Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Physical limitations of the electroluminescence mechanism in terbium-based light emitters: matrix and layer thickness dependence

The physical limits of downscaling the SiO₂ thickness of rare earth implanted metal–oxynitride–oxide–semiconductor-based light emitters are explored by investigating the drop down of the electroluminescence power efficiency with decreasing SiO₂ thickness of Tb-implanted devices. It will be experimentally shown that there is a dark zone with an extension of about 20 nm behind the injecting interface in which the hot electrons have not yet gained enough kinetic energy in order to excite the Tb³⁺ luminescence centers. In addition, replacing the host matrix SiO₂ by SiON results in a decrease of power efficiency by two orders of magnitude what is consistent with the experimental data about the hot energy distribution in these media.     

Electron-excited luminescence of SiO2 layers on silicon

An analysis of cathodoluminescence and electroluminescence spectra of Si-SiO₂ structures suggests a conclusion concerning the processes involved in excitation of the luminescence centers generated in the UV spectral region and their localization. The electroluminescence observed in this region of the spectrum is generated in excitation of luminescence centers localized in the immediate vicinity of the Si-SiO₂ phase boundary. In the case of cathodoluminescence, the observed emission bands at ??4.3 and ??2.7 eV appear in excitation of the luminescence of silylene centers at the Si-SiO₂ phase boundary.     

Role of masking oxide on the silicon surface on defect formation in SIMOX structures

The properties of Si/SiO₂ structures produced by oxygen implantation into silicon (SIMOX technology) are investigated by the high-frequency C-V method and by the electroluminescence (EL) method. The existence of electrically active and luminescence centers in the oxide layer near the interface is established. The effect of a SiO₂ masking layer on the silicon surface on defect formation in the SIMOX structure is elucidated. The dependence of the concentration of the electrically active and luminescence centers on the thickness of the masking layer is found.     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Electroluminescence of silicon nanocrystals in MOS structures

We have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a substoichiometric SiO_x (x<2) thin film deposited by plasma-enhanced chemical vapor deposition. After deposition the samples were annealed at high temperature (>1000 °C) to induce the separation of the Si and the SiO₂ phases with the formation of Si nanocrystals embedded in the insulating matrix. We observed at room temperature a quite intense electroluminescence (EL) signal with a peak at ∼850 nm. The EL peak position is very similar to that observed in photoluminescence in the very same device, demonstrating that the observed EL is due to electron–hole recombination in the Si nanocrystals and not to defects. The effects of the Si concentration in the SiO_x layer and of the annealing temperature on the electrical and optical properties of these devices are also reported and discussed. In particular, it is shown that by increasing the Si content in the SiO_x layer the operating voltage of the device decreases and the total efficiency of emission increases. These data are reported and their implications discussed. Received: 31 August 2001 / Accepted: 3 September 2001 / Published online: 17 October 2001     

Optical Properties and Structure of Electroluminescent SiOx:Tb Films

Using nondestructive optical methods (measurement of transmission spectra in the visible and IR regions and multiangular ellipsometry), we have studied the structural changes in SiO _x :Tb films subjected to high–temperature annealing in air, which are responsible for the appearance of electroluminescence in light–emitting structures based on them. It has been established that the appearance of green electroluminescence in such a film (upon annealing of the film in air at temperatures of 600–800°C) is due to the structural changes in its matrix, leading to partial disproportionation of the thermally deposited SiO _x :Tb film (as a result, the film represents a mixture of several phases – Si, SiO_x, and SiO₂). Films showing blue electroluminescence (annealing temperature 1000°C) are characterized by a higher content of oxygen, a better compactness, and a better macroscopic homogeneity in comparison with films showing green electroluminescence. It is also shown that the thermal cycling accompanying the annealing leads to the appearance of birefringence and scattering in SiO _x O:Tb films. It is anticipated that the annealing–stimulated structural changes taking place at both the micro– and the macrolevel should cause changes in the local surroundings of the luminescence center and in the conditions for heating the charge carriers exciting the luminescence centers.     

