Session T28: Semiconductors

Interaction-driven versus disorder-driven transport in ultra-dilute GaAs two-dimensional hole systems

It is well-known that the insulating behavior in the two-dimensional metal-to-insulator transition demonstrates a finite temperature conduction via hopping. Recently, however, some very strongly interacting higher purity two-dimensional electron systems at temperatures $T\rightarrow 0$ demonstrate certain nonactivated insulating behaviors that are absent in more disordered systems. Through measuring in dark the $T$-dependence of the conductivity of ultra-high quality 2D holes with charge densities down to $7\times10^{8}$ $cm^{-2}$, an approximate power-law behavior is identified. Moreover, for the lowest charge densities, the exponent exhibits a linearly decreasing density-dependence which suggests an interaction-driven nature. Such an electron state is fragile to even a slight increase of disorder which causes a crossover from nonactivated to activated conduction. The non-activated conduction may well be an universal interaction-driven signature of an electron state of strongly correlated (semiquantum) liquid.   

Abruptness improvement of the interfaces of AlGaN/GaN superlattice by cancelling asymmetric diffusion

Interface abruptness has been an important issue in the construction of quantum wells as active layer in optoelectronic devices, which is extremely crucial in achieving stronger quantum confinement and consequently higher emission efficiency. The interfacial sharpness is highly associated with the crystal structure as well as the elemental transition. However, few studies have been done focusing on the elemental diffusion effect at the interface. In this work, the accurate determination was approached to the elemental inter-diffusion depth across the GaN/Al$_{0.5}$Ga$_{0.5}$N interfaces by using transmission electron microscopy, Auger electron microscopy, and X-ray diffraction. The GaN/Al$_{0.5}$Ga$_{0.5}$N superlattice was grown by metalorganic chemical vapor deposition (MOCVD) at high growth temperature (1070 $^{\circ}$C). The results showed that the Al diffusion at the upper and lower interfaces of Al$_{0.5}$Ga$_{0.5}$N barrier appears an asymmetric behavior, which is 0.62 and 0.99 nm, respectively. Such will lead to the gradient interfacial region and asymmetric quantum well, affecting the carrier quantum confinement. To improve the abruptness of the interface and to modify the asymmetric diffusion, self-compensation pair technique was proposed and introduced to the growth of the lower Al$_{0.5}$Ga$_{0.5}$N/GaN interface, blocking the Al downward diffusion. Fist-principles simulations also showed that the structural relaxation at the strained heterointerface influences the electronic structure as well as elemental diffusion.   

Local Structure of Amorphous GaNAs Alloys Across the Composition Range

Typically only dilute (up to $\sim$10\%) highly mismatched alloys (HMAs) can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. Recently, we overcame the miscibility gap of the GaN$_{1-x}$As$_{x}$ system using low temperature molecular beam epitaxy (LT-MBE) and successfully synthesized alloys over a wide composition range. In the intermediate composition range (0.10 $<$ x $<$ 0.75) the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder. The significant dangling bond density may help explain the difficulty of doping the material.   

Characterization of GaN grown on Si substrate with sputtering AlN buffer layer by molecular beam epitaxy

This study reports the characterization of GaN grown on Si substrate with sputtering AlN buffer layer by plasma-assisted molecular beam epitaxy. Structural properties were measured by X-ray diffraction measurement and transmission electron microscopy. XRD spectrum showed the sputtering buffer layer which was mainly (100)$_{AlN}$ and followed by the GaN epilayer which contained poly \textbf{\textit{M}}-plane GaN and other structural planes such as (002)$_{GaN}$ (\textbf{\textit{c}}-plane), and (101)$_{GaN}$. In TEM analysis, we demonstrated GaN of \textbf{\textit{c}}- and \textbf{\textit{A}}-plane grains. The pure \textbf{\textit{M}}-plane GaN was also found in the form of grain and the high-resolution images showed clear atomic arrangement. Besides, the diffraction pattern with multi \textbf{\textit{M}}-plane GaN was attributed to several \textbf{\textit{M}}-plane GaN crystals which were grown in different orientations on in-plane surface. In addition, optical properties measured by photoluminescence and cathodoluminescence measurement both showed two main peaks at 3.2 eV and 3.4 eV, indicated that zinc-blende and wurtzite structure exist in the GaN layer at the same time.   

Characterization of InGaN/GaN Quantum Well grown on GaN microdisk using $\gamma $-LiAlO$_{2}$ substrate by Plasma-assisted Molecular Beam Epitaxy

The InGaN/GaN quantum wells grown on GaN microdisks by plasma-assisted molecular beam epitaxy (PAMBE) have been investigated. The optical properties and micro-structure of InGaN/GaN quantum wells were studied by Cathodoluminescence (CL) and transmission electron microscope (TEM). According to the observation of high-resolution TEM, we obtained the high quality of InGaN/GaN quantum wells grown on GaN micro-disk. The In-ratio (x) of In$_{x}$Ga$_{1-x}$N is $\sim $8{\%} determined by the measurement of energy-dispersive X-ray spectroscopy (EDX). The optical gap of In$_{0.08}$Ga$_{0.92}$N was measured to be $\sim $3eV determined by CL measurement, which is consistent with the calculation of bowing parameters and the measurement of EDX.   

