InGaP Surface Research Articles

Transport and electronic structure properties of MBE grown Sn doped Ga2O3 homo-epitaxial films

In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1 × 1016 to 2.8 × 1019 cm−3. The UID film with an electron density of 2.1 × 1016 cm−3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.

Gallium phosphide conformal film growth on in-situ tri-TBP dry-cleaned InGaP/GaAs using atomic hydrogen ALD

Nearly stoichiometric (high P/Ga ratio and low oxygen content) ∼ 20 nm GaP epi-layers were conformally deposited on the side-walls of InGaP layers using an atomic H ALD process with triethyl gallium (TEGa) and tri-tertiary butyl phosphine (tri-TBP). The key for uniform nucleation is an in-situ tri-TBP dry-clean in 40 mTorr Ar and 160 mTorr H2 without acidic wet-clean to generate a particle-free and damage-free InGaP surface which induces local epitaxy growth of homogeneous GaP thin films. The in-situ tri-TBP dry-clean removed the native oxide of InGaP, surface particles, and the residual impurities simultaneously. It is hypothesized that the key to the process is in-situ formation of PHx(C4H9)y surface adsorbates from atomic H and tri-TBP surface reactions. In-situ AES, cross-sectional STEM image, ex-situ XPS, AFM, and EDS confirmed the formation of high-quality GaP thin films on InGaP/GaAs with a high-quality interface and smooth surface morphology.

Two-dimensional carrier gas at a polar interface without surface band gap states: A first principles perspective

We present first principles calculations of the interface between GaN and strained AlN, using a slab model in which polarization is compensated via surface fractional-charge pseudo-hydrogen atoms. We show that an interface two-dimensional carrier electron or hole gas emerges naturally in response to different compensating surface charges, but that this need not involve in-gap surface states.

Origin of the exotic electronic states in antiferromagnetic NdSb

Using angle resolved photoemission spectroscopy measurements and first principle calculations, we report that the possible unconventional 2q antiferromagnetic (AFM) order in NdSb can induce unusual modulation on its electronic structure. The obvious extra bands observed in the AFM phase of NdSb are well reproduced by theoretical calculations, in which the Fermi-arc-like structures and sharp extra bands are originated from the in-gap surface states. However, they are demonstrated to be topological trivial. By tuning the chemical potential, the AFM phase of NdSb would go through a topological phase transition, realizing a magnetic topological insulator phase. Hence, our study sheds new light on the rare earth monopnictides for searching unusual AFM structure and the potential of intrinsic magnetic topological materials.

Orbital Shift-Induced Boundary Obstructed Topological Materials with a Large Energy Gap.

Boundary obstructed topological phases caused by Wannier orbital shift between ordinary atomic sites are proposed, which, however, cannot be indicated by symmetry eigenvalues at high symmetry momenta (symmetry indicators) in bulk. On the open boundary, Wannier charge centers can shift to different atoms from those in bulk, leading to in‐gap surface states, higher‐order hinge states or corner states. To demonstrate such orbital shift‐induced boundary obstructed topological insulators, eight material candidates are predicted, all of which are overlooked in the present topological databases. Metallic surface states, hinge states, or corner states cover the large bulk energy gap (e.g., more than 1 eV in TlGaTe2) at related boundary, which are ready for experimental detection. Additionally, these materials are also fragile topological insulators with hourglass‐like surface states.

Evolution of the Electronic Structure of Ultrathin MnBi2Te4 Films.

Ultrathin films of intrinsic magnetic topological insulator MnBi2Te4 exhibit fascinating quantum properties such as the quantum anomalous Hall effect and the axion insulator state. In this work, we systematically investigate the evolution of the electronic structure of MnBi2Te4 thin films. With increasing film thickness, the electronic structure changes from an insulator type with a large energy gap to one with in-gap topological surface states, which is, however, still in drastic contrast to the bulk material. By surface doping of alkali-metal atoms, a Rashba split band gradually emerges and hybridizes with topological surface states, which not only reconciles the puzzling difference between the electronic structures of the bulk and thin-film MnBi2Te4 but also provides an interesting platform to establish Rashba ferromagnet that is attractive for (quantum) anomalous Hall effect. Our results provide important insights into the understanding and engineering of the intriguing quantum properties of MnBi2Te4 thin films.

