Bolometric nanowires

In this project we will explore the thermal and electronic properties of Ge-nanowires for bolometric applications like faster and more sensitive infrared detectors and infrared cameras. Per definition, a bolometer consists of an absorptive element, which is connected to a thermal reservoir through a thermal link and a thermometer. Proportional to the incident radiation intensity, any radiation will raise the temperature of the absorptive element above that of the reservoir and the temperature change is measured using an attached thermistor. Ge nanowires, and especially horizontally grown nanowires embedded between two electrical contacts are perfect natural buildings blocks for bolometers. Ge nanowires have an extremely small thermal mass and the same highly temperature dependent electrical resistance like bulk Ge-bolometers. Thus, they are expected to be extremely fast and are good candidates for infrared camera applications. The electrical contact pads of the nanowires are located on the substrate and automatically act as thermal reservoir. This has the advantage that the thermal conductance between the active parts of the nanowires and the contacts can be engineered using appropriate wire widths or, more advanced, using appropriate artificial nano-size constrictions in the wires. In detail, we want to explore the thermal and electronic properties of Ge-nanowires in order to study their principal advantages for infrared detector applications. The limits of sensitivity shall be studied in dependence of various technological parameters such as nanowire diameter, length, doping levels and type. Nanowires overgrown with oxides and other dielectric materials shall be studied, too because the trap assisted surface transport properties can possibly be engineered to control the temperature sensitivity and the photoelectric response of the wires. Additional infrared absorber coatings, such as Gold- black or other commercial absorbers will help to increase the detector response. Finally, the performance of single nanowire detectors and nanowire-grass detectors shall be studied and compared. Nanowire grass-detectors are fabricated by overgrowing a lithographically defined array of closely spaced contact pads, which will be interconnected by the nanowires after the overgrowth process. Compared to single wire detectors, nano grass- detectors should have smaller impedances, higher speed, and larger signals. In addition, they should be more robust and more reproducible so that camera applications should become feasible.

The increasing demand for compact and mobile sensors intensifies the need for highly miniaturized device architectures. This is further supported by the downscaling of modern integrated circuits, very memorably summarized in the so called Moore's Law. Today, this development results in quasi-1D transistor structures also described as nanowires. In this context, the goal of the bolometric nanowires project was to investigate CMOS compatible germanium nanowires as broad-band infrared sensors, based on the principle of a bolometer. The ability of bolometers to operate as uncooled broadband infrared sensor makes them a versatile tool with manifold field applications including chemical analysis, thermography and observation of astronomical phenomena. For this type of detection, the energy of the electromagnetic radiation is converted into heat by an absorber and transferred to a thermometer. In the case of a semiconducting thermometer, this heating leads to a generation of charge carriers and a resulting measurable increase in conductivity. The extremely small thermal mass of nanowires combined with the highly temperature dependent electrical resistance of germanium, makes them a perfect building block for a high performance bolometer device. Further, the simple integration of germanium into silicon based circuits combined with an architecture close to modern transistors, allows for an easy realization of a future detector. The structure used in this project is based on a silicon nitride membrane on which individual germanium nanowires are electrically contacted. The backside of the membrane is equipped with an impedance matched Pt absorber heating up the membrane under IR radiation. In a first basic characterization, the temperature coefficient of resistance was determined for germanium nanowires of different doping and surface configuration in the range of 4 K to 296 K. Subsequently, the amplitude and speed of the bolometric response characteristic was studied over the entire temperature range. Simulations based on the finite element method - later on confirmed by experiments - showed a strong dependence of the bolometer characteristics on the size of the membrane. The bolometer was then benchmarked for its performance characteristics at room temperature. Further, it was demonstrated that the performance of the bolometer can be significantly improved by designing a parallel circuit consisting of several nanowires. For such parallelization we explored a top down process based on a GeOI platform technology resulting in scalable and well reproducible Al-Ge-Al heterostructures providing a promising framework for a future highly miniaturized broadband IR sensor with a wide field of possible applications. These Al-Ge-Al heterostructures with atomically abrupt interfaces appeared to be a promising material combination for other applications ranging from quantum ballistic transport, single electron transistors, to even active plasmonic devices.