Defect acceptor and donor in ion-bombarded GaN
Mladen Petravic, Victoria A. Coleman, Ki-Jeong Kim, Bongsoo Kim, and Gang Li
Citation: Journal of Vacuum Science & Technology A 23, 1340 (2005); doi: 10.1116/1.1991869
View online: http://dx.doi.org/10.1116/1.1991869
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Defect acceptor and donor in ion-bombarded GaN
Mladen Petravic and Victoria A. Coleman
Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
Ki-Jeong Kim and Bongsoo Kim
Department of Physics and Pohang Accelerator Laboratory, Pohang University of Science
and Technology, Pohang, Kyungbuk 790-784, Korea
Gang Li
ShenZhen Fangda GuoKe Optronics Technical Co. Ltd., FangDa Town, Longjing, Xili, Nanshan, ShenZhen,
China 518055
共Received 7 March 2005; accepted 31 May 2005; published 22 July 2005兲
We have employed synchrotron-based core level photoemission measurements and near-edge x-ray
absorption fine structure spectroscopy to identify and characterize nitrogen interstitials in p-type
GaN, created by nitrogen bombardment. From absorption measurements around the nitrogen K edge
we have identified nitrogen interstitial levels within the band gap, in good agreement with
theoretical predictions. The reduction in band bending determined from photoemission
measurements was explained by the acceptor-like character of these defects. Argon bombardment
produces nitrogen vacancies and the metallic Ga phase at the surface, which will produce the
increased band bending and pinning of the surface Fermi level closer to the conduction band
minimum. © 2005 American Vacuum Society. 关DOI: 10.1116/1.1991869兴
I. INTRODUCTION
The study of point defects in GaN and related nitride
semiconductors has been attracting considerable interest over
the past decade, motivated primarily by the important application of these materials in blue and ultraviolet optoelectronic and high-frequency microelectronic devices.1 It is essential to determine the properties of point defects such as
vacancies, interstitials, or antisites, that are present in the
material or introduced into it during various processing steps,
as these defects may control some vital properties of nitride
semiconductors, ranging from the type of conductivity to the
dopant diffusion. For example, it has been established, both
theoretically2,3 and experimentally,4 that native donor defects
such as nitrogen vacancies, can be responsible for the n-type
conductivity of as-grown undoped GaN.
Several intrinsic defects in GaN have been identified and
characterized theoretically.2,3,5–7 However, while donor centers such as nitrogen vacancies, VN, are relatively well understood and documented, the properties of acceptor states
are much less studied and understood. In addition, there are
not many reports in literature on direct experimental observation and characterization of point defects in GaN. For example, high-pressure optical measurements have shown only
indirectly the presence of VN in n-type GaN samples,8 while
Hall effect measurements on GaN samples have been consistent with the donor nature of VN.4 Only recently, a direct
experimental identification and characterization of interstitial
Ga by optical detection of electron paramagnetic resonance
has been reported for n-GaN samples.9
In the previous studies,4,9 point defects in GaN were created by high-energy electron irradiation. However, point
defects such as VN or nitrogen interstitials, NI, are
known to form easily under ion-bombardment of nitride
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J. Vac. Sci. Technol. A 23„5…, Sep/Oct 2005
semiconductors.10 Both NI and VN can be created simultaneously within the collisional cascade by displacement of
nitrogen atoms from their original crystalline sites to interstitial positions. In contrast to electron bombardment, one
expects a different vacancy/interstitial ratio for different
types of ions used for sputtering. While, for example, argon
bombardment may preferentially remove nitrogen from
GaN,11,12 thus creating an excess of nitrogen vacancies, nitrogen bombardment may increase the nitrogen content in
the near-surface region13 and possibly create more nitrogen
interstitials.
