United States

Abstract Body: Control over dopant concentration and distribution is essential to engineering nanowire-based devices. However, controllable n-type doping has remained challenging for GaAs, which is the foundation for many important III-V heterostructures. In particular, silicon behaves as a p-type dopant in GaAs nanowires grown by a vapor-liquid-solid (VLS) process in molecular beam epitaxy (MBE) conditions. Recently, it was shown that Si can act as a donor when incorporated in a catalyst free vapor-solid (VS) growth process,[1] highlighting the importance of understanding the growth and dopant incorporation mechanisms to controlling conductivity and carrier type. Here we present evidence that Si stabilizes the nanowire facets necessary for nanowire elongation, leading to uniform doping in a core region and enhanced doping at the nanowire surface. The growth conditions used here produce crystalline clusters of GaAs on Si (111) substrates in the absence of a Si flux. The introduction of Si leads to a high yield of uniform vertical nanowires (Figure 1(b)). We used atom probe tomography (APT) to measure the Si distribution in three dimensions and resolve variations in doping across the diameter of the nanowire. The reconstructions in Figure 2(a) and Figure 3(a) show a portion of the GaAs nanowire and the AlGaAs passivation layer (the entire diameter is not within the field of view). The dopant concentration increases from the nanowire center to the edge and peaks near the interface between GaAs core and AlGaAs shell (Figure 2(b)), indicating that different incorporation rates may be associated with distinct facets whose prevalence is influenced by the Si dopant itself. Additionally, Si-enriched regions and clusters appear at the intersection of {110} facets of some nanowires (Figure 3(b)), which maybe be linked to variations in surface faceting. The above observations suggest that Si alters the surface energy of GaAs and stabilizes the facets that promote nanowire growth via preferential incorporation. Si has previously been observed to induce twin-defect mediated growth in GaAs,[2] which may also be related to modification of surface energies. Surface etching to remove Si enriched layers provides a pathway to uniformly doped n-type GaAs nanowires that are free of catalyst impurities. References: [1] Ruhstorfer, D., et al. Demonstration of n-type behavior in catalyst-free Si-doped GaAs nanowires grown by molecular beam epitaxy. Applied Physics Letters, 2020, 116, 052101. [2] Keitaro Ikejiri, et al, Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapor-phase epitaxy. Nanotechnology, 2008, 19, 265604.

EMC 2021

Si Dopants Enable Catalyst-Free n-Type GaAs Nanowire Growth by Modifying Facet Energies

vapor-solid growth

atom probe tomography

doping

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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technical paper

Abstract Body: A wearable neuromorphic electronic system, that can learn and interpret the non-structural biometric information at extremely low-power, has been brought great attention because of its applicability as the intelligent device that can easily attach onto the human body or any rough surface [1-3]. With this reason, organic-based artificial synaptic devices have been proposed as a potential candidate for wearable neuromorphic applications due to its inherent mechanical flexibility and the material (or device form) variability for the desired functionalities [2,3]. In this study, we designed 1D fiber-shaped multi-synapses comprising ferroelectric organic transistors fabricated on a 100 mm Ag wire and utilized them as multi-synaptic channels in an e-textile neural network for wearable neuromorphic applications. The device mimics diverse synaptic functions, including short- and long-term plasticity with 80 states and spike rate- and timing-dependent plasticity. It exhibited excellent reliability even under 6,000 repeated input stimuli and mechanical bending stress. Various NOR-type textile arrays are formed simply by cross-pointing 1D synapses with Ag wires, where each output from individual synapse can be integrated and propagated without undesired leakage. Notably, the 1D multi-synapses achieved up to ~90% and ~70% recognition accuracy for MNIST and electrocardiogram patterns, respectively, even in a single-layer neural network, and almost maintained regardless of the bending conditions. References [1] Park, Y.; Park, M.-J.; Lee, J.-S. Adv. Funct. Mater. 2018, 1804123 [2] van de Burgt, Y.; Lubberman, E.; J. Fuller, E.; T. Keene, S.; C. Faria, G.; Agarwal, S.; J. Marinella, M.; Talin, A. A.; Salleo A. Nat. Mater. 2017, 16, 414-418 [3] Jang, S.; Jang, S.; Lee, E.-H.; Kang, M.; Wang, G.; Kim, T.-W. ACS Appl. Mater. Interfaces 2019, 11, 1071-1080 [4] *S. Ham et al. Sci. Adv. 2020, 6, eaba1178 [Published]

