Research On Current Transport And Reliability Of GaN Based Light Emitting Diodes

?-nitride semiconductors represented by gallium nitride?GaN?are the third generation semiconductor materials after silicon and gallium arsenide.They usually exhibit excellent properties such as wide bandgap,high breakdown electric field,large electron mobility and good thermal stability.As direct bandgap compound semiconductors,GaN,AlN,InN and their multi-component alloys A1_x In_yGa_1-x-y-x-y N have typical bandgap values varying from 0.7 eV to 6.2 eV,corresponding to a wide wavelength range from the infrared to the deep ultra-violet spectra,and making them very suitable for fabricating GaN-based light-emitting diodes?LEDs?with high efficiency.Compared with traditional illumination sources such as filament and halogen lamps,GaN-based LEDs have outstanding advantages such as lower power consumption,longer life,smaller size,faster response and less environmental pollution,and thus have giant application markets in the solid-state illumination,information display,automotive illumination,biomedical field,detection,optical communication and photonic integration.Therefore,China has paid great attention to the development of GaN-based LED technologies and related industries in the past decades.Although the GaN-based LED technologies have achieved significant progress by far,their large-scale commercial applications in some fields are still restricted by various electrical reliability problems,including the excessive leakage current,the progressive current degradation and breakdown behavior under high reverse bias,and the efficiency droop behavior under large injection current.Therefore,in-depth studies of these reliability issues and their underlying physical mechanisms have important scientific and practical significance for futher promoting applications of GaN-based LEDs in the fields of illumination,display,visible light communication and detection.In this thesis,the in-depth and systematic testing and analysis of electrical reliabilities are carried out to reveal the internal physical mechanisms of GaN-based LEDs,and the feasible methods to improve the devices'performance are proposed based on the research results.The main research content and conclusions are summarized as follows.1.By combining the variable temperature electrical measurements with data fitting,the reverse current transport mechanism of the GaN-based blue LEDs is studied.Based on the detailed analysis of variable temperature current-voltage?T-I-V?data of GaN-based blue LEDs,a novel combined tunneling-hopping transport model is developed,which can explain comprehensively the bias-and temperature-dependence of the reverse leakage current.This model depicts that,the current is majorly limited by the transport process in the depletion region near the neutral N-side,where electrons are readily to tunnel from the valence band into the localized defect states near the quasi Fermi level(E_fn),then hop continuously via the localized states within a few kTs along a relatively constant E_fn,and finally arrive at the conduction band.At T<200 K,the electron transportation shows a weaker temperature dependence that can be fitted very well by the low-field variable range hopping?VRH?equation,while at T>200 K,the temperature dependence can be described by the nearest-neighbor hopping?NNH?mechanism.2.By using the advanced testing techniques such as the emission microscopy?EMMI?,the optical beam induced resistance change?OBIRCH?,the scanning electron microscopy?SEM?and the focused ion beam?FIB?,the progressive current degradation and electrical breakdown behavior of GaN-based blue LEDs under high reverse bias stress are studied in details.The increasing reverse leakage current shows a linear dependence on the increasing accumulative number of the electroluminescence“hot spots”?namely the failure spots?,and the time-to-failure for each failure site obyes approximately the classical Weibull distribution with a slopes of about 0.67 and 4.09,corresponding to the infant and the wear-out failure periods,respectively.The current breakdown of the LEDs is mainly the transient discharge breakdown caused by the surface defects.The detailed breakdown path starts form the P-electrode,and finally the high leakage current reaches the N-electrode along the paths of the surface defects and the vertical defects on the mesa side.3.By combining the variable temperature optical measurements with numerical simulations,the influence of temperature on the efficiency droop behavior in the GaN-based blue LEDs is also investigated.The hole freeze-out effect in the P-GaN is found having a strong effect on the forward current transport behavior,which leads to a limited hole density and a nonuniform hole distribution among the quantum wells.Then,by analyzing the variable temperature optical measurement data,the effect of temperature on the internal physical mechanism of efficiency droop is clarified.At T>200 K,the Auger effect remains mainly responsible for the efficiency droop behavior since the diffusion-recombination mechanism dominates.At T<200 K,however,due to the enhanced Coulomb screening field caused by the low hole density,the excessive electrons can readily tunnel into the P-GaN side,compensate Mg acceptors directly and produce non-radiative recombinations.As a result,the electron leakage current becomes the dominant role leading to the efficiency droop at low temperatures.