AVS1996 Session EM-WeA: Novel Opto-electronic Materials and Processing

Wednesday, October 16, 1996 2:00 PM in Room 204A

Wednesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	EM-WeA-1 Recent Advances in III-V Heteroepitaxy for Photonic Devices J. Cunningham (Bell Laboratories) We report on efforts that exploit heteroepitaxy of III-V materials so as to enable a new class of photonic devices. Two specific examples illustrating this are the i) monolithic integration of GaAs/AlGaAs modulators to Si containing pre-existing IC’s and ii) monolithic integration of InP based materials on GaAs and Si for 1.55\mu\m devices. Each material integration is currently driven technologically by strategic telecommunication applications such as terabit capacity photonic switching for the former and fiber access to the home for the latter. From a heteroepitaxial consideration both examples contain a comparable lattice mismatch, of about 5%, but differ fundamentally in that GaAs on Si also destroys crystal symmetry due to the broken inversion symmetry. In the above examples we report on the initial stages of epitaxy and how their differences connect to heteroepitaxy considerations. These investigations are based on RHEED, Auger spectroscopy and Xray diffraction. We have further identified this stage of epitaxy to be crucial to a photonic device’s capacity to modulate light signals as measured with voltage dependent absorption spectroscopy. For example, formation of a nearly perfect staircase of bilayer steps on Si surfaces is required in order that 850 nm modulators grown on Si produce the same optical modulation performance obtained on GaAs substrates. No such requirement is found for InP/GaAs. In fact here, heteroepitaxial healing beyond the interface is much accelerated compared with GaAs/Si as is determined by RHEED diffraction analysis. Furthermore,the status of how photonic applications can enhacened by III-V/Si-electronics is discussed along the various successful forms of integration technologies curently available.
2:40 PM		EM-WeA-3 Synthesis and Characterization of Epitaxial Sn\sub X\Ge\sub 1-x\ Alloy Thin Films G. He, H. Atwater (California Institute of Technology); G. Jellison (Oak Ridge National Laboratory) The group IV metastable Sn\sub x\Ge\sub 1-x\ alloy system is an interesting semiconductor material with potential applications in the fabrication of Si-based infrared optoelectronic devices. Band structure calculations have suggested that the Sn\sub x\Ge\sub 1-x\ alloys have direct energy gaps continuously tunable from 0.55 eV to 0 eV with very small electron effective masses for compositions x from 0.2 to 0.6. However, synthesis of Sn\sub x\Ge\sub 1-x\ alloy films in the direct gap composition range by conventional thin film growth techniques have not been successful due to the severe surface segregation of Sn during the film growth. We report the synthesis of epitaxial Sn\sub x\Ge\sub 1-x\/Ge/Si<001> with compositions up to x=0.34 by electron cyclotron resonance (ECR) ion-assisted molecular beam epitaxy in the substrate temperature range of 120 to 200 degree C. The ECR ion source produces 30-50 eV Ar\super +\ ions with ion to atom flux ratios of the order of unity. The high flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as x-ray rocking curve indicated that the Sn\sub x\Ge\sub 1-x\ alloy films are epitaxial, and Rutherford backscattering spectra confirmed the Sn\sub x\Ge\sub 1-x\ alloy compositions and indicated an absence of Sn segregation. Fourier transform infrared transmission spectroscopy in the spectral range of 0.2 eV to 0.9 eV showed decreased infrared transmittance with increasing Sn concentrations from x=0 to 0.30 relative to Ge layers of the same thickness, which is consistent with decreased band gap. Transmission measurements were also performed on Sn\sub x\Ge\sub 1-x\ samples of the same composition but different thickness ranging from 50 nm to 300 nm to study the optical cavity effects in the Sn\sub x\Ge\sub 1-x\ layers. Spectroscopic ellipsometry measurements in the 1.5 eV to 5 eV range indicated a red shift of the fundamental gap with increasing Sn composition. Results of infrared photoconductivity measurements at 77K will also be discussed.
3:00 PM		EM-WeA-4 Growth of GeSi Short-period Superlattices by Chemical Vapor Deposition D. Greve, A. Mocuta (Carnegie Mellon University) Short-period superlattices are of interest for optical emitters and detectors in the near-infrared region. So far, such structures have been grown by MBE either at very low substrate temperatures or by using surfactants to prevent surface segregation of germanium. In this talk, we report on the growth of such structures using ultra-high vacuum chemical vapor deposition. By growing at temperatures near 550 C, much of the surface will be hydrogen-terminated, which reduces germanium segregation similarly to surfactants. Germanium- silicon superlattices with periods of 23 and 38 \Ao\ have been grown from germane and silane at 550 C. Substrates were (100) silicon either with or without a graded buffer layer. The compositions obtained show that the growth rates are considerably different from those of thick films, suggesting that the surface composition continuously changes due to segregation. We will report on characterization of superlattices by X-ray diffraction and Raman spectroscopy. The degree of intermixing between Ge and Si layers as a function of growth conditions will be discussed.
3:20 PM	Invited	EM-WeA-5 Active Devices with Organic Materials A. Dodabalapur (Bell Laboratories) Some classes of organic\polymeric materials have very interesting electrical and optical properties which has made possible the realization of very efficient visible light-emitting diodes and thin film transistors with adequate mobilities. This talk will review the important characteristics of both materials and devices, and highlight those properties which make organics so interesting. The optical processes in organic LED materials and their modification through microcavity effects will be explained. The optical characteristics of a photopumped 'plastic' laser will be briefly described. The electrical and morphological characteristics of organic transistors and the rationale behind the choice of materials for achieving relatively high field-effect mobilities will be discussed.
