M. K. Rathi

Nanoscale selective growth and optical characteristics of quantum dots on III-V substrates prepared by diblock copolymer nanopatterning

As an alternate Quantum Dot (QD) fabrication method to self-assembled SK mode QDs, diblock copolymer nano-patterned QDs were investigated. By employing selective growth of QDs on diblock copolymer nano-patterned masks, independence from the problematic wetting layer and controllability on QD size and distribution associated with SK growth mode QDs were realized. The diblock copolymer nano-patterned masks were fabricated using a diblock copolymer template and a dielectric mask, and InxGa1-xAs QDs were selectively grown on patterned GaAs and InP substrates by Metalorganic Chemical Vapor Deposition (MOCVD). The optical properties from diblock copolymer patterned QDs on III-V substrates were investigated at low temperature.

Progress Towards Intersubband Quantum-Box Lasers for Highly Efficient Continuous Wave Operation in the Mid-Infrared

Intersubband quantum-box (IQB) lasers, which are devices consisting of 2-D arrays of ministacks (i.e., 2-4 stages) intersubband QB emitters have been proposed as alternatives to 30-stage quantum-cascade (QC) devices, for efficient room-temperature (RT) emission in the mid-infrared (4-6 µm) wavelength range. Preliminary results include: 1) the design of devices for operation with 50% wallplug efficiency at RT; 2) realization of a novel type of QC device: the deep-well (DW) QC laser, that has demonstrated at λ µm low temperature sensitivity of the threshold current, a clear indication of suppressed carrier leakage; 3) the formation of 2-D arrays at nanopoles by employing nanopatterning and dry etching; 4) the formation of 40 nm-diameter, one-stage IQB structures on 100 nm centers by preferential regrowth via metal-organic vapor phase epitaxy (MOVPE).

Controlled growth of InGaAs/InGaAsP/InP Quantum Dots using diblock copolymer lithography and selective area MOCVD growth

Diblock copolymer nanopatterning and selective growth is utilized to produce InGaAsP/In0.53Ga0.47As/InGaAsP Quantum Dots on InP substrates, demonstrating RTPL near 1.6 mum. Electroluminescence near 1.25 mum is achieved from ridge-waveguide devices.

Selective MOCVD growth of InGaAs/GaAs and InGaAs/InP quantum dots employing diblock copolymer nanopatterning

The conventional approach to fabricate semiconductor based QDs is based on the Stranski-Krastnow (SK) growth mode, which has enjoyed considerable success in device applications. However, the SK QD approach is complicated by the randomness of the QD size distribution and inherent presence of the wetting layer. Carrier leakage to the wetting layer has been identified as one of the underlying causes for low optical gain and high temperature sensitivity in diode lasers. To fully exploit the potential advantages of ideal Quantum Dots (i.e. full 3D carrier confinement), elimination of the wetting layer and a uniform mono-modal QD size distribution is needed. Nanopatterning with selective MOCVD QD growth has potential for achieving a higher degree of control over the QD formation, compared with the SK process. Furthermore, the problematic wetting layer states are eliminated and improved optical gain is expected. The QD patterning is prepared by dense nanoscale (20-30 nm diameter) diblock copolymer lithography, which consists of perpendicularly ordered cylindrical domains of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) matrix. For selective MOCVD growth, a dielectric template mask was utilized and the polymer patterning is transferred on it. The resulting GaAs QD densities are larger than 5×1010/cm2, comparable to SK growth mode, with a nearly monomodal QD size distribution. Variable temperature PL has been used to characterize the optical properties of capped InGaAs QDs on GaAs (λ ~ 1.1 μm) and InP (λ ~ 1.5 μm) substrates.

High antimony content GaAs1−zNz–GaAs1−ySby type-II “W” structure for long wavelength emission

GaAs1−zNz–GaAs1−ySby type-II “W” structures were studied for long wavelength (1300–1600 nm) applications. These structures were grown on a GaAs substrate using metal-organic vapor phase epitaxy. The antimony and nitrogen compositions in the pseudomorphic GaAs1−ySby and GaAs1−zNz were estimated by separately growing GaAs1−ySby–GaAs and GaAs1−zNz–GaAs strained superlattices. X-ray studies indicate that a maximum of y=0.37 antimony can be incorporated in the pseudomorphic GaAs1−ySby film grown using triethyl gallium (TEGa), trimethyl antimony (TMSb) and arsine (AsH3) at the growth temperatures employed. A postgrowth anneal was used to improve the emission intensity but leads to shifts in the emission wavelength. An emission wavelength as long as 1.47 μm was realized using a GaAs1−zNz–GaAs1−ySby–GaAs1−zNz structure.

Selective growth and chracterization of InGaAs Quantum Dots on patterned InP substrates utilizaing a diblock copolymer template

To realize the theoretical advantages of Quantum Dots (QDs) as the active region for diode lasers, the selective growth of QDs on patterned substrates were investigated. The substrate nanopatterning and QD formation was realized by diblock copolymers combined with selective MOCVD growth. Using a CF4 reactive ion etch (RIE) and a sacrificial SiNx mask, diblock copolymer nanopattern was trans ferred to InGaAs QDs on top of prepared InGaAsP / InP substrates and then QDs were grown selective ly by MOCVD. Since growth temperature of patterned QDs is free from QD formation mechanism, unlike self-assembled QDs, two samples were grown at relatively higher temperatures (550 °C and 630 °C) and a significant improvement in photoluminescence (PL) was observed from the sample grown at higher temperature.

GaAsSb-GaAsN -based type-II ‘W’ structures for mid-IR emission

GaAsSb-GaAsN-based type-II ‘W’ structures have been studied for mid-IR (1.3–1.6µm) emission. Post growth annealing increases the photoluminescence (PL) intensity of the structures. Temperature dependent PL show a charge localization effect due to presence of nitrogen.

N-rich and dilute-nitride GaN x (AsSb) 1-x on InP substrates

GaAsNSb alloys have been demonstrated using MOCVD growth over a wide span of nitrogen composition. Dilute-nitride alloys hold potential for mid-IR emission using GaAsSbN/GaAsSb type-II QWs.