Robert E. Stahlbush

Post‐irradiation cracking of H2 and formation of interface states in irradiated metal‐oxide‐semiconductor field‐effect transistors

Molecular hydrogen is alternately introduced into and removed from the gate oxide of irradiated metal‐oxide‐semiconductor field‐effect transistors at room temperature by changing the ambient between forming gas (10/90% H2/N2) and nitrogen. Using charge pumping, it is observed that H2 causes a simultaneous buildup of interface states and decrease of trapped positive charge. The results are explained by a reaction sequence in which H2 is cracked to form mobile H+, which under positive bias drifts to the Si/SiO2 interface, and reacts to produce a dangling‐bond defect. The rate limiting step over most of the time domain studied is the cracking process. Two types of cracking sites are modeled by molecular orbital calculations: oxygen vacancies (E’ centers) and broken bond hole traps (BBHTs). Initial‐ and final‐state energies, as well as the activation energies, are calculated. The calculations indicate that the latter is the more likely H2 cracking site. The combined experimental and theoretical results sugges...

Electron and hole trapping in irradiated SIMOX, ZMR and BESOI buried oxides

Shallow electron and deep hole trapping in the buried oxides of SIMOX (separation by implantation of oxygen), ZMR, and BESOI (bond and etchback silicon-on-insulator) material are examined. By irradiating the oxides with X-rays at cryogenic temperatures, 40-50 K, hole motion is frozen and electrons are trapped. The oxide charge is determined by C-V measurements. Following the cryogenic irradiation, the electrons are detrapped by field stressing (tunneling) or by annealing (thermal excitation). Hole trapping is examined by annealing after the trapped electrons are removed by field stressing. Substantial shallow electron and deep hole trapping distributed uniformly through the oxide is observed for all buried oxides that are processed above about 1100 degrees C. A comparison to thermal oxides grown at 850 degrees C and annealed at 1300 degrees C with and without a polysilicon capping layer shows that the top silicon layer significantly increases trap formation. These results indicate that the oxide defects responsible for the electron and hole trapping are produced by chemically reducing the oxide and producing defects such as Si-Si pairs. >

X‐ray rocking curve measurement of composition and strain in Si‐Ge buffer layers grown on Si substrates

The level of strain and the fraction of Ge in SiGe layers grown on Si can be found rapidly and unambiguously using double‐crystal x‐ray diffraction and a simple application of the linear elasticity theory combined with Vegard’s law. The method gives excellent results for 0.4‐μm‐thick buffer layers of SiGe/Si containing 5%–50% germanium. It is shown that lattice relaxation rises abruptly at x(Ge)≥15%, and that some strain remains for x(Ge) as high as 50%.

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.

Use of atomic layer epitaxy buffer for the growth of InSb on GaAs by molecular beam epitaxy

A 300 A buffer layer of InSb grown by atomic layer epitaxy at a substrate temperature of 300 °C at the GaAs/InSb interface has been employed to grow epitaxial films of InSb having bulk‐like properties. The reduction of the defects in the top InSb film has been observed with cross‐sectional transmission electron microscopy and channeling Rutherford backscattering spectroscopy. The optimum substrate temperature for the primary InSb layer growth was 420 °C with an atomic flux ratio of Sb to In of 1.4 and a growth rate of 1 μm/h. The best 5‐μm‐thick InSb layers had x‐ray rocking curve widths of 100 s, 77 K n‐type carrier concentrations in the low 1015/cm3 range, and 77 K carrier mobilities greater than 105 cm2/V s. Mesa isolated photodiodes had carrier lifetimes of 20 ns, in comparison to 200 ns observed in bulk InSb having a similar carrier concentration. An unexplained, weak free‐electron spin resonance transition has been observed in these films.

Basal plane dislocation reduction in 4H-SiC epitaxy by growth interruptions

The paths of basal plane dislocations (BPDs) through 4H-SiC epitaxial layers grown on wafers with an 8° offcut were tracked using ultraviolet photoluminescence imaging. The reduction of BPDs by conversion to threading edge dislocations was investigated at ex situ and in situ growth interrupts. For ex situ interrupts, BPDs are imaged after each of several growths. The wafer remains in the reactor for in situ interrupts and BPDs are imaged after the growth is finished. For in situ interrupts, a combination of temperature, propane flow, and duration has been determined, which achieve a BPD reduction of 98%.

Turning of Basal Plane Dislocations During Epitaxial Growth on 4° off-axis 4H-SiC

Epitaxial layers were grown on 4° off-axis 4H-SiC substrates by hot-wall chemical vapor deposition. The reduced off-cut angle resulted in lower basal plane dislocation (BPD) densities. The dependence of BPD reduction on growth conditions was investigated using ultraviolet photoluminescence (UVPL) imaging. With this method, it was found that the dislocations were converting to threading edge dislocations throughout the thickness of the film. A high (≥ 97%) conversion efficiency was found for all films grown with this orientation. A conversion of 100% was achieved for several films without pre-growth treatments or growth interrupts.

Effect of oxidation and reoxidation on the oxide-substrate interface of 4H- and 6H-SiC

X-ray photoelectron spectroscopy and sputter depth profiling were used to investigate SiO2 grown on 4H- and 6H-SiC with and without a reoxidation procedure. The oxides grown and oxide-substrate interfaces formed on 4H and 6H were similar in chemistry but different from Si(100). Reoxidation changes the structure of the oxide and the abruptness of the oxide-substrate interface. We propose a model for SiC oxidation where a transition layer containing Si–Si bonds is produced between the oxide and the SiC substrate.

Glide and multiplication of basal plane dislocations during 4H‐SiC homoepitaxy

Basal plane dislocations (BPDs) are an important category of extended defects in SiC epilayers. They act as nucleation sites for single layer Shockley-type stacking faults which account for the degradation of the bipolar devices operating under forward bias. It is well documented that most of the BPDs in the SiC epilayers propagate from the substrates. However, two characteristic types of BPDs were suggested to be due to either nucleation or multiplication during epitaxy, including interfacial dislocations and short BPD arrays connected to the epilayer surface by threading segments. Combining molten KOH etching, plan-view transmission x-ray topography, and photoluminescence mapping, both types are determined to be two parts of one defect produced by the sideway glide of a BPD under the influence of shear stress. During the glide, the down-step end of the BPD frequently produces a series of short BPD segments at the moving growth front. These BPD segments will grow into an array of dislocation half loops. ...

Interaction of hydrogen with defects in a-SiO2

Abstract In this paper, the current understanding of the role of hydrogen in passivating and creating defects in irradiated MOS samples is reviewed. The first ab initio calculation of the energies of activation and reaction of the dissociation of H 2 molecules at the E′ 1 defect in α-quartz, and for a simple model for the E′ γ defect in a-SiO 2 is prestented. The effects of various improvements to standard Hartree-Fock theory, including perturbative configuration interaction and improved basis sets, were studied. It is found that the qualitative picture of H 2 dissociation is insensitive to these improvements. The estimates of the energies of reaction and activation in the adiabatic approximation are 0.25 eV and 0.78 eV, respectively. The latter energy is roughly twice the activation energy reported by Li et al. The probable sources of error are explored and it is concluded that quantum effects (zero point energy and tunneling are probably crucial for modeling this dissociation reaction. The first ab initio study of the preferred conformation of the proton in a-SiO 2 and a preliminary semiempirical calculation of the activation energy for proton hopping are also presented.