Gas Switching Sequence Research Articles

Characterization of InAs/GaSb superlattices grown by MOCVD with atomic resolution

High-angle annular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) in a Cs-corrected scanning transmission electron microscope were utilized to analyze the interfacial atomic structure of InAs/GaSb superlattices (SLs) grown by metalorganic chemical vapor deposition (MOCVD) on InAs substrates. Despite high growth temperature, narrow interface (IF) widths of less than 2.5 monolayers (MLs) and 3.8 MLs were extracted from HAADF and EELS, respectively, indicating that the IF quality of MOCVD-grown InAs/GaSb SLs is comparable to those grown by molecular beam epitaxy. GaAs-type IFs are considered to account for the narrow IF width. In addition, GaSb-on-InAs IFs were found to be sharper and more strained than InAs-on-GaSb IFs, which is correlated with the special gas supply and switching sequence during MOCVD growth. The strain profile deduced from the HAADF image suggests that little Sb is incorporated into InAs sublayers and 7% In is incorporated into GaSb sublayers.

Effect of interfacial AsH3 surge treatment on GaInP/GaAs dual-junction solar cells grown by metal–organic vapor phase epitaxy

The effect of a modified PH3-to-AsH3 gas switching sequence with an AsH3 surge treatment on GaInP/GaAs dual-junction solar cells was studied. With AsH3 surge treatment, both the material quality with an absence of defects and device performance was preserved, but without it, deterioration in both of these characteristics was observed, which originated in the As/P interface. It was found that defects with an inhomogeneous III–metal composition were formed at the interface and developed into a V-shaped dislocation which penetrated the GaInP top cell. The results suggest that sufficient arsenic supply is necessary prior to As-based material growth on GaInP.

High-Performance Mid-Wavelength InAs/GaSb Superlattice Infrared Detectors Grown by Production-Scale Metalorganic Chemical Vapor Deposition

We demonstrate high-performance mid-wavelength p-i-n infrared detectors based on InAs/GaSb type-II superlattices (SLs) grown by a production-scale metalorganic chemical vapor deposition system. Through a specially-designed gas switching sequence and the use of GaAs-type of interfacial layers, strain-balanced SLs with excellent material quality were obtained, as evidenced by atomic force microscopy, photoluminescence, X-ray diffraction, and transmission electron microscopy. The processed devices, with a cutoff wavelength around 4.8 μm, exhibited a R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> A of 2.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> Ωcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a peak responsivity of 1.4 A/W at 77 K. A specific detectivity of 7.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cmHz1/2/W was achieved at 3.7 μm. These values are comparable with those reported for InAs/GaSb SL detectors with similar cutoff wavelengths grown by molecular beam epitaxy.

(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

We report novel luminescent materials based on group III-V compound semiconductor nanowires. Semiconductor nanowires are essentially one-dimensional rods with length of several microns and diameter below 100 nm. Hence, nanowires exhibit interesting quantum confinement and carrier transport properties. Nanowires are grown using metal seed particles by the vapor-liquid-solid (VLS) process in a molecular beam epitaxy or metalorganic chemical vapor deposition system. By varying the material deposition during growth, axial or radial nanowire heterostructures and p-n junctions may be formed for various device applications including light emitting diodes, lasers, and photodetectors. Due to the large surface area to volume ratio of a nanowire, lattice mismatch strain may be accommodated by elastic distortion of the nanowire without detrimental misfit dislocations, which gives a much greater ability to perform bandgap engineering in nanowires as compared to thin films. Hence, unique heterostructures are possible in nanowires that would be impossible in thin films, opening up new device applications and possibilities in condensed matter physics. We will report our recent work on the photoluminescence properties of InAsP/InP nanowires. InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown in the middle of the InP nanowires, creating a multiple quantum dot structure or superlattice. The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). Photoluminescence (PL) from the quantum dot structure could be tuned by the InAsyP1-y composition (y), or by the size of the quantum dot via the quantum confinement effect. Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots. To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.The growth mechanism of the quantum dots were inferred from the InAsP and InP segment lengths as a function of nanowire diameter. Short InAsP segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAsP and InP grew by diffusive transport of adatoms from the nanowire sidewalls. The present study offers a manner to engineer the lengths of InAsP quantum dots embedded in InP barriers to better control the PL or CL emission. A novel group III and V gas switching sequence is presented to improve compositional control of the QD.

