8:20AM, L1
"Optical Characterization of Organic Thin Films for OLEDs:" F.G. CELII, L.S. Swanson, S.J. Jacobs, A. J. Purdes, Corporate Research Laboratories, Texas Instruments, Inc., M.S. 147, PO Box 655936, Dallas, TX 75265
Emissive organic light-emitting diodes (OLEDs) hold much promise as low-voltage, lightweight light sources for a variety of applications, including full-color displays. Typical OLEDs consist of thin layers (~500-1000 Å) of evaporated and/or polymerized organics. For instance, a typical OLED is based on green emission of a layer of this (8-hydroxyl) quinoline aluminum, or Alq3, which is 400-800 Å thick. The accuracy and repeatability of layer thicknesses is an important manufacturing issue to which optical diagnostics can be applied.
In this work, we report the optical characterization of thin, evaporated films of Alq3 and N,N'-diphernyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'diamine, or TPD, a common hole-transport layer used in OLEDs. While PL and UV/vis absorption spectra are typically reported for these films, we have obtained and analyzed variable-angle spectroscopic ellipsometry (VASE) spectra of thin Alq3 and TPD films following growth. Analysis of the ex situ VASE data yields accurate thickness values as well as the optical constants of the materials.
Thin organic and metal layers were prepared by evaporation in a bell-jar system, pumped to ~2 x 10-6 Torr. Films were deposited simultaneously on various substrates (Si, Al-coated Si, glass) so that multiple characterization techniques could be employed on similar films. Single organic layers of 500-5000 Å were prepared on Si for determination of optical constants using SE, and for thickness verification by SEM. Reflection-mode FTIR was performed using the Al-coated Si substrates, while photoluminescence (PL) measurements were made on the glass and Si samples. OLEDs were prepared using sequential deposition of blanket Alq3 and TPD films on ITO-coated glass substrates, followed by masked deposition of Mg/Ag alloy contact and Ag capping layers. Electroluminescence with emission peaked near 540 nm was observed.
Variable-angle SE (VASE) spectra were obtained using a phase-modulated SE system at angles of incidence in the 70-80deg. range. Spectral analysis of the tan [[psi]] and cos [[Delta]] data, using the WVASE software,1 was performed in two stages: first, SE data from the transparent wavelength regions of the films ([[lambda]]<550 nm for TPD;[[lambda]]<650 nm for Alq3) were fit to a Cauchy model of the optical functions to determine the layer thickness; second, the entire spectral region was then fit (point-by-point) with the optical constants unconstrained by a functional form. For improved accuracy, the data from all acquired angles were fit simultaneously. In addition, a multi-sample technique2 was employed in which data from layers of different thickness (either from different regions of the same sample, or from different samples) were fit simultaneously.
The optical constants determined from analysis of the VASE data will be reported. We will also present the results of on-going optical characterization by FTIR, PL, and UV/vis absorption. The data will be discussed in terms of extension to analysis of films on ITO/glass substrates, and for prospects for in situ SE monitoring.
_____________________________________
1The WVASE analysis software is a product of the J.A.Woollam Co.,
Lincoln, NE.
2C.M. Herzinger, H.D. Yao, P.G. Snyder, J.A. Woollam, F.G. Celii,
Y.-C. Kao and B. John, J. Appl. Phys., 77, 4677 (1995).
8:40AM, L2+
"Electrical Characteristics of Si Nanocrystallite-Based Electroluminescent Devices:" T.A. BURR, K.D. Kolenbrander, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139
Nanocrystalline silicon provides a platform on which to develop Si-based optoelectronics. As part of our continuing research in this field, we have constructed and characterized electroluminescent devices based on Si nanocrystallite thin films. The films were deposited using a pulsed laser ablation supersonic expansion technique and exhibit visible photoluminescence (PL) and electroluminescence (EL) once the surface is sufficiently passivated. Our simplest device design is a tri-layer heterostructure in which and emitter layer, consisting of Si nanocrystallites embedded in a native oxide matrix, is sandwiched between two Al and transparent ITO (indium tin oxide) electrodes. The devices produced light with output powers on the order of ~10 nW and ~10-3% efficiency. Although EL emission intensity is weak, device design and fabrication parameters are not yet optimized and these data demonstrate that Si nanostructures have strong potential for future application.
