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application for isothermal capacitance transient spectroscopy:of tunneling in semiconductor-insulator interfacesE. C. Paloura,a)J. Lagowski, and H. C. GatosMassachusetts nstitute of Technology,Department of Materials Scienceand Engineering, Cambridge,Massachusetts 2139(Received 29 June 1990; accepted for publication 15 October 1990)The GaAs-insulator interface is characterized by deep level transient spectroscopy (DLTS)and isothermal capacitance transient spectroscopy (ICTS). It is demonstrated thatwhile DLTS can only detect transients with temperature-dependent emission rates, ICTS candetect temperature-independent phenomena as well. The GaAs-insulator interface ischaracterized by two electron traps, with activation energies 0.67 and 0.23 eV, respectively,and a tunneling component that is detected only by ICTS. This tunneling component,which dominates the ICTS spectrum at 80 K < T< 180 K and is characterized by afield-induced barrier reduction given by AE, = 1.4 x 10 - 3q a, is attributed to puretunneling.

Deep level transient spectroscopy (DLTS) has beenused for the characterization of deep traps ands. However, the temperature scanning re-for data acquisition can induce errors in the mea-density distributions and limits the applicationDLTS to defects with temperature-dependent emissionFurthermore, tunneling cannot be resolved in DLTS

not result in clearly defined peaks indifferential capacitance (AC) versus temperature ( 7)increases the background DLTS sig-( ICTS),4 which has been proposed as an alternative char-technique, relies on the same principle withbut takes place under isothermal conditions.

Here, we apply DLTS and ICTS for the characteriza-the GaAs-insulator interface. It is demonstratedis capable of detecting and analyzing tunnelingthat, due to their temperature-independent emis-rate, are transparent to DLTS. The insulator wason n-type (100) GaAs by direct exposure to thea nitrogen glow discharge. Nitrogenation wasout at 500C at 13.56 MHz, with 100 W of rfDetails on the plasma process and the measurementreported earlier.56The essential feature of DLTS is the ability to set anwindow such that the measurement appara-when it sees a transient with a ratethis window. If we define the normalized correlationLTS signal as S(T) = [C(t,) - C(t,)]/AC(O) and as-exponential transients, then S(T) = exp( - t,/r){ 1 - exp[ ( t2 - t,)/r]}, where AC(O) is the device ca-acitance at t=O, t, and t2 are the gating times, and r is themission time constant. This function goes through a max-imum for rmax = ( t2 - t,)/ln( t2/tl) and results in a peakin the AC vs T plot. However, the tunneling emission rateof carriers trapped in insulator traps, located at Ei belowthe insulator conduction band, is temperature independent

Permanent address: Aristotle University of Thessaloniki, Department ofPhysics, 54006 Thessaloniki, Greece.

and is given by5 e,(Ei,z) =a exp[ - (2z/h) (2m*Ei)12],where a is a constant, m* is the electron effective mass inthe insulator, z is the tunneling distance, and h is Plancksconstant. During a DLTS scan e,( E,z) is constant and thetemperature-dependent correlation signal S(T) does notgo through a maximum. Therefore, tunneling componentscannot be detected in the AC vs T plot resulting from aDLTS temperature scan. On the other hand, the ICTSsignal is generated by sampling the transient at differenttimes ti and t&, where K> 1 is a fixed parameter. Theconcentration and emission rate of a deep level in n-typematerial can be directly calculated from the time (t,,,)and the amplitude (S,,,) at the maximum signal, using7NT=Smax[2No(ln K/K - l)KK/(K- 1 and en= (l/r) = ( 1 t,,,) K2 (In K/K - 1) , where NT and No are theconcentrations of the deep level and the free carriers, re-spectively.The DLTS spectrum from a metal-insulator-semiconductor (MIS) capacitor with a 640-A-thick insu-lating film is shown in Fig. 1 (in dashed line). It shows amajor peak, which is attributed to interface states, with apronounced tail towards the low-temperature range. Theindividual transients used for the construction of the DLTSspectrum were exponential. The identification of this peakwith interface states has been established as follows: ( 1)this peak is characteristic of the DLTS spectrum of MIScapacitors and it is not present in the DLTS picture of thesubstrate either in the as-grown state or after the nitroge-nation; (2) the peak exhibits a filling pulse height depen-dence that is characteristic of interface traps.2 The activa-tion energy and capture cross section of the interface trapsare 0.53 eV and 1.64~ 10 - I5 cm2, respectively.ICTS analysis was performed on capacitance transientsrecorded over the temperature range 80-370 K, under thesame bias conditions with DLTS. The evolution of repre-sentative ICTS spectra with temperature is shown in Fig.2, where AC is shown as a function of log r. The corre-sponding activation energy plot is shown in the inset. Inthe range 80-180 K a tunneling component, with a prac-tically temperature-independent emission rate, dominatesthe spectrum, while the levels associated with the peaks in

