United States

There have been studies of understanding and controlling the charge transport through molecular junction wince it gives fundamental grounds that helps to gain a novel idea to realize numerous potential molecular functional devices. [1,2] There are some key factors considered in molecular charge transport, such as molecular barrier which is directly related to molecular orbital levels, coupling strength, asymmetric factor, and molecular configuration and conformation. Much research has been reported to obtain deeper insights of such key factors under certain controlled conductions with the effect of the external stimuli applied on the molecular junction such as light irradiation [3], electrochemical gating [4], electrical field [5], and mechanical stress such by tip loading force [6]. Molecular heterojunction with two-dimensional (2D) semiconductor provides various key factors for the charge transport through the heterojunction; however, the effect of molecular tilt configuration on the molecular heterojunction have never been studied. Here, we present the effect of molecular tilt configuration on the molecule/mono-layer (1L) 2D semiconductor heterojunction by tip loading force with conductive atomic force microscopy (C-AFM) technique. With various tip loading force (1-30 nN), the molecular tilt configuration affects differently for the three different interfaces in Au/molecule/1L-2D semiconductor/Au heterojunction that the transition voltage spectroscopy (TVS), which is a conventional tool to analyze charge transport through molecular junction, rectification ratio (RR), non-linearity (NL) and possible array size are demonstrated. Molecule/2D semiconductor heterojunction system has been reported as molecular diode (with RR) and selector (with NL) and it is a novel platform owning so much various potential in, that understanding the charge transport by the effect of external stress on the heterojunction will help developing such future molecular scale functional devices. [1] L. A. Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin, L. Jones, D. L. Allara, J. M. Tour, P. S. Weiss, Science 271, 1705 (1996),. [2] H. Song, Y. Kim, Y. H. Jang, H. Jeong, M. A. Reed, T. Lee, Nature 462, 1039 (2009). [3] G. Reecgt, C. Lotze, D. Sysoiev, T. Huhn, K. J. Franke, ACS Nano 10, 10555 (2016). [4] R. A. Wong, Y. Yokta, M. Wakisaka, J. Inukai, Y. Kim, Nat. Commun. 11, 4194 (2020). [5] J. Shin, S. Yang, Y. Jang, J. S. Eo, T.-W. Kim, T. Lee, C.-H. Lee, G. Wang, Nat. Commun. 11, 1 (2020). [6] G. Wang, T. -W. Kim, G. Jo, T. Lee, J. Am. Chem. Soc. 131, 5980 (2009).

EMC 2023

Effect of Molecular Tilt Configuration in Molecular Heterojunction with Two-Dimensional Semiconductor

2d semiconductor

molecular heterojunction

molecular electronics

technical paper

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). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Many semiconductor devices (e.g., light emitters and photovoltaics) utilize heterojunctions of doped and undoped layers or depend on gradients of electronic doping to control charge transport. Doping of conjugated polymers requires the introduction of small molecules dopants which undergo charge transfer reactions with the polymer. The rate of the doping reaction and diffusion of the dopant controls the overall doping level. Conjugated polymers can be electrically doped by strong acids, but the details of the reaction mechanism and subsequent stability are not understood. Here, we show a clear kinetic isotope effect in the doping of thin films of poly(3-hexylthiophene) (P3HT) by bis(trifluoromethane)sulfonimide (HTFSI) from solution indicating that this doping process is limited by proton transfer to the polymer. The subsequent charge transfer reaction occurs rapidly between polymer segments in the solid state, thus circumventing the need to match redox potentials of dopants and conjugated polymers. Dynamic secondary ion mass spectrometry (DSIMS) of doped films show that H/D is retained in doped films after the doping process. Complementary X-ray photoelectron spectroscopy and DSIMS depth profiling of dopant concentrations show definitive evidence of enrichment at the P3HT surface. These surface-limited concentration profiles suggest that diffusivity of dopants vary inversely with dopant concentration due to doping-induced changes to the structure of the conjugated polymer.

