Optical Diagnostics and Imaging - Applied Optics Fundamentals and Device Applications - page 298

temperatures to determine if temperature variations are a factor affecting
the performance of the device.
The long-term effects of time and temperature on waveguide and device
operation are difficult to predict theoretically. It is usually necessary to
experimentally determine such performance data by conducting life tests on
the various devices after they are fabricated.

6.4 Measurementof N D ( d )Using
Capacitance-VoltageTechnique
The capacitance-voltage (C-V) technique is an effective method for determin-
ing the dopant profile as well as the width of carrier compensated guiding
layers, Schottky barriers, p-n junctions, and heterojunctions. The thickness
of the depletion layer is determined from the capacitance of a non-injecting
metal contact on the guide surface. The voltage V p required to form a deple-
tion layer of thickness d in a conducting layer of carrier concentration N D and
ϵ r is given by

2

qN d

D

V = (6.8)

p

2 ε

r

If the bulk breakdown field E c of the semiconductor is exceeded during the
measurement, current flows, the accuracy of the technique is immediately
seriously impaired, and the measurement rapidly fails. This occurs when

qN d

D

E = ε (6.9)

c

r

and limits d to the value

= ε r

E
qN

c

d (6.10)

D

where E c is approximately 6 × 10 5 V/cm for N D = 10 17 . This limits the measur-
able layer thickness to the values indicated in Figure 6.5. Chemical etching
sequences are required to characterize deeper layers. Figure 6.6 shows an
automated CV measurement system.