Thermoreflectance characterization of β-Ga<sub>2</sub>O<sub>3</sub> thin-film nanostrips

doi:10.1364/OE.18.016360

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Thermoreflectance characterization of β-Ga₂O₃ thin-film nanostrips

Ching-Hwa Ho, Chiao-Yeh Tseng, and Li-Chia Tien »View Author Affiliations

Optics Express, Vol. 18, Issue 16, pp. 16360-16369 (2010)
http://dx.doi.org/10.1364/OE.18.016360

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– Abstract
1. Introduction
2. Experimental details
3. Results and discussion
4. Conclusions
– References and links

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Abstract

Nanostructure of β-Ga₂O₃ is wide-band-gap material with white-light-emission function because of its abundance in gap states. In this study, the gap states and near-band-edge transitions in β-Ga₂O₃ nanostrips have been characterized using temperature-dependent thermoreflectance (TR) measurements in the temperature range between 30 and 320 K. Photoluminescence (PL) measurements were carried to identify the gap-state transitions in the β-Ga₂O₃ nanostrips. Experimental analysis of the TR spectra revealed that the direct gap (E₀) of β-Ga₂O₃ is 4.656 eV at 300 K. There are a lot of gap-state and near-band-edge (GSNBE) transitions denoted as E_D3, E_W1, E_W2, E_W3, E_D2, $E_{DB}^{ex}$ , E_DB, E_D1, E₀, and E₀′ can be detected in the TR and PL spectra at 30 K. Transition origins for the GSNBE features in the β-Ga₂O₃ nanostrips are respectively evaluated. Temperature dependences of transition energies of the GSNBE transitions in the β-Ga₂O₃ nanostrips are analyzed. The probable band scheme for the GSNBE transitions in the β-Ga₂O₃ nanostrips is constructed.

1. Introduction

The use of ultraviolet (UV) portion light in optical absorber (such as solar cell) and optical emitter (such as white lighting) is crucial for sustainable energy and solid-state lighting fields. For optical absorption and emission of the UV lights, wide-band-gap material plays an important role for such an achievement. In the solar-energy related field, the I-III-VI wide-band-gap material of CuAlS₂ (~3.49 eV) was proposed to act as a window material for cascade thin-film solar cell to improve quantum efficiency in the UV region [1

M. Caglar, S. Ilican, and Y. Caglar, “Structural, morphological and optical properties of CuAlS₂ films deposited by spray pyrolysis method,” Opt. Commun. 281(6), 1615–1624 (2008). [CrossRef]

C. H. Ho, “Thermoreflectance characterization of band-edge excitonic transitions in CuAlS₂ ultraviolet solar-cell material,” Appl. Phys. Lett. 96(6), 061902 (2010). [CrossRef]

]. On the other hand, for the solid-state white lighting, the III-V nitride as GaN [3

B. Monemar, “Luminescences in III-nitrides,” Mater. Sci. Eng. B 59(1-3), 122–132 (1999). [CrossRef]

], II-VI compound as ZnO [4

C. H. Ho, Y. J. Chen, H. W. Jhou, and J. H. Du, “Optical anisotropy of ZnO nanocrystals on sapphire by thermoreflectance spectroscopy,” Opt. Lett. 32(18), 2765–2767 (2007). [CrossRef] [PubMed]

], and III-VI compound as Ga₂O₃ [5

S. C. Vanithakumari and K. K. Nanda, “A one-step method for the growth of Ga₂O₃-Nanorod-based white-light-emitting phosphors,” Adv. Mater. 21(35), 3581–3584 (2009). [CrossRef]

G. Sinha and A. Patra, “Generation of green, red and white light from rare-earth doped Ga₂O₃ nanoparticles,” Chem. Phys. Lett. 473(1-3), 151–154 (2009). [CrossRef]

] are some of the promising candidates. Among them, β-Ga₂O₃ possesses the largest gap and hence the widest tunable spectral range as comparing to those of the other wide-band-gap semiconductors.

β-Ga₂O₃ nanostructures have recently received growing attentions in fabrication of white-light broadband emitter [7

Y. P. Song, H. Z. Zhang, C. Lin, Y. W. Zhu, G. H. Li, F. H. Yang, and D. P. Yu, “Luminescence emission originating from nitrogen doping of β-Ga₂O₃ nanowires,” Phys. Rev. B 69(7), 075304 (2004). [CrossRef]

–9

L. Dai, X. L. Chen, X. N. Zhang, A. Z. Jin, T. Zhou, B. Q. Hu, and Z. Zhang, “Growth and optical characterization of Ga₂O₃ nanobelts and nanosheets,” J. Appl. Phys. 92(2), 1062–1064 (2002). [CrossRef]

] and nanophotonic switch [10

C. H. Hsieh, L. J. Chou, G. R. Lin, Y. Bando, and D. Golberg, “Nanophotonic switch: gold-in-Ga₂O₃ peapod nanowires,” Nano Lett. 8(10), 3081–3085 (2008). [CrossRef] [PubMed]

] due to their better performance in optics. The abundance of gap states naturally existed inside the nanowires [7

E. Nogales, B. Méndez, and J. Piqueras, “Cathodoluminescence from β-Ga₂O₃ nanowires,” Appl. Phys. Lett. 86(11), 113112 (2005). [CrossRef]

], nanobelts and nanosheets [9

] of β-Ga₂O₃, which may emit a wide spectral range of defect emissions from 1.8 to 3.4 eV. Energy band gap is the most important parameter which dominates main optical absorption and emission in semiconductors. The direct band gap of β-Ga₂O₃ has only been probed by the optical-absorption related methods of transmittance and excitation to date [11

H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga₂O₃ ,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]

,12

K. Shimamura, E. G. Villora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped β-Ga₂O₃ single crystals,” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]

]. The deviation of choosing energy range in the absorption edge of β-Ga₂O₃ may be a factor for determining the band gap. Besides, the wide-band-gap character (~4.656 eV) of the β-Ga₂O₃ nanostructures may arise certain difficulty in making the direct-band-gap emission by using PL because a very short-wavelength pumping laser is needed for the experiment. Therefore, an effective method that can determine and verify the direct band-edge nature of the β-Ga₂O₃ nanostructures is more essential to be implemented.

