A new computer-aided simulation model for polycrystalline silicon film resistors

A general transport theory for the I–V characteristics of a polycrystalline film resistor has been derived by including the effects of carrier degeneracy, majority-carrier thermionic-diffusion across the space charge regions produced by carrier trapping in the grain boundaries, and quantum mechanical tunneling through the grain boundaries. Based on the derived transport theory, a new conduction model for the electrical resistivity of polycrystalline film resitors has been developed by incorporating the effects of carrier trapping and dopant segregation in the grain boundaries. Moreover, an empirical formula for the coefficient of the dopant-segregation effects has been proposed, which enables us to predict the dependence of the electrical resistivity of phosphorus-and arsenic-doped polycrystalline silicon films on thermal annealing temperature.Phosphorus-doped polycrystalline silicon resistors have been fabricated by using ion-implantation with doses ranged from 1.6 × 10¹¹ to 5 × 10¹⁵/cm². The dependence of the electrical resistivity on doping concentration and temperature have been measured and shown to be in good agreement with the results of computer simulations. In addition, computer simulations for boron-and arsenic-doped polycrystalline silicon resistors have also been performed and shown to be consistent with the experimental results published by previous authors.     

Influence of laser recrystallization and hydrogen annealing on electrical characteristics of polysilicon

 Asenic ions are implanted with doses of 5×10~(11)—5×10~(15)/cm~2 into LPCVD polysilicon films on SiO_2 isolating substrate.The polysilicon films have been recrystallized with CW Ar~+ laser before implantation.Electrical measurements show that the resistivity is lowered and the mobility is increased significantly at low doping concentration(～10~(17)As~+/cm~3).Plasma hydrogen annealing is performed on laser-recrystallized samples.The electrical characteristics of plasma hydrogen annealed samples are close to that of single crystalline silicon.It is found that the resistivity decreases from 1.2 Ω.cm to 0.45 Ω.cm,the mobility rises from 62 cm~2/V.s to 271 cm~2/V.s,the electrical activation energy reduces from 0.03 eV to -0.007 eV and the trapping state density at the grain boundary drops from 3.7×10~(11)/cm~2 to 1.7×10~(11)/cm~2.Based on the existing theoretical models for conduction in polysilicon, a new formula for large grain polysilicon has been proposed,with the help of which,a good agreement between theory and experimental results is achieved within the doping concentration range from 10~(16)/cm~3 to 10~(20)/cm~3.     

A model for conduction in polycrystalline silicon—Part II: Comparison of theory and experiment

In the preceding paper [1], a new phenomenological model for the electrical conduction in polycrystalline silicon was developed. Electrical conduction in polycrystalline silicon was shown to be controlled by dopant segregation, carrier trapping, and carrier tunneling through the grain boundaries. In this paper, the theoretical model is compared to experiment. The electrical behavior of polycrystalline silicon is shown to be influenced by the properties of the grain boundaries. In arsenic and phosphorus-doped polycrystalline-silicon films the grain boundaries are best modeled by rectangular barriers with a height of 0.66 eV and an approximate width of 7 Å. The width of the grain-boundary barriers and the density of carrier trapping states are found to be weak functions of the dopant species and sample processing. The resistivity is found to be a strong function of dopant concentration, dopant species, and processing history at low and intermediate dopant concentrations, and the model can be used to predict this behavior.     

Electrical properties of uncontaminated PbTe films on mica substrates prepared by molecular beam deposition

Uncontaminated PbTe films were prepared by molecular beam deposition under clean conditions in a uhv environment and the film properties were measured in situ. The carrier concentration was found to be determined by source conditions and values between 10¹⁶ cm^?3 (intrinsic level) and n = 5 × 10¹⁸cm^?3 could be obtained in a controllable manner. A low temperature anneal enabled bulk value Hall mobilities (1750 cm² V^?1 sec^?1 at 300 K) to be obtained at room temperature and above which indicated that surface scattering in the films was predominantly specular. The mobility at low temperatures (down to 100 K) was limited by small potential barriers located at the double-positioned grain boundaries which were present in the film. Field effect measurements indicated the potential barriers arose from a continuous distribution of band gap states situated in the grain boundaries. These states had a fairly uniform density (? 10¹²cm^?2 (kT)^?1) but there was some increase towards the conduction band edge. They also limited the field effect mobility (μ_FE) to ?0.5 bulk value, giving μ_FE ? 800cm² (volt sec)^?1 for films with carr concentrations above 5 × 10¹⁷ cm^?3. By exposure to low pressures of oxygen the carrier concentrations in annealed n-type films could be reduced to near intrinsic values with no associated degradation in the electrical properties. This indicated that the films were not compensated with the native p-type defect.     

