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.     

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.     

Cu3Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10^-7 Ω cm² is obtained on n-type GaAs with doping concentrations of～ 1 x 10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ε₁-Cu₃Ge (average lattice parameters: a₀= 5.30Å, b₀= 4.20Å, c₀= 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of theξ and ε-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.     

Molecular beam epitaxial growth of P-ZnSe:N using a novel plasma source

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (N_A—N_D) of around 1 × 10¹⁸ cm⁻³ was reproducibly achieved. The activation ratio ((N_A—N_D)/[N]) of p-ZnSe:N with N_A—N_D of 1.2 × 10¹⁸ cm⁻³ was found to be as high as 60%, which is the highest value so far obtained for N_A—N_D ∼ 10¹⁸ cm⁻³. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high N_A—N_D. On leave from Sumitomo Electric Industry Ltd. On leave from Sony Corp.     

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.     

Electrical properties of be-implanted GaA1-xPx

Hall effect and resistivity measurements on Be implanted GaAs_1-xP_x(x～0.38) indicate that essentially 100% doping efficiency may be obtained for normal Be concentrations after a 900°C anneal using either SiO₂ or Si₃N₄ as an encapsulant. The temperature dependence of hole mobility in these samples exhibits impurity banding effects similar to those reported in heavily Zn doped GaAs. Hall effect measurements in conjunction with successive thin layer removal techniques indicate there is no significant diffusion of the implanted Be during anneal for a fluence of 6×10¹³ ions/cm².     

Electrochemical deposition and characterisation of CdTe thin films from an ethylene-glycol-based bath

CdTe films were deposited on Ni and conducting glass (SnO₂) substrates from an ethylene-glycol-based bath by galvanostatic and potentiostatic methods. The film composition and electrical properties depend on parameters such as working electrode potential current density, deposition temperature, substrate type and post-deposition treatments. It is possible to improve the grain size and stoichiometry of the film by post-deposition heat treatment in air. The conductivity type was determined from the photocurrent-working electrode potential behaviour of the film. Dark capacitance measurements in a 0.5 M H₂SO₄ solution at 10 kHz showed a linear behaviour, from which the flatband potential V_fb= −0.365 V vs. a saturated calomel electrode (SCE) and the doping density N_D = 1.35 × 10¹⁸ cm⁻³ were determined. © 1997 John Wiley & Sons, Ltd.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.     

Growth and characterization of GaSb bulk crystals with low acceptor concentration

GaSb bulk single crystals with low acceptor concentration were grown from a bismuth solution by the traveling heater method. The result is isoelectronic doping by Bi which gives a variation of the opto-electronic properties as a function of grown length as well as a pronounced microscopic segregation. Photoluminescence spectra at 4K show a decrease of the natural acceptor during growth, which is confirmed by Hall measurements. The electrical properties of this isoelectronic doped GaSb are hole concentrations and mobilities of N_A − N_D = 1.7 × 10¹⁶ cm⁻³ and μ = 870 cm²Vs at room temperature and N_A-N_D = 1 × 10¹⁶ cm⁻³ and μ = 4900 cm²/Vs at 77K, respectively. The lowest p-type carrier concentration measured at 300K is N_A − N_D = 3.3 × 10¹⁵ cm⁻³     

Characteristics of GZO thin films deposited by sol–gel dip coating

Zinc oxide (ZnO) films were deposited by sol–gel dip coating using the acrylamide route. The films were doped with different concentrations of gallium in the range 250–1200 ppm. The films exhibited hexagonal structure. The grain size decreased from 100 to 10 nm as the dopant concentration increased. The resistivity of the doped samples decreased from 10³ to 3×10⁻³ Ω cm. The band gap value shifted towards the short-wavelength region as the dopant concentration increased. XPS studies indicated doping of Ga in ZnO     

Electrical phenomena in a metal/nanooxide/p +-silicon structure during its transformation to a resonant-tunneling diode

To investigate and develop novel silicon-based electronic components, the electro-physical effects in a metal-insulator-semiconductor (MIS) structure with nanometer size parameters, gained by enhancement of the silicon doping level up to N _A ～ 10¹⁹ cm^?3 and reduction of the oxide thickness down to 0.4–4.0 nm, have been studied. As a result of such changes, the MIS nanostructure satisfies necessary and sufficient conditions for the electron resonant tunneling that can be observed at relatively low (some volts) reverse biases. Thereby a MIS capacitor can be transformed into a resonant-tunneling diode with substantial extension of its properties and functions.     

