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A new 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
502
Journal of Crystal Growth 101 (1990) 502—506 North-Holland
A NEW 1.47 eV DEFECT-LUMINESCENCE BAND IN MOCVD-GROWN CdTe ON (100) GaAs Chikara ONODERA and Tsunemasa TAGUCHI Department of Electrical Engineering~Faculty of Engineering~Osaka University, Suita, Osaka 565, Japan
We have studied the temperature dependence of emission intensity and peak energy, photoluminescence excitation and time-resolved spectra (TRS) of the LO-phonon replicated 1.47 eV band in CdTe films grown on (100) GaAs substrates by a low-pressure metalorgamc chemical vapour deposition (MOCVD) method. The TRS studies indicate that there is no observable energy shift toward lower photon energy after time delay. The excitation spectrum represents an exciton resonant peak at 1.598 eV, suggesting that the contribution of free excitons to the 1.47 eV band is predominant. This evidence is also revealed by the thermal quenching of the emission intensity which shows that a thermal liberation of excitons from the localized center takes place. We therefore propose that the observed 1.47 eV band looks like the Y line series, attributable to recombination of excitons at an extended defect, which has been already observed in ZnSe.
1. Introduction A high-quality epitaxial layer of CdTe has been required as a buffer layer for subsequent growth of CdHgTe heterostructures on GaAs substrates [1]. Thus it is very important to characterize the quality of the epitaxial layers of CdTe, in which the thickness above 1.0 ~tm is the minimum buffer layer thickness for the high-quality CdHgTe layer growth [1,2]. It has been argued, so far, from TEM results with (100) CdTe on (100) GaAs by metalorganic chemical vapour deposition (MOCVD) that at about 1 ~tm thickness the strains in the layer appear to be completely relaxed and above 1.3 ~tm the density of dislocations decreases greatly, and that no evidence of an oxide or a foreign interface layer is found [3,4]. We have recently discovered broad-emission bands with many LO-phonon sidebands, in the vicinities of 1.47 and 1.42 eV with about 1.5 ~tm thickness films, in addition to intense excitomc lines at 4.2 K [5,6]. We tentatively suggest [7] that the band consists of two individual emission bands which include the well-known 1.42 eV [8] and weak (S = 0.2) LO-phonon-replicated 1.476 eV bands (hereafter, 1.47 eV band). The 1.47 eV band is similar to that observed in MBE-grown CdTe/InSb and has been ascribed to electron— hole recombination at extended defects [9]. How0022-0248/90/$03.50 © 1990
—
ever, Leopold et al. [10] have found for the MBE growth that this band is much stronger in (100) layers grown on (100) GaAs substrate including a thin surface oxide layer. In addition, Choyke et al. Eli] and Taguchi et a!. [6,7] have put forward several arguments to interpret the nature of the 1.47 eV band, so far. In order to investigate the detailed nature of the 1.47 eV band in CdTe epitaxial layers, we have measured the temperature dependence of emission intensity, peak position, photoluminescence (PL) excitation and time-resolved spectra of CdTe films on (100) GaAs substrates. This paper is concerned with the luminescence properties of the 1.47 eV band and suggests that the band is principally different from the ordinary 1.42 eV band [8] which originates from a donor— acceptor pair transition encompassing a complex acceptor.
2. Low-pressure MOCVD method and PL characterization We have developed a low-pressure metalorganic chemical vapour deposition (MOCVD) apparatus where the growth was carried out at 1—5 Ton, and as a Te precursor, dimethyltelluride (DMTe) was used and precracked at 500 °C.
