Si-based materials and devices for light emission in silicon

We report on the fabrication and performances of extremely efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1.54 μm electroluminescence (EL) at 300 K with a 10% external quantum efficiency, comparable to that of standard light-emitting diodes using III–V semiconductors. Er excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light-emitting MOS devices have been fabricated using Er-doped silicon rich oxide (SRO) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 1%. In these devices, Er pumping occurs by energy transfer from the Si nanostructures to the rare-earth ions. Finally, we have also fabricated MOS structures with Tb- and Yb-doped SiO₂ which show room temperature EL at 540 nm (Tb) and 980 nm (Yb) with an external quantum efficiency of a 10% and 0.1%, respectively.     

Intraband photoresponse of SiGe quantum dot/quantum well multilayers

In this contribution we study the intravalence band photoexcitation of holes from self-assembled Ge quantum dots (QDs) in Si followed by spatial carrier transfer into SiGe quantum well (QW) channels located close to the Ge dot layers. The structures show maximum response in the important wavelength range 3–5 μm. The influence of the SiGe hole channel on photo- and dark current is studied depending on temperature and the spatial separation of QWs and dot layers. Introduction of the SiGe channel in the active region of the structure increases the photoresponsivity by up to about two orders of magnitude to values of 90 mA/W at T=20 K. The highest response values are obtained for structures with small layer separation (10 nm) that enable efficient transfer of photoexcited holes from QD to QW layers. The results indicate that Si/Ge QD structures with lateral photodetection promise very sensitive large area mid-infrared photodetectors with integrated readout microelectronics in Si technology.     

Origin of the defect states at ZnS/Si interfaces

Electrical characterisation of silicon surfaces contaminated by a zinc-sulphide overlayer has been carried out by forming Schottky diodes on the silicon after the ZnS has been etched off. The techniques include current-voltage, capacitance-voltage, and deep-level transieni spectroscopy. The Schottky diodes show clear memory of the presence of the ZnS overlayer and the electrical characteristics are far from ideal. Five deep levels in the sub-surface region of the silicon are detected, corresponding to the Zn⁺, Zn⁺⁺, S^–, S^–– states and probably to a Zn–B complex (p-type). Diffusion of the zinc and sulphur into the silicon is therefore confirmed and this diffusion is thought to create a compensated layer at the interface. These impurity states control the electrical characteristics of the surface in these diodes.     

Optimising dislocation-engineered silicon light-emitting diodes

This article presents a study of the possibilities of optimising the electroluminescence (EL) efficiency of dislocation-engineered silicon light-emitting diodes (DELEDs). The diodes were produced by implantation of boron in n-type (100)Si wafers, at a constant ion energy and fluence, of 30 keV and 1×10¹⁵ ions/cm², respectively. The density and the areal coverage by dislocation loops were varied by applying different annealing times in a rapid thermal processing, from 30 s to 60 min. It is shown that the EL efficiency is directly correlated to the number and areal coverage by the loops. The highest population of loops, ∼5×10⁹ /cm², and an areal coverage of around 50% were achieved for 1–5 min annealing. This loop distribution results in optimal DELEDs, having the highest EL response and the largest increase of EL intensity with operating temperature (80–300 K). The results of this work confirm a previously introduced model of charge-carrier spatial confinement by a local stress induced by the edge of the dislocation loops, preventing carrier diffusion to non-radiative recombination centres and enhancing radiative transitions at the silicon band edge. PACS 85.60.Jb; 78.60.Fi; 61.72.Tt     

Silicon LEDs emitting in the band-to-band transition region: Effect of temperature and current strength

The parameters of silicon light-emitting diodes (LEDs) prepared through boron implantation into n-Si, followed by annealing at 700–1200°C, were studied. The maximum room-temperature internal quantum efficiency of electroluminescence (EL) in the region of band-to-band transitions was estimated as 0.4% and reached at an annealing temperature of 1100°C. This value did not vary more than twofold within the operating temperature range 80–500 K. The EL growth and decay kinetics was studied at various currents. Following an initial current range of nonlinear dependence, the EL intensity scaled linearly with the current. It is shown that interpretation of this result will apparently require a revision of some present-day physical concepts concerning carrier recombination in silicon diodes.

