Publications

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In April 5-7, 2022, Sandia National Laboratories hosted a second virtual workshop to further explore the potential for developing AI-enhanced co-design for microelectronics (AICoM). This second piece in an ongoing workshop series again brought together two themes. The first theme, co-design for next generation microelectronics, was drawn from the 2018 Department of Energy Office of Science (DOE SC) “Basic Research Needs for Microelectronics” (BRN) report (DOE/SC, 2018, 2021), which called for a “fundamental rethinking” of the traditional design approach to microelectronics, in which subject matter experts (SMEs) in each microelectronics discipline (materials, devices, circuits, algorithms, etc.) work near-independently. Instead, the BRN called for a non-hierarchical, egalitarian vision of co-design, wherein “each scientific discipline informs and engages the others” in “parallel but intimately networked efforts to create radically new capabilities.” The second theme, exploiting and advancing artificial intelligence (AI) to support co-design for microelectronics, acknowledges the continuing breakthroughs in AI that are currently enhancing and accelerating solutions to traditional design problems in materials synthesis and processing, circuit design, and electronic design automation (EDA).

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We demonstrate the generation of a cold-atom ensemble within a sub-millimeter diameter hole in a transparent membrane, a so-called “membrane MOT”. With a sub-Doppler cooling process, the atoms trapped by the membrane MOT are cooled down to 10 μ K. The atom number inside the unbridged/bridged membrane hole is about 10 4 to 10 5, and the 1 / e2-diameter of the MOT cloud is about 180 μ m for a 400 μ m-diameter membrane hole. Such a membrane device can, in principle, efficiently load cold atoms into the evanescent-field optical trap generated by the suspended membrane waveguide for strong atom-light interaction and provide the capability of sufficient heat dissipation at the waveguide. This represents a key step toward the photonic atom trap integrated platform (ATIP).

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In this study, the Raman biaxial stress coefficients KII and strain-free phonon frequencies ω0 have been determined for the E2 (low), E2 (high), and A1 (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E2 (high) mode of AlN is recommended for the residual stress analysis of AlN due to its high sensitivity and the largest signal-to-noise ratio among the studied modes. The E2 (high) Raman biaxial stress coefficient of -3.8 cm-1/GPa and strain-free phonon frequency of 656.68 cm-1 were then applied to perform both macroscopic and microscopic stress mappings. For macroscopic stress evaluation, the spatial variation of residual stress was measured across an AlN-on-Si wafer prepared by sputter deposition. A cross-wafer variation in residual stress of ∼150 MPa was observed regardless of the average stress state of the film. Microscopic stress evaluation was performed on AlN piezoelectric micromachined ultrasonic transducers (pMUTs) with submicrometer spatial resolution. These measurements were used to assess the effect of device fabrication on residual stress distribution in an individual pMUT and the effect of residual stress on the resonance frequency. At ∼20 μm directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of ∼100 MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film.

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Ultra-wide-bandgap aluminum gallium nitride (AlGaN) possesses several material properties that make it attractive for use in a variety of applications. This chapter focuses on power switching and radio-frequency (RF) devices based on Al-rich AlGaN heterostructures. The relevant figures of merit for both power switching and RF devices are discussed as motivation for the use of AlGaN heterostructures in such applications. The key physical parameters impacting these figures of merit include critical electric field, channel mobility, channel carrier density, and carrier saturation velocity, and the factors influencing these and the trade-offs between them are discussed. Surveys of both power switching and RF devices are given and their performance is described including in special operating regimes such as at high temperatures. Challenges to be overcome, such as the formation of low-resistivity Ohmic contacts, are presented. Finally, an overview of processing-related challenges, especially related to surfaces and interfaces, concludes the chapter.

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We present an implementation that can keep a coldatom ensemble within a sub-millimeter diameter hole in a transparent membrane. Based on the effective beam diameter of the magneto-optical trap (MOT), d = 400 mm-hole diameter, we measure the atom number that is 105 times higher than the predicted value using the conventional d6 scaling rule. Atoms trapped by the membrane MOT are cooled down to 10 mK with sub- Doppler cooling process and can be potentially coupled to the photonic/electronic integrated circuits that can be fabricated in the membrane device by taking a step toward the atom trap integrated platform.

