Self-Switching Diodes for Zero-Bias Terahertz Detection

Doctoral thesis, 2015

The self-switching diode (SSD) has been investigated as a potential terahertz detector in recent years. The SSD accomplishes a non-linear current-voltage relation through a field-effect, which enables detection at zero bias from microwave up to terahertz frequencies. In this work, SSDs were realised in two new materials; InAs and graphene. InGaAs SSDs were also fabricated. The effect of geometrical and material parameters on SSD detector performance under zero bias were for the first time described by an analytical model, and confirmed by experiment. InAs SSDs were fabricated in an InAs/AlSb heterostructure. A noise equivalent power (NEP) of 150-200 pW/Hz½ was observed at 2-315 GHz with a 50 Ω source. InGaAs SSDs fabricated in an InGaAs/InAlAs heterostructure exhibited an NEP of 40-150 pW/Hz½ at 2-315 GHz with a 50 Ω source, the lowest NEP reported for any SSD in this frequency range. An InAs SSD integrated with a spiral antenna and silicon lens demonstrated an NEP of less than 4.4 nW/Hz½ at 600 GHz. SSDs require a deep submicron isolation pattern which presents a major fabrication challenge in the InAs/AlSb heterostructure due to the oxidation sensitivity of the thick AlSb buffer layer. An SSD process was developed that allowed the fabrication of InAs/AlSb isolation patterns with feature sizes down to 35 nm. Graphene SSDs were demonstrated for the first time. The SSD was based on epitaxial graphene on silicon carbide. A flat NEP of 2.4 nW/Hz½ was measured from 1 to 67 GHz using a 50 Ω source. An analytical model of the SSD was derived to explain the influence of geometrical and material parameters. The model predicted that lowering the two-dimensional electron gas carrier concentration in the heterostructure increased the responsivity and reduced the NEP of the SSD zero-bias detector. Device simulations and measurements confirmed the predictions. Further, the model, together with experiments, showed that to minimize NEP there is an optimum number of channels and optimal channel length. The frequency dependence of SSD detectors was described using a small-signal equivalent circuit, which reproduced the measured responsivity up to 315 GHz. The highest cut-off frequency of InAs SSDs was estimated to 775 GHz.