Vincent Gambin

Long-wavelength GaInNAs(Sb) lasers on GaAs

The boom in fiber-optic communications has caused a high demand for GaAs-based lasers in the 1.3-1.6-/spl mu/m range. This has led to the introduction of small amounts of nitrogen into InGaAs to reduce the bandgap sufficiently, resulting in a new material that is lattice matched to GaAs. More recently, the addition of Sb has allowed further reduction of the bandgap, leading to the first demonstration of 1.5-/spl mu/m GaAs-based lasers by the authors. Additional work has focused on the use of GaAs, GaNAs, and now GaNAsSb barriers as cladding for GaInNAsSb quantum wells. We present the results of photoluminescence, as well as in-plane lasers studies, made with these combinations of materials. With GaNAs or GaNAsSb barriers, the blue shift due to post-growth annealing is suppressed, and longer wavelength laser emission is achieved. Long wavelength luminescence out to 1.6 /spl mu/m from GaInNAsSb quantum wells, with GaNAsSb barriers, was observed. In-plane lasers from these samples yielded lasers operating out to 1.49 /spl mu/m, a minimum threshold current density of 500 A/cm/sup 2/ per quantum well, a maximum differential quantum efficiency of 75%, and pulsed power up to 350 mW at room temperature.

Multiple-quantum-well GaInNAs-GaNAs ridge-waveguide laser diodes operating out to 1.4 /spl mu/m

In this letter, results from a ridge waveguide laser diode (LD) structure, with three GaInNAs quantum wells (QWs) and GaNAs barriers, are presented. The sample was grown by solid source molecular beam epitaxy with an RF plasma nitrogen source. These devices differ from previously reported GaInNAs QWs LDs that used GaAs as the barrier material. The introduction of nitrogen into the barriers reduces the spectral blue shift caused by post-growth annealing. Long wavelength emission out to 1.405 /spl mu/m was observed. The devices exhibited threshold current densities as low as 1.5 kA/cm/sup 2/, high differential efficiency of 0.67 W/A, and a maximum output power of 350 mW.

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.

Nitrogen-related electron traps in Ga(As,N) layers (⩽3% N)

Capacitance spectroscopy is used to examine the compositional dependence of deep levels in Si-doped Ga(As,N) layers grown on GaAs. We find two predominant electron traps at about 0.80 and 1.1 eV above the valence band edge EV, which do not depend on composition. For N contents above 0.1% N, the concentration of the acceptor-like gap level at EV+1.1 eV strongly increases and leads to a distinct reduction of the donor doping efficiency in Ga(As,N) layers. Based on theoretical prediction, this electron trap is tentatively associated with a split interstitial defect containing a nitrogen and an arsenic atom on the same As lattice site [(AsN)As]. The trap at EV+0.80 eV likely corresponds to nitrogen dimers, i.e., two N atoms on a single As site [(NN)As]. When approaching the critical layer thickness, this electron trap is increasingly generated during growth. The dimer defect can be removed by rapid thermal annealing at 720 °C after growth, in contrast to the stable bulk level at EV+1.1 eV. By the formation of...

Thermal conduction inhomogeneity of nanocrystalline diamond films by dual-side thermoreflectance

Thin diamond films of thickness near 1 μm can have highly nonuniform thermal conductivities owing to spatially varying disorder associated with nucleation and grain coalescence. Here, we examine the nonuniformity for nanocrystalline chemical vapor deposited diamond films of thickness 0.5, 1.0, and 5.6 μm using picosecond thermoreflectance from both the top and bottom diamond surfaces, enabled by etching a window in the silicon substrate. The extracted local thermal conductivities vary from less than 100 W m−1 K−1 to more than 1300 W m−1 K−1 and suggest that the most defective material is confined to within 1 μm of the growth surface.

