Minority electron mobilities in pf-In0.ssGac4, As have been measured with the zero field time-of-flight technique. The room-temperature (297 K) minority electron mobilities for p+-In,, 53Gac47A~ doped 0.9 and 3.1 x 10” cmm3 are found to be 2900 and 3300 cm* V-’ s-l, respectively. These are the first measurements to demonstrate enhancement in minority-carrier mobility as doping is increased for heavily doped Ines3Gae.4+s. This enhancement in mobility as doping is increased is similar to that observed in p+-GaAs, which has been attributed to reductions in plasmon and carrier-carrier scattering between minority electrons and majority holes

Hamm, R. A.

Harmon, E. S.

Lovejoy, Michael L.

Lundstrom, Mark S

Melloch, Michael R

Ritter, D.

English

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Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Faculty PublicationsDepartment of Electrical and ComputerEngineering1993Minority-carrier mobility enhancement in p+InGaAs lattice matched to InPE. S. HarmonPurdue UniversityMichael L. LovejoyPurdue UniversityMichael R. MellochPurdue University, melloch@purdue.eduMark S. LundstromPurdue University, lundstro@purdue.eduD. RitterAT&T Bell Laboratories, Murray Hill, NJSee 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.Harmon, E. S.; Lovejoy, Michael L.; Melloch, Michael R.; Lundstrom, Mark S.; Ritter, D.; and Hamm, R. A., "Minority-carriermobility enhancement in p+ InGaAs lattice matched to InP" (1993). Department of Electrical and Computer Engineering FacultyPublications. Paper 102.https://docs.lib.purdue.edu/ecepubs/102AuthorsE. S. Harmon, Michael L. Lovejoy, Michael R. Melloch, Mark S. Lundstrom, D. Ritter, and R. A. HammThis article is available at Purdue e-Pubs: https://docs.lib.purdue.edu/ecepubs/102Minority-carrier mobility enhancement in p+ InGaAs lattice matched to InPE. S. Harmon, M. L. Lovejoy, M. R. Melloch, and M. S. LundstromD. Ritter and R. A. HammCitation:  63, (1993); doi: 10.1063/1.109974View online: http://dx.doi.org/10.1063/1.109974View Table of Contents: http://aip.scitation.org/toc/apl/63/5Published by the American Institute of PhysicsMinority-carrier mobility enhancement in p+ InGaAs lattice matched to InP E. S. Harmon, M. L. Lovejoy,a) M. R. Melloch, and M. S. Lundstrom School of Electrical Engineering, Purdue University, West Lafayette, Indiaea 47907-1285 D. Ritterb) and R. A. Hamm AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 8 April 1993; accepted for publication 13 May 1993) Minority electron mobilities in pf-In0.ssGac4, As have been measured with the zero field time-of-flight technique. The room-temperature (297 K) minority electron mobilities for p+-In,, 53Gac47A~ doped 0.9 and 3.1 x 10” cmm3 are found to be 2900 and 3300 cm* V-’ s-l, respectively. These are the first measurements to demonstrate enhancement in minority-carrier mobility as doping is increased for heavily doped Ines3Gae.4+s. This enhancement in mobility as doping is increased is similar to that observed in p+-GaAs, which has been attributed to reductions in plasmon and carrier-carrier scattering between minority electrons and majority holes. this technique, the photodiodes described above are pho- Ino.s3Gac47As has emerged as a very important mate- rial for use in heterojunction bipolar transistor (HBT) structures lattice matched to InP substrates. In0.s3Gac,As has a number of properties that make it a good choice for HBTs: it has an extremely small electron effective mass and a very large separation between the I? and L valleys in the conduction band that can allow for higher velocity overshoot.’ To accurately design and analyze InGaAs based heterojunction bipolar transistors (HBTs), it is im- portant to understand the minority-carrier transport prop- erties of heavily doped p-Ins5sGau4,As. Here, we present the first measurements of an enhancement in minority- carrier mobility for increasing doping in Inc,,Gac,,As. The photodiode films used for this study were grown by the metalorganic molecular-beam epitaxy (MOMBE) technique. Arsine and phosphine served as group V sources, trimethylindium and triethylgallium as the group III sources, and thermal beams of Be and Sn derived from elemental sources as p- and n-type dopants, respectively. The grow& apparatus and procedures were previously described. ’ The films consisted of an InP buffer layer grown on the InP substrate, a thin n-&,s,Ga,,,As base region, and a thick p-In0.s3Gaa.47As emitter region. A thin InP window was grown on top of the emitter region to insure that the surface recombination velocity at the het- erointerface was less than lo4 cm/s, and a thin In0.s3Gas4,As cap was placed on top of the window to facilitate ohmic contact formation (see Fig. 1). The 500 x 500 pm* diode mesas were wet etched using 4: 1: 10 H3P04 :H202 :H20 to selectively etch