Dmitri Kotchetkov

Thermal conductivity of GaN films: Effects of impurities and dislocations

We report details of the calculation of the lattice thermal conductivity κ in wurtzite GaN. Numerical simulations are performed for n-type wurtzite GaN with different density of silicon dopants, point defects and threading dislocations. Using the material specific model we verified the experimentally observed linear decrease of the room-temperature thermal conductivity with the logarithm of the carrier density n. The decrease was attributed mostly to the increased phonon relaxation on dopants. Our calculations show that the increase in the doping density from 1017 to 1018 cm−3 leads to about a factor of 2 decrease in thermal conductivity from 1.77 W/cmu2009K to 0.86 W/cmu2009K. We have also established that the room-temperature thermal conductivity in GaN can be limited by dislocations when their density is high, e.g., ND>1010u2009cm−2. The obtained results are in good agreement with experimental data. The developed calculation procedure can be used for accurate simulation of self-heating effects in GaN-based devices.

Effect of dislocations on thermal conductivity of GaN layers

We report calculation of the lattice thermal conductivity in wurtzite GaN. The proposed model is material specific and explicitly includes phonon relaxation on threading dislocations and impurities typical for GaN. We have found that a decrease of the dislocation density by two orders of magnitude in GaN leads to a corresponding increase of the thermal conductivity from 1.31 to 1.97 W/cmu200aK. This theoretical prediction is in very good agreement with experimental data obtained from scanning thermal microscopy. The developed model can be used for thermal budget calculations in high-power density GaN devices.

Heat Removal in Silicon-on-Insulator Integrated Circuits With Graphene Lateral Heat Spreaders

Graphene was recently proposed as a material for heat removal owing to its extremely high thermal conductivity. We simulated heat propagation in silicon-on-insulator (SOI) circuits with and without graphene lateral heat spreaders. Numerical solutions of the heat-propagation equations were obtained using the finite-element method. The analysis was focused on the prototype SOI circuits with the metal-oxide-semiconductor field-effect transistors. It was found that the incorporation of graphene or few-layer graphene (FLG) layers with proper heat sinks can substantially lower the temperature of the localized hot spots. The maximum temperature in the transistor channels was studied as function of graphenes thermal conductivity and the thickness of FLG. The developed model and obtained results are important for the design of graphene heat spreaders and interconnects.

The Effect of Defects and Dopants on Thermal Conduction in GaN Films

We present a theoretical investigation of the effects of dislocations, impurities and dopants on the thermal conductivity of GaN layers. It is shown that the experimentally observed decrease of the room-temperature thermal conductivity with increasing doping density is a result of enhanced phonon relaxation on silicon dopant atoms. Scattering of acoustic phonons on free carriers plays a relatively minor role in GaN. The functional dependence of the thermal conductivity on doping density is in good agreement with experiment. A developed model can be used for thermal budget calculation in high-power density GaN devices.

Study of Cronin effect and nuclear modification of strange particles in d-Au and Au-Au collisions at 200 GeV in PHENIX.

Effects of strangeness on nuclear modification in d-Au and Au-Au collisions at 200 GeV are studied, in order to quantify the effects of quark content and mass. Measurements of ratios of the yields in central collisions to the yields in peripheral collisions are performed for A baryon and Φ meson. Found results show little dependence of particle suppression or enhancement on mass and strange content, but rather prominent difference in nuclear modification between mesons and baryons.

Carrier-Density Fluctuation Noise and the Interface Trap Density in GaN/AlGaN HFETs

The interface trap density N T is extracted from the experimental low-frequency noise data for GaN/AlGaN heterostructure field-effect transistors (HFETs). The trap density is determined based on the carrier-density fluctuation formalism. We show that the value of N T approximately defines the noise response of different GaN/AlGaN HFETs fabricated on the same wafer and it weakly depends on the gate bias. The dependence is due to the non-uniformity of the trap distribution. A model for computer simulation of the low-frequency noise in GaN devices is proposed.

Thermal conduction through diamond - silicon heterostructures

The interest to silicon - diamond structures was recently renewed motivated by industrys needs for composite substrates and better thermal management. In this work we investigated thermal conductivity and thermal boundary resistance (TBR) of ultrananocrystalline (UNCD) and microcrystalline diamond (MCD) films on silicon. The measurements were carried out using the transient plane source (TPS) technique. It was found that most of the silicon - synthetic heterostructures are rather resistive thermally with the TBR values of up to ∼ 10−6 m2K/W at room temperature. We established an importance of the trade-off between the structures characterized by the ultra-small diamond grain size with smooth silicon-diamond interface and those with larger grain size but rougher interface. It is shown that composite Si/Diamond wafers are promising at the elevated temperatures characteristic for operation of state-of-art electronic devices. The knowledge of TBR and heat conduction through silicon - diamond heterostructures is important for further development of composite substrates for electronic and optoelectronic industries.