Impact of critical processes on HgCdTe diode performance and yield

HgCdTe detector performance and yield are strongly dependant on CdZnTe substrate and HgCdTe epilayer properties, and on key device processes, especially for 8–12 μm application. Due to the correlation and optimization between these figures and diode performance, AIM has developed a mature HgCdTe technology for superior detector performance and high production rate. To meet high yield and performance for long wavelength (LW) HgCdTe diodes, dislocation densities of < 1 × 10^t cm⁻² both in substrate and epilayer have to be ensured. By a unique AIM substrate growth process, dislocation densities of 2 × 10⁴-9 × 10⁴ cm⁻² are achieved for all substrates and epilayers (100% yield). The etch pit density (EPD) on 〈111〉 epilayers is revealed by an AIM proprietary etching procedure. One critical effect is the dislocations in the diode area, which can originate from the substrate and epilayer growth and the subsequent device processes, respectively. Our studies have shown that device processes can cause additional dislocations in the diode area. Diode yield was clearly improved by a combination of wet and dry etching for diode contact etching.     

Misfit dislocations in limited inhomogeneous media: A review

Elastic fields of translation and misfit dislocations (resp. TD's and MD's) are investigated for two types of inhomogeneous materials.First, a multilayer formed by parallel heterointerfaces or free surfaces and containing one periodic array of interfacial MD's. In the more simple case of a thin layer on a substrate, analytic solutions can be found for the displacement field u relative to a single or a hexagonal periodic array of MD's. In the less simple case of a thin bicrystal and a layer sandwiched between two semi-infinite media, explicit solutions can still be extracted. For the general case of a multilayer material which involves a number N>2 heterointerfaces, analytic expressions become intractable. Solutions can nevertheless be obtained from the numerical inversion of linear equations which express the limiting boundary conditions along the heterointerfaces.Second, a thin bicrystal containing an heterointerface perpendicular to the two free surfaces of a foil. Starting from the elastic field of an edge TD parallel to the free surfaces, it is shown how to derive that of (i) a coherent heterointerface, (ii) a semicoherent heterointerface containing a single MD, and (iii) two close and parallel coherent heterointerfaces. Numerical calculations and graphs illustrate the solutions for metals (Al, Al₂Cu), semiconductor materials (CoSi₂, Si, GaAs), thin foils of the GaAs/Si type and a / superalloy.     

Faster Diffusion of Oxygen Along Dislocations in (La,Sr)MnO3+δ Is a Space-Charge Phenomenon

In displaying accelerated oxygen diffusion along extended defects, (La,Sr)MnO_3+δ is an atypical acceptor-doped perovskite-type oxide. In this study, ¹⁸O/¹⁶O diffusion experiments on epitaxial thin films of La_0.8Sr_0.2MnO_3+δ and molecular dynamics (MD) simulations are combined to elucidate the origin of this phenomenon for dislocations: Does diffusion occur along dislocation cores or along space-charge tubes? Transmission electron microscopy studies of the films revealed dislocations extending from the surface. ¹⁸O penetration profiles measured by secondary ion mass spectrometry indicated (slow) bulk diffusion and faster diffusion along dislocations. Oxygen tracer diffusivities obtained for temperatures 873 ≤ T [K] ≤ 973 were over two orders of magnitude higher for dislocations than for the bulk. The activation enthalpy of oxygen diffusion along dislocations, of (2.95 ± 0.21) eV, is surprisingly high relative to that for bulk diffusion, (2.67 ± 0.13) eV. This result militates against fast diffusion along dislocation cores. MD simulations confirmed no accelerated migration of oxide ions along dislocation cores. Faster diffusion of oxygen along dislocations in La_0.8Sr_0.2MnO_3+δ is thus concluded to occur within space-charge tubes in which oxygen vacancies are strongly accumulated. Reasons for and the consequences of space-charge zones at extended defects in manganite perovskites are discussed.     

