WO2017180633A1

WO2017180633A1 - Method to achieve active p-type layer/layers in iii-nitride epitaxial or device structures having buried p-type layers

Info

Publication number: WO2017180633A1
Application number: PCT/US2017/027025
Authority: WO
Inventors: Yuuki ENATSU; Chirag Gupta; Stacia Keller; Umesh K. Mishra; Anchal AGARWAL
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2016-04-11
Filing date: 2017-04-11
Publication date: 2017-10-19
Anticipated expiration: 2018-10-11
Also published as: US20190181329A1; US11588096B2

Abstract

An optoelectronic or electronic device structure, including an active region on or above a polar substrate, wherein the active region comprises a polar p region. The device structure further includes a hole supply region on or above the active region. Holes in the hole supply region are driven by a field into the active region, the field arising at least in part due to a piezoelectric and/or spontaneous polarization field generated by a composition and grading of the active region.

Description

METHOD TO ACHIEVE ACTIVE P-TYPE LAYER/LAYERS IN III-NITRIDE EPITAXIAL OR DEVICE STRUCTURES HAVING BURIED P-TYPE LAYERS

CROSS REFERENCE TO RELATED APPLICATIONS

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

U.S. Provisional Patent Application No. 62/321,194, filed on April 11, 2016, by Yuuki Enatsu, Chirag Gupta, Stacia Keller, and Umesh Mishra, entitled

"METHOD TO ACHIEVE ACTIVE P-TYPE LAYER/LAYERS IN III-NITRIDE EPITAXIAL OR DEVICE STRUCTURES HAVING BURIED P-TYPE LAYERS," Attorney's Docket No. 30794.615-US-P1 (UC Ref. 2016-770-1); and

U.S. Provisional Patent Application No. 62/415,807, filed on November 1, 2016, by Chirag Gupta, Anchal Agarwal, Umesh Mishra, and Stacia Keller, entitled "METHOD TO ACHIEVE TUNNEL JUNCTION LED AND LASER IN III- NITRIDE BY METAL ORGANIC CHEMICAL VAPOR DEPOSITION

(MOCVD)," Attorney's Docket No. 30794.640-US-P1 (UC Ref. 2017-241-1);

all of which applications are incorporated by reference herein.

This application is related to the following co-pending and commonly- assigned U.S. patent applications:

U.S. Patent Application Serial No. 15/344,377, filed on November 4, 2016, by

Srabanti Chowdhury, Jeonghee Kim, Chirag Gupta, Stacia Keller, Silvia H. Chan, and Umesh K. Mishra, entitled "TRENCHED VERTICAL POWER FIELD EFFECT TRANSISTORS WITH IMPROVED ON-RESISTANCE AND BREAKDOWN VOLTAGE," (2014-718) which application:

claims the benefit under 35 U.S.C. Section 119(e) of U.S Provisional Patent application Serial No. 62/250,741 filed November 4, 2015 by Chirag Gupta, Umesh K. Mishra, Silvia H. Chan, and Stacia Keller, entitled "DEVICE STRUCTURE TO ACHIEVE ENHANCEMENT MODE OPERATION ALONG WITH LOW ON- RESISTANCE AND HIGH BREAKDOWN VOLTAGE IN A SEMICONDUCTOR DEVICE," Attorney's Docket No. 30794.605-US-P1 (UC Ref. 2016-249-1); is a continuation in part of PCT international application No.

PCT/US 15/31041, filed May 15, 2015, by Srabanti Chowdhury, Jeonghee Kim, Chirag Gupta, Stacia Keller and Umesh K. Mishra, entitled "TRENCHED

VERTICAL POWER FIELD-EFFECT TRANSISTORS WITH IMPROVED ON- RESISTANCE AND BREAKDOWN VOLTAGE," Attorney's docket No.

30794.550-WO-U1 (2014-718-2), which application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned U.S. Provisional Patent Applications:

U.S. Provisional Patent Application Serial No. 61/993,759, filed on May 15, 2014, by Umesh Mishra, Stacia Keller, and Srabanti Chowdhury, entitled

"GALLIUM NITRIDE (GAN) BASED VERTICAL METAL OXIDE

SEMICONDUCTOR (MOS) TRANSISTORS AND JUNCTION FIELD EFFECT TRANSISTORS (JFETS)", attomey's docket No. 30794.550-US-Pl (2014-718-1);

U.S. Provisional Patent Application Serial No. 62/075,556, filed on November 5, 2014, by Srabanti Chowdhury, Chirag Gupta, Stacia Keller and Umesh K. Mishra, entitled "SUPERJUNCTION CURRENT APERTURE VERTICAL ELECTRON TRANSISTOR FOR ULTRA-LOW ON-RESISTANCE", attorney's docket No. 30794.579-US-P1 (2015-302-1); and

U.S. Provisional Patent Application Serial No. 62/075,560, filed on November 5, 2014, by Jeonghee Kim, Stacia Keller, Srabanti Chowdhury and Umesh K. Mishra, entitled "TRENCHED VERTICAL POWER FIELD-EFFECT TRANSISTORS WITH IMPROVED ON-RESISTANCE AND BREAKDOWN VOLTAGE", attorney's docket No. 30794.580-US-P1 (2015-303-1);

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention is related to methods and devices achieving active p- type layers. 2. Description of the Related Art.

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

There is a need in the art for methods and structures for improving the performance of III -nitride devices. The present invention satisfies this need.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention allow for the first time both surface and buried p-type regions without the need for doping the materials with an acceptor. Here, material polarization in a graded layer is used to create regions of negative charge. Holes are extracted from separate regions to neutralize the negative charge. These regions could be regions doped with acceptors, or also metals, or a combination thereof.

Furthermore, illustrative embodiments of the present invention allow, for the first-time, tunnel junction device epitaxial layers to be entirely grown via Metal Organic Chemical Vapor Deposition (MOCVD). A tunnel junction is formed between the graded region and an n type region which may or may not be graded itself.

Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The graded region may be connected to a separate region from which holes could be extracted. These regions could be regions doped with acceptors, or also metals, or a combination thereof, or a component of another tunnel junction. These separate regions may or may not be active regions.

An illustrative embodiment comprises depositing a polar p region, or active region comprising a polar p region, on or above a polar substrate, wherein the polar p region (e.g., polar p-type region) includes a graded layer/region having a polar orientation or a region with an abrupt interface or a series of multiple interfaces. A hole supply region is deposited on, above, or below the polar p region. Holes in the hole supply region are driven by a field into the active region and the field comprises or arises at least in part due to a piezoelectric and/or spontaneous polarization field generated by the polar p region (e.g., a composition and grading of the graded layer).

The method can comprise depositing one or more layers on the polar p region; at least partially etching one or more of the layers to form an etched region; and depositing the hole supply region, comprising a doped p-type region, in the etched region.

The method can comprise depositing one or more layers on the hole supply region, the hole supply region comprising a p-type region; at least partially etching the p-type region and one or more of the layers to form an etched region; and activating the p-type region in the etched region. The method can further comprise growing a further p-type region in the etched region, wherein the activating comprises activating the p-type region grown in the etched region.

The hole supply region can comprise a p-type region that is etched or selectively regrown so that it does not completely cover the graded layer.

In one or more embodiments, the layers on the polar p region or the hole supply region are Ill-nitride layers, and the p-type region and/or the further p-type region are p-type GaN. The hole supply region can comprise or consist essentially of metal such as gold, silver, titanium, nickel, palladium, platinum, etc. In one embodiment comprising a device structure that does not include a p-type doped region, the graded layer comprises a steep grade adj acent the metal, the steep grade enabling tunneling of holes from the metal into the active region.

