Thin films of Laf3 deposited on Si or GaAs substrates have been observed to form blocking contacts with very high capacitances. This results in comparatively‐hysteresis‐free and sharpC‐V (capacitance‐voltage) characteristics for MIS structures. Such structures have been used to study the interface states of GaAs with increased resolution and to construct improved photocapacitive infrared detectors

Crouch, R. K.

Miller, W. E.

Moriarty, John A.

Seiber, B. A.

Sher, A.

Tsuo, Y. H.

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VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassPhysics Publications Dept. of Physics1979LaF3 insulators for MIS structuresA. SherCollege of William and MaryW. E. MillerNASA-Langley Research CenterY. H. TsuoCollege of William and MarySee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/phys_pubsPart of the Physics CommonsSher, A., Miller, W.E., Tsuo, Y.H., et al. LaF3 insulators for MIS structures. Applied Physics Letters, 34, 799 (1979).Copyright © 1979 AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Physics at VCU Scholars Compass. It has been accepted for inclusion in PhysicsPublications by an authorized administrator of VCU Scholars Compass. For more information, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/phys_pubs/48AuthorsA. Sher, W. E. Miller, Y. H. Tsuo, John A. Moriarty, R. K. Crouch, and B. A. SeiberThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/phys_pubs/48LaF 3 insulators for MIS structures A. Sher College of William and Mary, Williamsburg, Virginia 23185 W. E. Miller NASA-Langley Research Center, Hampton, Virginia 23665 Y. H. Tsuo and John A. Moriarty College of William and Mary, Williamsburg, Virginia 23185 R. K. Crouch NASA-Langley Research Center, Hampton, Virginia 23665 8. A. Seiber Virginia Commonwealth University, Richmond, Virginia 23284 (Received 8 January 1979; accepted for publication 23 February 1979) Thin films of LaF1 deposited on Si or GaAs substrates have been observed to form blocking contacts with very high capacitances. This results in comparatively-hysteresis-free and sharp C - V (capacitance-voltage) characteristics for MIS structures. Such structures have been used to study the interface states of GaAs with increased resolution and to construct improved photocapacitive infrared detectors. PACS numbers: 73.40.Qv, 73.60.Hy Lanthanum fluoride (LaF3) is a fast ionic conductor with exceptional polarization properties. I When this materi-al is deposited on a substrate bye-gun evaporation, the re-sulting film possesses thin dipole layers (- 50-100 A.) at its surfaces2 which produce large capacitive effects. In MIS structures, the film also acts as a blocking contact for elec-tronic conduction as long as the breakdown voltage of the device is not exceeded. The effective capacitance of the film is that of an insulating layer 100-200 A. thick with a dielec-tric constant of 14 (the bulk value of LaF 3)' This capcitance is independent of the film's actual thickness as long as the measurement frequency lies below a characteristic value cor-responding to the RC time constant of the LaF3. For a typi-cal 250-A. film at room temperature, we have established that the characteristic frequency is above 100 kHz. At high-enough frequencies, or at low temperatures where the ionic conduction ceases, the film's capacitance is expected to de-crease to its geometrical value. Measured C- V (capacitance-voltage) characteristics are illustrated for two Si samples in Fig. 1 and for two GaAs samples in Fig. 2. No visible hysteresis is observed in the Si We have deposited LaF3 films on freshly prepared bare Si (n type, with a carrier concentration n ~ 2 X 1014 cm-3), on freshly prepared bare GaAs (n type, with n ~ 3 X 1015 cm-3), and on native-oxide-coated Si and GaAs to form composite insulators.3 The native oxide is thermally grown Si02 on the former and anodized GaAs on the latter. Back Ohmic con-tacts are formed for the Si samples by first e-beam depositing an Al film then sintering at 550°C for 10 min in flowing N2 gas, and for the GaAs samples bye-beam depositing a film of Ge-Au-Ni alloy then sintering at 450°C for 10 min in flow-ing forming gas. Next, the LaF3 layer and a 125-A.-thick transparent Au front contact are deposited to complete the MIS structure. Finally, the sample is annealed at 400 °C in a N2 atmosphere for 1 h. 250 200 '" E 150 u L;: ~ « "-u 100 50 Si f -100KHz f • 1KHz 500.0. LaF3 Aa o.3cm 2 f·IKHz .-::--,..,.---"-----, . " .... :,_~ " 250 A 5i02 • 250A LoF3 I: j/ :1 II il 1/ /I BIAS (V) A. 0 003 cm 2 FIG. I. The C- V characteristics for two Si samples. The ramp rate for all curves is - 50 m V /sec. 799 Appl. Phys. Lett. 34(11 J, 1 June 1979 0003-6951/79/110799-03$00.50 © 1979 American Institute of Physics 799  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.58 On: Fri, 24 Apr 2015 18:03:37250 200 150 N E u "-lL c: ~ U 100 50 Go As 250 Ii. LoF3 -A=0.3cm2 i , ~ ,'i ': " 250 A native o"lde + 500A La F:\ :: A:.O.3cm2 II 'I f:. I KHz '. " " f;" " " " ,~J ___ -_-____ -_-_-::_~~o:--- _o~<' ·3 ·2 ·1 o BIAS (V) f :. 