GaSb Valence Band Research Articles

Hybridization between the ligand p band and Fe−3d orbitals in the p-type ferromagnetic semiconductor (Ga,Fe)Sb

(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature ($T_{\rm C}$) is above room temperature. However, the origin of ferromagnetism with high $T_{\rm C}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb including the Fe-3$d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in (Ga$_{0.95}$,Fe$_{0.05}$)Sb observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The Fe-3$d$ resonant ARPES spectra demonstrate that the Fe-3$d$ IB crosses the Fermi level ($E_{\rm F}$) and hybridizes with the VB of GaSb. These observations indicate that the VB structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb is consistent with that of the IB model which is based on double-exchange interaction between the localized 3$d$ electrons of the magnetic impurities. The results indicate that the ferromagnetism in (Ga,Fe)Sb is formed by the hybridization of the Fe-3$d$ IB with the ligand $p$ band of GaSb.

Proposal of an Intrinsic-Source Broken-Gap Tunnel FET to Reduce Band-Tail Effects on Subthreshold Swing: A Simulation Study

High source doping is essential in tunnel FETs (TFETs) to support high electric fields necessary to provide sufficient drive currents. High doping is associated with band-tails in the density of states that decay exponentially into the bandgap, comparable with the decay of the carrier density above the band edge due to Fermi-tails. This is expected to limit the subthreshold swing (SS) and OFF-current in the TFETs. In this paper, we analyze the effects of band-tails on the subthreshold characteristics of III–V TFETs based on GaSb/InAs heterojunction. It is shown that the decay constant comparable with room-temperature thermal energy, $k_{B}T = 26$ meV, can result in significant degradation of SS in TFETs from their ideal value. Electrically doping the intrinsic source could be an alternative, but not only does it increase the coupling between the gate and source contact due to proximity but also complicates the fabrication process. As a solution, we propose an undoped-source GaSb/InAs broken-gap TFET in an i-i-n configuration, which achieves similar performance to that of a corresponding p-i-n TFET while having exactly the same geometry. In the proposed device, the high drive current is supported by the broken-gap tunnel junction, while the Fermi-level pinning near the valence band of GaSb helps in realizing an ohmic source contact for the carriers in the valence band, which eventually participate in interband tunneling at the broken-gap tunnel junction in the ON-state.

Characterization of interface defects in ALD Al2O3/p-GaSb MOS capacitors using admittance measurements in range from kHz to GHz

Atomic layer deposited (ALD) Al2O3/p-type GaSb Metal–Oxide–Semiconductor (MOS) capacitors are studied with capacitance–voltage (C–V) and conductance–voltage (G–V) measurements using AC signal frequencies covering the range from kHz to GHz. The potential and limitations of the measurements at GHz frequencies for oxide and interface defect characterization are described. The effect of bulk oxide traps in communication with the GaSb valence band via hole tunneling is highlighted. Modeling indicates that the C–V and G–V frequency dispersions observed in the accumulation, flat-band and depletion regions of the Al2O3/p-GaSb MOS capacitors are due to combined contributions of bulk-oxide traps and interface traps.

