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Photoluminescence spectra of Cl-doped CdTe crystals
Journal of Crystal Growth 186 (1998) 354—361
Photoluminescence spectra of Cl-doped CdTe crystals Hwa-Yuh Shin*,1 Cherng-Yuan Sun Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC Received 12 September 1997
Abstract A detailed analysis of photoluminescence (PL) spectra has been done to identify the origins of PL emissions in solution-grown Cl-doped CdTe crystals. In the exciton emission region, a sharp peak at 1.590 eV and a broad peak at 1.586 eV are observed in the Cl-doped CdTe crystals and their intensities are enhanced with increasing amount of Cl. On the basis of the temperature-dependence measurements, the two features are attributed to the recombination of excitons bound to two different acceptors. From the variations in the PL spectra of the Cl-doped CdTe crystals before and after Cd annealing, it is demonstrated that the feature at 1.590 eV is associated to exciton bound to complex acceptor formed with a Cd vacancy and two Cl donors (V -2Cl ), and the feature at 1.586 eV is associated to exciton bound to another C$ T% Cl-related complex acceptor of (V -Cl ). The ionization energies for (V -2Cl ) and (V -Cl ) complex acceptors are C$ T% C$ T% C$ T% found to be about 45 and 120 meV, respectively. ( 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Cadmium Telluride (CdTe), due to its large average atomic number (Z"50) and its high forbidden band gap (E "1.5 eV at room temperature), is an ' important material for the fabrication of c-ray detectors to be operated at room temperature. To realize this application, high-resistivity CdTe crystals with good charge transport properties are required. Generally, this is achieved by compensating the Cd vacancies (V ), the main intrinsic defects in C$ CdTe crystals, with incorporation of chlorine (Cl)
* Corresponding author. 1 Also at: Institute of Nuclear Energy Research, P.O. Box 3-11, Lung-Tan 325, Taiwan, ROC.
into CdTe [1,2]. Chlorine is usually thought to act as hydrogenic donors by substituting for tellurium as well as to form acceptor-like complex centers in CdTe. In the past years, photoluminescence (PL) spectroscopy has been extensively used to investigate the nature of PL lines for the Cl-doped CdTe crystals and the energy levels of the Cl-related defects have also been deduced consequently [3—7]. Nevertheless, some controversies are still not dissolved completely, for example, ambiguity in the nature of the characteristic lines both at 1.586 and at 1.590 eV in the exciton emission region. Saminadayar et al. [5] have studied the PL spectra of Cl-doped CdTe crystals in the region ranging from 776 to 784 nm and, with the backing from the electron paramagnetic resonance (EPR) data, have tentatively proposed that the origin of PL lines W at 1.586 eV and G at 1.590 eV may be related to
0022-0248/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 5 3 9 - 3
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
complexes such as donor-acceptor pair (DAP) with a distant-pair effect or acceptor-like complexes (A2~,D`). Using data from PL emission intensity versus laser excitation intensity, Suzuki et al. [6] have confirmed that, like other three nearby PL lines D , G and AC6, the PL line W is excitonic in 1 1 nature, not (DAP) as proposed by Saminadayar et al., and, in association with the data from time-offlight (TOF) measurements, have concluded that the line W at 1.587 eV is due to a hole trap composed of a Cd vacancy and a Cl on Te site, i.e. (V -Cl ), at 135 meV above the valence band, and C$ T% the line G at 1.591 eV is due to a electron trap formed by a Cd-vacancy and two Cl on Te sites, (V -2Cl ), at 30 meV below the conduction band. C$ T% However, a further study by the same group, Seto et al. [7], on PL spectra in the region ranging from 770 to 900 nm, except the emission region from 790—820 nm, of Cl-doped CdTe crystals, have manifested an entirely different picture. They have shown that the sharp PL peak at 1.5903 eV is the key emission line for the high-resistivity, and have suggested its origin can be associated with an acceptor-like defect composed of a Cd-vacancy and a Cl on Te site, (V -Cl ), with an ionization enC$ T% ergy of about 120 meV. The broad PL line W at 1.586 eV has not been discussed in this paper presented by Seto et al.. In this paper, the PL spectra of undoped and Cl-doped CdTe crystals are presented, and all the characteristic lines in the spectra are tentatively designated from the beginning for the convenience of identification, and hence, of discussion. Then temperature-dependence experiments and thermal annealing in Cd atmosphere are carried out in order to confirm the correctness of these tentative designations and, consequently, to clarify the above-mentioned discrepancies in interpretation. The experimental data from the temperature dependence of the PL spectra are utilized also to obtain the ionization energies of the Cl-related defect levels.
2. Experimental procedure Undoped and Cl-doped CdTe crystals were grown by the Temperature Gradient Solution
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Growth (TGSG) method [8]. In order to obtain high-purity CdTe crystals, the starting materials used were 6 N Cd and 6 N Te that were purified further by multipass zone-refining. Acting as the compensating dopant, various Cl concentrations ranging from 10 to 1000 ppm by nominal weight were added to the starting materials in the form of CdCl . Thermal annealing was performed at 2 600°C for 48 h in a sealed quartz tube which was replenished with Cd atmosphere, and then followed by cooling slowly to room temperature. The electrical properties of the samples were measured at room temperature by using the van der Pauw technique. All the samples used in the PL measurements were mechanically polished and then chemically etched in a 2% bromine—methanol solution for about 5 min to remove surface damages. The samples were placed in a cryostat with a controlled temperature ranging from 9 to 300 K. Photoluminescence was obtained by exciting the samples with a 5 mW He—Ne laser (j"632.8 nm), and the resulting spectra were detected with a cooled photomultiplier tube through an 1 m SPEX monochromator.
