Patent Application
20080237616
This application is based upon and claims the benefit of priority from prior Japanese Patent Application P2007-082000 filed on Mar. 27, 2007; the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a semiconductor light emitting device and a method for manufacturing the same.
2. Description of the Related Art
In a semiconductor light emitting device, such as a light emitting diode (LED) and the like, the refractive index of a semiconductor material of the semiconductor light emitting device is greater than air or a resin which is in contact with the semiconductor material. For this reason, total reflection occurs on an interface between the semiconductor material and the air or the resin so as to extremely decrease the light extraction efficiency of the semiconductor light emitting device. In order to increase the light extraction efficiency, various techniques, such as device shape processing, surface texture structure, and a photonic crystal, have been developed.
As one of such techniques, a technique for improving the light extraction efficiency by using interference has been reported (refer to JP-A 2004-207742 (KOKAI)). In a gallium nitride (GaN) based LED, a light output in a vertical direction can be intensified by interference with a reflection light from an electrode of the LED. For example, in the GaN based LED fabricated on a sapphire substrate, the light extraction efficiency from the sapphire substrate to the air is increased or decreased depending on the distance between an active layer serving as a light emitting layer and electrodes provided on a surface of a GaN layer. That is, the light extraction efficiency is increased when the reflection light from the electrode on the surface of the GaN layer and the light emitted in the vertical direction in the GaN layer get a constructive interference with each other. However, as a consequence of the total reflection on an interface between the GaN layer and the sapphire substrate, a value of the light extraction efficiency cannot be increased.
Furthermore, since a sapphire substrate is used, a flip-chip structure is employed, in which both of a p-side electrode and an n-side electrode of the LED are formed on a surface of the GaN layer opposite to the sapphire substrate. As a result, there is a problem in that a package assembly is difficult. Additionally, since the current is forced to flow horizontally in a narrow channel, series resistance between the electrodes also increases.
A conductive GaN substrate may be used instead of the sapphire substrate to provide a structure that enables a current to flow vertically between the electrodes. By using a GaN substrate, it is possible to provide the electrodes on front and back surfaces of the LED and to decrease series resistance between the electrodes. However, when the electrodes are provided on the front and back surfaces, it is impossible to extract a light from portions of the electrodes. Therefore, it is difficult to use the interference effect of a light reflected from the bottom electrode so as to intensify the light in the vertical direction of the LED, as mentioned above. Accordingly, in a typical semiconductor light emitting device, it is difficult to satisfy both requirements of low series resistance and high light extraction efficiency. Thus, it is difficult to achieve a semiconductor light emitting device having high performance.
A first aspect of the present invention inheres in a semiconductor light emitting device including an active layer radiating a light having a wavelength λ; a first semiconductor layer of a first conductivity type provided on the active layer, the first semiconductor layer having a first principal surface, a second principal surface and side surfaces, the first principal surface in contact with the active layer, the second principal surface facing the first principal surface, and the side surfaces connected to the second principal surface, each of the side surfaces having a bevel angle in a range from about 45 degrees to less than 90 degrees with respect to the second principal surface; a second semiconductor layer of a second conductivity type provided under the active layer; and a first electrode provided under the second semiconductor layer, wherein a distance d between the active layer and the first electrode depends on the wavelength λ and a refractive index n of the second semiconductor layer.
A second aspect of the present invention inheres in a method for manufacturing a semiconductor light emitting device including growing an active layer on a front surface of a first semiconductor layer having a first conductivity type; growing a second semiconductor layer of a second conductivity type on the active layer; forming a first electrode on the second semiconductor layer; forming a second electrode on a back surface of the first semiconductor layer; and dividing the first and second semiconductor layers into a chip having side surfaces, each of the side surfaces having a bevel angle in a range from about 45 degrees to less than 90 degrees with respect to the back surface, wherein a distance between the active layer and the first electrode depends on a wavelength λ and a refractive index n of the second semiconductor layer.
Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified.
An LED chip (20, 2 and 22) of a semiconductor light emitting device according to an embodiment of the present invention includes a first electrode 20, a semiconductor layer 2, and a second electrode 22, as shown in
The first semiconductor layer (10, 12) includes a semiconductor substrate 10, and a buffer layer 12 on a front surface (first principal surface) of the semiconductor substrate 10. The first electrode 20 is disposed on a front surface of the contact layer 18. The second electrode 22 is disposed on a back surface (second principal surface) of the semiconductor substrate 10 so as to face the first electrode 20 across the semiconductor layer 2. The active layer 14 is a light emitting layer of the semiconductor light emitting device.
