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Photoluminescence from AlGaAs-GaAs single quantum wells grown on variously oriented GaAs substrates by MBE
Journal of Crystal Growth 81(1987) 85—90 North-Holland, Amsterdam
85
PHOTOLUMINESCENCE FROM AIGaAs-GaAs SINGLE QUANTUM WELLS GROWN ON VARIOUSLY ORIENTED GaAs SUBSTRATES BY MBE Toshiaki FUKUNAGA, Takeshi TAKAMORI and Hisao NAKASHIMA Optoelectronics Joint Research Laboratory, 1333 Kamikodanaka, Nakahara-ku, Kawasaki 211, Japan
Photoluminescence (PL) measurements at low temperatures have been carried out for A1GaAs—GaAs single quantum well (SQW) structures grown on (100) , (110), (111)B, (311)A and (311)B GaAs substrates by MBE. A mirror smooth surface was obtained under each optimal growth condition except for (111)B and (110) samples. The (110) surface is cloudy due to the columnar growth. PL results suggest that the height of column is in the order of 10 nm. The film on the (111)B surface Consists of a lot of trigonal pyramids, which give a rather broad line width of PL peaks from a SQW compared with the (100) sample. The high optical quality for both (311)A and (311)B SQW’s, which is comparable to that for (100) SQW’s, is achieved in optimal growth conditions. PL study determines the heavy-hole effective mass of about 0.5m 0 for the [311] direction.
1. Introduction Photoluminescence (PL) spectra of AlGaAs— GaAs quantum well (QW) structures have much information on the abruptness of heterointerfaces and the growth mechanisms [1—3].The epitaxial growth on differently orientated substrates will exhibit the different electrical and optical properties and also different growth processes due to orientation-dependent band structures and surface atom configurations. Recently Wang [4] has reported that surface morphology of the (Nil) (N ~ 2) molecular beam epitaxial (MBE) layers is excellent and two-dimensional carrier mobility in modulation doped AIGaAs/GaAs heterostructures grown on these high index planes is cornparable to that for (100) plane. The physical properties of A1GaAs—GaAs QW’s with (100) index have commonly been studied. The optical properties of AIGaAs—GaAs QW’s with other indices have been scarcely studied except for the (211) case reported by Subbanna et al. [5]. In this paper, the optical properties and surface morphology of AlGaAs—GaAs single quantum well (SQW) structures grown on (100), (110), (111)B, (311)A and (311)B GaAs substrates by MBE have been investigated by low temperature PL measurements and Nomarski differential interference contrast (NDIC) microscopy, respectively,
The mirror smooth and high quality (311)A and (311)B samples, which were comparable to those of (100) one, were achieved in optimal growth conditions. The heavy-hole effective mass for the [3111direction was estimated from PL results. The growth process difference among various planes is discussed.
2. Experimental The SQW structures were simultaneously grown side by side on Si doped GaAs substrates with three indices of (100), (110) and (111)B or (100), (311)A and (311)B. GaAs substrates were oriented within ±0.5°of the (111)B and (110) planes and ±0.1°of the others. The sample structure consisted of (a) GaAs buffer layer (500 nm); (b) thin Al~Ga1 ~As marker layer; (c) GaAs growth rate check layer (500 nm); (d) Al5Ga1~As—GaAs SQW’s with five or three different well widths separated by 45 rim thick Al 5Ga1 ~As barriers; (d) GaAs cap layer (5 nm). The GaAs growth rate was determined by measuring the thickness of GaAs growth rate check layer with a scanning electron microscope image for cleaved facets. Then the well widths were estimated from the growth time of each well. The GaAs growth rate in the present experiments was about 500 nm/h. Two
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
-
—
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/ Photoluminescence from A/GaAs
different growth conditions for the (100), (110) and (l11)B series were chosen: the growth temperatures (T~)were 600 and 650°Cunder As4/Ga beam pressure ratio of 100, corresponding to a flux ratio (f) of about 5. We also employed six different growth conditions for the (100), (311)A and (311)B series, i.e. two different 7’s of 600 and 650°C under three different F’s of about 2.5, 5 and 10. The surface morphology was observed by NDIC microscopy. PL measurements were carried out at 77 and 1.8 K, using an argon ion laser (514.5 nm) line as an excitation source. The excitation power
—
GaAs SQ W’s
~
T~I.8K
‘
x60
—
j\J~j~j (110) x2
—
x20 0
(lll)B
2
density was about 5 W/cm The AlAs mole fraction x was determined from the 1.8 K PL peak energy of the neutral acceptor bound exciton (A°—X)in Al~Ga1_5Asbarrier. .
