arXiv subjects

Andrew G. Norman

Publications and source records attributed to Andrew G. Norman.

7 recordsLinked to original sources

arXiv2026

Toward Tunable Magnetic Dirac Semimetals: Mn Doping of Cd$_3$As$_2$

Magnetic impurities provide a route toward increasing functionality in electronic materials, often enabling new device concepts and architectures. In the case of topological semimetals, dilute magnetic doping presents a particularly attractive approach for inducing a Dirac to Weyl phase change via time reversal symmetry breaking. However, efforts to realize changes in the electronic structure have been limited by challenges in incorporating magnetic impurities into crystals with sufficiently high electron mobilities to detect them via transport or spectroscopic techniques. Here, we demonstrate incorporation of Mn into Cd$_3$As$_2$ Dirac semimetal thin films grown by molecular beam epitaxy (MBE). Using As-rich growth conditions and [001] oriented thin films, Mn compositions of >10% are achieved. Films contain uniform distributions of Mn with no evidence of secondary phases and exhibit electron mobilities greater than 10,000-30,000 cm$^2$/Vs up to 5% Mn. An evolution in the magnetization behavior along with the emergence of a second quantum oscillation frequency at low Mn concentrations provide preliminary evidence of Mn-induced changes in the electronic structure that are consistent with a Weyl phase. This work demonstrates the potential of magnetically doping topological semimetal thin films and a pathway for synthesizing them.

cond-mat.mtrl-sci↗

arXiv2023

Thin film TaAs: developing a platform for Weyl semimetal devices

MX monopnictide compounds (M=Nb,Ta, X = As,P) are prototypical three-dimensional Weyl semimetals (WSMs) that have been shown in bulk single crystal form to have potential for a wide variety of novel devices due to topologically protected band structures and high mobilities. However, very little is known about thin film synthesis, which is essential to enable device applications. We synthesize TaAs(001) epilayers by molecular beam epitaxy on GaAs(001) and provide an experimental phase diagram illustrating conditions for single phase, single-crystal-like growth. We investigate the relationship between nanoscale defects and electronic structure, using angle-resolved photoemission spectroscopy, Kelvin probe microscopy and transmission electron microscopy. Our results provide a roadmap and platform for developing 3D WSMs for device applications.

cond-mat.mtrl-sci↗

arXiv2020

$Mg_xZn_{1-x}O$ contact to $CuGa_3Se_5$ absorber for photovoltaic and photoelectrochemical devices

$CuGa_3Se_5$ is a promising candidate material with wide band gap for top cells in tandem photovoltaic (PV) and photoelectrochemical (PEC) devices. However, traditional CdS contact layers used with other chalcopyrite absorbers are not suitable for $CuGa_3Se_5$ due to the higher position of its conduction band minimum. $Mg_xZn_{1-x}O$ is a transparent oxide with adjustable band gap and conduction band position as a function of magnesium composition, but its direct application is hindered by $CuGa_3Se_5$ surface oxidation. Here, $Mg_xZn_{1-x}O$ is investigated as a contact (n-type buffer or window) material to $CuGa_3Se_5$ absorbers pretreated in $Cd^{2+}$ solution, and an onset potential close to 1 V vs RHE in 10 mM hexaammineruthenium (III) chloride electrolyte is demonstrated. The $Cd^{2+}$ surface treatment changes the chemical composition and electronic structure of the $CuGa_3Se_5$ surface, as demonstrated by photoelectron spectroscopy measurements. The performance of $CuGa_3Se_5$ absorber with $Cd^{2+}$ treated surface in the solid-state test structure depends on the Zn/Mg ratio in the $Mg_xZn_{1-x}O$ layer. The measured open circuit voltage close to 1 V is promising for tandem PEC water splitting with $CuGa_3Se_5$/$Mg_xZn_{1-x}O$ top cells.

cond-mat.mtrl-sci↗

arXiv2020

Heteroepitaxial integration of ZnGeN2 on GaN buffers using molecular beam epitaxy

