- Email: [email protected]
Improvement of optical properties of gas source MBE grown GaPAlP short period superlattices
...^....:::*:::~"~[i~'-:#.":~:i:~>'~::::::::.:... ~.~.::. . . . . . . . . . . . . . . . . . . :...:'.~
a~ surface science ELSEVIER
Applied Surface Science 92 (1996) 566-570
Improvement of optical properties of gas source MBE grown GaP/AlP short period superlattices J.H. Kiln, H. Asahi *, K. Asami, T. Ogura, K. Doi, S. Gonda The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, lbaraki, Osaka 567, Japan Received 12 December 1994; accepted for publication 2 March 1995
Abstract
Optical properties of ( G a P ) m / ( A I P ) n normal and (GaP),~(AIP)n (GaP),,z(AIP),2(m = m, + m 2, n = n I + n 2, m, >t m 2, n, >t n 2) modulated superlattices (SLs) grown by gas source molecular beam epitaxy are studied. Optical properties of the normal SLs are improved with increasing growth temperature. The highest photoluminescence (PL) intensity and the narrowest full width at half maximum (9 meV) are obtained for 640°C growth. It is found that the PL intensity variation with measuring temperature is slower for the 640°C grown SLs than for the 600°C grown SLs. Modulated SLs show strong dependence of the PL intensity and wavelength on the modulated SL structure. The PL intensity of the (m I + n I = odd, m 2 + n 2 - odd) modulated SLs shows slower temperature variation than that of the normal SLs. At 30 K, the PL intensity of the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL. Electroluminescence (EL) emission was measured at 130-300 K for preliminary fabricated light emitting diodes having a GaP/AlP (9, 4)(4, 3) modulated SL as the active layer. The temperature variations of PL and EL wavelengths confirm that the EL emission originates from the GaP/AlP modulated SL active layer.
1. Introduction
G a P / A l P short period superlattices (SLs) which consist of two indirect band gap semiconductors, Gap and AlP, exhibit a direct band gap nature due to zone-folding and band-mixing effects [1]. Theoretical calculations show that (Gap)m/(A1P) n SLs exhibit band gap energies in the range of 2.40-2.06 eV depending on the period (m, n) [2]. We have already reported the gas source molecular beam epitaxy (MBE) growth of (GaP)m/(A1P) . short period SLs and their optical properties [3-6].
* Corresponding author. Fax: +81 6 877 6447.
Several groups have also reported the growth of G a P / A l P SLs by metalorganic vapor phase epitaxy (MOVPE) [7-9], where the growth temperatures ranged from 720 to 880°C. In Raman scattering measurements, Asahi et al. [3] pointed out that the absorption coefficients of these G a P / A l P SLs have relatively small values compared with normal direct band gap semiconductors, which agrees with theoretical calculations [2]. Weisbuch et al. reported the improvement of the structural and optical properties of G a A s / G a I _ x AI x As multiquantum well (MQW) structures grown with variation of the growth temperature [10]. Ikeda et al. have reported the improvement of optical transition properties of the S i / G e short period SLs by introducing modulated SL structures by theoretical calculations [11 ].
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 9 6 - 0
J.H. Kim et al./ Applied Surface Science 92 (1996) 566-570 unit ~_ i
period 20
_1
m
13
.1=
n
-
7
_-
[(GaP)m/(A1P)n] SL, [ (GaP)ml/(A1P)nl, (GaP)m2/(AIP)n2 ]SL (m+n=20) m = ml+m2, n = nl+n2 (ml_> m2, nl_> n2) Fig. 1. Schematic structures of the (GaP)I3/(ALP) 7 normal and the GaP/AlP modulated superlattices.
In this paper, to improve the optical properties of GaP/AlP SLs, we have studied the growth temperature dependence and the effect of the SL structure modulation. The growth temperature dependence is studied in the range 600-660°C. To improve the optical transition properties of GaP/AlP SLs, we propose here the modulated SL structures: (GAP),,, (ALP), (Gap),.:(AIP),2SLs (m -- m I + m2, n = n I + n2, m I > / m 2 , n I >/ n 2) as shown in Fig. 1. These modulated SL structures are similar to the (Si),,,(Ge),,(Si),.,(Ge),,. . . . SLs (m' + n' + m" + n" + ... = 10) studied by Ikeda et al. [1 l]. The intention of our modulated SLs is the enhancement of the band-mixing effect to improve the direct band gap nature by introducing structural modulation to normal (GaP),,/(A1P),SLs. Eiectroluminescence (EL) emission characteristics are also described for fabricated light emitting diodes (LED) having a G a P / A l P modulated SL as the active layer.
