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Evidence for vertical superlattices grown by surface selective growth in MOMBE (CBE)
Journal of Crystal Growth 124 (1992) 186—191 North-Holland
~
CRYSTAL GROWTH
Evidence for vertical superlattices grown by surface selective growth in MOMBE (CBE) H. Heinecke, B. Baur, A. Miklis, R. Matz, C. Cremer and R. Höger Siemens Corporate Research and Development, Otto Hahn Ring 6, D-W-8000 München 83, Germany
This report shows for the first time the simultanous growth of InP/GaInAsP heterostructures having vertical and horizontal interfaces, demonstrated with multi (5) quantum well structures. The surface selective growth (SSG) in metalorganic MBE (MOMBE or CBE) enables on the one hand selective area epitaxy and on the other hand it allows to obtain vertical side walls at selectively grown structures. In addition SSG can provoke that the GaInAsP material composition is different on horizontal (100) and vertical (011) planes. This effect is utilized in order to allocate the results from focused photoluminescence measurements to the different types of superlattices. It is discussed that the proposed growth technique includes an attractive way to produce quantum wire systems besides the realization of vertical superlatices.
1. Introduction In metalorganic MBE (MOMBE) the growth mechanism is mainly determined by the kinetics of the surface reactions. Consequently the nature of the substrate type can play an important role in controlling growth rate and material composition. The selective area epitaxy (SAE), where during MOMBE no growth at all takes place on the surface of a dielectric mask, is a subcase of surface selective growth (SSG). There are several reports on the SAE of GaAs [1,2], InP [3,41 and GalnAs [4—6]using MOMBE. An important result was that in contrast to metalorganic vapour phase epitaxy (MOVPE) neither the growth rate nor the material composition is affected when GaInAs is grown only locally [3,71, since gas phase diffusion effects are not present in the MOMBE process. In order to implement the SAE into modern device concepts for the lateral integration of heterostructure devices, it is desireable to eliminate the (111) crystal planes often observed at the boundaries of the grown structures (e.g., see refs. [6,7]). We have recently shown that the use of slightly misoriented substrates enables the SAE 0022-0248/92/$05.00 © 1992
—
of GaInAsP/InP homo- and heterostructures having vertical sidewalls at the edges of the masks [81. We were also able to demonstrate that the quaternary material composition is not affected by the shape of the mask [8,91. There are also some recent reports in the literature on vertical side wall growth or overgrowth using atomic layer epitaxy in MOVPE or cloride VPE or conventional MOVPE on (111) substrates [10—121.These studies concentrated on the GaAs based materials systems and the vertical planes are achieved by the surface controlled reaction mechanism as in MOMBE. The advantage of the MOMBE method used here is that the growth speed is significally higher than in ALE systems and that it is applicable to ternary and quaternary InP based materials where ALE seems to be difficult to achieve. The intention of this study was to use the vertical side wall growth in SAE by MOMBE as a base for a new type of GaInAsP/InP vertical superlattice structure. In addition the orientation dependence of the quaternary material composition [9,13]was utilized for preselecting anisotropic superlattices in vertical and horizontal direction grown in a single run. This means that both
Elsevier Science Publishers B.V. All rights reserved
H. Heinecke eta!.
/ Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
187
aspects of SSG can be used to grow GaInAsP/InP vertical superlattices. The presented method is an attractive solution to produce lateral heterojunctions like vertical superlattices where the process can be more easily controlled than in the wellknown growth on vicinal planes with pregiven terrace lenghts as introduced by Petroff et al. [14,151. 2. Experimental procedure
For the growth of the InP/GaInAsP structures we used AsH3, PH3, TEG and TMI as starting materials. For details of the growth equipment and material growth, see refs. [16—18]. Substrates were (100) 2°off towards (110) InP wafers. The lateral Si02 structure for the SAE process used here dimension of 700 700 2separated by ahave freeasubstrate length of x about p~m 300 ~.tm.For the sample fabrication process see ref. [8]. For SEM inspection the grown structures were cleaved through the centre of the oxide
Fig. 1. SEM micrograph of 2 ~m thick, cleaved InP ridge looking on the (011) plane at the mask edge.