For example, for the semiconductor substrate 10, an n-type (first conductivity type) semiconductor substrate, such as a GaN substrate, may be used. For the buffer layer 12, an n-type growth layer, such as a GaN layer, may be used. For the active layer 14, a quantum well (QW) layer, such as indium gallium nitride (InGaN) layers, may be used. For the contact layer 18, a p-type (second conductivity type) growth layer, such as a GaN layer, may be used. For the first electrode 20, a metal, such as silver (Ag), aluminum (Al), gold (Au), and palladium (Pd), and an alloy containing such metal as a main component may be used. For the first electrode 20, Ag, which is a high reflection film material, or a Ag alloy including a metal, such as Pd, rhodium (Rh), Au, copper (Cu), neodymium (Nd), titanium (Ti), magnesium (Mg), zinc (Zn), and In, is desirable. For the second electrode 22, a laminated metal film, such as a Ti/platinum (Pt)/Au film, may be used.
Note that the first conductivity type and the second conductivity type are conductivities opposite to each other. Specifically, if the first conductivity type is n-type, the second conductivity type is p-type, and, if the first conductivity type is p-type, the second conductivity type is n-type. In the following description, for convenience, n-type conductivity is set as the first conductivity type, and p-type conductivity is set as the second conductivity type. However, p-type may be set as the first conductivity type and n-type may be set as the second conductivity type.
As shown in
In a cross section perpendicular to the back surface of the semiconductor substrate 10, respective side surfaces 40a, 40b, 40c and 40d of the semiconductor substrate 10 are bevel planes having bevel angles Θ to planes parallel to the back surface of the semiconductor substrate 10. The distance between the active layer 14 and the first electrode 20 is denoted as “d”.
The distance d corresponds to a physical thickness of the contact layer 18 provided between the active layer 14 and the first electrode 20. For example, the refractive index of the semiconductor material GaN of the contact layer 18 is denoted by “n” and a center wavelength of light emitted from the active layer 14 is denoted by “λ”. The distance d is provided so that a value of (n×d/λ) is about 0.4. Specifically, when the wavelength λ is about 450 nm and the refractive index of GaN for the wavelength λ is about 2.47, the distance d is about 72 nm. In addition, the product of (n×d) is an optical thickness for the contact layer 18.
Furthermore, the bevel angles Θ of the side surfaces 40a to 40d of the semiconductor substrate 10 are about 57 degrees. The lights emitted from the active layer 14 are extracted from the inclined side surfaces 40a to 40d and enter the external resin member. As shown in
A light extraction efficiency η of the extracted light, in the resin member 58 from the semiconductor layer 2 shown in
In the calculation of the light extraction efficiencyη, interference effect between the light La emitted towards the semiconductor substrate 10 from the active layer 14 and the light Lb reflected from the first electrode 20 should be considered. Here, Ag is used as the first electrode 20. A complex refractive index of Ag is about (0.055-2.42i). When the lights La, Lb are superimposed with each other, the respective lights La, Lb may be intensified or attenuated by each other due to a constructive or a destructive interference. Thus, the light extraction efficiency η is varied depending on the distance d. Additionally, the light extraction efficiency η is also varied depending on an angle of Θ light extraction plane. As shown in
For an LED, which is manufactured by using a sapphire substrate, as a comparative example, the light extraction efficiency has been calculated. As shown in
As shown in
On the other hand, light distributions radiated to the sapphire substrate 110 and the air under a condition B shown in
The value of (n×d/λ) corresponding to the condition B is not changed so much, even when the LED is surrounded by the resin instead of air, and even when the light is extracted from the inclined side surfaces of the sapphire substrate but not in the vertical direction. This is because, when the light is extracted from the inclined side surfaces of the sapphire substrate, the operational effect of the total reflection at the interface between the GaN layer and the sapphire substrate is still achieved. Calculation results of the light extraction efficiency n of the LED surrounded by the resin are shown in
Furthermore, in the comparative example, a sapphire substrate 110 is used. Thus, a flip-chip structure in which the first electrode 20 and the second electrode 22 are formed on the same side may be used. Since levels of the surfaces of the first and second electrodes 20, 22 are different, package assembly may be difficult. Moreover, it is impossible to flow a current vertically between the first and second electrodes 20, 22. Thus, the series resistance may be increased.
