Youdou Zheng

On the reverse gate leakage current of AlGaN/GaN high electron mobility transistors

In this work, we include the polarization effect within the AlGaN barrier into calculation of the near-surface electrical field ES underneath the Schottky contact metal which determines the field-dependent characteristics of reverse gate leakage current of AlGaN/GaN high electron mobility transistors. High-frequency capacitance-voltage measurement combined with electrostatic analysis is used to estimate ES as a function of reverse bias voltage. The resultant log(I/ES) versus ES curves over a temperature range from 293 to 453 K agree well with the predicted model of Frenkel–Poole (FP) emission of electrons up to the conductive states of threading dislocations. Around zero bias, the reverse polarization-field-induced FP emission current is balanced by a forward defect-assisted tunneling current, both of which follow the same temperature dependent characteristics.

Mesoporous iron oxide directly anchored on a graphene matrix for lithium-ion battery anodes with enhanced strain accommodation

A continuous mesoporous iron oxide nanofilm was directly formed on graphene nanosheets through the in situ thermal decomposition of Fe(NO3)3·9H2O and was anchored tightly on the graphene surface. The lithiation-induced strain was naturally accommodated, owing to the constraint effect of graphene and the mesoporous structure. Hence, the pulverization of the iron oxide nanofilm was effectively prevented.

The impact of initial growth and substrate nitridation on thick GaN growth on sapphire by hydride vapor phase epitaxy

Abstract Nucleation and the very initial stages of thin film formation, and their impact on the GaN thick film properties, were studied for the hydride vapor phase epitaxy (HVPE) growth technique by affecting changes in the growth supersaturation and substrate–GaN interfacial energy. Through these studies, an optimized growth procedure for HVPE was developed based on separate conditions for the initiation of growth and the subsequent thick film formation. A two-step growth process was applied to grow GaN on bare sapphire without buffer layers. In the first step, a high growth rate has been used to obtain rapid initial growth and film coalescence on the sapphire substrate. Sapphire nitridation was also used to enhance the initial film growth. In the second step, low growth rate and higher NH 3 partial pressures, conditions that generally favor a high surface mobility and lateral growth rates, has led to improved materials, in terms surface morphology, carrier concentration, and optical properties. This study leads to a wider process window for HVPE GaN growth on sapphire substrates relative to the single step processing approaches.

Forward tunneling current in GaN-based blue light-emitting diodes

Forward tunneling current in InGaN/GaN multiquantum-well blue light-emitting diodes grown on sapphire substrate was studied by temperature-variable current-voltage (I-V) measurement. All semilog I-V curves obtained in the temperature range from 100 to 300 K exhibit two successive linearly dependent regions at low forward bias. The corresponding slopes appear to be insensitive to temperature, which indicates a dominant defect-assisted tunneling process. It is found that the tunneling current varies approximately as a function of ∼exp(−βEg+λeV), where β and λ are constants independent of temperature and voltage. The temperature- and voltage-dependence of forward tunneling current are explained by thermally induced band gap shrinkage and bias-induced route change of diagonal tunneling, respectively. The likely tunneling entities involved in the forward tunneling process are also discussed.

Raman and photoluminescence of ZnO films deposited on Si (111) using low-pressure metalorganic chemical vapor deposition

The highly c-oriented ZnO films were epitaxially grown on n-type Si (111) substrate at the temperature range of 340–460 °C using the low-pressure metalorganic chemical vapor deposition method. All films exhibit a pronounced (002) peak for ZnO, indicative of the strong c-axis oriented characteristic. The ZnO film grown at 400 °C shows the best structural quality along with the largest lateral grain size, well supported by the narrowest full width at half maximum of ZnO (002) peak about 0.19° in x-ray diffraction. However, the temperature dependence of the vibrational modes at 436 and ∼563 cm−1 in Raman spectra revealed a low density of oxygen vacancies in the films grown at low temperatures, which is supposed to determine the photoluminescence (PL) properties. At low temperatures, the narrow ultraviolet (UV) near band emission dominated the PL spectrum with a very weak low energy tail near the band. High temperature (up to 460 °C) would cause serious oxygen deficiency, resulting in the weak broad UV band w...

Electrical instability of amorphous indium-gallium-zinc oxide thin film transistors under monochromatic light illumination

This work was supported by the State Key Program for Basic Research of China under Grant Nos. 2010CB327504, 2011CB922100, 2011CB301900; the National Natural Science Foundation of China under Grant Nos. 60825401, 60936004, 11104130, BK2011556, and BK2011050.

Direct observation of transferred‐electron effect in GaN

We report the direct observation of transferred‐electron effect in unintentionally doped GaN epilayers grown by metalorganic chemical vapor deposition. The negative differential resistivity (NDR) was observed from the current‐electric field characteristics in GaN using a metal‐semiconductor‐metal (M‐S‐M) system. The threshold field for the onset of NDR was independent of the spacing of M‐S‐M fingers, and was measured to be 1.91×105 V/cm for GaN with an n‐type carrier concentration of 1014 cm−3. This value is very close to the value obtained from theoretical simulation. This observation is an experimental evidence of transferred‐electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn‐effect devices using GaN materials.

Suppression of compensation from nitrogen and carbon related defects for p-type N-doped ZnO

In this letter, the authors performed a comprehensive study on suppression mechanism of compensation from nitrogen and carbon related complex defects in N-doped ZnO grown by metal-organic chemical vapor deposition. The chemical bonding information of donorlike substitutional complex defects, (NN)O and (NC)O, were restrained with low N/O ratio, leading to the conduction type conversion. High epitaxial temperature has more suppressing effect on the formation of desired acceptor NO than that of (NC)O, as evident by the decreasing hole concentration. Upon utilization of such suppression effect, this study provides a promising route to realize p-type ZnO.

Enhanced bias stress stability of a-InGaZnO thin film transistors by inserting an ultra-thin interfacial InGaZnO:N layer

Amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) having an ultra-thin nitrogenated a-IGZO (a-IGZO:N) layer sandwiched at the channel/gate dielectric interface are fabricated. It is found that the device shows enhanced bias stress stability with significantly reduced threshold voltage drift under positive gate bias stress. Based on x-ray photoelectron spectroscopy measurement, the concentration of oxygen vacancies within the a-IGZO:N layer is suppressed due to the formation of N-Ga bonds. Meanwhile, low frequency noise analysis indicates that the average trap density near the channel/dielectric interface continuously drops as the nitrogen content within the a-IGZO:N layer increases. The improved interface quality upon nitrogen doping agrees with the enhanced bias stress stability of the a-IGZO TFTs.

Precipitation of Cu and Fe in Dislocated Floating-Zone-Grown Silicon

Precipitation behaviors of Cu and Fe in dislocated Floating-zone-grown silicon crystals are investigated by means of transmission electron microscopy (TEM) and the electron-beam-induced-current (EBIC) technique. Cu precipitation on dislocations is affected significantly by the cooling rate of a specimen after contamination. Cu develops precipitate colonies at some special sites on dislocations and does not decorate other parts of dislocations if the specimen is cooled slowly. These preferential precipitation sites are suggested to be non-dissociated edge-type dislocation segments. The fast cooling of a specimen leads to that Cu precipitates on all of dislocations. Fe decorates all of dislocations uniformly, irrespective of the cooling rate of a specimen.