Impurity Freeze-out Research Articles

Optical measurements of solid nitrogen solubility in hydrogen at cryogenic temperatures

The solubility of impurities in hydrogen at cryogenic temperatures is essential information for the design of liquid hydrogen plants. However, large data scatter and inconsistent predictions of impurity solubility by thermodynamic models mean that expensive engineering design margins are required to avoid blockages in cryogenic equipment. Here we report the combination of a stereomicroscope, commercial cryocooler and pressure cell to directly visualise solid impurity freeze-out and melting in stirred hydrogen at low temperatures and elevated pressures. Scattered literature data for nitrogen's solubility in hydrogen are compared to new solid-fluid equilibrium (SFE) measurements for a 1000 ppm N2 in H2 mixture at (1.8–5) MPa, and (42–52) K. These results were used to improve solubility models implemented in the ThermoFAST software package, achieving a three-fold decrease in the root-mean-square deviations between the SFE predictions and the data, helping lower the risk of impurity freeze-out in hydrogen liquefaction.

Massively parallel FDTD-FBMC simulations of nonlinear hole dynamics in silicon at cryogenic temperatures driven by intense EM THz pulses

THz spectroscopy with intense electromagnetic (EM) pulses offers unique avenues to study the charge carrier dynamics in semiconductors. Quantitative analysis of the experiment requires a detailed microscopic model that is based on the wave propagation within the sample and on the strongly nonparabolic dispersion relation of the valence bands. With a finite difference time domain (FDTD) solver for the EM wave in conjunction with a full band Monte Carlo (FBMC) model for the holes the generation of higher harmonics by the nonlinear hole response in the p-doped silicon sample can be simulated. Due to impurity freeze-out at the cryogenic temperature the hole density is low and the FDTD solver can be decoupled from the FBMC model by a perturbation approach. This enables the independent processing of a very large number of holes on a computer cluster with nearly 100% computational efficiency resulting in a very low level of stochastic noise, which is necessary to achieve a reasonable signal to noise ratio for the faint higher harmonics. The simulations show that at THz frequencies and cryogenic temperatures strong generation of higher harmonics is only possible, if the band structure is nonparabolic.

Measurement and Remediation of Impurity Freeze-Out in Hydrogen Liquefaction with Microwave Resonance Sensors

Measurement and Remediation of Impurity Freeze-Out in Hydrogen Liquefaction with Microwave Resonance Sensors

Experimental study of impurity freeze-out in ternary methane + ethane + benzene mixtures with applications to LNG production

The freeze-out of impurities in LNG production can pose significant operational risks and lead to costly blockage-induced plant shutdowns. Study of ternary and higher-order mixtures, which are more analogous to LNG, present the critical tests of thermodynamic models in terms of their utility and predictive accuracy. In this work, a visual CryoSolids apparatus was upgraded to allow analytical measurements of solvent composition in multi-component systems where a solid phase is present at equilibrium. The analytical system, which included a ROLSI sampling valve and capillary together with a gas chromatograph, was successfully commissioned and used to measure melting temperatures and solvent compositions of a ternary mixture containing methane, ethane, and benzene at temperatures down to 125 K and pressures up to 6 MPa. The effect on the solubility of benzene by adding ethane to the solvent was investigated by varying the ethane mole fraction from 0 to 0.96. The resulting temperature at which benzene melted into the liquid solvent decreased from 246 K at 4.7 MPa to 125 K at 5.7 MPa. The comparison of results with the ThermoFAST model showed that it could describe the new data with a deviation less than 1 K for ethane liquid phase mole fractions from 0 <xC2< 0.5, which increased to less than 4 K for 0.85 <xC2< 0.97. This result indicates that ThermoFAST satisfactorily represents SFE conditions in LNG mixtures of industrial relevance but needs improvement to cover wider ranges of composition.

