A METHOD FOR GROWING HIGH-QUALITY HETEROEPITAXIAL MONOCLINIC GALLIUM OXIDE CRYSTAL

Abstract

Disclosed is a method for growing a high-quality heteroepitaxial β-Ga2O3 crystal by specifically using low-pressure chemical vapor deposition (LPCVD) method in the field of chemical vapor deposition, wherein said method includes the process steps of; preparing the substrate having hexagonal surfaces cut in different directions with inclinations such that the inclination angle is in a range between 2° and 10°; physically carrying the vapor obtained from Gallium heated in the second zone to the pump/sample by means of Argon gas; driving oxygen into the system with a separate ceramic or refractory metal tube and vertically transferring it onto the surface of the sample directly over the substrate; creating the core layer of β-Ga2O3 on the surface such that the ratio of Ga:O surface atoms on the growing surface is in a range between 10:1 and 1:10 so as to ensure that the surface atoms of Ga and O create the β-Ga2O3 crystal on the heated substrate; growing the core region of β-Ga2O3 at a thickness between 5 nm-2000 nm and at the growth rate between 10 nm/h-500 nm/h; maintaining the growing process on the core layer created in the previous step such that the β-Ga2O3 growth rate is in a range between 100 nm/h and 10 μm/h.

Claims

1. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 (beta Gallium Oxide) crystal by means of low-pressure chemical vapor deposition (LPCVD) method, wherein it comprises the process steps of; a) Preparing the substrate having hexagonal surfaces cut in different directions with inclinations such that the inclination angle is in a range between 2° and 10°, b) Physically carrying the vapor obtained from Gallium heated in the second zone (2) to the pump/sample by means of a carrier noble gas, c) Driving oxygen into the system with a separate ceramic or a refractory metal tube and transferring it onto the sample directly over the substrate from a distance of 0.1-4 cm and at an angle of 0°-90°, d) Creating the core layer of β-Ga.sub.2O.sub.3 on the surface such that the ratio of Ga:O surface atoms on the growing surface is in a range between 10:1 and 1:10 so as to ensure that the surface atoms of Ga and O create the β-Ga.sub.2O.sub.3 crystal on the heated substrate, e) Growing the core region of β-Ga.sub.2O.sub.3 at a thickness between 5 nm-2000 nm and at the growth rate between 10 nm/h-500 nm/h, f) Maintaining the growing process on the core layer created in the previous step such that the β-Ga.sub.2O.sub.3 growth rate is in a range between 100 nm/h and 10 μm/h.

2. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein the ratio of Ga:O surface atoms on the growing surface in the process step f) is in a range between 8:1 and 1:4.

3. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 2, wherein; the ratio of Ga:O surface atoms on the growing surface in the process step f) is 2:3.

4. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; the ratio of Ga:O surface atoms on the growing surface in the process step d) is 2:3 respectively.

5. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; n-type or p-type layers are obtained by using different doping elements and/or molecules.

6. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 5, wherein; doping elements used are selected from Ge, Sn, Si, and/or molecules used are selected from N.sub.2, H.sub.2, SiCl.sub.4.

7. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; the substrate is (0001) sapphire or (0001) SiC.

8. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein, the substrate is a (0001) oriented sapphire inclined by 6° in the <11-20> direction.

9. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; the carrier noble gas is Argon (Ar).

10. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; ceramic or refractory metal tube is a quartz tube.

11. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; it comprises the process step of maintaining the sample temperature at 925° C. and adjusting the Ga crucible temperature to 795° C. in step d).

12. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; it comprises the process step of increasing the Ga crucible temperature to 920° C. in step f).

13. A method for growing a heteroepitaxial β-Ga.sub.2O.sub.3 crystal according to claim 1, wherein; it comprises the process step of g) vaporizing solid source Ge inside the system or driving SiCl4 gas into the system by mixing it with a carrier gas (noble gasses) for n-type doping of grown β-Ga.sub.2O.sub.3 subsequent to the step f).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 illustrates the system geometry of the low-pressure chemical vapor deposition (LPCVD).

