The University of Notre Dame (UND) is developing a new deep ultraviolet (UV) light LED, GaN Quantum Dots (QD).
Aluminum nitride deep UV LEDs are widely used in water treatment, disinfection, integrated biosensors, solid state lighting, and lithography. “The use of low power, light weight and durable deep UV light sources will appear in many other areas.â€
The research team's leader, Debdeep Jena, also sees market potential in the low-threshold laser field: "Quantum points require lower injection currents than quantum wells (QW) or double heterojunction lasers because the dimensions are reduced. In very wide band gaps In semiconductors, free carriers do not easily appear due to doping, so quantum dot active regions are attractive for electron-injected deep UV lasers."
At present, the external quantum efficiency (EQE) of aluminum nitride (AlGaN) QW LEDs is very low (the maximum efficiency in the wavelength region of 250 nm is only a few percentage points) because of the following challenges. The main challenge is the difficulty of injecting a sufficient amount of electrons and holes into the active region. Another challenge is that the hole source can be trapped in gallium nitride or very low aluminum content of AlGaN (band gap is narrower than luminescent radiation), which means that the luminescent electrons are heavily absorbed by the p-type contact.
The researchers believe that GaN QD has two advantages over AlGaN QW: "Three-dimensional confinement allows electrons and holes to be free from heat and causes misalignment and non-radiative recombination. Single layer thickness makes the luminescence process It is more immune to the quantum-limited Stark effect (QCSE). When the InGaN/GaN quantum well band structure is subjected to a polarization field, its band structure is subject to change and tilt. At this time, the wave function of the quantum well to the carrier ( Wave function ) produces spatial confinement effects, redistribution of electrons and holes, called quantum confinement of the Stark effect, and causes the energy of the radiation recombination to decrease, so it will exhibit a red-shift in the luminescence spectrum. Phenomenon, which in turn affects the quantum efficiency of multiple quantum wells.)
Constraints of such electron and hole energy levels in quantum dot devices can increase the band gap, resulting in higher energy, shorter wavelength photons. Electron injection and hole injection are achieved by tunneling through the phenomenon of electron penetration barriers, that is, by calibrating the conduction band and valence band of the electron and hole energy levels of the gallium nitride QD, instead of drifting with conventional devices. Process - diffusion (diffusion / diffusion). Tunnel penetration can avoid the problem of self-heating effect.
In order to solve the problem of p-type hole injection contact, UND researchers use polarization doping, so that AlGaN with wider band gap can be used instead of GaN. This technique uses changes in polarization to enhance the activation of magnesium doping. Generally, high aluminum content AlGaN has a very high activation energy, which eliminates hole density and conductivity.
Semiconductor material growth is grown on thick sapphire-based aluminum nitride by plasma enhanced molecular beam epitaxy. The growth temperature of the AlN nucleation layer and the buffer layer was 730 °C. It is also necessary to make the active region of the compressive strained gallium nitride quantum dots because the lattice mismatch of aluminum nitride is 2.4%.
Photoluminescence of gallium nitride quantum dots varies with growth time and gallium flux. The growth time was reduced from 35 seconds to 25 seconds, and the gallium flux was 6.2x10-8 Torr, which reduced the peak wavelength from 2700 nm to 246 nm. And there will be a second peak in the longer wavelength case, which is a greater reduction in QD at shorter wavelengths. The gallium flux is reduced to 5.6x10-8 Torr, the growth time is 25 seconds, and the blue light drift wavelength is extended to 238 nm.
This is a 2.6x10-7 Torr rich gallium flux technique, but if the growth time is 12 seconds and then 45 seconds, the PL peak wavelength will be as short as 234 nm and the corresponding photon energy will be 5.3 eV. The bulk gallium nitride is near UV (~ 365 nm).
Researchers attribute this shorter wavelength problem to this interrupt process, which means that quantum desorption leads to smaller and more quantum dots. The QD height in the 2.7 nm aluminum nitride carrier was 0.58 nm.
The LED active area has 8 layers of QD, two devices use a 25 second growth mode, and the other uses an intrusive technique that generates growth for 12 seconds and then 25 seconds.
The n-type electron injection region contains 225 nm of silicon-doped AlGaN, and the 117 nm p-type hole injection region is realized by different techniques. One of the 25 second growth devices used a conventional uniform magnesium doped Al0.5Ga0.5N layer (Sample I). The other two samples were doped with polarization (magnesium doped AlxGa1-xN with x-grading by changing the aluminum flux during growth). The grading of the second 25-second device was reduced from 0.5 to 0.25 (sample II). Lesson 3 Interventional growth devices used 0.97-0.77 grading (Sample III).
300μm x 300μm LED is made by etching station, which exposes titanium, aluminum, nickel and gold on the n-type surface, and makes a thin nickel-gold transparent current diffusion electrode on the n-type surface, and deposits titanium p-type Contact pad.
Figure 1: GaN/AlN QD UV LED cross-section: Sample I: 73% n-AlGaN, 50% p-AlGaN; Sample II: 80% n-AlGaN, 50%–25% p-AlGaN; Sample III: 77% n-AlGaN, 97%–77% p-AlGaN.
Sample III devices also block electron overcurrent (Figures 1 and 2). In fact, the luminescence of the p-type implanted region of the sample I device is larger than that of QD. The sample II device improves luminescence due to QD, but some of the radiation in the p-type region is converted into a longer wavelength, accompanied by leakage.
The sample III device solves the leakage and downconversion problems with a single deep UV peak wavelength of 243 nm (5.1 eV). Leakage is blocked by a thicker carrier, which is resolved by wideband gaps and by blocking QD photon reabsorption.
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