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Abstract
We demonstrated a (GaIn)2O3 films based UV photodetector with a planar photoconductor structure, and investigated the photoresponse properties of the fabricated devices. The (GaIn)2O3 films are of ohmic contact with a Au/Ti electrode. The fabricated photodetectors show relatively high photoresponsivity of more than 0.1 A/W. The turn-on wavelength of the photodetectors varied from 285 to 315 nm with the increase of the indium content from 0.25 to 0.49. The properties of the films were also further investigated. The films are of a (−201) oriented monoclinic phase with a high transmittance of more than 90% in the visible region and smooth surfaces without phase separation. The absorption edges of the films shift toward a longer UV wavelength region with the increase of indium content. The above results suggest that wavelength selective UV detectors can be realized based on these films.
© 2017 Optical Society of America
1. Introduction
Photo detection in the ultraviolet (UV) region has drawn extensive attention owing to its various applications such as chemical, environmental and biological analysis or monitoring, flame and radiation detection, astronomical studies, and optical communications. There exists four regions of UV light including UVA (wavelength range 400~320 nm), UVB (320~280 nm), UVC (280~200 nm), and far UV (200~10 nm) [1, 2]. Each subdivision has different effects on earth. For example, UVA radiation stimulates the photosynthesis, and has some role in the synthesis of some vitamins. However prolonged exposures may lead to sunburn and premature aging [3]. UVB is very detrimental for living species. It causes skin cancer, acute sunburns and cataracts. However, it activates the vitamin D [4]. UVC is the most dangerous. It can ionize the DNA thereby cause severe mutations [5]. Therefore, it is crucial to have photosensitive receptors working in different portions of the UV radiation.
Commercially available solar-blind optical devices such as Si and GaAs based UV photodetectors are not truly solar-blind, as additional visible-light blocking filters are required due to the narrow bandgaps of the semiconducting materials. Therefore, they are bulky, fragile, and only operational under large bias conditions. These limitations have triggered research into the development of alternate wide-bandgap materials to replace Si and GaAs [6, 7]. In recent years, UV photodetectors based on wide bandgap alloy semiconductors such as AlGaN, ZnMgO have attracted intensive attentions because of their tunable bandgap. The variable bandgap from 3.3 to 7.7 eV [8] enables MgZnO UV detectors working in UV radiation from 160 to 375 nm; The AlGaN UV detectors can be worked in UV radiation from 200 to 365 nm according to their variable bandgap from 3.4 to 6.2 eV [9]. On another aspect, the AlGaInO system has variable bandgap from 3.6 to 9.9 eV [10] and is of chemically and thermally stable. These properties have enabled AlGaInO system as a new candidate for UV detectors working in wider UV radiation from 125 to 375 nm. However, the investigation on AlGaInO system is still in its infancy and most of the UV detectors were based on Ga2O3 substrate [11, 12], films [13, 14] and nano-structures [15, 16]. Only Kokubun et al. have tried to realize a (GaIn)2O3 UV photodetector with indium content lower than 0.2 [17]. Recently we have obtained bandgap tunable AlGaInO films by pulsed laser deposition method (PLD) [18–20]. In this paper, the authors demonstrated that AlGaInO films can be used for UV photodetector devices. The photodetectors were fabricated based on (GaIn)2O3 films with indium content from 0.25 to 0.49. Their turn-on wavelength can be varied from 285 to 315 nm by changing the indium content. The films are of ohmic contact with Au/Ti electrode and the fabricated photodetectors show relatively high photoresponsivity.
2. Experiment
The films were deposited at room temperature by PLD and then annealed in air. The laser energy at the target surface was 225 mJ. α-Al2O3 (0001) was used as substrate. Facing the substrate, (GaIn)2O3 bulks (diameter of 20 mm) with different In content (weight ratio of In2O3/(Ga2O3 + In2O3): 0.2, 0.3 and 0.5) were used as targets. The pulsed laser with a frequency of 2 Hz was irradiated with a target-substrate distance of 40 mm. The O2 pressure was set as 0.1 Pa. Post-annealing was carried out with a quartz tube furnace in air ambient. The samples were placed on a quartz boat. The annealing temperature was 900 °C and the annealing time was 12 hours.