[0023] FIG. 2 illustrates the FE-SEM surface view of β-Ga.sub.2O.sub.3 layers grown on sapphire at a Ga crucible temperature of 795° C.

[0024] FIG. 3 illustrates the XRC measurement of β-Ga.sub.2O.sub.3 layers grown on sapphire at Ga crucible temperature of 795° C.

[0025] FIG. 4 illustrates the FE-SEM surface view of β-Ga.sub.2O.sub.3 layers grown on sapphire, at a Ga crucible temperature of 920° C.

[0026] FIG. 5 illustrates the XRC measurement of β-Ga.sub.2O.sub.3 layers grown on sapphire, at a Ga crucible temperature of 920° C.

DESCRIPTION OF THE REFERENCE NUMERALS

[0027]

TABLE-US-00002 NO Part/Section Name 1 First Zone 2 Second Zone 3 Third Zone 4 Pump 5 First Crucible 6 Second Crucible

DETAILED DESCRIPTION

[0028] The present disclosure discloses the process steps of a method that is based on the chemical vapor deposition method and more specifically, the process steps of a method for growing high-quality heteroepitaxial β-Ga.sub.2O.sub.3 crystal by using low-pressure chemical vapor deposition method.

[0029] FIG. 1 illustrates the system geometry of the method utilized in the present disclosure. The system comprises a 3-zone furnace. In the system, argon gas physically carries the vapor, which is obtained from Gallium (Ga) heated in the second zone (2), towards the pump (4), namely, towards the sample. Oxygen is driven into the system in a separate quartz tube and transferred directly onto the surface of the sample in a vertical direction on the substrate. Thus, Ga and O surface atoms create the β-Ga.sub.2O.sub.3 crystal on the substrate that is heated to an appropriate temperature. Preferably, the substrate used herein is a (0001) oriented sapphire inclined by 6° in <11-20> direction. The substrate's inclination by specific degrees (2°-10°) has a deterministic effect on rendering the growing β-Ga.sub.2O.sub.3 layer a single crystal.

[0030] In the implementation of the inventive system, the entire system is held inside a single furnace with different heating zones at a high temperature. Keeping different metal crucibles in different zones allows for achieving the desired vapor pressures, thereby providing the opportunity for doping and alloying processes. As shown in FIG. 1, metal vapors are conveyed to distribution lines by means of noble gasses like Ar, and they are homogeneously distributed onto the sample by creating pixels at certain intervals. Analogously, O.sub.2 (Oxygen gas) may also be transferred to the substrate through a different channel from designated pixel positions either directly or by being diluted with a carrier gas. In addition to these lines, it is also possible to drive gasses, which will ensure n or p-type doping (e.g., SiC.sub.4, H.sub.2, etc.), either through parallel lines or by mixing it directly into the line that is carrying O.sub.2. The sample platform is a rotatable disk and is capable of holding a plurality of substrates. In the state of the art, such distribution structures are available as close-coupled showerhead chemical vapor deposition systems. In the system, carrier gas or oxygen diluted with a carrier gas is driven into the system from a position that is in close proximity of the center of the rotatable disk platform, thereby providing a radial flow for the gasses from the center of the disk towards the outer circumference thereof.

