Deep ultraviolet photodetector based on nickel oxide / gallium oxide heterojunction

Abstract

Provided is a deep ultraviolet photodetector based on a nickel oxide / gallium oxide heterojunction, comprising: an n-type β-gallium oxide substrate; an n-type β-gallium oxide epitaxial layer epitaxially grown on the n-type β-gallium oxide substrate; a p-type nickel oxide thin film formed on the n-type β-gallium oxide epitaxial layer; a lower electrode forming an ohmic contact with the n-type β-gallium oxide substrate; and a spiral upper electrode pattern formed on the p-type nickel oxide thin film.

Description

Nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector

[0001] The present invention relates to a self-powered deep ultraviolet (DUV) photodetector.

[0002] Recently, the demand for deep ultraviolet (DUV) solar-blind photodetectors has increased significantly due to their critical applications in various fields such as environmental monitoring, flame detection, space exploration, and secure communications. Among these, arc detectors are critical devices used to detect electric arcs or faults in electrical systems, particularly in environments where safety and protection are paramount. Electric arcs typically occur when unintended current flows through the air due to short circuits, insulation failures, or incorrect connections. Electric arcs emit a unique UV light signature within the 200nm to 280nm UV-C band, which can be detected by optical sensors specifically designed to detect these emissions. UV-C photodetectors can detect the presence of electric arcs much faster and more reliably than conventional methods such as temperature detection, acoustic detection, and visual monitoring, providing a high-sensitivity approach for early fault detection.

[0003] Solar blocking photodetectors sensitive only to ultraviolet rays with wavelengths less than 280 nm are particularly useful for detecting high-energy ultraviolet rays while avoiding interference from visible light and infrared rays. Among the materials used to manufacture solar blocking photodetectors, wide-bandgap semiconductors such as beta-gallium oxide (β₂O₃) are receiving significant attention due to their excellent properties, including an ultra-wide bandgap of 4.8 to 4.9 eV and outstanding thermal and chemical stability. Additionally, β₂O₃ is suitable for mass production due to the advantage of being able to grow single crystals at a relatively low cost. However, β-Ga₂O₃ faces significant challenges related to p-type doping, making it difficult to integrate into homojunction structures. Therefore, alternative p-type materials must be used to fabricate pn heterojunctions in devices utilizing β-Ga₂O₃.

[0004] Among numerous candidate materials, nickel oxide (NiO), a p-type wide bandgap semiconductor, has emerged as an ideal counterpart to β-Ga2O3 in heterojunction-based photodetectors due to its high electrical conductivity, stability, and tunable optical properties. As a p-type material, NiO forms a robust pn heterojunction with β-Ga2O3, offering several advantages such as facilitating efficient charge separation and improving overall device performance. The combination of NiO and β-Ga2O3 not only enhances UV sensitivity but also enables self-powering, eliminating the need for an external power source and thereby reducing energy consumption.

[0005] Although NiO / β-Ga2O3 heterojunctions hold promising potential for solar-blocking deep ultraviolet (DUV) light detection, optimizing key device performance parameters such as responsivity, detectivity, photocurrent, and the photo-to-dark current ratio (PDCR) remains a significant challenge. In particular, enabling ultra-high photocurrent is critical for detecting stronger arcs and extending the detectable distance. As the distance from the light source increases, the amount of light incident on the arc detector decreases, leading to a reduction in the photocurrent generated by the photoreaction. Consequently, higher photocurrent is essential for effective arc detection over longer distances.

[0006] (Patent Document 0001) Korean Published Patent Application No. 10-2022-0068811

[0007] The present invention aims to provide a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector capable of generating a relatively large photocurrent for DUV of the same intensity.

[0008] According to a preferred embodiment of the present invention, a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector is provided, comprising an n-type β-gallium oxide substrate, an n-type β-gallium oxide epitaxial layer grown on the n-type β-gallium oxide substrate, a p-type nickel oxide thin film formed on the n-type β-gallium oxide epitaxial layer, a lower electrode forming an ohmic contact with the n-type β-gallium oxide substrate, and a helical upper electrode pattern formed on the p-type nickel oxide thin film.

[0009] In one embodiment, the upper electrode pattern may be a rectangular spiral formed by alternately arranging vertical metal lines and horizontal metal lines having progressively decreasing lengths so that one end of each is connected.

[0010] In one embodiment, the lengths of the first vertical metal line and the first horizontal metal line, each connected at one end, are the same, and the lengths of the second vertical metal line and the second horizontal metal line, connected at the other end of either the first vertical metal line and the first horizontal metal line, are the same, but the length of the second horizontal metal line may be 2 / 3 of the length of the first horizontal metal line.

