Three-Dimensional Quantum Confinement of Charge Carriers in Self

Letter pubs.acs.org/NanoLett

Three-Dimensional Quantum Confinement of Charge Carriers in SelfOrganized AlGaN Nanowires: A Viable Route to Electrically Injected Deep Ultraviolet Lasers S. Zhao,† S. Y. Woo,‡ M. Bugnet,‡ X. Liu,† J. Kang,† G. A. Botton,*,‡ and Z. Mi*,† †

Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada ‡ Department of Materials Science and Engineering, Canadian Centre for Electron Microscopy, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada S Supporting Information *

ABSTRACT: We report on the molecular beam epitaxial growth and structural characterization of self-organized AlGaN nanowire arrays on Si substrate with high luminescence efficiency emission in the deep ultraviolet (UV) wavelength range. It is found that, with increasing Al concentration, atomic-scale compositional modulations can be realized, leading to three-dimensional quantum confinement of charge carriers. By further exploiting the Anderson localization of light, we have demonstrated, for the first time, electrically injected AlGaN lasers in the deep UV band operating at room temperature. The laser operates at ∼289 nm and exhibits a threshold of 300 A/cm2, which is significantly smaller compared to the previously reported electrically injected AlGaN multiple quantum well lasers. KEYWORDS: GaN, nanowire, LED, laser, ultraviolet, Anderson localization, quantum dot ince the first invention of semiconductor lasers in 1962,1,2 tremendous progress has been made in electrically injected semiconductor lasers in the visible, infrared, and terahertz wavelength ranges.3−9 In the rich deep ultraviolet (UV) spectrum, however, it has remained difficult to realize electrically injected semiconductor lasers or efficient light emitting diodes (LEDs).10−25 Bridging such a deep UV gap will underpin one of the emerging revolutions in photonics, i.e., the replacement of conventional mercury lamps by an efficient solid-state UV light source for a broad range of applications, including water purification, disinfection, biochemical detection, medical diagnostics, and materials processing, to name a few. In this context, AlGaN-based materials, with a direct energy bandgap in the range of 3.4−6.1 eV, have been intensively studied. Optically pumped AlGaN quantum well lasers that can operate in the UV−B (280−315 nm) and UV− C (100−280 nm) bands have been demonstrated, and the threshold power is generally in the range of 100 kW/cm.2,26−30 Such relatively high threshold is directly related to the unique properties of conventional AlGaN materials. Due to the large bandgap and large effective mass for both electrons and holes, it generally requires carrier densities on the order of 1019 cm−3 to reach transparency conditions (Ntr), i.e., the onset of population inversion in conventional deep UV AlGaN quantum well lasers.31 Accordingly, the transparency current density is given by,

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© 2015 American Chemical Society

Jtr =

qdNtr ητ i

(1)

For a carrier density of 1 × 1019 cm−3, an active region thickness d = 50 nm, carrier lifetime τ = 0.5 ns, and carrier injection efficiency ηi = 50%, the transparency current density Jtr is estimated to be around 30 kA/cm2, which sets the lower limit for the threshold current density of a deep UV AlGaN quantum well laser. Such a high current density can lead to severe carrier leakage, electron overflow, Auger recombination, and device heating effect, which make it extremely challenging to achieve lasing under electrical injection in the deep UV spectral range. Moreover, the device performance is severely limited by the presence of extensive dislocations and defects and the extremely poor p-type conduction in conventional AlGaN planar structures. The transparency carrier concentration, and ultimately the lasing threshold, can be drastically reduced by modifying the density of states (DOS) using quantum-confined nanostructures, such as quantum dots and quantum wires.32,33 With the use of such low-dimensional nanostructures, semiconductor lasers with significantly enhanced gain and differential gain have been proposed and experimentally demonstrated, leading to Received: May 30, 2015 Revised: October 9, 2015 Published: November 5, 2015 7801

