Layers Of InAs Quantum Dots Research Articles

Room-Temperature Operation of InP-Based InAs Quantum Dot Laser

A ridge waveguide quantum dot (QD) laser with a stripe width of 15 μm was fabricated by using the seven-stacked InAs QD layers based on the InAlGaAs-InAlAs material system on InP [001] substrate. Room-temperature lasing operation was observed at 1.501 μm, which is the first observation from the InAs QDs with the InAlGaAs-InAlAs structure. The characteristic temperature of the InAs QD laser calculated from the temperature dependence of threshold current density was 135 K in the temperature range from 200 K to room temperature.

Heterostructures for achieving large responsivity in InAs/GaAs quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIPs) offer normal-incidence detection, low dark current, high operating temperature, and multi-wavelength detection. In an effort to get better responsivity in QDIPs at higher operating temperatures, we have grown, fabricated, and characterized two different QDIP heterostructures. The first is a device with 70 quantum dot layers. As a result, the peak responsivity is measured to be 0.12 A/W for Vbias=2.0 V and T=175 K. The second device has ten layers of InAs quantum dots embedded in an AlAs/GaAs superlattice. As a result of the subsequent large dot density and increased carrier confinement, the peak responsivity is measured to be 2.5 A/W for Vbias=−1.5 V and T=78 K.

High-Temperature Operation of InAs–GaAs Quantum-Dot Infrared Photodetectors With Large Responsivity and Detectivity

We have optimized the growth of multiple (40-70) layers of self-organized InAs quantum dots separated by GaAs barrier layers in order to enhance the absorption of quantum-dot infrared photodetectors (QDIPs). In devices with 70 quantum-dot layers, at relatively large operating biases (/spl les/-1.0 V), the dark current density is as low as 10/sup -5/ A/cm/sup 2/ and the peak responsivity ranges from /spl sim/0.1 to 0.3 A/W for temperatures T=150 K-175 K. The peak detectivity corresponding to these low dark currents and high responsivities varies in the range 6/spl times/10/sup 9//spl les/D/sup */(cm/spl middot/Hz/sup 1/2//W)/spl les/10/sup 11/ for temperatures 100/spl les/T(K)/spl les/200. These performance characteristics represent the state-of-the-art for QDIPs and indicate that this device heterostructure is appropriate for incorporation into focal plane arrays.

Structural and optical properties of heterostructures with InAs quantum dots in an InGaAsN quantum well grown by molecular-beam epitaxy

Results obtained in a study of the structural and optical properties of GaAs-based heterostructures with InAs quantum dot layers overgrown with InGaAsN quantum wells are presented. Transmission electron microscopy has been applied to analyze how the thickness of the InGaAsN layer and the content and distribution of nitrogen in this layer affect the size of nanoinclusions and the nature and density of structural defects. It is shown that the size of InAs nanodomains and the magnitude of the lattice mismatch in structures containing nitrogen exceed those in nitrogen-free structures. A correlation between the luminescence wavelength and the size and composition of nanodomains is demonstrated. Furthermore, a correlation between the emission intensity and defect density in the structure is revealed.

Self-assembled quantum dots as probes for Landau-level spectroscopy

We report on capacitance–voltage measurements of an inverted two-dimensional electron gas (2DEG) with a layer of self-assembled InAs quantum dots in its immediate vicinity. A variable frequency approach enables us to determine the position of the Quantum dots-induced charging peaks for the first six electrons per dot in magnetic fields up to 10 T . Using the first s-electron as an energetic reference we monitor the oscillations of the 2DEG Fermi energy as a function of the applied magnetic field. Because in the present experiment, also the energy scale of the oscillations can be determined a detailed comparison with theoretical models becomes possible. We find good agreement at low and intermediate fields, when a Gaussian density of states with a constant background is assumed. The discrepancy found for highest fields investigated suggests that more sophisticated models are necessary in this regime.

