Method of manufacturing p-type gallium oxide by intrinsic doping, the thin film obtained from gallium oxide and its use

Abstract

The inventive method provides for a method of p-type doping of Ga.sub.2O.sub.3 without adding impurity elements. Embodiments allow for significant simplification relative to extrinsic impurity element doping, and thus offers a reduced fabrication cost while being more temperature resistant since the defect dopants require higher temperatures to alter their impact. Certain methods disclosed provide for p-type gallium oxide formation via intrinsic defect doping, without requiring the addition of impurity elements which provide significant simplification relative to the existing state of the art approaches providing more temperature and radiation resistance, while offering a reduced fabrication cost.

Claims

1. A method for the fabrication of wide bandgap p-type Ga.sub.2O.sub.3 (α, β, ε or κ) by defect doping with gallium vacancies, involving the growth of gallium oxide on a substrate, using a conventional thin film growth technique, by acting only on the oxygen stoichiometry, and through controlling the growth temperature, the nature/pressure of the oxidant gas or the growth rate, such that p-type gallium oxide is formed via intrinsic defect doping, without requiring the addition of impurity elements.

2. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 1 in which the substrate can be sapphire (Al.sub.2O.sub.3) (with a, c, r or m orientation), strontium titanate (SrTiO.sub.3); magnesium oxide (MgO), silicon (Si), gallium oxide (Ga.sub.2O.sub.3), silicon carbide (SiC-4H or SiC-6H), nickel oxide (NiO), copper oxide (CuO or Cu.sub.2O), zinc oxide (ZnO), gallium nitride (GaN), aluminium nitride (AlN), glass, quartz, a metal, graphene, graphite, graphene oxide, or a polymer.

3. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 1 in which the growth temperature can be either held constant or varied over a temperature range between 300 and 1500° C. and in which the partial pressure of oxidant gas can be held constant or varied (according to the level/profile of doping that is desired) between 10.sup.−2 and 10.sup.−11 Torr.

4. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 1 in which the oxidant atmosphere can be gaseous molecular oxygen, water vapor, gaseous ozone, gaseous nitrogen monoxide or dioxide (NO or NO.sub.2) or atomic oxygen from a plasma source.

5. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 1 which involves the use of a buffer layer, which can be in an oxide or nitride material, wherein the thickness of the buffer layer is between 1 to 500 nm.

6. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 1 which involves a post-anneal (in-situ or ex-situ) in order to reinforce the activation of the p-type doping in the gallium oxide, wherein the anneal occurs over a duration between 10 ns and 10 hours.

7. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 6 which involves an anneal.

8. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 6 in which the laser anneal can be realised with UV light (continuous or pulsed) having a wavelength under that corresponding to the bandgap of Ga.sub.2O.sub.3 (an energy of ≥5 eV).

9. The method for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 6 in which the lamp anneal can be done with a halogen lamp or an incandescent lamp or a discharge lamp, with an annealing temperature which can be up to 1500° C., and, in which, the energy density can be over 100 J/cm.sup.2 and last between 100 μs and 300 seconds.

10. A thin film of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) with a thickness between 1 nm and 10 μm that is obtained by the growth of gallium oxide on a substrate via intrinsic defect doping, without requiring the addition of impurity elements according to the fabrication method of claim 1.

11. Use of a p-type Ga.sub.2O.sub.3 (α, β, ε or κ) thin film, obtained by the fabrication method of claim 1, in Ga.sub.2O.sub.3 based p-n junctions.

12. Use of a p-type Ga.sub.2O.sub.3 (α, β, ε or κ) thin film, obtained by the fabrication method of claim 1, as a p-type transparent electrode, in UVC photodetectors, in high frequency switches, in high temperature electronics, in power electronics, in thermoelectrics, in radiation resistant electronics (e.g. betavoltaics) and in space electronics.

13. The method-for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 7 wherein the anneal is a thermal anneal, a laser anneal, a lamp anneal, or combinations thereof.

