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Photoluminescence study of a bulk vapour grown CdTe crystal
Journal of Crystal Growth 220 (2000) 30}38
Photoluminescence study of a bulk vapour grown CdTe crystal D.P. Halliday*, M.D.G. Potter, J.T. Mullins, A.W. Brinkman Department of Physics, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK Received 14 February 2000; accepted 7 August 2000 Communicated by R.B. James
Abstract Low-temperature photoluminescence has been used to pro"le the distribution of impurities and defects through a CdTe boule grown at 7003C by the new `multi-tubea vapour-growth technique. The PL spectrum is dominated by an acceptor bound exciton band at 1.590 eV. We also observe eA and DAP recombination. The Y luminescence band at 1.477 eV is also present and there are no other deep level bands observed. The identi"cation of the luminescence bands is con"rmed by temperature and intensity-dependent measurements and also by the strength of the phonon coupling. The PL data is used to map out the distribution of impurities and dislocations throughout the boule and supports previously published X-ray and defect etching data. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.C; 71.55.G; 81.05.E Keywords: CdTe; Photoluminescence; II}VI semiconductors; Defect luminescence; Bulk crystals
1. Introduction Cadmium telluride is a technologically important semiconductor with a range of applications which require high quality, defect-free substrates. There are di$culties in doping many II}VI compounds because of self-compensation from native defects, however CdTe is one of a small number of wide bandgap II}VI compounds which can be readily doped p- and n-type [1]. CdTe is used as a nuclear c detector because of the large average
* Corresponding author. Tel.: #44-191-374-2551; fax: #44191-374-3848. E-mail address: [email protected] (D.P. Halliday). Present address: Thomas Swan and Co. Ltd., Button End, Harston, Cambridge CB2 5NX, UK.
atomic number, Z"50, and its relatively large room temperature fundamental energy gap (1.49 eV). This application requires high-resistivity material with a high-mobility lifetime product [2]. The resistivity is often limited by the presence of contaminants from the crystal growth technique. Another application is for infra-red detectors with Hg Cd Te layers. CdTe also has high electroV \V optic coe$cients and a signi"cant non-linear response to two photon absorption in the near infrared making it a contender for non-linear optical devices [3]. It has a signi"cant potential for application as an e$cient absorber layer in thin "lm photovoltaic devices [4]. Recently, there has been interest in using CdTe as a photorefractive material [5]. Epitaxial growth of CdTe has been attempted on a range of substrate materials in an attempt to
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 5 5 - 7
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
produce layers of su$cient quality and reduced defect density for these wide range of applications. Substrates used have included: GaAs [6], InSb [7] and sapphire [8,9]. The lattice mismatch between CdTe and GaAs is 14.6% which leads to a high density of dislocations being formed at the substrate-epilayer interface which can propagate through the epitaxial layer reducing the e!ectiveness of this material for device applications. There is thus a need to produce highquality CdTe substrate boules which can be used for the applications discussed above. A review of the growth of II}VI substrates is given by Sato [10]. The growth of 50 mm diameter CdTe crystals has recently been reported using a new `multi-tubea seeded vapour-growth technique [11]. This technique separates the source and growth regions, linking them with a calibrated #ow restrictor in a semi-open arrangement which results in controlled di!usionless transport. Crystal growth occurs on a seed wafer supported by a quartz pedestal as in the Markov technique [12] and results in boules of substantial size. The growth occurs at a low temperature (7003C) compared to the melting point (10903C) producing a higher quality crystal with a reduced defect density since the critical resolved shear stress increases with decreasing temperature. Reduction in thermal strain and background contamination are also bene"ts of the reduced growth temperature. In addition, the CdTe phase diagram favours stoichiometric growth as the growth temperature is lowered. In this paper, we present a detailed analysis of a CdTe boule grown with this new vapour growth method using photoluminescence spectroscopy. A preliminary study of this crystal has been reported [13]. Brie#y, this has shown that there is a small amount of strain in these crystals, the broadening in the X-ray h!2h scans was 2 arcsec indicating a residual strain of 10\. The X-ray triple axis rocking curves showed broadening attributed to a mosaic of unstrained sub-grains. Defect etch pit studies showed a decreasing dislocation density with increasing vapour growth onto the substrate, with an average value of 6;10 cm\.
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2. Experimental procedure Growth took place as described previously [11,13] on a 49 mm diameter Cd Zn Te wa fer, the majority of which was a +1 1 1 ,B grain. This was polished using a bromine/methanol solution. The growth rate was controlled by independently controlling the source and seed plate heaters resulting in a maximum growth rate of around 200 lm h\. The heating and cooling rates were designed to ensure that the source #ow exceeded the loss rate from the crystal at all times. Flow modelling showed that the Cd and Te partial pressures would result in a Te excess of 2;10\ at% [13]. The main +1 1 1 ,B grain was cut into horizontal and vertical slices and polished in preparation for optical spectroscopy. The PL was excited using the 458 nm line of an argon ion laser with typical excitation densities of 10 mW cm\. The samples were mounted in a closed cycle helium cryostat which can be controlled between 10 and 300 K. The spectrometer is a grating spectrometer with a 1024 element photodiode array detector. The peak energies of our PL lines have an accuracy of $0.5 meV.
3. Results and discussion Fig. 1 presents a schematic diagram of the boule showing the location of the PL samples. After cutting, the slices were hydroplane polished to remove surface damage. Fig. 2 shows a PL spectra from a section close to the centre of the boule, which is representative of the PL spectra obtained from most of the sample. The fundamental energy gap at 10 K is 1.606 eV [14]. The spectrum is dominated by a sharp peak at 1.590 eV. This is the groundstate acceptor bound exciton decay (AX). This acceptor bound exciton feature dominates in the PL spectrum observed from a range of as grown CdTe samples which are usually p-type [15]. There has been some question about the acceptor centre responsible for this emission. Following a series of measurements in magnetic "eld, a PL feature at 1.5896 eV was assigned to the ground-state recombination of an exciton bound at a Cu acceptor [16]. Taguchi et al. observed a bound exciton line at
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D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
Fig. 1. Schematic diagram of the boule showing the relative location of the PL samples.
Fig. 2. Photoluminescence spectrum from a position in the centre of the boule.
1.5901 eV which they performed Zeeman measurements on. This PL feature was found to have C symmetry resulting from a complex along the J 11 1 12 direction. This was later con"rmed as being due to recombination of an exciton localised at the chlorine A centre which is a cadmium vacancy chlorine donor centre (V }Cl ) [15]. More ! 2 recently, the feature at 1.590 has been assigned to an exciton bound at a di!erent acceptor complex consisting of a Cd vacancy and two Cl donors (V }2Cl ) [17,18]. The identi"cation of the ! 2
acceptor centre responsible for the 1.590 eV (AX) emission is still unclear. The energy of the (AX) feature in our sample agrees with this centre. The weak feature at 1.595 eV is the donor bound exciton decay line which is close to the well accepted value for the (DX) recombination at a shallow chlorine donor [19]. There is also a shoulder visible at 1.599 eV which is the ground-state free exciton decay emission. The peak at 1.569 eV is the LO phonon replica of the dominant (AX) transition giving a LO phonon energy of 21 meV in agreement with published values of 21.3 meV [14]. The peak at 1.577 eV is the LO phonon replica of the FE transition. The features at 1.549 and 1.541 eV are the electron acceptor (eA) and donor acceptor pair (DAP) transitions. The nature of the centre responsible for this emission has been identi"ed by Shin [17]. The peak at 1.549 eV is a free-to-bound eA transition between a free electron and an acceptor complex consisting of a cadmium vacancy and two shallow donors (V }2D). The corresponding DAP ! transition between a hole bound at this acceptor complex and an electron bound at a shallow donor is at 1.541 eV giving a shallow donor biding energy of 7 meV. Phonon replicas of these transitions are visible at 1.528 eV (eA}LO) and 1.519 eV (DAP}LO). It is worth noting from this data that Haynes Rule is not valid for acceptors in CdTe, this behaviour has been previously observed [20] and is generally the case for acceptors in direct band gap semiconductors [21]. Finally, there is a broad luminescence band around 1.477 eV which is due to the Y-centre. Table 1 lists the energy and FWHM of each of these peaks along with their assignments as discussed above. The values of the FWHM for the FE and DX transitions are more di$cult to determine as these features are much weaker. Typical values for the excitonic FWHM on the best quality Bridgeman material would be about an order of magnitude less. The inhomogeneous broadening of the excitonic features observed in this sample will be principally due to the strain. The laser excitation intensity dependence of the dominant (AX) peak is shown in Fig. 3. The PL intensity is obtained from the integrated area of the PL band. The PL intensity is described by the function I&LI where L is the laser power. The "t to our data gives k"1.2 which agrees with previous
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38 Table 1 PL peak energies, FWHM and assignment Energy (eV)
FWHM (meV)
Assignment
1.599 1.595 1.590 1.577 1.569 1.549 1.541 1.528 1.519 1.477 1.456 1.434
3 3 3 4 4 5 5 7 8 11 19 15
FE DX AX FE}LO AX}LO eA DAP eA}LO DAP}LO Y Y}LO Y}LO
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some rather unusual features for a deep level emission band: a weak phonon coupling and a relatively broad zero}phonon line. It is common with deep level PL features to observe a series of transitions to lower energy from the main peak which are the result of coupling to LO phonons, and which can be very pronounced in polar compound materials. It results from the interaction between the out of phase motion of the two atoms in the sphalerite unit cell producing a localised electric dipole distortion which can couple to the electronic transition in the long wavelength limit. At low temperatures phonon-related transitions are generally observed only to lower energies, a Stokes process. The relative intensity of the phonon satellites gives a measure of the strength of the coupling between the electron and phonon. The intensity of these phonon assisted transitions is described by a Poisson distribution SL I(hl!n u )"e\1 , n! where hl is the zero phonon line energy of the electronic transition. S is the Huang}Rhys coupling parameter which, in this approximation, is the mean number of phonons emitted during the transition [24] and assumes the phonons all have the same energy. The spectra can be modelled by assigning a lineshape of the form
Fig. 3. Laser intensity dependence for the luminescence intensity of the dominant AX feature.
