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Nova Crystals takes InGaAs-on-silicon avalanche photodiodes to the market

Fiber-optic components manufacturer Nova Crystals claims to have developed an InGaAs-on-silicon APD with a difference. Touted at this year s OFC as "the world s most sensitive semiconductor for metro and long-haul optical networks", the 1300-1600 nm InGaAs-on-Si APD is said to have a record low excess noise, high gain bandwidth, and the same dark current and responsivity as commercial InP-based APDs.

Nova is gearing up to supply the APDs in sample quantities by the third quarter of this year. This has been achieved by developing a wafer-scale bonding technology that the company says successfully combines the light absorption advantages of InGaAs with the beneficial physical properties of silicon.

Overcoming InP obstacles

InGaAs-on-InP APDs are widely used in high-sensitivity 2.5 Gbit/s and 10 Gbit/s fiber-optic receivers, but InP material limitations restrict gain-bandwidth to around 80 GHz and lead to a high excess noise factor. Previous attempts to combine InGaAs with silicon, rather than InP, have failed because lattice mismatches have led to high dark currents, which are fatal to the device s operation. However, Yu-Hwa Lo and colleagues at Nova Crystals and the University of California, San Diego claim to have overcome these problems by using a proprietary wafer fusion technique that reduces stresses at the bonding interface.

The silicon part of the researchers photodiode consists of an n+ substrate, a 1 µm n-epitaxial layer and a thin p+-epitaxial layer, while the III-V layers comprise a 1 µm undoped InGaAs layer and two InGaAsP-on-InP p+ contact layers. To fuse the III-V and Si wafers, the researchers first grew the III-V layers onto an InP substrate. Each substrate was then rinsed in hydrofluoric acid, placed in contact and heated at 650 °C for half an hour.

Once the wafers had bonded, the InP substrate was etched away leaving only the epitaxial device layers on the silicon substrate. The device operates by absorbing 1300-1600 nm light within the InGaAs layer. Photo-generated electrons are then injected into the silicon multiplication region.

Although they were unable to give extra details on the wafer fusion technique, the researchers say that while van der Waals forces hold the surfaces together on initial contact, prolonged heating leads to stronger covalent forces that bond the surfaces (see figure 1).

Wafer fusion success

"By optimizing the temperature and pressure cycles [during bonding] we have reduced thermal stresses at the interface," said Lo. "This prevents the generation of threading crystal dislocations, reduces tensile shear stress that can cause microcracks, and also eliminates peel stress that leads to debonding."

According to the researchers the lower thermal stresses are responsible for the device s dark current of 35 nA, while the use of silicon enables a record low excess gain noise factor of 2.2, both at a photomultiplication gain of 10. At a gain of 50, the excess noise factor is only 2.8, compared with a value of 26 for conventional InP APDs (see figure 2).

The researchers have also reported gain-bandwidths of more than 140 GHz - with the potential to extend to 400 GHz - as well as a temperature coefficient of less than 0.03 V/°C. Compared with conventional InP-based APDs, the InGaAs-on-Si devices are expected to have a receiver sensitivity that is 5 dB higher, and a similar responsivity.

"Yields have been considerably improved by reducing the thermal stress at the bonding interface, and are at similar levels to those achieved during InP-based manufacturing," said Lo. "We can achieve wafer-scale manufacturing and the use of silicon substrates makes testing and packaging easier."

Nova Crystals is already beginning to work towards faster 10 Gbit/s and 40 Gbit/s versions of the InGaAs-on-Si photodiode.

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