Effects of Plasma-$\hbox{PH}_{3}$ Passivation on Mobility Degradation Mechanisms of $\hbox{In}_{0.53} \hbox{Ga}_{0.47}\hbox{As}$ nMOSFETs

In this paper, we investigated the inversion-layer scattering mechanisms of HfAlO In_0.53Ga_0.47As nMOSFETs with a plasma-PH₃ passivation layer to understand the physical origins of mobility enhancement compared with a nonpassivated device. It has been found in low E_eff that the mobility enhancement caused by the plasma- PH₃ passivation is due to the reduction in Coulomb scattering caused by reduction in D_it in the upper half of the band gap, as shown from the plasma-PH₃ reaction, which involves P-for-As exchange reaction that reduces the As vacancy sites. Plasma- PH₃ passivation also results in reduction of the phonon scattering caused by soft optical phonons in the HfAlO, which has weak temperature dependence. This is due to the thicker passivation layer of the plasma-PH₃ -passivated device compared with the interfacial layer present in the nonpassivated device. Plasma- PH₃ passivation also helps to reduce the interface dipole scattering caused by fluctuating dipoles at the HfAlO/ In_0.53Ga_0.47As interface, which may be attributed to the interdiffusion of elements from HfAlO and In_0.53Ga_0.47As . In addition, it is found that effective channel mobility is decreased as gate length reduces until sub-100 nm, due to increased effects of neutral scattering of charges near the source/drain as well as the effect of ballistic transport, thus possibly degrading mobility with further device scaling.

I. Introduction

The III-V compound semiconductor is one of the attractive materials for the substrate of MOSFETs due to their higher intrinsic mobility compared with Si. Demonstration of -based MOSFETs fabricated with various in situ and ex situ deposited high- dielectrics, with and without a passivation layer, has been reported [1]–[4]. While the long-channel MOSFET characteristics are promising, exhibiting unpinned Fermi level at the –dielectric interface and high peak values of inversion channel mobility, its channel mobility has still been lower than the expected bulk mobility achievable in . Hence, this shows the possibility of further improvement in the device interface to maximize mobility. In order to realize mobility enhancement, the scattering mechanisms responsible for the mobility degradation must be understood. So far, several scattering mechanisms have been proposed, and they include mechanisms such as interface roughness, interface dipole scattering [5], remote soft optical phonon scattering, polar optical phonon scattering from itself, and remote Coulomb scattering due to charges in the oxide and/or in the interface [6]. In this paper, we first studied the mobility degradation mechanisms present in HfAlO nonpassivated and plasma--passivated devices. Second, the factors causing the improvement in mobility of the plasma--passivated device over the nonpassivated device would be explained. Lastly, we will present evidence to show that the effective mobility in the sub-100-nm channel device is further decreased compared with long-channel devices, thus possibly degrading device performance with further scaling. Fig. 1.

Process flow of the self-aligned channel MOSFET with plasma--passivation process.