Recently theorized hybrid II-IV-N{_2} / III-N heterostructures, based on current commercialized (In,Ga)N devices, are predicted to significantly advance the design space of highly efficient optoelectronics in the visible spectrum, yet there are few epitaxial studies of II-IV-N{_2} materials. In this work, we present heteroepitaxial ZnGeN{_2} grown on GaN buffers and AlN templates. We demonstrate that a GaN nucleating surface is crucial for increasing the ZnGeN{_2} crystallization rate to combat Zn desorption, extending the stoichiometric growth window from 215 {\degree}C on AlN to 500 {\degree}C on GaN buffers. Structural characterization reveals well crystallized films with threading dislocations extending from the GaN buffer. These films have a critical thickness for relaxation of 20 nm - 25 nm as determined by reflection high energy electron diffraction (RHEED) and cross-sectional scanning electron microscopy (SEM). The films exhibit a cation-disordered wurtzite structure, with lattice constants a = 3.216 Å {\pm} 0.004 Å and c = 5.215 Å {\pm} 0.005 Å determined by RHEED and X-ray diffraction (XRD). This work demonstrates a significant step towards the development of hybrid ZnGeN{_2}-GaN integrated devices.

cond-mat.mtrl-sci↗

arXiv2015

Solar energy conversion properties and defect physics of ZnSiP$_2$

Implementation of an optically active material on silicon has been a persistent technological challenge. For tandem photovoltaics using a Si bottom cell, as well as for other optoelectronic applications, there has been a longstanding need for optically active, wide band gap materials that can be integrated with Si. ZnSiP$_2$ is a stable, wide band gap (2.1 eV) material that is lattice matched with silicon and comprised of inexpensive elements. As we show in this paper, it is also a defect-tolerant material. Here, we report the first ZnSiP$_2$ photovoltaic device. We show that ZnSiP$_2$ has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III-V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP$_2$ are shallow and the minority carrier lifetime is 7 ns. These favorable results encourage the development of ZnSiP$_2$ and related materials as photovoltaic absorber materials.

cond-mat.mtrl-sci↗

arXiv2015

III-V/Si Wafer Bonding Using Transparent, Conductive Oxide Interlayers

We present a method for low temperature plasma-activated direct wafer bonding of III-V materials to Si using a transparent, conductive indium zinc oxide interlayer. The transparent, conductive oxide (TCO) layer provides excellent optical transmission as well as electrical conduction, suggesting suitability for Si/III-V hybrid devices including Si-based tandem solar cells. For bonding temperatures ranging from 100$^{\circ}$C to 350$^{\circ}$C, Ohmic behavior is observed in the sample stacks, with specific contact resistivity below 1 $Ω$cm$^2$ for samples bonded at 200$^{\circ}$C. Optical absorption measurements show minimal parasitic light absorption, which is limited by the III-V interlayers necessary for Ohmic contact formation to TCOs. These results are promising for Ga$_{0.5}$In$_{0.5}$P/Si tandem solar cells operating at one sun or low concentration conditions.

cond-mat.mtrl-sci↗

arXiv2015

Combinatorial Insights into Doping Control and Transport Properties of Zinc Tin Nitride

ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate \emph{n}-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn(1+x)Sn(1-x)N(2) over the ranges 35-340 degrees C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10^(18) cm^(-3). Films grown at 230 degrees C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10^(18) cm^(-3)) and high mobility (8.3 cm^(2) V^(-1) s^(-1)). Furthermore, we report evidence of a Burstein-Moss shift widening the apparent bandgap as cation composition becomes increasingly Sn-rich, and tunable carrier density as a function of cation composition (lower carrier density for higher Zn content), which suggests the formation of defect complexes. Collectively, these findings provide important insight into the fundamental properties of the Zn-Sn-N material system, and also highlight the potential to utilize ZnSnN2 for photovoltaics.

cond-mat.mtrl-sci↗