Optimization of Gas-Switching Sequence for InGaAs/GaAsP Superlattice Structures Using In situ Wafer Curvature Monitoring

By high-accuracy in situ curvature measurement during the growth of InGaAs/GaAsP superlattice structures by metal organic vapor phase epitaxy, we have successfully observed the effect of thin GaAs insertion layers between InGaAs wells and GaAsP barriers on strain control. By analyzing curvature transients, we found that an inadequate gas-switching sequence induces the carry over of indium from the InGaAs layer to the overlying GaAs insertion layer. The resulting carry-over layer has an estimated thickness of 0.6 nm and adversely affects the average strain of the structure. Through consideration of the kinetics of surface atoms, it has been revealed that an optimized gas-switching sequence with a 1 s hydrogen purge after the growth of InGaAs wells is effective for preventing the carry over.

Optimized interfacial management for InGaAs/GaAsP strain-compensated superlattice structure

In this paper, the interfacial management for InGaAs/GaAsP strain-compensated superlattice (SL) structures with thin wells and barriers and a large number of hetero-interfaces has been investigated. By means of X-ray diffraction (XRD) measurement and high-accuracy in-situ curvature measurement, we found that the hetero-interfaces with substantial lattice mismatch tend to generate interfacial defects, which can be mitigated by the insertion of ultrathin GaAs interlayers. However, an inadequate gas-switching sequence induces unintended atomic content near the hetero-interfaces and in the GaAs insertion layers, which influences the average strain of the structure. It was found effective to extend the stabilization time for the arsenic and phosphorus mixture before GaAsP barrier growth to 3s and to insert an 1s hydrogen purge after InGaAs well growth. Static photoluminescence (PL) and time-resolved photoluminescence (TRPL) results were also used to evaluate the crystal quality of grown structures.

Control of In Surface Segregation and Inter-Diffusion in GaAs on InGaP Grown by Metal–Organic Vapor Phase Epitaxy

In order to fabricate abrupt heterointerfaces of the GaAs/InGaP system by metal–organic vapor phase epitaxy (MOVPE), we studied the In atom distribution by X-ray photoelectron spectroscopy (XPS). The systematic XPS depth profile analyses revealed that the InGaP surface contains an excess amount of In atoms owing to surface segregation. The excess In atoms diffuse into the GaAs layer and cause compositional mixing at the interface of GaAs on InGaP. In order to suppress the interdiffusion and surface segregation of In atoms into GaAs on InGaP, we have developed a novel gas switching sequence for growing GaAs on InGaP. That is, after the growth of InGaP, only tertiarybutylphosphine (TBP) was introduced, and after stopping the supply of TBP, trimethylgallium (TMGa) was pre-introduced to the reactor before the growth of GaAs. Then tertiarybutylarsine (TBAs) was allowed to flow to initiate GaAs growth. This novel gas switching sequence contributed to the formation of abrupt heterointerfaces of GaAs on InGaP.