InP MQW
_________________________________
tnP MQW
_________________________________
IriP
InP Si0 2
S02
Fig. 2. SEM micrographs of selectively grown InP/GalnAsP MQW structure looking on a cleaved and stained (Oil) plane. Note the vertical sidewalls also in the MQW region (for details, see text). (a) Si02, view surface/cross section; (b) InP, view cross section.
188
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/ Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
island and stained. The layers were characterized by 300 K photoluminescence. The green line of an argon laser was focused on the surface of the sample, the luminescence light was dispersed by a one meter monochromator and detected with a Ge detector by conventional lock in technique. The sample position was scanned by a remote piezo ceramic driven manipulator. 3. Growth of vertical planes During the kinetic growth studies on InP and GaInAsP we found that the phosphorus sticking coefficient and incorporation is enhanced when using (100) wafers misoriented towards (110) [9,13,18].These phenomena increase the effective V/Ill ratio at the growing surface. This is an important factor for obtaining vertical side walls such as shown in fig. 1. In order to suppress the kinetic limitations on the (111) planes, high V/Ill ratios are mandatory as observed earlier during LP MOCVD growth of GaAs on (111) surfaces [19]. We found that the growth of more than 2
j.~mthick structures with vertical sidewalls is reproducibly possible (see fig. 1) when reducing the InP growth rate to about 0.5 ~.tm/h and using an input V/Ill ratio of 9—12 during growth on off oriented substrates towards (110). It can be concluded that under these experimental conditions, the kinetically controlled rate on the (111) planes is higher than the actual rate on the (100) and (011) planes. This is confirmed by the result that the transfer of these in MOMBE developped parameters also enables the SAE of InP with vertical side walls and planar surfaces in LP MOVPE [20].A grown InP ridge as shown in fig. 1 can be used as a base for a combination of vertical and lateral heterojunctions, the entire structure grown without etching or growth interruption. 4. GalnAsP/InP superlattice structures Fig. 2 shows SEM micrographs of the edge of a selectively grown InP/GaInAsP MQW structure looking on a cleaved and etched (011) plane.
Fig. 3. SIM niicrographs of samples as in 11g. 2 hut having thicker wells and higher \~ Ill ratio during gro~th:(a) view cross section at vertical superlattice; (b) view cross section at a (111) plane (for details, see text).
H. Heinecke et aL
/
Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
The Si02 mask can be recognized on the left hand sides of the micrographs. A one ~m thick InP buffer layer having vertical sidewalls was grown in the Si02 windows as starting layer for the Ga0 11In089As0~P077/InPmultiquantum well sequence. The thickness of the 5 wells are 100 A each separated by 300 A InP barriers. The structure is capped by a 0.25 ~m thick InP layer. Fig. 2a shows the smooth surface of the structure at both planes. The horizontal MQW structure is well resolved up to the transition edge to the lateral growth region (see fig. 2). The resolution of the micrographs in the area of the vertical sidewalls is not sufficient for a clear separation of the wells. The lower lateral growth rate and the different etching characteristics for both planes require higher resolution than available by SEM inspection. However, fig. 2 demonstrates that the vertical and horizontal interfaces to the MOW region are clearly parallel to each other. The slightly wavy bending of the horizontal lines is caused by the SEM under higher magnification. This can be recognized in the micrograph of a different structure with a well thickness of 200 A in fig. 3a. The lateral growth rate for this structure was increased by a higher V/Ill ratio. This effect and the precise analysis on the geometrical data of the vertical MOW’s is presently under investigation using TEM [211.We reported earlier [8]that contaminations due to the etching process can affect the growth mechanism at the Si02 mask edge. Fig. 3b shows such an example where (111) crystal planes are formed. The (111) planes are also created during the growth under low V/Ill ratios (see discussion in section 3). Whether there is a connection between both plenomena is not clear, but it has to be mentioned that for well prepared structures we got satisfactory vertical sidewalls in more than 90% of the experiments, 5. Optical characterization Fig. 4 shows the room temperature photoluminescence of a MOW structure like in fig. 3a. The spectra are recorded in three different distances from the oxide mask. Position 1 represents the luminesence measured 150 ~m apart from the
189
3~K~ InP/GinASP MOW: hoiizontal/vertlcal
I
Position 1
/
/~\\~~
S ~
/
,i Position 2/
~/
~‘-~
Position 3
/
9500
10000
t\
/
/
~
\
10500 11000 11500 Wavelength ($~
12000
Fig. 4. Room temperature focused photoluminescence spectra measured at various position on the structure from fig 3a. Distances in lateral direction from the edge; position 1:150 jsm; position 2: 5 ~tm; position 3:0 ~m.
