Atomic Layer Molecular Beam Epitaxy Research Articles

Superluminescent diodes using quantum dots superlattice

We report a broadband superluminescence diode using InGaAs/GaAs self-assembled quantum dots structure grown by atomic layer molecular beam epitaxy. This two-section SLD consists of weakly guided-rib-waveguide gain section (4 μm wide) butt-connected to a broad-area photon absorber (50 μm wide) to minimize optical feedback and to suppress lasing action. The fabricated device produces a low ripple spectrum (<0.3 dB) with a spectral bandwidth of 135 nm at a peak wavelength of 1210 nm under continuous wave operation (20 °C) at 105 mA current injection.

Lanthanum aluminate by atomic layer deposition and molecular beam epitaxy

A comparison of physical and electrical characteristics of lanthanum aluminate (LAO) dielectrics formed by atomic layer deposition (ALD) and molecular beam epitaxy (MBE) is investigated in this study. Physical characteristics of LAO deposited with these two deposition methods are compared using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). Capacitors with TaN and TaSiN gate electrodes were fabricated to assess electrical properties of these LAO films. ALD LAO was deposited using La[N(SiMe3)2]3,Al(CH3)3 and water at 225–275 °C. ALD LAO is stable against Pt and TiN∕W metal gates up to 800 °C. After annealing at 900 °C, interactions between metal gates and dielectrics are observed resulting in nonfunctional devices. MBE LAO film was deposited at room temperature or 200–400 °C in two ways: Using single element targets (La, Al) or a compound target. The LAO∕TaN stack deposited with single element targets showed significant Si up diffusion from the substrate to the dielectric and the metal gate electrode. In addition, nitrogen diffusion from the metal gate into the dielectric was detected. The LAO∕TaN stack deposited with a compound target showed improved thermal stability. No metal out-diffusion and only very slight Si out diffusion was detected after a 900 °C anneal. LAO capacitors show well-behaved capacitance-voltage and leakage current density-voltage characteristics. These results indicate that the method by which lanthanum aluminate films are deposited strongly influenced their thermal stability. Of all the films examined, the most stable films are deposited by MBE using a compound target.

Structure and thermal stability of InAs/GaAs quantum dots grown by atomic layer epitaxy and molecular beam epitaxy

The structure and thermal stability of self-assembled InAs/GaAs quantum dots (QDs) grown by atomic layer epitaxy (ALE) and molecular beam epitaxy (MBE) QDs were studied using high-resolution electron microscopy with in situ heating experiment capabilities. The ALE QDs were larger and more regular shaped than MBE QDs because of the higher diffusivity of In compared with that of InAs in MBE growth. The QDs were found to form a lens-shaped structure with side facets in the early stage of growth. Upon capping with a GaAs layer, however, the QD apexes flattened because of the diffusion of In and As from the QDs. The structural behavior of QDs at elevated temperatures was observed directly on the atomic scale by in situ heating experiments within TEM. The in situ high-resolution electron microscopy revealed that the uncapped ALE and MBE QDs remained stable up to 580 °C. However, above 600 °C, the QDs collapsed due to the diffusion and evaporation of In and As from the QDs.

Low-frequency noise characteristics of InGaAs quantum-dot infrared photodetector structures grown by atomic layer molecular-beam epitaxy

Optical and electrical characteristics of n–i–n InGaAs/GaAs quantum-dot (QD) infrared photodetectors are reported. In particular, the low-frequency excess electrical noise is measured at room temperature and analyzed in conjunction with the optical properties of the structure. The three stackings of QD were formed by atomic layer molecular-beam epitaxy and highly Si-doped, and AlGaAs current-blocking layer was also included to reduce the dark current. The power-dependent photoluminescence (PL) spectra at 300 K indicates that there are at least three confined states in the QD. The photo-current was observed only at low temperatures (10 K) at wavelengths between 3 and 9 μm with three peaks. The dark current was relatively large and asymmetric at low temperatures. At room temperature the dark current was symmetric and ohmic. The 1/ f-like low-frequency noise spectral density exhibited an almost quadratic current dependence giving a large value of the Hooge parameter of the order of unity. The relatively low-growth temperature for the AlGaAs current blocking layer and the high doping at the quantum dots seem to generate a considerable amount of defects and result in low-temperature photodetection and a large low-frequency noise density.

