Quantum Dot Region Research Articles

Single-Period InAs–GaAs Quantum-Dot Infrared Photodetectors

In this letter, we investigate the performance of single-period InAs-GaAs quantum-dot (QD) infrared photodetectors, in which the single-period QD is sandwiched by different thicknesses of the undoped GaAs confinement layers. By using a 5-nm p-type GaAs layer as a current blocking barrier, the investigated three devices exhibit no response, the highest response, and the medium response, respectively. It is attributed to three different electron occupancy situations in the QD region resulted from the various locations of the Fermi level. A higher barrier for the thermionic emission current is observed for the device with a lower Fermi level in the QD structure. It is attributed to the acceptor-like behavior of the depleted QD such that a higher barrier height would be observed.

Dependence of Linewidth Enhancement Factor on Duty Cycle in InGaAs–GaAs Quantum-Dot Lasers

The linewidth enhancement factor (alpha-factor) of InGaAs-GaAs quantum-dot (QD) lasers is studied as a function of the current pulse duty cycle. In this letter, we demonstrate the impact that current pulse duty cycle has on measured -factor in QD lasers due to increased thermal effects. Below threshold, while comparable quantum-well lasers blue-shift under all conditions, the QD lasers demonstrate an unusual sensitivity of chirp to duty cycle of the current, with a blue-shift at low duty cycles, and a red-shift at high duty cycles and dc current, which may lead to negative -factor. The assumption of Gaussian gain distribution and linear gain shift with temperature is used to analyze the -factor at different duty cycle currents. The result agrees well with the positive, zero, and negative alpha-factors reported for QD lasers. The effect of duty cycle is quite significant and can shift the alpha-factor substantially going from pulse to dc. This is due to the low thermal conductivity of QD regions and increased thermal effects in those devices.

1.55 μ m InAs quantum dots grown on a GaAs substrate using a GaAsSb metamorphic buffer layer

The use of a GaAsSb metamorphic buffer layer (MBL) is demonstrated to significantly enhance the room-temperature photoluminescence intensity for 1.55μm metamorphic InAs∕GaAs quantum dots (QDs) in comparison with a conventional InGaAs MBL. A dramatic reduction of QD photoluminescence emission efficiency above 1.5μm has been observed at room temperature when the indium composition in the InxGa1−xAs MBL is increased over x=0.25. By using a GaAsSb buffer instead of InGaAs, we demonstrate a strong enhancement of photoluminescence intensity of InAs∕GaAs QDs. The effects of the GaAsSb MBL can be understood in terms of smoothing the surface morphology of the buffer layer and, hence, suppressing the formation of dislocations in the QD region. These results suggest an alternative approach to developing GaAs-based light sources in the telecommunication-wavelength range near 1.55μm.

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.

Effects of rapid thermal annealing on the emission properties of highly uniform self-assembled InAs∕GaAs quantum dots emitting at 1.3μm

The authors report the effects of rapid thermal annealing (RTA) on the emission properties of highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3μm grown on GaAs substrate by metal organic chemical vapor deposition. Postgrowth RTA experiments were performed under N2 flow at temperatures ranging from 600to900°C for 30s using GaAs proximity capping. Surprisingly, in spite of the capping, large blueshifts in the emission peak (up to about 380meV at 850°C) were observed (even at low annealing temperatures) along with enhanced integrated photoluminescence (PL) intensities. Moreover, pronounced peak broadenings occurred at low annealing temperatures (<700°C), indicating that RTA does not always cause peak narrowing, as is typically observed with traditional QDs with large inhomogeneous PL linewidths. The mechanism behind the large peak blueshift was studied and found to be attributed to the as-grown QDs with large size, which cause a larger dot-barrier interface and greater strain in and near the QD regions, thereby greatly promoting Ga–In intermixing across the interface during RTA. The results reported here demonstrate that it is possible to significantly shift the emission peak of the QDs by RTA without any additional procedures, even at lower annealing temperatures.

Analysis of the reduced thermal conductivity in InGaAs∕GaAs quantum dot lasers from chirp characteristics

The thermal conductivity of self-organized quantum dot (QD) active regions is estimated by measurements of wavelength chirp with injected current as a function of the current pulse duty cycle both below and above threshold. A simple model which separates out thermal and charge carrier chirps is used to estimate the thermal conductivity of the QD active region. With this model, the thermal conductivity of the InGaAs QD active region is estimated to be ∼0.1W∕mK, about two orders of magnitude less than that of the bulk material. This is consistent with theoretical predictions of the reduced thermal conductivity of QD regions.