Elsevier Science Publishers B.V. (North-Holland)
C. Onodera, T. Taguchi
/ New
1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
DMTe is easily dissoluble R (= CH3) and RTe at an elevated cracking temperature, and thus RTe chemical beams will react with DMCd gas. The (100) GaAs substrates are etched in boiled HC1 and then rinsed in a chemical solution of 5H2S04, 1H202 and 1H20. The growth temperature was at 300—350°C.DMCd was diluted with He gas with a concentration of 0.13%. The [DMTeJ/[DMCd] mol ratio was about 1—2. The film thickness was about 1.2 ~sm. Photoluminescence spectra measurements were carried out at 4.2 K using a 441.6 nm He—Cd laser and a 1 m Jobin—Yvon singlegrating monochromator. The temperature dependence was studied using a temperature-variable cryostat. Time-resolved spectral measurements were done using an N2 laser (pulsewidth 1 ns and repetition rate 10 Hz) and a boxcar integrator,
3. Results and discussion Fig. 1 shows a typical PL spectrum at 4.2 K of a MOCVD-grown CdTe layer (1.2 ~smthickness), grown on a (100) GaAs substrate. In addition to sharp and well-resolved excitonic lines which indude the free-exciton (Eu) line at 1.596 eV, the neutral-donor-bound exciton (D°,X) line at 1.5934 eV and the neutral-acceptor-bound exciton (A°,X) line at 1.5905 eV, and LO-phonon (hwLo = 21 meV)-replicated broad band is seen with a zerophonon line at 1.476 eV. Here, it is noted that the intensity of the 0-phonon line is stronger than the ______________________________________ CdTe/(100)GaAs He-Cd Iaser44l.6nm 42 K
~47~V
/1~ ~
2
/ ,/isi
/ /
/
/
/
~x) 150 1.60 PHOTON ENERGY (eV) obtained Fig. 1. PLatspectrum 4.2 K excited of CdTe by (1.2 a 441.6 pm mu thick) He—Cd on (100) laserGaAs and excitation spectrum. 1.40
503
1- or 2-LO phonon sidebands. This spectral lineshape is similar to that observed in the MBE-grown CdTe on InSb [9]. Dean et a!. [9] have thus claimed that this band has a similar behaviour like the Y band in ZnSe. We have designated this band as the 1.47 eV band [5]. This feature is peculiar because the electron—LO phonon coupling strength is about 0.2, that is smaller than 1 [9]. An emission peak at 1.518 eV and a shoulder at 1.489 eV are unknown in nature at present, but, are not due to emission of the GaAs substrate. The PL excitation spectrum is also given in this figure. Its main peak is located around 1.598 eV, which corresponds to the free-exciton peak. However, this spectrum continues above the band-gap region because of the overlapping of the excitation spectrum for the ordinary 1.42 eV band. We have already predicted that the 1.47 eV band overlaps with the 1.42 eV band which has been known as the 1.42 eV donor—acceptor pair band [8]. Namely, this means that there are two individual bands centered at about 1.45 eV [7]. Fig. 2 shows the temperature dependence of the 1.47 eV band at temperatures between 4.2 and 71.5 K. With increasing temperature, the 0-LO phonon line (arrow) of the 1.47 eV band decreases in intensity, accompanied by an energy shift to lower photon energy. On the other hand, the broad band at the lower-energy side does not steadily decrease in intensity. Above 70 K, only a single broad band is seen around 1.42 eV. It is therefore clear that the 1.47 eV band is faster decreasing in intensity than the 1.42 eV band. With further increasing temperature, the 1.42 eV band is thermally quenched with an activation energy of about 170 meV which is in good agreement with the previous value [8] in the bulk 1.42 eV band. Theoretical PL lineshapes were drawn (dashed curves) as a function of temperature using the following equations:
1(E) =A~exp(-S)~ 1 X ~,
— —
I
a’
1 7:-
1
+
~
—
L~O+
~. ~2/Y 2 1~LO1
‘
(1)
504
C. Onodera, I Taguchi
/
New 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
CdTe/(100)GoAs
4.2K
~.
1
_______
~..
~ 214K44.5K
1, -
-
~-
I.’:
-- -
uJ
50.0K .
59 ~
_______
—
71.5K _______
—
2o 1 45 1 1 PHOTON ENERGY (eV) Fig. 2. Temperature dependence of the 1.47 eV band in the temperature range of 4.2 to 71.5 K. The dashed curves are drawn using eq. (1).