data [22]. This superlinear dependence on laser power supports the assignment of this feature to an exciton recombination process. The observed intensity dependence arises from the interaction between the di!erent radiative recombination pathways associated with near band edge shallow impurity and exciton states [22]. In the majority of the PL spectra a broad band with phonon replicas is observed around 1.477 eV. This is the Y luminescence band. It was "rst identi"ed by Dean [23]. This band is characterised by
1 , 1#(hl!E #nE )/C *to each phonon-assisted transition. An analysis of 20 PL spectra recorded from di!erent positions and depths throughout the boule gave the parameters shown in Table 2 which agree with other published values [23,25]. The relatively small S parameter indicates a reduced coupling to the lattice and we can infer from this that the recombination occurs at an extended defect complex. Dean postulated that this luminescence feature was due to recombination of excitons bound to extended dislocations [23]. Seto has observed a strong correlation between the intensity of the PL band and the dislocation density [26]. This has been con"rmed by Hildebrandt [25] who has shown using Vickers microindentation
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D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
Table 2 Analysis of Y luminescence Energy of ZPL (eV)
FWHM (meV)
Energy of LO (eV)
FWHM (meV)
Energy of LO (eV)
FWHM (meV)
Average LO energy
S
1.477
10
1.456
19
1.434
15
21
0.36
that the Y luminescence band originates from radiative decay of excitons bound to Te(g) glide dislocations which strongly supports this assignment of an exciton localised at a dislocation. Many deep level PL bands have been observed in CdTe in this energy region from 1.3 to 1.5 eV. Often, there can be such a large number of features close together that only a very broad featureless band is observed [26,27] often referred to as the `defect banda. As there are a range of possible recombination centres in this spectral region, care must be taken to identify the centre responsible for the PL emission. The chlorine A centre also produces a PL band with a zero phonon line at 1.478 eV which is the result of a donor acceptor recombination between a chlorine substitutional donor and the A centre, a cadmium vacancy chlorine donor complex (V }Cl ) [28]. However this is ! 2 characterised by a much larger Huang}Rhys factor of S"2.2 giving a di!erent PL spectrum from the one observed here. Other examples of defects and their related PL bands in this region are discussed in a review by Neumark [29]. The temperature dependence of the PL spectra is shown in Fig. 4. As the temperature is increased, the luminescence is quenched due to thermally excited non-radiative transitions. An Arrhenius plot of the spectra is shown in Fig. 5. It is clear from Fig. 4 that as the temperature is increased the relative intensity of the PL features decreases. This is true for all the features apart from the peak at 1.549 eV and its associated phonon replica at 1.528 eV. The intensity of this feature does not alter signi"cantly over the temperature range studied. The temperature variation is shown more clearly in Fig. 5 where the intensity dependence has been split into three different groupings for clarity. These are (a) a fast intensity decay, (b) a slow intensity decay and (c) a roughly constant intensity. This type of decay is commonly modelled using a theoretical expression
of the form I " 2
I , 1#a exp(!E /k ¹)#a exp(!E /k ¹) this simulates the presence of two thermal activation processes and is appropriate for bound exciton and donor acceptor pair recombination processes [30]. I is the zero temperature intensity of the PL band, a and a are constants related to the relative energy density of states and E and E are the thermal activation energies of the low temperature and high temperature quenching processes. Table 3 lists the values obtained from the "ts plotted in Fig. 5. The dominant AX feature has "tting parameters of E "6 meV and E "14 meV. The value of E compares with the energy required to localise a free exciton at a neutral acceptor (9 meV) which is obtained from the energy separation between the FE and AX lines. The other notable feature is the very small change in the intensity of the eA line and the corresponding phonon replica. This results from the relatively large acceptor ionisation energy (50 meV). The temperature dependence of the Y luminescence band has been described more realistically using an expression of the form [25] I . I " 2 1#a¹ exp(!E/k ¹) This is a result of a detailed balance analysis involving capture of a free exciton at the defect site and thermal dissociation of bound exciton states [31]. This particular description is more consistent with experimental observations that deep acceptor bound exciton luminescence is more rapidly quenched than donor bound exciton luminescence in CdTe and allows for a non-trivial lattice relaxation associated with deep acceptors in CdTe. Using this temperature dependence we obtain a "t of 10 meV to our data. This will be a measure of the
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
35
Fig. 5. Arrhenius plot for the luminescence intensity of the major PL features. Table 3 Analysis of temperature dependence of luminescence using models described in text Fig. 4. Temperature dependence of the PL spectra.
energy required to localise the exciton at the dislocation. The uncertainty on this value is much larger as an acceptable "t to the data can be obtained using a range of energies by adjusting the values of I and a. There are a number of di!erences between this spectra and other PL spectra commonly observed from CdTe material. It is unusual to observe strong well-resolved bound exciton band edge luminescence and clear Y luminescence in the same material. Generally, either one or the other is resolved (see for example, Shin [18]). Seto [26] observes sharp excitonic features and Y luminescence in the same samples but the Y band is 1}5;10\ times weaker. In addition, we observe a prominent (AX) feature at 1.590 eV which is either due to recombination of an exciton at the chlorine A centre [15] or
Peak energy (eV)
E (meV)
E (meV)
1.590 1.577 1.569 1.541 1.519 1.477
5.9 0.5 6.9 6.5 4.0 10
14.0 9.2 28.5 30.0 30.7 *
the (V }2Cl ) centre [18]. Formation of either of ! 2 these centres would be favoured in this material where a slight Te excess is predicted from the growth conditions [13]. However, as there is a very low chlorine concentration in this material the formation of the A centre is more likely than the isoelectronic centre involving two chlorine atoms. Given that there is no luminescence from the chlorine A centre at 1.48 eV we know that the chlorine concentration must be very small in this
36
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
vapour-grown material. Seto observes the appearance of both the chlorine related (AX) line and the chlorine A centre luminescence after introducing chlorine at a concentration of 2 ppm [32]. Using PL to study a series of solution grown CdTe crystals doped with Cl, it was found that the chlorine A centre PL band at 1.48 eV only appeared when the chlorine concentration was in the range of 25}100 ppm [18]. This would suggest that the concentration of chlorine is between these two values. The source of the chlorine contamination is unknown but it is possible that chlorine was present in the seed crystal. We can also infer that the concentration of copper is extremely low in this material as the Cu AX line is not present. Fig. 6 shows a plot of the variation of the PL spectrum as a function of position through the boule. This spectrum was recorded using a slice towards the centre of the boule from the seed to the surface. There are several trends which can be observed from this plot. There is a blue shift of the dominant AX peak into the seed. The maximum shift is 15 meV. Using the results of PL at 4.2 K on a series of ZnTe}CdTe mixed crystals, we would expect a shift of 25.4 meV for a Zn concentration of 4% [33]. There must have been some di!usion of Zn through the crystal during the growth process. There is also a large deep level band at 1.51 eV in the CdZnTe seed. This band is not present in the CdTe epilayer and its origin is uncertain. To visualise the data more clearly, we have plotted the intensity variation of the principal PL peaks in Fig. 7. The intensity is obtained by "tting to obtain the area under the PL spectra. In Fig. 7a the total intensity and the intensity for each of the four major PL features is shown. The AX peak follows the general form of the total intensity as expected. The eA and DAP intensity pro"les also follow the general form of the total intensity. The intensity pro"le of the Y luminescence band is di!erent from the others. At 2 mm from the bottom of the seed plate the Y luminescence band increases to a maximum value of 400 counts whereas, the other features drop in intensity, for example the AX band drops to 700 counts from a maximum of 5000 counts. This corresponds to a region of the crystal where there was a large dislocation density and a number of cracks were visible under an
Fig. 6. Plot showing the variation of the PL spectra through the boule from seed to surface.
