Material Design of Green-Light-Emitting Semiconductors: Perovskite-Type Sulfide SrHfS3

Article abstract of DOI:10.1021/jacs.8b13622

A current issue facing light-emitting devices is a missing suitable material for green emission. To overcome this, we explore semiconductors possessing (i) a deep conduction band minimum (CBM) and a shallow valence band maximum (VBM), (ii) good controllability of electronic conductivity and carrier polarity, and (iii) a directly allowed band gap corresponding to green emission. We focus on early transition metal (eTM)-based perovskites. The eTM cation's high and stable valence state makes its carrier controllability easy, and the eTM's nonbonding d orbital and the anion's p orbital, which constitute the deep CBM and shallow VBM, are favorable to n- and p-type doping, respectively. To obtain a direct band gap, we applied a scheme that folds the bands constituting the VBM at the zone boundary to the zone center where the CBM appears. Orthorhombic SrHfS₃ was chosen as the candidate. The electrical conductivity was tuned from 6 × 10^-7 to 7 × 10^-1 S·cm^-1 with lanthanum (La) doping and to 2 × 10^-4 S·cm^-1 with phosphorus (P) doping. Simultaneously, the major carrier polarity was controlled to n type by La doping and to p type by P doping. Both the undoped and doped SrHfS₃ exhibited intense green photoluminescence (PL) at 2.37 eV. From the PL blue shift and short lifetime, we attributed the emission to a band-to-band transition and/or exciton. These results demonstrate that SrHfS₃ is a promising green-light-emitting semiconductor.

Full text of DOI:10.1021/jacs.8b13622

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Article
Materials Design of Green Light-emitting
Semiconductors: Perovskite-type Sulfide SrHfS
3
Kota Hanzawa, Soshi Iimura, Hidenori Hiramatsu, and Hideo Hosono
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Mar 2019
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K. Hanzawa et al.,
Materials Design of Green Light-emitting Semiconductors:
Perovskite-type Sulfide SrHfS3
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1,2,*
and Hideo Hosono1,2
Kota Hanzawa, Soshi Iimura, Hidenori Hiramatsu,
1Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo
Institute of Technology, Mailbox R3-3, 4259 Nagatsuta-cho, Midori-ku, Yokohama
2
26-8503, Japan
2Materials Research Center for Element Strategy, Tokyo Institute of Technology,
Mailbox SE-1, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
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K. Hanzawa et al.,
Abstract
A current issue facing light-emitting devices is a missing suitable material for green
emission. To overcome this, we explore semiconductors possessing (i) a deep
conduction band minimum (CBM) and shallow valence band maximum (VBM), (ii) a
good controllability of electronic conductivity and carrier polarity, and (iii) a
direct-allowed band gap corresponding to a green emission. We focus on early
transition metal (eTM)-based perovskites. The eTM cation’s high and stable valence
state makes its carrier controllability easy, and the eTM’s non-bonding d-orbital and the
anion’s p-orbital, which constitute the deep CBM and shallow VBM, are favourable for
n- and p-type doping, respectively. To obtain a direct band gap, we applied a scheme
that folds the bands constituting the VBM at the zone boundary to the zone centre where
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the CBM appears. Orthorhombic SrHfS was chosen as the candidate. Electrical
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conductivity was tuned from 6×10−7 S·cm−1 to 7×10−1 S·cm−1 with lanthanum
(
La)-doping and to 2×10−4 S·cm−1 with phosphorus (P)-doping. Simultaneously, the
major carrier polarity was controlled into n-type by La-doping, and into p-type by
P-doping. Both the undoped and doped SrHfS3 exhibited an intense green
photoluminescence (PL) at 2.37 eV. From the PL blue shift and short lifetime, we
attributed the emission to a band-to-band transition and/or exciton. These results
demonstrate that SrHfS is a promising green light-emitting semiconductor.
