Etch Pit Counting Research Articles

Spatial correlation of defect-selective etching and dark luminescence spots in Al x Ga1−x N

Defect-selective etching with molten Ba(OH)2/MgO etch drops was performed on c-plane AlGaN layers covering the entire composition range between GaN and AlN. Regardless of the aluminum content, the etchant produced shallow, hexagonal etch pits with depth-to-diameter ratios of 1/10–1/100. Two predominant types of etch pits were observed, which differed in size. In addition, the etch rate decreased from the center to the edge of the area exposed to the etch drops, providing a radially symmetric variation in etch pit size. For all AlGaN compositions, the positions of the etch pits correlate perfectly with the positions of the dark luminescence spots in cathodoluminescence measurements. Areas on the AlGaN samples that were not exposed to the etching procedure showed identical dark spots with the same size and density as those in the etched regions. Additionally, the density of etch pits and dark spots corresponded to the density of threading dislocations in the AlGaN layers. These observations demonstrate that the density of threading dislocations in c-plane AlGaN layers can be determined by destructive defect-selective etching with Ba(OH)2/MgO and etch pit counting, as well as by nondestructive counting of the dark spots in cathodoluminescence images.

Investigation of Doping Processes to Achieve Highly Doped Czochralski Germanium Ingots

Highly doped germanium (HD-Ge) is a promising material for mid-infrared detectors, bio-sensors, and other devices. Bulk crystals with a doping concentration higher than 1018 cm−3 would be desirable for such device fabrication technologies. Hence, an effective method needs to be developed to dope germanium (Ge) ingots in the Czochralski (Cz) growth process. In this study, a total of 5 ingots were grown by the Cz technique: two undoped Ge ingots as a reference and three doped ingots with 1018, 1019 , and 1020 atoms/cm3 respectively. To obtain a uniform p-type doping concentration along the crystal, co-doping of boron-gallium (B-Ga) via the Ge feed material was also attempted. Both B and Ga are p-type dopants, but with a large difference in their segregation behavior (contrary segregation profile) in Ge, and hence it is expected that the incorporation of dopants in the crystal would be uniform along the crystal length. The distribution of the dopants followed the Scheil-predicted profile. The etch pit density maps of the grown crystals showed an average dislocation density in the order of 105 cm−2. No increase in the overall etch pit count was observed with increasing dopant concentration in the crystal. The grown highly doped Ge crystals have a good structural quality as confirmed by x-ray diffraction rocking curve measurements.

Microstructural Evolution of Mechanically Deformed Polycrystalline Silicon for Kerfless Photovoltaics

Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.

Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40–30nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting.

From plastic to elastic stress relaxation in highly mismatched SiGe/Si heterostructures

We present a detailed experimental and theoretical analysis of the epitaxial stress relaxation process in micro-structured compositionally graded alloys. We focus on the pivotal SiGe/Si(001) system employing patterned Si substrates at the micrometre-size scale to address the distribution of threading and misfit dislocations within the heterostructures. SiGe alloys with linearly increasing Ge content were deposited by low energy plasma enhanced chemical vapour deposition resulting in isolated, tens of micrometre tall 3D crystals. We demonstrate that complete elastic relaxation is achieved by appropriate choice of the Ge compositional grading rate and Si pillar width. We investigate the nature and distribution of dislocations along the [001] growth direction in SiGe crystals by transmission electron microscopy, chemical defect etching and etch pit counting. We show that for 3 μm wide Si pillars and a Ge grading rate of 1.5% μm−1, only misfit dislocations are present while their fraction is reduced for higher Ge grading rates and larger structures due to dislocation interactions. The experimental results are interpreted with the help of theoretical calculations based on linear elasticity theory describing the competition between purely elastic and plastic stress relaxation with increasing crystal width and Ge compositional grading rate.

Дослідження дисперсії швидкості пружних хвиль в опромінених кристалах LiF

The influence of a preliminary deformation (the residual strain e = 0.65%) and x-ray irradiation to exposure doses of 0–800 R on the frequency dependence of the sound velocity, v(f), in LiF crystals in the frequency interval from 7.5 to 232.5 MHz and at room temperature has been studied using the pulsed technique. By extrapolating the results obtained for v(f) to the low-frequency interval and using the well-known theoretical relations, the coefficient of dynamic viscosity B and the dislocation density Λ were found to be independent of the irradiation dose. At the same time, the absolute values of B were found to be lower and the values of Λ higher by an order of magnitude than the corresponding values obtained with the use of the most reliable techniques, such as the methods of high-frequency internal friction and etch pit counting, respectively.

