RF-sputter deposition of Zn-Ge nitride thin films

Article abstract of DOI:10.1016/S0038-1098(99)00389-0

Nitride Zn-Ge thin films were deposited by reaction sputtering. Black conducting (Zn_1-xGe_x)₃N_2+δ with x≤0.27 was obtained in a compositional range below 29 wt% Ge against the total metal content. Greenish pale yellow ZnGeN₂ was observed in a range of 30-60 wt% Ge. This crystallized in a hexagonal lattice of a comparable size to the isoelectronic GaN. Its solid solution range seems to be very narrow and its band gap was estimated to be about 3.1 eV. Ge₃N₄, like amorphous (Ge_1-yZn_y)₃N_4-y with y≤0.43 films, shows pale yellow color above the compositional range of 60 wt% Ge.

Full text of DOI:10.1016/S0038-1098(99)00389-0

PERGAMON
Solid State Communications 112 (1999) 513–515
www.elsevier.com/locate/ssc
RF-sputter deposition of Zn–Ge nitride thin films
1,
*
S. Kikkawa , H. Morisaka
ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 6 July 1999; accepted 4 August 1999 by F.J. DiSalvo
Abstract
Nitride Zn–Ge thin films were deposited by reaction sputtering. Black conducting ꢀZn1ϪxGex†3N2ϩd with x Յ 0:27 was
obtained in a compositional range below 29 wt% Ge against the total metal content. Greenish pale yellow ZnGeN2 was
observed in a range of 30–60 wt% Ge. This crystallized in a hexagonal lattice of a comparable size to the isoelectronic
GaN. Its solid solution range seems to be very narrow and its band gap was estimated to be about 3.1 eV. Ge3N4, like amorphous
ꢀGe1ϪyZny†3N4Ϫg with y Յ 0:43 films, shows pale yellow color above the compositional range of 60 wt% Ge. ᭧ 1999 Elsevier
Science Ltd. All rights reserved.
Keywords: A: Semiconductors; B: Chemical synthesis; C: Crystal structure and symmetry; D: Electronic transport; D: Optical properties
1. Introduction
to control both crystal lattice size and electronic structure.
ZnGeN2 is a derived binary nitride from GaN substituted
with its neighboring II and IV group elements. Its crystal
structure originally assumed to be derived from wurtzite [4].
Neutron diffraction showed an ordering of Zn and Ge in a
tetrahedral site in ZnGeN2 with a monoclinic crystal lattice
of a ˆ c ˆ 0:3167 nm; b ˆ 0:5194 nm; b ˆ 118Њ530 [5].
The crystal structure is of the NaFeO2 type with space
group Pna21. ZnGeN2 will be either a candidate material
for a blue laser emitter itself substituting GaN or a substrate
material for the GaN thin film growth when its defect
density will be very low. Its preparation and properties
have been studied less [4–6]. A brief report is available
on the optical and electrical properties of ZnGeN2 thin
film grown in an open flow HCl–N2 atmosphere on Zn
and Ge elements [6]. Both Zn3N2 and Ge3N4 are end
members of nitrides in a Zn–Ge binary system. Only their
crystal structures have been reported [7–9].
Structural feature is important for a semiconductor thin
film. Efforts have been made on homoepitaxial growth of
GaN on the small bulk GaN crystals. Dislocation density
was less than 106 cmϪ2 with the residual doping level of
1017 cmϪ3 [1]. The reduction of dislocation density is very
critical for a blue laser application. The range of substrates
available for use in group III nitrides is limited. Growth of
GaN bulk crystal is still in a preliminary research level and
is not yet commercially available. The most commonly used
substrate for the nitride devices has been a sapphire, which
has a large lattice mismatch to the nitride resulting in a
defect density of 108ϳ11 cmϪ2. ZnO did not show any
improvement in the GaN quality from the films on sapphire
or GaAs [2]. Low-temperature buffers were required to
improve nucleation behavior of GaN on LiGaO2 and InN
on LiAlO2 [2].
Crystal structure has been refined on the wurtzite GaN.
The hexagonal lattice sizes were a ˆ 0:3190 nm and c ˆ
0:5189 nm [3]. Isoelectronic replacement is possible on
the group III nitrides to II–IV analogues. It has advantages
In the present investigation, rf-sputter deposition was
applied to obtain thin films of ZnGeN2, zinc and germanium
nitrides, respectively. Zn, Ge and their composite targets
were sputtered in nitrogen atmosphere. The products were
characterized in structure, electrical and optical properties.
* Corresponding author. Tel.: ϩ 81-6-6879-8452; fax.: ϩ 81-6-
6879-8452.
E-mail address: kikkawa@sanken.osaka-u.ac.jp (S. Kikkawa)
1 This research was partly supported by NEDO International Joint
Research Program on Advanced Nitrides (97MB2).
2. Experimental
Films were grown on glass or Si wafer substrates without
0038-1098/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(99)00389-0

