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InP multi quantum wells by Raman spectroscopy, X-ray diffractometry and photoluminescence
c~D
~
CRYSTAL GROWTH
ELSEVIER
Journal of Crystal Growth 145 (1994) 813—818
Characterization of the interface abruptness of 1n053Ga047As/InP multi quantum wells by Raman spectroscopy, X-ray diffractometry and photoluminescence J. Geurts
a,* J. Finders a J• Woitok a D. Gnoth a A. Kohl b K. Heime b j Physikalisches In.stitut, RWTHAachen, Sommerfeldstrasse 28, D-52056 Aachen, Germany b Institutfür Haibleitertechnik, RWTHAachen, Templergraben 55, D-52056 Aachen, Germany a
Abstract For lattice-matched InGaAs/InP multi quantum well structures, the interface abruptness was investigated by a combination of X-ray diffractometry, Raman spectroscopy and photoluminescence. The focus was on the effects of the gas switching parameters at the InGaAs-to-InP interface, especially the PH3 and H2 purging times. Ternary InAsP and quaternary InGaAsP interface layers due to carry-over and exchange effects were directly identified. Their thicknesses drastically depend on the PH3 purging time. H2 purging affects the interface quality to some degree, but it has only minor effects on the chemical composition at the interfaces. 1. Introduction InGaAs/InP multi quantum wells (MQWs) are of great interest for optoelectronics in the near infrared and for electronic applications such as high electron mobility transistors (HEMTs) [1]. As a suitable growth technique for these MQWs, metalorganic vapour phase epitaxy (MOVPE) is often used. However, in MOVPE the switching of the reactive gases easily leads to interface layers, whose thickness and composition may drastically depend on the switching sequence of the reactive gases. For optimum device performance the switching sequence of the reactive gases has to be adjusted. In previous studies, we have shown that the interface layers are directly detected in Ra-
*
Corresponding author.
man spectroscopy from their characteristical lattice vibrations which lead to additional modes [2]. The composition of the interface layers is deduced from the frequencies and/or the intensity ratio of these modes. An unambiguous quantitative determination of composition as well as thickness of the interface layers is obtained by combining the Raman results with those from double-crystal X-ray diffraction (DCXD). In the latter technique a number of well defined satellites are observed besides the 0-order peak of the MQW. From the 0-order peak, the mean perpendicular lattice constant of the MQW can be calculated in a straightforward manner, while the angular separation of the satellite peaks yields the MQW periodicity [31.The interface layer thickness and composition are obtained from a fit of the satellite peak intensities, based on a dynamical X-ray scattering model. The Raman- and Xray derived results allow the interpretation of a
0022-0248/94/$07.OO © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00278-T
814
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
variation of the E1—H1 transition energy in photoluminescence in terms of changes of interfacelayer thickness and composition. In this study, we focus on the InGaAs-to-InP interface. It is well known that at this interface an As carry-over effect may occur: the incorporation of residual As atoms into the following InP layer leads to the formation of an intermediate InAsP layer [4]. To avoid this carry-over effect, purging the reactor with either H2 or PH3 prior to the InP layer growth seems effective. However, cxtensive PH3 purging might lead to P-incorporation into the freshly grown ternary InGaAs layer, resulting in quaternary InGaAsP. H2 purging might lead to a degradation of the InGaAs surface. In order to study such effects on the interface properties, we apply a combination of Raman spectroscopy, photolurninescense and X-ray diffractometry to characterize a series of samples for which the H2 and PH3 purging times at the InGaAs-to-InP interface were systematically varied. The results of these optical techniques will be compared to electronic performance data of the quantum well structures.
2. Experimental procedure The MOWs were grown on InP(100) substrates at 910 K with a total gas pressure of 20 hPa and a gas flux velocity of 1.2 rn/s in a horizontal quartz reactor, using the precursors TMIn, TMGa, PH3 and AsH3. The MOWs consisted of 10 times 2 nm In0 53Ga047As and 7 nm InP. After growing the MOW a 15 nm InP capping layer was deposited. At the InP-to-InGaAs interface we used a constant switching sequence: after switching off the TMIn source, the surface was stabilized for 1 s with PH3. Immediately after this the reactor was purged for 1 s with AsH3 before the start of the InGaAs growth. The Raman spectra were recorded in near-back-scattering geometry. The scattered light was analysed using a double monochromator (Spex 1403) equipped with an optical multichannel analyser (EG&G 1462) and a GaAs photomultiplier tube (RCA-31034). As a light source we used the 488 nm line of an Ar~ ion laser, which was focussed by a cylindrical
lens. The input power was 100 mW. For this laser line the penetration depth is about 100 nm, which is equal to the stack thickness. Thus all single quantum wells and their possible interface layers contribute to the Raman signal. We used a slit width of 150 ,am, giving a spectral resolution of about 1 cm Photoluminescence was excited using the 514 nm line of the Ar~ion laser. The PL spectra were taken at 90 K. X-ray diffraction measurements were carried out on a home-made double-crystal diffractorneter with a Si (004) monochrornator crystal. The MQWs were analysed using the symmetrical (004) substrate reflection with Cu-Ka1 radiation. The data were collected in ~9—2@scans with a step width of 0.002°,while the detector aperture was about 0.07°. .
