GB2543682A

GB2543682A - Epitaxial structure for improving efficiency drop of GaN-based LED

Info

Publication number: GB2543682A
Application number: GB1700942.4A
Authority: GB
Inventors: Ju Jing; Ma Houyong; Li Qiming; Xu Huiwen; Sun Chuanping
Original assignee: Enraytek Optoelectronics Co Ltd
Current assignee: Enraytek Optoelectronics Co Ltd
Priority date: 2014-07-24
Filing date: 2015-07-20
Publication date: 2017-04-26
Anticipated expiration: 2035-07-20
Also published as: CN104134732A; CN104134732B; DE112015003419T5; GB2543682B; WO2016011924A1; GB201700942D0

Abstract

Proposed is an epitaxial structure for improving the efficiency drop of an LED. The epitaxial structure comprises a substrate (10), and a GaN underlying layer, a superlattice stress relief layer (50), a multi-quantum well layer (60), a P-type InGaN insertion layer (70), a P-type electron blocking layer (80) and a P-shaped GaN layer (90) which are stacked in sequence on the substrate. The P-type InGaN insertion layer (70) is inserted between the last potential barrier of the multi-quantum well layer (60) and the P-type electron blocking layer (80), an In component of the P-type InGaN insertion layer (70) gradually increases from close to the multi-quantum well layer (60) to the electron blocking layer (80) and uses pulsed Mg-doping. As such, leakage of electrons to a P end can be reduced, and at the other hand, injection from a hole to an active region can be enhanced. The problem of the efficiency drop of a GaN-based LED can be solved, and the luminescence efficiency at a large current condition can be improved.

Description

EPITAXIAL STRUCTURE FOR IMPROVING EFFICIENCY DROP OF

GaN-BASED LED

TECHNICAL FIELD

The present invention relates to the manufacturing of gallium nitride (GaN)-based blue light-emitting diodes (LEDs) and, in particular, to an epitaxial structure that reduces efficiency droop in an LED.

BACKGROUND

Light-emitting diodes (LEDs) are solid-state semiconductor light-emitting devices utilizing semiconductor p-n junctions as light-emitting means which can directly convert electricity to light. Gallium nitride (GaN)-based high brightness LEDs are currently cutting-edge and the focus of the optoelectronics field or industry. At present, indium gallium nitride (InGaN)-based LEDs and GaN-based LEDs have gained dramatic improvements in light-emission efficiency. However, high-power GaN-based LEDs suffer from significant efficiency droop, i.e., a rapid decrease in internal quantum efficiency (IQE) upon a high incoming current. Previously, a number of mechanisms have been proposed to try to explain this phenomenon, including electric field polarization, electron leakage, non-uniform distribution of active region carriers, Auger non-radiative recombination, etc. In view of the current research, low hole-injection efficiency and leakage of electrons toward the p-terminal is one of the possible causes of efficiency droop at a high current.

In order to address insufficient electron blockage, some researchers have proposed electron blocking layers (EBLs). However, due to the presence of a polarized electric field at the heterojunction, such a conventional EBL will slopes downward and is hence still incapable of blocking the leakage of electrons towards the p-terminal when a high current flows into the device. In addition, such conventional EBL tends to have a wide forbidden band gap which hinders the injection of holes into the multiple quantum well layer.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an epitaxial structure that reduces efficiency droop in a GaN-based LED and improves its light-emission efficiency under a high-current condition by, upon a high driving current, on one hand, better blocking the leakage of electrons toward a p-type terminal of the LED and, on the other hand, intensifying the injection of holes in a multiple quantum well layer thereof.

This objective is attained by an epitaxial structure for improving efficiency droop of a GaN-based LED according to the present invention. The epitaxial structure includes a substrate and, sequentially stacked on the substrate, a GaN underlayer, a superlattice layer for stress relief, a multiple quantum well layer, a p-type InGaN interlayer, a p-type electron blocking layer and a p-type GaN layer.

Further, the p-type InGaN interlayer may be pulse-doped with Mg and may have an In content varying gradually from 0% to 7%. Moreover, the p-type InGaN interlayer may have a thickness of from 3 nm to 12 nm. Further, the Mg dopant may have a concentration of from lei8 cm'3 to le 19 cm'3.

Further, the electron blocking layer may be formed from a p-AlGaN layer or a superlattice structure formed of p-AlGaN and p-GaN layers, and the p-type electron blocking layer may have a thickness of from 30 nm to 80 nm.

Further, the p-type GaN layer may be doped with Mg of a concentration of from lel9 cm'3 to 6e20cm"3 and may have a thickness of from 30 nm to 50 nm.

