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Preparation And Optical Properties Of GaN-based And ZnTe Semiconductor

Posted on:2019-11-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:H Y LvFull Text:PDF
GTID:1368330545453664Subject:Microelectronics and Solid State Electronics
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In recent years,the development of material growth technology and the improvement of semiconductor device,promote the synthesis of semiconductor materials and the device development.And a lot of research results have been achieved.?-? compound semiconductor materials such as GaN and its alloys(InGaN,AlGaN,AlInGaN and so on),and ?-? compound semiconductor materials represented by Zn-based compound(such as ZnTe and so on)have been developed rapidly.On the one hand,as a typical representative of ?-? compound semiconductor materials,GaN and its alloys are all direct bandgap materials.By deliberately tuning their constituent ratio,their band gap can be altered from 0.7 eV(InN)to 6.28 eV(AIN),covering a wavelength from the infrared to the ultraviolet.Moreover,GaN has been the preferred materials in the field of semiconductor optoelectronic and microelectronic devices,due to its high emission efficiency,high electronic mobility,high thermal conductivity,very high hardness,low permittivity,stable chemical properties,and resistant to radiation and high temperature.Especially,GaN has been widely used in light emitting diodes(LEDs),laser diodes(LDs),ultraviolet detectors,high electron mobility transistors(HEMTs)and so on.This inspires the interest in the research of GaN material and its alloys.As the key part of GaN based optoelectronic devices,InGaN/GaN multiple quantum wells(MQWs)acting as the active areas has been the research topic in the field of semiconductor optoelectronics.First,In-rich area(quasi-quantum center),caused by In composition fluctuation in InGaN alloy,can formed potential well.And the localized carriers in the quantum well are considered to prevent them from reaching non-radiative recombination sites.Second,there is a strong strain-induced piezoelectric polarization field in the conventional c-plane InGaN/GaN active region due to lattice and thermal expansion coefficient misfit between GaN and InN.The piezoelectric polarization field results in a quantum-confined Stark effect(QCSE),which can cause a decrease in wave function overlap between the electron and the hole in a QW and a decrease in recombination eficiency.Because of the immaturity of the growth process,the imperfection of structure parameters and lack of proper substrate(for example,there is a large mismatch(13%)between GaN and sapphire.),the epilayers(including the GaN buffer layer,n-GaN layer,GaN barriers,InGaN wells,p-GaN layer and so on)contains a high density of the dislocations,defects and impurities.These factors acting as nonradiative recombination centers will have a great influence on the emission efficiency.Therefore,to further study the optical properties and dynamics of the carriers in the InGaN/GaN MQW is instructive to improve the luminous efficiency of this structure and its relavant structure.Moreover,as a core part of HEMT devices,investigation of electrical properties and defect characterization of AlGaN/GaN heterostructures are also hot topics in this field.On the other hand,ZnTe as a typical representative in II-VI compound semiconductors and has the band gap of 2.26 eV at room temperature which is a wide band semiconductor and its band structure is also of direct optical transition type.ZnTe can serve as an excellent material for green LEDs which makes a strong supplement to luminous materials of GaN-based LEDs,and has extensive application prospects in green LED,THz detector,optical waveguide,solar cell and modulator.As a result,it is of great theoretical and practical significance to study ZnTe materials and the related devices.At present,although great progress has been made in the study of ZnTe,there are still many problems in fabrication of ZnTe and the related light emitting devices,such as low quality and low emission efficiency of ZnTe epilayers.Therefore,it is necessary to study the preparation and optical properties of high quality ZnTe bulk and epilayers.The main research works in this paper are as follows:(a)GaN epilayers,InGaN/GaN MQWs,AlGaN/GaN heterostructures have been prepared using metal organic chemical vapor deposition(MOCVD);(b)ZnTe epilayer and bulk have been prepared using metal organic vapor phase epitaxy(MOVPE)and vertical