J. W. Pomeroy

• MSci
• Research Associate at University of Bristol

Thick metamorphic buffers are considered indispensable for III‐V semiconductor heteroepitaxy on large lattice and thermal‐expansion mismatched silicon substrates. However, III‐nitride buffers in conventional GaN‐on‐Si high electron mobility transistors (HEMT) impose a substantial thermal resistance, deteriorating device efficiency and lifetime by t...

A electroluminescence (EL) based methodology has been devised to screen AlGaN/GaN Super-Lattice Castellated Field Effect Transistors (SLCFETs). EL intensity captured during off-state stressing has been correlated with an increase in gate leakage current after stress. Two off-state constant-voltage stress conditions were used, both applied over a st...

An electroluminescence (EL) based method is introduced for micrometer-spatial resolution quantitative threshold voltage mapping across transistors, illustrated on GaN HEMTs. The threshold voltage determined using the optical method is confirmed to be consistent with the conventional electrical method that averages a whole device. With this approach...

Advanced millimeter-wave (mm-wave) transceiver systems, including future mm-wave 5G and 6G mobile networks, are of great interest to support high-data-rate communications (e.g., 10 Gb/s or higher) and backhaul communications with >50 Gb/s. Since the E band and beyond can also support multigigahertz bandwidths, there is also growing interest in phas...

Characterizing electric fields in semiconductor devices using electric field-induced second-harmonic generation (EFISHG) has opened new opportunities for an advanced device design. However, this new technique still has challenges due to the interference between background second-harmonic generation (SHG) and EFISHG generated light. We demonstrate t...

Thick metamorphic buffers are perceived to be indispensable for the heteroepitaxial integration of III-V semiconductors on silicon substrates with large thermal expansion and lattice mismatches. However, III-nitride buffers in conventional GaN-on-Si high electron mobility transistor (HEMT) heterostructures impose a substantial thermal resistance, t...

Advancing Silicon (Si) technology beyond Moore's law through 3D architectures requires highly efficient heat management methods compatible with foundry processes. While continued increases in transistor density can be achieved through 3D architectures, self‐heating in the upper tiers degrades the performance. Self‐heating is a critical problem for...

An innovative Electroluminescence (EL) method is introduced for precisely probing and mapping local threshold voltage variations within devices. Consistency of the optical to electrical determination of threshold voltage is demonstrated. We can probe variations in threshold voltage along the gate finger with 1 µm spatial resolution, and less than 1...

Thermal interface materials are crucial to minimize the thermal resistance between a semiconductor device and a heat sink, especially for high-power electronic devices, which are susceptible to self-heating-induced failures. The effectiveness of these interface materials depends on their low thermal contact resistance coupled with high thermal cond...

The transient thermal response of GaN high-electron-mobility transistors (HEMTs) is studied using transient thermal thermoreflectance (TTR) measurements. The change in the thermal impedance at various ambient temperatures is observed and analyzed using finite-element (FE) simulation and thermal equivalent circuits. A temperature-dependent Cauer mod...

AlGaN/GaN-based Superlattice Castellated Field Effect Transistors (SLCFET) are the foundation for high power RF amplifiers and switches for future radars, although their reliability is not yet well understood. Transistor latching is observed in GaN transistors for the first time. At the latching condition, drain current (ID) transits from an OFF-St...

Hot electron effects have been studied in multichannel AlGaN/GaN-based Superlattice Castellated Field Effect Transistors (SLCFET). Current-Voltage (I-V) and simulations could identify channel position-dependent impact ionization within the device fins. Electroluminescence (EL) emission is observed simultaneously with the impact ionization, whose sp...

High thermal conductivity and an appropriate coefficient of thermal expansion are the key features of a perfect heat spreader for electronic device packaging, especially for applications with increased power density and the increasing demand for higher reliability and semiconductor device performance. For the past decade, metal-diamond composites h...

We report on the growth of monoclinic β- and orthorhombic κ-phase Ga2O3 thin films using liquid-injection metal-organic chemical vapor deposition on highly thermally conductive 4H-SiC substrates using gallium (III) acetylacetonate or tris(2,2,6,6-tetramethyl-3,5-heptanedionato) gallium (III). Both gallium precursors produced the β phase, while only...

Heterostructures of Ga[Formula: see text]O[Formula: see text] with other materials such as Si, SiC or diamond, are a possible way of addressing the low thermal conductivity and lack of p-type doping of Ga[Formula: see text]O[Formula: see text] for device applications, as well as of improving device reliability. In this work we study the electrical...

