Current collapses were studied, which were observed in A1GaN/GaN high electron mobility transistors （HEMTs） with and without InGaN back barrier （BB） as a result of short-term bias stress. More serious drain current collapses were observed in InGaN BB A1GaN/GaN HEMTs compared with the traditional HEMTs. The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse. The surface states may be the primary mechanism of the origination of current collapse in A1GaN/GaN HEMTs for short-term direct current stress.
Electrical properties of In_x Al_（1-x）N/AlN/GaN structure are investigated by solving coupled Schr（o｜¨）dinger and Poisson equations self-consistently.The variations in internal polarizations in In_xAl_（1-x）N with indium contents are studied and the total polarization is zero when the indium content is 0.41.Our calculations show that the twodimensional electron gas（2DEG） sheet density will decrease with increasing indium content.There is a critical thickness for AIN.The 2DEG sheet density will increase with In_xAl_（1-x）N thickness when the AIN thickness is less than the critical value.However,once the AIN thickness becomes greater than the critical value,the 2DEG sheet density will decrease with increasing barrier thickness.The critical value of AIN is 2.8 nm for the lattice-matched In_（0.18）Al_（0.82）N/AlN/GaN structure.Our calculations also show that the critical value decreases with increasing indium content.
In this paper, InGaN/GaN multiple quantum well solar cells （MQWSCs） with an In content of 0.15 are fabricated and studied. The short-circuit density, fill factor and open-circuit voltage （Voc） of the device are 0.7 mA/cm2, 0.40 and 2.22 V, respectively. The results exhibit a significant enhancement of Voc compared with those of InGaN-based hetero and homojunction cells. This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing Voc of an In-rich InxGal-~N solar cell. The device exhibits an external quantum efficiency （EQE） of 36% （7%） at 388 nm （430 nm）. The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.
A novel three-dimensional lattice model of tetrapod-like zinc oxide whisker（T-ZnOw） resin matrix composite with a coordination number of 12 is constructed based on the special structure of T-ZnOw;the percolation phenomenon of the system is simulated by the Monte Carlo method,and the percolation threshold is obtained at 23.2%.The critical mixing ratio of T-ZnOw is calculated by considering the practical factors,and the result basically agrees with the reported one.Theoretical calculation shows that the critical mixing ratio mainly depends on the L/D ratio of T-ZnOw, and is also related to the size of T-ZnOw as well as the preparation method of the composite.The microwave absorbing mechanism of T-ZnOw composite is discussed,and conductivity loss and point discharge caused by the polarization effect are regarded to be two important means of energy dissipation.
A homemade 7×2 inch MOCVD system is presented.With this system,high quality GaN epitaxial layers,InGaN/GaN multi-quantum wells and blue LED structural epitaxial layers have been successfully grown. The non-uniformity of undoped GaN epitaxial layers is as low as 2.86%.Using the LED structural epitaxial layers, blue LED chips with area of 350×350μm2 were fabricated.Under 20 mA injection current,the optical output power of the blue LED is 8.62 mW.
Yellow and blue luminescence in undoped GaN layers with different resistivities are studied by cathodoluminescence.Intense yellow and blue luminescence bands are observed in semi-insulating GaN,while in n-GaN the yellow luminescence and blue luminescence bands are very weak.The stronger yellow and blue luminescences in semi-insulating GaN are correlated to the higher edge-type dislocation density.The scanning cathodoluminescence image reveals strong defect-related luminescence at the grain boundaries where the dislocations accumulate.It is found that the relative intensity of the blue luminescence band to the yellow luminescence band increases with the cathodoluminescence beam energies and is larger in n-GaN with a lower density of edge-type dislocations.An approximately 3.35eV shoulder next to the near-band-edge peak is observed in n-GaN but not in semi-insulating GaN.A redshift of the near-band-edge peak with cathodoluminescence beam energy is observed in both samples and is explained by internal absorption.
A solar cell with a novel structure is investigated by means of the analysis ofmicroelectronic and photonic structure （AMPS）. The power conversion efficiency is investigated with the variations in interface recombination ve- locity, thicknesses of p-type layer, intrinsic layer, n-type layer, and doping density. Results show that it is available and preferable in theory to employ a-SiC：H as a window layer in p-a-SiC：H/i-a-Si：H/n-μc-Si solar cells, and provide a new approach to improving the power conversion efficiency of amorphous silicon solar cells.
