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In this paper, we studied the effect of temperature and mask margin size on optical emission and growth rate enhancement (GRE) of InGaN grown by metal organic chemical vapor deposition (MOCVD) and nano-selective-area growth (NSAG) on AlN-buffered Si(111). For all mask geometries and temperatures, NSAG produced 90% single-crystal InGaN nanopyramids with smooth facets, perfect selectivity, and 1.2 times the indium composition enhancement (23% and 33% for 800 °C and 780 °C NSAG, respectively) as found in non-NSAG planar growth at the same conditions. The vapor phase diffusion model was found to be insufficient to predict NSAG GRE, and we propose an explanation combining mechanisms from the vapor phase diffusion with surface migration models. A two-peak emission was noted for all NSAG. The total and relative intensities of the two peaks was found to be strongly dependent upon both temperature and local indium precursor concentration during growth, the latter of which varies based on mask margin size. In NSAG grown at lower temperature and with higher local indium precursor concentration, the bluer of the two peaks was more dominant and the overall emission intensity was higher. InGaN nanopyramids were chemically uniform, ruling out phase separation as origin of the double-peak. We propose an explanation based on the sudden transition from strained to relaxed growth moderated by temperature and local indium precursor concentration.
© 2017 Optical Society of America
1. Introduction
The tunability of the fundamental bandgap of indium gallium nitride (InGaN) across the full range of the visible spectrum has led to the development of a variety of optoelectronic devices including blue, green, red and white light-emitting diodes [1], blue and green laser diodes [2,3], and solar cells [4–7]. For these applications, thick, high-quality, and In-rich InGaN material is required. However, whatever the approach used (MQW, bulk layer), the active layer is plagued by phase separation and/or a high density of dislocations, which usually act as nonradiative recombination centers and sources of leakage current paths in III-nitride thin films. These detrimental phenomena are present in growth on lattice-mismatched GaN templates [8–10] and are even more pronounced on silicon substrates due to the large lattice mismatch with InGaN as reported on Ref. [11–13].
However, significant progress has been reported on using InGaN nanostructure fabrication, in the form of InGaN nanowires [14], InGaN nanorings and nanoarrays [15], and InGaN/GaN core-shell structure nanorods [16, 17], to make high-quality InGaN films and/or freestanding nanowires with tunability across a wide range of compositions. Nano selective area growth (NSAG) has recently been proposed as a technique for significantly extending the critical layer thickness of pseudomorphic growth in mismatched heterostructures [18–20]. NSAG exploits three-dimensional stress relief mechanisms that are available at the nanoscale to reduce the strain energy in lattice-mismatched material systems without creation of dislocations, leading to higher indium incorporation and thickness of the InGaN layer on GaN/Al2O3 [21, 22] or as reported recently on AlN/Si(111) [23].
This work continues the effort to develop NSAG of InGaN on AlN/Si(111), and focuses in particular on understanding the effect of both mask geometry and growth temperature on structural and optical properties of the nanostructures. To that end, we report on MOCVD growth by NSAG of InGaN nanopyramids with different sizes and optical wavelength emission on AlN/Si(111) wafer by controlling the growth temperature and SiO2 mask design (Fig. 1).
Fig. 1 NSAG geometry (a) SEM image of a pattern of exposed HSQ (dark is SiO2) on a AlN/Si(111) substrate; the 1 μm margin pattern is shown. α and β are the only dimensions varied between different mask patterns. The inset shows a zoom on the right-hand side of the apertured region. All mask patterns use the same aperture size and spacing. (b, c and d) SEM image of 800 °C NSAG InGaN on the 1 μm-margin mask (α = 1 μm, β = 8 μm), 4 μm-margin mask (α = 4 μm, β = 2 μm) and 16 μm-margin mask (α = 16 μm, β = 2 μm), respectively.
