I. Crupi

Light absorption in silicon quantum dots embedded in silica

The photon absorption in Si quantum dots (QDs) embedded in SiO2 has been systematically investigated by varying several parameters of the QD synthesis. Plasma-enhanced chemical vapor deposition (PECVD) or magnetron cosputtering (MS) have been used to deposit, upon quartz substrates, single layer, or multilayer structures of Si-rich-SiO2 (SRO) with different Si content (43–46 at. %). SRO samples have been annealed for 1 h in the 450–1250 °C range and characterized by optical absorption measurements, photoluminescence analysis, Rutherford backscattering spectrometry and x-ray Photoelectron Spectroscopy. After annealing up to 900 °C SRO films grown by MS show a higher absorption coefficient and a lower optical bandgap (∼2.0 eV) in comparison with that of PECVD samples, due to the lower density of Si–Si bonds and to the presence of nitrogen in PECVD materials. By increasing the Si content a reduction in the optical bandgap has been recorded, pointing out the role of Si–Si bonds density in the absorption proce...

How far will silicon nanocrystals push the scaling limits of NVMs technologies

For the first time, memory devices with optimized high density (2E12#/cm/sup 2/) LPCVD Si nanocrystals have been reproducibly achieved and studied on an extensive statistical basis (from single cell up to 1 Mb test-array) under different programming conditions. An original experimental and theoretical analysis of the threshold voltage shift distribution shows that Si nanocrystals have serious potential to push the scaling of NOR and NAND flash at least to the 35 nm and 65 nm nodes, respectively.

Self-assembled silver nanoparticles for plasmon-enhanced solar cell back reflectors: correlation between structural and optical properties

The spectra of localized surface plasmon resonances (LSPRs) in self-assembled silver nanoparticles (NPs), prepared by solid-state dewetting of thin films, are discussed in terms of their structural properties. We summarize the dependences of size and shape of NPs on the fabrication conditions with a proposed structural-phase diagram. It was found that the surface coverage distribution and the mean surface coverage (SC) size were the most appropriate statistical parameters to describe the correlation between the morphology and the optical properties of the nanostructures. The results are interpreted with theoretical predictions based on Mie theory. The broadband scattering efficiency of LSPRs in the nanostructures is discussed towards application as plasmon-enhanced back reflectors in thin-film solar cells.

Electroluminescence and transport properties in amorphous silicon nanostructures

We report the results of a detailed study on the structural, electrical and optical properties of light emitting devices based on amorphous Si nanostructures. Amorphous nanostructures may constitute an interesting system for the monolithic integration of optical and electrical functions in Si ULSI technology. In fact, they exhibit an intense room temperature electroluminescence (EL), with the advantage of being formed at a temperature of 900 °C, while at least 1100 °C is needed for the formation of Si nanocrystals. Optical and electrical properties of amorphous Si nanocluster devices have been studied in the temperature range between 30 and 300 K. The EL is seen to have a bell-shaped trend as a function of temperature with a maximum at around 60 K. The efficiency of these devices is comparable to that found in devices based on Si nanocrystals, although amorphous nanostructures exhibit peculiar working conditions (very high current densities and low applied voltages). Time resolved EL measurements demonstrate the presence of a short lifetime, only partially due to the occurrence of non-radiative phenomena, since the very small amorphous clusters formed at 900 °C are characterized by a short radiative lifetime. By forcing a current through the device a phenomenon of charge trapping in the Si nanostructures has been observed. Trapped charges affect luminescence through an Auger-type non-radiative recombination of excitons. Indeed, it is shown that unbalanced injection of carriers (electrons versus holes) is one of the main processes limiting luminescence efficiency. These data will be reported and the advantages and limitations of this approach will be discussed.

High-efficiency silicon-compatible photodetectors based on Ge quantum dots

We report on high responsivity, broadband metal/insulator/semiconductor photodetectors with amorphous Ge quantum dots (a-Ge QDs) as the active absorbers embedded in a silicon dioxide matrix. Spectral responsivities between 1–4 A/W are achieved in the 500–900 nm wavelength range with internal quantum efficiencies (IQEs) as high as ∼700%. We investigate the role of a-Ge QDs in the photocurrent generation and explain the high IQE as a result of transport mechanisms via photoexcited QDs. These results suggest that a-Ge QDs are promising for high-performance integrated optoelectronic devices that are fully compatible with silicon technology in terms of fabrication and thermal budget.

