Fabrication and characterization of GaAs quantum well buried in AlGaAs/GaAs heterostructure nanowires

We developed a growth method for forming a GaAs quantum well contained in an AlGaAs/GaAs heterostructure nanowire using selective-area metal organic vapor phase epitaxy. To find the optimum growth condition of AlGaAs nanowires, we changed the growth temperature between 800 and 850 °C and found that best uniformity of the shape and the size was obtained near 800 °C but lateral growth of AlGaAs became larger, which resulted in a wide GaAs quantum well grown on the top (1 1 1)B facet of the AlGaAs nanowire. To form the GaAs quantum well with a reduced lateral size atop the AlGaAs nanowire, a GaAs core nanowire about 100 nm in diameter was grown before the AlGaAs growth, which reduced the lateral size of AlGaAs to roughly half compared with that without the GaAs core. Photoluminescence measurement at 4.2 K indicated spectral peaks of the GaAs quantum wells about 60 meV higher than the acceptor-related recombination emission peak of GaAs near 1.5 eV. The photoluminescence peak energy showed a blue shift of about 15 meV, from 1.546 to 1.560 eV, as the growth time of the GaAs quantum well was decreased from 8 to 3 s. Transmission electron microscopy and energy dispersive X-ray analysis of an AlGaAs/GaAs heterostructure nanowire indicated a GaAs quantum well with a thickness of 5−20 nm buried along the 〈1 1 1〉 direction between the AlGaAs shells, showing a successful fabrication of the GaAs quantum well.     

AlGaAs/GaAs/AlGaAs quantum wells as a sensitive tool for the MOVPE reactor environment

We present in this work a simple quantum well (QW) structure consisting of GaAs wells with AlGaAs barriers as a probe for measuring the performance of arsine purifiers within a metalorganic vapour phase epitaxy system. Comparisons between two different commercially available purifiers are based on the analysis of low-temperature photoluminescence emission spectra from thick QWs, grown on GaAs substrates misoriented slightly from (1 0 0). Neutral excitons emitted from these structures show extremely narrow linewidths, comparable with those that can be obtained by molecular beam epitaxy in an ultra-high vacuum environment, suggesting that purifications well below the 1 ppb level are needed to achieve high quality quantum well growth.     

GaAs/AlGaAs quantum wire lasers fabricated by cleaved edge overgrowth

We have used the molecular beam growth technique which we call "cleaved edge overgrowth" to fabricate quantum wire lasers, in which 1D quantum confinement is entirely defined by the growth process. The active region of our lasers consists of atomically precise quantum wires that form at the T-shaped intersections of 7 nm wide GaAs quantum wells grown along the [001] crystal axis and after an in situ cleave along the [110] crystal axis. The origin of the quantum mechanical bound state is the relaxation of quantum well confinement at this intersection. The high degree of structural perfection achievable in this way allows the observation of stimulated optical emission from the lowest exciton state in optically as well as in electrically pumped devices. The formation of a linear p-n junction in which the quantum wires are embedded is achieved by doping with Be and Si in the two orthogonal growth directions. Efficient current injection into the wires is demonstrated by the almost complete suppression of optical emission from the quantum well states as well as by threshold currents as low as 0.4 mA for uncoated devices at 1.7 K.     

A difference between initial growth stages of the AlGaAs/GaAs and GaAs/AlGaAs heterostructures produced by contact replacement of solutions

The method of liquid epitaxial growth of GaAs/AlGaAs/GaAs heterostructures when the change of solutions occurs due to pushing off one melt by another is discussed. It has been shown theoretically and experimentally that the initial stages of film growth after the change of the binary Ga As melt on the ternary Al Ga As melt differ from that when the Ga As solution pushes off the Al Ga As liquid. The difference is caused by the inequality of the diffusion coefficients of As and Al in a multicomponent Al Ga As liquid (D_Al > D_As). As a result, the growth of an AlGaAs film begins immediately in the case when the Al Ga As solution pushes off the Ga As liquid but in the opposite case the dissolution of an underlying AlGaAs solid is unavoidable and depends little on degree of a saturation of the Ga As washing solution. These peculiarities must be taken into account in discussions of abruptness and other properties of LPE-grown AlGaAs/GaAs and GaAs/AlGaAs heterojunctions.     

