- Email: [email protected]
As4 pressure dependence of the interface flatness of GaAsAl0.3Ga0.7As quantum wells grown on (411) A GaAs substrates by MBE
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applied
surface science
!!!I ELSEVIER
Applied Surface Science
113/I 14 (1997) 73-78
As, pressure dependence of the interface flatness of GaAs/Al,.,Ga,,As quantum wells grown on (411) A GaAs substrates by MBE K. Shinohara a.*, K. Kasahara a, S. Shimomura a, A. Adachi b, Y. Okamoto ‘, N. Sano d, S. Hiyamizu a-e d Fact&v ofEngineering b R&D
Dil’ision,
Nissin Electric
Science, Osaka fJnii,ersity,
’ Research and Headquarters ’ Fact&
Kubota LTD., Amagasaki,
of Science. Kwunsei-Cakuin
’ Research Centrrfor
Toxonaka
Company Ltd., Umezu-Takase-cho,
Extreme Materials.
University.
560, Japan
Ukyo-ku, Kyoto 615, Japart H.vogo 661. Japan
Nishinomiya,
H~ogo 662 Japan
Osaka UniL,ersity, Ttjyonaka.
Osuka 560, Japan
Abstract Arsenic pressure dependence of the interface flatness of GaAs/AlGaAs (Al content of 0.27-0.29) quantum wells (QWs) grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) was investigated. Effectively atomically flat (41llA interfaces could be realized under a low V/III ratio (V/III = 71, but the interfaces decreased in flatness under an increased V/III ratio (V/III 2 11). The linewidth of the photoluminescence peak increases with increasing the V/III ratio during the growth of the GaAs QW layers, but does not increase so much with increasing V/III ratio during the growth of the AlGaAs barrier layers, indicating that the flatness of an AlGaAs/GaAs upper interface of the GaAs QWs grown on (4ll)A substrates is more strongly affected by the high V/III ratio than that of a GaAs/AlGaAs lower interface of the GaAs QWs. The excess fluctuation in the GaAs well width (AL,) which results from the roughness of the upper interface caused by the high V/III ratio increases parabolically as a function of the GaAs well width (L,,). This result was explained in terms of the formation of microsteps consisting of (3 11)A and (5 11)A microfacets on the averaged (4 11)A GaAs surface. Sizes of the (31 IlA and (5 1llA microfacets increase in proportion with the GaAs well width CL,,).
1. Introduction Formation of atomically flat GaAs/AlGaAs interfaces over a macroscopic area is very important for applications to quantum devices such as resonant tunneling hot electron transistors. GaAs/AIGaAs quantum wells (QWs> grown on (41 ])A-oriented GaAs substrates by molecular beam epitaxy (MBE)
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have been reported to show extremely flat interfaces over a large area [l-4]. GaAs/Al,,Ga,,,As QWs with well width of 2.4 nm grown on the (41l)A substrate show a very narrow photoluminescence (PL) peak (FWHM = 5.5 meV at A = 709.2 nm) at 4.2 K [I] whose linewidth is the same as that for growth-interrupted QWs grown on (100) GaAs substrates [5,61. There always exist two or three PL peaks from a single QW on the conventional (100) substrate, which is due to a lateral variation of the well thickness by i one monolayer, while the
0 1997 Elsevier Science B.V. All rights reserved.
14
K. Shinohara
et al./Applied
Surface Science 113/
GaAs/Al,,Ga,,,As QWs grown on (411)A substrates exhibit only one sharp PL peak from each QW over an extra ordinarily large area (N 1 cm’), indicating the formation of effectively atomically flat (41l)A GaAs/AlGaAs interfaces over a wafer-size area (‘super-flat interfaces’) [2]. The super-flat interfaces can be realized under a low V/III ratio, but MBE growth of n-type Si-doped GaAs with a low compensation ratio on the (411)A GaAs substrate requires an increased V/III ratio (V/III 2 11) [7]. Hence, the different V/III ratios must be used for growing, for instance, a GaAs/AlGaAs resonant tunneling diode (RTD) on the (4 11)A substrate which has the super-flat interfaces and n-type GaAs layers [S]. In this paper, we study for the first time how an upper interface of AlGaAs on GaAs or a lower interface of GaAs on AlGaAs in a GaAs/AlGaAs QW grown on (411)A GaAs substrate degrades in flatness under an increased V/III ratio (V/III 2 11).