Compared to the prior art, the present invention offers mainly the following benefits: as the p-type InGaN interlayer formed between the multiple quantum well layer and the p-type electron blocking layer has a gradually varying In content, a polarized electric field resulting from lattice mismatch between a GaN potential barrier layer and the interlayer can be reduced; and InGaN has a narrower forbidden band gap compared to the conventional electron blocking layers, which results in an increase in hole injection efficiency, prevention of electrons from leaking toward the p-terminal and increased light-emission efficiency of the GaN-based LED when operating at a high current.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cutaway view schematically illustrating an epitaxial structure that reduces efficiency droop in a GaN-based LED in accordance with an embodiment of the present invention.

Fig. 2 is a flow chart graphically showing a process for fabricating an epitaxial structure that reduces efficiency droop in a GaN-based LED in accordance with an embodiment of the present invention.

Figs. 3 to 6 are cutaway views schematically illustrating a process for fabricating an epitaxial structure that reduces efficiency droop in a GaN-based LED in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Epitaxial structures that reduce efficiency droop in GaN-Based LEDs according to the present invention will be described in greater detail in the following description which presents preferred embodiments of the invention and is to be read in conjunction with the accompanying drawings. It is to be appreciated that those of skill in the art can make changes in the invention disclosed herein while still obtaining the beneficial results thereof. Therefore, the following description shall be construed as being intended to be widely known by those skilled in the art rather than as limiting the invention.

For simplicity and clarity of illustration, not all features of the disclosed specific embodiments are described. Additionally, descriptions and details of well-known functions and structures are omitted to avoid unnecessarily obscuring the invention. The development of any specific embodiment of the present invention includes specific decisions made to achieve the developer’s specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art.

The present invention will be further described in the following paragraphs by way of example with reference to the accompanying drawings. Features and advantages of the invention will be more apparent from the following detailed description, and from the appended claims. Note that the accompanying drawings are provided in a very simplified form not necessarily presented to scale, with the only intention of facilitating convenience and clarity in explaining a few illustrative examples of the invention.

As mentioned in the Background section, under the effect of a high incoming current, an excess of electrons which are abundant in the active region will leak to the p-terminal. At the same time, holes that are relatively great in effective mass are injected into the active region in a non-uniform manner in which most of them are trapped in the potential wells nearer to the p-terminal.

Referring to Fig. 1, in order to overcome these problems, the embodiment provides an epitaxial structure that reduces efficiency droop in a GaN-based LED.

The epitaxial structure includes a substrate 10 and, sequentially stacked on the substrate 10, a GaN underlayer, a superlattice layer 40 for stress relief, a multiple quantum well layer 50, a p-type InGaN interlayer 70, a p-type electron blocking layer 80 and a p-type GaN layer 90.

The p-type InGaN interlayer 70 may be pulse-doped (delta-doped) with magnesium (Mg) and include an In content gradient of from 0 % to 7 %. The p-type InGaN interlayer 70 may have a thickness between 3 nm and 12 nm, for example, 8 run, and a Mg dopant concentration in the range of from lei8 cm'3 to lel9cm'3. Delta-doping of Mg allows a high activation rate of Mg in the p-type InGaN interlayer 70 and less leakage of Mg to a last barrier layer of the multiple quantum well layer 60, thereby preventing its performance degradation under low currents. When the epitaxial structure is used to fabricate a chip operating at a low current, the p-type InGaN interlayer 70 may also not be doped with Mg. A portion of the p-type InGaN interlayer 70 that comes into contact with the multiple quantum well layer 60 has an In content of 0%, while a portion of the p-type InGaN interlayer 70 in contact with the subsequently formed electron blocking layer 80 has an In content of 7%. The remainder of the p-type InGaN interlayer 70 includes an In content gradient of from 0 % to 7 %. The In content gradient of the p-type InGaN interlayer 70 can help in reducing a polarized electric field resulting from lattice mismatch between the last barrier layer of the multiple quantum well layer 60 and the p-type InGaN interlayer 70. Additionally, InGaN has a relatively narrow forbidden band gap which, on one hand, raises a potential barrier for electrons to leak to the p-terminal and, on the other hand, lower a potential barrier for the injection of holes into the N-underlayer. As a result, higher hole injection efficiency, prevention of electron leakage toward the p-terminal and increased light-emission efficiency can be obtained.

According to this embodiment, unsatisfactory efficiency under a high current can be improved singly by the p-type InGaN interlayer 70. Therefore, it has the advantages of a simple process and easy achievability.

Referring to Fig. 2, a method for fabricating an epitaxial structure that reduces efficiency droop in a GaN-based LED according to an embodiment includes the following steps.

In S100: a substrate 10 is provided and a GaN buffer layer 20 is grown on the substrate, wherein the GaN buffer layer 20 has a thickness of about 15 nm to 50 nm, as shown in Fig. 3.

