Solid State Molecular Beam Epitaxy Research Articles

Enhancement of photoluminescence of GaAsBi quantum wells by parabolic design of AlGaAs barriers

Influence of barrier material and structure on carrier quantum confinement in GaAsBi quantum wells (QWs) is studied comprehensively. Single- and multi-QW structures were grown using solid-state molecular beam epitaxy with conventional rectangular, step-like and parabolically graded AlGaAs barrier designs. It was discovered that room temperature photoluminescence is increased by more than 50 times in the GaAsBi QWs with parabolically graded barriers (PGBs) if compared to standard rectangular and step-like structures. The enhancement of photoluminescence was reproducible within the range of growth parameters. The carrier localization and increase of trapping efficiency in GaAsBi QWs is responsible for observed enhancement in radiative properties of PGB structures. The random potential field fluctuations for carriers were increased up to 44 meV due to the blurred well-barrier interface causing the conditions for Bi content and/or well width variations. Due to the impact of self-organizing effects on the reproducibility of optical properties, the GaAsBi QWs with AlGaAs PGBs open the window for fabrication of 1.0–1.55 μm wavelength emission lasers based on GaAsBi quantum structures.

Electron irradiation study of room-temperature wafer-bonded four-junction solar cell grown by MBE

For application in space environments, the effect of 1-MeV electron irradiation on wafer-bonded GaInP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid state molecular beam epitaxy was studied. After exposure to 1-MeV electron irradiation at 1×15 e/cm2, an end of life remaining factor of approximately 85% was obtained. The wafer bonding interface was studied by spectral response and transmission electron microscopy. 1-MeV electron irradiation was conducted on the individual InGaAsP and InGaAs single junction cell, respectively. The degradation of the four-junction cell was mainly due to damage on the InGaAsP and InGaAs subcells rather than the bonding interface.

Effects of buffer layer and back-surface field on MBE-grown InGaAsP/InGaAs solar cells

Solid-state molecular beam epitaxy (MBE)-grown InGaAsP/InGaAs dual-junction solar cells on InP substrates are reported. An efficiency of 10.6% under 1-sun AM1.5 global light intensity is realized for the dual-junction solar cell, while the efficiencies of 16.4 and 12.3% are reached for the top InGaAsP and bottom InGaAs cells, respectively. The effects of the buffer layer and back-surface field on the performance of solar cells are discussed. High device performance is achieved in the case of a low concentration of oxygen and weak recombination when InGaAs buffers and InP back-surface field layers are used, respectively.

GaNAs/InGaAs Superlattice Solar Cells with High N Content in the Barrier Grown by All Solid-State Molecular Beam Epitaxy**Supported by the National Natural Science Foundation of China under Grant No 61274134, the University of Science and Technology Beijing Talents Start-up Program under Grant No 06105033, and the International Cooperation Projects of Suzhou City under Grant No SH201215.

We demonstrate nearly 1 eV GaN0.03As0.97/In0.09Ga0.91As strain-compensated short-period superlattice solar cells by all solid-state molecular beam epitaxy. The optimal period thickness for the superlattice growth is achieved to realize high structural quality. Meanwhile, the annealing conditions are optimized to realize a photoluminescence (PL) at a low temperature. However, no PL signal is detected at room temperature, which could be reflected by a lower open-circuit voltage of the fabricated devices. The GaN0.03As0.97/In0.09Ga0.91As superlattice solar cells show a reasonably-high short-circuit current density (Jsc) of over 10 mA/cm2. Furthermore, a concentration behavior is measured, which shows a linear relationship between Jsc and concentration ratios. The extrapolated ideality factor and saturated current density by the concentration action are in good agreement with that extracted by the dark case of the p-i-n diodes.

