Nanocrystalline/microcrystalline thin films prepared at relatively low temperatures by plasma-enhanced chemical vapor deposition (PECVD), in particular hydrogenated microcrystalline Si films (μc-Si:H), have attracted an increasing attention not only as potential materials for thin film solar cells, but also as active layers in thin film transistor arrays for flat panel displays. This paper reviews recent progress in the investigation of these materials; preparation methods, structural and optical properties, and electronic transports. Emphasis is placed on the understanding of the growth mechanism of μc-Si:H films as well as the microscopic characterization of the film structure.

We review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 Å in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ∼0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.

Porous semiconductors constitute a class of material that exhibit surprising properties, are quite easy to fabricate, but are however fragile, complex, and difficult to characterise. This tutorial review extracts specific topics from the large knowledge base now available on porous Si that are deemed relevant to other porous semiconductors beginning to receive study. It also highlights topics where controversy is now resolved, where problems remain, and where further effort could be focused.

Molecular dynamics simulations are carried out for silicon clusters to analyze the effect of heating and cooling on their structural properties. The calculations are based on the Kaxiras and Pandey potential which has been derived from a microscopic calculation with the density functional method. Results for the cluster formation are presented for different rates of cooling and heating. Our simulations clearly show the dependence of the patterns on the cooling and heating rates as well as the way in which these clusters evolve. Our results indicate that the structures are most stable when the rate of loss of thermal energy is slow and the potential energy is lowest.

The atomic and electronic structures of Si6H2m and Si6H+ 2m+1 clusters have been investigated in the framework of density-functional theory. For both neutral and ionized clusters we found the structure to belong to one of four distinct structural families. A molecular-orbital picture of hydrogenation is presented. From the calculated formation energies of these clusters, we infer the relative stability of the different structural families discussed.

The occurence of confinement effects for amorphous Si and Si:H clusters is investigated theoretically in an empirical tight binding treatment. The results show that one must consider three categories of states: strongly localized, weakly localized and delocalized. In all cases a substantial blue shift with reduction in size is obtained, of comparable magnitude for amorphous a-Si:H and crystalline clusters. These results are compared with available experimental data

We studied the effect of H, O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10–25 Å are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the absorption threshold. These results are important for the discussions concerning absorption and luminescence in porous Si.

Micro and nano-structures have opened a new area in materials research since they present interesting phenomena such as efficient luminescence and localization of carriers. An important example of these new materials is porous silicon (PS). It is considered that the quantum confinement is an essential cause of the opto-electronic properties of PS [1], thus microscopic analysis should be performed. We have developed a supercell model to study PS with a tight-binding Hamiltonian, where an sp 3 s* basis set is used. In an otherwise perfect silicon structure empty columns of atoms are produced and passivated with hydrogen atoms [2]. In this work we calculate the dielectric function and compare it against experimental data for bulk c-Si, ultrathin c-Si films and PS. We discuss the importance of considering the relaxation of the electron wavevector (k) conservation in order to include disorder effects in PS.

Pressure effects on silicon nanocrystallites are calculated using semi-empirical tight-binding and ab-initio local density calculations. Using the confinement model in porous silicon a red shift of the luminescence energy with increasing pressure is obtained. Quantum confinement in BC8 phase silicon nanocrystallites obtained after release of high pressure are also studied. It increases the cluster gap and also enhances the electron-hole radiative recombination rate.

The use of porous silicon (PS) in fabricating optoelectronic devices is in progress. However, the performance of such applications still needs to be improved. In particular, in solar cells heat must be dissipated to avoid a decay in their efficiency and one of the properties of PS that must be evaluated is the effective thermal conductivity. It is well known that the thermal conductivity of silicon is temperature dependent. Thus we cannot use a standard effective medium approach to obtain its effective thermal response. In this work, we extend the averaging volume and surface methods [1] to consider nonlinear effects in the effective transport coefficients. We model PS as composed of c-Si cylindrical columns covered by different overlayers (i.e. a-Si or SiO2) immersed in another medium. In our model the effective thermal conductivity has an explicit dependence on the temperature gradient. We present a parametric analysis of the model, compare it with the c-Si behavior and evaluate the importance of the nonlinear contribution.

High-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. We have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdSxSe1 x, CdTe, ZnS, ZnSe) in fused silica, Al2O3 and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.

We have experimentally studied the photoluminescence (PL) properties of Si clusters in SiO2 glassy matrices. Si clusters in the SiO2 matrices were fabricated by Si+ ion implantation into SiO2 glasses and then thermally annealed in forming gas. Broad PL peaks are observed in the visible spectral region at room temperature. Resonantly excited PL spectra indicate that the strong coupling of excitons and stretching vibrations of the Si-0 bonds causes the broad luminescent spectra. It is concluded that the interaction between electronic and vibrational excitations controls the luminescent emission and the observed dynamics.

