Direct Measurement of Quantum Confinement Effects at Metal to Quantum-Well Nanocontacts

Model metal-semiconductor nanostructure Schottky nanocontacts were made on cleaved heterostructures containing GaAs quantum wells (QWs) of varying width and were locally probed by ballistic electron emission microscopy. The local Schottky barrier was found to increase by ∼0.140 eV as the QW width was systematically decreased from 15 to 1 nm, due mostly to a large (∼0.200 eV) quantum-confinement increase to the QW conduction band. The measured barrier increase over the full 1 to 15 nm QW range was quantitatively explained when local "interface pinning" and image force lowering effects are also considered

Anomalous current transport in Au/low-doped n-GaAs Schottky barrier diodes at low temperatures

The current-voltage characteristics of Au=low doped n-GaAs Schottky diodes were determined at various temperatures in the range of 77-300 K. The estimated zero-bias barrier height and the ideality factor assuming thermionic emission (TE) show a temperature dependence of these parameters. While the ideality factor was found to show the T0 effect, the zero-bias barrier height was found to exhibit two different trends in the temperature ranges of 77-160 K and 160-300 K. The variation in the flat-band barrier height with temperature was found to be -(4.7±0.2)× 104 eVK-1, approximately equal to that of the energy band gap. The value of the Richardson constant, A∗∗, was found to be 0.27 Acm-2K-2 after considering the temperature dependence of the barrier height. The estimated value of this constant suggested the possibility of an interfacial oxide between the metal and the semiconductor. Investigations suggested the possibility of a thermionic field-emission-dominated current transport with a higher characteristic energy than that predicted by the theory. The observed variation in the zero-bias barrier height and the ideality factor could be explained in terms of barrier height inhomogenities in the Schottky diode

Study of Optical and Structural Characteristics of Ceria Nanoparticles Doped with Negative and Positive Association Lanthanide Elements

This paper studies the effect of adding lanthanides with negative association energy, such as holmium and erbium, to ceria nanoparticles doped with positive association energy lanthanides, such as neodymium and samarium. That is what we called mixed doped ceria nanoparticles (MDC NPs). In MDC NPs of grain size range around 6 nm, it is proved qualitatively that the conversion rate from Ce 4+ to Ce 3+ is reduced, compared to ceria doped only with positive association energy lanthanides. There are many pieces of evidence which confirm the obtained conclusion. These indications are an increase in the allowed direct band gap which is calculated from the absorbance dispersion measurements, a decrease in the emitted fluorescence intensity, and an increase in the size of nanoparticles, which is measured using both techniques: transmission electron microscope (TEM) and X-ray diffractometer (XRD). That gives a novel conclusion that there are some trivalent dopants, such as holmium and erbium, which can suppress Ce 3+ ionization states in ceria and consequently act as scavengers for active O-vacancies in MDC. This promising concept can develop applications which depend on the defects in ceria such as biomedicine, electronic devices, and gas sensors

Multivalley electron conduction at the indirect-direct crossover point in highly tensile-strained germanium

As forward-looking electron devices increasingly adopt high-mobility low-band-gap materials, such as germanium (Ge), questions remain regarding the feasibility of strain engineering in low-band-gap systems. Particularly, the Ge L-Γ valley separation (∼150 meV) can be overcome by introducing a high degree of tensile strain (ε ≥ 1.5%). It is therefore essential to understand the nature of highly strained Ge transport, wherein multivalley electron conduction becomes a possibility. Here, we report on the competitiveness between L- and Γ-valley transport in highly tensile-strained (ε ∼ 1.6%) Ge/In0.24Ga0.76 Asheterostructures. Temperature-dependent magnetotransport analysis reveals two contributing carrier populations, identified as lower- and higher-mobility L- and Γ-valley electrons (in Ge), using temperature-dependent Boltzmann transport modeling. Coupling this interpretation with electron-cyclotron-resonance studies, the effective mass (m*) of the higher-mobility Γ-valley electrons is probed, revealing m* = (0.049 ± 0.007)me. Moreover, a comparison of empirical and theoretical m* indicates that these electrons reside primarily in the first-two quantum sublevels of the Ge Γ valley. Consequently, our results provide an insight into the strain-dependent carrier dynamics of Ge, offering alternative pathways toward efficacious strain engineering

High-quality InAsyP1−y step-graded buffer by molecular-beam epitaxy

Relaxed, high-quality, compositionally step-graded InAsyP1-y layers with an As composition of y=0.4, corresponding to a lattice mismatch of similar to1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4x10(6) cm(-2) within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1-xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1-y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III-V devices. (C) 2003 American Institute of Physics