3. Results and discussion The ability of Raman spectroscopy to identify the interface layers is demonstrated in Fig. 1. Here the typical net contribution from the interface layers to the Raman spectrum of an InGaAs/InP MOW grown with a long PH3 purging time (t = 7 s) is plotted. It was obtained by subtracting the measured Raman spectra of bulk-like InGaAs and bulk InP from the Raman spectrum of the MOW. The remaining structures belong to InAs-like, GaAs-like, InP-like and GaP-like longitudinal optical vibration modes (in the following abbreviated as In—As, Ga—As, In—P and Ga—P
Cia-As
In-P~ GP
In-As
220
240
260
280
330
340
350
360
370
frequency shift (1/cm) Fig. 1. Contribution of the In—As, Ga—As, Ga—P and In—P lattice vibrations from the interface layers to the Raman spectrum of an lnGaAs/InP MOW for 7 s PH3 purging time.
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
mode, respectively). Such modes are characteristic for ternary and quaternary compounds such as
06 0
InAsP and InGaAsP [5]. Their frequencies and relative intensities reflect the composition of the interface layers. In this manner we can distin-
effects guish between and quaternary ternary InGaAsP InAsP tocarry-over exchange reactions. spectrum in Fig.due 1due is to identified as quaternaryThe InGaAsP. Further details about its vibration modes have been published elsewhere [2]. The systematic variation of the PH3 purging time between 1 and 20 s (without prior 2 purging) results in clear effects on the interface layer, as can be deduced from the corresponding Raman peaks. For very short PH3 purging times (1 s) a characteristic In—As vibration near 225 cm’ is observed. It stems from InAsP, formed due to the As carry-over, leading to As incorporation into the following InP layer. For longer purging times, this carry-over effect is strongly reduced and the 225 cm~ Raman peak disappears. Instead, the exchange of As by P in the freshly grown InGaAs leads to the formation of quaternary InGaAsP. Since forthe InGaAsP 1 and Ga—As the modeIn—As near mode near 240 cm 260 cm-i have the best-defined peak shapes, as can be seen in Fig. 1, we employed them for the estimation of the quaternary layer thickness. For this purpose we evaluated their intensity, normalized to the intensity of the Ga—As vibration of the InGaAs wells. The development of these normalized In—As and Ga—As intensities as a function of the PH3 purging time is shown in Fig. 2. A distinct increase is observed, followed by a saturation for purging times beyond 10 s. Also shown in Fig. 2 is the dependence of the In—As vibration of ternary InAsP, which essentially vanishes for times beyond 4 s. In DCXD, the existence of interface layers is essentially reflected in a net strain of the nominally lattice-matched
815
05
E
~OIn~AsI~ • ___________
In-As .-~.
.
Pft3purgingtimet8(s)
Fig. 2. Normalized intensities of the In—As and Ga—As modes in the Raman spectra of the InGaAsP and InAsP interface layers for different PH3 purging times at the InGaAs-to-InP interface.
The exchange-induced InGaAsP layer results in a reduction of the effective well width, since it grows at the expense of the InGaAs well layer. The reduction of the quantum well width should lead to an upward shift of the electronic energy levels and therefore to an increase of the E1—H1 transition energy in photoluminescence. The plot in Fig. with 3 indeed shows PH an increasing transition energy increasing 3 purging time. This underscores the importance of well-controlled interfaces when the MOWs are applied in optoelectronics. From the optical analysis results described above we conclude that for a PH3 purging time of about 3 s the formation of InAsP due to As carry-over is essentially suppressed, while the formation of InGaAsP owing to exchange reactions
=
MQW, which is observed from the 0-order peak position. With increasing PH3 purging, the strain gradually varies from compressive (InAsP-induced) to tensile (InGaAsP-induced). An InGaAsP saturation thickness of 4 monolayers was obtained from the combination of the Raman data with DCXD results [6].
