Martienssen W., Warlimont H. (Eds.). Handbook of Condensed Matter and Materials Data

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652 Part 4 Functional Materials

4.1.3 II–VI Compounds

4.1.3.1 Beryllium Compounds

A. Crystal Structure, Mechanical and Thermal Properties

Tables 4.1-94 – 4.1-99.

Table 4.1-94 Crystal structures of beryllium compounds

Crystal Space group Crystal system Structure type Figure

Beryllium oxide BeO C

mc hex Wurtzite Fig. 4.1-130

Beryllium sulﬁde BeS T

fcc Zinc blende Fig. 4.1-131

Beryllium selenide BeSe T

F43m fcc Zinc blende Fig. 4.1-131

Beryllium telluride BeTe T

fcc Zinc blende Fig. 4.1-131

Fig. 4.1-130 The wurtzite lattice (a and c are the lattice

parameters)

Table 4.1-95 Lattice parameters of beryllium compounds

Crystal Lattice parameters (nm) Temperature (K) Remarks

Beryllium oxide BeO a = 0.26979 X-ray diffraction

c =0.4380

Beryllium sulﬁde BeS a = 0.48630 (5) RT X-ray diffraction

Beryllium selenide BeSe a = 0.51520 (23) RT Epitaxial ﬁlm, X-ray diffraction

Beryllium telluride BeTe a = 0.56270 (15) 300 Epitaxial ﬁlm, X-ray diffraction

Table 4.1-96 Elastic constants c

(in GPa) of beryllium oxide

Crystal c

Remarks

Beryllium oxide BeO 460.6 126.5 88.5 491.6 147.7 Ultrasonic data, RT

Fig. 4.1-131 The zinc blende lattice (a is the lattice param-

eter)

Part 4 1.3

Semiconductors 1.3 II–VI Compounds 653

Table 4.1-97 Densities of beryllium compounds

Crystal Density (g/cm

)

Beryllium oxide BeO 3.01

Beryllium sulﬁde BeS 2.36

Table 4.1-98 Melting point T

of beryllium oxide

Crystal Melting point T

Beryllium oxide BeO 2507

◦

B. Electronic Properties

Tables 4.1-100 – 4.1-102.

Table 4.1-100 First Brillouin zones of beryllium com-

pounds

Crystal Figures

Beryllium oxide BeO Fig. 4.1-132

Beryllium sulﬁde BeS Fig. 4.1-133

Beryllium selenide BeSe Fig. 4.1-133

Beryllium telluride BeTe Fig. 4.1-133

∆

Fig. 4.1-132 The Brillouin zone of the wurtzite lattice

Table 4.1-101 Band structures of beryllium compounds

Crystal Figure Remarks

Beryllium oxide BeO Fig. 4.1-135

Beryllium sulﬁde BeS Fig. 4.1-134

Beryllium selenide BeSe Fig. 4.1-136

Beryllium telluride BeTe Fig. 4.1-137

According to theory, BeS is an indirect-gap material with the

highest valence band edge at Γ and the lowest conduction

band edge at X.

According to theory, BeSe is an indirect-gap material with

the highest valence band edge at Γ and the lowest conduc-

tion band edge at X.

Table 4.1-99 Linear thermal expansion coefﬁcient α of

beryllium compounds

Crystal Expansion Tempera-

coefﬁcient ture (K)

α(K

−1

)

Beryllium oxide BeO 9.7×10

−6

373

Beryllium telluride BeTe 7.66 (21) ×10

−6

300

∆

Fig. 4.1-133 The Brillouin zone of the zinc blende and rock

salt lattices

–5

–10

–15

E (eV)

X U,K L K ZΓΓ∆ΣΛWX

BeS

Wave vector k

Fig. 4.1-134 Band structure of beryllium sulﬁde

Part 4 1.3

654 Part 4 Functional Materials

E (Ry)

Wave vector k

A 0,0,

M 0, ,0

2π

Γ(0,0,0)