In this paper, the whole gap-state and near-band-edge (GSNBE) transitions of β-Ga₂O₃ nanostrips are characterized by temperature-dependent thermoreflectance (TR) measurements in the temperature range of 30 to 320 K. Modulation spectroscopy such as TR is a powerful technique for the characterization of direct interband transitions in semiconductors [13

F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. Rep. 10(7-8), 275–374 (1993). [CrossRef]

]. The derivative-like nature of modulation spectra suppresses uninteresting background effects and emphasizes the transition features localized in direct transitions. According to the low-temperature TR spectra of the β-Ga₂O₃ nanostrips, two band-edge transitions of E₀ and E₀′ can be detected by the TR measurements. The energy values are E₀ = 4.726 eV and E₀′ = 4.978 eV at 30 K. The energy separation of the band-edge transitions E₀ and E₀′ may come from the valence-band splitting of the β-Ga₂O₃ nanostrips by crystal field. Temperature-dependent PL measurements were also carried out to identify the below-band-gap defect transitions of the β-Ga₂O₃ nanostrips from 30 to 320 K. Based on the experimental analyses, the whole band-edge and defect structures of β-Ga₂O₃ nanostrips were thus constructed.

2. Experimental details

The β-Ga₂O₃ nanostrips were grown by a vapor transport method driven by vapor-liquid-solid (VLS) mechanism [14

R. S. Wagner and W. C. Ellis, “Vapor-liquid-solid mechanism of single crystal growth,” Appl. Phys. Lett. 4(5), 89–90 (1964). [CrossRef]

]. A horizontal tube furnace was used for the growth. Molten gallium (0.5g, 99.9999%) was the source material and put into a boat placed at the center of the quartz tube. A thin layer of gold (5 nm) was pre-deposited on silicon (001) as the catalyst by sputtering. Substrates were placed in another boat and positioned downstream. The quartz tube was evacuated and the furnace was heated to 900°C at a rate of 10°C/min. Highly pure oxygen carried by argon gas was fed into the quartz tube. The growth temperature was 600–700°C with a background pressure of 0.5 Torr. Typical growth time was 2 h. After the growth, samples were then cooled down under the same ambient gas.

TR measurements of the β-Ga₂O₃ nanostrips were carried out using indirect heating manner with a gold-evaporated quartz plate as the heating element [15

C. H. Ho, H. W. Lee, and Z. H. Cheng, “Practical thermoreflectance design for optical characterization of layer semiconductors,” Rev. Sci. Instrum. 75(4), 1098–1102 (2004). [CrossRef]

,16

C. H. Ho, M. C. Tsai, and M. S. Wong, “Characterization of indirect and direct interband transitions of anatase TiO₂ by thermoreflectance spectroscopy,” Appl. Phys. Lett. 93(8), 081904 (2008). [CrossRef]

]. The thin sheet-type sample was closely attached on the heating element by silicone grease. The on-off heating disturbance was necessary to avoid any increase of temperature on sample. An 150 W xenon-arc lamp filtered by a PTI 0.2 m monochromator provided the monochromatic light. The average spot size of the incident light was 0.5 mm². The reflected and scattered lights of the sample were collected and detected by a Hamamatsu H3177-51 photomultiple tube (PMT) module and the signal was recorded from an EG&G model 7265 dual phase lock-in amplifier. A closed-cycle cryogenic refrigerator with a thermometer controller facilitated the temperature-dependent measurements.

The PL measurements were carried out in a spectral measurement system in which a Triax 320 imaging spectrometer equipped with three gratings of 600, 1200, and 2400 groves/mm acted as the optical dispersion unit. A photomultiple tube attached to the outside slit of the spectrometer utilized for the optical detection. AC phase-sensitive detection (PSD) was implemented using a lock-in amplifier to improve the signal-to-noise (S/N) ratio. A Q-switched diode-pumped solid-state laser of 266 nm acted as the pumping light source. The measurements were done in the temperature range between 30 and 320 K with a temperature stability of about 0.5 K or better. A RMC model 22 closed-cycle cryogenic refrigerator equipped with a model 4075 digital thermometer controller facilitated the temperature dependent measurements.

3. Results and discussion

Figure 1 shows the scanning electron microscope (SEM) images (top view and side view) and X-ray diffraction pattern of the as-grown β-Ga₂O₃ thin-film nanostrips. The β-Ga₂O₃ nanostrips have the widths of tens to hundreds nanometers grown on Si substrate. The nanostrips and nanobelt-like β-Ga₂O₃ are not closely dense and which deposited on the Si substrate with an average thickness of ~210 μm. One single nanostrip with width of ~55 nm was also magnified and displayed in the inset of Fig. 1(a) for indication and comparison. The X-ray diffraction pattern presented in Fig. 1(c) reveals narrow linewidth and clear peak features for the as-grown gallium oxide thin film. This result indicates high crystal quality for the gallium oxide nanostrips. The X-ray peak features in Fig. 1 also respectively indexed to the β phase of monoclinic Ga₂O₃. The lattice constants of the β-Ga₂O₃ thin-film nanostrips are determined to be a = 12.23 Å, b = 3.04 Å, c = 5.81 Å, and β = 103.83°, respectively.

Fig. 1 SEM images of (a) top view and (b) side view of the as-grown β-Ga₂O₃ thin-film nanostrips. (c) X-ray diffraction pattern of β-Ga₂O₃ nanostrips.

