GaN Core–Shell Single Nanowire ... - ACS Publications

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Nonpolar InGaN/GaN Core−Shell Single Nanowire Lasers Changyi Li,† Jeremy B. Wright,‡ Sheng Liu,‡,§ Ping Lu,‡ Jeffrey J. Figiel,‡ Benjamin Leung,‡ Weng W. Chow,‡ Igal Brener,‡,§ Daniel D. Koleske,‡ Ting-Shan Luk,‡,§ Daniel F. Feezell,† S. R. J. Brueck,† and George T. Wang*,‡ †

Center for High Technology Materials, University of New Mexico, 1313 Goddard Street SE, Albuquerque, New Mexico 87106, United States ‡ Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: We report lasing from nonpolar p-i-n InGaN/ GaN multi-quantum well core−shell single-nanowire lasers by optical pumping at room temperature. The nanowire lasers were fabricated using a hybrid approach consisting of a topdown two-step etch process followed by a bottom-up regrowth process, enabling precise geometrical control and high material gain and optical confinement. The modal gain spectra and the gain curves of the core−shell nanowire lasers were measured using micro-photoluminescence and analyzed using the HakkiPaoli method. Significantly lower lasing thresholds due to high optical gain were measured compared to previously reported semipolar InGaN/GaN core−shell nanowires, despite significantly shorter cavity lengths and reduced active region volume. Mode simulations show that due to the core−shell architecture, annularshaped modes have higher optical confinement than solid transverse modes. The results show the viability of this p-i-n nonpolar core−shell nanowire architecture, previously investigated for next-generation light-emitting diodes, as low-threshold, coherent UV−visible nanoscale light emitters, and open a route toward monolithic, integrable, electrically injected single-nanowire lasers operating at room temperature. KEYWORDS: Nonpolar, core−shell, nanowire, laser, InGaN, GaN

S

heterostructured nanowires have exhibited optically pumped lasing, as a result of the small gain volume of the quantum disk based active region. In comparison, radial quantum wells (QWs) grown on the nanowire sidewalls can easily provide one or more orders of magnitude greater active region volume, allowing for optically pumped lasing to be achieved in single aaxis oriented triangular core−shell nanowires with semipolar InGaN/GaN QWs.37 Recently, c-axis oriented hexagonal core−shell GaN nanowires with nonpolar InGaN/GaN multiple quantum wells (MQWs) have been heavily studied for next generation LEDs38−47 and solar cells.48 In contrast to a-axis or m-axis oriented GaN-based nanowires grown using metal catalysts by the vapor−liquid−solid (VLS) approach,49−51 c-axis oriented nanowires can be grown catalyst-free via selective area growth (SAG)29,52−54 or etched from standard c-plane GaN epilayers using top-down approaches,11,19,28,55−57 enabling much more precise control over arrangement, yield, diameter, and alignment. Additionally, InGaN/GaN quantum wells grown on

emiconductor single nanowire lasers have garnered heavy interest as potential low threshold,1 low power requirement, compact and integrable coherent light sources for wideranging applications, such as very large scale photonic integrated circuits,2 high-speed communications,3 optical probes, and sensing.4 Because of the advantages of the IIInitride (AlGaInN) semiconductor materials system, including their direct bandgaps, broad spectral range from the UV to near-infrared,5 low surface state density,6 and large exciton binding energies,7 GaN-based single-nanowire lasers have been among the most heavily studied.1−3,8−17 However, most single nanowire lasers demonstrated thus far consist of homogeneous, single composition materials (e.g., GaN, GaAs, or ZnO).10−12,16−21 Thus, the lasing emission is limited to specific wavelengths corresponding to the fundamental bandgap. More importantly, for achieving monolithic, integrable electrically injected single nanowire light-emitting didoes (LEDs) and lasers, a p-i-n diode heterostructure architecture is likely necessary. Spontaneous emission from full p-i-n junction nanowire LEDs has been demonstrated with axial InGaN/GaN quantum wells/ disks.22−31 Although lasing has been demonstrated from random ensembles of axially heterostructured nanowires32,33 or ordered photonic crystal arrays,34−36 to date no single axial © XXXX American Chemical Society

