Electron microscope studies of InxGav1−xAs/GaAs/Si grown by metalorganic chemical vapor deposition

The microstructure of In_xGa_1−xAs/GaAs (5 nm/5 nm, x < 0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450‡C, and subsequently annealed at 750‡C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.     

Electron-beam doping-electron radiation effects in impurity overlayers and semiconductor substrates

3-9 MeV electrons were used to introduce impurity Ge atoms into Si wafers from Ge sheets, which are in contact with a Si surface at 20-60‡C in water bath. Concentration-dependent diffusivities of ∼10-18-10^-14 cm²sec^-1 for Ge in Si were measured. Activation energies of sputtering yield for Ge and of the diffusivity of Ge in Si are estimated to be ∼0.3 eV and ∼0.58 eV, respectively. In a case of hot (∼250‡C) irradiation in ∼1x10^-3 Torr vacuum, also the similar concentration profiles of impurity atoms in the substrates were observed.     

MOCVD epitaxial growth of single crystal GaN,AlN and AlxGa1-xN

Ga begins to deposit from a stream of trimethylgallium (TMG) in H₂ at a minimum temperature of 475‡C. Addition of sufficient amounts of NH₂ results in the growth of textured polycrystalline GaN on basal plane sapphire substrates above 500‡C. A minimum temperature of 800‡C is required for the epitaxial growth of GaN on the substrate. Under similar conditions, but with the TMG replaced with trimethylaluminum (TMA), polycrystalline A1N begins forming at 400‡C (in the absence of NH₃,, the TMA starts pyrolyzing at 300‡C), but single crystal growth of A1N requires a temperature of at least 1200‡C. Epitaxial single crystal layers of Al_x Ga_1-x N can be grown in the temperature range 800−1200‡C, tne minimum temperature being approximately proportional to x, but preferential deposition of A1N on the hot walls of the reactor (>400‡C) precludes precise control of the alloy composition. This predeposition of A1N can be retarded by keeping the walls below 400‡C by using a water-cooled jacket, by rapid flow-rates, or by injecting the TMA through a nozzle close to the surface of the substrate.     

Process parameters,orientation, and functional properties of melt-processed bulk Y-Ba-Cu-O superconductors

This study compared the microstructure, texturing, and functional properties (critical currents) of YBa₂Cu₃O_7-x-based bulk pellets that were prepared by the quench-melt-growth-process (QMGP), melt-textured growth, and conventional solid-state reaction approaches. Using two x-ray diffraction methods, θ-2θ, and rocking curves, we found that the individual grains of two melt-processed pellets exhibited remarkable preferred orientational alignment (best rocking curve width < 3.2‡). However, the direction of the preferred orientation among the grains was random. Among the three types of bulk materials studied, the QMGP sample was found to have the best J_c values, ≈4500 A/cm² at 77K in a field of 2 kG, as determined from superconducting quantum interference device magnetic data.     

Liquidus isotherms,solidus lines and LPE growth in the Te-rich corner of the Hg-Cd-Te system

Liquidus isotherms for the Hg_1−xCd_xTe primary phase field in the Te-rich corner of the Hg-Cd-Te ternary system have been determined for temperatures from 425 to 600‡C by a modified direct observational technique. These isotherms were used to help establish conditions for the open-tube liquid phase epitaxial growth of Hg_1−xCd_xTe layers on CdTe_1−ySe_y substrates. Layers with x ranging from 0.1 to 0.8 have been grown from Te-rich HgCdTe solutions under flowing H₂ by means of a horizontal slider technique that prevents loss of Hg from the solutions by evaporation. Growth temperatures and times of 450–550‡C and 0.25–10 min, respectively, have been used. The growth solution equilibration time is typically 1 h at 550‡C. Source wafers, supercooled solutions, and (111)-oriented substrates were employed in growing the highest quality layers, which were between 3 and 15 Μm thick. Electron microprobe analysis was used to determine x for the epitaxial layers, and the resulting data, along with the liquidus isotherms, were used to obtain solidus lines. In addition to EMP data, optical transmission results are given. This work was sponsored by the Department of the Air Force and the U. S. Army Research Office.     

