2. Research background of UV LEDs

The development of semiconductor light sources operating in the DUV region, such as DUV LEDs and laser diodes (LDs), is an important subject because these devices are required for a wide variety of applications. Figure 1 gives an overview of these applications, divided into three wavebands. Potential applications for UVC and UVB lights are in sterilization, water purification, medicine and biochemistry, agriculture, and as light sources for high-density optical memory. UVA together with UVB and UVC lights also have potential for curing, adhesives, printing and coating [1, 2].

disinfection. As shown in Figure 2, the wavelength range covered by AlGaN LEDs is from

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 129

The direct transition energy range of AlGaN covers the region from 6.2 eV (AlN) to 3.4 eV [2]. Figure 3 shows the bandgap of the wurtzite (WZ) AlInGaN material system, as well as, the lasing wavelengths of several kinds of gas lasers. AlGaN is a direct transition semiconductor having an emission wavelength range from 200 to 360 nm. Both p- and n-type conductivities are obtained in DUV wavelength range. AlGaN is physically hard and suitable for long lifetime devices. Also, the material is free from harmful elements, i.e., As, Hg and Pb. Therefore, AlGaN

Several research groups have started the research on AlGaN-based UV LEDs with wavelength below 360 nm, between 1996 and 1999 [3–5]. In the US, the effort, directed at DUV light sources, was driven by DARPA's Semiconductor Ultraviolet Optical Sources (SUVOS) program. The sub-300 nm DUV LEDs were achieved by a group at the University of South Carolina between 2002 and 2006 [6–8]. The shortest wavelength (210 nm) LED using an AlN emitting layer was reported by a group at NTT in 2006 [9]. We started research into AlGaNbased DUV LEDs in 1997, and reported the first efficient DUV (230 nm) photoluminescence (PL) from AlGaN/AlN QWs [10], and a 330 nm-band AlGaN-QW UV LED on SiC in 1999 [4]. We have also developed highly efficient UV LEDs by incorporating In into AlGaN [1, 11, 12]. We demonstrated cw operation with powers of several mWs for 340–350 nm InAlGaN-QW

The development of 260–280 nm AlGaN DUV LEDs performed in 2005–2010 was an important step in the progress toward sterilization applications. High IQEs in AlGaN and quaternary InAlGaN QWs were achieved in 2007 [15–17], by developing a low-threading dislocation density (TDD) AlN buffer layers on sapphire substrates utilizing a pulse-flow growth method. EIE was significantly increased by introducing a multi-quantum barrier (MQB) [18]. Wide

is considered to be the most appropriate semiconductor to develop a DUV LED [2].

Figure 2. Classification of UV light and the wavelength range achieved by AlGaN DUV LEDs.

UV LEDs on both GaN single-crystal substrates [13] and sapphire substrates [14].

UVA to UVC.

Figure 2 shows the wavelength range of UV light from UVA to UVC, and possible wavelength range of DUV LEDs developed by AlGaN. As well known, UVA light causes sunburn and UVB light is dangerous light, which causes skin cancer or cataracts. Indicated curve in Figure 2 with peak wavelength at 265 nm in UVC waveband is known as sterilization effects curve, which well matches to the absorption spectrum of DNA (deoxyribonucleic acid). The wavelength between 260 and 280 nm is effective for sterilization, water purification and surface

Figure 1. Potential applications of DUV LEDs and LDs.

Figure 2. Classification of UV light and the wavelength range achieved by AlGaN DUV LEDs.

AlN crystals on sapphire substrates, as well as, by improving electron injection efficiency (EIE)

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

In Section 2, the background to this research, including device applications, history and the current status of DUV LEDs, is described. In Section 3, we describe the development of the crystal growth techniques undertaken in order to obtain high-quality AlN and AlGaN crystals. The realization of devices with high IQE and fabrication of the LEDs are dealt with in Sections 4 and 5, respectively. We discuss ways in which EIE and LEE can be improved, and the future

The development of semiconductor light sources operating in the DUV region, such as DUV LEDs and laser diodes (LDs), is an important subject because these devices are required for a wide variety of applications. Figure 1 gives an overview of these applications, divided into three wavebands. Potential applications for UVC and UVB lights are in sterilization, water purification, medicine and biochemistry, agriculture, and as light sources for high-density optical memory. UVA together with UVB and UVC lights also have potential for curing,

Figure 2 shows the wavelength range of UV light from UVA to UVC, and possible wavelength range of DUV LEDs developed by AlGaN. As well known, UVA light causes sunburn and UVB light is dangerous light, which causes skin cancer or cataracts. Indicated curve in Figure 2 with peak wavelength at 265 nm in UVC waveband is known as sterilization effects curve, which well matches to the absorption spectrum of DNA (deoxyribonucleic acid). The wavelength between 260 and 280 nm is effective for sterilization, water purification and surface

and light extraction efficiency (LEE).

prospects for DUV LEDs, in Sections 6, 7 and 8, respectively.

2. Research background of UV LEDs

adhesives, printing and coating [1, 2].

Figure 1. Potential applications of DUV LEDs and LDs.

disinfection. As shown in Figure 2, the wavelength range covered by AlGaN LEDs is from UVA to UVC.

The direct transition energy range of AlGaN covers the region from 6.2 eV (AlN) to 3.4 eV [2]. Figure 3 shows the bandgap of the wurtzite (WZ) AlInGaN material system, as well as, the lasing wavelengths of several kinds of gas lasers. AlGaN is a direct transition semiconductor having an emission wavelength range from 200 to 360 nm. Both p- and n-type conductivities are obtained in DUV wavelength range. AlGaN is physically hard and suitable for long lifetime devices. Also, the material is free from harmful elements, i.e., As, Hg and Pb. Therefore, AlGaN is considered to be the most appropriate semiconductor to develop a DUV LED [2].

