GaAs Spacer Layer Research Articles

High-performance 1.3 µm InAs/GaAs quantum-dot lasers with low threshold current and negative characteristic temperature

A high-growth-temperature GaAs spacer layer (HGTSL) is shown to significantly improve the performance of 1.3 µm multilayer InAs/GaAs quantum-dot (QD) lasers. The HGTSL inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved performance of QD lasers. To further reduce the threshold current density and improve the room-temperature characteristic temperature (T0), the high-reflection (HR) coating and p-type modulation doping have been incorporated with the HGTSL technique. A very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A cm−2 are achieved for a three-layer device with a 1 mm HR/HR cavity, while a very low threshold current density of 48 A cm−2 and a negative T0 are achieved in the p-doped lasers.

Observation and Modeling of a Room-Temperature Negative Characteristic Temperature 1.3-$\mu$m p-Type Modulation-Doped Quantum-Dot Laser

A room-temperature negative characteristic temperature (T0 ) and ultralow threshold current density (Jth) of 48 Amiddotcm-2 are demonstrated for a 1.3-mum InAs quantum dot laser. These characteristics are obtained by combining a high-growth-temperature GaAs spacer layer with p-type modulation doping of the quantum dots in multiple layer dot-in-a-well structures. Through a comparison of p-doped and undoped devices, a photon coupling mechanism is proposed to account for the different temperature dependences of Jth for the two devices. Numerical simulations based on a rate equation model, which includes photon coupling between ground and excited quantum dot states, are performed. The simulations are able to account for the very different temperature-dependent Jth behavior of the doped and undoped device

Surface photovoltage spectroscopy and photoluminescence study of vertically coupled self-assembled InAs∕GaAs quantum dot structures

Multilayer self-assembled InAs∕GaAs quantum dot (QD) structures with varying GaAs spacer layer (SL) thickness are systematically investigated using surface photovoltage spectroscopy (SPS) and photoluminescence (PL). The optical transitions are more clearly visible in the room-temperature SPS spectra and exhibit more features in comparison to PL. The enhanced electronic coupling effect for thinner SL results in the appearance of an additional excited state lying higher than the second excited quantum dot state. A peculiar feature denoted as QDX below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is lower than that of vertically coupled ones. The transition blueshifted with a decrease of the SL thickness indicates that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain. The intensity ratio between the ground state and QDX in the temperature dependent PL spectra is found to be associated with the processes of enhanced thermal carrier transfer between QDs and tunneling of carriers between QDs in adjacent layers for a stack due to a reduction in SL thickness. The results demonstrate considerable diagnostic value of SPS for the nondestructive characterization of vertically coupled self-assembled QD structures.

Improved surface morphology of stacked 1.3μm InAs∕GaAs quantum dot active regions by introducing annealing processes

The authors report a simple but effective way to improve the surface morphology of stacked 1.3μm InAs∕GaAs quantum dot (QD) active regions grown by metal-organic chemical vapor deposition (MOCVD), in which GaAs middle spacer and top separate confining heterostructure (SCH) layers are deposited at a low temperature of 560°C to suppress postgrowth annealing effect that can blueshift emission wavelength of QDs. By introducing annealing processes just after depositing the GaAs spacer layers, the authors demonstrate that the surface morphology of the top GaAs SCH layer can be dramatically improved. For a model structure of five-layer QDs, the surface roughness with the introduced annealing processes (IAPs) is reduced to about 1.3nm (5×5μm2 area), much less than 4.2nm without the IAPs. Furthermore, photoluminescence measurements show that inserting the annealing steps does not induce any changes in emission wavelength. This dramatic improvement in surface morphology results from the improved GaAs spacer surfaces due to the IAPs. The technique reported here has important implications for realizing stacked 1.3μm InAs∕GaAs QD lasers based on MOCVD.

