Abruptness Of Heterointerfaces Research Articles

Evaluation of GaAs solar cells grown under different conditions via hydride vapor phase epitaxy

In this study, we characterize the GaAs solar cells grown under different conditions using a custom-built atmospheric-pressure hydride vapor-phase epitaxy (HVPE) reactor. Under typical HVPE growth involving the decomposition of AsH3 to Asx, the growth rate is considerably dependent on the temperature and is limited to ~10 μm/h at a low deposition temperature of 660 °C because of the large kinetic barrier of 198 kJ/mol. Herein, we grow GaAs with a considerably lower kinetic barrier of 7 kJ/mol by suppressing the decomposition of AsH3 in the reactor. Further, we grew GaAs solar cells at an extremely high growth rate of 120 μm/h using uncracked AsH3. However, the open-circuit voltage (VOC) was reduced from 1.0 V for the cells grown at 8 μm/h under typical growth using Asx to 0.95 V for the cells grown at 120 μm/h using hydride-enhanced growth. The reduction in VOC was attributed to the modulation of both the doping profile and the abruptness of heterointerfaces. A small amount of residual gases, which presents at the growth surface after the growth of the p-InGaP back surface field layer, is prominently incorporated during the growth of the p-GaAs base layer upon hydride-enhanced growth due to the fast kinetics.

(Invited) Fast and Slow Light-Emitting Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures (NSs) produce photoluminescence (PL) and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast (PL lifetime <20 ns) and hence more efficient SiGe light-emitting devices. The results presented here demonstrate that in such Si/SiGe 3D NSs with a nominal Ge concentration approaching ~35% the PL peaked near 0.78 eV strongly depends on the Si/SiGe heterointerface abruptness. In other Si/SiGe NS/quantum-well samples with a Ge concentration approaching ~40%, we find two PL bands peaked at ~0.8 eV and ~0.9 eV at low temperatures. The PL peaked at 0.8 eV rises and decays slowly, and it quickly saturates as the excitation intensity increases. In contrast, the PL peaked at 0.9 eV shows a much shorter lifetime and exhibits a linear dependence versus excitation intensity. The slow/delayed PL at 0.8 eV is attributed to carrier recombination at the SiGe NS/Si transition layer while the faster and more efficient PL at 0.9 eV is associated with SiGe quantum wells. More complicated and similarly fast (~10-7 s) decays are observed at very high excitation intensities due to electron-hole droplet formation. The physics of carrier recombination in these Si/SiGe NSs is discussed.

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.

Influence of Heterointerface Abruptness on Electrorefractive Effect in InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well

An InGaAs/InAlAs five layer asymmetric coupled quantum well (FACQW) is expected to produce a large electrorefractive index change and is a promising structure for high-performance phase modulators. However, because an FACQW is composed of ultrathin layers, poor abruptness in heterointerface may deteriorate the electrorefractive effect. We therefore investigated theoretically the influence of abruptness of heterointerfaces in an FACQW on the electrorefractive effect of the FACQW. In the analysis, it is assumed that the composition profile in a heterointerface is exponential, and the thickness of the transition layer L is defined. With the increase in L, the absorption edge is blue-shifted, which deteriorates the electrorefractive index change of the FACQW especially in the case of L≥3 monolayer (ML). In addition, we propose an improved FACQW structure, which is expected to exhibit a large electrorefractive index even in the case of L=3 ML.

Heterointerface quality of InGaP–GaAs superlattices determined by photopumping, x-ray analysis, and transmission electron microscopy

Data are presented on the room-temperature, photopumped, continuous laser operation of multiple-well InGaP–GaAs quantum-well heterostructures and superlattices grown by low-pressure metalorganic chemical vapor deposition. Continuous laser operation at room temperature indicates high material quality and minimal nonradiative recombination at the heterointerfaces. The lasing wavelength near the GaAs band gap is consistent with weak bandfilling, and small quantum confinement of electrons and large confinement of holes (mh&amp;gt;me, ΔEv&amp;gt;ΔEc). Double-crystal x-ray diffraction and transmission electron microscopy data confirm the high material quality and heterointerface abruptness.

Stability and interface abruptness of InxGa1−xN/InyGa1−yN multiple quantum well structures grown by OMVPE

The abruptness of hetero-interfaces in InGaN multiple quantum well structures is shown to degrade when a high temperature growth follows growth of the multiple quantum well (MQW) region, as is generally required for the growth of full device structures. We have analyzed MQW samples both with and without high temperature GaN “cap” layers, using x-ray diffraction (XRD), grazing incidence x-ray reflection (GIXR), and photoluminescence. While all of these techniques indicate a degradation of the MQW structure when it is followed by growth at high temperature, GIXR is shown to be especially sensitive to changes of heterointerface abruptness. GIXR measurements indicate that the heterojunctions are less abrupt in samples that have high temperature cap layers, as compared to samples with no cap layer. Furthermore, the degree of roughening is found to increase with the duration of growth of the high temperature cap layer. The degradation of the heterointerfaces is also accompanied by a reduction in the intensity of satellite peaks in the x-ray diffraction spectrum.

