It has been an elusive goal for decades to combine the directly tunable bandgap of III–V compounds with advanced silicon fabrication technology on a CMOS platform to achieve a seamless integration of active photonic components with silicon electronics. Our nanophotonic projects focus on exploring scaled III–V active components monolithically integrated on silicon as emitters and detectors for applications in communication and IoT.
Integrating photonic devices
To integrate photonic devices, we are exploiting and expanding a fabrication technique called Template-Assisted Selective Epitaxy (TASE) invented at IBM [ More ]. This technique is ideally suited for photonic applications that require micrometer-scaled features, and that may benefit from the flexibility with regard to the choice of materials and growth sequence as well as from the high level of integration this affords. We want to exploit the potential for realizing different types of cavities on silicon and in-plane with silicon passives.
To integrate III–V-based optical devices, we have extended TASE and developed the virtual substrate and direct cavity approaches, which are two new ways to enable the monolithic integration of III–V material optical devices on silicon.
Microdisk laser integrated on silicon. Scale 3 µm.
In the direct cavity approach, III–V material is grown from a confined Si area of a Si wafer into a cavity. Growth first proceeds vertically, then it expands horizontally. The grown crystal has a mushroom-like shape. We used direct cavity growth to fabricate 1 μm large hexagonal disks that lase at around 835 nm at room temperature and feature an extremely high temperature stability.
From 10 to 300 K, the lasing peak shifts by only 6 nm from 830 to 836 nm, which translates to a remarkably high characteristic temperature of 1500 K — about one order of magnitude greater than usually found in III–V silicon photonic lasers. Lasing thresholds were found to scale with a reduction of disk diameter down to about 2 pJ/pulse [ More ].
GaAs microdisk laser fabricate dusing the direct cavity approach.
(a) Cross section of microdisk (b).
(b) Top view of a GaAs microdisk 1 µm in diameter.
(c) Photoluminescence spectra of a GaAs microdisk at T = 80 K.
Virtual substrate approach
In the virtual substrate approach, III–V material is integrated via a two-step templated growth process. First, a thin lateral µm2-sized virtual substrate is grown via TASE. Next, oxide is deposited and patterned to form a second template for the growth of the gain material in vertical direction from the virtual substrate. GaAs microring and disk lasers have been demonstrated with the virtual substrate approach emitting at 880 nm at room temperature [ More ].
We recently extended the fabrication process from binary GaAs to ternary InGaAs, which allows emission wavelengths above the Si absorption edge and hence opens the possibility of using Si for passives. We demonstrated the integration of InGaAs microdisk and ring lasers up to 200 K at emission wavelengths around 1150 nm [ More ]. The composition control in ternary compounds is more challenging, which can result in multi-crystalline structures.
III–V semiconductors on Si tandem solar cells
Silicon-based solar cells are reaching their theoretical efficiency limit. This limit can be overcome by combining the current silicon photovoltaic technology with higher-bandgap materials, such as III–V semiconductors. However, material costs of III–V semiconductors is a limiting factor for mass production. Nanowires offer a cost-effective (minimum material consumption) solution without compromising absorption or performance.
The image at right shows a schematic drawing and a scanning electron microscope image of III–V nanowires grown with the TASE process on top of a silicon solar cell to form a multi-junction solar cell for next-generation photovoltaic technology [ More ].