Energy barriers at interfaces of high-mobility semiconductors with insulating oxides

Extended electron states in insulating oxide layers on Si and high-mobility semiconductors The electron energy band spectrum is of prime importance when attempting to engineer insulating stacks for application in metal-oxide semiconductor structures for practical applications. In particular, transition from traditional silicon to semiconductors with higher mobility (Ge, GaAs, InGaAs, InP, graphene…) poses significant challenges in terms of interface band alignment characterization as neither the role of interface dipoles nor the energy bands associated with hardly avoidable inter-layers are known at present. Moreover, scaling the insulating layers down to a thickness of a few nanometers requires understanding of general trends in the interaction of electron states originating from atoms of different sort forming a complex oxide. The investigation of these complex interfaces requires separate characterization of the density of states close to the edges of the conduction band (CB) and the valence band (VB), respectively. Experimentally, the latter can be achieved by studying Internal PhotoEmission (IPE) of electrons and holes from a known semiconductor crystal. In this way, the IPE spectroscopy is expected to deliver fundamental information on the development of electron bands in the insulating nano-layers grown on the surfaces of high-mobility semiconductors and, very desirably, may indicate possible ways to optimize them. The study of semiconductor interfaces requires samples of highest quality fabricated using the state-of-the-art microelectronics technology. Nowadays, two groups of materials promise gains over silicon technology sufficient to justify such technological efforts. These are the group IV semiconductors (SiGe, Ge, graphene) and narrow-gap AIIIBV alloys (GaAs, InGaAs, InP, InAs). The CB/VB offset energies and the corresponding interface band offsets are of crucial technological importance because they determine the energy barriers the charge carriers encounter at the interfaces and, in turn, the ultimate insulating performance of the semiconductor/oxide stack. The physical origin of the interface barriers include several components like bulk electron spectrum of contacting solids, the electron density of states in the interlayer(s), the interface charges and dipoles. Establishing the relative importance of the indicated factors represents the major fundamental issue to be addressed in the work. Another type of semiconductor materials emerging as areas of important innovations are the narrow-gap metal oxides like NiO and VO2, which poses unique electronic and optical properties. The bandgap of these materials is determined by the spin-correlated electron states which enables IPE analysis of the spin-related features in the electron density of states. For instance, the metal-insulator (Mott) transition in VO2 is observed at T=68 °C which allows direct monitoring of the valence states evolution by electron photoemission into insulating layer (SiO2 or Al2O3). These experiments are may shed light at the fundamental mechanisms of interface barrier formation. It is currently planned to conduct the band offset determination by IPE at the interfaces of semiconductors of the mentioned types with most widely applied oxide insulators (Al2O3, HfO2, ZrO2) as a function of surface treatment, crystallographic orientation, and interface doping with foreign atoms. These oxide insulators were extensively characterized in the past on Si surfaces that allows meaningful comparison. This research is planned in extensive international collaboration network that includes IMEC (Belgium), MDM Laboratories (Italy), Tyndall Microelectronics Center (Ireland), as well as other research establishments. Working plan: Year 1. In the first year it is planned to concentrate the analysis of interfaces of GaAs and InP with various oxides. By varying semiconductor composition and crystallographic orientation of its surface one can estimate the importance of composition-sensitive interface dipoles as compared to the contribution of bulk electron bands and interlayers. At the moment, two batches of samples on InP and GaAs with different surface orientation and treatment are fabricated using Al2O3 deposition and are being in the process of IPE measurements. Year 2. In the second year the impact of interface doping with foreign atoms on the band alignment will be studied. The systems to be analyzed are the interfaces of different insulating oxides with Ge, GaAs or GaAs interfaces. Again, the possibility of interface dipole formation will be in the focus of the investigation. Year 3. In the third year, the analysis of various metal oxides (NiOx, TiOx, VOx) is planned with as major goal gaining fundamental understanding of the electronic structure in the non-conductive and conductive states. The influence of the oxide composition on the band energies will also be addressed aiming at possible chemical identification of the major factors affecting electrical behavior of the oxides. Year 4. Based on the systematic analysis of the obtained results, further exploration of electronic structure of materials will be conducted. The work can be extended to other materials like graphene provided reproducible technology of its growth on insulating substrates will become available. Preparation of the PhD thesis is planned for the second half of the year.Chapter 1 Introduction 1.1 Band alignment in semiconductor heterojunctions 1.2 Band offset determination using spectroscopy of internal photoemission Chapter 2 Physics and Experimental Realization of Internal Photoemission 2.1 Basics of internal photoemission spectroscopy 2.2 Experimental procedures Chapter 3 Band Alignment at Interfaces of Ge and Ge-Based Semiconductors with Oxide Insulators 3.1 Band alignment at interfaces of Ge with Al2O3 and HfO2 3.2 Band alignment at interfaces of amorphous Al2O3 with Ge1-xSnx- and strained Ge-based channels 3.3 Conclusions Chapter 4 Band Alignment at III-V/Oxide Interfaces 4.1 Introduction 4.2 Oxide/arsenide interfaces 4.3 In and Ga phosphide/oxide interfaces 4.4 Gallium and indium antimonides/oxide interfaces 4.5 Comparison of interface barriers at interfaces of In and Ga phosphides,arsenides and antimonides with Al2O3 4.6 Transitivity of band offsets between semiconductor heterojunctions and oxide insulators 4.7 Conclusions Chapter 5 Interface Barriers at Interfaces of Transition Metal Oxides 5.1 Band offsets at interface of atomic-layer deposited TaSiOx insulators with Si, InP and In0.53Ga0.47As 5.2 Band alignment at interfaces of NiO layers grown on Al2O3 and SiO2 using metallo-organic chemical vapor deposition 5.3 Band alignment of vanadium di- and pento-oxides at interfaces with SiO2 and Al2O3 5.4 Conclusions Chapter 6 Summary and Outlook 6.1 General conclusions 6.2 Future worknrpages: 145status: publishe