Characterization of Smooth CdTe(111) Films by the Conventional Close-Spaced Sublimation Technique

Thin epitaxial CdTe films were grown on CdTe(111)B substrates by the close-spaced sublimation (CSS) technique and were characterized over a range of experimental parameters. The source temperature was varied between 480°C and 540°C, maintaining an average constant source–substrate temperature difference ΔT of ∼130°C. Helium was used as a carrier gas at pressures between 2 Torr and 10 Torr. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to analyze the film morphology and structure. Growth rates ranging from 1 μm/h to 4 μm/h were observed, based on profilometer thickness measurements. The addition of a pre-growth heat treatment step and post-growth annealing treatment resulted in smooth CdTe(111) films. An evolution in growth morphology was demonstrated with SEM images and film quality was confirmed with XRD.     

Maskless selective area growth of InP on Sub-μm V-groove patterned Si(001)

Low pressure metalorganic chemical vapor deposition of InP onexactly oriented Si(OOl) substrates with a periodic V-groove pattern of periodicity ≤1.2 μm using a two temperature growth sequence (400 and 640°C) is reported. Planar InP layers with extremely low defect density of 7 × 10⁴ cm⁻² are obtained. For InP on V-grooves of width g ≤1.0μm, a planar surface is formed after less than 1 μm of growth. Formation or suppression of antiphase domains (APDs) is a function of the widths of the (OOl)-oriented ridges. For s ≤1 μm, epilayers are single domain and the direction is oriented parallel to the grooves. At 400°C, nucleation starts homogeneously on {111}-sidewallsand (001)-facets. While heating up to 640°C, InP migrates into the grooves, depleting almost completely the (001)-facets. During growth of the main layer, first the V-grooves are filled up. Subsequently (001)-ridges are overgrown laterally or voids are formed on top of them. This mechanism is responsible for both planarization and APD-suppression. The surface migration length of InP on Si(001) at 640°C is estimated to be ≈0.5 μm.     

Monte Carlo Modeling of VLWIR HgCdTe Interdigitated Pixel Response

Increasing very long-wave infrared (VLWIR, λ _c ≈ 15 μm) pixel operability was approached by subdividing each pixel into four interdigitated subpixels. High response is maintained across the pixel, even if one or two interdigitated subpixels are deselected (turned off), because interdigitation provides that the preponderance of minority carriers photogenerated in the pixel are collected by the selected subpixels. Monte Carlo modeling of the photoresponse of the interdigitated subpixel simulates minority-carrier diffusion from carrier creation to recombination. Each carrier generated at an appropriately weighted random location is assigned an exponentially distributed random lifetime τ _i, where 〈τ _i〉 is the bulk minority-carrier lifetime. The minority carrier is allowed to diffuse for a short time dτ, and the fate of the carrier is decided from its present position and the boundary conditions, i.e., whether the carrier is absorbed in a junction, recombined at a surface, reflected from a surface, or recombined in the bulk because it lived for its designated lifetime. If nothing happens, the process is then repeated until one of the boundary conditions is attained. The next step is to go on to the next carrier and repeat the procedure for all the launches of minority carriers. For each minority carrier launched, the original location and boundary condition at fatality are recorded. An example of the results from Monte Carlo modeling is that, for a 20-μm diffusion length, the calculated quantum efficiency (QE) changed from 85% with no subpixels deselected, to 78% with one subpixel deselected, 67% with two subpixels deselected, and 48% with three subpixels deselected. Demonstration of the interdigitated pixel concept and verification of the Monte Carlo modeling utilized λ _c(60 K) ≈ 15 μm HgCdTe pixels in a 96 × 96 array format. The measured collection efficiency for one, two, and three subelements selected, divided by the collection efficiency for all four subelements selected, matched that calculated using Monte Carlo modeling.     