0.50

0.25

– 0.25

– 0.50

– 0.75

–1.00

–1.25

–1.50

BeO

Fig. 4.1-135 Band structure of beryllium oxide

Table 4.1-102 Energy gaps of beryllium compounds

Crystal

Band gap

Remarks

Beryllium oxide BeO 10.585 eV

Beryllium sulﬁde BeS >5.5 eV Absorption

Beryllium selenide BeSe 5.6eV RT

Beryllium telluride BeTe E

g, ind

2.8eV E

g, dir

4.1eV 300 K optical reﬂection, ellipsometry

–5

–10

–15

E (eV)

X U,K L K ZΓΓ∆ΣΛWX

BeSe

Wave vector k

Fig. 4.1-136 Band structure of beryllium selenide

–5

–10

–15

X U,K L K ZΓΓ∆ΣΛWX

E (eV)

BeTe

Wave vector k

Fig. 4.1-137 Band structure of beryllium telluride

Part 4 1.3

Semiconductors 1.3 II–VI Compounds 655

C. Transport Properties

Table 4.1-103.

Table 4.1-103 Electronic transport in beryllium compounds, general description

Crystal Transport properties

Beryllium oxide BeO BeO, synonym beryllia, is an electrical insulator like a ceramic, but conducts heat like a metal. The

electrical resistivity is larger than 10

Ω cm

Beryllium sulﬁde BeS The electrical conductivity of polycrystalline BeS disks was found to be p-type. This is attributed to

Be vacancies or S interstitials

Beryllium telluride BeTe BeTe, like ZnTe, tends towards p-type conductivity. Using plasma-assisted nitrogen doping during

molecular-beam epitaxy, very high p-type concentrations above 10

−3

could be achieved; n-type

doping of BeTe has not yet been reported

D. Electromagnetic and Optical Properties

Table 4.1-104.

Table 4.1-104 Dielectric constants ε of beryllium compounds

Crystal Temperature (K) ε(0) ε(∞) Method Remarks

Beryllium oxide BeO 300 ε

parall

7.65 ε

perpend

6.94 Reﬂectance Crystalline BeO

Beryllium selenide BeSe 300

Beryllium telluride BeTe RT 7 Infrared reﬂection

BeO is transparent up to 9.4 eV (vacuum UV) and quite resistant to UV damage.

The interest in BeSe originates from the fact that ternary BeZnSe and quaternary BeMgZnSe alloys can be lattice-matched to

silicon substrates and can also be used as base materials for blue–green-emitting ZnSe-based laser diodes grown on GaAs. The

ternary alloy has a band gap of approximately 4 eV if lattice-matched to silicon.

BeTe is used in efﬁcient pseudomorphic p-contacts for ZnSe laser diodes.

4.1.3.2 Magnesium Compounds

A. Crystal Structure, Mechanical and Thermal Properties

Tables 4.1-105 – 4.1-111.

Table 4.1-105 Crystal structures of magnesium compounds

Crystal Space group Crystal system Structure type Figure

Magnesium oxide MgO O

Fm3m fcc NaCl Fig. 4.1-138

Magnesium sulﬁde MgS O

Fm3m fcc NaCl Fig. 4.1-138

Magnesium selenide MgSe O

Fm3m fcc NaCl Fig. 4.1-138

Magnesium telluride MgTe C

mc hex Wurtzite Fig. 4.1-130

Part 4 1.3

656 Part 4 Functional Materials

Fig. 4.1-138 The rock salt lattice

Table 4.1-106 Lattice parameters of magnesium compounds

Crystal Lattice parameters (nm) Temperature (K) Remarks

Magnesium oxide MgO a = 0.4216 298 X-ray diffraction, powder samples

Magnesium sulﬁde MgS a = 0.5203 300 X-ray diffraction

Magnesium selenide MgSe a = 0.546 300 X-ray diffraction

Magnesium telluride MgTe a = 0.45303 300

c =0.74056

Table 4.1-107 Densities of magnesium compounds

Crystal Density Temperature

(g/cm

) (K)

