5. AlGaN- and InAlGaN-based UVA-UVC LEDs

We fabricated AlGaN and InAlGaN MQW DUV LEDs on low-TDD AlN templates [15–26]. Figure 11 shows a schematic diagram of the structure of an AlGaN-based DUV LED fabricated on a sapphire substrate. Table 1 shows typical design values for the fraction of Al (x) in the AlxGa1xN wells, in the buffer and barrier layers, and in the electron-blocking layers (EBLs) used for 222–273 nm AlGaN-MQW LEDs. Large compositions of Al in AlGaN were used to obtain DUV LEDs operating at short wavelengths, as shown in Table 1. The detail layer structures and device geometries of the LEDs are described in Ref. [15, 17]. The output power was measured using a Si photodetector located behind the LED sample. The photodetector was calibrated by measuring the luminous flux from a flip-chip LED. The forward voltages

(Vf) of the bare wafer and the flip-chip samples were approximately 15 and 8 V, respectively,

Table 1. Typical design values of the fraction of Al (x) in the AlxGa1xN wells, the buffer and barrier layers, and the

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 137

Figure 12 shows the electroluminescence (EL) spectra of the AlGaN and InAlGaN MQW LEDs measured at RT. We obtained single-peak operations for LED samples with emission wavelength from 222 to 351 nm. Figure 13 shows the EL spectra of a 227 nm AlGaN LED on a log scale [19]. The deep-level emissions with wavelengths at around 255 and 330–450 nm are more

Figure 12. Electroluminescence (EL) spectra of fabricated AlGaN and InAlGaN MQW LEDs with emission wavelengths

between 222 and 351 nm, all measured at room temperature (RT) with injection currents of around 50 mA.

with an injection current of 20 mA.

electron-blocking layers (EBLs) used for 222–273 nm AlGaN-MQW LEDs.

Figure 11. Schematic structure of a typical AlGaN-based DUV LED fabricated on a sapphire substrate.


to maintain the crystal quality. We obtained high-quality InAlGaN with a high amount of Al (>45%) by using epitaxy at a relatively low growth rate, i.e., 0.03 μm/h. The intensity of the light at 280 nm emitted by an InAlGaN QW at RT was increased by a factor of 5 by reducing

136 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

We fabricated AlGaN and InAlGaN MQW DUV LEDs on low-TDD AlN templates [15–26]. Figure 11 shows a schematic diagram of the structure of an AlGaN-based DUV LED fabricated on a sapphire substrate. Table 1 shows typical design values for the fraction of Al (x) in the AlxGa1xN wells, in the buffer and barrier layers, and in the electron-blocking layers (EBLs) used for 222–273 nm AlGaN-MQW LEDs. Large compositions of Al in AlGaN were used to obtain DUV LEDs operating at short wavelengths, as shown in Table 1. The detail layer structures and device geometries of the LEDs are described in Ref. [15, 17]. The output power was measured using a Si photodetector located behind the LED sample. The photodetector was calibrated by measuring the luminous flux from a flip-chip LED. The forward voltages

Figure 11. Schematic structure of a typical AlGaN-based DUV LED fabricated on a sapphire substrate.

the growth rate from 0.05 to 0.03 μm/h.

5. AlGaN- and InAlGaN-based UVA-UVC LEDs

Table 1. Typical design values of the fraction of Al (x) in the AlxGa1xN wells, the buffer and barrier layers, and the electron-blocking layers (EBLs) used for 222–273 nm AlGaN-MQW LEDs.

(Vf) of the bare wafer and the flip-chip samples were approximately 15 and 8 V, respectively, with an injection current of 20 mA.

Figure 12 shows the electroluminescence (EL) spectra of the AlGaN and InAlGaN MQW LEDs measured at RT. We obtained single-peak operations for LED samples with emission wavelength from 222 to 351 nm. Figure 13 shows the EL spectra of a 227 nm AlGaN LED on a log scale [19]. The deep-level emissions with wavelengths at around 255 and 330–450 nm are more

Figure 12. Electroluminescence (EL) spectra of fabricated AlGaN and InAlGaN MQW LEDs with emission wavelengths between 222 and 351 nm, all measured at room temperature (RT) with injection currents of around 50 mA.

