Polarization Effects In GaN-based Resonant Tunneling Diodes

In recent years,the terahertz(THz)frequency range has been receiving considerable attention because of its many applications,such as ultrahigh-speed wireless communications,spectroscopy,and imaging.Since the THz frequency lies between the light wave and the millimeter wave,both optical and electronic devices are being investigated for THz sources.For semiconductor single oscillators,p-type germanium laser and THz quantum cascade lasers are discussed from the point of view of optics,while the two terminal devices such as impact-ionization-avalanche-transit-time diodes(IMPATT)and Gunn diode as well as the resonant tunneling diode(RTD)are studied from the perspective of electronic devices.RTD utilizes quantum mechanical tunneling,which enables RTD to operate at higher oscillation frequencies.As a result,RTD is widely regarded as the fastest solid-state electronic device.The earliest RTD electronic devices are based on GaAs materials.However,due to the properties of arsenide materials,the output power of RTD was only microwatts,which greatly limited its application.In recent years,the third-generation semiconductor materials represented by GaN have attracted extensive attention due to their wide band gaps,high mobility,high thermal conductivity and other excellent properties.GaN-based RTD devices emerge at the historic moment.However,there are strong polarity and high defects in group III nitride materials,which makes it difficult for GaN-based RTD diodes to maintain stable negative resistance characteristics at room temperature.Until recently,nitride heterojunction based RTD devices have achieved stable,repeatable resonant tunneling characteristics at room temperature,but the repeatability of NDR remains limited.Based on this,the advantages of RTD devices based on the heterogeneous junction of group III nitrides should be developed and explored as much as possible.At the same time,we should study the genesis and mechanism of the adverse factors such as polarity and defects in devices,and propose a scheme to suppress these adverse factors,which constitute the core of this paper.Through the research,this paper mainly comes to the following conclusions:1.This work presents a GaN-based RTD simulation model based on Non-Equilibrium Green Function(NEFG).Firstly,the correlation coefficient of the mobility,velocity field,ionization and recombination models embedded in commercial Silvaco-Atlas numerical simulators are fitted by Matlab mathematical method using the theoretical data obtained by the comprehensive Monte Carlo method and the actual measured experimental data of group III nitride materials.In addition,the polarity and defect limiting the application of III nitride are also modeled.On the basis of these material models,a GaN-based RTD device is successfully modeled by a planar NEFG electron transport model.In the device simulation,the potential and electron distribution inside the device are obtained by solving the Schrodinger-Poisson coupling equation with trap charge.The current density and the transmission coefficient of the RTD device can also be obtained through the planar NEFG and used to characterize the macro and micro parameters of the RTD device.These parameters provide a quantitative basis for the research of RTD device characteristics,which is also the basis of the follow-up research.The simulation results show the Negative Differential Resistance(NDR),which depends on the quantum well,barrier layer and other parameters.2.In this work,the Metal Organic Chemical Vapor Deposition(MOCVD)epitaxy technology is used to grow AlGaN/GaN heterojunction on homogeneous self-supporting GaN substrate,and the overall quality and thickness of the epitaxial layer are characterized by X-ray diffraction(XRD)and scanning transmission electron microscope(STEM).Then GaN-based RTD is made by traditional semiconductor device manufacturing process and tested.The test results show that there is a clear NDR at the forward scan,but the peak current density declines obviously after multiple scans,and the I-V characteristics show an asymmetry when the forward voltage scan is performed on the device.Finally,the numerical model is used to analyze the experimental results,which reveale that it is the polarity that cause the strong built in electric field to change the barrier structure and electron distribution in the effective region of RTD,while the barrier structure and electron distribution in the effective region affect the transmission coefficient and ionization rate of defects.Therefore,we propose that the characteristics of the device can be improved by inhibiting polarity.3.In this work,a RTD based on nonpolar AlGaN/GaN heterostructure is proposed.The growth of AlGaN/GaN epitaxial layer in nonpolar direction makes the polar electric field and epitaxial growth direction perpendicular,thus avoiding the negative influence of polarity on device characteristics.Firstly,the nonpolar GaN material is modeled,and then the nonpolar device model is successfully established based on the material model to obtain the negative differential characteristics of the diode.It is found that nonpolar RTD can obtain higher peak current density,peak-to-valley current ratio(PVCR)and lower peak voltage than those of polar RTD.Due to the lack of polarity,nonpolar RTD has symmetrical double potential barrier,so it can obtain symmetrical I-V curve under positive and negative bias,which extends the application of RTD.Finally,it is proposed that the disappearance of the built-in electric field prevents the free electrons from being swept away in the effective region of RTD devices,thus suppressing the possibility of ionization of defect traps and improving the repeatability of the I-V curve of RTD devices in the nonpolar direction.4.A polarization-matched AlInN/GaN heterojunction RTD structure is proposed.In the direction of polarity,the polarization matching AlInN of GaN is used as the potential barrier layer of RTD to eliminate the built-in electric field and improve the I-V characteristics of the device.First,the material model of AlInN is successfully simulated,and then the device model is established.Then,the negative differential resistance characteristics is obtained by solving the Schrodinger-Poisson equations with defect trap charge and the planar NEGF.The results show that the polarization-matched RTD can achieve symmetric I-V characteristics,increase the intrinsic response frequency and improve the repeatability of I-V characteristics.5.This work presents the AlGaN/GaN heterostructure with tri-barrier structure.Through the introduction of the third barrier,two negative differential resistance region appear in the I-V characteristic curve of RTD,which extends the application of RTD devices to more fields.Using the existing material and device models,the tri-barrier RTD negative differential resistance is obtained by solving the Schrodinger-Poisson equations with trap charge and the planr NEGF.Finally,it is proposed that the number of NDR regions,PVCR and peak current in each NDR region can be effectively adjusted by adjusting the width of potential barrier layer,Al component of potential barrier layer and the width of sub-quantum well,which increases great flexibility for our practical application...