where S is the LO phonon coupling constant, I~ and 1~are the intensity of 1-LO and 0-LO phonon lines, respectively, A is a fitting parameter, E0 is the position of the 0-LO phonon line, y is the damping constant and is a function of temperature, and LO is t e p onon energy meV). For the 1.47 eV band, the S value is 0.16, while for the 1.42 eV band, we prefer the S value to be 1.7. These values are in good agreement with the experimental S values obtained by eq. (2). In order to fit the experimental curves, each emission intensity of the 1(E) for the 1.42 and 1.47 eV bands was analyzed by the dashed curves as shown in fig. 2. From the theoretical curves, it is understood that around 50 K the intensity of the 1.47 eV band decreases, when compared to that of the 1.42 band. Furthermore, the phonon replicas of theeV1.47 eV band are smeared out above 50 K due to thermal broadening. (—
Fig. 3 shows the temperature dependence of the emission intensity of the 1.47 eV band. We see that there are at least two thermal dissociation processes. From 35 to 50 K, the estimated activation energy is about 14 meY, while above 60 K, the activation energy is estimated to be about 125 meV. As understood from the temperature dependence of the Y line in ZnSe [12], the first process is derived from the direct dissociation of the free exciton itself, trapped at a certain localized impurity which can form the radiative recombination center of the 1.47 eV transition. The second process is considered to be probably due to the thermal release of free excitons bound to the center, since the locahzation energy is obtamed to be about 130 meV from the energy difference between the peak position and the band gap of CdTe at 4.2 K. This implies to us that the 1.47 eV band exhibits a typical bound-exciton characteristic. Fig. 4 shows the temperature dependence of the peak position of the 1.476 eV line and also mdidates the temperature dependence of the energy band gap of CdTe bulk material. The behaviour is consistent with the temperature dependence of the energy band gap of the bulk CdTe [13]. Fig. 5 shows the TRS of the CdTe on (100) GaAs excited by an N 2 laser at 4.2 K. After
~
50
TEMPERATURE(K)
2
10
-
..
1
/
~ 10
20 25 2f~TEM~E~ATURE 30 35 40 45 REC!pROc Fig. 3. Temperature dependence of the 1.476 eV 0-LO phonon line intensity.
C. Onodera, I Taguchi
/ New 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
505
1606 1604
1602 ~ 1600 1598 ~1.596
1478
w ~476
147GeVBQod)Zero LU
<1.474
Q_ 1472 hoe) 1.470
1468 1466 20
30
40
50
60
TEMPERATURE K C
Fig. 4. Temperature dependence of the 1.47 eV band: the energy shift of the 1.476 eV line and the energy band gap [13].
pulsed excitation, the 1.476 eV 0-LO phonon line is not shifted toward the lower-photon energy and the associated 1-LO phonon sideband is seen as indicated by an arrow with increasing time delay,
N2 toser 337mm 421K CdTe/GoA~, TRS of the 1,47GeVBcnd /,
-
-
1 LU
7,~,t\o
It is evident that the 1.47 eV band is not due to a donor—acceptor pair transition. However, the 0-LO phonon line is rapidly decreased in intensity with time. After 1 ~ts delay, another LO-phonon-replicated broad band appears; a line at 1.452 eV ordinary 1.42 eV band, and all its phonon sidebands are originate shifted towardthe the0-phonon lower-energy which may inside the with time delay. Thisfrom is consistent with line the preyto a donor—acceptor pair. The decay of the 1.476
Ops
ousline assignment thatthan the 1.42 band is eV ascribed From eV these was findings, faster wethat suggest ofeVthe that 1.42 the 1.47 band. eV
3~ 6ps
ordinary 1.42 eV band. band a characteristic Theis same behaviour band of thedifferent 1.476 eVfrom line the of CdTe as the Y line of ZnSe has been first reported by Dean et a!. [9]. Until now, it has been suggested that the 1.47 eV band shows the following properties:
-9~ >I.-
z
lOps l4ps 2Ops
23ps
_______________________
1.40 1.45 1.50 PHOTON ENERGY (eV)
Fig. 5. Time-resolved spectra of the 1.47 and 1.42 eV bands at 4.2 K.
(1) a weak LO-phonon coupling strength, viz. smaller than unity [7,9]; (2) dependence on the film thickness [11]; (3) dependence on orientation and on the presence of oxygen impurities [7,10]; (4) dependence on growth temperature and on various Te alkyl sources [6,7].
