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The kinetics of exciton photoluminescence in mercuric iodide
Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
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The kinetics of exciton photoluminescence in mercuric iodide X.M. Wen a,b,c,*, P. Xu b, N. Ohno c a Department of Physics, Yunnan University, Kunming, Yunnan 650091, People's Republic of China Department of Physical Optics, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia c Academic Frontier Promotion Center, Osaka Electro-Communication University, Osaka, Neyagawa 572-8530, Japan b
Received 11 February 2002; accepted 5 March 2002
Abstract Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor±acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV. q 2002 Published by Elsevier Science Ltd. PACS: 78.47. 1 P; 78.55.Hx; 29.40.Wk
1. Introduction Mercuric iodide single crystal in the red tetragonal modi®cation with a space group D4h [15] is a direct band-gap semiconductor [1]. It is a high potential material for Xray, g-ray detectors and photocells because of its relatively high atomic number of the constituent elements, large bandgap at room temperature and good photosensitivity [2,3]. Enhancement in the performance for mercuric iodide detectors is particularly useful because comparable energy resolution is not available with other materials at room temperature. Besides, mercuric iodide detectors exhibit exceptionally high resistance to lattice damage from highenergy radiation. Research is presently being undertaken to improve the growth process and to determine the importance of impurities and defects on carrier transport [4±6]. However, the quality of mercuric iodide single crystal has not reached a high enough level because of the low carrier * Corresponding author. Address: Department of Physics, Yunnan University, Kunming, Yunnan 650091, People's Republic of China. E-mail address: [email protected] (X.M. Wen).
mobility which is due to high defect concentration in sample. It is still very dif®cult to control the native defects, dopants and inadvertent impurities during the growth of the crystal although fully packed detectors with pre-ampli®er are now commercially available. Because of its intrinsic importance for the development of radiation detectors, the defect structure of mercuric iodide has been investigated with different techniques [7±9]. Photoluminescence (PL) spectroscopy provides a nondestructive technique for the analysis of impurities and lattice defects in semiconductors and it has become a major method in probing the nature of defects in mercuric iodide. PL spectrum of mercuric iodide has been extensively studied by several groups [10±26]. Novikov et al. [11,12] studied PL of mercuric iodide at low temperatures and observed three primary luminescence bands. Several sharp lines near band-gap were ascribed to annihilation of the free and bound excitons. Merz et al. [5] studied sublimation and doping experiments on PL at low temperatures and discussed the origins of the luminescence bands due to the stoichiometry in the crystal. Akopyan et al. [17,18] studied emission bands for mercuric iodide crystals with different
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Fig. 1. Steady state PL spectrum near band-gap of mercuric iodide at 10 K.
method growth and treatment. PL spectrum was studied by Bao et al. [21] with high resolution at low temperature and at least 26 emission lines were found in the wavelength region between 529 and 540 nm. Exciton luminescence spectra above 100 K were also studied by Goto et al. [22± 24]. However, the prevalence of the various anomalies observed, their precise nature in various crystals and the in¯uence of each on performance have remained unclear. Their origins and any interrelationships have proven elusive as well [4]. In the present study, time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. TRPL experiment reveals that band-gap luminescence comprises fast and slow decay components and shows different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to a defect in the anion sublattice. The temporal behavior of the tail band is described with rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor±acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region with two-photon excitation, which shows the band-gap at room temperature is about 2.125 eV.
2. Experimental Red HgI2 single crystals used for this work were grown from the saturated acetone solution of HgI2 at room temperature [20,25,26]. Firstly, the saturated acetone solution was made with analytical grade mercuric iodide. Then the solution was placed in an ultra-clean room to evaporate
isothermally (T 293 K). It needs several days to grow the single crystals in sizes of several millimeter, as grown surface has a crystallographic plane perpendicular to the c-axis. The samples were stored in a dry air ambience. TRPL measurements were performed using a streak camera in conjunction with a 25 cm spectrometer employing a 300 lines/mm grating. A frequency-doubled modelocked Ti/Sapphire laser was used for excitation (400 nm wavelength, 1.5 ps pulse width, 0.6 mW average power and 18 kHz repetition rate). The HgI2 crystal sample was mounted on the cold ®nger of a helium-cycled cryostat for PL measurement. The laser beam was incident onto the surface at an angle of 608 to the normal. In order to avoid super-saturation in the CCD units of streak camera a ®lter of transmissivity 10% was used to weaken the excitation. The energy density on the sample is 0.3 mJ/cm 2 per pulse. Two-photon PL was carried out using a Leica TCS MD microscope. The excitation laser was a mode-locked Ti/ Sapphire laser (Coherent Mira 900) producing 150 fs pulses at wavelength between 720 and 920 nm and the power on sample surface was about 100 mJ/cm 2.
