Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.     

Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy

The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As₂ flux ratio. Introducing a small amount of Sb (<∼1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of >1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality.     

Shallow donors and deep levels in GaAs grown by atomic layer molecular beam epitaxy

We correlate the Si concentration measured by secondary ion mass spectrometry (SIMS) and the net donor concentration in GaAs:Si grown by atomic layer molecular beam epitaxy (ALMBE); Si was supplied during: (a) both the As and the Ga subcycles, (b) the As subcycle, and (c) the Ga subcycle; the layers were grown at temperatures in the 300-530°C range. The results show that Si incorporation and its compensation depend on the Si-supply scheme and that the extent of compensation decreases with the growth temperature. We also study the deep levels in the ALMBE GaAs grown under the above conditions. Our results show the occurrence of M1, M3 and M4 levels with concentrations that are: (i) essentially independent of both the Si supply scheme and the ALMBE growth temperature, (ii) close to those of MBE GaAs grown at 600°C, and (iii) up to 2 orders of magnitude lower than that of GaAs prepared by molecular beam epitaxy (MBE) at similar temperatures.     

Quantum dot nanostructures and molecular beam epitaxy

In order to fulfil the requirements of the information society there is a growing demand for nanoelectronic devices with new or largely improved performances; these devices are based on low-dimensional carrier systems, and in particular on zero-dimensional ones, that have peculiar properties as compared to the three- and two-dimensional counterparts.

In this paper we review and discuss the basic features of the Molecular Beam Epitaxy growth of quantum dots that are very interesting archetypes of zero-dimensional nanostructures; quantum dots can be obtained by the three-dimensional growth of self-assembled nanoislands that takes place during the preparation of structures based on highly lattice-mismatched materials. Aspects of the morphological, electronic and optical properties of quantum dots will be reviewed and it will be shown how the energy of confined levels for carriers is determined by design and growth parameters of nanostructures and how quantum dot emission wavelengths can be tuned in the windows of optoelectronic and photonic interest, such as that at 0.98, 1.31 and 1.55 μm. An overview of quantum dot devices will be given, with particular attention paid to the quantum dot laser, unarguably the most important application of quantum dots so far.     

Optoelectronic devices by gas source molecular beam epitaxy

Different structures for optoelectronic devices have been grown by gas source molecular beam epitaxy. Strained layer multiquantum well structures with quarternary wells exhibit low threshold current density (J_th=510 A/cm²) and high T₀ value (90 K). A two step growth procedure enables the realization of the vertical structure distributed feedback laser with very good control of the coupling coefficient between laser mode and engraved gratings. Finally, the realization of a buried heterojunction laser is demonstrated in a three-step epitaxial process and a new lift-off procedure avoiding the use of a dielectric mask. Preliminary devices show power emission up to 40 mW with a threshold current I_th=60 mA.     

p-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI₂H₂) as a carbon source. In the low 10¹⁹ cm⁻³ range, almost all carbon atoms are electrically activated and at 9×10¹⁹ cm⁻³, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.     

Growth temperature dependence in molecular beam epitaxy of gallium arsenide

Epitaxial layers of GaAs were grown on GaAs(100) at substrate temperatures ranging from 400° to 600°C by molecular beam epitaxy. Surface structures of the substrate and the epitaxial layers were investigated by means of low-energy electron diffraction. Two new structures of c(4 × 4) and c(8 × 8) were observed from layers grown at the low temperature of 400°C. The electrical and optical properties of layers doped with Si were investigated by measurement of Hall effect and photoluminescence as a function of growth temperature. It is found that a semi-insulating layer is grown below a critical temperature, and the layer is useful as a buffer layer for GaAs FET's. Variation of carrier concentration was observed near the interface between layers grown at different temperatures under a constant Sn beam flux. The effect is attributed to defect-induced segregation of Sn.     

New frontiers of molecular beam epitaxy with in-situ processing

The in-situ process combines film growth and device fabrication steps which take place under an ultra-high vacuum (UHV) or a controlled ambient environment. Processing, materials, and devices produced using this technique with molecular beam epitaxy (MBE) are reviewed.     

