Frequency responses of modulated electroluminescence(EL)of light-emitting diodes were measured using a testing setup.With increasing frequency of the ac signal,the relative light intensity(RLI)clearly decreases.Furthe...Frequency responses of modulated electroluminescence(EL)of light-emitting diodes were measured using a testing setup.With increasing frequency of the ac signal,the relative light intensity(RLI)clearly decreases.Furthermore,a peculiar asynchrony between the RLI and ac small-signal is observed.At frequencies higher than 10 kHz,the RLI clearly lags behind the ac signal and the absolute value of the lagging angle is nearly proportional to the signal frequency.Using the classical recombination model of light-emitting diodes under ac small-signal modulation,these abnormal characteristics of modulated EL can be clearly explained.展开更多
GaN films on sapphire substrates are obtained using the metal-organic chemical vapor deposition growth technique.We present two methods to reduce the GaN wafer bowing caused by the mismatch of the thermal expansion co...GaN films on sapphire substrates are obtained using the metal-organic chemical vapor deposition growth technique.We present two methods to reduce the GaN wafer bowing caused by the mismatch of the thermal expansion coefficients(TECs)between the film and the substrate.The first method is to use coating materials on the back side of the substrate whose TECs are smaller than that of the GaN films.The second is to cut grooves on the back side of the sapphire substrate and filling the grooves with appropriate materials(e.g.,tungsten,silicon nitride).For each method,we minimize wafer bowing and even reduce it to zero.Moreover,the two methods can reduce stress concentration and suppress the propagation of cracks in the GaN/sapphire structure.展开更多
A thin film of GaN with the thickness of 1.0μm was grown onα-Al2O3 substrate by metal organic chemical vapour disposition and then a thick GaN film with thickness of 12μm was grown in the halide vapour phase epitax...A thin film of GaN with the thickness of 1.0μm was grown onα-Al2O3 substrate by metal organic chemical vapour disposition and then a thick GaN film with thickness of 12μm was grown in the halide vapour phase epitaxy system.Some macro-pyramids appeared on the surface of the sample.The macro-pyramids made the surface of the GaN film rough,which was harmful to the devices made by GaN materials.These defects changed the distribution of carrier concentration and affected the optical properties of GaN.The step height of the pyramids was about 30-40 nm measured by atomic force microscopy.A simple model was proposed to explain the macro-pyramid phenomenon compared with the growth spiral.The growth of the macro-pyramid was relative to the physical conditions in the reaction zone.Both increasing growth temperature and low pressure may reduce the pyramid size.展开更多
Using solid-phase regrowth technique,Pd/Ge contact has been made on the GaN layer,and very good ohmic behavior was observed for the contact.The Photoluminescence(PL)spectra for different structures formed by the Pd/Ge...Using solid-phase regrowth technique,Pd/Ge contact has been made on the GaN layer,and very good ohmic behavior was observed for the contact.The Photoluminescence(PL)spectra for different structures formed by the Pd/Ge contact,GaN layer,sapphire substrate,and mirror were studied,and a defect-assisted transition was found at 450nm related to Ge impurity.The results show that the microcavity effect strongly influences the PL spectra of the band-gap and defect-assisted transitions.展开更多
P-type GaN was grown on AI3O2 substrate by low pressure metalorganic vapor phase epitaxy without any post treatment.The p-type conduction of as-grown GaN has the typical characteristics of a light-doped or high-compen...P-type GaN was grown on AI3O2 substrate by low pressure metalorganic vapor phase epitaxy without any post treatment.The p-type conduction of as-grown GaN has the typical characteristics of a light-doped or high-compensation semiconductor,that is a hole concentration of 2.2× 10^(17)cm^(-3) at 77K,which changes to n-type with an electron concentration of 2.7 × 10^(17) cm^(-3) at room temperature.After thermal annealing under a N2 ambient,it showed the characteristics of a heavy-doped semiconductor and the hole concentration was enhanced reaching a hole concentrations of 4.2× 10^(17) at 77K and 5.7 × 10^(17) cm^(-3) at room temperature.展开更多
A joint optimal sensing-transmission time duration and power allocation scheme has been proposed to maximize the energy efficiency for cooperative relay network. In particular, observing that the spectrum sensing and ...A joint optimal sensing-transmission time duration and power allocation scheme has been proposed to maximize the energy efficiency for cooperative relay network. In particular, observing that the spectrum sensing and data transmission time duration lies within a strict interval, the joint optimal solutions of our proposed scheme are achieved by sequential optimization method. Numerical evaluation results reveal that the relay-assisted transmission using our proposed scheme significantly outperforms the non-relay transmission in terms of the network energy-efficiency.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos.60876035,50901050,11004148 and 11004149.
