Novel Semiconductor Alloys based on GaSb for domestic PV

Novel semiconductor alloys based on GaSb

(for domestic thermophotovoltaics?)M. K. Rajpalke1, W. M. Linhart1, M. Birkett1, K. M. Yu2, T. S. Jones3,

J. Kopaczek4, J. Misiewicz4, R. Kudrawiec4, M. J. Ashwin3, TimVeal1

1Stephenson Institute for Renewable Energy and Department of Physics, Univ. of Liverpool, UK2Lawrence Berkeley National Labs, Berkeley, USA3Department of Chemistry, University of Warwick, Coventry, UK4Institute of Physics, Wrocław Univ. of Technology, Wrocław, Poland

Outline…

Summary of research on semiconductor materials for photovoltaics

Thermophotovoltaics

Molecular beam epitaxy of GaSb1-xBix alloys

Structural and compositional characterisation of GaSb1-xBix alloys

FT-IR absorption, photoreflectance and PL studies of GaSb1-xBix

alloys

Band structure of GaSb1-xBix alloys

Conclusions

2

Semiconductor materials for photovoltaics

3

Three main areas of research in progress:

Earth abundant semiconductors for sustainable TW scale solar PV

High mobility transparent conducting oxides

Semiconductors for improved efficiency thermophotovoltaics

4

Optical absorption spectroscopy of CuSbS2 thin films

DFT, Scanlon et al. (2012) Samples from Zaketayev et al., NREL

5

1020

1021

10

1005

1

7

43

2

Ele

ctr

on m

obili

ty (

cm

2V

-1s

-1)

Electron concentration (cm-3)

normal IIS

IIS with remote screening

In2O

3 undoped

In2O

3:Mo

In2O

3:Sn

1

Mo-doping of In2O3 for high mobility

Mo+ not Mo3+

CVD samples from Parkin group, UCL

Photovoltaics

Conversion efficiency

Eg

cb

vbEF

hn

Ener

gy

Conversion efficiency

Eg

cb

vbEF

hn

p-type n-type

hn

One electron per photon Eg = energy available from each

K. Durose

Power at ground level is about 1000 W/m2

Thermophotovoltaics (TPV)

Solar photovoltaics can be seen as a special case of the more general thermophotovoltaics (TPV).

TPV systems consist of a heat source, an absorber/emitter material, a spectral filter and a

photovoltaic cell. In solar PV, the heat source is the sun!

Solar PV versus TPV

The Sun is about 1.5x1011 m from the Earth.

So this is the emitter to PV cell distance in solar PV.

The incident power density from the Sun on Earth at ground

level is about 103 W/m2.

Power output from solar PV is about 150 W/m2

In TPV, the emitter is typically 10-1 m from the PV cell.

The incident power density onto PV cell can be 106 W/m2.

Power outputs can be higher than 104 W/m2.

TPV applications

TPV has no moving parts and is quiet. The applications of TPV and associated heat

sources are wide ranging and include:

(i) combined heat and power (CHP) generation from domestic boilers, so-called

micro-CHP, providing central heating and converting waste heat into

electricity;

(ii) CHP in remote (off-grid) locations to avoid the need for a separate generator;

(iii) hybrid cars, where the gas powered engine exhaust heat is converted by a TPV

device to charge the batteries which power the electric drive;

(iv) recovery of waste heat from nuclear power sources, especially for space

applications and submarines;

(v) military use to partly replace batteries or to charge them using logistic fuels;

and

(vi) recovery of waste energy from high temperature industries such as glass, steel

and cement.

TPV History

TPV originated in the 1960s

It has had mixed fortunes ever since

Interest seems to periodically fluctuate largely based around US military funding

Recent revival including interest from MIT and IMEC

Sleeping giant of low carbon energy? Or already extinct?

13

Feasibility of TPV for combined heat and power application in

residential buildings?

M. Bianchi et al., Appl. Energy 97 (2012) 704

Assuming gas boiler as heat source and

10% efficient TPV cell in a house in Northern

Italy:

• 35% reduction in purchased electricity

• 10 % total energy saving

• 10 year monetary payback period

Water heating load curve Winter heating load curve

14

Winter: thermal load is

high, so TPV electricity

production is high and

exceeds demand – sell to

grid

Summer: thermal load is

low, so TPV production is

low and below demand

(Here’s where solar PV is a

big help.)

