CN103337568B - Strained super lattice tunnel junction ultraviolet LED epitaxial structure and preparation method thereof - Google Patents

Abstract

The invention provides a strained superlattice tunnel junction ultraviolet LED epitaxial structure and a preparation method thereof. The epitaxial structure is from top to bottom: an epitaxial growth substrate, an AlN buffer layer, an n-type AlGaN layer, a multilayer quantum well, and an electron barrier Layer, strained superlattice, n-type degenerately doped AlGaN layer, n-type Si-doped AlGaN layer; strained superlattice includes p-type AlGaN layer and AlyGa1 _{-yN /} AlxGa1 _{-xN .} The energy band of AlGaN moves to the low-energy direction under the action of the polarization electric field as a whole, and the superlattice structure contacts with heavily doped n-type AlGaN to form a p-AlGaN/SSL/n*-AlGaN tunnel junction, p-type AlGaN Under the action of an external electric field, the electrons in the valence band tunnel to the n-type AlGaN side through the tunnel effect, forming holes in the p-type AlGaN. The material of the surface layer is changed from p-type AlGaN to n-AlGaN, which avoids the problem of p-type ohmic contact.

Description

Strained superlattice tunnel junction ultraviolet LED epitaxial structure and its preparation method

【Technical field】

The invention belongs to the technical field of semiconductor light-emitting diodes, and in particular relates to an ultraviolet LED epitaxial structure and a preparation method thereof which utilize a strained superlattice tunnel junction structure to enhance the performance of a p-type nitride material.

【Background technique】

Ultraviolet light emitting diode (light emitting diode, hereinafter referred to as LED), because of its short wavelength, high photon energy, and uniform beam, has important applications in the fields of physical sterilization, high color rendering index lighting, and high-density optical storage. At present, a large number of researches have made important breakthroughs in crystal quality, high Al composition and short-wavelength structure design, and successfully prepared deep ultraviolet LED devices below 300 nanometers to achieve milliwatt-level power output and reliability. great progress has been made.

However, AlGaN materials with high Al composition will reduce carrier concentration and carrier mobility. With the increase of Al composition, the acceptor activation energy of Mg atoms increases linearly, making the p-type doping activation rate very low, and the hole concentration at room temperature is very low, so the preparation of p-type ohmic contacts becomes very difficult. A good ohmic contact determines the electrical injection efficiency, which directly affects the overall performance of semiconductor devices.

At present, in order to increase the carrier concentration of the p-type layer and reduce the p-type ohmic contact resistance, the technology used is mainly to use the p-type layer of the superlattice structure, and then grow a p-type GaN cap on the p-type AlGaN. layer as an ohmic contact layer and is achieved by optimizing the annealing conditions. However, these methods can only optimize the p-type ohmic contact within a certain range, and its resistivity is still several orders of magnitude higher than that of the n-type ohmic contact, and the thick p-type GaN cap layer will cause photon absorption loss.

A low-resistance GaN/InGaN/GaN tunnel junction structure, degenerately doping both n-type GaN and p-type GaN, growing a layer of n-type GaN on p-type GaN, and using the strong electric field generated by interface polarization charges Make the GaNpn junction above the quantum well meet the conditions of the tunnel diode, so that the electrons in the p-type GaN valence band enter the n-type GaN conduction band through the tunneling effect, leaving a large number of holes in the p-type GaN valence band. The contact electrodes on the upper surface are directly made on the n-type GaN surface on the p-type GaN, that is, the positive and negative contact electrodes are all realized under the n-type GaN structure, which not only avoids p-type ohmic contact, but also achieves low resistivity. goals while maintaining device performance integrity. However, this method still uses heavy doping to achieve degeneracy, thereby obtaining a tunnel junction, but the difficulty of p-type nitride doping still exists.