2. Experiment
In this work, nano-selective area growth (NSAG) of InGaN using exposed hydrogen silsesquioxane (HSQ) masks was conducted on AlN/Si(111) substrates. Our process begins with silicon (111) substrates, which were pre-coated with 200-nm non-crystalline AlN by physical vapor deposition. Then a 100 nm thick negative-tone resist HSQ was spin-coated and patterned using electron-beam lithography, leaving a hydrogenated SiO2 mask. A tetramethyl ammonium hydroxide etch was then used to open nano-holes with a diameter of around 75 nm. An explanation of the mask design and the specific mask geometries investigated can be found in Fig. 1. A full description of the mask patterning process can be found elsewhere [21]. Three different geometries, (α = 16 μm, β = 2 μm), (α = 4 μm, β = 2 μm), and (α = 1 μm, β = 8 μm), were used, and they will be referred to in this article by their margin size α (For this scale, β, the size of the aperture zone, had no visible effect on emission or growth rate). Each of these three mask geometries was applied to both of two sets of wafers so that two different growth temperatures, 780 and 800 °C, could be investigated. MOCVD NSAG was performed in a T-shaped reactor [24] using trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) for the gas precursors under nitrogen ambient, with a reactor pressure of 100 Torr. A 20 nm thick GaN layer was first grown on the AlN buffer layer with a growth temperature of 1000 °C. Then, 100 nm thick InGaN was grown with 5000 V/III ratio. Those conditions were optimized for producing In0.20Ga0.80N and In0.28Ga0.72N at 800 °C and 780 °C growth temperature respectively.
The surface morphology of the nanostructures were studied using field emission scanning electron microscopy (SEM). The structural properties were investigated using aberration corrected scanning transmission electron microscopy (STEM) along with energy-dispersive X-ray (EDX) analysis, operating at 200 kV with a probe current of 150 pA, and a probe size of 0.12 nm at the full width at half maximum (FWHM). The samples were prepared for STEM using focused ion beam (FIB) thinning and ion milling. In order to preserve the sample surface during FIB preparation, a surface coating consisting of a 50 nm - thick layer of carbon, followed by 100 nm of silicon nitride (Si3N4) was applied. The samples were cleaned using an argon plasma cleaner before the STEM imaging along the <1100> zone axis.
The optical properties were investigated by low temperature (LT), 77 K, depth resolved cathodoluminescence (CL) by tuning the electron beam energy from 3 to 7 keV with beam current in the range of 200 to 260 pA. The LT CL measurements were perfomed in a SEM using a liquid nitrogen cooled module. The CL emission is collected using a parabolic mirror and analyzed by a spectrometer with a focal length of 320 mm, a 300 grooves/mm grating and a 1024 x 256 liquid nitrogen cooled CCD camera leading to a spectral resolution of 0.06 nm.
3. Results and discussion
Figure 1(b)–1(d) shows the NSAG InGaN on the three mask geometries studied, with perfectively selective growth on the patterned area, without any polycrystalline deposits on the masks. The insets of Fig. 2(a)–2(c) are zoomed in views of the InGaN nanopyramids, showing smooth, faceted, and uniform morphology, with 90 % of the heterostructures being single crystal, thus, presenting better morphological quality as compared to the planar InGaN which exhibits 3D growth with V-pits and trench defects evidenced by SEM in Fig. 1(b)–1(d). We can also clearly remark a growth enhancement of the outermost nanopyramids which is most readily apparent for the larger margins (Fig. 2(b) and 2(c) insets). Closer examination reveals that the majority of this growth enhancement takes place on the out-facing facets. A statistical analysis of the size of nanopyramids was performed using SEM images. The size of a nanopyramid is calculated by the area of inclined surface surrounded by a flat border. The histograms in Fig. 2(a)–2(c) show the results for the 1, 4 and 16 μm margin-masked NSAG, respectively. The histograms each show two clearly defined size groups, the smaller-size distribution arising from inner nanopyramids and the larger arising from the outer.
Fig. 2 Nanopyramid size analysis for all 3 margin sizes for 800 °C NSAG. (a,b and c) Size analysis for the 1, 4 and 16 μm margin growth, respectively. Figures have the same X-axis. (d) Simulated growth rate enhancement ratio for GaN and InN on the 4 μm margin growth based on the vapor phase diffusion model. X = 0 is the center of the NSAG area.
