Electron beam-induced current in the temperature range from 304 to 404 K was employed to measure the minority carrier diffusion length in metal–organic chemical vapor deposition-grown p-Ga2O3 thin films with two different concentrations of majority carriers. The diffusion length of electrons exhibited a decrease with increasing temperature. In addition, the cathodoluminescence emission spectrum identified optical signatures of the acceptor levels associated with the VGa–VO++ complex. The activation energies for the diffusion length decrease and quenching of cathodoluminescence emission with increasing temperature were ascribed to the thermal de-trapping of electrons from VGa–VO++ defect complexes.

β-Ga2O3 is an emerging fourth-generation power electronics platform with a wide-bandgap of ∼4.8 eV and a high breakdown field (8 × 106 V cm−1).1–6 It is becoming increasingly attractive due to its applications in high-power electronics, true solar-blind UV detection, and optoelectronic devices.2,3,7–10 Undoped β-Ga2O3 tends to be n-type due to unintentional donor impurities such as Si. Intentionally, n-type β-Ga2O3 can be obtained by adding controlled amounts of impurities such as Si, Sn, and Ge, which is well documented.3,5 Carrier transport characterization revealed impurity bands and the hopping mechanism of electrical transport in such doped films.11–14 Low-temperature electron mobilities up to 796 cm2/V s13 have been reported. The incorporation of doped layers in devices such as Schottky diodes and field-effect transistors (FETs), including metal–oxide–semiconductor FETs (MOSFETs), and their ability to withstand high energy particle radiation have been explored.15–24 Replicating these results to achieve p-type conductivity in β-Ga2O3 has proven very difficult due to factors such as doping asymmetry, high compensation of acceptors, the high ionization energy of acceptor levels, and hole-trapping at O(I) and O(II) sites.25–30 Despite these difficulties, native p-type conductivity was demonstrated at high temperatures in undoped β-Ga2O3.31,32 It was observed that native p-type conductivity is achievable by creating a significant number of native acceptors (VGa) and suppressing the compensation due to native donors (VO). The thermodynamic balance required to weaken the self-compensation in undoped β-Ga2O3 was achieved by adjusting the growth temperatures and oxygen partial pressures during the deposition of Ga2O3 on sapphire substrates by Metal–Organic Chemical Vapor Deposition (MOCVD).32,33

p-type β-Ga2O3 is a relatively recent discovery and an uncharted territory in terms of minority carrier transport and luminescence characterization as well as their temperature dependences. Knowledge of minority carrier (electrons) transport properties in p-type β-Ga2O3 is essential for achieving bipolar technology on the gallium oxide platform. In this report, the diffusion length of minority carriers (electrons), cathodoluminescence, and their temperature dependence are studied in p-type β-Ga2O3 with two different majority carrier (holes) concentrations.

Undoped β-Ga2O3 samples, analyzed in this study, were grown in an RF-heated horizontal MOCVD reactor with separate inlets to avoid premature reactions in the manifold between oxygen and organometallics precursors. Trimethylgallium (TMGa) and 5.5 N pure oxygen were used as gallium and oxygen sources, respectively. Argon was used as the carrier gas (cf. Ref. 32). The β-Ga2O3 layer was grown on a c-oriented sapphire substrate using Ga/O ratio and growth temperature as 1.4 × 10−4 and 775 °C, respectively. Two different total reactor pressures of 30 and 38 Torr and variable growth rates (gallium and oxygen precursor fluxes) were used to create two different native defect (VGa and VO) concentrations in the Ga2O3 films, leading to the different values of p-type conductivity. The difference between the total reactor pressures for the deposition of the two samples is due to a change in the oxygen partial pressure. The concentration of native defects responsible for p-type conductivity is sensitive to the oxygen partial pressure. The epitaxial layer thickness was ∼450 nm. X-ray diffraction scans revealed highly textured films of gallium oxide in the β-Ga2O3 phase with a monoclinic space group (C2/m) symmetry. Hereinafter, the sample grown under 30 Torr total reactor pressure will be labeled A and that grown under 38 Torr will be labeled B.