930 .
.
8
(s) Fig. 3. E1—H1 transition energies from photoluminescence spectra for different PH3 purging times at the InGaAs-to-InP interface. PH, purging time t6
816
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818 05
5.970
~0.4I I
2ç
= 0,3 ~ ‘5
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_________________ • • 0 •
——
In-As Ga-As
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960
~ 1) =
955
•
•
I
0
2940
0
10
5
15
= 930
U U
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2 purging time t~(s) Fig. 4. Dependence of the normalized intensities of the In—As and Ga—As interface layers modes on the H2 purging time at the InGaAs-to-InP interface.
is still minor. Besides, we observed from a series of HEMTs that the electrical performance (e.g., fT~ ‘DSS’ grnext) is significantly improved for these growth conditions [7]. Since PH3 purging at the InGaAs-to-InP interface inevitably leads to the formation of a ternary InAsP or quaternary InGaAsP interface layer, H2 purging may be an alternative to suppress the As carry-over. However, since there is no group-V stabilizing during this growth interruption, a degradation of the InGaAs surface may occur during the H2 purging. In the same manner as for PH3 purging, the H2 purging time was optimized using the combination of Raman spectroscopy, PL and X-ray diffractometry as a sensitive monitor for detecting the interface layers. For this study the H2 purging time was varied between 0 and 20 s, while the PH3 purging before the start of the next InP layer growth was kept constant at a value of 1 In Fig. 4 the intensity of the In—As and Ga—As vibrations of the interface layers, normalized to the Ga—As mode of the quantum well, is plotted as a function of the H2 purging time. In contrast to PH3 purging, variation of the H2 purging time does not lead to a significant change of the mode intensities. In particular, the intensity of the In— As mode, which is very sensitive to As carry-over into the InP, is not altered by the H2 purging. The PL spectra show distinct peaks at two discrete E1—H1 transition energies, owing to indicates the high quality of the interfaces. The monolayer fluctuations of the well thickness. This H2 purging-time dependence of both transition energies is plotted in Fig. 5. They show only minor variations within a range of 10 meV, which is in good agreement with the constant well width, ~.
_____________________________________________________
W
5
‘5
a
20
H2 purging time t~(s)
Fig. 5. Dependence of the E1—H1 transition energies from PL on the H2 purging time at the InGaAs-to-InP interface.
which could be deduced from the Raman spectroscopy results. Furthermore, we did not observe any degradation of the InGaAs layers for increasing H2 purging time in Raman spectroscopy. For all purging times the observed FWHM of the Ga—As lattice vibration of the InGaAs wells is comparable to the FWHM observed for optimized bulk-like InGaAs layers, thus indicating the good crystalline quality of the InGaAs layers. This is in full agreement with DCXD results. The X-ray profile in Fig. 6 shows well-defined and narrow satellites as well as distinct “Pendellösung” fringes, reflecting an excellent crystalline quality, in-depth homogeneity and interface sharpness of the MOW. Besides from the position of the satellites in X-ray diffractometry, the periodicity of the MOW was also deduced from the frequency of the folded acoustic phonons in Raman spectroscopy. The folded phonons are a result of the reduction of the Brillouin zone for MOWs due to the in-
nP (004)
0
-~
290
30
oo
330
(degree)
Fig. 6. X-ray diffraction profile for a H2 purging time of 4 s at the InGaAs-to-InP interface.
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
•
817
thickness of d 2 nm we observe this mode near 270 cm 1, whereas for bulk-like InGaAs layers of thethe same composition, the frequency of the Ga— in MOW [8]. For MOWs with a nominal well As mode lies at 273 cm’ (at 90 K). For some samples a weak structure near 264 cm1 is observed, which we attribute to the L0 3 mode. Scattering from the second confined Ga—As mode (m 2, L02) is symmetry-allowed in 100 =
~
-1 ‘
-2
2 .3 +3
0
10
20
30
40
50
60
=
frequency shift (1/cm)
.