Received: October 26, 2016 Revised: January 5, 2017

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Figure 1. Schematic diagram of the InGaN/GaN core−shell nanowire laser fabrication process, SEM and TEM images of the InGaN/GaN core− shell nanowire laser. (a) Silica microspheres deposited on the n-type GaN film. (b) n-type GaN nanowires with tapered and rough sidewalls after ICP dry etch. (c) Straight and smooth sidewalls of n-type GaN nanowires are created after AZ400 K wet etch. (d) Shell layers (n-GaN layer, InGaN/ GaN MQW, AlGaN electron blocking layer, and p/p+ GaN capping layer) are grown on the n-type GaN nanowire template. (e) SEM image of a core−shell nanowire transferred onto a Si3N4/Si substrate. (f,g) High-resolution cross-sectional STEM images of a core−shell nanowire.

cores, a ∼7 μm thick c-plane Si-doped n-type c-plane GaN epitaxial film grown on a c-plane sapphire substrate by metal− organic chemical vapor deposition (MOCVD) was utilized as the starting material. Three micrometers diameter silica microspheres were deposited onto the GaN epilayer19,55 to act as the dry-etch mask. Next, a chlorine-based inductively coupled plasma (ICP) etch was employed to transfer the circular silica patterns into the GaN film, forming ∼7 μm long n-type GaN nanowires with rough and tapered sidewalls. In order to remove the ICP-induced damage of the material, obtain straight and smooth sidewalls, and tune the nanowire diameter, a KOH-based crystallographically selective wet etch was applied by soaking the dry-etched sample in a AZ 400K photoresist developer at 65 °C. Because of the crystallographically selective etch, n-GaN nanowires with smooth and straight sidewalls were created after the wet etch process19 with lengths of ∼7 μm and diameters of ∼150 nm. After the etch process, the n-GaN nanowire template was returned to the MOCVD reactor to subsequently regrow the 5-period InGaN/ GaN MQWs, AlGaN electron blocking layer, and p-GaN layer. Figure 1e shows a scanning electron microscope (SEM) image of a core−shell nanowire. Consistent with previous work,47,48 a pointed tip due to six semipolar {101̅1} facets that form during the regrowth step is observed. A scanning transmission electron microscopy (STEM) image of a core−shell nanowire crosssection is shown in Figure 1f, revealing a hexagonal crosssection and 5 pairs of {101̅0} m-plane InGaN/GaN MQWs. As seen in Figure 1f,g, the nanowire sidewalls exhibit atomically smooth sidewalls, which is important for reducing optical loss and lowering the lasing threshold. The thicknesses of the InGaN quantum wells and the GaN quantum barriers as measured from Figure 1g are approximately 4.5 nm and 5.5− 6.0 nm, respectively. The AlGaN layer and p-GaN layer have thicknesses of ∼3.5 and 12 nm, respectively. An In composition in the InGaN QWs of ∼8% (92% Ga) is measured using STEM energy-dispersive X-ray spectroscopy (EDS) mapping. The core−shell nanowires were dry transferred onto a Si3N4/ Si substrate using a cotton swab for μ-PL characterization at room temperature. The optical setup has been described in

