Christopher M. Matthews

Georgia Institute of Technology

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W. Alan Doolittle

Georgia Institute of Technology

Zachary Engel

Georgia Institute of Technology

Habib Ahmad

Georgia Institute of Technology

Evan A. Clinton

Georgia Institute of Technology

Keisuke Motoki

Georgia Institute of Technology

Stephanie Elizabeth Sandoval

Georgia Institute of Technology

Mengkun Tian

Georgia Institute of Technology

Matthew M. Schneider

Los Alamos National Laboratory

Bairav S. Vishnugopi

University of Michigan–Ann Arbor

Partha P. Mukherjee

Purdue University West Lafayette

Cooperative Institutions

Georgia Institute of Technology

Sandia National Laboratories

United States Naval Research Laboratory

Sandia National Laboratories California

Arizona State University

University of Virginia

Oak Ridge National Laboratory

Purdue University West Lafayette

University of California, Los Angeles

University of Atlanta

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Realization of homojunction PN AlN diodes

Journal of Applied Physics (2022)

Habib Ahmad Zachary Engel Christopher M. Matthews Sang Ho Lee W. Alan Doolittle

Aluminum nitride (AlN) is an insulator that has shown little promise to be converted to a semiconductor via impurity doping. Some of the historic challenges for successfully doping AlN include a reconfigurable defect formation known as a DX center and subsequent compensation that causes an increase in dopant activation energy resulting in very few carriers of electricity, electrons, or holes, rendering doping inefficient. Using crystal synthesis methods that generate less compensating impurities and less lattice expansion, thus impeding the reconfiguration of dopants, and using new dopants, we demonstrate: (a) well behaved bulk semiconducting functionality in AlN, the largest direct bandgap semiconductor known with (b) substantial bulk p-type conduction (holes = 3.1 × 1018 cm−3, as recently reported in our prior work), (c) dramatic improvement in n-type bulk conduction (electrons = 6 × 1018 cm−3, nearly 6000 times the prior state-of-the-art), and (d) a PN AlN diode with a nearly ideal turn-on voltage of ∼6 V for a 6.1 eV bandgap semiconductor. A wide variety of AlN-based applications are enabled that will impact deep ultraviolet light-based viral and bacterial sterilization, polymer curing, lithography, laser machining, high-temperature, high-voltage, and high-power electronics.

Homojunction

10.1063/5.0086314

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Citations (34)

Advancements in High Indium Content Alinn Grown Via Metal Modulated Epitaxy and Application Towards Polar/Non-Polar Optical Devices

Meeting abstracts/Meeting abstracts (Electrochemical Society. CD-ROM) (2019)

Zachary Engel Evan A. Clinton Christopher M. Matthews W. Alan Doolittle

AlInN has been a topic of recent study due to its unique property among III-nitrides of lattice matching to GaN at a composition of 18% indium, making it a strong candidate for power electronic and optoelectronic applications. A widely unexplored area is the application of AlInN towards visible/Near-IR optical devices. AlInN has a tunable bandgap, spanning the infrared to ultraviolet range, of 0.7 to 6.1eV. At a composition of about 70% indium AlInN has a perfect bandgap, 1.7eV, for tandem solar cells with silicon and for various optical communications applications. Challenges exist with the growth of AlInN as a result of the large lattice parameter mismatch and differences in growth regimes between the two binary components. At the low temperatures required for the growth of AlInN the aluminum adatoms have a low mobility, often leading to lateral phase separation in the film. Metal Modulated Epitaxy (MME) offers a good solution to the growth issues of AlInN. This flux modulated technique allows for growth under metal rich conditions, increasing surface diffusion lengths of the Aluminum while limiting droplet formation and terminating in a dry surface suitable for devices. For comparison ~100 nm thick high indium content AlInN samples were grown using nitrogen rich (0.8 III/V ratio) MBE and MME using a III/V ratio of 1.3 with a dose designed to prevent surface segregation (details supplied at the conference). Both films were grown cold at 375 degrees C to limit phase separation The MME sample showed improvement over the MBE sample in crystal quality and surface roughness, with the XRD (0002) reflection FWHM improving from 783 arcsec to 184 arcsec, the XRD (10-15) reflection FWHM improving from 2456 arcsec to 1421 arcsec, and the AFM surface roughness improving from 1.52nm rms to 0.882nm rms between the MBE and MME samples respectively. The carrier concentrations of both samples were found to be about 1E19 cm -3 via hall measurement implying that like InN, In-rich AlInN may suffer from excessive residual electron concentrations and/or surface fermi-level pinning. Optical measurements will be presented at the conference. The same MME growth conditions were used to grow 100nm of indium rich AlInN directly on a (111) p-silicon substrate. Simple indium contacts were used to contact the Si and AlInN. Vertical conduction and rectification between the silicon and AlInN layers were observed in this large area (~0.25x0.25 cm 2 ) device. Understanding of the limitations of and physics governing this promising vertical polar/non-polar heterojunction will be given at the conference. Additionally, results from various growth conditions will be detailed. Figure 1