When a material is placed under a strain system and elastically strained, the energy band changes. Among them, the positive strain component causes the overall movement of the conduction band and the valence band, and the movement amounts are: ΔE _c = a _c (ε _xx +ε _yy +ε _zz ), ΔE _v =a _v (ε _xx +ε _yy +ε _zz ), where a _c and a _v are the hydrostatic pressure deformation potentials of the conduction band edge and valence band edge of the semiconductor material, respectively, and ε is the strain caused by the normal pressure in each direction.

In the case of a large lattice mismatch (7% to 9%), as long as the thickness of each layer of the crystal is thin enough, the stress formed by the lattice mismatch can be adjusted by the elastic strain of each layer, so that it can grow Strained Superlattice (SSL) without misfit dislocations. Using the superlattice structure can reduce the defect state density caused by dislocations and increase the effective carrier concentration; in addition, by controlling the stress field generated by the strain of the superlattice, the conduction band bottom and valence band of the semiconductor material can be quantitatively adjusted. The overall movement of the top is of great significance for the realization of non-degenerate tunneling junctions.

【Content of invention】

The technical problem to be solved by the present invention is to provide a tunnel junction ultraviolet LED epitaxial structure and a preparation method to increase the hole carrier concentration of the p-type AlGaN layer and solve the ohmic contact bottleneck problem.

In order to achieve the above object, the present invention adopts the following technical solutions:

A strained superlattice tunnel junction ultraviolet LED epitaxial structure, including an epitaxial growth substrate, and an AlN buffer layer, an n-type AlGaN layer, a periodic multilayer quantum well, an electron blocking layer, and a strained growth substrate sequentially grown on the epitaxial growth substrate Superlattice, n-type degenerately doped AlGaN layer, n-type Si-doped AlGaN cap layer; wherein, the strained superlattice includes a p-type AlGaN layer and a single layer or multilayer _AlyGa layer continuously grown on it _1-y N/Al _x Ga _1-x N, where y>x>0.65.

As a preferred embodiment of the present invention, the material of the multilayer quantum wells is Al _0.65 Ga _0.35 N/Al _0.7 Ga _0.3 N, grown alternately.

As a preferred embodiment of the present invention, the material of the electron blocking layer is Al _0.8 Ga _0.2 N/Al _0.77 Ga _0.23 N.

As a preferred embodiment of the present invention, the single-layer or multi-layer AlyGa1 _{-yN /} AlxGa1 _{- xN} is co-doped with Mg and Si, and the co-doped doping form shows a δ distribution.

As a preferred embodiment of the present invention, the Al composition in AlxGa1 _{-xN in} the superlattice is higher than the Al composition in the quantum well material to ensure that the photons emitted by the active layer are not absorbed.

As a preferred embodiment of the present invention, the doping concentration of the n-type degenerately doped AlGaN layer is greater than 1×10 ¹⁹ cm ^-3 , so that its energy band is in a degenerate state, which facilitates the formation of tunneling junctions.

As a preferred embodiment of the present invention, the doping concentration of the n-type Si-doped AlGaN cap layer is greater than 1×10 ¹⁸ cm ^-3 , and is used as an anode ohmic contact layer for making devices.

A preparation method for preparing a strained superlattice tunnel junction ultraviolet LED epitaxial structure, comprising the following steps:

(1) Prepare an AlN buffer layer on the epitaxial growth substrate, the thickness of the AlN buffer layer is 10nm～500nm;

(2) growing a Si-doped n-type Al _0.77 Ga _0.23 N layer on the epitaxial growth substrate on which the AlN buffer layer has been grown;

(3) Alternately grow multilayer quantum wells on the n-type Al _0.77 Ga _0.23 N layer; the material of the multilayer quantum wells is Al _0.65 Ga _0.35 N/Al _0.7 Ga _0.3 N;

(4) growing a multi-layer electron blocking layer on the topmost layer of the multi-layer quantum well, and the material of the electron blocking layer is Al _0.8 Ga _0.2 N/Al _0.77 Ga _0.23 N;

(5) Growth of p-type Al _0.77 Ga _0.23 N and Al _0.77 Ga _0.23 N/AlN strained superlattice (SSL) on the electron blocking layer;

(6) Grow a thin layer of heavily doped n-type Al _0.77 Ga _0.23 N on the Al _0.77 Ga _0.23 N/AlN strained superlattice (SSL), and then grow a thin layer on the heavily doped n-type Al _0.77 Ga _0.23 N The layer is lightly doped with n-type Al _0.77 Ga _0.23 N.

