We have observed lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs/A1a,Gae,As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/S.-Such long lifetimes in GaAs doped at N,= 1.3 X 10” cme3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies

Ahrenkiel, R. K.

Levi, D. H.

Lundstrom, Mark S

Lush, Gregory Benedict

MacMillan, H. F.

Melloch, Michael R

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Faculty PublicationsDepartment of Electrical and ComputerEngineering1992Microsecond Lifetimes and Low InterfaceRecombination Velocities in Moderately Doped n-GaAs Thin FilmsGregory Benedict LushPurdue UniversityD. H. LeviNational Renewable Energy Laboratory, Golden, COH. F. MacMillanVarian Research Center, Palo Alto, CAMichael R. MellochPurdue University, melloch@purdue.eduMark S. LundstromPurdue University, lundstro@purdue.eduSee next page for additional authorsFollow this and additional works at: https://docs.lib.purdue.edu/ecepubsPart of the Electrical and Computer Engineering CommonsThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Lush, Gregory Benedict; Levi, D. H.; MacMillan, H. F.; Melloch, Michael R.; Lundstrom, Mark S.; and Ahrenkiel, R. K., "MicrosecondLifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films" (1992). Department of Electrical andComputer Engineering Faculty Publications. Paper 95.https://docs.lib.purdue.edu/ecepubs/95AuthorsGregory Benedict Lush, D. H. Levi, H. F. MacMillan, Michael R. Melloch, Mark S. Lundstrom, and R. K.AhrenkielThis article is available at Purdue e-Pubs: https://docs.lib.purdue.edu/ecepubs/95Microsecond lifetimes and low interface recombination velocities in moderatelydoped n-GaAs thin filmsG. B. Lush, M. R. Melloch, and M. S. LundstromD. H. Levi and R. K. AhrenkielH. F. MacMillanCitation:  61, (1992); doi: 10.1063/1.108190View online: http://dx.doi.org/10.1063/1.108190View Table of Contents: http://aip.scitation.org/toc/apl/61/20Published by the American Institute of PhysicsMicrosecond lifetimes and low interface recombination velocities in moderately doped n-GaAs t G. B. Lush, M. R. Melloch, and M. S. Lundstrom School of Electrical Engineering, Purdue University, West Lafayette, Indiana 47907 D. H. Levi and R. K. Ahrenkiel National Renewable Energy Laboratory, Golden, Colorado 80401 H. F. MacMillan Varian Research Center, Palo Alto, California 94303 (Received 17 March 1992; accepted for publication 11 September 1992) We have observed lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs/A1a,Gae,As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/S.-Such long lifetimes in GaAs doped at N,= 1.3 X 10” cme3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies. Thin-film GaAs solar cells, where the active volume is separated from the substrate, offer a potential method of achieving a 35% eficient solar cell,’ a priority of the pho- tovoltaic community. Such high efficiencies in GaAs re- quire ultralong lifetimes which have thus far been achieved only in nominally undoped material unsuitable for bipolar devices.24 For moderately doped GaAs, however, lifetimes on the order of 1 ps are theoretically possible if photon recycling is enhanced by removing the light-absorbing sub- strate.5 We report here our observation of lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs double heterostructure membranes obtained by removing the sub- strate by chemical etching. We attribute our ultralong life- times to enhanced photon recycling.3”-8 We also demon- strate high-quality interfaces, with the upper limit of the recombination velocity being 25 cm/s and a more accurate estimate being S < 12 cm/s. Figure 1 shows the basic structure of the double het- erostructures (DHs) grown for this study. Five DHs with GaAs active layer thicknesses ranging from 0.25 to 10 ,um were grown at 740 “C by metalorganic chemical vapor dep- osition in a horizontal reactor. The n-GaAs active layers were doped with selenium to a concentration of N,= 1.3 X 10” cmw3. Alc3Gaa7As cladding layers, doped to ND =2X 1018 cmV3, provide surface passivation and carrier confinement, and the A10~8,Gae,,A~ layer acts as an etch stop.’ An area of the substrate typically 2-3 mm in diam- eter was removed by wet etching using granular citric acid dissolved in deionized water and H202.9’*0 The Alo~,,GacI,As was then removed with hydrofluoric acid diluted to 5% by volume with deionized water. The DH membrane was therefore still attached at its periphery to the substrate which was easily handled by tweezers. As depicted in Fig. 1, photoluminescence (PL) decay was ob- served both on the membrane area and on the adjacent region where the substrate remained intact. PL decays were observed using time-correlated single-photon count- ing techniques described in more detail previously. “,” The illumination source was a 10 ps full width at half maxi- mum, mode locked laser focused to a 0.8 mm diam circle at a continuous incident power of 0.2 mW, with the exception of the 0.25 pm thick DH for which the incident power was 0.05 mW. With a repetition rate of 80 kHz, the laser gen- erated < 1016 carriers/cm3, so the DHs were always in low injection. The PL spectra were observed in the same sys- tem using standard photon counting techniques. The longest lifetime observed in these samples was 1.2 ps for the 10 pm DH. This decay constant, rnn, is fully 3 1 times the expected radiative lifetime computed using r, = l/BN,, where B is 2x lo-‘s cm3/s.’ (Although pub- lished values of the B-coefficient vary over an order of magnitude,‘,13 used value.14) B=~x 10-t’ cmm3/s is the most commonly Figure 2 shows the measured decays for the 5 pm DIE. Without the substrate backing [curve (a)], rnH is 1.05 ps for the initial half of the decay time. Curve (a) in Fig. 2 n-GaAs Active Layer FIG. 1. Structure of double heterostructure (DH) showing hole etched in substrate and the two locations illuminated individually by the laser. 2440 Appl. Phys. Lett. 61 (20). 16 November 1992 0003-6951/92/452440-03$03.00 @ 1992 American Institute of Physics 2440 1 o3 -57 -E I.- 2 x lo2 2-i 2 s c - ii 10' l a- .-?, . . -.. 0 0.5 1 1.5 2 2.5 3 3.5 4 Time Qs) 8000 8200 8400 8600 8800 9000 Wavelength (A) FIG. 2 Photoluminescence decays for 5 pm DH illuminated in the two locations depicted in Fig. 1. Without the substrate [curve (a)], the decay constant, ron, is 1.05 ps, 27 times the expected radiative lifetime. Even the DH still backed by the substrate [curve (b)] has a decay constant enhanced nearly ten times over the supposed radiative limit. FIG. 3. PL spectra showing the 12-fold increase in intensity exhibited by the GaAs membrane [curve (a)] over that of the region [curve (b)] still backed by the absorbing substrate. shows evidence of the saturation of Shockley-Read-Hall centers, ” as initially the PL decays more slowly than it does toward the end of the plot. All the lifetimes reported here are for the initial parts of the curves; this more closely corresponds to the injection level appropriate for devices of interest, such as solar cells. Even with the substrate intact Ecurve &)I, TJJH is 360 ns, nearly ten times the supposed radiative limit. These ultralong decay constants are due to photon recycling, the reabsorption of photons emitted dur- ing radiative recombination events.’ Self-absorption gener- ates new electron-hole pairs which increases the observed lifetime. The effective radiative lifetime in a DH can then be written as Tag= +q,., where 4, Asbeck’s recycling cofac- torp6 is the inverse of the average probability that an iso- tropically emitted photon escapes the DH. Previous work- ers3*16 have computed 4,~ 11 for a 5 pm DH still attached to its substrate, which is consistent with our result, assum- ing that some nonradiative recombination occurs. The ef- fective radiative lifetime of the 5 pm membrane is further enhanced over the region still attached to the substrate because the back side of the DH is now more reflective. The critical angle, 0,,, of the back interface of the DHs still attached to their substrates is about 72”, whereas for the back of the membranes 0,,= 17”.16 Thus, simply re- moving the substrate increases 4, because the isotropically emitted photons are better trapped, increasing their prob- ability of being reabsorbed. This method of enhancing 7,ff has been proposed as a means of increasing GaAs solar cell efficiency.’ Our results demonstrate that if one is able to control nonradiative recombination, ultralong lifetimes are possible, even at this moderately high doping concentra- tion. ted that might normally pass into the substrate are now totally internally reflected and eventually reabsorbed. The resulting electron-hole pair might later emit a photon to- ward the front interface during a radiative recombination event. The result of this recycling process is that there are more “opportunities” for photons to exit the front face and be detected.” Figure 4 plots ran versus DH thickness for all the samples, with and without substrates. There is a stronger thickness dependence to ran for the standard DHs over roH for the membranes because the highly reflective inter- faces in the membranes trap most of the photons, so the probability of being reabsorbed is essentially independent of DH thickness. Therefore, #r is more strongly dependent on the critical angle of the interfaces than on DH thickness w. For the standard DHs, 4,. is a stronger function of w because the back interface is less reflective so an average photon in a thicker DH is more likely to be reabsorbed +z E 3 1 I I I J2 0 2 4 6 8 ---IO DHThickness(~m) Figure 3 shows the PL spectra for the 