Low-temperature photoluminescence analysis of the γ-irradiation effect on the defect structure in Ge-doped CdTe single crystals

Author's Accepted Manuscript

Low-temperature photoluminescence analysis of the γ-irradiation effect on the defect structure in Ge-doped CdTe single crystals Iu. Nasieka, L. Rashkovetskyi, M. Boyko, V. Strelchuk, Z. Tsybrii, B. Danilchenko, L. Shcherbak

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S0022-2313(13)00384-0 http://dx.doi.org/10.1016/j.jlumin.2013.06.046 LUMIN11995

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Journal of Luminescence

Received date: 29 January 2013 Revised date: 10 June 2013 Accepted date: 27 June 2013 Cite this article as: Iu. Nasieka, L. Rashkovetskyi, M. Boyko, V. Strelchuk, Z. Tsybrii, B. Danilchenko, L. Shcherbak, Low-temperature photoluminescence analysis of the γ-irradiation effect on the defect structure in Ge-doped CdTe single crystals, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2013.06.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1    1  2 

Low-temperature photoluminescence analysis of the γ-irradiation effect on the defect structure in Ge-doped CdTe single crystals

3 

Iu. Nasiekaa, L. Rashkovetskyia , M. Boykoa, V. Strelchuka, Z. Tsybriia, B. Danilchenkob and

4 

L. Shcherbakc

5  6  7  8  9 

a

Lashkarev Institute of semiconductor physics, NAS of Ukraine, 41 Pr. Nauki, Kyiv, Ukraine

b

Institute of physics, NAS of Ukraine, 46 Pr. Nauki, Kyiv, Ukraine

c

Yuriy Fedkovych Chernivtsi National University, 2 Str. Kotsiubinskogo, Chernivtsi, Ukraine Keywords: low-temperature photoluminescence, CdTe crystals, Ge-doping, γ-irradiation,

Compton’s phenomenon, ionization, Gaussian decomposition.

10 

Abstract

11 

An influence of different doses (10–500 kGy) γ-irradiation on the low-temperature

12 

photoluminescence (LTPL) of germanium-doped (Ge-doped) CdTe crystals was investigated.

13 

The following results were obtained: even at the lowest doses (equal to 10kGy) γ-irradiation

14 

leads to the substantial modification of the LTPL of Ge-doped CdTe crystals namely decrease in

15 

the intensities of the bound exciton emission lines - D0X, A0X, VDX and increase in the

16 

intensities of impurity-related emission lines – band to double acceptor transitions line edA and

17 

donor-acceptor pairs recombination line DA. Such changes in the LTPL spectra can be explained

18 

by the radiation-induced changes in the concentration of the corresponding luminescence centers

19 

due to their interaction with fast electrons created corresponding to Compton’s phenomenon.

20 

However at the increase in the dose of γ-irradiation the effect of saturation was observed in the

21 

dose dependencies of the intensities and Huang-Rhys factor of the all emission lines. Mentioned

22 

feature can indicate that studied luminescence centers are formed by the defects with different

23 

nature and some of that have increased radiation stability.

24  25 

2    1 

Introduction

2 

Cadmium telluride is a promising material for different possible applications in research or

3 

industry [1-3]. But the main field of the CdTe application is manufacturing of the high-sensitive

4 

radiation detectors which have the ability to perform energy-dispersive spectroscopy of high

5 

energy radiation such as X-rays, γ-rays and other types of ionizing radiation [2-5]. The

6 

increasing interest in the use of II-VI compounds, in particular Ge-doped CdTe single crystals, in

7 

the devices of room-temperature X- and γ-rays detectors is caused by their high resistivity, good

8 

signal-to-noise ratio and μτ-product [4, 5]. High resistive CdTe can be manufactured by

9 

introducing of impurities which create deep levels and can compensate intrinsic defect

10 

complexes. The introducing of Ge atoms converts the intrinsically conductive p-type CdTe to

11 

semi-insulating by the formation of a Ge2+/3+ related deep donor level that is responsible for the

12 

compensation of the native acceptors cadmium vacancies (Cd-vacancies) [4-7]. During the

13 

growth process Ge dopant atoms also substitute vacancies in the cadmium sublattice. It is

14 

assumed by formation of the complexes (Ge+VCd-)0 or (Ge+VCd2-)-.

