Origin of void defects in Hg1−xCdxTe grown by molecular beam epitaxy

Characterization of defects in Hg_1−xCd_xTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg_1−xCd_xTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm⁻². Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.     

Formation and control of defects during molecular beam epitaxial growth of HgCdTe

Void defects were demonstrated to form away from the substrate-epifilm interface during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride substrates. These were smaller in size compared to voids which nucleated at the substrate-epifilm interface, which were also observed. Observations of void nucleation away from the substrate-epifilm interface were related to the respective growth regimes active at the time of the void nucleation. Once nucleated, voids replicated all the way to the surface even if the flux ratios were modified to prevent additional nucleation of voids. For a significant number of films, void defects were observed co-located with hillocks. These voids were usually smaller than 1 μm and appeared almost indistinguishable from unaccompanied simple voids. However, these void-hillock complexes displayed a nest of dislocation etch pits around these defects upon dislocation etching, whereas unaccompanied voids did not. The nests could extend as much as 25 μm from the individual void-hillock complex. The density of dislocations within the nest exceeded 5×10⁶ cm⁻², whereas the dislocation density outside of the nest could decrease to <2×10⁵ cm⁻². The void-hillock complexes formed due to fluctuations in growth parameters. Elimination of these fluctuations drastically decreased the concentrations of these defects.     

Electromigration on void formation of Sn3Ag1.5Cu FCBGA solder joints

The electromigration on void formation and failure mechanism of FCBGA packages under a current density of 1 × 10⁴ A/cm² and an environmental temperature of 150 °C was investigated. Two solder/substrate combinations of Sn3Ag1.5Cu with Cu-OSP and Cu/Ni/Au were examined. A conservative failure criterion was adopted to predict the failure of package, and SEM was used to observe in situ microstructural change and failure modes.Failure was mainly attributed to void occupation along UBM/solder interfaces by the side of cathode chip of bumps with downward electron flow. The current crowding was the cause for void initiation from the entrance corner of electron flow. Two specific void locations were identified at IMC/solder and UBM/IMC interfaces, and both can co-exist in the same specimen but in different bumps. No coupling mode of void was found. Since there is a discrepancy of diffusion rate between solder and IMC layers, current density results in more voids between them. A current density of 1 × 10⁴ A/cm² was found as a dominant factor that was high enough for void pattern at IMC/solder interface. However, the void formation at the UBM/IMC interface was generally induced by the UBM consumption due to the high temperature of 150 °C that dominates the void morphology crucially at UBM/IMC interface.     

Probing into Reverse Bias Dark Current in Perovskite Photodiodes: Critical Role of Surface Defects

Minimizing reverse bias dark current density (J_dark) while retaining high external quantum efficiency is crucial for promising applications of perovskite photodiodes, and it remains challenging to elucidate the ultimate origin of J_dark. It is demonstrated in this study that the surface defects induced by iodine vacancies are the main cause of J_dark in perovskite photodiodes. In a targeted way, the surface defects are thoroughly passivated through a simple treatment with butylamine hydroiodide to form ultrathin 2D perovskite on its 3D bulk. In the passivated perovskite photodiodes, J_dark as low as 3.78 × 10^-10 A cm^-2 at -0.1 V is achieved, and the photoresponse is also enhanced, especially at low light intensities. A combination of the two improvements realizes high specific detectivity up to 1.46 × 10¹² Jones in the devices. It is clarified that the trap states induced by the surface defects can not only raise the generation-recombination current density associated with the Shockley–Read–Hall mechanisms in the dark (increasing J_dark), but also provide additional carrier recombination paths under light illumination (decreasing photocurrent). The critical role of surface defects on J_dark of perovskite photodiodes suggests that making trap-free perovskite thin films, for example, by fine preparation and/or surface engineering, is a top priority for high-performance perovskite photodiodes.     

