P.J. McNally

Room-temperature ultraviolet luminescence from γ-CuCl grown on near lattice-matched silicon

We have probed the luminescence properties of a wide-band-gap, direct band-gap optoelectronic material, grown on closely lattice-matched silicon substrates, namely, γ-CuCl on Si. This material system is compatible with current Si or GaAs-based electronic/optoelectronic technologies. Polycrystalline epitaxy of CuCl can be controlled such that it maintains an orientation similar to the underlying Si substrate. Importantly, chemical interactions between CuCl and Si are eliminated. Photoluminescence and cathodoluminescence results for CuCl, deposited on either Si (100) or Si (111), reveal a strong room-temperature Z3 excitonic emission at ∼387nm. We have developed and demonstrated the room-temperature operation of an ultraviolet electroluminescent device fabricated by the growth of γ-CuCl on Si. The application of an electrical potential difference across the device results in an electric field, which promotes light emission through hot-electron impact excitation of electron-hole pairs in the γ-CuCl. Since th...

Low Temperature Growth GaAs on Ge

In this work, low temperature growth of GaAs epitaxial layers on Ge substrates by metalorganic vapor phase epitaxy has been studied. The experiments show that a growth temperature of 530°C and a V/III ratio of 3.5 result in smooth GaAs surfaces. Atomic force micrographs do not show any anti-phase boundaries on the surface of GaAs grown on a misoriented substrate. X-ray diffraction curves show that the layer tilt is reduced as the growth temperature is lowered. Synchrotron X-ray topography reveals very low threading dislocation densities of 300 cm-2 for the GaAs epitaxial layers. Additionally, no misfit dislocations are observed. If a single layer is deposited at low temperature, secondary ion mass spectrometry shows a considerably reduced arsenic diffusion into Ge. When an additional layer is deposited at higher temperature on top of the initial low temperature layer, a substantial increase for the deep concentration-dependent arsenic diffusion is found.

Hybrid organic–inorganic spin-on-glass CuCl films for optoelectronic applications

Cuprous halides are among the most studied inorganic materials for excitonic related linear/non-linear optical processes due to their large excitonic binding energies (∼190 and 108meV for CuCl and CuBr, respectively). In recent years, we have studied CuCl thin films deposited by vacuum evaporation and sputtering techniques on a variety of substrates. We now report on the extension of this research to the deposition of CuCl nanocrystals on flexible substrates via a spin-on technology. In this study, we present the synthesis, deposition and characterization of CuCl nanocrystals embedded in organic polysilsesquioxane (PSSQ) films on a variety of substrates via the spin coating method. The nanocrystals were synthesized by a complexation‐reduction‐precipitation mechanism reaction of CuCl2 · 2H2O, alpha D-glucose and de-ionized (DI) water with a PSSQ based solution as the host matrix material. The deposited films were heated at 120 ◦ C for durations between 1 and 24h in vacuo. The room temperature UV‐Vis absorption spectra for all hybrid films, except the as-deposited film, showed both Z1,2 and Z3 excitonic absorption features. Room temperature photoluminescence measurements of all heated films reveal very intense Z3 excitonic emission at 3.221eV. Room temperature x-ray diffraction (XRD) of the as-deposited films gave no evidence of the crystallization of CuCl. However, after heating the films, XRD confirmed the preferential growth of CuCl nanocrystals whose average size is ≈25‐45nm in the � 111� orientation. The CuCl hybrid films showed bright electroluminescent emission at 384nm when subjected to an ac voltage of about 100V peak to peak. (Some figures in this article are in colour only in the electronic version)

Examination of mechanical stresses in silicon substrates due to lead–tin solder bumps via micro-Raman spectroscopy and finite element modelling

Due to the fact that semiconductor devices have decreased significantly in geometry and increased enormously in electronic design complication, flip-chip packaging technology was launched to increase input/output count, improve electrical performance, reduce packaging size and be cost effective. The Intel®Pentium®III microprocessor uses the popular ball grid array (BGA) packaging technique. BGA is one of the most common flip-chip packaging techniques used for microprocessor applications. However, mechanical stresses induced by the flip-chip process are major concerns for the reliability of such devices. Micro-Raman spectroscopy (μRS) is a powerful technique for investigating the spatial extent of strain fields in microelectronic devices. In this study, the strain fields imposed on the underlying silicon substrate due to the lead–tin solder bump process in BGA packaging have been investigated in pre- and post-reflowed samples using μRS and finite element modelling (FEM). For pre-reflowed samples, an approximate uniaxial compressive stress of 200 MPa is developed near the edge of the under bump metallization (UBM). However, a tensile stress up to ~300 MPa is found for post-reflowed samples. Two-dimensional (2D) plane strain FEM has also been performed. The magnitudes and spatial distribution of the stresses after the reflow process are in good agreement with the micro-Raman results.