Magnesium oxide MgO 3.576 298

Magnesium sulﬁde MgS 2.86

Magnesium selenide MgSe 4.21

Magnesium telluride MgTe

Table 4.1-109 Elastic constants c

(in 10

dyn/cm

) of magnesium oxide

Crystal c

Temperature (K) Remarks

Magnesium oxide MgO 2.971 (6) 0.965 (7) 1.557 (2) 293 Single crystals

Table 4.1-110 Linear thermal expansion coefﬁcient α of magnesium oxide

Crystal Expansion coefﬁcient α(K

−1

) Temperature (K) Remarks

Magnesium oxide MgO 9.84 × 10

−6

283 Capacitance cell

See Fig. 4.1-139 for the temperature dependence.

0 40 80 120 160 200 240 280 320

T (K)

α (×10

–6

–1

)

BaO

MgO

Fig. 4.1-139 MgO, BaO. Linear thermal expansion co-

efﬁcient α vs. temperature. Open circles, [1.120]; ﬁlled

circles, [1.121]

Table 4.1-108 Melting point T

of magnesium oxide

Crystal Melting point T

Magnesium oxide MgO 2827

◦

Part 4 1.3

Semiconductors 1.3 II–VI Compounds 657

Table 4.1-111 Phonon frequencies/wavenumbers of magnesium compounds

Crystal ν

Temperature Symmetry Remarks

(THz) (THz) (cm

−1

) (cm

−1

) (K) point

Magnesium oxide MgO 12.05 21.52 300 Γ Thin ﬁlms, polarized

reﬂectance

Magnesium telluride MgTe 235 292 300 Raman spectroscopy

B. Electronic Properties

Tables 4.1-112 – 4.1-114.

Table 4.1-112 First Brillouin zones of magnesium com-

pounds

Crystal Figure

Magnesium oxide MgO Fig. 4.1-133

Magnesium sulﬁde MgS Fig. 4.1-133

Magnesium selenide MgSe Fig. 4.1-133

Magnesium telluride MgTe Fig. 4.1-132

Table 4.1-113 Band structures of magnesium compounds

Crystal Figure

Magnesium oxide MgO Fig. 4.1-140

Magnesium sulﬁde MgS Fig. 4.1-141

Magnesium selenide MgSe Fig. 4.1-142

Magnesium telluride MgTe Fig. 4.1-143

Disagreement exists about the position of the minimum of

the lowest conduction band. Some authors place the mini-

mum at Γ

and anticipate a direct gap; others claim that the

minimum is at X

, indicating an indirect gap.

1.0

0.8

0.6

0.4

0.2

– 0.2

– 0.4

– 0.6

– 1.2

– 1.4

– 1.6

– 1.8

XL ΓΣΛ∆ KΓ

– 2.8

– 4.8

E (Ry)

Wave vector k

MgO

∆

25'

∆

25'

∆

Fig. 4.1-140 Band structure of magnesium oxide

Part 4 1.3

658 Part 4 Functional Materials

1.8

1.2

0.6

– 0.6

– 1.2

WXLΓ

E (eV)

Wave vector k

MgS

Fig. 4.1-141 Band structure of magnesium sulﬁde, rock salt

structure

–4

–8

–12

ΓΓ

E (eV)

Wave vector k

X U(K) XL W

DOS (arb. units)

MgTe

0.04

0.08

0.12

Fig. 4.1-143 Band structure of magnesium telluride

1.8

1.2

0.6

– 0.6

– 1.2

WXLΓ

E (Ry)

Wave vector k

MgSe

Fig. 4.1-142 Band structure of magnesium selenide, rock

salt structure

Part 4 1.3

Semiconductors 1.3 II–VI Compounds 659

Table 4.1-114 Energy gaps of magnesium compounds

Crystal Band gap (eV) Temperature (K) Remarks

Magnesium oxide MgO E

gx, dir

7.672 (10) 85 Exciton states

g, theor

7.9 (7) 0 Temperature dependence of microwave conductivity

Magnesium sulﬁde MgS 4.5 77 Zinc blende structure

Magnesium selenide MgSe 4.05 300 Zinc blende structure

Magnesium telluride MgTe 3.49 RT Epitaxial ﬁlms, photoluminescence, zinc blende structure

