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Hydrides as materials for semiconductor electronics
S.Zh. Karazhanov abc; A. G. Ulyashin c; P. Vajeeston a; P. Ravindran a
a
Centre for Materials Sciences and Nanotechnology, Department of Chemistry, University of Oslo, Blindern,
N-0315 Oslo, Norway b Physical-Technical Institute, 700084 Tashkent, Uzbekistan c Institute for Energy
Technology, N-2027 Kjeller, Norway
Online Publication Date: 01 June 2008
To cite this Article Karazhanov, S.Zh., Ulyashin, A. G., Vajeeston, P. and Ravindran, P.(2008)'Hydrides as materials for semiconductor
electronics',Philosophical Magazine,88:16,2461 — 2476
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Philosophical Magazine
Vol. 88, No. 16, 1 June 2008, 2461–2476
Hydrides as materials for semiconductor electronics
S.Zh. Karazhanovabc*, A.G. Ulyashinc, P. Vajeestona and P. Ravindrana
a
Centre for Materials Sciences and Nanotechnology, Department of Chemistry, University of
Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway; bPhysical-Technical Institute,
2B Mavlyanov St., 700084 Tashkent, Uzbekistan; cInstitute for Energy Technology,
P.O. Box 40, N-2027 Kjeller, Norway
Downloaded By: [Karazhanov, S.Zh.] At: 06:18 29 September 2008
(Received 3 April 2008; final version received 22 July 2008)
Systematic studies using density functional theory have shown that some hydrides
possess the features of semiconductors. These features include larger fundamental
band gap, well dispersed bottom-most conduction band and/or top-most valence
band, small electron/hole effective masses and small intrinsic carrier concentration. It is demonstrated that depending upon the composition, hydrides possess
a wide range of band gap values and hence they can be regarded as materials
for narrow to wide band gap semiconducting applications. The possibility of
designing hydride-based p–n junctions, and also their advantages as well as
deficiencies compared to existing oxide semiconductors, are discussed. Replacing
oxide-based semiconductors by hydrides can help to avoid problems such as
formation of an oxide layer, band offsets, large concentration of defect states at
the interface between the oxide and semiconductor, etc. Moreover, hydrides can
be regarded as an alternative to conventional semiconductors and hence can be
used in future-generation electronic devices called ‘‘hydride electronics’’.
Keywords: hydrides; semiconductors; applications of hydrides; semiconductor
electronics; hydride electronics
1. Introduction
Several applications of hydrides have been found so far, e.g. switchable mirrors [1–3],
energy storage [4,5], rechargeable batteries [5], etc. The unique behaviour of hydrogen is
the subject of discoveries such as universal alignment of hydrogen levels in solids [6],
hydrogen-induced crystallization of amorphous silicon [7], amorphization of crystalline
solids [8], metal–insulator transition accompanied by pronounced optical changes [1–3],
metallization of semiconductor surfaces [9,10], conversion of Pauli paramagnets or
diamagnets into ferri-, ferro- or antiferromagnets, enhancement/suppression of ferromagnetism, alteration of magnetic anisotropy, development of spin-glass structures,
suppression [11]/appearance [12] of superconductivity, etc. Recently, this long list has been
extended with the application of hydrides as transparent conducting materials [13]. Since
the discovery of p-type conductivity in highly transparent thin film of CuAlO2þx [14],
studies of transparent conducting oxides (TCOs) have emerged as a new field in
*Corresponding author. Email: smagul.karazhanov@ife.no
ISSN 1478–6435 print/ISSN 1478–6443 online
ß 2008 Taylor & Francis
DOI: 10.1080/14786430802360362
http://www.informaworld.com
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S.Zh. Karazhanov et al.
optoelectronic device technology, the so-called ‘‘transparent electronics’’ or ‘‘invisible
electronics’’ [14–20].
So far the hydrides have been intensively studied for their potential application as
hydrogen storage materials. From these studies it is found that hydrogen deficiency can
cause metal to insulator phase transition [1–3,21–23] and the band gap of hydrides varies
from 0 to 46 eV [1–3,24]. The metal–insulator transition has been detected in YHx, LaHx
[2,25–27], CeHx [28] and Mg2NiH4 [23,29–31]. The transparency of insulating hydrides to
visible light was utilized recently for switchable mirrors [1–3]. In these studies, special
attention was paid to accelerating the kinetics of the hydrogenation/dehydrogenation
processes and to increase the hydrogen content in the hydrides. However, there is no
intentional study about the reverse process of how to slow kinetics of the hydrogenation/
dehydrogenation processes, which would be more important for electronic device
applications of the hydrides.
Less attention has been paid to doping and electrical properties of hydrides. However,
a more detailed study [32,33] on this subject is available for LiH where studies with Mg, In,
Tl, Sn, Sb, and Bi doping have been made. Since LiH has a wide band gap (Eg ¼ 4.99 eV
[34]) the above impurities are colour centres that form deep energy levels in the band gap,
thus modulating the luminescent properties of LiH and reducing its photoconductivity.