Compared with the prior art, the invention has the following beneficial effects: the invention utilizes the strain field generated by the superlattice structure to affect the energy band structure. In the superlattice energy band structure, the energy band of AlGaN moves to the lower energy direction as a whole under the action of the polarization electric field. After the superlattice structure is in contact with heavily doped n-type AlGaN, a p-AlGaN/SSL/n*-AlGaN tunnel junction is formed, and electrons in the valence band of p-type AlGaN tunnel to the On the n-type AlGaN side, holes, that is, hole carriers, are formed in the p-type AlGaN. At the same time, the surface material is changed from p-type AlGaN to n-AlGaN, which avoids the problem of p-type ohmic contact. The n-type contact electrode is used as the anode of the LED device, which greatly reduces the resistivity and improves the current spreading performance, thereby enhancing the device. performance.

【Description of drawings】

Fig. 1 is a schematic diagram of the epitaxial structure of an ultraviolet LED containing an SSL tunnel junction according to the present invention.

Fig. 2 is a schematic diagram of the structure of the electron blocking layer.

Fig. 3 is a schematic diagram of the AlyGa1 _{-yN /} AlxGa1 _{- xN} strained superlattice structure.

Fig. 4 is a schematic diagram of energy bands for forming a tunnel junction.

Among them, 1-epitaxial growth substrate, 2-AlN buffer layer, 3-n-type Si-doped AlGaN layer, 4-periodic multilayer quantum well, 5-multilayer thickness gradient structure electron blocking layer, 6-p-type AlGaN layer, Al _y Ga _1-y N/Al _x Ga _1-x N superlattice, 601-tunnel junction, 7-n-type degenerate Si-doped AlGaN layer, 8-n-type Si-doped AlGaN layer, 51- Al _0.8 Ga _0.2 N, 52-Al _0.77 Ga _0.23 N, 61-Al _y Ga _1-y N, 62-Al _x Ga _1-x N.

【detailed description】

The invention provides a method for manufacturing a tunnel junction-enhanced ultraviolet LED epitaxial structure, which uses the strain field generated by the superlattice to move the energy band of the AlGaN material in the SSL structure to a low-energy direction as a whole, and forms p-AlGaN/ SSL/n*-AlGaN tunnel junction, providing hole carriers. Include at least the following steps:

Using MOCVD (Metal-organicChemicalVaporDepositiong, metal organic compound chemical vapor deposition) or PAMBE system for epitaxial growth:

1) Prepare an AlN buffer layer 2 on the epitaxial substrate 1, with a thickness of 10 nm to 500 nm;

2) growing a Si-doped n-type Al _0.77 Ga _0.23 N layer on the substrate 1 on which the AlN buffer layer 2 has been grown;

3) five multiple quantum well layers 4 are alternately grown on the n-type AlGaN layer 3; the material of the multiple quantum well layers is Al _0.65 Ga _0.35 N/Al _0.7 Ga _0.3 N;

4) Multi-layer electron blocking layers 5 are grown on the topmost quantum well layer, and the thickness of the electron blocking layers 5 gradually changes. The material is Al _0.8 Ga _0.2 N/Al _0.77 Ga _0.23 N.

5) growing p-type Al _0.77 Ga _0.23 N and Al _0.77 Ga _0.23 N/AlNSSL on the electron blocking layer;

6) Grow a thin layer of heavily doped n-type Al _0.77 Ga _0.23 N on Al _0.77 Ga _0.23 N/AlNSSL, and then grow a layer of lightly doped n-type Al on heavily doped n-type Al _0.77 Ga _0.23 N _0.77 Ga _0.23 N.