15 

In order to utilize Ge-doped CdTe crystals as detectors, their behavior under intense

16 

radiation fields still needs to be investigated. Therefore, in the present work an influence of

17 

different doses of γ-irradiation on the impurity-defect states in the Ge-doped CdTe crystals will

18 

be investigated using the method of low-temperature photoluminescence (LTPL). The processes

19 

of the interaction of the radiation with intrinsic defects, in particular attributed with Ge-dopant

20 

atoms in studied crystals, will be discussed.

21 

1. Experimental part

22 

Our investigations were performed for Ge-doped CdTe crystals grown by vertical

23 

Bridgman technique. The concentration of Ge dopant atoms in the solid phase was (2 – 3)×1016

24 

cm-3. An initial components Cd and Te for the growth process were of 6N purity. Investigated

25 

Ge-doped CdTe crystals were of p-type conductivity with the room-temperature resistivity ρ ≈

3    1 

109 Ohm·cm and ρ → ∞ at T = 5 K (the conductivity of the crystals at 5 K is determined by

2 

excess electrons and holes). From the boles of 40 mm in the diameter the wafers of size 20×20×2

3 

mm3 were cut. Ge-doped CdTe wafers were of (111) orientation. Damaged during cutting

4 

process, surface layers were mechanically polished with the different fraction diamond pastes

5 

then etched in the bromine-methanol [8]. As a result the wafers had the surface sharpness equal

6 

to 0.1 μm. Such prepared Ge-doped CdTe wafers (crystals) were irradiated at room temperature

7 

with the different doses of γ-irradiation in the range of 10-500 kGy. Corresponding γ-quanta

8 

fluxes were Nγ = 1.69×1015 – 8.45×1016 quanta/cm2. 60Co source with the photon energy of about

9 

1.2 MeV was used.

10 

For the verification of the material quality of the initial samples IR-Fourier spectrometer

11 

Perkin Elmer Spectrum BX II was used. The spectra of optical transmittance were recorded in

12 

the range of the wavelengths λ = 1.5 – 25 μm. The method of LTPL was used as a main tool for

13 

the investigation of gamma-stimulated changes of the structural properties of Ge-doped CdTe

14 

single crystals. He-Ne laser with λex = 632.8 nm was used as an excitation source. All

15 

photoluminescence measurements were done at 5 K. For keeping at such temperature, samples

16 

were loaded in A-240 Optical Helium bath-flow cryostat with Unified Thermoregulated

17 

Cryogenic

18 

photomultiplier with antimony-cesium emitter. To reduce an electronic noises photomultiplier

19 

was cooled with liquid nitrogen. 

Systems

(UTRECS).

Photoluminescence

signals

were

registered

using

20 

2.

Results and discussion

21 

Spectral dependence of the optical transmittance in the two points of initial Ge-doped

22 

CdTe samples of (111) orientation was shown in the figure 1. One can see the studied samples

23 

have relatively high optical transmittance T = 65 – 68 % in all spectral range. Such value of the

24 

optical transmittance, which is close to the theoretical possible one for the CdTe single crystals

25 

[6], indicates high crystalline quality of the studied crystals. In our case such high transmittance

4    1 

of the studied crystals can be attributed to the compensating and stabilizing effect of the Ge

2 

impurity.

3 

The first LTPL measurements show us that the general feature of the γ-irradiation effect on

4 

the photoluminescence of the studied crystals is the similar and synchronous decrease of the

5 

amplitudes of all emission lines observed in the spectra for all crystals irradiated with different

6 

doses. Such effect can be explained by increasing in the concentration of radiationless

7 

luminescence centers. Therefore, with the aim to minimize the influence of such centers on the

8 

photoluminescence lines amplitudes we have normalized all photoluminescence spectra. Figure 2

9 

presents normalized LTPL spectrum of the Ge-doped CdTe crystals and its decomposition on an

10 

elementary Gaussians [1, 9]. The solid line is an experimental spectrum, while dots designate a

11 

theoretical Gaussians. As usual LTPL spectrum of initial sample consists of three spectral ranges

12 

– excitonic (1.56 – 1.6 eV), impurity (1.46 – 1.56 eV) and deep level defects (1.3 – 1.46 eV)

13 

ranges. These ranges have complex structure and it is appropriate to describe them separately.