2. MBE growth of GaAs/ AlGaAs QWs (411)A GaAs substrates were degreased and chemically etched by sulfuric acid etchant solution (HZS04:H,02:H20 = 5:l:l) prior to MBE growth. GaAs/AlGaAs QWs with the Al content of 0.270.29 were grown on the (411)A substrates at the substrate temperature CT,) of 580°C by using a Nissin RB-2001G MBE system. The substrate temperature was monitored by a pyrometer, which was calibrated by the melting point of Al (660°C). The three different V/III ratios (7, 11, 13 in pressure) were used for MBE growth of GaAs QW layers and/or AlGaAs
114 (1997) 73-78
barrier layers. Seven samples with the same QWs structure were grown under the different growth conditions of the V/III ratio. The growth rates of GaAs and AlGaAs were about 1.24 pm/h and 1.73 pm/h, respectively. The substrates were rotated at 30 rpm during MBE growth. Typical GaAs well widths CL,,) were 1.5, 3.0, 4.5, 6.0, 8.9 and 14.9 nm, and AlGaAs barrier layers were 24.2 nm thick. A buffer layer of GaAs (124 nm)/AlGaAs (24 nm)/GaAs (248 nm)/AlGaAs (24 nm)/GaAs (62 nm) was grown on another buffer layer of AlGaAs (61 nm)/GaAs (25 nm)/AlGaAs (242 nm)/GaAs (25 nm)/AlGaAs (61 nm)/GaAs (62 nm). The growth rates of GaAs and AlGaAs, and an Al content of the AlGaAs were obtained within the error limit of 3% from the cross-sectional observations of thicknesses of these buffer layers by atomic force microscope (AFM) as shown in Table I. The obtained Al contents of AlGaAs layers were in good agreement with those determined by the RHEED (reflection high-energy electron diffraction) oscillations for GaAs and AlAs layers on a (100) GaAs substrate.
3. Photoluminescence measurements Fig. 1 shows PL spectra at 18 K from seven GaAs/AlGaAs QWs samples with the Al content of 0.27-0.29 grown on the (411)A GaAs substrates at T, = 580°C. The wavelength of an excitation laser (He-Cd) was 325 nm and the excitation power was 2 mW. The excitation laser beam was focused on an area of a sample surface with a diameter of about
Table 1 Growth rates of GaAs QW layers and AlGaAs barrier layers, Al content of AlGaAs barrier layers for the seven samples (#a-#g) in this work, GaAs well width (a nominal well width of 1.5 nm) calculated from the growth rate (GR) of GaAs and the well width deduced from observed PL peak energy Sample #a #b #c #d #e #f #g
GR of AlGaAs (@m/h)
Al content
( pm/h)
Well width deduced from the growth rate (nm)
Well width deduced from PL peak energy (nm)
1.24
I .73
1.23
I.73 1.I3 1.72 1.74 1.73 1.73
0.28 0.29 0.21 0.28 0.29 0.28 0.28
1.49 I .48 1.51
1so 1.52 1.67 1.53
CR of GaAs
1.26 1.24 I .23 1.24 I .24
1.49 1.48 I .49 I .49
1.64 I .41 I .57
K. Shinohara et a/./Applied II
*
8
’
1’1
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IT
“,
Surjace Science 113/114
I”‘,
GaAs/Alo,3Gao,7AsQWs on (411)A (18K) 1.5nm
3.0 nm
6.0nm
14.9nm
bulk
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(GaAa, AlGaAs)
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Wavelength (nm) Fig. I Photoluminescence spectra at 18 K from seven &As/AlGaAs QWs samples (#a-#gl grown on (4lllA GaAs substrates under the condition of three different V/III ratios (low (71, medium (Ill, high (13)) for MBE growth of GaAs QW layers and/or AlGaAs barrier layers.
200 pm. Fig. l(a) shows PL spectrum of the QWs (sample #a) grown at the low V/III ratio (7) for both GaAs QW layers and AlGaAs barrier layers. (‘L’ (‘M’, ‘H’) in Fig. 1 represents MBE growth under the low (medium, high) V/III ratio of 7 (11, 131.) Six sharp peaks at 679.6 nm (full width at half maximum, FWHM, of 7.6 meV), 721.9 nm (5.2 meV), 749.2 nm (4.6 meV), 768.0 nm (3.0 meV), 788.0 nm (1.7 meV), 804.9 nm (0.9 meV) correspond to the luminescence from QWs with well widths of L,, = 1.5, 3.0, 4.5, 6.0, 8.9 and 14.9 nm, respectively. Since these FWHMs are almost the same as the best values reported so far 111, it is likely that the (411)A super-flat interfaces of GaAs/AlGaAs QWs are realized in this sample. Fig. l(b), (c) show PL spectra from the QWs grown at the medium V/III ratio of 11 (sample #b) and at the high V/III ratio of 13 (sample #cl, respectively, for both the GaAs QW layers and the AlGaAs barrier layers. In Fig. l(c), six broad peaks are observed at longer wavelengths compared with those from the sample #a with the super-flat interfaces. Since the thickness of GaAs QW layers were almost the same for each sample (1.48- 1.5 1 nm> as indicated in Table 1, this
75
(1997) 73-78
red-shift is considered to result from some amount of lateral variation of the GaAs well width of the sample #c. The difference of the peak energy of the 1.5 nm thick QW between samples #a and #c is 19 meV, which corresponds to the increase of the QW layer thickness of about 0.2 nm (12% of the well thickness which is about one order of magnitude larger than the thickness variation due to the change of growth rate) in the sample #c. This result indicates that the uniformity of the QW layer thickness as well as the interface flatness of the GaAs QWs grown on (411 )A GaAs substrates is very sensitive to the V/III ratio and the (411)A interfaces easily degrade under a slightly increased V/III ratio. Similar red-shift of PL peaks is seen for sample #e (Fig. l(e)), of which GaAs QW layers were grown at the high V/III ratio. When the high V/III ratio is used for the GaAs QW layers (Fig. l(c), (e)), FWHMs as well as red-shifts of PL peaks become large due to the enhanced interface roughness (or the degraded uniformity of the QW layer thickness). Fig. 2 shows the V/III ratio dependence of the FWHM of the PL peak observed at 18 K from the 3.0 nm thick QW grown on the (411)A GaAs substrates at T, = 580°C. The FWHM increases from 5.2 meV to 9.9 meV with increasing the V/III ratio
15
I
. GsAs/A10,Gao ,As QW on (4ll)A
(L,, = 3.0 nm) (#b)
18K I
0
10
5
V/III
ratio (in pressure)
Fig. 2. V/III ratio dependence of the full-width-at-half-maximum (FWHM) of the photoluminescence peak (18 Kl of the 3.0 nm thick QWs for samples of #a-#g grown on (4ll)A GaAs substrates.
