This application claims the benefit of Korean Application No. 2000-78543 filed Dec. 19, 2000 in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor light-emitting device having a resonant cavity structure for emitting light perpendicularly to the plane of an active region and a method of manufacturing the same. More particularly, the present invention relates to a semiconductor light-emitting device in which a central axis of an upper electrode window, through which resonated light is emitted, and a central axis of a current aperture of an oxidized layer are automatically aligned, and a method of manufacturing the same.
2. Description of the Related Art
Semiconductor light-emitting devices, first developed by General Electric (GE) in 1962, are designed to recombine electrons with holes by applying forward current across a PN junction in a compound semiconductor to generate light having a wavelength corresponding to band gap energy determined according to a structure of the semiconductor.
Semiconductor light-emitting devices are divided according to a process of emitting light into a light emitting diode, which emits incoherent light using spontaneous emission, and a semiconductor laser, which emits coherent light using stimulated emission.
Semiconductor lasers are divided according to the positions of their reflectors. For example, a Fabry-Perot semiconductor laser has reflectors that are positioned at opposite sides of a chip. A Vertical Cavity Surface Emitting Laser (VCSEL) having a resonant cavity structure has reflectors that are horizontally positioned within a chip.
VCSELs do not need an optical system to correct the shape of a beam because they emit a nearly circular Gaussian beam in a direction in which semiconductor material layers are stacked. In addition, since the size of VCSELs is small, a plurality of lasers can be integrated on a single semiconductor wafer. Therefore, VCSELs have a wide range of optical applications such as optical communication, electronic calculators, audio-video devices, laser printers, laser scanners and medical instruments.
FIGS. 1A through 1E show a conventional method of manufacturing a VCSEL. FIG. 1A shows that a lower reflector layer 13, an active layer 15, a pre-oxidized layer 17, and an upper reflector layer 19 are sequentially stacked on a substrate 10. Here, the substrate 10 is formed of, for example, a semiconductor material having n-type impurities. The lower reflector layer 13 is doped with impurities of the same type as the substrate 10. For example, the lower reflector layer 13 is formed by stacking 20-30 layers of n-type GaAs, in which the ratio of Ga to As is different in each layer, on top of the substrate 10. The upper reflector layer 19 is formed of the same semiconductor material as the lower reflector layer 13 but contains the opposite type of impurities to those contained in the lower reflector layer 13. In other words, the upper reflector layer 19 is formed of p-type GaAs. The pre-oxidized layer 17 is subjected to a horizontal oxidation process in vapor.
FIG. 1B shows that a plurality of VCSEL posts I, II and III and spaces 21 are formed after a dry etching process and through which light will be independently radiated. Referring to FIG. 1C, when an oxidation atmosphere is provided after the spaces 21 are formed, the pre-oxidized layer 17 is oxidized horizontally from its outside to its inside, thereby forming horizontally oxidized high-resistance portions 18 and current apertures 17a which are not oxidized.
Subsequently, as shown in FIG. 1D, the spaces 21 are filled with polyimide fillings 23 in order to prevent the posts I, II, and III from being damaged during a lapping process. Then, the polyimide fillings 23 are planarized to be level with the surroundings. Afterwards, the resultant structure is turned over and most of the substrate 10 is removed by a lapping process.
FIG. 1E shows that upper electrodes 25 having a window 25a is formed on the VCSEL posts I, II and III and the polyimide fillings 23. Finally, a lower electrode 27 is formed on a bottom surface of a lapped substrate 10a, thereby completing the manufacture of a VCSEL. VCSELs having the above structure may be used as a single chip array structure or may be cut at each polyimide portion to be used separately.
According to a conventional technique shown in FIG. 2A, the current apertures 17a formed by a horizontal oxidation process after the posts were formed and the window 25a of the upper electrodes 25 formed by a photolithographic process are not exactly aligned. Therefore, an alignment error exists where a central axis 16 of the window 25a and a central axis 14 of the aperture 17a deviate from each other. Such an alignment error results in a loss of emitted light and hinders formation of an exact Gaussian beam, thereby degrading the electro-optical characteristics of a VCSEL.
To take into account of an alignment error, FIG. 2B shows a VCSEL designed by way of xe2x80x9celectrode pulling.xe2x80x9d Here, the upper electrodes 25 are formed beyond the region of the current apertures 17a between the high-resistance portions 18. However, a current path 30 is lengthened and the overall device resistance is increased. Alternatively, FIG. 2C shows a VCSEL design according to xe2x80x9celectrode pushing.xe2x80x9d In this case, the upper electrodes 25 are formed to extend over the current apertures 17a between the high-resistance portions 18. Because an upper electrode window size 34 is smaller than a current aperture size 32, a loss of emitted light occurs.
Therefore, it is necessary to exactly align the central axis of an upper electrode window and the central axis of a current aperture.
To solve the above and other problems, it is an object of the present invention to provide a semiconductor light-emitting device with improved electro-optical characteristics by exactly aligning a central axis of an upper electrode window and a central axis of a current aperture, and a method of manufacturing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided a semiconductor light-emitting device having a post that is composed of a plurality of layers including at least one pre-oxidized layer on a substrate, and an upper electrode on the post, the semiconductor light-emitting device is manufactured according to an embodiment of the present invention by forming the post by etching, by way of self-alignment using the upper electrode, and horizontally oxidizing the pre-oxidized layer by a predetermined distance from a sidewall of the post.
According to another embodiment of the present invention, the post is formed using the upper electrodes as a guide during an etching process to align a central axes of a window defined by the upper electrodes and a central axes of a current aperture of the pre-oxidized layer.
According to an aspect of the invention, during the etching process, a sidewall of the pre-oxidized layer included in the post is exposed, and the pre-oxidized layer is horizontally oxidized by an oxidizing process to a predetermined distance from the sidewall.
According to another aspect of the invention, when a diameter of the post is about 60 xcexcm, about 45-50 xcexcm of the pre-oxidized layer is oxidized, a portion of the pre-oxidized layer oxidized by the oxidizing process becomes a high-resistance portion, and a portion of the pre-oxidized layer unoxidized during the oxidizing process becomes the current aperture through which current or light passes, and since the post is formed by way of self-alignment using the upper electrode, and the current aperture is formed to correspond to the exposed sidewall of the post, the central axis of the window of the upper electrode and the central axis of the current aperture are automatically aligned, such that, due to the exact alignment between the window and the current aperture, the electro-optical characteristics of the Vertical Cavity Surface Emitting Laser (VCSEL) are improved.
In the present invention, the window of the upper electrode is passivated by a photoresist in order to avoid damage during the etching process. Simultaneously, the surface of the upper electrode is partially or entirely passivated by the photoresist. When the surface of the upper electrode is partially passivated and even if a portion of the upper electrode exposed during the etching process is damaged, the remaining portion of the upper electrode protected by the photoresist is sufficient to demonstrate conductivity as an electrode.