1 Versa tile strain relief pathways in epitaxial films of 001 -oriented PbSe on III-V substrates Brian B. Haidet1 Jarod Meyer2 Pooja Reddy2 Eamonn T. Hughes1 and Kunal Mukherjee2

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Versatile strain relief pathways in epitaxial films of (001)-oriented PbSe on III-V substrates

Brian B. Haidet1, Jarod Meyer2, Pooja Reddy2, Eamonn T. Hughes1, and Kunal Mukherjee2

1 Materials Department, University of California Santa Barbara, Santa Barbara, CA, 93106, USA

2 Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA

ABSTRACT

PbSe and related IV-VI rocksalt-structure semiconductors have important electronic properties that may be controlled by

epitaxial strain and interfaces, thus harnessed in an emerging class of IV-VI/III-V heterostructures. The synthesis of such

heterostructures and understanding mechanisms for strain-relief is central to achieving this goal. We show that a range of

interfacial defects mediate lattice mismatch in (001)-oriented epitaxial thin films of PbSe with III-V templates of GaAs, InAs,

and GaSb. While the primary slip system {100}<110> for dislocation glide in PbSe is well-studied for its facile glide properties,

it is inactive in (001)-oriented films used in our work. Yet, we obtain nearly relaxed PbSe films in the three heteroepitaxial

systems studied with interfaces ranging from incoherent without localized misfit dislocations on 8.3% mismatched GaAs, a

mixture of semi-coherent and incoherent patches on 1.5% mismatched InAs, to nearly coherent on 0.8% mismatched GaSb.

The semi-coherent portions of the interfaces to InAs form by 60° misfit dislocations gliding on higher order {111}<110> slip

systems. On the more closely lattice-matched GaSb, arrays of 90° (edge) misfit dislocations form via a climb process. The

diversity of strain-relaxation mechanisms accessible to PbSe makes it a rich system for heteroepitaxial integration with III-V

substrates.

I. INTRODUCTION

IV-VI rocksalt narrow bandgap semiconductors have long

been important materials in infrared optoelectronics [1–3] and

thermoelectrics, [4,5] and more recently have received

attention as topological crystalline insulators [6,7] and for

spin qubits [8] as a part of the broader class of quantum

materials. Part of the attraction of IV-VI materials for infrared

optoelectronics comes from an uncommon mix [9] between

metallic, covalent, and ionic bonding, or perhaps even a

distinct bonding character, [10] that brings about novel

properties such as high static dielectric constants, high

refractive indices, and low Auger recombination. [3,11–14]

Due to the unique bonding and low growth temperatures

necessary for epitaxy, semiconductors like PbSe are quite

different from predominately covalently bonded zincblende

III-V semiconductors like InAs despite similar bandgaps.

Device fabrication with single crystal PbSe

traditionally requires epitaxy on expensive and fragile native

IV-VI substrates or hygroscopic and electrically insulating

BaF2 substrates, both also poor conductors of heat. [15] These

issues with substrate scalability and properties have renewed

interest in epitaxial integration of PbSe and related IV-VI

alloys with commercially available III-V, II-VI, Si, and Ge

substrates. [16–21] IV-VI/III-V heterostructures not only

provide a potential path to epitaxial films on large area

substrates and materials already used in optoelectronics, but

also provide new ways to manipulate and control electronic

properties using strain and charge at the

heterointerface. [18,22] Fabricating a class of devices that

harness the combined properties of IV-VI and III-V materials

requires an in-depth understanding of the defects that form

due to the lattice constant, thermal expansion, crystal

kunalm@stanford.edu

structure, and bonding mismatch between these two classes of

materials.

Dislocations are the most common defect for

accommodating lattice and thermal expansion mismatch

strain in epitaxial systems, but dislocations in PbSe behave

differently than dislocations in other rocksalt materials like

NaCl due to a significantly higher lattice polarizability. [23]

Most notably, the primary slip system in PbSe is {100}<110>

instead of the more typical {110}<110> slip system of NaCl,

with important implications for epitaxy. [24] An

overwhelming majority of IV-VI heteroepitaxial growth has

been geared towards (111)-oriented films prepared on (111)-

oriented cubic substrates like BaF2 or Si where the (100) glide

planes are inclined and capable of relieving mismatch strain.

Films of reasonable quality may be achieved with both lattice

mismatch and thermal-expansion mismatch via (111)-

oriented growth. On the other hand, the primary slip system

feels no resolved shear for in-plane strain on a (001)-oriented

film. Hence, the most technologically significant orientation

in III-V and SiGe epitaxy is also the orientation that has no

conventional way to relax thermal-expansion-mismatch or

lattice-mismatch strain in IV-VI films. Thermal-expansion

mismatch is particularly difficult to manage in the IV-VI/III-

V (001)-oriented heteroepitaxial system (e.g. αPbSe=19 ppm/K

and αGaAs=5.8 ppm/K at 300K) and the post-growth cooldown

ultimately leads to cracking in thick PbSe. [25]