Fiber-Shaped Multi-Synapses Enabling an Electronic-Textile Neural Network for Wearable Neuromorphic Applications

Abstract Body: Newly emerged wide bandgap semiconductor (WBG) β-Ga2O3 has been extensively researched for power electronics, RF electronics and optoelectronics due to its large bandgap of 4.7- 4.9 eV and high breakdown field of ~ 8 MV/cm. Due to the absence of p-type Ga2O3, most of demonstrated β-Ga2O3 devices are unipolar devices such as high electron mobility transistors (HEMTs) and Schottky barrier diodes. To overcome this limitation, other p-type materials, such as GaN and NiO, have been investigated to produce Ga2O3 based p-n heterojunctions. Among these p-type materials, p-GaN has attracted especial interests since GaN is also a WBG semiconductor and has a clear crystallographic relationship to β-Ga2O3 for epitaxial growth. In addition, both β-Ga2O3 and GaN can be produced in the industrial standard tools, i.e., the metalorganic chemical vapor deposition (MOCVD). The growth of high-quality GaN on Ga2O3 by MOCVD has been recently reported. β-Ga2O3/GaN p-n heterojunction via mechanically exfoliation also showed decent rectifying behaviors. However, the breakdown capability of the β-Ga2O3 based p-n heterojunctions is still far from expectations due to the lack of effective edge terminations. Here we design and simulate mesa based edge terminations for the vertical β-Ga2O3/GaN p-n heterojunctions using SILVACO TCAD simulator. The model was first calibrated by the experimental results, including the conduction and valance band offsets of the heterojunction and electrical properties. The device structure consisted of n+-Ga2O3 n-contact layer, 5 µm unintentionally doped (UID) β-Ga2O3 drift layer and 500 nm p-GaN layer. The ideal breakdown voltage (BV) of the heterojunction was 1.37 kV, while the BV of the reference device without effect mesa edge termination dropped dramatically to 300 V due to the electric field crowding at the device edge. Therefore, it is critical to design and optimize the mesa edge termination to improve the BV of the β-Ga2O3/GaN p-n heterojunctions. Three mesa edge termination structures were investigated in this work: beveled mesa, step mesa, and deeply-etched mesa. The beveled mesa angle θ is the key parameter in determining the device BV. When θ was varied from 15° to 90°, the highest BV of 1224 V was obtained for θ =15°. For the step mesa edge termination, the width (W), depth (D), and the number of steps define the mesa structure. It was found that the BV of the devices increased with increasing number of steps. The combination of (W, D) values of (0.5,0.5) and (0.5,1.0) and (1.0,1,0) was investigated, where all the values are in µm. The highest value was obtained for W = 0.5µm, D = 1.0µm and the number of steps = 5. Smaller W and larger D led to better BV. The electric field distributions of devices with different mesa edge terminations were compared at –300V. Without effect mesa edge termination, the peak electric field at the junction edge was ~ 4.2 MV/cm. By incorporating the mesa edge termination techniques, the electric field crowding at the device edge was mitigated, and the peak electric fields were reduced to 1.8 MV/cm and 2.4 MV/cm for the beveled and step mesa edge terminations, respectively. For the deeply-etched mesa structure, it was found that a more uniform electric field distribution was observed when the mesa depth was increased from 0.5,1.0, 2.0, to 3.0 µm, where the peak electric fields were decreased from 2.1, 1.8 ,1.5 to 1.2 MV/cm, respectively. In summary, this work presents a comprehensive design and optimization of mesa edge terminations for the wide bandgap vertical β-Ga2O3/GaN p-n heterojunctions, which is first of its kind. Three mesa structures were explored, including beveled mesa, step mesa and deeply-etched mesa. It was found that these mesa edge terminations can alleviate the electric field crowding at the junction edge and increase the device BV. This work can provide critical guidance for the development of high-performance β-Ga2O3 based bipolar high voltage and high power devices.

