TIPOS DE ADSORBENTES 1 ZEOLITAS

Linear and circular transmission lines were used in order to determine the characteristics of a range of electrical contacts.

Linear transmission lines consist of rectangular metal contact pads of constant dimensions fabricated at intervals with varied spacing, as shown in Figure 3-22. CTLM lines are similar to TLMs, but with circular rather than rectangular contact pads, in increasingly spaced concentric circles. The devices in this work had contact separations from 2 to 512 μm with intermediate values being powers of 2.

Current-Voltage (I-V) measurements are obtained at each contact separation distance, from which the electrical resistance can be extracted and contact type (Schottky, Ohmic). At smaller contact separations the resistance of the contact dominates the overall behaviour, at longer contact separations material resistance dominates. The contact separation is plotted against determined electrical resistance. By extrapolating to the resistance at zero separation, a value for the contact resistance can be obtained.

3.9.1 TLM and CTLM Device Fabrication

The device processing steps are shown in the series of diagrams in Figure 3-23. Prior to any device fabrication, the sample surface is cleaned to remove any surface contaminates. With silicon-germanium materials surface cleaning is performed with dilute HF acid, however the chemical sensitivity of Ge1-xSnx alloys makes HF acid

too aggressive. Therefore ~1 × 2 cm sections of Ge1-xSnx epilayer samples are

cleaned by being immersed in warm acetone and vibrated in an ultrasonic bath, then removed and dried with nitrogen gas.

Figure 3-22 Schematic diagram of the TLM structure. The metal contact pads are of constant dimensions (thickness, width w and lenglth l) at increasing seperation between contacts (d1 < d2 < d3). The mesa structure

Subsequently ‘primer’ is pooled onto the sample surface then dried with nitrogen gas, this step aids in ensuring any surface contamination is removed. A negative photoresist (AZ5124E) is then pooled onto the sample surface until completely covered. The sample is spun at high speed to produce a thin uniform coating of the photoresist, then heated (baked) on a hot plate for ~1 minute.

A glass plate which has been patterned with chrome with front contact mask, is cleaned with acetone. The mask is then aligned to sample, with the TLM lines orientated along the crystal planes. The mask is brought into physical contact with the sample surface. Physical contact is necessary to obtain the best possible definition of the mask features onto the sample. The sample surface is exposed to UV light for ~1 sec. Longer exposures can facilitate higher definitions, allowing for smaller contact separations, but can lead to the degradation of the contact pads. After the front contact mask exposure the sample is baked for a second time, using similar conditions the bake. The entire sample, without any mask, is then exposed to UV for ~10 seconds. The sample is then submerged into a wet chemical developer for 1 minute, which removes any photoresist which has not exposed to UV.

The sample is then transferred to an electron-beam evaporator. The evaporator sputters a target material in a vacuum-like environment, in this work the metal for the electrical contact pads, the sputtered material then coats sample with an even coating with a deposition thickness which is controllable with a fairly high precision. After metal deposition, the sample is immersed in warm acetone and vibrated in an ultrasonic bath for ~1 min. This removes any metal deposited on the surface not in the area defined by the contact mask. The sample is then rinsed in deionized water and dried with nitrogen gas.

The contacts pads have now been fabricated, to produce the mesa structure additional processing steps are undertaken. The sample is again cleaned with the primer, dried with nitrogen gas and then a positive photoresist is pooled onto the sample surface and spun for a thin, homogeneous coating and then the sample is baked for several minutes on a hot plate.

The sample is then mounted into the mask aligner and a glass plate patterned mesa mask loaded. The mesa mask completely shadows each individual device, the metal contacts and the uncoated sample between contacts, but leaves material between the devices exposed. The mask is aligned onto features previously made and the sample is exposed to the UV source. The sample is then immersed in a chemical developer for ~1 minute then rinsed in DI water and dried with nitrogen gas, leaving the photoresist covering only the areas of the sample which are not to be exposed to UV light.

The sample is then loaded into the resistive ion etcher, where a radio-frequency pulsed plasma is used to etch the sample surface. The devices, which are covered by several microns of photoresist, are protected from the plasma and are not damaged by the process. The uncoated sample material between the devices is etched by the plasma down to the substrate, removing the Ge1-xSnx epilayer and Ge buffer. The

samples are then cleaned in acetone, removing the remaining photoresist, and then rinsed in DI water and dried with nitrogen gas.

The height of the mesa is measured with a profilometer to ensure that sufficient material has been etched to completely remove the epilayer and buffer layer in order to electrically isolate each individual device.

A

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Figure 3-23 TLM device fabrication steps. A) Clean structure B) Spin coat negative resist C) Pattern front contacts D) Develop photoresist E) E-beam deposition of contact metal F) Lift off excess metal in acetone bath G) Spin coat positive resist H) Pattern mesa structure I) Develop mesa structure J)