A --BB --C TERNARY SEMICONDUCTORS -BIV -CV2 TERNARY SEMICONDUCTORS FOR...

AII-BIV-CV2 TERNARY SEMICONDUCTORS FOR PHOTOVOLTAICS

AAIIII--BBIVIV--CCVV2 2 TERNARY SEMICONDUCTORS TERNARY SEMICONDUCTORS FOR PHOTOVOLTAICSFOR PHOTOVOLTAICS

Belarusian State University of Informatics and Radioelectronics Belarusian State University of Informatics and Radioelectronics (Minsk, Belarus)(Minsk, Belarus)

A. V. Krivosheeva, V.A. V. Krivosheeva, V. L.L. Shaposhnikov,Shaposhnikov, V.V. E.E. BorisenkoBorisenko

F.F. Arnaud Arnaud DD’’AvitayaAvitaya , , J.J.--L.L. LazzariLazzariCentre Interdisciplinaire de Nanoscience de Marseille Centre Interdisciplinaire de Nanoscience de Marseille

(Marseille, France)(Marseille, France)

Search for alternative and renewable sources of energy requires materials of low cost with new electronic, transport and opticalproperties;

Stable II-IV-V2 compounds weakly lattice mismatched with silicon, germanium or gallium arsenide can be potential candidates for photovoltaic devices;

Some of II-IV-V2 compounds may replace more expensive CuInSe2 in photovoltaic applications.

MOTIVATIONMOTIVATION

- To find materials with a lattice constant, closest to the silicon, gallium arsenide or germanium substrate

- To determine the optical properties (dielectric constants and absorption coefficients) of II-IV-V2 semiconductors

APPROACHAPPROACH

PROBLEMSPROBLEMS

Materials for solar cells should possess high absorption coefficient and have large gap to absorb mass of the visible light spectrum;

Be- and most of Mg-containing compounds were not experimentally synthesized. Their electronic and optical properties are still unknown.

GGA-PAW approximation (VASP code) – structural optimization and calculation of lattice parameters

FLAPW method (WIEN2K code) – simulation of band structures and optical properties

Chalcopyrite type tetragonal crystal structure with 16 atoms wasused for simulation

METHODMETHOD

STRUCTURESTRUCTUREAAIIIIBBIVIV AAIIIIBBIVIVCCVV

22

STRUCTURESTRUCTURE

Si Ge Sn5.0

5.2

5.4

5.6

5.8

6.0

6.2

X

GaA

s

a, A

ng

Si

BeXP2

BeXAs2

MgXP2

MgXAs2

ZnXP2

ZnXAs2

CdXP2

CdXAs2

Theoretically computed lattice constants of some ch alcopyrites with variation of group IV element

Be-compounds

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

NГ

Ene

rgy,

eV

BeSiAs2

T

N

Г

BeGeAs2

T

N

Г

BeGeP2

T NГ

Ene

rgy,

eV

BeSiP2

T

ELECTRONIC PROPERTIES

Eg=1.07 eV Eg=0.62 eV

Eg=1.20 eV Eg=0.85 eV

-6 -4 -2 0 2 4-6 -4 -2 0 2 40.0

0.5

1.0

1.5

2.0

Ge-s As-p

BeGeAs2BeSiAs2

PD

OS

, sta

tes/

eVEnergy, eV

Si-s As-p

Valence band near Fermi level is formed primary by p states of As. Conduction band near the gap is forme d primary by s electrons of Si or Ge

Total DOS

Projected DOS

Band structures and DOS demonstrate semiconducting nature of Be-compounds. No experimental data is ava ilable.

-6 -4 -2 0 2 4-6 -4 -2 0 2 40

5

10

15

20

BeGeAs2

BeSiAs2

DO

S, s

tate

s/eV

Energy, eV

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

E||x E||z

ε 2

BeSiAs2

E||x E||z

BeGeAs2

E||x E||z

Energy, eV

BeGeP2

E||x E||z

Energy, eV

ε 2

BeSiP2

OPTICAL PROPERTIES

Equal starts and similar shape of εεεε2222 curves for both directions of light polarization –anisotropy of εεεε2222 is practically absent.