When preparing the tunnel junction, the corresponding material, dopant and doping concentration can be selected according to the required characteristics of the tunnel junction. If the tunnel junction material is prepared by AlGaN/AlNSSL and n-type AlGaN, the doping form of Mg and Si is co-doped with δ distribution, and Si element is used as the dopant of AlGaN/AlNSSL and n-type AlGaN respectively, and the p-type AlGaN region The thickness is relatively thin.

In the above-mentioned 2) step, the proportion of the Al component of n-type Al _0.77 Ga _0.23 N is properly selected according to the quantum well center material, but it must be higher than the Al component in the quantum hydrazine to ensure that the active layer emits photons Not absorbed.

In the above step 3), the proportion of the Al component of the reflective center material of the multiple quantum well layer Al _0.65 Ga _0.35 N is determined by the required wavelength. If Al _0.65 Ga _0.35 N is selected as the quantum well luminescence center material, the center wavelength of the luminescence is 248nm.

In step 5) above, the number and thickness of Al _0.77 Ga _0.23 N/AlN superlattice layers are changed according to the Al composition of p-type AlGaN, and the number of layers of AlGaN/AlN superlattice is changed between 1-10 layers . The thickness of each layer varies between 0.5-2nm.

The structure and preparation method of the present invention will be further described below in conjunction with the accompanying drawings and specific examples. In the specific device design and manufacture, the ultraviolet LED structure proposed by the present invention will be modified and adapted to some of its structures and materials according to the needs of the application field and the implementation of the process.

The embodiment describes in detail the preparation process of the strained superlattice tunnel junction enhanced ultraviolet LED according to the present invention, and specifically explains the relevant parameters.

(1) Sapphire is selected as the epitaxial growth substrate, and an AlN buffer layer 2 is first prepared on the sapphire substrate 1 along the [0001] direction with a thickness of 10nm-500nm.

(2) An n-type Si-doped AlGaN layer 3 , a periodic multilayer quantum well 4 , and a multilayer electron blocking layer 5 are grown sequentially on the low-temperature AlN buffer layer 2 .

(3) After growing the electron blocking layer 5, continue to grow p-type AlGaN, and co-doped single-layer or multi-layer Al _y Ga _1-y N/Al _x Ga _1-x with Mg and Si doping forms showing a δ distribution N-strained superlattice 6. Its structure is shown in Figure 3. The Al composition in Al _x Ga _1-x N in the superlattice is higher than that in the quantum well material. The size relationship of the Al composition of the two materials of the superlattice is y>x>0.65. The thicknesses of AlxGa1 _{- xN} and _{AlyGa1 -yN} in the superlattice are both between 0.5nm and 2nm. The number of layers of the Al _y Ga _1-y N/Al _x Ga _1-x N strained superlattice is between 1 and 20.

(4) On the AlyGa1 _{-yN /} AlxGa1 _{- xN} strained superlattice 6, grow a layer of heavily doped n-type Si-doped AlGaN layer 7 with a doping concentration greater than 1×10 ¹⁹ cm ^-3 , so that its energy band is in a degenerate state, which is conducive to the formation of tunneling knots. Its thickness is between 1nm-500nm

(5) On the degenerately doped n-type AlGaN layer 7, grow a Si-doped n-type AlGaN layer with a lower doping concentration, the doping concentration is greater than 1×10 ¹⁸ cm ^-3 , as the anode for making devices Ohmic contact layer.

As shown in Figure 2, the electron blocking layer is a multi-layer AlGaN structure. The barrier layer width is a gradual structure. The number of layers of the electron blocking layer is 1-10 layers, and the thickness is 2nm-10nm. According to different requirements, the thickness of the electron blocking layer can be fixed or gradually changed.