The thickness of all the films was about 150–200 nm as determined by a stylus type surface profilometer. The structural of the films was investigated by X-ray diffraction (XRD) on a PANalytical X’Pert PRO system using CuKα emission line at room temperature. The transmittance of the films was measured using a double-beam spectrometer. Surface morphologies were obtained by an atomic force microscope (AFM) in the contact mode using Digital instruments Nanoscope, Veeco, MMAFMLN-AM.
The photodetectors were of metal-semiconductor-metal (MSM) structure made from the (GaIn)2O3 films. Interdigital Ti/Au fingers are deposited onto the (GaIn)2O3 films as electrodes using a shadow mask by electron beam evaporation. The Ti/Au fingers are 3 mm in length and 0.1 mm in width, and the inter electrode spacing is 0.1 mm. The thickness of the Ti/Au electrodes is about 50/50 nm. The schematic diagram of the photodetector is shown in Fig. 1.
The current–voltage (I–V) characteristics and time-dependent photoresponse of the (GaIn)2O3-based photodetectors were measured in air using Hokuto denko HAL-3001 potentiostat connected to the photodetector devices. The time-dependent photoresponse measurement of (GaIn)2O3 photodetectors with nominal indium content (indium in the target) 0.2 and 0.3 were performed at a constant voltage of 10 V while that of photodetectors with nominal indium content 0.5 were performed at a constant voltage of 5 V. The current range of the potentiostat for the photodetectors with nominal indium content 0.2, 0.3 and 0.5 was set as 1 μA, 1 μA and 1 mA, respectively. A Xe lamp with a monochromator was employed as the excitation source.
3. Results and discussion
Figure 2 shows the XRD 2θ/θ patterns for (GaIn)2O3 films with nominal indium content from 0.2 to 0.5. All of the films show monoclinic structure (−201) oriented diffraction patterns. The (−201) peak of the films with nominal indium content of 0.2, 0.3 and 0.5 is located at 2θ value about 18.67°, 18.55° and 18.40°, respectively. The peak position has shifted toward lower angle with the increase of indium content, indicating indium atoms has incorporated into the monoclinic lattice. No phase separation has been observed in these films. According to Vegard’s Law, the indium content in the films has been calculated. The (−201) peak of monoclinic Ga2O3 and GaInO3 located at 18.95 o (PDF#43-1012) and 18.39 o (PDF#21-0334) has been used as references. The calculated indium content in the films with nominal value of 0.2, 0.3 and 0.5 is about 0.25, 0.36 and 0.49, respectively. The calculated indium content in the films correspond well with that of the nominal content, indicating stoichiometric transfer from the targets to the films by the PLD process. It is noticeable that film with indium content of 0.49 shows reduced peak intensity, indicating the degradation of crystal quality. The degradation of crystallinity with increasing indium content is contribute to the higher inclusion level of indium, which has also been observed in (AlGa)2O3 films [20].
AFM measurements were performed to study the surface morphologies of (GaIn)2O3 films. The images are shown in Fig. 3 over a scale of 4 μm × 4 μm. The surfaces of films with indium content of 0.25 and 0.36 are composed of uniformly distributed and tightly packed grains with triangle shape as shown in Fig. 3(a) and 3(b). The surface of film with indium content of 0.49 appears ellipse grain with unclear boundary as shown in Fig. 3(c). The difference in morphology of the (GaIn)2O3 films indicates the different element content of these films. The RMS values of these films measured by AFM are less than 5 nm, indicating the good surface quality of these films.
Fig. 3 AFM surface morphologies of (GaIn)2O3 films with indium content of (a) 0.25, (b)0.36 and (c)0.49.
