Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-08-29
2004-08-24
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S088000, C257S090000, C257S091000, C257S093000, 57
Reexamination Certificate
active
06781157
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a structure of a monolithic array-type light emitting device having a plurality of light emitting parts in one device (chip) and a process for producing the same, and more particularly to a monolithic array-type light emitting device suitable particularly as a light source for printers.
2. Prior Art
Printers of a xerography system utilizing light emitting diodes (LEDs) have been put to practical use.
In this system, LEDs satisfying emission wavelength and emission intensity required based on the light receiving sensitivity of a photoreceptor should be selected. LED arrays using GaAsP as a main material of p
junction in the light emitting device and LED arrays using GaAlAs as a main material of p
junction in the light emitting device have been adopted for practical use.
For emission output which is one of important characteristics of LED, in order to provide products having the highest possible emission output, an attempt has been made to efficiently take out light from one direction using the so-called “Bragg-type” multilayered reflection film. The semiconductor multilayered reflection film comprises a plurality of layers of high-refractive index &lgr;/4n films and low-refractive index &lgr;/4n films wherein &lgr; represents emission wavelength of LED and n represents refractive index. In this case, light, which has been emitted from the active layer and directed to the substrate side, is reflected from the multilayered reflection film and exits from the top surface of the device. This can improve the light takeout efficiency.
The provision of the multilayered reflection film, however, poses a new problem of increased resistance of the device. Specifically, in the AlGaAs material used in the multilayered reflection film, the band offset of the valence band at the interface of junction between the high-refractive index film and the low-refractive index film is so large that the injection of holes is not easy, leading to significantly increased resistance of the device. The increased resistance of the device leads to increased forward voltage which adversely affects characteristics of the device and results, for example, in increased power consumption and generation of heat.
Further, in conventional AlGaAs-based LEDs, when AlGaAs materials are adopted in the multilayered reflection film provided between the substrate and the active layer, in terms of the reflectance, it is considered that the light takeout efficiency increases with increasing the refractive index difference and the number of pairs. This had led to a tendency toward the adoption of GaAs in the thin layer on the high-refractive index side while adopting Al
y
Ga
1−y
As, wherein 0<y≦1, in the thin layer on the low-refractive index side. The use of GaAs as the high-refractive index film for the purpose of providing higher reflectance, however, further increases the band offset at the interface of each hetero junction constituting the multilayered reflection film, resulting in significantly increased resistance of the device.
The wavelength of the LED array for LED printers is determined by materials used in pn junction in LED. When GaAsP or AlGaAs is used as the material for the pn junction, the wavelength of LED is in many cases 700 to 800 nm. This is because a wavelength region, which provides high emission output, is selected according to materials. On the other hand, in the case of LED printers, the wavelength sensitivity of a photoreceptor, which receives light emitted from LED, is also an important parameter, and the emission wavelength is restricted by a combination of the light emitting device with the photoreceptor. For LED printers, there is an increasing demand for higher speed and higher resolution, and LED arrays with higher output are required. At the present time, however, the above-described materials do not always satisfy the demand at emission efficiencies determined by the materials per se.
Accordingly, high emission output is desired in AlGaAs-based LED arrays for LED printers because the emission output has direct influence on printing speed. To satisfy this requirement, the realization of LED arrays having high output while providing enhanced light takeout efficiency through the adoption of the multilayered reflection film, double hetero (DH) structure or the like is necessary.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide a light emitting device of an LED array which, while utilizing the advantage of improved light takeout efficiency realized by providing a Bragg-type multilayered reflection film, can suppress an increase in resistance of the device attributable to the provision of the multilayered reflection film and, in its turn, an increase in forward voltage, and can realize high emission efficiency and high output on the whole.
The above object can be attained by an LED array-type light emitting device wherein, in an AlGaAs-based LED array device having a Bragg-type multilayered reflection film comprising a plurality of reflection layers, the aluminum (Al) composition ratio of AlGaAs constituting the layers of the multilayered reflection film is specified to make reflection efficiency higher than the case where a high-refractive index GaAs layer is adopted as one reflection layer in the multilayered reflection film, and to suppress an increase in resistance of the device attributable to the provision of the Bragg-type multilayered reflection film and, in its turn, to suppress an increase in forward voltage, thereby realizing high output.
Specifically, according to the first feature of the invention, a monolithic array-type light emitting device comprising a plurality of light emitting parts in one device, wherein: each of said light emitting parts comprises a light emitting diode having a laminate structure comprising an n-type GaAs substrate and, epitaxially grown on the n-type GaAs substrate in the following order, an n-type GaAs buffer layer, an n-type laminated reflection film formed of layer pairs each comprising AlGaAs layers different from each other in Al (aluminum) composition ratio, an n-type AlGaAs lower cladding layer, a p-type or undoped AlGaAs active layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer; and each of the layer pairs constituting the laminated reflection film is a multilayered film of an Al
X1
Ga
1−X1
As layer and an Al
X2
Ga
1−X2
As layer where X1 and X2 each represent Al composition ratio, wherein hetero junction has been formed between the Al
X1
Ga
1−X1
As layer and the Al
X2
Ga
1−X2
As layer, wherein the Al composition ratio is X1<X2 and the refractive index is n1>n2 where n1 represents the refractive index of the Al
X1
Ga
1−X1
As layer and n2 represents the refractive index of the Al
X2
Ga
1−X2
As layer, wherein the Al
X1
Ga
1−X1
As layer and the Al
X2
Ga
1−X2
As layer satisfy X1≧X and X2≧X where X1 and X2 are as defined above and X represents Al composition ratio in Al
X
Ga
1−X
As constituting the active layer, and wherein the Al
X1
Ga
1−X1
As layer satisfies Eg
X1
≧E&lgr; wherein Eg
X1
represents the band gap energy of the Al
X1
Ga
1−X1
As layer and E&lgr; represents emission wavelength energy.
In the light emitting device according to the first feature of the invention, the density of the light emitting parts is preferably not less than 240 dpi (dots per inch).
In any one of the above light emitting devices, preferably, the size of a light emitting region in each LED constituting the light emitting parts is 50×50 &mgr;m or less.
In any one of the above light emitting devices, preferably, an electrode contact layer having a size of not more than 10×50 &mgr;m is provided on each of the light emitting regions, an ohmic contact part is provided on a part of the top of each of the contact layers, and anodes for respective wirings are drawn respectively from the ohmi
Koizumi Genta
Kunitake Eiichi
Noguchi Masahiro
Conte James B.
Hitachi Cable Ltd.
Tran Mai-Huong
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