[0011] In one embodiment, the upper electrode pattern may include a nickel-chromium alloy thin film deposited on the p-type nickel oxide thin film and an aluminum-silicon alloy thin film deposited on the nickel-chromium thin film.

[0012] In one embodiment, the upper electrode pattern may include a Li-doped nickel oxide thin film deposited on the p-type nickel oxide thin film, a nickel-chromium alloy thin film deposited on the Li-doped nickel oxide thin film, and an aluminum-silicon alloy thin film deposited on the nickel-chromium thin film.

[0013] In one embodiment, the thickness of the Li-doped nickel oxide thin film may be 150 nm.

[0014] In one embodiment, a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector can detect light with a wavelength of 222 nm.

[0015] The nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to the present invention can generate a relatively large photocurrent for DUV of the same intensity.

[0016] The present invention is described below with reference to embodiments illustrated in the accompanying drawings. For the sake of understanding, identical reference numerals have been assigned to identical components throughout the entire set of accompanying drawings. The configurations illustrated in the accompanying drawings are merely exemplary embodiments implemented to explain the present invention and are not intended to limit the scope of the invention. In particular, the accompanying drawings may exaggerate some of the elements depicted in the drawings to aid in understanding the invention.

[0017] FIG. 1 is a diagram exemplarily illustrating a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0018] FIGS. 2a to 2d are exemplary drawings illustrating the upper electrode pattern of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector.

[0019] FIG. 3 is a graph of the IV characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0020] FIG. 4 is a graph showing the on current, off current, and on / off ratio of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0021] FIGS. 5a to 5d are graphs of photocurrent and PDCR of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0022] FIGS. 6a to 6d are graphs showing the reactivity and detectability of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0023] FIGS. 7a to 7e are graphs showing the photoresponse characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0024] FIGS. 8a to 8d are graphs showing the rise and fall times of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0025] FIG. 9 is a diagram exemplarily illustrating a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0026] FIG. 10 is a graph of the IV characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0027] FIG. 11 is a graph showing the on current, off current, and on / off ratio of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0028] FIGS. 12a to 12d are graphs of photocurrent and PDCR of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0029] FIGS. 13a to 13d are graphs showing the reactivity and detectability of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0030] FIGS. 14a to 14e are graphs showing the photoresponse characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0031] FIGS. 15a to 15d are graphs showing the rise and fall times of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0032] FIGS. 16a to 16e illustrate, in an exemplary manner, the process of manufacturing a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector.

[0033] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0034] Terms such as "first," "second," etc., may be used to describe various components, but said components should not be limited by said terms. These terms are used solely for the purpose of distinguishing one component from another.

[0035] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, terms such as "comprising" or "having" are intended to specify the presence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0036] When an element, such as a layer, region, or substrate, is described as existing "on" or extending "onto" another element, that element may be directly on or extending directly onto the other element, or there may be an intermediate intervening element. On the other hand, when an element is described as being "directly on" or extending "directly onto" another element, no other intermediate elements exist. Furthermore, when an element is described as being "connected" or "coupled" to another element, that element may be directly connected or directly coupled to the other element, or there may be an intermediate intervening element. On the other hand, when an element is described as being "directly connected" or "directly coupled" to another element, no other intermediate elements exist.

[0037] Relative terms such as "below," "above," "upper," "lower," "horizontal," "lateral," or "vertical" may be used herein to describe the relationship of one element, layer, or region to another element, layer, or region as illustrated in the drawings. These terms should be understood as intended to encompass other orientations of the device in addition to the orientation depicted in the drawings.

[0038] Hereinafter, embodiments of the present invention will be described in detail with reference to the relevant drawings. Throughout the attached drawings, identical or similar elements are referred to using the same reference numerals.

[0039] FIG. 1 is a diagram exemplarily illustrating a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0040] Referring to FIG. 1, a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector (10) may include an n-type β-gallium oxide substrate (100), an n-type β-gallium oxide epitaxial layer (110), a lower electrode (120), a p-type nickel oxide thin film (130), and an upper electrode pattern (140).

[0041] An n-type β-gallium oxide substrate (100) comprises an n-type β-gallium oxide epitaxial layer (110) grown epitaxially on its upper surface. The β-gallium oxide substrate (100) doped with an n-type dopant is formed from single-crystal β-gallium oxide (β2O3). The n-type dopant may be, for example, tin (Sn), and the concentration of Nd-Na is approximately 4.0 x 10⁻⁶. 18 cm -3 The thickness of the doped β-gallium oxide substrate (100) can be about 581 μm.