[0031] The inventive heteroepitaxial β-Ga.sub.2O.sub.3 crystal growing method with low-pressure chemical vapor deposition (LPCVD) method comprises process steps of: [0032] a) Preparing the substrate having hexagonal surfaces cut in different directions with inclinations such that the inclination angle is in a range between 2° and 10°, [0033] b) Physically carrying the vapor obtained from Gallium heated in the second zone (2) to the pump (4)/sample by means of the carrier gas (noble gasses), [0034] c) Driving oxygen into the system with a separate ceramic or a refractory metal tube and transferring it onto the substrate directly from a distance of 0.1-4 cm and at an angle of 0°-90°, [0035] d) Creating the core layer of β-Ga.sub.2O.sub.3 on the surface such that the ratio of Ga:O surface atoms on the growing surface is in a range between 10:1 and 1:10 so as to ensure that the surface atoms of Ga and O create the β-Ga.sub.2O.sub.3 crystal on the heated substrate, [0036] e) Growing the core region of β-Ga.sub.2O.sub.3 at a thickness between 5 nm-2000 nm and at the growth rate between 10 nm/h-500 nm/h, [0037] f) Maintaining the growing process on the core layer created in the previous step such that the β-Ga.sub.2O.sub.3 growth rate is in a range between 100 nm/h and 10 μm/h.

[0038] The substrate used herein is (0001) sapphire or (0001) SiC. (0001) sapphire and (0001) SiC have a similar surface atomic packing. In fact, high-quality β-Ga.sub.2O.sub.3 structures were obtained on sapphire as (−201) β-Ga.sub.2O.sub.3 complies with these planes. Although not in terms of cost, SiC provides a substantial motivation in terms of thermal conductivity. As stated before, the thermal conductivity of high-power electronic devices is an essential limiting factor.

[0039] Preferably, the substrate used in the present disclosure is a (0001) oriented sapphire inclined by 6° in <11-20> direction. Thus, it is ensured that the β-Ga.sub.2O.sub.3 layer that is being grown is a single crystal.

[0040] Preferably, Argon (Ar) is used as a carrier gas (noble gas) in the present disclosure. Moreover, a ceramic or refractory metal tube, which allows for driving oxygen into the system, is preferably made of quartz.

[0041] Heteroepitaxially obtained optimal β-Ga.sub.2O.sub.3 layer is fundamentally dependent on two conditions. First, nucleation is performed by maintaining a low growth rate. This value is in a range between 10 nm and 500 nm. The nucleation stage is disclosed in d) and e) steps given above.

[0042] Second, the adatom (surface atom) density ratio on the growing surface in step f) is in a range between 8:1 and 1:4 for Ga and O respectively. Preferably, this ratio is 2:3.

[0043] Furthermore, in the present disclosure, preferably the ratio of Ga:O surface atoms on the growing surface in step d) is 2:3 respectively.

[0044] A characteristic of the inventive method for growing heteroepitaxial β-Ga.sub.2O.sub.3 crystal is that it comprises the process step of maintaining the sample temperature at 925° C. and adjusting the Ga crucible temperature to 795° C. in step d). Moreover, it further comprises the process step of raising the Ga crucible temperature to 920° C. in step f).

[0045] When the sample temperature was maintained at 925° C., Ar flow at 300 sccm, the distance between the Ga crucible and the sample was determined as 23 cm and when the Ga crucible temperature was adjusted as 795° C. by using the system shown in FIG. 1, scanning electron microscope surface images were obtained as shown in FIG. 2 and the oscillation curve scan was measured as shown in FIG. 3. Deviations in terms of atomic steps can be observed quite clearly. The XRC FWHM value, which is a value that indicates the crystal quality, was obtained at a record-high value of 0.049. This value proves the technical effects of the present disclosure with technical data.

[0046] When the Ga crucible temperature was raised to 920° C. under the same growing conditions, the growth rate increased from 100 nm/h to 1000 nm/h, and the surface morphology (FIG. 4) evolved to much smoother and fully aligned atomic steps. The main factor here is that the 4 sccm's of O.sub.2 flow provides Oxygen surface atom at much higher numbers when compared to the Ga surface atom number obtained from the Ga vapor at 795° C. As a matter of fact, Ga surface atom numbers at 920° C. were increased and Oxygen and Gallium surface atoms were obtained at relatively similar rates. Thus, the effect of the value that is close to the optimal Ga:O surface atom ratio, which is the first condition for high-quality growing, was observed. Even though surface morphology was better, the XRC FWHM value increased to 0.158°. Thus, the low growth rate requirement, which is the second condition for achieving optimal heteroepitaxy, was satisfied. As a matter of fact, the growth rate increased from 100 nm/h to 1000 nm/h upon the temperature increase of Ga as mentioned above and the reason behind this is that surface atoms form bonds by creating defects without even finding appropriate growing positions.