Band Alignment Measurement of Tunneling Field Effect Transistor Heterojunction

The electrical performance of a tunnel field effect transistor (TFET) depends critically on the band offset at their semiconductor heterojunction. Historically it has been rather difficult to experimentally determine how the electronic band is aligned at the interface. In this talk, I will present and discuss an experimentally unique measurement design to ascertain a complete energy band alignment of the heterojunction using internal photoemission (IPE) spectroscopy. Examples of all three types (I, II, and III) of the band alignment of technologically relevant TFET heterojunction will be presented. Due to small band offsets, IPE usually requires high intensity light sources such as free-electron laser which is not easily accessible. However, it will be shown by using conventional IPE with an experiment procedure that was specifically designed and tailored to enhance sensitivity of electron photoemission from each semiconductor component of the heterojunction over a large band gap insulator, we were able to successfully characterize band alignments of W / Al2O3 / InGaAs / InAs / InP and W / Al2O3 / n+-InAs / p+-Al0.45Ga0.55Sb TFET heterojunction [1]. In both cases, the measurement technique was possible only when the larger band gap semiconductor is on top of the other with the thickness of each layer that was specifically designed to control light absorption and penetration depth. As a results, at the oxide flat-band condition, the barrier height from the top of the InGaAs/InAs valence band and the top of the InP valence band to the bottom of the Al2O3 conduction band is determined to be 3.5 and 2.8 eV, respectively, resulting a 0.7 eV valence band offset at InGaAs / InP interface (Fig. A) . Similarly, for W/ Al2O3 / n+-InAs / p+-Al0.45Ga0.55Sb structure, a 0.4 eV valence band offset at the n+-InAs / p+-Al0.45Ga0.55Sb interface was observed (Fig. B). We have further advanced the method to a more elaborate approach which simultaneously resolves both valence and conduction band offsets at the heterojunction interface without the required layer configuration used in the above case of InGaAs/InAs and InAs/p+ AlGaSb. In the traditional IPE measurement, the hole photocurrent (if present) from semiconductor is negligible compared to the electron photocurrent from the semi-transparent gate (usually a thin metal) due to the much lower quantum yield for hole emission. Recently, we first reported using graphene as transparent electrode to measure the hole photoemission [2]. The two main reasons for graphene as transparent electrode to enhance the hole photocurrent and make it measurable are as follows. First, the high (~97.7%) broad range light transmittance of graphene allows most of the light to reach to the emitter and thus increase external quantum efficiency of the hole emission. Secondly, the availability of large area graphene and its high conductivity make the collection of the emitted carriers more efficient. This new method is demonstrated on a broken gap (type III) InAs / GaSb TFET. The barrier height from the top of InAs and GaSb valence band to the bottom of Al2O3 conduction band is inferred from electron emission whereas hole emissions reveal the barrier height from the top of Al2O3 valence band to the bottom of InAs and GaSb conduction band. As a result, the offset at the broken gap InAs / GaSb interface was uniquely determined and is summarized in Fig. C.This new IPE approach can be extended to determine band alignment of the other technologically relevant heterojunction and 2-dimentional materials as well.

Schottky barrier height reduction for metal/n-GaSb contact by inserting TiO2 interfacial layer with low tunneling resistance

Fermi level pinning near GaSb valence band edge leads to high Schottky barrier height for metal/n-type GaSb contacts. However, this effect can be alleviated by depinning of the Fermi level with the introduction of thin interfacial dielectric. In this paper, the use of TiO2 allows depinning of the Fermi level without introducing excessive tunneling resistance due to the low conduction band offset, estimated by synchrotron radiation photoemission spectroscopy. It is shown the insertion of TiO2 results in reduction in Schottky barrier height and greater than four orders of magnitude increase in current density for metal contacts on n-type GaSb.

Band offsets at the (1 0 0)GaSb/Al 2O 3 interface from internal electron photoemission study

From electron internal photoemission and photoconductivity measurements at the (1 0 0)GaSb/Al 2O 3 interface, the top of the GaSb valence band is found to be 3.05 ± 0.10 eV below the bottom of the Al 2O 3 conduction band. This interface band alignment corresponds to conduction and valence band offsets of 2.3 ± 0.10 eV and 3.05 ± 0.15 eV, respectively, indicating that the valence band in GaSb lies energetically well above the valence band of In x Ga 1− x As (0 ⩽ x ⩽ 0.53) or InP.

Measurement of the GaSb surface band bending potential from the magnetotransport characteristics of GaSb–InAs–AlSb quantum wells

Low-temperature magnetotransport measurements on GaSb∕InAs∕AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, EFS, of undoped molecular-beam-epitaxy-grown GaSb. EFS is pinned around 0.2eV above the top of the GaSb valence band when the GaSb cap layer width is greater than around 900Å. For smaller GaSb cap widths, EFS decreases with the GaSb width. The undoped GaSb∕InAs∕AlSb heterostructure’s Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well.

