WO2020018709A1

WO2020018709A1 - Method for the fabrication of electrically-conductive semiconductor layers

Info

Publication number: WO2020018709A1
Application number: PCT/US2019/042272
Authority: WO
Inventors: Christian J. ZOLLNER; Burhan SAIFADDIN; Abdullah ALMOGBEL; Michael Iza; James S. Speck; Shuji Nakamura; Steven P. Denbaars
Original assignee: The Regents Of The University Of California
Priority date: 2018-07-18
Filing date: 2019-07-17
Publication date: 2020-01-23

Abstract

A method for treating nitride-based semiconductor layers. The method includes using a dopant-containing substrate material for growth of the layers, and then annealing the layers and substrate such that the substrate acts as a diffusion source for the dopant atoms. The method includes treating the layers in some condition, such as a controlled ambient and/or thermal annealing, such that the dopant atoms diffuse from the substrate into the layers to improve some desired characteristic such as electrical conductivity.

Description

METHOD FOR TOE FABRICATION OF

ELECTRICALLY -CONDUCTIVE SEMICONDUCTOR LAYERS

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned application:

U.S. Provisional Application Serial No. 62/700,077, filed on July 18, 2018, by Christian J. Zollner, Burhan K. Saifaddin, Abdullah Almogbel, Michael Iza, James S. Speck, Shuji Nakamura and Steven P. DenBaars, entitled“METHOD FOR THE FABRICATION OF ELECTRICALLY -CONDU CTIVE SEMICONDUCTOR LAYERS,” attorneys’ docket number G&C 30794.0686USP1 (UC 2018-762-1); which application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Fi el d of the Inv ention.

This invention relates to a method for the production of an electrically - conductive semiconductor layers via high temperature annealing, wherein the substrate material acts as a source for the dopant element in the annealed material.

2. Description of the Related Art.

(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x] A list of these different publications ordered according to these reference numbers can be found below- in the section entitled“References.” Each of these publications is incoiporated by reference herein.)

The usefulness of gallium nitride (GaN), aluminum nitride (AIN), and their ternary and quaternary compounds incorporating gallium, aluminum and indium (AlGaN, InGaN, AllnGaN), has been well established for the fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques comprising molecular beam epitaxy (MBE), metalorganic chemical vapor deposition

(MOCVD), and hydride vapor phase epitaxy (HYPE).

Additionally, the development of AiGaN and AIN for short wavelength devices enabled nitride-based light emitting diodes (LEDs) and laser diodes (LDs) to overtake many other research ventures. Additionally, A!GaN and GaN have found wide popularity in power electronics applications, and AIN is expected to have many benefits in transistors and other electronic devices due to its high breakdown field and electron mobility. Consequently, AiGaN and AIN based materials and devices have become the dominant material system used for ultraviolet light semiconductor applications, and are of great research and industrial interest for electronics applications of many kinds.

However, the growth of highly conductive AIN and AiGaN with high aluminum content remains a challenge. These materials, along with all other III- mtride-based semiconductor materials, can be made conductive or insulative by doping with Si, Ge, Mg, or some other hetero valent impurity'. Growing materials doped with such impurities is challenging due to reduced material quality'

(smoothness, dislocation content) and lo solid solubility. Another common method for doping semiconductors is ion implantation; however, this is well known to damage the material and is limited in achievable penetration depth and uniformity. On the other hand, annealing semiconductor samples ex-situ has been shown to increase material quality. [1 -7]

SUMMARY OF THE INVENTION

In this disclosure, a method of doping AIN or AiGaN films by post-growth annealing is proposed, wherein the substrate material acts as a source for dopant atoms. By annealing at high temperatures, bulk diffusion of the dopant atoms from the substrate into the films can produce heavily-doped films of high crystalline quality, which may not be achievable by in-situ doping during growth or doping by ion implantation.

