The role of interfacial segregation and microstructure in interdiffusion between aluminum and silicon

In this investigation, we studied the interdiffusion behavior between polycrystalline silicon (poly-Si) and aluminum where the poly-Si was doped with antimony via ion implantation. Post sintering sample microstructure was determined by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and optical microscopy, and composition was determined by Rutherford Backscattering Spectrometry (RBS) and Scanning Auger Microscopy. As-implanted samples showed interdiffusion during sintering (465° C, dry N₂) independent of Sb concentrations up to 1.1 × 10²¹ cm⁻³ near the Al/Si interface. In samples where the implantation damage was annealed out prior to sintering, interdiffusion is inhibited when the Sb concentration at the interface was above a threshold concentration of 7.3 × 10¹⁹ cm⁻³. This threshold concentration is lower if the segregation of Sb is preserved prior to metallization. We propose that interdiffusion is inhibited by dopant (Sb) passivation of interfacial Si defects, the sites where interdiffusion is believed to initiate. MIT work supported by NSF contract 84-18718-DMR.     

Temperature measurement in a silicon carbide light emitting diode by raman scattering

Raman spectra of the transverse-optic phonon mode from a light-emitting layer of a SiC diode have been measured. The phonon peak broadens and shifts to lower frequency with the rise of temperature when the injected current is increased. The frequency shift was compared with a result for bulk reference measured separately at various temperatures. We found that the temperature of the light-emitting layer reached 350°C at a current density of ∼200 A/cm².     

Recrystallization and sulfur diffusion in CdCl2-treated CdTe/CdS thin films

The role of CdCl₂ in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl₂. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl₂ treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl₂ at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe_1−xS_x region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained. © 1997 John Wiley & Sons, Ltd.     

Electronic states on silicon surface after deposition and annealing of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiO _x interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiO _x interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiO _x interface upon annealing. This process is caused by oxygen diffusion from the SiO _x film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiO _x film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.     

Interfacial Reactions Between Pb-Free Solders and In/Ni/Cu Multilayer Substrates

This study investigates the interfacial reactions between Sn-3.0wt.% Ag-0.5wt.%Cu (SAC) and Sn-0.7wt.%Cu (SC) on In/Ni/Cu multilayer substrates using the solid–liquid interdiffusion bonding technique. Samples were reflowed first at 160°C, 180°C, and 200°C for various periods, and then aged at 100°C for 100 h to 500 h. The scalloped Cu₆Sn₅ phase was formed at the SAC/In/Ni/Cu and SC/In/Ni/Cu interfaces. When the reflowing temperatures were 160°C and 180°C, a ternary Ni-In-Sn intermetallic compound (IMC) was formed when the samples were further aged at 100°C. This ternary Ni-In-Sn IMC could be the binary Ni₃Sn₄ phase with extensive Cu and In solubilities, or the ternary Sn-In-Ni compound with Cu solubility, or even a quaternary compound. As the reflow temperature was increased to 200°C, only one Cu₆Sn₅ phase was formed at the solder/substrate interface with the heat treatment at 100°C for 500 h. Mechanical test results indicated that the formation of the Ni-In-Sn ternary IMC weakened the mechanical strength of the solder joints. Furthermore, the solid–liquid interdiffusion (SLID) technique in this work effectively reduced the reflow temperature.     

Interdiffusion of the p-InP with Au−Zn,Ti/Au,Pd/Au,Ti/Pd/Au at interface and their electrical properties

 In this report,the interdiffusion between the p-InP with Au-Zn,Ti/Au,Pd/Au and Ti/Pd/Au at interface have been investigated by Auger electron spectroscopy and electron spectroscopy for chemical analysis.The surface morphology for the heat treatment are observed with scanning electron microscopy. It is found that the indiffusion of Au is easier than that of Pd and Ti and the outdiffusion of In is easier than that of P.The combination state of In and Au is formed during the heat treatment of p-InP/Au-Zn. The effects of the alloying temperature and time on the specific contacts resistance of p-InP/Au-Zn system are studided.The low specific contact resistance,p_c=2.4-2.7×10~(-4)Ω-cm~2,is obtained when alloying at 450℃ for 2 min or at 350℃ for 30 min. These results indicate that the specific contact resistance strongly depend on the“interdiffusion degree”.The Zn in Au-Zn ahoy distributes onto the most surface layer of p-InP/Au-Zn system during evaporation process and heat treatment.It may be one of the reasons for the higher specific contacts resistance.     

Cathodoluminescence spectroscopy of deep defect levels at the ZnSe/GaAs interface with a composition-control interface layer

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.     