Fig. 7. Raman spectrum in the region of the folded acoustic modes
creased periodicity length (in our case 9 nm). Therefore, their eigenfrequencies and peak widths are very sensitive to changes or inhomogeneities of the periodicity length. A Raman spectrum of a MOW in the region of the folded acoustical phonons is shown in Fig. 7. The very narrow peaks clearly indicate the homogeneity of the MOW periodicity. From the peak positions we obtained a periodicity length of 9.0 nm, which agrees very well with the value of 8.91 nm which we obtained from DCXD results of the same sample. We therefore conclude that our gas switching procedure allows the fabrication of reproducible and well-controlled interfaces, The abruptness of the interfaces is further confirmed by the observation of confined optical modes in the InGaAs wells. Confinement of the optical vibrations is commonly observed in superlattices when the frequencies of the optical modes of both constituents do not match, i.e. the optical modes of one material cannot propagate in the other one. For InGaAs/InP MOWs, the Ga—As mode of the InGaAs wells cannot propagate in the InP layers, since its frequency is below the frequencies of the optical modes of InP. Because of this confinement, the InGaAs wells of thickness d should give rise to confined optical Ga—As modes with wave vector values k mir/d (m 1, 2, ...). Their frequencies w are determined by the phonon dispersion relation w(k) for bulk InGaAs. The frequency of the first confined Ga— As mode (m 1, L01 mode), which is the main feature in the Raman spectra of InGaAs layers for 100(010,001)100 polarization, depends characteristically on the thickness of the InGaAs layers
.
.
(010,010)100 polarization. Indeed, in this configuration we observe a peak at 267 cm while the LO~vanishes. The appearance of well-defined Ga—As confined modes up to the third order in the InGaAs layers indicates the abruptness of the interfaces. A gradual transition of the group III and group V compositions between InGaAs well and InP barrier would allow a coupling between the modes of both layers and therefore disturb phonon confinement. The intensity of the L02 and L03 modes of the InGaAs well varied with H2 purging time. This result indicates that the confined modes are also sensitive to other effects than changes of the interface thickness. Such effects may, e.g., be charged defects or interface states. Recently we proved by frequency dependent C—V measurements the dependence of trap concentration on the switching sequence in quantum well structures [9]. The role of interface states on the intensity variations of the confined modes is subject of further investigations. Effects of the 2 purging time were also observed in the intensity of the “Pendellosung” fringes which occur in the X-ray diffraction profiles (see Fig. 6). The most pronounced fringes occur for 2 purging between 4 and 7 s. These observations mdicate that the best interfaces result for these H2 purging times. .
—
,
4. Conclusions
=
=
In conclusion, we have shown from the combination of X-ray diffractometry, Raman spectroscopy and photoluminescence that the purging times of PH3 as well as of H2 are important parameters for the InGaAs-to-InP interfaces. PH3 purging reduces the As carry-over into the InP layers, but increasing the PH3 purging time leads
818
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
to an increased exchange of As by P in the InGaAs layers, resulting in quaternary InGaAsP. For purging times around 3 s, the carry-over and exchange effects are in the same order. The effects of H2 purging on the chemical composition at the interfaces is far less pronounced. However, minor differences in the Raman spectra and X-ray diffraction profiles indicate purging effects on the interface quality. Best results were obtained for H2 purging between 4 and 7 S.
References [1] A. Mesquida Küsters, A. Kohl, R. Muller, V. Sommer and K. Heime, IEEE Electron Device Lett. EDL-14 (1992) 36. [2] J. Finders, M. Keuter, D. Gnoth, J. Geurts, J. Woitok, A.
Kohl, R. Muller and K. Heime, Mater. Sci. Eng. B 21 (1993) 161. [3] R. Meyer, M. Hollfelder, H. Hardtdegen, B. Lengeler and H. Lüth, J. Crystal Growth 124 (1992) 583. [4] J.M. Vandenberg, A.T. Macrander, R.A. Hamm and MB. Panish, Phys. Rev. B 44 (1991) 3991. [5] T.P. Pearsall, R. Charles and iC. Portal, AppI. Phys. Lett. 42 (1983) 436. [6] J. Finders, D. Gnoth, M. Keuter, J. Geurts, J. Woitok, A. Kohl, R. Muller and K. Heime, in: Proc. ICFSI-4, 1993 (World Scientific, Singapore, 1994) p. 538. [71A. Kohl, A. Mesquida Kiisters, R. Muller, S. Brittner, K. Heime, J. Finders, M. Keuter and J. Geurts, in: Proc. Conf. on InP and Related Materials, Paris, 1993. [8] J. Finders, J. Geurts, D. Gnoth, A. Kohl and K. Heime, to be published [9] A. Kohl, A. Mesquida Kiisters, S. Brittner, K. Heime, J. Finders, D. Gnoth, J. Geurts and J. Woitok, in: Proc. Conf. on InP and Related Materials, Santa Barbara, CA, 1994, in press.