Displayed in Fig. 2 are the TR and PL spectra of the GSNBE transitions in the β-Ga₂O₃ nanostrips at 30 and 300 K, respectively. The TR measuerements were carried out in a wide energy range of 1.25 to 6 eV. The dashed lines are the experimental data and hollow-circle lines are least-square fits to a Lorentzian line-shape function appropriate for the interband transitions expressed as [11

H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga₂O₃ ,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]

,17

D. E. Aspnes, in Handbook on Semiconductors , edited by M. Balkanski, (North Holland, Amsterdam, 1980).

\frac{Δ R}{R} = Re [\sum_{i = 1}^{n} A_{i}^{} e^{j φ_{i}^{}} {(E - E_{i}^{} + j Γ_{i}^{})}^{- 2}],

(1)

where

A_{i}^{}

and

φ_{i}^{}

are the amplitude and phase of the line shape, and

E_{i}^{}

and

Γ_{i}^{}

are the energy and broadening parameter of the interband transitions of β-Ga₂O₃ nanostrips. The fits yield transition energies are indicated by arrows in Fig. 2. There are a lot of GSNBE transitions denoted as E_D3, E_W,

E {}_{D B}^{e x}

, E_DB, E₀, and E₀′ could be detected in the TR spectrum at 30 K while the

E {}_{D B}^{e x}

feature was not clearly detected in the TR spectrum at 300 K due to the thermal ionization and broadened effect contributed to the excitonic transition. There are two band-edge transitions of E₀ and E₀′ can be detected in the TR spectra at 30 K and 300 K. The energies obtained from the line-shape fits of Eq. (1) are E₀ = 4.726 eV and E₀′ = 4.978 eV at 30 K, and E₀ = 4.656 eV and E₀′ = 4.908 eV at 300 K, respectively. The energy separation of E₀ and E₀′ is about 0.252 eV, which is in agreement with the crystal-field splitting energy that determined by polarized transmittance spectra of E||b and E⊥c for an undoped β-Ga₂O₃ single crystal [12

K. Shimamura, E. G. Villora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped β-Ga₂O₃ single crystals,” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]

]. The E₀ and E₀′ features may come from the Γ_V top → Γ_C bottom as well as split Γ_V → Γ_C bottom at Γ point for β-Ga₂O₃. The E₀ and E₀′ transitions also demonstrate an energy-redshift behavior as well as an amplitude-reduced character with respect to the increase of temperatures from 30 to 300 K, such as the general semiconductor behavior. It also verifies that the E₀ and E₀′ features are the band-to-band transitions. The transition energies of E_DB obtained from the line-shape fits of Eq. (1) are 3.410 eV at 30 K and 3.373 eV at 300 K, respectively. The temperature-energy shift of E_DB shows less sensitive to those of the E₀ and E₀′ transitions. The E_DB transition feature may come from the Γ_V top → a defect donor band (DB) constructed by oxygen vacancy (V_O) inside the β-Ga₂O₃ nanostrips. The energy of the

E {}_{D B}^{e x}

feature at 30 K by TR is 3.362 eV. It also presented in a sharp exciton peak in the PL spectrum of β-Ga₂O₃ nanostrips at 30 K. The

E {}_{D B}^{e x}

emission can be assigned as the bound excitonic luminescence from the ground-state exciton level below the defect donor band DB to the Γ_V top. The transition energies of the lower-energy TR features of E_D3 and E_W analyzed by Eq. (1) are about 2.390 ± 0.008 eV and 2.616 ± 0.008 eV with the temperatures rising from 30 to 300 K. The insensitive character of the temperature-energy shift of the E_D3 and E_W features indicate that they are coming from the defect-to-defect transitions. The defect transitions are not sensitive to the lattice thermal expansion of β-Ga₂O₃ nanostrips when temperature is changed. As shown in Fig. 2, there is also a defect transition denoted as E_D2 can be detected in the PL spectra while it is so weak and not clearly observed in the TR spectra at 30 and 300 K. This result is due to the E_D2 defect transition may come from a lower-electron-density defect state (such as Ga vacancy V_Ga) to a higher-electron-density defect state (such as O vacancy V_O) to render low E_D2 transition amplitude in TR but high E_D2 luminescence intensity in PL.

Fig. 2 The experimental TR and PL spectra of β-Ga₂O₃ nanostrips between 1 and 6 eV at 30 and 300 K. The hollow-circle lines are the least-square fits to Eq. (1).

Temperature-dependent PL spectra of the β-Ga₂O₃ nanostrips are depicted in Fig. 3 in the temperature range between 30 and 320 K. There are several defect related transition features of E_D3, E_W1, E_W2, E_W3, E_D2, and

E_{DB}^{ex}

can be detected in the low-temperature PL spectrum of 30 K. The E_D3 feature can also be clearly detected in the TR spectra of Fig. 2 from 30 to 300 K. The energies of the E_D3 feature at various temperatures are nearly invariant to a value of about 2.390 ± 0.008 eV. It should come from a defect-to-defect emission by the donor-to-acceptor recombination. The E_D3 feature can be assigned as a defect transition between the DB bottom by V_O and the higher-energy state of acceptor defect band by V_Ga′. There are also three defect emissions of E_W1, E_W2 and E_W3 observed in the low-temperature PL spectra of the β-Ga₂O₃ nanostrips. They present temperature insensitive of energy shift. Their energy values are E_W1≈2.555 eV, E_W2≈2.615 eV, and E_W3≈2.715 eV at 30 K and the energy spacings are about 60 meV for E_W2-E_W1 and 100 meV for E_W3-E_W2, respectively. The defect transition features of E_W1, E_W2 and E_W3 look quite similar to previous peculiar peak structures in the absorption spectra of the thin single crystals of β-Ga₂O₃ near the band edge at low temperatures [18

L. Binet and D. Gourier, “Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga₂O₃ ,” Appl. Phys. Lett. 77(8), 1138–1140 (2000). [CrossRef]

]. These specific peak structures come from some of the acceptors assembled into low-dimensional defect clusters, in which quantum size effect occurred near the valence-band edge. The defect-clusters model for donor (larger-size cluster) and acceptor (small-size cluster) has been proposed for β-Ga₂O₃ [19

L. Binet and D. Gourier, “Origin of the blue luminescence of β-Ga₂O₃ ,” J. Phys. Chem. Solids 59(8), 1241–1249 (1998). [CrossRef]

]. The formation of acceptor cluster requires the association of vacancies V_O and V_Ga to from V_O-V_Ga pairs. In that case with quantum size effect, the quantization of transition energies for the acceptor clusters can be expressed as

E_{W n} = E_{W 0} + [\sum_{d = 1}^{d_{\max}} π^{2} \cdot ℏ^{2} / (2 m^{*} \cdot L^{2})] \cdot n_{d}^{2}, n
                        d = 1, 2, 3, \dots