P -doped 1.3μm InAs∕GaAs quantum-dot laser with a low threshold current density and high differential efficiency

A modification of the thickness of the low-growth-temperature component of the GaAs spacer layers in multilayer 1.3μm InAs∕GaAs quantum-dot (QD) lasers has been used to significantly improve device performance. For a p-doped seven-layer device, a reduction in the thickness of this component from 15to2nm results in a reduced reverse bias leakage current and an increase in the intensity of the spontaneous emission. In addition, a significant reduction of the threshold current density and an increase of the external differential efficiency at room temperature are obtained. These improvements indicate a reduced defect density, most probably a combination of the selective elimination of a very low density of dislocated dots and a smaller number of defects in the thinner low-growth-temperature component of the GaAs spacer layer.

Dependence of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs double quantum dots grown by using the Indium-flush procedure

Potential applications of quantum dots (QDs) have driven extensive efforts to grow high-quality QDs on semiconductor substrates by using various techniques [1]. Optoelectronic quantum devices utilizing QDs can be fabricated because QDs have discrete artificial atomic energy levels [2]. Thus, the microstructural and the optical properties of self-assembled QD systems have been particularly attractive for many years because of the potential application of QDs to optoelectronic devices [3–8]. In particular, the optical properties of QDs have been widely investigated because of interests in both fundamental physical properties and promising applications, such as QD lasers [9, 10], QD infrared photodetectors [11, 12], and QD memory devices [13] operating at lower current and at higher temperature. Among the many quantum dot structures, coupled quantum dot structures consisting of two smaller band-gap wells, which are different in band-gap energy and are separated by a thin embedded barrier, are currently receiving considerable attention for promising applications in electronic and optoelectronic devices [14–18]. Since the microstructural and the optical properties of coupled QDs are very important for optoelectronic devices based on QD structures, studies concerning the physical properties are still necessary in order to fabricate high-efficiency coupled QD devices. Many works on the formation and the physical properties of InAs/GaAs QDs grown by using the Stranski–Krastanov (S–K) growth mode have been performed. Even though some works on the sizes and the shapes of vertically stacked self-assembled QDs grown by using an indium-flush procedure have been performed [19–21], studies concerning the dependence of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs closely coupled double QDs grown by using the indium-flush method have not yet been reported. While the size of the upper QD in a coupled double QD grown by using the S–K mode is larger than that of the lower QD due to a strain field [22], the sizes of the two QDs in the coupled QDs grown by using the S–K mode, together with an indium-flush procedure, are almost the same. Since the top and the bottom dots of a double QD having similar sizes and shapes is important for investigating the coupling effects of double QDs and their promising applications in electronic and optoelectronic devices, the indium-flush method, which is an effective way to control precisely the sizes and the thicknesses of the QDs, should be very useful for investigating the coupling behavior in double QDs [19]. Furthermore, very few works on the dependence of the activation energy of double QDs on the spacer thickness have been done. This letter reports the dependences of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs double QDs with different GaAs spacer layers grown by using molecular beam epitaxy (MBE) with an indium-flush method. Transmission electron microscopy (TEM) measurements were performed to characterize the microstructural properties of the InAs/GaAs double QDs, and photoluminescence (PL) measurements were carried out in order to investigate the dependences of the full width at half maximum (FWHM), the peak position of the interband transitions, and the activation energy on the I. Y. Jung AE Y. M. Park AE Y. J. Park AE J. I. Lee Nano-Device Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Investigation of multilayer electronic vertically coupled InAs∕GaAs quantum dot structures using surface photovoltage spectroscopy

Using room-temperature surface photovoltage spectroscopy, we have characterized several 30-layer stacked self-assembled InAs∕GaAs quantum dot (QD) structures with different spacer layer (SL) thicknesses. Signals from every relevant portion of the samples, including QDs, wetting layer, and GaAs barrier have been observed. The strain-induced field for thinner SL is responsible for a significant modification of the band structure, possibly resulting in the appearance of an additional excited state lying higher than the second excited QD state. A peculiar feature below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is significantly lower than that of vertically coupled ones. The spectra show blueshifted features with a decrease of the SL thickness, indicating that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain.