In,,sGas4,As and 3:l H3P04:HC1 to selectively etch the InP window region. The thickness of the InGaAs region was measured with an Alpha Step profilometer. Ohmic contact to the emitter re- gion was made with Au:Ti:Au metalization shadowing less than 2% of the diode mesa, and the device was mounted in a high-speed package to minimize circuit effects. The zero field time-of-flight4 (ZFTOF) technique was used to measure the minority electron diffusivity 0,. In ‘)Present address: Sandia National Laboratories, Organization 1322, Al- buquerque, NM 87185. “Present address: Technion Institute, Haifa, Israel. toexcited with <0.5 ps full width at half-maximum (FWHM) pulses from a Coherent Mira 9OOf Ti:sapphire laser pumped with 14 W from a Coherent Innova 415 argon-ion laser. A Conoptics Pockels cell is used to reduce the laser repetition rate to -5 kHz in order to allow suf- ficient time for the transient response to decay before the next pump pulse arrives. Excitation wavelengths of both 720 and 440 nm were used in this study, where 440 nm was obtained by doubling the Ti:sapphire output at 880 nm using a 2-mm-thick P-BaB204 (BBO) crystal. The optical pulse generates electron hole pairs near the top surface of the emitter region of the photodiode. The electrons then diffuse in a zero-field environment to the edge of the de- pletion region and are collected by the junction. To extract D,, the measured transient response is fit to a one- dimensional numerical simulation of transport in the P-I~.~~G~Q~~As emitter region with fitting parameters be- ing diffusivity ( D,), lifetime, and surface recombination velocity (S,) at the top InP/InGaAs interface. The use of InP as a passivation layer for the InGaAs region ensures that Sf< lo4 cm/s, which minimizes the effect of S, on the transient response. The ZFTOF transient responses for the InGaAs structures studied here are sensitive to both the cap Window I p - InCaAs N’ W* Emitter n - InCaAs 1x10“ 2500 h B.&Se I InP n+ Substrate FIG. 1. ZFTOF diode structures used for mobility measurements. N* is the emitter doping density and WY is the emitter width. For Ny=O.9 X 10” cmm3, w*=2.2 pm and for N*=3.1X 10” cme3, wY’1.7 ym. 636 Appl. Phys. Lett. 63 (5), 2 August 1993 0003-6951/93/63(5)/636/3/$6.00 ~-0 1993 American Institute of Physics 636 time (psec) FIG. 2. Example 297 K ZFT’OF response for a p-InGaAs photodiode doped 3.1 X 10” cmm3 with an excitation wavelength of 440 nm. Included in this plot are numerical simulation results with mobility=3300 cmZV-Is-’ and lifetime=O.l ns. Also included are simulations with f 15% change in mobility. minority electron lifetime and diffusivity. The minority electron mobility is obtained from the diffusivity with the Einstein relation. An example of the measured and simu- lated ZFTOF response for the InGaAs sample doped 3.1 X 10” cmm3 is presented in Fig. 2. Steady-state measurements of the internal quantum ef- ficiency (IQE) versus wavelength of the diode structure were also performed to give independent information about the diffusion length under the assumption of low Sf.’ Ac- curate modeling of the IQE requires accurate knowledge of the absorption coefficient of InGaAs; unfortunately, the reported absorption coefficients for InGaAs vary by as much as 50%.6p7 For the InGaAs photodiodes used in this study, the IQE is very sensitive to the diffusion length as shown in Fig. 3. Including the uncertainty in the absorp- tion coefficient, we estimate the accuracy of the diffusion length determination by IQE to be =!=20%. Combining both the IQE results and the ZFTOF re- sults, we obtain room-temperature (297 K) minority elec- tron mobilities of 2900 and 3300 ~~~ V-’ s-l for the hole concentrations of 0.9 and 3.1 x 1019 cmm3, respectively, us- ing an excitation wavelength of 440 nm. For excitation at 720 nm, we obtain approximately 6% lower mobilities. For both ZFTOF excitation wavelengths we obtain effective 80 -s 60- wavelength (nm) FIG. 3. Internal quantum efficiency (IQE) measurements for the In- GaAs ZFTOF diode doped 3.1 X 10” cmn3. The solid lines are theoretical IQE results for the diffusion lengths (L) of 0.80, 0.95, and 1.10 pm, assuming the front surface recombination velocity is low ( < lo4 cm/s). lifetimes of 0.3 and 0.1 ns for doping densities of 0.9 and 3.1 x lOI cmm3, respectively. The small discrepancy be- tween the 720 and 440 nm results may be attributed to uncertainties in the absorption coefficient at 720 nm and systematic experimental error in the time zero determina- tion. There are three major sources of errors that can result in uncertainties in the mobility measurements. First, un- certainties in the lifetime may cause significant error in the measured mobility. The 20% uncertainty in the IQE mea- surement of the diffusion length, when combined with the ZFTOF measurements, results in