S. P. Duttagupta

Photoluminescence and electroluminescence in partially oxidized porous silicon

Abstract The results of photoluminescence (PL) and electroluminescence (EL) studies from partially oxidized porous silicon (POPS) layers are presented. The PL from POPS is stable, peaks at 600-570 nm and its temperature dependence can be fitted by an exponential law with an activation energy Ea ≈ 10 meV. The current-voltage characteristics of Au-(POPS)-crystalline silicon (c-Si) structures follow a power law I ≈ Vn. When the index n becomes higher than 2, electroluminescence (EL) is found. The EL peaks at 760 nm and is stable for more than 100 hours of operation. The intensity of the EL is a linear function of current for all measured structures up to current density J ≈ 1 A/cm2. Our results suggest that partially oxidized porous silicon is more useful for device applications than freshly anodized porous silicon which has unstable properties or than fully oxidized porous silicon in which transport is poor.

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.

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.

Post-Anodization Implantation and CVD Techniques for Passivation of Porous Silicon

Proper surface passivation is critical for achieving stable, efficient PL from light-emitting porous silicon (LEPSi). As-anodized LEPSi is passivated by hydrogen which desorbs at a temperature as low as 400 °C. For device purposes, it is necessary that porous Si can tolerate at least 450 °C for post anodization annealing/metallization steps. We have established that, if the hydrogen at the surface is substituted by oxygen, the resulting Si-O x passivation is significantly more stable. One way of achieving this is to implant low energy/low dose oxygen to form a thin coating of SiO 2 on the surface. Post implantation FTIR data report the absence of Si-H peaks. XPS data indicate the formation of nearly stoichiometric SiO 2 at the surface. Similar results were achieved by implanting with nitrogen to form Si 3 N 4 . As an alternative to implantation, we have deposited thin capping layers of SiO 2 , Si 3 N 4 and SiC by plasma-enhanced chemical vapor deposition (PECVD) which resulted in a similar degree of passivation. Wafers were pre-treated at 400 °C to remove hydrogen from the surface. Finally, we carried out a low-pressure CVD (LPCVD) oxide deposition on LEPSi. Post implantation/CVD annealing was done at temperatures up to 600 °C. In most cases, little or no change was observed in the resultant PL intensities.

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.

Confocal scanning beam laser microscope/macroscope: applications requiring large data sets

A new confocal scanning beam laser microscope/macroscope is described that combines the rapid scan of a scanning beam laser microscope with the large specimen capability of a scanning stage microscope. This instrument combines an infinity-corrected confocal scanning laser microscope with a scanning laser macroscope that uses a telecentric f*(theta) laser scan lens to produce a confocal imaging system with a resolution of 0.25 microns at a field of view of 50 microns to 5 microns at a field of view of 75,000 microns. The frame rate is 3 seconds per frame for a 512 X 512 pixel image, and 45 seconds for a 2048 X 2048 pixel image. Changes made in the instrument to increase the image capture from 512 X 512 pixels to 2048 X 2048 pixels are described. Applications discussed focus on three important advantages of the instrument over a confocal scanning laser microscope: an extremely wide range of magnification, the ability to record very large data sets, and the ability to image very large specimens. Examples are presented from imaging of fibers in paper, latent fingerprint detection, and reflected-light and photoluminescence imaging of porous silicon.

A New Confocal Scanning Laser MACROscope/Microscope Applied to the Characterization of Solar Cells

Scanning stage microscopes have traditionally been used to provide high resolution, large area photocurrent mapping of solar cells and detectors. This imaging method, while very useful in the characterization and quality control of solar cells, is unfortunately slow (image acquisition takes several minutes). This paper describes a confocal scanning beam MACROscope-Microscope which can image specimens from 25 x 25 {micro}m up to 7.5 x 7.5 cm in size, a zoom factor of 3,000, using reflected light, photoluminescence, and optical beam induced current in less than 10s. Resolutions range from 0.25 to 10 {micro}m laterally and 0.5 to 300 {micro}m axially depending upon whether microscope of MACROscope mode is used. This instrument can therefore be used to characterize and provide quick and efficient quality control for solar cells and detectors at a microscopic and macroscopic level. A brief description of the MACROscope-Microscope is given. The MACROscope-Microscope`s many abilities are highlighted by showing various reflected-light, photoluminescence and optical beam induced current images from CdZnS/CuInSe{sub 2} thin film solar cells and porous silicon devices.