Integration of β-Ga 2 O 3 with high thermal conductivity materials such as diamond has been considered due to β-Ga 2 O 3 's low and anisotropic thermal conductivity, reaching only 27 W m –1 K –1 . However, the effect of crystallographic orientation on thermal interface resistance has not been studied extensively, which is relevant for potential dev...

The ever‐increasing power density is a major trend for electronics applications from dense computing to 5G/6G networks. Joule heating and resulting high temperature in the device channel due to the increased power density results in performance degradation and premature failure. Diamond integration near the hot spot can spread the heat by increasin...

We characterized the electric field distribution of GaN-on-GaN p–n diodes with partially compensated ion-implanted edge termination (ET) using an electric field induced second harmonic generation technique (EFISHG). The distributed electric field from the anode to the outer edge of the ET demonstrates the effectiveness of the ET structure. However,...

Direct bonded Si-on-SiC is an interesting alternative to silicon-on-insulator (SOI) for improved thermal management in power conversion and radio frequency applications in space. We have used transient thermoreflectance and finite element simulations to characterize the thermal properties of direct bonded Si-on-4H–SiC samples, utilizing a hydrophob...

An improved analysis of the interfacial toughness using nanoindentation induced blistering of thin films on stiff substrates is demonstrated on GaN-on-diamond. The Hutchinson-Suo analysis requires accurate measurement of blister dimensions, conventionally measured using 2-D line-scans from 3-D topographical maps. The new meteorology overcomes short...

The drain current temperature dependence is an efficient way to determine the channel temperature in semiconductor devices; however, it has been challenging to use due to the potential interference of trapping effects. A trapping tolerant method is proposed, illustrated here for Ga2O3 MOSFETs, making in situ temperature measurements possible, allow...

The design space of Ga₂O₃-based devices is severely constrained due to its low thermal conductivity and absence of viable p-type dopants. In this work, we discuss the limits of operation of a novel Ga₂O₃-Al₂O₃-diamond-based super-junction device concept, which can alleviate the constraints associated with Ga₂O₃-based devices. The improvements achie...

In this paper, we investigate the charge trapping in power AlGaN/GaN high electron mobility transistors which occurs in ON-state operation (V DS = 40 V, V GS = 0 V, I DS = 0.18 A mm⁻¹). By analysing the dynamic ON-resistance (R ON) after OFF-state and ON-state stress in devices with different SiN x passivation stoichiometries, we find that this cha...

We investigate the accuracy and reliability of temperature mapping using scanning thermal microscopy (SThM) in contact and PeakForce tapping mode on the example of a GaN-on-SiC high electron mobility transistor (HEMT). HEMT steady-state and transient surface temperatures are extracted from SThM measurements to study the method’s accuracy and transi...

Electric fields drive the degradation of wide-bandgap semiconductor devices. However, directly mapping the electric field inside an active device region remains challenging. Here we show that electric-field-induced second harmonic generation can be used to map the electric field in the device channel of GaN-based high-electron-mobility transistors...

Future GaN based radio frequency (RF) high-electron-mobility-transistors (HEMTs) can enable increased areal power dissipation by, for example, integrating GaN device layers with high thermal conductivity diamond substrates. To maximise the benefit of the ultra-high power density electronic devices, improved package level cooling methods are needed...

Reliable operation of high power GaN amplifiers at maximum performance relies on the mutual optimization of several design parameters constrained by a defined thermal budget. On high thermal conductivity, substrates, such as SiC and diamond, undergo small changes within the design that can lead to drastic changes in channel temperature. We utilize...

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stress...

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes s...

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stress...

Carbide forming interlayers, such as amorphous silicon nitride, are typically used for GaN-on-diamond heterogenous integration. This interlayer has a low thermal conductivity, introducing an additional extrinsic interfacial thermal resistance. It may therefore be advantageous to omit this layer, directly bonding GaN-to-diamond (van der Waals bond)....

We report a method of growing a diamond layer via chemical vapour deposition (CVD) utilizing a mixture of microdiamond and nanodiamond seeding to give a low effective thermal boundary resistance (TBReff) for heat-spreading applications in high-frequency, high-power electronic devices. CVD diamond was deposited onto thin layers of both GaN and AlN o...

The trend of miniaturization and rapid progress in the cost-competitive microelectronic industry require high resolution, fast, accurate and cost-effective thermal characterization techniques. These techniques aid the assessment of reliability and performance bench-marking of new device designs for the realistic operation conditions. We present a t...

GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm × 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. At...

The trend of electronic device miniaturization, from the micro-scale to the nano-scale, presents a temperature measurement challenge. The available techniques have limitations in terms of either resolution, calibration, acquisition time or equipment cost. Here we demonstrate a thermography technique called hyperspectral quantum rod thermal imaging...