Investigation of InGaAsP solar cells grown by solid-state molecular beam epitaxy

We report on the study of InGaAsP solar cells grown by solid-state molecular beam epitaxy (MBE) on InP. The effect of growth temperature on device performance is investigated. Under standard one-sun air-mass 1.5 global (AM1.5) illumination, an efficiency of 18.8% has been obtained for In0.78Ga0.22As0.48P0.52 single-junction solar cells grown at high temperature. Time-resolved photoluminescence (PL) results demonstrate that the In0.78Ga0.22As0.48P0.52 optical quality is greatly improved in the case of a high growth temperature. A longer PL decay time of In0.78Ga0.22As0.48P0.52/InP in contrast to In0.78Ga0.22As0.48P0.52/In0.52Al0.48As indicates that InP is more promising as the back surface field for future solar cell performance improvements.

III-V Nanostructures Grown by Molecular Beam Epitaxy for High Efficiency Solar Cells

Semiconductor nanostructures have demonstrated great potential in energy harvesting due to their unique optical and electrical properties compared to their bulk counterparts. Particularly, self-assembled semiconductor nanostructures have received much attention in the last decade. For example, zero-dimensional (0D) quantum dots (QDs) with discrete energy levels like atoms have been proposed to realize high efficient photovoltaic devices that employ novel concepts including hot carriers, intermediate band, and multiple exciton generation. In addition, one-dimensional (1D) nanowires (NWs) can not only facilitate effective charge separation but also form nanophotonic structures that enable enhanced light absorption through light trapping. Moreover, the small footprint of NWs has a high tolerance to thermal and lattice mismatch and hence, integration of materials with different structural properties is possible to implement complex device architecture for high efficiency solar cells. In this presentation, we introduce our developments in nanomaterial growth by using solid-state molecular beam epitaxy and the applications of these self-assembled semiconductor nanostructures in photovoltaic applications. Emphasis will be put on design of III-V QDs for intermediate band solar cells as well as growth of III-V NWs on silicon for cost-effective high efficient solar cells.

GaAs tunnel junction grown using tellurium and magnesium as dopants by solid-state molecular beam epitaxy

We report a GaAs tunnel junction grown by all-solid-state molecular beam epitaxy (MBE), using tellurium (Te) and magnesium (Mg) as n- and p-type dopants, respectively. The growth conditions, including V/III ratio, and growth rate, growth temperature, were optimized. Through these optimizations, Te- and Mg-doped GaAs with high carrier concentrations as well as good mobilities were obtained. A GaAs tunnel junction with a peak current density of 21 A/cm2 was demonstrated.

(Invited) Integration of High-κ Dielectrics on Epitaxial (100), (110) and (111) Germanium for Multifunctional Devices

With continued scaling of Si CMOS technology, new channel materials and device architectures are needed to address power consumption and constrained high-speed operation. With the demonstration of n-channel III-V transistors, the development of equivalent p-channel transistor is mandatory to realize energyefficient CMOS logic. The bulk transport properties of Ge make it an ideal candidate for p-channel transistor. The ultra-high hole mobility Ge can be realized through a III-V/Ge/III-V transistor configuration with different surface orientations would enable much faster switching, thus addressing dynamic power consumption, while the superior high- κ gate dielectric and larger bandgap barrier would help minimize OFF-state leakage. This paper discusses the i) in-situ growth of III-V/Ge/III-V heterostructure, ii) tailor-made surface orientations of Ge enable to achieve both high-hole and high-electron mobilities, and iii) band offsets of high- κ dielectrics on crystallographic oriented epitaxial (100)Ge, (110)Ge, and (111)Ge layers using solid state molecular beam epitaxy.

Comprehensive study on molecular beam epitaxy-grown InAs sub-monolayer quantum dots with different capping combinations

