Effects of Annealing in a Partially Reducing Atmosphere on Sputtered Al-Doped ZnO Thin Films

Aluminum-doped ZnO (AZO) films have been deposited by room-temperature radio frequency (RF) magnetron sputtering onto fused-quartz substrates using a ZnO:Al₂O₃ (98:2 wt.%) target. A post-deposition anneal in an N₂ flow has been shown to improve the electrical and optical properties of as-deposited AZO. All films were polycrystalline and exhibited a hexagonal wurtzite structure with the c-axis oriented perpendicular to the substrate. An increase in the estimated crystallite size was observed after annealing. Electrical resistivity was reduced from 3.7 × 10⁻³ Ω cm for as-deposited layers to 7.1 × 10⁻⁴ Ω cm for annealed films. An average optical transmittance >85% for annealed films was routinely measured. X-ray photoelectron spectroscopy measurements indicated that the surfaces of all layers investigated were oxygen deficient, and the density of oxygen vacancies was found to increase following the anneal.     

A rapid growth of aligned carbon nanotube films and high-aspect-ratio arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we report an efficient process to grow well-aligned CNT films and high-aspect-ratio CNT arrays with very high area distribution density (>1600 μm⁻²). Chemical vapor deposition (CVD) was invoked to deposit highly aligned CNTs on Al₂O₃/Fe coated silicon substrates of several square centimeter area using ethylene as the carbon source, and argon and hydrogen as carrier gases. The nanotubes grew at a high rate of ∼100 μm/min. for nanotube films at 800°C, while the nanotube arrays grew at ∼140 μm/min. even at 750°C, due to the base growth mode. The CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs are of high purity and form densely aligned arrays with controllable size and height. The as-grown CNT structures have considerable potential for thermal management and electrical interconnects for microelectronic devices.     

Low-Resistivity Ru-Ta-C Barriers for Cu Interconnects

Ru-Ta-C films deposited on silicon substrates were evaluated as barriers for copper metalization. The films were prepared by magnetron cosputtering using a Ru target and a Ta-C target. Compositions and structure of resultant films were optimally tuned by the respective deposition power of each target. The fabricated Ru-Ta-C films were characterized via four-point probe measurement, x-ray diffractometry, field-emission electron probe microanalysis, and transmission electron microscopy. Failure temperature was evaluated by the sudden rise in electrical resistivity after annealing the Cu/Ru-Ta-C/Si sandwich films, and a reference bilayer Cu/(5 nm Ru)/(5 nm Ta-C)/Si scheme. The optimal compositions were 10 nm Ru₇₇Ta₁₅C₇ and (5 nm Ru)/(5 nm Ta-C), both of which showed failure temperature of 650°C for 30 min and electrical resistivity less than 150 μΩ cm. Because of their high thermal stability and low electrical resistivity, both Ru-Ta-C and Ru/Ta-C films are promising barriers for Cu metalization.     

Nanostructured Interfaces for Thermoelectrics

Temperature drops at the interfaces between thermoelectric materials and the heat source and sink reduce the overall efficiency of thermoelectric systems. Nanostructured interfaces based on vertically aligned carbon nanotubes (CNTs) promise the combination of mechanical compliance and high thermal conductance required for thermoelectric modules, which are subjected to severe thermomechanical stresses. This work discusses the property require- ments for thermoelectric interface materials, reviews relevant data available in the literature for CNT films, and characterizes the thermal properties of vertically aligned multiwalled CNTs grown on a candidate thermoelectric material. Nanosecond thermoreflectance thermometry provides thermal property data for 1.5-μm-thick CNT films on SiGe. The thermal interface resistances between the CNT film and surrounding materials are the dominant barriers to thermal transport, ranging from 1.4 m² K MW⁻¹ to 4.3 m² K MW⁻¹. The volumetric heat capacity of the CNT film is estimated to be 87 kJ m⁻³ K⁻¹, which corresponds to a volumetric fill fraction of 9%. The effect of 100 thermal cycles from 30°C to 200°C is also studied. These data provide the groundwork for future studies of thermoelectric materials in contact with CNT films serving as both a thermal and electrical interface.     