14 

Firstly, it is worth noting there are no emission peaks in the deep levels defects range of the

15 

LTPL spectra of initial and irradiated samples. Such fact indicates relatively high material

16 

quality of the studied samples. Therefore, further we will not show and discuss the spectral range

17 

of 1.3 – 1.46 eV. So, the excitonic range of the LTPL spectrum of initial sample consists of free

18 

exciton (FE) emission peak at 1.5964 eV with the full width on half maximum (FWHM) - w = 3

19 

meV, neutral donor bound exciton (D0X) emission peak at 1.5927 eV and w = 2.6 meV, shallow

20 

neutral acceptor bound exciton (A0X) at 1.5891 eV and w =2.8 meV and the emission peak

21 

attributed to the exciton bound on the complex Cd-vacancy – donor (VCd-D) centered at 1.5864

22 

eV and w = 4 meV (VDX line). The phonon replicas of the FE and A0X emission peaks also

23 

were registered, see figure 2. The phonon energy is 21.5 meV [1, 9 and 10]. The impurity range

24 

of the LTPL spectrum of initial sample consists of band to double acceptor (edA) emission line

25 

at 1.5577 eV and 1.5536 eV (there were pointed photon energies determine the spectral region of

5    1 

the double acceptor) [11, 12] and donor-acceptor pair (DA) emission peak at 1.5468 eV and w =

2 

12 meV. According to the Gaussian decomposition both acceptors included in the double

3 

acceptor complex separately have emission lines with w = 3 meV and 4 meV respectively. The

4 

phonon replicas of the edA and DA emission peaks were registered too, see figure 2. The phonon

5 

energy is equal to 21.5 meV [1, 9, 13-15].

6 

As it is known, FE emission peak is caused by annihilation of the free excitons. D0X peak

7 

is mainly caused by the neutral germanium atoms – GeCd0, where GeCd0 is a donor in CdTe. The

8 

donor type of the GeCd impurities provided by two unbound bindings of the Te neighbor atom

9 

which under specific conditions, such as high energy particles flux, can leave its site with

10 

formation of Frenkel’s pairs VTe-Tei [4, 6, 7, 16]. A0X is determined by the Cd-vacancies (VCd).

11 

However, in the bulk of the Ge-doped CdTe crystals energetically different Cd-vacancies do

12 

exist [17-19]. Therefore, we supposed that the centers VCd2- are responsible for the A0X emission

13 

line. For the same reasons VDX line can be determined by the VCd2- and GeCd0 as a donor. The

14 

lines eA1 and eA2 are the top and the bottom levels of the double acceptor which can be caused

15 

by the neutral Cd-vacancy (VCd0). As it follows from the works [19-23] DA emission is most

16 

probably caused by the recombination in the donor-acceptor pairs formed Ge-related donors such

17 

as GeCd2+/3+ and neutral Cd-vacancy acceptors.

18 

Figure 3 shows the influence of the minimal dose of γ-irradiation on the LTPL spectra of

19 

Ge-doped CdTe single crystals. As it is seen from the spectrum of the irradiated crystal such

20 

dose leads to the decrease in the intensities of the bound exciton emission lines - D0X, A0X,

21 

VDX and increase in the intensities of impurity-related emission lines – band to double acceptor

22 

transitions line edA and donor-acceptor pair recombination line DA. Such changes in the LTPL

23 

spectra can be explain by the radiation-induced changes in the concentration of the

24 

corresponding luminescence centers due to their interaction with γ-quanta.

6    1 

A probability of the appearance of the atom displacement or creation of other radiation-

2 

induced defect in the result of direct interaction of γ-quanta with the crystalline material

3 

nucleuses is very small. Therefore, it is worth noting the main effect of the defect creation in this

4 

case is attributed to the influence of the fast electrons which were induced as a result of a

5 

photoelectric effect and Compton’s phenomenon [24-26]. It is generally known, in CdTe based

6 

compounds γ-irradiation leads to the creation of different charged Cd-vacancies and related

7 

interstitial atoms [13, 15, 17 and 18]. We have not performed a direct experiment that proves the

8 

radiation-induced acceptors nature, however, the high probability of the assumption pointed

9 

above is confirmed by the following facts. Firstly, the concentration of radiation-induced Cd-

10 

vacancies in irradiated crystals is somewhat higher than the concentration of tellurium Te-

11 

vacancies. The latter induce the luminescence line with the energy peak position at 1.1 eV in

12 

CdTe at 4.2 K [1, 2 and 27]. In our LTPL measurements we did not observe the lines with

13 

pointed peak positions as well as emission lines which can be attributed to interstitial Te atoms.

