Electrically active point defects in Mg implanted n-type GaN grown by metal-organic
chemical vapor deposition
G. Alfieri, V. K. Sundaramoorthy, and R. Micheletto

Citation: Journal of Applied Physics 123, 205303 (2018); doi: 10.1063/1.5029254
View online: https://doi.org/10.1063/1.5029254
View Table of Contents: http://aip.scitation.org/toc/jap/123/20
Published by the American Institute of Physics

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JOURNAL OF APPLIED PHYSICS 123, 205303 (2018)

Electrically active point defects in Mg implanted n-type GaN grownby metal-organic chemical vapor deposition
G. Alfieri,1 V. K. Sundaramoorthy,1 and R. Micheletto2
1

ABB Corporate Research, Segelhofstrasse 1K, 5405 Baden-D€
attwil, Switzerland
Department of Nanosystem Sciences, Yokohama City University, 236-0027 Yokohama, Japan

2(Received 12 March 2018; accepted 10 May 2018; published online 24 May 2018)
Magnesium (Mg) is the p-type doping of choice for GaN, and selective area doping by ion
implantation is a routine technique employed during device processing. While electrically active
defects have been thoroughly studied in as-grown GaN, not much is known about defects generated
by ion implantation. This is especially true for the case of Mg. In this study, we carried out an electrical characterization investigation of point defects generated by Mg implantation in GaN. We havefound at least nine electrically active levels in the 0.2–1.2 eV energy range, below the conduction
band. The isochronal annealing behavior of these levels showed that most of them are thermally stable up to 1000  C. The nature of the detected defects is then discussed in the light of the results foundin the literature. Published by AIP Publishing. https://doi.org/10.1063/1.5029254

I. INTRODUCTION

While n-type conductivity in GaN can be routinely
achieved by Si-doping, p-type doping efficiency is still an
issue. Magnesium is typically employed for p-type doping,and it is incorporated during growth. Mg occupies a Ga-site(MgGa) and gives rise to an acceptor level at 0.2 eV above
the valence band edge (EV).1,2 The reported activation ratio
is 1%–2%,3,4 meaning that high concentrations of Mg must
be incorporated into GaN in order to obtain, at room temperature, a hole concentration that is acceptable for device
applications. To complicate matters even further, H passivation, as well as compensation due to nitrogen vacancies(VN)5 and Mg interstitials (Mgi),1 must be taken into accountwhen dealing with Mg doped GaN.
Selective area doping is a common process in semiconductor device manufacturing for the formation of pþ regions,and GaN devices are no exception.6,7 It is known that ion
implantation leads to the formation of point defects which
can give rise to electrically active levels. Electrically active
point defects can undermine the correct functionality of thedevice, which is why post-implantation heat treatments are
required to anneal out such defects. For device processing,pþ regions are formed by multiple ion implantation (boxprofile implantation) and such defects are also found at the
implantation tail of the box-profile where concentrations ashigh as 1015 cm3 have been reported.8
Positron annihilation spectroscopy (PAS) studies9 haveshown that VN and gallium vacancies (VGa), formed after
Mg implantation, form clusters [(VGa)nVN] after annealing at
1000  C. However, Lardeau-Falcy et al.10 have shown thatan annealing temperature of at least 1000  C is necessary to
remove the presence of point defects generated by ion
implantation. Recent investigations on Mg implanted GaN
have reported that the density of deep levels was reduced
after annealing at 800  C.9 Nevertheless, devices that underwent annealing at 1400  C, with the use of AlN capping,showed a rather low breakdown voltage (300 V) despite
0021-8979/2018/123(20)/205303/6/$30.00

the thickness of the drift layer (10 lm). One of the causes
was attributed to the presence of a highly defective layer at
the junction.7
As evident from the above-mentioned literature, despite
the technological importance of Mg implantation for GaN
power devices, it is still not known what electrically active
defects arise after Mg implantation and how thermally stablethese defects are. In order to shed light on this topic, we present an electrical characterization and isochronal annealing
study of Mg implanted GaN.

