Uniform Dots Research Articles

Visible InGaN/GaN Quantum-Dot Materials and Devices

General properties of III-V nitride-based quantum dots (QDs) are presented, with a special emphasis on InGaN/GaN QDs for visible optoelectronic devices. Stranski-Krastanov GaN/AlN dots are first discussed as a prototypical system. It is shown that the optical transition energies are governed by a giant quantum-confined Stark effect, which is the consequence of the presence of a large built-in internal electric field of several MV/cm. Then we move to InGaN/GaN QDs, reviewing the different fabrication approaches and their main optical properties. In particular, we focus on InGaN dots that are formed spontaneously by In composition fluctuations in InGaN quantum wells. Finally, some advantages and limitations of nitride laser diodes with active regions based on InGaN QDs are discussed, pointing out the requirements on dot uniformity and density in order to be able to exploit the expected quantum confinement effects in future devices.

Inside Front Cover: Inkjet Printing of Luminescent CdTe Nanocrystal–Polymer Composites (Adv. Funct. Mater. 1/2007)

AbstractSchubert and co‐workers have performed a detailed investigation on ink‐jet printing of well‐defined dots of luminescent CdTe nanocrystals (NCs) embedded in a poly(vinyl alcohol) matrix, as reported on p. 23, and subsequently made studies of their morphology and photoluminescence. The inside cover shows a photograph of an ink‐jet‐printed combinatorial library of differently sized CdTe NCs emitting at different wavelengths, and a 3D profilometer image of an array of printed dots.Inkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.

Evolution of self-assembled lateral quantum dot molecules

Self-assembled lateral InAs quantum dot molecules (QDMs) are grown by solid-source molecular beam epitaxy (MBE) using the thin-capping-and-regrowth MBE process. Thin capping of GaAs on as-grown InAs quantum dots (QDs) at low temperatures leads to nanohole templates. The shape, size and depth of nanoholes are controlled by capping thickness. Subsequent regrowth with different amounts of InAs on the templates result in nano-propeller QDs with different blades’ dimensions. We showed that the length of the propeller blades is controlled by either the capping temperature or the thickness of capping layer. When the regrowth thickness reaches 1.2 ML, different self-assembled lateral QDMs are formed depending on the shape of nanoholes. The dot uniformity and the dot size of all QDM samples are confirmed by photoluminescence (PL) measurements at 77 K. In addition, by ramping the regrowth temperature while the In shutter is open after the deposition of the capping layer, we are able to investigate the formation of nano-propeller QDs at the beginning phases and to propose a model to explain the evolution.

Fabrication of nanostructures with laser interference lithography

Maskless nanostructure fabrication by laser interference lithography (LIL) using Lloyd's mirror interferometer is investigated. With the usage of 100 nm thickness high contrast i-line positive photoresist PFI-88 A6, discrete uniform dot pattern is achieved with stable dose. The edge quality is improved by anti-reflective coating (ARC) between the substrate and the photoresist to minimize the interference of vertical standing waves. Using AZ-BARLi-II 90 (ARC), 100 nm uniform dot pattern with smooth and round edge quality is demonstrated. Three regions of laser intensity distributions and vertical standing wave schematics are described. This nanopatterning technology has potential applications in high density data storage as well as nano-device fabrication.

Inkjet Printing of Luminescent CdTe Nanocrystal–Polymer Composites

AbstractInkjet printing is used to produce well‐defined patterns of dots (with diameters of ca. 120 μm) that are composed of luminescent CdTe nanocrystals (NCs) embedded within a poly(vinylalcohol) (PVA) matrix. Addition of ethylene glycol (1–2 vol %) to the aqueous solution of CdTe NCs suppresses the well‐known ring‐formation effect in inkjet printing leading to exceptionally uniform dots. Atomic force microscopy characterization reveals that in the CdTe NC films the particle–particle interaction could be prevented using inert PVA as a matrix. Combinatorial libraries of CdTe NC–PVA composites with variable NC sizes and polymer/NC ratios are prepared using inkjet printing. These libraries are subsequently characterized using a UV/fluorescence plate reader to determine their luminescent properties. Energy transfer from green‐light‐emitting to red‐light‐emitting CdTe NCs in the composite containing green‐ (2.6 nm diameter) and red‐emitting (3.5 nm diameter) NCs are demonstrated.

