Method for forming a thin film comprising an ultrawide bandgap oxide semiconductor

Abstract

A method is disclosed for depositing a high-quality thin films of ultrawide bandgap oxide semiconductors at growth rates that are higher than possible using prior-art methods. Embodiments of the present invention employ LPCVD deposition using vapor formed by evaporating material as a precursor, where the material has a low vapor pressure at the growth temperature for the thin film. The vapor is carried to a reaction chamber by an inert gas, such as argon, where it mixes with a second precursor. The reaction chamber is held at a pressure that nucleation of the precursor materials occurs preferentially on the substrate surface rather than in vapor phase. The low vapor pressure of the material gives rise to growth rates on the substrate surface that a significantly faster than achievable using prior-art growth methods.

Claims

1. A method comprising: forming a first layer on a substrate, wherein the first layer includes a first material that is substantially crystalline and is an ultrawide bandgap oxide semiconductor, and wherein the first layer is formed by operations comprising: evaporating a second material to form a first vapor; providing the first vapor to a reaction chamber; providing a first precursor comprising a third material to the reaction chamber; reacting the first vapor and the first precursor to nucleate the first material; controlling a first temperature of the substrate within the range from approximately 750 C. to approximately 1000 C.; and controlling a pressure in the reaction chamber to mitigate homogeneous nucleation of the first material.

2. The method of claim 1 wherein the pressure is controlled such that it is less than or equal to approximately 50 Torr.

3. The method of claim 2 wherein the pressure is controlled such that it is within the range of approximately 1 Torr to approximately 50 Torr.

4. The method of claim 1 further comprising providing the second material such that it has a vapor pressure that is less than or equal to 10 Torr at the first temperature.

5. The method of claim 1 further comprising providing the substrate such that it comprises a fourth material that has a lattice constant that is within 10% of the lattice constant of the first material.

6. The method of claim 5 further comprising providing the substrate such that the fourth material is selected from the group consisting of sapphire, native (010) Ga.sub.2O.sub.3, native (201) Ga.sub.2O.sub.3, native (001) Ga.sub.2O.sub.3, native (100) Ga.sub.2O.sub.3, gallium nitride (GaN), magnesium oxide (MgO), nickel oxide (NiO), and silicon carbide (SiC).

7. The method of claim 1 wherein the first layer is formed such that the first material is gallium oxide.

8. The method of claim 7 further comprising providing the second material as high-purity gallium.

9. The method of claim 8 further comprising providing the first precursor such that the third material is high-purity oxygen.

10. The method of claim 1 further comprising: forming a first electrical contact that is electrically coupled with the first layer; providing a second electrical contact that is electrically coupled with the substrate; wherein the substrate, first layer, first electrical contact, and second electrical contact collectively define at least a portion of an electronic device.

11. The method of claim 10 wherein the electronic device is selected from the group consisting of a Schottky diode, a substantially solar-blind photodetector, and a sensor.

12. The method of claim 1 wherein the first layer is formed such that it includes a first dopant, and wherein the first layer has a doping concentration that is within the range of 110.sup.15 cm.sup.3 to 910.sup.19 cm.sup.3.

13. The method of claim 1 wherein the first layer is formed such that the first material is selected from the group consisting of lithium gallium oxide, indium oxide, aluminum gallium oxide, and indium gallium oxide.

14. The method of claim 1 wherein the first vapor is provided to the reaction chamber via a carrier gas flowing through a conduit that is fluidically coupled with the reaction chamber, the carrier gas being an inert gas.

15. The method of claim 14 wherein the first precursor is provided to the reaction chamber via the conduit.

16. A method comprising: forming a first layer on a substrate, wherein the first layer includes a first material that is substantially crystalline and is an ultrawide bandgap oxide semiconductor, and wherein the first layer is formed by operations comprising: conveying a first vapor of the first material to a reaction chamber via a conduit that is fluidically coupled with the reaction chamber; providing a first precursor comprising a second material to the reaction chamber; reacting the first vapor and the first precursor in the reaction chamber at a growth temperature; and controlling a pressure in the reaction chamber such that the pressure is less than or equal to 50 Torr.

