xAs Structures Research Articles

Relaxed Inx.Ga1−xas Graded Buffers Grown With Organometallic Vapor Phase Epitaxy on GaAs

AbstractInxGa1−xAs structures with compositionally graded buffers were grown with organometallic vapor phase epitaxy (OMVPE) on GaAs substrates and characterized with plan-view and cross-sectional transmission electron microscopy (PV-TEM and X-TEM), atomic force microscopy (AFM), and x-ray diffraction (XRD). The results show that surface roughness experiences a maximum at growth temperatures where phase separation occurs in In.Gal.,As. The strain fields from misfit dislocations induce this phase separation in the <110> directions. At growth temperatures above and below this temperature, the surface roughness decreases significantly; however, only growth temperatures above this regime ensure nearly complete relaxed graded buffers with the most uniform composition caps. With the optimum growth temperature for grading InxGa1−x,As determined to be 700 °C, it was possible to produce In0.33Ga0.67As diodes on GaAs with threading dislocation densities < 8.5 × 106/cm2.

Compositional mapping of semiconductor structures by friction force microscopy

Topographic and chemical mapping of materials at high resolution define the goals of a microscope. Force microscopy can provide methods for simultaneous topography and chemical characterization of materials. Here we describe the use of the atomic force microscope to map chemical variations of semiconductor samples. Chemical maps of semiconductor InP/InGaAs alloys have been determined with 3 nm spatial resolution while 10% changes in indium composition are resolved in InxGa1−xAs structures. The present resolution is limited by the tip’s curvature radius, cantilever lateral force constant, and the total applied force. Theoretical calculations predict lateral compositional resolutions of about 1 nm.

InAs monolayers and the controlled introduction of deep levels in AlGaAs alloys

InAs monolayers and submonolayers have been introduced by means of molecular beam epitaxy in AlxGa1−xAs structures. Our photoluminescence and photoluminescence excitation measurements represent the first results on heterostructures with In sheets in AlxGa1−xAs. It is argued that these results together with their tight-binding modeling here presented provide a valid framework to implement a new class of heterostructures based on engineered deep centers.

Local quantification of the composition in GaAs/Al xGa 1− xAs structures by thickness fringe analysis

The increase of the complexity in semiconductor structures raises more and more the need for local evaluation techniques. For example, laser structures with graded-index waveguides are now widespread, but the characterisation of the shape of the gradient is still empirical. In this study, we use transmission electron microscopy (TEM) for the local determination of the chemical composition of GaAs/Al x Ga 1− x As semiconductor laser structures, by quantifying the variation of the thickness fringe profiles with the aluminium content on wedge shaped samples. Contrary to former studies, the presented method uses experimental profiles for calibration. The quantification procedure is thus greatly simplified since no simulation program is needed. Measurements can be performed over distances of more than 1 μm with a spatial resolution of 1 nm, at any depth of the sample. The precision obtained is about ±0.03 for the aluminium content x, and the results show the potential of the method for the assessment of semiconductor layer structures.

Influence of the DX center on capacitance-voltage profiling for Si δ -doped AlxGa1−xAs

Si δ-doped AlxGa1−xAs structures grown by metalorganic chemical vapor deposition have been studied by capacitance-voltage (C-V) measurements against temperature and frequency. The potential lowering due to the ionized DX centers is responsible for hysteresis and abnormal phenomena in C-V curves at low temperatures. Two electron-accumulation peaks are observed in C-V profiles measured in the temperature range of 200–300 K. One is related to the occupancy of localized DX centers, and the other is related to the two-dimensional electron gas at the planar layer of dopants. The apparent C-V profile, which is widely used to determine the dopant distribution, also depends on the measuring frequency. This behavior is interpreted as an artifact caused by the emission kinetics of the DX centers localized in the dopant plane.

Evaluation of ohmic contacts to GaAs(100) and (111)B using InAs/GaAs short period superlattice and InGaAs intermediate layers

The nature of InAs/GaAs short period superlattice (SPS) and InxGa1−xAs based ohmic contacts, grown by molecular-beam epitaxy, on GaAs(100) and (111)B surfaces have been investigated. The resistive nature of the contacts formed on GaAs(100), using an intermediate InAs/GaAs SPS between the metal and the substrate were found to be comparable with those obtained with an equivalent In0.33Ga0.67As interlayer. A model is described that accounts for the ohmic contact to the InxGa1−xAs structure and may apply to the SPS. Unlike the GaAs(100) configurations, similar contacts to GaAs(111)B were found to differ greatly in their resistive nature; this is a likely consequence of the laminar growth mode and poor crystalline quality of InAs epilayers grown on this substrate orientation, coupled to a higher barrier at the InAs–GaAs(111)B interface.

