Surface Versus Impurity-Doping Contributions in InAs Nanocrystal Field Effect Transistor Performance

The electrical functionality of an array of semiconductor nanocrystals (NCs) depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs NC thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped NC-based field effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NC-based FETs. As-synthesized InAs NC-based FETs show n-type characteristics assigned to the contribution of the surface electron accumulation layer that can be considered as an actual electron-donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NC FETs show enhanced n-type conduction as expected from the Cu impurity location as an interstitial n-dopant in InAs NCs. The simulated curves reveal the additional contribution from electrons within an impurity sub-band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC-based FETs. It provides guidelines for control of doping of NC arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs toward their future utilization as building blocks in bottom-up prepared optoelectronic devices.