506
C. Onodera, I Taguchi
/ New 1.47 eVdefect-luminescence band in
It is therefore concluded, on the basis of the above-mentioned evidence and our present experimental results, that the 1.47 eV band originates from the recombination of excitons bound to a localized center. Following our previous reports [5—7]and the assignment of Dean et al. [9], we would like to propose that the localized center of the 1.47 eV band, which can bind excitons, is ascribed to structural defects such as stacking faults and/or dislocations and associated defects [2—4].
4. Conclusions The luminescence properties of the 1.47 eV band, having a zero-LO phonon line at 1.476 eV, have been extensively investigated by means of the temperature dependence and time-resolved spectra. We have suggested that the 1.47 eV band indicates a different nature of luminescence transitions as compared to the ordinary 1.42 eV band which has been attributed to donor—acceptor pairs. It is tentatively considered that this band results from recombination of excitons bound to extended defects, as supported by the results of ZnSe
Acknowledgements One of the authors (T.T.) thanks R. Tsunoda and K. Matsumoto of Nippon Sanso Corp. and R.
MOCVD-grown CdTe on (100) GaAs
Takamatsu of Trichemical Research Laboratory for supplying DMCd and DMTe, respectively. He is also much indebted to the Research Institute for Production Development (Nissha Foundation) for financial support of the MOCVD project.
References [1] I.B. Bhat, Mater. Res. Soc. Symp. Proc. 127 (1988) 34. [2] J. Petruzzello, D. Olego, S.K. Ghandhi, N.R. Tasker and I. Bhat, Appl. Phys. Letters 50 (1987) 1423. [3) P.D. Brown, J.E. Hails, G.J. Russel and J. Woods, AppI. Phys. Letters 50 (1987) 1144. [4] R.L. Chou, M. Lin and KS. Chon, J. Crystal Growth 94 (1989) 551. [5] K. Ohba, T. Taguchi, Ch. Onodera, Y. Hiratate and A. Hiraki, Japan. J. Appl. Phys. 28 (1989) 1246. [6] K. Ohba, Ch. Onodera, T. Taguchi, Y. Hiratate and A. Hiraki, in: Proc. Intern. Conf. on Defects in Semiconductors, Yokohama, Sept. 1989 (to be published). [7] T. Taguchi and M. Suita, Japan. J. Appl. Phys. 28 (1989) 1889. [8] T. Taguchi, J. Shirafuji and Y. Inuishi, Japan. J. AppI. Phys. 34 (1973) 5514. [9] PJ. Dean, G.M. Williams and G.B. Blackmore, J. Phys. Phys.)J.M. 17 (1984) 2291.and M.L. Wroge, Appi. [10] D. D.J.(Appl. Leopold, Bathngall Phys. Letters 49 (1986) 1473. [11] W.J. Choyke, MG. Burbe, Z.C. Feng, M.H. Hanes and A. Mascarenhas, Mater. Sci. Forum 10—12 (1986) 769. [12] T. Taguchi, T. Kusao and A. Hiraki, J. Crystal Growth 59 (1985) 477. [13] D.J. Chadi and M. Balkanski, Phys. Rev. B5 (1972) 3058.
Journal of Crystal Growth 101 (1990) 502—506 North-Holland
A NEW 1.47 eV DEFECT-LUMINESCENCE BAND IN MOCVD-GROWN CdTe ON (100) GaAs Chikara ONODERA and Tsunemasa TAGUCHI Department of Electrical Engineering~Faculty of Engineering~Osaka University, Suita, Osaka 565, Japan
We have studied the temperature dependence of emission intensity and peak energy, photoluminescence excitation and time-resolved spectra (TRS) of the LO-phonon replicated 1.47 eV band in CdTe films grown on (100) GaAs substrates by a low-pressure metalorgamc chemical vapour deposition (MOCVD) method. The TRS studies indicate that there is no observable energy shift toward lower photon energy after time delay. The excitation spectrum represents an exciton resonant peak at 1.598 eV, suggesting that the contribution of free excitons to the 1.47 eV band is predominant. This evidence is also revealed by the thermal quenching of the emission intensity which shows that a thermal liberation of excitons from the localized center takes place. We therefore propose that the observed 1.47 eV band looks like the Y line series, attributable to recombination of excitons at an extended defect, which has been already observed in ZnSe.
