The room temperature growth of two-dimensional van der Waals (2D-vdW) materials is indispensable for state-of-the-art nanotechnology. Low temperature growth supersedes the requirement of elevated growth temperatures accompanied with high thermal budgets. Moreover, for electronic applications, low or room temperature growth reduces the possibility of intrinsic film-substrate interfacial thermal diffusion related deterioration of the functional properties and the consequent deterioration of the device performance. Here, we demonstrated the growth of ultrawide-bandgap boron nitride (BN) at room temperature by using the pulsed laser deposition (PLD) process, which exhibited various functional properties for potential applications. Comprehensive chemical, spectroscopic and microscopic characterizations confirmed the growth of ordered nanosheet-like hexagonal BN (h-BN). Functionally, the nanosheets show hydrophobicity, high lubricity (low coefficient of friction), and a low refractive index within the visible to near-infrared wavelength range, and room temperature single-photon quantum emission. Our work unveils an important step that brings a plethora of potential applications for these room temperature grown h-BN nanosheets as the synthesis can be feasible on any given substrate, thus creating a scenario for "h-BN on demand" under a frugal thermal budget.

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena.

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band XPS, FTIR, Raman) and microscopic (AFM and STEM) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (234 V) as compared to GaN (168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step towards bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

Army Research Laboratory (ARL) is developing radio frequency (RF) field-effect-transistors (FETs) on hydrogen-terminated, single-crystal diamond surfaces. By employing advanced fabrication methods, we achieve state-of-the-art device performance with gate lengths below 100 nm. We are exploring methods to improve the stability of fabricated FETs, which is critical for maturation of the technology and its commercial acceptance. DC and RF measurement data will be reviewed and discussed within the framework of improving device yield and reliability.

High power radio frequency (RF) transfer-doped diamond field effect transistors (FETs) are being fabricated at the Army Research Laboratory (ARL). To implement these into radar systems we have a parallel effort to extract accurate compact models from their measured DC and RF data. At this early stage we are using the commercially available Angelov model and will discuss fitting the model parameters and how their parameter values differ from GaN and GaAs FETs. Results indicate good model prediction of measured results in some cases. Also, model extraction can indicate areas of the device that needs greater attention for improved performance such as the access region resistance. Furthermore, in the saturation region of operation these transistors exhibit a hole saturation velocity of 5 × 106 cm/s obtained from extracted model parameters.

In this report, the thermal performance of a hydrogen (H)-terminated diamond field-effect transistor (FET) is investigated using Raman spectroscopy and electrothermal device modeling. First, the thermal conductivity (κdiamond) of the active diamond channel was determined by measuring the temperature rise of transmission line measurement structures under various heat flux conditions using nanoparticle-assisted Raman thermometry. Using this approach, κdiamond was estimated to be 1860 W/m K with a 95% confidence interval ranging from 1610 to 2120 W/m K. In conjunction with measured electrical output characteristics, this κ was used as an input parameter for an electrothermal device model of an H-terminated diamond FET. The simulated thermal response showed good agreement with surface temperature measurements acquired using nanoparticle-assisted Raman thermometry. These diamond-based structures were highly efficient at dissipating heat from the active device channel with measured device thermal resistances as low as ∼1 mm K/W. Using the calibrated electrothermal device model, the diamond FET was able to operate at a very high power density of 40 W/mm with a simulated temperature rise of ∼33 K. Finally, the thermal resistance of these lateral diamond FETs was compared to lateral transistor structures based on other ultrawide bandgap materials (Al0.70Ga0.30N, β-Ga2O3) and wide bandgap GaN for benchmarking. These results indicate that the thermal resistance of diamond-based lateral transistors can be up to ∼10× lower than GaN-based devices and ∼50× lower than other UWBG devices.

Surface induced transfer doping (SITD) is a novel, highly efficiency doping technique that is being used to invoke the p-type surface conductivity of intrinsic diamond for high-frequency, high-power electronic devices. In the SITD process, a high electron affinity (EA) thin film acceptor layer is interfaced with the hydrogenated diamond surface with negative electron affinity (NEA) to induce the effective p-type doping on the diamond surface. Overall, device performance of the SITD doped devices is contingent on the type and quality of the interface between the acceptor layer and hydrogenated diamond surfaces. Motivated by this, our internal theoretical modeling efforts based on a hybrid approach of machine learning and first principle calculations have focused on performing bottom-up design of novel acceptor layers with higher stability and improved device performance, e.g., doped TMOs and 2D layer. In this talk, recent results from our predictive modeling effort will be presented.

Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which could be unreachable experimentally. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (001), (013), (1 1 0), (1 1 3), and (1 1 1) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation on these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (001), (110), and (113) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity on the diamond surfaces, a key requirement for high-field, high-frequency device applications.