Bridgman,respectively;(c)the samples mentioned above are measured,characterized and analyzed by means of photoluminescence(PL),electroluminescence(EL),Hall measurement and transmission electron microscope(TEM).The details are as follows:1.PL properties of InGaN/GaN MQWs.The carrier generation,transfer and recombination mechanism of the MOCVD-grown InGaN/GaN MQWs is investigated.At excitation powers below 3 mW,the InGaN matrix-related PM emission and the In-rich QDs-related PD emission are observed in the full PL spectrum due to strong phase separation,as confirmed by HRTEM.The temperature-dependent behavior of peaks PD and PM at 0.5 ?W indicates that the recombination process of the MQWs contains not only the carrier relaxation process followed by the carrier thermalization process inside individual components of the phase structure with increasing temperature,but also the carrier transfer process between these two phase-separated structures.With increasing excitation power(from 0.5 ?W to 50 mW),the PD peak disappeares at about 3 mW,while the temperature behavior of the PM peak energy(linewidth)gradually evolves from a strong "S-shaped"("W-shaped")temperature dependence into a weak"S-shaped"(an approximately "V-shaped")relationship,until becoming an inverted"V-shaped"(a monotonically increasing)temperature dependence.This indicates that with increasing excitation power the carrier localization effect gradually reduces and the QD-related transition is submerged by the significantly enhanced InGaN matrix-related transition,while the carrier thermalization effect is gradually enhanced and became predominant at high excitation powers.2.EL properties of InGaN/GaN MQWs,to achieve high-performance InGaN-based LEDs that emit photons at a fixed wavelength,two blue InGaN/GaN MQW-based LED samples,with different indium contents and well widths,have been grown,and their EL spectra investigated over the injection current range of 0.001-200 mA and the temperature range of 6-300 K.The current dependences of the EL peak energy and linewidth at different temperatures show that,at the low temperature of 6 K and in the initial current range,the recombination process of the MQWs is dominated by the filling effect of the low-energy localized states for sample A with its higher indium content and smaller well width,but by the Coulomb screening effect of the QCSE for sample B with its lower indium content and larger well width.These behaviours can be mainly attributed to that fact that the higher indium content in the MQWs induces a stronger piezoelectric field in the MQWs,leading to the more significant electron leakage and a relatively significant carrier localization effect.Also,at temperatures above about 80 K,the stronger piezoelectric field of sample A is also confirmed by its significant Coulomb screening effect although it has a smaller well width.Moreover,compared with sample B,the significant efficiency droop of sample A also further confirms the conclusion that the latter has larger electron leakage and/or overflow than the former,due to the stronger piezoelectric field and smaller well width.3.Photoelectric properties of AlGaN/GaN heterojunction.In this study,GaN epitaxial layer and AlGaN/GaN heterojunction were prepared on c-plane sapphire substrate by MOVPE method.The PL properties of GaN epitaxial layer,and the influence of AlGaN barrier thickness on the optoelectronic properties of AlGaN/GaN heterojunction,are investigated by means of PL and Hall effect measurement.For the GaN epitaxial layer structure,the origin of various kinds of PL peak are observed and identifide,including emission peaks represented by ground state FXA and excited state FXB of the free excitons of GaN epilayers,free-exciton ground state emission peak(FXA),first excited state emission peak(FXB),and several emission bands related to the defects such as Blue Band(BL)and Yellow Band(YL).On the other hand,for the AlGaN/GaN heterojunction structure,PL measurement results show that as the thickness of the AlGaN barrier layer increases,the peak position of FXA gradually blue-shifts,the intensity of the first-order phonon replica(FXA-1LO)gradually decreases,while the impurity-related emission intensity(such as BL and YL,etc.)gradually increases.This is because the biaxial stress and the crystalline quality of the GaN layer are affected by the AlGaN barrier thickness.Moreover,Hall measurement results show that,as the thickness of AlGaN barrier layer increases,the concentration of 2DEG increases,but the mobility first increases and then decreases.This