Figure 4 shows the transmission spectra of (GaIn)2O3 films with indium content from 0.25 to 0.49. The average transmittance of all the films in the visible region is more than 90%. No multi-absorption-edge, a typical characteristic of phase separation, is observed for all the samples, which confirms that phase separation does not occur in these samples [21]. The absorption edges of the (GaIn)2O3 films shift toward longer wavelength region with the increase of indium content. The transmission spectra are converted into (αhν)2~hν plot (α denotes absorption coefficient) as shown in the insert of Fig. 4. It can be observed that (αhν)2 as a function of hν fits the straight line well at the photon energy range around 4.2-5.0 eV. The bandgap of the films was obtained by extrapolating the linear part of (αhν)2~hν to the horizontal axis. The derived typical bandgap values of (GaIn)2O3 films with indium content of 0.25, 0.36 and 0.49 are 4.81, 4.69 and 4.32 eV, respectively. The variation of bandgap values promise that multi-color photodetectors can be obtained on these films. The variation of bandgap with the indium content corresponds well with our previous work [18], verifying the calculated indium content by XRD results using Vegard’s Law.
In order to verify the UV detectivity of the (GaIn)2O3 films, MSM structure photodetectors with different indium content were fabricated. (Ga0.75In0.25)2O3 photodetector is selective as a representative to characterize the performance of the photodetectors. Figure 5 shows the I–V characteristics under dark conditions and illuminations of 250, 300 and 400 nm wavelengths of (Ga0.75In0.25)2O3 photodetector. A clear linear relationship between the current and the applied voltage is observed in Fig. 5(a), indicating the ohmic property of the contact. The dark current is below 1 nA in Fig. 5(b), which is below the limitation of our measurements. Under a bias of 5V, the detector shows negligible current under the illumination of 400 nm. With the illumination wavelength decreases to 300 nm, the current begins to increase with the voltage. The current shows a sharp jump as the device is exposed to the 250 nm light. It should be noticed that all of the investigated (GaIn)2O3 films showed ohmic contact characteristics with the Au/Ti electrodes, irrespective of their indium contents. Guo et al. [13] have found Au/Ti electrodes were ohmic contact with the as-grown β-Ga2O3 films and Schottky contact with the annealed films. They speculated that the conversion from ohmic-type contact to Schottky-type contact is related to the change of oxygen vacancies concentration in annealed films. The ohmic contact of (GaIn)2O3 films in this work should also relate to the oxygen vacancies, because the as-deposited (GaIn)2O3 film under room temperature has shown plenty of oxygen vacancies [22].
Fig. 5 Typical I-V characteristics curves of the (GaIn)2O3 photodetectors with the (a) linear and (b) logarithmic coordinate in the dark, under 200, 300 and 400 nm lights.
Figure 6 shows the typical time-dependent photoresponse of the fabricated (Ga0.75In0.25)2O3 photodetector devices to 250 nm illumination. Upon UVC (250 nm) illumination, the current instantaneously increases for the detector. When the light turned off, the current decreases rapidly. The rise and decay time constant are analyzed by using the following equation [6]:
where is the steady-state photocurrent, t is the time, A and B are constants, and τ is the relaxation time constant. τr and τf denote the rise and decay time constants, respectively. The fitting is shown as the dashed lines in Fig. 6. The total rise time and decay time of the detector are about 8.8 and 1.1 s, respectively. Both the rise and decay time constants have two components (fast and slow), which are attributed to band-to-band transition and band-to-deep-level transition [6]. The slow response time makes it difficult for the photocurrent to recover to the initial value after UV illumination, namely, the persistent photocurrent (PPC), which is frequently observed in ZnO-based materials [23]. The long lifetime of our device can reasonably be attributed to the presence of oxygen vacancies (Vo), which has been evidenced in our previous paper [22]. Under UV illumination, Vo will transition to its charge states, Vo+ or Vo++, and release conducting electrons. However, after illumination, these charged states relax back to their neutral states only with difficulty, owing to the lattice deformation potential induced by the ionization process, causing a clear PPC. The β-Ga2O3 thin film as well as β-Ga2O3 nanowires based photodetectors showed similar degree of time constants with that of us 7,18, indicating similar intrinsic mechanism of the PPC effects of these photodetectors.Fig. 6 Typical experimental data and fitted curves of the rise and decay process of (Ga0.75In0.25)2O3 photodetectors.
