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Research Of The Mechanisms For Polarization Coulomb Field Scattering In GaN-Based Electron Devices

Posted on:2015-02-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:C B LuanFull Text:PDF
GTID:1268330431955403Subject:Microelectronics and Solid State Electronics
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With the excellent physical and chemical properties of the GaN material, such as wide band gap, high saturated electron drift velocity, high critical breakdown electric field, chemical stability, GaN heterostructure field-effect transistors (HFETs) are suitable for the production of anti-radiation, high frequency, high power, high density integration of electronic devices and blue, green, ultraviolet optoelectronic devices, these have a wide range of applications in the national economy and national defense construction, such as wireless communications, satellite, radar, automotive electronics, aerospace, nuclear industry and military electronics.Although the performance of GaN-based HFETs has been developed to a high level, there are still many problems in the investigation of GaN-based HFETs, such as the current collapse, device reliability and many other issues. The materials and devices parameters of the GaN-based HFETs, such as two-dimensional electron gas (2DEG) electron density,2DEG electron mobility, the polarization charge density of the barrier layer, the Schottky barrier height, subthreshold swing, on/off current ratio will directly affect the frequency and the power characteristics of the HFET devices. Therefore, the systematic study of the characteristic parameters of the GaN-based HFETs is essential to improve the frequency and the power performance of the devices. Zhao et al and Lv et al point out that, in AlGaN/GaN and AlGaN/AlN/GaN HFETs, the polarization Coulomb field scattering, the polar optical-phonon scattering and the interface roughness scattering are the main scattering mechanisms. But the presence of the polarization Coulomb field scattering is only confirmed from the experiment, it is crucial to establish the theoretical model of the polarization Coulomb scattering at low electric field in AlGaN/AlN/GaN HFETs. Also, the study of the relationship between the polarization Coulomb field scattering and the characteristic parameters (such as subthreshold swing and on/off current ratio) in AlGaN/AlN/GaN HFETs is necessary to improve the device characteristics. In addition, the parameters of the InAlN/AlN/GaN HFETs (such as2DEG electron mobility and the barrier height) are also made a detail study in this dissertation. The main contents of the dissertation are listed below.1. The theoretical model of the polarization Coulomb field scattering and the application of the polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs.(a) Influence of the side-Ohmic contact processing on the polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs.Using the measured Capacitance-Voltage (C-V) cures and the Current-Voltage (I-V) characteristics for the circular and rectangular AlGaN/AlN/GaN HFETs with side-Ohmic contacts and normal-Ohmic contacts, the relationship between the2DEG electron mobility and the gate voltage can be calculated when the source-drain voltage VDS is0.1V. It is shown that the2DEG electron mobility decreased with the increasing gate voltage in almost all the AlGaN/AlN/GaN HFETs with side-Ohmic contacts, but the2DEG electron mobility increased with the increasing gate voltage in the small gate area AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Based on the measured scanning electron microscope with energy dispersive spectrometer (SEM-EDS) spectrum and the scattering mechanisms in AlGaN/AlN/GaN HFETs (polarization Coulomb field scattering, polar optical-phonon scattering and interface roughness scattering), the above phenomenon was explained and the conclusion can be made that the polarization Coulomb field scattering caused by the polarization charge density variation at the AlGaN/AlN/GaN interfaces is closely related to the Ohmic-contact processing, and the side-Ohmic contact processing greatly weakens the polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs.(b) Theoretical model of the polarization Coulomb field scattering in strained AlGaN/AlN/GaN HFETs.Base on the origin of the polarization Coulomb field scattering, the distribution of the polarization charges in the AlGaN barrier layer can be gotten, then the perturbation potential of the polarization Coulomb field scattering can be calculated and the theoretical model of the polarization Coulomb field scattering caused by the polarization charge density variation at the AlGaN/AIN interface can be obtained. And the theoretical values for the electron drift mobility, which were calculated using the Matthiessen’s rule that includes polarization Coulomb field scattering, piezoelectric scattering, polar optical-phonon scattering and interface roughness scattering, are in good agreement with our experimental values. Thus, the conclusion can be made that the developed theoretical model for polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs is correct, and it is also shown that polarization Coulomb field scattering is an important carrier scattering mechanism in AlGaN/AlN/GaN HFETs.(c) Influence of polarization Coulomb field scattering on the subthreshold swing in depletion-mode AlGaN/AlN/GaN heterostructure field-effect transistors.Used the measured capacitance-voltage (C-V) curves and current-voltage (I-V) curves for the prepared AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs), the relationship between polarization Coulomb field scattering (PCF scattering) and the subthreshold swing for depletion mode (D-mode) AlGaN/AlN/GaN HFETs has been investigated. It was found that the S value (S=(бlg(IDS)/бVGs)-1) of subthreshold swing is smaller for the device with stronger polarization Coulomb field scattering, and the S value decreases by more than26%for the D-mode AlGaN/AlN/GaN HFET samples. The reason is attributed to the big gradient of the mobility and the gate-source bias curve which is generated by the polarization Coulomb field scattering.2. Research of the meachnnisms for polarization Coulomb field scattering in InAlN/AlN/GaN heterostructure field-effect transistors.