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Ion-bombardment-induced reduction in vacancies and itsenhanced effect on conductivity and reflectivity in hafnium nitridefilms
Zhiqing Gu1 • Jiafu Wang1 • Chaoquan Hu1 • Xiaobo Zhang1 • Jianchen Dang1 •
Sam Zhang2 • Jing Gao1 • Xiaoyi Wang3 • Hong Chen4 • Weitao Zheng1
Received: 8 December 2015 / Accepted: 24 July 2016
� Springer-Verlag Berlin Heidelberg 2016
Abstract Although the role of ion bombardment on elec-
trical conductivity and optical reflectivity of transition
metal nitrides films was reported previously, the results
were controversial and the mechanism was not yet well
explored. Here, we show that proper ion bombardment,
induced by applying the negative bias voltage (Vb), sig-
nificantly improves the electrical conductivity and optical
reflectivity in rocksalt hafnium nitride films regardless of
level of stoichiometry (i.e., in both near-stoichiometric
HfN1.04 and over-stoichiometric HfN1.17 films). The
observed improvement arises from the increase in the
concentration of free electrons and the relaxation time as a
result of reduction in nitrogen and hafnium vacancies in the
films. Furthermore, HfN1.17 films have always much lower
electrical conductivity and infrared reflectance than
HfN1.04 films for a given Vb, owing to more hafnium
vacancies because of larger composition deviation from
HfN exact stoichiometry (N:Hf = 1:1). These new insights
are supported by good agreement between experimental
results and theoretical calculations.
1 Introduction
The Group-IVB, VB and VIB transition metal nitride
(TMNx, TM = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) are well
known as a class of fascinating and technologically
important materials in the fields of electrochemical power
sources, microelectronic devices, cutting- and machining-
tool industries [1–3]. They have attracted much attention
because of their excellent electrical conductivity [4], high
infrared reflectance [5], large free-carrier concentrations
[6] coupled with other properties such as diffusion resis-
tance [7], thermal stability [8], high hardness [9, 10],
abrasion [11] and corrosion resistance [12]. This makes
them competitive for use as electrodes in fuel cell and Li-
ion microbatteries [13], alternative plasmonic materials [6],
solar control coatings on windows [14], decorative coatings
[15], gate electrode for MOS technology [16], conductive
diffusion barrier layers [17, 18], thin film resistors [19]
highly reflecting back-contacts in solar cells [20] and light-
emitting-diode (LED) devices [21]. In these applications,
the electrical and optical properties of the films are of
utmost importance, as they determine the ultimate perfor-
mances of the devices. It is very crucial, therefore, to
understand how to control them.
Studies have revealed [22, 23] that ion bombardment via
applying negative bias voltage (Vb) during film deposition
Electronic supplementary material The online version of thisarticle (doi:10.1007/s00339-016-0308-0) contains supplementarymaterial, which is available to authorized users.
& Chaoquan Hu
cqhu@jlu.edu.cn
& Weitao Zheng
wtzheng@jlu.edu.cn
1 School of Materials Science and Engineering, Key
Laboratory of Mobile Materials, MOE, and State Key
Laboratory of Superhard Materials, Jilin University,
Changchun 130012, China
2 School of Mechanical and Aerospace Engineering, Nanyang
Technological University, 50 Nanyang Avenue,
Singapore 639798, Singapore
3 Key Laboratory of Optical System Advanced Manufacturing
Technology, Changchun Institute of Optics, Fine Mechanics
and Physics, Chinese Academy of Sciences,
Changchun 130033, China
4 Department of Control Science and Engineering, Jilin
University, Changchun 130025, China
123
Appl. Phys. A (2016) 122:776
DOI 10.1007/s00339-016-0308-0
is an effective way to alter the electrical and optical
properties in TMNx films. Although extensive researches
on the influence of Vb on electrical conductivity and optical
reflectivity have been carried out and numerous results
reported [22–24], the following three questions still remain
unanswered: (1) what is the fundamental role that the Vb
plays in electrical conductivity and optical reflectivity? The
reported results are controversial. Some argue that the Vb
causes increase in the electrical conductivity and optical
reflectivity [24–26], while others claim the opposite
[22, 27, 28]. Clearly, further studies are needed to clear the
mist and elucidate the effect, especially in d-HfNx films as
no systematic investigations on these films are conducted.
