A wide ultraviolet spectra response photodetector based on epitaxial growth of highly-oriented ε-Ga2O3 crystal on diamond substrate

Abstract In recent years, diamond has shown great potential in solar-blind ultraviolet (UV) photodetection due to its ultrawide bandgap (∼ 5.5 eV) and other superior semiconductor properties. However, the response region of diamond photodetector is usually smaller than 230 nm, which cannot cover the whole solar-blind region from 200 to 280 nm. In this work, an ε-Ga2O3/diamond photodetector with wide spectra responsivity from 210 (or even lower) to 260 nm was fabricated. High-quality ε-Ga2O3 film with columnar crystal was epitaxially grown on single crystalline CVD diamond substrate by pulse laser deposition (PLD). TEM characterization revealed that the ε-Ga2O3 film grew along the <001> orientation on diamond (100) substrate. The deep ultraviolet (DUV) photodetector based on the ε-Ga2O3/diamond structure showed a high light-to-dark ratio over 5.7 × 104 and good linear response to the incident light power density from 10 to 400 mW/cm2. Moreover, compared to other photodetectors, the fabricated ε-Ga2O3/diamond photodetector achieved high responsivity and wide spectra response region from 210 to 260 nm, with high solar-blind rejection ratio of 104 (R240/R280) and 165 (R210/R280), respectively. The extension of spectra region with high responsivity of the ε-Ga2O3/diamond photodetector can be attributed to the thin thickness of ε-Ga2O3 film (around 200 nm) and parts of the DUV light were absorbed by diamond. The high responsivity and wide spectra response region indicate the fabricated ε-Ga2O3/diamond photodetector can be used for the detection of ultraviolet in the most of the DUV region.


Introduction
The deep ultraviolet (DUV) with wavelengths from 200 to 280 nm is commonly referred to as the solar-blind UV range, as solar radiation in this wavelength region is effectively blocked by the ozone layer in the Earth's atmosphere [1][2][3].Owing to the low natural background, photodetectors operating in solar-blind region have high signal-to-noise ratios and low false alarm rates.Therefore, solar-blind photodetectors have many civil and military applications such as biological/chemical, sterilization and disinfection, missile tracking and environmental monitoring [4][5][6].Unfortunately, most of the commercial semiconductor DUV photodetectors are made of silicon (Si), and the narrow bandgap (1.12 eV) makes Si photodetector vulnerable to visible light interference during DUV detection, thus limiting its applications.Diamond has emerged as a promising material for solar-blind photodetection due to its wide bandgap (E g : ~ 5.5 eV) and high radiation hardness, which can promise intrinsic invisible/solar blindness [7,8].However, due to the limitation of the band gap and UV adsorption edge, diamond photodetectors only show response to DUV light with wavelengths smaller than 230 nm.Therefore, to realize the full (or nearly full) wavelength and high response to DUV light in a single photodetector, a semiconductor with smaller bandgap is necessarily to be employed to form heterostructure with diamond.
Gallium oxide (Ga 2 O 3 ), as another ultrawide bandgap semiconductor, has also been widely used for DUV photodetection due to its wide bandgap (E g : ~ 4.9 eV), and other fantastic physical and electrical properties [9,10].However, due to the decrease of adsorption coefficient (α) in the short wavelength region (< 240 nm) [11,12], the high spectra response regions of the Ga 2 O 3 photodetectors are usually between 240 and 260 nm [13][14][15], which correspond exactly to the irresponsive region of diamond photodetectors in DUV region.Therefore, the combination of Ga 2 O 3 and diamond has gained much interest in recent years, leading to the development of different fabrication methods of Ga 2 O 3 /diamond photodetectors [16,17].Kim et al. transferred the β-Ga 2 O 3 nanolayer onto the diamond substrate through mechanical exfoliation and fabricated a β-Ga 2 O 3 /diamond photodetector [16].However, the fragility of β-Ga 2 O 3 film limits the transferring of large-scale β-Ga 2 O 3 films.Therefore, Chen et al. fabricated a β-Ga 2 O 3 /diamond photodetector by growing β-Ga 2 O 3 film on diamond substrate through plasma enhanced chemical vapor deposition (PECVD) [17].However, due to the low symmetric monoclinic structure of β-Ga 2 O 3 (space group: C2/m) and lattice mismatch between β-Ga 2 O 3 and diamond, the β-Ga 2 O 3 film had a poor crystalline quality with a full width at half maximum (FWHM) of the rocking curve of over 2°, which leaded to a low responsivity (0.2 mA/W).
