The minority hole diffusivity, or equivalently the hole mobility, was measured in n+GaAs with the zero‐field time‐of‐flight technique. The minority hole mobility was measured for the donor doping range of 1.3×1017 cm−3 to 1.8×1018 cm−3 and was found to vary from 235 to 295 cm2/V s. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8×1018 cm−3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnp‐heterojunction bipolar transistors

Ahrenkiel, R. K.

Lovejoy, Michael L.

Lundstrom, Mark S

Melloch, Michael R

English

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Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Faculty PublicationsDepartment of Electrical and ComputerEngineering1992Minority Hole Mobility in n+ GaAsMichael L. LovejoyPurdue UniversityMichael R. MellochPurdue University, melloch@purdue.eduMark S. LundstromPurdue University, lundstro@purdue.eduR. K. AhrenkielNational Renewable Energy Laboratory, Golden, COFollow 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.Lovejoy, Michael L.; Melloch, Michael R.; Lundstrom, Mark S.; and Ahrenkiel, R. K., "Minority Hole Mobility in n+ GaAs" (1992).Department of Electrical and Computer Engineering Faculty Publications. Paper 94.http://dx.doi.org/doi: 10.1063/1.108108Minority hole mobility in n+GaAsM. L. Lovejoy, M. R. Melloch, and M. S. LundstromR. K. AhrenkielCitation:  61, (1992); doi: 10.1063/1.108108View online: http://dx.doi.org/10.1063/1.108108View Table of Contents: http://aip.scitation.org/toc/apl/61/22Published by the American Institute of PhysicsMinority hole mobility in n+GaAs M. L. Lovejoy, M. R. Melloch, and M. S. Lundstrom School of Electrical Engineering, Purdue University, West Lafayette, Indiana 47907 R. K. Ahrenkiel NationaI Renewable Energy Laboratory, Golden, Colorado 80401 (Received 22 June 1992; accepted for publication 22 September 1992) The minority hole diffusivity, or equivalently the hole mobility, was measured in n”GaAs with the zero-field time-of-flight technique. The minority hole mobility was measured for the donor doping range of 1.3 X 1017 cmU3 to 1.8~ 1018 cm-’ and was found to vary from 235 to 295 cm’/V s. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8X 10” cm-3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnp-heterojunction bipolar transistors. Recent measurements of minority hole diffusivity, or equivalently minority hole mobility, in n+GaAs showed that minority hole mobility was greater than the majority hole mobility in comparably doped p-type GaAs.‘*2 This result is in striking contrast with results for minority elec- tron mobilities which have been found to be lower than mqjority electron mobilities3-6 and also contrasts with a recent theoretical prediction of low minority hole mobili- ties.7 In degenerately doped silicon, however, both minor- ity hole and electron mobilities have been found to be greater than majority mobilities.8,9 For bipolar device en- gineering there is a great need for minority mobility data in nCGaAs. In this letter, we report direct measurements of the zero-field minority hole mobility for doping densities from 1.3~ 10” to 1.8~ 10’s cmp3. The zero field time-of-flight (ZFTOF) technique was used to measure the minority hole diffusivity, Dp6 In this technique a specially designed photodiode is photoexcited with a high-speed laser (3-5 ps FWHM). Carriers are generated near the surface and diffuse in a zero-field envi- ronment to the pn-junction that is 1 pm from the surface. A mode-locked YAG laser, synchronously pumping a dye laser containing Rhodamine 6G dye was used for photo- generation. The excitation wavelength is 600 nm, which corresponds to an absorption length 0.2 pm. To extract D,,, the measured transient response is fit to a numerical sim- ulation.” Steady-state internal quantum efficiency (IQE) analysis was also performed on the same films used in the ZFTOF study in order to investigate the surface recombi- nation velocity and the diffusion length. Devices for this study were grown on (100) p+GaAs substrates by molecular beam epitaxy (MBE) in a Varian GEN-II system. Silicon and beryllium are the n- and p-type