Low Lattice Temperatures Research Articles

Phonon growth characteristics in one-dimensional cylindrical quantum wires at low lattice temperatures

The phonon growth characteristics in a cylindrical quantum wire structure has been theoretically analysed here under the condition of low lattice temperature. The present theory takes into account the effects of finite barrier height, energy band non-parabolicity of the material and the non-zero values of the transverse components of the phonon wave vector. At the low lattice temperatures again, the interaction of the non-equilibrium electrons with acoustic phonons becomes inelastic and consequently the phonon distribution cannot be approximated by the simple equipartition law. Hence the full form of the Bose-Einstein distribution for the phonon ensemble is taken into consideration. The numerical results that follow from the present theory for the wire samples of GaAs/Ga0.7Al0.3As and Ga0.47In0.53As/InP turn out to be quite different from what follows in the traditional simplified framework.

A comprehensive study on the Hall mobility of an ohmic ensemble of two-dimensional electrons confined in a quantum well of a heterostructure at low lattice temperatures

In the present study, the characteristic dependence of the Hall mobility upon the temperature of a degenerate ensemble of two-dimensional electrons in a well of heterostructure were calculated in broader detail, for the ohmic field region, at low temperatures. Apart from the electronic interactions with the remote ionized impurities and the surface roughness, the study also duly considered some factors that are often ignored for mathematical simplicity. These include the inelasticity of the electron–phonon collisions and the true phonon distribution. The results thus obtained for heterostructures such as AlGaAs/GaAs, AlGaN/GaN, and AlInSb/InSb show that the consideration of the above details significantly changes the overall characteristics, and makes them agree quite well with the experimental data.

Generation of microwave harmonics by non-ohmic surface layers in quantum wells of compound semiconductors at low lattice temperatures

It is well known that the free electron ensemble in the material of a semiconductor structure, at times, may exhibit electrical nonlinearity, when an effectively large electric field is applied. Under the condition, when a microwave field is also superimposed at the input, the efficiency of generation of second harmonic has been estimated here, using the electrical nonlinearity of the surface layers in infinite triangular quantum wells of compound semiconductors at low lattice temperatures. The nonlinear electrical characteristics are obtainable in the widely used framework of the field-dependent effective electron temperature approximation. Some of the explicit low-temperature features, like the degeneracy of the ensemble, and their interaction with the piezoelectric phonons, are taken into account. The efficiency characteristics which are thus obtained for some practically used modulation doped heterostructures of compounds, like InSb, GaAs, and GaN, seem to be interesting. Moreover, the specific low-temperature features are found to effect significant changes in the efficiency characteristics of the layers of such compounds. How the present analysis leaves much scope for further refinement is also discussed.

A novel deep gate power MOSFET in partial SOI technology for achieving high breakdown voltage and low lattice temperature

A novel deep gate power MOSFET in partial SOI technology for achieving high breakdown voltage and low lattice temperature

Effects of inelastic interaction of the non-equilibrium electrons with the neutral impurities on the non-ohmic characteristics of a compensated semiconductor compound at low lattice temperatures

Low lattice temperature has some distinct features which are hardly observed at high temperatures. These features significantly change the electron transport characteristics of a semiconductor. The inelasticity of the collision of non-equilibrium electrons with the neutral impurities is one of such features. The effects of this inelasticity on the average rate of loss of energy of such an electron have been studied here for a non-degenerate and compensated semiconductor compound, adopting the well-known standard method. The results for the electric field dependence of the effective temperature of the electrons have also been obtained here from the solution of the energy balance equation of the electron-lattice system. The numerical data for InSb, InAs and GaSb, which follow from the present analysis, reveal that the effects are significant. The results are seen to be in satisfactory agreement with the experimental observations.

Structural Optimization of a Junctionless VSTB FET to Improve its Electrical and Thermal Performance

This article provides an insight into the electrical and thermal performance improvement of a junctionless vertical super-thin body (JL VSTB) FET based on dimensional and material optimization in its shallow-trench isolation (STI) wall and contact/side isolations with the aid of a properly calibrated TCAD tool. The lowering in STI wall thickness ( tSTI ) and contact isolations’ height ( hi ) effectively improves off-state current ( Ioff ) and on current ( Ion ), respectively. More interestingly, by inducing fringing field effect on channel, the material of STI wall and side isolations exhibits a crucial effect on Ioff and Ion , respectively. On the other hand, the thermal analysis based on the self-heating effect (SHE) reveals that compared to hi , tSTI has a much more prominent control over the thermal response of the device. Such a nature originates because unlike hi , tSTI -variation directly causes dimensional change in the principal heat sink (substrate) of the device. In addition, an excellent improvement in all the thermal parameters is shown by employing high thermal conductivity material in STI/all isolations. Finally, based on low lattice temperature profile, the most suitable sets of tSTI and hi for low-power (LP) and high-performance (HP) applications are suggested in accordance with Ioff and Ion characteristics, respectively.