Slightly different location of the main peaks for E||a and E||c at higher photon energies (about 4 eV)

Direct gap and rapid increase of εεεε2 curves at about 2.0 eV in BeSiAs 2 and BeSiP 2

All Be-containing compounds are characterized by low value of dipole matrix element of first direct transition (at the Gamma point), thus they have low oscillator strength and thereby they are not suitable for light emitting devices.

( ) ( ) ( )( )3

32

22

22

22

8

πω−−δ

ωπ=ωε ′

′≠ ′ Ω′∑∑∫

kk

dEEP

m

e kn

kn

nn nnn h

Dielectric function

( ) ( ) ( )1 2iε ω = ε ω + ε ω

( ) ( )∫∞

ω−ω′ω′ω′ω′ε

π+=ωε

022

21

21

dP

OPTICAL PROPERTIES

T. Markvart, Luis Castañer, Practical handbook of photovoltaics: fundamentals and applications.- Elsevier (2003), p. 984

Absorption coefficient

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

E||x E||z

α, 1

04 cm

-1

BeSiAs2

E||x E||z

BeGeAs2

E||x E||z

Energy, eV

BeGeP2

E||x E||z

Energy, eV

α, 1

04 cm

-1

BeSiP2

Absorption coefficients of BeGeAs 2 demonstrate similar order of magnitude with CuInSe 2

0 2 4 6 8 10102

103

104

105

106 E||x

BeSiAs2

BeGeAs2

BeSiP2

BeGeP2

α, c

m-1

Energy, eV

Mg-compounds


-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

NÃ

Ene

rgy,

eV

MgSiP2

T

N

Ã

MgSiAs

2

T

N

Ã

MgGeAs2

T NÃ

Ene

rgy,

eV

MgGeP2

T

Eg=1.37 eV Eg=1.26 eV

Eg=1.52 eV Eg=0.57 eV

Magnesium-containing compounds are found to be direct-gap semiconductors, except MgGeP 2 which is quasi-direct gap semiconductor. Here the VBM is located at the Γ-point whereas the CBM at the Γ-point and Γ-T direction have practically equal values.

OPTICAL PROPERTIES

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

E||x E||z

ε 2

MgSiP2

Energy, eV Energy, eV

E||x E||z

MgGeAs2

E||x E||z

MgGeP2

E||x E||z

ε 2

MgSiAs2

The dielectric functions of Mg-compounds show more anisotropic behavior especially for the photon energies higher than 4 eV. Both Si-containing direct-gap compounds are characterized by low value of the dipole matrix element of the first direct transition as well as MgGeP2. MgGeAs 2 is found to have a large value of dipole matrix element (0.209 for E||c) and its ε2 spectra is characterized by abrupt threshold and strong increase at about 0.6 eV that corresponds to the value of estimated band gap.

Dielectric function

OPTICAL PROPERTIES

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

Energy, eV

E||x E||z

α, 1

04 cm

-1

MgSiAs2

Energy, eV

E||x E||z

MgGeAs2

E||x E||z

MgGeP2

E||x E||z

α, 1

04 cm

-1

MgSiP2 Absorption coefficient

MgSiP 2 has the lowest value of the absorption coefficient while other Mg–IV–V2 compounds possess the values comparable with GaAs and GaP. Moreover, the values of α are higher than those for CuInSe 2 especially at photon energies higher than 2 eV.

Zn-compounds


-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

NГ

Ene

rgy,

eV

ZnSiAs2

T

N

Г

ZnGeAs2

T

N

Г

ZnGeP2

T NГ

Ene

rgy,

eV

ZnSiP2

T

10 20 30 40 50

0.00.40.81.21.62.02.4

Eg,

eV

atomic number

calc. ref.

