Metamorphic In(0.20)Ga(0.80)As p-i-n photodetectors grown on GaAs substrates for near infrared applications

The growth and performance of top-illuminated metamorphic In(0.20)Ga(0.80)As p-i-n photodetectors grown on GaAs substrates using a step-graded In(x)Ga(1-x)As buffer is reported. The p-i-n photodetectors display a low room-temperature reverse bias dark current density of ~1.4×10(-7) A/cm(2) at -2 V. Responsivity and specific detectivity values of 0.72 A/W, 2.3×10(12) cm·Hz(1/2)/W and 0.69 A/W, 2.2×10(12) cm·Hz(1/2)/W are achieved for Yb:YAG (1030 nm) and Nd:YAG (1064 nm) laser wavelengths at -2 V, respectively. A high theoretical bandwidth-responsivity product of 0.21 GHz·A/W was estimated at 1064 nm. Device performance metrics for these GaAs substrate-based detectors compare favorably with those based on InP technology due to the close tuning of the detector bandgap to the target wavelengths, despite the presence of a residual threading dislocation density. This work demonstrates the great potential for high performance metamorphic near-infrared InGaAs detectors with optimally tuned bandgaps, which can be grown on GaAs substrates, for a wide variety of applications.

Chemically resolved scanning tunneling microscopy imaging of Al on p-type Al0.1Ga0.9As(001)‐c(2×8)∕(2×4)

The ability to chemically differentiate individual subsurface Al and Ga atoms, when imaging the Al0.1Ga0.9As(001)-c(2x8)(2x4) surface with scanning tunneling microscopy (STM), has been observed for the first time. In filled-state STM images first layer As atoms bonded to second layer Al atoms appear brighter than those bonded to second layer Ga atoms. This effect is only observed experimentally with p-type Al0.1Ga0.9As grown on p-type GaAs substrates and has been computationally modeled with density functional theory (DFT) calculations. It is hypothesized that chemical specificity is not observed on n-type material because the extra surface charge given to first layer As atoms by second layer Al atoms adds negligibly to the filled-state density of the surface, thus preventing the visualization of chemical specificity with filled-state STM imaging. The ability to distinguish whether first layer As atoms are bonded to second layer Ga and/or Al atoms in STM images shows that small differences in bond ionicity affect the local electronic structure of the material.

Structure and dynamics of fluorescently labeled complex fluids by fourier imaging correlation spectroscopy

We present a method of Fourier imaging correlation spectroscopy (FICS) that performs phase-sensitive measurements of modulated optical signals from fluorescently labeled complex fluids. FICS experiments probe the time-dependent trajectory of a spatial Fourier component of the fluid particle density at a specified wave number k, and provide a direct route to the intermediate scattering function. The FICS approach overcomes signal sensitivity problems associated with dynamic light scattering, while offering a means to acquire time-dependent information about spatial distributions of fluorescent particles, superior in efficiency to direct imaging methods. We describe the instrumental setup necessary to perform FICS experiments, and outline the theory that establishes the connection between FICS observables and statistical mechanical quantities describing liquid state dynamics. Test measurements on monolayer suspensions of rhodamine labeled polystyrene spheres are detailed.

Measurement of the dynamic structure function of fluorescently labeled complex fluids by fourier imaging correlation spectroscopy

Wave-number-resolved time-correlation functions of density fluctuations are determined from purely incoherent fluorescence signals using a new method, Fourier imaging correlation spectroscopy (FICS). We demonstrate the application of FICS to chemical and biological samples with ultrahigh signal sensitivity and time resolution superior to direct imaging methods. We outline the theory that establishes the connection between FICS observables and statistical mechanical quantities describing liquid state dynamics.