Effect of deposition period on structural and optical properties of InGaAs/GaAs quantum dots formed by InAs/GaAs short-period superlattices

Structural and optical properties of In 0.5Ga 0.5As/GaAs quantum dots (QDs) grown at 510 °C by atomic layer molecular beam epitaxy technique are studied as a function of n repeated deposition of 1-ML-thick InAs and 1-ML-thick GaAs. Cross-sectional images reveal that the QDs are formed by single large QDs rather than closely stacked InAs QDs and their shape is trapezoidal. In the image, existence of wetting layers is not clear. In 300 K-photoluminescence (PL) spectra of InGaAs QDs ( n = 5 ) , 4 peaks are resolved. Origin of each peak transition is discussed. Finally, it was found that the PL linewidths of atomic layer epitaxy (ALE) QDs were weakly sensitive to cryostat temperatures (16–300 K). This is attributed to the nature of ALE QDs; higher uniformity and weaker wetting effect compared to SK QDs.

Raman scattering from InGaAs/GaAs quantum dot structures grown by atomic layer molecular beam epitaxy

We report Raman scattering studies of optical phonons in InGaAs/GaAs quantum dot (QD) structures grown by atomic layer molecular beam epitaxy to explore formation of QDs and relaxation of strain. The QDs were grown by alternate supply of InAs and GaAs with deposition periods of n=3, 5, and 7. Raman scattering reveals longitudinal optical (LO) and transverse optical phonon responses related to the sample structure. Importantly, a Raman response at ∼237 cm −1 was observed. This Raman response is attributed to the InAs-like LO phonons of InGaAs QDs, indicating clear evidence of the formation of the QDs. Moreover, both the GaAs and the GaAs-like LO phonon energies are shifted downward with increasing n from 3 to 7, suggesting that the strain relaxation occurs and the QDs grow in size.

Evaluation of atomic exchange in GaAs∕GaP interfaces by tetragonal distortion measurements

The quantification of interdiffusion processes in the interfaces of GaAs quantum wells, with thickness ranging from one to six atomic layers, grown by atomic layer molecular beam epitaxy in a GaP matrix is addressed. The atomic row positions have been determined from high resolution transmission electron microscopy images. The distortion of distances and angles between neighboring rows with respect to those expected in the strained structure has been related to changes in chemical composition. This method appears more efficient for this system than other methods based on the analysis of the atomic row contrast. Results show that interdiffusion occurred at the GaAs∕GaP interfaces. The arsenic profiles in the quantum well suggest that the main contribution of interdiffusion comes from atomic exchange. The experimental profiles have been fitted to a simple model of segregation giving segregation coefficients around 0.45. The results are compared to simulations based on Raman measurements performed on the same samples.

Thermal annealing effect on InAs/InGaAs quantum dots grown by atomic layer molecular beam epitaxy

The effect of rapid thermal annealing on the InAs quantum dots (QDs) grown by atomic layer molecular beam epitaxy and capped with InGaAs layer has been investigated using transmission electron microscopy and photoluminescence (PL). Different from the previously reported results, no obvious blueshift of the PL emission of QDs is observed until the annealing temperature increases up to 800°C. The size and shape of the QDs annealed at 750°C have hardly changed indicating the relatively weak Ga/In interdiffusion, which is characterized by little blueshift of the PL peak of QDs. The QD size increases largely and a few large clusters can be observed after 800°C RTA, implying the fast interdiffusion and the formation of InGaAs QDs. These results indicate that the delay of the blueshift of the PL peak of QDs is correlated with the abnormal interdiffusion process, which can be explained by two possible reasons: the reduction of excess-As-induced defects and the redistribution of In, Ga atoms around the InAs QDs resulted from the sub-monolayer deposition of InGaAs capping layer.

Hydrogenation of strain engineered InAs/In x Ga 1− x As quantum dots

InxGa1−xAs/InAs/InxGa1−xAs structures have been grown by atomic layer molecular beam epitaxy on top of a GaAs buffer and substrate. In these structures, the thickness d and/or the In composition x of the lower InxGa1−xAs confining layer control the strain in self-assembled quantum dots. This strain engineering has allowed achieving emission energies as low as 1.5 μm at low T, with a rapid quenching of the photoluminescence (PL) signal at high T. Hydrogen irradiation of these structures leads to an increase in the PL efficiency, higher in samples with higher x, with a blue shift in the peak energy. A higher concentration of non radiative defects in confining layers richer in indium is responsible for the observed PL quenching, more than an increased thermal escape of carriers toward the InxGa1−xAs barriers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral composition modulation in strain compensated (GaInP) m(GaInAs) m short-period superlattices grown on (0 0 1) InP by atomic layer molecular beam epitaxy

