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Preparation And Optical Property Of Ⅲ-V Compound Semiconductor Materials Of GaN Epitaxial Film And InAs Quantum Dots

Posted on:2007-01-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:J H YangFull Text:PDF
GTID:1118360185455312Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
This article mainly reported on the III-V group compoundssemiconductor grown by the method of molecular beam epitaxy, and the effectof preparation technology and geometric sample structure on the opticalproperties were studied, which will provide new type semiconductor materialsfor the low-cost and high-powered semiconductor laser and LBD. Two kindsof III-V group compounds, GaN epitaxical film and InAs quantum dots, wereinvestigated. Samples used in this investigation were grown in an EPI930 solidsource molecular beam epitaxy system. Their preparation technology andoptical property were studied, and a effective measure were used to improvetheir surface morphology and enhance their optical performance.1. GaN epitaxical filmIn recent years gallium nitride and related III–V nitride semiconductorshave shown suitability for high-temperature, high-power electronic devicesand blue-ultraviolet light emitters due to their strong binding energies andlarge direct energy band gaps. Considerable effort is being devoted toimproving the quality of epitaxial layers, material characterization methodsand techniques for device processing. However, the lack of a lattice-matchedsubstrate makes it difficult to achieve defect free epitaxial layers. There is alarge lattice mismatch of 14.83%, between GaN and the most commonly usedsubstrate, sapphire. Heteroepitaxial layers, in the form of thethermodynamically stable hexagonal wurtzite crystal structure have beenobtained by metal-organic vapour phase epitaxy (MOVPE) and molecularbeam epitaxy (MBE). One key advantage of MBE over MOVPE is thecapability of controlling the growth process in real time at the monolayer scale,through the in situ reflection high-energy electron diffraction (RHEED).However, the growth of high-quality GaN for device fabrication still raisesmany questions and requires further research. As an example, the sapphiresurface nitridation procedure before growth effects the morphological andoptical properties of thick GaN layers. Furthermore, the nucleation layer andthe growth conditions of the GaN buffer layer, especially the N/Ga-ratio, areof importance to achieve high-quality optical and electrical properties andsmooth surface morphology. All these problems should be further investigated.Therefore, the optimized conditions for preparing GaN epitaxical film byMBE were studied, and the surface morphology and optical properties of themgrown under different conditions were also systematically investigated.On one hand, a series of GaN layers were grown with a turbo pumped,solid source Varian GEN II Modular MBE system on (0001)-oriented sapphiresubstrates. the correlation between the low temperature optical properties, freecarrier concentration and the surface morphology of thick GaN on sapphire(0001) were studied by changing the Ga-flux, purifying N2 and introducingAlN buffer layer while keeping all other parameters constant. Thisstep-by-step modification of the growth conditions successfully separatedimpurity and defect related donors and acceptors, revealing their effects on theoptical properties.The low-temperature PL spectra were dominated by the neutraldonor-bound excitons (D°,X) transition, at 3.472–3.474 eV. The FWHMvalues as a function of the growth rate correlated to the SEM revealed surfacemorphology. For Ga-rich growth conditions, rougher surface morphology wascorrelated to better optical properties. FWHM and defect concentration weredecreasing with the increase of Ga flux, while the surface roughness changedin the opposite trend. That is to say that the improving PL properties wereassigned to the reduced defect related intensities, because of the decreasingdefect related donor concentrations from 1×1020cm-3 to 6×1018cm-3. Thenarrowest linewidth of 41 meV was obtained for highest surface roughnessunder strongly Ga-rich conditions.The linewidth could be further reduced by purification of the N2-flowduring MBE-growth, giving a PL linewidth down to 20meV. This is a strongindication of a direct correlation between optical properties and theincorporation of impurities, such as oxygen and carbon. In the PL spectrumthe neutral acceptor bound excitonic (A°,X) intensity and the peak area at3.41–3.43 eV decreased. Both are directly attributed to carbon acceptors. Thelower oxygen concentration resulted in a decreased free electron concentration,from 6×1018cm-3 to 1×1018cm-3.The introduction of the AlN buffer further reduced the linewidth of themain PL peak. This was mainly due to a vanishing low-energy shoulder region,affected by the (A°,X) transition, directly attributed to defect related acceptorstates, as Ga-antisite/Ga-vacancy complex, originating from the substrate-layerinterface. About one order of magnitude reduction in the free electronconcentration, from 1×1018cm-3to1×1017cm-3, is a result of lower defect donorconcentration.On the other hand, the effects of the Al introduction in the GaN on theiroptical, electrical and even morphological properties were studied. And thelayer quality was assessed by photoluminescence, Hall Effect measurementsand high-resolution SEM. The results showed that the morphology and theoptical property were improved step-by-step with the changes of the