To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V–1 s–1 (5.11×1011 cm–2) was obtained in a sample with a 20 nm thick Ge strained channel

Dziuba Z.

E. H. C. Parker

M. Myronov

O. A. Mironov

S. Koh

T. E. Whall

T. Irisawa

Y. Shiraki

English

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Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001)
                    p

Myranov, Maksym

Irisawa, T.

Mironov, O. A.

Koh, S.

Shiraki, Y.

Whall, Terry E.

Parker, Evan H. C.

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  University of Warwick institutional repository: http://go.warwick.ac.uk/wrapThis paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): M. Myronov, T. Irisawa, O. A. Mironov, S. Koh, Y. Shiraki, T. E. Whall, and E. H. C. ParkerArticle Title: Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures Year of publication: 2002 Link to published version: http://dx.doi.org/10.1063/1.1473690 Publisher statement: None     APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 17 29 APRIL 2002Extremely high room-temperature two-dimensional hole gas mobility inGeÕSi0.33Ge0.67 ÕSi001 p-type modulation-doped heterostructuresM. Myronova)Department of Physics, University of Warwick, Coventry CV4 7AL, United KingdomT. IrisawaDepartment of Applied Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, 113-8656, JapanO. A. MironovDepartment of Physics, University of Warwick, Coventry CV4 7AL, United KingdomS. Koh and Y. ShirakiDepartment of Applied Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, 113-8656, JapanT. E. Whall and E. H. C. ParkerDepartment of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom~Received 27 November 2001; accepted for publication 26 February 2002!To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas~2DHG! that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67 /Si~001! p-typemodulation-doped heterostructures, the magnetic field dependences of the magnetoresistance andHall resistance at temperature of 295 K were measured and the technique of maximum entropymobility spectrum analysis was applied. This technique allows a unique determination of mobilityand sheet carrier density of each group of carriers present in parallel conducting multilayerssemiconductor heterostructures. Extremely high room-temperature drift mobility ~at sheet carrierdensity! of 2DHG 2940 cm2 V21 s21 (5.1131011 cm22) was obtained in a sample with a 20 nmthick Ge strained channel. © 2002 American Institute of Physics. @DOI: 10.1063/1.1473690#There has been considerable attention directed towardsrealizing high-speed p-type field-effect transistors ~FETs! inSi-based materials, because the performance ofcomplementary–metal–oxide–semiconductor ~CMOS! typecircuits is limited by p-type devices due to their lower holemobility. In order to enhance the hole mobility, several typesof Si/Ge heterostructures have been intensively studied.1Among them, the SiGe/pure-Ge channel/SiGe heterostruc-ture is considered to provide the highest mobility since theeffective hole mass decreases with an increase in Ge contentand there is no alloy scattering which may dominate trans-port in SiGe alloys. Strained pure-Ge channel modulation-doped ~MOD! structures with very high hole mobility weresuccessfully fabricated on Si substrates by several groups.2,3Hock et al.4 performed mobility spectrum analysis ~MSA! toextract the mobility of holes only in a pure-Ge channel ex-cluding the effects of parallel conductions and they obtainedmobility of 1665 cm2 V21 s21 ~at 831011 cm22) at roomtemperature ~RT!, which was the highest value until Madhaviet al.5 reported Hall mobility of 1700 cm2 V21 s21 ~at 7.931011 cm22!. It was achieved by reducing the contributionby parasitic conducting layers in Ge/Si0.3Ge0.7 MOD hetero-structures. However, this value is still lower than the bulk Geone (1900 cm2 V21 s21) in spite of modification of the bandstructure caused by compressive strain, i.e., decreasing theeffective hole mass and increasing the band splitting betweenheavy-hole and light-hole bands.a!Electronic mail: m.myronov@warwick.ac.uk3110003-6951/2002/80(17)/3117/3/$19.00Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject toWe previously reported that pure-Ge channel MODstructures with very high mobility could be grown by utiliz-ing a low-temperature buffer technique,2,6 which providedvery high quality SiGe strain relaxed buffer layers with highGe contents. The highest measured Hall mobility at RT was1320 cm2 V21 s21. However, the mobility only in thepure-Ge channel in our multilayer samples should be muchhigher because the effects of parallel conduction were neverexcluded in our measurements. Then, in order to extract theintrinsic mobility in the strained Ge channel of our samplesand to investigate the amount and the origin of parallel con-ductions, we carried out MSA. It was found that the mobili-ties of holes in the strained Ge channel layers were extremelyhigh and beyond the bulk Ge value.Samples were grown by solid-source molecular beamepitaxy on n-type ~5–10 V cm!, ~100!