Structural changes on annealing of MBE grown (Ga, In)(N, As) as measured by X-ray absorption fine structure

Abstract GaInNAs grown on GaAs has recently been found to optically emit at wavelengths longer than previously possible with material grown epitaxially on GaAs substrates. To improve radiative efficiency, material is annealed after growth, after which the band gap is found to significantly blueshift. Structural changes that occur during this annealing process were studied using fluorescent X-ray absorption fine structure. By comparing the absorption data with lattice parameter and band structure simulations, it was found that the number of In atoms surrounding N atoms increased after the high-temperature annealing. According to ab initio simulations this ordering will increase the band gap of the GaInNAs alloy. We believe the reduction of free energy drives the reordering process towards increasing In–N coordination.

Ga(As,N) layers in the dilute N limit studied by depth-resolved capacitance spectroscopy

Deep carrier traps in the upper half of the band gap of Ga(As,N) layers in the dilute N limit (⩽0.1%) are examined by depth-resolved capacitance spectroscopy on n-type Ga(As,N)/GaAs heterojunctions grown by molecular-beam epitaxy. Distinct compositional fluctuations are revealed in the deep-level spectra. Native point defects are predominantly formed in regions with larger N content. High concentrations of electron traps near the surface control the properties of as-grown Ga(As,N) layers and lead to strong carrier depletion and frequency- as well as temperature-dependent capacitance (admittance dispersion). The related defects at the surface can be removed by rapid thermal annealing.

Use of transmission electron microscopy in the characterization of GaInNAs(Sb) quantum well structures grown by molecular beam epitaxy

The quaternary GaInNAs alloy is a very promising material system for lasers in the 1.2–1.6 μm range with application in telecommunication fiber-optic networks. While good quality laser material has been demonstrated at 1.3 μm, pushing the emission beyond 1.5 μm by adding up to 40% In and 2% N has been unsuccessful. Recently, the addition of small amounts of Sb has put this alloy back on track for the 1.5 μm challenge by dramatically improving the luminescence efficiency of the material. In this work, high-resolution transmission electron microscopy (TEM), energy-filtered TEM, dark-field TEM, and energy-dispersive x-ray spectroscopy were used to structurally characterize both GaInNAs and GaInNAsSb quantum well structures. The results provide insight into the role of antimony in improving the optical properties of the material, namely reducing the local compositional fluctuations of In.

Long wavelength GaInNAs(Sb) lasers on GaAs

Presents new techniques for developing long wavelength post-annealed GaInNAs(Sb) materials grown by solid source MBE (SSMBE) with a RF plasma nitrogen source. Rather than utilizing conventional GaAs barriers between GaInNAs quantum wells (QWs), we have grown GaNAs barriers. This design reduces the blue-shift of the emission spectrum due to decreased nitrogen out-diffusion. Moreover, the decreased carrier confinement of GaNAs further supports longer emission wavelength compared to GaAs barrier. Additional benefit of GaNAs barriers is a reduction in the overall compressive strain of the active region because GaNAs is under tensile-strain.

High-efficiency multiple-quantum-well GaInNAs/GaNAs ridge-waveguide diode lasers

We present a new structure with nitrogen incorporation in barrier and new material with antimony for developing post-annealed long wavelength material. This new structure and new material result in a shift of the post-annealed luminescence out to 1.6 um. The new structure, nitrogen in barrier, reduces the blue-shift of the emission spectrum by suppressing nitrogen out-diffusion from the quantum wells (QWs) and decreasing carrier confinement between barriers and QWs. GaNAs or GaNAsSb barriers can also reduce the overall strain of the active region because the high indium mole fraction QWs are compressively strained and the barriers with nitrogen are tensely strained. By adding small amount of antimony, we were able to incorporate up to 46% indium. We will present results of high efficiency long wavelength multiple QW GaInNAs ridge-waveguide laser diodes using GaNAs barriers. We will also show GaInNAsSb QWs with GaNAsSb barriers ridge waveguide laser emitting at 1.465 um. We have observed photoluminescence up to 1.6 um with different indium and antimony concentrations. Our GaInNAs and GaInNAsSb ridge waveguide laser diodes have broad emission spectra covering 1.27 um to 1.465 um with pulsed operation up to 90 degree(s)C. The maximum output power at room temperature, under pulsed operation was 350 mW with a differential efficiency of 0.67 W/A.