Ion-assisted molecular beam epitaxy of GaAs on Si(100)

GaAs was grown by molecular beam epitaxy (MBE) and ion-assisted MBE on Si(100) substrates. Three-dimensional (3D) island nucleation, observed during MBE growth, was eliminated during ion-assisted MBE when the ion energyE was >25 eV and the product ofE and the current densityJ was ≈6-12 eV mA/cm². IncreasingEJ to ≈15 eV mA/ cm² resulted in excessive ion damage. Decreasing the substrate temperature from 280 to 580° C during ion-assisted MBE yielded a slight decrease in surface roughness, and flatter surfaces were obtained for lower As⁴/Ga flux ratios. The suppression of 3D island nucleation led to an improvement in the crystalline perfection of thicker GaAs films. For example, the x-ray diffraction rocking-curve full-width-at-half-maximum values for 0.5 μm thick films grown at 380° C decreased from 1700 arcsec to 1350 arcsec when ion irradiation was used during nucleation. IAMBE allowed nucleation of thin, relatively flat-surfaced GaAs films even at 580° C, resulting in FWHM values of 1850 arcsec for 0.14 /μm thick films.     

Fabrication and Characterization of MOVPE-Grown CdTe-on-Si Heterojunction Diode-Type Gamma-Ray Detectors

We report on the growth of very thick (>260 μm) high-crystalline-quality single-crystal CdTe epitaxial films on (211) Si substrates in a metalorganic vapor-phase epitaxy reactor, and the development of gamma ray detectors and their radiation detection properties. Films were grown with a high growth rate varying from 40 μm/h to 70 μm/h. A heterojunction diode was fabricated by growing a 90-μm-thick CdTe layer on an n ⁺-Si substrate, which exhibited good rectifying behavior and had a low reverse bias leakage current of 0.18 μA/cm² at 100 V bias. The diode clearly demonstrated its gamma radiation detection capability by resolving energy peaks from the ²⁴¹Am radioisotope during room-temperature measurements. By cooling the diode detector to −30°C, the leakage current could be reduced by three orders of magnitude from the room-temperature value. At this operating condition dramatic improvements in the pulse height spectrum were observed.     

Molecular beam epitaxial HgCdTe material characteristics and device performance: Reproducibility status

Extensive material, device, and focal plane array (FPA) reproducibility data are presented to demonstrate significant advances made in the molecular beam epitaxial (MBE) HgCdTe technology. Excellent control of the composition, growth rate, layer thickness, doping concentration, dislocation density, and transport characteristics has been demonstrated. A change in the bandgap is readily achieved by adjusting the beam fluxes, demonstrating the flexibility of MBE in responding to the needs of infrared detection applications in various spectral bands. High performance of photodiodes fabricated on MBE HgCdTe layers reflects on the overall quality of the grown material. The photodiodes were planar p-on-n junctions fabricated by As ion-implantation into indium doped, n-type, in situ grown double layer heterostructures. At 77K, diodes fabricated on MBE Hg_1−xCd_xTe with x ≈ 0.30 (λ_co ≈ 5.6 μm), x ≈ 0.26 (λ_co ≈ 7 μm), x ≈ 0.23 (λ_co ≈ 10 μm) show R₀A products in excess of 1 x 10⁶ ohm-cm², 7 x 10⁵ ohm-cm², and 3 x 10² ohm-cm², respectively. These devices also show high quantum efficiency. As a means to assess the uniformity of the MBE HgCdTe material, two-dimensional 64 x 64 and 128 x 128 mosaic detector arrays were hybridized to Si multiplexers. These focal plane arrays show an operability as high as 97% at 77K for the x ≈ 0.23 spectral band and 93% at 77K for the x ≈ 0.26 spectral band. The operability is limited partly by the density of void-type defects that are present in the MBE grown layers and are easily identified under an optical microscope.     

Interaction Between Ni and Cu Across 95Pb-5Sn High-Lead Layer

Ni/95Pb-5Sn/Cu ternary diffusion couples were used to investigate the cross-interaction between Ni and Cu across a layer of 95Pb-5Sn solder. High-lead solder layers with a thickness of 100 μm or 400 μm were electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited high-lead solder surface. The diffusion couples were then aged at 150°C to 250°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high-temperature process such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that massive spalling also occurred during aging even though reflow was not used. The massive spalling began with the formation of microvoids. When the microvoids had congregated into large enough voids, intermetallic compounds (Cu₃Sn) started to spall from the interface. This spalling phenomenon occurred sooner with increasing temperature and decreasing solder volume.     

