Structural , Optical , and Electrical Characterization of β-Ga 2 O 3 Thin Films Grown by Plasma-Assisted Molecular Beam Epitaxy Suitable for UV Sensing

Facultad de Ingenieŕıa, Universidad Autónoma de Baja California, Blvd. Benito Juárez s/n, 21280 Mexicali, BC, Mexico Instituto de Ingenieŕıa, Universidad Autónoma de Baja California, Blvd. Benito Juárez s/n, 21280 Mexicali, BC, Mexico Materials Science, Engineering and Commercialization Program, Texas State University, San Marcos, TX 78666, USA Ingram School of Engineering, Texas State University, San Marcos, TX 78666, USA


Introduction
Ultraviolet radiation (UV) covers the wavelengths shorter than 400 nm in the electromagnetic spectrum.e effect of UV radiation depends on the wavelength and can lead to some benefits as in the case of UV-A (400-320 nm) that stimulates photosynthesis, while exposure to other wavelengths can cause skin cancer (UV-B 320-280 nm) and be a risk factor for some eye diseases.e most energetic part of the UV spectrum, UV-C (280-10 nm) causes damage and degradation of organic materials [1][2][3].As a result, the development of miniature, fast, highly sensitive, selective, stable, and robust UV detectors is the subject of intense studies [4].
In this work, we report on the preparation and characterization of β-phase Ga 2 O 3 thin films grown by plasmaassisted molecular beam epitaxy (PAMBE) at various growth conditions.e quality of the grown films was inferred from reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD).Current-voltage (I-V) measurements were carried out in dark, as well as under visible and UV light illumination to study the possibility of using β-phase Ga 2 O 3 in deep-UV sensors.

Materials and Methods
β-Ga 2 O 3 thin films were grown on single-side polished c-plane sapphire substrates by PAMBE in an oxide MBE chamber.
e substrates were degreased in acetone and thermally cleaned for 15 min at 850 °C in the chamber followed by an oxygen plasma exposure for 30 minutes.A chamber base pressure of <5 × 10 −10 Torr was obtained by a combination of turbomolecular and cryogenic pumps.During the growth, the oxide chamber cryopanels were cooled with liquid nitrogen.e films were grown using a well-known method from evaporated Ga and oxygen provided from a radio frequency (RF) plasma source equipped with an ion deflector [19].e oxygen flow rate was varied from 1.0 to 1.5 sccm using RF plasma power of 200 and 300 W. e elemental evaporated Ga was provided from an effusion cell with a fixed temperature of 800 °C (thermocouple temperature).e substrate temperature was varied from 650 to 750 °C as measured using a thermocouple.All samples were grown for 60 minutes with the surface quality monitored in real time in situ using RHEED.
e as-grown samples were analyzed by XRD (Bede System diffractometer) to determine the crystalline quality of the films.e thickness and the optical constants of the films were determined by spectroscopic ellipsometry measurements using a J. A. Woollam M-2000 variable angle ellipsometer.Transmission spectra of the samples were measured by the UV-Vis spectrometer Shimadzu 2600.For electrical characterization, a group of samples with 70 × 70 µm Al contacts with spacing of 30 µm between them was prepared by thermal evaporation.I-V measurements of lateral current between two adjacent contacts were carried out in dark as well as under UV-C (254 nm), UV-A (365 nm), and visible light illumination (532, 650 nm) using a Keithley 4200 Semiconductor Characterization System.

