Effects of growth interruption on the optical properties of AIGaAs/GaAs and GaAs/InGaAs quantum wells grown by atmospheric pressure organometallic chemical vapor deposition

In this study, the effects of growth interruptions on Al_0.17Ga_0.83As/GaAs and GaAs/ In_xGa_1-xAs quantum wells (QWs) grown by organometallic chemical vapor deposition (OMCVD) were assessed using low-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Growth interruption times were varied between 60, 10, and 0 sec. For both material systems, as the interruption time was reduced, the ground-state QW transition energies increased, while the linewidths of the peaks decreased. For the Al_0.17Ga_0.83As/GaAs structures, 5 K PL data suggests that the incorporation of impurities is enhanced by longer growth interruption times. In addition, as the interruption time was reduced, the energy separation between the 5 K PL and PLE peaks (Stokes shift) decreased, and was as low as 2.6 meV for no interruption. For GaAs/In_0.11Ga_0.89As samples, 2 K PL data indicated that the incorporation of donor species was not a function of the growth interruption time.     

Gas-Source molecular beam epitaxy and characterization of inGaAs/lnGaAsP quantum well structures on InP

In this paper, we report the effect of using a group-V residual source evacuation (RSE) time on the interfaces of InGaAs/lnGaAsP quantum wells (QWs) grown by gas-source molecular beam epitaxy. High-resolution x-ray rocking curve and low-temperature photoluminescence (PL) were used to characterize the material quality. By optimizing the RSE time, a PL line width at 15K as narrow as 6.6 meV is observed from a 2 nm wide single QW, which is as good as or better than what has been reported for this material system. Very sharp and distinct satellite peaks as well as Pendellosung fringes are observed in the x-ray rocking curves of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y multiple QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. Theoretical considerations of the PL linewidths of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y single QWs show that for QW structures grown with the optimized RSE time, the PL linewidth is mainly due to alloy scattering, whereas the contribution from interface roughness is small, indicating a good interface control.     

Two Distinct Origins of Highly Localized Luminescent Centers within InGaN/GaN Quantum‐Well Light‐Emitting Diodes

The high light‐output efficiencies of In_xGa_1‐xN quantum‐well (QW)‐based light‐emitting diodes (LEDs) even in presence of a large number of nonradiative recombination centers (such as dislocations) has been explained by localization of carriers in radiative potential traps, the origins of which still remain unclear. To provide insights on the highly efficient radiative traps, spectrally resolved photoluminescence (PL) microscopy has been performed on green‐light‐emitting In_0.22Ga_0.78N QW LEDs, by selectively generating carriers in the alloy layers. PL imaging shows the presence of numerous inhomogeneously distributed low‐band‐gap traps with diverse radiative intensities. PL spectroscopy of a statistically relevant number of individual traps reveals a clear bimodal distribution in terms of both band‐gap energies and radiative recombination efficiencies, indicating the presence of two distinct classes of carrier localization centers within the same QW sample. Disparity in their relative surface coverage and photoemission “blinking” characteristics suggests that the deep traps originate from local compositional fluctuations of indium within the alloy, while the shallow traps arise from nanometer‐scale thickness variations of the active layers. This is further supported by Poisson–Schrödinger self‐consistent calculations and implies that radiative traps formed due to both local indium content and interface‐morphology‐related heterogeneities can coexist within the same QW sample.     

Study of optical characteristics of structures with strongly strained In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

Results of photoluminescence (PL) studies of heterostructures with strongly strained InxGa_{1 ? x} As quantum wells (QWs) are presented. It is shown that the dependence of the PL intensity on the QW thickness has a maximum whose position depends on the composition of the In _x Ga_{1 ? x} As solid solution. The PL wavelength at the maximum intensity is 1.13 µm at a QW thickness of 60 µm at a QW thickness of 50 Å for x = 0.39 and 0.42, respectively.     

High performance AlyGa1−yAs-GaAs-lrxGa1−xAs quantum well lasers defined by silicon-oxygen impurity-induced layer disordering

