Evaluation of indium phosphide based ultrafast optoelectronic
Doctoral thesis, UCL (University College London).
The use of opto-electronic devices for ultrafast switching applications is a practical alternative to all electronic devices due to their operating ceiling. The harnessing of femto-second laser pulses to momentarily switch devices has been the centre of research for over twenty years since the pioneering work of Auston. Though the design of the electrodes of an Auston switch has not altered to any great effect, the light absorbing material has been the primary occupation of researchers. The ultrafast behaviour of these materials is due to the sub picosecond quenching of photogenerated carriers caused by deep trapping levels pinned between the valence and conduction bands of the material. The creation of these carrier traps is due to the presence of non-stoichiometric features in the semiconductor lattice structure. There have been several attempts to create such material. Originally based on Silicon on Sapphire materials, the most successful ultrafast photoconductors were found to be low temperature grown Gallium Arsenide. However this large bandgap material requires cumbersome gas or titanium sapphire lasers as the pulsed light source. Of great interest are the Indium Phosphide based materials which can harness the 1550 nm wavelength technology of optical telecommunications where erbium fibre and solid state mode-locked lasers have been developed which are low cost, compact and could enable on-chip integration. One successful approach to achieve 1550 nm absorbing ultrafast photoconductors has been the use of high energy ion irradiation of Indium Gallium Arsenide (In0.53Ga0.47As) lattice matched to Indium Phosphide substrates. There has been research of proton and heavy ion irradiation of 1550 nm wavelength absorbing materials; but no ultrafast switching devices have been fabricated from lighter ion irradiation. The advantages of this method are the higher defect concentrations achievable compared to proton irradiation and the minimising chemical changes to the material substrates which have been observed with heavy ion irradiation. The implanted ion chosen in this project was Nitrogen because of its mass and inert behaviour. In order to demonstrate the ultrafast behaviour of the Nitrogen ion implanted InGaAs, and to show that it is a practical alternative to LT-GaAs based devices, a set of ultrafast photoconductive sampling switches were designed, fabricated and evaluated. This thesis describes the design, fabrication and evaluation of InP based ultrafast switches capable of sampling waveforms up to 20 GHz. The principle mechanisms involved in the ultrafast quenching of photocarriers was investigated and the optimum design for the photoconductive switch determined. An equivalent circuit of the switch was devised and its expected performance modelled with regard to the on and off state resistances. Using transform mapping techniques, the switch capacitance and waveguide dimensions were calculated. The switches were fabricated using wet etching and metal lift-off techniques prior to evaluation of the pre-irradiated devices. Once the expected behaviour of the pre-implanted switch had been characterised, the switches were implanted by high energy nitrogen ions. These implanted devices were then evaluated and their ultrafast characteristics confirmed. With a carrier recombination time of 5 picoseconds (FWHM) being measured, this is the first time that deep nitrogen implantation has been used to create ultrafast InP based switches.
|Title:||Evaluation of indium phosphide based ultrafast optoelectronic switches|
|Additional information:||Permission for digitisation not received|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Electronic and Electrical Engineering|
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