Unraveling the Atomic Structure of GaN(0001) Pseudo-1$\times $1: Surface-Electron-Gas Mediated Dimer Ordering

Gallium nitride based light emitting devices have seen a steep rise of attention in recent years because of their potential in revolutionizing the current lighting industry. Of great importance in advancing the material technology is the understanding of the GaN(0001) surface, onto which most of the commercial devices are grown. We report a novel dimer-based model for Ga-rich GaN(0001), which exhibits an intriguing reconstruction known as the pseudo-1$\times $1. To unravel its atomic structure, we have cooled the surface to cryogenic temperatures while monitoring the reconstruction by reflection high-energy electron diffraction and scanning tunneling microscopy. Upon cooling, the pseudo-1$\times $1 phase transforms into a new phase consisting of buckled Ga-dimers on the surface. This observation then strongly suggests the existence of Ga dimers also at the room-temperature surface, although only partially ordered compared to the low temperature case. Combined with first-principles calculations, we propose a new model for the pseudo-1$\times $1 consisting of dimers that are spaced $\sim $1.9 nm (6a) apart along $<$11\underline {2}0$>$ azimuths, while being randomly distributed along other directions. The mechanism of this partial ordering is identified to be through the surface electron gas residing within the bulk band-gap.   

The electrical properties of epitaxial p-type ZnO films grown on GaAs (001)

Wurtzite ZnO epitaxial layers with both p-type and n-type characteristics are grown on n-type GaAs(001) substrates by pulsed laser deposition (PLD). The local electrical properties of the ZnO layers were investigated by electrostatic force microscope (EFM), Kevin force microscopy (KFM), and conducting atomic force microscopy (CAFM). Local work function difference of $\sim $125.5 meV was observed on p-type ZnO layer from KFM and EFM measurements due to the un-uniform diffusion of As atoms from the GaAs substrate upon thermal annealing. We also found the work function of p-type ZnO is larger than that of n-type ZnO layer, in which the difference varied from 368.4 to 493 meV. The rectifying junction under p-ZnO/n-GaAs configuration and Ohmic contact under metal probe (Ti)/p-ZnO configuration were both observed by CAFM.   

Mapping the band profile across the Gd$_{2}$O$_{3}$/GaAs(100) hetero-interface by using scanning tunneling microscopy

Direct measurements of atomic-scale electronic structure at nm-thick epitaxial Gd$_{2}$O$_{3}$ gate oxides on GaAs have been performed using cross-sectional scanning tunneling microscopy and spectroscopy. Both scanning tunneling spectroscopy and analysis of the local electronic states across the gate oxides suggest the Ga-O terminated hetero-interface. In addition, along with the theoretical modeling, the band offsets for both conduction and valence states are identified. A unique combination of STM and STS successfully provides direct information on the interfacial band profile and band offsets across the model high-$\kappa$/III-V system in the work.   

Phase-dependence and manipulation of coherent oscillations of a single-electron wavefunction in a dynamic quantum dot

Surface acoustic waves (SAWs) in a GaAs heterostructure generate dynamic quantum dots, each capable of carrying a single electron through a gated potential landscape. At the SAW velocity (~2800 m/s), the change in potential due to a 100nm surface gate will occur in a period of 40ps in the rest frame of the dot. This high-speed modulation of the potential, far beyond the experimental limit of fast gate-switching, allows for the observation of coherent single-electron dynamics. Previous work has shown that an abrupt shift in dot confinement will cause an electron to oscillate unitarily from side to side. This excitation was measured non-invasively via a tunnel barrier, and good agreement was found between measurements and simulations of the dot dynamics. We present here the results of further work in which we examine the coherence length and phase-dependence of the single-electron oscillations. Through the use of a time-dependent model we also study surface-gate arrangements which may be used to manipulate the electron dynamics mid-stream.   

Equi-spin-splitting distribution near the minimum-spin-splitting surface under biaxial strains in bulk wurtzite materials

The spin-splitting energies in biaxially strained bulk wurtzite materials are calculated using the linear combination of atomic orbital method, and the equi-spin-splitting distributions in $k$-space near the minimum-spin-splitting (MSS) surfaces are illustrated. These data are compared with those derived analytically using the two-band \textbf{k} $\mathbf{\cdot}$ \textbf{Dp} (2KP) model. It is found the results from these two methods are in good agreement for small $k$. However, the ellipsoidal MSS surface under compressively biaxial strain predicted by the 2KP model does not exist, due to the data points are far from the $\Gamma $ point in this case. As a result, from compressively to tensilely biaxial strain, only three types of shapes of the MSS surface exist in the wurtzite Brillouin zone; that is, a hyperboloid of two sheets, a hexagonal cone and a hyperboloid of one sheet.   