The lateral composition modulation of strain compensated (GaInP) m (GaInAs) m short-period superlattices (SPSL) has been investigated. Transmission electron microscopy results show that long-range lateral composition modulation is formed in m=2 SPSL grown at 400°C by atomic layer molecular beam epitaxy. The evolution of photoluminescence (PL) peak with temperature shows a blue shift with increasing temperature; in addition, low-temperature PL is strongly polarized. These properties are attributed to the lateral composition modulation. Quantum wires (QWRs) were formed using (GaInP) m (GaInAs) m SPSL as quantum wells in a multiquantum well heterostructure. The formation of QWR is confirmed by their PL properties.

Tuning the wetting layer in the InGaAs/AlGaAs quantum dots

We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled In 0.5Ga 0.5As/Al x Ga 1− x As quantum dot (QD) structures, grown by atomic layer molecular beam epitaxy. A blue shift of the energy transition of both the QD and the wetting layer (WL) is found when increasing the Al content in the barrier. Nevertheless, the energy separation between 0D and 2D levels increases with x and, therefore, the coupling between the fundamental QD state and the WL electronic level can be tuned by increasing the barrier band gap. Since the WL state acts as the intermediate level on which escape and thermalization occur, we found that the larger separation between QD and WL states reduces the QD thermal quenching and enhances the QD radiative efficiency at high temperature.

Investigation of defects created by growth of InAs quantum dots in GaAs

Defects created by self-organized InAs quantum dot growth in a GaAs matrix were investigated. Growth of the quantum dots was found to enhance strongly the tendency towards inhomogeneous growth. The inhomogeneity seems to stem from point defect clusters generated in the vicinity of a molecular beam epitaxy (MBE)/atomic layer MBE (ALMBE) interface. The point defects identified are characteristic of MBE and ALMBE GaAs layers. The InAs quantum dot states and interface states are not clearly detected since they are located in a depleted layer surrounding the point defects clusters.

Effects of Growth Sequence on Optical and Structural Properties of InAs/GaAs Quantum Dots Grown by Atomic Layer Molecular Beam Epitaxy

ABSTRACTThe influence of growth sequence on optical and structural properties of InAs/GaAs quantum dots (QD) grown by atomic layer epitaxy was investigated systematically. It is found that growth interruption (GI) after In is more effective than non-GI after In in reducing the density of coalescent dots, and reducing the dot width distribution of the QDs. Meanwhile, dot densities are approximately doubled by non-GI after In. GI after As reduces dot height distribution compared with non-GI after As. Generally, GI after In plays a more critical role than GI after As does in formation of the QDs. The sample grown with In/GI/As/GI sequence shows the lowest 300 K-photoluminescence (PL) linewidth (∼30 meV), high PL peak separation between ground and 1st excited level (∼80 meV). From the result, it is known that In/GI/As/GI is the favorable growth sequence among the sample sets. Temperature dependence of PL linewidth shows that the In/GI/As/GI sample is insensitive to cryostat temperature and it is attributed to weak wetting effect. Thinner wetting layers shown in a cross-sectional TEM image supports this.

Electronic Coupling Effects on the Optical Properties and Carrier Dynamics of InAs Quantum Dots

Vertically aligned InAs/GaAs quantum dot structures were investigated. They were grown by atomic layer molecular beam epitaxy, with 10 layers and wedged spacer thickness d varying between 7.7 and 12.5 nm at steps of 0.2 nm. The effects of electronic coupling on the fundamental transition energy and decay dynamics were studied by time-resolved and continuous-wave photoluminescence (PL). The energy of the PL peak position decreases with increasing d, while the corresponding lifetime increases with d. This is attributed to the interplay of the electronic delocalization along the column and the Coulomb interaction between electrons and holes. Furthermore, a bell-shaped behaviour of the PL lifetime as a function of emission energy was observed inside the inhomogeneously broadened PL band.

