AVS1997 Session EM-MoP: Issues in Optoelectronics
Monday, October 20, 1997 5:30 PM in Room Exhibit Hall 1
Monday Afternoon
Time Period MoP Sessions | Topic EM Sessions | Time Periods | Topics | AVS1997 Schedule
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EM-MoP-1 Effect of Atomic Hydrogen on Er Luminescence for AlN
S.J. Pearton, C.R. Abernathy, J.D. MacKenzie (University of Florida, Gainesville); U. Hommerich, X. Wu (Hampton University); J.M. Zavada (US Army Research Office); R.G. Wilson, R.N. Schwartz (Hughes Research Laboratories) AlN doped with Er during Metal Organic Molecular Beam Epitaxy has been plasma hydrogenated in-situ at 200-250°C using an ECR source. By isotopic substitution of 2H for 1H, we have found that using Secondary Ion Mass Spectrometry profiling a 30 min hydrogenation treatment can incorporate ~2x1019 atoms.cm-3 to depths ≥ 1 micron. The intensity of the 1.54 micron Er3+ luminescence is increased by a factor of ~5 by the 200°C hydrogenation, and this affect is thermally stable to 600°C, indicating a binding energy of >3eV for hydrogen at defects in the AlN which would normally either be recombination centers or provide an alternative de-excitation path for the Er. We have previously found that AlN provides the best resistance to thermal quenching of Er luminescence of any semiconductor due to its wide bandgap, and together these results suggest that AlN(Er) may be a promising material for optical control of devices such as light-triggered SiC or GaN thyristors for power switching applications, where a fiber-transmitted signal from a temperature-tolerant material is necessary in controlling power distribution grids. Hydrogen does not leave the AlN until ~800°C and presumably forms an intermediate state such as H2 or larger clusters prior to evolution from the surface, and again this stability is among the best for any semiconductor. |
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EM-MoP-2 Comparison of ICP and ECR Cl2 and CH4/H4 Etching of III-Nitrides
C.B. Vartuli, J.W. Lee, Y.-B. Hahn, S.M. Donovan, C.R. Abernathy, S.J. Pearton (University of Florida, Gainesville); R.J. Shul (Sandia National Laboratories); C. Constantine (Plasma Therm, Inc.) Patterning of the InGaAlN system by dry etching is critical for mesa formation in LEDs and lasers, and for trench formation in high power nitride thyristors. We have previously shown that high density plasma sources such as ICP and ECR produce etch rates typically five times higher than conventional reaactive ion etching systems. In this current work we will describe a comparison of etch rates, etch selectivities, feature anisotropy, surface stoichiometry changes and damage in GaN, AlN, InN, InGaN and InAlN in Cl2-based (Cl2/Ar, Cl2/N2, Cl2/H2) or Ch4/H2-based ICP and ECR systems. If the experiments are performed on the same reactor platform, with either ICP or ECR source in place, rates are typically slightly higher in the former (eg. ~7000Å.min-1 for GaN in Cl2/H2 at -100Vdc bias and 750WICP source power, compared to ~6200Å.min-1 under the same conditions with the ECR source). However the typical ICP reactor generally has a much larger diameter rf-powered chuck, leading to lower developed dc biases per watt of applied chuck power, and this leads to lower nitride etch rates because the removal mechanism is ion-driven under almost all conditions. Selectivities up to 10 for InN over GaN and AlN are obtained with CH4/H2, with values of 4-5 for InN and GaN over AlN in Cl2-based mixtures. Dielectric or metal masks are the best choice for high ion density conditions, although photoresist is suitable at moderate source powers (<750W) in both tools. Ion-induced damage appears to be much less severe compared to other III-V materials, but we typically see preferential loss of nitrogen from all nitrides, leading to creation of thin (<100Å) n+ surface layers. Annealing at 400°C can restore the initial doping levels in GaN provided that N2 was part of the etch chemistry. |
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EM-MoP-5 Identity and Concentrations of the Plasma Etch Products of the III-V Nitrides
G.A. Gaddy, S.F. Webb, R. Blumenthal (Auburn University) The III-V Nitride semiconductors have experienced a virtual explosion of interest, as their potential for fabricating blue and UV LED's and laser diodes has begun to be realized. However, the exceptional chemical stability of these materials, particularly to wet chemical etching, has created a critical need for alternative processes that can create the structures necessary for device fabrication. A wide variety of etching methods (RIE, ECR, ICP, CAIBE and LE4) have recently been demonstrated to be capable of etching the III-V nitrides with widely varying results. Although other chemistries have been explored, the etching of GaN and AlN has been based primarily on chlorine chemistries, with the etch products assumed to be GaClx, NClx and NHx, when hydrogen is added to the mixture. Due to the involatility of InCl3, the etching of InN has primarily been attempted using CH4:H2:Ar mixtures, which presumably generate a volatile In(CHx)y product. Despite the extensive efforts to develop processes based on these specific chemistries, there had previously been no direct experimental evidence to support the existence of any of these proposed etch products in the plasma environment. In this work, we report the first experimental determinations of the identities and concentrations of etch products within an ECR plasma during the etching of the III-V nitrides for a range of plasma chemistries including Cl2, Cl2: H2 and CH4:H2:Ar plasmas. Measurements of the concentrations of the products within the plasma are obtained using supersonic pulse, plasma sampling mass spectrometry, which has been shown recently to be capable of detecting the direct products of silicon etching in chlorine plasmas. |
