22 July 2014

Working toward lasers for on-chip global interconnects

Tokyo Institute of Technology has claimed the first room-temperature continuous-wave (RT-CW) operation of a lateral-current-injection (LCI) gallium indium arsenide phosphide (GaInAsP) membrane Fabry-Perot (FP) laser bonded to a silicon substrate [Daisuke Inoue et al, Appl. Phys. Express, vol7, p072701, 2014].

The researchers believe their work could lead to ultralow-power-consumption lasers for on-chip optical interconnections. The aim would be to use such a laser in conjunction with on-chip waveguides to replace copper global interconnects that suffer from signal delays and Joule heating effects.

Figure 1: (a) Schematic structure of membrane laser on Si substrate and (b) initial heterostructure.

The semiconductor heterostructure for the laser (Figure 1) was grown on n-type indium phosphide (InP) substrate using molecular beam epitaxy (MBE). The five quantum wells (QWs) in the active region consisted of 1% compressively strained 6nm Ga0.22In0.78As0.81P0.19 in 0.15% tensile-strained 10nm Ga0.26In0.74As0.49P0.51 barriers. The 15nm optical confinement layers (OCLs) were also of GaInAsP.

Before the laser was transferred to the silicon substrate, the lateral-current-injection structures were created using reactive ion etching and a two-step organometallic vapor phase epitaxy (OMPVE) of the n-InP and p-InP contact regions on either side of the 0.7μm-wide device mesa stripe.

The top of the structure was covered in 1μm of silicon dioxide. The host silicon substrate was prepared with a 2μm layer of spin-coated benzocyclobutene (BCB) adhesive, pre-cured at 210°C in nitrogen. The laser wafer was flipped onto the host silicon and bonded at 25kPa pressure at 130°C, followed by hard-curing at 250°C for an hour in nitrogen.

Selective etching was used to removed the InP substrate and etch stop layer. The p+-GaInAs was also removed except for the p-electrode region. The p-InP cap layer was etched away from the n-electrode region. Finally, the titanium/gold (Ti/Au) electrodes were applied.

The structure was cleaved into Fabry-Perot lasers of various cavity lengths. A 350μm-long device had a threshold current of 2.5mA (1100A/cm2 density) in room-temperature continuous-wave operation. The external differential quantum efficiency was 22% from each facet (44% in total). The output power was 1.1mW at 10mA.

By analyzing the performance of lasers with different cavity lengths, the researchers estimate an internal quantum efficiency (IQE) of 75% and a waveguide loss of 42/cm. The internal quantum efficiency is close other vertical lasers and LCI membrane lasers on semi-insulating InP produced previously by the researchers. The team comments: “This indicates that non-radiative carrier recombination at both the top and bottom surfaces of the membrane structure are almost negligible, which is attributed to 50nm-thick InP cap layers as well as high-quality buried heterostructure interfaces prepared by two-step OMVPE re-growth. In addition, this high internal quantum efficiency indicates that there is minimal leakage current through the p-InP cap layer to the n-InP layer.”

The waveguide loss, however, is significantly higher than the 5.1/cm achieved for LCI lasers on semi-insulating InP at Tokyo Institute of Technology. The researchers suggest that scattering losses may have arisen from the side-wall roughness of the re-growth interface between the mesa stripe and the InP cladding layers.

Figure 2: Spectrum of LCI membrane FP laser with 370μm cavity length and 0.7μm stripe width. Bias at twice threshold current of 2.5mA.

The emission wavelength was around 1565nm (Figure 2).

Tags: LCI GaInAsP membrane FP laser GaInAsP MBE

Visit: http://iopscience.iop.org/1882-0786/7/7/072701/

The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.