Above-bandgap optical properties of biaxially strained GeSn alloys grown by molecularbeam epitaxyVijay Richard D’Costa, Wei Wang, Qian Zhou, Eng Soon Tok, and Yee-Chia Yeo

Citation: Applied Physics Letters 104, 022111 (2014); doi: 10.1063/1.4862659 View online: http://dx.doi.org/10.1063/1.4862659 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Compositional dependence of optical critical point parameters in pseudomorphic GeSn alloys J. Appl. Phys. 116, 053520 (2014); 10.1063/1.4892105 Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy Appl. Phys. Lett. 103, 041908 (2013); 10.1063/1.4816660 Structural and optical characterization of SixGe1x ySny alloys grown by molecular beam epitaxy Appl. Phys. Lett. 100, 141908 (2012); 10.1063/1.3701732 Investigation of the direct band gaps in Ge1xSnx alloys with strain control by photoreflectance spectroscopy Appl. Phys. Lett. 100, 102109 (2012); 10.1063/1.3692735 Increased photoluminescence of strain-reduced, high-Sn composition Ge1 x Sn x alloys grown by molecularbeam epitaxy Appl. Phys. Lett. 99, 181125 (2011); 10.1063/1.3658632

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Above-bandgap optical properties of biaxially strained GeSn alloys grownby molecular beam epitaxy

Vijay Richard D’Costa,1,a) Wei Wang,1 Qian Zhou,1 Eng Soon Tok,2 and Yee-Chia Yeo1,b)

1Department of Electrical and Computer Engineering, National University of Singapore, Singapore 1175832Department of Physics, National University of Singapore, Singapore 117551

(Received 12 November 2013; accepted 3 January 2014; published online 16 January 2014)

The complex dielectric function of biaxially strained Ge1�xSnx (0� x� 0.17) alloys grown on Ge

(100) has been determined by spectroscopic ellipsometry from 1.2 to 4.7 eV. The effect of

substitutional Sn incorporation and the epitaxial strain on the energy transitions E1, E1þD1, E00, and

E2 of GeSn alloys is investigated. Our results indicate that the strained GeSn alloys show Ge-like

electronic bandstructure with all the transitions shifted downward due to the alloying of Sn. The

strain dependence of E1 and E1þD1 transitions is explained using the deformation potential theory,

and values of �5.4 6 0.4 eV and 3.8 6 0.5 eV are obtained for the hydrostatic and shear deformation

potentials, respectively. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862659]

GeSn offers exciting possibilities for bandgap and strain

engineering in the group-IV alloy system. Theory and

experiments indicate that GeSn is a direct bandgap material

for Sn composition ranging from 6 to 11%.1–4 Enhanced

absorption in the near infrared region is observed as substitu-

tional Sn is incorporated in Ge.5–7 In addition to the alloying

effect of Sn in Ge, biaxial strained GeSn epitaxially grown

on a substrate with a lower lattice constant is of great interest

for high performance complementary metal oxide semicon-

ductor devices.8–10 GeSn p-metal-oxide-semiconductor field-

effect transistors (MOSFETs) with channel mobilities higher

than Ge have been demonstrated using compressively

strained GeSn alloys.9,10 GeSn has the potential to enable

co-integration of infrared optoelectronic and electronic devi-

ces based solely on group-IV materials.

Recently, the structural and physical properties of

Ge1�xSnx (0� x� 0.145) pseudomorphically grown on Ge

(100) by molecular beam epitaxy (MBE) were reported.11

Ge1�xSnx light emitting diode with Sn content as high as 8%

was also reported.12,13 Photoluminescence has also been

observed in Ge1�xSnx (x� 0.06–0.09) quantum wells grown

on Ge.14 However, research efforts on MBE-grown GeSn

alloys have mainly focused on the indirect bandgap and the

lowest direct bandgap.2,3,15 The higher energy transitions are

also affected by composition and strain. The electronic band-

structure of biaxially strained alloys above 1 eV where the

various energy transitions E1, E1þD1, E00, and E2 are

observed at high symmetry points of the Brillouin zone16 has

not been studied experimentally. Previous reports focused on

the compositional dependence of dielectric function and

energy transitions in relaxed GeSn alloys grown by chemical

vapour deposition (CVD).4 A study on the lowest direct gap

E0 in strained Ge suggests that one may have to revisit the

deformation potentials for higher transitions in Ge.17

Deformation potentials for E0 in GeSn (Ref. 15) are known,

but no data are available for transitions above E0. The deter-

mination of E1, E1þD1, E00, and E2 in strained Ge-rich

GeSn alloys will allow us to gain insight into deformation

potentials which are important for predicting the energy

shifts resulting from strain.