ZnGeP-As-Sb2

P As Sb

Band gap behavior of ZnGeP-As-Sb 2 compounds

ZnSiP 2: Eg=1.38 (d)

ZnSiAs 2 : Eg=0.89 (d)

ZnGeP2 : Eg=1.16 (qd)

ZnGeAs 2 : Eg=0.10 (i)

Eg=0.89 eV

Eg=1.38 eV Eg=1.16 eV

Eg~0.1 eV

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

E||x E||z

ε 2

ZnSiAs2

E||x E||z

ZnGeAs2

E||x E||z

Energy, eV

ZnGeP2

E||x E||z

Energy, eV

ε 2

ZnSiP2

OPTICAL PROPERTIES

ZnSiAs 2 compound is characterized by a large value of dipole matrix element (0.277 for E||c) and its εεεε2 spectrum is described by strong increase at about 1 eV

Similar behaviour can be observed for ZnGeP2 (|Pzz|2 = 0.234) with εεεε2 curve starting at 1.2 eV in spite of the fact that this compound was found to have a quasi-direct gap

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

E||x E||z

α, 1

04 cm

-1

ZnSiAs2

E||x E||z

ZnGeAs2

E||x E||z

Energy, eV

ZnGeP2

E||x E||z

Energy, eV

α, 1

04 cm

-1

ZnSiP2

OPTICAL PROPERTIES



0 2 4 6 8 10

104

105

106 E||x

Energy, eV

ZnSiAs2

ZnGeAs2

ZnSiP2

ZnGeP2

α, c

m-1

Absorption coefficients of ZnSiAs 2 and ZnGeP 2 demonstrate similar order of magnitude with CuInSe 2

Cd-compounds

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

3

4

NГ

Ene

rgy,

eV

CdSiAs2

T

N

Г

CdGeAs2

T

N

Г

CdGeP2

T NГ

Ene

rgy,

eV

CdSiP2

T


10 20 30 40 50

0.0

0.4

0.8

1.2

1.6

Eg,

eV

atomic number

calc.

ref.

CdGeP-As-Sb2

P As Sb

Band gap behaviour of CdGeP-As-Sb 2 compounds

Eg=0.38 eV

Eg=1.42 eV Eg=0.59 eV

Eg~0.1 eV

-4 -2 0 2 40

20

40

60

80

100

DO

S, s

tate

s/eV

Energy, eV

CdGeAs2

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

0 2 4 6 8 100

5

10

15

20

25

30

E||x E||z

ε 2

CdSiAs2

E||x E||z

CdGeAs2

E||x E||z

Energy, eV

CdGeP2

E||x E||z

Energy, eV

ε 2

CdSiP2

OPTICAL PROPERTIES

Cd-based materials provide similar optical properties as compared with Zn-based ones.

CdSiAs 2 is characterized by a large value of dipole matrix element (0.242 for E||c) and its εεεε2 spectrum is described by strong increase at about 0.4 eV.

CdGeP2 has a |Pzz| 2 value of 0.213, its εεεε2 curve starts at about 0.6 eV.

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

0 2 4 6 8 100

50

100

150

200

E||x E||z

α, 1

04 cm

-1

CdSiAs2

E||x E||z

CdGeAs2

E||x E||z

Energy, eV

CdGeP2

E||x E||z

Energy, eV

α, 1

04 cm

-1

CdSiP2

OPTICAL PROPERTIES



0 2 4 6 8 10101

102

103

104

105

106 E||x

Energy, eV

CdSiAs2

CdGeAs2

CdSiP2

CdGeP2

α, 1

04 cm

-1

Absorption coefficients of CdSiAs 2 and CdGeP 2 demonstrate similar order of magnitude with CuInSe 2

GENERAL REMARKSGENERAL REMARKS

One of the possible ways to increase the quantum efficiency of solar cell is multilayered heterostructures consisting of semiconducting materials with different band gaps.