3. Results and discussion The electrical properties of various CdTe samples used in this study are listed in Table 1. The as-grown undoped samples revealed p-type conduction with electrical resistivity of 9.3]102 ) cm and carrier concentration of 1.3]1014 cm~3. The as-grown Cl-doped samples also manifested p-type conduction, but their electrical resistivities were boosted up with increasing Cl concentration and reached a highest value of 3.4]108 ) cm in the 300 ppm Cl-doped sample and stayed almost at this value beyond 300 ppm, indicating clearly the compensation effect of Cl in CdTe. However, when the annealing in Cd atmosphere was carried out on a Cl-doped p-CdTe sample, its conduction type was converted to n-type with carrier concentration of 4.1]1017 cm~3, and its resistivity was lowered drastically down to 6]10C2 ) cm. Several reports have pointed out that the electrical properties of Cl-doped p-CdTe crystals are converted to n-type,
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Table 1 The electrical properties of various CdTe samples used in this study Sample
Conductivity
Resistivity () cm)
Carrier concentration (cm~3)
Undoped as-grown Cl-doped as-grown (10 ppm) Cl-doped as-grown (25 ppm) Cl-doped as-grown (100 ppm) Cl-doped as-grown (300 ppm) Cl-doped as-grown (500 ppm) Cl-doped as-grown (1000 ppm) Cl-doped Cd-annealed (100 ppm)
p p p p p p p n
9.3]102 7.6]103 5.7]106 2.7]107 3.4]108 3.0]108 3.3]108 6.0]10~2
1.3]1014 1.1]1014
because Cl atoms are activated to become hydrogenic donors by the Cd-saturated annealing [5,7,9]. For the convenience of discussion, the PL spectra in this study are divided into three regions: (1) the exciton emission region, for the wavelengths shorter than 785 nm, (2) the edge emission region, for the intermediate wavelengths, and (3) the deep level emission region, for the wavelengths longer than 820 nm [10,11]. As a reference, the typical PL spectrum of an undoped as-grown CdTe crystal is shown in Fig. 1a. In the exciton emission region, a prominent line at 1.589 eV (780.0 nm) designated as (A°,X) is attributed to the recombination of an a exciton bound to a neutral acceptor “a”. In the edge emission region, the PL spectrum shows a pair of doublet at 1.548 eV (800.6 nm) and at 1.541 eV (804.5 nm), designated as (e,A°) and (DAP) , a a respectively. The origin of these lines have been discussed in our previous report [12], and the acceptor “a” has been recognized to be the complex formed by a Cd-vacancy and two unidentified donors, (V -2D). The LO-phonon replicas of C$ (e,A°) and (DAP) , designated as (e,A°) -1LO a a a and (DAP) -1LO, are also observed at energies of a about one LO-phonon energy (21 meV) [13] below the doublet. It should be noted that the spectrum does not reveal any features in the deep level emission region which, according to many other reports, is an indication of high crystalline quality [14—16]. PL properties of the as-grown CdTe crystals doped with different amounts of Cl are shown in spectra for (b) 10 ppm, (c) 25 ppm, (d) 100 ppm,
4.1]107
Fig. 1. PL spectra of p-CdTe (a) undoped, (b) 10 ppm Cldoped, (c) 25 ppm Cl-doped, (d) 100 ppm Cl-doped, (e) 300 ppm Cl-doped.
and (e) 300 ppm of Fig. 1. The spectra for 500 and 1000 ppm are very similar to Fig. 1e, so are not shown here. We tentatively designate all the
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
characteristic peaks that appear in these spectra, and then attempt to verify the correctness of these designations later in this article. When the Cl concentration is equal to or less than 25 ppm, as shown in Fig. 1b and Fig. 1c, the (A°,X) line of the una doped crystals at 1.589 eV is still discernible, while two new features at 1.593 eV (778.3 nm) and 1.590 eV (779.5 nm) are noticed to emerge out in the exciton emission region. When the Cl concentration is equal to or more than 100 ppm, as shown in Fig. 1d and Fig. 1e, the (A°,X) line is no longer a visible, while the features at 1.593 and 1.590 eV become larger and, furthermore, another new feature at 1.586 eV (781.6 nm) comes into view in the exciton emission region. The features at 1.593, 1.590, and 1.586 eV have been designated by Saminadayar et al. [5] to be D , G, and W, respec1 tively, and their intensities are progressively enhanced with increasing amounts of Cl doping, while the (A°,X) line of the undoped crystals at a 1.589 eV disappears drastically. The feature at 1.593 eV may be indisputably designated as (D°,X), and has been known to be due to the recombination of excitons bound to Cl as neutral hydrogenic donors [5,7]. Understandably, the peak of (D°,X) grows as the doping concentration of chlorine is increased. It should be noted that a peak of (D°,X) at 1.592 eV in the PL spectra of a undoped crystals, which is too small to be marked out in Fig. 1a, is naturally not perceptible also in the spectra of the Cl-doped samples. From the experimental results of temperature dependence, to be discussed later in the following paragraph, it can be demonstrated that both Cl-related features at 1.590 and at 1.586 eV are due to the recombination of excitons bound to neutral acceptors “b” and “c”, respectively, and thus the two features may be tentatively designated here as (A°,X) and (A°,X) , corb c respondingly. In the edge emission region, it can be seen from the vicissitudes of the spectral lines in Fig. 1 that both peaks of the doublet in the undoped crystals, i.e., (e,A°) and (DAP) , are lowered progressively a a with increasing amount of Cl, and (e,A°) is dimina ished faster than (DAP) ,. At the same time, a pair a of new doublet at 1.559 eV (795.1 nm) and at 1.553 eV (798.2 nm), tentatively designated here as (e,A°) and (DAP) , respectively, appears in Fig. 1d, b b
357
the spectrum of the 100 ppm Cl-doped crystal. The features of (e,A°) and (DAP) are not observed in b b the Cl-doped samples with concentration below 25 ppm. However, the new doublet is replaced by a broad band at the same position in Fig. 1e, the spectrum of the 300 ppm Cl-doped crystal. Again, in the temperature-dependence experiment, it can be demonstrated that this broad band is actually composed of the (e,A°) transition and the (DAP) b b recombination. By comparing the spectra in Fig. 1d and Fig. 1e, it can be seen that the intensities of the doublet are intensified with increasing Cl concentration. Since the doublet appears only in the spectra of Cl-doped crystals, we may conclude that the doublet is strongly associated with Clrelated acceptors. In the deep-level emission region, a broad band around 1.477 eV (839.2 nm) is observed in the Cldoped crystals with concentration above 100 ppm, together with its LO-phonon replica. This broad band has been observed by many authors in the Cl-doped CdTe crystals [3,4,11], and it may be elucidated to be caused by a donor-acceptor recombination between a Cl donor and the soT% called A center which behaves like an acceptor. Hofmann et al. [4] have demonstrated strongly on the basis of Optically Detected Magnetic Resonance (ODMR) experiments that the structure of the “A” center is in the form of (V -Cl ). Hence, C$ T% this broad band may be presumed to be a conjunction of a doublet designated tentatively here as (e,A°) #(DAP) , in which the subscript “c” represc c ents that the band is related to the acceptor (V C$ Cl ). Like the doublet (e,A°) and (DAP) in the T% b b edge emission region as mentioned above, the intensity of the (e,A°) #(DAP) band is also c c augmented with increasing amounts of Cl doping. This is because that the concentrations of both Cl donors and (V -Cl ) acceptors are simultaT% C$ T% neously increased as the amount of Cl doping is increased. In order to investigate the nature of the species “b” and to demonstrate the broad band in the edge emission region of Fig. 1e being composed of (e,A°) transitions and (DAP) recombination, temb b perature-dependence measurements were performed for the CdTe crystals doped with 300 ppm Cl. The experimental results are presented in Fig. 2.
