InP structures by selective area MOMBE

j........ C R Y S T A L OROWTH

ELSEVIER

Journal of Crystal Growth 175/176 (1997) 1186 1194

Lateral coupling of InP/GalnAsP/InP structures by selective area MOMBE M. Wachter a, U. Sch6ffel a, M. Schier b, H. H e i n e c k e a'* a University of Ulm, Department of Semiconductor Physics, D-89069 Ulm, Germany bSiemens AG, Corporate Research and Development, D-81730 Munich, Germany

Abstract Lateral couplings of InP/GalnAsP/InP structures selectively grown by metalorganic molecular beam epitaxy (MOMBE) are presented. The heterostructures were grown by either using the hydrides AsH 3 and PH3 or tertiarybutylphosphine and tertiarybutylarsine as group V precursors in a prototype multiwafer (MOMBE) system. The base heterostructures of the first epitaxy were patterned with SiO2 mask stripes and trenches were reactive-ion etched. Heterostructures were selectively filled in by a second growth run forming lateral heterojunctions of different quaternary materials. The structures exhibit a bright photoluminescence and a sharp transition at the boundary of the locally grown material indicating a high crystal quality up to the lateral junction with only a minor change in the PL wavelength ( < 2 nm). These optimized lateral couplings were applied to laser/waveguide butt-couplings. Optical coupling losses were determined by means of reactive-ion etched waveguides across cascades of coupled GalnAsP layers with an emission wavelength of 2~ ~ 1050 nm. Values as low as 0.12 dB/coupling were determined in cut-back measurements.

1. Introduction The realization of photonic integrated circuits (PICS) is a field of growing interest [1-3]. The monolithic integration of devices like lasers, waveguides and photodetectors require some lateral coupling of heterostructures. Lateral couplings can be achieved by overgrowth of patterned semiconductor surfaces [4, 5]. A more attractive pathway for the production of PICS with lateral couplings is the embedded selective-area epitaxy

*Corresponding author: Fax: + 49 731 502-6106; e-mail: [email protected].

(SAE), due to the planarity of the structures and a higher degree of freedom in the design of the device structures [1, 2, 6]. The true SAE of I n P / G a l n A s P heterostructures is only achieved in metalorganic growth systems [2, 7]. The lateral couplings of heterostructures by SAE have been realized in metalorganic vapour phase epitaxy (MOVPE) [1, 8-10] and in metalorganic molecular beam epitaxy (MOMBE) [3, 11]. In M O V P E the gas-phase-concentration gradients and diffusion processes lead to a growth-rate enhancement and to a change of the material composition starting at a distance larger than 50 I.tm from the masked area. This behaviour is a function of the growth parameters and depends on the mask

0022-0248/97/$17.00 Copyright (t3 1997 Elsevier Science B.V. All rights reserved P l l S0022-0248(96)0 1027- 5

M. Wachter et al. / Journal of Crystal Growth 175/176 (1997) 1186-1194

geometry and the aspect ratio of the masked to the unmasked area [2, 12-14]. In MOMBE the growth is determined by the adsorption, diffusion and desorption processes of the molecules on the semiconductor surface only. The variation of the material composition on the (1 0 0) surface at a transition of a growth to a nongrowth area is not significantly affected [6] and is rather independent of the aspect ratio [2, 15]. In the case of embedded SAE, the growth starts in a tub at various crystal planes. The growth rate on these side walls has to be optimized for planar infills. The published couplings achieved by selective area MOMBE so far were all grown by using a tilted molecular beam geometry for the group III precursors [3, 11]. Due to shadowing and side wall growth effects there is only a small growth parameter window to achieve a good lateral contact without gaps or planar layers with no growth rate enhancement at the transition area for the use of a tilted beam geometry. Therefore, we have here applied a perpendicular molecular beam geometry for reducing the lateral growth rate [16]. In terms of production, the replacement of hydrides by less dangerous precursors like tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP) is of growing interest [17, 18]. High-quality heterostructure couplings are obtained with both classes of precursors. The optimization of the embedded SAE-process yields extremely low-loss waveguide couplings applicable in PICs.

2. Experimental procedure The samples were grown in a multiwafer production-type MOMBE system (Riber Epineat). The precursors trimethylindium (TMI), triethylgallium (TEG) for the group III elements, and thermally precracked arsine (ASH3) or TBAs and phosphine (PH3) or TBP for the group V elements are injected into the growth chamber. The group III molecular beams are created by gas injectors having an angle of 90 ° or 50° with respect to the substrate surface. Due to the lower lateral growth all the growth experiments presented in this study were carried out by using the perpendicular molecular beam geometry [16].