3. Results and discussion 3.1. Band-gap photoluminescence at low temperatures Steady-state PL spectrum of HgI2 is shown in Fig. 1; the vertical bars and the relevant notations indicate the emission peaks. The strongest line appears in l 2 532.5 nm that was ascribed to radiative annihilation of bound excitons localized at vacancy of iodide [4,12,18,21]. Another bound exciton line l x, which was generally assumed to be due to radiative annihilation of bound excitons localized at vacancy of mercuric, is much weaker than l 2 so that it cannot be identi®ed clearly in our experiment. The short wavelength structures locating between 529.6 and 531.3 nm were generally assumed to be free exciton lines [12,18,21]. The free exciton line l 1 531.3 nm as shown in Fig. 1 corresponds to free exciton ground state [4]. A phonon replica line, l 2-LO 535.6 nm, obviously appears in our experiment. There were 21 phonon replica lines of free and bound excitons in the region from 532.6 to 540 nm in the high resolution spectrum [21]. A weak luminescence tail band appears obviously between 540 and 557 nm. The band of 560 nm that was discussed in Refs. [17,18] did not appear for our sample. The correlation among all lines and bands was studied by earlier authors. Exciton line l x and 560 nm band were due to the same crystal lattice defect (mercuric vacancy) [18]. In our experiment, both exciton line l x and 560 nm band do not appear, which shows that our sample is excess mercuric. In the case of radiative detector the best material would be those mercuric iodide crystals which have luminescence spectrum characterized by weak 560 nm band [5]. At the same time both l 2 and the tail band were assumed to
X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
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Fig. 4. Rise-time near band-gap and temporal behavior of the tail band.
Fig. 2. Near band-gap luminescence of HgI2 between 10 and 30 K.
originate from the anion defect, a strong l 2 line is accompanied by an obvious tail band for our sample. Fig. 2 shows near band-gap PL between 10 and 30 K. Intensity of bound exciton is much stronger at 10 K and it decreases more notably than that of free exciton with increasing temperature. Intensity of peak l 2 at 30 K is
about 4% that of 10 K and it cannot be detected at 40 K. Intensity of l 1 lowers to a much smaller scale than that of l 2 and its intensity at 30 K is about 60% of that at 10 K. As for lifetime, at 10 and 20 K, free and bound excitons reveal non-exponential decay. We can acquire the lifetime of different decay components through bi-exponential ®tting, as shown in Fig. 3. At 10 K, the decay of bound excitons is much slower than that of free excitons and it accelerates notably with increasing temperature. According to the ®tting, decay of bound and free excitons consists of the different lifetime components below 20 K. The difference between the fast and slow components decreases obviously with increasing temperature and the decay becomes single exponential decay at 30 K. At the same time, decay of bound excitons is almost as fast as that of the free excitons at 30 K. Comparing with fast component, the slow component is obviously dependent on temperature. At 30 K, the slow is nearly as fast as that of the fast so that two kinds of decay components merge into one, that is, the decay become exponential. The phonon replica has slower decay than free and bound excitons and their decay signi®cantly accelerates with increasing temperature. The intensity contribution of the fast component mainly distributes in free exciton region and the slow does in both bound exciton and phonon replica. 3.2. The tail band
Fig. 3. Lifetime of slow and fast decay components between 10 and 20 K.
Relatively, the temporal behavior of the tail band is quite different. Fig. 4 shows the rise-time of near band-gap luminescence and the temporal behavior of the tail band, where the rise-time is de®ned as the time rising to the maximum intensity. The solid round is rise-time originating from experiment; the square and triangle are the time constant t and t 2 from decay curve ®tting according to Eq. (3), respectively; and the empty diamond is the rise-time calculated based on time constant t and t 2. The rise-time is obviously shorter in the free and bound exciton region
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Fig. 5. The red band luminescence at different temperatures.
