Low noise transistor microwave amplifiers
Diverse experiments depend on microwave signal processing technology, ranging from searching for the origin of fast radio bursts to investigating the nature of dark matter to the development of near-term quantum computers. While this technology is now relatively mature, present transistor amplifiers remain around a factor of 5 above the standard noise limit set by quantum mechanics. Although superconducting amplifiers operating at the quantum limit are available and in widespread use, they have a number of practical limitations that prohibit their use in many applications.
Our group is developing transistor microwave amplifiers with outstanding low-noise performance from the microwave into the millimeter-wave spectrum. These transistors are based on InGaAs/InAlAs quantum wells. Our research specifically focuses on incorporating atomic layer etching into the fabrication process so as to enable highly-scaled devices with unprecedented noise performance above 100 GHz. Such devices would be an enabling technology for a space-based observatory for the Next-Generation Event Horizon Telescope for imaging black holes.
Microscopic image of a discrete GaAs HEMT in our cryogenic probe station that enables measurements of microwave noise and S parameters. We use this approach to determine the physical origin of microwave noise in the devices and how it may be mitigated by materials science considerations and new nanofabrication processes.
Measured noise temperature versus physical temperature (black line) and that predicted assuming the drain (output) noise temperature is fixed at the room temperature value. The measurements indicate the drain noise generator must exhibit a temperature dependence, constraining the physical origin of the drain noise.
Novel nanofabrication processes - atomic layer etching
Detectors and emerging quantum hardware rely on nanofabrication processes to transform a uniform substrate into a complex spatial architecture that endows it with its function. Despite the capabilities of these processes, it is now clear that the chemical and structural quality of the resulting structures are insufficient to meet future scientific needs. The core problem is the precision gap between material growth and removal: while methods exist to synthesize materials with near-perfect atomic quality, analogous methods to remove material do not. Typical etching methods such as wet etches or reactive ion etching leave up to nanometer-scale surface roughness and a poor quality surface region that extends tens of Angstroms into the bulk. Thus, the initial Angstrom-scale quality of the substrate is degraded by nearly two orders of magnitude, indicating a fundamental gap in precision between material growth and removal. This precision gap leads to a variety of deleterious effects that are now the primary limiting mechanism for diverse figures of merit for devices such as electromagnetic resonators.
Atomic layer etching (ALE) is a method that has potential to mitigate the nanofabrication precision gap. ALE can be roughly viewed as the inverse of atomic layer deposition in that it consists of two or more self-terminating reactions at the surface of a substrate. Unlike in ALD, though, ALE leads to the removal, rather than deposition, of a monolayer. In the simplest two-step process, the first step weakens the bonds at the top monolayer of material, for instance by fluorination. The second step serves to remove this modified surface compound and can be accomplished by various schemes. In directional ALE, ions in a low energy plasma are directed to the surface, dislodging primarily the modified monolayer. In isotropic ALE, another reactant volatilizes the modified surface which is subsequently purged.
We are developing atomic layer etching processes which will enable subtractive manufacturing of electronic and quantum devices with atomic precision for the first time. We are applying these methods to fabrication of various devices, including low-noise InGaAs transistor amplifiers, single-photon detectors and qubits based on patterned superconducting thin films, and quantum photonic devices based on thin-film lithium niobate.
Schematic of an ALE process. The surface is fluorinated with an SF6 plasma, and the resulting modified surface products are removed by ligand exchange.
EDS image from cross-sectional TEM samples of an Al film (left) and ALE/ALD treated Al. In the processed sample, the native oxide has been removed and replaced with AlF3.
Thermal laser epitaxy
Molecular beam epitaxy is a workhorse method for the growth of crystalline thin films of unparalleled quality. However, the flux for material growth generally relies on heating of various elements to temperatures where the vapor pressure is high enough to produce adequate flux. These temperatures are limited to around 2000 °C for effusion cells and 2500-3000 °C using electron beam heating. The temperature restrictions limit the synthesis space for growing materials with refractory elements, and electron beam heating incurs practical complications and may degrade the purity of the film.
Thermal Laser Epitaxy (TLE) is a new technique which has potential to overcome these limitations. TLE exploits the recent availability of economical, high-power lasers typically used for welding and cutting in industry. These lasers are used to heat the source targets directly while using the bulk of the material as its own crucible. Compared to electron beam heating, laser heating allows for improved flux stability, higher purity films as the crucible is eliminated, and simpler implementation owing to the simplicity of light optics over electron optics. Most importantly, TLE enables exceedingly high temperatures of the source targets to be realized (above 3000 °C), facilitating access to unexplored regimes of synthesis space.
Our lab is developing a TLE system to enable the synthesis of ultra-pure quantum materials incorporating refractory elements. We are most interested in quantum materials such as topological semimetals, non-centrosymmetric superconductors, or others, which are composed of refractory elements such as TaAs, NbSe2, and WTe2. This system will be the first of its kind in the United States and will lead to disruptive advances in the synthesis of epitaxial quantum materials with transformative implications for quantum science and technology.
(Left) Vapor pressure versus temperature for various elements. The approximate equivalent deposition rate in monolayers per second (ML/s) is indicated. Refractory elements need exceedingly high temperatures above 3000 °C for typical flux rates. (Right) Schematic of TLE concept, in which the flux for growth from material sources is obtained by laser heating of the source targets.
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