Investigation On Strain-compensated AlInGaN Superlattice Materials And Its Applications In Optoelectronic Devices

Owing to their excellent physical properties such as high temperature resistance,radiation resistance,high breakdown electric field,high electron mobility,and the advantages of direct-band gap covering the spectrum range from near-infrared to ultraviolet,the ?-nitrides have been successfully applied in light emitting diodes,lasing diodes and high power microwave devices.The study of polarization engineering and the fabrication of optoelectronic devices with high efficiency are always the hotspot in ?-nitrides' science and technology.Due to the lack of inversion symmetry,the crystal structure of ?-nitrides exhibit strong spontaneous polarization.Meanwhile,the lattice-misfit strain of cause piezoelectric polarization for multi quantum wells(MQWS)and other GaN-based heterostructure.The built-in electrostatic fields induced by the spontaneous and piezoelectric polarizations cause the tilt of energy band,which decrease the spacial overlapping of electron and hole wave function in MQWS and thus reduce the probability of electron-hole recombination,consequently decrease the internal quantum efficiency of MQWS.The AlxInyGa1-x-yN quaternary alloy system has attracted intense attention,for the strain-compensated effect will bring performance improvement in GaN-based optoelectronic devices.The growth conditions of AlInGaN film are extremely rigorous,which will lead to large spatial strain fluctuation,strong compositional pulling effect and high surface defect density.Thus,it is difficult to obtain high quality epitaxial materials and high-efficiency optoelectronic devices.This dissertation is focused on two major issues:the elimination of piezoelectric polarization and the fabrication and application of AlInGaN superlattice materials.Starting from the study on polarization effect of high-quality MQWs,the method to solve the piezoelectric polarization problem by fabricating AlInGaN superlattice materials has been investigated,and the applications of AlInGaN superlattice materials in MQWs and Schottky contact structures are further realized.The main content and conclusions are listed as follows:1.High-quality InGaN/GaN MQWs with smooth interfaces are obtained using GaN substrate,which effectively inhibit the formation of dislocation defects and In clusters.The photoluminescence characteristics of InGaN/GaN MQWs with different well widths are investigated under polarization.Temperature-dependent PL measurements demonstrate that the photoluminescence of the narrower InGaN/GaN MQWs(2.7 nm)is always dominated by the radiative recommbination of ground state due to stronger quantum confinement effect on excitons,while the wider MQWs(3.2nm)exhibits multi-level photoluminescence.It is found that the emission peaks of the wider MQWs exhibit S-shape shift with increasing temperatures,while the narrower MQWs(2.7nm)exhibit anomalous inverted "V"reversal from blueshift to redshift,indicating that the deeper local energy level forming in the narrower MQWs can effectively restrict the relaxation processes of carriers to other localized states.The excitation-dependent PL measurements confirm that the piezoelectric polarization induced by lattice mismatch exist in both InGaN/GaN MQWs.The luminescence decay lifetimes of the wider and narrower MQWs determined by fitting TRPL spectra are 2.227ns and 1.528ns,respectively.The stronger piezoelectric polarization causes the wider MQWs has longer luminescence decay lifetime.It is confirmed that the piezoelectric polarization play a significant role in the luminescence performance of high-quality InGaN/GaN MQWs based on GaN substrate.2.The effects of variation of growth conditions on the surface morphology,growth models and defect formation mechanisms of AlInGaN single film layer are systematically investigated.It is found that the growth model of AlInGaN single films change from three-dimensional island to two-dimensional step with the increase of growth temperature.The AFM and SEM results show that the segregation and desorption of In atoms dominate the nucleation mechanism of V-pits on the surface of AlInGaN single layer films.The growth conditions of AlInGaN single layer films are very rigorous,the In segregation and misfit-strain can easily result in the occurrence of spatial compositional fluctuation and high-density surface defects.Therefore,it is urgent to explore and develop new structures of AlInGaN materials.3.The advantages and feasibility of preparing high-quality AlInGaN materials through short period AlGaN/InGaN superlattices are investigated and analyzed.The superlattice structure can avoid alloy disordering and spatial compositional fluctuation for AlInGaN superlattice material,and thus has good in-plane uniformity.The design scheme and calculation method of AlInGaN superlattice material matched with the lattice constant of InGaN template layer are given.High-quality AlGaInN superlattice materials have been obtained by growing short period AlGaN/InGaN superlattices by MOCVD.The XRD measurements show that AlGaInN superlattice materials have smooth interfaces and high compositional uniformity.The Al/In ratio of AlGaInN superlattice matched with In0.14Ga0.86N template layer is 3.9:1.The PL measurements show that the PL intensity of AlGaInN superlattice materials increases remarkably,which can be attributed to the decreasing densities of nonradiative recombination centers arising from the lattice-match between AlGaInN and InGaN template layer.4.The InGaN/AlInGaN MQWs using AlInGaN superlattice as lattice-matched barrier are fabricated.XRD test show that the AlInGaN superlattice barriers achieve a well lattice match with the InGaN wells?SEM test show that the V-pits density of the InGaN/AlInGaN MQWs is far lower than that of the InGaN/GaN MQWs.The excitation-dependent PL measurements at room temperature show that the shift of InGaN/AlInGaN MQWs peak positions is negligible when the excitation power densities increase from 2W/cm2 to 4×102W/cm2,indicating that the absence of piezoelectric polarization in InGaN/AlInGaN MQWs.The PL intensity of InGaN/AlInGaN MQWs is over three times higher than that of strained InGaN/GaN MQWs at room temperature under the excitation power density of 4×102W/cm2.Temperature-dependent PL measurements confirm the distinct reducing of strain-induced nonradiative recombination centers in InGaN/AlInGaN MQWs,and the internal quantum efficiency of InGaN/AlInGaN MQWs(76.1%)is much higher than that of InGaN/GaN MQWs(21%).5.The Pt/Au Schottky contacts to AlInGaN superlattice samples with different background carrier concentrations are fabricated.The correlation between the background carrier concentrations and the current transport mechanisms and barrier characteristics of Pt/Au/AlInGaN Schottky contacts are investigated by current-voltage(I-V)characteristics and analysized based on the TE and TFE models.Electrical measurements show that the high density of donor-like VN defect produced by lower growth temperature(770?800?)can enhance the background carrier concentrations,and also result in the thinning of the depletion layer and thus lead to the reduction of SBH and allowing electrons to tunnel,and the defects-assisted tunneling transport and thermionic emission constitute the current transport mechanisms in the Schottky.It is also found that higher growth temperature(830?)inhibits the formation of VN,and the lattice match between AlInGaN and GaN avoids the formation of piezoelectric polarization-induced charges,the carrier concentrations decrease by two orders of magnitude.The results obtained from the TE and TFE models are almost identical,which indicating that the defects-assisted tunneling transport plays a very minor role and thus the thermionic emission is the dominant current transport mechanism in the Schottky contact to strain-balanced AlInGaN superlattice sample with low background carrier concentrations.