Results and Discussion
Real-time RHEED was used to monitor the crystallinity and surface quality during the film growth.Figure 1(a) shows a RHEED pattern of pure c-plane sapphire substrate prior to growth presenting a diffuse background.After formation of the Ga 2 O 3 film, the RHEED pattern exhibited a streaky threefold reconstruction, indicating high-degree of crystallinity in the grown films as is shown in Figure 1(b).
e film thicknesses were determined by Ellipsometry using the Cauchy model [20,21].
e Cauchy model assumes that the refractive index n can be represented by a slowly varying function of the wavelength n � A + B/λ 2 + C/λ 4 , where A, B, and C are fitting parameters that determine the index.e thicknesses were determined by fitting the experimental results only in the 300-1000 nm range, where the films are transparent.
e obtained results for the thickness, roughness, and the refractive index at λ � 632.8 nm of the grown samples are shown in Table 1.e dependence of the film thickness on the substrate temperature is shown in Figure 2(a), while Figure 2(b) presents the dispersion curves of the refractive index.
e reduction of thickness at high temperature is a result of increased desorption of Ga 2 O molecules on the growing surface whereas higher oxygen flux suppresses the desorption resulting in higher growth rates.For films grown at oxygen flux of 1.0 sccm, the refractive index shows a tendency to increase with the substrate temperature, while for films grown at flux of 1.5 sccm, the dependence of n on T is very weak.
XRD results confirmed the formation of the β-phase single crystal Ga 2 O 3 on sapphire with excellent crystallinity.e X-ray diffractograms were measured in the 10-70 °2θ range.Figure 3 shows XRD spectra of films grown with an oxygen flux of 1.5 sccm and RF plasma power of 300 W at three different substrate temperatures, 650, 700, and 750 °C.ree well-defined peaks at 18.9 °, 38.4 °, and 59.2 °can be distinguished, corresponding to diffractions of (−201), (−402), and (−603) planes of β-Ga 2 O 3 .Besides the peaks associated to β-Ga 2 O 3 and the substrate, no other significant peaks were found in the spectra, thus excluding the existence of another phase.e intensity of the XRD peaks shows a weak dependence on the growth temperature; higher intensity is observed at lower substrate temperature.Similar results were obtained for films grown at the same  4), a result that is in agreement with the thickness dependence on the temperature (Figure 2).e UV-Vis spectroscopy results revealed that strong absorption of β-Ga 2 O 3 starts at ∼270 nm, which corresponds to UV-C radiation.Figure 5 shows the UV-Vis spectra of sapphire substrate as well as of β-Ga 2 O 3 thin lms grown with RF plasma power of 200 and 300 W at two temperatures, 700 and 750 °C.Sapphire is a highly transparent material and exhibits higher transmittance compared to the sapphire/ β-Ga 2 O 3 samples.e samples grown with an RF plasma power of 200 W exhibit a lower transmittance than that of samples grown at 300 W, which does not depend on the deposition temperature.In contrast, the increase of the substrate temperature for samples grown at 300 W leads to higher transmittance.e transmittance values shown in Figure 5 are  lower than previously reported [14,22,23] because of light scattering by the unpolished side of the sapphire substrate.
From the UV-Vis results, the optical bandgaps of the lms were determined using the power law for direct bandgap semiconductors [22][23][24][25][26]: where α is the absorption coe cient, hν is the photon energy, and E g is the bandgap.e optical absorption coe cients were calculated using the ellipsometrically determined lm thicknesses.E g was estimated graphically by regression analysis of     4 Advances in Materials Science and Engineering the linear part of the (αhν) 2 versus hν dependence (Figure 6).e obtained values for lms grown at 300 W are shown in Table 2 and are similar to previously reported values [27][28][29].
A schematic representation of the structure used in the electrical characterization is shown in Figure 7. I-V measurements were carried out between top Al contacts on Ga 2 O 3 using voltage sweeps from 0 to ± 10 V in dark, as well as under UV light illumination.Symmetric characteristics were obtained at positive and negative voltages (Figure 8), excluding the formation of a Schottky contact between the Al contact and the thin lms.e slight asymmetry is an instrumentation e ect, which most likely results from the probe station or from small variation of the characteristics of the two source-monitor units (SMUs) of Keithley 4200 SCS.
e results in Figure 8 were obtained using SMU1 as a "Force" and SMU2 as a "Ground" for measurements at positive and at negative voltages.By changing the SMU1 to "Ground" and SMU2 to "Force," the absolute values of the current measured at negative voltages coincide with the current at positive voltages (inset in Figure 8).In the voltage range used, the photocurrent under 254 nm UV illumination reaches values ∼5 times higher than the dark current in both directions.
In order to prove that the observed photocurrent is exclusively due to UV-C excitation, the samples were exposed to illumination with light sources of 650 nm (red), 532 nm (green), and 366 nm (UV-A).In Figure 9, a plot of the electrical behavior of β-Ga 2 O 3 thin lms under di erent illuminations is shown proving that the photocurrent was generated only under UV-C exposure.e I-V results are in agreement with the UV-Vis measurements where strong absorption occurs at wavelengths shorter than 270 nm.

Conclusions
β-Ga 2 O 3 thin lms with high crystal quality were grown successfully on c-plane sapphire substrate by plasmaassisted molecular beam epitaxy.In this work, the combined e ect of substrate temperature, RF plasma power, and oxygen ux was studied showing that at a xed O 2 ux the thickness of the grown lm decreases with the increase of the temperature.Ellipsometry measurements revealed that, at oxygen ux of 1.0 sccm, the refractive index increases with the substrate temperature, while at ux of 1.5 sccm this dependence is very weak.e samples grown at 300 W RF plasma power exhibited higher transmittance than those grown at 200 W. Optical measurements showed that β-Ga 2 O 3 starts to  e electrical measurements under UV-C illumination showed that the grown thin films of β-Ga 2 O 3 are excellent candidates for deep-ultraviolet sensing.

Figure 2 :Figure 3 :
Figure 2: (a) Film thickness dependence on the substrate temperature at uxes of 1.5 and 1.0 sccm; (b) dispersion curves of the refractive index.

Figure 5 :
Figure 5: UV-Vis spectra of sapphire substrate (circles) and β-Ga 2 O 3 thin lms grown at 700 °C with RF plasma power of 200 and 300 W (squares and triangles) and at 750 °C at the same powers (diamonds and crosses).

Figure 7 :
Figure 7: Schematic representation of the structure used in the I-V measurements.

Figure 8 :
Figure8: Dark current and photocurrent measured under 245 nm UV illumination.e inset gure shows I-V characteristics measured using SMU1 as "Force" at positive and as "Ground" at negative voltages.

Figure 9 :
Figure 9: I-V characteristics of Ga 2 O 3 thin lms in dark and under di erent

Table 1 :
ickness, roughness, and refractive index at λ 632.8 nm obtained using the Cauchy model.

Table 2 :
Optical bandgaps of β-Ga 2 O 3 thin lms grown at 300 W at di erent temperatures and oxygen uxes.