3. Growth of high-quality AlN on sapphire substrates

range emissions from 222 to 351 nm were demonstrated in AlGaN and InAlGaN LEDs [17–21]. We began to improve LEE of UVC LEDs by introducing a transparent p-AlGaN contact layer and a reflective p-type electrode [22–24]. We also developed commercially available DUV LED

Figure 3. Relationship between the direct transition bandgap energy and the lattice constant of the wurtzite (WZ)

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

Sensor Electronic Technology (SET) developed the first commercially available LEDs with wavelengths ranging between 240 and 360 nm [27–28]. They reported a maximum EQE of 11% for a 278 nm LED in 2012 [28]. They also did detailed investigations into the properties of

Since 2010, many companies have started developing UVC LEDs aiming at sterilization applications. Nikkiso has developed highly efficient UVC LEDs [32–34] and reported EQEs of over 10% [32]. They improved the LED properties by introducing an encapsulating resin that does not deteriorate under UVC radiation [34]. Crystal IS developed efficient 265 nm LEDs on bulk AlN substrates fabricated by a sublimation method [35, 36], and Tokuyama developed UVC LEDs on a thick transparent AlN layer grown, also on bulk AlN substrates, by hydride vapor phase epitaxy (HVPE) [37–40]. Nichia has developed high wall-plug efficiency (WPE) UVC LEDs [41, 42] using a lens bonding technique [42]. Also, M. Kneissl's group in the Technical

modules to be used for sterilization in 2014 [25, 26].

InAlGaN material system and the lasing wavelengths of various gas lasers.

AlGaN epilayers and UVC LED devices [29–31].

In order to obtain low-TDD, crack-free AlN buffer layer with atomically flat surface on sapphire, we introduced an 'ammonia (NH3) pulsed-flow multilayer (ML) growth method [15]. Figure 4 shows a schematic view of the growth control method and a typical gas flow sequence using pulsed and continuous gas flows.

The samples were grown on sapphire (0001) substrates at 76 Torr by metal-organic chemical vapor deposition (MOCVD). First, an AlN nucleation layer and a 'buried' AlN layer were deposited, both by NH3 pulsed-flow growth. The pulsed-flow mode is effective for initial high-quality AlN growth on sapphire because of the increased migration of the precursor. After the growth of the first layers, we introduced a continuous-flow mode AlN growth to reduce the surface roughness. By repeating the pulsed- and continuous-flow modes, we can obtain crack-free, thick AlN layers with atomically flat surfaces. By maintaining Al-rich growth conditions, we can obtain stable Al (+c) polarity, which is necessary for suppressing polarity inversion from Al to N. The detailed growth conditions are described in Ref. [15, 19]. The advantages in comparison with former approaches [46, 47] are that the method is in-situ process, and low TDD AlN can be obtained without the need for AlGaN layers, yielding a device structure with minimal DUV absorption.

Figure 5 shows the full-width at half maximum (FWHM) of X-ray diffraction (10–12) ω-scan rocking curves (XRC) for various stages in the ML-AlN growth. This was reduced from 2160 to 550 arcsec by executing the pulsed-flow mode twice. Figure 6 shows atomic-force microscope

Figure 4. Gas flow sequence and schematic view of the growth control method used for the NH3 pulsed-flow multilayer (ML)-AlN growth technique.

(AFM) images of the surface of ML-AlN on sapphire at various stages of growth. We can see that the surface improves as more layers are grown, ending with an atomically flat surface. The typical root-mean-square (RMS) of the surface roughness was 0.16 nm, as seen in Figure 6.

Figure 7. (a) Schematic diagram and (b) cross-sectional TEM image of an AlGaN/AlN template including a 5-step ML-

Figure 6. AFM images of the surface of the ML-AlN layer with an area of 5 <sup>5</sup> <sup>μ</sup>m<sup>2</sup> at various stages in the growth.