[0042] An n-type β-gallium oxide epitaxial layer (110) is epitaxially grown on the upper surface of an n-type β-gallium oxide substrate (100). The concentration of Nd-Na in the n-type β-gallium oxide epitaxial layer (110) is approximately 1.0 x 10¹⁶ cm⁻¹. -3 The thickness may be about 5.3 μm. The n-type β-gallium oxide epitaxial layer (110) can be deposited by, for example, HVPE (Halide vapor phase epitaxy), MOCVD (Metalorganic chemical vapor deposition), Mist CVD, MBE (Molecular Beam Epitaxy), PLD (Pulsed laser deposition), etc.

[0043] The lower electrode (120) is an ohmic contact layer formed on the lower surface of an n-type β-gallium oxide substrate (100).

[0044] A p-type nickel oxide thin film (130) is formed on an n-type β-gallium oxide epitaxial layer (110). The p-type nickel oxide thin film (130) can be formed by RF sputtering a nickel oxide target in a mixed gas atmosphere of argon and oxygen to deposit it on the n-type β-gallium oxide epitaxial layer (110) to a thickness of about 20 nm.

[0045] The upper electrode pattern (140) is formed on the p-type nickel oxide thin film (130). The upper electrode pattern (140) can be formed by sequentially depositing a nickel-chromium alloy (NiCr) thin film and an aluminum-silicon alloy (Al-Si) thin film on the p-type nickel oxide thin film (130), for example, by RF sputtering. Here, the thickness of the NiCr thin film may be about 200 nm, and the thickness of the Al-Si thin film may be about 600 nm. The upper electrode pattern (140) is described in detail with reference to FIG. 2.

[0046] FIGS. 2a to 2d are exemplary drawings illustrating the upper electrode pattern of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector.

[0047] The upper electrode pattern (140a) exemplified in FIG. 2a is circular. The diameter of the upper electrode pattern (140a) is approximately 300 μm. Meanwhile, the upper electrode pattern (140a) can also be formed with diameters of approximately 600 μm and approximately 1,000 μm. The circumference, i.e., the perimeter, of the upper electrode pattern (140a) with a diameter of approximately 300 μm is approximately 0.094 cm, and the area is approximately 0.000707 cm² 2 am.

[0048] The upper electrode pattern (140b) exemplified in FIG. 2b is a ring. The inner diameter of the upper electrode pattern (140b) is approximately 1,500 μm, and the thickness of the metal line corresponding to the ring is approximately 25 μm. The upper electrode pattern (140b) is formed on the outer side of the ring and may include a plurality of pad areas (140b') for wiring. The perimeter of the upper electrode pattern (140b), taking into account the plurality of pad areas (140b'), is approximately 1.09 cm, and the area is approximately 0.00192 cm² 2 am.

[0049] The upper electrode pattern (140c) exemplified in FIG. 2c is a square grid. The upper electrode pattern (140c) includes an outer metal line defining a first square area, the first square area is divided into four second square areas by vertical and horizontal metal lines, and a circular pad area with a diameter of about 1,000 μm may be formed at the location where the vertical metal line and the horizontal metal line intersect. The length of one side of the first square area is about 4,500 μm, and the thickness of the outer metal line, the horizontal metal line, and the vertical metal line may be about 250 μm. The perimeter of the upper electrode pattern (140c), including the circular pad area, is 4.93 cm, and the area is 0.0679 cm² 2 Meanwhile, the ratio of the area where no metal lines are formed to the area where vertical and horizontal metal lines are formed is approximately 32:68.

[0050] The upper electrode pattern may be spiral. The upper electrode pattern (140d) illustrated in FIG. 2d is a rectangular spiral, and may also be formed as a circular spiral, a polygonal spiral, etc. The upper electrode pattern (140d) is configured by alternately arranging vertical metal lines and horizontal metal lines having progressively decreasing lengths so that their respective ends are connected. In the rectangular spiral upper electrode pattern (140d) illustrated in FIG. 2d, the length of the vertical metal line and horizontal metal line (first pair) connected at their respective ends is the same as the first length (approx. 4,333 μm), and the length of the vertical metal line and horizontal metal line (second pair) connected to the other end of either of the first pair of vertical metal lines and horizontal metal lines is the same as the second length (approx. 3,000 μm), but the second length is about 2 / 3 of the first length, so the first length and the second length are different. The perimeter of the upper electrode pattern (140d) is 6.53 cm, and the area is 0.107 cm. 2 Meanwhile, the ratio of the area where no metal lines are formed to the area where vertical and horizontal metal lines are formed is approximately 57:43.

