Showing papers by "David J. Meyer published in 2016"

Highly repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown by molecular beam epitaxy

Abstract: AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm2 and a peak-to-valley current ratio of ≈1.15 across different sizes.

Epitaxial Lift-Off and Transfer of III-N Materials and Devices from SiC Substrates

Abstract: In this paper, electrical characterization results of N-polar GaN high-electron-mobility transistors that have been released from a 6H-SiC wafer and manually transferred to a Si wafer using a novel epitaxial lift-off (ELO) technique are presented. This recently developed ELO method uses a thin sacrificial layer of Nb2N, a hexagonal epitaxial conductor with less than 1% lattice mismatch to 4H- and 6H-SiC, to serve as the template for III-N device heterostructure growth. Measured results of transferred devices indicate that electron transport properties and low power density electrical performance are nominally unchanged relative to values measured before release. This technique has several advantages over competing ELO techniques, such as the well-known smart cut method, including bonding-ready released material with atomically-smooth backsides (≤ 0.5 nm rms), easy substrate reclaim with indefinite recycling potential, and a transfer process that can be performed after full front-side device processing and yield screening has been completed.

Characterization of molecular beam epitaxy grown β-Nb2N films and AlN/β-Nb2N heterojunctions on 6H-SiC substrates

Abstract: β-Nb2N films and AlN/β-Nb2N heterojunctions were grown by molecular beam epitaxy (MBE) on 6H-SiC. The epitaxial nature and β-Nb2N phase were determined by symmetric and asymmetric high-resolution X-ray diffraction (HRXRD) measurements, and were confirmed by grazing incidence diffraction measurements using synchrotron photons. Measured lattice parameters and the in-plane stress of β-Nb2N on 6H-SiC were c = 5.0194 A, a = 3.0558 A, and 0.2 GPa, respectively. The HRXRD, transmission electron microscopy, and Raman spectroscopy revealing epitaxial growth of AlN/β-Nb2N heterojunctions have identical orientations with the substrate, abrupt interfaces, and bi-axial stress of 0.88 GPa, respectively. The current finding opens up possibilities for the next generation of high-power devices that cannot be fabricated at present.

Demonstration of GaN HyperFETs with ALD VO 2

Abstract: Owing to strong electron-electron interactions, transition metal oxide materials can exhibit multiple phases with vastly different electronic, magnetic, structural, and thermal properties. Reversible control of the transitions between these phases by electronic means can give rise to completely novel devices which can provide new functionalities and help to overcome limits of traditional semiconductor devices [1, 2]. VO 2 is a transition metal oxide material that exhibits a metal-insulator transition (MIT) at a temperature of ∼67 C [3]. Recently, by coupling VO 2 to the source of traditional semiconductor MOSFET devices, hybrid-phase-transition-FET (hyper-FET) devices were demonstrated [4]. These HyperFETs showed steep switching slope less than the room-temperature Boltzmann switching limit of ∼60 mV/dec [4]. GaN based electronics has emerged as an enabler of high-speed and high-power RF and microwave electronics [5], and is currently being investigated intensively for next-generation high-voltage power electronics [6,7], as well as steep-switching based low-power digital electronics [8]. In this work, we combine ALD-grown VO 2 with III-Nitride high-electron mobility transistors (HEMTs) to realize GaN-VO 2 HyperFETs, demonstrating steep-switching behavior in a platform that is amenable to integration and scaling.

Electrical characterisation of epitaxial AlN/Nb2N heterostructures grown by molecular beam epitaxy

Abstract: Initial electrical characterisation of epitaxial AlN/Nb2N heterostructures grown by molecular beam epitaxy on 6H-SiC substrates is presented. Metal–insulator–metal (MIM) devices of varying diameter were used for current–voltage and capacitance–voltage (C–V) measurements at various temperatures. From C–V measurements, a dielectric constant of 10.1 was extracted along with a linear dependence of capacitance on temperature of 0.02 nF/cm2/°C. Leakage current was determined to be consistent with a Poole–Frenkel conduction mechanism at electric fields >1.25 MV/cm with an extracted trap ionisation energy of 0.82 eV. Electric field breakdown ranged between 3.4 and 5.6 MV/cm with some dependence on the diameter of the MIM device. These initial findings demonstrate the potential for high-power devices based on AlN.

Polarization-mediated Debye-screening of surface potential fluctuations in dual-channel AlN/GaN high electron mobility transistors

Abstract: A dual-channel AlN/GaN/AlN/GaN high electron mobility transistor (HEMT) architecture is proposed, simulated, and demonstrated that suppresses gate lag due to surface-originated trapped charge. Dual two-dimensional electron gas (2DEG) channels are utilized such that the top 2DEG serves as an equipotential that screens potential fluctuations resulting from surface trapped charge. The bottom channel serves as the transistor's modulated channel. Two device modeling approaches have been performed as a means to guide the device design and to elucidate the relationship between the design and performance metrics. The modeling efforts include a self-consistent Poisson-Schrodinger solution for electrostatic simulation as well as hydrodynamic three-dimensional device modeling for three-dimensional electrostatics, steady-state, and transient simulations. Experimental results validated the HEMT design whereby homo-epitaxial growth on free-standing GaN substrates and fabrication of same-wafer dual-channel and recessed-gate AlN/GaN HEMTs have been demonstrated. Notable pulsed-gate performance has been achieved by the fabricated HEMTs through a gate lag ratio of 0.86 with minimal drain current collapse while maintaining high levels of dc and rf performance.

