Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from ?100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from ?80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.     

Maximum Active Concentration of Ion-Implanted Phosphorus During Solid-Phase Epitaxial Recrystallization

In this paper, we showed that the maximum active P concentration of approximately 2 times10²⁰ cm^-3 exists during solid-phase epitaxial recrystallization (SPER). This maximum active concentration is close to the reported values for other active impurity concentrations during SPER. We introduced the concept of an isolated impurity that has no neighbor impurities with a certain lattice range. Assuming that impurities interact with three or four neighbor impurities, we can explain the activation phenomenon during SPER. According to our model, the isolated P concentration N _iso has a maximum value of approximately 2 times10²⁰ cm^-3 at a total impurity concentration of approximately 10²¹ cm^-3, and it decreases with a further increase in total impurity concentration. Deactivation occurs after the completion of SPER with increasing annealing time, and the active impurity concentration decreases with time but is always higher than the maximum diffusion concentration N _{Diff max}. We also observed that N _{Diff max} is independent of the annealing time despite nonthermal activation in the high-concentration region. We evaluated the dependence of N _{Diff max} on annealing temperatures. We think that this N _{Diff max} can be regarded as the electrical solid solubility N _Esol that the active impurity concentration reaches in thermal equilibrium. We observed the transient enhanced diffusion (TED) after the completion of SPER, and that, the deactivation process continues during and after TED, and the corresponding diffusion coefficient is still much higher than that in thermal equilibrium even after TED has finished, which suggests that the deactivation process releases point defects.     

Hot Electrons in THz Quantum Cascade Lasers

We compare the electrical power dependence of the lattice temperature and the electronic temperature of GaAs/Al_xGa_1-xAs THz quantum cascade lasers (QCLs) with different active region schemes, as extracted by the analysis of microprobe band-to-band photoluminescence experiments. Thermalized non-equilibrium distributions are found in all classes of QCLs. While in the case of bound-to-continuum structures all subbands share the same temperature, the upper laser level of active regions based on the resonant-phonon scheme heats up by ΔT ~ 100 K with respect to lower energy levels. The comparison among samples with different Al mole fractions show that the use of smaller x values leads to larger electronic temperatures.     

A Consistent Description of Scaling Law for Flux Pinning in ${hbox{Nb}}_{3}{hbox{Sn}}$ Strands Based on the Kramer Model (Corrected)*

There are a lot of experimental reports on the scaling of flux pinning in the form of F = F_mb^1/2(1 - b)², with b = B/B_c2.The temperature dependence of F_m is approximately proportional to B'.2 , whereas the strain dependence of Fm is reported to be proportional to the upper critical field Bc2. In this work, we re-analyze our previous data with the Kramer model including the pin-breaking dynamic pinning force (F_p) for a low field region. It is shown that the extrapolated upper critical field B_c2*, strongly depend on the ratio between the mean of the parameter K_p for F_p (p>) and the parameter K, for the flux line lattice shearing pinning force F_s. It is found that the strain dependence of F_m at 4.2 K is approximately proportional to (B_c2*)^1.5. We further compare the data with the prediction of our recent scaling theory based on Eliashberg theory of strongly coupled superconductors. It is shown that the strain dependence of F_m at 4.2 K is proportional to BC₂ ^5/2 kappa^-2, consistent with the temperature dependence of Fm. Moreover, this model agrees reasonably well even with the data in a high compressive strain region (<-0.8%).     

Thermal Behavior Investigation of Terahertz Quantum-Cascade Lasers

This paper investigates the heat conduction behavior of a terahertz (THz) quantum-cascade laser (QCL) active region by measuring its temperature using in-situ microprobe band-to-band photoluminescence (PL) technique. The heat resistance of different regions in QCL structure is derived from the temperature measurement. Experimental results show that thinning the substrate from 300 $mu{hbox {m}}$ thick to 140 $mu{hbox {m}}$ lowers thermal resistance of the device by 21%, which helps achieving continuous-wave operation. A thermodynamic differential equation was numerically solved and the temperature profiles and thermal behavior of various regions within actively biased QCL devices under various conditions were obtained. The simulation confirms the measured results that the substrate accounts for 59% of the total device thermal resistance.      

半导体薄片激光器窗口散热模式的热效应

基于垂直外腔面发射半导体激光器窗口散热模式的传热模型,用有限元法计算了不同条件下量子阱有源区的温度变化,建立了量子阱最高温度的等效热阻模型和计算公式,并通过拟合确定了热阻模型的相关参数.计算表明量子阱最高温度与抽运功率存在线性关系,与光斑面积近反比关系,窗口散热片可显著降低量子阱有源区温度和温度的不均匀度.等效热阻模型表明由于半导体晶片内热流在径向难以扩散,热传导中存在较大串联热阻,使得散热片热扩散能力趋于饱和,其中碳化硅的散热性能约为金刚石的75%.     

