The application of molecular beam epitaxy to the growth of infrared sensor structures employing either metallic or semiconducting silicides is being investigated. Thin, heavily Ga-doped layers have been used to modify the cut-off wavelength of Schottky diodes in CoSi2 on Si(111). The cut-off wavelengths of n-type diodes have been decreased from 2.1 to 1.4 pm, while the cut-off wavelengths of p-type diodes have been increased from 3.5 to 5.0 Rm. In addition, epitaxial growth of CrSi2, a small bandgap semiconductor which might be used as an intrinsic detector, is being explored. Initial results show that CrSi2 forms islands on Si(111) which commonly exhibit two different epitaxial relationships with the substrate. These orientations are related by a 30° rotation about the surface normal.
Gallium nitride (GaN) is a compound semicon-ductor with a direct, wide bandgap (3.5 eV at 300K) and a large saturated electron drift veloc-ity. This unique combination of properties pro-vides the potential for fabrication of short wave-length (near UV and blue) semiconductor lasers, LEDs and detectors as well as transit-time-limited (IMPATT, etc.) microwave power amplifiers from this material. However, all GaN previously pro-duced has possessed a high n-type carrier concen-tration which has limited its potential. This phenomenon has been almost invariably associated with the presence of nonequilibrium nitrogen va-cancies. This paper reports growth of GaN by mod-ified MBE techniques in order to reduce the nitro-gen vacancies. The primary advantages of these MBE-based techniques are low growth temperature and high nitrogen activity. A standard effusion cell was used for the gallium source, but for ni-trogen, a microwave glow discharge was used to ac-tivate the nitrogen prior to deposition. The films were deposited on (100) 3-SiC and (0001) Al203 substrates. Growth results and preliminary film characterization will be presented.
Growth of epitaxial dielectrics of II-A fluorides on Si and compound semiconductors has generated considerable research interest in recent years. In general, MBE and vacuum deposition techniques are being employed for the epitaxial growth of II-A fluorides on semiconductor substrate maintained between about 400 and 900°C during the deposition of dielectric material. With the typical deposition rate of about 1-2 Å/s, 8-10 minutes are required to deposit 1000 Å of dielectric material. This type of prolonged heating may not be suitable for future submicron and three-dimensional Si integrated circuits, as well as for compound semiconductor devices. We have developed a new reduced thermal budget (product of processing temperature and time) processing technique for the deposition of epitaxial dielectric films on Si and compound semiconductors. In this process, epitaxial dielectric is deposited in the e-beam system at room temperature and subsequently subjected to in-situ rapid isothermal processing by using incoherent light sources incorporated in the e-beam system. In this paper, electrical and structural characteristics of thin epitaxial dielectric films on Si are described.
The metalorganic chemical vapor deposition (MOCVD) of InSb using tri-neopentylindium (InNp3) is reported. The newly synthesized InNp3 has several advantages, e.g., easy to prepare, easy to purify, and non-pyrophoric, over the commonly used tri-ethylindium (TEIn) and tri-methylindium (TMIn). We report the MOCVD growth of InSb using InNp3. The InSb films were examined by double crystal x-ray diffraction, scanning electron microscope (SEM), and x-ray energy dispersive analysis. The results indicate that the deposited InSb films are stoichiometric and single crystal. The crystal quality and surface morphology are comparable to similar films grown from TMIn.
Epitaxial CdTe has been grown on both (100) GaAs/Si and (111) GaAs/Si substrates. A combination of molecular beam epitaxy and metal organic chemical vapor deposition have been employed to achieve this growth. The GaAs layers are grown on Si substrates by molecular beam epitaxy, followed by the growth of CdTe on GaAs/Si substra by metalorganic chemical vapor deposition. X-ray diffraction, photoluminescence and scanning electron microscopy have been used to characterize the CdTe films.
Molecular beam epitaxy has been used to grow both GaAs, ,Sb films lattice-matched to InP and GaAsi_ Sb /GaAs strained layer superlattices oh.6aArgubstrates. Films grown on InP substrates are ofa composition inside the well-known solid-phase miscibility gap for this alloy. Because these films are metastable, they exhibit an unusual microstructure which includes both ordering and clustering effects. Nevertheless, we have obtained low-temperature photoluminescence linewidths of under 8 meV. This represents the best linewidth for this material reported to date. Correlations between film microstructure and the optical quality of these alloys have been observed. Strained layer GaAsi_xSb /GaAs superlattices grown on GaAs substrates have been characterized by x-ray di3i'fraction, photoluminescence, optical absorption, and photoreflectance. Structural parameters as determined by x-ray diffraction have been used in an envelope function superlattice band structure model to estimate the band offsets and indicate that the superlattices are Type II, with a large valence band discontinuity.