For photovoltaic applications we are interested in stable compounds having a direct gap (absorption coefficients and effective masses are higher for direct gaps as compared to indirect ones).

If we want eliminate expensive, rare, dangerous or too toxic elements it is better do not consider Cd-based and P containing compounds, as phosphorous is highly volatile, red-phosphorous burns, and phosphinegas is highly toxic.

Among materials of II-IV-V2 class arsenic compounds are easier to be obtained experimentally in comparison with nitrogen and phosphorous based ternaries.

Zn(Si, Sn)As2 family should possess lattice constants close to GaAs one and have band gap suitable for photovoltaics.

MgSiP2, ZnSiP2, ZnSiAs 2, CdSiP2, CdSiAs 2, CdGeP2, having direct gaps with the values more than 1 eV and possessing good match ing with Si, Ge and GaAs substrates, can find application in photovoltai cs and may compete with the more expensive CuInSe 2

d0.850.10ZnGeAs 2

qd2.21.16ZnGeP2

d1.70.89ZnSiAs 2

d2.31.38ZnSiP 2

i–0.62BeGeAs 2

i–0.85BeGeP2

d–1.07BeSiAs 2

d–1.20BeSiP 2

Exp.Calc.Transition

Band gap, eVCompound

Theoretical and experimental gap values

Blue fields – good lattice matching with Si, yellow fields – with Ge and GaAs

i0.530.10CdGeAs 2

d1.80.59CdGeP2

d1.60.38CdSiAs 2

d2.41.42CdSiP 2

d–0.57MgGeAs 2

qd–1.52MgGeP2

d–1.26MgSiAs 2

d2.01.37MgSiP 2

Exp.Calc.Transition

Band gap, eVCompound



Si Ge Sn

0.0

0.5

1.0

1.5

X

Eg, e

V

MgXAs2; ZnXAs

2; CdXAs

2

MgXP2; ZnXP

2; CdXP

2

Band-gap values in dependence of variation of group IV element

1.0 1.5 2.0 2.5 3.0 3.50.2

0.3

0.4

0.5

R

Photon energy, eV

MgSnAs2

ZnSnAs2

ZnSiAs2

CdGeAs2

CdSiAs2

CuInSe2

GaAs GaP

1200

1000

800

600

400

λ, nm

1.0 1.5 2.0 2.5 3.0 3.5

2

4

6

8

10

1200

1000

800

600

400

α, 1

05 cm

-1

Photon energy, eV

λ, nm

Averaged absorption coefficients of II–IV–V 2 ternaries in comparison with GaAs, GaP and CuInSe 2

Averaged reflectance spectra of II–IV–V 2 ternaries in comparison with GaAs, GaP and CuInSe 2


All investigated AII-BIV-CV2 materials are found to be semiconductors;

Most of materials (except Be-containing compounds) have the direct-band gap located at the Γ-point with the gaps ranging from about 0.1 to 1.5 eV. The band gaps in P-containing compounds are higher than those in As-containing ones;

An increase of the atomic number of the group IV elements leads to the decrease of the energy of the start point of ε2 curve for all chalcopyrites studied;

The static dielectric function (ε1(0)) remains practically the same for both light polarizations having the same order of magnitude in these materials;

MgGeAs 2, MgSnP2, MgSnAs 2, ZnSiAs 2, ZnGeP2, ZnSnP2, CdSiAs 2 and CdGeP2 were found to have valuable dipole matrix elements of the first direct transitions that make them promising for light-emission applications;

ZnSiAs 2 and ZnSnAs 2 seem to be the most suitable for photovoltaic applications as they have large absorption coefficients, optimal band gaps and lattice constants matched to GaAs substrate.

CONCLUSIONSCONCLUSIONS

Thank you for your Thank you for your attention!attention!

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