less than 15% uncer- tainty in mobility due to -30% error in lifetime estima- tion. Second, photon recycling’ where additional electron- hole pairs are generated throughout the device due to the absorption of photons emitted during radiative recombina- tion events may also be significant for these samples. Nu- merical simulations4 of photon recycling in the structures used in this study indicate that the effect of photon recy- cling on the measured mobility is minimal, while the effect on lifetime may be as large as a factor of 2. Thus the lifetimes presented here should be vietied as effective life- times that have been enhanced by photon recycling. The final source of error results from the assumption that the carrier transport is entirely diffusive. The carriers are gen- erated with about 1.0 and 2.0 eV excess energy for 720 and 440 nm excitation, respectively. If these “hot” carriers do not thermalize quickly then the assumption of entirely dif- fusive transport would not be valid. Femtosecond carrier relaxation measurements in pf-GaAs (Refs. 9 and 10) show that electrons rapidly thermalize in <OS ps and de- cay to the lattice temperature with an - 1 ps time constant. Similar results appear to hold for I~.,,G~,,As.” Thus we do not expect the large excess energy of the electrons to have a significant effect on the measured mobility. As a comparison, the measured mobilities in p+- IQ.~~G%.~~As are about 2.1 and 1.2 times larger than the measured mobilities in ~+-G~As,‘~‘*~ which may most likely be attributed to the smaller electron mass in In0.53Ga,,47As. These results also indicate that, like p’- Ga-% p+-%.&%.47 As shows an enhancement in minor- ity electron mobility as doping density is increased for de- generately doped material.12’13 Lowney and Bennett14 theoretically demonstrate that a rise in minority electron mobility in pf-GaAs would be expected as plasmon and carrier-carrier scattering between minority electrons and majority holes are reduced at high hole concentrations. Plasmon scattering is reduced because the hole plasmon frequency becomes too high for interaction with the elec- trons, and carrier-carrier scattering is suppressed because degeneracy reduces the number of states available to re- ceive a scattered hole. In conclusion, we have measured minority electron mobilities in p+-In,,53C;%,47 As of 2900 and 3300 cm2 V-’ s-l for hole concentrations of 0.9 and 3.1 X i0” cm -3, respectively. These are the first measurements of minority electron mobility in heavily doped Ino.53G%.47As that demonstrate an enhancement in the minority electron mobility for increasing doping. These results indicate that Harmon et al. 637 637 Appl. Phys. Lett., Vol. 63, No. 5, 2 August 1993 the transport of electrons across the base of an InGaAs HBT may be thermally limited. The diffusion velocity (2!? width) for a 5OO-A-wide base doped -3 x 10” cm would be 3.4~ lo7 cm/s, which is considerably larger than the thermal velocity of electrons in InGaAs. This work was supported by the Indiana Business Modernization and Technology Corporation through the Purdue Optoelectronic Center, and the Department of En- ergy University Research Instrumentation Program. ‘R. Sakamoto, K. Akai, and M. Inoue, IEEE Trans. Electron Devices 36, 2344 (1989). *D. Ritter, R. A. Hamm, M. B. Panish, J. M. Vandenberg, D. Gershoni, S. D. Gunapala, and B. F. Levine, Appl. Phys. Lett. 59, 552 (1991). ‘R. A. Hamm, D. Ritter, H. Ten&in, M. B. Panish, and M. Geva, Appl. Phys. Lett. 58, 2378 (1991). 4M. L. Lovejoy, M. R. Melloch, R. K. Ahrenkiel, and M. S. Lundstrom, Solid-State Electron. 35, 25 1 ( 1992). 5L. D. Partain, M. S. Kuryla, L. M. Fraas, P. S. McLeod, and J. A. Gape, J. Appl. Phys. 61, 5150 (1987). 6H. Burkhard, H. W. Dinges, and E. Kuphal, I. Appl. Phys. 53, 655 (1982). ‘F. R. Bather, J. S. Blakemore, J. T. Ebner, and J. R. Arthur, Phys. Rev. B 37, 2551 (1988). so. von Roos, J. Appl. Phys. 54, 1390 (1983). ‘W. Z. Lin, R. W. Schoenlein, J. G. Fujimoto, and E. P. Ippen, IEEE J. Quantum Electron. QE-24, 267 (1988). “T. Furuta and A. Yoshii, Appl. Phys. Lett. 59, 3607 (1991): “H Kurz, W. Kuett, K. Seibert, and M. Strahen, Solid-State Electron. 3i, 447 (1988). I’D. M. Kim, S. Lee, M. I. Nathan, A. Gopinath, F. Williamson, K. Beyzavi, and A. Ghiasi, Appl. Phys. Lett. 62, 861 (1993). 13E. S. Harmon, M. L. Lovejoy, M. R. Melloch, M. S. Lundstrom, T. J. de Lyon, and J. M. Woodall, Appl. Phys. Lett. 63, 536 (1993). 14J. R. Lowney and H. S. Bennett, J. Appl. Phys. 69, 7102 (1991). 638 Appl. Phys. Lett., Vol. 63, No. 5, 2 August 1993 Harmon et al. 638 

Minority-carrier mobility enhancement in p+ InGaAs lattice matched to InP

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Minority-carrier mobility enhancement in p+ InGaAs lattice matched to InP

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