Energy loss rate and non-ohmic characteristics of a degenerate surface layer of compound semiconductors at low lattice temperatures

The rate of energy loss of the non-equilibrium electrons and their non-ohmic mobility characteristics have been analyzed here for a degenerate ensemble of two dimensional electron gas in a well of compound semiconductor, taking due account of some of the low temperature features. In place of the Fermi Dirac distribution function, a well-tested alternative form of the distribution function has been used here for mathematical simplicity. The numerical results thus obtained here for the wells of InSb, GaAs and GaN are compared with some available theoretical and experimental data. Our theoretical results are seen to be in quite reasonable agreement with the experimental data when some qualitative comparison is made between them. The results being interesting, the present work stimulates further studies in the same field. • At low temperatures, non-ohmic transport in quantum surfaces changes significantly. • Theory of energy loss rate and non-ohmic mobility are developed at low temperature. • In place of the Fermi-Dirac an alternative distribution is used for simplicity. • Degenerate surface layers of compounds like InSb, GaAs and GaN are studied. • Agreement with available experimental data for GaAs has been reasonable.

Terahertz luminescence and photoconductivity associated with the impurity electron transitions in GaAs/AlGaAs quantum wells

The photoconductivity and photoluminescence spectra of GaAs/AlGaAs quantum wells doped with shallow donors are studied at low lattice temperatures. The optical electron transitions between the first electron subband and donor ground states, as well as between the excited and ground donor states, are revealed in the terahertz photoluminescence and photoconductivity spectra. The temperature evolution of the impurity-related photocurrent in the terahertz spectral range is also studied.

Acceptor-related terahertz and infrared photoconductivity in p-type GaAs/AlGaAs quantum wells

Photoconductivity in GaAs/AlGaAs quantum well nanostructures doped with acceptors was studied at low lattice temperatures in the infrared and terahertz spectral ranges. The photocurrent spectra revealed features that can be attributed to the hole transitions from the ground acceptor state to the excited acceptor states and size-quantized subbands.

Low Dimensional Magnetic Lattice and Room Temperature Magneto(di)electric Effect in Polyanion Ruddlesden-Popper Iron Oxides.

We have shown a new design strategy which exploits different oxyanions in a Ruddlesden-Popper (RP)-type phase to modulate the local crystal structure and magnetic lattice. Material (Sr4Fe2(SO4)0.5O6.5) with the larger voluminous oxyanion (SO4, S-O distance = 1.49 Å) as separating blocks between magnetic FeO layers shows a two-dimensional magnetic lattice. A three-dimensional magnetic lattice and spin reorientation transition is observed for the Sr4Fe2(CO3)O6, having CO3 (C-O distance = 1.25 Å), a smaller oxyanion, as a separating layer. Using mixed oxyanions (SO4 and CO3) in the central perovskite block of the RP3 phase, we have demonstrated a facile strategy to modulate the local crystal structure. The modulated displacement of the magnetic cations, which can break the local centrosymmetry, is suggested to originate the magnetodielectric effect near the magnetic ordering temperatures (higher than room temperature). Further, all CO3 containing samples show magnetodielectric coupling below room temperature due to the spin reorientation transition. The room temperature magnetodielectric effect coupled to the targeted local modulation of the crystal structure by oxyanions (in the absence of second-order Jahn-Teller active "distortion centers") opens a new door to the design of new multifunctional materials with the possibility for the room temperature application.

Homogeneous Paraexciton Dynamics at Ultralow Temperatures by Numerical Simulations

This paper presents a theoretical investigation of the relaxation behavior of paraexcitons including exciton–phonon and exciton–exciton collisions as relaxation processes. Paraexcitons have been modeled as a homogeneous gas within cuprous oxide. Special care has been given to the evolution of the distribution function with low and high density of the paraexciton gas and the cooling process. The total working procedure has been described by the Boltzmann equation which is solved numerically using MATLAB. The analysis of the relaxation behavior has been done for the temperatures between 0.1 and 3 K. The numerical calculations show that for very low lattice temperatures (T ≪ 1 K), the process of thermalization is slow, and at 0.1 K, the paraexcitons might not reach the lattice temperature within their finite lifetime. For all the temperatures in the investigated range, when the paraexciton density is significantly higher than the critical density, a high peak of paraexciton occupation number at or near zero energy indicates the occurrence of Bose–Einstein condensation. Therefore, the calculations indicate that the condensation may take place.

