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Research Paper

Applied Science and Convergence Technology 2017; 26(4): 79-85

Published online July 31, 2017


Copyright © The Korean Vacuum Society.

Stimulated Emission with 349-nm Wavelength in GaN/AlGaN MQWs by Optical Pumping

Sung-Bock Kim* , Sung-Bum Bae , Young-Ho Ko , Dong Churl Kim , and Eun-Soo Nam

Electronics and Telecommunications Research Institute, Daejoen 305-700, Korea

Correspondence to:E-mail: sbk@etri.re.kr

Received: June 14, 2017; Revised: July 28, 2017; Accepted: July 28, 2017

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

The crack-free AlGaN template has been successfully grown by using selective area growth with triangular GaN facet. The triangular GaN stripe structure was obtained by vertical growth rate enhanced mode with low growth temperature of 950°C and high growth pressure of 500 torr. The lateral growth rate enhanced mode of AlGaN for crack-free and flat surface was also investigated. Low pressure of 30 torr and high V/III ratio of 4400 were favorable for lateral growth of AlGaN. It was confirmed that the 4 μm -thick Al0.2Ga0.8N was crack-free over entire 2-inch wafer. The dislocation density of Al0.2Ga0.8N was as low as ~7.6 × 108 /cm2 measured by cathodoluminescence. Based on the high quality AlGaN with low dislocation density, the ultraviolet laser diode epitaxy with cladding, waveguide and GaN/AlGaN multiple quantum well (MQW) was grown by metalorganic chemical vapor deposition. The stimulated emission at 349 nm with full width at half maximum of 1.8 nm from the MQW was observed through optical pumping experiment with 193 nm KrF laser. We also have fabricated the deep ridge type ultraviolet laser diode (UV-LD) with 5 μm-wide and 700 μm-long cavity for electrical properties. The turn on voltage was below 5 V and the resistance was ~55 Ω at applied voltage of 10 V. The amplified spontaneous emission spectrum of UV-LD was also observed from pulsed current injection.

Keywords: Crack-free AlGaN, Dislocation density, Selective area growth, Stimulated emission, Ultraviolet laser diode, Amplified spontaneous emission

Ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs) have numerous applications in bio-/chemical and environment fields, UV curing and material processing. AlGaN based UV-LEDs and LDs provide light emission at a wavelength shorter than 365 nm corresponding to the band gap of GaN. However, poor structural quality of high Al contents AlGaN heterostructure grown on lattice mismatched substrate cause serious difficulties in fabrication of UV emitters [13]. The UV-LDs require a more complex structure, thicker layers and lower dislocation density than LEDs to satisfy all the requirements of suitable optical and electrical confinements as well as high emission efficiency. Lee et al. reported the critical thickness of AlGaN with various Al-mole fractions. It was impossible to grow more than 100nm thickness without any crack for AlGaN with 20% AlN [2]. It is difficult to reduce dislocations and suppress crack generation for fabrication of laser diodes with a shorter wavelength on sapphire substrate. Some growth techniques have been suggested to obtain an AlGaN template with crack-free surface and lower dislocation density. Amano and Akasaki’s research group in Meijo University reported the dislocation density and strain could be released by growing AlGaN on periodically grooved GaN template. The crack-free AlGaN with low threading dislocation density was grown on grooved GaN by combining the LT-AlN interlayer and hetero-epitaxial lateral overgrowth (hetero-ELO) [46]. They succeeded in an electrically pulsed operation of UV-LD at 350.9 nm, for the first time, using a GaN/AlGaN MQW active layer grown on the thick and crack-free AlGaN with a low threading dislocation density. Yoshida et al. obtained the thick-AlxGa1-xN (x = 0.2–0.3) with low dislocation density by hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO) with triangular GaN seed crystal [7,8]. The dislocation density of crack-free AlGaN was observed between 1.8–6.0 × 108 /cm2 from cathodoluminescence (CL) dark spots [9]. From these results of low dislocation AlGaN grown on inclined GaN facet, the various UV-LDs between 336–360 nm with AlGaN/AlGaN or GaN/AlGaN MQWs were demonstrated [913].