Understanding the electrical transport properties of nanocrystalline Si films and the carrier injection mechanism at the nanocrystal/electrode interface are critical in improving EL. Consequently, we have conducted a systematic study probing the conduction processes in a series of electrode/nanocrystallite layer/electrode double heterojunction devices. Three electrode systems were evaluate: Al/Al, Al/ITO, and Al/Si. We have found that electrode material and polarity of applied bias determine whether the devices operate in the interface limited or carrier transport limited regimes. Our results show the room temperature I-V characteristics of the Al/Si system are rectifying and governed by carrier injection across the nanocrystallite/c-Si interface heterojunction, at least in the reverse bias direction. In contrast, the Al/Al and Al/ITO devices give rise to dc I-V traces which are non-ohmic, non-rectifying and have near inversion symmetry. This result would not be inconsistent with a carrier injection mechanism such as tunneling or thermionic emission at the heterojunction barriers, since the work function of Al (4.26 eV) and TO (4.5-4.7 eV) are very close. However, the I-V behavior was shown to be essentially independent of temperature (over a temperature range of 4 to 300 K), which implies that the process is not thermally activated. This strong dependence on applied voltage, but weak temperature dependence indicates that some form of tunnel emission (either of trapped carriers in the nanocrystallite film or at the nanocrystal/electrode interface) may be the carrier injection/transport mechanism. Above 80 K, there is a slight increase in current density with increasing temperature and a corresponding increase in light output observed. This linear relationship between output power and current is compatible with a system limited by transport between nanocrystallites. Carrier transport mechanisms and the temperature regimes in which they are dominant will be discussed.
In the Al/ITO system, there is no Schottky or p-n junction in the nanocrystalline Si layer. Under such conditions, radiative recombination can only occur during the drift of the injected carrier from anode to cathode. Our studies suggest that this transit time ([[tau]]t, the time required for carriers to traverse the film) limits the forward bias (FB) device performance for all three of the electrode systems. Under ac modulation, the FB current increases exponentially with the time constants (an estimate of [[tau]]t) on the order of 30-80 us. The temperature dependent I-V behaviors of all three systems confirm this result.
9:00AM, L3
"Blue/Violet Emission From Epitaxial CeO2 Films on Silicon Substrates:" A.H. MORSHED, M.E. Moussa*, S.M. Bedair, Electrical and Computer Engineering Dept., North Carolina State University, Box 7911, NCSU, Raleigh, NC 27695; *Visiting from Air Defense College, Alexandria, Egypt
Photoluminescence of cerium oxide films grown epixatially on Si(111) substrates and excited by a 325 nm He-Cd laser beam is reported. A sharp emission peak at about 400 nm coincident in energy with charge transfer transitions from the Ce 4f band to the valence band of CoO2 was observed with potential applications in optical devices on silicon substrates.
CoO2 is a rare-earth insulating oxide with a face centered cubic fluorite crystal structure closely matched to silicon. Thin films of the oxide were epitaxially deposited on Si(111) substrates by puled laser ablation of 99.999% CeO2 targets under ultra high vacuum conditions. Previous studies have indicated that the dominant defects in CeO2 crystals are oxygen vacancies. X-ray diffraction indicated a non-stoichiometry of the as-grown films of about CeO1.97. The as-grown films shows a broad photoluminescence spectrum peaked at about 560 nm (2.6 eV) that might be due to oxygen vacancies while luminescence due to the 4f to valence band transitions is suppressed. Furnace annealing in O2 did not improve the photoluminescence emission properly. However, we found that rapid thermal annealing in Ar gives a fairly sharp and strong emission in the violet/blue spectrum. The emission strength increased with the thickness of the CeO2 film. A much broader and weaker peak at about 800 nm (1.55 eV) was observed in the luminescence of the treated films while barely detectable in other samples. Rapid thermal annealing could have changed the charge state of a large portion of the remaining oxygen vacancies from a singly charged state which is expected to be dominant at low temperatures and high oxygen partial pressures to a doubly charged state which is located deeper in the band gap of the material. Doubly charged vacancy traps communicate less effectively with the 4f band by electron-phonon interaction, thus draining less electrons from the band.
We will report on the optical properties of these CeO2 films and their dependence on the growth and thermal treatment conditions, and the potential applications of the films in optical device fabrication on silicon substrates.
9:20AM, L4+
"Strong Room Temperature Emission at [[lambda]]=1.54 um from Erbium Nanoparticles in a Silicon Matrix:" A. THILDERKVIST, J. Michel, S.-T. Ngiam, K.D. Kolenbrander, L.C. Kimerling, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
Erbium doped silicon is the most promising materials candidate for an integrated silicon light source at [[lambda]]=1.54 um to be used in optical interconnection. We report strong room temperature (RT) photoluminescence from erbium nanoparticles embedded in a matrix of Si. The Er nanoparticles were produced by a pulsed laser ablation supersonic expansion technique and incorporated in a Si matrix which was simultaneously laser evaporated onto a Si-wafer. This method provides a convenient way of achieving large concentrations of Er in different hosts. After annealing the Si:Er films at 500deg.C for 30 min in Ar, Er is optically activated by forming complexes with oxygen and strong photoluminescence at [[lambda]]-1.54 um is recorded. The luminescence, due to an intra-4f transition (4I13/2-4I15/2) of the Er3+ ion, exhibits a remarkable stability over a wide range of temperature contrary o, e.g., Er implanted crystalline Si which shows an intensity reduction at [[lambda]]=1.54 um of three order magnitude when the temperature is increased from 4 K to 300 K. For the Si:Er films reported here, the integrated intensity is only reduced by 50% in the same temperature range, and no reduction is recorded between 75 K and 300 K. A similar temperature dependence is observed for the lifetime of the excited state. At 4 K the lifetime is 120 us and it decreases to 40 us at 75 K. For higher temperatures, the linewidth of the 15.4 um band increases but the integrated intensity and the lifetime stay constant. The external quantum efficiency of the Si:Er films at RT is in the range 10-4 - 10-3, nearly independent of laser power.
Before anneal, the samples show strong luminescence at u=1.57 um (C-line) due to C-O pairs in the Si-substrate, crated by the impact of hot Si particles on the wafer during deposition. Luminescence from CS-SiI-CS centers at [[lambda]]=1.28 um (G-line) in the film is also observed. This center only exists in a crystalline environment. Glancing angle X-ray results confirm that the film is polycrystalline. Depositing only Er nanoparticles does not result in any luminescence, showing that the Si matrix plays a significant role in the excitation process. When the samples are excited with a laser from the backside of the Si-wafer, Er-related luminescence is still recorded. It is thus concluded that the Er centers are excited by minority carrier recombination in the polycrystalline Si matrix at the Er complexes. The possibilities of achieving electroluminescence are presently being investigated and device structures have been fabricated. Results from electrical characterization of the Si:Er films will be presented.
9:40AM, L5+
"MBE Growth and Characterization of Europium-Doped CaF2 Layers on Silicon Substrates for Blue Light Emitting Devices:" TATHAGATA CHATTERJEE, Xiao-Ming Fang, Patrick J. McCann, School of Electrical Engineering, University of Oklahoma, Norman, OK
The ability to grow nearly lattice-matched epitaxial fluoride layers on silicon substrates has allowed fabrication of optoelectronic devices on silicon in which the fluoride layers functioned as a buffer between the optical material and the silicon [1]. Very few attempts, however, have been made to use the fluoride layer itself as the active optoelectronic material. In this work, we propose a materials system, europium-doped CaF2, that has the potential for fabricating blue light emitting devices on silicon. The layers are prepared by co-evaporating CaF2 and elemental europium in a standard MBE system. This technique [2], which offers precise control of Eu ion concentration and location, has been used to grow high crystalline quality layers with thicknesses between 1500 Å and 1 um on (100) and (111) oriented silicon substrates. Strong room temperature photoluminescence from these layers has been observed where the zero phonon line associated with europium's 4f65d -> 4f7 transition at 413 nm is Stokes and anti-Stokes broadened to yield emission that spans the violet to blue regions. Electrical characterization of these layers shows that current densities up to 102 amps/cm2 can be sustained through this material over a duration of minutes without causing catastrophic breakdown. The ability to withstand large current densities makes these layers possible candidates for electron injection and impact excitation. Results obtained from dual dielectric structures with SiO2 as an intermediate second dielectric between the substrate and the Eu:CaF2 layer will also be presented and the significance of the second dielectric will be discussed. An overview of future work towards understanding the impact excitation mechanism and realizing a practical electroluminescent device will conclude the presentation.
_____________________________________
1] A. Fach, C. Maissen, J. Masek, S. Teodoropol, and H. Zogg, "IV-VI
on Fluoride/Si Structures for IR-Sensor Applications", Mat. Res. Soc. Symp.
