A novel approach for highly activated p+ diffusion layer formation in germanium by pre-heating oxygen desorption before ion implantation

Silicon semiconductor devices have remarkably been improved by the process scaling and introduction of some boosting technologies such as high-k/metal gate and strained silicon. However further improvement is quite difficult from the view point of mobility of silicon material. To overcome this problem, an increasing amount of attention has been devoted to the investigation of some higher carrier mobility materials such as SiGe, Ge or III-V materials. Among those materials, due to higher carrier mobility of Ge, it is promising as one of the candidates for high-performance devices with low-power and high-speed operation [1]. However several challenges still remain for the fabrication of Ge MOSFETs. For junction formation, it is difficult to form shallow N
⁺
/P junction in Ge due to the anomalous high diffusion coefficient of donor ions. On the other hand, forming shallow P
⁺
/N junction is relatively easy, however the high activation is difficult. To address these problems, some processes have been proposed such as aluminum doping [2], group-III metal doping [3] and optimization of Flash Lamp Annealing (FLA) [4]. In this study, we focused on the effect of oxygen remaining in the surface region of Ge substrate. Generally, a large amount of solid-solved oxygen is contained in Ge substrate. It is known that the oxygen inside Ge crystal acts as a Thermal Double Donor (TDD) [5]. TDD will inhibit the activation of acceptor ions in Ge and result in increase in the sheet resistance (Rs) of p
⁺
diffusion layer formed into the surface region of the Ge substrate. In order to improve the activation, the Ge substrate is pre-heated at high temperature in the vacuum chamber before ion implantation to control the oxygen concentration in the surface region. Oxygen atoms in the Ge substrate can easily out-diffuse at the initial stage of ion implantation. Furthermore implantation is carried out at relatively low temperature to avoid TED of boron. In this paper, we report the results of BF
2
implantation into Ge substrate with pre-heating process.