Volume 26, Number 4, April 1997

This Month Featuring: Regular Issue Papers. View April 1997 Table of Contents.


Role of Interfacial-Charge in the Growth of GaN on -SiC
1--Department of Physics, Peking University, Beijing 100871, People's Republic of China and Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504. 2--Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504.

6H-SiC, GaN, substrate
The growth of GaN and AlN films on (0001) substrates of 6H-SiC has produced high-quality opto-electronic films. The SiC surface at the interface with GaN or AlN is either Si-terminated or C-terminated, and the Si-terminated interface is known to be the better substrate, producing higher-quality films. The polarity of the interface is important, as recognized by Sasaki and Matsuoka. We propose that the main relevant parameter for characterizing the interface and its potential for producing high-quality opto-electronic GaN or AlN films is the interfacial charge, which leads to the best films when the charge is positive and relatively large. The positive charge reduces the size of the N ions at the interface and hence improves the local lattice match. (The charges are approximately -0.45 |e| and +0.55 |e| on the interfacial N and Si atoms, respectively.) Therefore, while the polarity of the interface is important, the polarity's effect on the local lattice mismatch is what leads to a high-quality interface. These ideas are consistent with XPS data and are supported by electronegativity arguments, by calculations for ordinary mono-bonded GaN/SiC superlattices (with N-Si and Ga-C interfaces) and by computations for superlattices with tri-bonded interfaces. We predict that the tri-bonded N-Si interface of GaN/SiC should produce excellent GaN and AlN films.

InAsSb/InTlSb Superlattice: A Proposed Heterostructure for Long Wavelength Infrared Detectors
1--Department of Electrical Engineering, North Carolina A&T State University, Greensboro, NC 27411. e-mail address: 2--Department of Electrical Engineering, North Carolina A&T State University, Greensboro, NC 27411. 3--Department of Physics, Emory University, Atlanta, GA 30322.

InAsSb/InTlSb, long wavelength infrared (LWIR) detector, superlattice
A novel superlattice (SL) heterostructure, comprising of InTlSb well and InAsSb barrier lattice matched to InSb, is proposed for long wavelength 8-12 µm detectors. Improvements in the InTlSb epilayers' structural quality are expected, as it will be sandwiched between higher quality zinc-blende InAsSb epilayers. Preliminary energy band calculations of 30Å InAs0.07Sb0.93/100Å In0.93Tl0.07Sb SL show the band alignment favorable to type I with three heavy-hole subband confinement in the valence band and a partial electron subband confinement in the conduction band due to the small conduction band offset. Including the effect of strain indicates significant changes in the band offsets, with optical bandgap essentially unaltered. The optical band gap of this SL was computed to be 0.127 eV (9.7 µm) at 0K, indicating its potential for long wavelength applications.

Deposition of Tantalum Oxide Films by Dual Spectral Source Assisted Metalorganic Chemical Vapor Deposition (MOCVD)
1--Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634-0915. 2--Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634-0915. e-mail address: 3--Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695.

Dielectrics, metalorganic chemical vapor deposition (MOCVD), rapid isothermal processing (RIP)
Dual spectral source assisted metalorganic chemical vapor deposition (MOCVD) is an ideal technique for the deposition of high dielectric constant materials. Tungsten halogen lamps and a deuterium lamp are used as the sources of optical and thermal energy. In this paper, we have reported the deposition and characterization of tantalum penta oxide films. Ta2O5 films were deposited at 660°C for 15 min and annealed at 400° for 1 h. The leakage current densities of 10.6 nm thick films are as low as 10-10 A/cm2 for gate voltage under 4V. To the best of our knowledge, these are the best results reported to date by any researcher. The high energy photons used in the in-situ cleaning and deposition process play an important role in obtaining high quality films of Ta2O5.

Chemical Beam Epitaxial Growth of InP Using EDMIn and BPE
College of Engineering, University of Utah, Salt Lake City, Utah 84112.

