Arrays of Two-Dimensional Islands Formed by Submonolayer Insertions: Growth, Properties, Devices

Strassburg M., Schulz O., Pohl U.W., Bimberg D., Klude M., Hommel D.

II-VI-semiconductor-based, green-light-emitting laser diodes with significantly improved characteristics are reported. Threshold current densities of 42 A/cm/sup 2/ are obtained. Novel contacts also lead to a 25% reduction in the threshold voltage and to an increased device lifetime by a factor of 24 as compared to previous results.

Kovsh A.R., Zhukov A.E., Livshits D.A., Egorov A.Y., Ustinov V.M., Maximov M.V., Musikhin Y.G., Ledentsov N.N., Kop'ev P.S., Alferov Z.I., Bimberg D.

AlGaAs/GaAs diode lasers with the active region based on a dense array of self-organised InAlAs-InAs quantum dots have been fabricated. 3.5 W output power for both facets with a peak conversion efficiency of 45% in a 100 µm-wide stripe with uncoated facets were obtained.

Straßburg M., Schulz O., Pohl U.W., Bimberg D., Itoh S., Nakano K., Ishibashi A.

Implantation-induced disordering is applied to define laterally structured waveguiding in ZnCdSe single quantum well lasers. Owing to the intermixing-induced lateral step of the refractive index, the emission characteristics are significantly improved. A reduction in the threshold current density from 276 to 96 A/cm2 is achieved.

Kato E., Noguchi H., Nagai M., Okuyama H., Kijima S., Ishibashi A.

A II-VI laser diode lifetime of >140 h at 40/spl deg/C, as well as /spl sim/400 h at 20/spl deg/C, has been achieved for a ZnCdSe-ZnSSe-ZnMgSSe separate-confinement heterostructure laser diode under continuous-wave operation with a constant output power of 1 mW. Progress has been made towards increasing the basic reliability of ZnMgSSe-based wide bandgap laser diodes by reducing point defects.

Lott J.A., Ledentsov N.N., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop'ev P.S., Alferov Z.I., Bimberg D.

Ground state lasing is reported for vertical cavity lasers containing three-period InGaAs/GaAs vertically coupled quantum dot active regions. The structures include selectively oxidised AlO current apertures and AlO/GaAs reflectors. Experimental devices emitting near 1.0 µm operate continuous wave at 20°C with threshold currents

Shernyakov Y.M., Bedarev D.A., Kondrat'eva E.Y., Kop'ev P.S., Kovsh A.R., Maleev N.A., Maximov M.V., Mikhrin S.S., Tsatsul'nikov A.F., Ustinov V.M., Volovik B.V., Zhukov A.E., Alferov Z.I., Ledentsov N.N., Bimberg D.

Low threshold current density (J/sub th/=65 A/cm/sup 2/) operation near 1.3 /spl mu/m at room temperature (RT) is realised for lasers using InAs-InGaAs-GaAs quantum dots (QDs). The lasing occurs via the QD ground state for cavity length L>1 mm. The differential efficiency is 40% and internal losses are 1.5 cm. The characteristic temperature near RT is 160 K.

Tsatsul'nikov A.F., Krestnikov I.L., Lundin W.V., Sakharov A.V., Kartashova A.P., Usikov A.S., Alferov Z.I., Ledentsov N.N., Strittmatter A., Hoffmann A., Bimberg D., Soshnikov I.P., Litvinov D., Rosenauer A., Gerthsen D., et. al.

Coherent ultrathin GaAsN insertions are formed in a GaN matrix by predeposition of an ultrathin GaAs layer on a GaN surface, followed by annealing in an NH3 atmosphere and overgrowth with GaN. During the overgrowth, most of the As atoms are substituted by N, with a dense array of coherent GaAsN nanodomains with lateral sizes of about 3-4 nm formed in the GaN matrix. We report a green luminescence due to GaAsN insertions, surviving at high observation temperatures and excitation densities.

Litvinov D., Rosenauer A., Gerthsen D., Ledentsov N.N.

