BISMUTH-DOPED SEMI-INSULATING GROUP III NITRIDE WAFER AND ITS PRODUCTION METHOD

Abstract

The present invention discloses a semi-insulating wafer of GaxAlyIn1-x-yN (0≦x≦1, 0≦x+y≦1) which is doped with bismuth (Bi). The semi-insulating wafer has the resistivity of 104 ohm-cm or more. Although it is very difficult to obtain a single crystal ingot of group III nitride, the ammonothermal method can grow highly-oriented poly or single crystal ingot of group III nitride having the density of dislocations/grain boundaries less than 105 cm−2. The invention also disclose the method of fabricating the semi-insulating group III nitride bulk crystals and wafers.

Description

BACKGROUND

1. Field of the Invention

The invention is related to a semiconductor wafer used for various devices including electronic devices such as transistors. More specifically, the invention is on a compound semiconductor wafer composed of group III nitride.

2. Description of the Existing Technology

(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)

Gallium nitride (GaN) and its related group III nitride alloys are the key material for various electronic devices such as microwave power transistors and solar-blind photo detectors. However, the majority of these devices are grown epitaxially on heterogeneous substrates (or wafers), such as sapphire and silicon carbide since GaN wafers are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end electronic devices, such high-power microwave transistors.

To solve all fundamental problems caused by heteroepitaxy, it is indispensable to utilize group III nitride wafers sliced from bulk group III nitride crystal ingots. For the majority of devices, GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow GaN crystal ingots. Currently, majority of commercially available GaN wafers are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is a vapor phase method, which has a difficulty in reducing dislocation density less than 10⁵ cm⁻².

To obtain high-quality GaN wafers of which density of dislocations and/or grain boundaries is less than 10⁵ cm⁻², a new method called ammonothermal growth has been developed [1-6]. Recently, high-quality GaN wafers having density of dislocations and/or grain boundaries less than 10⁵ cm⁻² can be obtained by the ammonothermal growth. However, a GaN ingot grown by the ammonothermal method typically shows n-type conductivity, which is not favorable to high-electron mobility transistors (HEMT). Due to high-frequency operation, conductive substrate causes high-level of capacitance loss through the substrate. To improve the performance of transistors, semi-insulating wafers are strongly demanded.

SUMMARY OF THE INVENTION

The present invention discloses a semi-insulating wafer of Ga_xAl_yIn_1-x-yN (0≦x≦1, 0≦x+y≦1) which is doped with bismuth (Bi). The semi-insulating wafer has the resistivity of 10⁴ ohm-cm or more. Although it is very difficult to obtain a single crystal ingot of group III nitride, the ammonothermal method can grow highly-oriented polycrystalline ingot of group III nitride having the density of dislocations/grain boundaries less than 10⁵ cm⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

FIG. 1 is a typical process flow to fabricate a Bi-doped semi-insulating group III nitride wafer.

FIG. 2 is a picture of a Bi-doped GaN bulk crystal.

In the figure each number represents the followings:

• • 1. a Bi-doped GaN bulk crystal

DETAILED DESCRIPTION OF THE INVENTION

Overview

The semi-insulating substrate of the present invention provides suitable properties for fabrication of GaN-based high-frequency transistors such as high electron mobility transistors (HEMT). Due to lack of single crystalline substrates of group III nitride, the GaN-based high-frequency transistors have been fabricated on so-called heteroepitaxial substrates such as sapphire, silicon and silicon carbide. Since the heteroepitaxial substrates are chemically and physically different from the group III nitride, the device contains high density of dislocations (10⁸˜10¹⁰ cm⁻²) generated enerated at the interface between the heteroepitaxial substrate and the device layer. Such dislocations deteriorate performance and reliability of devices, thus substrates composed of single crystalline group III nitride such as GaN and AlN have been developed. Currently, the majority of commercially available GaN substrates are produced using hydride vapor pressure epitaxy (HVPE), a process in which it is difficult to reduce the dislocation density to less than 10⁵ cm⁻². Although the dislocation density of HVPE-GaN substrates is a few orders of magnitude lower than GaN film on heteroepitaxial substrates, the dislocation density is still a few orders of magnitude higher than typical silicon devices in electronics. To achieve higher device performance, lower dislocation density is required. To attain dislocation density less than 10⁵ cm², ammonothermal growth, which utilizes supercritical ammonia, has been developed to date. Currently, the ammonothermal method can produce GaN wafers with dislocation density less than 10⁵ cm². However, the ammonothermal growth typically produces crystals with a high-level of electrons, thus the substrate is n type or n+ type. In order to use the low-defect ammonothermal wafers for transistor applications, it is necessary to increase the resistivity.

