Ex situ Ohmic contacts to n-InGaAs

Ex situ Ohmic contacts to n-InGaAs Ashish Baraskara兲 Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 Mark A. Wistey Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556 Vibhor Jain, Evan Lobisser, Uttam Singisetti, and Greg Burek Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 Yong Ju Lee Technology Manufacturing Group, Intel Corporation, Santa Clara, California 95054 Brian Thibeault Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 Arthur Gossard Department of Electrical and Computer Engineering and Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106 Mark Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 共Received 22 February 2010; accepted 24 May 2010; published 19 July 2010兲 The authors report ultralow specific contact resistivity 共␳c兲 in ex situ Ohmic contacts to n-type In0.53Ga0.47As 共100兲 layers, with an electron concentration of 5 ⫻ 1019 cm−3. They present the ␳c obtained for molybdenum 共Mo兲 contacts to n-type In0.53Ga0.47As, with the semiconductor surface cleaned by atomic H before metal deposition. The authors compare these data with the ␳c obtained for contacts made without atomic H cleaning. After exposure to air during normal device processing, the semiconductor surface was prepared by UV-ozone exposure plus a dilute HCl etch and subsequently exposed to thermally cracked H. Mo contact metal was deposited in an electron beam evaporator without breaking vacuum after H cleaning. Transmission line model measurements showed a contact resistivity of 共1.1⫾ 0.9兲 ⫻ 10−8 ⍀ cm2 for the Mo/ In0.53Ga0.47As interface. This ␳c is equivalent to that obtained with in situ Mo contacts 关␳c = 共1.1⫾ 0.6兲 ⫻ 10−8 ⍀ cm2兴. Ex situ contacts prepared by UV-ozone exposure plus dilute HCl 共without any atomic H exposure兲 result in ␳c = 共1.5⫾ 1.0兲 ⫻ 10−8 ⍀ cm2. © 2010 American Vacuum Society. 关DOI: 10.1116/1.3454372兴 I. INTRODUCTION Very low resistance metal-semiconductor contacts are required for submillimeter-wave electronics; III-V bipolar and field-effect transistors require a specific contact resistivity 共␳c兲 of less than 1 ⫻ 10−8 ⍀ cm2 to achieve simultaneous 1.5 THz current-gain 共f t兲 and power gain 共f max兲 cutoff frequencies.1,2 Contact resistivity strongly depends on semiconductor surface preparation prior to metal deposition.3 In situ metal deposition immediately after semiconductor growth avoids surface oxidation and contamination, and contact resistivity 共␳c兲 as low as 共1.1⫾ 0.6兲 ⫻ 10−8 ⍀ cm2 has been so obtained.4 But, transistor fabrication process flows often require that contacts be formed after the semiconductor has been exposed to air. To obtain low contact resistivity with such ex situ contacts, surface preparation requires considerable attention. Ti/Pt/Au contacts deposited on Ar+ sputtered n-InGaAs result in ␳c = 4.3⫻ 10−8 ⍀ cm2.5 ␳c = 4.1 a兲 Electronic mail: ashish.baraskar@ece.ucsb.edu C5I7 J. Vac. Sci. Technol. B 28„4…, Jul/Aug 2010 ⫻ 10−8 ⍀ cm2 共after correcting for metal resistance兲 was obtained with TiW contacts to n-InGaAs; in those samples the semiconductor surface was oxidized with UV generated ozone and subsequently treated with dilute HCl prior to metal deposition.3 III-V semiconductor surfaces contaminated by oxides and carbon compounds can also be cleaned by atomic H.6 After exposure to atomic H, atomically clean GaAs surfaces have been confirmed with reflection high energy electron diffraction 共RHEED兲.7,8 Atomic H has been used to clean surfaces prior to semiconductor regrowth9 by molecular beam epitaxy 共MBE兲. Atomic H cleaning is carried out at lower temperatures than conventional thermal oxide desorption and hence causes less surface roughening due to less group V desorption. While thermal cleaning under group V flux is usually carried out at 550 ° C for InP 共100兲 and InGaAs 共100兲, and 580 ° C for GaAs 共100兲, surface cleaning with atomic H can be achieved at 390 ° C for these semiconductors.10,11 How- 1071-1023/2010/28„4…/C5I7/3/$30.00 ©2010 American Vacuum Society C5I7 C5I8 Baraskar et al.: Ex situ Ohmic contacts to n-InGaAs C5I8 ever, considerable surface indium loss has been observed after prolonged atomic H cleaning of InGaAs alloys.11 We here report ␳c for ex situ Mo contacts to n-type In0.53Ga0.47As, with an electron concentration of 5 ⫻ 1019 cm−3. We studied two surface cleaning techniques: one involving UV-ozone/HCl treatment and the other involving UV-ozone/HCl/H treatment. We compare the ␳c obtained for these two techniques. II. EXPERIMENT The experiments used three vacuum chambers connected under ultrahigh vacuum 共UHV兲, the first containing a H cracking cell and a substrate heater, the second containing the RHEED system used for surface characterization, and the third containing an electron beam evaporator for metal deposition. The semiconductor epilayers were grown by a Gen II solid source MBE system. A 150 nm undoped In0.52Al0.48As layer was grown on a semi-insulating InP 共100兲 substrate, followed by 100 nm of silicon 共Si兲 doped In0.53Ga0.47As. The samples were grown at a 420 ° C substrate temperature with a 1400 ° C Si cell temperature. The active carrier concentration, mobility, and sheet resistance were obtained from Hall measurements by placing indium 共In兲 contacts on samples. The samples were removed from vacuum for a long period 共395 days兲; this permits the surface to oxidize, as would occur during normal device processing. The samples were then exposed to UV-ozone for 30 min and then treated with 1:10 HCl: H2O and de-ionized 共DI兲 water rinse for 1 min each. The samples were then immediately loaded into the vacuum chamber. On a first set of samples, without breaking vacuum, 20 nm of Mo was deposited in an electron beam evaporator without any further surface treatment. A second set of samples was exposed to thermally cracked H for times ranging from 20 to 40 min and temperatures ranging from 375 to 420 ° C. The filament temperature of the H cracking cell was maintained at 2200 ° C. The chamber pressure during H cleaning was maintained at 10−6 torr. RHEED patterns were recorded along the 关110兴 and 关1̄10兴 azimuths after H cleaning. The samples were then transferred to the electron beam evaporator where 20 nm Mo was deposited. To extract the specific contact resistivity, the samples were processed into TLM structures.12 For the TLM structures 共Fig. 1兲, Ti 共20 nm兲/Au 共500 nm兲/Ni 共50 nm兲 contact pads were patterned using optical photolithography and liftoff after an e-beam deposition. The Au layer is 500 nm thick to reduce interconnect resistance. Mo was then dry etched in a SF6 / Ar plasma using Ni as an etch mask. The TLM structures were then isolated using mesas formed by photolithography and a subsequent wet etching with 1 : 1 : 25: H3PO4 : H2O2 : DI water. A scanning electron microscope 共SEM兲 image of the TLM pattern is shown in Fig. 2. Resistances were measured using a four-point 共Kelvin兲 probe technique on an Agilent 4155C semiconductor parameter analyzer.4 In the Kelvin probing structure 共Fig. 2兲, the observed resistance, Rmeasured = 2␳c / WLT + ␳sLgap / W + Rmetal, contains a small contribution Rmetal from the sheet resistivity J. Vac. Sci. Technol. B, Vol. 28, No. 4, Jul/Aug 2010 FIG. 1. Cross-section schematic of the metal-semiconductor contact layer structure used for contact resistivity 共␳c兲 measurements. 共␳m / Tm兲 of the contact metal. Here ␳c is the metalsemiconductor contact resistivity, ␳s is the semiconductor sheet resistivity, LT = 冑␳c / ␳s is the transfer length, ␳m is the bulk metal resistivity, and Tm is the contact metal thickness. The dimensions W and Lgap are defined in Fig. 2. Rmetal is determined from separate measurements of ␳m / Tm and from numerical finite-element analysis of the contact geometry. Rmetal changes the contact resistivity data by less than 5% for TLM structures with W = 25 ␮m. The sheet resistance of the semiconductor between the contacts does not change after being exposed to SF6 / Ar plasma