In the present article we report on observation of VN and
NI and identify their role in the band structure of p-GaN,
using synchrotron-based photoemission and absorption techniques. We have employed both photoemission spectroscopy
around the valence edge and core levels, and near-edge x-ray
absorption fine-structure 共NEXAFS兲 spectroscopy around the
nitrogen K edge to determine changes in the electronic structure of bombarded surfaces and to identify the defects responsible for these changes.
II. EXPERIMENT
The sample used in this study was a p-type 共Mg-doped,
5 ⫻ 1019 Mg/ cm3兲 GaN共0001兲 epilayer, 2 ␮m thick and
grown on a sapphire substrate by metallorganic chemical vapor deposition. This sample, transported in air to the synchrotron facility without receiving any chemical or ion-beam
precleaning prior to measurements, is referred to as “asgrown” sample.
All measurements were performed in an ultrahigh vacuum
chamber attached to the 2B1 beam line of the Pohang Light
Source, equipped with a hemispherical electron analyzer
共Gammadata SES 100兲 for photoemission measurements and
0734-2101/2005/23„5…/1340/6/$22.00
©2005 American Vacuum Society
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Petravic et al.: Defect acceptor and donor in ion-bombarded GaN
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FIG. 1. Relative concentration ratio of Ga and N, determined from the Ga 3d
and N 1s core-level photoemission, from ion-bombarded p-GaN surface:
closed circles for 2 keV Ar+ bombardment for 15 min and open 共closed兲
triangles for 2 keV N+2 bombardment for 15 共30兲 min.
a low-energy ion-gun 共placed at the distance of ⬃20 cm
from the sample surface兲 for sample bombardment with Ar+
and N+2 ions at normal incidence.
Photoemission and NEXAFS were measured at room
temperature with an energy resolution of ⬃3000. Photoemission was taken around both the valence band and Ga 3d core
level, and separately, the N 1s core level. NEXAFS spectra
were recorded in the total electron yield 共TEY兲 mode around
the nitrogen K edge at various angles of incidence as measured from the sample surface. The photon energy scale was
calibrated against the C 1s peak at 284.6 eV or the Au 4f 7/2
peak at 84 eV. The position of the Fermi level was determined by synchrotron-based ultraviolet photoelectron spectroscopy on a clean gold film in electrical contact with the
GaN samples.
The photoemission spectra were normalized to the photoelectron current of the primary photon, measured simultaneously on a Au grid, and then simulated with several sets of
mixed Gaussian–Lorentzian functions with Shirley background subtraction.11,14 The raw NEXAFS data were also
normalized prior to fitting procedure and then fitted with
Gaussian line shapes in order to simulate the resonant transitions to bound final states observed in the near-edge
spectra.11,15
III. RESULTS AND DISCUSSION
A. Surface composition
Changes in surface stoichiometry of ion-bombarded GaN
can be described by the Ga to N ratio, determined from the
photoemission measurements around Ga and N core levels.
In general, Ga/ N has been calculated from integrated emission intensities of Ga 3d and N 1s core levels. However, this
ratio is relative as it does not take into account photoionization cross sections and transmission functions of the spectrometer. Thus, in our measurements, we have normalized
the integrated ratio of Ga 3d and N 1s to the ratio obtained
from the as-grown sample.
FIG. 2. Normalized photoemission spectra for the valence band and Ga 3d
levels from as grown p-type GaN and surfaces bombarded by 2 keV N+2
ions.
The normalized ratio of the integrated intensities of Ga 3d
and N 1s emission curves is plotted in Fig. 1 as a function of
ion impact energy. Sputtering with argon ions is known to
remove nitrogen from the GaN surface, thus creating nitrogen vacancies and generating gallium rich surfaces. In addition, a prolonged sputtering may lead to the formation of
metallic Ga clusters, or even a thin Ga film on the
surface.11–13 In both cases, the Ga/ N ratio is expected to
increase with Ar+ energy following the preferential loss of
nitrogen from the surface. For example, in the present experiment, the Ga/ N ratio on the surface bombarded with
2.5 keV Ar+ was about three times higher than the corresponding ratio on the as-grown surface.
