April 2008
EPL, 82 (2008) 17006
doi: 10.1209/0295-5075/82/17006
www.epljournal.org
Semiconducting hydrides
S. Zh. Karazhanov1,2(a) , A. G. Ulyashin3 , P. Ravindran1 and P. Vajeeston1
1
Center for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo
P.O. Box 1033, Blindern, N-0315 Oslo, Norway
2
Physical-Technical Institute - 2B Mavlyanov St., 700084 Tashkent, Uzbekistan
3
Institute for Energy Technology - P.O. Box 40, NO-2027 Kjeller, Norway
received 13 November 2007; accepted in final form 6 February 2008
published online 11 March 2008
PACS
PACS
PACS
71.20.-b – Electron density of states and band structure of crystalline solids
71.55.-i – Impurity and defect levels
71.15.Mb – Density functional theory, local density approximation, gradient and other
corrections
Abstract – Using first-principles density functional calculations with AlH3 , KMgH3 , LiMgH3 ,
NaBeH3 , NaMgH3 , and RbMgH3 as model systems we have analyzed the band structure of
hydrides. It is shown that hydrides can possess the features of semiconductors with n- and/or
p-type electrical conductivity. We have found that carrier effective masses of some hydrides are
almost the same as those of commonly known semiconductors. The present study suggests that
the Mg impurities substituting Al form a shallow acceptor level in the band gap of AlH3 , which
can provide holes and cause p-type electrical conductivity. From studies of optical properties we
have found that even if Mg impurities of about 1.3 × 1021 cm−3 concentration substitute the Al
site, AlH3 can still be transparent in the visible spectra. This result opens up the door for the
application of hydrides in the future generation of optoelectronic devices.
c EPLA, 2008
Copyright Hydrides are found to be an important class of materials
due to their potential use for energy storage/economy [1–6].
The basic attention to study hydrides has so far been
focused on the acceleration of kinetics for the hydrogenation/dehydrogenation processes at moderate temperatures
and on the increase of the weight percentage of hydrogen. So those hydrides characterized by relatively slow
kinetics for hydrogenation/dehydrogenation processes
and high decomposition temperature do not present much
interest for hydrogen economy. However, these hydrides
have recently been suggested [7] to find unique exciting
applications in optoelectronics. But the proposed applications depend on their stability with respect to doping,
solubility of shallow donors and acceptors, and electrical
as well as optical properties. Furthermore, the hydrides
are widely available in powder form, which is required
for hydrogen storage/economy, but is not preferable for
electronic device technology. For the latter applications
crystalline or amorphous hydrides are more preferable. At
the moment this is an open problem and comprehensive
theoretical and experimental studies are needed. Nevertheless, some preliminary supporting experimental evidences
for device applications of the hydrides already existed.
(a) E-mail:
smagul.karazhanov@ife.no
It is found that like many wide band gap semiconductors some hydrides have a large fundamental band gap,
e.g., Mg2 NiH4 [3,8], MgH2 [9], AlH3 [10], LiH [11], LaH3 ,
YH3 [1,12], Mg(AlH4 )2 , Mg(BH4 )2 [13,14], Ca(AlH4 )2 [15]
and AAlH5 (A = Be, Ca, Sr) [16], etc. Further, the band
gap has been found to be modulated by H stoichiometry [1,17,18]. Bottommost conduction band (CB) and
topmost valence band (VB) of some hydrides are well
dispersive [7,10,11,19], which indicates that they can
be capable of electrical conductivity. Carrier concentration and electrical conductivity in YH2.9 [20] have been
measured to be ∼ 2.32 × 1019 cm−3 and ∼ 70 Ω−1 cm−1 ,
respectively. Except shiny metallic and transparent states
in Mg2 NiHx a third state characterized by high electrical
conductivity ∼ 1.6 × 104 Ω−1 cm−1 , low reflection (∼25%)
and no transmission has been found [21], which corresponds to ∼75% absorption of incident light. This state is
believed to be formed [3] due to the reversible double layering of metallic Mg2 NiH0.3 and semiconducting Mg2 NiH4 .
Therefore, the layers can be regarded as a semiconductor
n+ -n junction.
The literature survey shows that there are some studies about defects in hydrides from the point of view of
their effect on hydrogenation/dehydrogenation kinetics as
well as electrochromic feature. But no attention has yet
17006-p1
S. Zh. Karazhanov et al.
6
6
4
4
2
Energy (eV)
been paid to the question as to the effect of the impurities
on electrical conductivity of hydrides. Calculated effective
masses of some hydrides are found [7,19] to be almost the
same as those of semiconductors. Moreover, the ab initio
calculations on Ca and Mg impurities substituting Al in
orthorhombic AlH3 show [7] shallow acceptor levels thus
indicating the tendency to contribute to p-type electrical
conductivity. Impurities of Si substituting Al in hexagonal
AlH3 can form shallow donor level resulting in n-type electrical conductivity [7]. In addition, even at high concentrations of Mg and Ca (∼ 6.1 × 1020 cm−3 ), α-AlH3 has
been shown [7] to remain transparent to the visible light.
