Lattice-mismatched Epitaxy Research Articles

Relaxed GaP on Si with low threading dislocation density

We demonstrate a two-step procedure for the growth of relaxed GaP on pseudomorphic GaP/Si templates with a threading dislocation density (TDD) of 1.0–1.1 × 106 cm−2. In lattice-mismatched epitaxy, suppressed dislocation nucleation and unimpeded dislocation glide during relaxation are both critical to achieve a low TDD. Our two-step growth process realizes the former by initiating growth with a thin, low growth temperature (Tgrowth) layer and the latter with a subsequent high-Tgrowth layer. In optimizing the low-Tgrowth layer thickness, we find a trade-off where too little thickness does not suppress dislocation nucleation, while too much thickness takes away the advantage of higher dislocation velocity at high-Tgrowth. Dislocation pileups and trenches are identified as heterogeneous features with a very high local TDD that commonly arise during single-step growth of GaP on Si at high-Tgrowth, and two-step growth virtually eliminates their formation. Overall, this work shows that after initiation at low-Tgrowth, subsequent epitaxy can be performed at high-Tgrowth while avoiding rampant dislocation nucleation, as well as formation of trenches and dislocation pileups.

Anomalous tilting in InGaAs graded buffers from dislocation sources at wafer edges

Uniform strain relaxation and defect densities across the entire wafer are important aspects of lattice-mismatched or metamorphic epitaxy of semiconductors. Achieving this requires an understanding of the sources of dislocations that lead to relaxation. We show that wafer edges are a major source of misfit dislocations in InGaAs graded buffers on GaAs substrates. AlInP solar cells fabricated from regions on the wafer affected by edge sources have a markedly lower performance due to a local increase in the threading dislocation density. Spatially resolved x-ray reciprocal space maps and electron backscattered diffraction maps of the InGaAs compositionally graded buffer reveal anomalous epi-layer tilting on 2-inch substrates from these dislocation sources at the wafer edge. The tilting abruptly changes direction partially through the graded buffer growth as the dislocations from the wafer edge appear in the x-ray sampling areas. We speculate that anisotropic misfit dislocation glide in III-V materials in combination with non-uniform stress states during growth biases slip system activity. These experiments demonstrate that lattice-mismatched epitaxy requires additional controls over the growth environment to achieve uniform strain relaxation for wafer scale heterogeneous integration.

An InP-Based Mid-Wave Infrared Up-Converter Utilizing Cascade Carrier Transportation

Semiconductor-based up-conversion infrared (IR) photodetectors have the advantages of pixelless imaging and being free from thermal mismatch between photodetector and read-out integrated circuit. Profited by cascade carrier transportation, cascade IR up-converters (CIUPs) provide a fresh idea to ease the contradiction between dark current and responsivity. For GaAs-based mid-wave IR CIUP, a strained InGaAs/AlGaAs material system is an essential configuration, in spite of the limitation of lattice-mismatched epitaxy. An InGaAs/InAlAs material system lattice-matched to InP substrate is a promising alternative to the strained InGaAs/AlGaAs structures. In this letter, an InP-based mid-wave IR CIUP is demonstrated with a 4.7- $\mu \text{m}$ peak response wavelength and 1.19- $\mu \text{m}$ peak emission wavelength. For the up-conversion system, the dark-current-limited detectivity reaches $1.1\times 10^{11}$ Jones at 78 K and 1.3 V CIUP bias, and the background-limited infrared performance condition is achieved at 107 K with the detectivity of $1.2\times 10^{10}$ Jones.

Conductance slope and curvature coefficient of InGaAs/GaAsSb heterojunctions at varying band alignments and its implication on digital and analog applications

We assess InGaAs/GaAsSb heterojunctions at varying band alignments for applications in both tunnel field effect transistors (TFETs) as well as for nonlinear analog components such as millimeter wave detectors. We use conductance slope measurements as a fundamental figure of merit, as it is not affected by the three-terminal parasitics of subthreshold-slope in a TFET and represents the ideal subthreshold slope intrinsic to the junction in the absence of three-terminal parasitics. We prove that conductance slope/subthreshold slope is not equivalent to curvature coefficient, indicating that it is actually easier to exceed the thermal limit of curvature for analog applications than it is to exceed the subthreshold slope limit for digital applications. In addition, we show that no published heterojunction that exceeds the curvature limit would be capable of exceeding the subthreshold slope limit. We experimentally demonstrate the formation of epitaxial InGaAs/GaAsSb heterojunctions at varying band alignments accomplished using lattice-mismatched epitaxy with graded buffers. We show a dependence of conductance slope on material quality, adding further proof that “steepness” is limited by materials defects and inhomogeneity. We demonstrate that the conductance slope does not depend on temperature for type-II band alignment, adding further proof that TFETs, which show strong temperature dependencies, are dominated by thermal parasitics. Finally, we develop and demonstrate an InGaAs/GaAsSb heterojunction system integrated on an InP platform with a record 76 mV/decade conductance slope and 43 V−1 curvature coefficient near zero-bias, with the capability of up to 60 V−1 curvature coefficient.