DOI: 10.1021/acs.nanolett.5b02133 Nano Lett. 2015, 15, 7801−7807

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Nano Letters room-temperature threshold current densities as low as a few A/cm2.8 Furthermore, in such low-dimensional nanostructures, the formation of defects and dislocations can be significantly reduced, due to the effective strain relaxation. Such quantum dot lasers are generally grown in the conventional Stranski− Krastanov mode, wherein a relatively large lattice mismatch is required. Quantum-dot-like nanoclusters can also be induced by phase separation. For example, the presence of In-rich nanoclusters has been commonly observed in InGaN-based quantum well lasers, and the resulting carrier localization has been identified as one of the major factors contributing to the excellent performance of GaN-based quantum well lasers operating in the near-UV, blue, and blue-green spectral ranges.3,34−36 To date, however, there are very few reports on the formation of self-organized (Al)GaN quantum dots in the deep UV spectral range,31,37−41 partly due to the relatively small lattice mismatch (a maximum of 3% between GaN and AlN), prohibiting the realization of electrically injected quantum dot lasers in the deep UV band. In this Letter, we have investigated the structural and optical characteristics of AlGaN nanowires grown directly on Si substrate by utilizing a self-organized molecular beam epitaxial (MBE) growth process. We demonstrate that, with increasing Al incorporation, quantum dot/dash-like nanostructures can be formed in nearly defect-free AlGaN nanowires. The presence of extensive atomic-scale Ga-rich nanoclusters is revealed by scanning transmission electron microscopy (STEM) studies, which can lead to strong 3D quantum confinement due to the energy band fluctuation. Albeit such atomic-scale Ga-rich AlGaN behaves like quantum dots, the formation mechanism is beyond the conventional Stranski−Krastanov or Volmer− Weber growth modes. Our detailed analysis shows that the formation is likely related to the interplay between the chemical ordering and nonuniform incorporation of Ga and Al atoms at the nanowire growth front, partly due to the irregular shapes of nanowires. We have further demonstrated electrically injected lasers that can operate, for the first time, in the deep UV band at room-temperature, wherein photons are confined in the nanowire arrays by the Anderson localization of light. For a lasing wavelength of 289 nm, the threshold current density was estimated to be 300 A/cm2, which is nearly 2 orders of magnitude lower than the previously reported AlGaN quantum well lasers.42 In this work, catalyst-free AlGaN nanowires were grown directly on Si substrates by a Veeco Gen II radio frequency plasma-assisted MBE system under nitrogen rich conditions.43,44 The Si wafers were cleaned by standard solvents prior to loading into the system. The Si surface oxide was thermally desorbed at 770 °C in situ. Before growth initiation, a thin Ga seeding layer was utilized to promote the nanowire formation. Schematically shown in Figure 1a, the nanowire structure consists of Si-doped GaN (∼250 nm), p−i−n AlGaN junction (each layer ∼100 nm), and a thin (∼10 nm) Mgdoped GaN contact layer. The growth conditions include a nitrogen flow rate at 1.0 standard cubic centimeter per minute (sccm), a forward plasma power of 350 W, Ga beam equivalent pressure (BEP) in the range of 5 × 10−9 to 7 × 10−8 Torr, and Al BEP in the range of 8 × 10−9 Torr to 4 × 10−8 Torr. The growth temperatures for GaN and AlGaN layers are 780 and 800 °C, respectively. The Si and Mg dopant concentrations for n- and p-AlGaN regions were estimated to be 1 × 1018 cm−3 and 1 × 1020 cm−3 based on calibrations from Si and Mg-doped epilayers grown under similar conditions, respectively. The Mg

Figure 1. (a) Schematic of AlGaN nanowire heterostructure grown on Si substrate. (b) SEM image of self-organized AlGaN nanowire arrays formed on Si.