Improved performance of 1.3-/spl mu/m In(Ga)As quantum-dot lasers by modifying the temperature profile of the GaAs spacer layers

We demonstrate greatly improved threshold current and modal gain performance of 1.3-μm quantum-dot (QD) lasers by growing the GaAs spacer layers in two parts, where subtle modification of the spacer layer growth temperature profoundly influences the QDs. Measurements reveal significantly increased ground state (1.3 μm) modal absorption by increasing the growth temperature of the second part of the spacer layer. Improved modal gain is achieved, enabling continuous-wave lasing from a 1-mm-long uncoated laser, even without p-doping. This approach provides a potentially viable route to superior edge-emitting lasers and GaAs-based 1.3-μm vertical-cavity surface-emitting lasers.

Pinholelike defects in multistack 1.3μm InAs quantum dot laser

The pinholelike defects often observed in multistack InAs quantum dot structures on GaAs have been investigated comprehensively. Due to the high surface stress of InAs quantum dots, the overgrowth of GaAs and InGaAs capping layers on InAs quantum dots is far from conformal and leads to the formation of these defects. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to completely eliminate the pinholelike defects in multistack quantum dot structures. Ridge-waveguide 1.32μm InAs quantum dot lasers prepared by this method exhibit internal quantum efficiency as high as 62%.

Improvement in Linearity of Novel InGaAsN-Based High Electron Mobility Transistors

We have fabricated InGaAsN-based high electron mobility transistors (HEMTs) using InGaAsN as the channel layer. An extremely large gate-voltage swing (GVS) up to 4.2 V can be achieved by utilizing the large conduction band offset between the GaAs spacer layer and the InGaAsN channel layer. However, the poor channel mobility and current density as a result of nitrogen-induced electrically active defects limit the transconductance (gm) performance. Attempts using various annealing temperatures have demonstrated that better device characteristics can be obtained via rapid thermal annealing at 700 °C. In this study, we investigate the effect of nitrogen-induced traps on the basis of Hall measurements and device characterizations of HEMTs. The improvement in GVS in the annealed samples is also discussed. Despite the relatively poor gain, InGaAsN HEMTs with excellent linearity performance after proper thermal annealing are expected to be compatible for novel InGaAsN-based optoelectronics integral circuits (OEICs).

Probing the intermixing in In(Ga)As∕GaAs self-assembled quantum dots by Raman scattering

We show that Raman scattering is a sensitive technique for probing the degree of Ga intermixing in In(Ga)As∕GaAs self-assembled quantum dots (QDs). The shifts of the QD phonon frequency that we observe are explained by the modification of the strain due to Ga incorporation into the QDs from the GaAs matrix during growth. Using an elastic continuum model, we estimate the average In content of the dots from the QD phonon frequency. The varying amount of intermixing in QDs grown with different In compositions, QD layer thicknesses, growth temperatures, and stacking spacer layer thicknesses are investigated. The Raman data indicate that Ga intermixing is larger for QD samples with low In(Ga)As coverage thickness and∕or high growth temperature and, in multilayered systems, for samples with small GaAs spacer layers.

The vertical coupling effect on the electronic states in self-assembled InAs/GaAs quantum dots

A series of self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy to investigate the dependence of transition energy on GaAs spacer layer thickness. The latter was varied of 60, 45, 30, 15, and 10 monolayers (MLs) for the five different samples. The photoluminescence (PL) measurements were carried out. The electronic states in coupled self-assembled InAs/GaAs QDs are investigated through PL properties with the aid of the theoretical calculation. First the energy levels of electrons and holes are calculated by solving the three-dimensional Schrodinger equation by considering the vertical coupling effect between a finite numbers of QDs. Based on the results the energies transitions between electrons and holes levels are calculated. Modification of PL spectra by increasing number of layers was found and attributed to an increasing vertical coupling. The PL full-width at half-maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 MLs. Moreover, the observed behavior PL lines is analyzed theoretically.