Figure 7 shows (a) a schematic illustration of the structure and (b) a cross-sectional transmission electron microscope (TEM) image of an AlGaN/AlN template including ML-AlN buffer layer grown on a sapphire substrate. The typical FWHMs of the (10–12) and (0002) XRCs of the ML-AlN were approximately 330 and 180 arcsec, respectively. This was grown in a 3 2 inch MOCVD reactor [25]. The minimum corresponding FWHMs obtained using a 1 2 inch MOCVD reactor were approximately 290 and 180 arcsec, respectively. The minimum edge-

observed in the TEM image. To further reduction of TDD, we introduced an AlN epitaxial lateral overgrowth (ELO) technique on a patterned sapphire substrate (PSS) and obtained

Figure 8 shows a cross-sectional TEM image of an AlGaN multi-quantum well (MQW) of a 227 nm DUV LED fabricated on a ML-AlN buffer. In order to suppress the spontaneous

, respectively, as

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 133

and screw-type dislocation densities were below 5 108 and 4 107 cm<sup>2</sup>

.

4. Increasing the internal quantum efficiency (IQE)

TDDs of the order of 10<sup>7</sup> cm<sup>2</sup>

AlN buffer layer grown on a sapphire substrate.

Figure 5. FWHM of the X-ray diffraction (10–12) ω-scan rocking curve (XRC) at various stages in the growth of the ML-AlN layer.

#### Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 133

process, and low TDD AlN can be obtained without the need for AlGaN layers, yielding a

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

Figure 5 shows the full-width at half maximum (FWHM) of X-ray diffraction (10–12) ω-scan rocking curves (XRC) for various stages in the ML-AlN growth. This was reduced from 2160 to 550 arcsec by executing the pulsed-flow mode twice. Figure 6 shows atomic-force microscope

Figure 4. Gas flow sequence and schematic view of the growth control method used for the NH3 pulsed-flow multilayer

Figure 5. FWHM of the X-ray diffraction (10–12) ω-scan rocking curve (XRC) at various stages in the growth of the ML-

device structure with minimal DUV absorption.

(ML)-AlN growth technique.

AlN layer.

Figure 6. AFM images of the surface of the ML-AlN layer with an area of 5 <sup>5</sup> <sup>μ</sup>m<sup>2</sup> at various stages in the growth.

Figure 7. (a) Schematic diagram and (b) cross-sectional TEM image of an AlGaN/AlN template including a 5-step ML-AlN buffer layer grown on a sapphire substrate.

(AFM) images of the surface of ML-AlN on sapphire at various stages of growth. We can see that the surface improves as more layers are grown, ending with an atomically flat surface. The typical root-mean-square (RMS) of the surface roughness was 0.16 nm, as seen in Figure 6.

Figure 7 shows (a) a schematic illustration of the structure and (b) a cross-sectional transmission electron microscope (TEM) image of an AlGaN/AlN template including ML-AlN buffer layer grown on a sapphire substrate. The typical FWHMs of the (10–12) and (0002) XRCs of the ML-AlN were approximately 330 and 180 arcsec, respectively. This was grown in a 3 2 inch MOCVD reactor [25]. The minimum corresponding FWHMs obtained using a 1 2 inch MOCVD reactor were approximately 290 and 180 arcsec, respectively. The minimum edgeand screw-type dislocation densities were below 5 108 and 4 107 cm<sup>2</sup> , respectively, as observed in the TEM image. To further reduction of TDD, we introduced an AlN epitaxial lateral overgrowth (ELO) technique on a patterned sapphire substrate (PSS) and obtained TDDs of the order of 10<sup>7</sup> cm<sup>2</sup> .

#### 4. Increasing the internal quantum efficiency (IQE)

Figure 8 shows a cross-sectional TEM image of an AlGaN multi-quantum well (MQW) of a 227 nm DUV LED fabricated on a ML-AlN buffer. In order to suppress the spontaneous

Figure 8. Cross-sectional TEM image of the quantum well region of an AlGaN MQW DUV-LED.

We observed a considerable increase in the DUV emission from AlGaN-QWs by fabricating them on low TDD AlN templates [16, 17]. Figure 9 shows the photoluminescence (PL) spectra of AlGaN QWs with emission peaks at around 255 nm fabricated on ML-AlN measured at room temperature (RT). We used a 244 nm Ar-ion second-harmonics generation (SHG) laser for the excitation of the sample. The excitation power density was approximately 200 W/cm2

Figure 10. PL intensity of AlGaN-QWs as a function of the FWHM of the XRC (10–12) of AlGaN buffers measured at

The PL intensity significantly increases with narrower FWHM. We can see from Figure 9 that