567
the growth temperature dependence of the optical properties. We have also grown modulated SLs as shown in Fig. 1: (m t, nl)(m 2, n 2 ) = ( 1 0 , 4)(3, 3), (9, 4)(4, 3), (8, 4)(5, 3), and (7, 4)(6, 3). To study the SL structure modulation effect and to compare the optical properties of the modulated SLs with those of normal SLs, the (GaP)|3/(A1P) 7 normal SLs as well as the modulated SLs were grown at 640°C in the same growth series. They were characterized through PL and EL measurements. PL was measured in the temperature range 4.2-105 K by using a He-Cd laser (325 nm) as an excitation light source. EL was measured under pulsed condition with a pulse width of 2 /zs and a repetition of 1 kHz in order to suppress the Joule heating. The temperature dependence of EL was measured in the temperature range from 130 to 300 K.
3. Results and discussion
3.1. Growth temperature dependence Growth temperature dependence of the optical properties of (GAP) ll/(A1P) 3 normal SLs was studied in the range from 600 to 660°C. Fig. 2 shows the growth temperature dependence of the PL intensity. The PL intensity remarkably increases with increasing growth temperature. In each growth series, improvement by a factor of over 10 was observed by increasing the growth temperature from 600 to 640°C. The decrease of the PL intensity at 660°C is considered to be partly caused by the additional outgassing of impurities from the substrate holder and around it. For the 640°C growth, we have observed the narrow" ~ lOOO (GaP)I ff(AJP)3
4.'~ K
loo q)
2. Experimental GaP/AlP SLs were grown on S-doped (001) GaP substrates by gas source MBE. Elemental Ga and AI and thermally cracked PH 3 were used as Group III and Group V sources, respectively. (GaP)It/(AIP) 3 normal SLs were grown at 600°C-660°C to study
.J rt 1
rr
o Series I • Series 11
.1 580
600
620
640
660
680
Growth Temperature ('C)
Fig. 2. Growth temperature dependence of the 4.2 K PL peak intensity for the (GaP)ll/(ALP) 3 SL.
568
J.H. Kim et a l . / Applied Surface Science 92 (1996) 566-570 610
o
E c
4.2K
o ,s~,I
[3 (13.7) • (!,4){4,3) 0 (8,4){$,3)
10
• Sede=I]
(o~,odd) ~ , ( ~ , ~ - ~ (even,even) .01
I
570
-J
n
(13,7)
(10,4)(3.3)(9.4)(4.3) (8,4)(5,3) (7,4)(6.3) SupedattJce Structure
Fig. 3. PL peak wavelength of the (GaP)I3/(ALP) 7 and the (GaP),nI(AIP), (GaP),n2(AIP)n2 modulated superlattices (mt + m 2 = 13. n I + n 2 = 7).
est PL full width at half maximum (FWHM) of 9 meV. It was found that the PL intensity variation with measuring temperature was slower for the 640°C grown SLs than for the 600°C grown SLs. It is considered that the improvements observed here result from the decrease of residual impurities and defects and the formation of the smooth G a p / A l P interfaces with increasing growth temperature, and it is concluded that high temperature growth is favorable to obtain good quality G a p / A l P SLs and that the optimum growth temperature is 640°C in the present growth system. 3.2. Modulated SLs
We have studied the dependence of PL wavelengths at 4.2 K for the modulated SL structures. A blue shift of the PL peak was observed with decreasing period m] + n I (wider period) of modulated SLs, as shown in Fig. 3, indicating that the PL emission energy is mainly determined by the wider period. We have found a strong dependence of the PL intensity
=
3(
4.2K
o Sc~r~l
._~
o I @ i'r
(13,7) {10,4)(3,3)(9,4)(4.3) (a,4)(5,3) (7.4)(6,3)
Superlattice Structure Fig. 4. Relative PL intensities of the (GaP)I 3/(ALP) 7 normal and the (GaP),,,(AIP),,(GaP)=2(AIP),2 modulated superlattices.