10.1149/ma2019-02/29/1271

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Improved crystallographic order of ScAlN/GaN heterostructures grown at low temperatures under metal rich surface conditions

Journal of Applied Physics (2024)

Keisuke Motoki Zachary Engel Timothy M. McCrone Huijin Chung Christopher M. Matthews

Sc0.18Al0.82N/GaN with state-of-the-art x-ray diffraction figures of merit grown by metal modulated epitaxy under metal-rich conditions and a low substrate temperature of 400 °C is demonstrated to have improved crystalline order [250 arc sec for the (0002) reflection and 469 arc sec for the (101¯5)] compared to a previous state-of-the-art sample grown at a more conventional temperature of 650 °C. While both samples show a columnar structure, the higher substrate temperature sample has a good symmetric rocking curve (RC) of 229 arc sec, but unlike the lower temperature sample, the RC of the (101¯5) asymmetric reflection could not be measured, indicating a more columnar structure common among ScAlN films. Local lattice constant maps (LLCMs) from 4D-STEM depict abrupt strain relaxation within ∼2 nm from the ScAlN/GaN interface for the sample grown at Tsub = 400 °C. Since these LLCMs suggest a lattice mismatch in the a-lattice constant, and since the films show a sudden roughening, the composition for lattice match to GaN may be less than the accepted 18%–20% Sc, consistent with the average GaN lattice match from lattice constant values reported in the literature of 12%. Compared to traditional III-Nitrides, ScAlN films have substantially more screw and mixed-type threading dislocations, suggesting substantial shear forces that result in significant twist and distortion leading to orthorhombic diffraction patterns as viewed from plan-view TEM in the Tsub = 650 °C sample. These results offer the possibility of ScAlN integration into low-thermal-budget processes including CMOS but further indicate that structural understanding of ScAlN remains lacking.

Wide-bandgap semiconductor

10.1063/5.0176344

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InGaN based solar cells with GaN tunnel junction contacts

Ehsan Vadiee Evan A. Clinton Heather McFavilen Zachary Engel Christopher M. Matthews

InGaN/GaN multi-quantum well solar cells with highly-doped GaN p-n homo tunnel junction contacts are grown with a hybrid method where the active regions were grown with metal organic chemical vapor deposition and the TJ contacts were grown by plasma-assisted molecular beam epitaxy. Different solar cells with different TJ contacts were demonstrated by this method. The best solar cell featuring TJ exhibits a low leakage current density of 7.5×10 ^-9 mA/cm ² , an open-circuit voltage (VOC) of _~ 2.2 V, and short circuit-current density (JSC) of _~ 0.9 mA/cm ² . Correlations are made between different characterization methods to draw conclusions about the behavior of the devices.

10.1109/pvsc.2018.8547327

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Controlling surface adatom kinetics for improved structural and optical properties of high indium content aluminum indium nitride

Journal of Applied Physics (2020)

Zachary Engel Evan A. Clinton Christopher M. Matthews W. Alan Doolittle

A non-traditional, for AlInN, method of controlling adatom kinetics and a low temperature growth condition were employed to improve the quality of high indium content aluminum indium nitride films. Metal-rich surfaces were used to enhance adatom mobility and compensate for the low growth temperature (Tsub ≤ 400 °C) effect of reducing surface diffusion lengths. The metal-rich approach resulted in 12 times lower x-ray diffraction full-width at half-maximum rocking curve figures of merit when compared to literature. In addition to promising photoluminescence emission, these results indicate improved structural quality over other reported approaches. AlInN films with ∼70% indium content were characterized via x-ray diffraction, atomic force microscopy, and photoluminescence spectroscopy with each technique indicating an optimal growth temperature of 350 °C. Al0.3In0.7N grown above 400 °C exhibited phase separation and a reduction in quality, while samples grown colder were predominantly single-phase and displayed improved photoluminescence at ∼1.45 eV. The photoluminescence spectra suggest emission from quantum wire-like structures with dimensions ranging from 15 to 18 nm. These low-temperature, metal-rich findings for high indium content AlInN are promising for future long-wavelength III-nitride optical devices.