The effect of annealing temperature on the carrier concentration OF Hg0.6Cd0.4Te

The carrier concentration dependence of Hg_0.6Cd_0.4Te on annealing temperature for the Hg and Te saturation condi-tions is presented in this paper. At low annealing tempera-tures, T_A < 350‡ C, residual donor impurities apparently limit the carrier concentration. In contrast, at higher annealing temperatures, 350‡ C < T < 700‡ C, the stoichio-metric acceptor density is increased such that the residual donors are compensated and the material is converted to p-type with an acceptor density as large as 10¹⁷ cm⁻³ . An empirical expression describing this dependence of the acceptors on annealing temperature is given for both the Hg and Te saturation condition: P (Hg saturation) = 1.46 × 10²² exp (−0.84/kT_A), P (Te saturation) = 1.90 × 10¹⁸ exp (−0.15/kT_A). Supported in part by Air Force Materials Laboratory, WPAFB, Ohio under Contract AF61533-C-74-5041.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Recent status on high temperature superconducting Bi2Sr2CaCu2O8+x wire development at NYSIS: 1-90 meter length Jcs and 3 meter diameter ring furnace design

The status of long length, Bi₂Sr₂CaCu₂O_8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport J_cs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000–40,000 A/cm² for 1, 4–15, and 40–90 meter length tapes, respectively. The decrease in J_c as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J_c to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13^w × 10^h × 1000¹ cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.     

Electrical characterization of very-narrow-gap bulk HgCdTe single crystals by variable magnetic field hall measurements

Variable-magnetic-field Hall measurements (0 to 1.5 T) are performed on very-narrow-gap bulk-grown Hg_1−xCd_xTe single crystals (0.165 ≤ x ≤ 0.2) at various temperatures (10 to 300K). The electron densities and mobilities are obtained within the one-carrier (electrons) approximation of the reduced-con-ductivity-tensor scheme. The present data together with the selected data set reported by other workers exhibit a pronounced peak when the electron mobility is plotted against the alloy composition x-value which has been predicted to be due to the effective-mass minimum at the bandgap-crossing (E_g ≈ 0). The observed position (x ≈ 0.165), height (≈4 x 10² m²Vs), and width (≈0.01 in x) of the mobility-peak can be explained by a simple simulation involving only ionized-impurity scattering. A lower bound of the effective mass is introduced as a fitting parameter to be consistent with the finiteness of the observed electron mobility and is found to be of the order of 10⁻⁴ of the mass of a free electron.     

Structures and physical properties of sputtered amorphous SiC films

Optical and electrical properties have been measured for amorphous SiC films prepared by rf sputtering in a pure Ar atmosphere with a sintered 6H-SiC target. The absorption edge E₀ determined from the relation of αhΝ = B(hΝ-E₀)² ranged from 1.45 to 1.80 eV depending on the film thickness and the substrate temperature. The room temperature electrical conductivity is in the range of 5.4×10⁻¹¹ and 1.4×10⁻⁵ Ω⁻¹cm⁻¹. The absorption edge decreases and the conductivity increases with increasing film thickness. The absorption edge shifts to shorter wavelengths (blue shift) and the conductivity decreases during annealing below 400‡C for 60 min, whereas the absorption edge shifts to the longer wavelength side (red shift) and the conductivity increases during annealing at 800‡C It is proposed that the two annealing processes cause structural changes in amorphous SiC films, one of which involves removal of defects or voids while the other involves rearrangement or rebonding of the component atoms.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Annealing studies of Be-implanted GaAs0.6P0.4

