- Email: [email protected]
Temperature dependence of excitonic luminescence from high-quality ZnS epitaxial layers
ELSEVIER
Journal of Crystal Growth 184/185 (1998) 1110-1113
Temperature dependence of excitonic luminescence from high-quality ZnS epitaxial layers Seiji Nakamura, Takashi Sakashita, Yoichi Yamada*, Tsunemasa Taguchi Department of Electrical and Electronic Engineering, Yamaguchi University, 2557 Tokiwadai, Ube, Yamaguchi 755, Japan
Abstract Excitonic properties of high-quality ZnS epitaxial layers grown by low-pressure metalorganic chemical vapor deposition have been studied by means of temperature-dependent photoluminescence spectroscopy. The excitonic luminescence spectrum was dominated by the radiative recombination of free excitons. Both light-hole and heavy-hole free-exciton luminescence were observed with line widths of 2.6 and 3.3 meV, respectively, at 4 K. The excitonic luminescence was observed even at room temperature (300 K). The line width broadening of the free-exciton luminescence with increase in temperature was analyzed in terms of exciton-LO-phonon and exciton-acoustic-phonon scattering processes. 0 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf Keywords;
Photoluminescence; ature dependence
ZnS; Light-hole
free exciton; Heavy-hole
1. Introduction Among II-VI compound semiconductors, ZnS is one of the key materials for applications in shortwavelength optoelectronic devices, especially for ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes (LDs) because of its direct and wide band gap of 3.73 eV at room temperature [l], For high performance of UV LEDs and LDs, the growth of high-quality ZnS epitaxial layers and the fabrication of quantum-well structures are essen-
*Corresponding author. Fax: [email protected].
+
81 836 35 9449;
e-mail:
0022-0248/98/%19.00 c 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00774-4
free exciton; Luminescence
line width; Temper-
tial. Conductivity control is also necessary for such applications. However, there have been few reports on p-type conductivity control of ZnS. The difficulty in p-type doping stems from carrier compensation by residual donor-like impurities and defects. Therefore, in order to prevent such compensation effects, it is necessary to obtain highquality undoped ZnS epitaxial layers for doping. Recently, the growth and optical characterization of ZnS epitaxial films have been reported by several authors [24]. We have also reported on the optical properties of ZnS epitaxial layers grown by low-pressure metalorganic chemical vapor deposition (MOCVD). The photoluminescence (PL) spectrum of our ZnS epitaxial layer was dominated
S. Nakamura et al. /Journal
by the radiative recombination of free excitons even at low temperature as well as at low excitation density, and no notable luminescence band associated with self-activated (SA) or other deep centers was detected [5,6]. Very recently, we have succeeded in growing high-quality ZnS epitaxial layers. Both light-hole and heavy-hole free-exciton luminescence was observed with line widths of 2.6 and 3.3 meV, respectively, at 4 K. In addition, owing to the reduction of residual impurity contamination, the luminescence intensity of bound excitons was much weaker than that of free excitons. We will mainly describe in the present paper the temperature dependence of excitonic luminescence from high-quality ZnS epitaxial layers.
2. Experimental
1111
of Crystal Growth 1841185 (1998) 1110-1113
D”,X) 325w~v&&&nm~328
(AoX
1
procedure 325
High-quality cubic-structured ZnS epitaxial layers were grown on (1 0 0)-oriented GaAs substrates by means of low-pressure MOCVD using all gaseous sources. High-purity dimethylzinc (DMZn) at a concentration of 1.06% in He gas and hydrogen sulfide (H,S) at a concentration of 10.8% in H, gas were used as Zn and S sources, respectively. These gases were supplied to a reaction chamber directly through the piezo valves of mass flow controllers and air operated valves. The growth conditions of high-quality ZnS epitaxial layer used in the present work were as follows: a growth temperature of 500°C a growth pressure of about 0.5 Torr, and a VI/II flow ratio of 18. The layer thickness was about 2.0 urn. As the excitation source for the PL measurement, we used a Xe-Cl excimer laser (308 nm) in order to perform band-to-band excitation. The repetition rate and the pulse width were 100 Hz and 2.5 ns, respectively. The excitation-power density was as low as 0.2 kW/cm2. The sample temperature was varied from 4 to 300 K.