[0051] FIGS. 3 to 8d are values ​​measured in a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector having an upper electrode pattern (140) formed by stacking a NiCr thin film and an Al-Si thin film. Hereinafter, the photodetector having the upper electrode pattern (140a) of FIG. 2a is denoted as P1, the photodetector having the upper electrode pattern (140b) of FIG. 2b is denoted as P2, the photodetector having the upper electrode pattern (140c) of FIG. 2c is denoted as P3, and the photodetector having the upper electrode pattern (140d) of FIG. 2d is denoted as P4.

[0052] FIG. 3 is an IV characteristic graph of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment. Referring to FIG. 3, the IV characteristic curves are measured at P1 to P4 in the range of about -4V to about 4V in a state without light (hereinafter dark state). From the measured IV characteristic curves, it can be seen that P1 to P4 all have pn junction rectification characteristics regardless of the shape of the upper electrode pattern.

[0053] FIG. 4 is a graph showing the on current, off current, and on / off ratio of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0054] Referring to FIG. 4, when examining the on-current, off-current, and on-off ratio of P1 to P4, the on-off ratio decreased inversely proportional to the pattern size. This is because the increase in the area of ​​the upper electrode patterns (140a to 140d) has a greater effect on the off-current, i.e., reverse leakage current, than on the on-current. Generally, in an ideal, defect-free pn junction diode, no current flows under reverse bias; however, in actual devices, defects are always present. An increase in electrode area necessarily entails an increase in the region where defects can occur. Consequently, the reverse leakage current caused by defects increases in proportion to the electrode area. The off-current of P2 is 1.82 x 10⁻¹⁰ -10 It is A (I-1.5V), and the ON current is measured at 0.33A, so the ON-OFF ratio is approximately 1.8 x 10⁻⁶ 9 It showed the largest value.

[0055] FIGS. 5a to 5d are graphs of the photocurrent and photocurrent-to-dark current ratio (hereinafter PDCR) of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment, wherein the PDCR is measured by applying 0V and about 5V biases to P1 to P4 and irradiating with light of about 254nm wavelength of various intensities.

[0056] Referring to FIG. 1 in conjunction with FIG. 5a and FIG. 5b, for both 0V and 5V biases, P1 to P4 exhibit a tendency for the photocurrent to increase as the light intensity increases. At the same light intensity, a higher ratio of electrode patterns per unit area—in other words, a longer perimeter of the electrode—tends to generate a relatively larger photocurrent. This is because the depletion region, which receives UV-C light to generate electron-hole pairs (EHPs), is formed over a wider area around the electrode, allowing more EHPs to be generated and reach the electrode. Approximately 1,000 μW / cm² at a wavelength of 254 nm. 2 When investigated, the photocurrent of P4 with a 0V bias applied was approximately 0.11μA, and with a 5V bias applied, it was approximately 0.73μA, showing a relatively large value compared to other photodetectors. The photocurrent at 5V bias increased by about 6.6 times compared to 0V. This is because the applied bias forms a stronger electric field in the depletion region, causing EHPs to detach rapidly while reducing the recombination probability. Subsequently, the detached charge carriers are accelerated by the externally applied voltage, improving the carrier collection efficiency at the electrode, and consequently, the photocurrent increases. The strength of the electric field formed in the depletion region can be explained as follows.

[0057] [Mathematical Formula 1]

[0058]

[0059] [Mathematical Formula 2]

[0060]

[0061] Here, E is the electric field strength applied to the depletion region, W is the width of the depletion region, Vbi is the built-in voltage, Vbias is the externally applied bias voltage, ε is the permittivity of the semiconductor, q is the charge of an electron, and NA and ND are the doping concentrations of the P-type and N-type materials. As can be seen in Equation 1, E is inversely proportional to W, but in Equation 2, W increases by the power of the externally applied voltage and is smaller than the increase in the externally applied voltage, so consequently, the magnitude of E increases proportionally to Vbias.

[0062] Referring to Figures 5c and 5d together, although the photocurrent itself is greater for P4 than for P2, the PDCR of P2 is the highest because the dark current of P2 is much smaller than that of P4. The PDCR of P2 is approximately 1,000 μW / cm² at a wavelength of 254 nm. 2 When examined, 8.5x10 at 0 V 3 , 9.3x10 at 5V 1 It was measured as.

[0063] FIGS. 6a to 6d are graphs showing the reactivity and detectability of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment, wherein the reactivity and detectability are calculated by applying 0V and about 5V biases to P1 to P4 and irradiating with light of about 254nm wavelength of various intensities.

[0064] Reactivity can be calculated as follows.

[0065] [Mathematical Formula 3]

[0066]

[0067] Here, R is the reactivity, Jphoto is the photocurrent density, JDark is the dark current density, and P is the intensity of irradiated light per unit area.

