Lattice-matched Composition Research Articles

Pinpointing lattice-matched conditions for wurtzite ScxAl1−xN/GaN heterostructures with x-ray reciprocal space analysis

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite ScxAl1−xN layers on (0001) GaN to be x = 0.14 ± 0.01. 100 nm thick ScxAl1−xN films (x = 0.09–0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions, assuming the c-lattice parameter of ScAlN films coherently strained on GaN increases linearly with Sc-content determined independently by Rutherford backscattering spectrometry [Dzuba et al., J. Appl. Phys. 132, 175701 (2022)]. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (101¯5) reflection reveal that ScxAl1−xN films with x = 0.14 ± 0.01 are coherently strained with the GaN substrate, while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300 nm thick Sc0.14Al0.86N layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this Sc0.14Al0.86N film is 106 arc sec and corresponds to the lowest value reported in the literature for wurtzite ScAlN films.

Sub-bandgap optical absorption processes in 300-nm-thick Al1−xInxN alloys grown on a c-plane GaN/sapphire template

We investigate the sub-bandgap optical absorption (SOA) in 300-nm-thick Al1−xInxN alloys used in cladding layers of edge-emitting laser diodes and distributed Bragg reflectors of vertical-cavity surface-emitting lasers. Al1−xInxN alloys, with indium content x ranging from 0.114 to 0.185, were grown by metal-organic chemical vapor deposition on a c-plane GaN/sapphire template. SOAs on 300-nm-thick thin films were characterized using photothermal deflection spectroscopy (PDS). Thermal emission, such as nonradiative recombination with phonon emission, is the dominant energy relaxation process occurring after SOA in Al1−xInxN alloys. The absorption coefficient of the SOA was estimated to be 0.6–7.0 × 103 cm−1 in these samples by combining PDS and spectroscopic ellipsometry. The drastic increase in the SOA, when x exceeded the lattice-matched composition of the GaN/sapphire template, indicates that impurities, vacancy-type defects, and their complexes with increasing x are possible candidates that result in SOA in Al1−xInxN alloys.

Thermodynamics of the solid-liquid phase equilibrium of a binary system: Effect of a chemical reaction in the liquid and epitaxial strain in the solid

We introduce two thermodynamics models of solid-liquid phase equilibrium of a binary system in the presence of (a) a chemical reaction in the liquid involving only one of the binary components and, (b) strain-inducing epitaxial conditions on the solid in addition to the chemical reaction in the liquid. These models show the existence of the counter intuitive behavior that epitaxy may promote recrystallization of the solid in spite of the reaction driving dissolution. The conditions for this recrystallization behavior are determined by the value of the epitaxial lattice matching composition, the temperature and the overall composition of the binary solid-liquid system.

GaSb-based >3 µm laser diodes grown with up to 2.4% compressive strain in the quantum wells using strain compensation

We describe the growth of GaSb-based diode lasers with quinary AlInGaAsSb alloy waveguide/barriers and highly strained InGaAsSb type-I quantum wells, designed to emit at ~3.2µm at room temperature. To increase the compressive strain in the QWs above the pseudomorphic limit, we employ strain compensation in which the lower portion of the quinary alloy waveguide is slightly arsenic-rich as compared to the lattice-matched composition. Using this scheme, we fabricated a set of laser devices with incrementally increasing compressive strain in the quantum wells. With compressive strain reaching as high as ~2.4%, the device performance corroborates the notion that adding compressive strain in the quantum wells helps improve hole confinement and suppresses carrier leakage during operation near room temperature.

Analysis of InAl(Ga)N/GaN wet-etching by structural, morphological and electrical methods

Wet etching of InAl(Ga)N/GaN structures has been studied in detail by means of Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy and capacitance–voltage profiling (C–V). The samples used for the study were grown on three different substrates (sapphire, silicon carbide and silicon(111)). Nearly lattice-matched compositions were measured for all the samples. We obtained different etching rate depending on the homogeneity and root-mean-square roughness of the surface as well as the underlying substrate, attributing the difference possibly to the presence of threading dislocation in the sample. The study interest is correlated to the possibility to control at a very precise level the thickness etching of the material, making it possible to fabricate normally-off recessed gate high-electron-mobility-transistors.