Si02. A single peak identical with the data from a large area reference sample is observed having a maximum intensity at 1.044 ~m wavelength. Position 2 was measured after moving the sample in lateral direction towards the edge, so that the focused excitation laser beam was positioned in a distance of approximately 5 ~m from the oxide edge. The major recombination is still at a gap wavelength of 1.044 Mm, but an additional peak shows up at A = 1.145 ~m. Directly on the edge of the structure (position 3 in fig. 4) we observed a significant increase of the luminescence at 1.145 ~m and the peak at the shorter wavelength develops a shoulder above 1.05 ~m. For the allocation of the observed recombinations presented in fig. 4, we recorded the photoluminescence intensity at the peak wavelengths from fig. 4 as a function of the lateral position of the laser spot on the surface (see fig. 5). In order to determine the lateral information diameter of the focused photoluminescence set-up, we measured the luminescence intensity decay at the cleaved edge of a reference MOW structure. The data from fig. 5a yield a spatial resolution of approximately 10—12 ~m. In fig. 5b the luminescence intensity originating from the horizontal MQW shows a compara-
190
H. Heinecke et aL
a
/ Evidence for vertical superlartices grown by SSG in MOMBE (cBE)
300K PL InP/GalnA~P MOW:
)~=1.042~im
\Cleaved edge
______________________ b ~ 1.Q45~m X=1.l45~m
\~ ______________________
C ~
A= 1.053km
X= 1.145~m
0
ib
20
30
Lateral position (~im)—.
Fig. 5. Room temperature photoluminescence intensity at fixed measurement wavelengths versus lateral position on the sample (Si02 area with no material growth on the right hand side): (a) measurement at a cleaved edge for the determination of the resolution of the measurement; (b) (c) measurements on the structure of fig. 3a (for details, see text).
ble decay (Si02 area, right-hand side in fig. 5) when scanning over the edge of the grown structure from fig. 3a as in the case of the cleaved edge (see fig. 5a). Consequently, it can be concluded from the similar shape of the spectra in fig. 4 and the comparison of figs. 5a and Sb that the horizontal MOW at the grown edge behaves like the cleaved structure, which means that there are no lateral inhomogeneities concerning material composition and thickness at the grown edge of these MOWs. The luminescence at A = 1.145 ~m is localized in the signal decay area of the horizontal MQW, so that this luminescence must originate from the vertical MQW. Due to the substrate orientation dependence of the quaternary material composition [9,13,18], it was expected that under the selected growth conditions, the GaInAsP grown on the (011) plane should
have a comparable gap wavelength at around 1.12 ~m, as observed on (100) exactly oriented wafers before [9]. Since the horizontal surface is (100) 2° towards (110), the gap wavelength of the horizontal MOW is below the nominal value of A5 = 1.05 ~m due to quantum size effects. The orientation dependence of the material composition, which is best pronounced for the materials around Ga0 i~ In089As023P077, support the result that the luminescence at 1.145 M~is originating from the vertical MOW. Consequently, the SSG was used in this specific experiment to separate the growth phenomena at the vertical and horizontal interfaces. The slight increase of the luminescence intensity at A = 1.045 ~m in fig. 5b at the edge of the structure is probably due to a third recombination wavelength, as indicated by the shoulder in the PL spectrum in position 3 of fig. 4. Fig. Sc shows the results as in fig. Sb, but having the spectrometer tuned to the above mentioned shoulder (A = 1.053 Mm) and with the intensity expanded by a factor of 2 with respect to fig. 5b.. The peak at the edge of the structure is increased, which is an indication for a third recom.