Vertical Wide Bandgap β-Ga2O3/GaN p-n Heterojunction with Mesa Based Edge Termination

Abstract Body: β-Ga2O3 is of high interest for high voltage and high-power DC and microwave applications. Point defects and extended structural defects in bulk crystal and epitaxy layer are likely responsible for poor device performance. The fundamental properties of instability under high reverse bias (device degradation mechanism) are still poorly understood. Here, changes of I-V and C-V curves of schottky diodes based on MOVPE films and UID crystals can be observed at relatively low temperature after a high current density (0.1 mA/cm2) stress. About 1.6um-thick UID Ga2O3 epitaxial film were grown by MOVPE on (010)-oriented NCT Fe-doped substrate using triethyl gallium, diluted silane and oxygen precursors. UID crystals were grown by Czochralski method. The defect properties for the pristine and stressed devices will be investigated by deep level transient spectroscopy (DLTS).

Temperature Dependent Current Stress of β-Ga2O3 Schottky Devices

Abstract Body: β-Ga2O3 has become a promising semiconductor for high power applications. High-quality dielectrics are crucial for enabling high performance β-Ga2O3 field-effect transistors (FETs) especially because achieving p-type doping does not seem feasible for this material system. The material properties of high-quality gate dielectric include high dielectric constant, negligible gate leakage, low density of interface and bulk traps, and large breakdown field. Recently, aluminum silicon oxide (AlSiO) has been proposed as a high-performance and reliable gate dielectric for GaN-based devices [1], [2]. Previous studies showed that the alloying of Al2O3 with silicon to form AlSiO has the potential to combine the merits of both SiO2 and Al2O3 thus realizing low density of interface traps (Dit), high conduction/valence band offset and high breakdown strength [3], [4]. The promising results of AlSiO as a dielectric for GaN-based devices motivated us to expand its applications to Ga2O3. In this work, the interface and bulk properties of aluminum-silicon-oxide (AlSiO) dielectric grown by metal-organic chemical vapor deposition (MOCVD) on (001) β-Ga2O3 were investigated systematically using a deep UV-assisted capacitance-voltage methodology. The AlSiO dielectric, with a silicon composition of 40%, was grown by metal organic chemical vapor deposition (MOCVD) in a close coupled showerhead chamber. The AlSiO deposition temperature was 700 °C, and the TMAl, Si2H6, and O2 flows were 3.2 μmol/min, 3.2 μmol/min, and 4.4 mmol/min, respectively. AlSiO with various thicknesses (10 nm, 20 nm, and 30 nm) were deposited on three different Ga2O3 samples treated by UV-ozone followed by an HF dip. This improved surface preparation with a combination of UV-ozone and wet chemical treatment reduced near-interface traps resulting in a negligible hysteresis. The fast trap density was extracted to be 2.18×1011 cm-2, and the slow trap density excluding the contribution from fast traps was calculated to be 2.73×1011 cm-2. A net positive interface fixed charge of 1.56×1012 cm-2 was measured. The Dit was extracted accurately by accounting for dielectric bulk traps employing deep UV-assisted C-V method and physical models. An average interface state density of 6.63 ×1011 cm-2 eV-1 and AlSiO bulk trap density of 4.65×1017 cm-3 eV-1 were quantified, which is half of that for Al2O3 deposited by atomic layer deposition (ALD). In addition, a high dielectric breakdown field of ~7.8 MV/cm and more effective suppression of gate leakage were achieved on these devices compared with ALD-Al2O3 on similar metal-oxide-semiconductor (MOS) structures. The negligible hysteresis, lower interfacial trap density, low leakage current and high breakdown electric field achieved on AlSiO/Ga2O3 MOSCAPs reveals MOCVD AlSiO as a promising gate dielectric for high-performance Ga2O3 devices. [1] S. H. Chan et al., Jpn. J. Appl. Phys., vol. 55, no. 2, p. 021501, Feb. 2016. [2] D. Kikuta et al., J. Vac. Sci. Technol., vol. 35, no. 1, p. 01B122, Jan. 2017. [3] C. Gupta, et al., IEEE Electron Device Lett., vol. 38, no. 11, pp. 1575–1578, Nov. 2017. [4] I. Sayed et al., Appl. Phys. Lett., vol. 115, no. 17, p. 172104, Oct. 2019.

