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InP superlattices grown by gas source MEE (migration enhanced epitaxy)
Journal of Crystal Growth 127 (1993) 194—198 North-Holland
o~o,
CRYSTAL GROWT H
Atomically controlled InGaAs/InP superlattices grown by gas source MEE (migration enhanced epitaxy) Hajime Asahi, Teruaki Kohara, Ravi Kant Soni, Kumiko Asami, Shuichi Emura and Shun-ichi Gonda The Institute of Scient4fic and Industrial Research, Osaka University, 8-], Mihogaoka, Ibaraki, Osaka 567, Japan
Atomically controlled InGaAs/InP SL structures having different types of heterointerfaces are grown on (OOl)InP substrates at 350°Cby gas source MEE (migration enhanced epitaxy). RHEED intensity traces exhibit the same shape at the positions of the same type of heterointerfaces, indicating the formation of the desired heterointerfaces. The Raman spectrum from the SL, having only the InAs-type heterointerfaces, is characterized by the absence of GaP-like LO phonon clearly suggesting the formation of only the InAs-type heterointerfaces, while the SL having InGaP-type interfaces indeed shows the presence of GaP-like LO phonon peak. 4.2 K photoluminescence (PL) spectra for the InGaAs/InP quantum well (OW) structures show a very narrow line width comparable to the narrowest line width reported so far. Furthermore, the PL peak energy variation with well thickness clearly depends on the heterointerface type.
1. Introduction The atomic arrangement at the interfaces of superlattices/ quantum well (SL/QW) structures influences their physical properties. To control the physical properties of SL/QW structures, the atomically controlled growth of the heterointerface is necessary. Lattice-matched InGaAs/InP SL/QW structures consist of two group III and two group V atoms and have two different types of heterointerfaces (InGaP and InAs types) depending on the growth sequence even when ideally formed. Recently, Vandenberg et al. confirmed the existence of these two different types of heterointerfaces with high-resolution X-ray diffraction [1].
ergy shift is observed for the OW structures prepared by different growth sequences at the interfaces [2,3], suggesting that the QW PL peak energy critically depends on the composition of the heterointerface. The optical and electrical qualities of the SL/QW structures are also influenced by the precise atomic arrangement at the interfaces. These situations are common in all of the SL/QW structures having two group III and two group V atoms, such as InGaP/GaAs, InAs/ GaSb and InAs/AlSb. To control the physical properties of InGaAs/ InP SL/QW structures, we have proposed the gas source MEE growth method, the combination of gas source MBE and migration enhanced epitaxy (MEE), where phosphine (PH3) and arsine
Under the usual growth condition, InGaAs/ InP heterointerfaces have uncontrolled interfaces, because the changeover of As and P flows is necessary and the desorption of P (As) from InP (InGaAs) surface and the incorporation of As (P) into InP (InGaAs) layer near the interface are easy to occur owing to high growth temperature. In fact, photoluminescence (PL) peak en-
(AsH3) are used as group V sources, and have already reported the gas source MEE growth of InP, InGaAs and InGaAs/InP heterostructures on (001) InP substrates [4,5]. In this paper, we report the gas source MEE growth of atomically controlled InGaAs/InP superlattices and their optical properties. Atomically controlled growths are monitored with RHEED intensity oscilla-
0022-0248/93/$06.00 © 1993
—
Elsevier Science Publishers B.V. All rights reserved
H. Asahi eta!. / Atomically controlled lnGaAs /InP SLs grown by gas source MEE
tions. Grown layers are characterized by FL and Raman scattering measurements.