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H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
Fig. 2. Temperature dependence of the PL spectra of 300 ppm Cl-doped CdTe.
In the exciton emission region, the (D°,X) line at 1.593 eV is decreased with increasing sample temperature, while both peaks of the (A°,X) line at b 1.590 eV and the (A°,X) line at 1.586 eV are rec duced also but much more rapidly. Although Saminadayar et al. [5] have pointed out that the shape of the (A°,X) line is much broader than an c acceptor-related bound exciton line, nevertheless, we believe that both of the lines at 1.590 and at 1.586 eV are due to acceptor-bound exciton emissions. The reason is that the intensities of both (A°,X) and (A°,X) lines are diminished rapidly b c with increasing temperature and disappear at a temperature around 30 K, while the (D°,X) line although decays also but remains and becomes broader. It is known that the (A°,X) exciton is more
localized than the (D°,X) exciton, and thus the former interacts with LO phonons more strongly [13,17]. Consequently, the more strongly interacting (A°,X) exciton should be expected to be more effectively dependent on temperature than the (D°,X) exciton. In the edge emission region of the PL spectra of 300 ppm Cl-doped CdTe as shown in Fig. 2, two peaks are overlapped together to become a broad band between 1.55 and 1.56 eV, and remain so during the entire temperature process. As the sample temperature increases, the intensity of the peak at the low-energy side decreases, while that of the peak at the high-energy side increases. This result is similar to the case of GaAs in which the low-energy peak is assigned to (DAP) recombinations and the high-energy peak to (e,A°) transitions, both of them are related to the same acceptor [18]. The explanation for this phenomenon is that with increasing sample temperature the electron trapped at the shallow donor level is excited to the conduction band, so that the intensity of the (DAP) band decreases and (e,A°) band becomes increasingly dominant. Therefore, we assign the peak at the low-energy side to be (DAP) and the peak at the b high-energy side to be (e,A°) , both of them are b related to the same acceptor designated “b”. From the energy position of the (e,A°) transition and the b value of the energy gap at 9 K, E "1.604 eV [19], ' the ionization energy of the acceptor “b” can be derived to be about 45 meV. The acceptor levels around E #0.045 eV in CdTe : Cl has been exten7 sively studied by many workers, and it is now generally accepted that the acceptor level is due to a complex formed by a Cd-vacancy and two Cl donors on Te sites, (V -2Cl ) [3,20,21]. C$ T% In the deep-level emission region of the PL spectra of 300 ppm Cl-doped CdTe as shown in Fig. 2, the (e,A°) #(DAP) band around 1.477 eV c c moves slightly to higher-energy side with increasing temperature. The band shift is also due to that at low temperature the (DAP) recombination is c dominant, whereas at high temperature the higher energy peak, attributed to the (e,A°) transition, is c progressively enhanced. This behavior is similar to the cases of the doublet, (e,A°) and (DAP), due to the species “b” or to the species “a” as explained in the previous paragraph, and provides indeed
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
Fig. 3. Cd annealing effect on PL spectrum of 100 ppm Cldoped CdTe; (a) as-grown, (b) Cd annealed.
