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Simultaneous photo-excitation of Li acceptor and shallow donors in ZnSe
Physica B 302–303 (2001) 277–281
Simultaneous photo-excitation of Li acceptor and shallow donors in ZnSe H. Nakata*, K. Yamada, Y. Itazaki, T. Ohyama Department of Physics, Graduate School of Science, Osaka University, 1-16 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
Abstract We observed the donor-assisted infrared absorption of Li acceptor in bulk ZnSe. Two kinds of donors, Liint and/or GaZn, make transition from 1S to 2S state during photo-excitation of Li acceptor. The transition energies are estimated at 6.3 and 20.2 meV for Liint and GaZn, respectively. Fano resonances of the valence band state with the simultaneous excited states of a Li acceptor and donors were also observed. # 2001 Elsevier Science B.V. All rights reserved. PACS: 78.30.Fs; 71.55.Gs Keywords: ZnSe; Infrared; Donor; Acceptor
1. Introduction Impurities in a semiconductor can be excited by the infrared absorption and the phonon emission is possible with the absorption. For example, intervalley phonons have been observed for deep impurities in Ge and Si [1,2]. We propose a new mechanism, i.e., donor-assisted infrared absorption of an acceptor. Here donors in the vicinity of the acceptor are excited when the acceptor absorbs infrared radiation. Such a process has never been reported until now. There are two processes analogous to it, more specifically, donor–acceptor (DA) recombination [3] and two-electron transition of a bound exciton [4]. For DA recombination, an electron on the donor and a hole on the *Corresponding author. Tel.: +81-6-6850-5757; fax: +81-66850-5764. E-mail address: [email protected] (H. Nakata).
acceptor recombine each other under the favor of the overlapping of their wavefunctions. For twoelectron transition of a bound exciton, an electron and a hole in the bound exciton recombine remaining the excitation of another electron to the 2S states. Donor-assisted infrared absorption of an acceptor needs interaction between the donor and the acceptor which is realized in DA recombination. The excitation of the donor during infrared absorption of the acceptor is a kind of Auger process which arises in the recombination of the bound exciton. In our case, the remote Coulomb interaction between a donor and an acceptor induces donor excitation. We have reported the infrared absorption of Li acceptors in ZnSe [5,6]. Large number of absorption lines were observed and most of them have been identified. All the absorption lines are due to 1S to 2P transition of a Li acceptor, because almost the same spectra have been observed for different samples. The absorption lines above
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 4 1 - 0
278
H. Nakata et al. / Physica B 302–303 (2001) 277–281
110 meV are broadened and distorted by Fano effect which is resonance phenomenon between the valence band state and the composite state containing the excited state of a Li acceptor, an LO phonon and others [7]. The distinct line observed at 72.9 meV was assigned to the transition from 1S3/2 ground state to 2P3/2 excited state [8]. The next prominent lines are the doublet of the transition from 1S3/2 ground state to 2P5/2 excited states. The small anisotropy of the valence band in ZnSe is responsible for the tiny splitting of these lines. We estimated the Luttinger parameters from the peak position of the doublet and the heavy hole mass along [1 1 1] direction [9]. In addition, we estimated the ionization energy of a Li acceptor to be 110.2 meV from the resonance energy of 1S3/2 to 2P1/2 which is well described by effective mass approximation. Several peaks have remained unidentified. Most of them are replica of 2P5/2 doublet accompanied with elementary excitations of 6.3 meV. We identified the excitation as 1S to 2S transition of a Li interstitial donor. The other unidentified peaks are assigned to the simultaneous excitation of a GaZn donor and a Li acceptor.
2. Experimental The samples we used in this study were bulk ZnSe grown by the solid growth method. The size of the sample was 4 4 1 mm3. Infrared absorption was carried out by a Fourier-transformed infrared spectrometer (Analect; AQS20) with a HgxCd1 xTe detector. The sample was attached on the cold finger of the cryostat which contains liquid helium.