[0047] Briefly, two-stage growing is necessary for β-Ga.sub.2O.sub.3 heteroepitaxial CVD growth. In the first stage, nucleation is ensured at approximately 100 nm/h with the lowest number of defects, while the growing process continues at the rate of 1000-3000 nm/h on the high-quality nucleated p-Ga.sub.2O.sub.3 layer.

[0048] As different doping elements (Ge, Sn, Si, etc.) and molecules thereof (N.sub.2, H.sub.2, SiCl.sub.4, etc.) may be used in both growing stages, the growing process may also be performed in an undoped manner.

[0049] In an embodiment of the present disclosure, the process step of; g) vaporizing solid source Ge inside the system or driving SiCl.sub.4 gas into the system by mixing it with a carrier gas (noble gasses) for n-type doping of grown β-Ga.sub.2O.sub.3 is performed subsequent to the step f) in the inventive growing method.

[0050] X-ray diffraction signal width XRC FWHM values, which indicate the crystal quality achieved by various research groups, are listed in Table 2. As one can understand, an FWHM value of 0.049.sup.0 that is much lower than the values in the literature was obtained by means of the inventive modified low-pressure vapor deposition system. This value was obtained by means of the growing method disclosed in the present disclosure. The lowest FWHM values obtained by means of other methods and/or obtained from grown heteroepitaxial Ga.sub.2O.sub.3 layers as disclosed in other patent documents are around 0.4-0.5. This value indicates that the inventive method allows for obtaining layers, defect densities of which are quite low. In fact, this value is of such high quality that it can be compared with some β-Ga.sub.2O.sub.3 layers grown in clusters and it is a little over the approximate FWHM (0.014.sup.0) value of optimized β-Ga.sub.2O.sub.3 substrates grown with record quality and in clusters. This particular difference is a natural result of dislocations that form subsequent to the growing process on foreign substrates, and it indicates a sufficient crystal quality for the devices to be manufactured.

TABLE-US-00003 TABLE 2 X-ray diffraction oscillation curve FWHM values and growth rate of β-Ga2O3 layers grown on (0001) sapphire with different growing systems. The inventive Standard method PECVD MOCVD HVPE MBE LPCVD LPCVD Growth 0.58 0.75 6 <0.12 6 0.1 or Rate higher (μm/h) XRC 0.8 0.6 1.48 0.68 0.47 0.049 FWHM

[0051] The product obtained by means of the inventive method can be used in electric vehicle charging stations (at 600-1200V voltage-100 A current), in the operation of defense industry products operating with high electrical power, for example, in the production of transistors and diodes that can work under very high voltage (20-30 kV) and current (1000-3000A) in the production of electromagnetic cannons (railguns).

[0052] Moreover, it can also be used in the manufacturing of electronic devices that are used for connecting solar panel farms and wind turbines.

[0053] Furthermore, it can also be used in the manufacturing of solar-blind photodetectors, in detectors that enable missile tracking, and for transferring data in underwater communications.

ABBREVIATIONS USED IN THE DESCRIPTION

[0054] LPCVD: Low-Pressure Chemical Vapor Deposition [0055] CVD: Chemical Vapor Deposition [0056] PECVD: Plasma-Enhanced Chemical Vapor Deposition [0057] MOCVD: Metal-Organic Chemical Vapor Deposition [0058] HVPE: Halogen Vapor-Phase Epitaxy [0059] MBE: Molecular Beam Epitaxy [0060] β-Ga.sub.2O.sub.3: Beta Gallium Oxide