Sol-gel prepared beta-Ga2O3 thin films for ultraviolet photodetectors

Sol-gel prepared β-Ga2O3 thin films for ultraviolet photodetectorsYoshihiro Kokubun, Kasumi Miura, Fumie Endo, and Shinji NakagomiCitation: Appl. Phys. Lett. 90, 031912 (2007); doi: 10.1063/1.2432946View online: /10.1063/1.2432946View Table of Contents: /resource/1/APPLAB/v90/i3Published by the American Institute of Physics.Related ArticlesLarge enhanced perpendicular magnetic anisotropy in CoFeB/MgO system with the typical Ta buffer replaced by an Hf layerAIP Advances 2, 032151 (2012)Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growthJ. Appl. Phys. 112, 043503 (2012)Photoexpansion and nano-lenslet formation in amorphous As2S3 thin films by 800nm femtosecond laser irradiationJ. Appl. Phys. 112, 033105 (2012)Thickness effects on the magnetic and electrical transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on MgO substratesAppl. Phys. 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Lett.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsSol-gel prepared␤-Ga2O3thinfilms for ultraviolet photodetectors Yoshihiro Kokubun,a͒Kasumi Miura,Fumie Endo,and Shinji NakagomiDepartment of Information Technology and Electronics,Faculty of Science and Engineering,Ishinomaki Senshu University,1Shinmito,Minamisakai,Ishinomaki,Miyagi986-8580,Japan͑Received4November2006;accepted15December2006;published online19January2007͒␤-Ga2O3thinfilms have been prepared on͑0001͒sapphire substrates by the sol-gel method.X-ray diffraction showed that␤-Ga2O3polycrystallinefilms were formed at heat-treatment temperatures above600°C.With increasing heat-treatment temperature above900°C,the lattice constants of the␤-Ga2O3films decreased,while the band gap increased.Planar geometry photoconductive detectors based on the sol-gel prepared␤-Ga2O3thinfilms have been fabricated.They showed the photoresponse only for the wavelengths shorter than270nm,which correspond to the solar-blind region.The peak wavelength in the spectral response depended on the heat-treatment temperature in the sol-gel process.©2007American Institute of Physics.͓DOI:10.1063/1.2432946͔Monoclinic structured gallium oxide͑␤-Ga2O3͒is a wide band gap͑ϳ4.9eV͒semiconductor1and is a chemi-cally and thermally stable material.Therefore,␤-Ga2O3has attracted interest as a material for gas sensors,2phosphors,3 transparent conductors,4and transparent electronic devices.5 Moreover,␤-Ga2O3is the promising candidate for the ultra-violet͑UV͒photodetector that is blind to wavelengths above 280nm,the so-called solar-blind photodetector.It is appli-cable to aflame detector,which is useful for applications in flame safeguard andfire control areas.Although solar-blind photodetectors based on AlGaN with high Al composition have been intensively investigated,6AlGaN photodetectors tend to show degraded characteristics with increasing Al composition.Recently,Feng et al.7have demonstrated that ␤-Ga2O3nanowires are promising materials for realizing solar-blind photodetectors.However,UV photodetectors based on␤-Ga2O3thinfilms have not been investigated.At present,␤-Ga2O3films are prepared by various meth-ods such as sputtering,8chemical vapor deposition͑CVD͒,9 pulsed laser deposition,10spray pyrolysis,11and sol-gel method.12In recent years,the sol-gel method has been ex-tensively used to prepare various kinds of oxidefilms,such as TiO2,13LiNbO3,14and ZnO15,16films.The sol-gel method is more convenient and less expensive and has such general advantages as easier composition control and superior uni-formity.However,there have been few detailed reports on the preparation of␤-Ga2O3films by the sol-gel method ex-cept for the applications to gas sensors and phosphors,par-ticularly from the viewpoint of the influence of the prepara-tion conditions on thefilm properties.In this letter,we describe the sol-gel prepared␤-Ga2O3films as ultraviolet photodetectors,relating to the heat-treatment temperature in the sol-gel process.