Differential resistivity and Hall effect measurements and secondary ion mass spectrometry (SIMS) are used to study the annealing behavior of Be-implanted GaAs_0.6P_0.4. Results similar to that previously reported for Be-implanted GaAs are observed, including outdiffusion of Be into the Si₃N₄ encapsulant during 900‡C annealing of high dose implants. Nearly all (85–100%) of the Be remaining after a 900‡C, 1/2 hr anneal is electrically active. However, the electrical activation at low annealing temperatures (600–700‡C) is much lower in GaAs_0.6P_0.4 than in GaAs. A substantial amount of diffusion is observed even for the low fluence Be implants in GaAs_0.6P_0.4 annealed at 900‡C, indicating a greater dependence of the diffusion on defect-related effects in the ternary. This work was supported by the Joint Services Electronics Program (U.S. Army, U.S. Navy, U.S. Air Force) under Contract DAAB-07-72-C-0259, by Monsanto Company, and by the Naval Electronic Systems Command. On leave at Cornell University, Department of Electrical Engineering, Ithaca, NY 14853.     

Current controlled liquid phase epitaxial growth of Hgl−xCdxTe

Using the technique of current controlled liquid phase epitaxy we have grown crystalline layers of Hg_l-xCd_xTe from a Hg-melt. Best results were obtained on (111) oriented substrates. The dominant mechanism of solute transport is electromigration rather than the Peltier effect. Composition (x ≈0.1) and homogeneity across a sample surface (Δ x ≈ 0.03) was determined by the electrolyte electroreflectance method. Work supported by AFOSR Grant 74-2714B     

Understanding the phase relations and cation disorder in LRE1+xBa2− xCu3O7+δ

Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE_1+xBa_2-xCu₃O_7+δ (LRE123ss), with increasing substitution of the LRE³⁺ for the Ba²⁺ with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO₂). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 > × > 0.1, there is an increase in T_c when the high temperature annealing (T ∼ 940‡C) is performed in low PO₂ (> 0.1 bar). The maximum T_c occurs at a doping level of ∼x < 0.05 for Nd and Gd. When annealing is performed in 1 bar PO₂, there is a gradual decrease in T_c with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.     

Solution growth of Indium-Doped silicon

Indium-doped silicon has been grown from indium-rich solutions using a gradient-transport solution growth process. The growth temperatures were varied from 950‡ to 1300‡C to determine the solubility limits of indium in silicon. The maximum indium concentration obtained was 1.6×10¹⁸/cm³ at a growth temperature of 1300‡ but indications are that the maximum solubility is 2. 5×10¹⁸/cm³. The growth process is described by one-dimensional diffusion limited transport and predicts growth rates in excess of lcm/day at 1300‡C. Infrared absorption measurements were used to monitor the indium, oxygen and carbon concentrations, in addition to the shallower .indium: X defect found in the crystals. The solution-grown crystals were found to have a lower concentration of this shallower defect than melt grown crystals of the same indium concentration. The oxygen and carbon concentrations increased with the increased growth temperatures suggesting a solubility limited value. The shallower indium: X defect also increased with growth temperature, but the concentration was significantly lower than typically found in melt-grown crystals. The peak optical cross-section for indium was also determined to be 5.3×10^-17cm² from these measurements. This work was sponsored in part by the Defense Advanced Research Projects Agency under Order No. 3211, monitored by NV & EOL under Contract No. DAAK70-77-C-0194.     

Sintering behavior of a reactively bonded thick film gold ink

The sintering behavior of thick film reactively bonded Au films was studied using electrical re-sistance measurement and scanning electron microscopy. Isothermal firings were conducted at 350‡ to 850‡C for times up to three hours. The sintering process consists of three portions: 1) Burnoff of the binding resin at 350‡ to 450‡C. 2) A second stage involving neck growth between the gold particles with an activation energy of ∼ 15 Kcal/mole and a time exponent of 5.5 to 6.5 corresponding to surface diffusion control and 3) a third stage of densificat ion by pore annihila-tion and grain growth. The activation energy in stage III of 35–45 Kcal/mole agrees closely with bulk diffusion control. Electrical resistance change appears to be a useful method for studying particularly the initial stages of sintering.     

