Aero-Ga2o3 Nanomaterial Electromagnetically Transparent from Microwaves to Terahertz for Internet of Things Applications

Communication Aero-Ga2O3 Nanomaterial Electromagnetically Transparent from Microwaves to Terahertz for Internet of Things Applications

Tudor Braniste 1,* , Mircea Dragoman 2, Sergey Zhukov 3, Martino Aldrigo 2, Vladimir Ciobanu 1 , Sergiu Iordanescu 2 , Liudmila Alyabyeva 3 , Francesco Fumagalli 4 , Giacomo Ceccone 4, Simion Raevschi 5, Fabian Schütt 6, Rainer Adelung 6, Pascal Colpo 4, Boris Gorshunov 3 and Ion Tiginyanu 1,7,* 1 National Center for Materials Study and Testing, Technical University of Moldova, Stefan cel Mare av. 168, 2004 Chisinau, Moldova; [email protected] 2 National Institute for Research and Development in Microtechnologies (IMT Bucharest), Erou Iancu Nicolae Street 126A, 077190 Voluntari, Romania; [email protected] (M.D.); [email protected] (M.A.); [email protected] (S.I.) 3 Laboratory of Terahertz Spectroscopy, Center for Photonics and 2D Materials, Moscow Institute of Physics and Technology (State University), 9 Institutskiy per., 141701 Dolgoprudny, Russia; [email protected] (S.Z.); [email protected] (L.A.); [email protected] (B.G.) 4 Joint Research Centre (JRC), European Commission, Via E. Fermi 2749, 21027 Ispra, Italy; [email protected] (F.F.); [email protected] (G.C.); [email protected] (P.C.) 5 Department of Physics and Engineering, State University of Moldova, Alexei Mateevici str. 60, 2009 Chisinau, Moldova; [email protected] 6 Institute for Materials Science, Kiel University, Kaiserstr. 2, D-24143 Kiel, Germany; [email protected] (F.S.); [email protected] (R.A.) 7 Academy of Sciences of Moldova, Stefan cel Mare av. 1, MD-2001 Chisinau, Moldova * Correspondence: [email protected] (T.B.); [email protected] (I.T.); Tel.: +373-22-509-920 (I.T.)

Received: 4 May 2020; Accepted: 27 May 2020; Published: 29 May 2020

Abstract: In this paper, fabrication of a new material is reported, the so-called Aero-Ga2O3 or Aerogallox, which represents an ultra-porous and ultra-lightweight three-dimensional architecture made from interconnected microtubes of gallium oxide with nanometer thin walls. The material is fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods followed by decomposition of sacrificial ZnO and oxidation of GaN which according to the results of X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) characterizations, is transformed gradually in β-Ga2O3 with almost stoichiometric composition. The investigations show that the developed ultra-porous Aerogallox exhibits extremely low reflectivity and high transmissivity in an ultrabroadband electromagnetic spectrum ranging from X-band (8–12 GHz) to several terahertz which opens possibilities for quite new applications of gallium oxide, previously not anticipated.

Keywords: aero-Ga2O3; ultra-porous nanomaterial; extremely low reﬂectivity; electromagnetically transparent nanomaterial; X-band and terahertz frequencies

1. Introduction The materials transparent for a certain electromagnetic bandwidth are key components for many industries such as aeronautic, space, telecommunications, etc. [1,2]. They are called radomes and are conﬁgured in the form of various enclosures depending on the applications; their role is to protect antennas from various agents such as rain, snow, dust, heat, etc. The radomes can be seen, for example,

Nanomaterials 2020, 10, 1047; doi:10.3390/nano10061047 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1047 2 of 10 in any airport or on top of high buildings. The new wireless communication systems imply that very high frequencies will be used such as 0.1 THz for 5 G and 10 THz for 6 G [3]. There are materials which are transparent in the THz region, such as high resistivity (HR) semiconductors (silicon, boron nitride, gallium arsenide, germanium) or dielectrics (quartz, sapphire, fused silica, diamonds). However, for Internet of Things (IoTs) applications [4], which are the backbone of 5G and 6G communications, very small and lightweight enclosures are required to protect antennas, since the dimensions of antennas are reduced to tens of microns and even lower, comparable to the dimensions of a human hair. Thus, the existing materials for radomes at airports, telecommunications and in many other applications could not be directly reused for tiny IoT due to the requirements of dimensions and weight and there is a real need to ﬁnd new transparent materials for the applications involved. The wide bandgap β-Ga2O3 semiconductor is studied intensively for power electronics [5]. Please note that along with β-Ga2O3, there are other Ga2O3 polymorphs, all with a small index of refraction (less than 2) [6], indicating that gallium oxide could be a promising candidate for use in transparent electronics. However, little is known about RF properties of Ga2O3 since only a few results on Ga2O3 transistors are reported (see [7] and the references therein). Taking into account that three-dimensional architectures consisting of networks of low-dimensional structures prove to be among the multifunctional materials most promising for new applications in electronics and biomedicine [8,9], we developed an ultra-porous architecture made from interconnected microtubes of β-Ga2O3 with nanometer thin walls, carried out its structural characterization and experimentally demonstrated that the new ultra-lightweight nanomaterial, called aero-Ga2O3 or Aerogallox, is highly transparent and exhibits extremely low reﬂectivity in the X-band and THz region, up to 3 THz, thus disclosing a novel application of Ga2O3 previously not anticipated.