文摘Frequency responses of modulated electroluminescence(EL)of light-emitting diodes were measured using a testing setup.With increasing frequency of the ac signal,the relative light intensity(RLI)clearly decreases.Furthermore,a peculiar asynchrony between the RLI and ac small-signal is observed.At frequencies higher than 10 kHz,the RLI clearly lags behind the ac signal and the absolute value of the lagging angle is nearly proportional to the signal frequency.Using the classical recombination model of light-emitting diodes under ac small-signal modulation,these abnormal characteristics of modulated EL can be clearly explained.
基金Supported by the National Natural Science Foundation of China under Grant Nos 10872003,10932001 and 61006035the Foundation for the Author of National Excellent Doctoral Dissertation of China under Grant No 2007B2+1 种基金the Research Fund for the New Teacher Program of the State Education Ministry of China under Grant No 200800011011the Scientific Research Foundation for the Returned Overseas Chinese Scholars State Education Ministry of China,and China Postdoctoral Science Foundation under Grant No 20090460168.
文摘GaN films on sapphire substrates are obtained using the metal-organic chemical vapor deposition growth technique.We present two methods to reduce the GaN wafer bowing caused by the mismatch of the thermal expansion coefficients(TECs)between the film and the substrate.The first method is to use coating materials on the back side of the substrate whose TECs are smaller than that of the GaN films.The second is to cut grooves on the back side of the sapphire substrate and filling the grooves with appropriate materials(e.g.,tungsten,silicon nitride).For each method,we minimize wafer bowing and even reduce it to zero.Moreover,the two methods can reduce stress concentration and suppress the propagation of cracks in the GaN/sapphire structure.
基金Supported by the National Natural Science Foundation of China under Grant Nos.69789601 and 69876002.
文摘A thin film of GaN with the thickness of 1.0μm was grown onα-Al2O3 substrate by metal organic chemical vapour disposition and then a thick GaN film with thickness of 12μm was grown in the halide vapour phase epitaxy system.Some macro-pyramids appeared on the surface of the sample.The macro-pyramids made the surface of the GaN film rough,which was harmful to the devices made by GaN materials.These defects changed the distribution of carrier concentration and affected the optical properties of GaN.The step height of the pyramids was about 30-40 nm measured by atomic force microscopy.A simple model was proposed to explain the macro-pyramid phenomenon compared with the growth spiral.The growth of the macro-pyramid was relative to the physical conditions in the reaction zone.Both increasing growth temperature and low pressure may reduce the pyramid size.
基金Supported by the National Natural Science Foundation of China under Grant No.19774010Peking University Foundation。
文摘Using solid-phase regrowth technique,Pd/Ge contact has been made on the GaN layer,and very good ohmic behavior was observed for the contact.The Photoluminescence(PL)spectra for different structures formed by the Pd/Ge contact,GaN layer,sapphire substrate,and mirror were studied,and a defect-assisted transition was found at 450nm related to Ge impurity.The results show that the microcavity effect strongly influences the PL spectra of the band-gap and defect-assisted transitions.
基金Supported by the National Natural Science Foundation of China under Grant No.69476025.
文摘P-type GaN was grown on AI3O2 substrate by low pressure metalorganic vapor phase epitaxy without any post treatment.The p-type conduction of as-grown GaN has the typical characteristics of a light-doped or high-compensation semiconductor,that is a hole concentration of 2.2× 10^(17)cm^(-3) at 77K,which changes to n-type with an electron concentration of 2.7 × 10^(17) cm^(-3) at room temperature.After thermal annealing under a N2 ambient,it showed the characteristics of a heavy-doped semiconductor and the hole concentration was enhanced reaching a hole concentrations of 4.2× 10^(17) at 77K and 5.7 × 10^(17) cm^(-3) at room temperature.
基金supported by the Key Research of China Southern Power Grid Company(K201029.3)
文摘A joint optimal sensing-transmission time duration and power allocation scheme has been proposed to maximize the energy efficiency for cooperative relay network. In particular, observing that the spectrum sensing and data transmission time duration lies within a strict interval, the joint optimal solutions of our proposed scheme are achieved by sequential optimization method. Numerical evaluation results reveal that the relay-assisted transmission using our proposed scheme significantly outperforms the non-relay transmission in terms of the network energy-efficiency.
