Carrier localization and defect-insensitive optical behaviors of ultraviolet multiple quantum
wells grown on patterned AlN nucleation layer
Item Type Article
Authors Chen, Li;Dai, Yijun;LI, LIANG;Jiang, Jiean;Xu, Houqiang;Li, Kuang- Hui;Ng, Tien Khee;Cui, Mei;Guo, Wei;Sun, Haiding;Ye, Jichun Citation Chen, L., Dai, Y., Li, L., Jiang, J., Xu, H., Li, K., … Ye, J. (2021).
Carrier localization and defect-insensitive optical behaviors of ultraviolet multiple quantum wells grown on patterned AlN nucleation layer. Journal of Alloys and Compounds, 861, 157589.
doi:10.1016/j.jallcom.2020.157589 Eprint version Post-print
DOI 10.1016/j.jallcom.2020.157589
Publisher Elsevier BV
Journal Journal of Alloys and Compounds
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Link to Item http://hdl.handle.net/10754/666910
Carrier localization and defect-insensitive optical behaviors of ultraviolet multiple quantum wells grown on patterned AlN nucleation layer
Li Chen1,2#, Yijun Dai1,2#, Liang Li1,2, Jiean Jiang1,2, Houqiang Xu1,2, Kuang-hui Li3, Tien Khee Ng3, Mei Cui1,2, Wei Guo1,2*, Haiding Sun4*, and Jichun Ye1,2*
1Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, Zhejiang, China
2University of Chinese Academy of Sciences, Beijing, 100049, China
3King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
4School of Microelectronics, University of Science and Technology of China, Hefei, Anhui 230026, China
#These authors contribute equally to this work
*Corresponding authors: [email protected], [email protected], [email protected]
Abstract
In this work, we report a significantly boosted defect-insensitive ultraviolet emission from InGaN/GaN multiple quantum wells (MQWs) grown on patterned AlN nucleation layer (NL) compared to samples grown on uniform NL. Carrier localization is clearly illustrated for MQWs grown on patterned AlN NL as evidenced by an S-shape profile from temperature-dependent photoluminescence characterization. The underlying mechanism for the carrier localization is demonstrated to correlate with partial relaxation of compressive strains inside epitaxial thin films. This work illustrates that carrier localization can be achieved in MQWs with very low indium content by the adoption of patterned NL during growth, and provides a promising route towards the realization of high-efficiency ultraviolet emitter.
Keywords: III-nitride, multiple-quantum-well, nanostructure patterning, carrier localization, strain relaxation
Introduction
Ultraviolet light-emitting-diode (UV-LED) has been recognized as one of the best candidates to replace the conventional UV lamp for the wide applications such as UV curing, water disinfection and chemical/biomedical diagnosis.[1, 2] As the building block of UV-LEDs, the crystalline quality and structure design of the
III-nitride thin film are critically important.[3] In the past decades, great progress has been made in heteroepitaxially growing UV-LED thin films on highly mismatched substrates such as sapphire[4], which is up to now one of the mostly accepted substrates due to its large size, low cost and mature device fabrication technology.
Prior to the high-temperature growth of UV-LED structure by the technique of metal-organic chemical vapor deposition (MOCVD), a nucleation layer (NL) is usually deposited on the substrate to release the strains induced by lattice mismatch.[5, 6] Among all types of NLs, AlN is universally utilized as it supports a high-quality III-nitride thin film.[7] The AlN NL plays an important role in filtering the dislocations during high temperature epitaxial growth. A transition from 3D nucleation to 2D step-flow growth mode is usually achieved with improved crystalline quality and stronger luminescence intensity.[8] Since extended and point defects are usually linked to non-radiative recombination centers inside epitaxial thin films, higher dislocation density usually leads to degradation of internal-quantum-efficiency (IQE) and jeopardize device performance of UV-LEDs.[9]
Therefore, great efforts have been put in reducing dislocations in III-nitride thin film.[3, 10, 11]
Except for dislocations, performance of UV-LEDs also reply on other factors such as the design of the multiple-quantum-wells (MQWs) and the modulation of light polarization,[12] both of which received relatively less attention so far. A typical example to show the importance of these factors is that, regardless of high concentration of dislocations in InGaN alloys, IQE of InGaN-based blue LEDs was found to be rather high. This is commonly attributed to localization of indium atoms in the QWs that prevents carriers from reaching defect-related nonradiative recombination sites.[13, 14] Due to the solid phase immiscibility of In and Ga atoms, phase separation was reported to occur in InGaN thin film and MQW regions of the blue LEDs.[15] This discovery has actually paved the way for the development of high-brightness and high-efficiency blue LEDs which has been widely used in full-color display and solid-state lighting for decades.[16] In comparison to that, indium localization effect was seldom reported in UV-LEDs due to much lower indium content of less than 5%, and consequently more difficult in forming In-rich clusters.[17, 18] Herein, the promotion of phase separation and carrier localization are strongly desired in order to improve the optical properties of UV-LEDs.
