Chemical vapor deposition of germanium nitride on group-iii nitride semiconductors

Abstract

Germanium nitride films are deposited in-situ on group-III nitride semiconductor in a growth chamber to prevent the group-III nitride semiconductor's surface from reacting with ambient constituents, e.g., surface oxidation processes, in order to ensure stable properties of resulting semiconductor devices fabricated from an epitaxial layer stack. Using germanium nitride instead of the commonly-used silicon nitride for surface passivation of group-III nitride semiconductor eliminates any negative side effects associated with the use of silicon nitride, such as difficulties in reactor chamber cleaning, particle formation and yield losses. Specifically, the lower thermal stability of germanium in comparison to silicon nitride eases the removal of germanium nitride from reactor chamber parts, as it can be removed via thermal evaporation. Germanium nitride can also be used as a dielectric layer.

Description

[0001] CHEMICAL VAPOR DEPOSITION OF GERMANIUM NITRIDE ON GROUP-III NITRIDE SEMICONDUCTORS

[0002] CROSS REFERENCE TO RELATED APPLICATION

[0003] This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned application:

[0004] U.S. Provisional Application Serial No. 63 / 626,369, filed on January 29, 2024, by Stacia Keller, Vineeta Muthuraj, Steven P. DenBaars, and Umesh Mishra, entitled ‘CHEMICAL VAPOR DEPOSITION OF GERMANIUM NITRIDE ON GROUP-III NITRIDE SEMICONDUCTORS,” attorneys’ docket number G&C 30794.0854USP1 (UC-2024-879-1); which application is incorporated by reference herein.

[0005] BACKGROUND OF THE INVENTION

[0006] 1. Field of the Invention.

[0007] This invention relates to a method of depositing germanium nitride on group- ill nitride semiconductors.

[0008] 2. Description of the Related Art.

[0009] In semiconductor device fabrication, silicon nitride (Si3N4) films are often used for surface passivation of group-III nitride semiconductors to prevent surface oxidation. However, the use of silicon nitride for surface passivation has negative side effects, such as difficulties in reactor chamber cleaning, particle formation and yield losses.

[0010] Thus, there is a need in the art for improvements to the fabrication methods used for group-III nitride semiconductors. The present invention satisfies this need. SUMMARY OF THE INVENTION

[0011] In the present invention, germanium nitride films are deposited in-situ on a group-III nitride semiconductor in a growth chamber to prevent the semiconductor's surface from reacting with ambient constituents, e.g., surface oxidation processes, in order to ensure stable properties of resulting semiconductor devices fabricated from an epitaxial layer stack.

[0012] Using germanium nitride instead of the commonly used silicon nitride for surface passivation of group-III nitride semiconductor eliminates any negative side effects associated with the use of silicon nitride, such as difficulties in reactor chamber cleaning, particle formation and yield losses.

[0013] Specifically, the lower thermal stability of germanium in comparison to silicon nitride eases the removal of germanium nitride from reactor chamber parts, as it can be removed via thermal evaporation. For applications where the surface passivation layer on top of a group-III nitride semiconductor heterostructure needs to be taken off, germanium nitride can be removed via thermal evaporation as well.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figs. 1(a), 1(b), 1(c) and 1(d) are schematic structures of an epitaxial layer stack for N-polar GaN transistors and a transistor device using germanium nitride.

[0016] Figs. 2(a) and 2(b) are schematic structures of an epitaxial layer stack for Ga- polar GaN transistors and a transistor device using germanium nitride.

[0017] Fig. 3 is a flowchart showing the process flow for using germanium nitride as a protection layer for fabrication procedures requiring regrowlh.

[0018] Fig. 4 is a graph of Growth Rate (A / s) vs. Growth Temperature (°C) showing the dependence of the germanium nitride grow th rate on the deposition temperature.