Electrical and optical properties of LaHx, YHx and Mg2NiH4 have also been studied
systematically. It is found that electrical conductivity and carrier concentration can be as
high as 70 1 cm1 and 2.32 1019 cm3, respectively, for YH2.9 [26]. In addition to shiny
metallic and transparent states, Mg2NiHx is found [35] to exhibit a third state characterized by high electrical conductivity (1.6 104 1 cm1), low reflection (25%) and no
transmission corresponding to absorption of 75% of the incoming light. This intriguing
state is found [23] to be caused by the self-organized and reversible double layering of
metallic Mg2NiH0.3 and semiconducting Mg2NiH4. This double layer can be regarded as
a semiconductor nþ–n junction. Some studies have been made on the effect of defects in
hydrides on hydrogenation/dehydrogenation kinetics as well as electrochromic features.
Almost no attention has been paid to the question as to the effect of impurities on
electrical conductivity of hydrides. In this paper, the electronic structure and optical
properties of bulk and doped hydrides are described and the possibility reported
of utilizing them as semiconductor materials in future-generation electrical devices.
The application of hydrides in semiconductor electronics is expected to solve a number of
problems related to the interface, band gap engineering, etc.
2. Methods
Our study is based on density functional theory (DFT) within the generalized-gradient
approximation using the projected-augmented-wave method implemented in the VASP
package [36,37]. The Perdew–Wang exchange-correlation functional [38] has been used.
The pseudopotentials were generated in accordance with the projector-augmented wave
(PAW) method [39,40]. For primitive unit cells the self-consistent calculations were
performed using a 10 10 10 mesh of special k-points. For studies of defects, a 2 2 2
supercell has been used for all types of the lattices considered. All configurations were fully
relaxed using the conjugate gradient method. A plane-wave cut-off energy of 500 eV was
used for all the calculations. The convergence was achieved when the forces acting on the
Philosophical Magazine
2463
atoms were smaller than 10 meV Å1 and the total energy difference between two
consecutive iterations was 5106 eV. The imaginary part of the optical dielectric function
has been derived by a summation over all allowed transitions from occupied to unoccupied
states for energies much higher than those of the phonons. This is further used to derive
the reflectivity and absorption coefficients. More details about the optical calculations are
discussed elsewhere [41].
From the calculated total density of states (DOS), N(E) as a function of energy, E, the
effective DOS in the conduction band (CB), Nc, and in the valence band (VB), Nv, have
been estimated [42]:
Z
1 1
Ec E
Nc ¼
exp
NðE ÞdE,
ð1Þ
V0 Ec
kT
Downloaded By: [Karazhanov, S.Zh.] At: 06:18 29 September 2008
Nv ¼
1
V0
Z
Ev
exp
1
E Ev
NðE ÞdE,
kT
ð2Þ
where k is the Boltzmann constant, T is the temperature, and Ec and Ev are the energies
corresponding to the bottom-most CB and top-most VB, respectively. V0 is the volume
of the unit cell. Equations (1) and (2) were used to calculate the intrinsic carrier
concentration, ni:
pffiffiffiffiffiffiffiffiffiffiffi
Eg
ni ¼ Nc Nv exp ,
ð3Þ
2kT
where Eg ¼ Ec Ev is the fundamental band gap. Knowledge of ni makes it possible to
estimate [43] the diffusion potential, VD, of the p–n junction with concentration of shallow
donors Nd and shallow acceptors Na:
kT
Na Nd
ln
VD ¼
:
ð4Þ
q
n2i
This is one of the important parameters for semiconductor p–n junction-based devices. The
density of states mass for electrons, mde, and holes, mdh, were calculated from the Equation
Nc,v
2mde, dh kT 3=2
¼2
:
h2
ð5Þ
3. Results
3.1. Electronic structure and electrical parameters of undoped hydrides
Results in the literature and the present calculations of the electronic structure and optical
properties of hydrides with/without defects and impurities show that several hydrides are
insulators with a large fundamental band gap (see Figure 1). The calculated band gaps
presented in Figure 1 are underestimated because of the well known deficiency of DFT.
Real band gaps are therefore expected to be larger than those presented in Figure 1.
Analysis of Figure 1 shows that the calculated band gap for Ca4Mg4FeH63, Mg2RuH4,
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2464
S.Zh. Karazhanov et al.
