Nader M. Kalkhoran

Visible electroluminescence from porous silicon np heterojunction diodes

We report the preparation of silicon‐based visible light‐emitting diodes, configured as heterojunctions between porous silicon (formed by electrochemical etching of p‐type silicon wafers), and n‐type indium tin oxide (ITO). The transparent ITO film allows light emission through the top surface of the device, under a forward electrical bias of several volts across the junction. Photogenerated currents are observed under reverse biases. A tentative model for this electroluminescence is presented, based on injection of minority carriers through a narrow interphase region into the porous silicon structure, where radiative recombination occurs.

Current injection mechanism for porous‐silicon transparent surface light‐emitting diodes

We present a model for the injection of minority carriers into porous silicon films which results in visible dc electroluminescence. A thin interfacial dielectric region is postulated between the surface of the porous silicon layer and a transparent conductive oxide on the surface, which allows alignment of states between the two corresponding conduction bands of these materials under bias, and hence, overlap of electron wave functions and the passage of a tunneling current. Interface state densities are calculated and a parasitic nonradiative shunt current through such states is discussed.

Strong room‐temperature infrared emission from Er‐implanted porous Si

This communication demonstrates a strong, room‐temperature (RT), infrared (IR) (1.54 μm) emission from Er‐implanted red‐emitting (peaked at 1.9 eV) porous silicon (Er:PSi). Erbium was implanted into porous Si, bulk Si, and quartz with a dose of 1015/cm2 at 190 keV and annealed for 30 minutes in N2 at temperatures ranging from 500 °C to 900 °C under identical conditions. No RT IR emission was observed from Er implanted quartz and silicon after annealing at 650 °C (although after annealing at 900 °C very weak emission was observed from quartz at 9 K). The highest RT emission intensity at 1.54 μm was from Er:PSi with a peak concentration of 1.5×1020/cm3 and annealed at 650 °C. Even the luminescence intensity from Er:PSi annealed at 500 °C was 26 times higher than that observed from Er‐implanted quartz at 400 keV and annealed at 900 °C. A reduction in photoluminescence (PL) intensity of about a factor of two from Er:PSi over the 9 to 300 K temperature range was observed which is consistent with Er in wide ban...

Photoluminescence spectra from porous silicon (111) microstructures: Temperature and magnetic‐field effects

Visible and near‐infrared (IR) photoluminescence emission spectra (0.9–3.0 eV) from p‐type porous Si(111) microstructures are reported as a function of temperature and magnetic field. The visible peak located at 1.84 eV at 4 K shifted to ∼1.56 eV at 575 K where it disappeared; the intensity reached a maximum value at ∼150 K. The photoluminescence spectrum showed no measurable shift in the peak position with magnetic field from 0 to 15 T. Strong IR intrinsic band‐to‐band emission above and below the bulk silicon band gap at ∼1.09 eV at 300 K was observed. This luminescence was found to be enhanced by two orders of magnitude or more over the IR spectrum from an unanodized wafer.

Optoelectronic applications of porous polycrystalline silicon

We report visible light emission from porous structures formed in bulk and thin‐film polycrystalline silicon materials by anodic etching in an HF:ethanol solution. Our results indicate photoluminescence (PL) peaks at wavelengths between 650 and 655 nm and with intensities comparable to those typically obtained from porous samples of single‐crystal silicon. The analyses of the surface morphology of porous polycrystalline silicon (PPSI) layers suggest that the etch rate could be preferentially greater at the grain boundaries. We have illuminated PPSI films formed on quartz substrates from both the front and rear of the samples and have measured PL emission from the same corresponding sides. Luminescent polycrystalline silicon films offer the possibility of integrating a novel Si‐based flat‐panel display along with the recently developed thin‐film transistor (TFT) driver circuitry on a glass substrate. In addition, nanostructures originating from polycrystalline silicon substrates may enable low‐cost fabrica...