3
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Introduction
Light-emitting semiconductors are key components in practical optoelectronic
devices such as light-emitting diodes, laser diodes, and solar cells. Although
light-emitting technologies have widely been applicable, there is a strong demand on
next-generation materials to exhibit higher performance in terms of light brightness,
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quantum efficiency, and colour accuracy . Currently, GaN- and GaAs-based IIIb-Vb
semiconductors have commercially been employed in light-emitting devices. However,
they have a serious issue where the emission quantum efficiency drastically decreases in
a particular wavelength region around green and yellow, that is, it decreases from
~
70%–80% in the red- and blue-light regions to ~15% in the green region (~530 nm) or
20% in the yellow region (~570 nm)2, 3. This is called the ‘green gap problem’ and
~
exists because there is currently no green-light source bright enough to realize the
next-generation optoelectronic devices. Therefore, it is strongly required to explore new
semiconductor materials with a high quantum efficiency of emission in the green
wavelength region (E = 2.18−2.50 eV, λ = 495−570 nm) and n- and p-type dopability.
g
To meet these demands, we consider three basic electronic characteristics of
semiconductors; (i) a deep conduction band minimum (CBM; large electron affinity)
and shallow valence band maximum (VBM; small ionization potential) to achieve both
n- and p-type dopings, respectively, (ii) controllability of electronic conductivity and
carrier polarity, and (iii) a direct allowed band gap to realize highly efficient light
emission.
Recently, our research group proposed that early transition metal (eTM)-based
semiconductors with a high symmetry crystal structure can satisfy both the first and
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−6
second requirements . Usually, the empty d-orbital of the eTM and p-orbital of the
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K. Hanzawa et al.,
anion form a strong antibonding state, so that the conduction band (CB) of the
eTM-based compound possesses a too shallow energy level (i.e., too small electron
affinity) for electron doping (Fig. 1a). However, by introducing a deep non-bonding
state of the d-orbital of eTM, we demonstrated that a deep CBM is formed at a specific k
point, which enables doping with an electron and reduces the effective mass. Figure 1b
shows an example of the d-orbital configuration in cubic perovskite at the Γ point (k =
0
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). At this point, the phase of the t2g orbitals of transition metals (e.g., eTM in
A-eTM-X , where A and X denote a cation and an anion, respectively) is invariant from
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one unit cell to the next. This configuration of the d-orbital can form neither a bonding
nor anti-bonding state with the neighbouring p-orbitals of anions owing to their
symmetry constraint. As a result, the deep unoccupied non-bonding state consists of the
CBM.
The use of eTMs has advantages in terms of controllability of the electronic
conductivity and carrier polarity. The eTM cations tend to occupy a high coordination
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+
number site owing to their large valence states, such as Zr , which suppresses the
formation of a cation vacancy. In addition, the stable valence state inhibits the
localization of the generated electron at the eTM cation site through the change of
valence state. These characteristics are in contrast to those of post-transition metal
(
pTM, e.g., Cu, In, and Sn) cations, which are constituents of conventional oxide
semiconductors. These semiconductors suffer from a poor doping controllability owing
to their low and variable valence changes of the cation7−9.
Therefore, eTM-based cubic perovskites A-eTM-X seem to be promising
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candidates in terms of the dopability and controllability of the carrier density. A unique
feature of the cubic perovskite is that their CBM and their VBM are composed of
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non-bonding states. Figure 1c depicts the orbital configuration of a fully
electron-occupied p-orbital of anion X at the R point. At the R point of the primitive
cubic structure, the phase of each orbital in the unit cell is inverted by the primitive
translation along the x, y, or z directions. This configuration of p orbitals can form
neither a bonding nor anti-bonding state with the neighbouring d-orbitals of eTM.
Consequently, this shallow energy level of the non-bonding state pushes up the VBM
and makes it easy to dope the hole carriers.