Figure 1. Fundamental band gap of metal and complex hydrides: (1–4) MgH2, BeH2, LiAlH4,
NaAlH4; (5–9) KAlH4, RbAlH4, CsAlH4, LiBH4, NaBH4; (10–14) KBH4, RbBH4, CsBH4, LiGaH4,
NaGaH4; (15–19) KGaH4, RbGaH4, CsGaH4, Li3AlH6, Na3AlH6; (20–24) K3AlH6, LiMgH3,
NaMgH3, KMgH3, RbMgH3; (25–29) CsMgH3, LiBeH3, NaBeH3, KBeH3, RbBeH3; (30–34) CsBeH3,
BeAlH5, MgAlH5, CaAlH5, SrAlH5; (35–39) BaAlH5, MgB2H8, MgAl2H8, CaB2H8, CaAl2H8; (40–44)
Ba6Mg7H26, BaMgH4, Ca19Mg8H54, Ca4Mg3H14, Ca4Mg4FeH63; (45–49) CaMgNiH4, Cs2MgH4,
Cs3MgH5, K2MgH4, LiMg2RuH6; (50–54) Mg2RuH4, Mg3ReH7, Rb2MgH4, Rb3MgH5,
Rb4Mg3H10; (55–59) SrMg2FeH8, SrMgH4, SrMgNiH4, Yb4Mg3H14, Sr2Mg3H10; (60–65) LiH,
NaH, KH, RbH, CsH, CuH; (66–69) BaLiH3, CaCaH3, RbCaH3, SrLiH3; (70–72) -AlH3, -AlH3,
-AlH3. Circles denote experimentally found hydrides, whereas triangles correspond to hypothetical,
theoretically predicted hydrides. The background shades distinguish the opaque (light) and transparency (darker) energy ranges. The darkest background indicates the most desirable transparency
range. (A colour-coded version is available online).
CuH and BaLiH3 is smaller than 1.5 eV. It should be noted that the small band gap solids
often possess the feature of easy bipolar doping, i.e. doping with shallow donors and
acceptors provide materials with n- and p-type conductivities. Hence, one can design a p–n
junction from such solids. Also the well dispersed nature of the bottom-most CB and
top-most VB shows that these materials are expected to have good electrical conductivity.
Such hydrides might be interesting for application in p–n junction-based electronic devices
such as solar cells, photodetectors, diodes, transistors, etc.
Analysis of the band structure of some of the hydrides shows that the bottom-most CB
for LiH, NaH, KH, RbH, CsH, CsCaH3, RbCaH3, SrLiH3, SrMg2FeH8 and NaAlH4, and
top-most VB for BaMgH4 and Mg2RuH4, and both the bottom-most CB and top-most VB
for BaLiH3, KMgH3, LiAlH4, Na3AlH6, SrMgH4 and MgH2 are well dispersed. Figure 2
demonstrates band dispersion for MgH2, LiH, CsCaH3 and BaLiH3 as well as that for the
well-known semiconductors Si and GaAs. It can be seen that the bottom-most CB and
top-most VB for MgH2, CsCaH3 and BaLiH3 are well dispersed, similar to that of Si and
GaAs. Consequently, the present study suggests that carrier transport through the CB
and/or VB for these hydrides is possible. So it is expected that some hydrides are capable
of electrical current transport similar to well-known semiconductors.
2465
Philosophical Magazine
10
4
4
0
5
CsCaH3
2
MgH2
−4
0
0
−8
Energy (eV)
Γ X M
Γ
Z
Γ
A R Z
X M
Γ R
M
L
Γ
X
K
Γ
X
K
Γ
10
4
4
5
0
2
LiH
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Si
−2
−5
−4
BaLiH3
0
0
−2
−5
X
Γ
L U X W L KW
GaAs
−8
−12
Γ
X M
Γ R
M
L
Γ
Figure 2. Band structure of MgH2, LiH, CsCaH3, BaLiH3, Si and GaAs. The Fermi level is set
to zero.
The band gap values for solids calculated using DFT are systematically underestimated. One of the ways to correct the DFT deficiency is to shift the CB states rigidly up
to the experimentally determined location. So, for the band dispersions in Figure 2,
we have made use of this so-called scissor operation for correcting the band gaps
calculated by DFT. For CsCaH3 and BaLiH3, no experimentally measured band-gap
values are available in the literature, and hence for these two compounds alone we have
displayed the band structure without the scissor operation.
One of the parameters characterizing transport properties of solids is carrier effective
masses. The effective masses have been calculated for MgH2, LiH, KMgH3 and BaLiH3
(Table 1). Analysis shows that the effective masses for hydrides are of the same order of
magnitude as those of the well-known semiconductors (Si and GaAs) and TCOs (ZnO and
In2O3). These results indicate that the electrical conductivity of hydrides can be the same as
that of semiconductors and TCOs. However, systematic studies on hydrides are required
on this aspect, in particular their carrier mobility compared with the semiconductors is
important for device applications.
