Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

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1 Approaches for high internal quantum efficiency green In light-emitting diodes with large overlap quantum wells Hongping Zhao,,3,4 Guangyu Liu, Jing Zhang, Jonathan D. Poplawsky, 2 Volkmar Dierolf, 2 and Nelson Tansu,5 Center for Optical Technologies, Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania 85, USA 2 Department of Physics, Center for Optical Technologies, Lehigh University, Bethlehem, Pennsylvania 85, USA 3 Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, Ohio 446, USA 4 hongping.zhao@case.edu 5 tansu@lehigh.edu Abstract: Optimization of internal quantum efficiency (IQE) for In quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered In QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered In QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the In sub-layers will not affect the performance of the staggered In QWs detrimentally. The growths of linearly-shaped staggered In QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of In QWs LEDs and other approaches to reduce dislocation in In QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in In QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer. 2 Optical Society of America OCIS codes: (23.367) Light-emitting diodes; (23.25) Optoelectronics; (4.42) Multiple quantum well. References and links. M. H. Crawford, LEDs for solid-state lighting: performance challenges and recent advances, IEEE J. Sel. Top. Quantum Electron. 5(4), 28 4 (29). 2. D. D. Koleske, A. J. Fischer, A. A. Allerman, C. C. Mitchell, K. C. Cross, S. R. Kurtz, J. J. Figiel, K. W. Fullmer, and W. G. Breiland, Improved brightness of 38 nm light emitting diodes through intentional delay of the nucleation island coalescence, Appl. Phys. Lett. 8(), (22). 3. J. Han and A. V. Nurmikko, Advances in AlGaInN blue and ultraviolet light emitters, IEEE J. Sel. Top. Quantum Electron. 8(2), (22). 4. M. Kneissl, D. W. Treat, M. Teepe, N. Miyashita, and N. M. Johnson, Continuous-wave operation of ultraviolet In/InAl multiple-quantum-well laser diodes, Appl. Phys. Lett. 82(5), (23). 5. D. Queren, A. Avramescu, G. Brüderl, A. Breidenassel, M. Schillgalies, S. Lutgen, and U. Strauß, 5 nm electrically driven In based laser diodes, Appl. Phys. Lett. 94(8), 89 (29). 6. K. Okamoto and Y. Kawakami, High-efficiency In/ light emitters based on nanophotonics and plasmonics, IEEE J. Sel. Top. Quantum Electron. 5(4), (29). 7. H. Zhao, J. Zhang, G. Liu, and N. Tansu, Surface plasmon dispersion engineering via double-metallic Au/Ag layers for III-nitride based light-emitting diodes, Appl. Phys. Lett. 98(5), 55 (2). 8. J. Henson, A. Bhattacharyya, T. D. Moustakas, and R. Paiella, Controlling the recombination rate of semiconductor active layers via coupling to dispersion-engineered surface plasmons, J. Opt. Soc. Am. B 25(8), (28). 9. X. Li, S. Kim, E. E. Reuter, S. G. Bishop, and J. J. Coleman, The incorporation of arsenic in by metalorganic chemical vapor deposition, Appl. Phys. Lett. 72(6), (998).. T. Jung, L. K. Lee, and P.-C. Ku, Novel epitaxial nanostructures for the improvement of In LEDs efficiency, IEEE J. Sel. Top. Quantum Electron. 5(4), (29).. X. Li, S. G. Bishop, and J. J. Coleman, epitaxial lateral overgrowth and optical characterization, Appl. Phys. Lett. 73(9), 79 8 (998). # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A99

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Lett. 96(22), 225 (2). 92. H. J. Kim, S. Choi, S.-S. Kim, J.-H. Ryou, P. D. Yoder, R. D. Dupuis, A. M. Fischer, K. W. Sun, and F. A. Ponce, Improvement of quantum efficiency by employing active-layer-friendly lattice-matched InAlN electron blocking layer in green light-emitting diodes, Appl. Phys. Lett. 96(), 2 (2).. Introduction III-Nitride semiconductors have significant applications for solid state lighting and lasers [ 2], power electronics [2], thermoelectricity [22,23], and solar cells [24 26]. The In quantum wells (QWs) employed as active region for nitride light-emitting diodes (LEDs) covers the wide spectral regime from near ultraviolet to near infrared [ 2]. As the emission wavelength of the In QWs extends from blue to green and red spectral regimes, the # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A994