is because ionization energy of the donor state on the surface,polarization field on the interface and varies of scattering mechanisms are affected by the AlGaN barrier thickness.4.PL properties of ZnTe epilayer and ZnTe bulk crystalIn this work,we investigate the excitation power and temperature-dependent PL spectra of the ZnTe epilayer grown on(100)GaAs substrate and ZnTe bulk crystal over the excitation power range of 2 ?W to 40 mW and the temperature range of 6 K to 300 K.The sharp bound excitonic emission peak(Ia)and the absence of deep structural defect-related emissions(Y lines)indicate that both the structures are of good structural quality.Furthermore,in contrast to the ZnTe bulk crystal,although the ZnTe epilayer shows a weaker emission peak Ia(possibly due to As atoms diffusing from the GaAs substrate into the ZnTe epilayer and/or because of the strain-induced degradation of crystalline quality of the ZnTe epilayer),neither the DAP emission nor e-A emission is observed in the ZnTe epilayer.All of the results indicate that by further optimizing the growth process,it is possible to obtain a high-quality ZnTe heteroepitaxial layer,which is comparable to the ZnTe bulk crystal or ZnTe homoepitaxial layer.The main innovations of this study are as follows:(1)Temperature-dependent photoluminescence(PL)of phase-separated InGaN quantum wells is investigated over a broader excitation power range.The temperature-dependent behavior of PL spectrum indicates that the recombination process of the MQWs contains not only the carrier relaxation process followed by the carrier thermalization process inside individual components of the phase structure with increasing temperature,but also the carrier transfer process between these two phase-separated structures.Moreover,with increasing excitation power the carrier localization effect gradually reduces and the QD-related transition is submerged by the significantly enhanced InGaN matrix-related transition,while the carrier thermalization effect is gradually enhanced and became predominant at high excitation powers.The experimental results are expected to provide useful guidance for the fabrication of high-performance blue or green LEDs and LDs with high-quantum efficiency.(2)Two InGaN/GaN multiple quantum well(MQW)-based blue LEDs emitting photons at approximately the same wavelength,with different indium contents and well widths,are grown,and the injection current dependences of their electroluminescence(EL)peak energies and linewidths at different temperatures are investigated.The results show that,compared with sample B,with the lower indium content and larger well width,sample A,with the higher indium content and smaller well width,shows significant electron leakage(or overflow),which is attributed to the stronger piezoelectric field induced by the higher indium content in the MQWs and the stronger quantum-confne efect(QCE)induced by the smaller well width.This indicates that exploring the proper In content and well width is necessary to produce efficient InGaN/GaN MQW-based LEDs;(3)The influence of AlGaN barrier thickness on the optoelectronic properties of AlGaN/GaN heterojunction by means of PL measurement.The results show that,as the AlGaN barrier thickness increases,both the 2DEG-related emission and the dopant-related(or defect-related)emission increase,indicating that the 2DEG concentration increases,while the the GaN crystal quality deteriorates.This conclusion is also confirmed by the Hall test results:with the increase of AlGaN barrier thickness,the 2DEG concentration monotonically increases,but the mobility first increases and then decreases.This shows that exploring a suitable thickness of AlGaN barrier layer is necessary for preparing high-performance AlGaN/GaN-based HEMTs devices;(4)The temperature-and power-dependences of photoluminescence of ZnTe epilayer and bulk crystal are investigated.Results show that the ZnTe epitaxial layer has a crystal quality comparable to that of the ZnTe bulk crystal.This indicates that it is entirely possible to grow high quality ZnTe heteroepitaxial layer structure on GaAs substrate by further optimizing the MOCVD growth process and sample structure parameters.The results provide an important solution for the preparation of high quality ZnTe.
Keywords/Search Tags:photoluminescence(PL), electroluminescence(EL), InGaN/GaN MQWs, localization effect, quantum confined Stark effect(QCSE), AlGaN/GaN heterojunction, ZnTe epilayer, ZnTe bulk crystal
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