Thomas G. Stoebe

Influence of OH− ions on infrared absorption and ionic conductivity in lithium fluoride crystals

Abstract The presence of OH − ions in LiF crystals is found to depress the ionic conductivity by up to two orders of magnitude compared with crystals of the same doping level but which are relatively free of OH − . The presence of OH − in these crystals gives rise to two types of infrared absorption spectra: a broad line at 2.68 μ in “pure” crystals and a sharp line spectrum between 2.71 and 2.83 μ in Mg-doped crystals. The properties associated with these absorption spectra are discussed, and the results explained in terms of the formation of Mg 2+ −OH − complexes which remove vacancies from the system and thus depress the conductivity.

Effect of Impurities on the Mechanical Behavior of MgO Single Crystals

The effect of impurities on the critical resolved shear stress (CRSS) of magnesium oxide single crystals has been determined using compression testing in crystals containing up to 40 mole ppm Fe3+ and up to 500 mole ppm of Ni+2 impurities. In pure crystals containing small amounts of Fe2+, the value of 0.73±0.07 kg/mm2 is determined as the CRSS, and two distinguishable strain hardening regions are identified. The effect on CRSS on Fe3+ content and of isochronal and isothermal aging is also studied; precipitation hardening is observed on aging even for very low concentrations of Fe3+. Heat treatments have very little effect on Ni+2‐doped crystals, and yielding studies show that Fe3+ is much more significant than Ni2+ in hardening MgO.

Optical absorption and luminescent processes in thermoluminescent CaSO4:Dy

Optical absorption in irradiated and non-irradiated single crystals of CaSO4 and CaSO4:Dy is studied. The intrinsic defect centres SO4-, SO3-, SO2-, O3-, and O- are identified in irradiated samples. However, these centres are generally observed only if stabilised by Dy impurity ions within the structure. Dy2+ absorption is also observed in irradiated doped samples. Luminescence via energy transfer to Dy ions is corroborated by this study. Areas where these results may be used to develop further understanding of the thermoluminescence process in CaSO4:Dy are discussed.

Distribution of hydroxide ions in doped alkali halide crystals

Abstract The available information concerning the distribution of hydroxide ions in alkali halide crystals is reviewed. The various possible reactions among the hydroxide ions and divalent cation impurities and their accompanying charge compensating cation vacancies are given and discussed in terms of the available ionic conductivity and optical absorption data. The possible hydroxide ion complexes are discussed in terms of ESR results. Complexes containing hydroxide ions, each on their own anion site, with cation impurities and vacancies on adjacent sites are indicated as being the most probable to occur. Different geometries of such a complex are discussed in terms of the available experimental data.

Hydroxyl ions and the 200−nm absorption band in Mg− and Ti−doped thermoluminescent LiF single crystals

A study of the distribution of the optical absorption coefficient of the 200−nm band in Mg− and Ti−doped LiF single−crystal boules, grown from monocrystalline starting materials of varying purity, indicates that this absorption band is due to titanium−hydroxyl complexes. The measurements of ionic conductivity and infrared absorption on the starting materials and in doped crystals indicate that each titanium ion probably interacts with several hydroxyl ions to form these complexes, and that the association of hydroxyl ions is stronger with titanium ions than with magnesium ions. Since earlier work has shown that the 200−nm band is involved in the thermoluminescent recombination process, it is indicated that hydroxide impurities may be essential constituents in dosimetry grade LiF.

The relation between deformation and thermoluminescent defect centers in LiF (TLD‐100)

Plastic deformation after exposure to ionizing radiation is shown to increase the F‐band absorption in LiF (TLD‐100), while decreasing the absorptions at 310 and 380 nm which are related to thermoluminescent traps. The rate of change with deformation of the three bands indicates that F centers are created when 310‐ and 380‐nm absorption centers are destroyed. The results are interpreted in terms of the removal of F centers from the defect complexes causing the 310‐ and 380‐nm absorption bands; this implies that the F center may be an integral part of the thermoluminescent traps in this material.

An electron spin resonance study of thermoluminescence mechanisms in CaSO4:Dy

Mechanisms of thermoluminescence (TL) in CaSO4:Dy are studied using electron spin resonance (ESR) in single-crystal samples. The ESR analysis indicates the presence of several variations of a distorted SO4- centre locally stabilised by a nearest neighbour Ca vacancy. This centres structure, which has not been reported previously, relates to several centres observed in this system and could also explain results in related compounds. The presence of the Dy impurity, which increases the concentration of Ca vacancies due to charge neutrality requirements, enhances both the concentration of these centres and the intensity of the major TL peaks near 220 and 350 degrees C. The specific role of these and other previously reported centres in the TL of this material is followed using heat treatments. This enables one to identify centres that change in the range of the observed TL peaks as well as centres that seem to be related to one another by charge transfer or rearrangement. The behaviour of the observed centres and their relationship to the TL behaviour of the system reemphasises the complexity of this system.

Optical absorption and thermoluminescence in LiF TLD‐100

The thermoluminescent (TL) properties of many LiF samples have demonstrated a strong influence on trace impurities and impurity content on the TL process. Hence, the general validity of any particular model must be tested against its applicability in a standard material such as Harshaw LiF (TLD‐100). In this paper the validity of the use of Harshaw LiF(54) by Mayhugh et al. in their model development for the TL process in LiF is demonstrated by comparing the TL and optical properties of LiF(54) with TLD‐100. The specific properties of optical absorption bands at 310 and 380 nm, the F band near 250 nm, and the Z3 band near 225 nm are intercompared with observed TL peaks in both materials. Both the gamma and uv‐exposure behavior of the Z3 center demonstrates a direct relationship between Z3 and TL peak 10, while no direct conversion between Z3 and the 310‐nm band is observed in TLD‐100. These results do not support recent models identifying Z3 with TL peak 6 and Z2 with the 310‐nm band (and TL peak 5); this...

Role of hydroxide impurities in the thermoluminescent behavior of lithium fluoride

The influence of OH ion impurities on thermoluminescent sensitivity and supralinearity in LiF:Mg, Ti is analyzed. Available evidence is shown to be consistant with the presence of Ti‐OH and Mg‐OH complexes. The track interaction model is used to explain the data, with competing centers decreasing sensitivity and supralinearity at high concentrations.

A comparative study of selective carbon doping in MOCVD GaAs using trimethylarsenic and arsine

Carbon incorporation in GaAs epitaxial layers grown by low pressure metalorganic chemical vapor deposition (MOCVD), using trimethylgallium (TMGa) as the gallium source and trimethylarsenic (TMAs) and AsH3 as the arsenic sources, has been studied over a wide range of growth parameters. Carbon incorporation is identified by secondary ion mass spectroscopy (SIMS), Hall measurement, and C-V analysis. Active carbon levels between 2 × 1015 cm-3 and 7 × 1020 cm-3 are obtained. The carbon incorporation is more sensitive to the partial pressure of TMGa than of TMAs in the growth temperature range 500 ~ 610° C. Carbon incorporation is increased as growth temperatures are decreased to 500° C for growth pressures near 10000 Pa. Results indicate that surface-adsorbed methyl radicals from the dissociation of TMGa controls carbon incorporation in this temperature range.