Characterisation of Dislocation-Content in Multicrystalline-Silicon Wafers

A major performance limiting factor of multicrystalline silicon wafers is structural defects, mainly dislocations, reducing solar cell efficiency. Dislocations are formed during crystallisation process. Characterization of dislocation-content is necessary both to optimise the crystallisation and to eliminate bad wafers before cell processing. We developed two techniques to characterise dislocations: conventional etch-pit counting modified for full size wafers using a new etch-recipe and a novel etch-pit counting algorithm. Secondly we developed a technique to estimate the dislocation content directly from photoluminescence images of as-cut wafers.

Unexpected Dominance of Vertical Dislocations in High‐Misfit Ge/Si(001) Films and Their Elimination by Deep Substrate Patterning

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

Epitaxy of In0.01Ga0.99As on Ge/offcut Si (001) virtual substrate

In0.01Ga0.99As thin films free of anti-phase domains were grown on 7° offcut Si (001) substrates using Ge as buffer layers. The Ge layers were grown by ultrahigh vacuum chemical vapor deposition using ‘low/high temperature’ two-step strategy, while the In0.01Ga0.99As layers were grown by metal-organic chemical vapor deposition. The etch-pit counting, cross-section and plane-view transmission electron microscopy, room temperature photoluminescence measurements are performed to study the dependence of In0.01Ga0.99As quality on the thickness of Ge buffer. The threading dislocation density of Ge layer was found to be inversely proportional to the square root of its thickness. The threading dislocation density of In0.01Ga0.99As on 300nm thick Ge/offcut Si was about 4×108cm−2. Higher quality In0.01Ga0.99As can be obtained on thicker Ge/offcut Si virtual substrate. We found that the threading dislocations acted as non-radiative recombination centers and deteriorated the luminescence of In0.01Ga0.99As remarkably. Secondary ion mass spectrometry measurement indicated as low as 1016cm−3 Ge unintended doping in In0.01Ga0.99As.

Structural defects and microindentation analysis of zone melted Bi2Te3−xSex whiskers

The dislocation density and microhardness of Bi2Te3−xSex (x=0–0.3at% Se) grown by the zone melting method have been investigated. We also have got the whiskers of Bi2Te3−xSex at the end of ampoule during the growth process. SEM was characterized for surface analysis of the grown whisker. The length of the grown whiskers was around 10mm in the direction of the crystallographic c-axis. Concentric pairs of dislocation triangle were observed on the as-grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by the chemical etching technique. Dislocation etching was achieved on all crystal planes examined using a saturated solution of citric acid and nitric acid as the etchant. The dislocation etchant has been found to give reproducible etch-pits on the cleavage surface. The use of citric acid and nitric acid proved to be especially advantageous for the basal plane, producing etched pits suitable for dislocation etch pit counting. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the (001) faces of Bi2Te3−xSex.

Depth-dependent etch pit density in Ge epilayer on Si substrate with a self-patterned Ge coalescence island template

Ge epilayer with low dislocation density is prepared on a low temperature self-patterned Ge coalescence island template on Si substrate by ultra-high vacuum chemical vapor deposition. The depth profile of dislocation density in the Ge epilayer measured by etch-pit counting indicates that the dislocation density decreases drastically when the thickness of Ge layer is larger than 270 nm, and then depends weakly on the further increase of Ge thickness. The reduction of dislocations is ascribed to the Ge islands coalescence and lateral growth during deposition of Ge at higher temperature. 1.05-μm-thick Ge epilayer on Si with a dislocation density of the order of 10 6 cm − 2 and root-mean-square surface roughness of 0.45 nm are achieved without any additional thermal treatments.

