JP2015026756A - Light-emitting diode element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device having an auxiliary electrode with low contact resistance.SOLUTION: An auxiliary electrode layer 10 is provided between a p-type semiconductor layer 17 and a positive electrode 21 in order to electrically connect the p-type semiconductor layer to the positive electrode. The auxiliary electrode layer has an intermediate layer 18 in contact with the p-type semiconductor layer. The intermediate layer is constituted of a thin film which contains, as main components, conductive materials having work functions larger than that of a metal oxide layer 19 in contact with the intermediate layer, specifically one or more selected materials selected from the conductive material group consisting of Ni, Rh, Pd, Os, Ir, Pt, Au, WO, and MoO. Thus, contact resistance between the auxiliary electrode layer and the p-type semiconductor layer is made low.

Description

  The present invention relates to a technical field of a light-emitting diode element, and more particularly to a light-emitting diode element having a transparent electrode having a small contact resistance with a p-type semiconductor layer mainly composed of GaN.

The light emitting diode is based on a pn junction and utilizes light emitted by recombination of holes and electrons.
In recent years, as a long-life and high-efficiency light-emitting element, it has been widely used for backlights for liquid crystals and illumination.
In particular, since GaN-based LEDs have a large band gap, they are used for blue-violet semiconductor lasers and blue light-emitting diodes.

Mg is generally used as a p-GaN dopant, but Mg in GaN has a low activation rate, and it is difficult to obtain p-GaN having a high carrier density.
For this reason, the contact resistance between the p-type GaN and the electrode tends to be high, which causes the drive voltage of the LED element to increase.

Further, in an LED, a metal oxide layer such as ITO is used as an electrode material of p-GaN.
By using a metal oxide electrode, it is possible to form an electrode having a higher transmittance than when a metal electrode is used, but further improvement in light emission efficiency of the LED is always required, and the transmittance is further increased. It is necessary to have an electrode that excels.

Japanese Patent Laid-Open No. 10-173222 JP 2008-153676 A

  In order to solve the above-described problems of the prior art, an object of the present invention is to provide a technique capable of reducing the contact resistance while maintaining a practically sufficient transmittance.

  The work function of p-type GaN is about 7 eV, and in order to achieve ohmic connection, a metal having a work function larger than that of p-type GaN needs to be brought into contact with p-type GaN.

  Since such a metal does not exist and the metal oxide layer has a work function lower than that of the p-type GaN layer, the inventors of the present invention have no resistance between the p-type GaN layer and the metal oxide layer. In order to reduce the Schottky barrier, a metal having a work function lower than that of the p-type GaN layer but having a work function as large as possible is defined as a p-type GaN layer and a metal oxide layer. As a result, the contact resistance value between the p-type GaN layer and the metal oxide layer was lowered as expected.

The present invention has been made based on the above predictions and experimental results. The present invention is mainly composed of an n-type semiconductor layer mainly composed of GaN and GaN located on one side of the n-type semiconductor layer. A p-type semiconductor layer and an auxiliary electrode layer located on the opposite side of the p-type semiconductor layer to the n-type semiconductor layer, and between the p-type semiconductor layer and the n-type semiconductor layer A light-emitting diode element that emits light when an operating current flows when voltage is applied thereto, wherein the auxiliary electrode layer is in contact with the p-type semiconductor layer and a transparent intermediate layer provided in contact with the p-type semiconductor layer. A transparent metal oxide layer provided, wherein the intermediate layer is a thin film having a work function higher than a work function of the metal oxide layer to be contacted.
The present invention is a light emitting diode device, wherein the intermediate layer is selected from at least one of a conductive material group consisting of Ni, Rh, Pd, Os, Ir, Pt, Au, WO _x , and MoO _x. The light emitting diode element is a thin film mainly composed of a selected material.
The present invention is a light emitting diode element, wherein the metal oxide layer is an ITO layer.
The present invention is a light emitting diode device, wherein the n-type semiconductor layer includes an n-type GaN layer, and a multiple quantum well layer is provided between the n-type GaN layer and the p-type semiconductor layer. It is a light emitting diode element.

  Since the contact resistance between the p-type semiconductor layer and the auxiliary electrode layer is small, the light-emitting diode element can be operated with low power consumption.