Effect of thermal treatment on the properties of YBa2Cu3O7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes

Detailed studies were conducted on YBa₂Cu₃O_7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes. Ytterbium not only provided an excellent ohmic contact to YBa₂Cu₃O_7-x thin films, but also improved the zero resistance temperature after thermal treatment of the contacts. Superconducting thin films with zero resistance temperature of around 90 K with extremely sharp resistivity transitions, contact current density greater than 1.3 x 10³A/cm², and specific contact resistivity as low as 10^-9 ohm · cm² at 77 K were achieved on yttrium stabilized ZrO₂ substrates. This occurred after contact deposition and thermal treatment at 450° C in pure oxygen for 20 min.     

Low resistivity as-deposited ohmic contacts to 3C-SiC

The contact resistivities of Al and Ti ohmic contacts to n-type 3C-SiC were measured using the circular TLM method. The surface doping concentration under the contact was increased by ion-implantation of nitrogen into SiC. The contact resistivity was observed to decrease with increasing surface doping concentration for both Al and Ti contacts. The minimum value for the contact resistivities for Aland Ti contacts was 1.4x 10^-5and 1.5 x 10^-5 ω cm², respectively, at the surface doping concentration of 3 x 10²⁰ cm^-3 without any annealing of the contacts. These values are an order of magnitude lower than previously reported minimum values for as-deposited ohmic contacts on n-type 3C-SiC.     

Nanoscale FeS2 (Pyrite) as a Sustainable Thermoelectric Material

We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS₂ materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS₂ samples showed higher electrical conductivity, from 0.02 S cm^?1 for particles with 70 nm grain size up to 3.1 S cm^?1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K^?1 at RT for the sample with 70-nm grains down to 101 μV K^?1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m^?1 K^?1 at RT to a minimal value of 1.2 W m^?1 K^?1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m^?1 K^?1) but improved the electrical conductivity (41 S cm^?1) and Seebeck coefficient (-129 μV K^?1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m^?1 K^?1) as well as to an improved electrical conductivity (26 S cm^?1) and Seebeck coefficient (110 μV K^?1). The ZT value of FeS₂ was increased by a factor of five by Co doping and by a factor of three by Se doping.     

Effect of doping concentration on the grain boundary trap density and threshold voltage of polycrystalline SOI MOSFETs

PolySOI MOSFETs have been fabricated on undoped and doped polycrystalline silicon films and characterized to study the effect of doping on grain boundary passivation. The grain boundary trap density (N_ST) and threshold voltages have been extracted experimentally to evaluate the extent of grain boundary passivation by the dopants. Charge sheet model based on the effective doping concentration has been employed to analytically estimate the threshold voltages using the experimentally determined grain boundary trap density and grain size (L_g) as model parameters. The variation of threshold voltages with increasing doping concentration for the range of N_A ? (N_ST/L_g) has been studied both by simulation and experiments and the results are presented. Analytically estimated threshold voltages and experimental results show that the threshold voltage falls with increase in the dopant concentration and that this effect is indeed due to the reduction in N_ST as a result of the grain boundary passivation by the dopants.     

Electrical properties of substitutionally doped cvd amorphous silicon

Electrical properties of phosphorus doped amorphous Si (a-Si) prepared by chemical vapor deposition (CVD) have been investigated. The gaseous impurity ratio R = Np_PH3/ N_SiH4, was varied from 2×10^-6 to 1.1×10^-2. When the ratio R exceeds R_C =4.2 ×10^-4, the room temperature conductivity is dominated by the conduction in the extended states and rapidly increases with R up to 10^-1 (Ω-cm)^-1 at the ratio R = 1.1×10^-1. It is found that phosphorus doping below R_C results in the compensation of native defects or dangling bonds and that an efficient shift of the Fermi level as well as a narrowing of the tailing width of the extended states take place by doping above R_C. The magnetoresistance is explained as a modification of the spin-flip relaxation time of localized electrons by external magnetic field.     

