Nitride-based semiconductor devices have tremendous applications in solid-state lighting [1
], lasers [10
], photovoltaic [15
], thermoelectricity [18
], and terahertz photonics [21
]. 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
], 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 [23
]; 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
], as follows: (1) InGaN QW with AlGaN δ-layer [24
], (2) staggered InGaN QW [25
], (3) type-II QW [31
], (4) strain-compensated InGaN-AlGaN QW [32
], (5) InGaN-delta-InN QW [34
], and (6) InGaN QW with novel AlInN barrier design [35
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
]. 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
] 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
]. 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
] and arsenide-based QDs [46
The MOCVD epitaxy of the self-assembled InGaN QDs emitting in the 510-520-nm region has been reported in reference [44
]. 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
. The S-K growth mode of InGaN QDs [42
] resulted in relatively low density range (mid 109
up to high 109
), 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
] have led to high-performance lasers with high QD density (high 1010
) 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
]. 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
]. 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
]. The ideal QDs obtained by the SAE approach [52
], in particular realized by employing diblock copolymer lithography [55
], 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
]. Previous studies on the SAE of InGaN QDs have been pursued by using electron-beam lithography [58
], and anodized aluminum oxide (AAO) template [62
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 SiNx 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.