The calculated total DOS, N(E) (Figure 3), shows that the top-most VB is sufficiently
broad and that there are no sharp peaks. This indicates that the VB electrons in the
outermost shells are not tightly bound to their atoms. Hence, covalency plays
a considerable role in chemical bonding. As a result, the hole mobility, favourable for
current transport, is expected to be large in these compounds. Using the calculated N(E )
values, the effective DOS corresponding to the CB as well as VB and the intrinsic carrier
concentration at T ¼ 300 K, have been estimated using Equations (1)–(3). This has been
done for BaLiH3, KMgH3, MgH2 and LiH, the semiconductors Si and GaAs, as well as
for ZnO (see Table 2). Analysis shows that the magnitude of Nc and Nv calculated for Si,
f
0.19
(X ! U)
0.56
(Z ! A)
0.28
(X ! W)
0.01
(X ! )
0.14
(X ! M)
0.13
(X ! )
0.09
(R ! )
0.12
(R ! M)
0.12
(X ! )
0.44
(Z ! )
KMgH3
LiH
0.77
(R ! M)
0.19
(M ! X)
1.16
(M ! )
BaLiH3
0.27a
( ! F)
0.32a
( ! L)
-AlH3
0.48b
( ! L)
0.67a
( ! Y)
2.96a
( ! Z)
0.60b
( ! L)
-AlH3
1.30a
( !S)
-AlH3
0.79
0.541d
( ! L)
0.27 0.217J
( ! X)
1.09
(1.08)c
Si
0.76 0.45–1.03k
0.52–4.00l
( ! L)
0.44 0.34–0.48j
0.34–0.80k
( ! X)
0.53
( ! K)
0.047
(0.067)d
GaAs
VASP-PAW [13]. bVASP-PAW [44]. cExperiment [45]. dKP theory, semi-empirical results [46]. eVASP-PAW [47].
FP-LMTO [48]. gExperiment [49]. hVASP-PAW [50]. iExperiment [51]. jExperiment [52]. kExperiment [53]. lKP theory [53].
a
mh
mc
MgH2
0.54e
(0.59)g
( ?A)
0.35f
( ?A)
2.74e
(0.59)g
( kA)
2.27
( kA)f
0.15e
(0.22)g
ZnO
0.23h
(0.30)i
In2O3
Table 1. Effective masses (in units of the electron rest mass m0) of electrons, mc, and holes, mh, for some hydrides, conventional semiconductors
and TCOs.
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S.Zh. Karazhanov et al.
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Philosophical Magazine
N(E) (States eV−1 f.u.−1)
1.6
MgH2
EF
4
1.2
3
0.8
2
0.4
1
EF
3
Si
EF
2
1
−6 −4 −2 0
LiH
1.2
2
4
−6 −4 −2 0
6
EF
5
BaLiH3
2
4
−6 −4 −2 0
6
EF
3
GaAs
2
4
6
4
6
EF
4
0.8
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KMgH3
2
3
2
0.4
1
1
−6 −4 −2 0
2
4
6
−6 −4 −2 0 2 4
Energy (eV)
6
−6 −4 −2 0
2
Figure 3. Total DOS for MgH2, LiH, KMgH3, BaLiH3, Si and GaAs. The Fermi level is set to zero.
GaAs and ZnO deviates considerably from those determined experimentally, which shows
that the DFT calculations do not predict the density of states properly. Intrinsic carrier
concentrations have also been calculated using the theoretically and experimentally
determined values of Nc and Nv. However, experimentally determined band gap values
have been used for both cases. The calculated intrinsic carrier concentration ni for Si,
GaAs and ZnO is found to be smaller than the corresponding experimental values. But the
error in the estimation of ni is much smaller than that for the estimation of Nc and Nv.
Using the values of Nc and Nv, the DOS masses for CB and VB electrons, mde and mdh,
have been calculated and compared with available experimental data (Table 2). Analysis of
Table 2 shows that the calculated mde and mdh are considerably underestimated.
Based on dominant contribution of the s- or d-electrons to the top-most VB and
bottom-most CB, the TCOs can be classified as type-s, or type-d [60]. In general, the
semiconductors and TCOs have mainly contributions by p electrons of anions in the
top-most VB. Since in hydrides the hydrogen atoms are anions, the origin of their topmost VB are often related to s electrons. To demonstrate this we have calculated the
orbital and site projected density of states (PDOS) for all the compounds considered in
the present study. The PDOS for a hydride (BaLiH3) and semiconductors (Si and ZnO)
are shown in Figure 4. The PDOS for BaLiH3 show that the s-electrons contribute not
only to the bottom-most CB but also the top-most VB. This is the distinguishing feature of
most of the hydrides compared to the TCOs, where the top-most VB are commonly
originated from p-/d-electrons and the bottom-most CB from s-electrons. The dominant
contribution of s electrons to the top-most VB of hydrides originates from the hydrogen
atoms due to the fact that hydrogen is in the anionic state in these hydrides.
2468
S.Zh. Karazhanov et al.