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EM-MoP-6 Cross-Sectional Scanning Tunneling Microscopy Study of the Spontaneously Ordered GaxIn1-xP Alloy
N. Liu, K.-J. Chao, C.K. Shih (University of Texas, Austin); A. Mascarenhas, J.M. Olson (National Renewable Energy Laboratory) Spontaneously long-range ordering has been found in the GaxIn1-xP alloy which is epitaxially grown on the GaAs (001) substrate under certain growth conditions. By using cross-sectional scanning tunneling microscopy, we have been investigating the GaxIn1-xP alloy. A GaxIn1-xP alloy, which is not spontaneously long-range ordered, has been resolved atomically. Although, lack of long-range ordering, the GaxIn1-xP alloy is found ordered locally. The spontaneously long-range ordered GaxIn1-xP alloy is currently under investigation and the results will be presented. |
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EM-MoP-8 Passivation of ZnCdTe Surfaces by Oxidation in Low Energy Atomic Oxygen
H. Chen (Fisk University); M.A. George (University of Alabama, Huntsville); K. Chattopadhyay (Fisk University); J.C. Gregory (University of Alabama, Huntsville); K.-T. Chen, A. Burger (Fisk University); P. Nag (University of Alabama, Huntsville); R.B. James (Sandia National Laboratories) We have established a method of surface passivation of ZnCdTe employing low energy atomic oxygen which results in an effective reduction of the interstrip surface leakage current. Detector grade crystals were polished, chemically etched, and two Au strip contacts, similar to those used in gamma-ray imaging systems, were deposited. The 10 µm wide interstrip surface was then exposed to atomic oxygen bombardment at the UAH Thermal Atomic Oxygen Facility. This system generates atomic oxygen species with an effective energy range of 0.5 - 1.0 eV. The dependance of the oxide thickness on the exposure time was determined, providing mechanistic information about the formation and subsequent growth of the oxide layers and the kinetics of the oxidation process. Atomic Force Microscopy and angle-resolved XPS measurements showed that, compared with previous methods of oxidation, oxide layers having an improved morphology were obtained. A description of the UAH atomic oxygen facility and the results of film characterization, including current-voltage measurements and gamma-ray detector performance studies will be presented. |
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EM-MoP-10 Intrinsic and Extrinsic Defects in CuIn1-xGaxSe2
D.J. Schroeder, A. Rockett (University of Illinois, Urbana-Champaign) Recent environmental and energy resource concerns have increased interest in renewable energy sources such as photovoltaic devices. CuIn1-xGaxSe2 (CIGS) heterojunction devices work well especially when the CIGS is a polycrystalline material. Stable solar cells with efficiencies >17% and modules with efficiency >11% have been produced. To understand the performance limitations and to model the results it is necessary to have a complete understanding of the effect of lattice defects on hole transport and how impurities affect cell performance. The effect of intrinsic and extrinsic point defects on carrier transport in CIGS epitaxial layers are described. The effects of Ga and Se content, impurities such as Na and Fe, and the effect of ion damage have been determined. The films were produced using a hybrid sputtering and evaporation process on GaAs substrates. Temperature dependent Hall-effect provided mobility, resistivity, and carrier concentration. Ga was found to increase acceptor density but not to change the depth of the predominant acceptor or mobility of the holes. The room temperature behavior was unchanged by changes in film composition or Ga content over a wide range. Impurities studied also appear to have little effect on mobility but can change compensation significantly leading to an apparent increase in hole concentration, an increase in low temperature mobility, and to changes in photoconductivity lifetime. Ion damage was found to decrease carrier mobility without changing carrier lifetime. The results effectively explain many aspects of observed limitations to the performance of solar cells based on CIGS. |
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EM-MoP-11 Electrical, Optical and Structural Properties of Indium-Tin-Oxide (ITO) Thin Films Deposited by rf Sputtering on PET Substrates
