Independent control of valence- and conduction-band states in composite quantum wells

The driving force for the rapid development of two-dimensional semiconductor structures, such as quantum wells, is their potential for various electronic and opto-electronic applications. The advent of modern epitaxial techniques allows the growth of semiconductors with a precision down to a single atomic layer. Due to the intense research in epitaxial growth, it is even possible to fabricate the quantum well structures for lattice mismatched materials as well, as long as the critical thickness is not exceeded. Under such circumstances, strain becomes an important design tool to tune the band-structure in a quantum well. This thesis focuses on type-II asymmetric composite quantum well structures in which the valence and conduction bands can be tuned independently by engineering the strain. The composite well region consists of GaxIn1-xAs and InAsyP1-y layers. These materials are chosen because of the different conduction and valence band offsets with respect to InP barrier material. The resulting potential profile is highly asymmetric and it leads to real space separation of electrons and holes in these structures even without the application of an external electric field. The electrons are confined in the InAsyP1-y part of the composite well while the holes are confined in the GaxIn1-xAs well. We have shown that adjusting the strain in the structure can control the zero field separation of electrons and holes. The strain can also modify the valence band ground-state. By putting more tension on the GaxIn1-xAs part of the composite well the valence band ground-state changes from heavy-hole to light-hole character. In a conventional type-I quantum well, the application of an external electric field pulls the electrons and holes to the opposite sides of the quantum well. This results in a red shift of the transition energy together with a decrease in absorption (Quantum confined Stark effect). However, a blue shift of the transition energy is observed, if the electric field pushes the electrons and holes close to one another. This results in an enhancement of the electron-hole overlap which leads to an increase in oscillator strength. Due to the initial electron-hole separation in our composite quantum well structures, an applied electric field can either push the carriers close to one another or pull them further away. So, depending on the direction of the applied electric field either a blue or a red Stark shift of the transition energy is observed. The blue shifting quantum well structures offer potential advantages for advanced opto-electronic systems such as self-electro-optic effect devices (SEEDs). The photocurrent measurements carried out on our composite quantum wells at 100 K shows a large asymmetric Stark shift (blue and red) of the ground-state transition energy. Due to the non-zero dipole moment present in the system, the observed Stark shift is quite linear at lower electric fields, which changes to quadratic field dependence at higher fields. A blue shift of 35 meV together with an enhancement in oscillator strength has been achieved in a "strain balanced" structure (GaxIn1-xAs under 1.3 % tension and InAsyP1-y under 1.3% compression) at an applied electric field of 106 kV/cm. We have been the first to demonstrate that a blue shift of this magnitude can be realised in III/V composite quantum wells. Such systems clearly have attractive possibilities for advanced optoelectronic devices based on the blue shifted quantum confined Stark effect. By varying the width and composition of the GaxIn1-xAs and InAsyP1-y layers, the energy of the groundstate and the degree of separation of electron and hole wave functions can be controlled. This can be used to optimise both operating wavelength and the magnitude of the blue shift for practical devices. All the measurements agree with our theoretical calculations. Our results show that one can choose material combinations with optimum confinement characteristics for the electrons and holes individually. In addition to the electric field induced blue shift, these composite wells (undoped and unbiased) also exhibit a blue shift of the transition energy with optical excitation. When the structure is illuminated the photo-created electrons and holes are accumulated in spatially separated regions. Electrons are captured in the InAsyP1-y well and holes are trapped in the GaxIn1-xAs part of the composite well. The observed blue shift of the transition energy with increase in optical excitation density is related to the build-up of photogenerated carriers causing band-structure modifications. These results were compared with the selfconsistent calculations of the band-structure incorporating the contribution of the exchange and correlation effects. This reveals that the dominant mechanism causing the optically induced blue shift is the change of the electrostatic potential due to the build-up of spatially separated electrons and holes. We have also shown that one can control over the maximum achievable blue shift by controlling the strain in the structure. The observed effect has great promise for all-optical switching applications. We have already seen that by controlling the strain one can change the character of the valence band in these composite wells. Polarisation of the luminescence collected from the cleaved side of the samples gives valuable information on the character of the valence band ground-states. Information on excited-states is also extracted by polarisation dependent photoluminescence excitation (PLE) measurements at low temperatures. These results agree with our model calculations. The low temperature photoluminescence spectrum from the "strain balanced" sample shows a side-band on the low energy side of the exciton peak. This is a longitudinal-optical (LO) phonon side-band. The presence of the LO-phonon peak in the photoluminescence spectrum suggests that the excitons are localised in the alloy fluctuations. The PLE from this sample also shows a large Stokes shift of 13.5 meV. The presence of the LO-phonon peak together with the large Stokes shift is a clear indication of exciton localisation at the quantum well plane at low temperatures. In order to probe the extent of localisation we have performed photoluminescence measurements in magnetic fields. The intensity of the photoluminescence from the "strain balanced" sample decreases up to 4 T and then saturates. The transition energy shows a quadratic diamagnetic shift. Both are indications of localisation. We have extracted the extent of localisation from the best fit to the quadratic variation of the transition energy, and found that it is approximately 7 nm. For both the lattice matched (GaxIn1-xAs lattice matched to InP) and "strain balanced" structure, the InAs0.42P0.58 part of the composite well is always under 1.3 % compression. The difference arises from the GaxIn1-xAs part of the composite well. As we change the Ga content from 0.47 to 0.67, the diamagnetic shift changes from linear to a quadratic behaviour. This clearly indicates the localisation caused in the GaxIn1-xAs part of the composite well and it increases with increase in the Ga content. For the "strain balanced" structure, the photoluminescence line shape at low temperature and excitation intensity as well as the evolution of its peak energy with temperature are characteristics of the localised excitons induced by potential fluctuations. The peak energy of the photoluminescence spectrum of this sample shows a deviation from the temperature induced bandgap modification of the quantum well materials. It shows an S-shaped temperature dependence with a blue shift of 13 meV at low temperatures. The full width at half maximum of the photoluminescence peak from this sample shows an inverted S-shaped dependence with increase in temperature. This anomalous behaviour toget with the dependence of line shape with temperature clearly indicates the presence of localised states in this structure. The estimated localisation energy is 13 meV. This value of the localisation energy is comparable to the Stokes shift of 13.5 meV observed in this sample. These results strongly support the results on localised states by PLE and magnetooptic measurements done on this system. However, this localisation does not affect the device operation at room temperature. The temperature dependent photoluminescence of the "strain balanced" sample shows that the phonon side-band is completely washed away at temperature above 100 K. This shows that there is no in-plane localisation above 100