[0019] Fig. 5 is a graph of Growth Rate (A / s) vs. NHs Flow (SLM) showing the dependence of the germanium nitride growth rate on the ammonia (NH3) flow. Fig. 6 is a graph of Growth Rate (A / s) vs. Pressure (kPa) showing the dependence of the germanium nitride growth rate on the pressure in the growth chamber.

[0020] DETAILED DESCRIPTION OF THE INVENTION

[0021] In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which 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.

[0022] Overview

[0023] The present invention discloses germanium nitride films that are deposited in- situ on group-III nitride semiconductors, such as N-polar group-III nitride semiconductor and metal-polar or Ga-polar group-III nitride semiconductor. Germanium nitride films are especially important for the fabrication of N-polar group-III nitride semiconductor heterostructures because of the higher chemical reactivity of N-polar compared to metal-polar (e.g., Ga-polar) surfaces. The thin germanium nitride surface layers prevent any reaction with ambient constituents after removal of the semiconductor wafer from a growth chamber, preserving the properties of the grown heterostructure. In addition, thin germanium nitride surface layers ease the processing procedure of N-polar group-III nitride semiconductor wafers and save processing costs.

[0024] The need of surface passivation of group-III nitride structures was first discovered in conjunction with the fabrication of GaN based transistors composed of AlGaN / GaN heterostructures . In undoped structures the internal electric fields resulting from the polarization when grown in the typical c-direction predominantly result in the formation of a 2-dimensional electron gas (2DEG) at the AlGaN / GaN interface. The current understanding is that the electrons forming the 2DEG are supplied from states on the AlGaN surface. Therefore, any changes of the AlGaN surface led to a change in the charge density' of the 2DEG. To obtain stable transistor operation, methods were developed to prevent any interaction of ambient constituents with the semiconductor’s surface by capping the surface with a passivation layer, typically silicon nitride (SisN4).

[0025] As a result, the passivation with Si.fNr not only prevented unwanted surface reactions, but also reduced the relaxation and cracking of the tensile-strained AlGaN layer on top of the GaN base layer (AIN has a smaller lattice constant than GaN).

[0026] In order to take full advantage of these two effects, the S13N4 layers are deposited in-situ on top of the group-III nitride semiconductor’s surface immediately' after the group-III nitride semiconductor’s growth without any ambient exposure of the group-III nitride semiconductor's surface, typically within the growth chamber of the group-ill nitride semiconductor.

[0027] If no longer needed, the S13N4 cap layers are etched off ex-situ via additional external processing steps.

[0028] The deposition of Si.^Nr layers inside of the growth chamber used for the group-III nitride semiconductor, however, can lead to particle formation as it can be difficult to remove S13N4 from the chamber’s parts. Particle formation leads to lower process yields.

[0029] In this new process, the ty pical SisN4 cap layers are replaced by germanium nitride layers (GesN4).

[0030] Germanium nitride has previously been used as a dielectric on germanium, often formed via nitridation of the germanium crystal surface. Germanium nitride has a band gap of 4.7 eV, it is resistive like SisN4, and therefore does not interfere with electrical testing of group-III nitride semiconductor heterostructures.

[0031] The lower thermal stability of germanium, as compared to silicon nitride, enables the easy removal of GesN4 deposits in the group-III nitride semiconductor’s growth chamber via thermal evaporation. In addition, thin germanium nitride surface layers can be removed from the group-III nitride semiconductor’s surface by thermal desorption within the grow th chamber. This is attractive when a group-III nitride semiconductor sample needs to be processed prior to continuation of the growth of the remaining layer structure, for example.