A model for conduction in polycrystalline silicon—Part I: Theory

A new phenomenological model for the electrical conduction in polycrystalline silicon is developed. The combined mechanisms of dopant segregation, carrier trapping, and carrier reflection at grain boundaries are proposed to explain the electrical conduction in polycrystalline silicon. The grain boundaries are assumed to behave as an intrinsic wide-band-gap semiconductor forming a heterojunction with the grains. Thermionic emission over the potential barriers created within the grains due to carrier trapping at the grain boundaries and then tunneling through the grain boundaries is proposed as the carrier transport mechanism. A generalized current-voltage relationship is developed which shows that the electrical properties of polycrystalline silicon depend on the properties of the grain boundaries.     

Electrical characterization of semiconducting diamond thin films and single crystals

Naturally occurring semiconducting single crystal (type IIb) diamonds and boron doped polycrystalline thin films were characterized by differential capacitance-voltage and Hall effect measurements, as well as secondary ion mass spectroscopy (SIMS). Results for natural diamonds indicated that the average compensation for a type IIb diamond was >17%. Mobilities for the natural crystals varied between 130 and 564 cm²/V·s at room temperature. The uncompensated dopant concentration obtained by C-V measurements (2.8 ± 0.1 × 10¹⁶ cm⁻³) was consistent with the atomic B concentration measured by SIMS performed on similar samples (3.0 ± 1.5 x 10¹⁶ cm⁻³). Measurement of barrier heights for three different metals (platinum, gold, and aluminum) found essentially the same value of 2.3 ± 0.1 eV in each case, indicating that the Fermi level was pinned at the diamond surface. Polycrystalline semiconducting diamond thin films demonstrated a complex carrier concentration behavior as a function of dopant density. This behavior may be understood in terms of a grain boundary model previously developed for polycrystalline silicon, or by considering a combination of compensation and impurity band conduction effects. The highest mobility measured for a polycrystalline sample was 10 cm²/V·s, indicating that electrical transport in the polycrystalline material was significantly degraded relative to the single crystal samples.     

Fluorine behavior in BF 2 + implanted polysilicon films subjected to rapid thermal annealing

The physical and electrical properties of BF ₂ ⁺ implanted polysilicon films subjected to rapid thermal annealing (RTA) are presented. It is found that the out diffusion ofF and its segregation at polysilicon/silicon oxide interface during RTA are the major causes ofF anomalous migration. Fluorine bubbles were observed in BF ₂ ⁺ implanted samples at doses of 1×10¹⁵ and 5×10¹⁵ cm⁻² after RTA.     

Selenium implantation in GaAs

The dependence of carrier concentration and mobility profiles on the dose of 400 keV Se ions implanted into Cr-doped semi-insulating GaAs, and on the annealing temperature has been studied for doses ranging from 3 × 10¹²/cm² to 2 × 10¹⁵/cm² and for annealing temperatures between 800 and 1000°C. Sputtered aluminum oxy-nitride and silicon nitride films were used as encapsulants for protection of the implanted surface during annealing treatments. The carrier profiles exhibited deep tails for implantations along both random and {110} planar directions. It was found that annealing temperatures of 900°C or above were necessary to obtain high carrier density and mobility values for implantation doses above 1 × 10¹⁴/cm². Samples encapsulated with aluminum oxy-nitride films exhibited 3 to 4 times higher carrier concentration values and also slightly higher mobility values than those encapsulated with silicon nitride films. The maximum carrier concentration obtained was about 4 × 10¹⁸/cm³ with aluminum oxy-nitride films as the encapsulant.     

The dopant density and temperature dependence of electron mobility and resistivity in n-type silicon

Traditional analysis of electron mobility in n-type silicon neglects the effect of electron-electron scattering in the mobility calculations. As a result, theory fails to conform with experiment when dopant density exceeds 2 × 10¹⁶ cm^?3. In this work, an improved theoretical model for computing mobility and resistivity as functions of dopant density and temperature has been developed for n-type silicon. The model has been applied to phosphorus-doped silicon for dopant densities from 10¹³ to 10¹⁹ cm^?3, and temperatures between 100 and 500 K. The mobility was calculated analytically by appropriately combining lattice, ionized impurity and neutral impurity scattering contributions. The effect of electron-electron scattering was incorporated empirically for dopant densities greater than 2 × 10¹⁶ cm^?3. Additionally, the anisotropic scattering effect was included in the mobility formulations. Resistivity measurements on seven phosphorus-doped silicon wafers with dopant densities from 1.2 × 10¹⁴ to 2.5 × 10¹⁸ cm^?3 were carried out for temperatures from 100 to 500 K. Electron mobility at 300 K was deduced from resistivity and junction C-V measurements for dopant densities from 10¹⁴ to 10¹⁸ cm^?3. Agreement between theoretical calculations and experimental data for both electron mobility and resistivity of phosphorus-doped silicon was within ±7% in the range of dopant densities and temperatures studied.     

