V. I. Ozhogin

Isotope effect in the thermal conductivity of germanium single crystals

The thermal conductivity of chemically, structurally, and isotopically highly pure germanium single crystals is investigated experimentally in the temperature range from 2 to 300 K. It is found that the thermal conductivity of germanium enriched to 99.99% 70Ge is 8 times higher at the maximum than the thermal conductivity of germanium with the natural isotopic composition.

High purity isotopically enriched 70Ge and 74Ge single crystals: Isotope separation, growth, and properties

[sup 70]Ge and [sup 74]Ge isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity [sup 70]Ge and [sup 74]Ge single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of very small weight ([similar to]4g). A [sup 70]Ge and a [sup 74]Ge crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both [sup 70]Ge and [sup 74]Ge crystals contain low electrically active chemical net-impurity concentrations of [similar to]2[times]10[sup 12] cm[sup [minus]3], which is two orders of magnitude better than that of [sup 74]Ge crystals previously grown by two different groups.[sup 1,2] Isotopic enrichment of the [sup 70]Ge and the [sup 74]Ge crystals is 96.3% and 96.8%, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition less than 10[sup 11] cm[sup [minus]3] gallium, aluminum, and indium were found in the [sup 70]Ge crystal.

Neutron transmutation doping of isotopically engineered Ge

We report a novel approach for obtaining precise control of both p‐ and n‐type dopant concentrations in bulk Ge single crystals. High‐purity Ge single crystals of controlled 74Ge/70Ge isotope composition ratios were grown and subsequently doped by the neutron transmutation doping (NTD) technique. The resulting net‐impurity concentrations and the compensation ratios were precisely determined by the thermal neutron fluence and the [74Ge]/[70Ge] ratios of the starting Ge materials, respectively. Application of NTD to seven crystals with 0≤[74Ge]/[70Ge]≤4.34 lead to p‐type Ge:Ga,As with compensation ratios in the range 0–0.76. The ability to grow crystals with accurately controlled Ge isotope mixtures allows us to obtain ratios anywhere between 0 and 1 for both p‐ and n‐type doping.

Neutron Transmutation Doped Natural and Isotopically Engineered Germanium Thermistors

We report on the development, fabrication and performance of a new class of thermal sensors for far IR and millimeter wave detection. These devices consist of small single crystal samples of ultra-pure, natural or isotopically engineered germanium which have been doped by the neutron transmutation doping (NTD) technique. The concentrations of the acceptor and donor dopants (NA,ND) can be accurately controlled with this technique. They depend on the thermal neutron fluence, the neutron absorption cross sections and the atomic fractions of 70Ge (for the Ga acceptors) and 74Ge (for the As donors), respectively. The values of NA and ND and their ratio result in a predictable resistivity of the Ge crystals down to temperatures of a few milliKelvin. The excellent control of the resistivity down to very low temperatrues, together with the development of ohmic contacts working at the lowest temperatures, allows the fabrication of high sensitivity bolometer arrays with over 100 pixels and highly uniform response.

Three-phonon coupled excitations in an antiferromagnet

Results of an experimental observation of the generation of three-phonon coupled excitations in an α-Fe2O3 high-temperature antiferromagnet in a transverse electromagnetic pumping field are presented. The underthreshold and superthreshold generation regimes are studied depending on the phase shift of pumping with respect to the phase of the three-phonon acoustic field correlator. The results agree well with the theory of the resonant nonlinear interaction of quasi-phonons with an electromagnetic field.

Low temperature hopping conduction in neutron transmutation doped isotopically enriched70Ge: Ga single crystals

The temperature dependence of variable range hopping resistivity ρ in neutron transmutation doped (NTD) isotopically enriched70Ge:Ga samples is reported. Five samples with compensation ratios K less than 0.001 and Ga concentrations between 3×1016 and 1.77×1017 cm−3 were studied. All samples investigated show the ln ρ∝T−1/2 dependence in the temperature range below 1.5K. As thermistor materials NTD70Ge:Ga samples are found to have more than factor of two higher sensitivity than commonly used natural NTD Ge in the temperature range between 0.2K and 1K. Our results are compared with theoretical predictions for variable range hopping conduction.

Normal processes of phonon-phonon scattering and the drag thermopower in germanium crystals with isotopic disorder

A strong dependence of the thermopower of germanium crystals on the isotopic composition is experimentally found. The theory of phonon drag of electrons in semiconductors with nondegenerate statistics of current carriers is developed, which takes into account the special features of the relaxation of phonon momentum in the normal processes of phonon-phonon scattering. The effect of the drift motion of phonons on the drag thermopower in germanium crystals of different isotopic compositions is analyzed for two options of relaxation of phonon momentum in the normal processes of phonon scattering. The phonon relaxation times determined from the data on the thermal conductivity of germanium are used in calculating the thermopower. The importance of the inelasticity of electron-phonon scattering in the drag thermopower in semiconductors is analyzed. A qualitative explanation of the isotope effect in the drag thermopower is provided. It is demonstrated that this effect is associated with the drift motion of phonons, which turns out to be very sensitive to isotopic disorder in germanium crystals.

Selenium double donors in neutron transmutation doped, isotopically controlled germanium

Abstract Far infrared photoconductivity and absorption measurements were performed on isotopically controlled 76 Ge samples that were neutron irradiated to produce 77 Se through double beta decay. The spectra exhibit ground state to bound excited state transitions which place the first ionization level of Se at E c -0.2688 eV. Hall effect measurements on compensated Ge:Se single crystals yield the second ionization level in the lower half of the band gap at E v +0.17 eV. Our experiments offer the first unambiguous identification of the deep donor level formed by single Se atoms on Ge lattice sites and verify earlier findings.

Normal and “Anomalous” isotope shifts of phonon frequencies in mono- and polyisotopic germanium crystals

The shifts of the phonon frequencies in 70Ge and 74Ge monoisotopic crystals have been studied using inelastic neutron scattering. It has been shown that the frequency shifts for all of the branches are normal, i.e., inversely proportional to √M. At the same time, the gap between acoustic and optical frequencies in monoisotopic crystals appears to be much wider (by about 2 meV in the [111] direction) than that in natural polyisotopic crystals containing “isotopic defects.” In this case, the phonon frequencies undergo “anomalous” shifts; i.e., the oscillation frequency of the heavier isotope is higher than that for the lighter isotope. The effect is in qualitative agreement with old theoretical calculations of the effect of mass defects on the phonon spectra of diatomic lattices.