Blue cathodoluminescence from thulium implanted AlxGa1−xN and InxAl1−xN

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted Al_xGa_1−xN with different AlN contents (in the range 0≤x≤0.2) and from implanted In_xAl_1−xN with different InN contents (x=0.13 and 0.19) close to the lattice match with GaN. The Tm³⁺ emission spectrum depends critically on the host material. The blue emission from Al_xGa_1−xN:Tm peaks in intensity for an AlN content of x0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 C. The blue emission from In_0.13Al_0.87N:Tm, annealed at 1200 C, is more than ten times stronger than that from Al_xGa_1−xN:Tm, x≤0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.     

Bound polaron in a wurtzite GaN/AlxGa1 − xN ellipsoidal finite-potential quantum dot

A variational method is used to study the ground state of a bound polaron in a weakly oblate wurtzite GaN/Al_xGa_1 − xN ellipsoidal quantum dot. The binding energy of the bound polaron is calculated by taking the electron couples with both branches of LO-like and TO-like phonons due to the anisotropic effect into account. The interaction between impurity and phonons has also been considered to obtain the binding energy of a bound polaron. The results show that the binding energy of bound polaron reaches a peak value as the quantum dot radius increases and then diminishes for the finite potential well. We found that the binding energy of bound polaron is reduced by the phonons effect on the impurity states, the contribution of LO-like phonon to the binding energy is dominant, the anisotropic angle and ellipticity influence on the binding energy are small.     

About some optical properties of AlxGa1−xN /GaN quantum wells grown by molecular beam epitaxy

Wurtzitic nitride quantum wells grown along the (0001) axis experience a large Stark effect induced by the differences of spontaneous and piezoelectric polarizations between the well and barrier materials. In Al_xGa_1−xN/GaN quantum wells, due to the adverse actions of quantum confinement, that blue-shifts transition energies, and of the Stark field, that red-shifts them, the transition energies are nearly independent of barrier compositions at a particular well thickness (L₀2.6 nm), at least for x≤0.3. The effect of alloy fluctuations is then minimal, as reflected by a minimum in the quantum well luminescence linewidth when LL₀ for wells grown by molecular beam epitaxy on silicon or sapphire substrates. We use this effect to estimate the average variances of well widths and alloy composition fluctuations. Both results are in good agreement with, respectively, a scanning tunneling microscopy study of GaN (0001) surfaces, and estimates based on the lateral extent of the quantum well excitons.We then discuss the optical properties of the Al_xGa_1−xN barrier material, with particular emphasis on the symmetry of the valence band maximum (Γ₉ or Γ₇). We show that it may play an important role in the apparent barrier luminescence efficiency. We analyse the possible consequences of the barrier Γ₉–Γ₇ crossover on the Al_xGa_1−xN/GaN quantum well properties.     

Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

Electron scattering on disordered double-barrier GaAs–AlxGa1−xAs heterostructures

We present a novel model to calculate vertical transport properties such as conductance and current in unintentionally disordered double-barrier GaAs–Al_xGa_1−xAs heterostructures. The source of disorder comes from interface roughness at the heterojunctions (lateral disorder) as well as spatial inhomogeneities of the Al mole fraction in the barriers (compositional disorder). Both lateral and compositional disorder break translational symmetry along the lateral direction and therefore electrons can be scattered off the growth direction. The model correctly describes channel mixing due to these elastic scattering events. In particular, for realistic degree of disorder, we have found that the effects of compositional disorder on transport properties are negligible as compared to the effects due to lateral disorder.     

High performance AlxGa1−xN-based avalanche photodiodes

We report high performance solar-blind photodetectors with reproducible avalanche gain as high as 820 under ultraviolet illumination. The solar-blind photodetectors have a sharp cut-off around 276 nm. We improved the device performance by designing different epitaxial wafer structure with thinner active multiplication layer. We compare the resulting fabricated devices from these wafers in terms of dark current, photoresponse, avalanche gain performances.     