In order to obtain the crack-free AlGaN template, we have grown selectively the triangular facetted GaN using vertical growth enhanced mode. Subsequently, thick-Al0.2Ga0.8N layer with low dislocation density on facetted GaN layer was deposited by lateral growth enhanced mode. Finally, UV-LD epitaxy structures including cladding layers, waveguide layers with GaN/AlGaN MQWs, and contact layer were grown on high quality AlGaN template. We fabricated the deep-ridge waveguide type UV-LD by conventional photolithography and ICP-RIE dry etching. We measured the stimulated emission by optical pumping and the amplified spontaneous emission (ASE) by electrical current injection. These results demonstrate that the high quality thick-AlGaN template grown by selective area growth (SAG) provides a possibility for AlGaN based deep-UV applications such as UV-LEDs, UV-LDs and UV-detectors.

High-quality AlGaN templates were prepared by SAG on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The triangular GaN facet structure was selectively grown on GaN buffer layer, which was periodically masked by SiNx stripes. The stripes masks with a width of 2 μm and a spacing of 2 μm were deposited along the <11̄00> axis on the GaN layer. The triangular shaped GaN with {112̄2} plane was grown at 500 torr and 950–1040°C. The crack-free and flat Al0.2Ga0.8N was grown on the facetted GaN under various V/III ratio and growth temperature at 1030°C. Next, we grew UV-LD epitaxial structure on a crack-free 6 μm-thick n- Al0.2Ga0.8N template. The structure on AlGaN template was composed of a 400 nm-thick n- Al0.2Ga0.8N cladding layer, a 100 nm-thick Al0.13Ga0.87N waveguide, three pairs GaN (2 nm)/Al0.1Ga0.9N (8 nm) MQWs, a 90 nm-thick Al0.13Ga0.87N waveguide, a 30 nm-thick Al0.3Ga0.7N electron blocking layer (EBL), a 500 nm p-A0.2lGa0.8N cladding layer, and a 30 nm-thick p+-GaN contact layer. The schematic structure of UV-LD is shown in Fig. 1.

The structural evolution of GaN and AlGaN epilayers with various growth conditions was measured by scanning electron microscopy (SEM). Surface morphology and optical uniformity were observed by Nomarski microscope and photoluminescence (PL) mapper. To evaluate spatial-resolved optical properties and dislocation density of thick AlGaN template, we carried out the CL measurement combined with the field emission-SEM.

We fabricated the UV-LD samples for optical pumping, which have double side mirrors of cleaved facets and cavity length of about 1 mm. The stimulated emission of UV range was measured by using 193 nm krF excimer laser. An optical pumping experiment was constituted in an edge emission geometry. The pump beam was focused on the surface by a cylindrical lens and the edge emission was detected from the side. The deep-ridge waveguide type-UV-LD with 5 μm-wide and 700 μm-long cavity was fabricated by conventional photolithography and dry etching. A Ni/Au was evaporated on the p+-GaN contact layer, while a Ti/Al/Ti/Au was deposited on the n-Al0.2Ga0.8N contact layer. The laser cavity mirrors were formed by cleaving. Electroluminescence (EL) measurement was performed at room temperature under the pulsed condition.

The high-quality and crack-free Al0.2Ga0.8N templates were deposited by the MOCVD SAG. The triangular GaN stripe between masks was grown by a vertical growth enhanced mode. During formation of triangular GaN stripe by SAG, the dislocations were bent from the vertical direction to the perpendicular direction of facetted planes. Next, a flat thick-AlGaN layer was grown on triangular facetted GaN by a lateral growth enhanced mode. The dislocations were propagated laterally in lateral growth mode. From the control of dislocation propagation depended on growth mode, we expect to obtain the high quality AlGaN with low dislocation density.