Low-noise photon counting with a radio-frequency quantum-dot field-effect transistor

We present photon counting experiments with a single-photon detector based on a field-effect transistor gated by a layer of InAs quantum dots. A cryogenic radio-frequency amplifier is used to convert the photon-induced steps in the source-drain current of the transistor into voltage peaks. We measure a maximum photon detection efficiency of 0.14%, corresponding to internal quantum efficiency of 10%. The dark count rate is less than 10−8 ns−1 when the efficiency is 0.045%.

Electrical transport and low frequency noise characteristics of Au/n-GaAs Schottky diodes containing InAs quantum dots

Au/n-GaAs Schottky diodes, containing layers of InAs quantum dots (QDs), are investigated by measuring the forward current–voltage characteristics in the temperature range of 77–300 K and low frequency noise at room temperature. The zero-bias barrier height decreases and the ideality factor increases with decreasing temperature, and the ideality factor was found to follow the T0-effect. The departure from the ideal thermionic-emission diffusion model was interpreted in terms of inhomogeneous Schottky contact with a Gaussian distribution of barrier heights. The excess current at small biases, observed in diodes containing layers of InAs QDs, was attributed to small patches of reduced barrier height. In the diode without QDs, the noise intensity SI shows 1/f behaviour and is proportional to I2F, which is explained by modulation of the barrier height due to trapping processes in interface states. In diodes containing InAs QDs, SI shows 1/fγ (with γ < 1) behaviour and is proportional to I2F in the high current range, which is explained by generation of band tail states with exponential energy distribution in the GaAs layer due to the QD formation. In the low current range, SI increases faster than I2F due to contribution to the noise of patches of reduced barrier height.

Wavelength tuning of InAs quantum dots grown on InP (100) by chemical-beam epitaxy

We report on an effective way to continuously tune the emission wavelength of InAs quantum dots (QDs) grown on InP (100) by chemical-beam epitaxy. The InAs QD layer is embedded in a GaInAsP layer lattice matched to InP. With an ultrathin GaAs layer inserted between the InAs QD layer and the GaInAsP buffer, the peak wavelength from the InAs QDs can be continuously tuned from above 1.6 μm down to 1.5 μm at room temperature. The major role of the thin GaAs layer is to greatly suppress the As/P exchange during the deposition of InAs and subsequent growth interruption under arsenic flux, as well as to consume the segregated surface In layer floating on the GaInAsP buffer layer.

Ballistic Electron Emission Luminescence of InAs Quantum Dots Embedded in a GaAs/AlxGa1−xAs Heterostructure

ABSTRACTBallistic electron emission luminescence (BEEL) is a further development of ballistic electron emission microscopy (BEEM) combining three-terminal hot electron injection and interband radiative recombination in direct-gap semiconductor materials. By using a planar tunnel-junction emitter rather than a STM tip, a spectrographic analysis of the induced electroluminescence can be performed with the help of much higher current injection level. We demonstrate the operational principle of BEEL in a GaAs/AlxGa1−xAs heterostructure with a layer of InAs quantum dots (QDs) as the optical active layer. The wavelength-resolved BEEL spectra from planar tunnel-junction devices disclose the QD luminescence as a peak near 1.34 eV accompanied with a linear quantum-confined Stark shift. At higher collector voltage, luminescence from bulk states of GaAs peaked at 1.48 eV is observed. The spectrally integrated BEEL intensity as a function of collector voltage fits well with the results from STM tip injection, which is measured in a single-photon-counting mode. Measurement of ballistic electron current spectroscopy is made possible by freezing out the thermionic leakage current at low temperatures. Our results indicate that it is feasible to simultaneously acquire topographic, electronic and photonic information of buried light-emitting semiconductor heterostructures.