[0032] Example Structures

[0033] Examples for using germanium nitride as protection and / or surface passivation layer include but are not limited to the following:

[0034] 1. N-polar group-III nitride layer structures, because of the higher chemical reactivity of N-polar compared to Ga-polar surfaces, as shown in Figs. 1(a), 1(b), 1(c) and 1(d), which are schematic structures of an epitaxial layer stack for N- polar GaN transistors and a transistor device using germanium nitride. Figs. 1(a) and 1(b) are schematic structures comprising an epitaxial stack of GaN or Al GaN 100, AlGaN 101, GaN 102, and germanium nitride 103, wherein Fig. 1(b) also shows the 2DEG 104 and includes a source 105, drain 106 and gate 107. such that the GaN 102 is an active region of the device, and the germanium nitride 103 is a dielectric or passivation layer between the gate 107 and the active region. Figs. 1(c) and 1(d) are schematic structures comprising an epitaxial stack of AlGaN or AIN 108, GaN 109, and germanium nitride 110, wherein Fig. 1(d) also shows the 2DEG 111 and includes a source 112, drain 113 and gate 114, such that the GaN 109 is an active region of the device, and the germanium nitride 110 is a dielectric or passivation layer between the gate 114 and the active region.

[0035] 2. For Ga-polar high electron mobility transistors (HEMTs): GesN4 on AlGaN / GaN, AlN / GaN, AlInN / GaN, AlGalnN / GaN, AlScN / GaN or AlGaScN / GaN HEMTs are examples, as shown in Figs. 2(a) and 2(b), which are schematic structures of an epitaxial layer stack for Ga-polar GaN transistors and a transistor device using germanium nitride. These barrier / channel structures can be not only on GaN base layers, but also AlGaN or AIN. The channel can be any group-III nitride semiconductor material. Examples for HEMTs with a channel other than GaN are AlN / AlGaN or AlGaN / InGaN. Figs. 2(a) and 2(b) are schematic structures comprising an epitaxial stack of GaN or Al GaN or AIN 200, GaN 201, Al GaN 202, and germanium nitride 203, wherein Fig. 2(b) also shows the 2DEG 204 and includes a source 205, drain 206 and gate 207, such that the AlGaN 202 is an active region of the device, and the germanium nitride 203 is a dielectric or passivation layer between the gate 207 and the active region.

[0036] 3. For group-III nitride optoelectronics, including but not limited to, light emitting diodes, laser diodes, photodetectors, solar cells, or other electronic or optoelectronic devices, of any wavelength or frequency operation range.

[0037] 4. For photonic applications, for example, a waveguide on a group-III nitride semiconductor surface may be comprised of germanium nitride.

[0038] 5. The germanium nitride of these various devices may have a thickness in a range of 0.2 - 500 nm, 0.2-100 nm, 0.20 - 25 nm, or 0.2-325 nm.

[0039] 6. For preserving the surface / regrowth interface, which is of interest, in particular, for regrowth on Al-containing lay ers, which otherwise form strong surface oxides that do not evaporate during heating of the sample. This is shown in Fig. 3. which is a flowchart showing the process flow' for using germanium nitride as a protection layer w'hen fabricating a semiconductor device with procedures requiring regrowth, including the steps of: growing a group-III nitride semiconductor, such as an AlGaN wafer, in a growth chamber, such as a metal-organic chemical vapor deposition (MOCVD) growth chamber (300), depositing a germanium nitride layer on or above a surface of the group-III nitride semiconductor in-situ in the growth chamber (301), w herein the deposited germanium nitride prevents any reaction of the surface of the group-III nitride semiconductor with ambient constituents after removing the resulting sample comprised of the group-III nitride semiconductor and the germanium nitride from the MOCVD growth chamber (302), storing or processing the sample, e.g., etch mesas or tiles, etc. (303), reloading the stored or processed sample in the MOCVD growth chamber to remove the germanium nitride (304), heating the sample in the MOCVD grow th chamber to remove the germanium nitride layer (305), and then depositing additional layers on the sample in the MOCVD growth chamber (306). Using this process, the surface of the group-III nitride semiconductor is never exposed to air. and no surface reactions with ambient constituents can occur which could alter the surface. As a result, the need for extensive cleaning of the surface of the group-III nitride semiconductor prior to regrowth is eliminated.