Injection electroluminescence from CdTe p-n junctions prepared by LPE

CdTep-n junction diodes were prepared by LPE using CdCl₂ as a solvent. Excess cadmium was added to the CdCl₂-CdTe solution. Capacitance-voltage characteristics show that the diode structure is ofp-i-n type. Injection electroluminescence spectra reveal that radiative transitions occur mainly in thep-type region; relevant recombination centers are discussed in connection with those in a previous paper on the photoluminescence of CdTe:P crystals. Temperature dependences of the electroluminescence spectra were explained taking into account a change in sites where electrons radiatively recombine.     

The formation of new adsorption centers on platinized SiO2 surfaces

Kinetic and spectral characteristics of luminescence and excitation of luminescence of magnesium phthalocyanine (MgPc) molecules adsorbed on silicon dioxide (SiO₂) are studied. They are found to be affected by finely divided platinum (Pt) present at the surface and hydration. The deposition of a Pt catalyst on SiO₂ leads to the formation of new centers. Adsorption of MgPc molecules at these centers increases the lifetime of excited states of the former. Luminescence of charge transfer complexes and the protonized form of phthalocyanine is detected at the platinized surface of silicon dioxide.     

Tunneling recombination luminescence under excitation of PbWO4:Mo crystals in the defect-related absorption region

Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO₄:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed.     

X-ray luminescence of disperse SiO2 and porous silicon

We carry out a comparison between the luminescence spectra (photo-and x-ray luminescence) of porous silicon and disperse SiO₂, which by its physical characteristics is most similar to oxide films on porous silicon. The photoluminescence of porous silicon was also investigated using fluorescence (excitation by a nitrogen laser) and metallographic microscopes. We found that the natures of the luminescence centers of porous silicon and disperse SiO₂ are identical. A porous layer on single-crystal silicon ensures the creation of a highly branched surface of oxide film. Luminescence centers are located on its inner (as viewed from the porous silicon) surface. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 247–251, March–April, 1998.     

High field effects in insulating CdF2: Gd crystals

CdF₂ crystals doped with Gd, before any conversion to semiconductor, exhibit at low temperatures electrical current and electroluminescence with d.c. voltage applied.The electrical conduction, attributed to electrons, is bulk limited and the i = i (V, T) characteristics are interpreted as a Poole-Frenkel conduction.The electroluminescence, due to intrinsic and impurity emission, is attributed to the impact process of field accelerated electrons, which give rise to electron-hole pair formation and impurity centre excitation.     

Room-temperature photoluminescence in amorphous SrTiO3– the influence of acceptor-type dopants

Room-temperature photoluminescence (PL) was observed in undoped and 2 mol % Cr-, Al- and Y-doped amorphous SrTiO₃ thin films. Doping increased the PL, and in the case of Cr significantly reduced the associated PL wavelength. The optical bandgaps, calculated by means of UV–vis absorption spectra, increased with crystallinity and decreased with the doping level. It was considered that yttrium and aluminum substituted Sr²⁺, whereas chromium replaced Ti⁴⁺. It is believed that luminescence centers are oxygen-deficient BO₆ complexes, or the same centers with some other defects, such as oxygen or strontium vacancies, or BO₆ complexes with some other defects placed in their neighborhood. The character of excitation and the competition for negatively charged non-bridging oxygen (NBO) among numerous types of BO₆ defect complexes in doped SrTiO₃ results in various broadband luminescence peak positions. The results herein reported are an indicative that amorphous titanates are sensitive to doping, which is important for the control of the electro-optic properties of these materials. The probable incorporation of Cr into the Ti site suggests that the existence of a double network former can lead to materials displaying a more intense photoluminescence. Received: 20 November 2001 / Accepted: 22 November 2001 / Published online: 27 March 2002     

Scanning near-field cathodoluminescence microscopy of an Si+ implanted and thermally annealed SiO2 layer

A scanning near-field cathodoluminescence microscope (SNCLM) is successfully used to image the cathodoluminescence of an Si⁺ implanted and thermally annealed submicronic SiO₂ layer. Owing to the subwavelength resolution of the system a “cross-sectional” cathodoluminescence image was obtained. The intensity image profile shows that sample luminescence results from the whole SiO₂ layer confirming a preceding electroluminescence study. Sample luminescence is attributed to point defects generated into the whole SiO₂ layer during Si⁺ ion implantation and thermal annealing.     

