A PVD-grown undoped ε-Ga2O3 thin film and its preparation method
By depositing an alumina diffusion barrier layer on a substrate and growing an ε-Ga2O3 thin film using a pure gallium oxide target, the problem of undoped growth was solved, and a high-performance undoped ε-Ga2O3 thin film was achieved. This film was then applied to photodetectors, improving their photoelectric performance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HARBIN INSTITUTE OF TECHNOLOGY (SHENZHEN) (INSTITUTE OF SCIENCE AND TECHNOLOGY INNOVATION HARBIN INSTITUTE OF TECHNOLOGY SHENZHEN)
- Filing Date
- 2024-05-10
- Publication Date
- 2026-06-02
Smart Images
Figure CN118668161B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of semiconductor materials technology, and in particular to a PVD-grown undoped ε-Ga2O3 thin film and its preparation method. Background Technology
[0002] Most reports on gallium oxide materials focus on the growth of β-Ga₂O₃, while research on ε-phase growth is limited. As of September 2023, a search on Web of Science revealed 4802 articles on β-phase gallium oxide, but only 270 on ε-Ga₂O₃. Regarding gallium oxide preparation methods, β-phase gallium oxide is easily prepared in various forms, including bulk, granular, thin film, and nanowire materials. However, preparing pure ε-phase gallium oxide is difficult. Currently, the mainstream preparation method is mostly chemical vapor deposition (CVD). Although CVD can epitaxially produce ε-Ga₂O₃ thin films without doping, it still has some unavoidable drawbacks, such as lower doping control precision compared to PVD, less flexible film structure, and the toxicity of organic precursors.
[0003] Compared to chemical vapor deposition (CVD), the preparation of pure ε-Ga₂O₃ using physical vapor deposition (PVD) is of greater research value. Furthermore, the two different deposition processes have different thermodynamic and kinetic principles, leading to differences in the way thin films are obtained and their properties. In the field of PVD epitaxial growth of ε-Ga₂O₃ thin films, molecular beam epitaxy (MBE) and PLD techniques are two feasible methods for achieving growth. To date, In / Sn doping has become a necessary condition for the preparation of ε-Ga₂O₃ via PVD.
[0004] It is worth noting that the presence of precursors results in a higher background impurity concentration in films prepared by CVD methods. Compared to chemical vapor deposition, physical vapor deposition (PVD) is more likely to yield ε-phase with fewer impurities. However, to date, no literature or patents have demonstrated that physical vapor deposition can achieve the growth of a pure ε-phase without doping. Summary of the Invention
[0005] To address the above technical problems, this invention discloses a PVD-grown undoped ε-Ga2O3 thin film and its preparation method, achieving the growth of Ga2O3 thin films without doping with pure ε phase.
[0006] The technical solution adopted by this invention is as follows:
[0007] A method for preparing undoped ε-Ga2O3 thin films by PVD growth includes the following steps:
[0008] Step S1: Prepare the substrate and target material and place them into the main deposition chamber of the physical vapor deposition equipment;
[0009] Step S2: Heat the substrate to above 650°C, introduce oxygen as the growth atmosphere, and use a gallium oxide target with an atomic content of 0.7%-3% tin to deposit the first thin film on the substrate.
[0010] Step S3: Deposit an aluminum oxide layer on the surface of the first thin film as a diffusion barrier layer;
[0011] Step S4: In the PVD equipment, using a pure gallium oxide target, the substrate is heated to above 650°C, and oxygen is introduced as the growth atmosphere gas to deposit a thin film on the surface of the aluminum oxide layer; then cooled to room temperature to obtain an undoped ε-Ga2O3 thin film.
[0012] Using this technique, undoped pure ε-Ga2O3 films were grown. The photodetector fabricated with this film showed a significant improvement in photo-dark current ratio (PDCR) compared to tin-doped ε-phase gallium oxide films.
[0013] As a further improvement of the present invention, step S3 utilizes atomic layer deposition (ALD) to construct an alumina diffusion barrier layer to prevent the diffusion of tin elements from affecting the growth of the second layer of undoped pure ε phase gallium oxide film.