Specific contact resistivity of indium contacts to n-type CdTe

Indium alloyed to n-type CdTe of about 10¹⁶ cm^-3 electron concentration provides a contact resistivity of about 7 x 10^-3 ohm cm². This is achieved by alloying for 10 minutes at 150-450‡C in a sealed ampoule with an overpressure of cadmium. If the alloying is done in an open tube H₂ flow without a Cd vapor overpressure, alloying temperatures above 250‡C cause the contact resistance to rise as cadmium vacancies increase the compensation in the CdTe. Further improvement of the contact resistivity to 1 x 10^-3 ohm cm is obtained by a 900‡C diffusion of In into the n-CdTe (electron concentration 10¹⁶ cm^-3 before the diffusion).     

Minority carrier lifetime in indium-doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x ≈ 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as ≈2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of τ_n0 and τ_p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.     

Dependence of electrical properties of polycrystalline cvd si films on grain size and impurity doping concentrationa,b

The electrical properties of sets of simultaneously grown p-type polycrystalline Si films, deposited by SiH₄ pyrolysis on polycrystalline high-purity alumina substrates and B-doped during growth, were determined by Hall-effect measurements in the temperature range 77-420K as functions both of impurity doping concentration N (~10^l5 to ~10²⁰cm⁻³) and average grain size (≈1 to ≈125μm) in the film. Room temperature data showed rapidly increasing resistivities and rapidly decreasing free-carrier concentrations for doping below a critical concentration N_m and distinct mobility minima at that concentration, with the value of N_m being larger the smaller the average grain size. Measurements as a function of sample temperature showed the intergrain barrier height E_b, decreasing from a maximum value of ~0.4eV at the critical concentration to very small values (~0.01eV) for concentrations above 10¹⁹cm⁻³, with a functional dependence close to E_b ∝l/N^1/2 and E_b for any given concentration being larger the smaller the average grain size. Results are interpreted in terms of the grain-boundary trapping model. Trapped carrier densities in the grain boundaries were calculated to range from ~5×l0¹¹cm⁻² at N≈N_m to ~5×l0¹²cm⁻² for N>10¹⁹cm⁻³, the density being higher the smaller the grain size, and evidence was found for an energy distribution of traps in the Si bandgap, rather than a fixed density at a single discrete energy level. The observed relationship between N_m and average grain size nearly coincides with that of the model for films with ~lμm grain size but sharply departs from it for larger grain sizes, indicating probable applicability of the model for grain sizes up to that range. ^aThis work was supported by the U.S. Department of Energythrough its San Francisco Operations Office under Contract DE-AC03-79ET23045 and monitored by the Solar Energy Research Institute, Golden, CO. bThese results were first described at the 22nd Electronic Materials Conference, Ithaca, NY, June 21–27, 1980, Paper No. M4.     

Stability of various doping species in trans-polyacetylene

Polyacetylene, a simple conjugated organic polymer, a material of strong interest if doped by various chemical species such as iodine or antimony pentafluoride : its electrical conductivity increases by more than twelve orders of magnitude. By increasing the dopant concentration one obtains a semiconductive or a metallic regime. Such properties allow us to make electronic material and then devices by convenient n or p type doping of polyacetylene. However,it is necessary to determine the thermal stability of the various dopant species. In this paper, we compared the stability of iodine, SbF₅ and CF₃SO₃H dopants by studying the mass loss and the electrical conductivity decrease at various temperatures from 20‡C up to 180‡C. It is shown that only SbF₅ is a relatively stable dopant up to 80‡C, a temperature which may be reached in some active electronic devices. Raman spectroscopy allowed us to measure the ratio of I₋₃ to I₋₅ ions during the desorption process and to propose a possible chemical reaction which produces gaseous iodine from the active dopant species.