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Although non-bonding deep d- and shallow p-orbitals levels are favourable for
doping of electrons and holes, respectively, the indirect band gap in the cubic perovskite
definitely inhibits an efficient light emission (Fig. 1d). Thus, we introduce another
strategy, that is, band folding, to obtain a direct band gap suitable for light emission.
Figure 1e and 1f show the relationship between the size of the unit cell in real space
and the Brillouin zone (BZ) in reciprocal space10, 11. Here, we consider how the band
structure of a one-dimensional s-orbital chain changes by choosing the unit cell
containing one orbital or two orbitals. In the case of a one orbital cell with a lattice
parameter of a, a single s-band appears in the first BZ ranging from −π/a to π/a. By
doubling the unit cell size to a’ = 2a, the first BZ halves. Consequently, bands located
outside the first BZ ranging from −π/2a to π/2a (i.e., located in the second BZ) are
folded back to the first BZ and the number of bands doubles. This relation tells us that
the VBM at the R point of cubic perovskite can be transferred to the Γ point by
choosing a proper superstructure. Note that such band folding we suppose occurs only
when the number of superstructures is an even multiple of a primitive unit cell.
In a perovskite-type structure, a superstructure is introduced by tilting of the
eTM-X octahedra. Indeed, it is known that octahedron tilting and the resultant
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K. Hanzawa et al.,
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superstructure are well governed by the tolerance factor , therefore we can find an
optimum electronic structure with the direct-type transition by choosing constituent
elements with different ionic radii.
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In this paper, we investigate the electronic and optical properties of ternary alkali
earth metal (AE)-eTM-S systems with an orthorhombic structure whose direct band gap
corresponds to a colour ranging from green to red. Through screening by first-principles
calculations and experiments, we identify that SrHfS is a promising candidate for the
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green light emission among the AE-eTM-S systems. We then experimentally
demonstrate that SrHfS is a novel n- and p-type dopable semiconductor exhibiting an
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intense green emission at room temperature.
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Figure 1. Design of n- and p-type dopable semiconductors with direct band gaps in
eTM-based perovskite-type compounds. (a) Schematic energy diagram of diatomic
molecule. The configurations of non-bonding (b) d-orbitals at Γ point and (c) p-orbitals
at R point in cubic perovskite-type structure. The former constitutes CBM, while the
latter constitutes VBM. Red cloverleafs and green dumbbells represent d-orbitals of
eTM and p-orbitals of X (anion), respectively. (d) Schematic band structure of cubic
perovskite-type compounds. Black solid curves denote their bands at CBM and VBM.
Red band at Γ point is a band transferred from R point after band folding. The inset
illustrates the BZ of primitive cubic cell. (e) Two different unit cells in one-dimensional
s-orbital chain containing one orbital (top) and two orbitals (bottom). (f) Band structures
of one-orbital unit cell (top) and two-orbital unit cell (bottom). For the two-orbital
model, bands in the second BZ are folded back to the first BZ of the one-orbital model.
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K. Hanzawa et al.,
Results and discussion

Electronic structures of AE-eTM-S3
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From screening based on our concepts above, we chose AE-eTM-S (AE = Ca, Sr or
3
Ba, eTM = Zr or Hf) owing to their bandgaps suitable for green light emission. The
crystal structure shown in Fig. 2a forms an orthorhombic perovskite-type structure with
a space group of Pnma. Their lattice constructs a √2 × 2 × √2 superstructure of a cubic
perovskite-type cell, so that bands at the R point of the cubic lattice are folded to the Γ
point of the orthorhombic lattice. The relationship between formation of the
superstructure and the band folding is explained in Fig. S1. Figure 2b and 2c
respectively show the electron density maps at the CBM and VBM of SrHfS as an
3
example. At the CBM, a d-orbital-like electron density appears at the Hf site, while at
the VBM, a p-orbital-like population is also visible at the S site. The other orbital
contributions are negligibly small both for the CBM and VBM, which indicates that the
non-bonding state in cubic perovskite is preserved in this orthorhombic structure.