Investigation of Angular Spectrum of Scattered Inert Gas Ions from the InGaP (001) Surface

It has been shown that this method is quite suitable for surface studies and diagnostics of many component materials. The values of the azimuthal angle of distribution of Ne, Ar and Xe ions scattered from InGaP (001) <110> are obtained. The relationship between the spatial variables of the scattered beam (mainly azimuthal angular spectra) and the type of ions has been established. The correlation between focusing properties of surface semichannel with a type of bombardment ion at the different angle of incidence has been shown. 

Analysis of InGaP(001) surface by the low energy ion scattering spectroscopy

Ion scattering spectroscopy, which is a variation of low energy ion scattering (LEIS) that employs glancing scattering angles, is performed on InGaP(001) surfaces. LEIS energy distribution are simulated by computer simulation along the <110> and <ī10> direction, and the match of the positions of the flux peaks shows that the top three atomic layers are bulk-terminated. A newly observed feature are identified as a minimum in the multiple scattering when the ion beam incidence is along a low index direction. Calculated trajectories of scattered ions. This new method for analysis of large-angle LEIS data was shown to be useful for accurately investigating complex surface structures.

Methodological Issues in First-Principle Calculations of CH3NH3PbI3 Perovskite Surfaces: Quantum Confinement and Thermal Motion.

Characterization and control of surfaces and interfaces are critical for photovoltaic and photocatalytic applications. In this work, we propose CH3NH3PbI3 (MAPI) perovskite slab models whose energy levels, free of quantum confinement, explicitly consider the spin–orbit coupling and thermal motion. We detail methodological tools based on the density functional theory that allow achieving these models at an affordable computational cost, and analytical corrections are proposed to correct these effects in other systems. The electronic state energies with respect to the vacuum of the static MAPI surface models, terminated in PbI2 and MAI atomic layers, are in agreement with the experimental data. The PbI2-terminated slab has in-gap surface states, which are independent of the thickness of the slab and also of the orientation of the cation on the surface. The surface states are not useful for alignments in photovoltaic devices, while they could be useful for photocatalytic reactions. The energy levels calculated for the MAI-terminated surface coincide with the widely used values to estimate the MAPI alignment with the charge transport materials, i.e., −5.4 and −3.9 eV for valence band maximum and conduction band minimum, respectively. Our study offers these slab models to provide guidelines for optimal interface engineering.

High-Speed MOVPE Growth of InGaP Solar Cells

Impacts of reactor pressure and growth temperature on the qualities and properties of InGaP epitaxial layers grown in a narrow-channel horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor were investigated with a growth rate of 10 μm/h. The roughness of InGaP was improved remarkably when the pressure was increased from 6 to 10 and 15 kPa. The reason is most likely related to the absolute pressure of P and the migrations of In and Ga adatoms on the wafer surface. Owing to the gas-phase reactions and desorption of P from InGaP, the surface of InGaP became rough, and the pit dislocation was increased with an increase in growth temperature. Although a low temperature was beneficial in order to obtain a smooth InGaP surface, the carrier lifetime in InGaP was shortened as a drawback. Consequently, InGaP n-on-p solar cells grown at a low temperature suffered from the degradations of both short-circuit current density and open-circuit voltage. Background impurities and defect densities in low temperature grown InGaP were attributed to these deteriorations. At the optimized growth temperature of 650 °C and the reactor pressure of 15 kPa, the growth rate of InGaP was accelerated to 20 and 30 μm/h. InGaP solar cells were successfully fabricated with the rate of 30 μm/h that opened up the way for the fabrication of III–V multijunction solar cells in the high-speed MOVPE reactor.

Spectromicroscopic measurement of surface and bulk band structure interplay in a disordered topological insulator

Topological insulators are bulk semiconductors that manifest in-gap massless Dirac surface states due to the topological bulk-boundary correspondence principle [1-3]. These surface states have been a subject of tremendous ongoing interest, due both to their intrinsic properties and to higher order emergence phenomena that can be achieved by manipulating the interface environment [4-11]. Here, angle resolved photoemission (ARPES) spectromicroscopy and supplementary scanning tunneling microscopy (STM) are performed on the model topological insulator Bi2Se3 to investigate the interplay of crystallographic inhomogeneity with the topologically ordered bulk and surface band structure. Quantitative analysis methods are developed to obtain key spectroscopic information in spite of a limited dwell time on each measured point. Band energies are found to vary on the scale of 50 meV across the sample surface, enabling single-sample measurements that are analogous to a multi-sample doping series (termed a "binning series"). Focusing separately on the surface and bulk electrons reveals a nontrivial hybridization-like interplay between fluctuations in the surface and bulk state energetics.