Imaging of electron standing waves in InAs/GaSb superlattices by low‐temperature scanning tunneling spectroscopy

The local density of states at cleaved cross-sectional surfaces of InAs/GaSb superlattices were investigated by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) with atomic resolution. Broken-gap energy band profiles (the overlap of the conduction band of InAs and the valence band of GaSb) were clearly seen in the LT-STS spectra as a function of growth direction. Clear images of electron standing waves confined in the superlattice were also obtained. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Influence of GaSb Layer Thickness on the Band Gap of InAs/GaSb Type-II Superlattices for Mid-Infrared Detection

ABSTRACTThe effect of small changes in GaSb layer thickness on the photoresponse spectrum of InAs/GaSb superlattices (SLs) designed for mid-infrared detection was systematically investigated. The samples were grown by molecular beam epitaxy with precisely calibrated growth rates. The basic SL used for this study consisted of 40 periods of InAs (20.5 Å)/GaSb (X Å), where the nominal value for X was adjusted from 18 to 27 Å in four different samples. An InSb-like interface (IF) was inserted between the layers to balance the SL strain. By decreasing the GaSb width, the photoresponse cut-off wavelength (λc) was adjusted from 4.03 μm to 4.55 μm, i.e., the SL energy band gap is being decreased. This decrease in the energy separation between the first heavy hole band (HH1) and the first conduction band (C1) as the GaSb layer is narrowed is counter intuitive. However, this experimental trend can be explained by a modified envelope function approximation (EFA) calculation that includes the effect of in-plane asymmetry at InAs/GaSb interfaces. As expected, the HH band is pushed away from the top of the GaSb valence band as the GaSb layer width becomes narrower. However, at the same time the C1 band is significantly broadened by the increased wave function overlap of the electron states in the InAs layer. The trend to smaller band gap with narrower GaSb layers and other effects of the design changes on the photoresponse spectrum are discussed.

Charge-collection dynamics of AlSb-InAs-GaSb resonant interband tunneling diodes (RITDs) [for MOBILE logic circuits

Time-resolved charge-collection measurements performed on AlSb/InAs/GaSb resonant interband tunneling diodes (RITDs) with pulsed laser excitation exhibit complex behavior as a function of the device operating point. A model considering conventional charge-collection principles in combination with transient band-bending effects is proposed to describe the experimental results. In the proposed model, a transient distortion of the band structure of the device (transient band bending) induced by holes trapped in the GaSb valence band well modulates the DC current through the RITD. The manner in which this transient modulation is manifested in the experimental observable depends sensitively on the operating point of the device, giving rise to qualitatively different temporal signatures under different DC bias conditions.

Broken-gap heterojunction in the p-GaSb-n-InAs1−x Sbx(0≤x≤0.18) system

Epitaxial InAs1−x Sbx layers with the Sb content 0≤x≤0.18 were grown by metalorganic vapor phase epitaxy (MOVPE) on p-GaSb and n-InAs substrates. The photoluminescence (PL) spectra of the heterostructures were measured at T=77 K. The experimental PL data were used to study variation of the bandgap as a function of the InAsSb solid solution composition. The energy difference between the GaSb valence band top and the InAs0.82Sb0.18 conduction band bottom was calculated. It was established that GaSb/InAs1− x Sbx with 0≤x≤0.18 represents a broken-gap heterojunction of type II.

A Theory of Charge Transport Due to Electron?Hole Recombination in Type II Semiconductor Quantum Well Devices

Activationless electron-hole generation and recombination in type II semimetallic single and double quantum wells is studied. An 8 x 8 k . p matrix Hamiltonian, and the transfer matrix method, is used to calculate the recombination rates for quantum wells made from InAs and GaSb. This model includes the non-parabolicity and anisotropy of the energy band structure. Because the conduction band of InAs lies beneath the valence band of GaSb, the generation and recombination processes do not require extra excitation particles such as phonons or photons. The effect of externally applied electric and magnetic fields is also included in the calculation of the recombination rates and both n-type and p-type single and double quantum wells are considered.