Figure 2d shows the band structure of SrHfS . It is obvious that SrHfS has a
3
3
direct-type band gap at the Γ point owing to the band folding, as explained in Fig. S1.
Band structures of the other AE-eTM-S compounds are displayed in Fig. S2. To check
3
whether the optical transition from the CBM to VBM is symmetrically allowed or not,
we calculated the corresponding matrix momentum elements (Fig. S3). The finite and
large value at the Γ point indicates that the band-edge transition is allowed. The
calculated effective masses of electrons in SrHfS were 0.40, 0.26, and 0.43 m for Γ–X,
3
0
Γ–Y, and Γ–Z, respectively, while those for holes were 0.70, 0.19, and 0.61 m , where
0
m represents the effective mass of a free electron. For comparison, the calculated
0
effective masses of other AE-eTM-S are summarized in Table S1. Figure 2e shows the
3
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density of states (DOS) of SrHfS . The CB consists of Hf-5d orbitals and the Sr-4d is
3
located at a higher energy than the CB of Hf-5d orbitals. Because Sr-4d orbitals do not
hybridize with Hf-5d orbitals, the CBM is determined almost only by Hf-5d orbitals.
The VBs are composed mainly of S-3p orbitals. These results indicates that the
electronic structures of CBM (i.e., d orbitals of an eTM) and VBM (i.e., p orbitals of an
anion) are as we expected.
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Figure 2f summarizes the band alignments of AE-eTM-S3 and other
semiconductors that exhibits both n- and p-type conductivity. The energy levels of
CBM and VBM for conventional semiconductors (i.e., Si and GaAs13, 14) are located
1
3
within the typical doping limit region, as shown by the yellow area . Note that the
energy levels of CBM and VBM of AE-eTM-S are both located around the boundary or
3
inside of the limit region, which suggests that n- and p-type dopings should be possible.
The calculated band gaps of CaHfS , SrHfS , and BaHfS were 2.21, 2.18, and 1.90 eV,
3
3
3
respectively, while those of CaZrS , SrZrS , and BaZrS were 1.95, 1.94, and 1.72 eV,
3
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1
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respectively, which are close to the calculated values reported previously . Results of
the band gap calculations for other AE-eTM-S are also summarized in Table S1.
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Because the band gaps of AE-HfS are more suitable for a green emission than the
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Zr-system, we chose the Hf-system, and investigated the optoelectronic properties.
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Figure 2. Electronic structures of AE-eTM-S . (a) Crystal structure of orthorhombic
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perovskite-type sulfide AE-eTM-S . Blue, red, and green spheres correspond to AE,
3
eTM, and sulfur ions, respectively. Charge density maps at (b) CBM and (c) VBM of
SrHfS3 calculated using the Heyd–Scuseria–Ernzerhof (HSE) hybrid-functional.
Vertical colour scale bar indicates the intensity of the charge density. (d) Band structure
of SrHfS projected by maximally localized Wannier functions. (e) Density of states
3
(
DOS). Black, blue, red, and green lines correspond to total DOS and contributions
from Sr 4d-, Hf 5d-, and S 3p-orbitals, respectively. (f) Band alignment of AE-eTM-S3
and three n- and p-type dopable semiconductors. Except for AE-eTM-S , all data are
3
experimental results taken from Refs 5, 13, 14. The yellow area denotes a conventional
carrier dopable range located from −4 to −6 eV with respect to Evac13.
