PIasma-Enhanced Atomic Layer Deposition of Amorphous Ga2O3 for SoIar-BIind Photodetection

Ze-Yu Fan | Min-Ji Yang | Bo-Yu Fan | Andra? Mavri? | Nadiia Pastukhova | Matjaz VaIant | Bo-Lin Li | Kuang Feng | Dong-Liang Liu | Guang-Wei Deng | Qiang Zhou | Yan-Bo Li

Abstract—Wide-bandgap gallium oxide (Ga2O3) is one of the most promising semiconductor materials for solar-blind(200 nm to 280 nm) photodetection.In its amorphous form,amorphous gallium oxide (a-Ga2O3) maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature,thus it is compatible with Si integrated circuits (ICs) technology.Herein,the a-Ga2O3 film is directly deposited on pre-fabricated Au interdigital electrodes by plasma enhanced atomic layer deposition (PE-ALD) at a growth temperature of 250 °C.The stoichiometric a-Ga2O3 thin film with a low defect density is achieved owing to the mild PE-ALD condition.As a result,the fabricated Au/a-Ga2O3/Au photodetector shows a fast time response,high responsivity,and excellent wavelength selectivity for solar-blind photodetection.Furthermore,an ultra-thin MgO layer is deposited by PE-ALD to passivate the Au/a-Ga2O3/Au interface,resulting in the responsivity of 788 A/W (under 254 nm at 10 V),a 250-nm-to-400-nm rejection ratio of 9.2×103,and the rise time and the decay time of 32 ms and 6 ms,respectively.These results demonstrate that the a-Ga2O3 film grown by PE-ALD is a promising candidate for high-performance solar-blind photodetection and potentially can be integrated with Si ICs for commercial production.

1.Introduction

The solar-blind spectral region refers to the wavelength range of 200 nm to 280 nm because sunlight cannot reach the Earth’s surface due to the absorption by ozone in the atmosphere[1].Solar-blind photodetectors,which are sensitive to light in the range of 200 nm to 280 nm,have many military and civilian applications,such as missile warning,space communications,flame detection,biological/chemical analysis,and imaging[2]-[6].The commercially available Si-based photodetector shows very low responsivity (＜0.1 A/W)in the solar-blind region because of the low penetration depth of high-energy ultraviolet photons and its small bandgap (1.1 eV) requires additional optical filters to achieve solar-blind photodetection[7].Therefore,semiconductors with an intrinsic wide bandgap (＞4.5 eV) are promising candidates for solar-blind photodetection.These wide-bandgap semiconductors include AlN,AlxGa1-xN,MgxZn1-xO,and Ga2O3[8]-[11].AlN cannot detect wavelengths longer than 210 nm due to its extremely wide bandgap (6.1 eV)[8].The AlxGa1-xN film is normally grown on a Si substrate,which leads to a high dislocation density and film stress because of its inherent large lattice mismatch (～17%).Besides,the high growth temperature (～1300 °C) for AlxGa1-xN induces thermal strain due to the excessive heat mismatch (～53%),which further leads to the increase of the defect density[9].The synthesis of wurtzite MgxZn1-xO with the bandgap in the solar-blind region is challenging because of the phase segregation problem[10].On the other hand,Ga2O3has a direct bandgap of～4.8 eV and shows excellent chemical,thermal,and radiation stability,which is ideally suited for solar-blind photodetection[11].Besides,Ga2O3can be synthesized with simple physical or chemical vapor deposition methods at a relatively low cost.All of these make Ga2O3an ideal candidate for fabricating solar-blind deepultraviolet (DUV) photodetectors.