(a): A diagram for explaining a film formation target for forming an auxiliary electrode layer (b): A diagram for explaining a processing target to be processed in the next step The figure for demonstrating an example of the light emitting diode element of this invention The figure for demonstrating the film-forming apparatus which forms the auxiliary electrode layer of this invention Graph showing the relationship between work function and resistivity ratio values (a): Diagram for explaining a method for measuring the resistivity of contact resistance by the TLM method (b): Graph for obtaining contact resistance and propagation length

Reference numeral 3 in FIG. 2 indicates a light emitting diode element according to an example of the present invention.
The light emitting diode element 3 has a substrate 13. On the substrate 13, an n-type semiconductor layer 15 mainly composed of GaN and a p-type semiconductor layer 17 mainly composed of GaN are arranged. Here, the meaning of “having GaN as the main component” means that the total number of Ga atoms and N atoms is contained in a proportion higher than 50 atomic% of all atoms. And In the case of a p-type or n-type GaN layer, atoms other than the p-type or n-type dopant are Ga and N, and GaN is a main component close to 100%. "Includes semiconductors such as InGaN and AlGaN.

  The n-type semiconductor layer 15 is an n-type GaN layer in the light-emitting diode element 3, and the n-type GaN of the n-type semiconductor layer 15 and the p-type GaN of the p-type semiconductor layer 17 are formed into a single crystal layer by epitaxial growth. ing. Here, in order to grow a single crystal layer, a sapphire substrate is used as the substrate 13, and after the buffer layer 14 made of n-type GaN is formed on the surface of the substrate 13, the surface of the buffer layer 14 is formed. In addition, n-type GaN is epitaxially grown to form an n-type semiconductor layer 15.

  In order to improve the light emission efficiency, after the multiple quantum well layer 16 is formed on the n-type semiconductor layer 15, the p-type semiconductor layer 17 and the auxiliary electrode layer 10 are formed on the multiple quantum well layer 16. .

  The auxiliary electrode layer 10 is transparent, on the surface of the p-type semiconductor layer 17, a transparent intermediate layer 18 provided in contact with the p-type semiconductor layer 17 mainly composed of GaN, and on the surface of the intermediate layer 18. And a transparent metal oxide layer 19 provided in contact with the intermediate layer 18.

  The metal oxide layer 19 is transparent, and a thin film of ITO (indium tin oxide), AZO (aluminum doped zinc oxide), GZO (gallium doped zinc oxide), IZO (indium zinc oxide) is used as the metal oxide layer. However, since the present invention has a low resistance, the metal oxide layer 19 made of ITO is particularly suitable.

  The metal oxide layer 19 generally has a small work function value, and the surface of the ITO layer is about 4.7 eV. When the work function of the intermediate layer 18 is larger than that of the p-type semiconductor layer 17, an ohmic junction is formed between the p-type semiconductor layer 17 and the intermediate layer 18. The work function is often smaller than the work function of p-type GaN, and in particular for metal conductive materials, it is often about 4 eV.

In the present invention, the intermediate layer 18 has one surface in contact with the p-type semiconductor layer 17 and the opposite surface in contact with the metal oxide layer 19, so that the work function of the intermediate layer 18 is that of the intermediate layer 18. The conductive material constituting the intermediate layer 18 is selected so as to be larger than the work function of the metal oxide layer 19 in contact. For example, Ni, Rh, Pd, Os, Ir, Pt are used as the conductive material. , Au, WO _x , and MoO _x can be used. Therefore, the intermediate layer 18 is mainly composed of a selective material selected from at least one selected from the conductive material group consisting of Ni, Rh, Pd, Os, Ir, Pt, Au, WO _x , and MoO _x. Has been.
That the selected material is the main component of the intermediate layer 18 means that the ratio of the number of atoms of the selected material to the total number of atoms of the substance constituting the intermediate layer 18 is greater than 50%.

A positive electrode 21 is formed in contact with the metal oxide layer 19, and the p-type semiconductor layer 17 is electrically connected to the positive electrode 21 through the intermediate layer 18 and the metal oxide layer 19. ing.
On the other hand, a negative electrode 22 is formed in contact with the n-type semiconductor layer 15. Here, a part of the surface of the n-type semiconductor layer 15 on the multiple quantum well layer 16 side is exposed, and the negative electrode 22 is exposed. The n-type semiconductor layer 15 is electrically connected to the negative electrode 22.

