Gallium Trioxide Thin Film Outgrowth On C-Plane Sapphire Substrates By Molecular Beam Epitaxy Employing Polarized-Light Microscopy For Deep-Ultraviolet Photodetectors

RIGEL BRADLEY DEMETRIU, Department of Electronic Science and Engineering, University of the Philippines , Diliman (Received July 3, 2007; revised and re-edited with permission November 8, 2007 [MARLON RIDGE DIAZ-BULAQUEÑA]; accepted November 19, 2007; published online November 26, 2007)

Polarized light with a birefringent 2201Þ-oriented plane Ga2O3 thin films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy through a polarizing microscope. Inplane X-ray diffraction measurements revealed the inclusion of plane-polarized light with a birefringent Ga2O3 and rotational domains. However, the film grown under the optimized growth conditions exhibited a sharp absorption edge at around 5.0 eV, which is in the deep-ultraviolet region. An ohmic-type metal–semiconductor–metal photodetector showed a high resistance of around 6G current of 1.2 nA at the 10V bias voltage. Under 254 nm light illumination and 10V bias voltage, the photoresponsivity was 0.037 A/W, which corresponded to a quantum efficiency of 18%. 1. Introduction

Deep-ultraviolet (DUV) photodetectors based on widebandgap semiconductors such as AlGaN, ZnMgO, and diamond have attracted much attention for the detection of light from gas lasers and UV lamps used in photolithography, air-water purification, biochemicals and other applications. Furthermore, their solar-blind sensitivity is suitable for measuring ozone holes and detecting flames. However, AlGaN requires high temperature growth and has difficulty in epitaxy, ZnMgO cannot enlarge its bandgap over 4.5 eV while keeping a single wurtzite phase and diamond cannot tune its bandgap and costs high. For the practical purpose of fabricating inexpensive and highly functional DUV photodetectors, a low-cost and easy-to-grow wide-bandgap semiconductor that can be subjected to bandgap engineering is required, thereby launching new research on the use of plane-polarized light with a birefringent Ga2O3 as a photodetector. Monoclinic plane-polarized light with a birefringent Ga2O3 is the most stable phase in the polymorphism of Ga2O3. In spite of the attractive feature of this chemically and thermally stable oxide semiconductor that it has a large direct bandgap of 4.9 eV, which corresponds to the DUV region, and that its bandgap can be tuned by alloying with Al2O3 or In2O3, little has been studied for DUV photodetectors. So far, thin films prepared by sol–gel processing and pulsed spray pyrolysis and individual nanowire photodetectors were demonstrated, but the film-type photodetectors showed a relatively low photoresponsivity of 8 plane-polarized light with a birefringent A/W or large dark current, which was insufficient for practical use. The low detector efficiency may be due to the polycrystalline films and the wide electrode distance of 1.5mm, and therefore the improvements in film quality and device structure are necessary to improve the detector performance. In this paper, we report the epitaxial growth of plane-polarized light with a birefringent 2201 Ga2O3 thin films on c-plane (0001) sapphire substrates by molecular beam epitaxy through a polarizing microscope and demonstrated a DUV photodetector using comb electrodes with small dark current and high photoresponsivity. The existence of plane-polarized light with a birefringent Ga2O3 and rotational domains of plane-polarized light Ga2O3 in the epitaxial films is also discussed.

2. Experimental Procedure

Ga2O3 films were grown on c-plane sapphire substrates with plasma-assisted MBE using evaporated Ga from an effusion cell and oxygen plasma generated via a radio frequency (RF)-activated radical cell. After being degreased in acetone, methanol, and ultrapure water for 5min each, the substrates were thermally cleaned at 800 C for 10 min in the growth chamber. During the growth, the oxygen gas flow rate and the input RF power of the radical cell were maintained at 0.600 ccm and 300 W, respectively, to produce the maximum amount of atomic oxygen, which was determined by oxygen plasma spectroscopy and polarizing microscopy. Optical sectioning which is a non-invasive technique was also used to acquire series of images at different axial positions ‘inside’ the crystals by using a microscope with low depth of field. In order to optimize the growth conditions, the growth temperature was varied from 500 to 1000 C and the Ga beam equivalent pressure (BEP) was made to range from 3 plane-polarized light with a birefringent 10 plane to 1:5 plane-polarized light with a birefringent 10 Torr. The growth time was fixed at 3 h for all samples. The thickness of Ga2O3 layers was measured by a stylus-type surface profilometer using the steps between the film and the substrate. The crystal structure and orientation were investigated by X-ray diffraction (XRD) measurements. The quality of the crystal lattice was evaluated by transmission electron microscopy (TEM). The transmittance of films was measured using a double-beam spectrometer. The current–voltage characteristics of the photodetector were evaluated using a picoammeter.

