The Advantage of Molecular-Beam Epitaxy for Device Applications

8 The Advantage of Molecular-Beam Epitaxy for Device Applications

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 1, 2016 | http://pubs.acs.org Publication Date: October 2, 1985 | doi: 10.1021/bk-1985-0290.ch008

A. Y. Cho AT&T Bell Laboratories, Murray Hill, NJ 07974

Continuous striving to improve the performance of semiconductor devices motivated much research in semiconductor material science. In turn, a new material may generate new devices and new technology. Molecular beam epi taxy may hold the key to producing the next generation microwave and optoelectronic devices with "band-gap engineering." Monolithic integration and in situ dry processing will be unique developments of MBE for the coming years.

The most important feature of molecular beam epitaxy (MBE) is itsflexibilityin growing vari ous compounds and structures with precise doping and compositional profiles accurate to atomic dimensions. This is one of the reasons that MBE has led in the technology forefront such as with the first fabrication of multilayered structures with dimensions smaller than a carrier diffusion length 0,2). Unique electrical (3,4) and optical (5) properties of MBE grown layers that do not exist in bulk semiconductors have led to developments such as quantum well lasers, (6,7) modulation doped high electron mobility devices, (8-10) and separately controlled absorp tion and multiplication region (SCAM) photodiodes (11). In this communication, we would like to discuss the advantages of MBE for device fabrication, the present status, and future developments. Advantages of MBE Molecular beam epitaxy is conducted in an ultra high vacuum environment (12,13). The film material (constituent) can therefore be started or stopped abruptly to grow thin layers with abrupt changes in doping and compositional profiles. Most of the devices made to date resulted from the fact that semiconductor materials were grown in one reactor and the metallization was done in another. Between these two steps, more than 30A of oxide is usually formed on the sur face of the semiconductor. Since MBE is a vacuum process, many of these device fabrication processes can be incorporated with the MBE system as shown in Fig. 1. For instance, for the purpose of making smaller devices with higher yields, non-alloyed ohmic contacts, (14,15) Schottky barrier diodes, 06,17) insulating layers, (18-20) and in situ masking (21-24) can all be performed in the MBE system. A combination of MBE and ion implantation may produce precise dimensions in both lateral and vertical directions (25). The epitaxial growth rate with MBE can be varied from 0.1 μπι/hr to 10 μιη/hr. The advantage of a slower growth rate com pared to that of chemical vapor deposition or liquid phase epitaxy is that one can control the layer thickness more precisely. The fast production rate of MBE depends upon its fast 0097-6156/85/ 0290-0118$06.00/ 0 © 1985 American Chemical Society

In Integrated Circuits: Chemical and Physical Processing; Stroeve, Pieter; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Molecular-Beam Epitaxy for Device Applications


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turnaround time. For example, one may load six to twelve wafers with 2 inch diameter in a commercial MBE system, and the time required for exchanging substrates is less than five minutes. No bakeout or gas purging is required for MBE between sample exchanges. One may therefore prepare twelvefieldeffect transistor (FET) wafers or six laser wafers in a day. This one-day production may keep a twenty person processing line occupied for one week. Molecular beam epitaxy is very versatile. One may grow an FET, a laser, a mixer diode, a bipolar transistor, a photodetector, and a varactor in one sequence and in one day without modifying either the sample holder or the deposition geometry. The epitaxial growth is not under thermodynamic equilibrium and therefore a large range of abrupt changes in doping profile and compositional profile are possible. Furthermore, nucleation on a foreign substrate is more readily accomplished than with other growth techniques. For instance, the growth for a lattice mismatched system will proceed even when the strain energy exceeds the Gibbs energy. The controlled strained superlattice is another new development with MBE. One other important point is the conservation of film materials. For the growth of GaAs, ten grams of Ga will prepare more than 100 FET wafers. A point that becomes of increasing concern is operational safety. MBE may be operated with solid sources (Ga, As, Al, P, Si, Sn, Be, ...) with refill load-lock for ovens without using toxic gases such as arsine or phosphine. However, there is some advantage in using the latter for Ga Inj_ AS|_ Py which will be dis cussed in a following section. X