.001
>
.0001
~
.00001
¢r
20
40
60
80
100 120
Temperature (K)
Fig. 5. Temperature variation of relative PL intensities of the (GaP)]3/(ALP) 7 normal and modulated superlattices.
on the modulated SL structure (Fig. 4). The modulated SLs of (m I + nj -- odd, m 2 + n 2 = odd) periods show stronger PL intensities than those of ( m I -kn I = even, m 2 + n 2 = even) periods. Fig. 5 shows the temperature dependence of the PL intensity. The results for the (13, 7) normal SL, and the (9, 4)(4, 3) modulated (odd, odd) SL and the (8, 4)(5, 3) modulated (even, even) SL are shown. It was found that the (odd, odd) modulated SL does not only have a stronger PL intensity than those of the other SLs at 4.2 K, but also shows a slower temperature variation of the PL intensity. At 30 K, the PL intensity of the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL, as can be seen in Fig. 5. Although the 4.2 K PL intensity for the (even, even) modulated SL is weaker than that of the normal SL, the temperature variation is slower and the intensity keeps its strength even at higher temperatures. Ikeda et al. [11] conducted theoretical calculations on S i / G e modulated SLs and showed the improvement of the dipole transition probability. We believe that improvements of the optical properties observed here resulted from the enhancement of the transition probability due to the modulated SL structures, although theoretical verification for the G a P / A l P modulated SLs is needed. The reason for the difference in PL intensity between the (odd, odd) and (even, even) modulated SL structures is also not clear at present because of the absence of theoretical calculations. However, the strong dependence of the PL intensity on the modulated SL structures was clearly observed with good reproducibility. It implies that proper modulated periods of GaP/AlP SLs must enhance the optical properties of G a P / A l P SLs.
J.H. Kim et a l . / Applied Surface Science 92 (1996) 566-570
569
IO0
3.3. EL properties Based on the above results, we preliminarily fabricated a LED having a GaP/AlP (9, 4)(4, 3) modulated SL as the active layer (Fig. 6). From the fabricated LED, we have observed the EL light emission at room temperature. The emission wavelength was about 572 nm. To investigate the origin of the EL emission, the temperature dependence of the EL was measured in the range of 130-300 K. Figs. 7a and 7b show the temperature dependence of the EL intensity and wavelength, respectively. In Fig. 7b, open and closed circles show the temperature variation of the PL wavelength for the (9, 4)(4, 3) modulated SL in the range 4.2-105 K and that of the EL wavelength for the LED at 130-300 K. The PL peak shifts to the longer wavelength side with increasing temperature in the range 4.2-25 K, but shifts to shorter wavelengths at temperatures between ~ 25 and ~ 50 K, followed by a red shift again. This temperature variation can be explained by bound exciton emissions at low temperatures and free exciton emissions at high temperatures. The blue shift observed again over 80 K is attributed to emission due to band-to-band transitions. The EL wavelength agrees with the PL wavelength within a growth run distribution ( ~ 5 nm--- 12 meV) in Fig. 3. By considering that the donor and acceptor activation energies in GaP are larger than 50 meV, this result indicates that the EL emissions at room temperature come from the near band-to-band transition in the (9, 4)(4, 3) modulated SL active layer.
0.25 lain 1As 0.3 Ixm aP 0.8 Ixm • : GaP/AlP SL 0.5 V-m tP 0.3 l.trn ed (001) GaP n t 0.25 p.m
Fig. 6, Schematic drawing of the structure of a LED having a G a P / A l P SL active layer.
"~
10
d
I
o
•... :"i
(9.4)(4,3) St. ~
tr
50
layer•
100 150 200 250 300
Temperature(K) 575 o PL • EL
(b) q
570
e e
Q
~'8:Po o NpDO N'° (9,4)(4,3)SL mivelt~t 50
100 150 200 250 300
Temperature (K)
Fig. 7. Temperature variations of (a) EL intensity and (b) wavelength of the LED having a (9, 4)(4, 3) modulated SL active layer. Results for the PL wavelength are also shown by open circles for comparison.
The observed EL intensity is still high even at room temperature. Optimization of the modulated SL structures will further improve the EL intensity.