2. Materials and Methods

The technological route for the fabrication of aero-Ga2O3 is as follows. Initially the aero-GaN was obtained by growing an ultra-thin layer of GaN on sacrificial ZnO templates [10]. The ZnO templates represented networks of interpenetrated ZnO microtetrapods obtained using the flame transport synthesis approach, as previously described in ref. [11]. GaN was grown in a hydride vapor phase epitaxy (HVPE) horizontal reactor containing distinct source and reaction zones. Metallic gallium as well as ammonia (NH3, 99.99%, EG No. 231-653.3), hydrogen chloride (HCl, 99.9995%) and hydrogen (H2, 99.999%) acquired from Geschaftsbereich Linde Gas, Germany, were used as source materials and carrier gases during the growth process. In the source zone, at high temperature (T = 850 ◦C) the GaCl is formed as a result of chemical reactions between gaseous HCl and liquid Ga. The gaseous GaCl and NH3 reacted with each other in the react zone, where initially the temperature was kept at 600 ◦C for 10 min to initiate nucleation of GaN on the surface of ZnO microtetrapods, and then increased up to Tg = 850 ◦C for 10 min to produce GaN layer. In the process of GaN growth, the HCl, NH3 and H2 flow rates were equal to 15, 600 and 3600 smL/min, respectively. In the process of HVPE growth of GaN, the ZnO sacrificial template is being decomposed due to corrosive atmosphere at high temperature leading to the formation of microtubular structures representing aero-GaN [10]. Previously, we demonstrated the high crystalline quality of the resulting GaN microtubes as well as the existence of ZnO traces (at the level of about 2%) on the inner surface of GaN microtube walls [10,12]. At the final step of the technological route, the aero-GaN is subjected to annealing in air at 950 ◦C for 60 min and, as a result, is transformed into aero-Ga2O3 or Aerogallox. The aero-GaN and aero-Ga2O3 thin films crystal structure and phases were investigated using a Bruker AXS D8 Advance X-ray diffractometer (XRD, Bruker Italia S.r.l., Milano, Italy) in a standard θ–2θ Bragg–Brentano configuration with a monochromatic Cu Kα1 (λ = 0.15406 Å) radiation. A 40 kV beam voltage and 40 mA beam current were used. For acquisition a linear position-sensitive semiconductor detector (LYNXEYE, Bruker Italia S.r.l., Milan, Italia) in 0D-mode was used, beam optics were Göbel mirror, 6 mm slit, Soller 2.5◦; detector optics were 6 mm slit, Soller 2.5◦. Diffraction Nanomaterials 2020, 10, 1047 3 of 10

pattern data were collected between 20◦ and 50◦ with step lengths of 0.025◦, the sample was measured in powder form. NanomaterialsThe chemical 2020, composition10, x FOR PEER REVIEW at surfaces was studied by means of X-ray Photoemission Spectroscopy,3 of 11 XPS (AXIS ULTRA, DLD Kratos Analytical, Manchester, UK) equipped with a monochromatic Al The chemical composition at surfaces was studied by means of X-ray Photoemission Kα sourceSpectroscopy, (hν = 1486.6 XPS (AXIS eV) operating ULTRA, DLD at 150 Kratos W (h—Planck’s Analytical, Manchester constant, ν,—frequency). UK) equipped Spectra with a were 2 recordedmonochromatic from a 100 Al Kα100 sourceµm analysis (hν = 148 area6.6 and eV) at operating 160 eV (survey) at 150 W pass (h—Planck’s energy, whereas constant, coreν— level × 7 spectrafrequency). were recorded Spectra were using recorded pass energy from a of 100 20 × eV. 100 Operating µm2 analysis pressure area and was at 160 6 eV10 (survey)Pa. Prior pass to the × − measurement,energy, whereas the surfacescore level werespectra sputtered were recorded using using an pass Ar+ energybeam operatedof 20 eV. Operating at 2 keV pressure and 0.84 wasµ A for 2 min.6 × Surface10−7 Pa. Prior charging to the was measurement, compensated the usingsurfaces low were energy sputtered (~5 eV)using electrons an Ar+ beam and adjustedoperated usingat 2 the chargekeV balance and 0.84 plate µA for on 2 the min instrument.. Surface charging Three was diff erentcompensated spots were using analyzed low energy for ( each~5 eV) sample. electrons All the spectraand wereadjusted processed using the with charge CasaXPS balance (ver. plate 2.3.20). on the Spectrainstrument. were Three calibrated different setting spots hydrocarbonwere analyzed C1s at for each sample. All the spectra were processed with CasaXPS (ver. 2.3.20). Spectra were calibrated 285.0 eV. The surface composition was evaluated from the survey spectra, after a Tougaard U3-type setting hydrocarbon C1s at 285.0 eV. The surface composition was evaluated from the survey spectra, background subtraction, using relative sensitivity factors provided by the manufacturer. Peak fitting after a Tougaard U3-type background subtraction, using relative sensitivity factors provided by the wasmanufacturer. performed with Peak no fitting preliminary was performed smoothing. with Symmetricno preliminary Gaussian–Lorentzian smoothing. Symmetric (70% Gaussian Gaussian– and 30%Lorentzian Lorentzian) (70% product Gaussian functions and 30% were Lorentzian) used to product approximate functions the were line shapesused to ofapproximate the fitting the components. line shapesFor the ofmicrowaves the fitting components. and terahertz characterizations of the material, bulk samples were prepared in the formFor ofthe rectangular microwaves pelletsand terahertz (20 mm characterizations10 mm 2 mm).of the material, The freestanding bulk sample sampless were prepared were exposed × × to electromagneticin the form of rectangular radiation. pellets The microwave(20 mm × 10 mm characterization × 2 mm). The freestanding of the aero-Ga samples2O3 pellets were exposed was carried out byto electromagnetic means of a VNA radiation. (Vector The Network microwave Analyzer) characterization connected of the to aero a WR90-Ga2O waveguide-based3 pellets was carried set-up suitableout by for means measurements of a VNA (Vector in the X-bandNetwork (i.e., Analyz 8.2–12.4er) connected GHz) [13to ].a WR90 A schematics waveguide of-based the waveguide set-up is suitable for measurements in the X-band (i.e., 8.2–12.4 GHz) [13]. A schematics of the waveguide is presented in Figure1, where a = 22.86 mm and b = 10.16 mm. Since the dimensions of the cavity are presented in Figure 1, where a = 22.86 mm and b = 10.16 mm. Since the dimensions of the cavity are a a bit larger than the aero-Ga2O3 pellet, we made use of a supporting flange to fix the sample in the bit larger than the aero-Ga2O3 pellet, we made use of a supporting flange to fix the sample in the cavity,cavity, as to as avoid to avoid electromagnetic electromagnetic radiation radiation due to to imperfect imperfect coupling coupling between between the two the waveguides, two waveguides, hencehence affecting affecting the the reliability reliability of of the the performed performed measurements.

