GaAsN/AlAs/AlGaAs double barrier quantum wells grown by molecular beam epitaxy as an alternative to infrared absorption below 4 μm

In this work, we report on the design, growth and characterization of GaAsN/AlAs/AlGaAs double barrier quantum well infrared detectors to achieve intraband absorption below 4 μm. Due to the high effective mass of N-dilute alloys, it is common for these N-containing double barrier quantum well structures to have more than one bound state within the quantum well, enabling the possibility of achieving multispectral absorption from these confined levels to the quasi-bound. Based on a transfer matrix calculation we will study the influence of the potential parameters, in particular the well width and the introduction of a GaAs spacer layer in between the N-well and the AlAs barriers. We will compare the case in which there are two confined levels with the case in which only one level is bound, like in the conventional AlGaAs/AlAs/GaAs structures. On the basis of the simulation, we have grown and characterized some N-containing double barrier detectors. Moreover, an optimization of the post-growth annealing treatments of the GaAsN quantum well structures has also been performed. Finally, room temperature absorption measurements of both as-grown and annealed samples are presented and analyzed.     

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.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Fabrication and characterization of n-ZnO on p-SiC heterojunction diodes on 4H-SiC substrates

We report on the growth and characterization of n-ZnO/p-4H-SiC heterojunction diodes. Our n-ZnO layers were grown with radical-source molecular beam epitaxy (RS-MBE) on p-4H-SiC epilayers, which was previously prepared in a horizontal hot-wall reactor by chemical vapour deposition (CVD) on the n-type 4H-SiC wafers. Details on the n-ZnO growth on 8-off 4H-SiC wafers, the quality of the layers and the nature of realized p–n structures are discussed. Mesa diode structures were fabricated. Al was sputtered through a circle mask with diameter 1 mm and annealed to form Ohmic contacts to p-SiC. Ohmic contacts to the n-ZnO were formed by 30 nm/300 nm Ti/Au sputtered by electron beam evaporation. Electrical properties of the structures obtained have been studied with Hall measurements, and current–voltage measurements (I–V). I–V measurements of the device showed good rectifying behavior, from which a turn-on voltage of about 2 V was obtained.     

Effects of composition grading at heterointefaces and layer thickness variations on Bragg mirror quality

GaAs/AlAs Bragg mirrors on GaAs with varied number of layer pairs were grown, by molecular beam epitaxy (MBE), to be applied for semiconductor saturable absorber mirrors (SESAMs) and intensity modulators. Due to the random variation of the growth rate, substrate surface roughness, and interdiffusion at the interfaces, precise control of the growth conditions of deposited layers poses a serious problem. Usually, thickness variations and composition grading at the heterointefaces result in variations of the mirror reflectivity. In this paper, the high resolution X-ray diffraction (HRXRD), optical reflectance, Rutherford backscattering/channelling (RBS), supported by numerical evaluation methods were employed to determine both the exact thickness of each layer and the composition grading at the interface between succeeding layers of GaAs/AlAs-based mirrors. To reduce ambiguity and to speed up the analysis, the rocking curves and RBS spectra were simulated concurrently, using results of one simulation to verify the others. This process was carried out until the best fit between experimental and calculated curves was achieved. The complementary use of both methods results in improved sensitivity and makes the whole process of evaluation of the thickness variation of each layer and the size of the composition grading at the interfaces less time consuming.     

Spin resonance spectroscopy of grown-in defects in Ga(In)NP alloys

We employ the optically detected magnetic resonance (ODMR) technique to study and identify important grown-in defects in Ga(In)NP grown by molecular-beam epitaxy (MBE). Several types of defects were revealed from ODMR studies. The dominant defects were found to be related to Ga interstitials, evident form their characteristic hyperfine interaction arising from the spin interaction between the electron and the Ga nucleus. Some other as yet unidentified intrinsic defects were also found to be commonly present in the alloys. The effects of growth conditions (ion bombardment, N₂ gas flow, etc.) and post-growth rapid thermal annealing on the formation of these defects were studied in detail, shedding light on the formation mechanism of defects.     

气源MBE外延自组装GeSi量子点的光致荧光

利用气源分子束外延技术(MBE)制作了GeSi自组装量子点样品.利用原子力显微镜(AFM)和光致荧光(PL)光谱研究了该量子点的形貌和光学性质.气源MBE在较低温度下生长的量子点材料具有较高的量子点覆盖度.200K以下载流子以局域激子形式束缚在量子点中,激子束缚能约为17 meV.升温至200 K,载流子的输运过程发生...     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Structural characterization of the epitaxially grown core–shell ZnTe/ZnMgTe nanowires

We report the method of the epitaxial growth of the core–shell ZnTe/ZnMgTe nanowires. The morphology and the crystal structure of several samples grown in different processes have been studied by scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction methods. It was shown that the ZnMgTe shell growth was clearly epitaxial with a good crystal quality. The average lattice spacing of the ZnTe cores and ZnMgTe shells have been calculated and Mg content in the shells has been estimated. It was documented that growing the shell lattice mismatched to the core induces the strain in the core. The model of the strain creation mechanism has been proposed. The presence of a shell with a larger energy gap than that of the core results in a strong emission in the spectral region near the band edge.     

Magneto-photoluminescence study of type-II charge confinement in epitaxially grown GaInP2

We have studied the photoluminescence from GaInP₂ grown by molecular beam epitaxy as a function of laser power, magnetic field and temperature. We show that the single luminescence peak observed in such samples arises from weakly bound (type-II) excitons in which the electrons are localised and the holes are free, and that it is the same as the additional low-energy peak typically observed in strongly CuPt-ordered GaInP₂ grown by chemical vapour deposition techniques. We propose that the electron is confined in In-rich regions of the sample and the hole is delocalised by coupling between heavy- and light-hole bands.