C. C. Wang

Designer Ge quantum dots on Si: A heterostructure configuration with enhanced optoelectronic performance

An otherwise random, self-assembly of Ge quantum dots (QDs) on Si has been controlled by nano-patterning and oxidation to produce QDs with desired sizes, locations, and depths of penetration into the Si substrate. A heterostructure consisting of a thin amorphous interfacial oxide between the Ge QD and the Si substrate is shown to improve crystalline quality by de-coupling the lattice-matching constraint. A low dark current density of 1.1 μA/cm2 and a high photocurrent enhancement up to 35 000 and 1500, respectively, for 1.5 mW incident illumination at 850 nm and 1160 nm was measured on our Ge QD-based metal-oxide-semiconductor photodiodes.

Size-tunable strain engineering in Ge nanocrystals embedded within SiO2 and Si3N4

We report a unique ability to control the sign and size of the stress within Ge nanocrystals or nanodots fabricated using a complementary metal-oxide-semiconductor-compatible process within SiO2 and Si3N4 layers. Very large (as much as 4.5%), size-dependent compressive and tensile strains can be generated depending on whether the dot is embedded within either a Si3N4 or a SiO2 layer. Raman measurements reveal significant anharmonicity for smaller Ge dots and possible distortions of the diamond cubic lattice as evidenced by the measured Grunesien parameters and confirmed by their transmission electron diffraction patterns. Two completely different mechanisms are proposed to explain the formation of the tensile and compressive strain states, respectively.

High quality multifold Ge/Si/Ge composite quantum dots for thermoelectric materials

We present an effective approach to grow high-quality thin film of composite quantum dots (CQDs) as a building block for thermoelectric materials, in which 3 times the usual Ge deposition can be incorporated within a 3-fold CQD. Selective chemical etching experiments reveal that a thin Si inserted layer in the CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying. Such thin-film-like CQD materials are demonstrated to exhibit reduced thermal conductivity κ⊥ with respect to the conventional QDs, perhaps as a consequence of enhanced diffusive phonon scattering from the high Si/Ge interface density and enhanced local alloying effect.

The pivotal role of SiO formation in the migration and Ostwald ripening of Ge quantum dots

We report a unique, cooperative mechanism that involves the interplay of Ge, Si, and Oxygen interstitials enabling an unusual Ostwald ripening and migration behavior of Ge nanocrystallites and quantum dots (QDs) embedded within a SiO2 matrix. In the presence of high Si interstitial fluxes with no supply of oxygen interstitials, the oxide surrounding the Ge QDs is decomposed by the Si interstitials, creating the volatile SiO reaction product and hence voids that enable the Ge QDs to grow by Ostwald ripening. When both Si and Oxygen interstitials are present in high concentrations, the Ostwald ripened Ge QD is further able to migrate towards the source of the Si interstitials. The QD movement occurs by virtue of the fact that the SiO created in front of the QD combines with O interstitials to regenerate SiO2 behind the Ge QD on its migration path. Thus, SiO influences the migration and Ostwald ripening behavior of the Ge QDs via a unique “Destruction-Construction” mechanism.

CMOS-Compatible Generation of Self-Organized 3-D Ge Quantum Dot Array for Photonic and Thermoelectric Applications

We demonstrate a CMOS-compatible scheme, selective oxidation of SiGe pillars, for creating well-organized 3-D Ge quantum dot (QD) array by guiding QDs migration along the oxidation path and thus placing them on targeted locations where the ultimate oxidation occurs. Stacked QDs exhibit tunable luminescence over the visible and possess low thermal conductivity, showing promise for nanophotonic and energy conversion devices.

