APPLIED PHYSICS LETTERS 91, 211108 共2007兲
Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors Rajat Sharma, Yong-Seok Choi,a兲,b兲 Chiou-Fu Wang,b兲 Aurélien David, Claude Weisbuch, Shuji Nakamura, and Evelyn L. Hub兲 Materials Department, University of California, Santa Barbara, California 93106, USA
共Received 23 July 2007; accepted 12 October 2007; published online 21 November 2007兲 We report on the realization of highly efficient InGaN microcavity light-emitting diodes incorporating a high index contrast air-gap distributed Bragg reflector 共DBR兲. Detailed analysis deduces an effective cavity length of ⬃500 nm and cavity mode orders of 5 and 6 for measured Fabry-Pérot fringes. A value reflectivity of ⬃70% was determined for the 4.5 period air/ Al0.08Ga0.92N DBR through the analysis of cavity finesse based on the angle-resolved photoluminescence 共PL兲 data. A fivefold improvement in light extraction efficiency was verified by electrical probing as well as angle-resolved PL measurements. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2805028兴 III-nitride based optoelectronic devices1 have demonstrated outstanding optical performance, producing highly efficient sources for solid state lighting.2 Furthermore, the advances in III-nitride processing have led to the high index contrast, air-gap photonic devices such as air-gap distributed Bragg reflectors 共DBR兲,3 photonic crystal nanocavities with InGaN quantum wells,4 and high-Q InGaN microdisk lasers.5 The high index contrast between air and the III-nitride material ensures strong Bragg scattering as well as tight optical confinement, allowing a compact geometry for highly integrable, versatile optoelectronic devices as was demonstrated in other material systems.6 The air-gap DBR provides higher reflectivity per DBR period, alleviating the growth engineering of numerous DBR stacks,7–10 which is necessitated by the limited index contrast of III-nitride materials. The air-gap DBR can facilitate the development of efficient microcavity light-emitting diodes 共LEDs兲 and vertical-cavity surfaceemitting lasers 共VCSELs兲. Regarding LED applications, conventional GaN-based planar LEDs bear some attributes of a “weak” microcavity11 that is formed between two low-reflectivity mirrors, i.e., a top GaN/air and a bottom GaN/sapphire interfaces. The low reflectivity and the relatively large cavity thickness give rise to multiple cavity modes 共an order of ⬃40兲, which are more characteristic of a planar waveguide rather than of a microcavity. As a result, vertical emission of light in the planar geometry is limited by the large fraction of emission undergoing total internal reflection. On the other hand, a thin LED structure, which is comprised of a half-wavelength thick active and an underlying 4.5 period air-gap/GaN DBR, can improve the light extraction efficiency. Along with the direct benefit of preventing parasitic loss to substrate, the confined LED geometry ensures light emission into a finite number of cavity modes. Therefore, the modal emission characteristics, such as a resonant wavelength, bandwidth, and emission profile, can be engineered to significantly enhance one mode in preference to the others, yielding improved light extraction and control of the emission pattern. a兲
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[email protected] Also at California NanoSystems Institute, UCSB.
b兲
0003-6951/2007/91共21兲/211108/3/$23.00
In this letter, the achievement of strong microcavity effects is obtained through the formation of air-gap DBRs beneath the InGaN quantum well 共QW兲 active layer embedded within a GaN / AlGaN membrane, as shown in Fig. 1共a兲. The air-gap DBR comprised of 4.5 period air/Al0.08Ga0.92N layers can allow in theory a high reflectivity 共⬃99% 兲. The high index contrast between air and Al0.08Ga0.92N ensures a broader stop band as well as smaller penetration length in the DBR as compared to III-nitride DBRs. The starting material was grown by metal-organic chemical-vapor deposition on a GaN template on c-plane sapphire.3 The DBR region is initially comprised of the high-quality Al0.08Ga0.92N and InGaN superlattice sacrificial layers, as depicted in Fig. 1共a兲. The unique control over the selective removal of InGaN sacrificial layers was obtained by the photoelectrochemical 共PEC兲 etching, utilizing its bandgap selectivity3,12 as well as defect sensitivity.13 For electroluminescence 共EL兲 measurements, samples were prepared to have the 5-m-diameter concentric p contact placed on top of the 12-m-diameter microcavity layer.
FIG. 1. 共Color online兲 共a兲 A schematic diagram of the air-gap DBR LED, 共b兲 a wafer structure, 共c兲 a thick-cavity LED emission, and 共d兲 the emission from a microcavity LED with the air-gap DBR. The continuous-wave current density was ⬃1 kA/ cm2 in both cases.
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FIG. 2. 共Color online兲 共a兲 The angle-resolved PL spectra for the thick cavity emitter and the microcavity emitter with an air-gap DBR. 共b兲 The normalized PL spectra.