Group-III vacancy inducedInxGa1−xAsquantum dot interdiffusion

The impact of group-III vacancy diffusion, generated during dielectric cap induced intermixing, on the energy state transition and the inhomogeneity reduction in the $\mathrm{InGaAs}∕\mathrm{GaAs}$ quantum-dot structure is investigated. We use a three-dimensional quantum-dot diffusion model and photoluminescence data to determine the thermal and the interdiffusion properties of the quantum dot. The band gap energy variation related to the dot uniformity is found to be dominantly affected by the height fluctuation. A group-III vacancies migration energy ${H}_{m}$ for $\mathrm{InGaAs}$ quantum dots of $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was deduced. This result is similar to the value obtained from the bulk and $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum-well materials confirming the role of ${\mathrm{SiO}}_{2}$ capping enhanced group-III vacancy induced interdiffusion in the $\mathrm{InGaAs}$ quantum dots.

Long-range-ordered CdTe∕GaAs nanodot arrays grown as replicas of nanoporous alumina masks

Long-range-ordered CdTe nanodot arrays with controlled size and density were grown on GaAs substrates by using molecular-beam epitaxy with ultrathin nanoporous alumina masks. The CdTe∕GaAs nanodot arrays were grown as replicas of the self-assembled porous alumina masks in spite of the large lattice mismatch between GaAs and CdTe. Using ultrathin alumina masks (ca. 200nm in thickness), we fabricated CdTe nanodot arrays with uniform dot sizes in the ranges of 35nm (with a density of ∼2.5×1010cm−2) and 80nm (with a density of ∼8.1×109cm−2). This is the report on controlling both the size and the density of II-VI/III–V heterostructure semiconductor nanodots.

Optimizing the growth of 1.3-μm InAs/InGaAs dots-in-a-well structure: Achievement of high-performance laser

The structural and optical properties of single- and multi-layer GaAs-based 1.3-μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized to improve the performance of quantum dot (QD) lasers. A strong dependence of the InAs QD density and the QD emission wavelength on the In composition of InGaAs well has been demonstrated for single-layer samples. The optimum In composition in the well for 1.3-μm applications is obtained to be 15%. The effects of growth temperature of the spacer layers (SPLs) are investigated using a 5-layer structure. It is found that the growth temperature of GaAs SPLs has a pronounced effect on both the structural and optical properties of the InAs QDs. Dislocations are introduced in the second and subsequent QD layers for samples where the GaAs SPLs are deposited at a low temperature of 510 °C. By incorporating a high-temperature (580 °C) growth step for the GaAs SPLs, dislocation formation is inhibited and a high level of inter-layer dot uniformity is achieved. An extremely low continuous wave room-temperature threshold current density of 39 A/cm 2 is demonstrated for an as-cleaved 5-layer device with emission at 1.306 μm and ground-state operation up to 100 °C.

Controlling the properties of InGaAs quantum dots by selective-area epitaxy

Selective growth of InGaAs quantum dots on GaAs is reported. It is demonstrated that selective-area epitaxy can be used for in-plane bandgap energy control of quantum dots. Atomic force microscopy and cathodoluminescence are used for characterization of the selectively grown dots. Our results show that the composition, size, and uniformity of dots are determined by the dimensions of the mask used for patterning the substrate. Properties of dots can be selectively tuned by varying the mask dimensions. A single-step growth of a thin InGaAs quantum well and InGaAs quantum dots on the same wafer is demonstrated. By using a single-step growth, dots luminescing at different wavelengths, in the range 1150–1230nm, in different parts of the same wafer are achieved.

Phosphorus-mediated growth of Ge quantum dots on Si(0 0 1)

The influence of pre-deposited phosphorus atoms on the formation of self-assembled Ge quantum dots (QDs) on Si(0 0 1) is investigated by solid-source molecular beam epitaxy (MBE) and atomic force microscopy (AFM). AFM images show that P atoms have a great influence on the size and density of Ge QDs. In the presence of 0.1 ML P atoms, highly uniform self-assembled Ge QDs with a mean base size of 32 nm and an areal density of 1.4×10 11 cm −2 are obtained. These QDs are dome shaped with a size distribution width at half-maximum of 8 nm, which have a higher aspect ratio and are more uniform in size than dots obtained with the presence of C atoms. The strain effect on the nucleation and growth of Ge dots induced by P atoms is discussed. In situ annealing reveals the non-metastable nature of the as-grown uniform dots and they are formed under highly kinetically limited condition.