Tailoring of high-temperature photoluminescence in InAs∕GaAs bilayer quantum dot structures

Temperature-dependent photoluminescence is investigated in bilayer InAs∕GaAs quantum dot structures with constant InAs deposition θ1 in the seed layer, but variable deposition θ2 in both the second layer and the GaAs spacer layer. It is shown that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperature photoluminescence and strongly influences carrier relaxation channels. We report that radiative recombination and carrier capture efficiency by the quantum dots in the second layer can be tailored using the deposition θ2 and the GaAs spacer thickness.

Control of InAs∕GaAs quantum dot density and alignment using modified buffer layers

We have investigated the patterning effects of GaAs buffers during the growth of InAs∕GaAs quantum dot (QD) superlattices (SLs). One-, five-, and ten-period QD SLs were deposited on GaAs buffer layers grown at 580°C and/or 500°C, with various annealing steps. High-temperature-grown buffers consist of relatively flat surfaces, while low-temperature-grown buffers contain “mound-like” features elongated along the [11¯0] direction. Isotropic distributions of QDs are observed for QD growth on flat buffers. Interestingly, QD alignment along the [11¯0] direction is observed for QD SL growth on buffers containing mounds. This anisotropic QD alignment is enhanced as the number of QD SLs increases and is dependent on the density of mounds. For flat buffers, the density of QDs decreases with stacking, consistent with the model of Tersoff [J. Tersoff, C. Teichert, and M. G. Lagally, Phys. Rev. Lett. 76, 1675 (1996)]. However, for buffers containing mounds, this effect is compensated by an increase in QD density. We propose a mechanism for QD nucleation, based upon buffer layer patterning leading to an undulated In-enriched GaAs spacer layer following the initial layer of QDs.

Control on self-organization of InGaAs/GaAs(100) quantum-dot chains

The spontaneous formation of long chains of quantum dots during the growth of InGaAs/GaAs multiple layers has been reported recently. The effects of In content and spacer on the evolution of dotchains are investigated in the present work. By reducing the In content in the InGaAs layer, the quantum dots in chains are more connected and finally arrays of quantum wires would form. By changing the GaAs spacer layer thickness, the vertical and also lateral spacing between dotchains can be continually tuned. The capability to insert a thick layer of AlGaAs as part of the spacer layer enables us to fabricate InGaAs quantum-dot chains without vertical electronic coupling. The achieved control of self-assembly of organized InGaAs quantum dots may be advantageous for novel optoelectronic applications.

Effects of strain and confinement on the emission wavelength of InAs quantum dots due to aGaAs1−xNxcapping layer

A GaAsN capping layer grown on InAs quantum dots (QDs) induces a strong redshift of the emission wavelength and extends it beyond $1.3\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. We investigated this effect systematically by changing the nitrogen content in the GaAsN layer, varying the thickness of this layer, and embedding a GaAs spacer layer between the GaAsN layer and the QDs. The samples were grown on GaAs(001) substrates by plasma-assisted solid-source molecular beam epitaxy (MBE). Additionally, we simulated the band structure and the electron and hole energy levels based on $6\ifmmode\times\else\texttimes\fi{}6\phantom{\rule{0.3em}{0ex}}\mathbf{k}\mathbf{∙}\mathbf{p}$ calculations, including strain and piezoelectric effects. We found that the wavelength extension is caused by the decrease of the confining energy barrier for the electron wave function in the QDs due to the lower conduction band energy of the GaAsN layer with respect to GaAs. The strain inside the QDs is almost unaffected by the overgrowth with the tensilely strained GaAsN layer. The insertion of a GaAsN layer below the QDs yields only a very small change in wavelength compared to the effect produced by a GaAsN capping layer. This difference is attributed to a reduced QD volume due to the growth on GaAsN that is suggested in cross-sectional scanning tunneling microscopy (XSTM) measurements. The blueshift due to this structural change of the QDs compensates for the redshift that is induced by the decreased confinement.

