Optical and electrical characterizations of ZnSe self-organized quantum dots grown by molecular beam epitaxy

Optical and electrical properties of ZnSe self-organized quantum dots were investigated using photoluminescence, capacitance-voltage, and deep level transient Fourier spectroscopy techniques. The temperature dependence of photoluminescence was employed to clarify the mechanism of photoluminescence thermal quenching processes in ZnSe quantum dots. A theoretic fit on considering a two-step quenching processes well explained the experimental data. The apparent carrier concentration profile obtained from capacitance-voltage measurements exhibits an accumulation peak at the depth of about 100nm below the sample surface, which is in good agreement with the location of the quantum dot layer. The electronic ground state of ZnSe quantum dots is determined to be about 0.11 eV below the conduction band of ZnS, which is similar to that obtained by simulating the thermal quenching of ZnSe photoluminescence.

Study of GaN thin films grown on intermediate-temperature buffer layers by molecular beam epitaxy

A detailed characterisation study of GaN thin films grown by rf-plasma molecular beam epitaxy on intermediate-temperature buffer layers (ITBL) was carried out with Hall, photoluminescence (PL) and deep-level transient Fourier spectroscopy (DLTFS) techniques. The unique feature of our GaN thin films is that the GaN epitaxial layers are grown on top of a double layer that consists of an ITBL, which is grown at 690 degreesC, and a conventional low-temperature buffer layer deposited at 500 degreesC. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm(2)V(-1)S(-1) for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. The DLTFS results suggest a three-order-of-magnitude reduction in the deep level at E-c-0.40 eV in the device fabricated with the GaN films grown on an ITBL thickness of 1.25 mum in comparison with the control device without an ITBL. Our analyses indicate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material, resulting in an improvement in the optoelectronic properties of the films. (C) 2002 Elsevier Science BN. All rights reserved.

Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Growth and characterization of GaInNAs/GaAs by plasma-assisted molecular beam epitaxy

We have studied the growth of GaInNAs/GaAs quantum well (QW) by molecular beam epitaxy using a DC plasma as the N sourer. The N concentration was independent of the As pressure and the In concentration, but inversely proportional to the growth rate. It was almost independent of T, over the range of 400-500 degreesC, but dropped rapidly when T-g exceeded 500 degreesC. Thermally-activated N surface segregation is considered to account for the strong falloff of the N concentration. As increasing N concentration, the steep absorption edge of the photovoltage spectra of GaInNAs/GaAs QW became gentle, the full-width at half-maximum of the photoluminescence (PL) peal; increased rapidly, and a so-called S-shaped temperature dependence of PL peak energy showed up. All these were attributed to the increasing localized state as N concentration. Ion-induced damage was one of the origins of the localized state. A rapid thermal annealing procedure could effectively remote the localized state. (C) 2001 Elsevier Science D.V. All rights reserved.

Quality improvement of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The optimum growth condition of GaInNAs/GaAs quantum wells (QWs) by plasma-assisted molecular beam epitaxy was investigated. High-resolution X-ray diffraction and photoluminescence (PL) measurements showed that ion damage drastically degraded the quality of GaNAs and GaInNAs QWs and that ion removal magnets can effectively remove the excess ion damage. Remarkable improvement of PL intensity and obvious appearance of pendellosung fringes were observed by removing the N ions produced in the plasma cell. When the growth rate increased from 0.73 to 1.2 ML/s, the optimum growth temperature was raised from 460 degreesC to 480 degreesC and PL peak intensity increased two times. Although the N composition decreased with increasing growth rate, degradation of optical properties of GaInNAs QWs was observed when the growth rate was over 0.92 ML/s. Due to low-temperature growth of GaInNAs QWs, a distinctive reflection high-energy electron diffraction pattern was observed only when the GaAs barrier was grown under lower As-4 pressure. The samples with GaAs barriers grown under lower As-4 pressure (V/III ratio about 24) exhibited seven times increase in PL peak intensity compared with those grown under higher As-4 pressure (V/III ratio about 50). (C) 2001 Elsevier Science B,V. All rights reserved.

Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy

Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy were studied. Photoluminescence measurements on a series of samples with different well widths and N compositions were used to evaluate the effects. The intermixing of GaNAs and GaAs layers was clearly enhanced by the presence of a SiO2-cap layer. However, it was strongly dependent on the N composition. After annealing at 900 degreesC for 30 s, a blueshift up to 62 meV was observed for the SiO2-capped region of the sample with N composition of 1.5%, whereas only a small blueshift of 26 meV was exhibited for the bare region. For the sample with the N composition of 3.1%, nearly identical photoluminescence peak energy shift for both the SiO2-capped region and the bare region was observed. It is suggested that the enhanced intermixing is mainly dominated by SiO2-capped layer induced defects-assisted diffusion for the sample with smaller N composition, while with increasing N composition, the diffusion assisted by interior defects become predominant. (C) 2001 American Institute of Physics.

