Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1997-12-29
2001-04-24
Bovernick, Rodney (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S096000, C438S043000, C438S044000
Reexamination Certificate
active
06222866
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser used as a light source for optical communication, an optical recording apparatus, a laser printer or the like, a method of producing the same and a surface emitting semiconductor laser array.
2. Related Prior Art
As a relatively simple and easy method of producing a surface emitting laser, there has been proposed a method in which a surface emitting laser structure layer is stacked on a flat GaAs substrate and thereafter ions (protons) are injected into a region, where a current is not made to flow, to make the region electrically insulated, a active region is defined and a current constriction structure is formed (Japanese Published Unexamined Patent Application No. Hei 4-226093).
This method has a fault that, while a surface of an element is flat and an electrode and the like are easy to be formed, an optical wave guiding (index guiding) structure which is important in a laser structure is not formed. This laser is called a gain guiding surface emitting laser but laser characteristics are low, for example, a threshold current is high, a quantum efficiency is low and the like. Besides, it has a problem of a thermal lens effect that a micro fluctuation of a refractive index arises in the bulk of a laser under influence of heat generation and such a micro fluctuation causes gradual increase in an optical output, so that the output is hard to be constant. Moreover, it has another problem that it is required that a bias current is always made to flow in order to realize a high speed modulation, so that power consumption is large.
As a method of forming a current constriction structure without usage of proton implantation or the like, there has been proposed a method in which a surface emitting laser structure layer is formed in a layered manner on a surface of a substrate processed in a concave shape (Japanese Published Unexamined Patent Application No. Hei 5-226778).
This method is to utilize a phenomenon that a silicon doped layer grown on a (
111
) oriented surface of a substrate which is processed in a concave profile whose bottom is in the shape of a flat regular triangle comprises n-type and p-type layers respectively grown on the two kinds of surface, that is flat and sloped surfaces. That is to say, after a n-type multilayer mirror is grown, a n-type active layer is grown. Since an active layer on the flat surface is of n-type and that on the sloped surface is of p-type, an active layer is surrounded by a pn junction in a lateral direction. A surface emitting laser structure is completed by growing a p-type multilayer mirror thereon.
Even in this method, in the same way as that of the above mentioned example, an index guiding structure is not formed but a gain guiding surface emitting laser is formed, so that there is a problem that laser characteristics are low.
In this structure, since a region of a pn junction which has the minimum band gap is a corner region between the flat and sloped surfaces of the active layer, an electron and a hole are subjected to recombination to emit light. In the case of a surface emitting laser, since recombination desirably occurs in the middle portion of the flat surface, there arises a problem that a laser of this structure has low characteristics.
There has been proposed an index guiding laser to solve a problem of a gain guiding laser (Electronics Letters, Vol. 31, No. 11, 1995, pp. 886 to 888). This laser has a current constriction structure and a light wave guiding structure formed by selectively oxidizing only an AlAs layer inserted near an active layer to make it electrically isolated. After growth of a laser structure on a GaAs substrate
1
, a circular cylinder of 100 &mgr;m to 10 &mgr;m in diameter or an angular prism of a similar size is formed in a protruding manner by etching to expose a sectional surface of an AlAs layer. Thus etched substrate is heated in a water vapor atmosphere to oxidize the AlAs layer on the outer side surface thereof. An oxidation of the AlAs layer advances from the outer side surface of the circular cylinder or angular prism to the center thereof while an oxide is formed in a doughnut shape. An oxidizing AlAs forms aluminum oxide
18
. This has a high insulating property and thereby blocks a current, so that a current flows only through an aperture of an unoxidized region in a doughnut-shaped aluminum oxide. Since aluminum oxide has a low refractive index and thus there is formed a light wave guide along a light emitting direction (a longitudinal direction), the light wave guide being a region of the aperture of the doughnut working as a core. This structure is shown in FIG.
7
. In this laser element, a current is made to flow between an n-type electrode
13
formed on a rear side of a n-type substrate and a p-type electrode
12
formed on a top end surface of the laser.
An index guiding surface emitting laser using such selective oxidation of AlAs has excellent laser characteristics such as a low threshold current, high quantum efficiency, good high speed modulation property and the like. However, since the circular cylinder or angular prism is as high as 3 &mgr;m or more in a protruding manner, formation of an electrode on the top end surface is difficult. Moreover, since the top end surface is generally tens of &mgr;m square (a circle of tens of &mgr;m in diameter) or smaller in area and has a light emitting apparatus opened in the central portion, wire bonding cannot be applied to a metal electrode on the top end surface. For that reason, an interconnect to the electrode is extended over a surface of the substrate and an electrode pad is formed for wire bonding at the other end of the interconnect located in a part other than a part which a laser element occupies. Such a high protrusion is a great disturbance to form an electrode interconnection in its process. There has been a great hardship in film formation for an electrode, coating of resist, exposure to light, electrode etching and the like over a step having a rise of 3 &mgr;m or more in a concavity and convexity profile with a good yield.
As described above, a conventional surface emitting laser has suffered from serious problems.
Besides, in the case of a matrix interconnection surface emitting laser array, such problems are more serious.
The matrix interconnection laser array has a structure wherein surface lasers are two dimensionally arrayed and individual arrays are driven by positive electrodes and negative electrodes interconnected in row and column directions. When a degree a integration of a laser is higher, since individual interconnection of electrodes on lasers cannot be realized, matrix interconnection becomes necessary. In matrix interconnection, grooves of several &mgr;m or more in depth have to be formed by etching in gaps between adjacent columns of lasers in order that they are electrically isolated between any adjacent two thereof regardless of a gain guiding structure or a index guiding structure. Negative electrodes are formed in parallel to the grooves and positive electrodes are formed on the negative electrodes in directions perpendicular to the array columns with an insulating film interposing between them. It has been found from our experiments that formation of matrix interconnection on such a deep concavity and convexity profile is a very difficult task.
Even in the most advanced LSI process, a depth of a concavity and convexity profile is generally on the order of 1 &mgr;m. There has been no proposal of a technique in which interconnection of electrodes are formed across a surface having a concavity and convexity profile and as can be seen from such circumstances, such a electrode interconnection technique is extremely difficult. As a matrix interconnection surface emitting laser array, an example in which a surface emitting laser is fabricated by matrix interconnection with ion implantation has been proposed (AT & T Bell Laboratories, IEEE Photonics Technology Letters, Vol., No. 8, Aug
Bovernick Rodney
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
Stahl Michael J
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