(2) What governs the Vb-induced evolution in electrical
conductivity and optical reflectivity? Even though the
phenomenon has been reported previously where electrical
conductivity and optical reflectivity in TMNx films depend
strongly on Vb, the mechanism is unknown. (3) How will
the effect of Vb change with change of chemical stoi-
chiometry in the film? It is known that non-stoichiometric
TMNx films were usually obtained experimentally because
point defects exist when x deviates from unity [29, 30]. It
has been a conjuncture that Vb has different influence when
the stoichiometry is different, and that, in turn, affects
electrical and optical properties differently. To date, how-
ever, no research on this has been attempted yet.
With the above three questions in mind, we prepared
near-stoichiometric (x & 1.04) and over-stoichiometric
(x & 1.17) rocksalt hafnium nitride films (d-HfNx) at dif-
ferent Vb and studied the effect of ion bombardment on
structure, electrical conductivity and optical reflectivity of
the films by experimental measurements in combination of
the first-principle calculations. The experimental and the-
oretical results agree well, proving that: (1) in both HfN1.04
films and HfN1.17 films, proper Vb promotes significant
increase in both electrical conductivity and infrared
reflectance; too low or high Vb results in negative effect;
(2) the changes in electrical conductivity and infrared
reflectance with Vb arise from the variation in vacancies
concentration, because the formation of vacancies causes
reduction in the concentration of free electrons and electron
relaxation time; (3) although Vb has similar effect on both
HfN1.04 and HfN1.17 films, the electrical conductivity and
infrared reflectance of HfN1.04 film are always higher than
that of the HfN1.17 film at a given Vb. The mechanisms
behind these phenomena are discussed in detail.
2 Experimental and computational details
d-HfNx films were simultaneously deposited on both
single-crystal Si (001) and optical glass substrates by
radio frequency (RF) reactive sputtering. A pure Hf
target was used in the mixed discharge gases of Ar and
N2. The distance between the target and substrate holder
was fixed at 55 mm, and the chamber was evacuated by
a turbomolecular pump to 4 9 10-4 Pa prior to depo-
sition. Before being introduced into the deposition
chamber, Si (001) and glass substrates were cleaned
ultrasonically in acetone, alcohol and distilled water
ether, consecutively. The flow rate of Ar and N2 was
controlled by independent mass flow controllers (MT50-
2 J). During the deposition, the argon flow rate, RF
power, work pressure and substrate temperature were
kept at 80 sccm, 150 W, 1.0 Pa and 200 �C, respec-
tively. The nitrogen flow rate was kept at 3.6 and
6.0 sccm, respectively. Different negative bias voltages
(Vb) ranging from -6 V (floating) to -200 V were
applied to the substrate during deposition.
X-ray diffraction (XRD) measurements were taken in
grazing-incidence (GIXRD) modes by a Bruker D8tools
X-ray diffractometer using Cu Ka as the incident radi-
ation. The stoichiometry x and valence band spectra of
the films were determined by X-ray photoelectron
spectroscopy (XPS) measurements (VG ESCALAB
MKII), in which a monochromatized Al Ka (1486.6 eV)
X-ray source was used, and Ar? cleaning procedure
lasting 180 s was applied to all samples prior to XPS
quantitative analysis to remove the adventitious carbon
and absorbed oxygen from the sample surface. The
microstructure and surface morphology of the films were
characterized using a high-resolution transmission elec-
tron microscope (HR-TEM, JEOL TEM-2010), field-
emission scanning electron microscope (FE-SEM, JEOL
JSM-6007F) and atomic force microscopy (AFM,
VEECO). Raman measurements were taken using a
785-nm line Ar? laser excitation with a laser current of
0.5 A and an accumulation time of 30 s under the fre-
quency range of 50–1200 cm-1. The dc conductivity r0was obtained by four-point probe electrical conductivity
measurements. The reflectivity spectrum in the range of
300–2200 nm was obtained by a PerkinElmer Lambda
900 Ultraviolet–Visible–near-infrared (UV–Vis–NIR)
spectrometer. The substrate has no influence on the
optical reflectivity measurements due to the high thick-
ness of films. The complex dielectric function, concen-
tration n and relaxation time s of free electrons were
obtained by fitting the reflectance spectra using a Drude–
Lorentz model [5]. The details of Drude–Lorentz fitting
of optical reflectivity spectra were reported elsewhere
[5]. The thicknesses of the films were determined by
using a Dektak3 surface profile measuring system.