Materials genomics of thin film strain relaxation by misfit dislocations

We summarize the development and implementation of a “process simulator” for modeling thin film strain relaxation by injection of misfit dislocations. The process simulator, initially developed for GexSi1−x/Si(100) lattice-mismatched epitaxy, integrates elasticity and dislocation theory with experimental measurements of kinetic parameters describing dislocation nucleation, propagation, and interactions. This enables predictive simulation of the development of misfit dislocation arrays during growth and thermal annealing sequences. Further, in the spirit of the materials genome initiative, we show how once a relatively complete description is built for one materials system, extension to a related system may be implemented using a greatly reduced data set. We illustrate this concept by translation of the simulator for GexSi1−x/Si(100) epitaxy into predictive simulation for the GexSi1−x/Si(110) system (which has quite different dislocation microstructure and kinetics) using greatly reduced data sets for the latter system and incorporating data refinement methods to extract unknown kinetic parameters. This sets the platform for extension of these methods to a broader set of strained layer systems.

Nucleation Sequence of InAs Quantum Dots on Cross-Hatch Patterns

InAs quantum dots (QDs) are grown via molecular beam epitaxy on cross-hatch pattern (CHP) templates that result from lattice-mismatched epitaxy of In(x)Ga(1-x)As on (100)-GaAs substrates. Growth of InAs on low-(x = 0.10) and medium-(x = 0.13) mismatch CHPs with InAs thickness grading from sub- to beyond critical thickness show different stages of QD nucleation that is dictated mainly by surface steps. Tangential surface stress fields arising from the buried network of (110) misfit dislocations (MDs) at the InGaAs/GaAs interface are simulated in two dimensions and found to have a direct correlation to QD height at various locations, implying sequential QD nucleation at the surface intersection of the glide plane of dislocation T-section, cross-hatch intersection, threading dislocation, [1-10] MD line, and [110] MD line, followed by nucleation on the flat areas. Deviations from this nominal sequence is possible due to material anisotropy and are accounted for in the stress calculation by dislocation-specific scaling factors.

Analysis of buried heterointerfacial hydrogen in highly lattice-mismatched epitaxy on silicon

We realized the epitaxial growth of a Sr layer on Si(111) with an atomically abrupt heterointerface – in spite of its large lattice mismatch (12%) with Si – by introducing a monoatomic layer of H on Si. In order to identify the buried H, we carried out a combination analysis involving neutron reflectometry and resonant nuclear reaction of 1H(15N,αγ)12C analysis. We found different neutron reflectivity profiles resulting from a contrast variation between the H and D atoms at the buried heterointerface. Furthermore, the depth γ-ray intensity profiles revealed that the H at the heterointerface acts as an effective buffer layer that enables it to manage the highly mismatched epitaxy on Si.

Optimization of buffer layers for lattice-mismatched epitaxy of Ga xIn 1−xAs/InAs yP 1−y double-heterostructures on InP

We optimize InAs y P 1− y buffer layers and compositional grades for lattice-mismatched heteroepitaxy of Ga x In 1− x As/InAs y P 1− y double-heterostructures on InP. The strains of the active and buffer layers depend on the bulk misfit difference between these layers. The misfit difference is adjusted to eliminate strain in the active layer, thus avoiding misfit dislocations and surface topography that would otherwise form to relieve strain. The optimized structure uses an “overshoot” with respect to the conventional design in the misfit and As composition of the InAs y P 1− y buffer. Nearly optimized heterostructures typically show excellent structural quality and extended minority-carrier lifetimes.

Self-forming silicide/SiGe-based tube structure on Si(001) substrates

Silicide/SiGe-based tube structures have been fabricated onto silicon by precise transformation from two-dimensional structures to three-dimensional objects. By using the strain in a pair of lattice-mismatched epitaxy layers, a method was developed to create the tube structure by their release from a substrate. The tube structures combining semiconductor (SiGe) and metallic silicide (NiSi 2) may find applications in advanced devices.