Technical Description of the Invention

The current invention of semi-insulating substrate is expected to maximize the benefit of low-dislocation group III substrates by the ammonothermal growth. To obtain semi-insulating GaN-based substrates, doping of transition metals has been proposed [7]. However, the disclosed method is focused on HVPE and not applicable for high-quality bulk crystal having dislocation density less than 10⁵ cm⁻². To attain low-dislocation group III nitride bulk crystals, bulk growth method such as ammonothermal method must be used. However, the doping efficiency of transition metals is not efficient in the ammonothermal growth. Therefore, we searched for a new dopant which is compatible with the ammonothermal method and will realize the semi-insulating characteristic in the group III-nitride substrate. Through our research we found Bi is suitable for doping in the ammonothermal growth.

A typical process flow for preparing Bi-doped semi-insulating group III nitride wafer is presented in FIG. 1. By using ammonothermal method or other bulk growth method (such as flux method, high-pressure solution growth), a Bi-doped group III nitride bulk crystal is grown. The Bi-doped group III nitride bulk crystal should be large enough to be sliced into wafers. Also the Bi-doped group III nitride bulk crystal should be highly oriented poly or single crystalline with density of dislocations and/or grain boundaries less than 10⁵ cm⁻². The Bi-doped group III nitride bulk crystal is sliced into wafers with a multiple wire saw, and then the wafers are finished by grinding, lapping and polishing. By using bulk growth method such as ammonothermal method, dislocation density less than 10⁵ cm⁻² can be attained. For commercial use, substrate larger than 1″ in diameter and thicker than 200 microns is favorable. The surface on which a device layer or structure is formed is polished to the atomic level. The polishing process typically consists of grinding with diamond tool, lapping with diamond slurry and polishing with colloidal silica. The finished wafer optionally can have a protective layer to protect the finished surface from chemical, physical and/or mechanical damage.

EXAMPLE 1

Bulk crystal of GaN was grown with the ammonothermal method using polycrystalline GaN as a nutrient, supercritical ammonia as a solvent, sodium (4.5 mol % to ammonia) as a mineralizer, and elemental Bi (6% to nutrient) as a dopant. The elemental Bi was placed together with the mineralizer. The growth temperature was between 500 to 600° C. and the growth was extended to 4 days. More than 200 micron-thick GaN was grown on a c-plane GaN seed crystal [FIG. 2]. The size of the crystal was approximately 10 mm². The crystal quality was characterized with X-ray diffraction. The full-width half maximum (FWHM) of 002 diffraction was 231 arcsec, and FWHM of 201 diffraction was 485 arcsec. Although diffraction from other planes such as 011, 110, 013, 020, 112, 004, 022, 121, 006 are not measured, similar number of FWHM is expected. Even with Bi doping, the crystal quality was not deteriorated significantly. The resistivity of the GaN crystal was measured with two-probe method. The separation of the proves was 2.5 mm and the applied voltage was 10 V. The current through the circuit was under the detection limit (10 micro A) of the ampere meter. From this experiment, we estimated the resistivity of 10⁴ ohm-cm or higher. As shown in FIG. 2, the crystal color was dark brown or black. Obviously, the optical absorption coefficient is higher than 10 cm⁻¹ throughout the visible wavelength range. Although the dislocation density was not measured on this particular crystal, we estimate the dislocation density at about 10⁵ cm⁻² or less based on the X-ray FWHM.

Advantages and Improvements

The current invention provides a semi-insulating III nitride wafers having dislocation density lower than 10⁵ cm². Bi is an efficient dopant to grow semi-insulating group III nitride bulk crystal by ammonothermal growth. Even with heavy doping of Bi, the crystal quality was not deteriorated significantly. Wafers fabricated by slicing the Bi-doped group III nitride bulk crystal is expected to be suitable for electronic devices such as HEMT.

Possible Modifications

Although the preferred embodiment describes GaN substrates, the substrate can be group III nitride alloys of various composition, such as AlN, AlGaN, InN, InGaN, or GaAlInN.

