Selective area epitaxy of ultra-high density InGaN quantum dots by diblock copolymer lithography
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Highly uniform InGaN-based quantum dots (QDs) grown on a nanopatterned dielectric layer defined by self-assembled diblock copolymer were performed by metal-organic chemical vapor deposition. The cylindrical-shaped nanopatterns were created on SiN x layers deposited on a GaN template, which provided the nanopatterning for the epitaxy of ultra-high density QD with uniform size and distribution. Scanning electron microscopy and atomic force microscopy measurements were conducted to investigate the QDs morphology. The InGaN/GaN QDs with density up to 8 × 1010 cm-2 are realized, which represents ultra-high dot density for highly uniform and well-controlled, nitride-based QDs, with QD diameter of approximately 22-25 nm. The photoluminescence (PL) studies indicated the importance of NH3 annealing and GaN spacer layer growth for improving the PL intensity of the SiN x -treated GaN surface, to achieve high optical-quality QDs applicable for photonics devices.
KeywordsDiblock Copolymer Atomic Force Microscopy Measurement Selective Area Epitaxy Strong Quantum Effect
anodized aluminum oxide
atomic force microscopy
metal-organic chemical vapor deposition
selective area epitaxy
scanning electron microscope.
Nitride-based semiconductor devices have tremendous applications in solid-state lighting [1, 2, 3, 4, 5, 6, 7, 8, 9], lasers [10, 11, 12, 13, 14], photovoltaic [15, 16, 17], thermoelectricity [18, 19, 20], and terahertz photonics [21, 22]. Nitride-based InGaN quantum wells (QWs) are typically employed as active regions in energy-efficient and reliable light-emitting diodes (LEDs) for solid-state lighting. However, the large spontaneous and piezoelectric polarization fields in III-Nitride material lead to a significant charge separation effect [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35], which in turn results in low internal quantum efficiency of green-emitting nitride-based LEDs, and high threshold current density in nitride lasers. Nonpolar nitrides were employed to remove the polarization field ; however, the development of nonpolar InGaN QWs is relatively limited due to high substrate cost and less mature epitaxial techniques. Recent approaches to improve the LED internal quantum efficiency by employing novel InGaN QWs with improved electron-hole wavefunction overlaps have been reported [24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35], as follows: (1) InGaN QW with AlGaN δ-layer , (2) staggered InGaN QW [25, 26, 27, 28, 29, 30], (3) type-II QW , (4) strain-compensated InGaN-AlGaN QW [32, 33], (5) InGaN-delta-InN QW , and (6) InGaN QW with novel AlInN barrier design .
The pursuit of quantum dot (QD)-based active regions for optoelectronic and photovoltaic devices is very important because of the stronger quantum effects in the nanostructures [36, 37, 38, 39]. The three-dimensional potential boundaries deeply localize carriers, and thus the overlap of the electron-hole wavefunctions is greatly enhanced. The strain field from the large lattice mismatch of InGaN/GaN is released in three dimensions for QD nanostructures so that the non-radiative recombination centers and defects can significantly be reduced. Besides, QD design enables high In-content InGaN epitaxy, which enlarges the coverage of emission spectrum and enriches the design of QD-based active region. The QDs can be implemented in intermediate-band solar cells [40, 41] to greatly enhance the efficiency over the whole solar spectrum.
Two conventional approaches for realizing the QD structure include (1) etching technique and (2) self-assembled epitaxy based on Stranski-Kastranow (S-K) growth mode [42, 43, 44, 45, 46, 47, 48, 49, 50]. The approach to obtain QD by etching techniques suffers from surface roughness and significant surface recombination issues. The S-K growth mode has been employed by both molecular beam epitaxy and metal-organic chemical vapor deposition (MOCVD) technique for the epitaxy of nitride-based [42, 43, 44, 45] and arsenide-based QDs [46, 47, 48].
The MOCVD epitaxy of the self-assembled InGaN QDs emitting in the 510-520-nm region has been reported in reference . The use of the self-assembled growth technique of InGaN QDs led to QDs with circular base diameter of 40 nm and an average height of 4 nm, and the QD's density was measured as 4 × 109 cm-2. The S-K growth mode of InGaN QDs [42, 43, 44, 45] resulted in relatively low density range (mid 109 up to high 109 cm-2), nonuniformity in QD distribution, and the existence of wetting layer. In contrast to InGaN-based QDs, S-K growths of In(Ga)As/GaAs QDs [46, 47, 48] have led to high-performance lasers with high QD density (high 1010 cm-2) and uniform QD distribution.
Another important obstacle preventing one from fully exploring the radiative and gain properties of the QD structure from S-K growth mode is the inherent presence of the wetting layer [36, 37, 38, 49, 50]. Several recent studies have shown that the strain fields in the wetting layer from the S-K-grown QDs reduces the envelop function overlap and recombination rate in QD's active region [36, 37, 38]. The wetting layer also serves as a carrier leakage path because of the coupling of wetting-layer states with localized QD states, which leads to the increase of threshold current in laser devices.
To eliminate of the detrimental wetting layer as well as fully control the formation of QDs, an alternative to achieve the growth of arsenide-based and nitride-based QDs devices by utilizing selective area epitaxy (SAE) [51, 52, 53, 54, 55, 56, 57]. The ideal QDs obtained by the SAE approach [52, 53, 54, 55, 56, 57], in particular realized by employing diblock copolymer lithography [55, 56, 57], have comparable QD density to that of S-K growth mode, but potentially have better device performance because of the removal of the wetting layer and better carrier confinement [55, 56, 57]. Previous studies on the SAE of InGaN QDs have been pursued by using electron-beam lithography [58, 59, 60, 61], and anodized aluminum oxide (AAO) template .