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al1−xGaxAs

The use of focused ion beam implantation doping of an inverted GaAs/Al_1−xGa_xAs heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10¹¹ cm⁻² and an electron mobility of 4.8×10⁵ cm²/V s, as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.     

Optical-field profiles in InxGa1−xN -MQW laser structures

In this paper, we calculated the optical fields for In_xGa_1−xN-multiquantum well (MQW) laser structures. Two different optical cavities are compared, the conventional step separate confinement heterostructure (Step) and a graded-index (GRIN) structure with a parabolic variation of the Al content in the Al_xGa_1−xN guide layers. A comparison is made regarding the confinement factor, near- and far-field patterns. An anomalous behavior for the confinement factor is observed in the structure, and it can be eliminated by choosing an appropriated combination of the layer’s thicknesses forming the waveguide. For Al_xGa_1−xN, an improved expression for the refractive index is presented, which shows better agreement with experimental data than previously reported expressions.     

Raman response of GaAs/AlAs superlattices with AlxGa1−xAs intralayers

We present a theoretical investigation on the effects induced on the Raman spectra of a binary (001) GaAs/AlAs superlattice by an intentionally deposited ternary Al_xGa_1−xAs intralayer. We investigate how the intralayer chemical composition and position affect the GaAs-like response of the host system.     

Analysis of electron mobility versus temperature after photoexcitation in Si-doped AlxGa1−xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples and compared with calculated data using the background acceptor density and the alloy scattering potential as free parameters. The possibility of discriminating between negative- or positive-U electron correlation energy for the DX centre has been investigated through a careful analysis of mobility versus temperature curves relating to different photoexcited electron densities. A crucial role of the acceptor density to explain the experimental data within the positive-U model has been evidenced.     

MOVPE growth study of BxGa(1−x)N on GaN template substrate

BGaN materials with good structural quality and surface morphology have been successfully grown on GaN template substrates by low pressure metal organic vapour phase epitaxy. TEB and NH3 were used as precursors of boron and nitrogen respectively. All the growths were performed under 100% N₂ process gas. Boron concentration was estimated by HRXD measurements combined with SIMS analysis. Single-crystal layers of BGaN with B content as high as 3.6% have been obtained.     

Interface-related effects on confined excitons in GaAs/AlxGa1−xAs single quantum wells

Confined excitons in non-abrupt GaAs/Al_xGa_1−xAs single quantum wells are studied. The graded interfaces are described taking into account fluctuations in their thickness a and positioning with respect to the abrupt interface picture. Numerical results for confined (0,0),(1,1) and (0,2) excitons in GaAs/Al_0.3Ga_0.7As quantum wells show that while the interfacial fluctuations produce small changes (<0.5 meV) in the exciton binding energies, the confined exciton energies can be red- or blue-shifted as much as 25 meV for wells with mean width of 50 Å and 2 ML wide interfaces.     

Formation of AlxGa1−xAs periodic array of micro-hexagonal pillars and air holes by selective area MOVPE

A two-dimensional (2D) periodic array having air/semiconductor interfaces can be applied to photonic crystals (PCs), which are expected to control spontaneous emission and optical transports in the next-generation devices. In this paper, we report on the selective area metal-organic vapor phase epitaxial (SA-MOVPE) growth of a Al_xGa_1−xAs 2D periodic array on a GaAs (1 1 1)B substrate for application to 2DPCs having GaAs/AlGaAs heterostructures. Al_xGa_1−xAs (x=0, 0.25 and 0.50) growth was carried out on triangular lattice array of hexagonal GaAs openings and hexagonal SiN_x masks. A uniform Al_0.50Ga_0.50As hexagonal pillar array and a GaAs hexagonal air-hole array with a 1 μm-period were successfully obtained. The important growth parameter for uniform 2DPC structure formation by SA-MOVPE was clarified. Furthermore, we describe the successful demonstration of a 400 nm-period pillar array and an air-hole array, which corresponds to the optical communication wavelength λ=1.3–1.55 μm. The results indicate that SA-MOVPE method is very promising for the formation of uniform semiconductor 2DPCs without the occurrence of process-induced damages.     