Effect of source-supply interruptions on the interface abruptness in gas source molecular beam epitaxy grown InGaAs/InP heterostructures

The effect of source-supply interruptions on the InGaAsInP heterointerface abruptness in gas-source molecular-beam epitaxy grown lattice-matched InGaAs/InP has been investigated. It is confirmed by observing high-resolution transmission electron microscope (HRTEM) images and photoluminescence (PL) peak energy shifts that the thickness of the InGaAs well becomes narrower as the source-supply interruption (SSI) becomes longer. When the SSI is not optimized, an interface layer forms, consisting of one to several strained monolayers as confirmed by PL and HRTEM. When the SSI is optimized, Auger electron microscopy, PL, and HRTEM reveal abrupt heterointerface formation.

Growth of abrupt InGaAs(P)/In(GaAs)P heterointerfaces by gas source molecular beam epitaxy

We demonstrate a novel method for the growth of abrupt InGaAs(P)/In(GaAs)P heterojunctions by gas source molecular beam epitaxy. We find that exposure of freshly grown InP to an As flux during growth interruption between layers of different compositions results in the substitution of surface P atoms and As atoms, thereby generating a strained transition layer at each interface. By assuring a group-III stabilized surface during interruption required to grow InGaAs(P)/In(GaAs)P heterointerfaces, As/P substitution can be avoided, thereby resulting in improved interface quality. Heterointerface abruptness was examined by double-crystal x-ray diffraction and low temperature photoluminescence. The results show that the interfaces grown with the modified switching sequence are considerably more abrupt than those obtained using conventional sequences where As/P interdiffusion extends over several monolayers.

MOVPE growth of short-period superlattices of AlP-GaP and its application for light-emitting diodes

Short-period superlattices of AlP–GaP with different periods have been grown by metalorganic vapor phase epitaxy using triethylaluminum, triethylgallium and phosphine as the sources, and they have been characterized by X-ray diffraction and photoluminescence measurements. As a result, it has been found that the samples of the present study are better in heterointerface abruptness than those of the previous study, where trimethylaluminum and trimethylgallium were used as Al and Ga sources. Light-emitting diodes consisting of the AlP–GaP superlattices have been fabricated as an application of this superlattice system.

Growth of Silicon-Germanium Alloys by Atmospheric-Pressure Chemical Vapor Deposition at Low Temperatures

ABSTRACTDichlorosilanc and germane were used to grow silicon-germanium alloys at temperatures as low as 550°C at atmospheric pressure. Germanium mole fractions as high as 44% were obtained and the layers exhibit smooth surface morphology. Silicon-gcrmanium/silicon multilayers with abrupt hctero-intcrfaccs have been achieved. Cross Section Transmission Electron Microscopy, (XTEM) and High Resolution X-Ray Diffraction, (HRXRD) characterization of the hetero-interface abruptness will be presented. Recent results on two-dimensional (2-D) hole mobility structures grown by this technique will also be reported. Selective growth of silicon-germanium on oxide patterned silicon wafers was also demonstrated. A significant feature of the selective deposition is the lack of faceting at the oxide sidcwall, which has been commonly observed in high temperature silicon growth.

Visible laser diodes grown by molecular beam epitaxy.

High-quality AlGaInP lavers lattice-matched to a GaAs substrate have successfully been grown by solid source MBE with a dimeric phosphorus beam. The bulk crystal quality and the abruptness of heterointerfaces were studided by measuring a photoluminescence spectrum. Net acceptor concentration in Be doped AlInP reaches saturation above -1×1018 cm-3. A 10-μm wide stripe-geometry GaInP/AlInP laser diode is capable of stable CW operation at room temperature. The maximum output power of 12 mW was achieved.

Characterization of Heterointerface Abruptness by Luminescence Method

Characterization of Heterointerface Abruptness by Luminescence Method

Characterization of GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition

The electrical and structural properties of GaAs layers grown directly by metalorganic chemical vapor deposition on Si substrates oriented 2° off (100) toward [011] are reported. The uniformity of minority-carrier lifetime in the 2 in.- diam heteroepitaxial wafers is comparable to that in bulk GaAs of the same doping density (2×1016 cm−3). Selective etching of the GaAs layer reveals an etch pit density of ∼108 cm−2, consistent with plan view transmission electron microscopy which shows a defect density of ∼108 cm−2. Rapid annealing at 900 °C for 10 s does not significantly alter the heterointerface abruptness, and at the same time the crystalline quality of the GaAs improves slightly. The deep level concentration in the as-grown layer is ∼1013 cm−3 as determined by capacitance spectroscopy. Finally, the activation characteristics of low dose Si implants (3×1012 cm−2 at 60 keV) are similar to those in high quality bulk GaAs.

Sensitivity of X-ray diffractometry for strain depth profiling in III–V heterostructures

The use of the fine structure of X-ray rocking curves for the characterization of the abruptness of heterointerfaces is presented for the particular example of Ga1−x Al x As/GaAs heterojunctions. The sensitivity of the method is discussed in detail for the 400 reflection with two different wavelengths (1.2378 and 1.5410 Å). It is shown that the conclusions can be extended to other reflections and wavelengths provided that the ratio χ hi /χ hr depending on both the absorption and the strength of the reflection is kept below 10−1.

Abruptness of InGaAs/InP heterointerface grown by liquid phase epitaxy

The heterointerface abruptness of the InGaAs/InP system grown by liquid phase epitaxy (LPE) has been evaluated by secondary ion mass spectrometry (SIMS), Raman and C − V analyses. From these analyses, it is found that the interface abruptness decreases with lowering the growth temperature, due to the formation of a metastable InGaAsP intermediate layer in the InGaAs/InP heterointerface.