17. The method of claim 16 further comprising controlling the growth temperature within the range from approximately 750 C. to approximately 1000 C.

18. The method of claim 16 further comprising: forming a first electrical contact that is electrically coupled with the first layer; and providing a second electrical contact that is electrically coupled with the substrate; wherein the substrate, first layer, first electrical contact, and second electrical contact collectively define at least a portion of an electronic device.

19. The method of claim 18 wherein the electronic device is selected from the group consisting of a Schottky diode, a substantially solar-blind photodetector, and a sensor.

20. The method of claim 16 wherein the first layer is formed such that the first material is selected from the group consisting of lithium gallium oxide, indium oxide, aluminum gallium oxide, and indium gallium oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a schematic drawing of a cross-sectional view of an ultrawide bandgap oxide semiconductor structure in accordance with an illustrative embodiment of the present invention.

(2) FIG. 2 depicts a growth system suitable for forming an ultrawide bandgap oxide semiconductor structure in accordance with the present invention.

(3) FIG. 3 depicts a method for forming an ultrawide bandgap oxide semiconductor structure in accordance with the illustrative embodiment.

(4) FIG. 4 shows a cross-sectional scanning-electron-microscope image of a -Ga.sub.2O.sub.3 thin film grown on c-sapphire substrate in accordance with the present invention.

(5) FIG. 5 shows a plot of photoluminescence excitation (PLE) spectra of a Si-doped -Ga.sub.2O.sub.3 thin film in accordance with the present invention at a plurality of temperatures, where the grown layer has a doping concentration of 2.510.sup.18 cm.sup.3.

(6) FIG. 6 shows plots of PLE peak positions as a function of temperature for layers in accordance with the present invention having different doping concentrations.

(7) FIG. 7 shows a plot of the optical reflectance spectra of Si-doped -Ga.sub.2O.sub.3 thin films with various carrier concentrations ranging from low-10.sup.17 to low-10.sup.19 cm.sup.3.

(8) FIG. 8 depicts a schematic drawing of a cross-sectional view of a high-power electronics device in accordance with the present invention.

(9) FIGS. 9A-B depict measured current-voltage (I-V) curves for device 800 in the forward-biased and reverse-biased directions, respectively.

DETAILED DESCRIPTION

(10) FIG. 1 depicts a schematic drawing of a cross-sectional view of an ultrawide bandgap oxide semiconductor structure in accordance with an illustrative embodiment of the present invention. Structure 100 comprise substrate 102 and layer 104. For the purposes of this Specification, including the appended claims, the term ultrawide bandgap oxide semiconductor is defined as an oxide semiconductor having a bandgap greater than 3.4 eV (i.e., the bandgap of GaN).

(11) FIG. 2 depicts a growth system suitable for forming an ultrawide bandgap oxide semiconductor structure in accordance with the present invention. Growth system 200 includes crucible 202, reaction chamber 204, heating system 204, and vacuum system 208.

(12) FIG. 3 depicts a method for forming an ultrawide bandgap oxide semiconductor structure in accordance with the illustrative embodiment. Method 300 begins with operation 301, wherein substrate 102 is placed in reaction chamber 204. Method 300 is described herein with continuing reference to FIGS. 1 and 2.

(13) Substrate 102 is a conventional c-sapphire substrate. In some embodiments, substrate 102 is a different substrate having a lattice constant that is within 10% of the lattice constant of -Ga.sub.2O.sub.3. Substrate materials suitable for use in embodiments of the present invention include, without limitation, native (010) Ga.sub.2O.sub.3, native (201) Ga.sub.2O.sub.3, native (001) Ga.sub.2O.sub.3, native (100) Ga.sub.2O.sub.3, gallium nitride (GaN), magnesium oxide (MgO), nickel oxide (NiO), silicon carbide (SiC) and the like.