Improving crystal quality of InGaAs∕GaAs quantum dots by inductively coupled Ar plasma

The crystal quality of InGaAs∕GaAs quantum dots (QDs) is substantially improved without redistribution of composition using inductively coupled Ar plasma exposure. After plasma exposure, the QDs exhibit an increase in photoluminescence intensity by a factor of 1.7 while keeping the peak wavelength unshifted, and the band gap blueshift after rapid thermal annealing is suppressed, denoting an improvement in thermal stability. The time-resolved photoluminescence shows an increase in carrier lifetime from 735to1140ps by plasma exposure, indicating the mechanism of grown-in defects reduction in the QD regions.

Influence of doping density on the normal incident absorption of quantum-dot infrared photodetectors

The influences of doping densities at the quantum-dot (QD) region for 30-period InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The InAs∕GaAs QDIPs with a lower doping density can operate at high responsivity and high background-limited-performance temperature. Also observed is the decreasing photocurrent ratio of s∕p-polarized lights for the QDIPs with increasing QD doping density. Compared to the similar photocurrent ratio of s∕p-polarized lights for the GaAs∕AlGaAs quantum-well infrared photodetectors at different applied voltages, the observed voltage-dependent response ratio for QDIPs is attributed to the strong scattering characteristics of QDs occupied with photoexcited electrons for electron transport through the QD region.

Nonradiative centers in InAs quantum dots revealed by two-wavelength excited photoluminescence

We studied nonradiative recombination centers in MBE-grown InAs/GaAs quantum dot (QD) structures with photoluminescence (PL) peak energies between 1.12 and 1.29 eV by the scheme of two-wavelength excitation. Temporally chopped below-gap excitation (BGE) light of 0.75 eV was superposed on a CW above-gap excitation light of 1.59 eV on the sample surface, and the resultant PL intensity change due to the BGE (BGE effect) was measured. Observed 25–35% decrease in PL intensity at 80–90 K implies a first discrimination of a pair of nonradiative centers activated at 0.75 eV inside the band gap of the QD region. Due to trap-filling, the BGE effect showed saturation with increasing BGE power. Its temperature dependence below 50 K was different from that of quantum wells, reflecting carrier dynamics peculiar to the QD.

Nonlinear elasticity in III-N compounds:Ab initiocalculations

We have studied the nonlinear elasticity effects in zinc-blende and wurtzite crystallographic phases of III-N compounds. Particularly, we have determined the pressure dependences of elastic constants in InN, GaN, and AlN by performing ab initio calculations in the framework of plane-wave pseudopotential implementation of the density-functional theory. The calculations have been performed employing two exchange-correlation functionals, one within the local density approximation and the other within the generalized gradient approximation. We have found that ${\mathrm{C}}_{11}$, ${\mathrm{C}}_{12}$ in zinc-blende nitrides and ${\mathrm{C}}_{11}$, ${\mathrm{C}}_{12}$, ${\mathrm{C}}_{13}$, ${\mathrm{C}}_{33}$ in wurtzite nitrides depend significantly on hydrostatic pressure. Much weaker dependence on pressure has been observed for ${\mathrm{C}}_{44}$ elastic constant in both zinc-blende and wurtzite phases. Further, we have examined the influence of pressure dependence of elastic constants on the pressure coefficient of light emission, $d{E}_{E}∕dP$, in wurtzite $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ quantum wells. We have shown that the pressure dependence of elastic constants leads to a significant reduction of $d{E}_{E}∕dP$ in nitride quantum wells. Finally, we have considered the influence of nonlinear elasticity of III-N compounds on the properties of hexagonal nitride quantum dots (QDs). For typical wurtzite $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ QDs, we have shown that taking into account pressure dependence of elastic constants results in the decrease of volumetric strain in the QD region by about 7%. Simultaneously, the average $z$ component of the piezoelectric polarization in the QDs increases by $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MV}∕\mathrm{cm}$ compared to the case when linear elastic theory is used. Both effects, i.e., decrease of volumetric strain as well as increase of piezoelectric field, decrease the band-to-band transition energies in the QDs.

