Specific resistivity of a metal-n GaAs OHMIC contact with an intermediate N+ layer

The specific contact resistivity ρ_c of a metal-n GaAs structure, over a wide range of carrier concentrations in the intermediate layer (5 × 10¹⁸–5 × 10²⁰ cm^?3) and in the substrate (10¹⁵?10¹⁷ cm^?3) is calculated. The results which are presented graphically demonstrate the dependence of ρ_c on metal-semiconductor barrier height from 0.2 eV–0.8 eV. A comparison with the measured data of an alloyed AuGeNi-n GaAs system, suggests that the calculations corresponding to a concentration of 5 × 10¹⁸ cm^?3 in the layer and barrier height of 0.4 eV give the best fit based on the experimental observations. It is hoped that the results can be used as a guideline in developing ohmic contacts of ultra low resistance values.     

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.     

Liquid phase epitaxial growth and characterization of high purity lattice matched GaxIn1-xAs ON <111>B InP

High purity Ga_xIn_1-x As has been grown lattice matched on <111>B InP by liquid phase epitaxy. Silicon, the residual impurity, is purged from the melt by long time baking in a hydrogen atmosphere with slight ambient water vapor concentration. Epitaxial layers with a net electron concentration as low as 3.5 × 10¹⁴ cm^-3 and a liquid nitrogen mobility of 70,000 cm²/Vsec have been grown. The room temperature mobility is shown to be significantly higher than GaAs over a wide range of net electron concentrations useful for device applications, with the highest value of μ₃₀₀ = 13,800 cm²/Vsec on a sample with n = 1.9 × I0¹⁵ cm_-3     

electrical properties of the proton-irradiated semi-insulating GaAs:Cr

The n-p conversion of the conduction type and a decrease in resistivity to 10² Ω cm at 300 K were revealed upon proton irradiation (5 MeV, 300 K, D≈2×10¹⁷ cm^?2) of semi-insulating GaAs:Cr (ρ≈(3–4)×10⁸ Ω cm). Temperature dependences of ρ for heavily irradiated samples indicate a hopping conduction in the temperature range of 400–120 K, with the transition to the conduction with variable-range hopping at T≤120 K. The effects of electronic switching were found in low-resistivity proton-irradiated GaAs:Cr at about 20 K. The isochronous annealing of radiation defects in the temperature range of 20–750°C was investigated.     

Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory

We have investigated the contact resistivity of GeCu₂Te₃ (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρ_c of as-deposited amorphous GCT to W was 3.9×10⁻² Ω cm². The value of ρ_c drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρ_c of 4.8×10⁻⁶ Ω cm². The ρ_c contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge₂Sb₂Te₅ (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell.     

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.     

On the Thermoelectric Properties of Zintl Compounds Mg3Bi2−x Pn x (Pn = P and Sb)

A series of Zintl compounds Mg₃Bi_2-x Pn _x (Pn = P and Sb) have been synthesized by the solid-state reaction method. While Sb can be substituted to a level as high as x = 1.0, P can be substituted only up to x = 0.5. The thermoelectric potential of these compounds has been evaluated by measuring resistivity (ρ), Seebeck (α) and Hall coefficients, and thermal conductivity between 80 K and 850 K. The measured resistivity and Seebeck coefficient values are consistent with those expected for small-bandgap semiconductors. Hall measurements suggest that the carriers are p type with concentration (p) increasing from ~10¹⁹ cm^?3 to ~10²⁰ cm^?3 as the Bi content is increased. The Hall mobility decreases with increasing temperature (T) and reaches a more or less similar value (~45 cm²/V s) for all substituted compositions at room temperature. Due to mass defect scattering, the lattice thermal conductivity (κ _L) is decreased to a minimum of ~1.2 W/m K in Mg₃BiSb. The power factor (α ²/ρ) is found to be rather low and falls in the range 0.38 mW/m K² to 0.66 mW/m K². As expected, at a high temperature of 825 K, the total thermal conductivity (κ) of Mg₃BiSb reaches an impressive value of ~1.0 W/m K. The highest dimensionless figure of merit (ZT) is realized for Mg₃BiSb and is ~0.4 at 825 K.     