137 Appl. Phys. Lett. 58 (2). 14 January 1991 0003-6951/91/020137-03$02.00 @ 1991 American Institute of Physics 137Downloaded 08 Mar 2007 to 132.234.251.211. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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0200 250 Tempe%re 350 400( K

FIG. 1. Fitting of the DLTS peak using the activation energies and cap-ture cross sections calculated from the ICTS analysis (dashed line: DLTS;solid lines: ICTS) .the temperature range 200- 370 K (e, < 1 s - *> are theinterface states that are also responsible for the peak in t heDLTS spectrum. The very small temperature dependenceof the emission rate observed in the range 80-l 80 K is mostprobably due to the temperature dependenceof the carrierthermal velocity.Analysis of the ICTS data indicates that two discretetraps are present at the interface, with the following char-acteristics: AE, = 0.23 eV, crl = 1.8 x lo- 2o cm2,NT1 = 4.5X 10a2 = 3.9x 10-l: cm

-2 ev- and AE, = 0.67 eV,cm2, Nn = 6.5 X 1Ol4 cm - 2 eV - , re-spectively. These values for AE, u, an d NT were used to fitthe DLTS peak with satisfactory results, as shown in Fig.1 (solid li nes). T he fitting parameter u sed was aI and thebest fit was obtained for (TV 8 X 1 0 - 2o m2. For the fittingof the DLTS peak, AC was calculated using2S(T)=[C(t,) - C(r,)/ AC(O) and N,,= (ACE,N,Ci)/[3kTCi,(t,)ln(t,/t,)], where ZV,, s the interface state con-centration, Ci, is the insulator capacitance, AC is the dif-ferential capacitan ce at the peak, and E, is thesemiconducto r dielectric constant. The position of the dis-crete interface level at EC- 0.67 eV is consistent with theenergy distribution of the GaAs/oxide8?9 and GaAs/Si3N4 interface states previously reported. It is responsi-ble for the Fermi level pinning near midgap a nd has beenattributed to the As vacan cy created during the formationof an As-surface rich layer. The discrete level at 0.23 eVand the tunneling compone nt at low temperatures have not

FIG. 2. ACTS spectrum as a function of temperature. T he inset shows thecorresponding activation energy plot of 15~ vs lO/T.

t I106-l-.-L,400 600 a03 1000 1200E . : V,k~rn)~

FIG. 3, Field enhan cement of the tunneling emission rate.been reported i n the past. At the moment, we cannot offera detailed identification of the level at 0.23 eV but it see msplausible to attribute it to surface damage due to theplasma treatment.12

