Molecular beam epitaxy of vertically compact AlxGa1 − xAsGaAs laser-HEMT structures for monolithic integration

j. . . . . . . .

CRYSTAL GROWTH

Journal of Crystal Growth 175/176(1997) 898-902

ELSEVIER

Molecular beam epitaxy of vertically compact AlxGal - xAs/GaAs laser-HEMT structures for monolithic integration A. Gaymann*, J. Schaub, W. Bronner, N. Grtin, J. Hornung, K. K6hler Fraunhofer-lnstitut fur Angewandte Festkiirperphysik, Tullastrasse 72, D-79108 Freiburg, Germany

Abstract Vertically compact AlxGal _xAs/GaAs laser structures with Alo.3Gao.vAs/AIAsSPSL-cladding layers were grown on top of HEMT structures by MBE and investigated with regard to series resistance. We found an exponential dependence of series resistance Rs with AlAs and Alo.3Ga0.vAs SPSL-layer thicknesses at ambient temperature. Laser-HEMT structures for monolithic integration were grown and lasers were processed. For 3 × 200 ~tm2 3-QW-lasers, suitable for high-frequency performance, threshold currents of 20 mA and series resistances below 12 ~) were obtained. The slope of the linear regression of the lth values as a function of mesa width yielded a low threshold current densityjth of 480 A/cm 2. Reduction of the p-cladding thickness from 600 nm down to 450 nm shows no increase in threshold current density if the p-dopants are kept from diffusing into the active region. This clearly demonstrates that laser structures for monolithic integration can be designed very compact without loosing performance.

I. Introduction The monolithic integration of laser and electronic circuits represents a promising device technology to meet the growing demand for future information data transfer systems. An ideal semiconductor for short-range optical communication is the GaAs/AlxGax_xAs system since even sophisticated vertical structures can be routinely grown by molecular beam epitaxy (MBE). However,

*Corresponding author. Fax: +49 761 5159200, e-mail: [email protected].

in the case of monolithic integration, an increased complexity of the layer structure may lead to restrictions for parameters like growth temperatures, doping profiles and layer thicknesses which can result in lower device performance. Growth of the low-temperature H E M T structure on top of the highly doped cap of the laser structure is excluded since it results in poor high-frequency performance. A processed laser structure overgrown in an second epitaxial run is also excluded due to poor crystal quality of the overgrown layer at the mesa edges [1]. Therefore, we grow the laser structure on top of the H E M T structure. This is done during one epitaxial run. A scheme of the

0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 8 7 7 - 9

A. Gaymann et al. / Journal of Crystal Growth 175/176 (1997) 898- 902

vertical layer structure is given in Fig. 1. In order to obtain an easy to manufacture HEMT process, a vertically compact design of the laser structure is necessary. This can be achieved by a moderate thickness of the cladding layers with high aluminum content realized by short period A10.3Gao.7 As/AlAs superlattices (SPSLs). In this paper three points will be addressed: Firstly, the influence of the layer sequence of the SPSLs on the series resistance of the laser diode is studied. Secondly, DC data of lasers grown on top of the HEMT structures are reported. Finally, a successful reduction of p-cladding layer thickness is reported.

2. Growth of heterostructures and results All samples were grown by MBE on 2 in GaAs substrates in a Varian Modular Gen II. The aluminum content in the ternary compound was x = 0.3. Layers with AI mole fractions between 0 and 0.3 were realized by GaAs/Alo.3Gao.TAS short-period superlattices (SPSLs) and those with x > 0.3 by Al0.3Gao.7As/AIAs SPSLs. The growth rate for GaAs was 1.25 I.tm/h. It was controlled by reflection high-energy diffraction (RHEED) before and after the growth. The actual composition of the ternary compound, and by that, the aluminum contents and the growth rates of all other compositions were determined by additional RHEED measurement of the Alo.3Gao.vAs compound. The knowledge of the actual thicknesses of the various layers is important for the wet etching step during laser processing. The As4/Ga beam equivalent pressure was below 20. Si and Be are used as n- and pdopants, respectively. Doping concentrations are related to GaAs. Calibration of dopants was done with Hall effect measurements of GaAs layers. For studying the influence of the AlAs and Alo.3Gao.7As layer thicknesses in the cladding layers on the series resistance of the laser diode, samples with different SPSL-layer thicknesses were grown. The layer sequence of the test samples was the same as the structure displayed in Fig. 1. The mean A1 mole fraction of the cladding layers was 80%. AlAs- and Alo.aGao.7As-layer thicknesses varied from 2 to 6 nm and 0.8 to 2.4 nm,

