Suyun Zhang

Confinement factor and absorption loss of AlInGaN based laser diodes emitting from ultraviolet to green

Confinement factor and absorption loss of AlInGaN based multiquantum well laser diodes (LDs) were investigated by numerical simulation based on a two-dimensional waveguide model. The simulation results indicate that an increased ridge height of the waveguide structure can enhance the lateral optical confinement and reduce the threshold current. For 405 nm violet LDs, the effects of p-AlGaN cladding layer composition and thickness on confinement factor and absorption loss were analyzed. The experimental results are in good agreement with the simulation analysis. Compared to violet LD, the confinement factors of 450 nm blue LD and 530 nm green LD were much lower. Using InGaN as waveguide layers that has higher refractive index than GaN will effectively enhance the optical confinement for blue and green LDs. The LDs based on nonpolar substrate allow for thick well layers and will increase the confinement factor several times. Furthermore, the confinement factor is less sensitive to alloys composition of waveguide and cladding layers, being an advantage especially important for ultraviolet and green LDs.

Thermal annealing behaviour of Pt on n-GaN Schottky contacts

The structural evolution and temperature dependence of the Schottky barrier heights of Pt contacts on n-GaN epilayer at various annealing temperatures were investigated extensively by Rutherford backscattering spectrometry, x-ray diffraction measurements, Auger electron spectroscopy, scanning electron microscopy and current–voltage measurements. The temperature dependence of the Schottky barrier heights may be attributed to changes of surface morphology of Pt films on the surface and variation of nonstoichiometric defects at the interface vicinity. Experimental results indicated the degradation of Pt contacts on n-GaN above 600uC.

Syntheses and crystal structures of a series of alkaline earth vanadium selenites and tellurites.

Six new novel alkaline-earth metal vanadium(V) or vanadium(IV) selenites and tellurites, namely, Sr(2)(VO)(3)(SeO(3))(5), Sr(V(2)O(5))(TeO(3)), Sr(2)(V(2)O(5))(2)(TeO(3))(2)(H(2)O), Ba(3)(VO(2))(2)(SeO(3))(4), Ba(2)(VO(3))Te(4)O(9)(OH), and Ba(2)V(2)O(5)(Te(2)O(6)), have been prepared and structurally characterized by single crystal X-ray diffraction analyses. These compounds exhibit six different anionic structures ranging from zero-dimensional (0D) cluster to three-dimensional (3D) network. Sr(2)(VO)(3)(SeO(3))(5) features a 3D anionic framework composed of VO(6) octahedra that are bridged by SeO(3) polyhedra. The oxidation state of the vanadium cation is +4 because of the partial reduction of V(2)O(5) by SeO(2) at high temperature. Ba(3)(VO(2))(2)(SeO(3))(4) features a 0D [(VO(2))(SeO(3))(2)](3-) anion. Sr(V(2)O(5))(TeO(3)) displays a unique 1D vanadium(V) tellurite chain composed of V(2)O(8) and V(2)O(7) units connected by tellurite groups, forming 4- and 10-MRs, whereas Sr(2)(V(2)O(5))(2)(TeO(3))(2)(H(2)O) exhibits a 2D layer consisting of [V(4)O(14)] tetramers interconnected by bridging TeO(3)(2-) anions with the Sr(2+) and water molecules located at the interlayer space. Ba(2)(VO(3))Te(4)O(9)(OH) exhibits a one-dimensional (1D) vanadium tellurite chain composed of a novel 1D [Te(4)O(9)(OH)](3-) chain further decorated by VO(4) tetrahedra. Ba(2)V(2)O(5)(Te(2)O(6)) also features a 1D vanadium(V) tellurites chain in which neighboring VO(4) tetrahedra are bridged by [Te(2)O(6)](4-) dimers. The existence of V(4+) ions in Sr(2)(VO)(3)(SeO(3))(5) is also confirmed by magnetic measurements. The results of optical diffuse-reflectance spectrum measurements and electronic structure calculations based on density functional theory (DFT) methods indicate that all six compounds are wide-band gap semiconductors.

Syntheses, Crystal Structures, and Properties of Five New Transition Metal Molybdenum(VI) Selenites and Tellurites