High-performance three-layer 1.3-/spl mu/m InAs-GaAs quantum-dot lasers with very low continuous-wave room-temperature threshold currents

The combination of high-growth-temperature GaAs spacer layers and high-reflectivity (HR)-coated facets has been utilized to obtain low threshold currents and threshold current densities for 1.3-μm multilayer InAs-GaAs quantum-dot lasers. A very low continuous-wave (CW) room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are achieved for a three-layer device with a 1-mm HR/HR cavity. For a 2-mm cavity, the CW threshold current density is as low as 17 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for an HR/HR device. An output power as high as 100 mW is obtained for a device with HR/cleaved facets.

A study of the low‐energy interference oscillations in photoreflectance of GaAsSb/GaAs quantum well structures

AbstractWe present a room temperature photoreflectance (PR) investigation of GaAsSb/GaAs/AlGaAs quantum well (QW) structures grown on GaAs substrates by molecular beam epitaxy. We encounter strong low energy interference oscillations (LEIO), which tend to obscure any PR signals arising from the QW. By varying the angle of incidence, we exploit the LEIO to estimate the refractive index of the layer responsible and thus identify it. Furthermore, we demonstrate the ability to model the lineshapes of the observed LEIO accurately, through calculations of the Seraphin coefficients. This then allows us to deduce the actual thickness of the layer responsible, which is found to be close to the nominal value. These calculations in turn suggest a way to avoid the LEIO – by shortening the laser excitation wavelength. This successfully reveals good PR signals from the QW region, yielding several transition wavelengths, which we then compare with a theoretical model, assuming a small type II offset between GaAsSb QW and GaAs spacer layers. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Microstructure of (Ga,Mn)As∕GaAs digital ferromagnetic heterostructures

We report on the microstructure of (Ga,Mn)As digital ferromagnetic heterostructures grown on GaAs (001) substrates by low-temperature molecular-beam epitaxy. The Mn concentration and the As4∕Ga beam equivalent pressure (BEP) ratio are varied in the samples containing periods of Mn sheets separated by thin GaAs spacer layers. Transmission electron microscopy studies reveal that decreasing the Mn doping concentration and reducing the BEP ratio lead to smaller composition fluctuations of Mn and more homogeneous (Ga,Mn)As layers with abrupt interfaces. Planar defects are found as the dominant defect in these heterostructures and their density is related to the magnitude of the composition fluctuation. These defects show a noticeable anisotropy in the morphologic distribution parallel to the orthogonal [110] and [11¯0] direction. Along the [11¯0] direction, they are stacking faults, which are preferentially formed in V-shaped pairs and nucleate at the interfaces between (Ga,Mn)As and GaAs layers. Along the [110] direction, the planar defects are isolated thin twin lamellae. The character of the planar defects and their configuration are analyzed in detail.

Diffusion mechanisms of indium and nitrogen during the annealing of InGaAs quantum wells with GaNAs barriers and GaAs spacer layers

In this article, we discuss two indium diffusion mechanisms that are present during the rapid thermal annealing of InxGa1−xAs quantum wells (x=0.18, 0.22, and 0.26) with GaNyAs1−y barriers (y=0.6 or 1.2%). Samples were grown with and without a GaAs spacer layer in between the quantum well and barrier. The dominant mechanism is dependent on the amount of thermal energy applied during the annealing process. At low annealing times and temperatures, we have observed that In-Ga intra-diffusion entirely within the quantum well is dominant. For the higher times and temperatures, In-Ga inter-diffusion between the quantum well and barrier becomes dominant. These observations were confirmed by high-resolution x-ray diffraction and the peak emission wavelengths were measured by room-temperature photoluminescence. We have also observed that nitrogen had diffused from the GaNAs barriers into the InGaAs quantum wells in all of our annealed samples. In addition, the commonly observed indium-content dependent diffusion in GaInNAs-based systems was not observed with InGaAs∕GaNAs-based structures.