(a) InP growth
195
Tsub= 350°C
In
3sec
3.8sec
2. Experimental procedure
~
3~ec
3sec
I.m_..I,.i
~
I
I
~
I
~I
(O.25sccm)
The gas source MBE system (ANELVA GBE83O) evacuated by an oil diffusion pump with a liquid nitrogen trap was used in this experiment. Elemental indium (In) and gallium (Ga), and phosphine (PH 3) and arsine (AsH3) were used as group III and group V sources, respectively. PH3 and AsH3 were introduced into the growth chamber through mass flow controllers (MFCs) ad gas cracker cells (cracking temperature 900°C). The substrates used were Fe-doped (001)InP. After thermal cleaning, InP buffer layers, 60 nm thick, were grown at 480°Cwith a PH3 flux of 0.80 SCCM by conventional gas source MBE method. The PH3 and AsH3 gas flows were rapidly supplied or interrupted by using electronically operated two-way valves located close to the gas cracker cells. The gas source MEE growth was carried out at 350°Cwith PH3 and AsH3 fluxes of 0.25 and 0.15 SCCM, respectively, PL measurement was carried out at 4.2 K using a He—Ne gas laser (632.8 nm, 15 mW) as an excitation light source. Raman scattering spectra were measured at room temperature in a backscattering geometry. The excitation source was an Ar-ion laser (488.0 nm, 500 mW). The propagation and polarization of the incident laser beam were along the [1001and [0101directions of the crystal, respectively,
3. Results and discussion 3.1. Gas source MEE growth of InP and InGaAs The gas source MEE growth of InP and InGaAs was already reported elsewhere [4,51. Here, we summarize the results briefly. It was found that even at a substrate temperature as low as 350°C, the desorption of the some amount of phosphorus from the InP surface occurs when the PH3 flow is interrupted, although the RHEED
(b) InGaAs growth
35O °~
Tsub=
ri
In,Ga 2
‘~‘
Ash3
I
‘~
I U
4.5sec
~2sec ~‘m~.’
I
ii
I
(O.l5sccm)
Fig. 1. Deposition sequences for the gas source MEE growth of (a) InP and (b) InGaAs.
pattern is still showing the (2 X 4) reconstructions. On the other hand, with a PH3 flow of 1 SCCM at 350°C,the (2 X 4) patterns became diffused (2 X 1) showing an additional adsorption of P on the surface. As a result, the perfectly alternating supply of In and P was achieved only when PH3 is supplied with a proper interruption time before the supply of In. Fig. la shows the deposition sequence used here for the growth of InP. The PH3 interruption time is 3s; 3.8 s corresponds to the supply time of 1 atomic layer of In atoms. Under this deposition sequence, the RHEED intensity oscillation continued with the same amplitude for a long period. In the growth of InGaAs, the proper interruption time ( 1 s) between the In,Ga supply and the AsH3 supply was necessary in order to achieve the perfectly alternating supply of In,Ga and As. This indicates that at low ‘ub of 350°Cit takes more than 1 s for In,Ga atoms to migrate to the proper sites. Although the interruption of the AsH3 supply before the In,Ga supply was not necessary, the supply of a proper amount of AsH3 was important. The (2 x 4) reconstruction was observed at 350°Cwhen a proper AsH3 flow was supplied, while the (2 x 3) reconstruction was observed when an excess AsH3 flow was supplied. The RHEED intensity oscillation during the growth of InGaAs under the (2 X 3) condition persisted only a short duration. Under the (2 x 4)
H. Asahi et a!. / Atomically controlled inGaAs
196