1KHz FIG. 2. The C- V characteristics for two GaAs samples. The ramp rate for both curves is - 50 m V /sec. characteristics. There is a small amount of hysteresis in the GaAs characteristics, but this is considerably less than that reported for anodized native-oxide layers alone" The GaAs devices began to leak above the largest positive voltages shown (- 1 Y) in Fig. 2. The Si sample with a 500-A. layer of LaF3 also began to leak above 0.8 Y, and the apparent onset of saturation above 0.4 Y is, in fact, due to the onset of this leakage instead. The C- V characteristic for the composite-insulator-covered Si sample was measured at both 1 and 100 kHz, as indicated in Fig. I. In each case a saturation capacitance (the insulating-layer value) of 117 nF /cm2 and a maximum/min-imum capacitance ratio of 23 was obtained. Since the theo-retical capacitance ofa 250-A. layer ofSi02 is 136 nF/cm2, we infer that the 250 A. of LaF3 contributes - 840 nF/cm2. Table I. Equivalent circuit parameters for GaAs MIS structures. 2S0-A Native oxide + SOO-A LaF, Filter \0" \0-2 \0-' \00 <P (cm" sec-') l.!3x \0" 1.43 X 10" 8.S9x 10" 8.76x 10" Cd (nF) 5.03 5.05 5.17 5.37 T, (sec) 0.38 0.27 0.097 0.012 Co (nF) 85 Ro(il) 28.3 C, (nF) 105 N, (states/ eV cm') 2.9x 10" T,,(sec) 0.411 K, (cm2) 9.23 X 10-" C, (nF) 0.12 T, (sec) 1.7x 10-4 800 Appl. Phys. Lett., Vol. 34, No. 11, 1 June 1979 10 2 r--=---,-----,------,------, filter ell [Photons/cm2 - sec1 o 10° 876.,0'2 o 10- ' 859 .. 10" o 10-2 I 43 II 10 II & 10-3 113)(10 '0 t::.. DARK o 10 10'~---~----~---_+---~ ., o 10 ·2 10 f (Hz) FIG. 3. The series capacitance C, and dissipation factor D as a function of frequency and photon flux for the GaAs sample with a 2S0-A-thick LaF, insulator. The dots are data points and the solid curves are fits to these points determined by inserting the parameters in Table I into analytic ex-pressions for C, and D for the circuit shown in the inset. This corresponds to - 75-A surface dipole layers or an effec-tive thickness for the LaF, film of -150 A. (In other sam-ples, effective thicknesses of -120 A have been found.) The slight frequency dependence of the total capacitance at small bias voltages is most likely due to Si-Si02 interface-state ef-fects,5.6 but there may also be a contribution from slow sur-face states in the LaF). I The C-V characteristic for the composite-insulators-covered GaAs sample in the depletion region is qualitatively similar to that of native-oxide-covered n-type GaAs. 4 For a negative-voltage ramp, the capacitance falls below its equi-librium high-frequency value. When the ramp is reversed, the capacitance rises to its equilibrium value (reaching it at Dark 5.571 3.00 10-\ 1.13 X 10'0 5.579 2.20 320 13.8 170.8 3.5X \0" 2.81 5.27X \0-12 0.068 2.SX 10' 2S0-ALaF, 10'2 1.43 X 10" 5.764 0.970 \0" 8.S9X 10" 5.962 0.202 100 8.76x 10" 6.224 0.0215 Sher eta!. 800  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.58 On: Fri, 24 Apr 2015 18:03:37- 0.5 V in Fig. 2) and maintains this value back to zero bias. There are a number of possible explanations for this effect. Previous workers have speculated on the existence of bulk traps7 or a spatially extended interface region between the GaAs and the native oxide4 to account for the phenomenon. Since replacing the native oxide by LaF3 removes the effect, our results suggest that bulk traps are not the primary mechanism and that the proper explanation is linked to the properties of the insulating layer. The effectively thin insulating layers permitted by the use of LaF3 would seem to have many potential device appli-cations, e.g., CCO's with lower voltages or smaller areas needed to store a given charge and more-sensitive larger-dynamic-range varactors. Our direct interest in these struc-tures, however, has been stimulated by two other types of application. The first is as an aid to the fundamental study of interface states, especially in GaAs where such states are not well characterized. The higher insulator capacitances permit higher resolution of interface-state effects in electrical mea-surements than otherwise possible. The second application is to improve photocapacitive MIS infrared detectors. 