In this work, we report a defect-insensitive ultraviolet emission from InGaN/GaN MQWs grown on patterned AlN NL. Enhanced photoluminescence (PL) intensity and higher radiative recombination rate are observed in MQWs grown on patterned AlN NL despite higher dislocation density. Carrier localization effect of the MQWs is unambiguously demonstrated by temperature-dependent PL characterization. The initiation of carrier localization is further confirmed to be related with the relaxation of bi-axial strains.
Experiment
InGaN/GaN UV-MQWs with indium content of 5% was grown on 2-inch c-plane
sapphire substrate via an Aixtron Cruis I MOCVD. Trimethylaluminum (TMA), triethylgallium (TEG) and ammonia (NH3) were used as precursors of Al, Ga and N, respectively. Hydrogen (H2) was used as the carrier gas. Prior to MOCVD growth, 20 nm thick AlN NL was blanket deposited on planar sapphire substrate through magnetron sputtering technique. For conventional UV-MQW as reference sample, sapphire substrate with as-deposited AlN NL was directly loaded into the MOCVD chamber for high temperature (HT) epitaxy. For the investigated UV-MQW sample, sapphire substrate with AlN NL is uniformly coated with polystyrene (PS) spheres of 2 µm via a large-area micro-propulsive injection method. The hexagonal pattern of closely-packed PS spheres is transferred onto the AlN NL by reactive-ion-etching (RIE) using Cl2/BCl3 gases. The remaining PS spheres were removed by sonication in acetone solution, and the patterned AlN NL was reloaded into MOCVD chamber for subsequent epitaxial growth. Note that both investigated sample and reference sample are grown in the same run with exactly the same growth condition. Therefore, surface morphologies, optical properties and strain conditions of samples should be directly correlated with the underlying AlN NL. The structure consists of 3.5 µm GaN template grown on top of either uniform or patterned AlN NL, 8 pairs of In0.05Ga0.95N/GaN MQWs with thickness of 3 nm (quantum well, QW) and 11 nm (quantum barrier, QB), respectively, and finally a 5 nm AlGaN cap layer was deposited on the top. GaN template was grown in H2 ambiance with temperature of 1000 ℃ (3D GaN, 0.5 µm) and 1050 ℃ (2D GaN, 3 µm), while MQWs were grown under N2 ambiance with temperature of 800 ℃ for QW and 900 ℃ for QB.
AlGaN cap was deposited on top with N2 carrier gas under the temperature of 1000 ℃. The surface morphology of the MQW structure was characterized by a Veeco Dimension 3100 V atomic force microscopy (AFM). Steady-state PL study was performed using a Coherent Ar-F (193 nm) excimer laser at various pumping powers, and signal was collected by a Horiba iHR550 spectrometer. Temperature-dependent PL analysis was performed in a closed-cycle Helium cryostat, enabling the temperature to be adjusted down to 20 K. Time-resolved PL (TRPL) was performed by CNI FL-213-Pico 213 nm picosecond diode laser and recorded by a Time-Correlated Single-Photon Counting (TCSPC) system. Cathodoluminescence (CL) measurement was carried out by a FEI Quanta FEG 250 in combination with a Gatan MonoCL setup. Dislocation densities of both samples were characterized using a point-focused high-resolution X-ray (Cu Kα1) diffractometer (HRXRD, Bruker D8 Discover) equipped with a four-bounce symmetric Ge (220) monochromator.
Reciprocal space mapping (RSM) measurements were taken by PANalytical X’Pert3 MRD XL XRD configuration. Weak beam dark field TEM images were acquired under two beam condition at an acceleration voltage of 300 kV using a FEI Titan ST microscope system. Instead of Focused Ion Beam milling, which causing the Ga ion implantation and amorphous damage, Ar+ ion milling procedure with much lower milling energy was used for TEM cross-section sample preparation. The two samples were bonded face to face and polish to a thickness of about 30 μm for Ar+ ion milling.
