United States

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

EMC 2023

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

algan/gan

qwip

mocvd

photodetector

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. 

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.

Demonstration of MOCVD Based 'in situ Ga Etching’ of β-Ga2O3 Using TEGa

There is an ever-increasing demand for energy-efficient multifunctional sensors or transducers based on low-cost and environmentally acceptable material systems. This demand is a direct result of the remarkable improvement of current technology, which has led to an increase in energy consumption. In this context, we describe a self-assembled vertically aligned β–Ga2–XWXO3 nanocomposite (GWO-VAN) architecture-assisted ultrafast deep-ultraviolet (DUV) photodetection on a silicon platform. Silicon has traditionally served as the core of traditional electronics. By carefully controlling the growth settings, it was possible to demonstrate the growth of W-enriched vertical β–Ga2–XWXO3 nanocolumns contained within a W-deficient β–Ga2–XWXO3 matrix on a silicon substrate. The presence of a nanocomposite of β–Ga2–XWXO3 with a high structural and chemical quality has been confirmed by detailed structural and morphological investigations. In addition, absorption and photoluminescence spectroscopy are able to describe photo-absorption dynamics as well as the recombination process through the use of various donor-acceptor energy levels. The GWO-VAN architecture makes it possible to have equally scattered nanoregions with asymmetric donor energy state distribution, which ultimately results in the formation of build-in potential at those vertical β–Ga2–XWXO3 interfaces. Under UV irradiation, the heterostructure displays the characteristics of a photovoltaic cell. A responsivity of ~30 A/W is seen alongside an ultrafast response time of ~350 microseconds under transient triggering. The corresponding detectivity and efficiency values are 7.9×1012 Jones and 1.4×104 %, respectively. The implications of the aforementioned approach are enormous in terms of designing electronic devices for extreme environment applications and have immense potential to demonstrate a significant improvement in low-cost UV photodetector technology.

Vertically Aligned β–Ga2–XWXO3 Nanocomposites for Ultrafast Deep-UV Photodetectors

(MgxGa1-x)2O3 alloys with bandgap energy from ~5.0 to ~6.0 eV can provide opportunities for optoelectronics in deep ultraviolet spectral range and electronics with extremely high critical field strength. It is important to grow high-quality (MgxGa1-x)2O3 alloys with varied Mg compositions and understand their structural, optical and electrical properties. In this project, (MgxGa1-x)2O3 ternary alloy thin films of different Mg concentrations were grown by using oxygen plasma assisted molecular beam epitaxy (MBE). The atomic ratios of Mg and Ga from sample 1 to sample 20 were recorded from 0 at.%/100 at.% to 100 at.%/ 0 at.% separately, which were measured by energy-dispersive X-ray spectroscopy (EDX). The lattice structures, which have transformed from β-phase in Ga-rich materials, to beta and rock salt mixture phase in high-Ga high-Mg alloys, and to pure rock salt phase in Mg-rich alloys, were investigated by high resolution x-ray diffraction (HR-XRD) and reciprocal space mapping (RSM). The change of film quality and lattice parameter with different Mg at.% was revealed. Surface morphology was characterized by atomic force microscopy (AFM), and the change of surface roughness with Mg at.% was recorded. Ultrawide bandgap absorption and transmittance spectra were measured using a high-performance UV-Vis-NIR spectrophotometer. Bandgap tuning from 5.03 eV to 5.89 eV was achieved, and all samples have the transmittance over 90% in the visible spectral range.

Investigation of Phase Transition and Bandgap Engineering in (MgxGa1-x)2O3 Thin Films Grown by Molecular Beam Epitaxy

Ultra-wide bandgap semiconductors are promising for many applications such as power electronics and deep-UV photonics. With a bandgap of ~4.9 eV, Gallium Oxide (Ga2O3) has drawn much attention recently. To improve film quality, a low-temperature buffer layer is commonly used. In this research, wider bandgap β-phase Magnesium Gallium Oxide (MgGaO) alloys were grown in a plasma-assisted molecular beam epitaxy system. The relationship between growth temperature and thickness of low-temperature buffer layer and the quality of active layer was studied by X-ray diffraction (XRD) and by analyzing metal-semiconductor-metal (MSM) photodetector devices based on these films. Low-temperature homo-buffer layers with a growth temperature of 300-500 °C and a thickness of 0-40 nm, respectively, were deposited on c-sapphire substrates, followed by two-hour active layer growth at a temperature of 650 °C. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to analyze the surface morphology. Energy dispersive X-ray spectroscopy (EDX) was used and the average composition for Ga and Mg is 85 at. % and 15 at. %, respectively. Bandgaps were extracted from the Tauc plot with a value of 5.1 eV for all samples. X-ray diffraction results show all samples are oriented -phase MgGaO. The X-ray rocking curve measurement shows that the full width at half maximum (FWHM) increases as the buffer layer thickness increases, and this may be due to the poor quality of the buffer layer. Current-voltage measurement for the interdigitated electrode photodetectors shows that the current increases as the low-temperature buffer layer increases. Photocurrent decay time increases as the buffer layer thickness increases, indicating that the quality of the active layer improves. Recombination of photo-generated electrons and holes is reduced with fewer defects in the active layer, resulting in a larger current under illumination and a longer photocurrent decay time.