Preparation and Characterization of CdTe for Solar Cells,Detectors, and Related Thin-Film Materials

Cadmium telluride (CdTe) thin films were prepared by the close-space sublimation (CSS) technique, using 99.99% pure CdTe powder as the evaporant. Films were then annealed at 400°C for 30 min and were later dipped in Cu(NO₃)₂-H₂O solution at 80 ± 2°C. After immersion these films were again annealed at 400°C for 1 h to ensure the Cu diffusion into the films. X-ray diffraction (XRD) results confirmed the formation of a new compound copper telluride and a change in the morphology was observed by scanning electron microscopy (SEM). The DC electrical resistivity reduced from 10⁶ Ω-cm for as-deposited to 10⁻³ Ω-cm for 15 h immersed film. As the wt.% of Cu increased, the mobility increased to some extent, while the carrier concentration showed a systematic increase. The film thickness and optical parameter such as refractive index, absorption coefficient, and the optical band gap were deduced by fitting the optical transmittance in the wavelength range 300 to 3000 nm. The transmission decreased with increasing immersion time of films in the␣solution. The Cu concentration was recorded as 0.9 wt.% for 3 min to 56.6 wt.% for 15 h immersed samples using an electron microprobe analyzer (EMPA). In the next step, ITO/CdS/CdTe heterojunctions with 10.9% solar cell efficiency were fabricated on glass slides.     

Comparison of MBE Growth of InSb on Si (001) and GaAs (001)

We describe the epitaxial growth of InSb films on both Si (001) and GaAs (100) substrates using molecular-beam epitaxy and discuss the structural and electrical properties of the resulting films. The complete 2 μm InSb films on GaAs (001) were grown at temperatures between 340°C and 420°C and with an Sb/In flux ratio of approximately 5 and a growth rate of 0.2 nm/s. The films were characterized in terms of background electron concentration, mobility, and x-ray rocking curve width. Our best results were for a growth temperature of 350°C, resulting in room-temperature mobility of 41,000 cm²/V s.  For the growth of InSb on Si, vicinal Si(001) substrates offcut by 4° toward (110) were used. We investigated growth temperatures between 340°C and 430°C for growth on Si(001). In contrast to growth on GaAs, the best results were achieved at the high end of the range of T _S =  C, resulting in a mobility of 26,100 cm²/V s for a 2 μm film. We also studied the growth and properties of InSb:Mn films on GaAs with Mn content below 1%. Our results showed the presence of ferromagnetic ordering in the samples, opening a new direction in the diluted magnetic semiconductors.     

Conductor Formation Through Phase Transformation in Ti-Oxide Thin Films

The resistance and transmittance of Ti-oxide thin films sputtered on quartz substrates were studied. The electrical and optical properties can be changed by varying the percentage of O₂ introduced during the sputtering. The lowest resistivity for the sputtered Ti-oxide thin film was 2.30 × 10⁻² Ω cm for 12.5% O₂, which was obtained after annealing at 400°C in ambient oxygen. The results of x-ray photoelectron spectroscopy (XPS) curve-fitting indicate that the Ti-oxide thin film contained both Ti₂O₃ and TiO₂ phases during deposition. The Ti₂O₃ phase was transformed into the stable TiO₂ phase during annealing. The Ti₂O₃-TiO₂ phase transformation initiated the substitution reaction. The substitution of Ti⁴⁺ ions in the TiO₂ phase for the Ti³⁺ ions in the Ti₂O₃ phase created the free electrons. This Ti₂O₃-TiO₂ phase transformation demonstrates the potential mechanism for conduction in the annealed Ti-oxide thin films. The transmittance of the annealed Ti-oxide thin films can be as high as approximately 90% at the 400 nm wavelength with the introduction of 16.5% O₂. This result indicates that the annealed Ti-oxide thin films are excellent candidates for use as transparent conducting layers for ultraviolet (UV) or near-UV light-emitting diode (LED) devices.     