condition, the RHEED intensity oscillation continued with the same amplitude for a long period, This suggests that at low ‘~uh the desorption of As does not occur and that the amount of supplied As is needed to be precisely controlled. Fig. lb shows the deposition sequence used here for the growth of InGaAs. 3.2. Gas source MEE growth of InGaAs /InP superlattices Based on the above results, the gas source MEE growth of InGaAs/InP superlattices was studied. Fig. 2a shows RHEED intensity traces during the gas source MEE growth of (InGaAs)m(InP)n superlattices at Tsub of 350°C (m n 10 in the upper trace and m n 11 in the lower trace). The arrows indicate the heterointerface positions. The RHEED intensity oscillations continued with the same amplitude for over 1 h. In particular, at the positions of heterointerfaces, the RHEED intensity trace exhibited completely the same shapes at any time, as can be seen in fig. 2a. The RHEED intensity trace during the growth of InP and InGaAs also showed its own particular shape for the individual layer growth. When growing the InGaAs/InF heterostructures, heterointerfaces are classified into 4 different types. They are As—In type and InGa—P type =
=
=
As-In
P-InGa (InP),
5
As-In
(a)
=
/ InP SLs grown by gas source MEE
in the transition from lnGaAs layer to InP layer, and P—InGa type and In—As type in the transition from InP layer to InGaAs layer. Fig. 2b shows RHEED intensity traces during the growth of (InGaAs)rn (InP)~superlattices (m 9.5, ‘~ 7.5 in the upper trace, and m 9.5, n 10.5 in the lower trace) having different combinations of heterointerfaces from those in fig. 2a. The RHEED intensity traces at the individual heterointerface positions and in the individual layers are the same as the individual ones in fig. 2a. It is noteworthy that the RHEED intensity traces in fig. 2 show the transition region of about 1.5 MLs for all of the different types of heterointerfaces. This means that the intermixing of As—P atoms during growth does not occur in this growth method, and indicates that the information of RHEED intensity oscillation contains that of nearly 1.5 MLs. Similar results were also obtamed for the different sets of m and n in (InGaAs)m(InP)n superlattices. =
=
3.3. Characterization of InGaAs /InP quantum well structures Fig. 3 shows Raman spectra from the InGaAs/InP OW structures having different types of heterointerfaces. All samples have 2, 4, 7, 10 and 15 ML thick InGaAs OW layers separated by 25 MLs of InP layers. Sample (a) has P—InGa and
InGa-P
~(InGaAs),st
InGa-P
In-As (InP),,
InGa-P
(b)
(lnP)~ (InGaAs)~
InGa-P
As-In
~(InGaAs),, ~
120 TIME(sec 1
P-InGa
(InP)~5 ~(InGaAsJ
(lnP)~ (lnGaAs)n~ m
=
=
n-As (InP)~,
240
As-In
~InGaAs)~,~
120 TIME(sec)
240
Fig. 2. RHEED intensity traces during the gas source MEE growth of (InGaAs),/InP), superlattices having tailored heterointerfaces at L.5h of 350°C: (a) m=n = 10, and m = n=11; (b) m=9.5, n= 7.5, and m = 9.5, n = 10.5. The arrows indicate heterointerface positions.
H. Asahi et a!. / Atomically controlled InGaAs /InP SLs grown by gas source MEE
As—In type heterointerfaces at the transition from the InP layer to the InGaAs layer and at the transition from the InGaAs layer to the InP layer respectively. Sample (b) has P—InGa and InGa—P type heterointerfaces. Sample (c) has In—As and InGa—P types, and the sample (d) has In—As and As—In types. Four peaks are observed in the frequency range of 200—400 cmi. They correspond to the InAs-like (221.1 cm 1), GaAs-like (268.7 cm’), InP-like (353.8 cm1) and GaP-like (384.7 cm~) LO phonon peaks, respectively, as indicated by arrows in fig. 3. Samples (a), (b) and (c) have InAs, GaAs, InP and GaP bonds. The GaP bonds exist only at the InGa—F type heterointerfaces. Sample (d) has InAs, GaAs and InP bonds. The Raman spectrum for sample (d) shows InAs-like, GaAs-like and InP-like LO phonon peaks. However, the GaP-like LU phonon peak could not be observed. This strongly mdicates that tailored heterointerfaces can be grown by gas source MEE at 350°C. Fig. 4 shows the 4.2 K PL spectrum from the InGaAs/InP QW structure wafer. The sample has P—InGa type and As—In type heterointer-
InAs-Uke LO
lnP-like LO
GaA 1s-Uke ~
GaP-likeLO
(a) >‘—
z
1.2 1.1 1.0 1) lnGaAs/InP OW I— 4.2 K
197
ENERGY (eV) 0.9 0.8
Z
0.7
lnGaAs reference
15ML 4ML 1OML x~ 2ML 7ML
>-
XiS
(I)
I
134 I—
a. tO
1.2 1.4 1.6 WAVELENGTH (Fm)
1.8
Fig. 4. 4.2 K PL spectra for the gas source MEE grown InGaAs/InP QWs showing very narrow line width comparable to the narrowest one reported so far (ref. [6]).
