Evaporation of Crystals - Evaporation of Germanium-mono-sulfide Cleavage Plane

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Evaporation of Crystals - Evaporation of Germanium-mono-sulfide Cleavage Plane
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Kristallverdampfung - Verdampfung der Germaniummonosulfid-Spaltfläche
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E 2791
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Winckler, Erika
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Film, 16 mm, LT, 200 m ; F, 18 1/2 min

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With the interference-contrast-microscope it is possible to observe low steps which occur on a GeS-cleavage face during evaporation. The surface evaporates at locations where the crystal structure is disturbed: at the cleavage steps and at distinct dislocations. At first the cleavage steps are completely evaporated if they are not oxidized. Subsequently terraced pits are formed. Steps spread from the center of the pits over the surface homogeneously. The origins of the steps are locally fixed (at dislocations) or appear at random locations when the steps are formed by dislocation loops or by stacking faults. The shape of the pits is rhombic at low temperatures and oval at high temperatures. In addition, rectangular pits randomly appear at different locations at high temperatures. With increasing temperature stationary rhombic step patterns appear temporarily dendritic. Herringbones are formed by intersecting steps of different stationary centers. Precipitated impurities (GeS[2]) affect the spreading steps if the surface deviates from the ideal cleavage plane. Many details of the step patterns are explained by still pictures which show chemically etched examples. A ball model and an animation illustrate the relation between evaporation patterns and the crystal structure.
Keywords evaporation of crystals crystal evaporation germanium monosulfide / cleavage plane crystal lattice decomposition lattice structure / crystal step dislocation screw dislocation semi crytsal sites evaporation pattern precipitation / germanium disulfide etching pit, rhombic argon monocrystal / germanium disulfide single crystal / germanium disulfide lattice model heating chamber sources / dendritic protective gas cleavage plane evaporation dislocations / crystals

