R.A.D. Mackenzie

Compositional homogeneity of metalorganic chemical vapor deposition grown III‐V compound semiconductor epilayers

The compositional homogeneity of epitaxially grown layers of GaInAs, AlInAs, and GaAlInAs on InP substrates has been investigated using pulsed laser atom probe techniques. All the material characterized was shown by transmission electron microscopy to have a fine‐scale contrast variation, however only some of the samples were found, using atom probe techniques, to show distinct deviations from compositional uniformity. The average composition measured from each of the layers was that of the lattice‐matched composition intended during growth, however the composition varied locally, on a scale of typically 10–20 nm, from the mean composition by up to 5 at. %.

Atom probe analysis and field emission studies of silicon

Single ungridded silicon tips have been studied by field emission current–voltage characterization, field emission imaging, field ion imaging, and pulsed laser atom probe (PLAP) analysis. Changes in field emission characteristics were observed as a thin contaminant layer (oxygen, hydrogen, carbon, and carbon monoxide) formed on the surface of clean field evaporated silicon tips. Removal of the contaminant layer by pulsed laser‐assisted field desorption restored the original field emission characteristics. Clean silicon tips that were oxidized in air began to emit at half the voltage required for unoxidized tips. As the voltage was increased, a sudden irreversible increase in field emission current was observed. PLAP analysis and field emission imaging showed that this effect was caused by the disruption of the oxide layer shortly after the onset of field emission. After the oxide disruption, field emission occurred from regions of sharp local geometry around the circumference of the tip. Removal of these ...

Field emission and atom probe field ion microscope studies of palladium‐silicide‐coated silicon emitters

Palladium‐silicide‐coated field emitters were prepared by palladium thermal evaporation onto silicon tips, followed by annealing at 675 K. Pulsed laser atom probe analysis showed a clean near stoichiometric, Pd2Si layer with a thin silicon‐enriched surface. In spite of the higher work function of palladium silicide compared to silicon, enhanced emission was obtained from the silicide‐coated tips. Field ion and field emission imaging of the tips showed that this enhanced emission was due to the roughness of the silicide layer. Pulsed laser atom probe analysis of a palladium silicide tip exposed to air for four days showed that the silicon‐rich surface layer was converted to an oxidized layer 2–4 monolayers thick. The field emission characteristics of silicide tips exposed to air were similar to those observed previously in oxidized silicon tips. After an initial current burst which corresponded to the disruption of the oxide layer, intense emission was observed from sharp areas around the circumference of ...

CHARACTERIZATION OF GRIDDED FIELD EMITTERS

A system combining field emission microscopy, field ion microscopy, atom probe, and current–voltage (I–V) measurement facilities has been developed to study gridded field emitter devices, either in the form of single tips or arrays. (I–V) measurement results, field emission images, and field ion images of a single tip and an array of 1500 tips are presented here, of which field ion images of the gridded emitters are reported for the first time. The first atom probe results from a gridded array are also given here, demonstrating that chemical composition analysis of gridded field emission cathodes is possible.

Ultrahigh resolution characterisation of compound semiconductors using pulsed laser atom probe techniques

Abstract The pulsed laser atom probe has been used to characterise III-V semiconductor epilayer and quantum-well structures. The combination of high spatial resolution and good chemical specificity makes it possible to observe very fine scale composition variations in epilayer materials, and also to assess the roughness of individual interfaces in quantum-well material.

Atom probe field ion microscope studies of palladium silicide on silicon

Palladium silicide coated silicon specimens have been formed by thermally evaporating palladium onto clean field evaporated n-type silicon (100) tips. The thin palladium overlayer is reacted to form palladium silicide by thermal annealing at 675 K. The tips were studied by field ion imaging, pulsed laser atom probe and field emission current-voltage characterisation. These analyses were carried out in a modified atom probe field ion microscope which allows all these techniques to be employed without breaking vacuum. The stoichiometry of the 2–3 nm thick silicide layer was close to that expected for Pd2Si. All analyses showed a thin silicon rich layer on top of the palladium silicide. Fowler-Nordheim plots of the field emission characteristics of silicide coated tips were obtained as the layer was removed by field evaporation. Analysis of the slopes and intercepts of these plots indicates that the silicide coated tips have a rougher surface than the silicon tips. This was confirmed by field ion microscope and field emission microscope images which showed localised electron emission from the sharpest areas of the tip. PLAP analysis of a tip exposed to air for 4 days showed an oxidised silicon rich surface layer which was 2–4 monolayers thick.

Development of a FEM, FIM-AP system for studying gridded vacuum microelectronic devices

Abstract We have modified a conventional FIM-AP system specifically for studying vacuum microelectronic (VME) devices, which includes facilities for field emission microscopy (FEM), field ion microscopy (FIM) atom probe microanalysis (AP) and current-voltage measurements. This is the first integrated experimental system for the characterisation of VME devices on the atomic scale. In this paper the behaviour of a typical gridded field emitter in different operation modes is discussed using a simple simulation model, and is compared with that of a conventional FIM tip. The simulation shows that gridded emitters still preserve the projection geometry of FIM and FEM which is essential to the interpretation of FIM and FEM images. The design considerations for an ideal FIM/FEM/AP system for studying VME devices are also discussed. A wide view angle detector and low voltage operation are the two key elements. Our system has been tested on silicon-based VME devices. The results show that in-situ chemical analysis and FIM images with atomic resolution are achieved. The geometrical field factor k of the tested VME devices is found to be in the range of 1–2. This confirms that gridded VME devices have much better field enhancement factors than conventional field emitters.

Characterization of nanometer-scale compositional variations using field ion microscopy and the position sensitive atom probe

Abstract The position sensitive atom probe, POSAP, is shown to be capable of resolving complicated nanometer-scale structures in multiphase materials. The combination of high spatial resolution with high chemical specificity makes the POSAP ideal for characterizing many nanostructured materials. It has been applied to a wide range of metallic and semiconductor systems and is now being used in the characterization of nanocrystalline materials.

Pulsed Laser Atom Probe Analysis of III-V Compound Semiconductor Epilayers.

The pulsed laser atom probe has been used to characterise the fine scale chemistry of a range of III-V ternary and quaternary compound semiconductors (GaInAs, AlInAs, GaAlInAs) grown, using MOCVD techniques, on indium phosphide substrates. It has been observed that there are fine scale chemical fluctuations in some specimens on a scale of typically 10–20 nm. The fluctuations appear to be a result of localized clustering of the group III components in the epilayer. In quaternary material there is evidence for different degrees of clustering for different components. It is suggested that this compositional fluctuation is a consequence of clustering occuring above a miscibility gap. The existence of a TEM contrast mechanism inherent to the material has the effect of making TEM an unreliable indicator of fine scale compositional variations in these systems.