-oriented Si substrates.Relaxed Si0.33Ge0.67 buffer layers were grown by the two-step low temperature ~LT!-buffer technique. In the first step,a 500 nm Si0.73Ge0.27 layer was grown at 600 °C on 50 nmLT-Si layers. Then, a 500 nm Si0.33Ge0.67 layer was grown at500 °C on 50 nm LT-Si0.73Ge0.27 layers. LT-Si layers andLT-Si0.73Ge0.27 layers were grown at 400 and 300 °C, respec-tively. Next, 10 nm B-doped layers (;231018 cm23), 20nm spacer layers, a Ge channel, and 40 nm capping layerswere grown successively on the buffer. The channel thick-ness in sample A was 7.5 nm and in sample B was 20 nm andthe growth temperatures were 350 and 300 °C, respectively.Lowering of the growth temperature was needed to grow athicker channel without strain relaxation.2First, the Hall mobility and sheet carrier density wereobtained by combining Hall and resistivity measurements in7 © 2002 American Institute of Physics AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp3118 Appl. Phys. Lett., Vol. 80, No. 17, 29 April 2002 Myronov et al.the temperature range of 10–300 K. Samples were fabricatedin Hall-bar geometry. Ohmic contacts were formed byevaporating AuGa. The sample with a thinner Ge channelshowed better magnetotransport properties at 10 K. The Hallmobility of the two-dimensional hole gas ~2DHG! ~at thesheet carrier density! that formed in the Ge channel ofsamples A and B measured at 10 K was 15 770 cm2 V21 s21(1.1131012 cm22) and 7570 cm2 V21 s21 (9.5531011 cm22), respectively. At room temperature the situa-tion was the reverse. The sample with a thicker Ge channelshowed better Hall mobility. The Hall mobility ~at the sheetcarrier density! measured at 295 K in samples A and Bwas 1110 cm2 V21 s21 (2.9731012 cm22) and1440 cm2 V21 s21 (2.2831012 cm22), respectively.Samples A and B were optimized during differentgrowth experiments for their best low and room temperatureHall mobilities, respectively.2 We present two samples toshow that the structure ~sample A! optimized to get higherHall mobility at low temperatures does not show higher mo-bility at room temperature in comparison with the structure~sample B! optimized for higher Hall mobility at room tem-perature. We stress this point to contrast our results to theresults reported by Madhavi et al.,5 who used thicker bufferlayers and reported no pronounced channel thickness ~in therange of 10–15 nm! effects on the Hall mobility.The sheet carrier density determined from Hall effecttechnique measurements is pHall5(e3RH)21. This equationFIG. 1. Mobility spectrum ~a! as the result of the sxx(B) and sxy(B) fits ~b!measured at 295 K for sample A. The number of magnetic field points in ~b!is 50 and the number of carriers in ~a! is 300. Total deviation squared x258.831025.Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject tois valid for one type of carrier only. At low temperatures it istrue and corresponds to the value of 2DHG that formed inthe strained Ge channel when all parasitic conductions andbackground impurities were frozen out. In our case it is invery good agreement with the values predicted by numericalsolution of a one-dimensional ~1D! Schro¨dinger–Poissonsimulation. With an increase in the temperature from 20 to300 K conduction in parasitic parallel layers occurs. In thiscase the average mobility and average sheet carrier densityof all conducting layers are measured by the Hall effect tech-nique and the simple equation pHall5(e3RH)21 no longerrepresents the sheet carrier density in the Ge channel. Tosolve this problem the magnetic field dependences of themagnetoresistance and Hall resistance at 295 K were mea-sured and MSA was applied. MSA was proposed by Beckand Anderson,7 and was further developed by Dziuba et al.,8by Antoszewski et al.,9 by Vurgaftman et al.,10 and by Kiat-gamolchai et al.11,12 as a useful technique for analyzingmulti-carrier galvanomagnetic phenomena. MSA is the trans-formation of the electrical conductivity tensor versus themagnetic field into the conductivity density versus the mo-bility spectrum. The technique allows a unique determinationof the mobility and sheet carrier density of each group ofcarriers present in parallel conducting multilayers of semi-conductor heterostructures. The magnetic field dependencesof the magnetoresistance and Hall resistance were measuredat 295 K as the magnetic field (B) was swept continuouslyFIG. 2. Mobility spectrum ~a! as the result of the sxx(B) and sxy(B) fits ~b!measured at 295 K for sample B. The number of magnetic field points in ~b!is 50 and the number of carriers in ~a! is 300. Total deviation squared x257.431025. AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp3119Appl. Phys. Lett., Vol. 80, No. 17, 29 April 2002 Myronov et al.Downloaded 06 JTABLE I. Average Hall mobility and sheet carrier density of Ge/Si0.33Ge0.67 /Si(001) multilayered heterostruc-tures at 295 K in comparison with the 2DHG drift mobility and sheet carrier density for the strained Gep-channel only extracted by ME-MSA.SampleGe channelthickness~nm!mHall(cm2 V21 s21)pHall(cm22)md ~2DHG!(cm2 V21 s21)ps ~2DHG!