Low temperature molecular beam epitaxy growth of cubic GaCrN

Cubic GaCrN layers are grown on MgO(0 0 1) substrates at 350–700 °C by plasma-assisted molecular beam epitaxy. Substrate temperature dependences of their structural and magnetic properties were systematically studied. It is found that the solubility limit of Cr atoms in cubic GaCrN is dramatically improved by the low temperature (350 °C) growth, though crystalline quality becomes poorer. It is also observed that the magnetic ordering increases with Cr content in the low Cr content region, but after showing highest ordering it decreases in the high Cr content region. The Cr content range showing ferromagnetic behavior increases with lowering substrate temperature. However, the magnetization vs. magnetic field curve shows “emaciated” hysteresis for the low temperature grown samples.     

Stability of GaAs oxide under metalorganic molecular beam epitaxy process

Well-defined oxide of GaAs can be used as a mask material for selective-area metalorganic molecular beam epitaxy (MOMBE) of GaAs. In this study, the reaction between triethylgallium (TEG) and the GaAs oxide layer was studied using a quadrupole mass spectrometer (QMS) and an atomic force microscope (AFM). Results of the QMS observation showed that TEG was reflected on the GaAs oxide surface until the start of desorption of the GaAs oxide, and the GaAs oxide layer was desorbed from the wafer after a large time delay from the start of TEG supply. AFM images showed that many holes appeared on the GaAs oxide surface during the desorption of the GaAs oxide. The effect of incident TEG upon the stability of the GaAs oxide mask is discussed.     

Growth of EuTe islands on SnTe by molecular beam epitaxy

Semiconductor magnetic quantum dots are very promising structures, with novel properties that find multiple applications in spintronic devices. EuTe is a wide gap semiconductor with NaCl structure, and strong magnetic moments S=7/2 at the half filled 4f⁷ electronic levels. On the other hand, SnTe is a narrow gap semiconductor with the same crystal structure and 4% lattice mismatch with EuTe. In this work, we investigate the molecular beam epitaxial growth of EuTe on SnTe after the critical thickness for island formation is surpassed, as a previous step to the growth of organized magnetic quantum dots. The topology and strain state of EuTe islands were studied as a function of growth temperature and EuTe nominal layer thickness. Reflection high energy electron diffraction (RHEED) was used in-situ to monitor surface morphology and strain state. RHEED results were complemented and enriched with atomic force microscopy and grazing incidence X-ray diffraction measurements made at the XRD2 beamline of the Brazilian Synchrotron. EuTe islands of increasing height and diameter are obtained when the EuTe nominal thickness increases, with higher aspect ratio for the islands grown at lower temperatures. As the islands grow, a relaxation toward the EuTe bulk lattice parameter was observed. The relaxation process was partially reverted by the growth of the SnTe cap layer, vital to protect the EuTe islands from oxidation. A simple model is outlined to describe the distortions caused by the EuTe islands on the SnTe buffer and cap layers. The SnTe cap layers formed interesting plateau structures with easily controlled wall height, that could find applications as a template for future nanostructures growth.     

Selective growth and other applications of hydrogen-assisted molecular beam epitaxy

The usefulness of atomic hydrogen in molecular beam epitaxy has been demonstrated, centering around selective growth. Atomic hydrogen is effective for low-temperature cleaning of substrates, surfactant effects such as restrain of island growth and suppression of the surface migration of the adatoms and selective growth on masked or V-grooved substrates. These effects are dependent on substrate temperatures. The selective growth of GaAs has been successfully demonstrated at the conventional growth temperature and growth rate with the aid of atomic hydrogen. The main mechanism of the selective growth is the re-evaporation of Ga and As from mask materials such as SiN_x or SiO₂. Selective growth has also been observed on low-index crystal facets. On (111)A and (110) facets, no GaAs was deposited in the presence of atomic hydrogen, the flux of which is approximately the same as that of Ga. GaAs quantum wire structures have been fabricated on the substrates with V-shaped grooves. The efficient capture and confinement of carriers into wire regions have been observed by photolumenescence.     

Carbon doping into GaAs using combined ion beam and molecular beam epitaxy method

Carbon (C) doping by combined ion beam and molecular beam epitaxy (CIBMBE) was investigated. In this technique, mass-analyzed C ions (¹²C⁺) are accelerated at low energies of 30 to 1000 eV and are irradiated onto growing GaAs substrate. Doping concentration control in CIBMBE can be very stably accomplished by simply adjusting the ion beam current density, which is independent of growth conditions of host materials. Experiments on systematic variation of C⁺ ion acceleration energy (E_C+) indicated that, in the energy range of E_C+<170 eV, net hole concentration (|N_A-N_D|) increases slightly as E_C+ increases. The highest |N_A-N_D| is obtained at E_C+ = 170 eV under the constant C⁺ ion beam current density. For E_C+>170 eV, |N_A-N_D| decreases dramatically with increasing E_C+, which can be explained in terms of enhanced sputtering effect. Although no evidence of damages induced by ion irradiation is shown for low E_C+ range of ≤170 eV, trace of damages is apparently observed for E_C+>170 eV.     

Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

MgO films grown on yttria-stabilized zirconia by molecular beam epitaxy

MgO films were grown on (0 0 1) yttria-stabilized zirconia (YSZ) substrates by molecular beam epitaxy (MBE). The crystalline structures of these films were investigated using X-ray diffraction and transmission electron microscopy. Growth temperature was varied from 350 to 550 °C, with crystalline quality being improved at higher temperatures. The MgO films had a domain structure: (1 1 1)[1 1 2¯]_MgO(0 0 1)[1 0 0]_YSZ with four twin variants related by a 90° in-plane rotation about the [1 1 1]_MgO axis. The observed epitaxial orientation was compared to previous reports of films grown by pulsed laser deposition and sputtering and explained as resulting in the lowest interface energy.     

Low temperature GaAs grown by gas source molecular beam epitaxy

Low temperature growth of GaAs by gas source molecular beam epitaxy (GSMBE) is investigated. Reflection high energy electron diffraction is used to monitor the low temperature buffer (LTB) growth and anneal conditions. Growth at low temperatures with dimeric arsenic is more sensitive to the V/III flux ratios and substrate temperatures than with As₄ used in solid source MBE. Temperature dependent conductivity and deep level transient spectroscopy measurements are presented to observe trap outdiffusion from the LTB into subsequently grown FET channels. Low temperature photoluminescence spectra show degradation of quantum well properties when LTBs are grown with increasing V/III flux ratios.     

RHEED monitoring of AlN epitaxial growth by plasma-assisted molecular beam epitaxy

We have studied the epitaxial growth of AlN layers by plasma-assisted molecular beam epitaxy (MBE) on 6H–SiC substrate. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth by the observation of the 2D–3D growth transition, respectively, in Al- and N-rich conditions. Special attention was given to the elimination of the Al droplets which often form in Al-rich conditions. Different growth procedures are proposed to avoid the appearance of these droplets while keeping a 2D growth. Each of the procedure gives AlN epilayers with identical crystalline quality and low surface roughness as measured, respectively, by X-ray diffraction and atomic force microscopy.     

Growth mechanisms of GaAs nanowires by gas source molecular beam epitaxy

GaAs nanowires were grown on GaAs (1 1 1)B substrates in a gas source molecular beam epitaxy system, using self-assembled Au particles with diameters between 20 and 800 nm as catalytic agents. The growth kinetics of the wires was investigated for substrate temperatures between 500 and 600 °C, and V/III flux ratios of 1.5 and 2.3. The broad distribution of Au particles enabled the first observation of two distinct growth regimes related to the size of the catalyst. The origins of this transition are discussed in terms of the various mass transport mechanisms that drive the wire growth. Diffusion of the growth species on the 2-D surface and up the wire sidewalls dominates for catalyst diameters smaller than ∼130 nm on average, while direct impingement on the catalyst followed by bulk diffusion through the Au particle appears to sustain the wire growth for larger catalyst diameters. A change in wire sidewall facets, indicating a probable transition in the crystal structure, is found to be primarily dependent on the V/III flux ratio.     

Improvements in silicon doping of InP and GaInAs in metalorganic molecular beam epitaxy

We have investigated the Si doping of InP and GaInAs in metalorganic molecular beam epitaxy (MOMBE) by using a conventional Si effusion cell. In order to reduce the formation of SiC promoted by the background gases in MOMBE, we introduced a liquid nitrogen cooled baffle between the cell and the mechanical shutter. The results show that the passivating reaction can be substantially suppressed by a proper treatment of the source cell. The doping efficiency remains constant over a long period of operation corresponding to a large total layer thickness (>100 μm). The comparison of SIMS analysis with Hall data reveals an electrical activation of Si in InP up to 100% and about 65% for Si in GaInAs. These results and the investigations on doping profiles show that Si is a suitable donor in InP and GaInAs in the MOMBE process.