Browsing by Subject "Gallium arsenide semiconductors"
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Item Carrier dynamics in quantum dot and GaAs-based quantum dot cascade laser(2004-05) Cao, Chuanshun, 1972-; Deppe, Dennis G.Self-organized quantum dots provide unique atomic-like density of states and have important applications in semiconductor lasers. Energy relaxation of charge carriers in quantum dots is important for understanding the physics of devices fabricated from these artificially structured materials. Because the charge carriers relax through discrete energy levels, quantum dots provide a means to study the charge carrier interactions in the semiconductor crystal to unprecedented detail. The physics of the charge carrier relaxation is also expected to be substantially modified from that of the bulk semiconductor because of the modification of the electronic density of states. In this dissertation, we will present our work on the simulation of carrier dynamics in the quantum dot and the application of quantum dots in quantum cascade laser. Based on time-resolved InGaAs quantum dot PL measurement, a fourelectron-level quantum dot energy structure model is built up and rate equations are used to simulate the carrier distribution and relaxation in the quantum dot ensemble. By comparing simulated PL intensity and risetime vs. excitation level curves with the experiment results, we conclude that when carriers are excited into the quantum dots wetting layer, the random carrier capture process is dominated by exciton capture. And the spin blocking effect must be considered to explain relative strong first excited state emission under very low excitation level. Quantum cascade laser using quantum dot as its active region is proposed and studied experimentally. Possible advantages of quantum dot cascade laser over quantum well cascade lasers include lower threshold, high efficiency, vertical cavity surface emitting, etc. InAlAs quantum dot with ground state emission close or shorter than GaAs band edge emission has been developed and used as the active region of GaAs based quantum cascade lasers where lattice matched GaAs/AlGaAs superlattice is used for the carrier selective tunneling. Double plasmon-enhanced and Al-free waveguide is designed to form a thin, low loss and high confining waveguide for the mid-infrared emission. Crystal growth, device processing and characterization at cryogenic temperature have been performed.Item Characterization of gallium arsenide nitride and indium gallium arsenide nitride novel semiconductors(Texas Tech University, 1999-05) Francoeur, SébastienIn this thesis, important properties of gallium arsenide nitride (GaAsN) and indium gallium arsenide nitride (InGaAsN) novel semiconductors are presented. Using several experimental techniques such as x-ray diffraction, photocurrent, photoluminescence, effusion, and secondary ion mass spectrometry, several characteristic properties are measured, analyzed, compared to theoretical predictions, and discussed. Single layers of GaAsN were grown on GaAs (001) substrates. Single crystals with nitrogen concentrations of more than 5% were achieved. The incorporation of nitrogen in GaAs does not seem to be limited by an intrinsic solubihty limit, but by the growth conditions. The stability against phase separation was estimated from thermal treatment and was found to be highly satisfactory. Effusion experiments on GaAsN revealed that the increase in photoluminescence efficiency upon annealing is partly due to the removal of an important number of nitrogen atoms {^ 10^^ cm~^) incorporated on non-substitutional lattice sites. An accurate determination of the bandgap dependence on nitrogen concentration was made and compared with calculated dependencies. The bowing coefficient is found to be concentration dependent: it is close to 18 eV at low nitrogen concentrations while it is approximately 14 eV at nitrogen concentrations of 3.6%. Samples grown with active nitrogen provided by DMHy instead of plasma-cracked N2 show better luminescence efficiency, probably due to a higher background doping for DMHy-samples and/or the production of defects by high energy nitrogen ions for N2-samples. The formation of the bandgap of GaAsN is investigated for samples with low nitrogen concentrations. Low temperature photoluminescence yielded complex spectra of highly efficient radiative recombination centers similar to nitrogen pairs in GaP. These centers are tentatively attributed to exciton recombination bound to a pair of nitrogen related complexes. Quantum wells of GaAsN were also grown on GaAs. The room temperature photoluminescence from two sets of multiple quantum wells (MQW) were analyzed. Using the valence band offset as a free parameter, the duplication of the experimental results with calculated optical transition energies yielded a type-II natural band alignment between GaAsN and GaAs. It was found the valence band of GaAsN shifts to lower values at a rate of approximately 50 meV per percent nitrogen. The accuracy and