.
bination path at the edge. The micrographs in
figs. 2 and 3 reveal that just at the junction area of the horizontal and vertical MOWs, short (111) planes are formed. Consequently the GaInAsP material composition and well thickness must be different in this area and quantum wire systems can be grown along such edges. In order to achieve insight into the growth effects on the (111) planes we have analysed the structure from fig. 3b in the same manner as the vertical MOWs. The results on the luminescence versus lateral position are shown fig. 6. The luminescence of the (111) planes was found to be at A = 1.072 ~m, and is also located in the signal decay of the horizontal MOW (A = 1.042 Mm). The increase of the intensity at A = 1.042 M~ at the edge is produced by the rising luminescence at A = 1.072 ~m, since the 300 K luminescence is broad enough to interfere at A = 1.042 urn. On the basis of the data of figs. 5 and 6, it can be suggested that quantum wire systems are formed in these areas where the vertical and horizontal structures joint
together.
H. Heinecke et aL
/ Evidence for vertical superlattices grown l~SSG in MOMBE (CBE)
191
erate only a thin nonabsorbing vertical interlayer between passive and active regions.
300K PL InP/GaInAsP MOW: honzontall(111) plane
The thickness of this interlayer can even be
designed by the input V/Ill ratio during MOMBE growth. ~
[1] N. Vodjdani, A. Lemarchand and H. Paradan, J. Physique
~
References
0
ib
20 30 Lateral position (i~m)~
40
Fig. 6. Room temperature photoluminescence intensity at fixed measurement wavelengths versus lateral position on the sample of fig. 3b.
6. Conclusions The presented results show that by surface selective growth in MOMBE horizontal and vertical heterostructures can be grown simultanously. The material interfaces involved are spatially separated from etched surfaces. The fully grown vertical interfaces should have a nonpolar surface as a basis for the vertical superlattices with still
unknown material aspects in the InP/GaInAsP system used. In the joint area of the vertical and horizontal heterostructures, quantum wire systems (for e.g., electron waveguides or transistors) can be fabricated along the edges. The perfection of selective area epitaxy in MOMBE enables the growth of quantum wire devices. In addition, this technique allows the full wafer processing of spatially seperated device structures without etched surfaces in the near of sensitive regions of these devices Another aspect from the application point of view is that the results indicate for the first time that in SAE of GaInAsP by MOMBE the material composition in lateral direction remains unaffected on a Mm scale. This is an important result, e.g. for the integration of lasers with waveguides using butt coupling in a selective area growth technique. The device concept has to to!-
43 (1982) C5—339~. [21 H. Heinecke, A. Brauers, F. Grafahrend, C. Plass, N. Pütz, K. Werner, M. Weyers, H. Lüth and P. Balk, J. Crystal Growth 77 (1986) 303. [3] 0. Kayser, J. Crystal Growth 107 (1991) 989. [4] D.A. Andrews, M.A.Z. Rejman Green, B. Wakefield and G.J. Davies, Appi. Phys. Letters 53 (1988) 97. [5] W.T. Tsang, L. Yang, MC. Wu and Y.K. Chen, Electron. Letters 27 (1991) 3. [6] H. Sugiura, R. Iga, T. Yamada and T. Toriyama, Japan. J. AppI. Phys. 30 (1991) L1089. [7] G.J. Davies, W.J. Duncan, P.J. Skevington, C.L. French and J.S. Foord, Mater. Sci. Eng. B 9 (1991) 93. [8] H. Heinecke, B. Baur, R. Schimpe, R. Matz, C. Cremer, R. Hbger and A. Miklis, J. Crystal Growth 120 (1992) 376. [9] H. Heinecke, Mater. Sci. Eng. B 9 (1991) 83. [10] A. Usui, Mater. Res. Soc. Symp. Proc. 198 (1990) 183. [11] D.B. Gladden, W.D. Goodhue, C.A. Wang and G.A. Lincoln, J. Electron. Mater. 