Figure 2(d) shows the predicted GRE profile for GaN and InN in and near the 4 μm margin mask based on the 3D vapor phase diffusion model. We compute the (normalized) concentration of the active species around the mask by solving the Laplace diffusion equation in the domain. The main parameters of the model that control the GRE are the mask size and the ratio between (D) the diffusion coefficient in the vapor phase and (Ks) a sticking rate constant. The ratio D/Ks has unit of length and can be viewed as an effective diffusion length of the active species in the domain. A full description of the model with the boundary conditions for the calculation can be found elsewhere [25]. This model predicts that the growth enhancement of the outer nanopyramids, dominated by GaN enhancement due to the TMIn saturation, should only be around 1 % greater than for the inner nanopyramids. Indeed, for our TMIn-saturated conditions, growth rate enhancement ratio for InGaN will be proportional to the growth rate enhancement ratio for GaN. Figure 2(d) shows the simulated ratio (red) for the 4-micron margin size and 2-micron mask opening. We can take the ratio for GaN at the inner and outer nanopyramids from the min (X = 0) and max (X = ± 1). For this mask margin size, the inner and outer nanopyramids have GaN growth rate enhancement ratio = 1.25 and 1.27, respectively, a difference of 1.5 %. Additionally, this difference should occur gradually, over the scale of microns, which is not in agreement with the drastic difference in size observed between the outer nanopyramids and the inner nanopyramids just next to them (Fig. 2(a)). Analogous simulation for the 1 and 16 μm margin growth shows the same gradual diminishing away from the outer pyramids. Clearly, vapor phase diffusion is insufficient to explain the growth enhancement in the NSAG regions, though it does predict the thickness profile observed for our field at the mask edges (measured by AFM, not shown).
The drastic effect of mask margin size on outer nanopyramid GRE suggests that the growth of these structures is dominated not by vapor phase diffusion but by surface migration of precursor species across the mask, i.e., species that reach the mask migrate in a random lateral direction until they come across existing InGaN material into which they can adsorb. This explains why larger mask margins result in more exaggerated outer nanopyramids: more mask surface means more species touch down where they cannot adsorb, creating a large supply of migrating atoms to feed outer nanopyramid growth. The significant size difference between the very outer nanopyramids (around 150 nm wide) and the inner pyramids just next to them suggests a surface migration length of around 300 nm on 60 degree inclined semi-polar InGaN facets.
CL analysis was performed for all margin sizes on both the 800 °C and 780 °C NSAG and unpatterned field growth. Figure 3 summarizes the results. Firstly, we remark that the low-temperature growth emits significantly redder than the higher-temperature growth, which is expected due to the decreasing bond strength ratio between InN and GaN with increasing temperature [26,27]. Secondly, NSAG material is found to have 1.2 times enhancement of In incorporation compared to the field for both temperatures and all margin sizes.
Fig. 3 CL summary of representative InGaN nanopyramids registred at 5 keV in log10 scale. (a) Fits of CL spectra of the 800 °C InGaN nanopyramids grown on the 3 different margin sizes and of the unpatterned field growth. Solid curves are total fits, and the dashed and dotted curves are deconvoluted fits of the blue-shifted and redder peaks, respectively, and the dotted and dashed green curves are deconvoluted fits of the primary and defect band peaks of the unpatterned field growth, respectively. The field growth was measured very far from the masked regions in scanning mode instead of spot mode for nanopyramids. (b) Deconvoluted CL fits for the 780 °C InGaN nanopyramids and field.
The planar (non-NSAG) InGaN exhibits two luminescence bands, which are centered at 503 nm and 675 nm for the 800 °C-grown sample and 562 nm and 725 nm for 750 °C-grown sample. The short wavelength peaks can be attributed to the near band edge emission of InGaN with an In incorporation equal to 18.5 % and 24.5 % estimated according to the work of Orsal et al. [28]. This is close to x-ray diffraction measurements which determine an incorporation of 19.5 % and (26.5 ± 1.5) %. The other luminescence band spanning from 550 nm to 800 nm originates probably from the localization of excitons at potential minima in In-rich InGaN areas confirming the expected Indium content fluctuations in planar InGaN.
