Suppression of vacancy aggregation by silicon-doping in low-temperaturegrown Ga1−xCrxN
A. Yabuuchi, M. Maekawa, A. Kawasuso, Y.-K. Zhou, S. Hasegawa et al.
Citation: Appl. Phys. Lett. 102, 142406 (2013); doi: 10.1063/1.4801426
View online: http://dx.doi.org/10.1063/1.4801426
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APPLIED PHYSICS LETTERS 102, 142406 (2013)

Suppression of vacancy aggregation by silicon-dopingin low-temperature-grown Ga12xCrxN
A. Yabuuchi,1,a) M. Maekawa,1 A. Kawasuso,1 Y.-K. Zhou,2 S. Hasegawa,2 and H. Asahi2
1Advanced Science Research Center, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki,
Gunma 370-1292, Japan
2
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan(Received 23 January 2013; accepted 27 March 2013; published online 9 April 2013)
Effect of Si doping on low-temperature grown GaCrN films has been investigated by positronannihilation spectroscopy. In undoped GaCrN films grown at 540  C, vacancy clusters with sizes of
V6–V12 were found to be responsible for positron trapping. Such vacancy clusters were considerably
suppressed in Si-doped GaCrN films grown at 540  C, although divacancies (VGa VN ) still survived.
The Si-doping may be one possible way to suppress vacancy aggregation during low temperature
C 2013 American
crystal growth, and the further methods to remove divacancies are required. V
Institute of Physics. [http://dx.doi.org/10.1063/1.4801426]
Transition-metal-doped semiconductors have attracted
much attention as diluted magnetic semiconductor (DMS)
materials.1–4 For use in devices, high-Curie-temperature ferromagnetism is an important property. Among variousDMSs, the Curie temperature of chromium-doped GaN has
been reported to be higher than 400 K (Refs. 5 and 6) andhence a promising candidate operating at room temperature.
In addition, GaN-based DMSs can also potentially emit bluelight. The combination of the ferromagnetic and opticalproperties of GaCrN will lead to spintronic applications,such as circular polarization light emitting devices.
To exploit DMSs as spintronic devices, an improvement inmagnetization per unit volume is necessary. Two approaches
have been utilized in improving the magnetization:7 Si-dopingof GaCrN to enhance carrier-induced ferromagnetism and low
temperature molecular beam epitaxy (MBE) growth to increase
Cr atom concentration without formation of CrN precipitates.
However, in the second method, GaCrN crystallinity will bedegraded due to the introduction of vacancy defects during theMBE growth. These vacancy defects will act as carrier traps or
non-radiative recombination centers. Maintaining crystallinity
while suppressing vacancy defects in low-temperature growthis important for real device applications. In this study, vacancy
defects in low-temperature-grown GaCrN films containing nosecondary phases have been probed by a slow positron beam at
different doping conditions.
Positron annihilation spectroscopy is a powerful technique to detect vacancy defects in crystalline solids.8–10 In a
perfect lattice, positrons annihilate at the interstitial region
because positrons are repelled from ion cores. When vacancy
defects exist, positrons tend to be trapped there and annihilate preferentially with valence electrons having lower
momenta, compared to core electrons. Consequently, the intensity in the peak (tail) region of the Doppler broadening ofannihilation radiation (DBAR) spectrum, that is associateda)

Present address: Research Institute of Instrumentation Frontier (RIIF), National
Institute of Advanced Industrial Science and Technology (AIST), 1-1-1
Umezono, Tsukuba, Ibaraki 305-8568, Japan. atsushi.yabuuchi@aist.go.jp

0003-6951/2013/102(14)/142406/4/$30.00

mainly with valence (core) electrons, increases(decreases) as
compared to the perfect lattice.8–10
The samples used in this study were GaN-cap (33 nm)/GaCrN (500 nm)/GaN-buffer (33 nm) structures. They weregrown by a radio-frequency (RF) plasma-assisted MBE
method on commercial GaN (2 lm)/Al2O3 (0001) templates
grown by metal-oxide chemical vapor deposition (MOCVD).
The GaN-buffer layers were prepared at 700  C. The GaCrN(with Cr concentration of 0.7%) layers were grown at 540  Cwith a growth rate of 200 nm/h. This growth temperature is
low enough to prevent the CrN formation occurring at higher
growth temperatures, e.g., 700  C. The Ga flux, N2 flow rate,
RF plasma power, and Cr cell temperature were maintained
constant at 1  107 Torr, 1.5 SCCM, 300 W, and 960  C,
respectively. Si-doped GaCrN film was also prepared. The Si
cell temperature was set at 1100  C. The cap layers weregrown at 540  C.
The crystallinity of the GaCrN samples was evaluated by
X-ray diffraction (XRD) measurements. Monoenergetic positron beams with incident energy (E) of 0.2–25 keV wereimplanted into the samples, and the DBAR measurements
were performed. The obtained DBAR spectra were characterized in terms of an S parameter, which is defined as the numberof annihilation events over an energy range of 511 6 0:8 keV
divided by the total number of events in the 511 keV line. Thevalue of S increases when positrons are trapped at vacancy
defects. All the S parameters were normalized to that obtained
from a MOCVD GaN template. To examine the electronpositron momentum distribution in detail, coincidence DBAR
measurements were also performed.
To interpret experimental DBAR spectra, theoretical
DBAR spectra were calculated within the local density
approximation method.11 The valence electron wavefunctions
were calculated with the projector augmented-wave (PAW)
method12 using the ABINIT4.6.4 code.13 The potentials andprojectors were generated using the ATOMPAW code.14 The
valence electron configurations were 3d 10 4s2 4p1 ; 2s2 2p3 ;3s2 3p6 3d5 4s1 , and 3s2 3p2 for Ga, N, Cr, and Si atoms, respectively. A supercell, including 32 Ga and 32 N atoms with a2  2  2 conventional wurtzite unit cell, was constructed as