21(1992)109. [12] S. Ando, S.S. Chang and T. Fukui, J. Crystal Growth 115 (1991) 69. [13] H. Heinecke, B. Baur, R. Hdger, B. Jobst and A. Miklis, J. Crystal Growth 124 (1992) 170. [14] P.M. Petroff, AC. Gossard and W. Wiegmann, Appl. Phys. Letters 45 (1984) 620. [15] P.M. Petroff, J. Crystal Growth 95 (1989) 260. [16] H. Heinecke, B. Baur, R. Höger and A. Miklis, J. Crystal Growth 105 (1990) 143. [17] H. Heinecke, B. Baur, R. Höger, A. Miklis and R. Treichler, J. Crystal Growth 111 (1991) 599. [18] H. Heinecke, B. Baur, N. Emeis and M. Schier, J. Crystal Growth 120 (1992) 140. [19] H. Heinecke, E. Veuhoff, N. Pütz, M. Heyen and P. Balk, J. Electron. Mater. 13 (1984) 815. [20] E. Veuhoff, H. Heinecke, J. Rieger, H. Baumeister, R. Schimpe and S. Prdhl in: Proc. 4th Intern. Conf. on InP and Related Materials, Newport, RI, 1992, in press. [21] H. Heinecke, S. Schiid, B. Baur, A. Miklis and H. Cerva, to be published.
~
CRYSTAL GROWTH
Evidence for vertical superlattices grown by surface selective growth in MOMBE (CBE) H. Heinecke, B. Baur, A. Miklis, R. Matz, C. Cremer and R. Höger Siemens Corporate Research and Development, Otto Hahn Ring 6, D-W-8000 München 83, Germany
This report shows for the first time the simultanous growth of InP/GaInAsP heterostructures having vertical and horizontal interfaces, demonstrated with multi (5) quantum well structures. The surface selective growth (SSG) in metalorganic MBE (MOMBE or CBE) enables on the one hand selective area epitaxy and on the other hand it allows to obtain vertical side walls at selectively grown structures. In addition SSG can provoke that the GaInAsP material composition is different on horizontal (100) and vertical (011) planes. This effect is utilized in order to allocate the results from focused photoluminescence measurements to the different types of superlattices. It is discussed that the proposed growth technique includes an attractive way to produce quantum wire systems besides the realization of vertical superlatices.
1. Introduction In metalorganic MBE (MOMBE) the growth mechanism is mainly determined by the kinetics of the surface reactions. Consequently the nature of the substrate type can play an important role in controlling growth rate and material composition. The selective area epitaxy (SAE), where during MOMBE no growth at all takes place on the surface of a dielectric mask, is a subcase of surface selective growth (SSG). There are several reports on the SAE of GaAs [1,2], InP [3,41 and GalnAs [4—6]using MOMBE. An important result was that in contrast to metalorganic vapour phase epitaxy (MOVPE) neither the growth rate nor the material composition is affected when GaInAs is grown only locally [3,71, since gas phase diffusion effects are not present in the MOMBE process. In order to implement the SAE into modern device concepts for the lateral integration of heterostructure devices, it is desireable to eliminate the (111) crystal planes often observed at the boundaries of the grown structures (e.g., see refs. [6,7]). We have recently shown that the use of slightly misoriented substrates enables the SAE 0022-0248/92/$05.00 © 1992
—
of GaInAsP/InP homo- and heterostructures having vertical sidewalls at the edges of the masks [81. We were also able to demonstrate that the quaternary material composition is not affected by the shape of the mask [8,91. There are also some recent reports in the literature on vertical side wall growth or overgrowth using atomic layer epitaxy in MOVPE or cloride VPE or conventional MOVPE on (111) substrates [10—121.These studies concentrated on the GaAs based materials systems and the vertical planes are achieved by the surface controlled reaction mechanism as in MOMBE. The advantage of the MOMBE method used here is that the growth speed is significally higher than in ALE systems and that it is applicable to ternary and quaternary InP based materials where ALE seems to be difficult to achieve. The intention of this study was to use the vertical side wall growth in SAE by MOMBE as a base for a new type of GaInAsP/InP vertical superlattice structure. In addition the orientation dependence of the quaternary material composition [9,13]was utilized for preselecting anisotropic superlattices in vertical and horizontal direction grown in a single run. This means that both