dopants, respectively. The device structure used in’ this study is shown in Fig. 1 and described in Table I. As shown in the figure, the emitter surface is passivated with Al,.,,G~,,,As to provide a low surface recombination ve- locity at the emitter-AlGaAs interface, thereby eliminating it as a fitting parameter in the transient analysis. A thin 1 pm emitter layer was chosen to minimize recombination and reabsorption erects and to maximize the sensitivity to diffusivity. Its thickness was accurately controlled by mon- itoring reflection high energy electron diffraction oscilla- tions during film growth. Mesa isolation by wet etching with NH4:H202:DI (10:3.5:500) was used to fabricate the diodes of area 500x500 ,um.2 An alloyed AuGe:Ti:Au contact to the emitter was electron beam evaporated and alloyed at 400 “C for 90 s. Electrical contact to the sub- strate was facilitated by the alloyed indium layer formed during substrate mounting for MBE. To minimize circuit effects, the devices were packaged in a high-speed pack- age.6 Film properties were characterized by Hall mobility measurements and secondary ion mass spectroscopy (SIMS) analyses. Prior to the Hall measurements, the thick cap was etched off the Hall devices. The majority carrier Hall mobilities shown in Table I were found to agree with the upper bound of reported experimental data.” (A Hall factor of 1 was assumed in the Hall anal- yses.) SIMS analysis yielded consistent values of total sil- icon concentration, which are also shown in Table I. Sili- con is an amphoteric dopant in GaAs, and these data imply compensation ratios ( NA/ND) between 0.11 and 0.33. The SIMS analysis also verified that the emitter layer thickness was correct and that the emitter doping profile was uni- form. Our previously reported measurement at the highest doping level was performed with a large portion of the junction ( -54%) shadowed by the emitter contact met- allization and by a black wax that was applied over the mesa perimeter.’ Covering the perimeter is necessary be- I-GaAs cap 150 nm “-%*,G%.7,AS 40 nm n-GaAs emitter p-GaAs base 1 w p-GaAs Substrate FIG. 1. Cross section of zero field time-of-flight diode used for measure- ments of minority hole diffusivity. 2683 Appl. Phys. Lett. 61 (22). 30 November 1992 0003-6951/92/472683-02$03.00 @  1992 American Institute of Physics 2683 TABLE I. The base and emitter concentrations, compensation level in the emitter, and majority carrier Hall mobility in the emitter for n-GaAs devices for measuring the minority carrier hole diffusivity. Concentration Base Emitter Emitter Compensation Hall ( x lV6 Hall SIMS ratio cmm3) (X 10” cm ‘) mobility ( x 10” cm-‘) WA/N,) (cm*N s) 18 1.3 1.5 0.14 4650 0.7 2.7 3.0 0.11 4150 18 6.8 8.5 0.22 3170 1.4 18 25 0.33 2390 larger than the majority hole mobility while at the lower doping levels they are comparable. Over the investigated concentration range the minority hold diffusivity is roughly constant, but the majority hole diffusivity varies by nearly a factor of two. The theoretical prediction of the minority carrier hole mobility is much smaller than our measured results, which suggests that more work is needed to understand the important scattering mechanisms in n+GaAs. cause generation along the perimeter can significantly disl tort the measured signal. This is a particular problem for these devices because the cap layer is thick; consequently, generation in the emitter is lower than generation on the perimeter, where the photon flux is not attenuated by the cap layer. Simulations using a lumped element RC trans- mission line model6 indicate rapid lateral signal propaga- tion in the sample with the emitter doped at 1.8X10’* cmd3. However, this is not the case for the devices with emitters doped in the mid-lOi crnm3 range. Therefore low levels of shadowing were necessary. With all devices used in this study, contacts were moved off the mesa onto poly- imide layers and metallization was used to shadow the perimeter thereby reducing the shadowing to < 8%. For the three lower doped samples, steady-state IQE analysis showed that essentially all carriers generated in the nS GaAs emitter layers were collected, so the ZFTOF analysis contained only one fitting parameter, Dp. For the sample with emitter doped at 1.8X 1018 cm- 3 two fitting parameters were necessary, Dp and rp. The resulting mi- nority hole mobilities are shown in Fig. 2 along with a curve representing the majority hole mobility, a theoretical prediction of the minority hole mobility by Lowney and Bennett,’ and a measurement of the minority hole mobility at 4~ lOI8 cme3 by Slater.’ For the doping concentration of 1.8X IO’* cme3, the minority hole mobility is 30% minority hole Slatar et al. -- - majority hole” -Lowney and Bennett’ 1 o18 Doping (cme3) FIG. 2. The minority hole diffusivities resulting from zero field timeof- Right measurements. Also shown is a line representing the majority hole mobility, theoretical prediction of the minority hole mobility by Lowney and Bennett, J. Appl. Phys. 69, 7102 (1991), and a measurement of the minority hole mobility at 4~ IO’* cm1113 by Slater et al, IEEE Electron Device Lett. 12, 54 (1991). Because recombination was minimal in the three lower doped emitters, 7p could only be deduced for the most heavily doped emitter of 1.8 X lOI cm.-.3. IQE fits for the most heavily doped emitter yielded a diffusion length of 1.4-1.7 pm. Combining this with the diffusion coefficient implies a lifetime of 3.3-4.8 ns, which agrees with the value deduced from the transient analysis and with the expected radiative limited lifetime for n + GaAs doped to 1.8 X 10” cm-3 11 In summary, the minority hole diffusivity in silicon doped n+GaAs was found to be roughly constant over the doping range of 1.3 x lOI to 1.8 x lo’* cme3. At the high- est doping concentration Dp was found to be 6.0 cm’/s, which corresponds to a room-temperature zero-field mo- bility of 230 cm2ff s. This is 30% higher than the majority hole mobility at this doping. At the lowest doping concen- tration, Dp was found to be 7.5 cm’/s, which corresponds to a room-temperature zero-field mobility of 295 cm-/V s. These results have important implications for the design of GaAs devices such as solar cells and pnp-heterojunction bipolar transistors. This work was supported by the National Science Foundation, The Indiana Business Modernization and Technology Corporation, and by the National Renewable Energy Laboratory. Author Lovejoy acknowledges sup- port from the AT&T Ph.D. Scholarship Program. The au- thors thank B. M. Keyes and D. J. Dunlavy for helpful discussions and technical assistance in the ZFTOF mea- surements. ‘M. L. Lovejoy, B. M. Keyes, M. R. Melloch, R. K. Ahrenkiel, and M. S. Lundstrom, Inst. Phys. Conf. Ser. 99 ( 1991). ‘D. B. Slater, Jr., P. M. Enquist, F. E. Najjar, M. Y. Chem. J. A. Hutchby, A. S. Morris, and R. J. Trew, IEEE Electron Device Lett. 12, 54 (1991). “M. I-Nathan, W. P. Dumke, K. Wrenner, S. Tiwari, S. L. Wright, and K. A. Jenkins, Appl. Phys. Lett. 52, 654 (1988). 4T. Furuta, H. Taniyama, and M. Tomizawa, J. Appl. Phys. 67, 293 (1990). “M. L. Lovejoy, B. M. Keyes, M E. Klausmeier-Brown, M. R. Melloch, R. K. Ahrenkiel, and M. S. Lundstrom, Extended Abstracts of the 22nd (1990) International Conference on Sotid State Devices and Materials (Japan Society of Applied Physics, Tokyo, 1990), Part I, p. 613. 6M. L. Lovejoy, M. R. Melloch, R. K. Ahrenkiel, and M. S. Lundstrom, Solid-State Electron. 35, 251 (1992). “I. R. Lowney and H. S. Bennett, J. Appl. Phys. 69, 7102 (1991). ‘A. J. Dziewior and A. D. Silber, Appl. Phys. Lett. 35, 170 (1979). ‘S. E. Swirhun, J. A. Del Alamo, and R M. Swanson, IEEE Electron Device Lett. EDL-7, 168 (1986). “M. Ilegems, The Technology and Physics of Molecular Beam Epitaxy, edited by E. H. C. Parker (Plenum, New York, 1985), Chap. 5, p. 83. “M. S. Lundstrom, M. E. Klausmeier-Brown, M. R. Melloch, R. K. Ahrenkiel, and B. M. Keyes, Solid-State Electron. 33, 693 (1990). 2684 Appl. Phys. Lett., Vol. 61. No. 22, 30 November 1992 Lovejoy et al. 2664 

Minority Hole Mobility in n+ GaAs

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Minority Hole Mobility in n+ GaAs

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