1
4
x20
x60
f~ WAVELENGTH
~s. (nm)
Fig. 2. Typical photoluminescence (PL) spectra at 1.8 K from (100), (110) and (111)B samples grown at 650°C under As4/Ga flux ratio of about 5. The arrows indicate the calculated PL peak energies from quantum wells.
3. Results and discussion Fig. 1 shows the surface morphology of (100), (HO) and (111)B layers grown at 650°C. The dense V shaped terraces in the [110] direction are formed on the (110) surface due to the columnar growth. These features are similar to those reported by Wang [6]. A lot of small trigonal pyramids are formed on the (111)B surface. These features are also similar to those of GaAs layers reported by Cho [7]. These pyramids become larger and more dense with decreasing T~.The morphology of the (110) surface is less dependent on
than that of the (111)B surface. The different surface features among three orientations may be caused by the different anisotropic surface atom migration length. Typical PL spectra of A1041Ga0 59As—GaAs SQW’s grown on (100), (110) and (111)B substrates at = 650°C are shown in fig. 2. Each layer contained three wells, of thickness 1.2, 5.8 and 23 nm. It can be expected that three types of
a [110]
50gm
Fig. 1. Nomarski differential interference contrast image of (a) (1(8)). (h) (110) and Cc) (III )B growth at 650°Cunder As4/Ga flux ratio of about 5.
T. Fukunaga et a!.
/ Photo/uminescence from A/GaAs
PL bands originating from Al0 45Ga0 59As barrier, three QW’s and GaAs layer appear in the PL spectra. The sharp (A°—X)and broad conduction band to neutral acceptor transition (e—A°) of A1041Ga059As for the (100) sample are observed at 609 nm and 622 nm, respectively. Three free exciton lines from three QW’s appear at a wavelength of 650 to 815 nm and a neutral donor bound exciton is observed at 816 nm. The PL bands the at the wavelength than PL 818 spectrum, nm come from GaAs layer. Inlonger the (110) however, the emission bands from the A1GaAs barrier do not appear and the full width at half maximun (FWHM) of the PL peaks from QW’s whose well width (Lw) is less than 10 nm is much broader than that for the other samples and becomes sharp at L~ 23 rim. This suggests that the height of colunm shown in fig. lb is in the order
—
87
GaAs SQ W’s
fluctuation originating from the hillocks as shown in fig. ic also broadens the FWHM of PL peaks from QW’s. The PL peak energies from QW’s for the (11 1)B sample are always lower than those for the (100) one. Since the GaAs growth rate and AlAs mole fraction of barrier in the (11 1)B sample are the same as those for the (100) case, it can be expected that the heavy-hole effective mass in the [111] direction (mhh[lll]) thatinin the 100]).is larger The than arrows the [100] direction (111)B and (100)(mhh[ spectra indicate the calculated energies of each well based on a simple rectangular well of the experimentally-determined thickness with corrections for the nonparabolicity of conduction band [8] and exciton binding energy (9 meV) [9] using the 60% conduction band offset of the total band gap discontinuity [10,11], mhh[IOO] 0.34m 0 [10,11], m~[111] 0.67m0 [12]. In this calculation it is assumed that the anisotropy of mhh in A1GaAs is the same as that in GaAs. The experimental PL peak energies in the (100) spectra well agree with the calculated ones. The slight discrepancy in (111)B spectrum between calculated and experimental ones may come from well width fluctuation. Fig. 3 shows the surface morphology of (100),
=
=
=
of 10 nm. The absence of A1GaAs PL emission is probably caused by the incomplete alloying during MBE growth [6]. The FWHM’s of the (A°—X)and (e—A°)bands from A1045Ga059As and three peaks from QW’s for the (11 1)B sample are broader than those for the (100) one. This may be caused by impurity incorporation and defect formation. The well width
a
c
b -
50~zm
Fig. 3. Nomarski differential interference contrast image of (a) (100), (b) (311)A and (c) (311)B growth under As4/Ga flux ratio of about 2.5. The growth temperatures of upper and lower photographs are 650 and 600°C,respectively.