(a) The influence of different gate areas on the2DEG electron mobility in In0.18Al0.82N/AlN/GaN HFETs.Using the measured capacitance-voltage curves of Ni Schottky contacts with different areas and the current-voltage characteristics for the rectangular and circular In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, the relationship between the2DEG electron mobility and the gate voltage can be calculated. It is shown that the2DEG electron mobility decreased with the increasing gate voltage in the In0.18Al0.82N/AlN/GaN HFETs with large gate area, but the2DEG electron mobility increased with the increasing gate voltage in the In0.18Al0.82N/AlN/GaN HFETs with small gate area. And the2DEG electron mobility increased with the increasing gate area at a certain gate voltage. Based on the measured scanning electron microscope with energy dispersive spectrometer (SEM-EDS) spectrum and the scattering mechanisms in In0.18Al0.82N/AlN/GaN HFETs (polarization Coulomb field scattering, polar optical-phonon scattering and interface roughness scattering), the above phenomenon was explained and the conclusion can be made that the Ohmic contact processing and the gate bias cause the irregular distribution of the polarization charges at the In0.18Al0.82N/AlN interface which generates the polarization Coulomb field, and the polarization Coulomb field scattering has an important influence on the2DEG electron mobility in both our rectangular and circular In0.18Al0.82N/AlN/GaN HFET devices as same as in AlGaN/AlN/GaN HFET devices.(b) Enhanced effect of side-Ohmic contact processing on the2DEG electron density and electron mobility of In0.17Al0.83N/AlN/GaN heterostructure field-effect transistors.Using the measured Capacitance-Voltage (C-V) cures and the Current-Voltage (I-V) characteristics for the circular and rectangular In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts and normal-Ohmic contacts, the relationship between the2DEG electron mobility and the gate voltage can be calculated when the source-drain voltage VDS is0.1V. It is shown that the2DEG electron densities of In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts are increased by more than twice compared with the2DEG electron density of the In0.17Al0.83N/AlN/GaN HFETs with normal-Ohmic contacts. And the trend of the2DEG electron mobility changed with the gate voltage for all the In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts is slower than that for the In0.17Al0.83N/AlN/GaN HFETs with normal-Ohmic contacts. Based on the measured SEM-EDS spectrum and the scattering mechanisms in In0.17Al0.83N/AlN/GaN HFETs (polarization Coulomb field scattering, polar optical-phonon scattering and interface roughness scattering), the above phenomenon was explained and the conclusion can be made that the polarization Coulomb field scattering (PCF) is closely related to the normal-Ohmic contact processing, and the PCF was weakened by the side-Ohmic contact processing in In0.17Al0.83N/AlN/GaN HFETs as same as in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner In0.17Al0.83N barrier layer, the influence of the gate bias on the PCF in In0.17Al0.83N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Moreover, the2DEG electron density in the In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts is increased by more than twice compared with the2DEG electron density in the In0.17Al0.83N/AlN/GaN HFETs with normal-Ohmic contacts.(c) A method to extract In0.18Al0.82N/AlN/GaN heterostructure Schottky barrier heights from forward Current-Voltage curves.From the forward current-voltage (Ⅰ-Ⅴ) characteristics of the circular and rectangular In0.18Al0.82N/AlN/GaN Schottky diodes, the flat-band voltage (V0) was analyzed and obtained. Then, the Schottky barrier heights of the prepared circular and rectangular Schottky diodes have been analyzed and calculated by self-consistently solving Schrodinger’s and Poisson’s equations. The calculated Schottky barrier heights for the prepared circular and rectangular Schottky diodes agree well with the photocurrent measured values.3. Comparison for the carrier mobility between the Ⅲ-Ⅴ nitrides and AlGaAs/GaAs heterostructure field-effect transistorsUsing the measured capacitance-voltage curves of Ni/Au Schottky contacts with different areas and the current-voltage characteristics for the AlGaAs/GaAs, AlGaN/AlN/GaN and In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, the two-dimensional electron gas (2DEG) electron mobility for the prepared HFETs was calculated and analyzed. It was found that there is obvious difference for the variation trend of the mobility curves between the Ⅲ-Ⅴ nitride HFETs and the AlGaAs/GaAs HFETs. In the Ⅲ-Ⅴ nitride HFETs, the variation trend for the curves of the2DEG electron mobility with the gate bias is closely related to the ratio of gate length to drain-to-source distance. While the ratio of gate length to drain-to-source distance has no effect on the variation trend for the curves of the2DEG electron mobility with the gate bias in AlGaAs/GaAs HFETs. The reason is attributed to the polarization Coulomb field scattering in the Ⅲ-Ⅴ nitride HFETs.
Keywords/Search Tags:AlGaN/AlN/GaN heterostructures, InAlN/AlN/GaN heterostructures, two-dimensional electron gas, electron mobility, polarization charge, polarizationCoulomb field scattering, Schottky barrier height, subthreshold swing
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