Enn Velmre

Laterally pumped GaAs/AlGaAs quantum wells as sources of broadband terahertz radiation

In this work we consider lateral current pumped GaAs/AlGaAs quantum wells as sources of incoherent terahertz radiation. The lateral field heats the electrons in a two-dimensional quantum layer and increases the population of higher subbands, hence also increasing the radiation power generated in spontaneous intersubband emission processes. Digitally graded quasi-parabolic and simple square quantum wells are considered, and the advantages of both types are discussed. Calculations at lattice temperatures of 77 K and 300 K, for electric fields up to 10 kV/cm, show that the optical output power of ∼100−200 W/m2 may be achieved for the 7 THz source. The main peak of the spectrum, at 7 THz, of the quasi-parabolic quantum well exceeds the black body radiation at 300 K by approximately a factor of two and by two orders of magnitude at 77 K.

High Phonon-Drag Thermoelectric Efficiency of SiC at Low Temperatures

Results confirming the high values of Seebeck coefficient phonon-drag component Sph in SiC are discussed and the relevant consequences to the thermoelectric figure-of-merit ZT=T S 2  / are studied. The fact that Sph increases at low temperatures as T 2.4 similarly for low-doped p-6H- SiC, Si and Ge is demonstrated. Materials comparison criterion Sph  / meff, based on heat conductivity, lattice mobility and carrier effective mass is offered and Sph strength proportions 0.1 : 1 : 5 : 10 for Ge, Si, SiC and C (diamond) are proposed. Related to that high T500K phonon- drag domination limit is obtained for SiC. It is shown that the proportionality Sph , yielding ZT , could reverse some concepts accepted in thermoelectric material development. The best-case calculations predict good (>0.1) and excellent (>1) ZT values with steep temperature-dependence T 3…4 for low temperatures 50200K if sufficiently high electrical conductivity  could be ensured. The last condition may be feasible for n-SiC with relatively shallow donor levels but difficult to achieve in the case of p-SiC with relatively deep acceptor levels.

Modeling of Lattice Heat Conductivity and Thermopower in SiC Considering the Four-Phonon Scattering Processes

The lattice heat conductivity in low-doped SiC is studied in the wide temperature range 50÷1200 K in order to estimate the strength coefficient a4P for 4-phonon scattering processes mechanism for later application in both heat conductivity and Seebeck coe fficient models. Heat conductivity calculations confirm the domination of the 4-phonon mechanism and t he secondary role of the 3-phonon Umklapp mechanism. The obtained a4P ≈1.5⋅10 −22 s/K gives very satisfactory result in Seebeck coefficient phonon drag component modeling. Introduction. To achieve accuracy in electrothermal device simulation the rel iabl models for the lattice heat conductivity κL and thermopower (Seebeck coefficient) S are needed. To model these material parameters the phonon transport and relevant scattering proc esses should be analyzed. The phonon flux is determining κL and the partial momentum transfer from phonons to electron and hole subsystems (phonon drag effect) increases remarkably the electron and hole Seebeck coefficient Sn,p values, especially at low temperatures. In SiC, according to the hi g heat conductivity and relatively low carrier mobilities, the phonon drag effect is approximately 5 ÷10 times stronger than in silicon [1]. Respectively in SiC the phonon drag component of the Seebeck coeffici ent typically (except low doped material of 10 15 cm range) dominates over classic diffusion component for all temperature range of practical application [1]. More detailed investigation predicts that parameters κL and S are strongly influenced by the 4phonon scattering processes (4-phonon mechanism). Below we will evaluate the s rength coefficient a4P for 4-phonon mechanism from experimental κL(T) in low-doped 6H-SiC in order to use this a4P later in improved theoretical model of Seebeck coefficient phonon drag component (see [1]) . Theoretical background and model description. The phonon drag effect was predicted theoretically for metals by Gurevich in 1946 [2,3] and then observed experi mentally in semiconductors by Frederikse, Geballe and Hull [4-7]. Frederikse [4] s howed also that the strength of this drag is proportional to the ratio of phonon and carrier mean free paths lph/lcarr which rises rapidly at lower temperatures [8]. Herring in his seminal papers [9,10] analyzed the role of lowenergy (i.e. long-wave, LW) phonons for κL and S and proposed a theory for lph calculation considering the 3-phonon LW mechanism. However, soon it was discovered [6,7] tha t Herring’s model provides obviously too long lph and, therefore, significantly overestimates the phonon drag contribution to the thermopower Sph. Our recent investigations reveal the same situation in the case of SiC [1,8,11], Si and Ge. Herring’s model predicts also too strong te mperature dependence Sph ∼T −3.5 [8,11] what disagrees with the observed experimental results Sph ∼T −1.8 ...−2.5 [1,7,8,12]. This indicates that some important phonon scattering processes reducing lph value have been omitted in Sph modeling. In order to overcome this problem, we decided to consider in the Sph model beside the Herring’s type 3-phonon processes also the 4-phonon processes introduced into the κL analysis by Pomeranchuk [13]. Detailed characterization of 3-phonon and 4-phonon processes is giv n, e.g. in Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 391-394 doi:10.4028/www.scientific.net/MSF.433-436.391