76
K. Shinohara
et al. /Applied
Surjkace Science I I3 / I14 (1997) 73-78
during the growth of both the GaAs QW layers and the AlGaAs barrier layers. When the low V/III ratio (7) was used for the AlGaAs barrier layers, the FWHM is improved a little (sample #d and #e). On the other hand,when the low V/III ratio is used for the GaAs QW layersthe FWHM is much improved and it becomes almost as small as that of the sample #a with the super-flat interfaces. This tendency can be seen for other QWs with the different well widths. This result indicates that the upper interface of the GaAs/AlGaAs QWs grown on the (411)A GaAs substrates is more strongly affected by the high V/III ratio for the GaAs QW layers than the lower interface of the QWs with the AlGaAs barrier layers grown at the high V/III ratio. In order to quantitatively investigate how the upper interface roughens by an increased V/III ratio for GaAs QW layer, we estimated the fluctuation in the GaAs well width from the observed PL linewidth (r= FWHM). The broadening of the PL linewidth is caused by (1) the thermal excitation <&,>, (2) the interface roughness (&,s,) and (3) the impurities inside and/or outside the QWs crimp). Since the factors except for cough are considered to be common for all samples, we assume that the increase in FWHM (A K,,“sh = FWHM - FWHM(#a)) for samples of #b-#g from that of sample #a with the super-flat interfaces is due to an excess interface roughness. Then, an excess fluctuation in the well width (AL, > for samples of #b-#g is calculated by using the following equation:
QW layers in sample #f, AL, does not increase with increasing L,, . The interfaces of other two samples (#b and #d) degrade in flatness, because the medium V/III ratio was used for the GaAs QW layers and AL, increases parabolically with increasing L,,. This result suggests that the upper interface roughens more with increasing GaAs well width when the GaAs QW layers were grown under an increased V/III ratio. In order to further understand the A L,-L,, relation, we calculated AL, as a function of L,, by using a simple model of the interfaces. It has been reported by Yamaguchi et al. [9] that microfacets of (3 11)A and (5 11)A are observed on a (411)A GaAs surface under an increased V/III ratio with STM. Hence, we assume that the upper interface of a GaAs QW with a given L,, grown at an increased V/III ratio consists of microsteps formed by (31 1)A and (51 l)A, while the lower interfaces are super-flat and that the size of each microfacet of (3 11)A and (5 11)A increases in proportion with the GaAs well width L,, as illustrated in Fig. 4(a). We also use the method proposed by Singh et al. [lo] to determine the probability of distribution of the local well width over the extent of the exciton (R,,). In our model (Fig. 4(c)), the height of the microstep (h) is in proportion with the lateral size of the microstep (D),
-z 5
12
-7 1.0-
a (1)
f
5 3
and E is the transition
I”““’
06
sample#b
o
sample#d
0
sample #f
------
,“a:,.5 !’
Calculation
8’ ,' ,' /'
(2)
where E, and E, are the quantized energy levels of electrons and holes in the GaAs quantum wells measured from the corresponding band edge, Eg the band gap of GaAs and E,, the binding energy of an exciton. Fig. 3 shows the excess fluctuation (AL,) as a function of the well width (L,,) for three samples of #b (O), #d (0) and #f (0) and broken lines show the calculated results which will be discussed later. Since the low V/III ratio was used for the GaAs
l
-
,’
,’
energy given by
E = E, + E, + Eg - E,, ,
I
1 “““7 l
a = 1.1,"
,' I I I I' /' , I I ' ,' 8' ,,
Well width (nm) Fig. 3. Excess fluctuation (AL,)of function of the well width CL,,) #MO), #d(O) and #ff 0).