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
SrHfS with an appropriate band gap for green light emission
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To examine the optical and carrier transport properties of AE-HfS , we first
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prepared the undoped polycrystalline samples. AE-HfS was previously prepared
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through the direct sulfurization of a mixture of AE-CO and HfO . However, we
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employed a conventional solid-state reaction using binary sulfide precursors to obtain
high-density bulk (relative density >80%), which are advantageous especially for
reliable carrier transport property measurements. Figure 3a and 3b show the X-ray
diffraction (XRD) patterns of SrHfS and BaHfS . They indicate that SrHfS and
3
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3
BaHfS3 were successfully synthesized, which was crosschecked by chemical
composition analysis using an electron micro-probe analyser. For synthesis of AE = Ca,
however, we could not obtain the CaHfS phase by the same method. From the Rietveld
3
analysis on the XRD patterns, we observed that major parts of the obtained bulk
consisted of SrHfS (purity = 94.0 mol%) and BaHfS (79.1 mol%), and the minor
3
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impurities were HfO and AE-S for both samples. Insets in Fig. 3a and 3b show
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photographs of the SrHfS and BaHfS polycrystalline bulk. The sample colours were
3
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vivid yellow and deep orange for SrHfS and BaHfS , respectively. Note that both
3
3
polycrystalline bulk samples were quite stable in ambient air atmosphere despite that
they are sulfides.
We measured the diffuse reflectance at room temperature in the visible–
near-infrared wavelength region to examine the optical band gap. The raw spectra of the
diffuse reflectance and the Kubelka–Munk plots are displayed in Fig. S4a and S4b.
Figure 3c summarizes the calculated and observed optical band gaps of AE-HfS . Even
3
though the calculated band gaps were slightly underestimated, they reproduced well the
observed optical band gaps. The observed gaps of SrHfS and BaHfS were respectively
3
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.32 (λ = 534 nm) and 2.06 eV (λ = 602 nm), which was consistent with the sample
colours, as shown in the inset photographs of Fig. 3a and 3b. Because the band gaps of
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SrHfS and BaHfS respectively correspond to the green- and orange-light emission
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wavelengths, we focussed on SrHfS hereafter.
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Figure 3. XRD patterns of (a) SrHfS and (b) BaHfS . Each inset is a photo of the
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polycrystalline bulk. (c) Optical band gaps of AE-HfS . Red circles and blue squares
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represent observed optical and calculated band gaps, respectively. Vertical colour bar
represents colours of the absorbed light. The raw diffuse reflectance spectra are shown
in Fig. S4. The dashed lines indicate the green-light region, which is the main target of
this study.
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
Electronic transport properties of SrHfS3
Next, we performed electron and hole dopings of SrHfS to examine the carrier
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controllability (i.e., control of carrier polarity and concentration). Among the several
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candidates for appropriate dopants, we chose La (1.16 Å ) to act as a donor at the Sr
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2−
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(
1.26 Å )-site and P (2.12 Å ) as an acceptor at the S (1.84 Å )-site, owing to their
suitable ionic radii. The nominal concentrations of La were set to 0.5%, 1%, 2%, 3%,
4%, and 6%, while those of P-substitution were 1%, 2%, 2.5%, 3%, 4%, and 4.5%.
Hereafter, the doping concentrations are defined as the nominal concentrations.
Before carrier transport property measurements, we confirmed replacement of
La and P at the Sr- and S-sites by XRD. Figure 4a shows the doping concentration
dependence of the lattice volume (V). The change in each lattice parameter is
summarized in Fig. S5a for a-, S5b for b-, and S5c for c-axes. In the case of La
3
3
substitution, V gradually decreased from 461.43 Å (undoped) to 461.09 Å (6%
La-doped) owing to the smaller ionic radius of La than Sr. However, for P substitution,
3
V expanded to 461.79 Å (4.5% P-doped) because of the larger ionic radius of P than S.
This structure variation suggests that La and P progressively occupied the Sr- and
S-sites with an increase in the doping concentrations, respectively.