II. EXPERIMENTAL DETAILS

1 lm thick Si-doped GaN epilayers (net donor concentration, Nd, 3–5  1016 cm3, [C]  1  1016 cm3) grown bymetal-organic chemical vapor deposition (MOCVD) on GaN
substrates were purchased from NTT (Japan). Samples werecleaned by a mixture of H2O:HCl, and after being rinsed
with deionized water, a Ti/Al (40/150 nm) bilayer was
deposited on the backside. Rapid thermal annealing (RTA)
for 5 min at 550  C leads to the formation of ohmic contacts.
Samples were then implanted with 200 keV Mg ions and adose of 1011 cm2, at room temperature. This dose, which is
low for device fabrication, was chosen in order that the
implanted region could simulate the implant tail region of abox-profile. For this reason, GaN epilayers have retained
their original n-type conductivity. After implantation,
Schottky contacts were formed by e-beam deposition of Ni(70 nm) on the frontside of the samples.
Electrical characterization was carried out by capacitance voltage (C-V) and Fourier-transform deep level transient spectroscopy (FT-DLTS).11 The reverse bias (Vr) for
FT-DLTS measurements was set to –5 V, to cover the
implantation profile, pulsed to 0 V by employing a filling
pulse of 1 ms and a period width of 0.2 s. FT-DLTS was performed in the 100–650 K temperature range, always startingfrom 100 K.

123, 205303-1

Published by AIP Publishing.

205303-2

Alfieri, Sundaramoorthy, and Micheletto

An isochronal annealing study of the electrically active
traps was performed between 100 and 1000  C, with a time
step of 1 min, by using RTA in vacuum. This choice was
motivated by the fact that RTA treated Mg doped GaN leads
to a similar hole concentration, like a traditional furnace, but
has higher hole mobility.12 For samples annealed up to400  C, the highest measurement temperature coincided withthe annealing temperature. Heat treatments from 600  C
were performed by first removing the Ni metal contacts byaqua regia and the Ti/Al bilayer by HF 5% aqueous
solution.
III. RESULTS AND DISCUSSION

Figure 1(a) displays the Mg implantation profile and thevacancy concentrations obtained by full-cascades Monte
Carlo simulations.13 The displacement energies of Ga and N
are 39 eV and 17 eV, respectively.14 In Fig. 1(a), Nd, obtainedby C-V measurements on the as-grown and implanted samples, is also shown. As can be seen, the Mg implantation profile has a maximum concentration of 4  1015 cm3 at0.25 lm, while the concentrations of both VGa and VN are
almost three orders of magnitude higher.
After implantation, the Nd profile is detected beyond theimplanted Mg profile, at a depth of 0.5–0.6 lm. After annealing at 300  C, Nd is slightly changed, but only after annealing
at 400  C, Nd is reactivated near the implanted area. Annealing
at 500  C results in the recovery of Nd to the as-grown conditions. This is more evident after heat treatments at 800  C.
The reason why Nd is only detected outside the implantedarea, after implantation, can be explained in terms of formationof compensating centers. Figure 1(b) shows 1/C2 versus Vr forthe as-grown, as-implanted, and annealed samples. As it canbe seen, the C2-Vr relationship, for the as-grown sample, islinear in agreement with the uniform Nd distribution over thestudied depth range. After implantation, the capacitance isreduced and C2–Vr is not linear anymore. As a matter of fact,
close to the surface (low Vr), the capacitance is independent of

