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e report the on-wafer characterization of S-parameters and microwave noise temperature (T50) of discrete metamorphic InGaAs high electron mobility transistors (mHEMTs) at 40 and 300 K and over a range of drain-source voltages (VDS). From these data, we extract a small-signal model (SSM) and the drain (output) noise current power spectral density (Sid) at each bias and temperature. This procedure enables Sid to be obtained while accounting for the variation of SSM, noise impedance match, and other parameters under the various conditions. We find that the noise associated with the channel conductance can only account for a portion of the measured output noise. Considering the variation of output noise with physical temperature and bias and prior studies of microwave noise in quantum wells, we hypothesize that a hot electron noise source (NS) based on real-space transfer (RST) of electrons from the channel to the barrier could account for the remaining portion of Sid. We suggest further studies to gain insights into the physical mechanisms. Finally, we calculate that the minimum HEMT noise temperature could be reduced by up to ∼50% and ∼30% at cryogenic temperature and room temperature, respectively, if the hot electron noise were suppressed. 

Lithium niobate (LiNbO3, LN) is a ferroelectric crystal of interest for integrated photonics owing to its large second-order optical nonlinearity and the ability to impart periodic poling via an external electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is presently limited by propagation losses arising from surface roughness and corrugations. Atomic layer etching (ALE) could potentially smooth these features and thereby increase photonic performance, but no ALE process has been reported for LN. Here, we report an isotropic ALE process for x-cut MgO-doped LN using sequential exposures of H2 and SF6/Ar plasmas. We observe an etch rate of 1.59±0.02 nm/cycle with a synergy of 96.9%. We also demonstrate that ALE can be achieved with SF6/O2 or Cl2/BCl3 plasma exposures in place of the SF6/Ar plasma step with synergies of 99.5% and 91.5%, respectively. The process is found to decrease the sidewall surface roughness of TFLN waveguides etched by physical Ar+ milling by 30% without additional wet processing. Our ALE process could be used to smooth sidewall surfaces of TFLN waveguides as a postprocessing treatment, thereby increasing the performance of TFLN nanophotonic devices and enabling new integrated photonic device capabilities. 

We report the isotropic plasma atomic layer etching (ALE) of aluminum nitride using sequential exposures of SF6 plasma and trimethylaluminum [Al(CH3)3]. ALE was observed at temperatures greater than 200 °C, with a maximum etch rate of 1.9 Å/cycle observed at 300 °C as measured using ex situ ellipsometry. After ALE, the etched surface was found to contain a lower concentration of oxygen compared to the original surface and exhibited a ∼35% decrease in surface roughness. These findings have relevance for applications of AlN in nonlinear photonics and wide bandgap semiconductor devices. 

Microwave loss in superconducting TiN films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications, or the etch rate lacks the desired control. Furthermore, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF6/H2 plasma. For certain ratios of SF6:H2 flow rates, we observe selective etching of TiO2 over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 Å/cycle at 150°C to 3.2 Å/cycle at 350°C using ex situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits. 

We report wafer characterization of the S-parameters and microwave noise temperature of discrete GaAs and GaN HEMTs over a temperature range of 20 - 300 K. The measured noise temperature (T50) exhibits a dependence on physical temperature that is inconsistent with a constant drain temperature, with Td for the GaAs and GaN devices changing from ~ 2000 K and ~2800 K at room temperature to ~ 700 K and ~ 1800 K at cryogenic temperatures, respectively. The observed temperature dependence is qualitatively consistent with that predicted from a theory of drain noise based on real-space transfer of electrons from the channel to the barrier.