(cm22)A 7.5 1110 2.9731012 2540 5.1231011B 20 1440 2.2831012 2940 5.1131011from 211 up to 111 T and the reversd. After that an aver-age was taken and the magnetoresistance and Hall resistancewere converted into conductivity tensor components sxx(B)and sxy(B) followed by a maximum entropy-MSA ~ME-MSA! fit procedure.11,12 It is worth pointing out that the ME-MSA approach does not require any preliminary assumptionsabout the number of different types of carriers and this aspectis very important for transport phenomena analysis in semi-conductor structures.Mobility spectra as a result of the conductivity tensorcomponent sxx(B) and sxy(B) fits of samples A and B arepresented in Figs. 1 and 2, respectively. The fitted magneticfield dependences of sxx(B) and sxy(B) are in very goodagreement with the measured ones for both samples ~seeFigs. 1 and 2!. For both samples the mobility spectrum con-sists of four peaks. The small peaks with negative mobilityaround 21500 cm2 V21 s21 in the spectrum show the exis-tence of electrons, which may come from a Si n-type sub-strate or a thin Si tensile strained cap layer on the surface.Two merged peaks around ;300 cm2 V21 s21 correspond tothe group of carriers in B-doped Si0.33Ge0.67 layers and toSi0.33Ge0.67 and Si0.73Ge0.27 layers in the two-step LT buffer.These parallel conductions significantly reduce not only theHall mobility but also device performance. Work on optimi-zating the doping profile and reducing the background impu-rities to decease these parallel conductions is now underway.The peaks with the highest mobilities in the spectra corre-spond to the 2DHG that formed in the strained Ge channels.In contrast to the mobilities of carriers in the parallel con-ducting layers, the mobilities of 2DHG increase with a de-crease in temperature ~further details of this will be pub-lished!. Also, the increase in conductivity of 2DHG with adecrease in temperature was observed. Others peaks showedconductivity freeze out behavior with a decreasing in tem-perature and could be considered to be from parasitic chan-nels in our structures. The drift mobility ~at the sheet carrierdensity! of 2DHG at 295 K extracted from the mobility spec-trum is 2540 cm2 V21 s21 (5.1231011 cm22) and2940 cm2 V21 s21 (5.1131011 cm22) for samples A and B,respectively. These are, we believe, the highest publishedvalues of drift mobility of 2DHG formed in the Ge strainedchannel at room temperature and much higher than the bulkul 2009 to 137.205.202.8. Redistribution subject toGe mobility. This indicates that strain increases the hole mo-bility in the Ge channel probably by reducing the effectivehole mass and increasing the band splitting between heavy-hole and light-hole bands to reduce intervalley scattering. Tothe best of our knowledge, this is one of the first reports toshow that the mobility of strained Ge exceeds that of thebulk. The results of the Hall and resistivity measurementsand MSA at 295 K for both samples are summarized in TableI. The higher mobility of sample B is consistent with theresults of the Hall measurements, and may be caused by thedifference in interface roughness scattering due to the lowergrowth temperature and the thicker Ge channel in sample B.In conclusion, we characterized pure-Ge strained chan-nel p-type MOD heterostructures by mobility spectrumanalysis and found that the drift mobility ~at the sheet carrierdensity! of holes in Ge channel layers reached2940 cm2 V21 s21 (5.1131011 cm22) at 295 K which ex-ceeded the bulk Ge value. This result clearly indicates thatstrain really does increase the carrier mobility.This work was supported in part by the Grant-in-Aid forScientific Research on Priority Areas ~No. 11232202! of theMinistry of Education, Culture, Sports, Science and Technol-ogy and by the International Collaboration Program of theJapan Society for the Promotion of Science.1 F. Schaffler, Semicond. Sci. Technol. 12, 1515 ~1997!.2 T. Irisawa, H. Miura, T. Ueno, and Y. Shiraki, Jpn. J. Appl. Phys., Part 140, 2694 ~2001!.3 Y. H. Xie, D. Monroe, E. A. Fitzgerald, P. J. Silverman, F. A. Thiel, and G.P. Watson, Appl. Phys. Lett. 63, 2263 ~1993!.4 G. Hock, M. Gluck, T. Hackbarth, H. J. Herzog, and E. Kohn, Thin SolidFilms 336, 141 ~1998!.5 S. Madhavi, V. Venkataraman, and Y. H. Xie, J. Appl. Phys. 89, 2497~2001!.6 T. Ueno, T. Irisawa, Y. Shiraki, A. Uedono, and S. Tanigawa, Thin SolidFilms 369, 320 ~2000!.7 W. A. Beck and J. R. Anderson, J. Appl. Phys. 62, 541 ~1987!.8 Z. Dziuba and M. Gorska, J. Phys. III 2, 99 ~1992!.9 J. Antoszewski, D. J. Seymour, L. Faraone, J. R. Meyer, and C. A. Hoff-man, J. Electron. Mater. 24, 1255 ~1995!.10 I. Vurgaftman, J. R. Meyer, C. A. Hoffman, D. Redfern, J. Antoszewski, L.Faraone, and J. R. Lindemuth, J. Appl. Phys. 84, 4966 ~1998!.11 S. Kiatgamolchai, Ph.D. thesis ~Warwick University, Coventry, UK,2000!, p. 191.12 S. Kiatgamolchai, O. A. Mironov, V. G. Kantser, M. Myronov, Z. Dziuba,E. H. C. Parker, and T. E. Whall ~unpublished!. AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

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Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

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