(Invited) Differential Hall Effect Metrology (DHEM) Sub-Nm Profiling and Its Application to Dopant Activation in n-Type Ge
Authors: Ramesh Pranav , Saraswat Krishna C. , Joshi Abhijeet , Basol Bulent M , Wang Larry , Buyuklimanli Temel 
Published: May 2020

With silicon CMOS technology approaching its limits due to continued device scaling, many potential new channel materials have been proposed. Germanium shows considerable promise due to having higher electron and hole mobilities compared to silicon. In addition, the electron and hole mobilities are similar, making it a viable option for CMOS technology. High performance Ge PMOS has been demonstrated, but NMOS has lagged behind due to the difficulty in making low resistance contacts to n-type Ge. Contact resistance is a strong function of the carrier concentration at the semiconductor surface, and it is extremely difficult to achieve high electron activation in Ge. Point defects in Ge are known to behave as acceptor sites and reduce the overall electron activation, particularly near the surface. Minimization of these defects is essential to achieving high performance Ge NMOS. Techniques such as fluorine passivation and co-doping with Sb and P have been shown to reduce the defect density and improve dopant activation, but the impact of these methods in the near-surface regime has largely been uncharacterized. Accurate analysis of dopant activation within 5 nm of the surface is critical to understanding the effectiveness of such processes on improving contact resistance.

Differential Hall Effect Metrology (DHEM) is a powerful method that provides dopant activation depth profiles at sub-nm resolution. The method employs electrochemical means of reducing the electrically active thickness of a semiconductor film and determining the sheet resistance and mobility of the thinned down layer using Van der Pauw/Hall effect type measurements. Repeating this process and using differential equations, depth profiles of mobility and carrier concentration are obtained [1].

We have previously used the ALProTM tool employing the DHEM technique to study the effect of Ge co-doping with Sb and P on near-surface dopant activation [2]. In Ge with only P-doping, the active dopant concentration was ~5x1018cm-3 at the surface despite the P chemical concentration being ~1x1020cm-3, indicating that only 5% of the implanted dopants were electrically active. With co-doping of Sb and P, the active electron concentration increased to ~2x1019cm-3 with a similar P chemical concentration. Prior work has shown similar increases in activation with co-doping but did not capture the significant decrease in electron concentration by 4x within 10 nm of the surface. Here, we further study the effects of the implantation and annealing process on near-surface dopant activation in n-type Ge. Samples were prepared by growing 3 µm thick epi-Ge layers on Si substrates. Sample 1 was capped with 20 nm SiO2 and Sample 2 was capped with 20 nm Al2O3 to minimize implant damage and control implant depth. Both samples were implanted first with Sb (dose: 6x1014cm-2, energy: 65 keV) followed by P (dose: 6x1014cm-2, energy: 90 keV). Post-implant anneal was done at 500°C for 10 s in N2 to activate the dopants. ALProTM and SIMS measurements were done to characterize active dopant concentration and total chemical dopant concentration, respectively.

The Al2O3 capped sample (Sample 2) showed significantly lower dopant activation than the SiO2 capped sample (Sample 1) throughout the depth of the profile (Figure 1). Active electron concentration at the surface was lower by nearly an order of magnitude, decreasing from ~2x1019cm-3 to ~2x1018cm-3. Additionally, the peak concentration decreased from 1x1020cm-3 to 5x1019cm-3 at a depth of ~30 nm in both cases. The variations in the carrier profile for the Al2O3 capped sample also indicate greater process variation. SIMS profiles show that there is significant diffusion of Al into the Ge matrix after implant and anneal. Al chemical concentration is 1x1020cm-3 at the Ge surface and steadily decays up to a depth of more than 300 nm. This indicates that there is significant damage caused during the implantation process and the activation anneal is not enough to remove these defects. Al atoms then diffuse from the Al2O3 capping layer into the highly defective Ge substrate. Al is a p-type dopant in Ge and effectively counter dopes the Sb and P implants to reduce overall electron concentration in the Al2O3 capped sample. It is important to note the high resolution DHEM technique was able to resolve this important phenomenon right near the surface of the material.

References:

1. Abhijeet Joshi and Bulent Basol, “ALPro System: An Electrical Profiling Tool for Ultra-thin Film Characterization”, 2017 Frontiers of Characterization and Metrology for Nanoelectronics (FCMN), pgs.170-172.

2. Ramesh, et. al., “Near-Surface Sub-nm Resolution Activation Profiles in P and Sb+P Doped Ge”, 2019 Frontiers of Characterization and Metrology for Nanoelectronics (FCMN), pgs. 193-195.

Figure 1