The holes driven into the graded layer/polar p region can form a hole gas in the graded Ill-nitride active region of a variety of devices. For example, the hole gas can be the gated channel in a field effect transistor or a current aperture vertical electron transistor. In another example, the holes driven into the active region of the light emitting diode recombine with electrons in the active region to emit the light emitted by the light emitting device or diode.

In various examples, a thickness and doping of the hole supply region, and composition and grading of the graded layer in the active region, can be selected (1) such that the field creates a negative charge of e.g., at least 8 x 10¹⁹ cm^-3 in the active region, (2) such that the field creates a hole density of e.g., at least 10¹⁹ cm^-3 in the active region, and/or (3) to obtain a desired hole concentration in the active region, e.g., such that a diode comprising the device structure has a turn on voltage of no more than 5.0 Volts, and/or such that the holes neutralize at least 20%, at least 50%, or at least 90% of the negative charge created by the field.

One or more embodiments of the present invention further disclose a light emitting diode, comprising a device structure including an active region, wherein the active region comprises graded polar materials and has a polar orientation; and a hole supply region, wherein holes in the hole supply region are driven by a field into the active region, the field arising at least in part due to a piezoelectric and/or

spontaneous polarization field generated by a composition and grading of the active region.

The LED can be an inverted LED with the p-GaN layer not at the surface. For example, the device can comprise an inverted or flip-chip light emitting diode and the hole supply region can be a p-GaN layer buried beneath a surface of the light emitting diode. For example, the p-GaN layer can be between the active region and a substrate, submount, or sacrificial substrate, and the active region can be between the p-GaN layer and an n-type GaN layer.

Further examples of the devices enabled by one or more embodiments of the invention include Light Emitting Diodes (LEDs) without acceptor doping in the active p-type region (p-region), buried active p-regions and devices based on this availability, such as tunnel junction based LEDs and lasers, and vertical electronic devices that need buried p-regions, such as Current Aperture Vertical Electron Transistors (CAVETs), Trench Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), and super-junction devices.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 illustrates a method and device structure according to a first embodiment of the present invention.

Figure 2 illustrates a method and device structure according to a second embodiment of the present invention.

Figure 3 illustrates a method and device structure according to a third embodiment of the present invention.

Figure 4 illustrates a method and device structure according to a fourth embodiment of the present invention.

Figure 5 illustrates a method and device structure according to a fifth embodiment of the present invention.

Figure 6 illustrates a method and device structure according to a sixth embodiment of the present invention.

Figures 7(a)-7(d) illustrate methods to fabricate acceptor free device structures according to embodiments of the present invention.

Figure 8(a) illustrates a device structure according to one or more

embodiments of the invention, and Figure (8b) illustrates device performance of embodiments of the present invention.

Figure 9 illustrates a device structure, and performance, of a light emitting diode fabricated according to one or more embodiments of the present invention.

Figures 10(a)- 10(b) illustrate emission from the device structure of Figure 9. Figures 1 l(a)-l 1(d) illustrate cross-sections of tunnel junction LED structures according to one or more embodiments of the invention. Figures 12(a)- 12(d), 13 (a)- 13(d) and 14(a)- 14(d) are cross sectional schematics illustrating devices according to one or more embodiments of the present invention wherein the hole supplying region is a p-type region.

Figures 15(a)-15(d) and 16(a)-16(d) are cross sectional schematics illustrating devices according to one or more embodiments of the present invention wherein the hole supplying region is a metal region.

Figures 17(a)- 17(d) show cross-sectional schematics illustrating devices according to one or more embodiments of the present invention wherein the hole supplying region is a p-type region in contact with multiple graded Ill-nitride layers.

Figures 18(a)- 18(d) show cross-sectional schematics illustrating devices according to one or more embodiments of the present invention wherein the hole supplying region is a metal region in contact with multiple graded Ill-nitride layers.

Figures 19(a)- 19(d) show cross-sectional schematics illustrating devices according to one or more embodiments of the present invention wherein the hole supplying region is a metal region and the graded Ill-nitride layer includes a sharp grade.

Figure 20 is a flowchart illustrating a method of fabricating a device according to one or more embodiments of the present invention.

Figure 21 is a flowchart illustrating a method of fabricating a device according to one or more embodiments of the present invention.

Figures 22(a)-22(f) show structural properties and surface morphologies of the compositionally un-doped graded In_xGai-_xN layers, (a)— (c) Triple axis x-ray diffraction scans: (top line) measurement data (bottom line) simulation data, (d)— (f) AFM images of the compositionally un-doped graded In_xGai-_xN layer. The scan area is 55 μm ².

Figures 23(a)-23(c) show temperature-dependent Hall effect measurements (100K— 300K) on 100 nm-thick un-doped, compositionally graded In_xGai-_xN layers with different gradations of the InN mole fraction (0— 5%, 10%, 20%) and Mg doped Ino.05Gao.95N layer: (a) hole concentration, (b) resistivity, and (c) hole mobility.

Figures 24(a)-24(b) show temperature-dependent Hall effect measurement results (50K— 300K) for three un-doped, compositionally graded In_xGai-_xN layers (x:0→0.05) with different thicknesses (100, 300, 800 nm) and Mg doped Ino.05Gao.95N layer: (a) hole concentration and (b) hole mobility.

Figures 25(a) is a schematic p-n diode structure of sample number 8 and Figure 25(b) shows the 1/C² and depletion width as a function of voltage for the p-n diode with 600nm thick compositionally graded In_xGai-_xN layer (x:0→0.05). The solid line corresponds to 1/C², the dotted line is the depletion width. The carrier concentration of the compositionally graded In_xGai-_xN layer was extracted from the dashed line.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to 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.

Technical Description

It is desirable to remove the need for high quality Magnesium (Mg) doped regions to produce p-type regions in photonic devices such as III-N based light emitters and detectors. The reason is that high quality p-GaN has to be grown at higher temperatures than what some of the constituent materials in the device stack would like to be subjected to. In addition, it is difficult to achieve high hole concentrations, because Mg is a relatively deep acceptor with an activation energy of ~ 1 10— 160 meV in GaN (and even higher values in AlGaN, for example). In recent times, vertical GaN power devices have also gained increased attention, owing to their potential to provide higher breakdown voltage without increasing specific on-resistance. Numerous vertical GaN power devices/transistors have been demonstrated in the past few years [1 -9]. Almost all of the above listed GaN power devices utilize a buried p-GaN layer for various purposes, namely normally-off operation, threshold voltage control, high breakdown voltage, and reduction in source-drain leakage, etc. However, these devices suffer from the inefficiency of dopant based p-type doping in buried p-GaN layers. For example, the p-type doped concentration cannot be increased sufficiently in U-MOSFET type devices [5-9], because the p-type GaN layer is located close to the channel. The high activation energy of the most commonly used p-type dopant, Mg (in GaN, -160 meV), results in only about 1% ionization of Mg atoms at room temperature. This problem is exacerbated by passivation of the dopant (Mg) by forming complexes with hydrogen during regrowth or processing steps. This severely impedes device design, growth and fabrication space.

As described herein, using polarization to create negative charges, that are then neutralized by holes from a hole supplying region, is a breakthrough that can improve photonic device efficiencies across the spectrum of light emission as well as improve the performance of electronic devices.