In the past few years,Ga2O3has been extensively used for fabricating various types of solar-blind photodetectors and their properties have been summarized in several recent reviews[12]-[15].Most Ga2O3DUV solar-blind photodetectors were based on monoclinic β-Ga2O3films,which were epitaxially grown on sapphire substrates by molecular beam epitaxy (MBE),chemical vapor deposition (CVD),or pulsed laser deposition(PLD) processes.The solar-blind photodetection characteristics mainly depend on the crystalline quality of the β-Ga2O3film,influenced by the thermal-lattice match with the substrate,growth temperature,deposition rate,and annealing conditions[16].In thin-film β-Ga2O3-based photodetectors,high responsivity could be achieved through the introduction of oxygen vacancies in the β-Ga2O3thin film.However,the existence of oxygen vacancies also leads to an extremely slow response time of a few seconds.For instance,a typical β-Ga2O3metal-semiconductor-metal (MSM) photodetector achieved the responsivity of 153 A/W with the rise time and the decay time of 5.0 s and 10.3 s,respectively[17].Besides,the formation of monoclinic β-Ga2O3phase requires a high processing temperature (＞650 °C),causing a drastic increase of the thermal budget and limiting the selection of substrates[18]-[21].In contrast,the amorphous gallium oxide (a-Ga2O3) thin film can be deposited by relatively simple chemical and physical vapor deposition methods,such as radiofrequency (RF) sputtering and atomic layer deposition (ALD) on almost any substrates at a low growth temperature[22]-[25].Compared with RF sputtering,ALD is advantageous due to its precise thickness control,high conformality,outstanding step coverage,and uniformity for the preparation of ultrathin layers[26],[27].In addition,nearly all of a-Ga2O3thin films prepared through the ALD method are stoichiometric with decent electrical and optical properties,which are important factors for achieving high-performance solar-blind photodetection[28].

In this work,we demonstrate a solar-blind photodetector with high responsivity and a rapid response time based on amorphous a-Ga2O3films deposited by plasma enhanced atomic layer deposition (PE-ALD).The low processing temperature (250 °C) of the PE-ALD method makes it possible to directly deposit the a-Ga2O3film on pre-fabricated Au interdigital electrodes,thus eliminating the possibility to contaminate the a-Ga2O3film during the post device fabrication process.Under the 254-nm DUV illumination at the 10-V bias,the fabricated Au/a-Ga2O3/Au MSM solar-blind photodetector shows high responsivity (R) of 579 A/W and a clear cut-off wavelength at 280 nm.TheR250 nm/R400 nmrejection ratio is as high as 1.5×103and the rise time and the decay time are as quick as 42 ms and 8 ms,respectively.Furthermore,it is found that the passivation of the Au/a-Ga2O3/Au interface with an ultra-thin MgO layer further improves the responsivity to 788 A/W and theR250 nm/R400 nmrejection ratio to 9.2×103,while the rise time and the decay time are decreased to 32 ms and 6 ms,respectively.These results demonstrate the advantages of PE-ALD in the deposition of functional MgO/a-Ga2O3layers for the fabrication of high-performance solar-blind photodetectors that potentially can be integrated with Si integrated circuits (ICs) for commercial applications.

2.ExperimentaI Section

2.1.Thin-FiIm Growth and Device Fabrication

First,quartz glass substrates were ultrasonically cleaned in soap water,deionized water,acetone,and isopropanol for 15 min each.Second,the interdigital electrodes consisted of Cr and Au with thicknesses of 10 nm and 60 nm,respectively,were fabricated by photolithography,electron beam evaporation (Angstrom Engineering AMOD),and lift-off processes.Each device comprised 25 pairs of interdigital electrodes whose length,width,and spacing were 200 μm,4 μm,and 4 μm,respectively,resulting in an effective illumination area of 4×10-4cm2.Then,a-Ga2O3thin films with a thickness of～100 nm were deposited on the substrates with interdigital electrodes by PE-ALD (Picosun R-200 Advanced).Triethylgallium (TEG,99.99%) was used as the Ga precursor.Oxygen (O2) plasma generated remotely by a microwave plasma generator at 3000 W under 50-sccm O2(99.999% purity) flow was used as the O source.The TEG source temperature was maintained at 25 °C and the substrate temperature was 250 °C.The PE-ALD sequence consisted of 0.5-s TEG exposure,5-s N2purge,12-s O2plasma exposure,and 5-s N2purge.This sequence was repeated for 2000 cycles to complete the deposition.For devices with the MgO passivation layer,MgO was deposited before the deposition of the a-Ga2O3film.Bis(cyclopentadienyl)magnesium (MgCp2) was used as the Mg precursor and its temperature was maintained at 120 °C.The PE-ALD sequence for the deposition of MgO consisted of 1.6-s MgCp2pulse,5-s N2purge,12-s O2plasma exposure,and 5-s N2purge.The deposition was completed after 10 cycles to achieve a thickness of～1 nm.Si substrates were also used to determine the thickness of the deposited a-Ga2O3and MgO films by ellipsometry and scanning electron microscopy (SEM),and the composition of the films by X-ray photoelectron spectroscopy (XPS).After deposition,all samples were annealed in air at 300 °C for 1 h before the material and device characterizations.This air annealing process is necessary for improving the photocurrent stability of the devices (refer to Fig.S1 in Supplementary).