Heating of a degenerate electron ensemble in a well of compound semiconductors at low lattice temperatures

At low lattice temperatures, an ensemble of electrons exhibits some specific features which are hardly observed for higher lattice temperatures. Considering a two-dimensional ensemble of non-equilibrium electrons in a quantum well of compound semiconductors, the dependence of the effective electron temperature upon the electric field has been analysed under the condition when the lattice temperature is low. The aim of the analysis has been to assess how does the degeneracy of the ensemble and the piezoelectric interaction of the electrons, result in changes in the electron temperature characteristics at low lattice temperatures. Numerical results are obtained for quantum wells of InSb, GaAs and GaN which are potentially used for the device purpose. The assessment is made through the comparison of the results obtained from the present analysis with other theoretical and experimental results which are available in the literature. The analysis predicts that the low-temperature features result in significant changes in the characteristics. In the absence of appropriate experimental data in the literature, any conclusive comparison of the theoretical results of the present analysis with the experiment could hardly be made. However, a rather cursory comparison for the wells of GaAs with the indirectly obtained effective temperature characteristics of the electrons shows some qualitative similarity, particularly for the regime of low electric fields. The results of the present approximate analysis being interesting, it holds promise for further work in the same field.

Diffusion properties of electrons in GaN crystals subjected to electric and magnetic fields

We studied the diffusion coefficient of hot electrons of GaN crystals in moderate electric (1...10 kV/cm) and magnetic (1...4 T) fields. Two configurations, parallel and crossed fields, are analysed. The study was carried out for compensated bulk-like GaN samples at different lattice temperatures (30...300 K) and impurity concentrations (10^16..10^17 cm^{-3}). We found that at low lattice temperatures and low impurity concentrations, electric-field dependencies of the transverse-to-current components of the diffusion tensor are non-monotonic for both configurations, while the diffusion processes are greatly controlled by the magnetic field. With an increase of the lattice temperature or the impurity concentration, the behaviour of the diffusion tensor becomes more monotonous and less affected by the magnetic field. We showed that such behaviour of the diffusion processes is due to the distinct kinetics of the hot electrons in polar semiconductors with strong electron-optical phonon coupling. We suggest that measurements of the diffusion coefficient of the electrons subjected to electric and magnetic fields facilitate the identification of features of different electron transport regimes and the development of more efficient devices and practical applications.

Energy loss in nano-dimensional wire structures due to piezoelectric interaction at low temperatures

The theory of energy loss rate of non-equilibrium electrons due to inelastic interaction with intravalley piezoelectric phonons in a nano-dimensional semiconductor wire has been developed under the condition of low lattice temperatures, without recourse to the standard approximations which are usually made when developing theories at relatively higher temperatures. Such approximations are hardly valid when the lattice temperature is low. As expected, compared to the well-known earlier results, the theory here, reveals a complex and altogether different dependence of the energy loss rate upon the carrier energy and the lattice temperature. The numerical results have been obtained for narrow channel wires of GaAs and GaN. On comparison with other available results, it is revealed that the finite energy of piezoelectric phonons and the full form of the phonon distribution bring about quite significant changes in the energy loss characteristics at low temperatures.

An analysis of phonon emission as controlled by the combined interaction with the acoustic and piezoelectric phonons in a degenerate III–V compound semiconductor using an approximated Fermi–Dirac distribution at low lattice temperatures

Compound semiconductors being piezoelectric in nature, the intrinsic thermal vibration of the lattice atoms at any temperature gives rise to an additional potential field that perturbs the periodic potential field of the atoms. This is over and above the intrinsic deformation acoustic potential field which is always produced in every material. The scattering of the electrons through the piezoelectric perturbing potential is important in all compound semiconductors, particularly at the low lattice temperatures. Thus, the electrical transport in such materials is principally controlled by the combined interaction of the electrons with the deformation potential acoustic and piezoelectric phonons at low lattice temperatures. The study here, deals with the problem of phonon growth characteristics, considering the combined scattering of the non-equilibrium electrons in compound semiconductors, at low lattice temperatures. Beside degeneracy, other low temperature features, like the inelasticity of the electron–phonon collisions, and the full form of the phonon distribution have been duly considered. The distribution function of the degenerate ensemble of carriers, as given by the heated Fermi–Dirac function, has been approximated by a simplified, well-tested model. The model which has been proposed earlier, makes it much easier to carry out analytically the integrations without usual oversimplified approximations.