As another major Ga 2 O 3 polymorph, ε-Ga 2 O 3 is the second most stable phase and has many fantastic properties, like ferroelectric polarization [18,19].Recently, the crystal structure of ε-Ga 2 O 3 has been demonstrated to be composed of orthogonal nanoscale (about 5-10 nm) ordered κ-Ga 2 O 3 cells (space group: Pna21) arranged 120° next to each other [20].Therefore, the crystal structure of ε-Ga 2 O 3 is regarded as orthographic (space group: Pna21) rather than the hexagonal (space group: P63mc) as previously thought.The orthorhombic crystal structure promises ε-Ga 2 O 3 more suitable for epitaxial growth on hexagonal and cubic semiconductor substrates [21][22][23].Therefore, due to better lattice compatibility, the high crystallographic symmetry indicates that it is possible to grow high quality ε-Ga 2 O 3 crystal on single crystalline diamond substrates.In our previous work, an ε-Ga 2 O 3 /diamond photodetector with high thermal stability was fabricated by growing crystalline ε-Ga 2 O 3 film on the diamond substate [24].However, till now, there are few reports on the epitaxial growth of ε-Ga 2 O 3 on diamond substrate, as well as the photodetector based on ε-Ga 2 O 3 /diamond heterostructure.
In this work, an ε-Ga 2 O 3 /diamond photodetector with nearly full spectra response in DUV region was fabricated.The high quality ε-Ga 2 O 3 film was epitaxially grown on single crystalline diamond substrate by pulsed laser deposition (PLD).The crystal qualities of the ε-Ga 2 O 3 film on the diamond substrate were detailly characterized by XRD and TEM, which revealed that the ε-Ga 2 O 3 film was composed of a series of columnar crystals with in-plane rotational domains.Besides, the detailed structure of ε-Ga 2 O 3 /diamond interface was determined by a combination of high-resolution scanning TEM and first principles calculations.Based on the obtained ε-Ga 2 O 3 /diamond heterostructure, a DUV photodetector was fabricated by depositing Ti/Au electrodes on ε-Ga 2 O 3 surface.Compared to other Ga 2 O 3 photodetectors, the fabricated ε-Ga 2 O 3 /diamond photodetector showed high spectra response from below 210 nm to the wavelength about 260 nm.The fabrication of ε-Ga 2 O 3 /diamond photodetector with broad and high spectra response will push the investigation of the integration of ultrawide bandgap heterostructures in the future.

Materials and experiments
The CVD diamond (100) substrate with size of 7 × 7 × 0.3 mm 3 used in this work is grew in our lab by micro-plasma chemical vapor deposition (MPCVD).The Raman peak center of the CVD diamond locates at about 1332.3 cm −1 and the full width at half maximum (FWHM) is about 2.1 cm −1 .The ε-Ga 2 O 3 film was deposited on diamond substrate by PLD system (SKY Technology Development Co., Ltd.Chinese Academy of Science) using a 248 nm KrF excimer laser.During ε-Ga 2 O 3 growth, the oxygen pressure and growth temperature were maintained at 20 mTorr and 800 °C, respectively.The thickness of ε-Ga 2 O 3 film can be controlled by changing the deposition cycling times.An ε-Ga 2 O 3 /diamond photodetector with metal-semiconductor-metal (MSM) structural interdigital electrodes (200 μm gap between electrodes) was then fabricated by depositing Ti (30 nm)/Au (50 nm) on the grown ε-Ga 2 O 3 surface through electron beam deposition system (SKY Technology Development Co., Ltd.Chinese Academy of Science).An in-situ thermal annealing at about 450 °C was performed in vacuum to achieve good Ohmic contact between electrodes and substrate.