It is an object of the present invention to provide a method to achieve active p- active buried p-layers in polar materials, including but not limited to Ill-nitride device structures, and also create a new class of light emitters and electronic devices with p- doped regions and metals used only for hole supply and separated from p-active regions.

Active buried p-GaN layers can be achieved by utilizing the unique property of III-N material systems having piezoelectric and/or spontaneous polarization fields on polar planes. By engineering these polarization fields, a three dimensional electron or hole gas is achieved, which can result in dopant-free doping of III-N layers. The doping and polarity of the charge can be controlled by choosing suitable alloy composition and grading.

First Example

Figures l(a)-l(b) illustrate the p-type doped layer 100 in the epitaxial stack

102 is replaced by one or multiple graded (Ga,Al,In,B)N layers 104 followed by a p- type doped layer 106. The p-type doped layer 104, if passivated, is then activated by usual p-GaN activation techniques such as thermal annealing. The active p-type doped layer 106 provides holes to the graded (Ga,Al,In,B)N layers 102, resulting in the graded (Ga,Al,In,B)N 102 acting as an active p-type GaN (p-GaN) layer or a graded p-(Ga,Al,In,B)N. The holes present in the graded (Ga,Al,In,B)N layers are field-driven into the graded layers 104 from the active p-type doped layer 106 and need not contain the dopant species. This allows the graded (Ga,Al,In,B)N layers 104 to remain active even after subsequent re-growths in a hydrogen rich environment. The substrate 108, one or more layers 110 or 1... .m between the substrate 108 and graded layer 104, and one or more layers 112 or -m+1... n are also shown. Layer m+1... n indicates n layers where n is an integer and layer 1... m indicates m layers where m is an integer. Second Example

Figures 2(a)-2(d) illustrate the p-type doped layer 200 in the epitaxial stack 202 is replaced by one or multiple graded (Ga,Al,In,B)N layers 204 followed by the rest of desired epitaxial structure 206, resulting in buried graded (Ga,Al,In,B)N layers 204. Thereafter, the layers 206 deposited/grown above graded (Ga,Al,In,B)N layers are selectively removed/etched from a part of (or completely from) the wafer so as to form an etched region 208. Subsequently, in the etched region 208, doped p-GaN 210 is regrown and activated. The active p-type doped layer 210 provides holes to the buried graded (Ga,Al,In,B)N layers 204, resulting in the graded (Ga,Al,In,B)N 204 acting as an active p-GaN layer or an active p-(Ga,Al,In,B)N layer throughout the entire wafer 202. The substrate 212 and one or more layers 214 or 1... .m are also shown.

Third Example

Figures 3(a)-3(d) illustrate the p-type doped layer 300 in the epitaxial stack

302 is replaced by one or multiple graded (Ga,Al,In,B)N layers 304 followed by a p- type doped layer 306. The p-type doped layer 306 may or may not be activated. Subsequently, the rest of the desired epitaxial structure 308 is grown/deposited, resulting in buried graded (Ga,Al,In,B)N 304 and p-type doped layers 306.

Thereafter, the layers 308 deposited/grown above p-type doped layer and a part of (or complete) p-type doped layer are selectively removed/etched from a part of (or completely from) the wafer 302 to form an etched region 310 exposed to air.

Subsequently, in the etched region 310, the remaining part of the doped p-GaN 306 is activated. Thus, the active p-type doped layer 306 would provide holes to the buried graded (Ga,Al,In,B)N layers 304, resulting in the graded (Ga,Al,In,B)N 304 acting as an active p-GaN layer or an active p-(Ga,Al,In,B)N layer throughout the entire wafer 302. The p-type doped layer 306 is added in this structure to simplify processing complexity.

The substrate 312 and one or more layers 314 or 1... .m between the substrate 312 and graded layer 304 are also shown.

Fourth Example

Figures 4(a)-4(d) illustrate the p-type doped layer 400 in the epitaxial stack 402 is replaced by one or multiple graded (Ga,Al,In,B)N layers 404 followed by a p- type doped layer 406. The p-type doped layer 406 may or may not be activated. Subsequently, the rest of the desired epitaxial structure 408 is grown/deposited resulting in buried graded (Ga,Al,In,B)N 404 and p-type doped layers 406.

Thereafter, the layers 408 deposited/grown above p-type doped layer 406 and a part of (or complete) p-type doped layer 406 are selectively removed/etched from a part of (or completely from) the wafer 402. Subsequently, in the etched region, doped p-GaN 410 is regrown and activated. The active p-type doped layer 410 provides holes to the buried graded (Ga,Al,In,B)N layers 404, resulting in the graded (Ga,Al,In,B)N 404 acting as an active p-GaN or an active p-(Ga,Al,In,B)N layer 404 throughout the entire wafer 402. The p-type doped layer 406 is added in this structure to simplify processing complexity. The substrate 412 and one or more layers 414 or 1... .m between the substrate 412 and graded layer 404 are also shown.

Fifth Example

Figures 5(a)-5(b) illustrate, in order to achieve the active surface layer, the p- type doped layer 500 is replaced by one or multiple graded (Ga,Al,In,B)N layers 502 followed by a p-type doped layer 504 (which may be exposed to air). The p-type doped layer 504, if passivated, is then activated by usual p-GaN activation techniques such as thermal annealing. The active p-type doped layer 504 provides holes to the graded (Ga,Al,In,B)N layers 502, resulting in the graded (Ga,Al,In,B)N 502 acting as an active p-GaN layer or an active p-(Ga,Al,In,B)N layer 502. The holes present in the graded (Ga,Al,In,B)N layers 502 are field-driven into the graded layers from the active p-type doped layer and the graded layer does not need to contain the dopant species.

The substrate 504 and one or more layers 506 or 1... .m between the substrate

504 and graded layer 502 are also shown.

Sixth Example

Figures 6(a)-6(d) illustrate the p-type doped layer 600 is replaced by one or multiple graded (Ga,Al,In,B)N layers 602 in order to achieve the active surface layer. Thereafter, the layers deposited/grown above graded (Ga,Al,In,B)N layers 602 are selectively removed/etched from a part of (or completely from) the wafer 604.

Subsequently, doped p-GaN 606 is regrown and activated either selectively on a part of, or completely on, the wafer 604. The selectively regrown active p-type doped layer 606 confined to a particular region provides holes to the graded (Ga,Al,In,B)N layers 602, resulting in the graded (Ga,Al,In,B)N 602 acting as an active p-GaN layer or an active p-(Ga,Al,In,B)N layer throughout the entire wafer 604. The p-GaN 606 may be exposed to air.

Seventh Example

In another embodiment of the invention, the graded materials could have multiple graded junctions with different grading profiles to provide varying hole concentrations related to the nature of the grading profile.

Figures 7(a)-7(d) illustrate one particular embodiment wherein a sharp grade

700 is incorporated adjacent to the surface 702 to enable tunneling of holes from the metals 704 into the graded regions 700, 706 directly without the need of a p-type doped region 708. Figure 7(d) illustrates an acceptor free structure with holes (a new invention with multiple opportunities). Another example is also shown in Figure 9.

Prototype demonstration

Figure 8(a) illustrates a device structure according to one or more

embodiments of the invention, comprising Ti/Au contacts, bulk GaN substrate 800, UID GaN 802a, n-GaN 802b, graded InGaN 804, p⁺ InGaN 806, and Ni/Au contacts.

