化合物半导体表面的电子控制化学稳定性

Effects of humid environment on the degradation of semiconductors were studied  to understand the role of the surface charge on material stability. Two distinctly different  semiconductors with the Fermi level stabilization energy EFS located inside the conduction (CdO)  and valence band (SnTe) were selected and effects of an exposure to 85℃ and 85% relative humidity conditions on their electrical properties were investigated. Undoped CdO films with bulk  Fermi level EF below EFS and positively charged surface are very unstable. The stability greatly  improves with doping when EF shifts above EFS and the surface becomes negatively charged. This  charge-controlled reactivity is further confirmed by the superior stability of undoped p-type SnTe  with EF above EFS. These distinct reactivities are explained by the surface attracting either the  reactive OH-  or passivating H+  ions. The present results have important implications for  understanding the interaction of semiconductor surfaces with water or in general with ionic  solutions.

The progressing miniaturization and increasing surface to volume ratio in modern  semiconductor devices greatly emphasizes the importance of the semiconductor surface stability  under various environmental conditions. In addition, in some instances, e.g., sensors or water  splitting devices, the surface plays a role essential in the device operation and cannot be separated  from the ambient. Therefore, there is a need to fully understand the factors affecting chemical  reactions on semiconductor surfaces induced by different environments.

Different factors have been identified to impact the stability of compound semiconductors upon  moisture exposure. The nature and the strength of the chemical bonds are one of the main factors  that determine the material stability and the degradation process. It is well known that a prolonged  exposure of weakly bound perovskite-type hybrid materials to moisture results in the material  decomposition1,2 . Also, Hisaka et. al3  found that the degradation of AlGaAs/InGaAs pseudomorphic high-electron-mobility transistors started with the oxidization reaction at the drain  side corner under high humidity conditions. Comizzoli et. al.4  found that the performance  degradation of their avalanche photodiodes upon moisture exposure began with the corrosion of  the SiN passivation layer.

Numerous studies have shown that surface Fermi energies of semiconductor materials are  pinned at the Fermi stabilization energy which is a common energy EFS reference located at about  4.9 eV below the vacuum level. Although in most semiconductors is located inside the band  gap, in some instances  EFS the can fall into either the conduction or the valence band, as shown  in Figure 1. In these extreme cases, the semiconductors  EFS show a clear proclivity for the n-type or ptype doping. The surface Fermi energy is pinned by dangling-bond-like defects whose nature.

Fig1

Consequently, the surface electrical charge can be controlled by a judicious choice of the  material, doping type and the doping level. Thus a surface of undoped CdO with located  deeply in conduction band12 is positively charged whereas the surface of undoped SnTe with  located in the valence band16 is negatively charged. The surface charge can also be controlled by  shifting of the Fermi energy with an intentional doping.