(14) Chamber 204 is a conventional LPCVD reaction chamber that is operatively coupled to one or more conventional support systems, including heating system 206 and vacuum system 208.

(15) At operation 302, vacuum system 208 controls the pressure in chamber 204 at a pressure that fosters nucleation of gallium oxide on substrate surface 106 (i.e., heterogeneous nucleation) rather than reaction in the gas phase (i.e., homogeneous nucleation). In the depicted example, chamber 204 is evacuated to approximately 4 Torr; however, in some embodiments, the pressure in vacuum system 208 is controlled at a different pressure. Preferably, however, the pressure in chamber 204 is held to less than 50 Torr. In some embodiments, the pressure in chamber 204 is controlled to a different pressure within the range of approximately 1 Torr to approximately 50 Torr.

(16) One skilled in the art will recognize that, during the chemical vapor deposition process, precursor vapors transported to the growth chamber can be consumed by homogeneous (nuclei are formed in vapor form before being deposited) or heterogeneous (nuclei are formed on the substrate) nucleation. In the prior art, gallium oxide CVD growth is performed at high pressure (i.e., greater than 50 Torr). Unfortunately, growth at pressures above 50 Torr promotes vapor-phase reactions that reduces the film growth rate on substrate. It is an aspect of the present invention that, by maintaining a low pressure in chamber 204 during growth, the likelihood of undesirable homogeneous nucleation in the gas phase is reduced. For the LPCVD growth of Ga.sub.2O.sub.3 using metallic gallium as precursor, a growth pressure of 50 Torr or less is preferred.

(17) At operation 303, heating system 206 heats substrate 102 to approximately 900 C. One skilled in the art will recognize, after reading this Specification, that other growth temperatures can be used without departing from the scope of the present invention. Preferably, the growth temperature is controlled within the range from approximately 750 C. to approximately 1000 C.

(18) At operation 304, material 210 is vaporized to provide vapor 212 in conduit 218. Material 210 is vaporized by heating crucible 202.

(19) Preferably, material 210 is a material that is characterized by a vapor pressure at the growth temperature that is less than or equal to 10 Torr. In the depicted example, material 210 is high-purity gallium, typically in pellet form. For the purposes of this Specification, including the appended claims, the term high purity is defined as having a purity of at least 99.9999%. Embodiments of the present invention derive particular advantages over the prior art by employing a low-vapor-pressure precursor material in the LPCVD growth of layer 104. The vapor pressure of a material is dependent on is material properties and its temperature. Prior-art MOCVD growth processes typically employ precursors of Trimethylgallium (TMGa) and/or Triethylgallium (TEGa), which have vapor pressures of approximately 40 Torr at 10 C. and approximately 5.10 Torr at 20 C., respectively.

(20) In contrast to the prior art, the vapor pressure of vaporized gallium is very low (approximately 3.55 mTorr at 900 C.). Due to this low vapor pressure, at the growth temperatures in accordance with the present invention (e.g., 780-950 C.), the probability of vapor-phase reaction between gallium and oxygen is relatively low.

(21) In some embodiments, material 210 is a material other than pure gallium.

(22) It should be noted that methods in accordance with the present invention are suitable for depositing materials other than ultra-wide bandgap semiconductors. Examples of other materials suitable for deposition in accordance with the present invention include, without limitation, ultrawide bandgap oxide semiconductors, such as lithium gallium oxide (LiGaO.sub.2), etc. LiGaO.sub.2 is an example of an ultrawide bandgap material with bandgap of about 5.6 eV. During deposition of LiGaO.sub.2, therefore, material 210 would comprise high-purity lithium. In similar fashion to gallium, lithium has a relatively low vapor pressure (at 1000K, the vapor pressure of Li is approximately 0.76 Torr). Other sources of vapor 212 that can be used as precursor include lithium carbonate (Li.sub.2CO.sub.3). As a result, LPCVD growth methods in accordance with the present invention are applicable to grow LiGaO.sub.2, and similar materials, on a variety of substrates (e.g., sapphire, GaN, Ga.sub.2O.sub.3, MgO, NiO, SiC and etc.).

