Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Dopant introduction into semiconductor region
Reexamination Certificate
2003-05-13
2004-08-17
Quach, T. N. (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Dopant introduction into semiconductor region
C438S046000, C438S047000
Reexamination Certificate
active
06777257
ABSTRACT:
RELATED APPLICATION
This application claims the priorities of Japanese Patent Application No. 2002-143783 filed on May 17, 2002, 2002-143713 filed on May 17, 2002, 2002-143784 filed on May 17 2002, 2002-143782 filed on May 17, 2002, 2002-287421 filed on Sep. 30, 2002 and 2003-126650 filed on May 1, 2003.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of fabricating a light emitting device, and a light emitting device.
2. Description of the Related Art
Light emitting devices having the light emitting layer portion thereof composed of (Al
x
Ga
1-x
)
y
In
1-y
P (where, 0≦x≦1 and 0≦y≦1, referred to AlGaInP alloy, or more simply AlGaInP, hereinafter) alloy can achieve a high luminance by adopting a double heterostructure in which a thin AlGaInP active layer is sandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer which have a larger band gap.
Referring now to AlGaInP light emitting device, an n-type GaAs buffer layer, an n-type AlGaInP cladding layer, an AlGaInP active layer, and a p-type AlGaInP cladding layer are stacked in this order on an n-type GaAs substrate, to thereby form a light emitting layer portion having a double heterostructure Current supply to the light emitting layer portion is generally accomplished via a metal electrode formed on the surface of the device. Since the metal electrode functions as a light interceptor, it is typically formed so as to cover only the center portion of the main surface of the light emitting layer portion, so that the light can be extracted from the peripheral area having no electrode formed thereon.
In this case, reduction in the area of the metal electrode is advantageous in ensuring a larger light leakage area formed around the electrode, and in improving the light extraction efficiency. Although a number of efforts have been made to increase the amount of extractable light by effectively spreading current within the device based on various designs of the electrode shape, any of them could not exempt from increase in the electrode area, and fell in a dilemma that consequent reduction in the light leakage area undesirably limits the amount of light extraction. Another problem resides in that carrier concentration of the dopant, that is conductivity, is generally suppressed at a slightly lower level in order to optimize light emitting recombination of the carriers within the active layer, and this makes the current less likely to spread in the in-plane direction. This means undesirable concentration of current density into the area covered with the electrode, and reduction in substantial amount of extractable light from the light leakage area. A general method for solving this problem is to dispose a current spreading layer having a raised carrier concentration, and consequently having a low resistivity, between the cladding layer and electrode. On the other hand, another possible constitution relates to that a thick current spreading layer is disposed on the back side of the device so that the layer is used also as the substrate (while the current spreading layer in this case may be assumed as a conductive substrate, it is to be defined in the specification that the layer conceptually belongs to the current spreading layer in a broad sense). In most conventional cases, such current spreading layer has been formed by the metal organic vapor-phase epitaxy process (also occasionally referred to as the MOVPE process) together with the light emitting layer portion
The current spreading layer provided in the light emitting device is generally designed so as to increase the thickness thereof to some extent in order to sufficiently spread the current in the in-plane direction, and typically formed with a larger thickness than the light emitting layer portion has. The MOVPE process, however, is slow in the layer growth rate, needs considerably long time for growing the current spreading layer to a sufficient thickness, and thus raises problems in degraded production efficiency and increased costs. Organo-metallic compounds used as Group III element sources in the MOVPE process are generally expensive. Moreover, it is necessary in the MOVPE process to use Group V element sources (e.g., AsH
3
, PH
3
) in a great excess (10 to several hundred times) of Group III element sources in order to improve the crystallinity, and this raises another disadvantage from the viewpoint of costs
The current spreading layers grown by the MOVPE process are likely to contain residual H (hydrogen) and C (carbon) derived from the organo-metallic molecules. For the case where the current spreading layer is designed to have a conductivity type of p-type by doping with Zn (zinc) or Mg (magnesium), the residual H in particular is known to bond with Zn or Mg (Zn in particular) to lower the acceptor activity, so that a relatively large amount of Zn or Mg must be added in order to ensure a sufficient conductivity required for the current spreading layer. Addition of Zn or Mg in a large amount will, however, raise the problems below.
The light emitting devices lower their luminance as the current supply thereto is prolonged. Assuming now that the emission luminance measured immediately after the start of current supply to the device at a constant current is defined as the initial luminance, and the emission luminance which decreases with the elapse of cumulative current supply time is traced. In this case, a time required for the emission luminance to reach a predetermined limit luminance, or a ratio of emission luminance after the elapse of evaluation current supply time with respect to the initial luminance (referred to as “device life”, hereinafter) under a constant evaluation current supply time (e.g., 1,000 hours) can be used as a kind of index for evaluating the device life. Excessive increase in the Zn concentration in the current spreading layer, in particular in a portion adjacent to the light emitting layer portion, tends to accelerate the degradation of the device life.
Another problem is that the electrode in the light emitting device can act as a light interceptor, and application of a drive voltage to the electrode raises the in-device current density in a portion directly under the electrode or around, but lowers it in the circumferential area around the electrode from which the light is to be extracted. The light extraction efficiency is thus likely to degrade. In one possible countermeasure, a current blocking layer is formed as being buried in the current spreading layer directly under the electrode so as to allow the current to bypass in the current spreading layer out from the electrode formation area, to thereby successfully raise the light extraction efficiency. The formation of such current blocking layer inevitably increases the number of process steps, and further degrades the production efficiency.
Formation of the current blocking layer as being buried in the current spreading layer can raise another problem when the current spreading layer covering the current blocking layer Is formed by the MOVPE process or the LPE (liquid-phase epitaxy) process, since the resultant current spreading layer tends to have on the surface thereof a large step with a dulled shape of the underlain current blocking layer, while being associated with crystal defects. Such step and defects are causative of connection failure with the electrode, or degradation of efficiency and yield ratio of the wire bonding process since they may serve as a factor of detection error in the image processing for wire bonding to the electrode.
It is therefore a first subject of the invention to provide a method of fabricating a light emitting device capable of forming the current spreading layer in an efficient manner. A second subject resides In providing a light emitting device having an improved device life even if Zn or Mg is used as a p-type dopant. A third subject resides in a method of fabricating a light emitting device capable of forming a current spreading layer having a current blocking layer
Shinohara Masayuki
Yamada Masato
Quach T. N.
Shin-Etsu Handotai & Co., Ltd.
Snider Ronald R.
Snider & Associates
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