Effect of thin strain-compensated Al0.6Ga0.4P layers on the growth of multiple-stacked InP/In0.5Al0.3Ga0.2P quantum dots

Multiple-stacked InP self-assembled quantum dots (SAQD or QD) were grown on an In_0.5Al_0.3Ga_0.2P matrix lattice-matched on a GaAs (001) substrate using metalorganic chemical vapor deposition. Cathodoluminescence (CL) scanning electron microscopy, and transmission electron microscopy were employed to characterize the optical, morphological, and structural properties of the grown QDs. We found that the CL line width broadens and the surface becomes rough with an increase in the number of stacked QD layers in the structure. However, by introducing thin tensile-strained Al_0.6Ga_0.4P layers in the middle of In_0.5Al_0.3Ga_0.2P spacer layers to compensate the compressive strain of the InP QD layers, the CL and morphology are significantly improved. Using this technique, 30-stacked InP/In_0.5Al_0.3Ga_0.2P QD structures with improved CL properties and surface morphology were realized.     

Effect of Si doping on the photoluminescence properties of AlGaInP/GaInP multiple quantum wells

The influence of Si doping on the photoluminescence (PL) properties of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P multiple-quantum-wells (MQWs) was studied. For the samples without p-type layers, the PL peak wavelength from (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs did not vary when Si was doped in MQWs, the PL peak intensity did not change obviously and the PL FWHM broadened. We consider that Si doping results in worse interface quality of (Al_0.3Ga_0.7)_0.5In_0.5P/Ga_0.5In_0.5P MQWs. However, for the full light-emitting diode (LED) structure samples, the PL intensity of MQWs obviously increased when Si was doped in MQWs. The PL intensity from MQWs with Si-doped barriers was about 13 times stronger than that of undoped MQWs. The PL intensity from MQWs with Si-doped barriers and wells was strong as 28 times as that of undoped MQWs. The reasons are discussed.     

Properties and use of ln0.5(AlxGa1-x)0.5P and AlxGa1-x as native oxides in heterostructure lasers

Data are presented demonstrating the formation of native oxides from high Al composition In_0.5(Al_xGa_1-x)_0.5P (x≳ 0.9) by simple annealing in a “wet” ambient. The oxidation occurs by reaction of the high Al composition crystal with H₂O vapor (in a N₂ carrier gas) at elevated temperatures (≥500° C) and results in stable transparent oxides. Secondary ion mass spectrometry (SIMS) as well as scanning and transmission electron microscopy (SEM and TEM) are employed to evaluate the oxide properties, composition, and oxide-semiconductor interface. The properties of native oxides of the In_0.5(Al_xGa_1-x)_0.5P system are compared to those of the Al_xGa_1-xAs system. Possible reaction mechanisms and oxidation kinetics are considered. The In_0.5(Al_xGa_1-x)_0.5P native oxide is shown to be of sufficient quality to be employed in the fabrication of stripe-geometry In_0.5(Al_xGa_1-x)_0.5P visible-spectrum laser diodes.     

Characterizations of InxAlyGa1−x−yN alloy systems grown on GaN substrates by molecular-beam epitaxy

In_0.05Al_0.10Ga_0.85N epilayers and Al_0.10Ga_0.90N epilayers have been grown on bulk GaN single crystals and GaN templates by radio-frequency (RF) molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra at different temperatures ranged from 8 to 300 K were measured for these epilayers. The decreasing rates of PL peak intensity of the In_0.05Al_0.10Ga_0.85N epilayers were smaller than those of the Al_0.10Ga_0.90N epilayers. The fluctuations of emission intensities were not observed in the In_0.05Al_0.10Ga_0.85N epilayers by cathodoluminescence observations at 77 K. Our results indicate that In-related effects exist in InAlGaN quaternary alloys on substrates with low-dislocation densities, however, expect that the localization effect related to In-segregation is weak.     

Super-flat interfaces in In0.53Ga0.47As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In_0.53Ga_0.47As/In_0.52Al_0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In_0.53Ga_0.47As/In_0.52Al_0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of AlxInyGa1−x−yN quaternary alloys grown on sapphire substrates by molecular-beam epitaxy