In these experiments impurity-induced layer disordering (IILD) utilizing chemical reduction of SiO₂ by Al (from Al_0.8Ga_0.2As) is employed to generate Si and O to effect layer disordering. The SiO₂-Al_0.8Ga_0.2As reaction is studied with respect to annealing ambient. By controlling the extent of disordering via As₄ overpressure, closely spaced (∼1μm) Si-O IILD buried heterostructure lasers can be optically coupled or uncoupled. Direct observation of O incorporation into the buried layers is shown using secondary ion mass spectroscopy (SIMS). The thermal stability of separate-confinement Al_yGa_1−yAs-GaAs-In_xGa_1−xAs quantum well heterostructure (QWH) laser crystals is investigated using SIMS, transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The data show that the thermal stability of a strained-layer In_0.1Ga_0.9As quantum well (QW) is strongly dependent upon: (1) the layer thickness and heterointerfaces of the Al_yGa_1−yAs-GaAs waveguide layers located directly above and below the QW, (2) the type of surface encapsulant employed, and (3) the annealing ambient. Narrow single-stripe (<2μm) lasers fabricated via Si-O diffusion and layer disordering exhibit low threshold currents (I_th ∼ 4 mA) and differential quantum efficiencies,η, of 22% per facet under continuous (cw) room-temperature operation.     

Strain relaxation in InGaAs lattice engineered substrates

Strain relaxation of hypercritical thickness In_xGa_1−xAs layers has been observed during lateral oxidation of underlying AlAs layers. Strain relaxation of In_xGa_1−xAs layers was studied as a function of indium composition and the AlAs oxidation temperature. It is proposed that the enhanced strain relaxation is due to two factors. The first is enhanced motion of threading dislocations due to stresses generated during the lateral oxidation process. The second is the porous nature of the In_xGa_1−xAs/Al₂O₃ interface that minimizes the interaction of threading dislocations with existing misfit dislocation segments. The extent of strain relaxation increases with increasing oxidation temperature, whereas the efficiency of strain relaxation was found to decrease with increasing indium composition. The efficiency of strain relaxation upon oxidation can be improved by reducing the misfit dislocation density at the In_xGa_1−xAs/AlAs interface prior to oxidation and by changing the nature of the In_xGa_1−xAs/Al₂O₃ interface.     

Photoluminescence study of AlGaAs/GaAs/AlGaAs double quantum wells separated by a thin AlAs layer

Photoluminescence (PL) spectra of Al_0.21Ga_0.79As/GaAs/Al_0.21Ga_0.79As double quantum wells (DQWs) separated by a thin AlAs barrier have been studied in the temperature range 77–300 K. The well width was varied from 65 to 175 Å, and the thickness of the AlAs barrier was 5, 10, or 20 Å. In the case of a sufficiently thin (5, 10 Å) AlAs barrier, the energy spectrum of QW states is considerably modified by coupling between the QWs. This effect shifts the main spectral peak of PL, and specific features associated with the splitting of the ground state into symmetric and asymmetric states are observed in the spectra at higher temperatures. The DQW structure with a 20-Å-thick AlAs barrier is a system of two uncoupled asymmetric Al_0.21Ga_0.79As/GaAs/AlAs QWs. The energy levels in double coupled QWs were calculated as functions of the well width and AlAs barrier thickness, and good correlation with the experimentally observed energies of optical transitions was obtained.     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

Critical Layer Thickness in Exponentially Graded Heteroepitaxial Layers

Exponentially graded semiconductor layers are of interest for use as buffers in heteroepitaxial devices because of their tapered dislocation density and strain profiles. Here we have calculated the critical layer thickness for the onset of lattice relaxation in exponentially graded In _x Ga_1?x As/GaAs (001) heteroepitaxial layers. Upwardly convex grading with \( x = x_{\infty } \left( {1 - {\rm e}^{ - \gamma /y} } \right) \) was considered, where y is the distance from the GaAs interface, γ is a grading length constant, and x _∞ is the limiting mole fraction of In. For these structures the critical layer thickness was determined by an energy-minimization approach and also by consideration of force balance on grown-in dislocations. The force balance calculations underestimate the critical layer thickness unless one accounts for the fact that the first misfit dislocations are introduced at a finite distance above the interface. The critical layer thickness determined by energy minimization, or by a detailed force balance model, is approximately \( h_{\rm{c}} \approx <Exponentially graded semiconductor layers are of interest for use as buffers in heteroepitaxial devices because of their tapered dislocation density and strain profiles. Here we have calculated the critical layer thickness for the onset of lattice relaxation in exponentially graded In _x Ga_1−x As/GaAs (001) heteroepitaxial layers. Upwardly convex grading with x = x_￥ ( 1 - e ^{- g/y} ) x = x_{\infty } \left( {1 - {\rm e}^{ - \gamma /y} } \right) was considered, where y is the distance from the GaAs interface, γ is a grading length constant, and x _∞ is the limiting mole fraction of In. For these structures the critical layer thickness was determined by an energy-minimization approach and also by consideration of force balance on grown-in dislocations. The force balance calculations underestimate the critical layer thickness unless one accounts for the fact that the first misfit dislocations are introduced at a finite distance above the interface. The critical layer thickness determined by energy minimization, or by a detailed force balance model, is approximately h_c ? < h_{\rm{c}} \approx < Although these results were developed for exponentially graded In _x Ga_1−x As/GaAs (001), they may be generalized to other material systems for application to the design of exponentially graded buffer layers in metamorphic device structures such as modulation-doped field-effect transistors and light-emitting diodes.     

