Ultraviolet quantum well lasers

Ultraviolet quantum well lasers

CHAPTER Ultraviolet quantum well lasers 6 Che-Hao Liao, Haiding Sun, Xiaohang Li King Abdullah University of Science and Technology (KAUST), Advanc...

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CHAPTER

Ultraviolet quantum well lasers

6

Che-Hao Liao, Haiding Sun, Xiaohang Li King Abdullah University of Science and Technology (KAUST), Advanced Semiconductor Laboratory, Thuwal, Saudi Arabia

1. Ultraviolet quantum well lasers-UVA and UVB Wide bandgap III-nitride semiconductors have attracted great attention due to the numerous applications in ultraviolet (UV) range such as bio-chemical analysis and material processing. Ultraviolet photonic devices such as photodetectors, light-emitting diodes (LEDs), and laser diodes (LDs) have attracted considerable interest recently because of their potential applications including air and water purification, disinfection of medical tools, medical diagnostics, dermatology, bio-agent detection, critical communications, high-density optical storage, currency screening, UV curing and photolithography as alternatives to the low-efficiency, toxic, bulky mercury lamps and gas lasers [1e3]. Each application requires different wavelength range of UV light [4,5] and typically the UV spectrum is divided into three bands, they are UVA band (400e320 nm), UVB band (320e280 nm), and UVC band (<280 nm), respectively [6]. In the UVA region, many applications have been identified. Within the wavelength region that longer than 365 nm, laser diodes are already commercialized with reasonable performance. Such kind of devices can be used as a phosphor pumping sources for a broad-spectrum emission to produce white light, which also can be used as the light sources for inducing fluorescence in the detection of counterfeit note. Another application in the industry is UV curing for UV glue or coating process, and those UV devices serve as ultraviolet light sources and possess the advantages of less heat production and longer lifetime. The applications below the wavelength 365 nm region are in a large variety of biomedical systems. Biomolecules such as elastin, NADH and collagens have strong absorption in this spectrum range, which can be used for indicating the presence of molecules by inducing fluorescence. Moreover, a source with emission wavelength of 340 nm is required for typical immunoassay analyzer [7]. In the UVB region, besides bio-fluorescence applications, UV LEDs and LDs have potential for skin condition treatment, such as psoriasis. Research shows that broadband UVB in the treatment of psoriasis is less effective and the risk of sunburn and erythema of the non-affected skin is higher. In order to provide an effective treatment without any damaging from the short

Nanoscale Semiconductor Lasers. https://doi.org/10.1016/B978-0-12-814162-5.00006-6 Copyright © 2019 Elsevier Inc. All rights reserved.

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wavelength exposure, UV LDs, being inherently narrow band light sources, would be the ideal choice in such an application [6,7].

1.1 AlGaN-based ultraviolet LDs e GaN QW and AlGaN QW active layer The UV LDs research began in the 1970s [8]. Optically pumped stimulated emission at low temperature (2 K) from GaN single-crystal needle was first observed in 1971, and its low threshold power (0.3 MW/cm2) and a high gain (105 cm1) indicated a great potential for laser fabrication [9]. Stimulated emission at room temperature from an optically pumped GaN film was then first observed in 1990, with an optical pump power threshold of 0.7 MW/cm at emission wavelength around 370 nm2 [10,11]. The first GaN/AlGaN double heterostructures (DHs) stimulated emission with optically pumped at room temperature were realized in 1993, with an optical pump power threshold of 0.1 MW/cm2 at a wavelength of 368.2 nm [12]. After that, the GaN/AlGaN separate confinement heterostructures (SCHs) single quantum well with optically pumped stimulated emission at RT was realized in 1996, with a strong UVA emission at 365 nm and low optical pump power threshold around 90 kW/cm2 [13]. However, the electric pumped UV LDs encountered a lot of challenges. The lasing by current injection from binary GaN active layer was not observed until 2001. The GaN single quantum-well (SQW) LDs emission under pulsed current injection at wavelength of 366.9 nm and under continuous-wave (CW) operation at wavelength of 369.0 nm (output power of 2 mW) at room temperature has been achieved by S. Nagahama et al. [14]. In 2003, Kneissl et al. demonstrated UV emission at a wavelength around 360 nm from AlGaN multiple-quantum-well (MQW) LDs under pulsed current injection with low threshold current density (23 kA/ cm2) at RT [15]. In 2004, Akasaki et al. obtained partially low-dislocation density and crack-free GaN/AlGaN MQW on a grooved GaN template by using the combination of heteroepitaxial lateral overgrowth and low temperature AlN (LT-AlN) interlayer deposition as show in Fig. 6.1A and B [16,17]. Based on this technique,

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p-GaN SiO p-AI Ga N cladding layer p-AI Ga N blocking layer i-AI Ga N guiding layer GaN/AI

Ga

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FIG. 6.1 (A) Schematic structure of GaN/AlGaN MQW UV LD grown on a grooved GaN template. (B) Cross-sectional SEM images of heteroepitaxial lateral overgrowth n-AlGaN layer and the UV LD.

