Temperature-dependent Raman-active phonon modes and electron−phonon coupling in β-Ga2O3 microwire

The lattice vibration and electron-phonon coupling (EPC) in β-Ga2O3 microwire are systematically investigated. The β-Ga2O3 microwire that is (020)-oriented shows 14 Raman peaks, with all their FWHM narrower than those of (100)-oriented β-Ga2O3 bulk single crystal. As the temperature increases from 80 to 300 K, most Raman-active phonon modes are blueshifted, while a few modes are first blueshifted and then redshifted. The photoluminescence mainly originates from the recombination of self-trapping exciton and the quantitative analysis reveals that there exists quite strong EPC in β-Ga2O3 microwire and the Huang–Rhys factor is up to Sʹ ≈ 14.

The lattice vibration and electron-phonon coupling (EPC) in β-Ga 2 O 3 microwire are systematically investigated.The β-Ga 2 O 3 microwire that is (020)-oriented shows 14 Raman peaks, with all their FWHM narrower than those of (100)-oriented β-Ga 2 O 3 bulk single crystal.As the temperature increases from 80 to 300 K, most Raman-active phonon modes are blueshifted, while a few modes are first blueshifted and then redshifted.The photoluminescence mainly originates from the recombination of self-trapping exciton and the quantitative analysis reveals that there exists quite strong EPC in β-Ga 2 O 3 microwire and the Huang-Rhys factor is up to Sʹ ≈ 14. © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd Supplementary material for this article is available online B eta-phase gallium oxide has attracted much attention due to its low theoretical cost, high breakdown field strength, high electron mobility, and large absorption coefficient in the deep-UV band.][3] β-Ga 2 O 3 bulk single crystal, thin film and other materials made of β-Ga 2 O 3 have been extensively studied.Recently, high-quality β-Ga 2 O 3 microwire has been grown by CVD, 4,5) optical vapor supersaturated precipitation 6) and exfoliation. 7,8)The distinct high aspect ratio of the microwire gives it an unexpectedly high absorption cross-section, enhancing its capacity for sufficient photon harvest. 1,9)As a result, the photodetectors based on 1D β-Ga 2 O 3 microwire exhibit better photoelectric performance than those based on thin films or bulk material. 10)Wang et al. reported a self-powered solar-blind UV photodetector based on PEDOT:PSS/β-Ga 2 O 3 organic/Inorganic p-n junction, which exhibits a high responsivity up to 2.6 A•W −1 and a narrow bandpass response of only 17 nm in width. 5)The selfpowered solar-blind photodetector based on individual ZnO-Ga 2 O 3 heterostructures fabricated by Zhao et al. shows a high rejection ratio. 11)espite the excellent performance of the photodetectors based on β-Ga 2 O 3 microwire, there is still insufficient knowledge of the fundamental properties of the microwire.Kumar et al. studied the effect of deposition time on the morphological, structural and optical properties of β-Ga 2 O 3 nano-/micro-wires grown by CVD. 12)López et al. investigated the effect of Li-doping on the morphology and physical properties of β-Ga 2 O 3 microrod. 13)However, important aspects of β-Ga 2 O 3 microwire, such as the lattice vibration, optical properties and their temperature-dependent properties, still remain unclear.Raman spectroscopy and PL spectroscopy, which have been widely used to study the fundamental properties of β-Ga 2 O 3 bulk single crystal and powders, are powerful tools to investigate these essential properties.In a Raman spectroscopy study of β-Ga 2 O 3 , Dohy et al. clarified lattice vibration modes by approximation, and found that the Raman shifts of all peaks exhibit redshifts as the temperature increases. 14)1][22][23] Generally, the UV emission is attributed to the recombination of self-trapping exciton (STE), independent of the doping elements.15) The blue emission (BE) in undoped β-Ga 2 O 3 samples originates from the excitation of defects in crystal. 