Surface-Confined Brønsted Acidic Doping of Conjugated Polymers Thin Films

The paper reports on a novel process flow to manufacture conductive organic electrodes from highly conductive doped PEDOT:PSS polymer films that can be patterned and display good adhesion to oxidized Si wafers as well as to flexible substrates, such as Mylar. It is shown that multiple depositions of PEDOT:PSS increase the electrical conductivity by more than two orders of magnitude without increasing the film thickness of PEDOT:PSS significantly. An exponential dependence between sheet resistance and the number of PEDOT:PSS coatings is found. Additional coatings, in excess of 6 coatings provide diminishing returns to the lowering of the sheet resistance. The dependence of the sheet resistance Rsq(n) as function of the number of PEDOT:PSS coatings, n, can be described by the formula Rsq(n)=230×exp[-(n-1)×1.28] kΩ. The multiple coating methods can be further optimized by selecting a low spinning speed (e.g. 1500 rpm) for the initial coatings and a high spinning speed (e.g. 3000 rpm) for the final coatings. The reason for the decrease in the sheet resistance is the densification of the conductive PEDOT ribbons at the bottom part of the PEDOT:PSS layer. The electrical conductivity of PEDOT:PSS can be increased by another two orders of magnitude by doping it with Cu nanoparticles (Cu NP) when coated on the surface of a soft-baked PEDOT:PSS film. However, if Cu NP are dispersed in the aqueous solution of PEDOT:PSS then the impact on the sheet resistance is nil. It is found that both kinds of conductivity enhancement are not additive. The doping with Cu nanoparticles is desirable, as it provides the surface of an organic electrode with readily ionized copper ions useful for several applications including Cu filament formation in organic switching layers of a resistive RAM (ReRAM) memory cell. Adhesion of PEDOT:PSS to oxidized Si wafers and to BoPET (Mylar) has been ensured by applying an oxygen plasma cleaning step prior to spin coating. The manufactured high conductivity PEDOT:PSS film can be patterned using a sacrificial metal layer with subsequent etching of PEDOT:PSS in oxygen plasma, followed by the removal of the patterned segments of the sacrificial metal layer in an aqueous acid solution on both Si wafers and flexible Mylar substrates. A single PEDOT:PSS layer has a thickness of around 60 nm depending on the spinning speed and has sheet resistance (Rsq) of 230 kΩ. The same deposition applied 6 times results in a PEDOT:PSS layer of 90 nm thickness (instead of 6×60 nm=360 nm) and Rsq=980 Ω. A surface doping of 1xPEDOT:PSS with Cu NP lowers the sheet resistance from 230 kΩ to 60 kΩ, whereas in the case of multiple coatings, the 6xPEDOT:PSS with the same Cu doping lowers the Rsq from 980 Ω to 6.2 Ω. Thus, with reference to one PEDOT:PSS layer, the sheet resistance has been reduced by a factor of 4×104. The methods described above can be further combined with the methods of in situ doping of PEDOT:PSS with inert metal NP such as Ag and Au NP, that are largely immune to the oxidation reactions in the aqueous solutions, and with acid treatment such as nitric or phosphoric acid. It has been found that all the various methods to increase the conductivity mentioned above are not additive and, in some cases, work at cross-purposes with one another. Therefore, a tailored optimum combination of the available methods requires a detailed understanding of their respective mechanisms. Careful morphological analysis performed by dint of optical microscope picture and surface roughness plots by AFM reveals a close interplay between the thickness of the PEDOT:PSS film, the density of the PEDOT:PSS particles - here controlled by the multiple coatings -, and the size of the Cu NP. The size of Cu NP should be smaller than the film thickness, and the density of PEDOT:PSS particle should be neither too small nor too high to accommodate the Cu NP in the PSS spaces between the conductive PEDOT ribbons to achieve the lowest sheet resistance.

Manufacturing of Highly Conductive Organic PEDOT:PSS Films for Electronic Devices