M. Bianchi et al., Appl. Energy 97 (2012) 704

Temperature dependence of black-body spectrum

Planck’s law

Solar PV versus TPV

For solar PV, where the equivalent black body temp is 5777 K, need

semiconductors with band gap of 1.5 eV (0.83 µm)

Si (1.1 eV), CdTe (1.44 eV), CuInGaSe2 (1.0-1.7eV) are typically used.

Si is abundant, but has an indirect band gap.

For TPV, typically have 1000 - 1600 K heat source often from burning

hydrocarbons , so need semiconductor with band gap of 0.4-0.7 eV (3.1-1.8

µm)

GaSb (0.73 eV) is frequently used, but its band gap is slightly too high.

Ge (0.67 eV) has also been used, but has an indirect band gap.

Need lower band gap materials. GaInAsSb has been used but miscibility gap is a

problem at compositions which give the right band gaps.

New materials?

Introduction – dilute nitrides and GaNxSb1-x

17

Incorporation of a small amount of N atoms in III-V

semiconductors leads to a large band gap reduction.

This phenomenon is relatively

unstudied in antimonides.

Offers the possibility of tuning

the band gap with lattice matching

via the incorporation of In or Bi.

The properties of GaNxSb1-x

(and also Ga1-yInyNxSb1-x and

InNSb) have recently been determined.

18J. Wu et al., Semicond. Sci. Technol. 17 860 (2002)

Band gap tuning using dilute amounts of nitrogen

Band gap reduction by substitution of nitrogen atoms on a few per cent of the anion sites in III-V semiconductors is well known, particularly for GaNxAs1-x and GaInNAs.

Nitrogen introduces localised level in the band structure which interacts with host conduction band to form two non-parabolic subbands.

The N level and conduction band interaction is described as band anticrossing.

Perturbation theory applied to the nitrogen localized states and the host extended states gives:

19

Band gap tuning using dilute amounts of nitrogen

The band anticrossinginteraction results in the lowest subband moving to lower energy as the amount of N is increased.

This enables the band gap to be tuned by controlling the amount of N.

Of course, the band gap of GaN is greater than that of GaAs, so initially adding N was expected to result in a larger band gap.

J. Wu et al., Semicond. Sci. Technol. 17 860 (2002)

Introduction – dilute nitrides and GaNxSb1-x

20

• Incorporation of a small amount of N atoms in III-V

semiconductors leads to a large band gap reduction.

• This phenomenon is relatively

unstudied in antimonides.

• Offers the possibility of tuning

the band gap with lattice matching

via the incorporation of In or Bi.

• The properties of GaNxSb1-x

(and also Ga1-yInyNxSb1-x and

InNSb) have recently been determined.

• However, GaNSb does not exhibit PL.

Introduction – dilute bismides and GaSb1-xBix

21

Incorporation of a small amount of Bi atoms in III-As semiconductors has been

shown to lead to a large band gap reduction, 84-90 meV/Bi%, in GaAsBi.

Gowth of InSbBi ternary alloys dates back to 1969

– it was the first “dilute bismide” alloy [eg. 1,2]

GaSb1-xBix grown by liquid phase epitaxy with

x < 0.01 [3]

Very few reports on growth of epitaxial GaSbBi

alloys [4,5]. Previous MBE material had lattice

contraction as Bi was added rather than expected

expansion.

Optical properties of GaSbBi are not yet established.

[1] J. L. Zilka. et.al. J. Appl. Phys. 51,1549 (1980).

[2] J. J. Lee.et.al. Appl. Phys. Lett. 70 , 3266 (1997)

[3] P. Gladkov.et.al. Journal of Crystal Growth 146 ,319 (1995)

[4] S.K. Das, et.al. Infrared Physics & Technology 55 (2012) 156

[5] Y.Song, et.al. J. Vac. Sci. Technol. B 30, 02B114 (2012)

C.A. Broderick, M.Usman,

S.J. Sweeney and E.P. O'Reilly,

Semicond. Sci. Technol. 27

094011 (2012)

GaAsBi

8-14 micron atmospheric transmission window

3-5 micron atmospheric transmission window

3-5 and 8-14 micron wavelengths are used for remote gas sensing, night vision, bio-medical imaging for diagnosis in healthcare and detection in optical spectroscopy.