the GaAs well width as a for GaAs QWs samples of
K. Shinohara et al./Applied
(a)
Surface Science 113/ 114 (1997) 73-78
(b)
under the high V/III ratio
II
under the low V/III ratio
Fig. 4. Model of the interfaces of GaAs/AlGaAs QWs. Degraded (41 l)A interface consists of microsteps formed by (311)A and (51 l)A microfacets and the size of each microstep increases in proportion with the thickness of the GaAs QW layer.
and a simple relation exists between ( D) and the height (h) given by h = 0.04 X D.
the lateral size
(3)
The probability (P> of the distribution of the local well width in the region of extent of the exciton can be obtained with the well width CL,,):
Fig. 5 shows the calculated L, - L,, in the case for the microsteps of D = 1.O, 2.0, 3.0 inset shows calculated AL, as
P as a function of lateral size of the and 4.0 nm and the a function D from
’ (4)
and L,=L,,+h[(C,-C,O)
-(C,-Cl)],
(5)
where C,O (C,“) represents the mean concentration of the convex (3 11)A/(5 11)A microsteps (the concave (3 1 l)A/(511)A microsteps) on the upper interfaces over a full-wafer area. C, CC,> corresponds the mean concentrations of the convex (3 1 l)A/(5 11)A microsteps (the concave (3 1 l)A/(5 11)A microsteps) on the AlGaAs/GaAs interfaces over the region of the exciton extent. C: and C,O are the same (0.5) to keep the mean well width constant over the wafer area. L, is a local well width which the excitons feel in a QW with the mean well width of L,, as shown in Fig. 4(a).
p
1.0
$ -2
0.6
a 0.6
0.4 0.2 0.0 -0.05
O.CMl
0.05
Lw- Lo 0-N Fig. 5. Calculated probability (P) of the distribution of the local well width as a function L, - L,, in the case of the lateral size of the microstep of D = 1.0, 2.0, 3.0 and 4.0 nm. The inset shows AL, as a function of D deduced from the P -(L, - L,,) curve.
78
K. Shinohara et al./Applied
Surface Science 113/
the P - (L, - L,,) curve. AL, increases parabolically with increasing D and this behavior is very similar to the observed AL, - L,, curves (Fig. 31, indicating that the lateral size of the microsteps is in proportion with the well width. Broken lines in Fig. 3 show the calculated AL, - L,, curves using D = LyXLWO’where cr = 0.5 N 1.5. The calculated result for (Y= 1.1 is in good agreement with the experimental data. The fluctuation in the well width of GaAs QWs grown under an increased V/III ratio can be explained by the formation of the (3 1 l)A/(5 11)A microsteps of which size increases in proportion with the well width.
I14 (1997) 73-78
it remains to be almost zero even when the high V/III ratio was used for the AlGaAs barrier layers. This increase of the excess fluctuation (AL,) was explained in terms of the formation of the microsteps formed by (3ll)A and (511)A microfacets by using a simple model where the sizes of (311)A and (5ll)A microfacets increase in proportion with the GaAs QW layer thickness (L,, 1.
Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture.
4. Conclusion Arsenic pressure dependence of the interface flatness of GaAs/AlGaAs QWs (Al content of 0.270.29) grown on (4ll)A GaAs substrates by MBE have been investigated for the first time. (41 1)A super-flat interfaces of the QWs could be realized under the low V/III ratio (7), but the interfaces degraded in flatness under an increased V/III ratio (V/III 2 11). E ven when the slightly high V/III ratio (11) was used for growing the GaAs QW layers, the upper interfaces of the QWs degraded in flatness. On the other hand, when the high V/III ratio (13) was used for growing AlGaAs barrier layers, the lower interfaces did not degrade in flatness so much, indicating that the upper interfaces are more strongly affected by the increased V/III ratio than the lower interfaces. Furthermore, the excess fluctuation AL, of the well width for the GaAs QWs grown under various conditions of V/III ratio was estimated from the increase in the PL linewidth from that of the QWs with the super-flat interfaces (sample #a). AL, increases parabolically with increasing well width L,, when the increased V/III ratio (11, 13) was used for a GaAs QW layer, while
References ill S. Shimomura,
A. Wakejima, A. Adachi, Y. Okamoto, N. Sano, K. Murase and S. Hiyamizu, Jpn. J. Appl. Phys. 32 (1993) L1728. Dl S. Hiyamizu, S. Shimomura, A. Wakejima, S. Kaneko, A. Adachi, Y. Okamoto, N. Sano and K. Murase, J. Vat. Sci. Technol. B 12 (1994) 1043. [31 S. Shimomura, S. Kaneko, T. Motokawa, K. Shinohara, A. Adachi, Y. Okamoto, N. Sano, K. Murase and S. Hiyamizu, J. Cryst. Growth 150 (1995) 409. [41 S. Shimomura, K. Shinohara, T. Kitada, S. Hiyamizu, Y. Tsuda, N. Sano, A. Adachi and Y. Okamoto J. Vat. Sci. Technol. B 13 (1995) 696. 151M. Tanaka, H. Sakaki, J. Yoshino and T. Furuta, Surf. Sci. 174 (1986) 65. l61 R.F. Kopf, E.F. Schubert, T.D. Harris and R.S. Becker, Appl. Phys. Lett. 58 (1991) 631. 171K. Shinohara, T. Motokawa, K. Kasahara, S. Shimomura, N. Sano, A. Adachi and S. Hiyamizu, Semicond. Sci. Technol. I1 (1996) 125. Dl S. Shimomura, K. Shinohara, K. Kasahara, T. Motokawa, A. Adachi, Y. Okamoto, N. Sano and S. Hiyamizu, Solid-State Electron. 40 (1996) 417. 191 H. Yamaguchi, T. Yamada and Y. Horikoshi, Jpn. J. Appl. Phys. 34 (1995) L1490. [IO] J. Singh. K.K. Bajaj and S. Chaudhuri, Appt. Phys. 44 (1984) 805.