5 pm DH with and without the substrate intact. The intensity of the peak without the substrate is [curve (a)] 12 times that of the peak observed with the substrate [curve (b)]. The increase in PL peak intensity is due to the decrease in 0,, for pho- tons escaping out the back of the DH. Many photons emit- FIG. 4. DH decay constants vs DH thickness, with [curve (b)] and without [curve (a)] the substrate. Under both circumstances, ran in- creases with increasing thickness due both to stronger self-absorption in thicker DHs and to the decreasing contribution from the interfaces to the total recombination. Also shown [curve (c)] is the enhancement of ran, defined as the thin-film decay constant divided by the decay constant observed with the substrate intact. 2441 Appl. Phys. Lett., Vol. 61, No. 20, 16 November 1992 Lush et a/. 2441 0.8 , I 1 t , 1 0 12 3 4 5 6 7 8 2/w (pnY1) FIG. 5. Plot of l/rou vs 2/w for the DHs membranes. The slope of the plot yields an interface recombination velocity of less than 12 cm/s. before reaching the largely transparent back interface, as compared to a photon in a thinner DH. These consider: ations explain why the enhancement factor, defmed as rnn without the substrate divided by rnn with the substrate for the same thickness, steadily increases with decreasing DH thickness. The long lifetimes observed in thin films make it easy to deduce the interface recombination velocity because in- terface recombination is more important than it was with the substrate intact. If SW/D < 1, where D is the minority carrier diffusivity, recombination in a DH can be described by 18,19 &=&+E, (1) where r,,ulk is the bulk lifetime. One can obtain an upper limit to the interface recombination velocity S by attribut- ing all the recombination of the thinnest DH membrane to the interfaces. In this case S=u1/2ron, and since for the 0.25 pm DH, rDn= 500 ns, the upper limit of S is 25 cm/s. One can obtain a more accurate value of S by plotting l/~nn vs 2/w, as shown in Fig. 5. Equation ( 1) suggests that the data in such a plot should lie on a straight line with slope S. However, r,,,lk changes with w due to photon recycling SO plots Of l/Ton vs 2/w always display negative curvature when the material is dominated by radiative re- combination and photon recycling.*’ A least-squares fit to the data points of the three thinnest DH membranes yields S=12 cm/s, while a line drawn through the points repre- senting the two thinnest DH membranes has a slope of S < 11 cm/s. We  believe these numbers to be more reason- able values for the interface recombination velocity. To conclude, we have measured lifetimes longer than 1 ,us in thin-film GaAs double heterostructure membranes with active regions doped to N,= 1.3 X 10” cme3. These ultralong lifetimes are possible because of a combination of enhanced photon recycling produced by the removal of the backing substrate and minimal nonradiative recombination in the bulk and at the interfaces. We  deduced an absolute upper limit of S=25 cm/s for the interface recombination velocity and a more reasonable estimate of S < 12 cm/s. Such long lifetimes suggest that thin-film GaAs solar cells designed to enhance photon recycling may represent a vi- able way of achieving higher cell efficiencies. The authors would like to thank R. Alferos for his assistance in growing the double heterostructures at the Varian Research Center. This work was supported by the National Renewable Energy Laboratory and by Varian As- sociates. ‘G. Lush and M. Lundstrom, Solar Cells, 30, 337 (1991). *D. J. Wolford, G. D. Gilliland, T. F. Kuech, L. M. Smith, J. Martinsen, J. A. Bradley, C. F. Tsang, R. Venkatasubramanian, S. K. Ghandi, and H. P. Hjalmarson, J. Vat. Sci. Technol. 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I4 R K Ahrenkiel, Minorify Carrier Lifetime in Compound Semiconduc- . . ton (Academic, New York, 1988), Chap. 1, p. 3. “R. K. Ahrenkiel, B. M. Keves, and D. J. Dunlavy, J. Appl. Phys. 70, 225 (1991). _ ‘“K L. Miller, M.S.E.E. thesis, University of Colorado, at Boulder, 1989. “I. ‘Schnitzer, E. Yablanovitch, C. Caneau, and T. J. Gmitter, Appl. Phys. Lett. (to be published). ‘*R. K. Ahrenkiel, Solid-State Electron. 35, 239 (1992). I9 R. J. Nelson, J. Vat. Sci. Technol. 15, 1475 (1978). “G. B. Lush, H. F. MacMillan, B. M. Keyes, R. K. Ahrenkiel, M. R. Melloch, and M. S. Lundstrom, Conference Rec., 22nd IEEE Photovol- fait Specialists Conference (IEEE, New York, 1991), p. 189. 2442 Appl. Phys. Lett., Vol. 61, No. 20, 16 November 1992 Lush et a/. 2442 

Microsecond Lifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films

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Microsecond Lifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films

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