Density functional theory (DFT) calculations were per-
formed to explore the effect of N and Hf vacancies on
the electronic structure of d-HfNx. The details of com-
putation methods were reported elsewhere [5].
776 Page 2 of 10 Z. Gu et al.
123
3 Results and discussion
3.1 Vb-induced changes in vacancy concentrations
Figure 1a shows a typical HRTEM lattice image of HfNx
films, wherein well-crystallized nanograins with a size of
*16 nm are uniformly distributed on the film surface and
a highly ordered crystal lattice is observed from the
enlarged image of nanograin (Fig. 1b). The measured
interplanar distances match well with the (111) and (200)
plane spacings of perfect rocksalt hafnium nitride, indi-
cating that the films have the rocksalt structure (d-HfNx).
Figure 1c displays a typical SEM cross-sectional image of
d-HfNx films with a thickness of *1.0 lm on Si (001)
substrate. There are no obvious pinholes or surface cracks
in the film. Figure 1d exhibits a typical AFM image of d-HfNx films with an area of 2.0 lm 9 2.0 lm, in which a
very flat and smooth surface with nanoscale island-like
features is observed. The HRTEM, SEM and AFM mea-
surements are in good agreement, proving that the present
d-HfNx films are smooth [root-mean-square (RMS)
roughness below 3.34 nm], compact and well-crystallized,
desirable for the following investigation on their structure,
electrical and optical properties.
Figure 2a, b plots the GIXRD spectra for near-stoi-
chiometric HfN1.04 and over-stoichiometric HfN1.17 films
deposited at different substrate bias voltages (Vb), wherein
the five diffraction peaks (2h & 33�, 39�, 57�, 68� and 72�)attributed to the (111), (200), (220), (311) and (222) in d-
HfNx phase appear simultaneously, indicating that two sets
of d-HfNx samples remain crystallized in the rocksalt
structure as the absolute value of Vb increases from 6
(floating) to 200 V. The results agree well with the
HRTEM measurements. The lattice parameters (a) ob-
tained from GIXRD spectra for d-HfNx films deposited at
different Vb are shown in Fig. 2c, d. interestingly enough,
as the absolute value of Vb increases from 6 (floating) to
200 V, the a for both sets of samples first increases and
then decreases and reaches maximum values at
Vb = -80 V (x & 1.04) and Vb = -120 V (x & 1.17),
respectively. The lattice constant a for the two sets of
samples exhibits the same evolution with Vb, pointing to
the changes in a may have the same microscopic origins. In
our previous investigation [31, 32], we proved that the
introduction of N and Hf vacancies in the d-HfNx films
caused a decrease in a due to lattice contraction. Hence, the
observed changes in a (Fig. 2c, d) may indicate that as Vb
is applied to the substrates the concentrations of vacancies
in HfN1.04 and HfN1.17 films first reduce and then increase,
and arrive at minimum values at Vb = -80 V and
-120 V, respectively. This is further confirmed by Raman
and dielectric function spectra of these samples.