Electrical Properties of MOCVD-Grown AlSiO Gate Dielectric on (001) β-Ga2O3

Abstract Body: Vanadium dioxide (VO2) is well-known for its temperature-driven metal-to-insulator transition between the monoclinic (M1) phase and the rutile (R) phase, which is accompanied with abrupt changes in electrical and optical properties. These intriguing changes have attracted great interest and promises a lot of potential applications. However, the phase transition of bulk VO2 usually occurs around 68 °C with significant thermal hysteresis induced by the latent heat of the first-order transition. The relatively high phase transition temperature Tc and broad hysteresis width ΔTc are adverse to its practical application. Therefore, the modulation of its phase transition behavior is quite necessary. Accumulating evidence suggests that the strain state plays an important role in the phase transition behavior of VO2 and it has been confirmed that the specific strain between VO2 and TiO2 can significantly reduce Tc[1, 2]. Based on this, in order to finely modulate the phase transition behavior and further investigate its relationship with strain state, we inserted TiO2 epitaxial film between VO2 film and the (100) α-Al2O3 substrate as the buffer layer. By controlling the thickness of both VO2 film and TiO2 buffer layer, the variational strain state can be introduced and further lead to the change of lattice parameters and phase transition behavior of VO2 film. Specifically speaking, when both VO2 and TiO2 layers are thin (~10 nm), these two layers are highly strained to the Al2O3 substrate. Due to inconsistent lattice mismatch between [100] and [010] direction, the lattice parameter a and b of VO2 will be stretched and compressed respectively, leading to the suppression of phase transition. If increasing the thickness of TiO2 layer to 200 nm and maintaining the VO2 thickness of 10 nm, due to the lattice relaxation, TiO2 will be relaxed to the rutile phase. As the result, the structure of VO2 layer which is still strained to TiO2 will change synchronously and exhibit the sharper phase transition with reduced Tc of ~34 °C. This phenomenon is similar to the VO2 film epitaxially grown on TiO2 substrate. Lastly, by further increasing the VO2 thickness from 5.5 to 50 nm, the VO2 layer will also tend to be relaxed to bulk M1 phase and the Tc can be continuously adjusted in a wide range from 34 to 58 °C. More interestingly, when the VO2 is thicker than 30 nm, the ΔTc is extremely narrowed to ~0.4 °C and the lattice parameter c exceeds the bulk value correspondingly. We consider the strain-induced lattice parameter change can modify both of the Tc and ΔTc. And by controlling the strain state, the phase transition behavior of VO2 can be finely designed. These results may be useful for the design of VO2-based functional material and device. [1] Y. Muraoka, Z. Hiroi, Appl. Phys. Lett. 80, 583-585 (2002) [2] N. B. Aetukuri et al., Nat. Phys. 9.10, 661-666 (2013)

Reduction of the Tc and Suppression of the △Tc of VO2 Epitaxial Films on TiO2 Buffered Sapphire Substrate