A.K. Kulkarni, K.H. Schulz, T.-S. Lim, M. Khan (Michigan Tech University) Tin doped indium-oxide thin films (about 100nm thick) are rf sputter deposited on unheated PET (poly ethylene terephthalate) substrates. During deposition, the partial pressure of oxygen is varied from 5% to 20% with respect to the total chamber pressure (Ar + O2) to observe the changes in the sheet resistivity and the refractive index of these thin films. The sheet resistivities were obtained from four-point probe measurements and the refractive indices were calculated from the wavelength and intensity of the quarter-wave reflection peaks. A minimum sheet resistivity of 2.13×10-4 ohm-cm is obtained for a sample fabricated with 15% of oxygen partial pressure. The refractive index shows a maximum of 2.07 for a sample fabricated with 10% oxygen partial pressure. The x-ray diffraction (XRD) data indicate polycrystalline films with grain orientations predominantly along <222>, <400>, <440> and <622> directions. The changes in the electrical and optical properties of these ITO thin films with respect to oxygen partial pressure can be explained on the basis of the corresponding structural changes observed in XRD and TEM data. |
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EM-MoP-12 Scanning Probe Microscopy Investigations of Rubbed Polyimide Thin Films
C.L.H. Devlin, S. Chiang (University of California, Davis) Polyimide films are used to align liquid crystals in liquid crystal displays. In order to be effective at aligning the liquid crystals the films must first be rubbed with cloth. The characterization of this rubbing process is the subject of much current research. We have used tapping mode atomic force microscopy (TMAFM) on rubbed thin films of polyimide to elucidate the effects of rubbing on the surface. We have seen not only scratches, but troughs which are microns across that cut through the depth of the film. We have also observed small-scale ridges and smoothing in rubbed films. Very thin films may be completely penetrated by the rubbing cloth to the substrate beneath. The effect of these different features on the alignment of liquid crystals will be discussed. We have also investigated these films using scanning tunneling microscopy (STM). |
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EM-MoP-13 Electron-Hole Interaction in an Organic Molecular Semiconductor
C.I. Wu, A. Rajagopal, A. Kahn (Princeton University) We have used photoemission, inverse photoemission and electron energy loss spectroscopy (PES, IPES, EELS) to determine the on-site attractive electron-hole interaction energy Ee-h in organic molecular semiconductors. The PES-IPES combinations yields a gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the singly positive and negative charged molecule. The localization of electron and holes in the molecular solids reduces screening of the excess charge and leads to an increase in the measured HOMO-LUMO gap relative to that of the neutral molecule obtained by EELS or photon absorption through on-molecule excitation (Frenkel-like exciton). The difference is Ee-h. We find Ee-h = 1.0±0.2 eV in the archetype organic molecular semiconductor PTCDA, thus larger than the "band width" estimated to be smaller than 0.4 eV. Correlation effects in PTCDA are therefore important, consistent with its molecular nature, but limited by the strong π-electron coupling between overlapping molecules in the solid. Similar measurements are presently being performed on other molecular semiconductors like α-NPD and Alq3. |
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EM-MoP-14 Polymer Layer Ordering of Polyaniline Derivatives in PLED Devices
R.C. Advincula (University of Alabama); C.W. Frank (Stanford University); W. Knoll (Max Planck Institute for Polymer Research, Germany); D. Roitman, J. Sheats, R. Moon (Hewlett-Packard) The use of self-assembled polyaniline derivatives for PLED devices is investigated primarily as a hole-injecting layer on the anode surface (ITO coated glass). The layer ordering and self assembly process is facilitated using the alternate polyelectrolyte deposition process. This involves the use of counter polyelectrolytes that allow multilayer build-up depending on the solution and deposition conditions as initially reported by Decher 1 and Rubner2. Spectroscopy and microscopy is used to probe the layer order and integrity, foremost of which is surface plasmon spectroscopy and atomic force microscopy. Different solution and pairing conditions resulted in varrying degrees of order and properties for these layers. The objective of these iterations being to determine the layer properties towards efficient PLED device performance. Using current-voltage (J-V) and luminance-voltage (L-V) measurements, the efficiency and lifetime is compared to previously spin-casted systems. The improvements are compared from systems using the same luminescent polymer (specifically MEH-PPV) and fabrication methodology. The effect of parameters such as layer number, pairing system, annealing, etc. are discussed as they relate to the device. Overall, the presence of these films of the order of 20 nm improves the device performance and stability. While these results are important towards the eventual objective of an efficient device, effort is made to understand the mechanism of this improvement and the importance of layer order at the nm scale. This will involve investigating the effects of polymer chain orientation and complexation with the counter polyelectrolytes on a layer by layer basis.
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