Internal Photoemission in Semiconductor Heterostructures with Engineered Barriers: Impact of Interface Dipoles

With the size of modern electronic devices reduced to a few nanometer scale, their electronic performance is largely determined by the properties of surfaces or interfaces. In particular, this concerns the interface energy barriers that essentially control electron transport properties of heterogeneous material systems. Thus, the ability to control and design the barrier heights represents an important milestone along the path to desired functionality and performance of the devices. However, the current understanding of the physical mechanisms determining the electron energy band alignment at the interface between two solids appears to be significantly incomplete, particularly, when considering the possible existence and impact of additional dipole component(s).

The main goal of the present work consists of obtaining spectroscopic information regarding origin, conditions of formation, and basic physical properties of the interface dipoles formed at the interfaces of semiconductors and metals with oxide insulators. Here, we present and discuss the experimental results of the interface-sensitive internal photoemission (IPE) spectroscopy studies of the interfaces between semiconductors (Si and Si-doped Ge) or various metals (Pt, Au, TiN, Ru, Ir, Ge-doped CuTe) and different oxide insulators, such as SiO2, Al2O3, HfO2, HfAlO, SrTiO3, ZrO2, SrO and Sc2O3. Results of this research reveal the importance of different barrier-related components, such as interlayers, dipoles, charges, etc., in this way indicating the possibilities and limits of different barrier engineering approaches. We demonstrate the difference between semiconductor/insulator and metal/insulator interfaces regarding the dipole formation and also show how different barrier variation mechanisms can be experimentally distinguished using the IPE method.

Furthermore, metal/insulator barriers are evaluated in relationship to the effective work-function (EWF) determination at the metal interface, which is used in advanced microelectronic devices, e.g., for the threshold voltage control in metal-gate transistors, resistive-switching stacks, etc. The metal EWF was found to vary significantly (up to 1 eV) depending on the several factors, ranging from the contacting insulator composition to the annealing and deposition conditions. Furthermore, we addressed the issue of lateral uniformity of electrodes by demonstrating ability of IPE spectroscopy to characterize and quantify inhomogeneous interface barriers. 

The results clearly confirm the place of IPE technique as one of the most reliable, straightforward and non-destructive methods for interface barrier quantification.