Desarrollo de las declaraciones de la Visión y Misión

InGaAs and p-Ge (2DHG) devices were fabricated in the semiconductor physics lab-oratory in Cambridge, and the clean room in the London Centre for Nanotechnology (LCN). The majority of the processing were carried out in the semiconductor physics clean room. The sputtering of Nb and e-beam evaporation were carried out at the clean room in LCN.

3.1.1 Cleaving and Cleaning Wafers

The wafers are cleaved into desired size depending on how many Hall bars are required.

Typical Hall bar used in this project is 2 mm x 2 mm. Cleaving the wafer requires making short scribes at the end of the wafer to define the chip size, then cleaving by applying pressure to either side of the score, the chip will cleave along the major axis (high mobility direction). Once the desired wafer is cleaved, it is cleaned in an ultrasonic bath in acetone and then propanol for 5 minutes, and dried with N2. For Ge Wafers a Disco Dicer was used to cut the desired wafer size, as Ge wafers due to the crystal structure is extremely difficult to cleave using a diamond scriber.

3.1.2 Photolithography

Photolithography is an essential method for creating the Hall bars. In this project positive resist was used where the ultra violet (UV) light exposed areas are etched away by the developer. Before each photolithography step the samples were pre-baked on a hot plate at 125 C to remove any humidity from the samples.

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3.1.2.1 Photolithography for Etching

To create a mesa pattern for the Hall bar, Shipley Microposit S1805 resist is spun on to the sample at 5.5 krpm for 45 seconds and post baked for 1 min at 115 C on a hot plate. The photoresist is passed through a 0.2 µm particle filter and applied to the chip using a glass syringe. After the post bake the mesa pattern (fig. 3.1) is exposed with UV for 3.5 s, and developed using Shipley MF319 developer for 42 s.

The etch solution is usually mixed and left for an hour to stabilise, before patterning.

It is prepared by mixing 1:8:120 - H2SO4:H2O2:H2O in a plastic beaker. The wet etch rate for the In_0.75Ga_0.25As wafers was ⇠ 6 nm/s. As the quantum well lies about 150 nm deep, it was etched about 180nm. This has worked well for the devices fabricated.

Figure 3.1: The mesa pattern used to create the Hall bar, from the J-11 mask.

Using a Dektak the etch depth was measured. This is a surface contact measurement technique, where a low force stylus is dragged across a surface. First the thickness of the photoresist is measured. Once the required depth is etched, the sample is cleaned in acetone and rinsed in propanol. The sample is then remeasured to obtain the etched thickness without the photoresist.

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3.1.2.2 Photolithography for metallisation (Ohmic patterning)

For ohmics patterning Shipley Microposit S1813 is used, this is a thicker photoresist.

The sample is heated for 2 minutes at 125 C to remove humidity and solvents, and the resist is spun at 5.5 krpm for 45 s. The sample is patterned by applying 6.5 s of UV light.

Before developing, the chip is soaked in chlorobenzene for 1.5 min, which hardens the top of the resist for an undercut, this makes sure that the hardened surface is developed more slowly creating an undercut. The sample is then developed in Shipley MF319 developer for 1.5 mins, rinsing in clean DI water, drying with N2.

As Hydrochloric acid etches InGaAs, the sample is put into 40% bu↵ered hy-droflouric acid for 10 s to remove oxides which can cause problems for lift o↵.

3.1.3 Ohmics evaporation and thermal annealing

The ohmic metal used for the samples is a eutectic of Au, Ge, and Ni. Where Au makes the electric contact, and germanium helps to di↵use and dope the surface and nickel helps with adhesion. A tungsten boat is used for the AuGeNi alloy slugs. 500 mg tends to result in 150 nm thickness of metal. The sample is placed in a metal plate that holds the wafer securely facing the source boat. Once the AuGeNi slug and the sample is in place, a bell jar is placed on, and the pump is started, to create a vacuum inside the bell jar. Once the pressure is below 1 x 10 ⁶ mbar, current is passed through the boat, which heats up the slug, thermally evaporating the alloy. The rate and thickness is monitored by the quartz crystal monitor. This monitor works by resonating the quartz crystal at a certain frequency, where its frequency of oscillation changes as its mass changes, which is easily calibrated in terms of frequency against deposition thickness for the metal in use. Once the evaporation begins the shutter is opened manually from the side of the evaporator. As nickel evaporates at higher temperature, the current is increased towards the end of the evaporation to get the remaining metal out of the boat. Once the rate is zero, the shutter is closed and the current is slowly decreased.

After the evaporation is complete, the system is kept under vacuum for another twenty minutes for cool down, before taking out the samples. The samples are placed in a jar containing acetone and left for an hour before lift-o↵. Using a glass pipette, acetone is squirted onto the sample which lifts of the metal, leaving metal only on the exposed places. The lift-o↵ is observed in a petri dish containing IPA, under the microscope to make sure all the metal has been lifted o↵, and once it is, it is blown dry with N₂. While lifting o↵ the metal, a practice anneal run is activated on the RTP 600S