Indium nitride

10.1063/1.5142295

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Prospectives for AlN electronics and optoelectronics and the important role of alternative synthesis

Applied Physics Letters (2023)

W. Alan Doolittle Christopher M. Matthews Habib Ahmad Keisuke Motoki Sang Ho Lee

Future applications for emerging AlN semiconductor electronics and optoelectronics are facilitated by emerging doping technologies enabled by low temperature, non-equilibrium epitaxy. Defect and impurity compensation can be reduced by controlling the surface chemistry with reducing compensating vacancy concentrations being a key driver for lower temperature growth. Contrary to common understanding, low temperature, metal-rich vacuum processes are shown to have higher diffusion lengths than high temperature nitrogen-rich methods. This feature can be utilized to inhibit silicon-DX center formation without compromises in crystal quality. First principles calculations identify the valence split-off band as the dominant hole band contributing to impurity band formation (as opposed to the heavy and light hole bands in other nitrides). This anomalous band structure causes an impurity band to form at dopant concentrations similar to GaN even though AlN has a deeper isolated acceptor energy and results in hole mobilities that are substantially higher than possible in GaN. AlN hole concentrations of ∼4.4 × 1018 cm−3 and 0.045 Ω cm resistivity and electron concentrations of ∼6 × 1018 cm−3 and ∼0.02 Ω cm resistivity are shown and offer substantial promise for future generations of AlN bipolar electronic and optical devices.

Wide-bandgap semiconductor

Acceptor

10.1063/5.0156691

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Citations (26)

at]Hard-switching reliability studies of 1200 V vertical GaN PiN diodes

MRS Communications (2018)

Oleksiy Slobodyan Thomas G. Smith Jack Flicker Stephanie Elizabeth Sandoval Christopher M. Matthews

Leakage (economics)

10.1557/mrc.2018.204

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Structural and Electrochemical Evolution of Alloy Interfacial Layers in Anode-Free Solid-State Batteries

Stephanie Elizabeth Sandoval John Lewis Bairav S. Vishnugopi Douglas Nelson Matthew M. Schneider

“Anode-free” solid-state batteries feature high energy density since there is no anode active material upon assembly. While beneficial effects of interfacial layers have previously been demonstrated, the mechanisms through which they influence lithium plating/stripping are unclear. Here, we reveal the evolution of 100-nm silver and gold interfacial layers during anode-free lithium plating/stripping using electrochemical methods, electron microscopy, and modeling. The alloy layers significantly improve Coulombic efficiency and resistance to short circuiting, even though the alloys form solute regions or particulates that detach from the current collector as lithium grows. In-situ electrochemical impedance spectroscopy shows that the alloy layers return to the interface and mitigate contact loss at the end of stripping, avoiding a critical vulnerability of anode-free cells. The enhanced contact retention is driven by increased Li uniformity that promotes spatially even stripping, as well as local alloy delithiation in response to current concentrations to homogenize current and diminish voiding.

Stripping (fiber)

Current collector

Plating (geology)

10.26434/chemrxiv-2023-b9x1p

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Structural and Electrochemical Evolution of Alloy Interfacial Layers in Anode-Free Solid-State Batteries

Stephanie Elizabeth Sandoval John A. Lewis Bairav S. Vishnugopi Douglas Nelson Matthew M. Schneider

“Anode-free” solid-state batteries feature high energy density since there is no anode active material upon assembly. While beneficial effects of interfacial layers at the anode-solid electrolyte interface have previously been demonstrated, the mechanisms through which they influence lithium plating/stripping are unclear. Here, we reveal the evolution of 100-nm silver and gold interfacial layers during anode-free lithium plating/stripping using electrochemical methods, electron microscopy, synchrotron micro x-ray computed tomography, and modeling. The alloy layers significantly improve Coulombic efficiency and resistance to short circuiting, even though the alloys form solute regions or particulates that detach from the current collector as lithium grows. In-situ electrochemical impedance spectroscopy shows that the alloy layers return to the interface and mitigate contact loss at the end of stripping, avoiding a critical vulnerability of anode-free cells. The enhanced contact retention is driven by uniform Li thickness that promotes spatially uniform stripping, as well as local alloy delithiation in response to current concentrations to homogenize current and diminish voiding.

Stripping (fiber)

Current collector

Plating (geology)

10.26434/chemrxiv-2023-b9x1p-v3

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Impact and mitigation of surface oxides on aluminum nitride substrates

Applied Surface Science (2024)

Christopher M. Matthews Keisuke Motoki Habib Ahmad Zachary Engel Sang Ho Lee

10.1016/j.apsusc.2024.162208

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