optical microscope. It coincides with the seed crystal interface where vapour growth commenced. As noted earlier, the intensity of the Y luminescence band has been related to the number of dislocations present in the crystal so we have a clear indication from this intensity variation that there is an increased density of dislocations present in the crystal at this point. Fig. 7b shows a plot of the intensity ratio of each of the four main luminescence bands to the total intensity. This illustrates more clearly the trends discussed above. The AX peak has a ratio which varies smoothly from 0.4 at the bottom of the seed to 0.7 at the surface with a region 2 mm from the bottom of the seed where the ratio drops to 0.3. The eA and DAP intensity ratios vary between 0.01 and 0.08. These curves have the same general shape. The only discernable trend is a decrease in the intensity ratio moving towards the top surface of the crystal. This is most likely due to the increasing dominance of the Y luminescence band in this region. Fig. 7c shows a plot of the PL intensity ratio for the Y luminescence band to the AX luminescence band. We know that the Y luminescence band is due to recombination of excitons bound at dislocations. If we were to normalise this Y luminescence band to the free exciton intensity, we would then have a measure of the
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
37
reported X-ray data on these samples [13]. It clearly indicates that the dislocation density is lowest in the vapour-grown CdTe material. 4. Conclusion Photoluminescence spectra have been observed from a CdTe bulk crystal produced by low-temperature vapour growth. All the main luminescence features have been identi"ed and used to characterise the crystal. These are (DX) at 1.595 eV, (AX) at 1.590 eV, eA at 1.549 eV, DAP at 1.541 eV and the Y luminescence band at 1.477 eV. Luminescence mapping has shown the variation of defects and impurities throughout the crystal, in particular the dislocation density distribution is obtained from the Y luminescence band. This shows a clear improvement in crystal quality in the bulk of the grown material compared to the seed and interface regions. We estimate the Cl concentration to be between 25 and 100 ppm based on a comparison of PL from other CdTe crystals. Acknowledgements
Fig. 7. Plot of the PL intensity as a function of depth through boule. (a) Plot of total integrated intensity and intensity of the dominant PL features. (b) Plot of ratio of the four dominant PL features to the total integrated intensity. (c) Plot of intensity ratio of Y luminescence band to (AX) band. Note the log scale on the y-axis in each case.
We acknowledge "nancial support from the European Community under Brite-Euram contract BRE2.CT94.0609 for the crystal growth programme. MDGP acknowledges EPSRC for a studentship. References
dislocation density. In the PL spectra the free exciton decay is extremely weak (see Fig. 2). We have instead used the intensity ratio of the Y band to the AX peak. Whilst not having exactly the same intensity distribution as the free exciton decay Fig. 7c will give a systematic means of plotting the dislocation density as a function of depth through the crystal. We note that there is a large peak in the dislocation density 2 mm from the bottom of the seed as indicated earlier. We also observe a minima in the dislocation density at 3.5 mm into the sample and an overall reduction in the dislocation density of the as grown material compared to the seed and interface regions. This is consistent with previously
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[9] N.V. Sochinskii, V. Munoz, V. Bellani, L. Vina, E. Dieguez, E. Alves, M.F. daSilva, J.C. Soares, S. Bernardi, Appl. Phys. Lett. 70 (1997) 1314. [10] K. Sato, Y. Seki, Y. Matsuda, O. Oda, J. Crystal Growth 197 (1999) 413. [11] J.T. Mullins, J. Carles, N.M. Aitken, A.W. Brinkman, Crystal Growth 208 (2000) 211. [12] E.V. Markov, A.A. Davydov, Inorg. Mater. 7 (1971) 413. [13] N.M. Aitken, M.D.G. Potter, D.J. Buckley, J.T. Mullins, J. Carles, D.P. Halliday, K. Durose, B.K. Tanner, A.W. Brinkman, J. Crystal Growth 199 (1999) 984. [14] G. Nimtz, Cadmium telluride, in: O. Madelung, M. Schulz, H. Weiss (Eds.), Landolt-BoK rnstein, Springer, Berlin, 1982, pp. 225}230. [15] T. Taguchi, C. Onodera, Mat. Sci. Forum 65}66 (1990) 235. [16] E. Molva, L.S. Dang, Phys. Rev. B 27 (1983) 6222. [17] H.Y. Shin, Y. Sun, Mater. Sci. Eng. B 52 (1998) 78. [18] H.Y. Shin, C.Y. Sun, J. Crystal Growth 186 (1998) 354. [19] K. Saminadayar, J.M. Francou, J.L. Pautrat, J. Crystal Growth 72 (1985) 236. [20] J.P. Chamonal, E. Molva, J.L. Pautrat, Solid State Commun. 43 (1982) 801. [21] P.J. Dean, Prog. Crystal Growth Characterization Mater. 5 (1982) 89.
