John M. MaCaulay

ELECTRON FIELD EMISSION FROM DIAMOND AND OTHER CARBON MATERIALS AFTER H2, O2 AND CS TREATMENT

This letter reports, diamond field emitters, Cs treated, air stable, that emit electrons at the lowest reported field, <0.2 V μm−1. Field emission from B‐, Li‐, P‐, and N‐doped diamonds and carbonized polymer was characterized as a function of surface treatment. A treated with an O2 plasma, coated with Cs, heated, and exposed to O2 exhibited increased emission for all samples except for B‐doped diamond. The best emission was obtained from N‐doped diamond samples, followed by carbonized polymer, the Li‐doped, and polycrystalline P‐doped diamond. Li‐ and N‐doped samples treated with Cs were stable in laboratory air for several days. This stability of the surface‐activated diamond is believed to be due to the formation of a diamond–O–Cs salt. If the sample is treated with a H2 plasma instead of an O2 plasma, the Cs‐enhanced emission degrades with heat and exposure to O2. Subbands formed by Li and N impurities are believed to be responsible for this enhanced emission. The surface treatment on N‐doped diamond ...

Luminescence and structural study of porous silicon films

The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as‐prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≊690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggest...

Pore morphology and the mechanism of pore formation in n‐type silicon

Porous layers formed in n‐type silicon are characterized by pores of square cross section oriented perpendicular to the (100) plane. The pore walls are defined by the {011} planes in the [100] zone with secondary pores propagating from the main pores in the 〈010〉 and 〈001〉 directions. The pores are located in an array with a characteristic spacing. The characteristic pore morphology during stable pore propagation is explained in terms of a tunneling mechanism due to a high‐field region created by the curvature at the pore front. The pore spacing is determined by the distance at which the region between the pores becomes fully depleted.

Influence of stress on the photoluminescence of porous silicon structures

The blueshifting of photoluminescence spectra of porous silicon structures formed in p‐type silicon is shown to be related to stresses in the porous material. A characteristic cellular structure, with varying length scale, is observed in the high porosity films due to high surface stresses. The cellular structure is not formed during the secondary open‐circuit etching procedure itself but occurs during evaporation of the electrolyte after removal of the porous silicon from the etching solution.

Cesiated thin-film field-emission microcathode arrays

The effect of adsorbed cesium on the performance of thin‐film field‐emission microcathode arrays with molybdenum tips is discussed. A reduction in the voltage required to extract an average of 1 nA per tip from 50 to 15 V is reported. Cesiated arrays have exhibited stable performance when operated continuously for up to 650 h at array current densities up to 100 A/cm2, with minimal leakage current. Using the Fowler–Nordheim theory, analysis of current‐voltage characteristics of a heavily cesiated emitter array gives values for the emitting area, work function, and geometrical factor.

Crystallographic aspects of pore formation in gallium arsenide and silicon

Abstract The structure of porous layers formed in n-type GaAs is characterized and compared with the more familiar structure of porous n-type Si. Pores in n-type GaAs run in 〈111〉a directions and have triangular or hexagonal cross-sections; their size and degree of branching depend on the doping level and current density. The characteristic differences between porous GaAs and porous Si are explained by a model in which we consider the bonding configuration of atoms on steps. Kinks and terraces on the interior of the pore and the spatial ditribution rate-limiting electrochemical reactions.

Characterization of arsenic doping profile across the polycrystalline Si/Si interface in polycrystalline Si emitter bipolar transistors

The combination of Z‐contrast scanning transmission electron microscopy, transmission electron microscopy, and secondary ion mass spectroscopy allows the most accurate determination, to date, of the As doping profile across the polycrystalline Si/Si interface of an npn polycrystalline Si emitter bipolar transistor. We measure a peak in the As doping profile which is coincident with the polycrystalline Si/Si crystallographic interface and is approximately 40 A full width at half‐maximum. There is a uniform As dopant level in the polycrystalline Si emitter of 2×1020 cm−3 and an estimated maximum As concentration of 5×1020 cm−3 in the peak at the interface.

Current gain enhancement in bipolar transistors by low‐energy ion beam modification of the polycrystalline silicon emitter

The current gain of polycrystalline silicon emitter transistors has been improved by modification of the polycrystalline silicon/silicon interface using a low‐energy ion beam. The base current is reduced by a factor of 3 while maintaining ideal current‐voltage characteristics. No change in the collector current and the emitter series resistance is observed.