[0068] Detectability can be calculated as follows.

[0069] [Mathematical Formula 4]

[0070]

[0071] Here, D is detectability, R is reactivity, e is the electron charge, and JDark is the dark current density.

[0072] Both reactivity and detectability actually decreased as the intensity of the irradiated light increased. This is because while increasing light intensity generates more EHPs, it simultaneously causes transient self-heating due to Joule heating, which also promotes carrier scattering and EHP recombination. As the separated EHPs flow inside the detector, heat is generated due to resistance, which raises the internal temperature of the detector and makes lattice vibrations more active. Consequently, carrier scattering and EHP recombination are further promoted. Equations 5 and 6 represent the correlation between temperature and lattice vibrations.

[0073] [Mathematical Formula 5]

[0074]

[0075] [Mathematical Formula 6]

[0076]

[0077] Here, n(w, T) is the average number of particles at temperature T with frequency w in the Bose-Einstein distribution, ħ is Planck's constant (ħ = h / 2π, which is the angular frequency of a phonon), kB is Boltzmann's constant, T is the absolute temperature, and Elattice is the energy of lattice vibrations in the solid.

[0078] In Equation 5, as temperature increases, the average number of particles increases; according to Equation 6, an increase in the average number of particles ultimately leads to an increase in the energy of lattice vibrations. Furthermore, reactivity increased further when a bias of approximately 5V was applied, because the increase in photocurrent was greater than the increase in dark current at 5V bias. Detectability, however, decreased at 5V bias, because the rate of increase in dark current was greater than the rate of increase in reactivity at 5V bias. Reactivity showed the highest values ​​at 0V for P1 (28.9 mA / W) and at 5V bias for P2 (292 mA / W), while detectability for P2 was 1.5 x 10⁻¹⁰ at 0V. 12 Jones, 5.8x10 at 5V bias 11 Jones showed the largest value.

[0079] FIGS. 7a to 7e are graphs showing the photoresponse characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0080] Referring to Figures 7a and 7b, the time-dependent photocurrent is shown when light of approximately 254 nm is irradiated while 0 V and approximately 5 V biases are applied to P4. It can be seen that the photocurrent increases slightly during light irradiation, which is because carriers trapped in the trap are released from the trap and move to the electrode.

[0081] Referring to Figures 7c and 7d, the photocurrent over time is shown when light at approximately 222 nm is irradiated with 0 V and approximately 5 V biases applied to P4. Since the wavelength range of light where the maximum peak is detected upon arc occurrence is approximately 220 nm to approximately 230 nm, the photoresponse characteristics at 222 nm were additionally verified to assess the practical applicability for arc detection. At a wavelength of approximately 222 nm, the photoresponse is approximately 1,000 μW / cm² 2When P4 was irradiated, a photocurrent of approximately 0.16 μA at 0 V and 0.88 μA at 5 V was measured. This is about 1.5 times higher than the photocurrent measured at 254 nm, which means that the photodetector with the top electrode pattern including P4 responds more strongly to 222 nm light, and thus shows that it can be applied more effectively for arc detection.

[0082] Referring to Fig. 7e, the photocurrent is shown when 222, 254, and 365 nm light is irradiated onto P4 at 0 V with the same light intensity. The rejection ratios for 222 nm (UV-C) and 365 nm (UV-A) are approximately 3.4 x 10⁻⁶. 1 It can be confirmed that it can selectively react only in the UV-C region.

[0083] FIGS. 8a to 8d are graphs showing the rise time and decay time of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment, wherein the rise time and decay time are determined by applying 0V and approximately 5V biases to P4 and using light of approximately 222nm wavelength at approximately 1,000μW / cm² 2 It was measured by investigating the intensity.

[0084] Rise time and fall time can be defined as the time taken for the photocurrent to increase from 10% to 90% and the time taken to decrease from 90% to 10%, respectively. To analyze the response time, the following mathematical formula was used.

[0085] [Mathematical Formula 7]

[0086]

[0087] [Mathematical Formula 8]

[0088]

[0089] Here, Idark is the dark current, A is a constant, τr is the rise time, and τd is the decay time.

[0090] The rise and fall times were analyzed using non-linear curve fitting. The rise and fall times were measured to be approximately 26.0 ms and 32.0 ms at 0 V, and approximately 40.0 ms and 74.3 ms at 5 V bias. Both the rise and fall times increased at 5 V bias, which is because when more photocurrent flows due to the applied voltage, more self-heating occurs, which makes interactions with traps more active and acts as a factor that increases the reaction time. Additionally, the fall time was greater than the rise time at both 0 V and 5 V, which is because wide-bandgap materials such as gallium oxide have a deep donor trap density, allowing carriers to be released relatively slowly upon recombination.

