Germanium Bicrystals Research Articles

Atomic structure and electronic properties of the ?37(610) and ?29(520) [001] tilt grain boundaries in Ge

400 kV high resolution electron microscopy (HREM), deep level transient spectroscopy (DLTS) and steady state electrical measurements have been applied to ∑37(610) and ∑29(520) [001] tilt grain boundaries (GBs) in germanium bicrystals. The atomic boundary structures were revealed by experimental HREM images taken under different defocus conditions. Later, structure models were refined by means of a trial-and-error method applying alternatively the image simulation and the molecular static calculation of relaxed structures. The structures were shown to be consistent with the modified structural unit model. Although the structures are different for the two GBs studied, DLTS data and steady state measurements were found to be quite similar for both GBs. Thus, the results point to the extrinsic origin of localized deep states at the GBs. The analysis of DLTS spectra indicates the impurity segregation at the boundary, e.g., the formation of vacancy-type oxygen complexes of a donor-like state at Ec-0.21 eV, which results in the fluctuation of the potential barrier. Defects in the GBs—like facets, atomic steps and secondary grain boundary dislocations—which are characteristic of both boundaries can act as nuclei to the impurity segregation.

Two-Dimensional Transport Properties of Germanium Bicrystal at Low Temperature

Two-dimensional properties of grain boundaries in germanium bicrystals are studied at liquid helium temperature ranging from 1.6 to 4.2 K. The experimental results indicate that the magnetoresistivity for 8° tilt angle increases exponentially with the magnetic field intensity square in weak magnetic field and with the linear magnetic field intensity in strong magnetic field. The phenomena are explainable with the percolation theory.

Grain boundary states in germanium

Abstract We have investigated the electrical properties of large-angle grain boundaries in n- and p-type germanium bicrystals. Measurements of the two-dimensional conductivity in the boundary plane and of the thermally activated conductance across the boundary were performed on the same specimen for the first time. A sequence of measurements was chosen which allowed us to exclude contamination or other experimental differences as the reason for a fundamental discrepancy between earlier results of different authors. From a new analysis we are able to show that all experimental results are in fact consistent with the same model of a two-dimensional quasi-metallic hole band for the grain boundary states. In the neutral state, the Fermi level of this band is very close to the edge of the valence band with a few milli-electron-volts as an upper limit.

The relation between the structure and mechanical properties of A σ = 51 tilt boundary in germanium

A series of Germanium bicrystals containing a 16° [011] tilt boundary has been deformed in high-temperature creep and analyzed by both high-resolution lattice-fringe imaging and conventional transmission electron microscopy. Before deformation, the structure of the boundary consists of a linear array of Lomer dislocations. Subgrain boundaries are produced by the deformation and intersect the original grain boundary. Absorption of lattice dislocations causes the interfacial structure to change along the boundary from one subgrain to another. Deviations from the Σ = 51 coincidence structure are accommodated by secondary dislocations. The secondary dislocations have non-primitive DSC-Burgers vectors. Upon entering the grain boundary, the deformation-produced lattice dislocations decompose into secondary dislocations. Their motion in the interface is associated with grain boundary migration and grain boundary sliding. However, the latter process could not be identified macroscopically and, therefore, appears to make a much smaller contribution to the total strain than it does in metals. It is also directly shown that screw dislocations with Burgers vector in direction of the tilt axis can pass through the boundary.

Secondary dislocations in [011] tilt boundaries in germanium

Abstract The structures of a series of tilt grain boundaries in germanium bicrystals and the interaction of deformation-induced lattice dislocations with these boundaries have been studied using transmission electron microscopy. High-resolution techniques were used to determine the Burgers vectors of the secondary dislocations. The bicrystals were grown from the melt using the Czochralski method to produce the low-Σ grain boundaries: Σ = 11, 33, 41 and 51. The Burgers vector of the secondary dislocations was usually not the primitive Burgers vector which might have been expected by applying O-lattice analysis. This interpretation is consistent with the Burgers vectors determined for secondary dislocations produced by lattice dislocations dissociating on entering the grain boundaries.