(2)

where E_W0 is the energy difference between the exciton level below DB and the top of the acceptor quantum well (referred to Fig. 7 ), and d_max = 1, 2, or 3 for a one- (1D), two- (2D), or three-dimensional (3D) potential well. The parameters of m* and L are, respectively, the hole effective mass for the acceptor clusters and the well width. The E_W1, E_W2 and E_W3 emission features shown in Fig. 3 are hence assigned as the defect luminescences from the DB exciton level to n = 1, n = 2, and n = 3 quantum-well states for the acceptor clusters by V_O-V_Ga pairs in β-Ga₂O₃ nanostrips. The acceptor state formed by V_O-V_Ga pair is not like that of V_Ga which is empty of electron. The V_Ga can obtain electrons from the V_O in forming the V_O-V_Ga pairs. This result can be evident in Fig. 2 that the E_W feature (by V_O-V_Ga pairs) can be detected in the TR spectrum at 30 K while the E_D2 TR feature (by V_Ga) is absent. As shown in Fig. 3 at 30 K, the analyses of energy values and energy spacings for E_W1, E_W2, and E_W3 using Eq. (2) render the values of E_W0≈2.535 eV and π²⋅

ℏ^{2}

/(2m*⋅L²) ≈20 meV are determined. The energies of the E_W1, E_W2 and E_W3 features make the acceptor clusters much agree with the 1D model (d_max = 1, quantum dot) that similar to those observed in the absorption spectra below the band edge [18

L. Binet and D. Gourier, “Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga₂O₃ ,” Appl. Phys. Lett. 77(8), 1138–1140 (2000). [CrossRef]

]. Assume the value of hole effective mass m* of the acceptor clusters is set equal to the electron rest mass (1⋅m₀ ≈9.1 × 10⁻³¹kg), the value of π²⋅

ℏ^{2}

/(2m*⋅L²) ≈20 meV renders a well width of the cluster size of L ≈2.2 nm. The size is in reasonable agreements with the previous electron-microscopy observation of the defect clusters with the domain sizes from ~2.2 nm to ~2.8 nm in the annealed and non-annealed β-Ga₂O₃ [20

E. G. Víllora, Y. Marukami, T. Sugawara, T. Atou, M. Kikuchi, D. Shindo, and T. Fukuda, “Electron microscopy studies of microstructures in β-Ga₂O₃ single crystals,” Mater. Res. Bull. 37(4), 769–774 (2002). [CrossRef]

]. The temperature variation of the E_W1, E_W2 and E_W3 features in Fig. 3 also indicates that the E_W1 feature is gradually ionized and combined with the well top with temperatures rising up to 140 K. The E_W3 feature is not observed with T ≥ 260K because the n_d = 3 well level is approximately merged with the valence band by thermal effect.

Fig. 3 Temperature-dependent PL spectra of the β-Ga₂O₃ nanostrips between 30 and 320 K.

Fig. 7 Representative band-structure scheme of the gap-state and near-band-edge transitions in β-Ga₂O₃ nanostrips.

Also shown in Fig. 3, one prominent defect exciton peak of

E {}_{D B}^{e x}

of ~3.362 eV was observed in the PL spectrum of the β-Ga₂O₃ nanostrips at 30 K. The

E {}_{D B}^{e x}

feature shows a slight energy red-shift behavior and a line-shape broadened character with respect to the increase of temperatures. The energy red-shift behavior as compared to that of E_D2, E_W1-E_W3, and E_D3 (i.e. defect to defect transition) is due to the

E {}_{D B}^{e x}

emission is originated from the recombination of a DB exciton level to the valence band. With the energy lower than the

E {}_{D B}^{e x}

emission, it also shows several small emission peaks periodically repeated and extended below the main

E {}_{D B}^{e x}

peak. The character of equal-energy spacing for the small peaks indicates that they are coming from the optical-phonon replicas such as those observed in the PL emission of III-V nitride compounds at low temperatures [21

C. H. Ho and J. W. Lee, “Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy,” Opt. Lett. 34(23), 3604–3606 (2009). [CrossRef] [PubMed]

]. Figure 4(a) shows the magnification of energies near the

E {}_{D B}^{e x}

peak. The character of the small periodical peaks of 1O, 2O, 3O, etc. reveals that they are coming from the optical-phonon replicas that reproduced by the main

E {}_{D B}^{e x}

peak. The energy spacing of the phonon replicas in Fig. 4(a) is about 43 meV. In order to evaluate the origin of the phonon replicas, Raman spectroscopy of the thin-film β-Ga₂O₃ nanostrips is carried out at room temperature. Figure 4(b) displays the Raman spectrum of the β-Ga₂O₃ nanostrips sample between 100 and 700 cm⁻¹ at room temperature. There are essentially three groups of vibration modes for the optical phonons of β-Ga₂O₃ [22

D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga₂O₃ ,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]

–24

Y. H. Gao, Y. Bando, T. Sato, Y. F. Zhang, and X. Q. Gao, “Synthesis, Raman scattering and defects of β-Ga₂O₃ nanorods,” Appl. Phys. Lett. 81(12), 2267 (2002). [CrossRef]

]. As shown in Fig. 4(b), the group C (> 580 cm⁻¹) contains two high-frequency modes of 650 cm⁻¹ and 627 cm⁻¹ with A_g symmetry that are contributed by the GaO₄ tetrahedra related optical phonons [22

D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga₂O₃ ,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]

,23

R. Rao, A. M. Rao, B. Xu, J. Dong, S. Sharma, and M. K. Sunkara, “Blushifted Raman scattering and its correlation with the [110] growth direction in gallium oxide nanowires,” J. Appl. Phys. 98(9), 094312 (2005). [CrossRef]

]. The group B possesses three main peak modes of 346 cm⁻¹ (A_g), 415 cm⁻¹ (A_g), and 472 cm⁻¹ (A_g), which are coming from the in-plane Ga₂O₆ octahedra related optical modes. For the group A (< 230 cm⁻¹), three low-frequency modes of 200 cm⁻¹ (A_g), 170 cm⁻¹ (A_g), and 145 cm⁻¹ (B_g) are found, which are coming from the vibration modes by libration and translation of tetrahedra-octahedra chains in β-Ga₂O₃ [22

D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga₂O₃ ,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]

,23

]. The energy spacing of phonon replicas of

E {}_{D B}^{e x}

in Fig. 4(a) is about 43 meV, matches well with the optical mode of group B near the 346 cm⁻¹ peak (~42.9 meV). This mode is dominant in the in-plane symmetric Ga-O vibration mode [22

D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga₂O₃ ,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]

] in the β-Ga₂O₃ nanostrips. The phonon-replicas emissions had ever been found in the cathodoluminescence (CL) spectra of N-doped β-Ga₂O₃ nanowires with an energy separation of 20 meV at 88 K [8

E. Nogales, B. Méndez, and J. Piqueras, “Cathodoluminescence from β-Ga₂O₃ nanowires,” Appl. Phys. Lett. 86(11), 113112 (2005). [CrossRef]

]. The phonon energy matches well with the optical modes of group A in Fig. 4(b). The difference in optical-phonon replicas (i.e. 43 meV vs. 20 meV) is maybe due to their arising from different growth directions of nanostructures and hence different symmetric modes of the vibrated phonons are enhanced in the nanostrips or nanowires.