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S. Kikkawa, H. Morisaka / Solid State Communications 112 (1999) 513–515
direction of anti-bixbyte type structure. The crystallinity
was reduced both at 1.3 Pa in 40 W and at 0.5 Pa in 20 W.
Film thickness was in the range of 0:6–1:4 mm in a 2 h
deposition. A columnar texture was observed in its cross-
section by SEM. Zn3N2 thin films were of the n-type semi-
conductors with an electrical resistivity of 0.44 V cm at
room temperature. The carrier density was 1:98 × 1018 and
its mobility was 72.1 cm2/(V s). The films were easily
oxidized to ZnO in an ambient atmosphere in a month.
Germanium nitride was prepared at 1.3 Pa in the range of
30–40 W for 3 h. Thin films of germanium nitride were
electrically insulating and pale yellow in color. X-ray
diffraction showed a hallow pattern in a 2u range of
20–40Њ corresponding to Ge3N4 like amorphous. The amor-
phous patterns showed some similarity to the reported data
for a- and b-Ge3N4 on the films deposited at 40 and 30 W,
respectively (JCPDS 11-69, 38-1374). Local structures
similar to the high temperature a- and low temperature b-
phases are present in the respective films because of the
different energy supply during the deposition. The former
showed a smaller optical absorption gap than the latter: 2.6
and 2.7 eV for the films deposited at 40 and 30 W for
180 min, respectively. They were 3.0 and 3.2 eV for the
respective films deposited for 90 min. The local structure
may slightly change with the deposition duration as well
as the applied rf-power. The film was stable at 500ЊC in
an ammonia flow but decomposed to metallic Ge at 700ЊC.
Germanium content could be controlled in the film depos-
ited using the composite target with various Zn/Ge area
ratios, in which Ge chips were distributed on a Zn metal
target. The composition control was difficult in an opposite
configuration of the composite target, in which Zn chips
were easily changed to nitride to reduce severely the deposi-
tion rate of zinc nitride. Films were deposited at 1.3 Pa in
30–40 W. Black ꢀZn1ϪxGex†3N2ϩd solid solution was
obtained in a range below 29 wt% Ge against the total
metal content. The composition corresponds to x ˆ 0:27:
This gradually became X-ray amorphous with an increasing
x and electrically resistive; 8:52 × 108 V cm at x ˆ 0:27:
ZnGeN2 was observed on the films deposited in the range
of 30 ϳ 60 wt% Ge. Crystallinity was poor in the brown
film obtained at 30 wt% Ge as shown in Fig. 1. Greenish
pale yellow ZnGeN2 crystallized well with an increase of Ge
content changing the crystalline orientation. The X-ray
diffraction patterns could be indexed in a hexagonal lattice.
Solid solution range was assumed to be very narrow because
the lattice parameters changed very little against the Ge
content. ZnGeN2 crystallized very well and showed the
smallest unit cell volume of 4:64 × 10Ϫ2 nm3 with a ˆ
0:3213 nm and c ˆ 0.5191 nm at the Ge content of
52 wt%. It corresponds to Zn=Ge ˆ 1:0 in the molar ratio.
The ZnGeN2 crystal lattice has been reported as monoclinic
with a ˆ c ˆ 0:3167 nm; b ˆ 0:5191 nm; b ˆ 118Њ530 in
neutron diffraction [5], and as hexagonal with a ˆ
0:3193 nm; c ˆ 0:5187 nm [4] Their unit cell volumes are
Fig. 1. X-ray diffraction pattern of ZnGeN2 thin films prepared with
various Ge wt% against total metal content: (a) 30; (b) 45; (c) 51; (d)
52; and (e) 57%.
their heating in a rf-magnetron sputter deposition equipment
(Anelva SPF-210H). Sputter gas was nitrogen in a 6N
purity. Target materials were Zn or Ge in a 5N purity of
100 and 76 mm in diameter, respectively. Zn/Ge composi-
tional ratio was controlled by changing distribution of small
Ge chips ꢀ5 × 5 mm2† on the Zn target. X-ray diffraction was
measured using Rigaku diffractometer both for powder and
for thin film with a monochromatized CuKa radiation gener-
ated with a rotary target. 2u angle was scanned with an X-
ray incident angle u ˆ 1Њ in the thin film diffractometry.
Texture was observed by Hitachi scanning electron micro-
scope S-2150 with Horiba EMAX-2770. Optical absorption
was measured with a UV–visible spectrometer, Jasco 670
on the films deposited on silica glass substrate. Electrical
properties were investigated on the films of 10 × 10 mm2
using gold lead wire with indium electrodes applying
ResiTest 8300.
3. Results and discussion
It is necessary to obtain preparation conditions of both
end members in the use of the composite target. Both the
applied rf-power and sputter gas pressure were adjusted for
depositions of Zn and Ge nitrides, respectively. Zinc nitride
was prepared in a region with 0:5 ϳ 1:3 Pa of sputter gas
pressure and 20 ϳ 40 W of rf-power. Zn target itself was
destroyed due to a severe nitridation in higher conditions.
Metallic Zn deposited at 0.5 Pa in 30 W. A black Zn3N2 thin
film was obtained at 1.3 Pa in the range of 20–40 W (JCPDS
35-762). It showed a preferred orientation in the ͗100͘
4:51 × 10Ϫ2 and 4:58 × 10Ϫ2 nmϪ3
; respectively. The