(23) At operation 305, carrier gas 214 is flowed through conduit 218 to carry vapor 212 to chamber 204 where it mixes with precursor 216. In the depicted example, carrier gas 214 is high-purity argon and precursor 216 is high-purity oxygen; however, other carrier and precursor gasses can be used without departing from the scope of the present invention.

(24) The selection of the precursors is typically based on the consideration of the desired growth rate, reaction temperature, impurity levels and crystallinity of the resultant -Ga.sub.2O.sub.3 films. Although the vapor pressure of evaporated gallium is relatively low (as discussed above) as compared to that of the trimethylgallium (TMGa) or GaCl precursors used in prior-art growth methods, the high efficiency of the reaction between gallium and oxygen leads to a high growth rate of -Ga.sub.2O.sub.3.

(25) At operation 306, layer 104 is grown on surface 106 of substrate 102. In the depicted example, layer 104 is a layer of unintentionally doped -Ga.sub.2O.sub.3.

(26) Embodiments of the present invention are afforded significant advantages over prior-art growth methods, including: i. an ability to grow very high-purity materials with low background doping and impurity levels due to the availability of high-purity precursors and relatively low growth pressure; or ii. a more efficient reaction process, which enables a wider growth window and high growth rates of epitaxial -Ga.sub.2O.sub.3; or iii. a mitigated need for the use of hazardous gases; or iv. lower cost; or v. scalability; or vi. compatibility with industrial standards and large-scale manufacturing; or vii. any combination of i, ii, iii, iv, v, and vi.

(27) It should be noted that, although the illustrative embodiment yields a thin-film layer that is an unintentionally doped layer of -Ga.sub.2O.sub.3, other layers can be grown without departing from the scope of the present invention. Alternative layers in accordance with the present invention include, without limitation, n-type doped -Ga.sub.2O.sub.3 thin films, semi-insulating -Ga.sub.2O.sub.3 thin films, and the like.

(28) In some embodiments, a dopant gas is included in chamber 204 during the growth of layer 104 to dope the layer as it is grown. For example, in some embodiments, SiCl.sub.4 is added to yield an n-doped layer.

(29) Using the exemplary growth conditions described above, thin-film growth rates of 0.8-10 m/hour are achieved. In some embodiments, a lower growth pressure (<1 Torr) used to achieve faster growth rates.

(30) X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature dependent Hall measurement and Raman spectroscopy analysis indicate that high-material-quality -Ga.sub.2O.sub.3 thin films result from the inventive method. The growth orientation of the thin film is determined by the growth substrate, while the thin-film growth rate depends primarily on growth conditions and source flow rates. LPCVD growth of -Ga.sub.2O.sub.3 thin films also show that the film growth rate has a dependence on the substrate material.

(31) FIG. 4 shows a cross-sectional scanning-electron-microscope image of a -Ga.sub.2O.sub.3 thin film grown on c-sapphire substrate in accordance with the present invention.

(32) Substrate 402 is a conventional c-sapphire substrate and is analogous to substrate 102 described above.

(33) Layer 404 is an unintentionally doped (UID) thin film that was grown for 1 hour at 900 C. Careful review of image 400 shows that the thickness of layer 404 is approximately 6 microns, which corresponds to a growth rate of 6 microns/hr.

(34) FIG. 5 shows a plot of photoluminescence excitation (PLE) spectra of a Si-doped -Ga.sub.2O.sub.3 thin film in accordance with the present invention at a plurality of temperatures, where the grown layer has a doping concentration of 2.510.sup.18 cm.sup.3. The PLE spectra were collected at an emission wavelength of .sub.emission=415 nm. Careful review of plot 500 evinces that the PLE peaks redshift with increasing temperature: 257 nm (T=77K) to 266.4 nm (T=298K).