5 internal quantum efficiency (IQE) in In QWs LEDs decreases significantly due to () high dislocation density results from the lattice mismatch between the sapphire substrate and / In leading to large non-radiative recombination rate, and (2) charge separation from the polarization fields in the QW leading to reduction of the electron-hole wavefunction overlap (Γ e_hh ) and radiative recombination rate (R sp ) in particular for green-emitting QWs. Recent works have reported the growths of highly-abrupt interfaces between In QWs and barriers with no indium clustering [27]. In addition, the existence of spontaneous and piezoelectric polarization fields in In QWs [28 34] led to charge separation effect, which in turn impacts both the radiative recombination rate and carrier dynamics in the QWs [35 37]. In order to maximize the external quantum efficiency of nitride LEDs, the optimizations of both extraction efficiency and IQE of the LEDs are of great importance. Various methods have been pursued to optimize the light extraction efficiency in nitride LEDs [5 2]. It is important to note that the reduction in dislocation densities in nitride alloy is also instrumental in reducing the non-radiative recombination rate in the LEDs, which will also lead to enhancement in the radiative efficiency of the devices [38 42]. Recently, the elimination of V-defect in In QWs had been reported resulting in enhancement in the IQE of the LEDs [38,39]. Recent works had reported a threading dislocation density reduction in blue- and green-emitting In LEDs grown on nanopatterned sapphire substrates [4,42]. Recently, several approaches have been proposed to suppress the charge separation issue by employing novel QWs with improved electron-hole wavefunction overlap (Γ e_hh ) such as () nonpolar In QWs [43 45], (2) staggered In QW [46 58], (3) In QW with δ- Al layer [59,6], (4) type-ii In QW [6 63], (5) In-delta-InN QW [64], and (6) triangular QWs [65]. These approaches used various methods to engineer the In QWs in order to obtain structures with large optical matrix elements, which in turn lead to significantly improved radiative recombination rate. Other approach based on strain compensated In QW design [66] had also been proposed to enhance the IQE of the LED, and recent experimental works [67] showed good agreement with theory. Recently, the use of staggered QWs [46 58] had also been implemented in ZnO / ZnMgO QWs structures [68]. In this paper, we investigated the approaches based on quantum well structures with large overlap design to enhance the internal quantum efficiency (IQE) for In QWs based LEDs. The optimization of both radiative efficiency and current injection efficiency for In QWs is crucial for the enhancement of the IQE of the QWs. Staggered In QWs are analyzed as active region for nitride LEDs with enhanced electron-hole wavefunction overlap (Γ e_hh ). The effect of abruptness of interface in staggered In QWs on Γ e_hh and radiative recombination rate (R sp ) is studied. In addition, the effect of current injection efficiency on efficiency droop in In QWs is studied. Novel structures designed by employing thin large band gap barrier materials surrounding the In QWs to suppress the efficiency droop are discussed. The optimization of both radiative efficiency and current injection efficiency in In QWs enables the realization of high internal quantum efficiency for nitride based LEDs emitting in the green wavelength region and longer. The organization of this paper is presented as follows. Section 2 introduces the concept of band structure engineering for In QWs with large electron-hole wavefunction overlap / large momentum matrix element. In Section 3, the staggered In QWs for high internal quantum efficiency