Improvement of Heterointerface Properties of GaAs Solar Cells Grown With InGaP Layers by Hydride Vapor-Phase Epitaxy

It is crucial to develop a low-cost growth system to implement large-scale terrestrial modules in III–V solar cells. Hydride vapor-phase epitaxy (HVPE) has emerged as a low-cost alternative for the fabrication of III–V solar cells. However, it is difficult to utilize HVPE to fabricate abrupt GaAs-on-InGaP heterointerfaces. During growth interruption, indium segregation and exchange between arsenic and phosphorous atoms occur easily on the InGaP surface. The growth sequence is dependent on the reactor configuration. In this study, we investigated the growth sequence in a triple-chamber HVPE system to improve the abruptness of the heterointerfaces of GaAs solar cells. It was found that a short interruption of growth under flowing phosphine is critical in eliminating the InGaAsP interlayer at the interface between the p-InGaP back surface layer and the p-GaAs base layer. This improved cell efficiency to 20.87% for the upright cell and to 22.10% for the inverted front-junction cell.

Topological band engineering of graphene nanoribbons.

Topological insulators are an emerging class of materials that host highly robust in-gap surface or interface states while maintaining an insulating bulk1,2. Most advances in this field have focused on topological insulators and related topological crystalline insulators3 in two dimensions4-6 and three dimensions7-10, but more recent theoretical work has predicted the existence of one-dimensional symmetry-protected topological phases in graphene nanoribbons (GNRs)11. The topological phase of these laterally confined, semiconducting strips of graphene is determined by their width, edge shape and terminating crystallographic unit cell and is characterized by a [Formula: see text] invariant12 (that is, an index of either 0 or 1, indicating two topological classes-similar to quasi-one-dimensional solitonic systems13-16). Interfaces between topologically distinct GNRs characterized by differentvalues of [Formula: see text] are predicted to support half-filled, in-gap localized electronic states that could, in principle, be used as a tool for material engineering11. Here we present the rational design and experimental realization of a topologically engineered GNR superlattice that hosts a one-dimensional array of such states, thus generating otherwise inaccessible electronic structures. This strategy also enables new end states to be engineered directly into the termini of the one-dimensional GNR superlattice. Atomically precise topological GNR superlattices were synthesized from molecular precursors on a gold surface, Au(111), under ultrahigh-vacuum conditions and characterized by low-temperature scanning tunnelling microscopy and spectroscopy. Our experimental results and first-principles calculations reveal that the frontier band structure(the bands bracketing filled and empty states) of these GNR superlattices is defined purely by the coupling between adjacent topological interface states. This manifestation of non-trivial one-dimensional topological phases presents a route to band engineering in one-dimensional materials based on precise control of their electronic topology, and is a promising platform for studies of one-dimensional quantum spin physics.

The role of surface states during photocurrent switching: Intensity modulated photocurrent spectroscopy analysis of BiVO4 photoelectrodes

Intriguing photo-electrochemical characteristics of BiVO4 photoelectrodes studied in pH-neutral aqueous solutions are reported herein. Indeed, we have observed photocurrent polarity switching, as put in evidence by cyclic voltammetry under chopped illumination conditions. Such unusual behavior was analyzed in detail using Intensity Modulated Photocurrent Spectroscopy (IMPS). At potentials where positive photocurrent was observed, the expected shape of IMPS was recorded, starting in quadrant (IV) at high-frequency (HF) and reaching quadrant (I) at low-frequency (LF) with two well defined semicircles. Surprisingly, in the negative photocurrent region, IMPS started in quadrant (II) at HF and ended in quadrant (III) at LF. Such highly infrequent features were interpreted here as the rotation of the IMPS spectra around the origin of the plot, due to the sign switch of the photocurrent. A model that takes into account the existence of in-band energy states at the surface of BiVO4 has been used in order to account for the experimental results. It was found that (i) the surface state capacitance; (ii) the relaxation time constant associated to surface states; and (iii) the density of in-gap surface states, were all showing a well-marked maximum in the nearby value of the switch potential. This suggests that surface states are more influent in the nearby where photocurrent switch occurs.