~
CRYSTAL GROWTH
ELSEVIER
Journal of Crystal Growth 145 (1994) 813—818
Characterization of the interface abruptness of 1n053Ga047As/InP multi quantum wells by Raman spectroscopy, X-ray diffractometry and photoluminescence J. Geurts
a,* J. Finders a J• Woitok a D. Gnoth a A. Kohl b K. Heime b j Physikalisches In.stitut, RWTHAachen, Sommerfeldstrasse 28, D-52056 Aachen, Germany b Institutfür Haibleitertechnik, RWTHAachen, Templergraben 55, D-52056 Aachen, Germany a
Abstract For lattice-matched InGaAs/InP multi quantum well structures, the interface abruptness was investigated by a combination of X-ray diffractometry, Raman spectroscopy and photoluminescence. The focus was on the effects of the gas switching parameters at the InGaAs-to-InP interface, especially the PH3 and H2 purging times. Ternary InAsP and quaternary InGaAsP interface layers due to carry-over and exchange effects were directly identified. Their thicknesses drastically depend on the PH3 purging time. H2 purging affects the interface quality to some degree, but it has only minor effects on the chemical composition at the interfaces. 1. Introduction InGaAs/InP multi quantum wells (MQWs) are of great interest for optoelectronics in the near infrared and for electronic applications such as high electron mobility transistors (HEMTs) [1]. As a suitable growth technique for these MQWs, metalorganic vapour phase epitaxy (MOVPE) is often used. However, in MOVPE the switching of the reactive gases easily leads to interface layers, whose thickness and composition may drastically depend on the switching sequence of the reactive gases. For optimum device performance the switching sequence of the reactive gases has to be adjusted. In previous studies, we have shown that the interface layers are directly detected in Ra-
*
Corresponding author.
man spectroscopy from their characteristical lattice vibrations which lead to additional modes [2]. The composition of the interface layers is deduced from the frequencies and/or the intensity ratio of these modes. An unambiguous quantitative determination of composition as well as thickness of the interface layers is obtained by combining the Raman results with those from double-crystal X-ray diffraction (DCXD). In the latter technique a number of well defined satellites are observed besides the 0-order peak of the MQW. From the 0-order peak, the mean perpendicular lattice constant of the MQW can be calculated in a straightforward manner, while the angular separation of the satellite peaks yields the MQW periodicity [31.The interface layer thickness and composition are obtained from a fit of the satellite peak intensities, based on a dynamical X-ray scattering model. The Raman- and Xray derived results allow the interpretation of a
0022-0248/94/$07.OO © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00278-T
814
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
variation of the E1—H1 transition energy in photoluminescence in terms of changes of interfacelayer thickness and composition. In this study, we focus on the InGaAs-to-InP interface. It is well known that at this interface an As carry-over effect may occur: the incorporation of residual As atoms into the following InP layer leads to the formation of an intermediate InAsP layer [4]. To avoid this carry-over effect, purging the reactor with either H2 or PH3 prior to the InP layer growth seems effective. However, cxtensive PH3 purging might lead to P-incorporation into the freshly grown ternary InGaAs layer, resulting in quaternary InGaAsP. H2 purging might lead to a degradation of the InGaAs surface. In order to study such effects on the interface properties, we apply a combination of Raman spectroscopy, photolurninescense and X-ray diffractometry to characterize a series of samples for which the H2 and PH3 purging times at the InGaAs-to-InP interface were systematically varied. The results of these optical techniques will be compared to electronic performance data of the quantum well structures.
2. Experimental procedure The MOWs were grown on InP(100) substrates at 910 K with a total gas pressure of 20 hPa and a gas flux velocity of 1.2 rn/s in a horizontal quartz reactor, using the precursors TMIn, TMGa, PH3 and AsH3. The MOWs consisted of 10 times 2 nm In0 53Ga047As and 7 nm InP. After growing the MOW a 15 nm InP capping layer was deposited. At the InP-to-InGaAs interface we used a constant switching sequence: after switching off the TMIn source, the surface was stabilized for 1 s with PH3. Immediately after this the reactor was purged for 1 s with AsH3 before the start of the InGaAs growth. The Raman spectra were recorded in near-back-scattering geometry. The scattered light was analysed using a double monochromator (Spex 1403) equipped with an optical multichannel analyser (EG&G 1462) and a GaAs photomultiplier tube (RCA-31034). As a light source we used the 488 nm line of an Ar~ ion laser, which was focussed by a cylindrical
lens. The input power was 100 mW. For this laser line the penetration depth is about 100 nm, which is equal to the stack thickness. Thus all single quantum wells and their possible interface layers contribute to the Raman signal. We used a slit width of 150 ,am, giving a spectral resolution of about 1 cm Photoluminescence was excited using the 514 nm line of the Ar~ion laser. The PL spectra were taken at 90 K. X-ray diffraction measurements were carried out on a home-made double-crystal diffractorneter with a Si (004) monochrornator crystal. The MQWs were analysed using the symmetrical (004) substrate reflection with Cu-Ka1 radiation. The data were collected in ~9—2@scans with a step width of 0.002°,while the detector aperture was about 0.07°. .