By contrast, sputtering with nitrogen ions may drastically
suppress the reduction in nitrogen concentration at the surface 共as evident from Fig. 1兲 as the nitrogen from the ion
beam replaces the preferentially removed nitrogen and restores N–Ga bonds.13 In addition, a fraction of impinging
nitrogen ions, together with atoms displaced from their original crystalline sites, may remain embedded in the material at
interstitial positions, creating surfaces that are N rich.
B. Photoemission and band bending
One of the most accurate methods of determining shifts in
the surface Fermi level and corresponding band bending is
the measurement of the energy position of some characteristic core-level peaks.16 In the case of Ga-based compound
semiconductors, such as GaAs or GaN, the shift in the Ga 3d
peak position is often used for determination of band
bending.14,17–19
In Fig. 2 we show normalized photoemission spectra for
the valence band and Ga 3d core level taken by 70 eV photons from the as-grown surface of p-GaN, and from the same
surfaces after 2 keV N+2 or Ar+ bombardment. We first note
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Petravic et al.: Defect acceptor and donor in ion-bombarded GaN
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FIG. 3. Schematics of the bands of the as-grown p-GaN surface and surfaces
bombarded with 2 keV N+2 or Ar+ ions.
that the valence band maximum 共VBM兲 of the as-grown pGaN sample is located at ⬃1.65 eV below the Fermi level.
This value is within the range of 1.4– 1.7 eV reported in
Refs. 20 and 21, but is higher than the value of 1.1– 1.2 eV
from Ref. 19. Indeed, one can expect the difference in the
VBM position on “clean” and contaminated GaN surfaces, as
different surface states and/or surface oxides can cause the
error up to 0.5 eV in position of the VBM.18,21
The position of the VBM on the as-grown surface from
Fig. 2 indicates a downward surface band bending of about
1.35 eV. Here, we have assumed an activation energy of
0.3 eV for Mg dopants in p-GaN,19,22 that would place the
Fermi level at 0.3 eV above the VBM for the flatbands. This
situation is shown schematically in Fig. 3.
In Fig. 4, we display the photoemission curves around the
Ga 3d peaks. All emission peaks were deconvoluted into
several doublets with a spin-orbit splitting of 0.44 and
branching ratio of 0.60.11,14 The Ga 3d emission peaks from
as-grown samples were fitted with two or three doublets
originating from either GaN 共A; Ga from GaN兲, gallium oxynitride 共B; Ga from GaOxNy兲 or gallium oxide 共C; Ga from
GaxOy兲, with the binding energy 共BE兲 of B and C shifted
towards higher binding energy by ⬃0.7 and ⬃1.2 eV,
respectively.11,13 All shifts mentioned in the present article
are related to the Ga 3d5/2 position of the bulk component A.
The zero position on the relative binding energy scale corresponds to the bulk component A of the as-grown sample.
The presence of peaks B and C in Fig. 4 indicates the
presence of gallium oxides and oxynitrides on the GaN surface even after the ion bombardment. Indeed, our photoemission spectra exhibit a strong emission from O 1s levels 共not
shown兲. It is well known that GaN samples grown by metalorganic chemical vapor deposition may contain a large
amount of oxygen.23 In addition, the high reactivity of GaN
surface to H2O may also be the source of oxygen contamination on bombarded surfaces, even at the characteristic base
pressure during bombardment in our experiments of
FIG. 4. Normalized photoemission spectra for Ga 3d levels 共open circles兲
from Fig. 2, plotted on a relative binding energy scale, together with numerical fits 共solid and broken lines兲.
10−9 Torr.11 On the other hand, a strong C 1s emission from
the as-grown surface 共not shown兲 is completely absent in
photoemission from ion-bombarded surfaces.