Figure 4b shows the electrical conductivities at room temperature as a function
of each doping concentration. Before the measurements, we confirmed the ohmic
contacts using Au electrodes for La- and undoped SrHfS , and Pt electrodes for P-doped
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SrHfS . Although undoped SrHfS exhibited a very low conductivity of 6×10 S·cm ,
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the conductivities sharply increased with both La- and P-substitutions. In the case of La,
−
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the conductivity reached 7×10 S·cm at a La doping of 6%; while in the case of P, it
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reached 2×10 S·cm at a P doping of 4%.
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We checked the carrier polarity by thermopower measurements for both La-
and P-doped SrHfS . The raw data, that is, the detected thermoelectromotive forces
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(
ΔV) as a function of the difference in temperature (ΔT), are shown in Fig. S6. Probably
owing to the high electrical resistance, we could not obtain a clear linear relation for 1%
and 2% P-doped SrHfS . Figure 4c shows the Seebeck coefficients obtained from the
3
slopes in Fig. S6. For all La- and P-concentrations, negative and positive Seebeck
coefficients were observed, respectively, which indicates that La- and P-substitutions
effectively supplied electrons and holes into SrHfS . The negative Seebeck coefficients
3
varied from −450 up to −230 μV/K with an increase in La concentration from 0.5% to
6
%, while the positive Seebeck coefficients varied from +880 to +60 μV/K with P
concentrations from 2.5% to 4.5%, which implies that carrier concentration increased
with an increase in doping levels. The above results of electrical conductivity and
Seebeck coefficients represent that carriers were continuously and widely activated
along with the change in the cell volumes and the doping concentrations. These
electronic transport properties demonstrate the good controllability of the carrier
polarity as well as the concentrations in SrHfS by intentional impurity doping.
3
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Figure 4. Electron- and hole-doping effects on SrHfS . (a) Lattice volume (V), (b)
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electrical conductivity (σ), and (c) Seebeck coefficients (S) of La- (blue squares), P- (red
circles), and undoped (black triangle) SrHfS . Horizontal axis represents nominal
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doping concentrations of La and P. All these measurements were performed at room
temperature. The V was calculated from each axis lattice parameter shown in Fig. S5.
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
Intense green luminescence characteristics in undoped and doped SrHfS3
Finally, we performed photoluminescence (PL) measurements to examine the
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optical properties. Figure 5a shows PL spectra of undoped SrHfS at 30–300 K. SrHfS
3 3
exhibited intense green emission (peaking at E = 2.37 eV, λ = 523 nm at 300 K) at all
temperatures measured, where no other emission peaks were detected. We confirmed
good agreement of the emission photon energy at 300 K with the band gap energy
estimated from diffuse reflectance (2.32 eV, see Figs. 3c and S4). The full width at half
maxima were 0.18 and 0.25 eV at 30 and 300 K, respectively. Note that the green
emission could be seen by the human eye even at room temperature (i.e., 300 K) as
shown in the inset picture of Fig. 5a, which suggests that the quantum efficiency of
emission should be high. To elucidate the origin of the emission, we investigated the
variation in the peak energy at different temperatures and constructed the decay curve.
Figure 5b shows the temperature dependence of the emission peak energy. The energy
increased from 2.37 eV at 300 K to 2.40 eV at 30 K; that is, a blue shift was clearly
observed and the shift was well reproduced using an empirical model for the band
19
edge . From the decay curves, we estimated the emission lifetime to be 7.6 ns at 300 K
and 3.7 ns at 30 K (see Fig. S7), which are comparable or shorter than the detection
limit of our PL measurement system (i.e., pulse width of the excitation laser was 7−8
ns). This result indicates that the actual life time of the emission was shorter than 7 ns.