Figure 8(b) illustrates one important demonstration, using semiconductor barriers 806 formed on a nominally graded III-N material 804 (in this case graded InGaN 804 grown on n-type GaN 802, wherein the graded InGaN 804 is graded from an In composition of 0% to 5% over a distance of 200 nm), showing three distinct regions of operation. A first device example, wherein the barrier is a metal layer Ni/Au formed directly (0 nm) on the graded region 804, exhibits low current and resistive characteristics. A second device example, comprising the barrier that is a 4 nm thick p-type material grown on the graded region 804, exhibits barrier limited current transport but with a high tum-on voltage of 6.8 volts. A third device example, comprising the barrier 806 that is a 40 nm thick p-type cap layer formed on the material stack as illustrated in Figure 8(a), shows excellent barrier limited transport with an expected turn-on voltage of 3.2 volts. This shows that the p-type region needs to have and provide enough holes to satisfy/achieve near charge neutrality in the graded region.

Figure 9 illustrates a second set of devices, wherein a two-step grade was employed with first a nominal 0-5% InGaN grade 900 over 100 nm grown on a conventional multi quantum well structure 902 used for blue LEDs. This layer 900 was followed by a sharper grade 904, from 5% In to 25% In over 3 nm, to generate an expected negative charge concentration of 8.1 * 10¹⁹ cm^-3. No p-type region was grown and this structure was Mg-free. Pd/Au metal was deposited on the structure to form the LED. (See Figures 7(a)-7(d) for the material and device structure). As shown in the photo of Figure 10(b), blue emission was obtained from the structure, verifying that electron-hole recombination was occurring in the LED active region and that the Mg-free structure was successful in producing holes.

Tunnel Junction Examples

In devices (such as photonic devices) it is desirable to remove the need for molecular beam epitaxy (MBE) growth on top active LED/Laser epitaxial layer structure in a tunnel junction Light Emitting Diode (LED). The MBE growth is required to keep the p-type doped region active. It is commonly known that during Metal Organic Chemical Vapor Deposition (MOCVD) growth of subsequent layers above p-type layer, the p-type dopant, Mg forms complex with hydrogen and gets passivated and therefore affects LED performance drastically. Partial activation by etching through top layers to the p-type layer is also insufficient to utilize the full potential of tunnel junction LED/Laser.

It is an object of the present invention to provide a method to achieve tunnel junction devices (e.g., LED, laser, or electronic devices) in Ill-nitride by metal organic chemical vapor deposition (MOCVD). Tunnel junction LEDs, lasers, and electronic devices could be achieved by utilizing the unique property of III-N material systems having piezoelectric and/or spontaneous polarization fields on polar planes. Here, material polarization is used to create regions of negative charge. A tunnel junction is formed between the graded region and an n type region which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The graded region may be connected to a separate region from which holes could be extracted. These regions could be: regions doped with acceptors, metal regions, a combination thereof, or a component of another tunnel junction. These separate regions may or may not be active regions.

Figures 1 l(a)-l 1 (d) a p-type doped layer 1100 is replaced by one or multiple graded (Ga,Al,In,B)N layers 1 102. The doping, composition, and thickness of the layers 1 102 can be modified/designed to achieve desirable device characteristics. In this structure, a tunnel junction is formed between the graded region 1102 and an n type region 1104 which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region 1104 to the valence band of the graded region 1 102. The substrate 1 108 and one or more layers 1 1 10 (e.g., active layers including electron blocking layer (EBL)) between the substrate 1108 and graded region 1 102 are also shown.

Figures 12(a)- 12(d) illustrate a p-type doped layer 1200 in an epitaxial stack 1202 is replaced by one or multiple graded (Ga,Al,In,B)N layers 1204 followed by the rest of desired epitaxial structure (including n-type doped layer 1206), resulting in buried graded (Ga,Al,In,B)N layers 1204. Thereafter, the layers 1206

deposited/grown above graded (Ga,Al,In,B)N layers are selectively removed/etched from a part of (or completely from) the wafer 1202 to form an etched region.

Subsequently, in the etched region, doped p-type material 1208 (which may or may not be III-N) is regrown and activated (if needed). The active p-type doped layer 1208 provides holes to the buried graded (Ga,Al,In,B)N layers resulting in graded

(Ga,Al,In,B)N 1204 acting as the active p-GaN layer throughout the entire wafer. The substrate 1210 and one or more layers 1212 (e.g., active layers including electron blocking layer (EBL)) between the substrate 1210 and graded region 1204 are also shown.

Figures 13(a)-13(d) and 14(a)-14(d) show the regrown/re-deposited p-type layer 1300, 1400 on the graded layer 1302, 1402 may have a distance of di and d2 from two adjacent layers 1304a, 1304b, 1404a, 1404b. The value of di and d2 can be either positive, negative, zero, or a combination thereof to obtain desired device design. The substrate 1410 and one or more layers 1412 (e.g., active region) between the substrate 1410 and graded region 1402 are also shown.

In the epitaxial structures shown in Figures 2(a)-2(d) and Figures 12(a)- 12(d), the doping, composition, and thickness of the layers can be modified/designed to achieve desirable device characteristics. In these structures, a tunnel junction is formed between the graded region and an n type region which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The separate p-type regions shown may or may not be active regions.

Figures 15(a)- 15(d) and 16(a)- 16(d) illustrate another embodiment of the present invention. In the epitaxial stack 1500, 1600 a p-type doped layer 1502, 1602 is replaced by one or multiple graded (Ga,Al,In,B)N layers 1504, 1604 followed by the rest of desired epitaxial structure 1506, 1606 resulting in buried graded

(Ga,Al,In,B)N layers 1504, 1604. Thereafter, the layers 1506 (e.g., n-type doped layer 1606) deposited/grown above graded (Ga,Al,In,B)N layers 1504, 1604 are selectively removed/etched from a part of (or completely from) the wafer 1500, 1600 to form an etched region 1508, 1608. Subsequently, in the etched region 1508, metal 1510 is deposited which may have a distance of di and d2 from two adjacent layers 1506. The value of di and d2 can be either positive, negative, zero, or a combination thereof, to obtain desired device design (15(a)-15(d) and 16(a)-16(d)). In the epitaxial structures shown in Figures 15(a)-15(d) and 16(a)-16(d), the doping, composition, and thickness of the layers can be modified/designed to achieve desirable device characteristics. In these structures, a tunnel junction is formed between the graded region and an n type region which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The separate metal regions shown may or may not be active regions. The substrate 1512/1612 and one or more layers 1514/1614 (e.g., active region including electron blocking layer EBL) between the substrate 1512/1612 and graded region 1504/1604 are also shown.

Figures 17(a)- 17(d) illustrate another embodiment of the present invention. In the epitaxial stack 1700 in Figures 17(a)- 17(d), a p-type doped layer 1702 is replaced by one or multiple graded (Ga,Al,In,B)N layers 1704 followed by the rest 1706 of desired epitaxial structure resulting in buried graded (Ga,Al,In,B)N layers 1704.