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respectively. The total thicknesses of the cladding layer was 600 nm. The Be concentration in the p-cladding layer was chosen to 1 x 10 la cm -3 to emphasize the effect of the short period layer thicknesses on Rs. Diode mesas with a cavity length of 200 ~tm and widths from 3 to 32 ~tm were fabricated by means of wet etching. Electrical contacts were obtained by evaporation of ohmic metals. Series resistance was determined by recording I-V curves of fabricated diodes at ambient temperature. Rs is the differential resistance for diode voltages above 2 V. Fig. 2 shows the measured Rs data for cavity widths of 3 ~tm (full diamonds) and 16 ~tm (full triangles). The y-axis is chosen in logarithmic scale. We found an exponential dependence of series resistance Rs on AlAs and Alo.3Gao.TAs SPSL-layer thicknesses. This functional relation can be understood by taking into account tunneling processes of holes through the AlAs barriers. (The vertical hole transport in the cladding layer should be the main contribution to series resistance since vertical electron transport contributes much less to R~ due to the much higher mobility.) Thus, if the AlAs barrier thickness increases the tunneling probability

A. Gaymann et al. / Journal of Crystal Growth 175/176 (1997) 898-902

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should decrease exponentially, e.g., the series resistance should increase similarly. However, at room temperature not only tunneling processes occur but also vertical transport in the p-doped superlattice driven by thermionic emission is present. With increasing AlAs barrier thickness, the Alo.3Gao.vAs quantum well width increases, too, in order to keep a mean A1 mole fraction of 80%. This results in a higher energy difference between QW level and valence band edge. Thus, this second contribution also leads to higher series resistance with thicker AlAs layers since the number of carriers crossing the barriers gets smaller. However, the details of the vertical transport can only be discussed based on results for the temperature dependence of R~ and lies beyond the scope of this paper. As a result, acceptable R~ values can be obtained even with low Be doping of the cladding layers if the thicknesses of the SPSL layers are sufficiently thin. Laser-HEMT structures for monolithic integration were grown as follows (see Fig. 1): First, the two-sided h-doped QW HEMT structure was grown at a substrate temperature Ts = 520°C measured by pyrometer. This growth temperature is sufficiently low to suppress Si segregation into the two-dimensional electron gas (2DEG) channel. After growth of a thin etchstop layer the laser

structure was grown. The Ts was raised to 660°C for the growth of the n-graded layer, the n-cladding layer and the separate confinement layers with the laser active region. This active region consists of three 7.4 nm thick quantum wells which are separated by 7.2 nm thick Alo.25Gao.vsAs barriers. At the beginning of the lower confinement layer, Ts was lowered for the growth of a few monolayers to prevent Si from segregating into the quantum wells. The p-cladding layer and p-graded layer were grown at slightly reduced temperatures of 625°C and 590°C, respectively. Finally, the heavily-doped GaAs cap layer was grown at T~ = 520°C. For the AlAs and Alo.3Ga0.vAs layers in the SPSL-claddings of the actual laser-HEMT structure we choose a thickness of 3 nm and 1.2 nm, respectively. These values seem reasonable with regard to a limitation of shuttering times and frequency. The Si-doping concentration of the n-buffer, n-graded layer, and n-cladding layer was 4 x 1018 cm -3. Resistivities for higher p-doping concentrations were estimated from Fig. 2 on the supposition that the mobility is only weakly dependent on NB, in the considered doping regime and that the p-cladding layer and the p-graded layer contribute most to Rs. In order to obtain a series resistance of 15 f~ the doping concentration of the p-cladding was set to 1 x 1018 cm -3 for the first 300nm and raised to 2x1018cm -3 for the growth of the remaining 300 nm. This doping profile leads to an effective lowering of Be diffusion into the active region for two reasons. First, the Be concentration is held below the solubility limit of 2 x l 0 ~ a c m -3 in Alo.sGao.zAs. Solubility limits of Be in AlxGa~ _ xAs as a function of A1 mole fraction were found in Be doped samples by depth profiling with Secondary Ion Mass Spectrometry [2]. Our investigations have clearly demonstrated the ability of Be to diffuse through the complete p-cladding layer if the intended doping concentration in the cladding layer is beyond these upper limits. It was also shown that in this case this diffusion process cannot be suppressed by using low growth temperatures. As a second point, if the Be concentration is below the solubility limit like in the laser-HEMT structure reported here, Be diffusion can be reduced by lowering the growth temperature Ts. For that

A. Gaymann et al. / Journal of Crystal Growth 175/176 (1997) 898-902

reason Ts values of 625°C and 590°C were chosen for the p-cladding and the p-graded layer, respectively. The p-graded layer was doped with 8x1018cm -3. Laser diodes with fixed cavity length and various widths were processed by means of wet etching and evaporation of ohmic metals. Details of the process technology can be found in Ref. [3]. Finally, laser mirrors were fabricated by either reactive ion etching (RIE) or chemical assisted ion-beam etching (CAIBE). These two methods have proved to be highly reproducible and have yielded AlxGal _xAs/GaAs lasers with almost the same threshold currents as lasers with cleaved facets. lth and Rs were determined by on-wafer measurements. Here, Rs is the differential resistance for a diode current 5 mA above threshold. The lasing wavelength was 850 nm. The quality and homogeneity of the laser mirrors were tested by measuring 40 identical lasers which were located on a diameter of the wafer. We found a scattering of the Ith data points of less than 10%. Values for /th and R~ for mesa widths between 3 and 32 ~tm are shown in Fig. 3. For 3 x 200 lim 2 3 QW-lasers we found threshold currents /th below 20 mA and series resistances below 12 ft. The slope of the linear regression of the Ith values yielded a threshold current density Jth of 480 A/cm 2. This is almost the same value as obtained for high-speed AlxGal_~As/GaAs lasers grown in our laboratory on top of a 1 ~tm buffer