Atomic distribution was acquired by High-angle annular dark field (HAADF) imaging and microscopic energy dispersive X-ray spectroscopy (EDS) mapping in Talos
F200x 200 kV TEM.
Results and discussion
AFM images of patterned and uniform AlN NL deposited on sapphire substrates are shown in Figure 1(a) and (b), respectively. The height of the circular pattern is 25 nm, in good agreement with the targeted AlN NL thickness. Smooth sidewalls are observed at the edges of the patterned AlN NL, and grainy crystal structures are shown in as-deposited sample. Root-mean-square (rms) roughness of the uniform AlN NL is 1.2 nm, proving to be a good buffer layer for subsequent epitaxial growth. After MOCVD growth, surface morphologies of the samples grown on patterned and uniform AlN NL are characterized again, and the images are shown in Figure 1(c) and (d), respectively. The rms roughness of the epitaxial structure grown on patterned AlN NL is 1.61 nm, and surface roughness of sample grown on uniform AlN NL is 0.43 nm. The former structure exhibits a spiral growth mode with sparsely distributed step-bunching as indicated on the bottom right of Figure 1(c). Nevertheless, no correlation between the underlying periodic AlN NL and the top step-bunching features is identified. The surfaces of both samples are mirror flat as seen by eye.
Figure 1. AFM images of the patterned (a) and uniform (b) AlN NL prior to MOCVD growth; Morphologies of the UV MQWs on patterned (c) and uniform (d) AlN NL.
RT PL spectra were collected from MQWs grown on both patterned and uniform AlN NL. Figure 2 (a) shows that PL intensity of UV-MQW grown on patterned AlN NL is two-fold stronger than that grown on uniform AlN NL under the same pumping power of ~ 20 kW/cm2. The full-width-half-maximum (FWHM) of the PL peaks are 14.2 nm and 12.0 nm, respectively. Figure 2 (b) and (c) show the log-scale power-dependent PL spectra of MQWs grown on patterned and uniform AlN NL, respectively. The pumping power ranges from ~0.2 kW/cm2 to ~50 kW/cm2, which
are rather low in order to mitigate carrier flooding effect inside the MQWs. The peak positions of these two samples are 388 and 385 nm with only 3 nm in difference.
Interestingly, no obvious peak shift as a function of pumping power is identified, indicating the suppressed quantum-confined-stark-effect for both samples. This can be ascribed to, firstly, good crystalline quality of the epitaxial layers and, secondly, carrier screening effect inside the MQWs under the pulsed laser pumping. The occurrence of small “side peak” at ~390 nm for MQW grown on uniform AlN NL might possibly due to amplified spontaneous emission from the cavity formed by the smooth top surface and GaN/sapphire interface, as a superlinear increase of the peak intensity versus pumping power is identified.
Figure 2. (a) RT PL spectra of UV-MQWs grown on different substrates under pumping power of 20 kW/cm2; Power-dependent PL spectra of UV-MQWs grown on patterned (b) and uniform (c) AlN NL.
IQE measurement was performed by taking PL spectra under room temperature and low temperature. And the IQE value can be simply calculated by the ratio of integrated PL intensity at RT to that at 9K, under the assumption that IQE at 9K is 100%
due to frozen non-radiative recombination centers. The IQE values for MQWs grown on patterned and uniform AlN NL are 68.5% and 55.7%, respectively under the same pumping power. This strongly confirms that MQWs grown on patterned AlN NL exhibit higher radiative recombination rate.
Figure 3. Comparison of IQE values estimated from the RT and LT-PL spectra of MQWs grown on patterned (a) and uniform (b) AlN NL.
To investigate the carrier dynamic properties of the UV-MQW samples, TRPL of the InGaN/GaN MQWs grown on patterned and uniform AlN NL were performed at approximately 10 K, and the decay curves are shown in Figure 4. The TRPL decay curves were analyzed using a two-term exponential decay model shown by equation (1) to depict the multiple recombination characteristics.
