Ultrathin Ga Research Articles

Epitaxial growth of ultrathin gallium films on Cd(0001)

Growth and electronic properties of ultrathin Ga films on Cd(0001) are investigated by low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that Ga films exhibit the epitaxial growth with the pseudomorphic 1 × 1 lattice. The Ga islands deposited at 100 K show a ramified shape due to the suppressed edge diffusion and corner crossing. Furthermore, the majority of Ga islands reveal flat tops and a preferred height of three atomic layers, indicating the electronic growth at low temperature. Annealing to room temperature leads to not only the growth mode transition from electronic growth to conventional Stranski–Krastanov growth, but also the shape transition from ramified islands to smooth compact islands. Scanning tunneling spectroscopy (STS) measurements reveal that the Ga monolayer exhibits metallic behavior. DFT calculations indicate that all the interfacial Ga atoms occupy the energetically favorable hcp-hollow sites of the substrate. The charge density difference analysis demonstrates that the charge transfer from the Cd substrate to the Ga atoms is negligible, and there is weak interaction between Ga atoms and the Cd substrate. These results shall shed important light on fabrication of ultrathin Ga films on metal substrates with novel physical properties.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal.

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D β-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.

The structure and superconductivity of ultrathin Ga films on GaN substrate: A first-principles calculations

A free-standing ultrathin Ga film with hexagonal symmetry is unstable due to its incompatibility with the chemical aromatic rule. Recently, two or three monolayers Ga films have been grown on GaN substrate and exhibit superconductivity [Zhang et al., Phys. Rev. Lett. 114, 107003 (2015) and Xing et al. Science 350, 542 (2015)]. To elucidate the role of the GaN substrate, the properties of slab consisting of ultrathin Ga films and GaN substrate are calculated from first-principles. The results show that the stable interface structure is ultrathin Ga films deposited on N-terminated GaN substrate. Covalent and metallic bonding coexist in the deposited ultrathin Ga films and this peculiar feature contributes to their stability. For two monolayers Ga film deposited on GaN substrate, the obtained electron-phonon coupling λ is about 1.18 and the estimated superconducting temperature (5.1 K) is in excellent agreement with the experimental value, suggesting that it is a conventional BCS-type two-dimensional superconductor. For the deposited three monolayer Ga film, our calculations of the phonon structures show that it is dynamically unstable. While for one monolayer Ga film deposited on GaN substrate, due to the deposited metallic Ga layer with partial covalent character and meanwhile having the behavior of phonon softening, the obtained electron-phonon coupling λ (2.31) and the estimated superconducting transition temperature (9.4 K) are much larger than the case of the deposited two monolayer Ga film.

Nanoscale superconductivity of γ-Ga islands grown by molecular beam epitaxy

We report on the preparation and superconductivity of metastable γ-Ga islands on Si (111) substrate. The Ga grows in a typical Volmer-Weber mode at a low temperature of 110 K, resulting in formation of Ga nanoislands at various sizes with the identical γ-phase. In-situ low temperature scanning tunneling spectra reveal quantized electronic states in ultrathin Ga islands. It is found that both the lateral size and thickness of the Ga islands strongly suppress the superconductivity. Due to substantial surface energy contribution, the transition temperature T c scales inversely with the island thickness and the minimum thickness for the occurrence of superconductivity is calculated to be two monolayers.

Annealing effect on magnetic anisotropy in ultrathin (Ga,Mn)As

We investigated the effect of low temperature annealing on magnetic anisotropy in 7-nm ultrathin Ga0.94Mn0.06As devices by measuring the angle-dependent planar Hall resistance (PHR). Obvious hysteresis loops were observed during the magnetization reversal through the clockwise and counterclockwise rotations under low magnetic fields (below 1000 Gs, 1 Gs = 10−4 T), which can be explained by competition between Zeeman energy and magnetic anisotropic energy. It is found that the uniaxial anisotropy is dominant in the whole measured ferromagnetic range for both the as-grown ultrathin Ga0.94Mn0.06As and the annealed one. The cubic anisotropy changes more than the uniaxial anisotropy in the measured temperature ranges after annealing. This gives a useful way to tune the magnetic anisotropy of ultrathin (Ga,Mn)As devices.

