Julian Anaya

Thermal conductivity of ultrathin nano-crystalline diamond films determined by Raman thermography assisted by silicon nanowires

The thermal transport in polycrystalline diamond films near its nucleation region is still not well understood. Here, a steady-state technique to determine the thermal transport within the nano-crystalline diamond present at their nucleation site has been demonstrated. Taking advantage of silicon nanowires as surface temperature nano-sensors, and using Raman Thermography, the in-plane and cross-plane components of the thermal conductivity of ultra-thin diamond layers and their thermal barrier to the Si substrate were determined. Both components of the thermal conductivity of the nano-crystalline diamond were found to be well below the values of polycrystalline bulk diamond, with a cross-plane thermal conductivity larger than the in-plane thermal conductivity. Also a depth dependence of the lateral thermal conductivity through the diamond layer was determined. The results impact the design and integration of diamond for thermal management of AlGaN/GaN high power transistors and also show the usefulness of the nanowires as accurate nano-thermometers.

Thermal conductivity of bulk GaN—Effects of oxygen, magnesium doping, and strain field compensation

The effect of oxygen doping (n-type) and oxygen (O)-magnesium (Mg) co-doping (semi-insulating) on the thermal conductivity of ammonothermal bulk GaN was studied via 3-omega measurements and a modified Callaway model. Oxygen doping was shown to significantly reduce thermal conductivity, whereas O-Mg co-doped GaN exhibited a thermal conductivity close to that of undoped GaN. The latter was attributed to a decreased phonon scattering rate due the compensation of impurity-generated strain fields as a result of dopant-complex formation. The results have great implications for GaN electronic and optoelectronic device applications on bulk GaN substrates.

Thermal characterization of polycrystalline diamond thin film heat spreaders grown on GaN HEMTs

Polycrystalline diamond (PCD) was grown onto high-k dielectric passivated AlGaN/GaN-on-Si high electron mobility transistor (HEMT) structures, with film thicknesses ranging from 155 to 1000 nm. Transient thermoreflectance results were combined with device thermal simulations to investigate the heat spreading benefit of the diamond layer. The observed thermal conductivity (κDia) of PCD films is one-to-two orders of magnitude lower than that of bulk PCD and exhibits a strong layer thickness dependence, which is attributed to the grain size evolution. The films exhibit a weak temperature dependence of κDia in the measured 25–225 °C range. Device simulation using the experimental κDia and thermal boundary resistance values predicts at best a 15% reduction in peak temperature when the source-drain opening of a passivated AlGaN/GaN-on-Si HEMT is overgrown with PCD.

Effect of grain size of polycrystalline diamond on its heat spreading properties

The exceptionally high thermal conductivity of polycrystalline diamond (>2000 W m−1 K−1) makes it a very attractive material for optimizing the thermal management of high-power devices. In this paper, the thermal conductivity of a diamond sample capturing grain size evolution from nucleation towards the growth surface is studied using an optimized 3ω technique. The thermal conductivity is found to decrease with decreasing grain size, which is in good agreement with theory. These results clearly reveal the minimum film thickness and polishing thickness from nucleation needed to achieve single-crystal diamond performance, and thus enable production of an optimal polycrystalline diamond for heat-spreading applications.

The effects of grain size and grain boundary characteristics on the thermal conductivity of nanocrystalline diamond

Molecular dynamics simulation was used to study the effects of each grain dimension and of grain boundary characteristics on the inter-grain thermal boundary resistance (TBR) and intragrain thermal conductivity of nanocrystalline diamond. The effect of the grain boundaries perpendicular to the heat flow was studied using a multiple slab configuration, which greatly reduced the artifacts associated with the heat source/sink. The TBR between the slabs was found to be more sensitive to the atomic arrangement at the boundary than to the tilt angle between the slabs. When the atomic arrangement at the interface was altered from the minimum energy configuration, the TBR increased by a factor of three, suggesting that a sub-optimal interface quality between the grains could play a large role in reducing the thermal conductivity of nanocrystalline diamond. The thermal conductivity between the boundaries was found to be similar to the bulk value, even when the boundaries were only 25 nm apart. The effect of grain ...

