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Optimization of p-dopant profiles for GaInAsP MQW laser structures in MOMBE
Journal of Crystal Growth 195 (1998) 660—667
Optimization of p-dopant profiles for GaInAsP MQW laser structures in MOMBE P. Kro¨ner , H. Baumeister , J. Rieger , E. Veuhoff *, M. Popp , H. Heinecke Department of ZT KM 4, Siemens Corporate Technology, D-81730 Munich, Germany Department of Semiconductor Physics, University of Ulm, D-89069 Ulm, Germany
Abstract For industrial device fabrication gaseous dopant sources are preferred in metalorganic growth technologies. Both in metalorganic vapor phase epitaxy (MOVPE) and metalorganic molecular beam epitaxy (MOMBE/CBE) diethylzinc is used. For doping of InP based ridge waveguide laser structures the critical parts are the active region with GaInAsP confinement layers, MQW layers and the p-type region with InP spacer layer, GaInAsP etch stop layer and InP cladding layer. Growing Zn doped InP in between GaInAsP layers leads to a significant Zn diffusion into the adjacent GaInAsP layers. This effect is much more pronounced in MOMBE than in MOVPE. It is demonstrated how this effect is reduced by insertion of additional intermediate layers with appropriate dopant concentration, and by compensating Si co-doping in the spacer and adjacent layers taking advantage of the Fermi level effect. Applying these techniques, reduced threshold current densities and improved high temperature performance can be obtained. The results are discussed by models of dopant diffusion in MOMBE in comparison to MOVPE. 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Fd; 81.15.Gh; 81.15.Hi; 85.30.!z Keywords: MOMBE; CBE; MOVPE; GaInAsP; Doping; MQW lasers; SIMS
1. Introduction For advanced optical communication systems high performance long wavelength lasers are key components. It has successfully been demonstrated
* Corresponding author. Fax: #49 89 636 41658; e-mail: [email protected].
that laser characteristics, such as high temperature performance, can significantly be improved by employing a strained layer multiple quantum well (SL-MQW) structure as an active region, and these InP based SL-MQW laser structures are routinely produced by metalorganic vapor phase epitaxy (MOVPE) today [1—3]. Generally, growth of strained layers is facilitated at lower growth temperatures [4]. As in metalorganic molecular beam epitaxy (MOMBE/CBE) growth temperatures for
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 6 4 2 - 3
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
InP based compounds are about 100—150°C lower than in MOVPE, a higher strain can be applied in MOMBE grown SL-MQW structures [5]. Thus strain compensated MQW laser structures were fabricated by MOMBE with a high temperature device performance superior to best MOVPE data published so far [5,6]. The dopant profile in a laser structure has a significant effect on device performance, as discussed in recent reports [7—9]. Especially in the p-type doped region of the device structure a reduced carrier concentration caused by dopant outdiffusion into adjacent layers may result in a high electron leakage current from the active device region which is detrimental to high temperature performance. On the other hand, an increased p-dopant level adjacent to the active region for improved quantum efficiency may cause an increase in threshold current due to Zn diffusion into the active region. Therefore, a careful optimization of the p-type dopant profile in complex laser structures is required. In MOMBE the same gaseous dopant sources as in MOVPE should be applied, since gaseous sources are easier to handle compared to solid sources. Moreover, the industrial acceptance of the MOMBE process is increased, when the same dopant sources as in qualified MOVPE production lines are used [10,11]. Following these arguments, in MOMBE Zn should be the preferred p-type dopant. In MOMBE, however, Zn appears to exhibit a more pronounced tendency for diffusion in InP based device structures than in MOVPE [12,13]. The metal clad ridge waveguide (MCRW) structure is often chosen as a laser structure, since this structure offers both a simple fabrication technology and excellent long-term device reliability [14,15]. Such a laser structure consists of layers with different material composition in the p-type region, which may enhance Zn diffusion. In this paper the Zn diffusion in MCRW SL-MQW laser structures is investigated. The effect of Zn diffusion on device performance is considered. It is described how Zn diffusion can be reduced. The diffusion mechanisms are discussed and compared with diffusion mechanisms in analogous laser structures grown by MOVPE.