faces. The PL peaks from each OW layer as well as the InGaAs reference layer grown between the OW structure and the InP substrate are observed. The full widths at half maximum (FWHMs) for the 2, 4, 7 and 10 MLs OW layers are 12, 8, 7 and 6 meV, respectively. These values are comparable to the narrowest FWHMs reported so far [61. It is noteworthy that the gas source MEE InGaAs/InP QW structures were grown at ‘~ub as low as 350°C,while those re7ub ~ 500°C.This indicates highgrown quality ported in the ref. very [6] were at of OW structures grown by gas source MEE. Fig. 5 shows the PL energy shift for the InGaAs/InP OW structures having different cornbination of heterointerfaces as a function of InGaAs well thickness. They are clearly dependent on the heterointerface types and can be divided
z
tures having InGaP type heterointerfaces at both ~
into three interfaces those having (P—InGa, groups. InGaP The InGa—P), type firstatgroup one and interface the is OW second strucand is InAs type at another interface (P—InGa, As—In) ~
4~ RAMAN SHIFT (cml Fig. 3. Raman spectra from the InGaAs/InP quantum well structures having different types of heterointerfaces: (a) P InGa and As—In; (b) P—InGa and InGa—P; (c) In—As and InGa—P; (d) In—As and As—In type interfaces. InAs-like, GaAs-like, InP-like and GaP-like LO phonons are indicated by arrows,
and (In—As, InGa—P). The third is those with InAs type interfaces at both interfaces (In—As, As—In). This figure indicates that the InGaP type interface increases the energy level of QWs, while the InAs type interface decreases the energy level. This is reasonable to the theoretical consideration on the energy shift. The InGaP interface will form higher potential than that of InP and will
198
H. Asahi eta!.
/ Atomically controlled InGaAs /InP SLs grown by gas source MEE
0.5 InGaAslInP OW 4.2 K 0.4
•
P-InGa, InGa-P
0
~
P-lnGa,As-In
A
In As, InGa-P
~
InGaAs/InP quantum well structures, having only InAs-type heterointerfaces, are characterized by the absence of GaP-like LU phonons clearly suggesting the formation of only the InAs type heterointerfaces, while structures having InGaP type LU phonon peak. Consequently, it is concluded interfaces indeed show the presence of a GaP-like
~0.3 ~~~A,As-In U)
w
0.2
Z
w —j
0.
0.1
that the bred heterointerfaces InGaAs/InP heterostructures can be grown by having gas source taiMEE at 350°C. Furthermore, 4.2 K FL spectra for the InGaAs/InP OW structures showed a very narrow line width comparable to the narrowest line width reported so far. The PL peak energy variation with well thickness clearly depends on the heterointerface type. This result also supports the formation of atomically controlled interfaces.
0.0 0
10 20 NUMBER OF InGa ATOMIC LAYERS
Fig. 5. InGaAs well thickness dependence of PL energy shift for the InGaAs/InP QW structures having different combination of heterointerfaces.
reduce the bond length of InGaAs near the interface. These effects both increase the energy level of OWs. This result also supports the formation of atomically controlled interfaces. A comparison with the experimental energy shift and the theoretical one will be reported elsewhere.
4. Summary We have studied the gas source MEE growth of InGaAs/InP SL/OW structures at 350°C. RHEED intensity traces exhibited the same shape at the positions of the same type of heterointerfaces, indicating the formation of the desired heterointerfaces. The Raman spectra from the
Acknowledgement This work was supported in part by Grant-inAid for Scientific Research on Priority Area, “Electron Wave Interference Effects in Mesoscopic Structures” and the Scientific Research Grant-in-Aid #02650014 from the Ministry of Education, Science and Culture of Japan.