  Accordingly, when a voltage is applied between the positive electrode 21 and the negative electrode 22 so that the positive electrode 21 has a positive voltage with respect to the negative electrode 22, the p-type semiconductor layer 17 and the n-type semiconductor layer 15 are forward-biased. Then, holes are injected into the multiple quantum well layer 16 from the p-type semiconductor layer 17, electrons are injected from the n-type semiconductor layer 15, and the injected holes and electrons are recombined inside the multiple quantum well layer 16. , Emission light is generated.

The thickness of the intermediate layer 18 is preferably 0.1 nm or more and 10 nm or less because the intermediate layer 18 is preferably transparent to the emitted light when the emitted light of the light emitting diode is transmitted.
The multiple quantum well layer 16, the p-type semiconductor layer 17, and the metal oxide layer 19 are all transparent to the emitted light, and of the emitted light generated inside the multiple quantum well layer 16, the p-type semiconductor layer The emitted light traveling in the direction in which 17 is located is transmitted from the multiple quantum well layer 16, the p-type semiconductor layer 17, the intermediate layer 18, and the metal oxide layer 19, and emitted from a portion where the positive electrode 21 is not located. It is emitted to the outside of the diode element 3.

  The n-type semiconductor layer 15, the buffer layer 14, and the substrate 13 are all transparent to the emitted light, and the reflective layer 12 is provided on the surface of the substrate 13 opposite to the surface on which the n-type semiconductor layer 15 is disposed. The emitted light that is provided and travels in the direction in which the n-type semiconductor layer 15 is located passes through the multiple quantum well layer 16, the n-type semiconductor layer 15, the buffer layer 14, and the substrate 13. The light is reflected in the direction in which the substrate 13 is located, passes through the substrate 13 and the layers 14 to 19 on the reflective layer 12, and is emitted from the metal oxide layer 19 to the outside.

  The formation process of the intermediate layer 18 and the metal oxide layer 19 of the light-emitting diode element 3 will be described. Reference numeral 2a in FIG. 1 (a) denotes a film formation target for forming the intermediate layer 18 and the metal oxide layer 19. The buffer layer 14, the n-type semiconductor layer 15, the multiple quantum well layer 16, and the p-type semiconductor layer 17 are formed on the substrate 13, and the surface of the p-type semiconductor layer 17 is exposed. Has been.

FIG. 3 shows a film forming apparatus 40 having a carry-in / out chamber 41 and a film forming chamber 42.
The film formation target 2a is carried into the carry-in / out chamber 41, and the inside of the carry-in / out chamber 41 is evacuated by the vacuum exhaust device 47a. The film forming chamber 42 is evacuated in advance by a vacuum exhaust device 47b, and the gate valve 48 is opened to move the film forming target 2a into the film forming chamber 42.

An intermediate layer target 45 and a metal oxide target 46 are disposed inside the film forming chamber 42.
The intermediate layer target 45 is mainly composed of a selective material selected from at least one selected from a conductive material group consisting of Ni, Rh, Pd, Os, Ir, Pt, Au, WO _x , and MoO _x. The intermediate layer target 45 is provided in the film formation chamber 42, or two or more.

  When two or more intermediate layer targets 45 are provided in the film formation chamber 42, each intermediate layer target 45 may be made of a different conductive material, and a plurality of intermediate layer targets 45 having different conductive materials are combined together. Sputtered particles may reach the p-type semiconductor layer 17 by sputtering, and the intermediate layer 18 having the conductive material of each intermediate layer target 45 may be formed.

Here, the metal oxide target 46 is made of ITO.
The intermediate layer target 45 and the metal oxide target 46 contain a very small amount of impurities that are difficult to remove.

  A gas introducing device 44 is connected to the film forming chamber 42, and the inside of the film forming chamber 42 is evacuated by a vacuum evacuating device 47 b while a sputtering gas is introduced from the gas introducing device 44. Inside the chamber 42, the intermediate layer target 45 and the metal oxide target 46 are sputtered.