1. Introduction A high-quality epitaxial layer of CdTe has been required as a buffer layer for subsequent growth of CdHgTe heterostructures on GaAs substrates [1]. Thus it is very important to characterize the quality of the epitaxial layers of CdTe, in which the thickness above 1.0 ~tm is the minimum buffer layer thickness for the high-quality CdHgTe layer growth [1,2]. It has been argued, so far, from TEM results with (100) CdTe on (100) GaAs by metalorganic chemical vapour deposition (MOCVD) that at about 1 ~tm thickness the strains in the layer appear to be completely relaxed and above 1.3 ~tm the density of dislocations decreases greatly, and that no evidence of an oxide or a foreign interface layer is found [3,4]. We have recently discovered broad-emission bands with many LO-phonon sidebands, in the vicinities of 1.47 and 1.42 eV with about 1.5 ~tm thickness films, in addition to intense excitomc lines at 4.2 K [5,6]. We tentatively suggest [7] that the band consists of two individual emission bands which include the well-known 1.42 eV [8] and weak (S = 0.2) LO-phonon-replicated 1.476 eV bands (hereafter, 1.47 eV band). The 1.47 eV band is similar to that observed in MBE-grown CdTe/InSb and has been ascribed to electron— hole recombination at extended defects [9]. How0022-0248/90/$03.50 © 1990
—
ever, Leopold et al. [10] have found for the MBE growth that this band is much stronger in (100) layers grown on (100) GaAs substrate including a thin surface oxide layer. In addition, Choyke et al. Eli] and Taguchi et a!. [6,7] have put forward several arguments to interpret the nature of the 1.47 eV band, so far. In order to investigate the detailed nature of the 1.47 eV band in CdTe epitaxial layers, we have measured the temperature dependence of emission intensity, peak position, photoluminescence (PL) excitation and time-resolved spectra of CdTe films on (100) GaAs substrates. This paper is concerned with the luminescence properties of the 1.47 eV band and suggests that the band is principally different from the ordinary 1.42 eV band [8] which originates from a donor— acceptor pair transition encompassing a complex acceptor.
2. Low-pressure MOCVD method and PL characterization We have developed a low-pressure metalorganic chemical vapour deposition (MOCVD) apparatus where the growth was carried out at 1—5 Ton, and as a Te precursor, dimethyltelluride (DMTe) was used and precracked at 500 °C.
Elsevier Science Publishers B.V. (North-Holland)
C. Onodera, T. Taguchi
/ New
1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
DMTe is easily dissoluble R (= CH3) and RTe at an elevated cracking temperature, and thus RTe chemical beams will react with DMCd gas. The (100) GaAs substrates are etched in boiled HC1 and then rinsed in a chemical solution of 5H2S04, 1H202 and 1H20. The growth temperature was at 300—350°C.DMCd was diluted with He gas with a concentration of 0.13%. The [DMTeJ/[DMCd] mol ratio was about 1—2. The film thickness was about 1.2 ~sm. Photoluminescence spectra measurements were carried out at 4.2 K using a 441.6 nm He—Cd laser and a 1 m Jobin—Yvon singlegrating monochromator. The temperature dependence was studied using a temperature-variable cryostat. Time-resolved spectral measurements were done using an N2 laser (pulsewidth 1 ns and repetition rate 10 Hz) and a boxcar integrator,
3. Results and discussion Fig. 1 shows a typical PL spectrum at 4.2 K of a MOCVD-grown CdTe layer (1.2 ~smthickness), grown on a (100) GaAs substrate. In addition to sharp and well-resolved excitonic lines which indude the free-exciton (Eu) line at 1.596 eV, the neutral-donor-bound exciton (D°,X) line at 1.5934 eV and the neutral-acceptor-bound exciton (A°,X) line at 1.5905 eV, and LO-phonon (hwLo = 21 meV)-replicated broad band is seen with a zerophonon line at 1.476 eV. Here, it is noted that the intensity of the 0-phonon line is stronger than the ______________________________________ CdTe/(100)GaAs He-Cd Iaser44l.6nm 42 K
~47~V
/1~ ~
2
/ ,/isi
/ /
/
/
/
~x) 150 1.60 PHOTON ENERGY (eV) obtained Fig. 1. PLatspectrum 4.2 K excited of CdTe by (1.2 a 441.6 pm mu thick) He—Cd on (100) laserGaAs and excitation spectrum. 1.40
503