Polarization-mediated Debye-screening of surface potential fluctuations in dual-channel AlN/GaN high electron mobility transistors

Abstract: A dual-channel AlN/GaN/AlN/GaN high electron mobility transistor (HEMT) architecture is proposed, simulated, and demonstrated that suppresses gate lag due to surface-originated trapped charge. Dual two-dimensional electron gas (2DEG) channels are utilized such that the top 2DEG serves as an equipotential that screens potential fluctuations resulting from surface trapped charge. The bottom channel serves as the transistor's modulated channel. Two device modeling approaches have been performed as a means to guide the device design and to elucidate the relationship between the design and performance metrics. The modeling efforts include a self-consistent Poisson-Schrodinger solution for electrostatic simulation as well as hydrodynamic three-dimensional device modeling for three-dimensional electrostatics, steady-state, and transient simulations. Experimental results validated the HEMT design whereby homo-epitaxial growth on free-standing GaN substrates and fabrication of the same-wafer dual-channel and reces...

Suppression of surface-originated gate lag by a dual-channel AlN/GaN high electron mobility transistor architecture

Abstract: A dual-channel AlN/GaN high electron mobility transistor (HEMT) architecture is demonstrated that leverages ultra-thin epitaxial layers to suppress surface-related gate lag. Two high-density two-dimensional electron gas (2DEG) channels are utilized in an AlN/GaN/AlN/GaN heterostructure wherein the top 2DEG serves as a quasi-equipotential that screens potential fluctuations resulting from distributed surface and interface states. The bottom channel serves as the transistor's modulated channel. Dual-channel AlN/GaN heterostructures were grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. HEMTs fabricated with 300 nm long recessed gates demonstrated a gate lag ratio (GLR) of 0.88 with no degradation in drain current after bias stressed in subthreshold. These structures additionally achieved small signal metrics ft/fmax of 27/46 GHz. These performance results are contrasted with the non-recessed gate dual-channel HEMT with a GLR of 0.74 and 82 mA/mm current collapse wi...

Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy

Abstract: The sensitivity of the surface morphology and microstructure of N-polar-oriented InAlN to variations in composition, temperature, and layer thickness for thin films grown by plasma-assisted molecular beam epitaxy (PAMBE) has been investigated. Lateral compositional inhomogeneity is present in N-rich InAlN films grown at low temperature, and phase segregation is exacerbated with increasing InN fraction. A smooth, step-flow surface morphology and elimination of compositional inhomogeneity can be achieved at a growth temperature 50 °C above the onset of In evaporation (650 °C). A GaN/AlN/GaN/200-nm InAlN heterostructure had a sheet charge density of 1.7 × 1013 cm−2 and no degradation in mobility (1760 cm2/V s) relative to 15-nm-thick InAlN layers. Demonstration of thick-barrier high-electron-mobility transistors with good direct-current characteristics shows that device quality, thick InAlN layers can be successfully grown by PAMBE.

Method for fabricating suspended mems structures

Abstract: A process for fabricating a suspended microelectromechanical system (MEMS) structure comprising epitaxial semiconductor functional layers that are partially or completely suspended over a substrate. A sacrificial release layer and a functional device layer are formed on a substrate. The functional device layer is etched to form windows in the functional device layer defining an outline of a suspended MEMS device to be formed from the functional device layer. The sacrificial release layer is then etched with a selective release etchant to remove the sacrificial release layer underneath the functional layer in the area defined by the windows to form the suspended MEMS structure.

I2 basal stacking fault as a degradation mechanism in reverse gate-biased AlGaN/GaN HEMTs

Abstract: Here, we present the observation of a bias-induced, degradation-enhancing defect process in plasma-assisted molecular beam epitaxy grown reverse gate-biased AlGaN/GaN high electron mobility transistors (HEMTs), which is compatible with the current theoretical framework of HEMT degradation. Specifically, we utilize both conventional transmission electron microscopy and aberration-corrected transmission electron microscopy to analyze microstructural changes in not only high strained regions in degraded AlGaN/GaN HEMTs but also the extended gate-drain access region. We find a complex defect structure containing an I2 basal stacking fault and offer a potential mechanism for device degradation based on this defect structure. This work supports the reality of multiple failure mechanisms during device operation and identifies a defect potentially involved with device degradation.

Metallic β-Nb2N Films Epitaxially Grown by MBE on Hexagonal SiC Substrates

Abstract: RF-plasma MBE was used to epitaxially grow 4 - 100-nm-thick metallic β-Nb2N thin films on hexagonal SiC substrates. When the N/Nb flux ratios are greater than one, the most critical parameter for high-quality β-Nb2N is the substrate temperature. The X-ray diffraction (XRD) of films grown between 775 °C and 850 °C demonstrates pure β-Nb2N phase formation which was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy measurements. Using the (0002) and (2131) XRD peaks of a β-Nb2N film grown at 850 °C reveals a 0.68% lattice mismatch to the 6H-SiC substrate. This suggests that β-Nb2N can be used for high-quality metal/semiconductor heterostructures that cannot be fabricated at present.

Investigation of N-Polar AlGaN/GaN and InAlN/GaN Thin Films Grown by MBE

Abstract: 1. Department of Physics, Arizona State University, Tempe, AZ 85287 2. National Research Council Postdoctoral Fellow, resident at the Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375 3. Electronics Science & Technology Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375 4. Sotera Defense Solutions, 2121 Cooperative Way, Suite 400, Herndon, VA 20171-5393

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors.

Abstract: Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm2/V∙sec for devices with a sheet charge density of 1.7 x 1013 cm-2.