RESEARCH ARTICLE | APRIL 18 2014

Hydride vapor phase epitaxy and characterization of highquality ScN epilayers 
Yuichi Oshima; Encarnación G. Víllora; Kiyoshi Shimamura

J. Appl. Phys. 115, 153508 (2014)
https://doi.org/10.1063/1.4871656

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30 January 2024 07:23:41

JOURNAL OF APPLIED PHYSICS 115, 153508 (2014)

Hydride vapor phase epitaxy and characterization of high-quality
ScN epilayers
n G. Vıllora, and Kiyoshi Shimamura
Yuichi Oshima,a) Encarnacio
Environment and Energy Materials Research Division, National Institute for Materials Science, 1-1 Namiki,
Tsukuba, Ibaraki 305-0044, Japan(Received 7 March 2014; accepted 3 April 2014; published online 18 April 2014)
The heteroepitaxial growth of ScN films was investigated on various substrates by hydride vapor
phase epitaxy (HVPE). Single crystalline mirror-like ScN(100) and ScN(110) layers weresuccessfully deposited on r- and m-plane sapphire substrates, respectively. Homogeneous
stoichiometric films (N/Sc ratio 1.01 6 0.10) up to 40 lm in thickness were deposited. Their
mosaicity drastically improved with increasing the film thickness. The band gap was determined by
optical methods to be 2.06 eV. Impurity concentrations including H, C, O, Si, and Cl were
investigated through energy dispersive X-ray spectrometry and secondary ion mass spectrometry.
As a result, it was found that the presence of impurities was efficiently suppressed in comparison
with that of HVPE-grown ScN films reported in the past, which was possible thanks to thehome-designed corrosion-free HVPE reactor. Room-temperature Hall measurements indicatedthat the residual free electron concentrations ranged between 1018–1020 cm3, which was markedly
lower than the reported values. The carrier mobility increased monotonically with the decreasing
in carrier concentration, achieving the largest value ever reported, 284 cm2 V1 s1 at
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4871656]
n ¼ 3.7  1018 cm3. V
I. INTRODUCTIONa)

Electronic mail: OSHIMA.Yuichi@nims.go.jp

0021-8979/2014/115(15)/153508/8/$30.00

115, 153508-1

C 2014 AIP Publishing LLC
V

30 January 2024 07:23:41

ScN is a group III transition metal nitride semiconductor,
which crystallizes in the cubic rock-salt crystal structure witha lattice constant of 4.505 Å.1 Similar to other transition metal
nitrides, ScN exhibits excellent physical properties, such as
high hardness and high thermodynamic stability, which arecomparable to those of TiN.1–3 ScN is reported to have a
direct band gap of 2.1–3.2 eV, and an additional indirect band
gap of 0.9–1.3 eV has been predicted.3–5 Since ScN(111) lattice matches to wurtzite-GaN(0001) and cubic-GaN(111), it
has been used as a buffer layer or interlayer to reduce thethreading dislocation density in GaN epilayers.6–9 In addition,
GaxSc1xN alloys10–16 and the ScN/GaN hetero-junction17
are being investigated to improve the performance of
GaN-based devices. Furthermore, among the transition metal
nitrides, ScN possesses an anomalous high thermoelectric
factor,18,19 which make it potential candidate material forhigh temperature thermoelectric applications. ScN is also
promising as a host material of ferromagnetic semiconductors
due to its high manganese solubility and an estimated highCurie temperature.20,21 AlxSc1xN alloys shows an exceptionally large piezoelectric response,22,23 and meta-stable hexagonal ScN is expected to have even greater piezoelectric
constant, comparable to that of ferroelectric perovskites.24
ScN is thus a very attractive material, which has a great
potential for various applications.
The growth of ScN thin film has been investigatedmainly by molecular beam epitaxy (MBE), sputtering, andhydride vapor phase epitaxy (HVPE). The growth temperature is below 900  C in most of the cases. The typical growthrate is below 0.5 lm/h, and the film thickness is usually