VNA Port 2 S22 / S21 Coaxial cable Port 1 WR90 S / S 11 12 waveguide cavity

Cavity for b aero-Ga2O3 Coaxial cable pellet a

bʹ

aʹ t

FigureFigure 1. Microwave 1. Microwave experimental experimental set-upset-up for for X X-band-band characterization characterization of the of aero the aero-Ga-Ga2O3. 2O3.

ForFor terahertz terahertz measurements measurements the the samplessamples were were fixed fixed on on metallic metallic holders holders covering covering 6 mm 6 aperture, mm aperture, the radiationthe radiation passing passing through through the the sample sample in in free free space. space. SamplesSamples with different different densities densities from from 70 70 to to 110 1103 mg/cm3 of aero-Ga2O3 were investigated. Room-temperature terahertz spectra of complex mg/cm of aero-Ga2O3 were investigated. Room-temperature terahertz spectra of complex dielectric –1 permittivitydielectric εpermittivity*(ν) = ε’(ν )ε*(+ν)i ε=”( εν’()ν) were + iε”( measuredν) were measured in the in range the rangeν = 10–100 ν = 10–100 cm –1cmin in a quasi-opticala quasi- optical arrangement (in an open space with no waveguides used) with the help of commercial time- arrangement (in an open space with no waveguides used) with the help of commercial time-domain domain spectrometer TeraView TPS 3000 (TeraView, Cambridge, UK). The spectra of the real and spectrometerimaginary TeraView parts of TPSdielectric 3000 permittivity(TeraView, Cambridge, are determined UK). in The the spectra transmission of the real geometry and imaginary via partsmeasuremen of dielectricts of permittivity the complex aretransmission determined coefficient in the (amplitude transmission and geometry phase) of the via plane measurements-parallel of the complexsamples; standard transmission expressions coeffi forcient electrodynamics (amplitude and of a phase)plane-parallel of the layer plane-parallel are used [14 samples;]. In addition, standard expressionsthe spectra for of electrodynamicstransmission coefficients of a plane-parallel of the same samples layer were are usedmeasured [14]. at In frequencies addition, up the to spectra7000 of cm−1 using a standard Fourier-transform spectrometer Bruker Vertex 80v.

Nanomaterials 2020, 10, 1047 4 of 10

1 transmission coeﬃcients of the same samples were measured at frequencies up to 7000 cm− using a standard Fourier-transform spectrometer Bruker Vertex 80v.

Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 11 3. Results and Discussion 3. Results and Discussion 3.1. Structural Characterization of Aero-Ga2O3 3.1. Structural Characterization of Aero-Ga2O3 Figure2 depicts scanning electron microscope (SEM) images of interpenetrated networks of ZnO Figure 2 depicts scanning electron microscope (SEM) images of interpenetrated networks of ZnO microtetrapods (a), aero-GaN (b) and the obtained aero-Ga O (c). The inset pictures are the respective microtetrapods (a), aero-GaN (b) and the obtained aero-Ga2O3 (c).2 The3 inset pictures are the respective photos ofphotos the of pellets the pellets of 20 of mm20 mm ×10 10 mmmm × 22 mm mm (L (L× W ×W H), whereH), whereone can oneeasily can distinguish easily distinguishthe the × × × × change inchange color in ofcolor the of materialthe material after after each each technological technological step step (white, (white, yellow, yellow, white). white).

FigureFigure 2. SEM 2. SEM images imagesof of ( a)) initial initial ZnO ZnO template, template, (b) intermediate (b) intermediate aero-GaN, and aero-GaN, (c) resulted aero and- (c) resulted aero-GaGa2O2O3 3nanomaterial. nanomaterial. The The inset inset pictures pictures represent represent the photographs the photographs of the pellet samples of the of pellet ZnO, GaN, samples of ZnO, and Ga2O3 respectively. GaN, and Ga2O3 respectively. Figure 3 illustrates the XRD spectra of aero-GaN (Figure 3a) used for the preparation of aero- Figure3 illustrates the XRD spectra of aero-GaN (Figure3a) used for the preparation of aero-Ga 2O3, Ga2O3, the mixture phase of GaN and Ga2O3 (Figure 3b) that was obtained by thermal treatment of the mixtureaero-GaN phase at T of = 800 GaN °C andin air Gafor 21O h,3 and(Figure the completely3b) that transformed was obtained aero- byGa2 thermalO3 (Figure treatment 3c) obtained of aero-GaN at T = 800after◦C 1 inh of air treatment for 1 h, at and 950°C the of completely aero-GaN samples. transformed The XRD aero-Gareflections2O at3 32.25°,(Figure 36.8°3c) and obtained 48.12° after 1 h of were assigned to wurtzite GaN planes (010), (011) and (012), respectively, while the reflections at treatment at 950◦C of aero-GaN samples. The XRD reflections at 32.25◦, 36.8◦ and 48.12◦ were assigned to wurtzite30.3°, GaN 31.7°, planes33.5°, 35.3°, (010), 38.4°, (011) 43.2°, and 45.9° (012), and 48.9° respectively, were assigned while to beta the phase reflections of monoclinic at 30.3Ga2O3, 31.7 , 33.5 , planes (−110), (002), (−1−11), (1−11), (202), (600), (1−12) and (−510), respectively. Comparing the◦ ◦ ◦ 35.3 , 38.4 , 43.2 , 45.9 and 48.9 were assigned to beta phase of monoclinic Ga O planes ( 110), (002), ◦ different◦ patterns,◦ ◦ the qualitative◦ trend of GaN oxidative phase change occurring at different2 3 stages − ( 1 11),of(1 the 11),thermal (202), synthesis (600), processes (1 12) can and be (appreciated.510), respectively. While both nitride Comparing and oxide the phases diff erentco-exist patterns, the − − − − − qualitativeafter trend the thermal of GaN step oxidative at T = 800°C phase in air change for 1 h (both occurring reflection at dipeaksfferent sets stagesfor wurtzite of the GaN thermal and synthesis processesmonoclinic can be appreciated.Ga2O3 are found Whilein the diffraction both nitride pattern), and only oxide reflections phases from co-exist the oxide after remain the after thermal step at annealing at T = 950°C. Formation of monoclinic Ga2O3 in thin structures was already observed for T = 800◦annealingC in air fortemperatures 1 h (both as reflection low as 750 peaks°C [15]. sets for wurtzite GaN and monoclinic Ga2O3 are found in the diffraction pattern), only reflections from the oxide remain after annealing at T = 950◦C. Formation of monoclinic Ga2O3 in thin structures was already observed for annealing temperatures as low as 750 ◦C[15]. Figure4 shows the principal results of β-Ga2O3 surface chemical analysis by means of XPS. Survey spectra (Figure4a) show that the main elements left in the sample after annealing are Ga and O with low level of C and Zn present as impurities. Observed gallium main emission lines are Ga 2p (shown in Figure4b), Ga 3p and Ga 3d (not shown) and several Auger lines in the 400–600 eV regions. All Ga doublets show energy shifts with respect to metallic gallium binding energy (BE), consistent with literature reported values for β-Ga2O3 [16,17]. Data show that O 1s peak (Figure4c) need to be resolved using two components. The main component BE at 530.98 eV can be attributed to Ga-O bonding in the oxide while the higher BE contribution at 533.17 eV can be assigned to O-vacancies sites and/or Ga suboxides [18–20]. This peak presence is related to the Ar ions sputtering process used to clean the sample surface in order to remove adventitious carbon contamination. Presence of reduced Ga can be seen also in the low energy satellite peak (at BE 19.16 eV) of Ga 3d (not shown). However, sputter cleaning of the sample was necessary for reliable estimation of atomic concentrations; before Ar+ bombardment C atoms surface concentration was estimated to be around 11% while after cleaning dropped down to 2%. Measured Ga/O ratio is thus 0.62 with an estimated relative error of 6.21% (stoichiometric value 0.67). The presence of Zn 2p doublet (Figure4d) suggests some residual ( <1 at. %) of metallic impurities left from the sacrificial ZnO matrix used in the fabrication process. It is to Nanomaterials 2020, 10, 1047 5 of 10 be noted that the chemical composition study using energy dispersive X-ray analysis disclosed the stoichiometric composition of the Ga2O3 nanoarchitecture, at the same time traces of Zinc at the level of aboutNanomaterials 1.5 at. % were2020, 10, shown. x FOR PEER REVIEW 5 of 11

