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The initial stages of the nucleation of cubic (c-) GaN in heterophase epitaxy on a Si v-groove are investigated. Growth of GaN on a nanoscale {111}-faceted v-groove fabricated into a Si(001) substrate proceeds in the hexagonal (h-) phase that induces a secondary v-groove replicating the substrate topography with two opposing {0001} facets. The secondary v-groove is then orientationally mismatched at the junction of the h-GaN facets (h -h junction) resulting in structural instability. This instability is relieved either by the formation of voids that reduce the actual junction area or by the transition to c-phase (h-c transition) suppressing further extension of the h-h junction. The distribution of voids that is locally affected by the island growth mode of h-GaN on Si(111) and the imperfection in the groove geometry impacts the initial stage of heterophase epitaxy. Primarily, The h-c transition is observed as a non-local phenomenon; it occurs homogeneously and simultaneously along the bottom of the entire secondary groove and forms a one-dimensional (1D) seed layer except for some interruptions where the h-h junction is defected by gaps or incomplete voids. Between these interruptions, epitaxy retains a single crystal but results in a series of c-GaN nanodots on the seed layer with large fluctuation in size and spacing. The adatom incorporation observed in this heterophase epitaxy is a 1D analog to the wetting of a substrate followed by the self-assembly in conventional quantum dot epitaxy. The surface morphology of the c-GaN nanodots is governed by the faceting mostly composed of (001)- and (11n)-orientations and the roughening between these facets that ultimately affect the morphology of the final top surface of the c-III-N. The interruptions interfere with the homogeneity of the h-c transition and can cause antiphase defects and mosaicity. Based on experimental results, a solution to improve these issues is proposed.

InSe, a member of the layered materials family, is a superior electronic and optical material which retains a direct bandgap feature from the bulk to atomically thin few-layers and high electronic mobility down to a single layer limit. We, for the first time, exploit strain to drastically modify the bandgap of two-dimensional (2D) InSe nanoflakes. We demonstrated that we could decrease the bandgap of a few-layer InSe flake by 160 meV through applying an in-plane uniaxial tensile strain to 1.06% and increase the bandgap by 79 meV through applying an in-plane uniaxial compressive strain to 0.62%, as evidenced by photoluminescence (PL) spectroscopy. The large reversible bandgap change of ~239 meV arises from a large bandgap change rate (bandgap strain coefficient) of few-layer InSe in response to strain, ~154 meV/% for uniaxial tensile strain and ~140 meV/% for uniaxial compressive strain, representing the most pronounced uniaxial strain-induced bandgap strain coefficient experimentally reported in 2D materials. We developed a theoretical understanding of the strain-induced bandgap change through first-principles DFT and GW calculations. We also confirmed the bandgap change by photoconductivity measurements using excitation light with different photon energies. The highly tunable bandgap of InSe in the infrared regime should enable a wide range of applications, including electro-mechanical, piezoelectric and optoelectronic devices.

The density of states optical absorption edge in polarized Ga1-xInxN/GaN quantum wells (x ≤0.14) is studied through photoluminescence excitation spectroscopy and effective mass theory. Within the vicinity of the main absorption edge in the range of 3.0–3.3 eV several extrema are resolved. On the basis of experimental polarization field values and established parameters a model of the electronic band structure in the quantum wells is derived. Both predicted interband transition energies as well as relative matrix elements provide a good description of the observations and a level assignment. We thus identify a dominance of e1lh1 and e1hh1 transitions with increasing competition by e1hh2 for higher x. Due to the strong polarization effects, the latter is an allowed transition and paired with the high lying ionization threshold to the valence band. These results provide a quantitative basis for the further identification of the electronic band structure in polarized heterostructures.

The authors report on photothermal deflection spectroscopy (PDS) experiments for InGaAs/InP heterostructures. The apparatus is simple, yet sufficiently sensitive to register the heating by photon absorption for a single quantum well on a substrate with considerable thermal mass (thickness approximately=400 mu m). The PDS measurement accounts for the non-radiative relaxation after optical excitation. The extent to which the spectral dependence of the PDS signal at room temperature follows the absorption spectrum allows an estimation of the fraction of non-radiative processes. The large dynamic range of PDS and the ease of calibration of the signal permit one to measure quantum well absorption in the proper units.

Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N(+) implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 × 10(16) cm(-2)), the InGaN MQWs exhibit a high stability against ion beam mixing.

Linearly polarized light emission is analyzed in nonpolar light-emitting diodes (LEDs) covering the blue to green spectral range. In photoluminescence, m-plane GaInN/GaN structures reach a polarization ratio from 0.70 at 460 nm to 0.89 at 515 nm peak wavelength. For a-plane structures, the polarization ratio is 0.53 at 400 nm and 0.60 at 480–510 nm. In electroluminescence the polarization ratio is 0.77 at 505 nm in 350×350 µm2m-plane devices at 20 mA. Such a device should allow 44% power saving compared with nonpolarized c-plane LEDs combined with a polarizing filter, as commonly used in LED-backlit liquid crystal displays.

The polarization properties of doped Ga0.85In0.15N/GaN multiple quantum well structures are investigated under the application of external bias voltages. From Franz-Keldysh oscillations at and above the barrier band edge, the electric field in the barriers is determined. From the bias dependence, we derive the polarity and an offset of 0.51 MV/cm of the internal field. This is attributed to the piezoelectric polarization in the well region of magnitude P = -e z (0.009 - 0. 014) C/m2. We find that Si doping at typical concentrations of 3 ×1018 cm-3 cannot screen the polarization effects on the length scale typical for quantum wells and superlattices but induces large potential barriers.

The electronic bandstructure of strained Ga1-x InxN wells between barriers of GaN is found to exhibit an unusual Stark-ladder controlled mainly by the piezoelectric dipole across the strained layer. In luminescence and reflection spectroscopy four distinct steps including a strong redshift with respect to the thin film band gap are identified. Huge piezoelectric fields F ≤1 MV/cm are derived directly from Franz-Keldysh oscillations and interband transitions between carriers originating on opposite sides of the well. For the largest strain and electric field, a Stark-like ladder is identified. This provides important details for the interpretation of the electronic band structure in group-III nitride heterostructures.

Michihiko KARIYA, Shugo NITTA, Shigeo YAMAGUCHI, Hisaki KATO, Tetsuya TAKEUCHI,Christian WETZEL, Hiroshi AMANO andIsamu AKASAKIDepartment ofElectrical and Electronic Engineering, Meijo University, 1-501Sl1iogamagucl1i, Tempaku-ku.Nagoya 468-8502.Japan(Received April 23, 1998; accepted for publication May 7,1998)Ah_xlnxN layers grown on GaN by metalorganic vapor phase epitaxy (MOVPE) have been structurally studied usingwand

The etch pit density of organometallic vapor phase epitaxy (OMVPE)-grown GaN on sapphire was discovered to reduce drastically by the insertion of either a low-temperature-deposited AlN buffer layer or GaN buffer layer between high-temperature-grown-GaN on sapphire.