Proc. 299, 279 (1994).
2] X.M. Fang, T. Chatterjee, P.J. McCann, W.K. Liu, M.B. Santos, W.
Shan, J.J. Song, "Eu-Doped CaF2 Grown on Si(100) Substrates by Molecular Beam
Epitaxy", Applied Physics Letters 67, 1981 (1995).
10:20AM, L6+
"Evidence for Size and Surface Control of Visible Photoluminescence From Silicon Nanocrystallites:" E. WERWA, A.A. Seraphin, S.T. Ngiam, K.D. Kolenbrander, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139
There has been a great deal of recent interest in light emission from nanocrystalline silicon materials. The advantages of silicon based light emitters stem from their processability and the potential for integration of optoelectronic and microelectronic devices on a single wafer. There has been much debate about the identity of the light emitting species in nanostructured silicon, however, with the two most popular models being quantum confined states within the nanocrystallites and localized states at the crystallite surface (or the crystalline core/oxide interface, in the case of oxidized materials). We report here on our work on self-supporting thin films of agglomerated silicon nanocrystallites synthesized by a novel pulsed laser ablation supersonic expansion. These films have been observed to photoluminesce in the visible portion of the spectrum under UV laser excitation. We have undertaken several studies to determine the role of the crystallite size and the nature and degree of surface passivation in the emission process. We have found that size impacts the emission wavelength and that surface passivation affects only the emission intensity of the materials.
We have examined the role of nanocrystallite size using three methods. First, films were chemically processed using HF/HNO3 mixtures, and acid etch/oxide regrowth steps to reduce the size of the crystalline cores. These treatments resulted in a blue shifting of the luminescence peak wavelength. Second, experimental parameters of the pulsed laser ablation deposition process, such as the length of the cluster/gas interaction channel (in which the nanocrystallites grow) and the timing of the carrier gas pulse (which controls the amount of gas present at the time of ablation, and important factor in cluster growth), were varied so as to modify the mean particle size of the crystallites being generated. Photoluminescence (PL) spectra of these materials also exhibit a blue shift for films deposited using conditions designed to reduce the mean particle size. Third, we have sought to reduce both the mean crystallite size and the distribution of sizes in the films by introduction a mechanical velocity selection apparatus into the deposition system. This velocity selector takes advantage of the fact that as the nanocrystallites are formed, they have a range of propagation speeds through the system that is related to their range of sizes. When deposited with velocity selection parameters designed to allow only the fastest crystallites (i.e., smallest particle sizes) to be collected, films exhibit PL spectra with peak wavelengths in the same region as those observed of chemically size reduced nanocrystallite films and with a much narrower spectral width, consistent with narrowed size distribution and a smaller particle size. Thus, we have seen a relationship between particle size and emission wavelength, suggesting a quantum confinement origin to the light emission properties.
The role of surface passivation has also been investigated. The PL intensity has been correlated with surface passivation through comparisons of the emission from unpassivated gas phase clusters, vacuum aged films, as deposited films (all of which have little to no surface passivation), and atmospherically aged films (with excellent surface passivation by a native oxide). Materials with no surface passivation demonstrate no PL while those with surface passivation exhibit PL. Extended dips in 48 w/o HF have been used to progressively improve the surface passivation and enhance PL intensity, indicating a correlation between degree of surface passivation and luminescence intensity. Films have also been passivated with Iodine through dips in Iodine:Methanol solutions. X-ray Photoelectron Spectroscopy (XPS) reveals that Iodine binds to the surface silicon and passivates non-radiative dangling bonds without changing the emission wavelength. The passivation results indicate that surface passivation is necessary to tie up dangling bonds and eliminate non-radiative recombination, but that the chemical species performing the passivation is not related to the emission wavelength.
10:40AM, L7
Improved Porous Silicon for Visible Light-Emitting Devices: P.M. FAUCHET, L. Tsybeskov, S.P. Duttagupta, K.D. Hirschman*, Department of Electrical Engineering, University of Rochester, Rochester, NY 14627; *Also Department of Microelectronic Engineering, Rochester Institute of Technology
The integration of silicon microelectronic circuits with optoelectronic and optical components, in particular light emitting devices (LEDs), is faced with severe problems of reliability and manufacturability due to the lack of silicon-based LEDs that operate at room temperature. Until recently, room-temperature LEDs made of porous silicon did not seem to be promising for commercial applications for several reason: (1) the efficiency was 0.01%, (2) irreversible degradation occurred in minutes, (3) the modulation speed was in the kHz regime, and (4) the compatibility with a Si fabrication line environment was not demonstrated.
