Method for growing a gallium oxide layer on a substrate and semiconductor wafer

Abstract

The substrate is measured by AFM at 300 × 10 -12 polishing the substrate to a roughness of 0.1 mm or less; etching the substrate for 12 to 20 minutes using an aqueous solution of phosphoric acid (H3PO4) having a concentration of at least 85% by volume; heat treating the substrate at 850 to 950°C in an atmosphere consisting essentially of high-purity oxygen; and metalorganic vapor phase epitaxy (MOVPE) using a showerhead that directs a gas flow toward the surface of the substrate. 1. A method for growing a layer of β-Ga2O3 on a substrate in an apparatus for metalorganic chemical vapor phase epitaxy, comprising: setting a distance between a showerhead and a surface of a substrate; wherein the gas flow comprises triethylgallium (TEGa), high-purity oxygen, and high-purity argon; and growing a layer of β-Ga2O3 on the substrate, the distance between the showerhead and the surface of the substrate being greater than 10 mm and less than 15 mm; the gas flow rate of the high-purity argon being greater than or equal to 3000 sccm and less than or equal to 10000 sccm; and the molar ratio of the high-purity oxygen to the triethylgallium (TEGa) being greater than 300 and less than 400.

Description

[Technical Field]

[0001] Field The present invention relates to a method for growing a layer of β-Ga2O3 having excellent electrical properties on a substrate, and to a substrate having a layer consisting essentially of β-Ga2O3. [Background technology]

[0002] background The semiconductor material β-Ga2O3 has an extremely wide band gap of approximately 4.9 eV, a theoretically predicted critical electric field of up to 8 MV / cm, and an electron mobility of 200 cm at room temperature. 2 V -1 s -1 As a result, the Baliga figure of merit of β-Ga2O3 is 2-3 times higher compared to other power electronic semiconductors such as SiC and GaN. The demonstration of high-quality bulk β-Ga2O3 single crystals grown from the melt using various techniques such as the Czochralski method, floating zone method, edge-defined film-fed growth (EFG), and vertical Bridgman method is one of the key advantages that has attracted great interest from both academia and industry for its potential in low-cost mass production and growth of homoepitaxial films.

[0003] Homoepitaxially grown β-Ga2O3 thin films have been investigated on various substrate orientations by several growth methods, including molecular beam epitaxy (MBE), metalorganic vapor phase epitaxy (MOVPE), halide vapor-phase epitaxy (HVPE), and low-pressure chemical vapor deposition (LPCVD). Among all the growth techniques mentioned above, MOVPE (or MOCVD) epitaxy is the industrially preferred growth method due to its advantages of relatively high growth rate and application scenarios for mass production. In the development of MOVPE-grown β-Ga2O3 thin films, great success has been demonstrated for the (010) orientation to reach high mobility without intentional doping. However, for the (100) orientation, the low-doped region (10 17 cm -3 (less than 100%) has not yet been successfully demonstrated due to epitaxy challenges such as structural defects and growth mode control, which limits its application in vertical power devices.

[0004] Chou et al. (T.-S. Chou, P. Seyidov, S. Bin Anooz, R. Gruneberg, T. Thi Thuy Vi, K. Irmscher, M. Albrecht, Z. Galazka, J. Schwarzkopf, and A. Popp, AIP Adv. 11, 115323 (2021)) show that a lower VI / III ratio plays an important role in the formation of a stable Ga wetting layer (or Ga adlayer) on the (100) β-Ga2O3 surface, which helps maintain the desired step-flow growth mode even at micrometer-level thicknesses.

[0005] The conventional intuitive approach to tune the VI / III ratio is by adjusting the flow rates of the Ga precursor and oxidant species. However, due to the complex vapor-phase transport mechanism and the insufficient precision of mass flow control in MOVPE reactors, the exact VI / III ratio can vary significantly with a small change in any single parameter, resulting in a narrow growth window and limited room for further optimization. Summary of the Invention [Problem to be solved by the invention]

[0006] Assignment Description The inventors of the present invention have noticed that when layers of intentionally undoped β-Ga2O3 are grown with a thickness greater than 2 μm, parasitic particles become evident.