1. Ultraviolet quantum well lasers-UVA and UVB

they reported the lasing under pulsed current injection with an emission wavelength of 350.9 nm at RT [18]. In 2007, Yoshida successfully achieved GaN/AlGaN MQW UV LDs crack-free on entirely 2-in. sapphire substrate by using a hetero-facet-controlled ELOG (hetero-FACELOG) technique [19,20]. After the deposition of a LT buffer layer on sapphire substrate, a GaN layer about 2.5-mm-thick was grown at growth temperature of 1050  C. Triangular GaN seed crystals were selectively grown at 900  C with a height of 6 mm by the facet-controlled epitaxy growth on GaN layer, which was periodically patterned mask by 3 mm-width SiO2 stripes along the h1100i axis. The AlGaN overgrown-layer was laterally overgrown at 1150  C subsequently on the GaN facets. Finally, GaN/AlGaN MQW UV LDs were grown on the overgrown AlGaN layer. The UV LDs lased under pulsed current operation in the emission wavelength range from 361.6 to 355.4 nm at RT [19]. By optimizing this hetero-FACELOG technique, Yoshida et al. succeeded in pushing the lasing wavelength further toward a shorter wavelength region. The AlGaN MQW UV LDs lasing by pulsed electrical pumped at a wavelength of 342 nm and even shorter emission at 336 nm were demonstrated in 2008 [21,22]. Fig. 6.2A shows the UV laser diode structure comprised of a Al0.14Ga0.86N guiding layer (GDL) with thickness of 90 nm, Al0.04Ga0.96N/Al0.14Ga0.86N MQWs, and a 120 nm-thick Al0.14Ga0.86N GDL for 342 nm UV LD. In order to push further toward a shorter lasing wavelength (to 336 nm), the AlN mole fractions were increased by 2% in MQW and each GDL compared to those layers in 342 nm UV LD as shown in Fig. 6.2B. The external quantum efficiency (IQE) of the 336 nm AlGaN MQW UV laser is about 1.1% and the peak output power is 3 mW [22]. Fig. 6.2C shows a series of lasing spectra of UV LDs with different MQW design including GaN/AlGaN MQW and AlGaN MQW. For GaN/AlGaN MQW UV LDs the lasing wavelengths are 359.6 and 354.4 nm and for AlGaN MQW UV LDs the lasing wavelengths are 342.3, 339.5 and 336.0 nm, all UV LDs were operated at room temperature under the pulsed-current mode [23]. CTL (B) p-AI p-GaN Ga N CLL

0.3 0.7 p-AIGaN EBL GDL MQWs GDL n-AI0.3Ga0.7N CLL n-AI0.3Ga0.7N CTL AI0.3Ga0.7N SiO GaN Buffer layer Sapphire substrate

This work 336 nm UV-AII LD

Previous work 342 nm UV-AI LD

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AI0.16Ga0.84N AI0.14Ga0.86N AI0.06Ga0.94N AI0.04Ga0.96N AI0.16Ga0.84N AI0.14Ga0.86N

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FIG. 6.2 (A) Schematic structure of AlGaN MQW UV LD using hetero-FACELOG. (B) Schematic structures of an AlGaN MQW UV LDs lasing at wavelengths of 336 and 342 nm. (C) A series of lasing spectra of UV LDs consist of GaN/AlGaN MQW and AlGaN MQW.

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1.2 High power AlGaN-based ultraviolet LDs The pulsed operation of GaN/AlGaN MQW UV LD with a vertical structure has been demonstrated by Aoki et al. in 2015. The vertical UV laser diode was grown on c-plane bulk GaN substrate with vertical current path as shows in Fig. 6.3A. The lasing from the one side of uncoated facet with an emission wavelength of 356.6 nm, and the output power peak up to 10 mW under pulsed current operation at room temperature (with a pulse width of 10 ns and a repetition rate of 5 kHz). Fig. 6.3B shows the vertical far-field pattern (FFP) and horizontal FFP of the UV LD on GaN substrate. The FFPs of similar UV device grown on sapphire substrate by using the epitaxial lateral overgrowth (ELO) are also shown in Fig. 6.3C for comparison. In contrast with both vertical and horizontal FFPs of the UV LD grown on GaN substrate shown Gaussian-like shapes, both distorted FFPs of the UV device on sapphire substrate can be observed. It is expected that the GaN bulk substrate could provide not only a beneficial thermal management solution when comparing to the sapphire substrate but also a cleaved mirror facet leading to a better far-field pattern. The experiment results of Aoki et al. have confirmed and showed that UV LD on GaN bulk substrate has better non-distorted FFPs than that on sapphire substrate. It is believed that the interference between direct and reflected beam caused the deteriorating of both vertical and horizontal FFPs from UV device on sapphire substrate, which reflected beam is from the etched mirror facet with the residual terrace underneath [24]. In 2016, Taketomi et al. have successfully achieved a peak power over 1 W in 338.6 nm of AlGaN MQW UV LD at room temperature under pulsed operation (with 4 ns pulse duration and 5 kHz repetition rate), which is the highest power that AlGaN-based UV-LDs ever obtained. Fig. 6.4A shows the high power UV laser diode structure which grown on c-plane GaN bulk substrate. The cavity length and

FIG. 6.3 (A) Schematic structure of a vertical GaN/AlGaN MQW UV LD on GaN bulk substrate. (B) FFPs of UV LD on GaN bulk substrate with the cleaved mirror facets. (C) FFPs of UV LD on sapphire substrate with the etched mirror facets. Insets are the photos of their far-field patterns (FFPs).

1. Ultraviolet quantum well lasers-UVA and UVB

FIG. 6.4 (A) Schematic structure of AlGaN MQW UV LD on c-plane GaN bulk substrate. (B) LeI curve of the high-power UV LD under pulsed operation at RT. (C) Lasing spectra of the high-power UV LD under different pulsed current injection at RT.

ridge width of the vertical conductive structure and broad-area of the UV-LD were 600 and 50 mm. The differential external quantum efficiency (EQE) of the UV LD was evaluated about 8.5% and the threshold current density of 38.9 kA/cm2 can be estimated from Fig. 6.4B. The UV laser diode at 338.6 nm lasing wavelength under pulsed injection current of 12A at RT with light output power over than 1 W has been obtained as show in Fig. 6.4C. The vertical device structure created a new way toward the high power UV Laser diodes [25].