The low-temperature tunneling component could beattributed to pure I:unneling, phonon-assi sted tunneling orFrenkel-Poole (Fl?) field-enhanced emission. Phonon-assisted tunneling cannot contribute significantly since thephenomenon is observed at very low temperatures and it istemperature independent. When FP tunneling dominatesthe conduction, the temperature and field (E) depen denceof the emission rate is=AT2 exp[ - (ET given by e,( T,E)- AE,)/kT]=e,(T) exp(AE&kT),where AE,=q(qE/reg,) I2 (in eV) is the field-inducedbarrier reduction and A is a constant. Therefore, the rela-tive i ncrease of the emission rate associated with the de-crease of the barrier by AEE is (e,/e,,) = exp( - AE,/kT). A seri es of isothermal transients was recorded withvarious biases ( - 2 to - 8 V with a 2 V step) a nd aconstant filling pulse height of 8 V. The correspondingemission rate was obtained us ing the ICTS analysis. In-deed, the experimental results show the expect ed field de-pendence, as shown in Fig. 3. The obtained value for thebarrier reduction is 1.4X lo- 3 qa, which is slightlylarger than the e xpected value ( AE, = 2.2 x 10 - 4q a),but still well within the FP regime. However, the FP effectrequires thermal e,mission of carriers over the effectivelylower potential barriersurrounding the trapped charge andit is therefore expecte d to contribute at high temperaturesand high fields. In our study, the tunneling is observed attemperatures well below room temperature, where thermalemission of the trapped carriers cannot be significant, andits emission rate appears to be temperature independent.Furthermore, Lagowski et al. I3 have shown that a pro-nounced electric field enhancement of the emission rate isobservedn diodeswith large leakage currents. Therefore,it could be concluded that the origin of the observed phe-nomen on is pure tunneling. Finally, the leakage currentdependenceof the maximum capacitance at the ICTS p eakis given by I4 S,,, -I (efl/e,, + )zc,) , where nc, = Ip,/qAis the electron capture rate, 1, is the leakage current den-sity, A is the area of the MIS capacitor, and q is the elec-tron charge. Thus, the observed temperature dependence

138 Appl. Phys. Lett., Vol. 58, No. 2, 14 January 1991 Paloura, Lagowski. and Gates 138Downloaded 08 Mar 2007 to 132.234.251.211. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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the maximum capacitance at the ICTS tunneling peak ised to the temperature dependence of the leakageIn summary, by applying ICTS for the characteriza-the GaAs-insulator interface, w e demonstrate itsfor detection and characterization of both tem-

This technique i s suitable for the characterization ofstructures and has a wider range of applications thanwhile still retaining its attractive features, i.e., itssensitivity, and its spectroscopi c nature. From

observed tunneling component in the ICTS spectra.National Science Foundation is gratefully acknowl-d for financial support.D. V. Lang, J. Appl. Phys. 45, 3023 ( 1974).Yamasaki, M. Yoshida, and T. Sugano, Jpn. J. Appl. Phys. 18, 113(1979).

3D. Vuillaume, J. C. Bourgoin, and M. Lannoo, Phys. Rev. B 34, 1171(1986).

4H. Okushi and Y. Tokumaru, Jpn. J. Appl. Phys. 20, Suppl. 20-1, 261(1981).E. C. Paloura, K. Nauka, J. Lagowski, and H. C. Gatos, Appl. Phys.Lett. 49, 97 (1986).

E. C. Paloura, Ph.D. thesis, M.I.T., 1988, Department of MaterialsScience and Engineering, Cambridge, MA 02139.M. C. Chen, D. V. Lang, W. C. Dautremont-Smith, A. M. Sergent, andJ. P. Harbison, Appl. Phys. Lett. 44, 790 (1984).

*E. Kamieniecki, T. E. Kazior, J. Lagowski, and H. C. Gatos, J. Vat.Sci. Technol. 17, 1041 (1980).9K. N. Bhat, S. K. Ghandhi, and J. M. Borrego, J. Appl. Phys. 57,4657(1985).F. L. Schuermeyer and H. P. Singh, J. Vat. Sci. Technol. 19, 421

(1981). C. Y. Wu and M. S. Lin, J. Appl. Phys. 60, 2050 ( 1986).*N. M. Johnson, F. A. Ponce, R. A. Street, and R. J. Nemanich, Phys.Rev. B 35, 4166 (1987).J. Lagowski, D. G. Lin, H. C. Gatos, J. M. Parsey, Jr., and M. Ka-minska, Appl. Phys. Lett. 45, 89 (1984).14E. K. Kim, H. Y. Cho, S.-K. Min, S. H. Choh, and S. Namba, J. Appl.Phys. 67, 1380 ( 1990).

Appt. Phys. Lett., Vol. 58, No. 2, 14 January 1991 Paloura, Lagowski, and Gatos 139Downloaded 08 Mar 2007 to 132.234.251.211. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp