The spectral range of quaternary AlGaInN/AlGaN MQWs extends from UV to IR. Nitride-based green and blue LEDs reveal a high efficiency for the further application. Integrating LEDs of three element colors can perform white light. The optical properties of GaN MQWs are very sensitive to the growth conditions of MQWs. The ununiformity is not fabrication desired but needs to prevent, which is necessary to understand and to precisely control through its growth condition for manufacture the LED. In this work the sample has a luminescence varied from orange to purple across the whole wafer. In this work, the correlations between optical and structural properties in these samples have been studied by means of Transmission Electron Microscopy (TEM), energy dispersive X-ray spectrometry (EDS), and cathodoluminescence (CL) measurements.

We present full-scale relativistic pseudopotential calculations of the first-order susceptibility in p-type SiGe/Si multiple quantum well structures with a view to exploring the suitability of such systems for infrared applications in the 3-5yrn and 8-15itm ranges. A derivation of an expression for the linear susceptibility, or absorption, is given and the frequency dependence of the linear response due to transitions between the valence minibands is determined. The microscopic origin of the absorption is demonstrated for both parallel and normal incident light. Comparisons between calculated and experimental results are presented and shown to be in good agreement. The effects of changing well width, temperature, doping concentration and germanium concentration in the well are considered. We also consider Auger recombination and discuss the possibility of engineering the miniband structure in order to prevent certain Auger processes occuring, Preliminary results from full scale Auger calculations are also presented.

Laser Diodes (LD) have numerous applications for industry, military, medicine and communications. The first visible LD was invented in 1962 by Nick Holonyak, emitted at 710 nm (red). In 1990s, Shuji Nakamura invented the blue and green Light Emitting Diodes (LED) and later LDs. The production of LDs emitting between 532- 632 nm has been severely lagging behind the rest of the visible spectrum. Yellow and orange LDs are still not accessible due to the lack of successfully grown material with high optical efficiency. AlGaInP is the quaternary compound semiconductor used to grow green to red LEDs and red LDs. At a material composition that is supposed to lase below 630 nm, the optical efficiency becomes low due to the oxygen-related defects associated with high Al content. The quantum well intermixing (QWI) is a post-growth process that is applied to laser structure to tune the wavelength of laser. Until now, there are limited reports on successful intermixing of InGaP/InAlGaP laser structures while maintaining the crystal quality. In this work, we introduced a novel intermixing process that utilizes the high strain induced by the dielectric film during annealing to initiate the intermixing. We deposited SiO2 capping by plasma-enhanced chemical vapor deposition (PECVD) onto the InGaP/InAlGaP laser structure emitting at 635 nm, and then annealed the structure up to 950 Celsius for different periods of time, resulting in an astonishing 100 nm blueshift. This blueshift allowed us to produce an unprecedented shorter wavelength orange lasers emitting at 608 nm. For low degree of intermixing, we have noticed an increase in the intensity of the photoluminescence (PL) signal. The improvement in the PL signal was translated to a reduction in threshold current. We implemented the technique on an LED structure with Al-rich QWs emitting at 590 nm. Significant increase in the PL intensity (20 folds) was observed. By analyzing the improved structure, we observed reduction in oxygen contamination. This may represent a solution to the oxygen-related defect. The thesis opens the door for major steps forward in GaInP/AlGaInP structures for manufacturing efficient optoelectronic devices in the green, yellow and orange visible range.

Approved for public release, distribution is unlimited / In this thesis, the performance of a step quantum well infrared photodetector (QWIP), which was designed to detect a laser spot at 1.05 m æ and IR radiation between 9.5 m æ to 10.5 m æ, was evaluated. In the far infrared (FIR), the maximum responsivity of the test detector at 10 K was measured at 1.02 A/W with a peak wavelength of 10.3 æm under a negative bias of 0.83 V. The D* at background limited infrared performance (BLIP) was measured at 9 8.0 10 cm Hz /W with 180o field of view. The BLIP temperature was found to be about 55 K. In addition, the FIR detection wavelength found shift with the amount of bias across the device. The amount of shift observed was 0.21 meV/KVcm-1 which is due to linear Stark effect associated with the step quantum well. The quantum well infrared detectors made of square quantum wells found to have an order of magnitude lower Stark shift originated from second order effects. This suggests that the step well infrared detector can have applications in tunable wavelength infrared detectors. / Major, Republic of Singapore Navy

Neste trabalho, investigamos a influência da estrutura de energia das minibandas dos estados eletrônicos ocupados no tempo de recombinação em poços quânticos múltiplos (MQW) fracamente acoplados de GaAs / AlGaAs. Um dos melhores métodos para estudar o efeito da estrutura energética consiste em medir o tempo de recombinação eletrônica em função de parâmetros expostas à influência externa que afeta a estrutura energética, por isso, aplicamos um campo magnético externo. O espectro da emissão de fotoluminescência foi composta pelas contribuições das minibandas da banda de condução, Γ – Γ e Γ – XZ. Observou-se um aumento notável do tempo de recombinação quando o campo magnético causou a despopulação da minibanda de maior energia, Γ – XZ. O efeito observado é atribuído à variação induzida pelo campo magnético na densidade dos estados eletrônicos. / In the present work, we investigate the influence of the miniband energy structure of the populated electron states on the recombination time in GaAs/AlGaAs weakly coupled multiple quantum wells (MQW). The best method to study the effect of the energy structure is to measure the recombination time in the same sample subject to external influence which affects the energy structure, therefore, we apply an external magnetic field. The photoluminescence emission was composed of the contributions from the Γ – Γ and Γ – XZ conduction band minibands. Remarkable enhancement of the recombination time was observed when the magnetic field caused depopulation of the higher energy Γ – XZ miniband. The observed effect is attributed to the magnetic field induced variation of the electron density of states.

