Study of GaInNAs Epilayers Using Optical Methods
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Photovoltaic devices that convert sun's energy into electricity have the potential to influence energy needs on a global scale. A major limitation of single junction solar cells is that only photons with energy slightly above the bandgap are absorbed efficiently. One of the methods is to split the energy of the incoming spectrum into multiple bands each of which is absorbed separately for more efficient collection. That is why multijunction solar cells formed from III-V compound semiconductors are the highest efficiency photovoltaic devices today. To achieve this goal, researchers stack a number of junctions made of different materials with the highest gap material at the top and the lowest at the bottom since each material is transparent to photons with energy smaller than its bandgap. Kurtz  predicted an improvement in the performance of multijunction solar cells if a fourth material with bandgap in the 1.0eV-1.05eV range is included between the GaAs (bandgap = 1.42 eV) and Ge (bandgap = 0.67 eV) in the solar cell. In order for this fourth material to be easily incorporated into the GaInP/ GaAs/Ge triple junction device, it must also be lattice matched to germanium. Since it is preferred to grow multijunction solar cells monolithically lattice matching is required making the options for the 1 eV material rather limited. The most promising material for the fourth junction is currently GaInNAs. This is the reason why this thesis concentrates on the study of this material. In this thesis, we have conducted PL, optical pumping, magneto-PL, reflectance and transmission spectroscopic studies of undoped and p-type doped GaInNAs epilayers. The objective of these studies is to investigate the following phenomena in our samples: (a) Localized excitons and free excitons at low temperatures in GaInNAs epilayers: The exciton localization at low temperatures in undoped GaInNAs epilayers results in the S-shape of the PL peaks versus temperature plot. On the other hand, the dominance of free excitons in heavily doped p-type samples removes this S-shape of PL peaks versus temperature. The results presented in this thesis show that an optical pumping experiment is an effective method for differentiating between free and localized exciton recombination in GaInNAs epilayers. (b) Conduction Band to Acceptor level (CB[arrow right]A) transition in Be doped GaInNAs epilayers: The incorporation of Be-acceptors has been demonstrated previously in order to passivate background donor impurities. The inclusion of Be-acceptors during growth results in a new PL feature associated with acceptor-to-conduction band transitions at low temperatures. In this thesis, we describe the results of a study of Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge PL spectra at T = 7 K contain two features; the first is associated with the free exciton while the second with the conduction band to acceptor (CB[arrow right]A) transition. From the energies of the two PL features, as well as the exciton binding energy in GaInNAs we determined the Be-acceptor binding energy to be equal to 42 meV. (c) Spin dynamics in GaInNAs epilayers: The spin dynamics in semiconductors have been extensively studied during the last forty years. The spin lifetime of carriers is strongly limited by spin relaxation processes. It has been reported that the rapid thermal annealing on GaInNAs samples reduces drastically the spin relaxation time.  We have carried out Hanle measurements of the electron spin lifetime T S in optically pumped annealed GaInNAs epilayers in order to verify the results of Ref. 3. For the annealed undoped GaInNAs sample, we determine the electron spin lifetime T S to be 15 ps at T =50 K,. As the sample temperature increases, T S decreases ( T S = 7 ps at T = 150 K). Our Hanle results are in agreement with the T S values measured by Lombez et al. using time-resolved photoluminescence spectroscopy.