4.4 Cryogenic Measurements
Cryogenic measurements refer to characterisations carried out under low temperature typically ranging from liquid helium (4.2K) or liquid nitrogen (77.3K) to below room temperature (298K). These measurements are used for determining a number of physical parameters and mechanisms in semiconductor materials and devices such as bandgap, barrier heights, defect levels, current transport mechanisms etc. In this study, the lowest temperature was limited to 77.3K, the boiling point of nitrogen.
A Oxford Instruments variable temperature liquid nitrogen cryostat, model DN1704, in
conjunction with a temperature controller, model ITC4, also manufactured by Oxford
Instruments, constituted the major parts of the experimental set-up. A schematic diagram of
the cryostat is shown in Figure 4.7:
Figure 4.7: Schematic of the Oxford Instruments Cryostat, Model no. DN1704
The cryostat consists of an inner cylindrical sample chamber surrounded by an outer jacket;
the outer jacket is pumped to a relative vacuum for thermal isolation during measurements.
T05 headers containing the test device are mounted on a metallic rod insert which enters the
cryostat from the top, placing the device into the sample space at the bottom. The insert has
11 BNC connectors: 10 for the T05 header and 1 for an additional temperature sensing silicon
diode placed in close proximity to the test device a few millimeters above the T05 header.
A heat exchanger with heater and temperature sensor sits slightly above the sample.
Following the insertion of the test device, the inner chamber is pumped down and flushed with
dry nitrogen; the procedure is repeated several times to remove any water vapour which
would otherwise condense at sub-zero temperature and may short out electrical contacts.
Temperature control is achieved by setting the target on the ITC4 temperature controller and
adjusting the liquid nitrogen flow rate into the heat exchanger; it is opened progressively for
lowering the temperature or closed for raising the temperature.
The temperature sensing diode mounted on the insert rod is first calibrated against the
temperature reading of on the ITC4 controller display. This is achieved by passing a 10mA
forward current from a Keithley source through the diode and measuring the voltage drop
across it with a digital volt meter. The current is deliberately kept low to minimise the effects
of local heating from the sensing diode. At a constant forward current, the voltage drop
across a diode is inversely proportional to the ambient temperature according to the diode
equation. This is illustrated by the experimental results as shown in
Figure 4.8.
Figure 4.8: Graph of voltage drop across the temperature sensing diode vs. ambient
temperature while the forward current was kept at a constant value of 10mA
In addition to calibrating the DVM reading versus the temperature for the sensing diode, it is
important to establish the time required to reach a target temperature. This is shown by the
measured results in Figure 4.9.
Figure 4.9: Graph showing the time required to reach a target temperature
It is seen that the approximate time to reach a steady target temperature varies between 25 to
35 minutes. This time is independent of the direction i.e. the time taken to reach a higher
temperature takes roughly the same as that for a lower temperature.
4.4.1 Experimental Set-up
© 1998: Shabbir A. Bashar (in accordance with paragraph 8.2d, University of London
Regulations for the Degrees of M.Phil. and Ph.D., October 1997). The Copyright of
this thesis rests with the author, and no quotation from it or information derived
from it may be published without the prior written consent of the author.
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