Properties and applications of SiGe nanodots

The use of SiGe strained layers for BiCMOS technology has been prevailing in the market place for high-speed and high-frequency mixed mode applications. The advantages of using SiGe are due to its compatibility with mainstream Si CMOS processing. SiGe has also been gradually making its way to the mainstream strained layer CMOS as well. In this paper, we will discuss SiGe quantum structures, which have the unique properties due to their nanosize effects and the presence of strain. We will describe the growth of SiGe quantum dots on Si. First, for dot growth, issues such as the growth conditions and their effects on the dot morphology will be reviewed. We will focus on the dependence of growth temperature on the size and also show the mechanisms of uniform dot formation. The basic issues such as vertical correlation and dot morphology evolution will be addressed in relation to the critical thickness of Ge quantum dot superlattices. Both interband and intersubband photoluminescence properties of Ge quantum dot superlattices will be discussed as functions of dot size, excitation power, growth temperature, superlattice period, and Ge concentration. We will also discuss quantum dot pin photodetectors for 1.3–1.55 μm for integration with Si technology for communications applications. Potential light emission from dots will be described and the characteristics of fabricated LED devices will be reported. Likewise, we will show the results of intersubband properties of boron-doped and modulation-doped Ge quantum dot superlattices in reference to intersubband absorption as studied by FTIR. We will describe the quantum dot pip photodetectors for 3–5 and 8–12 μm applications.

Quantitative Analysis in Nanoliter Wells by Prefilling of Wells Using Electrospray Deposition Followed by Sample Introduction with a Coverslip Method

In contrast to performing assays on a substrate using immobilization techniques, wet analysis in nanoliter-sized wells allows quantitative monitoring of enzyme-based reactions by measuring luminescence with time. However, a suitable dispensing method is required to accurately deposit stabilized enzyme solutions into nanoliter wells in such a manner that the enzyme activities are preserved prior to and during measurements. Furthermore, an efficient method is required to add sample liquid to these wells in such a manner that evaporation of assay liquid is completely prevented during sample introduction and monitoring. A powerful methodology is presented in this paper allowing quantitative analysis of enzyme-based reactions in identical nanoliter volumes on-chip. In a first step, picoliter amounts of protein solutions are deposited as uniform dry dots into wells using our reported electrospraying technique (Moerman, R.; Frank, J.; Marijnissen, J. C. M.; Schalkhammer, T. G. M.; van Dedem, G. W. K. Anal. Chem. 2001, 73, 2183-2189.). The silicon chips are then stored at temperatures as low as -80 degrees C. At the time of analysis, a sample solution is slid into the wells using a coverslip. With the edge of the coverslip, sample solution is pushed across the wells at a speed of 1.5-2.5 cm/s to prevent carryover of reagents to neighboring wells. Evaporation of assay liquid from the wells is prevented because the coverslip seals the wells and "bonds" to the chip by adhesion forces. Electrospraying appears to be an excellent method to deposit enzyme solutions containing up to 20% (w/v) of trehalose without being hampered by clogging of the capillary or splashing of droplets. After being sprayed on-chip (silicon nitride), the enzymes pyruvate kinase and lactate dehydrogenase remained stable for a period of 1.5-2 months at a storage temperature of -20 degrees C. The coverslip method completely prevented evaporation for minutes up to hours allowing monitoring of enzyme-based reactions in arrays of nanoliter wells.

Some analytical results in phase coherent transport in quantum wire

A quantum wire of uniform cross section (but with eventual disorder) with three regions: dot, left lead, and right lead, is considered. Assuming that the same unitary transformation diagonalizes all unit cells of this wire, we propose a new method for the calculation of the Green's function (GF) and the coherent transmission coefficient. In particular, the problem of a uniform dot (simple cubic uniform dot attached to the simple cubic wire), with all onsites equal and all hoppings equal is solved fully analytically. The energy and dot-length dependence of the GF, local density of states (LDOS), the transmission coefficient and bound state energies are also derived.

A silicon-molecular hybrid memory device

We have fabricated MOS memory devices based on coil–rod–coil triblock molecules actingas quantum dots. Uniform molecular dots result in a discrete shift in the threshold voltageat room temperature, which is indicative of single-electron effects. Molecular scalability andlow-power operation make the silicon-molecular hybrid device an attractive candidate fornext-generation electronic devices.

Synthesis of uniform GaN quantum dot arrays via electron nanolithography of D2GaN3

We demonstrate the deposition of periodic arrays of uniformly sized GaN quantum dots onto a SiOx substrate. The dots are deposited using a nanolithography technique based on a combination of electron-beam-induced chemical vapor deposition and single-source molecular hydride chemistries. Under appropriate deposition conditions, we can deposit uniform dots of height 5 nm and full widths at half-maxima of 4 nm. The dot size is controlled by the spatial distribution of secondary electrons leaving the substrate surface. The smallest, most uniform void-free dots are created via nanolithography of molecules adsorbed on the substrate surface.

