The Research Of Characteristic Parameters For AlGaN/GaN Heterostructure Materials And Devices

Recently AlGaN/GaN heterostructure field-effect transistors (HFETs) have been the focus of intense research in order to achieve performance of high tempreture, high frequency, and high power. Except for the intrinsic excellent physical properties and chemical stabilities of GaN, such as wide band gap, high breakdown electric field, high electron saturated velocity and high thermal conductivity, AlGaN/GaN heterostructure have large band offset and strong polarization, which make the sheet carrier concentration of the two dimensional electron gas (2DEG) up to-1013/cm2without intentional doping. AlGaN/GaN HFETs based on AlGaN/GaN heterostructures have many excellent performances, such as high transconductance, high saturated current, high cutoff frequency and high breakdown voltage.Although the performance of AlGaN/GaN HFETs has been improved to a high level and just been utilized commercially, there are still many problems in the investigation of AlGaN/GaN HFETs, such as the converse piezoelectric effect, the reliability of the AlGaN/GaN HFETs working in high temperature, the strain of AlGaN barrier layer and the difference between the performance predicted by AlGaN/GaN heterostructure and the practical device performance et al. These problems limit the commercial prosess. The characteristic parameters for AlGaN/GaN heterostructure materials and devices such as2DEG electron mobility, polarization charge density, the stress of AlGaN/GaN, the distribution of2DEG and Schottky barrier height and so on determine the frequence and power devices. Therefore, it is very important to investigate the characteristic parameters for AlGaN/GaN heterostructure materials and devices. The work in this dissertation mainly includes the scattering mechanism of2DEG electron mobility in AlGaN/AlN/GaN HFETs, extraction of AlGaN/GaN (AlGaN/AlN/GaN) heterostructure Schottky barrier heights and density of the polarization charge in AlGaN/GaN heterostructure from forward current-voltage (Ⅰ-Ⅴ) characteristics, and the influence of thermal stressing on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts. The main conclusions of the dissertation are listed below.1. The scattering mechanism of2DEG electron mobility in AlGaN/AlN/GaN HFETs.(a) The influence of different gate areas on the2DEG electron mobility.According to the Ohm Law, we deduced the I-V relationship in the linear region of the circular and rectangular AlGaN/AlN/GaN HFETs. The2DEG electron mobility at the source-drain bias of100mV under different gate biases was calculated using the measured capacitance-voltage (C-V) and I-V characteristics of the circular and rectangular AlGaN/AlN/GaN HFETs with different gate areas. It is shown that with increasing the drain-source bias the2DEG electron mobility increases for the small gate area but decreases for the big gate area. We explained these phenomena with polarization Coulomb field scattering, LO phonon scattering and the interface roughness scattering. In addition, we compared the calculated2DEG electron mobility in AlGaN/AlN/GaN HFETs and AlGaN/GaN HFETs with the same size. It's was found that the polarization Coulomb field scattering has a stronger influence on the electron mobility of the2DEG in the AlGaN/GaN HFET devices than that in the AlGaN/AlN/GaN HFET devices. This is attributed to the AlN interlayer in the strained AlGaN/AlN/GaN heterostructures to enlarge the average distance between the2DEG electrons and the polarization charges which generate the polarization Coulomb field.(b) The influence of different drain-source distance on the2DEG electron mobility.The2DEG electron mobility at the source-drain bias of100mV under different gate biases was calculated using the measured C-V and I-V characteristics of the rectangular AlGaN/AlN/GaN HFETs with different drain-source distance and gate areas. It is shown that the2DEG scattering mechanism in AlGaN/AlN/GaN HFETs is determined by the ratio of the gate length to the drain-source distance, not affected by the drain-source distance. (c) The influence of different drain-source distance and gate areas on the2DEG electron mobility in the linear region (higher drain-source bias).The average2DEG electron mobility at different source-drain biases and gate biases was calculated using the measured C-V and I-V characteristics of the rectangular AlGaN/AlN/GaN HFETs with different drain-source distance and gate areas. It is shown that the former scattering mechanism in AlGaN/AlN/GaN HFETs can commendably explain the variety of the average2DEG electron mobility at different source-drain biases and gate biases with source-drain