Exciton wave function, binding energy, and lifetime in InAs/GaSb coupled quantum wells

We studied theoretically excitons in narrow-coupled InAs/GaSb quantum wells where there is not any overlap between the InAs conduction subband and the GaSb valence subband. In this case, excitons do not exist in the equilibrium and the luminescence of pumped excitons can be observed. We calculated the exciton binding energy making use of the variational method. The resulting binding energy is around 4 meV. We also calculated the exciton lifetime for both radiative recombination and nonradiative recombination. Due to the unique band alignment of InAs/GaSb, the recombination can happen via two channels: first, mixing of the conduction band of InAs with the valence band of GaSb and second, electron tunneling from InAs conduction band to GaSb conduction band and hole tunneling from GaSb valence band to InAs valence band. The fastest recombination process is the radiative process in the second channel. The lifetime varies with the well widths from 45 ps for narrow wells to 400 ps for wider wells.

Evidence for electron-hole hybridization in cyclotron-resonance spectra of InAs/GaSb heterostructures

Cyclotron-resonance (CR) experiments have been performed on InAs/GaSb/AlSb double structures, in which electron and hole layers are separated by AlSb barriers. The strength of the electron-hole coupling is altered by varying the barrier width. Our data reveal that properties of InAs/GaSb systems with AlSb barriers thinner than 1.5 nm are entirely determined by interlayer tunneling and the CR features in such samples can be explained by hybridization between states in the InAs conduction band and in the GaSb valence band.

High magnetic field far-infrared spectroscopy of spatially separated electron–hole layers

Cyclotron resonance (CR) experiments have been performed on InAs/GaSb structures, in which InAs (electron) and GaSb (hole) layers are separated by AlSb barriers. Our data reveal that properties of InAs/GaSb systems with AlSb barriers thinner than 1.5 nm are entirely determined by interlayer tunneling and the CR features in such samples can be explained by hybridization between states in the InAs conduction band and in the GaSb valence band. The hybridization gap has been estimated from CR spectra for two samples with different degrees of electron–hole coupling. The CR spectra of widely separated electron–hole layers (with barriers thicker than 2 nm) are characterized by pronounced spin splitting. In addition to multiple spin–split CR lines new lines have been observed at high magnetic fields (11–13 T).

InAs/GaSb interfaces; the problem of boundary conditions

We present here a derivation of phenomenological boundary conditions for envelope functions at the interface between an InAs layer and a GaSb layer. The overlap of the valence band of GaSb with the conduction band of InAs leads to a coupling between electrons with total angular momentum 1/2 and holes with total angular momentum 3/2. The method that we use to match these wave-functions is that of the minimizing of the total energy of the system including the surface energy. We consider the different spinor characters of the states explicitly, and derive the required boundary conditions, which are not dependent on any specific microscopic model. The proposed approach is general, and may be used to obtain boundary conditions for other complicated cases of interfaces.

Semimetal-semiconductor transition in InAs [sbnd]GaSb heterostructures

In a heterostructure consisting of neighboring layers of InAs and GaSb in a large band-gap host like AlSb, the bottom of the InAs conduction band lies ∼150meV below the top of the GaSb valence band. However, because the electron and hole subbands are weakly coupled for non-zero values of the wavevector k ∥ along the layer, an anticrossing of the levels occurs at k ∥ = k c, where k 2 c = (2μ/ħ 2) (Δ−ε v 0−ε c 0). Here Δ is the band overlap, ε c 0 and ε v 0 are the lowest electron and hole subband energies, and μ is the reduced mass. The splitting is in the order of 5–10 meV. For an intrinsic material, the Fermi level lies in this gap, so that no semimetallic state occurs. If the InAs layers are sufficiently wide, or if an electric field can be applied across the heterostructure (by fabricating front and back gates) it is possible to have the first excited electron subband ε c 1 realize an anticrossing with ε v 0 as well. Then a true semimetallic state can occur with a circle of electrons in the ε c 1 subband near k ∥ = 0, and a ring of holes near the anticrossing in the hybridized valence subband. A semimetal-to-semiconductor transition should occur, which is driven by shifting the single-particle subband energies with an applied electric or magnetic field.

