摘要:

A one step method for the production of quantum dots is presented. The method exploits the mismatch strain of molecular beam epitaxy (MBE) deposited InGaAs on GaAs to induce a transition from the two‐dimensional growth mode to the three‐dimensional (Stranski–Krastanow) growth mode. The cluster size is limited to quantum dimensions by precisely controlling the amount of InGaAs that is deposited in order to cause the growth mode transition. Very narrow lateral size distributions with standard deviations of 14% on the dot area have been obtained. Smooth MBE growth of GaAs over these clusters produces a layer of quantum dots, whose high quality and uniformity has been observed with transmission electron microscopy, atomic force microscopy, and photoluminescence (PL). The quantum dot PL intensity is enhanced compared to a reference quantum well. Resonances in photoluminescence excitation (PLE) spectra suggest that the density of states in these dots has minima close to zero between the quantum states, as expected for a zero‐dimensional system .These PLE peaks shift with the detecting energy, showing that by changing the detecting energy, we are sampling different sizes of dots. The temperature dependence of the PL indicates that the onset energy of thermal quenching of quantum dots is enhanced by a factor of 2, as compared to a quantum well, due to the additional confinement. In all samples, there is virtuallynooverlap between the PL emission and the first PLE peak of the quantum dots.

ISSN:1071-1023    

DOI:10.1116/1.587794

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Quantum wire structures of GaAs/AlGaAs have been grown by metalorganic vapor phase epitaxy in V grooves using pre‐etched corrugated substrates and have been characterized by high‐resolution transmission electron microscopy. Low‐temperature cathodoluminescence (CL) identifies luminescence peaks with the spatial distributions of the different recombinations, achieving a top view spatial resolution of ≊0.2 μm in the CL images. Principally we report how a scanning tunneling microscope (STM) induces local luminescence in the sample structure, and we spectrally resolve STM‐induced luminescence for the tip in different positions relative to the wires. We have recorded the luminescence from a single wire and observed band‐filling effects resulting from varying levels of excitation into a wire. We have demonstrated the difference between recombination of electron‐hole pairs generated in CL and the recombination of injected holes from the STM tip with a thermalized distribution of accumulated electrons in scanning tunneling luminescence.  

ISSN:1071-1023    

DOI:10.1116/1.587795

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Within a valence force field framework, we calculate the critical thickness of a film that is lattice mismatched to the substrate on which it is epitaxially deposited. A capping layer that is lattice matched to the substrate is known to enhance the critical thickness. We calculate the efficiency of the capping layer as a function of its thickness. We also demonstrate that this efficiency can be improved by using a capping layer that is not lattice matched to the substrate. When the capping layer is undesirable, another way of enhancing the critical thickness is to use an oppositely strained intermediate (OSI) layer between the substrate and the film. To maximize efficiency, the OSI layer lattice constant should be larger (respectively, smaller) than that of the substrate if the film is in tension (respectively, compression). We demonstrate that a single OSI layer has no significant effect on the critical thickness. A microscopic description of the strain on all of system is provided, and the mechanism of strain compensation is explained.

ISSN:1071-1023    

DOI:10.1116/1.587796

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

The microscopic surface features were observed during the molecular‐beam epitaxy of InxGa1−xAs on InP (100) substrates by scanning microprobe reflection high‐energy diffraction in real time as a function ofxfrom 0.41 to 0.96. In the case of In0.41Ga0.59As (the lattice mismatch of the strained layer wasf=−0.77%, where the minus sign represents a smaller lattice constant of an epitaxial layer than that of a substrate), three‐dimensional island growth was observed from the start of growth. On the other hand, two‐dimensional layer‐by‐layer growth was maintained during growth of InxGa1−xAs withx=0.45–0.75 (f=−0.47 to +1.48%, where the plus sign represents a larger lattice constant of an epilayer than that of a substrate). While the growth of compressive epilayers withfbetween +0.36% and +1.48% proceeded, a surface crosshatched morphology was observed after the growth of certain film thicknesses, which were dependent on the lattice mismatch. A rough textured morphology was observed instead of a crosshatched morphology for those epilayers with a lattice mismatch greater than +2.29% (In0.87Ga0.13As), where three‐dimensional island growth was favored over the layer‐by‐layer growth mode. Transmission electron microscope observations showed that the presence of a surface crosshatched pattern was correlated with the presence of 60° mixed misfit dislocations. The result indicates that the thickness at which the crosshatched line appears represents the critical layer thickness corresponding to misfit dislocation generation.