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2. Experimental procedure The MOMBE system for growth of the SLMQW laser structures was described in detail previously [11]. Trimethylindium, triethylgallium, phosphine, arsine, diethylzinc (DEZn) and disilane were used as precursors. A carrier gas was not required. The molecular beams were pressure controlled and injected via a vent/run valve configuration enabling an extremely precise flow control and fast source switching. Twin sources for the growth species were applied. Therefore, MQW growth could be performed without growth interruptions. The growth temperature was adjusted within $1°C by an automated infrared pyrometer control technique. A growth temperature of 500°C was used for all but the Zn doped layers. For Zn doping DEZn was injected without precracking, and the growth temperature was reduced to 450°C for efficient dopant incorporation [13]. Single layers were characterized by standard techniques (Hall measurements, photoluminescence and double crystal X-ray diffractometry). Dopant profiles were analyzed by secondary ion mass spectrometry (SIMS). The sequence of the layers for the MCRW test laser structures is given in Table 1. The active region contains a separate confinement heterostructure (SCH) with an SL-MQW structure consisting of 5 quaternary wells (well thickness d "6 nm, 1% 5 compressive strain) and quaternary barriers (d "10 nm) yielding an emission wavelength j"1.3 lm [16]. Information on the nominal carrier concentrations (c.c.) and dopants is also given in the table. Focusing on the p-type region above the active region it can be recognized that an additional GaInAsP layer is inserted, which serves as etch stop layer for convenient processing. This layer is 50 nm in thickness separated from the MQW region by a p-doped InP layer (thickness: d"100 nm). For a test of the quality of the structure, broad area lasers (100;400 lm) and from some selected samples MCRW lasers (2.2; 350 lm) were fabricated. MOMBE laser structures are compared with MCRW laser structures grown in a low pressure MOVPE system (100 mbar) at 600 and 650°C using the same dopant precursors. For a test of thermal stability, selected MOMBE laser structures were annealed in a low
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
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Table 1 Layer sequence of the MCRW test laser structure with nominal carrier concentrations (c.c.) Layer
Material
Function
19 18 17 16 15 14 13 4—12 3 2 1 0
InP GaInAs InP GaInAsP InP GaInAsP GaInAsP MQW GaInAsP GaInAsP InP InP
Cap Contact Cladding Etch stop Spacer SCH SCH Active SCH SCH Cladding Substrate
j (lm)
1.66 1.15 1.05 1.10 1.3 1.10 1.05
d (nm)
c.c. (cm\)
Dopant
150 200 1600 50 100 100 35 5;6/4;10 35 100 '0.8 '3000
10 2;10 5;10 5;10 5;10 5;10 5;10 — 5;10 5;10 5;10 3;10
Zn Zn Zn Zn Zn Zn Zn — Si Si Si S
Fig. 1. Zn depth profiles measured by SIMS in MOMBE test lasers with and without GaInAsP etch stop layer (solid and dashed lines, resp.). Arsenic signal included (in arbitrary units). Zn penetration depth and relative Zn accumulation indicated in the profile.
pressure MOVPE system for 1 h under phosphine atmosphere at 600 and 650°C, temperatures typical for an MOVPE overgrowth process.
3. Results and discussion Zn depth profiles in test laser structures with and without the quaternary etch stop layer are shown in Fig. 1. The SIMS data include the As depth
profile (in arbitrary units) to mark the heterointerfaces. Without the etch stop layer a Zn profile with a steep decrease at the heterointerface to the active region can easily be achieved (dotted line) provided that a nominally undoped intermediate InP layer is used to compensate Zn diffusion effects, as previously described in detail [10,13]. With etch stop layer (layer 16 in Table 1) two effects are observed (solid line). A Zn accumulation occurs in this etch stop layer. Additionally, Zn moves into
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
the nominally undoped SCH layer (layer 14) and accumulates there. The penetration depth (defined by the width, where the Zn concentration has dropped to 1/e of the maximum concentration at the interface) and the relative Zn accumulation (defined by the ratio of the maximum Zn concentration in the SCH layer to the minimum concentration in the spacer layer) are indicated in the figure. Both effects, Zn outdiffusion to the etch stop layer and to the SCH layer, result in a Zn depletion in the InP spacer layer (layer 15). This depletion is not observed without etch stop layer, because then a virtually unlimited Zn supply from the thick InP cladding layer is available to compensate Zn outdiffusion to the SCH layer. Both Zn depletion in the spacer and Zn accumulation in the SCH layer may result in deteriorated device performance. Indeed MCRW lasers fabricated from a structure with such a low Zn level of about 3;10 cm\ in the spacer layer failed at elevated temperatures (at 85°C). Part of the Zn accumulation in the etch stop layer is probably caused by segregation. The preceding InP spacer layer is Zn doped so that Zn may accumulate at the growing surface, and this segregation yields a higher Zn concentration in the following quaternary layer. Analogous segregations effects were addressed for beryllium as acceptor in InP based heterostructures in hydride source molecular beam epitaxy and in MOMBE [17,18]. The Zn accumulation is more pronounced in the etch stop layer, since here in addition to segregation, diffusion from the cladding and spacer layers (layers 17 and 15) comes into effect. An explanation for the Zn accumulation in the SCH layer, however, is not that straightforward, since the Zn precursor beam flow is turned on after growth of this layer (layer 14) so that Zn is moving backwards opposite to growth direction. From previous MOMBE and MOVPE studies it is known that in InP layers a higher concentration of Zn interstitials is formed than in ternary and quaternary layers, where Zn mainly occupies substitutional sites yielding higher free carrier concentrations in these layers than in InP [12,13]. It appears that this high concentration of Zn interstitials in InP leads to an enhanced Zn diffusion backwards into the quaternary SCH layer, where Zn is immediately consumed on substitutional
