J. P. Pelz

Spatially-resolved spectroscopic measurements of Ec − 0.57 eV traps in AlGaN/GaN high electron mobility transistors

Simultaneous temperature-dependent measurements of resistance transients (RTs) and spatially resolved surface potential transients were made after bias switching on AlGaN/GaN high electron mobility transistors (HEMTs). We find an Ec − 0.57 eV trap, previously correlated with HEMT degradation, located in the GaN buffer and not in the AlGaN barrier or at the AlGaN surface. The amplitude of the Ec − 0.57 eV trap in RTs depends strongly on the Fe-concentration in the GaN buffer. Filling of this trap occurs only under bias conditions where electric fields penetrate into the GaN buffer.

Instrumentation for direct, low frequency scanning capacitance microscopy, and analysis of position dependent stray capacitance

We describe instrumentation for scanning capacitance microscopy (SCM), using an atomic force microscope, that is designed to make calibrated, low frequency measurements of tip–sample capacitance and spreading resistance of thin dielectric films. We also characterize spatial variations in stray capacitance Cstray that must be understood before such measurements can be interpreted. Existing SCM circuits are generally optimized for measuring dC/dV, and not for calibrated low frequency measurements of absolute capacitance. Our circuit uses a commercially available current amplifier and low frequency (∼5 kHz) lock-in detection. This circuit adds an inverted, constant amplitude current to suppress the stray displacement current from the large (∼0.5 pF) stray capacitance Cstray between the sample and the mm-sized cantilever–probe assembly. The capacitance noise spectrum is ∼0.35 aF/Hz and is flat down to 1 Hz measurement frequency, with an integrated noise <5 aF integrated over a 1–160 Hz bandwidth. We have also...

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.

Elevated temperature oxidation and etching of the Si(111) 7×7 surface observed with scanning tunneling microscopy

Scanning tunneling microscopy has been used to study the initial oxidation of the Si(111) 7×7 surface over the temperature range 400–600 °C. Oxygen‐induced etching of the surface is observed for low O2 pressures (<10−7 Torr) at temperatures between 500 and 600 °C, causing monolayer‐deep holes in the surface and the retraction of step edges. The newly exposed surfaces often reconstruct into metastable 5×5 or 9×9 structures, as well as the stable 7×7 structure.

Coverage-dependent self-organization: from individual adatoms to adatom superlattices

The coverage-dependent self-organization of Ce-adatoms on a Ag(111) surface is studied by scanning tunnelling microscopy at temperatures ranging from 3.9 to 10 K. At a coverage of 0.03% of a Ce monolayer individual Ce-adatoms and Ce dimers are observed, the mutual interatomic distances of which are clearly related to multiples of the Fermi wavelength λF/2, reflecting surface-state-mediated electronic interactions. At a coverage of 0.2% the formation of chains and small islands with hexagonal structure prevails. At a coverage of 1% a hexagonal superlattice with a periodicity of 3.2 nm is observed. At a coverage of 2% the superlattice of Ce-adatoms is found to be compressed, showing an interatomic distance of 2.2 nm. At higher coverage the number of dimers increases considerably and the superlattice collapses into compact islands. An increase in the temperature towards about 10 K at a coverage of 1% also causes the collapse of the hexagonal Ce superlattice. These experimental findings are rationalized within the electron scattering model of Hyldgard and Persson, which specifically takes into account the electronic surface-state on Ag(111). The experimentally derived two-body interaction potential is able to account for the observed phenomena as a function of concentration and temperature.

Ballistic electron emission microscopy study of Schottky contacts on 6H- and 4H-SiC

We have performed ballistic-electron emission microscopy (BEEM) on (Pd,Pt)/(6H,4H)-SiC(0001) Schottky contacts. Schottky barrier heights (SBHs) determined from the BEEM data using the Bell–Kaiser model are 1.27 eV/1.34 eV (Pd/Pt) for 6H- and 1.54 eV/1.58 eV for 4H-SiC. Our measurements also give the first direct evidence of a second conduction band minimum (CBM) on 4H-SiC about 0.14 eV above the overall CBM. Spatial inhomogeneity of SBHs were examined and shown to be no larger than the fitting error due to the system noise. Additionally, enhanced ballistic transmittance was observed over a region intentionally stressed by injecting high kinetic energy (10 eV above EF) hot electrons using BEEM in the Pt/4H-SiC sample.

Scanning capacitance microscopy for thin film measurements

We have used direct, low-frequency scanning capacitance microscopy measurements to characterize variations in thin, dielectric films with up to 1 nm thickness and ~200 nm lateral resolution. This technique may be used on metallic as well as semiconducting substrates because it does not rely upon d C/d V measurements. We also find that the sensitivity of capacitance to film thickness can be enhanced by an aqueous meniscus that typically forms between the atomic force microscope tip and the sample surface. Further, we quantified the nanometre-scale capacitance of the tip–meniscus–sample system that is sensitive to variations in film thickness by making simultaneous capacitance and cantilever deflection measurements. This capacitance is used along with an average film thickness to quantify variations in film thickness.

Nm-scale measurements of fast surface potential transients in an AlGaN/GaN high electron mobility transistor

Fast, bias-induced, surface potential transients (SPTs) and conductance transients (CTs) were simultaneously measured on an AlGaN/GaN high electron mobility transistor. SPTs measured near the drain-side gate edge and CTs have nearly the same shape and are well-fit with two exponentials having room-temperature time constants of 4.2 ms and 36 ms, likely indicating emission from two trap species. Kelvin probe force microscopy was used to measure SPTs. Electrostatic simulations of SPT amplitudes, which account for the measured probe/sample geometry, are consistent with a uniform trapped surface charge density of 7 × 1012 electrons/cm2 extending 200 nm from the drain-side gate edge.

Effect of inclined quantum wells on macroscopic capacitance-voltage response of Schottky contacts: Cubic inclusions in hexagonal SiC

Finite-element calculations of Schottky diode capacitance-voltage (C-V) curves show that an array of subsurface inclined quantum wells (QWs) produce negligible change in shape and slope of C-V curves, but significantly reduce the intercept voltage. This is particularly important for hexagonal SiC, in which current- or process-induced cubic inclusions are known to behave as electron QWs. These calculations naturally explain the surprisingly large effect of cubic inclusions on the apparent 4H–SiC Schottky barrier determined by C-V measurements, and together with the measured C-V data indicate the QW subband energy in the inclusions to be ∼0.51eV below the host 4H–SiC conduction band.