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AlGaN polarization-doped field-effect transistors were characterized by DC and pulsed measurements from room temperature to 500 °C in ambient. DC current-voltage characteristics demonstrated only a 70% reduction in on-state current from 25 to 500 °C and full gate modulation, regardless of the operating temperature. Near ideal gate lag measurement was realized across the temperature range that is indicative of a high-quality substrate and sufficient surface passivation. The ability for operation at high temperature is enabled by the high Schottky barrier height from the Ni/Au gate contact, with values of 2.05 and 2.76 eV at 25 and 500 °C, respectively. The high barrier height due to the insulatorlike aluminum nitride layer leads to an ION/IOFF ratio of 1.5 × 109 and 6 × 103 at room temperature and 500 °C, respectively. Transmission electron microscopy was used to confirm the stability of the heterostructure even after an extended high-temperature operation with only minor interdiffusion of the Ni/Au Schottky contact. The use of refractory metals in all contacts will be key to ensure a stable extended high-temperature operation.

The precise patterning of front-side mesas, backside vias, and selective removal of ternary alloys are all needed for power device fabrication in the various wide bandgap (AlGaN/GaN, SiC) and ultrawide bandgap (high Al-content alloys, boron nitride, Ga2O3, diamond) semiconductor technologies. The plasma etching conditions used are generally ion-assisted because of the strong bond strengths in these materials, and this creates challenges for the choice of masks in order to have sufficient selectivity over the semiconductor and to avoid mask erosion and micromasking issues. It can also be challenging to achieve practical etch rates without creating excessive damage in the patterned surface. The authors review the optimum choices for plasma chemistries for each of the semiconductors and acknowledge the pioneering work of John Coburn, who first delineated the ion-assisted etch mechanism.

Research results for AlGaN-channel transistors are reviewed as they have progressed from low Al-content and long-channel devices to Al-rich and short-channel RF devices. Figure of merit (FOM) analysis shows encouraging comparisons relative to today's state-of-the-art GaN devices for high Al-content and elevated temperatures. Critical electric field (EC), which fuels the AlGaN transistor FOM for high Al-composition, is not measured directly, but average gate-drain electric field at breakdown is substantially better in multiple reported AlGaN-channel devices compared to GaN. Challenges for AlGaN include the constraints arising from relatively low room temperature mobility dominated by ternary alloy scattering and the difficulty of making low-resistivity Ohmic contacts to high Al-content materials. Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼2 × 10-6ωcm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. The Al-rich transistors promise to be a very broad and exciting field with much more progress expected in the coming years as this technology matures.

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Gate length dependent (80 nm–5000 mm) radio frequency measurements to extract saturation velocity are reported for Al0.85Ga0.15N/Al0.7Ga0.3N high electron mobility transistors fabricated into radio frequency devices using electron beam lithography. Direct current characterization revealed the threshold voltage shifting positively with increasing gate length, with devices changing from depletion mode to enhancement mode when the gate length was greater than or equal to 450 nm. Transconductance varied from 10 mS/mm to 25 mS/mm, with the 450 nm device having the highest values. Maximum drain current density was 268 mA/mm at 10 V gate bias. Scattering-parameter characterization revealed a maximum unity gain bandwidth (fT) of 28 GHz, achieved by the 80 nm gate length device. A saturation velocity value of 3.8 × 106 cm/s, or 35% of the maximum saturation velocity reported for GaN, was extracted from the fT measurements.

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AlGaN-channel high electron mobility transistors (HEMTs) were operated as visible- and solar-blind photodetectors by using GaN nanodots as an optically active floating gate. The effect of the floating gate was large enough to switch an HEMT from the off-state in the dark to an on-state under illumination. This opto-electronic response achieved responsivity > 108 A/W at room temperature while allowing HEMTs to be electrically biased in the offstate for low dark current and low DC power dissipation. The influence of GaN nanodot distance from the HEMT channel on the dynamic range of the photodetector was investigated, along with the responsivity and temporal response of the floating gate HEMT as a function of optical intensity. The absorption threshold was shown to be controlled by the AlN mole fraction of the HEMT channel layer, thus enabling the same device design to be tuned for either visible- or solar-blind detection.