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sites maintaining this Zn diffusion flow, which results in the observed Zn accumulation in the SCH layer. Several steps can be taken to reduce the Zn depletion in the spacer layer. (i) The Zn diffusion into the active region may be reduced by a slight compensating n-type doping of the adjacent SCH layer taking advantage of the Fermi level effect of diffusion [12,19,20]. (ii) Intermediate InP layers with appropriate dopant concentration can additionally be inserted. (iii) The problem can be attacked in the spacer layer itself, where the technique of donor co-doping may be employed to decrease Zn outdiffusion [21]. A comparison with a Zn profile in a similar MCRW laser structure grown by MOVPE may elucidate the Zn diffusion phenomenon. Typical SIMS depth profiles are given in Fig. 2. The MOVPE structures were Zn doped starting from SCH layer 13. A similar procedure for MOMBE structures would result in a further Zn accumulation yielding increased threshold currents. Therefore, for MOMBE growth the Zn precursor beam flow was turned on for growth of the spacer layer 15 and subsequent layers, as mentioned above. At first sight the Zn distribution in the MOVPE structure appears to be rather similar to the distribution in the MOMBE structure. Both in the etch stop layer and in the SCH layer a Zn accumulation is detected analogous to MOMBE. However, there are quantitative differences. The Zn accumulation in the MOMBE structure is stronger than in the MOVPE structure. This behavior is expected, as part of the Zn accumulation is attributed to segregation, and in MOMBE due to the much lower growth temperature the Zn surface concentration should be higher than in MOVPE, where Zn easily desorbes at the high temperature. Furthermore, a significant reduction of Zn diffusion in the SCH layer can be realized at the lower MOVPE growth temperature. Such a profile is suited for excellent high temperature device operation. The decrease of the Zn concentration in the SCH layer appears to be steeper for the MOMBE than for the MOVPE profile. In MOMBE the Zn diffusion in InP is much more pronounced than in MOVPE in spite of the low MOMBE growth temperature. On the other hand for ternary and
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Fig. 2. Typical Zn depth profiles in MCRW laser structures grown by MOMBE and MOVPE. Arsenic SIMS signal included (in arbitrary units).
quaternary compounds the Zn diffusion appears to be weaker in MOMBE than in MOVPE. Hence in MOMBE InP/GaInAsP heterostructures the effects of Zn depletion in InP layers and Zn accumulation in GaInAsP layers appear to be stronger than in MOVPE so that in MOMBE specific procedures have to be introduced to compensate these effects. In the first approach a slight intentional n-type doping background in the p-SCH layer (layer 14) is used to affect Zn diffusion into this layer according to the Fermi level effect of diffusion [12,19,20]. With this technique the solubility and charge state of point defects are modified resulting in a decreased impurity diffusion. An analysis of SIMS data reveals that indeed the Zn penetration depth into the SCH layer can be reduced by increasing the Si background level in the SCH layer, as summarized in Fig. 3. The maximum Si beam flow (in beam equivalent pressure, BEP) would yield an electron concentration of approximately (6—7);10 cm\ in the SCH layer, and the relationship is linear [10]. The Zn accumulation at the heterointerface spacer/SCH layer (layers 14/15),
however, cannot be avoided, as shown in Fig. 3 (right y-axis). In the second approach increased Zn precursor beam flows in the spacer layer (layer 15) and in the cladding layer (layer 17) close to the heterojunction were offered to compensate Zn depletion, and in a third approach donor co-doping in the spacer layer was applied to reduce Zn outdiffusion from this layer. For delta-doping in MBE GaAs it was reported that diffusive broadening of Be acceptor spikes is completely avoided by the presence of co-deposited Si [21]. Analogous effects are found with Zn and Si co-doping in the present study. Fig. 4 shows SIMS depth profiles along with the dopant flow distributions. The Si beam flow would result in an electron concentration of (1—2);10 cm\. It can be recognized that a higher DEZn beam flow is used in the first 200 nm of the p-cladding layer preventing a Zn depletion in this cladding layer. Indeed a flat Zn profile is achieved in the cladding layer. A comparison with the MCRW structure in Fig. 1 reveals the reduced Zn loss in the co-doped spacer layer and no accumulation in the SCH layer. The laser exhibited
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
665
Fig. 3. Effect of intentional Si background doping (in beam equivalent pressure, BEP) on Zn penetration depth into SCH layer (left y-axis, square symbols) and on relative Zn accumulation at the heterointerface spacer/SCH layer in MOMBE (right y-axis, circle symbols); data taken from SIMS analysis.