Defect structure of epitaxial CrxV1 − x thin films on MgO(001)

Epitaxial thin films of CrxV1−x over the entire composition range were deposited on MgO(001) by molecular beam epitaxy. The films exhibited the expected 45° in-plane rotation with no evidence of phase segregation or spinodal decomposition. Pure Cr, with the largest lattice mismatch to MgO, exhibited full relaxation and cubic lattice parameters. As the lattice mismatch decreased with alloy composition, residual epitaxial strain was observed. For 0.2≤x≤0.4 the films were coherently strained to the substrate with associated tetragonal distortion; near the lattice-matched composition of x=0.33, the films exhibited strain-free pseudomorphic matching to MgO. Unusually, films on the Cr-rich side of the lattice-matched composition exhibited more in-plane compression than expected from the bulk lattice parameters; this result was confirmed with both x-ray diffraction and Rutherford backscattering spectrometry channeling measurements. The effect of thermal expansion mismatch on strain in the heterostructure was estimated. High resolution transmission electron microscopy was utilized to characterize the misfit dislocation network present at the film/MgO interface. Dislocations were found to be present with a non-uniform distribution, which is attributed to the Volmer–Weber growth mode of the films. The CrxV1−x/MgO(001) system can serve as a model system to study the fundamentals of defect formation in bcc films.

Compositional instability in InAlN/GaN lattice-matched epitaxy

The InxAl1−xN/GaN system is found to show compositional instability at the lattice-matched composition (x = 0.18) in epitaxial layers grown by metal organic chemical vapor deposition. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1−xN layer. The V-grooves coalesce at ∼200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. Transmission electron microscopy suggests that the structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.

Raman scattering and cathodoluminescence characterization of near lattice-matched InxAl1−xN epilayers

We present a Raman scattering and cathodoluminescence study of a set of InxAl1−xN/GaN epilayers with InN fractions around the lattice-matched composition. We observed the A1(LO) and InN-like E2 modes of the alloy, whose frequencies are in good agreement with theoretical predictions, but we were unable to detect the AlN-like E2 mode. The InN-like E2 mode did not exhibit noticeable frequency shifts in the studied samples. This is explained by the presence of residual strain in the pseudomorphic InxAl1−xN films. A luminescence peak that shifts to lower energies with an increasing InN fraction was observed at energies above the band edge of the GaN substrate. The cathodoluminescence peak energy is lower than expected, indicating a large band-gap bowing in these alloy layers.

Lattice-matched AlInAs–InGaAs mid-wavelength infrared QWIPs: characteristics and focal plane array performance

The AlInAs/InGaAs material system is promising for mid-wavelength infrared (MWIR) and multi-band quantum well infrared photodetectors (QWIPs) as a lattice-matched alternative to the strained AlGaAs/InGaAs system. In this paper, we report a large format (640 × 512) AlInAs/InGaAs QWIP focal plane array (FPA) with 4.9 µm cut-off wavelength and assess the performance of this material system for MWIR QWIP applications at both pixel and large format FPA level. We also experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by changing only the quantum well (QW) width at the lattice-matched composition. The cut-off wavelength is shifted from 4.15 to 4.9 µm when the QW width is decreased from 30 to 23 Å, in which case a very broad spectral response (Δλ/λp = 32%) with a reasonably high peak detectivity (5.5 × 1010 cm Hz1/2 W−1, f/1.5) is achievable. The AlInAs/InGaAs QWIP FPA yielded excellent sensitivity with a noise equivalent temperature difference as low as 22 mK and a background limited performance (BLIP) temperature as high as 115 K with f/1.5 aperture and 300 K background. The results clearly demonstrate the potential of this material system for completely lattice-matched dual- or multi-band QWIP FPAs for third-generation thermal imagers.

Solid-solution strengthening in ordered In x Ga1 − x P alloys

Ordered In x Ga1 − x P alloys have been grown on to GaAs(001) substrates by metal–organic vapour-phase epitaxy. Lattice-matched compositions of alloys (In x Ga1 − x P with x ≈ 0.5) were used in order to produce epitaxial layers free of structural defects (threading dislocations). Growth temperature and substrate orientation were adjusted to control the degree η, of order, of the alloys in the range 0.3–0.5. Nanoindentation tests were carried out to measure the mechanical response of the heteroepitaxial layers. Transmission electron microscopy was used to characterize the structure as well as the plastic zones generated into the specimen by nanoindentation. Comparison of the alloys with binary references (InP and GaP bare substrates) showed strengthening, with the hardness and flow-stress values that are much larger than those of InP and of the same order of magnitude as those of GaP.

Intersubband absorption saturation in InGaAs-AlAs-AlAsSb coupled quantum wells

This paper gives saturation (switching) energy (I/sub S/) estimates in an all-optic switch material based on absorption saturation in InGaAs-AlAs-AlAsSb coupled-double quantum wells (cDQWs). A model based on density matrix theory for a 4-level system is used to simulate the pulsed excitation conditions of the experiment in the communication wavelength region. The theoretical estimates are compared with experimentally determined values. A comparison of the I/sub S/ in two different cDQWs (one with an indium composition in the well of about 53% and without an AlAs stopping layer at the well-barrier interface and another with 72% indium and an AlAs stopping layer) clearly shows that the samples with high indium content are of a better quality compared to those with lattice-matched indium composition (53%). An order-of-magnitude reduction in the I/sub S/ in an all-optic switch based on cDQWs with high indium content is reported.