J. Appl. Phys. 123, 205303 (2018)the voltage, suggesting that implantation leads to a compensated layer. This layer is gradually removed by annealing( 500  C), and the linearity of C2-Vr is restored meaningthat the compensating species are either annealed out or indiffuse in the bulk. Interestingly, the temperature (500  C) at
which Nd approaches the as-grown profile (or C2-Vr becomeslinear) is the same temperature at which the electron paramagnetic resonance (EPR) signal of VGa was reported to anneal
out.15 Furthermore, it was recently reported that at 500  C,
the carrier concentration of Mg implanted metal-organic vapor
phase epitaxy (MOVPE) GaN recovers to the as-grownlevels.16
In order to identify those defects responsible for compensation, Saarinen et al.17 have shown that VGa is the main compensating center in N-rich MOCVD GaN. This is also
confirmed by theory that predicts a lower formation energy ofthe single negative charge state of VGa in N-rich rather thanGa-rich conditions.18 However, its formation energy is positive (2.5 eV) and this makes [VGa] limited. For this reason,
it can be thought that it is not VGa alone the reason for carrier
compensation but rather VGa jointly to other defects, such as
interstitials, antisites, or VGa-related complexes.
Ga or N antisites (GaN and NGa) or even N interstitials(Ni) and VGa behave as acceptors in n–type GaN.5 However,the formation energy of antisites and interstitials is so high(5 eV), in n–type GaN, that their concentration is negligible. It can be thought that divacancies (VGaVN) can form
after implantation. These behave as acceptors.19 However,PAS measurements9 on Mg implanted GaN have shown that
VGaVN forms only after high temperature heat treatments(1000  C). VGa(VN)2 were found after implantation,9 but
nothing is known about their donor/acceptor nature. Carbon,
which is also present in high concentrations in our samples,
can also form complexes with either VGa or VN, but suchcomplexes possess very high formation energies.20 A moreplausible hypothesis would be the formation of Si-related
complexes. As a matter of fact, it was shown that a
VGa-complex, formed with Si, gives rise to an acceptor in

FIG. 1. (a) Simulated Mg and VGa, VN profiles, as obtained by SRIM (solid/dashed lines). This is plotted together with the net-donor profiles in the as-grown,
as-implanted, implanted, and annealed GaN epilayers, at 300, 400, 500, and 800  C (squares). The Mg implanted profile was multiplied by a factor of 100, forclarity. (b) Measured C2-Vr characteristics for the as-grown, as-implanted, and 500 and 800  C annealed samples.

205303-3

Alfieri, Sundaramoorthy, and Micheletto

J. Appl. Phys. 123, 205303 (2018)

n-type GaN and has a negative formation energy21 meaningthat it can be very abundant in n-type GaN.
Figure 2 shows the DLTS spectra of the as-grown, asimplanted, and annealed GaN epilayers. In the as-grown
MOCVD GaN [Fig. 2(a)], two traps are found at 0.20 and
0.56 eV below the conduction band edge (EC) and labeled M1
and M2, respectively. M1 has a concentration of 1013 cm3and a capture cross section of 1016 cm2, while M2 has a concentration of 4.2  1014 cm3 and a capture cross section of
1.5  1016 cm2. In addition, although not shown here,
annealing the as-grown sample at higher temperatures(>500  C) also revealed the presence of only M1 and M2.
After implantation [Fig. 2(a)], another level is found, labeled
M3, at EC – 0.63 eV (6  1017 cm3 and 3.5  1016 cm2). It isinteresting to note that the M1 level is not detected after
implantation. Annealing at 300  C [Fig. 2(b)] leads to the formation of another level, M4 (EC – 0.70 eV, 1  1016 cm3, and1014 cm2). The DLTS peak labeled M4 spans on a temperature range of 200 K, meaning that it may be formed by theoverlapping of different contributions, e.g., the M3 level. In
order to uncover such contributions, we applied thefollowing:11
S¼

NT Cst TW =s
2p=TW
ðe
 1Þ
;
2
TW ND
1=s þ ð2p=TW Þ2(1)

where NT, Cst, and TW are the trap concentration, the steadycapacitance under the reverse bias condition, and the period
width, respectively, while s ¼ rvth1NC eEC ET =kT with r, vth, NC,
ET, and k being the capture cross section, thermal velocity,
effective density of states, activation energy, and Boltzmann
constant, respectively.
Figure 2(b) shows that the sum (red solid line) of the M3
and M4* (EC – 0.69 eV and 3.8  1016 cm2) levels (reddashed lines), obtained by using Eq. (1), reproduces the M4
DLTS signal quite well. Subsequent heat treatments at 400  C[Fig. 2(c)] reveal the presence of a broader DLTS peak.
This peak is labeled M5 (EC – 0.68 eV, 1.3  1016 cm3, and1016 cm2) and it is too broad to be assigned to a single level.