3. Results and discussion Fig. 1 shows infrared absorption spectra of the bulk ZnSe at different temperatures. In comparison with the assignment of photoluminescence (PL) data by Tew et al., the peak observed at the lowest energy peak was identified with the transition of 1S3/2 to 2P3/2 state of Li acceptor [8]. The number of suffix denotes the total angular
Fig. 1. Infrared absorption spectra of Li acceptor in ZnSe at different temperatures.
momentum of the state following the notation by Baldereschi and Lipari [10]. The absorption peaks with the highest intensities observed at 90 meV forms 2P5/2 doublet as is the case of acceptors in Ge [11]. The doublet is split into 2P5/2 (G7 ) and 2P5/2 (G8 ) states by the crystal field with cubic symmetry. The transition to another 2P state, i.e., 2P1/2 was observed at 107.3 meV. From the peak position of 2P5/2 doublet and 2P1/2, we have obtained Luttinger parameters and the ionization energy of Li acceptor by referring the heavy hole mass along [1 1 1] direction obtained from microwave cyclotron resonance [9]. We could not use the peak position of 2P3/2 because the 2P3/2 state with a large ionization energy cannot be well described by effective mass approximation. The calculation was done in the framework of Baldereschi and Lipari’ theory [10,11]. The Luttinger parameters g1 , g2 , g3 are estimated at 6.44, 2.58 and 2.74, respectively. The small difference g2 g3 indicates the slight anisotropy of the valence band. The ionization energy of 1S3/2 state is reduced to be 110.2 meV. The estimated anisotropy of the valence band is very small and in good agreement with the result of angle dependence of the resonance magnetic field in microwave cyclotron resonance [9]. Another typical feature is the appearance of transitions associated with an LO phonon, one of which is 2PLO 3/2 observed at 103 meV. Strong
H. Nakata et al. / Physica B 302–303 (2001) 277–281
LO-phonon coupling is responsible for the birth of the transition. The peaks, which appear above the ionization energy of the Li acceptor are broad and distorted by Fano effect, which is a quantum interference effect between the valence band state and the composite state containing LO phonons and the excited states of the Li acceptor. Two unidentified peaks are observed around 100 meV. They should be the replica of the 2P5/2 doublet associated with the elementary excitation of which characteristic energy is 6.3 meV. Several replicas are associated with the same excitation and some of them accompany LO phonons as shown in Fig. 2. In order to clarify the origin of this elementary excitation, we performed the absorption measurements at different temperatures. The absorption peaks with 6.3 meV excitation diminish swiftly with increasing temperature. It means that the excitation has a small ionization energy. One of the candidates for the excitation is a shallow donor. The shallow donor described by effective mass approximation has the ionization energy of 27 meV in ZnSe [4]. As for the shallow donor, the possible excitation is 1S–2S transition whose energy is about 20 meV. This value is much larger than 6.3 meV. But there is one possibility which gives rise to such small characteristic energy. It is extremely shallow donor of Li interstitial (Liint). Its ionization energy is estimated to be about 15 meV from the PL measurement [12]. We speculate that the 1S–2S transition of Liint is responsible for the 6.3 meV excitation. If the 2S state of the Liint is described by effective mass approximation, the ionization energy of the 2S state is 7.1 meV. As a result, the ionization energy of Liint is estimated at 13.4 meV which is close to the value obtained from the PL measurement. In PL measurements for the same sample, we have observed the DA recombination line on Liint donor–Li acceptor. The existence of this DA line justifies the above-mentioned model. Satoh et al. observed optically detected cyclotron resonance (ODCR) in the same ZnSe sample [13]. If Liint fulfils the role for the elementary excitation related to the infrared absorption of Li acceptor, the shallow donor described by effective mass approximation can play the same role. The absorption peak at 114 meV is the replica of 2P5/2
279
Fig. 2. Infrared absorption spectra of Li acceptor just below the ionization energy of the acceptor.