Fig. 4 (a) The low-temperature PL spectrum and phonon replicas of β-Ga₂O₃ nanostrips near the

E {}_{DB}^{ex}

defect emission. (b) The Raman spectrum of β-Ga₂O₃ thin-film nanostrips.

Figure 5 shows the temperature-dependent TR spectra of the β-Ga₂O₃ thin-film nanostrips between 30 and 320 K. The dashed lines are the experimental data and solid lines are fitted to Eq. (1). The obtained transition energies are shown by arrows and their variation traces indicated by dotted lines. The E_D3 and E_W2 features reveal nearly temperature-invariant character of energy shift with temperatures rising from 30 to 320 K due to their intrinsic property of defect-to-defect transition. The E_DB related transitions show slightly red-shift behavior owing to that E_DB is coming from Γ_V → DB. The band-edge transitions of E₀ and E₀′ are nearly identical to the values of 4.726 and 4.978 eV from 30 to 77 K. The unshifted behavior is similar to the absorption edge measurement for a bulk β-Ga₂O₃ crystal at 4-77 K [11

H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga₂O₃ ,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]

]. For T>77 K, the energies of E₀ and E₀′ show gradually red shift with the increase of temperature such as the general semiconducting behavior. It is noticed that an additional defect feature of E_D1 presents only in the TR spectra of temperature range from 100 to 260 K. The E_D1 feature cannot be clearly detected at low temperatures of 30-77 K because it maybe comes from the V_Ga defect acceptor level, which is initially empty of electron. For T = 100-260 K, the E_D1 feature is detectable and its energy is nearly invariant to a value of ~4.232 eV. This result indicated that it is a defect-to-defect transition. The E_D1 can be assigned as the transition of V_Ga to the highest defect state in DB by either a V_O′ or a Ga_i located near the conduction-band edge. For T≥300 K, The V_O′ or Ga_i defect state is nearly ionized and merged with the conduction band that reduces the TR amplitude of the E_D1 feature as shown in Fig. 5.

Fig. 5 Temperature-dependent TR spectra of the β-Ga₂O₃ nanostrips between 30 and 320 K.

Temperature dependences of transition energies of the E_DB, E₀ and E₀′ features obtained by the temperature-dependent TR spectra of Fig. 5 are depicted in Fig. 6 . The solid lines are fitted to a Bose-Einstein expression E_i(T) = E_iB-a_iB⋅{1 + 2/[exp(θ_iB/T)-1]}, where i is the respective transition feature, a_iB represents the strength of the electron-phonon interaction and Θ_iB corresponds to the average phonon temperature. The fitted values of E_iB, a_iB and Θ_iB for the β-Ga₂O₃ nanostrips are 5.209 ± 0.102 eV, 230 ± 103 meV, and 560 ± 100 K for E₀′ transition, 4.950 ± 0.100 eV, 224 ± 100 meV, and 560 ± 100 K for E₀ transition, and 3.528 ± 0.101 eV, 115 ± 55 meV, and 560 ± 100 K for the E_DB transition, respectively. The values of a_iB for the E₀ and E₀′ (band-to-band transition) are about doubled to that of E_DB with valence band to DB transition. It indicates that the variation speed of temperature-energy shift for E₀ and E₀′ is faster than that of E_DB as those in Fig. 6. The average phonon temperature Θ_iB for the E_DB, E₀ and E₀′ transitions is 560 ± 100 K. The phonon energy value (by kT) agrees well with the phonon replicas of ~43 meV that induced by the group B Raman mode in the β-Ga₂O₃ nanostrips as shown in Fig. 4(b).

Fig. 6 Temperature dependences of transition energies of E₀′, E₀, and E_DB transitions in β-Ga₂O₃ nanostrips.

Based on the experimental analyses of TR and PL measurements, the near-band-edge structure of β-Ga₂O₃ nanostrips is thus constructed and depicted in Fig. 7. The transition energies of defect and band-edge transitions of β-Ga₂O₃ at 30 K are also listed for comparison. The top of valence band (Γ_V) is consisted of oxygen 2p orbital and the lowest of conduction band (Γ_C) constructed by Ga 4s electrons [11

H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga₂O₃ ,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]

,25

K. Yamaguchi, “First principles study on electronic structure of β-Ga₂O₃ ,” Solid State Commun. 131(12), 739–744 (2004). [CrossRef]

] to render a direct gap E₀ of 4.726 eV at 30 K. The valence-band top separates into higher Γ_V and lower Γ_V by crystal-field splitting with the energy of ~0.252 eV. The E₀′ transition comes from the lower split Γ_V → Γ_C. The defect donor band (DB) is mainly consisted by oxygen-vacancy series levels V_O as well as the highest defect donor of V_O′ or Ga_i. The acceptors of β-Ga₂O₃ nanostrips may have Ga vacancies V_Ga and V_Ga′ to form an acceptor band and V_Ga-V_O vacancy-pair clusters to form near 1D quantum-well structure with size of L~2.2 nm. Experimental TR and PL results indicated that there is an excitonic level with a binding energy of ~40-50 meV located below the DB band. All the transition assignments and transition energies of the E_D3, E_W1, E_W2, E_W3, E_D2,

E_{DB}^{ex}

, E_DB, E_D1, E₀, and E₀′ transitions are indicated distinctly in Fig. 7. An overall gap-state and near-band-edge picture for the β-Ga₂O₃ nanostrips is hence being realized by the experimental study.