[0014] As a further improvement of the present invention, in step S3, the output pulse frequency is 5Hz, and the total number of pulse lasers is 2000-6000 times.
[0015] As a further improvement of the present invention, in step S2, the tin atom ratio content of the gallium oxide target doped with tin is 0.8-2.5%. Further, the tin atom ratio content of the gallium oxide target doped with tin is 0.9-1.5%.
[0016] As a further improvement of the present invention, in step S2, the partial pressure of oxygen is 2 mtorr to 3 mtorr, and the initial energy of the laser is 450 mJ.
[0017] As a further improvement of the present invention, in step S2, the output pulse frequency is 5Hz and the total number of pulse lasers is 4000.
[0018] As a further improvement of the present invention, the thickness of the alumina layer in step S3 is 2-4 nm. Specifically, the thickness of the alumina layer is 3 nm.
[0019] As a further improvement of the present invention, in step S4, the cooling rate is 5-15℃ / min. The cooling rate is 10℃ / min.
[0020] As a further improvement of the present invention, the substrate is a (0001) crystal plane sapphire substrate.
[0021] As a further improvement of the present invention, step S1 includes: polishing the substrate on both sides, immersing it in acetone for ultrasonic cleaning, removing it and cleaning it sequentially with anhydrous ethanol and deionized water, and drying it with pure nitrogen gas.
[0022] The present invention also discloses a PVD-grown undoped ε-Ga2O3 thin film, which is prepared by the PVD-grown undoped ε-Ga2O3 thin film preparation method described in any one of the above.
[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:
[0024] The technical solution of this invention utilizes a pulsed laser deposition system to fabricate an undoped ε-Ga₂O₃ photodetector, which exhibits a significantly higher PDCR value than tin-doped ε-Ga₂O₃ devices. Furthermore, the undoped ε-Ga₂O₃ device obtained by this invention can achieve a photocurrent of 0.626 μA, a dark current of 0.278 pA, and a PDCR of 22517. Under the same conditions, the PDCR of an epitaxially doped ε-Ga₂O₃ device is only 733.
[0025] The thin film obtained by using the technical solution of this invention can be used to prepare a solar-blind ultraviolet detector, which can then be applied to military applications such as missile early warning and tracking, ultraviolet communication, and port fog-breaking navigation, as well as civilian applications such as ozone hole monitoring, ultraviolet intensity monitoring for disinfection and sterilization, high-voltage corona detection, and ultraviolet monitoring for forest fire prevention. Attached Figure Description
[0026] Figure 1 The diagram shows a comparison of the thin film structures of Example 1 and the comparative example of the present invention; wherein, (a) is the doped-barrier-undoped thin film structure of Example 1, (b) is the doped-undoped thin film structure of the comparative example, and (c) is an XRD comparison diagram of Example 1 and the comparative example.
[0027] Figure 2 The XRD results are for the doped-barrier-undoped ε-Ga2O3 thin films grown on sapphire in Examples 1-3 of this invention.
[0028] Figure 3 The images are TEM cross-sectional views of the doped-barrier-undoped ε-Ga2O3 thin films grown on sapphire in Examples 1-3 of the present invention; (a), (b), and (c) correspond to Examples 1, 2, and 3, respectively.
[0029] Figure 4 The images show TEM diffraction patterns of undoped ε-Ga2O3 thin films grown on sapphire in Examples 1-3 of this invention; (a), (b), and (c) correspond to Examples 1, 2, and 3, respectively.
[0030] Figure 5 This is a comparison diagram of the tin content in the ε-Ga2O3 thin film region of the doped-barrier-undoped structure grown on sapphire in Example 1 of the present invention. Detailed Implementation
[0031] The preferred embodiments of the present invention will be described in further detail below.