␤-Ga2O3films were prepared on͑0001͒sapphire sub-strates by the sol-gel method.The sapphire substrates were degreased in ultrasonically agitated organic solvents prior to film preparation.As a starting material,gallium isopropoxide was used.2-Methoxyethanol and monoethanolamine were used as the solvent and stabilizer,respectively.Gallium isopropoxide wasfirst dissolved in a mixture of 2-methoxyethanol and monoethanolamine at60°C.The mo-lar ratio of monoethanolamine to gallium isopropoxide was maintained at1.0and the concentration of gallium isopro-poxide was0.4mol/l.After the solution was stirred for1h, a homogeneous and transparent solution was obtained.The films were spin coated on the substrates at3000rpm.After spin coating,the substrates werefirst heated at90°C in air for10min to evaporate the solvent,and then at300°C for 20min to eliminate the organic component in thefilm.This procedure was repeated six times.Thefilms were then heat treated at400–1200°C in air for1h.The thickness of the films was around150–200nm.The structural properties of thefilms were studied by x-ray diffraction͑XRD͒in the␪−2␪mode using Cu K␣ra-diation.Optical properties were evaluated using ultraviolet-visible transmission spectroscopy.Planar geometry photo-conductive detectors were fabricated by forming two Au Ohmic contacts with1.5mm spacing on the␤-Ga2O3film surface by vacuum evaporation.Spectral photoresponse was measured using a monochromator with a xenon-arc lamp as the optical excitation source.Photocurrent was measured un-der a10V bias condition with an Advantest R8340A ultra-high resistance meter.The photocurrent was also normalized to allow for incident light intensity variation as a function of wavelength.Figure1shows the spectral responses of the photocon-ductive detectors fabricated by using␤-Ga2O3films prepareda͒Electronic mail:kokubun@isenshu-u.ac.jpFIG. 1.Spectral responses of the photoconductive detectors based on␤-Ga2O3films prepared at various temperatures.APPLIED PHYSICS LETTERS90,031912͑2007͒0003-6951/2007/90͑3͒/031912/3/$23.00©2007American Institute of Physics90,031912-1at various heat-treatment temperatures in the sol-gel process.The measurements were carried out by illuminating the top surface of the film as illustrated in the inset of Fig.1.The photocurrent increases as the wavelength is scanned from 200nm.In the films prepared at 600and 800°C,the pho-tocurrent reaches its peak value at 250nm and then falls sharply.For wavelengths longer than 280nm,no photocur-rent is observed.The peak wavelength of photocurrent shifts toward shorter wavelengths with increasing heat-treatment temperature above 900°C.The peak value of the photocur-rent slightly increased with increasing heat-treatment tem-perature up to 1000°C,but it abruptly decreased at 1200°C.The maximum responsivity was about 8ϫ10−5A/W for the film prepared at 1000°C.The decrease of photocurrent at wavelengths less than the peak wave-length can be ascribed to the strong absorption near the film surface,where the lifetime for the recombination of photo-generated carriers may be smaller than in the bulk.17All samples showed no photoresponse for wavelengths longer than 280nm.This result indicates that ␤-Ga 2O 3films have potential for solar-blind photodetectors such as flame sen-sors.In order to investigate the cause for the peak wavelength shift in the spectral response due to the heat-treatment tem-perature,optical absorption,and XRD measurements were performed.The optical transmission spectra for the ␤-Ga 2O 3films showed high transmittance above 80%in the wavelength region above 280nm independent of the heat-treatment temperature.Figure 2shows ͑␣h ␯͒2vs h ␯plots,where ␣and h ␯are the absorption coefficient and photon energy,respectively.The absorption coefficient increases rapidly at the photon energy range around 4.9–5.6eV de-pending on the heat-treatment temperature,and ͑␣h ␯͒2as a function of h ␯fits the straight line quite well.The result