Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
1999-02-08
2002-08-27
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S046000, C438S497000, C438S507000, C117S054000, C117S104000
Reexamination Certificate
active
06440765
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for fabricating an infrared-emitting light-emitting diode in which an active layer sequence is applied on a semiconductor substrate, preferably composed of GaAs. The active layer sequence emits infrared (IR) radiation during operation of the light-emitting diode and has, proceeding from the semiconductor substrate, a first AlGaAs cover layer, a GaAs and/or AlGaAs containing active layer and a second AlGaAs cover layer.
Light-emitting diodes (LEDs) are used as transmission elements particularly in optical communications technology. Depending on the desired wavelength of the emitted light, use is made of different semiconductor systems whose respectively underlying semiconductor materials also entail different fabrication methods each having independent technological approaches to problems. In the visible spectral region with wavelengths of about 400 to 800 nm, use is made of an AlGaInP alloy system in which the desired wavelength from a relatively wide color range can be defined by setting the aluminum content. It is necessary to distinguish between the latter case and the case of light-emitting diodes which operate with longer wavelengths in the infrared region and, as a rule, are based on an AlGaAs system in which wavelengths of the emitted light of above about 800 nm can be achieved by setting the aluminum content in the typical range from about 10% to 30%. The present invention is concerned with the fabrication of such infrared-emitting light-emitting diodes based on an AlGaAs system. In the case of the infrared-emitting light-emitting diodes to date, the entire layer sequence to be applied to the GaAs substrate has been fabricated by the LPE (Liquid Phase Epitaxy) method. In the LPE method, in order to form an epitaxial layer, a melt which is supersaturated at a specific temperature and is composed of the correspondingly desired doping materials is brought into contact with the substrate crystal made of GaAs, with the result that an epitaxial layer of GaAs or AlGaAs is formed on the substrate crystal during subsequent cooling. In this way, it is possible to grow single-crystal layers of different compositions in accordance with the phase diagrams. The liquid phase epitaxy method has relatively high growth rates and, therefore, is also suitable for the fabrication of relatively thick epitaxial layers. The fact that the relatively thick AlGaAs LPE layers deposited according to the previous method lead to mechanical stresses and hence to warped epitaxial wafers, which are difficult to process in subsequent technological steps, is regarded as disadvantageous precisely in connection with the fabrication of the layer sequence in infrared-emitting light-emitting diodes of the AlGaAs system of interest here. For this reason, the method used to date for fabricating infrared-emitting light-emitting diodes in the wafer composite is limited to-semiconductor substrates having a wafer diameter of at most two inches.
In a refinement of the invention, the infrared-emitting light-emitting diodes are equipped with a so-called coupling-out layer for the purpose of improving the optical coupling-out efficiency of the light radiated from the light-emitting diode. In this case, the optical coupling-out efficiency is initially determined to a large extent by the layer structure of the underlying semiconductor material. If the actual generation of light takes place in a thin layer, designated in summary as the active layer, and includes a so-called active zone, for example as in the case of light-emitting diodes with a double hetero-layer' structure, or a light-generating zone formed around a pn junction, as in the case of homo-pn light-emitting diodes, then cover layers disposed on both sides and having a low degree of absorption for the emitted wavelength can distinctly improve the coupling-out efficiency. For this purpose, in the course of fabricating the layer structure, it is necessary to apply relatively thick semiconductor layers such that they are set with a higher energy gap than in the light-generating zone (active layer). The fabrication of such structures is currently produced in situ just using the liquid phase epitaxy method (LPE) or by a combination of different vapor phase epitaxy methods (VPE). In the latter case the light-generating structure is produced, as a rule, by a metal organic chemical vapor deposition (MOVPE) method and the thick coupling-out layer is produced by the VPE method. The comparatively low deposition rates constitute a disadvantage of using a MOVPE method for fabricating LED structures with such thick coupling-out layers. The combination of the MOVPE method and the VPE method has been previously employed for the purpose of fabricating relatively thick window layers having thicknesses of approximately more than 10 &mgr;m and/or coupling-out layers, preferably made of GaAs, which enable a particularly high increase in the coupling-out efficiency. It is also an intention of the invention to specify a new method combination which enables LED structures with thick coupling-out layers to be produced. A difficulty in this context consists, inter alia, in the fact that both the composition and the fabrication conditions for the coupling-out layer also influence the external efficiency since the extent of damage to the sensitive layer structures (double hetero-layer in the case of an active zone, or pn junction) is determined by the aforementioned parameters.
The fabrication of light-emitting diodes in the visible spectral region of wavelengths from 555 to 620 nm on the basis of an AlGaInP alloy system has been disclosed in U.S. Pat. No. 5,233,204 and in the reference by K. H. Huang et al., Appl. Phys. Lett. 61 (9), Aug. 31, 1992, pages 1045-1047. These light-emitting diodes include a layer sequence which, proceeding from a GaAs substrate, has a first AlGaInP cover layer of the n-type, an active zone made of AlGaInP of the n-type and a second AlGaInP cover layer of the p-type. The two cover layers each have a thickness of about 800 nm. The active zone has a thickness of about 500 nm. The first AlGaInP cover layer, the active zone, and the second AlGaInP cover layer are fabricated by the metal organic chemical vapor deposition (MOCVD) method. Furthermore, it is known to form an optically transparent coupling-out layer made of GaP, AlGaAs, or GaAsP with a thickness of at least 15 &mgr;m above the second AlGaInP cover layer. It being possible for the optically transparent coupling-out layer to be fabricated by the vapor phase epitaxy (VPE) method. In this case, the coupling-out layer serves to increase the effective light radiation of the light-emitting diode by increasing the quantity of light emitted laterally and by reducing the quantity of light absorbed by the light-absorbing substrate. For this purpose, the coupling-out layer has a thickness of at least 0.06 times the width of the light-emitting diode. In this case, the optical refractive index of the coupling-out layer is chosen such that the proportion of light that is absorbed within the light-emitting diode is reduced. The proportion of light that is absorbed within the light-emitting diode is determined by the angle of total reflection.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for fabricating an infrared-emitting light-emitting diode that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which, even with the deposition of relatively thick AlGaAs layers, the mechanical stresses generated are reduced, and which is also suitable, accordingly, for the problem-free fabrication of a multiplicity of light-emitting diodes in the wafer composite with wafer diameters of more than two inches.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating an infrared-emitting light-emitting diode, which includes: applying an active layer sequence on a semiconductor substrate, the ac
Nirschl Ernst
Sedlmeier Reinhard
Stath Norbert
Greenberg Laurence A.
Jr. Carl Whitehead
Locher Ralph E.
Perkins Pamela E
Siemens Aktiengesellschaft
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