Fig. 4. Zn depth profile in an MOMBE MCRW laser structure co-doped with Si. Arsenic SIMS signal included (in arbitrary units). Zn and Si dopant beam flow distributions on top (in beam equivalent pressure, BEP). BEP would result in an electron concentration of 1 (1—2);10 cm\.
improved high temperature performance. There are several possible models for a reduced Zn diffusion in the presence of Si. Both Zn and Si occupy group III lattice sites, and Si>—Zn\ next-nearest-
neighbour pairing may occur in analogy to the interaction of Si and Be in GaAs [22]. Such a complex is probably characterized by a low diffusion coefficient. Alternatively, the presence of Si on
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P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
a substitutional group III lattice site may increase the solubility of Zn on a substitutional lattice site. Or the presence of Si may simply inhibit the transfer of Zn from a substitutional site to the rapidlydiffusing interstitial lattice state. Hence this effect of co-doping is not unexpected. However, it should be taken into account that this co-doping technique results in a compensation of the carrier concentration, which may be a drawback. Therefore, the compensating Si level should be kept as low as possible. It should be pointed out that annealing of the MOMBE MCRW laser structure under MOVPE growth conditions does not affect the Zn doping profile, whereas a higher MOVPE growth temperature increases Zn diffusion during growth (see Fig. 2). This result has important consequences for applications, as it indicates that a basic MOMBE laser structure can be overgrown in an MOVPE process without degradation of the dopant profile. Additionally, the result suggests that diffusion during growth is determined by other mechanisms than diffusion by post-growth annealing. For a deeper insight into diffusion mechanisms surface analytical tools should be applied in MOMBE.
4. Conclusions Zn diffusion in InP based device structures has been studied in MOMBE and compared with MOVPE data. A similar Zn accumulation behavior in quaternary layers is observed for both technologies. Quantitative differences, however, can be recognized, since the MOMBE process is more surface sensitive and is performed at a lower growth temperature. Hence Zn redistribution by segration and diffusion is more pronounced in MOMBE affecting threshold current and high temperature performance of ridge waveguide lasers. This difference in Zn dopant behavior in MOMBE and MOVPE makes an individual dopant beam flow optimization for MOMBE heterostructures necessary, since MOVPE dopant flow parameters can not directly be transferred to MOMBE. For optimized Zn dopant profiles in MOMBE several steps can be taken to compensate Zn outdiffusion from InP to adjacent GaInAsP layers: the
Zn precursor beam flow should be increased in the InP layers adjacent to the etch stop layer so that in spite of Zn outdiffusion a flat dopant profile can be obtained. It is expected that a GaInAsP spacer layer with a low Ga and As content instead of the standard InP spacer should help to reduce outdiffusion due to a lower concentration of Zn interstitials in such a quaternary layer. Comparing the technique of using the Fermi level effect of diffusion with the technique of co-doping, the latter appears to be better suited to achieve appropriate Zn profiles in laser structures: a sufficient Zn concentration in the spacer layer can be maintained by using additional low Si doping in the InP spacer layer. Thus waveguide lasers with improved performance can be fabricated.
Acknowledgements The authors would like to thank Bernd Borchert, Roland Gessner and Bernhard Stegmu¨ller for laser characterization and for stimulating discussions. Part of this work was performed under contract 01 BP 468/8 of the Photonics Program of the German Ministry of Education and Research (BMBF).
References [1] P.J.A. Thijs, L.F. Tietmeijer, J.J.M. Binsma, T. van Dongen, IEEE J. Quantum Electron. QE-30 (1994) 477. [2] E. Veuhoff, in: P.H. Holloway, G.E. McGuire (Eds.), Handbook of Compound Semiconductors, Noyes Publications, Park Ridge, NJ, 1995, p. 29. [3] R.L. Moon, J. Crystal Growth 170 (1997) 1. [4] G.L. Price, Phys. Rev. Lett. 66 (1991) 469. [5] H. Sugiura, J. Crystal Growth 164 (1996) 434. [6] J.F. Carlin, A.V. Syrbu, C.A. Berseth, J. Behrend, A. Rudra, E. Kapon, Appl. Phys. Lett. 71 (1997) 13. [7] R.F. Kazarinov, M.R. Pinto, IEEE J. Quantum Electron. 30 (1994) 49. [8] G.L. Belenky, C.L. Reynolds, R.F. Kazarinov, V. Swaminathan, S.L. Luryi, J. Lopata, IEEE J. Quantum Electron. 32 (1996) 1450. [9] S. Seki, K. Yokoyama, IEEE Photon. Technol. Lett. 10 (1998) 200. [10] M. Popp, H. Heinecke, H. Baumeister, E. Veuhoff, J. Crystal Growth 176 (1997) 1247. [11] E. Veuhoff, J. Crystal Growth 188 (1998) 231.