[0040] These process steps may include the following alternatives: the growing of the group-III nitride semiconductor and the depositing of the germanium nitride may be performed in-situ in the growth chamber; the germanium nitride may be deposited without the group-III nitride semiconductor being exposed outside the growth chamber following the growing of the group-ill nitride semiconductor; the depositing of the germanium nitride may be performed in a separate chamber of the growth chamber from the grow ing of the group-III nitride semiconductor; and the depositing of the germanium nitride may be performed ex-situ following the growing of the group-III nitride semiconductor in the growth chamber.

[0041] The heat treatment to evaporate the germanium nitride layer is preferentially performed in nitrogen carrier gas or nitrogen-ammonia gas mixtures to minimize damage of the group-III nitride semiconductor underneath the germanium nitride.

[0042] Alternatives and Modifications

[0043] In one example, a template or substrate for a group-III nitride device comprises an aluminum-containing nitride, such as an AlGaN or AIN substrate or template, and a germanium nitride cap layer is deposited directly on the aluminum- containing nitride. The AlGaN or AIN substrate or template is later used as a base layer or a template for the deposition of epitaxial layer structures for electronic or optoelectronic devices. Prior to the deposition of device layers, the sample is heated inside the nitride growth chamber, and the germanium nitride film evaporates from the AIN or AlGaN template or substrate, leaving a pristine surface behind. The end result is a group-III nitride device (transistor or optoelectronic) on a surface of an AlGaN or AIN substrate or template, or on a surface of a heterostructure comprising AlGaN / GaN, wherein the AlGaN or AIN has at least one of a threading dislocation density, relaxation, stress, or surface states characterized by passivation / protection using a germanium nitride cap layer that was removed by evaporation prior to depositing the group-III nitride device on a surface in an oxygen deficient or oxygen free environment, wherein the environment is free of silicon and / or free of an organic compound, and other contaminants, and the AlGaN or AIN is not oxidized. In the absence of the germanium nitride cap layer, an Al-containing oxide layer would form on the AlGaN or AIN substrate or template, which does not evaporate when heating the sample to typical growth temperatures, leading to re-nucleation and threading dislocation formation in any regrown layers.

[0044] In another example, a group-III nitride device (e.g., substrate, template, transistor, or optoelectronic device) is fabricated having a passivation layer comprising germanium nitride on AlGaN, AIN or GaN, an indium-containing nitride such as InN or InGaN, or a scandium-containing nitride such as AlScN or AlGaScN, or a yttrium containing nitride such as A1YN or AlGaYN.

[0045] In yet another example, a high electron mobility transistor (HEMT) is fabricated, comprising: a source and drain contact to an N-polar heterostructure comprising an AlGaN, AIN or other (Al,Ga,In,Sc,Y)N barrier confining a GaN channel; a gate; and a dielectric comprising germanium nitride between the gate and the heterostructure. The dielectric may further comprise silicon nitride on the germanium nitride, where a thickness of the silicon nitride and the germanium nitride is selected to tailor a stress of the heterostructure and dielectric properties of the layer stack comprising germanium nitride and silicon nitride.

[0046] In still another example, the germanium nitride can be used as dielectric layer on group-III nitride transistors. Replacing silicon nitride by germanium nitride affects the interface state density at the semiconductor / dielectric interface and influences the stress in the dielectric layer.

[0047] The germanium nitride could be applied as single dielectric layer or in combination with silicon nitride. On top of the group-III nitride semiconductor surface, the following layers can be deposited, for example:

[0048] 1. Depositing silicon nitride on or above the germanium nitride;

[0049] 2. Depositing silicon nitride on or above the group-III nitride semiconductor before depositing the germanium nitride on or above the silicon nitride; and

[0050] 3. Depositing alternating layers of the germanium nitride and silicon nitride as a multi-layer stack.