Transmission of an obliquely incident electron wave through GaAs-AlxGa1−xAs structures: Application to an electron wave filter

Transmission of an obliquely incident electron wave through GaAs-AlxGa1−xAs structures is analyzed using a generalized transmission matrix approach. Angle dependence is investigated quantitatively. Concepts of critical angle are discussed for step barrier structures. It is verified that the transmission probability is a function of longitudinal energy. Consequences of resonant tunneling of a non-normal incident electron wave through a double barrier heterostructure (DBHS) are discussed. It is found that the angle of incidence largely affects the probability of tunneling of electrons through DBHS. Application to an electron wave filter is discussed.

Minority-carrier recombination kinetics and transport in ‘‘surface-free’’ GaAs/AlxGa1−xAs double heterostructures

We have measured room-temperature band-to-band recombination decay kinetics in superior quality GaAs heterostructures, and have observed the longest lifetime (2.5 μs) observed for any GaAs/AlxGa1−xAs structure to date. Additionally, using a novel time-resolved optical photoluminescence imagining technique, analogous to the Haynes–Shockley experiment, we have also measured room-temperature minority-carrier transport in this series of ‘‘surface-free’’ GaAs/Al0.3Ga0.7As double heterostructures, measurements only possible in high-quality samples with long lifetimes and intense photoluminescence. We find the transport to be diffusive with diffusion lengths of ≳100 μm. Further, we find, for thick structures, minority-carrier transport is hole-dominated ambipolar diffusion, as expected for high-purity n-type material. However, for thinner structures, we find that the minority-carrier transport is time dependent, changing from ambipolar diffusion at early times, as in thick structures, to electron-dominated diffusion at later times. We show that these structures become effectively p-type modulation doped due to the relative ‘‘impurity’’ and thickness of the AlxGa1−xAs compared to the GaAs. As a result, the minority-carrier species changes from holes to electrons for decreasing GaAs layer thicknesses. Cumulatively, we show the band-to-band recombination decay kinetics and carrier transport results to be in excellent qualitative and quantitative agreement. Moreover, our results are in excellent agreement with electrical transport measurements of electron and hole mobilities. Finally, with our measured room-temperature lifetimes and minority-carrier transport measurements versus GaAs layer thickness, we accurately calculate the interface recombination velocity for these structures, with the result S∼40 cm/s, among the lowest ever reported for any GaAs/AlxGa1−xAs structure.

The effects of silicon and beryllium on the interdiffusion of GaAs/ AlxGa1−xAs and InxGa1−xAs/GaAs quantum well structures

The effects of silicon and beryllium at doping levels of up to 1019 cm−3 on the interdiffusion of GaAs/AlxGa1−xAs and InxGa1−xAs/GaAs quantum wells after annealing have been studied using photoluminescence. It was found that for beryllium concentrations up to 2.5 ×1019 cm−3 and for silicon doping concentrations up to 1018 cm−3, no change in the interdiffusion coefficients could be measured. For a silicon doping concentration of 6×1018 cm−3 a dramatic degradation of the material quality was observed following annealing at 750 °C for 15 s. This resulted in the luminescence from the well disappearing and the appearance of deep level luminescence related to donor-gallium vacancy complexes and arsenic antisite defects. From these results we suggest that the position of the Fermi level plays no role in the intermixing of III-V heterostructures and that most of the enhanced intermixing observed in silicon-doped GaAs/AlxGa1−xAs structures is related to silicon relocation at very high doping levels.

Optical investigation of the two-dimensional hole energy spectrum in GaAs/AlxGa1−xAs heterojunctions

We present the results of optical studies on the energy spectrum of a two-dimensional hole gas (2DHG) in a GaAs/AlxGa1−xAs structure. The photoluminescence (PL) line shape in the 2DHG is investigated as a function of temperature by heating the holes by a current flow through the 2D hole channel. The line shape of the PL from the 2DHG as a function of temperature is calculated by taking into account the real band structure and the hole–hole final-state interaction. By comparing experiment and theory, it is found that the special features of the band structure predicted theoretically explain the experimental data.