In this thesis, optical studies of c-plane InGaN/GaN quantum well (QW) structures are presented. The effects of a Si-doped underlayer (UL) on the optical properties of multiple quantum well (MQW) structures are investigated. The QW photoluminescence (PL) emission peak energy and radiative recombination rate decrease and increase respectively with increasing number of QWs. These observations are attributed to the increasing net electric field across the MQW structure as the strength of the surface polarisation field, which acts in the opposite sense to the piezoelectric polarisation fields across the QWs, reduces with increasing distance of the UL from the sample surface. This leads to a reduction in the electron-hole recombination energy and wavefunction overlap. It is also shown that the internal quantum efficiency of the MQW structures may decrease with increasing number of QWs due to the reducing radiative recombination rate, which could indicate that carrier losses due to thermionic emission or interface recombination are mitigated by the inclusion of an UL. Optical studies of single QW structures containing Si-doped ULs with different net electric fields across the QW are presented. The net electric field across the QW is changed by varying the thickness of the GaN cap layer. The full width at half maximum of the emission peak increases with increasing net electric field across the QW. This is attributed to the increasing variation in electron ground state energies due to the role of the electric field in the localisation of electrons at quantum well width fluctuations. For one sample, a smaller Huang-Rhys factor compared to the rest of the samples is calculated. The non-exponential PL decays detected on the low energy side of the QW emission peak from this sample are also of a different shape to the other PL decays detected at all energies for the other samples. This may be due to the reversal of the net electric field across these QW regions. Observations of a broad emission band on the high energy side of single QW structures at high excited carrier densities are presented. This band occurs in the carrier density regime at which the efficiency droop is observed. The emission band is attributed to higher energy weakly localised or delocalised electron and hole states that are populated following the saturation of the localised ground states. PL decay curves detected across this emission band exhibit plateaus where the PL intensity remains constant until the higher energy emission has decayed. These are similar to decays observed in semiconductor quantum dots, which are characteristic of Pauli state blocking.

Neste trabalho, investigamos a influência da estrutura de energia das minibandas dos estados eletrônicos ocupados no tempo de recombinação em poços quânticos múltiplos (MQW) fracamente acoplados de GaAs / AlGaAs. Um dos melhores métodos para estudar o efeito da estrutura energética consiste em medir o tempo de recombinação eletrônica em função de parâmetros expostas à influência externa que afeta a estrutura energética, por isso, aplicamos um campo magnético externo. O espectro da emissão de fotoluminescência foi composta pelas contribuições das minibandas da banda de condução, Γ – Γ e Γ – XZ. Observou-se um aumento notável do tempo de recombinação quando o campo magnético causou a despopulação da minibanda de maior energia, Γ – XZ. O efeito observado é atribuído à variação induzida pelo campo magnético na densidade dos estados eletrônicos. / In the present work, we investigate the influence of the miniband energy structure of the populated electron states on the recombination time in GaAs/AlGaAs weakly coupled multiple quantum wells (MQW). The best method to study the effect of the energy structure is to measure the recombination time in the same sample subject to external influence which affects the energy structure, therefore, we apply an external magnetic field. The photoluminescence emission was composed of the contributions from the Γ – Γ and Γ – XZ conduction band minibands. Remarkable enhancement of the recombination time was observed when the magnetic field caused depopulation of the higher energy Γ – XZ miniband. The observed effect is attributed to the magnetic field induced variation of the electron density of states.

GaAs-based nitride is found to be sensitive to growth conditions and ex-situ annealing processes. The critical thickness is almost one order thicker than the theoretical prediction by force balance model. The growth process could be sped up by the nitrogen incorporation itself, while the nitrogen incorporation could be affected by Beryllium doping. The incorporated nitrogen atoms partly occupy substitutional sites for Arsenic. Some nitrogen atoms are at interstitial sites. Annealing could drastically increase the optical quality of GaAs-based nitrides. As an end of this paper, some preliminary results of InGaAsN/GaAsN/AlGaAs laser diodes are also presented. / Singapore-MIT Alliance (SMA)

GaAs-based InGaAsN/GaAs quantum well is found to be very sensitive to growth conditions and ex-situ annealing processes. Annealing could drastically increase the optical quality of GaAs-based InGaAsN/GaAs quantum well. As an end of this paper, some results on InGaAsN/GaAsN/AlGaAs laser diodes are also presented. / Singapore-MIT Alliance (SMA)

The three wave mixing process was modeled in GaAs:AlGaAs superlattices using two new modeling tools that were developed in the course of this work: A 2D beam propagation tool for optimizing quasi-phase matching gratings, and a 1D iterative beam propagation tool for determining the output powers and threshold of optical parametric oscillators of arbitrary geometries. The 2D tool predicts close to 80% enhancement of conversion e ciency by phase matching near 800 nm compared to 775 nm, which was the originally designed operation wavelength. The model also predicts resonant behaviour for an abrupt grating pro le. The 1D tool was used to determine the threshold conditions for para- metric oscillation for di erent geometries. The performances of di erent phase matching approaches in AlGaAs were quantitatively compared. The model also indicated the need for pulsed operation to achieve reasonably low threshold powers in AlGaAs waveguides.