1.0 (010) (a)

0.5

(011)

(002)

(012) 0.0 1.0 (b)

(011)

(010)

(002)

0.5 (002)

(-110)

(012)

(202)

(-1-11)

(600)

(-602)

(1-12) 0.0 1.0

Normalized Intensity (a. u.) (c)

(002)

(-110)

(1-11)

0.5

(202)

(1-12)

(-1-11)

(600)

(-602)

0.0 20 25 30 35 40 45 50 2 theta (°)

Figure 3. XRD spectra of aero-GaN (a), mixture of GaN and Ga2O3 phases (b) and aero-Ga2O3 (c). Figure 3. XRD spectra of aero-GaN (a), mixture of GaN and Ga2O3 phases (b) and aero-Ga2O3 (c). Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 11 Figure 4 shows the principal results of β-Ga2O3 surface chemical analysis by means of XPS. Survey spectra (Figure 4a) show that the main(a) elements left in the sample after annealing are Ga(b) and Gallium 2p O with150 low level of C and Zn present as impurities. Observed gallium main emission lines are Ga 2p (shown in FigureEl. 4b), At. Ga (%) 3p and Ga 3d (not shown) and several2 Auger lines in the 400–600 eV regions.

)

) O 59.9+0.7 45

P 0

Ga2p M

(

( All Ga doublets Gashow37.2+0.7 energy shifts with respect45 to metallic gallium binding energy (BE), consistent

45 (% cpb.) (% -2

Residual Residual

3 Res. STD = 1.05

M C 2.1+0.7 M

23 cps 2 3 M

with literature reported values for M β-Ga O [16,17]. Data show that O 1s peak (Figure 4c) need to be

)

3 2

3 100 Zn 0.8+0.7 L Ga Center (eV) FWHM (eV) A(%)

M 60

2 resolved using two components.23 23 The main component BE at 530 Ga.98 2p 3/2eV (I) can 1189.09 be attributed 1.87 43.74 to Ga-O

M M

Ga L Ga

23 23

Ga L Ga Ga 2p 1/2 (I) 1144.97 1.87 20.55

cps

O1s

x 10

M M

3 2

3 bonding( in the oxide while the higher BE contribution at 533.17 eV Ga 2pcan 3/2 (II)be 1119.31 assigned 2.80 to 18.86 O-vacancies 40 Ga 2p 1/2 (II) 1156.41 2.80 8.99

O KLO

O KLO sites and/or Ga suboxidesL Ga L Ga [18–20]. This peak presence is related to the Ar ions sputtering process used 50 x10 Ga 2p satellte 1136.53 12.18 7.54

(

Zn2p

Ga3s Ga3d to clean the sample surface in order to removeGa3p adventitious carbon contamination. cps Bgd PresenceFit of 20 reduced Ga can be seen also in the lowC1s energy satellite peak (at BE 19.16 eV) of Ga 3d (not shown).

Intensity However,0 sputter cleaning of the sample was necessary for reliable estimation of atomic concentrations;1200 before900 Ar+ bombardment600 300 C atoms0 surface Intensity concentration1150 1140 was estimated1130 to1120 be around 11% while after cleaningBinding droppedEnergy (eV) down to 2%. Measured Ga/O ratioBinding is thus 0.62Energy with (eV) an estimated (c) relative errorOxygen of 1s 6.21% (stoichiometric value 0.67). The presenceZinc of 2p Zn 2p doublet (Figure 4d) suggests(d) 2 2 some 0 residual (<1 at. %) of metallic impurities left from0 the sacrificial ZnO matrix used in the

(% cpb) (%

fabricationResidual -2 process. It is to be notedRes. that STD the= 0.85 chemical composition-2 study using energy dispersiveRes. STD = 0.89 X-

(% cpb) (%

Residual

) ray analysis12 cpsdisclosed Bgd the Fitstoichiometric composition of the Ga2O cps3 nanoarchitecture, Bgd at the same time Center (eV) FWHM (eV) A(%) ) Center (eV) FWHM (eV) A(%)

traces cps of Zinc at the level of about 1.5 at. % were shown. O 1s 530.98 1.82 81.40 10 Zn 2p 3/2 1022.18 2.14 67.22

O 1s 533.17 3.15 18.60 cps Zn 2p 1/2 1045.22 2.14 32.78 9 (Vac.) 3

x10

(

x10 ( 9 6

Intensity

540 536 532 528 Intensity 1050 1040 1030 1020 Binding Energy (eV) Binding Energy (eV)

Figure 4. (a) XPS survey spectra of β-Ga2O3 powder with attribution of the principal emission lines, Figure 4. (a) XPS survey spectra of β-Ga2O3 powder with attribution of the principal emission lines, table in the inset shows the derived atomic abundancies. (b–d) High resolution XSP spectra with table in the inset shows the derived atomic abundancies. (b–d) High resolution XSP spectra with resolved peak components and normalized residuals, of the Ga 2p (b), O 1s (c) and Zn 2p (d) regions. resolved peak components and normalized residuals, of the Ga 2p (b), O 1s (c) and Zn 2p (d) regions.