In S200: a non-doped GaN layer 30 and a GaN layer 40 n-doped with silicon (Si) are sequentially formed over the GaN buffer layer 20. A total thickness of the non-doped GaN layer 30 and the n-doped GaN layer 40 can range from 1.5pm to 4.5 pm such as, for example, 3 pm.

In S300: a superlattice layer 50 for stress relief is formed on the n-doped GaN layer 40, as shown in Fig. 4.

The superlattice layer 50 can consist of periodic pairs of an InGaN layer having an In content varying in the range of 0% to 7% and a GaN layer. The superlattice layer 50 may have 3 to 20, for example, 10, such periodic pairs.

In S400: a multiple quantum well layer 60 is formed on the superlattice layer 50, as shown in Fig. 5.

The multiple quantum well layer 60 may consist of periodic pairs of a potential well layer and an overlying potential barrier layer. The multiple quantum well layer 60 may have 5 to 18, for example, 8, such periodic pairs. Each potential well layer may be formed of InGaN and have a thickness in the range of from 2 nm to 5 nm. Each potential barrier layer may be formed of GaN and have a thickness in the range of from 6 nm to 14 nm. In the multiple quantum well layer 60, all the other potential barriers than the last barrier layer (i.e., the potential barrier layer adjacent to a subsequently formed p-type InGaN interlayer 70) may be n-doped with silicon (Si) to an extent of lel7 cm'3 to 2el8cm'3.

In S500, a p-type InGaN interlayer 70 is formed on the multiple quantum well layer 60, as shown in Fig. 7.

The p-type InGaN interlayer 70 may be pulse-doped (delta-doped) with Mg to a concentration of 2el8 cm'3 to lel9cm"3 and have a thickness between 3 nm and 12 nm, for example, 8 nm

In S600: an electron blocking layer 80 and a p-type GaN layer 90 are sequentially formed over the p-type InGaN interlayer 70, thereby completing the epitaxial structure, as shown in Fig. 1.

The electron blocking layer 80 overlying the p-type InGaN interlayer 70 may be a p-GaN layer doped with aluminum (Al) (p-AlGaN), a p-type GaN layer (p-GaN) or a superlattice structure formed of a combination of them (p-AlGaN/GaN), and the electron blocking layer 80 has a thickness ranging from 30 nm to 80 nm, for example, 50 nm. The electron blocking layer 80 can increase the ability to block the leakage of electrons toward the p-terminal and thereby further improve the light-emission efficiency.

The p-type GaN layer 90 overlying the electron blocking layer 80 may be GaN p-doped with Mg of a concentration ranging from lel9 cm 3 to 6el9 cm 3. The p-type GaN layer 90 may have a thickness between 30 nm and 50 nm such as, for example, 40 nm, thereby completing the epitaxial structure.

In summary, in the epitaxial structures that reduce efficiency droop in GaN-based LEDs according to the embodiments of the present invention, because the p-type InGaN interlayer formed between the multiple quantum well layer and the p-type electron blocking layer has an In content varying in a gradual manner, a polarized electric field resulting from lattice mismatch between the GaN potential barrier layer and the interlayer can be reduced. In addition, compared to the conventional electron blocking layers, InGaN has a narrower forbidden band gap which results in an increase in hole injection efficiency, prevention of electrons from leaking toward the p-terminal and increased light-emission efficiency of the GaN-based LEDs when operating at a high current.

The foregoing description presents merely a preferred embodiment of the present invention and does not limit the scope of the invention in any way. All equivalent substitutions or modifications made to the subject matter disclosed herein by those of ordinary skill in the art without departing from the scope of the present invention fall within the scope of the invention.

Claims

1. An epitaxial structure for improving efficiency droop of a GaN-based light-emitting diode, the epitaxial structure comprising a substrate and, sequentially stacked on the substrate, a GaN underlayer, a superlattice layer for stress relief, a multiple quantum well layer, a p-type InGaN interlayer, a p-type electron blocking layer and a p-type GaN layer.

2. The epitaxial structure of claim 1, wherein the p-type InGaN interlayer is pulse-doped with Mg.

3. The epitaxial structure of claim 1, wherein the Mg dopant has a concentration of from lei 8 cm'3 to lel9cm'3.

4. The epitaxial structure of claim 1, wherein the p-type InGaN interlayer has an In content varying gradually from 0% to 7%.

5. The epitaxial structure of claim 1, wherein the p-type InGaN interlayer has a thickness of from 3 nm to 12 nm.

6. The epitaxial structure of claim 1, wherein the p-type electron blocking layer is formed from a p-AlGaN layer or a superlattice structure formed of p-AlGaN and p-GaN layers, and the p-type electron blocking layer has a thickness of from 30 nm to 80 nm.

7. The epitaxial structure of claim 1, wherein the p-type GaN layer is doped with Mg of a concentration of from lel9 cm"3 to 6e20cm'3 and has a thickness of from 30 nm to 50 nm.