A Study of (In,Ga,Al)As/GaAs Quantum-Dot Heterostructures by X-ray Diffraction and Total Reflection

The structure of (In,Ga,Al)As/GaAs stacks with one to three layers of self-assembled InAs quantum dots is studied by high-resolution x-ray diffractometry and x-ray reflectometry. It is shown that the quantum dots are almost pyramidal in shape. It is found that (i) a first InAs layer, 2.7 ML thick, is invariably produced with a faceted surface and (ii) vertical coupling of quantum dots and superlattice formation do occur (with three quantum-dot layers). It is demonstrated that combining the two methods provides researchers with more reliable structural data.

Manipulation of the structural and optical properties of InAs quantum dots by using various InGaAs structures

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-level spacing was largely related to the change in the QD shape and size, especially the aspect ratio (height/width), which was confirmed by cross-sectional TEM images.

Magnetic-field-induced recovery of resonant tunneling into a disordered quantum well subband

We investigate how the disorder produced by a layer of self-assembled InAs quantum dots affects the electronic states of a GaAs quantum well incorporated in a double barrier resonant tunneling diode. The disorder strongly suppresses the resonant peak in the current due to electron tunneling into the lowest energy subband of the quantum well. However, the peak is recovered by the application of an in-plane magnetic field, which provides a means of tuning the in-plane momentum of the tunneling electrons. Analysis of these data and of Landau level formation when the field is applied perpendicular to the quantum well plane provides information about the length-scale of disorder potential in the well.

Nonlinear charging effect of quantum dots in ap−i−ndiode

The current through a $p\ensuremath{-}i\ensuremath{-}n$ diode containing a layer of self-assembled InAs quantum dots in the intrinsic region is investigated. A series of peaks is observed in the differential conductance below the flat band regime which we attribute to electron tunneling in the quantum dot and wetting layer states, combining the carrier recombination and the carrier relaxation effects. This phenomenon is investigated numerically on the basis of a master equation model. Criteria for the observability of the charging of individual quantum dots are discussed. A key point is the presence of Coulomb screening as otherwise the long-range interdot interactions smear out the energy level spectrum.

Effect of thin GaAs interface layer on InAs quantum dots grown on InGaAs/InP using metalorganic vapor phase epitaxy

InAs self-assembled quantum dots (QDs) embedded within an InGaAs quantum well have been grown on InP substrate by low-pressure metalorganic vapor phase epitaxy. We find out that the underlying InGaAs layer could affect the dot growth dramatically in terms of size distribution and luminescence efficiency. After inserting a thin GaAs interface layer between the underlying InGaAs and the InAs QD layer, improved dot size uniformity and strong room temperature photoluminescence (PL) up to 2 μm were observed. In the case of InAs QDs with no GaAs layer, one has to reduce InAs QD layer thickness in order to obtain room temperature PL. These results suggest that InAs from the underlying InGaAs layer contribute to the InAs QD formation, and cause the InAs QDs to be non-uniform, but a thin GaAs interface layer could effectively block the migration of In atoms from the InGaAs layer toward InAs QDs, and therefore lead to more uniform QD formation with better luminescence efficiency.

Shot noise in tunneling through a quantum dot array

The shot noise suppression in a sample containing a layer of self-assembled InAs quantum dots has been investigated experimentally and theoretically. The observation of a non-monotonic dependence of the Fano factor on the bias voltage in a regime where only few quantum dot ground states contribute to the tunneling current is analyzed by a master equation model. Under the assumption of tunneling through states without Coulomb interaction this behaviour can be qualitatively reproduced by an analytical expression.

Tunable InAs quantum-dot lasers grown on (100) InP

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP were grown on (100) InP substrate to form a laser diode. Two methods were used to achieve tuning: changing the laser cavity length or varying the temperature. Stimulated emission between ∼1.51 and ∼1.64 μm was observed depending on the cavity length and the QD barrier height. The threshold current density was decreased for longer cavities to a value as low as 49 A/cm2 at 77 K. The relation between temperature and lasing peak wavelength was measured to be ∼0.21 nm/K leading to room temperature lasing at ∼1.61 μm.