14. The method-for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 7 wherein the anneal is at a fixed temperature or at a variable temperature.

15. The method-for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 7 wherein the atmosphere for the anneal can be molecular oxygen, NO.sub.2, NO, ozone, N.sub.2, Ar, Kr or air.

16. The method-for the fabrication of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) according to claim 8 wherein the length of exposure is between 10 ns and 1 s, and wherein the energy density of the light source can be between 1 mJ/cm.sup.2 and 1 kJ/cm.sup.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) In order to exploit the growth process for p-type gallium oxide, one can use, for example, a thin film deposition tool such as molecular beam epitaxy, DC magnetron sputtering, laser ablation (sometimes called pulsed laser deposition (PLD)) or chemical vapor deposition (CVD) or metal organic CVD or liquid phase deposition or sol gel or atomic layer deposition.

(2) The substrate can be sapphire (Al.sub.2O.sub.3), strontium titanate (SrTiO.sub.3); magnesium oxide (MgO), nickel oxide (NiO), copper oxide (CuO or Cu.sub.2O), silicon (Si), gallium oxide (Ga.sub.2O.sub.3), silicon carbide (SiC-4H or SiC-6H), zinc oxide (ZnO), gallium nitride (GaN), aluminium nitride (AlN), glass, quartz, a metal, graphene, graphite, graphene oxide, a polymer, etc. The substrate is cleaned and brought to a sufficiently high temperature in order to degas. The oxidant atmosphere can be gaseous molecular oxygen, water vapor, gaseous ozone, gaseous nitrogen monoxide or dioxide (NO or NO.sub.2) or atomic oxygen from a plasma source. The background pressure can be held constant or varied (according to the level/profile of doping that is desired) between 10.sup.−2 and 10.sup.−11 Torr.

(3) The growth temperature can be either held constant or varied over a temperature range between 300 and 1500° C. according to the desired level of doping (higher temperature reduces the doping level). The film thickness can be between 1 nm and 10 microns. A typical growth rate is about 0.5 nm per minute.

(4) A buffer layer can be used in order to increase the carrier concentration and/or mobility in the p-type Ga.sub.2O.sub.3 layer, or in order to increase the conductivity of the p-Ga.sub.2O.sub.3. The thickness of the buffer layer can be between 1 nm and 500 nm. In order to boost the activation of the p-type doping, a post-anneal step can be used. The anneal can be thermal or by laser or by lamp. In the case of a thermal anneal the anneal can be at a fixed or variable temperature between 600 and 1500° C. The thermal anneal can last between 10 minutes and 10 hours. The atmosphere for the anneal can be molecular oxygen, NO.sub.2, NO, ozone, N.sub.2, Ar, Kr or air.

(5) The laser anneal (continuous wave or pulsed) can be realised with UV light having a wavelength under that corresponding to the bandgap of Ga.sub.2O.sub.3 (i.e. an energy≥5 eV). The length of exposure can be between 10 ns and 1 s. The energy density of the light source can be between 1 mJ/cm.sup.2 and 1 kJ/cm.sup.2.

(6) In the case of a lamp anneal the source is often a halogen lamp. This technology is relatively flexible in terms of temperature rise and thus allows large thermal dynamic of the anneal. The annealing temperature can be up to 1500° C. The energy density can be over 100 J/cm.sup.2 and last between 100 μs and 30 seconds.

(7) The growth of a thin film of p-type Ga.sub.2O.sub.3 in the current state-of-the-art is achieved by introducing a p dopant such that the Ga in the Ga.sub.2O.sub.3 layer is replaced by the said dopant. For the current invention there is no introduction of a dopant; the procedure is based on varying only the oxygen stoichiometry, through control of growth conditions, for instance, growth temperature and/or the nature/pressure of the oxidant gas and/or the growth rate etc. With the procedure of the invention, p-type gallium oxide is formed via intrinsic defect doping, without requiring the addition of impurity elements. This approach represents a significant simplification relative to the existing state of the art approaches, and thus offers a reduced fabrication cost. Moreover, the p-type doping of Ga.sub.2O.sub.3 obtained in this way is more temperature and radiation resistant since the defect dopant distributions are not altered as readily as impurity defect distributions at elevated temperature.

