Thermal Neutron Transmutation Doped Gallium Oxide Semiconductor

Abstract

A germanium (Ge)-doped gallium oxide (Ga.sup.2O.sup.3) semiconductor material and method of making are provided. In embodiments, a method of making the Ge-doped Ga.sup.2O.sup.3 semiconductor material includes: subjecting a Ga.sub.2O.sub.3 semiconductor material to neutron irradiation comprising a higher thermal neutron content than fast neutron content, thereby producing a Ge-doped Ga.sub.2O.sub.3 semiconductor material; and annealing the Ge-doped Ga.sub.2O.sub.3 semiconductor material at a temperature of at least 700? C. in an atmosphere of nitrogen gas, thereby generating an electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor material.

Claims

1. A method of making a germanium (Ge)-doped gallium oxide (Ga.sub.2O.sub.3) semiconductor material, comprising: subjecting a Ga.sub.2O.sub.3 semiconductor material to neutron irradiation comprising a higher thermal neutron content than fast neutron content, thereby producing a Ge-doped Ga.sub.2O.sub.3 semiconductor material; and annealing the Ge-doped Ga.sub.2O.sub.3 semiconductor material at a temperature of at least 700? C. in an atmosphere of nitrogen gas, thereby generating an electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor material.

2. The method of claim 1, wherein the neutron irradiation is performed using a thermal neutron to fast neutron ratio of at least 25:1.

3. The method of claim 1, wherein the neutron irradiation has an energy of less than or equal to 0.5 electron volts (eV).

4. The method of claim 1, wherein the neutron irradiation has an energy of less than or equal to 0.025 electron volts (eV).

5. The method of claim 1, wherein the n-type Ge-doped Ga.sub.2O.sub.3 semiconductor material has a Ge concentration between 10.sup.13 Ge atoms/cm.sup.3 and 10.sup.18 Ge atoms/cm.sup.3.

6. The method of claim 1, wherein the n-type Ge-doped Ga.sub.2O.sub.3 semiconductor material has a Ge concentration between 10.sup.16 Ge atoms/cm.sup.3 and 10.sup.18 Ge atoms/cm.sup.3.

7. The method of claim 1, wherein the n-type Ge-doped Ga.sub.2O.sub.3 semiconductor material has a resistivity of between 0.05 and 10.sup.10 Ohm.Math.cm.

8. The method of claim 1, further comprising capping a surface of the Ge-doped Ga.sub.2O.sub.3 semiconductor material with a protective material prior to the annealing to protect against surface reconstruction during the annealing.

9. The method of claim 1, wherein the annealing is performed via multicycle rapid thermal annealing (MRTA) using a series of rapid heating and cooling pulses.

10. The method of claim 1, wherein the Ga.sub.2O.sub.3 semiconductor material is free of iridium (Ir).

11. The method of claim 1, further comprising growing the Ga.sub.2O.sub.3 semiconductor material.

12. The method of claim 1, wherein the neutron irradiation transmutes Ga-69 isotopes and Ga-71 isotopes in the Ga.sub.2O.sub.3 semiconductor wafer or boule to respective unstable isotopes Ga-70 and Ga-72, wherein the unstable isotopes Ga-70 and Ga-72 decay over a period of time to produce respective stable isotopes Ge-70 and Ge-72, thereby producing the Ge-doped Ga.sub.2O.sub.3 material in the form of a wafer of boule, and wherein the period of time is based on the half-life of Ga-72.

13. An electrically conductive n-type germanium (Ge)-doped gallium oxide (Ga.sub.2O.sub.3) semiconductor wafer or boule having an electrical resistivity of between 0.05 and 10.sup.10 Ohm.Math.cm.

14. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 wafer or boule of claim 13, wherein the conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule is free of iridium (Ir).

15. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 wafer or boule of claim 13, further comprising a Ge concentration between 10.sup.13 Ge atoms/cm.sup.3 and 10.sup.18 Ge atoms/cm.sup.3.

16. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 wafer or boule of claim 13, further comprising a Ge concentration between 10.sup.16 Ge atoms/cm.sup.3 and 10.sup.18 Ge atoms/cm.sup.3.

17. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule of claim 13, formed by: subjecting a Ga.sub.2O.sub.3 semiconductor wafer or boule to neutron irradiation comprising a higher thermal neutron content than fast neutron content, thereby producing a Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule; and annealing the Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule at a temperature of at least 700? C. in an atmosphere of nitrogen gas, thereby generating the electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule.

18. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule of claim 17, wherein the neutron irradiation is performed using a thermal neutron to fast neutron ratio of at least 25:1.

19. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule of claim 17, wherein the annealing is performed via multicycle rapid thermal annealing (MRTA) using a series of rapid heating and cooling pulses.

20. The electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule of claim 17, wherein the neutron irradiation transmutes Ga-69 isotopes and Ga-71 isotopes in the Ga.sub.2O.sub.3 semiconductor wafer or boule to respective unstable isotopes Ga-70 and Ga-72, and wherein the unstable isotopes Ga-70 and Ga-72 decay over a period of time to produce respective stable isotopes Ge-70 and Ge-72, thereby producing the Ge-doped Ga.sub.2O.sub.3 wafer or boule, and wherein the period of time is based on the half-life of Ga-72.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Aspects of the present invention are described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention.

[0009] FIG. 1 shows a flowchart of an exemplary method in accordance with aspects of the present invention.

[0010] FIG. 2 shows a diagram of the exemplary method of FIG. 1.

DETAILED DESCRIPTION

[0011] Aspects of the present invention relate generally to doping semiconductor materials and, more particularly, to doping gallium oxide (Ga.sub.2O.sub.3) using thermal neutron transmutation. In embodiments, a method is provided for generating a germanium (Ge)-doped Ga.sub.2O.sub.3 semiconductor substrate. Implementations of the invention produce controllably-low-doped ?-Ga.sub.2O.sub.3 wafers for high-voltage power applications.

[0012] Advantageously, implementations of the invention reduce the generation of defects in a Ga.sub.2O.sub.3 substrate during neutron transmutation doping (NTD) by utilizing neutron irradiation having a high thermal neutron to fast neutron ratio followed by a high-temperature annealing process. Embodiments of the invention result in an electrically conductive Ga.sub.2O.sub.3 substrate uniformly doped with Ge.

[0013] In implementations, thermal neutron reactions with the stable isotopes of gallium-69 (Ga-69) and gallium-71 (Ga-71) in a Ga.sub.2O.sub.3 substrate result in neutron capture to produce the unstable isotopes gallium-70 (Ga-70) and gallium-72 (Ga-72), which decay into respective stable germanium-70 (Ge-70) and germanium-72 (Ge-72) shallow donors. When the donor concentration (Np) is higher than the total concentration of intentional acceptors (e.g., N, Fe, Zn, etc.) and unintentional acceptors (e.g., V.sub.Ga) acceptors (collectively N.sub.A), a net excess electron concentration n (n=N.sub.D?N.sub.A) will result in an n-type doped semiconductor.

[0014] Unlike previous attempts to dope costly GaN boules, embodiments of the invention exploit the availability of affordable melt-grown boules of Ga.sub.2O.sub.3 as a viable route to produce large-area, uniformly doped substrates of any orientation for vertical power device applications. Currently, conductive Ga.sub.2O.sub.3 substrates doped via silicon (Si) or tin (Sn) donors can be doped n-type at levels of no less than 10.sup.17 electrons/cm.sup.3, necessitating the growth of thick, low-doped (<10.sup.16 cm.sup.?3) epitaxial layers for vertical power devices. The growth of thick epitaxial layers will inevitably results in the evolution of point and extended defects towards the epitaxial layer surface, leading to difficulty in maintaining high crystal quality epitaxial ?-Ga.sub.2O.sub.3 beyond a few micrometers. Advantageously, embodiments of the invention result in n-type Ga.sub.2O.sub.3 substrates doped in the 10.sup.15-10.sup.16 electrons/cm.sup.3 range, similar to that of Si substrates. Such substrates will greatly reduce the necessity of tens of micrometers thick epitaxial layers and greatly increase the relevance of Ga.sub.2O.sub.3 for power electronics applications. Additionally, unlike neutron irradiation of GaN, implementations of the present invention do not result in a semiconductor material including residual radioactive carbon-14 (C-14).

[0015] FIG. 1 shows a flowchart of an exemplary method in accordance with aspects of the present invention. In embodiments, at step 101, a method of the invention includes obtaining or growing a Ga.sub.2O.sub.3 semiconductor substrate. In implementations, a ?-Ga.sub.2O.sub.3 semiconductor substrate is grown utilizing a known method, such as the Float Zone, Czochralski, Vertical Bridgman and edge-defined film-fed growth (EFG) methods.