A detailed study of the electrical transport properties for the above-referenced highly resistive (close to stoichiometric) Ga2O3 samples has been performed. Ohmic contacts were prepared with silver paint at the four corners of the sample. Hall effect measurements were conducted in a van der Pauw configuration in the 500–850 K temperature range for magnetic fields perpendicular to the film plane varying from −1.6 to 1.6 T using a high impedance high-temperature custom-designed measurement setup. Resistivities at highest measured T = 850 K were found to be ρ (A) = 1.2 × 103 Ω cm and ρ (B) = 1.3 × 104 Ω cm. Hall effect measurements demonstrated (cf. Refs. 31 and 32) the positive sign for majority carriers in both samples, thus confirming the p-type conductivity. The free hole concentrations and mobilities at 850 K were estimated as follows: p = 5.6 × 1014 cm−3 and μ = 8.0 cm2 V−1 s−1 for sample A and p = 2.7 × 1013 cm−3 and μ = 16 cm2 V−1 s−1 for sample B. Temperature-dependent measurements were possible to perform only down to 520 K (p = 2.0 × 1010 cm−3) for sample A and only to 620 K for sample B (p = 7.1 × 1010 cm−3) due to the samples’ high resistivity. The difference in hole concentrations is due to the difference in growth conditions (total reactor pressure and the ratio of gallium–oxygen precursor fluxes), resulting in the variation of electrical compensation degree K = NA/ND, i.e., the ratio of native acceptor to native donor concentrations. Ni/Au (20/80 nm) asymmetrical pseudo-Schottky contacts were created on the film with lithography/liftoff techniques for further analysis.

Electron Beam-Induced Current (EBIC) and cathodoluminescence (CL) measurements were performed in situ in a Phillips XL-30 Scanning Electron Microscope (SEM) to characterize the diffusion length (L) of minority carriers (electrons) and luminescence behavior of the samples, respectively. The measurements were carried out in the 304–404 K temperature range using a Gatan MonoCL2 temperature-controlled stage integrated into the SEM. For both EBIC and CL measurements, the electron beam energy was kept at 10 keV. The EBIC line scans were obtained in a planar configuration (Fig. 1). The EBIC signal was amplified with a Stanford Research Systems SR 570 low-noise current amplifier and digitized with a Keithley digital multimeter (DMM) 2000 controlled by a personal computer (PC) using homemade software. CL measurements were carried out using a Gatan MonoCL2 attachment to the SEM. Spectra were recorded with a Hamamatsu photomultiplier tube sensitive in 150–850 nm range and a single grating monochromator (blazed at 1200 lines/mm).

FIG. 1.

A schematic diagram of the sample structure and experimental setup.

FIG. 1.

A schematic diagram of the sample structure and experimental setup.

Close modal

EBIC line-scans were used to extract diffusion length, L, from the following equation:34,35

Cx=C0xαexpxL,
(1)

where C(x) is the EBIC signal at distance x from the Schottky junction, C0 is a scaling constant, x is the distance of the electron beam from the Schottky barrier, and α is the linearization parameter, related to surface recombination velocity. The coefficient α was set at −0.5, corresponding to the low influence of surface recombination. Since the carrier concentration is low in both samples, the Schottky barrier depletion width is significantly larger than L and, therefore, the approach outlined in Ref. 36 was used. Figures 2(a) and 2(b) show the raw EBIC signals and a fit with xα exp(−x/L) used in extracting L for samples A and B, respectively. The temperature dependence of L for samples A and B is shown in Fig. 3. L decreased with increasing temperature, with values for samples A and B at 304 K of 1040 and 8506 nm, respectively. At 404 K, L reduced to 640 and 6193 nm, respectively. Relatively large values of L are partially due to the shallow majority carrier concentration. Within the current temperature range of measurements, the origin of L decrease is likely due to phonon scattering.37 Reported values of L for minority carrier (holes) in n-type β-Ga2O3 are within 50–600 nm,20,21,38–41 lower than those of electrons measured in this work for minority carrier electrons. A likely reason could be the large effective mass for holes (18–25 m0).42 It is worth noting that a similar dependence of L on temperature was found for n-type β-Ga2O3, but it is attributed to scattering on ionized impurities due to heavy Si doping.41 The activation energy for the temperature dependence of L is given by43,44

LT=L0expΔEL,T2kT,
(2)

where L0 is a scaling constant, ΔEL,T is the thermal activation energy, k is the Boltzmann constant, and T is the temperature. The activation energy pertaining to the reduction of L with temperature is 67 and 113 meV for samples A and B, respectively. A detailed discussion regarding the origin of ΔEL,T is given later in the text.