As shown in FIG. 3 , a co-doped multilayer Al _y Ga _1-y N/Al _x Ga _1-x N strained superlattice structure in which the doping forms of Mg and Si are in a δ distribution is used. Among them, the AlyGa _{1-y N} layer is in a state of tensile strain, while the _{AlxGa 1-} xN layer is in a state of compressive strain, thus forming in the AlyGa _{1-y N} and _{AlxGa 1-} xN layers respectively The strain-polarized electric field along the [0001] and [000-1] directions. The electric field will cause the conduction band and valence band of AlGaN in the superlattice to move to the lower energy direction as a whole. Referring to Fig. 4, when the valence band energy level of p-type AlGaN is higher than the conduction band energy level of n-type AlGaN on the other side of the strained superlattice, and the thickness of the strained superlattice layer is very thin, a tunnel junction 601 is formed At this time, the electrons in the valence band of the p-region can enter the conduction band of the n-region through this tunnel junction, leaving holes in the p-region.

Referring to Fig. 1-Fig. 4, it is the epitaxial structure of strained superlattice tunnel junction ultraviolet LED described in the present invention, including:

- epitaxial growth substrate 1;

—AlN buffer layer 2, located on the epitaxial growth substrate 1;

- n-type AlGaN layer 3, located on the AlN buffer layer 2;

- a periodic multilayer quantum well 4, located on the n-type AlGaN layer 3;

- the electron blocking layer 5 is located on the multilayer quantum well 4;

—p-type AlGaN layer, co-doped multilayer AlyGa1 _{-yN /} AlxGa1 _{- xN} strained superlattice 6 with doping forms of Mg and Si in δ distribution, located on the electron blocking layer 5 ;

- n-type degenerately doped AlGaN layer 7, located on the multilayer AlyGa1 _{-yN /} AlxGa1 _{- xN} strained superlattice 6;

- An n-type Si-doped AlGaN cap layer 8 on the n-type degenerately doped AlGaN layer 7 .

In the present invention, the AlN buffer layer is used as the initial layer on the epitaxial growth substrate, and then the n-type AlGaN layer, the multi-quantum well region, the electron blocking layer, the p-type AlGaN layer, the _{AlyGa1 -yN} / Al _x Ga _1-x N strained superlattice, n-type degenerately doped AlGaN layer, n-type AlGaN cap layer. Using the co-doped multilayer AlyGa1 _{-yN /} AlxGa1 _{- xN} strained superlattice with the doping form of Mg and Si in the form of δ distribution to modify the energy band, so that a large number of electrons enter it through the tunnel junction The n-type AlGaN leaves a large number of hole carriers in the p region, providing sufficient hole carrier concentration for the active region to emit light. In addition, the n-type AlGaN structure on the upper surface is used as the anode contact layer of the device, which solves the problem of ohmic contact, reduces the turn-on voltage of the device, and improves the current expansion capability. The ultraviolet LED chip prepared by the method gets rid of the difficulty of p-type layer doping and the difficulty of making ohmic contact, and greatly improves the performance of the ultraviolet LED device.

Advantages of the present invention include:

1) Using the stress field generated by the co-doped multilayer AlGaN/AlNSSL structure in which the doping form of Mg and Si is δ distributed, the energy band of AlGaN in SSL is shifted to the low energy direction as a whole, so that it is compatible with p-type AlGaN and heavy The n-type AlGaN doped to a degenerate state collectively forms a tunnel junction. This method uses stress to realize the tunnel junction structure, rather than relying on pure Mg element heavy doping to realize the degenerate state.

2) A large number of electrons tunnel from the valence band of the p-type material to the conduction band of the n-type material through the tunnel junction, which will leave a large number of hole carriers in the p-type layer. This method solves the problem of difficult doping of p-type material with Mg element and improves the carrier concentration.

3) The n-type AlGaN is covered on the surface of the p-type layer, and the anode ohmic contact can be directly made on the n-type material, which greatly improves the ohmic contact performance, reduces the contact resistance, and improves the current expansion capability.