The annealing behavior of silicon implanted SiO2 layers is studied using continuous and time-gated photoluminescence (PL). Two PL emission bands are observed. A band centered at 560 nm is present in as implanted samples and it is still observed after 1000 °C annealing. The emission time is fast (0.2 -2 ns). A second band centered at 780 nm further increases when hydrogen annealing was performed. The emission time is long (1 μs - 0.3 ms).

In previous work we reported on the observation of continuously tunable photoluminescence in Si+-implanted SiO2-films with moderate intensities. In this paper we demonstrate improved performance of such samples. The photoluminescence intensity increases abruptly up to two orders of magnitude when the anneal temperature is elevated to values higher than 1000…1100°C. This strong photoluminescence degrades less than that of porous silicon. Very fine tunability in the spectral range from 2.1 eV to 1.3 eV is achieved in samples implanted with a graded dose. In the analysis of the results we try to distinguish between the contributions of the Si-nanocrystals and of the oxide related defects.

We present a systematic analysis of SiOx alloys films containing Ge nanocrystals prepared by dc magnetron sputtering. Increasing the sputtering power (50–175W) reduces the average Ge nanocrystal size exponentially down to ∼2nm. Broad band photoluminescence spectra are observed in the visible at room temperature centered at ∼3eV or/and ∼2eV. Neither the 3eV nor the 2eV luminescence can be correlated to the change in the size. Excluding the quantum-confined origin, the presence of a luminescence center located in the inhomogeneous strain field of the Ge nanocrystal surface is discussed.

We report nanometer-sized silicon germanium (SiGe) alloy crystallites prepared by excimer laser ablation in constant-pressure inert gas. Size distribution of the SixGel x ultrafine particles decreases with decreasing x under fixed conditions of deposition such as ambient gas pressure. Raman scattering spectra of the deposited SiGe ultrafine particles show three peaks intrinsic to crystalline SiGe alloys, and the linewidths of these peaks broaden due to the reduced size of the crystallites. Furthermore, a visible photoluminescence (PL) band with a peak at around 2.2 eV is obtained at room temperature after an annealing process.

The luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal Nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal Nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar in magnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected in a quantum confined system.

Applying laser ablation technique, we have synthesized two types of SiO2 films that include nanometer-sized Si particles. One is synthesized by alternative deposition of Si nanoparticles layers and SiO2 layers. The synthesized film exhibits red photoluminescence (PL) with a peak energy below 1.5 eV. The other is synthesized by annealing at 1000°C of SiO x films, which are formed by laser ablation in diluted O2 gas. We find that there is a narrow range of composition for efficient red PL. Based on the experimental results, we tentatively discuss a possible model for the origin of the red PL.

Weblike aggregates of coalesced Si nanocrystals are produced by a laser vaporization -controlled condensation technique. SEM micrographs show particles with ∼ 10 nm diameters but the Raman shift suggests the presence of particles as small as ∼ 4 nm. FTIR of the freshly prepared particles shows weak peaks due to the stretching, bending and rocking vibrations of the Si-O-Si bonds indicating the presence of a surface oxidized layer SiOx (x<2).

The particles show luminescence properties that are similar to those of porous Si and Si nanoparticles produced by other techniques. The nanoparticles do not luminesce unless, by exposure to air, they acquire the SiOx passivated coating. They show a short-lived blue emission characteristic of the SiO2 coating and a biexponential longer-lived red emission. The short lifetime component of the red emission, about 12 μs, does not depend on emission wavelength. The longer-lived component has a lifetime that ranges from 90 to over 130 μs (at 300 K), increasing with emission wavelength. The results are consistent with the quantum confinement mechanism as the source of the red photoluminescence.

Magnetic properties of photoluminescing spark-processed silicon (sp-Si) have been investigated for the first time. Contrary to the diamagnetic signal known for bulk silicon, sp-Si displays a paramagnetic resonance as well as a ferromagnetic hysterisis loop. The paramagnetic resonance was studied using an EPR system and showed a high concentration of at least two distinct paramagnetic centers. One center can be eliminated by annealing in Ultra-High Purity nitrogen for 30 minutes at 600 °C. Measurements utilizing a SQUID magnetometer revealed that sp-Si displays ferromagnetic ordering with a saturization magnetization occuring at low fields. This is attributed to the high density of paramagnetic centers. Temperature dependent measurements were performed to establish possible links between magnetic properties and the luminescence of sp-Si.