8 As an illustration of the first application, we have plot-ted in Fig. 3 our measurements of the frequency and optical fl ux ( (/I ) variation of the total series capacitance C s and dissi-pat ion factor D of the LaF3-coated GaAs sample under zero applied bias voltage and subject to illumination on the front surface. The measured values of Cs and D are observed to be independent of the wavelength ofthe incident light as long as the absorption depth of the semiconductor remains within an order of magnitude of the depletion-layer thickness. The data in Fig. 3 was all taken at a wavelength of 0.820 /lm. The solid lines in Fig. 3 are parameterized fits to the experimental points obtained from the equivalent circuit shown in the in-set. Most of the circuit elements have a simple physical inter-pretation: Ro is the sheet resistance of the front Au contact; Co is the insulator capacitance; Cd is the depletion-layer ca-pacitance of the semiconductor; and C; and R; are the inter-face-state capacitance and resistance, respectively. The re-maining circuit elements CI and R, represent a yet undetermined process, but probably a secondary one associ-ated with the insulator-GaAs interface. The isolation of C; and R; allows one to immediately infer the interface-state density at the Fermi level, N;, by the relationships N;= C/eA, where A is the device area, and also the interface-state re-sponse time constant Only Cd and 7; vary significantly with light intensity. The former increases with (/I because electron-hole pairs created by the photons absorbed in the depletion layer are separated by its large electric field, thereby decreasing its thickness. The interface-state response rate 7;- 1 increases linearly with (/I, 7;- 1 = 7;; 1 + K;(/I, because light-generated holes are driven to the interface by the depletion-layer electric field, thus providing a fast exit 801 Appl. Phys. Lett., Vol. 34, No. 11, 1 June 1979 mechanism for electrons localized in interface states. Nu-merical values of the fitted circuit parameters for both the LaF3-coated and composite-insulator-coated GaAs samples are given in Table I. In the general case, one can identify four features of our MIS structures and measurement technique which lead to improved resolution of interface-state effects: (i) The large values of Co permitted by the effectively thin insulating layers maximize the interface-semiconductor contribution to the measured electrical quantities. (ii) The use of low-carrier-concentrations semiconduc-tors keeps Cd relatively small, so there is less shunt effect on the C;, R; and CI, R I legs of the circuit. The carrier concentra-tions of our samples are significantly lower than those nor-mally used previously.6 (iii) The measurement of Cs and D as a function of opti-cal flux helps to identify C; and R;. (iv) The comparison of the behavior of different insula-tors helps to distinguish different physical mechanisms, es-pecially bulk and interface effects. To this list we could also add the temperature and bias voltage variation of Cs and D. We expect, for instance, that such measurements will be use-ful in refining our understanding of the GaAs interface-state properties and ultimately providing a complete profile of the interface density of states. In our second application, photocapacitive MIS in-frared detectors8 that operate at room temperature have been built from both LaF3-covered and composite-insulator-covered Si and GaAs. These new detectors have unopti-mized detectivities at 13 Hz of - 2 X 1013 W-I cm Hz'l2 for Si and - 1 X 1013 W-I cm Hz'l2 for GaAs. The former number represents an order-of-magnitude improvement over both our initial Si devices, 8 which used Si02 insulating layers, and conventional photovoltaic Si detectors. 9 Additional details on this application will also be presented elsewhere. We wish to thank W.R. Feltner who grew the Si02 lay-ers, and T.C. Steele for his help with sample preparation. 'A Sher, R. Solomon, K. Lee, and M.W. Muller, Phys. Rev. 144, 593 (1966); R. Solomon, A Sher, and M.W. Muller, J. Appl. Phys. 37, 3427 (1966). 'C.O. Tiller, A.C. Lilly, and B.C. LaRoy, Rev. B 8, 4787 (1973); B.C. LaRoy, A.E. Lilly, and C.O. Tiller, J. Electrochem. Soc. 120, 1668 (1973). 'The oxide-coated Si was supplied to us by W.R. Feltner of the Electronics Development Division of the NASA Marshall Space Flight Center. The GaAs, purchased from Applied Materials, is a 20-Jim-thick Te-doped epi-taxiallayer on an n' substrate. 'E. Kohn and H.L. Hartnagel, Solid-State Electron. 21, 409 (1978). 'The physics of MIS devices is reviewed by S.M. Sze, Physics of Semicon-ductor Devices (Wiley, New York, 1969), pp. 425-504. 'A. Goetzberger, E. Klansmann, M.J. Schulz, Crit. Rev. Solid State Sci. 6, 1 (1976). 'C.R. Zeisse, L.J. Messick, and D.L. Lile, J. Vac. Sci. Technol. 14, 857 (1977). gA. Sher, R.K. Crouch, S.S.-M. Lu, W.E. Miller, and J. A. Moriarty, Appl. Phys. Lett. 32, 713 (1978). 'H. Levinstein, in Semiconductors and Semimeta/s, edited by R.K. Willard-son and A.C. Beer (Academic, New York, 1970), Vol. 5, p. 10. Sher etat. 801  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.58 On: Fri, 24 Apr 2015 18:03:37

LaF3 insulators for MIS structures

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LaF3 insulators for MIS structures

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