between 529 and 540 nm except the bound exciton line and its phonon replica. This suggests that the tail band has different luminescent mechanism from free and bound excitons. The tail band was observed by Akopyan et al. and they found that any treatment of a mercuric iodide sample resulting in enhancement of the bound exciton line l 2 tended to increase the intensity of the tail band and vice versa. It can be assumed that the band origins from the anion sublattice and there should exist a strong interaction among them because the width of the tail band is about 17 nm wide. Under the assumption that the tail band originates from the non-radiative decay of bound excitons (located at iodide vacancies), its temporal behavior should be described with rate equations dN1 1 1 2 1 t.0 1 N dt t1 t 1 dN2 1 1 N1 2 N dt t t2 2
t.0
2
where N1 and N2 are the population of the corresponding bound excitons and iodide sublattice, the leaping rates of the ground are 1/t 1 and 1/t 2, respectively, the rate of bound excitons leaping non-radiatively to the sublattice is 1/t . With the initial condition N1 0 const: N0 ; N2 0 0 and general approximation t ! t 1, t 2, the solution of the equations will be N2 t N0 e2t=t2 2 e2t=t
3
Fitting result based on Eq. (3) reveals a very good coincidence with the measurement between 540 and 557 nm [26]. Experiment curve cannot be ®tted well at shorter wavelength than 540 nm, which suggests the luminescent mechanism of tail band is different from that of excitons and their phonon replica. Beyond 557 nm luminescent intensity is too weak to ®t correctly, which indicates the band ends at 557 nm. In the entire band the time constant t and t 2 at various wavelengths has little difference with about t 60 ps and t 2 320 ps, which is shown in Fig. 4.
This is in accordance with experimental result, the ®lled circle. The luminescence becomes so weak and ¯uctuant that the rise-time cannot be acquired accurately from the experiment curve at long wavelength side of tail band. The relaxation mechanism of free exciton±polaritons is mainly non-radiative decay such as trapped into impurities or defects. At the same time electron±hole pairs created by pulse excitation do transfer non-radiatively from free exciton±polaritons to bound excitons within a short time. The lifetime depends on both radiative and non-radiative decay. For the free exciton±polaritons, the observed decay time does not just indicate the radiative lifetime, but the total trapping rate into bound excitons. The short lifetime means the non-radiative decay is very ef®cient and the intensity of free excitons is much weaker than that of bound excitons, which suggests that radiation is far from the main relaxation mechanism and the dominant decay mechanism of free excitons is transferring into bound excitons. Bound excitons will also be converted into free exciton±polariton, but the probability is expected to be quite small at low temperature because their binding energies are estimated as 4±5 meV. At 10 K dominant decay of the free excitons is nonradiative, whereas that of the bound excitons is mainly radiative. The decay time in the bound-exciton luminescence is thus supposed to be the radiative lifetime of the excitons localized at defects. At low temperature excitons are mostly in localized states and the dominant decay mechanism of bound excitons is radiation. For bound excitons, non-radiative decay is very inef®cient so that their decay is pretty slower, which causes a very strong luminescence, very long lifetime and long rise-time. With increasing temperature, excitons begin to populate mobile states and they may diffuse until they interact with defects or impurities and decay non-radiatively. Excitons become more diffusive at higher temperatures, which allows them to encounter more defects or impurities and accelerate to decay. The intensity of bound excitons lowers remarkably because non-radiative decay of bound excitons intensi®es gradually with increasing temperature. The intensity decreases greatly between 20 and 30 K, which suggests non-radiative decay increases notably above 20 K. The conversion into the bound excitons is the dominant decay mechanism all along for the free excitons so that the radiative intensity is not greatly affected with increasing temperature. 3.3. The red band Fig. 5 shows the red band PL spectrum with different temperatures. Its central wavelength locates at 630.9 nm with FWHM 35.5 nm at 10 K. The lifetime of the band is much longer than that of near band-gap, on the order of microsecond. Intensity decreases notably with increasing temperature, which results mainly from thermal quench increasing. As shown in Fig. 5, the intensity decrease is not even with the same temperature interval and a signi®cant decrease takes place between 20 and 30 K, which
X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
Fig. 6. Decay curves of red band at different temperatures.