Epitaxial growth of GdN on silicon substrate using an AlN buffer layer

We report on the epitaxial growth of the intrinsic ferromagnetic semiconductor GdN on Si (1 1 1) substrates buffered by a thick AlN layer, forming a heteroepitaxial system with promise for spintronics. Growth is achieved by depositing Gd in the presence of unactivated N₂ gas, demonstrating a reactivity at the surface that is sufficient to grow near stoichiometric GdN only when the N₂:Gd flux ratio is at least 100. Reflection high-energy electron diffraction and X-ray diffraction show fully (1 1 1)-oriented epitaxial GdN films. The epitaxial quality of the films is assessed by Rutherford backscattering spectroscopy carried out in random and channelling conditions. Magnetic measurements exhibit a Curie temperature at 65 K and saturation magnetisation of 7 μ_B/Gd in agreement with previous bulk and thin-film data. Hall effect and resistance data establish that the films are heavily doped semiconductors, suggesting that up to 1% of the N sites are vacant.     

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE has been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of Al_xGa_1−xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.     

CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

Effect of an applied electric current in epitaxial growth of GaAs layer on patterned GaAs substrate

Selective epitaxial growth of a GaAs layer on SiN_x masked Si-doped semi-insulating (1 0 0) GaAs substrate was performed by current-controlled liquid-phase epitaxy (CCLPE) in the conventional liquid-phase epitaxy. Experiments were carried out with and without the application of electric current. Surface morphology of (1 0 0) facet of the grown layer and the vertical and lateral growth rates were significantly improved under applied electric current. A thick layer of about 330 μm was achieved at relatively low growth time of 6 h with a current density of 20 Acm⁻². The epitaxial growth is realized by both electromigration of the solute and supercooling under a constant rate of furnace cooling. The dislocation density of the grown layer was significantly reduced, compared with that of the substrate (4×10⁴ cm²).     

MOVPE growth of GaAs on Ge substrates by inserting a thin low temperature buffer layer

High quality GaAs layers have been grown by low pressure MOVPE on Ge(001) and Ge(001) 9° off oriented in [110] direction by using a thin low temperature (LT) GaAs layer. Investigations of the initial growth step were performed at different V/III ratios and temperatures. To show the good buffer layer quality solar cell structures were grown on off oriented n‐Ge(001) and n‐GaAs(001) substrates. The surface morphology was studied by atomic force microscopy which showed the step‐flow growth mode on 1.2 µm thick GaAs/Ge structures. The crystalline qualities of this structures and the smooth surface morphology were investigated by double crystal X‐ray diffraction (XRD) and atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Metalorganic vapour phase epitaxy growth of InGaAs/GaAs strained layer quantum wells beyond the pseudomorphic limit

Low pressure metalorganic vapour phase epitaxy growth and characterization of strained InGaAs/GaAs quantum well structures beyond the pseudomorphic limit are reported. Photoluminescence and X-ray diffraction measurements of these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the layers.     

In-situ and ex-situ characterization of GaAs/AlAs quantum well structures using spectroscopic ellipsometry

Spectroscopic ellipsometer is used to monitor the MBE growth of quantum well structures. Real time monitoring of the growth enabled the measurement of growth rate and correlation with RHEED oscillations. The growth of a single GaAs/AlAs quantum well is also monitored in real time using multiple wavelengths. Interface roughness of the interrupted “inverted” AlAs/GaAs interface was also monitored with SE. Under our growth conditions, we measure approximately a 2 ML interfacial region at the inverted interface. A correlation with photoluminescence is also discussed.     

X-ray diagnostics of P-HEMT AlGaAs/InGaAs/GaAs structures

The structural characteristics of the P-HEMT AlGaAs/InGaAs/GaAs heterostructure have been studied by high-resolution X-ray diffractometry. The parameters of the heterostructure layers were determined by simultaneous analysis of the X-ray reflection curves for the (004) and (113) crystallographic planes. Interface diffusion has been established for the In_yGa_1?yAs quantum well and the Al_xGa_1?x As spacer layer, which are characterized by reconstructed profiles of the lattice parameter distribution and anisotropic distribution of random displacements in the layer plane and in the perpendicular direction.     