Enhancement-mode Al0.7Ga0.3N-channel high electron mobility transistors (HEMTs) were achieved through a combination of recessed etching and fluorine ion deposition to shift the threshold voltage (VTH) relative to depletion-mode devices by +5.6 V to VTH = +0.5 V. Accounting for the threshold voltage shift (ΔVTH), current densities of approximately 30 to 35 mA/mm and transconductance values of 13 mS/mm were achieved for both the control and enhancement mode devices at gate biases of 1 V and 6.6 V, respectively. Little hysteresis was observed for all devices, with voltage offsets of 20 mV at drain currents of 1.0 × 10-3mA/mm. Enhancement-mode devices exhibited slightly higher turn-on voltages (+0.38 V) for forward bias gate currents. Piecewise evaluation of a threshold voltage model indicated a ΔVTH of +3.3 V due to a gate recess etching of 12 nm and an additional +2.3 V shift due to fluorine ions near the AlGaN surface.

This work exhibits the ability to shift the threshold voltage of an Al0.45Ga0.55N/Al0.3Ga0.7N high electron mobility transistor through the implementation of a 100 nm thick p-Al0.3Ga0.7N gate. A maximum threshold voltage of +0.3 V was achieved with a 3 μm gate length. In addition to achieving enhancement-mode operation, this work also shows the capability to obtain high saturated drain current (>50 mA/mm), no gate hysteresis, high ION,MAX/IOFF,MIN ratio of >109, and exceptionally low gate leakage current of 10-6 mA/mm even under high forward bias of Vgs = 8 V.

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The thickening behavior of aluminum scandium nitride (Al_0.88Sc_0.12N) films grown on Si(111) substrates has been investigated experimentally using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy, and residual stress measurement. Al_0.88Sc_0.12N films were grown with thicknesses spanning 14 nm to 1.1 um. TEM analysis shows that the argon sputter etch used to remove the native oxide prior to deposition produced an amorphous, oxygen-rich surface, preventing epitaxial growth. XRD analysis of the films show that the A1ScN(002) orientation improves as the films thicken and the XRD A1ScN(002) rocking curve full width half maximum decreases to 1.34 q for the 1.1 pm thick film. XRD analysis shows that the unit cell is expanded in both the a- and c-axes by Sc doping; the a-axis lattice parameter was measured to be 3.172 ± 0.007 A and the c-axis lattice parameter was measured to be 5.000 ± 0.001 A, representing 1.96% and 0.44% expansions over aluminum nitride lattice parameters, respectively. The grain size and roughness increase as the film thickness increases. A stress gradient forms through the film; the residual stress grows more tensile as the film thickens, from -1.24 GPa to +8.5MPa.

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We present heavy ion and proton data on AlGaN high-voltage HEMTs showing single event burnout (SEB), total ionizing dose, and displacement damage responses. These are the first such data for materials of this type. Two different designs of the epitaxial structure were tested for SEB. The default layout design showed burnout voltages that decreased rapidly with increasing LET, falling to about 25% of nominal breakdown voltage for ions with LET of about 34 MeV · cm2/mg for both structures. Samples of the device structure with lower AlN content were tested with varying gate-drain spacing and revealed an improved robustness to heavy ions, resulting in burnout voltages that did not decrease up to at least 33.9 MeV · cm2/mg. Failure analysis showed that there was consistently a point, location random, where gate and drain had been shorted. Oscilloscope traces of terminal voltages and currents during burnout events lend support to the hypothesis that burnout events begin with a heavy ion strike in the vulnerable region between gate and drain. This subsequently initiates a cascade of events resulting in damage that is largely manifested elsewhere in the device. This hypothesis also suggests a path for greatly improving the susceptibility to SEB as development of this technology goes forward. Testing with 2.5-MeV protons showed only minor changes in device characteristics.

