Resonant tunneling diodes (RTDs) are next-generation candidates for core THz generation technologies, with proven quasi-optical tunable emission capability, with centre frequencies of 0.1 - 1.98 THz at cca. 1mW, when coupled into a suitable monolithically integrated antenna. For this purpose, the strained InGaAs/AlAs/InP material system is approaching technological maturity, with its offering of high electron mobility, suitable conduction band offsets, and very low resistance contacts. However, the epitaxially thin layers used for RTDs, realise devices with current densities in excess of 10 mAμm-2 and electric fields approaching that of the breakdown of the material. As a high current density is a traditional indicator of performance for these oscillators, it is now increasingly important to grow crystalline layers with near-atomic perfection. In previous work, we showed how the inclusion of a nominally identical, un-doped electrically neutral copy of the RTD double barrier - quantum well (QW) system, leads to the observance of a type-II QW emission in addition to the type-I emission from the active region QW. This could be used to establish the quasi-bound elastic energy, whose level is directly correlated to the peak voltage of the N-shape I-V characteristic. Here we extend this approach with the addition of high-resolution X-ray diffractometry and low-temperature photoluminescence spectroscopy. Through a step-by-step process of curve fitting, comparing to simulation and results, we can comment on the quality and thickness of the ternary InGaAs alloy interfaces surrounding the AlAs barriers. These findings are confirmed with scanning transmission electron microscopy
Resonant tunnelling diodes (RTDs) are a strong candidate for future wireless communications in the THz region, offering compact, room-temperature operation with Gb/s transfer rates. We employ the InGaAs/AlAs/InP material system, offering advantages due to high electron mobility, suitable band-offsets, and low resistance contacts. We describe an RTD emitter operating at 353GHz, radiating in this atmospheric transmittance window through a slot antenna. The fabrication scheme uses a dual-pass technique to achieve reproducible, very low resistivity, ohmic contacts, followed by accurate control of the etched device area. The top contact connects the device via the means of an air bridge. We then proceed to model ways to increase the resonator efficiency, in turn improving the radiative efficiency, by changing the epitaxial design. The optimization takes into account the accumulated stress limitations and realities of reactor growth. Due to the absence of useful in-situ monitoring in commercially-scalable metal-organic vapour phase epitaxy (MOVPE), we have developed a robust non-destructive epitaxial characterisation scheme to verify the quality of these mechanically shallow and atomically thin devices. A dummy copy of the active region element is grown to assist with low temperature photoluminescence spectroscopy (LTPL) characterisation. The resulting linewidths limits the number of possible solutions of quantum well (QW) width and depth pairs. In addition, the doping levels can be estimated with a sufficient degree of accuracy by measuring the Moss-Burstein shift of the bulk material. This analysis can then be combined with high resolution X-ray diffractometry (HRXRD) to increase its accuracy.