9 Specifically, the present invention discloses a method for annealing a nitride- based semiconductor film, which is grown on a substrate containing one or more atomic species that function as a dopant (donor or acceptor) in the nitride-based semiconductor film.

The annealing may comprise heating the substrate and film to a temperature greater than about 1000°C. The treatment may also comprise exposing the substrate and film to an inert environment, or to an ambient or low-pressure atmosphere that contains some argon (Ar), hydrogen (H₂), nitrogen (N₂), oxygen (O2), ammonia (NH3), or some other forming gas, or some other process gas. Using this treatment, various layers of the nitride-based semiconductor film can be made conductive via the bulk diffusion of dopant atoms from the substrate into the film.

The substrate may be comprised of silicon carbide (SiC), magnesium aluminate spinel (MgAkCri), magnesium oxide (MgO), or any other substrate material containing oxygen (O), silicon (Si), germanium (Ge), zinc (Zn), magnesium (Mg), iron (Fe), phosphorous (P), boron (B), sulfur (S), selenium (Se), beryllium (Be), or fluorine (F), or any other possible dopant atoms.

The semiconductor film may be comprised of one or more nitride-based layers. The terms“nitride-based” or“PI-niirides” or“nitrides” refer to any alloy composition of the (Ga, Al,In,B)N semiconductors having the formula Ga«ALInyB_zN where:

0 £ n £ 1, 0 < x < 1, 0 £y < 1, 0 < z < 1, and n + x +y + 2 = 1.

The nitride-based layers may be grown using deposition methods comprising conventional chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), sputtering, atomic layer deposition (ALD), evaporation under vacuum or controlled ambients, ion beam deposition (IBD), hydride vapor phase epitaxy (HVPE), metalorgamc chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).

The nitride-based layers may be grown in any crystallographic direction, such as on a conventional c-plane oriented nitride-based semiconductor crystal, or on a nonpolar plane, such as a-plane or m-plane, or on any semipolar plane, such as {20-21 }, {11-22} and {10-11 }.

The present invention also discloses a material having enhanced electrical (n- type) conductivity when processed using the method.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG 1 is a flowchart describing the process steps used in one embodiment of the invention.

FIG. 2 is a graph of Si (crn-3) vs. etch depth (a.u.) that plots secondary -ion mass spectrometry data for annealed AIN films on SiC substrates, wherein, after annealing at 1700° C for 60 min, an Si concentration of 2el9 cnr³ or greater is achievable.

DETAILED DESCRIPTION OF TOE INVENTION

In the foll owing description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and m winch is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Overview

The present invention describes a method for treating a semiconductor film containing one or more nitride-based layers, where the fact that the film is grown on a substrate containing some dopant atoms allows for diffusion of dopant atoms from the substrate into the film at high temperatures. High-temperature annealing will not degrade the quality of substrate, and has actually been reported to increase the crystal quality of the semiconductor film as well. Doping the semiconductor film allows for higher dopant concentrations, better crystal quality, and eliminates surface damage associated with doping during growth and ion implantation, respectively.

In this disclosure, the following terms are defined:

® The term“treatment” refers to the placement of the sample in a condition such as high-temperature annealing and/or exposure to some process gas with the goal of improving or changing some material or device characteristic.

* The term“high temperature” refers to substrate temperatures greater than 1 QQQ°C. For practical purposes, in one embodiment, high temperature refers to temperatures of l000°C to 2500°C.

Temperatures above 2500°C require specialized equipment and processes, and therefore are not practical for most applications.

® The term“process gas” refers to any gas commonly used in

semiconductor processing, such as nitrogen, argon, ammonia, hydrogen, oxygen, a forming gas, or another process gas, etc.

The current state of the art in nitride heteroepitaxy involves growing nitride- based layers on foreign substrates, such as sapphire (most common) and Si (less common). This invention uses substrates capable of high-temperature annealing and which contain some dopant atoms (SiC, MgAbCri, MgO, and other dopant- containing oxide, semi -insulating, or semi-conducting substrates). The present inventi on provides a means of enhancing the electri cal characteristi cs of nitride- based layers by doping these materials via bulk-diffusion during high-temperature annealing.