In this Letter, we report the dielectric function and the

above-bandgap electronic transitions of strained GeSn alloys

grown using MBE. Spectroscopic ellipsometry was used to

obtain the complex dielectric function of the alloys. The

dielectric function of the material contains sharp features

which are related to E1, E1þD1, E00, and E2 in the joint den-

sity of states in the electronic bandstructure of the material.

The compositional dependence of E1, E1þD1, E00, and E2 is

determined using a critical point analysis.18 The determina-

tion of dielectric function will not only elucidate the elec-

tronic bandstructure of strained GeSn alloys but will also

enable accurate process metrology for the fabrication of opti-

cal and electronic devices.

Crystalline Ge1�xSnx (0� x� 0.17) were grown directly

on Ge (100) substrates in a solid-source MBE system at the

National University of Singapore (NUS) with a base pressure

of 3� 10�10 Torr. The substrates were 4 in. diameter,

single-side polished, and As-doped with a doping level of

1� 1017 cm�3. The Ge wafer was cleaned using a diluted

hydrofluoric (DHF) etch and loaded immediately into the

load-lock chamber. The wafer was pre-heated to 630 �C to

remove the germanium oxide. The substrate temperature was

adjusted to between 100 �C and 170 �C to enable growth of

high quality Ge1�xSnx films with thicknesses ranging from

200 nm to 25 nm as x varied from 0.02 to 0.17. The alloy

composition was tuned by varying Ge and Sn fluxes.

The samples were investigated for their crystalline qual-

ity, composition, strain, and thickness by high-resolution X-

ray diffractometry (HR-XRD) at Singapore Synchrotron

Light Source (SSLS). Figure 1(a) shows the x-2h curves for

three GeSn alloys. The peak at lower 2h angle in each (004)

scan corresponds to GeSn alloys, and it shifts to a lower

angle with increasing Sn composition, indicating incorpora-

tion of Sn in the Ge lattice. We determined the GeSn thick-

nesses from the Pendellosung fringes which are observed

in all our samples, indicating coherent epitaxial films.11

The thicknesses were also independently confirmed by

a)Electronic mail: elevrd@nus.edu.sgb)Electronic mail: eleyeoyc@nus.edu.sg

0003-6951/2014/104(2)/022111/4/$30.00 VC 2014 AIP Publishing LLC104, 022111-1

APPLIED PHYSICS LETTERS 104, 022111 (2014)

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cross-sectional transmission electron microscopy (XTEM).

(115) reciprocal space mapping (RSM) for a representative

sample is shown in Figure 1(b) from which the in-plane

strain ek and the Sn composition are determined.11,19 Both

Ge and GeSn peaks have the same value of reciprocal lattice

vector along [110] Qk, indicating that the epitaxial GeSn

layer is fully strained to Ge. As previously reported for

strained Ge1�xSnx alloys,11 the critical thickness to obtain

high quality or pseudomorphic alloys decreases as a function

of x. Ge0.83Sn0.17 alloy, which is also the thinnest film,

appears to have larger full-width at half-maximum (FWHM)

compared to the thicker alloys with lower Sn content. Using

Scherer’s formula, we verified that the FWHM in (004)

curves is mainly due to the finite thickness of our films and

not caused by the presence of non-uniformities and defects.11

Figure 1(c) shows an XTEM image for the Ge0.98Sn0.02

alloy, indicating that the film is of uniform thickness with a

sharp interface between the film and the substrate. High

magnification XTEM images for Ge0.98Sn0.02, Ge0.92Sn0.08,

and Ge0.83Sn0.17 alloys are shown in Figures 1(d)–1(f),

respectively. In addition, atomic force microscopy (AFM)

showed that the root mean square (RMS) roughness of

Ge0.92Sn0.08 alloy is less than 1 nm (not shown here).

