Porous Silicon Diodes Research Articles

Doping influence on microwave detection by metal–porous silicon contacts

We present the study of voltage signal rise across both additionally doped and undoped porous silicon diode sensors exposed to microwave radiation. The doped ones exhibit fast, of the nanosecond order, response times, but lower voltage–power responsivity values as compared to similar diodes but containing no porous layer. Insertion of porous surface layer into the undoped samples can significantly enhance their responsivity, however, they demonstrate much slower, of the order of tens of microseconds, response to microwave-modulating pulse. Microwave radiation induces voltage signals of opposite polarity in different types of the porous samples. Models all based mainly on hot carrier effects are exploited to explain the experimental results. Possible aspects of application are discussed as well.

Improved quasiballistic electron emission from a nanocrystalline Si cold cathode with a monolayer-graphene surface electrode

The quasiballistic electron emission from a nanocrystalline porous silicon (nc-Si) diode is drastically enhanced by using a monolayer-graphene film as the surface electrode. Due to little scattering losses in monolayer-graphene, the electron emission efficiency at room temperature is increased up to 6.3% that is considerably higher than that in the case of conventional thin metal films. The peak energy of emitted electrons can be tuned by the applied voltage while keeping narrow energy dispersion. The energy distribution becomes more monochromatic at a low temperature of around 150 K. Monolayer-graphene acts as a highly transparent nanogrid for quasiballistic hot electrons.

Porous Silicon NDR-Based High Power RF Oscillator Diode

A high power oscillation occurring on a porous silicon diode with an obvious dc N-shaped negative differential resistance (NDR) at room temperature is presented for the first time. The voltage oscillation swing was up to 12.4 V and oscillation frequency was 101.3 kHz, which belongs to the radio frequency range. The oscillation was based on a porous silicon superlattice diode structure, which was observed to provide a stable NDR with a high peak-to-valley current ratio as high as 7.9 for dc current–voltage characteristics at room temperature. A superlattice diode structure was formed by an electrochemical etching method on an n-type (100) silicon wafer with an alternating etching current. In addition, a dc constant current source was used as the power supply for the diode oscillation. In particular, two steady operating points were observed for the dc NDR current–voltage characteristics of the circuit load line. The system produced a steady oscillation between these two operating points in a harmonic form.

Theory of quasiballistic transport through nanocrystalline silicon dots

A model to describe the underlying physics of high-energy electron emission from a porous silicon diode is presented. The model is based on an atomistic tight-binding method combined with semiclassical Monte Carlo simulation. It well reproduces essential features of experimental findings. An initial acceleration region is shown to play a crucial role in generating quasiballistic electron emission.

Quasi-ballistic electron transport through silicon nanocrystals

Monte Carlo simulation of electron transport through a silicon nanocrystal array is performed to clarify the mechanism of high-energy electron emission from a porous silicon diode. The electronic states are calculated within an empirical tight-binding approximation. In the Monte Carlo simulation, optical-phonon emission and elastic tunneling processes are taken into account. It is found that initial electron acceleration, which is enhanced due to the discrete nature of lower energy levels, plays an essential role in generating high-energy electron emission.

Stabilization of nano‐crystalline porous silicon electroluminescence by high pressure water vapor annealing

AbstractStabilization of electroluminescence (EL) from nanocrystalline porous silicon diodes has been achieved by using high pressure water vapor annealing (HWA) of nanocrystalline porous silicon (PS). HWA is performed at 260 °C and a pressure of 1.3 MPa. Devices treated by HWA show no degradation in EL intensity and diode current density under dc operation at constant voltage. The high stability is attributed to the high quality of the oxide surrounding the Si nanocrystals after HWA. This oxide is mostly unstrained and consists of a very uniform network. Furthermore, the Si/SiO2 interfacial defect density is very much reduced by HWA in comparison to that of conventional as‐prepared PS. The electrical characteristics of the diodes are also improved by HWA by reduced charge carrier trapping and charging. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stable electroluminescence of nanocrystalline silicon device activated by high pressure water vapor annealing

