News Article
Direct integration of III-V lasers on silicon
Compared to their previous work which demonstrated pulsed laser operation, the French IES group have incorporated in the laser a layer which has a dual function as an etch-stop and an ohmic contact layer.
Researchers from the University of Montpellier and CNRS (France) have realised the direct epitaxial integration of a III-V semiconductor laser diode on a silicon substrate.
The diode fabricated by the nanoMIR group of Institut d’Electronique du Sud (IES) operates under continuous wave (cw) above 30 °C. It emits several mW of output power at a 2 µm wavelength.
IES says this work represents a breakthrough towards the direct integration of III-V semiconductor materials and devices (transistors, lasers, LEDs, photodetectors) on a silicon platform. This, they point out, is a pre-requisite to emerging applications such as III-V CMOS logics, photonic integrated circuits, on-chip optical communications or system-on-chip/system-in-package integrated sensors.
Figure:CW L-I-V characteristics of a “top-top” LD. Right axis: output power vs injected current for different temperatures. Left axis: voltage-current curve at 20 ° C. The inset shows the lasing spectrum taken at 20 °C under 300 mA cw injection.
There is currently a lot of research in progress which is aimed at integrating III-V semiconductor alloys and devices on silicon in order to combine their extraordinary intrinsic properties with very advanced silicon technology.
Althoughremarkable advancements have been made in the last couple of years with die bonding of InP-based devices, it remains unclear whether this technique can be used on a large scale. Heterogeneous epitaxy in contrast would definitely allow large-scale fabrication and direct integration, but it has proven difficult due to the conjunction of large lattice-, thermal-, and polarity mismatches. The work of IES demonstrates that GaSb-based compounds are good candidates to solve these problems.
Epitaxial structures were produced by MBE on a tilted (001)-oriented silicon substrate. In contrast to other III-V/Si systems, under appropriate growth conditions, strain relaxation in the GaSb/Si system is known to occur by formation of defects which remain confined close to the GaSb/Si interface. This allows the realisation of good quality heterostructures without resorting to thick or composite buffer layers.
Compared to their previous work which demonstrated pulsed laser operation, the IES group enhanced the laser by adding a layer with a dual role: an etch-stop layer and an ohmic contact layer.
The chips were then processed with a “top-top” contact configuration which avoids driving the current through the defective GaSb/Si interface. A turn-on voltage of 0.8 eV was measured at 20 °C very close to the active zone bandgap of 0.6 eV. Several mW of output power under cw operation were measured with uncoated facets. CW operation was achieved up to 35 °C limited by the experimental setup.
The wavelength of these laser diodes is 2 µm. However the IES group has previously demonstrated emission with Sb-based lasers in the entire range from 1.5 µm up to 3.3 µm. They point out that the present results can thus be extended to this spectral zone. Further work will focus on this aspect as well as on reducing the optical losses which remain high as compared to homoepitaxial lasers.
More details of this work can be read in the paper “Continuous-wave operation above room temperature of GaSb-based laser diodes grown on Si” by Reboul et al, Applied Physics Letters, 99, 121113 (2011).