1.3 AlInGaN-based ultraviolet LDs e InGaN QW and AlInGaN QW active layer Relationship between the lattice constant and the energy bandgap of quaternary InAlGaN material system are show in Fig. 6.5A, meanwhile, various kinds of gas lasers are also listed according to their lasing wavelengths [3]. InAlGaN material system with wide UV emission range covers the UV lasing spectra of various solid-state and gases lasers. In 2001, Nagahama et al. demonstrated the quaternary active layer AlxInyGa(1xy)N UV LDs grown on epitaxial lateral overgrown GaN (ELOG) template by MOCVD. They studied the dependence of Al and In mole fractions and laser diodes optical characteristics in UV emission region as show in Fig. 6.5B and C. The Al0.03In0.02Ga0.95N single quantum well (SQW) UV LD (In content about 2%) emission a wavelength of 366.4 nm at room temperature under pulsed current injection. In contrast to the Al0.03In0.03Ga0.94N SQW UV LD (In content around 3%) was demonstrated continuous-wave (CW) operation under room temperature (25  C) [26]. They also investigated the Laser diode characteristics of three active layer types of UV LDs: (1) InGaN ternary SQW (2) AlInGaN quaternary SQW and (3) GaN binary SQW structures. Active layers of InGaN LDs (382 nm) and GaN LDs (369 nm) were demonstrated the CW operation, while Al0.03In0.02Ga0.95N LDs (366.4 nm) were demonstrated the pulsed-current operation

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FIG. 6.5 (A) Lattice constant and energy bandgap of AlInGaN material system and the lasing wavelengths of various gas lasers. (B) Emission wavelength and threshold current density of AlInGaN SQW LDs with respect to the mole fraction of Al and (C) the mole fraction of In.

at RT [27]. The estimated lifetime of AlInGaN UV LD is around 500 h under CW operation at RT with 2 mW output power. In 2003, Kneissl et al. realized the electrical pumping UV InGaN/AlInGaN MQW LDs under continuous-wave operation. The UV LD device lasing at a wavelength of 373.5 nm under CW operation with an output powers over 1 mW [28]. At the same time, Masui et al. demonstrated the AlInGaN SQW UV LDs with CW operation in 365 nm at room temperature. The lifetime of the UV LDs was estimated about 2000 h at 3 mW output power [29]. In 2004, optically pumped AlInGaN separate confinement heterostructure (SCH) MQW UV laser diodes at 340 nm in lasing wavelength with a threshold peak power about 800 kW/cm2 were demonstrated and characterized by He et al. [30]. Recently, the effect in optical performance by simulation of AlInGaN MQW UV LDs with a polarization-matched AlInGaN electron blocking layer (EBL) was investigated. The polarizationmatched Al0.25In0.08Ga0.67N electron blocking layer was introduced into UV laser diode for reducing the polarization effect in the active region. The results revealed that such UV LDs with AlInGaN EBL possess higher optical power intensity and lower threshold current than those with AlGaN EBL [31].

1.4 High power InGaN-based ultraviolet LDs III-nitride based Laser diodes grown on sapphire substrates usually suffered high threading dislocation density, around 108-1010 cm2, which resulted in short lifetime even under the low power CW operation still below a few hours. By introducing the ELOG technology which is epitaxial lateral overgrown GaN on sapphire substrate, the nitride-based LDs’ lifetime was dramatically improved [32]. However, the UV LDs is difficult to fabricate with a shorter lasing wavelength because higher Al mole fraction induced more dislocations and cracks in the epi-layer by AlGaN and GaN lattice coefficient mismatched. In 2006, Kozaki et al. successfully

1. Ultraviolet quantum well lasers-UVA and UVB

FIG. 6.6 (A) Schematic structure of basic high-power GaN-based LD on n-GaN free-standing substrate. (B) Lasing spectra of InGaN/AlGaN SQW UV LDs under CW operation at 20 mW. (C) Light output powerecurrent (LeI) and voltageecurrent (IeV) curves of UV LDs under CW operation at room temperature.

fabricated a high-power InGaN/AlGaN UV LD lasing at 375 nm in wavelength by applying n-type GaN free-standing substrate, a basic device structure of high-power nitride-based LD on GaN free-standing substrate as show in Fig. 6.6A. The active layer of this high-power UV LD consisted of InGaN single quantum well (SQW) and AlGaN quantum barrier. The laser cavity of InGaN/AlGaN SQW UV LD was formed by cleaving along the m-plane (1100) facet, and its front facet was coated with low-reflectivity film as well as the rear facet was coated with high-reflectivity films. Fig. 6.6B shows the InGaN/AlGaN SQW UV LD lasing spectra around 375 nm under room temperature CW operation at an output power of 20 mW. The voltageecurrent (IeV) and light output powerecurrent (LeI) curves of UV LDs under CW operation at room temperature is show in Fig. 6.6C. The lifetime test result of this UV LDs was estimated a lifetime longer than 10,000 h [33]. Pulsed current-injected operation InGaN/AlInGaN MQW UV LDs were demonstrated on bulk AlN substrates with low dislocation density by Kneissl et al. in 2007. The AlInGaN heterostructures were grown by MOCVD as show in Fig. 6.7A. By

FIG. 6.7 (A) Schematic structure of InGaN/InAlGaN UV LD grown on bulk AlN substrate. (B) TM and TE emission spectra of UV LD above threshold at room temperature. (C) Pulsed room-temperature LI curve for UV LD emitting at 371 nm.