concentration in AlGaN nanowires, however, may be lower, due to the Mg surface desorption at the growth temperature of AlGaN nanowires. The typical SEM image taken with a 45degree angle of such AlGaN nanowires is shown in Figure 1b. It is seen that the nanowires possess relatively high uniformity in terms of the nanowire length and diameter. Detailed studies further suggest such AlGaN nanowires have nitrogen polarity (see Figure S1 in Supporting Information). Optical properties of AlGaN nanowires were studied using temperature variable photoluminescence (PL) spectroscopy. Shown in Figure 2a are the PL spectra measured at room-

Figure 2. (a) Room-temperature PL spectra of AlGaN nanowires with low Al concentration (Sample A) and high Al concentration (Sample B). (b) Variations of the internal quantum efficiency (IQE) of Sample B vs excitation power. The inset shows the semi-logarithmic plot under low excitation powers.

temperature for two representative nanowire samples, including a low Al concentration and high Al concentration sample, denoted as Samples A and B, respectively. Sample A was excited by a 266 nm diode-pumped solid-state laser, while sample B was excited by a 193 nm ArF excimer laser. The collected PL emission was spectrally resolved and detected by a photomultiplier tube detector. Compared to Sample A, the spectral line width of Sample B is about twice broader, which is consistent with the inhomogeneous broadening introduced by atomic-scale compositional modulations, i.e., quantum-dot/ dash-like nanostructures in AlGaN nanowires (to be described later). The luminescence efficiency was further estimated by comparing the integrated PL intensity at room-temperature with that measured at 10 K, assuming the luminescence efficiency at 10 K to be unity. In practice, it is noted that the luminescence efficiency at 10 K also varies with the excitation power and other factors; therefore it is necessary to examine the efficiency over a broad range of excitation powers. Shown in Figure 2b is the efficiency of AlGaN nanowires as a function of 7802

DOI: 10.1021/acs.nanolett.5b02133 Nano Lett. 2015, 15, 7801−7807

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structure is consistent with previous studies.15,43,44 The formation of such core−shell nanowire structures can significantly suppress nonradiative surface recombination and increase the carrier injection efficiency of nanowire LEDs and lasers.45,46 Shown in Figure 3c is a high-magnification image taken from the marked region in Figure 3a. The aforementioned Al-rich AlGaN shell near the nanowire surface (dark region) can be observed; in addition, this Al-rich AlGaN shell is further surrounded by a few atomic-plane thick Ga-rich AlGaN outermost shell. One other important feature to point out is, for such a low Al concentration, the nanowire bulk region shows mostly uniform compositional distribution. With increasing Al concentration, the presence of Al-rich AlGaN shell near the surface region can also be observed (see Supporting Information, Figure S2). However, in contrast to Sample A (low Al concentration sample), strong atomic-scale compositional fluctuations were formed within the AlGaN layers in Sample B (high Al concentration sample). Illustrated in Figure 4b is a high-magnification HAADF image from the pAlGaN region (boxed region in Figure 4a) of one AlGaN nanowire from Sample B, showing strongly modulated image intensity along the c-axis direction that can be attributed to local changes in Ga-concentration. The presence of extensive atomic-scale Ga-/Al-rich modulations along the growth direction can be clearly observed in the detailed view in Figure 4c. Further chemical evidence validating the compositional origin of the observed Z-contrast intensity modulations is provided by EELS-SI carried out at atomic-resolution. The Gamap and the concurrently acquired ADF signal in Figure 4d show a distinct enrichment in the Ga-signal that corresponds to the local increase in the ADF signal within single c-planes. An overlay of the integrated intensity line profiles of the ADF signal (blue) and the Ga-signal (red) in Figure 4e further emphasizes their unequivocal correspondence. The localized variations in the Ga-concentration are estimated to be ∼5−10 at.%. In addition, it should be noted that the Ga-rich AlGaN bands (brighter regions) are not continuous along the lateral direction (perpendicular to the growth direction), as exemplified in Figure 4c. Such Ga-rich AlGaN regions have sizes varying from a single atomic layer (0.25 nm) to 2 nm along the growth direction, and lateral sizes varying from 2 to 10 nm. Moreover, both a- and m-plane orientation views of the p-AlGaN segment (see Supporting Information, Figure S3) were used to deduce the approximate lateral dimensions of the Ga-rich regions. The observed lateral discontinuities suggest that the Ga-rich regions make up a small fraction of the nanowire diameter in one direction and may occupy a substantial portion of the projected thickness within the nanowire diameter (in order to be detectable). This indicates that the atomic-scale compositional fluctuations possess quantum dot/dash-like structural characteristics within the nanowires. It is further noted that such atomic-scale compositional modulations can be also observed in i- and n-AlGaN regions (see Supporting Information, Figure S2). The formation process of such quantum dot/dash-like features is likely related to the interplay between spontaneous chemical ordering and anisotropic atom migration from the irregular top/lateral surfaces of nanowire arrays. Previously, the spontaneous formation of long-range ordering of Al-rich and Ga-rich layers was observed in nanowires47,48 and thin films,49 which was explained by the significantly different binding energies between Ga−N and Al−N. Such spontaneous