The evaluation of mechanical stresses developed in underlying silicon substrates due to electroless nickel under bump metallization using synchrotron X-ray topography

The switch-over to the use of flip-chip Si integrated circuit bonding techniques has been driven by a need to develop higher power and lower voltage devices, capable of carrying larger currents with greater reliability. With the increased use of solder bump interconnections, an understanding of the behaviour of commonly used electroless nickel under bump metallization (UBM) layers is becoming ever more crucial. The aim of this paper is to evaluate the usefulness of white beam synchrotron X-ray topography (WBSXRT) for non-destructive evaluation of the induced mechanical stresses on Si substrates for different Ni(P) based UBM sizes and thicknesses. It is shown that WBSXRT is a powerful tool for non-destructively mapping strain and/or defect distributions within the underlying silicon substrate. Using this technique, it was also found that the crystalline misorientation induced in the underlying silicon is increased for larger UBM diameters. Stress magnitudes in the Si substrate directly under the UBM can reach values as high as 260MPa.

B-Spline X-Ray Diffraction Imaging — Rapid non-destructive measurement of die warpage in ball grid array packages

Abstract Next generation “More than Moore” integrated circuit (IC) technology will rely increasingly on the benefits attributable to advanced packaging ( www.itrs.net [1] ). In these increasingly heterogeneous systems, the individual semiconductor die is becoming much thinner (25 to 50xa0μm, typically) and multiple dies can be stacked upon each other. It is difficult to assess non-destructively, non-invasively and in situ the stress or warpage of the semiconductor die inside these chip packages and conventional approaches tend to monitor the warpage of the package rather than the die. This paper comprises an account of a relatively new technique, which we call B-Spline X-Ray Diffraction Imaging (B-XRDI) and its application, in this instance, to the non-destructive mapping of Si semiconductor die lattice misorientation inside wire bonded encapsulated Low-profile Fine-pitch Ball Grid Array (LFPGA) packages. B-XRDI is an x-ray diffraction imaging technique which allows the user to reconstruct from a series of section x-ray topographic images a full profile of the warpage of the silicon semiconductor die inside such a chip package. There is no requirement for pre-treatment or pre-processing of the chip package and we show that synchrotron-based B-XRDI mapping of wafer warpage can be achieved with angular tilt resolutions of the order of 50xa0μradxa0≈xa00.003° in times as short as 9–180xa0s (worst case X–Y spatial resolutionxa0=xa0100xa0μm) for a full 8.7xa0mmxa0×xa08.7xa0mm semiconductor die inside the fully encapsulated LFBGA packages. We confirm the usefulness of the technique by correlating our data with conventional warpage measurements performed by mechanical and interferometric profilometry and finite element modelling (FEM). We suggest that future developments will lead to real-time, or near real-time, mapping of thermomechanical stresses during chip packaging processes, which can run from bare wafer through to a fully encapsulated chip package.

Electroluminescence of γ-CuBr thin films via vacuum evaporation depositon

γ-CuBr is a I–VII wide band gap mixed ionic–electronic semiconducting material with light emitting properties suitable for novel UV/blue light applications. Its structural and physical properties allow for vacuum deposition on a variety of substrates and herein we report on the deposition of γ-CuBr on Si and indium tin oxide coated glass substrates via vacuum evaporation with controllable film thickness from 100 to 500 nm. Temperature dependent photoluminescence characteristics of these γ-CuBr films on Si (1 0 0) reveal familiar Zf and I1 excitonic features. A thin film electroluminescent device using a γ-CuBr active layer was fabricated and room temperature electroluminescence was obtained for γ-CuBr for the first time. CuBr features relating to known excitonic (Zf, 3.1 eV) emissions were observed as well as a number of previously unknown emissions at 3.81, 3.02, 2.9, 2.75 and 2.1 eV. We speculate on the origins of these peaks and attribute them to the presence of monovalent Cu+ generated during ac excitation.