Abstract Body: The addition of boron into conventional III-V alloys reduces their lattice constants, expanding the materials options for near-infrared photonics on GaAs1 and offering a path towards direct-bandgap III-V materials lattice-matched to Si.2 B-III-V active region emitters and detectors on GaAs and Si could have a profound impact on technologies such as telecommunications, laser radar, and photovoltaics. However, the addition of boron into conventional III-Vs forms a highly-mismatched alloy and their material properties alloy are underexplored. Other highly-mismatched constituents in conventional III-V alloys, most notably nitrogen and bismuth, significantly perturb the host band structure, resulting in a technologically-advantageous reduction in the bandgap, while undesirably degrading material properties required for high-performance devices. The addition of B to conventional III-V materials is predicted to degrade material properties less severely than other highly mismatched alloys, since boron is predicted to have a higher solid solubility.3 This suggests that superior quality material and higher levels of incorporation may be achievable with boron. This work investigates the effects of dilute boron incorporation on the electronic band structure of III-V alloys to determine their ultimate device potential. Dilute-nitrides have been found to suffer a significant reduction in carrier mobility, which was attributed to alloy scattering from large variations in the band edges due to band anticrossing.4 However, we previously observed a weaker reduction in carrier mobility in the dilute-borides as compared to dilute-nitride alloys and the carrier mobility of p-type doped B0.05Ga0.95As can be at around 180 cm2/Vs.2,5 We performed theoretical calculations of the band structure using the density functional theory (DFT) package Vienna Ab initio Simulation Package (VASP) with HSE06 hybrid functionals. We observed a splitting of the conduction band (CB) and a slight perturbation of the lower CB (relative to GaAs) when boron was incorporated substitutionally into (In)GaAs. The perturbed CB showed increased electron effective mass and density of states, which could be leveraged for low-threshold diode lasers with high differential gain, as well as low-noise avalanche photodiodes with suppressed electron-initiated impact ionization. Additionally, we observed similar conduction band perturbation when boron was incorporated as B-B split interstitials into GaAs. Detailed investigation of boron-related defects in BGa(In)As and their effects on the band structure is underway and will be reported. Experimental characterization of the CB splitting in BGa(In)As alloys, grown by solid-source molecular beam epitaxy, will also be reported. This work was performed in part at the University of Texas Microelectronics Research Center, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (No. ECCS-1542159). This work was also supported by the National Science Foundation (Award Nos. ECCS-1933836, DMR-1508646, and CBET-1438608). The authors also acknowledge the Learning, Exploration, Analysis, and Processing (LEAP) cluster at Texas State University for computing resources. 1 El-Jaroudi et al. Appl. Phys. Lett. (2012) 2 McNicholas et al. 60th Electronic Materials Conference (2018) 3 Hart and Zunger. Phys. Rev. B. (2000) 4 Fahy and O’Reilly. Appl. Phys. Lett. (2003) 5 McNicholas et al. Manuscript in preparation