Thereafter, the layers 1706 deposited/grown above graded (Ga,Al,In,B)N layers 1704 are selectively removed/etched from a part of (or completely from) the wafer 1700. Subsequently, in the etched region, doped p-type material 1708 (which may or may not be III-N) is regrown and activated (if needed). The active p-type doped layer 1708 provides holes to the buried graded (Ga,Al,In,B)N layers resulting in graded

(Ga,Al,In,B)N to act as active p-GaN layer throughout entire wafer. Figure 17(b) shows the regrown p-type layer 1708 is grown in such a way that the regrown p-type region 1708 makes at least one contact to each graded (Ga,Al,In,B)N layer 1704 in one active region 1710. In the epitaxial structure 1700 shown in Figures 17(a)- 17(d), doping, composition and thickness of the layers can be modified/designed to achieve desirable device characteristics. In this structure, multiple tunnel junctions are formed between the graded region 1704 and an n type region 1706 which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The separate p-type region 1708 shown in Figure 17(b) may or may not be an active region. The substrate 1712 and one or more layers 1714 between the substrate 1712 and graded region 1704 are also shown. Layers l ..k comprise k one or more layers 1804,1806 where k is an integer.

Figures 18(a)-18(d) illustrate an epitaxial stack 1800, wherein a p-type doped layer 1802 is replaced by one or multiple graded (Ga,Al,In,B)N layers 1804 followed by the rest 1806 of desired epitaxial structure resulting in buried graded

(Ga,Al,In,B)N layers 1804. Thereafter, the layers 1806 deposited/grown above graded (Ga,Al,In,B)N layers 1804 are selectively removed/etched from a part of (or completely from) the wafer 1800. Subsequently, in the etched region, metal 1808 is deposited. Figures 18(a)- 18(d) shows the metal 1808 is deposited in such a way that the deposited metal 1808 makes at least one contact to each graded (Ga,Al,In,B)N layer 1804 in one active region 1810. The substrate 1812 and one or more layers 1814 between the substrate 1812 and graded region 1804 are also shown.

Figures 19(a)- 19(d) illustrate another embodiment of the invention, wherein the p-type doped layer 1900 is replaced with graded materials 1902, 1904 having multiple graded junctions with different grading profiles to provide varying hole concentrations related to the nature of the grading profile.

In one particular embodiment a sharply graded layer 1902 is incorporated adjacent to the surface 1906 to enable tunneling of holes from the metals 1908 into the graded regions 1902, 1904 directly without the need of a p-type doped region

1900. This can be an acceptor free structure with holes, which is a new invention with multiple opportunities.

In another embodiment of the present invention, the regrown p-type region as shown in Figures 2(a)-2(d) and Figures 12(a)-12(d) may be used as an active LED region.

Process Steps

Figure 20 is a flowchart illustrating a method of fabricating a device according to one or more embodiments of the present invention. The method comprises one or more of the following steps.

Block 2000 represents depositing a polar p region (e.g., polar p-type region) comprising a graded layer 1 102, 1204, 1302, 1402, 1504, 1604, 1704, 1804 (having a polar orientation) and/or an abrupt interface or multiple interfaces between layers with a polarization discontinuity. In one embodiment, the step comprises depositing an active region comprising the polar p region, the polar p region comprising the graded layer or graded polar materials 104, 204, 304, 404, 502, 602, 700, 706, 804 (having a polar orientation) or the polar p region comprising an abrupt interface or multiple interfaces between layers with a polarization discontinuity.

In one or more embodiments, the polar p region is deposited on or above a substrate or base layer.

In one or more embodiments, the polar orientation is a c-plane (e.g., N-polar or Ga polar orientation) and the graded layer/graded polar materials are Ill-nitride layers grown on a Ill-nitride substrate such as Gallium Nitride (GaN) 800. Ga-polar refers to layers having a c⁺ or (0001) plane orientation, and N-polar refers to layers having a c- or (000-1) plane orientation. Other examples of substrates 108 include, but are not limited to, foreign substrates such as sapphire. In one example, the base layer comprises GaN on sapphire.

The graded layer can be grown on part of a device structure (e.g., active region and electron blocking layer) grown on the substrate.

Block 2002 represents depositing one or more layers 206, 406, 1206, 1404, 1506, 1706, 1806, 1902, 1904 on, above, or below the polar p region (e.g., the graded layer 1102, 1204, 1302, 1402, 1504, 1604, 1704, 1804) and/or the active region comprising the polar p region (e.g., the graded polar materials/layer 104, 204, 304, 404, 502, 602, 700, 706, 804).

In one or more embodiments, the one or more layers comprise a hole supply region. In one or more embodiments, the hole supplying region comprises or consists essentially of a p-type doped region 210, 306, 406, 410, 504, 606, 1208, 1400, 1300, 1708 such as a Ill-nitride region (e.g., p-type GaN). In yet further embodiments, the hole supplying region comprises/consists essentially of metal 704, 1510, 1600, 1808 such as, but not limited to, gold (Au), silver (Ag), titanium (Ti), nickel (Ni), palladium (Pd), platinum (Pt), etc or combinations thereof. In other embodiments, one or more embodiments, the hole supplying region comprises a metal region and/or a p- type doped region. In one embodiment wherein the hole supplying region comprises a metal, the active region or graded Ill-nitride layer comprises a steep or sharp composition grade 700, 904, 1806, 1902, adjacent the metal 704, Ni/Au, Pd/Au, 1908 wherein the steep or sharp composition grade enables tunneling of holes from the metal into the active region or the graded Ill-nitride layer. Examples of the sharp grade include, but are not limited to, an InGaN region having an indium composition graded from 5%-25% over a thickness of 5 nanometers or less. In one or more examples, the sharp grade creates a negative charge concentration of at least 8.1 ^χ 10¹⁹ cm^-3. In yet further embodiments, the graded Ill-nitride layer comprises a layer underneath the sharp grade and the layer underneath the sharp grade has an indium composition graded from 0% to 5% over a thickness of at least 100 nm.

In one or more embodiments, the one or more layers on the graded Ill-nitride layer are n-type layers 1206 (e.g., n-type Ill-nitride such as, but not limited to, n-type GaN).

Block 2004 represents optionally depositing one or more further layers 308,

408 on the hole supplying region 306, 406.

Block 2006 represents optionally at least partially etching the layers (formed in Blocks 2002 and 2004 on the graded III -nitride layer) to form an etched region 310, 1508, 1608.

Block 2008 represents optionally depositing/growing material 210, 410, 606,

1208, in the etched region and on the graded Ill-nitride layer.

In one embodiment where the hole supply region 306 406 is deposited in Block 2002, the material deposited in the etched region comprises a p-type region 410 (e.g., p-type GaN).

In one embodiment where an n-type layer 1206, 1606 is deposited in Block

2002, the material 210, 1208, 1600 deposited in the etched region comprises a hole supplying region as described above. In one or more embodiments, a tunnel junction is formed between the graded Ill-nitride layer 1204, 1604 and the n-type region/layer 1206, 1606. In one or more embodiments, the hole supplying region comprises a component of a tunnel junction.

Block 2010 represents optional further processing steps, including, but not limited to, formation of contacts.

Block 2012 represents the end result, a device or device structure comprising an active region 902, 104, 204, 304, 404, 502, 602, 700, 706, 804 (comprising graded polar materials) and/or graded region (e.g., graded Ill-nitride region/layer) 1102, 1204, 1302, 1402, 1504, 1604, 1704, 1804, wherein the active region or the graded region is on or above a polar substrate; and a hole supply region 210, 306, 406, 410, 504, 606, 1208, 1400, 1300, 1708, 704, 1510, 1600, 1808 on or above the active region 902 and/or the graded layer. Holes in the hole supply region are driven by a field into the active region 902 and/or the graded layer 1 102, 1204, 1302, 1402, 1504, 1604, 1704, 1804, 104, 204, 304, 404, 502, 602, 700, 706, 804 and the field arises at least in part due to a piezoelectric and/or spontaneous polarization field generated by the polar p region (e.g., the piezoelectric and/or spontaneous polarization field generated by a composition and grading of the polar p region including the graded layer/ graded polar materials). The holes driven into the active region may (for example) form a hole gas in the active region.

