Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

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Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.

Determination of piezoelectric fields in strained GaInN quantum wells using the quantum-confined Stark effect

This survey deals with 1/f noise in homogeneous semiconductor samples. A distinction is made between mobility noise and number noise. It is shown that there always is mobility noise with an /spl alpha/ value with a magnitude in the order of 10/sup -4/. Damaging the crystal has a strong influence on /spl alpha/, /spl alpha/ may increase by orders of magnitude. Some theoretical models are briefly discussed none of them can explain all experimental results. The /spl alpha/ values of several semiconductors are given. These values can be used in calculations of 1/f noise in devices. >

1/f Noise Sources

Semiconductor Power Devices

There can be little doubt that the Monte Carlo method for semiconductor device simulation has enormous power as a research tool. It represents a detailed physical model of the semiconductor material(s), and provides a high degree of insight into the microscopic transport processes. However, if the authority ascribed to Monte Carlo models of devices at 1/spl mu/m feature size is to be maintained for devices below O.1/spl mu/m, modelling of the fundamental physics must be further improved. And if the Monte Carlo method is to be successful as a semiconductor device design tool, the device model must be made more realistic. Success in the industrial sector depends on this, but also on achieving fast run-times optimisation - where the scope and need for ingenuity is now greatest.

The Monte Carlo method for semiconductor device simulation

Avalanche noise measurements have been performed on a range of homojunction GaAs p/sup +/-i-n/sup +/ and n/sup +/-i-p/sup +/ diodes with "i" region widths, /spl omega/ from 2.61 to 0.05 /spl mu/m. The results show that for /spl omega//spl les/1 /spl mu/m the dependence of excess noise factor F on multiplication does not follow the well-established continuous noise theory of McIntyre [1966]. Instead, a decreasing noise factor is observed as /spl omega/ decreases for a constant multiplication. This reduction in F occurs for both electron and hole initiated multiplication in the thinner /spl omega/ structures even though the ionization coefficient ratio is close to unity. The dead-space, the minimum distance a carrier must travel to gain the ionization threshold energy, becomes increasingly important in these thinner structures and largely accounts for the reduction in noise.

Avalanche multiplication noise characteristics in thin GaAs p/sup +/-i-n/sup +/ diodes

Photomultiplication, initiated by both pure electron and pure hole injection, has been measured in submicrometer Si p+-i-n+ and n+-i-p+ diodes with intrinsic region thicknesses w between 0.1 and 0.8 mum, at temperatures between 15 and 420 K. A local analysis is used to extract the values of effective ionization coefficients. Values of bulk ionization coefficients, alpha and beta, are then deduced and parameterized in an extended form of Chynoweth's expression to cover their dependence on both electric field and temperature. Multiplication at various temperatures can be recovered from these bulk coefficients by using a simple dead space correction. beta falls faster with temperature than alpha so that the ionization coefficient ratio, k=beta/alpha, decreases with temperature. Decreasing w reduces this temperature sensitivity, which is weaker than in GaAs, possibly because of the softer ionization threshold in Si

Temperature Dependence of Impact Ionization in Submicrometer Silicon Devices

A Monte Carlo (MC) model has been used to estimate the excess noise factor in thin p/sup +/-i-n/sup +/ GaAs avalanche photodiodes (APD's). Multiplication initiated both by pure electron and hole injection is studied for different lengths of multiplication region and for a range of electric fields. In each ease a reduction in excess noise factor is observed as the multiplication length decreases, in good agreement with recent experimental measurements. This low noise behavior results from the higher operating electric field needed in short devices, which causes the probability distribution function for both electron and hole ionization path lengths to change from the conventionally assumed exponential shape and to exhibit a strong dead space effect. In turn this reduces the probability of higher order ionization events and narrows the probability distribution for multiplication. In addition, our simulations suggest that fur a given overall multiplication, electron initiated multiplication in short devices has inherently reduced noise, despite the higher feedback from hole ionization, compared to long devices.

A Monte Carlo investigation of multiplication noise in thin p/sup +/-i-n/sup +/ GaAs avalanche photodiodes

Avalanche multiplication and noise in 1.0, 0.5, 0.1, and 0.05 μm GaAs p+-i-n+ diodes have been calculated for both electron and hole initiated multiplication using a simple model which incorporates a randomly-generated ionization path length (RPL) and a hard-threshold dead space. We find that the mean multiplication obtained using this RPL model is in excellent agreement, even for the shortest structure, with that obtained from an analytical-band structure Monte Carlo (MC) model, which incorporates soft-threshold effects. However, it predicts slightly lower avalanche noise in the shorter devices. This difference results from the narrower ionization path length probability distribution and larger dead space of the hard-threshold RPL model at high electric fields as compared to the more realistic distribution function associated with the relatively sophisticated MC model.

A simple model to determine multiplication and noise in avalanche photodiodes

The electron and hole multiplication coefficients, M/sub e/ and M/sub h/, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, /spl alpha/ and /spl beta/, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 /spl mu/m down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M/sub e//M/sub h/ ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w/spl les/0.1 /spl mu/m) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (/spl ges/1/spl times/10/sup 3/ kV/cm).

G.J. Rees

Papers

Avalanche multiplication noise characteristics in thin GaAs p/sup +/-i-n/sup +/ diodes

Temperature Dependence of Impact Ionization in Submicrometer Silicon Devices

A Monte Carlo investigation of multiplication noise in thin p/sup +/-i-n/sup +/ GaAs avalanche photodiodes

A simple model to determine multiplication and noise in avalanche photodiodes

Investigation of impact ionization in thin GaAs diodes