Effects of Zn Doping on Optical Properties of Polycrystalline β-Ga₂O₃

Abstract

In this study, Zn-doped Ga₂O₃ polycrystalline samples were prepared by solid-phase sintering, and the effects of Zn doping on the optical properties of Ga₂O₃ were investigated. It is found that the introduced Zn ions disrupted the Ga-O bonds and formed Zn_Ga, altering the Ga-O vibration modes and causing a blue shift in the related Raman mode. From near-infrared to visible light-range was a transparent region for Zn-doped Ga₂O₃. The fundamental optical bandgap underwent a decrease with increasing Zn doping content, primarily due to the p-d orbital hybridization of the O 2p and Zn 3d orbitals causing an upward shift valence band maximum and band renormalization effect-induced band-tails. The recombination of electrons at donor levels (V_O) and holes at acceptor levels (V_Ga or V_O-V_Ga) gave rise to blue-green luminescence. Zn doping increased the concentration oxygen vacancies (V_O), resulting in significant blue-green luminescence enhancement in Zn-doped Ga₂O₃. Additionally, Zn doping resulted in a noticeable reduction in the red luminescence of Ga₂O₃, which may be attributed to Zn doping suppressing nitrogen incorporation from the air during high-temperature preparation processes.

1. Introduction

In recent years, with the unceasing development of semiconductor science and technology, great progress has been achieved in both the integration density and processing speed of microelectronic devices. However, traditional semiconductors face critical limitations in meeting the operational demands of high-frequency, high-power systems and optoelectronic device applications, primarily due to the inherent disadvantages of their narrow band-gap width and low breakdown voltage characteristics. Consequently, new-generation wide-bandgap compounds, such as gallium nitride, silicon carbide, and gallium oxide, have become a predominant research focus in advanced semiconductor physics. Among these compounds, monoclinic gallium oxide (β-Ga₂O₃) is a wide-bandgap oxide semiconductor material with a bandgap of approximately 4.9 eV [1,2,3,4]. It exhibits a high breakdown electric field and high transparency in the visible and near-ultraviolet light ranges, along with excellent thermal and chemical stability. These distinctive properties render β-Ga₂O₃ useful for extensive applications in high-power electrical devices [5], gas sensors [6], solar-blind photodetectors [7,8], Schottky barrier diodes [9], transparent electrodes [10], photovoltaic cells [11], and so on.

The excellent performance of β-Ga₂O₃-based devices is closely related to the intrinsic properties of β-Ga₂O₃ and doping works as an indispensable strategy for further property optimization. Numerous effects have been made in this area. N-type doping can be readily achieved by introducing Sn or Si ions into β-Ga₂O₃. Wang et al. used metal-organic chemical vapor deposition (MOCVD) to deposit uniform Sn-doped β-Ga₂O₃ thin films on β-Ga₂O₃(100) substrates, reducing the resistivity by more than eight orders of magnitude [12]. In addition, Si-doped Ga₂O₃ thin films fabricated by PLD (Pulsed Laser Deposition) have exhibited over 90% transparency in the visible region, with a carrier concentration of 9.1 × 10¹⁹ cm⁻³, a conductivity of 732 Scm⁻¹, and a mobility of 26.5 cm²V⁻¹s⁻¹ [13]. As the Si doping concentration increases, the formation of secondary phases (SiO₂) leads to an increase in the bandgap [13]. Based on these studies, n-type Ga₂O₃ is readily available through the existence of native defects such as oxygen vacancies, acting as shallow donors [14].

However, realizing viable p-type doping of Ga₂O₃ still remains an enormous challenge in Ga₂O₃ research and applications. It is mainly because Ga₂O₃ contains numerous intrinsic defects such as gallium vacancies (V_Ga) and gallium–oxygen vacancy pairs (V_Ga-V_O), which often result in deep acceptor levels, making it difficult for holes to enter the valence band [15]. Low dopant solubility limits (<5 × 10¹⁷ cm⁻³ for Mg in β-Ga₂O₃) [16] may be another important reason. Even so, it has been verified that divalent ion incorporation, such as with Mg²⁺ or Zn²⁺, enables Ga₂O₃ to display p-type semiconductor behavior. Li et al. used first-principle calculations to study the electronic structure and optical properties of intrinsic Ga₂O₃ and Zn-doped Ga₂O₃, finding that Zn-doped Ga₂O₃ has the potential to serve as a typical p-type semiconductor [17]. Besides this, Mg-doped Ga₂O₃ thin films were deposited on MgO substrates by MOCVD, observing a degradation in crystallinity after doping. These films exhibit excellent transmittance in the visible light region, and the optical bandgap can be modulated within the range of 4.87 to 5.22 eV [18]. Vasyltsiv et al. enhanced the electrical conductivity of p-type doped β-Ga₂O₃ by changing the Mg doping concentration. It is discovered that when the Mg-doping concentration is 0.05%, the conductivity was more than 1000 times greater than that of a 1% Mg-doping concentration [19].