Growth of AlxGa1−x as by metalorganic chemical vapor deposition using trimethylgallium and trimethylamine alane

High-quality Al_xGa_1−xAs layers with aluminum arsenide contentx up to 0.34 have been grown in a low pressure metalorganic chemical vapor deposition (MOCVD) system using trimethylgallium (TMG), trimethylamine alane (TMAA) and arsine. The carbon content in these films depended on growth conditions but was in general lower than in those obtained with trimethylaluminum (TMA) instead of TMAA in the same reactor under similar conditions. Unlike TMA grown layers, the TMAA grown Al_xGa_1−xAs layers, (grown at much lower temperature—down to 650° C), exhibited room temperature photolu-minescence (PL). Low temperature (25 K) PL from these films showed sharp bound exciton peaks with a line width of 5.1 meV for Al_0.25Ga_0.75As. A 39 period Al_0.28Ga_0.72As (5.5 nm)/GaAs (8.0 nm) superlattice grown at 650° C showed a strong PL peak at 25 K with a line width of 5.5 meV attesting to the high quality of these layers.     

Novel In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistors on GaAs substrate with InxGa1−xP graded buffer layers

Compositionally graded In_xGa_1−xP (x=0.48→x=1) metamorphic layers have been grown on GaAs substrate by solid source molecular beam epitaxy using a valved phosphorus cracker cell. Three series of samples were grown to optimize the growth temperature, V/III ratio and grading rate of the buffer layer. X-ray diffraction (XRD) and photoluminescence (PL) were used to characterize the samples. The following results have been obtained: (1) XRD measurement shows that all the samples are nearly fully strain relaxed and the strain relaxation ratio is about 96%; (2) the full-width at half-maximum (FWHM) of the XRD peak shows that the sample grown at 480°C offers better material quality; (3) the grading rate does not influence the FWHM of XRD and PL results; (4) adjustment of the V/III ratio from 10 to 20 improves the FWHM of XRD peak, and the linewidth of PL peak is close to the data obtained for the lattice-matched sample on InP substrate. The optimization of growth conditions will benefit the metamorphic HEMTs grown on GaAs using graded InGaP as buffer layers.     

Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates

We report on the growth of Al_0.25Ga_0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5° and 1.4° are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5° and 1.4° off [0001] toward [1[`1] 00] [1\overline{1} 00] . Aluminum (Al) incorporation in the Al _x Ga_1−x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.     

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.     

Electron transport in an In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As/In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As quantum well with a δ-Si doped barrier in high electric fields

The electron conduction in a two-dimensional channel of an In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well (QW) with a δ-Si doped barrier has been investigated. It is shown that the introduction of thin InAs barriers into the QW reduces the electron scattering rate from the polar optical and interface phonons localized in the QW and increases the electron mobility. It is found experimentally that the saturation of the conduction current in the In_0.53Ga_0.47As channel in strong electric fields is determined by not only the sublinear field dependence of the electron drift velocity, but also by the decrease in the electron concentration n _s with an increase in the voltage across the channel. The dependence of n _s on the applied voltage is due to the ionized-donor layer located within the δ-Si doped In_0.52Al_0.48As barrier and oriented parallel to the In_0.53Ga_0.47As QW.     

Weak-beam stereomicrography of defects in GaAs/In x Ga1−x As interfaces

Weak-beam stereomicrography was used to image dislocation arrays at the two interfaces in GaAs/In _x Ga_1−x As/GaAs sandwich structures. For samples with mismatch equal to 1.8%, separate dislocation arrays were found at each interface. The measured dislocation density at the upper interface was only 10% of the density found at the lower interface. This is attributed to reduced mismatch at the upper interface due to strain in the In_0.25Ga_0.75As layer. Also, the dislocation spacing asymmetry along the [01l] and [01l] directions is exhibited at each interface. In the upper interface, it is attributed to growth on asymmetrically strained In_0.25Ga_0.75As. Stereo-imaging of samples with higher mismatch (f = 2.9%) showed 3-dimensional details of dislocation half-loop nucleation that leads to a semiorthogonal array with increasing In_0.4Ga_0.6As layer thickness.     