88
T. Fukunaga eta!.
/ Photo/uminescence from A/GaAs—GaAs
(311)A and (311)B layers as a function of growth temperature under P. 2.5. As shown in fig. 3a the (100) surface grown at 600°C is featureless, but the honeycomb features are formed at high 1, which is probably caused by faster reevaporaton rate of surface atoms. The (311)A surface grown at 600°Cis excellent, but a lot of_features like a bird-footprint are formed for the [011] direction at high 7. The surface morphology does not depend on F but on 7. The (311)B surface grown at 650°C is excellent, as shown in fig. 3c,but a lot of circular tips with a long tail in the [011] direction are formed at 600°C. The (311)B surface morphology strongly depends on and F~.Since the surface morphology is improved at high 1 and low F~,the roughness on (311)B surface may be caused by suppression of surface atom migration due to the formation of strong Ill—As bond. Typical PL spectra from five A1~Ga15As(x —
=
—
0.24)—GaAs SQW’s of (100), (311)A and (311)B substrates grown at 600°C and 650°C under ~ 2.5 are shown in figs. 4a and 4b, respectively, Details of the layers are given in table 1. The FWHM’s of the PL peaks from SQW’s in the —
I
I
~ (3106
4
~
=
=
=
=
=
=
650°C are narrower than those in the other ones under the same growth conditions, all peaks except for the longest wavelength one have clear tails towards the longer wavelength. The (e—A°) transition in the largest well of a (311)B sample clearly appears at 827 nm, which is not observed =
I
I
(b)
T~I.8K
(100)
LJ~~~JJLt
~:iijj~
~
700 750 WAVELENGTH
=
I
T-I.8K
(100)
(3fl)A spectrum at 650°C are slightly narrower than those at 600°C. This indicates that the line width of the PL peaks does not directly depend on the macroscopic surface morphology but on the microscopic heterointerface roughness. The overall PL features of (100) and (311)A depend little on 1. The FWHM’s of the PL peaks from SQW’s in the (311)B spectrum at 1 600°C are much broader than those at 650°C. The intensity of the (e_A°)-likeband in A1GaAs (610 nm) and GaAs (830 nm) at 600°C is larger than that at 650°C. The large PL spectrum difference between 600°C and 650°Cis perhaps caused by differences in impurity incorporation, defect formation and the well width fluctuation originating from the circular tips shown in fig. 3b. Although the FWHM’s of the PL peaks from SQW’s in the (31l)B spectrum at
I
(a)
SQW’s
(311)6
800 (nm)
WAVELENGTH
(nm)
Fig. 4. Typical photoluminescence spectra (PL) at 1.8 K from (100), (311)A and (311)B samples grown at (a) 600°Cand (b) 650°C under As4 /Ga flux ratio of about 2.5. The arrows indicate the calculated PL peak energies from quantum wells.
T. Fukunaga et aL
/ Photoluminescence from A/GaAs
—
89
GaAs SQ W’s
Table 1 Well widths (Lw) and Al mole fraction (x) for A1~Ga
1~As—GaAs QW structures used for fig. 4
Growth temperature (° C)
Substrate orientation (100) X
650
0.247
600
0.242
(311)A
Lw (nm) 1.4 2.8 3.5 11.2 22.4
X
1.5 3.0 6.0 12.0 24.0
0.246
0.243
1.45 2.9
x 0.224
Lw (nm) 1.55 3.3 6.6 12.2 24.4
0.215
1.6 3.2 6.4 12.8 25.6
5.8
=
15 X(I00) S (311 )A 0(311 )B 10
Lw (nm) 1.45 2.9 5.8 11.6 23.2
11.6 23.2
in the other samples. These facts also suggest that the (311)B surface is radical to incorporate impurities. The arrows in figs. 4a and 4b indicate the calculated energies of each well in the foregoing way, using m~[3l1] 0.5m0. The experimental PL peak energies well agree with the calculated ones, except for the (311)B sample grown at 600°C, which may come from well width fluctuation. The PL peak energy originating from the electron to light-hole transition at 77 K spectra depended
T1.8K
(311)B
‘
.
little on the orientation. This indicates that the anisotropy of the light-hole effective mass is smaller than that of the heavy-hole effective mass. Typical FWHM’s of PL peaks from SQW’s in (100), (311)A and (311)B samples are shown in fig. 5 as a function of well width. The well width dependence of FWHM’s in the (311)B sample is similar to that in the (100) sample. This indicates that the microscopic roughness in the (31 l)B heterointerface is similar to that in (100) heterointerface. The FWHM of a (311)A sample is slightly larger than that of the others at each well width. Since the extrinsic luminescence intensity in the (311)A spectra is no more than that in the (100) spectra, the line width difference comes from the microscopic surface roughness. The surface atom migration length on the (311)A surface may be smaller than that on the (100) surface.