102, 142406-1

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Yabuuchi et al.

Appl. Phys. Lett. 102, 142406 (2013)

FIG. 1. XRD curves for the undoped and Si-doped GaCrN samples.

a GaN crystal. For vacancy defects, the lattice relaxationswere introduced by the molecular dynamics simulation installed in the code. The cut-off energy of the plane wave basis
set was 60 Ryd. The core electron wavefunctions were represented by the Slater function parameterized by Clementi andRoetti.15 A self-consistent positron wavefunction was calculated based on the two-component density functional theory inorder to minimize the energy functional.16 The BoronskiNieminen enhancement factor was adopted. The DBAR spectra were obtained by convoluting one-dimensional angularcorrelation of the annihilation radiation spectra obtained fromthe momentum density with the Gaussian resolution function
having a half width of 3:92  103 m0 c.
Figure 1 shows the XRD h-2h scan curves for theundoped and Si-doped GaCrN samples. Two diffraction peaks
were observed at around 41:7 from sapphire (0006) and
34:6 from GaCrN and GaN (0002), respectively. No obvious
secondary phase, such as CrN, was detected in these samples.
Figure 2 shows the S parameters for the undoped andSi-doped GaCrN samples as a function of incident positronenergy. The increase in all S parameters in the low positronenergy range (E < 3 keV) is related to surface effects, i.e., it
is caused by positron annihilation or positronium formation atthe sample surface.17–19 S parameters at 3 < E < 15 keV correspond to the GaCrN layers. The S parameter of the undoped
GaCrN sample is significantly higher than that of theMOCVD GaN template suggesting the existence of vacancy
clusters. Whereas, in the Si-doped GaCrN sample, the S parameter is considerably suppressed, although it is still higher
than that of the MOCVD GaN template. These results indicatethat the size and/or number of vacancy defects in the low-temperature-grown GaCrN are reduced by Si-doping. Probably,Si atoms, which occupy Ga sites as well as Ga vacancies,20
reduce the size and/or number of vacancy defects.
Figure 3 shows the coincidence DBAR spectrum
obtained for the MOCVD GaN template at E ¼ 8 keV. The
calculated DBAR spectra for perfect GaN lattice, Ga vacancy (VGa ), and N vacancy (VN ) are also shown. The experimental spectrum is well reproduced by the theoretical curvefor the perfect GaN lattice. The theoretical curve for VGa isFIG. 2. S parameters as a function of incident positron energy obtained for
undoped and Si-doped GaCrN samples. The S parameter of MOCVD GaN
template is also plotted. All the S parameters are normalized to that obtainedfor MOCVD GaN. The upper horizontal axis denotes the average implantation depth corresponding to the incident positron energy. The top of the figure schematically shows the structure of the samples.

sufficiently different from that for the perfect lattice. Thismeans that at least the MOCVD GaN template does not contain vacancy defects, which are larger than Ga vacancies.
The theoretical curve for VN is rather similar to that for theperfect lattice. Therefore, although it is difficult to argue
about the presence of N vacancies, the GaN template may be
used as the reference sample.
Figures 4(c) and 4(d) show the coincidence DBAR spectra obtained for the undoped and Si-doped GaCrN samples at
E ¼ 8 keV. To see the detailed spectrum shape, the original
spectra were normalized to that of the MOCVD GaN template. These measured DBAR spectra also indicate that

FIG. 3. Coincidence DBAR spectrum obtained for MOCVD GaN template
at E ¼ 8 keV. The calculated DBAR spectra for GaN lattice, Ga vacancy,and N vacancy are depicted by curved lines. The upper horizontal axis
denotes the gamma-ray energy corresponding to the electron momentum.