The character of Cd distribution and the morphology of CdSe layers with nominal thicknesses between 0.7 and 3.6 ML in a ZnSe matrix were studied by conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using plan-view and cross-section TEM samples. The Cd distribution was determined on an atomic scale by two different techniques. The first method is based on the measurement of local lattice parameters from zone-axis HRTEM images. The second technique relies on the evaluation of Fourier amplitudes derived from off-axis lattice fringe images. Continuous ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ layers are observed, which are significantly broadened compared to the nominal thicknesses. The layers contain Cd-rich inclusions (small islands) with a size of less than 10 nm, and regions with a lower Cd concentration. With increasing nominal CdSe thickness, the Cd concentration and the island density increase. In addition, larger islands with a density two orders of magnitude below the small island density and a shape asymmetry are found in the 3.6-ML CdSe layer. The results are discussed with respect to the impact of the averaging effect caused by the finite TEM foil thickness on the measured Cd content in the Cd-rich islands and the surrounding region.

Kurtz E., Schmidt M., Baldauf M., Wachter S., Grün M., Litvinov D., Hong S.K., Shen J.X., Yao T., Gerthsen D., Kalt H., Klingshirn C.

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.

Türck V., Rodt S., Stier O., Heitz R., Pohl U.W., Engelhardt R., Bimberg D.

We analyze the line-broadening mechanisms of single CdSe quantum dot (QD) emission lines. A jitter in the emission energy of individual CdSe QDs is reported for the first time. The jitter is caused by the quantum-confined Stark effect induced by the randomly fluctuating charges of defects in the vicinity of the QDs. These random processes lead to a broadening of the emission lines and usually inhibit the determination of a true homogeneous line width. On the other hand, identical jitter allows the unambiguous assignment of groups of emission lines to single QDs. A strong thermal broadening of the QD emission lines is observed. From our observations, parameters of the phase relaxation due to acoustic and LO phonon scattering, which is the main line broadening mechanism, are derived.

Türck V., Rodt S., Stier O., Heitz R., Engelhardt R., Pohl U.W., Bimberg D., Steingrüber R.

The quantum confined Stark effect is observed for quantum dots (QD's) exposed to randomly fluctuating electric fields in epitaxial structures. These fields, attributed to charges localized at defects in the vicinity of the QD's, lead to a jitter in the emission energies of individual QD's. This jitter has typical frequencies of below about 1 Hz and is characteristic for each QD thus providing a unique means to unambiguously identify the emission spectra of single QD's. Up to eight lines are identified for individual QD's and attributed to excitonic, biexcitonic, and LO-phonon-assisted transitions. The intensity of the LO-phonon replica is surprisingly large corresponding to Huang-Rhys factors of about one.

Strassburg M., Deniozou T., Hoffmann A., Heitz R., Pohl U.W., Bimberg D., Litvinov D., Rosenauer A., Gerthsen D., Schwedhelm S., Lischka K., Schikora D.

Schikora D., Schwedhelm S., As D.J., Lischka K., Litvinov D., Rosenauer A., Gerthsen D., Strassburg M., Hoffmann A., Bimberg D.

Yu P., Langbein W., Leosson K., Hvam J.M., Ledentsov N.N., Bimberg D., Ustinov V.M., Egorov A.Y., Zhukov A.E., Tsatsul’nikov A.F., Musikhin Y.G.

We have studied the polarization of surface and edge-emitted photoluminescence (PL) from structures with vertically coupled ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots (QD's) grown by molecular beam epitaxy. The PL polarization is found to be strongly dependent on the number of stacked layers. While single-layer and 3-layer structures show only a weak TE polarization, it is enhanced for 10-layer stacks. The 20-layer stacks additionally show a low-energy side-band of high TE polarization, which is attributed to laterally coupled QD's forming after the growth of many layers by lateral coalescence of QD's in the upper layers. While in the single, 3- and 10-layer stacks, both TE polarized PL components are stronger than the TM component, the [110] TE component is weaker than the TM component in the 20-layer stack. This polarization reversal is attributed to an increasing vertical coupling with increasing layer number due to increasing dot size.

Engelhardt R., Pohl U.W., Bimberg D., Litvinov D., Rosenauer A., Gerthsen D.

Efficient resonant excitonic waveguiding is achieved in laser structures, grown by metallorganic chemical vapor deposition, with stacked CdSe quantum islands which were separated by ternary ZnSSe barriers. Plastic relaxation within the stack is shown to be suppressed by adjusting the sulfur content in the barriers to compensate the strain. Excitonic lasing with low threshold intensities is demonstrated well above room temperature with Ith77 K=0.8 kW/cm2 and Ith300 K=55 kW/cm2.