GaAs 0.5 Sb 0.5 grown on InP by molecular beam epitaxy (MBE) has been characterized by .. transmission electron microscopy and variable temperature Hall measuremts 0 Unintentionally doped layers are p-type with measured hole concentrations of 1-3 x 10 16 cm-3 at 300K. The behavior of the Hall coefficient as a function of temperature is found to be well-described in the temperature range 30K<T<300K by a two-acceptor model. Intentional Si doping of the GaAs ,Sbn layers leads to relatively uncompensated p-type conductivity. Hole mobilities in these njers are generally less than 100 cm /Vs at 300K as a result of compositional fluctuations which result from the metastable nature of these films. In addition, hole mobilities are anisotropic and reflect an unusual asymmetry in the film microstructure which can be observed by transmission electron microscopy.
Partially relaxed GaInAs layers grown on (001) GaAs substrates by Metalorganic Chemical Vapor Deposition are studied using x-ray rocking curve (XRC) and double crystal topography, energy dispersive x-ray analysis (EDAX), and Nomarski phase contrast microscopy. Epilayers of 1 - 7 gm thickness are grown on various buffer layers. Epilayers grown on a plain GaAs buffer layer and on a graded GaInAs buffer layer contain many line defects (LD) and show cross-hatched patterns on the surface. The layer grown on a strained layer superlattice buffer layer is free of LD's and of the cross-hatched patterns. All the layers are relaxed by differing amounts along the two <110> directions. The XRC and EDAX measurements of the ternary layer compositions agree reasonably well. The mean spacing of misfit dislocations from XRC and the LD spacing from topography agree in the order of magnitude with the electron microscopy measurements by others. The XRC data on x-ray strain, elastic strain, and the misorientation angle between the epilayer and substrate are also presented.
Thin, pure, hard carbon films about 1 pm thick have been deposited on a water-cooled copper plate installed in a continuous-wave (cw) plasma source with graphite walls. The transparent, glassy films were observed after several thousand multisecond pulses of intense hydrogen discharges. The properties of these films and the operating conditions of the plasma, source are described. Many ion beam and rf sources are available in the Fusion Energy Division of the Oak Ridge National Laboratory (ORNL). The potential application of these plasma sources to growing large, thick diamond films is discussed.
Diamond is an excellent candidate material for use in electronic and wear resistant coating applications due to its hardness, strength, thermal conductivity, high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include high power/high frequency devices and devices to be utilized in high temperature, chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films using low pressure gases has created the potential for growing thin films for such electronic devices or wear resistant coatings. In this research, diamond thin films grown on silicon by microwave plasma enhanced chemical vapor deposition were characterized by a variety of materials analysis techniques including secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and infrared spectroscopy (IR). This paper reports the characterization of these polycrystalline diamond films and discusses the impurities, bonding, and structure of the as-grown diamond films.
There are several extreme properties) of diamond which make it a desireable material for technological applications. Its hardness, which is related to its large elastic moduli, is well known. Less widely recognized are its high thermal conductivity and its low thermal expansion coefficient, which suggest its use in high temperature applications. Its large bandgap of 5.5 eV suggests uses in optical electronics in the VUV region, but the indirect nature of the gap has contributed to the lack of active investigation of diamond as an optical component. More important in this regard is the problem of fabrication of diamond components: the stable form of carbon is graphite, with the cubic (and hexagonal) forms of carbon being metastable. This metastability is primarily a limitation only in the fabrication process; once tetrahedrally-bonded carbon is formed it is exceedingly "stable", even up to high temperatures.
The recent progress in the area of vapor deposition of diamond has been encouraging. An overview of the various methods which have been used has been given by Moustakas et al. 2 Moreover, fabrication of operating p-n diodes on the related material BN by Mishima et al.,3 constructed by growing n-type BN on p-type seed crystals, suggests that the construction of diamond-based electronic components may not be far away. For applications involving the epitaxial growth of diamond on nearly lattice-matched materials, it is crucial to understand the effects of the various uniaxial strains which may occur. A question of special importance is: are there physically realizable strains which will lead to an indirect-to-direct gap inversion, making it more amenable to application in electronic/optical devices. In this paper we provide the initial results of our investigation of these questions.