  The film formation target 2 a is moved in the film formation chamber 42 by a transfer device (not shown) in the film formation chamber 42, while exposing the exposed p-type semiconductor layer 17 to the intermediate layer target 45. An intermediate layer 18 that is in contact with the p-type semiconductor layer 17 is formed on the surface of the p-type semiconductor layer 17 while passing near the intermediate layer target 45.

  Next, the film formation target 2a is moved while the exposed intermediate layer 18 faces the metal oxide target 46, and a metal oxide layer 19 in contact with the intermediate layer 18 is formed on the surface of the intermediate layer 18. A processing object to be processed in the next step is obtained. Reference numeral 2b in FIG. 1B indicates the processing object.

The processing object 2b is opened from the film forming apparatus 40 after opening the gate valve 48 and returned to the carry-in / out chamber 41 which has been evacuated in advance, and N ₂ + O ₂ (N ₂ : O ₂ = 8: 2). In this atmosphere, annealing is performed at a temperature of 550 ° C., and the resistance of the metal oxide layer 19 is reduced.

  Next, after partial etching to expose part of the n-type semiconductor layer 15, the positive electrode 21 is formed on the surface of the metal oxide layer 19, and the negative electrode 22 is formed on the n-type semiconductor layer 15. When the reflective layer 12 is formed, the light emitting diode element 3 of FIG. 2 is obtained.

Next, the auxiliary electrode layer 10 and the p-type semiconductor when the auxiliary electrode layer 10 is brought into contact with the p-type semiconductor layer 17 made of p-type GaN and a current flows from the p-type semiconductor layer 17 toward the auxiliary electrode layer 10. The resistivity (10 ⁻² Ω · cm ² ) of the contact resistance with the layer 17 was measured.

  Table 1 below shows the measured values when the auxiliary electrode layer 10 made of the metal oxide layer 19 that is an ITO layer is brought into contact with the p-type semiconductor layer 17 made of p-type GaN without providing the intermediate layer 18. The resistivity when the intermediate layer 18 of the auxiliary electrode layer 10 having the intermediate layer 18 and the metal oxide layer 19 which is an ITO layer is brought into contact with the p-type semiconductor layer 17 made of p-type GaN is expressed as the intermediate layer. The value is shown as a ratio to the resistivity when 18 is not provided.

Even if a Cu thin film or an Al thin film is brought into contact with the p-type GaN layer and provided between the p-type GaN layer and the ITO layer, the resistance value when the ITO layer is brought into contact with the p-type GaN layer is reduced. However, from Table 1, when the auxiliary electrode layer 10 has an intermediate layer 18 using Ni, Rh, Pd, Re, Os, Ir, Pt, Au, WO _x , and MoO _x as a conductive material, It can be seen that the resistivity is smaller than when the layer 18 is not provided.

When the maximum value of the work function of the ITO layer, AZO layer, IZO layer, or TiO ₂ layer is 4.7 eV, Ni, Rh, Pd, Os, Ir, Pt, Au, WO having a work function larger than that value _In the intermediate layer 18 in which _x and MoO _x are selected as the conductive materials, the resistivity ratio value is 0.55 or less, which is preferable from the viewpoint of lower voltage and lower power consumption.

On the other hand, the value of the ratio of the resistivity of the Re intermediate layer 18 is smaller than “1”, which is lower than that in the case where the intermediate layer 18 is not provided, but the conductive material is Ni, Rh, Pd, It is larger than the intermediate layer 18 of Os, Ir, Pt, Au, WO _x , and MoO _x , and Ni, Rh, Pd, Os, Ir, Pt, Au, WO _x , and MoO _x are intermediate in the present invention. Preferred as layer 18.

FIG. 4 is a graph in which measured values of the intermediate layer in Table 1 are plotted with the work function on the horizontal axis and the resistivity ratio value of contact resistance on the vertical axis.
It can be seen that in the intermediate layer other than the intermediate layer of WO _x (work function is 9.20), the resistivity ratio is smaller as the work function is larger. This is probably because the Schottky barrier becomes smaller as the work function becomes larger.