[0032] Example 1
[0033] A method for preparing undoped ε-Ga₂O₃ thin films based on physical vapor deposition, more specifically, a method for growing doped ε-Ga₂O₃ thin films using pulsed laser deposition. The basic principle of this method is to generate high-energy pulsed laser light using a pulsed laser generator, which bombards the target surface. When the target surface reaches an ablation threshold, a plasma plume is formed, evaporating a large number of electrons, plasma, atoms, molecules, and clusters. These particles propagate along the target's normal direction, macroscopically manifesting as the formation of a specific plume under a specific atmosphere, ultimately adhering to and growing on the substrate surface to form a thin film.
[0034] The pulse deposition system mainly consists of a laser and a deposition chamber; it includes a two-stage vacuum system with a coarse mechanical pump and a turbomolecular pump to maintain a chamber vacuum of approximately 10. -7 The system consists of a vacuum system with a vacuum level of mbar, a laser generation system composed of a high-voltage power supply and a thyratron, a heating system composed of resistance wires, and a series of mechanical transmission systems.
[0035] The entire deposition process can be viewed as two stages: the interaction between the laser and the target, and the deposition of high-energy molecules on the substrate in the form of adsorbed atoms, which solidifies and grows into a thin film. In the first stage, the laser generates KrF pulsed laser under high pressure, which is then transmitted to the surface of the target in the cavity through an optical mirror system, generally making the angle between the target and the incident laser 45°.
[0036] Specifically, it includes the following process steps:
[0037] (1) Preliminary preparations
[0038] First, a double-sided polished 5mm×5mm sapphire substrate was ultrasonically cleaned in acetone for 8 minutes. After removal, residual acetone was removed by washing with anhydrous ethanol, followed by rinsing with deionized water for 5 minutes, and then dried with high-purity nitrogen. The substrate was fixed to the center of the sample holder using silver paste with butyl acetate, and then the sample holder and substrate were placed on a 120°C heating stage to solidify the silver while the butyl acetate evaporated. The sintered gallium oxide target was placed in the target holder and fed into the deposition chamber. The prepared target was surface-treated with 2000-grit sandpaper to obtain a smooth surface, and then purged with nitrogen to avoid SiC particle residue on the sandpaper.
[0039] (2) Place the sample
[0040] The target and sample holder are introduced into the main deposition chamber after passing through the transition injection chamber. First, the vacuum in the transition chamber is broken by sequentially shutting down the molecular pump and then the mechanical pump. When the vacuum level in the transition chamber drops to 1000 mbar, the chamber door is opened, and the sample or target is placed in. The sample is secured to the transfer rod using a mechanical threaded connection. The chamber door is then closed, and the mechanical pump is turned on. When the vacuum level in the injection chamber drops below 10 mbar, the molecular pump is turned on to further evacuate the chamber. When the vacuum level in the transition injection chamber reaches 5*10... -5 At mbar, open the gate valve between the transition chamber and the main chamber, and use the sample transfer rod to send the sample holder into the main deposition chamber. Set the required substrate growth temperature using the TSST's built-in resistance thermometer heating method, and heat the sample to 650℃ at a rate of 20℃ / min. The target placement method is similar to that of the sample holder, but without a heating process. After placing the target holder, raise and lower it to a position 45mm from the target-substrate distance of the sample holder, awaiting subsequent thin film deposition.
[0041] (3) Thin film growth
[0042] When the substrate is heated to 650℃, 99.999% oxygen is introduced as the growth atmosphere. The oxygen partial pressure is controlled by adjusting the gas concentration and the frequency of vacuum valve operation to achieve 2 mtorr or 3 mtorr. The initial laser energy is set to 450 mJ. Using a laser energy meter, the energy reaching the target surface after optical path loss is measured to be 115 mJ. The ablation surface area of the laser in this experiment is 0.12 cm². 2 Therefore, its energy density is approximately 1 J / cm³. 2 .
[0043] The first thin film deposition used a gallium oxide target with a 3% atomic ratio of tin. The laser pulse frequency was controlled by software at 5 Hz, with a total of 4000 pulses, to begin deposition, resulting in a thickness of 100 nm. The intermediate alumina diffusion barrier layer was deposited using atomic layer deposition (ALD), achieving a 3 nm thick layer after 24 cycles. The second thin film deposition used a pure gallium oxide target. The laser pulse frequency was controlled by software at 5 Hz, with a total of 2000 pulses, to begin deposition, resulting in a thickness of 50 nm.