[0007] On the one hand, such particles induce harmful acceptor-like structural defects in the layers, which reduce mobility (and in some cases even make Hall measurements impossible). On the other hand, in vertical device structures, they increase the reverse leakage current and thus reduce the blocking ability of the Schottky barrier. Therefore, detailed investigation and process optimization are required to understand the origin of such parasitic particles and how to completely eliminate them.

[0008] It is an object of the present invention to provide an improved method that does not exhibit these problems. It is also an object of the present invention to provide an improved material that also overcomes these problems. [Means for solving the problem]

[0009] These objects are solved by the methods and products set forth in the claims. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 10 compares optical microscope images of films grown under different showerhead distances: (a) 80 mm and (b) 15 mm. [Figure 2] FIG. 1 shows the electron hole mobility of several samples of different origins using different techniques. DETAILED DESCRIPTION OF THE INVENTION

[0011] Detailed Description In Figure 1, it can be clearly seen that the shorter the distance from the showerhead to the surface of the substrate (b), the fewer particles are generated on the surface. The scale shown in the lower right corner of both Figures 1a and 1b represents a length of 500 μm. The values ​​represented by open circles in Figure 2 have layer thicknesses less than 2 μm. The electron Hall mobility values ​​represented by filled squares represent experimental samples with larger layer thicknesses (much greater than 2 μm) using methods known in the art. However, the filled circles represent electron Hall mobility values ​​at larger layer thicknesses fabricated using the method of the present invention. The dash-dotted line represents the empirical behavior of Hall mobility versus electron concentration.

[0012] 5×10 16 cm -3 Layers of β-Ga2O3 with a doping concentration less than 0.01% (hence referred to as intentionally undoped) and a (100) crystal orientation were grown on Mg-doped semi-insulating and Si-doped conducting substrates, both (100), fabricated by the Czochralski method.

[0013] Preferably, the substrate is polished. More preferably, the roughness after polishing is 300 pm or less as measured by atomic force microscope (AFM), where the measurement is the root mean square of the deviation of the average height measured by AFM.

[0014] Preferably, after polishing, the substrate is etched in an aqueous solution of phosphoric acid (H3PO4, preferably 85% by volume or more) for at least 12 minutes and not more than 20 minutes, with best results achieved with 15 minutes to remove layers damaged during polishing.

[0015] After polishing, the substrate is preferably heat treated at a temperature of 850°C to 950°C, preferably for at least 1 hour, in an atmosphere consisting essentially of high purity oxygen to exhibit a terraced surface structure.

[0016] The inventors of the present invention used a metalorganic vapor phase epitaxy system comprising a vertical showerhead low-pressure reactor with a rotating susceptor (Structured Materials Industries, Inc., USA) that was utilized to grow n-type β-Ga2O3 thin films. An example of the equipment used can be found in U.S. Pat. No. 6,289,842.

[0017] Preferably, triethylgallium (TEGa) is used as the organometallic precursor of Ga, while preferably high-purity O2 (5N) is used as the oxidant and high-purity argon (5N) is used as the carrier gas.

[0018] The preferred proportional gas flow rate of high purity argon is between 3000 and 10000 sccm, and more preferably between 5000 and 8000 sccm.

[0019] Preferably, the molar ratio of high purity oxygen to triethylgallium is greater than 300 and less than 400.

[0020] Preferably, the growth temperature is above 800°C, preferably above 825°C, and below 850°C.

[0021] Preferably, after growth of the layer, the wafer is heat treated at a temperature 10 K above and less than 30 K above the growth temperature used for a period of 2 to 4 hours.

[0022] Preferably, the ambient atmosphere for this heat treatment consists essentially of argon. After the heat treatment process is completed, the second growth step is preferably initiated, allowing the thickness of the layer to be increased without compromising the quality of the grown layer.