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FIG. 6.8 (A) Schematic structure of the UV LD on (20-21) GaN substrate. (B) Symmetric on-axis RSM of the UV LD. X-ray beam was incident along the [10-14] c-axis projection. (C) Schematic diagram of reciprocal space lattice illustrating the effect of epi-layer tilt and partial relaxation on the structure.

using bulk AlN substrate with an off-axis orientation less than 0.5 , a smooth surface morphology can be obtained. Fig. 6.7B and C show the UV LD lasing wavelength around 370 nm under pulsed current operation with a low threshold current density about 13 kA/cm2 and an output power up to 300 mW (with 6.7% quantum efficiency) was achieved. The InGaN/AlInGaN MQW UV LDs grown on bulk AlN substrates has demonstrated the feasibility to realize the nitride laser diodes for the UV spectral region [34].

1.5 Semipolar plane ultraviolet LDs e (20-21) QW active layer In 2012, Haeger et al. demonstrated an electrically injected semi-polar (20-21) InGaN/AlGaN UV laser diode grown by low pressure MOCVD on n-type AlGaN buffer layer which is relaxed as show in Fig. 6.8A. The stresses are relaxed at the n-AlGaN/GaN interface by misfit dislocations which were generated from the glide of threading dislocations (TDs) propagated along the basal plane (0001). Defects are confined at the n-AlGaN/GaN interface leads to the capability of the following high Al composition films growth with low TDs densities which are less than 108 cm2. The semi-polar InGaN/AlGaN UV LD lasing at 384 nm and its threshold current density is 15.7 kA/cm2. The reciprocal space mapping (RSM) as Fig. 6.8B and C show that each epi-layer of UV LD was coherent to the relaxed n-AlGaN buffer layer and identify the only relaxation happened at the n-AlGaN/GaN interface without any secondary relaxation at other interfaces (active region or p-AlGaN). UV Laser diodes grown on semi-polar with a n-AlGaN buffer relaxed provide an alternative choice of template to grow [35]. In 2013, Sawicka et al. also demonstrated InGaN MQW UV LDs lasing at 388 nm which was grown on semi-polar (20-21) GaN substrates by PA-MBE, plasma-assisted molecular beam epitaxy, under metal-rich condition. After growth, a smooth surface morphology with atomically flat was demonstrated, and the excellent crystal quality was corroborated by TEM, transmission electron microscopy.

2. Ultraviolet quantum well lasers-UVC

The semi-polar (20-21) InGaN MQW UV LD threshold current density is 7.2 kA/ cm2 and the threshold voltage is 10.8 V at room temperature [36].

1.6 Summary At present, the development of UV laser diodes is moving toward shorter wavelengths. However, it is also more difficult to achieve electrical pumping whether pulsed-current stimulated or continuous-wave operation for UV LDs. Many challenges must be overcome to achieve high-performance UV LDs. First, the high densities of defects and dislocations in the quantum-wells (active region) of UV LDs which will decrease their internal quantum efficiency (IQE), resulting in low external quantum efficiency (EQE). Appropriate substrates with high transparency, high electrical and thermal conductivities are required to improve the UV laser diodes performance. Second, the difficulty involved in p-type doping of p-AlGaN layer will reduce the hole injection efficiency and increase the series resistance, which leads to an increased threshold and reduced efficiency. Additionally, the device fabrication processes such as etching, thinning, and cleaving will increase losses and reduce the efficiency of UV LDs. Finally, a suitable laser structure design is required to improve the efficiency of UV laser diodes, such as vertical current path design for high power UV LDs.

2. Ultraviolet quantum well lasers-UVC Ultraviolet C (UVC) lasers, emitting at a wavelength of 200e280 nm, are crucial technology that can be applied for strategic areas such as non-line-of-sight optical communications, biochemical detection/sensing, material processing (UV curing), and air/water sterilization. Nowadays, the primary UVC sources are gas lasers (excimer laser) and solid-state lasers. Other methods of producing lasers emitting in the UVC spectrum include frequency conversions that are implemented by nonlinear optical processes, such as second, third, fourth, and fifth harmonic generation and sum frequency generation. Although high optical output power can be obtained at short wavelengths using these laser sources, they possess limited portability and reliability, require high power consumption, and utilize harmful constituents. The semiconductor UVC lasers are compact, portable, reliable, and harmless devices. The high Al-content AlGaN alloys have been recognized as one of the most promising material systems to fulfill this promise which possesses large optical bandgap, chemical resistance, and high-temperature capabilities [37]. However, the development of such semiconductor lasers has been lagging behind mainly because of a series of materials challenges facing in the AlGaN alloys as the wavelength moves shorter. First of all, with increasing the Al content, the AlGaN alloys tend to become more defective due to the low mobility of Al atoms which leads to a smaller diffusion length during the epitaxial growth, creating large amount of defects in the epilayers [38]. Those defects or dislocations will lead to large leakage

