Cross-Section Scanning Tunneling Microscopy of InAs/GaSb Superlattices

Cross-Section Scanning Tunneling Microscopy of InAs/GaSb Superlattices Cecile Saguy A. Raanan, E. Alagem and R. Brener Solid State Institute. Technion, Israel Institute of Technology, Haifa 32000.Israel O. Klin, S. Grossman and E. Weiss. SCD-SemiConductor Devices. P.O.Box 2250 Dept. 99. Haifa 31021. Israel

Aim of the work MBE Growth of the InAs/GaSb heterostructures In-situ cleave in UHV STM system Atomic resolution imaging of the SL interfaces: roughness evaluation, atom intermixing, layer quality and thickness Connection between the heterostructure growth conditions, the structural and chemical composition, at atomically scale, of the interface and the optical and electronic properties of the devices.

OUTLINE Material InAs/GaSb heterostructures MBE growth- difficulties Why XSTM is the method of choice to get structural and chemical information at an atomic scale on the SL? Cross Section Scanning Tunneling Microscopy (XSTM) Why doing STM on InAs/GaSb cleaved surface? Cleave of InAs/GaSb heterostructures: Results and Interpretation Conclusion

The 6.1Å family GaSb InAs AlSb Lattice constant 6.10 A 6.06 A 6.14 A M. Razeghi et al., Physics Procedia 3 (2010) 1207 Close lattice match enables the growth of high quality heterostructures with low density of defects on GaSb substrates

Applications of The 6.1Å family Design of opto-electronic devices in the short/ mid/ and long wavelength infrared regimes InAs/GaSb-based heterojunctions are interesting for Mid Wavelength Infrared Regime (MWIR) photon detection.. InAs/AlSb based heterostructures are utilized for high frequency field effect transistors.

InAs/GaSb based superlattices ev Broken gap alignment (CB of InAs is 0.15eV lower than the VB of GaSb). M. Razeghi et al., Physics Procedia 3 (2010) 1207 Excellent wavelength tunability as a function of layer thicknesses. Possibility to achieve effective band gap narrower than that of InAs itself.

Difficulties associated with the Molecular Beam Epitaxy (MBE) growth of InAs/GaSb heterojunctions Both anion (As, Sb) and cation (In, Ga) change at the interface!

Difficulties associated with the Molecular Beam Epitaxy (MBE) growth of InAs/GaSb heterojunctions The interfaces are strained due to different bond lengths of InSb and GaAs. Too fast growth rate will increase interface roughness Too high growth temperature or layer thickness reduction will enhance intermixing of isovalent atoms. Material Bond length Lattice constant GaSb 2.64 A 6.10 A InAs 2.63 A 6.06 A GaAs 2.43A 5.65A InSb 2.81 A 6.48 A

Difficulties associated with the MBE growth of InAs/GaSb SL Interface disorder affects directly the quality of the SLbased devices. Interface roughness reduces the carrier mobility. Atom intermixing at the interface alters the band structure. Need for a method providing precise composition of interface. XSTM was demonstrated to be the method of choice to determine, at atomic resolution level, the composition and roughness of individual layers as well as the interfacial bonding.

Ultra high vacuum Omicron Variable temperature STM/AFM system

Ultra high vacuum Omicron Variable temperature STM/AFM system In-situ cleaver Preparation chamber STM chamber

Scanning Tunneling Microscopy

Tunneling current ( E, x) : s : t T ( E, ev 0 2m 2, x) V exp( 2 z ( x)) Density of sample states Density of tip states T( E, ev, x) V 2m 2 exp( 2 z ( x)) A variation of z of 0.1 nm implies a variation of I of one order of magnitude!

STM scan in constant current mode.

STM at positive and negative bias V>0: The carriers injected from the tip to the unoccupied states in the CB contribute to the current. V>0: The carriers injected from the occupied states of the VB to the tip contribute to the current.