[0091] FIG. 9 is a diagram exemplarily illustrating a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment. Descriptions identical to those in FIG. 1 are omitted, and only the differences are described.

[0092] In a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector (11), an upper electrode pattern (145) is formed on a p-type nickel oxide thin film (130). The upper electrode pattern (145) can be formed by sequentially depositing a Li-doped nickel oxide thin film (Li-doped NiO), a NiCr thin film, and an Al-Si thin film onto the p-type nickel oxide thin film (130), for example, by RF sputtering. Here, the thickness of the Li-doped nickel oxide thin film may be about 150 nm, the thickness of the NiCr thin film may be about 200 nm, and the thickness of the Al-Si thin film may be about 600 nm. The upper electrode pattern (145) has the pattern illustrated in FIGS. 2a to 2d.

[0093] A Li-doped nickel oxide thin film was integrated into a conventional photodetector structure as a P+ layer. Inserting a P+ layer between a p-type nickel oxide thin film (semiconductor) and a NiCr thin film (metal) reduces the Fermi level difference at the junction and relaxes band bending, allowing for the formation of excellent ohmic contact. This enables carriers generated in the depletion region to move more efficiently to the electrode, thereby allowing a higher photocurrent to flow.

[0094] FIGS. 10 to 15d are values ​​measured in a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector (11) having an upper electrode pattern (145) formed by stacking a Li-doped p-type nickel oxide thin film, a NiCr thin film, and an Al-Si thin film. Hereinafter, P1 is a photodetector (11) having an upper electrode pattern of the same shape as FIG. 2a, P2 is a photodetector (11) having an upper electrode pattern of the same shape as FIG. 2b, P3 is a photodetector (11) having an upper electrode pattern of the same shape as FIG. 2c, and P4 is a photodetector (11) having an upper electrode pattern of the same shape as FIG. 2d.

[0095] FIG. 10 is a graph of the IV characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0096] Referring to FIG. 10, it can be seen that all of P1 to P4 have rectification characteristics. Meanwhile, unlike the trend observed in the IV characteristic graph (Fig. 3) measured in the nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector (10), the leakage current of P4 became smaller than the leakage current of P3. This appears to be because P4 is a helical electrode, which uniformly distributes the path of the electric field and current, thereby reducing the activation of local defects and traps and minimizing current flow in the defect region to lower the leakage current, whereas P3 is a grid-type electrode, which concentrates the electric field near the intersection point, thereby locally activating defects and traps, and consequently, the current flows intensively near the intersection point, and thus is more affected by the defects, resulting in an increase in leakage current.

[0097] FIG. 11 is a graph showing the on current, off current, and on / off ratio of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0098] Referring to Fig. 11 together with Fig. 4, the on-currents of P1 to P4 show a similar trend to the case without Li-doped p-type nickel oxide thin films (Fig. 4), whereas the off-currents increased relatively significantly in P1 and P3, and decreased relatively significantly in P2 and P4. As a result, the on-off ratio of P4 became larger than that of P3.

[0099] FIGS. 12a to 12d are graphs of photocurrent and PDCR of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0100] Referring to Figs. 12a and 12b, the tendency for a longer electrode perimeter to generate a relatively larger photocurrent at the same photointensity was also confirmed in P1 to P4, which contained Li-doped p-type nickel oxide thin films. Approximately 1,000 μW / cm² at a wavelength of 254 nm. 2When investigated, the photocurrent of P4 with a 0V bias applied showed the largest value at approximately 2μA, and P3 with a 5V bias applied showed the largest value at approximately 4.6μA. When compared to the photocurrent measured in the photodetector (10) without the Li-doped p-type nickel oxide thin film (Figs. 5a and 5b), it can be confirmed that the photocurrents of P3 and P4 increased significantly at both 0V and 5V biases.

[0101] Meanwhile, referring to FIG. 12c and FIG. 12d together, at 0V, the PDCR of P3 is greater than that of P4, but at 5V bias, at all photointensities, the PDCR of P4 is greater than that of P3. This is because the photocurrent of P3 is greater than that of P4, but the dark current of P4 is much smaller than that of P3.

[0102] FIGS. 13a to 13d are graphs showing the reactivity and detectability of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0103] Similar to the trends observed in Figs. 6a to 6d, in P1 to P3, where Li-doped p-type nickel oxide thin films are added, both reactivity and detectability tend to decrease as the intensity of the irradiated light increases. On the other hand, the reactivity of P4 remains constant despite the increase in light intensity, and in particular, when a 5V bias is applied, the detectability of P4 is 400 μW / cm² 2 At light intensities above this level, it continuously increases.