In this presentation, we will address each of the four points and show that improvements in the preparation and processing of LEPSi, together with better device design, make porous silicon LEDs attractive. (1) Through improvements in device design and passivation of porous silicon, we have demonstrated power efficiencies of 0.1% at voltages and currents compatible with Si microelectronic circuitry. (2) By using high temperature oxidation and providing better heat sinking, we have manufactured LEDs that show no sign of degradation for weeks. (3) We have achieved modulation speeds in excess of 1 Mhz. (4) Our LEDs are fabricated using standard processing steps found in Si fabrication lines, such as high temperature treatments, and we will describe our efforts at integrating porous Si LEDs with simple Si microelectronic circuitry.
10:40AM, L7
"Improved Porous Silicon for Visible Light-Emitting Devices:" P.M. FAUCHET, L. Tsybeskov, S.P. Duttagupta, K.D. Hirschman*, Department of Electrical Engineering, University of Rochester, Rochester, NY 14627; *Also Department of Microelectronic Engineering, Rochester Institute of Technology
There has been a great deal of recent interest in light emission from nanocrystalline silicon materials. The advantages of silicon based light emitters stem from their processability and the potential for integration of optoelectronic and microelectronic devices on a single wafer. There has been much debate about the identity of the light emitting species in nanostructured silicon, however, with the two most popular models being quantum confined states within the nanocrystallites and localized states at the crystallite surface (or the crystalline core/oxide interface, in the case of oxidized materials). We report here on our work on self-supporting thin films of agglomerated silicon nanocrystallites synthesized by a novel pulsed laser ablation supersonic expansion. These films have been observed to photoluminesce in the visible portion of the spectrum under UV laser excitation. We have undertaken several studies to determine the role of the crystallite size and the nature and degree of surface passivation in the emission process. We have found that size impacts the emission wavelength and that surface passivation affects only the emission intensity of the materials.
We have examined the role of nanocrystallite size using three methods. First, films were chemically processed using HF/HNO3 mixtures, and acid etch/oxide regrowth steps to reduce the size of the crystalline cores. These treatments resulted in a blue shifting of the luminescence peak wavelength. Second, experimental parameters of the pulsed laser ablation deposition process, such as the length of the cluster/gas interaction channel (in which the nanocrystallites grow) and the timing of the carrier gas pulse (which controls the amount of gas present at the time of ablation, and important factor in cluster growth), were varied so as to modify the mean particle size of the crystallites being generated. Photoluminescence (PL) spectra of these materials also exhibit a blue shift for films deposited using conditions designed to reduce the mean particle size. Third, we have sought to reduce both the mean crystallite size and the distribution of sizes in the films by introduction a mechanical velocity selection apparatus into the deposition system. This velocity selector takes advantage of the fact that as the nanocrystallites are formed, they have a range of propagation speeds through the system that is related to their range of sizes. When deposited with velocity selection parameters designed to allow only the fastest crystallites (i.e., smallest particle sizes) to be collected, films exhibit PL spectra with peak wavelengths in the same region as those observed of chemically size reduced nanocrystallite films and with a much narrower spectral width, consistent with narrowed size distribution and a smaller particle size. Thus, we have seen a relationship between particle size and emission wavelength, suggesting a quantum confinement origin to the light emission properties.
The role of surface passivation has also been investigated. The PL intensity has been correlated with surface passivation through comparisons of the emission from unpassivated gas phase clusters, vacuum aged films, as deposited films (all of which have little to no surface passivation), and atmospherically aged films (with excellent surface passivation by a native oxide). Materials with no surface passivation demonstrate no PL while those with surface passivation exhibit PL. Extended dips in 48 w/o HF have been used to progressively improve the surface passivation and enhance PL intensity, indicating a correlation between degree of surface passivation and luminescence intensity. Films have also been passivated with Iodine through dips in Iodine:Methanol solutions. X-ray Photoelectron Spectroscopy (XPS) reveals that Iodine binds to the surface silicon and passivates non-radiative dangling bonds without changing the emission wavelength. The passivation results indicate that surface passivation is necessary to tie up dangling bonds and eliminate non-radiative recombination, but that the chemical species performing the passivation is not related to the emission wavelength.