Electromigration-induced UBM consumption and the resulting failure mechanisms in flip-chip solder joints

Eutectic PbSn flip chip solder joint was subjected to 5×10³ A/cm² current stressing at 150°C and 3.5 × 10⁴ A/cm² current stressing at 30°C. The under bump metallurgy (UBM) on the chip was sputtered Ni/Cu, and the substrate side was a thick Cu trace. It was shown through in-situ observation that the local temperature near the entrance of electrons from the Al interconnect to the solder became higher than the rest of the joint. The accelerated local Ni UBM consumption near the entrance was also observed. Once the Ni was consumed at a location, a porous structure formed, and the flow of the electrons was blocked there. It was found that the formation of the void and the formation of the porous structure were competing with each other. If the porous structure formed first, then the void would not be able to nucleate there. On the other hand, if the void could nucleate before the UBM above lost its conductivity, then the joint would fail by the void formation-and-propagation mechanism.     

Influence of silicon processing in atomic hydrogen on the formation of local molten regions as a result of pulsed irradiation with optical photons

The effect of intense atomic hydrogen flux on the defect density in the surface layer of single-crystal silicon is studied. It is shown that the formation of local molten regions by pulsed-light heating of Si samples and further analysis of the local melting pattern can be an efficient tool for determining the number of defects introduced by the processing in atomic hydrogen. It was found that the processing conditions in atomic hydrogen with an exposure dose lower than 2.7 × 10¹⁷ cm^?2 do not change the number of defects in Si; in contrast, conditions with an exposure dose above 3.6 × 10¹⁸ cm^?2 significantly increase the defect density. The increase in the number of defects can be caused by the interaction of atomic hydrogen with the Si surface.     

MBE growth of HgCdTe on silicon substrates for large format MWIR focal plane arrays

HgCdTe p-on-n double layer heterojunctions (DLHJs) for mid-wave infrared (MWIR) detector applications have been grown on 100 mm (4 inch) diameter (211) silicon substrates by molecular beam epitaxy (MBE). The structural quality of these films is excellent, as demonstrated by x-ray rocking curves with full widths at half maximum (FWHMs) of 80–100 arcsec, and etch pit densities from 1 10⁶ to 7 10⁶ cm⁻². Morphological defect densities for these layers are generally less than 1000 cm⁻². Improving Hg flux coverage of the wafer during growth can reduce void defects near the edges of the wafers. Improved tellurium source designs have resulted in better temporal flux stability and a reduction of the center to edge x-value variation from 9% to only 2%. Photovoltaic MWIR detectors have been fabricated from some of these 100mm wafers, and the devices show performance at 140 K which is comparable to other MWIR detectors grown on bulk CdZnTe substrates by MBE and by liquid phase epitaxy.     

Low-Temperature,Strong SiO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> Covalent Wafer Bonding for III–V Compound Semiconductors-to-Silicon Photonic Integrated Circuits

We report a low-temperature process for covalent bonding of thermal SiO₂ to plasma-enhanced chemical vapor deposited (PECVD) SiO₂ for Si-compound semiconductor integration. A record-thin interfacial oxide layer of 60 nm demonstrates sufficient capability for gas byproduct diffusion and absorption, leading to a high surface energy of 2.65 J/m² after a 2-h 300°C anneal. O₂ plasma treatment and surface chemistry optimization in dilute hydrofluoric (HF) solution and NH₄OH vapor efficiently suppress the small-size interfacial void density down to 2 voids/cm², dramatically increasing the wafer-bonded device yield. Bonding-induced strain, as determined by x-ray diffraction measurements, is negligible. The demonstration of a 50 mm InP epitaxial layer transferred to a silicon-on-insulator (SOI) substrate shows the promise of the method for wafer-scale applications.     

Defects Passivation via Potassium Iodide Post-Treatment for Antimony Selenosulfide Solar Cells with Improved Performance

Antimony selenosulfide (Sb₂(S,Se)₃) has been emerging as a promising light absorber in the past few years owing to tunable bandgap (1.1–1.7 eV), high absorption coefficient (>10⁵ cm⁻¹) and excellent phase and environmental stability. However, the efficiency of Sb₂(S,Se)₃ solar cells lags far behind the Shockley–Queisser limit. One of the critical obstacles originates from various extrinsic and intrinsic defects. They mostly locate in the deep energy levels and are prone to form recombination centers, inhibiting the improvement of device performance. Herein, surface post-treatment via potassium iodide is introduced to fabricate high-quality Sb₂(S,Se)₃ films and solar cells. The surface post-treatment not only manipulates the crystal growth process to form compact films with larger grain size but also forms better band alignment and inhibits the formation of deep-level defects antimony antisite (Sb_Se), thus improving the quality of heterojunction. Consequently, the resultant Sb₂(S,Se)₃ solar cells achieve a champion power conversion efficiency  of 9.22%. This study provides a new strategy of passivating deep-level intrinsic defects via surface post-treatment for high-efficiency Sb₂(S,Se)₃ solar cells.     