3. Results and discussion Fig. 1 shows the excitonic region of the PL spectrum at 4 K taken from the ZnS epitaxial layer used in the present work. It is found from our separate
335
330 WAVELENGTH
(nm)
Fig. 1. Excitonic photoluminescence spectrum at 4 K taken from a high-quality ZnS epitaxial layer. The inset shows a Boltzmann deconvolution of light-hole (EL”) and heavy-hole (EHH) splitting of the free exciton.
reflectance measurement that the dominant luminescence line at 326.24 nm (3.7994 eV) is due to the radiative recombination of free excitons (denoted by Ex). The spectrum also includes the radiative recombination of excitons bound to neutral donors [denoted by (D”,X)] and neutral acceptors [denoted by (A’,X)]. The origin of both the donor and the acceptor species is not clear at the present stage. The weak luminescence lines observed at 329.87 nm (3.7576 eV) and 333.80 nm (3.7133 eV) are attributed to LO-phonon replicas of the freeexciton line (denoted by Ex-1LO and Ex-2L0, respectively). Furthermore, the luminescence line associated with the y1= 2 excited states of the free exciton is clearly observed at 323.89 nm (3.8269 eV). Therefore, the binding energy of the free exciton is estimated to be 36.7 meV on the basis of the energy separation between the n = 1 ground and the n = 2 excited states. It is noted here that the luminescence intensity of both the (D’,X) and the (A’,X) lines relative to that of the Ex line is much weaker than that of our previous samples [5,6].
1112
S. Nakamura et al. /Journal
of Crystal
The inset in Fig. 1 shows the enlarged spectrum of the free-exciton luminescence (Ex line). It can clearly be seen from the inset that the free-exciton luminescence consists of two components. Because of the larger thermal expansion coefficient of ZnS (6.7 x lop6 K-i) as compared to that of GaAs (5.8 x 10m6 K-l), there exists biaxial tensile strain in the ZnS epitaxial layer. Then, the strain splits the uppermost degenerate valence band into heavy-hole and light-hole bands. Therefore, the lower-energy and the higher-energy components are attributed to the radiative recombination of light-hole and heavy-hole free excitons (ELH and Euu), respectively. A Boltzmann deconvolution of light-hole and heavy-hole splitting of the free exciton is shown in the inset. The luminescence line widths of the light-hole and heavy-hole free excitons are estimated to be 2.6 and 3.3 meV, respectively, and the energy separation between the lighthole and the heavy-hole free excitons is estimated to be 3.8 meV. On the basis of a bimetallic strip model [7], the tensile strain in the ZnS epitaxial layer was derived analytically and the resultant energy splitting between the light-hole and the heavy-hole bands was estimated to be 4.6 meV. This value approximately agrees with the experimental value of 3.8 meV. Fig. 2 shows the excitonic PL spectra taken from the ZnS epitaxial layer at 4, 70, 150, and 300 K. With increasing temperature, thermal quenching of the bound-exciton luminescence [(D”,X) and (A’,X) lines] occurs to a marked extent compared to that of the free-exciton luminescence. In addition, the increase in the luminescence intensity of the heavyhole exciton relative to that of the light-hole exciton is observed. This is due to the thermal population from light-hole to heavy-hole exciton states. The luminescence can be observed even at room temperature (300 K). We consider that the room-temperature luminescence is still excitonic in origin on the basis of the temperature-dependent peak position which will be shown in Fig. 3a. Fig. 3a and Fig. 3b show the temperature dependence of the peak position and the line width (full-width at half-maximum) of the light-hole free-exciton luminescence, respectively. The peak position shifts towards the lower-energy side with increasing temperature. This energy shift reflects
Growth 1841185 (1998) 1110-1113
ELH
EHH
12 A,
150K
I
II,,,,,.,,,,It
325
c
I,
330
335
WAVELENGTH Fig. 2. Temperature-dependent spectra taken from a high-quality
340
(nm)
excitonic photoluminescence ZnS epitaxial layer.