Spectroscopic ellipsometry measurements were carried

out at room temperature using a rotating analyzer ellipsome-

ter manufactured by J. A. Woollam Co.20 The ellipsometric

angles Psi and Delta were acquired at 10 meV energy steps

for angles of incidence 65� and 75� from 1.2 to 4.7 eV. A ref-

erence Ge film grown on Ge substrate was characterized

along with the GeSn alloys. The samples were modeled as a

three-layer system containing a Ge substrate, the GeSn film,

and a surface layer. The surface layer was modeled as a

GeO2 layer.21 The Ge substrate used in growth experiments

was characterized separately, and the dielectric function

obtained was used in tabulated form in the model. The com-

plex dielectric function of GeSn film is modeled using a dis-

persion model as developed by Herzinger and Johs following

the theoretical work of Kim and Garland.22,23 The model

takes into account the full analytic form of joint density of

states and is thus capable of describing the measured dielec-

tric function very accurately. Four parametric oscillators are

needed to describe the four critical point transitions E1,

E1þD1, E00, and E2 which are observed in the measured

data range. The data are processed using the procedure fol-

lowed in Ref. 4 which gives us the non-parametric or point--

by-point fit dielectric function in addition to the

parametrized dielectric function. The agreement between the

two is excellent confirming the Kramers-Kronig consistency

of the non-parametric function. It is to be noted here that an

isotropic dispersion model fully accounts for the observed

dielectric response of strained GeSn alloys although aniso-

tropic optical response is expected under the influence of

biaxial strain. This behavior is also observed in other

strained systems including Si and SiGe and is expected as

the acquired spectroscopic ellipsometry data correspond to

the in-plane component of dielectric function for the experi-

mental conditions used.24–26 Figure 2 shows the dielectric

function of GeSn alloys. The dielectric function of strained

GeSn alloys is very similar to Ge and shifts to the left with

respect to Ge with increasing Sn concentration, indicating

the alloying effect of Sn on the electronic bandstructure of

pure Ge.4

The parameterized dielectric function obtained for the

strained alloys is very useful for monitoring film thickness

and composition during device fabrication. For critical point

analysis, the non-parametric dielectric function is preferred to

the parameterized dielectric function.4 The non-parametric

dielectric function is numerically differentiated twice using

17 Savitzky-Golay coefficients corresponding to a polynomial

of order 5. We ensured that the number of smoothing coeffi-

cients used did not distort the line shape. The derivatives of

the experimental dielectric function were fitted using4

d2edE2¼X

j

AjeiUj

½E� Ej þ iCj�2; (1)

where Aj is the amplitude for transition j, Uj is the phase

angle, Ej is the critical point energy, and Cj is the broadening

FIG. 1. HRXRD results, showing (a) (004) x–2h scans of selected GeSn

alloys, (b) (115) RSM of a Ge0.83Sn0.17 alloy. Qk and Q? are the reciprocal

vectors along [110] and [001] and are expressed in reciprocal lattice units

(rlu). The wavelength of X-ray radiation used was 1.538 A. (c) Low magnifi-

cation XTEM image for a Ge0.98Sn0.02 alloy. High magnification XTEM

images showing the interface between Ge1�xSnx and Ge, (d) x¼ 0.02, (e)

x¼ 0.085, and (f) x¼ 0.17. The virtually defect-free interface indicates the

good crystalline quality of GeSn epitaxially grown on Ge.

FIG. 2. (a) Real and (b) imaginary parts of the complex dielectric function

of selected strained Ge1�xSnx alloys and a relaxed Ge film. The legend

applies to both plots.

022111-2 D’Costa et al. Appl. Phys. Lett. 104, 022111 (2014)

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parameter. The summation covers all the four transitions E1,

E1þD1, E00, and E2. The imaginary and real parts of the

dielectric function are fitted simultaneously by a least-square

procedure which follows the Levenberg-Marquardt algo-

rithm.27 The procedure was first verified on a reference Ge

film grown on a Ge substrate. The critical point parameters

for this film were found to be in excellent agreement with

the bulk Ge values reflecting the accuracy of our procedure

as well as the quality of our Ge film.18 The critical point

analysis of a GeSn alloy is shown in Figure 3. The sharp fea-

tures in the derivative spectra correspond to E1, E1þD1, E00,

and E2 which are extracted from a fit using Eq. (1).

