Jiangnan Dai

Advantages of AlGaN-Based 310-nm UV Light-Emitting Diodes With Al Content Graded AlGaN Electron Blocking Layers

In order to improve the performance of deep ultraviolet light-emitting diodes (UV LEDs), the effects of different electron blocking layers (EBLs) on the performance of AlxGa1-xN-based deep UV LEDs at 310 nm have been studied through a numerical simulation. The simulation results show that the adoption of EBLs is critical to improve the device performance. In comparison with a conventional structure using EBL with constant Al composition (0.7), the device structure with an Al-content graded AlxGa1-xN (from 0.9 to 0.4 in the growth direction) EBL possesses numerous advantages such as lower working voltage, higher internal quantum efficiency, and less efficiency droop under high-current injection. By detailedly analyzing the profiles of energy band diagrams, distributions of carrier concentration, and electron current density, the advantages of Al-content graded AlxGa1-xN EBL are attributed to the resulting lower resistivity, higher barrier for electron leakage, and simultaneously reduced barrier for hole injection compared with the conventional EBL with constant Al composition.

Tunability of intersubband transition wavelength in the atmospheric window in AlGaN/GaN multi-quantum wells grown on different AlGaN templates by metalorganic chemical vapor deposition

The properties of intersubband transition in AlGaN/GaN multi-quantum wells (MQWs) grown on different AlGaN templates by metalorganic chemical vapor deposition are investigated. The strain states of GaN wells are studied by Raman spectra and reciprocal space mappings, which shows that the GaN wells are compressively strained and the compressive strain is increased when the Al mole composition is varied from 0 to 0.3. The Fourier transform infrared spectrometer results show that the intersubband transition wavelength in the AlGaN/GaN MQWs can be tuned from 5.14 μm to 4.65 μm when the Al mole composition of the AlGaN template is increased. The results can be attributed to the quantum confined Stark effect.

Effect of growth temperature of an AlN intermediate layer on the growth mode of AlN grown by MOCVD

High-temperature AlN (HT-AlN) films inserted with a thin AlN intermediate layer were grown on c-sapphire directly by MOCVD, and the influence of growth temperature of the AlN intermediate layer on the growth mode of HT-AlN was investigated. The results showed that with the temperature of the AlN intermediate layer increasing from 470 to 670 °C, the growth mode was changed from three-dimensional (3D) growth to two-dimensional (2D) growth. However, when the temperature was further increased to 870 °C, the growth mode was changed into 3D growth again. This suggested that the thin AlN intermediate layer could alter the growth mode of the HT-AlN film by varying the growth temperature, which provides a possible way to improve the morphology and quality of AlN films. The reasons responsible for this phenomenon were probed by Raman spectral, reciprocal space map and growth evolutions of AlN, and a model was developed to explain the effects of IM-AlN on the growth mode of HT-AlN.

Nonpolar a-plane ZnO films grown on GaN/sapphire templates with SiNx interlayer by pulsed laser deposition

Nonpolar a-plane ZnO films were deposited for the first time on nearly lattice-matched a-plane GaN templates using an in situ SiNx interlayer by the pulsed laser deposition method. The symmetric and asymmetric reciprocal space mappings reveal the broadening effects of the reciprocal lattice points and the residual biaxial in-plane tensile strain of a-plane ZnO to be 0.335% and 0.055% along the [0 0 0 1] c-axis and p-axis, respectively. The photoluminescence spectrum at 85 K is dominated by neutral donor-bound excitons and free-electron-to-bound (e-A0) emissions; relatively intense LO-phonon replicas of (e-A0) have also been observed in a-plane ZnO. Temperature-dependent PL spectra have also been discussed to identify the origin of the emission peaks. Up to fourth A1(LO) phonon mode can be observed to be enhanced significantly from the resonant Raman spectrum.

Anisotropic optical polarization dependence on internal strain in AlGaN epilayer grown on Al x Ga1−x N templates

Anisotropic optical polarization of AlGaN has been one of the major challenges responsible for the poor efficiency of AlGaN-based ultraviolet light emitting diodes (UV LEDs). In this work, we experimentally investigated the effect of internal strain on the optical polarization of AlGaN epilayers which were pseudomorphically grown on Al x Ga1?x N templates with Al composition changing from 0.1 to 0.42. High-resolution x-ray diffraction and reciprocal space mapping were conducted to determine the crystal quality and strain status. Polarization-dependent photoluminescence (PL) measurement was performed to study the degree of polarization (DOP) of light emission from lateral facet of the AlGaN epilayer. The result showed that the DOP increased from???0.69 to???0.24 with the in-plane strain changing from tensile status (1.19%) to compressive status (?0.70%) and it exhibited a strong dependence of the DOP on the strain. These results demonstrated that the compressive in-plane strain could facilitate TE mode emission from AlGaN, which providing a potential way to enhance the surface light emission of AlGaN-based UV LEDs via strain management of the active region.