4. Conclusions

Optical characterization of gap-state and near-band-edge transitions in β-Ga₂O₃ nanostrips by using temperature-dependent TR measurements was firstly reported in the temperature range between 30 and 320 K. PL experiment was carried out to verify and identify the property of the gap-state and near-band-edge transitions in the β-Ga₂O₃. Transition origin, transition energy, optical-phonon behavior, and temperature dependence of gap-state and near-band-edge transitions in β-Ga₂O₃ nanostrips are characterized via the experimental analyses of TR, PL, and Raman spectroscopy. The whole band-edge structure below and above the band gap of β-Ga₂O₃ nanostrips was thus constructed.

Acknowledgments

The authors would like to acknowledge the financial support from the National Science Council of the Republic of China under the grant Nos. NSC 98-2221-E-011-151-MY3 and NSC97-2112-M-259-008-MY2.

References and links

1.	M. Caglar, S. Ilican, and Y. Caglar, “Structural, morphological and optical properties of CuAlS₂ films deposited by spray pyrolysis method,” Opt. Commun. 281(6), 1615–1624 (2008). [CrossRef]
2.	C. H. Ho, “Thermoreflectance characterization of band-edge excitonic transitions in CuAlS₂ ultraviolet solar-cell material,” Appl. Phys. Lett. 96(6), 061902 (2010). [CrossRef]
3.	B. Monemar, “Luminescences in III-nitrides,” Mater. Sci. Eng. B 59(1-3), 122–132 (1999). [CrossRef]
4.	C. H. Ho, Y. J. Chen, H. W. Jhou, and J. H. Du, “Optical anisotropy of ZnO nanocrystals on sapphire by thermoreflectance spectroscopy,” Opt. Lett. 32(18), 2765–2767 (2007). [CrossRef] [PubMed]
5.	S. C. Vanithakumari and K. K. Nanda, “A one-step method for the growth of Ga₂O₃-Nanorod-based white-light-emitting phosphors,” Adv. Mater. 21(35), 3581–3584 (2009). [CrossRef]
6.	G. Sinha and A. Patra, “Generation of green, red and white light from rare-earth doped Ga₂O₃ nanoparticles,” Chem. Phys. Lett. 473(1-3), 151–154 (2009). [CrossRef]
7.	Y. P. Song, H. Z. Zhang, C. Lin, Y. W. Zhu, G. H. Li, F. H. Yang, and D. P. Yu, “Luminescence emission originating from nitrogen doping of β-Ga₂O₃ nanowires,” Phys. Rev. B 69(7), 075304 (2004). [CrossRef]
8.	E. Nogales, B. Méndez, and J. Piqueras, “Cathodoluminescence from β-Ga₂O₃ nanowires,” Appl. Phys. Lett. 86(11), 113112 (2005). [CrossRef]
9.	L. Dai, X. L. Chen, X. N. Zhang, A. Z. Jin, T. Zhou, B. Q. Hu, and Z. Zhang, “Growth and optical characterization of Ga₂O₃ nanobelts and nanosheets,” J. Appl. Phys. 92(2), 1062–1064 (2002). [CrossRef]
10.	C. H. Hsieh, L. J. Chou, G. R. Lin, Y. Bando, and D. Golberg, “Nanophotonic switch: gold-in-Ga₂O₃ peapod nanowires,” Nano Lett. 8(10), 3081–3085 (2008). [CrossRef] [PubMed]
11.	H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga₂O₃ ,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]
12.	K. Shimamura, E. G. Villora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped β-Ga₂O₃ single crystals,” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]
13.	F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. Rep. 10(7-8), 275–374 (1993). [CrossRef]
14.	R. S. Wagner and W. C. Ellis, “Vapor-liquid-solid mechanism of single crystal growth,” Appl. Phys. Lett. 4(5), 89–90 (1964). [CrossRef]
15.	C. H. Ho, H. W. Lee, and Z. H. Cheng, “Practical thermoreflectance design for optical characterization of layer semiconductors,” Rev. Sci. Instrum. 75(4), 1098–1102 (2004). [CrossRef]
16.	C. H. Ho, M. C. Tsai, and M. S. Wong, “Characterization of indirect and direct interband transitions of anatase TiO₂ by thermoreflectance spectroscopy,” Appl. Phys. Lett. 93(8), 081904 (2008). [CrossRef]
17.	D. E. Aspnes, in Handbook on Semiconductors , edited by M. Balkanski, (North Holland, Amsterdam, 1980).
18.	L. Binet and D. Gourier, “Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga₂O₃ ,” Appl. Phys. Lett. 77(8), 1138–1140 (2000). [CrossRef]
19.	L. Binet and D. Gourier, “Origin of the blue luminescence of β-Ga₂O₃ ,” J. Phys. Chem. Solids 59(8), 1241–1249 (1998). [CrossRef]
20.	E. G. Víllora, Y. Marukami, T. Sugawara, T. Atou, M. Kikuchi, D. Shindo, and T. Fukuda, “Electron microscopy studies of microstructures in β-Ga₂O₃ single crystals,” Mater. Res. Bull. 37(4), 769–774 (2002). [CrossRef]
21.	C. H. Ho and J. W. Lee, “Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy,” Opt. Lett. 34(23), 3604–3606 (2009). [CrossRef] [PubMed]
22.	D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga₂O₃ ,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]
23.	R. Rao, A. M. Rao, B. Xu, J. Dong, S. Sharma, and M. K. Sunkara, “Blushifted Raman scattering and its correlation with the [110] growth direction in gallium oxide nanowires,” J. Appl. Phys. 98(9), 094312 (2005). [CrossRef]
24.	Y. H. Gao, Y. Bando, T. Sato, Y. F. Zhang, and X. Q. Gao, “Synthesis, Raman scattering and defects of β-Ga₂O₃ nanorods,” Appl. Phys. Lett. 81(12), 2267 (2002). [CrossRef]
25.	K. Yamaguchi, “First principles study on electronic structure of β-Ga₂O₃ ,” Solid State Commun. 131(12), 739–744 (2004). [CrossRef]

OCIS Codes
(160.4760) Materials : Optical properties
(160.6000) Materials : Semiconductor materials
(300.6380) Spectroscopy : Spectroscopy, modulation
(300.6470) Spectroscopy : Spectroscopy, semiconductors