indicates that the absorption in this energy region is due to the direct transition.The direct band gap was obtained from the intersection of the straight line and horizontal axis in Fig.2.The band gap of the films prepared at heat-treatment tem-peratures below 800°C is 4.95eV,which is approximately in agreement to the reported value for the ␤-Ga 2O 3films prepared by the pulsed laser deposition.10However,the bandgap increases with increasing heat-treatment temperature above 900°C.The enlargement of the band gap is consistent with the peak shift in the spectral response of photocurrent.Figure 3shows XRD patterns of the films prepared at various heat-treatment temperatures.At 400°C,no peaks appeared except for sapphire-related diffraction peaks,indi-cating that the prepared film was amorphous.In the films prepared at heat-treatment temperatures above 600°C,dif-fraction peaks corresponding to the Ga 2O 3monoclinic phase were observed,indicating that ␤-Ga 2O 3polycrystalline films were formed.The diffraction peaks indexed as ͑2¯01͒and higher order diffractions were dominant.The full width at half maximum ͑FWHM ͒values and the peak intensities of the diffractions for the ␤-Ga 2O 3films decrease and increase with increasing heat-treatment temperature,respectively.The crystallite sizes estimated from the FWHM values of the XRD peaks using Scherrer formula increased from 15to 40nm with increasing heat-treatment temperature.Each diffraction peak position gradually shifts toward higher angles with increasing heat-treatment temperature above 900°C.The lattice constants of the ␤-Ga 2O 3films were esti-mated by using the software CELLCALC ,18which is a program that calculates unit cell parameters from observed d value and hkl data by the method of least squares.Figure 4shows the heat-treatment temperature dependence of the lattice con-stants of the ␤-Ga 2O 3films.At heat-treatment temperatures below 800°C,the a -,b -,and c -axis lattice constants of the sol-gel prepared ␤-Ga 2O 3films are comparable to the stan-dard values for bulk ␤-Ga 2O 3͑a =12.23Å,b =3.04Å,and c =5.80Å͒.19However,they decrease with increasing heat-treatment temperature above 900°C.The diffusion of Al from the alumina substrates during postannealing at higher temperatures above 1000°C has been observed in the ␤-Ga 2O 3films prepared by sputtering 8and CVD.9Therefore,the observed decrease in the lattice constants at higher heat-treatment temperatures is supposed to be caused by the dif-fusion of Al from the sapphire substrates during the heat-treatment in the sol-gel process.Since the ionic radius of Al 3+͑0.54Å͒is smaller than that of Ga 3+͑0.62Å͒,the lat-tice constant would be reduced if Al substitutes for Ga site in the ␤-Ga 2O 3lattice.If Al is incorporated into the ␤-Ga 2O 3lattice as discussed above,the band gap would increasewithFIG.2.Square of absorption coefficient as a function of photon energy for ␤-Ga 2O 3films prepared at varioustemperatures.FIG.3.X-ray diffraction patterns of ␤-Ga 2O 3films prepared at various temperatures.the Al content because Al 2O 3has a wider band gap than Ga 2O 3.Therefore,the conclusion based on the lattice con-stant is consistent with the results obtained from the optical measurements.In conclusion,␤-Ga 2O 3films have been prepared on ͑0001͒sapphire substrates by the sol-gel method.␤-Ga 2O 3polycrystalline films were obtained at heat-treatment tem-peratures above 600°C.With increasing heat-treatment tem-perature above 900°C,the lattice constants decreased while the band gap increased,which are ascribed to the Al diffu-sion from the sapphire substrate during the heat-treatment ing the sol-gel prepared ␤-Ga 2O 3thin film,asolar-blind UV photodetector possessing the sensitivity only for the wavelengths shorter than 270nm was demonstrated.The peak in the spectral response shifted toward shorter wavelengths with increasing heat-treatment temperature above 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