18) Themission in the green and red bands, and several emissions in the blue band, are dependent on the doping elements and their concentration.19,20) In addition, Cheng et al. found that the electron-phonon coupling (EPC) in β-Ga 2 O 3 bulk single crystal is much stronger than in GaN, GaAs and other semiconductor materials through the analysis of the variable-temperature PL spectroscopy.24) Although there has been significant progress, a more comprehensive understanding of β-Ga 2 O 3 microwire is crucial and necessary for its device application.In this work, the lattice vibration, optical properties and temperature-dependent properties of β-Ga 2 O 3 microwire are systematically investigated by X-ray diffraction, Raman scattering and PL spectroscopy.The crystalline properties of the microwire are obtained by XRD analysis.The Raman-active lattice vibration modes are analyzed through Raman spectroscopy, and the effect of temperature on Raman peak position is studied through variable-temperature Raman spectroscopy. ThePL properties and the electron-phonon coupling in the microwire are investigated by PL spectroscopy and variabletemperature PL spectroscopy.It is found that the PL of the microwire is mainly the UV emission caused by the recombination of STE, while the BE is bright at low temperatures.In addition, the electron-phonon (e-p) interaction in the crystal lattice is explored through the analysis of the variable-temperature PL spectroscopy, in which the large parameter Sʹ (Huang-Rhys factor) acquired here reveals the existence of a strong EPC in the crystal lattice of β-Ga 2 O 3 microwire.These results provide an effective reference for further understanding the fundamental properties of β-Ga 2 O 3 microwire.
The β-Ga 2 O 3 microwire investigated here is synthesized via the CVD method. 4,5,25)In Fig. 1(a), we present a micrograph of the β-Ga 2 O 3 microwire.The diameter of the microwire is around 7.5 μm and its length is 5 mm.The Raman spectrum of β-Ga 2 O 3 microwire at RT is shown by the red line in Fig. 2, where 14 peaks are obtained by Lorentz fitting.The Raman shift and FWHM are summarized in Table SI.The Raman spectrum of (100)-plane β-Ga 2 O 3 bulk single crystal under the same experimental configuration is shown by the blue line in Fig. 2 and the fitting results are summarized in Table SI.All the FWHM of the Raman peaks of β-Ga 2 O 3 microwire are much narrower than their corresponding FWHM of bulk single crystal, indicating the higher crystalline quality and higher symmetry of the crystal structure of the microwire.
To further understand various properties of β-Ga 2 O 3 , we use the approximation method proposed by Dohy et al. to define the crystal structure of β-Ga 2 O 3 as the chains of Ga I O 4 -tetrahedra and the "isolated" Ga II 3+ ions among these chains, 14) under which the vibrations of β-Ga 2 O 3 crystal can be divided into 15 vibrations (see Tables II, IV, and Fig. 3  We now separately analyze the corresponding FWHM of Raman peaks of the microwire in three different frequency ranges.In the low frequency (<250 cm −1 ), the FWHM of the Raman peaks of the microwire are much narrower than those of bulk single crystal except the A g (3) peak, and even the FWHM of the B g (1) peak of the microwire is narrower than 50% of that of bulk single crystal.The A g (1)   , B g (2) and A g (2) modes represent the symmetry of the translations of Ga I (O) 4 -tetrahedra chains along the b-axis, a-axis and the axis perpendicular to the b and c axes in Fig. 1(c), respectively.The B g (1) peak represents the symmetry of the translations of Ga II 3+ chains along the b-axis.Therefore, the Ga I (O I ) 2 groups, Ga I (O II ) 2 groups and Ga II 3+ ions of the microwire have rather high symmetry.The FWHM of the A g (3)   peak of the microwire is close to the FWHM of bulk single crystal, and this peak represents the libration of Ga I O 4 -tetrahedra chains.Thus, the libration in the microwire is more similar to that in bulk single crystal.In the middle-frequency range (250 ∼ 500 cm −1 ), the FWHM of the Raman peaks of the microwire are also much narrower, where the FWHM of B g (3) and A g (6) of the microwire are even narrower than 75% of those of bulk single crystal.The B g (3) mode represents the symmetry of the rotations of the two O I in Ga I (O I ) 2 groups around the b axis in opposite directions.The A g (6) peak represents the symmetry of the rock of the two O I in Ga I (O I ) 2 groups around the b-axis and the translations of Ga II 3+ along the axis perpendicular to the b and c axes.This also indicates that the symmetry of Ga I (O I ) 2 groups and Ga II 3+ in microwire is higher.For the high-frequency band (>500 cm −1 ), all the FWHM of Raman peaks of the microwire are narrower than those of bulk single crystal but wider than 90% of those of bulk single crystal.Considering that these Raman modes represent the stretching and bending of Ga I O 4 -tetrahedra, one can conclude that these vibrations about the Ga I O 4 -tetrahedra in microwire are not drastically different to those in bulk single crystal.