10:40AM, L7
Improved Porous Silicon for Visible Light-Emitting Devices: P.M. FAUCHET, L. Tsybeskov, S.P. Duttagupta, K.D. Hirschman*, Department of Electrical Engineering, University of Rochester, Rochester, NY 14627; *Also Department of Microelectronic Engineering, Rochester Institute of Technology
The integration of silicon microelectronic circuits with optoelectronic and optical components, in particular light emitting devices (LEDs), is faced with severe problems of reliability and manufacturability due to the lack of silicon-based LEDs that operate at room temperature. Until recently, room-temperature LEDs made of porous silicon did not seem to be promising for commercial applications for several reason: (1) the efficiency was 0.01%, (2) irreversible degradation occurred in minutes, (3) the modulation speed was in the kHz regime, and (4) the compatibility with a Si fabrication line environment was not demonstrated.
In this presentation, we will address each of the four points and show that improvements in the preparation and processing of LEPSi, together with better device design, make porous silicon LEDs attractive. (1) Through improvements in device design and passivation of porous silicon, we have demonstrated power efficiencies of 0.1% at voltages and currents compatible with Si microelectronic circuitry. (2) By using high temperature oxidation and providing better heat sinking, we have manufactured LEDs that show no sign of degradation for weeks. (3) We have achieved modulation speeds in excess of 1 Mhz. (4) Our LEDs are fabricated using standard processing steps found in Si fabrication lines, such as high temperature treatments, and we will describe our efforts at integrating porous Si LEDs with simple Si microelectronic circuitry.
11:00AM, L8+
"Fabrication of Low-Loss Polycrystalline Silicon Waveguides:" ANURADHA M. AGARWAL, James S. Foresi, Ling Liao, L.C. Kimerling, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 13-4126, 77 Massachusetts Ave., Cambridge, MA 02139
Photonic integrated circuits in silicon require optical waveguides composed of materials which are compatible with silicon VLSI technology. Polycrystalline silicon (polySi), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. The lowest reported optical loss for polySi in the literature is 350dB/cm. By engineering the waveguide fabrication process to reduce losses by light scattering and absorption, we have achieved a world record transmission loss of 15dB/cm at [[lambda]]=1.54cm. This level of performance is the first to meet the requirements for on chip optical clock distribution.
Our first polySi waveguide loss measurements gave a value of 77dB/cm in as-deposited polySi structures. We have determined that the main source of loss are light scattering by rough surfaces and absorption at dangling bond-sites. We have reduced the surface scattering loss by 40dB/cm by fabricating waveguides in smooth recrystallized amorphous silicon or Chemo-Mechanically Polished (CMP) polySi. Both techniques lead to a decrease in RMS roughness from a value of 19-20nm to 4-6nm. The surface roughness is measured by Atomic Force Microscopy (AFM) and reflectance spectrophotometry. We have achieved a record low loss by hydrogen passivation of electronic defect states to reduce absorption. By means of Electron-Cyclotron Resonance (ECR) hydrogenation, we have reduced bulk losses at [[lambda]]=1.54um to 15dB/cm. The dependence of the residual bulk absorption losses on size, structure, and quality of grains and grain boundaries, is investigated by means of Transmission Electron Microscopy (TEM). We obtain a grain size of 0.4um at the lowest deposition temperature (560deg.C) compared to the grain size of 0.1um for the highest temperature (625deg.C). Our measurements indicate that the loss is independent of the grain size in this range.
We conclude with a physical explanation for the residual loss of 15dB/cm and provide a systems overview of waveguide material compatibility for silicon optical interconnection. Other compatible waveguide materials include SiO2 and Si3N4. However, the dielectric contrast (optical confinement) in each of these cases is much lower than that afforded by the polySi/SiO2 system. These materials, in addition, do not provide a common platform for integrated photonic devices consisting of emitters, waveguides and detectors in the same material. Crystalline silicon in the form of BESOI (Bonded and Etched-back Silicon On Insulator) or SIMOX (Separation by Implantation of Oxygen) structures can provide the common platform but only polySi offers the flexibility in processing for a multilevel interconnection option.