Low‐Temperature Conductivity of a Weakly Disordered, Strongly Anisotropic Two‐Dimensional System with Electron—Electron Interaction. Application to Germanium Bicrystals and to Surface Superlattices

AbstractIn the framework of a simple model for the electron gas in the grain boundary (inversion layer) of a semiconductor bicrystal or in a surface superlattice the quantum corrections to the conductivity tensor due to the electron—electron interaction are calculated. The results have a typical two‐dimensional form for w ≫ kBTħ/τ and show a crossing‐over to typical one‐dimensional behaviour for w ≪ kBTħ/τ. Here w⟂ is the transverse overlap integral betwween neighbouring dislocation tubes, τ the relaxation time due to scattering on the disorder. Also the weak localization corrections are calculated.

Low‐Temperature Magnetoresistance in Medium‐Angle Grain Boundaries in Germanium

AbstractMagneto‐transport properties of grain boundaries in germanium bicrystals are studied in the liquid helium temperature range. For samples with 7° and 8° tilt angle an exponential magnetic field dependence is observed, which is analyzed in terms of two‐dimensional hopping conduction. The acceptor concentration N in the grain boundary can be deduced by comparing different magnetic field orientations. A surprising temperature dependence of N is obtained. In addition, the activation energy is determined as about 0.5 meV, typical for the hopping process, and decreases slightly with increasing magnetic field.

Twin boundaries in GaAs grown on Ge

Interest in the structure of grain boundaries in semiconductors has been increasing for many reasons including the important role of boundaries on the electrical behavior of polycrystalline materials. GaAs is of interest because of its particular electrical properties such as its direct band gap and high electron mobility. Specific bicrystals of GaAs were produced on substrates cut from Czochralski-grown germanium bicrystals in a low-pressure organometallic vapor- phase epitaxy system. When the growth surface was parallel to the (110) plane, many microtwins were produced in the GaAs epilayer. The structure of twin boundaries in GaAs can take two forms depending on the relative polarities of the two adjoining grains because GaAs has a sphalerite structure which is a face-centered cubic lattice with a two atom basis. The polarity of the two grains meeting at the grain boundaries can be determined by using the dynamical coupling effects of high-order Laue-zone reflections on the (200) and (200) diffracted beams.

Growth of GaAs bicrystals

Bicrystals of gallium arsenide have been grown by the technique of organometallic, vapor phase epitaxy on substrates cut from Czochralski-grown germanium bicrystals. Both 〈110〉 and 〈001〉 tilt boundaries have been grown. Experimental illustrations of the Σ=3 and Σ=5 grain boundaries are presented. The grain boundaries tend to facet along well-defined planes but closely reproduce the misorientation of the seed bicrystal.

Quantum magneto-transport in p-type inversion layers of germanium bicrystals

A p-type inversion layer adjacent to a grain boundary in medium angle tilt germanium bicrystals behaves like a two-dimensional disordered system. Superimposed on a random potential is a periodic one , generated by a lattice of edge dislocations. At helium temperatures both resistivity and Hall coefficient increase logarithmically with decreasing temperature. The transverse magneto-resistance shows a logarithmic B dependence in wide ranges of the magnetic field. Many of the experimental data can be explained qualitatively in terms of recently developed many-body theories.

Transport along grain boundaries in germanium bicrystals at low temperatures

After a short discussion of previous data on transport properties along grain boundaries in germanium bicrystals it is pointed out that the system is suitable to study the metal-insulator transition. Complications in the interpretation of transport data arise because the conducting layers are p-type and two-dimensional. The magneto-transport data of space charge layers adjacent to tilted grain boundaries show anomalous behaviour which can be attributed to electron-electron interaction effects. Under usual conditions in [100] medium angle tilted bicrystals two hole subbands are occupied. This is the result of self-consistent subband calculations in the Hartree approximation.