3. Results and discussion The ability of Raman spectroscopy to identify the interface layers is demonstrated in Fig. 1. Here the typical net contribution from the interface layers to the Raman spectrum of an InGaAs/InP MOW grown with a long PH3 purging time (t = 7 s) is plotted. It was obtained by subtracting the measured Raman spectra of bulk-like InGaAs and bulk InP from the Raman spectrum of the MOW. The remaining structures belong to InAs-like, GaAs-like, InP-like and GaP-like longitudinal optical vibration modes (in the following abbreviated as In—As, Ga—As, In—P and Ga—P
Cia-As
In-P~ GP
In-As
220
240
260
280
330
340
350
360
370
frequency shift (1/cm) Fig. 1. Contribution of the In—As, Ga—As, Ga—P and In—P lattice vibrations from the interface layers to the Raman spectrum of an lnGaAs/InP MOW for 7 s PH3 purging time.
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
mode, respectively). Such modes are characteristic for ternary and quaternary compounds such as
06 0
InAsP and InGaAsP [5]. Their frequencies and relative intensities reflect the composition of the interface layers. In this manner we can distin-
effects guish between and quaternary ternary InGaAsP InAsP tocarry-over exchange reactions. spectrum in Fig.due 1due is to identified as quaternaryThe InGaAsP. Further details about its vibration modes have been published elsewhere [2]. The systematic variation of the PH3 purging time between 1 and 20 s (without prior 2 purging) results in clear effects on the interface layer, as can be deduced from the corresponding Raman peaks. For very short PH3 purging times (1 s) a characteristic In—As vibration near 225 cm’ is observed. It stems from InAsP, formed due to the As carry-over, leading to As incorporation into the following InP layer. For longer purging times, this carry-over effect is strongly reduced and the 225 cm~ Raman peak disappears. Instead, the exchange of As by P in the freshly grown InGaAs leads to the formation of quaternary InGaAsP. Since forthe InGaAsP 1 and Ga—As the modeIn—As near mode near 240 cm 260 cm-i have the best-defined peak shapes, as can be seen in Fig. 1, we employed them for the estimation of the quaternary layer thickness. For this purpose we evaluated their intensity, normalized to the intensity of the Ga—As vibration of the InGaAs wells. The development of these normalized In—As and Ga—As intensities as a function of the PH3 purging time is shown in Fig. 2. A distinct increase is observed, followed by a saturation for purging times beyond 10 s. Also shown in Fig. 2 is the dependence of the In—As vibration of ternary InAsP, which essentially vanishes for times beyond 4 s. In DCXD, the existence of interface layers is essentially reflected in a net strain of the nominally lattice-matched
815
05
E
~OIn~AsI~ • ___________
In-As .-~.
.
Pft3purgingtimet8(s)
Fig. 2. Normalized intensities of the In—As and Ga—As modes in the Raman spectra of the InGaAsP and InAsP interface layers for different PH3 purging times at the InGaAs-to-InP interface.
The exchange-induced InGaAsP layer results in a reduction of the effective well width, since it grows at the expense of the InGaAs well layer. The reduction of the quantum well width should lead to an upward shift of the electronic energy levels and therefore to an increase of the E1—H1 transition energy in photoluminescence. The plot in Fig. with 3 indeed shows PH an increasing transition energy increasing 3 purging time. This underscores the importance of well-controlled interfaces when the MOWs are applied in optoelectronics. From the optical analysis results described above we conclude that for a PH3 purging time of about 3 s the formation of InAsP due to As carry-over is essentially suppressed, while the formation of InGaAsP owing to exchange reactions
=
MQW, which is observed from the 0-order peak position. With increasing PH3 purging, the strain gradually varies from compressive (InAsP-induced) to tensile (InGaAsP-induced). An InGaAsP saturation thickness of 4 monolayers was obtained from the combination of the Raman data with DCXD results [6].