     

Single-layered light-emitting diodes possessing methoxy-modified pyrazoloquinoline dyes in poly-N-vinylcarbazole matrix

A principal opportunity to operate by current-voltage and electroluminescent-current dependences for the single-layered light-emitting diodes (LED) on the basis of the polyvinylcarbazole (PVK) polymers doped by methoxy-substituted pyrazoloquinoline (PQ) emitting dye chromphore is demonstrated. The principal light-emitting parameters in the architecture ITO/PQ: PVK/Ca (Al) were investigated. The maximally achieved quantum efficiency of the investigated LED was equal to about 0.87% and corresponded to the brightness about the 44 Cd/m². The absence of blue shift for the electroluminescence (EL) compared to PL may indicate on an absence of near-the-surface exciton diffusion for the methoxy-PQ contrary to the phenyl-methyl-substituted PQ. The quantum chemical calculations have shown principal role of the dye chromophore state dipole moments in the observed carrier kinetics determining the EL.     

Determination of the gap distribution in YBa2Cu3O7 using a far-infrared reflection-Fabry-Pérot device

This paper reports on far-infrared measurements of YBa₂Cu₃O₇ films oriented with the c-axis perpendicular to the surface, by using a silicon reflection Fabry-Pérot interferometer as a multireflection device. From these we could derive the dielectric function, the refractive index, the field penetration depth and the surface impedance of the material. The one order of magnitude higher sensitivity of the method compared to a direct reflectance measurement allowed to find an almost continuous gap distribution in the 70–215 cm^–1 region together with a separate gap at about 330 cm^–1. A quasizero gap absorption is found down to 20 cm^–1 even at low temperatures (10 K).     

Characterization of ITO/porous silicon LED structures

The electrical behavior and the electroluminescence (EL) obtained from n- and p-type ITO/porous silicon LEDs have been characterized simultaneously at different temperatures. Stability and aging in air were investigated, and means for avoiding their detrimental effects in the experiments are suggested. The dominating current carrying mechanism responsible for visible light emission in both substrate types has been identified to be Fowler–Nordheim tunneling. This emphasizes the contribution of embedded nanoparticles (quantum dots) rather than the role of nanowires in efficient EL.     

Temperature‐dependent electroluminescence intensity in green and blue (In,Ga)N multiple‐quantum‐well diodes

The electroluminescence (EL) intensity has been investigated of green and blue (In,Ga)N multiple‐quantum‐well diodes grown on c ‐plane sapphire over a wide temperature range and as a function of current between 0.01 mA and 10 mA. The EL intensity of the green diode with p‐(Al,Ga)N electron blocking layer does not show low‐temperature quenching, especially at low injection levels, previously observed for the blue (In,Ga)N quantum‐well diodes. This finding rules out possi‐ bilities that the freeze‐out of holes at deep Mg acceptor levels and the failure of hole injections through the p‐(Al,Ga)N layer are directly responsible for the EL quenching at temperatures below 100 K. Variations of the EL efficiency with current level suggest that capture/escape efficiencies of injected carriers by the wells play an important role for the determination of EL external quantum efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal annealing effects on I–V–T characteristics of sputtered Cr/n-GaAs diodes

