Inductively Coupled Plasma Etching in ICl- and IBr-Based Chemistries. Part II: InP, InSb, InGaP, and InGaAs

A parametric study of Inductively Coupled Plasma (ICP) etching of InP, InSb, InGaP, and InGaAs has been carried out in ICl/Ar and IBr/Ar chemistries. Etch rates in excess of 3.1 for InP, 3.6 for InSb, 2.3 for InGaP, and 2.2 m/min for InGaAs were obtained in IBr/Ar plasmas. The ICP etching of In-based materials showed a general tendency: The etch rates increased substantially with increasing ICP source power and rf chuck power in both chemistries, while they decreased with increasing chamber pressure. The IBr/Ar chemistry typically showed higher etch rates than ICl/Ar, but the etched surface morphologies were fairly poor for both chemistries.     

Bulk and shear mechanical loss of titania-doped tantala

We report on the mechanical loss from bulk and shear stresses in thin film, ion beam deposited, titania-doped tantala. The numerical values of these mechanical losses are necessary to fully calculate the Brownian thermal noise in precision optical cavities, including interferometric gravitational wave detectors like LIGO. We found the values from measuring the normal mode mechanical quality factors, Q's, in the frequency range of about 2000-10,000 Hz, of silica disks coated with titania-doped tantala coupled with calculating the elastic energy in shear and bulk stresses in the coating using a finite element model. We fit the results to both a frequency independent and frequency dependent model and find ${?}_{\mathrm{shear}}=(8.3\pm 1.1)\times {10}^{?4}$, ${?}_{\mathrm{bulk}}=(6.6\pm 3.8)\times {10}^{?4}$ with a frequency independent model and ${?}_{\mathrm{shear}}\left(f\right)=(5.0\pm 0.7)\times {10}^{?4}+(5.4\pm 1.1)\times {10}^{?8}f$, ${?}_{\mathrm{bulk}}\left(f\right)=(11\pm 2.8)\times {10}^{?4}?(8.7\pm 4.7)\times {10}^{?8}f$ with a frequency dependent (linear) model. The ratio of these values suggest that modest improvement in the coating thermal noise may be possible in future gravitational wave detector optics made with titania-doped tantala as the high index coating material by optimizing the coating design to take advantage of the two different mechanical loss angles.     

Low temperature growth of AlGaAs by MOMBE (CBE) using trimethylamine alane

In order to gain further insight into the surface chemistry of AlGaAs growth by metalorganic molecular beam epitaxy, we have investigated the deposition behavior and material quality of AlGaAs grown at temperatures from 350 to 500°C using trimethylamine alane (TMAA), triethylgallium (TEG) and arsine (AsH₃). Though the Al incorporation rate decreases with decreasing temperature, Ga-alkyl pyrolysis, and hence Ga incorporation rate, declines more rapidly. Thus the Al content increases from X_AlAs = 0.25 at 500°C to X_AlAs = 0.57 at 350°C. Below 450°C, the Ga incorporation rate appears to be determined by the desorption of diethylgallium species, rather than interaction with adsorbed AlH₃. Similarly, carbon incorporation is enhanced by 2 orders of magnitude over this temperature range due to the increasingly inefficient pyrolysis of the Ga-C bond in TEG. Additionally, active hydrogen from the TMAA1, which normally is thought to getter the surface alkyls, is possibly less kinetically active at lower growth temperatures. Contrary to what has been observed in other growth methods, low growth temperatures produced a slight decrease in oxygen concentration. This effect is likely due to reduced interaction between Ga alkoxides (inherent in the TEG) and the atomic hydrogen blocked Al species on the growth surface. This reduction in oxygen content and increase in carbon concentration causes the room temperature PL intensity to actually increase as the temperature is reduced from 500 to 450°C. Surprisingly, the crystalline perfection as measured by ion channeling analysis is quite good, χ_min≤5%, even at growth temperatures as low as 400°C. At 350°C, the AlGaAs layers exhibit severe disorder. This disorder is indicative of insufficient Group III surface mobility, resulting in lattice site defects. The disorder also supports our conclusions of kinetically limited surface mobility of all active surface components.     

Positive vibrational entropy of chemical ordering in FeV

Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03 kB/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.