3.2. Characterization of Aero-Ga2O3 at Microwaves.

The S-parameters at the two ports of the VNA were measured to provide the reflection (S11/S22) and transmission (S12/S21) coefficients at/between the two ports, respectively. The measured S- parameters of the Aerogallox sample (Figure 2c) with the density of 110 mg/cm3 are depicted in Figure 5.

(a) (L) (R) (b) (L) (R) Reflection Transmission Reflection Transmission -10 0 -10 0

-20 -0.625 -20 -0.625

-30 -1.25 -30 -1.25

Reflection (dB) -40 -1.88 Reflection (dB) -40 -1.88

Transmission (dB) Transmission

Transmission (dB)Transmission

-50 -2.5 -50 -2.5 8.2 9.04 9.88 10.72 11.56 12.4 8.2 9.04 9.88 10.72 11.56 12.4 Frequency (GHz) Frequency (GHz)

Figure 5. Microwave measurements in X-band for the two cases: (a) without the aero-Ga2O3 pellet and

(b) with the aero-Ga2O3 pellet, in terms of reflection (left vertical axis, solid black and blue curves) and transmission (right vertical axis, solid red and pink curves).

Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 11

(a) (b) Gallium 2p 150 El. At. (%) 2

)

) O 59.9+0.7 45

P 0

Ga2p M

(

( Ga 37.2+0.7 45

45 (% cpb.) (% -2

Residual Residual

3 Res. STD = 1.05

M C 2.1+0.7 M

cps M

)

3 2

3 100 Zn 0.8+0.7 L Ga Center (eV) FWHM (eV) A(%)

M 60

23 23 Ga 2p 3/2 (I) 1189.09 1.87 43.74

M M

Ga L Ga

23 23

Ga L Ga Ga 2p 1/2 (I) 1144.97 1.87 20.55

cps

O1s

x 10

M M

3 2

3 ( Ga 2p 3/2 (II) 1119.31 2.80 18.86 40 Ga 2p 1/2 (II) 1156.41 2.80 8.99

O KLO

Ga L Ga L Ga

50 x10 Ga 2p satellte 1136.53 12.18 7.54

(

Zn2p

Ga3s Ga3d Ga3p cps Bgd Fit 20

C1s

Intensity 0 1200 900 600 300 0 Intensity 1150 1140 1130 1120 Binding Energy (eV) Binding Energy (eV) Oxygen 1s (c) Zinc 2p (d) 2 2 0 0

(% cpb) (%

Residual -2 Res. STD = 0.85 -2 Res. STD = 0.89

(% cpb) (%

Residual

) 12 cps Bgd Fit cps Bgd Center (eV) FWHM (eV) A(%) ) Center (eV) FWHM (eV) A(%) cps O 1s 530.98 1.82 81.40 10 Zn 2p 3/2 1022.18 2.14 67.22

O 1s 533.17 3.15 18.60 cps Zn 2p 1/2 1045.22 2.14 32.78 9 (Vac.) 3

x10

(

x10 ( 9 6

Intensity

540 536 532 528 Intensity 1050 1040 1030 1020 Binding Energy (eV) Binding Energy (eV)

Figure 4. (a) XPS survey spectra of β-Ga2O3 powder with attribution of the principal emission lines, table in the inset shows the derived atomic abundancies. (b–d) High resolution XSP spectra with Nanomaterialsresolved 2020peak, 10components, 1047 and normalized residuals, of the Ga 2p (b), O 1s (c) and Zn 2p (d) regions. 6 of 10

3.2. Characterization of Aero-Ga2O3 at Microwaves. 3.2. Characterization of Aero-Ga2O3 at Microwaves. The S-parameters at the two ports of the VNA were measured to provide the reflection (S11/S22) and transmissionThe S-parameters (S12/S21 at) the coefficient two portss at/between of the VNA the were two measured ports, respectively. to provide the The reﬂection measured (S 11 S/-S22) and 3 parameterstransmission of (Sthe12 / AerogalloxS21) coeﬃcients sample at / (betweenFigure 2c) the with two the ports, density respectively. of 110 mg/cm The measured are depicted S-parameters in Figofure the 5. Aerogallox sample (Figure2c) with the density of 110 mg /cm3 are depicted in Figure5.

(a) (L) (R) (b) (L) (R) Reflection Transmission Reflection Transmission -10 0 -10 0

-20 -0.625 -20 -0.625

-30 -1.25 -30 -1.25

Reflection (dB) -40 -1.88 Reflection (dB) -40 -1.88

Transmission (dB) Transmission

Transmission (dB)Transmission

-50 -2.5 -50 -2.5 8.2 9.04 9.88 10.72 11.56 12.4 8.2 9.04 9.88 10.72 11.56 12.4 Frequency (GHz) Frequency (GHz)

FigureFigure 5. 5.MicrowaveMicrowave measurements measurements in X in-band X-band for the for two the cases: two cases: (a) without (a) without the aero the-Ga aero-Ga2O3 pellet2O and3 pellet and (b(b) )with with the the aero aero-Ga-Ga2O23O pellet,3 pellet, in interms terms of ofreflection reﬂection (left (left vertical vertical axis, axis, solid solid black black and andblue blue curves) curves) and andtransmission transmission (right (right vertical vertical axis, axis, solid solid redred andand pink curves). curves).