930 .
.
8
(s) Fig. 3. E1—H1 transition energies from photoluminescence spectra for different PH3 purging times at the InGaAs-to-InP interface. PH, purging time t6
816
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818 05
5.970
~0.4I I
2ç
= 0,3 ~ ‘5
r
I
F- •
_________________ • • 0 •
——
In-As Ga-As
o
4J
>,
960
~ 1) =
955
•
•
I
0
2940
0
10
5
15
= 930
U U
•
H
2 purging time t~(s) Fig. 4. Dependence of the normalized intensities of the In—As and Ga—As interface layers modes on the H2 purging time at the InGaAs-to-InP interface.
is still minor. Besides, we observed from a series of HEMTs that the electrical performance (e.g., fT~ ‘DSS’ grnext) is significantly improved for these growth conditions [7]. Since PH3 purging at the InGaAs-to-InP interface inevitably leads to the formation of a ternary InAsP or quaternary InGaAsP interface layer, H2 purging may be an alternative to suppress the As carry-over. However, since there is no group-V stabilizing during this growth interruption, a degradation of the InGaAs surface may occur during the H2 purging. In the same manner as for PH3 purging, the H2 purging time was optimized using the combination of Raman spectroscopy, PL and X-ray diffractometry as a sensitive monitor for detecting the interface layers. For this study the H2 purging time was varied between 0 and 20 s, while the PH3 purging before the start of the next InP layer growth was kept constant at a value of 1 In Fig. 4 the intensity of the In—As and Ga—As vibrations of the interface layers, normalized to the Ga—As mode of the quantum well, is plotted as a function of the H2 purging time. In contrast to PH3 purging, variation of the H2 purging time does not lead to a significant change of the mode intensities. In particular, the intensity of the In— As mode, which is very sensitive to As carry-over into the InP, is not altered by the H2 purging. The PL spectra show distinct peaks at two discrete E1—H1 transition energies, owing to indicates the high quality of the interfaces. The monolayer fluctuations of the well thickness. This H2 purging-time dependence of both transition energies is plotted in Fig. 5. They show only minor variations within a range of 10 meV, which is in good agreement with the constant well width, ~.
_____________________________________________________
W
5
‘5
a
20
H2 purging time t~(s)
Fig. 5. Dependence of the E1—H1 transition energies from PL on the H2 purging time at the InGaAs-to-InP interface.
which could be deduced from the Raman spectroscopy results. Furthermore, we did not observe any degradation of the InGaAs layers for increasing H2 purging time in Raman spectroscopy. For all purging times the observed FWHM of the Ga—As lattice vibration of the InGaAs wells is comparable to the FWHM observed for optimized bulk-like InGaAs layers, thus indicating the good crystalline quality of the InGaAs layers. This is in full agreement with DCXD results. The X-ray profile in Fig. 6 shows well-defined and narrow satellites as well as distinct “Pendellösung” fringes, reflecting an excellent crystalline quality, in-depth homogeneity and interface sharpness of the MOW. Besides from the position of the satellites in X-ray diffractometry, the periodicity of the MOW was also deduced from the frequency of the folded acoustic phonons in Raman spectroscopy. The folded phonons are a result of the reduction of the Brillouin zone for MOWs due to the in-
nP (004)
0
-~
290
30
oo
330
(degree)
Fig. 6. X-ray diffraction profile for a H2 purging time of 4 s at the InGaAs-to-InP interface.
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
•
817
thickness of d 2 nm we observe this mode near 270 cm 1, whereas for bulk-like InGaAs layers of thethe same composition, the frequency of the Ga— in MOW [8]. For MOWs with a nominal well As mode lies at 273 cm’ (at 90 K). For some samples a weak structure near 264 cm1 is observed, which we attribute to the L0 3 mode. Scattering from the second confined Ga—As mode (m 2, L02) is symmetry-allowed in 100 =
~
-1 ‘
-2
2 .3 +3
0
10
20
30
40
50
60
=
frequency shift (1/cm)
.
