GaInP Quantum Dots Research Articles

Excitons in InP, GaP, and GaxIn1−xP quantum dots: Insights from time-dependent density functional theory

Colloidal quantum dots (QDs) of group III-V are considered as promising candidates for next-generation environmentally friendly light emitting devices, yet there appears to be only limited understanding of the underlying electronic and excitonic properties. Using large-scale density functional theory with the hybrid B3LYP functional solving the single-particle states and time-dependent density functional theory accounting for the many-body excitonic effects, we have identified the structural, electronic and excitonic optical properties of InP, GaP and GaInP QDs containing up to a thousand atoms or more. The calculated optical gap of InP QD appears in excellent agreement with available experiments, and it scales nearly linearly with the inverse diameter. The radiative exciton decay lifetime is found to increase surprisingly linearly with increasing the dot size. For GaP QDs, we predict an unusual electronic state crossover at diameter around 1.5 nm whereby the nature of the lowest unoccupied molecular orbital (LUMO) state switches its symmetry from $\Gamma_{5}$-like at larger diameter to $\Gamma_{1}$-like at smaller diameter. After the crossover, the absorption intensity of the band-edge exciton states is significantly enhanced. Finally, we find that Vegard's law holds very well for GaInP random alloyed quantum dots down to ultra-small sizes with less than a hundred atoms. The obtained energy gap bowing parameter of this common-cation compound in QD regime appears positive, size-dependent and much smaller than its bulk parentage. The volume deformation, dominating over the charge exchange and structure relaxation effects, is mainly responsible for the QD energy gap bowing. The present work provides a road map for a variety of electronic and optical properties of colloidal QDs in group III-V that can guide spectroscopic studies.

Investigation of Gainp Quantum Dots in the AlGaInP-Based Light Emitting Diodes

Recently, AlGaInP-based light emitting diodes (LEDs) have experienced an impressive evolution in both device performance and market volume. However, development of new applications is required in order to realize their full potential in areas such as use as a light source for auto focusing in digital cameras, special illumination for particular functions in agriculture, and in full color displays. To enlarge their utility in these applications, it is necessary to fabricate and understand a new structure capable of emitting longer wavelengths of around 700 nm. In particular, AlGaInP heterostructure LEDs are lattice-matched with respect to the GaAs substrate, which limits the emitting spectrum to around 650 nm at the longer peak wavelength side. To fabricate an LED structure capable of emitting at a 700 nm peak wavelength, the composition (x) of GaxIn1-xP material in the active layer requires a compressive strain of larger than 1 %. This large lattice mismatch, however, causes significant problems in terms of both growth and device properties due to the formation of defects. To overcome these problems, it is necessary to relieve the well strain via the formation of islands, referred to as a Stranski-Krastanow (S-K) growth mode, in order to prevent the generation of dislocations [1,2]. However, in AlGaInP-based LEDs emitting at a 700 nm peak wavelength, the effects of well strain on the epitaxial growth and the realization of device performance has yet to be extensively studied. In this study, we investigate the behaviors of morphological and optical characteristics on the composition of Ga0.33In0.67P material and demonstrate the performance of a device emitting at around 700 nm using quantum dot (QD)-based LEDs. Figure 1

Structural and Optical Investigation of GaInP Quantum Dots According to the Growth Thickness for the 700 nm Light Emitters

We investigate Ga0.33In0.67P quantum dot structures appropriate for special lighting applications in terms of structural and optical behaviors. The Ga0.33In0.67P materials form from 2-dimentional to 3-dimensional dots as the nominal growth thickness increases from 0.5 nm to 6.0 nm, indicating a Stranski-Krastanov growth mode. As the ambient temperature is increased to 300 K, the PL spectrum of the B-type dots is annihilated quickly because the large dot size induces a defect-related nonradiative recombination process. In contrast, the PL spectrum of the A-type dots is well maintained to 300 K. These data indicate that the Ga0.33In0.67P material is appropriate for an active layer of 700 nm light emitters.