below 1 lm except the case of HVPE. Although there are no
lattice-matched substrate materials, the growth of single
crystalline ScN films has been reported on sapphire, MgO,
and Si.25–32 In general, heteroepitaxial growth on highly
lattice-mismatched substrates leads to significant mosaicity,
involving high density of crystal defect such as dislocations.
There are a few reports on the structural quality of ScN
films,27,30 and the best reported FWHM of X-ray rocking
curve is 0.35 , which is still far from the quality of conventional, high quality semiconductors, such as Si, GaAs, andGaN. On the other hand, in order to use semiconductors indevice applications, generally their conduction type and carrier concentration need to be controlled precisely. In the caseof ScN, however, such control has not been realized yet.
Nominally un-doped ScN usually shows strong n-type conduction, with a high residual carrier concentration ranging
between 1019 to 1022 cm3.29–32 The plausible origins of thedonors are impurities and crystal defects such as nitrogenvacancies. However, their contribution has not been fully
clarified in detail yet. In order to exploit the full potential of
ScN and to pave the road to practical use, it is essential toclarify and control the effect of crystal defects and impurities
by the growth of high-quality and high-purity crystals.
In this work, we have employed HVPE as the growthtechnique of ScN. HVPE is a kind of chemical vapor deposition (CVD) technique, which was first reported by Tietjen
and Amic in 1966 as a growth method of III-V semiconductors such as GaAsP.33 Nowadays, HVPE is widely used forthe mass production of freestanding GaN single crystal
wafers.34,35 The method is characterized by a fast growthrate and the high crystalline quality of resulting layers.
Usually, the equilibrium constant of HVPE is far smaller
than those of MBE and metalorganic vapor phase epitaxy(MOVPE). Such small equilibrium constant allows one to153508-2

Oshima, Vıllora, and Shimamura

schematically shown in Fig. 1. A water-cooled quartz tube isemployed as the outer tube. Inside, a pBN-coated graphite
tube is coaxially located. The graphite tube is inductively
heated by an RF-coil around the quartz tube to create a hotzone inside. Between both quartz and graphite tubes, there is
a pBN tube radiation shield, which prevents the overheatingof the inner quartz wall. Ar-gas is purged to avoid the exposure of the inner quartz wall to the corrosive materials.
Inside the graphite tube, the inner flow channel, the substrateholder, and the Sc container are composed of corrosionresistant materials, such as tungsten and SiC.
ScN was grown in the home-made HVPE reactor atatmospheric pressure using scandium chloride and NH3(99.9999% pure) as source materials. No intentional doping
was carried out. Scandium chloride was formed upstream in
the reactor by the reaction between solid Sc metal (99.99%
pure) and HCl gas (99.999% pure). The partial pressures ofthe HCl (PHCl) and NH3 (PNH3) were varied between
0.124–1.24 kPa and 6.2–19.3 kPa, respectively. The saturated
vapor pressure of ScCl3, one possible product formed by thereaction between Sc and HCl, reaches 100 kPa at 1037  C.39
Therefore, in order to prevent the condensation of ScCl3, thetemperature of the reactor was kept constant at a bit higher
value 1050  C. Usually, nitrogen gas is used as a carrier gas
in HVPE. However, N2 and Sc react directly with each otherto form ScN at high temperatures.40 Instead of N2, we
employed Ar as carrier gas.
We have also investigated which substrates are appropriate for the heteroepitaxial growth of ScN. Sapphire, MgO,
Yttrium-stabilized zirconia (YSZ), GaN, and AlN were taken
under consideration. In this investigation, PHCl and PNH3
were fixed to be 0.31 kPa and 10.6 kPa, respectively. All depositions were carried out at 1050  C for 5 min using the same
growth recipe. The ScN films were directly grown on each
substrate without any buffer layer to compensate for latticemismatch.
B. Characterization methods

The crystal orientation of HVPE-grown ScN films wasinvestigated by X-ray diffraction x-2h scan and pole figure
measurements. The surface morphology was observed bymeans of Nomarski microscopy and scanning electron
microscopy (SEM). The growth rate was determined bycross-sectional SEM. The N/Sc composition ratio and concentrations of impurities were estimated by Rutherford backscattering spectrometry (RBS), EDX, and secondary ion

II. EXPERIMENTAL METHOD
A. HVPE growth of ScN

Generally, quartz and alumina are the principal materials to construct a HVPE reactor. However, as was stated in
the former section, these materials are corroded by scandium
and scandium chloride, the source materials of ScN in
HVPE.37,38 Therefore, we have designed our own homemade HVPE reactor avoiding the use of quartz and alumina
in the high-temperature zone. The structure of the reactor isFIG. 1. Schematic of the HVPE reactor used for ScN growth.