is consistent with the situation near band-gap emission. FWHM of the band enlarges and central wavelength shifts towards longer wavelength with increasing temperature. This red band was studied by several groups and was ascribed to the recombination of donor±acceptor (D±A) pairs [17], i.e. donor-bound electrons to the hole-®lled acceptor. In the luminescence band of D±A pair recombination because of the Coulombic energy of interaction between the electron±hole pairs, the emission energy can be expressed by [27,28] E r Eg 2 ED 1 EA 1 e2 =1r
4
where Eg is the band-gap energy, ED and EA are the donor and acceptor binding energies, respectively, r is the D±A separation, 1 is low frequency dielectric constant. Therefore, a small separation r corresponds to high Coulomb energy, consequently appears in shorter wavelength within the band. Decay curve of 630 nm band is quite different from the near band-gap emission. Fig. 6 shows the decay curves of red band at different temperatures. The experiment reveals the decay curves approach approximately to straight in bilogarithmic coordinate. In other words, PL decay of this band can be approximately expressed with I t I0 t2n r
5
with n r; the decay rate and I0, a constant. The power decay of intensity shows the band is due to recombination of D±A pairs [34]. However, the approximate extent approaching to straight is not so good as some other semiconductors, such
2111
as ZnO [29], and the approximate extent is not the same for different temperatures. Fig. 6 shows the curves contort toward upward at low temperatures (below 30 K) and contort downward at higher temperatures (above 50 K). The best approaching appears at 40 K. The experiment suggests that the decay consists of two kinds of decay components that corresponds to two decay mechanisms and the two kinds of mechanisms are dominant at different temperature region, below and above 40 K, respectively. At 40 K, the two mechanisms nearly balance so that two decay rates approach to the same, which lead to a better straight. The inset of Fig. 6 shows variation of the decay rate with temperature. At low temperatures, below 40 K, decay rate decreases with increasing temperature and the larger decay rates appear at shorter wavelength. Above 40 K, decay rate increases with increasing temperature. The smallest decay rate appears at about 40 K. On the basis of experiment we can suppose that there exist two kinds of different mechanisms to control the recombination rate of D±A pairs and they are dominant below and above 40 K, respectively. At low temperature, 10 K, electrons signi®cantly occupy the shallowest traps. Wave function overlap between the delocalized shallowly trapped electrons and the deeply trapped hole is relatively larger, resulting in a higher recombination rate. With increasing temperature, shallow electron traps start to be thermally depopulated, which results in a decrease in the emission intensity since electrons now can reach the surface and recombine non-radiatively. Electron wave function is less delocalized in the slightly deeper electron trap and wave function overlap with the deeply trapped hole is reduced. This results in a decrease in the recombination rate with increasing temperature. The thermal detrapping of the shallowly trapped electrons decreases gradually with increasing temperature. Above 40 K the decrease becomes small. At the same time, the extra non-radiative decay paths that require thermal activation increase with increasing temperature. Above 40 K the extra non-radiative decay paths become gradually the dominant mechanism, resulting in the recombination rate which is controlled by the extra nonradiative decay so that the recombination rate n r increases with increasing temperature. On the other hand, with increasing temperature the thermal quenching increases, which causes luminescent intensity to decrease. According to experiment the thermal quench has a steep increase between 20 and 30 K and it is not even for the same temperature interval. 3.4. Two-photon PL at room temperature With increasing temperature, as most other semiconductors, band-gap energy of mercuric iodide shifts toward lower energy. Band-gap energy of mercuric iodide was measured with optical or electronic methods [10,20,30±33] at different temperatures. At room temperature the measurement value of 2.11 eV was reported with differing by as much
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Fig. 7. Two-photon luminescence of HgI2 at room temperature.
as 10%, although the large band-gap value was reported (,2.3 eV). Low-resolution two-photon PL spectrum is measured at room temperature (293 K) with fs laser excitation. A PL band is observed at yellow region and no other bands appear for entire visible region, as shown in Fig. 7. Its central wavelength locates at 584 nm and bandwidth is about 22 nm. The band is ascribed to exciton transition and its central wavelength corresponds to the band-gap, that is 2.125 eV. All the measurements for different excitation between 720 and 910 nm show the same spectra, including central wavelength and band width. Our measurement shows that the band-gap of red mercuric iodide at room temperature is 2.125 eV.
the room temperature band-gap of red HgI2 is 2.125 eV that is consistent with most other reports. Acknowledgements One of the authors (X.M. Wen) would like to acknowledge the ®nancial support of Osaka Electro-Communication University Foundation for Promotion of International Exchange on Research and Education. This work was partially supported by the Academic Frontier Promotion Project of the Ministry of Education, Science, Sports and Culture of Japan. Authors would like to thank Dr E. Kable of EMU (Electrical Microscopy Unit of Sydney University) for her kindly help for our experiment.
4. Conclusions We have studied TRPL of red mercuric iodide single crystal at low temperatures and measured the two-photon PL spectrum at room temperature. Near band-gap luminescence is ascribed to radiative annihilation of free and bound excitons and they comprises fast and slow decay component. For free excitons the dominant decay mechanism is converted into bound excitons. The conversion is very ef®cient so that they have much shorter lifetime than bound excitons, especially at low temperatures. At 10 K the dominant decay of bound excitons is radiation, which causes a much stronger intensity and slower lifetime compared with free excitons. Luminescence of bound exciton lowers notably with increasing temperature and disappears about 40 K. The tail band luminescence is ascribed to a defect in the anion sublattice and its temporal behavior may be described with the rate equations. Red band is ascribed to the radiative recombination of D±A pairs and its intensity may be expressed as I t t2n r : There exist two decay mechanisms to control the decay and they are dominant at different temperature regions. Two-photon PL spectrum is measured at room temperature (296 K) with fs laser excitation and the experiment shows
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The kinetics of exciton photoluminescence in mercuric iodide X.M. Wen a,b,c,*, P. Xu b, N. Ohno c a Department of Physics, Yunnan University, Kunming, Yunnan 650091, People's Republic of China Department of Physical Optics, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia c Academic Frontier Promotion Center, Osaka Electro-Communication University, Osaka, Neyagawa 572-8530, Japan b
Received 11 February 2002; accepted 5 March 2002
Abstract Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor±acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV. q 2002 Published by Elsevier Science Ltd. PACS: 78.47. 1 P; 78.55.Hx; 29.40.Wk
1. Introduction Mercuric iodide single crystal in the red tetragonal modi®cation with a space group D4h [15] is a direct band-gap semiconductor [1]. It is a high potential material for Xray, g-ray detectors and photocells because of its relatively high atomic number of the constituent elements, large bandgap at room temperature and good photosensitivity [2,3]. Enhancement in the performance for mercuric iodide detectors is particularly useful because comparable energy resolution is not available with other materials at room temperature. Besides, mercuric iodide detectors exhibit exceptionally high resistance to lattice damage from highenergy radiation. Research is presently being undertaken to improve the growth process and to determine the importance of impurities and defects on carrier transport [4±6]. However, the quality of mercuric iodide single crystal has not reached a high enough level because of the low carrier * Corresponding author. Address: Department of Physics, Yunnan University, Kunming, Yunnan 650091, People's Republic of China. E-mail address: [email protected] (X.M. Wen).
mobility which is due to high defect concentration in sample. It is still very dif®cult to control the native defects, dopants and inadvertent impurities during the growth of the crystal although fully packed detectors with pre-ampli®er are now commercially available. Because of its intrinsic importance for the development of radiation detectors, the defect structure of mercuric iodide has been investigated with different techniques [7±9]. Photoluminescence (PL) spectroscopy provides a nondestructive technique for the analysis of impurities and lattice defects in semiconductors and it has become a major method in probing the nature of defects in mercuric iodide. PL spectrum of mercuric iodide has been extensively studied by several groups [10±26]. Novikov et al. [11,12] studied PL of mercuric iodide at low temperatures and observed three primary luminescence bands. Several sharp lines near band-gap were ascribed to annihilation of the free and bound excitons. Merz et al. [5] studied sublimation and doping experiments on PL at low temperatures and discussed the origins of the luminescence bands due to the stoichiometry in the crystal. Akopyan et al. [17,18] studied emission bands for mercuric iodide crystals with different
0022-3697/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0022-369 7(02)00207-X
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X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
Fig. 1. Steady state PL spectrum near band-gap of mercuric iodide at 10 K.
method growth and treatment. PL spectrum was studied by Bao et al. [21] with high resolution at low temperature and at least 26 emission lines were found in the wavelength region between 529 and 540 nm. Exciton luminescence spectra above 100 K were also studied by Goto et al. [22± 24]. However, the prevalence of the various anomalies observed, their precise nature in various crystals and the in¯uence of each on performance have remained unclear. Their origins and any interrelationships have proven elusive as well [4]. In the present study, time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. TRPL experiment reveals that band-gap luminescence comprises fast and slow decay components and shows different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to a defect in the anion sublattice. The temporal behavior of the tail band is described with rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor±acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region with two-photon excitation, which shows the band-gap at room temperature is about 2.125 eV.
2. Experimental Red HgI2 single crystals used for this work were grown from the saturated acetone solution of HgI2 at room temperature [20,25,26]. Firstly, the saturated acetone solution was made with analytical grade mercuric iodide. Then the solution was placed in an ultra-clean room to evaporate
isothermally (T 293 K). It needs several days to grow the single crystals in sizes of several millimeter, as grown surface has a crystallographic plane perpendicular to the c-axis. The samples were stored in a dry air ambience. TRPL measurements were performed using a streak camera in conjunction with a 25 cm spectrometer employing a 300 lines/mm grating. A frequency-doubled modelocked Ti/Sapphire laser was used for excitation (400 nm wavelength, 1.5 ps pulse width, 0.6 mW average power and 18 kHz repetition rate). The HgI2 crystal sample was mounted on the cold ®nger of a helium-cycled cryostat for PL measurement. The laser beam was incident onto the surface at an angle of 608 to the normal. In order to avoid super-saturation in the CCD units of streak camera a ®lter of transmissivity 10% was used to weaken the excitation. The energy density on the sample is 0.3 mJ/cm 2 per pulse. Two-photon PL was carried out using a Leica TCS MD microscope. The excitation laser was a mode-locked Ti/ Sapphire laser (Coherent Mira 900) producing 150 fs pulses at wavelength between 720 and 920 nm and the power on sample surface was about 100 mJ/cm 2.
3. Results and discussion 3.1. Band-gap photoluminescence at low temperatures Steady-state PL spectrum of HgI2 is shown in Fig. 1; the vertical bars and the relevant notations indicate the emission peaks. The strongest line appears in l 2 532.5 nm that was ascribed to radiative annihilation of bound excitons localized at vacancy of iodide [4,12,18,21]. Another bound exciton line l x, which was generally assumed to be due to radiative annihilation of bound excitons localized at vacancy of mercuric, is much weaker than l 2 so that it cannot be identi®ed clearly in our experiment. The short wavelength structures locating between 529.6 and 531.3 nm were generally assumed to be free exciton lines [12,18,21]. The free exciton line l 1 531.3 nm as shown in Fig. 1 corresponds to free exciton ground state [4]. A phonon replica line, l 2-LO 535.6 nm, obviously appears in our experiment. There were 21 phonon replica lines of free and bound excitons in the region from 532.6 to 540 nm in the high resolution spectrum [21]. A weak luminescence tail band appears obviously between 540 and 557 nm. The band of 560 nm that was discussed in Refs. [17,18] did not appear for our sample. The correlation among all lines and bands was studied by earlier authors. Exciton line l x and 560 nm band were due to the same crystal lattice defect (mercuric vacancy) [18]. In our experiment, both exciton line l x and 560 nm band do not appear, which shows that our sample is excess mercuric. In the case of radiative detector the best material would be those mercuric iodide crystals which have luminescence spectrum characterized by weak 560 nm band [5]. At the same time both l 2 and the tail band were assumed to
X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
2109
Fig. 4. Rise-time near band-gap and temporal behavior of the tail band.
Fig. 2. Near band-gap luminescence of HgI2 between 10 and 30 K.
originate from the anion defect, a strong l 2 line is accompanied by an obvious tail band for our sample. Fig. 2 shows near band-gap PL between 10 and 30 K. Intensity of bound exciton is much stronger at 10 K and it decreases more notably than that of free exciton with increasing temperature. Intensity of peak l 2 at 30 K is
about 4% that of 10 K and it cannot be detected at 40 K. Intensity of l 1 lowers to a much smaller scale than that of l 2 and its intensity at 30 K is about 60% of that at 10 K. As for lifetime, at 10 and 20 K, free and bound excitons reveal non-exponential decay. We can acquire the lifetime of different decay components through bi-exponential ®tting, as shown in Fig. 3. At 10 K, the decay of bound excitons is much slower than that of free excitons and it accelerates notably with increasing temperature. According to the ®tting, decay of bound and free excitons consists of the different lifetime components below 20 K. The difference between the fast and slow components decreases obviously with increasing temperature and the decay becomes single exponential decay at 30 K. At the same time, decay of bound excitons is almost as fast as that of the free excitons at 30 K. Comparing with fast component, the slow component is obviously dependent on temperature. At 30 K, the slow is nearly as fast as that of the fast so that two kinds of decay components merge into one, that is, the decay become exponential. The phonon replica has slower decay than free and bound excitons and their decay signi®cantly accelerates with increasing temperature. The intensity contribution of the fast component mainly distributes in free exciton region and the slow does in both bound exciton and phonon replica. 3.2. The tail band
Fig. 3. Lifetime of slow and fast decay components between 10 and 20 K.
Relatively, the temporal behavior of the tail band is quite different. Fig. 4 shows the rise-time of near band-gap luminescence and the temporal behavior of the tail band, where the rise-time is de®ned as the time rising to the maximum intensity. The solid round is rise-time originating from experiment; the square and triangle are the time constant t and t 2 from decay curve ®tting according to Eq. (3), respectively; and the empty diamond is the rise-time calculated based on time constant t and t 2. The rise-time is obviously shorter in the free and bound exciton region
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X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
Fig. 5. The red band luminescence at different temperatures.