In the NSAG regions, the InGaN nanopyramids produce two-peak spectra comprised of one peak at the expected bandgap energy for relaxed InGaN, and a second, more intense blue-shifted peak. For 800 °C NSAG (Fig. 3(a)), the blue-shifted peak is at 555 nm and the redder peak is at 615 nm corresponding to an estimated In incorporation of 23.5 % and 29 %, respectively. In the 780 °C NSAG (Fig. 3(b)), the blue-shifted peak is at 620 nm and the redder peak is at 690 nm corresponding to an estimated In incorporation of 29.5 % and 35 %, respectively.
The vapor phase diffusion model predicts increased availability of TMIn precursors in the NSAG region resulting in an increased ratio of InN to GaN bond formation. However, both samples were grown at the saturation flow rate for TMIn, resulting in the uniform 1.2 times incorporation enhancement compared to the field that is consistent with the difference in incorporation expected between the c-plane and the (11.2) semi-polar facet [29]. We note however that the total and relative intensities of the two peaks in Fig. 3(b) do depend on margin size, specifically that larger margin sizes correspond to more dominant blue-shifted peaks and higher overall emission intensity. This is less obvious in Fig. 3(a) where the redder shifted peak, for small margin SAG regions seems to be more intense (or with the same intensity) with the bluer shifted peak. This could be due to the signal to noise ratio of the CL measurement which is relatively weak in this case leading to not accurate fit and deconvolution of the spectrum.
To investigate the source of this effect, we performed cross-sectional EDX mapping of the nanopyramids (Fig. 4(a)), and found that all nanostructures are chemically uniform, with 23 % and 33 % InN composition in the 800 °C and 780 °C-grown NSAG, respectively, corresponding roughly to the redder peaks in the characteristic two-peak emission. STEM in Fig. 4(b) reveals that the structures are defect free, and show no phase separation, ruling out what might be considered the obvious explanation. Mask margin size in this study is confirmed to have no effect on composition, despite the drastically different ratios of growth enhancement for InN and GaN predicted by vapor phase diffusion, confirming that the indium incorporation is at the saturation point for semi-polar growth.
Fig. 4 (a) EDX mapping of the (11.0) plane of a representative InGaN nanopyramid grown at 800 °C. The uniform In distribution is characteristic of all nanopyramids, though non-emitting cubic insertions are occasionally present. (b) Cross sectional HAADF-STEM images of the same nanopyramid.
Figure 5(a) shows depth-resolved CL analysis of a nanopyramid from the 780 °C NSAG. As the excitation profile recedes towards the surface for weaker e-beams, we remark that the ratio between the redder and blue-shifted peak intensity increases, indicating that the bluer-emitting region is deeper in the nanopyramid / nearer to the GaN interface. Figure 5(b) compares representative CL spectra from an outer and inner nanopyramid from the 16 μm margin mask for a very shallow 3 eV penetration depth. The wavelengths are close for both inner and outer nanopyramids in the same mask, despite the difference in size. The outer nanopyramids have a drastically higher redder / bluer peak ratio than do the inner nanopyramids, indicating that the redder-emitting region is significantly thicker here. From this information, we interpret the two-peak emission to be the result of a compressively strained, pseudomorphic InGaN region near the GaN interface that abruptly gives way to a relaxed growth mode without changing InN compostion.
Fig. 5 (a) Fits of depth-resolved CL spectra from 780 °C grown 16 μm margin InGAN NSAG. Solid curves are total fits, and the dashed and dotted curves are deconvoluted fits of the two nanopyramid peaks showing increasing predominance of the redder-emitting region closer to the surface. (b) Fits of 3 keV-excited (shallow) CL spectra for an inner (red curves) and outer (black curves) nanopyramid. Solid curves are total fits, and the dashed and dotted curves are deconvoluted fits of the two peaks showing an increased redder-peak intensity and much reduced bluer-peak intensity for the outer nanopyramid. (c) Total fits of CL spectra of the 780 °C field at various distances from the edge of the 16 μm margin mask edge. The red and blue dotted lines indicate the position of the redder and bluer peaks, respectively, of the two-peak emission, while the yellow line indicates the yellow defect band typical of 2D InGaN growth.