Microscopic defects on MBE grown LWIR Hg1−xCdxTe material and their impact on device performance

Long wavelength infrared molecular beam epitaxy (MBE) grown p-on-n Hg_1−xCd_xTe double layer planar heterostructure (DLPH) detectors have been characterized to determine the dominant mechanisms limiting their performance. Material defects have been identified as critical factors that limit 40K performance operability. This effort has concentrated on identifying microscopic defects, etch pit density (EPD) and relating these defects to the device performance. Visual inspection indicates defect densities as high as 10⁵ per cm² with a spatial extent as observed by atomic force microscope in the range of micrometers extending several micrometers beneath the surface. At high EPD values (greater than low 10⁶ cm⁻²) zero bias resistance (R₀) at 40K decreases as roughly as the square of the EPD. At 78K, however, measured R₀ is not affected by the EPD up to densities as high as mid-10⁶ cm⁻². Visual defects greater than 2–3 μm than ∼2 μm in size (micro-void defects) result in either a single etch pit or a cluster of etch pits. Large variations in a cross-wafer etch pit distribution are most likely a major contributor to the observed large spreads in 40K R₀. This study gives some insight to the present limitation to achieve higher performance and high operability for low temperature infrared applications on MBE grown HgCdTe material.     

Electromigration-Induced Void Formation at the Cu<Subscript>5</Subscript>Zn<Subscript>8</Subscript>/Solder Interface in a Cu/Sn-9Zn/Cu Sandwich

The electromigration that occurs in a Cu/Sn-9Zn/Cu sandwich was investigated for void formation at room temperature with 10³ A/cm². A focused ion beam revealed that voids nucleated at the intermetallic compound (IMC)/solder interface regardless of the electron flow direction. The needle-like voids initiated at the cathode Cu₅Zn₈/solder interface due to the outward diffusion of Zn atoms in the Zn-rich phase and expanded as a result of the surface diffusion of Sn atoms upon current stressing.     

Development and fabrication of two-color mid- and short-wavelength infrared simultaneous unipolar multispectral integrated technology focal-plane arrays

We report the development and fabrication of two-color mid-wavelength infrared (MWIR) and short-wavelength infrared (SWIR) HgCdTe-based focalplane arrays (FPAs). The HgCdTe multilayers were deposited on bulk CdZnTe (ZnTe mole fraction ∼3%) using molecular beam epitaxy (MBE). Accurate control of layer composition and growth rate was achieved using in-situ spectroscopic ellipsometry (SE). Epilayers were evaluated using a variety of techniques to determine suitability for subsequent device processing. These techniques included Fourier transform infrared (FTIR) spectroscopy, Hall measurement, secondary ion mass spectroscopy (SIMS), defect-decoration etching, and Nomarski microscopy. The FTIR transmission measurements confirmed SE’s capability to provide excellent compositional control with run-to-run x-value variations of ∼0.002. Nomarski micrographs of the as-grown surfaces featured cross-hatch patterns resulting from the substrate/epilayer lattice mismatch as well as various surface defects (voids and “microvoids”), whose densities ranged from 800–8,000 cm⁻². A major source of these surface defects was substrate particulate contamination. Epilayers grown following efforts to reduce these particulates exhibited significantly lower densities of surface defects from 800–1,700 cm⁻². Dislocation densities, as revealed by a standard defect-decoration etch, were 2–20×10⁵ cm⁻², depending on substrate temperature during epitaxy. The FPAs (128×128) were fabricated from these epilayers. Preliminary performance results will be presented.     