[0044] (4) Sampling
[0045] After film deposition is complete, stop the target movement and allow it to return to its initial position. Set the cooling rate to 10℃ / min to allow the substrate to return to room temperature. After completing the film deposition experiment, first, shut off the heating system and stop the oxygen supply to bring the gas pressure in the main chamber and transition chamber closer together. Next, open the gate valve, remove the sample holder through the sample transfer rod, and move it to the transition sampling chamber for vacuum sampling. During sampling, be careful to remove any residual silver paste from the sample back surface to avoid affecting subsequent experimental results. A mixture of acetone and alcohol can be used for cleaning until the sample back surface is clean. This completes the entire film deposition experiment.
[0046] The photodetector fabricated using the above method with undoped ε-Ga₂O₃ thin films exhibits a significantly higher PDCR value than tin-doped ε-Ga₂O₃ devices. Specifically, the undoped ε-Ga₂O₃ device fabricated using the above method has a photocurrent of 0.626 μA, a dark current of 0.278 pA, and a PDCR of 22517. In contrast, under the same conditions, the PDCR of an epitaxially doped 1% tin ε-Ga₂O₃ device is only 733.
[0047] Comparative Example 1
[0048] Based on Example 1, this Comparative Example 1 does not deposit an aluminum oxide layer as a diffusion barrier layer on the surface of the first thin film. Instead, it directly uses a pure gallium oxide target to deposit an undoped layer on the surface of the first thin film. Other process conditions are the same as in Example 1.
[0049] Schematic diagrams of the thin films obtained in Example 1 and Comparative Example 1 are shown below. Figure 1 (a) and Figure 1 As shown in (b), XRD tests were performed on both groups of samples, and the results are as follows. Figure 1 (c) It was found that the comparative example 1 without the barrier layer showed obvious β phase peaks, which indicates that the diffusion of tin element is dominant at this time (low tin element doping tends to form β phase), and that continuing to grow directly on the first layer thin film with high surface roughness is not conducive to the epitaxial growth of ε-Ga2O3 with regular hexagonal crystal system.
[0050] Comparative Example 2
[0051] Using existing technology, a pure gallium oxide target is used, the substrate is heated to above 650°C, and oxygen is introduced as the growth atmosphere to directly epitaxially deposit a gallium oxide thin film on a sapphire substrate.
[0052] Compared to Example 1, this comparative example shows that, firstly, ε-Ga₂O₃ in Example 1 exhibits good compatibility with substrates having hexagonal or quasi-hexagonal structures, such as (0001) oriented sapphire. However, since ε-Ga₂O₃ is a metastable phase and there is still a lattice mismatch with the substrate, it cannot be directly deposited for epitaxial growth, and an ε-Ga₂O₃ thin film cannot be obtained. Therefore, Sn doping is unavoidable. Specifically, in ε-Ga₂O₃, the ratio of tetrahedral to octahedral coordinated metal atoms is 1:3, and the octahedral coordination of Sn atoms stabilizes the ε phase. In Example 1, the alumina diffusion barrier layer is only 3 nm thick, which does not affect the surface state of the first high-quality ε-Ga2O3 film. Furthermore, the atomic layer deposition (ALD) film formation process is completed independently for each cycle, resulting in a very smooth film with a surface roughness that can be controlled to around 0.126 nm. In summary, the epitaxial growth of ε-Ga2O3 on the alumina diffusion barrier layer can be understood as a diffusionless homoepitaxial growth on the basis of the first ε-Ga2O3 layer.
[0053] Example 2
[0054] Based on Example 1, the difference in this implementation is that in step S2, the target material used is a gallium oxide target material with an atomic ratio of 1% tin, and the deposition thickness is 100 nm, while the other experimental parameters are the same.
[0055] Example 3
[0056] Based on Example 1, the difference in this implementation is that in step S2, the target material used is a gallium oxide target material with an atomic ratio of 1.5% tin, and the deposition thickness is 100 nm, while the other experimental parameters are the same.