(8) We will now give a descriptive example of a procedure for the fabrication of p-type Ga.sub.2O.sub.3 according to the present invention. In this, the p-type Ga.sub.2O.sub.3 is formed on a c-plane oriented sapphire (c-Al.sub.2O.sub.3) substrate by pulsed laser deposition (often referred to as PLD or laser ablation). The substrate is cleaned by sequential immersion in acetone, ethanol and deionised water followed by drying in a flow of dry nitrogen. It is then heated, under vacuum (<10.sup.−6 Torr), to a temperature between 700+/150° C. and held there for 30 minutes in order to degas. The growth is conducted with a Coherent LPX 200 KrF (248 nm) or ArF (193 nm) excimer laser (at a pulse repetition rate of 10+/−8 Hz and a pulse duration of 30+/−15 ns) and a solid source comprised of a compressed/sintered stoichiometric 4N powder of Ga.sub.2O.sub.3. The beam is focused on the target in order to give a power density of about 10.sup.8+/−5×10.sup.7 W/cm.sup.2.

(9) A uniform coverage of the two-inch-diameter c-sapphire wafer is obtained using an optical rastering of the incident laser beam on the target. The temperature of the Al.sub.2O.sub.3 target is maintained between 380 and 550° C. and the oxygen pressure during the growth is 10.sup.−4 Torr. In this way we obtain films of ε/κ-Ga.sub.2O.sub.3 of about 300 nm thick. The growth rate is about 5 nm/min. The films are then air annealed in a tubular furnace at 700+/−100° C. for 30 minutes in order to activate the concentration of p-type carriers. The Al.sub.2O.sub.3 substrate, on which the Ga.sub.2O.sub.3 is grown, has the advantage of being cheaper than the Ga.sub.2O.sub.3 bulk substrate that is typically used in the state-of-the-art, and it can be found in both larger formats and much larger production volumes. This makes the current invention's approach to p-type Ga.sub.2O.sub.3 fabrication cheaper and more suited to a rapid industrialisation.

(10) The invention also concerns p-type Ga.sub.2O.sub.3 (α, β, ε or κ) thin films obtained by the above process. The films obtained can have thicknesses between 1 nm and 10 micrometers according to the growth time.

(11) The invention also concerns the use of films of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) obtained by the above process in p-n junctions, in which the n-type and p-type regions are both made from intrinsically-doped Ga.sub.2O.sub.3 according to the above process.

(12) Moreover, the invention concerns the use of p-type Ga.sub.2O.sub.3 thin films (α, β, ε or κ), obtained by the above procedure in UVC photodetectors, p-type transparent electrodes, radiation resistant electronics (e.g. betavoltaics), high temperature electronics, thermoelectrics, power electronics, high frequency electronics, space electronics and other application domains. For example, the performance of a field effect transistor (FET), realised in a thin film of beta gallium oxide on insulator, were presented in the article “High Performance Depletion/Enhancement-Mode β-Ga.sub.2O.sub.3 on Insulator (GOOI) Field Effect Transistors” published in IEEE Electron Device Letters in January 2017.

(13) We know that the ultrawide bandgap of p-type Ga.sub.2O.sub.3 (α, β, ε or κ) makes it relatively efficient for use in high voltage/frequency switches and that this enhanced efficiency could help to reduce the energy consumption by replacing the silicon based devices which are currently used in high power/frequency switches.

(14) The study and structural/chemical analysis of a film of p-Ga.sub.2O.sub.3 obtained according to the current invention, as well as the electrical properties, were published in the article “p-type β-Ga.sub.2O.sub.3 oxide; A new perspective for power and optoelectronic devices” in Materials Today Physics.