excitation power for Sample B. It can be seen that the estimated efficiency stays nearly constant in the range of 70−80% over 2 orders of magnitude variations in the excitation power. Previous studies have shown that for AlGaN nanowire structures similar to sample A, the luminescence efficiency is ∼50%.44 The significantly higher efficiency of sample B is consistent with the presence of strong carrier confinement from quantum dot/ dash-like nanostructures (to be discussed in the following text). Detailed structural characterization of Samples A and B was further performed by high-resolution STEM. Figure 3a shows

Figure 3. Structural characterization of low-Al content AlGaN nanowires, Sample A. (a) STEM-HAADF image of one nanowire. (b) The pseudocolor EELS-SI map showing Ga and Al elemental distribution of the same nanowire, with weighted-principle components analysis (PCA) applied for noise reduction. (c) A highmagnification image from the boxed region in part a showing uniform Ga and Al distribution in the nanowire bulk region.

the high-angle annular dark-field (HAADF) Z-contrast image of a single AlGaN nanowire from Sample A taken with a double aberration corrected Titan Cubed 80−300 STEM under 200 kV. It is seen that the nanowire has a length of 600 nm and a top diameter of ∼50 nm, with an inversely tapered morphology. Electron energy-loss spectroscopy spectrum imaging (EELS-SI) was further performed to study the elemental distribution. Lateral profiles of the Ga-map suggest that a hexagonal crosssection with m-plane facets is maintained all along the nanowire. Shown in Figure 3b, the RGB pseudocolor image of Ga- and Al-signals suggests a low Al concentration in the core region and a high Al concentration in the shell region. The presence of an Al-rich AlGaN shell surrounding the nanowire 7803

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Figure 4. Structural characterization of high-Al content AlGaN nanowires, Sample B. (a) STEM-HAADF image of an overview of one nanowire, highlighting the n- and p-GaN segments in bright contrast. (b) High-magnification image from the p-AlGaN region (boxed in red dashed line in a), showing intensity modulations along the c-axis, indicative of local changes of Ga-composition. (c) Detailed view from the blue dashed box region in b, showing highly localized compositional modulations of alternating Ga-rich/Al-rich planes at the atomic-scale. (d) The Ga-map (displayed in temperature-scale) and concurrently acquired ADF signal from EELS-SI at atomic-resolution (boxed in green dashed line in b), showing a direct correspondence between the local increases in Ga-signal with the ADF signal within single atomic-planes. Weighted-principle components analysis (PCA) was applied for noise reduction. (e) Integrated intensity line profiles of the ADF signal (blue) overlaid with the Ga-signal (red) across the entire EELS-SI in d, further emphasizing that the HAADF intensity modulations are attributed to local enrichment of Ga-composition.