11:20AM, L9
"Picosecond Photoresponse of Carriers on Si:" ALBERT CHIN, K. Lee, W.J. Chen, Y.S. Zhang, S. Horng, J. H. Kao, Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan; Department of Mechanical Materials Engineering, National Yun-Lin Polytechnic Institute, Huwei, Taiwan; Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan
High speed integrated receiver is one of the key elements in fiber-optic communication. At present, a high speed III-V photodetector and Si IC are hybrid together as an integrated receiver. However, monolithic integration between high speed photodiodes and Si IC is essential to achieve better performance and mass production. In this abstract, we report the ps carrier lifetime on Si, which can be used in the monolithic integration of high speed photodetector and Si IC. To achieve this goal, we have studied the improved photoresponse in Si material and the LT-GaAs bonded on Si. It was reported that the ion-implanted process can generate high concentration of defects, which makes the As ion-implanted GaAs to achieve similar high speed photoresponse as LT-GaAs. Ps photoresponse of carriers have been measured using femtosecond transient reflectivity. A threshold implanted dose of 1E16 cm-2 is required to achieve picosecond carrier lifetime. Carrier lifetimes of 1.2 and 1.9 ps are measured from the as-implanted and 400deg.C annealed Si respectively. The relative intensity of photoresponse is also decreased by a factor of two after 400deg.C annealing. In contrast, there were no measurable photoresponse up to 1 ns for post-growth annealing above 600deg.C. The increased carrier lifetime after annealing is due to the reduced concentration of trap and recombination centers by the annealing effect. The measured carrier lifetimes are also strongly related to the sheet resistance. An eight fold increased sheet resistance after 400deg.C annealing may be due to the similar reduced hoping conduction that observed in LT-GaAs. Further evidence can also be measured from more than two orders of reduced sheet resistance as implanted dosage increased from 1E14 to 1E16 cm-2, where the concentrations of defects are increased with the increased implanted dosage. In order to explore the variety of ps photoresponse on Si, we have also studied the thermally bonded LT-GaAs on Si. One um thick LT-GaAs was grown at 250deg.C on a high temperature grown AlAs, and the GaAs substrate was lift-off from an IlAs after bonded Si, which is slightly better than the 1.2 ps value of Si ion-implanted Si. However, better thermal stability was achieved in LT-GaAs bonded Si. Detailed comparison of ion-implanted Si and LT-GaAs bonded Si will be presented.
11:40AM, L10
"Intrinsic and Extrinsic Luminescence in Si-rich Silicon Oxide Prepared by Oxidation of Porous Silicon:" L. TSYBESKOV, P.M. Fauchet, K.L. Moore, D. G. Hall, Department of Electrical Engineering and the Institute of Optics, University of Rochester, Rochester, NY 14627
Porous silicon, a form of Si made by anodization in an HF solution, contains nanocrystallites covered and passivated by Si-H bonds. It photoluminesces efficiently in the visible due to quantum confinement. Silicon-rich silicon oxide (SRSO) produced by high temperature oxidation of porous silicon is a novel composite material consisting of Si nanoclusters and an oxide tissue which displays attractive optoelectronic properties. This presentation is concerned with the intrinsic luminescence of undoped SRSO and its extrinsic luminescence after doping with impurities such as Er, Nd, Se and S.
When the oxidation is performed at a temperature T < 900deg.C, the intrinsic photoluminescence (PL) of SRSO is the familiar red band near 700 nm that is presumably due to quantum confinement in Si nanocrystals. When 900deg.C < T < 950deg.C, the intrinsic PL is in the infrared near 1.15 um. The low-temperature PL spectrum shows that this luminescence is due to phonon-assisted band-edge recombination within large Si grains that are presumably formed during the heat treatment. The most striking result is that the integrated PL spectrum is temperature-independent. When T > 950deg.C, the red/infrared PL disappears, consistent with the production of a porous glass.
SRSO can be doped with a large number of impurities, using electroplating
between the anodization and oxidation steps or using implantation before
anodization. SRSO implanted with S and Se exhibits room temperature PL near
1.4um and 1.2um respectively. Compared to previous reports of Er+
and Nd+ is strongly luminescent at 1.54 um and 1.06um respectively.
Compared to previous reports of Er+ doping of porous Si, we find
that the PL spectrum is free from a broadband background that can be associated
with dangling bonds in the porous Si matrix. The preparation and processing
parameters that control the PL intensity and the prospects for infrared
electroluminescent devices will be discussed.
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