The emission from surfaces bombarded with 2 keV N+2
ions requires an additional doublet 共D兲, shifted by 0.7 eV
towards lower BE, to achieve the best fit of experimental
curves 共see Fig. 4兲. We associate D with the production of
uncoordinated Ga atoms during ion bombardment. Namely,
the ion bombardment of GaN surface can cause breaking of
Ga–N bonds and the production of Ga atoms bonded to
fewer electronegative N atoms in the ion-damaged surface
region of GaN. The change in chemical environment of Ga
causes a spatial redistribution of valence electrons on the Ga
atom, creating more screening of the Ga nucleus and, consequently, a less attractive potential seen by core electrons. As
a result, the binding energy for core-electrons decreases,
which explains the position of D in Fig. 4. At the same time,
the valence band spectrum 共see Fig. 2兲 exhibits a new feature
at ⬃1.3 eV below the Fermi level, which we also attribute to
the presence of uncoordinated Ga 共Ga atoms bonded to fewer
electronegative N atoms兲 at the GaN surface.24
The Ga 3d emission after 2 keV Ar+ bombardment exhibits an additional peak, E, shifted towards a lower binding
energy by ⬃1.3 eV 共see Fig. 4兲. The shift in binding energy
of this component corresponds to the chemical shift from
elemental Ga11,25 and we attribute this peak to the formation
of metallic Ga on the Ar-bombarded GaN surface.
We also note in Fig. 2 that the feature observed in the
valence band spectrum of the N2-bombarded surface develops into a well pronounced peak in the case of argon bombardment. Therefore, we associate this peak with the development of a metallic Ga phase at the GaN surface under Ar
bombardment.
The most interesting feature in Figs. 3 and 4 is, however,
the shift of the Ga 3d peak in opposite direction on nitrogen-
J. Vac. Sci. Technol. A, Vol. 23, No. 5, Sep/Oct 2005
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Petravic et al.: Defect acceptor and donor in ion-bombarded GaN
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and argon-bombarded surfaces. The Ga 3d peak shifts towards a lower BE by 1.09 eV on the surface bombarded with
2 keV N+2 ions. This shift corresponds to movement of the
surface Fermi level closer to the VBM by 1.09 eV, i.e., to
less surface band bending, as shown schematically in Fig. 3.
On the other hand, the Ga 3d peak shifts towards a higher
BE by 1.0 eV after 2 keV Ar+ bombardment, indicating
movement of the surface Fermi level closer to the conduction
band minimum 共CBM兲 by 1.0 eV, or more surface band
bending.
Assuming the band gap of 3.4 eV for the p-GaN sample,1
the total band bending is 0.26 eV after 2 keV N+2 bombardment and 2.35 eV after 2 keV Ar+ bombardment 共see Fig. 3兲.
C. Surface donor states
Before offering a possible explanation for opposite shifts
of the Ga 3d peaks under nitrogen and argon bombardment,
we recall that some point defects may act as acceptors or
donors in GaN,2,3 affecting the band structure of p-GaN just
in the way shown in Fig. 3.26 The prime candidates for donors and acceptors in GaN are nitrogen vacancies, VN, and
nitrogen interstitials, NI, respectively. They are known as
common native defects in GaN,2,31 but can be also produced
very efficiently during ion bombardment.6
Nitrogen vacancies have been identified as shallow donors in several theoretical and experimental studies of defects in GaN.2,3,6,7 They may form singlet states about 0.8 eV
above the CBM.2 When purposely created at the surface during electron or ion bombardment, VN will increase the electron concentration in the surface layer4 and destroy the
charge neutrality in its vicinity. Positive charge of such surface donors can be compensated by an equal amount of negative space charge 共bulk acceptors in p-GaN兲 within a depletion layer, which will cause a downward band bending.26
Preferential sputtering of N from GaN surfaces under low
energy Ar+ bombardment has been documented quite well in
the literature.11–13 It produces nitrogen vacancies within the
range of Ar ions, and a metallic Ga phase at a higher Ar
implantation dose and/or energy. Our present results, shown
in Figs. 1 and 2, are in full agreement with this picture.