Aluminium incorporation for growth optimization of 1.3 μm emission InAs/GaAs quantum dots by molecular beam epitaxy

We have systematically studied the effects of incorporation of Al in the embedding layers of self-organized InAs quantum dots (QDs) grown on GaAs(0 0 1) by molecular-beam epitaxy on their density and structural and optical properties. When using an In 0.1Al x Ga 0.9− x As lower embedding layer ( x=0.001–0.1) we observe a large increase in QD density with increasing x. The increase is much larger than when increasing the In content in a conventional In x Ga 1− x As ( x=0–0.2) lower embedding layer. We attribute the differences to different nucleation mechanisms. Furthermore, introducing a thin InAlAs upper embedding layer results in strong 1.3 μm emission with a large energy separation between the ground and the first excited state transitions.

Microanalysis of Self-assembled InAs Quantum Dot Structures Grown for Infrared Detector Applications

ABSTRACTIn an effort to develop materials that are sensitive to mid and far infrared radiation, we examine InAs quantum dot/GaAs matrix multilayer structures grown by molecular beam epitaxy (MBE). Customized electrical and optical properties result from nanoscale-level manipulation of the dots' physical dimensions. The MBE growth temperature can be set to yield dots having the desired lateral dimension; however this leads to dots of insufficient vertical height. It is therefore necessary to grow the dots in a manner that allows independent control of the lateral and vertical dimensions. In this experiment, the vertical dimension is controlled by growing the dots in a multilayer structure with GaAs matrix layers. An initial layer of InAs quantum dots was grown on top of GaAs, followed by a few seconds short growth of GaAs, and then followed by the growth of another layer of InAs dots. The GaAs laterally surrounds, but does not bury, the InAs quantum dots. When the second layer of InAs dots is grown, they tend to self-organize directly on top of the exposed first layer of dots. We then grew a third layer of dots in the same manner. This effectively results in a pseudo-single layer of dots of the desired height which is then completely buried in GaAs. The goal is to develop structures that can be integrated into high operating temperature quantum dot infrared detectors (QDIPs) that have maximum sensitivity, robustness, and portability.

Hydrogen passivation of self assembled InAs quantum dots

A systematic study on the hydrogen passivation of nonradiative centers in InAs quantum dot’s grown on GaAs substrates is presented. The samples used in this study were grown by molecular beam epitaxy. The structures contain an InxGa1−xAs insertion layer between the InAs quantum dots layer and the GaAs cap layer. The thickness and In concentration of the InxGa1−xAs are varied to achieve the emission wavelength at 1.3 μm. The samples after the H2 plasma treatment show a significant increase of the photoluminescence intensity. The experimental results show that the quality of the InAs quantum dot structures does not degrade after the hydrogen (H2) plasma treatments. The enhancement of the photoluminescence intensity from the InAs quantum dots is thought to be due to the passivation of nonradiative centers like defects in the structures. High resolution x-ray diffraction rocking curves are used to correlate photoluminescence results.

Photoluminescence study of InAs quantum dots embedded in GaNAs strain compensating layer grown by metalorganic-molecular-beam epitaxy

Self-assembled InAs quantum dots (QDs) embedded in GaN0.007As0.993 strain compensating layers have been grown by metalorganic-molecular-beam epitaxy on a GaAs (001) substrate with a high density of 1×1011 cm−2. The photoluminescence properties have been studied for two periods of InAs quantum dots layers embedded in GaN0.007As0.993 strain compensating layers. Four well-resolved excited-state peaks in the photoluminescence spectra have been observed from these highly packed InAs QDs embedded in the GaN0.007As0.993 strain compensating layers. This indicates that the InAs QDs are uniformly formed and that the excited states in QDs due to the quantum confinement effect are well defined. This is explained by tensile strain in GaNAs layers instead of the usual GaAs layers to relieve the compressive strain formed in InAs QDs to keep the total strain of the system at a minimum.