Here the authors report a comprehensive study on InAs sub-monolayer quantum dots with different capping layers. After performing systematic optimization of InAs deposition and GaAs thickness, they grew three samples, namely A, B and C, using solid-state molecular beam epitaxy with identical architecture but different capping materials (2 nm of GaAs, InGaAs-GaAs, and InAlGaAs-GaAs, respectively). Photoluminescence emission peaks due to the ground state transition from the dots were observed at 898, 917, and 867 nm for samples A, B, and C, respectively. Narrow full-width half-maxima (19–32 meV) of the emission peaks indicates high uniformity of dot size distribution. Using the conventional Arrhenius plot, the authors calculated the thermal activation energies from temperature-dependent photoluminescence experiment for samples A, B, and C as 49, 112, and 109 meV, respectively. To complete the study, single-pixel photodetectors were fabricated from samples A, B, and C and temperature-dependent dark current variation with applied bias voltage was measured. Dark current was calculated to be in the range of 10−5–10−4 A/cm2 at a 0.5 V applied bias at 77 K. The activation energies calculated from temperature-dependent dark current measurement for samples A, B, and C were 75, 160, and 155 meV, respectively, and followed the trend observed in temperature-dependent photoluminescence measurements.

The investigation of GaInP solar cell grown by all-solid MBE

We report on the study of GaInP solar cell grown by solid-state molecular beam epitaxy (MBE) on GaAs. The effect of growth temperature on the device performance is investigated. Under the standard one-sun air–mass 1.5 global (AM1.5G) illumination, an efficiency of 16.6% has been obtained for GaInP single-junction solar cell grown at a high temperature. A worse device performance is observed with decreasing growth temperature. Temperature-dependent and time-resolved photoluminescence results demonstrate that the GaInP optical quality is greatly improved in the case of a high growth temperature. A long PL decay time of GaInP/AlInP structure indicates that AlInP is more promising as the back surface field for the future solar cell performance improvement.

Photoluminescence spectra of InAs quantum dots embedded in GaAs heterostructure

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively short durations (0–50s) at growth temperature (520°C). Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) studies indicate that the QD islands increase in size with the growth pause and correspondingly their density decreases. Photoluminescence (PL) studies were carried out on these QDs. Both ground and excited state PL spectra are reported. We present a formalism based on the quantum confinement model and a distribution of dot sizes to explain our results. A minimal set of parameters is employed whose numerical values are obtained from independent experiments and/or microscopic theories. Calculations based on our formalism satisfactorily account for observed PL spectra. The temperature dependence of the PL is also studied and models to explain it are explored.

Temperature dependent photoluminescence investigation of the effect of growth pause induced ripening in InAs/GaAs quantum dot heterostructures

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively shorter durations (0–50s) at the growth temperature (520°C). The islands are found to increase in size with the growth pause and correspondingly their density decreases. Though the photoluminescence spectra of the islands subjected to growth pause is found to follow conventional QD systems, a contradiction is noticed in the calculated values of the activation energy of the dots. We ascribed this contradiction due to the poor crystalline quality of the ripened QDs as a result of desorption and sublimation of indium during the pause at high growth temperature.

Graphene films grown on Si substrate via direct deposition of solid-state carbon atoms

Graphene films were grown on Si(1 1 1) substrates at different temperatures (600, 700 and 800 °C) by directly depositing solid-state carbon atoms through solid-state molecular beam epitaxy (SSMBE). The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that SiC precipitates primarily formed at 700 and 800 °C, as a small quantity of carbon atoms were deposited. When carbon atoms continued depositing, graphene films formed at 800 °C and amorphous or polycrystalline carbon thin films formed at 700 and 600 °C. We think that graphene films could be grown on Si(1 1 1) substrates because of the high activity of carbon atoms at 800 °C and weak interaction between carbon atoms and silicon atoms of substrate due to a SiC buffer layer stabilizing the surface of Si substrate.

Nano-crystalline Sb-based compound semiconductor formed on silicon

A self-assembled InGaSb quantum dot (QD) structure as a nano-structured Sb-based compound structure is fabricated on a silicon substrate using a solid-state molecular beam epitaxy (MBE). A small (dimensions of less than a few tenths of a nanometer) and high-density (>1010/cm2) InGaSb QD structure can be obtained on a Si wafer surface under optimal growth conditions at a low temperature, which is compatible for use with Si-CMOS processes. Using high-resolution transmission electron microscope (TEM) observation, a clear crystalline lattice image in the fabricated InGaSb QD and the interface between the InGaSb and the Si are successfully observed. Based on these results, we expect that a nano-crystalline Sb-based compound semiconductor on a Si surface will become a useful and novel material system with high crystalline quality, enabling the development of a new generation of integrated photonics and high-speed electron devices on the Si platform.