between 529 and 540 nm except the bound exciton line and its phonon replica. This suggests that the tail band has different luminescent mechanism from free and bound excitons. The tail band was observed by Akopyan et al. and they found that any treatment of a mercuric iodide sample resulting in enhancement of the bound exciton line l 2 tended to increase the intensity of the tail band and vice versa. It can be assumed that the band origins from the anion sublattice and there should exist a strong interaction among them because the width of the tail band is about 17 nm wide. Under the assumption that the tail band originates from the non-radiative decay of bound excitons (located at iodide vacancies), its temporal behavior should be described with rate equations dN1 1 1 2 1 t.0 1 N dt t1 t 1 dN2 1 1 N1 2 N dt t t2 2
t.0
2
where N1 and N2 are the population of the corresponding bound excitons and iodide sublattice, the leaping rates of the ground are 1/t 1 and 1/t 2, respectively, the rate of bound excitons leaping non-radiatively to the sublattice is 1/t . With the initial condition N1 0 const: N0 ; N2 0 0 and general approximation t ! t 1, t 2, the solution of the equations will be N2 t N0 e2t=t2 2 e2t=t
3
Fitting result based on Eq. (3) reveals a very good coincidence with the measurement between 540 and 557 nm [26]. Experiment curve cannot be ®tted well at shorter wavelength than 540 nm, which suggests the luminescent mechanism of tail band is different from that of excitons and their phonon replica. Beyond 557 nm luminescent intensity is too weak to ®t correctly, which indicates the band ends at 557 nm. In the entire band the time constant t and t 2 at various wavelengths has little difference with about t 60 ps and t 2 320 ps, which is shown in Fig. 4.
This is in accordance with experimental result, the ®lled circle. The luminescence becomes so weak and ¯uctuant that the rise-time cannot be acquired accurately from the experiment curve at long wavelength side of tail band. The relaxation mechanism of free exciton±polaritons is mainly non-radiative decay such as trapped into impurities or defects. At the same time electron±hole pairs created by pulse excitation do transfer non-radiatively from free exciton±polaritons to bound excitons within a short time. The lifetime depends on both radiative and non-radiative decay. For the free exciton±polaritons, the observed decay time does not just indicate the radiative lifetime, but the total trapping rate into bound excitons. The short lifetime means the non-radiative decay is very ef®cient and the intensity of free excitons is much weaker than that of bound excitons, which suggests that radiation is far from the main relaxation mechanism and the dominant decay mechanism of free excitons is transferring into bound excitons. Bound excitons will also be converted into free exciton±polariton, but the probability is expected to be quite small at low temperature because their binding energies are estimated as 4±5 meV. At 10 K dominant decay of the free excitons is nonradiative, whereas that of the bound excitons is mainly radiative. The decay time in the bound-exciton luminescence is thus supposed to be the radiative lifetime of the excitons localized at defects. At low temperature excitons are mostly in localized states and the dominant decay mechanism of bound excitons is radiation. For bound excitons, non-radiative decay is very inef®cient so that their decay is pretty slower, which causes a very strong luminescence, very long lifetime and long rise-time. With increasing temperature, excitons begin to populate mobile states and they may diffuse until they interact with defects or impurities and decay non-radiatively. Excitons become more diffusive at higher temperatures, which allows them to encounter more defects or impurities and accelerate to decay. The intensity of bound excitons lowers remarkably because non-radiative decay of bound excitons intensi®es gradually with increasing temperature. The intensity decreases greatly between 20 and 30 K, which suggests non-radiative decay increases notably above 20 K. The conversion into the bound excitons is the dominant decay mechanism all along for the free excitons so that the radiative intensity is not greatly affected with increasing temperature. 3.3. The red band Fig. 5 shows the red band PL spectrum with different temperatures. Its central wavelength locates at 630.9 nm with FWHM 35.5 nm at 10 K. The lifetime of the band is much longer than that of near band-gap, on the order of microsecond. Intensity decreases notably with increasing temperature, which results mainly from thermal quench increasing. As shown in Fig. 5, the intensity decrease is not even with the same temperature interval and a signi®cant decrease takes place between 20 and 30 K, which
X.M. Wen et al. / Journal of Physics and Chemistry of Solids 63 (2002) 2107±2113
Fig. 6. Decay curves of red band at different temperatures.