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Present Development of MBE One of the recent efforts in MBE for device fabrication has been to extend the growth to materials such as I ^ G a ^ A s , (26-28) In Alj_ As, (29,30) Ga Al Inj__ __ As, (31,32) and Ga Inj_ AS|_ Py (33-36). These alloy systems cover the band-gap energies (wavelengths) from 0.756 eV (1.65 μπι) to 1.55 eV (0.8 μπι) and are lattice matched to InP substrate at the same time. For the Ga AlyInj_ As system solid charges of Ga, Al, In, and As may be used (18) while the Ga Inj__ Asj__ P system "gas source" MBE appears to be the best choice (35,37-38). One of the most intriguing manners by which to achieve a laser operating at 1.55 Mm with Gag 47I11Q 53AS is to fabricate quantum-well lasers instead of the standard doubleheterostructure lasers which would operate at 1.65 μτη. The first successful preparation of a GaQ 47I11Q ^ A S / A I Q 4glng multiquantum-well laser was recently reported (39). The schematic diagram of the quantum-well laser structure and the spontaneous and stimulated emission characteristics are shown in Fig. 2. The active layer consisted of a series of GaQ 47I11Q 53AS wells separated by barriers of A1Q 4gIriQ 52^· The ^ S band-gap difference between wells and barriers, AEq — 0.7. eV and the large conduction band discontinuity, AE — 0.5 eV, (40) result in very efficient carrier confinement. For a sampte with 13 layers of GaQ 47I11Q 53As, 90A thick, separated by 12 layers of A1Q 4gInQ 52^* 30A, estimate the up shift in lasing energy to be 56 meV. Adding the heavy hole contribution of =10% of the above, the energy shift is expected to be about 63 meV. The emission peak in Fig. 2 is in agree ment with the calculated values. Figure 3 shows the light output as a function of current for this multiquantum-well laser. Threshold current density as low as 2.4 KA/cm^ was achieved (39). Excellent results of double heterostructure and separate confinement heterostructure lasers of Ga Inj_ Asj_yP lattice matched to InP and emitting at 1.5 μτη have been reported recently (36). These lasers were grown by gas source MBE utilizing the decomposition of AsH and P H at 900-1000 C as a source of AS2 and P2 molecules. The advantage of using gas sources is to reduce the frequency for group V element oven replenishment if solid group V sources were used. It is also more reproducible in achieving precise Α^/^2 ^ by the use of precise gas flow controls. Source tanks containing 250 gm of 100% PH3 and 50 gm of 100% x

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C o n c e p t u a l arrangement of an i n s i t u p r o c e s s i n g MBE system.