Electroluminescence (EL) and electrical properties of nanocrystalline porous silicon (PS) diodes have been sufficiently improved by introducing high pressure water vapor annealing into the active PS layer. The EL emission is significantly enhanced without affecting the operating voltage. In addition, the fabricated device shows no degradations in both the EL intensity and the diode current density under a dc operation. The EL spectra coincide well with the photoluminescence ones. The obtained high EL performance is presumably caused by complete passivation of nanocrystalline silicon surfaces by thin tunnel oxides with mostly unstrained uniform network and little interfacial trapping defects.

A porous silicon diode as a source of low-energy free electrons at milli-Kelvin temperatures

We have developed a porous silicon (PS) diode that yields free-electron currents with energies <0.1eV below 77 K. The power dissipated during emission is low so that pulses of electrons can be produced below 100 mK without raising the temperature of the system. Free electrons were generated in liquid He4 and He3 as well. The device was developed as a source of electrons for a quantum computing system using electrons on the surface of a dielectric film. The results suggest that a Poole-Frenkel type of mechanism accounts for the observed electric-field-enhanced conduction but the electron emission mechanism is not well understood in the present models of PS.

Stable electroluminescence from passivated nano‐crystalline porous silicon using undecylenic acid

Stabilization of electroluminescence from nanocrystalline porous silicon diodes has been achieved by replacing silicon-hydrogen bonds terminating the surface of nanocrystalline silicon with more stable silicon-carbon (Si-C) bonds. Hydrosilylation of the surface of partially and anodically oxidized porous silicon samples was thermally induced at about 90 °C using various different organic molecules. Devices whose surface have been modified with stable covalent bonds shows no degradation in the EL efficiency and EL output intensity under DC operation for several hours. The enhanced stability can be attributed to the high chemical resistance of Si-C bonds against current-induced surface oxidation associated with the generation of nonradiative defects. Although devices treated with 1-decene exhibit reduced EL efficiency and brightness compared to untreatred devices, other molecules, such as ethyl-undecylenate and particularly undecylenic acid provide stable and more efficient visible electroluminescence at room temperature. Undecylenic acid provides EL brightness as high as that of an untreated device. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Harvesting Betavoltaic and Photovoltaic Energy with Three Dimensional Porous Silicon Diodes

ABSTRACTConventional two-dimensional p-n diodes can be used for betavoltaic and photovoltaic energy conversion, but the device efficiency is limited by the planar geometry. We propose and demonstrate a novel three-dimensional diode geometry based on porous silicon. The 3D pore array provides two very important features: (1) the storage of the radioisotope energy source and (2) its extreme proximity to the p-n junction on each pore wall. The particle energy losses are thereby minimized prior to entering the conversion layer. In betavoltaics, our 3D betavoltaic device efficiency is 10 times that of a similar planar device. In photovoltaics, photons play the role of beta particles and photon trapping inside the pores enhances the conversion efficiency. Further fabrication and geometry optimization can result in practical, high performance devices.

Quasiballistic Electron Emission from Planarized Nanocrystalline-Si Cold Cathode

ABSTRACTMechanism of electron transport through planerized nanocrystalline-Si (nc-Si) cold cathode surface emitting devices was investigated. The energy distribution of electrons emitted from nc-Si emitter was obviously not Maxwellian, which was usually obtained at conventional cold cathode devices, but was similar to that from the nanocrystalline porous silicon diode emitter. These results strongly suggest that electrons are emitted quasiballistically from our devices and indicate that the planarized nc-Si layer play an important role in this high efficiency cold cathode emitter.

