A study of InAs based quantum structures for use in optoelectronic devices
Creators: name: Alradhi, Hayfaa
Publisher: name: Lancaster University
Year: 2018

description: Due to increasing demand of nanowires (NWs) in the areas of electrical and
      photonic devices applications, it is imperative to try to improve their
      properties that are found to degrade their device performances. This
      thesis provides an extensive investigation of InAs based NWs for use in
      the photodetection applications. To achieve this goal, the structural and
      optical properties of InAs NWs and InAs NW-based quantum materials (e.g.,
      InAsSb NWs, InAs/AlSb NWs and InAs/GaSb core-shell NWs) have been
      investigated.
      
      The NW samples were grown by molecular beam epitaxy (MBE); self-catalysed
      droplet epitaxy was used as the growth mechanism for them. To improve the
      optical properties of InAs NWs, we further optimised the MBE growth for
      the NWs based on our previous growth understanding, which resulted in InAs
      NWs give strong room temperature photoluminescence (PL) at room
      temperature. We also notified that the substrate doping type gives
      significant effect on PL emission. In order to improve the axial growth
      rate for the epitaxy of NW devices, we developed a new 3-step growth
      technique: step 1 – droplet seeding, step – 2: NW growth initiation, step
      - 3 NW growth at high growth rate. The above new growth method yielded
      high quality InAs NWs with much higher axial growth rate compared to
      conventional growth methods. This method offers cost-effectiveness and
      reduction in time consumption. The NW samples grown by this method
      demonstrated denser, longer and enhanced optical properties. 
      
      The thesis also studied the incorporation of Sb into the InAs NWs to
      synthesis InAsSb NWs which is very challenging but has many interesting
      device applications. Through comprehensive growth study, we demonstrated
      the realization of InAsSb NWs with increased Sb content through reducing
      growth rate of the NWs. For the first time, we produced optically active
      InAsSb NWs with 19% of Sb content which gives PL emission at a long
      wavelength of 5.1 훍m at 10 K. This achievement reveals that our InAsSb NWs
      could be used for infrared photodetectors and emitters operating in entire
      mid wavelength infrared spectral range (MWIR), e.g. 3-5 훍m.
      
      It is well known that NW structures suffer from severe surface states
      which degrade the resulting devices performance, due to the large surface
      to volume ratio. To overcome this problem, in this thesis, we developed
      several advanced NW-based quantum materials with a core-shell structure
      (heterojunction), e.g., the NW core is passivated with shell layer of
      different material. Two novel core-shell NW materials namely, InAs/AlSb
      and InAs/GaSb were grown. Transmission electron microscopy (TEM) and
      electron diffraction x-ray (EDX) confirmed the success of the core-shell
      structure. More importantly, the PL study indicates a massive enhancement
      in the PL emission, by 5 times in comparison with the bare InAs NWs.
      Temperature dependence PL measurements proved that surface states were
      significantly eliminated. Our core-shell NW structure is a great success
      in surface passivation.
      
      Mesa device of bare InAs NWs were fabricated and its current-voltage (I-V)
      were tested in room temperature. I-V measurements showed that the NW
      device is a working device with relatively high dark current of 0.0011A
      and negative photocurrent of (-8.9526×104A).
      
      Single NW field effect transistors (FET) were fabricated using bare InAs
      NWs and InAs/ AlSb core-shell NWs and operated as photodetectors. The dark
      current measurements reveal that the bare InAs NW device gives a high dark
      current 1.5×10-6 A, while the core-shell NW device has a much more
      supressed dark current of 
      
      2.8×10-8 A, which is 186 times less than bare InAs NW device. This is a
      further evidence of the surface passivation induced by the shell layer,
      which is important for fabricating photodetectors with high directivity.
      The photocurrent study show that the bare InAs device gives a photocurrent
      of 0.2 um leading to a signal-to-noise ratio of 13%, while core-shell NW
      devices exhibited anomalous photocurrent behaviour, e.g. The device gives
      a signal-to-noise ratio of 80%, which is 6 times higher than the bare InAs
      NW device. These striking device properties were attributed to the
      efficient surface passivation caused by the shell layer of the materials.
      The anomalous photocurrent behavior was attributed to the trap centres in
      the shell layer. Our study demonstrates the great potential of the
      core-shell structure in the use of highly efficient infrared
      photodetectors.