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Radiation Measurements 46 (2011) 1654e1657

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Radiation Measurements

journal homepage: www.elsevier .com/locate/radmeas

Alpha-particle response of an InSb radiation detector made of liquid-phaseepitaxially-grown crystal

Yuki Sato*, Yasunari Morita, Tomoyuki Harai, Ikuo KannoGraduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

a r t i c l e i n f o

Article history:Received 1 November 2010Received in revised form4 April 2011Accepted 7 April 2011

Keywords:InSbRadiation detectorLiquid-phase-epitaxy

* Corresponding author. Tel.: þ81 75 753 5844; faxE-mail address: Y.Sato@nucleng.kyoto-u.ac.jp (Y. S

1350-4487/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.radmeas.2011.04.011

a b s t r a c t

We fabricate a radiation detector using a p-type InSb crystal grown using the liquid-phase-epitaxy (LPE)method. The energy resolution for 5.5-MeV alpha particles was improved 2.4% from 2.9% through theapplication of bias voltage. This value represents a significant improvement over that of previouslyfabricated devices, 15e40%.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

For environmental preservation, an important method for det-ecting hazardous elements such as Li, Be, and Pb is the X-rayfluorescence analysis method. In X-ray fluorescence analysis, Sidetectors are generally used for X-ray detection. Si detectors are,however, not very suitable for detecting heavy elements, such as Pb,which emit high-energy K X-rays that are nearly 80 keV in energy.The small atomic number and density of Si result in an absorptionefficiency less than 3% for a 3-mm-thick Si(Li) detector (Gallagherand Cipolla, 1974). In addition, the K X-rays of light elements suchas Li and Be have energies below 100 eV. For the measurement oflow-energy X-rays, the energy resolution of the Si detectors isinsufficient. Photon detectors that have high detection efficiencyfor X-rays that have energies of several tens of keV and high-energyresolution for X-rays below 100 eV are necessary for the detectionof these hazardous elements.

Compound semiconductor InSb has advantages as a substratefor high performance photon detectors. The high atomic numbers(In: 49, Sb: 51) and high density (5.78 gcm�3) result in photonabsorption efficiency nearly three orders of magnitude greater thanthose of Si detectors. In addition, it has the smallest bandgap energy(0.165 eV at 290K) among developed semiconductor substrates,which brings twice the energy resolution of that of Si detectors(McHarris, 1986). Work has been done on the development of InSbdetectors using commercial InSb wafers (Kanno et al., 2002, 2003,

: þ81 75 753 5845.ato).

All rights reserved.

2004, 2005, 2006, 2007, 2008; Hishiki et al., 2005, 2006, 2007).In addition, it has been shown that InSb detectors can detect thealpha-particle emission of 241Amwith energy resolution that variesfrom 15 to 40% (Kanno et al., 2006).

The InSb detectors described above, however, could not operatewith an applied bias voltage, due to their small diode resistances. Inthis work, response of an InSb detector made using LPE InSb crys-tals to alpha particles is investigated. We show that the resistanceof the InSb detector as a diode is high enough to operate with biasvoltage.

2. Experiment

2.1. Detector fabrication

The LPE InSb crystal was grown on a 0.4-mm-thick InSbsubstrate (n-type: Galaxy Compound Semiconductor, Inc., U.S.A)and had a thickness of 115 mm. The electrical and growth propertiesof LPE InSb are described in the reference (Sato et al., 2010). UsingHall measurements, the electric conductivity of the LPE InSb crystalwas found to be p-type at temperatures below 140K. A schematic ofthe InSb detector is shown in Fig. 1. The LPE InSb crystal was cut toa dimension of 3 � 5 mm. For a rectifying contact, we patterneda 1-mm-diameter electrode on the epitaxial side of the LPE crystalusing photoresist. Both sides of the crystal were etched for 30 swith a mixture of hydrogen fluoride, hydrogen peroxide, andde-ionized water (48%HF:H2O2:H2O ¼ 1:1:3). To serve as a recti-fying contact, Al was deposited by heat evaporation with a thick-ness of 17 nm on the epitaxial side. The substrate surface of the LPE

Fig. 1. Schematic drawing of the InSb detector.

Bias Voltage(CANBERRA MODEL 3102D)

Charge sensitive Pre. Amp.(CANBERRA 2003BT)R1

MCA

Main Amp.(CANBERRA 2024)

PC

R2InSbdetector

Fig. 3. Block diagram of the electronic circuit used in the alpha-particle measurement.MCA refers to the multichannel analyzer. The internal resistances R1 and R2 in thepreamplifier were 10 MU and 100 MU, respectively.

Y. Sato et al. / Radiation Measurements 46 (2011) 1654e1657 1655

crystal was attached to a Cu plate using In solder as an Ohmiccontact. The edge of the LPE crystal was coated with an epoxy resinto adhere it to the Cu plate.