Figure 3a, b plots the Raman spectra for HfN1.04 and
HfN1.17 films deposited at different Vb. The first-order
acoustic (A) and optical (O) peaks at *110 and
*500 cm-1 occur simultaneously in all spectra, indicating
that all of the films have the rocksalt phase containing N
and Hf vacancies [31, 33]. As the absolute value of Vb
Fig. 1 A typical HRTEM
lattice image of d-HfNx films
(a) and enlarged image (b) ofthe detailed atomic arrangement
of the white square framed
region in a, as well as typicalSEM cross-sectional (c) andAFM (d) images of the films
Ion-bombardment-induced reduction in vacancies and its enhanced effect on conductivity and… Page 3 of 10 776
123
increases from 6 (floating) to 200 V, both intensities of
A and O peaks first decrease and then increase (Fig. 3c, d),
and at the same time their positions shift to lower fre-
quencies firstly and then to higher ones (Fig. 3e, f). The
intensities and position of A and O peaks for HfN1.04 and
HfN1.17 samples reach minimum values at Vb = -80 V
and -120 V, respectively. These results consistently prove
that as the absolute value of Vb increases the concentrations
of N and Hf vacancies in HfN1.04 and HfN1.17 films first
decrease and then increase, and they reach a minimum
value at Vb = -80 V in the x & 1.04 film and at
Vb = -120 V in the x & 1.17 film, respectively [31, 33].
This agrees well with results from the GIXRD spectra.
The imaginary parts of the dielectric function for the
HfN1.04 and HfN1.17 films deposited at different Vb are
plotted in Fig. 4a–c, f–h, respectively. Each curve is
composed of four absorption bands, the most dominant
arises from the contribution of interband transition
absorption related to free electrons, and another three
absorption bands centered at *4.50,*3.64 and*1.00 eV
originate from the contribution of interband transition
absorption corresponding to bound electrons. To
investigate the relationship between the three absorption
bands and vacancy defects, the total and partial density of
states (TDOS and PDOS) for defect-free HfN, N-vacancy-
containing Hf8N7 and Hf-vacancy-containing Hf7N8 is
calculated and shown in Fig. 4d, i. Based on these calcu-
lated DOS, the observed absorption band centered at
*4.50 eV that appeared in all d-HfNx films is attributed to
the intrinsic interband transition from the N2p electrons to
the Fermi level. The absorption band at *3.64 eV is
assigned to the interband transition from defect states of Hf
vacancies to the unoccupied states located around the
Fermi level (calculated energy separation is *3.85 eV),
while the absorption band at *1.00 is attributed to the
interband transition from defect states of N vacancies to the
unoccupied states located around the Fermi level (calcu-
lated energy separation is *1.10 eV). The simultaneous
appearance of N- and Hf-vacancy-induced absorption
bands means that both N and Hf vacancies exist in d-HfNx
films. Furthermore, as the absolute value of Vb increases
from 6 (floating) to 200 V, both relative integrated inten-
sity of N- and Hf-vacancy-induced absorption bands first
decrease and then increase, and reach minimum values at
Fig. 2 GIXRD spectra for
HfN1.04 and HfN1.17 films
prepared at different Vb (a, b),as well as lattice parameters
calculated from GIXRD spectra
for two sets of samples (c, d)
776 Page 4 of 10 Z. Gu et al.
123
Vb = -80 V (x & 1.04) or Vb = -120 V (x & 1.17),
respectively (Fig. 4e, j). These results are in good agree-
ment with the above GIXRD and Raman analysis,
demonstrating that N and Hf vacancies are formed in the
films deposited at different Vb. As the absolute value of Vb
increases gradually, the total concentration of N and Hf
vacancies first decreases and then increases.
In summary, the consistence between GIXRD, Raman
and dielectric function spectra proves that too low or too
high Vb causes a significant increase in concentration of
vacancies, while the proper Vb is energetically most
favorable to reduction in concentration of vacancies. In
HfN1.04 and HfN1.17 films, concentration of N and Hf
vacancies reaches a minimum value at Vb = -80 and
-120 V, respectively.