distance of100u m and60μ m, however, can not explain the one with source-drain distance smaller than20μ m. This abnormal phenomenon is due to the influence of the source-drain electric field on the distribution of the polarization charges.2. Extraction of AlGaN/GaN (AlGaN/AlN/GaN) heterostructure Schottky diode barrier heights from forward Ⅰ-Ⅴ characteristics.In this chapter we discussed two methods of extracting AlGaN/GaN (AlGaN/AlN/GaN) heterostructure Schottky barrier heights from forward Ⅰ-Ⅴ characteristics. The first method is based on the measured forward Ⅰ-Ⅴ characteristics and the two-diode model, using thermionic emission model the effective Richardson constant and Ni Schottky barrier height at zero bias has been analyzed and calculated by self-consistently solving Schrodinger's and Poisson's equations, and the correlation expression between the barrier height at zero electric field and that at zero bias has been derived for Schottky contacts on AlGaN/GaN (AlGaN/AlN/GaN) heterostructures. The barrier height at zero electric field is finally obtained with the above correlation expression. The second method is based on the flat-band voltage that was analyzed and obtained from the forward I-V characteristics. Then the correlation expression between barrier height at zero electric field and the flat-band voltage was obtained. The barrier height at zero electric field was finally calculated with the above correlation expression by self-consistently solving Schrodinger's and Poisson's equations.3. Extraction of the polarization charge density in AlGaN/GaN heterostructure from forward I-V characteristics.The relationship between the polarization charge and flat-band voltage obtained from Ⅰ-Ⅴ curve was analyzed and obtained. Then the polarization density can be simply calculated using the C-V measurement.4Influence of thermal stressing on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts.(a) Influence of thermal stressing on the strain of AlGaN barrier layer.Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated, and samples were then thermally treated with different temperature and different times. The polarization sheet charge density was obtained with C-V and I-V curves by self-consistently solving Schrodinger's and Poisson's equations. It's was found the changes for the polarization sheet charge density can be divided into two stages for the different thermal stressing times under600℃. In the first stage strain is present in the AlGaN barrier layer and is reduced as a function of stressing time, and in the second stage the AlGaN barrier layer has relaxed and therefore no strain is present. Besides, for the samples thermally treated for0.5hour the thermal stressing just affects the strain of the AlGaN barrier layer when the thermal stressing temperature is less than700℃. Once the temperature reaches800℃, the thermal stressing changes both spontaneous polarization and piezoelectric polarization due to huge diffusion of the contact metal atoms.(b) Influence of thermal stressing on the relative permittivity of the AlGaN barrier layer.Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated, and samples were then thermally treated with different temperature and different times. The relative permittivity of the AlGaN barrier layer was obtained with C-V. It was found that the relative permittivity of the AlGaN barrier layer is related to the piezoelectric and spontaneous polarization of the AlGaN barrier layer. The relative permittivity is in proportion to the strain of the AlGaN barrier layer. The relative permittivity and the strain reduce with the increased thermal stressing times until the AlGaN barrier totally relaxes, and then the relative permittivity is almost a constant. When the sample is treated at800℃for0.5h the relative permittivity is less than the constant due to huge diffusion of the contact mental atoms. Besides, it's found that thermal stressing with an appropriate stressing time and temperature can not only reduce the reverse saturation current, but also can reduce the relative permittivity of the AlGaN barrier layer and restrain the converse piezoelectric effect, and then can improve the performance of AlGaN/GaN heterostructure devices.(c) Influence of thermal stressing on other characteristic parameters of AlGaN/GaN heterostructure Schottky contacts.Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated, and samples were then thermally treated with different temperature and different times. With C-V and I-V curves the influence of thermal stressing on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts, such as Fermi level, the triangle quantum well depth (V2DEG) and the distribution of the2DEG were investigated.