Investigation of the effect of varying growth pauses on the structural and optical properties of InAs/GaAs quantum dot heterostructures

Effect of growth pause or ripening time on structural and optical properties of self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE ) technique with two different growth rates of InAs (0.032 MLs −1 and 0.197 MLs −1) has been investigated. The QD heterostructures were grown at 520 ∘C with InAs monolayer coverage of 2.7 ML. The results were explained on the basis of high angle annular dark field scanning transmission electron microscope (HAADF-STEM), scanning electron microscope (SEM) and photoluminescence (PL) measurements. Introduction of growth pause leads the QD system towards a thermodynamic equilibrium state which in turn makes interesting changes on the morphology of the samples. Coagulation of some smaller dots occurs because of ripening to produce evolved QDs and the dot density reduces with growth pause.

Integration of functional epitaxial oxides into silicon: From high-K application to nanostructures

The authors will present results for crystalline lanthanide oxides on silicon with the Ln2O3 composition (Ln=Pr, Nd, and Gd) in the cubic bixbyite structure grown by solid state molecular beam epitaxy. On Si(100) oriented surfaces, crystalline Ln2O3 grows as (110)-oriented domains, with two orthogonal in-plane orientations. Layers grown under best vacuum conditions often exhibit poor dielectric properties due to the formation of crystalline interfacial silicide inclusions. Additional oxygen supply during growth improves the dielectric properties significantly. Experimental results for Ln2O3-based metal-oxide-semiconductor (MOS) capacitors grown under optimized conditions show that these layers are excellent candidates for application as very thin high-K materials replacing SiO2 in future MOS devices. In addition, they will present a new approach for nanostructure formation which is based on solid-phase epitaxy of the Si quantum well combined with simultaneous vapor-phase epitaxy of the insulator on top of the quantum well. Ultrathin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by this approach on Si(111). Finally, the incorporation of crystalline Si islands into single-crystalline oxide layers will be demonstrated.

Epitaxial multi-component rare earth oxide for high-K application

We studied the growth and electrical properties of single crystalline mixed (Nd 1 − x Gd x) 2O 3 (NGO) thin films and compared the results with those of the binary Gd 2O 3 and Nd 2O 3 thin films, respectively. Epitaxial ternary NGO thin films were grown on Si(100) substrates using modified solid state molecular beam epitaxy. The films were characterized physically using various techniques. The capacitance equivalent oxide thickness of a 4.5 nm NGO thin film extracted from capacitance–voltage ( C– V) characteristics was 0.9 nm, which is lower than all values reported earlier for other crystalline oxides. The leakage current density and the density of interface traps were 0.3 mA/cm 2 at | Vg − V FB| = 1 V and 1.4 × 10 12/cm 2, respectively. These excellent electrical properties of NGO thin films demonstrate that such ternary oxides could be one of the promising candidates for gate dielectrics in the upcoming generations of complementary metal oxide semiconductor (CMOS) devices.

Photoluminescence Lifetime and Structure of Molecular Beam Epitaxy Porous Si1 − xGex Grown on Si

A study is presented of the local structure of intense visible light emitting porous SiGe with an initial Ge fraction of 30 at% grown on boron doped-Si substrates by Solid State Molecular Beam Epitaxy (SS-MBE). Analysis of the Extended X-ray Absorption Fine Structure (EXAFS) of the SS-MBE grown SiGe and their anodized porous counterparts processed under various conditions is used to obtain a better understanding of the visible light-emitting mechanism of porous SiGe. In addition, the photoluminescence decay dynamics of porous SiGe are examined and are found to differ in terms of speed and behaviour from porous Si. We interpret the origin of visible PL of the porous MBE SiGe films by considering the quantum confinement effect, as in the interpretation of PL from porous Si, and the evolution of the SiGe Si like-band structure.