Selective etch-back and growth of InGaAs ON (100) Fe:lnP by electroepitaxy

Selective etch-back prior to growth of InGaAs islands on SiO₂-masked (100)Fe-doped InP substrates was performed by electroepitaxy. The etch-back of the substrate and the growth of the layer was done at a constant furnace temperature of 640° C by passing a direct electric current from the melt to the substrate for etch-back and from the substrate to the melt for growth. The current density used was 1 to 20 A/cm² for a period from 15 to 60 min. The isolated InP regions were of various sizes (40 × 1000μm to 3000 × 3000μm), and different geometries (narrow and wide strips, square, circular). A uniform etch-back and uniform growth with excellent surface morphology was obtained on strips as wide as 200μm and on circles withd < 500μm. For islands with wider geometry, growth as well as etch-back were uniform up to 100–200μm from the periphery with excellent surface morphology. The etch-back and growth profiles are trapezoid-shaped and are not influenced by the difference in chemical activity between crystalline planes. The orientation dependence of the etch rate was {110} > {100} > {011} > {111} B > {111} A.     

Effects of S,Si, or Fe dopants on the diffusion of Zn in InP during MOCVD

Diffusion of Zn in InP during growth of InP epitaxial layers has been investigated in layer structures consisting of Zn-InP epilayers grown on S-InP and Fe-InP substrates, and on undoped InP epilayers. The layers were grown by metalorganic chemical vapour deposition (MOCVD) atT = 625° C andP = 75 Torr. Dopant diffusion profiles were measured by secondary ion mass spectrometry (SIMS). At sufficiently high Zn doping levels ([Zn] ≥8 × 10¹⁷ cm⁻³) diffusion into S-InP substrates took place, with accumulation of Zn in the substrate at a concentration similar to [S]. Diffusion into undoped InP epilayers produced a diffusion tail at low [Zn] levels, probably associated with interstitial Zn diffusion. For diffusion into Fe-InP, this low level diffusion produced a region of constant Zn concentration at [Zn] ≈ 3 × 10¹⁶ cm⁻³, due to kick-out of the original Fe species from substitutional sites. We also investigated diffusion out of (Zn, Si) codoped InP epilayers grown on Fe-InP substates. The SIMS profiles were characterised by a sharp decrease in [Zn] at the epilayer-substrate interface; the magnitude of this decrease corresponded to that of the Si donor level in the epilayer. For [Si] ≫ [Zn] in the epilayer no Zn diffusion was observed; Hall measurements indicated that the donor and acceptor species in those samples were electrically active. All these results are consistent with the presence of donor-acceptor interactions in InP, resulting in the formation of ionised donor-acceptor pairs which are immobile, and do not contribute to the diffusion process.     

Impulse Response Time Measurements in Hg<Subscript>0.7</Subscript>Cd<Subscript>0.3</Subscript>Te MWIR Avalanche Photodiodes

The response time of front-sided illuminated n-on-p Hg_0.7Cd_0.3Te electron avalanche photodiodes (e-APDs) at T = 77 K was studied using impulse response measurements at λ = 1.55 μm. We measured typical rise and fall times of 50 ps and 800 ps, respectively, at gains of M ≈ 100, and a record gain-bandwidth (GBW) product of GBW = 1.1 THz at M = 2800. Experiments as a function of the collection width have shown that the fall time is strongly limited by diffusion. Variable-gain measurements showed that the impulse response is first-order sensitive to the level of the output amplitude. Only a slight increase in the rise time and the fall time was observed with the gain at constant output amplitude, which is consistent with a strongly dominant electron multiplication. Comparisons of the experimental results with Silvaco finite element simulations confirmed the diffusion limitation of the response time and allowed the illustration of the transit time and RC effects.     

Metal organic chemical vapor deposition of superconducting YBa2Cu3O7-x thin films