Five new transition metal molybdenum(VI) selenites or tellurites, namely, TM(MoO(3))(SeO(3))(H(2)O) (TM = Mn, Co), Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O), Cu(2)(MoO(4))(SeO(3)), and Ni(3)(MoO(4))(TeO(3))(2), have been prepared and structurally characterized. They belong to five different types of structures. Mn(MoO(3))(SeO(3))(H(2)O) and Ni(3)(MoO(4))(TeO(3))(2) are non-centrosymmetric and crystallize in the orthorhombic space groups Pmc2(1) and P2(1)2(1)2(1), respectively, whereas Co(MoO(3))(SeO(3))(H(2)O), Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O), and Cu(2)(MoO(4))(SeO(3)) are centrosymmetric and crystallize in P1, C2/c, P2(1)/c, respectively. The Mo(6+) cations in Mn(MoO(3))(SeO(3))(H(2)O), Co(MoO(3))(SeO(3))(H(2)O), and Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O) are in severely distorted octahedral geometry whereas those in Cu(2)(MoO(4))(SeO(3)) and Ni(3)(MoO(4))(TeO(3))(2) are in a slightly distorted tetrahedral geometry. Second-Harmonic Generation (SHG) measurements revealed that (MoO(3))(SeO(3))(H(2)O) displays a moderate SHG signal of about 3 x KH(2)PO(4) (KDP) whereas the SHG response of Ni(3)(MoO(4))(TeO(3))(2) is much weaker than that of KDP.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement

Syntheses, crystal structures, magnetic and luminescent properties of two classes of molybdenum(VI) rich quaternary lanthanide selenites.

Hydrothermal reactions of lanthanide(III) oxide, molybdenum oxide, and SeO(2) at 230 °C lead to five new molybdenum-rich quaternary lanthanide selenites with two types of structures, namely, H(3)Ln(4)Mo(9.5)O(32)(SeO(3))(4)(H(2)O)(2) (Ln = La, 1; Nd, 2) and Ln(2)Mo(3)O(10)(SeO(3))(2)(H(2)O) (Ln = Eu, 3; Dy, 4; Er, 5). Compounds 1 and 2 feature a complicated three-dimensional (3D) architecture constructed by the intergrowth of infinite molybdenum selenite chains of [Mo(4.75)SeO(19)](5.5-) and one-dimensional (1D) lanthanide selenite chains. The structures of 3, 4, and 5 exhibit 3D network composed of 1D [Mo(3)SeO(13)](4-) anionic chains connected by lanthanide selenite chains. The molybdenum selenite chain of [Mo(4.75)SeO(19)](5.5-) in 1 and 2 is composed of a pair of [Mo(3)SeO(13)](4-) chains as in 3, 4, and 5 interconnected by a [Mo(1.75)O(8)](5.5-) double-strand polymer via corner-sharing. The lanthanide selenite chains in both structures are similar in terms of coordination modes of selenite groups as well as the coordination environments of lanthanide(III) ions. Luminescent studies at both room temperature and 10 K indicate that compound 2 displays strong luminescence in the near-IR region and compound 3 exhibits red fluorescent emission bands with a luminescent lifetime of 0.57 ms. Magnetic properties of these compounds have been also investigated.

Investigation on the compensation effect of residual carbon impurities in low temperature grown Mg doped GaN films

The influence of unintentionally doped carbon impurities on electrical resistivity and yellow luminescence (YL) of low-temperature (LT) grown Mg doped GaN films is investigated. It is found that the resistivity of Mg doped GaN films are closely related to the residual carbon impurity concentration, which may be attributed to the compensation effect of carbon impurities. The carbon impurity may preferentially form deep donor complex CN-ON resulting from its relatively low formation energy. This complex is an effective compensate center for MgGa acceptors as well as inducing YL in photoluminescence spectra. Thus, the low resistivity LT grown p-type GaN films can be obtained only when the residual carbon impurity concentration is sufficiently low, which can explain why LT P-GaN films with lower resistivity were obtained more easily when relatively higher pressure, temperature, or NH3/TMGa flow rate ratio were used in the LT grown Mg doped GaN films reported in earlier reports.

The investigation on carrier distribution in InGaN/GaN multiple quantum well layers

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer.

Effects of thin heavily Mg-doped GaN capping layer on ohmic contact formation of p-type GaN

The growth condition of thin heavily Mg-doped GaN capping layer and its effect on ohmic contact formation of p-type GaN were investigated. It is confirmed that the excessive Mg doping can effectively enhance the Ni/Au contact to p-GaN after annealing at 550 degrees C. When the flow rate ratio between Mg and Ga gas sources is 6.4% and the layer width is 25 nm, the capping layer grown at 850 degrees C exhibits the best ohmic contact properties with respect to the specific contact resistivity (rho(c)). This temperature is much lower than the conventional growth temperature of Mg-doped GaN, suggesting that the deep-level-defect induced band may play an important role in the conduction of capping layer.

Suppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodes

InGaN-based blue-violet laser diodes (LDs) suffer from electron leakage into the p-type regions, which could be only partially alleviated by employing the electron blocking layer (EBL). Here, a thin undoped InGaN interlayer prior to EBL is proposed to create an additional forbidden energy range above the natural conduction band edge, which further suppresses the electron leakage and thus improve the characteristics of LDs. Numerical device simulations reveal that when the proper composition and thickness of InGaN interlayer are chosen, the electron leakage could be efficiently eliminated without inducing any severe accumulation of electrons at the interlayer, resulting in a maximum output power of the device.