FIG. 2.

An example of the acquired EBIC line-scan from sample A (a) and sample B (b) at 304 K along with exp(−x/L)/x0.5 fit for extraction of the diffusion length.

FIG. 2.

An example of the acquired EBIC line-scan from sample A (a) and sample B (b) at 304 K along with exp(−x/L)/x0.5 fit for extraction of the diffusion length.

Close modal
FIG. 3.

Temperature dependence of the diffusion length for samples A and B. The inset shows the Arrhenius plot with a linear fit for extraction of the activation energy ΔEL,T.

FIG. 3.

Temperature dependence of the diffusion length for samples A and B. The inset shows the Arrhenius plot with a linear fit for extraction of the activation energy ΔEL,T.

Close modal

Raw CL spectra and their Gaussian decompositions at 304 K are presented in Figs. 4(a) and 4(b) for samples A and B, respectively. The CL spectra exhibit four characteristic luminescence bands: ultraviolet (UVL′ and UVL) at 375 and 415 nm, blue (BL) at 450 nm, and green (GL) at 520 nm. The UVL′ and UVL bands are commonly ascribed to recombination of self-trapped excitons, considering the absence of near band edge emission and their lack in β-Ga2O3 for sub-bandgap excitation.26,27,29,45–48 The self-localization of excitons occurs at O(I) and O(II) site, corresponding to UVL′ and UVL bands, respectively.47,49 Although, as has been shown from Electron Paramagnetic Resonance (EPR) measurements50 and confirmed by several independent EBIC studies on n-type β-Ga2O3,20–23,39,51,52 the self-localization of holes is unstable above 110 K, the optical signature of the self-trapped excitons persists in CL and photoluminescence (PL) measurements. Note that the relative contribution of UVL′ and UVL bands in both A and B samples is much lower than in n-type β-Ga2O3, found in earlier reports.26,47,49,53–58 The BL band arises from donor–acceptor pair recombination involving a VO donor and VGa or a (VO, VGa) complex as an acceptor. GL has several different origins, mentioned in the literature, and was observed with an array of various dopants, such as Mg,59 Si,54 and Er.60 In undoped β-Ga2O3, grown by floating zone technique, Víllora et al.61 ascribed GL to self-trapped excitons as it existed only for PL excitation energies below the bandgap. Moreover, this band was also observed in β-Ga2O3 nanoflakes, structurally consisting of a crystalline core and amorphous shell.62,63 In a recent study on β-Ga2O3 films on a c-plane sapphire substrate with (201) orientation, deposited with magnetron sputtering,64 the intensities of BL and GL were modulated by changing the oxygen flow rate, and the origin of the GL was attributed to the presence of isolated VGa. Furthermore, the presence of isolated VO did not independently play a role in enhancing BL, and the origin of BL was assigned to a defect complex involving VO and VGa. Given the abundance of isolated VGa acceptors and (VO, VGa) complexes in both samples, a relatively large contribution of both BL and GL to the CL emission spectrum is observed in this work. Binet and Gourier53 and Onuma et al.49 independently found a correlation between conductivity and concentration of the VO donors in n-type β-Ga2O3. In this case, since VO compensates the acceptors, and due to the high ionization energy of acceptors, p-type β-Ga2O3 has relatively high resistivity below 450 K.31,32 The presence of a rather large number of VGa acceptors and (VO, VGa) acceptor complexes, promoting p-type conductivity, was confirmed from the CL emission spectrum.

FIG. 4.

Normalized CL spectrum for sample A (a) and sample B (b) and their Gaussian decomposition into four bands: UVL′, UVL, BL, and GL.

FIG. 4.

Normalized CL spectrum for sample A (a) and sample B (b) and their Gaussian decomposition into four bands: UVL′, UVL, BL, and GL.

Close modal

The temperature dependence of the CL signal follows the form53 

I(T)=I0/(1+eΔECL/kT),
(3)

where I(T) is the integrated CL intensity, I0 is a constant, and ΔECL is the process activation energy. Figure 5 shows the Arrhenius plot of ln(I0/I(T) − 1). The process activation energy ΔECL, obtained from a linear fit of the temperature dependence depicted in Fig. 5, was 88 and 101 meV for samples A and B, respectively. The total CL intensity is used in Fig. 5 because the relative contributions of the individual luminescence bands remained approximately constant in the temperature range of the measurements. The activation energies ΔEL,T and ΔECL for sample A (67 and 88 meV, respectively) and sample B (113 and 101 meV, respectively) are comparable and can be attributed to a common origin.