etch for removing Mo.4 This validates the extraction of the contact resistivity 共␳c兲 from the observed lateral access resistivity 共␳H兲 and semiconductor sheet resistivity 共␳s兲. III. RESULTS AND DISCUSSION A diffuse 共1 ⫻ 1兲 RHEED pattern was observed on samples without any H clean. As the samples were exposed to atomic H and the exposure time and temperature are increased, a gradual improvement from a 共1 ⫻ 1兲 to a 共2 ⫻ 4兲 reconstruction was observed. Figure 3 shows the RHEED patterns recorded along the 关110兴 and 关1̄10兴 azimuths after 40 min of atomic H exposure at 420 ° C. The observed 共2 ⫻ 4兲 reconstruction indicates an As-rich or As-terminated surface.11,13,14 FIG. 2. SEM image of the TLM pattern used for the contact resistivity 共␳c兲 measurement. Separate pads were used for current biasing and voltage measurement. C5I9 Baraskar et al.: Ex situ Ohmic contacts to n-InGaAs C5I9 the resistance analysis assuming that one-dimensional current flow is appropriate.15 Hall measurements on the samples indicate an active carrier concentration, a mobility, and a sheet resistance of 4.8⫻ 1019 cm−3, 984 cm2 / V s, and 13 ⍀, respectively. The sheet resistance obtained with TLM measurements was 13.5 ⍀, which closely correlates with the sheet resistance obtained with Hall measurement. In separate experiments, samples doped at ⬃5 ⫻ 1019 cm−3 were exposed to air for 2, 36, and 395 days, and treated with UV-ozone/HCl/H. Mo was deposited and TLM structures were fabricated on these samples. Observed ␳c were 1.1, 1.1, and 1.0 ⍀ ␮m2, respectively. A separate set of samples, doped at ⬃5 ⫻ 1019 cm−3, exposed to air for 2 and 395 days, was treated with UV-ozone/HCl. Mo contacts were formed and TLM structures were fabricated on these samples. Observed ␳c were 1.4 and 1.5 ⍀ ␮m2, respectively. These data indicate that the duration of air exposure has little effect on ␳c given the surface preparation procedures employed. FIG. 3. 共Color online兲 RHEED patterns of the atomic H cleaned sample along the 关110兴 and 关1̄10兴 azimuths showing 共2 ⫻ 4兲 reconstructed surface. Figure 4 shows the variation of TLM test structure resistance with contact separation for samples with different surface treatments. From the TLM data, ␳c = 共1.1⫾ 0.9兲 ⫻ 10−8 and 共1.5⫾ 1.0兲 ⫻ 10−8 ⍀ cm2 were determined for samples with UV-ozone/HCl/H treatment and UV-ozone/HCl treatment, respectively. The contact resistivity here obtained is the lowest reported to date for ex situ contacts to n-type In0.53Ga0.47As and is comparable to 共1.1⫾ 0.6兲 ⫻ 10−8 ⍀ cm2 obtained for in situ Mo contacts to n-type In0.53Ga0.47As.4 For the TLM structures, the transfer length was found to be 280 nm, 2.8:1 larger than the N+ layer thickness; hence, 14 UV-O / HCl/ H clean 3 12 UV-O / HCl 3 Resistance (:) 10 8 6 4 2 0 0 5 10 15 20 25 Contact Separation (Pm) FIG. 4. Measured TLM resistance as a function of contact separation for ex situ molybdenum Ohmic contacts to n-In0.53Ga0.47As. JVST B - Microelectronics and Nanometer Structures IV. CONCLUSIONS In summary, we report ultralow contact resistivity with ex situ contacts to heavily doped n-type In0.53Ga0.47As. The contact resistivities obtained with UV-ozone/HCl cleaned and UV-ozone/HCl/H cleaned contacts were 共1.5⫾ 1.0兲 ⫻ 10−8 and 共1.1⫾ 0.9兲 ⫻ 10−8 ⍀ cm2, respectively. The contact resistivity obtained here is comparable to that obtained with in situ Mo contacts, suggesting the effective removal of surface contaminants. These ultralow resistance ex situ Mo contacts make them a potential candidate to be applied in highly scaled HBTs and other devices of near-terahertz bandwidths. 1 M. J. W. Rodwell, M. L. Le, and B. Brar, Proc. IEEE 96, 271 共2008兲. M. J. W Rodwell et al., Proceedings of the IEEE Compound Semiconductor Integrated Circuit Symposium, 2008. 3 Vibhor Jain, Ashish K. Baraskar, Mark A. Wistey, Uttam Singisetti, Zach Griffith, Evan Lobisser, Brian J. Thibeault, Arthur C. Gossard, and Mark. J. W. 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