Epitaxial Semiconductor Research Articles

Photoluminescence emission from a nanofabricated scanning probe tip made of epitaxial germanium

We discuss a new type of nanoscale photon source constituted by a micro-pyramid made of epitaxial semiconductor material (Ge) attached to a silicon cantilever optimized for Atomic Force Microscopy and shaped by means of low-current Focused Ion Beam (FIB) milling. The optical characterization of these tips demonstrates sizeable photoluminescence emission at the direct energy gap of Ge (1550nm). We have checked by independent measurements on the nano-patterned epitaxial material prior to the tip shaping process that the photoluminescence yield is not significantly affected by FIB milling, indicating that the optical properties of the material in this wavelength range are preserved. We envisage an application of this tool as a scanning probe in apertureless scanning near-field optical microscopy.

Multiparametric measurements of epitaxial semiconductor structures with the use of one-dimensional microwave photonic crystals

The possibility of measuring simultaneously the thickness of the substrate of a semiconductor structure and the thickness and conductivity of a highly doped epitaxial layer is shown in the case when the semiconductor layer plays the role of a distortion of the periodicity of a microwave photonic crystal. For measuring the mobility of free charge carriers in the highly doped epitaxial layer, a modified method of microwave magnetoresistance based on solving the inverse problem with the use of frequency dependences of the transmission and reflection coefficients measured under the action of the magnetic field in the its absence is proposed.

First-principles study on the structure stability and doping performance of double layer h-BN/Graphene

Using the firs-principles method based on density functional theory, we study the stability and doping performance of double h-BN/Graphene structure, here the exchange correlation potential is expressed through the local density approximation and the interactions between ions and electrons are described by the projective-augmented wave method. Because double layer h-BN/Graphene represents a kind of epitaxial semiconductor system, which can be applied to tunnel pressure sensor, the research is very meaningful. In order to improve the application of this special double layer structures, we often carry out the dopings of some atoms. Unlike previous research work, in which the dopings of the metals Au, Co, Mn and other atoms were took into account, we now mainly consider the dopings of the active metal atoms, such as the dopings of Li, Na, and K atoms. The band structure, electronic density of states, as well as the charge density and stability are studied on the double h-BN/Graphene structure after alkali metal doping. At the same time, bonding and electronic properties of double h-BN/Graphene are discussed under the different biaxial strain conditions. The results show that for the dopings of Li and K atoms, the structure deformation is very large, and the band structure of double h-BN/Graphene can show a small band gap at the K point in the first Brillouin zone, taking on a linear dispersion relation the same as that of the perfect graphene. We can tune the band gap by applying external strain and dopings of atoms, and find a new level appearing near the Fermi level after doping, which is mainly due to the contribution of N atoms. In addition, there exists charge transfer between Na atom and N and C atoms, and the material is converted into metal. We find obvious charge overlapping in the vicinity of Na atoms, these charge overlaps appearing around the Na and C atoms indicate the existence of covalent bond and this covalent bond also appears around the Na atoms and N atoms. We prove the existence of the chemical bonds by adopting the Bader charge analysis, which suggests that the C atoms in the lower graphene layer obtain 0.11 e and dopant atoms around the three N atoms obtain 0.68 e. We infer that the increasing of Na atom doping can increase the charge transfer, so the method of changing the substrate to increase the graphene layer charge density is very conducive to the application of graphene in electronic devices. Because the double h-BN/Graphene has been successfully synthesized, our calculations provide a theoretical basis for the further development and application of technology. We can expect that Na atom doped double h-BN/Graphene can be well applied to the future electronic devices.

ChemInform Abstract: Epitaxial Growth of Hetero‐Nanostructures Based on Ultrathin Two‐Dimensional Nanosheets

AbstractReview: 79 refs.

Surface stability and the selection rules of substrate orientation for optimal growth of epitaxial II-VI semiconductors

The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds.

Epitaxial growth of hetero-nanostructures based on ultrathin two-dimensional nanosheets.

Ultrathin two-dimensional (2D) nanosheets, such as graphene and MoS2, which are demonstrated to be fundamentally and technologically important in many applications, have emerged as a unique family of nanomaterials in chemistry and material science over the past decade. The single-crystalline nature and ultrathin thickness of these 2D nanosheets make them ideal templates for the epitaxial deposition of nanostructures, which offer many possibilities to engineer microsized 2D p-n hetero-junctions at atomic/nanometer scale. This Perspective aims to provide information on the epitaxial growth of hetero-nanostructures based on ultrathin 2D nanosheets. Various methods for the epitaxial growth of nanostructures based on ultrathin 2D nanosheets or in situ growth of lateral or vertical epitaxial 2D semiconductor hetero-nanostructures are introduced. The advantages of these 2D epitaxial hetero-nanostructures for some applications, such as electronics, optoelectronics, and electrocatalysis, are also presented. On the basis of the current status of 2D epitaxial hetero-nanostructures, the future prospects of this promising area are discussed.