Strong enhancement of terahertz response in GaAs/AlGaAs quantum well photodetector by magnetic field

A strong enhancement of terahertz (THz) response in a GaAs/AlGaAs quantum well photodetector (QWP) is observed under perpendicular magnetic field. Photocurrent spectra show that besides partial contribution from an increase in the detector differential resistance, improvement of photoconductive gain induced by electron localization and Landau quantization of the in-plane electron motion in quantum wells is the predominant underlying mechanism. This study sheds light on the transport mechanism in THz QWPs and provides a possible means for enhancing THz QWP’s response.

Enhanced Optical and Thermal Performance of Eutectic Flip-Chip Ultraviolet Light-Emitting Diodes via AlN-Doped-Silicone Encapsulant

This paper investigated the optical and thermal performance of the nitride-based ultraviolet light-emitting diodes fabricated by the eutectic flip-chip method. A new packaging structure was proposed by introducing a thin encapsulation layer doped with 0.4 wt% AlN nanoparticles (NPs) and uniform quartz lens simultaneously. Experimental results showed that the packaging structure proposed in this paper could significantly enhance the light output power, reduce the junction temperature, and increase the emission angle compared with the encapsulation layer consisting silicone only. When the NPs concentration increased from 0.1 to 0.4 wt%, the light output power increased from 7.6% to 17.4% at the forward current of 800 mA. Meanwhile, the junction temperature decreased by 5.7 °C, while the emission angle increased by 11.3°. What is more, it was found that the enhancement of light output power depended on the NPs concentration and showed the maximum at the concentration of 0.4 wt%. The enhanced light output power was attributed to the additional light scattering and the increased average refractive index resulted from the NPs introduced in the proposed package structure.

Improved Ohmic contacts to plasma etched n-Al0.5Ga0.5N by annealing under nitrogen ambient before metal deposition

The contact characteristics of Ti/Al and Ti/Al/Ni/Au contacts to as-grown, plasma etched and plasma etched + annealed in N2 n-Al0.5Ga0.5N epilayers were compared. After a rapid thermal annealing, both Ti/Al and Ti/Al/Ni/Au contacts to as-grown and plasma etched + annealed in N2 n-Al0.5Ga0.5N became truly Ohmic, whereas the contacts to plasma etched samples still remained rectifying. Surface atomic concentration analysis indicates the N vacancies resulting from plasma treatment act more as deep-level states rather than shallow donors. However, these deep-level states could be effectively removed by annealing the plasma etched n-Al0.5Ga0.5N under N2 ambient, and as a result, the Fermi level was elevated toward conduction band edge, facilitating the formation of Ohmic contacts.

Large-scale growth of density-tunable aligned ZnO nanorods arrays on GaN QDs

An effective approach for growing large-scale, uniformly aligned ZnO nanorods arrays is demonstrated. The synthesis uses a GaN quantum dot (QD) template produced by a self-assembled Stranski-Krastanow mode in metal organic chemical vapor deposition, which serves as a nucleation site for ZnO owing to the QDs high surface free energy. The resultant ZnO nanorods with uniform shape and length align vertically on the template, while their density is easily tunable by adjusting the density of GaN QDs, which can be adjusted by simply varying growth interruption. By controlling the density of ZnO nanorod arrays, their optical performance can also be improved. This approach opens the possibility of combining one-dimensional (1D) with 0D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, and interconnects.

Defect Reduction in AlN Epilayers Grown by MOCVD via Intermediate-Temperature Interlayers

In this work, significant reduction of the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates via metalorganic chemical vapor deposition has been obtained by insertion of thin intermediate-temperature interlayers (IT-ILs). The growth temperature of the IT-ILs ranged from 750°C to 950°C after the initial growth at high temperature of 1200°C. Detailed characterizations were performed to understand the mechanisms of the reduction in dislocation density. It is found that the relatively low growth temperature of the IT-ILs can modify the originally two-dimensional (2D) growth mode to a three-dimensional (3D) growth process and creates a high density of small islands at the interface. During the subsequent growth of the high-temperature AlN layer, the AlN islands initially coalesce to form larger grains as the growth proceeds, and some TDs present in the previous AlN epilayers are found to bend near the interlayer, leading to dislocation merging and annihilation.