Electrical and optical characterizations of β-Ga₂O₃:Sn films deposited on MgO (110) substrate by MOCVD

Abstract

Tin-doped β-Ga₂O₃ (β-Ga₂O₃:Sn) films doped with different tin concentrations were deposited on MgO (110) substrates by metal organic chemical vapor deposition (MOCVD) at 700 °C. The effect of doping on the structural, electrical and optical properties of the films was investigated. The 10% Sn-doped film exhibited the best electrical conductivity properties with the lowest resistivity about 5.21 × 10⁻² Ω cm, which is over ten orders of magnitude lower than the un-doped film. Micro-structural analysis revealed that the film with 10% Sn content had a clear in-plane relationship of β-Ga₂O₃ (100) ‖ MgO (110) with β-Ga₂O₃ (2̄01) ‖ MgO (111). The average transmittance of the samples in the visible range exceeded 87% and the optical band gap of the films varied from 4.12 to 4.80 eV.

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I. Introduction

β-type gallium oxide belongs to the group of the transparent conductive oxides (TCOs). Compared with the traditional TCOs such as ZnO,¹ SnO₂ (ref. 2) and In₂O₃,³ β-Ga₂O₃ has a wider band gap (E_g ∼ 4.8 eV (ref. 4 and 5)), which can be used in short wavelength light emitting devices,⁶ deep ultraviolet (UV) photodetectors⁷ and gas sensors.⁸ The crystal structure of β-Ga₂O₃ is monoclinic with space group C2/m. The lattice parameters are a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, β = 103.7° (PDF#43-1012).⁹ However, pure β-Ga₂O₃ films show poor conductivity due to the large energy band gap.¹⁰ It is needed to increase the conductivity by doping method. There have been many reports about the β-Ga₂O₃ materials doping with different elements. 10 mol% Sn-doped β-Ga₂O₃ single crystals with the resistivity of 1.5 × 10⁴ Ω cm were fabricated by the floating zone method.¹¹ The electrical conductivity of β-Ga₂O₃ single crystals can be intentionally controlled over three orders of magnitude by Si doping.¹² Europium-doped gallium oxide films were grown on sapphire substrates by pulsed laser deposition and were characterized using optical techniques.¹³ However, most of the researches were focused on the β-Ga₂O₃ bulk materials,¹⁴ polycrystalline films¹⁵ and nanowires.^16–18 Compared with the polycrystalline films and nanowires, single crystalline epitaxial β-Ga₂O₃:Sn films have many advantages such as better uniformity, stability and compactness. There were few reports about the electrical characterization of Sn-doped β-Ga₂O₃ epitaxial films, especially by means of MOCVD technique. In this letter, β-Ga₂O₃:Sn films with different Sn concentrations were fabricated on MgO (110) substrates at 700 °C by MOCVD. The results show that Sn is an effective n-type dopant for β-Ga₂O₃ films and the conductivity of the film can be reduced by over ten orders of magnitude by Sn doping. MgO (110) substrate is very suitable for growing β-Ga₂O₃ single crystal epitaxial films,¹⁹ and its price is much cheaper compared with gallium oxide wafer. MOCVD technique has the advantages of growth epitaxial films and precise control of films composition, which is suitable for both scientific research and industrial production. This work provides a feasible method for improving the performance of β-Ga₂O₃ film materials, thus it will be favorable for the application of Ga₂O₃ in fields of transparent electronic devices and UV light emitting devices. The structural, electrical and optical properties of the β-Ga₂O₃:Sn films have been investigated in detail.

II. Experimental details

The films were prepared on MgO (110) substrates (double-face polished, thickness: 0.5 mm) by a high vacuum MOCVD system with two separated gas flows. Trimethylgallium {Ga(CH₃)₃}, and tetraethyltin {Sn(C₂H₅)₄}, were used as the organometallic (OM) source. High purity N₂ (purity, 9N) passed through the OM bubblers and delivered the OM vapor to the reactor. High purity O₂ (purity, 5N) with the flow rate of 50 sccm was used as oxidant and injected into the reactor using a separate delivery line. The pressure of the reactor was kept at 20 Torr and the substrate temperature was 700 °C. The flow rate of Ga(CH₃)₃ was 3.64 × 10⁻⁶ mol min⁻¹. The flow rates of Sn(C₂H₅)₄ were kept at 3.60 × 10⁻⁸, 1.08 × 10⁻⁷, 1.80 × 10⁻⁷, 2.88 × 10⁻⁷, 3.60 × 10⁻⁷, 3.96 × 10⁻⁷ and 4.32 × 10⁻⁷ mol min⁻¹, corresponding to 1%, 3%, 5%, 8%, 10%, 11% and 12% (atomic ratio) Sn concentrations, respectively.