3.2 Vb-induced evolution in electrical conductivity
and its mechanism
Figure 5a, b plots the electrical conductivity (r0) for d-HfNx films prepared at different Vb, in which as the abso-
lute value of Vb increases from 6 (floating) to 200 V, the r0
for both sets of samples first increases and then decreases,
and reaches maximum values at Vb = -80 V (x & 1.04)
and Vb = -120 V (x & 1.17), respectively. These results
indicate that the proper Vb applied to the substrate during
deposition can promote a significant increase in r0.According to the expression of electrical conductivity
r0 = ne2s/me [22], r0 is determined by the electron
relaxation time s and the free electron concentration n. As
the absolute value of Vb increases from 6 (floating) to
200 V, both n and s first increase and then decrease, and
arrive at maximum values at -80 V (x & 1.04) and
-120 V (x & 1.17), which are shown in Fig. 5c, d. The
same Vb dependence of n, s and r0 indicates that the
observed change in r0 attributes to the evolution in n and s.Why does n experience a first increase and sequent
decrease with Vb (Fig. 5c, d)? The typical XPS valence
band spectra of the d-HfN1.04 films prepared at Vb = -6 V
(floating) and -80 V are plotted in Fig. 5e. There are two
peaks in the spectra, a relatively weak peak located near the
Fermi level and another strong peak centered at 5.0 eV.
According to calculated DOS (Fig. 4d, i), these two peaks
arise from Hf 5d states and hybridized states of N 2p and
Fig. 3 Raman spectra for HfN1.04 and HfN1.17 films prepared at different Vb (a, b), respectively, as well as the integrated intensity (c, d) andpositions (e, f) of first-order acoustic (A) and optical (O) peaks for the same samples
Ion-bombardment-induced reduction in vacancies and its enhanced effect on conductivity and… Page 5 of 10 776
123
Hf 5d, respectively. The occurrence of pronounced Hf
5d peaks near the Fermi level indicate that all d-HfN1.04
films are metal like, which is in good agreement with the
calculated DOS (Fig. 4d, i) and measured high r0 (Fig. 5a).It can be clearly seen from Fig. 5e that the film containing
a higher concentration of vacancies prepared at Vb = -6 V
has a much less DOS near the Fermi level than that with a
lower concentration of vacancies grown at Vb = -80 V.
This is in good agreement with the trend of measured
n (Fig. 5a), indicating that increased vacancies cause a
significant decrease in n. In order to understand the reason
that n of HfNx films decreases with increasing vacancies,
the difference between the spectra of two samples at
Vb = -6 V (floating) and Vb = -80 V is obtained and
shown in Fig. 5e. The original electrons of 0–2 eV near EF
in the film prepared at Vb = -80 V are transferred to the
new hybridized states around 2–5 eV in the film deposited
at Vb = -6 V, which is consistent with the calculated DOS
(Fig. 4i), indicating the formation of vacancies causes free
electrons near Fermi level being partly localized, and cre-
ating new Hfd- and Np-hybridized states. The similar
results of XPS valence band spectra are also found in the
HfN1.17 films (Fig. 5f). These valence band spectra of
HfN1.04 and HfN1.17 films are in good agreement with
theoretical calculation, proving that the first increase and
sequent decrease in n with Vb arises from a first reduction
and then increase in concentration of vacancies because
formation of vacancies causes free electrons near Fermi
level being partly localized.
s is the average time between two scattering events of the
free electrons [34], and in polycrystalline materials it is
directly related to point defects and grain boundaries, which
is determined by the formula 1/s & 1/sd ?1/sg [22], wheresd and sg are the relaxation times corresponding to point
defects and grain boundary scattering, respectively. In order
to investigate whether s is determined by point defects or
grain boundaries, the electron mean free path (le) is calcu-
lated by le = svF [32], where vF is the velocity at the Fermi
surface defined by vF & (�h/m*)(3p2N)1/3 according to the
free electron model [32]. Figure 5g displays le for HfN1.04
films deposited at different Vb, wherein the average grain
sizes are also given for comparison. It can be clearly seen that
le (0.3–1.2 nm) is much smaller than the grain size
(8–37 nm).Moreover, the trend of le is totally different from
variation of the grain size with Vb. These results clearly
indicate s and le in d-HfNx films are not significantly
Fig. 4 Imaginary parts of the dielectric function for HfN1.04 and
HfN1.17 films prepared at different Vb (a–c), (f–h). Density of states
(DOS) and partial density of states (PDOS) for d-HfNx containing N
vacancies (Hf8N7) and Hf vacancies (Hf7N8) as well as the perfect
HfN structure obtained by first-principles calculations (d, i). The ratioof integrated intensity of N vacancy (VN) and Hf vacancy (VHf)
interband absorption bands to that of intrinsic interband absorption
bands for HfN1.04 and HfN1.17 films prepared at different Vb (e, j)