In Figure5a,b we show only S 11 and S21, since S22 is identical with S11, and S12 with S21 thanks to reciprocity and symmetry of the scattering matrix (typical for passive components). One can notice that the reflection coefficient is better than 10 dB all over the band of interest, whereas the insertion − loss has a maximum value better than 0.12 dB in both cases without and with aero-Ga O . In other − 2 3 words, the presence of the aero-Ga2O3 inside the cavity between the two X-band waveguides does not affect at all the transmission of the microwave signal, which means the aero-Ga2O3 is completely (within our accuracy) electromagnetically transparent in the X-band.

3.3. Characterization of Aero-Ga2O3 in the Terahertz Region Two samples of Aerogallox with different densities were studied—the low density equals to 70 mg/cm3 and the high density was 110 mg/cm3. Comparative analyses with aero-GaN samples with the density of 15 mg/cm3 were performed. Broad-band terahertz-infrared transmissivity spectra for two samples with low and high densities are shown in Figure6a. As expected, more dense samples have lower transmissivity. A decrease in the transmission coefficient for frequencies growing up to 1 1 200 cm− is caused by intensive absorption between 200 and 800 cm− (see inset in Figure6a). Our attempts to extract information about absorption mechanisms in this range by measuring the spectra of reflection coefficient failed due to very low intensity reflected by the samples, i.e., extremely small 1 value of reflectivity, as discussed below. Below 200 cm− , there are two narrow absorption resonances 1 located at ~155 and ~176 cm− whose parameters (frequency position and intensity) do not depend 1 noticeably on the sample density. A few more resonances are observed above 800 cm− , most intensive 1 at ~3500 cm− . Origin of observed narrow absorptions seems to be related to the presence of impurities such as hydrogen [21]. Nanomaterials 2020, 10, 1047 7 of 10 Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 11

Aero-Ga O 2 3 1 70 mg/cm3 1 -1 10 10-1

10-2

Tr -3 10-2 10 3 10-4 110 mg/cm 1 10 100 1000 7000 -1 -3 (a)  (cm ) 10 Aero-GaN

Transmission coefficient 1.4 1 Aero-GaN

' 

(%) 0.1

1.2 0.01 0 50 100 150 200 3 -1 110 mg/cm (b)  (cm )

70 mg/cm3 Permittivity 1

''  '' Aero-GaN  0.02 0.8

110 mg/cm3 0.01 0.4

70 mg/cm3 (c) Permittivity Permittivity 0.00 0.0 0 40 80 120 160 200 -1 Wavenumber (cm )

Figure 6. Terahertz spectral characteristics of aero-Ga2O3 samples with two different densities of Figure70 mg6. /Terahertzcm3 and 110 spectral mg/cm characteristics3, and aero-GaN of sample aero-Ga with2O3 the samples density with of 15 two mg/ cmdifferent3 (data fromdensities [22]): of 70 3 3 3 mg/cmtransmission and 110 coe mg/cmfficient Tr, and(a), real aeroε0-GaN(b) and sample imaginary withε”( thec) parts density of dielectric of 15 permittivity. mg/cm (data Open from dots [22]): transmissionon panels (coefficientb) and (c) present Tr (a), THzreal dataε′ (b for) and permittivity. imaginary Inset ε” ( inc) panelparts ( aof): dielectric spectrum ofpermittivity. transmission Open 1 dotscoe onffi cient panels measured (b) and at frequencies (c) present up THz to 7000 data cm− for. Inset permittivity. in panel (b): Inset spectra in of panel reflection (a): coe spectrumfficient of transmissioncalculated coefficientbasing on measured measured spectra at frequencies of real and imaginary up to 7000 parts cm of−1. dielectric Inset in permittivity panel (b): usingspectra of reflectionstandard coefficient Fresnel expressions calculated [ 14basing]. on measured spectra of real and imaginary parts of dielectric permittivity using standard Fresnel expressions [14]. In Figure6b,c we present the spectra of real and imaginary permittivity of the two samples, together with the reflection coefficient. It is seen that THz characteristics of our aero-Ga2O3 material In Figure 6b,c we present the spectra of real and imaginary permittivity of the two samples, drastically differ from those of parent bulk Ga2O3 crystal as well as from any other bulk semiconductor. togetherFor example, with the typical reflection semiconductors coefficient. Ge, It is GaAs seen and that Si THz have characteristics refractive indexes of our at 300 aero GHz-Gan2O=33.99, material drastically3.59 and differ 3.43, respectively from those [23 of], while parent the bulk refractive Ga2O index3 crystal of aero-Ga as wellO , asn from1.07 (sample any other with bulk 2 3 ≈ semiconductor.density 70 mg For/cm examp3, Figurele,7), typical only slightly semiconductors exceeds that Ge, of vacuum GaAs and ( n = Si1). have From refractive Figure6b,c indexes one can at 300 GHzsee n = that 3.99 there, 3.59 is and pronounced 3.43, respectively difference [23 between], while terahertz the refractive response index of aero-Gaof aero-2GaO32andO3, n previously ≈ 1.07 (sample withstudied density aero-GaN 70 mg/cm [223, ].Figure While 7) in, only aero-Ga slightly2O3 real exceeds permittivity that of εvacuum0 is dispersionless (n = 1). From and Figure imaginary 6b,c one can permittivitysee that thereε” is is pronounced small and approaches difference zero between with frequencyterahertz decrease,response bothof aero indicating-Ga2O3 and absence previously of any absorption process, aero-GaN demonstrates strong increase of ε and ε” toward low frequencies. studied aero-GaN [22]. While in aero-Ga2O3 real permittivity ε0′ is dispersionless and imaginary The origin of corresponding pronounced absorption is associated with the polarizability of the 3D permittivity ε” is small and approaches zero with frequency decrease, both indicating absence of any architecture of mutually interpenetrated GaN aerotetrapods, with the ZnO-GaN interfaces, and finally absorption process, aero-GaN demonstrates strong increase of ε′ and ε” toward low frequencies. The origin of corresponding pronounced absorption is associated with the polarizability of the 3D architecture of mutually interpenetrated GaN aerotetrapods, with the ZnO-GaN interfaces, and finally with dynamics of relatively big complexes of tetrapods. No such contribution exists in the present Aerogallox nanomaterial. Aero-Ga2O3 is characterized by very small imaginary permittivity ε” and dielectric losses tanδ = ε”/ε′ (Figure 7a) that are responsible for radiation absorption, and by real part of permittivity ε′ and real part n of complex refractive index n* = n + ik (Figure 7b) close to