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under optimized growth conditions [4]. The DC performance of our laser grown on the HEMT structure can be compared with data from literature and with earlier results. Nichols et al. [5] obtained a threshold current density of 700 A/cm 2 and a series resistance of 50 f~ for a strained GaAs/AIGaAs/GalnAs MQW laser structure grown on top of a MESFET by MOCVD although much thicker cladding layers were used. The circuits operated with bandwidths as high as 3.4 GHz. A similar bandwidth of a MBE grown GalnAs/ GaAs MODFET/laser device was reached by Offsey et al. [6]. Typical threshold currents were 80 mA for the 20 x 800 ~tm 2 lasers. Earlier data of monolithic integrated laser-HEMT devices from our laboratory were reported by Hornung et al. in Ref. [7]. Typical results were a threshold current of 28mA and a series resistance of 22fl for a 4 × 360 I.tm 2 laser diode grown on top of a HEMT structure. Laser diodes and driver circuits showed performance up to data rates of 7.4 Gbit/s in a short-range optical data transfer setup. Hence, taking the actual vertical layer structure as a starting point it should be possible to obtain a fully integrated laser-HEMT device exceeding the bandwidth reported in Ref. [7]. Finally, attempts were made to make the laser structure even more compact. Laser-HEMT structures with different p-cladding thicknesses but identical doping profiles were grown. Cladding layers

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Fig. 4. Threshold currents of 3-QW-lasers grown on top of a HEMT structure with 450 and 600 nm p-cladding thicknesses.

902

A. Gaymann et al. / Journal of Crystal Growth 175/176 H997) 898-902

were doped as reported above with the exception of the p-graded and the cap layers which were doped throughout with 2 x 1018 cm -3. This was done to keep Be diffusion into the active region to a minimum. Fig. 4 shows the results for laser diodes with a cavity length of 200 ~tm and widths between 3 and 32 ~tm. Threshold current densities below 485 A/cm 2 were found for both p-cladding thicknesses of 600 and 450 nm, respectively. Rs were above 30 92 as expected. Nevertheless, no increase in jth w a s observed upon reducing the cladding layer thickness by 150 nm.

compact when the p-doping profile is chosen in a way that p-dopants are kept from diffusing into the active region.

Acknowledgements The authors wish to thank T. Jakobus and his process technology group. Especially, the work of C. Buchgeister, J. Daleiden, S. KluBmann, E. Olander and G. Schilli is acknowledged. The "Bundesministerium ffir Bildung, Wissenschaft, Forschung und Technologic" is acknowledged for financial support.

3. Conclusion Vertically compact AlxGal_xAs/GaAs laser structures grown on top of H E M T structures with Alo.3Gao.TAs/A1As SPSL-cladding layers were grown by MBE. We found an exponential dependence of series resistance Rs on AlAs and Alo.aGao.TAS SPSL-layer thickness at ambient temperature. L a s e r - H E M T structures suitable for high-frequency performance were grown and processed. A threshold current density of 480 A/cm 2 was reached for a 3-QW-laser grown on top of a low-temperature grown H E M T . L a s e r - H E M T structures grown with different p-cladding thicknesses showed no increase in threshold current density upon reducing the cladding layer by 150 nm. This clearly demonstrates that laser structures for monolithic integration can be designed even more

References I-1"] K. K6hler, Appl. Surf. Sci. 100/101 (1996) 383. [2] A. Gaymann, M. Maier, W. Bronner, N. Griin and K. K6hler, Mater. Sci. Eng. B, in press. [3] W. Bronner, J. Hornung, K. K6hler and E. Olander, in: Gallium Arsenide and Related Compounds, Eds. H.S. Rupprecht and G. Weimann (lOP, Bristol, 1994) p. 461. [-4] J.D. Ralston, I. Esquivias, S. Weisser,D.F.G. Gallagher,P.J. Tasker, E.C. Larkins, J. Rosenzweig, H.P. Zappe, J. Fleissner and D.J. As, SPIE Vol. 1680 (1992) 127. [5] D.T. Nichols, J. Lopata, W.S. Hobson, N.K. Dutta, P.R. Berger, D.L. Sivco and A.Y. Cho, Electron. Lett. 30 (1994) 490. I-6] S.D. Offsey,P.J. Tasker, W.J. Schaff,L. Kapitan, J.R. Shealy and L.F. Eastman, Electron. Lett. 26 (1990) 350. [7] J. Hornung, Z.-G. Wang, W. Bronner, E. Olander, K. K6hler, P. Ganser, B. Raynor, W. Benz and M. Ludwig, Electron. Lett. 29 (1993) 1694.