2.2.MateriaI Characterization

The a-Ga2O3film was analyzed by X-ray diffraction (XRD) with Thermo Scientific ARL EQUINOX 1000 in theθ-2θmode using a monochromatic Cu Kα source operated at 40 kV and 30 mA.The cross-sectional SEM image of the film was taken with Zeiss Crossbeam 340.The optical microscope image of the device was taken with Zeiss Axiocam 305 color.The ultraviolet-visible (UV-Vis) absorption spectrum was measured with a SHIMADZU UV-1900 spectrophotometer.The chemical state and elemental composition of the films were characterized by XPS (Thermo Fisher Scientific ESCALAB-250 Xi) with a monochromatic Al Kα X-ray source.The binding energy was corrected by setting the binding energy of the hydrocarbon C 1s peak to 284.8 eV.The scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS) and high-resolution transmission electron microscopy (HRTEM) results were obtained by using JEOL JEM2100F equipped with the STEM unit and an EDS detector (Oxford Instruments).For transmission electron microscopy (TEM) observations,the cross section was glued by the epoxy resin,processed by a standard mechanical sample preparation process,and finalized by Ar+ion-milling and polishing (PIPS II,Gatan) at grazing incidence (＜5°).The acceleration voltage for HRTEM was 200 kV and the spot size in the STEM mode was 1 nm.

2.3.Device Characterization

To control the gas atmosphere and shield the electrical noise,the devices were installed in a tailor-made metallic chamber with a gas inlet and outlet.The chamber was purged with dry air before measurement.TheI-Vcurves were recorded with a picoammeter/voltage source (Keithley 6487).A DUV fluorescent tube lamp emitting at 254 nm with the irradiance of～310 μW/cm2was used as the light source for theI-Vmeasurement.The irradiance of the light source was measured by an optical power meter (Ophir Nova II) with a high sensitivity thermal sensor (Ophir 3A).The time-dependent photoresponse curves were recorded with a high-speed picoammeter (Feimoto FTH2185) at a 100-kHz sampling rate under the illumination of chopped 255-nm light emitting diode (LED) (Thorlabs LED255J Optan) light.The spectral photoresponse curves were measured with a portable picoammeter (Feimoto FT1185) under the illumination of monochromatic light from a spectrophotometer (SHIMADZU UV-1900) equipped with a deuterium lamp.The responsivity was calculated using the emission irradiance spectrum of the deuterium lamp provided by SHIMADZU and then normalized.A 9-V battery was used as a low-noise voltage source for the time-dependent and spectral photoresponse measurement.

3.ResuIts and Discussion

Fig.1 (a) exhibits the cross-sectional SEM image of the a-Ga2O3thin film deposited on a Si substrate with 2000 PE-ALD cycles.The a-Ga2O3thin film has a uniform thickness of～100 nm.And on Au and quartz substrates,a-Ga2O3thin films with a similar thickness are observed (Fig.S2),demonstrating that different substrates have little effect on the growth of a-Ga2O3films due to the low-temperature self-limited PE-ALD growth process.Fig.1 (b) shows the XRD pattern of the a-Ga2O3thin film deposited on a synthetic quartz glass substrate.Besides a hump in the range of 10° to 30° originating from the amorphous quartz glass substrate,the absence of diffraction peak confirms the amorphous nature of the a-Ga2O3thin film deposited by PE-ALD.This is reasonable as a minimum temperature of 500 °C is required to crystallize Ga2O3into α phase[29],[30],which is far higher than the processing temperature of PE-ALD.The Tauc plot of the UV-Vis absorption spectrum shown in Fig.1 (c) reveals that the optical bandgap of the a-Ga2O3film deposited by PE-ALD is～4.95 eV,which is consistent with previously reported values for the a-Ga2O3film[31].The chemical state and elemental composition of the PE-ALD deposited a-Ga2O3film were analyzed by XPS.All the spectral features of the survey spectrum in Fig.1 (d) are attributed to the core levels or Auger lines of Ga and O,except for the C 1s peak.Fig.1 (e) shows that the Ga 2p3/2core level spectrum can be well-fitted with a single peak centered at 1118.1 eV,corresponding to a single Ga3+chemical state in the PE-ALD deposited a-Ga2O3film.The O 1s core level spectrum in Fig.1 (f) is deconvolved into two peaks centered at 530.9 eV and 532.1 eV,which are attributed to the O-Ga chemical bonding in the a-Ga2O3film and the carbonate(C=O)/hydroxyl(OH) species chemisorbed on the surface,respectively.After excluding the contribution from C=O/OH species,the O/Ga ratio was determined to be～1.48 by quantitative XPS analysis.The nearly stoichiometric O/Ga composition is consistent with the single Ga3+chemical state observed in Fig.1 (e).The chemical state and the O/Ga composition of the a-Ga2O3film deposited by PE-ALD are drastically different from those of the a-Ga2O3film deposited by sputtering.In sputter-deposited a-Ga2O3films,there are high-density Ga1+defects due to the reducing nature of the sputtering process and the O/Ga ratio is commonly less than 1[32].The differences of the chemical state and elemental composition for a-Ga2O3films deposited by different methods result in different solar-blind photodetection characteristics,as shown in Fig.2.