The crystal structure of ε-Ga 2 O 3 film grown on single crystalline diamond substrate was characterized by X-ray diffraction (XRD, D8 Advance, Bruker, USA), and the microstructure of the ε-Ga 2 O 3 film was characterized using an aberration-corrected transmission electron microscopy (TEM, Spectra 300, Themo Fisher Scientific, USA).The transmittance of ε-Ga 2 O 3 , diamond and ε-Ga 2 O 3 /diamond samples in the range from 190 to 800 nm were measured by UV-Visible spectrometer (Lambda 1050+, Perkin Elmer, USA).The I-V characteristic of the ε-Ga 2 O 3 /diamond photodetector was measured in a semiconductor parameter analyzer (Keithley 4200-SCS, Tektronix, USA) under the illumination of different ultraviolet light sources.For the scanning test, the voltage was increased from minus to positive values at a step of 0.2 V.The UV source used in this work is composed of a while light source (EQ99X LDLS, Energetiq, Holland) and spectrophotometer (iHR 320, Horiba, Japan).

Results and disscussion
XRD was performed to characterize the crystal phase and orientation of the as-grown Ga 2 O 3 film on diamond substrate.According to the θ-2θ scan in Figure 1(a), besides the (400) plane diffraction peak of diamond substrate located at 119.4°, five obvious diffraction peaks at 19.1°, 38.8°, 59.8°, 83.5° and 112.7° can be also observed, which belong to the (002), ( 004), ( 006), ( 008) and (0010) planes of ε-Ga 2 O 3 (space group: Pna2 1 ) [20].The appearance of the only (001) series diffraction peaks of ε-Ga 2 O 3 on the XRD spectra confirms that the as-grown ε-Ga 2 O 3 film on diamond substrate has high purity and high crystal orientation.As shown in Figure 1(b), the measured FWHM of the ε-Ga 2 O 3 (002) in the ω-scan mode is about 0.89°, which is much lower than the reported β-Ga 2 O 3 film on diamond substrate grown by low pressure chemical vapor deposition (LPCVD) [25].Figure 1(c) shows the comparison of the transmittance of diamond sample in the UV-visible region before and after ε-Ga 2 O 3 film deposition.The transmittance of diamond is around 70% in the visible region, approximating to the theoretical value of intrinsic diamond.After ε-Ga 2 O 3 deposition, when light is transmitted from ε-Ga 2 O 3 side, the transmittance of ε-Ga 2 O 3 /diamond sample is limited below 70% in the region from 300 to 800 nm due to the existence of diamond substrate.Moreover, two absorption edges at 275 nm and 228 nm can be observed, which belong to the absorption edges of intrinsic ε-Ga 2 O 3 film and diamond substrate, respectively.The insert Tauc plot analysis in Figure 1(c   In order understand the growth mechanism of ε-Ga 2 O 3 on diamond substrate, the microstructure of ε-Ga 2 O 3 /diamond interface was investigated by a combination of TEM characterizations and first principles calculations.The STEM image of the ε-Ga 2 O 3 /diamond interface is shown in Figure 3(a) where the (001) planes of ε-Ga 2 O 3 are parallel to the (001) planes of diamond.It can also be observed that there is not an obvious interface layer between diamond and ε-Ga 2 O 3 crystal.The suitable epitaxial growth of ε-Ga 2 O 3 on substrates with cubic and orthorhombic structures is due to the orthorhombic structure of ε-Ga 2 O 3 .It should be noted that the lattice constant a ε of ε-Ga 2 O 3 is 5.04 Å [20], which is very close to the facial diagonal length of the unit cell of cubic diamond: 2 a D = 5.01 Å.The lattice constant mismatch between a ε and 2 a D is smaller than 1%.However, for the lattice constant b ε of ε-Ga 2 O 3 , it follows: 3/2 2 a D < b ε < 2 2 a D , as shown in Figure S3 in Supplementary Information.Therefore, beside of 120°, other rotation angles may occur during ε-Ga 2 O 3 growth, while the pseudohexagonal 4H stacking of the O atoms is preserved, giving rise of the columnar structure of ε-Ga 2 O 3 on diamond substrate [20].The SAED pattern taken from the ε-Ga 2 O 3 /diamond interface is shown in Figure 3(b).The reflections of diamond marked by green circles and ε-Ga 2 O 3 marked by yellow circles show that the (001) planes of the ε-Ga 2 O 3 are parallel to the (001) planes of diamond, which matches well with the HRTEM image shown in Figure 3(a).