It is well established that compressed InGaN exhibits strongly blue-shifted emission [28], and similar two-peak emission has already been observed in 2D c-plane growth by Pereira et al. [30], who explained the effect as due to the transition from pseudomorphic to Stranski-Krastanow (SK)-like growth. Pieira et al. found that the transition thickness had an indirect relationship with InN incorporation, but Oliver et al. [31] found that increased TMIn and lower temperatures discourage the transition to SK-like growth in planar InGaN layers grown by MOCVD. We note that in the current work, a higher bluer / redder peak intensity ratio (refer back to Fig. 3) is found to be correlated with low temperature and high local TMIn concentrations (as predicted by the vapor phase diffusion model), suggesting that similar physics may be at play for the semi-polar NSAG, i.e., that these two growth conditions delay the transition to the relaxed growth mode, resulting in thicker compressively strained regions in these areas. The consequently thinner relaxed (redder emitting) regions shadow less of the excitation energy, resulting in the bluer peaks being more pronounced for the same nanopyramid size. In the 2D field, the blue-shifted emission is strong enough to be observed only near the 16 μm mask edge for the lower temperature growth, which, while the thickest planar region studied, is also that with the lowest temperature and highest local TMIn concentration. This relationship between temperature, TMIn concentration, InN % and growth-mode transition suggests that it’s not increased InN % that delays growth mode transition, but rather it’s higher TMIn concentration and lower temperature that drive both.
It’s worth noting that in Fig. 5(b), the presence of the bluer peak for the outer nanopyramid when 99 % of the 3 eV exciting electrons are absorbed in the first 10 nm of material indicates that our larger outer nanopyramids have significantly larger bluer-emitting regions than do their smaller neighbors even though they experience the same temperature and local TMIn concentration. This suggests that the increased presence of Indium atoms due to surface migration may have an effect similar to increased TMIn concentration.
Figure 5(c) shows the CL profile taken at several distances from the field edge. We remark that the red peak is strongest near the mask edge, where both GRE and local TMIn concentration were highest.
4. Conclusion
In conclusion, we performed NSAG of InGaN nanopyramids on 20-nm GaN NSAG on AlN-buffered Si(111) templates using three different mask geometries and two different growth temperatures, and found that the NSAG had consistently 1.2 times the Indium content of our un-patterned field growth. We also found that while the vapor phase diffusion model is useful for predicting In % and growth enhancement in micron-scale SAG, our NSAG growth enhancement is dominated by surface migration effects. We also found that depending on mask margin size and growth temperature, the relative and overall intensity of the two-peak emission can be controlled. Larger margin sizes and lower growth temperatures result in nanopyramids which emit more intensely overall and have a larger ratio between the blue-shifted and redder peak intensities. Our findings suggest that the mask margin’s role in the color ratio is directly related to the delay of the transition between pseudomorphic and relaxed growth modes because of increased local TMIn concentration due to vapor phase diffusion and increased local free Indium atoms due to surface migration. The increased TMIn concentration has this effect even when it does not enhance growth rate or composition. Total CL emission intensity was also observed to be directly related to the amount of available TMIn, though the mechanism underlying this finding still needs investigation.
The ability to produce multiple colors in the same growth step has fantastic potential in the production of single-growth-step micro-pixels. Our lower-temperature growth produced a green-emitting 2D region and a semi-polar NSAG region that emits both red and orange. The red-emitting region was found to originate from later growth, meaning that increasing growth time will enhance the red emission, while eventually deleting the orange emission. This will happen sooner in NSAG with lower local indium precursor concentration during growth, which can be controlled by margin size. This means that in one growth step, we can produce green and red-emitting micro-regions- only one color away from achieving full RGB in a single growth step. Furthermore, blue-emitting cubic InGaN has been grown in growth conditions similar to ours [32], even in the presence of hexagonal growth on the same wafer [33]. This ability to reach green and red emissions with the obtained compositions is also very important for tunable LEDs using InGaN since the normal MQW-based LEDs still have a big problem reaching these compositions without an abrupt deterioration of luminous efficiency due to a large piezoelectric field [34].
Funding
French National Research Agency (ANR) under the NOVAGAINS (ANR-12-PRGE-0014-02) ANR Project and GANEX Program (ANR-11-LABX-0014).
Acknowledgments
The authors would like to thank ANR for financial support.
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