Hangyang Chen

High density GaN/AlN quantum dots for deep UV LED with high quantum efficiency and temperature stability

High internal efficiency and high temperature stability ultraviolet (UV) light-emitting diodes (LEDs) at 308 nm were achieved using high density (2.5 × 109 cm−2) GaN/AlN quantum dots (QDs) grown by MOVPE. Photoluminescence shows the characteristic behaviors of QDs: nearly constant linewidth and emission energy, and linear dependence of the intensity with varying excitation power. More significantly, the radiative recombination was found to dominant from 15 to 300 K, with a high internal quantum efficiency of 62% even at room temperature.

Enhancement of p-type conductivity by modifying the internal electric field in Mg- and Si-δ-codoped AlxGa1-xN /AlyGa1-yN superlattices

National Natural Science Foundation [60827004, 60776066, 90921002]; Science and Technology program of Fujian and Xiamen of China

Enhanced Pockels effect in GaN∕AlxGa1−xN superlattice measured by polarization-maintaining fiber Mach-Zehnder interferometer

Six-period 4 nm GaN/10 nm AlxGa1-xN superlattices with different Al mole fractions x were prepared on (0001) sapphire substrates by low-temperature metal-organic chemical vapor deposition. The linear electro-optic (Pockels) effect was studied by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The measured electro-optic coefficients, gamma(13)=5.60 +/- 0.18 pm/V, gamma(33)=19.24 +/- 1.21 pm/V (for sample 1, x=0.3), and gamma(13)=3.09 +/- 0.48 pm/V, gamma(33)=8.94 +/- 0.36 pm/V (for sample 2, x=0.1), respectively, are about ten times larger than those of GaN bulk material. The enhancement effect in GaN/AlxGa1-xN superlattice can be attributed to the large built-in field at the interfaces, depending on the mole fraction of Al

Vacuum Rabi Splitting of Exciton-Polariton Emission in an AlN Film

The vacuum Rabi splitting of exciton–polariton emission is observed in cathodoluminescence (CL) and photoluminescence spectra of an AlN epitaxial film. Atomic force microscopy and CL measurements show that the film has an atomically flat surface, high purity, and high crystal quality. By changing the temperature, anticrossing behavior between the upper and lower polariton branch can be obtained in low temperature with a Rabi splitting of 44 meV, in agreement with the calculation. This large energy splitting is caused by strong oscillator strength, intrinsically pure polarization in wurtzite AlN semiconductor, and high fraction of free exciton in the sample. These properties indicate that AlN can be a potential semiconductor for the further development of polariton physics and polariton–based novel devices.

Modified pulse growth and misfit strain release of an AlN heteroepilayer with a Mg–Si codoping pair by MOCVD

We report the modified pulse growth method together with an alternating introduction of larger-radius impurity (Mg) for the quality improvement and misfit strain release of an AlN epitaxial layer by the metal–organic chemical vapour deposition (MOCVD) method. Various pulse growth methods were employed to control the migration of Al atoms on the substrate surface. The results showed that the pulse time and overlapping of V/III flux is closely related with the enhancement of the 2D and 3D growth mode. In order to reduce the misfit strain between AlN and sapphire, an impurity of larger atomic radius (e.g. Mg) was doped into the AlN lattice to minimize the rigidity of the AlN epilayer. It was found that the codoping of Mg–Si ultrathin layers could significantly minimize the residual strain as well as the density of threading dislocations.

Improved Characteristics of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Superlattice p-Type Doping

The photoelectric properties and physical mechanism of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) with the superlattice p-type doping layer (PSL) are studied numerically and compared with the Al-composition (50%) conventional p-type layer AlGaN-based DUV-LEDs. The extraordinary design of DUV-LEDs with varied barrier PSL has been investigated by the advanced physical model of semiconductor device (APSYS) software by comparing the internal quantum efficiency, light output power, electroluminescence intensity, distributions of carrier concentration, and energy band diagrams. As a result of hole injection augmentation and electronic leakage reduction, the property of AlGaN-based DUV-LED with the PSL has been enhanced significantly. Moreover, the 55%-Al-composition of the superlattice barrier p-type doping layer greatly reduces the effective potential height for holes in the valence band, which is beneficial for hole injection from the PSL. The new structure improves the properties of DUV-LED and shows remarkable output performance.

Origins and suppressions of parasitic emissions in ultraviolet light-emitting diode structures

863 program; National Nature Science Foundation [60827004, 90921002, 60776066]; Science & Technology Program of Fujian and Xiamen of China

Non-contact electrical detection of intrinsic local charge and internal electric field at nanointerfaces

A nanoscale non-contact electrical measurement has been developed based on Auger electron spectroscopy. This approach used the specialty of an Auger electron, which is self-generated and free from external influences, to overcome the technical limitations of conventional measurements. The detection of the intrinsic local charge and internal electric field for nanostructured materials was achieved with a resolution below 10 nm. As an example, the electrical properties at the GaN/AlGaN/GaN nanointerfaces were characterized. The concentration of the intrinsic polarization sheet charges embedded in GaN/AlGaN nanointerfacial layers were accurately detected to be -4.4 e nm(-2). The mapping of the internal electric field across the nanointerface revealed the actual energy-band configuration at the early stage of the formation of a two-dimensional electron gas.

Ideal square quantum wells achieved in AlGaN/GaN superlattices using ultrathin blocking-compensation pair

A technique for achieving square-shape quantum wells (QWs) against the intrinsic polar discontinuity and interfacial diffusion through self-compensated pair interlayers is reported. Ultrathin low-and-high % pair interlayers that have diffusion-blocking and self-compensation capacities is proposed to resist the elemental diffusion at nanointerfaces and to grow the theoretically described abrupt rectangular AlGaN/GaN superlattices by metal-organic chemical vapor deposition. Light emission efficiency in such nanostructures is effectively enhanced and the quantum-confined Stark effect could be partially suppressed. This concept could effectively improve the quality of ultrathin QWs in functional nanostructures with other semiconductors or through other growth methods.

Pockels effect in GaN/AlxGa1-xN superlattice with different quantum structures

The linear electro-optic (Pockels) effect of wurtzite gallium nitride (GaN) films and six-period GaN/AlxGa1-xN superlattices with different quantum structures were demonstrated by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55μm. The samples were prepared on (0001) sapphire substrate by low-temperature metalorganic chemical vapor deposition (MOCVD). The measured coefficients of the GaN/AlxGa1-xN superlattices are much larger than those of bulk material. Taking advantage of the strong field localization due to resonances, GaN/AlxGa1-xN SL can be proposed to engineer the nonlinear responses.