Argirusis N., Achilleos A., Alizadeh N., Argirusis C., Sourkouni G.

Infrared (IR) sensors are widely used in various applications due to their ability to detect infrared radiation. Currently, infrared detector technology is in its third generation and faces enormous challenges. IR radiation propagation is categorized into distinct transmission windows with the most intriguing aspects of thermal imaging being mid-wave infrared (MWIR) and long-wave infrared (LWIR). Infrared detectors for thermal imaging have many uses in industrial applications, security, search and rescue, surveillance, medical, research, meteorology, climatology, and astronomy. Presently, high-performance infrared imaging technology mostly relies on epitaxially grown structures of the small-bandgap bulk alloy mercury–cadmium–telluride (MCT), indium antimonide (InSb), and GaAs-based quantum well infrared photodetectors (QWIPs), contingent upon the application and wavelength range. Nanostructures and nanomaterials exhibiting appropriate electrical and mechanical properties including two-dimensional materials, graphene, quantum dots (QDs), quantum dot in well (DWELL), and colloidal quantum dot (CQD) will significantly enhance the electronic characteristics of infrared photodetectors, transition metal dichalcogenides, and metal oxides, which are garnering heightened interest. The present manuscript gives an overview of IR sensors, their types, materials commonly used in them, and examples of related applications. Finally, a summary of the manuscript and an outlook on prospects are given.

Kumar R., Maidaniuk Y., de Oliveira F.M., Mazur Y.I., Salamo G.J.

Alzeidan A., Cantalice T.F., Sautter K.E., Vallejo K.D., Simmonds P.J., Quivy A.A.

We studied the impact of surface reconstruction on the performance of infrared photodetectors containing InAs/GaAs submonolayer quantum dots (SMLQDs) grown by molecular beam epitaxy. Adjusting the substrate temperature before InAs deposition allowed us to move between the conventional c(4×4) reconstruction of the GaAs(001) surface, observed at low growth temperatures, and the (2×4) reconstruction which can only be stabilized at higher sample temperatures than those commonly used to deposit InAs. Photodetectors based on SMLQDs grown on such a (2×4) surface reconstruction outperformed those grown on a c(4×4) reconstruction and provided specific detectivities as high as 8.3×1011 cm Hz1/2 W-1 at 12 K. This increase in performance is due to the nucleation of true two-dimensional InAs islands which are the building blocks of SMLQDs and can only form on a (2×4)-reconstructed GaAs(001) surface.

Tomar A., Mourya S.K., Kumar R.

This chapter summarizes the progress of InAs submonolayer (SML) quantum dot (QD) based intermediate band solar cell (IBSC). A brief background of intermediate band solar cells (IBSC) will be presented. Different IBSC prototypes will be discussed. The importance of quantum dots (QDs) for IBSC prototyping will be illustrated. An alternative of the most extensively used Stranski-Krastanow (SK)-QDs named SML QDs will be introduced. The fabrication of SML-QD-based IBSC will be discussed from the material point of view. We will also discuss the physics behind the improved performance of these SCs. Important research in this field will be reviewed. Finally, the future direction will be suggested to further improve the performance.

Borrely T., Alzeidan A., de Lima M.D., Jacobsen G.M., Huang T.-., Yang Y.-., Cantalice T.F., Goldman R.S., Teodoro M.D., Quivy A.A.

The effects of growth conditions on InAs/GaAs submonolayer-quantum-dot solar cells are still a little explored topic, and the literature shows contradictory results regarding the efficiency of these devices. Through electrical and optical characterizations (photoluminescence, current-voltage curves, and external quantum efficiency) and self-consistent Schrödinger–Poisson simulations in the effective-mass approximation, we investigate how the reconstruction of the GaAs(001) surface prior to the deposition of InAs/GaAs submonolayer quantum dots influences the properties of these nanostructures and the performance of solar cells. Current-voltage characteristics and external quantum efficiency curves show that the use of the (2 × 4) surface reconstruction—instead of the commonly used c(4 × 4) surface reconstruction—leads to higher short-circuit current density and improved performance at room temperature. The (2 × 4) surface reconstruction also leads to enhanced photoluminescence intensity at low temperatures compared to the c(4 × 4) surface reconstruction. The simulations—which are based on previous cross-sectional scanning tunneling microscopy data of InAs/GaAs submonolayer quantum dots—indicate that neither type of submonolayer quantum dot can confine electrons, as they are too small and their In content is too low. However, the electron ground state is closer to being confined in the SMLQDs grown with the (2 × 4) surface reconstruction, as such nanostructures are surrounded by a thick InGaAs layer having a lower In content than for the other surface reconstruction. The discussion presented herein elucidates a contradiction between different reports found in the literature regarding the conversion efficiency of InAs/GaAs submonolayer-quantum-dot solar cells and indicates possible ways forward for achieving 3D electron confinement in these devices.

