Yong O. Wei

Low-threshold stimulated emission at 249 nm and 256 nm from AlGaN-based multiple-quantum-well lasers grown on sapphire substrates

Optically pumped deep-ultraviolet (DUV) lasing with low threshold was demonstrated from AlGaN-based multiple-quantum-well (MQW) heterostructures grown on sapphire substrates. The epitaxial layers were grown pseudomorphically by metalorganic chemical vapor deposition on (0001) sapphire substrates. Stimulated emission was observed at wavelengths of 256 nm and 249 nm with thresholds of 61 kW/cm2 and 95 kW/cm2 at room temperature, respectively. The thresholds are comparable to the reported state-of-the-art AlGaN-based MQW DUV lasers grown on bulk AlN substrates emitting at 266 nm. These low thresholds are attributed to the optimization of active region and waveguide layer as well as the use of high-quality AlN/sapphire templates. The stimulated emission above threshold was dominated by transverse-electric polarization. This work demonstrates the potential candidacy of sapphire substrates for DUV diode lasers.

Highly luminescent, high-indium-content InGaN film with uniform composition and full misfit-strain relaxation

We have studied the properties of thick InxGa1−xN films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the InxGa1−xN film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.

Demonstration of transverse-magnetic deep-ultraviolet stimulated emission from AlGaN multiple-quantum-well lasers grown on a sapphire substrate

We demonstrate transverse-magnetic (TM) dominant deep-ultraviolet (DUV) stimulated emission from photo-pumped AlGaN multiple-quantum-well lasers grown pseudomorphically on an AlN/sapphire template by means of photoluminescence at room temperature. The TM-dominant stimulated emission was observed at wavelengths of 239, 242, and 243 nm with low thresholds of 280, 250, and 290 kW/cm2, respectively. In particular, the lasing wavelength of 239 nm is shorter compared to other reports for AlGaN lasers grown on foreign substrates including sapphire and SiC. The peak wavelength difference between the transverse-electric (TE)-polarized emission and TM-polarized emission was approximately zero for the lasers in this study, indicating the crossover of crystal-field split-off hole and heavy-hole valence bands. The rapid variation of polarization between TE- and TM-dominance versus the change in lasing wavelength from 243 to 249 nm can be attributed to a dramatic change in the TE-to-TM gain coefficient ratio for the sa...

Simulations, Practical Limitations, and Novel Growth Technology for InGaN-Based Solar Cells

Indium gallium nitride (InGaN) alloys exhibit substantial potential for high-efficiency photovoltaics. However, theoretical promise still needs to be experimentally realized. This paper presents a detailed theoretical study to provide guidelines to achieve high-efficiency InGaN solar cells. While the efficiency of heterojunction devices is limited to ~11%, homojunction devices can achieve suitable efficiencies, provided that highly p-type-doped InGaN layers and thick, single-phase InGaN films can be grown. Thus, we have developed a novel growth technology that facilitates growth of p-type nitride films with greatly improved hole concentration and growth of InGaN without phase separation, offering promise for future high-efficiency InGaN solar cells.

Inverse-Tapered p-Waveguide for Vertical Hole Transport in High-[Al] AlGaN Emitters

We report a high-aluminum-containing ([Al] ~ 0.6) AlGaN multiple-quantum well (MQW) double-heterojunction (DH) emitter employing an inverse-tapered-composition AlGaN:Mg p-type waveguide grown on a c plane Al-polar AlN bulk substrate. Using numerical simulations, we have determined that the inverse-tapered p-type waveguide design is necessary for high [Al] containing p-n junction devices as any valence band discontinuity at the junction will limit the vertical hole transport and induce a larger voltage-drop across the structure. The fabricated ultraviolet MQW DH emitter can sustain a DC current of at least 500 mA and a pulsed current of at least 1.07 A, which corresponds to a current density of 10 and 18 kA/cm2 at maximum measured voltage of 15 and 20 V with the measured series resistance of 15 and 11 Ω, respectively.

Optically pumped low-threshold UV lasers

Recently, low-threshold optically-pumped DUV lasers containing AlGaN-based multiple-quantum wells (MQWs) have been demonstrated by homoepitaxial growth on c-plane bulk AlN substrates [1-5]. The bulk AlN substrates were used in these studies due to low-dislocation density and reduction of the lattice mismatch and thermal expansion difference between the AlN substrate and Al-rich AlGaN epitaxial layers, thus leading to high-quality active regions with relatively low-dislocation density. However, because of high cost, smaller area, and impurity absorption of the bulk AlN substrates today, it is much more desirable to grow DUV lasers on the vastly available and lower-cost sapphire substrates.

Structural and Optical Properties of AlGaN MQWs Grown by MOCVD Using One and Two TMG Sources

Quantum wells are used to confine the electron-hole pairs and thus increase the quantum efficiency. However, the growth of a good-quality quantum well (QW) is challenging. Many factors such as choice of substrate, growth temperature, and number of sources affect the structure of the QWs. In this study, QWs are grown by metal-organic chemical vapor deposition (MOCVD) at a temperature of 1155 C, with one and two trimethylgallium (TMG) sources. The differences between one and two TMG sources are schematically shown in Figure 1. In one TMG configuration, the growth conditions between a quantum well and a quantum barrier (QB) requires the TMG flow rate to be interrupted and changed. During this change, there is no Ga flowing onto the sample’s surface. In two TMG configuration, QW and QB are grown by two TMG sources with different flow rate. The active region growth switches between QW and QB without interruption. The aluminum content in the QWs and the QBs in both samples (using one and two TMG) are targeted to be 0.6 and 0.75, respectively. Their structures are studied by scanning transmission electron microscope (STEM) and the optical properties by cathodoluminescence (CL).