Roman Engel-Herbert

Comparison of methods to quantify interface trap densities at dielectric/III-V semiconductor interfaces

Methods to extract trap densities at high-permittivity (k) dielectric/III-V semiconductor interfaces and their distribution in the semiconductor band gap are compared. The conductance method, the Berglund intergral, the Castagne–Vapaille (high-low frequency), and Terman methods are applied to admittance measurements from metal oxide semiconductor capacitors (MOSCAPs) with high-k/In0.53Ga0.47As interfaces with different interface trap densities. The results are discussed in the context of the specifics of the In0.53Ga0.47As band structure. The influence of different conduction band approximations for determining the ideal capacitance-voltage (CV) characteristics and those of the MOSCAP parameters on the extracted interface trap density are investigated. The origins of discrepancies in the interface trap densities determined from the different methods are discussed. Commonly observed features in the CV characteristics of high-k/In0.53Ga0.47As interfaces are interpreted and guidelines are developed to obtain...

Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm2 V(-1) s(-1).

The study of quantum phenomena in semiconductors requires epitaxial structures with exceptionally high charge-carrier mobilities. Furthermore, low-temperature mobilities are highly sensitive probes of the quality of epitaxial layers, because they are limited by impurity and defect scattering. Unlike many other complex oxides, electron-doped SrTiO(3) single crystals show high (approximately 10(4) cm(2) V(-1) s(-1)) electron mobilities at low temperatures. High-mobility, epitaxial heterostructures with SrTiO(3) have recently attracted attention for thermoelectric applications, field-induced superconductivity and two-dimensional (2D) interface conductivity. Epitaxial SrTiO(3) thin films are often deposited by energetic techniques, such as pulsed laser deposition. Electron mobilities in such films are lower than those of single crystals. In semiconductor physics, molecular beam epitaxy (MBE) is widely established as the deposition method that produces the highest mobility structures. It is a low-energetic, high-purity technique that allows for low defect densities and precise control over doping concentrations and location. Here, we demonstrate controlled doping of epitaxial SrTiO(3) layers grown by MBE. Electron mobilities in these films exceed those of single crystals. At low temperatures, the films show Shubnikov-de Haas oscillations. These high-mobility SrTiO(3) films allow for the study of the intrinsic physics of SrTiO(3) and can serve as building blocks for high-mobility oxide heterostructures.

Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy approach

A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source (titanium tetra isopropoxide) for Ti. SrTiO3 films were investigated as a function of cation flux ratio on (001) SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Growth conditions for stoichiometric insulating films were identified. Persistent (>180 oscillations) reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed.

Nitrogen-passivated dielectric/InGaAs interfaces with sub-nm equivalent oxide thickness and low interface trap densities

We report on the electrical characteristics of HfO2 and HfO2/Al2O3 gate dielectrics deposited on n-In0.53Ga0.47As by atomic layer deposition, after in-situ hydrogen or nitrogen plasma surface cleaning procedures, respectively. It is shown that alternating cycles of nitrogen plasma and trimethylaluminum prior to growth allow for highly scaled dielectrics with equivalent oxide thicknesses down to 0.6 nm and interface trap densities that are below 2.5 × 1012 cm−2 eV−1 near midgap. It is shown that the benefits of the nitrogen plasma surface cleaning procedure are independent of the specific dielectric.

Analysis of trap state densities at HfO2/In0.53Ga0.47As interfaces

HfO2 was deposited on n- and p-type In0.53Ga0.47As by chemical beam deposition. Interface trap densities (Dit) and their energy level distribution were quantified using the conductance method in a wide temperature range (77 to 300 K). A trap level close to the intrinsic energy level caused the Dit to rise above 1013 cm−2 eV−1. The trap level at midgap gives rise to false inversion behavior in the capacitance-voltage curves for n-type channels at room temperature. The apparent decrease of the Dit close to the band edges is discussed.