Enhanced Curie temperature and nonvolatile switching of ferromagnetism in ultrathin (Ga,Mn)As channels

The integration of ferroelectric polymer gates on a Mn-doped GaAs magnetic channel provides a promising route for the persistent field-effect control of magnetic properties in high-quality diluted magnetic semiconductors (DMSs) that are otherwise incompatible with traditional oxide ferroelectrics. That control demands the thinnest possible DMS layers, for which to date the Curie temperature (${T}_{C}$) is severely depressed. Here we show that reducing the channel thickness from 7 to 3--4 nm by etching, followed by a brief 135 \ifmmode^\circ\else\textdegree\fi{}C anneal, does not degrade the ${T}_{C}$ (\ensuremath{\sim}70 K) of the 7-nm film. The channel thinning results in a dramatic threefold increase of the ${T}_{C}$ shift controlled by the ferroelectric polarization reversal. Furthermore, we obtain the same exponent $(\ensuremath{\partial}\mathrm{ln} {T}_{C}/\ensuremath{\partial}\mathrm{ln} R)\ensuremath{\equiv}\ensuremath{\gamma}\ensuremath{\approx}\ensuremath{-}0.3$ for all channels with different thicknesses, regardless of the technique used for ${T}_{C}$ determination. These results suggest that the ferromagnetic coupling in an ultrathin 3-nm channel is far from the two-dimensional limit and shows a rather bulklike behavior, similar to well-established 7-nm films.

Magnitude and sign control of lithography-induced uniaxial anisotropy in ultra-thin (Ga,Mn)As wires

We were able to control the magnitude and sign of the uniaxial anisotropy in 5-nm-thin (Ga,Mn)As wires by changing the crystallographic direction of the lithography-induced strain relaxation. The 1-μm-wide (Ga,Mn)As wires, oriented in [110] and [11¯0] directions, were fabricated using electron beam lithography. Their magnetic anisotropies were studied by a coherent rotation method at temperatures between 4.5 and 75 K. Depending on the orientation of the wire, the additional uniaxial anisotropy observed along the axis of the 1-μm-wide samples either increased or decreased the total uniaxial anisotropy.

Crystallization behaviors of an ultra-thin Ga–Sb film

Upon decreasing ultra-thin Ga16Sb84 films from 10 to 3 nm, the exponential increase in crystallization temperature and electrical resistance ratio are attributed to increased specific interface-energies and inhomogeneous interfacial strain at the interfaces. The specific interface-energies and strain also stabilize the Sb(Ga) phase and suppress phase-separation of the GaSb phase.

Modification of structure and optical property of ZnO nanowires by Ga ion beam

AbstractBased on the focused ion beam (FIB) technology, we have prepared ZnO nanowires containing periodic nano-sized structures by an ultra thin Ga ion beam. ZnO nanowires can keep a good crystal quality after Ga ion bombardment. The cathodoluminescence (CL) spectroscopy study of the Ga-doped ZnO nanowires at low temperatures shows that the Ga doping effect can largely suppress the green emission that may mainly originate from the defects on the surfaces of ZnO nanowires.

The origin and control of the sources of AMR in (Ga,Mn)As devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analysed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5 nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [A.W. Rushforth, et al., Phys. Rev. Lett. 99 (2007) 147207] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of the spin–orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.

AMR and magnetometry studies of ultra thin GaMnAs films

We have measured the Anisotropic Magnetoresistance (AMR) of ultra thin (5nm) Ga(0.95)Mn(0.05)As films. We find that the sign of the AMR can be positive or negative, which may depend on the direction of the current with respect to the crystal. At low temperatures, transport measurements and SQUID magnetometry suggest that the magnetisation has a component pointing out of the plane of the film.

In situ analysis of Ga-ultrathin films by TOF-LEIS

In the paper the ability of TOF-LEIS to monitor the growth of ultrathin Ga layers in situ is presented. The FWHM of the Ga peaks in the TOF-LEIS spectra showed a linear dependence on Ga coverage. The analysis of the Ga growth on two Si(111) substrates cleaned in two distinct ways (chemical etching and UHV thermal flashing) revealed changes in the Si peak evolution caused by different growth modes taking place on these two substrates. This has been proved by ex situ AFM measurements as well.

Defect characteristics of ZnO film grown on [formula omitted] sapphire with an ultrathin gallium wetting layer

The defect characteristics of ZnO film grown on ( 0 0 0 1 ) sapphire substrate using an ultrathin Ga wetting layer are investigated by transmission electron microscopy and X-ray diffractometry compared to that of the ZnO film without Ga. It was found that the defects of the ZnO film with a Ga layer were prominently reduced and a high-quality film was formed. Within this ZnO film, most defects near the interface are mixed-type dislocations that interact strongly, leading to a remarkable reduction of dislocations in the upper part of the epitaxial film with a total dislocation density of as low as 8×10 8 cm −2. Almost no pure screw dislocations were observed. Furthermore, the film exhibits a single domain structure, and no inversed domains were found. The role of the ultrathin gallium layer in the defect reduction and inversion domain suppression is discussed.

Polarity determination of ZnO thin films by electron holography

The polarity of the ZnO film grown on sapphire using an ultrathin Ga wetting layer has been investigated by electron holography. Spontaneous polarization of the ZnO film leads to localized charges in the surface, which change the potential distribution in the vacuum side of the film. The potential distribution depends on the nature of the bounded charges and change as a function of the distance from the film surface. By studying the dependence of the potential change on the distance from the film surface, the ZnO film with very thin Ga wetting layer is determined to have the [0001] polarity.