Using AlH3 , KMgH3 , LiMgH3 , NaBeH3 , NaMgH3 , and
RbMgH3 as examples, here we show that hydrides possess
the features of semiconductors, namely, well-dispersed
topmost VB, small carrier effective masses, p-type electrical conductivity, and finite fundamental band gap.
Vienna ab initio simulation package (VASP) [22,23] has
been used for the present study along with the generalizedgradient approximation of Perdew-Wang [24]. The interaction between ions and electrons is described using the
projector augmented wave (PAW) method. For primitive
unit cells the self-consistent calculations were performed
using a 10 × 10 × 10 mesh of special k-points. For studies
of defects 2 × 1 × 2 supercell have been used for all types
of the lattices considered. All configurations were fully
relaxed using the conjugate gradient method. The planewave cutoff energy of 500 eV was used for all the calculations. The convergence was achieved when the forces
acting on the atoms were less than 2 meV Å−1 and the
total energy difference between two consecutive iterations
was < 10−6 eV/cell. The convergence criteria and more
details about the optical calculations were discussed in
ref. [25]. The imaginary part of the optical dielectric function has been derived by summing all allowed transitions
from occupied to unoccupied states for energies much
higher than those of the phonons. It is used to derive the
reflectivity and absorption coefficients. More details about
the optical calculations were discussed in ref. [25].
We have studied the band structure and optical properties of cubic AlH3 (also known as β-AlH3 ), cubic KMgH3 ,
rhombohedral LiMgH3 , tetragonal NaBeH3 , orthorhombic
NaMgH3 , hexagonal RbMgH3 , and doped β-AlH3 . The
choice of the hydrides for the present study is based on
their large fundamental band gap [11], e.g., the band
gap for hexagonal AlH3 calculated by the density functional theory (DFT) using the GW approximation (“G”
stands for one-particle Green’s function as derived from
many-body perturbation theory and “W” for Coulomb
screened interactions) is 3.5 eV [11]. Furthermore, the
cubic AlH3 (β-AlH3 ) is energetically more stable than the
other polymorphs [10] and can be synthesized in bulk. It
exhibits dehydriding reactions in the temperature range of
60–200 ◦ C (ref. [26]), 330 ◦ C (ref. [27]), which is well above
the operable range of optoelectronic devices. According to
ref. [28], one can store AlH3 for several years without any
loss of hydrogen. However, the hydrogen decomposition
β−AlH3
2
0
0
-2
-2
-4
X
-4
Γ
L
UX W
L
Γ
KW
4
2
X M
Γ
R
M
Γ
R
M
4
2
RbMgH3
0
0
-2
-2
A
KMgH3
L M
Γ A
H K
Γ
NaBeH3
Γ
X M
Fig. 1: Band dispersion for β-AlH3 , KMgH3 , RbMgH3 , and
NaBeH3 . Fermi level is set at zero energy.
is reported to be enhanced by doping, particle size [29]
and vacancy concentration [29,30]. Anyhow, here we
use β-AlH3 , KMgH3 , NaBeH3 , and RbMgH3 as model
systems only to demonstrate that hydrides can possess
p-type conductivity and transparency to the visible part
of the solar spectrum. Thus, we leave the problem of
finding a more suitable hydride for the future studies.
Optimized structural parameters have been found in
refs. [5,6]. For doped AlH3 we have considered Mg impurity substituting Al (MgAl ) as a candidate for shallow
level acceptor-type defects. The primitive unit cell of cubic
AlH3 consists of 16 atoms, whereas for the studies of
defects we have considered a supercell with 64 atoms
derived from 2 × 1 × 2 of the conventional cell.
The band structure of the above-mentioned hydrides has
been studied. The results have been presented for AlH3 ,
KMgH3 , RbMgH3 , and NaBeH3 (fig. 1). It is found that
the calculated fundamental band gap is 2.6 eV for KMgH3 ,
1.5 eV for NaBeH3 and 2.5 eV for RbMgH3 . Commonly,
the fundamental band gap calculated from DFT is smaller
than that derived from optical measurements. So, one
can expect that the actual band gap for these materials will be larger than that listed above. The topmost
VB and the bottommost CB of KMgH3 , NaBeH3 , and
NaMgH3 are found to be well dispersive resulting in small
carrier effective masses, where the calculated directiondependent effective masses indicate that the conductivity
will be highly anisotropic. Among the hydrides considered for the present study, RbMgH3 and AlH3 posses
well-dispersed and nearly isotropic topmost VB as well
as bottommost CB. Here, we shall concentrate our attention on AlH3 . Figure 1 displays the band structure for
β-AlH3 where the CB minimum and VB maximum are
located at the X- and Γ-point, respectively, indicating
that AlH3 is an indirect band gap material. The calculated direct and indirect band gaps are 3.5 eV and 3.0 eV,
17006-p2
Semiconducting hydrides
-1
-1
-1
0.08
0.04
H
0.16
1.6
-1
0.12
EF
-1
Al
s
p
0.16
PDOS (States eV fu atom )
Total DOS (States eV fu )
EF
1.2
0.8
0.4
0.12
-2 -1
0.08
0.04
0 1 2 3
Energy (eV)
4
Fig. 3: Total DOS for Mg-doped β-AlH3 . The Fermi level is set
to zero energy.