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of optical modes in the active region as well as large leakage current which may damage the device operation. Second, it is still challenging to dope Al-rich AlGaN films with Mg-acceptors. Thus at this moment, the AlGaN with Al content more than 50% Al is quite resistive because of high ionization energy for the Mg-acceptors [39]. Lastly, the UVC laser structure has to be carefully designed because of the complexity of wave guiding layer due to the extreme small difference of refractive index between Al-rich AlGaN layers at such short wavelength [37]. Thus, up until now, the current-injected AlGaN thin-film UVC laser has yet to be demonstrated (below 300 nm). Based on pumping sources, there are mainly three approaches to generate UVC semiconductor laser: 1. Optically-pumped: It is the most straightforward way to get a UVC laser. Many groups around the world have demonstrated such semiconductor lasers emitting at sub 280 nm by using a 193 nm or 248 nm excimer laser [40e49]. However, it is impractical for actual application due to large size of the excimer laser. 2. Electrically-pumped: UVC semiconductor lasers have been demonstrated in the form of AlGaN nanowires [50e52]. However, the output powers of those random nanowire lasers are extremely small. The inefficient p-type doping for Al-rich AlGaN alloy, high-quality epilayer and proper waveguiding design are the primary challenges that hamper the development of high power electricallypumped UVC lasers. 3. Electron beam (E-beam) pumped: In such laser configuration, the electron-hole pair is generated by the energy transfer of high energy electrons emitted from an e-gun. A direct benefit of this approach is that both n- and p-type doping are not required high Al-content AlGaN films, alleviating the problems associated with carrier injection. The high power green, blue and infrared lasers pumped by ebeam have already been demonstrated [53,54]. The success of these devices demonstrates great potentials of achieving e-beam pumped high-power UVC laser. In this context, the recent exploration of AlGaN-based UVC lasers will be described, mainly focusing on two types of such lasers: (1) optically pumped thin film lasers on sapphire [40e43], bulk AlN substrate [44e46], and SiC [47,48]; (2) electrically pumped nanowire lasers on Si substrate [50e52]. Lastly, the potential applications of UVC laser will be elaborated.

2.1 UVC laser on sapphire Sapphire is a most common substrate for growing III-nitride devices, including LED and laser. Li et al. reported different optically-pumped Al-rich AlxGa1xN/AlyGa1yN multiple quantum well (MQW) UVC lasers grown by MOCVD on sapphire. The lowest thresholds were obtained from the lasers emitting at 249 nm and 256 nm [55]. The laser structures contain a AlN/sapphire template [56,57] followed by a AlxGa1xN grading waveguide layer, five to ten periods of AlxGa1xN/

2. Ultraviolet quantum well lasers-UVC

AlyGa1yN MQWs and a thin AlyGa1yN cap layer sequentially with smooth surface which is close to the lasers grown on AlN substrates [49]. The Fabry-Perot cavities were formed by cleaving and the lengths were in the range of 1.0e2.5 mm. Later, an ArF excimer laser (193 nm) was used to carry out the optically pumping studies at room temperature [55]. Fig. 6.9A shows a room temperature (RT) UVC edge-emitting laser operating at 249 nm on sapphire. Photoluminescence (PL) spectra of the laser with a cavity length of 1.7 mm under different pumping power densities are presented. In Fig. 6.9B, the intensities of PL spectral under different pumping power density demonstrated a threshold of w90 kW/cm2 and the spectral linewidth decreases with increasing pumping power density and reaches w1.6 nm after lasing. Additionally, a laser with a wavelength of 256 nm was also shown with a small threshold power of w61 kW/cm2 [55]. Furthermore, the light polarization of the laser emitting at 249 nm was also measured. Fig. 6.9C indicates the transverse electric (TE) and transverse magnetic (TM) emission spectra operating at pumping power densities about three times of the respective thresholds. This laser is strongly TE-polarized

FIG. 6.9 (A) Laser emission spectra under different pumping power. (B) The integrated spectral intensity and spectral linewidth versus pumping power density. (C) The laser emission spectra of the TE mode and TM mode of the 249-nm laser. Adapted with permission from Li XH, Detchprohm T, Kao T, Satter MM, Shen S, Yoder PD, Dupuis RD, Wang S, Wei YO, Xie H, Fischer AM, Ponce FA, Wernicke T, Reich C, Martens M, Kneissl M. Low-threshold stimulated emission at 249 nm and 256 nm from AlGaN-based multiple-quantum-well lasers grown on sapphire substrates. Applied Physics Letters 2014;105:141106.

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with the degree of polarization (P), defined as P ¼ (ITE  ITM)/(ITE þ ITM), is close to w1. The TE-dominant emission is particularly favorable for the application of vertical-external-cavity surface-emitting-laser (VECSEL) because TE polarized emission tends to propagate in a direction normal to the c-axis of the film therefore can be effectively confined in the active region by the distributed Bragg reflector (DBR). There is a unique optical property in the AlGaN alloy which will be addressed below. Unlike InGaN alloy, a study has shown that ternary AlGaN alloy tends to emit light in TM mode when the Al content increases in the active region [58]. The TE to TM transition is attributed to the reason that GaN and AlN have an opposite sign in the crystal field splitting, þ38 meV for GaN and 219 meV for AlN, respectively [59,60]. As a result, the order of the valence bands is different between AlN and GaN (G7 vs. G9), as shown in Fig. 6.10 with x ¼ 0, 0.25, and 1 in the AlxGa1xN alloy. This reversal ordering of the valence bands in AlGaN alloy due to the increase of Al-content implement certain selection rules in the optical transitions between conduction and valence band. Specifically, in GaN layer, the light is mainly TE polarized with the electric field vector E t c-direction. However, TM polarized is promoted when the light is emitted in AlN with E ǁ c [56,57]. As a result, the existence of such unique optical behavior in the AlGaN materials imposes additional challenge to the design, growth and fabrication of UVC surface emitting laser diode. However, this favorable TM-polarized light could be beneficial in the development of edge emitting UVC lasers. The transition from TE to TM mode occurs when the wavelength moves shorter. Recently, the TM-dominant optically-pumped AlGaN lasers [41] emit at 239, 242, and 243 nm with very low thresholds of 280, 250, and 290 kW/cm2 are reported, as shown in Fig. 6.11AeC, respectively. All three lasers are TM-polarized based on the polarization study. The peak position difference between the TE-polarized and TM-