[0016] Some growth methods have been shown to result in a Ga.sub.2O.sub.3 substrate including iridium (Ir). For example, Ir is a common unintentional impurity in Ga.sub.2O.sub.3 crystals synthesized via EFG and Czochralski techniques relying on the use of an Ir crucible. Upon neutron irradiation, the radioisotope Ir-192 will be produced via neutron capture. With a half-life of 73 days for the gamma rays resulting, Ir-192 is a strong gamma emitter and is thus undesirable from an electronic device safety perspective. Accordingly, in embodiments, the Ga.sub.2O.sub.3 semiconductor substrate is grown utilizing an iridium-free method to produce an iridium-free Ga.sub.2O.sub.3 semiconductor substrate. For example, the Ga.sub.2O.sub.3 semiconductor substrate may be grown by the float-zone (zone melting) method or vertical Bridgman method, another crucible-free or non-Ir crucible method, or another EFG or CZ technique where Ir incorporation in the Ga.sub.2O.sub.3 crystal is suppressed.

[0017] With continued reference to FIG. 1, at step 102 implementations of the invention include subjecting the Ga.sub.2O.sub.3 semiconductor substrate of step 101 to a primarily thermal neutron flux irradiation with a low flux of fast neutrons to produce a Ge-doped Ga.sub.2O.sub.3 semiconductor substrate over a period of time. In embodiments, the irradiation comprises a thermal neutron energy of less than or equal to 0.5 eV, or less than or equal to 0.025 eV, to encompass the slow, thermal, epithermal and cadmium neutron energy range.

[0018] In aspects of the invention, the period of time for step 102 to enable decay of Ga-70 and Ga-72 to Ge-70 and Ge-72, respectively, is nominally proportional to the decay half-life for the two isotopes, 21.1 minutes for Ga-70 and 14.1 hours for Ga-72. In embodiments, this period of time is a nominal quarantine period of about two weeks prior to material release from the reactor. In embodiments, the thermal neutron irradiation comprises a thermal neutron to fast neutron ratio of at least 25:1. In implementations, the thermal to fast neutron ratio can vary from 1:1 to higher than 25:1 depending on reactor placement, filtering, neutron energy spectrum, etc., however, a highest ratio is desirable. In implementations, a neutron moderator is utilized to thermalize neutron irradiation to the Ga.sub.2O.sub.3 semiconductor substrate resulting in the thermalization (i.e., reducing neutron energy) of any non-thermal (e.g., fast, epithermal, etc.) neutron irradiation. In general, a neutron moderator is a medium(s) that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons. Upon thermal neutron irradiation, Ga-69 and Ga-71 isotopes in the Ga.sub.2O.sub.3 semiconductor substrate transmute to the unstable isotopes Ga-70 and Ga-72, respectively. In turn, the unstable isotopes Ga-70 and Ga-72 decay over time to produce stable isotopes Ge-70 and Ge-72, respectively. Thus, the end-product of the thermal neutron irradiation of the Ga.sub.2O.sub.3 semiconductor substrate in accordance with embodiments of the invention results is a Ge-doped Ga.sub.2O.sub.3 semiconductor substrate.

[0019] In implementation, the Ga.sub.2O.sub.3 semiconductor substrate is irradiated for an appropriate period of time for the conditions (e.g., 82 hours when irradiating a large piece of Ga.sub.2O.sub.3 within a container in a neutron flux of 5?10.sup.13 neutrons per cubic centimeters per second (n/cm.sup.2/s)). One of ordinary skill in the art could select an appropriate time period and flux for a particular size of Ga.sub.2O.sub.3 semiconductor substrate, and the present invention is not intended to be limited to a particular NTD time or neutron flux conditions. In embodiments, the Ga.sub.2O.sub.3 semiconductor substrate is in the form of a wafer of boule (bulk crystal) having a size of between a 1 cm.sup.2 diameter and a 6 inch diameter. In the case of boules, the final product may be sliced into wafers. Various reactors may be utilized to implement step 102. One example of a reactor that may be utilized to provide thermal neutron irradiation in accordance with methods of the present invention is the University of Missouri Research Reactor (MURR). It should be understood that other reactors with high thermal neutron flux and low fast neutron flux are also capable of performing neutron transmutation doping.