Effect of cryogenic temperature characteristics on 0.18-μm silicon-on-insulator devices**Project supported by the National Natural Science Foundation of China (Grant Nos. 61176095 and 61404169) and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

The experimental results of the cryogenic temperature characteristics on 0.18-μm silicon-on-insulator (SOI) metal-oxide-silicon (MOS) field-effect-transistors (FETs) were presented in detail. The current and capacitance characteristics for different operating conditions ranging from 300 K to 10 K were discussed. SOI MOSFETs at cryogenic temperature exhibit improved performance, as expected. Nevertheless, operation at cryogenic temperature also demonstrates abnormal behaviors, such as the impurity freeze-out and series resistance effects. In this paper, the critical parameters of the devices were extracted with a specific method from 300 K to 10 K. Accordingly, some temperature-dependent-parameter models were created to improve fitting precision at cryogenic temperature.

Symmetrically linearised charge-sheet model for extended temperature range

The symmetrically linearised charge-sheet model, which forms the core of the industry standard compact MOSFET model is extended to a wide temperature range by including partial impurity freeze-out. The new model is verified by comparison with the exact results and includes the original formulation as a special case corresponding to complete impurity ionisation.

GEM operation in helium and neon at low temperatures

We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne and Ne+H 2 at temperatures in the range of 2.6–293 K. In He, at temperatures between 62 and 293 K, the triple-GEM structures often operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved by the Penning effect in the gas impurities released by outgassing. At lower temperatures, the gain–voltage characteristics are significantly modified probably due to the freeze-out of impurities. In particular, the double- and single-GEM structures can operate down to 2.6 K at gains reaching only several tens at a gas density of about 0.5 g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be reestablished in Penning mixtures of Ne+H 2: very high gains, exceeding 10 4, have been obtained in these mixtures at 50–60 K, at a density of 9.2 g/l corresponding to that of saturated Ne vapor near 27 K. The results obtained are relevant in the fields of two-phase He and Ne detectors for solar neutrino detection and electron avalanching at low temperatures.

Impact of LDD structures on the operation of silicon MOSFETs at low temperature

The impact of Lightly Doped Drain structures (LDD) on the electrical characteristics of Si MOS transistors is investigated from liquid helium up to room temperature. It is found that the presence of LDD regions strongly alters the ohmic drain current characteristics at low temperatures ( ≤ 100 K). This feature is attributed to the increase of the source-drain series resistance due to the impurity freeze-out which takes place in the LDD regions. In contrast, it is also found that, for sufficiently high drain and gate voltages, the drain current is only weakly affected by the presence of LDDs. The analysis of the drain current and output conductance characteristics allows us to demonstrate that the LDD resistance is indeed a non-linear function of the longitudinal electric field. The above LDD resistance dependence with electric field is interpreted in term of field assisted impurity ionization which occurs in the LDD regions. The mechanisms responsible for the field assisted impurity ionization are discussed with regard to the Poole-Frenkel effect and the shallow level impact ionization process. Moreover, a simple model for the field dependent LDD resistance is proposed and makes it possible to explain quantitatively the change of the output conductance with temperature for MOS devices operated at low and very low temperatures.

Brief review of the MOS device physics for low temperature electronics

A brief review of the main physical phenomena involved in the cryogenic operation of CMOS silicon devices down to liquid helium temperature is given. Going from solid state physics towards electrical engineering point of views, several aspects such as the quantification of the inversion layer, the electronic transport in the 2D electron or hole gases, the scattering mechanisms, the impurity freeze-out in the substrate or in the lightly doped source and drain regions, the field-assisted impurity and impact ionization phenomena, the influence of series resistance and other parasitic effects (kink effect, hysteresis, transient, …) which alter the device characteristics will be discussed. The short channel effects such as drain induced barrier lowering, punch through, velocity overshoot will also be addressed.