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AsH^ can prepare approximately 360 laser wafers. A schematic of the gas source cracking oven and tnc gas handling system is shown in Fig. 4. At a growth temperature of about 525 C, the accommodation coefficient of Ρ is less than that of As on the growing surface (35,41,42). Therefore, a higher Ρ to As ratio is required in the beam flux than that in the solid. This rela tionship is illustrated in Fig. 5 (Ref. 35). Broad area lasers (λ - 1.5 Mm) with room tempera ture pulsed threshold current densities of 2 KA/cm^ were reproducibly obtained (36). Another new development in M B E is to demonstrate the capability for simultaneously lattice matched graded composition structures in the quaternary material system (32). This "band-gap engineering" resulting from the graded composition opens a new window in our search for a new degree of freedom for device and structure design. Grading of the high field region and superlattices in an avalanche photodiode have been used to enhance the ionization rates ratio in the GaAs/AlGaAs system (43). Recently, the first vertical Npn AIQ 4gIriQ ^2^s/GaQ 47I1ÏQ 53AS heterojunction bipolar transistor with a graded emitter comprised of a quaternary layer of GaQ 47_ Α1 Ιι^ 53As was reported (32). Grading from GaQ 47I11Q 53AS to AIQ 4gInQ was achieved by simultaneously lowering the A l and raising the Ga oven temperature in such a manner as to keep the total group HI flux constant during the transition. The energy band diagram for the abrupt and graded transistors are shown in Fig. 6(a) and 6(b), respectively. The elimination of the spike at the heterojunction for the graded emitter case enhances the forward injection and results in an improvement of current gain from 140 to 280 as illustrated in Fig. 7(a) and 7(b), respectively. Molecular beam epitaxy has the ability to grow extremely sharp doping profiles with n+ and p-H layers. One therefore can design a separately controlled absorption and multiplication region (SCAM) photodiode (11). The electric field profile in the avalanche region is controlled by the hi-lo doping profile resembling a hi-lo IMPATT diode (44). The electric field at the heterojunction region for this structure is constant rather than linearly graded. This eliminates the problem of tunneling in the low gap layer. Together with the lower electric field in the gain region it becomes easier to achieve simultaneously both high gain and low dark current. For a typical structure, the high band-gap region is comprised of a 5000A undoped (10 /cm ) AlQ4gInQ avalanche layer, and a 5 0 0 À thick n+ spike doped to 5 x l 0 / c m , separated from the heterojunction interface by a low electric field AIQ 4gIriQ i-layer, 2000A thick. The absorption layer is 2 μτη of undoped GaQ4 InQ 3As (n PH

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GROWTH OF G a j j I n ^ A s ^ y P y BY MBE

F i g u r e 5. R e l a t i v e c o n c e n t r a t i o n of phosphorus i n the m o l e c u l a r beam as a f u n c t i o n of t h a t i n c o r p o r a t e d i n the s o l i d . (Adapted from Ref. 35.)

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F i g u r e 6. Schematic energy-band diagram under e q u i l i b r i u m f o r the Alo.48Ino.52As/Gao.47Ino.53As h e t e r o j u n c t i o n b i p o l a r t r a n s i s t o r w i t h (a) abrupt e m i t t e r and (b) graded e m i t t e r . Note the e l i m i n a t i o n of the c o n d u c t i o n band n o t c h through the use of a graded e m i t t e r . (Adapted from Ref. 32.)


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C H E M I C A L A N D PHYSICAL PROCESSING O F INTEGRATED CIRCUITS


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F i g u r e 7. Common-emitter c h a r a c t e r i s t i c s of the A I Q . 4 8 0 . 5 2 A s / G a 4 7 InQ 5 3 A S h e t e r o j u n c t i o n b i p o l a r t r a n s i s t o r w i t h ( a ) abrupt e m i t t e r and ( b ) ' g r a d e d e m i t t e r a t 300 K. (Adapted from Ref. 32.) 0m


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0.85 — 1.7 /im SCAM — A P D

DISTANCE F i g u r e 8. Schematic of a s e p a r a t e l y c o n t r o l l e d a b s o r p t i o n and m u l t i p l i c a t i o n r e g i o n s (SCAM) avalanche photodiode (APD) u s i n g h i g h low doping p r o f i l e s i n the avalanche r e g i o n t o produce c o n s t a n t electric field. (Adapted from Ref. 11.) Acknowledgments The author would like to thank M. B. Panish, F. Capasso, and K. Alavi for useful advice and discussions.

