JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 26, Number 8, August 1997 This Month Featuring: Regular Issue Papers and Letters. View August 1997 Table of Contents.

REGULAR ISSUE PAPERS

KEY WORDS Dielectric film, refractive index, tantalum pentoxide

The temperature dependence of refractive index of tantalum pentoxide (Ta₂O₅) dielectric films was investigated experimentally. The films were formed by a magnetron radio frequency sputtering technique on the Si substrates. After deposition, the film was fabricated into an antiresonant reflecting optical waveguide using a novel wet etching technique. The thermal variation of refractive index of Ta₂O₅ was characterized by measuring the index-vs-temperature coefficient of the antiresonant reflecting optical waveguide with a Mach-Zehnder interferometry system. The measured result was 2.3 x 10^-6 1/K at 632.8 nm from 298 to 328K. This result indicates that index-vs-temperature coefficient of the Ta₂O₅ dielectric films is about ten times less than those of conventional III-V semiconductors.

Low Resistance Contacts to p-CuInSe₂ and p-CdTe Crystals
M.K. RABINAL, I. LYUBOMIRSKY, E. PEKARSKAYA, V. LYAKHOVITSKAYA, and DAVID CAHEN
Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot, Israel-76 100

KEY WORDS CuInSe2, gold plating, ohmic contacts, p-CdTe

Room temperature formation of ohmic contacts by electroplating gold on chemically treated surfaces of p-CuInSe₂ and p-CdTe single crystals is reported. The effect of Br₂/methanol and KOH+KCN+H₂O treatments prior to plating was analyzed in the case of CuInSe₂. It is shown that the former treatment yields better ohmic contacts, with lower contact resistance, than the latter. While annealing these contacts made them highly non-ohmic, the method gives reasonably ohmic contacts on surfaces, that were purposely oxidized prior to contact preparation. In the case of p-CdTe stable, low resistance ohmic contacts were obtained at room temperature by electrochemical diffusion of Hg from solution, prior to gold plating. The treatment forms a highly degenerated p⁺-HgCdTe layer. The contacts, which have a very low contact to bulk resistivity ratio, were further improved by vacuum annealing.

Growth of Thick GaN Films on RF Sputtered AlN Buffer Layer by Hydride Vapor Phase Epitaxy
HEON LEE,^1,3 MASAAKI YURI,¹ TETSUZO UEDA,¹ JAMES S. HARRIS,¹ KYUSIK SIN²
1--Solid State Electronics Laboratory, Stanford University, Stanford, CA 94305. 2--Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. 3--Current address: Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 08817

KEY WORDS AlN buffer layer, GaCl3, GaN film, hydride vapor phase epitaxy (HVPE), minimum RBS channeling, NH3, Rutherford backscattering spectroscopy (RBS)

High crystalline quality thick GaN films were grown by vapor phase epitaxy using GaCl₃ and NH₃. The growth rate was in the range of 10~15 µm/h. GaN films grown at higher temperatures (960~1020°C) were single crystalline with smooth surface morphologies. No chlorine impurity was incorporated in these films during growth. The best crystalline quality and surface morphology of grown films was achieved by sputtering a thin AlN buffer layer, prior to growth. According to reflection high energy electron diffraction and atomic force microscopy measurements, as-sputtered AlN buffer layer was amorphous with root means square roughness of 0.395 nm and then crystallized during the GaN growth. This improved the GaN growth due to more uniform distribution of GaN nucleation. Rutherford backscattering channeling experiments produced the lowest value from the GaN film grown on a-Al₂O₃ with a 500Å AlN buffer layer at 1020°.