30 January 2024 07:23:41

supply high concentration gaseous source materials to realize
a high growth rate without parasitic gas phase reaction. In
addition, such small equilibrium constant enables the crystal
to grow under nearly thermal equilibrium condition, whichleads to the growth of high quality crystals.
The first HVPE of ScN was reported by Dismukes et al.
in 1970.31,32 They demonstrated the epitaxial growth of ScN
on r-plane sapphire substrate within 850–930  C. Besides
this temperature range, all samples were polycrystalline. The
single crystalline growth was possible at growth rates below
2.8 lm/h, and relatively thick films up to 20 lm wereobtained. Their nominally un-doped ScN films also showedn-type conductivity with high carrier concentrations in theorder of 1020–1021 cm3, which are comparable to those of
ScN films grown by MBE or sputtering. Their ScN films
contained chlorine and oxygen impurities in high concentrations, i.e., [Cl] ¼ 1.8  1020 cm3 and [O] ¼ 4.4  1019 cm3,
respectively. The high carrier concentration was attributed tothe presence of Cl ions, because their concentrations werecomparable. In spite of such high carrier concentration, thecarrier mobility was also relatively high, 151 cm2 V1 s1 at
n ¼ 1.1  1020 cm3, which is twice as high as that of silicon.
In recent years, Edger et al. tried to grow ScN on
6 H-SiC substrates by HVPE.36 Their ScN films contained
aluminum and chlorine as impurities in much higher concentrations, being even detectable by energy dispersive X-ray
spectrometry (EDX). In this case, the Al impurity was
assumed to originate from the reaction between the
scandium-chloride reactant and an alumina tube used to contain Sc metal in the HVPE reactor. The [Al] was as high as
1.5 at. % in their ScN film grown at 1000  C. By decreasing
the temperature of the alumina tube (source temperature)from 1000 to 800  C, the corrosion was suppressed and [Al]decreased. However, [Al] was still high (0.5 at. %) and[Cl] increased from 1.9 to 2.7 at. % due to the diminished Cl
desorption from the growth front.
As stated above, ScN is a very attractive material, and itis necessary to realize high quality epilayers with wellcontrolled electrical conductivity for device applications.
HVPE is, in principle, a promising growth method for ScN.
However, this technique needs to be improved in order toavoid the incorporation of impurities into the films, which is
caused by the chamber corrosion with the highly reactive
source materials. In this work, we demonstrate the growth ofhigh-quality, high-purity ScN epilayers by using an originally developed corrosion-resistant HVPE reactor. The crystalline, optical, and electrical properties of the resulting ScN
films are analyzed and discussed.

J. Appl. Phys. 115, 153508 (2014)

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J. Appl. Phys. 115, 153508 (2014)

mass spectrometry (SIMS). The mosaicity (tilt and twist) in
the films was evaluated through X-ray rocking curve measurements in symmetric and skew-symmetric geometry.
Transmittance and reflectance spectra were utilized to figureout the optical absorption coefficient and to determine theoptical band gap. Electrical properties (carrier concentrationand electron mobility) were investigated by Hall measurements by the van der Pauw method. All the measurements
were carried out at room temperature.
III. RESULTS AND DISCUSSION
A. Chemistry of ScN growth

Sc2 Cl6 þ 2NH3 ¼ 2ScN þ 6HCl;

B. Investigation of substrates for heretoepitaxyof ScN

Table I summarizes the results of ScN growth on various
substrates. Films grown on MgO, YSZ, GaN were peeled off
and broke into pieces after deposition. This behavior is
attributed to a substrate etching process at the interface,probably related to the presence of HCl gas in the growth
atmosphere.
Mechanically stable films were successfully grown on
sapphire and AlN. X-ray x-2h scan (not shown) revealed
that the ScN films grown on c- and a-plane sapphire and AlN
included domains with mixed out-of-plane orientations of(100) and (111). ScN films with single out-of-plane orientation were obtained on r- and m-plane sapphires, i.e.,(100)ScN//(10-12)sap and (110)ScN//(10-10)sap, respectively.
Figures 2(a) and 2(b) are the X-ray pole figures of theScN(100) and the ScN(110) films. The pole figure of theScN(100) film shows (111) poles with four-fold symmetry,
and that of the ScN(110) film shows (200) poles with
two-fold symmetry as expected for single in-plane orientation films. This proves that single crystalline films wereachieved on r- and m-plane sapphire. From the /-scan profiles of the epilayers and the substrates (not shown), following epitaxial relationships were elucidated:

ScN on r-plane sapphire : ð100ÞScN ==ð10  12Þsap and ½001ScN ==½11  20sap ;ScN on m-plane sapphire : ð110ÞScN ==ð10  10Þsap and ½001ScN ==½11  20sap :In the following, the characterization of these single
crystalline films is described.
C. Growth rate of ScN

Figure 3 shows the growth rate of ScN as a function of(a) PHCl, and (b) PNH3. The results of ScN(100) and ScN(110)were identical within the thickness accuracy estimated by
TABLE I. Results of HVPE growth of ScN on various substrates.
Substrate
SapphireMgOac-plane (0001)
a-plane (11–20)
m-plane (10–10)
r-plane (10–12)

(100)
(110)
YSZa (100)
GaN, c-plane (0001)
AlN, c-plane (0001)

Y2O3 (10 mol. %)—ZrO2 (90 mol. %)SEM. The growth rate first increases with increasing PHCl,
reaches maximum around PHCl ¼ 0.5 kPa, and then starts todecrease. The dependence on PNH3 is similar. The decrease in
growth rate probably results from a gas-phase reaction
upstream in the reactor that consumes scandium chloride andNH3 before they arrive at the substrate.
The next sections present and discuss the morphological,
structural, optical, and electrical properties of ScN films grownunder representative growth condition, PHCl ¼ 0.31 kPa andResultMixed orientations, (100) and (111)(110) single crystalline(100) single crystalline
Chemically unstable

Mixed orientations, (100) and (111)
FIG. 2. X-ray pole figures of ScN films grown on (a) r-plane sapphire and(b) m-plane sapphire.

30 January 2024 07:23:41

First of all, we discuss the chemical reactions by which
ScN is produced by HVPE. Dismukes et al. studied the vapor
phase reactions involved in ScN HVPE.32 They presumed
that ScCl2 is the predominant scandium chloride species
since only a dihalide is generated in the case of chemical
reaction between Cl2 and Y, being chemically similar to Sc.
In our case, however, the predominant scandium chloride
species would be (ScCl3)x, since the molar ratio of the supplied HCl and the decrease of Sc metal was 3:1. According toPatrikeev et al., ScCl3 and Sc2Cl6 co-exist in thermal equilibrium,39 and approximately 74% should be Sc2Cl6 at our reactor temperature 1050  C. Therefore, we presume that
following reactions are taking place simultaneously:

ScCl3 þ NH3 ¼ ScN þ 3HCl:

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J. Appl. Phys. 115, 153508 (2014)

FIG. 3. Growth rate of ScN as a function of (a) HCl partial pressure, (b)NH3 partial pressure.

PNH3 ¼ 10.6 kPa, corresponding to a growth rate of 5 lm/h.
The growth time was varied from 5 min to 8 h, thus the thickness ranged approximately between 0.4  40 lm.
D. Surface morphology of HVPE-grown ScN

E. Structural properties

Figures 6(a) and 6(b) show the thickness dependences oftilt and twist angles of our HVPE-grown ScN films. In the caseof ScN(100) films, we use the FWHMs of (200) and (131)rocking curves as the measure of the out-of-plane and in-planemosaicity, i.e., tilt and twist angles, respectively. Analogously,
we employed the FWHMs of (220) and (-240) reflections astilt and twist angles for ScN(110) films. Both (131) and (-240)
are inclined from the film surface by approximately 72 .
The tilt and twist angles monotonically decreased withincreasing film thickness. A similar quality improvement has

FIG. 4. Nomarski microscope surface images of: ScN (100) films with an approximate thickness of (a) 10 lm, (b) 20 lm, (c) 40 lm ScN (110) films with an approximate thickness of (d) 10 lm, (e) 20 lm, (f) 40 lm.