The crystal structure of ε-Ga 2 O 3 is complex, whose unit cell contains 16 Ga and 24 O atoms.The atoms in one unit cell can be clarified into 10 different layers vertical to the [001] direction.Thanks to the high-resolution STEM image, the atomic model of the ε-Ga 2 O 3 / diamond interface is shown in Figure 3(c).The atomic layers of ε-Ga 2 O 3 are labeled as 1-10.One should be aware that it is difficult to determine the atomic layer close to the diamond by TEM characterizations due to its limited resolution.In this case, the first principle calculation was used to analyze the detailed structure of the ε-Ga 2 O 3 /diamond interface.The ε-Ga 2 O 3 /diamond interface models containing 140 atoms (96 C atoms, 17 Ga atoms and 27 O atoms) were built for calculations.A vacuum space of 12 Å was left along the Z-direction.The calculation was carried out using the Vienna ab initio Simulation Package (VASP) based on density functional theory, and the Projector-Augmented Wave (PAW) method together with the Perdew Burke Ernzerhof (PBE) formulation were employed [26].The cutoff energy of the plane waves was set as 450 eV and the Monkhorst-Pack K-point mesh was 4 × 4 × 1.The diamond structure was fixed and the ε-Ga 2 O 3 structure was fully relaxed until the Hellmann-Feynman force on all atoms was less than 0.05 eV/Å.The calculated total energy of the ε-Ga 2 O 3 /diamond interface system is shown in Figure 3(d).The total energy varies from −986 eV to −980 eV when the O atoms of ε-Ga 2 O 3 contact with diamond substrate, lower than that when the A solar-blind photodetector based on the obtained ε-Ga 2 O 3 /diamond heterostructure was fabricated by depositing Ti (30 nm)/Au (50 nm) interdigital electrodes on ε-Ga 2 O 3 surface.As shown in Figure 4(a), the measured I-V curve was almost linear under no light illumination which indicates a good ohmic contact of Ti/Au electrodes on ε-Ga 2 O 3 surface.The zero point of the I-V curve shifted to the negative voltage because of charging and discharging on the highly insulating ε-Ga 2 O 3 film.At an applied voltage of 30 V, the ultralow dark current was about 4.3 × 10 −13 A as shown in Figure 4(b).When exposed to 240 nm ultraviolet light, the ε-Ga 2 O 3 /diamond photodetector (effective area: ~ 2.2 mm 2 ) achieved a photocurrent of about 2.5 × 10 −8 A for an applied bias of 30 V and a light-to-dark current ratio of more than 5.7 × 10 4 .The photoresponse of ε-Ga 2 O 3 /diamond photodetector under different incident light power densities is shown in Figure 4(c) where the photocurrent shows a linear relationship to the incident light power density from 10 mW/cm 2 to over 400 mW/cm 2 .Moreover, the time response of the ε-Ga 2 O 3 /diamond photodetector is shown in Figure 4(d).The measured rise time (τ up ) and decay time (τ decay ) of the photodetector are about 0.64 s and 0.13 s when the current increases from 10% to 90% and decreases from 90% to 10% of the maximum current, respectively.The rise and decay times of the photodetector are similar to the reported values of other Ga 2 O 3 / diamond photodetectors [17,24].