14 

Taking into account the latter we can suppose that decrease in the intensity of D0X emission line

15 

is induced by decreasing in the concentration of GeCd0 centers due their ionization by radiation

16 

induced high energy electrons created respectively to Compton’s effect or photoelectric effect

17 

[24, 28]. Ionization of the GeCd0 centers leads to creation of other donor centers - GeCd2+/3+.

18 

Decrease in the intensity of A0X emission line can be explained by decrease in the concentration

19 

of VCd2- due to their ionization and creation of the VCd0 acceptors [5-7]. Based on the latter we

20 

can conclude the superposition of the ionization both GeCd0 and VCd2- is responsible for the

21 

decreasing in the intensity of VDX emission line. As for the edA and DA emission lines their

22 

intensities increase is caused by the increase in the concentration of the radiation-induced VCd0

23 

acceptors and GeCd2+/3+ donors [4-7, 16].

24 

In our work we investigated an influence of different doses of γ-irradiation on the LTPL of

25 

Ge-doped CdTe. Figure 4 shows the dependencies of the excitonic emission line intensities on

7    1 

the γ-irradiation doses. One can see all intensities have similar decreasing character with the

2 

growth of irradiation dose. Such regularities of the changes in the intensities of excitonic

3 

attributed emission lines A0X, D0X and VDX can be explained by decreasing in the

4 

concentration of GeCd0 and VCd2- centers due to their further ionization by fast radiation-induced

5 

electrons. However, as we can see from the figure 4, at the highest (500 kGy) dose of γ-

6 

irradiation nonzero values of the excitonic emission line intensities are observed. Such fact

7 

indicates the existence of small quantity of centers with high radiation stability in the set of

8 

luminescence centers responsible for the A0X, D0X and VDX lines [5-7, 9, 28]. We believe that

9 

noted centers are probably caused by the defects of different comparatively to the majority of

10 

ones nature which determines their increased radiation stability.

11 

Figure 5 represents the intensities of edA and DA emission lines as functions of irradiation

12 

dose. One can see that the intensities of these lines firstly significantly increase with increase in

13 

the radiation dose (to the value equal to 50 kGy) then with further increase in the irradiation dose

14 

(to the 500 kGy) they remain virtually unchanged. Such type of dose dependencies indicates an

15 

existence of the saturation dose of about 50 kGy for the γ-irradiation in Ge-doped CdTe. In our

16 

case it means that in the dose range 50-500 kGy the crystalline structure and defects states

17 

changes very weak or remains unchanged. However, we did not investigate an influence of lager

18 

then 500 kGy dose of γ-irradiation on the defects structure of Ge-doped CdTe. Therefore, we do

19 

not exclude the possibility of the changing of the stable character of the radiation modified

20 

defects structure in the range 50 – 500 kGy to unstable one which means further increase or

21 

decrease in the concentration of corresponding luminescence centers at the doses lager than 500

22 

kGy.

23 

Here it should be noted, similar character of the dose dependencies of both components of

24 

double acceptor intensities indicates about correctness of the assumption that emission lines at

8    1 

1.5577 eV and 1.5536 eV have the same nature and are two components of the one double

2 

acceptor, see figure 5.

3 

The measure of the Huang-Rhis factor S (electron-LO-phonon coupling factor) [9, 21, 29]

4 

from the relative intensities of the LO-phonon (longitudinal optical phonon) replicas gives useful

5 

information about electronic structure of the center mediating recombination. Dependencies of

6 

Huang-Rhys factor of the different radiative transitions contained in the LTPL spectra of Ge-

7 

doped CdTe crystals on an irradiation dose are shown on the figures 6 and 7. From the figures

8 

we can see the value of S for the FE, A0X, edA and DA emission lines is much lower than 1 for

9 

the initial and γ-irradiated samples. This fact indicates weak electron-LO-phonon coupling in the

10 

corresponding luminescence centers [9, 21 and 29]. Also one can see the value of electron-LO-

11 

phonon coupling factor of all emission lines except DA have similar character. At first it

12 

increases then has weak dependence on a radiation dose. DA Huang-Rhys factor does not depend

13 

on a value of γ-irradiation dose. Such features indicate saving in the electron-LO-phonon

14 

coupling degree at high doses and confirm an idea about existence of defects with increased

15 

radiation stability.