Technical Description

The present invention describes a method for treating a nitride-based semiconductor film grown on a substrate material containing some dopant atoms. The annealing of the nitride-based semiconductor fil on the dopant-containing substrate results in higher concentration of the dopant atom in the film than can s often be achieved by doping during growth or via ion-implantation, which damages the crystal.

These films were grown using a commercially available MOCVD system. The mechanism of semiconductor film doping and conductivity enhancement is independent of, growth method, or processing condition particulars.

Using this method, nitride-based semiconductor films can be treated and exhibit enhanced dopant concentration and electrical conductivity. Thus, a wider variety of electronic and/or light-emitting structures is made possible, since growth or processing steps which would normally result m low dopant concentration and/or implantation damage can be avoided. This can result in improved device performance for nitride-based semiconductor films treated using the above described method.

Process Steps

FIG. 1 is a flowchart of process steps used in one embodiment of the present invention for treating one or more nitride-based semiconductor layers or films grown upon a dopant-containing substrate.

Block 100 represents the step of growing one or more nitride-based semiconductor layers or films on a substrate using any growth technique.

In alternative embodiments, the nitride-based semiconductor layers or films have a c-plane oriented surface, an a-plane oriented surface, an m-plane oriented surface, or a semi polar oriented surface. Also, in alternative embodiments, the substrate is comprised of silicon carbide (SiC), spinel i Y!gAbO ). magnesium oxide (MgO), or some other non-nitride substrate. Moreover, in alternative embodiments, the substrate contains one or more atomic species that function as dopant elements, including oxygen (O), silicon (Si), germanium (Ge), zinc (Zn), magnesium (Mg), iron (Fe), phosphorous (P), boron (B), sulfur (S), selenium (Se), beryllium (Be), and/or fluorine (F). In the embodiment described further herein, the nitride-based semiconductor film is an AIN film grown on a SiC substrate by MOCVD. As grown, the film is highly resistive due to the low Si concentration in the MOCVD grown material.

Block 101 represents the step of thermal annealing of the nitride-based semiconductor layers or films and the substrate. Specifically, this step involves heating a sample comprised of the nitride-based semiconductor layers or films and the substrate with a duration and temperature sufficient to allow the dopants to diffuse from the substrate into the nitride-based semiconductor layers or films.

In alternative embodiments, the heating occurs at a temperature of about 1000°C to about 2500°C, and at a pressure less than about 100 atmospheres.

Moreover, in alternative embodiments, the heating occurs in an inert environment, or in the presence of nitrogen (N₂), or in the presence of argon (Ar), or in the presence of ammonia (Nth), or in the presence of hydrogen (¾), or in the presence of oxygen (Oz), or in the presence of a forming gas, or in the presence of another process gas.

In the embodiment described further herein, annealing an A1N film up to 1000 nm thick for 60 minutes at about 1700°C is sufficient to increase the Si concentration in the film by two orders of magnitude, and to make the film electrically conductive.

Block 102 represents the step of further processing, such as additional growth, characterization, packaging, etc., to produce a semiconductor device. In one embodiment, the nitride-based semiconductor layers or films could be used as a conductive growth template for the growth of subsequent semiconducting layers, or could be characterized, or could be packaged into an electronic or opto-electronic device.

Experimental Results

FIG. 2 is a graph of Si (cm-3) vs. etch depth (a.u.) that plots secondary ion mass spectroscopy (SIMS) data showing the Si concentration for three samples comprised of AIN films grown on SiC substrates; (1) where the films are as-grown (with an Si concentration of 4el 7 cm ³), (2) following an anneal at 1700°C for 30 minutes, and (3) following an anneal at l700°C for 60 minutes (with an Si concentration of 2el9 cm ³ or greater).