Study on the structural, electrical and optical properties of Al-F co-doped ZnO thin films prepared by RF magnetron sputtering

Al and F co-doped ZnO(ZnO:(Al,F)) thin films on glass substrates are prepared by the RF magnetron sputtering with different F doping contents.The structural,electrical and optical properties of the deposited films are sensitive to the F doping content.The X-ray analysis shows that the films are c-axis orientated along the(002) plane with the grain size ranging from 9 nm to 13 nm.Micrographs obtained by the scanning electron microscope(SEM) show a uniform surface.The best films obtained have a resistivity of 2.16×10-3Ω·cm,while the high optical transmission is 92.0% at the F content of 2.46 wt.%.     

Atomic layer deposition of titanium nitride from TDMAT precursor

TiN was grown by atomic layer deposition (ALD) from tetrakis(dimethylamino)titanium (TDMAT). Both thermal and plasma enhanced processes were studied, with N₂ and NH₃ as reactive gases. Using an optimized thermal ammonia based process, a growth rate of 0.06 nm/cycle and a resistivity of 53 × 10³ μΩ cm were achieved. With an optimized plasma enhanced NH₃ process, a growth rate of 0.08 nm/cycle and a resistivity of 180 μΩ cm could be obtained. X-ray photo electron spectroscopy (XPS) showed that the difference in resistivity correlates with the purity of the deposited films. The high resistivity of thermal ALD films is caused by oxygen (37%) and carbon (9%) contamination. For the film deposited with optimized plasma conditions, impurity levels below 6% could be achieved. The copper diffusion barrier properties of the TiN films were determined by in-situ X-ray diffraction (XRD) and were found to be as good as or better than those of films deposited with physical vapor deposition (PVD).     

On the behavior of Bi in a CdTe lattice and the compensation effect in CdTe:Bi

CdTe crystals of two types have been grown by the vertical Bridgman method: (i) crystals doped with Bi to ～10¹⁸ cm^?3 and (ii) double-doped (Bi + Cl) crystals with a Bi concentration of ～10¹⁸ cm^?3 and a Cl concentration of ～10¹⁷ cm^?3. The temperature dependences of the resistivity, photoconductivity, and low-temperature photoluminescence are investigated for the crystals grown. Analysis has shown that doping with Bi (crystals of the first type) leads to compensation of the material. The resistivity of the CdTe:Bi samples at room temperature, depending on the doping level, is varied in the range of 10⁵–10⁹ Ω cm. The hole concentration is determined by the acceptor level at E _v + 0.4 eV in lightly doped CdTe:Bi samples and by the deep center at E _v + 0.72 eV in heavily doped CdTe:Bi samples. Double doping leads to inversion of the conductivity type and reduces the resistivity to ～1 Ω cm. Heavily doped CdTe:Bi crystals and double-doped crystals exhibit the presence of acceptors with an ionization energy of 36 meV, which is atypical of CdTe.     

Microstructural and electrical property evolution in an acceptor-dopant free positive temperature coefficient thermistor

The effect of varying sintering temperature in the range 1270–1430 °C on the resistivity–temperature characteristics of semiconducting BaTiO₃ based positive temperature coefficient of resistance thermistors containing a donor-dopant, but without acceptor doping, was investigated by impedance spectroscopy. As the sintering temperature was increased the specimen resistivity around the Curie temperature decreased, while the peak resistivity, obtained above the Curie temperature, remained approximately constant. The change in PTC behaviour with increasing sintering temperature is inconsistent with the standard double Schottky barrier model, but is explained in terms of grain size variations coupled with a, sintering temperature independent, grain boundary barrier layer thickness of 0.50±0.04 μm.