(35) FIG. 6 shows plots of PLE peak positions as a function of temperature for layers in accordance with the present invention having different doping concentrations. Plot 600 shows the dependence of the PLE peak positions as a function of temperature for three different Si-doped -Ga.sub.2O.sub.3 thin films having doping concentrations of 2.510.sup.18, 9.210.sup.18, 3.710.sup.19 cm.sup.3, respectively. The corresponding PLE spectra were collected at an emission wavelength of .sub.emission=372 nm.

(36) For all three films, the PLE peaks showed a similar trend. The band gap shrinks as the increase of temperature. Such dependence of band gap on temperature has been reported for other semiconductor materials. The shrinkage of band gap with increasing temperature occurs mainly due to two contributing factors: (i) Thermal dilation of crystal lattice which reduces the overlap between the electron wave functions of neighboring atoms; and (ii) Electron-phonon interaction at finite temperature which changes the bond energy of the electron. At moderate temperature, lattice phonons are excited in large numbers. They influence the electron bonding energy through various orders of electron-phonon interactions. The change of bonding energy in turn alter the optical bandgap.

(37) Optical reflectance spectroscopy was used to study the effects of doping concentration on the optical band gap of -Ga.sub.2O.sub.3 thin films grown on c-sapphire substrates.

(38) FIG. 7 shows a plot of the optical reflectance spectra of Si-doped -Ga.sub.2O.sub.3 thin films with various carrier concentrations ranging from low-10.sup.17 to low-10.sup.19 cm.sup.3. Careful review of plot 700 shows that the reflectance peaks for the samples shift toward shorter wavelengths as doping concentration increases (i.e., they blue shift with increasing concentration). In addition, the band gap shifts from 265.5 nm (n=3.6210.sup.17 cm.sup.3) to 259.9 nm (n=1.5210.sup.19 cm.sup.3) as the doping level increases. This result is in agreement with the results shown in plot 600. It is believed that the increase in band gap with increasing carrier concentration is due to the Burstein-Moss (BM) shift for a doped semiconductor.

(39) FIG. 8 depicts a schematic drawing of a cross-sectional view of a high-power electronics device in accordance with the present invention. Device 800 is a Schottky-barrier diode comprising substrate 802, layer 804, and contacts 806 and 808.

(40) Substrate 802 is analogous to substrate 102 described above. In the depicted example, substrate 802 is a -Ga.sub.2O.sub.3 (201) substrate; however, other materials can be used in substrate 802 without departing from the scope of the present invention.

(41) Layer 804 is analogous to layer 104 described above. In the depicted example, layer 804 is a -Ga.sub.2O.sub.3 thin film having a thickness of approximately 0.92 microns. In some embodiments layer 804 has a different suitable thickness.

(42) Contact 806 is a conventional multi-layer electrical contact suitable for electrically contacting layer 804. In the depicted example, contact 806 includes layers of platinum, titanium, and gold having thicknesses of approximately 15 nm, 5 nm, and 150 nm, respectively.

(43) Contact 808 is a conventional multi-layer electrical contact suitable for electrically contacting substrate 802. In the depicted example, contact 808 includes layers of titanium and gold having thicknesses of approximately 80 nm and 140 nm, respectively.

(44) FIGS. 9A-B depict measured current-voltage (I-V) curves for device 800 in the forward-biased and reverse-biased directions, respectively.

(45) Careful review of plots 900 and 902 reveals that device 800 has a built-in potential of approximately 0.55 eV, an ideality factor of approximately 1.18, a Schottky barrier height of approximately 0.99 eV, and a breakdown voltage of approximately 320 V.

(46) The present invention provides a new pathway to synthesize high-purity and high-crystalline quality homoepitaxial and heteroepitaxial -Ga.sub.2O.sub.3 thin films with and without intentional doping. Advancements of low cost LPCVD of UWBG -Ga.sub.2O.sub.3 with high material quality and reasonable growth rates will open up opportunities for low cost high power electronic devices and solar blind deep-UV photodetectors.