The properties of lattice vibration in the microwire are similar to those in bulk single crystal at RT, but the temperature-dependent properties of the microwire exhibit some differences.The variable-temperature Raman spectra and the fitting results are shown in Fig. SI and Table SII, respectively.Figure 3 shows the Raman shifts of these 15 Raman peaks as a function of temperature and a fit using the model calculation of the following form: 14,26,27) w w a a = --T T , 1 where ω 0 is the Raman shift at 0 K, while α 1 and α 2 are the firstand second-order temperature coefficients, respectively.The best-fitting parameters are listed in Table SIII.In general, as the temperature increases, the relative motion among  atoms in crystal will be enhanced, which will weaken the interaction between atoms and that between unit cells, leading to a decrease in the vibration frequency and redshifts of Raman peak position. 28)However, the experimental results show that the five peaks of the B g (1) , A g (2) , A g (3) , A g (4) and A g (6) modes first exhibit blueshifts and then redshifts, a phenomenon different to that of the corresponding Raman shifts of bulk single crystal, and their first-order temperature coefficients are negative.This is possibly due to the fact that the effect of temperature on interatomic spacing along the growth direction is different to that perpendicular to the growth direction.
The effect of lattice vibration on the transition of carriers also cannot be ignored.A strong EPC can result in the deep-UV-emitting device having a broadband emission, but a long response time to the photodetector.A suitable strength of EPC is thus significant for the design and application of photoelectric devices. 29)For this purpose, we carefully investigate the electron transition in β-Ga 2 O 3 microwire and the effect of lattice vibration on it using the PL spectroscopy and variable-temperature PL spectroscopy, respectively.We present the results as follows.
At RT, the luminescence is bright in the UV band and weak in the blue band, while it almost disappears in the green and red bands, similar to the luminescence excited in undoped β-Ga 2 O 3 bulk single crystal with great crystal quality.This reveals that there is no more defect level or impurity level in the band gap and the crystalline quality of the microwire is excellent.There are three fitted peaks acquired by Gaussian fitting at RT, corresponding to the photon energies of 3.84, 3.43 and 2.71 eV, respectively, as shown in Fig. 4(a).However, the BE is bright at 80 K [see Fig. 4(b)].
The photon energies of the two brightest peaks (3.84 and 3.43 eV) coincide with the photon energies of STE emission in previous reports. 22,23)The STE emission can be described simply with the configuration coordinate diagram 29,30) in Fig. 5(a).The curves g, f and s represent the energies of the ground state, free state and STE state, respectively, where A, Aʹ and Bʹ are the corresponding lowest points.After electrons are excited from A to Aʹ, they are carried to C by thermal agitation and then slide to Bʹ.These electrons then transition to B, which is vertical to Bʹ, and emit photons.Finally, the electrons come back to A by cooling transitions. 30,31)urthermore, the photon energy of BE is in the energy range of the emission for crystal defects.According to Binet et al., 18) an electron on one gallium-oxygen vacancy pair can be excited to one oxygen vacancy, leaving a hole on the original vacancy pair.The excited electron can be captured by the left hole through a tunneling process.When the trapped electron jumps back to the previous gallium-oxygen vacancy pair through transition, one photon will be emitted.Below 120 K, the BE is bright, but as the temperature increases its intensity quickly decreases.One reason is that the electron on the oxygen vacancy is excited into the conduction band or the hole on the gallium-oxygen vacancy pair is excited into the valence band at higher temperatures.Since there are fewer electrons and holes that can participate in recombination, the luminescence becomes weaker.