Elsevier Science Publishers B.V. All rights reserved
H. Heinecke eta!.
/ Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
187
aspects of SSG can be used to grow GaInAsP/InP vertical superlattices. The presented method is an attractive solution to produce lateral heterojunctions like vertical superlattices where the process can be more easily controlled than in the wellknown growth on vicinal planes with pregiven terrace lenghts as introduced by Petroff et al. [14,151. 2. Experimental procedure
For the growth of the InP/GaInAsP structures we used AsH3, PH3, TEG and TMI as starting materials. For details of the growth equipment and material growth, see refs. [16—18]. Substrates were (100) 2°off towards (110) InP wafers. The lateral Si02 structure for the SAE process used here dimension of 700 700 2separated by ahave freeasubstrate length of x about p~m 300 ~.tm.For the sample fabrication process see ref. [8]. For SEM inspection the grown structures were cleaved through the centre of the oxide
Fig. 1. SEM micrograph of 2 ~m thick, cleaved InP ridge looking on the (011) plane at the mask edge.
InP MQW
_________________________________
tnP MQW
_________________________________
IriP
InP Si0 2
S02
Fig. 2. SEM micrographs of selectively grown InP/GalnAsP MQW structure looking on a cleaved and stained (Oil) plane. Note the vertical sidewalls also in the MQW region (for details, see text). (a) Si02, view surface/cross section; (b) InP, view cross section.
188
H. Heinecke et al.
/ Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
island and stained. The layers were characterized by 300 K photoluminescence. The green line of an argon laser was focused on the surface of the sample, the luminescence light was dispersed by a one meter monochromator and detected with a Ge detector by conventional lock in technique. The sample position was scanned by a remote piezo ceramic driven manipulator. 3. Growth of vertical planes During the kinetic growth studies on InP and GaInAsP we found that the phosphorus sticking coefficient and incorporation is enhanced when using (100) wafers misoriented towards (110) [9,13,18].These phenomena increase the effective V/Ill ratio at the growing surface. This is an important factor for obtaining vertical side walls such as shown in fig. 1. In order to suppress the kinetic limitations on the (111) planes, high V/Ill ratios are mandatory as observed earlier during LP MOCVD growth of GaAs on (111) surfaces [19]. We found that the growth of more than 2
j.~mthick structures with vertical sidewalls is reproducibly possible (see fig. 1) when reducing the InP growth rate to about 0.5 ~.tm/h and using an input V/Ill ratio of 9—12 during growth on off oriented substrates towards (110). It can be concluded that under these experimental conditions, the kinetically controlled rate on the (111) planes is higher than the actual rate on the (100) and (011) planes. This is confirmed by the result that the transfer of these in MOMBE developped parameters also enables the SAE of InP with vertical side walls and planar surfaces in LP MOVPE [20].A grown InP ridge as shown in fig. 1 can be used as a base for a combination of vertical and lateral heterojunctions, the entire structure grown without etching or growth interruption. 4. GalnAsP/InP superlattice structures Fig. 2 shows SEM micrographs of the edge of a selectively grown InP/GaInAsP MQW structure looking on a cleaved and etched (011) plane.
Fig. 3. SIM niicrographs of samples as in 11g. 2 hut having thicker wells and higher \~ Ill ratio during gro~th:(a) view cross section at vertical superlattice; (b) view cross section at a (111) plane (for details, see text).