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667 [12] E. Veuhoff, H. Baumeister, J. Rieger, M. Gorgel, R. Treichler, J. Electron. Mater. 20 (1991) 1037. [13] E. Veuhoff, H. Baumeister, R. Treichler, M. Popp, H. Heinecke, J. Crystal Growth 164 (1996) 402. [14] M.-C. Amann, B. Stegmu¨ller, Appl. Phys. Lett. 48 (1986) 1027. [15] B. Stegmu¨ller, E. Veuhoff, J. Rieger, H. Hedrich, Electron. Lett. 29 (1993) 1691. [16] H. Baumeister, E. Veuhoff, M. Popp, H. Heinecke, J. Crystal Growth 188 (1998) 266. [17] M.B. Panish, R.A. Hamm, D. Ritter, H.S. Luftman, C.M. Cotell, J. Crystal Growth 112 (1991) 343.
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[18] D. Ritter, R.A. Hamm, M.B. Panish, M. Geva, Proc. 3rd Int. Conf. on Indium Phosphide and Related Materials, 1991, p. 363. [19] T.Y. Tan, U. Go¨sele, J. Appl. Phys. 61 (1987) 1841. [20] D.G. Deppe, N. Holonyak Jr., F.A. Kish, J.E. Baker, Appl. Phys. Lett. 50 (1987) 998. [21] J.J. Harris, J.B. Clegg, R.B. Beall, J. Castagne´, K. Woodbridge, C. Roberts, J. Crystal Growth 111 (1991) 239. [22] A. Mohades-Kassai, M.R. Brozel, R. Murray, R.C. Newman, Inst. Phys. Conf. Ser. 106 (1990) 471.
Optimization of p-dopant profiles for GaInAsP MQW laser structures in MOMBE P. Kro¨ner , H. Baumeister , J. Rieger , E. Veuhoff *, M. Popp , H. Heinecke Department of ZT KM 4, Siemens Corporate Technology, D-81730 Munich, Germany Department of Semiconductor Physics, University of Ulm, D-89069 Ulm, Germany
Abstract For industrial device fabrication gaseous dopant sources are preferred in metalorganic growth technologies. Both in metalorganic vapor phase epitaxy (MOVPE) and metalorganic molecular beam epitaxy (MOMBE/CBE) diethylzinc is used. For doping of InP based ridge waveguide laser structures the critical parts are the active region with GaInAsP confinement layers, MQW layers and the p-type region with InP spacer layer, GaInAsP etch stop layer and InP cladding layer. Growing Zn doped InP in between GaInAsP layers leads to a significant Zn diffusion into the adjacent GaInAsP layers. This effect is much more pronounced in MOMBE than in MOVPE. It is demonstrated how this effect is reduced by insertion of additional intermediate layers with appropriate dopant concentration, and by compensating Si co-doping in the spacer and adjacent layers taking advantage of the Fermi level effect. Applying these techniques, reduced threshold current densities and improved high temperature performance can be obtained. The results are discussed by models of dopant diffusion in MOMBE in comparison to MOVPE. 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Fd; 81.15.Gh; 81.15.Hi; 85.30.!z Keywords: MOMBE; CBE; MOVPE; GaInAsP; Doping; MQW lasers; SIMS
1. Introduction For advanced optical communication systems high performance long wavelength lasers are key components. It has successfully been demonstrated
* Corresponding author. Fax: #49 89 636 41658; e-mail: [email protected].
that laser characteristics, such as high temperature performance, can significantly be improved by employing a strained layer multiple quantum well (SL-MQW) structure as an active region, and these InP based SL-MQW laser structures are routinely produced by metalorganic vapor phase epitaxy (MOVPE) today [1—3]. Generally, growth of strained layers is facilitated at lower growth temperatures [4]. As in metalorganic molecular beam epitaxy (MOMBE/CBE) growth temperatures for
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 6 4 2 - 3
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
InP based compounds are about 100—150°C lower than in MOVPE, a higher strain can be applied in MOMBE grown SL-MQW structures [5]. Thus strain compensated MQW laser structures were fabricated by MOMBE with a high temperature device performance superior to best MOVPE data published so far [5,6]. The dopant profile in a laser structure has a significant effect on device performance, as discussed in recent reports [7—9]. Especially in the p-type doped region of the device structure a reduced carrier concentration caused by dopant outdiffusion into adjacent layers may result in a high electron leakage current from the active device region which is detrimental to high temperature performance. On the other hand, an increased p-dopant level adjacent to the active region for improved quantum efficiency may cause an increase in threshold current due to Zn diffusion into the active region. Therefore, a careful optimization of the p-type dopant profile in complex laser structures is required. In MOMBE the same gaseous dopant sources as in MOVPE should be applied, since gaseous sources are easier to handle compared to solid sources. Moreover, the industrial acceptance of the MOMBE process is increased, when the same dopant sources as in qualified MOVPE production lines are used [10,11]. Following these arguments, in MOMBE Zn should be the preferred p-type dopant. In MOMBE, however, Zn appears to exhibit a more pronounced tendency for diffusion in InP based device structures than in MOVPE [12,13]. The metal clad ridge waveguide (MCRW) structure is often chosen as a laser structure, since this structure offers both a simple fabrication technology and excellent long-term device reliability [14,15]. Such a laser structure consists of layers with different material composition in the p-type region, which may enhance Zn diffusion. In this paper the Zn diffusion in MCRW SL-MQW laser structures is investigated. The effect of Zn diffusion on device performance is considered. It is described how Zn diffusion can be reduced. The diffusion mechanisms are discussed and compared with diffusion mechanisms in analogous laser structures grown by MOVPE.