Jmerik V., Nechaev D., Semenov A., Evropeitsev E., Shubina T., Toropov A., Yagovkina M., Alekseev P., Borodin B., Orekhova K., Kozlovsky V., Zverev M., Gamov N., Wang T., Wang X., et. al.

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

Cantalice T.F., Alzeidan A., Jacobsen G.M., Borrely T., Teodoro M.D., Quivy A.A.

We used rapid thermal annealing and low-temperature photoluminescence to compare the optoelectronic properties of InAs/GaAs Stranski-Krastanov and submonolayer quantum dots. After annealing, the former showed large changes in their optical spectra, whereas the latter remained almost insensitive and had optical properties similar to those of an InGaAs quantum well. This contrast was attributed to the presence of In segregation during the submonolayer deposition that led to strong In–Ga intermixing and hindered the formation of the submonolayer quantum dots. The optical results point out that, in such conditions, the strain field in the InAs/GaAs system is too weak to effectively provide vertical alignment of the small two-dimensional islands nucleated in consecutive InAs submonolayers. To further improve the properties of submonolayer quantum dots, segregation should be reduced and an initial (2 × 4) surface reconstruction should be preferred, as it is the only one able to provide true two-dimensional InAs islands. • InAs/GaAs submonolayer quantum dots (SMLQDs) can provide excellent devices. • Indium segregation strongly affects SMLQDs and reduces their Indium content. • The internal strain field is too weak to provide SMLQDs with a full-height. • Segregation can be limited by decreasing the growth temperature of the SMLQDs. • A (2 × 4) surface reconstruction should provide better SMLQDs.

Christian Roca R., Kamiya I.

The 2D to 3D growth transition of stacked submonolayer (SML) InAs/GaAs nanostructures as a function of the number of SML InAs stacks is investigated by atomic force microscopy (AFM) measurements. It is found that critical amount of InAs per cycle decreases as the number of stacks increases. These results are analyzed in context of the balance between the average In content and the total deposited thickness in the SML stack.

Yan Q., Wang S., Guan X., He L., Sun K., Liang B.

Ultrathin InAs layers with different thicknesses, from 0.75 to 1.4 monolayer, are grown in the GaAs matrix by molecular beam epitaxy on GaAs (001) substrates. For sub-monolayer heterostructures, islands or segregations exist during the growth process. Taking advantage of the high spatial resolution of focused electron beams, cathodoluminescence measurements obtain a smaller excitation spot than conventional photoluminescence. Based on the change on the peak position, line width, and intensity, cathodoluminescence spectra indicate that the size, geometry, and roughness develop with the InAs content. Moreover, spatial discontinuities of ultrathin InAs layers are observed on spectrum images and transmission electron microscopy images. This research reveals the correlation between the optical and structural properties of ultrathin InAs layers.

Alzeidan A., Cantalice T.F., Vallejo K.D., Gajjela R.S., Hendriks A.L., Simmonds P.J., Koenraad P.M., Quivy A.A.