Fig.1.Material characterization of a-Ga2O3 thin film: (a) cross-sectional SEM image of a-Ga2O3 thin film deposited on Si substrate,(b) XRD pattern and (c) Tauc plot of UV-Vis absorption spectrum of a-Ga2O3 thin film deposited on quartz glass substrate,(d) XPS survey spectrum of a-Ga2O3 thin film deposited on Si substrate,and XPS core-level spectra of (e) Ga 2p3/2 and (f) O 1s.

Fig.2 (a) shows the optical microscopy image of the fabricated Au/a-Ga2O3/Au MSM photodetector.The device has an effective illumination area (S) of 4×10-4cm2with 25 pairs of interdigital electrodes whose length and spacing are 200 μm and 4 μm,respectively.Fig.2 (b) illustrates the semilogarithmic plots ofI-Vcurves of the Au/a-Ga2O3/Au photodetector measured in the dark and under the 254-nm DUV illumination at the irradiance (Plight) of～310 μW/cm2.At the 10-V bias,the dark current (Idark) value of the photodetector is～0.45 nA and the photocurrent (Ilight) is 65.5 μA,resulting in anIlight/Idarkratio of more than 1.4×105.The responsivity (R) of the photodetector is determined by the following equation:

By using the data from theI-Vcurves in Fig.2 (b),the responsivity of the Au/a-Ga2O3/Au photodetector is estimated to be 579 A/W at the 10-V bias (Fig.2 (c)),which compares favorably with other a-Ga2O3based photodetectors (Table S1 in Supplementary).The time-dependent photoresponse of the Au/a-Ga2O3/Au photodetector is shown in Fig.2 (d).The photocurrent shows a fast rise and decay,and excellent reproducibility upon switching on and off the DUV illumination with a short (0.1 s) time interval.With a highspeed picoammeter,it is possible to capture the rise and decay processes (Fig.2 (e)).The rise time and the decay time,defined as the time interval for the current to rise from 10% to 90% of the peak value and vice versa[33],are 42 ms and 8 ms,respectively.The time response of the Au/a-Ga2O3/Au photodetector is much faster than most a-Ga2O3based photodetectors (Table S1).For photoconductive type MSM photodetectors,there is usually a trade-off between the response time and responsivity.For instance,although high responsivity (4100 A/W) can be achieved using the sputter-deposited a-Ga2O3film with a high density of oxygen vacancies (O/Ga ratio～0.7),the rise time and the decay time are as long as 50 s and 400 s,respectively[34].In addition,the existence of high-density interbond defect states in a-Ga2O3also leads to photoresponse to lower-energy photons beyond the solar-blind region,resulting in a poor DUV/ultraviolet A(UVA) rejection ratio.In contrast,due to the more stoichiometric composition of the PE-ALD a-Ga2O3film(O/Ga ratio～1.48),our device exhibits significantly a shorter rise time and decay time while maintaining relatively high responsivity.The spectral photoresponse of the Au/a-Ga2O3/Au photodetector in Fig.2 (f)shows a clear cut-off at the edge of the solar-blind region.TheR250nm/R400nmrejection ratio is as high as 1.5×103,demonstrating excellent wavelength selectivity for solar-blind photodetection.The high responsivity,fast time response,and excellent wavelength selectivity demonstrate that the a-Ga2O3film deposited by PE-ALD with a low defect density is ideally-suited for solar-blind photodetection.

Fig.2.Device characterization of Au/a-Ga2O3/Au MSM photodetector: (a) optical microscope image,(b) I-V curves measured in the dark and under 254-nm DUV illumination at the irradiance of～310 μW/cm2,(c) responsivity,(d) transient photocurrent characteristics under chopped 255-nm LED light at 9-V bias,(e) rise and decay processes,and (f) spectral photoresponse.