Interface states density distribution in Au/n-GaAs schottky diodes on n-Ge and n-GaAs substrates

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Au/n-GaAs Schottky diodes on n-Ge substrates are investigated and compared with characteristics of diodes on GaAs substrates. The diodes show the non-ideal behavior of I-V characteristics with an ideality factor of 1.13 and barrier height of 0.735 eV. The forward bias saturation current was found to be large (3×10-10 A vs. 4.32×10-12A) in the GaAs/Ge Schottky diodes compared with the GaAs/GaAs diodes. The energy distribution of interface states was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height, though it is small. The interface states density was found to be large in the Au/n-GaAs/n-Ge structure compared with the Au/n-GaAs/n+-GaAs structure. The possible explanation for the increase in the interface states density in the former structure was highlighted

Doping dependence of the barrier height and ideality factor of Au/n-GaAs schottky diodes at low temperatures

The barrier height and ideality factor of Au/n-GaAs Schottky diodes grown by metal-organic vapor-phase epitaxy (MOVPE) on undoped and Si-doped n-GaAs substrates were determined in the doping range of 2.5×1015-1×1018 cm-3 at low temperatures. The thermionic-emission zero-bias barrier height for current transport decreases rapidly at concentrations greater than 1×1018 cm-3. The ideality factor also increases very rapidly at higher concentration and at lower temperature. The results agree quite well with thermionic field emission (TFE) theory. The doping dependence of the barrier height and the ideality factor were obtained in the concentration range of 2.5×1015-1.0× 1018 cm-3 and the results are well described using TFE theory. An excellent match between the homogeneous barrier height and the effective barrier height was observed which supports the good quality of the GaAs film. The observed variation in the zero-bias barrier height and the ideality factor can also be explained in terms of barrier height inhomogeneities in the Schottky diode

Breakdown characteristics of MOVPE grown Si-doped GaAs schottky diodes

The breakdown characteristics of Au/n-GaAs Schottky contacts on metal-organic vapor-phase epitaxy grown Si-doped n-GaAs were measured in the doping range of 6×1015-1.5×1018 cm-3. These results are compared with the experimentally measured breakdown voltages by several workers and also with the theoretical calculation predicted by Sze and Gibbons [Sze SM, Gibbons G. Appl. Phys. Lett. 1966;8:111]. Good agreement was observed between the measured data and the breakdown voltages by Sze and Gibbons in the high doping concentrations. The maximum depletion layer width is found to be in good agreement with the theoretical analysis by Sze and Gibbons. The breakdown voltage at higher doping concentration will be useful for the design and development of GaAs switching devices and the emitter-base region of bipolar transistors

Self-annihilation of antiphase boundaries in GaAs epilayers on Ge substrates grown by metal-organic vapor-phase epitaxy

The self-annihilation of antiphase boundaries (APBs) in GaAs epitaxial layers grown by low-pressure metal-organic vapor-phase epitaxy on Ge substrates is studied by several characterization techniques. Cross-sectional transmission electron microscopy shows that antiphase domain free GaAs growth on Ge was possible due to the proper selection of the growth parameters. The antiphase boundaries annihilate with each other after a thick 3 µm layer of GaAs growth on a Ge substrate as observed by scanning electron microscopy studies. Double crystal x-ray diffraction data shows a slight compression of GaAs on Ge, and the full width at half maximum decreases with increasing growth temperatures. This confirms that the APBs annihilate inside the GaAs epitaxial films. Low temperature photoluminescence measurements confirm the self-annihilation of the APBs at low temperature growth and the generation of APBs at higher growth temperatures

Electrical transport characteristics of Au/n-GaAs Schottky diodes on n-Ge at low temperatures

The current-voltage characteristics of Au/n-GaAs Schottky diodes grown by metal-organic vapor-phase epitaxy on Ge substrates were determined in the temperature range 80-300 K. The zero-bias barrier height for current transport decreases and the ideality factor increases at low temperatures. The ideality factor was found to show the T0 effect and a higher characteristic energy. The excellent matching between the homogeneous barrier height and the effective barrier height was observed and infer good quality of the GaAs film. No generation-recombination current due to deep levels arising during the GaAs/Ge heteroepitaxy was observed in this study. The value of the Richardson constant was found to be 7.04 A K-2 cm-2, which is close to the value used for the determination of the zero-bias barrier height