Table 2. Fundamental band gap (Eg, in eV) calculated by DFT and determined experimentally,
effective DOS (cm3) for electrons (Nc) and holes (Nv), intrinsic carrier concentration (ni, cm3),
density of states mass for electrons (mde) and holes (mdh) (in units of the free electron mass m0) and
diffusion potential (VD in eV) for hydrides, semiconductors and TCOs. Experimentally determined
band gap has been used in estimation of the intrinsic carrier concentrations.
Compound
Method
Eg
Nc
Nv
ni
mde
mdh
VD
BaLiH3
KMgH3
MgH2
Theory
Theory
Theory
Exp.a
Theory
Exp.b
Theory
Exp.c
Theory
Exp.d
Theory
Exp.e
Exp.f
1.23
2.36
3.71
5.6
2.95
4.64
0.70
1.12
0.54
1.42
0.74
3.44
2.3 1019
5.6 1017
2.8 1019
8.6 1019
1.9 1018
1.6 1018
2.1 109
1.6 102
7.1 1029
0.94
0.08
1.07
2.27
0.18
0.16
1.13
2.30
5.44
3.6 1017
1.3 1017
2.6 1022
0.06
0.03
4.66
4.0 1018
2.9 1019
1.5 1017
4.7 1017
1.8 1018
3.0 1018
6 1017
3.1 1019
1.3 1016
9.0 1018
6.4 1017
1.1 1019
6.2 108
1.2 1010
4.9 104
2.6 106
1.5 1011
8.0 1011
0.29
1.10
0.01
0.07
0.17
0.24
0.3–0.45
0.08
1.15
0.03
0.50
0.09
0.59
1.03
0.88
1.53
1.32
3.38
3.29
LiH
Si
ZnO
a
f
Experiment [54].
Experiment [59].
0.6
PDOS (States eV−1fu−1atom−1)
Downloaded By: [Karazhanov, S.Zh.] At: 06:18 29 September 2008
GaAs
Li
b
Experiment [55,56]. cExperiment [56].
EF
BaLiH3
s
p
d
0.4
0.2
EF
0.3
d
Experiment [57]. eExperiment [58].
Si
Si1
0.2
0.8
0.1
0.4
Si2
Ba
0.3
1.2
0.4
0.2
0.8
0.2
0.1
0.4
0.6
0.6
−2
H
0
2
4
Energy (eV)
6
ZnO
Zn
EF
1.2
O
−2
0
2
4
6
0.4
0.2
−2
0
2
4
6
Figure 4. Orbital and site projected DOS for BaLiH3, Si and ZnO. The Fermi level is set to zero.
The imaginary and real parts of the optical dielectric functions, "1(!) and "2(!), the
absorption coefficient, (!), and the reflectivity, R(!), calculated from DFT for the
undoped hydrides BaLiH3 and SrLiH3 as well as for the semiconductors Si and GaAs are
displayed in Figure 5. The results are compared with the corresponding experimental data
[61] for Si and GaAs. Locations of all the peaks in the spectral distribution of the optical
spectra calculated using DFT are shifted toward lower energies compared to those
measured experimentally. A rigid shift toward higher energies has been performed in order
2469
Downloaded By: [Karazhanov, S.Zh.] At: 06:18 29 September 2008
R(ω)
α(ω) (× 10−5, cm−1)
ε2(ω)
Philosophical Magazine
BaLiH3
5
4
3
2
1
SrLiH3
50
Si
40
Expt
Theory
GaAs
Expt
Theory
20
30
20
2.0
30
10
10
4
3
3
2
2
1
1
0.6
0.6
0.4
0.4
0.2
0.2
1.5
1.0
0.5
0.5
0.4
0.3
0.2
0.1
2
4
6
8
10
2
4 6 8 10
Energy (eV)
2
4
6
8
10
Figure 5. Imaginary part of the optical dielectric function, "2(!), absorption coefficient, (!)
(in cm1 divided by 105), and reflectivity, R(!), for BaLiH3, SrLiH3, Si and GaAs calculated by DFT
along with the experimental data [61] for Si and GaAs.
to correct the underestimation of the band gaps by DFT. In the thus obtained optical
spectra all peak locations agree fairly well with those determined experimentally. So the
present study suggests that the k-independent scissors operator can be applied to the
electronic structure to correct the band gap underestimated by DFT.
Analysis of the spectral distribution of the optical spectra shows that the magnitude of
the peaks corresponding to the fundamental absorption is overestimated compared to the
experimental data. This may be related to an overestimation of the optical matrix
elements, neglect of the Coulomb interaction between free electrons and holes (excitons),
local-field and finite lifetime effects. Furthermore, for calculations of the imaginary part of
the dielectric response function only direct optical transitions from occupied to unoccupied
states are considered. The experimental resolution will also smear out many fine features.