Behavior of germanium ion-implanted into SiO<Subscript>2</Subscript> near the bonding interface of a silicon-on-insulator structure

The properties of germanium implanted into the SiO₂ layers in the vicinity of the bonding interface of silicon-on-insulator structures are studied. It is shown that, under conditions of high-temperature (1100°C) annealing, germanium nanocrystals are not formed, while the implanted Ge atoms segregate at the Si/SiO₂ bonding interface. It is established that, in this case, Ge atoms are found at sites that are coherent with the lattice of the top silicon layer. In this situation, the main type of traps is the positive-charge traps; their effect is interpreted in the context of an increase in the surface-state density due to the formation of weaker Ge-O bonds. It is found that the slope of the drain-gate characteristics of the back MIS transistors increases; this increase is attributed to an increased mobility of holes due to the contribution of an intermediate germanium layer formed at the Si/SiO₂ interface.     

Evaluation of low-temperature interdiffusion coefficients in Hg-based superlattices by monitoring the E1 reflectance peak

We show that variations of the E₁ reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E₁ peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100°C for approximately six months, and then at 150°C for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern.     

An X-ray Radiography Study of the Effect of Thermal Cycling on Damage Evolution in Large-Area Sn-3.5Ag Solder Joints

There is a need for next-generation, high-performance power electronic packages and systems utilizing wide-band-gap devices to operate at high temperatures in automotive and electricity transmission applications. Sn-3.5Ag solder is a candidate for use in such packages with potential maximum operating temperatures of about 200°C. However, there is a need to understand the thermal cycling reliability of Sn-3.5Ag solders subject to such high-temperature operating conditions. The results of a study on the damage evolution occurring in large-area Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates with Au/Ni-P metallization subject to thermal cycling between 200°C and 5°C are presented in this paper. Interface structure evolution and damage accumulation were followed using high-resolution X-ray radiography, cross-sectional optical and scanning electron microscopies, and X-ray microanalysis in these joints for up to 3000 thermal cycles. Optical and scanning electron microscopy results showed that the stresses introduced by the thermal cycling result in cracking and delamination at the copper–intermetallic compound interface. X-ray microanalysis showed that stresses due to thermal cycling resulted in physical cracking and breakdown of the Ni-P barrier layer, facilitating Cu-Sn interdiffusion. This interdiffusion resulted in the formation of Cu-Sn intermetallic compounds underneath the Ni-P layer, subsequently leading to delamination between the Ni-rich layer and Cu-Sn intermetallic compounds.     

Evaluation of PZT thin films on Ag coated Si substrates

Fabrication characteristics of hybrid thin film components are investigated. Lead zirconate titanate (PZT) films, thickness 10 μm, are fabricated by using laser ablation on the Ag electrode (about 1 μm thick) which is deposited on 200 μm Si substrates by evaporation. Composition close to the target material is obtained in PZT films even in air and without substrate heating. Low surface energy in the Ag−Si system causes spheroidization of the Ag layer on the fresh Si substrate, but the surface can be modified by grinding and oxidization. Only some cavities exist at the interface. The interface between the Ag electrode and PZT layer is physically continuous, as revealed by electron microscopy. After annealing at 750°C for 2 h, the PZT layer consists of the rhombohedral perovskite phase with a fraction of the pyrochlore phase. Detrimental interdiffusion between Pb and Si occurs during annealing if the PZT thin film is directly on the Si substrate. This is retarded by the presence of the Ag layer.     

Influence of growth temperatures on the photoresponse of low temperature grown GaAs:As p-i-n diodes

We report a study of the sub-bandgap photoresponse of p-i-n photodetectors with 1 μm low-temperature intrinsic layers, and its dependence on the growth temperature of the intrinsic layer. Diodes with intrinsic layers grown near 250°C exhibit the highest photoresponse. The photoresponse decreases gradually as the growth temperature is raised above 250°C. For growth temperatures at or below 200°C, a drastic drop in the photoresponse is observed, along with degradation of crystal quality in the material. The extracted internal Schottky barrier heights are found to be within the range 0.7–0.8 eV.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

The adhesion strength of A lead-free solder hot-dipped on copper substrate

Eutectic Sn-Zn-Al solder alloy was used [composition: 91Sn-9(5Al-Zn)] to investigate the effects of dipping parameters such as the temperature, rate and time dipping on the adhesion strength between solder and substrate using dimethylammonium chloride (DMAHCl) flux. The optimum conditions for the highest adhesion strength (about 8 MPa) were determined as dipping at 350°C, and a rate of 10.8∼11.8 mm/s for 5∼7.5 min. A poor solder coating was obtained as dipped at 250°C. Some defects by non-wetting were found as dipped at a slow rate (slower than 8.2 mm/s). Quite different from the most tin-based solders for copper substrate, γ-Cu₅Zn₈ intermetallic compound particles were found by x-ray diffraction (XRD) analysis at the interface of solder and substrate as dipped at 300°C after pull-off test by etching out the unreacted solder layer. The morphology of the intermetallic compound formed was observed by scanning electron microscopy (SEM). The elements of Al (near Cu), Zn (near Sn) are enriched at the interface of solder and copper substrate as determined by the line scanning and mapping analysis.     