Saturation spectroscopy of electronic states in a magnetic field in InAs/AlxGa1−xSb single quantum wells

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al_0.5Ga_0.5Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n-level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al_0.1Ga_0.9Sb single quantum-well structure. Results show that the two lines are of different origin.     

Electronic structure of Ga1−xCrxN investigated by photoemission spectroscopy

We have obtained the Cr 3d-like energy states, which located in the band gap of GaN by means of resonant photoemission spectroscopy. In the difference spectrum between the valence band photoemission spectra of non-doped GaN and that of the Ga_0.937Cr_0.063N, we observed the new energy state, in band gap, consists of Cr 3d-like and N 2p-like component by strong hybridization.     

Effect of aliovalent ion substitution on the oxide ion conductivity in rare-earth pyrohafnates RE2−xSrxHf2O7−δ and RE2Hf2−x AlxO7−δ (RE=Gd and Nd; x=0, 0.1 and 0.2)

Oxide ion conductivity of the pure and aliovalent ion substituted rare-earth pyrohafnates in the series RE_2−xSr_xHf₂O₇ and RE₂Hf_2−xAl_xO₇ (RE=Gd and Nd; x=0–0.2) has been explored in the temperature range 400°C–700°C for the first time. It is seen that, conductivity is enhanced by doping 5 atom% Sr at the rare–earth site in these systems. Well defined impedance plots due to grain interior and grain boundary resistances were obtained in the Gd pyrohafnate with Sr substitution. The results of the conductivity variation for the pure, Sr and Al doped phases are explained on the basis of pyrochlore structure.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Time-resolved magneto-optical properties of InxGa1−xAs V-shaped single quantum wires

In this work we present a time-resolved magneto-luminescence investigation (up to 8 T) of InGaAs V-shaped quantum wires (QWRs) with different In content, as a function of temperature and the applied magnetic field. The states of the wires were investigated by CW PL and quantitatively compared with the results of a numerical solution of the two-dimensional Schrodinger equation. Time-resolved experiments performed in magnetic field at different temperatures indicate the existence of a competition between the electron confinement occurring in deep QWRs at low temperature, and the magnetic confinement prevailing in shallower QWRs.     

有效质量差异和电场对GaN/AlxGa1-xN球形量子点电子结构的影响

用平面波展开法对GaN/Al_xGa_1-xN球形量子点中类氢杂质态能级随量子点半径、Al组分以及结合能随Al组分的变化规律进行了详细讨论.计算了量子点内外有效质量差异对杂质态能级和结合能的修正,结果表明对于Al组分较高的GaN/Al_xGa_1-xN球形量子点,电子有效质量差异对杂质能级和结合能的修正不能忽略.考虑电子有效质量差异后,进一步具体计算了杂质结合能随量子点半 关键词： 球形量子点 平面波展开法 有效质量     

Structural and vibrational properties of dilute GaNxAs1−x(P1−x)

We report a comprehensive analyzes of the Fourier transform infrared (FTIR) absorption and Raman scattering data on the structural and vibrational properties of dilute ternary GaAs_1−xN_x,[GaP_1−xN_x] (x<0.03) alloys grown on GaAs [GaP] by metal organic chemical vapor deposition (MOCVD) and solid source molecular beam epitaxy (MBE). By using realistic total energy and lattice dynamical calculations, the origin of experimentally observed N-induced vibrational features are characterized. Useful information is obtained about the structural stability, vibrational frequencies, lattice relaxations and compositional disorder in GaNAs (GaNP) alloys. At lower composition (x<0.015) most of the N atoms occupy the As [P] sublattice {N_As[N_P]}—they prefer moving out of their substitutional sites to more energetically favorable locations at higher x. Our results for the N-isotopic shifts of local mode frequencies compare favorably well with the existing FTIR data.