Improving Quality of β–Phase MgGaO Thin Films by Using Low-Temperature Homo-Buffer Layer

Epitaxial lateral overgrowth (ELO) is an established technique for the reduction of the high threading dislocation density of GaN grown on sapphire substrates. ELO is typically carried out via a selective area growth process with stripes of SiO2 as a mask. While the benefits of ELO were instrumental in the development of early GaN optoelectronics, the phenomenon known as “wing tilt” is a longstanding drawback. So-named due to the crystallographic tilt induced in the “wing” regions above the SiO2 mask, the effect is associated with a tilt boundary at the coalescence boundary. While various approaches have been suggested to mitigate the impact of wing tilt, there is not currently a scheme to eliminate the effect. In this study, we demonstrate that wing tilt is a direct result of the use of the SiO2 mask, and that the tilt occurs even before coalescence. We also demonstrate GaN layers free of wing tilt using a mask-free facet-controlled ELO (FACELO) scheme. For this investigation, ammonothermally grown bulk GaN substrates were used. One template was prepared for traditional ELO with SiO2 stripes of 2 μm separated by windows of 2 μm for a period of 4 μm; another template was prepared with a mask-free pattern by etching trenches of 2 μm width and depth, leaving behind 2 μm stripes. Both templates had stripes oriented along the m-direction of GaN. As the first step in the ELO growth scheme, these templates were grown at a low temperature (950 °C) to promote the evolution of semipolar facets, resulting in triangular cross-sections over the stripes. The second growth step to achieve c-plane coalescence was then performed at a higher temperature (1100 °C). The samples were characterized at both steps via SEM and XRD, as well as TEM after coalescence. The (002) ω-rocking curve of the SiO2-grown triangles shows symmetric satellite peaks about the main peak characteristic of wing tilt. Additionally, the FWHM of the peak has increased from 40 arcsec for the substrate to 150 arcsec for the regrown structure. This result shows that even before coalescence, the growth of GaN over SiO2 results in a change in crystalline quality. After coalescence, the FWHM of the peak increases to 180 arcsec, a value suggesting a threading dislocation density in the high 108 cm-2 range. By contrast, the maskless sample shows no satellite peaks in the (002) ω-rocking curve, and at both steps the FWHM of the original template is maintained. The findings of the XRD are confirmed via TEM. In epitaxial layers above the SiO2 patterned sample, a significant density of dislocations can be seen. By contrast, the maskless sample showed no visible dislocations. We therefore conclude that wing tilt and its associated dislocations are caused by the presence of the SiO2 mask, and that by use of a maskless etched template, these deformations and defects can be avoided.

