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Register a new userbeta-Ga2O3 NEMS Oscillator for Real-Time Middle Ultraviolet (MUV) Light Detection
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We report on the first beta gallium oxide (beta-Ga2O3) crystal feedback oscillator built by employing a vibrating beta-Ga2O3 nanoresonator as the frequency reference for real-time middle ultraviolet (MUV) light detection. We fabricate suspended beta-Ga2O3 nanodevices through synthesis of beta-Ga2O3 nanoflakes using low-pressure chemical vapor deposition (LPCVD), and dry transfer of nanoflakes on microtrenches. Open-loop tests reveal a resonance of the beta-Ga2O3 device at ~30 MHz. A closed-loop oscillator is then realized by using a combined optical-electrical feedback circuitry, to perform real-time resonant sensing of MUV irradiation. The oscillator exposed to cyclic MUV irradiation exhibits resonant frequency downshifts, with a measured responsivity of $mathscr{R}$ = -3.1 Hz/pW and a minimum detectable power of delta Pmin = 0.53 nW for MUV detection.
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Beta-Ga2O3 has emerged as a promising candidate for electronic device applications because of its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is at least one order of magnitude lower than that of other wide bandgap semiconductors such as SiC and GaN. Thermal dissipation in electronics made from beta-Ga2O3 will be the bottleneck for real-world applications, especially for high power and high frequency devices. Similar to GaN/AlGaN interfaces, beta-(AlxGa1-x)2O3/Ga2O3 heterogeneous structures have been used to form a high mobility two-dimensional electron gas (2DEG) where joule heating is localized. The thermal properties of beta-(AlxGa1-x)2O3/Ga2O3 are the key for heat dissipation in these devices while they have not been studied before. This work reports the first measurement on thermal conductivity of beta-(Al0.1Ga0.9)2O3/Ga2O3 superlattices from 80 K to 480 K. Its thermal conductivity is significantly reduced (5.7 times reduction) at room temperature comparing with that of bulk Ga2O3. Additionally, the thermal conductivity of bulk Ga2O3 with (010) orientation is measured and found to be consistent with literature values regardless of Sn doping. We discuss the phonon scattering mechanism in these structures by calculating their inverse thermal diffusivity. By comparing the estimated thermal boundary conductance (TBC) of beta-(Al0.1Ga0.9)2O3/Ga2O3 interfaces and Ga2O3 maximum TBC, we reveal that some phonons in the superlattices transmit through several interfaces before scattering with other phonons or structural imperfections. This study is not only important for Ga2O3 electronics applications especially for high power and high frequency applications, but also for the fundamental thermal science of phonon transport across interfaces and in superlattices.
Beta gallium oxide (beta-Ga2O3) is an emerging ultrawide band gap (4.5 - 4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. beta-Ga2O3 thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal beta-Ga2O3 nanomechanical resonators using beta-Ga2O3 nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating beta-Ga2O3 circular drumhead structures, we demonstrate multimode nanoresonators up to the 6th mode in high and very high frequency (HF / VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Youngs modulus of E_Y = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ~40% upshift in frequency and ~90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable beta-Ga2O3 electronic, optoelectronic, and physical sensing devices.
Vertical $pn$ heterojunction diodes were prepared by plasma-assisted molecular beam epitaxy of unintentionally-doped $p$-type SnO layers with hole concentrations ranging from $p=10^{18}$ to $10^{19}$cm$^{-3}$ on unintentionally-doped $n$-type $beta$-Ga$_{2}$O$_{3}$(-201) substrates with an electron concentration of $n=2.0times10^{17}$cm$^{-3}$. The SnO layers consist of (001)-oriented grains without in-plane expitaxial relation to the substrate. After subsequent contact processing and mesa etching (which drastically reduced the reverse current spreading in the SnO layer and associated high leakage) electrical characterization by current-voltage and capacitance-voltage measurement was performed. The results reveal a type-I band alignment and junction transport by thermionic emission in forward bias. A rectification of $2times10^{8}$ at $pm1$V, an ideality factor of 1.16, differential specific on-resistance of 3.9m$Omegathinspace$cm$^{2}$, and built-in voltage of 0.96V were determined. The $pn$-junction isolation prevented parallel conduction in the highly-conductive Ga$_{2}$O$_{3}$ substrate (sheet resistance $R_{S}approx3thinspaceOmega$) during van-der-Pauw Hall measurements of the SnO layer on top ($R_{S}approx150$k$Omega$, $papprox2.5times10^{18}$cm$^{-3}$, Hall mobility $approx1$cm$^{2}$/Vs). The measured maximum reverse breakdown voltage of the diodes was 66V, corresponding to a peak breakdown field 2.2MV/cm in the Ga$_{2}$O$_{3}$-depletion region. Higher breakdown voltages that are required in high-voltage devices could be achieved by reducing the donor concentration in the $beta$-Ga$_{2}$O$_{3}$ to increase the depletion width as well as improving the contact geometry to reduce field crowding.
A comprehensive current-voltage (I-V) characterization is performed for three different Schottky contacts; Pt, Ni and Ti, to unintentionally doped (UID) {beta}-(Al0.19Ga0.81)2O3 grown by molecular beam epitaxy (MBE) on {beta}-Ga2O3 for temperatures ranging between 25C -300C. Reciprocal space mapping shows the (Al0.19Ga0.81)2O3 films are strained and lattice matched to the substrate. Schottky Barrier Height (SBH), ideality factor (n), and series resistance (Rs) are extracted from the I-V characteristics for the three types of metals and temperatures. Room temperature capacitance-voltage (C-V) measurements revealed fully depleted {beta}-(Al0.19Ga0.81)2O3 layer. Extracted room temperature SBHs after zero field correction for Pt, Ni and Ti were 2.39 eV, 2.21 eV, and 1.22 eV respectively. Variation of SBHs with metal clearly indicates the dependence on work function.
Ultraviolet (UV) plasmonics aims at combining the strong absorption bands of molecules in the UV range with the intense electromagnetic fields of plasmonic nanostructures to promote surface-enhanced spectroscopy and catalysis. Currently, aluminum is the most widely used metal for UV plasmonics, and is generally assumed to be remarkably stable thanks to its natural alumina layer passivating the metal surface. However, we find here that under 266 nm UV illumination, aluminum can undergo a dramatic photocorrosion in water within a few tens of seconds and even at low average UV powers. This aluminum instability in water environments critically limits the UV plasmonics applications. We show that the aluminum photocorrosion is related to the nonlinear absorption by water in the UV range leading to the production of hydroxyl radicals. Different corrosion protection approaches are tested using scavengers for reactive oxygen species and polymer layers deposited on top of the aluminum structures. Using optimized protection, we achieve a ten-fold increase in the available UV power range leading to no visible photocorrosion effects. This technique is crucial to achieve stable use of aluminum nanostructures for UV plasmonics in aqueous solutions.
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