Effect of B Distribution on the Band Structure of BGa(In)As Alloys

Abstract Body: III-V-bismide alloys have received significant attention over the past few decades due to the extreme bandgap reduction induced by the introduction of small concentrations of bismuth. However, dilute-bismides have struggled to achieve material quality commensurate with other III-V alloys, often attributed to the dramatically different growth temperatures of III-Bi and III-Sb/As materials. InSb1-xBix is a particularly promising candidate for accessing the midwave- to longwave-infrared (MWIR, LWIR respectively) with high-performance optical devices. Surprisingly, while GaSb1-xBix and InAs1-xBix have been widely studied, InSb1-xBix remains comparatively underexplored, with relatively few experimental investigations since the alloy was first synthesized nearly 50 years ago.1 In particular, growth in the “unity sticking” regime2 has not been demonstrated and photoluminescence (PL) has not been observed. In this work, we demonstrate high-quality InSb1-xBix films that exhibit a linear dependence of the bismuth concentration on the bismuth flux and, consequently, report the first PL from InSb1-xBix alloys. Epitaxial films, nominally 250 nm thick, were grown by solid-source molecular beam epitaxy on n-type (100) InSb substrates using a growth regime similar to that employed for high-quality GaSb1-xBix3 and InAs1-xBix4 synthesis. Samples were grown at a low substrate temperature of approximately 300°C with V/III flux ratios near stoichiometry. A comparatively fast growth rate of approximately 1 µm/hr was utilized to encourage bismuth incorporation. X-ray diffraction (XRD), PL, and atomic force microscopy (AFM) measurements were performed to characterize the bismuth content, optical quality, and surface roughness. X-ray diffraction measurements of the InSb1-xBix films suggested bismuth concentrations up to ~1.8%, assuming a lattice constant of 6.63 Å for InBi, which is reasonable given Rutherford backscattering spectrometry (RBS) measurements of both InAs1-xBix5 and InSb1-xBix.6 Photoluminescence measurements of the same InSb1-xBix films showed a red shift of PL associated with the InSbBi with increasing bismuth concentration. Specifically, PL measurements suggest a bandgap reduction of ~23 meV/% bismuth, which is comparable to previous reports of bandgap reduction estimated from optical absorption measurements of InSb1-xBix films.1,6 Temperature-dependent PL measurements of the highest Bi concentration film, containing 1.8% bismuth, showed a decrease in bandgap energy with increasing temperature that is consistent with a direct interband transition. AFM measurements of all films showed relatively smooth surfaces (RMS roughness of approximately 1 nm), however the highest concentration film exhibited large bismuth droplets. We expect droplet formation could be mitigated through optimization of the growth parameters. Thorough studies of the full growth space, particularly substrate temperature, as well as the effects of annealing on optical properties are in progress and will be presented. This work was supported by Lockheed Martin and NSF (ECCS-1933836). This work was performed in part at the University of Texas Microelectronics Research Center, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (No. ECCS-1542159). This work was also supported by an NSF Graduate Fellowship (RCW). *Corresponding Author: coreywhite@utexas.edu, Phone: 1-336-340-8587 1A. M. Jean-Louis, B. Ayrault, and J. Vargas Phys. Status Solidi B 34 (1969). 2S. Spruytte et al., MRS Internet J. Nitride Semicond. Res. 5 (2000). 3A. J. Ptak et al., J. Cryst. Growth 338 (2012). 4S. J. Maddox et al., 54th Electronic Materials Conf. (2012). 5S. J. Maddox et al., 55th Electronic Materials Conf. (2013). 6M. K. Rajpalke et al., Appl. Phys. Lett. 105 (2014).

Growth Optimization of InSbBi Alloys for Wavelength Extension on InSb

Abstract Body: Vacancy defects are unavoidable in 2D materials due to the challenges involved in fabricating defect-free lattice from experimental methods. While defects are known to degrade the mechanical properties of a lattice, they offer unique opportunities to tailor the electronic and thermal properties by controlling the mean free path of electrons and phonons. It is therefore important to develop a fundamental understanding of the role of defects in modulating the local electronic character of a lattice. Nevertheless, the understanding of defect-induced alteration of electronic properties (the electronic states for example) remains less understood, particularly under finite deformation. Although researchers have investigated the mechanical and electronic properties of pristine graphene under stress, there is little research on the chirality effects on electron distribution at defective sites in 2D materials under applied stress. In this work, we use the density functional theory simulations and the Mulliken charge population analysis to investigate the electronic behavior of an isolated defect in graphene under the uniaxial loading condition for five different chiralities of the lattice. We analyze how chirality and intensity of loading affect the mechanical properties, bond length and bond angle change, charge distributions, the density of states, and the partial density of states surrounding the vacancy. Our results show that there is a uniform pattern for bond length redistribution as well as electron redistribution at all orientation angles and that such bond and electron redistributions are directly correlated. During all simulations, we observe a bond reconstruction at the defective site at higher deformation. In terms of the orientation angle, we demonstrate that increased orientation angle yields changes in the strength, toughness, stiffness, bond distributions, and electronics surrounding a mono vacancy. We find that differing orientation angles result in different failure processes and speeds. Additionally, the redistribution at the defective site is chirality-dependent, and there is a significant localization of electronic states at the defective site. The pattern of density of states is deformation-dependent. This talk will present a comprehensive view on the specific effects of electron redistribution on mechanical and electronic characteristics of the defective lattice.