The crystalline structure and the epitaxial relationship of the deposited films were measured by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A Rigaku D/max-rB X-ray diffractometer with Cu Kα1 radiation was used for the XRD measurements. The HRTEM and selected area electron diffraction (SAED) measurements were performed on the cross-sectional sample using a Tecnai F20 transmission electron microscope at 200 kV. The cross-section and surface micrograph were obtained using a FEI Nova NanoSEM450 scanning electron microscope (SEM). The resistivity of the films was measured using a ZC-36 high resistance meter. The Hall mobility and carrier concentration were determined by the East changing ET9000 Hall measurement system. Optical transmittance measurements were performed with a TV-1901 double-beam UV-vis-NIR spectrophotometer in the wavelength range from 200 to 800 nm.

III. Results and discussion

Fig. 1 is the XRD patterns of β-Ga₂O₃:Sn films with various amounts of Sn-doping from 1% to 12 mol%. In addition to the strong substrate peaks of MgO (440) located at about 64°, a major diffraction peak of β-Ga₂O₃ (400) was observed, indicating an out-of-plane orientation of β-Ga₂O₃ (100) ‖ MgO (110). As the Sn content increased, the β-Ga₂O₃ (400) peak becomes weaker. For the 12% Sn doped sample, the β-Ga₂O₃ (400) diffraction peak cannot be seen clearly. The full width at half maximum (FWHM) of the β-Ga₂O₃ (400) peak is 0.8, 1.0, 1.2, 1.6 and 2.4° corresponding to the samples with Sn contents of 1%, 3%, 5%, 8% and 10%, respectively, which reveals the degradation of the crystalline quality of the films. The radius of Sn⁴⁺ is bigger than that of Ga³⁺, when some of the Ga³⁺ are replaced by Sn⁴⁺, the order of the β-Ga₂O₃ lattice is affected. So the crystalline quality of the films decreases as the increasing of Sn contents. Further, when too many Sn element into the film, gallium oxide lattice is destroyed and lead to the crystalline quality of film serious decline.

Fig. 1 XRD spectra of the Ga₂O₃:Sn films with different doping levels.

Fig. 2 exhibits the resistivity of the β-Ga₂O₃:Sn films as a function of Sn concentration. The undoped β-Ga₂O₃ film with the highest resistivity of about 10⁹ Ω cm is used as a comparison. With the increase of Sn content, the resistivity of the sample decreases rapidly. The minimum resistivity value of about 5.21 × 10⁻² Ω cm is obtained for the 10% Sn-doped sample. Then the resistivity increases as the Sn content further raise to 11% and 12%. The decrease of resistivity can be explained by the presence of the Sn⁴⁺ ion. When some of the Ga³⁺ ions in the lattice are replaced by Sn⁴⁺ or some of the Sn ions as interstitial atoms are located in the lattice, conduction electrons can be produced, thus the resistivity of films is reduced. However, as Sn concentration increases further (over 10%), the crystalline quality of the films becomes poor. Consequently, the defects inside the films lead to the enhancement of the carrier scattering effect, and also the doping efficiency drops. As a result, the electrical resistivity increased markedly. The variations of resistivity, carrier concentration and mobility for the samples with 10% and 11% Sn doping levels are summarized in Table 1. It can be seen that the carrier density and mobility both drop obviously as the Sn content increases from 10% to 11%, resulting in the increase, resulting in the increase of the resistivity.

Fig. 2 The resistivity of the Ga₂O₃ films with different Sn concentrations.

Table 1 The resistivity, carrier concentration and mobility of the samples with 10% and 11% Sn doping levels

Tin concentration (mol%)	Resistivity ρ (Ω cm)	Carrier concentration n (cm^−3)	Mobility μ (cm^2 V^−1 s^−1)
10%	5.21 × 10^−2	3.71 × 10^19	3.35
11%	4.99 × 10^−1	4.34 × 10^18	2.88

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The temperature dependent Hall measurements are performed on 10% Sn-doped film in vacuum (<10 Pa) and the results are shown in Fig. 3. The electrical properties of the film exhibit a semiconductor-like behavior. As the temperature rises from low to room temperature, the resistivity and Hall mobility decrease, and the carrier concentration increases. At low temperature, the Hall mobility stays at a high level, which can be attributed to the weak lattice scattering at the very low temperature. As the temperature rises up to room temperature, the lattice vibration aggravates, leading to the enhancement of phonon scattering and the decrease of Hall mobility. These results demonstrate that Sn is an effective n-type dopant for β-Ga₂O₃ films and the resistivity can be intentionally reduced by over ten orders of magnitude by Sn doping.

Fig. 3 Resistivity (ρ), carrier concentration (n) and Hall mobility (μ) of β-Ga₂O₃ films with 10% Sn doped as a function of reciprocal temperature.

The cross-section area and surface morphologies of the 10% Sn-doped β-Ga₂O₃ film are revealed by the SEM micrographs. From Fig. 4(a), the obvious boundary between the substrate and film can be observed, and the thickness of the film is about 470 nm. In Fig. 4(b), an orderly surface with well-defined crystallites is obtained. The boundaries of each crystalline particle can be seen clearly.

Fig. 4 The SEM image for the 10% Sn-doped β-Ga₂O₃ film: (a) the cross-section area and (b) surface.