Effect of Nitrogen Incorporation into InAs layer in InAs/InGaAs Self-Assembled Quantum Dots

We present the results of nitrogen incorporation into the InAs layer in InAs/InGaAs quantum dots (QDs). We show that this incorporation causes an asymmetric photoluminescence (PL) line shape and abnormally large redshift of the PL peak from the quantum dots as temperature increases. In addition, this incorporation causes a large series resistance and a rapid increase in reverse current at a bias corresponding to the QD region. This effect is due to the formation of a deep trap at 0.34–0.41 eV, which depletes the carriers in the QD region. This depletion gives rise to a geometric resistance-capacitance time-constant effect for the ac conductivity of the free electrons in the top GaAs layer. This trap markedly alters the emission properties of the QD structure. The ac conductivity of the QD structure is governed by the thermal activation of trapped electrons rather than the electron emission from the QD confined states. This incorporation significantly increases the electron emission time from the QD region and, thus, can be used intentionally to modify the emission properties of the QD structure.

Characterization of electron emission from relaxed InAs quantum dots capped with InGaAs

The effect of strain relaxation in a relaxed InAs quantum dot (QD) capped with InGaAs is investigated by admittance and deep-level transient spectroscopy (DLTS). Strain relaxation markedly increases the emission time in the QD region and extends carrier depletion to the bottom GaAs layer. The experimental data show the presence of relaxation traps in the QD region and the neighboring bottom GaAs layer. The electron emission from the QD region is governed by a trap located at 0.17–0.21 eV below the QD ground state. The electron-escape process is identified as thermal activation at high temperatures and direct tunneling at low temperatures from the trap. In the bottom GaAs layer near the QD, DLTS reveals a relaxation trap at 0.37–0.41 eV relative to the GaAs conduction band. The energy difference between these two traps is comparable to the QD ground-state energy relative to the GaAs conduction-band edge, suggesting that the two traps may be the same trap which is pinned to the GaAs conduction band. The considerable difference between their properties may result from different atoms surrounding the trap.

Lateral Averaging Effects on Surface Potential Measurements on InAs Dots Studied by Kelvin Probe Force Microscopy

Surface potential measurements on InAs quantum dots (QDs) were performed using Kelvin probe force microscopy (KFM) based on noncontact-mode atomic force microscopy. By the KFM measurements, the potential distribution well corresponding to the topography was successfully observed, and the observed potential difference between the InAs QD region and the surrounding wetting layer region was nearly proportional to the QD diameter. From two-dimensional simulations, in which we calculated the appropriate tip bias to minimize the electrostatic force working between the tip and the sample, we considered that the real dependence of the potential on the QD size still existed, although lateral averaging effects on the potential measurements should be considered to explain the potential observed by the actual KFM measurements. We also pointed out that a lateral averaging effect made the observed potential contrast indistinct, particularly when the size of the nanostructure was smaller than the tip radius.

Cathodoluminescence studies ofself-organized CdTe/ZnTe quantum dot structure grown by MBE:in-plane and in-depth properties of the system

We report the results of low-temperaturephotoluminescence, room-temperature cathodoluminescence (CL)and scanning CL and electron microscopy of self-organizedCdTe/ZnTe quantum dot (QD) structure. The in-depth profiling CLinvestigations were used to identify the microscopic origin ofthe CL emissions observed at 2.13, 2.0-2.1 and 2.25 eV.In particular, we distinguish between CL emissions originatingfrom the QD region of the structure and from the underlyingbuffer layers. Based on these measurements we assign the2.13 eV CL band to the wetting layer and the 2.0-2.1 eV band to the QDemission. From the study of the in-plane and in-depth CLcharacteristics we demonstrate large in-plane fluctuations ofthe CL intensity and discuss their origin.

Spatially and spectrally resolved imaging of GaAs quantum-dot structures using near-field optical technique

We present experimental result on spatially and spectrally resolved imaging of GaAs quantum dot (QD) structures using a near-field optical microscope. Three photoluminescence (PL) peaks which originate from QD, quantum well (QW), and bulk regions were observed at liquid-He temperature. Carriers were observed to diffuse effectively from the AlGaAs barriers to the QD and QW regions. The region of the intension carrier capture manifests itself by emitting a sharp PL peak, and is estimated to be about 300 nm.