In addition, the fitting results of PL spectra show that the FWHM of STE2 broadens with an increasing temperature [see Table SIV and Fig. 5(b)].In general, there are many factors that can cause the broadening of emission peaks, one of which is e-p interaction, which is dominated by the longitudinal optical (LO) phonon mode in β-Ga 2 O 3 .Considering only the contribution of LO phonon and neglecting the influence of other factors, the temperature dependence of FWHM of STE2 can be described by, 24) / where Γ 0 is the FWHM independent of temperature, Sʹ is a dimensionless parameter related to the strength of EPC, 〈ħω〉 is the energy of LO phonons, represents the thermal population of phonons and kB is the Boltzmann constant.The set of parameters Sʹ = 14.08, 〈ħω〉 = 38.4meV and Γ0 = 0.455 eV in Eq. ( 2) perfectly fits the FWHM results, as shown in Fig. 5(b).It is worth mentioning that the value of Sʹ is much larger than the value of 8.82 obtained in β-Ga 2 O 3 bulk single crystal, 24) indicating that the EPC in the 012004-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd crystal lattice in β-Ga 2 O 3 microwire is stronger than that in β-Ga 2 O 3 bulk single crystal.This is an important finding that cannot be overlooked in the application and study of photoelectric devices based on β-Ga 2 O 3 microwire.In this paper, we have systematically studied the crystal structure, optical properties and the temperature-dependent properties of β-Ga 2 O 3 microwire.The XRD spectrum shows that the crystal in the microwire grows along the [020] crystallographic direction.At RT, all the FWHM of all Raman peaks in the low-and middle-frequency range of β-Ga 2 O 3 microwire are much narrower than those of bulk single crystal, while the FWHM of all four Raman peaks in the high-frequency range of the microwire are narrower than that of the bulk single crystal but wider than 90%.These results reveal that the Ga I (O I ) 2 groups, Ga I (O II ) 2 groups and Ga II 3+ ions in microwire have higher symmetry, while there are few differences in the properties between the stretching and bending of Ga I O 4 -tetrahedra in microwire and in bulk single crystal.As the temperature increases, the B g (1) , A g (2) , A g (3) , A g (4) and A g (6) peaks first exhibit blueshifts and then redshifts, a phenomenon different to that for bulk single crystal, while the other ten Raman peaks are redshifted.For PL spectroscopy, the luminescence at RT is bright in the UV band and weak in the blue band, while it almost disappears in the green and red bands, indicating that the β-Ga 2 O 3 microwire has a high crystalline quality and there is no more defect level or impurity level in the band gap.At RT, the photon energies of STE1, STE2 and BE are 3.84, 3.43 and 2.71 eV, respectively.The variable-temperature PL spectroscopy indicates that the FWHM of STE2 broadens with increasing temperature, and the value of the Huang-Rhys factor is Sʹ = 14.08, higher than Sʹ = 8.82 in bulk single crystal, revealing the existence of a strong EPC in the crystal lattice of β-Ga 2 O 3 microwire.012004-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Figure 1 (
b) shows the XRD pattern of β-Ga 2 O 3 microwire.Referring to the β-Ga 2 O 3 XRD JCPDS card #76-0573, the XRD pattern indicates that the crystal in the microwire grows along the [020] crystallographic direction and the crystallinity of the (020)-plane is high.The unit cell diagram of this direction and the positions of unequivalent atoms are shown in Fig. 1(c).
in Ref. 14).They comprise three stretching modes, two bending modes, three deformation modes of Ga I O 4 -tetrahedra, three translations of Ga II 3+ , one libration and three translations of Ga I O 4 -tetrahedra chains.