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Title: The efficiency of strained and unstrained semiconductor light emitting devices
Author: Ring, William Sean
ISNI:       0000 0001 3519 523X
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1992
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In the work reported in this thesis we have used hydrostatic pressure to investigate the band structure related loss mechanisms of light-emitting diodes and strained and unstrained long wavelength laser diodes. In long-wavelength lasers there is an inherent temperature sensitivity problem which is attributed to two intrinsic temperature sensitive loss mechanisms, intervalence band absorption and Auger recombination. We have studied lattice-matched InGaAs active region multiple quantum well (MQW) laser diodes. We found that the reduction in the dimensionality of the electron gas does not affect the temperature sensitivity, contrary to initial predictions. We changed the barrier material to improve the conduction and valence band offsets and this was found to have no effect on the laser characteristics, indicating that the loss processes occur solely in the active region of the device. From the modelling of the laser characteristics of the devices we found that the loss mechanisms have similar values to those found in bulk material. We then looked at the incorporation of strain in the active region. The inclusion of strain, together with quantum confinement, can alter the valence band structure of a device and improve the laser characteristics. We investigated 1.8% compressively strained layer MQW lasers and found the external differential efficiency to be pressure independent. We compared the results with 1.55mum bulk lasers and concluded that strain had eliminated one of the loss mechanisms intervalence band absorption. The modelling of the device characteristics confirmed that intervalence band absorption was removed and that the Auger loss is also reduced in a strained-layer laser. Following the above evidence we investigated theoretically laser operation in the InAsxSbyP1-x-y material system lattice matched to InAs at a wavelength of 2.55mum. This is important since fluoride fibres are predicted to have their minimum absorption loss at this wavelength, and that it is an order of magnitude smaller than in the best silica fibres. We calculated the intervalence band absorption and Auger loss in this material system as a function of lasing wavelength. We modelled the laser characteristics of bulk double heterostructure lasers as a function of temperature and found that room temperature operation at the desired lasing wavelength could not be achieved. We concluded that strain would be advantageous in longer wavelength lasers. We have also looked at the role of higher lying satellite conduction band minima as a band structure loss. In AlxGa1-xAs light emitting diodes we find that the operating efficiency decreases as we increase the aluminum content. This is associated with the material becoming indirect as x increases. We have measured the differential efficiency of AlGaAs LEDs of varying compositions and as a function of hydrostatic pressure. We observed a dramatic decrease in the efficiency for compositions near the T-X crossover. We have modelled this and found that carriers are transferred to the X and L valleys as the pressure increases, which in turn reduces the efficiency. This shows the importance the satellite valleys can play in affecting the characteristics of short wavelength devices. From the above study we can conclude that the band structure plays an important role in determining the operating characteristics of devices and that the inclusion of strain in the active region of semiconductor laser diodes has a significant role to play.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Optics & masers & lasers