M. E. Twigg

Influence of MOVPE growth conditions on carbon and silicon concentrations in GaN

Abstract Impurity incorporation is studied as a function of metalorganic vapor phase epitaxy growth conditions. The same GaN growth conditions were used initially, resulting in films with approximately the same dislocation density, after which a single growth parameter was varied and the impurity concentrations measured using SIMS. The C concentrations were found to decrease with increasing growth temperature, pressure, and ammonia flow, and to increase with increasing H 2 carrier and trimethylgallium flow. The Si concentrations for both unintentionally doped (UID) and intentionally doped (ID) films increased with increasing growth pressure. The UID and ID Si concentrations varied inversely with the GaN growth rate, suggesting an independent source for UID Si within the reactor. Moreover, the NH 3 flow rate influenced the Si-doping concentration, even though the GaN growth rate remained constant. A H 2 /NH 3 etching mechanism is proposed to explain the growth parameter influence on the observed C and Si concentrations. The reduction in the ID Si concentrations at high NH 3 flows is explained by NH 3 site blocking, similar to that proposed for increased Ga vacancies at high NH 3 flows.

Enhanced GaN decomposition in H2 near atmospheric pressures

GaN decomposition is studied at metallorganic vapor phase epitaxy pressures (i.e., 10–700 Torr) in flowing H2. For temperatures ranging from 850 to 1050 °C, the GaN decomposition rate is accelerated when the H2 pressure is increased above 100 Torr. The Ga desorption rate is found to be independent of pressure, and therefore, does not account for the enhanced GaN decomposition rate. Instead, the excess Ga from the decomposed GaN forms droplets on the surface which, for identical annealing conditions, increase in size as the pressure is increased. Possible connections between the enhanced GaN decomposition rate, the coarsening of the nucleation layer during the ramp to high temperature, and increased GaN grain size at high temperature are discussed.

Structure of stacking faults formed during the forward bias of 4H-SiC p-i-n diodes

Using site-specific plan-view transmission electron microscopy (TEM) and light emission imaging, we have identified stacking faults formed during forward biasing of 4H-SiC p-i-n diodes. These stacking faults (SFs) are bounded by Shockley partial dislocations and are formed by shear strain rather than by the condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during p-i-n diode operation, suggesting the presence of a complex inhomogeneous strain field in the 4H-SiC layer.

Low-temperature cleaning processes for Si molecular beam epitaxy

Hydrogen‐terminated surface cleaning techniques of silicon substrates were investigated by using x‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). Either a 4% HF dip or an HF‐terminated abbreviated Shiraki clean was used as the cleaning technique. Shiraki‐cleaned samples were grown as control samples. XPS was used to measure the C, O, and F remaining on the surface at various stages of the cleaning/growth process, including after a 1 h bake at 200 °C prior to growth. XPS did not detect a significant difference in the adsorbate concentrations between the baked and unbaked samples. From SIMS, the lowest impurity concentrations at the epitaxial/substrate interface were achieved with the abbreviated Shiraki clean, approximately at the same levels as obtained with the standard Shiraki clean, 1.3×1013, 5.4×1012, 1.6×1010, and 4.2×1011/cm2 for C, O, F, and N, respectively. This was achieved without the 850 °C anneal required to desorb the ...

Surface segregation and structure of Sb-doped Si(100) films grown at low temperature by molecular beam epitaxy

Abstract Sb surface segregation and doping during Si(100) molecular beam epitaxy were studied for growth temperatures of 320–500°C. Surface segregation was analyzed by depth profiling with secondary ion mass spectrometry and the results indicate the existence of several distinct dopant concentration- and temperature-dependent surface segregation regimes: (1) For dilute Sb surface concentrations the measurements reveal a region where bulk and surface concentrations are linearly related, and the surface segregation is described by a constant. However, the experimentally determined temperature dependence of the segregation does not follow simple kinetics theory, and appreciable surface segregation is observed at temperatures ≤ 400°C. (2) At temperatures ≥ 350°C, the surface segregation reaches a maximum for Sb surface concentrations of 0.5 monolayers. (3) For surface concentrations near 1 monolayer, the surface segregation decreases with increasing surface Sb coverage due to dopant interaction within surface and subsurface layers. In cases where films were grown under very high dopant fluxes, we have identified cone-like defects and stacking faults that are the result of the apparent surface concentration exceeding 1 monolayer.