Observation of the topological surface state in the nonsymmorphic topological insulator KHgSb

Topological insulators represent unusual topological quantum states, typically with gapped bulk band structure but gapless surface Dirac fermions protected by time-reversal symmetry. Recently, a distinct kind of topological insulator resulting from nonsymmorphic crystalline symmetry was proposed in the KHgX ($X=\mathrm{As}$, Sb, Bi) compounds. Unlike regular topological crystalline insulators, the nonsymmorphic glide-reflection symmetry in KHgX guarantees the appearance of an exotic surface fermion with hourglass shape dispersion (where two pairs of branches switch their partners) residing on its (010) side surface, contrasting to the usual two-dimensional Dirac fermion form. Here, by using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of KHgSb on both (001) and (010) surfaces and reveal the unique in-gap surface states on the (010) surface with delicate dispersion consistent with the ``hourglass Fermion'' recently proposed. Our experiment strongly supports that KHgSb is a nonsymmorphic topological crystalline insulator with hourglass fermions, which serves as an important step to the discovery of unique topological quantum materials and exotic fermions protected by nonsymmorphic crystalline symmetry.

Evidence for in-gap surface states on the single phase SmB6(001) surface

Structural and electronic properties of the SmB6(001) single-crystal surface prepared by Ar+ ion sputtering and controlled annealing are investigated by scanning tunneling microscopy. In contrast to the cases of cleaved surfaces, we observe a single phase surface with a non-reconstructed p(1 × 1) lattice on the entire surface at an optimized annealing temperature. The surface is identified as Sm-terminated on the basis of spectroscopic measurements. On a structurally uniform surface, the emergence of the in-gap state, a robust surface state against structural variation, is further confirmed inside a Kondo hybridization gap at 4.4 K by temperature and atomically-resolved spatial dependences of the differential conductance spectrum near the Fermi energy.

High Reactivity of the ZnO(0001) Polar Surface: The Role of Oxygen Adatoms

Understanding the mechanism of water dissociation on metal oxide surfaces is of particular interest in catalytic reactions. In this work, the interaction of water with the ZnO(0001) polar surface is investigated, and the role of oxygen adatoms in water splitting is uncovered. The individual surface energies and electronic properties of ZnO polar surfaces are investigated on the basis of density functional theory calculations. The oxygen adatoms on the ZnO(0001) surface introduce in-gap surface states, resulting in a direct-to-indirect band gap transition. Water strongly interacts with oxygen adatoms to spontaneously form hydroxyl groups, recovering the direct band-gap characteristics of ZnO polar surfaces. Furthermore, water prefers to adsorb at the step edges of cavities after all of the oxygen adatoms are consumed, and the hydrogen-bonded network among water molecules triggers the dissociation of water at the edge sites, which is also confirmed by molecular dynamics calculations. Our results provide ato...

Bulk-boundary correspondence from the intercellular Zak phase

The Zak phase $\gamma$, the generalization of the Berry phase to Bloch wave functions in solids, is often used to characterize inversion-symmetric 1D topological insulators; however, since its value can depend on the choice of real-space origin and unit cell, only the difference between the Zak phase of two regions is believed to be relevant. Here, we show that one can extract an origin-independent part of $\gamma$, the so-called inter-cellular Zak phase $\gamma^{\mathrm{inter}}$, which can be used as a bulk quantity to predict the number of surface modes as follows: a neutral finite 1D tight-binding system has $n_s = \gamma^{\mathrm{inter}}/\pi$ (mod 2) number of in-gap surface modes below the Fermi level if there exists a commensurate bulk unit cell that respects inversion symmetry. We demonstrate this by first verifying that $\pm e\gamma^{\mathrm{inter}}/2\pi$ (mod $e$) is equal to the extra charge accumulation in the surface region for a general translationally invariant 1D insulator, while the remnant part of $\gamma$, the intra-cellular Zak phase $\gamma^{\mathrm{intra}}$, corresponds to the electronic part of the dipole moment of the bulk's unit cell. Second, we show that the extra charge accumulation can be related to the number of surface modes when the unit cell is inversion symmetric. This bulk-boundary correspondence using $\gamma^{\mathrm{inter}}$ reduces to the conventional one using $\gamma$ when the real-space origin is at the inversion center. Our work thereby clarifies the usage of $\gamma$ in the bulk-boundary correspondence. We study several tight binding models to quantitatively check the relation between the extra charge accumulation and the inter-cellular Zak phase as well as the bulk-boundary correspondence using the inter-cellular Zak phase.