is consistent with the situation near band-gap emission. FWHM of the band enlarges and central wavelength shifts towards longer wavelength with increasing temperature. This red band was studied by several groups and was ascribed to the recombination of donor±acceptor (D±A) pairs [17], i.e. donor-bound electrons to the hole-®lled acceptor. In the luminescence band of D±A pair recombination because of the Coulombic energy of interaction between the electron±hole pairs, the emission energy can be expressed by [27,28] E r Eg 2 ED 1 EA 1 e2 =1r
4
where Eg is the band-gap energy, ED and EA are the donor and acceptor binding energies, respectively, r is the D±A separation, 1 is low frequency dielectric constant. Therefore, a small separation r corresponds to high Coulomb energy, consequently appears in shorter wavelength within the band. Decay curve of 630 nm band is quite different from the near band-gap emission. Fig. 6 shows the decay curves of red band at different temperatures. The experiment reveals the decay curves approach approximately to straight in bilogarithmic coordinate. In other words, PL decay of this band can be approximately expressed with I t I0 t2n r
5
with n r; the decay rate and I0, a constant. The power decay of intensity shows the band is due to recombination of D±A pairs [34]. However, the approximate extent approaching to straight is not so good as some other semiconductors, such
2111
as ZnO [29], and the approximate extent is not the same for different temperatures. Fig. 6 shows the curves contort toward upward at low temperatures (below 30 K) and contort downward at higher temperatures (above 50 K). The best approaching appears at 40 K. The experiment suggests that the decay consists of two kinds of decay components that corresponds to two decay mechanisms and the two kinds of mechanisms are dominant at different temperature region, below and above 40 K, respectively. At 40 K, the two mechanisms nearly balance so that two decay rates approach to the same, which lead to a better straight. The inset of Fig. 6 shows variation of the decay rate with temperature. At low temperatures, below 40 K, decay rate decreases with increasing temperature and the larger decay rates appear at shorter wavelength. Above 40 K, decay rate increases with increasing temperature. The smallest decay rate appears at about 40 K. On the basis of experiment we can suppose that there exist two kinds of different mechanisms to control the recombination rate of D±A pairs and they are dominant below and above 40 K, respectively. At low temperature, 10 K, electrons signi®cantly occupy the shallowest traps. Wave function overlap between the delocalized shallowly trapped electrons and the deeply trapped hole is relatively larger, resulting in a higher recombination rate. With increasing temperature, shallow electron traps start to be thermally depopulated, which results in a decrease in the emission intensity since electrons now can reach the surface and recombine non-radiatively. Electron wave function is less delocalized in the slightly deeper electron trap and wave function overlap with the deeply trapped hole is reduced. This results in a decrease in the recombination rate with increasing temperature. The thermal detrapping of the shallowly trapped electrons decreases gradually with increasing temperature. Above 40 K the decrease becomes small. At the same time, the extra non-radiative decay paths that require thermal activation increase with increasing temperature. Above 40 K the extra non-radiative decay paths become gradually the dominant mechanism, resulting in the recombination rate which is controlled by the extra nonradiative decay so that the recombination rate n r increases with increasing temperature. On the other hand, with increasing temperature the thermal quenching increases, which causes luminescent intensity to decrease. According to experiment the thermal quench has a steep increase between 20 and 30 K and it is not even for the same temperature interval. 3.4. Two-photon PL at room temperature With increasing temperature, as most other semiconductors, band-gap energy of mercuric iodide shifts toward lower energy. Band-gap energy of mercuric iodide was measured with optical or electronic methods [10,20,30±33] at different temperatures. At room temperature the measurement value of 2.11 eV was reported with differing by as much
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Fig. 7. Two-photon luminescence of HgI2 at room temperature.
as 10%, although the large band-gap value was reported (,2.3 eV). Low-resolution two-photon PL spectrum is measured at room temperature (293 K) with fs laser excitation. A PL band is observed at yellow region and no other bands appear for entire visible region, as shown in Fig. 7. Its central wavelength locates at 584 nm and bandwidth is about 22 nm. The band is ascribed to exciton transition and its central wavelength corresponds to the band-gap, that is 2.125 eV. All the measurements for different excitation between 720 and 910 nm show the same spectra, including central wavelength and band width. Our measurement shows that the band-gap of red mercuric iodide at room temperature is 2.125 eV.
the room temperature band-gap of red HgI2 is 2.125 eV that is consistent with most other reports. Acknowledgements One of the authors (X.M. Wen) would like to acknowledge the ®nancial support of Osaka Electro-Communication University Foundation for Promotion of International Exchange on Research and Education. This work was partially supported by the Academic Frontier Promotion Project of the Ministry of Education, Science, Sports and Culture of Japan. Authors would like to thank Dr E. Kable of EMU (Electrical Microscopy Unit of Sydney University) for her kindly help for our experiment.
4. Conclusions We have studied TRPL of red mercuric iodide single crystal at low temperatures and measured the two-photon PL spectrum at room temperature. Near band-gap luminescence is ascribed to radiative annihilation of free and bound excitons and they comprises fast and slow decay component. For free excitons the dominant decay mechanism is converted into bound excitons. The conversion is very ef®cient so that they have much shorter lifetime than bound excitons, especially at low temperatures. At 10 K the dominant decay of bound excitons is radiation, which causes a much stronger intensity and slower lifetime compared with free excitons. Luminescence of bound exciton lowers notably with increasing temperature and disappears about 40 K. The tail band luminescence is ascribed to a defect in the anion sublattice and its temporal behavior may be described with the rate equations. Red band is ascribed to the radiative recombination of D±A pairs and its intensity may be expressed as I t t2n r : There exist two decay mechanisms to control the decay and they are dominant at different temperature regions. Two-photon PL spectrum is measured at room temperature (296 K) with fs laser excitation and the experiment shows
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