ToC Category:
Spectroscopy

History
Original Manuscript: May 17, 2010
Revised Manuscript: July 4, 2010
Manuscript Accepted: July 12, 2010
Published: July 20, 2010

Citation
Ching-Hwa Ho, Chiao-Yeh Tseng, and Li-Chia Tien, "Thermoreflectance characterization of β-Ga₂O₃ thin-film nanostrips," Opt. Express 18, 16360-16369 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-16360

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References

M. Caglar, S. Ilican, and Y. Caglar, “Structural, morphological and optical properties of CuAlS2 films deposited by spray pyrolysis method,” Opt. Commun. 281(6), 1615–1624 (2008). [CrossRef]
C. H. Ho, “Thermoreflectance characterization of band-edge excitonic transitions in CuAlS2 ultraviolet solar-cell material,” Appl. Phys. Lett. 96(6), 061902 (2010). [CrossRef]
B. Monemar, “Luminescences in III-nitrides,” Mater. Sci. Eng. B 59(1-3), 122–132 (1999). [CrossRef]
C. H. Ho, Y. J. Chen, H. W. Jhou, and J. H. Du, “Optical anisotropy of ZnO nanocrystals on sapphire by thermoreflectance spectroscopy,” Opt. Lett. 32(18), 2765–2767 (2007). [CrossRef] [PubMed]
S. C. Vanithakumari and K. K. Nanda, “A one-step method for the growth of Ga2O3-Nanorod-based white-light-emitting phosphors,” Adv. Mater. 21(35), 3581–3584 (2009). [CrossRef]
G. Sinha and A. Patra, “Generation of green, red and white light from rare-earth doped Ga2O3 nanoparticles,” Chem. Phys. Lett. 473(1-3), 151–154 (2009). [CrossRef]
Y. P. Song, H. Z. Zhang, C. Lin, Y. W. Zhu, G. H. Li, F. H. Yang, and D. P. Yu, “Luminescence emission originating from nitrogen doping of β-Ga2O3 nanowires,” Phys. Rev. B 69(7), 075304 (2004). [CrossRef]
E. Nogales, B. Méndez, and J. Piqueras, “Cathodoluminescence from β-Ga2O3 nanowires,” Appl. Phys. Lett. 86(11), 113112 (2005). [CrossRef]
L. Dai, X. L. Chen, X. N. Zhang, A. Z. Jin, T. Zhou, B. Q. Hu, and Z. Zhang, “Growth and optical characterization of Ga2O3 nanobelts and nanosheets,” J. Appl. Phys. 92(2), 1062–1064 (2002). [CrossRef]
C. H. Hsieh, L. J. Chou, G. R. Lin, Y. Bando, and D. Golberg, “Nanophotonic switch: gold-in-Ga2O3 peapod nanowires,” Nano Lett. 8(10), 3081–3085 (2008). [CrossRef] [PubMed]
H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga2O3,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]
K. Shimamura, E. G. Villora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped β-Ga2O3 single crystals,” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]
F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. Rep. 10(7-8), 275–374 (1993). [CrossRef]
R. S. Wagner and W. C. Ellis, “Vapor-liquid-solid mechanism of single crystal growth,” Appl. Phys. Lett. 4(5), 89–90 (1964). [CrossRef]
C. H. Ho, H. W. Lee, and Z. H. Cheng, “Practical thermoreflectance design for optical characterization of layer semiconductors,” Rev. Sci. Instrum. 75(4), 1098–1102 (2004). [CrossRef]
C. H. Ho, M. C. Tsai, and M. S. Wong, “Characterization of indirect and direct interband transitions of anatase TiO2 by thermoreflectance spectroscopy,” Appl. Phys. Lett. 93(8), 081904 (2008). [CrossRef]
D. E. Aspnes, in Handbook on Semiconductors, edited by M. Balkanski, (North Holland, Amsterdam, 1980).
L. Binet and D. Gourier, “Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga2O3,” Appl. Phys. Lett. 77(8), 1138–1140 (2000). [CrossRef]
L. Binet and D. Gourier, “Origin of the blue luminescence of β-Ga2O3,” J. Phys. Chem. Solids 59(8), 1241–1249 (1998). [CrossRef]
E. G. Víllora, Y. Marukami, T. Sugawara, T. Atou, M. Kikuchi, D. Shindo, and T. Fukuda, “Electron microscopy studies of microstructures in β-Ga2O3 single crystals,” Mater. Res. Bull. 37(4), 769–774 (2002). [CrossRef]
C. H. Ho and J. W. Lee, “Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy,” Opt. Lett. 34(23), 3604–3606 (2009). [CrossRef] [PubMed]
D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga2O3,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]
R. Rao, A. M. Rao, B. Xu, J. Dong, S. Sharma, and M. K. Sunkara, “Blushifted Raman scattering and its correlation with the [110] growth direction in gallium oxide nanowires,” J. Appl. Phys. 98(9), 094312 (2005). [CrossRef]
Y. H. Gao, Y. Bando, T. Sato, Y. F. Zhang, and X. Q. Gao, “Synthesis, Raman scattering and defects of β-Ga2O3 nanorods,” Appl. Phys. Lett. 81(12), 2267 (2002). [CrossRef]
K. Yamaguchi, “First principles study on electronic structure of β-Ga2O3,” Solid State Commun. 131(12), 739–744 (2004). [CrossRef]

Aoki, K.
Atou, T.
Bando, Y.
Binet, L.
Caglar, M.
Caglar, Y.
Chen, X. L.
Chen, Y. J.
Cheng, Z. H.
Chou, L. J.
Dai, L.
Dohy, D.
Dong, J.
Du, J. H.
Ellis, W. C.
Fukuda, T.
Gao, X. Q.
Gao, Y. H.
Golberg, D.
Gourier, D.
Ho, C. H.
Hsieh, C. H.
Hu, B. Q.
Ilican, S.
Jhou, H. W.
Jin, A. Z.
Kikuchi, M.
Lee, H. W.
Lee, J. W.
Li, G. H.
Lin, C.
Lin, G. R.
Lucazeau, G.
Marukami, Y.
Méndez, B.
Monemar, B.
Nanda, K. K.
Nogales, E.
Patra, A.
Piqueras, J.
Pollak, F. H.
Rao, A. M.
Rao, R.
Revcolevschi, A.
Sato, T.
Sharma, S.
Shen, H.
Shimamura, K.
Shindo, D.
Sinha, G.
Song, Y. P.
Sugawara, T.
Sunkara, M. K.
Tippins, H. H.
Tsai, M. C.
Ujiie, T.
Vanithakumari, S. C.
Villora, E. G.
Víllora, E. G.
Wagner, R. S.
Wong, M. S.
Xu, B.
Yamaguchi, K.
Yang, F. H.
Yu, D. P.
Zhang, H. Z.
Zhang, X. N.
Zhang, Y. F.
Zhang, Z.
Zhou, T.
Zhu, Y. W.

