C. Tivarus

Comparison of mixed anion, InAsyP1−y and mixed cation, InxAl1−xAs metamorphic buffers grown by molecular beam epitaxy on (100) InP substrates

The structural, morphological, and defect properties of mixed anion, InAsyP1−y and mixed cation, InxAl1−xAs metamorphic step-graded buffers grown on InP substrates are investigated and compared. Two types of buffers were grown to span the identical range of lattice constants and lattice mismatch (∼1.1–1.2%) on (100) InP substrates by solid source molecular beam epitaxy. Symmetric relaxation of ∼90% in the two orthogonal 〈110〉 directions with minimal lattice tilt was observed for the terminal InAs0.4P0.6 and In0.7Al0.3As overlayers of each graded buffer type, indicating nearly equal numbers of α and β dislocations were formed during the relaxation process and that the relaxation is near equilibrium and hence insensitive to asymmetric dislocation kinetics. Atomic force microscopy reveals extremely ordered crosshatch morphology and very low root mean square (rms) roughness of ∼2.2 nm for the InAsP relaxed buffers compared to the InAlAs relaxed buffers (∼7.3 nm) at the same degree of lattice mismatch with res...

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 ∼1.3% 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 4×106 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.

Spatial resolution of ballistic electron emission microscopy measured on metal/quantum-well Schottky contacts

Au Schottky contacts on cleaved AlGaAs∕GaAs∕AlGaAs quantum wells (QWs) were used as precise nanometer-scale apertures to quantify the spatial resolution of ballistic electron emission microscopy (BEEM). Both the amplitude and width of the measured average BEEM current profiles showed systematic dependencies on the QW width and Au film thickness, indicating surprisingly large BEEM resolutions of ∼12, ∼16, and ∼22nm for Au film thicknesses of 4, 7, and 15nm, respectively, but roughly independent of Au grain size. These measurements are consistent with theoretical models that include multiple hot-electron scattering at interfaces and in the bulk of the metal film.

Calculated potential profile near charged threading dislocations at metal/semiconductor interfaces

We have made finite element calculations of the expected potential profile around negatively charged threading dislocations (TDs) close to a metal–semiconductor interface, using a Pt contact on n-type GaN as a specific case. The potential was calculated as a function of the assumed linear density and energy level of TD-related acceptors. Our model shows good agreement with the model of Read [W. T. Read, Philos. Mag. 45, 775 (1954); 46, 111 (1954)] for an infinite dislocation, far from any interface. Assuming 1 acceptor/c-axis lattice spacing (c=0.52 nm), we found for our near-surface modeling that acceptors levels deeper than 1.3 eV below the conduction band minimum (CBM) should be charged all the way to the Pt/GaN interface. This should produce a significant local increase in the potential barrier and at the Pt/GaN interface and should be observable by ballistic electron emission microscopy (BEEM). In fact recent BEEM measurements by Im et al. on molecular beam epitaxy-grown GaN films [Phys. Rev. Lett. 8...

Relaxed InAsP layers grown on step graded InAsP buffers by solid source MBE

Si-doped InAs x P 1-x layers with As mole fractions ranging from 0.05 to 0.50 were grown on InAs x P 1-x step-graded buffer layers on InP substrates by solid source molecular beam epitaxy. The growth parameters consisted of a P:In flux ratio of 7:1, a growth temperature of ∼ 485°C, a growth rate of 2.2 A/s, and an As:In flux ratio of 0.37-2.36 for varying As mole fractions. The As mole fraction and the layer relaxation were determined using triple axis x-ray diffraction measurements. Near complete relaxation (>93%) was achieved for all Si-doped InAs x P 1-x epilayers. The structural morphology indicated that the InAs x P 1-x graded buffer layers were effective in relieving the lattice mismatch strain as evidenced by a well-developed crosshatch morphology and low rms surface roughness. The electron concentration, mobility, and Si donor activation energy for each InAs x P 1-x composition were determined using temperature dependent Hall measurements. At a constant electron carrier concentration of %3.5×10 16 cm -3 , the 300 K carrier mobility increased from 2700 to 4732 cm 2 /V-sec with increasing As mole fraction from 0.05 to 0.50.

Nanoscale Characterization of Metal/Semiconductor Nanocontacts

Ballistic Electron Emission Microscopy (BEEM) and finite‐element electrostatic modeling were used to quantify how “small‐size” effects modify the energy barrier at metal/semiconductor nanostructure nanocontacts, formed by making Schottky contacts to cleaved edges of GaAs quantum wells (QWs). The Schottky barrier height over the QWs was found to systematically increase with decreasing QW width, by up to ∼140 meV for a 1nm QW. This is mostly due to a large quantum‐confinement increase (∼200 meV for a 1nm QW), modified by smaller decreases due to “environmental” electric field effects. Our modeling gives excellent quantitative agreement with measurements for a wide range of QW widths when both quantum confinement and environmental electric fields are considered.

Avalanche ballistic electron emission microscopy with single hot-electron sensitivity

We discuss an implementation of ballistic electron emission microscopy (BEEM), in which the metallic or metal–insulator “stack” of interest is formed directly over an avalanche p–n diode. This allows nanometer-resolution studies of hot-electron transport through technologically important device stacks with up to single electron sensitivity and >10 kHz measurement bandwidth when the avalanche diode is cooled to <200 K.