30 January 2024 07:23:41

The surface of all the samples was mirror-like for humaneyes. Figures 4 and 5 show surface Nomarski microscope
images and SEM images (bird’s eye view) of the HVPEgrown ScN films. On the surface can be observed hillocks,
whose averaged diameter increases with the layer thickness.
As it will be seen in Sec. III E, the crystalline quality of the
ScN films improves with increasing film thickness. So, if
theses hillocks are reflecting the sub-grain structure of themosaic films, this increase in hillock size, i.e., decrease in
the density of hillocks, agrees well with the improvement of
the structural quality.
In SEM images of Fig. 5, ScN(100) surface appears
smoother than ScN(110). This can be explained by the difference in the surface diffusion barrier, which arises from thenumber of dangling bonds. In the case of ScN(100) surface,each atom will have only a single dangling bond, whereas
each atom of ScN(110) surface will have two dangling
bonds. Therefore, the surface diffusion barrier of ScN(100)should be lower than that of ScN(110). An analogous argument is found in the comparison of MBE-grown ScN(100)and ScN(111) surfaces.3 Our films, both ScN(100) and
ScN(110), exhibit a much smoother surface compared to the
HVPE-grown ScN films reported by Dismukes et al. and
Edger et al.32,36 This fact can also be explained from theview point of surface diffusion barrier. The ScN films of
Dismukes et al. and Edger et al. had surfaces of ScN(321)and ScN(111), respectively. On the both planes, each atom
has three dangling bonds and hence the surface diffusion barrier should be higher than those of ScN(100) and ScN(110).
In addition, our growth temperature is substantially higher
than those of the previous reports. Therefore, it is plausiblethat such a high growth temperature favors the surface diffusion, resulting thus in a smoother surface.

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J. Appl. Phys. 115, 153508 (2014)

FIG. 5. SEM images (bird’s eye-view) of: ScN (100) films with an approximate thickness of (a) 10 lm, (b) 20 lm, (c) 40 lm ScN (110) films with an approximate thickness of (d) 10 lm, (e) 20 lm, (f) 40 lm.

been observed in the case of GaN films. The interpretation
given to this phenomenon is that dislocations with opposite
Burgers vector gradually approach each other due to theattractive interaction between them, and then react with eachother to make a dislocation loop during thick film growth.41
The same mechanism would be possible in the case of ScN.
In order to confirm this, it will be necessary to perform thedirect observation of dislocations by cross-sectional TEM.
We have also found that ScN(110) films have a better
quality than ScN(100) ones when their thickness is the same.
The lattice mismatch between m-plane sapphire and two
periods of ScN(110) along [1–10]ScN is 2%, while thatbetween r-plane sapphire and three periods of ScN (100)along [010]ScN is 14%. The difference in the structuralquality is likely to be originated from this difference in lattice match.
F. Optical properties

Figure 7(a) shows the transmittance (T), the reflectance(R), and the absorption coefficient (a) of a ScN(110) film
(3.7 lm thick) as a function of photon energy (hv). Both T
and R show a clear oscillation due to the multiple reflections
on both sides. a was estimated from T and R by the method
described elsewhere.42 a shows a steep increase around
2.1 eV. No threshold was observed around 0.9–1.3 eV, where
the existence of the indirect transition is predicted. It should
be noted that the apparent a saturation above 2.1 eV is an

FIG. 7. (a) Transmittance and reflectance spectra of ScN (110), (b)Absorption coefficient in (ha)2 vs h
plot.

30 January 2024 07:23:41

FIG. 6. Thickness dependence of XRCFWHM of (a) ScN (100) and (b) ScN(110).

