Fabrication of InGaAs ridge quantum wires by selective molecular beam epitaxy and their characterization

InGaAs/InAlAs ridge quantum wires were successfully fabricated by selective molecular beam epitaxy (MBE) for the first time. Prior to wire fabrication, detailed data on selective growth characteristics were taken by using test structures. Then, triangular shaped InGaAs ridge quantum wires with a width of 300 å were fabricated, using the selectivity data. Photoluminescence (PL) measurements detected a strong and narrow peak from the wires which showed a blue shift of 159 meV with respect to the InGaAs band-gap. This value agrees excellently with the calculation.     

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.     

Control of ridge shape for the formation of nanometer-scale GaAs ridge quantum wires by molecular beam epitaxy

We have investigated the molecular beam epitaxial (MBE) growth mechanisms of nanometer scale GaAs ridge structures formed on patterned substrates and studied the way to control the widths of ridges and those of quantum wires grown on them. It is found that the width of the ridge structure decreases, as the growth temperature is reduced, reaching about 20 nm when grown below 580°C. The width of an AlAs ridge (10 nm at 570°C) is always found to be narrower than that of GaAs. A Monte Carlo simulation is performed to investigate the diffusion process of atoms in these ridge structures and indicates the important role of thermodynamical stability on the shape of a nanometer structure.     

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.     

Difference of anisotropic structures of InAs/GaAs quantum dots between organometallic vapor-phase epitaxy and molecular beam epitaxy

The anisotropies of the baseline in and [1 1 0] of InAs quantum dots (QDs) fabricated by molecular beam epitaxy (MBE) and organometallic vapor-phase epitaxy (OMVPE) are investigated. The structural and optical difference between QDs by MBE and OMVPE are investigated through an atomic force microscopy, a transmission electron microscopy, and a photoluminescence polarization measurement. It is found that the InAs QD structural anisotropy in MBE agrees with the individual growth rate anisotropy. Moreover, it is found that the mixture of the different structural anisotropies is unique in OMVPE at low growth temperature (440°C) and the growth mode is complex. From the photoluminescence polarization measurement, the InAs QD structures which mainly contribute to the optical property are decided by the plus and minus of the polarization degree of the ground state, and it is shown that the baseline anisotropy of the QDs mainly agrees with the growth rate anisotropy.     

Growth of Eu-doped GaN by gas source molecular beam epitaxy and its optical properties

Eu-doped GaN with various Eu concentrations were grown by gas source molecular beam epitaxy, and their structural and optical properties were investigated. With increasing Eu concentration from 0.1 to 2.2 at%, deterioration of the structural quality was observed by reflection high-energy electron diffraction, atomic force microscopy and X-ray diffraction. Such a deterioration may be caused by an enhancement of island growth and formation of dislocations. On the other hand, room temperature photoluminescence spectra showed red emission at 622 nm due to an intra-atomic f–f transition of Eu³⁺ ion and Fourier transform infrared spectra indicated an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the increasing behavior was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with relation to the defect.     

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.     

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.     

Anomalous temperature-dependent photoluminescence characteristic of as-grown GaInNAs/GaAs quantum well grown by solid source molecular beam epitaxy

The photoluminescence (PL) mechanisms of as-grown GaInNAs/GaAs quantum well were investigated by temperature-dependent PL measurements. An anomalous two-segmented trend in the PL peak energy vs. temperature curve was observed, which has higher and lower temperature-dependent characteristics at low temperature (5–80 K) and high temperature (above 80 K), respectively. The low and high-temperature segments were fitted with two separate Varshni fitting curves, namely Fit_low and Fit_high, respectively, as the low-temperature PL mechanism is dominated by localized PL transitions while the high-temperature PL mechanism is dominated by the e1–hh1 PL transition. Further investigation of the PL efficiency vs. 1/kT relationship suggests that the main localized state is located at 34 meV below the e1 state. It is also found that the temperature (80 K) at which the PL full-width at half-maximum changes from linear trend to almost constant trend correlates well with the temperature at which the PL peak energy vs. temperature curve changes from Fit_low to Fit_high.     

Carbon background in P-based III-V semiconductors grown by metalorganic molecular beam epitaxy using ethyl-metalorganic sources

Electrical properties of InP, GaP, InGaP and AlGaP, grown by metalorganic molecular beam epitaxy (MOMBE) using ethyl-metalorganics (triethyl-indium, -gallium and -alluminum), are discussed in connection with the carbon background. More C atoms are incorporated into epilayers in the order of AlGaP, GaP and InP. The C atoms act as accepters in AlGaP and GaP epilayers, but they probably work as donors in InP. InGaP showed highly resistive or compensated possibly due to the amphoteric nature of C atoms.     