These results strongly support that this intense emission originates from a band-to-band
transition and/or an exciton. Indeed, eTM-based compounds are more ionic than
pTM-based compounds owing to the lower electron negativity of eTMs than pTMs. In
addition, the electron effective mass of the d-orbital in eTMs is usually heavier than that
of the s-orbital in pTMs. These intrinsic natures of eTM-based semiconductors would be
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related to the efficient and bright emission, because both the ionicity and heavy electron
effective mass are essential to enhance the exciton binding energy. Furthermore, any
other emissions in the band gap (e.g., donor-acceptor pair emission) were not detected,
which suggests that the number of defects should be very low even in a polycrystalline
sample, probably as a result of the stable valence state of eTMs as we expected. This
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result is consistent with the low electrical conductivity (6×10 S·cm ) of the undoped
SrHfS (Fig. 4b).
3
We then investigated the PL behaviour of the doped samples to clarify their
potential as electroluminescence materials. Figure 5c summarizes the room temperature
PL spectra of doped (6% for La and 4% for P) and undoped SrHfS . In both La- and
3
P-doped SrHfS , only the band-edge and/or exciton emissions, similar to the undoped
3
SrHfS , were observed even at the high doping concentrations. Additionally, at other
3
doping concentrations, similar emission spectra were observed, as shown in Fig. S8a
(
La-doping) and S8b (P-doping). Moreover, in all the doped samples, the green
emissions were detected by human eyes at room temperature. It is noteworthy that
highly doped carriers of electrons or holes do not quench the emission. These features
denote that SrHfS can effectively emit green light even under the carrier-doped states.
3
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Figure 5. Optical emission properties of doped and undoped SrHfS . (a)
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Photoluminescence (PL) spectra of undoped SrHfS observed at 30−300 K. The inset is
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a picture of undoped SrHfS excited at room temperature. (b) Temperature dependence
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of emission peak energies. Red line is a fitting result using the following equation; E =
g
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E – αT /(T+β), where E is the E at 0 K and α and β denote material constants . (c) PL
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spectra of La 6%- (blue), P 4.5%- (red), and un-doped (black) SrHfS at 300 K.
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K. Hanzawa et al.,

Conclusions
We explored candidate compounds for green-light emitting electron- and
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hole-dopable semiconductors. To achieve good carrier dopability and controllability, we
first focused on eTM-based cubic perovskites, in which the high and stable valence state
of the eTM cation makes the carrier controllability easy, and the deep non-bonding
d-orbitals of eTM and shallow p-orbitals of the anion enable electron- and hole-doping,
respectively. To obtain the direct band gap in the perovskites, we applied the band
folding scheme that folds bands at the zone boundary to the zone centre. From the
screening using the first-principles calculations, we identified that AE-HfS has an
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optimum, allowed band gap for green emission and appropriate band alignment to n-
and p-type dopings. The experimentally estimated optical band gaps of synthesized
SrHfS and BaHfS were 2.3 eV (λ = 534 nm) and 2.1 eV, which correspond to green
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and orange emissions, respectively. The electrical conductivities of SrHfS were sharply
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−7
−1
−1
−1
3+
and widely changed from 6×10 S·cm at 0% to 7×10 S·cm at 6% La doping and
×10−4 S·cm−1 at 4% P doping. The Seebeck effect measurements confirmed that
3−
2
3
+
2+
3−
substituting La at the Sr -site successfully introduced n-type carriers, while the P
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−
substitution at the S -site exhibits p-type conductivity. From the PL measurements,
SrHfS exhibited an intense green PL at all observed temperatures (30−300 K), which
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originates from a band-to-band transition and/or an exciton. The PL could be seen by
the human eye even at room temperature and under carrier-doped states. Furthermore,
other emissions could not be detected other than the green emission even under the
doped conditions, which suggests that SrHfS should be strong against defects. These
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electronic and optical properties reveal that SrHfS is a promising candidate for novel
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green light-emitting semiconductors.
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Computational and Experimental Procedures

Theoretical calculations
Electronic structures of AE-eTM-S3 were calculated by using the HSE
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hybrid-functional with the HSE06 Vienna ab initio simulation package (VASP) code .