BPE, chemical beam epitaxy, EDMIn, InP
The effect of the growth temperature on the quality of InP grown by chemical beam epitaxy (CBE) using ethyldimethylindium (EDMIn) and bisphosphinoethane (BPE) are presented. The growth rate was nearly independent of growth temperature, BPE flow rate, and cracker cell temperature in the range from 700 to 900°C. Smooth and mirror-like surfaces were obtained for all of the samples grown at temperatures above 465°C. All of the InP samples were n-type. As the growth temperature increased, the net carrier concentration decreased and reached a minimum value of 3.2 x 1015 cm-3 at 485°C. The electron mobility increased with increasing growth temperature, reaching values of 3630 and 21800 cm2/Vs at 300 and 77K, respectively. The photoluminescence was found to depend strongly on the growth temperature. Excitonic luminescence was detected only for growth temperatures above 465°C. The intensity of the band edge emission is comparable to that of the acceptor related emission for layers grown at 465°C. At 485°C, the band-edge recombination is dominant and the acceptor related emission is barely observable. As the growth temperature increased from 465 to 485°C, the full width at half maximum of the bound exciton peak decreased from 6.8 to 3.5 meV at 14K. This trend was consistent with the decrease in the impurity concentration deduced from the Hall effect measurements.

Effects of Trimethylindium on the Purity of In0.5Al0.5P and In0.5Al0.5As Epilayers Grown by Metalorganic Chemical Vapor Deposition
J.C. CHEN,1 Z.C. HUANG,1,2 and K.J. LEE1,3 and RAVI KANJOLIA4
1--Department of Computer Science and Electrical Engineering, University of Maryland Baltimore County, Baltimore, MD 21228. 2--Also with Hughes-STX Corporation, 7701 Greenbelt Road, Suite 400, Greenbelt, MD 20770. 3--Also with Department of Electrical Engineering, University of Maryland, College Park, College Park, MD 20742. 4--Morton International, Inc., Danvers, MA 01923.

InAlP, InAlAs, metalorganic chemical vapor deposition (MOCVD), trimethylindium (TMIn)
In0.5Al0.5P lattice-matched to GaAs and In0.5Al0.5As lattice-matched to InP epilayers were grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD). The effect of trimethylindium on the purity of the as-grown layers was systematically studied using secondary ion mass spectroscopy (SIMS), deep level transient spectroscopy (DLTS), and capacitance-voltage (C-V) measurements. The SIMS results showed that oxygen is the main impurity in all layers and the oxygen concentration in InAlP was approximately one to four orders of magnitude higher than the oxygen concentration found in InAlAs when the same indium source was used, indicating that more oxygen was introduced by the phosphine source than by the arsine source. Two electron traps in the InAlP epilayers and four electron traps in the InAlAs epilayers were observed in this study. When a high-purity indium source was used, the best InAlP epilayer showed only one deep electron trap at 0.50 eV while the best InAlAs epilayer showed no deep levels measured by DLTS. In addition, we also found that a high concentration of oxygen is related to the high resistivity in both material systems; this suggests that semi-insulating (SI) materials can be achieved by oxygen doping and high quality conducting materials can only be obtained through the reduction of oxygen. The oxygen concentration measured by SIMS in the best InAlAs epilayer was as low as 3 x 1017 cm-3.

Characterization of Organic Thin Films for OLEDs Using Spectroscopic Ellipsometry
Semiconductor Research & Development/Technology, Texas Instruments, Inc., M/S 147, P.O. Box 655936, Dallas, TX 75265.

Organic films, organic light emitting diodes (OLEDs), spectroscopic ellipsometry
We report the optical characterization of thin, evaporated organic films used in fabrication of organic light emitting diodes (OLEDs): N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-1,1'biphenyl-4,4'diamine, or TPD, and tris (8-hydroxy) quinolato aluminum, or Alq3. In particular, we have obtained and analyzed spectroscopic ellipsometry (SE) data using a multi-sample approach, to determine the optical constants for Alq3 and TPD films over the wavelength range 250-850 nm. We show that bi-layer Alq3/TPD films on Si can be analyzed for individual layer thicknesses, even though the refractive index is nearly identical for these films in the visible region. Simulations of in situ monitoring are also presented, which show sub-nm thickness resolution for organic layer growth on a Si monitor wafer. SE has great utility for process control, either by ex situ or in situ thickness measurement.

Defect State Assisted Tunneling in Intermediate Temperature Molecular Beam Epitaxy Grown GaAs
1--Electrical and Computer Engineering Department, Drexel University, Philadelphia, PA 19104. Present address: Lucent Technologies, 330 S. Randolphville Rd., Piscataway, NJ 08854. 2--Electrical and Computer Engineering Department, Drexel University, Philadelphia, PA 19104.

Defect assisted tunneling, low-temperature-grown GaAs, metal-semiconductor-metal (MSM) detectors, Schottky contacts, thermal annealing
Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by defect states can dominate, at some bias ranges, current transport in Schottky contacts to unannealed GaAs material grown at the intermediate temperature range of about 400°C. The deep defect states are modeled by quantum wells which trap electrons emitted from the cathode before re-emission to semiconductor. Comparison of theory with experimental data shows defect states of energies about 0.5 eV below conduction band to provide the best fit to data. This suggests that arsenic interstitials are likely to mediate this conduction. Comparison is also made between as-grown material and GaAs grown at the same temperature but annealed at 600°C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers.