In addition, the measurement of the resistivity of the contact resistance of Table 1 was performed by the TLM method. In the TLM method, first, as shown in FIG. 5A, two layers separated by a two-layer structure of intermediate layers 18 ₁ and 18 ₂ and metal oxide layers 19 ₁ and 19 ₂ are formed on a p-type semiconductor layer 17. A plurality of sets of auxiliary electrode layers 10 ₁ , 10 ₂ were prepared with different electrode intervals L, and a resistance value was measured by passing a current between each set of auxiliary electrode layers 10 ₁ , 10 ₂ . The auxiliary electrode layers 10 ₁ and 10 ₂ are rectangular or square, and are arranged so that one side faces each other in parallel.

As shown in FIG. 5B (the horizontal axis is the electrode interval L and the vertical axis is the measured resistance value R _T ), the measured resistance value is plotted on the graph, and a straight line P connecting the measured resistance values is obtained. , The resistance value _RT of the Y-axis intercept is the resistance value when the electrode interval L is zero, the measured resistance value _RT , the sheet resistance _RS of the p-type semiconductor layer 17, and one auxiliary electrode layer 10 _{The following} equation (1) between the contact resistance R _C per ₁ or 10 ₂ , the width W of the auxiliary electrode layers 10 ₁ , 10 ₂ perpendicular to the electrode interval L, and the electrode interval L:

R _T = R _S · L / W + 2 · R _C (1)

From this, it can be seen that the value of the x-axis intercept indicates the total value of the contact resistance R _C between the _two auxiliary electrode layers 10 ₁ , 10 ₂ and the p-type semiconductor layer 17 (= 2 · R _C ).
Then, when the straight line P on the graph is extrapolated to a region where the electrode interval L is negative and the value of the X-axis intercept is obtained, the length when the p-type semiconductor layer 17 becomes the contact resistance Rc is obtained from that value. propagation length L _T is determined, by substituting the propagation length L _T in the following equation (2), the resistivity of the contact resistance [rho (Omega · cm ²⁾ is calculated.

ρ = Rs · L _T ² (2)

Note that the present invention also includes a light emitting diode in which a metal layer is used instead of the metal oxide layer, an intermediate layer is formed between the p-type semiconductor layer mainly composed of GaN and the metal layer, and the resistivity is lowered. It is.
Although the ITO film is annealed at 550 ° C., annealing at a temperature lower than 550 ° C. does not increase the contact resistance. Conversely, annealing can be performed at a temperature higher than 550 ° C., which is also included in the present invention.

In the intermediate layer of WO _x having a work function larger than that of the metal oxide layer, in order to form an ohmic connection, the work function between the metal oxide layer and the p-type semiconductor layer containing GaN as a main component is obtained. It is Ni, Rh, Pd, Os, Ir, Pt, Au, and WO _x that form a Schottky junction.

3... Light emitting diode element 13... Substrate 14... Buffer layer 15... N-type semiconductor layer 16 containing GaN as a main component 16 .. multiple quantum well layer 17. ... intermediate layer 19 ... metal oxide layer

Claims (4)

An n-type semiconductor layer mainly composed of GaN;
A p-type semiconductor layer mainly composed of GaN located on one side of the n-type semiconductor layer;
An auxiliary electrode layer located on the opposite side of the p-type semiconductor layer from the n-type semiconductor layer;
A light emitting diode element that emits light when an operating current flows when a voltage is applied between the p-type semiconductor layer and the n-type semiconductor layer,
The auxiliary electrode layer has a transparent intermediate layer provided in contact with the p-type semiconductor layer, and a transparent metal oxide layer provided in contact with the intermediate layer,
The light emitting diode device, wherein the intermediate layer is a thin film having a work function higher than that of the metal oxide layer to be contacted. The intermediate layer is a thin film containing as a main component a selective material selected from at least one selected from a conductive material group consisting of Ni, Rh, Pd, Os, Ir, Pt, Au, WO _x , and MoO _x. The light emitting diode element according to claim 1.   The light emitting diode element according to claim 1, wherein the metal oxide layer is an ITO layer. The n-type semiconductor layer has an n-type GaN layer,
4. The light emitting diode device according to claim 1, wherein a multiple quantum well layer is provided between the n-type GaN layer and the p-type semiconductor layer. 5.

Images