FIG. 6.10 The band structure near the G point of AlxGa1xN alloys for (A) x ¼ 0, (B) x ¼ 0.25, and (C) x ¼ 1. Adapted with permission from Li X, Wei YO, Wang S, Xie H, Kao TT, Satter M, Shen SC, Yoder PD, Detchprohm T, Dupuis RD, Fischer A, Ponce FA. Temperature dependence of crystalline quality of AlN layer grown on sapphire substrate by metalorganic chemical vapor deposition. Journal of Crystal Growth 2015;414:76e78.

2. Ultraviolet quantum well lasers-UVC

FIG. 6.11 Room temperature power dependent PL spectra of the (A) 239-nm, (B) 242-nm, and (C) 243-nm lasers. The insets show the respective light output intensity of stimulated emission as a function of excitation power density and their TE- and TM-polarized spectra above the respective threshold, showing TM-dominant stimulated emission for all three lasers. Adapted with permission from Li XH, Kao T, Satter MM, Wei YO, Wang S, Xie H, Shen S, Yoder PD, Fischer AM, Ponce FA, Detchprohm T, Dupuis RD. Demonstration of transverse-magnetic deep-ultraviolet stimulated emission from AlGaN multiple-quantum-well lasers grown on a sapphire substrate. Applied Physics Letters 2015;106:041115.

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Surface emission λpump: 193nm T: 300K

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500 1000 1500 Pumping power density (kW/cm2)

FIG. 6.12 (A) Surface emission spectra under different pumping power. (B) The light output intensity of surface emission as a function of pumping power density. Adapted with permission from Nam KB, Li J, Nakarmi ML, Lin LY, Jiang HX. Unique optical properties of AlGaN alloys and related ultraviolet emitters. Applied Physics Letters 2004;84:5264.

polarized emissions were nearly zero among the three lasers which indicates that the crossover of crystal-field split-off hole and heavy-hole valence bands [41]. Furthermore, Li et al. have achieved the first optically pumped UVC surface stimulated emission (parallel to the c-axis) from Al-rich AlGaN heterostructures grown on sapphire [61]. As shown in Fig. 6.12A, the value of full width at half maximum (FWHM) is 12 nm when the pumping power is low. When the pumping power is above a certain threshold (630 kW/cm2 in this case), the peak of the amplified spontaneous emission at l ¼ 257 nm with a small FWHM of 2 nm dominates the emission. The output optical power also transitioned from linear increase to super-linear increase with the pumping power (Fig. 6.12B). The UVC surface stimulated emission indicated high optical gain can be achieved with the AlGaN heterostructures. Surface stimulated emission can be deemed as a pre-effect of the surface emitting laser. Recently, Kao et al. have also demonstrated the first high reflectivity dielectric DBR for UVC lasers [62]. The reflectivity of the HfO2/SiO2 dielectric mirrors was larger than 92% and a stop-band of 60 nm was reported, as shown in Fig. 6.13A. The laser emits at 249 nm and it is predominately TE-polarized. The intensity of the light output as a function of the pumping power for the lasers with six and five pairs of mirrors together the one has no mirror are plotted in Fig. 6.13B. As they expected, the thresholds of the laser decrease from 250 to 180 kW/cm2 by deploying high-reflectivity mirrors. The internal loss of 2 cm1 and threshold gain of 10.9 cm1 is found. So far, both edge and surface emitting lasers on sapphires have been demonstrated via excimer laser pumping. In fact, RT lasing at UVC wavelength down to 214 nm has already been reported on a patterned sapphire substrate by pulsed lateral epitaxial overgrowth process in MOCVD [63].

2. Ultraviolet quantum well lasers-UVC

FIG. 6.13 (A) The room temperature power-dependent PL spectra of optically-pumped laser with dielectric mirrors coated on laser facets and the reflectivity spectra of 6- and 5-pairs HfO2/ SiO2 dielectric DBR mirrors. (B) Light-output intensity as a function of the optical pumping power density for the AlGaN-based MQW laser without the facet mirror coating (hollow circle in blue), after the rear-side mirror coating (solid square in red) and after the doublesided facet coating (hollow triangle in green). Adapted with permission from Northrup JE, Chua CL, Yang Z, Wunderer T, Kneissl M, Johnson NM, Kolbe T. Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells. Applied Physics Letters 2012;100:021101.

2.2 UVC laser on bulk AlN substrate Recently, optically-pumped UVC AlGaN lasers with low-thresholds also have been demonstrated on native AlN substrates [44,45]. The Al(Ga)N epitaxial layers grown on such substrates will have low dislocation density because of the relatively low threading dislocations and less lattice mismatch. Fig. 6.14 shows a few spectra of UVC lasers grown on native AlN substrates. The emission wavelength are in between 237 and 291 nm with threshold below 200 kW/ cm2 [44,64]. Besides, many other groups also reported optically-pumped lasers on AlN substrates with a threshold around 100e500 kw/cm2 [45,49,65]. So far, the lowest threshold power density of such laser is only 41 kW/cm2 and it emits at 266 nm, as shown in Fig. 6.15A. The PL spectra under different pumping power are shown in Fig. 6.15B. Such low threshold of UVC laser is comparable with the lasers emitting in the regime of visible spectral for which the MQWs compose of InGaN/GaN heterostructures. However, because of smaller wafer size and the high price of such AlN substrates, it is more favorable to grow lasers on cheaper sapphire substrates.

