Epsilon-near-zero (ENZ) materials have been key players in recent photonic applications due to their versatility in growth, excellent compatibility, and ability to be dynamically modulated. From a foundation in the recently developed carrier kinetic models of nonlinearities in ENZ materials, we discuss our efforts to realize scalable and high-quality Al:ZnO (AZO) films via a unique atomic layer deposition (ALD) process, and the use of AZO in both switching and frequency shifting applications. Throughout, we highlight the advantages and challenges that exist and conclude with an outlook for ENZ materials in the area of nonlinear optics.
Recently, materials with vanishing refractive index, near-zero-index (NZI), have garnered considerable amount of attention, primarily for their ability to exhibit enhanced light matter interaction, due to slow light affects. Furthermore, effects such as static light, enhanced nonlinearities and emission tailoring have made such materials a heavily researched area. Amongst them, transparent conducting oxides (TCOs), a class of materials that have vanishing index at technologically relevant near-IR spectral range, are increasingly being investigated, for their potential use in photonic circuits. Wide natural abundance of ZnO together with well-studied properties make ZnO-based TCOs, such as aluminum- and gallium-doped ZnO particularly attractive for NZI applications.
We report the direct current (DC) and microwave performance of BeMgZnO/ZnO heterostructure field effect transistors (HFETs) on sapphire substrates. The devices fabricated using Al2O3 as the gate dielectric with a gate length of 1.5 um and a gate width of 75 um exhibited a pinch-off voltage of −4.0 V and a maximum peak transconductance of 63 mS/mm. A current gain cutoff frequency fT of 5.0 GHz was achieved, highest among ZnO-based FETs. The corresponding electron velocity of above 1E7 cm/s estimated based on the gate transit time inches closer to the theoretical peak velocity in ZnO (3.5E7 cm/s). This value is significantly higher than the previously reported values in ZnO-based HFETs, which is attributed to the two-dimensional electron gas (2DEG) concentration at or near the resonance of longitudinal optical (LO)-phonon and plasmon frequencies as well as the improved quality of the heterostructure owing to optimized ZnO buffer growth and BeO and MgO alloying in the barrier. To probe the high-frequency response of the HFETs, extrinsic and intrinsic parameters of the small-signal equivalent circuit for the BeMgZnO/ZnO HFETs were investigated using the hybrid extraction method.
The subwavelength nature of the plasmonic resonances, observed in patterned thin metallic films, makes it an attractive choice for use in application such as security features, product branding and data storage and imaging. However, these have largely been limited by the practical issues of cost and robustness arising from the use of gold, silver or aluminum. Here we pattern TiN thin films, a well explored alternative plasmonic material, with sub wavelength apertures, arranged with hexagonal periodicity, which exhibit extraordinary transmission in the visible and near IR spectrum. These TiN structures are shown to withstand different levels of mechanical stresses, while gold doesn’t, making TiN an attractive platform for use as security features.
Transparent conducting oxides, such as Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) are attractive materials for high-performance plasmonic devices operating at telecommunication wavelengths. In this contribution, we compare the growth of epsilon-near-zero GZO and AZO films on sapphire by two different deposition techniques: molecular beam epitaxy (MBE) and atomic layer deposition (ALD). For MBE of GZO, a multiple buffer consisted of a high-temperature MgO layer, a low-temperature ZnO, followed by a high-temperature ZnO layer is employed to assure the crystalline quality of the GZO film. By controlling the growth parameters, including Ga doping level, VI/II ratio, substrate temperature, we are able to produce GZO films at 350 °C with electron mobility between 30 and 50 cm2/V.s, electron concentration up to 7×1020 cm-3, and resistivity down to 2.5×10-4 Ω.cm. For ALD of AZO, without using any buffer, by reducing the Al pulse duration, we are able to grow the AZO films under a large ratio of Al to Zn pulses of 1:6, which improves the activation of Al as an effective dopant. Hence AZO films with electron concentration above 7×1020 cm-3, electron mobility between 10 and 20 cm2/V.s, and resistivity below 6×10-4 Ω.cm have been obtained at 250 °C. The corresponding epsilon-near-zero point in the ALD-grown material was tuned down to 1470 nm. Our data indicate that the ALD method provides a low-temperature route to plasmonic TCOs for telecommunication wavelength range. Effect of electron mobility on optical loss and, therefore, plasmonic figure of merit is discussed.
Previously we have conducted researches on the molecular beam epitaxy (MBE) of Zn-polar BeMgZnO/ZnO heterostructure field effect transistor (HFET) structures on GaN templates, which exhibit both high two-dimensional gas (2DEG) density and high electron mobility with relatively low Mg contents via the strain modulation of Be and Mg co-incorporation. In this contribution, we report on the growth of the HFET structures directly on sapphire substrate by employing a buffer consisting of a rock-salt structure MgO layer, a low-temperature (LT) ZnO layer and a high-temperature (HT) ZnO layer. Compared with growth of O-polar on sapphire, in which a thin and wurtzite MgO buffer is deposited at high temperatures above 700 °C, the MgO buffer for Zn-polar growth has to be reduced to 450 °C, in order to obtain smooth interface and surface for the BeMgZnO/ZnO HFETs. The residual electron sheet concentration in Zn-polar ZnO layers is ~2×1012 cm-2 on GaN while semi-insulating Zn-polar ZnO layers on sapphire have been obtained via controlling the buffer growth conditions, which is vital to the realization of HFET device structures.
Accumulation of non-equilibrium hot longitudinal optical (LO) phonons limits the electron drift velocity for electronic devices operating under high electric field. Ultrafast decay of hot phonons can take place via plasmon-LO phonon resonance, which leads to fast electron energy relaxation and hence high electron drift velocity and optimum operation of the devices. This need motivates us to create heterostructures with 2DEG density close to the plasmon-LO phonon resonance region. Through incorporating a few percent of Be into the BeMgZnO barrier to switch the strain sign in the barrier from compressive to tensile, we have achieved 2DEG densities over a wide range in Zn-polar BeMgZnO/ZnO heterostructures with moderate Mg content (below 30%) grown by molecular beam epitaxy. We have obtained electron mobility of 250 cm2/Vs at room temperature (293 K) and 1800 cm2/Vs at 13 K in Be0.02Mg0.26ZnO/ZnO heterostructures. Via capacitance-voltage (CV) spectroscopy, we have explored the depth profiles of the apparent carrier density of samples grown under different conditions. The correlations between electrical properties and MBE growth parameters of Zn-polar BeMgZnO/ZnO heterostructures are discussed.
Growth of nonpolar and semi-polar GaN and GaN-based structures offers the opportunity to reduce quantum confined Stark effect and possibly increase indium incorporation, as compared to polar structures, for enhanced performance in green and longer wavelength light emitters. However, the development of the nonpolar and semi-polar GaN growth is hampered by the lack of suitable substrates. Silicon, despite its large thermal-expansion and lattice mismatch with GaN, provides the advantages of the availability of large-size wafers with high crystalline quality at low cost, good electrical conductivity, and feasibility of its removal through chemical etching for better light extraction and heat transfer. In this article, we overview the recent progress in epitaxial growth of nonpolar and semi-polar GaN-based structures on patterned Si substrates. Also discussed are structural and optical properties of the resulting material.
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