4:00 PM		EM-WeA-7 Chemical Vapor Deposition of Polymers used in Light Emitting Diodes K. Vaeth, K. Jensen (Massachusetts Institute of Technology) Electroluminescent polymers such as poly(phenylene vinylene) (PPV) and its derivatives offer an attractive alternative to inorganic semiconductors for materials used in large area flat panel displays. The major drawback of polymer based devices is short operating lifetimes (ca. 1000 hrs), which have been attributed to degradation of the polymer/metal interface, and bulk oxidation of the polymer. Consequently, control of the interface structure and impurities introduced during device fabrication and operation is critical for extending the device lifetime. Since PPV is not soluble in any known solvent, films are usually fabricated by solution processing a polymer precursor, followed by a thermal conversion to the fully conjugated polymer. Control of impurity levels with solution processing is difficult, and it is desirable to find alternative fabrication methods which offer better control over impurity incorporation into the film. In this research, chemical vapor deposition (CVD) of PPV is explored as an alternative to solution processing methods traditionally used to fabricate polymer-based LEDs. We have successfully fabricated high-quality PPV films from a from a p-dichloroxylene monomer by CVD, and characterized the film formation and thermal conversion in-situ with Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). Our results show that the preferential orientation of the phenyl group of vapor deposited PPV is in the plane of the polymer-metal interface. Ex-situ characterization of the vapor deposited PPV films with Fourier transform attenuated total reflection infrared spectroscopy (ATR), UV-VIS spectroscopy, and photoluminescence will be presented, and comparisons will be made to solution processed PPV films. The fabrication of LEDs in vacuum will also be described, and comparisons will be made between CVD and solution processed devices.
4:20 PM		EM-WeA-8 Transition to Crystalline Growth of an Organic Molecular Semiconductor on InAs(100) Studied by STM C. Kendrick, A. Kahn (Princeton University) We use scanning tunneling microscopy (STM) to study the mechanisms of crystalline growth for the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on In-terminated InAs(100). Previous work[1,2] has shown that the growth of molecular crystals on inorganic semiconductors is improved by substrate chemical passivation, however, the role of substrate morphology on film crystallinity has yet to be elucidated. To this end, we prepare In-terminated InAs(100) (4x2) surfaces which have a high degree of structural order and are morphologically very smooth, but which are un-passivated. PTCDA is incrementally deposited and characterized in UHV, with growth continued to a nominal thickness of 25ML. Two growth regimes are observed. Initially, the molecules interact strongly with the substrate, leading to two-dimensional growth up to 2-4ML and forming rows aligned along the top layer In dimers. The film shows evidence for multiple molecular layers with a high degree of coherence along the 4x direction of the substrate, but lacking a well-defined unit cell. Further PTCDA deposition results in the formation of crystalline clusters with various sizes and geometries which depend on the deposition conditions. The onset of this growth regime occurs at the coverage where the molecule-substrate interaction becomes weaker than the bulk intermolecular forces. We present the first report of molecularly resolved images from such thick (10-25ML), highly crystalline clusters and show that the unit cell dimensions and orientation are consistent with the (102) plane of bulk PTCDA, as previously proposed[2]. [1] H. Tada, T. Kawaguchi, and A. Koma, Appl. Phys. Lett. 61, 2021 (1992) [2] Y. Hirose, S. R. Forrest, and A. Kahn, Appl. Phys. Lett. 66, 944 1995)
4:40 PM		EM-WeA-9 Study of Effects of Chemistry, Surface States, and Confinement on the Wavelength and Lifetime of Emission from Si Nanocrystals F. Adar (ISA/SPEX Industries); E. Forsythe (Structured Materials Industries, Inc.); J. Mattheis, S. Atzeni (ISA/SPEX Industries, Inc.); D. Morton (Army Research Laboratories); G. Tompa (Structured Materials Industries, Inc.) In the following study, we shall report the light emission characteristics of Si nanocrystals in a Si oxide matrix. These materials have demonstrated significant photoluminescence (PL) and electroluminescence (EL), which have applications for electro-optic, LED, and detector devices. The origin of the light emission has been attributed to surface chemistries, direct band recombination, and other quantum confinement effects. The Si nanocrystal based films were grown by Low Pressure Chemical Vapor Deposition (LPCVD) and annealed in a nitrogen atmosphere at temperatures ranging from 875C to 1025C. The PL wavelength and lifetime as a function of particle size are reported, where the mean crystallite sizes were estimated by Raman spectroscopy. The particle sizes range from 6.5nm to 8nm depending on the annealing conditions. In addition, the Raman spectra show the presence of an amorphous Si phase in the films, which decreases as the annealing temperature increases. The PL emission has a strong red peak centered above 850nm as well as emission in the blue spectral region. Photoluminescent Excitation (PLE) spectroscopy results are also reported and show the red emission is consistent with absorption in Si nanocrystals. The PL lifetime has multi-exponential characteristics that demonstrate a correlation with the mean particle size. In summary, the light emission properties from Si nanocrystalline based films shows important characteristics that will assist in the development of devices from these materials.