Stacking fault in Bi2Te3 and Sb2Te3 single crystals

The structural characterizations of the Bi2Te3 and Sb2Te3 single crystals grown by the Zone melting method have been carried out by XRD and Chemical Etching process. The top free surface of as-grown Bi2Te3 and Sb2Te3 single crystal were observed under optical microscope. The particle size was calculated for a number of reflections using Scherrer’s formula by X-ray diffraction method. By the use of XRD data, the growth and deformation fault probability has been estimated for the grown single crystals. The presence of stacking fault in the lattice structure of the grown crystals was verified by the probability of growth and dislocation fault. Also new dislocation etchant has been developed by the successive trial–error method. Dislocation etching was achieved on (111) crystal plane examined using a saturated solution of citric acid and nitric acid as the etchant. The dislocation etchant has been found to give reproducible etch pits on the cleavage surface for an appropriate etching time 30s. The use of citric acid and nitric acid proved to be especially advantageous for the basal plane, producing etched pits suitable for dislocation etch pit counting.

Time-integrated monitoring of thoron progeny concentration around closed uranium mine sites in Japan

Thoron progeny concentrations were determined using the time-integrated method around closed uranium mine sites in Japan. Because the time-integrated radon progeny monitor developed by the authors has the function to detect (212)Po, time-integrated monitoring of thoron progeny concentration is also available with the monitor. Assuming that contribution of (216)Po is negligible, equilibrium equivalent concentration of thoron (EECTn) is theoretically calculated from the etch-pit counts by (212)Po. The annual averages of EECTn observed in the investigation area were about 0.2 Bq m(-3), and they had no remarkable differences from one another.

Effect of Mesa Shape on Threading Dislocation Density in Ge Epitaxial Layers on Si after Post-Growth Annealing

The effect of mesa shape on threading dislocation density in Ge epilayers on Si is studied. It is found from the etch-pit counting on the surfaces that the post-growth annealing reduces the threading dislocation densities in Ge mesa structures on Si and in the blanket Ge layers. The dry-etched mesas with vertical sidewalls show dislocation densities identical to those for the blanket layers. This indicates that the reduction in threading dislocation density in the dry-etched mesas is predominantly due to the annihilation of dislocations with opposite Burgers vectors, while the motion of dislocations to the mesa edges does not contribute. On the other hand, the selective-grown mesas with inclined (311) facet sidewalls show lower dislocation densities than those for the blanket layers, indicating that the dislocations can glide out beyond the mesa edges. Since the shear stress thermally induced in Ge is responsible for the dislocation glide motion, the present result suggests that the selective-grown mesas should be different from the dry-etched mesas in terms of the shear stress accumulated during the annealing. Strain simulations for these mesa structures show that the strength and distribution of the thermally induced shear stress are quite different near the top edges between the mesa structures. We discuss the relationship between the distribution of thermal stress and the reduction in dislocation density.

Influence of AlGaN/GaN superlattice inserted structure on the performance of InGaN/GaN multiple quantum well light emitting diodes

Investigations were conducted to explore the effect of Al 0.3Ga 0.7N/GaN short-period superlattice (SPSL)-inserted structures in the GaN under layer on the performance of In 0.2Ga 0.8N/GaN multiple quantum well (MQW) light emitting diodes (LEDs). The Al 0.3Ga 0.7N/GaN SPSL-inserted LEDs were found to exhibit improved materials and device characteristics including decrements in ideality factor and reverse leakage current. The results of etch pit counts reveal that SPSL-induced threading dislocation density reduction in the SPSL-inserted In 0.2Ga 0.8N/GaN MQW LED structures enables the improved LED performance.

Nanocavity Buffer Induced by Gas Ion Implantation in Silicon Substrate for Strain Relaxation of Heteroepitaxial Si1-xGex/Si Thin Layers

AbstractTo weight the importance of a nanocavity buffer in a SiGe deposition substrate, some P type (001) FZ Si wafers are implanted (A samples) or not (B samples) at room temperature with 5×1016 He+ cm–2 at 10keV. They are annealed at 700°C for one hour to form a nanocavity layer close to the Si surface. Then, the wafers are carefully chemically cleaned in a clean room to remove both organic and metallic impurities from the surface. They are coated either by 210 nm (A) or 430 nm (B) Si1−xGex (x=0.20±0.02) alloy grown at 575°C for 0.42 hour by low pressure chemical vapor deposition (LP-CVD) with a growth rate of 8 to 17 nm.mn−1. Both kinds of samples are studied by cross section transmission electron microscopy, X-rays diffraction, Rutherford backscattering, atomic force microscopy and etch pit counts. The association of these techniques demonstrates that the thin SiGe layer which is deposited on sample A is fully relaxed and that the threading dislocation density (estimated to hardly reach 4×103cm−2) is at least one order of magnitude lower than what is obtained so far using ion implantation assistance in SiGe layer growth on Silicon. The roughness of the SiGe surface is low enough to stand a further Si epitaxy. Nevertheless, the mechanism involved responsible for the threading dislocation annihilation and/or confinement is still unclear.