Al-rich AlGaN-channel high electron mobility transistors with 80-nm long gates and 85% (70%) Al in the barrier (channel) were evaluated for RF performance. The dc characteristics include a maximum current of 160 mA/mm with a transconductance of 24 mS/mm, limited by source and drain contacts, and an on/off current ratio of 109. fT of 28.4 GHz and fMAX of 18.5 GHz were determined from small-signal S-parameter measurements. Output power density of 0.38 W/mm was realized at 3 GHz in a power sweep using on-wafer load pull techniques.

This chapter discusses the motivation for the use of Ultra-Wide-Bandgap Aluminum Gallium Nitride semiconductors for power switching and radio-frequency applications. A review of the relevant figures of merit for both vertical and lateral power switching devices, as well as lateral radio-frequency devices, is presented, demonstrating the potential superior performance of these devices relative to Gallium Nitride. Additionally, representative results from the literature for each device type are reviewed, highlighting recent progress as well as areas for further research.

In this paper, we present heavy ion and proton data on AlGaN highvoltage HEMTs showing Single Event Burnout, Total Ionizing Dose, and Displacement Damage responses. These are the first such data for materials of this type. Two different designs of the epitaxial structure were tested for Single Event Burnout (SEB). The default layout design showed burnout voltages that decreased rapidly with increasing LET, falling to about 25% of nominal breakdown voltage for ions with LET of about 34 MeV·cm2/mg for both structures. Samples of the device structure with lower AlN content were tested with varying gate-drain spacing and revealed an improved robustness to heavy ions, resulting in burnout voltages that did not decrease up to at least 33.9 MeV·cm2/mg. Failure analysis showed there was consistently a point, location random, where gate and drain had been shorted. Oscilloscope traces of terminal voltages and currents during burnout events lend support to the hypothesis that burnout events begin with a heavy ion strike in the vulnerable region between gate and drain. This subsequently initiates a cascade of events resulting in damage that is largely manifested elsewhere in the device. This hypothesis also suggests a path for greatly improving the susceptibility to SEB as development of this technology goes forward. Lastly, testing with 2.5 MeV protons showed only minor changes in device characteristics.

Here, Al_0.26Ga_0.74N/GaN on SiC lateral Schottky diodes were fabricated with variable anode-to-cathode spacing and were analyzed for blocking and on-state device performance. On-chip normally-on High Electron Mobility Transistor (HEMT) structures were also fabricated for a comparison of blocking characteristics. The Schottky diode displayed an ideality factor of 1.59 with a Ni/AlGaN zero bias barrier height of 1.18 eV and a flat band barrier height of 1.59 eV. For anode-to-cathode spacings between 10 and 100 μm, an increase in median breakdown voltages from 529 V to 8519 V and median specific on-resistance (R_on-sp) from 1.5 to 60.7 mΩ cm² was observed with an increase in spacing. The highest performing diode had a lateral figure of merit of 1.37 GW/cm² corresponding to a breakdown voltage upwards of 9 kV and a R_on-sp of 59 mΩ cm². This corresponds to the highest Schottky diode breakdown voltage reported thus far with an Al_0.26Ga_0.74N/GaN lateral structure.

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In this paper we describe a method for controlling both the residual stress and the through-thickness stress gradient of aluminum nitride (AlN) thin films using a multi-step deposition process that varies the applied radio frequency (RF) substrate bias. The relationship between the applied RF substrate bias and the AlN residual stress is characterized using AlN films grown on oxidized silicon substrates is determined using 100 nm-1.5 μm thick blanket AlN films that are deposited with 60-100 W applied RF biases; the stress-bias relationship is found to be well described using a power law relationship. Using this relationship, we develop a model for varying the RF bias in a series of discrete deposition steps such that each deposition step has zero average stress. The applied RF bias power in these steps is tailored to produce AlN films that have minimized both the residual stress and the film stress gradient. AlN cantilevers were patterned from films deposited using this technique, which show reduced curvature compared to those deposited using a single RF bias setting, indicating a reduction of the stress gradient in the films.

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