β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin-channel MOSFETs were evaluated using both dc and pulsed I-V measurements. The reported pulsed I-V technique was used to study selfhe...

Growth of $>$100 $\mu$m thick diamond layers adherent on aluminium nitride with low thermal boundary resistance between diamond and AlN is presented in this work. Thermal barrier resistance(TBR$_{\mbox{\tiny{eff}}}$) was found to be in the range of 16 m$^2$K/GW which is a large improvement on the current state-of-the-art. While thick films failed t...

Heat extraction from novel GaN/AlGaN superlattice castellated field effect transistors developed as an RF switch is studied. The device thermal resistance was determined as 19.1 ± 0.7 K/(W/mm) from a combination of Raman thermography measurements, and gate resistance thermometry. Finite element simulations were used to predict the peak temperatures...

Growth of $>$100 $\mu$m thick diamond layer adherent on aluminium nitride is presented in this work. While thick films failed to adhere on untreated AlN films, hydrogen/nitrogen plasma treated AlN films retained the thick diamond layers. Clear differences in zeta potential measurement confirms the surface modification due to hydrogen/nitrogen plasm...

Hexagonal boron nitride (h-BN) has been predicted to exhibit an in-plane thermal conductivity as high as ~ 550 W m−1K−1at room temperature, making it a promising thermal management material. However, current experimental results (220–420 W m−1K−1) have been well below the prediction. Here, we report on the modulation of h-BN thermal conductivity by...

In this paper, we demonstrate Q-band power performance of carbon doped AlN/GaN high electron mobility transistors (HEMTs) using a deep sub-micrometer gate length (120 nm). With a maximum drain current density I D of 1.5 A/mm associated to a high electron confinement and an extrinsic transconductance g m of 500 mS/mm, this structure shows excellent...

Intrinsic limits to temperature-dependent substrate loss for GaN-on-Si technology, due to the change in resistivity of the substrate with temperature, are evaluated using an experimentally validated device simulation framework. Effect of room temperature substrate resistivity on temperature-dependent coplanar waveguide (CPW) line loss at various op...

From transmission electron microscope (TEM) observation, a 25 nm thick amorphous layer was confirmed at the diamond/Si bonding interface without annealing, the amorphous layer thickness decreased with the annealing temperature. No cracking even in nanometer scale occurred even after high-temperature annealing at 800 °C. From in-situ micro-Raman mon...

We demonstrate that GaN-on-diamond technology with an ultra-thin GaN buffer and interface layer offers excellent thermal resistance alongside good electrical performance. Two device sets were investigated, one with 354 nm thick GaN buffer and 17 nm thick interface layers, the other with 700 nm thick GaN buffer and 36 nm thick interface layers. The...

Intrinsic limits to temperature-dependent substrate loss for GaN-on-Si technology, due to the change in resistivity of the substrate with temperature, are evaluated using an experimentally validated device simulation framework. Effect of room temperature substrate resistivity on temperature-dependent CPW line loss at various operating frequency ban...

In this work, we investigate the bow of GaN-on-Si wafers with CVD diamond using the finite element modelling package COMSOL® Multiphysics. We demonstrate a series of models, that show the effects of wafer thicknesses of the layers and variation of the CVD diamond thickness caused by a nominal microwave plasma CVD system. A typical model of CVD diam...

β-Ga <sub xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is an attractive material for highvoltage applications and has the potential for monolithically integrated RF devices. A combination of Raman nanoparticle thermometry measurement and thermal simulation has been used to measure the...

GaN devices integrated with dissimilar substrates have transformed electronic and optoelectronic applications. However, an effective thermal resistance (TBReff) exists between the GaN layer and the dissimilar substrates typically, which can potentially cause a major heat transport bottleneck. A non-invasive method for monitoring the TBReff of bare...

Comparison between pulsed and CW large signal RF performance of field-plated $\beta $ -Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> MOSFETs has been reported. Reduced self-heating wh...

Current GaN-on-SiC RF electronic devices, although they already deliver outstanding performances are limited in power density mainly by the SiC substrate thermal conductivity. Integration of GaN with the highest thermal conductivity material known to mankind, diamond, offers the potential to increase power density in GaN devices by at least a facto...

There are huge differences in dynamic on-resistance Ron, also known as current-collapse, between current GaN power HEMT technologies. Here we illustrate this fact with dynamic Ron measurements on two commercially available devices from 2 different manufacturers, with one showing more than a factor of 2 increase in dynamic Ron after OFF-state drain...

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 investigate...

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 observ...