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:,:L
/.,., :,~..:..........i :.:. ..
. . . . . .._. ..:.::::
~::~.~~~
,c:.::....
__
applied
surface science
!!!I ELSEVIER
Applied Surface Science
113/I 14 (1997) 73-78
As, pressure dependence of the interface flatness of GaAs/Al,.,Ga,,As quantum wells grown on (411) A GaAs substrates by MBE K. Shinohara a.*, K. Kasahara a, S. Shimomura a, A. Adachi b, Y. Okamoto ‘, N. Sano d, S. Hiyamizu a-e d Fact&v ofEngineering b R&D
Dil’ision,
Nissin Electric
Science, Osaka fJnii,ersity,
’ Research and Headquarters ’ Fact&
Kubota LTD., Amagasaki,
of Science. Kwunsei-Cakuin
’ Research Centrrfor
Toxonaka
Company Ltd., Umezu-Takase-cho,
Extreme Materials.
University.
560, Japan
Ukyo-ku, Kyoto 615, Japart H.vogo 661. Japan
Nishinomiya,
H~ogo 662 Japan
Osaka UniL,ersity, Ttjyonaka.
Osuka 560, Japan
Abstract Arsenic pressure dependence of the interface flatness of GaAs/AlGaAs (Al content of 0.27-0.29) quantum wells (QWs) grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) was investigated. Effectively atomically flat (41llA interfaces could be realized under a low V/III ratio (V/III = 71, but the interfaces decreased in flatness under an increased V/III ratio (V/III 2 11). The linewidth of the photoluminescence peak increases with increasing the V/III ratio during the growth of the GaAs QW layers, but does not increase so much with increasing V/III ratio during the growth of the AlGaAs barrier layers, indicating that the flatness of an AlGaAs/GaAs upper interface of the GaAs QWs grown on (4ll)A substrates is more strongly affected by the high V/III ratio than that of a GaAs/AlGaAs lower interface of the GaAs QWs. The excess fluctuation in the GaAs well width (AL,) which results from the roughness of the upper interface caused by the high V/III ratio increases parabolically as a function of the GaAs well width (L,,). This result was explained in terms of the formation of microsteps consisting of (3 11)A and (5 11)A microfacets on the averaged (4 11)A GaAs surface. Sizes of the (31 IlA and (5 1llA microfacets increase in proportion with the GaAs well width CL,,).
1. Introduction Formation of atomically flat GaAs/AlGaAs interfaces over a macroscopic area is very important for applications to quantum devices such as resonant tunneling hot electron transistors. GaAs/AIGaAs quantum wells (QWs> grown on (41 ])A-oriented GaAs substrates by molecular beam epitaxy (MBE)
” Corresponding 8454632:
author. Tel.: +81-6.8506457: e-mail: [email protected].
Copyright PII SO1 69-4332(96)00863-X
0169-4332/97/$17.00
fax:
+ 81-6.
have been reported to show extremely flat interfaces over a large area [l-4]. GaAs/Al,,Ga,,,As QWs with well width of 2.4 nm grown on the (41l)A substrate show a very narrow photoluminescence (PL) peak (FWHM = 5.5 meV at A = 709.2 nm) at 4.2 K [I] whose linewidth is the same as that for growth-interrupted QWs grown on (100) GaAs substrates [5,61. There always exist two or three PL peaks from a single QW on the conventional (100) substrate, which is due to a lateral variation of the well thickness by i one monolayer, while the
0 1997 Elsevier Science B.V. All rights reserved.
14
K. Shinohara
et al./Applied
Surface Science 113/
GaAs/Al,,Ga,,,As QWs grown on (411)A substrates exhibit only one sharp PL peak from each QW over an extra ordinarily large area (N 1 cm’), indicating the formation of effectively atomically flat (41l)A GaAs/AlGaAs interfaces over a wafer-size area (‘super-flat interfaces’) [2]. The super-flat interfaces can be realized under a low V/III ratio, but MBE growth of n-type Si-doped GaAs with a low compensation ratio on the (411)A GaAs substrate requires an increased V/III ratio (V/III 2 11) [7]. Hence, the different V/III ratios must be used for growing, for instance, a GaAs/AlGaAs resonant tunneling diode (RTD) on the (4 11)A substrate which has the super-flat interfaces and n-type GaAs layers [S]. In this paper, we study for the first time how an upper interface of AlGaAs on GaAs or a lower interface of GaAs on AlGaAs in a GaAs/AlGaAs QW grown on (411)A GaAs substrate degrades in flatness under an increased V/III ratio (V/III 2 11).