.
l
.
.
= 2.7 x 10.’ meViK
0
100 TEMPERATURE
200
300
(K)
Fig. 3. Temperature dependence of the peak energy in (a) and the line width in (b) of the light-hole free-exciton luminescence taken from a high-quality ZnS epitaxial layer.
S. Nakamura et al. /Journal
of Crystal Growth 1841185 (1998) 1110-1113
the temperature dependence of the band-gap energy in ZnS. On the other hand, the luminescence line width is gradually increased with temperature, and the line width at 300 K is about 3.5 meV. Temperature-dependent broadening of luminescence line width, r(T), can be analyzed by making use of the following equation, taking account of the scattering processes with acoustic and LO phonons, and with ionized impurities [S]: T(T) =
r, + r,T
+
+ I)eXp( \
r
Lo
exp(fw&BV
- 1
1113
epitaxial layers grown by low-pressure MOCVD. In spite of a large LO-phonon energy of 42 meV, we observed the room-temperature excitonic luminescence from ZnS epitaxial layers. The binding energy of the free-exciton (light-hole free exciton) was estimated to be 36.7 meV on the basis of the energy separation between the n = 1 ground and the n = 2 excited states. The exciton-LO-phonon coupling constant (r,,) was estimated to be 100 meV, which was approximately 1.7 times as large as that for bulk ZnSe.
-(Eh)), lCB’
/
where r, is the inhomogeneous broadening, r, is a coefficient of the exciton-acoustic-phonon interaction, r,o is the exciton-LO-phonon coupling energy, ri is constant, hmLo is the LO phonon a proportionality factor which accounts for the density of impurity centers, and (&) is the binding energy averaged over all possible locations of the impurities. Using r, = 2.6 meV, r, = 2.7 x lop2 meV/K, r,o = 100 meV, and horo = 42 meV, the theoretical curve agrees fairly well with the experimental data as shown in Fig. 3b by the solid line. It is noted here that the contribution of the impurity scattering process to the line width broadening can be neglected (ri = 0 ). This means that the concentration of both donor and acceptor impurities in the present layer must be low. It is also noted that the line width broadening with temperature is dominated by the excitonLOphonon scattering process. It is well known that the exciton-LO-phonon coupling constant (rro) increases with the polarity of the material. The value for ZnS is estimated to be 100 meV, which is approximately 1.7 times as large as that in bulk ZnSe (60 meV) [9].
4. Conclusions We have studied the temperature dependence of the excitonic luminescence from high-quality ZnS
Acknowledgements This work was partly supported by the Electric Technology Research Foundation of Chugoku and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
References (Eds.), Optical [II Y. Yamada, in: T. Ogawa, Y. Kanemitsu Properties of Low-Dimensional Materials, Ch. 4, World Scientific, Singapore, 1995, p. 202. M. Di Blasio, D. Bouchara, J. Calas, PI A. Abounadi, M. Averous, 0. Briot, N. Briot, T. Cloitre, R.L. Aulombard, B. Gil, Phys. Rev. B 50 (1994) 11677. P. Prete, N. Lovergine, A.M. Mancini, [31 M. Fernandez, R. Cingolani, L. Vasanelli, M.R. Perrone, Phys. Rev. B 55 (1997) 7660. M T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers, J. Appl. Phys. 81 (1997) 2803. T. Yamamoto, S. Nakamura, T. Taguchi, [51 Y. Yamada, F. Sasaki, S. Kobayashi, T. Tani, Appl. Phys. Lett. 69 (1996) 88. T. Sakashita, K. Yoshimura, Y. Yamada, C61 S. Nakamura, T. Taguchi, Jpn. J. Appl. Phys. 36 (1997) L491. T.G. Read, D.R. Lamb, A.F.W. Wilc71 SD. Brotherton, loughby, Solid State Electron. 16 (1973) 1367. ca J. Lee, E.S. Koteles, M.O. Vassell, Phys. Rev. B 33 (1986) 5512. J. Ding, M. Hagerott, A.V. Nurmikko, c91 N.T. Pelekanos, H. Luo, N. Samarth, J.K. Furdyna, Phys. Rev. B 45 (1992) 6037.