X
0 5.
~
.
• c
0
I
10
20
WELL WIDTH (nm) Fig. 5. Typical full width at half maximum of photoluminescence peaks at 1.8 K in (100), (311)A and (311)B samples grown under As4/Ga flux ratio of about 2.5. The growth temperatures of (100) and the other samples are 600°C and 650°C,respectively,
4.Summary The surface morphology and optical properties of A1GaAs—GaAs SQW’s grown on variously onented GaAs substrates are investigated. The growth process highly depends on the substrate orientations and growth conditions caused by the different surface atom configurations. It is found that .
.
.
.
the (31l)B surface is radical to trap impunties. The high quality for (311)A and (311)B samples, which is comparable to that for a (100) sample, is
90
T Fukunaga et al.
/
Photoluminescence from A/GaAs — GaAs SQ W’s
achieved in optimal growth conditions. The heavy hole effective mass is also estimated from the PL 0.5m results as mhh[3ll] 0.
Acknowledgements The authors would like to thank Dr. T. Narusawa for his critical reading of this manuscript. The present research is supported by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.
References [1] L. Goldstein, Y. Horikoshi, S. Tarucha and H. Okamoto, Japan. J. App). Phys. 22 (1983) 1489.
[2] H. Sakaki, M. Tanaka and J. Yoshino, Japan. J. Appi. Phys. 24 (1985) L417. [3] T. Fukunaga, K.L.I. Kobayashi and H. Nakashima, Japan. J. Appl. Phys. 24 (1985) L510. [4] WI. Wang, Surface Sci. 174 (1986) 31. [5] S. Subbanna, H. Kroemer and J.L. Merz, J. AppI. Phys. 59 (1986) 488. [6] WI. Wang, J. Vacuum Sci. Technol. B1 (1983) 630. [7] A.Y. Cho, J. Appl. Phys. 41 (1970) 2780. [8] B.A. Vojak, W.D. Laidig, N. Holonyak, Jr. and M.D. Camras, J. App). Phys. 52 (1981) 621 [9] R.C. Miller, D.A. Kleinman, W.T. Tsang and A.C. Gossard, Phys. Rev. B24 (1981) 1134. [10] R.C. Miller, D.A. Kleinman and A.C. Gossard, Phys. Rev. B29 (1984) 7085. [11] K. Yamanaka, T. Fukunaga, N. Tsukada, K.L.I. Kobayashi and M. Ishii, AppI. Phys. Letters 48 (1986) 840. [12] P. Lawaetz. Phys. Rev. B4 (1971) 3460.
85
PHOTOLUMINESCENCE FROM AIGaAs-GaAs SINGLE QUANTUM WELLS GROWN ON VARIOUSLY ORIENTED GaAs SUBSTRATES BY MBE Toshiaki FUKUNAGA, Takeshi TAKAMORI and Hisao NAKASHIMA Optoelectronics Joint Research Laboratory, 1333 Kamikodanaka, Nakahara-ku, Kawasaki 211, Japan
Photoluminescence (PL) measurements at low temperatures have been carried out for A1GaAs—GaAs single quantum well (SQW) structures grown on (100) , (110), (111)B, (311)A and (311)B GaAs substrates by MBE. A mirror smooth surface was obtained under each optimal growth condition except for (111)B and (110) samples. The (110) surface is cloudy due to the columnar growth. PL results suggest that the height of column is in the order of 10 nm. The film on the (111)B surface Consists of a lot of trigonal pyramids, which give a rather broad line width of PL peaks from a SQW compared with the (100) sample. The high optical quality for both (311)A and (311)B SQW’s, which is comparable to that for (100) SQW’s, is achieved in optimal growth conditions. PL study determines the heavy-hole effective mass of about 0.5m 0 for the [311] direction.
