Estudio de la influencia de las trampas en el comportamiento DC y AC de nanodiodos GaN a temperaturas criogénicas y su aplicación como detectores de microondas

Zusammenfassung

The applications arising from electromagnetic radiation in the millimetre and submillimetre wave range (up to THz) have enormous potential for multitude of fields related to information and communication technology, medical diagnostics, industrial control, security scanners, etc. Hence the growing need to find devices with high frequency operation capabilities. In order to improve the performance of traditional technologies (mainly GaAs-based Schottky diodes), such as operating frequency or high power capabilities, new semiconductor devices with different architectures are being developed. In this work, an in-depth experimental analysis is carried out as a function of temperature T (10- 300 K), both in DC and AC regime, as well as RF power detection, in one of these novel structures: the Self-Switching Diode (SSD) based on an AlGaN/GaN heterostruture. To analyse the important impact of the traps that typically appear in the not fully mature GaN technology (either in the bulk or on the surface), experiments are carried out from basic measurement of the DC curves or impedance analysis, to more complex ones such as the extraction of the small-signal equivalent circuit, or the calculation of the figures of merit characterising the detection (responsivity and noise equivalent power) up to a frequency of 43.5 GHz. The experimental results are complemented, by the development of an analytical quasi-statistical (QS) model that predicts the detection performance from the I-V curves and, by numerical simulations using a Monte Carlo (MC) simulator to explain the physics behind the detection mechanism and to analyse the role of traps in the operation of SSDs. The detection capability of a device is determined by the non-linearity of its I-V curves, which is significant at zero bias in the case of SSDs. In addition, their planar geometry allows great flexibility in design to reduce parasitic capacitances and obtain competitive responsivity values up to quite high frequencies. Initially, the devices have been analysed at room temperature. The QS model based on the DC curves predicts responsivity values of several tens of V/W and noise equivalent powers (NEPQS 50) of only a few nW/Hz1/2. Moreover, these performances are improved by reducing the diode channel width. While β QS V,50 hardly changes with the length L or the number of parallel diodes N, the NEP is proportional to L and improves as N increases (it is proportional to 1/N). RF detection measurements (β RF V) confirm the QS model predictions and show a fairly frequency-independent result at least up to 43.5 GHz (limit of the measurement equipment). The small-signal equivalent circuit (SSEC) of the SSDs has also been obtained, which apart from the intrinsic R || C branch, needs new elements (associated to the traps behaviour) to reproduce the impedance and S-parameter measurements. These new elements are an inductor associated to the surface charges (typical of a device with a high surface-to-volume ratio) and an extra serial RC branch attributed to the bulk traps. Using this SSEC, we are able to determine an SSDs intrinsic operating frequency of more than 1 THz, confirming their excellent properties, but the crosstalk capabilities reduce this frequency to hundreds of GHz, making it necessary to optimise the design of the devices in order to improve their high-frequency performance. Finally, we analyse the SSD capacity to operate as memories and photodetectors. The DC curves of the SSDs exhibit a marked hysteresis cycle below 200 K, thus allowing to identify two well-differentiated currrent states for a given read bias. By applying enough positive/negative voltages (write pulses), it is possible to switch between the filled and empty states of the surface traps, and thus modulate the conductivity of the nanochannel. The analysis of current transients reveals very slow evolution times (of the order of tens or hundreds of seconds), thus demonstrating a memory retention time at 70 K of approximately 2 h. Regarding the photodetector capabilities, a maximum optical responsivity of 0.12 A/W has been obtained with the SSD with W=80 nm, with interesting dependencies on voltage, associated with the voltage induced charge/discharge of the surface states which is added to the light induced electron release.