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Appl. Phys. Lett. 102, 142406 (2013)

FIG. 4. Coincidence DBAR spectra obtained for(c) undoped and (d) Si-doped GaCrN films atE ¼ 8 keV. The original measured spectra are normalized to the spectrum of MOCVD GaN. The calculated DBAR spectra for the various defect species
are depicted by curved lines.

vacancy defects are present in both GaCrN samples. That is,an enhancement is seen in the low momentum region(p < 5  103 m0 c) and a suppression is observed in the highmomentum region (p > 10  103 m0 c). In the spectrum ofthe Si-doped GaCrN sample, an additional shoulder isobserved at around p ¼ 10  103 m0 c. This shows that thedefect species of the Si-doped GaCrN sample is differentfrom that of the undoped one.
To characterize vacancy defects in the GaCrN samples,as shown in Figs. 4(a) and 4(b), the DBAR spectra werecalculated assuming various defect species. These spectra
are normalized to that of perfect GaN lattice. In the case of
the undoped GaCrN sample [Fig. 4(c)], the experimental
spectrum is not reproduced considering small vacancies
shown in Fig. 4(b), because the amplitude of NðpÞ=Nref ðpÞ issignificantly enhanced. This indicates that the size of thevacancy defects contained in the undoped GaCrN sample islarger than V2. Therefore, we compare the experimental
DBAR spectrum with larger vacancy clusters shown in Fig.
4(a). Here, V6-planar denotes a six-membered ring-like vacancy cluster. In Fig. 4(c), the amplitudes of the calculated
spectra for V6, V6-planar, V8, and V12 around NðpÞ=Nref ðpÞ
are adjusted to 0.70, 0.80, 0.75, and 0.55 times, respectively.
The amplitude of the experimental spectrum is smaller thanthose of the calculated spectra for vacancy clusters. This
indicates that only a part of implanted positrons localizes atvacancy clusters and the others delocalize in the lattice. The
experimental spectrum is well reproduced with these calculation results. In particular, the experiment closely matches thecalculated results for V6 and V12.
In covalently bound materials, such as semiconductors,
vacancy clusters Vn that have particular magic numbers nhave been reported to be stable, because a low number ofdangling bonds (DBs) is considered to be favorable.21–24 In
wurtzite GaN crystals, V2, V4, V6, V8, V10, and V12 defects
have 6, 10, 10, 14, 18, and 18 DBs, respectively. This meansthat the total number of DBs is significantly reduced without

increasing the number of DBs in each vacancy cluster byreactions of V4 þ V2 ! V6 or V10 þ V2 ! V12 . That is, theV2, which has 6 DBs, can be eliminated by combining the V2
with the V4 (V10), which has 10 DBs (18 DBs) and becoming
the V6 (V12), which has the same 10 DBs (18 DBs). Hence,
during crystal growth, V4 and V10 defects will develop into
more stable V6 and V12 defects, respectively. This is also
consistent with the prediction that the vacancy clusters Vn
have particular magic numbers to minimize the number ofDBs. From these results, the major positron trapping centersin the undoped GaCrN sample can be identified as vacancy
clusters, and furthermore the observed experimental spectrum might be attributed to V6 or V12.
In the case of the Si-doped GaCrN sample [Fig. 4(d)],the experimental spectrum is not reproduced consideringlarge vacancy clusters. We calculated the theoretical DBAR
spectra assuming Ga vacancy (VGa ), N vacancy (VN ), Gavacancy-Cr-impurity (VGa -CrN ), Ga-vacancy-Si-impurity(VGa -SiN ), Ga-vacancy-Ga-antisite (VGa -GaN ), N-vacancyCr-impurity (VN -CrGa ), N-vacancy-Si-impurity (VN -SiGa ),N-vacancy-N-antisite (VN -NGa ) complexes, and divacancy(V2). VGa -CrN ; VGa -SiN , and VGa -GaN complexes might not
be potential candidates, because Cr and Si atoms have beenreported to substitute Ga sites in GaN (Refs. 20 and 25) anda VN ! VGa -GaN reaction cannot occur because the Ga-Ga
bond is weaker than the Ga-N bond.26 Therefore, the calculated spectra for VGa ; VN ; VN -CrGa ; VN -SiGa ; VN -NGa , and V2
are depicted in Fig. 4(b) and compared with the experiment[Fig. 4(d)]. In Fig. 4(d), the amplitude of the calculated spectrum for V2, around NðpÞ=Nref ðpÞ, is also adjusted by 0.58times. This again means that only a part of positrons wastrapped at divacancies. Consequently, the experiment isfound to be most well reproduced considering V2.
(The above-excluded candidates (VGa -CrN ; VGa -SiN , andVGa -GaN complexes) were also not compatible with theexperiment.) Thus, the major positron trapping centers in theSi-doped GaCrN sample can be identified as divacancies.

142406-4

Yabuuchi et al.

Ga-site-substituted Si are proposed to trap second-nearestneighbor Ga vacancies and consequently to form a VGa -SiGacomplex.27 The formation of the VGa -SiGa complexes will inhibit migration of VGa during crystal growth. Probably, thiscauses the suppression of vacancy aggregation, and as a result,the defects in the GaCrN layer stay in the form of divacancies.
In this study, defects in low-temperature-grown GaCrN
samples have been investigated by positron annihilation
spectroscopy. Vacancy clusters were detected in the undoped
GaCrN sample. These vacancy clusters could be effectively
removed by Si-doping; however, divacancies still survived.
In addition to Si-doping for reducing vacancy clusters, postgrowth annealing techniques for eliminating residual divacancies are needed to establish the low temperature growthtechnology of GaCrN films.
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