2. MBE growth of GaAs/ AlGaAs QWs (411)A GaAs substrates were degreased and chemically etched by sulfuric acid etchant solution (HZS04:H,02:H20 = 5:l:l) prior to MBE growth. GaAs/AlGaAs QWs with the Al content of 0.270.29 were grown on the (411)A substrates at the substrate temperature CT,) of 580°C by using a Nissin RB-2001G MBE system. The substrate temperature was monitored by a pyrometer, which was calibrated by the melting point of Al (660°C). The three different V/III ratios (7, 11, 13 in pressure) were used for MBE growth of GaAs QW layers and/or AlGaAs
114 (1997) 73-78
barrier layers. Seven samples with the same QWs structure were grown under the different growth conditions of the V/III ratio. The growth rates of GaAs and AlGaAs were about 1.24 pm/h and 1.73 pm/h, respectively. The substrates were rotated at 30 rpm during MBE growth. Typical GaAs well widths CL,,) were 1.5, 3.0, 4.5, 6.0, 8.9 and 14.9 nm, and AlGaAs barrier layers were 24.2 nm thick. A buffer layer of GaAs (124 nm)/AlGaAs (24 nm)/GaAs (248 nm)/AlGaAs (24 nm)/GaAs (62 nm) was grown on another buffer layer of AlGaAs (61 nm)/GaAs (25 nm)/AlGaAs (242 nm)/GaAs (25 nm)/AlGaAs (61 nm)/GaAs (62 nm). The growth rates of GaAs and AlGaAs, and an Al content of the AlGaAs were obtained within the error limit of 3% from the cross-sectional observations of thicknesses of these buffer layers by atomic force microscope (AFM) as shown in Table I. The obtained Al contents of AlGaAs layers were in good agreement with those determined by the RHEED (reflection high-energy electron diffraction) oscillations for GaAs and AlAs layers on a (100) GaAs substrate.
3. Photoluminescence measurements Fig. 1 shows PL spectra at 18 K from seven GaAs/AlGaAs QWs samples with the Al content of 0.27-0.29 grown on the (411)A GaAs substrates at T, = 580°C. The wavelength of an excitation laser (He-Cd) was 325 nm and the excitation power was 2 mW. The excitation laser beam was focused on an area of a sample surface with a diameter of about
Table 1 Growth rates of GaAs QW layers and AlGaAs barrier layers, Al content of AlGaAs barrier layers for the seven samples (#a-#g) in this work, GaAs well width (a nominal well width of 1.5 nm) calculated from the growth rate (GR) of GaAs and the well width deduced from observed PL peak energy Sample #a #b #c #d #e #f #g
GR of AlGaAs (@m/h)
Al content
( pm/h)
Well width deduced from the growth rate (nm)
Well width deduced from PL peak energy (nm)
1.24
I .73
1.23
I.73 1.I3 1.72 1.74 1.73 1.73
0.28 0.29 0.21 0.28 0.29 0.28 0.28
1.49 I .48 1.51
1so 1.52 1.67 1.53
CR of GaAs
1.26 1.24 I .23 1.24 I .24
1.49 1.48 I .49 I .49
1.64 I .41 I .57
K. Shinohara et a/./Applied II
*
8
’
1’1
”
IT
“,
Surjace Science 113/114
I”‘,
GaAs/Alo,3Gao,7AsQWs on (411)A (18K) 1.5nm
3.0 nm
6.0nm
14.9nm
bulk
0.9 nm
4.5 nm AA
(GaAa, AlGaAs)
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08 Jj
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#b:(M.M) - I
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. JL
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#f:(L,M)
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Wavelength (nm) Fig. I Photoluminescence spectra at 18 K from seven &As/AlGaAs QWs samples (#a-#gl grown on (4lllA GaAs substrates under the condition of three different V/III ratios (low (71, medium (Ill, high (13)) for MBE growth of GaAs QW layers and/or AlGaAs barrier layers.