nonpolar m-plane {101̅0} sidewalls of the c-axis oriented hexagonal nanowires eliminate the quantum-confined stark effect (QCSE), resulting in higher quantum efficiency and spectral stability.58 The reduction of QCSE also gives rise to an increased overlap of the electron and hole wave functions, leading to a larger momentum matrix element for nonpolar QWs.59 It is also measured that nonpolar QWs have lower transparency carrier density than c-plane QWs.60 As a result, the optical gain for nonpolar InGaN QWs is significantly higher (up to 10 times higher by calculation) than for polar c-plane QWs at a given charge carrier density, which is promising for low threshold laser diodes.59,60 It was also recently reported that nonpolar InGaN/GaN core−shell nanowires have Auger constants two orders of magnitude lower than those reported for planar c-plane MQWs,61 which could help to mitigate efficiency droop in LEDs. The hexagonal c-axis oriented nanowires also provide a more symmetric, uniform geometry compared to the a-axis or m-axis oriented nanowires that have an isosceles triangular cross-section consisting of two semipolar {1011̅ } facets and one polar (0001)̅ facet.51,62 These different sidewall facets have different polarization and structural properties and considerably different InGaN growth rates, further complicating efforts to realize practical devices from triangular a-axis and m-axis oriented GaN nanowires.37,62 Here, we demonstrate optically pumped, room-temperature lasing from individual p-i-n core−shell GaN/InGaN nanowires with nonpolar InGaN/GaN MQWs. The lasing characteristics were studied in detail by intensity-dependent micro-photoluminescence (μ-PL) experiments and analyzed using the Hakki-Paoli method and mode simulations. Lower lasing thresholds due to high optical gain, compared to previously reported semipolar MQW nanowire lasers,37 were realized. The results show the promise of nonpolar core−shell InGaN/GaN nanowires as integrable and low threshold coherent nanoscale light sources emitting in the UV−visible wavelengths. The p-i-n InGaN/GaN nonpolar core−shell nanowires were fabricated using a hybrid approach composed of a top-down two-step etch process and a subsequent regrowth process (Figure 1).19,48,55 To fabricate the template of n-GaN nanowire B

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Figure 2. Emission from a core−shell nanowire laser under optical pumping. (a) The L−L curve of a core−shell nanowire laser with a lasing threshold of ∼182 kW/cm2. Inset: log−log plot of the L−L curve showing an s-shaped curve. (b) Spectra of the core−shell nanowire laser optically pumped at various pump power densities. Far above threshold (794 kW/cm2), a 0.28 nm wide narrow lasing peak centered at 391 nm is observed. (c,d) CCD images of the emission from the core−shell nanowire laser pumped at 96 and 794 kW/cm2, respectively.

detail previously.10,17 A pulsed, 266 nm frequency-quadrupled Nd:YAG laser was utilized to optically pump the core−shell nanowires. The short pulse width (400 ps pulse width, 10 kHz repetition rate) of the pump laser minimizes heating effects during optical pumping. The pump laser beam transmits through a variable neutral density filter and is then focused to a spot with a diameter of ∼28 μm by a 50× UV objective and a lens. This spot size allows for uniformly pumping individual nanowires. The emission from the nanowires was collected by the same objective and analyzed by a fiber-coupled spectrometer. A charge coupled device (CCD) camera was used to capture the image of the emission from the nanowire lasers. The peak intensity of the emission from a single core−shell nanowire laser is plotted as a function of the pump power densities (light-in-light-out (L−L) curve) in Figure 2a. When the nanowire laser is pumped below ∼160 kW/cm2, the peak intensity increases linearly with a smaller slope as the pump power density increases. When the pump power density is greater than ∼200 kW/cm2, the curve remains linear but with a much higher slope. The output power P0 of a semiconductor laser under optical pumping is given by63 ⎞ hν ⎛ αm P0 = αηr βspηiA wire⎜ ⎟ L ⎝ ⟨αi⟩ + αm ⎠ hνp

(L < L th)

⎞ hν ⎛ αm P0 = αηiA wire⎜ ⎟ (L − L th) ⎝ ⟨αi⟩ + αm ⎠ hνp

(L > L th)