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J. Appl. Phys. 115, 153508 (2014)

FIG. 8. (a) RBS spectrum of ScN(100), (b) depth profile of atomic ratioof Sc, N, Al, O.

to that of HVPE-grown GaN. Regarding [H] and [Cl], reliable standards to determine absolute concentration were notavailable. We tentatively used the relative sensitivity factors(RSF) of GaN, since these are expected to be similar to thoseof ScN. Consequently, the actual [H] and [Cl] in Fig. 9 may
differ from the true values by a factor of 2–3. However, even
if the true value for [Cl] was 10 times larger than our tentative one ([Cl] ¼ 1.0  1018 cm3), this is still considerably
smaller than that reported by Dismukes et al., with
[Cl] ¼ 1.8  1020 cm3.32 These results indicate that the corrosion of the reactor materials is efficiently suppressed evenat the high growth temperature of 1050  C, which is substantially higher than those of previous HVPE reports. In addition, our high growth temperature could enhance thedesorption of chlorine from the growth surface and favor alow [Cl] in our ScN films.
H. Electrical properties

G. Composition and impurity concentration

We have investigated the N/Sc composition ratio of 1.2lm-thick ScN(110) film by RBS using Hþ and 4Heþ as incident ions. Nitrogen sensitive measurements are possible
when Hþ is utilized, while 4Heþ enables high depth resolution measurements. Figure 8(a) shows the RBS spectrum
obtained with Hþ as incident ion. The spectrum was accurately reproduced by assuming the existence of only Sc, N,
Al, and O, i.e., no impurity was observed by RBS. The estimated N/Sc ratio from this spectrum is 1.01 6 0.10.
Therefore, no significant deviation from the stoichiometricratio (N/Sc ¼ 1) was observed. Figure 8(b) is the depth profile of the composition calculated from the RBS spectrum(not shown) obtained with 4Heþ as incident ion. The N/Sc
ratio was confirmed to be stoichiometric along the depth
direction within the RBS accuracy.
No element except Sc and N was detected by EDX. In
order to obtain more detailed information about potential
impurities (namely H, Cl, C, O, Si, and W), SIMS was carried out. H and Cl are contained in HCl and NH3 gases. C,
O, Si, and W are constituents of the HVPE reactor materials.
As a result, all the elements except W were detected. Figure
9 shows the depth profile of the SIMS measurement. The
present [C] and [O] are approximately 7.9  1017 cm3 and8.8  1017 cm3, respectively. Even though these concentrations are significant, these are remarkably lower than thosereported by Dismukes et al., with [C] ¼ 1.7  1019 cm3 and[O] ¼ 4.4  1019 cm3.32 In the case of Si, there is no reporton ScN, and our value [Si] ¼ 1.6  1016 cm3 is comparable

The carrier concentration dependence of the electron
mobility is shown in Fig. 10. The represented data correspond to nominally un-doped samples with various thicknesses (between 0.4–40 lm), using the same growth recipe
but different deposition times. The carrier concentration
varies within two order of magnitude, no systematical correlation is found between this and the thickness or crystalline

FIG. 9. SIMS depth profile of ScN(100). Note that concentrations of H andCl are tentative ones calibrated with the RSFs of GaN. Therefore, these values may differ from the true ones by a factor of 2–3.

30 January 2024 07:23:41

experimental artifact caused by the decrease in transmittancesignal below the detection limit of our instrument for the relatively large film thickness. Therefore, a should be greater inreality. Figure 7(b) is the absorption coefficient spectrum in(hva)2 vs hv plot. The linear range correlates clearly with thedirect transition, and the band gap energy was determined tobe 2.06 eV by a linear fit. The data for ScN(100) films (notshown) show no significant difference with the results of
ScN(110) films.
The direct band gap of ScN is reported to lie in the rangeof 2.1–3.2 eV. Our band gap corresponds with the minimum
reported values. Moram et al. have demonstrated that oxygen
contamination leads to an increase in the direct band gap
energy of ScN.43 Therefore, our small energy gap is indicative of a low oxygen contamination in our samples. Indeed,
the oxygen concentration is found by SIMS to be in the orderof 1017 cm3, as it will be seen in Sec. III G.

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J. Appl. Phys. 115, 153508 (2014)

small to account for the carrier concentration divergence even
if the true values of [H] were 10 times larger than our tentativevalues. These results suggest that the dominant donor species
in our ScN films are not related to the elements listed in Table
II. According to Smith et al., nitrogen vacancies can also act as
donors.3 In order to clarify the actual origin of the residual carrier concentration, further investigations are required.
IV. SUMMARY

FIG. 10. Carrier concentration dependence of electron mobility.

TABLE II. Comparison of impurity concentrations (cm3) in ScN films
with largely different free electron concentrations.