References [1] J.M. Vandenberg, MB. Panish, H. Temkin and R.A. Hamm, Appi. Phys. Letters 53 (1988) 1920. [21 T.Y. Wang, E.H. Reihien, H.R. Jen and G.B. Stringfellow, J. AppI. Phys. 66 (1989) 5376. [3] H. Kamei and H. Hayashi, J. Crystal Growth 107 (1991) 567. [4] N. Takeyasu, H. Asahi, S.J. Yu, K. Asami, T. Kaneko and Gonda, J. Crystal Growth 111 (1991) 502. [51S. H. Asahi, T. Kohara, R.K. Soni, N. Takeyasu, K. Asami, S. Emura and S. Gonda, Appl. Surface Sci. 60/61 (1992) 625. [61W.T. Tsang, J. Crystal Growth 81(1987) 261.
o~o,
CRYSTAL GROWT H
Atomically controlled InGaAs/InP superlattices grown by gas source MEE (migration enhanced epitaxy) Hajime Asahi, Teruaki Kohara, Ravi Kant Soni, Kumiko Asami, Shuichi Emura and Shun-ichi Gonda The Institute of Scient4fic and Industrial Research, Osaka University, 8-], Mihogaoka, Ibaraki, Osaka 567, Japan
Atomically controlled InGaAs/InP SL structures having different types of heterointerfaces are grown on (OOl)InP substrates at 350°Cby gas source MEE (migration enhanced epitaxy). RHEED intensity traces exhibit the same shape at the positions of the same type of heterointerfaces, indicating the formation of the desired heterointerfaces. The Raman spectrum from the SL, having only the InAs-type heterointerfaces, is characterized by the absence of GaP-like LO phonon clearly suggesting the formation of only the InAs-type heterointerfaces, while the SL having InGaP-type interfaces indeed shows the presence of GaP-like LO phonon peak. 4.2 K photoluminescence (PL) spectra for the InGaAs/InP quantum well (OW) structures show a very narrow line width comparable to the narrowest line width reported so far. Furthermore, the PL peak energy variation with well thickness clearly depends on the heterointerface type.
1. Introduction The atomic arrangement at the interfaces of superlattices/ quantum well (SL/QW) structures influences their physical properties. To control the physical properties of SL/QW structures, the atomically controlled growth of the heterointerface is necessary. Lattice-matched InGaAs/InP SL/QW structures consist of two group III and two group V atoms and have two different types of heterointerfaces (InGaP and InAs types) depending on the growth sequence even when ideally formed. Recently, Vandenberg et al. confirmed the existence of these two different types of heterointerfaces with high-resolution X-ray diffraction [1].
ergy shift is observed for the OW structures prepared by different growth sequences at the interfaces [2,3], suggesting that the QW PL peak energy critically depends on the composition of the heterointerface. The optical and electrical qualities of the SL/QW structures are also influenced by the precise atomic arrangement at the interfaces. These situations are common in all of the SL/QW structures having two group III and two group V atoms, such as InGaP/GaAs, InAs/ GaSb and InAs/AlSb. To control the physical properties of InGaAs/ InP SL/QW structures, we have proposed the gas source MEE growth method, the combination of gas source MBE and migration enhanced epitaxy (MEE), where phosphine (PH3) and arsine
Under the usual growth condition, InGaAs/ InP heterointerfaces have uncontrolled interfaces, because the changeover of As and P flows is necessary and the desorption of P (As) from InP (InGaAs) surface and the incorporation of As (P) into InP (InGaAs) layer near the interface are easy to occur owing to high growth temperature. In fact, photoluminescence (PL) peak en-
(AsH3) are used as group V sources, and have already reported the gas source MEE growth of InP, InGaAs and InGaAs/InP heterostructures on (001) InP substrates [4,5]. In this paper, we report the gas source MEE growth of atomically controlled InGaAs/InP superlattices and their optical properties. Atomically controlled growths are monitored with RHEED intensity oscilla-
0022-0248/93/$06.00 © 1993
—
Elsevier Science Publishers B.V. All rights reserved
H. Asahi eta!. / Atomically controlled lnGaAs /InP SLs grown by gas source MEE
tions. Grown layers are characterized by FL and Raman scattering measurements.