2.2. Alpha particle measurements

To detect alpha particles using the LPE-InSb detector, thedetector was mounted on a Cu sample holder of a liquid-He flowcryostat (Helitran LT3: Advanced Research Systems, Inc., Pennsyl-vania, USA). A schematic of the experimental apparatus formeasuring the alpha particles is shown in Fig. 2. An 241Am (5 kBq)source was installed inside of the vacuum shroud of the cryostat.The distance between 241Am source and the InSb detector was15 mm. The alpha-particle measurements were performed withand without applying bias voltage at the temperature of 5.4K. Fig. 3shows a block diagram of the electronic circuit for the alpha-particle measurements. The energy spectra of the 241Am alphaparticles were measured using a multichannel analyzer. The biasvoltage was applied via the series connection of resistances ofa preamplifier (110 MU in total).

3. Results and discussion

The energy spectra of the alpha particles emitted by the 241Amsource with and without applying bias voltage are shown in Fig. 4.

Fig. 2. Schematic drawing of the experimental setup.

The energy resolution obtained, through the measurements inwhich the bias voltage was not applied, was 2.9%. This value issuperior to previously obtained values measured by InSb detec-tors made from commercially obtained InSb wafers, 15e40%.This is caused by the decreased number of recombination andtrapping of electrons and holes in the LPE-InSb crystal, becausethe carrier concentrations of LPE InSb crystal are smallerthan those of commercial InSb crystals (Kanno et al., 2006; Satoet al., 2010).

The peak channel numbers and energy resolutions are shown inFig. 5(a) and (b) as a function of the output voltage of the biassupply. When the bias voltage is applied, the peak channel numberincreased and energy resolution improved to 2.4%. In addition, byapplying bias voltage, the tail of the energy peak in Fig. 4 dis-appeared, because of the improved carrier collection efficiencycaused by the higher electric field.

In this detector, the thickness of the depletion layer with no biasvoltage was estimated to be a 2.3 mm (Sato et al., 2010). However,the travel range of the 5.5-MeV alpha particles was nearly 20 mm inInSb (Gobeli, 1956). Therefore, the alpha particles did not deposit all

Fig. 4. Energy spectra of the 241Am alpha particles (5.5 MeV) measured at 5.4K. Theshaping time of the main amplifier was 0.25 ms. The solid and dotted lines show theresults of the measurements without and with the 600-V bias supply output.

0.0 0 .2 0.4 0 .6 0.8 1 .0770

780

790

800

810

820

Pea

k C

hann

el N

umbe

r

Bias Supply Voltage (kV)

Bias Supply Voltage (kV)0.0 0 .2 0.4 0 .6 0.8 1 .0

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Ene

rgy

Res

olut

ion

(%)

a

b

Fig. 5. (a) Peak channel numbers as a function of the bias supply output for 5.5-MeValpha particles. The solid line shows the results of a linear fit. The error bars are smallerthan the size of the symbols. (b) The energy resolution as a function of the bias supplyoutput for 5.5-MeV alpha particles.

Fig. 6. Currentevoltage curve of the InSb detector measured at 4.2K.

Y. Sato et al. / Radiation Measurements 46 (2011) 1654e16571656

of their energy in the depletion layer. In addition, the absorptionefficiency of the 2.3-mm-thick InSb layer for 80 keV X-rays waspredicted to be about 0.4%, using themass attenuation coefficient ofIn and Sb (Hubbell and Seltzer, 1995). To improve the energyresolution for alpha particles and detection efficiencies for X-rays,a thicker depletion layer is necessary.

The thickness of the depletion layer is (Sze, 2001):

dy�23ðV0 þ VÞ

eNa

�1=2(1)

where V0 and V are the inherent voltage of the rectifying contactand added reverse voltage, Na is the accepter (donor) concentra-tion. 3 and e are the permittivity of InSb and elementary electriccharge, respectively. In this experiment, the added voltage, V, onthe detector was not equal to the output voltage of the bias supply,Vb. For the calculation of the depletion layer thickness, the addedvoltage V is necessary. The added voltage V is, however, hard toestimate, because the resistance of the InSb detector was very smallcompared with the internal resistance of the preamplifier.However, using the breakdown voltage, we can estimate themaximum thickness of the depletion layer. The breakdown voltagewas derived from the currentevoltage curve of Fig. 6, and wasapproximately�1 V. Themaximum thickness of the depletion layerwas estimated to be 4.7 mm, and the absorption efficiency for80-keV X-rays was predicted to be 0.8%.

According to Eq. (1), the thickness of the depletion layer isproportional to N�1=2

a . The 15-mm-thick depletion layer could beobtained if we can decrease the accepter concentration to the onetenth of the one of the present value for an LPE wafer when theadded voltage is �1 V. The absorption efficiency for 80-keV X-rayswould be 2.6%. This value is almost the same value that can beobtained from Si(Li) detectors that are one thousand times thicker.In addition, the increased breakdown voltage due to the decrease ofthe accepter concentration would effect a thicker depletion layer(Sze, 2001). As a result, a higher detection efficiency can be ach-ieved with InSb detectors than with using Si(Li) detectors.

4. Conclusion

A bias voltage was applied to an InSb detector made usingliquid-phase-epitaxy grown crystal that has a smaller impurityconcentration than commercially available crystals. When a biasvoltagewas applied, the energy resolution for 241Am alpha particlesimproved. Further efforts to decrease the impurity concentrationwill result in a thicker depletion layer and, consequently, a higherphoton absorption efficiency.

Acknowledgments

This work was supported by the Kyoto University Global COEProgram “Energy Science in the Age of Global Warming”.

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