Based on what is discussed above, current research efforts on Ga₂O₃ have primarily focused on thin-film and single-crystalline bulk configurations, whereas investigations on polycrystalline Ga₂O₃ remain notably scarce in the literature. Polycrystalline materials are low-cost and easily obtainable, rendering them highly valuable for applications. Besides this, in the polycrystalline state, the long-range order of the crystal is disrupted. This raises fundamental physical issues concerning polycrystalline Ga₂O₃, such as lattice vibrations, photoluminescence, and band transitions, which remain to be explored. Investigating these aspects would shed light into the fundamental physical processes and intrinsic nature of Ga₂O₃. Therefore, in this study, Zn-doped Ga₂O₃ polycrystalline samples were prepared using solid-phase sintering. Through a series of structural and optical characterizations, the effects of Zn doping on the crystal structure and optical properties of Ga₂O₃ were revealed.

2. Results and Discussion

2.1. The Effect of Zn Doping on the CRYSTAL Structure and Surface Morphology of β-Ga₂O₃

Figure 1a shows the XRD patterns of pure Ga₂O₃ and Zn-doped Ga₂O₃ samples. Firstly, the pure Ga₂O₃ sample exhibits a monoclinic Ga₂O₃ phase [20]. The peaks are in complete agreement with those of intrinsic Ga₂O₃ and no impurity peaks are observed in the XRD patterns of the Zn-doped samples, indicating that the Zn-doped samples retain the monoclinic crystal structure of Ga₂O₃. From the magnified scale of the XRD patterns (Figure 1b), it can be seen that the main diffraction peak positions of the Zn-doped samples shift to lower angles as the Zn concentration increases. This suggests that the lattice parameters gradually increase with the increasing Zn content. On account of the fact that the ionic radius of Zn²⁺ (0.74 Å) is slightly larger than that of Ga³⁺ (0.62 Å) [21], the substitution of Ga³⁺ ions by Zn²⁺ ions gives rise to an increase in lattice constants and the occurrence of lattice distortion. Therefore, it is indicated that Zn ions are indeed incorporated into the lattices of β-Ga₂O₃.

Figure 2 shows the surface morphology images of Ga₂O₃ doped with different concentrations of Zn. All samples exhibit a dense structure with uniform grain size. After Zn doping, the morphology of the sample grains changes little, but the grain size greatly decreases. This indicates that the incorporation of Zn may increase the crystal growth energy of Ga₂O₃, inhibiting grain growth and resulting in a microstructure with smaller grains.

2.2. The Influence of Zn Doping on the Phonon Vibrations of β-Ga₂O₃

As is well known, the primitive unit cell of β-Ga₂O₃ contains four Ga atoms and six O atoms, forming two tetrahedral (Ga_IO₄) and two octahedral (Ga_IIO₆) structures. Through analysis based on the C2/m space group, usually, there are 27 optical phonon modes at the Γ point [22]:

$${\Gamma }_{opt}=10A_g+5B_g+4A_u+8B_u$$

(1)

Among these, the $A_g$ and $B_g$ modes are Raman-active, while the $A_u$ and $B_u$ modes are infrared-active. The Raman spectra of the pure Ga₂O₃ and Zn-doped Ga₂O₃ polycrystalline samples are shown in Figure 3.