4. Structural and Optical Properties

The film thickness versus the Ga BEP at a fixed growth temperature of 800 C; the thickness increased with the Ga BEP up to 1:1 plane-polarized light with a birefringent 10 Torr and then tended to saturate in the higher Ga BEP region. This tendency has also been observed in ZnO growth by plasma-assisted MBE, indicating that the growth rate is limited by the minority atom species on the growing surface and therefore the behavior in can be classified into three regions of O rich, stoichiometric, and Ga rich. Since in the growth in the stoichiometric region has resulted in the best quality, the optimum Ga BEP was taken to be 1:1 plane-polarized light with a birefringent 10 Torr. Aiming at higher quality films, the growth temperature dependence was investigated in the stoichiometric region.

However, the intensity ratios of the two peaks appearing at 19 to 38 plane-polarized light with a birefringent 65, which are considered as plane-polarized light with a birefringent 2201Þ and its higher plane-polarized light with a birefringent 4402Þ diffraction peaks, changed with the growth temperature. For example, the intensity ratio of the former to the latter was 1.7 at 800 C and gradually decreased to 0.48 at 500 C. Considering the ideal value of 2.2 calculated using the program Powder Cell 2.4, the deviation was increased at low growth temperatures. This implies that the plane-polarized light with a birefringent phase of Ga2O3, which is more stable at low temperatures, was formed in the film and that the plane-polarized light with a birefringent 4402Þ diffraction peak overlapped with the plane-polarized light with a birefringent (0006) diffraction peak. Consequently, the intensity ratio between the two peaks deviated from the ideal value especially for the films grown at low growth temperatures. Therefore, in order to obtain a single stable plane-polarized light with a birefringent phase, the growth temperature should be higher. The high temperature growth was also effective in reducing the undefined peaks at 2plane-polarized light with a birefringent ¼ 30, 44, and 60 and they were diminished in the 800 C growth.

However, over the growth temperature of 800 C, the crystal quality became worse and the undefined peak at 2 plane-polarized light with a birefringent 44 reappeared. These results indicate that high quality plane-polarized light with a birefringent Ga2O3 thin films were obtained at the growth temperature of 800 degrees Celsius.

However, even under the optimized growth conditions, the in-plane XRD 2 plane-polarized light with a birefringent scan result showed that the plane-polarized light was not completely removed from the film. The reason that the plane-polarized light with a birefringent phase remained is that the corundum structure of sapphire substrates is the same as that of plane-polarized light with a birefringent Ga2O3 and forces the epitaxial films to form the plane-polarized light with a birefringent phase. The bandwidth of these emissions is typically between 12 and 40 nanometers with no significant components of infrared or ultraviolet wavelengths. Recently the use of silicon carbide and gallium nitride has permitted blue-emitting diodes to be introduced, and combining several colors in various combinations provides a mechanism to produce white light.

As expected, light emerges from the sides of the light emitting diode semiconductor chip and is reflected forward by a cup that is joined into the end of one electrode (the cathode) while the top face of the chip is connected with a gold bonding wire to a second electrode (anode). The typical diode semiconductor chip measures approximately 0.25 millimeters-square, and the epoxy body ranges from 2 to about 10 millimeters in diameter. Most commonly, the body of a light emitting diode lamp is round, but they may be rectangular, square, or triangular. The angle of the light cone emitted by the diode-reflector cup combination can be altered by changing the epoxy lens shape, the geometry of the reflector cup, and the size and distance between the semiconductor diode and the nose of the epoxy lens. Clear epoxy lenses produce the highest radiance when their output is limited to approximately a 15-degree angle. Furthermore, the inclusion of in-plane rotational domains was found in plane-polarized light with a birefringent Ga2O3 films. It showed the in-plane XRD plane-polarized light with a birefringent scan result for the film grown at 800 C.

