Junxue Ran

Simulation of In0.65Ga0.35N single-junction solar cell

The performances of In0.65Ga0.35N single-junction solar cells with different structures, including various doping densities and thicknesses of each layer, have been simulated. It is found that the optimum efficiency of a In0.65Ga0.35N solar cell is 20.284% with 5 × 1017 cm−3 carrier concentration of the front and basic regions, a 130 nm thick p-layer and a 270 nm thick n-layer.

Influence of AlN buffer layer thickness on the properties of GaN epilayer on Si(111) by MOCVD

The effect of thickness of the high-temperature (HT) AlN buffer layer on the properties of GaN grown on Si(111) has been investigated. Optical microscopy (OM), atomic force microscopy (AFM) and X-ray diffraction (XRD) are employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness of HT AlN buffer layer, and the optimized thickness of the HT AlN buffer layer is about 110nm. Together with the low-temperature (LT) AlN interlayer, high-quality GaN epilayer with low crack density can be obtained.

Theoretical design and performance of InxGa1?xN two-junction solar cells

The efficiencies of InxGa1-xN two-junction solar cells are calculated with various bandgap combinations of subcells under AM1.5 global, AM1.5 direct and AM0 spectra. The influence of top-cell thickness on efficiency has been studied and the performance of InxGa1-xN cells for the maximum light concentration of various spectra has been evaluated. Under one-sun irradiance, the optimum efficiency is 35.1% for the AM1.5 global spectrum, with a bandgap combination of top/bottom cells as 1.74 eV/1.15 eV. And the limiting efficiency is 40.9% for the highest light concentration of the AM1.5 global spectrum, with the top/bottom cell bandgap as 1.72 eV/1.12 eV.

Hydrogen sensors based on AlGaN/AlN/GaN HEMT

Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor.

Study on Mg memory effect in npn type AlGaN/GaN HBT structures grown by MOCVD

AlGaN/GaN npn heterojunction bipolar transistor structures were grown by low-pressure MOCVD. Secondary ion mass spectroscopy (SIMS) measurements were carried out to study the Mg memory effect and redistribution in the emitter-base junction. The results indicated that there is a Mg-rich film formed in the ongrowing layer after the Cp2Mg source is switched off. The Mg-rich film can be confined in the base section by switching off the Cp2Mg source for appropriate time before the end of base growth. Low temperature growth of the undoped GaN spacer suppresses the Mg redistribution from Mg rich film. The delay rate of the Mg profile in sample C with spacer growing in low temperature is about 56 nm/decade, which becomes sharper than 80 nm/decade of the samples A and B without low temperature spacer

AlGaN/AlN/GaN/InGaN/GaN DH-HEMTs with improved mobility grown by MOCVD

AlGaN/AlN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) structures with improved buffer isolation have been investigated. The structures were grown by MOCVD on sapphire substrate. AFM result of this structure shows a good surface morphology with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 ¿m× 5 ¿m. A mobility as high as 1950 cm2/Vs with the sheet carrier density of 9.89×1012 cm-2 was obtained, which was about 50% higher than other results of similar structures which have been reported. Average sheet resistance of 327 ¿/sq was achieved. The HEMTs device using the materials was fabricated, and a maximum drain current density of 718.5 mA/mm, an extrinsic transconductance of 248 mS/mm, a current gain cutoff frequency of 16 GHz and a maximum frequency of oscillation 35 GHz were achieved.

High quality GaN-based LED epitaxial layers grown in a homemade MOCVD system

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 × 350 μm2 were fabricated. Under 20 mA injection current, the optical output power of the blue LED is 8.62 mW.

Characteristics of InGaN Channel HEMTs Grown by MOCVD

The AlGaN/InGaN/GaN high electron-mobility transistors (HEMTs) structure was grown by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. The electron transport properties were investigated by variable temperature Hall effect measurements. The fabricated devices with gate length of 0.8mum and gate width of 120 mum show a transconductance of 136mS/mm and maximum drain current of 435mA/mm. The small signal properties were also achieved with the current gain cut-off frequency (fT) of 5.8GHz and the maximum frequency of oscillation (fMAX) of 17GHz

MOCVD grown AlGaN/AlN/GaN HEMT structure with compositionally step-graded AlGaN barrier layer

Unintentionally doped AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with compositionally step-graded AlGaN barrier layer were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The HEMT structure exhibited typical two-dimensional electron gas (2DEG) mobility of 1600cm2/Vs at room temperature and 6412cm2/Vs at 79K with almost equal 2DEG concentration of 1.0times1013/cm2. The 50mm HEMT wafer exhibited an average sheet resistance of 318.0Omega/square, with a good resistance uniformity of 0.89%. Atomic force microscopy (AFM) measurements revealed a smooth AlGaN surface with root-mean-square roughness (RMS) of 0.199nm and 0.295nm for scan area of 2mumtimes2mum and 5mumtimes5mum, respectively. A combined using of compositionally step-graded AlGaN barrier structure and AlN interlayer results in the high electrical performance and smooth surface of this heterostructure

Producing deep UV-LEDs in high-yield MOVPE by improving AlN crystal quality with sputtered AlN nucleation layer *

High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes (UV-LEDs). In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer (used as nucleation layer, SNL) by a high-yield industrial metalorganic vapor phase epitaxy (MOVPE). The full width half maximum (FWHM) of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of (002) and (102) peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power (LOP) of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.