The external quantum efficiency (EQE) of the photodetector is calculated by wherehis the Plank’s constant,cis the velocity of light,qis the elementary charge,andλis the wavelength of light.Under the 254-nm DUV illumination at the 10-V bias,the EQE value is as high as 2800 for the Au/a-Ga2O3/Au photodetector (Fig.S3).The EQE value of significantly higher than 1 suggests that there exists the photoconductive gain in the photodetector,whose origin has been well studied in semiconductor nanowire photodetectors[35].Due to the large surface-to-volume ratio and pinning of the Fermi level by the surface states,nanowires exhibit a depletion space charge layer near the surface.This surface depletion layer provides physical separation of electrons and holes and leads to a significantly enhanced photocarrier lifetime,which is the origin of the photoconductive gain.Similarly,in the ultrathin a-Ga2O3film,the surface depletion effect is expected to play an important role.As shown in Fig.S4 (a),a surface depletion layer could be formed due to the pinning of the Fermi level (EF) on the surface.The photogenerated holes migrate to the surface due to the upward band bending and get trapped by the surface states,leaving unpaired electrons moving freely inside the film.Under an external bias,the free electrons continue to circulate until they are annihilated by either recombination or trapping.The photoconductive gain (G) is achieved if the carrier lifetime (τ) is larger than the carrier transit time (τt) between the electrodes (G=τ/τt).According to this mechanism,the degree of surface depletion can affect the photoconductive gain,because a higher degree of surface depletion renders a better charge separation capability.To verify this,a MgO layer was applied on the surface of the a-Ga2O3layer to reduce the degree of surface depletion by partially passivating the surface defects (Fig.S4 (b)).MgO was selected because it is a common high refractory and high-kdielectric candidate for the passivation of metal-oxide semiconductor devices[36]-[38].As shown in Fig.S5,the photoconductive gain is indeed reduced in the fabricated MgO/a-Ga2O3/Au photodetector,while the response time becomes faster.

Instead of depositing the MgO layer on the top surface of the a-Ga2O3film,we speculate that the performance of the photodetector can be improved by inserting a MgO passivation layer between the Au electrode and the a-Ga2O3layer.As shown in Fig.S6 (a),the hole trap states at the a-Ga2O3/Au interface could become recombination centers for electrons migrating across this interface.By lowering the density of trap states by MgO passivation,the probability for the electrons to recombine at the interface is reduced (Fig.S6 (b)),thus further improving the responsivity of the photodetector.

The MgO thin layer with a thickness of～1 nm was deposited by PE-ALD before the deposition of the a-Ga2O3film.Figs.3 (a) to (d) show the STEM image and the corresponding EDS mapping results.These results show that the Ga and O elements distribute evenly in the a-Ga2O3layer with a uniform thickness of～100 nm.Despite the MgO layer was not directly observed in the STEM-EDS results due to its thin thickness and the reason that the Kα line of Mg (1.253 keV) lies very closely to the Kα line of Ga (1.098 keV),the HRTEM images in Figs.3 (e) and (f)indeed reveal an interfacial MgO layer between the Si substrate and the a-Ga2O3layer.The HRTEM image in Fig.3 (f) and the selected area electron diffraction (SAED) results in Fig.S7 further confirm the amorphous state of the a-Ga2O3layer.

Fig.3.Structure characterization of a-Ga2O3 film passivated by MgO: (a) STEM image;corresponding EDS elemental mappings of (b) Si,(c) Ga,and (d) O;(e)HRTEM images of a-Ga2O3/MgO layer deposited on Si substrate;(f) HRTEM image showing a-Ga2O3/MgO/Si interfaces.

Fig.4 (a) exhibits the XPS survey spectrum of the 1-nm MgO layer deposited on a Si substrate.The Mg 1s core-level spectrum in Fig.4 (b) is well-fitted with a single peak at 1304.8 eV,corresponding to a single chemical state of Mg2+[39].Fig.4 (c) exhibits the O 1s core-level XPS spectrum of MgO.Since the spectrum cannot be fitted with a single peak,two peaks centered at 532.1 eV and 532.2 eV were used to fit the spectrum,which could be assigned to the hydroxyl and surface-adsorbed carbonate species[40].The lattice oxygen (Mg-O bond) was not observed in the O 1s spectrum,most likely because the MgO thin layer was easily hydrolyzed during the transfer process between the PE-ALD deposition and the XPS characterization.Nevertheless,considering there was no OH source during the PE-ALD process and a relatively thick a-Ga2O3compact layer was deposited on top of the MgO layer,we believe that in the actual device the MgO layer maintained in its oxide form,instead of the hydroxide form.Figs.4 (d) to (f) exhibit the XPS core-level spectra of Ga 2p3/2and O 1s,and the Tauc plot of the UV-Vis spectrum of the a-Ga2O3film deposited on the MgO layer.The chemical state,elemental composition,and optical bandgap of the a-Ga2O3film were not affected by the underlying MgO layer,suggesting MgO only modified the interface properties of the Au/a-Ga2O3contact.