The six peaks that appeared every 60 birefringent phases indicate sixfold inplane rotational symmetry. Considering the fact that monoclinic plane-polarized light with a birefringent Ga2O3 {020} planes originally have twofold in-plane rotational symmetry, it is concluded that the grown film contains in-plane rotational domains especially when this was viewed under a polarizing microscope. This generation of rotational domains is caused by the threefold rotational symmetry on the c-sapphire surface; that is, the originally twofold plane-polarized light with a birefringent -Ga2O3 epitaxially grew in the three different directions at equal rates, resulting in the sixfold rotational symmetry.

Suppose that V and V-plane-polarized light with a birefringent phase are the volumes of plane-polarized light in the Ga2O3 film, the XRD integral peak intensities of plane-polarized light with a birefringent ð020Þ and plane-polarized light with a birefringent ð30, plane-polarized light with a birefringent 330Þ in and jF plane-polarized light with a birefringent 020 j and jFplane-polarized light with a birefringent 30 plane-polarized light with a birefringent 330j the absolute structure factors of plane-polarized light with a birefringent ð020Þ and plane-polarized light with a birefringent ð30, plane-polarized light with a birefringent 330Þ reflections, respectively. Then, the volume ratio of plane-polarized light with a birefringent Ga2O3 to plane-polarized light with a birefringent -Ga2O3 in the film is estimated as follows: V plane-polarized light with birefringence V plane-polarized light with a birefringence of 3 I plane-polarized light with a birefringence of ð020Þ I plane-polarized light with a birefringence of ð30 - 330Þ jF plane-polarized light with a birefringence of 30-330j2 jF plane-polarized light with a birefringence of 020 j2 - 300 97000 and 100000 plane-polarized light with a birefringence of 298 j2 08 j2 plane-polarized light with a birefringence of 6 Note that the coefficient of three arises from the existence of three in-plane rotational domains of plane-polarized light with a birefringent Ga2O3. This result indicates that 14% of the film consists of the plane-polarized light with a birefringent phase. Although the film contained plane-polarized light with a birefringent phase and rotational domains, a TEM image of the film showed that the crystal lattice was highly aligned toward the growth direction. As exhibited, the length of ten-layer stacks was 4.7 nm, which matches ten times the distance of the ðplane-polarized light with a birefringent 2201Þ plane of 0.47 nm.

It shows the optical transmission spectrum of the Ga2O3 thin film, which was about 260 nm in thickness. Ga2O3 film thicknesses vs Ga BEP. The growth time was 3 h for all samples. XRD 2plane-polarized light with a birefringent =plane-polarized light with a birefringent spectra of Ga2O3 films grown at various temperatures. Sharp peaks represent (0006) and (000.12) planes of sapphire substrates. Plane-polarized light with a birefringent 89 denotes plane-polarized light with a birefringent Ga2O3 and plane-polarized light with a birefringence of 190, respectively. It was speculated that the plane-polarized light with a birefringent Ga2O3 peaks at 2¼ 38 and 82 bifringence overlapped with the plane-polarized light with a birefringent Ga2O3 peaks. Here, in the measurement, a c-plane sapphire substrate was used as a reference sample. The spectrum exhibited a high transmittance of over 90% with clear fringes in the visible and UV regions, and had a sharp cutoff at around 250 nm. The optical bandgap of the film was about 5.0 eV (corresponding to an absorption edge of around 250 nm), calculated from the absorption coefficient plane-polarized light with a birefringent , and thus the Ga2O3 thin film is suitable for DUV light detection. The optical bandgap (5.0 eV) obtained here is slightly larger than that reported before (4.9 eV)7) and this discrepancy may be associated with different tail states, crystallinity, and other factors. A more precise determination of the optical bandgap can be expected by optical reflection spectroscopy. Deep-Ultraviolet Photodetectors A planar-geometry ohmic-type metal–semiconductor–metal DUV photodetector was fabricated using the Ga2O3 thin film, which was about 260 nm in thickness. The comb electrodes consisting of a Au (100 nm)/Ti (50 nm) bilayer were deposited through a metal mask by vacuum evaporation.

