C. Peng

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 ...

The frequency response of porous silicon electroluminescent devices

Under a constant forward bias, porous silicon light‐emitting devices (LEDs) produce stable electroluminescence (EL) that is detectable at applied voltages as low as 5 V and visible in daylight at higher voltages. The recombination dynamics of the EL are studied and correlated to the photoluminescence properties of light‐emitting porous silicon (LEPSi). The EL efficiency is related to the LEPSi properties and the device configuration. LEPSi LEDs with an EL efficiency of 0.01% have been achieved. The frequency response of the EL to a modulating ac bias is measured. For metal/LEPSi LEDs, the −3 dB frequency is determined by the carrier transit time which must be larger than the carrier lifetime to achieve efficient EL. For LEPSi pn junction LEDs, the −3 dB frequency is determined only by the carrier lifetime and can be in excess of 200 kHz.

Enhancement and suppression of the formation of porous silicon

We present the results of an investigation of various means to enhance or suppress the formation of porous silicon. The first method involves a lithographic process using silicon nitride to produce sub‐0.5 μm light emitting porous silicon (LEPSi) lines adjacent to fully protected silicon regions. The second method consists of amorphizing regions of the wafer prior to anodization with high energy/high dose ion implantation, followed by anodization and annealing. In this method, LEPSi is produced in the unimplanted regions only. Using focused ion‐beam implantation ∼100 nm patterns have been obtained. The third method utilizes low energy/low dose bombardment (ion milling/reactive ion etching) with argon ions prior to anodization. Under appropriate bombardment conditions, we have observed a strong enhancement of the formation rate of LEPSi, possibly due to the generation of a large number of defects on the wafer surface. Our results demonstrate that porous silicon light emitting diodes (LEDS) and silicon elec...

Photoluminescence imaging of porous silicon using a confocal scanning laser macroscope/microscope

This letter describes a confocal scanning beam macroscope/microscope that can image specimens up to 7 cm in diameter using both photoluminescence and reflected light. The macroscope generates digital images (512×512 pixels) with a maximum 5 μm lateral resolution and 200 μm axial resolution in under 5 s, and in combination with a conventional confocal scanning laser microscope can provide quality control at a macroscopic/microscopic level for porous silicon specimens, wafers, detectors, and similar devices. This combination of instruments can also be used as a method for evaluating preparation parameters involved in the manufacture of porous silicon. Various confocal and nonconfocal photoluminescence and reflected‐light images of porous silicon are shown using both a macroscope and a conventional confocal scanning laser microscope. A 3D profile of a porous silicon structure reconstructed from confocal slices is also shown.

Ion implantation of porous silicon

We have investigated the properties of light‐emitting porous silicon after ion implantation and successive annealing through continuous‐wave photoluminescence (CWPL) and time dependent photoluminescence (TDPL) spectroscopies. Implantation was performed with phosphorus, boron and silicon ions of different doses and energies. Low dose dopant implantation keeps or even increases the CWPL intensity and increases the TDPL decay time. High dose dopant implantation and silicon self‐implantation reduce the CWPL intensity and slightly decrease the TDPL decay time.

Optical characterization of light-emitting porous silicon

We report the results of an extensive optical characterization of the properties light-emitting porous silicon (LEPSi), using optical techniques such as Raman spectroscopy, FTIR, cw photoluminescence (PL) and time-resolved PL spectroscopy. Additional insight is obtained from several nonoptical techniques, such as optical and electron microscopy, atomic force microscopy, and various surface physics tools. We examine how to control the surface passivation of LEPSi and what the consequence for light emission are. Samples with widely different surface chemistry have been prepared by controlling the electrochemical processes during anodization or by selected post-anodization treatments such as low- and high- temperature oxidation. In particular, we discuss the relationship between the presence of Si-H, Si-O-H, and Si-O bonds, and the relative strengths of the red PL line have a microsecond(s) ec decay time and the blue PL having a Nsec decay time. These results are compared to the predictions of the leading models that have been proposed to explain the efficient room-temperature luminescence of porous silicon.

Comparative Study of Light-Emitting Porous Silicon Anodized with Light Assistance and in the Dark

In this study, we compare two different types of light emitting porous silicon (LEpSi) samples: LEpSi anodized in the dark (DA) and LEpSi anodized with light assistance (LA). On the basis of photoluminescence (PL), Raman, FTIR, SEM, spatially resolved reflectance (SRR) and spatially resolved photoluminescence (SRPL) studies, we demonstrate that the luminescence in LA porous silicon is strong, easily tunable, very stable and originates from macropore areas. These attractive properties result from passivation by oxygen in the Si-O-Si bridging configuration that takes place during electrochemical anodization. In addition, we have been able to correlate light emission with the presence of crystalline silicon nanograins.

Prospects for light-emitting diodes made of porous silicon from the blue to beyond 1.5 μm

Since the 1990 discovery that porous silicon emits bright photoluminescence in the red part of the spectrum, light-emitting devices (LEDs) made of light-emitting porous silicon (LEPSi) have been demonstrated, which could be used for optical displays, sensors or optical interconnects. In this paper, we discuss our work on the optical properties of LEPSi and progress towards commercial devices. LEPSi photoluminesces not only in the red- orange, but also throughout the entire visible spectrum, from the blue to the deep red, and in the infrared, well past 1.5 micrometers . The intense blue and infrared emissions are possible only after treatments such as high temperature oxidation or low temperature vacuum annealing. These new bands have quite different properties form the usual red-orange band and their possible origins are discussed. Different LED structures are then presented and compared and the prospects for commercial devices are examined.

Micron-Size and Submicron-Size Light-Emitting Porous Silicon Structures

We have developed three classes of techniques to produce micron-size and submicron-size light emitting porous Si (LEPSi) patterns and to protect the rest of the wafer. In the 1st class, LEPSi lines down to 2 µm width have been made using a photoresist/silicon nitride trilayer mask, followed by anodization. PL mapping of the structures indicates that the protected regions have not been etched. Using electron beam lithography sub-0.5 micron porous Si lines have been generated. In the 2nd class, formation of porous Si is inhibited by amorphizing Si using ion implantation followed by anodization and annealing. The crystallinity and electrical properties of the implanted region have been fully characterized after annealing. Using focussed ion-beam implantation, LEPSi patterns of the order of 100 nm have been obtained. The 3rd class consists of enhancing the formation of porous Si by a low energy/low dose bombardment (ion-milling) with argon ions prior to anodization. Under appropriate conditions, we have observed a strong enhancement of the formation rate of LEPSi where bombardment took place, possibly due to the generation of a large number of defects on the wafer surface.

Drift mobility measurements in porous silicon

Modulated electroluminescence (EL) measurements performed on a series of porous silicon (PSi) diodes are presented. The maximum response time of the devices scales with the square of the PSi layer thickness and inversely with the applied forward bias voltage. These scaling results indicate that the maximum response time is a carrier transit time from which a drift mobility {mu} of 10{sup {minus}4} cm{sup 2}/Vs is deduced at room temperature. Time-of-flight transport measurements on PSi are in qualitative agreement with this value for {mu}; in addition, they identify {mu} as the electron mobility and show that transport is dispersive, in contrast to the interpretation of the modulated EL experiments.