Precise structure control of GaAs/InGaP hetero-interfaces using metal organic vapor phase epitaxy and its abruptness analyzed by STEM

Fabrication of abrupt InGaP on GaAs (InGaP/GaAs) and GaAs on InGaP (GaAs/InGaP) hetero-interfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. Indium (In) surface segregation during InGaP growth also degrades the abruptness. Here, the MOVPE gas-switching sequence to fabricate atomically abrupt hetero-interfaces was optimized and the effects of this optimization on the hetero-interface abruptness were quantitatively evaluated using the Z-contrast method with scanning transmission electron microscopy (STEM). Results revealed that (a) in the fabrication of InGaP/GaAs hetero-interface, the GaAs top layer should be stabilized using As-source gas supply, and the excess As layer on GaAs should be terminated using an additional supply of Ga species, and (b) in the fabrication of GaAs/InGaP interface, the InGaP layer should be grown using the flow modulation method to suppress In surface segregation. In conclusion, the abruptness of hetero-interfaces of InGaP/GaAs and GaAs/InGaP was improved by using these optimized gas-switching sequences.

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3s at 610°C.

Structural and optical characterization of MOVPE grown InGaP/GaAs MQWs for advanced photovoltaic devices

The optimization of the electronic properties of InGaP/GaAs MQWs, to be inserted in multilayers heterostructure for novel photovoltaic devices, was performed by structural, optical and photoelectrical measurements. Different sequences of nominally undoped InGaP and GaAs alternated layers were grown by low-pressure metalorganic vapour phase epitaxy, employing tertiarybutylarsine and tertiarybutylphosphine as metalorganic precursors for the V-group elements. In order to minimize the As/P exchange effect, the interface In segregation, and to control the whole lattice matching, single and multi-quantum wells (MQWs) with different: (i) periods, (ii) well widths, (iii) growth temperatures, (iv) gas-switching sequences at the interfaces and (v) indium concentrations in the InGaP alloy, were prepared and investigated. The interface sharpness and the compositional fluctuation of thick MQW region containing up to 40 well-barrier sequences were investigated for the modelling, realization and evaluation of test structures based on low-dimensional systems for third generation solar cells.

Abrupt InGaP∕GaAs heterointerface grown by optimized gas-switching sequence in metal organic vapor phase epitaxy

Fabrication of abrupt InGaP∕GaAs heterointerfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. An optimized gas-switching sequence to fabricate heterointerface of InGaP on GaAs layer by MOVPE was previously developed in which the unstable top surface layer of GaAs is stabilized and the exchange of P and As between InGaP and GaAs layers is suppressed. In this study, the effect of this optimized gas-switching sequence was quantitatively evaluated by using scanning transmission electron microscopy (STEM). Changes in atomic composition from GaAs to InGaP at the interface at the atomic layer level were revealed by using Z-contrast method in STEM. Quantitative evaluation using the Z-contrast method confirmed that the abruptness of the GaAs∕InGaP interface was improved by this optimized gas-switching sequence.

Novel gas-switching sequence using group-III pre-flow (GIIIP) method for fabrication of InGaP on GaAs hetero-interface by MOVPE

InGaP/GaAs is an attractive material system to realize ultra fast electronic devices and optical devices. Fabrication of a monolayer abrupt hetero-interface by metal organic vapor phase epitaxy (MOVPE) is difficult, however, due to the rapid As/P exchange on the growth surface. Here, adsorption/desorption kinetics of group-V atoms on GaAs surface during a gas-switching sequence to form InGaP/GaAs structure was studied by using in situ kinetic ellipsometry. The in situ monitoring of the GaAs dielectric constant revealed that P diffuses easily and quickly into GaAs and that excess As adsorbed on the GaAs surface in MOVPE is difficult to desorb during the gas-switching sequence. Based on these results, we developed a novel gas-switching sequence in which an additional TMGa supply and a group-III pre-flow (GIIIP) were introduced for the first time. By optimizing this GIIIP sequence (i.e., by controlling the additional TMGa supply time), we successfully fabricated an abrupt interface of InGaP on GaAs. The effectiveness of this sequence was confirmed based on photo-luminescence measurements.