776 Page 6 of 10 Z. Gu et al.
123
influenced by grain boundary scattering. Considering the
contribution of grain boundary scattering to s is rather small,
we believe that the electron scattering processes that involve
point defects control s. This conclusion can be confirmed by
the strong dependence between the concentration of vacan-
cies and le whenever in HfN1.04 or HfN1.17 films. Hence, the
observed increase and then decrease in s and le arise from a
reduction and sequent increase in concentration of vacancies
because in polycrystalline d-HfNx films the main electron
scattering sites are vacancies.
As such, the HfN1.04 film deposited at Vb = -80 V and
HfN1.17 film grown at Vb = -120 V exhibit the highest r0because they contain the lowest concentrationofvacancies and
hence thehighestn and longest s. Furthermore, it canbeclearly
seen from Fig. 5a, b that r0 (1 9 104–30 9 104 Sm-1) of
HfN1.17 films are always lower than that (5 9 104–
55 9 104 Sm-1) of HfN1.04 films for a given Vb, which is
attributed to more Hf vacancies formed in HfN1.17 films
because of larger composition deviation from N:Hf = 1:1.
3.3 Vb-induced evolution in optical reflectivity
and its mechanism
Figure 6a, b displays the reflectance spectra of HfN1.04 and
HfN1.17 films deposited at different Vb. From Fig. 6c, d, as
the absolute value of Vb increases from 6 (floating) to
200 V, the cutoff wavelengths at the minimum reflectance
(kRmin) for both sets of samples first decrease and then
increase, and reach minimum values at Vb = -80 V
(x & 1.04) and Vb = -120 V (x & 1.17). This is in good
agreement with the trend of n (Fig. 5c, d), because a high
n leads to a high plasma energy (Ep) according to the
formula Ep � n1/2, and thus a small kRmin [5]. The average
reflectance in the infrared range of 1000–2200 nm (RIR)
first increases and then decreases with Vb, and arrives
maximum values at Vb = -80 V (x & 1.04) and
Vb = -120 V (x & 1.17), respectively (Fig. 6c, d). The
regular variation in the cutoff wavelengths and reflectance
also leads to the gradual changes in color and brightness of
d-HfNx films, which are clearly displayed in inset of
Fig. 6a, b. The changes in RIR with Vb indicate that the
proper Vb applied to the substrate during deposition can
promote a significant increase in RIR.
To explore the reason why the Vb significantly affects
the RIR, we have fitted the reflectance spectra of d-HfNx
films by the Drude–Lorentz model, wherein a series of
spectral fittings have been performed by changing the
number of Lorentz oscillators considered in the fitting
functions. The best fitting has been obtained by using one
Drude part and three Lorentz oscillators in the fitting
Fig. 5 Electrical conductivity r0 (a, b) and concentration of free
electrons n and relaxation time s (c, d) for HfN1.04 and HfN1.17 films
prepared at different Vb. The typical valence band spectra normalized
to the intensity of the respective Hf 4f core lines (e, f). The electron
mean free path (le) and average grain size for the samples prepared at
different Vb (g, h)
Ion-bombardment-induced reduction in vacancies and its enhanced effect on conductivity and… Page 7 of 10 776
123
process. It is found that the shape and peak position of
reflectivity spectra obtained by fitting are in good agree-
ment with the corresponding experimental spectra (see
Supporting Information Figs. A1 and A2), indicating that
the reflectivity spectra for d-HfNx films can be well fitted
by the Drude–Lorentz model and the observed evolution in
RIR with Vb is totally attributed to the changes in the
Lorentz and Drude parameters.
