The role of crystallographic defects in semiconductors for optoelectronic applications [electronic resource]
Keywords: Electrical Engineering.

Optoelectronic devices like solar cells and light-emitting diodes (LEDs) have become increasingly important in our lives because of their prevalent use in the fields of renewable energy and energy efficiency, respectively. Worldwide carbon emissions can be drastically reduced if the adoption of renewable energy and energy efficiency technologies is further driven through better performance or lower cost. Crystallographic defects play a critical role in determining the electronic and optical properties of semiconductors used in optoelectronics, and it is imperative that we achieve a better understanding of the impact of crystallographic imperfections on these semiconductors. In the first section of this work, the thermodynamic and electronic properties of the intrinsic point defects in solar energy conversion materials Cu2ZnSnSe4 and CuInSe2 have been investigated. This study is based on screened-exchange hybrid density functional theory. A comparison between the defect transition levels for Cu2ZnSnSe4 and CuInSe2 reveals that in Cu2ZnSnSe4, the Sn-on-Cu and Sn-on-Zn antisite defects can be recombination centers with defect states close to midgap, while the In-on-Cu antisite defect has a shallow defect level in CuInSe2. The resultant higher Shockley-Read-Hall recombination rate in Cu2ZnSnSe4 reduces the steady-state concentration of minority carriers and quasi-Fermi level separation under illumination. This may explain the origin of the low open-circuit voltage values for Cu2ZnSnSe4 solar cells compared to CuInSe2 solar cells. The second section of this work is an experimental study of the intrinsic point defects in Cu2ZnSn(S, Se)4 using a technique known as admittance spectroscopy. A thin-film solar cell based on Cu2ZnSn(S, Se)4 with power conversion efficiency of 1.04% has been demonstrated and extensively characterized. Admittance spectroscopy indicates the presence of a deep acceptor with an ionization activation energy of 190 meV with respect to the valence band maximum, which is attributed to the Cu-on-Zn antisite defect based on density functional theory calculations in the first section of this work. The relatively deep acceptor level results in carrier freeze-out at low temperatures, increasing the resistivity of Cu2ZnSn(S, Se)4 and reducing the power conversion efficiency of the solar cell. Temperature-dependent measurements of the Cu2ZnSn(S, Se)4 solar cell open-circuit voltage values indicate that the solar cell performance is limited by recombination of minority carriers at the absorber/buffer layer interface, either due to an unfavorable band offset between the absorber/buffer layers or a high concentration of recombination centers at the absorber/buffer layer interface. This is in striking difference with high-efficiency CuInSe2 solar cells, in which Shockley-Read-Hall recombination in the bulk depletion region is the predominant recombination mechanism. The third section of this work is a comprehensive study of the electronic properties of the earth-abundant semiconductor Cu3N and its potential as a solar energy conversion material, using density functional theory and experimental methods. Screened-exchange hybrid density functional theory indicates that among the dominant intrinsic point defects, copper interstitial defects behave as localized conduction band potential wells 0.45 eV in depth that efficiently capture electrons, and these potential wells exist in large concentrations considering the low formation energy of copper interstitials. Experimental results indicate the absence of photoconductivity and photogenerated current in Cu3N under illumination despite its high absorption coefficient and ideal conduction band offset with ZnS as a heterojunction partner layer. Electron capture at localized potential wells in the conduction band is the likely reason for the absence of photoconductivity in p-type Cu3N, and could be a major factor limiting the use of Cu3N as a material for thin-film photovoltaics. In the final section of this work, the use of reactive magnetron sputtering has been explored for the synthesis of III-nitride (AlN, GaN and InN) thin films and their alloys for optoelectronic applications. III-nitride thin films grown on c-plane sapphire (Al2O3) show excellent structural quality. X-ray diffraction measurements indicate extremely low densities of screw and mixed threading dislocations (10^5 cm-2), but relatively high densities of edge dislocations (10^10 cm-2). X-ray reflectivity measurements suggest that the III-nitride thin films were deposited in the layer-by-layer (Frank-van der Merwe) growth mechanism, with low root-mean-square surface roughness values of < 1 nm. Successful doping of GaN either p-type using Mg dopants or n-type using Si dopants has been demonstrated, as well as alloying of GaN to form phase-pure AlGaN and InGaN. Methods to reduce the densities of threading dislocations in GaN thin films have been investigated, and it has been shown that the densities of edge dislocations can be reduced by either increasing the film growth temperature or decreasing the film growth rate. Reactive magnetron sputtering is thus a promising technique for depositing III-nitride thin films for optoelectronic applications.