The compositional dependence of energy transitions in

the strained GeSn alloys is shown in Figure 4. E1, E1þD1,

E00, and E2 reduce with Sn composition but are elevated with

respect to relaxed GeSn alloys. E1þD1 shows larger devia-

tion between the experimental values and the relaxed values

compared to E1. Besides the alloying effect of Sn, the pres-

ence of biaxial strain will influence the critical point energy

values. We use the deformation potential theory to under-

stand the effect of strain on energy transitions.28,29 In this pa-

per, we focus on E1 and E1þD1. Strain analysis is not

established for E00 and E2 in Ge and SiGe. The deformation

potentials of E00 and E2 are not well studied, perhaps due to

an incomplete understanding of their origin.4,30 The expres-

sions for E1 and E1þD1 in GeSn alloys can be written as

E1ðx; eÞ ¼ E1ðxÞ þ D1ðxÞ=2þ EH �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiES

2 þ ðD1ðxÞ=2Þ2q

;

(2)

ðE1 þ D1Þðx; eÞ ¼ ðE1 þ D1ÞðxÞ � D1ðxÞ=2þ EH

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiES

2 þ ðD1ðxÞ=2Þ2q

; (3)

where EH ¼ ð2=ffiffiffi3pÞD1

1 1� c12=c11ð Þek gives the energy shift

due to the hydrostatic component of the strain and ES ¼�

ffiffiffiffiffiffiffiffiffiffið2=3

pÞD3

3ð1þ 2c12=c11Þek is the energy shift due to the

shear component of the strain. Linear interpolation between

Ge and a-Sn is used for the elastic constant ratio c12/c11.31

D11 and D3

3 are the hydrostatic deformation and shear defor-

mation potentials, respectively. The deformation potentials

for Ge are taken from Ref. 30 whereas experimental values

for a-Sn are not available yet. The compositional depend-

ence for E1(x), (E1þD1)(x), and D1ðxÞ is taken from Ref. 4.

Equations (2) and (3) indicate that the EH and

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiES

2 þ ðD1ðxÞ=2Þ2q

tend to cancel each other for E1 whereas

they both add up for E1þD1. This qualitatively explains the

observed behavior of E1 and E1þD1 in our strained alloys

and is analogous to what is found in strained SiGe alloys.26

Figure 5(a) shows that the theory (dotted line) using Ge de-

formation potentials accounts for observed E1 (triangles) but

overestimates E1þD1 (circles). It is quite possible that the

discrepancy between experiment and theory is caused by the

use of Ge deformation potentials for E1 and E1þD1. This is

not surprising because even the deformation potentials for

the widely studied E0 gap in Ge vary significantly in litera-

ture.17 It may be pointed out here that both the deformation

potentials for SiGe had to be adjusted to explain the strain

dependence of E1 and E1þD1.26 We analyze the average of

E1 and E1þD1 which contains only D11. Adding Eqs. (2) and

(3) and calling the average of the two transitions as �E1, we

obtain ( �E1(x,e) � �E1(x))¼DE¼EH as the energy shift due to

strain. We plot DE as a function of in-plane component of

strain in Figure 5(b). The dotted curve is the simulation of

experimental data using the Ge value for D11. It clearly shows

the discrepancy between experimental and theoretical

expression as observed for E1þD1. We model the experi-

mental data with D11 as a fitting parameter and obtain a value

of �5.4 6 0.4 eV. Figure 5(a) shows the resulting theoretical

curve (solid line) which is in very good agreement with the

FIG. 3. Numerical second derivatives of (a) real and (b) imaginary parts of

the dielectric function of a Ge0.92Sn0.08 alloy. Experimental data are plotted

in circles, and the model is plotted using solid lines.