The effect of packing density on luminescence of amorphous SiO<Subscript>2</Subscript> nanoparticles

Photoluminescence of amorphous SiO₂ nanoparticles compressed in the form of tablets is studied under exposure to UV radiation. The observed luminescence spectrum is a broad band extending from the excitation wavelength to 700 nm and with a maximum at ~470 nm. The spectrum can be decomposed into two Gaussian components with maxima at ~460 and ~530 nm. As the pressure applied for sample preparation increases, the integrated intensities of these bands change in opposite directions—the intensity of the short-wavelength band increases, while that of the long-wavelength band decreases. It is concluded that these bands are due to different luminescence centers of silicon dioxide located on the surface and in the bulk of SiO₂ nanoparticles.     

Size-dependent photoluminescence properties under vacuum ultraviolet excitation of Zn2SiO4:Mn nanophosphors

The Zn₂SiO₄:Mn²⁺ nanophosphors with different particle sizes were synthesized via the hydrothermal method by adjusting the concentrations of surfactant and the hydrothermal temperature. The behavior of the photoluminescence as a function of phosphor particle sizes under vacuum ultraviolet excitation was investigated. Higher critical quenching concentration with decreasing particle size of the Zn₂SiO₄:Mn²⁺ nanophosphors was observed. This is ascribed to the hindrance of energy transfer between luminescence centers under vacuum ultraviolet excitation. The prolonged decay time in smaller samples provides further evidence that the energy transfer confinement has an effect on the photoluminescence properties.     

Intrinsic defect related luminescence in ZrO2

The studies of ZrO₂ and yttrium stabilized ZrO₂ nanocrystals luminescence as well as yttrium stabilized single crystal luminescence and induced absorption showed that the intrinsic defects are responsible for luminescence at room temperature. These defects form a quasi-continuum of states in ZrO₂ band gap and are the origin of the luminescence spectrum dependence on the excitation energy. Luminescence centers are oxygen vacancies related but not the vacancies themselves. At room temperature, in ZrO₂, deep traps for electrons and holes exist. The oxygen vacancies are proposed to be the traps for electrons.     

Infrared electroluminescence from a Si MOS structure with Ge in the oxide

We report on room-temperature infrared electroluminescence (EL) from metal-oxide-semiconductor devices made from Si. We compare the luminescence from RF sputtered oxide films containing SiO₂ with and without Ge by using a composite target and luminescence from a SiO₂ layer made by rapid thermal oxidation. The sputtered films were annealed in the temperature range 600-900 °C. This densifies the films and is likely to reduce the concentration of defects. A luminescence peak located around 1150-1170 nm is observed at current densities as low as 0.1 A/cm². The corresponding photon energy is close to that of the Si band gap. In addition, we observe several broad luminescence bands in the range 1000-1750 nm. These bands get stronger with Ge in the SiO₂ film. Some of these bands have previously been suggested and are directly associated with Ge. Since we observe that the intensity is correlated with the presence of Ge while the mere presence of the bands is not, we discuss the EL bands being due to defects which concentration is influenced by Ge in the oxide.     

Estimation of the acceleration ability for electrons in SiO2 and the tunneling effect

Silicon dioxide (SiO₂) plays an important role in layered optimization scheme and solid-state cathodoluminescence (SSCL). Initially, it was believed that the SiO₂ layer would (i) generate extra interface states contributing to a number of primary electrons available for exciting the luminescent centers, and/or (ii) act as acceleration layer resulted in gaining high energy for those electrons that would tunnel into the luminescent layer to excite luminescent centers. Based on the brightness vs. voltage (B-V) measurements, we deem that the latter case, i.e. acceleration and tunneling, is the dominant mechanism. A detailed discussion in terms of electrons acceleration and tunneling as the main contributions to the enhancement of brightness is presented.