(G8 ) associated with 1S–2S transition of the shallow donor. The energy of the absorption peak is not the same as that of the original state. The quantum interference due to Fano resonance shifts the resonance peak and distorts the lineshape. We estimated the intrinsic energy level with the method proposed by Piao et al. as shown in Fig. 3 [14]. It is given by the intersection between the absorption curve and the line connecting the top and the bottom of the absorption peak. The energy of 1S–2S transition is estimated at 20.2 meV, which is just equal to that of 1S–2S transition of Ga donor [4]. As a matter of fact, the radiative recombination between Li acceptor and Ga donor was observed for the same sample [13]. Our model for the donor-assisted infrared absorption is consistent with the results of PL measurements. The reason why the replica of 2P5/2 (G7 ) with the excitation of a Ga donor have not been observed, is not clear now. The absorption peak at 121 meV is the simultaneous excitation of Li acceptor to 2P5/2 (G8 ) and Ga donor to 2S and Liint donor to 2S state. Moreover, it interferes with the valence band state because the final state lies above the ionization energy of Li acceptor. Another interesting feature is the tiny splitting of the peaks around 130 meV as shown in Fig. 4. These peaks are associated with LO phonon and
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H. Nakata et al. / Physica B 302–303 (2001) 277–281
The donor-assisted infrared absorption of acceptor is first proposed. As the similar process there is two-electron transition of donor-boundexciton complexes, where an electron-hole pair recombines each other and the remaining electron is excited to 2S state as Auger process. In this case, the first electron makes interband transition through the excitation of the second electron. In the case of donor-assisted infrared absorption of acceptor, a hole makes transition to the excited state by transfer of the donor electron.
4. Conclusion Fig. 3. Fano resonances between the valence band state and the composite state containing the excited donors and Li acceptor. Analysis is followed by Piao et al. [11].
Fig. 4. Infrared absorption spectra of Li acceptor. All the absorption peaks are Fano resonances with the valence band states.
the excitation of Liint donor. There are two types of intermediate state, one is associated with LO phonon and the other is with the excitation of Liint donor. The former has larger energy than the ionization energy of Li acceptor while the latter has smaller one. Fano resonance shifts the intermediate state owing to LO-phonon coupling and causes the small splitting.
We propose a new absorption process: donorassisted infrared absorption of acceptors. The donors in the vicinity of the acceptor make transition from 1S to 2S state during the 1S–2P transition of the acceptor by absorbing infrared light. Mutual interaction between a donor and an acceptor is realized in DA recombination in PL measurements. In our case, two kinds of donors, Li interstitial and Ga donor, are associated with the infrared absorption of Li acceptor. The ionization energy of Li interstitial estimated to be 13.4 meV, while that of Ga donor is 27.3 meV. In the energy region above the ionization energy of Li acceptor, we observed Fano resonance. One of typical examples is the quantum interference between the valence band state and the composite state containing 2P excited state of Li acceptor and the 2S excited state of Ga donor.
Acknowledgements We are grateful to Sumitomo Electric Co., Ltd. for supplying good quality samples. We also acknowledge K. Fujii and H. Kobori for useful discussion.
References [1] T.C. Chandler et al., Phys. Rev. B 26 (1982) 6588. [2] H.G. Grimmeiss et al., Phys. Rev. B 37 (1988) 6916.
H. Nakata et al. / Physica B 302–303 (2001) 277–281 [3] [4] [5] [6] [7] [8]
J.J. Hopfield et al., Phys. Rev. Lett. 10 (1963) 162. P.J. Dean et al., Phys. Rev. Lett. 18 (1967) 122. H. Nakata et al., Appl. Phys. Lett. 74 (1999) 3480. H. Nakata et al., Phys. Rev. B 60 (1999) 13269. U. Fano, Phys. Rev. 124 (1961) 1868. H. Tews et al., Phys. Rev. B 19 (1979) 5178.
[9] [10] [11] [12] [13] [14]
281
T. Ohyama et al., Jpn. J. Appl. Phys. 26 (1987) L136. A. Baldereschi, N.O. Lipari, Phys. Rev. B 8 (1973) 2697. A. Baldereschi, N.O. Lipari, Phys. Rev. B 9 (1974) 1525. M. Issiki et al., J. Cryst. Growth 117 (1992) 410. K. Satoh et al., J. Lum. 87–89 (2000) 1228. G. Piao et al., Solid Sate Commun. 75 (1990) 835.