Correlated metals as transparent conductors

The fundamental challenge for designing transparent conductors used in photovoltaics, displays and solid-state lighting is the ideal combination of high optical transparency and high electrical conductivity. Satisfying these competing demands is commonly achieved by increasing carrier concentration in a wide-bandgap semiconductor with low effective carrier mass through heavy doping, as in the case of tin-doped indium oxide (ITO). Here, an alternative design strategy for identifying high-conductivity, high-transparency metals is proposed, which relies on strong electron-electron interactions resulting in an enhancement in the carrier effective mass. This approach is experimentally verified using the correlated metals SrVO3 and CaVO3, which, despite their high carrier concentration (>2.2 × 10(22) cm(-3)), have low screened plasma energies (<1.33 eV), and demonstrate excellent performance when benchmarked against ITO. A method is outlined to rapidly identify other candidates among correlated metals, and strategies are proposed to further enhance their performance, thereby opening up new avenues to develop transparent conductors.

Demonstration of MOSFET-like on-current performance in arsenide/antimonide tunnel FETs with staggered hetero-junctions for 300mV logic applications

Type II arsenide/antimonide compound semiconductor with highly staggered GaAs<inf>0.35</inf>Sb<inf>0.65</inf>/In<inf>0.7</inf>Ga<inf>0.3</inf>As hetero-junction is used to demonstrate hetero tunnel FET (TFET) with record high drive currents (I<inf>ON</inf>) of 190µA/µm and 100µA/µm at V<inf>DS</inf>=0.75V and 0.3V, respectively (L<inf>G</inf>=150nm). In<inf>x</inf>Ga<inf>1−x</inf>As (x=0.53, 0.7) homo-junction TFETs and GaAs<inf>0.5</inf>Sb<inf>0.5</inf>/In<inf>0.53</inf>Ga<inf>0.47</inf>As hetero TFET with moderate stagger are also fabricated with the same process flow for benchmarking. Measured and simulated TFET performance is benchmarked with 40nm strained Si MOS-FETs for 300mV logic applications.

Highly Conductive SrVO3 as a Bottom Electrode for Functional Perovskite Oxides

Stoichiometric SrVO3 thin films grown by hybrid molecular beam epitaxy are demonstrated, meeting the stringent requirements of an ideal bottom electrode material. They display an order of magnitude lower room temperature resistivity and superior chemical stability, compared to the commonly employed SrRuO3 , as well as atomically smooth surfaces. Excellent structural compatibility with perovskite and related structures renders SrVO3 a high performance electrode material with the potential to promote the creation of new functional oxide electronic devices.

Metal-oxide-semiconductor capacitors with ZrO2 dielectrics grown on In0.53Ga0.47As by chemical beam deposition

Zirconium oxide films were grown by chemical beam deposition with zirconium tert-butoxide as the source on (2×4) reconstructed, n-type In0.53Ga0.47As surfaces obtained after As decapping. Optimized growth conditions yielded ZrO2/In0.53Ga0.47As interfaces that were free of second phases. Capacitance-voltage (CV) measurements with different top electrodes showed a frequency dispersion of less than 2% per decade in accumulation. The accumulation capacitance and horizontal position of the CV curve were independent of temperature, while the inversion capacitance was strongly temperature dependent. Flat band voltages correlated with the work function of the metal electrode.

Quantification of trap densities at dielectric/III-V semiconductor interfaces

High-frequency capacitance-voltage curves for capacitors with high-k gate dielectrics and III–V semiconductor channels are modeled. The model takes into account the low conduction band density of states, the nonparabolicity of the Γ valley, and the population of higher lying conduction band valleys. The model is used to determine interface trap densities (Dit) and band bending of HfO2/In0.53Ga0.47As interfaces with different Dit and with pinned and unpinned Fermi levels, respectively. Potential sources of errors in extracting Dit are discussed and criteria that establish unpinned interfaces are developed.