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Photoluminescence study of a bulk vapour grown CdTe crystal D.P. Halliday*, M.D.G. Potter, J.T. Mullins, A.W. Brinkman Department of Physics, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK Received 14 February 2000; accepted 7 August 2000 Communicated by R.B. James
Abstract Low-temperature photoluminescence has been used to pro"le the distribution of impurities and defects through a CdTe boule grown at 7003C by the new `multi-tubea vapour-growth technique. The PL spectrum is dominated by an acceptor bound exciton band at 1.590 eV. We also observe eA and DAP recombination. The Y luminescence band at 1.477 eV is also present and there are no other deep level bands observed. The identi"cation of the luminescence bands is con"rmed by temperature and intensity-dependent measurements and also by the strength of the phonon coupling. The PL data is used to map out the distribution of impurities and dislocations throughout the boule and supports previously published X-ray and defect etching data. 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55.C; 71.55.G; 81.05.E Keywords: CdTe; Photoluminescence; II}VI semiconductors; Defect luminescence; Bulk crystals
1. Introduction Cadmium telluride is a technologically important semiconductor with a range of applications which require high quality, defect-free substrates. There are di$culties in doping many II}VI compounds because of self-compensation from native defects, however CdTe is one of a small number of wide bandgap II}VI compounds which can be readily doped p- and n-type [1]. CdTe is used as a nuclear c detector because of the large average
* Corresponding author. Tel.: #44-191-374-2551; fax: #44191-374-3848. E-mail address: [email protected] (D.P. Halliday). Present address: Thomas Swan and Co. Ltd., Button End, Harston, Cambridge CB2 5NX, UK.
atomic number, Z"50, and its relatively large room temperature fundamental energy gap (1.49 eV). This application requires high-resistivity material with a high-mobility lifetime product [2]. The resistivity is often limited by the presence of contaminants from the crystal growth technique. Another application is for infra-red detectors with Hg Cd Te layers. CdTe also has high electroV \V optic coe$cients and a signi"cant non-linear response to two photon absorption in the near infrared making it a contender for non-linear optical devices [3]. It has a signi"cant potential for application as an e$cient absorber layer in thin "lm photovoltaic devices [4]. Recently, there has been interest in using CdTe as a photorefractive material [5]. Epitaxial growth of CdTe has been attempted on a range of substrate materials in an attempt to
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 5 5 - 7
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
produce layers of su$cient quality and reduced defect density for these wide range of applications. Substrates used have included: GaAs [6], InSb [7] and sapphire [8,9]. The lattice mismatch between CdTe and GaAs is 14.6% which leads to a high density of dislocations being formed at the substrate-epilayer interface which can propagate through the epitaxial layer reducing the e!ectiveness of this material for device applications. There is thus a need to produce highquality CdTe substrate boules which can be used for the applications discussed above. A review of the growth of II}VI substrates is given by Sato [10]. The growth of 50 mm diameter CdTe crystals has recently been reported using a new `multi-tubea seeded vapour-growth technique [11]. This technique separates the source and growth regions, linking them with a calibrated #ow restrictor in a semi-open arrangement which results in controlled di!usionless transport. Crystal growth occurs on a seed wafer supported by a quartz pedestal as in the Markov technique [12] and results in boules of substantial size. The growth occurs at a low temperature (7003C) compared to the melting point (10903C) producing a higher quality crystal with a reduced defect density since the critical resolved shear stress increases with decreasing temperature. Reduction in thermal strain and background contamination are also bene"ts of the reduced growth temperature. In addition, the CdTe phase diagram favours stoichiometric growth as the growth temperature is lowered. In this paper, we present a detailed analysis of a CdTe boule grown with this new vapour growth method using photoluminescence spectroscopy. A preliminary study of this crystal has been reported [13]. Brie#y, this has shown that there is a small amount of strain in these crystals, the broadening in the X-ray h!2h scans was 2 arcsec indicating a residual strain of 10\. The X-ray triple axis rocking curves showed broadening attributed to a mosaic of unstrained sub-grains. Defect etch pit studies showed a decreasing dislocation density with increasing vapour growth onto the substrate, with an average value of 6;10 cm\.
