Effect of pulsed laser action on hole-energy spectrum of $\mathrm{Ge}∕\mathrm{Si}$ self-assembled quantum dots

Space-charge spectroscopy has been used to study the hole energy spectrum of an array of Ge quantum dots (QD’s) coherently embedded in a Si matrix and subjected to a ruby laser $(\lambda =694\mathrm{nm})$ nanosecond irradiation ex situ. The laser energy density in a single pulse was near the melting threshold of the Si surface. The number of laser pulses was varied from 1 to 10, and the duration of each pulse was $80\mathrm{ns}$. From the capacitance-voltage characteristics, temperature- and frequency-dependent admittance measurements, the energies of holes confined in Ge QD’s were determined. The pulsed laser annealing was found to result in a deepening of the hole energy level relative to the bulk Si valence band edge and in a decrease of the hole energy dispersion. After the treatment with ten laser pulses, the spread of the hole energies due to varying sizes of the QD’s within the ensemble was reduced by a factor of about 2. The obtained results give evidence for a substantial reduction of the QD’s size dispersion and for a narrowing distribution of the hole energy levels stimulated by nanosecond laser irradiation. A possible explanation of the improved uniformity of QD’s sizes involves dissolving small size Ge QD’s in a Si matrix by pulsed laser melting of the Ge nanoclusters and their subsequent intermixing with surrounding solid Si.

Figure 1

The valence band profile of a Si Schottky diode with a layer of Ge quantum dots.

Figure 2

Raman spectra of Ge QD samples before and after pulsed laser annealing.

Figure 3

Capacitance-voltage characteristics measured at modulation frequency of $100\mathrm{kHz}$ for the reference and dot samples before and after pulsed laser processing. Experimental data are shown by symbols and model simulations by solid lines.

Figure 4

Temperature dependencies of the conductance measured at a bias voltage of $2\mathrm{V}$ and modulation frequency of $100\mathrm{kHz}$ for the reference and dot samples before and after pulsed laser treatment.

Figure 5

Contour plots of (a) temperature-frequency and (b) temperature-bias conductance mapping for as-grown dot sample.

Figure 6

The Arrhenius plots of the hole emission rate obtained from $G\text{--}T$ spectra with different bias voltages for the as-grown sample.

Figure 7

Bias dependent activation energies of hole emission rate before and after pulsed laser processing.