Thermal management of GaN-on-diamond high electron mobility transistors: Effect of the nanostructure in the diamond near nucleation region

The integration of diamond in ultra-high power GaN HEMT devices has demonstrated to be a very promising strategy to increase the device lifetime and their thermal management. Typically polycrystalline diamond films rather than single crystal diamond are used for this purpose, however for this material the thermal transport in the near-nucleation site is strongly affected by the small grain size and the accumulation of defects in this region. Here we modeled the phonon thermal transport in diamond, including the effect of the polycrystalline structure, showing that its thermal conductivity exhibits very different properties to those observed in single crystal diamond; namely, the thermal conductivity is severely reduced, the grain structure may induce anisotropy in the heat conduction and also a strong variation of the thermal conductivity from the nucleation and following the diamond growth direction is observed. All these features are included in a full thermal model of a GaN high power amplifier, showing their impact on the thermal management of the device. We show that including the full description of the polycrystalline diamond thermal conductivity is fundamental to accurately assess the thermal management of these devices, and thus to optimize their design.

Electromagnetic field enhancement effects in group IV semiconductor nanowires. A Raman spectroscopy approach

Semiconductor nanowires (NWs) are the building blocks of future nanoelectronic devices. Furthermore, their large refractive index and reduced dimension make them suitable for nanophotonics. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. Micro-Raman spectroscopy has been used as a characterization tool for semiconductor nanowires. The light/nanowire interaction can be experimentally assessed through the micro-Raman spectra of individual nanowires. As compared to both metallic and dielectric nanowires, semiconductor nanowires add additional tools for photon engineering. In particular, one can grow heterostructured nanowires, both axial and radial, and also one could modulate the doping level and the surface condition among other factors than can affect the light/NW interaction. We present herein a study of the optical response of group IV semiconductor nanowires to visible photons. The study ...

Barrier Layer Optimization for Enhanced GaN-on-diamond Device Cooling

GaN-on-diamond device cooling can be enhanced by reducing the effective thermal boundary resistance (TBReff) of the GaN/diamond interface. The thermal properties of this interface and of the polycrystalline diamond grown onto GaN using SiN and AlN barrier layers as well as without any barrier layer under different growth conditions are investigated and systematically compared for the first time. TBReff values are correlated with transmission electron microscopy analysis, showing that the lowest reported TBReff (∼6.5 m2 K/GW) is obtained by using ultrathin SiN barrier layers with a smooth interface formed, whereas the direct growth of diamond onto GaN results in one to two orders of magnitude higher TBReff due to the formation of a rough interface. AlN barrier layers can produce a TBReff as low as SiN barrier layers in some cases; however, their TBReff are rather dependent on growth conditions. We also observe a decreasing diamond thermal resistance with increasing growth temperature.

Mechanisms driving the catastrophic optical damage in high-power laser diodes

The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. The full sequence of the degradation follows different phases: in the first phase, a weak zone of the laser is incubated and the temperature is locally increased there; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption in a process driven by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this process. The influence of the progressive deterioration of the thermal conductivity of the laser structure as a result of the degradation during the laser operation will be addressed.

Degradation signatures of high power laser diodes

Rapid and catastrophic degradation of high power laser diodes occur because of the generation of extended defects inside the active parts of the laser structure during the laser operation. Local hot spots play a major role as actuators of the driven force leading to the formation of extended crystal defects. The laser power threshold for degradation is very sensitive to the packaging induced stress, and the thermal conductivity of the multilayer structure. The thermal conductivity of the QW and the barriers is suppressed by the low dimensionality but also by the quality of the interfaces being a major actor of the laser diode degradation. Modelling the thermal stresses induced by the hot spots in the active region of the diode permits to describe the degradation mechanisms of high power laser diodes.