MCT-on-Silicon Negative Luminescence Devices with High Efficiency

We used an InSb radiometric thermal imager to characterize the performance of 1″ × 1″ negative luminescent (NL) arrays. The devices grown on both CdZnTe (two arrays) and silicon (three arrays) as substrates have cut-off wavelengths ranging from 5.3 μm to 6.0 μm. The reverse-bias saturation current densities range from 0.3 A cm⁻² (λ _co = 5.3 μm) to 1 A cm⁻² (λ _co = 6.0 μm). The apparent array temperatures decrease by 37.9 K to 42.8 K under reverse bias, which corresponds to external NL efficiencies of 80–85%. Most of the inefficiency results from the non-ideal AR coating, whose reflectivity is ≈15% when weighted over the black body and atmospheric transmission spectra. It is highly encouraging that both the electrical and NL properties are slightly superior for the devices grown on silicon substrates.     

Ohmic Contact to High-Aluminum-Content AlGaN Epilayers

We report on the effect of dry etching and the combination of metal stacks used to form ohmic contacts on silicon-doped high-Al-content (>60%) n-AlGaN layers for deep-ultraviolet light-emitting diodes. The contact characteristics are compared for as-grown and plasma-etched n-AlGaN samples. The Ti/Al/Ti/Au contacts to as-grown n-AlGaN were linear, with a specific contact resistivity of 5 × 10⁻⁵ Ω-cm². The same metallic layer combinations yielded nonlinear contacts on the plasma-etched surface of the n-AlGaN layers. However, when Ni was used as the barrier layer instead of titanium, the contacts to plasma-etched AlGaN surfaces became linear, with a specific contact resistivity of 5 × 10⁻⁴ Ω-cm².     

Wetting and Soldering Behavior of Eutectic Au-Ge Alloy on Cu and Ni Substrates

Au-Ge-based alloys are interesting as novel high-temperature lead-free solders because of their low melting point, good thermal and electrical conductivity, and high corrosion resistance. In the present work, the wetting and soldering behavior of the eutectic Au-28Ge (at.%) alloy on Cu and Ni substrates have been investigated. Good wetting on both substrates with final contact angles of 13° to 14° was observed. In addition, solder joints with bond shear strength of 30 MPa to 35 MPa could be produced under controlled conditions. Cu substrates exhibit pronounced dissolution into the Au-Ge filler metal. On Ni substrates, the NiGe intermetallic compound was formed at the filler/substrate interface, which prevents dissolution of Ni into the solder. Using thin filler metal foils (25 μm), complete consumption of Ge in the reaction at the Ni interface was observed, leading to the formation of an almost pure Au layer in the soldering zone.     

Atmospheric plasma deposition of transparent semiconducting ZnO films on plastics in ambient air

Transparent zinc oxide (ZnO) thin films have been successfully synthesized on poly (methyl methacrylate) (PMMA), polycarbonate (PC), and polyethylene terephthalate (PET) substrates by atmospheric plasma deposition in ambient air at room temperature. The structural, optical and electrical properties of the ZnO films as well as their adhesion to the polymer substrates were investigated for various deposition conditions. The film surface exhibited a dome-shaped topography comprised of nanometer-sized grains. The size of both the domes and the grains became larger as the plasma power increased. The visible transmittance increased above 95% with decreasing plasma power. The resistivity exhibited a wide variation in the range of 10²–10⁸ ohm cm. The adhesion energies to PMMA varied from 0.2 to 1.5 J/m² with increasing plasma power. While a finer grain structure achieved with lower plasma power was preferable for higher transmittance, it resulted in lower adhesion to the plastic substrates. The study demonstrated the feasibility of depositing semiconducting transparent ZnO films on polymer substrates at low temperature in ambient air using atmospheric plasma deposition.     

Open tube vapor transport growth of Pb1−xSnxTe epitaxial films for infrared detectors

Epitaxial films of Pb_1−xSn_xTe have been grown by open tube vapor transport on (100) Pb_1−xSn_xTe substrates. The as-grown films are suitable for detector array fabrication with respect to both surface smoothness and electrical properties. Charge compositions from 1% excess metal to 1% excess Te were used. Growth rates up to 3–4 ym per hour were achieved. The asgrown carrier concentrations varied from 3 × 10¹⁶cm⁻³ to 3 × 10¹⁷cm⁻³ depending on growth temperature and charge composition. Schottky barrier detectors with semi-transparent electrodes were fabricated on as-grown layers with no surface preparation. Good uniformity of detector parameters was obtained with arrays of 20 to 40 elements. The array size is not limited by either substrate size or epitaxial quality. Resistance-area products on the order of 1 ohm-cm were obtained at 77 K for detectors with a 12 ym long wavelength cutoff. Quantum efficiencies for 8–12 urn radiation were 40–50%. Peak response and 50% cutoff occurred at 11 and 12 ym, respectively. Uniformity of cutoff wavelength across the arrays of ± 0.1 ym was obtained.     