Overall, the Raman spectra of both pure and Zn-doped Ga₂O₃ samples exhibit 10 A_g modes: $A_g$(1) at 112 cm⁻¹, $A_g$(2) at 168 cm⁻¹, $A_g$(3) at 199 cm⁻¹, $A_g$(4) at 321 cm⁻¹, $A_g$(5) at 346 cm⁻¹, $A_g$(6) at 416 cm⁻¹, $A_g$(7) at 473 cm⁻¹, $A_g$(8) at 630 cm⁻¹, $A_g$(9) at 655 cm⁻¹, and $A_g$(10) at 766 cm⁻¹, as well as one $B_g$(2) mode at 144cm⁻¹. These 11 Raman peaks are in complete agreement with the typical Raman peak positions of monoclinic Ga₂O₃ reported in the literature [22]. Comparing the Raman spectra of pure Ga₂O₃ with Zn-doped samples, the samples retain a monoclinic structure, which is consist with the XRD results. Most of the Raman peak positions do not show considerable shifts. However, when the doping concentration approaches 10 mol%, the doped sample shows a 2 cm⁻¹ blue shift of the Raman peak (201 cm⁻¹) relative to the corresponding peak of the pure sample at 199 cm⁻¹. The Raman peak at 201 cm⁻¹ is a typical Ga-O chain vibration mode. Due to the doping of Zn, Zn atoms replace some Ga atoms to form Zn_Ga, which disrupts the Ga-O bonds and alters their vibrational modes, resulting in a blue shift of the peak position.

2.3. The Impact of Zn Doping on the Fundamental Optical Properties of β-Ga₂O₃

To investigate the effect of Zn doping on the fundamental optical constants and optical bandgap of Ga₂O₃, spectroscopic ellipsometry (SE) measurements were performed on both pure and doped samples. The refractive index (n) obtained from SE as a function of the wavelength of the incident light in pure and Zn-doped Ga₂O₃ are showed in Figure 4. From the near-infrared to the visible light region, the refractive index gradually increases as the wavelength of the incident photons decreases; this indicates that it may be transparent for Zn-doped Ga₂O₃ polycrystalline samples this wavelength range. Generally, in the transparent region, the dispersion behavior can be depicted by the single oscillator Sellmeier model [23]:

$${n\left(\lambda \right)}^2-1={\displaystyle \frac{S_0{\lambda }_0}{1-{\left(\frac{{\lambda }_0}{\lambda }\right)}^2}}$$

(2)

where λ₀ is the average oscillator position and S₀ is the average oscillator strength. The refractive index n of the Zn-doped samples in the transparent region was well-fitted by a single-oscillator Sellmeier relationship, as shown in Figure 5, confirming that this region is transparent for Zn-doped Ga₂O₃ polycrystalline samples. As the wavelength of the incident photons further decreases into the ultraviolet region, the refractive index n rapidly increases and exhibits a distinct peak. This indicates that the Zn-doped Ga₂O₃ polycrystalline samples begin to absorb the incident light in this range, entering the weak or intrinsic absorption region. As usual, the peak in the refractive index n may be assigned to the transitions between the critical points (CP) [24]. The peaks of the refractive index n for the intrinsic and Zn-doped samples are located at 220, 250, 260, and 265 nm, respectively, suggesting that the critical point transition energy decreases with increasing doping concentrations. Given that the value of photon energy corresponding to the peaks of n is about 4–5 eV, which is in the range of the value of Ga₂O₃ band gap (4–5 eV), this CP point thus may be associated with the interband transition between the valence and conduction bands of Ga₂O₃. This suggests that the optical band-gap would shrink with increases in the Zn doping concentration.

To determine the optical bandgap of the samples, in terms of the formula α = 4πkλ, the absorption coefficient α can be obtained from the extinction coefficient k, which is derived from SE. According to the Tauc relationship [12], a fundamental optical absorption rule for semiconductors, the relationship between the optical bandgap Eg and the absorption coefficient α for pure and Zn-doped Ga₂O₃ polycrystalline materials in the ultraviolet/visible region is given by:

$${\left(\alpha h\nu \right)}^{\mathsf{\gamma }}=A\left(h\nu -E_g\right)$$

(3)

where α is the absorption coefficient, hν is the energy of the incident photons, and A is a material-related constant. For direct bandgap semiconductors, the exponent γ is two, while for indirect bandgap semiconductors, γ is 1/2.