Claims (9)

1. A strained superlattice tunnel junction ultraviolet LED epitaxial structure, characterized in that: comprising: an epitaxial growth substrate (1), and an AlN buffer layer (2) and an n-type AlGaN layer grown sequentially on the epitaxial growth substrate , periodic multilayer quantum well (4), electron blocking layer (5), strained superlattice (6), n-type degenerately doped AlGaN layer (7), n-type Si-doped AlGaN cap layer (8); Wherein, the strained superlattice (6) includes a p-type AlGaN layer and a single layer or multiple layers of Al _y Ga _1-y N/Al _x Ga _1-x N grown on it, wherein, y>x> 0.65. 2. The strained superlattice tunnel junction ultraviolet LED epitaxial structure according to claim 1, characterized in that: the material of the multilayer quantum wells is Al _0.65 Ga _0.35 N/Al _0.7 Ga _0.3 N, grown alternately. 3. The strained superlattice tunnel junction ultraviolet LED epitaxial structure according to claim 1, characterized in that: the material of the electron blocking layer is Al _0.8 Ga _0.2 N/Al _0.77 Ga _0.23 N. 4. The strained superlattice tunnel junction ultraviolet LED epitaxial structure as claimed in claim 1, characterized in that: said single-layer or multi-layer Al _y Ga _1-y N/Al _x Ga _1-x N is Mg and Si Co-doped, co-doped doping form is δ distribution. 5. The strained superlattice tunnel junction ultraviolet LED epitaxial structure as claimed in claim 1 or 4, characterized in that: the Al composition in the Al _x Ga _1-x N in the superlattice is higher than that in the quantum well material Al component to ensure that the photons emitted by the active layer are not absorbed. 6. The strained superlattice tunnel junction ultraviolet LED epitaxial structure according to claim 1, characterized in that: the doping concentration of the n-type degenerately doped AlGaN layer (7) is greater than 1×10 ¹⁹ cm ^-3 , So that its energy band is in a degenerate state, which is conducive to the formation of tunneling knots. 7. The strained superlattice tunnel junction ultraviolet LED epitaxial structure according to claim 1, characterized in that: the doping concentration of the n-type Si-doped AlGaN cap layer (8) is greater than 1×10 ¹⁸ cm ^-3 , As an anode ohmic contact layer for making devices. 8. The strained superlattice tunnel junction ultraviolet LED epitaxial structure as claimed in claim 1, characterized in that: in the strained superlattice structure, the Al _y Ga _1-y N layer is in a state of tensile strain, and the Al _x Ga _{1- The x} N layer is in a state of compressive strain. 9. A preparation method based on the strained superlattice tunnel junction ultraviolet LED epitaxial structure according to claim 1, characterized in that: comprising the following steps: (1) preparing an AlN buffer layer (2) on the epitaxial growth substrate (1), the thickness of the AlN buffer layer being 10 nm to 500 nm; (2) growing a Si-doped n-type Al _0.77 Ga _0.23 N layer on the epitaxial growth substrate (1) on which the AlN buffer layer (2) has been grown; (3) Alternately growing multilayer quantum wells (4) on the n-type Al _0.77 Ga _0.23 N layer; the material of the multilayer quantum wells is Al _0.65 Ga _0.35 N/Al _0.7 Ga _0.3 N; (4) growing a multilayer electron blocking layer (5) on the topmost layer of the multilayer quantum well, the material is Al _0.8 Ga _0.2 N/Al _0.77 Ga _0.23 N; (5) Growth of p-type Al _0.77 Ga _0.23 N and Al _0.77 Ga _0.23 N/AlN strained superlattice on the electron blocking layer; (6) Grow a thin layer of heavily doped n-type Al _0.77 Ga _0.23 N on the Al _0.77 Ga _0.23 N/AlN strained superlattice, and then grow a thin layer of heavily doped n-type Al _0.77 Ga _0.23 N Doped with n-type Al _0.77 Ga _0.23 N.