High-resolution X-ray diffraction (HR-XRD) with rocking curve, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy have been performed on high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using a molecular beam epitaxy (MBE) technique with aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. HR-XRD measurements confirmed the high crystalline quality of these alloys without any phase separation. The X-ray rocking curve of Al_xIn_yGa_1−x−yN films typically shows full widths at half maximum (FWHM) intensity between 14.4 and 28.8 arcmin. AFM measurements revealed a two-dimensional (2D) growth mode with a smooth surface morphology of quaternary epilayers. PL spectra exhibited both an enhancement of the integrated intensity and an increasing blueshift with increased Al content with reference to the ternary sample In_0.1Ga_0.90N. Both effects arise from Al-enhanced exciton localization. PL was used to determine the behavior of the energy band gap of the quaternary films, which was found to increase with increasing Al composition from 0.05 to 0.2. This trend is expected since the incorporation of Al increases the energy band gap of ternary In_0.1Ga_0.90N (3.004 eV). We have also investigated the bowing parameter for the variation of energy band gaps and found it to be very sensitive on the Al composition. A value of b=10.4 has been obtained for our quaternary Al_xIn_yGa_1−x−yN alloys.     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

Photoluminescence study on coarsening of self-assembled InAlAs quantum dots on GaAs (001)

Red-emission at ∼640 nm from self-assembled In_0.55Al_0.45As/Al_0.5Ga_0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy (MBE) has been demonstrated. We obtained a double-peak structure of photoluminescence (PL) spectra from quantum dots. An atomic force micrograph (AFM) image for uncapped sample also shows a bimodal distribution of dot sizes. From the temperature and excitation intensity dependence of PL spectra, we found that the double-peak structure of PL spectra from quantum dots was strongly correlated to the two predominant quantum dot families. Taking into account quantum-size effect on the peak energy, we propose that the high (low) energy peak results from a smaller (larger) dot family, and this result is identical with the statistical distribution of dot lateral size from the AFM image.     

Photoluminescence studies on InGaAlP layers grown by low-pressure metalorganic chemical vapor deposition

The optical properties for In_0.5(Ga_1-x Al _x )_0.5P (0 <x < 0.4) layers, grown by low-pressure Metalorganic Chemical Vapor Deposition, have been studied with photolominescence (PL) measurement. The PL intensity decreases with the increase of the Al composition (0 <x < 0.4). This dependence could not be accounted for only by the electron overflow from theΓ band to the X band. And the PL intensity is directly proportional to the excitation power at low temperature, below 50 K. On the other hand, the PL intensity is proportional to the second power of the excitation power at a high temperature range (>200 K). These results indicate that non-radiative recombination centers bound to theΓ band in In_0.5(Ga_1-x Al _x )_0.5P play a very important role in the radiation mechanism. PL dependence also shows these non-radiative recombination centers are thought to have strong relation to the aluminum substitution for In_0.5(Ga_1-x Al _x )_0.5P.     

Study of the long-wavelength optic phonons in AlGaInP and AlGaInAs

The investigation of the lattice dynamics of (Al_xGa_1−x)_yIn_1−yP quaternary semiconductor alloys lattice matched to GaAs has been made by Raman scattering. The Raman spectra exhibit three-mode behavior depending on the composition. A modified random element isodisplacement (MREI) model is generalized to the III-V (A_xB_1−x)_1−yC_yD-type quaternary alloys, describing the behavior of the optical phonons. The calculated result of two quaternary mixed crystals, (Al_xGa_1−x)_yIn_1−yP and (Al_xGa_1−x)_yIn_1−yAs, is in good agreement to the experimental data.     

The study of δ-doped InGaP/InGaAs/GaAs pseudomorphic high electron mobility transistor

An enhancement-mode pseudomorphic high electron mobility transistor (E-mode pHEMT) with In_0.49Ga_0.51P/In_0.25Ga_0.75As/GaAs structure is studied in this paper. The two-dimensional device simulator, MEDICI, is used to solve the Poisson's equation and the electron/hole current continuity equations. An optimized δ-doped InGaP/InGaAs pHEMT structure is found to be superior to the conventional AlGaAs/InGaAs pHEMT. It reveals that the maximum drain-source current (I_DS) goes up to 1600 mA/mm and transconductance (G_m) is 2120 mS/mm.     

Fabrication of InP-based InGaAs ridge quantum wires utilizing selective molecular beam epitaxial growth on (311)A facets

InP-based InGaAs/InAlAs ridge quantum wires were successfully fabricated by our new approach using selective molecular beam epitaxy (MBE). As the starting structures, array of InGaAs ridge structures composed of smooth (311)A facets were formed by MBE on mesa-patterned InP substrates. Prior to actual fabrication of the wires, MBE growth characteristics of In_0.53Ga_0.47As and In_0.52Al_0.48As layers on the starting structure were studied in detail. The results of growth experiments were then successfully applied to the fabrication of InGaAs ridge quantum wires with high spatial uniformity. Low temperature cathodoluminescence spectrum measured in response to spot excitation of wire region showed a strong light emission whose analysis indicated that it originates from InGaAs ridge quantum wire itself. In photoluminescence measurements, the emission from the wire had strong intensity even at room temperature, indicating that the wire crystal possesses excellent bulk and interface quality, and are largely free from nonradiative recombination centers.     