Electron Redistribution at Monovacancy in Graphene

Abstract Body: Beta-gallium oxide (β-Ga2O3) has emerged as an excellent candidate for transparent conducting oxides (TCO) applications such as ultraviolet photodetectors and optoelectronics due to its high electrical conductivity and transparency. To fulfill the extensive potential as an electronic material, β-Ga2O3 can be alloyed with Al2O3 for the tunability of a large bandgap up to 8.8 eV and modifiable photoresponse spectral range. The crystal structure of ground state Al2O3 is corundum phase (a), which is extensively different from the monoclinic phase of ground state Ga2O3. Density functional theory (DFT) calculations have denoted the monoclinic phase of (AlxGa1-x)2O3 is as an energetically preferred structure for up to 71% Al incorporation [1], despite the structural variations between these two materials. However, previous studies on the growth of (AlxGa1-x)2O3 heterostructures with high Al composition indicated impaired film quality or phase transformations [2], indicating the challenges in the controlling point and extended defects and achieving the theoretical maximum Al content while maintaining the monoclinic phase. These limitations have demonstrated the demand for direct characterization of Al incorporation and defects in beta-(AlxGa1-x)2O3 thin films. In this report, we investigated the atomic structure and defects in beta-(AlxGa1-x)2O3 thin films using scanning transmission electron microscopy (STEM). Our quantitative STEM analysis for beta-(AlxGa1-x)2O3 films grown by molecular beam epitaxy (MBE) and metal organic chemic vapor deposition (MOCVD) revealed two significant pieces of information for atomic-scale origins of growth characteristics and properties of the heterostructures. Our characterization indicated that 54% of the incorporated Al occupied the octahedrally coordinated Ga2 site, suggesting the non-equilibrium growth conditions lead to a higher energy structure. DFT calculations have shown that Al can be incorporated on the less thermally favorable tetrahedral Ga1 site due to its occupation in particular configurations on the surface and kinetic limitations. On the other hand, the formation of a planar defect perpendicular to the growth direction associated with a larger Al concentration corresponded to a local inclusion of the gamma phase. We have identified the local Al content reaches a critical value of about 50%, which could destabilize the monoclinic phase of (AlxGa1-x)2O3 films. DFT calculations have demonstrated these planar defects are thermally stable at higher Al compositions, and its formation reduces the in-plane stress of tensile-strained films. The projection of cation columns identically match with the recently observed divacancy-interstitial complex structures [3]. In summary, our STEM research offers essential atomic scale insights on exact control of impurity incorporation for semiconductor bandgap engineering, and eventually provides guidance to the future epitaxial growth of (AlxGa1-x)2O3 for high-performance power electronics applications. We acknowledge support by the Department of Defense, Air Force Office of Scientific Research GAME MURI Program (Grant No. FA9550-18-1-0479). [1] H. Peelaers, J. B. Varley, J. S. Speck, and C. G. Van De Walle, Structural and Electronic Properties of Ga2O3-Al2O3 Alloys, Appl. Phys. Lett. 112, 242101 (2018). [2] A. F. M. A. U. Bhuiyan, Z. Feng, J. M. Johnson, H. L. Huang, J. Sarker, M. Zhu, M. R. Karim, B. Mazumder, J. Hwang, and H. Zhao, Phase Transformation in MOCVD Growth of (AlxGa1-x)2O3 Thin Films, APL Mater. 8, 031104 (2020). [3] J. M. Johnson, Z. Chen, J. B. Varley, C. M. Jackson, E. Farzana, Z. Zhang, A. R. Arehart, H. L. Huang, A. Genc, S. A. Ringel, C. G. Van De Walle, D. A. Muller, and J. Hwang, Unusual Formation of Point-Defect Complexes in the Ultrawide-Band-Gap Semiconductor β-Ga2 O3, Phys. Rev. X 9, 041027 (2019).