C. Transport Properties

Electronic Transport, General Description. Magnesium

Oxide (MgO). Electrical transport measurements on al-

kaline earth oxides encounter several difﬁculties, such

as high resistance at low temperatures, a strong inﬂu-

ence of surface layers, and high-temperature thermionic

emission. The partly contradictory results depend con-

siderably on the purity and nature of the samples

(pressed porous powders, sintered samples, polycrystals,

and single crystals) and on the experimental conditions.

(bar)

σ (Ω

–1

)

–4

–5

–6

–7

–8

–9

–24

–20

–16

–12

–8

–4

MgO

1873 K

1773

1673

1573

1473

1373

1273 K

– –

100 Hz

Fig. 4.1-144 MgO. Electrical conductivity σ of a high-

purity single crystal at various temperatures vs. oxygen

partial pressure p

[1.122]. The numbers between the

curves for 1673 K and 1773 K indicate the slopes of the

dashed straight lines

As in several semiconducting oxides, the conduc-

tivity and the type of conduction are determined

by the oxygen partial pressure of the ambient

atmosphere. In high-purity MgO crystals, tempera-

tures below 1600 K favor ionic conduction, whereas

at temperatures above 2000 K, electronic conduc-

tion is increasingly predominant. At low oxy-

gen pressures (< 10

−3

mbar) the conduction is n-

type; at high pressures (> 10

−1

mbar) ionic and

electronic hole conduction seem to be superim-

posed.

See Fig. 4.1-144 for the temperature dependence of

the electrical conductivity of MgO and Fig. 4.1-145 for

the thermal conductivity of MgO.

–1

–2

–3

T (K)

4 × 10

–4

1014 × 10

–1

(W/cm K)

MgO

Fig. 4.1-145 MgO. Thermal conductivity κ vs. tempera-

ture [1.123]. Curve a, high-purity crystal; b, after additive

coloration in Mg vapor; c, after subsequent bleaching with

UV light

Part 4 1.3

660 Part 4 Functional Materials

D. Electromagnetic and Optical Properties

Table 4.1-115.

Table 4.1-115 Dielectric constant ε of magnesium compounds

Crystal ε(0) ε(∞) Temperature (K) Remarks

Magnesium oxide MgO 9.830 2.944 300 Single crystals

Magnesium sulﬁde MgS

Magnesium selenide MgSe 3.8 RT Zinc blende structure, Raman scattering

Magnesium telluride MgTe

4.1.3.3 Oxides of Ca, Sr, and Ba

A. Crystal Structure, Mechanical and Thermal Properties

Tables 4.1-116 – 4.1-123.

Table 4.1-116 Crystal structures of oxides of Ca, Sr, and Ba

Crystal Space group Crystal system Structure type Figure

Calcium oxide CaO O

Fm3m fcc NaCl Fig. 4.1-138

Strontium oxide SrO O

Fm3m fcc NaCl Fig. 4.1-138

Barium oxide BaO O

Fm3m fcc NaCl Fig. 4.1-138

Table 4.1-117 Lattice parameters of oxides of Ca, Sr, and Ba

Crystal Lattice parameters (nm) Temperature (K) Remarks

Calcium oxide CaO a =0.48110 (2) 297 Single crystal, X-ray diffraction

Strontium oxide SrO a =0.5159 Semiempirical calculation

Barium oxide BaO a =0.55363 (8) 300 Powder samples, negligible oxygen pressure

Table 4.1-118 Densities of oxides of Ca, Sr, and Ba

Crystal Density (g/cm

)

Calcium oxide CaO 3.335

Strontium oxide SrO 4.75

Barium oxide BaO 5.68

Table 4.1-120 Elastic constants c

of oxides of Ca, Sr, and Ba

Crystal c

(GPa) c

(GPa) Temperature (K) Remarks

Calcium oxide CaO 221.89 (60) 57.81 (66) 80.32 (10) 298 Calculated from sound velocities