4. Summary We have grown (GaP)m/(A1P) . normal and (GaP)m,(A1P).,(GaP)m2(AIP).2(m = m I + m 2, n = n, + n 2 , m l>~m 2,nl>~n 2) modulated SLs by gas source MBE. The optical properties of the normal SLs were improved with increasing growth temperature (600-660°C) and showed the best result for 640°C growth. In 4.2 K PL spectra for the SLs, the narrowest FWHM (9 meV) and highest PL intensity were obtained for 640°C growth. It was found that the PL intensity variation with measuring temperature is slower for the 640°C grown SLs than that for the 600°C grown SLs. In the modulated SLs, it was found that the PL from the (m I + n I = odd, m 2 + n 2 = odd) modulated SLs shows stronger intensity than that from the (even, even) modulated SLs and that they show slower temperature variation than that of the (13, 7) normal SLs. At 30 K, the PL intensity of
570
.1.1t. Kim et a l . / Applied Surface Science 92 (1996) 566-570
the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL. We have observed the EL emission from the LED having a GaP/AlP modulated SL as the active layer at 130-300 K. From the temperature variation of PL and EL wavelengths, it was confirmed that the EL emission originates from the Gap/AlP modulated SL. Further improvements of the PL and EL properties are expected by optimizing the modulated SL structures.
Acknowledgements This work was supported in part by a Grand-in-Aid for Scientific Research (No. 06236215) on Priority Areas from the Ministry of Education, Science and Culture. The authors would like to express their thanks to Dr. H. Gotoh of Mitsubishi Chemicals Ltd. for supplying GaP substrates.
References [1] U. Gnutzmann and K. Clausecker, Appl. Phys. 3 (1974) 9. [2] M. Kumagai, T. Takagahara and E. Hanamura, Phys. Rev. B 37 (1988) 898. [3] H. Asahi, K. Asami, T. Watanabe, S.J. Yu, T. Kaneko, S. Emura and S. Gonda, Appl. Phys. Lett. 58 (1991) 1407. [4] K. Asami, H. Asahi, T. Watanabe, M. Enokida, S. Gonda and Sg. Fujita, Appi. Phys. Left. 62 (1993) 81. [5] H. Asahi, K. Asami, R.K. Soni, M. Enokida, T. Watanabe, S. Emura, S. Gonda and Sg. Fujita, Inst. Phys. Conf. Ser. 129 (1992) 423. [6] K. Asami, H. Asahi, S.G. Kim, J.H. Kim, A. Ishida, S. Takamuku and S. Gonda, Appi. Phys. Lett. 64 0994) 2430. [7] A. Morri, H. Okagawa, K. Hara, J. Yoshino and H. Kumimoto, J. Cryst. Growth 124 0992) 772. [8] A. Sasaki, M. Kasn, T. Yamamoto and S. Noda, Jpn. J. Appl. Phys. 28 (1989) L1249. [9] X.L. Wang, A. Wakahara and A. Sasaki, Appl. Phys. Lett. 62 (1993) 888. [10] C. Weisbnch, R. Dingle, P.M. Pelroff, A.C. Gossard and W. Wiegmann, Appl. Phys. Lett. 38 (1981) 840. [11] M. Ikeda, K. Terakura and T. Oguchi, Phys. Rev. B 48 (1993) 1571.
a~ surface science ELSEVIER
Applied Surface Science 92 (1996) 566-570
Improvement of optical properties of gas source MBE grown GaP/AlP short period superlattices J.H. Kiln, H. Asahi *, K. Asami, T. Ogura, K. Doi, S. Gonda The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, lbaraki, Osaka 567, Japan Received 12 December 1994; accepted for publication 2 March 1995
Abstract
Optical properties of ( G a P ) m / ( A I P ) n normal and (GaP),~(AIP)n (GaP),,z(AIP),2(m = m, + m 2, n = n I + n 2, m, >t m 2, n, >t n 2) modulated superlattices (SLs) grown by gas source molecular beam epitaxy are studied. Optical properties of the normal SLs are improved with increasing growth temperature. The highest photoluminescence (PL) intensity and the narrowest full width at half maximum (9 meV) are obtained for 640°C growth. It is found that the PL intensity variation with measuring temperature is slower for the 640°C grown SLs than for the 600°C grown SLs. Modulated SLs show strong dependence of the PL intensity and wavelength on the modulated SL structure. The PL intensity of the (m I + n I = odd, m 2 + n 2 - odd) modulated SLs shows slower temperature variation than that of the normal SLs. At 30 K, the PL intensity of the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL. Electroluminescence (EL) emission was measured at 130-300 K for preliminary fabricated light emitting diodes having a GaP/AlP (9, 4)(4, 3) modulated SL as the active layer. The temperature variations of PL and EL wavelengths confirm that the EL emission originates from the GaP/AlP modulated SL active layer.