2.3 UVC laser on SiC and others Compared with sapphire, SiC has better thermal conductivity, extremely small lattice mismatch with AlN film, and more importantly, is much easier to be cleaved to form laser facets. Many attempts on growing UVC lasers on SiC have been

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FIG. 6.14 Spectra of optically pumped UVC AlGaN lasers on bulk AlN substrates. Optical excitation was performed with either a KrF (l ¼ 248 nm) or ArF (l ¼ 193 nm) excimer laser. Adapted with permission from Kneissl M, Rass J. III-nitride ultraviolet emitters. Switzerland: Springer; 2016.

FIG. 6.15 (A) Output power versus pump power density of AlGaN-based optically pumped laser emits at 266 nm. (B) Lasing spectra recorded at different excitation levels at room temperature. Adapted with permission from Kneissl M, Rass J. III-nitride ultraviolet emitters. Switzerland: Springer; 2016.

validated using conventional AlGaN MQWs [47,48]. Recently, a new design of UVC laser structure employing graded-index separate confinement heterostructure (GRINSCH) was implemented by Sun et al. using AlGaN thin films [66,67]. As mentioned earlier, for the electrically pumped laser diode, we face a tremendous challenge to obtain conductive Mg-doped p-type Al-rich AlGaN films [39]. Thus, any development of an electrically-injected UVC laser will highly depend on the p-doping efficiency in Al-rich AlGaN alloys. Recently, scientists proposed

2. Ultraviolet quantum well lasers-UVC

a method to use polarization-enhanced doping by compositionally graded AlGaN alloys to achieve homogeneous doping of AlGaN films [68]. In their approach, three dimensional (3D) carriers can be generated with the aid of polarization charges in the graded AlGaN films without the requirement of intentional impurities. A UVC laser design, which takes advantage of the polarization-enhanced doping in compositionally graded AlGaN films, is a structure on the basis of GRINSCH configuration. Traditional III-V compound (GaAs, InP) GRINSCH lasers with extremely low current thresholds were successfully commercialized [69,70]. Furthermore, it was shown that InGaN laser diodes with GRINSCH structures have only threshold current density of 3.5 kA/cm2 [71]. Sun et al. demonstrated the MBE growth and fabrication of UVC laser using a GRINSCH structure on SiC substrate, and the active region of the device had a 75 nm AlGaN film. The investigated GRINSCH laser is presented in Fig. 6.16A. The near-field optical mode profile and the vertical index of refraction and the optical mode are shown in Fig. 6.16B and C, respectively. The optical mode was confined well due to the graded AlGaN films, with an optical confinement factor of nearly 32.5%, as calculated in Fig. 6.16C. This number is a significant higher than the reported 1e3% for conventional AlGaN MQW lasers. A super-linear behavior of the PL intensity under different pumping fluence is shown in Fig. 6.16D and threshold fluence was established at about 14 mJ/cm2. The optical gain measurement shows the transparency threshold values occurs at 14 mJ/cm2 with a large net modal gain of 80 cm1 (Fig. 6.16D). Such low threshold has already been observed in many nanocrystal quantum dots based on various material system [72]. Such low threshold is mainly due to the presence of compositional fluctuations in the active region of the laser because it was intentionally grown by plasmaassisted MBE under Ga-rich conditions, featuring significant potential fluctuations in AlGaN films [48]. Moreover, Fig. 6.16E describes the energy band diagram of the device. This band structure shows that the formation of a p-n junction, attributing to the polarization-induced n- and p-type doping of the graded films on both side of the active area. The free carrier concentrations (electron and holes) of the junction are shown as well. It should be highlighted that a high level of doping (1018cm3) can be easily obtained without the addition of dopants during the growth. And later they reported the development of AlxGa1xN/AlyGa1yN MQW-based UVC stimulated emission (275 nm) having a GRINSCH configuration with an optimized active region design [65]. Because of polarization-enhanced doping of the compositionally graded layers, which automatically lead to the formation of pn junction, these results indicate that GRINSCH can be used for electrically injected UVC laser application.

2.4 UVC nanowire laser The recent development of AlGaN nanowire structures could offer an alternative to develop UVC lasers. Zhao et al. have shown that the high conductive p-type AlN and high Al-content AlGaN nanowires [73], leading to the generation of electrically pumped semiconductor lasers operating in the UVC bands which are attributed to

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(B)

(C) Optical Mode intensity(arb.unit)

50nm AIGaN graded up 75nm Bulk Al

Ga

N

50nm AlGaN graded down 500nm AIN cladding and buffer layer

40 35 30 25 20 15 10 5 0

(E)

0

10

20 30 40 50 pump fluence (μJ/cm )

60

120 80 40 0

Log(carrier conc/cm3)

(D)

Intensity @ 257 nm (arb. u.)