The discovery of YBCO superconductors has stimulated a great deal of scientific and technological research into thin films of these materials. Because the MOCVD technique is known to produce high quality films in the III/V and II/VI material groups, our approach has been to apply the method to superconducting thin films. Thin films were grown in a vertical high speed (0–2000 rpm) rotating disk reactor. The source materials were metalβ-diketonates kept at temperatures in excess of 100° in order to obtain growth rates of 0.3 to 0.5μm/hr. The precursors were transported to the chamber with a nitrogen carrier and injected separately in order to avoid any gas phase reactions. The chamber pressure was maintained at 76 Torr with an oxygen partial pressure of 38 Torr. A resistance heater was used to keep the substrate temperature at 500° YBa₂Cu₃O_7-x films were deposited simultaneously on a variety of substrates such as (100) MgO, (1-102) sapphire, (100) SrTiO₃ and (100) YSZ. Full XPS spectra were collected for the binary oxides. The scans demonstrate the existence of Y₂O₃, BaO, and CuO with the correct valence state for the metallic species. Energy dispersive analysis of x-ray (EDAX) was used to determine film compositions by comparing EDAX spectral intensity to a known superconducting standard. Appropriate changes were made in the precursor flows to correct the stoichiometry. The as-grown films were dark brown and semi-transparent. Cross-sectional SEM photomicrographs revealed an ordered columnar structure. After annealing at 950–980° however, the films on (100) SrTiO₃ appeared dull black and opaque. The surface morphology exhibited smooth large plate-like grains. X-ray data clearly display an orthorhombic phase, with c-axis perpendicular to the substrate surface. Four point resistance measurements for films on (100) SrTiO₃ show the onset of superconductivity at 90 K with a complete loss of resistance at 88 K. This sharp (≤2K) transition shows the high quality of these MOCVD grown YBCO films and are the first reported results from a large area (2 × 50 mm substrates) commercial reactor.     

Nanowires in the CdHgTe Material System

HgTe nanowires have been grown by molecular beam epitaxy (MBE). They are nucleated at Au particles on Si or GaAs substrates and subsequently self-organize and grow laterally on the surface into 20–50 nm wide, 0.5–1 μm long twisted, but single-crystal, wires. Further growth gives longer, wider, and straighter polycrystalline wires. When unimpeded by Au particles on the surface, the wires become straight and consist of segments of cubic 〈111〉 HgTe and hexagonal 〈001〉 Te parallel to the wire. Te nanowires and Au␣nanowires have also occasionally been formed. All attempts to grow CdHgTe on Si substrates with or without Au particles have resulted in polycrystalline layers. The phase diagrams and diffusion coefficients imply that CdHgTe or HgTe nanowires will not grow by the vapor–liquid–solid technique at the low MBE growth temperatures. SiO₂ functions as a mask for selective growth of HgTe, but not for CdHgTe.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.     

Microstructure evolution and magnetoresistance of the A-site ordered Ba-doped manganites

The microstructure, crystal structure, and magnetotransport properties of microsized and nanosized Badoped manganites have been investigated. A “two-step” reduction-reoxidation procedure has been used to obtain nanosized ceramic manganite Nd_0.70Ba_0.30MnO₃ (II). The parent microsized manganite Nd_0.70Ba_0.30MnO₃ (I) was prepared by usual ceramic technology in air. Then the sample was annealed in vacuum. The grain size of the reduced sample, determined by scanning electron microscopy, decreased from ∼5 μm down to ∼100 nm. To obtain the oxygen stoichiometry nanosized sample, the Nd_0.70Ba_0.30MnO_2.60 was again annealed in air. It is established that the (I) sample is a pseudocubic perovskite, whereas (II) is tetrahedral as a consequence of Nd³⁺ and Ba²⁺ ions as well as the ordering of oxygen vacancies. The (I) sample is a ferromagnet with T _C ≈ 140 K. It has metal-insulator transition at T _MI ≈ 135 K and a peak of magnetoresistance ∼50% in a field of 9 kOe. For the (II) sample, the critical points of phase transitions move to higher temperatures, T _C ≈ 320 K and T _MI ≈ 310 K. The magnetoresistance of the (II) sample at room temperature (T ≈ 293 K) is about 7% in a field of 9 kOe. The magnetotransport properties are interpreted in the framework of the nanosized effect. The text was submitted by the authors in English.     

Characterization of Hg0.7Cd0.3Te n- on p-type structures obtained by reactive ion etching induced p- to n conversion