Stress in Thin Layers Applied by Substrate Bending; Calibration and Potentialities

ABSTRACTUniaxial stresses up to 0.3GPa, both compressive and tensile, can be generated in thin epitaxial layers by substrate bending. We demonstrate the feasibility of the method and calibrate the stress, measuring piezoresistance at temperatures below the impurity freeze-out, on lightly B-doped silicon MBE layers, using the known expression for a surface stress in a bent cantilever beam. The ground state energy displacement of B in Si thus obtained can vary by ±5meV, suggesting potential applications of the technique in tunable devices, involving doped semiconductors or heterostructures.

A survey of MOS device physics for low temperature electronics

A review of the main physical phenomena involved in the cryogenic operation of CMOS Silicon devices down to liquid helium temperature is given. Going from solid state physics towards electrical engineering point of views, several aspects such as the quantification of the inversion layer, the electronic transport in the 2D electron or hole gases, the scattering mechanisms, the impurity freeze-out in the substrate or in the lightly doped source and drain regions, the field-assisted impurity and impact ionization phenomena, the influence of series resistance and other parasitic effects (kink effect, hysteresis, transient, …) which alter the device characteristics will be discussed. The short channel effects sush as Drain Induced Barrier Lowering (DIBL), punch through, velocity overshoot will also be addressed.

Approximations for carrier density in nonparabolic semiconductors

The authors propose simple, analytic approximations that describe properties of semiconductors with nonparabolic energy bands. The approach is based on the two-level kp model of the semiconductor statistics which includes effects of band nonparabolicity and carrier degeneracy. The new expressions are in the form of a correction to the classical, Boltzmann approximation of the semiconductor statistics and do not introduce any new adjustable parameters. These relations can be applied to both narrow- and wide-bandgap nonparabolic semiconductors. The authors calculate the low-frequency capacitance of a nonparabolic MIS structure and compare the results with an accurate numerical model and experimental measurements of a HgCdTe capacitor. The solution includes the contributions of both types of carriers and the effect of impurity freezeout. The new approximations should be useful for characterization and modeling of semiconductor devices with nonparabolic energy bands.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rigorous thermodynamic treatment of heat generation and conduction in semiconductor device modeling

A treatment of the self-heating problem is presented. It is based on the laws of phenomenological irreversible thermodynamics (e.g. Onsager's relations and conservation of total energy) and is also consistent with the physical models usually considered in the isothermal drift diffusion approximation. The classical isothermal device equations are extended and completed by a generalized heat-conduction equation involving heat sources and sinks which, besides Joule and Thomson heat, reflect the energy exchanged through recombination (radiative and nonradiative) and optical generation. Thus the extended model also applies to direct semiconductors (e.g., optoelectronic devices) and accounts for effects caused by the ambient light intensity. It fully allows for low temperature since the case of incomplete ionization of donors and acceptors (impurity freeze-out) is properly incorporated in the theory. A critical comparison with previous work is made, showing that, in the steady state, some of the heuristic models of heat generation, thermal conductivity, and heat capacity could indeed approximate the correct results within an error bound of 1-10%. In the transient regime, however, none of the models used previously seems to be reliable, particularly, if short switching times ( >

MOS device modeling at 77 K

The state of the art in self-consistent numerical low-temperature MOS modeling is reviewed. The physical assumptions that are required to describe carrier transport at low ambient temperatures are discussed. Particular emphasis is placed on the models for space charge (impurity freeze-out), carrier mobility (temperature dependence of scattering mechanisms at a semiconductor-insulator interface), and carrier generation-recombination (impact ionization). The differences with regard to the numerical methods required for the solution of low-temperature models compared to room-temperature models are explained. Typical results obtained with the simulator MINIMOS 4 are presented. These include comparisons of short-channel effects and hot-electron phenomena such as energy relaxation and avalanche breakdown at 77 K and 300 K ambient temperatures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Operation of poly bipolar transistors near liquid-helium temperatures (9 K)