[0104] FIGS. 14a to 14e are graphs showing the photoresponse characteristics of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to another embodiment.

[0105] In Figs. 14a and 14b, light with a wavelength of approximately 254 nm is applied to P4 at 100 μW / cm² 2 From 1,000μW / cm² 2When irradiated up to this point, the photocurrent generated by P4 with 0V applied increased from approximately 0.17μA to 2μA, and the photocurrent generated by P4 with 5V bias applied increased from 0.34μA to 3.9μA in proportion to the light intensity. Meanwhile, in Figs. 14c and 14d, light with a wavelength of approximately 222nm was applied to P4 at 1,000μW / cm² 2 When examined by intensity, the photocurrent generated by P4 with 0V applied is approximately 2.4μA, and the photocurrent generated by P4 with 5V bias applied is approximately 5.1μA. That is, the photocurrent generated at 222nm is about 1.3 times larger than the photocurrent generated at 254nm, which proves that it can respond more effectively to arc wavelengths of 220nm to 230nm.

[0106] Referring to Fig. 14e, the wavelength-dependent photocurrent and selectivity for the same optical intensity are shown. Approximately 1,000 μW / cm² at 0 V. 2 The selectivity ratio of 222 nm (UV-C) and 365 nm (UV-A) irradiated by intensity is 1.3 x 10⁻⁶ 3 Compared to the selectivity of a photodetector (10) without a Li-doped p-type nickel oxide thin film, it is about 40 times higher. This is because the addition of the Li-doped p-type nickel oxide thin film makes it more effective with short wavelength light, while at the same time having no significant effect on long wavelength light. In other words, by adding the Li-doped p-type nickel oxide thin film, the depletion region is thinned and the electric field is strengthened, thereby improving the charge collection efficiency. This is because it has a much greater effect at short wavelengths of 222 nm and 254 nm than at long wavelengths of 365 nm.

[0107] FIGS. 15a to 15d are graphs showing the rise and fall times of a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to one embodiment.

[0108] Compared to Figures 8a through 8d, both the rise time and fall time have decreased. This is because the charge transport characteristics were improved due to the Li-doped p-type nickel oxide thin film. The time constant τ is expressed as follows.

[0109] [Mathematical Formula 9]

[0110]

[0111] Here, R is the resistance of the device, and C is the capacitance of the device.

[0112] In addition, capacitance C is expressed as follows.

[0113] [Mathematical Formula 10]

[0114]

[0115] Here, ε is the dielectric constant of the semiconductor material within the depletion region, A is the electrode area, and d is the thickness of the depletion region. When a Li-doped p-type nickel oxide thin film is added, the thickness of the depletion region decreases, which causes C to increase slightly; however, since the rate of decrease in R due to the Li-doped p-type nickel oxide thin film in Equation 9 far exceeds the rate of increase in C, the time constant τ ultimately decreases. Additionally, during biasing, the rise time decreased and the fall time increased. This appears to be because the applied bias increased the electric field strength in the depletion region, causing the rise time to decrease, while the increased photocurrent intensified self-heating, causing the fall time to increase. The rise and fall times of P4 were measured to be approximately 16.0 ms and 10.6 ms at 0 V, and approximately 14.7 ms and 45.7 ms at 5 V bias.

[0116] FIGS. 16a to 16e illustrate, in an exemplary manner, the process of manufacturing a nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector.

[0117] In FIG. 16a, an n-type β-gallium oxide substrate (100) (hereinafter referred to as a wafer) having an n-type β-gallium oxide epitaxial layer (110) formed thereon is prepared. Foreign substances on the n-type β-gallium oxide substrate (100) are removed by washing with acetone, isopropyl alcohol (IPA), and deionized water (DI water), and then dried with N2 gas. Afterward, the n-type β-gallium oxide substrate (100) is heat-treated in a furnace at approximately 600°C for 1 hour.

[0118] In FIG. 16b, a lower electrode (120) is formed on the lower surface of a wafer. The lower electrode (120) can be formed by depositing, for example, a titanium (Ti) thin film and an Al-Si thin film in sequence, for example, by RF sputtering, on the lower surface of the wafer. Here, the thickness of the Ti thin film may be about 150 nm, and the thickness of the Al-Si thin film may be about 400 nm.

[0119] In FIG. 16c, a p-type nickel oxide thin film (130) is formed on the upper surface of the wafer. The p-type nickel oxide thin film (130) can be formed by RF sputtering a nickel oxide target in a mixed gas atmosphere of argon and oxygen to deposit it on an n-type β-gallium oxide epitaxial layer (110) to a thickness of about 20 nm.