Sputtered Cr/n-GaAs Schottky diodes have been prepared and annealed at 200 and 400 °C. The current–voltage (I–V) characteristics of the as-deposited and annealed diodes have been measured in the temperature range of 60–320 K with steps of 20 K. The effect of thermal annealing on the temperature-dependent I–V characteristics of the diodes has been investigated experimentally. The ideality factor and barrier height (BH) values for 400 °C annealed diode approximately remain unchanged from 120 to 320 K, and those of the as-deposited sample from 160 to 320 K. The departures from ideality at low temperatures have been ascribed to the lateral fluctuations of the BH. The BH values of 0.61 and 0.74 eV for the as-deposited and 400 °C annealed diodes were obtained at room temperature, respectively. A Richardson constant value of 9.83 A cm⁻² K⁻² for 400 °C annealed Schottky diode, which is in close agreement with the known value of 8.16 A cm⁻² K⁻² for n-type GaAs. Furthermore, T₀ anomaly values of 15.52, 10.68 and 5.35 for the as-deposited and 200 and 400 °C annealed diodes were obtained from the nT versus T plots. Thus, it has been seen that the interface structure and quality improve by the thermal annealing at 400 °C.     

Light emitting devices based on silicon nanostructures

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 μm. 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 EL with the advantage to be formed at a temperature of only 900 °C, remarkably lower than the temperature needed for the formation of Si nanocrystals (1100 °C or higher). To improve the extraction of the light, we coupled the emitting system with a 2D photonic crystal structure properly fabricated with ULSI technology to reduce the total internal reflection of the emitted light. We demonstrate that the extraction efficiency is increased by a factor of 4. Finally, the light emission from devices based on Er-doped Si nanoclusters has been studied and in particular we have investigated the luminescence quenching processes limiting quantum efficiency in these devices. In fact the carrier injection, that determines the excitation of Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. These data are presented and the implications on the device performances discussed.     

Two-Photon Raman scattering and high excitation luminescence in ZnTe

Two-Photon-Raman Scattering (TPRS) and the luminescence of ZnTe are investigated when the samples are highly excited with a tunable narrow-band dye-laser. In luminescence, one observes emission bands due to the well-known inelastic exciton-exciton scattering at intermediate excitation intensities, and the recombination radiation of an electronhole plasma (EHP) at the highest excitation levels. For the first time, TPRS is reported in ZnTe. From the change in the TPRS lines in magnetic fields up to 10T we deduce a diamagnetic shift of 1.2°10^–2 meV/T² of the free longitudinal exciton. This value is in good agreement with results obtained by other authors from reflection spectroscopy.     

Efficient visible electroluminescence from porous silicon diodes with low driven voltage

By using n-butylamine as carbon resource, diamond-like carbon film (DLCF) was deposited on the p-n porous silicon (PS) surface by means of a radio-frequency glow discharge plasma system. Electroluminescent (EL) spectra show that EL intensity of the passivated PS diodes increases by 4.5 times and 30-nm blue-shift of EL peak occurs compared with the diodes without treatment and both of them are stable while the passivated diodes are exposed to the air indoor. The current-voltage (Ⅰ-Ⅴ) characteristics exhibit that the passivated diodes have a smaller series resistance and a lower onset voltage. The EL intensityvoltage (ⅠEL-Ⅴ) relations of the PS devices with different DLCF thicknesses show that only medium DLCF thickness is optimum. These experimental phenomena have been explained based on Raman spectra and IR spectra of the diamond-like carbon films and IR spectra of the passivated PS samples.     

Optical properties of Stranski–Krastanov grown three-dimensional Si/Si1−xGex nanostructures

Detailed Raman and photoluminescence (PL) measurements are reported for Si/Si_1−xGe_x nanostructures grown by molecular beam epitaxy under near Stranski–Krastanov (S–K) growth mode conditions. In samples with x ranging from 0.096 to 0.53, we observe that an increase in the Raman signal related to Ge–Ge vibrations correlates with (i) a red shift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that for x>0.5 Ge atoms form nanometer size clusters with a nearly pure Ge core surrounded by a SiGe shell. Time-resolved PL measurements reveal a stretched-exponential long-lived PL component that is associated with compositional and dimensional fluctuations in the SiGe dots.     