A narrow photoluminescence linewidth of 19.2 meV at 1.35 mu m from In0.5Ga0.5As/GaAs quantum island structure grown by molecular beam epitaxy

A self-organized In0.5Ga0.5As/GaAs quantum island structure emitting at 1.35 mum at room temperature has been successfully fabricated by molecular beam epitaxy via cycled (InAs)(1)/GaAs)(1)monolayer deposition method. The photoluminescence measurement shows that a very narrow linewidth of 19.2 meV at 300 K has been reached for the first time, indicating effective suppression of inhomogeneous broadening of optical emission from the In0.5Ga0.5As island structure due to indium segregation reduction by introducing an AlAs layer and the strain reduction by inserting an In0.2Ga0.8As layer overgrown on the top of islands. The mound-like morphology of the islands elongated along the [1 (1) over bar0] azimuth are observed by the atomic force microscopy measurement, which reveals the fact that strain in the islands is partially relaxed along the [1 (1) over bar0] direction. Our results present important information for the fabrication of 1.3 mum wavelength quantum dot devices.

Influence of growth conditions on self-assembled InAs nanostructures grown on (001)InP substrate by molecular beam epitaxy

Self-assembled InAs nanostructures on (0 0 1) InP substrate have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). It is found that the morphologies and PL properties of InAs nanostructures depend strongly on the growth condition. For the same buffer layer, elongated InAs quantum wires (QWRs) and no isotropic InAs quantum dots (QDs) can be obtained using different growth conditions. At the same time, for InAs quantum dots, PL spectra also show several emission peaks related to different islands size. Theoretical calculation indicated that there are size quantization effects in InAs islands. (C) 2001 Elsevier Science B.V. All rights reserved.

Substrate dependence of InGaAs quantum dots grown by molecular beam epitaxy

A systematic study of self-organized In0.5Ga0.5As quantum dots (QDs) and islands grown by molecular beam epitaxy on (100) and (n11) A/B GaAs substrates is given, where n varies from 1 to 5. Low-temperature photoluminescence results show that the properties of the dots have a strong dependence on the substrate orientation as revealed by atomic force microscopy, consistent with the differences in size, shape, and distribution of QDs on different substrates. From (100) to (111) surface, the photoluminescence peak position of dots on B surfaces is found to blueshift more than that on A surfaces. QDs are also formed on (511) A surface. The positional distribution of these dots exhibits a wavy shape, which is related to the corrugated structure of this surface. Two kinds of islands are formed on (111) A surface, but further work is needed to explain the mechanism of these islands. (C) 2001 American Vacuum Society.

Room temperature 1.55 mu m emission from InAs quantum dots grown on (001)InP substrate by molecular beam epitaxy

Self-assembled InAs nanostructures on (0 0 1)InP substrate have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). It is found that the morphologies of InAs nanostructures depend strongly on the underlying alloy. Through introducing a lattice-matched underlying InAlGaAs layer on InAlAs buffer layer, the InAs quantum dots (QDs) can be much more uniform in size and great improvement in PL properties can be attained at the same time. In particular, 1.55 mu m luminescence at room temperature (RT) can be realized in InAs QDs deposited on (0 0 1)InP substrate with underlying InAlGaAs layer. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

Influence of substrate orientation on In0.5Ga0.5As/GaAs quantum dots grown by molecular beam epitaxy

In this paper, In0.5Ga0.5As quantum dots are fabricated on GaAs (100) and (n11)A/B (n = 3, 5) substrates by molecular beam epitaxy. Atomic force microscopy shows that the quantum dots on each oriented substrate are different in size, shape and distribution. In addition, photoluminescence spectra from these quantum dots are different in emission peak position, line width and integrated intensity. Auger electron spectra demonstrate that In concentration is larger near the surface than inside quantum dots, suggesting the occurrence of surface segregation effect during the growth of InGaAs dots. The surface segregation effect is found to be related to substrate orientation. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoluminescence and optical quenching of photoconductivity studies on undoped GaN grown by molecular beam epitaxy

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present. (C) 2000 Elsevier Science B.V, All rights reserved.

DX-like centers in n-type Al-doped ZnS1-xTex grown by molecular-beam epitaxy

Al-related DX-like centers were observed in n-type Al-doped ZnS1-xTex epilayers grown by molecular-beam epitaxy on GaAs substrates. The capacitance-voltage measurement, deep-level transient spectroscopy, and photoconductivity spectroscopy revealed that the behaviors of Al donors in ZnS1-xTex were similar to the so-called DX centers in AlxGa1-xAs. The optical ionization energies (E-i) and emission barriers (E-e) for the observed two Al-related DX-like centers were determined as E-i similar to 1.0 and 2.0cV and E-e similar to 0.21 and 0.39 eV, respectively. It was also shown that the formation of Al-related DX-like centers resulted in a significantly large lattice relaxation in ZnS1-xTex. (C) 2000 Elsevier Science B.V. All rights reserved.