Thermal resistance of light-emitting diodes

A detailed analysis of the heat flow in a light-emitting diode is carried out in the present paper. In the thermal model of a light-emitting diode, the heat flow from the active region throughout the area between it and the top contact, the nonuniform heat flux density distribution in the active region due to the current-spreading effect as well as the temperature dependence of the thermal conductivity of the semiconductor material, are taken into account. The solutions of the thermal conduction equation for a light-emitting diode are obtained for both the steady-state condition and the transient-state condition. The heat-spreading in the heat-sink is analyzed. The effect of LED construction parameters on its thermal resistance is illustrated in numerous figures.     

Comparison of one- and two-dimensional models of transistor thermal instability

The IC-VCElocus predicted by a one-dimensional model of thermal instability is compared with the IC-VCElocus predicted by a numerical model which accounts for nonuniform heat generation over the transistor area and also two-dimensional heat flow within the heat sink. Both models include the effects of distributed emitter and base ballast resistance, as well as the magnitude and temperature dependence of current gain. An important result obtained from this comparison is that the I-V loci predicted by the two models are very nearly the same, even though the temperature and power density profiles over the transistor die are distinctly different. It is the I-V locus which is of most practical interest, since it is one portion of the boundary of the forward safe operating area (SOA). The similarity of the I-V loci should allow one to use the simple one-dimensional model to predict a particular SOA, even though the assumptions under which the model was originally derived are not valid at the onset of thermal instability. Confirmation of this approach has been obtained by demonstrating good agreement between the measured safe operating area and that predicted by the one-dimensional model for both single- and double-diffused transistors. The predicted improvement due to the addition of discrete emitter resistors has also been verified by SOA measurements on actual devices. The device parameters which are important in determining SOA are the effective emitter and base resistances, the magnitude and temperature dependence of current gain, and the effective thermal resistance between the active region of the transistor and its heat sink. The quantitative dependence of SOA due to each of these parameters is described.     

Nonlinear thermal model of a heterojunction-based light-emitting diode

A mathematical nonlinear thermal model of a heterojunction-based light-emitting diode (LED) is considered; the model makes it possible to estimate the nonuniformity of the current and temperature distributions in the active region of the heterostructure with the LED efficiency and the temperature dependence of the thermal-conductivity coefficient in the structure taken into account. A numerical-analytical iteration method is used to solve a set of equations that includes solving a nonlinear time-independent equation of thermal conductivity with the density of electrical power converted to heat dependent on the LED efficiency and an equation of electrothermal feedback, under conditions of a constant value for the average current density over the active region of the structure. The results of theoretical and experimental studies into the dependence of the p-n junction-to-case thermal resistance on the forward current are represented for high-power light-emitting diodes.     

Calculation and analysis of distributions of current density and temperature over the area of the the InGaN/GaN structure of high-power light-emitting diodes

A thermal model of a high-power InGaN/GaN light-emitting diode is considered; the model takes into account the exponential temperature dependence of the current density and density of the power dissipated by the active region of the heterostructure. The iteration method has been used to solve a set of equations that includes a time-independent heat-conduction equation with adiabatic conditions of the second kind on the lateral boundaries of the heterostructure and thermogeneration equations under the condition of constant average current density in the active region of the structure. The dependences of maximal and average overheating over the active region of the structure on operation current and ambient temperature for two models of heat sinks, semi-infinite and limited, are determined.     

Influence of Radiative Energy Transfer on the Thermal Behavior of Bonded InGaAs/GaAs Lasers

Temperature characteristics and thermal resistance of InGaAs/GaAs laser diode (LD) is investigated from below to beyond the lasing threshold. Spectrally-resolved emission measurements show that the heat generated in the active region is induced by the radiative energy transfer of free carriers. Below the lasing threshold, nonradiative recombination induces large heat generation in the active region. Beyond the lasing threshold, Joules heating due to the series resistance of the LD dominates the heating response. The dependence of the associated thermal resistance on different bonding configurations and its correlation with the output power is also discussed. Epi-side up and epi-side down bonding of LDs reduces the device temperature by $sim$30% and $sim$50%, respectively.      

Detection of terahertz radiation with diode lasers

A standard commercial semiconductor is shown to be able to detect terahertz (THz) radiation at room temperature. A voltage variation across the active region of the device upon incident THz radiation is measured. The detected voltage signal scales linearly with the THz intensity measured with a Golay cell. A detailed analysis shows that thermal effects following the THz absorption by the carrier plasma play an important role in this detection process.     

Three-dimensional topography simulation model: etching andlithography

An etching model in which topography is derived by solving a modified diffusion equation is introduced. This model is simple and makes it possible to simulate three-dimensional (3-D) topography accurately and quickly. Based on this model, a 3-D topography simulator which can be applied in the development of photolithography and isotropic/anisotropic etching has been developed. With this simulator, it is possible to simulate the series processes and multilayer etching, such as contact hole and trench etching. By simulating photolithography, diffraction and standing-wave effects can be found clearly in the 3-D topography of the developed photoresist. In the case of an etching process which is restricted by diffusion, the dependence of the etch front topography on the window width of the mask is examined     

Numerical simulation of the thermal response of continuous-wave terahertz irradiated skin

We report a two-layer model to describe the thermal response of continuous-wave (CW) terahertz (THz) irradiated skin. Based on the Pennes bio-heat conduction equation, the finite element method (FEM) is utilized to calculate the temperature distribution. The THz wave with a Gaussian beam profile is used to simulate the photo-thermal mechanism. The simulation results show the dynamic process of temperature increasing with irradiation time and possible thermal damage. The factors which can affect temperature distribution, such as beam radius, incident power and THz frequency, are investigated. With a beam radius of 0.5 mm, the highest temperature increase is 3.7 K/mW.     