1. Introduction Photoluminescence (PL) spectra of AlGaAs— GaAs quantum well (QW) structures have much information on the abruptness of heterointerfaces and the growth mechanisms [1—3].The epitaxial growth on differently orientated substrates will exhibit the different electrical and optical properties and also different growth processes due to orientation-dependent band structures and surface atom configurations. Recently Wang [4] has reported that surface morphology of the (Nil) (N ~ 2) molecular beam epitaxial (MBE) layers is excellent and two-dimensional carrier mobility in modulation doped AIGaAs/GaAs heterostructures grown on these high index planes is cornparable to that for (100) plane. The physical properties of A1GaAs—GaAs QW’s with (100) index have commonly been studied. The optical properties of AIGaAs—GaAs QW’s with other indices have been scarcely studied except for the (211) case reported by Subbanna et al. [5]. In this paper, the optical properties and surface morphology of AlGaAs—GaAs single quantum well (SQW) structures grown on (100), (110), (111)B, (311)A and (311)B GaAs substrates by MBE have been investigated by low temperature PL measurements and Nomarski differential interference contrast (NDIC) microscopy, respectively,
The mirror smooth and high quality (311)A and (311)B samples, which were comparable to those of (100) one, were achieved in optimal growth conditions. The heavy-hole effective mass for the [3111direction was estimated from PL results. The growth process difference among various planes is discussed.
2. Experimental The SQW structures were simultaneously grown side by side on Si doped GaAs substrates with three indices of (100), (110) and (111)B or (100), (311)A and (311)B. GaAs substrates were oriented within ±0.5°of the (111)B and (110) planes and ±0.1°of the others. The sample structure consisted of (a) GaAs buffer layer (500 nm); (b) thin Al~Ga1 ~As marker layer; (c) GaAs growth rate check layer (500 nm); (d) Al5Ga1~As—GaAs SQW’s with five or three different well widths separated by 45 rim thick Al 5Ga1 ~As barriers; (d) GaAs cap layer (5 nm). The GaAs growth rate was determined by measuring the thickness of GaAs growth rate check layer with a scanning electron microscope image for cleaved facets. Then the well widths were estimated from the growth time of each well. The GaAs growth rate in the present experiments was about 500 nm/h. Two
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
-
—
86
T. Fukunuga et al.
/ Photoluminescence from A/GaAs
different growth conditions for the (100), (110) and (l11)B series were chosen: the growth temperatures (T~)were 600 and 650°Cunder As4/Ga beam pressure ratio of 100, corresponding to a flux ratio (f) of about 5. We also employed six different growth conditions for the (100), (311)A and (311)B series, i.e. two different 7’s of 600 and 650°C under three different F’s of about 2.5, 5 and 10. The surface morphology was observed by NDIC microscopy. PL measurements were carried out at 77 and 1.8 K, using an argon ion laser (514.5 nm) line as an excitation source. The excitation power
—
GaAs SQ W’s
~
T~I.8K
‘
x60
—
j\J~j~j (110) x2
—
x20 0
(lll)B
2
density was about 5 W/cm The AlAs mole fraction x was determined from the 1.8 K PL peak energy of the neutral acceptor bound exciton (A°—X)in Al~Ga1_5Asbarrier. .
1
4
x20
x60
f~ WAVELENGTH
~s. (nm)
Fig. 2. Typical photoluminescence (PL) spectra at 1.8 K from (100), (110) and (111)B samples grown at 650°C under As4/Ga flux ratio of about 5. The arrows indicate the calculated PL peak energies from quantum wells.
3. Results and discussion Fig. 1 shows the surface morphology of (100), (HO) and (111)B layers grown at 650°C. The dense V shaped terraces in the [110] direction are formed on the (110) surface due to the columnar growth. These features are similar to those reported by Wang [6]. A lot of small trigonal pyramids are formed on the (111)B surface. These features are also similar to those of GaAs layers reported by Cho [7]. These pyramids become larger and more dense with decreasing T~.The morphology of the (110) surface is less dependent on
than that of the (111)B surface. The different surface features among three orientations may be caused by the different anisotropic surface atom migration length. Typical PL spectra of A1041Ga0 59As—GaAs SQW’s grown on (100), (110) and (111)B substrates at = 650°C are shown in fig. 2. Each layer contained three wells, of thickness 1.2, 5.8 and 23 nm. It can be expected that three types of
a [110]
50gm
Fig. 1. Nomarski differential interference contrast image of (a) (1(8)). (h) (110) and Cc) (III )B growth at 650°Cunder As4/Ga flux ratio of about 5.
