A 1.65μm three-section distributed Bragg reflector （DBR） laser for CH4 gas sensors

A 1.65-μm three-section distributed Bragg reflector(DBR) laser for CH4 gas sensors is reported.The DBR laser has a wide tunable range covering the R3 and R4 methane absorption line manifolds.The wavelength tunability properties,temperature stability and laser linewidth are characterized and analyzed.Several advantages were demonstrated compared with traditional DFB lasers in harmonic detection.     

Characterization of surface roughness anisotropy on mismatched InAIAs/lnP heterostructures

We have studied surface roughness on mismatched In_0.65Al_0.35As epilayers of various thicknesses on (001) InP. The sample set spans the entire range from coherently strained to completely relaxed epilayers. As characterization tools, we have used atomic force microscopy (AFM), laser light scattering (LLS), and variable azimuthal angle ellipsometry (VAAE). AFM reveals that the surfaces are covered by densely packed ellipsoidal islands elongated along the [1-10] direction. The island size increases with layer thickness. Island anisotropy and the root mean square of the surface roughness increase with increasing thickness but decrease upon full lattice relaxation. LLS intensity displays a prominent azimuthal dependence that correlates well with the two-dimensional power spectrum of the surface topography, as predicted by theory. VAAE reveals a sinusoidal dependence of the ellipsometric parameter Δ on azimuthal angle. The amplitude of A correlates well with the short wavelength anisotropy of the surface power spectrum. Our work suggests that LLS and VAAE are fast, nondestructive, sensitive techniques for characterization of surface roughness in mismatched III-V heterostructures.     

Reliability of AllnAs/lnGaAs/lnP HEMT with WSi ohmic contacts

We have investigated the degradation mechanism of Al_0.48In_0.52As/In_0.53Ga_0.47As/ InP high electron mobility transistors (HEMTs) using WSi ohmic electrodes. Cross-sectional transmission electron microscopy (TEM) observation and en-ergy dispersive x-ray (EDX) analysis reveal impurities diffusion of gate electrode (titanium: Ti) and fluorine (F) in the AlInAs layer after a high temperature (T_a = 170°C operating life test for 500 h. The decrease of drain current (I_ds) during life test shows linear dependence on square root of aging time. It suggests that the degradation is controlled by a diffusion mechanism. Hence, the estimated degradation mechanism of this device is related with decrease of carrier concentration in the epitaxial layer by these diffused impurities. On the other hand, TEM and EDX show no degradation of WSi/InGaAs interface after aging. Therefore, the WSi electrode for this type of HEMT demonstrates excellent high stability under the accelerated operating life test.     

The preparation of CdSnP2/InP heterojunctions by liquid phase epitaxy from Sn-solution

Heterojunctions of n-type CdSnP₂ on p-type InP substrates are prepared by LPE from Sn-solutions. The significance of the tipping arrangement, of the initial solution composition and of the temperature program for preparing LPE layers of high quality is discussed. Heterodiodes cleaved from such LPE wafers electroluminesce under forward bias between 1 μm and 1.8 %m with internal quantum efficiencies approaching 1.8% at room temperature. When used as photovoltaic detectors, the quantum efficiency of these diodes surpasses that of Si for λ > 1.09 μ, and exceeds 1% for all wave-lengths between 0.96 and 1.3 μ.     

Field screening in (111)B InAsP/lnP strained quantum wells

Low-temperature photoluminescence measurements were performed on InAsP/InP strained quantum wells grown on InP (lll)B substrates by gas-source molecular beam epitaxy. The emission energy was observed to increase as the pump-power density increased. This was attributed to the screening of the internal piezoelectric field by photo-generated carriers. The energy shift was as large as 35 meV for an InAs_0.28P_0.72/InP quantum well with a lattice mismatch of ~0.9%. A similar structure with a smaller strain showed saturation of the energy shift with increasing pump-power density. We performed a model calculation which includes the quantum confined Stark effect, and this saturation was correlated with a flat-band structure of the quantum well due to the nearly complete screening of the built-in electric field.     

Lpe highly perfect ingaasp/lnp structure characterization by x-ray double crystal diffractometry

X-ray double crystal diffractometry has been used to assess the crystal quality of InGaAsP/ InP single heterostructures grown by liquid phase epitaxy. Diffraction profiles have been obtained in the parallel non-dispersive configuration, using Cu Kα₁ radiation, 004 symmetric reflection and a perfect InP crystal as a monochromator. Several structures, with different lattice mismatches, ranging from positive to negative values, have been investigated. The epilayer Bragg peak was found to be as narrow as theoretically predicted, if thickness effects are taken into account. Pendellosung fringes have been observed at the low angle side of the peak, thus the epilayer thickness could be measured. Finally, the sample curvature has been evaluated from the broadening of the substrate Bragg peak and, when the broadening was sufficiently large, a good agreement with that calculated from the elastic theory has been found. All the results demonstrate that the structures investigated are characterized by a very high crystal quality.     

Mode of growth in LP-MOVPE deposition of GalnAs/lnP quantum wells

GalnAs/lnP multiple quantum wells of 10 periods grown by low pressure metal organic vapor phase epitaxy were studied using a careful analysis of their photoluminescence (PL) spectra and of supporting x-ray data. The results demonstrate extremely precise control of the well width. The width of well and barrier can be reproducibly adjusted by a fraction of a monolayer. This allows one to distinguish between 3-dimensional (3-D) and 2-dimensional (2-D) growth, which determines the lateral extent of the atomically smooth interfacial areas. By varying the growth time per well a discontinuous energy shift of the PL peaks is obtained for wells with widths from 1 to 8 monolayers. We show that this discontinuous energy shift corresponds to a change in well width by one monolayer. It is also observed when growth at the upper interface is carried on without interruption and a thin quaternary film is deposited at the interface. From these results the presence of atomically flat surfaces during the growth is deduced; these give a strong indication of a 2-D mode of growth for an optimized set of parameters.