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Tandem Solar Cells Are Set To Benefit From Surface Activated Bonding

Surface activated bonding of GaAs and silicon substrates can yield high-quality heterojunctions, according to electrical characterization and microscopy.

A Japanese partnership between Osaka City University and NTT Photonics Laboratories has shown that surface-activated bonding is a promising approach for fabricating novel heterojunctions. This can be used in several devices, including tandem solar cells built from GaAs and silicon.

Surface activated bonding, which begins by firing of fast beams of argon atoms onto surfaces, has a significant advantage over conventional wafer bonding: It does not require a heating step to form high-quality electrical junctions.

Studies by researchers at UCLA have revealed that the electrical properties of a conventionally bonded GaAs/GaAs interface are strongly influenced by pre-bonding surface treatments and post-bonding annealing.

"They claimed that annealing in the range of 400 oC to 600 oC was essential for realising excellent characteristics," explains Naoteru Shigewaka from Osaka City University.

According to him, this annealing step has its downsides: It takes time; it may drive the diffusion of impurities through interfaces; and it can create mechanical defects, due to differences in substrate thermal expansion coefficients. 

Shigewaka and his co-workers have used surface activated bonding two form a GaAs-silicon heterojunction. This has been probed by current-voltage and capacitance-voltage measurements and scrutinized by field emission scanning electron microscopy and energy dispersive X-ray spectroscopy.

The surface-activated bonding tool involves the bombardment of substrates by a beam of fast argon atoms.

Devices were formed by taking a boron-doped silicon substrate and a silicon-doped GaAs substrate (resistivities of 0.1 Ωm and 0.002 Ωm, respectively, and carrier concentrations and 2.4 x 1017cm-3 and 1.1 x 1018cm-3, respectively) and cleaning them with acetone and ethanol in an ultrasonic bath for 5 minutes. After drying under nitrogen, substrates were loaded into a surface-activated bonding tool and their surfaces were activated by an argon fast atom beam.

After this step, pressing the substrates together for 60 s united them, and then Al/Ni/Au and AuGe/Ni/Ti/Au stacks were evaporated on the silicon and GaAs substrates, respectively. Rapid thermal annealing at 400 oC formed ohmic contacts.

Imaging with a field-emission scanning electron microscope showed that no structural defects, such as cracks, were present at the interface. What's more, the level of oxygen found in the interface is similar to that in the GaAs substrate, according to energy dispersive spectroscopy.

Plots of current as a function of voltage showed that the device had rectifying properties similar to those in conventional p-n junctions. For this heterojunction, the onset for the forward bias voltage was 0.38 V.

Capacitance-voltage measurements enabled calculations for the depletion layers. They are estimated to be 84 nm in silicon, and 18 nm in GaAs.

Two other features found in the current-voltage characteristics are an increase in current at higher values of reverse bias, and a gradient in these plots that shows very little variation with temperature. The researchers say that this behaviour can be explained with a trap-assisted tunnelling model, which incorporates a trap energy of 0.1 eV.

Thanks to the lack of any signs of oxygen at the interface, the team is arguing that its surface-activated bonding promises to enable the fabrication of high-quality, novel devices. They are now processing tandem cell structures formed by surface-activated bonding.

J. Liang et al. Appl. Phys. Express 6 021801 (2013) 

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