validity of these results is discussed. Ordering in GaAsN samples was investigated with x-ray diffraction and polarized photoluminescence. However, no evidence of long range ordering could be found. Single layers and multiple quantum wells made of InGaAsN were investigated. It was found that some nitrogen can be incorporated in InGaAs, but the nitrogen incorporation is apparently limited to very low nitrogen concentrations (^0.2%). This concentration is not enough to reach a emission wavelength of 1.3 /zm.Item Contact effects in light activated GaAs switches(Texas Tech University, 1985-05) Durkin, Peter SNot availableItem III-V channel MOS devices with atomic-layer-deposited high-k gate dielectrics : interface and carrier transport studies(2008-12) Shahrjerdi, Davood, 1980-; Banerjee, SanjayThe performance scaling of metal-oxide-semiconductor field-effect-transistors (MOSFETs) has been historically achieved through shrinking the gate length of transistors for over four decades. Addressing the current challenges with CMOS scaling, the 2005 edition of International Technology Roadmap for Semiconductors has predicted the need for so-called technology boosters involving new materials for the gate dielectric and the channel as well as innovative structures. Theoretical studies suggest that the incorporation of high-mobility channel materials such as germanium and III-Vs could outperform bulk Si technology in terms of switching characteristics. Hence, this has recently led to tremendous research activity to explore the prospects of III-V materials for CMOS applications. Nevertheless, technological challenges such as formation of highquality interface between gate dielectric and III-V channel have hindered the demonstration of enhancement-mode III-V MOSFETs. Hence, tremendous effort has been devoted to study the exact origin of Fermi level pinning at the oxide/III-V interface. On the other hand, the advent of high-k materials has opened up the possibility of exploring new channel materials, for which it is challenging to achieve high-quality interface analogous to that of SiO2 on Si. Lately, III-Vs have been extensively explored in order to find compatible gate dielectrics which can unpin the Fermi level at the interface. Amongst various schemes, atomic layer deposition of high-k dielectrics offers some unique advantages such as reduction of GaAs interfacial oxides upon high-k deposition through an appropriate choice of precursor chemistry. The chief focus of this dissertation is to develop a simple wet clean process prior to high-k deposition, suitable for III-V substrates. The impact of various chemical treatments of GaAs substrates on the properties of high-k/GaAs interface was studied through extensive material and electrical characterization methods. The suitability of the ALD-grown high-k gate dielectrics on GaAs for MOSFET fabrication was explored. Charge trapping was found to result in significant errors in mobility extraction in high-k GaAs interface, where the role of high-k is not well understood. Hence, pulsed I-V and QV measurements and galvanomagnetic effects were utilized in order to directly measure the inversion charge in the channel without being affected by the charge traps as much as possible. It was also found that the material studies on GaAs substrates can be readily extended to other III-V channels, such as InGaAs.Item Integration of thin film GaAs MSM photodetector in fully embedded board-level optoelectronic interconnects(2004) Lin, Lei; Chen, Ray T.Item Raman studies of heavily carbon doped GaAs(Texas Tech University, 1999-08) Seon, MoonsukOptical measurements, mainly Raman spectroscopy, are used to study GaAs heavily doped with carbon. Hole concentration in these samples ranges from 2.3 x 10 to 1x10 cm^20^-3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local vibrational mode (CAS LVM), and the coupled plasmon-LO phonon mode (LOPC). CAS LVM intensity is directly proportional to carrier concentration. This implies that CAS LVM intensity is a good carrier density indicator, even though its practical use is limited by its weakness. Only one phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically red shift as p increases even though the peak width of the mode stays constant. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the LO phonon we attribute to carbon size effect. Interestingly, the local mode intensity shows good correlation with that of LOPC mode as a function of p. Based on these results, the intensity of LOPC to that of the LO phonon is determined to be a good indicator of the carrier concentration. ILOPC/ILO decreases upon annealing, implying p reduction. Simultaneously, two peaks around 1375 and 1600 cm"' appear in all the annealed samples. The two peaks are assigned to carbon precipitates. From the observation, it is believed that carbon precipitates into a nanocrystalline graphite phase upon annealing. The crystal size of the carbon precipitates was estimated from the peak intensity ratio. Emission due to a conduction band to acceptor level