Analysis of Figure 5 shows that the absorption coefficient and reflectivity of LiBaH3
and SrLiH3 are fairly high only at photon energies 45 eV, which is beyond the visible
region of the optical spectra. However, for photons in the energy range 1.2 5 h! 5 2.5 eV
the absorption coefficient and reflectivity are within a reasonable limit to consider these
materials transparent to the visible spectrum. It should be noted that in Si and GaAs the
absorption coefficient is larger than that in LiBaH3 and SrLiH3.
3.2. Shallow level defects in hydrides
Application of a material for electric and optoelectronic devices depends critically on
dopability, which can be limited by the following three main factors [62]: (i) the desired
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S.Zh. Karazhanov et al.
Total DOS (States eV−1fu−1atom−1)
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MgH2:AlMg
EF
1.5
1.0
2
0.5
1
1.5
EF
LiH
2
0.5
1
Hi
MgLi
3
1.0
2
0.5
1
−6
CsBa
3
1.0
1.5
BaLiH3:SrLi
3
−3
0
3
−2
6
Energy (eV)
−1
0
1
2
Figure 6. Total DOS for MgH2:AlMg, LiH, Hi and BaLiH3:SrLi,CsBa,MgLi. The Fermi level is set
to zero.
shallow-level impurity may have low solubility in the host; (ii) even if it has good
solubility, its transition energy can be so deep that the defect cannot be ionized at
operating temperature; (iii) though the shallow level impurity possess good solubility, the
oppositely charged native defects or defect-impurity complexes can be formed when
shifting the Fermi energy. In this paper, we restrict ourselves by considering issue (ii) as to
whether an impurity will form shallow energy levels in the band gap of the hydrides and
leave options (i) and (iii) for systematic theoretical and experimental exploration in the
near future.
As the shallow-level defects and impurities can be the source for electrical conductivity
in semiconductors, we have calculated the electronic structure and optical properties with
defects and impurities for several hydrides: for CsH with VH and MgH, for MgH2 doped
with TiMg, AlMg, CuMg, LiMg and ScMg, for BaLiH3 doped with MgLi and CsMg, and for
MgH2 with structural point defects, such as VH, VMg, Hi, HMg and MgH, complex of
antisite defects HMg–MgH, and the defect impurity complexes Al–Hi. Subscripts indicate
the site with the defect/impurity (e.g. VH and Hi mean hydrogen vacancy and interstitial,
respectively). Figure 6 presents the total DOS for MgH2:AlMg, LiH, VH, Hi, and BaLiH3:
ScLi, CsBa, MgLi. We found that AlMg (Figure 6) and ScMg form states below the bottommost part of the CB in MgH2. These shallow-donor-states are not isolated from the CB
and hence they can contribute to the electrical conductivity. Defects such as TiMg, CuH, Hi
and VH form deep localized states that divide the large band gap of MgH2 into two smaller
parts. On the one hand, these types of defects can be important for modulation of the
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α(ω)
Philosophical Magazine
2.0
1.5
1.0
0.5
R(ω)
0.6
α(ω)
0.9
2.5
2.0
1.5
1.0
0.5
BaLiH3:SrLi
MgH2:AlMg
0.3
LiH
CsBa
Hi
MgLi
Rω
0.6
α(ω)
0.3
2.0
1.5
1.0
0.5
R(ω)
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0.9
0.4
0.3
0.2
0.1
2
4
6
8
2
10 12
Energy (eV)
4
6
8
10 12
Figure 7. Absorption coefficient (in cm1 divided by 105) and reflectivity spectra for MgH2:AlMg,
LiH, Hi, BaLiH3:SrLi, CsBa and MgLi.
optical properties, and, on the other hand, they can be responsible for band gap
engineering and insulator–semiconductor transitions.
LiH and VMg form delocalized broad band states at the top-most part of the VB in
MgH2. Therefore, both these defects can be used to create p-type conductivity. CsBa is
a shallow acceptor in BaLiH3 and can also be the source of p-type conductivity (Figure 6).
However, MgLi and MgBa form not only shallow acceptor states, but also very broad
bands at the bottom-most part of the CB. Hence, these defects can cause reduction of
transparency and can be used for modulation of the optical properties of BaLiH3.
As noted above, doping can affect not only conductivity but also transparency.
In order to establish this viewpoint we have studied absorption and reflectivity spectra for
BaLiH3:SrLi,CsBa,MgLi and that for MgH2:AlMg, CuH, LiH, Hi, VH, VMg for the electric
field E parallel to the crystallographic a, b, and c directions (Figure 7). From Figure 7 it is
seen that LiH doped MgH2 as well as SrLi and CsBa doped BaLiH3 remain transparent in
the visible spectra even at very high level of p-type doping, which in our case is equal to
2.0 1021 cm3 and 9.5 1020 cm3 for the MgH2 and BaLiH3 systems, respectively.
However, high level n-type doping reduces the transparency in the above listed hydrides.
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S.Zh. Karazhanov et al.