Key role of nanocrystalline feature in porous polycrystalline silicon diodes for efficient ballistic electron emission

The cold electron emission characteristics of nanocrystalline porous polysilicon (PPS) diodes have been investigated for two PPS diodes prepared under different conditions. The origin of the difference in the emission efficiency and electron energy distribution between the two samples is analyzed in relation to the respective photoluminescence properties. The photoluminescence spectrum in the efficient electron emitter consists of red and blue emission bands corresponding to nanocrystalline silicon and interfacial oxide, respectively, while that in the low-efficiency emitter consists only of a blue band. Too much oxidation results in a reduction of the emission efficiency due to increased electron scattering. It is evident that both the formation of nanocrystalline silicon and its appropriate surface oxidation are key issues for obtaining efficient ballistic emission. These results are consistent with the emission model that electrons are accelerated in the PPS layer by multiple tunneling through interfacial oxide barriers between interconnected silicon nanocrystallites.

Stabilization of porous silicon electroluminescence by surface passivation with controlled covalent bonds

Stabilization of electroluminescence (EL) from nanocrystalline porous silicon (PS) diodes has been achieved by replacing silicon–hydrogen bonds terminating the surface of nanocrystalline silicon with more stable silicon–carbon (Si–C) and silicon–oxygen (Si–O–C) bonds without significant effects on the electrical properties. The surface modification is performed by a thermal treatment of partially and anodically oxidized PS sample at about 90 °C with organic molecules: 1-decene, ethyl undecylenate, or n-caprinaldehyde. The porous silicon device whose surface has been modified with stable covalent bonds shows no degradation in the EL efficiency and EL output intensity under dc operation for several hours. The improved stability can be attributed to the high chemical resistance of Si–C and Si–O–C bonds against current-induced surface oxidation associated with the generation of nonradiative defects.

Efficient Visible Electroluminescence from Porous SiliconDiodes Passivated by Carbon Films

By using n-butylamine as a carbon resource, carbon film is deposited onthe p-n porous silicon (PS) surface with a radio-frequency glow dischargeplasma system. Raman spectra and infrared reflection (IR) spectra of thecarbon films indicate that there are amine-group and hydrogen atoms therein.The IR spectra of the passivated PS samples exhibit that the PS surfaces aremainly covered with Si-C, Si-N and Si-O bonds. Electroluminescence (EL)spectra show that the EL intensity of the passivated PS diodes increases greatlyand the blueshift of the EL peak occurs compared with the diodes withouttreatment. Both of these are stable while the passivated diodes areexposed to the air indoors. The I-V characteristics reveal that thepassivated diodes have a smaller series resistance and a lower onsetvoltage. The influence of the carbon film passivation on EL properties ofPS has also been discussed. The results have proven thatcarbon film passivation is a good way to enhance the PS luminescentintensity and stability.

Electron emission characteristics of the porous polycrystalline silicon diode

Abstract It is estimated that a porous polysilicon (PPS) diode with a structure of Au/PPS/n-type Si operates as an efficient stable surface emitting cold cathode. 2.0 μm of an non-doped polysilicon layer is formed on an heavily doped n-type silicon wafer and anodized in a solution of HF (50%):ethanol=1:1 under illumination by a 500 W tungsten lamp from a distance of 20 cm. The electron emission properties of the PPS diode were investigated as a function of anodizing condition such as anodizing current density. The electron emission trajectory was investigated, and it was also demonstrated their good uniformity in the emitting area.

Multifunctional properties of nanocrystalline porous silicon as a quantum-confined material

It is shown that the quantum properties of nanocrystalline silicon (nc-Si) appear in various ways. The most striking effect of quantum confinement in nc-Si is a significant bandgap widening. It causes efficient visible luminescence. To date, the external quantum and power efficiencies of red-band electroluminescence have been improved up to 1.1% and 0.4%, respectively, in porous silicon (PS) diodes. In the porosity-multilayer PS microcavity, nonlinear optical response due to a refractive index change of nc-Si was observed for photoexcitation and carrier injection. The PS diode also operates as a surface-emitting ballistic cold cathode. Under a high electric field, electrons can travel with an enlarged drift length, possibly via multiple tunneling through interfacial barriers between interconnected nc-Si. A strong carrier charging effect in nc-Si leads to a function as light-emissive nonvolatile information storage. An extremely lowered thermal conductivity due to complete carrier depletion makes it possible to use nc-Si as a thermally induced ultrasonic emitter without any mechanical vibrations. These are potentially applicable to integrated novel functional devices.