T. Fukunaga et a!.
/ Photo/uminescence from A/GaAs
PL bands originating from Al0 45Ga0 59As barrier, three QW’s and GaAs layer appear in the PL spectra. The sharp (A°—X)and broad conduction band to neutral acceptor transition (e—A°) of A1041Ga059As for the (100) sample are observed at 609 nm and 622 nm, respectively. Three free exciton lines from three QW’s appear at a wavelength of 650 to 815 nm and a neutral donor bound exciton is observed at 816 nm. The PL bands the at the wavelength than PL 818 spectrum, nm come from GaAs layer. Inlonger the (110) however, the emission bands from the A1GaAs barrier do not appear and the full width at half maximun (FWHM) of the PL peaks from QW’s whose well width (Lw) is less than 10 nm is much broader than that for the other samples and becomes sharp at L~ 23 rim. This suggests that the height of colunm shown in fig. lb is in the order
—
87
GaAs SQ W’s
fluctuation originating from the hillocks as shown in fig. ic also broadens the FWHM of PL peaks from QW’s. The PL peak energies from QW’s for the (11 1)B sample are always lower than those for the (100) one. Since the GaAs growth rate and AlAs mole fraction of barrier in the (11 1)B sample are the same as those for the (100) case, it can be expected that the heavy-hole effective mass in the [111] direction (mhh[lll]) thatinin the 100]).is larger The than arrows the [100] direction (111)B and (100)(mhh[ spectra indicate the calculated energies of each well based on a simple rectangular well of the experimentally-determined thickness with corrections for the nonparabolicity of conduction band [8] and exciton binding energy (9 meV) [9] using the 60% conduction band offset of the total band gap discontinuity [10,11], mhh[IOO] 0.34m 0 [10,11], m~[111] 0.67m0 [12]. In this calculation it is assumed that the anisotropy of mhh in A1GaAs is the same as that in GaAs. The experimental PL peak energies in the (100) spectra well agree with the calculated ones. The slight discrepancy in (111)B spectrum between calculated and experimental ones may come from well width fluctuation. Fig. 3 shows the surface morphology of (100),
=
=
=
of 10 nm. The absence of A1GaAs PL emission is probably caused by the incomplete alloying during MBE growth [6]. The FWHM’s of the (A°—X)and (e—A°)bands from A1045Ga059As and three peaks from QW’s for the (11 1)B sample are broader than those for the (100) one. This may be caused by impurity incorporation and defect formation. The well width
a
c
b -
50~zm
Fig. 3. Nomarski differential interference contrast image of (a) (100), (b) (311)A and (c) (311)B growth under As4/Ga flux ratio of about 2.5. The growth temperatures of upper and lower photographs are 650 and 600°C,respectively.
88
T. Fukunaga eta!.
/ Photo/uminescence from A/GaAs—GaAs
(311)A and (311)B layers as a function of growth temperature under P. 2.5. As shown in fig. 3a the (100) surface grown at 600°C is featureless, but the honeycomb features are formed at high 1, which is probably caused by faster reevaporaton rate of surface atoms. The (311)A surface grown at 600°Cis excellent, but a lot of_features like a bird-footprint are formed for the [011] direction at high 7. The surface morphology does not depend on F but on 7. The (311)B surface grown at 650°C is excellent, as shown in fig. 3c,but a lot of circular tips with a long tail in the [011] direction are formed at 600°C. The (311)B surface morphology strongly depends on and F~.Since the surface morphology is improved at high 1 and low F~,the roughness on (311)B surface may be caused by suppression of surface atom migration due to the formation of strong Ill—As bond. Typical PL spectra from five A1~Ga15As(x —
=
—
0.24)—GaAs SQW’s of (100), (311)A and (311)B substrates grown at 600°C and 650°C under ~ 2.5 are shown in figs. 4a and 4b, respectively, Details of the layers are given in table 1. The FWHM’s of the PL peaks from SQW’s in the —
I
I
~ (3106
4
~
=
=
=
=
=
=
650°C are narrower than those in the other ones under the same growth conditions, all peaks except for the longest wavelength one have clear tails towards the longer wavelength. The (e—A°) transition in the largest well of a (311)B sample clearly appears at 827 nm, which is not observed =
I
I
(b)
T~I.8K
(100)
LJ~~~JJLt
~:iijj~
~
700 750 WAVELENGTH
=
I
T-I.8K
(100)
(3fl)A spectrum at 650°C are slightly narrower than those at 600°C. This indicates that the line width of the PL peaks does not directly depend on the macroscopic surface morphology but on the microscopic heterointerface roughness. The overall PL features of (100) and (311)A depend little on 1. The FWHM’s of the PL peaks from SQW’s in the (311)B spectrum at 1 600°C are much broader than those at 650°C. The intensity of the (e_A°)-likeband in A1GaAs (610 nm) and GaAs (830 nm) at 600°C is larger than that at 650°C. The large PL spectrum difference between 600°C and 650°Cis perhaps caused by differences in impurity incorporation, defect formation and the well width fluctuation originating from the circular tips shown in fig. 3b. Although the FWHM’s of the PL peaks from SQW’s in the (31l)B spectrum at
I
(a)
SQW’s
(311)6
800 (nm)
WAVELENGTH
(nm)
Fig. 4. Typical photoluminescence spectra (PL) at 1.8 K from (100), (311)A and (311)B samples grown at (a) 600°Cand (b) 650°C under As4 /Ga flux ratio of about 2.5. The arrows indicate the calculated PL peak energies from quantum wells.