The reason is that the second band gap (i.e. the band gap after the n-type doping, which is
the energy difference between the nearest two bottom-most CBs) of these hydrides is not
sufficiently large. In order to consider these hydrides as TC materials the second band gap
should be 43.1 eV.
It is well known that one can easily introduce n-type conductivity in a number of wide
band gap materials such as TCOs. However, introduction of p-type conductivity in such
materials is problematic [20]. In contrast, we show below that the high level of p-type
doping in hydrides does not reduce their transparency, but on the other hand provide good
conductivity. However, a high level of n-type doping in hydrides will lead to loss
in transparency. This is one of the distinguishing features between TCOs and hydrides.
The calculated formation energies for AlMg, LiH, and Hi are equal to 2.2 eV, 1.1 eV,
and 3.3 eV, respectively, which indicate that AlMg, LiH can be abundant, whereas Hi is
unstable. A more detailed analysis about the doping limit in hydrides is to be studied
separately.
It should be noted that one cannot obtain quantitative results about the optical
properties from the present type of calculations owing to the limitation of DFT to predict
the optical properties of solids. However, the results from the present approach can be
considered as a lower bound and qualitatively predict the optical properties. First of all
our intention here is to show that hydrides are potential candidates for semiconducting
and transparent conducting applications. We strongly believe that the present report will
motivate scientists to find appropriate hydrides by systematic screening of existing
hydrides using time consuming approaches (e.g. GW approach). Our studies have shown
that the band structure of MgH2, BaLiH3, CsCaH3, etc, are more or less suitable for device
applications. Although upon heavy n-type doping the transparency of these hydrides can
be reduced, they may still be suitable for electronic device applications. Furthermore, at
lower concentrations of shallow donors (i.e. 51019 cm3) transparency should be within
a reasonable limit.
Knowledge about the concentration of shallow acceptors, donors, and intrinsic carriers
allows one to calculate the diffusion potential VD formed at the p–n junction using
Equation (4), which is one of the important parameters for p–n junction-based electronic
devices. Assuming Nd 1020 cm3 and Na 1015 cm3 one can find that VD 0.92 V for
BaLiH3. The value of this parameter is close to that of the nþ–p Si solar cells.
4. Electronic device applications of hydrides
In this section we analyse advantages, deficiencies and perspectives of using the hydrides
for electronic device applications. As can be seen from Figure 1, a large number of
hydrides possess wide band gap and can therefore be used as transparent windows for
a broad variety of electronic devices such as solar cells, light-emitting diodes, etc. Thus,
the wide band gap hydrides can be regarded as an alternative to conventional
transparent microelectronic materials, such as SiOx, SiNx or SiNx:H. In contrast to these
materials, hydrides are expected to have the advantage of providing good passivation of
interfaces in semiconductor devices similar to that of the a-Si:H due to high
concentration of hydrogen in their structure. More systematic investigations are
necessary to confirm the expected passivation behaviour of hydrides at the
semiconductor interfaces.
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Although wide band gap hydrides can be conductive, as was shown in Section 3.2, the
doping limit for these hydrides is not yet known and has to be studied systematically.
If effective high level doping and electrical conductivity can be realized for some of these
hydrides without compromising their transparency to the visible region of the solar
spectra, they can be regarded as an alternative to TCO materials. Nevertheless, at this
stage of investigations it is possible to claim that hydrides are more flexible materials than
SiNx or TCOs, which are widely used as antireflection coatings in solar cells. As is well
known, the optical properties of SiNx and TCOs and passivation of interface states are
controlled by modulation of the nitrogen or oxygen stoichiometry, which is rather
problematic. Upon using hydrides these features can relatively easily be controlled by
hydrogen content only. Furthermore, structural and morphological modifications of
hydrides can be done at much lower temperatures and pressures than SiNx and TCOs.
Hence, hydrides can be more suitable for low temperature synthesis technologies.
The use of TCOs in some devices results in the formation of an intermediate oxide
layer, large band-offset and defect states, which can limit the device performance [63,64].
If hydrides were to be used instead of the TCOs, no oxide layer would be formed and
there would be no need for buffer layers. Furthermore, hydrogen diffusion from hydrides
into semiconductors can be useful to passivate defect states at the interface and increase
their conductivity [6,9], which are important for improving the device performance.
Studies about stability, transparency and feasibility for the n- and p-type conductivity of
the hydride materials will be the subjects of detailed investigations in the near future.
Smaller band gap hydrides with well dispersed bottom-most CB and top-most VB can
be useful for replacing semiconductors used in devices such as solar cells. From this point
of view, hydrides can be regarded as novel class of materials for solar cells. The crystalline
nature and light weight of hydrides, as well as expected small concentration of defects
between the active solar cell and the hydrides present interest for third generation
photovoltaics. A lot of attention is currently focused worldwide on developing
environmentally friendly technologies, alternative energy sources, improving the
performance of solar cells, etc, and our finding about the semiconducting behaviour of
hydrides is directly relevant to these activities. Apart from the lighter weight of hydrides
compared to the conventional semiconductors, hydrogen is the most abundant element in
the universe. Consequently, hydrides are of great interest for terrestrial and space
applications of solar cells.
