Direct experimental verification of the arsenic-on-aluminum-site antisite donor model at AlAs-like bottom interfaces of AlSb/InAs/AlSb quantum wells, and quantitative attribution of the associated mobility degradation

Tuttle, Kroemer, and English discovered that the low-temperature electron mobility of AlSb/InAs/AlSb quantum wells grown by molecular beam epitaxy depends dramatically on which of the two possible interface bond configurations is grown at the bottom (substrate-side) InAs-on-AlSb interface: high mobilities occur only for InSb-like bottom interfaces, while AlAs-like bottom interfaces degrade mobility and raise the electron sheet density. They postulated that arsenic-on-aluminum-site antisite defects, formed when the aluminum-rich AlSb surface is exposed to arsenic flux during the interface crossover, act as charged donors. The subsequent literature has substantially established the origin of the electrons in nominally undoped wells (surface states plus deep donors in the AlSb barrier, of oxygen-related and/or antisite character) and the growth-parameter control of arsenic incorporation, and band-structure calculations support antisite donors at AlAs-like interfaces. However, the central conjecture has never been directly verified: no published measurement establishes the chemical identity, areal density, or ionization energy of interfacial arsenic-on-aluminum antisites, and the mobility degradation of AlAs-like-bottom wells has never been quantitatively decomposed into charged-antisite versus interface-roughness scattering. The problem asks for that direct identification, the quantitative scattering attribution, and resolution of the residual oxygen-versus-antisite ambiguity of the barrier deep donors.

Background

System and phenomenon. InAs (indium arsenide) sandwiched between AlSb (aluminum antimonide) barriers forms a deep quantum well: the conduction-band offset of about $1.3\ \mathrm{eV}$ confines electrons in the InAs layer into a two-dimensional electron gas (2DEG) even at nominally zero doping. Because InAs and AlSb share neither the cation (In vs. Al) nor the anion (As vs. Sb), the atomic plane at each InAs/AlSb interface can be bonded either In-Sb ("InSb-like") or Al-As ("AlAs-like"). Molecular beam epitaxy (MBE) — layer-by-layer growth from thermal beams of the constituent elements in ultra-high vacuum — selects the bond type through the timing of the group-V (As/Sb) and group-III (In/Al) shutter sequences at each interface. Tuttle, Kroemer, and English studied how this "interface sequencing" affects transport in AlSb/InAs/AlSb wells grown by MBE and found a strong asymmetry: high low-temperature Hall mobilities (between about $1.8\times10^{5}$ and $3.3\times10^{5}\ \mathrm{cm^{2}/Vs}$ in the best wells) occur only when the bottom (substrate-side, InAs-on-AlSb) interface is InSb-like; forcing that interface AlAs-like both degrades mobility and raises the electron sheet density. They postulated that growing the AlSb barrier surface aluminum-rich and then exposing it to As (to form the AlAs-like bond) incorporates As atoms on Al sublattice sites of the AlSb adjacent to the well — antisite defects, by analogy with the EL2 defect in GaAs (an As atom on a Ga site, which acts as a deep double donor). An arsenic atom on an Al site ($\mathrm{As_{Al}}$) likewise carries two excess valence electrons: when ionized it donates charge to the well and scatters the 2DEG through its Coulomb field.

The open questions as left by the source. The 1989 paper and its 1990 full-length companion left the model conjectural: the chemical identity, areal density, and ionization energy of the putative antisites had never been measured; the mobility degradation had not been decomposed into charged-impurity scattering from those defects versus the increased interface roughness that accompanies AlAs-like growth; the donors supplying the 2DEG in the high-mobility wells were known only to be displaced into the AlSb barrier, not identified; and the growth-parameter window controlling antisite formation was uncharacterized.