222 nm, which is the shortest wavelength ever reported for a QW LED, was achieved. The output power was 0.14 μW at an injection current of 80 mA, and the maximum EQE was

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 139

It has been reported that 'normal' c-axis (vertical) emission is difficult to obtain from an AlN (0001). This is because the optical transition between the conduction band and the top of the valence band is mainly only allowed for light that has its electric field parallel to the c-axis (E//c) [9]. The lateral propagation of the transvers-magnetic (TM) mode emission results in a significant reduction of LEE. Therefore, short wavelength AlGaN UVC LED shows a very low LEE. Several groups have reported on this [49, 50]. Banal et al. reported that the critical Al composition for 'polarization switching' could be expanded to approximately 0.82 by using very thin (<1.5 nm) AlGaN quantum wells on an AlN/sapphire template [49]. We investigated the variation in the spectrum of a 222 nm AlGaN QW LED with the angle of emission, and demonstrated that normal vertical emission can be obtained, even at short-wavelengths, for

6. Increasing the electron injection efficiency (EIE) by introducing an MQB

EIE into the QW is reduced due to the electron leakage caused by the low hole concentrations in the p-type AlGaN layers. The EIE reduction is particularly severe for LEDs with wavelength shorter than 260 nm, because an electron barrier height of an EBL becomes smaller [17]. We introduced a MQB [51, 52] to serve as an EBL, and consequently achieved a marked increase in

Figure 15 shows schematic illustrations of the electron flow for an AlGaN DUV LED with (a) a MQB EBL and (b) a conventional single barrier EBL. In usual case, we are using single barrier EBL for 250–280 nm UVC LEDs. However, the electron barrier height of the single barrier EBL is determined by the bandgap of the barrier material, and it is not sufficiently high for UVC LED with wavelength shorter than 260 nm. On the other hand, we can increase the 'effective' barrier height of the EBL by introducing MQB. Even electrons having higher energy above the MQB band-edge can be reflected by the multi-reflection effects of the MQB, and injected into

Figure 16 shows the electron transmittance through an AlGaN MQB and a conventional single barrier EBL for a 250 nm AlGaN LED calculated by a transfer-matrix method. It was shown, using barriers with thickness modulation, that the 'effective' barrier height of an AlGaN/

Figure 17 shows a schematic diagram of the structure of a 250 nm AlGaN QW DUV LED with an MQB EBL and a cross-sectional TEM image of a fabricated device. We carried out experiments to find an appropriate MQB structure, and found that the insertion of an initial thickbarrier is important for reflecting low energy electrons. We also found that thin barriers contribute to the reflection of higher-energy electrons. The optimum MQB comprised five layers of Al0.95Ga0.05N/Al0.77Ga0.23N with thicknesses of 7/4/5.5/4/4/2.5/4/2.5/4 nm, in which

AlGaN MQB is up to twice that of a conventional single-barrier EBL.

0.003%.

EIE [18].

LEDs with as much as 83% Al [20].

the QWs, resulting in higher EIE.

Figure 13. EL spectra on a log scale of a 227 nm AlGaN DUV LED for various injection currents.

than two orders of magnitude smaller than the main peaks. The output power of the 227 nm LED was 0.15 mW at an injection current of 30 mA, and the maximum EQE was 0.2% under pulsed operation at RT. Figure 14 shows (a) the EL spectra for various injection currents and (b) the current-output power (I-L) and current-EQE (ηext) (I-EQE) characteristics for a 222 nm AlGaN-MQW LED measured under pulsed operation at RT [20]. Single-peak operation at

Figure 14. (a) EL spectra for various injection currents and (b) the output power and EQE (ηext) vs current characteristics for a 222 nm AlGaN-MQW LED measured under pulsed operation at RT.

222 nm, which is the shortest wavelength ever reported for a QW LED, was achieved. The output power was 0.14 μW at an injection current of 80 mA, and the maximum EQE was 0.003%.

It has been reported that 'normal' c-axis (vertical) emission is difficult to obtain from an AlN (0001). This is because the optical transition between the conduction band and the top of the valence band is mainly only allowed for light that has its electric field parallel to the c-axis (E//c) [9]. The lateral propagation of the transvers-magnetic (TM) mode emission results in a significant reduction of LEE. Therefore, short wavelength AlGaN UVC LED shows a very low LEE. Several groups have reported on this [49, 50]. Banal et al. reported that the critical Al composition for 'polarization switching' could be expanded to approximately 0.82 by using very thin (<1.5 nm) AlGaN quantum wells on an AlN/sapphire template [49]. We investigated the variation in the spectrum of a 222 nm AlGaN QW LED with the angle of emission, and demonstrated that normal vertical emission can be obtained, even at short-wavelengths, for LEDs with as much as 83% Al [20].