Optimized strained In xGa 1−xAs structures for device application

Strained InGaAs/(AlGa)As quantum wells were grown on GaAs by molecular beam epitaxy for optimized device performance. Pseudomorphic growth, i.e., absence of relaxation and misfit dislocations, is conclusively verified by X-ray diffraction with asymmetric reflexes. Narrow photoluminescence linewidths and pronounced satellite peaks in symmetric diffraction geometry, usually taken as evidence for lateral lattice matching, do not suffice. Growth and structure parameters for pseudomorphic devices will be given.

Closed-loop control of growth of semiconductor materials and structures by spectroellipsometry

Using closed-loop ellipsometric feedback control, we have grown a variety of epitaxial AlxGa1−xAs structures by chemical beam epitaxy where compositions x vary continuously with thickness according to a given input function. Present limits are exemplified by a 200 Å parabolic quantum well structure grown using compositions determined from the outermost running ∼3 Å (∼1 monolayer) of depositing material.

Lattice tilt and dislocations in compositionally step-graded buffer layers for mismatched InGaAs/GaAs heterointerfaces

The strain relaxation and dislocation densities of a compositionally step-graded InGaAs/GaAs buffer layer have been investigated. The structural results were correlated with the electrical properties of an In0.3Ga0.7As/In0.29Al0.71As modulation doped heterostructure grown on top. The buffer was a three step InxGa1−xAs structure each layer 300 nm thick and each In composition step x=0.1. X-ray rocking curve analysis measured a tilt about an in-plane <110≳ axis of about 0.3° per step. At the top interface of the buffer the density of misfit dislocations evaluated by electron microscopy was comparable to the number required to account for this tilt. Thus, the 60° burgers vectors for the majority of dislocations along one in-plane line direction was restricted to a maximum of two rather than the usual four possibilities. The electron mobility in the two-dimensional electron gas channel showed a small assymetry (4%) in the two in-plane <110≳ directions and was highest in the direction parallel to the tilt axis. The sheet electron concentration was 1.2×1012/cm2 with peak mobilities of 9300 and 31 000 cm2/V s at room temperature and 77 K, respectively.

Electron effective mass and band-gap dependence on alloy composition of AlyGaxIn1−y−xAs, lattice matched to InP

AlyGaxIn1−y−xAs structures were prepared by molecular beam epitaxy to determine both the electron effective mass and band-gap dependence of the InP lattice-matched alloy. Cyclotron resonance and photoluminescence measurements were used, respectively. The effective mass obtained after nonparabolicity correction is m*=0.0403+0.0817y. The band-gap relationships obtained at 300 and 5.5 K are Eg (eV)=0.752+1.453y, and Eg (eV)=0.792+1.530y, respectively.

Growth of AlxGa1−xAs parabolic quantum wells by real-time feedback control of composition

Epitaxial AlxGa1−xAs structures whose compositions x vary continuously with thickness according to a given input function have been grown by chemical-beam epitaxy under closed-loop ellipsometric control. 200- and 500-Å parabolic quantum wells analyzed by photoreflectance and secondary-ion mass spectrometry, respectively, show that actual compositions follow target values to within 0.02 in x. Growth of the 200-Å profile was controlled using compositions ellipsometrically determined for the outermost running 3.1 Å (∼1 monolayer) of depositing material.

Intrinsic recombination and interface characterization in ‘‘surface-free’’ GaAs structures