Being this DLTS peak wider than that found after annealing at
300  C (250 K wide) means that it may be due to the overlapping of the M4 DLTS peak and other contributions. We
subtracted the two DLTS signals detected after 400 and
300  C. The result is shown as white circles in Fig. 2(c). By
using Eq. (1), it can be seen that this peak is given by the sum(solid red line) of two DLTS peaks, M5* and M5**, locatedat EC – 0.65 eV and EC – 0.72 eV, respectively (dashed redlines).
In Fig. 2(d), the DLTS spectrum of the Mg implanted
GaN annealed at 500  C is shown. Besides the M1 level, theM4 is also clearly detectable. A new level is found at
230 K and labeled M6 (EC – 0.51 eV, 5  1014 cm3, and7  1014 cm2). A shoulder peak is found at 400 K, but theestimation of its energy position in the bandgap is rather difficult at this stage, and another peak, at 440 K, labeled M7,
is detected. The latter has the same temperature position of
the M5** peak obtained by our fitting, so they may be
assigned to the same level (EC – 0.75 eV, 2.4  1015 cm3,
and 1  1016 cm2). At 540 K, a peak labeled M8 is
assigned to a level located at EC – 1.2 eV (6  1014 cm3 and5  1015 cm2). After annealing at 800  C [Fig. 2(e)], theM1, M6, M4, and M8 levels are still present. In addition, thepreviously detected shoulder peak is now more clearly visible and labeled M9 (EC – 0.67 eV, 1  1015 cm3, 2  1015cm2). Similar to the case of M7 (which is annealed out at
this stage), the M9 temperature peak position is like that ofM5*. For this reason, they may be assigned to the same electrically active level.
The last annealing stage [Fig. 2(f)] shows that M1(1.1  1016 cm3), M6 (2.6  1014 cm3), M4 (5  1014 cm3),
M8 (2  1015 cm3), and M9 (7  1014 cm3) are still persistent. We can also detect a peak which was earlier assigned to
the M2 level (4  1014 cm3), found in the as-grown material.
All the detected trap did not show electric field dependence,suggesting that they are of acceptor type. A summary of thedetected traps is reported in Table I.
The trap M1 has been detected in GaN grown by
MOCVD,22–24 molecular beam epitaxy (MBE),25 and halide

FIG. 2. DLTS spectra of the (a) as-grown and as-implanted, (b) 300  C, (c) 400  C, (d) 500  C, (e) 800  C, and (f) 1000  C annealed samples. White dots represent the difference between two DLTS signals whereas red and dashed solid lines represent the analytical fit of the DLTS peaks. The period width is set to 0.2
s for all measurements.

205303-4

Alfieri, Sundaramoorthy, and Micheletto

TABLE I. Labeling, energy position in the bandgap (eV), capture cross section (cm2), and conditions for detection of the levels found in this study.