31
2. Experimental procedure Growth took place as described previously [11,13] on a 49 mm diameter Cd Zn Te wa fer, the majority of which was a +1 1 1 ,B grain. This was polished using a bromine/methanol solution. The growth rate was controlled by independently controlling the source and seed plate heaters resulting in a maximum growth rate of around 200 lm h\. The heating and cooling rates were designed to ensure that the source #ow exceeded the loss rate from the crystal at all times. Flow modelling showed that the Cd and Te partial pressures would result in a Te excess of 2;10\ at% [13]. The main +1 1 1 ,B grain was cut into horizontal and vertical slices and polished in preparation for optical spectroscopy. The PL was excited using the 458 nm line of an argon ion laser with typical excitation densities of 10 mW cm\. The samples were mounted in a closed cycle helium cryostat which can be controlled between 10 and 300 K. The spectrometer is a grating spectrometer with a 1024 element photodiode array detector. The peak energies of our PL lines have an accuracy of $0.5 meV.
3. Results and discussion Fig. 1 presents a schematic diagram of the boule showing the location of the PL samples. After cutting, the slices were hydroplane polished to remove surface damage. Fig. 2 shows a PL spectra from a section close to the centre of the boule, which is representative of the PL spectra obtained from most of the sample. The fundamental energy gap at 10 K is 1.606 eV [14]. The spectrum is dominated by a sharp peak at 1.590 eV. This is the groundstate acceptor bound exciton decay (AX). This acceptor bound exciton feature dominates in the PL spectrum observed from a range of as grown CdTe samples which are usually p-type [15]. There has been some question about the acceptor centre responsible for this emission. Following a series of measurements in magnetic "eld, a PL feature at 1.5896 eV was assigned to the ground-state recombination of an exciton bound at a Cu acceptor [16]. Taguchi et al. observed a bound exciton line at
32
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
Fig. 1. Schematic diagram of the boule showing the relative location of the PL samples.
Fig. 2. Photoluminescence spectrum from a position in the centre of the boule.
1.5901 eV which they performed Zeeman measurements on. This PL feature was found to have C symmetry resulting from a complex along the J 11 1 12 direction. This was later con"rmed as being due to recombination of an exciton localised at the chlorine A centre which is a cadmium vacancy chlorine donor centre (V }Cl ) [15]. More ! 2 recently, the feature at 1.590 has been assigned to an exciton bound at a di!erent acceptor complex consisting of a Cd vacancy and two Cl donors (V }2Cl ) [17,18]. The identi"cation of the ! 2
acceptor centre responsible for the 1.590 eV (AX) emission is still unclear. The energy of the (AX) feature in our sample agrees with this centre. The weak feature at 1.595 eV is the donor bound exciton decay line which is close to the well accepted value for the (DX) recombination at a shallow chlorine donor [19]. There is also a shoulder visible at 1.599 eV which is the ground-state free exciton decay emission. The peak at 1.569 eV is the LO phonon replica of the dominant (AX) transition giving a LO phonon energy of 21 meV in agreement with published values of 21.3 meV [14]. The peak at 1.577 eV is the LO phonon replica of the FE transition. The features at 1.549 and 1.541 eV are the electron acceptor (eA) and donor acceptor pair (DAP) transitions. The nature of the centre responsible for this emission has been identi"ed by Shin [17]. The peak at 1.549 eV is a free-to-bound eA transition between a free electron and an acceptor complex consisting of a cadmium vacancy and two shallow donors (V }2D). The corresponding DAP ! transition between a hole bound at this acceptor complex and an electron bound at a shallow donor is at 1.541 eV giving a shallow donor biding energy of 7 meV. Phonon replicas of these transitions are visible at 1.528 eV (eA}LO) and 1.519 eV (DAP}LO). It is worth noting from this data that Haynes Rule is not valid for acceptors in CdTe, this behaviour has been previously observed [20] and is generally the case for acceptors in direct band gap semiconductors [21]. Finally, there is a broad luminescence band around 1.477 eV which is due to the Y-centre. Table 1 lists the energy and FWHM of each of these peaks along with their assignments as discussed above. The values of the FWHM for the FE and DX transitions are more di$cult to determine as these features are much weaker. Typical values for the excitonic FWHM on the best quality Bridgeman material would be about an order of magnitude less. The inhomogeneous broadening of the excitonic features observed in this sample will be principally due to the strain. The laser excitation intensity dependence of the dominant (AX) peak is shown in Fig. 3. The PL intensity is obtained from the integrated area of the PL band. The PL intensity is described by the function I&LI where L is the laser power. The "t to our data gives k"1.2 which agrees with previous
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38 Table 1 PL peak energies, FWHM and assignment Energy (eV)
FWHM (meV)
Assignment
1.599 1.595 1.590 1.577 1.569 1.549 1.541 1.528 1.519 1.477 1.456 1.434
3 3 3 4 4 5 5 7 8 11 19 15
FE DX AX FE}LO AX}LO eA DAP eA}LO DAP}LO Y Y}LO Y}LO
33
some rather unusual features for a deep level emission band: a weak phonon coupling and a relatively broad zero}phonon line. It is common with deep level PL features to observe a series of transitions to lower energy from the main peak which are the result of coupling to LO phonons, and which can be very pronounced in polar compound materials. It results from the interaction between the out of phase motion of the two atoms in the sphalerite unit cell producing a localised electric dipole distortion which can couple to the electronic transition in the long wavelength limit. At low temperatures phonon-related transitions are generally observed only to lower energies, a Stokes process. The relative intensity of the phonon satellites gives a measure of the strength of the coupling between the electron and phonon. The intensity of these phonon assisted transitions is described by a Poisson distribution SL I(hl!n u )"e\1 , n! where hl is the zero phonon line energy of the electronic transition. S is the Huang}Rhys coupling parameter which, in this approximation, is the mean number of phonons emitted during the transition [24] and assumes the phonons all have the same energy. The spectra can be modelled by assigning a lineshape of the form
Fig. 3. Laser intensity dependence for the luminescence intensity of the dominant AX feature.
