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Germanium nanocrystal density and size effects on carrier storage and emission
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Image of FIG. 1.
FIG. 1.

(a) TEM image (cross sectional view) of Ge NCs formed by annealing of an amorphous Ge layer (0.5 nm thickness) deposited on an ultrathin (5 nm thickness) during 30 min at and capped by 18 nm of amorphous silicon, (b) HRTEM image (cross sectional view) of a Ge NC where the distance between {111} plans are evidenced, and (c) a typical HRTEM image showing a truncated spherical form of an individual Ge-NC.

Image of FIG. 2.
FIG. 2.

(a) Band diagram of the structure showing the confinement in NC between silicon dioxide and layers; the conduction and valence band bendings are not shown here. (b) Energy band gap of spherical Ge NCs versus NC size; the full line corresponds to analytic expression combining Eqs. (1)–(3) extracted from Ref. 2.

Image of FIG. 3.
FIG. 3.

A schematic cross section of the studied samples.

Image of FIG. 4.
FIG. 4.

(a) characteristics for each studied sample at room temperature. Steps are obtained in curve for smallest NC mean size . represents the bias applied to the gate with respect to the substrate. (b) Temperature effects on characteristics for the sample .

Image of FIG. 5.
FIG. 5.

Typical result showing hysteresis in high frequency (1 MHz) characteristics for three temperatures. We notice the increase in when the temperature decreases. Band diagrams are quoted in the inset for the accumulation and inversion regimes showing the electron capture and emission processes.

Image of FIG. 6.
FIG. 6.

Electron exchange described by as a function of the deposited Ge layer thickness at different temperatures. Temperature effect is evidenced.

Image of FIG. 7.
FIG. 7.

Average number of electrons stored in each NC as a function of NC size for various temperatures.

Image of FIG. 8.
FIG. 8.

Emission efficiency plotted in an Arrhenius diagram as a function of 1000/T. Thermal activation energies are deduced and quoted in the inset as a function of NC mean diameter.

Image of FIG. 9.
FIG. 9.

Thermal activation energy as a function of NC band gap and diameter . is well fitted by the linear law: .


Generic image for table
Table I.

Parameters summarized in the table are the deposited Ge layer thicknesses , the mean Ge-NC diameter, and density of each studied sample.

Generic image for table
Table II.

The capacitance of individual spherical NC, its charging energy, and the threshold temperature from which the Coulomb blockade effect does occur are calculated for each sample. The results are given as a function of the mean diameter of NCs. The temperature quoted in the table allows us to expect the Coulomb blockade in and . Actually, we observe experimentally this process only in .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Germanium nanocrystal density and size effects on carrier storage and emission