Growth of High-Density Self-Aligned Carbon Nanotubes and Nanofibers Using Palladium Catalyst

In this paper we demonstrate vertical self-aligned growth of carbon nanotubes (CNT) and carbon nanofibers (CNF) using 1 nm of Pd as the catalyst material. Results were compared with those obtained using traditional catalysts (Co, Fe, and Ni). Pd is of interest as it has been demonstrated to be an excellent material for electrical contact to nanotubes. CNT were grown using plasma-enhanced chemical vapor deposition (PECVD) at 450°C to 500°C and using atmospheric-pressure chemical vapor deposition (APCVD) between 450°C and 640°C. The results were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. High-density (10¹¹ cm⁻² to 10¹² cm⁻²) self-aligned CNT growth was obtained using APCVD and Pd as the catalyst, while Co and Fe resulted in random growth. TEM revealed that the CNT grown by Pd with PECVD form large bundles of tubes, while Ni forms large-diameter CNF. It was found that the CNT grown using Pd or Ni are of low quality compared with those grown by Co and Fe.     

Laser processing of TiSi2 and CoSi2 thin films on silicon (100) substrates

We have investigated the formation of TiSi₂ and CoSi₂ thin films on Si(100) substrates using laser (wave length 248 nm, pulse duration 40 ns and repetition rate 5 Hz) physical vapor deposition (LPVD). The films were deposited from solid targets of TiSi₂ and CoSi₂ in vacuum with the substrate temperature optimized at 600° C. The films were characterized using x-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and four point probe ac resistivity. The films were found to be polycrystalline with a texture. The room temperature resistivity was found to be 16 μΩ-@#@ cm and 23 μΩ-cm for TiSi₂ and CoSi₂ films, respectively. We optimized the processing parameters so as to get particulate free surface. TEM results show that the silicide/silicon interface is quite smooth and there is no perceptible interdiffusion across the interface.     

Highly Ordered Vertical Silicon Nanowire Array Composite Thin Films for Thermoelectric Devices

The fabrication and characterization of silicon nanowire (NW) array/spin-on glass (SOG) composite films for thermoelectric devices are presented. Interference lithography was used to pattern square lattice photoresist templates over entire 2 cm × 2 cm n-type Si substrates. The photoresist pattern was transferred to a SiO₂ hard mask for a single-step deep reactive ion Si etch. The resulting Si NW arrays were 1 μm tall with 15% packing density, and the individual NWs had diameters of 80 nm to 90 nm with vertical sidewalls. The Si NW arrays were embedded in SOG to form a dense and robust composite material for device fabrication and thin-film characterization. The thermal conductivity of the Si NW/SOG composite film was measured to be a constant 1.45 ± 0.2 W/m-K from 300 K to 450 K. An effective medium model was then used to extract a thermal conductivity of 7.5 ± 1.7 W/m-K for the Si nanowires from the measured Si NW/SOG values. The cross-plane Seebeck coefficient of the Si NWs was measured to be −284 ± 26 μV/K, which is comparable to −310 μV/K for bulk Si. Power generation from the combined Si NW/SOG and substrate devices is also presented, and the maximum generated power was found to be 29.3 μW with ΔT of 56 K for a 50 μm × 50 μm device.     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

Preparation and Characterization of Ultrasonically Sprayed Zinc Oxide Thin Films Doped with Lithium