The methods described herein allow the formation of a hole supplying region and active region having various properties and various locations throughout the device. For example, the device:

• may be fabricated without providing acceptor dopants in the device structure and/or in active region, or so that the active region and/or the device structure does not contain a p-type dopant; and/or · may comprise the active region buried within the device structure; and/or

• may comprise the hole supply region buried or on a surface of the device; and/or • may comprise the hole supply region including a p-type region that is etched or selectively regrown so that it does not completely cover the active region.

In various examples, a thickness and doping of the hole supply region, and composition and grading of the active region, are selected:

• such that the field creates a negative charge of at least 10⁹ cm^-3 (e.g., 8 x 1 0¹⁹ cm^-3) in the active region; and/or

• such that the field creates a hole density of at least 10¹⁹ cm^-3 in the active region; and/or

• To obtain a desired hole concentration in the active region; and/or

• to obtain a diode comprising the device structure has a turn on voltage of no more than 5.0 Volts; and/or

• such that the holes neutralize the negative charge created by the field. In various examples, the one or more layers, the hole supply region, and the graded Ill-nitride layer are grown by Metalorganic Chemical Vapor Phase Deposition (MOCVD), so that the tunnel junction/tunneling part of the structure is formed without Molecular Beam Epitaxy. The p-type region of the device (including the hole supplying region) can be activated at various stages during the fabrication process (e.g., activating the p-type region in the etched region)

Device Structure Examples

Examples of devices enabled by one or more embodiments of the present invention include LEDs without acceptor doping in the active p-type region (p- region), buried active p-regions and devices based on this availability, such as tunnel junction based LEDs and lasers, and vertical electronic devices that need buried p- regions, such as Current Aperture Vertical Electron Transistors (CAVETs), Trench Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), and super-junction devices. Embodiments of the present invention could be used to replace the p-type layers/regions in the structures described in U.S. Provisional Patent Application No.62/250,741 (UC Ref. 2016-249-1), U.S. Patent Application Serial No. 15,344,377, and PCT international application No. PCT/US 15/31041 (UC Ref. 2014-718-2) cross- referenced above.

In one or more transistor embodiments, the graded Ill-nitride layer is the active region or (e.g., gated) channel layer (e.g., comprising the hole gas), or the graded Ill-nitride layer is part of the tunnel junction forming an electrical contact (e.g., tunnel junction contact) to the transistor, or the graded Ill-nitride layer is the current blocking layer in the transistor. Examples of transistors include field effect transistors, vertical electronic devices, such as, but not limited to, CAVETs, Trench Metal Oxide Semiconductor Field Effect Transistors, and super-junction devices such as CoolMOS Si devices. Such transistors may be useful in power electronics applications.

In one or more light emitting device embodiments, such as an LED or laser diode, the graded Ill-nitride layer is an active region of the device or part of the tunnel junction forming an electrical contact to the device. The holes driven into the active region recombine with electrons in the active region to emit the light emitted by the light emitting device.

Flip Chip Device

Figure 21 is a flowchart illustrating a method of fabricating a light emitting diode.

Block 2100 represents fabricating a device structure including an active region on a substrate. The active region comprises a polar p region, wherein the polar p region includes graded polar materials (having a polar orientation) and/or an abrupt interface or multiple interfaces between layers with a polarization discontinuity. The device structure further comprises a hole supply region, wherein holes in the hole supply region are driven by a field into the active region, the field arising at least in part due to a piezoelectric and/or spontaneous polarization field generated by the polar p region (e.g., by a composition and grading of the active region). In one example, the hole supply region comprises a p-GaN layer and the active region between the p-GaN layer and an n-type GaN layer.

Block 2102 represents flipping the device structure and mounting the side of the device structure opposite the substrate onto a submount.

Block 2104 represents the optional step of removing the substrate.

Block 2106 represents the end result, an inverted or flip-chip light emitting diode wherein the hole supply region (e.g., the p-GaN layer) is buried beneath a surface of the light emitting diode (i.e. the hole supply region or the p-GaN layer is not at a surface of the device).

Further Experimental Results

Table 1 lists the samples used to investigate the properties of the polarization- induced 3DHG.

All samples were grown on n-type bulk GaN (0001) substrates by metal organic chemical vapor deposition (MOCVD) at atmospheric pressure. The averag dislocation density of the bulk GaN substrates was 10⁶ cm^-2. Trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), ammonia (NH3) and bis- cyclopentadienyl-magnesium (Cp2Mg) were used as precursors for Ga, In, N and Mg, respectively. For samples 1 to 6, first, a 3 μm- thick semi-insulating layer (resistance ~10⁷ Ω) was grown at 1200°C in hydrogen (H2) carrier gas to prevent the current flow into the bottom conductive layer, bulk GaN substrate, during hall measurements. Thereafter, an un-doped graded In_xGai-xN layer was grown in nitrogen (N2) carrier gas. In this study, the InN mole fraction (Hall-series A) and the thickness in compositionally graded In_xGai-_xN layers (Hall-series B) were varied. For Hall-series A, the indium composition was changed from 0% to 5%, 10%, and 20% by growing at 855°C, 870 °C, and 890°C respectively (samples numbered 1, 2 and 3 in Table 1). The TMIn molar flow rate was increased from 0 mol/min to 4.19 X 10^-6 mol/min, while the TEGa flow rate was kept constant at 6.03 X 10^-6 mol/min. The thickness of the graded In_xGai-_xN layer was 100 nm. Here, the average composition of the graded In_xGai-N layers is lower than the Mg-doped In_xGai-_xN films used in previous studies [19], This was done to ease the growth process by limiting overall epi-layer strain. For Hall-series B, the thickness of the compositionally graded In_xGai-_xN layer thickness was varied from 100 nm to 800 nm, while the maximum indium

composition was kept constant at 5%. For all samples, the samples were capped with a 40 nm-thick Mg-doped Ino.05Gao.95N contact layer. The room temperature hole concentration and hole mobility of this contact layer was 4.2 X 10¹⁸ cm^-3 and 11.1 cm²/V s, respectively. For Hall-series A, the Cp2Mg flow rate was adjusted according to the indium concentration in the top contact layer to generate the same hole concentration in the contact layer for all samples [19]. For comparison, a reference sample with a 100 nm-thick Mg-doped Ino.05Gao.95N layer was grown. All samples were annealed at 700 °C in an N2/O2 ambient to activate the top Mg doped contact layer. The device fabrication process started with the deposition of Ni/Au electrodes as ohmic contacts. Thereafter, lithographic patterning and reactive ion etching (RIE) using BCI3/CI2 gases were carried out for mesa isolation in order to specify the device area. The indium composition of the compositionally graded In_xGai-_xN layer was evaluated by x-ray diffraction measurements. Van der Pauw-Hall effect measurements using a helium closed-cycle refrigerator system were carried out from 50K to 300K to examine the electrical properties of the un-doped compositionally graded In_xGai-_xN layers. We confirmed good ohmic contact characteristics and all samples exhibited p- type conductivity. In addition to Hall measurements, capacitance-voltage (C-V) measurements were performed on sample number 8 to demonstrate the presence of 3DHG within a compositionally graded In_xGai-_xN layer. For this sample, first, a 2 μιη-thick n⁺-GaN layer and a 100 nm-thick un-doped GaN layer were grown. The carrier concentrations in the n⁺-GaN layer and un-doped GaN layer were 5 X 10¹⁸ cm^-3 and 2 X 10¹⁵ cm^-3, respectively. Thereafter, a 600 nm-thick un-doped graded In_xGai- _XN layer was grown where the InN mole fraction was increased from 0% to 5%, followed by a 30 nm-thick Ino.o5Gao.95N:Mg (Mg: 8 X 10¹⁹ cm- ) contact layer. In this structure, the un-doped GaN layer was added to ensure the crystal quality and surface morphology of the subsequent graded In_xGai-_xN layer. After activating the sample in N2/O2 ambient, Ni/Au and Ti/Au were deposited by electron-beam evaporation on the top surface and the back surface, respectively. Mesa isolation was done using reactive ion etching (RIE). C-V measurements were carried out at a frequency of 1 MHz and at an ac modulation level of 0.1 V.