Although the preferred embodiment describes ammonothermal growth as a bulk growth method, other growth method such as high-pressure solution growth, flux growth, hydride vapor phase epitaxy, physical vapor transport, or sublimation growth can be used.

Although the preferred embodiment describes c-plane GaN, other orientations such as a-face, m-face, and various semipolar surface can also be used. In addition, the surface can be slightly miscut (off-sliced) from these orientations.

Although the preferred embodiment describes metallic Bi as a dopant, Bi doped group III nitride such as Bi doped polycrystalline GaN, Bi doped amorphous GaN, Bi doped single crystalline GaN can be used for better control of the doping concentration.

Although Bi-doped group III nitride crystals are discussed above, other elements such as sulfur, selenium, antimony, tin, or lead or any combination of these may act as a dopant instead of or in combination with bismuth for semi-insulating group III nitride crystals.

REFERENCES

The following references are incorporated by reference herein:

[1] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615.

[2] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,132,730.

[3] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 7,160,388.

[4] K. Fujito, T. Hashimoto, S. Nakamura, International Patent Application No. PCT/US2005/024239, WO07008198.

[5] T. Hashimoto, M. Saito, S. Nakamura, International Patent Application No. PCT/US2007/008743, WO007117689. See also US20070234946, U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.

[6] D'Eyelyn, U.S. Pat. No. 7,078,731.

[7] Vaudo, et al., U.S. Pat. No. 7,170,095.

Each of the references above is incorporated by reference in its entirety as if put forth in full herein, and particularly with respect to description of methods of making using ammonothermal methods and using these gallium nitride substrates.

What is disclosed by way of example and not by way of limitation is:

• • 1. A semi-insulating wafer comprising Ga_xAl_yIn_1-x-yN (0≦x≦1, 0≦x+y≦1) having a surface area more than 10 mm² and thickness more than 200 microns wherein the Ga_xAl_yIn_1-x-yN is doped with bismuth.
  • 2. A semi-insulating wafer of paragraph 1 wherein the resistivity of the wafer is higher than 10⁴ ohm-cm.
  • 3. A semi-insulating wafer of paragraph 1 or paragraph 2 wherein the crystal is fabricated by slicing a bulk ingot of Ga_xAl_yIn_1-x-yN (0≦x≦1, 0≦x+y≦1) grown in supercritical ammonia.
  • 4. A semi-insulating wafer of any of paragraph 1 through paragraph 3 wherein the wafer comprises highly-oriented poly or single crystal of Ga_xAl_yIn_1-x-yN (0≦x≦1, 0≦x+y≦1) with a density of line defects and grain boundaries less than 10⁵ cm⁻².
  • 5. A semi-insulating wafer of any of paragraph 1 through paragraph 4 wherein the wafer is GaN.
  • 6. A semi-insulating wafer of any of paragraphs 1-5 in which the Ga_xAl_yIn_1-x-yN doped with bismuth has a resistivity greater than about 10⁴ ohm-cm.
  • 7. A semi-insulating wafer of any of paragraphs 1-6 wherein the Ga_xAl_yIn_1-x-yN doped with bismuth has a density of dislocations and/or grain boundaries is less than 10⁵ cm⁻².
  • 8. A semi-insulating wafer of any of paragraphs 1-7 wherein the Ga_xAl_yIn_1-x-yN doped with bismuth is a layer upon a group III-nitride substrate.
  • 9. A semi-insulating wafer of any of paragraphs 1-7 wherein the wafer comprises, throughout the wafer, Ga_xAl_yIn_1-x-yN doped with bismuth.
  • 10. An electronic, optical, or opto-electronic device formed on a wafer of any of paragraphs 1-9.
  • 11. A method of growing a bismuth doped group III nitride bulk crystal having a surface area more than 10 mm² and thickness more than 200 microns comprising:
    • (a) charging group III containing nutrient in a high-pressure reactor;
    • (b) charging mineralizer in the high-pressure reactor;
    • (c) charging at least one seed crystal in the high-pressure reactor;
    • (d) charging bismuth-containing dopant in the high-pressure reactor;
    • (e) charging ammonia in the high-pressure reactor;
    • (f) sealing the high-pressure reactor;
    • (g) providing sufficient heat to the ammonia to create supercritical state of ammonia; and
    • (h) crystallizing group III nitride on the seed crystal.
  • 12. A method of growing bismuth doped group III nitride bulk crystal in paragraph 11 wherein the bismuth-containing dopant is selected from metallic bismuth and bismuth doped group III nitride.
  • 13. A method of growing bismuth doped group III nitride bulk crystal in paragraph 11 and paragraph 12 wherein the group III nitride crystallized on the seed crystal is GaN.
  • 14. A method of growing bismuth doped group III nitride bulk crystal from paragraph 11 through paragraph 13 wherein the seed crystal is GaN.
  • 15. A method of growing bismuth doped group III nitride bulk crystal from paragraph 11 through paragraph 14 wherein the mineralizer contains alkali metal.
  • 16. A method of growing bismuth doped group III nitride bulk crystal from paragraph 11 through paragraph 15 wherein the mineralizer contains sodium.
  • 17. A method of growing bismuth doped group III nitride bulk crystal from paragraph 11 through paragraph 16 wherein a sufficient amount of the bismuth-containing dopant is present such that the crystallized group III nitride is semi-insulating.
  • 18. A method of fabricating bismuth doped group III nitride semi-insulating wafers comprising the step of growing bismuth doped group III nitride bulk crystal as specified in paragraphs 11 through paragraph 17 and slicing of the bulk crystal.
  • 19. A method of fabricating bismuth doped group III nitride semi-insulating wafers in paragraph 18 further comprising polishing of the surface on which device is fabricated.