We still wish to leverage the high-quality surfaces

and technological relevance that come with thin (001)-

oriented layers on (001) III-V substrates. In this work, we

show how PbSe thin films beyond the critical thickness for

strain relief via dislocation formation but below the critical

thickness for cracking accommodate lattice-mismatch strain

during cube-on-cube epitaxy on (001)-oriented III-V

substrates of GaAs, InAs, and GaSb with starting lattice

constant mismatch at growth temperature of 8.3%, 1.5%, and

0.8%, respectively. We identify unusual defects that mediate

the mismatch between these two crystal structures and

demonstrate that PbSe is a versatile material with secondary

strain relaxation mechanisms for achieving good quality thin

films even when the primary slip system is not active. A more

complete understanding of strain relaxation in (001)-PbSe not

only facilitates integration schemes for thin films with

commercially available substrates, but also potentially

enables new means to tune electronic properties with atypical

interfacial structures.

II. METHODS

The PbSe samples on various III-V templates in this study

were synthesized using solid-source molecular beam epitaxy

(MBE). Figure 1 shows the basic structure of these samples

alongside the lattice-mismatch. (001)-oriented, nominally on-

axis III-V substrates of GaAs, InAs, and GaSb were prepared

prior to PbSe growth in a Veeco Gen III MBE system. The

substrate preparation involved oxide desorption under As or

Sb overpressure, followed by deposition of a homoepitaxial

layer. While we have previously grown PbSe directly on

GaSb(001), the film had multiple oriented nuclei and a

somewhat diffuse heterointerface. [17] Therefore, in this

work an additional 300 nm thick epitaxial InAs0.84Sb0.16 layer

followed with a very thin layer of strained InAs was deposited

on the GaSb substrate as rapidly as possible, at the ternary

deposition conditions, to seal the more reactive Sb species

below a less reactive surface. This allowed us to study strain

relaxation close to the GaSb lattice constant while preserving

an InAs-like surface chemistry that consistently yields purely

(001)-oriented PbSe films. The III-V templates were finally

arsenic-capped and transferred out of vacuum for PbSe

growth. PbSe films of 50–80 nm thickness were deposited on

GaAs, InAs, and InAs/InAsSb/GaSb templates using a Riber

Compact 21 MBE system. After desorbing the arsenic cap,

the III-V templates were exposed to PbSe flux at 400 °C for

20-30 seconds to prepare the surface for subsequent

nucleation and growth of PbSe at 320 °C and a growth rate of

2-3 nm/minute. [17] Only a single compound effusion cell

was used for PbSe, which likely results in Pb-rich n-type thin

film samples. In all cases, RHEED appears streaky across the

nucleation step, but we have previously noted during growth

on GaAs and InAs substrates that the growth mode is still of

the Volmer-Weber island type, just with very flat (001)-

oriented islands. [17]

The in-plane and out-of-plane lattice parameters and

film morphology are determined using coupled 2θ-ω scans,

reciprocal space maps (RSMs), and transverse scans collected

using triple-axis x-ray diffraction on a Panalytical X’Pert

instrument. The transverse scan is like a rocking curve

measurement but uses the monochromator on the detector

side, as opposed to a double-axis scan with a wide-open

detector for a classical rocking curve. [26] In the case of large

and moderate mismatch with GaAs and InAs, we focus

primarily on the film morphology determined by the

transverse scans, the atomic arrangement at the interface, and

dislocation network as much of the starting mismatch strain

is relaxed even for ultrathin films. On the other hand, we use

x-ray reciprocal space maps (RSMs) to more accurately study

strain relaxation in PbSe on the low-mismatch InAsSb/GaSb

template. We note that although PbSe has a larger bulk lattice

parameter than all the substrates studied here, we find the film

tensile strained at room temperature due to a large thermal

expansion mismatch between the PbSe and the III-V

substrate. We can ignore this thermal mismatch in the

analysis of relaxed strain as we assume thermal expansion

strain is unrelaxed during cool down in our thin films.

The atomic arrangement at interface and dislocations

are analyzed further in cross-section using a TFS Talos

scanning transmission electron microscope (STEM)

operating at 200 kV. A high-angle annular dark field

(HAADF) detector was used to collect image sequences at the

PbSe/III-V interface. These sequences were then drift-

corrected and stacked to resolve atomic columns. Individual

atomic column positions were measured by taking 1D line

traces parallel to the growth surface, convolving these traces

with a Gaussian curve representative of a single atomic

column, and locating peaks in the resulting smoothed signal.

Dislocations in the sample on InAsSb/GaSb are additionally

characterized in plan-view using electron channeling contrast

imaging (ECCI) in an Apreo-S scanning electron microscope

(SEM) at 30 kV.

III. RESULTS

III.A. Epitaxy on highly mismatched GaAs substrates

We find that PbSe grows epitaxially on GaAs (001) with a

conventional cube-on-cube orientation despite a severe 8.3%

compressive lattice mismatch at a growth temperature of 320

°C. This agrees with recent work showing (001)-oriented

PbSe films on GaAs with a different nucleation method. [18]

We have shown previously that PbSe nucleates as islands on

the substrate surface that eventually coalesce into a

Fig. 1. Sketch of the three PbSe/III-V epitaxial samples in this study.