Fig. 2 shows the evolution of GaN epitaxy with different growth temperature under growth pressure of 500 torr. The cross-sectional SEM images of GaN grown at 1040°C and 950°C were shown in Fig. 2(a) and 2(b), respectively. The GaN facetted structure grown at higher temperature (1040°C) during 20 minute had three facets of the (0001) flat top plane, the {112̄0} vertical facet, and the {112̄2} inclined facet. After 60 minute, these various facets merged laterally to (0001) flat plane because the growth rate of GaN on (0001) plane was slower than on other facets [14]. The result demonstrates that the lateral growth of GaN was enhanced at 1040°C under 500 torr.

On the other hand, the aspect of growth at low growth temperature (950°C) was quite different from at high growth temperature. Fig. 2(b) shows the evolution of GaN epilayer at lower growth temperature. The (0001) plane and the {112̄2} facet are observed without the {112̄0} vertical facet. After growing 20 minute, the vertical thicknesses of GaN (0001) at 950°C and 1040°C were 1.5 μm and 0.94 μm, respectively. This result implies that the vertical growth rate was enhanced at lower growth temperature. In progress of growth at 950°C, the {112̄2} facets developed dominantly and the (0001) facets were diminished. If the growth rates of each facets are defined as G<0001> and G<112̄2>, the G<0001> ≈ 2.7 μm/h and the G<112̄2> ≈ 0.5 μm/h at 950°C. This evolution of GaN epilayer shows that the vertical growth rate is faster than the lateral growth rate. Consequently, we obtained triangular GaN stripe at 950°C under 500 torr by using vertical growth enhanced mode of SAG. The results of facet formation at various growth temperatures were agreed very well with Hiramatsu’s previous work [15]. In their research, the morphological change should be considered by the stability of each surface which depends mainly on the surface energy and the stability of surface atoms.

In order to grow flat AlGaN on triangular GaN stripe, the lateral growth mode was required. For the investigation of AlGaN growth mode, we measured the relative ratio of lateral growth rate to vertical growth rate (L/V) as well as growth rate of AlGaN. The schematic illustration of AlGaN epilayer on triangular GaN stripe and the lateral to vertical growth rate (L/V = G<112̄2>/ G<0001>) is shown in inset of Fig. 3(a). In case of lateral growth enhanced mode (L/V > 1), the AlGaN facets on triangular GaN facet merge to (0001) AlGaN planar structure. However, if the lateral to vertical growth rate L/V is less than 1, the triangular structure could not be grown to merged planar structure. To obtain flat AlGaN surface, we have to take the growth condition of L/V > 1.

Fig. 3(a) shows the growth rate and relative ratio of lateral to vertical growth rate of AlGaN as a function of growth pressure. The growth temperature and V/III ratio were fixed at 1030°C and 1900, respectively. When the growth pressure was increased from 30 torr to 100 torr, the lateral to vertical growth ratio was decreased as well as decreased growth rate from 1080 nm/h to 370 nm/h. Although the relative ratio of lateral to vertical growth rate was less than 1, we found that the growth under low pressure was favorable for lateral growth of AlGaN. High growth pressure condition leads to lower L/V because high pressure made the surface of Ga-polarity unstable and N-polarity stable [15]. Based on these results, we selected low growth pressure of 30 torr with high growth temperature of 1030°C as Al0.2Ga0.8N ELO condition. We carried out the AlGaN growth with V/III ratio dependence by changing the NH3 flows from 7000 sccm to 16000 sccm. Fig. 3(b) shows AlGaN absolute growth rate and relative ratio of lateral to vertical growth rate as a function of the V/III ratios. For the low V/III ratio of 1900 (NH3 = 7000 sccm) the growth rate was over 1 μm/h and the L/V was measured as 0.82 (vertical growth enhanced mode). When the V/III ratio was increased to 4400 (NH3 = 16000 sccm), L/V was also increased to 1.15 (lateral growth enhanced mode) but growth rate was decreased to 0.45 μm/h. The planar (0001) AlGaN template should be grown under high V/III ratio condition. Relation between AlGaN growth conditions and relative ratio of lateral to vertical growth rate is summarized in Table 1.