Several studies of the shifts in band energies due to infinitesimal strains ("deformation potentials") have appeared. Most of these 4-8 have dealt only with homogeneous strains, although uniaxial strains have received some attention. 9, 10 Nielsen ll has carried out extensive studies of the stress-strain relationship in diamond under uniaxial deformations, but reported very little (recounted below) on the effects on the band structure.
A method which combines thermal and plasma dissociation of a methane/hydrogen gas mixture for low pressure chemical vapor deposition of diamond is described. A hot, thin-walled, refractory metal cathode is used to generate a high-current, low-voltage discharge. The substrates are located on the anode and immersed in the plasma emanating from the cathode tip. No auxiliary substrate heating is employed. Polycrystalline diamond particles and films are obtained on silicon (100) and molybdenum substrates at growth rates of 1-3 μm/hour. Data on the effects on diamond growth of the various pro-cess parameters along with some operating characteristics of the hollow cathode are presented.
Some of the commercial aspects of thin diamond films deposited by plasma enhanced chemical vapor deposition are reviewed. Diamond films deposited by the use of DC bias have been characterized by using a variety of techniques including Raman spectroscopy and scanning electron microscopy. The relationship between deposition conditions and film characteristics are discussed.
The many unique properties of diamond make it useful for laser optics. Diamond has a thermal stress parameter significantly higher than other materials. Laser damage thresholds of free-standing diamond film windows, diamond films deposited on silicon substrates and bare silicon substrates were measured. As expected the free-standing diamond films showed a high laser damage threshold. The film-substrate combination had a lower damage threshold which is attributed to film stress and conditions of film deposition. Dielectric breakdown induced by laser radiation may be the damage mechanism.
High-density interconnect (HDI) is a unique, novel, hybrid approach currently in the development stages at the GE Research and Development Center. Our approach uses polymer layer overlays laminated over bare chips mounted on a substrate. The over-lays are laser-patterned with copper to connect the chips and I/O. The advantages of the HDI hybrid approach can be summarized as follows: •The overlay layer makes the entire chip area available for interconnect lines.
•The interconnect has very high density: 2-mil pitch has been demonstrated.
•Via and line formation are under computer control. Thus, no patterning mask is used. Chip misalignment is accommodated by computer-adaptive writing.
•Copper is the conductor metallization.
•The chips can be almost touching.
•The interconnect technology accommodates any chip size and mixed chip technologies.
•The process is ideal for prototype or moderate volume production.
•The overlay can be removed and replaced without chip damage.
•Chips are mounted directly on the substrate for good heat dissipation.
Laser processing of polyimide dielectric layers for use in high-density interconnect structures was studied. A pulsed excimer laser was used to photoetch via holes and a CW argon ion laser operating at 351 nm was used to selectively deposit catalytic amounts of palladium on polyimide. Subsequent immersion of the irradiated samples in an electroless copper solution resulted in selective copper deposition.
The demands of increased circuit density and higher signal propagation speeds have combined to substantially increase the importance of micro-interconnect in the packaging of electronic devices. Interconnect and packaging technology have become important factors that limit the performance and reliability of microelectronic devices. To overcome these limitations, many groups 1-6 are considering new materials for microelectronic packaging applications. In this paper, the thermal and electrical properties of partially reactive cordierite and fully reactive cordierite substrates, which have been doped with glass and processed in different manners, will be reported. The dependence of the dielectric constant of the substrate on processing, composition, sintering temperature, and frequency of the applied field from 1KHz to 18GHz will be presented. The thermal conductivity of the substrate as a function of the processing and composition has also been studied. Preliminary data on the bonding of copper microstrips to the ceramic substrate will be reported.
Porous substrates of a low dielectric constant (K) ceramic such as cordierite offer the opportunity to produce substrates with very low composite K values. This is attractive in VLSI packages because a low K allows the package to operate at higher frequencies. However, the porosity causes problems with conduction line integrity and thermal conductivity. These problems could be relieved somewhat if the pores were filled with a second phase such as glass. Various mixing laws are available to predict the resultant K value in a two phase mixture and these have been applied with some success.
The research reported here involves a quantitative analysis of the volume fraction of porosity and pore size distribution in several cordierite compositions processed in various ways. These data are correlated with measurements of density and dielectric constant. They are also compared to several mixing laws to evaluate the relative fit between quantitative volume fraction measurements and density measurements. Finally, the results of this research are analyzed with respect to other recent research on porous substrate ceramics to identify the direction for future work.