Near-room temperature MWIR HgCdTe photodiodes limited by vacancies and dislocations related to Shockley-Read-Hall centres

Trap assisted tunnelling via traps located at dislocation cores as well as mercury vacancies are considered as the mechanisms of enhanced thermal generation of charge carriers in reverse-biased MWIR HgCdTe photodiodes operating with Peltier cooling. Field-induced reduction of trap activation energies increases thermal generation and creates conditions for large tunnelling currents. The model for LWIR devices published previously in Ref. [20], also explain experimental current-voltage characteristics of the MWIR photodiodes assuming great misfit dislocation density at graded gap interfaces between absorber and contact regions.     

Interface features of ultrasonic flip chip bonding and reflow soldering in microelectronic packaging

Ultrasonic features in the bonding area are of interest for researchers in the field of microelectronic packaging. In this study, the interface characteristics of bonding were examined using an XD7100 X‐ray instrument and a transmission electron microscope (TEM). It was seen that bubbles were usually detected at the interfaces of the reflow‐soldered flip chip (FC), but they were not found at the interfaces of ultrasonically formed FC, and so ultrasonic FC bonding is more reliable than reflow soldering. The strong mechanical effect of ultrasonic vibration activates the dislocations in the crystalline metal lattice. Dislocation diffusion is more prominent than crystal diffusion when the temperature is relatively low during ultrasonic bonding, and therefore the processes of ultrasonic bonding enhance by several orders of magnitude. This indicates that the mechanism of ultrasonic bonding is different from the melting mechanism of reflow soldering. Copyright © 2007 John Wiley & Sons, Ltd.     

The gauge theory of dislocations: A nonuniformly moving screw dislocation

We investigate the nonuniform motion of a straight screw dislocation in infinite media in the framework of the translational gauge theory of dislocations. The equations of motion are derived for an arbitrarily moving screw dislocation. The fields of the elastic velocity, elastic distortion, dislocation density and dislocation current surrounding the arbitrarily moving screw dislocation are derived explicitly in the form of integral representations. We calculate the radiation fields and the fields depending on the dislocation velocities.     

CdZnTe graded buffer layers for HgCdTe/Si integration

To investigate the potential benefits of compositional grading for dislocation control in CdTe/Si growth, Cd_1−xZn_xTe buffer layers with x graded smoothly from 1 to 0 have been deposited on Si (211) surfaces. Growth has been characterized using reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), and etch pit density measurements. XRD showed an increase in rocking curve full-width at half-maximum (FWHM) and global lattice tilt with decreasing x values. Tilt was also observed to increase as buffer growth temperature was increased. Final surface dislocation densities did not decrease below 7×10⁶ cm⁻². EPD surface dislocation measurements showed reduced dislocation densities and dislocation clustering along the and lines for CdTe cap layers grown on partially graded Cd_1−xZn_xTe buffer layers with slow compositional grading rates. Samples grown with faster grading rates showed higher final EPD values, with dislocations clustering along the and lines.     

High-temperature growth of very high germanium content SiGe virtual substrates

We have first of all studied (in reduced pressure–chemical vapour deposition) the high-temperature growth kinetics of SiGe in the 0–100% Ge concentration range. We have then grown very high Ge content (55–100%) SiGe virtual substrates at 850 °C. We have focused on the impact of the final Ge concentration on the SiGe virtual substrates’ structural properties. Polished Si_0.5Ge_0.5 virtual substrates were used as templates for the growth of the high Ge concentration part of such stacks, in order to minimize the severe surface roughening occurring when ramping up the Ge concentration. The macroscopic degree of strain relaxation increases from 99% up to values close to 104% as the Ge concentration of our SiGe virtual substrates increases from 50% up to 100% (discrepancies in-between the thermal expansion coefficients of Si and SiGe). The surface root mean square roughness increases when the Ge concentration increases, reaching values close to 20 nm for 100% of Ge. Finally, the field (the pile-up) threading dislocations density (TDD) decreases as the Ge concentration increases, from 4×10⁵ cm⁻² (1–2×10⁵ cm⁻²) for [Ge]=50% down to slightly more than 1×10⁵ cm⁻² (a few 10⁴ cm⁻²) for [Ge]=88%. For [Ge]=100%, the field TDD is of the order of 3×10⁶ cm⁻², however.