Xinhe Zheng

High-quality InGaN∕GaN heterojunctions and their photovoltaic effects

High-quality p-GaN∕i-In0.1Ga0.9N∕n-GaN heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellosung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.

Determination of twist angle of in-plane mosaic spread of GaN films by high-resolution X-ray diffraction

X-ray diffraction (XRD) measurements of Phi scan in different chi angle in skew geometry for GaN films have shown that the peak widths of Phi scan decrease with the increment of angle chi. The FWHM of omega scan also increases with the inclination angle and reaches to be equal until the reflection plane is perpendicular to the surface of the sample. Based on these measured results, we developed a method to determine directly the twist angle of un-doped and Si-doped GaN films grown on c-sapphire substrates by MOCVD

Improved thermal management of GaN/sapphire light-emitting diodes embedded in reflective heat spreaders

Using maskless lithography and electroforming techniques, we have demonstrated an enhanced performance of GaN/sapphire light-emitting diode (LED) embedded in a reflective copper heat spreader. The chip size and dominant wavelength of the blue emitter used in this research is 1×1 mm2 and 455 nm, respectively. The cup-shaped LED heat sink is electroformed on sapphire directly using the spin-coated photoresist coated with the Au/Cr/Ag mirror as a mold and dicing into the embedded LED with a Cu base dimension of 3×3 mm2, which effectively enhances the heat dissipation down to the metal frame and reaps the light flux generated from the side emission. With the aid of a reflective heat spreader, the encapsulated LED sample driven at 1 A yields the light output power of 700 mW and around 2.7-times increase in the wall-plug efficiency compared to that of the conventional GaN/sapphire LED. Infrared thermal images confirm the GaN/sapphire LED with more efficient heat extraction and better temperature uniformity. The...

Method for measurement of lattice parameter of cubic GaN layers on GaAs (001)

An extended technique derived from triple-axis diffraction setup was proposed to measure lattice parameters of cubic GaN(c-GaN) films. The fully relaxed lattice parameters of c-GaN are determined to be 4.5036+0.0004 Angstrom, which is closer to the values of a hypothetical perfect crystal. The speculated zero setting correction (Deltatheta) is very slight and within the range of the accuracy of measurement. Additionally, we applied this method to analyze strain of four different kinds of c-GaN samples. It is found that in-plane strain caused by large lattice mismatch and thermal expansion coefficients mismatch directly influence the epilayer growth at high temperatures, indicating that the relaxation of tensile strain after thermal annealing helps to improve the crystalline quality of c-GaN films and optical properties

Light extraction enhancement of InGaN light-emitting diode by roughening both undoped micropillar-structure GaN and p-GaN as well as employing an omnidirectional reflector

Light extraction enhancement of InGaN–GaN light-emitting diode (LED) is demonstrated with double-side roughening both on the p-GaN surface and the micropillar undoped GaN as well as an omnidirectional reflector via patterning sapphire substrate, wafer-bonding, laser lift-off, and chemical wet etching technologies. This device design enhances the light output power up to 77% compared to the conventional LED with a single roughened p-GaN on patterned sapphire substrate at an injection current of 350mA. Due to the employment of Si carrier, the junction temperature measurement at 350mA yields a 46.6°C lower than that of the conventional LEDs.

Comprehensive analysis of microtwins in the 3C-SiC films on Si(001) substrates

Microtwins in the 3C-SiC films grown on Si(0 0 1) by atmosphere pressure chemical vapor deposition (APCVD) were investigated in detail using X-ray four-circle diffractometry. The Phi scan shows that 3C-SiC films can grow on Si substrates epitaxially and epitaxial relationship is revealed as (0 0 1)(3C) (SiC)parallel to (0 0 1)(Si),[1 1 1](3C-SiC)parallel to [1 1 1](Si). Other diffraction peaks at about 15.8 degrees in x emerged in the pole figures of the (I 1 1) 3C-SiC. We performed the pole figure of (1 0 (1) over bar 0)h-SiC and the reciprocal space mapping from the (1 1 1) reciprocal lattice point of base SiC to the (0 0 2) point of microtwin for the first time, indicating that the diffraction peaks at 15.8 degrees in x result from not hexagonal SiC but microtwins of 3C-SiC, and twin inclusions are estimated to be around 1%

Enhanced Luminance Efficiency of Wafer-Bonded InGaN–GaN LEDs With Double-Side Textured Surfaces and Omnidirectional Reflectors

A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon-ODR-GaN LED with a small chip size of 250 mum times 500 mum can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm times 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN-sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.

Power-enhanced ITO omni-directional reflective AlGaInP LEDs by two-dimensional wavelike surface texturing

An n-side up-type AlGaInP multiple-quantum-well light-emitting diode (LED) has been developed by texturing the current spreading layer utilizing photolithography followed by an anisotropic etching process over n-AlGaInP grown by metalorganic chemical vapor deposition. The GaP-ITO-Ag omni-directional reflective LEDs with the wavelike textured surfaces provide a reasonable improvement in light output power and efficiency over the corresponding conventional structures. The luminous intensity of the surface-textured LED is 1.46 times greater than that of the conventional LED in the normal direction. The output power (at 350 mA) of the surface-textured LED is increased approximately 40% as compared with that of the conventional LED. Additionally, the optical simulation also presents a tendency towards the ray extraction ratio as the size of the wavelike hole changes, confirming a proper formation of pattern size associated with the fabrication process.

Effects of TMIn flow rate of barrier layer on the optical and structural properties of InxGa1-xN/InyGa1-yN multiple quantum wells

We reported the effect of trimethylindium (TMIn) flow rate in the barrier layer on the structural and optical properties of InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) grown on (0001)-oriented sapphire substrates by low-pressure metalorganic chemical deposition (LP-MOCVD). It was found that the change of TMIn flow rate dramatically influences the interface quality and optical properties. Temperature-dependent photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) measurements provide proof evidence that the increment of TMIn flow rate deteriorates the interfacial abruptness, decreases the emission wavelength (blue shift) and the efficiency of nonradiative recombination process

Photovoltaic Effects of InGaN/GaN Double Heterojunctions With p-GaN Nanorod Arrays

The p-GaN/In0.06Ga0.94N/n-GaN double heterojunctional solar cells with solely formed nanorod arrays of p-GaN have been fabricated on sapphire (0001). The p-GaN nanorod arrays are demonstrated to significantly reduce the reflectance loss of light incidence. A stress relief of the intrinsic InGaN region is observed from high-resolution X-ray diffraction analyses. The electroluminescence emission peak is blue shifted compared with the conventional solar cells. These results are reflected by the spectral dependences of the external quantum efficiency (EQE) that show a shorter cutoff wavelength response. The maximum EQE value is 55.5%, which is an enhancement of 10% as compared with the conventional devices.