Sridhar Govindaraju

Spatial correlation-anticorrelation in strain-driven self-assembled InGaAs quantum dots

We report evidence for the existence of anticorrelation in InGaAs∕GaAs self-assembled quantum dots (QDs). We found that, as a function of the spacer layer thickness, the QDs between the neighboring layers are either vertically correlated (at small spacer thickness) or anticorrelated (at larger spacer thickness). Moreover, in the case when the QDs are antialigned, the size distribution of individual quantum dots becomes more uniform. The implications of this work to the fundamental understanding of the self-assembly process, and the technological applications are discussed.

Growth of GaNAs by molecular beam expitaxy using a N2/Ar rf plasma

A high efficiency nitrogen rf plasma source has been used to grow GaNAs by diluting the N2 gas with Ar. This source (an EPI UniBulb™ source) was originally designed for use in the growth of pure nitrides at high growth rates. For growth of As-rich GaNAs, high concentrations of active nitrogen lead to the growth of GaN instead of a random alloy. In this work we demonstrate that a dilute N2/Ar mixture leads to GaNAs films where the amount of nitrogen incorporation varies directly with the percentage of N2 in the gas mixture. Films with high structural quality were grown, thus validating the use of this approach.A high efficiency nitrogen rf plasma source has been used to grow GaNAs by diluting the N2 gas with Ar. This source (an EPI UniBulb™ source) was originally designed for use in the growth of pure nitrides at high growth rates. For growth of As-rich GaNAs, high concentrations of active nitrogen lead to the growth of GaN instead of a random alloy. In this work we demonstrate that a dilute N2/Ar mixture leads to GaNAs films where the amount of nitrogen incorporation varies directly with the percentage of N2 in the gas mixture. Films with high structural quality were grown, thus validating the use of this approach.

Ion damage effects from negative deflector plate voltages during the plasma-assisted molecular-beam epitaxy growth of dilute nitrides

We studied the effects of ion damage on the optical properties of dilute nitrides grown by plasma-assisted molecular-beam epitaxy. A dual-grid retarding field ion energy analyzer was used to measure the ion flux and ion energy distribution at the substrate position from an Applied-EPI UniBuilb™ rf plasma cell. These changes were measured as the negative deflector plate voltage varied from 0 to −800V. The largest ion flux resulted with a −100V setting, while the greatest ion energies occurred with −200V. Deflector plate voltages more negative than −300V resulted in a significant reduction in both the ion flux and ion energy. The damage caused by these ions was determined by measuring the pre- and postanneal photoluminescence properties of Ga0.8In0.2N0.01As0.99 quantum wells. Comparable optical properties were possible with various combinations of ion fluxes and ion energies, which demonstrate how the ion flux and ion energy each impart an individual effect on the sample’s optical properties. An awareness o...

Diffusion mechanisms of indium and nitrogen during the annealing of InGaAs quantum wells with GaNAs barriers and GaAs spacer layers

In this article, we discuss two indium diffusion mechanisms that are present during the rapid thermal annealing of InxGa1−xAs quantum wells (x=0.18, 0.22, and 0.26) with GaNyAs1−y barriers (y=0.6 or 1.2%). Samples were grown with and without a GaAs spacer layer in between the quantum well and barrier. The dominant mechanism is dependent on the amount of thermal energy applied during the annealing process. At low annealing times and temperatures, we have observed that In-Ga intra-diffusion entirely within the quantum well is dominant. For the higher times and temperatures, In-Ga inter-diffusion between the quantum well and barrier becomes dominant. These observations were confirmed by high-resolution x-ray diffraction and the peak emission wavelengths were measured by room-temperature photoluminescence. We have also observed that nitrogen had diffused from the GaNAs barriers into the InGaAs quantum wells in all of our annealed samples. In addition, the commonly observed indium-content dependent diffusion i...

Molecular-beam epitaxy growth of Ga(In)NAs/GaAs heterostructures for photodiodes

The incorporation of nitrogen into GaInAs/GaAs heterostructures has received intense interest recently due to the large negative bowing parameter of the resultant alloy. This change in the band gap for GaInAsN makes it useful for near-infrared optoelectronic devices on the GaAs substrate. However, the effect of adding nitrogen into GaInAs is an important issue in the use of these alloys. This article focuses on the growth of GaInNAs/GaAs alloys for use in photodiodes. Under our growth conditions, we show that the incorporation of nitrogen into GaAs and Ga0.8In0.2As is linear with nitrogen in the growth chemistry up to approximately 3.5%. Photodiodes using GaNAs absorption regions show low dark currents and high quantum efficiencies with nitrogen values up to 1.75%.

Growth and characterization of Ga0.8In0.2(N)As quantum wells with GaNxAs1−x(x⩽0.05) barriers by plasma-assisted molecular beam epitaxy

GaInNAs has received significant attention due to the ability to achieve 1300 nm wavelengths on GaAs substrates. In most cases, nitrogen is added directly into the GaInAs quantum wells, which degrades their optical properties. In this work, we investigate how the placement of nitrogen in Ga0.8In0.2As/GaAs heterostructures affects their optical characteristics. The location of nitrogen was studied in three different structures: nitrogen in barriers alone, nitrogen in wells alone, and nitrogen in the wells and barriers. Our results show that placing nitrogen in the barriers instead of wells provides better luminescence properties. We also observed that the addition of a GaAs spacer at the barrier-well interfaces further improves the luminescence properties of the structure.

Inert gas maintenance for molecular-beam epitaxy systems

Molecular-beam epitaxy (MBE) most often involves the use of highly toxic and combustible materials, which may subject maintenance personnel to increased health risks. In our efforts to reduce these hazards, we describe the use of inert gas maintenance equipment and procedures that can be employed during the opening of MBE growth chambers. Our operations involve the use of nitrogen-purged glovebags that are sealed over the open port of the growth chamber, wherein applicable tasks are performed through appropriate gloveports of the glovebag. We also describe the associated equipment utilized inside of the glovebags, which aid in the removal of the substrate manipulator and effusion cells. The benefits of reducing the exposure of air to the growth chamber are observed after a bakeout of 145h, wherein the AsO partial pressure within the growth chamber was a factor of 10 lower due to our inert gas maintenance procedures than without. The use of these glovebags allows us to both terminate our bakeout approximat...