The Metalorganic Chemical Vapor Deposition Growth of AlAsSb and InAsSb/InAs Using Novel Source Materials for Infrared Emitters
R.M. BIEFELD, S.R. KURTZ, and A.A. ALLERMAN
Sandia National Laboratory, Albuquerque, NM 87185-0601

KEY WORDS InAsSb, lasers, mid-infrared, metalorganic chemical vapor deposition (MOCVD)

We have grown AlSb and AlAs_xSb_1-x epitaxial layers by metalorganic chemical vapor deposition (MOCVD) using trimethylamine or ethyldimethylamine alane, triethylantimony, and arsine. These layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. We examined the growth of AlAs_xSb_1-x using temperatures of 500 to 600°C, pressures of 65 to 630 Torr, V/III ratios of 1-17, and growth rates of 0.3 to 2.7 µm/h in a horizontal quartz reactor. We have also grown gain-guided, injection lasers using AlAsSb for optical confinement and a strained InAsSb/InAs multi-quantum well active region using MOCVD. The semi-metal properties of a p-GaAsSb/ n-InAs heterojunction are utilized as a source for injection of electrons into the active region of the laser. In pulsed mode, the laser operated up to 210K with an emission wavelength of 3.8-3.9 µm. The dependence of active region composition on wavelength was determined. We also report on the two-color emission of a light-emitting diode with two different active regions to demonstrate multi-stage operation of these devices.

Rectifying Characteristics of Heterostructures with Diamond and Diamond-Like Carbon Films
MING-RONG SHEN,¹ HAO WANG,¹ ZHAO-YUAN NING,¹ CHAO YE,¹ ZHAO-QIANG GAN,^1,and ZHAO-XING REN²
1--Department of Physics, Suzhou University, Suzhou, 215006, China. 2--Institute of Plasma Physics, Academia Sinica, Hefei, 230031, China

KEY WORDS Diamond-films, rectifying-heterostructures

Semiconducting diamond-like carbon (DLC) films were deposited on n-type Si substrates by the electrodeposition method, and diamond films were formed on the carbon film and/or a Si substrate directly by the hot-filament chemical vapor deposition method. Rectifying heterostructures of Al/DLC/Si/Al, Al/diamond/Si/Al, and Al/diamond/DLC/Si/Al were fabricated. The Al/DLC/Si/Al structure has a bulk resistivity of 6 x 10⁵ -cm and a rectification ratio close to two orders of magnitude. Experimental results demonstrate that the Al/diamond/DLC/Si/Al bilayer structure can significantly reduce the leakage current to about 10^-10 A and increase the rectification ratio by 3-5 orders of magnitude at 5 V, compared with that of the other two structures. Possible reasons are given for the changes of the rectifying characteristic.

Cryogenic Thermoelectric Cooler with a Passive Branch
C. CIPAGAUTA MINO,¹ J.W. COCHRANE,¹ E.H. VOLCKMANN,² and G.J. RUSSELL¹
1--Advanced Electronic Materials Group, School of Physics, The University of New South Wales, Sydney, NSW 2052 Australia. 2--Marlow Industries, Inc., 10451 Vista Park Road, Dallas, TX 75238-1645

KEY WORDS Bi0.88Si0.12, coolers, cryogenic thermoelectric, high-Tc superconductors

Cryogenic thermoelectric coolers have been fabricated from an active element (polycrystalline Bi_0.88Sb_0.12) and three different passive elements, a high-T_c superconductor (polycrystalline YBa₂Cu₃O_7-d), and lengths of high purity copper and aluminum wires. The results for a single couple show that for low, hot junction (sink) temperatures the superconducting element gives rise to maximum temperature drops of 6.6 degrees K at 70K and 7.3 degrees K at 75K in applied magnetic fields of 0.0 and 0.07 T, respectively. Temperature drops of ~9 and 11K, respectively, are expected for such a couple when form factors are taken into account. The copper couple with an applied magnetic field resulted in large cooling T values at T_hot = 150 and 293K, indicating the importance of metallic passive elements for intermediate sink temperatures. Performance curves for the superconductor based cryogenic thermoelectric cooler show promise with further improvement possible by the use of single crystalline Bi_0.85Sb_0.15.