Simultaneous photo-excitation of Li acceptor and shallow donors in ZnSe H. Nakata*, K. Yamada, Y. Itazaki, T. Ohyama Department of Physics, Graduate School of Science, Osaka University, 1-16 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
Abstract We observed the donor-assisted infrared absorption of Li acceptor in bulk ZnSe. Two kinds of donors, Liint and/or GaZn, make transition from 1S to 2S state during photo-excitation of Li acceptor. The transition energies are estimated at 6.3 and 20.2 meV for Liint and GaZn, respectively. Fano resonances of the valence band state with the simultaneous excited states of a Li acceptor and donors were also observed. # 2001 Elsevier Science B.V. All rights reserved. PACS: 78.30.Fs; 71.55.Gs Keywords: ZnSe; Infrared; Donor; Acceptor
1. Introduction Impurities in a semiconductor can be excited by the infrared absorption and the phonon emission is possible with the absorption. For example, intervalley phonons have been observed for deep impurities in Ge and Si [1,2]. We propose a new mechanism, i.e., donor-assisted infrared absorption of an acceptor. Here donors in the vicinity of the acceptor are excited when the acceptor absorbs infrared radiation. Such a process has never been reported until now. There are two processes analogous to it, more specifically, donor–acceptor (DA) recombination [3] and two-electron transition of a bound exciton [4]. For DA recombination, an electron on the donor and a hole on the *Corresponding author. Tel.: +81-6-6850-5757; fax: +81-66850-5764. E-mail address: [email protected] (H. Nakata).
acceptor recombine each other under the favor of the overlapping of their wavefunctions. For twoelectron transition of a bound exciton, an electron and a hole in the bound exciton recombine remaining the excitation of another electron to the 2S states. Donor-assisted infrared absorption of an acceptor needs interaction between the donor and the acceptor which is realized in DA recombination. The excitation of the donor during infrared absorption of the acceptor is a kind of Auger process which arises in the recombination of the bound exciton. In our case, the remote Coulomb interaction between a donor and an acceptor induces donor excitation. We have reported the infrared absorption of Li acceptors in ZnSe [5,6]. Large number of absorption lines were observed and most of them have been identified. All the absorption lines are due to 1S to 2P transition of a Li acceptor, because almost the same spectra have been observed for different samples. The absorption lines above
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 4 1 - 0
278
H. Nakata et al. / Physica B 302–303 (2001) 277–281
110 meV are broadened and distorted by Fano effect which is resonance phenomenon between the valence band state and the composite state containing the excited state of a Li acceptor, an LO phonon and others [7]. The distinct line observed at 72.9 meV was assigned to the transition from 1S3/2 ground state to 2P3/2 excited state [8]. The next prominent lines are the doublet of the transition from 1S3/2 ground state to 2P5/2 excited states. The small anisotropy of the valence band in ZnSe is responsible for the tiny splitting of these lines. We estimated the Luttinger parameters from the peak position of the doublet and the heavy hole mass along [1 1 1] direction [9]. In addition, we estimated the ionization energy of a Li acceptor to be 110.2 meV from the resonance energy of 1S3/2 to 2P1/2 which is well described by effective mass approximation. Several peaks have remained unidentified. Most of them are replica of 2P5/2 doublet accompanied with elementary excitations of 6.3 meV. We identified the excitation as 1S to 2S transition of a Li interstitial donor. The other unidentified peaks are assigned to the simultaneous excitation of a GaZn donor and a Li acceptor.
2. Experimental The samples we used in this study were bulk ZnSe grown by the solid growth method. The size of the sample was 4 4 1 mm3. Infrared absorption was carried out by a Fourier-transformed infrared spectrometer (Analect; AQS20) with a HgxCd1 xTe detector. The sample was attached on the cold finger of the cryostat which contains liquid helium.