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In the first epitaxy an InP/GalnAsP/InP double hetero(DH)structure was grown on a 2 inch InP wafer misoriented 2° towards the nearest (1 1 0) plane. This base structure was covered by different 120 nm-thick SiO2 masks. The mask consists either of SiO2 stripes with a width between 4 and 100 pm and a periodicity of 500 lam or of 15 pm wide SiO2 twin stripes separated by a gap ranging from 1.5 to 5 ~tm with a periodicity of 250 ~m. Trenches up to 2.7 ~m in depth were reactively ion etched into the base structure of the first epitaxy. Prior to the second SAE growth run, the samples were etched in H2SO4 : H202" H20 (3 : 1 : 6 in volume ratio) for 5 to 20 s just before introduction into the loading chamber. DH structures were filled in by the second growth run with a typical V/III input ratio of 10, a growth temperature of 495°C and a vertical growth rate of 0.5 pm/h. We have investigated lateral couplings between InP/GalnAsP/InP base structures with an emission wavelength of 26 ~ 1550 or 1050 nm and a filled in DH structure with 26 ,~ 1550 or 1050 nm. All the couplings were oriented in the [0 1 1] or [0 1 1] direction, which is the standard orientation for laser cavities in the production.

3. Results and discussion The test structure for the integration of lasers and waveguides is the coupling of a DH structure with 26 ~ 1550 nm and the infill DH structure with 26 ~ 1050 nm. The scanning electron microscopy (SEM) crosssection view in Fig. la shows a 5 pm wide selective infill (26 = 1048 nm) in between two 15 pm wide masked DH base structures (26 = 1510 nm). The SEM investigations reveal planar infill-layers and smooth contacts with lateral heterojunctions of the quaternary layer. The selective infill was grown under conditions where the (0 1 1) and (01T) planes are kinetically stable in planar SAE [7, 16, 19]. The perpendicular molecular beam geometry ensures a low lateral growth rate. In the case of a high lateral growth rate, the infilled layers are curved and the vertical growth rate enhancement in the vicinity of the junction facet leads to the formation of the significant "ears" in

M. Wachter et al. / Journal o f Crystal Growth 175/176 (1997) 1186-1194

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Fig. 1. Lateral junctions between selective infill [InP(d = 0.15 pm)/GalnAsP(d = 0.5 ~tm, 20 = 1048 nm)/InP(d = 0.15 ~tm)] and masked DH structure [InP(d = 0.8 pm)/GalnAsP(d = 0.2 ~tm, 26 = 1510 nm)/InP(d = 0.2 ~tm)] grown by using AsHs and P H 3. (a) SEM cross-section view; (b) Spatially resolved PL measurements at 300 K. Linescans of the PL intensity at the PL wavelength of the maximum and PL wavelength versus the lateral position of the laser spot.

the t r a n s i t i o n a r e a [2]. A n y h o w the lateral g r o w t h rate has to be high e n o u g h to a v o i d gaps between the infill a n d the side wall of the m a s k e d base structure. T h e p l a n a r i t y of the S A E layers s h o w n is i n d e p e n d e n t of the aspect ratio (here investigated from 2 to 22%) of the m a s k e d to the u n m a s k e d

surface a n d of the w i d t h of the m a s k e d ridges o r the width of the etched trenches. D u r i n g the g r o w t h of the I n P buffer layer a vertical I n P s e p a r a t i o n layer was g r o w n at the side wall of the t u b (see Fig. la). This I n P s e p a r a t i o n layer is a d j u s t a b l e in the thickness in the r a n g e from 10%

M. Wachter et al. / Journal of Crystal Growth 175/176 (1997) 1186-1194

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Fig. 2. Lateral junctions between selective infill [InP(d = 0.15 pm)/GalnAsP(d = 0.4 pm, )~c = 1047 nm)/InP(d = 0.15 p m ) a n d masked D H structure [InP(d = 0.8 p,m)/GalnAsP(d = 0.2 pm, )oc = 1540 nm)/InP(d = 0.2 I~m)] grown by using TBAs and TBP. (a) SEM cross-section view; (b) Spatially resolved PL measurements at 300 K, Linescans of the PL intensity at the PL wavelength of the m a x i m u m and PL wavelength versus the lateral position of the laser spot.

to 90% of the InP buffer thickness by changing the growth parameters of the buffer layer and the molecular beam geometry. In the micrograph shown in Fig. la this lateral thickness is 50%. The thickness of the InP separation layer is reduced for a lower vertical growth rate and a lower growth

temperature. For comparison, when using the tilted injection geometry the thickness of the InP separation layer is increased up to a factor of 2. An efficient butt-coupling of two D H structures requires a constant material composition of the quaternary infill up to the mask edge. We have