Figure 6 displays the (αhν)² dependence of the incident photon energy hν in Zn-doped samples, ranging from 0 to 10 mol%. The data were well fitted by the Tauc relationship. It was confirmed that both undoped and Zn-doped Ga₂O₃ polycrystalline materials remained direct bandgap semiconductors, with optical bandgaps of 5.2 eV, 4.6 eV, 4.5 eV, and 4.4 eV, respectively, which is consistent with the reported values in the literature (near 5 eV) [25]. As the Zn doping concentration increases, the optical bandgap Eg continuously decreased, which is in good agreement with the above evidence. This doping-induced bandgap narrowing phenomenon can be explained by the following two aspects: First-principle calculations indicate that the valence band maximum (VBM) of intrinsic Ga₂O₃ is formed by the 2p orbitals of oxygen (O), while the conduction band minimum (CBM) is formed by the 4s orbitals of gallium (Ga) [17]. When Zn ions are incorporated into Ga₂O₃ and replace Ga ions, the 2p orbitals of O in the VBM hybridize with the 3d orbitals of Zn, leading to p-d hybridization. This results in an overlap between the O 2p orbitals and Zn 3d orbitals, causing an upward shift in the energy of the VBM, while the CBM remains formed by the Ga 4s orbitals [25]. Consequently, part of the bandgap is constituted by the hybridized O 2p and Zn 3d orbitals and the Ga 4s orbitals [17,26]. Therefore, as the Zn doping concentration increases, the bandgap tends to shrink. Secondly, in the case of doping, particularly heavy doping, the random distribution of dopant ions can form band tails, resulting in band renormalization effects [26]. As the Zn doping concentration increases, the band tails and impurity bands expand, causing them to overlap and effectively narrowing the width of the intrinsic bandgap. It also contributes to reductions in the optical bandgap.

2.4. The Influence of Zn Doping on the Photoluminescence Properties of β-Ga₂O₃

Figure 7 presents the room-temperature PL spectra of Zn-doped Ga₂O₃ polycrystalline samples under excitation by a 325 nm laser. The PL spectrum of pure Ga₂O₃ consists of a broad blue-green emission band (400–600 nm) and a red emission band (620–850 nm). Typically, Ga₂O₃ exhibits three main emission bands: ultraviolet (UV) emissions, blue-green emissions, and red emissions. Harwig et al. [27] suggested that the UV emission arises from the recombination of electrons trapped at donor oxygen vacancies (V_O) with holes in the valence band. In the present case, due to the wavelength limitation of the excitation light, UV emissions were not observed. Broad blue-green emissions, commonly in the range of 350–620 nm, frequently occurred in various Ga₂O₃ nanomaterials [28,29]. This broad blue-green emission band is typically characteristic of bulk and nano-monocrystalline materials [30], originating from the recombination of donor–acceptor defect pairs. Under laser excitation, electrons in the valence band are excited to donor levels, and their subsequent relaxation leads to recombination with holes at acceptor levels, resulting in luminescence. The primary donor defects are oxygen vacancies (V_O), while the acceptors are gallium vacancies (V_Ga) or gallium–oxygen vacancy pairs (V_O-V_Ga) [31]. Therefore, the blue-green emission originates from the recombination of electrons at donor levels (V_O) and holes at acceptor levels (V_Ga or V_O-V_Ga). From Figure 7, it is also evident that, compared to pure Ga₂O₃, Zn doping considerably enhances the intensity of the blue-green emission in Ga₂O₃, but the emission peak position does not show an apparent shift. Divalent Zn ions act as acceptor dopants when the substitute trivalent Ga ions. To maintain electrical neutrality, the system tends to generate doubly charged oxygen vacancies for compensation. This process can be described by:

$$2\mathrm{Z}\mathrm{n}\mathrm{O}\stackrel{{\mathrm{G}\mathrm{a}}_2{\mathrm{O}}_3}{\to }2{\mathrm{Z}\mathrm{n}}_{\mathrm{G}\mathrm{a}}^{\prime }+{\mathrm{V}}_{\mathrm{O}}^{··}+{\mathrm{O}}_{\mathrm{O}}^{\times }+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2(\mathrm{g}\mathrm{a}\mathrm{s})$$

(4)

Thus, Zn doping increases the concentration of donor oxygen vacancies (V_O) compared to undoped cases, and the concentration of acceptor V_O-V_Ga pairs also increases, giving rise to the enhancement of blue-green emissions.

Figure 7. The PL spectrum (range from near−infrared to near-ultraviolet) of Zn−doped Ga₂O₃ with different doping concentration (Ga_2−XZn_XO₃, x = 0, 0.05 and 0.1).