Two-Photon Absorption in Organometallic Bromide Perovskites.

Organometallic trihalide perovskites are solution-processed semiconductors that have made great strides in third-generation thin film light-harvesting and light-emitting optoelectronic devices. Recently, it has been demonstrated that large, high-purity single crystals of these perovskites can be synthesized from the solution phase. These crystals' large dimensions, clean bandgap, and solid-state order have provided us with a suitable medium to observe and quantify two-photon absorption in perovskites. When CH3NH3PbBr3 single crystals are pumped with intense 800 nm light, we observe band-to-band photoluminescence at 572 nm, indicative of two-photon absorption. We report the nonlinear absorption coefficient of CH3NH3PbBr3 perovskites to be 8.6 cm GW(-1) at 800 nm, comparable to epitaxial single-crystal semiconductors of similar bandgap. We have leveraged this nonlinear process to electrically autocorrelate a 100 fs pulsed laser using a two-photon perovskite photodetector. This work demonstrates the viability of organometallic trihalide perovskites as a convenient and low-cost nonlinear absorber for applications in ultrafast photonics.

Microstructural evolution of a recrystallized Fe-implanted InGaAsP/InP heterostructure

Through the recrystallization of an amorphous heterostructure, obtained by MeV Fe ion implantation, we are able to tailor a standard epitaxial semiconductor material, a small gap InGaAsP/InP alloy, for photoconductive terahertz optoelectronics. Here, we report on microstructural changes occurring in the material over a broad range of rapid thermal annealing temperatures, using X-ray diffraction line profile analysis and transmission electron microscopy. Results show a complete amorphous transition of the heterostructure after multiple-energy implantations done at 83 K. Upon thermal annealing, multiple structural layers develop via solid phase epitaxy and solid phase recrystallization. The photoconductive InGaAsP layer becomes polycrystalline and submicron grained, with high crystalline volume fraction and apparent ⟨110⟩ texture. Many grains are elongated and internally faulted, with high densities of planar faults occurring on closed-packed (111) planes. The X-ray diffraction line broadening is anisotropic and evolves with rapid thermal annealing temperatures. At 500 °C, the X-ray coherent domain size estimate of 10 nm is aligned reasonably with electron microscopy made in faulted areas. Above 500 °C, a significant decrease of the planar fault density is detected. We discuss the influence of these microstructural changes happening with recrystallization temperatures on the ultrafast photoconductive response of Fe-implanted InGaAsP/InP.

Three-dimensional fabrication of free-standing epitaxial semiconductor nanostructures obtained by focused ion beam

We target the nanofabrication of free-standing nanostructures made of epitaxial semiconductor material layers of high crystallinity quality and high heterostructure complexity for optical applications at the nanoscale. Here we demonstrate the fabrication method in the case of epitaxial germanium grown on a silicon substrate but the method can be applied to any heterostructure material. The nanostructures are fabricated out of planar epitaxial wafers in the form of pillars with arbitrary section and high aspect ratio by electron-beam lithography and deep reactive-ion etching. The patterned SiGe structures are then released by focused ion-beam milling of the pillar base. In this way, they become free-standing and can be relocated on a suitable substrate by using a nanomanipulator. Microscopic characterizations are ongoing to verify that the high crystal quality typical of epitaxial layers grown on a large-area substrate is preserved throughout the different fabrication steps.

Epitaxial graphene contact electrode for silicon carbide based ultraviolet photodetector

We present the fabrication and characterization of graphene-semiconductor-graphene ultraviolet photodetector based on the rectifying character of Schottky junction at the interface between epitaxial graphene and SiC semiconductor. As-grown single layer epitaxial graphene is interdigitated as transparent conductive electrode to probe photo-generated charge carriers in a semi-insulating 4H-SiC substrate. The fabricated device exhibits the typical current-voltage characteristics of a conventional metal-semiconductor-metal type photodetector with low leakage current. Time-resolved photocurrent measurements suggest an excellent photocurrent reversibility and high response speed of the device. The measurements performed for different illumination wavelengths showed that the sample reveals higher responsivity values when it is exposed to the light with 254 nm wavelength. The obtained results imply that epitaxial graphene can be used readily as transparent conductive electrode for SiC based optoelectronic device applications.