Ferromagnetic semiconductor (In,Ga,Mn)As with Curie temperature above 100 K

We have grown (InyGa1−y)1−xMnxAs ferromagnetic semiconductor layers with Mn composition of x up to 0.13 on InP substrates by molecular beam epitaxy. Near the lattice-matched composition, i.e., y∼0.53, the Curie temperature increases linearly with the ferromagnetically effective Mn composition xeff, following the empirical equation TC=1300×xeff. We obtained Curie temperatures above 100 K when x is relatively high (x>0.1; xeff⩾0.08) and the hole concentration is of the order of 1019 cm−3.

Non-linear solid solution strengthening of InGaAs alloy

Solid solution strengthening of III–V semiconductors is one way to reduce dislocation density after crystal growth and to improve the mechanical behavior [1, 2]. Because of their minimal influence on the electrical behavior, isoelectronic dopings (atoms from columns III or V) have been used. Indium doping was reported to improve considerably GaAs mechanical strength [3, 4]. Nonetheless, the hardening mechanism is not yet well understood and depends on the temperature as well as on the doping type and concentration. The following hardening mechanisms have been mentioned: the electrical interaction between impurity clouds and dislocations, the enhanced bonding between the dopant and the Ga or As atoms, the effect of the size of the impurity, the stacking fault energy reduction which induces larger dislocation dissociation (Suzuki effect) and hardens their cross-slip [4]. Nonetheless, only a small range of impurity concentrations has been investigated as the dopants are usually added in concentrations less than 1019 cm−3. In this letter, we report measurements made on a InGaAs alloy with concentrations of In and Ga of the same order. We have compared the mechanical behaviors of this alloy with the behaviors of the binary compositions (InAs and GaAs). The latter were bulk materials. InGaAs alloys were as epitaxial layers. Therefore, we have used the nanoindentation test under low forces that is suited to the micron scale of the layers [5–7]. The plastic zone structure and size generated by nanoindentation were analyzed by transmission electron microscopy (TEM). This approach lets us get a better understanding of the alloy strengthening. 〈001〉 oriented Czochalski-grown single-crystals of undoped InAs and undoped GaAs were used as binary references. Heteroepitaxial InGaAs thick layers (thickness 9 μm) were grown on a (001) surface of InP by chemical beam epitaxy (CBE at 500◦C). The composition of the layer was Inx Ga1−x As with x = 0.516. This composition is very close to the lattice-matched composition (x = 0.532). The layer was just elastically relaxed (no dislocations were observed by TEM) and under a small tension (e=−0.107%). The samples were deformed by a Berkovitch diamond pyramid using a ND100 machine [7]. The tests were performed in the force-control mode of the machine. One of the sides of the pyramid was set parallel to one 〈110〉 crystallographic direction. Indentation arrays were made for subsequent relocalization in the TEM samples. Large indentations were used as markers. The maximum load was varied between 600 and 10 000 μN. For TEM, the undeformed side of the samples (back side) was mechanically and chemically thinned with a bromine-methanol solution until sufficiently thin to transmit the electron beam. Fig. 1 shows loading and unloading curves obtained for the three compositions under about 4500 μN maximum loads. During the indentor loading, both reversible and non reversible deformation occurred. During unloading, the reversible (elastic) deformation was recovered leaving a plastic deformation with a residual depth hr. InAs is the softest materials (largest hr) and is much softer than GaAs in good agreement with the hardness values reported in the literature (3.3 GPa compared to 7.5 GPa) [8]. However, the hardest material is InGaAs, extrapolation yields a Vickers hardness Hv of 14 GPa (Table I). We conclude that a linear dependence of Hv between InAs and GaAs compositions is not valid. In contrast, a much stronger solid solution hardening is obtained in the alloy. Sometimes, a discontinuity in the loading curve (popin) was observed in InAs and GaAs using light loads [9, 10]. Such pop-ins have been observed in several kinds of single-crystal materials [7, 11, 12]. This phenomenon has been attributed to the very poor defect density prior to the indenting test so that the onset of

Phase diagram calculation for epitaxial growth of GaInAs on InP considering the surface, interfacial and strain energies

In order to know the effects of the surface, interfacial and strain energies on the calculation of the phase diagram, these energies were calculated for the Ga x In 1− x As/InP structure and the Ga–In–As ternary phase diagram for the epitaxial growth of GaInAs on (1 1 1) InP was determined. The layer-thickness dependence of the liquidus temperature and solidus composition was determined. It was found that the liquidus and solidus phases were strongly influenced by these energies when the layer thickness was thinner than about 0.06 μm. The consideration of the effects of the surface, interfacial and strain energies is effective to explain the peculiar behavior of the experimental results near the lattice-matched composition ( x=0.47), which is called the latching effect.