The spectra response of the ε-Ga 2 O 3 /diamond photodetector under different ultraviolet wavelength is shown in Figure 5.As shown in Figure 5(a), with the decrease of incident ultraviolet wavelength, the photocurrent increased dramatically.Notably, the zero point shift disappeared on the I-V curves when the ε-Ga 2 O 3 /diamond photodetector was illuminated to the DUV light with wavelength smaller than 260 nm.It can be explained that when the illumination DUV wavelength was smaller than 260 nm, photocurrent would be generated in the ε-Ga 2 O 3 layer and the photo-generated carrier balanced the defects in ε-Ga 2 O 3 layer, leading to the disappearance of the zero point shift.The calculated responsivity is shown in Figure 5(b), in which the insert shows the logarithmic relationship of the responsivity of the ε-Ga 2 O 3 /diamond photodetector to incident wavelength at different applied voltages.For independent Ga 2 O 3 and diamond photodetectors, the peak responsivity usually locates at around 250 and 225 nm, respectively.However, in this work, the responsivity of the ε-Ga 2 O 3 /diamond photodetector did not reduce like other ε-Ga 2 O 3 photodetectors, when the incident wavelength decreased to 240 nm [27,28].Instead, a second increase tendency can be observed on the responsivity curve when the ultraviolet wavelength decreased below 230 nm.When the ultraviolet wavelength decreased to 210 nm, the responsivity achieved the maximum value of 0.28 A/W for voltage of 30 V. The high spectra responsivity region of ε-Ga 2 O 3 /diamond photodetector (ε-Ga 2 O 3 : ~200 nm) was extended to 260-210 nm or even lower.While for an ε-Ga 2 O 3 /diamond photodetector with 1 μm-thick ε-Ga 2 O 3 film, as shown in Figure S4, the peak responsivity of the ε-Ga 2 O 3 /diamond photodetector locates at around 250 nm, and with the decrease of illumination wavelength from 250 nm, the responsivity decreased dramatically, which was also reported in Ref. [24].Moreover, Figure 5(c,d) show the solar-blind and visible light rejection of the ε-Ga 2 O 3 /diamond under different applied voltages.It can be observed that all the rejection ratio curves show an increase tendency to the applied voltage from 5 V to 30 V. With the decrease of ultraviolet wavelength, the solar-blind rejection ratios of the ε-Ga 2 O 3 /diamond photodetector increases from 104 (R 240 /R 280 ) to 133 (R 225 /R 280 ) and 165 (R 210 /R 280 ) at 30 V, respectively.Meanwhile, the visible light rejection ratios increase from 738 (R 240 /R 280 ) to 946 (R 225 /R 280 ) and 1174 (R 210 /R 280 ) at 30 V, respectively.Such rejection values are much higher than the reported values of other Ga 2 O 3 /diamond photodetectors [17,24].The high solar-blind and visible-blind rejection ratios of the ε-Ga 2 O 3 /diamond photodetector indicate the good solar-blind properties of the proposed ε-Ga 2 O 3 /diamond photodetector.Moreover, compared to the Ga 2 O 3 /diamond photodetectors in Figure S4 and other reports, the wavelength response region of the proposed ε-Ga 2 O 3 /diamond photodetector extended from 230 nm to below 210 nm, which was never reported before.Considering the occurrence of two adsorption edges of ε-Ga 2 O 3 /diamond in the UV-Vis spectra in Figure 1(c), the wide spectra responsivity of the fabricated ε-Ga 2 O 3 /diamond photodetector is due to the ultraviolet response of diamond substrate.
The simulation of the optical intensity and electric field distribution in ε-Ga 2 O 3 /diamond photodetector with different ε-Ga 2 O 3 thickness were performed and compared.According to traditional physical theory, the light intensity decays exponentially with the light penetration depth.When the ε-Ga 2 O 3 film was 200 nm or thinner, the DUV light can transmit the ε-Ga 2 O 3 layer to the diamond substrate, which will generate the photocurrent in the diamond substrate (Figure S5).However, with the increase of ε-Ga 2 O 3 film thickness, nearly all the DUV light with wavelength smaller than 230 nm was adsorbed by the ε-Ga 2 O 3 layer, leading to the small or little photocurrent in diamond.Moreover, the distribution of the electric field in ε-Ga 2 O 3 and diamond layers also reveal that with the increase of ε-Ga 2 O 3 thickness, the electric field intensity becomes weaker in the diamond substrate, as shown in Figure 5(e,f) and Figure S6.Besides the above two reasons, quantum efficiency is also an important influence factor.The single crystalline diamond has much higher quantum efficiency than the columnar crystalline ε-Ga 2 O 3 layer in the DUV region.Therefore, the extension of the spectra response region of the ε-Ga 2 O 3 /diamond photodetector was caused by above many reasons.The wide spectra response region indicates the ε-Ga 2 O 3 /diamond photodetector can be used for photodetection in the most of the DUV region.