Each nucleation surface is arsenic-terminated to facilitate single-

crystal growth, but each surface also has a unique lattice constant.

PbSe layers on GaAs, InAs, and InAsSb-on-GaSb templates range

from 8.3% to 0.8% compressive mismatch.

contiguous film. [17] Prior to coalescence, these PbSe islands

on GaAs are faceted with non-polar low energy {100}

surfaces and edges, and after coalescence very flat (001)

surfaces are typical. Figure 2a shows triple-axis 2θ-ω coupled

scans of the (002) reflection from an 85 nm film of PbSe on

GaAs. Pendellösung fringes indicate a sharp interface. The

out-of-plane lattice parameter derived from this scan suggests

the film is only about 0.1–0.2% tensile strained in-plane,

relieving nearly all the compressive lattice-mismatch strain

during growth. Note the measured strain at room temperature

is complicated by a buildup of tensile strain (< 0.3%) induced

by thermal expansion mismatch between PbSe and GaAs

during cooldown, but regardless, any strain relief mechanism

that occurs during growth accommodates about 8%

mismatch.

Figure 2b shows a transverse scan for this film for the (002),

(004), and (006) reflections of PbSe. Interestingly, the (002)

transverse scan resolves both a narrow and a wide component,

the former barely visible in the (004) reflection and absent in

the (006) reflection. The narrow peak is also absent for all

reflections in the conventional open detector rocking curve

scan. Miceli et al. first discussed such a two-component

reflection in ErAs/GaAs films (coincidentally rocksalt on

zincblende). [27] They propose that the narrow component at

the center is an instrument broadening limited peak that

corresponds to a specular reflection or coherent scattering

from long-range ordering of atoms in the film, while the

second wider peak is the typical diffuse scattering arising

from short-range correlations in a mosaic-structured

film. [28] Later, other groups placed this interpretation on

firmer theoretical grounds on the basis of interfacial misfit

dislocation networks. [29,30] Two-component transverse

scan signatures have since been observed in a range of

epitaxial films, spanning metals to ceramics. [26,31]

The intensity of the coherent scatter component is

predicted to reduce with increasing order (hkl) of the

reflection, also seen in our experiments (and more clearly in

section III.B), but this expectation is for very thin films whose

thickness is on the order of the average misfit dislocation

spacing. [30] Remembering that the favored Burgers vector

of dislocations in PbSe is 𝑎

2〈110〉 (similar to III-V zincblende

materials), a semi-coherent interface with a square network of

edge misfit dislocations to relieve 8% strain would

correspond to a dislocation spacing of approximately 5 nm,

much smaller than the film thickness. More recent modeling,

however, predicts the coherent scatter component can remain

strong in films much thicker than the average misfit

dislocation spacing, but only if the misfit dislocations

themselves are well ordered. [32] Therefore, the fact that we

see the coherent peak even in an 85 nm thick film suggests a

high degree of order or periodicity in the defect structure at

the interface.

Cross-sectional STEM imaging of this film of PbSe

on GaAs (Fig. 3a) shows a columnar morphology evidenced

by numerous vertical features corresponding to either

threading dislocations or low-angle grain boundaries—

defects expected from island nucleation and coalescence. Fig.

3b shows a magnified view of the interface between PbSe and

GaAs, revealing a somewhat ordered defect structure

corresponding to the predicted 5 nm period. Remarkably, the

higher magnification Fig. 3c reveals this defect structure is

unlike a conventional array of misfit dislocations. We find the

periodic structure corresponds to a rippling of the initial PbSe

layer and/or the final layer of the arsenic-terminated

zincblende surface. A Burgers circuit reveals a net

displacement across the interface around each of these ripple

features, fulfilling the strain relaxation function of a

dislocation, yet the atoms are uniformly distributed without

any local strain of a typical dislocation core. The

displacement of 13 atomic columns of arsenic corresponds

Fig. 2. (a) Symmetric scans of the (002) reflection of PbSe thin

films of varying thickness on GaAs substrates, collected in a triple-

axis geometry. The film thickness is obtained via fitting to the

fringing (b) A transverse (triple-axis) rocking curve of the (002),

(004), and (006) reflections showing a classic two-component

peak: an instrument-resolution limited coherent Bragg reflection

above a defect-broadening peak. The intensity of the coherent

reflection reduces drastically upon increasing the magnitude of the

scattering vector.

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1Versatilestrainreliefpathwaysinepitaxialfilmsof(001)-orientedPbSeonIII-VsubstratesBrianB.Haidet1,JarodMeyer2,PoojaReddy2,EamonnT.Hughes1,andKunalMukherjee2*1MaterialsDepartment,UniversityofCaliforniaSantaBarbara,SantaBarbara,CA,93106,USA2DepartmentofMaterialsScienceandEngineering,StanfordUniversity...

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1 Versa tile strain relief pathways in epitaxial films of 001 -oriented PbSe on III-V substrates Brian B. Haidet1 Jarod Meyer2 Pooja Reddy2 Eamonn T. Hughes1 and Kunal Mukherjee2

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