Pyrolytic Laser CVD of Boron Nitride and Molybdenum

The first pyrolytic laser deposition of boron nitride and molybdenum is reported. The deposition from borazine and molybdenum pentachloride on variety of substrates is demonstrated including graphite, alumina and tungsten. The deposition of boron nitride on graphite produces very short fibers while its deposition on alumina results in uniform shallow dots. Molybdenum layers deposited on graphite at temperatures above 1150 °C are more fibrous and porous whereas films deposited at lower temperatures are flat and flaky. The morphology of each material has also been related to the operating parameters of the gas‐jet LCVD system.

1.3 μm lasers with AlInAs-capped self-assembled quantum dots

We report 1.3 μm room-temperature lasing for a self-assembled quantum dot (QD) laser in which the dots are capped by a thin AlInAs layer. The inclusion of the Al-containing layer is shown to improve the dot uniformity, and the larger carrier confinement increases the separation between the QD transitions. A RT threshold current density of 80 A cm−2 is obtained for a device containing three QD layers. The inclusion of the AlInAs layer allows the modification of the dot physical structure and electronic structure of the device, without any significant incorporation of nonradiative centers.

Temperature effect on the formation of uniform self-assembled Ge dots

The effect of the growth temperature on the formation of uniform self-assembled Ge dots on Si (001) substrates was studied. The ratio of pyramid dots to dome dots varies with the growth temperature from 500 to 700 °C. Temperature of 600 °C was optimum to form uniform self-assembled Ge dots, and is attributed to the enhanced diffusion kinetics. Highly uniform Ge dots with height deviation of ±3% were obtained at this growth temperature. Discontinuity in characteristic length was found in an Arrhenius plot between 600 and 625 °C, and it is due to intermixing of Si with Ge which occurred at high temperature.

Carrier recombination in aligned InAs/GaAs quantum dots grown on strain‐relaxed InGaAs layers

This thesis presents results of time-resolved photoluminescence experiments conducted on several different self-assembled InGaAs/GaAs and InAs/GaAs semiconductor quantum dot (QD) structures. Depending on the application in mind, different structural, electronic or optical properties have a different weight of importance. Fast carrier capture and relaxation is critical for QD based lasers, for example. In this thesis, the influence of surplus carriers, introduced through modulation-doping, is studied. It is shown that carrier capture is essentially unaffected whereas the intradot relaxation mechanisms, at least at low carrier concentrations, are fundamentally different. The phonon mediated cascade relaxation found in the undoped reference sample is replaced by efficient scattering with the built-in carriers in the case of the doped structures. Moreover, spin relaxation also depends on presence of extra carriers. During energy relaxation via carrier-carrier scattering, the spin polarization is preserved whereas in the undoped sample the strong interaction of relaxing carriers with LO phonons causes spin relaxation. The decay of the ground state spin polarization proceeds at the same rate for doped and undoped structures and is shown to be caused by acoustic phonons, even up to 300 K. While optimizing QD growth for specific applications, it is imperative to evaluate the influence of nonradiative recombination, which is most often detrimental. While misfit dislocations, deliberately introduced in the substrate, lead to the formation of laterally ordered, uniform dots, these samples are found to suffer from strong nonradiative recombination. Structures with different barrier thicknesses and numerical simulations indicate defects in the vicinity of the QDs as main origin of fast carrier trapping. On the other hand, it is shown that direct dot doping, compared to barrier doping or undoped structures, causes only minor degradation of the optical properties. Directly doped dots even exhibit a significantly weaker photoluminescence quenching with temperature, making them prospective for devices operating at room temperature. Finally, the superior proton radiation hardness of QD structures compared to quantum wells is demonstrated, which is due to the three-dimensional confinement. The increase of photoluminescence intensity at low to moderate doses is interpreted as an enhanced carrier transfer into the dots via the defects introduced into the material by the protons.

I–V characteristics of YBCO thin films with a periodic array of Ni dots

Enhancing the pinning force in cuprates can be achieved by externally introduced magnetic dots. We have used a novel nanochannel glass technique to create metal replica masks with sub-micron size holes that have a triangular lattice pattern. With this replica mask, magnetic dots with a periodic array were deposited onto the surface of YBCO thin films by a simple deposition. The I–V characteristics of an YBCO thin film strip with a uniform Ni dots have been measured. They were compared with that of a bare YBCO strip without any magnetic dots. The results show that as the magnetic field strength increases the critical current value of the strip with the magnetic dots reduces with a much slower pace in comparison with the values obtained from the bare sample.