FIG. 5.

Arrhenius plot of ln(I0/I(T) − 1) vs 1/(kT) from Eq. (1), where I is the integrated CL emission intensity, with the fit, used in the extraction of activation energy (ΔECL) for the thermal quenching process.

FIG. 5.

Arrhenius plot of ln(I0/I(T) − 1) vs 1/(kT) from Eq. (1), where I is the integrated CL emission intensity, with the fit, used in the extraction of activation energy (ΔECL) for the thermal quenching process.

Close modal

Temperature-dependent resistivity measurements between 300 and 850 K in our earlier study32 on deep VGa acceptor defects in similar Ga2O3 samples showed two activation energies due to temperature-activated processes. In the high temperature region (T > 400 K), the acceptors are ionized with an activation energy of 0.56 eV. However, for temperatures between 300 and 400 K, a shallower VGa–VO++ acceptor complex is present. The concentration of this complex strongly increases in off stoichiometric samples (after oxygen post-annealing) detectable by the Hall effect. The electrical activation energy has been determined as 0.17 eV (170 meV), and these complexes are responsible for forming an impurity band and hopping conductivity below 400 K.32 It is suggested in this study that the non-equilibrium electrons in as-grown (close to stoichiometry) Ga2O3 thin films, generated during the excitation with an electron beam, are captured by VGa–VO++ acceptor defect complexes. The thermal emission of these captured electrons is represented by activation energies extracted from EBIC and CL experiments. In other words, VGa–VO++ acceptor complexes are detectable by electron beam excitation even in close to stoichiometric samples when electrical measurements are insensitive, probably due to their insufficient concentration. Moreover, based on the discussion given above, the nature of the native defects probed with EBIC and CL in both samples is alike. The difference in the activation energies is primarily due to the difference in their concentration, which is governed by the oxygen partial pressure during the growth process. The process of non-equilibrium electron de-trapping in this work has an analogy with Mg-doped p-GaN, where the release of a non-equilibrium electron from deep acceptor levels is seen in the thermal activation of L.65 

In summary, EBIC and CL techniques were employed to understand the temperature dependence of the diffusion length of minority carriers and CL emission in p-type β-Ga2O3 with two different hole concentrations. Optical signatures of native acceptor defects (isolated VGa and VGa–VO2+ complex) were identified in the CL spectrum. In addition, the activation energies for change of L with temperature (ΔEL, T) and thermal quenching of CL intensity (ΔECL) were experimentally obtained as 67 and 88 meV for sample A and 113 and 101 meV for sample B within the temperature range of 304–404 K. Comparable values of ΔEL,T and ΔECL indicate a common origin for both processes, which is attributed to the thermal de-trapping of electrons from the VGa–VO++ acceptor level. The current development in the characterization of p-type β-Ga2O3 could serve a pivotal role in realizing bipolar gallium oxide devices.