Effects of Aluminum Sputtering Process Parameters on Via Step Coverage in Micro-Electronic Device Manufacturing
1--MOS 8 Process Engineering, Motorola, 3501 Ed Bluestein Blvd., Austin, TX 78721. 2--Department of Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO 80401.

Argon pressure, chamber configuration, PVD aluminum, step coverage, temperature
It has been observed that the step coverage achieved for aluminum deposition at 200°C on a multi-chamber (ultra-high vacuum) horizontal physical vapor deposition (PVD) system (System A) is electrically comparable to the step coverage on a single chamber, high vacuum (vertical) PVD system (System B) at 300°C under the optimized conditions of pre-clean process, sputtering power, argon pressure and chamber hardware configuration. In addition, it has been determined that the metal step coverage is relatively poor at temperatures higher than 200°C on System A, whereas the metal step coverage on System B is better at 300°C when compared with both substantially lower and higher temperatures. Since step coverage is a vital parameter in the manufacturing of sub-micron devices with high aspect ratio vias, the effect of the sputtering process parameters has been studied. This work investigates possible causes for the observed temperature effect and evaluates possible methods for improving the step coverage. The directionality of sputtering and film-substrate bonding are identified as two primary factors controlling step coverage.

Scanning Tunneling Potentiometry Study of Electron Reflectivity of a Single Grain Boundary in Thin Gold Films
Universität Göttingen, IV. Physikalisches Institut, Bunsenstr. 13, D-37073 Göttingen, Germany.

Grain boundary scattering, scanning tunneling microscopy (STM), scanning tunneling potentiometry (STP), thin film resistivity
Spatial variations of the local electric field in current-carrying thin gold films were studied with a scanning tunneling microscope on a nanometer scale. With a refined scanning tunneling potentiometry technique, it was possible to determine the local electric fields within single grains. At grain boundaries, we observe potential drops on length scales of less than 1 nm which exceed the potential difference within a grain greatly. We interpret our findings by applying a theory that models grain boundaries as barriers with a reflectivity R for the conduction electrons. With the assumption of isotropic background scattering within each grain, we determine the local current-density j(x,y) that passes a grain boundary. From that, we obtain the reflectivity of individual grain boundaries and find values of R = 0.7 to R = 0.9 which is much higher than expected from macroscopic experiments.

Influence of AlN Protective Film Thickness on the Hardness and Electrophotographic Properties of Organic Photoconductors
X.S. MIAO,1 Y.C. CHAN,1 C.K.H. WONG,1 D.P. WEBB,1 Y.W. LAM,1 K.M. LEUNG,2 and D.S. CHIU2
1--Department of Electronic Engineering, City University of Hong Kong, Hong Kong. 2--Department of Physics and Materials Science, City University of Hong Kong, Hong Kong.

AlN film, electrophotographic properties, microhardness, organic photoconductor
The deposition of a protective film to increase the hardness of an organic photoconductor (OPC) surface is an effective method to lengthen the lifetime of the OPC. In this work, AlN protective films were deposited onto OPC samples by rf reactive magnetron sputtering with low substrate temperature. The AlN films were deposited with optimized sputtering conditions and exhibited very high transmissivity in the visible wavelength range 300~800 nm. The films caused a remarkable increase in the hardness of the OPC surface, by between 32 and 62%. The acceptance voltage, dark decay rate, photodischarge rate, difference between the residual potential and the acceptance voltage of the OPC protected by AlN film were improved. These results show AlN is a suitable protective film for OPC.

A Comparison of the Critical Thickness for MBE Grown LT-GaAs Determined by In-Situ Ellipsometry and Transmission Electron Microscopy
1--Materials Directorate, 2941 P St. Ste. 1, Wright-Patterson AFB, OH 45433-6533. 2--The University of Texas, Microelectronics Research Center, MER 1.606/R9900 Austin TX 78712.