Hydrides have a large potential for applications in biocompatible semiconducting
devices. From this point of view, semiconductor device applications of hydrides are similar
to those of electrically conductive polymers suggested by Heeger [65], MacDiarmid [66]
and Shirakawa [67], which opened up polymer electronics. We expect that in the near
future hydrides will be used in electronic technology that will find a broad range of
applications.
5. Choice of materials
One of the important questions is which of the classes of hydrides would be more
preferable for use in semiconductor electronics. In the scientific literature (see, for
example, Vajeeston [24]) hydrides are mainly classified into metal hydrides and complex
hydrides depending on hydrogen content, operating temperature and hydrogen
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S.Zh. Karazhanov et al.
absorption/desorption kinetics. Hydrogen can be removed from metal hydrides easily,
even below 100 C. Such hydrides have relatively low weight percentage of stored hydrogen
(1.5 to 2.5 wt%) and they are opaque. This class of hydrides is interesting with respect
to hydrogen storage. Complex hydrides store more hydrogen (up to 20.8 wt% in Be(BH4)2
[5]), their operating temperature is very high (from 80 C to 600 C) and absorption/
desorption kinetics is much slower than metal hydrides. Because of these features complex
hydrides are not as popular as the metal hydrides for hydrogen storage/economy.
However, they would be more preferable for the semiconductor electronics. For the latter,
the hydrogen content is not as important as it is for hydrogen storage. Furthermore,
for the semiconductor electronics the following feature can be extremely important:
kinetics of the hydrogenation/dehydrogenation processes are required to be as slow as
possible. Nowadays finding a proper hydride for the electronic device applications is an
open problem and needs further investigations.
One of the very important problems with hydrides is that they typically react with air
and water. Therefore, in the proposed applications in semiconductor electronics hydrides
should be well protected from the environment. Here it should be noted that
semiconductor devices, e.g. solar cells, are protected from the environment by an external
layer. Moreover hydrides can be used not only for solar cells, but also for other possible
electronic device applications, such as a transparent conducting layer, antireflection
coating, buffer layer, etc. Upon using a hydride as a buffer layer, the hydride is always
protected by TCO from the environment, similar to the a-Si:H in the heterostructure TCO/
a-Si:H/Si. It is worth noting that hydrides are already widely used as ‘‘smart’’ windows
where the stability problem is solved at least for some of these materials.
6. Conclusion
From systematic studies using density functional theory we have shown that some
hydrides possess the features of wide band gap semiconductors, such as wide fundamental
band gap, well dispersed bottom-most conduction band and/or top-most valence band,
and small electron/hole effective masses. We have demonstrated that intrinsic carrier
concentration, carrier effective masses in some hydrides and diffusion potential at the
hydride based p–n junction are of the same order as those in the well known wide band gap
semiconductors. Further systematic studies for a large number of hydrides are necessary to
identify potential hydrides for electronic device applications. From studies of impurities
and defects in some selected hydrides, we have shown that some impurities form shallow
energy levels in the band gap, which is important for providing electrical conductivity. It is
demonstrated also that some structural point defects such as Hi can form deep energy
levels in the band gap, which are important for defect engineering of the wide band
gap hydrides. From the study of the effect of defects on optical properties of hydrides
it is shown that although some shallow-level defects can enhance electrical conductivity,
they can reduce transparency to the visible spectra. Kinetics of the hydrogen absorption/
desorption processes of some hydrides are found to be fast at ambient temperatures
and pressures [1,68–70] which would cause instability of electrical properties of the
material [24]. Other hydrides are found to have much slower hydrogenation/dehydrogenation even at elevated temperatures up to 400 C [5]. From analysis of features of metal
and complex hydrides it is suggested that the latter are more suitable for the proposed
Philosophical Magazine
2475
hydride-based electronic devices because the hydrogenation and dehydrogenation
processes in these compounds are not as fast as that in metal hydrides. The equilibrium
conditions for Si-induced destabilization are 1 bar at 490 C for LiH/Si [71] and 47.5 bar at
300 C for MgH2/Si systems [71], which remain outside the normal range. This indicates
that novel device applications of hydrides are not far from reality, since for a wide range of
semiconductor devices temperature of around 100 C and below is required. Further
systematic investigations regarding the stability of hydrides in an electronic device
environment are needed. We hope that hydrides will have a large impact on semiconductor
technology and electronics.
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Acknowledgments
This work has received financial and supercomputing support from the Research Council of Norway
within the FUNMAT and NANOMAT projects, as well as from the Academy of Sciences of
Uzbekistan. We are thankful to Dr K. Knizek, Dr R.Vidya, and Jo Gjessing for computationalpractical help.
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