We report a first-principles study of the structural and electronic properties of two-dimensional (2D) layer/hydrogen-terminated diamond (100) heterostructures. Both the 2D layers exhibit weak van-der-Waals (vdW) interactions and develop rippled configurations with the H-diamond (100) substrate to compensate for the induced strain. The adhesion energy of the hexagonal boron nitride (hBN) layer is slightly higher, and it exhibits a higher degree of rippling compared to the graphene layer. A charge transfer analysis reveals a small amount of charge transfer from the H-diamond (100) surface to the 2D layers, and most of the transferred charge was found to be confined within the vdW gap. In the graphene/H-diamond (100) heterostructure, the semi-metallic characteristic of the graphene layer is preserved. On the other hand, the hBN/H-diamond (100) heterostructure shows semiconducting characteristics with an indirect bandgap of 3.55 eV, where the hBN layer forms a Type-II band alignment with the H-diamond (100) surface. The resultant conduction band offset and valence band offset are 0.10 eV and 1.38 eV, respectively. A thin layer of hBN offers a defect-free interface with the H-diamond (100) surface and provides a layer-dependent tunability of electronic properties and band alignment for surface-doped diamond field effect transistors.

Wereport on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5.

We report on optimisation of the environmental stability and high temperature operation of surface transfer doping in hydrogen-terminated diamond using MoO 3 and V 2 O 5 surface acceptor layers . In-situ annealing of the hydrogenated diamond surface at 400 °C was found to be crucial to enhance long-term doping stability. High temperature sheet resistance measurements up to 300 °C were performed to examine doping thermal stability. Exposure of MoO 3 and V 2 O 5 transfer-doped hydrogen-terminated diamond samples up to a temperature of 300 °C in ambient air showed significant and irreversible loss in surface conductivity. Thermal stability was found to improve dramatically however when similar thermal treatment was performed in vacuum or in ambient air when the oxide layers were encapsulated with a protective layer of hydrogen silsesquioxane (HSQ). Inspection of the films by X-ray diffraction revealed greater crystallisation of the MoO 3 layers following thermal treatment in ambient air compared to the V 2 O 5 films which appeared to remain amorphous. These results suggest that proper encapsulation and passivation of these oxide materials as surface acceptor layers on hydrogen-terminated diamond is essential to maximise their environmental and thermal stability.

Initial results for diamond RF transistor technology are presented. Field Effect Transistors (FETs) were fabricated with gate lengths (L g ) ranging from 4μm to 50nm. The FETs have total gate width (W g ) of 40 or 120μm. L g =100 nm devices show DC drain current I D =600 mA/mm (V GS =-3V, V DS =-10V) with transconductance g m =140mS/mm (V GS =-0.3V, V DS =-4V). Small signal S-parameters were measured to evaluate the high-frequency performance of the diamond FETs. Extrinsic f t and f max were measured to be 41GHz and 44GHz, respectively. Load pull measurements were used to characterize the devices under large signal excitation. The L g =200nm, W g =40μm device, tested at 2GHz, shows peak efficiency of 30.5% at V DS =-5V. Both peak gain of 19.5dB and peak output power density of 0.66W/mm were achieved at V DS =30V. Biasing the device at V DS =-15V provides a trade off point for the large signal parameters - gain of ~15dB, efficiency of ~20%, and output power of ~0.5W/mm.

In recent years, large investments into the research of semiconducting two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have elucidated interesting device related physical phenomena such as valleytronics [1], 2D superconductivity [2], 2D excitonic effects [3] and vertical tunneling [4]. TMDs offer layer-dependent chemical tunability of electronic and optoelectronic properties governed by interlayer van der Waals (vdW) forces [5]. Because of their layered nature, these low-dimensional materials can be combined to form multifunctional heterostructure materials exhibiting entirely new physical systems offering new degrees of flexibility in designing electronics, optoelectronics and other novel devices [6], [7]. In the last couple of years, the focus in the 2D materials research have shifted from exploration of proof-of-concept devices using mechanically exfoliated materials to more advanced device processing using high-quality large-scale growth based on advanced scalable vdW-epitaxy techniques such as powder vapor deposition (PVD) and chemical vapor deposition (CVD).

Charge carrier trapping in diamond surface conduction field effect transistors (FETs) has been analyzed. For these devices two methods were used to obtain a negative electron affinity diamond surface; either plasma hydrogenation or annealing in an H2 environment. In both cases the Al2O3 gate dielectric can trap both electrons and holes in deep energy levels with emission timescales of seconds, while the diamond – Al2O3 interface traps exhibit much shorter time scales in the microsecond range. Capacitance-Voltage (CV) analysis indicates that these interface traps exhibit acceptor-like characteristics. Correlation with CV based free hole density measurements indicates that the conductance based interface trap analysis provides a method to quantify surface characteristics that lead to surface conduction in hydrogenated diamond where atmospheric adsorbates provide the acceptor states for transfer doping of the surface.

Abstract The emergence of graphene as a viable active material for electronic applications has sparked an explosion of research studying the fundamental electrical performance of it and other two-dimensional (2D) materials. As transistors continue to shrink in size, layered 2D materials, such as graphene, phosphorene, and the transition metal dichalcogenides, can fulfill the role as the active channel without the limitations primarily found with silicon. This discussion strives to provide an overview of the emerging research into the electrical behavior of 2D materials from simple field-effect transistors to state-of-the-art integrated circuits and devices. Experimental testing considerations and circuit design issues involving 2D material-based devices are also examined. The potential to harness 2D materials for digital logic circuits, radio frequency applications, and other novice device concepts is explored.