Optimization of the growth of the InGaP etch-stop layer by MOVPE for InGaP/GaAs HBT device application

InGaP has high etching selectivity to GaAs. It can be used as the etch-stop layer to easily fabricate the InGaP/GaAs heterojunction bipolar transistors (HBTs). This process will also increase the uniformity and manufacturability of the HBT devices. However, the InGaP etch-stop layer will increase the energy barrier height that electrons have to overcome flowing from the collector to the sub-collector, which will decrease the DC current gain of the device (T. Kobayashi, K. Taira, F. Nakamua, H. Kawai, J. Appl. Phys. 65 (1989) 4898). Therefore, the InGaP etch-stop layer has to be thin enough not to affect the device performance. But the spontaneous formation of the In xGa 1− x As y P 1− y intermixing layer between GaAs/InGaP during the metal organic vapour-phase epitaxy (MOVPE) growth will affect the InGaP layer thickness needed as an effective etch-stop layer. In this paper, very thin InGaP layer was achieved with the suppression of In x Ga 1− x As y P 1− y formation by the optimization of the growth temperature and the gas switching sequence time. An effective 20 Å InGaP etch-stop layer was grown at 575 °C with interruption time of 3 s. Both STEM and PL data proved that the intermixing In x Ga 1− x As y P 1− y layer was eliminated and from the selective etching experiment, this 20 Å InGaP layer can stand 45 s etching by the H 3PO 4:H 2O 2:H 2O=1:1:20 solution.

Effects of Gas switching sequences on GaAs/GaAs 1−ySb y superlattices

The interfacial structure of GaAs/GaAs 1− y Sb y superlattices (SL) was modified through changes in the interlayer gas switching sequence. Trimethyl gallium, trimethyl antimony, and arsine were used as the precursors in the growth of thin-layer SL structures. Sufficient antimony introduction during the growth pause prior to GaAs 1− y Sb y growth was required for obtaining abrupt interfaces with a minimal amount of compositional grading as determined by X-ray diffraction, as was the arsine introduction after alloy layer growth. In both cases, the specific amount of Sb or As deposition was found to produce an optimal structure with minimal interfacial grading. The antimony composition of the pseudomorphic alloy layers appear to be constrained by thermodynamic and kinetic limitations.

Effect of the growth sequence on the properties of InGaP/GaAs/InGaP quantum wells grown by LP-MOVPE from group-V metalorganic sources

Lattice-matched, single and multiple InGaP/GaAs/InGaP quantum wells (QWs) were grown at 600°C by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE), with the use of the tertiarybuthylarsine (TBAs) and tertiarybuthylphosphine (TBP) group-V sources. In order to enhance the interface abruptness, different gas switching sequences were exploited during the growth of the interface, and the best results were obtained by inserting a few monolayer-thick GaAsP interlayers (IL), at the direct GaAs-on-InGaP interface. Low-temperature photoluminescence (PL), high resolution X-ray diffraction, transmission electron microscopy and photoreflectance spectroscopy analysis were performed on the grown heterostructures, to correlate the adopted growth sequence with the interface properties and the QW optical transitions.Promising results were obtained, among which: (a) the suppression of the anomalous PL emission at low energy, (b) optical emission from the InGaP/GaAs/InGaP QWs, exhibiting a good correlation with theoretical expectations, (c) direct interface fluctuations within 1nm.

Cross-sectional scanning tunneling microscopy studies of lattice-matched InGaAs/InP quantum wells: variations in growth switching sequence

Cross-sectional scanning tunneling microscopy has been used to study variations in the source gas switching sequences in lattice-matched InGaAs/InP heterostructures grown by chemical beam epitaxy. Using finite element analysis, changes in well–barrier interface strain can be understood in terms of As/P exchange and As memory effect in the growth chamber. Results from annealed samples indicate a greater interdiffusion of group-V species than group-III species.