Journal of Crystal Growth 184/185 (1998) 1110-1113
Temperature dependence of excitonic luminescence from high-quality ZnS epitaxial layers Seiji Nakamura, Takashi Sakashita, Yoichi Yamada*, Tsunemasa Taguchi Department of Electrical and Electronic Engineering, Yamaguchi University, 2557 Tokiwadai, Ube, Yamaguchi 755, Japan
Abstract Excitonic properties of high-quality ZnS epitaxial layers grown by low-pressure metalorganic chemical vapor deposition have been studied by means of temperature-dependent photoluminescence spectroscopy. The excitonic luminescence spectrum was dominated by the radiative recombination of free excitons. Both light-hole and heavy-hole free-exciton luminescence were observed with line widths of 2.6 and 3.3 meV, respectively, at 4 K. The excitonic luminescence was observed even at room temperature (300 K). The line width broadening of the free-exciton luminescence with increase in temperature was analyzed in terms of exciton-LO-phonon and exciton-acoustic-phonon scattering processes. 0 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf Keywords;
Photoluminescence; ature dependence
ZnS; Light-hole
free exciton; Heavy-hole
1. Introduction Among II-VI compound semiconductors, ZnS is one of the key materials for applications in shortwavelength optoelectronic devices, especially for ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes (LDs) because of its direct and wide band gap of 3.73 eV at room temperature [l], For high performance of UV LEDs and LDs, the growth of high-quality ZnS epitaxial layers and the fabrication of quantum-well structures are essen-
*Corresponding author. Fax: [email protected].
+
81 836 35 9449;
e-mail:
0022-0248/98/%19.00 c 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00774-4
free exciton; Luminescence
line width; Temper-
tial. Conductivity control is also necessary for such applications. However, there have been few reports on p-type conductivity control of ZnS. The difficulty in p-type doping stems from carrier compensation by residual donor-like impurities and defects. Therefore, in order to prevent such compensation effects, it is necessary to obtain highquality undoped ZnS epitaxial layers for doping. Recently, the growth and optical characterization of ZnS epitaxial films have been reported by several authors [24]. We have also reported on the optical properties of ZnS epitaxial layers grown by low-pressure metalorganic chemical vapor deposition (MOCVD). The photoluminescence (PL) spectrum of our ZnS epitaxial layer was dominated
S. Nakamura et al. /Journal
by the radiative recombination of free excitons even at low temperature as well as at low excitation density, and no notable luminescence band associated with self-activated (SA) or other deep centers was detected [5,6]. Very recently, we have succeeded in growing high-quality ZnS epitaxial layers. Both light-hole and heavy-hole free-exciton luminescence was observed with line widths of 2.6 and 3.3 meV, respectively, at 4 K. In addition, owing to the reduction of residual impurity contamination, the luminescence intensity of bound excitons was much weaker than that of free excitons. We will mainly describe in the present paper the temperature dependence of excitonic luminescence from high-quality ZnS epitaxial layers.
2. Experimental
1111
of Crystal Growth 1841185 (1998) 1110-1113
D”,X) 325w~v&&&nm~328
(AoX
1
procedure 325
High-quality cubic-structured ZnS epitaxial layers were grown on (1 0 0)-oriented GaAs substrates by means of low-pressure MOCVD using all gaseous sources. High-purity dimethylzinc (DMZn) at a concentration of 1.06% in He gas and hydrogen sulfide (H,S) at a concentration of 10.8% in H, gas were used as Zn and S sources, respectively. These gases were supplied to a reaction chamber directly through the piezo valves of mass flow controllers and air operated valves. The growth conditions of high-quality ZnS epitaxial layer used in the present work were as follows: a growth temperature of 500°C a growth pressure of about 0.5 Torr, and a VI/II flow ratio of 18. The layer thickness was about 2.0 urn. As the excitation source for the PL measurement, we used a Xe-Cl excimer laser (308 nm) in order to perform band-to-band excitation. The repetition rate and the pulse width were 100 Hz and 2.5 ns, respectively. The excitation-power density was as low as 0.2 kW/cm2. The sample temperature was varied from 4 to 300 K.