transition (e.A) was observed from photoluminescence (PL) spectra. The band redshifts as carrier concentration increases due to band gap shrinkage. PL intensity of the (e.A) transition in annealed samples is drastically decreased. Carrier concentration reduction and the formation of nonradiative recombination centers are suggested as the cause of the behavior, and carbon precipitates observed in Raman spectra are suggested as the possible nonradiative recombination centers.Item The symmetry and thermal activation energy of the EL2 defect center in gallium arsenide(Texas Tech University, 1993-12) Yang, ShilianThe symmetry and thermal activation energy of a dominant defect center in n-type liquid encapsulated Czochralski grown gallium arsenide (GaAs) crystal, called EL2, is investigated experimentally by the deep level transient spectroscopy (DLTS) and uniaxial stress techniques. A brief presentation of what has been done and known about this defect center is given first. A numerical fitting method improved upon the standard DLTS analysis is then presented. This method deals with a situation where the standard rate window DLTS is no longer sufficient, and gives an EL2 thermal activation energy of 0.76 ± 0.001 eV, different from the 0.82 ± 0.006 eV obtained by the standard DLTS. The symmetry of the EL2 center is then investigated through a more sophisticated data analysis method. From the experimental capacitance transients data measured under uniaxial stress, which superficially appears compatible with Tj symmetry, we extract reproducible defect energy level splittings under uniaxial stress and conclude that EL2 has Csv symmetry, supporting the proposed EL2 structure as the arsenic antisite and arsenic interstitial pair, Asca — Asi, with Asi weakly bound to Asca.Item Theoretical study of GaAs-based quantum dot lasers and microcavity light emitting diodes(2004) Huang, Hua; Deppe, Dennis G.Quantum dots are semiconductor nanostructures that act as artificial atoms by confining electrons and holes in three-dimensions. Self-organized QDs sit “on top of” a wetting layer, so that the QDs’ zero-dimensional levels are electronically coupled to the wetting layer’s 2-D density of levels. A theoretical model is presented that is capable of describing both nonequilibrium carrier distributions in the QD zero-dimensional levels at low temperature, as well as quasiequilibrium distribution for higher temperature operation. Due to the larger thermalization rate out of higher energy QDs of the ensemble, rearrangement of the carrier distribution could occur, which produces asymmetric gain spectrum, and leads to the novel behavior that the threshold current density can in fact decrease with increasing temperature for a QD laser, and thus show a negative characteristic temperature coefficient. The narrowing of the full-wdith half-maximum (FWHM) of the spontaneous emission spectrum could occur due to this gain spectrum narrowing. At room temperature the carrier transport is so fast that the most important characteristics of QD lasers can be analyzed using quasi-equilibrium solutions to the rate equations. The closely spaced hole levels result in a thermal smearing of the hole population among many hole states, and cause a large fraction of injected holes that do not occupy the QDs’ ground states, so that injected carriers can be wasted by not optically coupling to the lasing mode. P-doping is proposed to build in excess holes and overcome the thermal effect of the close hole spacings. High characteristic temperature coefficient is predicted for these p-doped QD lasers, large modulation bandwidth with zero or negative chirp is also predicted. According to Fermi’s golden rule, the spontaneous emission probability of an active emitter is given by the electronic transition probability from the excited state to the ground state times the photon density of states. Therefore, by tailoring the surrounding environment of an active emitter, both the spontaneous emission rate and the direction of emission will be altered. We study a QD VCSEL with very small mode volume, and show that the modulation bandwidth has a strong dependence on the mode volume. The pulse response is also strongly mode volume sensitive, and at small mode volume the relaxation oscillation is greatly suppressed, even allowing multi-gigabit data transmission. In planar microcavity LEDs enhanced efficiency and narrower spectral linewidth are achieved through the Purcell effect. Quantum dots are useful to obtain the necessary electronic confinement to very small apertures and can provide a short spontaneous lifetime. To fully take advantage of the enhanced mode coupling provided by the microcavity, it is important to electronically confine the carriers to small optical mode volume, which can be achieved in apertured QD-MCLEDs. We study the performance of an apertured QD MCLED, and find that very high efficiency apertured QD-MCLED can be achieved with small mode volume and narrow QD inhomogeneous linewidth, while only a moderate cavity quality factor Q is needed.