I(t) = 𝐴𝑓exp(− 𝑡
𝜏𝑓) + 𝐴𝑠exp(− 𝑡
𝜏𝑠) (1)
𝐴𝑓 and 𝐴𝑠 are fast and slow peak intensities before decaying. 𝜏𝑓 and 𝜏𝑠 are decay lifetimes of fast and slow components, respectively. The lifetime values are obtained and summarized in Table 1. The lifetime is obviously lower for MQWs grown on patterned AlN NL. Considering that the measurement was taken at 10K where most non-radiative recombination centers are frozen, MQWs grown on patterned AlN NL exhibits a faster radiative recombination rate than that grown on uniform AlN NL, potentially indicating larger radiative recombination rate, and higher IQE for the former case. This result is in perfect agreement with stronger PL intensity as observed in Figure 2.
Figure 4. TRPL spectra recorded at 10K for UV MQWs grown on patterned and uniform AlN NL.
Table 1. Summary of fast and slow components of the decay lifetimes and decay coefficients.
𝐴𝑠 𝜏𝑠/ns 𝐴𝑓 𝜏𝑓/ns 𝜏𝑡𝑜𝑡𝑎𝑙/ns MQW on patterned
AlN NL 8.52×1013 0.76 7.47×1013 0.67 0.72
MQW on uniform
AlN NL 2.18×1010 1.14 1.91×1010 0.99 1.08
To provide a comprehensive understanding on the optical behaviors of the MQWs, panchromatic CL intensity mapping on both samples were performed as localized emission properties are strongly correlated with defects inside the MQWs.
In Figure 5, dark spots featuring threading dislocations (TDs) or other point defects, which act as non-radiative recombination centers in the MQW region, are visualized throughout the graph. The dark-spot-density (DSD) of MQWs grown on patterned AlN-NL is calculated to be 1×108 cm-2, much lower than the value of 5×108 cm-2 in MQW grown on uniform AlN-NL. CL results are consistent with RT PL spectra as well as TRPL data, demonstrating an improved optical property of MQWs grown on patterned AlN NL.
Figure 5. Panchromatic CL intensity mapping of MQWs grown on patterned AlN-NL (a) and uniform AlN-NL (b).
The crystalline qualities of the above two samples are examined by HRXRD rocking curves (RCs). RC scans of GaN (002) and (102) peaks are shown in Figure 6 (a) and (b), respectively. For (002) RCs, the full-width-half-maximum (FWHM) values are estimated to be 511 and 61 arcsec for samples grown on patterned AlN NL and uniform AlN NL. For (102) RCs, the FWHM values are 504 and 241 arcsec.
Since the FWHM value of the (002) RC scan is closely related to the screw type dislocation density whereas that of (102) RC scan correlates with edge type dislocations, dislocation densities of these two samples can be herein calculated. The screw and edge type dislocations of both samples are summarized in Table 2. It is clearly shown that compared to MQW grown on uniform AlN NL, the screw type dislocation of the MQW on patterned AlN NL increases by 70 times, and the edge type dislocation also increases by 2 times. This is quite surprising as the dislocation density obtained by HRXRD shows the opposite trend to the optical properties.
Intuitively, one would expect higher dislocation densities inside epitaxial thin film degrade the optical properties as carriers would recombine non-radiatively at those defects, leading to lower IQE and poorer PL intensity. The defect-insensitive optical properties or even reverse relationship between PL intensity and crystalline quality of the UV-MQWs suggests that the optical behaviors of MQWs grown on patterned AlN NL is significantly affected by other factors such as carrier localization effect. Under the circumstance of carrier localization, threading dislocations or point defects are screened by the small carrier diffusion length, leading to defect-insensitive optical behaviors.
Figure 6. HRXRD (002) (a) and (102) (b) RC scans of the GaN peak in the samples grown on patterned and uniform AlN NL
Table 2. Calculated screw and edge type dislocations throughout the GaN epitaxial layer in both samples
Screw (cm-2) Edge (cm-2) MQW on patterned ALN NL 5.7×108 1.4×109
MQW on uniform ALN NL 8.1×106 7.0×108
Herein, to demonstrate the proposed carrier localization effect in MQWs grown on patterned AlN NL, temperature-dependent PL characterizations were taken in the range between 30 K to 300 K, and the peak energies versus temperatures are shown in Figure 7. An “S-shape” profile is clearly observed for MQWs grown on patterned AlN NL, which is usually regarded as a manifestation of carrier localization due to the slight composition fluctuation in the InGaN QW. At low temperature region up to 55K, a slight redshift of peak energy corresponds to carrier’s relocation to potential minima.[19] When temperature increases beyond 55 K, a blue shift of peak energy corresponds to thermal population of the density of states, while with even higher temperature above 135 K, a drastic redshift of peak energy denotes as the bandgap shrinkage by Varshni equation.[20] In contrast, no obvious S-shape behavior is identified for MQWs grown on uniform patterned AlN NL. This strongly indicates a
lack of carrier localization effect in the latter case as the indium composition is too low for spinodal decomposition. In fact, carrier localization was widely reported in InGaN with high indium content above 15%.[21] Usually, the shrinkage of III-nitride bandgap can be described by the well-known Varshni empirical equation. Taking the term of statistical alloy broadening into consideration, Varshni relation can be revised as below.
