Recently, a white light emitting apparatus, in which a blue light emitting element that uses a nitride semiconductor element as a light emitting element and a phosphor are used, is generally used for a backlight of a large-sized liquid crystal television, a light source for lighting, and the like. A large number of white light emitting apparatuses are used for each of the products such as a large-sized liquid crystal television, lighting, and the like. Therefore, it is desired that the blue light emitting element being used for these products can be mass-produced with good quality and emit light with higher efficiency.
Such a nitride semiconductor light emitting element includes, for example, an n-type nitride semiconductor layer, a light emitting layer, and a p-type nitride semiconductor layer which are sequentially stacked on an insulating sapphire substrate. An n-side electrode and a p-side electrode for being connected to an external power source are formed on the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, respectively.
Since a sheet resistance of the p-type nitride semiconductor layer is generally higher than that of the n-type nitride semiconductor layer, a transparent electrode layer made of, for example, indium tin oxide (ITO), or the like is stacked substantially on the entire upper surface of the p-type nitride semiconductor layer for the purpose of assisting current diffusion inside the p-type nitride semiconductor layer, and the p-side electrode is formed on the transparent electrode layer. Such a transparent electrode layer transmits light from the light emitting layer, and functions as a current diffusing layer.
In a case where an insulation substrate such as a sapphire substrate is used for the nitride semiconductor light emitting element, the n-side electrode is not able to be formed on a rear surface of the substrate, and thus the n-side electrode is formed on a main surface on the same side as the p-side electrode being formed. For example, a region of a p-type semiconductor layer and the light emitting layer are partially removed by etching, the n-type nitride semiconductor layer is partially exposed, and the n-side electrode is formed on the exposed region.
However, in such a configuration, there is a problem in that current is concentrated on a part where the n-side electrode is formed in the n-type nitride semiconductor layer.
In addition, since the n-side electrode and the p-side electrode are formed to be relatively thick using a metal material such as Au, Al, Ni, or Ti, light emitted from the light emitting layer is reflected at a certain reflectivity without being transmitted through the n-side electrode or the p-side electrode. Therefore, there is a problem in that loss on a light extraction efficiency of the nitride semiconductor light emitting element is great due to light absorption by the n-side electrode or the p-side electrode.
Accordingly, in PTL 1, it is proposed that light extraction efficiency is improved by providing the transparent conductive film right under the n-side electrode and the p-side electrode in order to suppress light absorption by the n-side electrode or the p-side electrode.
However, when the nitride semiconductor light emitting element in which the electrodes are disposed as described above is mounted face-up, the n-side electrode and the p-side electrode require a certain area for wire-bonding. Particularly, in order to secure a certain area for connecting the n-side electrode, an exposed region of the n-type nitride semiconductor layer needs to be provided in accordance with the secured area, and compared to a light emitting element having a so-called an up-and-down electrode structure in which an electrode is formed on a rear surface of a substrate using a conductive substrate, if the same light output is intended to be obtained, there is a problem in that the size of chip may be large. In addition, in a case where the size of chip is not increased, since a light emitting area of the nitride semiconductor light emitting element is small-sized as the exposed region of the n-type nitride semiconductor layer is made large, a problem of deteriorating the light extraction efficiency of the nitride semiconductor light emitting element is generated, but there is no consideration for such a problem.
Regarding the light extraction efficiency, in PTL 1, under the n-side electrode and the p-side electrode, a first transparent electrode layer, which functions as the current diffusing layer and a second transparent electrode layer for suppressing light absorption by the n-side electrode or the p-side electrode are separately formed. Further, since the n-type semiconductor layer and the p-type semiconductor layer have a work function different from each other, different pretreatment processes for ohmic contact between each electrode and the transparent electrode layer is necessary under the n-side electrode and under the p-side electrode. Therefore, there is a problem in that the number of processes is increased, a manufacturing process is complicated, and a manufacturing cost is increased.
PTL 2 discloses a light emitting diode including a lower semiconductor layer which is formed on a substrate, an upper semiconductor layer which is disposed on the lower semiconductor layer such that at least a part of an edge region of the lower semiconductor layer is exposed, a first electrode which is formed above a part of the region of the upper semiconductor layer with an insulating layer in between and is formed for supplying current to the lower semiconductor layer, a second electrode which is formed above another part of the region of the upper semiconductor layer and is formed to supply current to the upper semiconductor layer, and an extending portion of the first electrode which extends from the first electrode and is formed to reach at least a part of the exposed lower semiconductor layer.
According to the light emitting diode of PTL 2, when the electrodes and the extending portion for supplying current to the lower semiconductor layer are formed, the electrodes and the extending portion are formed above the upper semiconductor layer with an insulating layer in between. Accordingly, reduction in the light emitting area caused by reduction in an area of the semiconductor layer, which is removed by mesa etching for forming the electrodes and the extending portion, can be suppressed.
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