A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Evaluation of mobility gaps and density of localized hole states in p-Ge/Ge1−x Six heterostructures in the quantum Hall effect mode

The temperature (0.1 K?T?20 K) and magnetic field (0 T?B?12 T) dependences of the longitudinal (ρ_xx) and Hall (ρ_xy) resistivities have been studied in detail for p-Ge/Ge_1?x Si_x (x=0.07) multilayer heterostructures with hole density p=(2.4–2.6)×10¹¹ cm^?2 and mobility μ=(1.1–1.7)×10⁴ cm² V^?1 s^?1. The energy spectrum parameters of two-dimensional (2D) hole gas in the quantum Hall effect mode have been determined. The mobility gap W=(2–2.5) meV and the background density of localized states g _c =(5–7)×10¹⁰ cm^?2 meV^?1 for the filling factors ν=1 and 2. The results are discussed in terms of long-range impurity potential models for selectively doped 2D systems.     

Specific Contact Resistance Measurement of Screen-Printed Ag Metal Contacts Formed on Heavily Doped Emitter Region in Multicrystalline Silicon Solar Cells

Multicrystalline silicon (mc-Si) wafers are widely used to develop low-cost high-efficiency screen-printed solar cells. In this study, the electrical properties of screen-printed Ag metal contacts formed on heavily doped emitter region in mc-Si solar cells have been investigated. Sintering of the screen-printed metal contacts was performed by a co-firing step at 725°C in air ambient followed by low-temperature annealing at 450°C for 15 min. Measurement of the specific contact resistance (ρ _c) of the Ag contacts was performed by the three-point probe method, showing a best value of ρ _c = 1.02 × 10^?4 Ω cm² obtained for the Ag contacts. This value is considered as a good figure of merit for screen-printed Ag electrodes formed on a doped mc-Si surface. The plot of ρ _c versus the inverse of the square root of the surface doping level (N _s ^?½ ) follows a linear relationship for impurity doping levels N _s ≥ 10¹⁹ atoms/cm³. The power losses due to current traveling through various resistive components of finished solar cells were calculated by using standard expressions. Cross-sectional scanning electron microscopy (SEM) views of the Ag metal and doped mc-Si region show that the Ag metal is firmly coalesced with the doped mc-Si surface upon sintering at an optimum firing temperature of 725°C.     

MeV energy sulfur implantation in GaAs and InP

Room temperature and elevated temperature sulfur implants were performed into semi-insulating GaAs and InP at variable energies and fluences. The implantations were performed in the energy range 1–16 MeV. Range statistics of sulfur in InP and GaAs were calculated from the secondary ion mass spectrometry atomic concentration depth profiles and were compared with TRIM92 values. Slight in-diffusion of sulfur was observed in both InP and GaAs at higher annealing temperatures for room temperature implants. Little or no redistribution of sulfur was observed for elevated temperature implants. Elevated temperature implants showed higher activations and higher mobilities compared to room temperature implants in both GaAs and InP after annealing. Higher peak electron concentrations were observed in sulfur-implanted InP (n ≈ 1 × 10¹⁹ cm⁻³) compared to GaAs (n ≈ 2 × 10¹⁸ cm⁻³). The doping profile for a buried n⁺ layer (n ≈ 3.5 × 10¹⁸ cm⁻³) of a positive-intrinsic-negative diode in GaAs was produced by using Si/S coimplantation.     

Air-stable polythiophene-based thin film transistors processed using oxidative chemical vapor deposition: Carrier transport and channel/metallization contact interface

Oxidative chemical-vapor-deposition (oCVD) provides a facile route to polymerize and deposit insoluble monomers in thin film form. Here, we report on oCVD polythiophene (PT)-based organic thin film transistors (OTFTs) that present both high mobility and excellent stability over time in air. The measured field effect mobility (μ_FE) is ∼0.02 cm²/V sec with the low threshold voltage between −1 V and 0.3 V. Additionally the PT OTFTs show no evidence of performance degradation after 3 months exposure in air. The transmission line model (TLM) enables the determination of the specific contact resistance (ρ_C) of oCVD PT channel/metallization interface and reveals that ρ_C is improved with increasing gate bias. The oCVD PT channel conductivity (σ_ch) and carrier density (p) were evaluated from more than 100 devices using TLM measurements and the relation of σ_ch = qpμ_FE. Carrier transport analysis suggests that the charge screening effect governs hole carrier mobility in the carrier density regime below approximately 10¹⁸/cm³ where an increase in carrier density leads to higher mobility. We also demonstrate photo-conductivity of oCVD PT through an increase in device on-state current and the field effect mobility when the PT OTFT is illuminated. Strategies to further enhance the performance of the materials and devices are also suggested.     