New Approach for the Synthesis of IV-VI Thermoelectric Thin Film Materials and Devices on Si: (211) PbSnSeTe/ZnTe/Si Heterostructures

AbstractLow-dimensional thin-film thermoelectric materials including superlattices and quantum-dot heterostructures have shown significant potential for improving the thermoelectric properties. Highly structured thin film materials such as these would be useful for integrated microdevices such as power MEMS and thermally triggered MEMS actuators. For reasons including scalability and ease-of-integration, silicon is the choice material for the substrate, however there is a lattice and a thermal-expansion mismatch. In this work, a new approach for the synthesis of integrated PbSnSeTe based thermoelectric thin film materials on silicon is demonstrated by employing an epitaxial buffer layer of II-VI compound telluride materials (e.g., ZnTe, CdTe) to help bridge the lattice and thermal expansion mismatches with silicon. This multilayer can be used for subsequent growth of thick films having low-dimensionality structures. We report the initial results from studying the structural and thermoelectric properties of simple solid-solution alloy PbSnSeTe thin films on ZnTe/Si heterostructures. Data from transmission electron microscopy and in-situ electron diffraction will be presented that shows that despite the large lattice mismatch with silicon and the three different crystal structures, unusually high structural quality PbSnSeTe has been obtained by matching the (211) lattice symmetry and the lattice spacing along the [110] directions of the ZnTe and PbSnSeTe. The structural quality of the PbSnSeTe was studied by measuring the dislocation density through etch-pit counting and x-ray diffraction. Results presented will show that a dislocation density as low as 1.2 × 106/cm2can be achieved by strategic lattice-matching between the buffer and thermoelectric layers. Electrical resistivity, doping density, and Seebeck coefficient values for solid-solution alloys without low-dimensional structuring will be shown to approach those of high-performance bulk.

Improvement in the characteristics of GaN-based light-emitting diodes by inserting AlGaN-GaN short-period superlattices in GaN underlayers

We report the influence of short-period superlattice (SPSL)-inserted structures in the underlying undoped GaN on the characteristics of GaN-based light-emitting diodes (LEDs). The measurements of current-voltage (I-V) curves indicate that GaN-based LEDs having pseudomorphic Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures exhibit improvements in device characteristics with the best LED being inserted with two sets of five-pair Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL structure. Based upon the results of etch pit counts, double-crystal X-ray diffraction measurements and transmission electron microscopic observations of the GaN-based LEDs, it was found that the Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures tended to serve as threading dislocation filters in the LEDs so that the improved I-V characteristics were achieved.

Reproducible defect etching of SiC single crystals

Etching in molten KOH is a usual method to reveal the defect density across a SiC wafer. Time stabilised etching rates are desirable for an objective assessment of the defect density via etch pit counting. The expected rate stabilising effect of oxygen, released from an addition of 2wt% KNO3 was not found. The observed increase of the etching rate with time in purely molten KOH could be masked by adding 20wt% K2CO3, a concentration roughly at the eutectic point.

Thermal etching of α-Zr single-crystal surfaces

Extensive thermal etching of α-Zr single crystals has been found to occur during high-temperature annealing (820°C) under ultra-high vacuum (< 1.0 × 10 −7Pa). Two grades of material were examined, Z1, high purity; and Z2, nominally pure: levels of the “surface active” element, Fe, were about 1 and 50 ppma, in Z1 and Z2, respectively. In Z1, strong faceting occurred on a high-index surface (8° off the (10 1 0) plane) and weak linear facets appeared on the (10 1 0) plane. In Z2, etch pits and linear groove defects formed on the (10 1 0) plane. Etch-pit formation may be promoted by Fe segregation to dislocations, Fe-rich precipitates were found at the bases of clusters of pits. Etch pit counts were consistent with dislocation densities of about 1.0 × 10 11/m 2. The (0002) plane remained comparatively flat, but Fe-rich, needle-shaped precipitates at 60° angles with each other were formed. An analysis of the results implies that the surface energies increase in the order (0002) < (10 11) < (10 10) , and that the etching mechanism is surface diffusion.