3. Results and discussion Fig. 1 shows the excitonic region of the PL spectrum at 4 K taken from the ZnS epitaxial layer used in the present work. It is found from our separate
335
330 WAVELENGTH
(nm)
Fig. 1. Excitonic photoluminescence spectrum at 4 K taken from a high-quality ZnS epitaxial layer. The inset shows a Boltzmann deconvolution of light-hole (EL”) and heavy-hole (EHH) splitting of the free exciton.
reflectance measurement that the dominant luminescence line at 326.24 nm (3.7994 eV) is due to the radiative recombination of free excitons (denoted by Ex). The spectrum also includes the radiative recombination of excitons bound to neutral donors [denoted by (D”,X)] and neutral acceptors [denoted by (A’,X)]. The origin of both the donor and the acceptor species is not clear at the present stage. The weak luminescence lines observed at 329.87 nm (3.7576 eV) and 333.80 nm (3.7133 eV) are attributed to LO-phonon replicas of the freeexciton line (denoted by Ex-1LO and Ex-2L0, respectively). Furthermore, the luminescence line associated with the y1= 2 excited states of the free exciton is clearly observed at 323.89 nm (3.8269 eV). Therefore, the binding energy of the free exciton is estimated to be 36.7 meV on the basis of the energy separation between the n = 1 ground and the n = 2 excited states. It is noted here that the luminescence intensity of both the (D’,X) and the (A’,X) lines relative to that of the Ex line is much weaker than that of our previous samples [5,6].
1112
S. Nakamura et al. /Journal
of Crystal
The inset in Fig. 1 shows the enlarged spectrum of the free-exciton luminescence (Ex line). It can clearly be seen from the inset that the free-exciton luminescence consists of two components. Because of the larger thermal expansion coefficient of ZnS (6.7 x lop6 K-i) as compared to that of GaAs (5.8 x 10m6 K-l), there exists biaxial tensile strain in the ZnS epitaxial layer. Then, the strain splits the uppermost degenerate valence band into heavy-hole and light-hole bands. Therefore, the lower-energy and the higher-energy components are attributed to the radiative recombination of light-hole and heavy-hole free excitons (ELH and Euu), respectively. A Boltzmann deconvolution of light-hole and heavy-hole splitting of the free exciton is shown in the inset. The luminescence line widths of the light-hole and heavy-hole free excitons are estimated to be 2.6 and 3.3 meV, respectively, and the energy separation between the lighthole and the heavy-hole free excitons is estimated to be 3.8 meV. On the basis of a bimetallic strip model [7], the tensile strain in the ZnS epitaxial layer was derived analytically and the resultant energy splitting between the light-hole and the heavy-hole bands was estimated to be 4.6 meV. This value approximately agrees with the experimental value of 3.8 meV. Fig. 2 shows the excitonic PL spectra taken from the ZnS epitaxial layer at 4, 70, 150, and 300 K. With increasing temperature, thermal quenching of the bound-exciton luminescence [(D”,X) and (A’,X) lines] occurs to a marked extent compared to that of the free-exciton luminescence. In addition, the increase in the luminescence intensity of the heavyhole exciton relative to that of the light-hole exciton is observed. This is due to the thermal population from light-hole to heavy-hole exciton states. The luminescence can be observed even at room temperature (300 K). We consider that the room-temperature luminescence is still excitonic in origin on the basis of the temperature-dependent peak position which will be shown in Fig. 3a. Fig. 3a and Fig. 3b show the temperature dependence of the peak position and the line width (full-width at half-maximum) of the light-hole free-exciton luminescence, respectively. The peak position shifts towards the lower-energy side with increasing temperature. This energy shift reflects
Growth 1841185 (1998) 1110-1113
ELH
EHH
12 A,
150K
I
II,,,,,.,,,,It
325
c
I,
330
335
WAVELENGTH Fig. 2. Temperature-dependent spectra taken from a high-quality
340
(nm)
excitonic photoluminescence ZnS epitaxial layer.
.
l
.
.