1. Introduction
G a P / A l P short period superlattices (SLs) which consist of two indirect band gap semiconductors, Gap and AlP, exhibit a direct band gap nature due to zone-folding and band-mixing effects [1]. Theoretical calculations show that (Gap)m/(A1P) n SLs exhibit band gap energies in the range of 2.40-2.06 eV depending on the period (m, n) [2]. We have already reported the gas source molecular beam epitaxy (MBE) growth of (GaP)m/(A1P) . short period SLs and their optical properties [3-6].
* Corresponding author. Fax: +81 6 877 6447.
Several groups have also reported the growth of G a P / A l P SLs by metalorganic vapor phase epitaxy (MOVPE) [7-9], where the growth temperatures ranged from 720 to 880°C. In Raman scattering measurements, Asahi et al. [3] pointed out that the absorption coefficients of these G a P / A l P SLs have relatively small values compared with normal direct band gap semiconductors, which agrees with theoretical calculations [2]. Weisbuch et al. reported the improvement of the structural and optical properties of G a A s / G a I _ x AI x As multiquantum well (MQW) structures grown with variation of the growth temperature [10]. Ikeda et al. have reported the improvement of optical transition properties of the S i / G e short period SLs by introducing modulated SL structures by theoretical calculations [11 ].
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 9 6 - 0
J.H. Kim et al./ Applied Surface Science 92 (1996) 566-570 unit ~_ i
period 20
_1
m
13
.1=
n
-
7
_-
[(GaP)m/(A1P)n] SL, [ (GaP)ml/(A1P)nl, (GaP)m2/(AIP)n2 ]SL (m+n=20) m = ml+m2, n = nl+n2 (ml_> m2, nl_> n2) Fig. 1. Schematic structures of the (GaP)I3/(ALP) 7 normal and the GaP/AlP modulated superlattices.
In this paper, to improve the optical properties of GaP/AlP SLs, we have studied the growth temperature dependence and the effect of the SL structure modulation. The growth temperature dependence is studied in the range 600-660°C. To improve the optical transition properties of GaP/AlP SLs, we propose here the modulated SL structures: (GAP),,, (ALP), (Gap),.:(AIP),2SLs (m -- m I + m2, n = n I + n2, m I > / m 2 , n I >/ n 2) as shown in Fig. 1. These modulated SL structures are similar to the (Si),,,(Ge),,(Si),.,(Ge),,. . . . SLs (m' + n' + m" + n" + ... = 10) studied by Ikeda et al. [1 l]. The intention of our modulated SLs is the enhancement of the band-mixing effect to improve the direct band gap nature by introducing structural modulation to normal (GaP),,/(A1P),SLs. Eiectroluminescence (EL) emission characteristics are also described for fabricated light emitting diodes (LED) having a G a P / A l P modulated SL as the active layer.
567
the growth temperature dependence of the optical properties. We have also grown modulated SLs as shown in Fig. 1: (m t, nl)(m 2, n 2 ) = ( 1 0 , 4)(3, 3), (9, 4)(4, 3), (8, 4)(5, 3), and (7, 4)(6, 3). To study the SL structure modulation effect and to compare the optical properties of the modulated SLs with those of normal SLs, the (GaP)|3/(A1P) 7 normal SLs as well as the modulated SLs were grown at 640°C in the same growth series. They were characterized through PL and EL measurements. PL was measured in the temperature range 4.2-105 K by using a He-Cd laser (325 nm) as an excitation light source. EL was measured under pulsed condition with a pulse width of 2 /zs and a repetition of 1 kHz in order to suppress the Joule heating. The temperature dependence of EL was measured in the temperature range from 130 to 300 K.