FIG. 4. Compositional dependence of (a) E1 and E1þD1 (b) E00 and E2 in

strained Ge1�xSnx alloys, plotted using square symbols. The solid curves

represent the compositional dependence of transition energies for the relaxed

alloys taken from Ref. 4.

FIG. 5. (a) Strain analysis of E1 (triangles), �E1 (squares), and E1þD1

(circles). The dotted curve represents calculated energy using the Ge value

of �8.6 eV for D11. The solid curve represents calculated energy using the

new D11 value of �5.4 eV for GeSn. (b) Energy shift DE for �E1 plotted as a

function of in-plane strain. The dotted curve represents the energy shift cal-

culated using the Ge value for D11. The solid line is a fit with DE¼D1

1f ðekÞ,where f ðekÞ ¼ ð2=

ffiffiffi3pÞ 1� c12=c11ð Þek.

022111-3 D’Costa et al. Appl. Phys. Lett. 104, 022111 (2014)

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experimental data for �E1 (squares). Note that D11 for GeSn

lies between pure Ge (�8.6 eV) and the theoretical value of

a-Sn (�3.8 eV), indicating that it is consistent with the

expected trend for D11.32 Eliminating the hydrostatic potential

from Eqs. (1) and (2), we obtain a value of 3.8 6 0.5 eV for

D33 of GeSn. Using the new values of D1

1 and D33, we

re-calculate the strain correction for E1þD1, which now

shows excellent agreement with the experimental data. E1

now overlaps with the theoretical curve calculated using the

Ge deformation potentials. It appears that the discrepancy

between experimental data and theory was primarily caused

by the use of Ge values to account for the strain effects in

GeSn. Our strain analysis for Ge-rich GeSn alloys indicates

that values of �5.4 eV and 3.8 eV for D11 and D3

3, respec-

tively, are required to account for the energy shifts caused by

strain in E1 and E1þD1.

In conclusion, we have determined the dielectric function

of biaxially strained GeSn alloys which indicate that the alloys

obey Ge-like bandstructure. The compositional dependence of

E1, E1þD1, E00, and E2 was determined using a critical point

analysis and interpreted in terms of the relaxed alloys. The de-

pendence of E1 and E1þD1 on strain was explained using the

deformation potential theory. We propose that a hydrostatic

potential value of �5.4 eV and a shear deformation potential

value of 3.8 eV can be used to calculate the energy shifts for

E1 and E1þD1 in strained GeSn alloys.

This work was supported by the National Research

Foundation under the Competitive Research Programme

(Grant No. NRF-CRP6-2010-4).

1W.-J. Yin, X.-G. Gong, and S.-H. Wei, Phys. Rev. B 78(16), 161203 (2008).2R. Ragan and H. A. Atwater, Appl. Phys. Lett. 77(21), 3418 (2000).3H. P. L. de Guevara, A. G. Rodriguez, H. Navarro-Contreras, and M. A.

Vidal, Appl. Phys. Lett. 84(22), 4532 (2004).4V. R. D’Costa, C. S. Cook, A. G. Birdwell, C. L. Littler, M. Canonico, S.

Zollner, J. Kouvetakis, and J. Menendez, Phys. Rev. B 73(12), 125207 (2006).5S. Su, B. Cheng, C. Xue, W. Wang, Q. Cao, H. Xue, W. Hu, G. Zhang, Y.

Zuo, and Q. Wang, Opt. Express 19(7), 6400 (2011).6V. R. D’Costa, Y. Fang, J. Mathews, R. Roucka, J. Tolle, J. Men�endez,

and J. Kouvetakis, Semicond. Sci. Technol. 24(11), 115006 (2009).7A. Gassenq, F. Gencarelli, J. Van Campenhout, Y. Shimura, R. Loo, G.

Narcy, B. Vincent, and G. Roelkens, Opt. Express 20(25), 27297 (2012).8O. Nakatsuka, Y. Shimura, W. Takeuchi, N. Taoka, and S. Zaima, Solid-

State Electron. 83, 82 (2013).

9G. Han, S. Su, C. Zhan, Q. Zhou, Y. Yang, L. Wang, P. Guo, W. Wei, C.