Effects of Rapid Thermal Annealing on Electrical Transport in Heavily Doped ZnO Thin Films Deposited at Different Substrate Temperatures

In this work, heavily doped ZnO thin films with carrier concentrations of 1.7 × 10²⁰–1.1 × 10²¹ cm^?3 were prepared on glass substrates using direct current magnetron sputtering combined with rapid thermal annealing (RTA). The effects of RTA on the electrical transport properties of the thin films were investigated. Results showed that the resistivities of the thin films deposited at low temperatures were markedly improved due to the increased mobilities and/or carrier concentrations. Temperature-dependent Hall measurements and theoretical calculations suggested that the influence of grain boundary scattering was negligible for all the samples and the mobility was mainly determined by ionized impurity scattering. The influence of crystallographic defects on the mobility could be effectively reduced via RTA when the carrier concentration was above 4.0 × 10²⁰ cm^?3, resulting in a mobility and resistivity close to the ionized impurity scattering theoretical estimation. The highest mobility of 46 cm² V^?1 s^?1 at the resistivity of 2.8 × 10^?4 Ω cm and the lowest resistivity of 2.6 × 10^?4 Ω cm were achieved for the RTA-treated 1 wt.% Al-doped ZnO and 5 wt.% Ga-doped ZnO thin films, respectively.     

Electrical conduction in implanted polycrystalline silicon

Undoped polycrystalline silicon (poly-Si) films, 0.5 μm thick, have been prepared at 700‡C by chemical vapor deposition (CVD) onto thermally oxidized n⁺-Si substrates. The impurity concentration was varied by implanting with As, P, and Sb ions, accelerated to 30 keV; total doses ranged from 2×10¹¹ to 3×10¹⁵ ions/cm². Sheet resistance measurements, spanning 8 orders of magnitude, were made as a function of implantation dose. A reduction of 6 orders of magntiude in poly-Si sheet resistance took place within the implantation dose range between 10¹² and 10¹⁴ ions/cm². Some samples also exhibited large reductions in sheet resistance following the standard heat treatment for Al contact sintering and surface state reduction, which is normally at 450‡C for 0.5 hr in H₂. Sheet resistance measurements were also made as a function of temperature in the range 0 to 315‡C. The effective activation energy for electrical conduction depends upon implantation dose. At low doses (2×10¹¹cm₋₂) the poly-Si is intrinsic, withE _A = 0.65 eV. At a dose of 10¹⁵ cm⁻², electrical conduction is a weak function of temperature, withE _A = 0.027 eV.     

Dose-rate effects in silicon-implanted gallium arsenide from low to high doses

For implantation of silicon dopant into gallium arsenide, sheet resistance and damage increase as the ion dose rate increases in the high-dose regime (>5.0 × 10¹³ cm⁻²). But, in the low-dose regime (<5.0 × 10¹² cm⁻²), although damage still increases with dose rate, the sheet resistance decreases. This qualitative difference implies that there must be a crossover point between the low- and high-dose regimes in the effect of damage and defect formation on dopant activation. This paper describes experiments in which damage and silicon dose were independently varied through the crossover point. Thermal wave, ion channeling, Hall effect measurements, and transmission electron microscopy were used to characterize structural and electrical changes that occur near the crossover. In GaAs implanted with silicon (²⁹Si⁺) at doses between 3 × 10¹² and 6 × 10¹³cm⁻², it is shown that electrical activation for low dose rates first begins to exceed that for high dose rates at a dose of 2 × 10¹³ cm⁻². Rapid growth of Type I dislocations also begins near this same dose, suggesting that there may be a link between defect formation and the crossover to negative dose-rate effects in the high-dose regime.     

Effects of oxygen on crystallization of amorphous silicon films and polysilicon TFT characteristics

Oxygen ions were implanted into the amorphous silicon film deposited at 540°C in order to study the effects of oxygen on the solid phase crystallization of silicon films. The resulting films were investigated using transmission electron microscopy, x-ray diffraction (XRD), and also by measuring the electrical characteristics of polycrystalline silicon thin film transistors (TFTs) fabricated in the crystallized films. The development of {111} texture as a function of annealing time is similar to films implanted with Si, with higher oxygen samples showing more texture. Transmission electron microscopy shows that the grain size of completely crystallized films varies little with oxygen concentration. The electrical performances of TFTs are found to degrade with increasing oxygen dose. The trap state density increases from 5.6 × 10¹²/cm² to 9.5 × 10¹²/cm² with increasing oxygen dose. It is concluded that for a high performance TFT, oxygen incorporation in the Si film should be kept to 10¹⁹/cm³ or less.     

Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×10⁴ V cm^?1. The electron and hole drift mobilities μ_e≈2×10^?3 cm² V^?1 s^?1 and μ_h≈6×10^?4 cm² V^?1 s^?1 were obtained at T=292 K and F=4×10⁴ V cm^?1. An exponential temperature dependence of drift mobility with activation energy of ～0.38 and ～0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ～0.03 eV.     

Sulphur doped silicon IR detectors

Experimental data on the electrical transport properties and photoconductive detector performance of sulphur doped silicon as a function of temperature are presented. Analysis of the data shows that the detector performance is determined by a donor level at 0.19 eV from the conduction band edge with an electron capture cross section of 2 × 10^?13 cm² and a peak photoionization cross section of 1 × 10^?16 cm². Photoconductivity has been observed at 95 K which may be associated with a centre 0.37 eV below the conduction band.     

InP-on-CdS thin film heterostructures

Thin films of InP were deposited on single crystals and thin films of CdS by the planar reactive deposition technique. Good local epitaxy was observed on single crystals of CdS as well as InP and GaAs. The electrical evaluation of unintentionally doped films on semi-insulating InP substrates show them to be n-type with room temperature electron concentrations ranging from 5 × 1016 cm⁻³ to 5 × 10¹⁷ cm⁻³ and mobilities up to 1350 cm²/Vsec. For films intentionally doped with Mn and Be, p-type films were obtained. For Mn doping (deep acceptor level), room temperature mobilities as high as 140 cm²/Vsec and free carrier concentrations as low as 5 × 10¹⁶ cm⁻³ (with dopant level of 3 × 10¹⁸ cm⁻³) were obtained. For Bedoped films, free carrier concentrations of about 5 × 10¹⁸ cm⁻³ and mobilities of 20 cm²/Vsec were found. Scanning electron microscope and microprobe pictures show appreciable interdiffusion between the InP/CdS thin-film pair for InP deposited at 450°C. The loss of Cd from the CdS and the presence of an indium-cadmium-sulfur phase at the InP/CdS interface were observed. Interdiffusion is alleviated for InP deposition at lower temperatures. Supported in part by ERDA and AFOSR.     

Sb2Te3 and Bi2Te3 Thin Films Grown by Room-Temperature MBE

Sb₂Te₃ and Bi₂Te₃ thin films were grown on SiO₂ and BaF₂ substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2?nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2?h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500?nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb₂Te₃ films at room temperature, i.e., low charge carrier density (2.6?×?10¹⁹?cm^?3), large thermopower (130???V?K^?1), large charge carrier mobility (402?cm²?V^?1?s^?1), and resulting large power factor (29???W?cm^?1?K^?2). Bi₂Te₃ films also showed low charge carrier density (2.7?×?10¹⁹?cm^?3), moderate thermopower (?153???V?K^?1), but very low charge carrier mobility (80?cm²?V^?1?s^?1), yielding low power factor (8???W?cm^?1?K^?2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Structural,Optical and Electrical Properties of Polycrystalline CuO Thin Films Prepared by Magnetron Sputtering

Cupric oxide thin films were deposited on silicon and sapphire substrates by reactive radio frequency magnetron sputtering at different substrate temperatures. The results showed that the CuO films were composed of different sizes of CuO nano-grains and the CuO films deposited on Si substrates showed a more dense and uniform surface than that deposited on Al₂O₃ substrates. It was noted that both the CuO films deposited on Si and Al₂O₃ substrates revealed only CuO related diffractions and the preferred orientation of the CuO films changed from (002) to (111) as the substrate temperature increased. Moreover, the carrier concentration was 1.141?×?10¹⁸ cm^?3 and the mobility was 0.401 cm²/v s at 450°C substrate temperature. The controllable electrical properties of the films can be achieved by the variation of crystal quality arising from the substrate temperature.     

The electrical characteristics of InP implanted with the column IV elements

A study of the electrical characteristics of InP implanted with C, Si, Ge and Sn demonstrates that all of these column IV elements are donors, although the net electrical activation achieved with the light ion C was only about 5%. Samples implanted at temperatures of 150–200°C generally had lower sheet resistivities, higher mobilities and except for high doses, higher sheet carrier concentrations than those done at room temperatures. Implants at 150–200°C with 1 × 10¹⁴cm^?2 of the heavier ions, Si, Ge or Sn, resulted in layers with sheet carrier concentrations of 7.8 × 10¹³, 5.6 × 10¹³ and 4.7 × 10¹³cm^?2, respectively. Carrier concentration profiles of samples implanted at 200°C with 1 × 10¹⁴cm^?2 of Si agreed reasonably well with LSS theory. Higher doses gave rise to substantial diffusion of the implanted Si, whereas room temperature implants showed poor activation near the surface.