Element
H
C
O
Si
Cl
a

Sample A
n ¼ 8.1  1019 (cm3)

Sample B
n ¼ 1.0  1019 (cm3)(1.6  1018)a
7.9  1017
8.8  1017
1.6  1016
(1.0  1018)a(2.3  1018)a
6.2  1017
2.9  1017
5.4  1016
(1.7  1018)a

Tentative values, calibrated with the RSFs of GaN.

ACKNOWLEDGMENTS

This work was partly supported by a Grant-in-Aid forYoung Scientists (B) No. 25790050 from Japan Society for
the Promotion of Science (JSPS). Part of this work was conducted for implementing the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) Elements Strategy
Initiative to Form Core Center of Japan. The authors wish to
thank Dr. Takaaki Mano for his great help in measuring Xray pole figures.
1

M. A. Moram, Z. H. Barber, C. J. Humphreys, T. B. Joyce, and P. R.
Chalker, J. Appl. Phys. 100, 023514 (2006).
2
D. Gall, I. Petrov, N. Hellgren, L. Hultman, J. E. Sundgren, and J. E.
Greene, J. Appl. Phys. 84, 6034 (1998).

30 January 2024 07:23:41

quality. Clearly, some unknown and uncontrolled factor isdominating the residual carrier concentration. It appears alsothat there is no significant difference between ScN(100) and
ScN(110). The data reported in the literature29,32 are shown
together for the sake of comparison. It is seen that the measured carrier concentrations are substantially lower than thereported ones, and that the electron mobility monotonicallyincreases with decreasing carrier concentration. This can bereasonably explained by assuming a decrease of carrier scattering on ionized donors.
In order to investigate the origin of the residual carrier
concentration, we compared the SIMS results between two
samples with largely different carrier concentrations (Table II).
Here, we denote the samples with high (n ¼ 8.1  1019 cm3)and low (n ¼ 1.0  1019 cm3) carrier concentration as samples
A and B, respectively. Again, it should be reminded that [H]and [Cl] are tentative values calibrated with the RSFs of GaN,
and therefore these could be different from their true values by
a factor of 2–3. Among the impurities listed in Table II, C andO are the only elements whose concentration is higher in sample A. However, the variances between sample A and B are too
small to explain the difference in carrier concentration. On theother hand, [Si] and [Cl] are larger in sample B. However, theyshould act as donor in ScN, and cannot be the cause of the carrier concentration difference. [H] is also greater in sample B,
but its contribution to the electrical properties of ScN is still
unknown. However, also in this case, the difference is too

Single crystalline ScN(100) and ScN(110) films are successfully grown on r- and m-plane sapphire substrates by
HVPE. The growth rate has a peak for the partial pressuresof both HCl and NH3, and the maximum growth rate was
5 lm/h. All the single crystalline samples had mirror-like surface with hillocks, which were visible by Nomarski microscope and SEM. The hillock size increased with increasing
the film thickness. The surface of ScN(100) appeared to besmoother than that of ScN(110), which can be originatedfrom the difference in surface diffusion barrier. The tilt andtwist angles for both ScN(100) and ScN(110) drastically
improved with increasing the film thickness. The structuralquality of ScN(110) was better than that of ScN(100) probably due to the better lattice matching to sapphire substrates.
The optical band gap was estimated to be 2.06 eV throughtransmittance and reflectance measurements. The N/Sc composition ratio was 1.01 6 0.10, i.e., stoichiometric within theaccuracy of RBS. EDX and SIMS revealed that impurity concentrations were successfully reduced compared to thereported values. This was achieved by a home-made corrosion-resistant HVPE reactor and a high growth temperature.
Our nominally un-doped samples show n-type conductivity
similar to the reported results. However, the residual carrier
concentration was markedly lower, ranging between
1018-1020 cm3. The carrier mobility increased with decreasing the carrier concentration, and reached 284 cm2 V1 s1 at
n ¼ 3.7  1018 cm3, which is the highest value ever reported.
It was found that the electrical properties (carrier concentration and mobility) are not directly affected by the structuralquality. Additionally, there is no clear evidence of a correlation between the impurity concentrations of H, C, O, Si, andCl and the electrical properties. Further work is required toclarify the dominant origin of the residual donors in order toenable the conductivity control.

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