A miniature laser made from a rolled-up tube of a thin semiconductor film looks set to offer silicon photonic integrated circuitry a new source of light. M. Hadi Tavakoli Dastjerdi and colleagues at McGill University in Canada say that their electrically-driven tube laser can be readily transferred onto a silicon platform without any performance degradation (Appl. Phys. Lett. 106, 021114; 2015).

Credit: AIP

The Canadian team first fabricated a 54-nm-thick planar heterostructure film (featuring InGaAs/InGaAsP quantum wells as the gain media) on an InP substrate by molecular beam epitaxy. A tensile-strained 15 nm In0.68Ga0.32As0.41P0.59 layer was first deposited on top of the ultrathin film to make it roll up into a tube following its removal from the InP substrate by wet etching.

Once removed from the substrate, the team used the rolled-up film to form a free-standing microtube (pictured) with a high Q-factor and Purcell factor. The microtube (with a length of ∼100 μm, a diameter of ∼5 μm and a wall thickness of ∼140 nm) was supported by two side pieces that served as n- and p-type contacts for the electrical injection current. The Q-factor of the microtube was estimated by numerical simulation to be ∼800 for a wall-confined (24,1) resonance mode and the Purcell factor was calculated to be ∼4.3. A small notch in the wall of the tube was used to control the direction of light emission out of the structure.

The current–voltage characterization of the microtube showed a turn-on voltage for current flow of ∼3 V, which is relatively high. It is thought that this is due to the large electrical resistance of the p-type contact. Ideally, the device should be annealed at ∼800–900 °C to properly activate the implanted Be ions in the p-type contact, but no rolling of the film was observed for annealing temperatures above ∼600 °C.

The electroluminescence was measured under a pulsed bias current at a temperature of 80 K. The emitted light was collected by an optical fibre and sent to a spectrometer. As the injection current into the tube was increased, the light emission around 1,485 nm showed a clear spectral narrowing for a current of ∼1.05 mA, which coincided with a discontinuous change in the strength of the electroluminescence — a clear signature of the onset of lasing.