Fig.4.XPS and UV-Vis characterization results: (a) XPS survey spectrum of MgO layer deposited on Si substrate;XPS core-level spectra of (b) Mg 1s and (c) O 1s for MgO layer on Si substrate;XPS core-level spectra of (d) Ga 2p3/2 and(e) O 1s of a-Ga2O3/MgO bi-layer deposited on Si substrate;(f) Tauc plot of UV-Vis absorption spectrum of a-Ga2O3/MgO bi-layer deposited on quartz glass substrate.

Fig.5 (a) shows the schematic structure of the a-Ga2O3/MgO/Au photodetector.The a-Ga2O3/MgO bilayer was deposited on the same type of Au interdigital electrodes shown in Fig.2 (a).TheI-Vcurves of the a-Ga2O3/MgO/Au photodetector measured in the dark and under the 254-nm DUV illumination at the irradiance of～310 μW/cm2are plotted in Fig.5 (b).With MgO passivation,the photocurrent at the 10-V bias is increased to 89.1 μA and theIlight/Idarkratio reaches 1.9×105.The responsivity is enhanced to 788 A/W at the 10-V bias under the 254-nm DUV illumination (Fig.5 (c)),which is 36% higher than that of the device without the MgO passivation layer.The corresponding detectivity (D*) of the device can be calculated by

Under the 254-nm DUV illumination at the 10-V bias,D*and EQE are calculated to be 1.27×1016J and 3800 (Fig.S3),respectively.Meanwhile,compared with the result in Fig.2 (d),the time-dependent photoresponse in Fig.5 (d) shows a faster response time under the same operating conditions.The detailed rise and decay processes in Fig.5 (e) reveal the rise time and the decay time for the a-Ga2O3/MgO/Au photodetector are shortened to 32 ms and 6 ms,respectively.Furthermore,the spectral photoresponse in Fig.5 (f) reveals that theR250 nm/R400 nmrejection ratio is enhanced to 9.2×103.These results demonstrate the beneficial effect of the MgO passivation layer on improving the responsivity,response time,and wavelength selectivity of the a-Ga2O3based solar-blind photodetector.

Fig.5.Device characterization of a-Ga2O3/MgO/Au solar-blind photodetector: (a) schematic diagram,(b) I-V curves measured in the dark and under 254-nm DUV illumination at the irradiance of～310 μW/cm2,(c) responsivity,(d) transient photocurrent characteristics under chopped 255-nm LED light at 9-V bias,(e) rise and decay processes,and (f) spectral photoresponse.

4.ConcIusions

The a-Ga2O3thin film deposited by PE-ALD was employed to fabricate the Au/a-Ga2O3/Au MSM solar-blind photodetector.The mild deposition condition of the PE-ALD process rendered a low defect density and nearly stoichiometric composition in the deposited a-Ga2O3thin film.As a result,the fabricated Au/a-Ga2O3/Au MSM photodetector exhibited a fast response time,high responsivity,and excellent wavelength selectivity for solar-blind photodetection.Furthermore,the performance of the device was improved by passivating the a-Ga2O3/Au interface with an ultra-thin MgO layer.The a-Ga2O3/MgO/Au solarblind photodetector achieved high responsivity of 788 A/W at the 10-V bias,a fast rise time and decay time of 32 ms and 6 ms,and a highR250 nm/R400 nmrejection ratio of 9.2×103.These results demonstrate the versatility of the PE-ALD method for the deposition of the a-Ga2O3DUV-absorbing layer and the MgO passivation layer to fabricate high-performance solar-blind photodetectors.The mild processing conditions also make it possible to directly integrate the solar-blind photodetectors with Si ICs for commercial applications.

SuppIementary

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jnlest.2022.100176.

DiscIosures

The authors declare no conflicts of interest.