Nanomaterials 2020, 10, 1047 8 of 10

with dynamics of relatively big complexes of tetrapods. No such contribution exists in the present Aerogallox nanomaterial. Aero-Ga O is characterized by very small imaginary permittivity ε” and Nanomaterials 2020, 10, x FOR PEER REVIEW 2 3 9 of 11 dielectric losses tanδ = ε”/ε0 (Figure7a) that are responsible for radiation absorption, and by real part of permittivity ε0 and real part n of complex refractive index n* = n + ik (Figure7b) close to 1. Both factors2 2 1. Both factors lead to a very low reflection coefficient (inset inh Figure2 6b)2i 푅h = [(푛 2− 1)2i+ 푘 ]/ lead to a very low reflection2 coefficient (inset in Figure6b) R = (n 1) + k / (n + 1) + k 2 2 2 2 2 [(푛 + 1) + 푘 2] ≈ [(√휀’ − 1) +2 푘 ] /[(√휀´ + 1) + 푘 ] − ≈ 2 2 that is as small as R ≈ 0.1% and can be made √ε0 1 + k / √ε + 1 + k that is as small as R 0.1% and can be made close to R 0.01%. close to R ≈− 0.01%. We can compare these extremely low≈ reflectivity values with those of ≈typical bulk We can compare these extremely low reflectivity values with those of typical bulk semiconductors, semiconductors,mentioned above, mentioned Ge (n = 3.99), above, GaAs Ge ( n(n= =3.59) 3.99), or siliconGaAs( n(n= =3.43); 3.59) corresponding or silicon (nreflection = 3.43); corresponding coefficients reflectionfall in coefficients the range 30–36%. fall in Along the range with high 30– value36%. ofAlong transmissivity, with high this value low reflectivity of transmissivity, of the material this low 2 3 reflectivitymay be of beneficial the materia for futurel may applications be beneficial of Gafor2 Ofuture3. applications of Ga O .

Frequency (THz)

-20.0 0.5 1.0 1.5 2.0 2.5 3.0 10 3 -2 (a) 110 mg/cm 10

10-3



k k -4 tan 3 10 70 mg/cm -3 10 tg 10-5 0 20 40 60 80 100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 1.06 (b) 110 mg/cm3 1.04

n 1.02 70 mg/cm3

1.00 0 20 40 60 80 100 -1 Wavenumber (cm ) Figure 7. Spectra of terahertz electrodynamic characteristics of aero-Ga O samples with two diﬀerent Figure 7. Spectra of terahertz electrodynamic characteristics of aero-Ga2 23O3 samples with two different 3 3 k a n b densitiesdensities of 70 of 70mg/cm mg/cm3 andand 110 110 mgmg/cm/cm :3: imaginary imaginary( k) ( anda) and real real( ) partsn (b) of parts complex of complex refractive refractive index n + ik and dielectric loss tangent tanδ (panel a, triangles). index n + ik and dielectric loss tangent tanδ (panel a, triangles). 4. Conclusions 4. Conclusions We developed a highly porous ultra-lightweight three-dimensional nanoarchitecture consisting of interconnectedWe developed microtubes a highly porous of Ga2O ultra3 with-lightweight nanometer thickthree walls,-dimensional and demonstrated nanoarchitecture that the gallium consisting β of interconnectedoxide skeleton is microtubes of crystalline of-phase Ga2O with3 with almost nanomet stoichiometricer thick composition. walls, and The demonstrated new nanomaterial that the galliumis shown oxide to skeleton exhibit ultra-low is of crystalline reﬂectivity β- andphase high with transparency almost stoichiometric in an extremely composition. wide range ofThe the new nanomaterialelectromagnetic is shown spectrum, to exhibit covering ultra the-low X-band reflectivity and THz and region, high uptransparency to 3 THz. The in an disclosed extremely novel wide properties of aero-Ga O open possibilities, in premiere, for the use of gallium oxide in IoT applications. range of the electromagnetic2 3 spectrum, covering the X-band and THz region, up to 3 THz. The disclosed novel properties of aero-Ga2O3 open possibilities, in premiere, for the use of gallium oxide in IoT applications.

Nanomaterials 2020, 10, 1047 9 of 10

Author Contributions: Conceptualization: I.T., M.D., B.G., R.A.; Data curation, T.B., S.Z., M.A., S.I., L.A., V.C., F.F., G.C.; Formal analysis, T.B., M.D., S.Z., B.G., M.A., F.F., G.C.; Funding acquisition, I.T., B.G., P.C., R.A; Investigation, T.B., S.Z., M.A., L.A., S.I., V.C., F.F., G.C., S.R.; Methodology, T.B., M.D., S.Z., B.G., I.T.; Project administration, I.T.; Resources, T.B., F.S., S.R.; Supervision, M.D., B.G, I.T.; Validation, T.B., S.Z., L.A., M.A., S.I., V.C., F.F., G.C., P.C., S.R.; Writing—original draft, M.D., T.B., B.G., I.T.; All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the support from the Ministry of Education, Culture and Research of the Republic of Moldova under the Grant #20.80009.50007.20, and to the European Union’s Horizon-2020 Research and Innovation Programme: H2020-EU.4.b—Twinning of research institutions, grant agreement No. 810652 “NanoMedTwin”. F.S. and R.A. acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) under contracts AD 183/27-1. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Mani, G.S. Radome Materials. In Microwave Materials; Murthy, V.R.K., Sundaram, S., Viswanathan, B., Eds.; Springer: Berlin, Germany, 1994; pp. 200–239. 2. Khatavkara, N.; Balasubramanian, K. Composite materials for supersonic aircraft radomes with ameliorated radio frequency transmission—A review. RCS Adv. 2016, 6, 6709–6718. [CrossRef] 3. Dang, S.; Amin, O.; Shihada, B.; Alouini, M.-S. What should 6G be? Nat. Electron. 2020, 3, 20–29. [CrossRef] 4. Asghari, P.; Rahmani, A.M.; Javadi, H.H.S. Internet of Things applications: A systematic review. Comput. Networks 2019, 148, 241–261. [CrossRef] 5. Zhou, H.; Zhang, J.; Zhang, C.; Feng, Q.; Zhao, S.; Ma, P.; Hao, Y. A review of the most recent progresses of state-of-art gallium oxide power devices. J. Semicond. 2019, 40, 011803. [CrossRef]