3. Results and discussion Fig. 1 shows infrared absorption spectra of the bulk ZnSe at different temperatures. In comparison with the assignment of photoluminescence (PL) data by Tew et al., the peak observed at the lowest energy peak was identified with the transition of 1S3/2 to 2P3/2 state of Li acceptor [8]. The number of suffix denotes the total angular
Fig. 1. Infrared absorption spectra of Li acceptor in ZnSe at different temperatures.
momentum of the state following the notation by Baldereschi and Lipari [10]. The absorption peaks with the highest intensities observed at 90 meV forms 2P5/2 doublet as is the case of acceptors in Ge [11]. The doublet is split into 2P5/2 (G7 ) and 2P5/2 (G8 ) states by the crystal field with cubic symmetry. The transition to another 2P state, i.e., 2P1/2 was observed at 107.3 meV. From the peak position of 2P5/2 doublet and 2P1/2, we have obtained Luttinger parameters and the ionization energy of Li acceptor by referring the heavy hole mass along [1 1 1] direction obtained from microwave cyclotron resonance [9]. We could not use the peak position of 2P3/2 because the 2P3/2 state with a large ionization energy cannot be well described by effective mass approximation. The calculation was done in the framework of Baldereschi and Lipari’ theory [10,11]. The Luttinger parameters g1 , g2 , g3 are estimated at 6.44, 2.58 and 2.74, respectively. The small difference g2 g3 indicates the slight anisotropy of the valence band. The ionization energy of 1S3/2 state is reduced to be 110.2 meV. The estimated anisotropy of the valence band is very small and in good agreement with the result of angle dependence of the resonance magnetic field in microwave cyclotron resonance [9]. Another typical feature is the appearance of transitions associated with an LO phonon, one of which is 2PLO 3/2 observed at 103 meV. Strong
H. Nakata et al. / Physica B 302–303 (2001) 277–281
LO-phonon coupling is responsible for the birth of the transition. The peaks, which appear above the ionization energy of the Li acceptor are broad and distorted by Fano effect, which is a quantum interference effect between the valence band state and the composite state containing LO phonons and the excited states of the Li acceptor. Two unidentified peaks are observed around 100 meV. They should be the replica of the 2P5/2 doublet associated with the elementary excitation of which characteristic energy is 6.3 meV. Several replicas are associated with the same excitation and some of them accompany LO phonons as shown in Fig. 2. In order to clarify the origin of this elementary excitation, we performed the absorption measurements at different temperatures. The absorption peaks with 6.3 meV excitation diminish swiftly with increasing temperature. It means that the excitation has a small ionization energy. One of the candidates for the excitation is a shallow donor. The shallow donor described by effective mass approximation has the ionization energy of 27 meV in ZnSe [4]. As for the shallow donor, the possible excitation is 1S–2S transition whose energy is about 20 meV. This value is much larger than 6.3 meV. But there is one possibility which gives rise to such small characteristic energy. It is extremely shallow donor of Li interstitial (Liint). Its ionization energy is estimated to be about 15 meV from the PL measurement [12]. We speculate that the 1S–2S transition of Liint is responsible for the 6.3 meV excitation. If the 2S state of the Liint is described by effective mass approximation, the ionization energy of the 2S state is 7.1 meV. As a result, the ionization energy of Liint is estimated at 13.4 meV which is close to the value obtained from the PL measurement. In PL measurements for the same sample, we have observed the DA recombination line on Liint donor–Li acceptor. The existence of this DA line justifies the above-mentioned model. Satoh et al. observed optically detected cyclotron resonance (ODCR) in the same ZnSe sample [13]. If Liint fulfils the role for the elementary excitation related to the infrared absorption of Li acceptor, the shallow donor described by effective mass approximation can play the same role. The absorption peak at 114 meV is the replica of 2P5/2
279
Fig. 2. Infrared absorption spectra of Li acceptor just below the ionization energy of the acceptor.