Optical properties of InAs/Al yGa 1− yAs/GaAs quantum dot structures

We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled InAs/Al y Ga 1− y As/GaAs quantum dot (QD) structures, grown by Atomic Layer Molecular Beam Epitaxy. We found a blue shift of the fundamental QD energy transition when increasing the Al content in the barrier. The comparison of the experimental data with previous findings and with a simple effective mass model suggests that the emission blue shift cannot be completely attributed to the increase of the confining barriers band gap. At the same time we show that the use of ALMBE allows a better control of the QD size distribution with respect to standard MBE growth. The increase of the barrier height enhances the QD radiative efficiency at high temperature, allowing to observe the QD emission up to T=430 K. Important pieces of information on the thermal activation of non radiative channels are obtained by comparing the QD PL temperature dependence with non-resonant and resonant excitation of the QD levels.

A growth method to obtain flat and relaxed In 0.2Ga 0.8As on GaAs (0 0 1) developed through in situ monitoring of surface topography and stress evolution

In this paper we develop a growth process for obtaining flat and relaxed In 0.2Ga 0.8As layers on GaAs (0 0 1). The process designed is based on the results obtained by in situ and real time characterization of surface morphology and layer relaxation. In particular, our results show that for growth temperatures T s⩽200°C the relaxation of In 0.2Ga 0.8As layers is inhibited and the morphology does not evolve to a crosshatched pattern. After growth thermal treatments of these low-temperature (LT) In 0.2Ga 0.8As layers induce the development of a very faint (rms=0.5 nm) crosshatched-like morphology. The relaxation process during the thermal annealing is strongly asymmetric and the layers present a high final strain state. By growing on top of the LT layer another In 0.2Ga 0.8As layer at higher temperature, relaxation is increased up to R≈70% and becomes symmetric. Depending on the growth process of the top layers morphology evolution differs, resulting in better morphologies for top layers grown by atomic layer molecular beam epitaxy (ALMBE) at T s=400°C. We have obtained 400 nm In 0.2Ga 0.8As layers with a final degree of relaxation R=70% and very flat surfaces (rms=0.9 nm).

Atomic equilibrium concentrations in (InGa)As quantum dots

In InxGa1−xAs quantum dots (QDs), the plot of peak emission energies versus the total amount of indium exhibits a well defined pattern, which is independent of the indium concentration. Moreover, photoluminescence spectra of InAs QDs grown by atomic layer molecular beam epitaxy (ALMBE) roughly coincide with those of In0.5Ga0.5As QDs grown by metalorganic vapor phase deposition. We suggest that the total amount of In rather than the nominal In concentration determines the emission energy of these two sets of QDs, and that In interdiffusion is rather strong in ALMBE growth.

Characterization of SnTe-doped InP grown by solid-source atomic layer molecular beam epitaxy

SnTe-doped InP layers were grown at low temperature by solid-source atomic layer molecular beam epitaxy. The samples were characterized by Hall measurements versus temperature, low temperature photoluminescence, x-ray diffraction, and secondary ion mass spectroscopy. The temperature of the SnTe effusion cell was varied from 320 to 440 °C, and the free electron concentration measured at room temperature ranged between 2.0×1016 cm−3 and 5.6×1018 cm−3 with the corresponding Hall mobility varying from 2320 to 1042 cm2/V s.

Study of GaAs spacer layers in InAs/GaAs vertically aligned quantum dot structures

We investigated vertically aligned InAs/GaAs QD structures, grown by atomic layer molecular beam epitaxy, with a number N of layers and with spacer thicknesses d. QD alignment and structure quality were checked by transmission electron microscopy. The dependencies of carrier capture, decay dynamics and existence of quenching channels on the design parameters N and d were studied by time resolved photoluminescence (PL), PL excitation (PLE) and PL temperature-dependent measurements. Our results show that the carrier capture and the radiative efficiency of the QDs are negatively affected by increasing the number of QD layers and by reducing the spacer thicknesses; this effect is likely to be related to the increase of defect concentrations in GaAs spacers, due to relaxation of an increasingly large strain.

Carrier transfer and photoluminescence quenching in InAs/GaAs multilayer quantum dots

A detailed study of the carrier transfer and photoluminescence (PL) quenching in stacked InAs/GaAs quantum dots (QDs) is presented. Vertically aligned QD structures, grown by atomic layer molecular beam epitaxy, with different numbers N of dot planes and different spacer thicknesses (d) were prepared and studied. The dependencies of carrier transfer from the GaAs barriers to the InAs QDs and of the PL quenching channels on the design parameters N and d have been identified by performing continuous wave and time-resolved PL measurements. We have found that both the radiative recombination and capture efficiency into the QDs are reduced by increasing N and by decreasing d, as a consequence of the deterioration of the interdot GaAs spacers induced by stacking.