Karl D. Hirschman

Nanocrystalline-silicon superlattice produced by controlled recrystallization

Nanocrystalline-silicon superlattices are produced by controlled recrystallization of amorphous-Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing at 600–1000 °C, and furnace annealing at 1050 °C. Transmission electron microscopy, Raman scattering, x-ray and electron diffraction, and photoluminescence spectroscopy show an ordered structure with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random. The luminescence from the nc-Si superlattices is demonstrated and studied.

Thermal crystallization of amorphous Si/SiO2 superlattices

Annealing of amorphous Si/SiO2 superlattices produces Si nanocrystals. The crystallization has been studied by transmission electron microscopy and x-ray analysis. For a Si layer thinner than 7 nm, nearly perfect nanocrystals are found. For thicker layers, growth faults and dislocations exist. Decreasing the a-Si layer thickness increases the inhomogeneous strain by one order of magnitude. The origin of the strain in the crystallized structure is discussed. The crystallization temperature increases rapidly with decreasing a-Si layer thickness. An empirical model that takes into account the Si layer thickness, the Si/SiO2 interface range, and a material specific constant has been developed.

Stable and efficient electroluminescence from a porous silicon‐based bipolar device

A complete process compatible with conventional Si technology has been developed in order to produce a bipolar light‐emitting device. This device consists of a layer of light‐emitting porous silicon annealed at high temperature (800–900 °C) sandwiched between a p‐type Si wafer and a highly doped (n+) polycrystalline Si film. The properties of the electroluminescence (EL) strongly depend on the annealing conditions. Under direct bias, EL is detected at voltages of ∼2 V and current densities J∼1 mA/cm2. The maximum EL intensity is 1 mW/cm2 and the EL can be modulated by a square wave current pulse with frequencies ν≥1 MHz. No degradation has been observed during 1 month of pulsed operation.

Carrier transport in porous silicon light‐emitting devices

This work presents a comprehensive investigation of carrier transport properties in light‐emitting porous silicon (LEPSi) devices. Models that explain the electrical characteristics and the electroluminescence properties of the LEPSi devices are developed. In metal/LEPSi devices, the forward current density–voltage (J–V) behavior follows a power law relationship (J∼Vm), which indicates a space charge current attributed to the carriers drifting through the high resistivity LEPSi layer. In LEPSi pn junction devices, the forward J–V behavior follows an exponential relationship (J∼eeV/nkT), which indicates that the diffusion of carriers makes a major contribution to the total current. The temperature dependence of the J–V characteristics, the frequency dependence of the capacitance–voltage characteristics, and the frequency dependence of the electroluminescence intensity support the models. Analysis of devices fabricated with a LEPSi layer of 80% porosity results in a relative permittivity of ∼3.3, a carrier ...

Diffusion barrier cladding in Si/SiGe resonant interband tunneling diodes and their patterned growth on PMOS source/drain regions

Si/SiGe resonant interband tunnel diodes (RITDs) employing /spl delta/-doping spikes that demonstrate negative differential resistance (NDR) at room temperature are presented. Efforts have focused on improving the tunnel diode peak-to-valley current ratio (PVCR) figure-of-merit, as well as addressing issues of manufacturability and CMOS integration. Thin SiGe layers sandwiching the B /spl delta/-doping spike used to suppress B out-diffusion are discussed. A room-temperature PVCR of 3.6 was measured with a peak current density of 0.3 kA/cm/sup 2/. Results clearly show that by introducing SiGe layers to clad the B /spl delta/-doping layer, B diffusion is suppressed during post-growth annealing, which raises the thermal budget. A higher RTA temperature appears to be more effective in reducing defects and results in a lower valley current and higher PVCR. RITDs grown by selective area molecular beam epitaxy (MBE) have been realized inside of low-temperature oxide openings, with performance comparable with RITDs grown on bulk substrates.

ROOM-TEMPERATURE PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE FROM ER-DOPED SILICON-RICH SILICON OXIDE

Porous silicon was doped by Er ions using electroplating and was converted to silicon-rich silicon oxide (SRSO) by partial thermal oxidation at 900 °C. The room-temperature photoluminescence (PL) at ∼1.5 μm is intense and narrow (⩽15 meV) and decreases by less than 50% from 12 to 300 K. The PL spectrum reveals no luminescence bands related to Si-bandedge recombination, point defects, or dislocations and shows that the Er3+ centers are the most efficient radiative recombination centers. A light-emitting diode (LED) with an active layer made of Er-doped SRSO (SRSO:Er) was manufactured and room temperature electroluminescence at ∼1.5 μm was demonstrated.

Stable photoluminescence and electroluminescence from porous silicon

Abstract By carefully controling the nanocrystallite surface passivation, it is possible to make light-emitting porous silicon essentially inert and to stabilize its photoluminescence. Using this material, which we call silicon-rich silicon oxide (SRSO), stable and efficient porous silicon light-emitting devices (LEDs) emitting in the visible have been manufactured. The materials optimization, device design, and device fabrication that have allowed us to achieve these goals are discussed. The electrical and optical properties of the LEDs are described and explained by a model for carrier transport and recombination. By changing the preparation and processing conditions and by doping the SRSO layer with impurities such as erbium, photoluminescence and electroluminescence at longer wavelengths have been demonstrated.

A Si‐based light‐emitting diode with room‐temperature electroluminescence at 1.1 eV

We have achieved room‐temperature electroluminescence (EL) at 1.1 eV from a light‐emitting diode with an active layer prepared by high‐temperature partial oxidation of electrochemically etched crystalline silicon. The EL is easily measurable under a forward bias ≥ 1 V and a current density <10 mA/cm2 and is only weakly temperature dependent from 12 to 300 K. The luminescence is due to Si band edge radiative recombination and originates from large silicon clusters within a nonstoichiometric silicon‐rich silicon oxide matrix.

Nanocrystalline silicon superlattices: fabrication and characterization

Abstract Rapid thermal annealing of amorphous Si/SiO 2 superlattices deposited by rf-sputtering and plasma oxidation forms Si nanocrystals while maintaining the superlattice structure. The crystal size is controlled by the thickness of the Si sublayers and can be varied from 12 nm to ∼2.5 nm. The strain in the c-Si/SiO 2 structure is released by annealing at 1050°C using a slow temperature ramp. After wet oxidation which reduces the size and completes the passivation of the crystals by SiO 2 , the room-temperature band-edge photoluminescence reaches an external quantum efficiency >0.1%.

Solvent Detection and Water Monitoring With a Macroporous Silicon Field-Effect Sensor

Integration of electrical and fluidic systems for the design and fabrication of a system-on-chip (SOC) capable of sensing various liquid phase solvents is reported. A monolithic integration strategy makes use of macroporous silicon (MPS) as a gateway to interface the electrical and fluidic domains. In this application, the MPS material, acting as a sensing membrane, is used in a flow-through structure to transport an analyte from fluidic channels on one side of the chip to sensing electrodes on the other. A fluid-oxide-semiconductor interface results in the modulation of a space charge region in the semiconductor where real-time measurements are used to detect and distinguish between the presences of various solvents. The fluidic system has delivered sample volumes as small as 2 mul. Selected test solvents (i.e. acetone, ethanol, isopropyl alcohol, methanol, and toluene) have generated a measured change in capacitance up to 11%. A practical application of this sensor was demonstrated by monitoring various concentrations of isopropyl alcohol in a water supply. Undiluted samples provide characteristic responses that can be used for signature identification. The sensing device has a high degree of reusability and does not require heating or other solvent removal methods often necessitated in other sensing devices