The research at UCF was supported, in part, by the NSF (Grant Nos. ECCS1802208 and ECCS2127916). The research at UCF and Tel Aviv University was supported partially by the US–Israel BSF (Award No. 2018010) and NATO (Award No. G5748). The work at UF was performed as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA), sponsored by the Department of the Defense, Defense Threat Reduction Agency (Award No. HDTRA1-20-2-0002), monitored by Jacob Calkins and by the NSF DMR (Grant No. 1856662) (J. H. Edgar). This work is a part of the “GALLIA” International Research Project, CNRS, France.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
S. I.
Stepanov
,
V. I.
Nikolaev
,
V. E.
Bougrov
, and
A. E.
Romanov
,
Rev. Adv. Mater. Sci.
44
,
63
(
2016
).
2.
M.
Kim
,
J.-H.
Seo
,
U.
Singisetti
, and
Z.
Ma
,
J. Mater. Chem. C
5
,
8338
(
2017
).
3.
S. J.
Pearton
,
J.
Yang
,
P. H.
Cary
,
F.
Ren
,
J.
Kim
,
M. J.
Tadjer
, and
M. A.
Mastro
,
Appl. Phys. Rev.
5
,
011301
(
2018
).
4.
Progress in semiconductor β-Ga2O3
,” in
Ultra-Wide Bandgap Semiconductor Materials
, edited by
M.
Liao
,
B.
Shen
, and
Z.
Wang
(
Elsevier
,
2019
), pp.
263
345
.
5.
M. J.
Tadjer
,
J. L.
Lyons
,
N.
Nepal
,
J. A.
Freitas
,
A. D.
Koehler
, and
G. M.
Foster
,
ECS J. Solid State Sci. Technol.
8
,
Q3187
(
2019
).
6.
J.
Zhang
,
J.
Shi
,
D.-C.
Qi
,
L.
Chen
, and
K. H. L.
Zhang
,
APL Mater.
8
,
020906
(
2020
).
7.
J. Y.
Tsao
,
S.
Chowdhury
,
M. A.
Hollis
,
D.
Jena
,
N. M.
Johnson
,
K. A.
Jones
,
R. J.
Kaplar
,
S.
Rajan
,
C. G.
Van de Walle
,
E.
Bellotti
,
C. L.
Chua
,
R.
Collazo
,
M. E.
Coltrin
,
J. A.
Cooper
,
K. R.
Evans
,
S.
Graham
,
T. A.
Grotjohn
,
E. R.
Heller
,
M.
Higashiwaki
,
M. S.
Islam
,
P. W.
Juodawlkis
,
M. A.
Khan
,
A. D.
Koehler
,
J. H.
Leach
,
U. K.
Mishra
,
R. J.
Nemanich
,
R. C. N.
Pilawa-Podgurski
,
J. B.
Shealy
,
Z.
Sitar
,
M. J.
Tadjer
,
A. F.
Witulski
,
M.
Wraback
, and
J. A.
Simmons
,
Adv. Electron. Mater.
4
,
1600501
(
2018
).
8.
H.
von Wenckstern
,
Adv. Electron. Mater.
3
,
1600350
(
2017
).
9.
M.
Higashiwaki
,
K.
Sasaki
,
H.
Murakami
,
Y.
Kumagai
,
A.
Koukitu
,
A.
Kuramata
,
T.
Masui
, and
S.
Yamakoshi
,
Semicond. Sci. Technol.
31
,
034001
(
2016
).
10.
K.
Akito
,
K.
Kimiyoshi
,
W.
Shinya
,
Y.
Yu
,
M.
Takekazu
, and
Y.
Shigenobu
,
Jpn. J. Appl. Phys.
55
,
1202A2
(
2016
).
11.
H. J.
von Bardeleben
and
J. L.
Cantin
,
J. Appl. Phys.
128
,
125702
(
2020
).
12.
E. B.
Yakimov
,
A. Y.
Polyakov
,
N. B.
Smirnov
,
I. V.
Shchemerov
,
P. S.
Vergeles
,
E. E.
Yakimov
,
A. V.
Chernykh