Fig. 7. Raman spectrum in the region of the folded acoustic modes
creased periodicity length (in our case 9 nm). Therefore, their eigenfrequencies and peak widths are very sensitive to changes or inhomogeneities of the periodicity length. A Raman spectrum of a MOW in the region of the folded acoustical phonons is shown in Fig. 7. The very narrow peaks clearly indicate the homogeneity of the MOW periodicity. From the peak positions we obtained a periodicity length of 9.0 nm, which agrees very well with the value of 8.91 nm which we obtained from DCXD results of the same sample. We therefore conclude that our gas switching procedure allows the fabrication of reproducible and well-controlled interfaces, The abruptness of the interfaces is further confirmed by the observation of confined optical modes in the InGaAs wells. Confinement of the optical vibrations is commonly observed in superlattices when the frequencies of the optical modes of both constituents do not match, i.e. the optical modes of one material cannot propagate in the other one. For InGaAs/InP MOWs, the Ga—As mode of the InGaAs wells cannot propagate in the InP layers, since its frequency is below the frequencies of the optical modes of InP. Because of this confinement, the InGaAs wells of thickness d should give rise to confined optical Ga—As modes with wave vector values k mir/d (m 1, 2, ...). Their frequencies w are determined by the phonon dispersion relation w(k) for bulk InGaAs. The frequency of the first confined Ga— As mode (m 1, L01 mode), which is the main feature in the Raman spectra of InGaAs layers for 100(010,001)100 polarization, depends characteristically on the thickness of the InGaAs layers
.
.
(010,010)100 polarization. Indeed, in this configuration we observe a peak at 267 cm while the LO~vanishes. The appearance of well-defined Ga—As confined modes up to the third order in the InGaAs layers indicates the abruptness of the interfaces. A gradual transition of the group III and group V compositions between InGaAs well and InP barrier would allow a coupling between the modes of both layers and therefore disturb phonon confinement. The intensity of the L02 and L03 modes of the InGaAs well varied with H2 purging time. This result indicates that the confined modes are also sensitive to other effects than changes of the interface thickness. Such effects may, e.g., be charged defects or interface states. Recently we proved by frequency dependent C—V measurements the dependence of trap concentration on the switching sequence in quantum well structures [9]. The role of interface states on the intensity variations of the confined modes is subject of further investigations. Effects of the 2 purging time were also observed in the intensity of the “Pendellosung” fringes which occur in the X-ray diffraction profiles (see Fig. 6). The most pronounced fringes occur for 2 purging between 4 and 7 s. These observations mdicate that the best interfaces result for these H2 purging times. .
—
,
4. Conclusions
=
=
In conclusion, we have shown from the combination of X-ray diffractometry, Raman spectroscopy and photoluminescence that the purging times of PH3 as well as of H2 are important parameters for the InGaAs-to-InP interfaces. PH3 purging reduces the As carry-over into the InP layers, but increasing the PH3 purging time leads
818
J. Geurts et al. /Journal of Crystal Growth 145 (1994) 813—818
to an increased exchange of As by P in the InGaAs layers, resulting in quaternary InGaAsP. For purging times around 3 s, the carry-over and exchange effects are in the same order. The effects of H2 purging on the chemical composition at the interfaces is far less pronounced. However, minor differences in the Raman spectra and X-ray diffraction profiles indicate purging effects on the interface quality. Best results were obtained for H2 purging between 4 and 7 S.
References [1] A. Mesquida Küsters, A. Kohl, R. Muller, V. Sommer and K. Heime, IEEE Electron Device Lett. EDL-14 (1992) 36. [2] J. Finders, M. Keuter, D. Gnoth, J. Geurts, J. Woitok, A.
Kohl, R. Muller and K. Heime, Mater. Sci. Eng. B 21 (1993) 161. [3] R. Meyer, M. Hollfelder, H. Hardtdegen, B. Lengeler and H. Lüth, J. Crystal Growth 124 (1992) 583. [4] J.M. Vandenberg, A.T. Macrander, R.A. Hamm and MB. Panish, Phys. Rev. B 44 (1991) 3991. [5] T.P. Pearsall, R. Charles and iC. Portal, AppI. Phys. Lett. 42 (1983) 436. [6] J. Finders, D. Gnoth, M. Keuter, J. Geurts, J. Woitok, A. Kohl, R. Muller and K. Heime, in: Proc. ICFSI-4, 1993 (World Scientific, Singapore, 1994) p. 538. [71A. Kohl, A. Mesquida Kiisters, R. Muller, S. Brittner, K. Heime, J. Finders, M. Keuter and J. Geurts, in: Proc. Conf. on InP and Related Materials, Paris, 1993. [8] J. Finders, J. Geurts, D. Gnoth, A. Kohl and K. Heime, to be published [9] A. Kohl, A. Mesquida Kiisters, S. Brittner, K. Heime, J. Finders, D. Gnoth, J. Geurts and J. Woitok, in: Proc. Conf. on InP and Related Materials, Santa Barbara, CA, 1994, in press.