(G8 ) associated with 1S–2S transition of the shallow donor. The energy of the absorption peak is not the same as that of the original state. The quantum interference due to Fano resonance shifts the resonance peak and distorts the lineshape. We estimated the intrinsic energy level with the method proposed by Piao et al. as shown in Fig. 3 [14]. It is given by the intersection between the absorption curve and the line connecting the top and the bottom of the absorption peak. The energy of 1S–2S transition is estimated at 20.2 meV, which is just equal to that of 1S–2S transition of Ga donor [4]. As a matter of fact, the radiative recombination between Li acceptor and Ga donor was observed for the same sample [13]. Our model for the donor-assisted infrared absorption is consistent with the results of PL measurements. The reason why the replica of 2P5/2 (G7 ) with the excitation of a Ga donor have not been observed, is not clear now. The absorption peak at 121 meV is the simultaneous excitation of Li acceptor to 2P5/2 (G8 ) and Ga donor to 2S and Liint donor to 2S state. Moreover, it interferes with the valence band state because the final state lies above the ionization energy of Li acceptor. Another interesting feature is the tiny splitting of the peaks around 130 meV as shown in Fig. 4. These peaks are associated with LO phonon and
280
H. Nakata et al. / Physica B 302–303 (2001) 277–281
The donor-assisted infrared absorption of acceptor is first proposed. As the similar process there is two-electron transition of donor-boundexciton complexes, where an electron-hole pair recombines each other and the remaining electron is excited to 2S state as Auger process. In this case, the first electron makes interband transition through the excitation of the second electron. In the case of donor-assisted infrared absorption of acceptor, a hole makes transition to the excited state by transfer of the donor electron.
4. Conclusion Fig. 3. Fano resonances between the valence band state and the composite state containing the excited donors and Li acceptor. Analysis is followed by Piao et al. [11].
Fig. 4. Infrared absorption spectra of Li acceptor. All the absorption peaks are Fano resonances with the valence band states.
the excitation of Liint donor. There are two types of intermediate state, one is associated with LO phonon and the other is with the excitation of Liint donor. The former has larger energy than the ionization energy of Li acceptor while the latter has smaller one. Fano resonance shifts the intermediate state owing to LO-phonon coupling and causes the small splitting.
We propose a new absorption process: donorassisted infrared absorption of acceptors. The donors in the vicinity of the acceptor make transition from 1S to 2S state during the 1S–2P transition of the acceptor by absorbing infrared light. Mutual interaction between a donor and an acceptor is realized in DA recombination in PL measurements. In our case, two kinds of donors, Li interstitial and Ga donor, are associated with the infrared absorption of Li acceptor. The ionization energy of Li interstitial estimated to be 13.4 meV, while that of Ga donor is 27.3 meV. In the energy region above the ionization energy of Li acceptor, we observed Fano resonance. One of typical examples is the quantum interference between the valence band state and the composite state containing 2P excited state of Li acceptor and the 2S excited state of Ga donor.
Acknowledgements We are grateful to Sumitomo Electric Co., Ltd. for supplying good quality samples. We also acknowledge K. Fujii and H. Kobori for useful discussion.
References [1] T.C. Chandler et al., Phys. Rev. B 26 (1982) 6588. [2] H.G. Grimmeiss et al., Phys. Rev. B 37 (1988) 6916.
H. Nakata et al. / Physica B 302–303 (2001) 277–281 [3] [4] [5] [6] [7] [8]
J.J. Hopfield et al., Phys. Rev. Lett. 10 (1963) 162. P.J. Dean et al., Phys. Rev. Lett. 18 (1967) 122. H. Nakata et al., Appl. Phys. Lett. 74 (1999) 3480. H. Nakata et al., Phys. Rev. B 60 (1999) 13269. U. Fano, Phys. Rev. 124 (1961) 1868. H. Tews et al., Phys. Rev. B 19 (1979) 5178.
[9] [10] [11] [12] [13] [14]
281
T. Ohyama et al., Jpn. J. Appl. Phys. 26 (1987) L136. A. Baldereschi, N.O. Lipari, Phys. Rev. B 8 (1973) 2697. A. Baldereschi, N.O. Lipari, Phys. Rev. B 9 (1974) 1525. M. Issiki et al., J. Cryst. Growth 117 (1992) 410. K. Satoh et al., J. Lum. 87–89 (2000) 1228. G. Piao et al., Solid Sate Commun. 75 (1990) 835.