Stress characterization of device layers and the underlying Si1-xGex virtual substrate with high resolution micro-Raman spectroscopy

Silicon–germanium (Si–Ge) epitaxially grown mismatched heterostructures are becoming increasingly important for high-frequency microelectronics applications. One option under serious consideration is that of using Si–Ge virtual substrates, i.e., compositionally graded layers designed to accommodate the lattice mismatch between the underlying Si substrate and the overlying active epilayers(s). This assists in the prevention of misfit dislocations that can impact adversely on the active device regions. The stress in both device silicon cap layers and the underlying Si1−xGex virtual substrates is characterized with high-resolution micro-Raman spectroscopy (μRS). The device layers of the samples studied composed of a 7-nm thick silicon channel, a 6-nm thick SiGe layer and were capped with a 7-nm thick silicon layer. The device layers are grown over a 1-μm thick constant composition Si0.70Ge0.30 virtual substrate capping layer, and the Si-Ge virtual substrate is grown on a p+-type (0 0 1) silicon wafer with a thickness of about 500 μm. μRS measurement results with a 488-nm Ar+ visible laser source indicate that the Si0.70Ge0.30 capping layer at the virtual substrate is fully unstrained, while the top silicon cap layer is in extremely high tension. The use of a 325-nm HeCd UV laser for the μRS measurements, which probes only a very small depth into the Si cap layer (approximately 9 nm) confirms this high tensile stress is in the top silicon cap layer. The tensile stress in the top silicon cap layer is estimated to be as large as 2.4 GPa by analyzing the shift of the Si Raman peak with respect to the standard strain-free silicon sample. The measured stress value is almost equal to the theoretically predicted tensile stress that should exist in the fully strained Si cap layer. This implies that the Si cap layer remains strained in samples with this structure.

Soft x-ray spectroscopic investigation of Zn doped CuCl produced by pulsed dc magnetron sputtering.

We report on a systematic investigation of the electronic properties of UV-light emitting Zn doped CuCl thin films implemented using near edge x-ray absorption fine structures (NEXAFS) and high-resolution x-ray photoemission spectroscopy. A clear shift of the valence band maximum towards higher binding energy by 0.2xa0±xa00.1xa0eV was observed in Zn doped CuCl as compared to undoped CuCl. This shift is in correlation with the increase in conductivity measured by the Hall effect measurements. A decrease in the optical band gap of CuCl film is also observed as a function of Zn doping. The profound changes in the full width at half maximum and the gradual disappearance of satellite features of Cu 2p core level photoemission as a function of Zn dopant are attributed to the reduced presence of the surface layer of Cu(2+) species with d(9) configuration in the doped films. These investigations help us to understand the doping mechanisms and underlying physics. The reduced presence of the Cu(2+) related surface layer as a function of Zn doping is also verified using NEXAFS.

Evaluation of conduction mechanism and electronic parameters for Au/organic?inorganic CuCl hybrid film/ITO structures

Hybrid materials are capable of combing organic and inorganic compounds into a nano-composite with unique characteristics. An example of such organic–inorganic CuCl hybrid films is studied here using a combination of organic polysilsesquioxane and inorganic CuCl; γ-CuCl is an ionic I–VII compound semiconductor material with the zincblende structure at room temperature. It has excellent ultraviolet (UV) emission properties at room temperature and is a promising candidate material for optoelectronic applications. The CuCl hybrid films were deposited on indium tin oxide (ITO)-coated glass by simple spin-coating techniques. Au/CuCl hybrid film/ITO structures were fabricated, and field-dependent electrical studies were carried out at room temperature in the range 2.5 × 105–3.5 × 106 V m−1. We confirm that the organic–inorganic CuCl film structure behaves as an effective single semiconducting medium, possessing bandstructure and other related properties. For electric field magnitudes up to 1.25 × 106 V m−1, an ohmic conduction mechanism was observed, and for field magnitudes >1.5 × 106 V m−1, Schottky emission conduction prevails in these structures. The electronic parameters were evaluated and an effective barrier height, ideality factor and series resistance were found to be 0.84 ± 0.05 eV, 1.12 ± 0.08 and 50 ± 2 MΩ, respectively, whereas the effective barrier height obtained from the C–V measurement was 1.05 ± 0.05 eV. This value is somewhat higher than the value obtained from the I–V measurement. This difference is likely caused by the presence of a thin intervening insulating layer between the hybrid film surface and the Schottky metal. The density distribution of the interface states decreases with an increase of the energy of the interface states. This organic–inorganic CuCl hybrid film behaves as an effective single semiconductor material structure, and a schematic energy-level diagram for the device is proposed.