Ultra Shallow Arsenic Junctions in Germanium Formed by Millisecond Laser Annealing

Abstract

Millisecond laser annealing is used to fabricate ultra shallow arsenic junctions in preamorphized and crystalline germanium, with peak temperatures up to 900°C. At this temperature, As indiffusion is observed while yielding an electrically active concentration up to 5.0 ϫ 10 19 cm −3 for a junction depth of 31 nm. Ge preamorphization and the consecutive solid phase epitaxial regrowth are shown to result in less diffusion and increased electrical activation. The recrystallization of the amorphized Ge layer during laser annealing is studied using transmission electron microscopy and spectroscopic ellipsometry.Germanium has received considerable attention in the last decade as a promising channel material for high performance logic applications. However, as scaling continues the source/drain resistance gains in importance. 1,2 Consequently, ultra shallow and low resistive junctions are required for the integration into very-large scale integration circuitry.Shallow, low resistive p-type junctions ͑using B or Ga as acceptors͒ can be attained using optimized solid phase epitaxial regrowth ͑SPER͒ and rapid thermal annealing ͑RTA͒ schemes. 3,4 In contrast, donors such as P or As show high concentration-enhanced diffusion; 5 a mechanism which has been consistently explained by mobile, negatively charged dopant-vacancy pairs. 6 The fabrication of ultra shallow junctions therefore requires a limited thermal budget ͑low temperature or fast anneal͒, while a high temperature is still needed to achieve a high electrically active concentration. These two competing requirements have led to the use of ultrafast heattreatment methods such as flash-assisted annealing 7-9 and millisecond laser thermal processing. Previously, laser annealed P junctions in Ge ͑X J Ͼ 70 nm͒ were investigated. 10,11 In this work, we investigate the feasibility of laser annealing to fabricate ultra shallow, low resistive As junctions in germanium. More specifically, the effects of the laser peak temperature and the combination with a preamorphization implant are studied.As junctions were fabricated using p-type, 300 mm, ͑100͒-oriented Si wafers on which a relaxed epitaxial Ge was grown ͑1.5 m thick, threading dislocation density Ϸ2 ϫ 10 7 cm −2 ͒ and capped with 2 nm of GeO 2 ͓which was stripped preceding secondary-ion mass spectroscopy ͑SIMS͒ and micro four-point probe tool ͑u4PP͒ analysis͔. The wafers received a boron well doping up to 3 ϫ 10 17 cm −3 , followed by a Ge preamorphization implant ͑PAI-Ge 20 keV, 2 ϫ 10 14 cm −2 ͒ on selected samples, yielding an amorphous layer of 25 nm. As was implanted at an energy of 5 keV ͑5 ϫ 10 14 cm −2 ͒. The samples then received millisecond laser annealing. The laser spot measures 1.1 cm ϫ 75 m and scans the wafer at a speed of 75 mm/s ͑two consecutive scans͒. A wafer preheating is applied ͑250°C͒ to reduce thermal stress arising from the localized laser heating. No absorber layer was deposited to assist in the laser anneal. Multiple regions ͑each measuring at least 5 ϫ 5 cm͒ were illuminated, whereby the laser ...

Diffusion ofn-type dopants in germanium

“…Similarly, high concentrations of electrically active As and Sb were obtained by laser annealing of As-and Sb-implanted Ge. 89 The short laser anneals favor solid phase epitaxial regrowth (SPER), i.e., the recrystallization of Ge layers that were amorphized by ion implantation either beforehand by means of Ge implantation or by dopant implantation itself. In particular, laser annealing is advantageous as even structural defects can be removed that are hardly dissolved by conventional thermal treatments (see, e.g., Bruno et al…”

N-type doped germanium contact resistance extraction and evaluation for advanced devices

“…Choosing the S/D architecture depends on ρ c , but also on dopant activation and diffusivity. The best reported active concentrations for P, As, and Sb are >10 20 , 5×10 19 , and >10 20 cm -3 respectively [20], [21], [22]. However the difference in P, As, and Sb diffusivities (D) is far more significant.…”

Processing of germanium for integrated circuits

“…Almost all literature dopant studies describe ion implanted B, Ga, P, As, or Sb into bulk Ge substrates. For the n-type dopants, the shallowest profiles have been produced by 5 keV As implants [72]. For the p-type dopants, the shallowest profiles have been produced by 2 keV B implants [73].…”

“…It has been demonstrated that dopant activation and contact resistance can be dramatically improved by LTA in the melt regime [72,[93][94][95][96][97][98]. Control of melt-depth is done by changing laser energy density.…”