Also 2-4 microns for thermophotovoltaics for waste heat recovery from glass and steel manufacturing and also from domestic heating systems.

Mid-infrared and long wavelength infrared applications

23

Dilute nitride antimonides and dilute bismide antimonides

GaBi

GaSb1-xBix

The dilute nitride arsenideshave been widely studied, but the dilute nitride antimonides have received relatively little attention.

N and In can be added to GaSb to form lattice-matched and narrower band gap GaInNSb.

Can also use N and Bi for lattice matching.

InSbBi is long studied but not well understood.

Very little has been done on GaSbBi

24

Conduction Band and Valence Band Anticrossing

Bi states located near the valence bands

anticrossing

N states located near the conduction

band anticrossing

25

Interaction between host and impurity p-like

states. Bi introduces 6 p-like localized states Interaction of localized N levels with

extended states of the conduction band.

Conduction Band and Valence Band Anticrossing

Molecular-Beam Epitaxy at Warwick

Gen II MBE system:

Group III cells: Al, Ga, In

Group V cells: As, Sb, BiN plasma source (250 – 500W, 0.01 – 0.5 sccm)

Dopant cells: Si , Te and BeGrowth on 50 mm diameter or ¼ 50mm diameter wafers

26

GaSb1-xBix Growth on GaSb (0 0 1) Substrates

27

GaSb buffer layer growth

Growth of GaSb1-xBix epilayers

Series 1, growth temp.-dependent:

Growth rate: 0.4 µm/h

Growth temp. range: 250-350°C

Series 2, growth rate-dependent:

Growth temps: 250 and 275°C

Growth rate range: 0.4 – 1.3 µm/h

GaSb (0 0 1)

GaSb buffer

GaSb1-xBix

100 nm

400 nm

Structural Characterisation of GaSb1-xBix

28

Omega-2 theta scan of GaSb1-xBix on GaSb (001)

Lattice expansion upon Bi incorporation

S.K. Das, et.al. Infrared Physics & Technology 55

(2012) 156

Y.Song, et.al. J. Vac. Sci. Technol. B 30, 02B114 (2012)

x=0.05 x=0.036

Lattice

contraction

Lattice

expansion

M. Rajpalke, T. D. Veal et al.,

Appl. Phys. Lett. 103, 142106 (2013)

Bismuth Content from RBS

29

RBS data from a film grown at 250°C

with x = 0.05.

The Bi content measured by RBS as a

function of growth temperature

>99% of the Bi atoms are substitutional on Sb site from channelling studies

M. Rajpalke, T. D. Veal et al., Appl. Phys. Lett. 103, 142106 (2013)

30

Structural Characterisation of GaSb1-xBix

Modelling of x-ray diffraction used to determine Bi-induced lattice

constant change and film thickness

30.20 30.25 30.30 30.35 30.40

GaSbBi 004

Inte

nsity (

Arb

.Units)

Omega (degrees)

GaSb0.964

Bi0.036

/GaSb

GaSb 004


0.00 0.02 0.04 0.06

6.096

6.099

6.102

6.105

Lattic

e c

onsta

nt (Å

)

Bi content, x, from RBS

Bismuth Content in GaSb1-xBix

31

Hybrid functional DFT with 1 Bi atom in 64 atom supercell gives 0.117% linear

expansion for Bi on the Sb site for a GaSb1−xBix alloy composition corresponding to

x = 0.03125

M. Rajpalke, D. O. Scanlon, T. D. Veal et al., Appl. Phys. Lett. 103, 142106 (2013)

Kinetic Modelling of Bi Incorporation in GaSb1-xBix

32

The bismuth content, x, is given by Z. Pan.et.al, Appl. Phys. Lett. 77, 214 (2000)

C. E. C. Wood. et.al, J. Appl. Phys. 53, 4230 (1982)

M. J. Ashwin, T. D. Veal et.al, AIP Advances 1,

032159 (2011)

M. J. Ashwin,T. D. Veal et.al, J. Appl. Phys.113,

033502 (2013)

K is the rate constant for Bi incorporation, αJGa, where JGa is the incident Ga flux

α is a constant

NGa is the Ga atom density

JBi is the incident Bi flux

D is the bismuth desorption rate constant given by

Bi atoms

T-dependence of Bi Incorporation in GaSb1-xBix

Bi content increases as temperature is reduced, before reaching plateau at low temp.