Enhanced Thermal Efficiency in Phase-Change Memory Cell by Double GST Thermally Confined Structure

A novel phase-change memory cell with a double- confinement structure was proposed and fabricated in this work. By having an additional bottom Ge₂Sb₂Te₅ layer under the electrically confined active region, the heat loss can be effectively prevented. The temperature uniformity over the active region significantly improves and so does the thermal efficiency. Therefore, a low I_RESET of about 0.3 mA and a reset power can be achieved. For the SET performance, a pulsewidth as low as 200 ns can be used without compromising the R_SET.     

Measurement of temperature in active high-power AlGaN/GaN HFETsusing Raman spectroscopy

We report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates. Micro-Raman spectroscopy was used to produce temperature maps with ≈1 μm spatial resolution and a temperature accuracy of better than 10°C. Significant temperature rises up to 180°C were measured in the device gate-drain opening. Results from a three-dimensional (3-D) heat dissipation model are in reasonably good agreement with the experimental data. Comparison of devices fabricated on sapphire and SiC substrates indicated that the SiC substrate devices had ~5 times lower thermal resistance     

Low threshold THz QC lasers with thin core regions

It is demonstrated that the active region thickness of THz quantum cascade lasers can be reduced by a factor of 2 without effects on the threshold current density and maximum operating temperature of the laser. Pulsed and continuous-wave operation, with a low threshold J_th= 71 A/cm², are obtained for a 5.86 mum-thick THz QC laser. The emission is peaked at lambdasime115 mum and the waveguide resonator is based on a metal-metal geometry     

Effects of Lateral Diffusion on the Temperature Sensitivity of the Threshold Current for 1.3-$mu{hbox {m}}$ Double Quantum-Well GaInNAs–GaAs Lasers

We present an experimental and theoretical investigation of the temperature dependence of the threshold current for double quantum well GaInNAs-GaAs lasers in the temperature range 10 degC-110 degC. Pulsed measurements of the threshold current have been performed on broad and narrow ridge wave guide (RWG) lasers. The narrow RWG lasers exhibit high characteristic temperatures (T₀) of 200 K up to a critical temperature (T_c), above which T₀ is reduced by approximately a factor of 2. The T0-values for broad RWG lasers are significantly lower than those for the narrow RWG lasers, with characteristic temperatures on the order of 100 (60) K below (above) T_c. Numerical simulations, using a model that accounts for lateral diffusion effects, show good agreement with experimental data and reveal that a weakly temperature dependent lateral diffusion current dominates the threshold current for narrow RWG lasers.     

A simulation study of IC layout effects on thermal management ofdie attached GaAs ICs

Power management and thermal characterization of integrated power amplifiers is crucial to the development of a number of advanced technologies including portable wireless applications. Reduction and or optimization of device operating temperatures and thermal characteristics is needed to control temperature activated failure phenomena. This paper presents the use of PATRAN, a three-dimensional (3-D) model builder and finite element method (FEM) solver as means of understanding the heat flow in integrated devices and optimizing the layout for thermal operation. The approach taken is to assume a priori knowledge of the heat generation region and decouple the semiconductor transport equations. This allows for solution of the heat equation over a sufficiently large region to be correct. After verifying the correctness of the assumption of the device temperature being relatively insensitive to the depth, thickness and shape of the heat generation region, the optimization of heat spreaders in a GaAs HBT process is presented. This optimization is performed as an example of how both the maximum temperature and temperature variation across the emitter can reduced by careful design of the emitter metallization. Finally, the use of PATRAN is presented for extracting a three resistor thermal model for two devices in close proximity     

Anti-geometric diffusion for adaptive thresholding and fast segmentation

We utilize an anisotropic diffusion model, which we call the anti-geometric heat flow, for adaptive thresholding of bimodal images and for segmentation of more general greyscale images. In a departure from most anisotropic diffusion techniques, we select the local diffusion direction that smears edges in the image rather than seeking to preserve them. In this manner, we are able rapidly to detect and discriminate between entire image regions that lie near, but on opposite sides of, a prominent edge. The detection of such regions occurs during the diffusion process rather than afterward, thereby side-stepping the most notorious problem associated with diffusion methods, namely, when diffusion should stop. We initially outline a procedure for adaptive thresholding, but ultimately show how this model may be used in a region splitting procedure which, when combined with energy based region merging procedures, provides a general framework for image segmentation. We discuss a fast implementation of one such framework and demonstrate its effectiveness in segmenting medical, military, and scene imagery.