green LEDs are presented. The effect of abruptness of interface in staggered In QWs is discussed in Section 4. Section 5 presents the epitaxy and characteristics of linearly-shaped staggered In QWs. Section 6 discusses the issues and approaches to address the efficiency droop in In QWs LEDs, as well as other important issues related to the internal quantum efficiency enhancement in In QWs LEDs. 2. Band engineering of In QWs for large electron-hole wavefunction overlap In conventional c-plane In QWs, the existence of strong electrostatic fields leads to strong energy band bending for both conduction band and valence band in the QW [28 34], which result in the spatial separation of the electrons and holes and reduce the electron-hole # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A995

6 wavefunction overlap (Γ e_hh ) [35 37]. The charge separation issue becomes more severe for In QWs with emission wavelength extended in the green and longer spectral regimes. Conventional QW Novel QW / QD y e y e Growth Direction y h Low electron-hole wavefunction overlap (G e_hh ) Large built-in Quantum Confined Stark Effect Low spontaneous emission rate and optical gain Large threshold carrier / current density y h High electron-hole wavefunctionoverlap (G e_hh ) Reduces Quantum Confined Stark Effect Enhances spontaneous emission rate and optical gain Reduces threshold carrier / current density Fig.. The concept of novel In QWs / QDs structures with improved electron-hole wavefunction overlap. To resolve the fundamental issue for the conventional In QW based LEDs due to the low electron-hole wavefunction overlap (Γ e_hh ), novel QW structures with enhanced overlap Γ e_hh are important to achieve active regions with large spontaneous emission radiative recombination rate (R sp ) [43 65]. As illustrated in Fig., by employing novel QW designs, the shift of the electron and hole wavefunctions toward the center of the QW leads to the reduction of the charge separation and enhancement of the electron-hole wavefunction overlap (Γ e_hh ). Based on the Fermi s golden rule, the transition matrix element is proportional to the square of the electron-hole wavefunction overlap ( Γ e_hh 2 ) [46]. Thus, with the enhanced Γ e_hh, the spontaneous emission radiative recombination rate will be enhanced. The improved optical matrix element leads to high R sp, which in turn enables the realization of high brightness nitride LEDs emitting in green spectral regime or beyond. 3. Staggered In QWs for high internal quantum efficiency green LEDs The purpose of using the staggered In QW design is to enhance the electron-hole wavefunction overlap (Γ e_hh ) by engineering the band lineups of the In QW, hence leading to an increase in the radiative recombination rate (R sp ) of the QW for LEDs application. The concept of staggered In QW has been introduced in references 46 58, which can be implemented as active regions in typical nitride-based LED devices [5 2]. In z Ga -z N In x Ga -x N In y Ga -y N In x Ga -x N In y Ga -y N In y Ga -y N Conventional In z Ga -z N QW (b) Two-Layer Staggered In x Ga -x N/In y Ga -y N QW (c) Three-Layer staggered In y Ga -y N/In x Ga -x N /In y Ga -y N QW Fig. 2. Schematics of the conventional In zga -zn- QW; (b) two-layer staggered In xga - xn / In yga -yn QW; and (c) three-layer staggered In yga -yn / In xga -xn / In yga -yn QW structures [47]. # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A996