Defects, ellipsometry, low temperature (LT) GaAs, molecular beam epitaxy (MBE)
The growth of low temperature (LT) GaAs by molecular beam epitaxy has been studied using ellipsometry. Different regimes of growth were observed in the data, depending on film thickness. Epitaxial growth of pseudomorphic LT-GaAs occurred immediately above the substrate, followed by a layer with changing dielectric properties. This upper layer can be modeled as a two-phase region consisting of epitaxial LT-GaAs and small grained, polycrystalline GaAs, which increases in volume fraction with increasing layer thickness. For sufficiently thick LT layers cross-sectional transmission electron microscopy analysis showed pyramidal defects that were composed primarily of highly twinned regions. The ellipsometry data showed a deviation from the homogeneous growth model at a thickness less than the thickness at which the pyramidal defects nucleated in all samples.

Study of Amorphous Ta2O5 Thin Films by DC Magnetron Reactive Sputtering
1--Center for Integrated Electronics and Electronic Manufacturing, Rensselaer Polytechnic Institute, Troy, New York, 12180. Also with Physics Department. 2--Center for Integrated Electronics and Electronic Manufacturing, Rensselaer Polytechnic Institute, Troy, New York, 12180. Also with Material Engineering Department.

dc magnetron reactive sputtering, stoichiometry, tantalum oxide thin films, x-ray photoelectron spectroscopy
The dc magnetron reactive sputtering deposition of tantalum pentoxide (Ta2O5) thin films was investigated. By combining Schiller's criterion and Reith's "target preoxidation" procedure, high quality Ta2O5 thin films were prepared at a high deposition rate of about 100Å/min. The deposited films were amorphous, with a refractive index around 2.07 and a dielectric constant of 20. An optical transmittance of 98.6% was obtained for a 4500Å thick film. The leakage current density is 5 x 10-9 A/cm2 at an electric field strength of 1 MV/cm and its breakdown field strength is above 2 MV/cm. The temperature coefficient of capacitance for capacitors fabricated using the deposited films is approximately +230 ppm/°C. X-ray photoelectron spectroscopy shows that the films are stoichiometric tantalum pentoxide, Ta2O5, and exhibit good stability.

Chemical Vapor Deposition and Characterization of Amorphous Teflon Fluoropolymer Thin Films
1--Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634-0915. 2--Department of Electrical and Computer Engineering and Materials Science and Engineering Program, Clemson University, Clemson, SC 29634-0915. e-mail 3-School of Textiles and Materials Science and Engineering Program, Clemson University, Clemson, SC 29634-1307. 4--School of Textiles, Clemson University, Clemson, SC 29634-1307.

Direct liquid injection, low dielectric constant (K) interconnect dielectrics, rapid isothermal processing (RIP), ultra violet (UV) light assisted processing
Dielectric materials with dielectric constant (K) less than the conventionally used SiO2 (K = 3.9) are gaining importance particularly for low voltage operated integrated circuits due to their ability to reduce signal propagation delays, power dissipation, and crosstalk when used as interconnect dielectrics. In this paper, we report for the first time ever a chemical vapor deposition technique for the deposition of the copolymeric amorphous Teflon fluoropolymer AF 1600 which is a new low K material with a value of K of about 1.93. The principle of direct liquid injection in an ultra-violet (UV) light assisted rapid isothermal processing system was followed. Different processing cycles were studied with and without the UV source to investigate the effect of high energy photons during processing on the film properties. Preliminary characterization results indicate that the processed films exceed the established dielectric performance standards required for future generation IC manufacturing. An improvement in film properties was observed when the UV source was used during processing.

InxGa1-xAs Ohmic Contacts to n-Type GaAs with a Tungsten Nitride Barrier
1--Division of Materials Science and Engineering, Department of Engineering Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan. 2--Mitsui Mining & Smelting Co. LTD., 2081 Karafune, Oaza, Omuta, Fukuoka 836, Japan. 3--Division of Materials Science and Engineering, Department of Engineering Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan.

Diffusion barrier, InxGa1-xAs, n-type GaAs, ohmic contact, radio frequency sputtering, tungsten nitride
Significant reduction of the contact resistance of In0.7Ga0.3As/Ni/W contacts (which were previously developed by sputtering in our laboratory) was achieved by depositing a W2N barrier layer between the Ni layer and W layer. The In0.7Ga0.3As/Ni/W2N/W contact prepared by the radio-frequency sputtering technique showed the lowest contact resistance of 0.2 mm after annealing at 550°C for 10 s. This contact also provided a smooth surface, good reproducibility, and excellent thermal stability at 400°C. The polycrystalline W2N layer was found to suppress the In diffusion to the contact surface, leading to improvement of the surface morphology and an increase in the total area of the InxGa1-xAs between metal and the GaAs substrate. These improvements are believed to reduce the contact resistance.

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