Atomic Scale Investigation of Aluminum Incorporation, Defects and Phase Stability in β-(AlxGa1-x)2O3 Films

Abstract Body: Controllable n-type doping in AlGaN and AlN is crucial for applications in electronics, optoelectronics, and power electronics. Si and recently Ge have been utilized as n-type dopants in AlxGa1-xN with low ionization energy for x<0.8 and x<0.5, respectively.1,2 However, the transition of the Si and Ge shallow donors to deep states occurs at x>0.8 and x>0.5, presenting challenges for novel applications employing Al rich AlGaN and AlN. While such a transition was previously observed in the AlGaAs system, it was confirmed to be an acceptor type DX-1. However, no such direct proof of the Si and Ge charge states in Al rich AlGaN and AlN exists. In this work we present direct evidence of single electron occupancy in the Ge deep state in AlxGa1-xN (x>0.5) i.e., Ge transitions to a deep neutral donor. Co-doping with the shallow donor Si (at x < 0.8) was used as a source of free electrons in Ge doped AlGaN grown via MOCVD to follow on the charge state of Ge. The measured carrier concentration (n) in Al rich AlGaN shows whether Ge is a shallow donor (n~[Si]+[Ge]), neutral deep donor (n~[Si]) or a compensating acceptor type DX-1 (n~[Si]-[Ge])). Temperature dependent carrier concentration measurements suggested an increase in the ionization energy of Ge: AlGaN with increase in Al content and a sudden drop in the carrier concentration was shown to occur at x>0.5 showing that Ge transitions to a deep state.1 Si remained a shallow donor with low ionization energy up to x~0.8. Therefore, both dopants are expected to be shallow donors in Al0.4Ga0.6N forming a d+ state. However, possible electron compensation in Si and Ge codoped Al0.65Ga0.35N where Ge transitions into a deep state, would directly show the charge sate of the deep state Ge, either D0 or DX-1. Carrier concentration measurements versus Al content under constant Si and Ge concentrations of 1×1019 cm-3 and 8×1018 cm-3, respectively, were realized at RT and 600°C. Negligible compensation and a temperature independent carrier concentration are observed in Al0.4Ga0.6N were both Ge and Si are expected to be in their shallow states. Interestingly, a reduction in carrier concentration at x~0.65 corresponding only to the Ge concentration was observed, indicating that Ge is not providing free electrons nor compensating the free electrons provided by Si. This is the signature of a deep donor having a higher ionization energy and not of an acceptor state. At higher temperatures, the ionization of the deep donor Ge provides higher carrier concentrations as expected for such a donor. Contrary to the formation of a stable DX-1, which is expected to pin the Fermi level at the (-/+) thermodynamic transition3, transition to a deep donor allows for the possibility of highly conducting AlN and Al rich AlGaN by degenerately doping the system. Hence, this work cast light on the true nature of donor related deep states in nitrides, further opening pathways for doping engineering and achievement of high conductivity in Al rich AlGaN and AlN. 1. Bagheri, P. et al. The nature of the DX state in Ge-doped AlGaN. Appl. Phys. Lett. 116, 222102 (2020). 2. Collazo, R. et al. Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications. Phys. Status Solidi C 8, 2031–2033 (2011). 3. Gordon, L., Lyons, J. L., Janotti, A. & Van de Walle, C. G. Hybrid functional calculations of D X centers in AlN and GaN. Phys. Rev. B 89, 085204 (2014).

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