7 Figure 2 shows the schematics of a conventional In z Ga -z N QW; (b) a two-layer staggered In x Ga -x N / In y Ga -y N QW and (c) a three-layer staggered In y Ga -y N / In x Ga -x N / In y Ga -y N QW structures, which are surrounded by the barriers [47]. Note that the three structures were designed with identical total QW thickness (d QW ) for comparison purpose. The studies of the characteristics of these staggered In QW LEDs were presented in references 47 and 48, and the analysis indicated that the use of green-emitting staggered In QW active region leads to significant enhancement in the radiative recombination rate by ~5-6 times for the optimized three-layer staggered In QW structure [47]. These results in turn correspond to ~ times enhancement in the expected increase in the radiative efficiency of the In QWs LEDs [47]. 4.5 x 4 4 Conventional In QW LED 4.5 x 4 4 Staggered In QW LED EL Intensity (a.u.) 3 2 Area = 5 m m x 5 m m I = 2 ma I = 5 ma I = ma I = 8 ma I = 5 ma I = 3 ma EL Intensity (a.u.) 3 2 Area = 5 m m x 5 m m I = 2 ma I = 5 ma I = ma I = 8 ma I = 5 ma I = 3 ma Wavelength (nm) (b) Wavelength (nm) Fig. 3. The electroluminescence spectra for conventional In QW and (b) three-layer staggered In QW LEDs emitting with peak wavelengths at nm, as function of injection current levels. Output Power (a.u.) In.2 Ga.79 N In.28 Ga.72 N 3-Layer staggered QW l peak ~ nm Area = 5 m x 5 m Room Temp EL 3-Layer Staggered In LED Conventional In LED Current Density (A/cm 2 ) Fig. 4. Light output power vs current density for conventional In QW and three-layer staggered In QW LEDs at λ~ nm, with the band lineups schematic of three-layer staggered In QW [48]. In order to carry out the experimental studies of the three-layer staggered In QW LEDs, the use of graded growth temperature technique in metalorganic chemical vapor deposition (MOCVD) was employed for the epitaxy of the active region [48]. The detail of the growth method was presented in reference 48. Figures 3 and 3(b) show the electrical luminescence (EL) for the conventional In QWs LED and three-layer staggered In QWs LED emitting at nm, measured under continuous wave (CW) operation at room # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A997

8 temperature [48]. Both devices were based on bottom-emitting square device, with area size of 5 μm x 5 μm. The enhancement of the peak EL for the three-layer staggered In LED as compared to the conventional LED is.8 times (.3 times) at I = ma (I = 2 ma). The FWHMs of the EL spectra for the three-layer staggered In QW LEDs were measured as larger [Fig. 3(b)], in comparison to those of the conventional LEDs. The larger FWHMs of the EL spectra were more obvious for increasing current density, which can be attributed to the less abrupt interfaces in the three-layer staggered In QW active region. The output powers versus the inject current density for conventional and three-layer staggered In LEDs measured under continuous wave operation were shown in Fig. 4. The output power of the three-layer staggered In QW LED was measured as ~2. times higher in comparison to that measured from conventional LEDs at high current density. The findings obtained from experiments are in good agreement with that predicted from theory..9.8 PL Intensity (a.u.) Conventional In QW (t=8.9ns) 3-Layer Staggered In QW (t=2.3 ns) t (ns) Fig. 5. Time resolved measurements on both 3-layer staggered In QW and conventional In QW LED samples, with peak emission wavelength at nm. The measurements were carried out by employing 43-nm excitation lasers with pulse duration of 25 ps [48]. In order to obtain the understanding of the carrier dynamic and recombination from the staggered In QW LEDs, the time resolved photoluminescence (TR-PL) measurements were performed for both conventional and three-layer staggered In QW LED samples emitting at nm [48]. The TR-PL measurements were carried out by utilizing a Nd:YAG laser that is doubled and tripled in frequency and whose ultraviolet output pumps an optical parametric generator / amplifier system that ultimately delivers short optical pulses with a wavelength than can be tuned from 42 nm up to 2 μm in the infrared. The excitation laser wavelength of 43-nm was employed for the measurements of the In QWs samples. The pulse duration, repetition rate, output power and beam diameter for the excitation laser were 25 ps, Hz, 3 μj / pulse, and ~3 μm, respectively. The emission from the In QW LEDs was detected by a photomultiplier tube (Thermo Oriel Instruments 775). The time evolution of photoluminescence from the In QWs was collected and displayed by the