(0001)6H-SiC

2.50

Active layer

Refractive Index

100nm AIN cladding layer

2.47 2.44 2.41 AIN

2.38

AIN

2.35 -1.0 -0.8-0.6-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Position (um)

25

hole electron Ec Ev

20 15

Graded down AlGaN

6 4 2 0

10 Graded up AlGaN

5 0

-2

Energy(eV)

(A)

net modal gain (cm )

156

-4 -6

-40

0.50

-80

-120

0

10

20 30 40 pump fluence (μJ/cm )

50

60

0.55

0.60

0.65

0.70

0.75

Distance (micro) Growth direction

FIG. 6.16 (A) Schematic of the GRINSCH laser. (B) Near field profile of the optical mode. (C) Refractive index profile and calculated intensity of the optical mode. (D) ASE intensity measured at the fixed wavelength of 257 nm (the peak wavelength) as a function of the pumping fluence and the entire set of measured net modal gain values as a function of the pumping fluence. (E) Simulated energy band diagram of the investigated GRINSCH. The polarization induced free carrier concentration in the p- and n-sides of the junction are also shown in this figure. Adapted with permission from Wunderer T, Chua CL, Northrup JE, Yang Z, Johnson NM, Kneissl M, Garrett GA, Shen H, Wraback M, Moody B, Craft HS, Schlesser R, Dalmau RF, Sitar Z. Optically pumped UV lasers grown on bulk AlN substrates. Physica Status Solidi C 2012;9:822 and Martens M, Mehnke F, Kuhn C, Reich C, Wernicke T, Rass J, Ku¨ller V, Knauer A, Netzel C, Weyers M, Bickermann M, Kneissl M. Performance characteristics of UVC AlGaN-based lasers grown on sapphire and bulk AlN substrates. IEEE Photonics Technology Letters 2014;26: 342.

the large Anderson localization of the emitted light from the nearly dislocation-free AlxGa1xN nanowires [50e52]. Fig. 6.17A and B schematically show the nanowire laser device and a SEM of nanowires grown by MBE, respectively. A 239 nm AlGaN nanowire laser device measured at room temperature is shown in Fig. 6.17C. A broad emission spectrum is measured under a low injection current and a sharp peak centered at 239 nm shows up at high injection current. The EL peak intensity under different injection currents is presented in Fig. 6.17D, which indicates a lasing threshold at 0.35 mA. Fig. 6.8E exhibits the decrease of FWHM from 1.7 to 0.9 nm near the threshold. In the figure,

2. Ultraviolet quantum well lasers-UVC

FIG. 6.17 (A) Schematic of AlGaN nanowire laser. (B) An SEM image of the nanowires with an inset showing the enlarged image. (C) The EL spectra of an AlGaN nanowire laser device under continuous wave (CW) biasing with different injection currents. (D) The integrated EL intensity as a function of the injection current for the lasing peak (filled circles) and a nonlasing cavity mode (open circles) from the boxed region in (C). The inset shows the LeI curve of the lasing peak in a logarithmic scale. (E) The EL spectral linewidth as a function of the injection current. Adapted with permission from Zhao S, Liu X, Wu Y, Mi Z. An electrically pumped 239 nm AlGaN nanowire laser operating at room temperature. Applied Physics Letters 2016;109:191106.

a non-lasing cavity mode is also presented at the wavelength of w267 nm. Shown in Fig. 6.17D, its EL intensity does not change above the threshold, further supporting the lasing action at 239 nm. This demonstration offers a possible approach to achieve semiconductor UVC laser diodes. However, significant efforts are required to further improve such laser performance since the output power of such laser is extremely small at this moment.

2.5 UVC laser application UVC light sources are among the most widely used light sources in civil, military, industrial, commercial, and space applications. Compact and stable UVC laser sources have been a long-standing need for planetary material analysis in space, forensics, curing, and sterilization. In addition, biomedical applications, optical data storage, metrology, Raman spectroscopy, laser cooling and trapping, laser inspection, and laser lithography can broadly benefit from efficient UVC sources [74e76]. The versatility comes from the similarity of photon energy with outer atomic electron energy levels, which makes it a perfect tool to probe the outer atomic electrons. The laser has high spatial and temporal coherence which enable

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applications in which light has to be highly focused to small spots or where the spectral quality is important. Below are a few potential applications: 1. Sterilization: The UVC light disrupts activities and damages structures of DNA molecules of pathogens, thus being germicidal. Moreover, it does not create chemical residuals that could cause environmental issues. In general, absorption of the DNA molecules is high in the deep UV spectrum [74]. This enables effective sterilization of water as well as air and surface. 2. Spectroscopic technologies: UV laser sources are the core components of many types of spectroscopy. For example, fluorescence spectroscopy (FS) is nondestructive and requires a minimal amount of sample preparation, which makes FS perfect for in-situ measurements of elemental, mineralogical, and organic compounds. UVC induced native fluorescence is proven to be very sensitive to condensed carbon and aromatic organics. The detection at or below 106 w/w (1 ppm) at < 100 mm spatial scales is possible. Other types of spectroscopy, like Raman spectroscopy and Raleigh scattering spectroscopy, can also benefit from a high power compact UVC source. 3. UV curing and additive manufacturing: High power UV light sources are also used in curing applications that will benefit from high intensity solid state UV light source. 3D printing also stands to benefit from light sources that can be focused to finer points and at high powers. 4. Non-line-of-sight (NLOS) communication: At the ground level, the atmospheric molecules often cause strong angle-independent scattering of the UVC light [77]. This creates an enormous amount of communication links from a source to a receiver. Thus they are insensitive to line-of-sight obstacles such as buildings, trees, and mountains, enabling NLOS communications.

2.6 Summary The development of optically-pumped AlGaN UVC laser on sapphire, native AlN substrates, and SiC are discussed. Furthermore, we elaborate the potential of Al(Ga)N nanowires for electrically injected UVC lasers. Lastly, we briefly discuss the applications of the UVC lasers.

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