661
2. Experimental procedure The MOMBE system for growth of the SLMQW laser structures was described in detail previously [11]. Trimethylindium, triethylgallium, phosphine, arsine, diethylzinc (DEZn) and disilane were used as precursors. A carrier gas was not required. The molecular beams were pressure controlled and injected via a vent/run valve configuration enabling an extremely precise flow control and fast source switching. Twin sources for the growth species were applied. Therefore, MQW growth could be performed without growth interruptions. The growth temperature was adjusted within $1°C by an automated infrared pyrometer control technique. A growth temperature of 500°C was used for all but the Zn doped layers. For Zn doping DEZn was injected without precracking, and the growth temperature was reduced to 450°C for efficient dopant incorporation [13]. Single layers were characterized by standard techniques (Hall measurements, photoluminescence and double crystal X-ray diffractometry). Dopant profiles were analyzed by secondary ion mass spectrometry (SIMS). The sequence of the layers for the MCRW test laser structures is given in Table 1. The active region contains a separate confinement heterostructure (SCH) with an SL-MQW structure consisting of 5 quaternary wells (well thickness d "6 nm, 1% 5 compressive strain) and quaternary barriers (d "10 nm) yielding an emission wavelength j"1.3 lm [16]. Information on the nominal carrier concentrations (c.c.) and dopants is also given in the table. Focusing on the p-type region above the active region it can be recognized that an additional GaInAsP layer is inserted, which serves as etch stop layer for convenient processing. This layer is 50 nm in thickness separated from the MQW region by a p-doped InP layer (thickness: d"100 nm). For a test of the quality of the structure, broad area lasers (100;400 lm) and from some selected samples MCRW lasers (2.2; 350 lm) were fabricated. MOMBE laser structures are compared with MCRW laser structures grown in a low pressure MOVPE system (100 mbar) at 600 and 650°C using the same dopant precursors. For a test of thermal stability, selected MOMBE laser structures were annealed in a low
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
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Table 1 Layer sequence of the MCRW test laser structure with nominal carrier concentrations (c.c.) Layer
Material
Function
19 18 17 16 15 14 13 4—12 3 2 1 0
InP GaInAs InP GaInAsP InP GaInAsP GaInAsP MQW GaInAsP GaInAsP InP InP
Cap Contact Cladding Etch stop Spacer SCH SCH Active SCH SCH Cladding Substrate
j (lm)
1.66 1.15 1.05 1.10 1.3 1.10 1.05
d (nm)
c.c. (cm\)
Dopant
150 200 1600 50 100 100 35 5;6/4;10 35 100 '0.8 '3000
10 2;10 5;10 5;10 5;10 5;10 5;10 — 5;10 5;10 5;10 3;10
Zn Zn Zn Zn Zn Zn Zn — Si Si Si S
Fig. 1. Zn depth profiles measured by SIMS in MOMBE test lasers with and without GaInAsP etch stop layer (solid and dashed lines, resp.). Arsenic signal included (in arbitrary units). Zn penetration depth and relative Zn accumulation indicated in the profile.
pressure MOVPE system for 1 h under phosphine atmosphere at 600 and 650°C, temperatures typical for an MOVPE overgrowth process.
