Aoki, Toru

Gnatyuk, Volodymyr

Kanemaru, Seigo

Mimura, Hidenori

Nagao, Masayoshi

Nakagawa, Masashi

Neo, Yoichiro

Yasumuro, Chiaki

English

Электронная библиотека БГУ

MICRO FIELD EMITTER DRIVE CdTe X-RAY IMAGER 
(приглашенный доклад) 
Toru Aoki, Masashi Nakagawa, Masayoshi Nagao*, Chiaki Yasumuro*, 
Seigo Kanemaru*, Volodymyr Gnatyuk, Yoichiro Neo, Hidenori Mimura 
Research Institute of Electronics, Shizuoka University, Japan 
*AIST, Japan 
We proposed a new addressing method that used a field emitter type 
electronic source for the radiation detector. And we have made it possible that 
this device operates at room temperature by using a cadmium telluride (CdTe) 
diode. In this addressing method, the electron energy from an electronic 
source in each pixel was highly uniform. Moreover, because the proposed 
device was actuated by the field emitter, it was expected to achieve the super-
high resolution X-ray imager. We used the carbon nanoneedle field emitter, 
and the proposed device operation was confirmed by verifying the principle 
about one pixel. 
The CdTe M-p-n diode and carbon nanoneedle field emitter were actually 
combined for the verification of the proposed principle that was the purpose of 
this study and it was experimentally confirmed. In the measurement system 
the carbon nanoneedle field emitter emitted electrons to the p-type CdTe side 
by applying the gate voltage (GV) of positive bias. At the same time, the n-
type CdTe side attracted electrons by applying the anode voltage (AV) of 
positive bias. As a result, p-type CdTe side was negatively electrified by field 
emission electrons. In this case, the flowed dark current in this circuit was 
measured. Furthermore, in addition to this state, X-ray was irradiated to the 
CdTe detector. An electron-hole pair was generated as a result of the 
photoelectric effect by interaction between CdTe and X-ray and then a hole 
was recombined with a field emission electron. As the signal current, we 
measured the flowed current which was changing by X-ray irradiation. In the 
experiment, field emission electrons continuously bombarded the p-type CdTe 
side but an X-ray source was turned on and off every 20 seconds. The X-ray 
current measurement was carried out by changing the X-ray tube current. The 
tube current related to the X-ray radiation value, so we confirmed the relation 
between the tube current and signal current. 
Fig. 1 shows the dark current measured when field emission electrons 
irradiated CdTe and at positive biased AV. If the dark current value is too 
large, the signal current is buried at radiation and X-ray is not detected. As it is 
seen from the results shown in Fig. 1, the anode current has a favorable small 
value at GV up to about 48 V. But the current was reversed from positive and 
negative at GV over 48 V. We have considered  that this reverse phenomenon
94
was caused by secondary electron emission. It happened because the number 
of secondary electrons emitted from CdTe exceeded the number of primary 
electrons irradiated to CdTe by the carbon nanoneedle field emitter. 
 
Fig. 1. The dark anode current as a function 
of applied anode voltage when field 
emission electrons irradiated CdTe 
 3 
 
 
Therefore, the value of AV was fixed at 20 V and it was used in the 
experiment. It experimented under the standard condition by using X-ray. The 
experimental results are shown in the Fig. 2 and 3. As a result of the 
experiment 1, we confirmed that the CdTe-based structure fabricated in this 
study was sufficiently effective for photoelectric conversion. 
 
 
 
0 10 20 30 40 50 60-1 
0 
1 
2 
Anode voltage, V
A
no
de
 c
ur
re
nt
, n
A
 
Fig. 2. The difference of the anode current in dark and 
X-ray irradiated conditions (experiment 1) 
1 
2 
100 200 300 
-1 
0 
Time, s
Si
gn
al
 c
ur
re
nt
, n
A
 
0 
95
Furthermore, Fig.3 shows that an increase in the tube current causes a rise 
in the signal current. Because the tube current corresponded to the X-ray 
intensity, this current dependence of the signal was evidence that the radiation 
detector was sensitive to X-ray. 
 
Fig. 3. The signal current as a function of the 
X-ray tube current 
 
0.8 
1 
 
 
In this study, we have proposed a new addressing method that used a field 
emitter type electronic source in order to fabricate super-high resolution 
radiation detectors in the near future. We carried out verification of the 
operation principle about one pixel by using the CdTe M-p-n diode and carbon 
nanoneedle field emitter, and demonstrated the capability of this device to 
operate as a photoelectric conversion detector. 
Secondary electron emission is due to work function and surface state of an 
Au electrode, so it is necessary to investigate electrode materials to apply 
higher AV and improve carrier collection efficiency. The results of this study 
have showed that the developed addressing method is promising for 
elaboration of two-dimension pixel devices. 
20 40 60 80 100
0.2 
0.4 
0.6 
0 
X-ray tube current, μA
Si
gn
al
 c
ur
re
nt
, n
A
 
96


Micro field emitter drive CdTe X-ray imager

http://elib.bsu.by/bitstream/123456789/31312/1/MICRO%20FIELD%20EMITTER%20DRIVE%20CdTe%20X-RAY%20IMAGER.Pdf

Micro field emitter drive CdTe X-ray imager

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