Nanocrystal metal-oxide-semiconductor memories obtained by chemical vapor deposition of Si nanocrystals

We have realized nanocrystal memories by using silicon quantum dots embedded in silicon dioxide. The Si dots with the size of few nanometers have been obtained by chemical vapor deposition on very thin tunnel oxides and subsequently coated with a deposited SiO2 control dielectric. A range of temperatures in which we can adequately control a nucleation process, that gives rise to nanocrystal densities of ∼3×1011 cm−2 with good uniformity on the wafer, has been defined. The memory effects are observed in metal-oxide-semiconductor capacitors or field effect transistors by significant and reversible flat band or threshold voltage shifts between written and erased states that can be achieved by applying gate voltages as low as 5 V. The program-erase window does not exhibit any change after 105 cycles on large area cells showing that the endurance of such a memory device which uses a thinner tunnel oxide is potentially much higher than that of standard nonvolatile memories. Moreover, good retention results are ...

Silicon-Based Light-Emitting Devices: Properties and Applications of Crystalline, Amorphous and Er-Doped Nanoclusters

In this paper, we summarize the results of an extensive investigation on the properties of MOS-type light-emitting devices based on silicon nanostructures. The performances of crystalline, amorphous, and Er-doped Si nanostructures are presented and compared. We show that all devices are extremely stable and robust, resulting in an intense room temperature electroluminescence (EL) at around 900 nm or at 1.54 mum. Amorphous nanoclusters are more conductive than the crystalline counterpart. In contrast, nonradiative processes seem to be more efficient for amorphous clusters resulting in a lower quantum efficiency. Erbium doping results in the presence of an intense EL at 1.54 mu m with a concomitant disappearance of the 900-nm emission. This suggests that under electrical pumping Er is excited through an efficient energy transfer from the silicon clusters which hence become dark. We have identified an Auger de-excitation of Er with trapped carriers as the main process competing with radiative emission and limiting EL efficiency. This process is particularly severe in presence of unbalanced carrier injection (electrons versus holes) and can be controlled in properly designed structures. These data are presented and their implications are discussed

Photonic-crystal silicon-nanocluster light-emitting device

We report on enhanced light extraction from a light-emitting device based on amorphous silicon nanoclusters, suitable for very-large-scale integration, and operating at room temperature. Standard low-cost optical lithography is employed to fabricate a two-dimensional photonic crystal onto the device. We measured a vertical emission with the extracted radiation enhanced by over a factor of 4, without the aid of any buried reflector. These achievements demonstrate that a cost-effective exploitation of photonic crystals is indeed within the reach of semiconductor industry and open the way to a new generation of nanostructured silicon devices in which photonic and electronic functions are integrated together.

Transient photoresponse and incident power dependence of high-efficiency germanium quantum dot photodetectors

We report a systematic study of time-resolved and power-dependent photoresponse in high-efficiency germanium quantum dot photodetectors (Ge-QD PDs), with internal quantum efficiencies greater than 100% over a broad wavelength, reverse bias, and incident power range. Turn-on and turn-off response times (τon and τoff) are shown to depend on series resistance, bias, optical power, and thickness (WQD) of the Ge-QD layer, with measured τoff values down to ∼40 ns. Two different photoconduction regimes are observed at low and high reverse bias, with a transition around −3 V. A transient current overshoot phenomenon is also observed, which depends on bias and illumination power.

Pd/Au/SiC Nanostructured Diodes for Nanoelectronics: Room Temperature Electrical Properties

Pd/Au/SiC nanostructured Schottky diodes were fabricated embedding Au nanoparticles (NPs) at the metal-semiconductor interface of macroscopic Pd/SiC contacts. The Au NPs mean size was varied controlling the temperature and time of opportune annealing processes. The electrical characteristics of the nanostructured diodes were studied as a function of the NPs mean size. In particular, using the standard theory of thermoionic emission, we obtained the effective Schottky barrier height (SBH) and the effective ideality factor observing their dependence on the annealing time and temperature being the signature of their dependence on the mean NP size. Furthermore, plotting the effective SBH as a function of the effective ideality factor we observe a linear correlation, indicating that the Au NPs act as lateral inhomogeneities in the Schottky diodes according to the Tungs model. Therefore, we can control the size, fraction of covered area, and surface density of such intentionally introduced inhomogeneities. The application of the Tungs model for the electronic transport in inhomogeneous Schottky contacts allow us to obtain, in particular, the homogeneous SBH. These nanostructured diodes are proposed as possible components of integrated complex nanoelectronic devices.