We review the Raman thermography technique, which has been developed to determine the temperature in and around the active area of semiconductor devices with submicron spatial and nanosecond temporal resolution. This is critical for the qualification of device technology, including for accelerated lifetime reliability testing and device design opti...

The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond wafers was studied using in situ focused ion beam cross-sectioning and scanning electron microscopy imaging. Microstructural studies revealed that the seeding conditions are a critical parameter to obtain an optimal material, allowing the manufacture of...

The nature of hot electron electroluminescence (EL) in AlGaN/GaN high electron mobility transistors is studied and attributed to Bremsstrahlung. The spectral distribution has been corrected, for the first time, for interference effects due to the multilayered device structure, and this was shown to be crucial for the correct interpretation of the d...

An experimental method to measure the gate metal temperature of GaN-based high electron mobility transistors is demonstrated. The technique is based on transient thermoreflectance measurements performed from the backside of the device. The thermoreflectance coefficient of the gate metal was calibrated by correlating the relative change of its optic...

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 si...

Achieving robust mechanical and thermal properties in GaN-on-diamond is critically important for reliable next-generation high-power electronics based on this material system. The work described here demonstrates that excellent stress management and interfacial strength has been achieved, as well as homogeneous interfacial thermal properties across...

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 si...

The thermal properties of GaN-on-diamond high-electron mobility transistor (HEMT) wafers from 25 °C to 250 °C are reported. The effective thermal boundary resistance between GaN and diamond decreases at elevated temperatures due to the increasing thermal conductivity of the amorphous SiNx interlayer, therefore potentially counteracting thermal runa...

Diamond has demonstrated excellent properties for thermal management of GaN electronics. In this presentation, we review our latest results on the diamond properties near the GaN-on-diamond interface, and the mechanical stability of the GaN-on-diamond interface and its properties.

The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power microwaveelectronic devices was studied using a micro-pillar based in situmechanical testing approach combined with an optical investigation of the stress and heat transfer across interfaces. We find the GaN/diamond interface to be thermo-mechanically...

Solid immersion lenses (SILs) are shown to greatly enhance optical spatial resolution when measuring AlGaN/GaN High Electron Mobility Transistors (HEMTs), taking advantage of the high refractive index of the SiC substrates commonly used for these devices. Solid immersion lenses can be applied to techniques such as electroluminescence emission micro...

Channel temperature is a key parameter for accelerated life testing in GaN HEMTs. It is assumed that self-heating is similar in RF and DC operations and that DC test results can be applied to RF operation. We investigate whether this assumption is valid by using an experimentally calibrated, combined electrical and thermal model to simulate Joule h...

Microwave and power electronics based on GaN enables the performance of systems and their safe operating area to be driven to ‘extremes’. One of the major issues that then arises is thermal management. This includes heat transfer limitations across interfaces, however also the need of incorporating novel high thermal conductivity materials such as...

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in...

Reducing GaN-on-diamond interfacial thermal resistance is crucial to maximize the thermal benefit of diamond substrates for high power transistor applications. In this work, we demonstrate a rapid, contactless transient thermoreflectance technique to assess the interfacial thermal resistance of as-grown GaN-on-diamond wafers.

GaN-on-Diamond wafers and transistors have emerged as a leading technology for use in next generation radar and microwave defense applications. This is due to diamond's excellent thermal properties. However, accurate measurement of the thermal properties of GaN-on-diamond wafers is challenging. This paper reviews state-of-the-art techniques and res...

Integration of chemical vapor deposited polycrystalline diamond offers promising thermal performance for GaN-based high power radio frequency amplifiers. One limiting factor is the thermal barrier at the GaN to diamond interface, often referred to as the effective thermal boundary resistance (TBReff). Using a combination of transient thermoreflecta...

The thermal conductivity of the GaN buffer layer in AlGaN/GaN devices can be determined by measuring the vertical temperature gradient through this layer. In this work, diamond micro-thermometers and standard micro-Raman thermography were used to determine the surface and volumetric depth average temperature, respectively, of the carbon-doped GaN b...

GaN in microwave and power electronics enables performances of systems and their safe operating area to be driven to 'extremes'. When this challenge is taken up, thermal management is one of the major issues to be addressed. This includes heat transfer limitations across interfaces, however also the potential of incorporating novel materials such a...

Low thermal resistance GaN-on-diamond wafers offer enhanced thermal management with respect to GaN-on-SiC devices. The GaN/diamond interfacial thermal resistance can contribute significantly to the total device thermal resistance and must therefore be minimized to gain the maximum benefit from GaN-on-diamond. A contactless thermoreflectance measure...