a proof to the presumption that the band around 1.477 eV is a combination of (e,A°) and (DAP) . c c The ionization energy of the acceptor “c” is estimated from the peak energy position of the (e,A°) #(DAP) band to be about 120 meV. c c Additional information can be acquired from investigating the effects of thermal annealing in a Cd-vapor saturated environment on the spectral peaks of the Cl-doped crystals. The annealing-induced alterations in the PL spectrum of the 100 ppm Cl-doped CdTe sample are rendered clearly in Fig. 3, where (a) is of the sample before annealing and (b) is of the sample after annealing. After the Cd annealing, it should be noted that the most prominent peak at 1.590 eV in the spectrum before annealing which is designated as the acceptor-bound exciton line (A°,X) and together with b
359
the doublet designated as (e,A°) and (DAP) are all b b eliminated simultaneously. On the contrary, the undistinguished bump at 1.586 eV in the spectrum before annealing which is designated as the acceptor-bound exciton line (A°,X) and together with c the broad band designated as (e,A°) #(DAP) , are c c all notably enhanced by the Cd annealing. In addition, the peak position of the (e,A°) #(DAP) band c c shifts slightly toward the higher-energy side. These consequences support duly the presupposed relationships that the (A°,X) line at 1.590 eV is indeed b associated with the doublet of (e,A°) at 1.559 eV b and (DAP) at 1.553 eV by the same acceptor “b”, b i.e., (V -2Cl ), while the (A°,X) peak at 1.586 eV is C$ T% c indeed associated with the (e,A°) #(DAP) band c c at 1.447 eV by the same acceptor “c”, i.e., (V -Cl ). C$ T% In the spectrum of Fig. 3b, the new feature at 1.565 eV (792.1 nm) identified as (A°,X) -1LO is c due to the one LO-phonon replica of the (A°,X) c line, according to their energy separation with respect to the location of the (A°,X) line. On the c other hand, the intensity of the (D°,X) line at 1.593 eV is obviously enhanced by the Cd annealing, and becomes the most prominent peak in Fig. 3b. The effect of Cd annealing on the PL lines can be interpreted as follows. By the Cd annealing, both (V -Cl ) and (V -2Cl ) complexes are dissoC$ T% C$ T% ciated into Cd vacancies and Cl hydrogenic donors, and then the Cd vacancies are filled by Cd atoms. Considering the time and energies needed for the migration of V and Cl , it is quite logical to C$ T% presume that (V -Cl ) complexes are more easily C$ T% formed in CdTe than (V -2Cl ) complexes. ThereC$ T% fore, (V -Cl ) complexes are created again during C$ T% the slow-cooling stage after the Cd annealing. As the consequence, the (A°,X) line at 1.590 eV disapb pears, and the (A°,X) line at 1.586 eV becomes c greatly enhanced. Furthermore, the fact that the (D°,X) line reaches prominence by the Cd annealing can be explained by the increase in the concentration of the isolated Cl donors. This is also the reason why the Cl-doped crystals change from ptype with high resistivity to n-type with low resistivity by the Cd annealing. In addition, the nature of the peak position of the (e,A°) #(DAP) band c c shifting towards higher-energy side due to Cd annealing is different from the temperature effects on
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the as-grown Cl-doped CdTe crystals as shown in Fig. 2 and explained in the last paragraph, and is believed to be caused by the increase in the Coulomb interaction energy which result from the increase in the donor concentration. It should be noted that this shifting is related to (DAP) only, c while (e,A°) has no influence on it. As the carrier c concentration is increased, the mean distance between the donor—acceptor pairs is decreased. Since the closer pairs result in higher Coulomb interaction energy, the (DAP) band shifts to higher-enc ergy side [22]. Returning to Fig. 1, the vicissitudes of the spectral peaks may be explained as follows. When the doping concentration of Cl is increased, the unidentified donors D in the species “a”, which is the complex (V -2D), is gradually replaced C$ by Cl as a hydrogenic donor before the Cl concentration attains to 100 ppm. Thus, the peak of the (A°,X) line at 1.589 eV is diminished and disapa pears eventually, while the peak of (A°,X) line b at 1.590 eV emerges and its magnitude seems reaching a steady value at the Cl doping of 100 ppm. Above 100 ppm, then, binding of excitons to Cl as neutral donors becomes more active and the formation of the (V -Cl ) complexes is also C$ T% initiated, thus both (D°,X) line at 1.593 eV and (A°,X) line at 1.586 eV are intensified. Since the c unidentified donors D seem to disappear eventually in the Cl-doped crystals, its nature may be considered to be of the intrinsic structural defects of CdTe.
and “c”, respectively. Moreover, the temperature dependence experiments also identify that the doublet in the edge emission region is due to (e,A°) b transition and (DAP) recombination, and that the b broad band in the deep level emission region is composed of both (e,A°) transition and (DAP) c c recombination. The nature of the acceptor “b” is believed to be the complex of (V -2Cl ), while the C$ T% acceptor “c” is considered to be the complex of (V -Cl ). The effects of Cd annealing on PL emisC$ T% sions from the Cl-doped CdTe crystals impart a definite evidence to support the fact that the 1.590 eV line can be attributed to an exciton trapped at the (V -2Cl ) complex acceptor, i.e., C$ T% (A°,X) , which is also responsible for the doublet in b the edge emission region, (e,A°) line and (DAP) b b line, while the 1.586 eV line is caused by an exciton trapped at another Cl related complex acceptor (V -Cl ), i.e., (A°,X) , which is also responsible for C$ T% c the broad band in the deep-level emission region, (e,A°) #(DAP) . The ionization energies of two c c complex acceptors (V -2Cl ) and (V -Cl ) are C$ T% C$ T% about 45 and 120 meV, respectively.
Acknowledgements The authors would like to thank Dr. S. M. Lan of the Institute of Nuclear Energy Research, ROC for fruitful discussions and comments.
References 4. Conclusions The effects of Cl-doping on the PL emissions in CdTe crystals grown by TGSG method have been studied. In the exciton emission region, two characteristic features at 1.590 and 1.586 eV can be found. In addition, a doublet located in the edge emission region and a broad band located in the deep emission region are also observed. The intensities of these features are enhanced as the Cl-concentration is increased. The temperature-dependence experiments strongly suggest that the features at 1.590 and 1.586 eV are caused by the recombination of excitons bound to two different acceptors, “b”
[1] B. Biglari, M. Samimi, J.M. Koebel, M. Hage-Ali, P. Siffert, Phys. Stat. Sol. A 100 (1987) 589. [2] C. Eiche, D. Maier, D. Sinerius, J. Weese, K. Benz, J. Honerkamp, J. Appl. Phys. 74 (1993) 6667. [3] T.A. Kuhn, W. Ossau, A. Waag, R.N. Biknell-Tassius, Landwehr, J. Crystal Growth 117 (1992) 660. [4] D.M. Hofmann, P. Omling, H.G. Grimmeiss, B.K. Meyer, K.W. Benz, D. Sinerius, Phys. Rev. B 45 (1992) 6247. [5] K. Saminadayar, J.M. Francou, J.L. Pautrat, J. Crystal Growth 72 (1985) 236. [6] K. Suzuki, S. Seto, A. Tanaka, M. Kawashima, J. Crystal Growth 101 (1990) 859. [7] S. Seto, A. Tanaka, Y. Masa, M. Kawashima, J. Crystal Growth 117 (1992) 271. [8] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng. B 41 (1996) 345. [9] N.V. Agrinskaya, M.V. Alecseenko, E.N. Arkadeva, O.A. Matveev, Sov. Phys.-Semicond. 9 (1975) 208.