To find out the main factors contributing to RIR of the
films deposited at different Vb, we have investigated the
role of each Drude and Lorentz parameter in changing the
reflectivity spectra by simulating the reflectivity spectra at
different parameters (see Supporting Information Figs. A3
and A4). It is clearly seen that all of Lorentz parameters
almost have no influence on RIR, indicating that the chan-
ges in the Lorentz parameters are not responsible for the
evolution in RIR. In contrast, the changes in n and s (Drudeparameters) cause RIR first rising and then falling, which is
consistent with the experimentally observed trend of RIR
with Vb (Fig. 6c–d), proving that n and s are the dominant
factors contributing to RIR. Hence, the first increase and
then decrease in RIR with Vb is attributed to the first
increase and sequent decrease in n and s.In order to understand why the RIR of d-HfNx film is
more dependent on the changes in Drude parameters (n and
s), we have compared the contribution between Drude
parameters and three Lorentz oscillators to the dielectric
function spectra in the infrared region of 1000–2200 nm,
which is plotted in Fig. 6e–h. It can be seen that Drude
parameters have a much more contribution to both real and
imaginary parts of the dielectric function than three Lor-
entz oscillators, which means that the dielectric function is
more sensitive to the variation in Drude parameters.
Therefore, the changes in n and s with Vb more easily
change the dielectric function and thus more significantly
influence RIR of d-HfNx film than three Lorentz oscillators.
This explains why there is a strong dependence between
Drude parameters (n and s) and RIR (see supporting
information Figs. A3 and A4).
In summary, the HfN1.04 film deposited at Vb = -80 V
and HfN1.17 film grown at Vb = -120 V exhibits the
highest RIR because they contain the lowest concentration
of vacancies and hence the highest n and longest s. Fur-thermore, it can be clearly seen from Fig. 6c, d that RIR
(30–70 %)of d-HfN1.17 films is always lower than RIR
(49–80 %)of d-HfN1.04 films (x & 1.04) at a given Vb,
which is due to more Hf vacancies formed in HfN1.17 films
because of larger composition deviation from N:Hf = 1:1.
4 Conclusions
1. Ion bombardment via applying the negative bias volt-
age (Vb) can cause reduction or more formation of Hf
and N vacancies in hafnium nitride films. Upon
reduction, increase in concentration of free electrons
Fig. 6 Reflectance spectra and photographs (a, b), and the average
reflectance in the infrared range of 1000–2200 nm (RIR) and the
cutoff wavelengths at the minimum reflectance (kmin) (c, d) for
HfN1.04 and HfN1.17 films prepared at different Vb. The typical real
parts e1 (e, f) and imaginary parts e2 (g, h) of the dielectric function in
the infrared range of 0.56–1.24 eV (1000–2200 nm) for the d-HfNx
samples
776 Page 8 of 10 Z. Gu et al.
123
and increase in electron relaxation time take place,
causing increase in electrical conductivity and optical
reflectance. This critical bias is -80 V for near-stoi-
chiometric HfN1.04 film and -120 V for over-stoi-
chiometric HfN1.17 film with corresponding electrical
conductivity of 5.5 9 105 Sm-1 (x & 1.04) and
3.0 9 105 Sm-1 (x & 1.17) and infrared reflectance
of 80 % (x & 1.04) and 70 % (x & 1.17). Further
increase in substrate bias results in formation of more
Hf and N vacancies, which in turn causes decrease in
concentration of free electrons and decrease in electron
relaxation time, resulting in both electrical conductiv-
ity and optical reflectance to deteriorate.
2. HfN1.17 films exhibit always much lower electrical
conductivity and infrared reflectance than HfN1.04
films for a given Vb, owing to more hafnium vacancies
because of larger composition deviation from HfN
exact stoichiometry (N:Hf = 1:1).
3. This study discovers that the electrical conductivity and
optical reflectivity of hafnium nitride films can be
enhanced via ion bombardment, which is useful in
improving the ultimate performance of transition metal
nitrides for important technological applications such as
electrical back-contacts for light-emitting devices, thin
film resistors and conductive diffusion barrier layers.
Acknowledgments The authors gratefully acknowledge the financial
support from National Natural Science Foundation of China (Grant
Nos. 51572104, 51102110 and 51372095), National Major Project for
Research on Scientific Instruments of China (2012YQ240264),
Technology Development Project (2015220101000836), Program for
studying abroad of China Scholarship Council. Also, the authors are
grateful for the support from the Academic Research Fund of Sin-
gapore (Tier 1, RG187/14).
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