[0057] The XRD comparison results of the thin film samples obtained in Examples 1 to 3 are as follows: Figure 2 As shown, it can be seen that the doping of the above three samples only produces peaks of ε-Ga2O3, and the peak intensity and half-peak width are similar, indicating that the crystallinity is relatively good.
[0058] The TEM results and diffraction patterns of the thin films obtained in Examples 1 to 3 are as follows: Figure 3 and Figure 4 As shown, with the increase of doping content, the orientation of the undoped ε-Ga2O3 grown in the upper layer gradually becomes disordered, and defects such as twins appear, and the diffraction pattern also gradually becomes irregular.
[0059] Similarly, Ti-Au electrodes (20nm-60nm) were deposited on the surfaces of three samples to fabricate MSM-structured photodetectors. The bandgap of aluminum oxide is higher than that of gallium oxide (4.9eV), which can completely shield the influence of 254nm ultraviolet light on the first tin-doped ε-Ga₂O₃ thin film. Therefore, the photoelectric test data are entirely characterizations of the undoped layer's performance. Testing the photo-dark current of the samples revealed that as the tin doping content increased, the dark current gradually increased. Although the photocurrent also increased, it was far less rapid than the increase in dark current. The devices fabricated with 1% and 1.5% doping achieved the best photo-dark current ratio (PDCR) exceeding 20,000, while the PDCR of the 3% doped device was 500-1000. Analysis of the tin content ratio in the ε-Ga₂O₃ thin film region, such as... Figure 5 As shown, this is related to excessive doping of Sn. When the Sn element used to stabilize the ε phase in the lower layer reaches saturation, the excess Sn element will remain on the film surface in the form of clusters, causing a change in the lattice stress state of the film surface. This, in turn, degrades the quality of the upper undoped homoepitaxial layer, generating a large number of defects. These defects are similar to n-type doping distributed in the upper film, increasing the current and degrading the overall performance.
[0060] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.
Claims
1. A method for preparing undoped ε-Ga2O3 thin films by PVD growth, characterized in that: Includes the following steps: Step S1: Prepare the substrate and target material and place them into the main deposition chamber of the physical vapor deposition equipment; Step S2: Heat the substrate to above 650°C, introduce oxygen as the growth atmosphere, and use a gallium oxide target with an atomic content of 0.9-1.5% tin to deposit the first thin film on the substrate. Step S3: Deposit an aluminum oxide layer on the surface of the first thin film as a diffusion barrier layer; Step S4: Using a pure gallium oxide target, the substrate is heated to above 650°C, and oxygen is introduced as the growth atmosphere gas to deposit a thin film on the surface of the aluminum oxide layer; then cooled to room temperature to obtain an undoped ε-Ga2O3 thin film.
2. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to claim 1, characterized in that: In step S4, the output pulse frequency is 5Hz, and the total number of pulse lasers is 2000-6000.
3. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to claim 1, characterized in that: In step S2, the partial pressure of oxygen is 2 mtorr~3 mtorr, and the initial laser energy is 450 mJ.
4. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to claim 1, characterized in that: In step S2, the output pulse frequency is 5Hz, and the total number of pulse lasers is 4000.
5. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to claim 1, characterized in that: The thickness of the alumina layer in step S3 is 2-4 nm.
6. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to claim 1, characterized in that: In step S4, the cooling rate is 5-15℃ / min.
7. The method for preparing undoped ε-Ga2O3 thin films by PVD growth according to any one of claims 1 to 6, characterized in that: The substrate is a (0001) crystal sapphire substrate; step S1 includes: polishing the substrate on both sides, immersing it in acetone for ultrasonic cleaning, taking it out and cleaning it in turn with anhydrous ethanol and deionized water, drying it and blowing it dry with pure nitrogen.
8. A PVD-grown undoped ε-Ga2O3 thin film, characterized in that: The undoped ε-Ga2O3 thin film was prepared by the PVD growth method as described in any one of claims 1 to 7.