Type I and Type II Alignment of the Light Hole Band in In_0.15Ga_0.85As/GaAs and in In_0.15Ga_0.85As/Al_0.15Ga_0.85As Strained Quantum Wells
E.M. GOLDYS,¹ H.Y. ZUO,¹ M.R. PHILLIPS,² C.M. CONTESSA,² M.R. VAUGHAN,³ and T.L. TANSLEY¹
1--Semiconductor Science and Technology Laboratories, Macquarie University, North Ryde, NSW 2109, Australia. 2--Microstructural Analysis Unit, University of Technology, Sydney, NSW 2007, Australia. 3--CSIRO Division of Radiophysics, P.O. Box 76, Epping, NSW 2121, Australia

KEY WORDS Band alignment, InGaAs/GaAs, strained quantum wells

We present results of photoluminescence and cathodoluminescence measurements of strained undoped In_0.15Ga_0.85As/GaAs and In_0.15Ga_0.85As/Al_0.15Ga_0.85As quantum well structures, designed to throw light on the current controversy over light-hole band alignment at low In content. We compare these data with theoretical calculations of the confined state energies within the eight band effective mass approximation. Our analysis shows that for In_0.15Ga_{0 85}As/GaAs, the observed two transitions are consistent with either type I or type II alignment of the light hole band for band offset ratios within the accepted range. In the case of In_0.15Ga_{0 85}As/Al_0.15Ga_{0 85}As, however, our results clearly indicate type II alignment for the light hole band. We derive the band offset ratio Q, defined here as Q = E_c/E_g where E_c is the conduction band offset and E_g is the bandgap difference between the quantum well and the barrier in the presence of strain, for the In_0.15Ga_0.85As/Al_0.15Ga_0.85As system to be Q = 0.83 and discuss it in the context of the common anion rule.

Nickel Filament Polymer-Matrix Composites with Low Surface Impedance and High Electromagnetic Interference Shielding Effectiveness
XIAOPING SHUI and D.D.L. CHUNG
Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 14260-4400

KEY WORDS Carbon, electromagnetic interference shielding, fibers, filaments, mechanical properties, nickel, polymer-matrix composites, surface impedance

The processing of nickel filaments of 0.4 µm diameter gives polyethersulfone-matrix composites with high electromagnetic interference shielding effectiveness, high reflection coefficient and low surface impedance at 1-2 GHz. With 7 vol.% nickel filaments, the composite exhibited shielding effectiveness 87 dB (compared to 90 dB for solid copper), surface impedance 1.2 (same as for solid copper), tensile strength 52 MPa, modulus 5 GPa, ductility 1.0%, and density 1.87 g/cm³.

Chemical-Mechanical Polishing of Parylene-N Films: Evaluation by X-Ray Photoelectron Spectroscopy and Atomic Force Microscopy
G.-R. YANG,¹ Y.-P. ZHAO,² JAN M. NEIRYNCK,³ SHYAM P. MURARKA,⁴ and RONALD J. GUTMANN³
1--Center for Integrated Electronics and Electronic Manufacturing, Rensselaer Polytechnic Institute, Troy, NY 12180. 2--Also in Department of Physics, Applied Physics, and Astronomy. 3--Also in Department of Electrical, Computer, and Systems Engineering. 4--Also in Department of Material Science and Engineering

KEY WORDS Atomic force microscopy (AFM), chemical-mechanical polishing (CMP), parylene-N films, x-ray photoelectron spectroscopy (XPS)

The surface quality of parylene-N(PA-N) films, as determined by x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), after chemical-mechanical polishing (CMP), is influenced mostly by two factors: quality of the as-deposited film and the slurry composition. The higher the quality of the as-deposited film (more specifically, less oxygen content), the higher the quality of the polished film. The XPS and AFM results show that PA-N film polished in 1% Al₂O₃ abrasive (0.3 µm particles), NH₄OH (2% by volume), and water, has better quality compared to the other slurries investigated. With high quality PA-N films, the film surface quality affected by CMP is relatively independent of polishing time, indicating that changes in surface chemistry occur in the initial seconds of polishing.