Microstructural characterization of damage in thermomechanically fatigued Sn-Ag based solder joints

Sn-Ag based solder joints of 100-μm thickness were thermomechanically fatigued between −15°C and +150°C with a ramp rate of 25°C/min for the heating segment and 7°C/min for the cooling segment. The hold times were 20 min at high temperature extreme and 300 min at the low temperature extreme. Surface damage accumulation predominantly consisted of shear banding, surface relief due to Sn-grain extrusion, grain boundary sliding, and grain decohesion usually near the solder/substrate interface. Small alloy additions were found to affect the extent of this surface damage accumulation.     

Process parameter dependence of impurity-free interdiffusion in GaAs/AlxGa1−xAs and InxGa1−yAs/GaAs multiple quantum wells

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Al_xGa_1−xAs and pseudomorphic In_y Ga_1−yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO₂ films of varying thickness, chemical-vapor-deposited SiO_xN_y films of varying composition, and spin-on SiO₂ films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusioninduced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO₂) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO₂ caps. A maximum concentration of [Ga] = 4–7 ×10¹⁹ cm⁻³ in the SiO₂ caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiO_xN_y cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In_0.2Ga_0.8As/GaAs interdiffusion coefficients, activation energies E_A and prefactors D_o were estimated to ranging from 3.04 to 4.74 eV and 5 × Kh⁻³ to 2 × 10⁵ cm²/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.     

The reactions between electroless Ni-Cu-P Deposit and 63Sn-37Pb flip chip solder bumps during reflow

This study investigates the interfacial reactions between electroless Ni-Cu-P deposit and 63Sn-37Pb solder bumps under various reflow conditions. The morphology of the intermetallic compounds formed at the Ni-Cu-P/Sn-Pb interface changes with respect to reflow cycle, reflow temperature, and reflow time. The (Ni,Cu)₃Sn₄ compounds with three different morphologies of fine grain, whisker, and polygonal grain form at the Ni-Cu-P/Sn-Pb interface after reflow at 220°C for 15 s. The whisker-shape and polygonal grains detach from the Ni-Cu-P deposit into the Sn-Pb solder during multiple reflows. The (Ni,Cu)₃Sn₄ compound grows rapidly when the reflow temperature is above the Ni-Sn eutectic temperature, 231°C. A continuous (Ni,Cu)₃Sn₄ layer forms after reflow at 220°C for 10 min. A 4.5 μm Ni-Cu-P deposit prevents the interdiffusion of Sn and Al atoms across the Ni-Cu-P deposit after 10 reflow cycles at 220°C for 15 s and after reflow at 220°C for 10 min.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

The effect of Zn1‐xMgxO buffer layer deposition temperature on Cu(In,Ga)Se2 solar cells: A study of the buffer/absorber interface

The effect of atomic layer deposition temperature of Zn_1‐xMg_xO buffer layers for Cu(In,Ga)Se₂ (CIGS) based solar cell devices is evaluated. The Zn_1‐xMg_xO films are grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in a temperature range of 105 to 180°C. High efficiency devices are produced in the region from 105 up to 135°C. At a Zn_1‐xMg_xO deposition temperature of 120°C, a maximum cell efficiency of 15·5% is reached by using a Zn_1‐xMg_xO layer with an x‐value of 0·2 and a thickness of 140 nm. A significant drop in cell efficiency due to large losses in open circuit voltage and fill factor is observed for devices grown at temperatures above 150°C. No differences in chemical composition, structure and morphology of the samples are observed, except for the samples prepared at 105 and 120°C that show elemental selenium present at the buffer/absorber interface. The selenium at the interface does not lead to major degradation of the solar cell device efficiency. Instead, a decrease in Zn_1‐xMg_xO resistivity by more than one order of magnitude at growth temperatures above 150°C may explain the degradation in solar cell performance. From energy filtered transmission electron microscopy, the width of the CIGS/Zn_1‐xMg_xO chemical interface is found to be thinner than 10 nm without any areas of depletion for Cu, Se, Zn and O. Copyright © 2008 John Wiley & Sons, Ltd.     

Low resistance pd/ge ohmic contacts to epitaxially lifted-off n-type GaAs film

A low resistance PdGe nonalloyed ohmic contact has been successfully formed to epitaxially lifted-off n-type GaAs films. The contact is made by lifting off partially metallized n-type GaAs films using the epitaxial lift-off method and bonding them to metallized Si substrates by natural intermolecular Van Der Waals forces. Low temperature sintering (200°C) of this contact results in metallurgical bonding and formation of the ohmic contact. We have measured specific contact resistances of 5 × 10⁻⁵ Ω-cm₂ which is almost half the value obtained for pure Pd contacts. Germanium forms a degenerately doped heterojunction interfacial layer to GaAs. Our experimental results show that germanium diffuses to the interface and acts as a dopant layer to n-GaAs film surface. Therefore, for epitaxially lifted-off n-type GaAs films, PdGe is a low resistance ohmic metal contact to use.