1- or 2-LO phonon sidebands. This spectral lineshape is similar to that observed in the MBE-grown CdTe on InSb [9]. Dean et a!. [9] have thus claimed that this band has a similar behaviour like the Y band in ZnSe. We have designated this band as the 1.47 eV band [5]. This feature is peculiar because the electron—LO phonon coupling strength is about 0.2, that is smaller than 1 [9]. An emission peak at 1.518 eV and a shoulder at 1.489 eV are unknown in nature at present, but, are not due to emission of the GaAs substrate. The PL excitation spectrum is also given in this figure. Its main peak is located around 1.598 eV, which corresponds to the free-exciton peak. However, this spectrum continues above the band-gap region because of the overlapping of the excitation spectrum for the ordinary 1.42 eV band. We have already predicted that the 1.47 eV band overlaps with the 1.42 eV band which has been known as the 1.42 eV donor—acceptor pair band [8]. Namely, this means that there are two individual bands centered at about 1.45 eV [7]. Fig. 2 shows the temperature dependence of the 1.47 eV band at temperatures between 4.2 and 71.5 K. With increasing temperature, the 0-LO phonon line (arrow) of the 1.47 eV band decreases in intensity, accompanied by an energy shift to lower photon energy. On the other hand, the broad band at the lower-energy side does not steadily decrease in intensity. Above 70 K, only a single broad band is seen around 1.42 eV. It is therefore clear that the 1.47 eV band is faster decreasing in intensity than the 1.42 eV band. With further increasing temperature, the 1.42 eV band is thermally quenched with an activation energy of about 170 meV which is in good agreement with the previous value [8] in the bulk 1.42 eV band. Theoretical PL lineshapes were drawn (dashed curves) as a function of temperature using the following equations:
1(E) =A~exp(-S)~ 1 X ~,
— —
I
a’
1 7:-
1
+
~
—
L~O+
~. ~2/Y 2 1~LO1
‘
(1)
504
C. Onodera, I Taguchi
/
New 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
CdTe/(100)GoAs
4.2K
~.
1
_______
~..
~ 214K44.5K
1, -
-
~-
I.’:
-- -
uJ
50.0K .
59 ~
_______
—
71.5K _______
—
2o 1 45 1 1 PHOTON ENERGY (eV) Fig. 2. Temperature dependence of the 1.47 eV band in the temperature range of 4.2 to 71.5 K. The dashed curves are drawn using eq. (1).
where S is the LO phonon coupling constant, I~ and 1~are the intensity of 1-LO and 0-LO phonon lines, respectively, A is a fitting parameter, E0 is the position of the 0-LO phonon line, y is the damping constant and is a function of temperature, and LO is t e p onon energy meV). For the 1.47 eV band, the S value is 0.16, while for the 1.42 eV band, we prefer the S value to be 1.7. These values are in good agreement with the experimental S values obtained by eq. (2). In order to fit the experimental curves, each emission intensity of the 1(E) for the 1.42 and 1.47 eV bands was analyzed by the dashed curves as shown in fig. 2. From the theoretical curves, it is understood that around 50 K the intensity of the 1.47 eV band decreases, when compared to that of the 1.42 band. Furthermore, the phonon replicas of theeV1.47 eV band are smeared out above 50 K due to thermal broadening. (—
Fig. 3 shows the temperature dependence of the emission intensity of the 1.47 eV band. We see that there are at least two thermal dissociation processes. From 35 to 50 K, the estimated activation energy is about 14 meY, while above 60 K, the activation energy is estimated to be about 125 meV. As understood from the temperature dependence of the Y line in ZnSe [12], the first process is derived from the direct dissociation of the free exciton itself, trapped at a certain localized impurity which can form the radiative recombination center of the 1.47 eV transition. The second process is considered to be probably due to the thermal release of free excitons bound to the center, since the locahzation energy is obtamed to be about 130 meV from the energy difference between the peak position and the band gap of CdTe at 4.2 K. This implies to us that the 1.47 eV band exhibits a typical bound-exciton characteristic. Fig. 4 shows the temperature dependence of the peak position of the 1.476 eV line and also mdidates the temperature dependence of the energy band gap of CdTe bulk material. The behaviour is consistent with the temperature dependence of the energy band gap of the bulk CdTe [13]. Fig. 5 shows the TRS of the CdTe on (100) GaAs excited by an N 2 laser at 4.2 K. After
~
50
TEMPERATURE(K)
2
10
-
..