Label

EC–ET (eV)Capture crosssection (cm2)16

M1

0.20 6 0.01

(1 6 0.03)  10

M2

0.56 6 0.01

(1.50 6 0.04)  1016

M3

0.63 6 0.02

(3.5 6 0.1)  1016

M4
M5
M6
M7

0.70 6 0.01
0.68 6 0.01
0.51 6 0.01
0.75 6 0.01

(1 6 0.02)  1016(1.6 6 0.08)  1016(5 6 0.05)  1014(1 6 0.1)  1016

M8

1.12 6 0.02

(5 6 0.1)  1015M90.67 6 0.03

(2 6 0.2)  1015

Comment
In as-grown andimplanted materials
Found in the as-grownmaterial
Found in the as-implantedmaterial
Stable up to 1000  C
Detected after 400  C
Detected after 300  C
Same as M5**. Anneals
out at 800  C
Found after 500  C. Stable
up to 1000  C
Same as M5*. Stable
up to 1000  Cvapor phase epitaxy (HVPE)-grown material26 and labeled
in different ways, in the past. Its nature has been attributed toeither a divacancy (VNVGa) or an impurity related complex.27 We observe that, despite being present in theas-grown material, M1 cannot be detected right after implantation, when Nd is detected away from the implanted region.
However, it can be found again after annealing at 400  Cand, for subsequent annealing steps, the M1 is always present with a rather constant concentration. If M1 were related
to a divacancy, it would be reasonable to expect an increasein the concentration for subsequent annealing steps, e.g.,when Nd is reactivated inside the implanted area. Recent
hybrid density functional calculation (DFT) showed that
divacancies in GaN form acceptors much deeper than EC –0.20 eV.19 For these reasons, the hypothesis of an impurity(C or H)-related complex might be favored. As a matter offact, a DFT study performed by Matsubara and Bellotti20
reported a level at EC – 0.22 eV for the CGaVN complex.
Similar to M1, M2 has also been detected in GaN grownby MOCVD22–24 and HVPE.26 In the present study, the M2level is detected in the as-grown material and in theimplanted and 1000  C annealed samples. However, contrary
to the case of M1, M2 might have been present at eachannealing step with its signal buried under the broader M4
and M5 DLTS peaks. Furthermore, M2 is stable up to
1000  C in both electron irradiated GaN, where it waslabeled D326 and, although not shown here, also in as-grown
GaN. Regarding the microscopic structure of M2, NGa or a
C-related defect has been suggested. Since this trap can also
be found in HVPE grown GaN, where C contamination isabsent, the former hypothesis seems to be more realistic.
The M3 level found after implantation is stable up to
1000  C. Chen et al.24 have found an electron beam evaporation induced level, labeled E4, at EC–0.65–0.70 eV close to
the M3 level. In our investigation, metal contacts were also
performed by electron-beam evaporation so it would be
tempting to identify the M3 level as E4. However, the shapeof E4 is much broader than that of M3 and we would expect
to find E4 also in the as-grown material, which is not theJ. Appl. Phys. 123, 205303 (2018)

case. In addition, M3 is detected when the Nd profile
is >0.5 lm away from the surface. Another possibility would
be the identification of M3 with the D3 level detected by
Haase et al.28 in N-implanted GaN. Similar to the D3 level,also M3 is found after implantation and it is thermally stable
up to 900  C.
The energy position and thermal stability of the D4 level
found in as-grown and electron irradiated HVPE grown
GaN26 are similar to those of the M4 level. However, the M4level is not found in as-grown MOCVD GaN, meaning thatits nature may be different from that of the D4 level.
Polyakov et al.29 have reported a level close to M4 in Cr
implanted GaN and related it to a defect located in theimplanted region. However, M4 is found after annealing at
300  C when Nd is away from the Mg-implanted area. Thissuggests that the level generated by Cr-implantation and theM4 level have a different microscopic structure. A similarargument holds for the M5 level, which is very close inenergy position to the M4 level, resulting from the overlapping of several contributions, like the M4, M5*, and M5**levels.
Heat treatments at 500  C result in the formation of theM6 level. No data on this electrically active level have beenreported in the literature. This, together with the facts that at
this annealing stage the Nd is reactivated inside the Mg
implanted area and that no significant diffusion of Mg occurs
up to 1060  C,30 might suggest that the M6 level could be a
Mg-related defect. However, DFT studies have shown thattypically, Mg-related defects, such as Mgi, MgGaVN,
MgGaH, and MgGaVNH, are either close to the valence band,
rather than to the conduction band, or do not yield any levelin the bandgap.1,31 Indeed, Duc et al.32 have found an acceptor trap in Mg-doped MOCVD-grown GaN at EVþ 0.57–0.60 eV and assigned it to a Mg-related defect. In
addition, Albrecht et al.33 have not found any Mg-related
defect in n-type GaN grown on sapphire implanted with the28
Mg radio-active isotope and annealed at 1000  C, by radiotracer DLTS. In the present investigation, although the roleof Mg cannot be completely ruled out, we find that the M6level could possibly be identified as the E2 level detectedin As-implanted and 800  C annealed GaN.34 Anothercandidate for the identification of M6 could also be theoxygen-carbon complex which has been reported to be at
EC – 0.57 eV.35
M7 is identified as M5** and this rules out the possibility of a Mg-related defect because, after annealing at 400  C,
Nd is still detected outside the Mg-implanted region. The M7level has a high thermal stability, similar to that of the D5I
trap which is detected after electron irradiation.26 Although
M7 is 0.1 eV shallower than D5I, they might share a common microscopic nature which for the case of the D5I level
was associated with an intrinsic defect.
Coming to the deepest level found in our study, the M8level, we rule out the possibility that it can be identified as
the D6 level.26 This is because, contrary to D6, the M8 levelhas a higher thermal stability. Similar to the case of M6, also
M8 is found when Nd reactivation is observed in theimplanted area. Analogously, even if theory does not predictthe presence of Mg-related defects close to EC, the role of