Ed = 1.75 eV and s = 6.5μs

33

GaNAs: Ed = 2.1 eV and s = 5 μs

GaNSb: Ed = 2.0 eV and s = 5 μs

Z. Pan.et.al, Appl. Phys. Lett. 77, 214 (2000)

M. J. Ashwin, T. D. Veal et.al, AIP Advances 1, 032159 (2011)

225 250 275 300 325 3500.00

0.01

0.02

0.03

0.04

0.05

Bi co

nte

nt,

x,

fro

m R

BS

Growth temperature (oC)

M. Rajpalke, T. D. Veal et al., Appl.

Phys. Lett. 103, 142106 (2013)

34

Growth rate-dependence of Bi incorporation in GaSb1-xBix

M. Rajpalke, M. J. Ashwin, T. D. Veal et al., J. Appl. Phys. 116, 043511 (2014).

FT-IR optical absorption studies of GaSb1-xBix

35

Optical absorption measurements and valence band anticrossing modelling

room temp. band gap varies from 720 eV for GaSb to 540 meV for GaSb0.95Bi0.05,

band gap reduction of 36 meV/%Bi.


Valence Band Anticrossing in GaSb1-xBix

36

Large band gap reduction with increasing Bi content

Not explained by the virtual crystal approximation

(VCA) alone

Need VCA lowering of CBM plus VBAC

M. Rajpalke, W. M. Linhart, T. D. Veal et al.,

Appl. Phys. Lett. 103, 142106 (2013)

T = 295 K

k.P method: K. Alberi et al.,

Phys. Rev. B 75, 045203 (2007)

37 K. Alberi et al., Phys. Rev. B 75, 045203 (2007)


38

Band gap reduction is 36.3 meV/%Bi

Virtual crystal approximation (VCA) accounts for 26.3 meV/%Bi (26 meV CBM, 0.3 meV, VBM)

Also need valence band anticrossing to give additional 10 meV/%Bi

GaBi has an inverted band structure with band gap of -2.14 eV (hybrid DFT, Scanlon & Buckeridge)

[Earlier GaBi band gap of -1.45 eV LDA+C DFT, A. Janotti et al., PRB 65, 115203 (2002).]

M. Rajpalke, D. O. Scanlon, J. Buckeridge, T. D. Veal et al., Appl. Phys. Lett. 103, 142106 (2013)

0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

GaBi

CBM

En

erg

y (

eV

)

Bi content in GaSbBi (%)

VBM

GaSb

Virtual Crystal Approximation (VCA)

39


15K

Exp.

Fit

Mod.

270K

15K(b) x=2.1%

PR

(a

rb.u

.)

270K

15K

GaSbBi(a) x=0.7%

GaSb1-x

Bix

GaSb

0.55 0.60 0.65 0.70 0.75 0.80 0.85

270K

(c) x=4.2%

Energy (eV)

Photoreflectance data confirms absorption

results and VCA and VBAC model also fits 15 K

PR data.

J. Kopaczek, R. Kudrawiec, T. D. Veal et al., Appl. Phys. Lett. 103, 261907 (2013)

40

DFT density of states of GaSbBi with 10%

Bi compared with GaSb, illustrating role of

Bi 6s states in lowering CBM

M. Polak, T. D. Veal et al., J. Phys. D.: Appl. Phys. 47, 355107 (2014)

DFT calculation of GaSb1-xBix band gaps

41

T-dependence of GaSb1-xBix band gap from PR

0.60 0.65 0.70 0.75 0.80

x16x16x16

x16

x16

x8

50K

70K

90K

110K

130K

150K

170K190K

210K

230K

250K

270K

290K

30K

Energy (eV)

Ph

oto

refle

cta

nce

(a

rb.u

.) 15K

x=0.7%

x2

x4

x8

x16

0.60 0.65 0.70 0.75 0.80

GaSb1-x

Bix

50K

70K

90K

110K

130K

150K

170K

190K

210K

230K

250K

270K

290K

30K

Ph

oto

refle

cta

nce

(a

rb.u

.)