Strontium oxide SrO 175.47 (37) 49.08 (35) 55.87 (13) 298

Barium oxide BaO 126.14(66) 50.03 (60) 33.68 (30) 298

Table 4.1-121 Linear thermal expansion coefﬁcient α of oxides of Ca, Sr, and Ba

Crystal Expansion coefﬁcient α(K

−1

) Temperature (K) Remarks

Calcium oxide CaO 15.2×10

−6

Single crystal, X-ray diffraction

Strontium oxide SrO 13.72 ×10

−6

293–593 Mean value, X-ray diffraction

Barium oxide BaO 12.8 (9) ×10

−6

200–400 Powder samples, X-ray diffraction

Table 4.1-119 Melting point T

of oxides of Ca, Sr, and Ba

Crystal Melting point T

(K)

Calcium oxide CaO 2900

Strontium oxide SrO 2930 (30)

Barium oxide BaO 2290 (30)

Part 4 1.3

Semiconductors 1.3 II–VI Compounds 661

Table 4.1-122 Heat capacity and Debye temperature of oxides of Ca, Sr, and Ba

Crystal Heat capacity c

Debye temperature Θ

Remarks

(J/mol K) (K)

Calcium oxide CaO 14.70 at 100 K 605 (2) Θ

(0) from heat capacity at low temperatures

42.21 at 300 K

Strontium oxide SrO 446 (5) Θ

from heat capacity at very high temperatures

Barium oxide BaO 47.28 at 298 K 370 (4) Θ

from heat capacity at very high temperatures

Table 4.1-123 Phonon frequencies at symmetry points for oxides of Ca, Sr, and Ba. Calcium oxide (CaO), 293 K;

strontium oxide (SrO), 300 K; barium oxide (BaO), 300 K. All data from inelastic thermal-neutron scattering

CaO SrO BaO

Mode Symmetry Frequency Mode Symmetry Frequency Mode Symmetry Frequency

point (THz) point (THz) point (THz)

q/q

max

= 0.2 16.40 (25)

Γ 7.02 (10)

Γ 4.32 (6)

Γ 8.8 (4)

Γ 14.47 (15)

Γ 13.02 (3)

X 12.00 (22)

L 3.33 (5)

L 1.99 (7)

X 9.4 (5)

L 7.14 (10)

L 4.88 (27)

X 6.48 (11)

L 8.35 (12)

L 6.58 (9)

L 12.94 (24)

B. Electronic Properties

Tables 4.1-124 – 4.1-127.

Table 4.1-124 First Brillouin zones of oxides of Ca, Sr, and Ba

Crystal Figure

Calcium oxide CaO Fig. 4.1-133

Strontium oxide SrO Fig. 4.1-133

Barium oxide BaO Fig. 4.1-133

Table 4.1-125 Energy gaps of oxides of Ca, Sr, and Ba

Crystal Band gap (eV ) Temperature (K) Remarks

Calcium oxide CaO E

g, dir

6.931 (6) 85 Exciton states

g, theor

7.8 0

Strontium oxide SrO E

g, opt

5.22 300

g, theor

6.4 0 Estimated from temperature dependence of conductivity

Barium oxide BaO E

g, theor

4.4 0 Estimated from temperature dependence of conductivity

Table 4.1-126 Effective masses of electrons for oxides of Ca, Sr, and Ba (in units of the electron mass m

)

Crystal Quantity Mass Temperature (K) Remarks

Calcium oxide CaO m

0.50 700 From Hall mobility

n, polaron

0.68 700–800 Polaron mass, Hall mobility

Strontium oxide SrO m

0.54 600–700 From Hall mobility

n, polaron

0.77 Polaron mass, Hall mobility

Barium oxide BaO m

0.59 600 From Hall mobility

n, polaron

0.86 500–700 Polaron mass, Hall mobility

Part 4 1.3