,
M.
Xian
,
F.
Ren
, and
S. J.
Pearton
,
J. Phys. D: Appl. Phys.
53
,
495108
(
2020
).
13.
A. K.
Rajapitamahuni
,
L. R.
Thoutam
,
P.
Ranga
,
S.
Krishnamoorthy
, and
B.
Jalan
,
Appl. Phys. Lett.
118
,
072105
(
2021
).
14.
Z.
Kabilova
,
C.
Kurdak
, and
R. L.
Peterson
,
Semicond. Sci. Technol.
34
,
03LT02
(
2019
).
15.
A. J.
Green
,
K. D.
Chabak
,
E. R.
Heller
,
R. C.
Fitch
,
M.
Baldini
,
A.
Fiedler
,
K.
Irmscher
,
G.
Wagner
,
Z.
Galazka
,
S. E.
Tetlak
,
A.
Crespo
,
K.
Leedy
, and
G. H.
Jessen
,
IEEE Electron Device Lett.
37
,
902
(
2016
).
16.
M. H.
Wong
,
K.
Sasaki
,
A.
Kuramata
,
S.
Yamakoshi
, and
M.
Higashiwaki
,
IEEE Electron Device Lett.
37
,
212
(
2016
).
17.
M.
Higashiwaki
,
A.
Kuramata
,
H.
Murakami
, and
Y.
Kumagai
,
J. Phys. D: Appl. Phys.
50
,
333002
(
2017
).
18.
G.
Yang
,
S.
Jang
,
F.
Ren
,
S. J.
Pearton
, and
J.
Kim
,
ACS Appl. Mater. Interfaces
9
,
40471
(
2017
).
19.
K.
Konishi
,
K.
Goto
,
H.
Murakami
,
Y.
Kumagai
,
A.
Kuramata
,
S.
Yamakoshi
, and
M.
Higashiwaki
,
Appl. Phys. Lett.
110
,
103506
(
2017
).
20.
S.
Modak
,
L.
Chernyak
,
S.
Khodorov
,
I.
Lubomirsky
,
J.
Yang
,
F.
Ren
, and
S. J.
Pearton
,
ECS J. Solid State Sci. Technol.
8
,
Q3050
(
2019
).
21.
S.
Modak
,
J.
Lee
,
L.
Chernyak
,
J.
Yang
,
F.
Ren
,
S. J.
Pearton
,
S.
Khodorov
, and
I.
Lubomirsky
,
AIP Adv.
9
,
015127
(
2019
).
22.
S.
Modak
,
L.
Chernyak
,
S.
Khodorov
,
I.
Lubomirsky
,
A.
Ruzin
,
M.
Xian
,
F.
Ren
, and
S. J.
Pearton
,
ECS J. Solid State Sci. Technol.
9
,
045018
(
2020
).
23.
S.
Modak
,
L.
Chernyak
,
A.
Schulte
,
M.
Xian
,
F.
Ren
,
S. J.
Pearton
,
I.
Lubomirsky
,
A.
Ruzin
,
S. S.
Kosolobov
, and
V. P.
Drachev
,
Appl. Phys. Lett.
118
,
202105
(
2021
).
24.
S.
Modak
,
L.
Chernyak
,
A.
Schulte
,
M.
Xian
,
F.
Ren
,
S. J.
Pearton
,
A.
Ruzin
,
S. S.
Kosolobov
, and
V. P.
Drachev
,
AIP Adv.
11
,
125014
(
2021
).
25.
Y.
Yan
and
S. H.
Wei
,
Phys. Status Solidi B
245
,
641
(
2008
).
26.
H.
Gao
,
S.
Muralidharan
,
N.
Pronin
,
M. R.
Karim
,
S. M.
White
,
T.
Asel
,
G.
Foster
,
S.
Krishnamoorthy
,
S.
Rajan
,
L. R.
Cao
,
M.
Higashiwaki
,
H.
von Wenckstern
,
M.
Grundmann
,
H.
Zhao
,
D. C.
Look
, and
L. J.
Brillson
,
Appl. Phys. Lett.
112
,
242102
(
2018
).
27.
S.
Marcinkevičius
and
J. S.
Speck
,
Appl. Phys. Lett.
116
,
132101
(
2020
).
28.
M. D.
McCluskey
,
J. Appl. Phys.
127
,
101101
(
2020
).
29.
Y. K.
Frodason
,
K. M.
Johansen
,
L.
Vines
, and
J. B.
Varley
,
J. Appl. Phys.
127
,
075701
(
2020
).
30.
T. D.
Gustafson
,
J.
Jesenovec
,
C. A.
Lenyk
,
N. C.
Giles
,