Figures 22(a), 22(c), and 22(e) show the triple axis x-ray ω-2θ scans and AFM images of samples 1, 2, and 4. For all samples, the experimental data matches well with the simulated curves Figures 22(a), 22(c), and 22(e), confirming the growth of compositionally graded In_xGai-_xN layers. Atomically flat surfaces with steps and without any V-defects were observed for all samples owing to the use of low dislocation bulk GaN substrates [20-21] as shown in Figures 22(b), 22(d), and 22(f). The measured root-mean-square roughness values were 0.76 nm, 0.98 nm and 1.17 nm for sample 1, 2 and 4 respectively.

Figures 23(a)-23(c) display the results of the temperature dependent Hall- effect measurements for sample 1, 2, 3 and 7. The following procedure was used to calculate the hole concentration: (1) The measured sheet carrier density, pi (cm^-2), was the sum value of sheet carrier density of graded In_xGai-_xN layer and Mg-doped InGaN contact layer. From pi , the sheet carrier density of Mg-doped InGaN contact layer, p2, was subtracted in order to obtain the net sheet carrier density of the graded InxGai-xN layer, p3=pl— p2. p2 was acquired using reference samples that having constant composition Mg-doped In_xGai-_xN layers (sample 7). (2) Then, p3 was divided by the graded InGaN layer thickness, d raded inGaN, to calculate the carrier density in graded InGaN layer, P raded inGaN(cm- )=p3/d_graded inGaN. (3) Thereafter, Pgraded inGaN added to the carrier concentration of Mg-doped InGaN contact layer, Pcontact InGaN=p2/dcontact InGaN. The Slim Value of Pgraded InGaN and Pcontact InGaN at each measured temperature was plotted in Figure 23(a). While the hole concentration of the reference sample with constant indium composition and Mg doping decreases exponentially with decreasing temperature due to carrier freeze-out [10] (black circles in Figure 23(a)), all compositionally graded In_xGai-xN samples exhibited a temperature independent hole concentration at less than -200K, thus confirming the presence of a 3-DHG. Note that these hall measurement results are total value of hole concentration in a Mg-doped InxGai-xN cap layer and a graded InxGai-xN layer. As a result, the similar temperature dependent behavior is observed from RT to -200K for Hall series A samples and the reference sample. To evaluate the gradation effect of InN mole fraction of the polarization doping on carrier concentration, the hole concentration at 100K was compared between samples graded to 5, 10, and 20% InN. Here, at 100K, the hole concentration would only comprise of holes from graded layer as the Mg doped contact layer suffers from carrier freeze-out.

Figure 23(a) shows that increasing the steepness of the grade, and thereby, increasing the rate of change of polarization charge across the graded layer, results in an increased hole concentration from 7.1 X 10¹⁷ cm^-3 to 3.9 X 10¹⁸ cm^-3. Even higher hole concentration may be achievable in N-polar compositionally graded In_xGai-_xN layers because of the higher indium incorporation efficiency in N- compared to Ga- polar films at the same TMIn flow rate [17]. The measured resistivities also show the different temperature dependent behavior of the un-doped compositionally graded In_xGai-xN samples compared to the reference sample with constant composition Ino.o5Gao.95N:Mg. Even though the layer resistivities at room temperature are almost identical (Figure 23(b)), the resistivity of the reference sample increases

monotonically with decreasing temperature from 0.14 Ω · cm to 1.94 Ω · cm. The different behavior was observed in the graded samples, where the resistivity initially increased as the temperature decreased, but then began to decrease at around 200— 250K. The increase of resistivity at the range of 250— 300K could be the impact of Mg-doped In_xGai-_xN contact layer. The decrease of resistivity at less than 250K indicates that there are holes induced by the polarization field in the graded In_xGai-_xN layer. Figure 2(c) shows the hole mobilities at different temperatures for the graded In_xGai-_xN samples. The increase in the hole mobility with decreasing indium composition could be attributed to the influence of alloy scattering [11].

Figures 24(a)-24(b) display the results of the temperature dependent Hall- effect measurements for series B. The hole concentration in all graded In_xGai-_xN samples was independent of the measurement temperature over a wide temperature range. The saturation hole concentration value at less than 150K was approximately inversely proportional to the thickness of the graded In_xGai-_xN layer (Figure 24(a)). The hole mobility increased with increasing thickness of the graded In_xGai-_xN layer (Figure 24(b)), an effect that was much more pronounced at low temperature. The temperature dependence of the hole mobility might be caused by alloy and phonon scattering. Further investigations are needed to clarify the contributions of the different scattering mechanisms.

Figure 25(a) shows p-n diode structure and the 1/C² and depletion width as a function of voltage measured for sample 8. The carrier concentration extracted from the raw C-V measurements, 1.37 X 10¹⁷ cm^-3, was similar to the value expected from the Hall measurements. The results confirmed that the 3DHG was confined within the graded layer region. The inventors believe that the non-linear region (between 0V to - 10V) in the C-V characteristics is due to the initial non-uniform incorporation of Indium in the graded layer. The structure comprises a bulk GaN substrate 2500, n- type GaN (n-GaN) 2502 (e.g., 1000 nm thick) on the substrate 2500, unintentionally doped (UID) GaN 2504 (e.g., lOOnm thick) on the n-GaN 2502, undoped graded InGaN 2506 with Indium (In) composition graded from 0 to 5% over a thickness (of e.g., 600 nm) on the UID-GaN 2504, magnesium (Mg) doped InGaN 2508 (e.g., 30 nm thick) on the graded InGaN 2506 (e.g., 30 nm thick), Ni/Au contacts on the Mg doped InGaN 2508, and Ti/Au contacts on the substrate 2500.

Thus, the existence of a 3DHG in un-doped and compositionally graded In_xGai-xN layers on Ga-polar GaN has been experimentally confirmed. The temperature dependent hall measurement results showed the saturation behavior of hole concentration in the un-doped compositionally graded In_xGai-xN layers at less than -200K. The 3DHG density was controlled by varying the indium composition and/or thickness of the graded In_xGai-_xN layer. The hole concentration in a 100 nm- thick un-doped, graded In_xGai-_xN layer where the indium composition was graded from x=0 to x=0.2 was as high as 3.9 X 10¹⁸ cm^-3 at 100K. The results of this study suggest that polarization-induced doping is an attractive way to enhance the hole concentration in group Ill-nitride hetero-structures and can contribute towards improving the performance of the next generation of GaN-based devices.