Lecroy LT584 oscilloscope ( GHz bandwidth). As shown in Fig. 5, the TR-PL measurements for both staggered (τ staggered = 2.3 ns) and conventional (τ conventional = 8.9 ns) In LEDs indicated a 35% reduction of carrier lifetime for the staggered In an QW LEDs. The TR-PL measurements for PL samples (with no p- cap layer) show the similar reduction of the carrier lifetime for the staggered In QWs. As compared with the conventional In QW LEDs, the staggered In QW LEDs show enhanced radiative efficiency, output power, and reduced carrier lifetime, which indicated that the increase in radiative recombination rate from the enhanced electron-hole wavefunction overlap is a dominant factor for the improved device performance. # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A998

9 Based on the following parameters, the carrier density in the In QW active region during the carrier lifetime measurement is estimated: () the 43-nm excitation laser power (3 μj / pulse), (2) the laser beam diameter (~3 μm), (3) the absorption coefficient for In material (α QW ~2 x 3 cm - ) [69], and (4) the total thickness of the In 4-QWs active region of 2 nm. Thus, the estimated carrier density in the In QW is in the range of mid- 9 cm -3. In order to estimate the monomolecular recombination coefficient (A) and Auger recombination coefficient (C) for the conventional and staggered In QWs, the total carrier lifetimes are calculated for both conventional In QW and three-layer staggered In QW emitting at ~525 nm at different carrier density as shown in Fig. 6. Based on the carrier lifetime measurements for both conventional (τ conventional = 8.9 ns) and staggered (τ staggered = 2.3 ns) In QWs, the estimated carrier density in In QW active region is about n ~5x 9 cm -3. In our analysis, we have made the assumption that the monomolecular coefficient (A) and Auger coefficient (C) for both conventional and staggered In QWs as identical. From Fig. 6, the estimated monomolecular recombination coefficient (A) and Auger coefficient (C) that match with the experimental measurements are A ~.6x 7 s - and C~5x -33 cm 6 s -. Based on the estimated monomolecular coefficient and Auger coefficient, the radiative carrier lifetimes (τ rad ) for both three-layer staggered In QWs and conventional In QWs emitting at λ ~ nm can be estimated as ~8.58 ns and ~39.35 ns, respectively, for carrier density n ~5x 9 cm -3. The finding indicates that the use of three-layer staggered In QWs leads to ~2.2 times increase in radiative recombination rate, in comparison to that measured for conventional In QWs. Total Carrier Lifetime (ns) A=.6x 7 s - C= 5x -33 cm 6 s - t Staggered ~ 9 ns t Conventional ~ 2.3 ns Conventional In QW Staggered In QW Carrier Density (x 8 cm -3 ) Fig. 6. Total carrier lifetime as a function of carrier density for conventional In QW and staggered In QW at monomolecular coefficient A =.6x 7 s - and Auger coefficient C = 5x -33 cm 6 s -. Note that the approaches of suppressing charge separation effect in In QWs discussed here are also applicable for In quantum dots (QDs) active regions [7 72]. Several approaches by engineering the In QDs have been pursued in order to achieve higher spontaneous emission rate by achieving structures with larger optical matrix elements [7 72]. In contrast to In QWs, one of key limitations for Al-based deep- and mid-uv LEDs and lasers is related to the valence band arrangement of heavy-hole (HH) and crystalfield split-off hole (CH) bands [73 75]. Thus, the optimization for Al QWs requires other method focusing on the use of novel QW structures that allow valence band lineups engineering in the active region [76]. # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A999