Low-Temperature Midinfrared Absorption in GaSe

We report the measurement of the temperature dependence of the absorption spectra of ε- GaSe over the temperature range 300 K to 5 K. Measurements have been made for both the e-ray (polarized parallel to the crystal’s c-axis) and the o-ray (polarized perpendicular to the c-axis), over the spectral range 4000 to 10 cm^?1. Nine absorption lines at 417, 440, 499, 546, 891, 945, 1015, 1093, 1270 cm^?1 were recorded at 300 K for the e- ray spectra. Some of these lines were identified using the results of a modified single layer, linear chain model of GaSe. The lines at 417, 440 and 499 cm^?1 were assigned to local impurity absorption originating from N, Mg and O, respectively. The weak lines at 945, 1015 and 1093 cm^?1 were assigned to hole transitions from the acceptor levels to the top of the valence band. Two absorption lines at 891 cm^?1 and 1270 cm^?1 were assigned to hole transitions from the quasi-local acceptor levels to the double degenerate valence sub-bands Γ₅ or Γ₆. The origin of lines recorded in the far IR absorption spectra at 20, 37 cm^?1 and 362 cm^?1 were also identified.     

Metallic conductivity over an acceptor band of lightly compensated copper-doped p-Hg0.78Cd0.22Te crystals

The conductivity and Hall effect of heavily doped p-Hg_0.78Cd_0.22Te:Cu crystals were studied in the temperature range of 4.2–125 K. The conductivity over the impurity band is of a metallic type for the acceptor concentration N _A>3.8×10¹⁷ cm^?3. The conductivity and the Hall coefficient governed by the delocalized charge carriers in the impurity band are independent of temperature. The sign of the Hall effect is positive in the metallic conductivity range. Near the metal-insulator transition point, the Hall mobility increases linearly with the acceptor concentration and is independent of the acceptor concentration at N _A>1.6×10¹⁸ cm^?3. The metallic conductivity is proportional to N _A in the concentration range under study at N _A<3.1×10¹⁸ cm^?3. The Anderson transition occurs at the Cu concentration N _A=1.4×10¹⁷ cm^?3 in the A ⁺ impurity band, which is formed by positively charged acceptors. Minimum metallic conductivity corresponding to this transition equals 5.1 Ω^?1 cm^?1. It is shown that ?₂ conductivity in the subthreshold region is defined by delocalized carriers in the upper Hubbard band only for fairly heavy doping (N _A>1.4×10¹⁷ cm^?3). For N _A<1.4×10¹⁷ cm^?3, the hopping conductivity is observed.     

A comparative study of selective carbon doping in MOCVD GaAs using trimethylarsenic and arsine

Carbon incorporation in GaAs epitaxial layers grown by low pressure metalorganic chemical vapor deposition (MOCVD), using trimethylgallium (TMGa) as the gallium source and trimethylarsenic (TMAs) and AsH₃ as the arsenic sources, has been studied over a wide range of growth parameters. Carbon incorporation is identified by secondary ion mass spectroscopy (SIMS), Hall measurement, and C-V analysis. Active carbon levels between 2 × 10¹⁵ cm^-3 and 7 × 10²⁰ cm^-3 are obtained. The carbon incorporation is more sensitive to the partial pressure of TMGa than of TMAs in the growth temperature range 500 ~ 610° C. Carbon incorporation is increased as growth temperatures are decreased to 500° C for growth pressures near 10000 Pa. Results indicate that surface-adsorbed methyl radicals from the dissociation of TMGa controls carbon incorporation in this temperature range.     