Possible Modifications and Variations

In one or more embodiments the polar orientation is a c-plane (e.g., N-polar or Ga polar orientation) and the device layers are Ill-nitride layers on a Ill-nitride substrate such as Gallium Nitride (GaN). Ga-polar refers to layers having a c⁺ or (0001) plane orientation, and N-polar refers to layers having a c- or (000-1) plane orientation.

However, the methods/devices of the present invention are not limited to III- nitride materials. For example, graded polar materials can also be used. Such graded polar materials can comprise, but are not limited to, binary, ternary, and quaternary compounds combining group III elements of the periodic table (Al, Ga, In, B) with group V elements of the periodic table (N, P, As, Sb, Bi) to form III-V materials such as (Al, Ga, In, B) (N, P, As, Sb, Bi).

The layers and structures described herein can be grown using growth techniques, including (but not limited to) metal-organic chemical vapor deposition and molecular beam epitaxy.

A thickness and doping of the hole supply region, and composition and grading of the active region, can be selected such that the field creates a negative charge of at least 8 x 10¹⁹ cm^-3 in the active region, such that the field creates a hole density of at least 10¹⁹ cm^-3 in the active region, to obtain a desired hole concentration in the active region, such that a diode comprising the device structure has a turn on voltage of no more than 5.0 Volts, and/or such that the holes neutralize at least 90% of the negative charge created by the field.

The device can be fabricated without providing acceptor dopants in the active region, or such that the active region does not contain a p-type dopant.

The device can be an inverted LED with the p-GaN layer not at the surface. For example, the device can comprise an inverted or flip-chip light emitting diode and the hole supply region can be a p-GaN layer buried beneath a surface of the light emitting diode. For example, the p-GaN layer can be between the active region and a substrate, submount, or sacrificial substrate, and the active region can be between the p-GaN layer and an n-type GaN layer.

Advantages and Benefits

The devices enabled by one or more embodiments of the invention include LEDs without acceptor doping in the active p-region, buried active p-regions and devices based on this availability, such as tunnel junction based LEDs and lasers, and vertical electronic devices that need buried p-regions such as CAVETs, Trench MOSFETs, and super-junction devices such as CoolMOS Si devices. In addition, since there is no need for growth of high quality p-regions, the temperature of the device stack could be reduced which is desirable for devices that prefer a low thermal budget such as green lasers. In addition, the method allows p-type doping of materials where traditional doping is hampered by a very high acceptor activation energy, as for example in aluminum and boron containing nitrides, representing materials for devices operating in the ultraviolet (UV) range of the electromagnetic spectrum.

It is another object of the present invention to provide a method to achieve a tunnel junction device, such as an LED, laser, or electronic device, in Ill-nitride by metal organic chemical vapor deposition (MOCVD). Here, material polarization is used to create regions of negative charge.

A tunnel junction is formed between the graded region and an n type region which may or may not be graded itself. Current flow across the junction occurs from the conduction band of the n region to the valence band of the graded region. The graded region may be connected to a separate region from which holes could be extracted. These regions could be regions doped with acceptors, or also metals, or a combination thereof, or a component of another tunnel junction. These separate regions may or may not be active regions.

The devices that this invention enables are tunnel junction LEDs, lasers, and electronic devices. Potential applications of one or more embodiments include applications in power electronics. Examples of devices include transistors or other devices useful in power electronics.

In addition, since there is no need for a separate MBE growth, this invention makes tunnel junction LEDs and lasers from deep UV through the visible into the infrared (IR) feasible with only MOCVD.

References

The following references are incorporated by reference herein:

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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. An optoelectronic or electronic device structure, comprising:

an active region on or above a polar substrate or base layer, wherein the active region comprises a polar p region comprising graded polar materials having a polar orientation or an abrupt interface or multiple interfaces between layers with a polarization discontinuity; and

a hole supply region on the active region, and wherein:

holes in the hole supply region are driven by a field into the active region, and the field arises at least in part due to a piezoelectric and/or spontaneous polarization field generated by a composition and grading of the active region.

2. A method of fabricating an optoelectronic or electronic device structure, comprising:

depositing a polar p region on or above a polar substrate or base layer, the polar p region comprising graded polar materials having a polar orientation or an abrupt interface or multiple interfaces between layers with a polarization

discontinuity; and

depositing a hole supply region on the polar p region, and wherein:

holes in the hole supply region are driven by a field into the polar p region, and

the field arises at least in part due to a piezoelectric polarization field, a spontaneous polarization field, or the piezoelectric and the spontaneous polarization field generated by the polar p region.

3. The method of claim 2, wherein the substrate or base layer is a c-plane Ill-nitride substrate.

4. The method of claim 2, wherein the substrate or base layer is GaN on sapphire.

5. The method of claim 2, wherein the hole supply region comprises an activated p-type layer.

6. The method of claim 2, further comprising:

depositing one or more additional layers on the polar p region;

at least partially etching one or more of the additional layers to form an etched region; and

depositing the hole supply region, comprising a doped p-type region, in the etched region.

7. The method of claim 6, wherein the additional layers on the polar p region are Ill-nitride layers and the doped p-type region is p-type GaN.

8. The method of claim 2, further comprising:

depositing one or more additional layers on the hole supply region, the hole supply region comprising a p-type region;

at least partially etching the p-type region and one or more of the additional layers to form an etched region; and

activating the p-type region in the etched region.

9. The method of claim 8, further comprising:

growing a further p-type region in the etched region, wherein the activating comprises activating the p-type region grown in the etched region.

10. The method of claim 2, wherein the hole supply region comprises an activated p-type layer on a surface of the device structure.

1 1. The method of claim 2, wherein:

the hole supply region comprises a p-type region that is etched or selectively regrown so that it does not completely cover the graded layer.

12. The method of claim 2, wherein the hole supply region comprises or consists essentially of metal.

13. The method of claim 12, wherein:

the graded layer comprises a steep grade adjacent the metal,

the steep grade enables tunneling of holes from the metal into the graded layer, and

the device structure does not include a p-type doped region.

14. The method of claim 2, further comprising depositing an active region comprising the polar p region, wherein the holes are driven into the active region so as to form a hole gas in the active region.

15. The method of the claim 2, further comprising depositing an active region comprising the polar p region, wherein the device structure is fabricated without providing acceptor dopants in the active region.

16. An optoelectronic or electronic device structure, comprising:

a polar p region on or above a polar substrate or base layer, the polar p region comprising graded polar materials having a polar orientation or an abrupt interface or multiple interfaces between layers with a polarization discontinuity; and

a hole supply region on the polar p region, and wherein:

holes in the hole supply region are driven by a field into the polar p region, and the field arises at least in part due to a piezoelectric and/or spontaneous polarization field generated by the polar p region.

17. The device structure of claim 16, wherein the hole supply region comprises or consists essentially of doped p-type GaN and/or a metal.

18. The device structure of claim 16, further comprising an active region including the polar p region, wherein the holes driven into the active region form a hole gas in the active region and the device structure is:

a transistor, or

a light emitting device, the holes recombining with electrons in the active region to emit light emitted by the light emitting device.

19. The device structure of claim 16, further comprising an active region including the graded layer, wherein:

a thickness and doping of the hole supply region, and composition and grading of the graded layer, are such that the holes neutralize negative charge in the polar p region created by the field.

20. The device structure of claim 16, wherein the device structure comprises a flip-chip light emitting diode and the hole supply region is a p-GaN layer buried beneath a surface of the light emitting diode.

21. The device structure of claim 16, wherein graded layer is an active region in the device structure or part of a tunnel junction forming an electrical contact to the device structure.