chemical ordering alone, however, cannot explain the formation of quantum dot/dash-like nanostructures. The macroscopic faceting in the high Al-content AlGaN nanowires was assessed from HAADF intensity profiles (see Supporting Information, Figure S3d), invalidating the presence of m-plane sidewall facets and revealing a-plane surfaces. In this work, due to the random nucleation and formation process as well as the shadowing/ coalescence effect of neighboring nanowires, self-organized AlGaN nanowires tend to develop into nonsymmetric shapes with such irregular sidewall facets. The nature of the surface facets strongly affects the diffusion of Al and Ga atoms along the sidewalls50 that can alter the supply of atoms to the top of the nanowire during the subsequent growth process. These effects, together with the difference in surface migration and desorption rates between Ga and Al atoms,48 can strongly modulate the competitive nature between Al- and Gaincorporation,40 driving the spontaneous chemical ordering process at the growth front. As such, quantum dot/dash-like nanostructures can be formed in self-organized AlGaN nanowire arrays, as illustrated in Figure 5. Effects of such compositional variations on the optical properties of AlGaN nanowires are manifested by the broad PL spectral line width as well as the extremely high luminescence efficiency (∼80%) at room-temperature, shown in Figure 2. Due to the large effective

Figure 5. Schematic illustration of the formation of AlGaN nanowires (nitrogen not shown here). The Al and Ga incorporation consists of impinging atoms and migrated atoms along the sidewalls, as illustrated. The irregular shapes of nanowires affect sidewall diffusion, and the difference in migration rates between Al and Ga atoms give highly nonuniform incorporation, which, together with the spontaneous chemical ordering at the growth front, leads to the formation of 3D quantum-confined nanostructure in AlGaN nanowires, illustrated in the inset.

mass of charge carriers in Al-rich AlGaN, the Bohr radii are only 1−2 nm, which is comparable to the size variations of the observed quantum dots and dashes. As a consequence, they can provide strong 3D quantum confinement. Moreover, such local 7804

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region of nanowire structures and the resulting efficient charge carrier hopping conduction.53−55 The electroluminescence (EL) spectra were measured at room temperature from the nanowire top surface. The output light was collected by a deep UV objective, spectrally resolved by a high-resolution spectrometer, and detected by a liquid nitrogen cooled CCD. The EL spectra under different injection currents are shown in Figure 6b. It is seen that, at low injection current (10 μA), only a very weak, broad emission spectrum (black curve) can be measured. As the injection current increases, an emission peak centered around 289 nm appears. It increases rapidly with injection current and becomes dominant at relatively high current densities. Figure 6c shows the integrated EL intensity of the 289 nm peak vs the injection current, which exhibits a clear threshold around 30 μA. The threshold current density was estimated to be 300 A/cm2 for a lasing area of 10 μm2. Variations of the background emission with increasing current are also shown in Figure 6c (black triangle), which was taken from the boxed region (spectral width ∼3 nm) in Figure 6b. Compared to the lasing peak at 289 nm, the integrated background emission shows a negligible increase above threshold, which is explained by the clamping of carrier concentration above threshold. The small increase of the background emission with increasing the injection current has been commonly measured from quantum dot lasers, due to the inhomogeneity of quantum dots and hot carrier effect.56 The inset of Figure 6c shows the L−I curve of the lasing peak at 289 nm in a logarithmic scale. Three distinct regions including spontaneous emission, amplified spontaneous emission, and lasing emission can be clearly observed, further providing an unambiguous evidence for the achievement of lasing. The lasing threshold of 300 A/cm2 is significantly smaller compared to the previously reported electrically injected quantum well lasers in the UV-A band (20 kA/cm2 at 336 nm).42 The extremely low lasing threshold is attributed to the drastically reduced transparency carrier density of 3D quantum-confined nanostructures, the nearly defect-free AlGaN core−shell nanowires, and the high Q optical cavity offered by Anderson localization. The derived line width vs injection current is shown in Figure 6d. It is seen that as the injection current increases, the line width decreases from 6 nm below threshold to 2.6 nm above threshold, further confirming lasing action. The relatively broad lasing line width (2.6 nm) is likely related to the presence of multiple modes in the random cavity and the broad gain spectrum due to the size dispersion of quantum dots/dashes. Lasing wavelength vs injection current was also investigated, illustrated in Figure 6e. The wavelength is nearly invariant above threshold, suggesting stable exciton emission. The device output power is estimated to be in the range of μW, which is largely limited by the light absorption of the p-GaN and p-metal contact layers. In conclusion, the spontaneous formation of quantum dot/ dash-like nanostructures in self-organized AlGaN nanowire arrays was unambiguously observed. The resulting 3D quantum confinement, together with the nearly defect-free nanowire structures, can drastically reduce the current density required for population inversion, leading to electrically injected AlGaN nanowire lasers with relatively low threshold at roomtemperature. This work sheds new light on the development of electrically injected small-scale deep UV lasers that were not previously possible.