Photon, Electron and Utlrasonic Emission from Nanocrystalline Porous Silicon Devices

ABSTRACTFor quantum-sized nanocrystalline silicon (nc-Si), various optical and electronic effects have been clarified in addition to a significant band gap widening. As typical examples of these induced effects, some emission properties of nanocrystalline porous silicon (PS) are described in this paper including the present status of application studies. The first one is electroluminescence (EL) of PS diodes. It is shown that following a drastic improvement in the external quantum and power efficiencies, stability has been significantly enhanced by the formation of covalent termination nc-Si surfaces. Next topic is the cold electron emission from PS diodes. When the nanostructure of the PS drift layer is appropriately controlled, injected electrons are accelerated ballistically toward the outer surface and emitted via tunneling through a thin-film top electrode perpendicular to the device surface as energetic electrons. As an efficient surface-emitting electron source, there are many advantages in this emitter over the conventional cold cathodes. The applicability of this emitter to either vacuum-type or solid-state flat-panel display is demonstrated. Finally, the usefulness of a PS device as a thermally induced ultrasonic emitter is presented on a basis of its fundamental characterizations. Technological potential of this emitter for functional acoustic devices is also discussed.

Efficient and ballistic cold electron emission from porous polycrystalline silicon diodes with a porosity multilayer structure

Based on the previous investigations of the cold electron emission from porous silicon and porous polycrystalline silicon (PPS) diodes, we have fabricated PPS diodes with a porosity multilayer structure and evaluated the emission characteristics. The multilayer structure has functions of reforming the electric field distribution and improving the thermal conductivity of the PPS layer. It is demonstrated here that the multilayered sample exhibits high emission current density of ∼0.8 mA/cm2 and high efficiency of ∼1%, which are close to values required for practical use. The emission is quite stable even with a high emission current density. Ballistic electron emission mechanism is discussed in relation to the energy distribution of emitted electrons.

Optical Accessibility of Light-Emissive Nanosilicon Memory

Optical memory functions in electroluminescence(EL)-emissive porous silicon (PS) diodes are described. The experimental PS diode is composed of an indium tin oxide (ITO) film, a PS layer, a p-type Si substrate and an Ohmic back contact. The PS layer was treated by rapid thermal oxidation (RTO) process. The RTO-PS diode shows stable on- and off-states which can be controlled by external bias voltages. The device can also store the optical information, and the stored information is erasable by photoillumination, since the bistable memory effect is based on field-induced hole injection and ejection into and from silicon nanocrystallites. The amount of the stored holes can be controlled by the incident light. The optical accessibility observed here suggests that the PS diode operates as a pixel-free image storage device.

Optical Accessibility of Light-Emissive Nanosilicon Memory

Optical memory functions in electroluminescence(EL)-emissive porous silicon (PS) diodes are described. The experimental PS diode is composed of an indium tin oxide (ITO) film, a PS layer, a p-type Si substrate and an Ohmic back contact. The PS layer was treated by rapid thermal oxidation (RTO) process. The RTO-PS diode shows stable on- and off-states which can be controlled by external bias voltages. The device can also store the optical information, and the stored information is erasable by photoillumination, since the bistable memory effect is based on field-induced hole injection and ejection into and from silicon nanocrystallites. The amount of the stored holes can be controlled by the incident light. The optical accessibility observed here suggests that the PS diode operates as a pixel-free image storage device.