Claims

1. A semi-insulating wafer comprising GaxAlyIn1-x-yN (0≦x≦1, 0≦x+y≦1) having a surface area more than 10 mm2 and thickness more than 200 microns wherein the GaxAlyIn1-x-yN is doped with bismuth.

2. A semi-insulating wafer of claim 1 wherein the resistivity of the wafer is greater than 104 ohm-cm.

3. A semi-insulating wafer of claim 1 wherein the wafer is fabricated by slicing a bulk ingot of GaxAlyIn1-x-yN (0≦x≦1, 0≦x+y≦1) grown in supercritical ammonia.

4. A semi-insulating wafer of claim 1 wherein the wafer comprises highly-oriented poly or single crystalline GaxAlyIn1-x-yN (0≦x≦1, 0≦x+y≦1) with a density of line defects and grain boundaries less than 105 cm−2.

5. A semi-insulating wafer of claim 1 wherein the wafer is GaN.

6. A semi-insulating wafer of claim 1 in which the GaxAlyIn1-x-yN doped with bismuth has a resistivity greater than about 104 ohm-cm.

7. A semi-insulating wafer of claim 1 wherein the GaxAlyIn1-x-yN doped with bismuth has a density of dislocations and/or grain boundaries is less than 105 cm−2.

8. A semi-insulating wafer of claim 1 wherein the GaxAlyIn1-x-yN doped with bismuth is a layer upon a group III-nitride substrate.

9. A semi-insulating wafer of claim 1 wherein the wafer comprises, throughout the wafer, GaxAlyIn1-x-yN doped with bismuth.

10. An electronic, optical, or opto-electronic device formed on a wafer of claim 1.

11. A method of growing a bismuth doped group III nitride bulk crystal having a surface area greater than 10 mm2 and thickness greater than 200 microns comprising: a. placing a group III containing nutrient in a high-pressure reactor;b. placing a mineralizer in the high-pressure reactor;c. placing at least one seed crystal in the high-pressure reactor;d. placing a bismuth-containing dopant in the high-pressure reactor;e. placing ammonia in the high-pressure reactor;f. sealing the high-pressure reactor;g. providing sufficient heat to the ammonia to create a supercritical state of ammonia; andh. crystallizing group III nitride on the seed crystal.

12. A method according to claim 11 wherein the bismuth-containing dopant is selected from metallic bismuth and bismuth doped group III nitride.

13. A method according to claim 11 wherein the group III nitride crystallized on the seed crystal is GaN.

14. A method according to claim 11 wherein the seed crystal is GaN.

15. A method according to claim 11 wherein the mineralizer contains alkali metal.

16. A method according to claim 11 wherein the mineralizer contains sodium.

17. A method according to claim 11 wherein a sufficient amount of the bismuth-containing dopant is present such that the crystallized group III nitride is semi-insulating.

18. A method of fabricating bismuth doped group III nitride semi-insulating wafers comprising slicing the bulk crystal formed by the method of claim 11 to form a wafer.

19. A method according to claim 18 and further comprising polishing a surface of the wafer on which a device is to be fabricated.