We have conducted a thorough photoluminescence (PL) and PL time-decay study of the interfacial passivating effects of metalorganic chemical vapor deposition prepared Al0.3Ga0.7As, n+ -doped GaAs, and Na2S surface barriers on epitaxial, high-purity (n−) GaAs structures. We observe 300-K radiative lifetimes, in such 10-μm structures, of 2.5 μs, 800 ns, and 150 ns, respectively. These are to be compared with lifetimes of ∼2 ns for a single, ‘‘bare’’ GaAs surface with an Al0.3Ga0.7As rear surface barrier, and ∼0.5 ns for unpassivated epitaxial GaAs. Accompanying radiative efficiencies are 103–104 higher in all of these epibased structures, and 102 higher for Na2S, than for corresponding bare GaAs surfaces. Further, from detailed PL lifetime studies versus GaAs thickness, we find the lowest interfacial recombination velocities reported for any GaAs/AlxGa1−xAs structure, to date, of ≲40 cm/s, and, correspondingly, 0–1800 cm/s for n+/n−/n+ all-GaAs homostructures. Thus, virtually ‘‘surface-free’’ structures are now achievable. In comparison, we find, at best, ∼5500 cm/s for Na2S, and typically 34 000 cm/s for bare GaAs surfaces. We conclude, on the basis of our detailed experimental study of a wide variety of samples, that these values provide truly reliable measures of surface recombination velocities for both surface types. We find an unambiguous determination of surface recombination velocities requires detailed examination of the minority-carrier recombination kinetics versus temperature, together with minority-carrier spatial transport properties. After demonstrating that minority-carrier recombination kinetics in our ideal structures are truly ‘‘intrinsic,’’ and thus wholly unaffected by extrinsic processes, we examine the temperature dependence of band-to-band and free-exciton recombination. We fully explain all intrinsic, free-carrier recombination found in each structure, for temperatures of 40–300 K through rate equations appropriate to each structure. Similar low-temperature (1.8–40 K) studies confirm dominant decay proceeds, here, by intrinsic free excitons.

Composition of selectively grown In xGa 1− xAs structures from locally resolved Raman spectroscopy

The composition of In x Ga 1− x As stripes of different width, selectively grown by low pressure MOVPE or MOMBE, was determined from the eigenfrequency of the Ga-As lattice vibration using Raman spectroscopy. In contrast to MOMBE, structures grown by MOVPE show a strong enhancement of the In content with decreasing width and increasing distances between the stripes. Comparative studies on MOVPE grown In x Ga 1− x P structures indicate a behavior similar to that observed in the case of In x Ga 1− x As.

Properties of DX Centers in AlxGa1−xAs and Effects on Heterojunction Devices

ABSTRACTIn this paper the effects of DX levels on the properties of AlxGa1−xAs and certain heterojunction device structures are summarized. Studies of alloy effects, variations of the local atomic environment near the donor, are reviewed and microscopic models of the DX center are discussed in terms of these and other recent experimental results.

Variation of strain in single and multilayer (In xGa 1−xAs) structures grown on Si(100), and Si(111), by molecular beam epitaxy

The difference in thermal expansion coefficients between GaAs and Si is known to induce a biaxial tensile strain, below room temperature, for GaAs epilayers grown on Si(100). We have performed photoluminescence (PL) measurements for GaAs layers grown on both Si(111) and misorientated Si(100) substrates by molecular beam epitaxy. These measurements indicate that the thermally-induced strain is greater for the (111) growth direction. Strained In xGa 1−xAs-GaAs multiple quantum well structures grown on Si(100), (MQW/Si), exhibit systematic decreases in low temperature PL transition energies when compared with similar structures grown on GaAs(100), (MQW/GaAs). The MQW structures incorporating greater strain (18.7% In content) show decreases in PL transition energies of 3–9 meV between MQW/Si and MQW/GaAs, while MQW structures containing less-strained layers (11% In content) exhibit 12–13 meV shifts to lower transition energies. It seems, therefore, that the effect of thermal strain becomes more pronounced at lower levels of coherency strain.

The effect of various buffer-layer structures on the material quality and dislocation density of high composition AlxGa1−xAs laser material grown by metalorganic chemical vapor deposition

The effect of various types of buffer layers on the generation and propagation of dislocations in epitaxial layers of high composition (x=0.85) AlxGa1−xAs grown by metalorganic chemical vapor deposition (MOCVD) on horizontal Bridgman (HB) and liquid-encapsulated Czochralski (LEC) substrates is examined. Bulk epilayers of high composition (x=0.85) AlxGa1−xAs and graded-barrier quantum-well laser structures with confining layers of the same composition were grown simultaneously on high-quality/low etch-pit density (EPD) HB substrates and comparatively lower quality/high EPD LEC substrates with one of four types of compositionally graded and/or superlattice buffer-layer structures. The bulk material was characterized by delineation and measurement of surface EPD and the observation of overall surface morphology. Data are also presented on the device characteristics of graded-barrier quantum-well laser diodes grown with these same buffer layers in order to determine the correlation between dislocation density and laser threshold current. The various buffer-layer structures were seen to be effective in reducing the defect density and improving the surface morphology of high composition epilayers grown on both HB and LEC substrates. The threshold-current density of the laser diodes, however, was independent of both the type of prelayer and/or substrate utilized.