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Evaporation of Crystals - Evaporation of Germanium-mono-sulfide from the Cleavage Plane Germanium-monosulfide crystals were grown from the vapor phase by the Pizzarello method. The crystals are cleaved with a razor blade. Extremely smooth planes result. The evaporation is observed at these planes.
The evaporation is investigated with a microscope. The process is filmed in time-lapse cinematography. The surface of the sample is observed with the interference contrast technique.
A 6 by 6 mm wafer of 0.5 mm thickness is loaded into the hot-stage furnace. Argon at atmospheric pressure is passed over the wafer. The sample is heated up to temperatures between 650 and 800 K within 15 min. The final temperature is held constant with an accuracy of 1 K. The
equipment has now been adjusted. The wafer evaporates at different
spots. The stepwise evaporation forms terraced oval pits.
Germanium monosulphide crystallizes into the orthorhombic structure. The structure is illustrated by a ball model. The blue spheres correspond to Germanium, the yellow spheres to Sulfur atoms, and the wires to covalent bonds.
Each atom of one kind is covalently bonded to three atoms of the other kind.
The resulting structure consists of puckered six-membered rings in the chair conformation.
The covalently bonded layers are connected by van-der-Waals bonds. Thus the layers can be easily separated by cleaving.
All the processes are observed at cleavage planes like this.
Here the cleavage plane appears as a projection of the model. Since the vapor consists of GeS monomers, GeS molecules are considered as evaporation units. GeS molecules in kink positions will evaporate preferentially. Close-packed steps are formed by traveling kinks which originate here at the crystals edges. We neglect the point defects along the steps.
Other sources of kinks are structural defects such as dislocations. Such defects may generate step patterns of high symmetry. For the sake of clarity the mechanism is oversimplified. The numerous macro-steps result from cleaving. When the wafer is heated the evaporation starts and continues at these cleavage steps. Irregular evaporation along the steps develops isolated terraces. The isolated terraces evaporate completely. A rather smooth and flat surface appears as a transient state.
Once more the initial state of evaporation. Nearly all steps maintain their step height during propagation over the surface. In the first stage of evaporation all terraces bounded by the cleavage steps disappear. The wafer has been exposed to air for two days. The evaporation starts at numerous spots along the cleavage steps. Propagation of the steps is seriously hindered since most of the kinks are pinned by oxygen. At various places flat-bottomed pits occur which spread across the surface. The pits are in many cases bounded by close-packed macro-steps.
The intersection of neighboring pits leads to large flat areas. Across those areas where the surface changes very irregularly the surface crumples up. Here the wafer evaporates beneath the surface.
This wafer has been in contact with air for over two days. Evaporation starts along the cleavage steps and at other places. Flat-bottomed pits are formed which spread preferentially into the direction with the smallest lattice constant. Grooves are formed along cleavage steps which are aligned close to the direction with the smallest lattice constant. Fast irregular changes of the patterns are caused by the evaporation beneath the surface. Pits and islands remain behind.
The morphology of the pits is revealed with a scanning electron microscope. The direction with the smallest lattice constant is termed 001.
The bounding steps are F-faces and orientated
in the 101 and 110 directions.
The preferred evaporation beneath the surface is exhibited by this SEM-micrograph. The oxidized layer prevented the surface from evaporating. The isolated terrace is elongated along 001. The bounding faces are the same F-faces as those found for the pits. The cleavage terraces have evaporated. The evaporation continues at distinct spots with the formation of pits. These flat-bottomed pits are observed only when the surface is flat and close to the exact cleavage plane. The initial micro-steps bunch near the source to form stable macro-steps with the contours of a rhombus. The shape of the rhombus is preserved during the spreading of the steps. The bright spots are ingrown precipitations of impurities. The propagation of the steps is not seriously affected by these precipitations. Steps are eliminated when neighboring pits intersect. New sources of shallow pits continually appear at random locations.
When the wafer was dipped into hot potash lye, these etch-pits were formed at dislocations. Thus the origin of that evaporation pit is located at a dislocation. The steps are only a few lattice constants in height and run parallel to F-faces. The structure defect might be an isolated one such as a dislocation loop or stacking fault since the origins of the flat-bottomed pits fluctuated locally.
Besides the shallow pits, pits with pointed bottoms are also observed. Initial micro-steps bunch into macro-steps and build up large symmetric terraces.
The origins of the pits stay at the same place or change continuously in one direction. Therefore, they are called stationary pits. The spreading steps have the same orientation as the shallow pits. The spreading is not noticeably affected by ingrown precipitated impurities which appear as bright spots.
A herringbone is formed by intersecting step trains which come from neighboring stationary pits. The step trains travel with remarkable steadiness in the direction of
the middle lattice constant. The trains and their sources are eliminated by irregular steps coming from other sources.
In many cases the steps of a stationary pit become irregular.
The intersection with either flat-bottomed pits or impurity precipitations might cause the serrated degeneration.
For a very high under-saturation the stationary pits develop dendritic step patterns. Steps spread very quickly in both directions of the smallest lattice constant. A groove is formed by steps which start at the centerline and spread slowly in opposite directions. Bunching of steps develops short-lived narrow terraces along the centerline.
The evaporation pattern is dramatically affected by the temperature. The stationary pits are now at 685 K. The temperature is suddenly increased by 50 K. The rhombic pattern of the pit changes to a dendritic one. Steps from the neighborhood move at high speed beyond the pit. When the final temperature is reached micro-steps in the center bunch to create curved macro-steps. The steps are rather smooth and spread with remarkable regularity across the surface. Shallow oval pits develop at random points along the terraces. These pits might arise from vacancy defects. The stationary oval pit is extended along 001.
The wafer was etched with hot potash lye. The etch-pits at the center points of the evaporation pits indicate dislocations.
For constant intermediate temperatures the steps of the stationary pit are rhombically arranged. The initially straight steps bunch while spreading to form rather irregularly shaped steps.
At temperatures as high as 750 K the evaporation occurs mainly at oval pits. In addition approximately rectangular pits are formed. With rising temperature the number of rectangular
pits increases enormously. The exposure frequency is increased from 1 f/s to 4 f/s. The temperature is 800 K which is about 130 K below the melting point. Possibly the pits are formed from vacancies.
Macroscopic ingrown precipitations affect the propagation of steps noticeably when the surface is misorientated to the ideal cleavage plane. Wedge-shaped elevated terraces are formed behind the precipitation.
Details on a precipitation are revealed with a scanning electron microscope. The precipitation consists of Ge-disulfide. On top of the disk-like precipitation a Ge-mono-sulfide layer is still intergrown.
The wedge-shaped terraces evaporate when the precipitations have disappeared.


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