Improved performance of Al/n<sup>+</sup>Ge Ohmic contact andGe n<sup>+</sup>/p diode by two-step annealing method

Abstract

Silicon based germanium devices are crucial parts of optoelectronic integration as CMOS feature size continuously decreases. Germanium has attracted increasing attention due to its higher electron and hole mobility, larger optical absorption coefficient as well as lower processing temperature than those of silicon. However, the high diffusion coefficient and low solid solubility about n-type dopant and relatively high thermal budget required for high n-type doping in Ge make it difficult to achieve high activation n-type doping and excellent n<sup>+</sup>/p shallow junction for source/drain in the nano-scaled n-MOSFET (here MOSFET stands for). The high activation concentration and shallow junction n-type doping in Ge are greatly beneficial to the scaled Ge n-MOSFET technology. In this work, the ohmic contact of Al/n<sup>+</sup>Ge and Ge n<sup>+</sup>/p junction fabricated by a combination of low temperature pre-annealing process and excimer laser annealing for phosphorus-implanted germanium are demonstrated. Prior to excimer laser annealing, the samplesare annealed at a relatively low temperature, which can heal the implantation damages preliminarily. Through the optimization of pre-annealing temperature and time, the low temperature pre-annealing step can play a critical role in annihilating the implantation damages and significantly suppressing phosphorus diffusion in the laser annealing process, resulting in a very small dopant diffusion length at a high activation level of phosphorus. Through the combination of ion implantation and two-step annealing technology, the specific contact resistivity (<i>ρ</i><sub>C</sub>) of Al/n<sup>+</sup>Ge Ohmic contact is measured by CTLM structure. The optimized annealing condition is 400 <sup>o</sup>C-10 min of low temperature annealing and 150 mJ/cm<sup>2</sup> of ELA. Under that annealing condition, the ρ<sub>C</sub> of the sample by two-step annealing is reduced to 2.61 × 10<sup>–6</sup> Ω·cm<sup>2</sup>, which is one order of magnitude lower than that by ELA alone (about 3.44 × 10<sup>–4</sup> Ω·cm<sup>2</sup>). The lower value of <i>ρ</i><sub>C</sub> for the sample with LTPA can contribute to the higher carrier concentration and better crystalline quality thanthat without LTPA, which is confirmed by SRP and TEM. Moreover, the rectification ratio of Ge n<sup>+</sup>/p junction diode is improved to 8.35 × 10<sup>6</sup> at ± 1 V, which is two orders of magnitudes higher than that by ELA alone. And a lower ideality factor of about 1.07 is also obtained than that by ELA alone, which indicates that the implantation damages can be repaired perfectly by two-step annealing method.