E(T) = E(0) −αT2
T+β− σ2
kBT (2)
where σ is the width of Gaussian distribution of the density of states and kB is Boltzman constant. The solid lines in the graph are the fitting curves according to equation 2, and σ of 13.5 meV and 3.2 meV were obtained. These values are considerably small compared to other reports,11, 12 which is directly related to low indium content and thus less carrier localization effect. However, MQWs grown on patterned AlN NL shows larger σ, qualitatively demonstrating the phase separation of indium by NL patterning. In fact, this work demonstrates that by proper growth modulation, carrier localization can also occur in InGaN MQWs with In content as low as 5%.
Figure 7. PL peak energy as a function of temperature for MQWs grown on patterned (a) and uniform (b) AlN NL with fitted curve representing Varshni relation
To provide a direct demonstration on carrier localization effect, STEM and EDS mapping was shown in Figure 8. By comparing the HAADF image with atomic Z contrast in Figure 8 (a), (b), (d) and (f), the interfaces of the MQWs differ dramatically in these two sample. The one grown on pattern AlN NL shows a rather
rough interface, while abrupt interface can be found in the MQWs with uniform AlN NL. The phase separation of indium can be observed more clearly by EDS mapping shown in Figure 8 (c) and (f), in which case the collection time are almost the same between these two samples. The InGaN/GaN MQWs grown on patterned AlN NL shows a much broader and uneven distribution of indium, while narrower and uniform indium content is observed for sample grown on uniform AlN NL. As a result, the HAADF and EDS mapping unambiguously demonstrated the existence of indium phase separation in MQWs on patterned AlN NL.
Figure 8. HAADF image of InGaN/GaN MQWs grown on patterned (a, b) and uniform AlN NL (d, e) as well as corresponding EDS mapping of indium distribution (c, f). The red arrows in (a) point out the position with low indium content in InGaN/GaN MQWs grown on patterned AlN NL.
As demonstrated above in Figure 2 to 4, UV-MQW adopting a patterned AlN NL exhibits superior luminescence intensity compared to that with uniform AlN NL despite much higher dislocation density. Furthermore, carrier localization effect was also clearly demonstrated by STEM images as illustrated above. However, at this point, the underlying mechanism that causing the carrier localization in MQW grown on patterned AlN NL is still not clear yet. To further investigate what are the driving forces, weak beam dark field (WBDF) TEM images were acquired under different reflection vectors g to observe the dislocations distribution throughout the whole structure (Figure 9). Threading dislocations with burgers vector of b=0002 and b=11-20 emerge from the sapphire substrate, where screw and mixed dislocations can be seen under g=0002 condition, and edge and mixed dislocations are visible under g=11-20 condition. For MQWs grown on patterned AlN NL, huge number of threading dislocations are illustrated at the bottom part of epitaxial thin film, which is in accordance with the large FWHM value of the XRD RC curve. Some of the
threading dislocations change their propagation directions from normal to parallel to the growth front, and finally annihilate. This is a strong indication of change in strain states. In comparison, dislocation distribution follows a different pattern in MQWs grown on uniform AlN NL. Threading dislocations randomly distribute throughout the epitaxial thin film, and there is no obvious bending of dislocations, suggesting a consistent strain state inside the thin film due to uniform AlN NL. To summarize, the different distributions of dislocations inside these two samples must originate from underlying AlN NL, from which strain conditions differ dramatically, and strongly influence the dislocation propagation behaviors.
Figure 9. Weak beam dark field TEM images of two MQW samples grown on different AlN NLs under different reflection vectors (a,b) g = 0002, and (c,d) g = 11-20, where screw with mixed type dislocations, and edge with mixed type dislocations can be observed respectively.