6. Pearton, S.J.; Yang, J.; Cary IV, P.H.; Ren, F.; Kim, J.; Tadjer, M.J.; Mastro, M.A. A review of Ga2O3 materials, processing, and devices. Appl. Phys. Rev. 2018, 5, 011301. [CrossRef] 7. Xia, Z.; Xue, H.; Joishi, C.; McGlone, J.; Kalaricka, N.K.; Sohel, S.H.; Brenner, M.; Arehart, A.; Ringel, S.;

Lodha, S.; et al. β-Ga2O3 delta-doped field-effecttransistors with current gain cutoff frequency of 27 GHz. IEEE Trans. El. Dev. 2019, 40, 1053–1055. 8. Jakus, A.E.; Secor, E.B.; Rutz, A.L.; Jordan, S.W.; Hersam, M.C.; Shah, R.N. Three-dimensional printing of high-content graphene scaffolds for electronics and biomedical applications. ACS Nano 2015, 9, 4636–4648. [CrossRef][PubMed] 9. Paulowicz, I.; Hrkac, V.; Kaps, S.; Cretu, V.; Lupan, O.; Braniste, T.; Duppel, V.; Tiginyanu, I.; Kienle, L.;

Adelung, R.; et al. Three-dimensional SnO2 nanowire networks for multifunctional applications: From high-temperature stretchable ceramics to ultraresponsive sensors. Adv. Electron. Mater. 2015, 1, 1500081. [CrossRef] 10. Tiginyanu, I.; Braniste, T.; Smazna, D.; Deng, M.; Schutt, F.; Schuchardt, A.; Stevens-Kalceff, M.A.; Raevschi, S.; Schurmann, U.; Kienle, L.; et al. Self-organized and self-propelled aero-GaN with dual hydrophilic/hydrophobic behaviour. Nano Energy 2019, 56, 759–769. [CrossRef] 11. Mishra, Y.K.; Kaps, S.; Schuchardt, A.; Paulowicz, I.; Jin, X.; Gedamu, D.; Freitag, S.; Claus, M.; Wille, S.; Kovalev, A.; et al. Fabrication of macroscopically flexible and highly porous 3D semiconductor networks from interpenetrating nanostructures by a simple flame transport approach. Part. Part. Syst. Charact. 2013, 30, 775. [CrossRef] 12. Dragoman, M.; Ciobanu, V.; Shree, S.; Dragoman, D.; Braniste, T.; Raevschi, S.; Dinescu, A.; Sarua, A.; Mishra, Y.K.; Pugno, N.; et al. Sensing up to 40 atm using pressure-sensitive aero-GaN. Phys. Status Solidi RRL 2019, 13, 1900012. [CrossRef] 13. Dragoman, M.; Braniste, T.; Iordanescu, S.; Aldrigo, M.; Raevschi, S.; Shree, S.; Adelung, R.; Tiginyanu, I. Electromagnetic interference shielding in X-band with aero-GaN. Nanotechnology 2019, 30, 34LT01. [CrossRef] [PubMed] 14. Born, M.; Wolf, E. Principles of Optics, 7th (expanded) ed.; Cambridge University Press: Cambridge, UK, 2003. 15. Zhang, X.; Huang, H.; Zhang, Y.; Liu, D.; Tong, N.; Lin, J.; Chen, L.; Zhang, Z.; Wang, X. Phase transition of

two-dimensional β-Ga2O3 nanosheets from ultrathin γ-Ga2O3 nanosheets and their photocatalytic hydrogen evolution activities. ACS Omega 2018, 3, 14469–14476. [CrossRef][PubMed] Nanomaterials 2020, 10, 1047 10 of 10

16. Bourque, J.L.; Biesingerb, M.C.; Baines, K.M. Chemical state determination of molecular gallium compounds using XPS. Dalton Trans. 2016, 45, 7678–7696. [CrossRef][PubMed] 17. Gibbon, J.T.; Jones, L.; Roberts, J.W.; Althobaiti, M.; Chalker, P.R.; Mitrovic, I.Z.; Dhanak, V.R. Band alignments

at Ga2O3 heterojunction interfaces with Si and Ge. AIP Advances 2018, 8, 065011. [CrossRef] 18. Huang, L.; Feng, Q.; Han, G.; Li, F.; Li, X.; Fang, L.; Xing, X.; Zhang, J.; Hao, Y. Comparison study of

β-Ga2O3 photodetectors grown on sapphire at diﬀerent oxygen pressures. IEEE Photonics J. 2017, 9, 2731625. [CrossRef]

19. Son, H.; Choi, Y.-J.; Hwang, J.; Jeon, D.-W. Inﬂuence of post-annealing on properties of α-Ga2O3 epilayer grown by halide vapor phase epitaxy. ECS J. Solid State Sci. Technol. 2019, 8, Q3024. [CrossRef] 20. Tak, B.R.; Dewan, S.; Goyal, A.; Pathak, R.; Gupta, V.; Kapoor, A.K.; Nagarajan, S.; Singh, R. Point defects

induced work function modulation of β-Ga2O3. Appl. Surf. Sci. 2019, 465, 973–978. [CrossRef] 21. Ritter, J.R.; Huso, J.; Dickens, P.T.; Varley, J.B.; Lynn, K.G.; McCluskey, M.D. Compensation and hydrogen

passivation of magnesium acceptors in β-Ga2O3. Appl. Phys. Lett. 2018, 113, 052101. [CrossRef] 22. Braniste, T.; Zhukov, S.; Dragoman, M.; Alyabyeva, L.; Ciobanu, V.; Aldrigo, M.; Dragoman, D.; Iordanescu, S.; Shree, S.; Raevschi, S.; et al. Terahertz shielding properties of aero-GaN. Semicond. Sci. Technol. 2019, 34, 12LT02. [CrossRef] 23. Kozlov, G.V.; Volkov, A.A. Millimeter and Submillimeter Wave Spectroscopy of Solids; Gruner, G., Ed.; Springer-Verlag: Berlin, Germany, 1997; p. 51.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).