InGaAs Quantum Dot Layers Research Articles

Emission Regimes of 1.06 μm Spectral Bandwidth Two-Sectional Lasers with Quantum Dot Based Active Layer

We have investigated two-sectional semiconductor lasers with an active region comprising five layers of InGaAs quantum dots, emitting in the spectral range near 1.06 μm. Regimes of passive mode-locking, passive Q-switching, and mode-locking with pulse modulated amplitude are realized. The transition conditions between generation regimes are investigated. The frequency tuning range with current increase in the Q-switched regime exceeds more than 4 times. The duration of the mode-locked pulses was 2 ps at the pulse repetition rate of 44.3 GHz.

Correlating Photoluminescence and Structural Properties of Uncapped and GaAs-Capped Epitaxial InGaAs Quantum Dots

The understanding of the correlation between structural and photoluminescence (PL) properties of self-assembled semiconductor quantum dots (QDs), particularly InGaAs QDs grown on (001) GaAs substrates, is crucial for both fundamental research and optoelectronic device applications. So far structural and PL properties have been probed from two different epitaxial layers, namely top-capped and buried layers respectively. Here, we report for the first time both structural and PL measurements from an uncapped layer of InGaAs QDs to correlate directly composition, strain and shape of QDs with the optical properties. Synchrotron X-ray scattering measurements show migration of In atom from the apex of QDs giving systematic reduction of height and enlargement of QDs base in the capping process. The optical transitions show systematic reduction in the energy of ground state and the first excited state transition lines with increase in capping but the energy of the second excited state line remain unchanged. We also found that the excitons are confined at the base region of these elliptically shaped QDs showing an interesting volume-dependent confinement energy scaling of 0.3 instead of 0.67 expected for spherical dots. The presented method will help us tuning the growth of QDs to achieve desired optical properties.

Режимы излучения двухсекционных лазеров спектрального диапазона 1.06 μm с активной областью на основе квантовых точек

AbstractWe have investigated two-sectional semiconductor lasers with an active region comprising five layers of InGaAs quantum dots, emitting in the spectral range near 1.06 μm. Regimes of passive mode-locking, passive Q-switching, and mode-locking with pulse modulated amplitude are realized. The transition conditions between generation regimes are investigated. The frequency tuning range with current increase in the Q-switched regime exceeds more than 4 times. The duration of the mode-locked pulses was 2 ps at the pulse repetition rate of 44.3 GHz.

Emission from quantum-dot high-\u03b2 microcavities: transition from spontaneous emission to lasing and the effects of superradiant emitter coupling

Measured and calculated results are presented for the emission properties of a new class of emitters operating in the cavity quantum electrodynamics regime. The structures are based on high-finesse GaAs/AlAs micropillar cavities, each with an active medium consisting of a layer of InGaAs quantum dots (QDs) and the distinguishing feature of having a substantial fraction of spontaneous emission channeled into one cavity mode (high β-factor). This paper demonstrates that the usual criterion for lasing with a conventional (low β-factor) cavity, that is, a sharp non-linearity in the input–output curve accompanied by noticeable linewidth narrowing, has to be reinforced by the equal-time second-order photon autocorrelation function to confirm lasing. The paper also shows that the equal-time second-order photon autocorrelation function is useful for recognizing superradiance, a manifestation of the correlations possible in high-β microcavities operating with QDs. In terms of consolidating the collected data and identifying the physics underlying laser action, both theory and experiment suggest a sole dependence on intracavity photon number. Evidence for this assertion comes from all our measured and calculated data on emission coherence and fluctuation, for devices ranging from light-emitting diodes (LEDs) and cavity-enhanced LEDs to lasers, lying on the same two curves: one for linewidth narrowing versus intracavity photon number and the other for g(2)(0) versus intracavity photon number.

Gain Measurement of Highly Stacked InGaAs Quantum Dot Laser with Hakki–Paoli Method

A 147-µm-long cavity laser diode with 19 InGaAs quantum dot layers was fabricated by the ultrahigh-rate molecular beam epitaxial growth technique, and its gain properties were investigated using the Hakki–Paoli method below the threshold current (Ith) of 111.5 mA. At an injection current of 100.3 mA (0.9Ith), the positive net modal gain was in the range between 1005 and 1043 nm, corresponding to a photon energy of 45 meV. The maximum net modal gain and maximum modal gain were 46.5 and 60.5 cm-1, respectively. A differential net modal gain of as high as 3.8 cm-1/mA was observed at 0.77 times the threshold current. No gain saturation appeared below the threshold current, and injection currents higher than 78.4 mA (≈0.7Ith) were required to obtain a net modal gain.

Microdisk cavity laser with InGaAs quantum dots on AlAs/GaAs distributed Bragg reflector

The lasing peak of λ=1090nm at T=110K was obtained for a microdisk cavity with three layers of InGaAs quantum dots integrated on AlAs/GaAs distributed Bragg reflector. The refractive indices and reflectance spectrum of AlAs/GaAs DBR at T=110K were derived and estimated. The lasing mode was subsequently confirmed as the whispering gallery mode with azimuthal mode number m=13 and could be explained as the valid of bottom DBR mirror.