How Hydrogen Terminated Diamond Acquires a Negative Electron Affinity Surface

Electron emission from the negative electron affinity (NEA) surface of hydrogen terminated, boron doped diamond in the [100] orientation is investigated using angle resolved photoemission spectroscopy (ARPES). ARPES measurements using 16 eV synchrotron and 6 eV laser light are compared and found to show a catastrophic failure of the sudden approximation. While the high energy photoemission is found to yield little information regarding the NEA, low energy laser ARPES reveals for the first time that the NEA results from a novel Franck-Condon mechanism coupling electrons in the conduction band to the vacuum. The result opens the door to the development of a new class of NEA electron emitter based on this effect.   

Laser Phase Separation of Si Rich Oxides: The Role of Composition

Continuous-wave laser annealing of Si-rich oxide thin films with varying Si content were performed in order to obtain Si nanocrystals (Si$_{nc}$) embedded in silica. The composition, irradiation times and power densities were investigated as well as the role of hydrogen in phase separation. Si$_{nc}$ in SiO$_{2}$ appear to be very promising for the realization of optical function as light emission or optical memory. Nanocrystaline Si finds also important utility in photovoltaics thanks to quantum confinement in the nanostructures offering a wider bandgap material which, in a tandem configuration, can allow a better use of the solar spectrum. Conventional techniques utilize high-temperature processing to obtain Si-SiO$_{2}$ phase separation. These processes are not compatible with mass production methods. An alternative approach capable of avoiding high temperature processing is the laser annealing of SiO$_{x}$ films. The structural effect due to annealing were investigated by Raman and photoluminescence spectroscopy. It has been shown that the size and amount of Si$_{nc}$ depends both on the oxygen content and on the laser power density. PECVD grown hydrogenated SiO$_{x}$ films were compared with sputtered films without hydrogen to identify its role for the phase separation.   

A Novel Technique for Detecting Trace Residues of Contamination on GaAs Surfaces before MBE Growth

To prepare a GaAs substrate for molecular beam epitaxial (MBE) growth, the nominal $\sim $ 3 nm native oxide is typically thermally desorbed in vacuum. To test the completeness of this desorption, we describe a technique, which combines MBE, thermal desorption, atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS), for detecting trace residues of contamination on (100) GaAs surfaces before MBE growth. At all desorption temperatures in the range 600 \r{ }C to 665 \r{ }C, our RHEED measurements show that the native oxide is largely desorbed within 2 min. However, the SIMS and AFM data indicate that a residue of sub-monolayer oxide invariably remains on the GaAs (100) surface, and tenaciously resists all further attempts at its removal by thermal desorption. Since thermal desorption of the native oxide has long been the standard technique before MBE growth, we suggest that all MBE growth of GaAs heterostructures has been through a partial monolayer of native oxide. We believe that this is the likely reason for the failure of high quality attempts at MBE growth of GaAs after lithographic patterning on a previously grown MBE structure.   

High-performance substrate-emitting quantum cascade ring lasers

An InP based mid-infrared quantum cascade laser with a ring-shaped cavity is demonstrated in room temperature continuous wave operation. The light is coupled out with a second order distributed feedback grating buried inside the cavity. The device is epilayer-down bonded to a heat spreader and the light is emitted through the substrate. The emission wavelength is around 4.85 $\mu $m with a high power output of 0.51 W. Single mode operation persists up to 0.4 W. Far field exhibits concentric ring features. Modal behavior is analyzed using the coupled mode theory, which suggests that the device operates in an extremely high order mode. Polarization measurement indicates that the beam is azimuthally polarized. This unique polarization state with high power output may find applications in tight focusing, optical tweezers, and material processing.   

3D Simulation of the Growth of Alloy Semiconductor Quantum Dots Considering Morphological and Compositional Coupling

Fabrication of quantum dots (QDs) with high density may be realized by self-assembly via heteroepitaxial growth of thin films. Since the electronic and optoelectronic properties of QDs are sensitive to size, morphology, strain and especially composition, it is of great importance to control their composition profiles and morphology, and engineer the strain in them. Since the growth is a dynamic process, which carries out via surface diffusion driven primarily by strain relaxation and entropy change due to chemical intermixing, a strong coupling between morphological and composition evolutions during this process leads to a rather complex dynamics, which has not been fully understood. In this work, a 3-D finite element model is developed, which is capable of modeling the formation, self-assembly and coarsening of hetero-epitaxial alloy islands by considering the coupling of morphological and compositional evolution. Several interesting experimental observations, such as fast coarsening kinetics; asymmetries in composition profile and island shape; lateral motion of alloy islands have been observed in our simulations. Our model predictions have painted a rather complete picture for the entire dynamic evolution during the growth of nanoscale heteroepitaxial islands.