Operation of bipolar transistors with poly emitters with current gain beta >25 at 77 K and beta >3 at 9 K has been demonstrated. In the temperature range of 300 K to 9 K, the plot of beta vs. temperature (T) exhibits a minimum at T equal to 40 K: decreasing monotonically from 300 K to 40 K, remaining unchanged between 40 K to 30 K, and increasing slightly from 30 K to 9 K. This behavior is attributed to freezeout of acceptor impurities in the base for T<40 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

N-channel enhancement-mode MOSFET characteristics from 10 to 300 K

Operation of MOSFET circuits at the liquid nitrogen temperature (77 K) has been suggested as a means of improving circuit and system performance. Previously reported work emphasizes mobility and threshold voltage at 77 K. However, small MOSFET's require several (≳10) parameters for circuit design. Since a full set of MOSFET model parameters have not been previously reported, it has not been established whether conventional models can be applied for MOSFET circuit design at 77 K. We present here the temperature dependence of a full set of MOSFET circuit model parameters for channel lengths from 2.5 to 8.5 µm and for temperatures ranging from 10 to 300 K. Temperatures below 77 K are of interest in evaluating effects of impurity freezeout and temperatures above 77 K are important since actual device temperatures will be above the ambient. Overall, we find that the mobility and the threshold voltage are the dominant temperature dependent parameters and that conventional I-V characteristics persist down to 77 K. Below 77 K, some new features appear in the I-V characteristics. However, the conventional behavior down to 77 K suggests that standard (circuit models can be used for circuits operating at 77 K. Such circuits would be about four times faster than at room temperature and, with liquid nitrogen cooling, would provide an order of magnitude higher power density for VLSI.

Behavior of electrically small depletion mode MOSFETs at low temperature

Impurity freezeout has a substantial effect on the threshold and subthreshold characteristics of large depletion mode devices. Recent experiments on short and narrow channel depletion mode devices demonstrate that geometry effects are independent of temperature and comparable to those of enhancement mode devices. Reduction of the electrical device size does not alter the degree of impurity freezeout. To predict the behavior of small channel enhancement and depletion mode devices, freezeout effects and geometry effects can be directly superimposed. A simple qualitative model is used to describe the conditions under which the superposition is valid.

Simulation of impurity freezeout through numerical solution of Poisson's equation with application to MOS device behavior

Incorporation of temperature dependencies in the one-dimensional Poisson's equation for use in numerical simulation of MOSFET threshold behavior from 350 to 50 K is discussed. Careful consideration has been given to accurate modeling of impurity freeze-out and temperature-dependent parameters. Examples of simulation of depletion-mode MOSFET's demonstrate the importance of proper modeling and show that impurity freezeout must be considered even at room temperature.

Temperature dependent threshold behavior of depletion mode MOSFETs: Characterization and simulation

During the study of depletion mode MOSFET behavior at low temperatures, unusual changes in the threshold characteristics of the devices were observed. First, the effectiveness of the donor implantation in producing a negative threshold voltage shift was significantly reduced. At the same time the substrate sensitivity was found to be substantially reduced. A third observation was the existence of an unusual structure in the subthreshold region of the device at low temperatures. Computer simulation is used to explore these observations and to demonstrate that they are caused by impurity freezeout as temperature is reduced. The computer simulation program, usable over the temperature range 50–350 K, is discussed, and a threshold definition suitable for numerical analysis of devices with arbitrary channel structures is developed.

Simple analytical models for the temperature dependent threshold behavior of depletion-mode devices

Threshold voltage shifts in ion-implanted depletion-mode MOSFETs depart substantially from the usual dose proportional shift of enhancement-mode devices. Analytic expressions for the relationship between threshold voltage shift and implanted donor dose and position are extended to include impurity freezeout at low temperatures, and a simple model for the observed low substrate sensitivity at low temperature is presented. Criteria to avoid parasitic subthreshold conduction in depletion-mode devices are also established using the threshold shift formulation.