From the study of AlGaN growth mode, we have grown the thick AlGaN template on triangular GaN stripe under the condition of lateral growth enhanced mode. The planar AlGaN template with (0001) plane was obtained after growing 6 hours under growth conditions of 1030°C, 30 torr and V/III ratio of 4400. The thickness of AlGaN template was about 4.4 μm from the top of GaN. Fig. 4 shows the Nomarski images of surface morphologies of thick-AlGaN grown on (a) triangular GaN stripe and (b) on AlGaN/GaN superlattice structure. It was measured that the AlGaN grown laterally on GaN facet had quite clear surface without any cracks as shown in Fig. 4(a). It was known well to obtain the crack-free epilayer with combining the superlattices as a strain relaxation layer and AlN inter layer [16,17]. As a reference, similar AlGaN layer was grown on AlGaN/GaN superlattices with AlN layer. However, a number of cracks were generated in AlGaN grown on superlattice as shown in Fig. 4(b). We confirmed that the lateral growth of AlGaN on facetted GaN is more effective to obtain the crack-free thick-AlGaN.

The PL was measured at room temperature as shown in Fig. 5(a). We observed at a peak wavelength of 325 nm with full width at half maximum (FWHM) of 10.1 nm. The fairly weak peak around 500 nm was known as yellow peak. From the peak wavelength, the Al-composition of AlGaN is as about 20%. This value of 20% Al-mole fraction agreed well with the Al-composition measured from XRD. The PL mapping image in inset also shows that compositional variation of AlGaN is quite uniform over the 2 inch wafer where the standard deviation of peak wavelength was measured as 0.68%. The CL spatial mapping was carried out on the surface of Al0.2Ga0.8N for estimating dislocation density as shown in Fig. 5(b). The dark spot in CL mapping indicates the region of poor emission intensity related with dislocation. In panchromatic CL mapping image, the density of threading dislocation was ~7.6 × 108 /cm2. While the dislocation density of conventional AlGaN with 20% Al-mole fraction is more than 1010 /cm2 [18,19]. Consequently, we had successfully grown crack-free Al0.2Ga0.8N template with low dislocation density.

By using the crack-free Al0.2Ga0.8N template of high crystal quality, we had grown LD full structure with n-Al0.2Ga0.8N cladding layer, Al0.13Ga0.87N waveguide layers including Al0.1Ga0.9N/GaN MQWs, Al0.3Ga0.7N EBL, p-Al0.2Ga0.8N cladding layer and p+-GaN contact layer as shown in Fig. 1. We carried out transmission electron microscopy (TEM) measurement to compare the grown epilayer to designed epilayer. The TEM images clearly indicated the realization of the design of UV-LDs grown on Al0.2Ga0.8N template. The concentrations of Mg and Si in p- and n-AlGaN cladding layers were evaluated as ~3 × 1019 /cm3 and ~4 × 1019 /cm3 by SIMS, respectively.

The CL occurs because the incident of a high electron beams onto a semiconductor generate electron-hole pairs. When an electron and a hole recombine, a photon is emitted. Because the electron beam penetrates into the semiconductor, the CL reflects the depth-resolved information [20]. The penetration depth is changed with acceleration voltage of electron beam. Fig. 6(a) shows the electron trajectory with different acceleration voltage by Monte-Carlo simulation. In acceleration voltage of 10 kV, most electrons arrived at AlGaN cladding layer. On the other hand, the electrons reached deeply the waveguide layers and MQWs in the acceleration voltage of 15 kV. Fig. 6(b) shows the depth-resolved CL spectra with different acceleration voltage. At low acceleration voltage of 10 kV, the unique CL peak related with AlGaN cladding layer was measured at ~323 nm (3.84 eV). This value is very similar to PL result of AlGaN template as shown in Fig. 5(a). In comparison, at higher voltage potential of 15 kV, additional two peaks were observed at ~339 nm and ~350 nm. The CL peak of 339 nm was originated from AlGaN waveguide layers. From luminescence peak, the Al mole-fraction of AlGaN waveguide is estimated as 13%. The largest peak at wavelength of ~350 nm came from GaN/AlGaN MQWs. Although less number of electrons reached the MQWs region, the peak from MQWs was the strongest. The result implies that the quality of UV-LD epilayer including MQWs on crack-free Al0.2Ga0.8N template is very good.