Energy (eV)

15K

x=2.1%

x2

x4

x5

x15

0.60 0.65 0.70 0.75 0.80

x6

x6

x2

x2

50K

70K

90K

110K

130K

150K

170K

190K

210K

230K

250K

270K

30K

Ph

oto

refle

cta

nce

(a

rb.u

.)

Energy (eV)

15K

x=4.2%

x2

x6


42

T-dependence of GaSb1-xBix band gap from PR

0 50 100 150 200 250 3000.60

0.65

0.70

0.75

0.80

0.85 Exp. data

Varshni fit

B-E fit

En

erg

y(e

V)

Temperature (K)

x=0.7%

x=2.1%

x=4.2%

x=0%

GaSb1-x

Bix

0 50 100 150 200 250 300

10

15

20

25

30 x=0.7%

x=2.1%

x=4.2%

Fit by Eq.(5)

(

me

V)

Temperature (K)

GaSb1-x

Bix

• Band gap variation with temperature for GaSbBi is similar to that for GaSb

• Broadening of PR signal (intrinsic lifetime, alloy scattering) for GaSbBi is 5-10 times lower

than for InGaAsBi – due to lower size mismatch between Sb and Bi compared to As and Bi.


43

Photoluminescence from GaSb1-xBix

Low and high energy PL peaks observed

Material quality superior to GaNSb

where no PL is seen

J. Kopaczek, R. Kudrawiec,

T. D. Veal et al., Appl. Phys.

Express 7, 111202 (2014)

44

High energy PL peak follows band gap from PR

Low energy PL peak is attributed to

recombination between CB and acceptor level

Origin of the acceptor level is unknown

Capacitance voltage measurements of carrier

density are plannedJ. Kopaczek, R. Kudrawiec,

T. D. Veal et al., Appl. Phys. Express 7, 111202 (2014)

45

Band gap reductions per 0.01Å lattice constant change

GaBi

GaSb1-xBix

In one sense, GaSbBi is one of the

most extreme highly mismatched

alloys.

Band gap reductions for 0.01Å

change of lattice constant:

210 meV for GaSbBi

105 meV for GaNSb

157 meV for GaNAs

125 meV for GaAsBi

This is largely a consequence of the relatively close proximity of the GaSb and

zinc-blende GaBi lattice parameters.


46

MBE growth of GaSbBi alloys up to 5% Bi incorporation

Lattice expansion observed, contrary to previous reports of MBE of GaSbBi

99% of the Bi atoms are substitutional from RBS

Reduction in band gap of 36meV/% Bi observed.

VCA and valence band anticrossing model fits absorption and PR data

Band gap change in GaSbBi is high per 0.01Å lattice constant change compared

with other highly mismatched alloys

GaSbBi alloys with band gaps in the range for domestic TPV (0.4 to 0.6 eV) have

been demonstrated.

Observation of photoluminescence suggests better quality than GaNSb. This is

expected due to better atomic size match between Sb and Bi than for Sb and N

Next steps: GaNSbBi alloys lattice matched to GaSb – only very small amounts of

N will be required, hopefully insufficient to turn the material to “cardboard”

Then doping and devices…

Conclusions:

47

Conclusions:

GaBi

GaSb1-xBix

0 25 50 75 100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

GaBi

CBM

En

erg

y (

eV

)

Bi in GaSbBi (%)

VBM

GaSb

GaSbBi

GaNSb

Acknowledgments

48

RBS measurements at Lawrence Berkeley National Lab were supported the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Calculations were funded by the EPSRC via the Centre for Innovation (EP/K000136/1 and EP/K000144/1) and the UK’s HPC Materials Chemistry Consortium (EP/F067496)

The work at Wroclaw is funded by the Polish National Science Centre (NCN grant no. 2012/07/E/ST3/01742)

Funding is acknowledged from University of Liverpool and the UK Engineering and Physical Sciences Research Council (Grant nos EP/G004447/2 and EP/H021388/1).

49

115 reciprocal space map of GaSbBi with 4.2% Bi

illustrating the in plane lattice parameter is strained

to that of the GaSb

GaSb 115

GaSbBi 115

Open symbols are measured lattice

constants of strained GaSbBi films.

Closed symbols are derived lattice

constants of fully relaxed GaSbBi, using

GaSb elastic constants in the dilute Bi limit

50

GaSb band structure

from hybrid functional

DFT

k.P band structure close

to the Gamma point