3. Results and discussion Zn depth profiles in test laser structures with and without the quaternary etch stop layer are shown in Fig. 1. The SIMS data include the As depth
profile (in arbitrary units) to mark the heterointerfaces. Without the etch stop layer a Zn profile with a steep decrease at the heterointerface to the active region can easily be achieved (dotted line) provided that a nominally undoped intermediate InP layer is used to compensate Zn diffusion effects, as previously described in detail [10,13]. With etch stop layer (layer 16 in Table 1) two effects are observed (solid line). A Zn accumulation occurs in this etch stop layer. Additionally, Zn moves into
P. Kro( ner et al. / Journal of Crystal Growth 195 (1998) 660–667
the nominally undoped SCH layer (layer 14) and accumulates there. The penetration depth (defined by the width, where the Zn concentration has dropped to 1/e of the maximum concentration at the interface) and the relative Zn accumulation (defined by the ratio of the maximum Zn concentration in the SCH layer to the minimum concentration in the spacer layer) are indicated in the figure. Both effects, Zn outdiffusion to the etch stop layer and to the SCH layer, result in a Zn depletion in the InP spacer layer (layer 15). This depletion is not observed without etch stop layer, because then a virtually unlimited Zn supply from the thick InP cladding layer is available to compensate Zn outdiffusion to the SCH layer. Both Zn depletion in the spacer and Zn accumulation in the SCH layer may result in deteriorated device performance. Indeed MCRW lasers fabricated from a structure with such a low Zn level of about 3;10 cm\ in the spacer layer failed at elevated temperatures (at 85°C). Part of the Zn accumulation in the etch stop layer is probably caused by segregation. The preceding InP spacer layer is Zn doped so that Zn may accumulate at the growing surface, and this segregation yields a higher Zn concentration in the following quaternary layer. Analogous segregations effects were addressed for beryllium as acceptor in InP based heterostructures in hydride source molecular beam epitaxy and in MOMBE [17,18]. The Zn accumulation is more pronounced in the etch stop layer, since here in addition to segregation, diffusion from the cladding and spacer layers (layers 17 and 15) comes into effect. An explanation for the Zn accumulation in the SCH layer, however, is not that straightforward, since the Zn precursor beam flow is turned on after growth of this layer (layer 14) so that Zn is moving backwards opposite to growth direction. From previous MOMBE and MOVPE studies it is known that in InP layers a higher concentration of Zn interstitials is formed than in ternary and quaternary layers, where Zn mainly occupies substitutional sites yielding higher free carrier concentrations in these layers than in InP [12,13]. It appears that this high concentration of Zn interstitials in InP leads to an enhanced Zn diffusion backwards into the quaternary SCH layer, where Zn is immediately consumed on substitutional
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sites maintaining this Zn diffusion flow, which results in the observed Zn accumulation in the SCH layer. Several steps can be taken to reduce the Zn depletion in the spacer layer. (i) The Zn diffusion into the active region may be reduced by a slight compensating n-type doping of the adjacent SCH layer taking advantage of the Fermi level effect of diffusion [12,19,20]. (ii) Intermediate InP layers with appropriate dopant concentration can additionally be inserted. (iii) The problem can be attacked in the spacer layer itself, where the technique of donor co-doping may be employed to decrease Zn outdiffusion [21]. A comparison with a Zn profile in a similar MCRW laser structure grown by MOVPE may elucidate the Zn diffusion phenomenon. Typical SIMS depth profiles are given in Fig. 2. The MOVPE structures were Zn doped starting from SCH layer 13. A similar procedure for MOMBE structures would result in a further Zn accumulation yielding increased threshold currents. Therefore, for MOMBE growth the Zn precursor beam flow was turned on for growth of the spacer layer 15 and subsequent layers, as mentioned above. At first sight the Zn distribution in the MOVPE structure appears to be rather similar to the distribution in the MOMBE structure. Both in the etch stop layer and in the SCH layer a Zn accumulation is detected analogous to MOMBE. However, there are quantitative differences. The Zn accumulation in the MOMBE structure is stronger than in the MOVPE structure. This behavior is expected, as part of the Zn accumulation is attributed to segregation, and in MOMBE due to the much lower growth temperature the Zn surface concentration should be higher than in MOVPE, where Zn easily desorbes at the high temperature. Furthermore, a significant reduction of Zn diffusion in the SCH layer can be realized at the lower MOVPE growth temperature. Such a profile is suited for excellent high temperature device operation. The decrease of the Zn concentration in the SCH layer appears to be steeper for the MOMBE than for the MOVPE profile. In MOMBE the Zn diffusion in InP is much more pronounced than in MOVPE in spite of the low MOMBE growth temperature. On the other hand for ternary and
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Fig. 2. Typical Zn depth profiles in MCRW laser structures grown by MOMBE and MOVPE. Arsenic SIMS signal included (in arbitrary units).