Exploiting long-range atomic ordering for the investigation of strain relaxation in lattice-mismatched epitaxy: A case study on CuPt B-type ordered (GaIn)P on GaAs

The great potential of exploiting growth-induced long-range atomic ordering for investigating strain relaxation in lattice-mismatched epitaxy is demonstrated by transmission electron microscopy (TEM) experiments performed on a CuPt B-type ordered (GaIn)P layer grown on top of a GaAs substrate. Due to the larger unit cell of the ordered structure compared to the zincblende structure of the disordered alloy some of the 60° misfit dislocations are no longer perfect dislocations, but are connected to antiphase boundaries (APBs) which were produced during the layer growth by dislocation glide. From the contours of these APBs in 〈1 1 0〉 cross-section TEM images, we are able to determine post-growth the layer thicknesses at which the individual 60° misfit dislocations were formed during the layer growth. This information is hardly available from any other technique. Apart from the problem of investigating a sufficiently large number of dislocations by TEM, this allows us to reconstruct the complete path of the strain relaxation. A striking feature revealed by the APBs is the simultaneous formation of many misfit dislocations with identical Burgers vectors at a layer thickness of about 12 times the critical thickness, indicating that dislocation multiplication took place during the layer growth.

Dislocation-independent mobility in lattice-mismatched epitaxy: application to GaN

Lattice-mismatched epitaxy produces a high concentration of dislocations ( N dis) in the interface region, and this region is often highly conductive, due to donor ( N D) decoration of the dislocations. Here we show that a simple postulate, N D= α( N dis/ c), where c is the lattice constant and α a constant of order 1–2, predicts a nearly constant low-temperature mobility, independent of N dis. This prediction is experimentally verified in GaN grown on Al 2O 3, and is also applied to other mismatched systems.

Epitaxy of 3d Metals on Semiconductors

Growth of fcc Mn on GaAs(001), as an example of the lattice-mismatched epitaxy of 3d metals on semiconductors, has been studied using reflection high energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and the high resolution transmission electron microscope (HRTEM). The result shows that the interface structure plays a critical role in the epitaxial growth of 3d metals on semiconductors. A new recipe is proposed to search for more epitaxially grown 3d metal phases.

Passive temperature compensation of uncooled GaInAsP-InP diode lasers using thermal stress

Strain arising from differential thermal expansion may be used to compensate the effects of rising temperature and stabilize the operating properties of diode lasers. We have demonstrated this using a 1.58-/spl mu/m GaInAsP-InP multiquantum-well (MQW) laser mounted on a bimetallic heatsink, obtaining nearly complete stabilization of the modal wavelength and a threshold current equivalent characteristic temperature of 133 K over the temperature range of 20/spl deg/C to 70/spl deg/C. The results are successfully modeled by a gain calculation that considers the effects of strain produced by both lattice-mismatched epitaxy and external uniaxial stress.

Dislocation Interactions And Their Impact On Electrical Properties Of GeSi-Based Heterostructures

AbstractSubstantial improvements in the quality of relaxed, heteroepitaxial GeSi layers have occurred in the past few years due to both the development of compositionally-graded buffer layers and to an improved understanding of strain-relaxation in lattice-mismatched epitaxy, which have led to low dislocation densities within these layers. As a result, high performance GeSi devices have been achieved which has fueled yet greater device design flexibility through the use of relaxed GeSi layers over a wider range of alloy compositions and lattice constants. A complete understanding of the properties of all defects that result from strain-relaxation is essential to realize high performance and reliable devices, especially as the lattice-mismatch strain becomes more severe. While the structural properties of dislocations in relaxed GeSi layers have been studied in detail, the electronic properties of these defects and their consequences on the overall electrical properties of relaxed GeSi layers are not well understood. In this paper, we show that for high quality (low dislocation density) relaxed layers, clusters of intrinsic point defects formed by dislocation interaction, rather than the dislocations themselves, now dominate the deep level spectrum. We further show that these defects reduce the electronic material quality for films grown by UHVCVD at low temperatures (650 °C) in comparison to films grown at temperatures > 800 °C even though all films have similarly low threading dislocation densities. Using DLTS, spreading resistance, EBIC and Hall effect measurements, these defects are shown to introduce a low concentration of hole traps from Ev, + 0.05 to Ev + 0.30 eV that compensate and type convert nominally undoped GeSi layers grown at 650 °C to p-type. Annealing studies indicate that these defects anneal out at ∼ 750 – 800 °C(2w, hich is accompanied by conductivity type reversal to ntype, elimination of compensation and increased diffusion length. These observations, in addition to the precise position of each energy level, closely correlate with earlier results on plasticallydeformed bulk Si, suggesting that these defect complexes are of intrinsic origin. We conclude that strain-relaxation in graded heterostructures causes considerable dislocation interaction which introduces point defect complexes associated with dislocations that, in addition to threading dislocation density, must now be considered as an important factor for achieving high electronic quality, relaxed GeSi layers.