10 4. Effect of abruptness of interface for staggered In QWs In this section, the effect of interface linear grading on the electron-hole wavefunction overlap and spontaneous emission radiative recombination rate will be analyzed for the staggered In QWs. Here we assume that the interface between the staggered In QWs sub-layers contains the transition In layer with linearly-graded In-content profile. Five staggered In QWs structures with various thicknesses of transition In layers (2-Å, 4-Å, 6-Å, 8- Å and -Å interface In-content linear-grading) are compared with the staggered In QWs with abrupt In-content interface (-Å interface In-content linear-grading). Figure 7 shows the energy band lineups and the corresponding electron and hole wavefunctions for the staggered 6-Å In.4 Ga.86 N / 8-Å In.3 Ga.7 N / 6-Å In.4 Ga.86 N QWs with: -Å interface In-content linear-grading, and (b) 6-Å interface In-content linear-grading. Energy (ev) self-consistent 6-Ǻ In.4 Ga.86 N Ψ hh Ψ e 6-Ǻ In.4 Ga.86 N G = 3.2 % Energy (ev) self-consistent 3-Ǻ In.4 Ga.86 N Ψ hh Ψ e 3-Ǻ In.4 Ga.86 N G = 32.5 % 6-Å Interface Linear-Grading - 8-Ǻ In.3 Ga.7 N 5 5 z (nm) - 2-Ǻ In.3 Ga.7 N (b) 5 5 z (nm) Fig. 7. Energy band lineups and electron, hole wavefunction for staggered In QWs with -Å interface In-content linear-grading; (b) 6-Å interface In-content linear-grading. From Fig. 7, we observed that the band lineups for the staggered In QWs with a linear gradient of the In-content linear-grading [Fig. 7(b)] are significantly-less abrupt as compared to those for the staggered In QWs with abrupt In-content interface [Fig. 7]. However, both electron and hole wavefunctions exhibited relatively similar distribution for both staggered In QWs with abrupt and less abrupt interfaces. Thus, the electron-hole wavefunction overlap (Γ e_hh ) for the staggered In QWs shown in Fig. 7(b) have relatively minor modification. Spontaneous emission spectrum (x 27 s - cm -3 ev - ) n = x 9 cm -3 2-Å Interface Linear-Grading 4-Å Interface Linear-Grading 6-Å Interface Linear-Grading 8-Å Interface Linear-Grading -Å Interface Linear-Grading 6-Ǻ In.4 Ga.86 N / 8-Ǻ In.3 Ga.7 N / 6-Ǻ In.4 Ga.86 N Photon Energy (ev) Fig. 8. R sp spectra for staggered In QWs with less abrupt interface by In-content lineargrading. # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A

11 To investigate the effect of the In-content linear-grading on the spontaneous emission radiative recombination rate (R sp ), the calculations of R sp were performed for the six staggered In QWs structures as discussed. The comparison of the R sp spectra calculated at the carrier density of n = x 9 cm -3 for staggered In QWs is shown in Fig. 8. From Fig. 8, we observe slight modification on the R sp spectra including slight modification on the spontaneous emission peak intensity and integrated intensity. Slight blue shifts in the peak wavelength are also observed for the staggered In QWs with less abrupt interfaces. Thus, the In-content with linear-grading between the In sub-layers in the staggered In QWs will not lead to the degradation of the QW performance. The graded growth temperature technique introduced for the metalorganic chemical vapor deposition (MOCVD) of the staggered In QWs [48,49] is expected to introduce less-abrupt interface in the sub layers of the QWs. However, our finding shows that the introduction of less-abrupt interface does not lead to any reduction in spontaneous emission rate in the QW, which shows that the use of graded growth temperature approach is practical for manufacturing of staggered QW LEDs. 5. Linearly-shaped staggered In QWs 5. MOCVD of linearly-shaped (LS) staggered In QWs Similar to the concept of three-layer staggered In QW, QWs designed with local minima in the center will result in the shift of both electron and hole wavefunction toward the center of the QW region, which leads to significant enhancement in the electron-hole wavefunction overlap (Γ e_hh ) and spontaneous emission rate. By controlling the growth temperature during the In QW layers epitaxy, various Incontent can be obtained in the In QWs. By using the current control mode to control the growth temperature during the staggered In QWs growths, the real growth temperature profile for the staggered In QWs shows stable temperature control as well as fast temperature modification. The ability