205303-5

Alfieri, Sundaramoorthy, and Micheletto

Mg in the nature of the M8 level cannot be completely ruledout. Yet, Matsubara and Bellotti35 have predicted that the
CNVGa defect yields a level at EC – 1.16 eV, very close to
the M8 level. As M8 is found in a region where a high concentration of VGa is present and since C is a common impurity in MOCVD-grown GaN, we consider that this would bea possible candidate for the microscopic structure of the M8level.
For the M9 level, similar considerations to the M7 level
hold true. In fact, the M9 level shares a similar temperature
position of the M5* level which suggests that they are indeed
the same level. This means that the involvement of Mg in
the nature of this level can be discarded. Further considerations on its nature cannot be put forward, due to the lack ofdata in the literature.
For most of the levels, the involvement of Mg in theirmicroscopic structure can be ruled out, as they are detected
when Nd is found outside the implanted region. However,this is not the case for M6 and M8. In light of this, we now
focus our discussion on these two levels. Figure 3 shows thedepth profile measurements performed on the 500  Cannealed sample, for M4, M6, and M8. The measurements
were performed by varying Vr and keeping the pulse height
constant. As the figure shows, the depth profiles of thesethree levels are located in the implanted region, in accordance with the behavior of Nd at this annealing stage. While
M4 seems to possess a localized distribution at around0.1 lm, M6 and M8 display a steep profile toward the surface. In particular, M8 has a shallower profile than M6 andthe absence of a localized profile around the implantation
maximum suggests that Mg may not be involved in its
atomic nature. On the other side, M6 also has a steep profile
toward the surface but it is not possible to say, whether or
not it is localized in the implanted region. If the profile keeps
increasing toward the surface, then the involvement of Mg
can be ruled out. However, if it shows a localized distribution inside the implanted region, then the involvement of Mg
can be possible.

FIG. 3. Depth profile of the implanted Mg and of the M4, M6, and M8 levels. Depth profile measurements were carried out on the 500  C annealed
sample. The depth profile of M6 was increased to improve visualization.
The Mg implantation profile was simulated by SRIM.

J. Appl. Phys. 123, 205303 (2018)

IV. CONCLUSIONS

Our study shows the presence of at least nine electrically
active levels in Mg implanted GaN. These are located in the
0.2–1.2 eV energy range, below the conduction band edge,
and their possible microscopic structure was discussed in thelight of previous results found in the literature. The isochronal annealing study showed that M1, M2, M4, M6, M8,
and M9 are thermally stable up to 1000  C. In particular,
after annealing at 1000  C, two levels labeled M1 and M8, at
EC – 0.22 eV and EC – 1.12 eV, respectively, are present
with concentrations ranging between 1014 and 1016 cm3.
This suggests that, when selective area doping is performedby ion implantation (e.g., box-profile), care should be taken
to the deeper region of the implant profile (implant tailregion) where such a defect could effectively trap charge carriers hindering the device functionality.
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