Strain relaxation in lattice-mismatched epitaxy

Growth of lattice mismatched layers epitaxially on top of each other is a challenge for today's semiconductor industry. For strained layer devices, it requires that the strained layers are not relaxed through the formation of misfit dislocations. For other device applications, such as GaAs on Si, it may be desirable that the overlayers are completely relaxed and all the misfit defects are confined to the lower interfaces. This article investigates the theories on strain relaxation mechanisms in face-centered-cubic (f.c.c.) systems. The tensile layers are found to initially relax through the formation of partial dislocations, at a critical thickness about half the thickness predicted by the conventional model based on the formation of perfect misfit dislocations. This poses a stringent condition on the maximum thickness useful for tensile layer devices. We also explain the difficulties involved in preparing thick strained layers with few threading dislocations and microtwins.

Absorption and population inversion in p-type InGaAs strained layers based on intervalence subband transition at FIR frequencies

A tensile strained p-InGaAs/InP heterostructure was employed to investigate the feasibility of THz-lasers experimentally. The internal strain induced by the lattice-mismatched epitaxy splits the degenerate valence bands and provides two separate subbands for laser action in the FIR. Strong absorption associated with the direct free hole transition between the LH- and HH-valence bands has been observed and the characteristics of the absorption spectra is discussed. It is shown that when the applied electric field is above 6 × 10 4 V/cm light holes are in streaming motion and the redistribution of holes could result in population inversion in the split valence subbands.

A Model for Lattice-Mismatched Epitaxy: A Continuum View

To date, primarily only idealized equilibrium models for the growth mode and strain relaxation of elastically strained overlayers have been proposed. Here we present a general continuum model for lattice-mismatched epitaxy. As molecular beam epitaxy is inherently a nonequilibrium growth process, surface diffusion kinetics is incorporated in the model. Additionally, a new strain relaxation mechanism in a dislocation-free film is considered. Experimental support for our view is obtained from measurements made by reflection high energy electron diffraction, scanning tunneling microscopy, and transmission electron microscopy on the growth of InGaAs on GaAs(100). These results demonstrate the strong effects which strain, surface diffusion kinetics, and surface energy have on growth mode. From analytical and numerical analysis in 1 + 1 dimensions, the interrelationship of such physical factors is revealed. Our improved understanding enables control over the growth behavior of strained-layer superlattices and heterostructures.

New approach to the strain relaxation mechanism in lattice-mismatched epitaxy

A new approach is presented for the calculation of the dependence of elastic strain on the thickness of CdTe(100) films on GaAs(100) substrates. The present results are obtained by assuming that the misfit dislocation density is in proportion to the strain. It is found that our model fits experimental results quite well. The model suggests that abrupt relaxation occurs at the interface between the film and substrate. The misfit dislocation density is calculated as a function of distance from the interface. Applications of our model to InAs/GaAs(100) and GaAs/Si(100) systems are also discussed.

Effect of coherent strain on hydrogenic acceptor levels in InyGa1−yAs/AlxGa1−xAs quantum well structures

The biaxial strain produced in lattice-mismatched epitaxy can have a substantial effect on the valence band structure. Theoretical results are presented for a hydrogenic acceptor in a quantum well under tensile and compressive strain. The acceptor level energy is a strong function of strain and could be used as a signature for the effect of strain on the valence band structure. Experimental studies are carried out on compressively strained InyGa1−yAs/ AlxGa1−xAs quantum well structures and the acceptor level energy is determined by photoluminescence measurements. Good agreement is found with the experiments.

Strain Relaxation Mechanisms in Lattice Mismatched Epitaxy

ABSTRACTThe relaxation of strained epitaxial layers by the introduction of misfit dislocations is reviewed. Current theoretical and experimental understanding of the nucleation, propagation and interaction of misfit dislocations are summarized. The ramifications for applicability of strained layer epitaxy to practical device structures are discussed.