On the basis of eqs 1 and 2, the output powers of a core− shell nanowire laser well below and well above threshold are linear functions of the pump power density with two different slopes. When the core−shell nanowire laser is pumped below threshold, the spontaneous emission dominates due to the insufficient modal gain, resulting in a generally lower radiative efficiency. In addition, only a small amount of the spontaneous emission is coupled to the lasing mode. Therefore, the spontaneous emission coefficient is much less than 1. Consequently, the slope of the L−L curve below threshold is much smaller than above threshold. The clear change of slope in the measured L−L curve of the core−shell nanowire laser is consistent with eq 1 and 2. By fitting the peak intensities after the slope change with a linear function, a lasing threshold of ∼182 kW/cm2 is calculated. The L−L curve is also plotted in a log−log scale shown in the inset of Figure 2a. The L−L curve is composed of two linear regions and a superlinear region in-between, which represents the transition from below threshold to above threshold. The s-shape of the L−L curve further verifies lasing emission from the core−shell nanowire laser, rather than amplified spontaneous emission.64,65 Figure 2b shows the spectra of the emission from the core− shell nanowire laser at various pump power densities. When the nanowire was optically excited at 96 kW/cm2, a broad-band spectrum with a full width at half-maximum (FWHM) of ∼14 nm centered at 397 nm was observed, corresponding to spontaneous emission. The corresponding CCD image (Figure 2c) also shows nearly spatially uniform emission across the nanowire laser without interference fringes, indicating incoherent emission from the nanowire. No blue shift of the peak wavelength was observed with increasing pump power, indicating emission is from the nonpolar InGaN/GaN MQWs. When the nanowire was pumped at 794 kW/cm2, a narrow-band lasing peak with a FWHM of ∼0.28 nm was measured. The small diameter (∼400−500 nm) of the core− shell nanowire laser cannot support well-confined whispering gallery modes. As a result, bright emissions at both ends of the nanowire laser are observed in the corresponding CCD image

(1)

(2)

where α is the nanowire absorption coefficient of the pumping laser; ηi and ηr are the injection efficiency and the radiative efficiency; βsp is the spontaneous emission coefficient, representing the percentage of the spontaneous emission coupled into the lasing mode; αm and ⟨αi⟩ are the mirror loss and the modal internal loss; hν and hνp are the photon energy of the emission and pumping laser; L and Lth are the pump power density and threshold pump power density. Awire is the illuminated area of the nanowire. C

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Nano Letters Table 1. Parameters of the Modal Gain versus Pump Power Density or Carrier Density Formula wire number

Γg0 for ⟨g⟩ (L) (cm−1)

Ltr (kW cm2)

Γg0 for ⟨g⟩ (N) (cm−1)

Ntr (1019 cm−3)

modal loss (cm−1)

threshold (kW/cm2)

1 2 3 4 5 6 7 semipolar MQW wire37

6136 5333 7132 13770 13227 10873 6380

234 142 131 122 256 266 209

15350 12870 17140 32980 32820 27100 15860

1.2 1.0 0.97 0.94 1.3 1.3 1.2

6500 6200 5300 7000 6600 6200 6600

598 323 258 182 360 422 520 ∼700−2000

(Figure 2d), implying a strongly guided Fabry−Perot mode rather than a whispering gallery mode, and the interference fringes indicate coherent emission. In order to evaluate the performance of the nonpolar InGaN/GaN core−shell nanowire lasers, the lasing thresholds and modal gains of seven nanowires were measured using the μ-PL setup, as shown in Table 1. Thresholds of these seven nanowires averaged ∼380 kW/cm2 and ranged from 182−598 kW/cm2. We note that this is significantly lower than the lasing thresholds reported for a-axis oriented, semipolar InGaN/GaN MQW nanowires by Qian et al. (∼2000 and ∼700 kW/cm2 for 13 MQW and 26 MQW structures, respectively). This is especially significant considering the much longer lengths (∼20−40 μm previously compared to ∼5.2−5.6 μm here) and greater number of MQWs (13 and 26 previously versus 5 here) for the semipolar MQW nanowires, which should result in ∼4− 8 times lower thresholds compared to the nonpolar MQW nanowires, other factors being equal.37 The lower lasing thresholds of the nonpolar InGaN/GaN MQW nanowires results from lower transparency carrier density and/or higher differential gain compared to the triangular semipolar InGaN/ GaN MQW nanowires. In order to further study the material gain properties and the onset of lasing, the modal gains minus the modal losses of the lasing modes of the core−shell nanowire lasers were measured using the Hakki-Paoli method (Figure 3 a).66,67 The resolution of the spectrometer (0.1 nm) is much smaller than the mode spacing (∼3 nm) of the core−shell nanowire lasers, allowing for completely resolving the longitudinal modes and calculating the modal gain. The modal gain can be written as a function of the carrier density ⎛N⎞ ⟨g ⟩ = Γg0 ln⎜ ⎟ ⎝ Ntr ⎠