As shown in
In the above-described description, the thickness of the active layer 14 is ignored in the calculation. Actually, when a multiple quantum well (MQW) layer is used, the light extraction efficiency η is varied depending on the position of the active layer. Thus, as for an average light extraction efficiency η, it may differs from the value shown in
In addition, the semiconductor light emitting device shown in
For example, as shown in
For example, the physical thickness and refractive index of the guide layer 15 are denoted as “da”, “na”, and the physical thickness and refractive index of the electron overflow preventing layer 16 are denoted as “db”, “nb”, and the physical thickness and refractive index of the contact layer 18 are denoted as “dc”, “nc”. An optical thickness of the second semiconductor layer (15, 16 and 18) is represented by (na×da+nb×db+nc×dc). An effective refractive index neff of the second semiconductor layer (15, 16 and 18) is defined as {(na×da+nb×db+nc×dc)/(da+db+dc)}. By using a distance (da+db+dc) between the active layer 14 and the first electrode 20 and the effective refractive index neff, it is possible to obtain a result similar to the dependence of the light extraction efficiency η on (n×d/λ), as shown in
Moreover, the second semiconductor layer may include a plurality of semiconductor layers, which includes i-th semiconductor layer of the semiconductor layers having a physical thickness di and a refractive index ni Here, i=1 to k, where k is an integer equal to or greater than 2. In such case, the thickness d between the active layer and the first electrode is defined by (d1+d2+ . . . +dk). An optical thickness of the second semiconductor layer is represented by (n1×d1+n2×d2+. . . +nk×dk), and an effective refractive index neff of the second semiconductor layer is defined as {(n1×d1+n2×d2+ . . . +nk×dk)/(d1+d2+. . . +dk)}. Thus, it is desirable to satisfy a condition of
0.3≦(n1×d1+n2×d2+ . . . +nk×dk)/μ≦0.5,
in order to ensure the interference effect with the reflection light from the first electrode 20.
A method for manufacturing a semiconductor light emitting device according to the embodiment of the present invention will be described below by using cross sectional views shown in
As shown in
The buffer layer 12 is doped with an n-type impurity, such as silicon (Si), germanium (Ge) and the like, at an impurity concentration of about 2×1018 cm−3. The guide layer 13 is grown at a film thickness of about 0.1 μm and doped with the n-type impurity at an impurity concentration of about 1×1018 cm−3. For the guide layer 13, the n-type In0.01Ga0.99N may be used. The growth temperature of the buffer layer 12 and the guide layer 13 is, for example, about 1000° C. to about 1100° C.
As for the active layer 14, an SQW layer is used, in which a quantum well and barrier layers sandwiching the quantum well are laminated. The quantum well layer is an undoped In0.2Ga0.8N having a film thickness of about 3.5 nm. Each barrier layer is an undoped In0.01Ga0.99N having a film thickness of about 7 nm. Alternatively, an MQW layer, in which quantum wells and barrier layers are alternately laminated, may be used for the active layer 14. The growth temperature of the active layer 14 is about 700° C. to about 800° C.
The guide layer 15 is grown with a film thickness da of about 40 nm. The electron overflow preventing layer 16 is grown with a film thickness db of about 10 nm and is doped with a p-type impurity, such as magnesium (Mg), zinc (Zn) and the like, with an impurity concentration between about 4×1018 cm−3 and about 1×1018 cm−3. The contact layer 18 is grown with a film thickness dc of about 25 nm, and doped with a p-type impurity, such as Mg, having ant an impurity concentration of about 1×1019 cm−3. The growth temperatures of the guide layer 15, the current block layer 16 and the contact layer 18 are about 1000° C. to about 1100° C.
As shown in
As shown in
As shown in
The distance between the active layer 14 and the first electrode 20 in the manufactured semiconductor light emitting device is (da+db+dc) as shown in
In the embodiment of the present invention, a light emitting device using a nitride based semiconductor is described. However, a light emitting device using another group III-V compound semiconductor or a group II-VI compound semiconductor, such as zinc selenide (ZnSe), zinc oxide (ZnO) and the like may be used.
Additionally, various kind of semiconductor layers are grown by MOCVD. However, the growth method for the semiconductor layer is not so limited. For example, it is possible to grow the semiconductor layers by molecular beam epitaxy (MBE) and the like.
Various modifications will become possible for those skilled in the art after storing the teachings of the present disclosure without departing from the scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2007-82000 | Mar 2007 | JP | national |