The transition layers in AlGaAs/GaAs and InGaAsP/GaAs heterostructures grown by LPE

AlGaAs/GaAs and InGaAsP/GaAs heterostructues were grown by liquid phase epitaxy (T_o = 800–770 °C), the transition layers (TLs) were measured with Auger profiling. The Auger profiling of the InGaAsP/GaAs heterostructures after a long heating (1–6 hours, 770 °C) has been made too. The analysis of the experimental data led to the conclusion that the fundamental values of the TL in above mentioned systems are about 10 and 1 to 2 nm, correspondingly. This difference between the minimum widths of the TL in AlGaAs/GaAs and InGaAsP/GaAs system is connected with the different speed of solid state diffusional processes which occur at the initial stpeps of liquid – solid contact.     

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al_xGa_1−xAs layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.     

The Back-Side Morphology of LPE-Grown AlGaAs/GaAs/AlGaAs Heterostructures after the Complete Removal of the GaAs Substrate by Selective Etching

As-grown surfaces of AlGaAs/GaAs/AlGaAs heterostructures were stuck down the glass disks and after that the GaAs substrates were removed completely by selective chemical etching. As a result the back-surface of the heterostructures became open to light. The influence of different factors on the back-surface planarity and on the density of such specific defects as holes in the GaAs layer have been investigated. It is shown that the density of holes can be decreased to a value less than 1 hole/cm² if a glove-box maintained under a pure N₂ atmosphere and connected with LPE installation is used.     

Optical investigation of the self-organized growth of InAs/GaAs quantum boxes

By studying the optical properties of highly strained InAs/GaAs multilayers as a function of the deposited quantity of InAs, a high resolution probing of the change from two-dimensional to three-dimensional morphology of the InAs layers has been performed. We show that the critical thickness for the onset of three-dimensional growth is very well defined for given growth conditions. The nucleation of the islands is, however, asynchronous, due to the spatial fluctuations of the deposited quantity of InAs. A fast initial growth of the InAs islands leads within a few seconds to a quasi-equilibrium morphology for InAs. Asynchronous nucleation and fast initial growth combined are shown to be the major origin of the size fluctuations of InAs quantum boxes in GaAs obtained by self-organized growth under standard molecular beam epitaxy (MBE) growth conditions.     

Growth of InP/InGaAs multiple quantum well structures by chemical beam epitaxy

InP/InGaAs multiple quantum well structures with up to 200 periods have been grown by CBE. These structures exhibit exceptional lateral uniformity, measured as ±1 Å in period, ±13 ppm in lattice mismatch and ±0.5 nm in wavelength across a 2 inch wafer. Good surface morphology, sharp interfaces and excellent growth control have all been demonstrated.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

Growth of GaAs/AlGaAs HBTs by MOMBE (CBE)

In this paper, we will discuss how the unique growth chemistry of MOMBE can be used to produce high speed GaAs/AlGaAs heterojunction bipolar transistors (HBTs). The ability to grow heavily doped, well-confined layers with carbon doping from trimethylgallium (TMG) is a significant advantage for this device. However, in addition to high p-type doping, high n-type doping is also required. While elemental Sn can be used to achieve doping levels up to 1.5×10¹⁹ cm^-3, severe segregation limits its use to surface contact layers. With tetraethyltin (TESn), however, segregation does not occur and Sn doping can be used throughout the device. Using these sources along with triethylgallium (TEG), trimethylamine alane (TMAA), and AsH₃, we have fabricated Npn devices with 2 μm×10 μm emitter stripes which show gains of ≥ 20 with either ƒ_t = 55 GHz and ƒ_max = 70 GHz or ƒ_t = 70 GHz and ƒ_max = 50 GHz, depending upon the structure. These are among the best RF values reported for carbon doped HBTs grown by any method, and are the first reported for an all-gas source MOMBE process. In addition, we have fabricated a 70 transistor decision circuit whose performance at 10 Gb/s equals or exceeds that of similar circuits made from other device technologies and growth methods. These are the first integrated circuits reported from MOMBE grown material.