Patterning Amorphous Fluoropolymer Films by Reactive Ion Milling
M.W. DENHOFF and MAE GAO
Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada KIA 0R6

KEY WORDS Fluoropolymer films, reactive ion milling

Amorphous fluoropolymer films have low dielectric constants and high chemical resistance and, so, have potential to be used as the insulator for high speed interconnects and as protection layers. Many applications would require high resolution patterning of the fluoropolymer film. We have found that these films are easily etched by reactive ion beam etching using an O₂/Ar gas mixture. High etching rates of 600 nm/min with a 0.2 mA/cm^-2, 500 eV ion beam were obtained. This technique allows good selectivity with typical underlayers such as Si, Au, and photoresist. We have also found that a short Ar ion milling of the fluoropolymer surface allows good wettability of the film by photoresist.

Determination of the Band Offset of GaInP-GaAs and AlInP-GaAs Quantum Wells by Optical Spectroscopy
H.C. KUO,¹ J.M. KUO,^1,2 Y.C. WANG,^1,2 C.H. LIN,^1,3 H. CHEN,^1,3 and G.E. STILLMAN¹
Department of Electrical and Computer Engineering, Microelectronics Laboratory Center for Compound Semiconductor Microelectronics, University of Illinois at Urbana-Champaign, Urbana, IL 61801. 2-Lucent Technology, Bell Laboratories, 700 Mountain Avenue, Murray Hill, NJ 07974. 3--Department of Material Science and Engineering, Material Research Laboratory University of Illinois at Urbana-Champaign, Urbana, IL 61801

KEY WORDS AlInP/GaAs, band offset, GaInP/GaAs, gas source molecular beam epitaxy (GSMBE), photoluminescence (PL), photoluminescence excitation (PLE), three-band Kane

We report the determination of band offset ratios, using photoluminescence excitation measurements, for GaInP/GaAs and AlInP/GaAs quantum wells grown by gas-source molecular beam epitaxy. To reduce the uncertainty related to the intermixing layer at heterointerfaces, the residual group-V source evacuation time was optimized for abrupt GaInP/GaAs (AlInP/GaAs) interfaces. Based upon thickness and composition values determined by double-crystal x-ray diffraction simulation and cross-sectional transmission electron microscopy, the transition energies of GaInP/GaAs and AlInP/GaAs quantum wells were calculated using a three-band Kane model with varying band-offset ratios. The best fit of measured data to calculated transition energies suggests that the valence-band offset ratio ([Gamma] band discontinuity) was 0.63 +/- 0.05 for GaInP/GaAs and 0.54 +/- 0.05 for AlInP/GaAs heterostructures. This result showed good agreement with photoluminescence data, indicating that the value is independent of temperature.

Deposition of High Purity Parylene-F Using Low Pressure Low Temperature Chemical Vapor Deposition
P.K. WU,^1,3 G.-R. YANG,^1,4 L. YOU,^1,4 D. MATHUR,^1,5 A. COCOZIELLO,^1,4 C.-I. LANG,^1,6 J.A. MOORE,^1,6 T.-M. LU,^1,4 and H. BAKRU²
1--Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180, 2--State University of New York at Albany, Albany, NY, 3--Also with Department of Physics, Southern Oregon University, Ashland, OR 97520. 4--Also with Department of Physics. 5--Also with Department of Chemical Engineering. 6--Also with Department of Chemistry

KEY WORDS Chemical vapor deposition, dielectric constant, Parylene-F

Parylene-F, poly(tetrafluoro-para-xylylene) (PA-F), has potential applications in microelectronics because of its high thermal stability and low dielectric constant. We found that a new precursor, 1,4-bis(trifluoromethyl) benzene (TFB) with a trace presence of , '-dibromo- , , ', '-tetrafluoro-p-xylene (DBX), can be used to produce PA-F films. PA-F films from this precursor are produced using a reaction line and a conventional deposition system. This process is simpler than previously reported processes and produces PA-F films with less impurities. The dielectric constant of the PA-F films produced by this process is 2.25 +/- 0.05 at 1 MHz. The deposition process and the material properties of the PA-F films produced are presented.