The main concern is the highly uniform Si concentration in the sample annealed at 1700°C for 60 minutes, with doping levels high enough for many (opto-)e!ectronic applications. The Si concentrations have been calibrated using ion- implanted AlGaN:Si reference samples. The SIMS etch depth profile is not quantitative, but the sharp spike in Si concentration marks the SiC substrate.

The sample thickness into which the Si has diffused to nearly uniform levels is known from other measurements (refiectometry, spectroscopic e!lipsometry, electron microscopy) to be about 800 nm. Thinner samples and longer anneal durations may allo higher Si concentrations

Note that, after 30 minutes of the anneal, the Si has only partially diffused into the sample. This is evidenced by the relatively steep slope in the etch depth profile as compared to that of the 60 minute annealed sample. The sense of the slope also confirms that the SiC substrate is acting as a diffusion source of Si atoms.

AIN samples annealed for 6, 12, 30, and 60 minutes (with thicknesses varying from 700 nm - 1000 nm) all showed enhanced electrical conductivity as measured using In-solder contacts. While a full Hall effect experiment has not been done due to equipment limitations, resistance-based results suggest resistivities on the order of 1 kQ-cm or less based on similar measurements on MOCVD-grown n- AlGaN reference samples of known resistivity and with similar Si concentrations.

Modifications and Alternatives

The present invention is intended to include the nitride-based semiconductor layers or films grown or processed using the steps described above.

The substrates employed may comprise SiC, MgAkOg MgO, or any other dopant-containing material. The substrate can be insulating, semi-insulating, or semiconducting. The nitride-based semiconductor layers or films may comprise a nitride alloy that contains some aluminum, such as AIN, AlInN, AlGaN, A1BN or AllnGaN, for example.

Also, the nitride-based semiconductor layers or films may be comprised of various thicknesses.

Advantages and Improvements

The present invention considers the substrate characteristics of the nitride- based semiconductor layers or films needed to ensure the presence of dopant elements and enhancement of electrical conductivity during high-temperature annealing treatment. The key advantage of the dopant-containing substrate and annealing is that the nitride-based semiconductor layers or films can be altered in some desirable way without the need for doping in-situ during growth or by ion implantation, which have many limitations discussed above.

It can be seen that using the substrate to act as a source for dopant atoms to diffuse from the substrate into the nitride-based semiconductor layers or films improves a characteristic of the nitride-based semiconductor layers or films after the annealing as compared the nitride-based semiconductor layers or films before the annealing. Specifically, the nitride-based semiconductor layers or films have an enhanced electrical conductivity after the annealing as compared to the nitride-based semiconductor layers or films before the annealing. Moreover, the nitride-based semiconductor layers or films are more heavily doped as compared to doping of the nitride-based semiconductor layers or films in-situ during growth or by ion implantation.

Increasing the dopant density and enhancing electrical conductivity (or some other desirable quality) by doping the samples in this way improve device efficiencies and allows for the development of new device types. Highly doped epitaxial layers are essential to fabricate high-quality, low-loss and reliable radiation emitting semiconductor device such as LEDs or LDs, and semiconductor electronics. References

The following references are incorporated by reference herein:

[1] Y. Lan and Y. Shi,“Effect of working pressure and annealing temperature on single-phase AIN films,” Mater. Lett., vol. 213, pp. 1-3, 2018.

[2] C.-Y. Huang et al.,“High-quality and highly-transparent AIN template on annealed sputter-deposited AIN buffer layer for deep ultra-violet light- emitting diodes,” AIP Adv., vol. 7, no. 5, p. 55110, 2017.

[3] H. Miyake, C. H. Lin, K. Tokoro, and K. Hiramatsu,“Preparation of high-quality AIN on sapphire by high-temperature face-to-face annealing,” I. Cryst. Growth, vol. 456, pp. 155-159, 2016.