A 193 nm KrF excimer laser was employed for optical pumping of the stimulated emission. The laser beam was focused by a cylindrical lens, so the excitation stripe was normal to the mirror facets of the laser sample. The edge emission from MQWs was measured from the sidewall of the sample through a fiber coupled spectrometer. The mirrors were fabricated by sequent processes of grinding/lapping, laser scribing and cleaving. The laser sample had the cavity length of about 1mm. Fig. 7(a) shows the edge emitting PL spectra of MQWs with various input optical power density at room temperature. Below pumping power density, there appeared a broad and weak peak due to a spontaneous emission. However, when the optical pumping power density increased over 600 kW/cm2, the peak became narrower and stronger, indicating a stimulated emission process. The stimulated emission peak for 770 kW/cm2 was measured at 349 nm with FWHM of 1.8 nm. The excitation power density dependence of the PL emission intensity is shown in Fig. 7(b). Sudden increase of emission intensity was observed around 600 kW/cm2. It was confirmed that the stimulated emission was obtained for larger excitation power than a threshold optical power density of ~580 kW/cm2.

As a reference, we had grown the same UV-LD structure on AlGaN template having AlGaN/GaN superlattice as a strain relaxation layer (see the Fig. 4(b)). In this sample, very weak and broad PL signal around 355 nm was observed. We could not observe the any stimulated emission in reference sample. These results indicate that the UV-LD grown on ELO-AlGaN template with facetted GaN has high optical properties by reducing efficiently dislocation density.

For EL characteristics, we fabricated the ridge waveguide type UV-LD with 5 μm-wide and 700 μm-long cavity. Total epitaxy thickness of UV-LD structure including AlGaN template and GaN buffer layer on sapphire substrate was over 13 μm. Because of the wafer bowing by strained thick-epilayer on sapphire substrate, the photolithography process using several masks was very difficult. The height difference due to bowing between center and edge of 2 inch wafer was about 150 μm. To reduce the bowing effect, we divided the 2 inch wafer into 15 × 15 mm2. Dry etching processes for mesa and ridge were carried out by ICP-RIE with Cl2 gas. N-metal and p-metal were deposited on mesa and on ridge, respectively. After SiNx passivation, Au as metal pad was deposited. The mirror facets with M-planes of GaN were fabricated by mechanical cleaving process. No highly reflective coatings were used for the mirror facets. After fabricating chip bar, the electrical measurement was carried out at constant temperature of 20°C as applying current by probes. The electrical properties of UV-LD were measured before fabricating mirror facets. We confirmed easily the UV-light emitted brightly from waveguide. The turn on voltage was below 5 V and the resistance also was about 55 Ω at applied voltage of 10 V.

We measured EL spectra under pulsed current injection with a pulse width of 10 ns and a repetition frequency of 1 kHz at room temperature shown in Fig. 8(a). The spectral measurements were carried out with the time-integrated detection by multi-channel analyzer. When the current is 500 mA, the spontaneous emission was observed. The center wavelengths of peaks did not change at ~355 nm under changing currents. Upon increasing the injection current to 2000mA, the integrated intensity of EL was increased linearly. Also the FWHM of EL spectrum had values of 12~14 nm shown in Fig. 8(b). The EL characteristics indicated the typical amplified spontaneous emission (ASE) under pulsed current mode at room temperature. We could not observe the strong and sharp lasing spectrum under any injection current. The device still worked as in LED-mode not LD-mode. It is necessary to improve the process of ridge waveguide formation and mirror facet formation. Optical loss is generated at rough ridge waveguide and imperfect mirror do not obtain sufficient optical gain. Above all, it is most important to reduce the dislocation which acts leakage current path for electrical driven UV-LD. The dislocation density of ~7.6 × 108 /cm2 in this work is still greater than in other research results [5,21].