ISSN:1071-1023    

DOI:10.1116/1.587797

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Reflectance anisotropy spectroscopy (RAS) was used to study metalorganic vapor phase epitaxial growth of GaAs(001). In previous studies, GaAs(001) surfaces have been identified asc(4×4)‐like under pregrowth arsenic stabilized conditions. During growth the RAS spectra showed significant differences with respect to the pregrowth spectra. Moreover, the RAS signal at a fixed photon energy has been shown to exhibit an oscillatory behavior when growth is initiated. The period of these oscillations has been identified as the time needed for the growth of exactly one monolayer. In this study, the dependence of the oscillations (amplitude and period) on temperature and trimethylgallium (TMGa) partial pressure is investigated and discussed together with the corresponding RAS spectra taken at V/III ratios between 5 and 300. Three different types of RAS spectra are identified during growth. Firstly, at high temperatures and low TMGa partial pressures, the surface anisotropy is nearly the same as in thec(4×4)‐like pregrowth state, with a minimum at 2.5 eV (type I spectra). This indicates that the surface during growth is essentially covered with a double layer of As. Secondly, at low temperatures or high TMGa partial pressures a different structure with a sharp minimum at 2.65 eV is observed (type III spectra). This might be related to a surface largely covered with TMGa fragments. Finally, intermediate between these extreme epitaxial conditions, RAS spectra with a minimum at 2.0 eV and nearly vanishing anisotropy around 2.5 eV are found. This indicates that the As dimers of the pregrowth surface have largely disappeared and a new surface structure, possibly consisting of Ga dimers, is now present. Oscillations have their maximum amplitude at the boundary between type II and type III spectra in apTMGa‐Tdiagram. At this boundary the growth rate becomes nonlinear and starts to saturate with increasing pressure of TMGa.

ISSN:1071-1023    

DOI:10.1116/1.587798

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

We have used scanning tunneling microscopy (STM) to study the initial stages of the growth of ZnSe by molecular‐beam epitaxy on the GaAs(001):Se‐(2×1) surface. The GaAs(001):Se‐(2×1) surface is formed by depositing Se onto the GaAs(001):Se‐(2×4) surface and annealing at above 520 °C. It is a highly ordered (2×1) array of Se dimers, and so is structurally similar to the Se‐terminated ZnSe(001)‐(2×1) surface, and might be expected to provide a good starting surface for two‐dimensional ZnSe growth. However, we find that ZnSe grows by the formation of three‐dimensional islands on the GaAs(001):Se‐(2×1) surface. Islands grow several layers high while much of the Se‐terminated GaAs surface remains uncovered. We have compared our STM images to a simple statistical model of growth and conclude that the sticking coefficient of ZnSe on the GaAs(001):Se‐(2×1) surface is about 1/5 of that on a continuous ZnSe film, whereas the sticking coefficient on top of the small ZnSe islands is close to that on a continuous ZnSe film. We can understand the low reactivity of the Se dimers on GaAs compared to Se dimers on ZnSe by considering electron counting. On the ZnSe surface, Se dimers are neutral, whereas on the GaAs surface Se dimers have a net positive charge which results in a lowering of the dimer energy, producing a relatively inert surface. We conclude that in order to grow structurally high quality ZnSe films on GaAs, which requires two‐dimensional growth, formation of a Se‐(2×1) structure on the GaAs(001) surface must be avoided.