Liquid encapsulated czochralski pulling of InP crystals

The growth of bulk indium phosphide crystals via liquid encapsulated Czochralski pulling from both stoichiometric and nonstoichiometric melts is described. Nominally un-doped crystals with carrier concentration N_D-N_A = 6 × 10¹⁵ cm⁻³ and Hall mobilities of 4510 cm²/Vsec at room temperature were grown. Also, we prepared Zn-or Cd-doped p-type crystals in the range 10¹⁶ ≤ N_A-N_D ≤ 10¹⁸ cm⁻³ with Hall mobilities ≤ 130 cm²/Vsec and Sn-doped n-type crystals in the range 4 × 10¹⁷ ≤ N_A-N_D ≤ 10¹⁸ cm^-3 with Hall mobilities ≤ 2400 cm²/Vsec. The dislocation density of LEC pulled InP crystals is typically ~ 10⁴ cm⁻².     

Scaling the Stiffness,Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Natural materials such as bone and tooth achieve precisely tuned mechanical and interfacial properties by varying the concentration and orientation of their nanoscale constituents. However, the realization of such control in engineered foams is limited by manufacturing‐driven tradeoffs among the size, order, and dispersion uniformity of the building blocks. It is demonstrated how to manufacture nanocomposite foams with precisely controllable mechanical properties via aligned carbon nanotube (CNT) growth followed by atomic layer deposition (ALD). By starting with a low density CNT forest and varying the ALD coating thickness, we realize predictable ≈1000‐fold control of Young's modulus (14 MPa to 20 GPa, where E ～ ρ ^2.8), ultimate compressive strength (0.8 MPa to 0.16 GPa), and energy absorption (0.4 to 400 J cm^–3). Owing to the continuous, long CNTs within the ceramic nanocomposite, the compressive strength and toughness of the new material are 10‐fold greater than commercially available aluminum foam over the same density range. Moreover, the compressive stiffness and strength equal that of compact bone at 10% lower density. Along with emerging technologies for scalable patterning and roll‐to‐roll manufacturing and lamination of CNT films, coated CNT foams may be especially suited to multifunctional applications such as catalysis, filtration, and thermal protection.     

A detailed calculation of the auger lifetime in p-type HgCdTe

There has been recent experimental evidence that showed, in heavily doped p-type HgCdTe, the lifetime may be limited by the Auger 7 recombination mechanism. We have performed a detailed calculation of both the Auger 7 and Auger 1 lifetimes as a function of Cd composition (x), temperature (T), and doping (N_A). Compared with those done 20 years ago, the depth and breadth of these calculations result in a significant increase in the accuracy of the predictions. We present here the Auguer 7 lifetime for two different compositions, x=0.305 and x=0.226 over a range of temperature extending from 60 K to 300 K and for acceptor doping from 10¹⁵ cm⁻³ to 10¹⁸ cm⁻³. The calculated results for MWIR (x=0.305) are in reasonably good agreement with recent experiments performed on MWIR HgCdTe at 77 K over a range of doping. In addition, we calculated γ (≡ τ_A7 /τ_A1) with the same doping, composition (x ≈ 0.22), for a range of temperatures (40–80 K) and found γ=3–6.     

Correlation of photoluminescence linewidths with carrier concentration in p-Ga0.52In0.48P

We find a statistically significant correlation between carrier concentration and the Lorentzian linewidth factor determined from a Voigt lineshape fit to roomtemperature photoluminscence (PL) measurements for partially ordered p-Ga_0.52In_0.48P epitaxially deposited by organometallic vapor-phase deposition on GaAs. The correlation is independent of the amount of ordering present in the material. For carrier concentrations over the range of ~10¹⁶ to 10₁₉ cm₋₃, PL provides rapid and nondestructive evaluation, with increasing accuracy above carrier concentration levels of 5 × 10¹⁷ cm₃.     

The role of spreading resistance profiling in manufacturing control and technology development

Spreading resistance profiling (SRP) has the unique ability to measure doping profiles through multiple junctions, to essentially unlimited depths and over a range of densities from 10¹⁰ to 10²¹ /cm³. This range and flexibility makes the technique ideally suited to find various device failure mechanisms that can arise during dopant processing, as well as aid New Technology Development (NTD).We show through case studies how SRP can reveal various failure mechanisms and illustrate its use in the development of new MOS technologies