To improve gallium nitride device performance for high-frequency RF applications, AppliCote is developing a high pressure laser doping process

Contact resistance and access resistance in deeply scaled FET devices greatly impact device performance at high frequency. This is of particular importance for GaN-based devices, which can achieve high power at high frequencies (>100 GHz).

Several processes have been developed to address these resistance issues, but these processes all have drawbacks.

For example, ion implantation can be used to increase the concentration of electrically active impurities in the source and drain regions of the device, but this process requires high temperatures for electrical activation, along with capping layers to prevent GaN decomposition.

In addition, implantation creates lattice damage that is difficult to remove via annealing and acts to compensate the dopants.

To improve GaN device performance for high-frequency RF applications, AppliCote is  developing a high pressure (greater than 500 psi gas/vapour precursor) laser doping process.

The procedure will introduce electrically active n-type impurities into the source and drain regions of a GaN device to reduce contact resistance and decrease access resistance from the metal contact to the two dimensional electron gas (2DEG) in the device.

Low pressure (less than 60 psi gas/vapor precursor) laser doping process that has been successfully used and reported previously with numerous materials, including GaN, SiC, silicon/SiGe, and silicon-based photovoltaics. 

Processing parameters for high pressure doping of silicon carbide have been developed (patent pending). Applicote plans to optimise processing conditions for doping GaN with silicon using a gaseous precursor and answer key questions about the electrical properties of the laser-doped GaN as well as process control and capabilities.

The high pressure laser doping process is a combination of a thermally driven process resulting from the interaction of the semiconductor with a high-power, short-duration laser pulse and a pressure driven process to increase dopant concentration to maximum solubility levels at deep depths while mitigating surface damage. 

The laser pulse results in an ultra-fast thermal ramp (1010 K/s) and impurity incorporation through decomposition of chemisorbed gas-phase source species and thermal diffusion of atoms into the crystalline lattice.

Impurity incorporation rate, diffusivity, and activation are all functions of the laser wavelength, power, and pulse time and precursor pressure.

Applicote has built a laser system and processing chamber for high pressure laser doping for rapid processing of substrates and simplification of the device fabrication process.