The Study Of GaN-based Heterostructures And Electronic Device

Due to the excellent physical and chemical properties such as large bandgap energy,high electron saturated velocity,high temperature resistence,high breakdown voltage,and strong anti-radiation,the ?-nitride based semiconductor materials have more wide applications in high temperature,high frequency and high power electronic devices than Si-MOSFET,AIGaAs/GaAs HEMT.Although the GaN-based electronic device has so much advantage,the stability of this device hampers it to commercial application.To improve the stability of the GaN-based high electron mobility transistor(HEMT),the based problems of the transport properties,the strain in the heterostructure with variable temperature and the degredation mechanisms under high temperature,high field should be studied in detail.In this paper,the GaN based heterostructure and electronic device were studied in theory and experiment.The main conclusions are listed as follows:1.Through the UV Raman seattering experiment and the ealculation of the dispersion relations of interface phonon modes,the Raman peaks corresponding to the interface phonon modes inAlGaN/GaN,AlInN/GaN heterostructures were observed in the UV Raman spectra under the role of resonant enhancement of electron transition from valence band to the first subband.Besides,the interface phonon mode at 600 cm-1 shows anormlous temperature dependence behavior(the interface phonon mode shows blue shift with increasing temperature).The internal physical reason for that is the special temperature dependence of the internal electric field.Due to the discontinuous of the spontaneous and piezoelectric polarization,there is very high built-in electric field(MV/cm)in the interface of the AlGaN/GaN heterostructure.The direction of the built-in electric field is[000-1].Due to the same direction of the built-in electric field and the electric dipole moment of the equivalent electric dipole in the material,the total electric dipole moment increased which mean that the distance between the positive and negative ions also increased.So the frequency of the lattiee vibration decreased.With increasing temperature,the built-in electrie field decreases.The influence of the built-in electric field on the interface phonon frequency decrease which mean that the interface phonon mode show blue shift in frequency.2.The thermal stress in the AlGaN/GaN heterostructure on sapphire substrate was calculated in theory based on the multilayer thermal mismatch model.The result shows that the tensile stress in the AlGaN barrier layer increases with increasing temperature.The stress in AlGaN layer with variable temperature can be got from the temperature dependent UV Raman spectra.The experiment result is consistent with the theory result,which proves the conclusion that the tensile stress in the AlGaN barrier layer increases with increasing temperature.This conclusion provides theroitical and experimental basis for the view that the strain relaxation of AlGaN barrier layer is the main factor for the degredation of AlGaN/GaN HEMT at high temperature.3.Using the thermal expansion coefficient difference between the dielectric and AlGaN/GaN heterostructure,the deposition of MgO film with larger thermal expansion coefficient than that of AlGaN ean prevent the increase of tensile strain in AlGaN with increasing temperature.Thus MgO/SiN double-media technology can not only retain the passivation effect of SiN medium,but also inhibit the increase of tensile strain in the AlGaN layer at high temperature.This technology can improve the reliability of AlGaN/GaN HEMT at high temperature.The MgO dielectric deposited on AlGaN/GaN heterostructure at high temperature induces external compressive stress according to the changement of the interface phonon mode in frequency and the 2DEG concentration.4.The degradation of the AlGaN/GaN HEMT and its recovery were characterized under Vgs=0 V,Vds=20 V on-state and Vgs=-10 V,Vds=20 V off-state bias stress.Under the on-state bias stress,the performance of the device shows degradation with increasing the stress time and the threshold voltage shifts toward positive voltage.After a few time,the performance of the device keep stable.The main degradation mechanism is hot-electron trapping effect.Differently,the performance of the device shows seriously degradation with increasing time under off-state bias stress.The threshold voltage keeps constant.The main degradation mechanism under this bias stress is gate electron injection effect.Compared to the degradation under on-state bias stress,the degradation under off-state bias stress is more serious.The reason for that is:the electrons in the gate inject into the surface states betvveen the gate and source under the high electric field,which make the channel under this area depleted.After removing the bias stress,the surface states doesn't release immediately the trapped electrons.So the 2DEG concentration is still very low,which make the performance of the device very weak.Besides,the degradation of device can recover after removal of stress for some time under these two kinds of stress,which also verified the degradation is not with the inverse piezoelectric effect.