A Green and Facile Way to Prepare Granadilla‐Like Silicon‐Based Anode Materials for Li‐Ion Batteries

A yolk‐shell‐structured carbon@void@silicon (CVS) anode material in which a void space is created between the inside silicon nanoparticle and the outer carbon shell is considered as a promising candidate for Li‐ion cells. Untill now, all the previous yolk‐shell composites were fabricated through a templating method, wherein the SiO₂ layer acts as a sacrificial layer and creates a void by a selective etching method using toxic hydrofluoric acid. However, this method is complex and toxic. Here, a green and facile synthesis of granadilla‐like outer carbon coating encapsulated silicon/carbon microspheres which are composed of interconnected carbon framework supported CVS nanobeads is reported. The silicon granadillas are prepared via a modified templating method in which calcium carbonate was selected as a sacrificial layer and acetylene as a carbon precursor. Therefore, the void space inside and among these CVS nanobeads can be formed by removing CaCO₃ with diluted hydrochloric acid. As prepared, silicon granadillas having 30% silicon content deliver a reversible capacity of around 1100 mAh g^?1 at a current density of 250 mA g^?1 after 200 cycles. Besides, this composite exhibits an excellent rate performance of about 830 and 700 mAh g^?1 at the current densities of 1000 and 2000 mA g^?1, respectively.     

3D Polydentate Complexing Agents for Passivating Defects and Modulating Crystallinity for High-Performance Perovskite Solar Cells

The grain boundaries (GBs)/surface defects within perovskite film directly impede the further improvement of photoelectric conversion efficiency (PCE) and stability of planar perovskite solar cells (PSCs). Herein, 3D phytic acid (PA) and phytic acid dipotassium (PAD) with polydentate are explored to synchronously passivate the defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups ( H₂PO₄) in the PA molecule afford six anchor sites to bind firmly with uncoordinated Pb²⁺ at the GBs/surface and modulate perovskite crystallization, thus enhancing the quality of perovskite film. Particularly, PAD containing an additional (K→PO) push–pull structure promotes the dominant coordination of phosphate group (PO) with Pb²⁺ and passivates halide anion defects due to the complexation of potassium ions (K⁺) with iodide ions (I^-). Consequently, the PAD-complexed PSCs deliver a champion PCE of 23.18%, which is remarkably higher than that of the control device (19.94%). Meanwhile, PAD-complexed PSCs exhibit superior moisture and thermal stability, remaining 79% of their initial PCE after 1000 h under continuous illumination, while the control device remain only 48% of their PCE after 1000 h. This work provides important insights into designing multifunctional 3D passivators for the purpose of simultaneously enhancing the efficiency and stability of devices.     

Surface gettering of background impurities and defects in GaAs wafers

The first data on surface gettering of background impurities and defects from the bulk of single-crystal undoped GaAs(111) wafers are reported. The wafers were 1.6 mm thick, with an initial electron density of (1–3)×10¹⁵ cm^?3 and a mobility of 1500–2000 cm²/(V s) at room temperature. The wafers were cut from single crystals grown by the Czochralski method from a nonstoichiometric As-enriched Ga-As melt. Gettering was carried out during thermal treatment of the wafers in hydrogen at 400–850°C, with the preliminary deposited layer of Y or SiO₂ 1000 Å thick. As a result of gettering, the charge carrier density decreased to 10⁸–10¹⁰ cm^?3, while the mobility increased to 7000 cm²/(V s).     

Particulates: A direct origin of oval defects in GaAs layers grown by molecular beam epitaxy

Based on micrographic as well as experimental analyses, we show that particulates which inadvertently adhere to the surface before the wafer is ready for growth are one of the most significant origins for the formation of oval defects on the GaAs layers grown by molecular beam epitaxy. We show that the density of surface particulates is proportional to that of airborne particles surrounding the wafer under preparation, especially during the drying and transferring process. With reduction of airborne particles, we simultaneously reduce the density of oval defects from a few thousand to about 200 cm^-2 for 1-μm thick layers.     