[0051] Using germanium nitride layers in combination with silicon nitride layers allows for tuning of stress as well as dielectric properties of the layer stack, including an interface state density between the semiconductor and dielectric films. Moreover, a device comprising a group-III nitride heterostructure may have a layer stack of germanium nitride and silicon nitride layers on the heterostructure, with the layer stack having a thickness tailoring the stress of the heterostructure and the dielectric properties of the layer stack.

[0052] In addition, a silicon nitride / germanium nitride layer stack can be used as mirror.

[0053] Instead of stacks of binary silicon and germanium nitride layers, silicon germanium nitride ternary alloy layers can be used.

[0054] This invention can be used for all group-III nitrides, including their alloys with other elements, as well as for other wide bandgap semiconductors, such as silicon carbide and gallium oxide, for example. The terms ‘‘group-III nitride” or “Ill-nitride” or “nitride” as used herein refer to any composition or material related to (B, Al, Ga, In, Sc, Y)(N, P, As, Sb) semiconductors. The germanium nitride films can be deposited on a group-III nitride semiconductor film having any polarity, e.g., polar c-plane layers, non-polar a-plane or m-plane layers, or semi-polar layers.

[0055] The germanium nitride layers can be deposited on other III-V semiconductors, such as (Al,Ga,In)(P,As,Sb).

[0056] The germanium nitride film can be deposited using any deposition technique, including but not limited to, chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD). molecular beam epitaxy (MBE) and atomic layer deposition (ALD).

[0057] While preferentially deposited in-situ in the same growth chamber as the group-III nitride material, the germanium nitride films can also be deposited ex-situ on top of a group-III nitride film, or in a separate chamber if using a multi-chamber tool.

[0058] The germanium nitride films can be deposited on or above sidewalls or other three-dimensional (3D) features of the group-III nitride semiconductor, e.g., mesas, pillars or stripes.

[0059] The low growth temperature of germanium nitride films makes them attractive for the passivation of temperature-sensitive group-III nitride layers such as InN and InGaN, for example.

[0060] In one example, the germanium nitride comprises GesN4 grown using isobutylgermane (IBGe) as a high vapor pressure germanium precursor and ammonia (NH3) as a nitrogen precursor. The IBGe precursor was previously used for Ge doping of group-III nitride films and did not show any harmful memory effect in MOCVD growth chambers.

[0061] Alternately, any other suitable germanium precursor can be used.

[0062] Experimental Results

[0063] To investigate the impact of the growth parameters on the germanium nitride growth rate, GeslSk layers were grown by MOCVD using IBGe as the germanium precursor and NH3 (ammonia) as the nitrogen precursor. The germanium nitride layers were grown on silicon (001) substrates and sapphire (0001) substrates with nitrogen as the carrier gas. Growth temperatures between 450 °C and 600 °C. and growth pressures between 20 kPa and 60 kPa, were investigated. The molar flow of IBGe was varied between 43 and 87 pmol / min, and the molar flow of NH3 was varied between 134 and 268 mmol / min. The thicknesses of the resulting GesN4 layers w ere measured using ellipsometry with samples grown on silicon. The relative conductivity of the layers was measured using pressed indium contacts on samples grown on sapphire. The germanium nitride layers were insulating as expected under a wide range of growth conditions.

[0064] The impact of the growth parameters on the germanium nitride growth rate is illustrated in Figs. 4, 5 and 6, wherein Fig. 4 is a graph of Growth Rate (A / s) vs. Growth Temperature ( C) showing the dependence of the germanium nitride growth rate on the deposition temperature; Fig. 5 is a graph of Growth Rate (A / s) vs. NH3 Flow7(SLM) show ing the dependence of the germanium nitride growth rate on the ammonia flow; and Fig. 6 is a graph of Growth Rate (A / s) vs. Pressure (kPa) showing the dependence of the germanium nitride growth rate on the pressure in the growth chamber.