Significantly Improved Mechanical Properties of Bi-Sn Solder Alloys by Ag-Doping
M. McCORMACK,^1,2 H.S. CHEN,¹ G.W. KAMMLOTT,¹ and S. JIN¹
1--Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974. 2--Current address: Emerson Company, Columbus, OH 43201

KEY WORDS Lead-free, mechanical properties, microstructure, solder

The addition of small amounts of Ag (less than ~0.5 wt. %) is found to significantly improve the ductility of the binary Bi-Sn eutectic solder. The ductility improvement, more than a threefold increase in tensile elongation, is observed even at a relatively high strain rate (0.01 s^-1). As the Bi-Sn binary eutectic alloy tends to fail catastrophically by brittle fracture at high strain rates, the reduced strain-rate sensitivity in the Ag-containing alloy is beneficial for improving solder reliability on sudden impacting as might be encountered during device assembly, shipping, or thermal shock/cycling. The observed increase in alloy ductility by Ag additions is attributed to a substantial refinement of the solidification microstructure.

LETTERS SECTION

KEY WORDS Excimer laser, grain engineering, laser recrystallization, polysilicon thin-film transistor (TFT)

The pulsed-laser recrystallization of polysilicon in recessed structures, consisting of a silicon film deposited on a patterned oxide layer on a heat sink, is investigated for the first time. The different thermal environments created by the recess area, when compared to those outside this area, causes the recessed silicon to cool first. The different cooling rates in the continuous silicon film creates lateral temperature gradients, producing large elongated grains. Additionally, the recessed structure can lead to different film microstructures within the recessed area compared to outside this area. This structure is therefore capable of grain engineering different microstructures for polysilicon.

Copper Metallizations for Integrated Circuits: TEM Analysis and Electrical Characterization
P. BRUSCHI, C. CIOFI, V. DATTILO, A. DILIGENTI, A. NANNINI, and B. NERI
University of Pisa, Dipartimento di Ingegneria dell'Informazione: Elettronica, Informatica, Telecomunicazioni., Via Diotisalvi 2, I-56126 Pisa, Italy

KEY WORDS Copper, interconnections, noise, reliability

Copper thin films have been deposited by means of three different techniques. Transmission electron microscopy analysis and electrical characterization revealed that the three types of film had different average grain size and resistivity. The two different contributions to the total resistivity, due to the scattering at the grain boundaries and at the intragranular defects, have been separated by using the Mayadas-Shatzkes theory. Microstructural analysis, electrical characterization, noise measurements, and lifetime tests have been performed in order to identify the deposition technique which leads to the most reliable interconnection lines.

Direct Evidence of Au Segregation in Laser Welded Au-Coated Invar Material
S.C. WANG,¹ S. CHI, ² K.C. HSIEH, ³Y.D. YANG,⁴ and W.H. CHENG⁴
1--Telecommunication Laboratories, 12, Lane 551, Min-Tsu Rd, Sec. 3, Yang-Mei, Taoyuan, Taiwan. 2--Institute of Electro-Optical Engineering, National Chiao Tung University, Kaohsiung, Taiwan. 3--Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan. 4--Institute of Electro-Optical Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan

KEY WORDS Au coating, Invar material, laser welding, optoelectronic packaging

Evidence of the Au segregation within the crack region in laser welded Au-coated Invar material for semiconductor laser packaging is investigated. Results obtained from the metallography, scanning electron microscope (SEM) mapping, energy dispersive spectrometer (EDS) line profile, and Auger electron spectroscopy find that the cracks in the welded joints occur around the Au rich boundaries. The SEM Au mapping and EDS line profiles show that Au accumulates at the crack region. This direct observation indicates that one of the primary causes of cracks in laser welded Au-coated materials is due to the segregation of Au in the final stage of solidification. Detailed knowledge of the defect formation mechanisms in laser welded Au-coated materials is important for the practical design and fabrication of reliable optoelectronic packaging.