H. Heinecke et aL
/
Evidence for vertical superlattices grown by SSG in MOMBE (CBE)
The Si02 mask can be recognized on the left hand sides of the micrographs. A one ~m thick InP buffer layer having vertical sidewalls was grown in the Si02 windows as starting layer for the Ga0 11In089As0~P077/InPmultiquantum well sequence. The thickness of the 5 wells are 100 A each separated by 300 A InP barriers. The structure is capped by a 0.25 ~m thick InP layer. Fig. 2a shows the smooth surface of the structure at both planes. The horizontal MQW structure is well resolved up to the transition edge to the lateral growth region (see fig. 2). The resolution of the micrographs in the area of the vertical sidewalls is not sufficient for a clear separation of the wells. The lower lateral growth rate and the different etching characteristics for both planes require higher resolution than available by SEM inspection. However, fig. 2 demonstrates that the vertical and horizontal interfaces to the MOW region are clearly parallel to each other. The slightly wavy bending of the horizontal lines is caused by the SEM under higher magnification. This can be recognized in the micrograph of a different structure with a well thickness of 200 A in fig. 3a. The lateral growth rate for this structure was increased by a higher V/Ill ratio. This effect and the precise analysis on the geometrical data of the vertical MOW’s is presently under investigation using TEM [211.We reported earlier [8]that contaminations due to the etching process can affect the growth mechanism at the Si02 mask edge. Fig. 3b shows such an example where (111) crystal planes are formed. The (111) planes are also created during the growth under low V/Ill ratios (see discussion in section 3). Whether there is a connection between both plenomena is not clear, but it has to be mentioned that for well prepared structures we got satisfactory vertical sidewalls in more than 90% of the experiments, 5. Optical characterization Fig. 4 shows the room temperature photoluminescence of a MOW structure like in fig. 3a. The spectra are recorded in three different distances from the oxide mask. Position 1 represents the luminesence measured 150 ~m apart from the
189
3~K~ InP/GinASP MOW: hoiizontal/vertlcal
I
Position 1
/
/~\\~~
S ~
/
,i Position 2/
~/
~‘-~
Position 3
/
9500
10000
t\
/
/
~
\
10500 11000 11500 Wavelength ($~
12000
Fig. 4. Room temperature focused photoluminescence spectra measured at various position on the structure from fig 3a. Distances in lateral direction from the edge; position 1:150 jsm; position 2: 5 ~tm; position 3:0 ~m.
Si02. A single peak identical with the data from a large area reference sample is observed having a maximum intensity at 1.044 ~m wavelength. Position 2 was measured after moving the sample in lateral direction towards the edge, so that the focused excitation laser beam was positioned in a distance of approximately 5 ~m from the oxide edge. The major recombination is still at a gap wavelength of 1.044 Mm, but an additional peak shows up at A = 1.145 ~m. Directly on the edge of the structure (position 3 in fig. 4) we observed a significant increase of the luminescence at 1.145 ~m and the peak at the shorter wavelength develops a shoulder above 1.05 ~m. For the allocation of the observed recombinations presented in fig. 4, we recorded the photoluminescence intensity at the peak wavelengths from fig. 4 as a function of the lateral position of the laser spot on the surface (see fig. 5). In order to determine the lateral information diameter of the focused photoluminescence set-up, we measured the luminescence intensity decay at the cleaved edge of a reference MOW structure. The data from fig. 5a yield a spatial resolution of approximately 10—12 ~m. In fig. 5b the luminescence intensity originating from the horizontal MQW shows a compara-
190
H. Heinecke et aL
a
/ Evidence for vertical superlartices grown by SSG in MOMBE (cBE)
300K PL InP/GalnA~P MOW:
)~=1.042~im
\Cleaved edge
______________________ b ~ 1.Q45~m X=1.l45~m
\~ ______________________
C ~
A= 1.053km
X= 1.145~m
0
ib
20
30
Lateral position (~im)—.