growthconditions, such as Ga-flux, different buffer layer and different degree ofoxygen contamination from the nitrogen gas. The smallest surface roughnesswas obtained at 0.11% Al, while the sample exhibited the lowest free electronconcentration (3×1017cm-3)and the highest electron mobility(140V.s/cm2).It is expected that the Al/O-ratio affect the incorporation in the alloys as wellas the free electron concentration, and the passivation of the oxygen providenot only the lowest free electron concentration but also the highest electronmobility.2. InAs quantum dotsInAs quantum dots(QDs) grown on GaAs substrates have been seen asone of the most promising candidates for use as a light source in optical fibers.InAs QDs grown on GaAs substrates not only emit light in the optical fiberwindow at 1.3 mm, but the most difficult problem, i.e., to form highlyreflecting distributed Bragg reflectors for verticalcavity surface emitting lasers,can also be easily solved when a GaAs substrate is used. There have been anumber of investigations on the properties of InAs QDs, such as electronicstructure, emission wavelength, relaxation and decay times, and theapplications of InAs QDs, i.e., laser devices. Experimental results show thatthe shape and size of InAs dots strongly depend on the growth conditions suchas substrate temperature, growth rate, cap layer, and nominal InAs thickness.Since the confinement energies of electrons and holes in InAs QDs aredetermined by the shape, size, and strain distribution of the dots, the emittingwavelength from InAs dots can be tuned by changing the growth conditions orusing a proper cap layer. In order to fabricate an InAs dot laser at the emissionwavelength of 1.3 μm, it is not only necessary to know how to tune theemission wavelength, but it is also important to enhance optical gain and thestable temperature (T0) of semiconductor laser, which need to be furtherinvestigated. Size fluctuation and gain saturation of QDs are the urgentproblems to be solved for the InAs QDs laser. The optical gain is directlyinfluenced by the low concentration of QDs. In order to increase theconcentration of QDs, several QDs layers have to be stacked for keepingenough gain. Moreover, the large energy separation between the ground andthe first-excited state could enhance the temperature stability of lasers. SoInAs QDs grown by MBE have been investigated, which will benefit to avoidthe multilayer growth, increase the concentration of QDs, tune the emissionwavelength and increase the energy separation between the ground and thefirst-excited state.A series of InAs quantum dots were grown in an EPI930 solid sourcemolecular beam epitaxy system on semi-insulating (001) GaAs substrates. Theeffects of the InxGa1-xAs cap layer, InxAl1-xAs tunneling barrier layer and theAl concentration in the In0.1AlxGa0.9-xAs lower embedding and the thickness ofIn0.2Al0.8As upper embedding layers on the emission efficiency and the intensityof the InAs QDs have been studied. The growth condition has been optimized.The emission of light with a wavelength of 1.3μm from InAs QDs has alsobeen achieved through changing the composition and the thickness of differentlayers. The results clearly illustrate that Growth conditions for bestluminescence intensity and smallest linewidth were found within narrowwindows of substrate temperature (500–520℃) and nominal InAs layerthickness (3.3–3.7monolayers). The emission wavelength of such InAs QDscapped by GaAs was around 1.24 mm. However, this is redshifted to 1.3 mmor more by capping the InAs QDs with a thin layer of InxGa1_xAs. Theresults show that both In content and thickness of the capping layer can beused to tune the emission wavelength.Through analyzing the emission of the InAs QDs with InxGa1-xAs cap layer,it can be seen that there indeed exist nonradiative centers both at the interfacebetween the InAs dots and surrounding layers and in the GaAs layers with theInxGa1-xAs layer covering the InAs dots, which can be suppressed byH-treatments.When a tunneling barrier of In0.2Al0.8As was introduced between the InAslayer and the GaAs cap layer, the intensity of the InAs QD emission increasesmore than an order of magnitude, which is due to that the tunneling barrierdepressed the interface defect so that the number of nonradiative centersdecreased. And the emission wavelength also can be tuned through changingthe thickness of the barrier layer. Our results show that the optimum thicknessof the In0.2Al0.8As layer is around 4–5ML.Moreover, with the changing of the Al concentration in the lowerembedding layer and the thickness of the In0.2Al0.8As cap layer, not only thewavelength have been tuned to 1.31μm, but the energy separation between theground and the first exited state transition of 108 meV have been realized. Tothe best of our knowledge, this results represents the largest energy separationyet report between the ground and the first excited state transitions in any kindof QD structures.These two materials mentioned in this article are both the investigationhotspot in the field of photoelectron. They are the potential materials tofabricate the low-cost and high-performance luminescence apparatus such assemiconductor laser, laser diode, photoinduced laser and so on. The researchesof this program not only provide the important theoretical and experimentalfoundation for the design and exploitation of these materials, but activelypromote the practical applications of them.
Keywords/Search Tags:Semiconductor
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