(a) InP growth
195
Tsub= 350°C
In
3sec
3.8sec
2. Experimental procedure
~
3~ec
3sec
I.m_..I,.i
~
I
I
~
I
~I
(O.25sccm)
The gas source MBE system (ANELVA GBE83O) evacuated by an oil diffusion pump with a liquid nitrogen trap was used in this experiment. Elemental indium (In) and gallium (Ga), and phosphine (PH 3) and arsine (AsH3) were used as group III and group V sources, respectively. PH3 and AsH3 were introduced into the growth chamber through mass flow controllers (MFCs) ad gas cracker cells (cracking temperature 900°C). The substrates used were Fe-doped (001)InP. After thermal cleaning, InP buffer layers, 60 nm thick, were grown at 480°Cwith a PH3 flux of 0.80 SCCM by conventional gas source MBE method. The PH3 and AsH3 gas flows were rapidly supplied or interrupted by using electronically operated two-way valves located close to the gas cracker cells. The gas source MEE growth was carried out at 350°Cwith PH3 and AsH3 fluxes of 0.25 and 0.15 SCCM, respectively, PL measurement was carried out at 4.2 K using a He—Ne gas laser (632.8 nm, 15 mW) as an excitation light source. Raman scattering spectra were measured at room temperature in a backscattering geometry. The excitation source was an Ar-ion laser (488.0 nm, 500 mW). The propagation and polarization of the incident laser beam were along the [1001and [0101directions of the crystal, respectively,
3. Results and discussion 3.1. Gas source MEE growth of InP and InGaAs The gas source MEE growth of InP and InGaAs was already reported elsewhere [4,51. Here, we summarize the results briefly. It was found that even at a substrate temperature as low as 350°C, the desorption of the some amount of phosphorus from the InP surface occurs when the PH3 flow is interrupted, although the RHEED
(b) InGaAs growth
35O °~
Tsub=
ri
In,Ga 2
‘~‘
Ash3
I
‘~
I U
4.5sec
~2sec ~‘m~.’
I
ii
I
(O.l5sccm)
Fig. 1. Deposition sequences for the gas source MEE growth of (a) InP and (b) InGaAs.
pattern is still showing the (2 X 4) reconstructions. On the other hand, with a PH3 flow of 1 SCCM at 350°C,the (2 X 4) patterns became diffused (2 X 1) showing an additional adsorption of P on the surface. As a result, the perfectly alternating supply of In and P was achieved only when PH3 is supplied with a proper interruption time before the supply of In. Fig. la shows the deposition sequence used here for the growth of InP. The PH3 interruption time is 3s; 3.8 s corresponds to the supply time of 1 atomic layer of In atoms. Under this deposition sequence, the RHEED intensity oscillation continued with the same amplitude for a long period. In the growth of InGaAs, the proper interruption time ( 1 s) between the In,Ga supply and the AsH3 supply was necessary in order to achieve the perfectly alternating supply of In,Ga and As. This indicates that at low ‘ub of 350°Cit takes more than 1 s for In,Ga atoms to migrate to the proper sites. Although the interruption of the AsH3 supply before the In,Ga supply was not necessary, the supply of a proper amount of AsH3 was important. The (2 x 4) reconstruction was observed at 350°Cwhen a proper AsH3 flow was supplied, while the (2 x 3) reconstruction was observed when an excess AsH3 flow was supplied. The RHEED intensity oscillation during the growth of InGaAs under the (2 X 3) condition persisted only a short duration. Under the (2 x 4)
H. Asahi et a!. / Atomically controlled inGaAs
196