Effective passivation of defects in Ge-rich SiGe-on-insulator substrates by Al2O3 deposition and subsequent post-annealing

A method of Al₂O₃ deposition and subsequent post-deposition annealing (Al₂O₃-PDA) was proposed to passivate electrically active defects in Ge-rich SiGe-on-insulator (SGOI) substrates, which were fabricated using Ge condensation by dry oxidation. The effect of Al₂O₃-PDA on defect passivation was clarified by surface analysis and electrical evaluation. It was found that Al₂O₃-PDA could not only suppress the surface reaction during Al-PDA in our previous work [Yang H, Wang D, Nakashima H, Hirayama K, Kojima S, Ikeura S. Defect control by Al-deposition and the subsequent post-annealing for SiGe-on-insulator substrates with different Ge fractions. Thin Solid Films 2010; 518: 2342-5.], but could also effectively passivate p-type defects generated during Ge condensation. The concentration in the range of 10¹⁶-10¹⁸ cm⁻³ for defect-induced acceptors and holes in Ge-rich SGOI drastically decreased after Al₂O₃-PDA. As a result of defect passivation, the electrical characteristics of both back-gate p-channel and n-channel metal-oxide-semiconductor field-effect transistors fabricated on Ge-rich SGOI were greatly improved after Al₂O₃-PDA.     

Controllable Thermochemical Generation of Active Defects in the Horizontal/Vertical MoS2 for Enhanced Hydrogen Evolution

As for 2D transition metal dichalcogenides, the creation of proper active defects concentrations is considered as the efficient strategy for improving hydrogen evolution performance. However, the synthesis methods of large-area MoS₂ catalysts with controllable active defects are limited, also for its working mechanism. Herein, thermochemical generation of active defects for MoS₂ catalysts has established by annealing sodium hypophosphite, in which the phosphine is spontaneously generated and chemically tailors the MoS₂ lattice. The defects formation is confirmed by the investigation of slightly-changed surface structure and unpaired electrons for the annealed samples. The hydrogen evolution reaction performances of horizontally/vertically grown MoS₂ films are improved by controlling reaction conditions, indicating the active defects could form in the basal plane and edges with retained crystal structure. The overpotential of MoS₂ samples converted from 10 nm Mo reduces from −520 to −265 mV with largely decreased Tafel slope. The electrochemical microreactor studies reveal the protons adsorption of active sites shows much more significant contribution, than interfacial charge transfer with the enhanced remarkable performance (−100 mV at 10 mA cm⁻²). This study presents the large-area synthesized strategy for MoS₂ based catalysts with controllable defects concentration and helps establish rational design principles for future MoS₂ family electrocatalysts.     

Tailored Silica Macrocellular Foams: Combining Limited Coalescence‐Based Pickering Emulsion and Sol–Gel Process

Solid‐sate monolithic macrocellular foams are synthesized by mineralizing the continuous phase of oil‐in‐water Pickering emulsions, used as templates, with the sol–gel process. For the first time, taking advantage of the limited coalescence phenomenon occurring in emulsions stabilized by solid particles, concentrated emulsions with calibrated drop size are produced, leading to the synthesis of monolithic foams with nearly monodisperse macroscopic voids. Such a strategy allows independent tuning of the macrocellular void diameters from 20 to 800 μm and the diameter of the windows connecting adjacent cells. The obtained macrocellular foams also bear micro‐ and mesoporosity, leading to Brunauer, Emmet and Teller (BET) surface area values between 700 and 900 m² g^?1 with a good mesopores monodispersity.     

ICP-induced defects in GaN characterized by capacitance analysis

The defects induced by inductively coupled plasma reactive ion etching (ICP-RIE) on a Si-doped gallium nitride (GaN:Si) surface have been analyzed. According to the capacitance analysis, the interfacial states density after the ICP-etching process may be higher than 5.4 × 10¹² eV⁻¹ cm⁻², compared to around 1.5 × 10¹¹ eV⁻¹ cm⁻² of non-ICP-treated samples. After the ICP-etching process, three kinds of interfacial states density are observed and characterized at different annealing parameters. After the annealing process, the ICP-induced defects could be reduced more than one order of magnitude in both N₂ and H₂ ambient. The H₂ ambient shows a better behavior in removing ICP-induced defects at a temperature around 500 °C, and the interfacial states density around 2.2 × 10¹¹ eV⁻¹ cm⁻²can be achieved. At a temperature higher than 600 °C, the N₂ ambient provides a much more stable interfacial states behavior than the H₂ ambient.