T. Fukunaga et aL
/ Photoluminescence from A/GaAs
—
89
GaAs SQ W’s
Table 1 Well widths (Lw) and Al mole fraction (x) for A1~Ga
1~As—GaAs QW structures used for fig. 4
Growth temperature (° C)
Substrate orientation (100) X
650
0.247
600
0.242
(311)A
Lw (nm) 1.4 2.8 3.5 11.2 22.4
X
1.5 3.0 6.0 12.0 24.0
0.246
0.243
1.45 2.9
x 0.224
Lw (nm) 1.55 3.3 6.6 12.2 24.4
0.215
1.6 3.2 6.4 12.8 25.6
5.8
=
15 X(I00) S (311 )A 0(311 )B 10
Lw (nm) 1.45 2.9 5.8 11.6 23.2
11.6 23.2
in the other samples. These facts also suggest that the (311)B surface is radical to incorporate impurities. The arrows in figs. 4a and 4b indicate the calculated energies of each well in the foregoing way, using m~[3l1] 0.5m0. The experimental PL peak energies well agree with the calculated ones, except for the (311)B sample grown at 600°C, which may come from well width fluctuation. The PL peak energy originating from the electron to light-hole transition at 77 K spectra depended
T1.8K
(311)B
‘
.
little on the orientation. This indicates that the anisotropy of the light-hole effective mass is smaller than that of the heavy-hole effective mass. Typical FWHM’s of PL peaks from SQW’s in (100), (311)A and (311)B samples are shown in fig. 5 as a function of well width. The well width dependence of FWHM’s in the (311)B sample is similar to that in the (100) sample. This indicates that the microscopic roughness in the (31 l)B heterointerface is similar to that in (100) heterointerface. The FWHM of a (311)A sample is slightly larger than that of the others at each well width. Since the extrinsic luminescence intensity in the (311)A spectra is no more than that in the (100) spectra, the line width difference comes from the microscopic surface roughness. The surface atom migration length on the (311)A surface may be smaller than that on the (100) surface.
X
0 5.
~
.
• c
0
I
10
20
WELL WIDTH (nm) Fig. 5. Typical full width at half maximum of photoluminescence peaks at 1.8 K in (100), (311)A and (311)B samples grown under As4/Ga flux ratio of about 2.5. The growth temperatures of (100) and the other samples are 600°C and 650°C,respectively,
4.Summary The surface morphology and optical properties of A1GaAs—GaAs SQW’s grown on variously onented GaAs substrates are investigated. The growth process highly depends on the substrate orientations and growth conditions caused by the different surface atom configurations. It is found that .
.
.
.
the (31l)B surface is radical to trap impunties. The high quality for (311)A and (311)B samples, which is comparable to that for a (100) sample, is
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T Fukunaga et al.
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Photoluminescence from A/GaAs — GaAs SQ W’s
achieved in optimal growth conditions. The heavy hole effective mass is also estimated from the PL 0.5m results as mhh[3ll] 0.
Acknowledgements The authors would like to thank Dr. T. Narusawa for his critical reading of this manuscript. The present research is supported by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.
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