Change in the electrical performance of GaAs solar cells with InGaAs quantum dot layers by electron irradiation

The radiation response of GaAs PiN solar cells with 50 stacked layers containing self-aligned In0.4Ga0.6As quantum dots (QDs) was compared to that of GaAs PiN solar cells with non QD layers. Those solar cells were radiated with 1MeV electrons up to 1×1016/cm2, and the change in their electrical characteristics under AM 0 was investigated using an in-situ measurement technique. After electron irradiation at 1×1016/cm2, the value of open circuit voltage (VOC) for the In0.4Ga0.6As 50 QD and the non QD solar cells remained 90 and 80% of the initial value, respectively. On the other hand, the values of short circuit current (ISC) and maximum power (PMAX) for the 50 QD solar cells became approximately 80 and 55% of the initial value by electron irradiation at 1×1016/cm2, respectively, although the non QD solar cells showed 95% for ISC and 63% for PMAX after the irradiation. The recovery of the electrical characteristics of both the InGaAs 50 QD and the non QD solar cells irradiated with electrons at 1×1016/cm2 were observed under AM0 light illumination at RT.

InGaP-based InGaAs quantum dot solar cells with GaAs spacer layer fabricated using solid-source molecular beam epitaxy

We report InGaP-based multistacked InGaAs quantum dot (QD) solar cells with GaAs spacer layers. We obtain a highly stacked and well-aligned InGaAs QD structure with GaAs spacer layers in an InGaP matrix grown by solid-source molecular beam epitaxy. The photoluminescence intensity of the InGaAs QDs in the InGaP matrix increases as the number of QD layers increases, which indicates the growth of a high-quality InGaP-based multistacked InGaAs QD structure. The short-circuit current density and the conversion efficiency of the InGaP-based QD solar cells increase as the number of InGaAs QD layers increases.

Spin-polarized multiexcitons in quantum dots in the presence of spin-orbit interaction

An efficient electron spin-relaxation mechanism has been observed in InAs quantum dots (QDs) that manifests itself as a sharp drop in the circular polarization of the light emitted by Fe spin-light emitting diodes, which incorporate a single layer of InGaAs QDs, for a narrow range of magnetic fields around 5 T. The underlying mechanism occurs when the QDs are occupied by three-electron-hole pairs forming a tri-exciton (3X) and is a two-step process. The first step involves the spin flip of one of the three electrons mediated by the spin-orbit interaction; in the second step the 3X relaxes to its ground state via phonon emission.

Miniband formation in InGaAs quantum dot superlattice

We report the formation of a miniband in a quantum dot (QD) superlattice with 20-stack InGaAs QD layers, which we confirmed by the excitation power dependence in photoluminescence (PL) measurements. The PL peak of a QD superlattice shifts to a shorter wavelength as the excitation power is increased. The blueshifted energy of the PL peak is 8 meV for a QD superlattice with an interdot spacing of 3.5 nm, whereas the blueshift is not observed for a multistacked QD structure with an interdot spacing of 15 nm. The blueshifted energy becomes larger as the interdot spacing is reduced.

An atomically resolved study of InGaAs quantum dot layers grown with an indium flushstep

In this cross-sectional scanning tunnelling microscopy study we investigate the indium flushmethod as a means to control the height of self-assembled InGaAs quantum dotsand wetting layers. The results show that application of an indium flush stepduring growth results in flattened dots and a reduced wetting layer of which theheight can be precisely controlled by varying the height of the first capping layer.

2D substrate imaging of a tapered laser cavity based on InGaAs quantum dots

Tapered cavities are an excellent solution to obtaining high brightness semiconductor laser sources for use in applications from frequency doubling to material processing. The high power density levels reached in such lasers can lead to nonlinear mechanisms related to the refractive index variations that de-stabilise the optical field distribution inside the laser cavity. These mechanisms are detrimental to the beam quality of the laser limiting its focussing ability. In this work we map the spatial distribution and evolution of the carrier density in tapered lasers as a function of injection current. A device with a taper length of 2.75 mm, taper angle of 6° and ridge length of 1.25 mm is used to demonstrate the principle. The active region consists of three layers of InGaAs quantum dots emitting around 950 nm. A window for imaging the spontaneous emission profile through the transparent GaAs substrate was formed by patterning the n-contact metal layer. The imaging was performed in the continuous wave regime to include the thermal-induced refractive index perturbation. The results show a non-uniform carrier and thermal distribution inside the cavity even at low current levels.

Vertical-cavity surface-emitting quantum-dot laser with low threshold current grown by metal-organic vapor phase epitaxy

Ground state lasing of electrically driven vertical-cavity surface-emitting lasers with a quantum-dot (QD) gain medium grown using metal-organic vapor phase epitaxy was realized. The devices use stacked InGaAs QD layers, placed in the field intensity antinodes of the cavity formed by selectively oxidized distributed Bragg reflectors. Devices with 3×3 QD layers demonstrate at 20°C a cw output power of 1.45mW at 1.1μm emission wavelength. The peak external efficiency was 45%, limited by lateral carrier spreading within the 4λ cavity and a reduction of the internal efficiency above 60°C. A minimum threshold current of 85μA was obtained from a device with a 1μm aperture.