The transparent p contact was made of 5 nm thick Pd and 10 nm thick Au layers. For the n contact, we used 10 nm thick Ti and 300 nm thick Pt layers. The concentric n contact was laterally separated from the microcavity region by about 5 m. Under electrical injection, holes are provided through a tapered electrical probe to the p contact on top of the microcavity region. Electrons are supplied through the partially undercut pedestal to the microcavity region. To demonstrate the role of the air-gap DBR, we have examined the output emission of the same sample before PEC etching 共thick cavity LED兲 and after PEC etching 共microcavity LED兲. The microcavity LED demonstrated significant improvement in output emission as compared to the thick cavity LED, as shown in Figs. 1共c兲 and 1共d兲. A more detailed understanding of microcavity effects is found through angle-resolved photoluminescence 共PL兲 measurements,14 carried out for both thick cavity and microcavity structures under identical excitation and detection conditions. Figure 2共a兲 shows a composite plot representing the angle-resolved PL spectra for a thick cavity 共the left from −90° to 0°兲 and a microcavity 共the right from 0° to 90°兲; 0° corresponds to the sample surface normal. Significantly higher output emission was also observed from the microcavity emitter with the air-gap DBR. The observed data show the strongest directional emission in the range of 50°–70° with respect to the surface normal. A simple estimate of the total enhancement in the light extraction along the 180° arc is obtained by integrating the PL intensity. This calculation yields an enhancement factor of 5.5 times for the microcavity emitter, as compared to the thick cavity emitter. Considering that 4% of the emitted light escapes from the top surface of the thick cavity emitter, the estimated enhancement
Appl. Phys. Lett. 91, 211108 共2007兲
suggests that approximately 22% of the light escapes from the top surface of the microcavity emitter with the air-gap DBR.15 This number is consistent with the performance of a microcavity LED bound by air on top, with a high reflectivity mirror on the bottom, given the cavity order mc that we deduce below.11,16 While the modification brought to the far-field pattern by the air-gap DBR is clear, most of the modal emission signature is dominated by the intrinsic lineshape of the QWs. In order to restore the microcavity signature, we estimate the intrinsic lineshape of each device by integrating the far-field spectra over all angles and normalize the far-field pattern by this lineshape, as shown in Fig. 2共b兲. This procedure approximately restores the optical response to the structure, as if it were excited by a “white” light source, i.e., a light source of a spectrally flat lineshape. The normalized PL spectrum for the thick cavity emitter shows multiple Fabry-Pérot fringes, which arise from interference of light waves due to multiple reflections at the GaN/air and GaN/sapphire interfaces. The spacing of the observed fringes was used to extract the cavity length of about 4.3 m, which is in agreement with the epitaxially grown GaN thickness. By comparison, the normalized PL spectrum of the microcavity emitter reveals a single dominant Fabry-Pérot fringe at the wavelengths of ⬃460 nm. In the spectrum, the low reflectivity of the GaN/ air interface is in part responsible for the broad low-finesse profile. The finite lateral extent of the structure can make broad spectral background due to the contribution of stray light. To complete our analysis of the microcavity emitters, we introduced a second, three-period SiO2 / Ta2O5 共75/ 50 nm兲 dielectric DBR stack to supplant the GaN/air interface by electron-beam evaporation.17 The dielectric DBR was designed to have a stop band centered at a wavelength of 440 nm with a peak reflectivity of 75%, which was confirmed by a reflectance measurement on the dielectric DBR simultaneously deposited on a sapphire wafer. The resulting structure is similar to the geometry used for VCSEL and we thus designate it as a VCSEL-type microcavity emitter. In the right panel of Fig. 3共a兲, the angle-resolved PL spectrum of the VCSEL-type emitter displays the angular dispersion of two Fabry-Pérot modes separated by ⬃70 nm in wavelength. The angular emission profile is now more uniform and spectrally narrow and therefore suitable for comparison with the theoretical dispersion relation, mc = nk0Leff cos , where mc is the cavity mode order, n the index of GaN, k0 the free wavevector of light, Leff the effective cavity thickness, and the angle of light in the cavity, respectively. From the fitting shown in Fig. 3共a兲, we have obtained an effective thickness Leff ⬇ 500 nm, and mode orders mc = 5 and 6 for the intense and the faint modes of the VCSEL-type microcavity, respectively. To extract the reflectivity of the air-gap DBR, we analyze the cavity finesse defined by F = / ␦, where = nk0Leff cos and ␦ is the full width at half maximum of the cavity peak as a function of . The contour plot of Fig. 3共a兲 was transformed to obtain the intensity plot as a function of wavelength and of , as shown in Fig. 3共d兲. Noting that the value of in units of corresponds to the microcavity order mc, we can identify individual Fabry-Pérot modes, as shown in Fig 3共b兲. We can confirm that F ⬇ 10 and mc = 5 for the Fabry-Pérot mode at the wavelength of ⬇ 470 nm, as shown in the inset of Fig 3共b兲. Therefore, we
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ments confirms the Leff of 500 nm and the mc of 5 for the dominant microcavity-assisted emission and the reflectivity of 70% for the air-gap DBR, respectively. The potential integration of photonic crystals to these devices will facilitate GaN-based optoelectronic applications toward more-efficient LEDs and low-threshold lasers, sensors, and optomechanical systems. This work was funded by Solid State Lighting and Display Center at the University of California Santa Barbara and DMEA 共H94003-04-2-0403兲. 1
FIG. 3. 共Color online兲 共a兲 The normalized PL spectra for the thick cavity emitter and the VCSEL-type microcavity emitter. The white curves are the fitted dispersion curves. 共b兲 The PL spectra presented as a function of .
can deduce the reflectivity of R1 = 兩r1兩2 ⬇ 71% for the fabricated air-gap DBR using the measured reflectivity of R2 = 兩r2兩2 ⬇ 75% for the dielectric DBR and the relation F ⬇ 冑r1r2 / 共1 − r1r2兲, where r1 and r2 are the amplitude reflection coefficients for the air-gap DBR and the dielectric DBR, respectively. The low reflectivity that we have observed can be attributed to the unoptimized PEC etching process as well as the presence of the pedestal in the air-gap DBR structure that has a finite lateral extent. In conclusion, we have demonstrated outstanding optical performance of the microcavity LED with the air-gap DBR. The high reflectivity, short effective cavity length, and simple epitaxial structure of the air-gap DBR microcavity LED provide significant advantages for enhanced light extraction with controlled directionality. Extensive analysis of the device performance based on angle-resolved PL measure-
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