This paper presents transport measurements on both vacancy doped and gold doped Hg_0.7Cd_0.3Te p-type epilayers grown by liquid phase epitaxy (LPE), with N_A=2×10¹⁶ cm⁻³, in which a thin 2 μm surface layer has been converted to n-type by a short reactive ion etching (RIE) process. Hall and resistivity measurements were performed on the n-on-p structures in van der Pauw configuration for the temperature range from 30 K to 400 K and magnetic field range up to 12 T. The experimental Hall coefficient and resistivity data has been analyzed using the quantitative mobility spectrum analysis procedure to extract the transport properties of each individual carrier contributing to the total conduction process. In both samples three distinct carrier species have been identified. For 77 K, the individual carrier species exhibited the following properties for the vacancy and Au-doped samples, respectively, holes associated with the unconverted p-type epilayer with p ≈ 2 × 10¹⁶ cm⁻³, μ ≈ 350 cm²V⁻¹s⁻¹, and p ≈ 6 × 10¹⁵ cm⁻³, μ ≈ 400 cm²V⁻¹s⁻¹; bulk electrons associated with the RIE converted region with n ≈ 3 × 10¹⁵cm⁻³, μ ≈ 4 × 10⁴ cm²V⁻¹s⁻¹, and n ≈ 1.5 × 10¹⁵ cm⁻³, μ ≈ 6 × 10⁴ cm²V⁻¹s⁻¹; and surface electrons (2D concentration) n ≈ 9 × 10¹² cm⁻² and n ≈ 1 × 10¹³ cm⁻², with mobility in the range 1.5 × 10³ cm²V⁻¹s⁻¹ to 1.5 × 10⁴ cm²V⁻¹s⁻¹ in both samples. The high mobility of bulk electrons in the RIE converted n-layer indicates that a diffusion process rather than damage induced conversion is responsible for the p-to-n conversion deep in the bulk. On the other hand, these results indicate that the surface electron mobility is affected by RIE induced damage in a very thin layer at the HgCdTe surface.     

Molecular beam epitaxy of in1-xGaxASyP1-y(y ≃2.2 ×) lattice matched to InP using gas cells

In this paper we report MBE growth of quaternary alloys In_1-xGa_xAs_yP_1-y (y≃2.2x, 0 <y < 1) lattice matched to InP, by using gas cells as sources of V elements. The growth is performed in a MBE system which can receive both gas and solid cells. An efficient pumping system made from association of cryo and turbo molecular pumps allows a background hydrogen pressure of ≃ 10^-5 Torr during the growth. Gas sources produce P₂ and As₂ flux from cracking of PH₃ and AsH₃. An accurate and flexible control of P₂ and As₂ flux is obtained by monitoring the gas cells by a mass selector system. Electron probe analysis and X-ray diffraction show that reproducible growth of homogeneous quaternary layers with a precise control of the composition can be achieved with gas cells. Possible contamination by residual impurities coming from the cracking furnace is investigated by low temperature photoluminescence (8 K). Typical spectrum (8 K) exhibits a narrow near band edge peak (FWHM = 17 meV) and a secondary peak located at 14 meV below the first one which is attributed to carbon acceptor impurity. Room temperature photoluminescence efficiency is equal to that obtained by LPE. For In_0.60Ga_0.40As_0.90Po_0.10 background carrier concentration isn = 10¹⁶cm^-3 with electron mobilities μ (300 K) = 6000 cm²/Vsec and μ (77 K) = 13000 cm²/Vsec.     

Low-Resistance Cu-Sn Electroplated–Evaporated Microbumps for 3D Chip Stacking

Low-resistance copper-tin (Cu-Sn) microbumps, with sizes varying from 5 μm × 5 μm to 20 μm × 20 μm and formed by electroplating–evaporation bumping (EEB) technology for three-dimensional integration of large-scale integrated chips, have been evaluated for their microstructure and electrical resistance. It was inferred from x-ray diffraction data that the formation of low-resistance Cu₃Sn intermetallic compound (IMC) is facilitated at higher bonding temperature. Electron probe microanalysis mapping showed that, even before bonding, Cu-Sn IMCs were formed at the interface between Cu and Sn, whereas they were sandwiched between the Cu of the upper and lower microbumps after bonding. Electron backscatter diffraction analysis revealed that the crystal orientation of Sn grains was sharply localized in the (100) orientation for physical vapor deposited (PVD) sample, while electroplated Sn film exhibited a mixed crystal orientation in all (100), (110), and (001) axes. A resistance value of ~35 mΩ per bump was obtained for Cu-Sn microbumps with area of 400 μm², which is several times lower than the resistance value reported for Cu-Sn microbumps fabricated by a pure electroplating method. The low resistance value obtained for EEB-formed Cu-Sn microbumps after bonding is explained by (i) the reduced surface roughness for evaporated Sn, (ii) the high degree of crystal grain orientation resulting from layer-by-layer growth in the PVD Sn, despite their smaller grain size, and (iii) the absence of impurity segregation at grain boundaries.     