Temperature dependence of electroluminescence from Si-based light emitting diodes with β-FeSi2 particles active region

Electroluminescence (EL) properties of Si-based light emitting diodes with β-FeSi₂ particles active region grown by reactive deposition epitaxy are investigated. EL intensity of β-FeSi₂ particles versus excitation current densities has different behaviors at 8, 77 K and room temperature, respectively. The EL peak energy shifted from 0.81 to 0.83 eV at 77 K with the increase of current density from 1 to 70 A/cm². Temperature dependence of the peak energy can be well fitted by semi-empirical Varshni's law with the parameters of α=4.34 e-4 eV/K and β=110 K. These results indicate that the EL emission originates from the band-to-band transition with the band gap energy of 0.824 eV at 0 K.     

The current and capacitance response of radiation-damaged silicon PIN diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of silicon p–i–n diodes have been investigated both prior to and after radiation-induced damage by 1 MeV neutrons. The results have been analysed and several rates of damage evaluated. The indication is mainly that radiation damage occurs only up to certain fluencies. Beyond these, the material becomes resistant to further damage. Thus, initial heavy radiation damage can be used to achieve radiation-hardness of detector diodes. This result is contrary to previous suggestions that continued irradiation renders the detectors inoperable but is in good agreement with our results on radiation-hardness induced by gold-doping.     

Room-temperature SiGe light-emitting diodes

In the present paper the electroluminescence of PIN diodes with either strained SiGe/Si or Ge islands in the i-region has been investigated experimentally and by quantitative modelling. The modelling helped to improve the diode structure. Consequently, diodes with strained Si_0.80Ge_0.20 could be improved so as to reveal emission up to room temperature, if the thickness was high enough. To overcome the thickness limitation due to plastic relaxation, we used selective epitaxy on small areas. We also present results for diodes with Ge islands in the active region. The internal quantum efficiency of light emitting diodes with strained SiGe was at room temperature 10⁻⁴, while diodes with islands emitted ten times less light.     

Electroluminescence of gallium phosphide in the visible spectrum region

The effect of doping impurities and the container material on the color and intensity of electroluminescence (EL) of gallium phosphide crystals grown from the melt in gallium is considered. It is found that the maximum efficiency of red EL is observed in crystals grown in quartz containers, and of green EL — in crystals produced in alundum containers having aluminum nitride coatings. The optimal doping-impurity concentrations are determined for producing diodes with red, yellow, and green radiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 12–16, April, 1973.     

High photo and electroluminescence efficiency oligothiophenes

We report photo and electroluminescence efficiency of a thiophene oligomer functionalized to enhance its solid-state efficiency. We find that the absolute PL quantum yield is up to 37% for spin-coated thin films of the compound. The material has been used as active material in organic light-emitting diodes (LEDs). EL efficiencies up to 0.9 cd/A are demonstrated in LEDs prepared with a blend indium tin oxide and Ca–Al electrodes.     

Absorption measurement of strained SiGe nanostructures deposited by UHV-CVD

In order to obtain a low band gap photocell based on the widely spread silicon technology, e.g. for thermophotovoltaics, SiGe nanostructures can be introduced into a monocrystalline silicon photocell. Beforehand, it is necessary to know the absorption coefficient of the SiGe quantum wells. On a silicon (1 0 0) substrate multiple Si/SiGe quantum well structures were grown by UHV-CVD. The Ge concentration and the well width were used as growth parameters. To obtain significant absorption, the experiment was set up to allow for 200 internal reflections.The total reflection of the light results in a standing electromagnetic wave. The absorption coefficient was obtained from the experimental data taking the geometry and the electric field distribution in the absorbing layer into account. The influence of well width and germanium content on the absorption was investigated with the goal of maximizing the absorption for photons with energies below the band gap energy of silicon. The measurement results are compared with a theoretical model, which takes the band structure of strained SiGe including confinement effects into account.