The mesh sampling in the Brillouin zone and cutoff energy were set to 5×4×5 and 600
eV, respectively, for all AE-eTM-S . The DOS of tetragonal ZrOS was also calculated as
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a normalizer for band alignments of AE-eTM-S with the HSE hybrid-functional, where
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the BZ sampling and cutoff energy were set to 6×6×4 and 600 eV, respectively. The
positions of CBM and VBM of AE-eTM-S were normalized by adjusting the energy
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levels of the core S 3s-band to that of ZrOS, where the energy levels were determined
by estimating the centre of gravity for the DOS peak of S 3s-states. Then, all the
normalized levels were shifted in parallel by adjusting the calculated VBM level of
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ZrOS to that experimentally observed . For SrHfS , BaHfS , and BaZrS , maximally
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localized Wannier functions as projection functions were constructed from the d-bands
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1
of AE and eTM, and the p-bands of S by using Wannier90 code ; while for CaHfS ,
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CaZrS , and SrZrS , they were from the d-bands of eTM and p-bands of S. The band
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3
structure and DOS shown in Fig. 2c and 2d, respectively, were depicted using those
Wannier functions. In Fig. S3, the band structures of AE-eTM-S calculated by VASP
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code is compared with those calculated by Wannier interpolation.

Sample preparation
Undoped polycrystalline AE-HfS (AE = Sr and Ba) was synthesized by
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conventional solid-state reaction from mixed precursors of AE-S and HfS . The
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mixtures were pelletized and heated at 1100 °C for 48 hours in Ar-sealed stainless-steel
tubes. For electron and hole dopings, we employed La S and SrP, respectively. The
2
3
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K. Hanzawa et al.,
chemical reaction formulae are as follows; (1−x)SrS + 0.5xLa S + HfS
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→
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3
Sr(1−x)La HfS + 0.5xS↑ and (1−y)SrS + ySrP + HfS → SrHfS(3−y)Py. All compounds
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used as precursors and dopants were prepared by ourselves. AE-S and HfS were
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synthesized by mixing ~1−2 mm dendritic pieces of AE metal (purity: 99.99%) or Hf
metal (99.99%) scraped from a metal-rod with sulfur powder (99.9999%) and by
heating at 750 °C and 950 °C, respectively. La S was prepared from scraped La metal
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3
(
99.9%) and sulfur at 950 °C, and SrP was synthesized from Sr metal and phosphorous
99.999%) at 750 °C.
(

Structure analysis
The crystal structure was analysed from powder X-ray diffraction (XRD) using a
Cu-Kα radiation source with a one-dimensional silicon strip detector. The structure
parameters (e.g., lattice parameter) and amounts of impurity phases were evaluated by
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2
Rietveld refinement using TOPAS code (Bruker AXS, version 4.2) . We crosschecked
the obtained phase by chemical compositions analysis using an electron probe micro
analyser with a wave dispersion detector, where AE-S and HfS were used as standard
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samples.

Optical properties
Diffuse reflectance spectra were obtained in the visible–near-infrared wavelength
region at room temperature. Then we performed the Kubelka–Munk transformation for
the obtained spectra to estimate the optical band gaps. To examine the emission
characteristics, PL spectra of undoped SrHfS were obtained in the temperature range of
3
3
0−300 K; whereas for La- and P-doped SrHfS , the temperature was set to 300 K. As
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the excitation source, we employed a third-harmonic Nd-doped YAG laser
(3ω-Nd:YAG, λ = 355 nm), whose pulse width was 7−8 ns. Decay curve was measured
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for evaluation of the emission lifetime with a gate delay between 0–20 ns.

Electronic transport properties
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We investigated the electrical conductivities by the four- and two-probe methods
for La- and P-doped SrHfS , respectively. For ohmic contacts, Au and Pt electrodes
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were deposited by DC sputtering on La- and P-doped SrHfS . Then, thermopower
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measurements were performed to determine the carrier polarity, where we deposited a
Pt electrode only on p-type SrHfS3.
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