Increasing Efficiency of Photoelectronic Conversion by Encapsulation of Photosynthetic Reaction Center Proteins in Arrayed Carbon Nanotube Electrode

The construction of efficient light energy converting (photovoltaic and photoelectronic) devices is a current and great challenge in science and technology and one that will have important economic consequences. Here we show that the efficiency of these devices can be improved by the utilization of a new type of nano-organized material having photosynthetic reaction center proteins encapsulated inside carbon nanotube arrayed electrodes. In this work, a generically engineered bacterial photosynthetic reaction center protein with specifically synthesized organic molecular linkers were encapsulated inside carbon nanotubes and bound to the inner tube walls in unidirectional orientation. The results show that the photosynthetic proteins encapsulated inside carbon nanotubes are photochemically active and exhibit considerable improvement in the rate of electron transfer and the photocurrent density compared to the material constructed from the same components in traditional lamella configuration.

Ohmic contacts in AlSb/InAs high electron mobility transistors for low-voltage operation

We report on the fabrication and characteristics of Pd/Pt/Au ohmic contacts that have been used in AlSb/InAs high electron mobility transistors (HEMTs) with low access resistance. The metalization exhibits minimal in-diffusion and a contact resistance of 0.08 Ω mm after a 175 °C hot-plate heat treatment. By comparison, AuGe/Ni/Pt/Au ohmic contact metalizations formed using a 300 °C rapid thermal anneal exhibit a contact resistance of 0.11 Ω mm, but with considerable Au in-diffusion. Using the Pd/Pt/Au contact, 0.6 μm gate-length AlSb/InAs HEMTs exhibit a low-field source-drain resistance of 0.47 Ω mm and a transconductance above 1 S/mm. After removal of the gate bonding pad capacitance from an equivalent circuit, an fTLg product of 38 GHz μm is obtained at VDS=0.4 V. HEMTs with a 60 nm gate length exhibit a low-field source-drain resistance of 0.35 Ω mm and a measured fT of 90 GHz at a drain voltage of only 100 mV. These fTLg and fT values are the highest reported for any field effect transistor at these ...

Electron channeling contrast imaging of atomic steps and threading dislocations in 4H-SiC

Direct imaging of atomic step morphologies and individual threading dislocations in on-axis epitaxial 4H-SiC surfaces is presented. Topographically sensitive electron images of the crystalline surfaces were obtained through forescattered electron detection inside a conventional scanning electron microscope. This technique, termed electron channeling contrast imaging (ECCI), has been utilized to reveal the configuration of highly stepped, homoepitaxial 4H-SiC films grown on 4H-SiC mesa structures. Individual threading dislocations have been consistently imaged at the core of spiral atomic step morphologies located on the 4H-SiC surfaces. The ability of ECCI to image atomic steps was verified by atomic force microscopy.

Nondestructive analysis of threading dislocations in GaN by electron channeling contrast imaging

Threading dislocations in metal-organic chemical-vapor grown GaN films were imaged nondestructively by the electron channeling contrast imaging (ECCI) technique. Comparisons between ECCI and cross-sectional transmission electron microscopy indicated that pure edge dislocations can be imaged in GaN by ECCI. Total threading dislocation densities were measured by ECCI for various GaN films on engineered 4H-SiC surfaces and ranged from 107to109cm−2. A comparison between the ultraviolet electroluminescent output measured at 380nm and the total dislocation density as measured by ECCI revealed an inverse logarithmic dependence.

A Si0.7Ge0.3 strained‐layer etch stop for the generation of thin layer undoped silicon

The use of a Si0.7Ge0.3 strained layer as an etch stop in silicon‐based materials is reported. The etch rates were characterized through silicon and a 60 nm Si0.7Ge0.3 strained layer. The etch rate through undoped silicon was 17–20 nm/min, while the etch rate through the Si0.7Ge0.3 layer was 1 nm/min. After annealing the wafer to 850 °C for 30 min, transmission electron microscopy was used to show that strain in the alloy layer was only partially relieved, and that generated misfit dislocations were confined to the strained Si0.7Ge0.3 layer. The etch rate through the strained layer increased to 1.7 nm/min after this treatment, and was still perfectly functional as an etch stop.