Electrocorticography (ECoG) is a critically important neuroscientific tool for recording neural activity in the cerebral cortex. Advances in recent years have produced high-resolution ECoG arrays, which have enabled brain-machine interface (BMI) applications such as speech and motor intention decoding systems. Alternatively, cortical activity can be studied using optical tools such as genetically encoded calcium indicators (GECIs) and optogenetics. Optical access to the brain is typically achieved with a small craniotomy covered by a transparent window. Electrophysiology and optical imaging provide complementary neural sensing capabilities: electrophysiological recordings have high temporal resolution, while optical imaging allows recording of genetically-defined populations at high spatial resolution. Combining these two modalities for simultaneous large-scale, multimodal sensing of neural activity across multiple brain regions can be very powerful. To this end, many research groups have developed transparent ECoG arrays that can be used simultaneously with optical recording methods in small rodents and nonhuman primates. These devices are made possible by using transparent conductive materials such as graphene or indium tin oxide (ITO). Alternatively, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a biocompatible transparent organic conductor which is routinely used to coat chronic neural interfaces in order reduce the interfacial impedance of the contact site. We have developed inkjet-printed transparent PEDOT:PSS microelectrodes which use a novel ring-shaped geometry to reduce interfacial impedance and improve charge injection. [1] This is accomplished by taking advantage of the volumetric capacitance in PEDOT:PSS. Ring-shaped microelectrodes with diameters ranging from 300 µm to 550 µm were fabricated by inkjet-printing PEDOT:PSS, encapsulating with Parylene C, and then exposing a contact site that is much smaller than the microelectrode outer diameter. This unique structure allowed the encapsulated portion of the microelectrode volume surrounding the contact site to participate in signal transduction, which reduced impedance and enhanced charge storage capacity. For the same size contact site, increasing the outer diameter of the encapsulated electrode from 300 to 550 μm reduced the impedance from 294 ± 21 to 98 ± 2kΩ, respectively, at 1 Hz. Similarly, the charge storage capacity was enhanced from 6 to 21 mC cm-2. The PEDOT:PSS microelectrodes provide a low-haze, high-transmittance optical interface, demonstrating their suitability for optical neuroscience applications. They remained functional after a million 1 V stimulation cycles, up to 600 μA of stimulation current, and more than 1000 mechanical bending cycles. We have integrated these inkjet-printed PEDOT:PSS electrodes into transparent cranial windows for multi-modal recording of activity from the cortex.[2] Implanted on transgenic mice expressing the Calcium (Ca2+) indicator GCaMP6f in excitatory neurons, these “eSee-Shells” provide a robust opto-electrophysiological interface for over 100 days. eSee-Shells enable simultaneous mesoscale Ca2+ imaging and electrocorticography acquisition from multiple brain regions covering 45 mm2 of cortex under anesthesia and in awake animals. These eSee-Shells allow either mesoscale imaging across the entire field of view (FOV) or random-access cellular level imaging of small FOVs throughout ≈45 mm2 of the cerebral cortex, an area that is an order of magnitude larger than previous transparent ECoG electrode arrays. [1] P. D. Donaldson and S. L. Swisher, Physica Status Solidi (a) 2022. [2] P. D. Donaldson et al., Adv. Healthcare Mater. 2022.

Transparent, Inkjet-Printed PEDOT:PSS Electrotrode Arrays for Large-Area Multimodal Neural Interfaces

We present vanadium (V) based ohmic contacts to high composition n-AlGaN grown on single crystal bulk aluminium nitride substrates by plasma-assisted molecular beam epitaxy (PAMBE). Metal-semiconductor contacts are integral for the performance of electronic devices such as transistors, light-emitting diodes (LEDs), lasers and photodetectors [1,2]. Low-resistance Ohmic contacts have proven difficult to achieve on n-AlGaN epilayers grown on single crystal bulk AlN substrates as compared to the ones grown on AlN-on-sapphire templates [3]. This is because the large density of threading dislocations allow for conduction pathways [4]. While the low dislocation density of the films grown on bulk substrates reduce optical losses, it also makes low resistance ohmic contacts difficult due to lack of such conduction channels. Since the bandgap of AlGaN increases with increasing Al composition, it is difficult to find suitable metals with low work functions for a low energy barrier in the conduction band. Therefore, an alternate way to make ohmic contacts to wide bandgap semiconductors is to heavily dope the semiconductor layers to thin down the depletion enough to enable tunneling of electrons. A Ti-based metal stack has been shown to work for low resistance ohmic contacts to low composition n-AlGaN [5]. More recently, vanadium based ohmic contacts have been achieved on entire AlGaN composition grown on sapphire wafers [6]. It is hypothesised, that through annealing, vanadium nitride (VN) forms at the junction and has a work function of 3.56 eV [7]. In this work we demonstrate V-based Ohmic contacts on Si-doped Al0.7Ga0.3N grown by PAMBE on bulk AlN. Degenerately doped AlGaN layers with Silicon doping concentrations [Si] = 3e19/cm3-9e19/cm3 were used. The metal-polar single-crystal AlN substrates were cleaned ex-situ using solvents and acids, as well as in-situ by Al-assisted cleaning [8,9]. A 500 nm thick buffer layer was grown in Al-rich conditions to separate the AlGaN layer from the substrate. Then a 400-440 nm thick AlGaN layer was grown in metal-rich conditions. Atomic steps were observed in the coherently strained epilayers that have an energy bandgap of 4.8 eV as measured by room temperature photoluminescence. The samples were blanket etched 70 nm using chlorine-based reactive ion etching to emulate the n-AlGaN contacts as required for a light-emitting diode or laser diode. Two metal stacks were explored. Bottom-up, they consist of V(20n m)/Al(80 nm)/V(40 nm)/Au(100 nm) and V(20 nm)/Al(80 nm)/Ni(40 nm)/Au(100 nm). They were deposited using an e-beam evaporator and annealed by rapid thermal annealing (RTA) for 30 seconds at 800oC in N2 ambience. Linear transfer length method (TLM) analysis was done to determine the specific contact resistance and sheet resistance for which the results are shown in Figure 2. Successive annealing steps were performed to explore the effect of annealing time on the contact resistance. We found that a 30 second anneal is enough to make a low resistance ohmic contact. For both samples V/Al/V/Au contact gave a lower specific contact resistance than the V/Al/Ni/Au contact. We find that only a very high doping (9e19/cm3) results in low resistance ohmic contacts in the 10-4 ohm.cm2 range. These optimised doping conditions and metal stacks are now available to be used for ultrawide bandgap photonic and electronic devices on homoepitaxial AlGaN. References: [1] AIP Advances 12, 035023 (2022) [2] Appl. Phys. Lett. 121, 222103 (2022) [3] Appl. Phys. Lett. 114, 113501 (2019) [4] Journal of Applied Physics 117, 245702 (2015) [5] Sampath, A., et al. "Metal Contacts to n-AlxGa1-xN." MRS Online Proceedings Library (OPL) 482 (1997) [6] Appl. Phys. Lett. 90, 062115 (2007) [7] Fomenko, V. S. (2012). Handbook of thermionic properties. Springer Science & Business Media [8] Applied Physics Letters 116.17 (2020): 17210 [9] Applied Physics Letters 116.26 (2020): 262102