Several research groups have started the research on AlGaN-based UV LEDs with wavelength below 360 nm, between 1996 and 1999 [3–5]. In the US, the effort, directed at DUV light sources, was driven by DARPA's Semiconductor Ultraviolet Optical Sources (SUVOS) program. The sub-300 nm DUV LEDs were achieved by a group at the University of South Carolina between 2002 and 2006 [6–8]. The shortest wavelength (210 nm) LED using an AlN emitting layer was reported by a group at NTT in 2006 [9]. We started research into AlGaNbased DUV LEDs in 1997, and reported the first efficient DUV (230 nm) photoluminescence (PL) from AlGaN/AlN QWs [10], and a 330 nm-band AlGaN-QW UV LED on SiC in 1999 [4]. We have also developed highly efficient UV LEDs by incorporating In into AlGaN [1, 11, 12]. We demonstrated cw operation with powers of several mWs for 340–350 nm InAlGaN-QW UV LEDs on both GaN single-crystal substrates [13] and sapphire substrates [14].

The development of 260–280 nm AlGaN DUV LEDs performed in 2005–2010 was an important step in the progress toward sterilization applications. High IQEs in AlGaN and quaternary InAlGaN QWs were achieved in 2007 [15–17], by developing a low-threading dislocation density (TDD) AlN buffer layers on sapphire substrates utilizing a pulse-flow growth method. EIE was significantly increased by introducing a multi-quantum barrier (MQB) [18]. Wide

University of Berlin recently carried out a series of studies on the properties of AlGaN epilayers and AlGaN and InAlGaN UV LEDs [2, 43–45]. The reported EQEs for AlGaN and

In spite of continuous efforts to develop an AlGaN DUV LED, its wall plug efficiency (WPE) is still as low as 3%, which is much lower than that of InGaN blue LEDs. The limited efficiency of

For InGaN QWs, high IQE more than 80% was already demonstrated. On the other hand, IQE at

sapphire. We need to develop further reduction of TDD of AlN, i.e. TDD of below 1108 cm<sup>2</sup>

in order to achieve more than 80% IQE. AlN single crystal wafers have advantages for high IQE, although they are expensive for commercially available DUV LEDs. The hole concentration of p-AlGaN used in UVC LEDs is as low as 11014 cm<sup>3</sup> owing to its deep acceptor levels, i.e., 240 (GaN)–590 meV (AlN). Electron overflow to the p-side layers results in the reduction of EIE for UVC LEDs. Since the hole density of p-type AlGaN is not very high, we use p-GaN for the contact layer. This results in a significant reduction in LEE, typically to below 8%, owing to

The usual value of EQE for 270 nm UVC LEDs obtained by our group is approximately 7%, which is determined by the IQE, EIE, and LEE of approximately 60, 80, and 15%, respectively. The technical issues to increase IQE, EIE and LEE are described in the following sections.

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

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

) AlN templates on

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

,

131

InAlGaN UVA-UVC LEDs up to 2015 are summarized in Figure 1.1 of Ref. [2].

1. The IQE of AlGaN is sensitive to TDD and still much lower than that of InGaN.

3. By the light absorption by p-GaN contact layer, the LEE is quite low.

around 50% is standard value after developing the low-TDD (5 108 cm<sup>2</sup>

3. Growth of high-quality AlN on sapphire substrates

sequence using pulsed and continuous gas flows.

2. Hole concentration of p-AlGaN is low and the carrier injection efficiency (IE) is low.

DUV LED is mainly due to the following three factors:

the strong absorption of DUV light.

Figure 3. Relationship between the direct transition bandgap energy and the lattice constant of the wurtzite (WZ) InAlGaN material system and the lasing wavelengths of various gas lasers.

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

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

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 University of Berlin recently carried out a series of studies on the properties of AlGaN epilayers and AlGaN and InAlGaN UV LEDs [2, 43–45]. The reported EQEs for AlGaN and InAlGaN UVA-UVC LEDs up to 2015 are summarized in Figure 1.1 of Ref. [2].

In spite of continuous efforts to develop an AlGaN DUV LED, its wall plug efficiency (WPE) is still as low as 3%, which is much lower than that of InGaN blue LEDs. The limited efficiency of DUV LED is mainly due to the following three factors:


For InGaN QWs, high IQE more than 80% was already demonstrated. On the other hand, IQE at around 50% is standard value after developing the low-TDD (5 108 cm<sup>2</sup> ) AlN templates on sapphire. We need to develop further reduction of TDD of AlN, i.e. TDD of below 1108 cm<sup>2</sup> , in order to achieve more than 80% IQE. AlN single crystal wafers have advantages for high IQE, although they are expensive for commercially available DUV LEDs. The hole concentration of p-AlGaN used in UVC LEDs is as low as 11014 cm<sup>3</sup> owing to its deep acceptor levels, i.e., 240 (GaN)–590 meV (AlN). Electron overflow to the p-side layers results in the reduction of EIE for UVC LEDs. Since the hole density of p-type AlGaN is not very high, we use p-GaN for the contact layer. This results in a significant reduction in LEE, typically to below 8%, owing to the strong absorption of DUV light.

The usual value of EQE for 270 nm UVC LEDs obtained by our group is approximately 7%, which is determined by the IQE, EIE, and LEE of approximately 60, 80, and 15%, respectively. The technical issues to increase IQE, EIE and LEE are described in the following sections.