Antimonide-based devices for thermophotovoltaic applications

Thermophotovoltaic (TPV) devices have been fabricated using ternary and quaternary layers grown by metalorganic vapor phase epitaxy (MOVPE) on GaSb substrates. GaInSb ternary devices were grown with buffer layers to accommodate the lattice mismatch, and GaInAsSb quaternary devices were grown with lattice-matched compositions. Improved devices are obtained when optical absorption occurs in the p-layer due to the longer minority carrier diffusion length. Thick emitter p/n devices are limited by surface recombination, with highest quantum efficiency and lowest dark current being achieved with epitaxially grown surface passivation layers on lattice-matched MOVPE quaternaries. Thin emitter/thick base, n/p devices are very promising since surface passivation is less critical than for p-emitter devices.

Photoluminescence decay measurements of ZnS xSe 1− x (0 < x < 0.12) epilayers on GaAs substrate grown by molecular beam epitaxy

The steady-state and time-resolved photoluminescence (PL) studies of ZnS x Se 1− x epilayers on GaAs substrate grown by molecular beam epitaxy around the lattice matching composition (0 < x < 0.12) are discussed. We have investigated the PL decay dynamics of ZnS x Se 1− x epilayers and found that the decay time of the ZnS x Se 1− x epilayer with sulfur composition closely lattice-matched with the substrate is longer than that of any other sample. This finding is interpreted as indicating that the defects induced by lattice mismatch act as nonradiative recombination centers and consequently reduce the PL lifetimes of the epilayers. These studies suggest that the lattice mismatch has a strong correlation with PL lifetimes of the ZnS x Se 1− x epilayers.

Optical study of good quality InGaP/GaAs quantum wells: Influence of the indium content around the lattice-matched composition

High structural and optical quality InxGa1−xP/GaAs quantum wells, with x from 0.51 to 0.45, have been successfully grown by atomic layer molecular beam epitaxy. In that compositional range, an important blue shift of the quantum well luminescence lines is observed, which is explained by an increase of the conduction band gap offset from compressive to tensile strain conditions. The luminescence intensity decreases with temperature above 20–30 K, which is attributed to impurities located at the interfaces and inside the quantum wells. The influence of the In content on the oscillator strength of the optical transitions is also evaluated.

A composition-dependent model for the complex dielectric function of In1 xGaxAsyP1− y lattice-matched to InP

Accurate and numerically efficient models for the complex dielectric function vs wavelength and material characteristics are essential for the use of nondestructive optical techniques such as spectroscopic ellipsometry or reflectometry. These optical techniques are commonly used for real-time and run-to-run monitoring and control of growth and etch processes to determine a material's composition and thickness. In this work, we discuss an improved composition-dependent model for the complex dielectric function for lattice-matched In1-xGaxAsyP1-y/InP systems valid over the entire composition range 0 ≤ y ≤ 1. We describe our model, which is an extension of the critical point parabolic band method and is based on the model proposed by Charles Kim et al. for the AxGa1-x/GaAs system. We demonstrate the quality of the model in fitting optical data for individual compositions and compare our results to other established models including the harmonic oscillator approximation and the model of Adachi. Using results obtained from the individual fits, we generate a composition-dependent model that is valid for the entire range of lattice-matched compositions. Also, we show how this model can be used to accurately determine the composition (±0.01) of an unknown material whose dielectric response has been obtained using spectroscopic ellipsometry or a similar technique.

Low-band-gap Ga0.5In0.5P grown on (511)B GaAs substrates

The band gap and microstructure of Ga0.5In0.5P have been shown to vary with deposition conditions. However, growth on (511)B GaAs substrates has been reported to give Ga0.5In0.5P with band gaps close to that of disordered material. It is shown here, that with appropriate selection of the growth parameters, Ga0.5In0.5P can be grown with low band gap and significant ordering on even the (511)B substrates, implying that surface steps play an important role in the ordering process. For the lattice-matched composition, a band gap of 1.83 eV was obtained using low growth temperature (575 °C), low growth rate (0.55 μm/h), and high phosphine pressure (5 Torr).

Growth of ZnS and ZnCdSSe alloys on GaP using an elemental sulfur source by molecular beam epitaxy

MBE growth of ZnS and ZnS-based alloys using an elemental sulfur source is reported. The initial growth process has been investigated by reflection high-energy electron diffraction, and crystalline quality is evaluated by X-ray rocking curve measurement. It is shown that crystalline quality is greatly improved by using ZnS buffer grown at a high substrate temperature. However, the quality of ZnSSe with nearly lattice-matched composition is still not perfect. The results are discussed in terms of relationship between growth process and crystalline quality.