3. Results and discussion
3.1. Growth temperature dependence Growth temperature dependence of the optical properties of (GAP) ll/(A1P) 3 normal SLs was studied in the range from 600 to 660°C. Fig. 2 shows the growth temperature dependence of the PL intensity. The PL intensity remarkably increases with increasing growth temperature. In each growth series, improvement by a factor of over 10 was observed by increasing the growth temperature from 600 to 640°C. The decrease of the PL intensity at 660°C is considered to be partly caused by the additional outgassing of impurities from the substrate holder and around it. For the 640°C growth, we have observed the narrow" ~ lOOO (GaP)I ff(AJP)3
4.'~ K
loo q)
2. Experimental GaP/AlP SLs were grown on S-doped (001) GaP substrates by gas source MBE. Elemental Ga and AI and thermally cracked PH 3 were used as Group III and Group V sources, respectively. (GaP)It/(AIP) 3 normal SLs were grown at 600°C-660°C to study
.J rt 1
rr
o Series I • Series 11
.1 580
600
620
640
660
680
Growth Temperature ('C)
Fig. 2. Growth temperature dependence of the 4.2 K PL peak intensity for the (GaP)ll/(ALP) 3 SL.
568
J.H. Kim et a l . / Applied Surface Science 92 (1996) 566-570 610
o
E c
4.2K
o ,s~,I
[3 (13.7) • (!,4){4,3) 0 (8,4){$,3)
10
• Sede=I]
(o~,odd) ~ , ( ~ , ~ - ~ (even,even) .01
I
570
-J
n
(13,7)
(10,4)(3.3)(9.4)(4.3) (8,4)(5,3) (7,4)(6.3) SupedattJce Structure
Fig. 3. PL peak wavelength of the (GaP)I3/(ALP) 7 and the (GaP),nI(AIP), (GaP),n2(AIP)n2 modulated superlattices (mt + m 2 = 13. n I + n 2 = 7).
est PL full width at half maximum (FWHM) of 9 meV. It was found that the PL intensity variation with measuring temperature was slower for the 640°C grown SLs than for the 600°C grown SLs. It is considered that the improvements observed here result from the decrease of residual impurities and defects and the formation of the smooth G a p / A l P interfaces with increasing growth temperature, and it is concluded that high temperature growth is favorable to obtain good quality G a p / A l P SLs and that the optimum growth temperature is 640°C in the present growth system. 3.2. Modulated SLs
We have studied the dependence of PL wavelengths at 4.2 K for the modulated SL structures. A blue shift of the PL peak was observed with decreasing period m] + n I (wider period) of modulated SLs, as shown in Fig. 3, indicating that the PL emission energy is mainly determined by the wider period. We have found a strong dependence of the PL intensity
=
3(
4.2K
o Sc~r~l
._~
o I @ i'r
(13,7) {10,4)(3,3)(9,4)(4.3) (a,4)(5,3) (7.4)(6,3)
Superlattice Structure Fig. 4. Relative PL intensities of the (GaP)I 3/(ALP) 7 normal and the (GaP),,,(AIP),,(GaP)=2(AIP),2 modulated superlattices.
.001
>
.0001
~
.00001
¢r
20
40
60
80
100 120
Temperature (K)
Fig. 5. Temperature variation of relative PL intensities of the (GaP)]3/(ALP) 7 normal and modulated superlattices.
on the modulated SL structure (Fig. 4). The modulated SLs of (m I + nj -- odd, m 2 + n 2 = odd) periods show stronger PL intensities than those of ( m I -kn I = even, m 2 + n 2 = even) periods. Fig. 5 shows the temperature dependence of the PL intensity. The results for the (13, 7) normal SL, and the (9, 4)(4, 3) modulated (odd, odd) SL and the (8, 4)(5, 3) modulated (even, even) SL are shown. It was found that the (odd, odd) modulated SL does not only have a stronger PL intensity than those of the other SLs at 4.2 K, but also shows a slower temperature variation of the PL intensity. At 30 K, the PL intensity of the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL, as can be seen in Fig. 5. Although the 4.2 K PL intensity for the (even, even) modulated SL is weaker than that of the normal SL, the temperature variation is slower and the intensity keeps its strength even at higher temperatures. Ikeda et al. [11] conducted theoretical calculations on S i / G e modulated SLs and showed the improvement of the dipole transition probability. We believe that improvements of the optical properties observed here resulted from the enhancement of the transition probability due to the modulated SL structures, although theoretical verification for the G a P / A l P modulated SLs is needed. The reason for the difference in PL intensity between the (odd, odd) and (even, even) modulated SL structures is also not clear at present because of the absence of theoretical calculations. However, the strong dependence of the PL intensity on the modulated SL structures was clearly observed with good reproducibility. It implies that proper modulated periods of GaP/AlP SLs must enhance the optical properties of G a P / A l P SLs.