Ohmic Contacts to Homoepitaxial Ultrawide Bandgap n-AlGaN Grown on Bulk AlN Substrates

Low specific contact resistivity ohmic contacts (ρc ~ 10-6 Ω-cm2) with thermal and chemical stability play an important role in III-nitrides based devices. A Ti-Al-Ni-Au multilayer metallization is the most commonly used scheme and requires high temperatures (>800oC) to form reliable ohmic contacts. There are a few reports on the surface microstructures and chemical composition of the Ti-Al based contacts [1-2]. The low contact resistivity has been explained via the formation of TiN and TiAl3 phases along with Al-Ni agglomerates surrounded by alloys of Al-Au, mainly AlAu2, Al2Au5 and AlAu4. We have investigated low ρc ohmic contacts (ρc ~ 10-7 Ω-cm2) formed on n-GaN using a Re-Al-Ni-Au multilayer stack annealed at 650-700oC [3]. In this work, we report the effect of Re layer thickness and annealing temperature on the microstructure of these contacts. Several samples with Re thickness (10, 30, 60 nm) were prepared and annealed at 450oC—800oC temperatures. The ρc was estimated using C-TLM measurements while AFM, GIXRD and SEM equipped with EDS were used to analyze the chemical composition and microstructures of the contacts. AFM images show surface microstructures as a function of annealing temperatures in the range of 500—800oC in steps of 50oC for 30 nm Re samples. The metals form almost a continuous film with signs of segregation into small clusters at an annealing temperature up to 500oC. The films show more extensive segregations into separate islands, forming larger agglomerates and the density of these islands keeps on reducing, with a concomitant increase in height with increasing temperature. The RMS roughness increases linearly with annealing temperature. AFM images of the surface microstructures as a function of Re thickness for the annealing temperature where lowest ρc is observed; (a) 10 nm at 650oC and (b) 30 and 60 nm at 700oC; shows that as the Re thickness increases, the size of the agglomerates increases, with the corresponding reduction in the density. The RMS surface roughness also increases with Re content. For contacts with 10 nm Re, the RMS roughness starts increasing slowly with increasing temperature from 450oC with a faster rate of increase beyond 600oC. For contacts with 30 nm Re, the surface roughness increases linearly over the entire temperature range 500—800oC. However, in case of contacts with 60 nm Re, the surface roughness starts increasing initally and then saturates around at ~170 nm. EDS elemental scans for Re, Al, Ni and Au for samples with Re thickness 10, 30 and 60 nm, annealed at temperatures for which ρc is minimum, shows that the Ni layer disintegrates to form isolated islands and also Al-Ni agglomerates. There is a thin continuous layer of Al over the entire surface with a few isolated patches devoid of aluminum. A continuous layer of Re underneath is observed across the surface. The dark spots observed in the Re scan are due to the thick agglomerates which absorb the signal from the underlying Re layer. The gold layer seems to get accumulated around the agglomerates. Glancing angle SEM image of the surface of 60 nm Re sample annealed at 650oC shows hills formed due to the Al-Ni agglomerates covered by Al-Au films spread across entire surface. A cross section view of one of the agglomerates shows a thick lump of Al-Ni in the centre of the hill, while rest of the surface is covered by an Al-Au layer. The Re layer is delineated from the top structure. The Re layer may form intermetalic phases of Re-Ni-Al in the center, while the different phases of Al-Re are formed on the entire surface. The combinations of phases of Al-Re, Re-Al-Ni, Al-Au along with a thin layer of Re-N formed between the Re and GaN interface gives the low ρc of this material system. [1] S. Ruvimov, et al., Appl. Phys. Lett., 69, 1556 (1996). [2] W. Macherzynski et al., Optica Applicata, XLIII, 67 (2013). [3] A. P. Shah, et al., J. Appl. Phys., 132, 075701 (2022).