Tuning the energy spectrum of InAs/GaAs quantum dots by varying the thickness and composition of the thin double GaAs/InGaAs cladding layer

It is shown that the ground state transition energy in quantum dots in heterostructures grown by atmospheric-pressure MOCVD can be tuned in the range covering both transparence windows of the optical fiber at wavelengths of 1.3 and 1.55 μm by varying the thickness and composition of the thin GaAs/In_xGa_1−x As double cladding layer. These structures also exhibit a red shift of the ground state transition energy of the In_xGa_1−x As quantum well (QW) as a result of the formation of a hybrid QW In_xGa_1−x As/InAs (wetting layer) between the quantum dots (QDs). The Schottky diodes based on these structures are characterized by an increased reverse current, which is attributed to thermally activated tunneling of electrons from the metal contact to QD levels. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 4, 2004, pp. 448–454. Original Russian Text Copyright ? 2004 by Karpovich, Zvonkov, Levichev, Baidus, Tikhov, Filatov, Gorshkov, Ermakov.     

Influence of indium doping on the formation of silicon-(gallium vacancy) complexes in gallium arsenide grown by molecular-beam epitaxy at low temperatures

Low-temperature photoluminescence (PL) studies of gallium-arsenide layers grown by molecular-beam epitaxy at low (200 °C) temperatures (LT GaAs) and doped with silicon or a combination of silicon and indium have been performed. The PL spectra of as-grown samples reveal a shallow acceptor-based line only. After annealing, an additional line at ∼1.2 eV appears, which is attributable to Si_Ga-V _Ga complexes. The activation energy of complex formation is found to be close to the activation energy of migration of gallium vacancies and is equal to 1.9±0.3 eV for LT GaAs: Si. It is found that doping with a combination of silicon and indium leads to an increase in the activation energy of formation of Si_Ga-V _Ga complexes to 2.5±0.3 eV. We believe that this increase in the activation energy is controlled by the gallium vacancy-indium interaction through local lattice deformations. Fiz. Tekh. Poluprovodn. 33, 1187–1191 (October 1999)     

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.     

湿法氧化Al0.85Ga0.15As时SiO2层的保护作用

本文研究了有无氧化硅保护层时Al0.85Ga0.15As层的高温湿法氧化。实验结果表明：氧化硅层对Al0.85Ga0.15As层的高温侧向湿法氧化速率基本无影响;被氧化区域SEM图像的衬度和有氧化硅保护层样品As拉曼峰的缺乏归因于被氧化区域中不存在氧化反应产物As,这有利于提高氧化层的热稳定性;有SiO2保护层样品的发光强度比无SiO2保护层样品的发光强度强的多,且具有SiO2保护层样品的发光峰位和半高全宽与氧化前的样品基本一致,而无SiO2保护层样品的发光峰位红移,半高宽展宽,这是由于氧化硅层阻止了GaAs盖层的氧化。     

Dislocation-Induced deep level states in In0.08Ga0.92As/GaAs heterostructures

We have performed luminescence experiments on In_0.08Ga_0.92As/GaAs heterointerfaces to explore the energy distribution of deep level states in the bandgap for two cases: (1) unrelaxed, pseudomorphic In_0.08Ga_0.92As films (200Å thick), which have few if any dislocations at the interface, and (2) partially relaxed In_0.08Ga_0.92As films (1000Å thick) which are expected to have a substantial interfacial dislocation density. A combined photoluminescence and cathodoluminescence technique is used which allows us to profile the sample luminescence through the buried interface region. Our results show the existence of deep level luminescent features characteristic of the GaAs substrate and features common to In_0.08Ga_0.92As and GaAs, as well as the existence of a deep level feature near 1 eV photon energy which undergoes a shift in energy depending upon the degree of strain relaxation in the In_0.08Ga_0.92As film. In addition, a deep level feature near 0.83 eV becomes prominent only in In_0.08Ga_0.92As films which have relaxed, and thus contain misfit dislocations at the interface. These deep level differences may be due to bandgap states associated with the intrinsic dislocation structure, impurities segregated at the dislocation, or bulk point defects, or threading dislocations generated during the strain relaxation. Previous work has determined that a deep level state 0.7 eV above the valence band edge would account for the electrical behavior of relaxed In_0.08Ga_0.92As/GaAs interfaces, which is in good agreement with the range of deep level transitions near 0.8 eV photon energy which we observe. These measurements suggest that photo- and cathodoluminescence measurements of deep level emission in these III-V semiconductors can provide a useful indicator of electrically active defect densities associated with misfit dislocations.