[0023] An atomic force microscope (Bruker Dimension Icon, USA) was used to characterize the surface roughness of the grown films. The surface roughness value is given as the root mean square of the deviation of the average surface.

[0024] The layer thickness and dopant concentration profiles were revealed by secondary ion mass spectrometry (SIMS) performed by RTG Mikroanalyse GmbH, Berlin. The electrical properties of unintentionally doped (UID) (100) β-Ga2O3 thin films were characterized by temperature-dependent resistivity and Hall measurements (HMS7504, Lake Shore) in the van der Paul configuration.

[0025] A metal stack of Ti (50 nm) / Au (150 nm) ohmic contacts was deposited by electron beam evaporation on several tens of -6 The sample was deposited on the corner in a vacuum at a pressure of 1000 mbar. Afterwards, the sample was subjected to rapid thermal annealing at 470 °C for 1 min in a N2 atmosphere to achieve low on-contact resistance. The temperature was varied from 20 K to 320 K, and excitation currents of 10 nA to 1 μA were used.

[0026] The inventors of the present invention investigated the effect of showerhead distance on the growth of β-Ga2O3 thin films.

[0027] It was found that by decreasing the showerhead distance from 80 mm to 15 mm while keeping the TEGa flow rate constant, the layer growth rate increased by almost a factor of two.

[0028] [Table 1]

[0029] Since growth under the conditions used is mass transport limited, the thin film growth rate was primarily limited by the TEGa flow rate.

[0030] The observed increase in thin film growth rate suggests that the smaller the showerhead distance, the less TEGa is lost in vapor phase transport from the showerhead to the susceptor.

[0031] The preferred distance between the showerhead and the surface of the substrate is greater than 10 mm and less than 15 mm.

[0032] The inventors of the present invention believe that the increase in growth rate indicates a smaller VI / III ratio near the surface compared to the larger showerhead distance, as the amount of oxygen remains constant, which is beneficial for the stability of Ga wetting layer formation and helps maintain the step-flow morphology during film growth.

[0033]

[0001] A desirable step-flow morphology along the direction could be observed under the growth conditions of the present invention.

[0034] The surface roughness (RMS) was measured to be less than 0.5 nm, which is smaller than that of MOVPE-grown (010) β-Ga2O3 of similar thickness in both small-scale (0.5 μm × 0.5 μm) and large-scale (5 μm × 5 μm) AFM images.

[0035] It is noted that the growth conditions of the present invention can be adapted to both Mg-doped and Si-doped substrates grown in the same chamber, enabling applications in lateral and vertical devices.

[0036] Furthermore, the present inventors have realized that a smaller showerhead distance appears to minimize undesired gas phase reactions within the chamber by reducing the precursor flight path, thereby resulting in fewer parasitic particles being formed and a significantly cleaner growth surface being observed.

[0037] An optical microscopy comparison of films grown under different showerhead distances is shown in FIG.

[0038] The inventors of the present invention have found that parasitic particles are more pronounced at larger film thicknesses (>2 μm), whereas such particles induce deleterious acceptor-like structural defects in the layer that reduce mobility (and in some cases even make Hall measurements impossible), rendering the material unusable for device fabrication.

[0039] In vertical device structures, the reverse leakage current becomes large and thus the blocking ability of the Schottky barrier is reduced. Therefore, detailed investigation and process optimization are required to understand the origin of such parasitic particles and how to completely eliminate them.

[0040] Another embodiment of the present invention is an inventive semiconductor wafer comprising a substrate having a front surface, wherein a semiconductor layer having a thickness d is disposed on the front surface and consists essentially of β-Ga 2 O 3 .

[0041] Preferably, the doping concentration of said layer is less than 5×10 as measured by Hall measurements at room temperature using a van der Pauw measurement setup. 16 cm -3 is less than.