Carrier quenching in InGaP/GaAs double heterostructures

Photoluminescence measurements on a series of GaAs double heterostructures demonstrate a rapid quenching of carriers in the GaAs layer at irradiance levels below 0.1 W/cm2 in samples with a GaAs-on-InGaP interface. These results indicate the existence of non-radiative defect centers at or near the GaAs-on-InGaP interface, consistent with previous reports showing the intermixing of In and P when free As impinges on the InGaP surface during growth. At low irradiance, these defect centers can lead to sub-ns carrier lifetimes. The defect centers involved in the rapid carrier quenching can be saturated at higher irradiance levels and allow carrier lifetimes to reach hundreds of nanoseconds. To our knowledge, this is the first report of a nearly three orders of magnitude decrease in carrier lifetime at low irradiance in a simple double heterostructure. Carrier quenching occurs at irradiance levels near the integrated Air Mass Zero (AM0) and Air Mass 1.5 (AM1.5) solar irradiance. Additionally, a lower energy photoluminescence band is observed both at room and cryogenic temperatures. The temperature and time dependence of the lower energy luminescence is consistent with the presence of an unintentional InGaAs or InGaAsP quantum well that forms due to compositional mixing at the GaAs-on-InGaP interface. Our results are of general interest to the photovoltaic community as InGaP is commonly used as a window layer in GaAs based solar cells.

Enhanced Low Bias Performance in Textured BiVO4 Photoanodes through Halide and Phosphate Treatments

Over the last couple of years, BiVO4 has merged as a promising photoanode material owing to its earth abundance, suitable band gap (2.4eV), high absorption coefficient and a reasonable hole diffusion length (100nm)[1]. The intrinsic electron mobility has been shown to be a bottleneck[2] in achieving high photocurrents with a standalone BiVO4 photoanode. Recently photocurrents as high as 5mA/cm2 at OER have been demonstrated using a heterojunction with WO3 rods[3]. This however does not solve the intrinsic electron transport problems of BiVO4 and hence enhancing the photoresponse at relatively low biases close to the flatband potential still remains a challenge and limits its application in a high efficiency tandem device. Here we report photocurrents in excess of 3mA/cm2 at applied biases as low as 0.6V vs RHE with an onset potential of 0.19V vs RHE for sulfite oxidation. A common route to improving the elctronic performance of BiVO4 is via addition of W or Mo as dopants. However, the addition of both W and Mo always results in an anodic shift in the onset potential, limiting the photoresponse at lower biases. In the this study, phosphate-based treatments of the fluorine-doped tin oxide are shown to eliminate the small rectifying junction existing at the back contact with BiVO4. Simultaneously, halide treatments were used to improve electron transport in undoped –BiVO4. Using results from our previous work[4], we have been able to successfully texture 300nm thin films with the previously mentioned phosphate and halide treatments to unity carrier separation at OER and over 80% at 0.6V RHE. Owing to the {004} texturing, these devices also demonstrate a record high catalytic activity for a standalone BiVO4 photoanode towards oxygen evolution. Such high carrier separation was achieved via halide treatment as well as surface texturing which reduces the density of in-gap surface defects states. UPS and XPS measurements were used to ascertain the nature and magnitude of these in-gap states. Impedance spectroscopy shows that the enhanced charge transfer on the surface of the textured photoelectrodes is the origin for the catalytic efficiency of these devices. This study hopes to pave the way towards a rational improvement in the electronic and catalytic properties of standalone undoped- BiVO4 as well as a new approach in designing highly efficient photoanodes for oxygen evolution. [1] Abdi F.F., Savenije T.J., May M.M., Dan B., Krol R., J.Phys.Chem.Lett., 2013,4,2752-2757 [2] Park Y., McDonald K.J., Choi K.S., Chem.Soc. Rev., 2013,422321-2337. [3] Shi X., Choi Y., Zhang K., Kwon J., Kim D.Y., Lee J., Ho L.S., Kim J.K., Park J.H., Nat. Comm., 5, 2014, 4775. [4] Nair V.V., Perkins C.L., Law M., Submitted. Figure 1