Energy bands in quantum confined silicon light-emitting diodes

Measurements of the temperature dependence of the current‐voltage characteristics of heterojunction light‐emitting diodes fabricated by depositing indium tin oxide onto the surface of electrochemically etched p‐type silicon (porous silicon) are presented, and the results are compared with those for adjacent devices formed on nonprocessed bulk silicon. The barrier height for the diodes which exhibit quantum confinement effects was determined to be 0.42 eV. Unlike the bulk silicon devices, the diodes prepared on porous silicon did not manifest a photovoltaic effect. These observations allow us to present a potential energy diagram for porous silicon heterojunction diodes which indicates barriers in both the conduction band and the valence band.

Er-Implanted Porous Silicon: a Novel Material for Si-Based Infrared LEDs

We have demonstrated a strong, room-temperature, 1.54 μm emission from erbium-implanted at 190 keV into red-emitting porous silicon. Luminescence data showed that the intensity of infrared (IR) emission from Er implanted porous Si annealed at ≤ 650°C, was a few orders of magnitude stronger than Er implanted quartz produced under identical conditions, and was almost comparable to IR emission from In 0.53 Ga 0 . 47 As material which is used for commercial IR light-emitting diodes (LEDs). The strong IR emission (much higher than Er in quartz) and the weak temperature dependency of Er in porous Si, which is similar to Er 3+ in wide-bandgap semiconductors, suggests that Er is not in SiO 2 or Si with bulk properties but, may be confined in Si light-emitting nanostructures. Porous Si is a good substrate for rare earth elements because: 1) a high concentration of optically active Er 3+ can be obtained by implanting at about 200 keV, 2) porous Si and bulk Si are transparent to 1.54 μm emission therefore, device fabrication is simplified, and 3) although the external quantum efficiency of visible light from porous Si is compromised because of self-absorption, it can be used to pump Er 3+ .

Challenges For Flat Panel Display Phosphors

Phosphors are a class of materials which emit visible light when impacted by either electrons or photons. Phosphors are the critical material in all self-emissive displays. The major display technologies which depend on phosphors are cathode ray tubes, flat cathode ray tubes (especially, field emission displays), thin film electroluminescent displays, and gas discharge plasma displays. Each of these technologies started with phosphors prepared in powder form, sprayed or screen printed onto a faceplate suitable for viewing. Electroluminescent displays have largely converted to thin film phosphors. It can be expected that, for many applications, the other competing technologies will also come to rely on more robust, high definition, thin film phosphors. Presently, full color displays must utilize several deposition and etching procedures to prepare the red, green, and blue pixels. Ion implantation of color centers is now paving the way for producing full color displays in a single host phosphor. We shall discuss the present limitations that compromise full color self-emissive displays, and present state-of-the-art solutions based on thin films and ion implantation.

Improvement of radiation hardness in fully-depleted SOI n-MOSFETs using Ge-implantation

This work demonstrates a well-controlled technique of channel defect engineering by implanting germanium into the channel of a silicon-on-insulator (SOI) MOSFET to generate subgap energy states. These subgap states act as minority-carrier lifetime killers to reduce parasitic bipolar effects. The Ge-implant also serves the dual purpose of positioning most of the subgap states in the back interface region which retard the total dose responses of off-state leakage and front-channel threshold voltage. >

Cobalt disilicide intercell ohmic contacts for multijunction photovoltaic energy converters

We have created thin buried films of low resistivity CoSi2 in silicon by ion implantation, and used them to provide intercell ohmic contacts for monolithically stacked multijunction photovoltaic energy converters. We have grown epitaxial silicon pn junction diodes by chemical vapor deposition onto the thin film of crystalline silicon formed over the CoSi2 layer after post‐implantation annealing. A single junction photovoltaic device with two CoSi2 contacts displayed an open circuit voltage of 0.60 V and a fill factor of 0.80, while a double junction tandem cell with three CoSi2 interconnects generated 1.2 V, under identical conditions of illumination with a Nd:YAG laser. These results indicate very low defect levels in the deposited silicon epitaxial layers, and excellent functioning of the CoSi2 interconnects.