Carbon reduction and antimony incorporation in InGaAsSb films grown by metalorganic molecular beam epitaxy using tris-dimethylaminoantimony

InGaAsSb layers nearly lattice-matched to InP were grown by metalorganic molecular beam epitaxy using tris-dimethylaminoantimony (TDMASb). Secondary-ion mass spectroscopy measurements revealed that TDMASb is useful not only as an Sb source but also as an additive that reduces the incorporation of C into the film from group-III metalorganic sources. In the room-temperature photoluminescence spectrum, the incorporation of Sb into InGaAs shifted the peak wavelength from 1.66 to 1.75 μm and, simultaneously, the peak intensity of InGaAsSb became more than twice that of InGaAs.     

Optimized InAs quantum effect device structures grown by molecular beam epitaxy

We have investigated InAs quantum effect devices based on both antimonides and arsenides. In an InAs quantum point contact device based on antimonides (InAs/AlGaSb), we have successfully reduced the leakage currents and observed quantum effects at around 77 K by optimizing the heterostructure growth and mesa-etched split-gate approach. Strained InAs quantum dots based on arsenides (AlInAs/AlAs/InAs/InGaAs/AlInAs) were successfully fabricated by MBE growth and mesa-etching. Blue-shifted photoluminescence was obtained from millions of quantum dots with an average lateral size of approximately 2000 å square.     

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.     

Surface diffusion kinetics of GaAs and AlAs metalorganic vapor-phase epitaxy

We have investigated the surface kinetics during metalorganic vapor-phase epitaxy (MOVPE), using high-vacuum scanning tunneling microscopy (STM) observation of two-dimensional (2D) nuclei and denuded zones. Using Monte Carlo simulations based on the solid-on-solid model, from 2D nucleus densities we estimated the surface diffusion coefficients of GaAs and AlAs to be 2 × 10⁻⁶ and 1.5 × 10⁻⁷ cm²/s at 530°C, and the energy barriers for migration to be 0.62 and 0.8 eV, respectively. The 2D nucleus size in the [110] direction was about two times larger than that in the [ 10] direction. The size anisotropy is caused primarily by a difference in the lateral sticking probability (P_s) between steps along the [ 10] direction (A steps) and steps along the [110] direction (B steps). The P_s ratio was estimated to be more than 3:1. Denuded zone widths on upper terraces were 2 ± 0.5 times wider than those on lower terraces. This showed that P_s at descending steps was 10 to 3 × 10² times larger than P_s at ascending steps.     

Se-doped AlGaAs grown on GaAs(111)A by molecular beam epitaxy

The electrical properties of Se-doped Al_0.3Ga_0.7As layers grown by molecular beam epitaxy (MBE) on GaAs(111)A substrates have been investigated by Hall-effect and deep level transient spectroscopy (DLTS) measurements. In Se-doped GaAs layers, the carrier concentration depends on the misorientation angle of the substrates; it decreases drastically on the exact (111)A surface due to the re-evaporation of Se atoms. By contrast, in Se-doped AlGaAs layers, the decrease is not observed even on exact oriented (111)A. This is caused by the suppression of the re-evaporation of Se atoms, by Se---Al bonds formed during the Se-doped AlGaAs growth. An AlGaAs/GaAs high electron mobility transistor (HEMT) structure has been grown. The Hall mobility of the sample on a (111)A 5° off substrate is 5.9×10⁴ cm²/V·s at 77 K. This result shows that using Se as the n-type dopant is effective in fabricating devices on GaAs(111)A.     

InGaAsP/InP multiple quantum wells grown by gas-source molecular beam epitaxy

We have grown In₁-_xGa_xAs_yP₁-_y/InP multiple quantum well structures with 1.3 μm excitonic absorption at room temperature by gas-source molecular beam epitaxy. In-situ composition determination in GaAs₁-_xP_x and InAs_xP₁-_x was carried out by measuring group-V-induced intensity oscillations of reflection high-energy electron diffraction. Based on the in-situ composition calibration for these ternary end members, Ga and As compositions in the quaternary compound, In₁-_xGa_xAs_yP₁-_y, were controlled successfully. Measurements by X-ray rocking curve, low-temperature photoluminescence and absorption spectroscopy indicate that high-quality In₁-_xGa_xAs_yP₁-_y/InP multiple quantum well samples were obtained.     

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.     

Low-temperature molecular beam epitaxy of GaAs: Influence of crystallization conditions on structure and properties of layers

A nontraditional approach to the control of GaAs properties via the introduction of an excessive amount of arsenic during growth of epitaxial layers under conditions of low-temperature molecular-beam epitaxy (LT-GaAs layers) is considered. The influence of excessive arsenic on the structure and properties of as-grown and annealed LT-GaAs layers is considered as well as the effect of layer doping on the “capture” of excess arsenic.