condition, the RHEED intensity oscillation continued with the same amplitude for a long period, This suggests that at low ‘~uh the desorption of As does not occur and that the amount of supplied As is needed to be precisely controlled. Fig. lb shows the deposition sequence used here for the growth of InGaAs. 3.2. Gas source MEE growth of InGaAs /InP superlattices Based on the above results, the gas source MEE growth of InGaAs/InP superlattices was studied. Fig. 2a shows RHEED intensity traces during the gas source MEE growth of (InGaAs)m(InP)n superlattices at Tsub of 350°C (m n 10 in the upper trace and m n 11 in the lower trace). The arrows indicate the heterointerface positions. The RHEED intensity oscillations continued with the same amplitude for over 1 h. In particular, at the positions of heterointerfaces, the RHEED intensity trace exhibited completely the same shapes at any time, as can be seen in fig. 2a. The RHEED intensity trace during the growth of InP and InGaAs also showed its own particular shape for the individual layer growth. When growing the InGaAs/InF heterostructures, heterointerfaces are classified into 4 different types. They are As—In type and InGa—P type =
=
=
As-In
P-InGa (InP),
5
As-In
(a)
=
/ InP SLs grown by gas source MEE
in the transition from lnGaAs layer to InP layer, and P—InGa type and In—As type in the transition from InP layer to InGaAs layer. Fig. 2b shows RHEED intensity traces during the growth of (InGaAs)rn (InP)~superlattices (m 9.5, ‘~ 7.5 in the upper trace, and m 9.5, n 10.5 in the lower trace) having different combinations of heterointerfaces from those in fig. 2a. The RHEED intensity traces at the individual heterointerface positions and in the individual layers are the same as the individual ones in fig. 2a. It is noteworthy that the RHEED intensity traces in fig. 2 show the transition region of about 1.5 MLs for all of the different types of heterointerfaces. This means that the intermixing of As—P atoms during growth does not occur in this growth method, and indicates that the information of RHEED intensity oscillation contains that of nearly 1.5 MLs. Similar results were also obtamed for the different sets of m and n in (InGaAs)m(InP)n superlattices. =
=
3.3. Characterization of InGaAs /InP quantum well structures Fig. 3 shows Raman spectra from the InGaAs/InP OW structures having different types of heterointerfaces. All samples have 2, 4, 7, 10 and 15 ML thick InGaAs OW layers separated by 25 MLs of InP layers. Sample (a) has P—InGa and
InGa-P
~(InGaAs),st
InGa-P
In-As (InP),,
InGa-P
(b)
(lnP)~ (InGaAs)~
InGa-P
As-In
~(InGaAs),, ~
120 TIME(sec 1
P-InGa
(InP)~5 ~(InGaAsJ
(lnP)~ (lnGaAs)n~ m
=
=
n-As (InP)~,
240
As-In
~InGaAs)~,~
120 TIME(sec)
240
Fig. 2. RHEED intensity traces during the gas source MEE growth of (InGaAs),/InP), superlattices having tailored heterointerfaces at L.5h of 350°C: (a) m=n = 10, and m = n=11; (b) m=9.5, n= 7.5, and m = 9.5, n = 10.5. The arrows indicate heterointerface positions.
H. Asahi et a!. / Atomically controlled InGaAs /InP SLs grown by gas source MEE
As—In type heterointerfaces at the transition from the InP layer to the InGaAs layer and at the transition from the InGaAs layer to the InP layer respectively. Sample (b) has P—InGa and InGa—P type heterointerfaces. Sample (c) has In—As and InGa—P types, and the sample (d) has In—As and As—In types. Four peaks are observed in the frequency range of 200—400 cmi. They correspond to the InAs-like (221.1 cm 1), GaAs-like (268.7 cm’), InP-like (353.8 cm1) and GaP-like (384.7 cm~) LO phonon peaks, respectively, as indicated by arrows in fig. 3. Samples (a), (b) and (c) have InAs, GaAs, InP and GaP bonds. The GaP bonds exist only at the InGa—F type heterointerfaces. Sample (d) has InAs, GaAs and InP bonds. The Raman spectrum for sample (d) shows InAs-like, GaAs-like and InP-like LO phonon peaks. However, the GaP-like LU phonon peak could not be observed. This strongly mdicates that tailored heterointerfaces can be grown by gas source MEE at 350°C. Fig. 4 shows the 4.2 K PL spectrum from the InGaAs/InP QW structure wafer. The sample has P—InGa type and As—In type heterointer-
InAs-Uke LO
lnP-like LO
GaA 1s-Uke ~
GaP-likeLO
(a) >‘—
z
1.2 1.1 1.0 1) lnGaAs/InP OW I— 4.2 K
197
ENERGY (eV) 0.9 0.8
Z
0.7
lnGaAs reference
15ML 4ML 1OML x~ 2ML 7ML
>-
XiS
(I)
I
134 I—
a. tO
1.2 1.4 1.6 WAVELENGTH (Fm)
1.8
Fig. 4. 4.2 K PL spectra for the gas source MEE grown InGaAs/InP QWs showing very narrow line width comparable to the narrowest one reported so far (ref. [6]).