Evolution of Surface Microstructure of Re-Al-Ni-Au Based Ohmic Contacts on N-Type GaN

Quantum well infrared photodetectors (QWIP) based on III-Nitrides are a subject of increasing interest due to their potential for applications in telecommunications, optical gas sensors, military, and imaging. These detectors rely on intersubband transitions (ISBTs) of an electron (hole) between the ground state and the first excited state in the conduction (valance) band. Large conduction band offset (CBO) of nearly 2 eV [1], [2] and valance band offset (VBO) of nearly 0.85 eV [3] in AlN/GaN QWIPs make this material system suitable for a wide range of ISBTs depending on Al composition and specifically in the spectral regions where GaAs-based QWIPs are limited due to their smaller bandgap offsets. Also, the longitudinal-optical (LO) phonon energy of GaN (92 meV) is more than two times higher than GaAs [4], which makes this material an ideal choice for room temperature detection application. By employing GaN-based QWIPs, ISBT energies can be tuned from a few meV to 2 eV based on QWIP design, allowing for full NIR to IR band tunability within a single material system. While III-N materials can be grown in cubic zincblende structure, only hexagonal wurtzite structure is thermodynamically stable. In this structure, lack of inversion symmetry along the (growth) c-axis, generates a spontaneous polarization (Psp) along this axis [5]. Additionally, at AlGaN/GaN heterointerface, the lattice mismatch causes strain leading to piezoelectric polarization [6]. The discontinuity of polarization at heterointerfaces results in an uncompensated sheet charge at the interface. Further due to the polarization selection rule, normal incidence absorption is not allowed in n-QWIPs [7], requiring a light coupler and adding to design complexity. To circumvent this issue, p-QWIPs are being considered. In this work, we report on the growth of 5 periods Mg-doped Al0.4Ga0.6N/GaN QWIP, by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire for a target 1.55 µm absorption. Critical to the performance of the QWIP are high controllability of layer thicknesses and composition in superlattice and interface quality. Sub-nanometer control of layer thicknesses is challenging in MOCVD growth due to the high temperature and the relatively poorer control of precursor flux on the sample surface, compared to other methods such as MBE. To overcome this, a pulsed growth method was used to achieve higher control of growth rate [8]. A TCAD-based physics model was implemented in the commercial Synopsys Sentaurus to model the energy band diagram, electric field and calculate the band offsets between AlxGa1-xN/GaN heterointerfaces by considering polarization charges. Based on the CBO and VBO in the QW, layers thicknesses and compositions can be adjusted to achieve desired absorption wavelength. X-ray diffraction and transmission electron microscope (TEM) are used to assess strain, interface quality, layer thicknesses, and aluminum composition. Transmission/reflection measurement is used to determine the absorption spectrum of the QWIP. [1]D. Cociorva et al., ‘Quasiparticle calculations of band offsets at AlN–GaN interfaces’, Solid State Commun, 2002 [2]A. N. Westmeyer et al., ‘Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys’, J Appl Phys, 2006 [3]G. Martin et al., ‘Valence-band discontinuities of wurtzite GaN, AlN, and InN heterojunctions measured by x-ray photoemission spectroscopy’, Appl Phys Lett, 1996 [4]K. Ahi, ‘Review of GaN-based devices for terahertz operation’, Optical Engineering, 2017 [5]H. Ünlü et al., ‘Band offsets in III-nitride heterostructures’, J Phys D Appl Phys, 2002 [6]S. Nicolay et al., ‘Strain-induced interface instability in GaN/AlN multiple quantum wells’, Appl Phys Lett, 2007 [7]B. F. Levine, ‘Quantum-well infrared photodetectors’, J Appl Phys, 1993 [8]P. Suvarna et al., ‘Design and growth of visible-blind and solar-blind III-N APDs on sapphire substrates’, J Electron Mater, 2013