Wing Tilt in ELO-Grown GaN

ScAlN alloys are promising due to their high spontaneous and piezoelectric polarizations and ferroelectric properties for various technologies including high electron mobility transistors (HEMTs), acoustic devices, etc. Despite the common belief that Molecular Beam Epitaxy (MBE) synthesis requires N-rich conditions to avoid metallic phase inclusions, our previous studies1,2 have demonstrated that the metal modulated epitaxy (MME) allows growth of state-of-the-art structural quality ScAlN in metal-rich conditions at a low substrate temperature (Tsub) of 400 °C by using precise control of metal adlayer buildup. Additionally, these previous studies found an extremely unusual feature wherein, a significant catalytic effect of Sc leads to more available nitrogen and thus, ~3x faster growth rates when Sc is present in large coverages on the surface. Accounting for this critical new feature, MME allows improved crystal quality by x-ray diffraction (XRD) figures of merit and surface roughness as low as 250 arcsec and 0.73 nm, respectively. The HEMT structure in the previous studies have shown an improved sheet resistance of 152 W/■, a mobility of 700 cm2/Vs, and a sheet charge of 5.9 x 1013 cm-2 which is near the theoretical limit, exhibiting MME’s great potential for ScAlN HEMT applications. Although the optimization of III/V ratio and growth temperature was done in the previous studies1,2, the structural properties of ScAlN are not well investigated yet especially for the MME grown films which involves low-growth temperature. The present transmission electron microscopy (TEM) study contrasts the unique characteristics of MME grown Sc0.2Al0.8N at low Tsub of 400 °C, and 650 °C without metallic inclusions and 600 °C samples that showed the metallic inclusions. Significant twist is present in the 600 °C grown sample that is absent in the 400 °C sample. MME Sc0.2Al0.8N were grown on MME GaN on hydride vapor phase epitaxy (HVPE) GaN templates on c-plane sapphire substrates as described elsewhere1,2, using plasma-assisted molecular-beam-epitaxy (PA-MBE). TEM and four-dimensional scanning TEM (4D-STEM) are employed to evaluate the structural properties. The cross-sectional sample is prepared by focused ion beam (FIB) under cryogenic temperature. TEM images close to the zone axis, and for the sample with Tsub = 400 °C show better film quality than the two higher temperature samples, having distinctive contrasts of dislocations. The 600 °C sample showed complicated and dense contrasts. This fact correlates with the previous results in XRD 2θ-ω scans indicating that the 600 °C sample had inclusions of Al3Sc phases, which are not present in the other two samples. AFM also showed smoother surface roughness for the 400 °C sample other samples.1,2 Local diffraction patterns (DPs) obtained from 4D-STEM taken for the 600 °C sample shows ScAlN is twisted ~5° along c-axis from underlying GaN, which is confirmed by twisting the sample around the c-axis in the measurement. Some local DPs show additional crystalline tilt by ~6° off the c-axis. These twist and tilt features are supported by the existence Moire fringes in the bulk of the film from TEM images observed in 600 °C sample, but not in 400 °C sample. In addition, multiple DP spots outside the wurtzite DPs are found in some regions, implying the inclusion of multiple phases, e.g., possibly rock-salt or Al3Sc phases, which could be from phase separation as revealed in XRD 2θ-ω scan in the previous studies.1,2 These features in 4D-STEM are not observed in 400 °C sample. The present study helps to understand the mechanisms of better crystal quality in the epitaxially grown ScAlN under low substrate temperatures. 1Z. Engel, et. al., J. Appl. Phys., 132, 185302, 2022. 2 Z. Engel, Doctoral Dissertation, Georgia Institute of Technology, Approved on Dec. 5th, 2022.

Structural Analysis and Observation of Tilts, Twists and Crystallographic Orientation of High-Quality, Metal Rich Grown Sc0.2Al0.8N

Manufacturing high-quality inorganic thin films on large-area flexible substrates is a key technology to utilize for flexible devices such as wearable sensors, flexible displays, and electronic devices. For the device fabrications, epitaxial thin films were grown on two-dimensional (2-D) layered materials such as graphene and h-BN. Since the 2-D materials offer weak bonding to the as-grown films, the exfoliation of epilayers from 2-D materials is easily feasible. In particular, remote epitaxy has recently been reported by molecular beam epitaxial growth of GaN thin films on the graphene buffer layer (GBL) on SiC substrates. This remote epitaxial growth method enables to grow the single-crystal thin films on graphene-coated polar substrates by maintaining the epitaxial relationship between the substrate and the epilayer because the graphene does not wholly screen the electrostatic potential of the substrate. However, metalorganic vapor-phase epitaxy (MOVPE), commercially used for manufacturing GaN devices, could not be used for the remote epitaxial growth and exfoliation of high-quality GaN thin films on graphene. During the MOVPE growth, graphene substrates are affected by H2 and NH3 ambient gases at high growth temperatures, resulting in the failure of GaN exfoliation from the graphene substrate. Here, we report a new strategy for MOVPE growth of transferable single-crystal GaN thin films on graphene/c-sapphire by using pulse operation of NH3 flow to prevent deterioration of graphene substrates during GaN growth. The basic strategy for epitaxial growth of transferable single-crystal GaN thin films on graphene/c-sapphire is to use pulse operation of NH3 flow to prevent deterioration of graphene substrates during GaN growth. In the NH3 pulsed-flow growth, the NH3 injection into the reactor was periodically interrupted while the trimethylgallium (TMGa) was continuously injected. By a controlled V/III ratio under the pulsed NH3 injection, the lateral growth of the GaN buffer islands was enhanced merging into a continuous film. By optimizing the growth parameters, including the duration and period of the pulsed ammonia flow, high-quality GaN films were grown on the graphene-coated sapphire substrates, easily lifted off, and transferred onto a foreign substrate by simple mechanical exfoliation using a tape. Furthermore, we investigated the role of the pulsed operation of ammonia flow as a critical factor for the fabrication of high-quality free-standing GaN films. The freestanding GaN films, exhibiting excellent single crystallinity and electrical and optical characteristics, were also applied for transferrable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and p-type GaN layers on the GaN films and transferring them onto Cu foil.

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

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

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