ISSN:1071-1023    

DOI:10.1116/1.587799

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Reflectance difference spectroscopy (RDS) is applied to the study of optical anisotropy in spontaneously ordered GaInP grown by metalorganic chemical‐vapor deposition. The degree of ordering in GaInP has been associated previously with a shift of the band‐gap energy ΔE0, and a crystal‐field valence‐band splitting ΔC. Theoretically and experimentally, both quantities are approximately proportional to the square of the order parameter, which varies from 0 to 1 for disordered and perfectly ordered GaInP, respectively. In this study, we examined a number of GaInP layers grown under conditions that yield a wide range of band‐gap energies. The main spectral feature in all samples is a bulk‐related, asymmetric peak atE0with a long tail that extends well belowE0and a sharp, high‐energy cutoff atE0+ΔC. The intensity of this peak is proportional to √ΔE0and is therefore linear with the order parameter. By annealing GaInP in PH3/H2mixtures, we find that the RD spectral features for energies betweenE0+ΔCand 3 eV are mainly surface induced. Evidence for a bulk‐related RDS peak atE1is also found.

ISSN:1071-1023    

DOI:10.1116/1.587800

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Ga0.51In0.49P layers were grown by organometallic vapor phase epitaxy on grooved GaAs(001) substrates misoriented by 9°. The formation of adjacent ordered and disordered domains has been observed on such substrates. The combination of both Raman and cathodoluminescence measurements allows for an independent determination of composition by analyzing the frequency positions of the optical phonon modes (with an accuracy of 1%) and of the band gap from the luminescence at the same position on the sample. Furthermore, ordered and disordered domains can be clearly identified from their different polarization selection rules observed in the Raman spectra. The cathodoluminescence spectra show shifts of the luminescence maxima of up to 100 meV for ordered and disordered domains of the same composition.

ISSN:1071-1023    

DOI:10.1116/1.587801

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

GaAs/InAs/GaAs heterostructures were grown by molecular beam epitaxy on vicinal GaAs (001) substrates. Effects of substrate misorientation on the early stage of InAs epitaxy, as well as the interaction between InAs and a GaAs overlayer, were studied by transmission and scanning electron microscopies and by photoluminescence measurements. The formation of InAs islands were observed after a few monolayer InAs deposition. Two major results were obtained in this study: (a) Upon deposition of a crystalline GaAs overlayer, InAs islands undergo a novel type of morphological transition, i.e., from disk‐shaped to ring‐shaped ones. (b) Substrate misorientation results in anisotropic effects on InAs island formation. In comparison with on‐axis or [110] tilted samples, substrate misorientation toward [11̄0]by up to 5° leads not only to reduction in InAs island density by a factor of 2, but also to the formation of InAs quantum dots. These results were found to be consistent with photoluminescence experiments.

ISSN:1071-1023    

DOI:10.1116/1.587802

出版商:American Vacuum Society    

年代:1994

数据来源: AIP

摘要:

Results of a systematic examination of InAs island formation on GaAs(100) as a function of deposition conditions and thickness are presented. A non‐Arrhenius dependence of the island density on substrate temperature and a decrease in the island density with increasing As4pressure at lower substrate temperatures is observed, indicating that currently popular frameworks of compound semiconductor molecular‐beam epitaxical growth and island formation mechanism(s) need to be enlarged. Plan‐view transmission electron microscopy (TEM) and use of the behavior of Moiré fringes provides the island size distribution and demarcation between coherent and incoherent islands. Photoluminescence (PL) behavior is shown to vary significantly with the growth conditions and to correlate to the attendant structural nature revealed by TEM. The issue of lateral quantum confinement (i.e., three‐dimensional islands as quantum boxes) is shown to be subtle and complex, calling for caution in interpreting PL behavior. The results suggest that a regular array of ‘‘quantum boxes’’ made of coherently strained volumes of uniform size may be achieved via growth on prepatterned mesas.  

ISSN:1071-1023    

DOI:10.1116/1.587803

出版商:American Vacuum Society    

年代:1994

数据来源: AIP