[0020] In NTD, the number of neutron captures per unit volume, N, is described by N=N.sub.T?.sub.c?, where N.sub.T is the number of target nuclei per unit volume, ?.sub.c is the capture cross section with units of cm.sup.2, and ? is the neutron fluence, n/cm.sup.2. The cross section represents the probability of interaction between the neutron and the nucleus. Neutron capture reactions occur at higher rates with low energy neutrons because the lower velocity allows greater interaction with the target nuclei. The capture cross section is inversely proportional to the neutron velocity at low energies. Reactor neutrons are parameterized into three groups based on energy. Fast neutrons have an energy greater than 100 kiloelectron volts (keV), epithermal neutrons have an energy between 0.5 electron volts (eV) and 100 keV, and thermal neutrons have an energy less than 0.5 eV. In implementations, NTD of Ga.sub.2O.sub.3 will occur when Ga-69 and Ga-71 undergo neutron capture reactions with thermal neutrons to produce unstable Ga-70 and Ga-72, respectively. The unstable isotopes Ga-70 and Ga-72 beta decay over a period of time to produce respective stable isotopes Ge-70 and Ge-72, thereby producing the Ge-doped Ga.sub.2O.sub.3 wafer or boule. The period of time for the unstable isotopes Ga-70 and Ga-72 to decay to produce the Ge-doped Ga.sub.2O.sub.3 wafer or boule (bulk crystal) is based on the half-life of Ga-72. Ga-70 and Ga-72 have half-lives of 21.1 minutes and 14.1 hours, respectively. Fast neutrons are unlikely to be captured, but cause knock-on damage through elastic scatter.

[0021] Thermal neutron irradiation is known to cause defects in Ga.sub.2O.sub.3. In order to address these defects, at step 103, implementations of the invention include annealing the Ge-doped Ga.sub.2O.sub.3 semiconductor substrate product of step 102 to obtain an electrically conductive Ge-doped Ga.sub.2O.sub.3 semiconductor substrate. In aspects of the invention, the electrically conductive Ge-doped Ga.sub.2O.sub.3 semiconductor substrate includes a Ge concentration of between 10.sup.13 and 10.sup.18 Ge per cubic centimeter (Ge/cm.sup.3). In certain embodiments, the electrically conductive Ge-doped Ga.sub.2O.sub.3 semiconductor substrate includes a Ge concentration of between 10.sup.16 and 10.sup.18 Ge/cm.sup.3. In aspects of the invention the electrically conductive neutron transmutation Ge-doped Ga.sub.2O.sub.3 semiconductor substrate has an electrical resistivity of between 0.05 and 10.sup.10 Ohm.Math.cm.

[0022] In implementations, the Ge-doped Ga.sub.2O.sub.3 semiconductor substrate product of step 102 is subjected to a high-temperature annealing process in an atmosphere of nitrogen gas (e.g., pure nitrogen gas) or forming gas. Forming gas is a mixture of hydrogen gas (H.sub.2) and nitrogen gas (N.sub.2), and is sometimes referred to as dissociated ammonia atmosphere. Forming gas typically has a ratio of H.sub.2 to N.sub.2 of 5:95%. In implementations, the annealing temperature is greater or equal to 700? C. In some embodiments, the annealing temperature is between 700? C. and 1600? C.

[0023] In accordance with embodiments of the invention, the Ge-doped Ga.sub.2O.sub.3 semiconductor substrate is annealed for a time period of between 10 seconds and 72 hours. In implementations, the annealing occurs in a chamber pressurized to between vacuum and atmospheric pressure. In embodiments, the annealing at step 103 may be implemented as a series of rapid heating and cooling pulses (e.g., up to ?600 K/s) at moderate pressures (e.g., ?10-100 bar) via multicycle rapid thermal annealing (MRTA) to access intermediate (?1250? C.) and high (1600? C.) states. In implementations, the MRTA occurs at a pressure near 35 bar, and with temperature pulses between 500 C and 1500 C at 300K/s. In aspects of the invention, step 103 includes a substep of capping a surface of the Ge-doped Ga.sub.2O.sub.3 semiconductor substrate with a protective layer (e.g., silicon nitride, silicon oxide, aluminum nitride, aluminum oxide, or other oxides or nitrides) during annealing to protect against surface reconstruction.

[0024] FIG. 2 shows a diagram of the exemplary method of FIG. 1. Specifically, a Ga.sub.2O.sub.3 semiconductor wafer or boule material 200 is shown prior to neutron irradiation. In embodiments, the Ga.sub.2O.sub.3 semiconductor wafer or boule material 200 is placed within a reactor chamber 202 and exposed to thermal neutron radiation from a neutron source 203 and moderator 204. As noted above, exposure to thermal neutron radiation transforms stable Ga isotopes to unstable isotopes, which in turn decay to stable Ge isotopes within the Ga.sub.2O.sub.3 semiconductor wafer or boule over a period of time to produce a Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule 206 having defects due to the neutron irradiation. The Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule 206 is then subjected to annealing in an oven 208 in accordance with embodiments of the invention to reduce defects and produce a final electrically conductive n-type Ge-doped Ga.sub.2O.sub.3 semiconductor wafer or boule 210.

[0025] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.