Determination of the polarity of ZnO thin films by electron energy-loss spectroscopy

The polarity of the ZnO film grown on sapphire using an ultra-thin Ga wetting layer has been investigated by electron energy-loss spectroscopy (EELS). The intensity of the oxygen K-edge in electron energy-loss spectrum from the ZnO film shows a prominent difference when the film orientation changes from the (0002) Bragg condition to the (000 2 ̄ ) Bragg condition. The EELS study reveals that the ZnO film with very thin Ga wetting layer has the [0001] polarity, which is further confirmed by the conventional convergent beam electron diffraction method.

Magnetization of ultrathin (Ga,Mn)As layers

Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin $({\mathrm{Ga}}_{0.95}{\mathrm{Mn}}_{0.05})\mathrm{As}$ layers. The thinner layers (below 250 \AA{}) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 \AA{}) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.

Growth of ultra-thin Ga and Ga 2O 3 films on Ni(1 0 0)

The growth of ultra-thin films of Ga 2O 3 on Ni(1 0 0) was investigated in the temperature range of 300–800 K by using Auger electron spectroscopy, low energy electron diffraction (LEED) and scanning tunneling microscopy. In addition, the growth of Ga at 300 K was also studied. For the formation of Ga 2O 3, first at 300 K, a 15 Å thick Ga layer was deposited on the Ni(1 0 0) surface. Oxygen adsorption until saturation leads to the formation of a thin amorphous Ga oxide on the top of a metallic Ga interlayer. Annealing up to 700 K leads to the formation of a well-ordered thin film of γ ′-Ga 2O 3 which is accompanied by a coalescence and ordering of the Ga 2O 3 islands. Large terraces are found which are separated by step heights of 2 Å. The LEED pattern shows a 12-fold ring structure, which originates from two domains with hexagonal structure, which are rotated by 90° with respect to each other. The lattice constant of the hexagonal unit cells is determined to be 2.8 Å.

Vibrational properties of ultrathin Ga 2O 3 films grown on Ni(1 0 0)

Growth and vibrational properties of ultrathin films of Ga 2O 3 on Ni(1 0 0) were investigated in the temperature range of 80–1200 K by using electron energy loss spectroscopy (EELS), Auger electron spectroscopy, and low energy electron diffraction (LEED). At 80 K, a 30 Å thick Ga layer was deposited on the c(2×2)-oxygen structure prepared on Ni(1 0 0). Afterwards, the Ga layer was oxidized with oxygen until saturation. At 80 K, both the Ga layer and the oxidized Ga layer are amorphous. After annealing to room temperature a energy loss at ∼640 cm −1 occurs. This loss is assigned to a vibrational excitation of Ga–O bonds in the Ga-oxide structure. Further annealing up to 700 K leads to the characteristic Fuchs–Kliever phonons of Ga 2O 3 at 305, 470 and 745 cm −1 in the EEL spectrum and the LEED pattern of Ga oxide shows a weak diffuse ring structure of domains with long-range order but random orientation with respect to the substrate. The lattice constant is determined to be 2.8 Å which corresponds to the distance between two O 2− ions in the oxide lattice.

Influence of the exchange reaction on the electronic structure of GaN/Al junctions

Ab-initio calculations have been used to study the influence of the interface morphology and, notably, of the exchange reaction on the electronic properties of Al/GaN (100) interfaces. In particular, the effects of interface structure (i.e. interfacial bond lengths, semiconductor surface polarity, and reacted intralayers) on the SBH at the Al/GaN (001) junction are specifically addressed. The electronic structure of the following atomic configurations have been investigated theoretically: (i) an abrupt, relaxed GaN/Al interface; (ii) an interface which has undergone one monolayer of exchange reaction; and interfaces with a monolayer-thick interlayer of (iii) AlN and (iv) Ga$_{0.5}$Al$_{0.5}$N. Intermixed interfaces are found to pin the interface Fermi level at a position not significantly different from that of an abrupt interface. Our calculations also show that the interface band line--up is not strongly dependent on the interface morphology changes studied. The p-type SBH is reduced by less than 0.1 eV if the GaN surface is Ga-terminated compared to the N-terminated one. Moreover, we show that both an ultrathin Ga$_x$Al$_{1-x}$N ($x$ = 0, 0.5) intralayer and a Ga$\leftrightarrow$Al atomic swap at the interface does not significantly affect the Schottky barrier height.

Composition changes in Ga xIn 1− xAs/InP superlattice growth by chemical beam epitaxy

The variation of compositions in ultra-thin Ga x In 1− x As/InP layers grown by chemical beam epitaxy (CBE) was investigated. The Ga atomic content of the grown Ga x In 1− x As was evaluated by an X-ray diffraction method. By comparing with theoretical simulation, we found a lack of Ga content in very thin layer growth. The reduction of Ga content ranged from 0.47 to 0.33 for 4-monolayer superlattice. Our result implies that there are some transition layers at the beginning of Ga x In 1− x As growth which should be eliminated.