On Hot Electron Transport in Si(100) 2DEG at Low Lattice Temperature

On Hot Electron Transport in Si(100) 2DEG at Low Lattice Temperature

The effect of stimulated interband emission on the impurity-assisted far-infrared photoluminescence in GaAs/AlGaAs quantum wells

Emission of far- and near-infrared radiations in the n-GaAs/AlGaAs quantum well nanostructures under interband photoexcitation of electron-hole pairs is studied at low lattice temperatures. Optical transitions of nonequilibrium electrons involving donor impurity states in quantum wells are revealed in far- and near-infrared emission spectra. Intensive optical pumping allows to observe near-infrared stimulated emission related to the radiative recombination of electrons from the ground donor state and holes from the valence subband in quantum wells. The possibility of the intensity increase of impurity-assisted far-infrared radiation due to effective depopulation of donor states with interband stimulated emission in quantum wells is demonstrated.

Heating of carriers as controlled by the combined interactions with acoustic and piezoelectric phonons in degenerate III-V semiconductors at low lattice temperature

In compound semiconductors which lack inversion symmetry, the combined interaction of the electrons with both acoustic and piezoelectric phonons is dominant at low lattice temperatures (~ 20K). The field dependence of the effective electron temperature under these conditions, has been calculated by solving the modified energy balance equation that takes due account of the degeneracy. The traditionally used heated Fermi-Dirac (F.D.) function for the non-equilibrium distribution function is approximated by some well tested model distribution. This makes it possible to carry out the integrations quite easily and, thus to obtain some more realistic results in a closed form, without taking recourse to any oversimplified approximations. The numerical results that follow for InSb, InAs and GaN, from the present analysis, are then compared with the available theoretical and experimental data. The degeneracy and the piezoelectric interaction, both are seen to bring about significant changes in the electron temperature characteristics. The scope for further refinement is discussed.

Piezoelectric interaction in controlling the effective electron temperature and the non-ohmic mobility characteristics in GaN and other III–V compounds at low lattice temperature

In a compound semiconductor that lacks inversion symmetry, the free electrons interact simultaneously with the piezoelectric and acoustic phonons. This combined interaction principally controls the electrical transport at low lattice temperatures. Again, at low temperatures, the electrons in some of the compounds may be significantly perturbed for comparatively low fields, say, even for a fraction of a Vcm−1 or so, which effectively seems to be high enough, and the material exhibits electrical nonlinearity. Such a perturbed ensemble then attains a field dependent effective electron temperature Te, which exceeds the lattice temperature TL. The relative importance of the piezoelectric interaction in controlling the field dependence of the effective electron temperature, and therefrom, the non-ohmic mobility characteristics have been analyzed here under the condition of low lattice temperature. The numerical results obtained for InSb, InAs, and GaN are studied in detail. When compared with the experiments, the results here seem to give the same qualitative picture with respect to the variation of the non-ohmic mobility with the electric field for the indium compounds. The results, being interesting, stimulate further work in the same field.

Application of kinetic flux vector splitting scheme for solving multi-dimensional hydrodynamical models of semiconductor devices

In this article, one and two-dimensional hydrodynamical models of semiconductor devices are numerically investigated. The models treat the propagation of electrons in a semiconductor device as the flow of a charged compressible fluid. It plays an important role in predicting the behavior of electron flow in semiconductor devices. Mathematically, the governing equations form a convection–diffusion type system with a right hand side describing the relaxation effects and interaction with a self consistent electric field. The proposed numerical scheme is a splitting scheme based on the kinetic flux-vector splitting (KFVS) method for the hyperbolic step, and a semi-implicit Runge–Kutta method for the relaxation step. The KFVS method is based on the direct splitting of macroscopic flux functions of the system on the cell interfaces. The second order accuracy of the scheme is achieved by using MUSCL-type initial reconstruction and Runge–Kutta time stepping method. Several case studies are considered. For validation, the results of current scheme are compared with those obtained from the splitting scheme based on the NT central scheme. The effects of various parameters such as low field mobility, device length, lattice temperature and voltage are analyzed. The accuracy, efficiency and simplicity of the proposed KFVS scheme validates its generic applicability to the given model equations. A two dimensional simulation is also performed by KFVS method for a MESFET device, producing results in good agreement with those obtained by NT-central scheme.