[4] H. Miyake et al.,“Annealing of an AIN buffer layer m N2-CO for growth of a high-quality AIN film on sapphire,” Appl. Phys. Express, vol. 9, no. 2, 2016.

15] H. Miyake, K. Matsumoto, A. Mishima, Y. Tomita, Y. Yano, and T. Tabuchi,“Characteristics of AIN layer on four-inch sapphire substrate by high- temperature annealing in nitrogen atmosphere,” Gall. Nitride Mater. Devices XIII, 1053204, 2018

[6] N. Susilo et al.,“AlGaN-based deep UV LEDs grown on sputtered and high temperature annealed AlN/sapphire,” Appl. Phys. Lett., vol. 112, no. 4, 2018.

[7] M. Nemoz, R Dagher, S. Matta, A. Michon, P. Vennegues, and I Brault,“Dislocation densities reduction in MBE-grown AIN thin films by high- temperature annealing,” J. Cryst. Growth, vol. 461, pp. 10-15, 2017.

Conclusion

This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method for treating a semiconductor layer, comprising:

(a) depositing at least one semiconductor layer on or above a substrate containing at least one dopant for the semiconductor layer: and

(b) heating a sample comprised of the semiconductor layer and substrate to diffuse the dopant from the substrate into the semiconductor layer.

2. The method of claim 1, wherein the treated semiconductor layer has a concentration of the dopant of about 2el 9 cm ³ or greater.

3. The method of claim 1, wherein the treated semiconductor layer has a resistivity of about 1 kO-cm or less.

4. The method of claim 1, wherein the heating occurs at a temperature of about 1()()0°C to about 2500°C.

5. The method of claim 1, wherein the heating occurs at a pressure less than about 100 atmospheres.

6. The method of claim 1 , wherein the heating occurs in an inert environment.

7. The method of claim 1, wherein the heating occurs in the presence of argon, hydrogen, nitrogen, oxygen, ammonia, a forming gas, and/or a process gas.

8 The method of claim 1, wherein the substrate is comprised of silicon carbide, spinel, magnesium oxide, or another non-nitride substrate. i _^

9. The method of claim 1, wherein the dopant comprises oxygen, silicon, germanium, zinc, magnesium, iron, phosphorous, boron, sulfur, selenium, beryllium, and/or fluorine.

10. The method of claim 1, wherein the semiconductor layer comprises a nitride-based semiconductor layer.

1 1. The method of claim 1, wherein the semiconductor layer has a c- plane oriented surface, an a-plane oriented surface, an m-plane oriented surface, or a semipolar oriented surface.

12. A semiconductor layer treated by the method of claim 1

13. A method, comprising:

treating one or more nitride-based semiconductor films grown upon a dopant-containing substrate by annealing the nitride-based semiconductor films and substrate, such that the substrate acts as a source for dopant atoms to diffuse from the substrate into the nitride-based semiconductor films to improve a characteristic of the nitride-based semiconductor films after the annealing as compared the nitride- based semiconductor films before the annealing.

14. The method of claim 13, wherein the annealing step comprises heating the nitride-based semiconductor films and substrate to a temperature greater than about 1000°C.

15. The method of claim 13, wherein the nitride-based semiconductor films are more heavily doped as compared to doping of the nitride-based semiconductor films in-situ during growth or by ion implantation.

16. One or more nitride-based semiconductor films treated by the method of claim 13.

17. A method, comprising:

annealing a nitride-based semiconductor film, which is grown on a substrate containing one or more atomic species that function as a dopant in the mtride-based semiconductor film, such that the atomic species diffuse from the substrate into the nitride-based semiconductor film.

18. The method of claim 17, wherein the annealing step comprises heating the nitride-based semiconductor film and substrate to a temperature greater than about 1000°C.

19. The method of claim 17, wherein the nitride-based semiconductor film has enhanced electrical conductivity after the annealing as compared to the nitride-based semiconductor film before the annealing.

20. A nitride-based semiconductor film annealed by the method of claim

17.