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h'L Wachter et al. /Journal of C~stal Growth 175/176 (1997) 1186-1194

investigated the material quality at the transition area by means of spatial-photoluminescence (g-PL) measurements at room temperature. The plot in Fig. lb shows linescans of the PL intensity dependent on the lateral position of the excitation laser spot (spot size 5 pm in diameter) on the (1 0 0) surface of the sample. The SAE quaternary layer exhibits a bright luminescence and a sharp decay at the boundary of the locally grown structure. This decay of the PL intensity at 2~ = 1048 nm coincides spatially with the increase at 2a = 1510 nm and vice versa. The measured decay /increase length of 4 - 5 pm coincides with the spot size of the excitation laser beam. Consequently, a sharp transition boundary is realized. The measured maximum PL wavelength at the lateral heterojunction in the coupling zone is very stable. The small deviation of + 2 nm is in the order of the wavelength error using 300 K PL for the rapidly decreasing intensity at the junction. At a distance of 5 gm, the PL wavelength of the infill is identical to the emission wavelength of a large-area-grown reference sample emphasizing the independence of the aspect ratio of the mask. Previous investigations by spatially resolved Xray diffraction of heterostructures selectively grown by M O M B E have shown a change of the lattice mismatch of less than 1.0x 10-4 in a transition width of 6 gm. p-PL measurements at these samples have demonstrated low variations in the PL wavelength too [20, 21]. We conclude from this knowledge and our P L results that the quaternary material composition with a high crystal quality is not affected up to the lateral junction and that the composition of the infill is identical to the reference sample. In comparison to the sample presented in the SEM micrograph in Fig. la, the filled in D H structure (2c = 1047 nm) in Fig. 2a was grown using TBAs and TBP as group V precursors. This infill shows a slight ear formation. The linescans of the PL intensity in the plot in Fig. 2b exhibits again a sharp transition of the quaternary materials with emission wavelengths of 2c; = 1047(infill) and 1540 nm (base structure). The measured wavelength shift of + 4 nm of the quaternary infill at the junction is slightly higher for the infill grown with TBAs and TBP. This example was grown with

Fig. 3. Lateral bun-coupling of a 2 SCH-MQW (,:tE = 1550nm) laser with a waveguide(2c = 1050nm) structure.

a vertical growth rate of 1 pm/h instead of the standard growth rate for SAE of 0.5 pm/h. Further experiments with the growth rate of 0.5 pm/h have shown a slight growth rate enhancement at the mask boundary too. This overgrowth and wavelength shift is probably caused by different surface diffusion processes, so that an individual optimization for the TBP/TBAs-process is required. The results of the optimization in the hydride process of the selective infill were applied for laser/waveguide butt-couplings. Fig. 3 presents a cross-section of a laser/waveguide butt-coupling grown with the conventional hydrides. There is a smooth contact between the strained multiquanturn-well laser structure with two separate confinement layers (2 SCH MQW) [22] and the waveguide infill [23]. The infill with a height of 2.5 pm shows neither any overgrowth over the mask nor ears at the junctions. During the growth of the entire selective infill the growth rate, the growth temperature and the V/III ratio was kept constant. This convenient growth procedure is only applicable in the perpendicular geometry [16, 241]. Masked islands of 100 pm x O00 ~tm were embedded by the waveguide D H structure with 2~ = 1050nm forming lateral couplings. Laserwaveguide chips were cleaved. The lasers (400 pm in length) are surrounded on three sides by the waveguide D H structure. The threshold current

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M. Wachter et al. / Journal of Crystal Growth 175/176 (1997) 1186-1194

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Fig. 4. Lateral junctions between selective infill [InP(d = 0.25 lam)/GalnAsP(d = 1.15 lam, )oo = 1100 nm)/InP(d = 0.4 lam)] and masked DH structure [InP(d = 0.5)/GalnAsP(d = 1.1/am, 2o = 1058 nm)/InP(d = 0.45 lam)] (a) SEM cross-section view; (b) Spatially resolved PL measurements at 300 K. Linescans of the PL intensity at the PL wavelength of the maximum and PL wavelength versus the lateral position of the laser spot.