Investigation of Charge Carrier Lifetime Temperature-Dependence in 4H-SiC Diodes

Recently published experimental results for 4H–SiC diodes up to 700 °C are used to deduce the hole lifetime temperature-dependence in n-base for high temperature range. The reverse recovery measurements are interpreted by the nonisothermal drift-diffusion simulator DYNAMIT. The uncertainties from lifetimes unknown behavior in emitter layers and consequences from possible nonuniform lifetime distribution in n-base are analyzed. Results show that up to temperature 400 °C nearly quadratic dependence of lifetime versus temperature τ ~ T 2 holds. At higher temperatures lifetime growth is accelerated approximately to quartic dependence τ ~ T 4.

Impact of phonon drag effect on Seebeck coefficient in p-6H-SiC : Experiment and simulation

The temperature dependence of Seebeck coefficient (S) for p-6H-SiC has been obtained. It increases from 2 up to 5.2 mV/K when temperature decreases from 400 down to 240 K. It is shown that phonon d ...

Numerical aspects of the development of Quantum Cascade Laser simulation software

One of the most efficient device producing coherent high power radiation in terahertz range in electromagnetic spectrum is Quantum Cascade Laser (QCL). During over a decade of history, together with intensive development of terahertz quantum cascade lasers, respecting simulation software tools has been developed too. The process of development of simulation software is a sophisticated task, because many fields of science meet there - quantum electronics, computing technologies, software development etc. The progress in computing power have made simulation tools much more handy, as calculations of non-complex quantum well heterostructures can be run on a personal computer during a considerable time. Simulation of Quantum Cascade Lasers with complex structures can still be very time-consuming. In this paper few strategies on how to improve and optimize QWWAD software tools are analysed, implemented and corresponding results presented. Improvement of eigenvalue problem solution gave 20 times faster algorithm in solving Schrödinger equation. Other ideas did not give remarkable success.

Numerical Investigation of SiC Devices Performance Considering the Incomplete Dopant Ionization

The temperature-dependences of ionized dopant concentration at different doping levels are generalized and the preconditions for thermal instabilities due to self-heating are studied. The nonisothermal simulations of forward-biased SiC structures over a wide temperature range are performed by using the drift-diffusion 1D-simulator DYNAMIT. Results show that the incomplete doping ionization will be an important effect if impurity activation energies exceed 0.1, 0.2 and 0.3 eV for doping levels 1019, 1018 and 1017cm−3, respectively. For appearance of S-shaped selfheated I/V curves the respective values must exceed 0.2, 0.3 and 0.4 eV. Strong influence of incomplete dopant ionization on forward I/V curves of realistic 4H-SiC and 6H-SiC p-i-n structures is predicted by simulations. At that the dominating role of the thick substrate layer is shown.

Modeling of Photon Recycling in GaN-Devices

The strength of recombination radiation reabsorption in GaN is discussed. For material comparisons a distance-dependent radiative recombination transfer function F(u) is introduced. In spite of high absorption rates of GaN, calculations predict ca. one order of magnitude higher photon recycling efficiency in GaN than in GaAs. Simulations of 2H-GaN p −i −n structures predict appearance of S-shaped forward I/V characteristics due to the generation of extra carriers in the base center. The study of GaN bipolar transistors shows that the radiative recombination will reduce the carrier lifetimes in the base and thereby restrict essentially the achievable current gains.

Failure prediction of power devices under reverse surge current conditions

GIGA is a new program to simulate electrothermal interaction within packaged semiconductor devices in one and two dimensions. In this paper, results of numerical modeling of a packaged rectifier element under reverse surge current conditions are presented. The internal behaviour of a device and the appearance of thermal instability conditions near the p-n junction are illustrated.