(3)

where Γ is the confinement factor, representing the percentage of the mode overlapping the gain medium, g0 is the gain coefficient of five quantum wells, Ntr is the transparency carrier density, and N is the carrier density. When the nanowire is pumped below lasing threshold, spontaneous emission dominates. Therefore, the carrier density and the resulting modal gain increase as a function of the pump power density A wireηi Vactivehνp

Figure 3. (a) Measured modal gain minus the modal loss (black) and output peak intensity (blue) plotted as a function of the pump power density. The red curve shows the fitting modal gain curve. The modal gain clamps at the lasing threshold indicated by the L−L curve, indicating the onset of lasing. (b) The gain spectra of a core−shell nanowire laser at five pump power densities. (c) The fitting modal gains vs pump power density for the seven nonpolar InGaN/GaN MQW core−shell nanowire lasers.37

L = AN + BN 2 + CN3

⎛ L⎞ ⟨g ⟩ = Γg0 ln⎜ ⎟ ⎝ L tr ⎠

(4)

where A, B, and C are the Shockley-Read-Hall recombination coefficient, the radiative recombination coefficient, and Auger recombination coefficient, respectively, and L is the pump power density. For convenience, the modal gain is also commonly expressed as a function of the pump power density

(5)

where Ltr is the transparency pump power density. On the other hand, when the nanowire is pumped above lasing threshold, stimulated emission dominates. Because of the fast stimulated emission process, excess carriers are consumed D

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Nano Letters during stimulated recombination. Thus, the carrier density becomes clamped near the threshold value. As a result, the modal gain quickly clamps and approaches to the total loss after passing the lasing threshold.63 The clamping of the measured modal gain shown in Figure 3a is consistent with the measured L−L curve, indicating lasing from the nanowire lasers. Figure 3b shows gain spectra of a core−shell nanowire laser at five different pump power densities measured by using the Hakki-Paoli method (further details available in the Supporting Information). Because of the larger separation of the quasi Fermi energies, the wavelength of the peak gain blue-shifted with increasing pump power density. For shorter wavelengths, higher modal gain was observed. However, there is no significant increase of the modal gain at longer wavelength (∼410 nm), suggesting that the modal gain at ∼410 nm is close to zero. Because the Hakki-Paoli method measures the difference between the modal gain and the modal loss, the modal loss, αi + αm, can be determined to be approximately 6500 cm−1 from the long wavelength part of the gain spectra. Using the measured modal loss, the parameters Γg0 and Ltr in eq 5 were calculated (shown in Table 1) by fitting the modal gain versus pump power density below lasing threshold using eq 5. Using the calculated parameters, the modal gain curves of the nonpolar core−shell nanowire lasers were plotted in Figure 3c. On the basis of the absorption coefficient of InGaN/GaN MQWs,68−70 the injection efficiency of optically pumped core− shell nanowire lasers was estimated as 0.4 using a finitedifference time-domain (FDTD) simulation (see Supporting Information for more details). With the estimated injection efficiency and a reasonable assumption of the ABC coefficients (A = 1 × 107 s−1, B = 2× 10−11 cm3/s, C = 1.5 × 10−30 cm6/s), transparency carrier densities Ntr consistent with planar InGaN QWs were calculated (shown in Table 1) by the Hakki-Paoli method.71,72 The results show that the Hakki-Paoli method can be a powerful tool for analyzing the gain properties of nanolasers, which due to the small dimensions are typically difficult to analyze using other techniques that are utilized for regular laser diodes. Various lasing thresholds of the nonpolar core−shell nanowire lasers were also observed. Assuming the required modal gain to reach the lasing threshold is 6000 cm−1 for five quantum wells, the required threshold pump power densities of the seven nanowire lasers were calculated based on Figure 3c and are plotted versus the threshold pump power density of the corresponding nanowire lasers in Figure 4a. Because a lower required pump power density implies a higher gain quality, the plot shows that a nanowire that has a higher gain quality corresponds to a lower lasing threshold. The modal losses of the core−shell nanowire lasers were also correlated to the measured lasing threshold, and no obvious relationship was observed, implying that the gain inhomogeneity is the dominant factor for the lasing threshold, most likely from inhomogeneous regrowth of the active region across nanowires. To further study the inhomogeneous material property, the linewidths of the spontaneous emission spectra were fitted using a Lorentzian function and plotted in Figure 4b. For nanowires with wider spontaneous emission linewidths, photons from spontaneous emission are spread into a wider wavelength range rather than concentrated in the lasing wavelength. Therefore, a lower spontaneous emission factor β would be expected. Because the lasing threshold is proportional to −β, a wider spontaneous emission linewidth results in a higher lasing threshold.