窗口层对三结电池双层减反膜的影响

窗口层的厚度对高效三层太阳能电池具有重要影响。本文针对太阳能电池的地面应用,优化了三节电池（Ga0.5In0.5P/In0.02Ga0.98As/Ge）的双层反射膜（SiO2/TiO2, SiO2/ZnS）。同时讨论了在双层减反膜优化结构下,变化Al0.5In0.5P窗口层厚度引起反射率的变化。     

Phase separation, effects of biaxial strain, and ordered phase formations in cubic nitride alloys

The thermodynamics as well as the energetics and the structural properties of cubic group-III nitrides alloys have been investigated by combining first-principles total energy calculations and cluster expansion methods. In particular results are shown for the ternary In_xGa_1−xN and the quaternary Al_xGa_yIn_1−x−yN alloys. Phase separation is predicted to occur at growth temperatures, for both fully relaxed alloys. A remarkable influence of an external biaxial strain on the phase separation, with the formation of ordered phase structures has been found for the InGaN alloy. These findings are used to clarify the origin of the light emission process in InGaN-based optoelectronic devices. Results are shown for the composition dependence of the lattice constant and of the energy gap in quaternary Al_xGa_yIn_1−x−yN alloys.     

Strain induced lateral ordering in Ga0.22In0.78As/Ga0.22In0.78P short period superlattices on (001) InP

The application of the strain induced lateral ordering process to the strain-compensated (Ga_0.22In_0.78As)_m (Ga_0.22In_0.78P)_m short period superlattices is investigated. The superlattices have been grown at low temperatures by solid source molecular beam epitaxy (MBE) on (001) InP. These superlattices have been used in multiquantum well heterostructures using InP as barriers. The anisotropic polarization of photoluminescence shows the existence of lateral modulation. Dark-field images using the 220 reflection gives modulated contrast in the superlattice layers. High-resolution transmission electron microscopy shows local variations of the interplanar spacing of the (200) planes as well as the angles they form with the (002) planes.     

Effect of substrate orientation on Zn-doping of AIGalnP grown by atmospheric pressure orgamometallic vapor phase epitaxy

Dependence of hole concentration ofp(Zn) — (Al_0.6Ga_0.4In_0.5P on substrate orientation was examined. It increases with tilting the substrate from (100) towards (511)A. Addition of ann-GaAs cap layer on top of the quaternary increases hole concentration while ap-GaAs cap shows little effect.     

TEM study of the effect of growth interruption in MBE of InGaP/GaAs superlattices

The influence of growth interruption during the MBE growth of (100) In_0.5Ga_0.5P/GaAs superlattices is investigated by cross-sectional TEM. A roughening of the growth front is observed during an interruption after the exchange of the group-V molecular beams. The roughening of growth front occurs due to a spontaneous change in the growth orientation of the superlattice from [100] to 〈311〉 directions. This change in growth orientation is characterized by an initial formation of V-shaped grooves with {311} facets on the GaAs growth front which eventually lead to the formation of regions of {311} superlattice structures. The direction of V-shaped grooves is along the [011] axis, which is parallel to the surface dangling bonds of the group V atoms in the unreconstructed (100) plane. The most critical stage for the spontaneous change of the growth orientation is the interruption after the growth of a GaAs layer with the P₂ flux.     

Properties of interfaces in GaInP solar cells

The effect of the properties of interfaces with Group-III phosphides on characteristics of GaInP solar cells has been studied. It is shown that the large valence band offset at the p-GaAs/p-AlInP interface imposes fundamental limitations on the use of p-AlInP layers as a wide-band-gap window in p-n structures of solar cells operating at ratios of high solar light concentration. It is demonstrated that characteristics of p-n solar cells can be, in principle, improved by using a double-layer wide-band-gap window constituted by p-Al_0.8Ga_0.2As and p-(Al_0.6Ga_0.4)_0.51In_0.49P layers.     

Bandgap and lattice constant of GaInAsP as a function of alloy composition

Published data for the composition dependence of the room-temperature bandgap (E_g) and lattice constant (a_o) in the pseudobinary Ga_yIn_1-yAs, Ga_yIn_1-yP, GaAs_xP_l-x, and InAs_xP_l-x systems have been used to derive the following equations for the quaternary Ga_yIn_l-yAs_x P_l-x, alloys: $$\begin{gathered} a_o ({\AA}) = 5.87 + 0.18x - 0.42y + 0.02xy \hfill \\ E_g (eV) = 1.35 - x + 1.4y - 0.33xy - (0.758 - 0.28x)y(1 - y) \hfill \\ - (0.101 + 0.109y) x(1 - x). \hfill \\ \end{gathered} $$ Available experimental data are in excellent agreement with these equations.