The performance of infrared photodetectors based on submonolayer quantum dots was investigated as a function of the arsenic flux. All the devices showed similar figures of merit and a very high specific detectivity above 1 × 10 11 cm Hz 1/2 /W at 12 K, despite the fact that cross-sectional scanning tunneling microscopy images pointed out a strong reduction in the density of such nanostructures with decreasing arsenic flux. This contrast is a consequence of the small size and low In content of the submonolayer quantum dots that lead to a strong delocalization of the electrons wave function and, therefore, reduce the advantage of samples having a very high density of quantum dots. A simple strain model showed that the properties of these nanostructures are limited by the lack of vertical alignment of the small two-dimensional InAs islands resulting from the strong segregation of In atoms. We have proposed some ways to improve the growth of submonolayer quantum dots and believe that, after further optimization, such nanostructures might provide devices with superior performance. • Submonolayer quantum dots can provide excellent infrared photodetectors. • Cross-sectional scanning tunneling microscopy shows strong Indium segregation. • Indium segregation is deleterious to submonolayer quantum dots. • Strain is too low in submonolayer quantum dots to provide stacking of islands. • (2×4) surface reconstruction should provide better submonolayer quantum dots.

Alzeidan A., de Cantalice T.F., Vallejo K.D., Simmonds P.J., Quivy A.A.

Two infrared photodetectors based on submonolayer quantum dots, having a different InAs coverage of 35% and 50%, were grown, processed and tested. The detector with the larger coverage yielded a specific detectivity of 1.13×10 11 cm Hz 1/2 W -1 at 12K, which is among the highest values reported in the literature for that kind of device.

Roca R.C., Kamiya I.

A direct comparison of the structures of 2D and 3D types of capped stacked submonolayer (SML) InAs nanostructures is evaluated by transmission electron microscopy (TEM). Results of the TEM observation of SML samples with three stacks of InAs unambiguously show a stark contrast between the structures of 2D and 3D SML nanostructures, where the 2D SML nanostructures exhibit a planar structure with thickness that is consistent with the deposited stack height, whereas the 3D SML nanostructures exhibit several-nm-high structures that exceed the height of the deposited stack. In addition, structural evolution at the 2D to 3D transition in uncapped SML nanostructures is investigated by atomic force microscopy (AFM). The AFM results clearly reveal that the 2D to 3D transition occurred during the deposition of the third (and last) InAs SML stack in the present samples, where the density of 3D structures increases in orders of magnitude with the deposited amount of InAs on the order of a tenth of a monolayer at the onset. This effectively bridges the gap between the 2D and 3D nanostructures elucidating the abrupt nature of the transition.

Hausen J., Herzog B., Nelde A., Meinecke S., Owschimikow N., Lüdge K.

The influence of optical feedback on semiconductor lasers has been a widely studied field of research due to fundamental interests as well as the optimization of optical data transmission and computing. Recent publications have shown that it is possible to induce a periodic pulsed like output in quantum-dot and quantum-well laser diodes utilizing the locking of the external cavity modes and the relaxation oscillation frequency. We present an in-depth analysis of this effect. We choose submonolayer quantum dots as a gain system, as these provide a relatively strong amplitude-phase coupling, which has proven to be very beneficial for these locking effects to occur. Introducing a new theoretical model we can correctly reproduce the essential features of the gain system and validate them by comparison to our experimental results. From this starting point we can further explore how the staircase behavior of the oscillation frequency with increasing pump current can be influenced by changing various laser parameters. The staircase behavior is induced by a reordering of the Hopf bifurcations giving birth to the regular pulsed-like oscillations.

Kocharovsky V.V., Mishin A.V., Seleznev A.F., Kocharovskaya E.R., Kocharovsky V.V.

Based on numerical simulation of the nonstationary working of a CW superradiant laser with a low-Q combined Fabry-Perot cavity with distributed feedback of waves, we describe a parametric mechanism of self-mode-locking caused by beats of two superradiant modes with a period half that of the cavity round-trip period and supporting the formation of a soliton of the electromagnetic field inside the cavity without using any additional mode-locking technique.

Kocharovskii V.V., Кочаровский В.В., Мишин А.В., Mishin A.V., Селезнев А.Ф., Seleznev A.F., Кочаровская Е.Р., Kocharovskaya E.R., Кочаровский В.В., Kocharovsky V.V.

На основе численного моделирования работы непрерывного нестационарного сверхизлучающего лазера с комбинированным низкодобротным резонатором Фабри-Перо с распределенной обратной связью волн дано описание параметрического механизма самосинхронизации мод, обусловленного биениями двух сверхизлучающих мод с периодом, вдвое меньшим периода обхода резонатора, и обеспечивающего формирование внутрирезонаторного солитона электромагнитного поля без использования какой-либо дополнительной техники синхронизации мод.