GaInNAs/GaAs uzun dalgaboylu kuantum kuyu lazerlerin teorik olarak incelemmesi

İİİ Abstract A THEORETICAL ANALYSIS OF GalnNAs/GaAs QUANTUM WELLS FOR LONG WAVELENGTH EMISSION ODUNCUO?LU, Murat Ph. D in Engineering Physics Supervisor: Assoc. Prof. Dr Beşire GÖNÜL July 2004, 136 pages III-N-V alloys of Gaı-xInxNyAsı-y display unusual physical properties compared to that of the conventional III-V semiconductors. While conventional semiconductors have a tendency of increasing bandgap energy with decreasing lattice constant, III-N-V exhibit quite different behaviour, an increase of N con centration causes a monotonic decrease of the bandgap, instead of an increase, towards that of cubic GaN. The work carried out in this thesis reveals that doping and pressure can cause the III-N-V systems having additional unusual properties compared to that of the III-V systems. The theoretical calculations have shown that the effective mass of the elec tron increases with nitrogen concentration. This behaviour is opposite to the conventional semiconductors. We have seen that the addition of nitrogen into InGaAs leads the GalnNAs system having a band alignment of that of the ideal case (deep conduction wells and shallow valence wells). Therefore, the use of GalnNAs as an active layer causes better carrier confinement and hence reduces the carrier leakage at high temperatures. We have presented model calculations to analyze the pressure dependence of band structure, effective mass, optical confinement factor, peak gain and the differential gain. These calculations have shown that although transparency car rier density increases with pressure, the threshold carrier density, can decrease due to the increased optical confinement factor. The loss mechanism in a semi conductor laser are strongly wavelength dependent and pressure can vary the bandgap. Therefore, we have used the pressure dependence of the above laser parameters to investigate the loss mechanism for a typical three competing laserIV devices emitting in the neighborhood of 1.3 /J,m. The variation of pressure de pendence of laser parameters indicates that aluminium and nitride based laser systems are superior to that of the phosphide based laser system and offers the Al- and N-based laser systems as ideal candidates for low threshold and high speed applications, respectively. We have found that the estimated variation of phonon-assisted Auger rates with pressure in N-based system has a slower de crease than that of the other two laser systems. The threshold carrier density nth in N-based system increases with pressure whereas it decreases in Al- and P-based laser systems. This opposite variation change the overall behaviour of the threshold current in these three competing laser systems. Our theoretical calculations indicate- a significant increase of the radiative to non-radiative re combination current in N-based laser system. This result highlights the intrinsic superiority of the N-based laser system. We have presented calculations which shows that doping has a reverse ef fect on gain characteristic of GalnNAs/GaAs laser system than that of the N-free corresponding laser system. So we have concluded, for the first time, that this is another unusual physical property of nitrogen containing alloys due to the nitrogen-induced modified conduction band. We have also shown that the gain characteristics of GalnNAs/GaAs laser system can be significantly improved by means of doping following the route of the high indium and low nitrogen con centration. We have seen that the optimal route of 1.3 fj,m wavelength GalnNAs would be the choice of the higher indium instead of the higher nitrogen due to the better matching of the effective masses, favorable band alignment, smaller trans parency carrier density, higher peak and differential gain and lower spontaneous emission factor. An accurate material gain calculation is a necessary ingredient for a full characterization of III-V and III-N-V laser systems. Material gain calculations, generally ignores the Coulomb interaction between electrons and holes. We have determined that the neglect of the Coulombic attraction between electrons and holes is an overestimation, since the linear gain is enhanced at least three orders of the magnitude when these interactions are taken into account. The optimization of III-N-V quantum well active layer for improved de vice performance requires a clear understanding of exciton behavior in quantum wells. An enhancement of the ground state heavy hole exciton have been calcu lated in Gaı-xInxNyAsı-.y / GaAs compared to that of the InxGai-xAs/GaAs. Moreover, we have seen that the binding energy of the exciton increases with increasing nitrogen concentration. The close examination of the In/N ratio de pendence of the band offsets and reduced exciton mass revealed the fact that low In / high N route must be chosen to get higher exciton binding energies in 1.3 /im laser systems. We have also compared the P-based laser seems as an ideal laser system due to the enhanced exciton binding energies. N-based lasers have intermediate binding energy values and Al- based lasers seem the worst one. Key words: GalnNAs/GaAs, nitrides, band anti-crossing model, pressure, bandgap renormalization, band offsets, exciton binding energy, gain, doping.