Control of indium surface segregation in GaAs layer on InGaP grown by MOVPE

ABSTRACTIn order to fabricate good hetero-interface of GaAs/InGaP system by metal-organic vapor phase epitaxy (MOVPE), we studied the indium atom distribution by X-ray photoelectron spectroscopy (XPS). The systematic XPS depth profile analyses of InGaP/GaAs revealed that InGaP surface contains an excess amount of indium atoms. The excess indium atoms diffuse into GaAs layer and cause surface segregation. We have developed a novel gas switching sequence to switch the growth from InGaP to GaAs. That is, after the growth of InGaP, trimethylgallium (TMGa) was pre-introduced to the reactor to terminate the excess amount of indium atoms on InGaP layer. Then tertiarybutylarsine (TBAs) was allowed to flow on the InGaP surface to initiate GaAs growth. This new sequence reduced the amount of indium atoms at the interface and contributed to suppress the inter-diffusion and surface segregation of indium atoms.

Characterization of InGaP/GaAs heterointerfaces grown by metal organic vapour phase epitaxy

In GaAs/InGaP/GaAs structures grown by metal organic vapor-phase epitaxy (MOVPE), the two heterointerfaces are not identical. Normal photoluminescence (PL) features corresponding to the band gaps of GaAs and InGaP are seen for InGaP layer grown on GaAs. However, an intense long-wavelength feature is observed if we grow GaAs on InGaP (inverted structure) while the features of InGaP and GaAs are suppressed. The nature of interfacial regions is investigated by using different gas switching sequences, which can influence the interfacial region composition. Significantly, we find anomalous PL features similar to those observed in the case of inverted structure if we briefly interrupt the growth of InGaP on GaAs and introduce AsH 3 during the growth interruption. Secondary-ion mass spectrometry (SIMS) measurements and preliminary results of the compositional analysis of the interfacial layers based on high resolution X-ray diffraction (HRXRD) and PL measurements suggest that a deleterious effect arises with the exposure of InGaP surface to AsH 3 and is attributed to the formation of an interfacial InGaAsP layer.

Kinetic ellipsometry measurement of InGaP/GaAs hetero-interface formation in MOVPE

In order to fabricate monolayer (ML)-abrupt hetero-interfaces, we investigated the surface phenomena during MOVPE using in situ kinetic ellipsometry. We studied the adsorption/desorption kinetics of group-V atoms on InGaP surface during gas-switching sequence to form InGaP/GaAs structure. When switching from InGaP growth to GaAs growth, first TMIn and TMGa supplies were stopped and excess amount of phosphorus was desorbed rapidly by stopping the TBP (tertiarybutylphosphine) supply. Purging monolayer phosphorus, which was bonded to In, or Ga required 0.8 s TBAs (tertiarybutylarsine) flow. No arsenic penetrated into the InGaP layer during TBAs introduction. Based on those observations, we devised optimum gas-switching sequence to obtain abrupt hetero-interface with ML unit. Photo-luminescence peak from the quantum well fabricated with this sequence shifted to shorter wavelength, showing the effectiveness of this sequence.

Real-time growth monitoring of InGaAs/InP-HBT structures with reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) was used for in-situ monitoring of MOVPE growth of InGaAs/InP-heterojunction bipolar transistors (HBT). Spectra were recorded during growth of differently doped InGaAs and InP layers in order to investigate the influence of the doping type and level on the spectra. The spectra of the growing InP layers show features similar to the phosphorous rich (2×1)-like surface. They reveal a strong effect of the doping on the RAS-signal around 4.5 eV photon energy, corresponding to the E 0′ transition of InP. This is probably due to the linear electro-optic effect at this transition. The InGaAs spectra are dominated by two prominent features around 2.3 eV and 4.0 eV. A correlation between the RAS-signal of the InGaAs layers and the doping type and level is visible at different wavelengths. Transients at different wavelengths were taken during growth, focusing the investigations on the changes in the RAS-signal occurring at the interfaces between InP, InGaAs and a quaternary InGaAsP layer. Characteristic features visible in the RAS transients during the gas switching sequences indicate the formation of interfacial layers.