density of a l a s e r / w a v e g u i d e c o m b i n a t i o n with a 300 ~tm-long w a v e g u i d e - D H structure is jth = 1.28 k A / c m 2. The t h r e s h o l d c u r r e n t density decreases to jth = 880 A / c m 2 if the w a v e g u i d e - D H structure in the direction of the laser cavity is cleaved off. This t h r e s h o l d c u r r e n t density is identical with the value of a reference s a m p l e before s e l e c t i v e - a r e a - g r o w t h processing [22, 23]. C o n s e -

quently, the etching a n d SAE g r o w t h of the waveguide have n o t d e g r a d e d the laser performance. In o r d e r to d e t e r m i n e q u a n t i t a t i v e l y the optical losses at such l a t e r a l c o u p l i n g s a wavegui d e / w a v e g u i d e i n t e g r a t i o n was processed. T h e S E M cross-section view in Fig. 4a gives an e x a m p l e of a c o u p l i n g of a n I n P / G a l n A s P ( 2 G = 1058 n m ) / I n P and an InP/GalnAsP(2G =

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M. Wachter et al. /Journal of Crystal Growth 175/176 (1997) 1186-1194

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1100 nm)/InP. In this example the masked structure was grown with TBAs and TBP. Such a structure allows for the investigation of the material composition at the transition areas by ~t-PL due to a difference in 2~ higher than the full width of maximum of the P L peak at 300 K and for the precise loss measurements at the butt-couplings

due to an approximately identical refractive index of both quaternary materials. This SAE infill exhibits absolutely planar layers, a thin InP separation layer (10% of the buffer layer thickness) and a smooth contact to the masked quaternary material. The Ix-PL data in Fig. 4b are comparable to the data of the example in Fig. lb, except for the lateral

M. Wachter et al. / Journal of Crystal Growth 175/176 (1997) 1186-1194

transition of an emission wavelength of ).6 = 1058 to 1100 nm. Again, there is only a shift of the PL wavelength at the junction of less than ___2 nm. These properties promise highly efficient buttcouplings. Fig. 5a gives a Nomarski interference micrograph of a top view on a series of reactive-ionetched-rib waveguides perpendicular across the lateral heterojunctions. The horizontal 8 and 12 Bmwide lines represent the heterostructure with 2c = 1058 nm from the first grown structure. In between, there is an infill of the DH structure with 26 = 1100 nm. We have etched two different types of rib waveguides as shown in the inset of Fig. 5b. In type 1 the wave is guided by a 3 p.m wide reactive-ion-etched ridge in the quaternary material. The second type of waveguide is obtained by a 4.5 Ixm-wide ridge etched in the InP cap layer. The precise determination of the loss of the waveguide/waveguide butt-couplings was achieved with the cut-back method using the TE polarized laser light with a wavelength of 1540nm. In Fig. 5b, the coupling loss of these different types of waveguides versus the number of butt-couplings is plotted. The small spread of data points for the two different waveguides of the same type in the case of type 1 and three in the case of type 2 over 12 cutbacks illustrates the high uniformity of the couplings. Some scatter of cleaved edge surface gives the major source of error in the measurement. Type 1 shows an optical loss of(0.22 ___0.05) dB/coupling in the cut-back measurements for a series of 24 butt-couplings and type 2 the best value of (0.12 ___0.04) dB/coupling for a series of 22 buttcouplings. The propagation loss of identical waveguides on a reference sample without any couplings was measured to be (0.59 _+ 0.06) dB/cm by the Fabry-Perot resonance method [25]. This low propagation loss of the waveguides are comparable to the best published data [26, 27] and is still included in the cut-back measurements.

4. Conclusion The embedded SAE grown lateral couplings of InP/GalnAsP/InP-DH structures have a constant material composition up to the lateral contact. The

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PL intensity is high up to the junction with a rapid decay at the junction given by the resolution of the laserspot of about 5 Ixm. We have presented the first lateral coupling of GalnAsP/InP heterostructures selectively grown by using the replacement precursors TBAs and TBP. The reactive ion etching process and the selective regrowth of waveguides in MOMBE allow for a high quality butt-coupling of laser and waveguides and does not degrade the threshold current densities of the M Q W lasers. The butt-coupling efficiency of 97% measured at a cascade of 22 couplings on a length over 5.5 mm, which is in the order of the dimensions of chips incorporating PICs, is very attractive for applications in the integration of photonic devices.

Acknowledgements The authors would like to thank E. Veuhoff, H. Baumeister, S. Illek, B. Schmidt from Siemens AG, P. Albrecht from Heinrich Hertz Institute, M. Popp and M. Keidler from University of Ulm for their support in processing and stimulating discussions. This work was partly funded by the German Ministery of Education and Research (BMBF) under contracts 01 BP 468/8 and 01 BM 412/2.

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