Further structural properties of the sample with 10% Sn content are studied by HRTEM. Fig. 5 shows the HRTEM and SAED micrographs of the interface between the film and substrate. The incident electron beam is parallel to the [11̄0] direction of the MgO substrate. In the HRTEM image, the spacings of the observed lattice planes in the film area are about 0.297 nm and 0.468 nm, corresponding to the β-Ga₂O₃ (400) and (2̄01) planes, respectively. For the substrate, the interplane spacings marked by the white lines are 0.144 and 0.234 nm which are consistent with MgO (440) and (222), respectively. The diffraction spots of β-Ga₂O₃ (400), β-Ga₂O₃ (2̄01), MgO (440), MgO (222) can be seen clearly in the SAED micrograph. The results of HRTEM and SAED show a clear orientation relationship of β-Ga₂O₃ (100) ‖ MgO (110) with β-Ga₂O₃ (2̄01) ‖ MgO (111).

Fig. 5 Cross-sectional HRTEM and SAED micrographs of the interface between the 10% Sn-doped β-Ga₂O₃ film and the MgO substrate.

Fig. 6 shows the XPS spectra for the β-Ga₂O₃:Sn film with 10% Sn content. Fig. 6(a) is the survey spectrum of the film, which indicates the peaks of C1s, Ga2p, O1s, Sn3d, Ga3s and Ga3d. Fig. 6(b–d) are the high resolution spectra of Ga2p, Sn3d and O1s peaks, respectively. In Fig. 6(b), two symmetrical peaks of Ga2p_1/2 located at 1144.1 eV and Ga2p_3/2 located at 1117.3 eV can be observed. The separation distance between these two peaks is about 26.8 eV, which is in good agreement with the binding energy of the Ga2p (Δ = 26.8 eV).^20,21 In Fig. 6(c), the tin core levels Sn3d_5/2 and Sn3d_3/2 observed at 486.4 and 494.8 eV, respectively, with a peak-to-peak separation of 8.4 eV confirm the formation of Sn.²² The O1s peak is located at about 530.5 eV. Using the Gaussian Fitting method, the peak can be resolved into two separate peaks. The signal at 530.5 eV is attributed to lattice oxygen in the β-Ga₂O₃:Sn film.^23,24 The other peak at 531.7 eV can be regard as the C/O or OH⁻ adsorbed species on the surface.²⁵ The compositional ratio of Sn-to-Ga is about 12%, which is larger than the expected value of 10%. The analysis above illustrates that the obtained film is Sn-doped β-Ga₂O₃.

Fig. 6 XPS spectra of the 10% Sn-doped β-Ga₂O₃ film.

The optical transmittance spectra for the β-Ga₂O₃:Sn films as a function of wavelength in the range of 200–800 nm are shown in Fig. 7(a). The average transmittance of all the films in visible rage exceeded 87%. For the direct band gap transition semiconductors, the absorption coefficient α and optical band gap (E_g) are related by αhν = A(hν − E_g)^1/2.^26,27 Where A is a constant, ν is the frequency of the incident photon and h is the Planck's constant. The optical band gap can be estimated by extrapolating the straight line portion of this plot to the energy axis. The optical gaps for the films as a function of the Sn content are shown in Fig. 7(b). As the Sn content increases, E_g decreases in initial with a minimum value of 4.12 eV obtained for the 10% Sn-doped sample, and then the E_g increases. The variation trend of E_g is consistent with that of resistivity. The decrease of E_g can be attributed to two reasons: first the band gap of SnO₂ is narrower than that of β-Ga₂O₃; second the band gap narrowing effect.^28,29 For the heavily doped semiconductors, the band tail and impurity band will be generated; as the doping concentration increases from 0 to 10%, the band tail and impurity band extended, which makes the band tail and impurity band overlap. As a result to the actual width of the forbidden band becomes narrower. But as the Sn concentration increases further (over 10%), the crystalline quality declined obviously, resulting in the broadening of the band gap.

Fig. 7 (a) The optical transmittance spectra of β-Ga₂O₃:Sn samples. The plot of (αhν)² as a function of photon energy hν are shown in the inset. (b) E_g of the β-Ga₂O₃:Sn films as a function of Sn content.

IV. Conclusions

In conclusion, β-Ga₂O₃:Sn films have been deposited on MgO (110) substrate by MOCVD at 700 °C. Sn is an effective n-type impurity and can reduce the resistivity by over ten orders of magnitude. The sample with 10% Sn content has the lowest resistivity of about 5.21 × 10⁻² Ω cm with a carrier density of 3.71 × 10¹⁹ cm⁻³. The micro-structure analysis of the 10% Sn-doped sample reveal a clear orientation relationship of β-Ga₂O₃ (100) ‖ MgO (110) with β-Ga₂O₃ (2̄01) ‖ MgO (111). The average transmittance of all the samples exceeded 87% in visible range. The film grown with 10% Sn content has the minimum optical band gap of about 4.12 eV. The variation trend of E_g is consistent with that of resistivity. The deposition of high quality β-Ga₂O₃:Sn film with excellent electrical properties can be used in many fields such as short wavelength light emitting devices, transparent transistors, quantum-well devices and solar cells.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant no. 51072102).

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