[0120] In FIG. 16d, a photolithography process is performed to form an upper electrode pattern. Specifically, a wafer having a p-type nickel oxide thin film (130) formed thereon is coated with a photoresist (PR). The PR used is a negative PR (DNR-L300, 86.7 cP) in which the areas exposed to UV are cured and the areas not exposed are removed. After the PR coating, a soft bake process is performed to increase the adhesion between the PR and the wafer before alignment and exposure. Next, the wafer is aligned with a photomask pattern and then exposed to UV light. Next, a hard bake is performed to fully cure the areas that are not yet fully cured immediately after exposure, thereby fixing the formed pattern. Next, the PR pattern on the wafer is formed by removing the PR not exposed to UV light using a solvent.

[0121] In FIG. 16e, an upper electrode pattern (140, 145) is formed on the upper surface of a wafer. The upper electrode pattern (140) can be formed by sequentially depositing a NiCr thin film and an Al-Si thin film on a p-type nickel oxide thin film (130), for example, by RF sputtering. Here, the thickness of the NiCr thin film may be about 200 nm, and the thickness of the Al-Si thin film may be about 600 nm.

[0122] Meanwhile, the upper electrode pattern (145) can be formed by depositing a Li-doped nickel oxide thin film (Li-doped NiO) on a p-type nickel oxide thin film (130), and then sequentially depositing a NiCr thin film and an Al-Si thin film on the Li-doped nickel oxide thin film, for example, by RF sputtering. Here, the thickness of the Li-doped nickel oxide thin film may be about 150 nm, the thickness of the NiCr thin film may be about 200 nm, and the thickness of the Al-Si thin film may be about 600 nm. The upper electrode pattern (145) has the pattern illustrated in FIGS. 2a to 2d.

[0123] Table 1 shows the deposition conditions of the lower electrode (120), the p-type nickel oxide thin film (130), and the upper electrode pattern (140, 145).

[0124] [Table 1]

[0125]

[0126] The metal thin film deposited on the PR is lifted off and removed to complete the upper electrode pattern (140, 145). After deposition or after lift-off, it can be post-heat treated by rapid thermal ablation (RTA) at about 300°C for 1 minute in an N2 atmosphere.

[0127] The foregoing description of the present invention is for illustrative purposes only, and those skilled in the art will understand that other specific forms can be easily modified without altering the technical spirit or essential features of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. In particular, the features of the present invention described with reference to the drawings are not limited to the structures depicted in specific drawings and may be implemented independently or in combination with other features.

[0128] The scope of the present invention is defined by the claims set forth below rather than by the detailed description above, and all modifications or variations derived from the meaning and scope of the claims and equivalent concepts thereof should be interpreted as being included within the scope of the present invention.

Claims

1. n-type β-gallium oxide substrate; An n-type β-gallium oxide epitaxial layer grown on the above n-type β-gallium oxide substrate; A p-type nickel oxide thin film formed on the above n-type β-gallium oxide epitaxial layer; A lower electrode forming an ohmic contact with the above n-type β-gallium oxide substrate; and A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector comprising a helical upper electrode pattern formed on the above p-type nickel oxide thin film.

2. A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to claim 1, wherein the upper electrode pattern is a rectangular spiral formed by alternately arranging vertical metal lines and horizontal metal lines having progressively decreasing lengths such that one end of each is connected.

3. In Claim 1, A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector, wherein the lengths of a first vertical metal line and a first horizontal metal line, each connected at one end, are equal, and the lengths of a second vertical metal line and a second horizontal metal line, connected at the other end of either the first vertical metal line and the first horizontal metal line, are equal, but the length of the second horizontal metal line is 2 / 3 of the length of the first horizontal metal line.

4. In claim 1, the upper electrode pattern is, A nickel-chromium alloy thin film deposited on the above p-type nickel oxide thin film; and A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector comprising an aluminum-silicon alloy thin film deposited on the nickel-chromium thin film.

5. In claim 1, the upper electrode pattern is, Li-doped nickel oxide thin film deposited on the above p-type nickel oxide thin film; A nickel-chromium alloy thin film deposited on the above Li-doped nickel oxide thin film; and A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector comprising an aluminum-silicon alloy thin film deposited on the nickel-chromium thin film.

6. A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector according to claim 5, wherein the thickness of the Li-doped nickel oxide thin film is 150 nm.

7. A nickel oxide / gallium oxide heterojunction-based deep ultraviolet photodetector of claim 5, which detects light having a wavelength of 222 nm.

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