Therefore, we argue that some donor-like VN states, introduced by Ar+ bombardment into the surface layer of pGaN, are responsible for the shift of the surface Fermi level
towards CBM and more band bending.
One should also consider the possible effect of the metallic Ga phase on the band bending at the surface of p-GaN. It
is well known that the deposition of metals causes band
bending on both n- and p-type semiconductors even at metal
coverage far below one monolayer.27 One of the models
共known as defect model兲 takes into account the defect states,
such as vacancies and antisites, that can be introduced at the
metal/semiconductor interface by metal deposition, in order
to explain the Fermi level pinning within the band gap.28 If
donor-like defects are introduced at the interface between the
metallic Ga and p-GaN, a depletion layer will form causing
more band bending and pinning of the Fermi level closer to
the CBM.
FIG. 5. N K-edge NEXAFS spectra from as-grown and nitrogen-bombarded
p-GaN, taken for photon impact angle of 30° and 90° as measured with
respect to the surface.
D. Surface acceptor states
On the other hand, nitrogen interstitials have been identified in several theoretical studies as acceptor-like defects
with energy levels within the energy gap.2,3,6,7 For example,
there are two high-symmetry positions for NI in GaN, with
two or six first neighbors, that introduce partially occupied
p-like states at 1.4 and 1.9 eV above the VBM, respectively.7
In addition, the most stable NI configuration in wurtzite GaN
is the split-interstitial structure 共with a partially occupied
level at 1.1 eV above the VBM兲, in which two N atoms share
the same substitutional lattice site.3,6 Finally, a channelcentered interstitial configuration introduces a doublet occupied by one electron at ⬃1.5 eV above the VBM.6 The presence of these acceptor-like interstitials would shift the
surface Fermi level closer to the VBM on p-type GaN, causing less band bending.
One can also treat the nitrogen interstitials at the surface
as negatively charged acceptor states that would introduce an
accumulation layer 共positively charged holes兲 below the
p-type GaN surface. Such an accumulation layer will reduce
the band bending or even cause an upward band bending on
p-type material.26
In general, the NI-type defect that introduces p states in
the energy gap of a semiconductor can be identify directly by
NEXAFS measurements around the N K edge.29 This is illustrated in Fig. 5 by typical NEXAFS spectra, recorded in
the TEY mode around the nitrogen K edge, from the asgrown p-GaN surface and the same surface bombarded with
2 keV N+2 .
First of all, we note in Fig. 5 the strong polarisation dependence of characteristic features in NEXAFS spectra of
the as-grown sample, characterized by some ␲* resonances
共peaks P1 and P3 that are more pronounced for an oblique
impact angle of photons兲 and ␴* resonances 共peaks P2, P4,
and P5, that are more pronounced for the normal photon
impact兲.11,15,30 After nitrogen bombardment, the NEXAFS
spectra become broader with less pronounced resonant transitions P1–P3. This is characteristic of the increased amount
of disorder within the surface region of GaN induced by ion
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Petravic et al.: Defect acceptor and donor in ion-bombarded GaN
FIG. 6. N K-edge NEXAFS spectra from as-grown and nitrogen-bombarded
GaN, taken for photon impact angle of 30°. The solid line is a numerical fit
of experimental curves 共open circles兲 with several Gaussians 共G兲 used to
simulate resonant transitions.
bombardment.29 In addition, two new resonant features,
showing a strong ␲ character, are clearly distinguished on
ion-bombarded surface 共peaks P6 and P7 in Fig. 5兲.