Figure 7. The PL spectrum (range from near−infrared to near-ultraviolet) of Zn−doped Ga₂O₃ with different doping concentration (Ga_2−XZn_XO₃, x = 0, 0.05 and 0.1).

As for the red emission in the PL spectrum of Ga₂O₃ polycrystals, a sharp and continuous red emission is observed in the 690–750 nm range in Figure 7, indicating the transitions between discrete energy levels [32,33]. The mechanism behind this red emission in Ga₂O₃ still remains uncertain and is highly debated. The previous literature has reported that red emissions often appear in Li-, Cr-, and Fe-doped Ga₂O₃ [34,35,36,37], originating from intra-transitions of outer orbital electrons of the doped ions. However, in the present case, the pure Ga₂O₃ oxide was not doped with such ions, so what causes the red emission? Related reports indicate that red emissions also exist in other forms of pure Ga₂O₃ oxides. For example, Ma et al. prepared pure Ga₂O₃ nanorods using a hydrothermal method and obtained a PL spectra almost identical to those seen in this case [38]. We note that N doping can also induce red emissions in Ga₂O₃, and the high-temperature calcination of Ga₂O₃ in an air atmosphere inevitably introduces trace amounts of N, which may be the cause of the red emission in the samples. T. Zhang et al. also observed sharp and continuous red emissions (690–750 nm) in pure Ga₂O₃ samples prepared at high temperatures and attributed it to trace nitrogen (N) introduced during the high-temperature (1350 °C) preparation process [39]. Here, Zn doping causes a noticeable reduction in the red emission of Ga₂O₃. This may be due to the Zn doping suppressing the introduction of N, which causes the weakening of red emissions. T. Zhang et al. also observed a reduction in red emissions in Ge-doped Ga₂O₃ [40].

3. Materials and Methods

A series of polycrystalline samples of Ga_2−XZn_XO₃ (x = 0, 0.05, 0.07, and 0.1) were synthesized by a solid-state reaction. The mixtures of ZnO (99.99%) and Ga₂O₃ (99.99%) with a stoichiometric ratio were ball-milled in deionized water for 8 h. The dried powders were pressed into the pellets. The pressed pellets were calcinated at 1050 °C for 10 h in air, then crushed and ball-milled again for 8 h, and finally sintered at 1500 °C in air for 6 h. After sintering, the samples’ surfaces were polished to achieve the desired smooth surface for subsequent crystalline structure, microstructure, and optical characterizations. The crystalline structure of the samples was checked by x-ray diffraction (D8 Discover X Series 2, Bruker Co., Beijing, China). The microstructure of the samples was observed by scanning electron microscopy (JSM7500SF, JEOL, Tokyo, Japan). The structural and phonon vibration characteristics of the samples were characterized using laser Raman spectroscopy (Renishaw inVia, Shanghai, China). Spectroscopic ellipsometry (V-VASE, J.A. Woollam, Lincoln, NE, USA) was employed to obtain the fundamental optical constants and band-gap width of the Ga₂O₃ polycrystalline samples. Photoluminescence (PL) spectroscopy was performed by a fluorescence spectrometer (Lab RAM HR 800UV, Horiba Jobin-Yvon, Paris, France) with a 325 nm laser excitation source.

4. Conclusions

In summary, the effects of Zn doping on the optical properties of Ga₂O₃ crystals are revealed. For Zn-doped Ga₂O₃, the introduced Zn ions disrupted the Ga-O bonds, altering the Ga-O vibration modes and causing a blue shift at the related Raman mode. The near-infrared and visible light spectrum lied within the transparent region. The fundamental optical bandgap decreased with increasing Zn doping concentrations. The shrinkage of the optical bandgap of the doped samples was due to the p-d orbital hybridization of the O 2p and Zn 3d orbitals, and the band renormalization effects induced band-tails. The blue-green luminescence came from the recombination of electrons at donor levels (V_O) and holes at acceptor levels (V_Ga or V_O-V_Ga). Zn doping increased the concentration of oxygen vacancies (V_O), leading to a great enhancement in the blue-green luminescence intensity of Ga₂O₃. However, Zn doping resulted in a noticeable reduction in the red luminescence of Ga₂O₃, which may be attributed to the Zn doping suppressing nitrogen incorporation from the air during high-temperature preparation processes.