1
/
~ 10
20 25 2f~TEM~E~ATURE 30 35 40 45 REC!pROc Fig. 3. Temperature dependence of the 1.476 eV 0-LO phonon line intensity.
C. Onodera, I Taguchi
/ New 1.47 eV defect-luminescence band in MOCVD-grown CdTe on (100) GaAs
505
1606 1604
1602 ~ 1600 1598 ~1.596
1478
w ~476
147GeVBQod)Zero LU
<1.474
Q_ 1472 hoe) 1.470
1468 1466 20
30
40
50
60
TEMPERATURE K C
Fig. 4. Temperature dependence of the 1.47 eV band: the energy shift of the 1.476 eV line and the energy band gap [13].
pulsed excitation, the 1.476 eV 0-LO phonon line is not shifted toward the lower-photon energy and the associated 1-LO phonon sideband is seen as indicated by an arrow with increasing time delay,
N2 toser 337mm 421K CdTe/GoA~, TRS of the 1,47GeVBcnd /,
-
-
1 LU
7,~,t\o
It is evident that the 1.47 eV band is not due to a donor—acceptor pair transition. However, the 0-LO phonon line is rapidly decreased in intensity with time. After 1 ~ts delay, another LO-phonon-replicated broad band appears; a line at 1.452 eV ordinary 1.42 eV band, and all its phonon sidebands are originate shifted towardthe the0-phonon lower-energy which may inside the with time delay. Thisfrom is consistent with line the preyto a donor—acceptor pair. The decay of the 1.476
Ops
ousline assignment thatthan the 1.42 band is eV ascribed From eV these was findings, faster wethat suggest ofeVthe that 1.42 the 1.47 band. eV
3~ 6ps
ordinary 1.42 eV band. band a characteristic Theis same behaviour band of thedifferent 1.476 eVfrom line the of CdTe as the Y line of ZnSe has been first reported by Dean et a!. [9]. Until now, it has been suggested that the 1.47 eV band shows the following properties:
-9~ >I.-
z
lOps l4ps 2Ops
23ps
_______________________
1.40 1.45 1.50 PHOTON ENERGY (eV)
Fig. 5. Time-resolved spectra of the 1.47 and 1.42 eV bands at 4.2 K.
(1) a weak LO-phonon coupling strength, viz. smaller than unity [7,9]; (2) dependence on the film thickness [11]; (3) dependence on orientation and on the presence of oxygen impurities [7,10]; (4) dependence on growth temperature and on various Te alkyl sources [6,7].
506
C. Onodera, I Taguchi
/ New 1.47 eVdefect-luminescence band in
It is therefore concluded, on the basis of the above-mentioned evidence and our present experimental results, that the 1.47 eV band originates from the recombination of excitons bound to a localized center. Following our previous reports [5—7]and the assignment of Dean et al. [9], we would like to propose that the localized center of the 1.47 eV band, which can bind excitons, is ascribed to structural defects such as stacking faults and/or dislocations and associated defects [2—4].
4. Conclusions The luminescence properties of the 1.47 eV band, having a zero-LO phonon line at 1.476 eV, have been extensively investigated by means of the temperature dependence and time-resolved spectra. We have suggested that the 1.47 eV band indicates a different nature of luminescence transitions as compared to the ordinary 1.42 eV band which has been attributed to donor—acceptor pairs. It is tentatively considered that this band results from recombination of excitons bound to extended defects, as supported by the results of ZnSe
Acknowledgements One of the authors (T.T.) thanks R. Tsunoda and K. Matsumoto of Nippon Sanso Corp. and R.
MOCVD-grown CdTe on (100) GaAs
Takamatsu of Trichemical Research Laboratory for supplying DMCd and DMTe, respectively. He is also much indebted to the Research Institute for Production Development (Nissha Foundation) for financial support of the MOCVD project.
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