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11. F. Capasso, K. Alavi, A. Y. Cho, P. W. Foy and C. G. Bethea, International Electron Dev ice Meeting Digest, p. 468, Washington, DC, December 5-7, 1983. 12. A. Y. Cho and J. R. Arthur, Progress in Solid State Chemistry, edited by G. Somorjai and J . O. McCaldin (Pergammon, New York, 1975), Vol. 10, p. 157. 13. A. Y. Cho, Thin Solid Film 100, 291 (1983). 14. P. A. Barnes and A. Y. Cho, Appl. Phys. Lett. 33, 651 (1978). 15. J. V. DiLorenzo, W. C. Niehaus and A. Y. Cho, J. Appl. Phys. 50, 951 (1979). 16. A. Y. Cho and P. D. Dernier, J. Appl. Phys. 49, 3328 (1978). 17. A. Y. Cho, E. Kollberg, H. Zirath, W. W. Snell, and M. V. Schneider, Elec. Lett. 18, 424 (1982). 18. H. C. Casey, Jr., A. Y. Cho, D. V. Lang, and E. H. Nicollian, J. Vac. Sci. Technol. 15, 1408 (1978). 19. R. F. C. Farrow, P. W. Sullivan, G. M. Williams, G. R. Jones, and D. C. Cameron, J. Vac. Sci. Technol. 19, 415 (1981). 20. C. W. Tu, S. R. Forrest, and W. D. Johnston, Jr., Appl. Phys. Lett. 43 569 (1983). 21. A. Y. Cho and F. K. Reinhart, Appl. Phys. Lett. 21, 355 (1972). 22. W. T. Tsang and A. Y. Cho, Appl. Phys. Lett. 32, 491 (1978). 23. A. Y. Cho and W. C. Ballamy, J. Appl. Phys. 46, 783 (1975). 24. W. T. Tsang and A. Y. Cho, Appl. Phys. Lett. 30 293 (1977). 25. K. Tabatabaie-Alavi, A. N. M. M. Choudhury, K. Alavi, J. Vlcek, N. Slater, C. G. Fon stad, and A. Y. Cho, IEEE Electron. Dev. Lett. EDL-3, 379 (1982). 26. Κ. Y. Cheng and A. Y. Cho, J. Appl. Phys. 53, 4411 (1982). 27. Y. Kawamura, Y. Moguchi, H. Asaki, and H. Nagai, Electron. Lett. 18, 91 (1982). 28. J. Massies, J. Rochette, P. Delescluse, P. Eitenne, J. Chevrier and Ν. T. Linh, Electron. Lett. 18, 758 (1982). 29. Κ. Y. Cheng and A. Y. Cho, J. Appl. Phys. 53, 4411 (1982). 30. M. A. Brummell, R. J. Nicholas, J. C. Portal, Κ. Y. Cheng, and A. Y. Cho, J. Phys. C: Solid State Phys. 16, L579 (1983). 31. K. Alavi, H. Temkin, W. R. Wagner and A. Y. Cho, Appl. Phys. Lett. 42, 254 (1983). 32. R. J. Malik, J. R. Hayes, F. Capasso, K. Alavi, and A. Y. Cho, IEEE Electron. Dev. Lett. EDL-4, 383 (1983). 33. A. Y. Cho, J. Vac. Sci. Technol. 16, 275 (1979). 34. W. T. Tsang, F. K. Reinhart and J. A. Ditzenberger, Appl. Phys. Lett. 41, 1094 (1982). 35. M. B. Panish and S. Sumski, J. Appl. Phys. (to be published). 36. M. B. Panish and S. Sumski, Appl. Phys. Lett, (to be published). 37. A. R. Calawa, Appl. Phys. Lett. 38, 701 (1981). 38. M. B. Panish, J. Electrochem. Soc. 127, 2729 (1980). 39. H. Temkin, K. Alavi, W. R. Wagner, T. P. Pearsall, and A. Y. Cho, Appl. Phys. Lett. 42, 845 (1983). 40. R. People, K. W. Wecht, K. Alavi, and A. Y. Cho, Appl. Phys. Lett. 43, 118 (1983). 41. J. R. Arthur and J. J. LePore, J. Vac. Sci. Technol. 6, 545 (1969). 42. C. T. Foxon, B. A. Joyce and M. T. Morris, J. Cryst. Growth 49, 132 (1980). 43. F. Capasso, J. Vac. Sci. Technol. B1, 457 (1983). 44. A. Y. Cho, C. N. Dunn, R. L. Kuvas, and W. E. Schroeder, Appl. Phys. Lett. 25, 224 (1974). 45. J. P. Faurie and A. Million, J. Cryst. Growth 54, 582 (1982). RECEIVED March 12, 1985


The Advantage of Molecular-Beam Epitaxy for Device Applications

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