Conclusion
In this work, an ε-Ga 2 O 3 /diamond photodetector with high spectra responsivity through the region from 210 nm to 260 nm was fabricated by epitaxially depositing ε-Ga 2 O 3 film on diamond substrate by PLD.The microstructure of ε-Ga 2 O 3 film gown on diamond has been shown to be the columnar crystal that rotating along the z axis.Due to the better lattice compatibility and crystal matching of ε-Ga 2 O 3 and diamond, the ε-Ga 2 O 3 film has good crystal quality with the FWHM of the ε-Ga 2 O 3 (002) rocking curve of 0.89°.In comparison to other independent photodetectors based on ε-Ga 2 O 3 or diamond materials, the fabricated ε-Ga 2 O 3 / diamond heterojunction photodetector achieved a significantly wide spectra response, from below 210 nm to about 260 nm.Moreover, the high rejection ratios for the responsivity for wavelengths of 280 nm and 400 nm were obtained, which demonstrated a high selectivity and sensitivity of the ε-Ga 2 O 3 /diamond photodetector in the most of the DUV region.The high responsivity and wide spectra region of the ε-Ga 2 O 3 /diamond photodetector shows great potential applications in DUV detection.

Figure 2 .
Figure 2. (a) low magnification cross-section tEm image of ε-Ga 2 o 3 film on diamond.(b) SEaD pattern taken from the region marked by the dashed circle in (a).(c) HRtEm image showing the grain boundary in the ε-Ga 2 o 3 film.(d) StEm image showing the ε-Ga 2 o 3 structure with the [010] zone axial at atomic resolution.(e) Enlarged image taken from the yellow rectangular region of (d). the insert in (e) shows the atomic model of ε-Ga 2 o 3 viewed along the [010] direction.the red and blue balls represent Ga and o atoms, respectively.

Figure 3 .
Figure 3. (a) StEm image of the ε-Ga 2 o 3 /diamond interface.(b) SaED pattern taken from the ε-Ga 2 o 3 /diamond interface.Diffraction spots marked by green and yellow circles represent diamond (with [110] zoon axis) and ε-Ga 2 o 3 (with [010] zoon axis), respectively.(c) atomic model of the ε-Ga 2 o 3 /diamond interface.the red and blue balls represent Ga and o atoms.the green and white balls represent the c atoms at the surface and inner of diamond substrate, respectively.(d) the calculated total energies of different atomic layers of ε-Ga 2 o 3 contacting with (001) diamond substrate.

Figure 4 .
Figure 4. (a) the I-V curve of the ε-Ga 2 o 3 /diamond photodetector, the insert is the schematic illustration of the fabricated ε-Ga 2 o 3 / diamond photodetector.(b) Photocurrent and dark current of the ε-Ga 2 o 3 /diamond photodetector.(c) Relationship of the photo response to the incident light power density.(d) time response of the fabricated ε-Ga 2 o 3 /diamond photodetector.

Figure 5 .
Figure 5. (a) Photocurrent and (b) responsivity of ε-Ga 2 o 3 /diamond photodetector under different incident wavelength ultraviolet light at different applied voltage, the insert figure in (b) shows corresponding logarithmic responsivity.the (c) solar-blind and (d) visible rejection ratio of the ε-Ga 2 o 3 /diamond photodetector at different applied voltages, respectively.a schematic illustration of the electric field intensity in ε-Ga 2 o 3 /diamond photodetector with (e) 200 nm and (f) 1 μm ε-Ga 2 o 3 layer.
Supporting Information: (I) Comparison of the UV-visible transmittance of ε-Ga 2 O 3 /diamond samples with different ε-Ga 2 O 3 thicknesses.(II) The diffraction spots in the SAED pattern of ε-Ga 2 O 3 film.(III) A potential contacting model of ε-Ga 2 O 3 on diamond substrate according to calculation.(IV) The photo responsivity of the ε-Ga 2 O 3 /diamond photodetector with thick ε-Ga 2 O 3 film.(V) Comparison of the optical intensity in ε-Ga 2 O 3 / diamond photodetector.(VI) Comparison of the electric field in ε-Ga 2 O 3 /diamond photodetector.