Summary of CTS-S15 Single Crystal Diamond Results
November 2010
Abstract: This report summarizes the results of measurements of single crystal diamond
CTS-S15. The report also compares CTS-S15 with CTS-S11.
CTS-S15
CTS-S15 is the first single crystal diamond grown under new conditions. This sample is a
small (roughly 4mm x 4mm) as-grown sample approximately 200um thick. The sample was
received in three pieces – two very small pieces and one large piece. All results below were
obtained using the largest piece of this sample. The sample varied in thickness from 160um
in the center to 232um on the edge. Figure 1 shows a photograph of the substrate and
growth surfaces. The remnants of the laser cutting are clearly visible on the substrate
surface; the stepped growth is clearly visible on the growth surface. There are no visible
pyramidal defects or any other gross defects on the growth surface. Figure 2 shows the edge
of the growth surface after metalization. Defects are observed in the edge region. Overall this
is the best growth surface we have seen for a single crystal sample.
Figure 1: Photograph of the (left) substrate/laser cut surface and the (right) growth surface
Figure 2: Photograph of the growth surface edge after metalization.
Initially, before metalization, the growth side had a resistance of 5M ohm and the substrate
side (laser cut side) had a resistance of 100 ohms. CTS-S15 was acid-cleaned three times
and oxygen plasma etched to remove any residual graphite-like material. After cleaning the
growth side had a resistance >20M ohms (unmeasurable) and the substrate side had a
resistance of 500 ohms. Since there was no observable resistance from the growth side to
the substrate side metalization was performed so the charge collection distance
measurements could be made quickly. CTS-S15 was metalized on opposite sides (faces) with
Cr/Au dots to ascertain the IV characteristics and the collection distance of the sample.
Figure 3 shows the IV characteristics of the CTS-S15 while it is being irradiated with a Sr-90
source. CTS-S15 exhibits very low leakage currents (less than 1nA even though the system
offsets have not been corrected), a quasi linear slope (indicating a bulk resistance of >2E12
ohms) and no soft breakdown up to voltages of 500V or electric fields of 2.5V/um. These
properties are indicative that CTS-S15 is a very good sample.
Figure 3: IV characteristics of single crystal sample CTS-S15 while under irradiation with 90-Sr.
In Fig. 4 we show the charge collection distance measurements of sample CTS-S15. The
charge collection distance is measured using a Sr-90 source and triggering on a sample
independent scintillator. This data in red is the first measurement of the sample i.e.
unpumped; the data in blue is the measurement after the sample has been pumped for 20 hrs
to passivate traps. The passivation process occurs when a Sr-90 electron passes through the
diamond creating eh pairs which move through the material under the influence of the
external electric field until they are captured by a trap rendering the trap inactive. Thus
pumping indicates the presence of trap. The charge collection distance is related to the mean
charge observed by the relationship:
Charge collection distance = ccd = [Mean charge]/[36e/um]
Figure 4: Charge collection distance versus Voltage for CTS-S15.
From Figure 4 we observe a collection distance of 42um at 500V. We note that at this voltage
the charge collection distance has not yet saturated. Moreover the pumped charge collection
distance of 42um represents 42/200 or 21% of the total thickness of the sample. This is thus,
by far, the best CTS scCVD sample grown. As far as both the IV measurement and charge
collection distance is concerned up to 500V this is an excellent sample.
Previously CTS-S11 was one of the best CTS scCVD samples grown. In order to compare
CTS-S15 with CTS-S11 one has to account for the difference in thickness of the various
samples. CTS-S15 is 200um thick while CTS-S11 is 950um thick. This is usually done by
plotting the collection distance versus electric field instead of applied voltage. In Figure 5 we
compare the collection distance of CTS-S15 with CTS-S11 as a function of Electric Field.
Figure 5: Charge collection distance of CTS-S15 and CTS-S11 as a function of applied electric field.
As Figure 5 indicates CTS-S15 and CTS-S11 have similar collection distances at similar
electric fields. However CTS-S11 is five times thicker than CTS-S15. Thus the conclusion is
CTS-S15 is five times better than CTS-S11, that is, if CTS-S15 was grown 5x thicker its
collection distance would be roughly 5x larger. The guess is that CTS-S15 was grown 2.5
times slower than CTS-S11 and this factor of 2.5 in growth rate translates into a factor of 5 in
collection distance. In any case it would be interesting to know the relative growth rates to
quantify this effect. It is also interesting to note there is a lot of room for improvement of CTSS15. The slow rise of the collection distance and the attainment of only 21% of the total
charge indicates the presence of traps. The best scCVD samples reach full (100%) charge
collection at an electric field of 0.2V/um.
Summary
CTS-S15 is the best CTS scCVD sample grown. That said the sample could be much better
since there are indications of traps in the material. Once way to reduce traps is to slow the
growth rate down. If the analysis above is correct another factor of 2 should yield a sample
with more than 50% charge collection. This sample is also an ideal candidate for removal of
a large amount of material from the substrate side to ascertain the charge collection distance
away from the substrate. This process would also increase the collection distance by
removing the poor material from the substrate.