200 pm. Fig. l(a) shows PL spectrum of the QWs (sample #a) grown at the low V/III ratio (7) for both GaAs QW layers and AlGaAs barrier layers. (‘L’ (‘M’, ‘H’) in Fig. 1 represents MBE growth under the low (medium, high) V/III ratio of 7 (11, 131.) Six sharp peaks at 679.6 nm (full width at half maximum, FWHM, of 7.6 meV), 721.9 nm (5.2 meV), 749.2 nm (4.6 meV), 768.0 nm (3.0 meV), 788.0 nm (1.7 meV), 804.9 nm (0.9 meV) correspond to the luminescence from QWs with well widths of L,, = 1.5, 3.0, 4.5, 6.0, 8.9 and 14.9 nm, respectively. Since these FWHMs are almost the same as the best values reported so far 111, it is likely that the (411)A super-flat interfaces of GaAs/AlGaAs QWs are realized in this sample. Fig. l(b), (c) show PL spectra from the QWs grown at the medium V/III ratio of 11 (sample #b) and at the high V/III ratio of 13 (sample #cl, respectively, for both the GaAs QW layers and the AlGaAs barrier layers. In Fig. l(c), six broad peaks are observed at longer wavelengths compared with those from the sample #a with the super-flat interfaces. Since the thickness of GaAs QW layers were almost the same for each sample (1.48- 1.5 1 nm> as indicated in Table 1, this
75
(1997) 73-78
red-shift is considered to result from some amount of lateral variation of the GaAs well width of the sample #c. The difference of the peak energy of the 1.5 nm thick QW between samples #a and #c is 19 meV, which corresponds to the increase of the QW layer thickness of about 0.2 nm (12% of the well thickness which is about one order of magnitude larger than the thickness variation due to the change of growth rate) in the sample #c. This result indicates that the uniformity of the QW layer thickness as well as the interface flatness of the GaAs QWs grown on (411 )A GaAs substrates is very sensitive to the V/III ratio and the (411)A interfaces easily degrade under a slightly increased V/III ratio. Similar red-shift of PL peaks is seen for sample #e (Fig. l(e)), of which GaAs QW layers were grown at the high V/III ratio. When the high V/III ratio is used for the GaAs QW layers (Fig. l(c), (e)), FWHMs as well as red-shifts of PL peaks become large due to the enhanced interface roughness (or the degraded uniformity of the QW layer thickness). Fig. 2 shows the V/III ratio dependence of the FWHM of the PL peak observed at 18 K from the 3.0 nm thick QW grown on the (411)A GaAs substrates at T, = 580°C. The FWHM increases from 5.2 meV to 9.9 meV with increasing the V/III ratio
15
I
. GsAs/A10,Gao ,As QW on (4ll)A
(L,, = 3.0 nm) (#b)
18K I
0
10
5
V/III
ratio (in pressure)
Fig. 2. V/III ratio dependence of the full-width-at-half-maximum (FWHM) of the photoluminescence peak (18 Kl of the 3.0 nm thick QWs for samples of #a-#g grown on (4ll)A GaAs substrates.
76
K. Shinohara
et al. /Applied
Surjkace Science I I3 / I14 (1997) 73-78
during the growth of both the GaAs QW layers and the AlGaAs barrier layers. When the low V/III ratio (7) was used for the AlGaAs barrier layers, the FWHM is improved a little (sample #d and #e). On the other hand,when the low V/III ratio is used for the GaAs QW layersthe FWHM is much improved and it becomes almost as small as that of the sample #a with the super-flat interfaces. This tendency can be seen for other QWs with the different well widths. This result indicates that the upper interface of the GaAs/AlGaAs QWs grown on the (411)A GaAs substrates is more strongly affected by the high V/III ratio for the GaAs QW layers than the lower interface of the QWs with the AlGaAs barrier layers grown at the high V/III ratio. In order to quantitatively investigate how the upper interface roughens by an increased V/III ratio for GaAs QW layer, we estimated the fluctuation in the GaAs well width from the observed PL linewidth (r= FWHM). The broadening of the PL linewidth is caused by (1) the thermal excitation <&,>, (2) the interface roughness (&,s,) and (3) the impurities inside and/or outside the QWs crimp). Since the factors except for cough are considered to be common for all samples, we assume that the increase in FWHM (A K,,“sh = FWHM - FWHM(#a)) for samples of #b-#g from that of sample #a with the super-flat interfaces is due to an excess interface roughness. Then, an excess fluctuation in the well width (AL, > for samples of #b-#g is calculated by using the following equation:
QW layers in sample #f, AL, does not increase with increasing L,, . The interfaces of other two samples (#b and #d) degrade in flatness, because the medium V/III ratio was used for the GaAs QW layers and AL, increases parabolically with increasing L,,. This result suggests that the upper interface roughens more with increasing GaAs well width when the GaAs QW layers were grown under an increased V/III ratio. In order to further understand the A L,-L,, relation, we calculated AL, as a function of L,, by using a simple model of the interfaces. It has been reported by Yamaguchi et al. [9] that microfacets of (3 11)A and (5 11)A are observed on a (411)A GaAs surface under an increased V/III ratio with STM. Hence, we assume that the upper interface of a GaAs QW with a given L,, grown at an increased V/III ratio consists of microsteps formed by (31 1)A and (51 l)A, while the lower interfaces are super-flat and that the size of each microfacet of (3 11)A and (5 11)A increases in proportion with the GaAs well width L,, as illustrated in Fig. 4(a). We also use the method proposed by Singh et al. [lo] to determine the probability of distribution of the local well width over the extent of the exciton (R,,). In our model (Fig. 4(c)), the height of the microstep (h) is in proportion with the lateral size of the microstep (D),
-z 5
12
-7 1.0-
a (1)
f
5 3
and E is the transition
I”““’
06
sample#b
o
sample#d
0
sample #f
------
,“a:,.5 !’