J. S.
McCloy
,
M. D.
McCluskey
, and
L. E.
Halliburton
,
J. Appl. Phys.
129
,
155701
(
2021
).
31.
E.
Chikoidze
,
A.
Fellous
,
A.
Perez-Tomas
,
G.
Sauthier
,
T.
Tchelidze
,
C.
Ton-That
,
T. T.
Huynh
,
M.
Phillips
,
S.
Russell
,
M.
Jennings
,
B.
Berini
,
F.
Jomard
, and
Y.
Dumont
,
Mater. Today Phys.
3
,
118
(
2017
).
32.
E.
Chikoidze
,
C.
Sartel
,
H.
Mohamed
,
I.
Madaci
,
T.
Tchelidze
,
M.
Modreanu
,
P.
Vales-Castro
,
C.
Rubio
,
C.
Arnold
,
V.
Sallet
,
Y.
Dumont
, and
A.
Perez-Tomas
,
J. Mater. Chem. C
7
,
10231
(
2019
).
33.
X.
Wang
,
T.
Liu
,
Y.
Lu
,
Q.
Li
,
R.
Guo
,
X.
Jiao
, and
X.
Xu
,
J. Phys. Chem. Solids
132
,
104
(
2019
).
34.
C. A.
Dimitriadis
,
J. Phys. D: Appl. Phys.
14
,
2269
(
1981
).
35.
L.
Chernyak
,
A.
Osinsky
,
H.
Temkin
,
J. W.
Yang
,
Q.
Chen
, and
M.
Asif Khan
,
Appl. Phys. Lett.
69
,
2531
(
1996
).
36.
V. K. S.
Ong
,
O.
Kurniawan
,
G.
Moldovan
, and
C. J.
Humphreys
,
J. Appl. Phys.
100
,
114501
(
2006
).
37.
N.
Ma
,
N.
Tanen
,
A.
Verma
,
Z.
Guo
,
T.
Luo
,
H.
Xing
, and
D.
Jena
,
Appl. Phys. Lett.
109
,
212101
(
2016
).
38.
A. Y.
Polyakov
,
I.-H.
Lee
,
N. B.
Smirnov
,
E. B.
Yakimov
,
I. V.
Shchemerov
,
A. V.
Chernykh
,
A. I.
Kochkova
,
A. A.
Vasilev
,
F.
Ren
,
P. H.
Carey
, and
S. J.
Pearton
,
Appl. Phys. Lett.
115
,
032101
(
2019
).
39.
A. Y.
Polyakov
,
N. B.
Smirnov
,
I. V.
Shchemerov
,
E. B.
Yakimov
,
S. J.
Pearton
,
C.
Fares
,
J.
Yang
,
F.
Ren
,
J.
Kim
,
P. B.
Lagov
,
V. S.
Stolbunov
, and
A.
Kochkova
,
Appl. Phys. Lett.
113
,
092102
(
2018
).
40.
E. B.
Yakimov
,
A. Y.
Polyakov
,
N. B.
Smirnov
,
I. V.
Shchemerov
,
J.
Yang
,
F.
Ren
,
G.
Yang
,
J.
Kim
, and
S. J.
Pearton
,
J. Appl. Phys.
123
,
185704
(
2018
).
41.
J.
Lee
,
E.
Flitsiyan
,
L.
Chernyak
,
J.
Yang
,
F.
Ren
,
S. J.
Pearton
,
B.
Meyler
, and
Y. J.
Salzman
,
Appl. Phys. Lett.
112
,
082104
(
2018
).
42.
M. M. R.
Adnan
,
D.
Verma
,
Z.
Xia
,
N. K.
Kalarickal
,
S.
Rajan
, and
R. C.
Myers
,
Phys. Rev. Appl.
16
,
034011
(
2021
).
43.
O.
Lopatiuk-Tirpak
,
L.
Chernyak
,
F. X.
Xiu
,
J. L.
Liu
,
S.
Jang
,
F.
Ren
,
S. J.
Pearton
,
K.
Gartsman
,
Y.
Feldman
,
A.
Osinsky
, and
P.
Chow
,
J. Appl. Phys.
100
,
086101
(
2006
).
44.
M.
Eckstein
and
H.-U.
Habermeier
,
J. Phys. IV
01
,
C6-23
(
1991
).
45.
S.
Yamaoka
and
M.
Nakayama
,
Phys. Status Solidi C
13
,
93
(
2016
).
46.
P.
Deák
,
Q.
Duy Ho
,
F.
Seemann
,
B.
Aradi
,
M.
Lorke
, and
T.
Frauenheim
,
Phys. Rev. B
95
,
075208
(
2017
).
47.
Q. D.
Ho
,
T.
Frauenheim
, and
P.
Deák
,
Phys. Rev. B
97
,
115163
(
2018
).
48.
J.
Lapp
,
D.