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361 [10] J. Gonzalez-Hernandez, E. Copez-Cruz, D.D. Allred, W.P. Allred, J. Vac. Sci. Technol. A 8 (1990) 3255. [11] J. Krustok, V. Valdna, K. Hjelt, H. Collan, J. Appl. Phys. 80 (1996) 1757. [12] H.Y. Shin, C.Y. Sun, Mater. Sci. Eng., to be published. [13] E. Cohen, R.A. Street, A. Muranevich, Phys. Rev. B. 28 (1983) 7115. [14] N.C. Giles-Taylor, R.N. Bicknell, D.K. Blanks, T.H. Myers, J.F. Schetzina, J. Vac. Sci. Technol. A 3 (1985) 76. [15] E. Molva, K. Saminadayar, J.L. Pautrat, E. Ligeon, Solid State Commun. 48 (1983) 955.
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[16] H.A. Mar, N. Salansky, J. Appl. Phys. 56 (1984) 2369. [17] Y.F. Chen, C.S. Tsai, Y.H. Chang, Y.M. Chang, T.K. Chen, Y.M. Pang, Appl. Phys. Lett. 58 (1991) 493. [18] P.B. Klein, J. Comas, S.G. Bishop, J. Appl. Phys. 50 (1979) 4327. [19] T. Taguchi, Phys. Stat. Sol. A 77 (1983) K115. [20] R.O. Bell, Solid State Commun. 16 (1975) 913. [21] M. Hage-Ali, P. Siffert, Nucl. Instrum. Methods A 322 (1992) 313. [22] E. Molva, J.P. Chamonal, J.L. Pautrat, Phys. Stat. Sol. B 109 (1982) 635.
Photoluminescence spectra of Cl-doped CdTe crystals Hwa-Yuh Shin*,1 Cherng-Yuan Sun Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC Received 12 September 1997
Abstract A detailed analysis of photoluminescence (PL) spectra has been done to identify the origins of PL emissions in solution-grown Cl-doped CdTe crystals. In the exciton emission region, a sharp peak at 1.590 eV and a broad peak at 1.586 eV are observed in the Cl-doped CdTe crystals and their intensities are enhanced with increasing amount of Cl. On the basis of the temperature-dependence measurements, the two features are attributed to the recombination of excitons bound to two different acceptors. From the variations in the PL spectra of the Cl-doped CdTe crystals before and after Cd annealing, it is demonstrated that the feature at 1.590 eV is associated to exciton bound to complex acceptor formed with a Cd vacancy and two Cl donors (V -2Cl ), and the feature at 1.586 eV is associated to exciton bound to another C$ T% Cl-related complex acceptor of (V -Cl ). The ionization energies for (V -2Cl ) and (V -Cl ) complex acceptors are C$ T% C$ T% C$ T% found to be about 45 and 120 meV, respectively. ( 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Cadmium Telluride (CdTe), due to its large average atomic number (Z"50) and its high forbidden band gap (E "1.5 eV at room temperature), is an ' important material for the fabrication of c-ray detectors to be operated at room temperature. To realize this application, high-resistivity CdTe crystals with good charge transport properties are required. Generally, this is achieved by compensating the Cd vacancies (V ), the main intrinsic defects in C$ CdTe crystals, with incorporation of chlorine (Cl)
* Corresponding author. 1 Also at: Institute of Nuclear Energy Research, P.O. Box 3-11, Lung-Tan 325, Taiwan, ROC.
into CdTe [1,2]. Chlorine is usually thought to act as hydrogenic donors by substituting for tellurium as well as to form acceptor-like complex centers in CdTe. In the past years, photoluminescence (PL) spectroscopy has been extensively used to investigate the nature of PL lines for the Cl-doped CdTe crystals and the energy levels of the Cl-related defects have also been deduced consequently [3—7]. Nevertheless, some controversies are still not dissolved completely, for example, ambiguity in the nature of the characteristic lines both at 1.586 and at 1.590 eV in the exciton emission region. Saminadayar et al. [5] have studied the PL spectra of Cl-doped CdTe crystals in the region ranging from 776 to 784 nm and, with the backing from the electron paramagnetic resonance (EPR) data, have tentatively proposed that the origin of PL lines W at 1.586 eV and G at 1.590 eV may be related to
0022-0248/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 5 3 9 - 3
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
complexes such as donor-acceptor pair (DAP) with a distant-pair effect or acceptor-like complexes (A2~,D`). Using data from PL emission intensity versus laser excitation intensity, Suzuki et al. [6] have confirmed that, like other three nearby PL lines D , G and AC6, the PL line W is excitonic in 1 1 nature, not (DAP) as proposed by Saminadayar et al., and, in association with the data from time-offlight (TOF) measurements, have concluded that the line W at 1.587 eV is due to a hole trap composed of a Cd vacancy and a Cl on Te site, i.e. (V -Cl ), at 135 meV above the valence band, and C$ T% the line G at 1.591 eV is due to a electron trap formed by a Cd-vacancy and two Cl on Te sites, (V -2Cl ), at 30 meV below the conduction band. C$ T% However, a further study by the same group, Seto et al. [7], on PL spectra in the region ranging from 770 to 900 nm, except the emission region from 790—820 nm, of Cl-doped CdTe crystals, have manifested an entirely different picture. They have shown that the sharp PL peak at 1.5903 eV is the key emission line for the high-resistivity, and have suggested its origin can be associated with an acceptor-like defect composed of a Cd-vacancy and a Cl on Te site, (V -Cl ), with an ionization enC$ T% ergy of about 120 meV. The broad PL line W at 1.586 eV has not been discussed in this paper presented by Seto et al.. In this paper, the PL spectra of undoped and Cl-doped CdTe crystals are presented, and all the characteristic lines in the spectra are tentatively designated from the beginning for the convenience of identification, and hence, of discussion. Then temperature-dependence experiments and thermal annealing in Cd atmosphere are carried out in order to confirm the correctness of these tentative designations and, consequently, to clarify the above-mentioned discrepancies in interpretation. The experimental data from the temperature dependence of the PL spectra are utilized also to obtain the ionization energies of the Cl-related defect levels.