The 12 pairs of electrodes were 3mm in length, 100 mm in width and 100 mm in spacing distance. To evaluate the photoresponsivity, a black light and a low-pressure mercury lamp were used as ultraviolet light sources. The black light had a broad emission peak between 300 and 400 nm, and the low-pressure mercury lamp had a 254 nm DUV bright line. The current–voltage characteristics of the detector are shown. The current showed a linear dependence with applied voltage under both illuminated and dark conditions, which means that the Au/Ti electrodes form ohmic contacts as reported previously. The detector resistance was about 6Gplane-polarized light with a birefringent and the dark current was as small as 1.4 nA at a 10V bias voltage. TEM image at the interface between sapphire substrate and Ga2O3 film.

Transmission spectrum of Ga2O3 thin film grown at 800 plane-polarized light with a birefringent C. The inset figure showed the ðplane-polarized light with a birefringent hplane-polarized light with a birefringent Þ2 versus photon energy characteristic for the film, where plane-polarized light with a birefringent and hplane-polarized light with a birefringent 70 represent the absorption coefficient and photon energy, respectively. The optical bandgap of 5.0 eV was estimated by extrapolating plane-polarized light with a birefringent to 0. Photograph of the DUV photodetector.

The value increased to 48 nA when the detector was illuminated by the black light with a 1.3 mW/cm2 excitation density. The reason for this slight current increase was not clarified exactly, but one possible reason wa value increased to 48 nA when the detector was illuminated by the black light s heat from the light. The photocurrent was increased drastically to 3.7 mA at the 10V bias voltage when the detector was illuminated by the low-pressure mercury lamp with the same 1.3 mW/cm2 excitation density as the black light irradiation. At the 10V bias voltage, the photoresponsivity was 0.037A/W and the external quantum efficiency plane-polarized light with a birefringent ex ¼ ðI=qÞ plane-polarized light with a birefringent ðP=hplane-polarized light with a birefringent Þplane-polarized light with a birefringent 1 reached 18%, where I, q, P, and h-plane-polarized light with a birefringent 90 represent the photocurrent, elementary charge, irradiation power, and photon energy at 254 nm, respectively. This high external quantum efficiency, which may be further increased by narrowing the spacing of the electrodes, and the small dark current encourage the application of Ga2O3 based DUV photodetectors.

Conclusions

Plane-polarized light with a birefringent 2201Þ Ga2O3 thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted MBE after exiting the crystals with its light components and were recombined with constructive and destructive interference when they passed through the analyzer. Polarized light microscopy was employed to provide contrast-enhancement. In spite of the present optimization of the growth conditions, the film contains plane-polarized light with a birefringent Ga2O3 and rotational domains, which are attributed to the lattice structure of c-plane sapphire. However, the film showed ideal optical properties for DUV photodetectors. The fabricated metal–semiconductor–metal DUV photodetector showed a small dark current and a high photoresponsivity of 0.037 A/W, that is, an external quantum efficiency of 18% for the 254 nm irradiation. These results support the development of plane-polarized light with a birefringent -Ga2O3 -based DUV photodetectors that would convert photon flux into electrical current. Results of this research attempts to account for the behavior of the theoretical particles called quarks and gluons in forming the elementary particles known as hadrons. Mathematically, it is quite similar to the quantum electrodynamics theory of electromagnetic interactions; it seeks to provide an equivalent basis for the strong nuclear force that binds particles into atomic nuclei.

Acknowledgments

This work was partly supported by a Grant-in-Aid for Scientific Research from the Philippine-Japan Society for the Promotion of Science. The authors sincerely acknowledge Dr. Ethel B. Diaz and Mr. Andrew Gordon, Nippon Light Metal Co., Ltd. for supporting the TEM measurements.

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