Calculation
8’ ,' ,' /'
(2)
where E, and E, are the quantized energy levels of electrons and holes in the GaAs quantum wells measured from the corresponding band edge, Eg the band gap of GaAs and E,, the binding energy of an exciton. Fig. 3 shows the excess fluctuation (AL,) as a function of the well width (L,,) for three samples of #b (O), #d (0) and #f (0) and broken lines show the calculated results which will be discussed later. Since the low V/III ratio was used for the GaAs
l
-
,’
,’
energy given by
E = E, + E, + Eg - E,, ,
I
1 “““7 l
a = 1.1,"
,' I I I I' /' , I I ' ,' 8' ,,
Well width (nm) Fig. 3. Excess fluctuation (AL,)of function of the well width CL,,) #MO), #d(O) and #ff 0).
the GaAs well width as a for GaAs QWs samples of
K. Shinohara et al./Applied
(a)
Surface Science 113/ 114 (1997) 73-78
(b)
under the high V/III ratio
II
under the low V/III ratio
Fig. 4. Model of the interfaces of GaAs/AlGaAs QWs. Degraded (41 l)A interface consists of microsteps formed by (311)A and (51 l)A microfacets and the size of each microstep increases in proportion with the thickness of the GaAs QW layer.
and a simple relation exists between ( D) and the height (h) given by h = 0.04 X D.
the lateral size
(3)
The probability (P> of the distribution of the local well width in the region of extent of the exciton can be obtained with the well width CL,,):
Fig. 5 shows the calculated L, - L,, in the case for the microsteps of D = 1.O, 2.0, 3.0 inset shows calculated AL, as
P as a function of lateral size of the and 4.0 nm and the a function D from
’ (4)
and L,=L,,+h[(C,-C,O)
-(C,-Cl)],
(5)
where C,O (C,“) represents the mean concentration of the convex (3 11)A/(5 11)A microsteps (the concave (3 1 l)A/(511)A microsteps) on the upper interfaces over a full-wafer area. C, CC,> corresponds the mean concentrations of the convex (3 1 l)A/(5 11)A microsteps (the concave (3 1 l)A/(5 11)A microsteps) on the AlGaAs/GaAs interfaces over the region of the exciton extent. C: and C,O are the same (0.5) to keep the mean well width constant over the wafer area. L, is a local well width which the excitons feel in a QW with the mean well width of L,, as shown in Fig. 4(a).
p
1.0
$ -2
0.6
a 0.6
0.4 0.2 0.0 -0.05
O.CMl
0.05
Lw- Lo 0-N Fig. 5. Calculated probability (P) of the distribution of the local well width as a function L, - L,, in the case of the lateral size of the microstep of D = 1.0, 2.0, 3.0 and 4.0 nm. The inset shows AL, as a function of D deduced from the P -(L, - L,,) curve.
78
K. Shinohara et al./Applied
Surface Science 113/
the P - (L, - L,,) curve. AL, increases parabolically with increasing D and this behavior is very similar to the observed AL, - L,, curves (Fig. 31, indicating that the lateral size of the microsteps is in proportion with the well width. Broken lines in Fig. 3 show the calculated AL, - L,, curves using D = LyXLWO’where cr = 0.5 N 1.5. The calculated result for (Y= 1.1 is in good agreement with the experimental data. The fluctuation in the well width of GaAs QWs grown under an increased V/III ratio can be explained by the formation of the (3 1 l)A/(5 11)A microsteps of which size increases in proportion with the well width.
I14 (1997) 73-78
it remains to be almost zero even when the high V/III ratio was used for the AlGaAs barrier layers. This increase of the excess fluctuation (AL,) was explained in terms of the formation of the microsteps formed by (3ll)A and (511)A microfacets by using a simple model where the sizes of (311)A and (5ll)A microfacets increase in proportion with the GaAs QW layer thickness (L,, 1.
Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture.
4. Conclusion Arsenic pressure dependence of the interface flatness of GaAs/AlGaAs QWs (Al content of 0.270.29) grown on (4ll)A GaAs substrates by MBE have been investigated for the first time. (41 1)A super-flat interfaces of the QWs could be realized under the low V/III ratio (7), but the interfaces degraded in flatness under an increased V/III ratio (V/III 2 11). E ven when the slightly high V/III ratio (11) was used for growing the GaAs QW layers, the upper interfaces of the QWs degraded in flatness. On the other hand, when the high V/III ratio (13) was used for growing AlGaAs barrier layers, the lower interfaces did not degrade in flatness so much, indicating that the upper interfaces are more strongly affected by the increased V/III ratio than the lower interfaces. Furthermore, the excess fluctuation AL, of the well width for the GaAs QWs grown under various conditions of V/III ratio was estimated from the increase in the PL linewidth from that of the QWs with the super-flat interfaces (sample #a). AL, increases parabolically with increasing well width L,, when the increased V/III ratio (11, 13) was used for a GaAs QW layer, while
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