Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-05-10
2001-05-29
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S014000
Reexamination Certificate
active
06239454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a heterostructure with a plurality of active material well layers alternately grown between a plurality of barrier layers and having a reduced net strain. More specifically, the present invention relates to an integrated semiconductor laser-modulator device exhibiting a reduced net strain, and to a process for manufacturing such a semiconductor device.
2. Description of the Related Art
During the manufacturing of heterojunction semiconductor devices, each individual layer is grown epitaxially using one of many known growth techniques such, for example, as metal-organic chemical vapor deposition (MOCVD). Several layers are usually grown when producing a multiple quantum well (MQW) semiconductor such as an MQW laser so that alternating layers of active material well layers and barrier material layers are grown. In addition, the alternating layers are bound between a pair of separate confinement layers (SCLs).
With respect to MQW semiconductor laser devices, it is known that a lattice mismatch between the alternating layers produces a strain in the active layer and affects the energy band structure of the quantum well and the wavelength of the light output from the quantum well. Furthermore, in line with continuing efforts to miniaturize and maximize the efficiency of semiconductor components, MQW semiconductor laser devices are being integrally designed with other element devices such as modulators. Referring to
FIG. 1
, a known integrated laser-modulator device
102
includes active layers
112
and barrier layers
110
comprising a plurality of quaternary layers bound between SCLs
118
grown on substrate
114
. The laser region
106
of this device is grown by a selective area growth (SAG) process in which the epitaxial growth of the layers is confined between two pads
116
comprising, for example, an oxide. The laser region
106
and the modulator region
104
of the device are grown simultaneously. The growth of the laser region
106
between the two oxide pads causes an increase in the growth rate (thickness) and strain in the laser region
106
as compared to the planar growth in the modulator region
104
. The resulting difference in thickness and strain causes the characteristic wavelength of the SAG or laser region to be longer than the characteristic wavelength of the modulator region.
It is also known that for satisfactory operation of an Electro-Modulated Laser or Electroabsorption-Modulated Laser (EML), which includes an integrated laser and modulator, a large change in strain is required in the well layer between the laser region and the modulator region of the device to effect the longer characteristic wavelength. However, the barrier and SCL layers do not require a large change in strain between the laser and modulator portions of the semiconductor device. It is also known that it is important to minimize the total amount of strain in the laser section. Each laser has a strain limit which is the maximum net strain that it can accommodate. The efficiency of a laser decreases as the net strain increases, so that the further the net strain is from the strain limit, the better. Designers must thus balance the requirement for a large change in strain between the laser and modulator with the relationship between the net strain and efficiency of the laser.
One prior art solution for affecting the strain is to carefully optimize the quaternary compositions used to form the semiconductor layers to achieve the desired characteristics of each layer. This procedure is however difficult to carry out because it generally requires time-consuming trial and error experimentation, yet yields only minimal improvements.
SUMMARY OF THE INVENTION
The present invention utilizes the differences in the thickness and strain characteristics that are produced in epitaxial layers using different gallium precursors in the formation of the compositions of the alternating epitaxial layers to reduce overall net strain of a semiconductor while maintaining proper operation. In a rudimentary embodiment of the present invention, the well layers of a multiple quantum well (MQW) configuration use a trimethylgallium (TMG) gallium precursor material and the barrier layers of the MQW configuration use a triethylgallium (TEG) gallium precursor material. This combination maximizes the strain in the well layers and simultaneously minimizes the overall or net strain of the semiconductor, an important and highly advantageous result because in certain semiconductor devices, the well layers are required to have as large a strain as possible. However, each semiconductor device also has a maximum strain limit which limits the total amount of strain that can be accommodated. In addition, the efficiency of the semiconductor decreases as the net strain increases. Therefore, the strain in the well layer should be kept large for proper operation of the semiconductor device while minimizing the strain in the barrier layer so as to maximize the operating efficiency of the device.
In a more specific embodiment, the semiconductor device is an electromodulated laser or electroabsorption-modulated laser (EML) (i.e., an integrated lasermodulator) having a laser region and a modulator region simultaneously grown on one substrate. The laser region is formed by a selective area growth (SAG) process in which the epitaxial growth of the barrier and well layers is confined between two oxide pads. As a consequence of the selective area growth process, the strain in the well layer of the laser region is greater than the strain of the same well layer in the modulator region. Satisfactory operation of the EML device requires a relatively large difference between the strain in the well layers of the laser region and the strain in the well layers of the modulator region. On the other hand, it is not required that there be a difference in strain between the barrier layers of the laser region and the barrier layers of the modulator region.
The net strain of the laser section is also a critical parameter. Each laser section can accommodate up to a maximum amount of strain, which is referred to as the strain limit. In addition, the efficiency of the laser decreases as the net strain in the laser section increases. Therefore, in designing an EML device, the strain difference between the wells of the laser section and the wells of the modulator section is advantageously maximized while simultaneously limiting the net strain of the laser section so that the efficiency of the device does not become unsatisfactory.
In accordance with the present invention, the difference between the thickness and strain characteristics of barrier and well layers formed from TMG and TEG gallium precurser materials is utilized to reduce the net strain by maintaining an adequate difference in strain between the well layers of the laser section and the well layers of the modulator section while minimizing the strain in the barrier layers of the laser. Using this inventive arrangement, the strain difference between the well layers of the laser region and the well layers of the modulator region may be maintained at a satisfactory level while reducing the net strain in the laser section and thereby increasing the efficiency of the EML device over those of the prior art. The prior art does not exploit the different characteristics of compositions grown from TEG and TMG gallium precursor materials to reduce the net strain in the laser section or in any semiconductor device having an MQW configuration.
The compositions used for the barrier layers and the well layers are preferably quaternary compositions of GaInAsP which is grown using the metal-organic chemical vapor deposition (MOCVD) process.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illu
Glew Richard W.
Grenko Judith A.
Chaudhuri Olik
Lucent Technologies - Inc.
Wille Douglas A.
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