Electric Conduction Phenomena of Two-Dimensional Hole Gas in Germanium Bicrystals

Electric Conduction Phenomena of Two-Dimensional Hole Gas in Germanium Bicrystals

TRANSIENT ELECTRICAL PROPERTIES OF LOW ANGLE TILT BOUNDARIES IN GROWN GERMANIUM BICRYSTALS

Transient electrical effects associated with carrier trapping and emission at dislocation walls in grown germanium bicrystals are presented, and compared to those obtained for dislocations in plastically deformed semiconductors.

Electric subbands in P-type germanium inversion layers

Shubnikov-de Haas oscillations have been observed in p-type inversion layers adjacent to the grain boundary in germanium bicrystals. The data are compared with self-consistent calculations of the subband structure in the Hartree approximation. The quantum oscillations arise from the light hole subband with m c=0.34 m 0.

Partial secondary dislocations in germanium grain boundaries

Abstract Interfacial dislocation networks have been studied in germanium bicrystals, using transmission electron microscopy. Periodic arrays of partial secondary dislocations, associated with a stacking-fault-like structure, have been observed in a near coincidence Σ=5 grain boundary in which the deviation from exact coincidence orientation is a 0·05° rotation about [131], close to the boundary normal [130]. The dislocation grid is made of a honeycomb network of two partial secondary dislocations and a perfect secondary dislocation crossed by a set of parallel partial secondary dislocations. When some diffracting vectors common to the two crystals are used, areas of different contrast, limited by the partial dislocations, appear suggesting that the boundary is formed by two interfacial domains.

Mise en evidence de dislocations secondaires dans des joints de flexion de bicristaux de germanium

High angle tilt boundaries in germanium bicrystals have been studied using transmission electron microscopy. Precisely controled deviation from coincidence orientation allows to point out secondary relaxations in Σ = 25, 37, 41 near coincidence boundaries.

The low-angle [011] tilt boundary in germanium I. High-resolution structure determination

Abstract Pure tilt boundaries (θ = 0·5 to 5°) in germanium bicrystals with a [011] common axis have been studied with respect to the tilt angle, θ, and the grain-boundary plane. Structural detail is revealed at a 3 A level by multi-beam interference using both a 100 and a 500 keV electron microscope. At this scale great complexity arises from dislocation splitting. Four types of grain boundaries are encountered : they are formed with dislocations having 1/2[011] Burgers vector, but also with very uncommon Burgers vectors such as [111] (split into three Frank dislocations) or 1/2[211] (split into three partials). The type of grain boundary is determined by both the tilt angle and the grain-boundary plane.

Kombination von galvanomagnetischen- und Feldeffekt-Messungen an Germanium-Bikristallen

The transport properties in a magnetic field of the p-type space charge region adjacent to 20° tilt grain boundaries in n-type germanium bicrystals were investigated as a function of a reverse bias voltage between bulk and grain boundary (field effect). The average HALL mobility at 77°K of the holes decreased with increasing bias, demonstrating the existence of a size effect caused by diffuse boundary scattering in connection with the small width of the space charge layer. The results obtained for symmetrical bias are in agreement with a simplified theory based on a constant space charge field and allow to estimate an average hole density. The transverse magnetoresistance also decreased with increasing field effect-voltage, as expected.

Field Dependence of Photoresponse in Germanium Bicrystals

Measurements of the photoresponse of germanium bicrystals are reported here with two specific results in mind: (1)The detailed response at higher optical frequencies. The measured transition steps at larger than band gap energies can be associated with the specific band structure of the dislocation plane taking the stress field into consideration. (2) The detailed bias or field dependence yields a linear law for the photovoltage. It is found that the electronic dislocation model, as used earlier, explains the linear field dependence of the photoresponse.

A Neutron Irradiation Test of Germanium Bicrystals

A neutron radiation test was performed on 12 germanium bicrystals of four different resistivities, to screen their radiation tolerance. Optical performance of four bicrystals was recorded intermittently during the test using a pulsed radiation-tolerant light source included in the test package. VI diode characteristics were also taken periodically throughout the test. The results obtained showed that the open circuit voltage response of the devices was degraded by 5 × 1013 nvtf and that their diode characteristics became linear in proportion to the donor concentration. The diode characteristics, however, were affected at a much lower value than anticipated, as shown by the zero bias conductance.