The normalized photocurrents of (GaIn)2O3 photodetectors with different indium content versus incident wavelength are shown in Fig. 7. As the indium content increases, the photocurrent begins to increase above the baseline value at successively longer wavelengths. The turn-on wavelength of detectors with indium content x of 0.25, 0.36 and 0.49 are nearly 285, 297 and 315 nm, respectively. The turn-on wavelength in this work is defined as the wavelength that the photocurrent rises 20% above the baseline (~dark) current. The variation of the turn-on wavelength with the indium content was summarized in the insert of Fig. 7. It is clear observed that the turn-on wavelength increased almost linearly with that of the indium content, indicating the turn-on wavelength can be tuned using (GaIn)2O3 films. On another aspect, we have observed relatively high responsivity of the (GaIn)2O3 film based photodetectors in this work. The photodetectors with indium content of 0.25, 0.36 and 0.49 showed relatively high photoresponsivity of about 0.1, 0.1 and 30 A/W under the illumination of 250 nm light as shown in Fig. 8. Kokubun et al. [17] have reported the responsivity of (GaIn)2O3 photodetectors with indium content lower than 0.2. The photodetectors showed responsivity lower than 0.01 A/W when indium content is lower than 0.1. The photodetector with indium content 0.2 showed responsivity about 0.2 A/W, which is similar with that of us. This is comparable or better than some of ZnMgO film based UV detectors reported in literature [4, 21, 24, 25]. The higher photo responsivity of the device with 0.49 indium content should be attributed to the long life time of photogenerated carriers induced by trap states in the film.
Fig. 7 Photo currents of (GaIn)2O3 film photodetectors versus incident wavelength. Insert shows the turn-on wavelength of the photodetectors as a function of indium content
As has been stated that AlGaInO system is a new candidate for UV detectors because of its variable bandgap from 3.6 to 9.9 eV and chemical and thermal stabilities. The tunable turn-on wavelength of the fabricated UV detector in this work has proved that feasibility. Although the fabricated UV detectors in this work showed similar degree of photo response time and responsivity with that of the AlGaN and ZnMgO based UV detectors, the AlGaInO based detectors have potential wider detection range than that of AlGaN and ZnMgO. Moreover, in order to realize detection wavelength tunable AlGaN and ZnMgO UV detectors, additional steps are indispensable in order to overcome the encountered challenges such as phase separation [26] and deteriorated film quality [1] during film growth. Although the (GaIn)2O3 films also exhibited phase segregation in many reports [11, 18, 27, 28], the obtained (GaIn)2O3 films in this work are of monoclinic phase just using post annealing process. The simplicity of post annealing process makes our films very suitable for device fabrication.
4. Conclusions
We demonstrated (GaIn)2O3 films based UV photodetector with a planar photoconductor structure, and investigated the photoresponse properties of the fabricated devices. Monoclinic structured (GaIn)2O3 films with indium content from 0.25 to 0.49 were obtained in present work. No phase separation was observed for all of the films from the results of XRD, AFM and transmittance measurements. The absorption edges of the (GaIn)2O3 films shift toward longer wavelength region with the increase of indium content. The photodetectors show relative high photoresponsivity. The turn-on wavelength of the (GaIn)2O3 photodetectors increases with the indium content, indicating color selective UV detectors can be realized.
Funding
National Natural Science Foundation of China (61764001, 61474031); Guangxi Key Laboratory of Precision Navigation Technology and Application (DH201701); Guangxi District Education Office projects to enhance the basic ability of young teachers (2017KY0201); Japan Society for Promotion of Science (JSPS) (KAKENHI Grant Numbers 16K06268 and 26630297).
Acknowledgements
This work was mainly performed in Saga University and the Kyushu University Cleanroom Laboratory Facility supported by the Ministry of Education, Culture, Sports, Science and Technology.
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