We have grown crack-free AlGaN template with low dislocation density over entire 2-inch wafer by using SAG and lateral growth on GaN facet. The triangular GaN stripe structure was obtained by vertical growth mode of SAG at low growth temperature of 950°C and high growth pressure of 500 torr. The structural properties of AlGaN grown on the triangular GaN structure were monitored with changing the growth conditions of pressure and V/III ratio. Low growth pressure of 30 torr and high V/III ratio of 4400 were favorable for AlGaN SAG due to high ratio of lateral to vertical growth rate. From the study of SAG mode, the AlGaN template had uniform Al-mole fraction of ~20% over entire 2-inch wafer and quite low dislocation density as ~7.6 × 108 /cm2.

Based on the high quality AlGaN, we have grown the UV-LD epitaxy with cladding, waveguide and GaN/AlGaN MQW by MOCVD. The luminescence radiated from Al0.2Ga0.8N cladding layer, Al0.13Ga0.87N waveguide layer and GaN/AlGaN MQWs was confirmed by depth resolved-CL experiment. We carried out optical pumping experiment using 193 nm KrF laser as excitation pump beam The stimulated emission at peak wavelength of ~349 nm with FWHM of 1.8 nm from the MQW was successfully measured with threshold optical power density of ~580 kW/cm2.

Finally, we have fabricated the deep ridge type UV-LD with 5 μm-wide and 700 μm-long cavity. The mirror facets of M plane were fabricated by cleaving. The turn on voltage of diode was below 5 V and the resistance was ~55 Ω at applied voltage of 10 V. The ASE spectrum of UV-LD was also observed from current injection. This study of crack-free AlGaN growth may provide key technology for highly-efficient optoelectronic devices below 365 nm.

This work was supported by Commercialization of ICT Technology Program (R-20160322-003458, 365 nm GaN-free UV-LED Epitaxy for Flip-chip funded by the Ministry of Science, ICT and Future Planning (MSIP, Korea).

The authors would like to thank Theeradetch Detchprohm and Russell Dupuis of Georgia Institute of Technology and Jae-Hyun Ryou of University of Houston for support of optical pumping experiments.

Fig. 1. The schematic structure of UV-LD on crack-free AlGaN template grown using triangular facetted GaN stripe on a sapphire substrate.
Fig. 2. Cross-sectional SEM images of the evolution of GaN epitaxy grown (a) at 1040°C and (b) at 950°C under growth pressure of 500 torr during 20 min and 60 min. The facet of GaN epilayer evolves to (0001) plane at high growth temperature of 1040°C. Otherwise, at low growth temperature of 950°C, the {112̄2} facets of GaN are remained by vertical growth rate enhancement.
Fig. 3. The growth rate and relative ratio of lateral and vertical growth rate of AlGaN (a) with various growth pressures and (b) with various V/III ratio under growth pressure of 30 torr. Inset in (a) illustrates AlGaN epitaxial layer on triangular GaN stripe. V and L mean vertical growth rate and lateral growth rate, respectively.
Fig. 4. Nomarski photographs of surface of AlGaN grown on (a) triangular GaN stripe structure and (b) AlGaN/GaN superlattice structure.
Fig. 5. (a) The PL spectrum of AlGaN template and its mapping of peak wavelength. (b) Panchromatic CL mapping image of the AlGaN template.
Fig. 6. (a) Electron trajectory in UV-LD with different acceleration voltage of 10 kV and 15 kV through Monte-Carlo simulation. At 10 kV, most electrons arrive at AlGaN cladding layer and do not penetrate until waveguide layers and MQWs. However the electrons accelerated by 15 kV can reach the lower AlGaN cladding layer. (b) Depth-resolved CL spectra with the acceleration voltage of 10 kV and 15 kV.
Fig. 7. (a) Edge emitting PL spectra of GaN/AlGaN MQWs below and above the lasing threshold at room temperature. (b) Emission intensity versus the pumping power density.
Fig. 8. (a) EL spectra of UV-LD at room temperature under various pulsed injection currents. (b) Integrated EL intensity and FWHM of EL spectrum versus the injection current.
Table. 1.

Conditions of lateral growth rate enhanced mode of AlGaN

Growth conditionsRelative ratio of lateral to vertical growth rate (L/V)
Growth temperature
Growth pressure
V/III ratio

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