Indium phase separation and composition fluctuation are known to be related to the biaxial strain in InGaN layer grown on GaN.[22] Researchers claimed that compressive strains can prevent phase separation due to coherence strain.[23] On the other hand, partial relaxation of bi-axial strains inside epitaxial thin film is believed to promote the indium localization effect.[19] RSM measurement was carried out in this work to make a connection between bi-axial strain and carrier localization. RSM graph of MQW grown on patterned AlN NL is shown in Figure 10 (a). Strong GaN template peak is located in the center. The 0st order MQW peak is hidden beneath the GaN peak, with Qx and Qz values being 0.2775 and 0.7412 rlu. On the other hand, 0st
order MQW peak and GaN template peak are well separated in MQWs grown on uniform AlN NL as shown in Figure 10 (b). The Qx and Qz values are 0.2799 and
0.7408 rlu. The average lattice constant a and c of the InGaN/GaN MQWs are calculated by Qx and Qz via equation (3):
3 x
a= Q , c=5 2Qz (3)
Using the calculated lattice constants a/c and fully relaxed lattice constants a0/c0, the strain states of MQWs are obtained by equation (4):
0 0
( )
( )
Al
xx yy
Al
a a x
= = a x− , 0
0
( )
( )
Al zz
Al
c c x
= −c x (4)
It is calculated that the compressive strains of MQWs grown on patterned and uniform AlN NL are 0.14% and 0.4%, respectively, indicating a more relaxed strain state in the former condition, and consequently enhanced carrier localization effect as previously demonstrated.
Figure 10. RSM of (105) peak of MQWs grown on patterned (a) and uniform (b) AlN NL
It is noted that patterned AlN NL is analogous to the patterned sapphire substrate (PSS) where 3D growth of III-nitride is promoted and strain relaxation is achieved.
However, since the AlN NL is too thin to initiate strong enough 3D growth mode, no obvious dislocation annihilation can be observed. In fact, higher dislocation density is shown for sample grown on patterned AlN NL from Figure 6, indicating that patterned AlN NL does not have a positive effect to improve the crystalline quality.
Additionally, patterned AlN NL is also not expected to enhance light scattering due to similar refractive index difference between AlN NL and epitaxial-grown GaN.
Nevertheless, patterned AlN NL indeed shows a strong influence on the control of bi-axial strain and polarity of III-nitride thin film. In our previous work, we have demonstrated that III-nitride grown on sapphire deposited with LT AlN exhibits metal
polarity, whereas thin film grown on nitridated sapphire shows nitrogen polarity.[6, 24]
In this study, however, the area of the exposed sapphire is relatively small compared to that of AlN NL. Furthermore, under the specific growth condition of low V/III ratio, N-polar growth is annihilated, if there is any. Above two conditions lead to the domination of III-polar thin film, and more importantly, relaxation of bi-axial strains through crystal coalescence. The relief of compressive strains inside MQWs would allow large indium atoms to segregate, which would not happen in conventional circumstances. Last of all, we emphasize again that, even though indium phase separation has been widely discussed in InGaN alloys, most of the reports are focused on LED with visible wavelength range with high indium content. In those cases, indium phase separation is usually accompanied with degradation of surface morphology, such as formation of V-defect, with concurrent formation of indium-rich interfacial zone at the sidewall of V-defect.[25] In this work, however, no V-defect is observed due to relatively low indium content in the MQW, and thus electrical properties of UV-LEDs based on patterned AlN NL will not be degraded.
Conclusion
In summary, we have proposed a novel technique in the realization of high UV emission through patterning of AlN NL prior to MOCVD thin film epitaxy. In spite of the high threading dislocation density in the UV-MQW grown on patterned AlN NL, the luminescence intensity and radiative recombination rate are found to be much higher than that grown on uniform AlN NL. A defect-insensitive optical behavior of MQW was demonstrated by comparing CL mapping with HRXRD. Carrier localization was unambiguously demonstrated by an S-shape profile in temperature-dependent PL characterization. The underlying reason is attributed to be the promotion of carrier localization effect caused by partial relaxation of compressive strains through nanopatterned AlN NL. Our study ambiguously demonstrated that the utilization of patterned AlN NL greatly benefits future design of high-efficiency UV light emitters.
Acknowledgment
This work was supported by National Key Research and Development Program of China (2016YFB0400802), National Natural Science Foundation of China (61974149, 61704176), Key Research and Development Program of Zhejiang Province (2019C01080, 2020C01145), and Ningbo Innovation 2025 Major Project (2018B10088, 2019B10121)
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