Design and Optimization of Room Temperature AlGaN/GaN Multi Quantum Well Infrared Photodetector by MOCVD for Near IR Range

We report on the use of reverse-graded heterojunction contacts to ultra-wide bandgap AlGaN to enable contact resistivity as low as 3.1 x 10-6 Ω.cm2 to a Al0.85Ga0.15N layers. This represents almost 1000X lower contact resistivity than that obtained in similar MOCVD-grown structures. Lateral transistors based on ultra-wide bandgap Al-rich AlGaN channels are promising for high frequency power amplifier applications as their higher electric breakdown fields enable improved Johnson Figure of Merit when compared with the GaN-based high electron mobility transistors [6][7]. However, a major challenge in the past has been the high contact resistance to ultra-wide bandgap AlGaN, a consequence of the significantly lower electron affinity for high Al-content AlGaN, which increases the Schottky barrier. The contact resistance leads to a lower current density and transconductance. Reverse graded top contacts have been shown to be a very promising approach to realize low contact resistance to AlGaN channels in recent studies [1][4][5][8][11]. In this work, we report on the design and demonstration of MOCVD-grown reverse graded AlGaN contact layer to achieve record low contact resistance – 0.7 Ω.mm on Al0.85Ga0.15N channel and analyze the contact resistance components based on two types of transfer length measurement (TLM) structures. Highly doped reverse graded contact layers were grown by MOCVD following Al0.85Ga0.15N channel, on which two types of TLM structures were fabricated. The epitaxial structure consists of 400 nm doped Al0.85Ga0.15N channel, 100 nm highly doped reverse graded contact layer – grading from Al0.85Ga0.15N to Al0.14Ga0.86N and 30 nm highly doped Al0.14Ga0.86N layer. Heavy doping in the reverse graded layer not only compensates the polarization-induced holes, but also enables good n+ ohmic contact on Al0.14Ga0.86N surface. Cross-sectional scanning transmission electron microscopy (STEM) images show very linear grading of composition and growth matches our design very well with energy dispersive spectroscopy (EDX) mapping. The fabrication flow started with ohmic metal deposition – Ti/Al/Ni/Au (20 nm/120 nm/30 nm/100 nm) by e-beam evaporator followed by rapid thermal annealing in N2 ambient for 30s. Mesa isolation was carried out after annealing. To understand the contact resistance from such structure, we fabricated two different types of structures for TLM – with and without recessing the contact layer by ICP/RIE dry etch. The components of contact resistance include metal-semiconductor (MS) resistance, sheet resistance of contact layer, and contact-to-channel resistance. From TLM results, the estimated MS resistance, sheet resistivity of contact layer and specific contact resistance are 0.023 Ω.mm, 181 Ω/sq, and 2.8×10-8 Ω.cm2 and the total contact resistance on recessed structure, sheet resistivity of doped Al0.85Ga0.15N channel and specific total contact resistance are 0.7 Ω.mm, 1.7 kΩ/sq and 3.14×10-6 Ω.cm2. Based on these estimates, we estimate a resistance of 0.59 Ω.mm between the reverse-graded contact layer and the channel layer, making it the dominant component in the total contact resistance. In summary, we have demonstrated MOCVD-grown reverse graded AlGaN contact layer on Al0.85Ga0.15N channel. Our work shows the promise of MOCVD-grown reverse graded contact structure on Al-rich AlGaN channel. References: [1] S. Bajaj et al., APL, 109, 133508 (2016). [2] H. Okumura et al., Jpn. J. Appl. Phys. 57, 04FR11 (2018). [3] A. M. Armstrong et al., Jpn. J. Appl. Phys. 57, 074103 (2018). [4] P. S. Park et al., IEEE EDL, 36, 226 (2015). [5] S. Bajaj et al., IEEE EDL, 39, 256 (2018). [6] X. Hu et al., IEEE EDL, 39, 1568 (2018). [7] H. Xue et al., 76th DRC, CA, June 24–27, 2017. [8] S. Muhtadi et al., APL, 110, 171104 (2017). [9] S. Muhtadi et al., APL, 110, 193501 (2017). [10] J. Lemettinen et al., IEEE EDL. 40, 1245 (2019). [11] T. Razzak et al., Electron. Lett. 54, 1351 (2018). [12] A. M. Armstrong et al., APL, 114, 052103 (2019).