quaternary compounds the Zn diffusion appears to be weaker in MOMBE than in MOVPE. Hence in MOMBE InP/GaInAsP heterostructures the effects of Zn depletion in InP layers and Zn accumulation in GaInAsP layers appear to be stronger than in MOVPE so that in MOMBE specific procedures have to be introduced to compensate these effects. In the first approach a slight intentional n-type doping background in the p-SCH layer (layer 14) is used to affect Zn diffusion into this layer according to the Fermi level effect of diffusion [12,19,20]. With this technique the solubility and charge state of point defects are modified resulting in a decreased impurity diffusion. An analysis of SIMS data reveals that indeed the Zn penetration depth into the SCH layer can be reduced by increasing the Si background level in the SCH layer, as summarized in Fig. 3. The maximum Si beam flow (in beam equivalent pressure, BEP) would yield an electron concentration of approximately (6—7);10 cm\ in the SCH layer, and the relationship is linear [10]. The Zn accumulation at the heterointerface spacer/SCH layer (layers 14/15),
however, cannot be avoided, as shown in Fig. 3 (right y-axis). In the second approach increased Zn precursor beam flows in the spacer layer (layer 15) and in the cladding layer (layer 17) close to the heterojunction were offered to compensate Zn depletion, and in a third approach donor co-doping in the spacer layer was applied to reduce Zn outdiffusion from this layer. For delta-doping in MBE GaAs it was reported that diffusive broadening of Be acceptor spikes is completely avoided by the presence of co-deposited Si [21]. Analogous effects are found with Zn and Si co-doping in the present study. Fig. 4 shows SIMS depth profiles along with the dopant flow distributions. The Si beam flow would result in an electron concentration of (1—2);10 cm\. It can be recognized that a higher DEZn beam flow is used in the first 200 nm of the p-cladding layer preventing a Zn depletion in this cladding layer. Indeed a flat Zn profile is achieved in the cladding layer. A comparison with the MCRW structure in Fig. 1 reveals the reduced Zn loss in the co-doped spacer layer and no accumulation in the SCH layer. The laser exhibited
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Fig. 3. Effect of intentional Si background doping (in beam equivalent pressure, BEP) on Zn penetration depth into SCH layer (left y-axis, square symbols) and on relative Zn accumulation at the heterointerface spacer/SCH layer in MOMBE (right y-axis, circle symbols); data taken from SIMS analysis.
Fig. 4. Zn depth profile in an MOMBE MCRW laser structure co-doped with Si. Arsenic SIMS signal included (in arbitrary units). Zn and Si dopant beam flow distributions on top (in beam equivalent pressure, BEP). BEP would result in an electron concentration of 1 (1—2);10 cm\.
improved high temperature performance. There are several possible models for a reduced Zn diffusion in the presence of Si. Both Zn and Si occupy group III lattice sites, and Si>—Zn\ next-nearest-
neighbour pairing may occur in analogy to the interaction of Si and Be in GaAs [22]. Such a complex is probably characterized by a low diffusion coefficient. Alternatively, the presence of Si on
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a substitutional group III lattice site may increase the solubility of Zn on a substitutional lattice site. Or the presence of Si may simply inhibit the transfer of Zn from a substitutional site to the rapidlydiffusing interstitial lattice state. Hence this effect of co-doping is not unexpected. However, it should be taken into account that this co-doping technique results in a compensation of the carrier concentration, which may be a drawback. Therefore, the compensating Si level should be kept as low as possible. It should be pointed out that annealing of the MOMBE MCRW laser structure under MOVPE growth conditions does not affect the Zn doping profile, whereas a higher MOVPE growth temperature increases Zn diffusion during growth (see Fig. 2). This result has important consequences for applications, as it indicates that a basic MOMBE laser structure can be overgrown in an MOVPE process without degradation of the dopant profile. Additionally, the result suggests that diffusion during growth is determined by other mechanisms than diffusion by post-growth annealing. For a deeper insight into diffusion mechanisms surface analytical tools should be applied in MOMBE.
4. Conclusions Zn diffusion in InP based device structures has been studied in MOMBE and compared with MOVPE data. A similar Zn accumulation behavior in quaternary layers is observed for both technologies. Quantitative differences, however, can be recognized, since the MOMBE process is more surface sensitive and is performed at a lower growth temperature. Hence Zn redistribution by segration and diffusion is more pronounced in MOMBE affecting threshold current and high temperature performance of ridge waveguide lasers. This difference in Zn dopant behavior in MOMBE and MOVPE makes an individual dopant beam flow optimization for MOMBE heterostructures necessary, since MOVPE dopant flow parameters can not directly be transferred to MOMBE. For optimized Zn dopant profiles in MOMBE several steps can be taken to compensate Zn outdiffusion from InP to adjacent GaInAsP layers: the
Zn precursor beam flow should be increased in the InP layers adjacent to the etch stop layer so that in spite of Zn outdiffusion a flat dopant profile can be obtained. It is expected that a GaInAsP spacer layer with a low Ga and As content instead of the standard InP spacer should help to reduce outdiffusion due to a lower concentration of Zn interstitials in such a quaternary layer. Comparing the technique of using the Fermi level effect of diffusion with the technique of co-doping, the latter appears to be better suited to achieve appropriate Zn profiles in laser structures: a sufficient Zn concentration in the spacer layer can be maintained by using additional low Si doping in the InP spacer layer. Thus waveguide lasers with improved performance can be fabricated.
Acknowledgements The authors would like to thank Bernd Borchert, Roland Gessner and Bernhard Stegmu¨ller for laser characterization and for stimulating discussions. Part of this work was performed under contract 01 BP 468/8 of the Photonics Program of the German Ministry of Education and Research (BMBF).
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