2. Experimental procedure Undoped and Cl-doped CdTe crystals were grown by the Temperature Gradient Solution
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Growth (TGSG) method [8]. In order to obtain high-purity CdTe crystals, the starting materials used were 6 N Cd and 6 N Te that were purified further by multipass zone-refining. Acting as the compensating dopant, various Cl concentrations ranging from 10 to 1000 ppm by nominal weight were added to the starting materials in the form of CdCl . Thermal annealing was performed at 2 600°C for 48 h in a sealed quartz tube which was replenished with Cd atmosphere, and then followed by cooling slowly to room temperature. The electrical properties of the samples were measured at room temperature by using the van der Pauw technique. All the samples used in the PL measurements were mechanically polished and then chemically etched in a 2% bromine—methanol solution for about 5 min to remove surface damages. The samples were placed in a cryostat with a controlled temperature ranging from 9 to 300 K. Photoluminescence was obtained by exciting the samples with a 5 mW He—Ne laser (j"632.8 nm), and the resulting spectra were detected with a cooled photomultiplier tube through an 1 m SPEX monochromator.
3. Results and discussion The electrical properties of various CdTe samples used in this study are listed in Table 1. The as-grown undoped samples revealed p-type conduction with electrical resistivity of 9.3]102 ) cm and carrier concentration of 1.3]1014 cm~3. The as-grown Cl-doped samples also manifested p-type conduction, but their electrical resistivities were boosted up with increasing Cl concentration and reached a highest value of 3.4]108 ) cm in the 300 ppm Cl-doped sample and stayed almost at this value beyond 300 ppm, indicating clearly the compensation effect of Cl in CdTe. However, when the annealing in Cd atmosphere was carried out on a Cl-doped p-CdTe sample, its conduction type was converted to n-type with carrier concentration of 4.1]1017 cm~3, and its resistivity was lowered drastically down to 6]10C2 ) cm. Several reports have pointed out that the electrical properties of Cl-doped p-CdTe crystals are converted to n-type,
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Table 1 The electrical properties of various CdTe samples used in this study Sample
Conductivity
Resistivity () cm)
Carrier concentration (cm~3)
Undoped as-grown Cl-doped as-grown (10 ppm) Cl-doped as-grown (25 ppm) Cl-doped as-grown (100 ppm) Cl-doped as-grown (300 ppm) Cl-doped as-grown (500 ppm) Cl-doped as-grown (1000 ppm) Cl-doped Cd-annealed (100 ppm)
p p p p p p p n
9.3]102 7.6]103 5.7]106 2.7]107 3.4]108 3.0]108 3.3]108 6.0]10~2
1.3]1014 1.1]1014
because Cl atoms are activated to become hydrogenic donors by the Cd-saturated annealing [5,7,9]. For the convenience of discussion, the PL spectra in this study are divided into three regions: (1) the exciton emission region, for the wavelengths shorter than 785 nm, (2) the edge emission region, for the intermediate wavelengths, and (3) the deep level emission region, for the wavelengths longer than 820 nm [10,11]. As a reference, the typical PL spectrum of an undoped as-grown CdTe crystal is shown in Fig. 1a. In the exciton emission region, a prominent line at 1.589 eV (780.0 nm) designated as (A°,X) is attributed to the recombination of an a exciton bound to a neutral acceptor “a”. In the edge emission region, the PL spectrum shows a pair of doublet at 1.548 eV (800.6 nm) and at 1.541 eV (804.5 nm), designated as (e,A°) and (DAP) , a a respectively. The origin of these lines have been discussed in our previous report [12], and the acceptor “a” has been recognized to be the complex formed by a Cd-vacancy and two unidentified donors, (V -2D). The LO-phonon replicas of C$ (e,A°) and (DAP) , designated as (e,A°) -1LO a a a and (DAP) -1LO, are also observed at energies of a about one LO-phonon energy (21 meV) [13] below the doublet. It should be noted that the spectrum does not reveal any features in the deep level emission region which, according to many other reports, is an indication of high crystalline quality [14—16]. PL properties of the as-grown CdTe crystals doped with different amounts of Cl are shown in spectra for (b) 10 ppm, (c) 25 ppm, (d) 100 ppm,
4.1]107
Fig. 1. PL spectra of p-CdTe (a) undoped, (b) 10 ppm Cldoped, (c) 25 ppm Cl-doped, (d) 100 ppm Cl-doped, (e) 300 ppm Cl-doped.
and (e) 300 ppm of Fig. 1. The spectra for 500 and 1000 ppm are very similar to Fig. 1e, so are not shown here. We tentatively designate all the
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
characteristic peaks that appear in these spectra, and then attempt to verify the correctness of these designations later in this article. When the Cl concentration is equal to or less than 25 ppm, as shown in Fig. 1b and Fig. 1c, the (A°,X) line of the una doped crystals at 1.589 eV is still discernible, while two new features at 1.593 eV (778.3 nm) and 1.590 eV (779.5 nm) are noticed to emerge out in the exciton emission region. When the Cl concentration is equal to or more than 100 ppm, as shown in Fig. 1d and Fig. 1e, the (A°,X) line is no longer a visible, while the features at 1.593 and 1.590 eV become larger and, furthermore, another new feature at 1.586 eV (781.6 nm) comes into view in the exciton emission region. The features at 1.593, 1.590, and 1.586 eV have been designated by Saminadayar et al. [5] to be D , G, and W, respec1 tively, and their intensities are progressively enhanced with increasing amounts of Cl doping, while the (A°,X) line of the undoped crystals at a 1.589 eV disappears drastically. The feature at 1.593 eV may be indisputably designated as (D°,X), and has been known to be due to the recombination of excitons bound to Cl as neutral hydrogenic donors [5,7]. Understandably, the peak of (D°,X) grows as the doping concentration of chlorine is increased. It should be noted that a peak of (D°,X) at 1.592 eV in the PL spectra of a undoped crystals, which is too small to be marked out in Fig. 1a, is naturally not perceptible also in the spectra of the Cl-doped samples. From the experimental results of temperature dependence, to be discussed later in the following paragraph, it can be demonstrated that both Cl-related features at 1.590 and at 1.586 eV are due to the recombination of excitons bound to neutral acceptors “b” and “c”, respectively, and thus the two features may be tentatively designated here as (A°,X) and (A°,X) , corb c respondingly. In the edge emission region, it can be seen from the vicissitudes of the spectral lines in Fig. 1 that both peaks of the doublet in the undoped crystals, i.e., (e,A°) and (DAP) , are lowered progressively a a with increasing amount of Cl, and (e,A°) is dimina ished faster than (DAP) ,. At the same time, a pair a of new doublet at 1.559 eV (795.1 nm) and at 1.553 eV (798.2 nm), tentatively designated here as (e,A°) and (DAP) , respectively, appears in Fig. 1d, b b
