClose Bi

Tsubs = 460oC

Langmuir Model for Bi Surface Coverage

Langmuir isotherm modified to include attractive Bi-Bi •interactions on surface

θ - Bi surface coverageP - Bi pressureU - binding energy of Bi to the surfaceε - lateral Bi-Bi interaction energyz - coordination number of Bi

expoU zb b

kTεθ+ =

1bP

bPθ =

+

1.0

0.8

0.6

0.4

0.2

0.0

Bi c

over

age

(ML)

800700600500400Substrate temperature(°C)

modified langmuirRHEED data

Temperature Dependence of Bi Coverage

U = 1 eVzε = 0.2 eV

Langmuir isotherm modified for lateral interactions•

Surface binding energy, vapour pressure, close to liquid Bi

Surfactant layer similar to liquid Bi

E. C. Young et al. J. Cryst. Growth 279, 316 (2005)

Effect of Bi on Nitrogen Incorporation

102

103

104

105

106

107

108D

iffra

cted

Inte

nsity

( cp

s )

6004002000-200θ ( arcsec )

Increasing Bi fluxIncreasing [N]

(004) x-ray diffraction peaks

Unexpected behaviour as both N, Bi are group V, competing for lattice sites

Effect of Bi on Nitrogen Incorporation

[ ] %58.0+∝ BiN θ

N content determined by x-ray diffraction•1.0

0.8

0.6

0.4

0.2

Nitr

ogen

Con

cent

ratio

n (%

)

14x10-6121086420Bismuth flux (Torr)

Data 460°CData 400°CModel 460°CModel 400°C

[ ] %40.0+∝ BiN θ

Possible mechanism: N content increases with Bi coverage• due to suppression of AsN re-evaporation (?)

Room Temperature Photoluminescence of InGaNAs:Bi QWs

1.0

0.8

0.6

0.4

0.2

0.0

RT

PL in

tens

ity (A

rb. u

nits

)

1400135013001250120011501100Wavelength (nm)

InGaNAs QWs (26% In; 1.1% N)

As GrownAnnealed at 730°C for 60s

With Bi (BEP ~10 -7Torr)

GaAs Substrate400 µm

InGaNAs QW

GaAs Cap~ 250 nm

low Bi flux ~ 10-7 torr•

Effect of Bi on PL SpectrumTemperature dependence of PL spectrum with and without Bi surfactant•

Low energy tail in PL reduced with Bi

0.54% N, no Bi 0.56% N, with Bi

Emission from Nx cluster states

Surfactant reduces density of localized N cluster statesD. Beaton, MSc Thesis UBC

Challenge of GaAsBi growth:

à avoid Bi and Ga droplet formation

5 x 5 µm scanVertical scale 350 nm

10 x 10 µm scanVertical scale 250 nm

Scat

tere

d Li

ght I

nten

sity

(au)

11x1031098765

Time (s)

5x10-7

4

3

2

1

Cham

ber Pressure (Torr)Open GaOpen Bi

Scattered light signalChamber pressure

In-situ Light Scattering Guides MBE Growth

As2 pressure

UV light scattering highly sensitive to metal (Ga, Bi) droplet formation•

Narrow process window•

Droplet formation

E. C. Young et al., Phys. Stat. Sol. (2007)

Diff

ract

ed In

tens

ity (

cps

)

200010000-1000-2000θ ( arcsec )

GaAs1-xBixx = 0.5%

GaAs1-xBixx = 1.9%

GaAs1-xBixx = 3%

Good Structural Quality

Vertical scale 3 nm rms roughness 0.205 nm Growth temp. 390 C !!

2 x 2 µm AFM

E. C. Young et al., PSS (2007)

14x103

12

10

8

6

4

2

0

RT

PL

Inte

nsity

(au)

14001300120011001000900800

Wavelength (nm)

GaAs1-xBix200 nm, x = 1.7%

GaAs1-xBix125 nm, x = 1.9%

GaAs1-xBix QW20 nm QW, x = 3.0%

10 layer MQW 5 nm InyGa1-yAs y = 20%

984 nm

1072 nm

962 nm

Bismides: Strong Room Temperature Photoluminescence

Efficient PL in low temperature grown material (T=360C), that is normally full of As antisites and poor electronic quality

Bismides

InGaAs QW

Ga GaBi

GaBiAs

GaBi As

In InN

InNN

GaN N

Ga

Light emitting center a •Bi cluster?

Indium cluster, light •emitting center in Ga(In)N

E. C. Young, PhD Thesis, UBC (2007)

Latticetemperature:

150 K175 K200 K225 K250 K275 K300 K

PL (a

rb. u

nits

)

1.30 1.35 1.40 1.45

200 K

Energy (eV)

PL (a

rb. u

nits

)

1.80 1.82 1.84 1.86 1.88

× 10

Latticetemperature:

5 K20 K40 K60 K80 K

100 K125 K150 K

PL (a

rb. u

nits

)

Energy (eV)

Small Spot Photoluminescence in Bismides

Conduction band to Heavy Hole

Split-off hole band

Substrate bandgap

Fluegel, Mascarenhas, NRELS-O hole band measure of spin-orbit splitting

1.30

1.40

1.50

1.60

1.80

1.85 (a)

Band gap PL

Spin-Orbit PL

150 KPL

Ene

rgy

(eV)

0.0 0.5 1.0 1.5 2.00.30

0.40

0.50

(b)

SO s

plitt

ing

(eV)

Bi concentration (%)

“Giant” Increase in Spin-Orbit Splitting

Fluegel, Mascarenhas et al. PRL 97, 067205 (2006)

Increase in spin-orbit splitting in parallel with reduction in bandgap

Frequency Dependent Conductivity

Bismide: Drude-like Nitride: non-Drude

Conductivity is dominated by electrons•

Bismide result similar to host GaAs, nitride rather different, consequent •of N affect on the conduction band. Explanation?

Measured 10 ps after optical injection of ~1018 cm-3 e-h pairs

Terahertz Measurements of Electron Mobility

D. Cooke, F. Hegmann et al APL 89, 122103 (2006)

1% N has drastic effect on electron mobility, Bi has comparatively little effect

Conclusions

Dilute bismide semiconductors belong to a class of semiconductor •alloys with a dilute, impurity-like component, analogous to dilute nitrides (GaAsN) etc.

Bi is an ideal surfactant (no incorporation) under certain conditions, •improves structure and electronic properties, enables low temperature growth

Need for calculations of spin-orbit splitting in bismides, conduction •band and valence band

Promising device applications •

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