J.H. Kim et a l . / Applied Surface Science 92 (1996) 566-570
569
IO0
3.3. EL properties Based on the above results, we preliminarily fabricated a LED having a GaP/AlP (9, 4)(4, 3) modulated SL as the active layer (Fig. 6). From the fabricated LED, we have observed the EL light emission at room temperature. The emission wavelength was about 572 nm. To investigate the origin of the EL emission, the temperature dependence of the EL was measured in the range of 130-300 K. Figs. 7a and 7b show the temperature dependence of the EL intensity and wavelength, respectively. In Fig. 7b, open and closed circles show the temperature variation of the PL wavelength for the (9, 4)(4, 3) modulated SL in the range 4.2-105 K and that of the EL wavelength for the LED at 130-300 K. The PL peak shifts to the longer wavelength side with increasing temperature in the range 4.2-25 K, but shifts to shorter wavelengths at temperatures between ~ 25 and ~ 50 K, followed by a red shift again. This temperature variation can be explained by bound exciton emissions at low temperatures and free exciton emissions at high temperatures. The blue shift observed again over 80 K is attributed to emission due to band-to-band transitions. The EL wavelength agrees with the PL wavelength within a growth run distribution ( ~ 5 nm--- 12 meV) in Fig. 3. By considering that the donor and acceptor activation energies in GaP are larger than 50 meV, this result indicates that the EL emissions at room temperature come from the near band-to-band transition in the (9, 4)(4, 3) modulated SL active layer.
0.25 lain 1As 0.3 Ixm aP 0.8 Ixm • : GaP/AlP SL 0.5 V-m tP 0.3 l.trn ed (001) GaP n t 0.25 p.m
Fig. 6, Schematic drawing of the structure of a LED having a G a P / A l P SL active layer.
"~
10
d
I
o
•... :"i
(9.4)(4,3) St. ~
tr
50
layer•
100 150 200 250 300
Temperature(K) 575 o PL • EL
(b) q
570
e e
Q
~'8:Po o NpDO N'° (9,4)(4,3)SL mivelt~t 50
100 150 200 250 300
Temperature (K)
Fig. 7. Temperature variations of (a) EL intensity and (b) wavelength of the LED having a (9, 4)(4, 3) modulated SL active layer. Results for the PL wavelength are also shown by open circles for comparison.
The observed EL intensity is still high even at room temperature. Optimization of the modulated SL structures will further improve the EL intensity.
4. Summary We have grown (GaP)m/(A1P) . normal and (GaP)m,(A1P).,(GaP)m2(AIP).2(m = m I + m 2, n = n, + n 2 , m l>~m 2,nl>~n 2) modulated SLs by gas source MBE. The optical properties of the normal SLs were improved with increasing growth temperature (600-660°C) and showed the best result for 640°C growth. In 4.2 K PL spectra for the SLs, the narrowest FWHM (9 meV) and highest PL intensity were obtained for 640°C growth. It was found that the PL intensity variation with measuring temperature is slower for the 640°C grown SLs than that for the 600°C grown SLs. In the modulated SLs, it was found that the PL from the (m I + n I = odd, m 2 + n 2 = odd) modulated SLs shows stronger intensity than that from the (even, even) modulated SLs and that they show slower temperature variation than that of the (13, 7) normal SLs. At 30 K, the PL intensity of
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the (9, 4)(4, 3) modulated SL is 2500 times stronger than that of the (13, 7) normal SL. We have observed the EL emission from the LED having a GaP/AlP modulated SL as the active layer at 130-300 K. From the temperature variation of PL and EL wavelengths, it was confirmed that the EL emission originates from the Gap/AlP modulated SL. Further improvements of the PL and EL properties are expected by optimizing the modulated SL structures.
Acknowledgements This work was supported in part by a Grand-in-Aid for Scientific Research (No. 06236215) on Priority Areas from the Ministry of Education, Science and Culture. The authors would like to express their thanks to Dr. H. Gotoh of Mitsubishi Chemicals Ltd. for supplying GaP substrates.
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