What later work established. Subsequent literature substantially settled two of these issues. On electron sources: the sheet density of nominally undoped wells depends on top-barrier thickness up to roughly $100\ \mathrm{nm}$ because the surface Fermi level is pinned by surface states about $850\ \mathrm{meV}$ below the AlSb conduction-band edge, so surface donors contribute strongly; independently, bulk MBE AlSb contains deep donors (attributed to oxygen incorporation, reducible to $5.5\times10^{15}\ \mathrm{cm^{-3}}$ by source purification), and transport measurements locate a barrier donor level roughly $170\ \mathrm{meV}$ above the InAs conduction-band minimum, consistent with anion-site antisites that density-functional calculations identify as low-energy intrinsic defects of AlSb. On growth control: Raman scattering (inelastic light scattering by lattice vibrations) shows strong group-V exchange — nominally AlAs interface layers become AlSb$_{1-x}$As$_x$, In-Sb bonds form preferentially, and even the residual As flux that bypasses a closed shutter converts AlSb barriers into dilute AlAsSb; migration-enhanced epitaxy (an MBE mode that alternates the group-III and group-V beams monolayer by monolayer, giving surface cations time to reach low-energy sites) yields high-quality structures of both interface types; atomic-resolution electron microscopy shows spontaneously formed AlAs-type interfaces spread over 2-3 monolayers. Interface-roughness scattering (elastic scattering from well-width fluctuations, characterized by a roughness amplitude of order one monolayer and a lateral correlation length $\Lambda\approx62\ \mathrm{\AA}$) was shown to limit the low-temperature mobility of narrow wells, and atomic-force microscopy confirmed that AlAs-like interfaces roughen the subsequent InAs growth.

What remains unresolved. No published measurement identifies the interfacial antisites themselves. A 2006 transport and optical study of AlAs-like-interface wells was consistent with antisite-As-related interface states but resolved no discrete spectral feature, leaving their energy positions, linewidths, density of states, and spatial distribution undetermined; no paramagnetic-resonance measurement (electron spin resonance of unpaired defect electrons, whose hyperfine spectrum identifies the atom's sublattice site), no local-vibrational-mode spectroscopy (vibrational fingerprints of a substitutional impurity atom at its host lattice site), and no sublattice-resolved imaging of $\mathrm{As_{Al}}$ at InAs/AlSb interfaces exists in the literature. Equally, no work quantitatively decomposes the mobility degradation of AlAs-like-bottom wells into charged-antisite versus roughness scattering, and the oxygen-versus-antisite attribution of the barrier deep donors remains open. These residuals define the open problem.

Problem Statement

Provide direct experimental verification (or refutation) of the arsenic-on-aluminum-site antisite ($\mathrm{As_{Al}}$) donor model for AlAs-like bottom interfaces of AlSb/InAs/AlSb quantum wells grown by molecular beam epitaxy, together with a quantitative account of the associated mobility degradation, specifically: (i) establish the chemical identity, areal density, and ionization energy of the donor defects created when the bottom (substrate-side, InAs-on-AlSb) interface is grown AlAs-like rather than InSb-like (that is, when the interfacial bonding plane is Al-As rather than In-Sb), where the identification is required to discriminate As atoms on the Al sublattice of the AlSb adjacent to the InAs well from the far more abundant isoelectronic As-on-Sb incorporation, from oxygen-related deep donors, and from interface roughness; (ii) quantitatively decompose the observed mobility degradation of AlAs-like-bottom wells into scattering from the ionized antisite charges versus the increased interface roughness known to accompany AlAs-like growth, over the well-width and density range of the original Tuttle, Kroemer & English experiments, with roughness parameters constrained by independent structural measurement; and (iii), as the connected residual of the donor-origin question, resolve the attribution of the remote deep donors that supply the two-dimensional electron gas in high-mobility wells (oxygen-related centers versus anion-site antisites in the AlSb barrier, level roughly $170\ \mathrm{meV}$ above the InAs conduction-band minimum per transport inference). These three deliverables form one objective — establish the microscopic defect origin of interface-sequencing-dependent electron density and mobility in this system — and the identification, the scattering decomposition, and the donor attribution must be mutually consistent. If the $\mathrm{As_{Al}}$ donor model is refuted, the identification instead reports the species that accounts for the interface-sequencing-dependent added charge and the decomposition reports the ionized-antisite channel at its measured (near-zero) weight, under the same consistency condition.

References

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