[0042] The Hall mobility of said layer measured at room temperature is preferably 120 cm 2 V -1 s -1 It is larger, and the thickness d is greater than 3 μm as measured by SIMS.

[0043] More preferably, the surface particle density on the semiconductor layer is 900 cm -2 The measurements are carried out by optical microscopy.

[0044] More preferably, the deviation of the layer thickness from the average layer thickness is less than 10% as measured by ellipsometry, and the layer has a surface roughness of 600 pm or less as measured by AFM, where the surface roughness is expressed as the root mean square of the deviation of the average height of the AFM measurement.

[0045] More preferably, the substrate has a surface roughness of 5×10 -2 It consists essentially of β-Ga2O3 with an electrical resistivity of less than Ωcm.

[0046] The present inventors have recognized that these substrates, having a preferred layer of β-Ga 2 O 3 , are highly suitable for use as starting materials for constructing vertical devices.

[0047] More preferably, the layer has a (100) crystal orientation. Most preferably, the layer thickness d is greater than 3 μm and less than 6 μm. Measurements can be easily carried out by SIMS measurements.

Claims

1. The substrate is measured by AFM. －１２ Polishing to a roughness of less than 100 mm At least 85% by volume of phosphoric acid (H ３ PO ４ ) etching the substrate for 12 to 20 minutes using an aqueous phosphoric acid solution having a concentration of 0.1 to 0.25; heat-treating the substrate at a temperature of 850°C to 950°C in an atmosphere consisting essentially of high-purity oxygen; 1. An apparatus for metalorganic chemical vapor phase epitaxy, comprising a showerhead for directing a gas flow toward a surface of the substrate, wherein the apparatus performs the metalorganic chemical vapor phase epitaxy by setting a distance between the showerhead and the surface of the substrate; The gas flow contains triethylgallium, high purity oxygen, and high purity argon as components, and deposits β-Ga on the substrate. ２ O ３ Grow a layer of the distance between the showerhead and the surface of the substrate is greater than 10 mm and less than 15 mm; the gas flow rate of the high-purity argon is 3000 sccm or more and 10000 sccm or less; The method of claim 1, wherein the molar ratio of said high purity oxygen to said triethylgallium is greater than 300 and less than 400.

2. 2. The method of claim 1, wherein the gas flow rate of the high purity argon is between 5000 sccm and 8000 sccm.

3. 2. The method of claim 1, wherein the growth temperature is above 800°C, preferably above 825°C, and below 850°C.

4. 2. The method of claim 1, wherein after growth of the layer, the wafer is heat treated at a temperature of more than 10 K and less than 30 K above the growth temperature for a period of 2 to 4 hours.

5. 5. The method of claim 4, wherein a second growth step is initiated after the heat treatment.

6. a substrate having a front surface; On the front surface, β-Ga ２ O ３ a semiconductor layer having a thickness d consisting essentially of The doping concentration of the layer measured by Hall measurements at room temperature using a van der Pauw measurement device is 5×10 １６ cm －３ is less than The hole mobility measured at room temperature is 120 cm ２ V －１ s －１ is larger than A semiconductor wafer characterized in that the thickness d measured by SIMS is greater than 3 μm.

7. The surface particle density on the semiconductor layer is 900 / cm ２ 7. The semiconductor wafer of claim 6, wherein the measurement is performed by optical microscopy.

8. 8. The semiconductor wafer of claim 6, wherein the thickness deviation from the average thickness measured by ellipsometry is less than 10% and the surface roughness of the layer measured by AFM is 600 pm or less.

9. The substrate was heated to 5×10 －２ β-Ga with electrical resistivity below Ωcm ２ O ３ 9. The semiconductor wafer according to claim 6, wherein the semiconductor wafer essentially consists of:

10. 10. The semiconductor wafer according to claim 6, wherein the crystal orientation of the layer is (100).

11. 7. The semiconductor wafer of claim 6, wherein the thickness d measured by SIMS is greater than 3 μm and less than 6 μm.

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