Effect of Pd Surface Roughness on the Bonding Process and High Temperature Reliability of Au Ball Bonds

A 0.3-μm-thick electrolytic Pd layer was plated on 1 μm of electroless Ni on 1 mm-thick polished and roughened Cu substrates with roughness values (R _a) of 0.08 μm and 0.5 μm, respectively. The rough substrates were produced with sand-blasting. Au wire bonding on the Ni/Pd surface was optimized, and the electrical reliability was investigated under a high temperature storage test (HTST) during 800 h at 250°C by measuring the ball bond contact resistance, R _c. The average value of R _c of optimized ball bonds on the rough substrate was 1.96 mΩ which was about 40.0% higher than that on the smooth substrate. The initial bondability increased for the rougher surface, so that only half of the original ultrasonic level was required, but the reliability was not affected by surface roughness. For both substrate types, HTST caused bond healing, reducing the average R _c by about 21% and 27%, respectively. Au diffusion into the Pd layer was observed in scanning transmission electron microscopy/ energy dispersive spectroscopy (STEM–EDS) line-scan analysis after HTST. It is considered that diffusion of Au or interdiffusion between Au and Pd can provide chemically strong bonding during HTST. This is supported by the R _c decrease measured as the aging time increased. Cu migration was indicated in the STEM–EDS analysis, but its effect on reliability can be ignored. Au and Pd tend to form a complete solid solution at the interface and can provide reliable interconnection for high temperature (250°C) applications.     

HgCdTe growth on (552) oriented CdZnTe by metalorganic vapor phase epitaxy

We report the growth of HgCdTe on (552)B CdZnTe by metalorganic vapor phase epitaxy (MOVPE). The (552) plane is obtained by 180 rotation of the (211) plane about the [111] twist axis. Both are 19.47 degrees from (111), but in opposite directions. HgCdTe grown on the (552)B-oriented CdZnTe has a growth rate similar to the (211)B, but the surface morphology is very different. The (552)B films exhibit no void defects, but do exhibit ∼40 μm size hillocks at densities of 10–50 cm⁻². The hillocks, however, are significantly flatter and shorter than those observed on (100) metalorganic vapor phase epitaxy (MOVPE) HgCdTe films. For a 12–14 μm thick film the height of the highest point on the hillock is less than 0.75 μm. No twinning was observed by back-reflection Laue x-ray diffraction for (552)B HgCdTe films and the x-ray double crystal rocking curve widths are comparable to those obtained on (211)B films grown side-by-side and with similar alloy composition. Etch pit density (EPD) measurements show EPD values in the range of (0.6–5)×10⁵ cm⁻², again very similar to those currently observed in (211)B MOVPE HgCdTe. The transport properties and ease of dopant incorporation and activation are all comparable to those obtained in (211)B HgCdTe. Mid-wave infrared (MWIR) photodiode detector arrays were fabricated on (552)B HgCdTe films grown in the P-n-N device configuration (upper case denotes layers with wider bandgaps). Radiometric characterization at T=120–160 K show that the detectors have classical spectral response with a cutoff wavelength of 5.22 μm at 120 K, quantum efficiency ∼78%, and diffusion current is the dominant dark current mechanism near zero bias voltage. Overall, the results suggest that (552)B may be the preferred orientation for MOVPE growth of HgCdTe on CdZnTe to achieve improved operability in focal plane arrays.     

Cross-Interaction between Ni and Cu across Sn Layers with Different Thickness

The Ni/solder/Cu material sequence is one of the most common material sequences in the solder joints of electronic packages. In this study, the Ni/Sn/Cu ternary diffusion couples were used to investigate the solder volume effect on the cross-interaction between Ni and Cu. Experimentally, a pure Sn layer with the thickness of 100–400 μm was electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited Sn surface. The diffusion couples were aged at 160°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the cross-interaction could occur in as short as 30 min. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 25–40 times higher than the Ni flux. Moreover, it was found that (Cu_1−x Ni _x )₆Sn₅ on the Ni side reduced the consumption rate of the Ni layer, and the cross-interaction also reduced the Cu₃Sn thickness on the Cu side.