-3 -2 -1 0 1 2
Energy (eV)
3
4
Fig. 2: Orbital and site projected DOS for β-AlH3 . The Fermi
level is set to zero energy.
respectively. As the bottommost part of the CB is less
dispersive, one cannot expect much contribution to electrical conductivity from CB electrons. However, the topmost
VB is much more dispersive than the bottommost CB indicating that the electrical current transport in AlH3 can be
determined basically by holes. The well-dispersed nature
of the topmost VB indicates the presence of a considerable covalent bonding between Al and H in agreement
with the density of states (DOS) and charge density analysis [10]. For the quantitative characterization of the band
dispersion the effective masses of holes (mh ) have been
calculated, which are equal to mh = 0.60m0 along the Γ-X
direction and mh = 0.48m0 along the Γ-L one demonstrating a slight anisotropy in the conductivity. However,
the magnitude of the masses is much smaller along Γ||A
or comparable along Γ ⊥ A than that for ZnO (2.74m0
(2.27m0 ) along Γ||A and 0.54m0 (0.35m0 ) along Γ ⊥ A
from the FP-LMTO (plane-wave pseudopotential [31])
method [32]).
In order to clarify the character of the bands in the vicinity of the fundamental gap in hydrides the orbital and site
projected DOS for AlH3 are plotted in fig. 2. The analysis
shows that the bottommost CB is basically contributed by
Al 3s-electrons, whereas the topmost VB is contributed by
H 1s-electrons. This is one of the distinguishing features
of some hydrides compared to conventional semiconductors where the topmost VB is commonly originated from
p-/d -electrons and the bottommost CB from s-electrons.
To our knowledge, there is no report on semiconductors
with s-type electrons at the topmost VB.
As pointed out in the above analysis of the band
structure (fig. 1) and effective masses, holes are capable
to contribute well to electrical conductivity in AlH3 .
However, upon heavy doping with shallow acceptors, not
only the conductivity can be enhanced but also the optical
properties can be changed considerably. We have also
studied structural properties, electronic structure, and
optical spectra of Mg-substituted AlH3 . From structural
studies we have found that the volume difference between
the ideal and Mg-doped β-AlH3 does not exceed 2%.
Furthermore, the calculated formation energy for the
Mg-doped β-AlH3 is −2.0 eV, which shows that such
configuration is energetically stable. Figure 3 displays the
total DOS for the Mg-doped AlH3 , which shows that the
Mg substitution for Al in AlH3 induces a shallow level
acceptor. Hence, MgAl can provide free holes and thus
contribute to electrical conductivity.
Figure 4 shows the calculated optical spectra for
AlH3 : MgAl , which indicates that despite heavy doping
with Mg, transparency has not been changed much in
the energy range 0–4 eV. Consequently, AlH3 can be
a well-conducting material and at the same time be
transparent to the visible range of the optical spectrum.
It may be noted that the band gap obtained from DFT
17006-p3
S. Zh. Karazhanov et al.
2.0
MgAl
Ideal
MgAl
Ideal
0.6
×10
1.2
−5
R(ω)
-1
α(ω) (cm )
1.6
0.4
0.8
0.2
0.4
4
8
12
16
4
8
12
16
Energy (eV)
Fig. 4: Absorption coefficient α(ω) and reflectivity R(ω) for
ideal and MgAl -doped β-AlH3 .
calculations is commonly underestimated due to the
artificial shift by the CB toward lower energies. So, in the
real case, β-AlH3 can be transparent in a wider energy
range of photons than that predicted from the present
optical calculations.
In conclusion, using DFT calculations we have demonstrated that several hydrides are capable of n- and p-type
electrical conductivity and at the same time transparent
to the visible range of the optical spectrum. This can
extend the class of semiconducting materials and be
important in the low-temperature optoelectronic device
manufacturing technology. Distinct from other semiconductors, hydrides can have added advantages such as
passivation of defects, improvement in interface problems,
avoiding of band off-sets, etc. On the one hand, the present
results extend the list of possible applications of hydrides
by revealing their semiconducting feature and, on the
other hand, we opened up the door for the application
of hydrides in the future generation of electronic devices
called “hydride electronics”. Studies about stability, transparency, and feasibility of the n- and p-type conductivity
for the hydrides are the subjects for detailed investigations
in the near future.
∗∗∗
This work has received financial and supercomputing
support from the Research Council of Norway within
NANOMAT project and from the Academy of Sciences
of Uzbekistan.
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17006-p5