data [22]. This superlinear dependence on laser power supports the assignment of this feature to an exciton recombination process. The observed intensity dependence arises from the interaction between the di!erent radiative recombination pathways associated with near band edge shallow impurity and exciton states [22]. In the majority of the PL spectra a broad band with phonon replicas is observed around 1.477 eV. This is the Y luminescence band. It was "rst identi"ed by Dean [23]. This band is characterised by
1 , 1#(hl!E #nE )/C *to each phonon-assisted transition. An analysis of 20 PL spectra recorded from di!erent positions and depths throughout the boule gave the parameters shown in Table 2 which agree with other published values [23,25]. The relatively small S parameter indicates a reduced coupling to the lattice and we can infer from this that the recombination occurs at an extended defect complex. Dean postulated that this luminescence feature was due to recombination of excitons bound to extended dislocations [23]. Seto has observed a strong correlation between the intensity of the PL band and the dislocation density [26]. This has been con"rmed by Hildebrandt [25] who has shown using Vickers microindentation
34
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
Table 2 Analysis of Y luminescence Energy of ZPL (eV)
FWHM (meV)
Energy of LO (eV)
FWHM (meV)
Energy of LO (eV)
FWHM (meV)
Average LO energy
S
1.477
10
1.456
19
1.434
15
21
0.36
that the Y luminescence band originates from radiative decay of excitons bound to Te(g) glide dislocations which strongly supports this assignment of an exciton localised at a dislocation. Many deep level PL bands have been observed in CdTe in this energy region from 1.3 to 1.5 eV. Often, there can be such a large number of features close together that only a very broad featureless band is observed [26,27] often referred to as the `defect banda. As there are a range of possible recombination centres in this spectral region, care must be taken to identify the centre responsible for the PL emission. The chlorine A centre also produces a PL band with a zero phonon line at 1.478 eV which is the result of a donor acceptor recombination between a chlorine substitutional donor and the A centre, a cadmium vacancy chlorine donor complex (V }Cl ) [28]. However this is ! 2 characterised by a much larger Huang}Rhys factor of S"2.2 giving a di!erent PL spectrum from the one observed here. Other examples of defects and their related PL bands in this region are discussed in a review by Neumark [29]. The temperature dependence of the PL spectra is shown in Fig. 4. As the temperature is increased, the luminescence is quenched due to thermally excited non-radiative transitions. An Arrhenius plot of the spectra is shown in Fig. 5. It is clear from Fig. 4 that as the temperature is increased the relative intensity of the PL features decreases. This is true for all the features apart from the peak at 1.549 eV and its associated phonon replica at 1.528 eV. The intensity of this feature does not alter signi"cantly over the temperature range studied. The temperature variation is shown more clearly in Fig. 5 where the intensity dependence has been split into three different groupings for clarity. These are (a) a fast intensity decay, (b) a slow intensity decay and (c) a roughly constant intensity. This type of decay is commonly modelled using a theoretical expression
of the form I " 2
I , 1#a exp(!E /k ¹)#a exp(!E /k ¹) this simulates the presence of two thermal activation processes and is appropriate for bound exciton and donor acceptor pair recombination processes [30]. I is the zero temperature intensity of the PL band, a and a are constants related to the relative energy density of states and E and E are the thermal activation energies of the low temperature and high temperature quenching processes. Table 3 lists the values obtained from the "ts plotted in Fig. 5. The dominant AX feature has "tting parameters of E "6 meV and E "14 meV. The value of E compares with the energy required to localise a free exciton at a neutral acceptor (9 meV) which is obtained from the energy separation between the FE and AX lines. The other notable feature is the very small change in the intensity of the eA line and the corresponding phonon replica. This results from the relatively large acceptor ionisation energy (50 meV). The temperature dependence of the Y luminescence band has been described more realistically using an expression of the form [25] I . I " 2 1#a¹ exp(!E/k ¹) This is a result of a detailed balance analysis involving capture of a free exciton at the defect site and thermal dissociation of bound exciton states [31]. This particular description is more consistent with experimental observations that deep acceptor bound exciton luminescence is more rapidly quenched than donor bound exciton luminescence in CdTe and allows for a non-trivial lattice relaxation associated with deep acceptors in CdTe. Using this temperature dependence we obtain a "t of 10 meV to our data. This will be a measure of the
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
35
Fig. 5. Arrhenius plot for the luminescence intensity of the major PL features. Table 3 Analysis of temperature dependence of luminescence using models described in text Fig. 4. Temperature dependence of the PL spectra.