Cleavage Technique 1- Polishing of back side to reduce sample thickness down to 150μm 2-Notch on the surface 3-In situ cleave in a vacuum better than 5x10-11 torrs 4-Transfer to STM chamber for XSTM topography [001] GaSb STM tip SL GaSb SL (100) GaSb SL 35nm

III-V compounds cleave perpendicular to the [001] growth direction exposing the (110) plane a=0.61 Sb Ga

Properties of III-V compound semi-conductor (110) cleavage surfaces No reconstruction The cleavage of the surfaces results in two broken bonds in each surface unit cell, which give rise to two surface states in the band gap. Relaxation of the surface displaces the surface anions outward relative to the surface cations. As a result of the elastic distortion of the surface, the surface states associated with the broken bonds are pushed out the band gap to the valence band (fully occupied states) and the conduction band (totally empty states). The empty states are localized on cations (In, Ga). The occupied states are localized on anions (As, Sb).

Atom-selective imaging At positive bias the image contrast is sensitive to group III atoms (Ga and In). At negative bias the image contrast is sensitive to group V atoms (As and Sb).

Results Results are obtained if: UHV (4x10-11 torrs) Large terraces, few steps on the cleaved surface The SL region (2 μm thick) is found without SEM. Good STM tip to get atomic resolution Anion image V<0

Electronic effects GaSb In As Sb Sb As VB max As sample tip 20nm Anion image V<0

Geometrical effects Anion image V<0 Material Bond length (A) Lattice const. (A) GaSb 2.64 6.10 InAs 2.63 6.06 GaAs 2.43 5.65 InSb 2.81 6.48

% of Sb atoms interchanging As atoms Statistics on the percentage of Sb atoms replacing As atoms in the InAs on GaSb interface 30 25 20 15 growth #1 growth # 2 10 5 0 1 2 3 4 5 As monolayer number from GaSb interface Anion image V<0

(110) Cross section at positive and negative bias Sb As InSb In Ga InSb InAs GaSb InAs GaSb V<0: Anion sublattice [001] V>0: Cation sublattice

Connection between growth sequence, cleavage plane and orientation of InSb bond [1-10] [100]

Connection between cleavage direction and InSb bond orientation: (110) cross section [001]

Connection between cleavage direction and InSb bond orientation: (1-10) cross section

Conclusion In-situ cleavage of III-V compound heterojunctions in UHV STM system was implemented at the Technion. XSTM provides, when performed at negative and positive bias and on both (110) and (1-10) cleaved surfaces a direct identification of the interface bondings at the non-common interface GaSb/InAs with atomic scale resolution. It allows optimization of the growth heterostructures by connecting between the interface structure, interface chemical composition and growth conditions.

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Sb in InSb bonds Sb As GaSb GaSb GaSb InAs InSb In In in InSb-like bonds [100] Superlattice GaSb

Cross incorporation: unintended substitution of one anion species for another during the MBE growth. Exchange: thermodynamically-favored substitution of the prevailing anion species that terminated a given III-V (001) surface reconstruction with another anion species from the vapor. Anion segregation: spatially graded composition along the growth direction in the near interfacial region.

Conclusion GaSb will appear brighter than InAs As substitution in the top layer Sb will appear darker Sb substitution in the top layer As will appears brighter The bright row along the GaSb on InAs heterojunction is associated to InSb like interface bonds. The InSb-like character of InAs on GaSb heterojunction is seen only in (1-10) face. The InSb-like character of GaSb on InAs heterojunction is seen only in (110) face. GaAs will appear lower than InAs (GaAs bond is less ionic than InAs one) InSb will appear higher than GaSb (InSb is more ionic than GaSb).

Electronic effects VB max GaSb In As Sb As p (ev) S (ev) (p+s)/2 (ev) As -7.91-17.33-12.62 Sb -7.24-14.80-11.02 sample tip Valence band maxima for InAs and GaSb and the dangling bond energy for As and Sb

Connection between growth sequence, cleavage direction and InSb bond orientation Growth direction Growth direction The InSb-like bonds at the heterojunction InAs/GaSb will have inor out- of- plane orientation depending on weather the cleavage face is (110) or (1-10).

Zoom on (110) surface Sb in InSb bonds Sb As GaSb GaSb Sb As Anion sublattice InSb Ga In Cation sublattice In in InSb-like bonds [001]