Study of GaInP Quantum Dots According to the Growth Thickness for the 700 nm Light Emitting Diodes

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Single-photon emission from a type-B InP∕GaInP quantum dot

Type-B InP∕GaInP quantum dots are expected to exhibit a type-II electronic structure. Evidence for this is provided by the variation in decay time of the ensemble as a function of excitation power density. Photon correlation measurements were subsequently performed on a single type-B InP∕GaInP quantum dot using a Hanbury-Brown and Twiss setup [Nature 178, 1447 (1956)]. Autocorrelation measurements were carried out under both continuous-wave and pulsed excitation conditions with single-photon emission observed in each case. The continuous-wave measurements display a pronounced antibunching dip at zero time delay while pulsed measurements enable the triggered generation of single photons on demand at a wavelength of approximately 750 nm.

Charging control of InP∕GaInP quantum dots by heterostructure design

Semiconductor quantum dots are often charged due to accumulation from a doped host material. Using low-temperature photoluminescence, we have studied the charging of single self-assembled InP dots in structures designed to control the electron population in a weakly n-type environment. By using designed heterostructures to position the Fermi level of the structure, not requiring electric fields or currents, we show that the electron accumulation can be reduced from approximately 18 electrons in the dot to approximately 8 electrons. In particular, we show that the single quantum dot luminescence spectrum of the Fermi-level pinned structure perfectly matches the low-energy part of the highly charged reference spectrum, a phenomenon predicted by the model for multiple charging of quantum dots.

Nanoindentation and near-field spectroscopy of single semiconductor quantum dots

Low-temperature near-field scanning optical microscopy was used to study the dependence of the emission spectra of single self-organized InAs on GaAs, InAs on AlAs and InP on GaInP quantum dots (QDs) on contact pressure exerted by a near-field optical fiber tip (nanoindentation). A large energy shift (up to 150 meV), broadening (up to 10 meV), and intensity increase (up to one order of magnitude) of single QD emission lines have been observed at tip compressions up to 70 nm. Ground state energy shift rates from 0.5 to 3.5 meV/nm have been measured for different aperture types (rounded and flat, metal coated and uncoated) and sizes (50--300 nm) in agreement with numerical calculations using Picus--Bir orbital-strain Hamiltonian. A reduction of the hydrostatic pressure coefficient due to a nonuniform In distribution in self-organized QDs has been observed. Anomalously strong lateral inhomogeneity of the local stress field has been observed.

Growth of self-assembled Ga xIn 1− xP quantum islands on GaP

Self-assembled GaInP quantum dots embedded in a GaP matrix were prepared by low-pressure metalorganic vapor-phase epitaxy using the Stranski–Krastanow growth mode. We present photoluminescence (PL) and atomic force microscopy studies of the formation process of three-dimensional quantum sized islands out of a two-dimensional GaInP quantum film. Up to 4×10 9 cm −2 coherently strained quantum islands were formed, emitting at 1.91 eV with a full-width at half-maximum of 38 meV (4 K). The dependence of the optical properties of the quantum dots on growth temperature and composition of the GaInP layer suggests, that the PL behavior is mainly influenced by the growth parameters of GaInP deposition, whereas the structure of the quantum islands and the nominal composition of the Ga x In 1− x P layer play only a minor role. These observations and temperature-dependent PL measurements indicate that the coherently strained quantum islands include regions with partially ordered GaInP domains. Operating under optimized growth conditions, we obtained a bright and narrow PL from the quantum dots up to room temperature. The observed strong PL at 1.87 eV (300 K) qualifies self-assembled GaInP quantum islands on GaP for applications in the visible part of the spectrum.

Red luminescence from strain-induced GaInP quantum dots

The strain of self-organized InP islands is used to induced quantum dots in near-surface GaInP/AlGaInP quantum wells. To obtain quantum dot luminescence in a widely tunable wavelength range of 630–700 nm, the composition and thickness of the GaInP quantum well is varied. The effect of different cap layer materials, i.e., GaAs, AlGaAs, GaInP, and AlGaInP on the InP island formation and quantum dot luminescence properties is investigated. The luminescence intensity ratio of the quantum dot peak to the quantum well peak is found to be highest when a GaAs cap is used.