energy required to localise the exciton at the dislocation. The uncertainty on this value is much larger as an acceptable "t to the data can be obtained using a range of energies by adjusting the values of I and a. There are a number of di!erences between this spectra and other PL spectra commonly observed from CdTe material. It is unusual to observe strong well-resolved bound exciton band edge luminescence and clear Y luminescence in the same material. Generally, either one or the other is resolved (see for example, Shin [18]). Seto [26] observes sharp excitonic features and Y luminescence in the same samples but the Y band is 1}5;10\ times weaker. In addition, we observe a prominent (AX) feature at 1.590 eV which is either due to recombination of an exciton at the chlorine A centre [15] or
Peak energy (eV)
E (meV)
E (meV)
1.590 1.577 1.569 1.541 1.519 1.477
5.9 0.5 6.9 6.5 4.0 10
14.0 9.2 28.5 30.0 30.7 *
the (V }2Cl ) centre [18]. Formation of either of ! 2 these centres would be favoured in this material where a slight Te excess is predicted from the growth conditions [13]. However, as there is a very low chlorine concentration in this material the formation of the A centre is more likely than the isoelectronic centre involving two chlorine atoms. Given that there is no luminescence from the chlorine A centre at 1.48 eV we know that the chlorine concentration must be very small in this
36
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
vapour-grown material. Seto observes the appearance of both the chlorine related (AX) line and the chlorine A centre luminescence after introducing chlorine at a concentration of 2 ppm [32]. Using PL to study a series of solution grown CdTe crystals doped with Cl, it was found that the chlorine A centre PL band at 1.48 eV only appeared when the chlorine concentration was in the range of 25}100 ppm [18]. This would suggest that the concentration of chlorine is between these two values. The source of the chlorine contamination is unknown but it is possible that chlorine was present in the seed crystal. We can also infer that the concentration of copper is extremely low in this material as the Cu AX line is not present. Fig. 6 shows a plot of the variation of the PL spectrum as a function of position through the boule. This spectrum was recorded using a slice towards the centre of the boule from the seed to the surface. There are several trends which can be observed from this plot. There is a blue shift of the dominant AX peak into the seed. The maximum shift is 15 meV. Using the results of PL at 4.2 K on a series of ZnTe}CdTe mixed crystals, we would expect a shift of 25.4 meV for a Zn concentration of 4% [33]. There must have been some di!usion of Zn through the crystal during the growth process. There is also a large deep level band at 1.51 eV in the CdZnTe seed. This band is not present in the CdTe epilayer and its origin is uncertain. To visualise the data more clearly, we have plotted the intensity variation of the principal PL peaks in Fig. 7. The intensity is obtained by "tting to obtain the area under the PL spectra. In Fig. 7a the total intensity and the intensity for each of the four major PL features is shown. The AX peak follows the general form of the total intensity as expected. The eA and DAP intensity pro"les also follow the general form of the total intensity. The intensity pro"le of the Y luminescence band is di!erent from the others. At 2 mm from the bottom of the seed plate the Y luminescence band increases to a maximum value of 400 counts whereas, the other features drop in intensity, for example the AX band drops to 700 counts from a maximum of 5000 counts. This corresponds to a region of the crystal where there was a large dislocation density and a number of cracks were visible under an
Fig. 6. Plot showing the variation of the PL spectra through the boule from seed to surface.
optical microscope. It coincides with the seed crystal interface where vapour growth commenced. As noted earlier, the intensity of the Y luminescence band has been related to the number of dislocations present in the crystal so we have a clear indication from this intensity variation that there is an increased density of dislocations present in the crystal at this point. Fig. 7b shows a plot of the intensity ratio of each of the four main luminescence bands to the total intensity. This illustrates more clearly the trends discussed above. The AX peak has a ratio which varies smoothly from 0.4 at the bottom of the seed to 0.7 at the surface with a region 2 mm from the bottom of the seed where the ratio drops to 0.3. The eA and DAP intensity ratios vary between 0.01 and 0.08. These curves have the same general shape. The only discernable trend is a decrease in the intensity ratio moving towards the top surface of the crystal. This is most likely due to the increasing dominance of the Y luminescence band in this region. Fig. 7c shows a plot of the PL intensity ratio for the Y luminescence band to the AX luminescence band. We know that the Y luminescence band is due to recombination of excitons bound at dislocations. If we were to normalise this Y luminescence band to the free exciton intensity, we would then have a measure of the
D.P. Halliday et al. / Journal of Crystal Growth 220 (2000) 30}38
37
reported X-ray data on these samples [13]. It clearly indicates that the dislocation density is lowest in the vapour-grown CdTe material. 4. Conclusion Photoluminescence spectra have been observed from a CdTe bulk crystal produced by low-temperature vapour growth. All the main luminescence features have been identi"ed and used to characterise the crystal. These are (DX) at 1.595 eV, (AX) at 1.590 eV, eA at 1.549 eV, DAP at 1.541 eV and the Y luminescence band at 1.477 eV. Luminescence mapping has shown the variation of defects and impurities throughout the crystal, in particular the dislocation density distribution is obtained from the Y luminescence band. This shows a clear improvement in crystal quality in the bulk of the grown material compared to the seed and interface regions. We estimate the Cl concentration to be between 25 and 100 ppm based on a comparison of PL from other CdTe crystals. Acknowledgements
Fig. 7. Plot of the PL intensity as a function of depth through boule. (a) Plot of total integrated intensity and intensity of the dominant PL features. (b) Plot of ratio of the four dominant PL features to the total integrated intensity. (c) Plot of intensity ratio of Y luminescence band to (AX) band. Note the log scale on the y-axis in each case.
We acknowledge "nancial support from the European Community under Brite-Euram contract BRE2.CT94.0609 for the crystal growth programme. MDGP acknowledges EPSRC for a studentship. References
dislocation density. In the PL spectra the free exciton decay is extremely weak (see Fig. 2). We have instead used the intensity ratio of the Y band to the AX peak. Whilst not having exactly the same intensity distribution as the free exciton decay Fig. 7c will give a systematic means of plotting the dislocation density as a function of depth through the crystal. We note that there is a large peak in the dislocation density 2 mm from the bottom of the seed as indicated earlier. We also observe a minima in the dislocation density at 3.5 mm into the sample and an overall reduction in the dislocation density of the as grown material compared to the seed and interface regions. This is consistent with previously
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