Evolution of epitaxial semiconductor nanodots and nanowires from supersaturated wetting layers.

In this tutorial we review recent progress in the design and growth of epitaxial semiconductor nanostructures in lattice-mismatched material systems. We focus on the Ge on Si model system after pointing out the similarities to III-V and other growth systems qualitatively as well as quantitatively. During material deposition, the first layers of the epitaxial film wet the surface before the formation of strain-driven three-dimensional nanostructures. In particular, we stress that the supersaturation of the wetting layer (WL), whose relevance is often neglected, plays a key role in determining the nucleation and growth of nanodots (NDs), nanodot-molecules and nanowires (NWs). At elevated growth temperatures the Ge reservoir in the planar, supersaturated WL is abruptly consumed and generates NDs with highly homogeneous sizes - a process mainly driven by elastic energy minimization. Furthermore, the careful control of the supersaturated Ge layer allows us to obtain perfectly site-controlled, ordered NDs or ND-molecules on pit-patterned substrates for a broad range of pit-periods. At low growth temperatures subtle interplays between surface energies of dominant crystal facets in the system drive the material transfer from the supersaturated WL into the elongating NWs growing horizontally, dislocation- and catalyst-free on the substrate surface. Due to the similarities in the formation of nanostructures in different epitaxial semiconductor systems we expect that the observation of the novel growth phenomena described in this Tutorial Review for Ge/Si should be relevant for other lattice-mismatched heterostructure systems, too.

Synthesis and characterization of single-crystal Cu(In,Ga)Se2nanowires: high Ga contents and growth behaviour

Precise control over the defect density, a high Ga content, and uniform stoichiometry are critical for controlling the physical and optical properties of Cu(In,Ga)Se2 (CIGS) nanowires (NWs). In this study, we investigated the synthesis of epitaxially grown, single-crystal CIGS NWs by a vapour-phase transport method using multiple sources of Ga2Se3, In2Se3, and Cu2Se as the precursors. No catalysts were employed, and r-cut Al2O3 substrates were used for the fabrication of the NWs. The synthesized CIGS NWs had a uniform composition along their length, and the NWs with the highest Ga/(In + Ga) content ratio (0.8) had a chalcopyrite structure. The bandgap energy of the CIGS NWs was higher than that of typical CIGS thin films grown by co-evaporation methods because of the high Ga content ratio. These single-crystal CIGS NWs offer an attractive platform for exploring various concepts related to hierarchical nanostructures and devices based on fully epitaxial semiconductor structures.

Liquid‐Phase Epitaxial Growth of Two‐Dimensional Semiconductor Hetero‐nanostructures

Although many two-dimensional (2D) hybrid nanostructures are being prepared, the engineering of epitaxial 2D semiconductor hetero-nanostructures in the liquid phase still remains a challenge. The preparation of 2D semiconductor hetero-nanostructures by epitaxial growth of metal sulfide nanocrystals, including CuS, ZnS and Ni3S2, is achieved on ultrathin TiS2 nanosheets by a simple electrochemical approach by using the TiS2 crystal and metal foils. Ultrathin CuS nanoplates that are 50-120 nm in size and have a triangular/hexagonal shape are epitaxially grown on TiS2 nanosheets with perfect epitaxial alignment. ZnS and Ni3S2 nanoplates can be also epitaxially grown on TiS2 nanosheets. As a proof-of-concept application, the obtained 2D CuS-TiS2 composite is used as the anode in a lithium ion battery, which exhibits a high capacity and excellent cycling stability.

Liquid‐Phase Epitaxial Growth of Two‐Dimensional Semiconductor Hetero‐nanostructures

AbstractAlthough many two‐dimensional (2D) hybrid nanostructures are being prepared, the engineering of epitaxial 2D semiconductor hetero‐nanostructures in the liquid phase still remains a challenge. The preparation of 2D semiconductor hetero‐nanostructures by epitaxial growth of metal sulfide nanocrystals, including CuS, ZnS and Ni3S2, is achieved on ultrathin TiS2 nanosheets by a simple electrochemical approach by using the TiS2 crystal and metal foils. Ultrathin CuS nanoplates that are 50–120 nm in size and have a triangular/hexagonal shape are epitaxially grown on TiS2 nanosheets with perfect epitaxial alignment. ZnS and Ni3S2 nanoplates can be also epitaxially grown on TiS2 nanosheets. As a proof‐of‐concept application, the obtained 2D CuS–TiS2 composite is used as the anode in a lithium ion battery, which exhibits a high capacity and excellent cycling stability.