faces. The PL peaks from each OW layer as well as the InGaAs reference layer grown between the OW structure and the InP substrate are observed. The full widths at half maximum (FWHMs) for the 2, 4, 7 and 10 MLs OW layers are 12, 8, 7 and 6 meV, respectively. These values are comparable to the narrowest FWHMs reported so far [61. It is noteworthy that the gas source MEE InGaAs/InP QW structures were grown at ‘~ub as low as 350°C,while those re7ub ~ 500°C.This indicates highgrown quality ported in the ref. very [6] were at of OW structures grown by gas source MEE. Fig. 5 shows the PL energy shift for the InGaAs/InP OW structures having different cornbination of heterointerfaces as a function of InGaAs well thickness. They are clearly dependent on the heterointerface types and can be divided
z
tures having InGaP type heterointerfaces at both ~
into three interfaces those having (P—InGa, groups. InGaP The InGa—P), type firstatgroup one and interface the is OW second strucand is InAs type at another interface (P—InGa, As—In) ~
4~ RAMAN SHIFT (cml Fig. 3. Raman spectra from the InGaAs/InP quantum well structures having different types of heterointerfaces: (a) P InGa and As—In; (b) P—InGa and InGa—P; (c) In—As and InGa—P; (d) In—As and As—In type interfaces. InAs-like, GaAs-like, InP-like and GaP-like LO phonons are indicated by arrows,
and (In—As, InGa—P). The third is those with InAs type interfaces at both interfaces (In—As, As—In). This figure indicates that the InGaP type interface increases the energy level of QWs, while the InAs type interface decreases the energy level. This is reasonable to the theoretical consideration on the energy shift. The InGaP interface will form higher potential than that of InP and will
198
H. Asahi eta!.
/ Atomically controlled InGaAs /InP SLs grown by gas source MEE
0.5 InGaAslInP OW 4.2 K 0.4
•
P-InGa, InGa-P
0
~
P-lnGa,As-In
A
In As, InGa-P
~
InGaAs/InP quantum well structures, having only InAs-type heterointerfaces, are characterized by the absence of GaP-like LU phonons clearly suggesting the formation of only the InAs type heterointerfaces, while structures having InGaP type LU phonon peak. Consequently, it is concluded interfaces indeed show the presence of a GaP-like
~0.3 ~~~A,As-In U)
w
0.2
Z
w —j
0.
0.1
that the bred heterointerfaces InGaAs/InP heterostructures can be grown by having gas source taiMEE at 350°C. Furthermore, 4.2 K FL spectra for the InGaAs/InP OW structures showed a very narrow line width comparable to the narrowest line width reported so far. The PL peak energy variation with well thickness clearly depends on the heterointerface type. This result also supports the formation of atomically controlled interfaces.
0.0 0
10 20 NUMBER OF InGa ATOMIC LAYERS
Fig. 5. InGaAs well thickness dependence of PL energy shift for the InGaAs/InP QW structures having different combination of heterointerfaces.
reduce the bond length of InGaAs near the interface. These effects both increase the energy level of OWs. This result also supports the formation of atomically controlled interfaces. A comparison with the experimental energy shift and the theoretical one will be reported elsewhere.
4. Summary We have studied the gas source MEE growth of InGaAs/InP SL/OW structures at 350°C. RHEED intensity traces exhibited the same shape at the positions of the same type of heterointerfaces, indicating the formation of the desired heterointerfaces. The Raman spectra from the
Acknowledgement This work was supported in part by Grant-inAid for Scientific Research on Priority Area, “Electron Wave Interference Effects in Mesoscopic Structures” and the Scientific Research Grant-in-Aid #02650014 from the Ministry of Education, Science and Culture of Japan.
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