to achieve very precise good control of the growth temperature by using the current control mode led to the ability to achieve higher degree of control in the Indium contents in the sub-layers of the staggered In QWs. Growth temperature TMIn flow In-content T 2 y # #2 In In T x #3 In T 2 y Growth Temperature ( o C) (b) LS- Staggered In QW Staggered In QW Time (S) Fig. 9. Schematics of the growth temperature, TMIn-flow rate, and In-content for the LS- staggered In QW [Fig. 9] and the corresponding real growth temperature profiles [Fig. 9(b)]. Figures 9 show the schematics of three different linearly-shaped (LS) staggered In QWs [LS- staggered QW (Fig. 9), LS-2 staggered QW (Fig. ) and LS-3 staggered QW (Fig. )]. The left figures [Figs. 9, and ] correspond to the schematics of the designed growth temperature profiles and the In-content profiles. Note that the TMIn flow was kept constant for all the experiments. The bottom figures [Figs. 9(b), (b) and (b)] show the real growth temperature profiles by MOCVD using the current control mode. By comparing Figs. 9,, with Figs. 9(b), (b) and (b), the real growth # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A

12 temperature profiles match very well with the schematics. In contrast to the LS staggered In QW, the conventional In QWs contain constant In-content for the In QW, where the growth temperature for the conventional In QW is constant. Growth temperature TMIn flow In-content T x # #2 In In T 2 y Growth Temperature ( o C) (b) LS-2 Staggered In QW Staggered In QW Time (S) Fig.. Schematics of the growth temperature, TMIn-flow rate, and In-content for the LS-2 staggered In QW [Fig. ] and the corresponding real growth temperature profiles [Fig. (b)]. Growth temperature TMIn flow In-content T 2 y # #2 In In T x #3 In T 2 y #4 In Growth Temperature ( o C) (b) LS-3 Staggered In QW Staggered In QW Time (S) Fig.. Schematics of the growth temperature, TMIn-flow rate, and In-content for the LS-3 staggered In QW [Fig. ] and the corresponding real growth temperature profiles [Fig. (b)]. During the MOCVD growth, TMGa / TEGa, TMIn, and NH 3 were used as gallium, indium, and nitrogen precursors, respectively. Cp 2 Mg and dilute SiH 4 were used as p- and n- type dopant sources, respectively. The growth of In active layer employs TMIn, TEGa and NH 3 as the precursors, and N 2 gas was employed as carrier gas. The V/III and [TMIn] / [III] molar ratios for the growth of In layers were kept constant at 27 and.56, respectively. Both conventional and LS staggered In QW LEDs emitting at nm were grown on 2.5 μm thick n-doped (n = 4x 8 cm -3 ) on c-plane double-side polished sapphire substrate, employing a low temperature 3-nm buffer layer. Both conventional and linearly-shaped staggered In QW structures consist of 4-period of In QWs with nm undoped barriers. The conventional In QW was grown at 75 C, and the LS staggered In QWs were grown at varied temperature profiles as shown in Figs. 9(b), (b) and (b) ranging between 738 C and 76 C. The thickness of the LS staggered In QW is calibrated as 3 nm, which is similar to that of conventional In QW. On top of the In QWs, 2 nm p- (p = 3x 7 cm -3 ) were employed as p-type layer. # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A2

13 5.2 Cathodoluminescence measurement of linearly-shaped staggered In QWs The luminescence characteristics of both conventional and linearly-shaped staggered In QWs samples were studied by power-density-dependent cathodoluminescence (CL) measurements performed at T = 3K. We utilized a kev electron beam in spot mode (area = 2. x -9 cm 2 ) to excite the In QWs active region. To study the effect of the excitation power on the CL intensity for both conventional and linearly-shaped staggered In QW LEDs, different excitation power density levels were applied on both conventional and linearly-shaped staggered In QW LED samples. The various excitation power densities for a constant volume were obtained by varying the electron beam current (with constant accelerating voltage of