InGaAs Based MSM Detector
 
Two Dimensional Simulation of Pulse and DC Light Response

The simulations were carried out for a 60 × 60 mm2 device with 3 mm finger width and 3 mm spacing between fingers. The physical properties of the InGaAs and InAlAs material systems were estimated with Matthiesen rule. Maxwell’s equation was used to solve the magnetic potential vector and electrostatic potential. Light intensity of the light source for the MSM detection, I, can be expressed as the modulus of a pointing vector, , and is also proportional to the square of the magnetic potential vector. The power dissipation in each node can be determined by taking the divergence of the pointing vector. The estimated power dissipation can be used to estimate the number of carriers generated in the semiconductor due to the fundamental absorption process. Thus the continuity equation equations can be solved with the estimated carrier generation rate.

Figure Caption: Electron and hole distributions at the end of the (left) 100 fs long light pulse (center) after 5ps from removal of light pulse (right) after 215 ps from removal of light pulse
It can be seen that electron and hole distributions follow the power dissipation profile very closely during the light exposure and right after the removal of the light. After the light pulse was switched off the carriers were swept away by the biases and terminated due to recombination. the electron was quick swept to the positive electrode due to the higher electron mobility. Hole mobility was the limiting factor for the MSM operation speed.

The rise time and FWHM of simulated pulse response are 14 ps and 44 ps,respectively. The measured data gives rise time of 20 ps, and FWHM of 140 ps. The simulated results showed reasonable agreement with measured data.
Figure Caption: Pulsed response of MSM photo detector
 
The knee voltage of the I-V curve was due to the band discontinuity between InAlAs and InGaAs. decrease in The knee voltage decereases as thinner Schottky Enahanced layer (SEL) used. As the thickness of SEL decreases, the tunneling current will increase and the knee would vanish altogether with SEL. The presence of ‘knee’ is also considered responsible for poor mixing efficiency.
Figure Caption: (left) Simulated DC responsivity of a 60 mm x 60 mm MSM photodetector with 3 mm finger width and 3 mm finger spacing for different SEL thicknesses at 14 mW of the light intensity. (right) Knee Voltage as a function of InAlAs layer SEL thickness.
 
Developed Submicron Inter-Digitated Finger Process To Reduce the Device Capacitance And Dark Current

PMMA based resist was used to pattern and form submicron inter-digitated fingers. The dark current of the MSM device with submicron inter-digitate fingers is an order less than that of the MSM device with 1 micron wide fingers.
Figure Caption: Normalized reflectance, transmittance, and absorbance of 2000 Å thick e-beam deposited (left) and sputtered ITO (right).
 
Comparison of E-beam and Sputter-Deposited ITO Films for 1.55 um Metal-Semiconductor-Metal Photo-Detector Applications
 
We compare the optical and electrical properties and surface morphologies for ITO films deposited by sputtering, e-beam evaporation and sputtered-e-beam deposited composite deposition. We also demonstrated a high yield process for lifting off sputter-deposited ITO for InGaAs-based MSMPD applications with standard toluene soaking of the resist before exposure during the lithography process.
 
Figure Caption:(Left) A cross section schematic of a multiple ZnO nanorod LED. (Right) A top view of multiple ZnO nanorods.
Typically, etch-back processes are used to make the patterns on sputtered films. However, the etch-back process introduces ion bombardment damage on the semiconductor during the sputter deposition stage. To avoid this problem, the standard positive photoresist lift-off technique had to be used to define the pattern of the composite ITO films. With the assistance of the conventional toluene soaking effect during lithography, 1µm line and space inter-digitated composite ITO fingers were successfully demonstrated.
Figure Caption: Optical microscopy image (left) and SEM image (right) of lifted-off 2000Å sputter ITO films
 
The dark current of these three devices were in a similar range, around 1.5 nA. The device with sputtered ITO did not show an appreciably higher dark current level, however it displayed an early breakdown voltage around 0.2V. This could result from the low bias voltage used for the ITO sputtering. The photo-response of both composite and sputtered ITO MSM showed more than double the photo response for the MSM device with Ti/Au fingers, since the dimension of the gaps and fingers of the MSM was the same. The shadow effect of the Ti/Au fingers reduces the photo response in those devices.
 
Figure Caption: I-V characteristics in the dark (Left) of the MSM devices and optical responses (Right).