Fig. 5. Room temperature photoluminescence intensity at fixed measurement wavelengths versus lateral position on the sample (Si02 area with no material growth on the right hand side): (a) measurement at a cleaved edge for the determination of the resolution of the measurement; (b) (c) measurements on the structure of fig. 3a (for details, see text).
ble decay (Si02 area, right-hand side in fig. 5) when scanning over the edge of the grown structure from fig. 3a as in the case of the cleaved edge (see fig. 5a). Consequently, it can be concluded from the similar shape of the spectra in fig. 4 and the comparison of figs. 5a and Sb that the horizontal MOW at the grown edge behaves like the cleaved structure, which means that there are no lateral inhomogeneities concerning material composition and thickness at the grown edge of these MOWs. The luminescence at A = 1.145 ~m is localized in the signal decay area of the horizontal MQW, so that this luminescence must originate from the vertical MQW. Due to the substrate orientation dependence of the quaternary material composition [9,13,18], it was expected that under the selected growth conditions, the GaInAsP grown on the (011) plane should
have a comparable gap wavelength at around 1.12 ~m, as observed on (100) exactly oriented wafers before [9]. Since the horizontal surface is (100) 2° towards (110), the gap wavelength of the horizontal MOW is below the nominal value of A5 = 1.05 ~m due to quantum size effects. The orientation dependence of the material composition, which is best pronounced for the materials around Ga0 i~ In089As023P077, support the result that the luminescence at 1.145 M~is originating from the vertical MOW. Consequently, the SSG was used in this specific experiment to separate the growth phenomena at the vertical and horizontal interfaces. The slight increase of the luminescence intensity at A = 1.045 ~m in fig. 5b at the edge of the structure is probably due to a third recombination wavelength, as indicated by the shoulder in the PL spectrum in position 3 of fig. 4. Fig. Sc shows the results as in fig. Sb, but having the spectrometer tuned to the above mentioned shoulder (A = 1.053 Mm) and with the intensity expanded by a factor of 2 with respect to fig. 5b.. The peak at the edge of the structure is increased, which is an indication for a third recom.
.
bination path at the edge. The micrographs in
figs. 2 and 3 reveal that just at the junction area of the horizontal and vertical MOWs, short (111) planes are formed. Consequently the GaInAsP material composition and well thickness must be different in this area and quantum wire systems can be grown along such edges. In order to achieve insight into the growth effects on the (111) planes we have analysed the structure from fig. 3b in the same manner as the vertical MOWs. The results on the luminescence versus lateral position are shown fig. 6. The luminescence of the (111) planes was found to be at A = 1.072 ~m, and is also located in the signal decay of the horizontal MOW (A = 1.042 Mm). The increase of the intensity at A = 1.042 M~ at the edge is produced by the rising luminescence at A = 1.072 ~m, since the 300 K luminescence is broad enough to interfere at A = 1.042 urn. On the basis of the data of figs. 5 and 6, it can be suggested that quantum wire systems are formed in these areas where the vertical and horizontal structures joint
together.
H. Heinecke et aL
/ Evidence for vertical superlattices grown l~SSG in MOMBE (CBE)
191
erate only a thin nonabsorbing vertical interlayer between passive and active regions.
300K PL InP/GaInAsP MOW: honzontall(111) plane
The thickness of this interlayer can even be
designed by the input V/Ill ratio during MOMBE growth. ~
[1] N. Vodjdani, A. Lemarchand and H. Paradan, J. Physique
~
References
0
ib
20 30 Lateral position (i~m)~
40
Fig. 6. Room temperature photoluminescence intensity at fixed measurement wavelengths versus lateral position on the sample of fig. 3b.
6. Conclusions The presented results show that by surface selective growth in MOMBE horizontal and vertical heterostructures can be grown simultanously. The material interfaces involved are spatially separated from etched surfaces. The fully grown vertical interfaces should have a nonpolar surface as a basis for the vertical superlattices with still
unknown material aspects in the InP/GaInAsP system used. In the joint area of the vertical and horizontal heterostructures, quantum wire systems (for e.g., electron waveguides or transistors) can be fabricated along the edges. The perfection of selective area epitaxy in MOMBE enables the growth of quantum wire devices. In addition, this technique allows the full wafer processing of spatially seperated device structures without etched surfaces in the near of sensitive regions of these devices Another aspect from the application point of view is that the results indicate for the first time that in SAE of GaInAsP by MOMBE the material composition in lateral direction remains unaffected on a Mm scale. This is an important result, e.g. for the integration of lasers with waveguides using butt coupling in a selective area growth technique. The device concept has to to!-
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