Thapa
,
J.
Huso
,
A.
Canul
,
M.
McCluskey
, and
L.
Bergman
,
Bull. Am. Phys. Soc.
65
,
245
(
2020
).
49.
T.
Onuma
,
S.
Fujioka
,
T.
Yamaguchi
,
M.
Higashiwaki
,
K.
Sasaki
,
T.
Masui
, and
T.
Honda
,
Appl. Phys. Lett.
103
,
041910
(
2013
).
50.
B. E.
Kananen
,
N. C.
Giles
,
L. E.
Halliburton
,
G. K.
Foundos
,
K. B.
Chang
, and
K. T.
Stevens
,
J. Appl. Phys.
122
,
215703
(
2017
).
51.
A. Y.
Polyakov
,
I.-H.
Lee
,
N. B.
Smirnov
,
A. V.
Govorkov
,
E. A.
Kozhukhova
,
N. G.
Kolin
,
A. V.
Korulin
,
V. M.
Boiko
, and
S. J.
Pearton
,
J. Appl. Phys.
109
,
123703
(
2011
).
52.
E. B.
Yakimov
,
A. Y.
Polyakov
,
I. V.
Shchemerov
,
N. B.
Smirnov
,
A. A.
Vasilev
,
P. S.
Vergeles
,
E. E.
Yakimov
,
A. V.
Chernykh
,
F.
Ren
, and
S. J.
Pearton
,
Appl. Phys. Lett.
118
,
202106
(
2021
).
53.
L.
Binet
and
D.
Gourier
,
J. Phys. Chem. Solids
59
,
1241
(
1998
).
54.
K.
Shimamura
,
E. G.
Víllora
,
T.
Ujiie
, and
K.
Aoki
,
Appl. Phys. Lett.
92
,
201914
(
2008
).
55.
S.
Yamaoka
,
Y.
Furukawa
, and
M.
Nakayama
,
Phys. Rev. B
95
,
094304
(
2017
).
56.
T. T.
Huynh
,
L. L. C.
Lem
,
A.
Kuramata
,
M. R.
Phillips
, and
C.
Ton-That
,
Phys. Rev. Mater.
2
,
105203
(
2018
).
57.
T.
Onuma
,
Y.
Nakata
,
K.
Sasaki
,
T.
Masui
,
T.
Yamaguchi
,
T.
Honda
,
A.
Kuramata
,
S.
Yamakoshi
, and
M.
Higashiwaki
,
J. Appl. Phys.
124
,
075103
(
2018
).
58.
Y.
Wang
,
P. T.
Dickens
,
J. B.
Varley
,
X.
Ni
,
E.
Lotubai
,
S.
Sprawls
,
F.
Liu
,
V.
Lordi
,
S.
Krishnamoorthy
,
S.
Blair
,
K. G.
Lynn
,
M.
Scarpulla
, and
B.
Sensale-Rodriguez
,
Sci. Rep.
8
,
18075
(
2018
).
59.
V.
Vasyltsiv
,
A.
Luchechko
,
L.
Kostyk
, and
B.
Pavlyk
, in
2019 XIth International Scientific and Practical Conference on Electronics and Information Technologies (ELIT)
(
IEEE
,
2019
).
60.
E.
Nogales
,
J. A.
García
,
B.
Méndez
,
J.
Piqueras
,
K.
Lorenz
, and
E.
Alves
,
J. Phys. D: Appl. Phys.
41
,
065406
(
2008
).
61.
E. G.
Víllora
,
M.
Yamaga
,
T.
Inoue
,
S.
Yabasi
,
Y.
Masui
,
T.
Sugawara
, and
T.
Fukuda
,
Jpn. J. Appl. Phys.
41
,
L622
(
2002
).
62.
X. T.
Zhou
,
F.
Heigl
,
J. Y. P.
Ko
,
M. W.
Murphy
,
J. G.
Zhou
,
T.
Regier
,
R. I. R.
Blyth
, and
T. K.
Sham
,
Phys. Rev. B
75
,
125303
(
2007
).
63.
G.
Pozina
,
M.
Forsberg
,
M. A.
Kaliteevski
, and
C.
Hemmingsson
,
Sci. Rep.
7
,
42132
(
2017
).
64.
Y.
Nie
,
S.
Jiao
,
S.
Li
,
H.
Lu
,
S.
Liu
,
S.
Yang
,
D.
Wang
,
S.
Gao
,
J.
Wang
, and
Y.
Li
,
J. Alloys Compd.
900
,
163431
(
2021
).
65.
L.
Chernyak
,
A.
Osinsky
,
V.
Fuflyigin
, and
E. F.
Schubert
,
Appl. Phys. Lett.
77
,
875
(
2000
).