compositional variations also induce strong perturbation to the energy band, due to the changes in the polarization fields. Previous studies have shown that such random nanowire arrays can function as a high Q optical cavity, due to the Anderson localization of light. Optically pumped lasing has been realized in GaN nanowires utilizing random cavities.51 More recently, we have demonstrated electrically injected AlGaN nanowire lasers operating at low temperatures.15,52 Our analysis suggests that such AlGaN nanowires, with optimized size and density, can lead to strong optical confinement (see Supporting Information). The optical cavity is formed due to the multiple scattering processes in the randomly distributed AlGaN nanowire arrays. The vertical optical confinement is further made possible by the inversely tapered geometry of nanowires (see Supporting Information), illustrated in Figures 1 and 4a. Shown in the inset of Figure 6a is the simulated electric field distribution of confined photons (λ = 290 nm) in the lateral dimension of near-randomly distributed AlGaN nanowires (see Supporting Information).

Figure 6. Lasing characteristics measured at room-temperature. (a) The I−V characteristics of AlGaN nanowire lasers. The inset shows the simulated electrical field distribution for wavelength at 290 nm. The area is 2.35 μm × 2.35 μm. (b) The EL spectra measured under different injection currents. (c) The L−I curve for the 289 nm lasing peak (solid blue circles) and the spontaneous emission (solid black triangles) from the boxed region in b. The inset shows the L−I curve of the 289 nm lasing peak in a logarithmic scale. (d and e) Line width and peak wavelength as a function of the injection current, respectively.

Electrically injected AlGaN nanowire lasers were subsequently fabricated using standard optical lithography and metallization techniques (see Supporting Information). No filling material was used during the fabrication process to avoid any light absorption. The devices were characterized under continuous-wave (CW) operation. The I−V characteristics measured at room temperature are shown in Figure 6a. It is seen that an excellent diode was formed with negligible leakage current. The device has a turn on voltage around 5 V. The excellent current−voltage characteristics measured here, compared to conventional planar devices, is due to the significantly enhanced dopant incorporation in the near-surface 7805

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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.5b02133. Additional structural information of AlGaN nanowires, laser device fabrication, and light confinement in selforganized AlGaN nanowire arrays (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 (905) 525-9140, ext 24767. *E-mail: [email protected]. Phone: +1 (514) 398-7114. Author Contributions

S.Z. and S.Y.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada and US Army Research Office under the grant W911NF-12-1-0477. Part of the work was performed in the McGill Nanotools-Microfab facility. SEM studies were performed in the Facility for Electron Microscopy Research, McGill University. STEM and EELS investigations were performed in the Canadian Centre for Electron Microscopy, a national facility supported by NSERC, the Canada Foundation for Innovation, and McMaster University.

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REFERENCES

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