Zinc oxide thin films doped with Li were deposited by ultrasonic spray pyrolysis (USP) at 350 ± 5°C on glass substrates from solutions of zinc acetate [Zn(CH₃COO)₂ · 2H₂O] and lithium acetate [C₂H₃LiO₂ · 2H₂O], in which the Li/Li + Zn ratios were 1 at.%, 3 at.%, and 5 at.%. The effects of the doping on the structural, optical, electrical, and morphological properties of the films were examined. X-ray diffraction patterns indicated that the undoped and Li-doped ZnO films had a polycrystalline hexagonal wurtzite structure with a (002) preferred orientation. The films showed optical transmission around 60–80% in the visible region of the spectrum. The films were found to be transparent in the wavelength range of 450–900 nm, with sharp ultraviolet absorption edges in the wavelength range of 350–450 nm. The absorption edge analysis revealed that the optical band gap energies for the films were between 3.24 eV and 3.29 eV, and the electronic transition was of the direct transition type. The width of the band tail states, which is connected to the localized states in the band gap, was estimated to be 82–113 meV by Urbach tail analysis. For study of the electrical properties of the films, Hall effect measurements, electrical conductivities, conductivity activation, and trap energies were investigated. The electrical measurements of the films were obtained in the dark, in vacuum, and in the temperature range of 10–300 K. Morphological studies for the films were carried out by scanning electron microscopy.     

A comparative study on the growth of InAlN films on different substrates

This work investigates the growth of InAlN films on Si (111), sapphire (001), GaAs (100) and glass substrates and compares the structural, morphological, electrical and optical properties of these films. One micron thick InAlN films were synthesized on these substrates at 300 °C by using reactive magnetron co-sputtering system. The structural analysis showed the formation of polycrystalline InAlN films on all the substrates having preferred orientation along (101) plane. The films grown on sapphire and silicon displayed better structural quality than the films grown on GaAs and glass. The morphological results revealed identical granular features on all the substrates with small variation in the grain size. The electrical resistivity of InAlN film on sapphire was the lowest one (8×10⁻³ Ω-cm) whereas the highest carrier concentration (8×10²⁰ cm⁻³) was obtained for the film deposited on glass. The energy band gap of InAlN films was determined through UV–vis absorption and reflectance spectroscopy. The band gap value obtained on the glass was slightly higher as compared to its value on the other substrates. The changes in InAlN properties on different substrates were explained on the basis of lattice mismatch, crystallite size, residual strain and orientation of the substrates.     

Influence of Surface Treatment and Annealing Temperature on the Formation of Low-Resistance Au/Ni Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-GaN

We have studied the influence of surface treatment and annealing temperature on the specific contact resistance of Au/Ni ohmic contacts to p-GaN with hole concentrations in the range of 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The sample with a hole concentration of 1 × 10¹⁸ cm⁻³, treated with the surface treatment HCl:H₂O = 3:1 solution and annealed at 500°C in a 90% N₂ and 10% O₂ atmosphere, yielded the lowest specific contact resistance of ~4 × 10⁻⁵ Ω cm² and ~2 × 10⁻⁷ Ω cm² at room temperature and at 150°C, respectively. To investigate the roles of interdiffusion between layer interfaces and the formation of NiO and nickel gallides, we examined the metallization stacks before and after annealing using high-resolution x-ray diffraction. We conclude that the nickel-gallide formation and the deterioration of the NiO layer are together responsible for the large deviation in contact resistances observed for samples annealed at various temperatures.     

Abnormal Failure Behavior of Sn-3.5Ag Solder Bumps Under Excessive Electric Current Stressing Conditions

Abnormal failure behavior of flip chip Sn-3.5Ag solder bumps with a Cu underbump metallurgy under excessive electric current stressing conditions is investigated with regard to electromigration lifetime characteristics and damage evolution morphologies. Abnormal behavior such as abrupt changes in the slope of the resistance versus stressing time curve correlate well with the changes in mean time to failure and the standard deviation with respect to␣the resistance increase ratio, which seems to be strongly related to highly␣accelerated electromigration test conditions of 120°C to 160°C and 3 × 10⁴ A/cm² to 4.6 × 10⁴ A/cm². This is closely related to changes in the damage evolution mechanism with time, even though the activation energy for electrical failure is primarily controlled by Cu diffusion through Cu-Sn intermetallic compound layers.