kev) from 2 na up to 8 na. CL Intensity (a.u.) Integrated CL Intensity (a.u.) T=3 K Conventional In QW LS- Staggered In QW LS-2 Staggered In QW LS-3 Staggered In QW CL current =2 na CL current =5 na Wavelength (nm) CL Voltage = kv T=3 K LS-2 Staggered In QW LS-3 Staggered In QW LS- Staggered In QW Fig. 2. CL spectra of conventional, LS- staggered, LS-2 staggered, and LS-3 staggered In QWs emitting at 46-48nm with various CL excitation currents; (b) Integrated CL intensity of conventional, LS- staggered, LS-2 staggered, and LS-3 staggered In QW versus CL excitation currents at T = 3K. Figure 2 shows the measured CL spectra plotted against CL pump current (shown for 2 na and 5 na) of the conventional, LS- staggered In QW, LS-2 staggered In QW, and LS-3 staggered In QW. The linearly-shaped staggered In QW samples exhibit improved peak luminescence by times of that of the conventional In QW. Note that the linearly-shaped staggered In QWs show slight blue shift as compared to the conventional In QW due to the ramping up of the growth temperature during the growths of QWs, which requires further optimization of the growth temperature profile to match the emission wavelength to that of the conventional In QWs. Integrated CL intensities for Conventional In QW (b) CL Current (na) # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A3

14 both QWs LEDs were obtained by integration of the CL spectra data over the photon energy [Fig. 2(b)]. The LS- (LS-2, and LS-3) staggered In QW exhibited improvement by (.8-2.7, and ) times of the integrated CL intensity as compared to that of the conventional QW. Both conventional and LS staggered In QWs LEDs show blue-shift in emission wavelengths as the pumping current increases due to the carrier screening effect. 5.3 Numerical Simulation of Linearly-Shaped Staggered In QWs To numerically study the effect of the linearly-shaped staggered In QWs on the spontaneous emission radiative recombination (R sp ), the R sp properties of LS-3 staggered In QW is calculated and compared to that of the conventional In QW. Figure 3 shows the band lineups for conventional 3-Å In.25 Ga.75 N QW and (b) LS-3 staggered In QW, with the design wavelength at λ~5 nm. The detailed structure for LS-3 staggered In QW is shown in Fig. 3(c), which comprises two side layers of 5-Å In.8 Ga.82 N layers and linearly-shaped In x Ga -x N layers (.8<x<.4). From Figs. 3 and 3(b), the electron-hole wavefunction overlap (Γ e_hh ) is significantly enhanced from 7.3% (conventional In QW) to 32.8% by utilizing the LS-3 staggered In QW. The enhancement of the Γ e_hh is due to the shift of both electron and hole wavefunctions toward the center of the QW region from the band lineups engineering. The use of LS staggered In QWs can be practically implemented by using graded growth temperature approach. 5 4 Ψ e G = 7.3% 5 4 Ψ e G = 32.8% Energy (ev) 3 2 Ψ hh 3-Å In.25 Ga.75 N Energy (ev) 3 2 Ψ hh LS-3 In z (nm) z (nm) (b) 5-Å In.8 Ga.82 N -Å In x Ga -x N (.8<x<.4 ) In.4 Ga.6 N -Å In x Ga -x N (c) Fig. 3. Band lineups and electron and hole wavefunction for conventional 3-Å In QWs and (b) LS-3 staggered In QW. (c) Schematic of the conduction band lineup without the polarization field for LS-3 staggered In QW with barriers, where the sub-layers at side contain 5-Å In.8Ga.82N, and the linearly-shaped -Å In xga -xn contain In layers with In-content linearly modified from.8 to.4. The spontaneous emission spectra are calculated for both conventional In QW and LS-3 staggered In QW. Figure 4 shows the spontaneous emission spectra for both two structures at carrier density from n = x 8 cm -3 up to n = 2x 9 cm -3. Both structures are designed emitting at the similar wavelength (~5 nm). From Fig. 4, we observe significant enhancement in the spontaneous emission at different carrier density. In addition, the LS-3 staggered In QW shows less blue-shift of the peak emission wavelength as the # $5. USD Received 9 May 2; accepted 22 Jun 2; published Jul 2 (C) 2 OSA 4 July 2 / Vol. 9, No. S4 / OPTICS EXPRESS A4