Figure 4. (a) Calculated required pump power densities (g = 6000 cm−1) of five quantum wells for the core−shell nanowire lasers and the corresponding threshold pump power density. (b) Calculated required threshold carrier densities (g = 6000 cm−1) of five quantum wells for the core−shell nanowire lasers and the corresponding linewidths of the spontaneous spectra.

Another factor that would affect the modal gain is the transverse confinement factors of the optical modes. An eigenmode solver (Lumerical MODE solution) was thus utilized to calculate the transverse modes and the corresponding confinement factors. Owing to the annular shaped active region, transverse modes with nodes at the center result in higher transverse confinement factors. For example, Figure 5

Figure 5. Mode profiles of (a) HE11 mode and (b) HE31 mode. The solid lines represent the outer boundary of the nanowire. The dashed lines represent the boundaries of the active region. Because of the annular geometry of the active region, annular shaped transverse modes have higher transverse confinement factors than the solid transverse modes.

shows that the mode profile of the HE31 mode better overlaps the active region than the HE11 mode does. Therefore, there is a significant variation of the transverse confinement factors (Table 2), which could play a major role in the modal gain. However, because the electric field parallel to the quantum wells obtains much higher gain than the one perpendicular to the quantum wells,63 the electric field profile should also be considered, which complicates the analysis of the transverse E

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subsidiary of Lockheed Martin Corporation for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Table 2. Transverse Confinement Factors of Transverse Modes mode

HE11

TE01

TM01

HE21

EH11

HE31

Γxy

0.106

0.193

0.180

0.186

0.207

0.237

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confinement factors. In addition, due to the difficulty of identifying the actual lasing mode, the transverse confinement factors were not correlated to the measured lasing thresholds here. In conclusion, lasing from single, nonpolar p-i-n core−shell InGaN/GaN MQW nanowires was demonstrated under optical pumping at room temperature. The nanowires were fabricated using a hybrid top-down two-step etch and bottom-up regrowth process, allowing for precise control over the nanowire geometry and high material quality. Significantly lower lasing thresholds due to high optical gain were measured compared to previous semipolar MQW nanowires, despite the much shorter cavity lengths and reduced active region volume. Detailed analysis of the lasing characteristics using Hakki-Paoli method suggest that the observed variations in lasing thresholds across different nanowires are due to inhomogeneous growth rather than differences in modal loss. While nonpolar InGaN/ GaN MQW core−shell nanowires have previously shown promise for next-generation LEDs, these results demonstrate that this architecture is also viable and attractive for nanolasers, providing a route forward for integrable, electrically injected single-nanowire UV−visible lasers for nanophotonic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04483. The principle of the Hakki-Paoli method and the estimation of the absorption coefficient of InGaN/GaN core−shell nanowires at 266 nm (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

George T. Wang: 0000-0001-9007-0173 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Sandia’s Laboratory Directed Research and Development program. C.L. and S.R.J.B. acknowledge funding from Sandia’s Solid-State-Lighting Science Energy Frontier Research Center, funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. This work was performed in part at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned F

DOI: 10.1021/acs.nanolett.6b04483 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.6b04483 Nano Lett. XXXX, XXX, XXX−XXX