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the spectrum of the 100 ppm Cl-doped crystal. The features of (e,A°) and (DAP) are not observed in b b the Cl-doped samples with concentration below 25 ppm. However, the new doublet is replaced by a broad band at the same position in Fig. 1e, the spectrum of the 300 ppm Cl-doped crystal. Again, in the temperature-dependence experiment, it can be demonstrated that this broad band is actually composed of the (e,A°) transition and the (DAP) b b recombination. By comparing the spectra in Fig. 1d and Fig. 1e, it can be seen that the intensities of the doublet are intensified with increasing Cl concentration. Since the doublet appears only in the spectra of Cl-doped crystals, we may conclude that the doublet is strongly associated with Clrelated acceptors. In the deep-level emission region, a broad band around 1.477 eV (839.2 nm) is observed in the Cldoped crystals with concentration above 100 ppm, together with its LO-phonon replica. This broad band has been observed by many authors in the Cl-doped CdTe crystals [3,4,11], and it may be elucidated to be caused by a donor-acceptor recombination between a Cl donor and the soT% called A center which behaves like an acceptor. Hofmann et al. [4] have demonstrated strongly on the basis of Optically Detected Magnetic Resonance (ODMR) experiments that the structure of the “A” center is in the form of (V -Cl ). Hence, C$ T% this broad band may be presumed to be a conjunction of a doublet designated tentatively here as (e,A°) #(DAP) , in which the subscript “c” represc c ents that the band is related to the acceptor (V C$ Cl ). Like the doublet (e,A°) and (DAP) in the T% b b edge emission region as mentioned above, the intensity of the (e,A°) #(DAP) band is also c c augmented with increasing amounts of Cl doping. This is because that the concentrations of both Cl donors and (V -Cl ) acceptors are simultaT% C$ T% neously increased as the amount of Cl doping is increased. In order to investigate the nature of the species “b” and to demonstrate the broad band in the edge emission region of Fig. 1e being composed of (e,A°) transitions and (DAP) recombination, temb b perature-dependence measurements were performed for the CdTe crystals doped with 300 ppm Cl. The experimental results are presented in Fig. 2.
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Fig. 2. Temperature dependence of the PL spectra of 300 ppm Cl-doped CdTe.
In the exciton emission region, the (D°,X) line at 1.593 eV is decreased with increasing sample temperature, while both peaks of the (A°,X) line at b 1.590 eV and the (A°,X) line at 1.586 eV are rec duced also but much more rapidly. Although Saminadayar et al. [5] have pointed out that the shape of the (A°,X) line is much broader than an c acceptor-related bound exciton line, nevertheless, we believe that both of the lines at 1.590 and at 1.586 eV are due to acceptor-bound exciton emissions. The reason is that the intensities of both (A°,X) and (A°,X) lines are diminished rapidly b c with increasing temperature and disappear at a temperature around 30 K, while the (D°,X) line although decays also but remains and becomes broader. It is known that the (A°,X) exciton is more
localized than the (D°,X) exciton, and thus the former interacts with LO phonons more strongly [13,17]. Consequently, the more strongly interacting (A°,X) exciton should be expected to be more effectively dependent on temperature than the (D°,X) exciton. In the edge emission region of the PL spectra of 300 ppm Cl-doped CdTe as shown in Fig. 2, two peaks are overlapped together to become a broad band between 1.55 and 1.56 eV, and remain so during the entire temperature process. As the sample temperature increases, the intensity of the peak at the low-energy side decreases, while that of the peak at the high-energy side increases. This result is similar to the case of GaAs in which the low-energy peak is assigned to (DAP) recombinations and the high-energy peak to (e,A°) transitions, both of them are related to the same acceptor [18]. The explanation for this phenomenon is that with increasing sample temperature the electron trapped at the shallow donor level is excited to the conduction band, so that the intensity of the (DAP) band decreases and (e,A°) band becomes increasingly dominant. Therefore, we assign the peak at the low-energy side to be (DAP) and the peak at the b high-energy side to be (e,A°) , both of them are b related to the same acceptor designated “b”. From the energy position of the (e,A°) transition and the b value of the energy gap at 9 K, E "1.604 eV [19], ' the ionization energy of the acceptor “b” can be derived to be about 45 meV. The acceptor levels around E #0.045 eV in CdTe : Cl has been exten7 sively studied by many workers, and it is now generally accepted that the acceptor level is due to a complex formed by a Cd-vacancy and two Cl donors on Te sites, (V -2Cl ) [3,20,21]. C$ T% In the deep-level emission region of the PL spectra of 300 ppm Cl-doped CdTe as shown in Fig. 2, the (e,A°) #(DAP) band around 1.477 eV c c moves slightly to higher-energy side with increasing temperature. The band shift is also due to that at low temperature the (DAP) recombination is c dominant, whereas at high temperature the higher energy peak, attributed to the (e,A°) transition, is c progressively enhanced. This behavior is similar to the cases of the doublet, (e,A°) and (DAP), due to the species “b” or to the species “a” as explained in the previous paragraph, and provides indeed
H.-Y. Shin, C.-Y. Sun / Journal of Crystal Growth 186 (1998) 354—361
Fig. 3. Cd annealing effect on PL spectrum of 100 ppm Cldoped CdTe; (a) as-grown, (b) Cd annealed.