Investigation of the surface elasticity of GaN by atomistic simulations and its application to the elastic relaxation of GaN nanoisland

Surface effect on epitaxial semiconductors has been widely investigated but limited to constant surface excess energy or surface stress so far. This work provides the first report on the complete surface elasticity tensors of wurzite GaN by atomistic simulations. As an application, this result is used in the investigation of the elastic relaxation of GaN nanoisland. It is demonstrated that surface elasticity becomes important to determine the overall property of the nanoisland when its size is lower than 10nm. This could hopefully help the fabrication and design of high quality nanostructures.

High-temperature continuous-wave laser realized in hollow microcavities.

Recently, an urgent requirement of ultraviolet (UV) semiconductor laser with lower cost and higher performance has motivated our intensive research in zinc oxide (ZnO) material owing to its wide direct band gap and large exciton binding energy. Here, we demonstrate for the first time continuous-wave laser in electrically-pumped hollow polygonal microcavities based on epitaxial ZnO/MgO-core/shell nanowall networks structures, and whispering gallery type resonant modes are responsible for the lasing action. The laser diodes exhibit an ultralow threshold current density (0.27 A/cm2), two or three orders of magnitude smaller than other reported UV-light semiconductor laser diodes to our knowledge. More importantly, the continuous-current-driven diode can achieve lasing up to ~430 K, showing a good temperature tolerance. This study indicates that nano-size injection lasers can be made from epitaxial semiconductor microcavities, which is a considerable advance towards the realization of practical UV coherent light sources, facilitating the existing applications and suggesting new potentials.

Photoelectrochemical etching of epitaxial InGaN thin films: self-limited kinetics and nanostructuring

We report here the characteristics of photoelectrochemical (PEC) etching of epitaxial InGaN semiconductor thin films using a narrowband laser with a linewidth less than ∼1nm. In the initial stages of PEC etching, when the thin film is flat, characteristic voltammogram shapes are observed. At low photo-excitation rates, voltammograms are S-shaped, indicating the onset of a voltage-independent rate-limiting process associated with electron-hole-pair creation and/or annihilation. At high photo-excitation rates, voltammograms are superlinear in shape, indicating, for the voltage ranges studied here, a voltage-dependent rate-limiting process associated with surface electrochemical oxidation. As PEC etching proceeds, the thin film becomes rough at the nanoscale, and ultimately the self-limiting etch kinetics lead to an ensemble of nanoparticles. This change in InGaN film volume and morphology leads to a characteristic dependence of PEC etch rate on time: an incubation time, followed by a rise, then a peak, then a slow decay.

Quantum-size-controlled photoelectrochemical fabrication of epitaxial InGaN quantum dots.

We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm).

Multi-spectral imaging with infrared sensitive organic light emitting diode.

Commercially available near-infrared (IR) imagers are fabricated by integrating expensive epitaxial grown III-V compound semiconductor sensors with Si-based readout integrated circuits (ROIC) by indium bump bonding which significantly increases the fabrication costs of these image sensors. Furthermore, these typical III-V compound semiconductors are not sensitive to the visible region and thus cannot be used for multi-spectral (visible to near-IR) sensing. Here, a low cost infrared (IR) imaging camera is demonstrated with a commercially available digital single-lens reflex (DSLR) camera and an IR sensitive organic light emitting diode (IR-OLED). With an IR-OLED, IR images at a wavelength of 1.2 µm are directly converted to visible images which are then recorded in a Si-CMOS DSLR camera. This multi-spectral imaging system is capable of capturing images at wavelengths in the near-infrared as well as visible regions.

Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor.

Superconducting circuits are exceptionally flexible, enabling many different devices from sensors to quantum computers. Separately, epitaxial semiconductor devices such as spin qubits in silicon offer more limited device variation but extraordinary quantum properties for a solid-state system. It might be possible to merge the two approaches, making single-crystal superconducting devices out of a semiconductor by utilizing the latest atomistic fabrication techniques. Here we propose superconducting devices made from precision hole-doped regions within a silicon (or germanium) single crystal. We analyse the properties of this superconducting semiconductor and show that practical superconducting wires, Josephson tunnel junctions or weak links, superconducting quantum interference devices (SQUIDs) and qubits are feasible. This work motivates the pursuit of 'bottom-up' superconductivity for improved or fundamentally different technology and physics.