In this study, we present the SAE of ultra-high density and highly uniform InGaN-based QDs on the nano-patterned GaN template realized by diblock copolymer lithography. The diblock copolymer lithography is ideal for device applications due to the adaptability to full wafer scale nanopatterning. All growths were performed by employing MOCVD on GaN templates grown on c-plane sapphire substrates. The distribution and size of QDs are well controlled, and the presence of the wetting layer is eliminated. Our photoluminescence (PL) studies under different template treatments and different growth conditions confirm the effect of SiN x deposition on the GaN template surface, as well as provide possible solutions to enhance luminescence from the QD samples.
It is to be noted that the use of SAE approach on dielectric nanopatterns defined by diblock copolymer process resulted in the growths of InGaN QDs without wetting layer, which potentially led to the increase in optical matrix element. In addition to the improved matrix element in the QD, the use of dielectric layers also serve as current confinement layer resulting in efficient carrier injection directly into the InGaN QDs arrays. The diblock copolymer lithography approach also leads us to very high-density patterning with excellent uniformity and low cost. In contrast, the use of AAO template leads to relatively non-uniform patterning, while the use of e-beam lithography leads to a high-cost approach.
Nanopatterning and SAE of InGaN QDs
The sample was then pretreated using PS-r-PMMA brush material followed by the deposition of cylindrical-shaped diblock copolymer PS-b-PMMA (Figure 1b) [55, 56, 57]. The brush material is made of random copolymer that would lead to non-preferential affinity to the both blocks of the self-organizing PS-b-PMMA copolymer , which enabled the formation of the cylindrical morphology on the diblock copolymer layer during the thermal annealing as a result of the microphase separation. After the UV exposure (λ = 254 nm) and chemical etching by acetic acid, the PMMA block was removed, leaving the PS block to form the patterned copolymer that was used as the polymer stencil (Figure 1c). Subsequently, the sample was made to undergo the reactive ion etching by CF4 plasma, and the nanopatterns were transferred from the copolymer layer to the underneath SiN x layer (Figure 1d). After the removal of the copolymer by O2 plasma and wet etching, the SiN x layer with the nanopatterns could serve as the mask in the following MOCVD process (Figure 1e). The details of the diblock copolymer-processing steps (Figure 1b-e) are described in references [55, 56]. The opening region where GaN template was exposed to the metal-organic source would enable the QD growth (Figure 1f). The remaining SiN x layer can also serve as an insulator between QDs within the active region of a device.
Structural and morphology characterizations
Earlier, studies have been carried out to obtain high density nitride-based QDs [63, 64]. Krestnikov et al.  reported the QD-like behavior in InGaN QW resulting from the In-clustering effect, and the density of the In-riched nanoisland within the QW layer was estimated to be in the range of 1011-1012 cm-2. The QD-like behavior in InGaN QW from the In-clustering effect resulted in relatively shallow QD/barrier systems. Tu et al.  reported the growth of InGaN QDs by employing GaN templates with SiN x treatment which resulted in template roughening, and this process leads to dot density of near 3 × 1011 cm-2 . However, the use of roughening approach leads to QD distribution with relatively non-uniform size distributions. Thus, the use of SAE approach in growing the InGaN QDs enabled them to grow highly uniform QDs with deep QD/barrier systems (i.e., with GaN or other larger bandgap barrier materials) and very high QD density (approx. 8 × 1010 cm-2).
The diameter of the QDs in our experiments was measured in the range of 22-25 nm, which is considered as relatively large QDs. The focus of the current studies is to investigate the various optimizations in the growth and annealing conditions for the development of the SAE technique for InGaN QDs with diblock copolymer lithography, and the current studies are focused on the dimensions of 20-25-nm diameter QDs. In order to obtain stronger quantum effects in the 3D carrier confinement, the QDs are preferably realized with smaller diameters (10-18 nm) . However, the 3D quantum effect in the carrier confinement still exists in the 20-25-nm QD diameter as discussed in the theoretical works in . Future optimization studies on the investigation of SAE InGaN QDs with smaller QDs diameter are of importance for achieving nanostructures with stronger 3D carrier confinement, and the optimization of this approach is required to achieve active regions with high optical quality for device applications.
PL studies and discussion
The SAE approach enabled the growth of ultra-high density InGaN QDs; however, no strong PL was observed from the InGaN/GaN QD samples. All the PL measurements were carried out by utilization of He-Cd laser with wavelength at 325 nm as the excitation source at room temperature. From our studies, we found that the surface treatment during the SiN x deposition could be the cause for the defect formation in the GaN surface, which results in poor luminescence from the SAE-grown QD samples. The surface treatment processes for the epitaxy of the QDs include SiN x deposition, and HF or CF4 plasma etching. A series of PL studies on the SAE-grown InGaN QDs were performed to identify and further understand the effects of various treatments on the PL of the samples, which will provide guidance in addressing these issues.
In summary, the selective area growths of InGaN QDs on dielectric patterns defined by the self-assembled diblock copolymer were carried out by MOCVD. The use of selective area approach resulted in ultra-high QD density of approx. 8 × 1010 cm-2, which represents the highest among the QD densities reported for highly uniform and well-controlled nitride-based QDs. PL studies of InGaN QDs and the QWs show that GaN spacer regrowth as well as annealing conditions can greatly improve the luminescence from QD samples. The availability of highly uniform and ultra-high density InGaN QDs formed by this approach potentially has significant impacts on developing high-efficiency LEDs for solid-state lighting, low threshold current density-visible diode lasers, and intermediate-band nitride-based solar cells.
The authors would like to acknowledge the funding supports received from the US National Science Foundation (ECCS #0701421, ECCS #1028490,DMR # 0907260 ), Class of 1961 Professorship Funds, and through ARO MURI W911NF-05-1-0262 (to Dr. John Prater).
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