Figure 10 shows the intensity of the PL peak at 255-nm measured at RT as a function of the FWHM of the XRC (10–12). Reducing the FWHM from 1400 to 500 arcsec increases the PL intensity by a factor of about 80. Between 500 and 800 arcsec the PL intensity increases rapidly. This rapid increase can be explained by a reduction in the non-radiative recombination rate as the distance between the TDs becomes greater compared with the carrier diffusion length in the QW. We obtained similar degrees of improvement for QWs operating at other wavelengths. The relationship between IQE and TDD in DUV AlGaN-QWs was also investigated

The quaternary alloy InAlGaN is also a strong candidate as a material for realizing DUV LEDs, since the inclusion of In leads to efficient UV emission as well as higher hole concentration. Just a few percent of In in AlGaN is needed to obtain high IQE, where the increases in efficiency are due to the In segregation effect, an effect previously investigated for the ternary alloy, InGaN.

We took up the challenge of developing the crystal growth technology needed to obtain high quality InAlGaN alloys operating at the 'sterilization' wavelength (280 nm) [17]. Growing crystals of InAlGaN with a high Al content is relatively difficult, because incorporating In becomes more challenging as the growth temperature is increased, which is necessary in order

We have described the advantages of the use of InAlGaN in Ref. [1, 11, 12, 14, 17].

the efficiency depends strongly on the edge-type TDDs.

in Ref. [27, 48].

room temperature.

.

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 135

Figure 9. Photoluminescence (PL) spectra of AlGaN QWs on ML-AlN templates with various FWHMs of the XRC (10–12) measured at room temperature.

polarization effects induced in the wells, very thin quantum well was used. It is important to obtain atomically smooth hetero-interfaces to achieve high IQE from such thin QWs. As shown in the cross-sectional TEM image in Figure 8, the three 1.3 nm-thick QWs have atomically-flat hetero-interfaces.

Figure 10. PL intensity of AlGaN-QWs as a function of the FWHM of the XRC (10–12) of AlGaN buffers measured at room temperature.

We observed a considerable increase in the DUV emission from AlGaN-QWs by fabricating them on low TDD AlN templates [16, 17]. Figure 9 shows the photoluminescence (PL) spectra of AlGaN QWs with emission peaks at around 255 nm fabricated on ML-AlN measured at room temperature (RT). We used a 244 nm Ar-ion second-harmonics generation (SHG) laser for the excitation of the sample. The excitation power density was approximately 200 W/cm2 . The PL intensity significantly increases with narrower FWHM. We can see from Figure 9 that the efficiency depends strongly on the edge-type TDDs.

Figure 10 shows the intensity of the PL peak at 255-nm measured at RT as a function of the FWHM of the XRC (10–12). Reducing the FWHM from 1400 to 500 arcsec increases the PL intensity by a factor of about 80. Between 500 and 800 arcsec the PL intensity increases rapidly. This rapid increase can be explained by a reduction in the non-radiative recombination rate as the distance between the TDs becomes greater compared with the carrier diffusion length in the QW. We obtained similar degrees of improvement for QWs operating at other wavelengths. The relationship between IQE and TDD in DUV AlGaN-QWs was also investigated in Ref. [27, 48].

The quaternary alloy InAlGaN is also a strong candidate as a material for realizing DUV LEDs, since the inclusion of In leads to efficient UV emission as well as higher hole concentration. Just a few percent of In in AlGaN is needed to obtain high IQE, where the increases in efficiency are due to the In segregation effect, an effect previously investigated for the ternary alloy, InGaN. We have described the advantages of the use of InAlGaN in Ref. [1, 11, 12, 14, 17].

We took up the challenge of developing the crystal growth technology needed to obtain high quality InAlGaN alloys operating at the 'sterilization' wavelength (280 nm) [17]. Growing crystals of InAlGaN with a high Al content is relatively difficult, because incorporating In becomes more challenging as the growth temperature is increased, which is necessary in order

polarization effects induced in the wells, very thin quantum well was used. It is important to obtain atomically smooth hetero-interfaces to achieve high IQE from such thin QWs. As shown in the cross-sectional TEM image in Figure 8, the three 1.3 nm-thick QWs have atomically-flat

Figure 9. Photoluminescence (PL) spectra of AlGaN QWs on ML-AlN templates with various FWHMs of the XRC (10–12)

Figure 8. Cross-sectional TEM image of the quantum well region of an AlGaN MQW DUV-LED.

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

hetero-interfaces.

measured at room temperature.

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 the growth rate from 0.05 to 0.03 μm/h.