Large reduction in thermal conductivity for Ge quantum dots embedded in SiO2 system

Thermal conductivity (k(T)) of Ge quantum dots (QDs) embedded in SiO2 was investigated at T = 100–400 K. The Ge QD/SiO2 system appears to have much lower k(T) than their counterparts of bulk Ge and SiO2, and the reduction factor increases with the surface-to-volume ratio of the QD in SiO2. Attendant to reduced magnitude includes delayed Umklapp decline and weaker dependence on temperature for k(T). Effective medium analysis suggests the reduction in k primarily comes from the decreased group velocity thanks to the QD inclusion that induces interfacial stress on SiO2, phonon confinement, and boundary scatterings.

The role of Si interstitials in the migration and growth of Ge nanocrystallites under thermal annealing in an oxidizing ambient

We report a unique growth and migration behavior of Ge nanocrystallites mediated by the presence of Si interstitials under thermal annealing at 900°C within an H2O ambient. The Ge nanocrystallites were previously generated by the selective oxidation of SiGe nanopillars and appeared to be very sensitive to the presence of Si interstitials that come either from adjacent Si3N4 layers or from within the oxidized nanopillars. A cooperative mechanism is proposed, wherein the Si interstitials aid in both the migration and coarsening of these Ge nanocrystallites through Ostwald ripening, while the Ge nanocrystallites, in turn, appear to enhance the generation of Si interstitials through catalytic decomposition of the Si-bearing layers.

The pivotal role of oxygen interstitials in the dynamics of growth and movement of germanium nanocrystallites

We report an unusual “symbiotic” behavior of oxygen interstitials acting in concert with Ge and Si interstitials at high temperature inducing morphology changes and autonomous migration of Ge nanocrystallites within SiO2/Si3N4 layers. The Ge nanocrystallites were originally generated by the selective oxidation of SiGe nano-pillars grown and lithographically patterned over buffer Si3N4 layers on Si substrates. The coalescence and movement of these Ge nanocrystallites appear to be very sensitive to the presence and flux of oxygen interstitials especially at the Ge nanocrystallite/buffer Si3N4 interface. A range of different morphologies are observed for Ge nanocrystallites that are directly attributable to the influence of oxygen interstitial concentration and consequently the interstitial Si and Ge concentrations. In combination with Si and Ge interstitials, oxygen interstitials activate the coalescence of sparsely-distributed Ge nanocrystallites and concurrently their migration towards the source of Si interstitials, i.e. the buffer Si3N4 layers, through catalytically-enhanced local decomposition and subsequent oxidation of both the SiO2 and Si3N4 buffer layers. We also show that these symbiotic effects are “tunable” by increasing the Ge content of the SiGe nano-pillars. Dense distributions of Ge nanocrystallites generated from high Ge content SiGe nano-pillars remain static and they show no changes in their morphology possibly because oxygen interstitials are simply unable to penetrate these clusters and consequently incapable of inducing symbiotic Si and Ge interstitial generation.

Designer Ge quantum dots Coulomb blockade thermometry

A Coulomb blockade (CB) thermometer has been experimentally demonstrated based on the temperature dependence of a Ge quantum-dot (QD) single-hole transistor (SHT). The Ge-QD SHT features distinctive current peaks/plateaus, sharp differential conductance (G_D) dips up to temperature 120K. The full-width-at-half minimum, V_1/2, of the G_D dips directly scale with temperature following the material parameter-independent equation of eV_1/2 ~ 5.44k_BT, providing the primary thermometric quantity. Also the depths of the G_D dips increases with 1/k_BT as expected from CB theory of ΔG_D/G_D0 = E_C/6k_BT. This experimental demonstration indicates that our Ge-QD SHT offers an effective building block for ultrasensitive CB primary thermometers with the detection temperature as high as 115K.

Size Effects on Phase Formation and Electrical Robustness of Nickel Silicide Nanowires

We reported that the morphology, phase formation, and electrical resistivity of the NixSiy NW have strong dependences on the geometrical size of the as-formed Si NW before silicidation. The electrical current stressing generated selfheating and thus local compressive stress buildup and relaxation near the locations where the NixSiy NW ruptured.