P. Wong, Z. X. Shen, B. Cheng, and Y. C. Yeo, in International ElectronDevices Meeting (IEEE, Washington, DC, 2011), p. 402.

10S. Gupta, Y. C. Huang, Y. Kim, E. Sanchez, and K. C. Saraswat, IEEE

Electron Device Lett. 34(7), 831 (2013).11N. Bhargava, M. Coppinger, J. P. Gupta, L. Wielunski, and J. Kolodzey,

Appl. Phys. Lett. 103(4), 041908 (2013).12J. P. Gupta, N. Bhargava, S. Kim, T. Adam, and J. Kolodzey, Appl. Phys.

Lett. 102(25), 251117 (2013).13H. H. Tseng, K. Y. Wu, H. Li, V. Mashanov, H. H. Cheng, G. Sun, and R.

A. Soref, Appl. Phys. Lett. 102(18), 182106 (2013).14A. A. Tonkikh, C. Eisenschmidt, V. G. Talalaev, N. D. Zakharov, J.

Schilling, G. Schmidt, and P. Werner, Appl. Phys. Lett. 103(3), 032106

(2013).15H. Lin, R. Chen, W. Lu, Y. Huo, T. I. Kamins, and J. S. Harris, Appl.

Phys. Lett. 100(10), 102109 (2012).16P. Y. Yu and M. Cardona, Fundamentals of Semiconductors: Physics and

Materials Properties (Springer-Verlag, Berlin, 1996).17J. Liu, D. D. Cannon, K. Wada, Y. Ishikawa, D. T. Danielson, S.

Jongthammanurak, J. Michel, and L. Kimerling, Phys. Rev. B 70(15),

155309 (2004).18L. Vi~na, S. Logothetidis, and M. Cardona, Phys. Rev. B 30(4), 1979

(1984).19H. Lin, R. Chen, Y. Huo, T. I. Kamins, and J. S. Harris, Thin Solid Films

520(11), 3927 (2012).20J. A. Woollam, B. D. Johs, C. M. Herzinger, J. N. Hilfiker, R. A.

Synowicki, and C. L. Bungay, in Optical Metrology, edited by G. A. Al-

Jumaily (Denver, Colorado, 1999), Vol. CR72, p. 3.21Y. Z. Hu, J. T. Zettler, S. Chongsawangvirod, Y. Q. Wang, and E. A.

Irene, Appl. Phys. Lett. 61(9), 1098 (1992).22B. Johs, C. M. Herzinger, J. H. Dinan, A. Cornfeld, and J. D. Benson, Thin

Solid Films 313–314, 137 (1998).23C. C. Kim, J. W. Garland, H. Abad, and P. M. Raccah, Phys. Rev. B

45(20), 11749 (1992).24D. E. Aspnes, J. Opt. Soc. Am. 70(10), 1275 (1980).25H. Lee and E. D. Jones, Appl. Phys. Lett. 68(22), 3153 (1996).26R. T. Carline, C. Pickering, D. J. Robbins, W. Y. Leong, A. D. Pitt, and A.

G. Cullis, Appl. Phys. Lett. 64(9), 1114 (1994).27W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery,

Numerical Recipes in C: The Art of Scientific Computing, 2nd ed.

(Cambridge University Press, New York, 1992).28F. H. Pollak, in Semiconductors and Semimetals, edited by P. P. Thomas

(Elsevier, 1990), Vol. 32, p. 17.29S. T. Pantelides and S. Zollner, in Optoelectronic Properties of

Semiconductors and SuperLattices, edited by M. O. Manasreh (Taylor &

Francis, New York, 2002), Vol. 15, p. 538.30P. Etchegoin, J. Kircher, M. Cardona, and C. Grein, Phys. Rev. B 45,

11721 (1992).31O. Madelung, in Landolt-B€orstein: Numerical Data and Functional

Relationships in Science and Technology, edited by O. Madelung

(Springer-Verlag, Berlin, 1985), Vol. 22a.32T. Brudevoll, D. S. Citrin, M. Cardona, and N. E. Christensen, Phys. Rev.

B 48(12), 8629 (1993).

022111-4 D’Costa et al. Appl. Phys. Lett. 104, 022111 (2014)

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