High-efficiency 1.3μmInGaAs∕GaAs quantum-dot microcavity light-emitting diodes grown by metalorganic chemical vapor deposition

We investigate the optical properties of quantum-dot (QD) microcavity light-emitting diodes (MCLED) operating at 1.3μm at room temperature. The active medium consists of a single layer of InGaAs quantum dots, directly grown in a GaAs matrix by metalorganic chemical vapor deposition. Electrical injected QD MCLEDs were fabricated by exploiting a hybrid technology, which employs epitaxial and oxide-based mirrors. Such technology allows us to preserve the QD properties and leads to a wider optical bandwidth of the hybrid top distributed Bragg reflector with respect to the fully epitaxial mirror, resulting in photon recycling of the overall QDs spectrum, thus improving the efficiency of the device. The devices exhibit bright electroluminescence peaked at 1.29μm, with a full width at half maximum of 10 meV. The room-temperature external quantum efficiency of such devices is 0.52%, higher than that reported in the literature for QD MCLEDs operating at 1.3μm at room temperature.

Morphology response to strain field interferences in stacks of highly ordered quantum dot arrays.

Twofold stacked InGaAs/GaAs quantum dot (QD) layers are grown on GaAs(001) substrates patterned with square arrays of shallow holes. We study the surface morphology of the second InGaAs QD layer as a function of pattern periodicity. Comparing our experimental results with a realistic simulation of the strain energy density E(str) distribution, we find that the second InGaAs QD layer sensitively responds to the lateral strain-field interferences generated by the buried periodic QD array. This response includes the well-known formation of vertically aligned QDs but also the occurrence of QDs on satellite strain energy density minima. Our calculations show that base size and shape as well as lateral orientation of both QD types are predefined by the E(str) distribution on the underlying surface.

Light emission tuning of In 0.5Ga 0.5As/In 0.05Ga 0.95As quantum dots by a two-dimensional photonic crystal

We report on the fabrication and characterization of a two-dimensional photonic crystal (PC) embedding two InGaAs quantum dot layers as active light emitters. Light confinement was provided by both a PC GaAs slab waveguide and a distributed Bragg reflector (DBR) located at the bottom of the guiding layer. The micro-photoluminescence characterization shows that the photonic crystal can be used to alter the quantum dot emission by modifying the photonic density of states.

Performance of lasers containing three, five and seven layers of quantum dots

The authors have analysed the performance of quantum dot lasers containing three, five and seven layers of InGaAs quantum dots. At room temperature the threshold current density of devices of length between 320 μm and 2000 μm, with uncoated facets, decreases with device length and is lower for higher layer numbers. They explain this behaviour by analysis of measured modal gain spectra, internal optical mode loss and the temperature dependence of the threshold current density. They find that increasing the number of dot layers can improve device performance because the optical mode loss does not increase while the modal gain at a particular wavelength does increase with more dot layers. However, at low temperatures <200 K they find that the three-layer structure exhibits the lowest threshold current density for devices of length 550 μm and longer. They conclude that the improved threshold current behaviour of the higher layer number devices at room temperature is not primarily due to intrinsic gain saturation but because the required modal gain can be achieved in high layer number samples with a lower quasi-Fermi level separation and hence a reduced population of higher lying energy states.

Electrically pumped InGaAs quantum dot ring and cylindrical cavity lasers

Operation of an electrically pumped, cylindrical cavity laser containing a stack of five self-assembled InGaAs quantum dot layers is reported. A threshold current density of 47 Acm-2 is obtained at 150 K, with whispering gallery mode laser emission observed at ~1.2 eV. Fabrication of a ring cavity via the removal of part of the non-lasing central region of the cylinder results in a 30% reduction in threshold current.

Modal gain and internal optical mode loss of a quantum dot laser

The modal gain spectra and internal optical mode loss of a semiconductor laser structure containing a single layer of InGaAs quantum dots have been measured independently and directly as a function of current density. The quantum dot gain exhibits no obvious polarization dependence. The maximum modal gain of (11±4) cm−1 obtained from the ground state of a single layer of quantum dots is in this case insufficient for lasing operation since the internal optical mode loss measured on the same sample is (11±4) cm−1. As expected laser emission is not observed from the dot ground state, but from the excited dot state or from the wetting layer depending on device length.

Dislocation-induced changes in quantum dots: Step alignment and radiative emission

A transition between two types of step alignment was observed in a multilayered InGaAs/GaAs quantum-dot (QD) structure. A change to larger QD sizes in smaller concentrations occurred after formation of a dislocation array. Cathodoluminescence (CL) spectra show a bimodal peak with lower energy peak enhancement when probing at lower e-beam energies. The two peaks separate as a result of QD interdiffusion. CL imaging and cross-sectional transmission electron microscopy showed contrast from a dislocation array formed at the interface between GaAs and the first InGaAs QD layer. Strong QD emission in the near infrared (800–1100 nm) was obtained despite the presence of dislocations.