16 

However, in the work [9] we reported about influence of γ-irradiation on the

17 

photoluminescence properties of Cd0.95Zn0.05Te crystals. It is worth noting for Cd0.95Zn0.05Te

18 

crystals the values of S fall with increase in the dose of γ-irradiation. In the present work we

19 

have increase of S values in the similar range of the γ-irradiation doses. The explanation of the

20 

mentioned fact can be the following. As-grown Ge-doped CdTe and Cd0.95Zn0.05Te crystals are

21 

characterized

22 

photoluminescence bands for these crystals formed by similar scheme of electron transitions are

23 

based on the luminescence centers with different systems of electron coupling with the

24 

corresponding crystalline lattices. Therefore, an influence of γ-irradiation on the experimentally

25 

obtained S for Ge-doped CdTe and Cd0.95Zn0.05Te crystals will be different.

by

the

different

impurity-defect

structures.

Obviously

corresponding

9    1 

Conclusions

2 

An analysis of the interaction of the ionizing radiation with the materials, perspective in

3 

the manufacturing of the room temperature radiation detectors and spectrometers, is important

4 

for the understanding of the features of the radiation-induced modification in the defects

5 

structure of the noted materials which can lead to the degradation of the spectrometric properties

6 

during the long-time operation under high radiation levels. From the performed investigation we

7 

can summarize that for Ge-doped CdTe crystals even at low doses of irradiation a substantial

8 

changes of the LTPL and accordingly in the defects structure are observed. We have shown that

9 

noted changes can be mainly attributed to the transformation and modification in the energy

10 

states of different type Cd-vacancies, Ge-dopant atoms in the host crystalline matrix and more

11 

complex defects which include the latter due to their ionization by the fast electrons induced by

12 

γ-radiation.

13 

However, it was obtained LTPL of the Ge-doped CdTe crystals includes the components

14 

with very weak sensitivity even to the high (equal to 500 kGy) doses of γ-radiation. The nature

15 

of pointed radiation stability is not fully understood yet. Therefore, the study of such

16 

luminescence centers can be in our next work correspondingly with the aim to find the possible

17 

ways of increasing of radiation stability of the detector-grade materials. References

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12   

Figure and captions

Figure 1. Optical transmittance spectra of as-grown Ge-doped CdTe crystal. 1 and 2 lines are attributed to the different dots on the sample surface.

1  2 

Figure 2. LTPL spectra of as-grown Ge-doped CdTe crystal with its decomposition on an

3 

elementary Gaussians.

13   

1  2  3 

Figure 3. LTPL spectra of as-grown (1) and γ-irradiated with the dose of 10 kGy (2) Ge-doped

4 

CdTe crystals.

5 

Figure 4. Dose dependencies of the intensities of the excitonic lines in as-grown and γ-

6 

irradiated Ge-doped CdTe single crystals.

14   

1 

Figure 5. Dose dependencies of the intensities of impurity related lines in as-grown and γ-

2 

irradiated Ge-doped CdTe single crystals.

3  4 

Figure 6. Dose dependencies of the Huang-Rhys factor obtained for the free exciton and neutral

5 

acceptor bound exciton lines from the corresponding LTPL spectra.

6  7 

Figure 7. Dose dependencies of the Huang-Rhys factor obtained for the donor-acceptor pairs and

8 

double acceptor emission lines from the corresponding LTPL spectra.

9 

15    1  2  3 

Keywords: low-temperature photoluminescence, CdTe crystals, Ge-doping, γ-irradiation, radiation detectors.

16    1 

•

detectors of ionizing radiation based on CdTe crystals.

2  3 

•

6  7  8 

In the article we analyze the γ-iradiation effect on the defects structure of CdTe:Ge crystals using the data of PL measurements.

4  5 

An article concerns the field of the manufacturing of uncooled semiconductor

•

An analysis of the PL data is provided in the terms of the changing of the intensities of corresponding emission lines as well as via Huang-Rhys factor.