Magnetotransport measurements on InAs-GaSb quantum wells with the application of hydrostatic pressure

We present a series of magnetotransport measurements on high mobility single InAs-GaSb semimetallic quantum wells using hydrostatic pressures up to 10 kbar and temperatures down to ~100mK. Clearly defined, hole-related peaks in pϱ xx close to integer electronic filling factors in the semimetallic regime, have a stronger temperature dependence than the electron Shubnikov-de Haas effect at millikelvin temperatures and with this additional structure, apparent in ϱ xx the quantum Hall plateaux in ϱ xy deviate from h ve 2 , where v is the filling factor. The decrease in the electron concentration, dn 0 dp is 3.0 × 10 10 cm −2 kbar −1 in the dark, and 4.0 × 10 10 cm −2kbar −1 in the dark following infrared illumination to reduce the Fermi energy. We relate this reduction in the electron and hole concentrations to the uncrossing of the InAs conduction band and the GaSb valence band and to the role played by a GaSb surface donor at high pressures. The fermi energy at the surface is pinned approximately 250 meV above the GaSb valence band edge.

High pressure studies of negative differential resistance in InAs/GaSb resonant conduction heterostructures

A single heterojunction of n-InAs/p-GaSb behaves as a two dimensional electron gas (2DEG) back to back with a two dimensional hole gas (2DHG), due to overlap between the GaSb valence band and the InAs conduction band. An unusual feature is the possibility of growing a ‘ light’ GaAs or a ‘ heavy’ InSb interface. The two interface types are predicted to give rise to different electronic and vibrational properties, while the existence of monolayer steps or interdiffusion could give rise to a mixed interface character. In this paper we demonstrate that vertical transport measurements as a function of pressure are a sensitive way of probing the electronic structure of the interface. On biasing InAs negative with respect to GaSb, electron transfer takes place from the 2DEG to the 2DHG, and the current initially increases with bias. At a critical bias, quantum confinement raises the energy of the 2DEG above that of the 2DHG and a region of Negative Differential Resistance (NDR) then occurs. The size of the NDR feature and the critical bias are both reduced by hydrostatic pressure, showing that the band overlap decreases. For a number of samples grown with a heavy interface by atmospheric pressure metalorganic vapour phase epitaxy the NDR vanishes at a threshold, P c ~ 12 ± 2 kbar. This threshold is consistent with a band overlap of 120 ± 20 meV and a shift of −10 meV/kbar. The NDR properties are found to be very sensitive to the detailed sequence of switching of the precursors during interface growth, as well as to the interface type.

Characterization of InAs‐GaSb type II superlattices grown by metal organic chemical vapor deposition

Abstract It was observed that the conduction‐band minimum of InAs is lower in energy than the valence‐band maximum of GaSb. This will cause the non‐rectification characteristic. Such unusual nonrectifying p‐n junction is the direct consequence of interpenetration between the GaSb valence band and the InAs conduction band, since the InAs‐GaSb superlattices can have a band gap smaller than that of either constituent materials. InAs‐GaSb superlattices have the modifiable band gap, but the absorption coefficient shall decrease rapidly when the period thickness is over 100 A(d 1=d 2). It has a good spectral response in 3–5 μm region, but bad in 8–14 μm. The new proposal offered a normal incidence valence intersubband transition at ∼10μm. It is potential to fabricate the long wavelength devices. In this paper, the InAs‐GaSb superlattices were grown by MOCVD. The quality is determined by x‐ray double crystal rocking curves. The period thickness is determined by High Resolution Transmission Electron Microscopy (H...