Adv. Mater.

S. C. Vanithakumari and K. K. Nanda, “A one-step method for the growth of Ga2O3-Nanorod-based white-light-emitting phosphors,” Adv. Mater. 21(35), 3581–3584 (2009). [CrossRef]

Appl. Phys. Lett.

C. H. Ho, “Thermoreflectance characterization of band-edge excitonic transitions in CuAlS2 ultraviolet solar-cell material,” Appl. Phys. Lett. 96(6), 061902 (2010). [CrossRef]
E. Nogales, B. Méndez, and J. Piqueras, “Cathodoluminescence from β-Ga2O3 nanowires,” Appl. Phys. Lett. 86(11), 113112 (2005). [CrossRef]
K. Shimamura, E. G. Villora, T. Ujiie, and K. Aoki, “Excitation and photoluminescence of pure and Si-doped β-Ga2O3 single crystals,” Appl. Phys. Lett. 92(20), 201914 (2008). [CrossRef]
R. S. Wagner and W. C. Ellis, “Vapor-liquid-solid mechanism of single crystal growth,” Appl. Phys. Lett. 4(5), 89–90 (1964). [CrossRef]
L. Binet and D. Gourier, “Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga2O3,” Appl. Phys. Lett. 77(8), 1138–1140 (2000). [CrossRef]
C. H. Ho, M. C. Tsai, and M. S. Wong, “Characterization of indirect and direct interband transitions of anatase TiO2 by thermoreflectance spectroscopy,” Appl. Phys. Lett. 93(8), 081904 (2008). [CrossRef]
Y. H. Gao, Y. Bando, T. Sato, Y. F. Zhang, and X. Q. Gao, “Synthesis, Raman scattering and defects of β-Ga2O3 nanorods,” Appl. Phys. Lett. 81(12), 2267 (2002). [CrossRef]

Chem. Phys. Lett.

G. Sinha and A. Patra, “Generation of green, red and white light from rare-earth doped Ga2O3 nanoparticles,” Chem. Phys. Lett. 473(1-3), 151–154 (2009). [CrossRef]

J. Appl. Phys.

L. Dai, X. L. Chen, X. N. Zhang, A. Z. Jin, T. Zhou, B. Q. Hu, and Z. Zhang, “Growth and optical characterization of Ga2O3 nanobelts and nanosheets,” J. Appl. Phys. 92(2), 1062–1064 (2002). [CrossRef]
R. Rao, A. M. Rao, B. Xu, J. Dong, S. Sharma, and M. K. Sunkara, “Blushifted Raman scattering and its correlation with the [110] growth direction in gallium oxide nanowires,” J. Appl. Phys. 98(9), 094312 (2005). [CrossRef]

J. Phys. Chem. Solids

L. Binet and D. Gourier, “Origin of the blue luminescence of β-Ga2O3,” J. Phys. Chem. Solids 59(8), 1241–1249 (1998). [CrossRef]

J. Solid State Chem.

D. Dohy, G. Lucazeau, and A. Revcolevschi, “Raman spectra and valence force field of single-crystalline β Ga2O3,” J. Solid State Chem. 45(2), 180–192 (1982). [CrossRef]

Mater. Res. Bull.

E. G. Víllora, Y. Marukami, T. Sugawara, T. Atou, M. Kikuchi, D. Shindo, and T. Fukuda, “Electron microscopy studies of microstructures in β-Ga2O3 single crystals,” Mater. Res. Bull. 37(4), 769–774 (2002). [CrossRef]

Mater. Sci. Eng. B

B. Monemar, “Luminescences in III-nitrides,” Mater. Sci. Eng. B 59(1-3), 122–132 (1999). [CrossRef]

Mater. Sci. Eng. Rep.

F. H. Pollak and H. Shen, “Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices,” Mater. Sci. Eng. Rep. 10(7-8), 275–374 (1993). [CrossRef]

Nano Lett.

C. H. Hsieh, L. J. Chou, G. R. Lin, Y. Bando, and D. Golberg, “Nanophotonic switch: gold-in-Ga2O3 peapod nanowires,” Nano Lett. 8(10), 3081–3085 (2008). [CrossRef] [PubMed]

Opt. Commun.

M. Caglar, S. Ilican, and Y. Caglar, “Structural, morphological and optical properties of CuAlS2 films deposited by spray pyrolysis method,” Opt. Commun. 281(6), 1615–1624 (2008). [CrossRef]

Opt. Lett.

Phys. Rev.

H. H. Tippins, “Optical absorption and photoconductivity in the band edge of β-Ga2O3,” Phys. Rev. 140(1A), A316–A319 (1965). [CrossRef]

Phys. Rev. B

Y. P. Song, H. Z. Zhang, C. Lin, Y. W. Zhu, G. H. Li, F. H. Yang, and D. P. Yu, “Luminescence emission originating from nitrogen doping of β-Ga2O3 nanowires,” Phys. Rev. B 69(7), 075304 (2004). [CrossRef]

Rev. Sci. Instrum.

C. H. Ho, H. W. Lee, and Z. H. Cheng, “Practical thermoreflectance design for optical characterization of layer semiconductors,” Rev. Sci. Instrum. 75(4), 1098–1102 (2004). [CrossRef]

Solid State Commun.

K. Yamaguchi, “First principles study on electronic structure of β-Ga2O3,” Solid State Commun. 131(12), 739–744 (2004). [CrossRef]

Other

D. E. Aspnes, in Handbook on Semiconductors, edited by M. Balkanski, (North Holland, Amsterdam, 1980).

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