= 2.7 x 10.’ meViK
0
100 TEMPERATURE
200
300
(K)
Fig. 3. Temperature dependence of the peak energy in (a) and the line width in (b) of the light-hole free-exciton luminescence taken from a high-quality ZnS epitaxial layer.
S. Nakamura et al. /Journal
of Crystal Growth 1841185 (1998) 1110-1113
the temperature dependence of the band-gap energy in ZnS. On the other hand, the luminescence line width is gradually increased with temperature, and the line width at 300 K is about 3.5 meV. Temperature-dependent broadening of luminescence line width, r(T), can be analyzed by making use of the following equation, taking account of the scattering processes with acoustic and LO phonons, and with ionized impurities [S]: T(T) =
r, + r,T
+
+ I)eXp( \
r
Lo
exp(fw&BV
- 1
1113
epitaxial layers grown by low-pressure MOCVD. In spite of a large LO-phonon energy of 42 meV, we observed the room-temperature excitonic luminescence from ZnS epitaxial layers. The binding energy of the free-exciton (light-hole free exciton) was estimated to be 36.7 meV on the basis of the energy separation between the n = 1 ground and the n = 2 excited states. The exciton-LO-phonon coupling constant (r,,) was estimated to be 100 meV, which was approximately 1.7 times as large as that for bulk ZnSe.
-(Eh)), lCB’
/
where r, is the inhomogeneous broadening, r, is a coefficient of the exciton-acoustic-phonon interaction, r,o is the exciton-LO-phonon coupling energy, ri is constant, hmLo is the LO phonon a proportionality factor which accounts for the density of impurity centers, and (&) is the binding energy averaged over all possible locations of the impurities. Using r, = 2.6 meV, r, = 2.7 x lop2 meV/K, r,o = 100 meV, and horo = 42 meV, the theoretical curve agrees fairly well with the experimental data as shown in Fig. 3b by the solid line. It is noted here that the contribution of the impurity scattering process to the line width broadening can be neglected (ri = 0 ). This means that the concentration of both donor and acceptor impurities in the present layer must be low. It is also noted that the line width broadening with temperature is dominated by the excitonLOphonon scattering process. It is well known that the exciton-LO-phonon coupling constant (rro) increases with the polarity of the material. The value for ZnS is estimated to be 100 meV, which is approximately 1.7 times as large as that in bulk ZnSe (60 meV) [9].
4. Conclusions We have studied the temperature dependence of the excitonic luminescence from high-quality ZnS
Acknowledgements This work was partly supported by the Electric Technology Research Foundation of Chugoku and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
References (Eds.), Optical [II Y. Yamada, in: T. Ogawa, Y. Kanemitsu Properties of Low-Dimensional Materials, Ch. 4, World Scientific, Singapore, 1995, p. 202. M. Di Blasio, D. Bouchara, J. Calas, PI A. Abounadi, M. Averous, 0. Briot, N. Briot, T. Cloitre, R.L. Aulombard, B. Gil, Phys. Rev. B 50 (1994) 11677. P. Prete, N. Lovergine, A.M. Mancini, [31 M. Fernandez, R. Cingolani, L. Vasanelli, M.R. Perrone, Phys. Rev. B 55 (1997) 7660. M T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers, J. Appl. Phys. 81 (1997) 2803. T. Yamamoto, S. Nakamura, T. Taguchi, [51 Y. Yamada, F. Sasaki, S. Kobayashi, T. Tani, Appl. Phys. Lett. 69 (1996) 88. T. Sakashita, K. Yoshimura, Y. Yamada, C61 S. Nakamura, T. Taguchi, Jpn. J. Appl. Phys. 36 (1997) L491. T.G. Read, D.R. Lamb, A.F.W. Wilc71 SD. Brotherton, loughby, Solid State Electron. 16 (1973) 1367. ca J. Lee, E.S. Koteles, M.O. Vassell, Phys. Rev. B 33 (1986) 5512. J. Ding, M. Hagerott, A.V. Nurmikko, c91 N.T. Pelekanos, H. Luo, N. Samarth, J.K. Furdyna, Phys. Rev. B 45 (1992) 6037.