S. Kikkawa, H. Morisaka / Solid State Communications 112 (1999) 513–515
515
4. Conclusion
Isoelectronic replacement of GaN was performed by
preparing ZnGeN2 thin films applying the rf-sputter deposi-
tion. Use of a composite target where Ge chips were distrib-
uted on a Zn metal target was significant to control the film
composition in the reaction sputtering in nitrogen atmo-
sphere. The product showed a slightly larger crystal lattice
with a ˆ 0:3213 nm and c ˆ 0:5191 nm than the previously
reported values. The values are also larger than the a ˆ
0.3190 nm and c ˆ 0.5189 nm of GaN. The obtained
product was an electrical insulator with a slightly smaller
optical band gap of ca 3.1 eV than 3.26 eV of GaN.
Acknowledgements
Fig. 2. Optical absorption spectra of ZnGeN2 thin films deposited
for (a) 20 and (b) 30 min.
Authors would like to thank Dr M. Takahashi and Ms K.
Adachi for their assistance in a preliminary data acquisition.
present value is slightly larger than the previous ones prob-
ably due to the presence of excess nitrogen absorbed during
the sputter deposition. The present product was an electrical
insulator. Vapor-grown ZnGeN2 film has been reported to be
an n-type semiconductor with electrical resistivities
between 0.3 and 0.4 V cm at room temperature [6]. Trace
doping of chlorine from its preparation atmosphere might
contribute for the electron doping. The present films showed
optical absorption spectra in Fig. 2. They were slightly
different with the film thickness but basically the same.
The band gap can be estimated to be about 3.1 eV. They
are comparable to a value of 3.26 eV for GaN [1]. The
reported value of 2.67 eV for ZnGeN2 might be related to
its impurity level [6]. The lattice parameters shrunk after its
annealing in ammonia for 1 h; a ˆ 0:3219 nm; c ˆ
0:5226 nm at 500ЊC and a ˆ 0:3214 nm; c ˆ 0:5129 nm at
700ЊC. They were also electrical insulators even after the
annealing. Pale yellow Ge3N4 like amorphous
ꢀGe1ϪyZny†3N4Ϫg film was obtained in a range above the
Ge content of 60 wt%. The composition corresponds to y ˆ
0:43: It was also electrically insulating.
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