Demonstration of 0.7 Ω.mm MOCVD-Grown Reverse Graded Contacts on Al0.85Ga0.15N Channel

Obtaining uniform silicon concentration, especially with low concentrations (ranging from 1x1016 cm-3 to 1x1018 cm-3) by molecular beam epitaxy (MBE) has been challenging due to the oxidation of silicon solid source in the oxide environment. In this work, Si doping of β-Ga2O3 (010) films by diluted disilane as the Si source is investigated using a hybrid plasma-assisted molecular beam epitaxy (hybrid-PAMBE). The impact of growth temperature, disilane source concentration, and disilane flow rate on Si incorporation was studied by secondary ion mass spectrometry. Uniform Si concentrations ranging from 3×1016cm-3 to 2×1019cm-3 are demonstrated. Si-doped β-Ga2O3 films with different silicon concentrations were grown on Fe-doped β-Ga2O3 (010) substrates. The electron concentration and mobility were determined using Van De Pauw Hall measurements. A high mobility of 135cm2/Vs was measured for an electron concentration of 3.4×1017cm-3 at room temperature.

Si Doping of β-Ga2O3 by Disilane via Hybrid Plasma-Assisted Molecular Beam Epitaxy

Ga2O3 has garnered significant interest due to its ultra-wide bandgap, high electron mobility, and large breakdown field. These properties exceed the current benchmarks set by state-of-the-art materials such as SiC and GaN and make it optimal for next-generation power devices. Still, it has been proposed that the properties of Ga2O3 can be extended further by alloying with Al to form (AlxGa1-x)2O3 which could raise the bandgap up to 8.6 eV. This goal presents a challenge for the most commonly researched phase, β-(AlxGa1-x)2O3, as it is structurally unstable at higher Al concentrations and transitions to the corundum structure that is favored for Al2O3. This structural mismatch limits the compositional range, thereby limiting the range of attainable bandgaps. In contrast, α-phase Ga2O3 has been shown to allow alloying over the full range of x, unlocking tunable bandgaps over the range of 5.3 - 8.6 eV. One of the techniques available for growing α-(AlxGa1-x)2O3 is molecular-beam epitaxy (MBE). MBE is a powerful and highly controllable growth technique for thin films with one drawback being its relatively slow growth rate. For high-quality β-Ga2O3 films grown by conventional MBE the growth rate may approach 0.2 μm/h. One method to circumvent the kinetic limitations of conventional MBE is the technique of suboxide MBE, which has been used to grow β-Ga2O3 thin films at rates exceeding 1 μm/h. This work investigates the application of suboxide MBE to the growth α-(AlxGa1-x)2O3 thin films. We show that suboxide MBE leads to improved growth kinetics for III-O compounds, and enables the epitaxial growth of high-quality α-Ga2O3 and α-(AlxGa1-x)2O3 thin films. This is demonstrated by the suboxide MBE growth of epitaxial α-(AlxGa1-x)2O3 with high structural quality over the full range of x at growth rates exceeding 1 μm/h. For this suboxide MBE study, gallium suboxide and elemental aluminum were the MBE sources and A-plane sapphire was used as the substrate. During each growth the distilled ozone pressure (5 x 10-6 Torr) was held constant while the gallium suboxide flux and aluminum flux were varied to control the film composition. We show that the film composition can be controlled directly by the relative ratios of the Ga2O and Al fluxes. Our films are shown to have high structural quality as revealed by the full width at half maximum (FWHM) of rocking curves of the α-(AlxGa1-x)2O3 films ranging from 11 - 15 arcseconds; these FWHMs are identical to those of the underlying sapphire substrates. The surfaces of the films are also demonstrated to be smooth with RMS roughnesses measured by atomic force microscopy ranging from 0.3 - 1.1 nm on α-(AlxGa1-x)2O3 films with thicknesses in the 17.8 - 47.8 nm range. Lastly, we confirm that the aluminum content of the α-(AlxGa1-x)2O3 films cover the full range of 0 < x < 1 with vacuum ultraviolet transmittance measurements showing that the bandgaps of our films shift with composition over the entire range of α-Ga2O3 to α-Al2O3.

