1. Field of the Invention
The present invention relates to an integrated semiconductor device and a method of manufacture thereof, and more particularly, the present invention relates to a buried heterojunction laser that is optically coupled to a ridge waveguide electro-absorption (EA) optical modulator and a method of manufacture thereof.
2. Description of the Related Art
It is known to fabricate an integrated semiconductor device having a laser and a waveguide EA modulator using a unitary III-V semiconductor substrate. The modulator can be formed either using a ridge waveguide or a buried mesa waveguide. The optical radiation output from the laser, for example, visible or infrared radiation, can then be coupled optically to the EA modulator, which is then used to high frequency modulate the optical radiation generated by the laser. Fabricating a laser and waveguide device on the same substrate gives significant advantages in terms of ensuring alignment between the laser and waveguide components of the device. The components can then use the same epitaxially grown current confinement layers, which helps to simplify the manufacturing process.
In the field of transmitter devices for fiber-optic communication, operation is required at optical wavelengths ranging from 1.3 to 1.6 μm. Such opto-electronic transmitter devices are therefore usually fabricated from a wafer grown from an n-InP substrate on which are grown a number of layers, including an undoped InGaAsP active layer, which can be either a bulk semiconductor or a multiple quantum well or dot structure sandwiched between an upper p-InP cladding layer and a lower n-InP buffer layer. A mask is applied to the upper cladding layer, and the surrounding layers are etched to leave a mesa structure. Buried heterostructure light emitting devices commonly have current confinement regions defined by areas of high resistivity to limit current flow. Such regions are grown to cover the sides of the mesa and to channel current to an optically active layer within the mesa structure.
A mask defining the mesa is then removed, and further layers are grown up to a p+-InGaAs ternary cap layer. The ternary cap layer has a relatively low resistance and narrow bandgap facilitating electrical contact, and so serves as a contact layer to which electrical contacts can be made.
In devices using InGaAsP/InP materials, current confinement regions have often been employed based on a reverse-biased p-n or n-p diode structure. Such structures provide high resistance to current flow, and low leakage currents. These devices can also be directly modulated, and are widely used in fiber optic communication systems across a range of operating temperatures and at frequencies up to about 2.5 GHz.
In recent years there has been an increasing demand for fiber optic communication links having a bandwidth in excess of 2.5 GHz, for example up to at least 10 GHz. EA modulators can be used to achieve higher operating frequencies, but further limitations to operating frequency arise when an EA modulator is formed with the laser on the same substrate using the same current confinement structure. At operating frequencies above 2.5 GHz, the performance of EA devices becomes limited by the capacitance of the blocking structures used by lasers to achieve good current blocking performance. A lower capacitance structure that permits the EA modulator to operate at a high frequency can have poorer current blocking performance or lead to higher threshold currents in the laser section. It can be possible to limit such temperature changes by the use of a thermo-electric cooler, but this adds to the complexity, power budget, and cost of the device.