a proof to the presumption that the band around 1.477 eV is a combination of (e,A°) and (DAP) . c c The ionization energy of the acceptor “c” is estimated from the peak energy position of the (e,A°) #(DAP) band to be about 120 meV. c c Additional information can be acquired from investigating the effects of thermal annealing in a Cd-vapor saturated environment on the spectral peaks of the Cl-doped crystals. The annealing-induced alterations in the PL spectrum of the 100 ppm Cl-doped CdTe sample are rendered clearly in Fig. 3, where (a) is of the sample before annealing and (b) is of the sample after annealing. After the Cd annealing, it should be noted that the most prominent peak at 1.590 eV in the spectrum before annealing which is designated as the acceptor-bound exciton line (A°,X) and together with b
359
the doublet designated as (e,A°) and (DAP) are all b b eliminated simultaneously. On the contrary, the undistinguished bump at 1.586 eV in the spectrum before annealing which is designated as the acceptor-bound exciton line (A°,X) and together with c the broad band designated as (e,A°) #(DAP) , are c c all notably enhanced by the Cd annealing. In addition, the peak position of the (e,A°) #(DAP) band c c shifts slightly toward the higher-energy side. These consequences support duly the presupposed relationships that the (A°,X) line at 1.590 eV is indeed b associated with the doublet of (e,A°) at 1.559 eV b and (DAP) at 1.553 eV by the same acceptor “b”, b i.e., (V -2Cl ), while the (A°,X) peak at 1.586 eV is C$ T% c indeed associated with the (e,A°) #(DAP) band c c at 1.447 eV by the same acceptor “c”, i.e., (V -Cl ). C$ T% In the spectrum of Fig. 3b, the new feature at 1.565 eV (792.1 nm) identified as (A°,X) -1LO is c due to the one LO-phonon replica of the (A°,X) c line, according to their energy separation with respect to the location of the (A°,X) line. On the c other hand, the intensity of the (D°,X) line at 1.593 eV is obviously enhanced by the Cd annealing, and becomes the most prominent peak in Fig. 3b. The effect of Cd annealing on the PL lines can be interpreted as follows. By the Cd annealing, both (V -Cl ) and (V -2Cl ) complexes are dissoC$ T% C$ T% ciated into Cd vacancies and Cl hydrogenic donors, and then the Cd vacancies are filled by Cd atoms. Considering the time and energies needed for the migration of V and Cl , it is quite logical to C$ T% presume that (V -Cl ) complexes are more easily C$ T% formed in CdTe than (V -2Cl ) complexes. ThereC$ T% fore, (V -Cl ) complexes are created again during C$ T% the slow-cooling stage after the Cd annealing. As the consequence, the (A°,X) line at 1.590 eV disapb pears, and the (A°,X) line at 1.586 eV becomes c greatly enhanced. Furthermore, the fact that the (D°,X) line reaches prominence by the Cd annealing can be explained by the increase in the concentration of the isolated Cl donors. This is also the reason why the Cl-doped crystals change from ptype with high resistivity to n-type with low resistivity by the Cd annealing. In addition, the nature of the peak position of the (e,A°) #(DAP) band c c shifting towards higher-energy side due to Cd annealing is different from the temperature effects on
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the as-grown Cl-doped CdTe crystals as shown in Fig. 2 and explained in the last paragraph, and is believed to be caused by the increase in the Coulomb interaction energy which result from the increase in the donor concentration. It should be noted that this shifting is related to (DAP) only, c while (e,A°) has no influence on it. As the carrier c concentration is increased, the mean distance between the donor—acceptor pairs is decreased. Since the closer pairs result in higher Coulomb interaction energy, the (DAP) band shifts to higher-enc ergy side [22]. Returning to Fig. 1, the vicissitudes of the spectral peaks may be explained as follows. When the doping concentration of Cl is increased, the unidentified donors D in the species “a”, which is the complex (V -2D), is gradually replaced C$ by Cl as a hydrogenic donor before the Cl concentration attains to 100 ppm. Thus, the peak of the (A°,X) line at 1.589 eV is diminished and disapa pears eventually, while the peak of (A°,X) line b at 1.590 eV emerges and its magnitude seems reaching a steady value at the Cl doping of 100 ppm. Above 100 ppm, then, binding of excitons to Cl as neutral donors becomes more active and the formation of the (V -Cl ) complexes is also C$ T% initiated, thus both (D°,X) line at 1.593 eV and (A°,X) line at 1.586 eV are intensified. Since the c unidentified donors D seem to disappear eventually in the Cl-doped crystals, its nature may be considered to be of the intrinsic structural defects of CdTe.
and “c”, respectively. Moreover, the temperature dependence experiments also identify that the doublet in the edge emission region is due to (e,A°) b transition and (DAP) recombination, and that the b broad band in the deep level emission region is composed of both (e,A°) transition and (DAP) c c recombination. The nature of the acceptor “b” is believed to be the complex of (V -2Cl ), while the C$ T% acceptor “c” is considered to be the complex of (V -Cl ). The effects of Cd annealing on PL emisC$ T% sions from the Cl-doped CdTe crystals impart a definite evidence to support the fact that the 1.590 eV line can be attributed to an exciton trapped at the (V -2Cl ) complex acceptor, i.e., C$ T% (A°,X) , which is also responsible for the doublet in b the edge emission region, (e,A°) line and (DAP) b b line, while the 1.586 eV line is caused by an exciton trapped at another Cl related complex acceptor (V -Cl ), i.e., (A°,X) , which is also responsible for C$ T% c the broad band in the deep-level emission region, (e,A°) #(DAP) . The ionization energies of two c c complex acceptors (V -2Cl ) and (V -Cl ) are C$ T% C$ T% about 45 and 120 meV, respectively.
Acknowledgements The authors would like to thank Dr. S. M. Lan of the Institute of Nuclear Energy Research, ROC for fruitful discussions and comments.
References 4. Conclusions The effects of Cl-doping on the PL emissions in CdTe crystals grown by TGSG method have been studied. In the exciton emission region, two characteristic features at 1.590 and 1.586 eV can be found. In addition, a doublet located in the edge emission region and a broad band located in the deep emission region are also observed. The intensities of these features are enhanced as the Cl-concentration is increased. The temperature-dependence experiments strongly suggest that the features at 1.590 and 1.586 eV are caused by the recombination of excitons bound to two different acceptors, “b”
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