Epitaxial Growth of α-(AlxGa1-x)2O3 by Suboxide Molecular-Beam Epitaxy at 1 μm/h

In this work, we report on the use of Triethyl Gallium for ‘in-situ Ga etching’ of β-Ga2O3 inside an MOCVD reactor. We demonstrate a maximum etch rate of ~8.5 μm/hr at a substrate temperature of 1000 °C and Ga flux of 140.47 μmol/min. In addition, low surface roughness of 2.9 nm was also obtained at a Ga flux of 6.05 μmol/min and substrate temperature of 800 °C. The ultra-wide band gap semiconductor β-Ga2O3 (band gap=4.7 eV, breakdown field=8 MV/cm) has made rapid strides in terms of substrates, material quality and device performance over the last decade. Ga2O3 devices with performance exceeding that of SiC has already been demonstrated [ref]. Several promising device structures in β-Ga2O3 require the use of 3-D structures like fins and trenches. For fabrication of such structures in β-Ga2O3, dry etching using BCl3/Ar has been widely used as the standard process. However, a key challenge when using dry etching is the induced subsurface damage. This has been shown to be significant in Ga2O3 resulting in charge depletion, reduction in channel mobility and increased sidewall roughness. In addition to dry etching, wet etching using HF, H3PO4 and KOH has also been reported. However, wet etching is found to mostly result in angled sidewalls which are not ideal. A promising etch technique for Ga2O3 is the use of suboxide formation upon exposure to Ga (Ga (s)+ Ga2O3 (s) Ga2O (g)) at sufficiently high temperatures. Suboxide based etching of β-Ga2O3 performed in an MBE chamber was previously shown to result in low damage etching of Ga2O3 along with vertical sidewalls for fins and trenches [1]. In this work, we studied ‘in-situ Ga etching’ performed inside an MOCVD reactor using Triethyl Gallium (TEGa) precursor. At sufficiently high substrate temperature, TEGa undergoes cracking resulting in the deposition of Ga on the substrate surface while the remaining hydro-carbon group (CxHy) get pumped out of the chamber. The deposited Ga reacts with Ga2O3 forming volatile suboxide, resulting in etching. We studied the effects of various parameters like TEGa flow rate, substrate temperature (Tsub) and substrate orientation ((001) and (010)) on etch characteristics like etch rate and surface roughness. Substrate temperatures ranged from 650 °C to 1000 °C to determine the effect on etch rate and surface roughness. Pressure was held constant at 15 Torr throughout the experiments. Samples with a constant etch depth of ~300 nm was used to measure surface roughness using AFM. Gallium flux spanned from 3.63 μmol/min to 140.47 μmol/min to determine its effect on the previously stated etch characteristics. The etch rate was found to increase with substrate temperature from 1.8 m/hr at Tsub=700 to 3.7 m/hr at Tsub=1000 . The surface roughness of etched surfaces also decreased with increase in Tsub for both (010) and (001) Ga2O3 surfaces. As expected, etch rate was found to monotonically increase with TEGa flow rates both at Tsub=800 and Tsub=1000 . At a substrate temperature of 1000 and TEGa molar flow of 140.5 μmol/min, etch rates exceeding 8.5 m/hr was achieved. However, RMS roughness ((010) β-Ga2O3 surface) was also found to increase at high TEGa flow rates. Lowest surface roughness was obtained at low TEGa flow rates (<12 μmol/min) and Tsub=800 with the lowest obtained RMS roughness of ~2.8 nm. Patterned etching using SiO2 hard mask was also demonstrated with a total etch depth of 2 m. In conclusion, we demonstrate MOCVD-based ‘in-situ Ga etching’ using TEGa as the Ga source. Etch rates as high as 8.5 μm/hr was obtained at a substrate temperature of 1000 and TEGa flow rate of 140.47 μmol/min. In addition, low surface roughness <3 nm and patterned etching using SiO2 as the hard mask is also demonstrated. This work was supported by faculty startup fund from Arizona State University. References: [1] Kalarickal, N. K., Applied Physics Letters, 119(12), 123503.

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