RESEARCH REVIEW Laser fulfils optical
interconnect requirements Quaternary laser combines a small footprint with sufficient power and a low threshold current
DATA TRANSFER RATES from one silicon chip to another are held back by the copper wires that link them. To unlock this bottleneck, engineers in many different groups have been striving hard to try and develop the III-V lasers that are suitable for deployment in optical interconnects.
Now, Daisuke Inoue from Tokyo Institute of Technology, is claiming that he and his co-workers have broken new ground, fabricating the first lasers satisfying the requirements for on-chip optical interconnection. “Previously reported devices satisfying all of these requirements – such as in-plane integrability, small footprint, sufficient output power and low threshold current – cannot be found.”
Inoue and his colleagues from Tokyo Institute of Technology have fabricated GaAsInP-based lasers with a lateral- current-injection architecture and attached them to a silicon wafer using
the polymer benzocyclobutene (BCB). Use of BCB has been claimed to compromise performance, by preventing efficient heat extraction from the laser.
Inoue accepts that BCB has a low thermal conductivity, but argues that this does not necessarily have to hamper laser performance: “According to [our thermal] analysis, if ultra-low power consumption operation is achieved, the poor thermal dissipation characteristic will have a small effect on the lasing characteristics.”
Lasers were formed by first depositing etch stop layers – 300 nm-thick GaInAs and 100 nm-thick InP – on an InP substrate, and then adding a stack of epitaxial layers, which included a highly doped p-type GaInAs contact layer, a strain-balanced active region with five Ga0.22
In0.78 As0.81 P0.19 quantum wells, and
an un-doped, 50 nm-thick InP cap. The membrane core layer of this MBE-grown structure has a total thickness of 220 nm.
Engineers then performed a two-step MOCVD regrowth to create a lateral p-n junction. This involved forming 7 μm- wide, 50 nm-thick stripes on the epitaxial structure, carrying out a reactive ion etch, and filling trenches with n-type InP. Etching away this InP region from one side of every laser structure, and filling it with p-type InP, created devices with lateral current injection.
To unite lasers to the silicon substrates, the team deposited a 1 μm-thick film of SiO2
onto the epiwafer by plasma
CVD, and bonded this to the silicon host substrate, which was topped with a spin- coated, 2 μm-thick film of BCB that had been pre-cured at 210 °C in a nitrogen atmosphere. Wafer bonding involved applying a pressure of 25 kPa to the wafers while they were held at 130 °C, followed by hard-curing at 250 °C for one hour in a nitrogen atmosphere.
Polishing and selective wet chemical etching removed the InP substrate, the etch stop layers, and then the contact layer in all areas apart from the p-electrode region. After removing the p-type InP cap on the n-electrode region, Ti/Au electrodes were added on the heavily doped p-type GaInAs contact and the n-type InP.
It took an hour to remove the 600 μm- thick InP substrate by chemical polishing and wet etching. “We are considering to re-use the InP substrate by applying a smart-cut technique,” says Inoue.
Cleaving the wafer formed Fabry-Perot lasers with a 350 μm-long cavity and a 0.7 μm stripe-width had a 2.5 mA threshold current, a front facet external differential quantum efficiency of 22 percent, and a 1.1 mW output at a 10 mA injection current. Since near-facet and far-facet light output were identical, the differential quantum efficiency of the device was 44 percent. Inoue says that the introduction of a distributed feedback structure will drive down the threshold current to below 100 μA, leading to a high efficiency for optical interconnects.
“Our target on the performance of the lasers will soon be realised,” explains Inoue, who reveals that the team’s next plan is to implement the semiconductor membrane optical interconnection on CMOS large-scale-integration chips.
Scanning electron microscopy image of GaInAsP/InP membrane, distributed-feedback lasers integrated with waveguides and detectors.
D. Inoue et. al. Appl. Phys Express 7 072701 (2014) Copyright Compound Semiconductor Issue VI 2014
www.compoundsemiconductor.net 67
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