Finally, in Fig. 6 we show NEXAFS spectra taken at 30°
from both as-grown and N2-bombarded p-GaN, which have
been deconvoluted into a number of Gaussians 共only three
are shown in Fig. 5 for the as-grown sample兲 to simulate
several resonant electron transitions from the N 1s initial
state to the final, low lying states of p symmetry in the conduction band or within the energy gap.11,15
Keeping the position of original Gaussians G1–G3 at
fixed energies, the good fit of spectrum from the ionbombarded sample was possible only by introduction of several new Gaussian functions: 共i兲 a sharp resonance, G4,
emerging around 0.2 eV below the original peak G1 and
around 1 eV above the absorption edge, 共ii兲 a resonance G5
at 397.4 eV, i.e., 2.1 eV below the absorption edge, and finally 共iii兲 a resonance G6 at 398.1 eV 共1.4 eV below the
absorption edge兲. The presence of these additional curves
and their peak positions were determined from the derivative
of the experimental curve, while the absorption edge of
399.5 eV was taken at the inflection point of the leading
edge of NEXAFS spectrum of the as-grown sample.
To identify the origin of new resonances G5 and G6, we
note that their energy positions fall below the nitrogen absorption edge, clearly indicating the location of the final
empty states of p symmetry for these resonant transitions
within the energy gap of GaN. For a gap of 3.4 eV, the
energy position of G5 and G6 is at 1.3 and 2.0 eV above the
VBM, respectively. As these values are within the range of
calculated levels for NI states in GaN,2,3,6,7 we associate
peaks G5 and G6 to nitrogen interstitials formed by lowenergy nitrogen bombardment. We stress again that our pho-
1344
toemission measurements 共see Figs. 2–4兲 are in full agreement with the acceptor-like character of these defects.
It is interesting to mention here that some molecules, such
as CO, O2, or C2H4, chemisorbed on metals, can also act as
acceptors or donors 共they form bonds with the surface atoms
in which electrons are transfer from the surface to the molecule or vice versa兲.31 Acceptor bonds exhibit a strong ␲
character in NEXAFS 共they are known as “␲*-acceptor
bonds”兲,31 quite similar to acceptor-like nitrogen interstitials
in our NEXAFS measurements from Figs. 5 and 6.
Turning finally to the resonance G4 in Fig. 6, we first
argue that the buildup of interstitial nitrogen in GaN under
nitrogen bombardment eventually produces molecular nitrogen, N2, trapped at interstitial positions. Therefore, we associate the sharp resonance G4 with the molecular nitrogen
formed in GaN under N+2 bombardment. We note that this
assignment is in agreement with some previous NEXAFS
studies found in literature. For example, a spectrum from a
thin amorphous GaN films prepared by an ion-assisted deposition using 0.5 keV nitrogen beam exhibits a strong peak at
position similar to G4.32 This peak has been associated with
the presence of nitrogen bubbles in amorphous GaN film, but
no explanation was given for the origin of this effect.32 A
very intensive resonant peak at position similar to G4, that
has been tentatively associated with the molecular nitrogen,
has also been observed in NEXAFS spectra of InSb and InAs
bombarded by 500 eV N+2 .33
IV. CONCLUSION
In conclusion, our NEXAFS measurements around the N
K edge of p-GaN have provided the evidence for existence
of interstitial nitrogen in samples bombarded by low-energy
nitrogen ions. From the shift of the surface Fermi level towards the VBM, measured by core-level photoemission, we
have determined the acceptor-like character of nitrogen interstitials in p-GaN. On the other hand, low-energy argon
bombardment produces nitrogen vacancies that act as donors, causing more band bending in p-GaN and pinning of
the surface Fermi level closer to the CBM.
ACKNOWLEDGMENTS
This work was supported by the Australian Synchrotron
Research Program, which is funded by the Commonwealth
of Australia under the Major National Research Facilities
Program. Experiments at the Pohang Light Source were also
supported through the X-ray-particle-beam Nanocharacterization Program by MOST and POSTECH Foundation, Korea.
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JVST A - Vacuum, Surfaces, and Films
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