[0065] References

[0066] The following publications are incorporated by reference herein:

[0067] [1] U.S. Patent Application Publication No. 2016 / 0163846 Al, entitled Metalorganic chemical vapor deposition of oxide dielectrics on n-polar group-III nitride semiconductors with high interface quality7and tunable fixed interface charge,” filed on January 28, 2016, as Application Serial No. 15 / 009,509, by Xiang Liu, Umesh K. Mishra, Stacia Keller. Jeonghee Kim. Matthew Laurent, Jing Lu, Ramya Yeluri, Silvia H. Chan. [2] J. Derluyn et al., Improvement of AlGaN / GaN high electron mobility transistor structures by in situ deposition of a SisN4 surface layer, J. Appl. Phys.98, 054501 (2005). Conclusion

[0068] 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 fabricating a semiconductor device, comprising: growing group-III nitride semiconductor in a growth chamber; and depositing germanium nitride on or above a surface of the group-III nitride semiconductor; wherein the deposited germanium nitride prevents any reaction of the surface of the group-III nitride semiconductor with ambient constituents after removal from the growth chamber.

2. The method of claim 1, wherein the growing of the group-III nitride semiconductor and the depositing of the germanium nitride are performed in-situ in the growth chamber.

3. The method of claim 2, wherein the germanium nitride is deposited without the group-III nitride semiconductor being exposed outside the growth chamber following the growing of the group-III nitride semiconductor.

4. The method of claim 2, wherein the depositing of the germanium nitride is performed in a separate chamber of the growth chamber from the growing of the group-III nitride semiconductor.

5. The method of claim 1, wherein the depositing of the germanium nitride is performed ex-situ following the growing of the group-III nitride semiconductor in the growth chamber.

6. The method of claim 1, wherein the germanium nitride is deposited on or above sidew alls or other three-dimensional features of the group-III nitride semiconductor.

7. The method of claim 1, further comprising depositing silicon nitride on the germanium nitride.

8. The method of claim 1 , further comprising depositing silicon nitride on or above the group-III nitride semiconductor before depositing the germanium nitride on or above the silicon nitride.

9. The method of claim 8, further comprising depositing alternating layers of the germanium nitride and silicon nitride.

10. The method of claim 1 , further comprising depositing ternary silicon germanium nitride.

11. The method of claim 1, further comprising: removing a sample comprised of the group-III nitride semiconductor and the germanium nitride from the growth chamber; storing or processing the sample; reloading the sample into the growth chamber; heating the sample in the grow th chamber to remove the germanium nitride; and depositing additional layers on the sample in the growth chamber.

12. The method of claim 1, wherein the germanium nitride comprises GesN4 grown using isobutylgermane (IBGe) as a high vapor pressure germanium precursor and ammonia (NH3) as a nitrogen precursor.

13. The method of claim 1, wherein the group-III nitride semiconductor comprises N-polar group-III nitride semiconductor.

14. The method of claim 1, wherein the group-III nitride semiconductor comprises metal-polar or Ga-polar group-III nitride semiconductor.

15. The method of claim 1 , wherein the device is a transistor, high electron mobility transistor (HEMT), light emitting diode, laser diode, photodetector, solar cell, or other electronic or optoelectronic device, of any wavelength or frequency operation range.

16. The method of claim 1, wherein the group-III nitride semiconductor is an active region of the device, and the germanium nitride is a dielectric or passivation layer between a gate and the active region.

17. A semiconductor device, comprising: a group-III nitride semiconductor grown in a growth chamber; and germanium nitride deposited on or above the group-III nitride semiconductor; wherein the germanium nitride prevents any reaction of the group-III nitride semiconductor with ambient constituents.

18. The device of claim 17, wherein the germanium nitride is a dielectric or passivation layer between a gate and an active region of the device.

19. A device, comprising: a template or substrate for a group-III nitride device comprising: an aluminum-containing group-III nitride; and a germanium nitride cap layer directly on the aluminum-containing group-III nitride.

20. The device of claim 19, wherein the aluminum-containing group-III nitride comprises AlGaN or AIN.

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