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Electronic characterization of n-ScNp Si heterojunctions

F. Perjeru, X. Bai, and M. E. Kordescha)

Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701

Received 13 August 2001; accepted for publication 27 November 2001

We report the electronic characterization ofn-ScN in heterojunctions, including deep level transientspectroscopy of electrically active deep levels. ScN material has been grown by plasma assisted

physical vapor deposition on commercialp

Si substrates. Currentvoltage and capacitancevoltage measurements indicate that the built-in voltages for the heterojunctions used in this study areapproximately 1.74 and 0.40 eV, as seen by electrons and holes. Deep level transient spectroscopyresults show the presence of an electronic trap with activation energy ECET0.51 eV. The traphas a higher concentration ( 1.2 1.61013 cm3) closer to the ScN/Si interface. 2002 AmericanInstitute of Physics. DOI: 10.1063/1.1447323

Transition-metal nitrides are well known for their re-markable physical properties including high hardness andmechanical strength, chemical inertness, and electrical resis-tivity that varies from metallic to semiconducting. They arewidely studied for applications such as hard wear-resistantcoatings, diffusion barriers, and optical coatings.1

Theoretical calculations indicate that ScN has an indirectband gap of 0.90 eV and a direct one of 2.20 eV 2 but experi-mentally the direct gap at 2.2 eV dominates, the indirect gaphas yet to be confirmed. ScN crystallizes in cubic form rocksalt structure 3 with lattice constant a4.48 . ScNmaterial has been grown by a variety of techniques.4 8

In this study the presence of deep levels in ScN materialwas investigated using deep level transient spectroscopy DLTS . Schottky barriers could not be formed on ScN,6

while ScN p n homo-junctions do not have well-behavedcurrentvoltage (I V) characteristics i.e., large leakage

currents and low breakdown voltages .8

ScNSi p n hetero-junctions were fabricated for this study.The ScN material used in this study was grown by

plasma assisted physical vapor deposition PAPVD , as de-scribed in Ref. 3, on pSi substrates. The Si substrateshave been cleaned in HF:de-ionized water 1:1 then rinsedin de-ionized water and dried in inert gas prior to ScN depo-sition.

The ohmic contacts were fabricated using dc-sputteredPd for ScN and evaporated Al for Si thickness 1000 .Details on contact formation can be found in Ref. 7.Currentvoltage measurements were conducted at 300 K us-ing a Keithley 236 source measure unit. Capacitance

voltage and DLTS measurements were performed in vacuumat a pressure of 1103 Torr and at a frequency of 1 MHzusing a SULA Technologies deep level transient spectrom-eter. The samples were heated indirectly using a power resis-tor while the cooling process was done using liquid nitrogen.The sample temperature, needed for Arrhenius analysis, wasmeasured with a thermocouple and a Eurotherm temperaturecontroller.

From Hall effect measurement for samples grown underthe same conditions as the ones used for this study, it was

found that ScN films were n type with free carrier concen-tration of ( 1.0 5.0)1015 cm3. The Si substrates were ptype doped with boron, with a free carrier concentration of2.01018 cm3. The difference in the doping level of almostthree orders of magnitude simplifies the DLTS analysis in

that the space charge region will mostly spread into the ScNside and spreading of the space charge region into the Si sidewould be negligible.

A typical set of ScN/Si I V and C V characteristics isdisplayed in Fig. 1. Ideality factors were in the range of3.23.5, with turn-on voltages for these junctions in therange of 2.63 V, and series resistances of approximately300 k. From logarithmic characteristics, barriers of 1.61.7eV, at 300 K, were calculated.

From C V measurements, the carrier concentration forScN was calculated to be 2.01015 cm3 in agreement withHall data and uniformly distributed in the bulk of ScN. Val-ues of 1.74 and 0.40 eV, seen by electrons and holes, respec-tively, were obtained for the built-in voltages.

The Arrhenius plot was obtained from DLTS data9 takenusing reverse bias voltage of2 V, filling pulse of0 V,and filling time duration of 1 ms. For every temperature,approximately 20 DLTS signals were recorded and averaged.The temperature range used in this study was 280420 K.The sample temperature was kept constant during data acqui-sition (within 0.5 K), with a temperature rate change of10 s/K. DLTS scans while the sample was cooled down have

a Electronic mail: kordesch@ohio.edu

FIG. 1. I V and C V characteristic for ScN/Si heterojunctions. Idealityfactors of 3.2 3.5, turn-on voltages of 2.63 V, and series resistances ofapproximately 300 k were calculated. From logarithmic characteristics inset, left , barriers of 1.61.7 eV, at 300 K, were extracted.

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 6 11 FEBRUARY 2002

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been taken as well, in order to estimate any shift in the DLTSpeak with increasing/decreasing temperature. The rate win-dows used in this study were 0.86, 0.43, 0.215, and 0.086ms, corresponding to emission rates of 11 627.90, 1162.79,4651.16, and 2325.58 s1. Only for these rates could aDLTS peak be seen in the temperature range investigated.

Typical DLTS graphs are displayed in Fig. 2. An elec-tronic trap with concentration of 1.2 1.61013 cm3 is

found in the sample, with an activation energy of EcET0.510.03 eV assuming a temperature independent cap-ture cross section . In order to estimate the capture crosssection for this trap, a value for the effective electron massfor ScN is necessary, which has not been measured nor cal-culated to this date.

The concentration of the trap has been estimatedusing10,11 the equation NT2 ND (C/C0), where NT rep-resents the trap concentration, ND is the free carrier concen-tration, C0 is the junction capacitance at the applied reversebias, C is the DLTS signal.

The concentration of the trap has been investigated byvarying the spreading of the space-charge region in the ScNmaterial, using different values for the reverse bias, and isshown in Fig. 3. The rate window 0.86 ms , filling pulse(0 V) and filling time 1 ms were kept constant, while thereverse bias was changed from 4 to 1 V. This corre-sponds to approximately 600 m from ScN/Si interface. Toinvestigate the electric-field dependence of the trap, the du-ration of the filling pulse was varied while all other param-eters were kept constant.

The fact that these junctions are characterized by idealityfactors greater than 1 suggests that interface roughness dueto the way the ScN grows and the lattice mismatch with Siplays an important role in degradation of the I V

properties.12

Assuming an effective electron mass equal to the elec-

tron mass (9.11031 Kg), the Fermi level position forthese samples can be calculated using Boltzmannapproximation,13 and would be situated at 0.24 eV below theconduction band. In order to be able to estimate the barrierand the band bending of the conduction band at the interfaceof ScN/Si, one needs to know the electron affinity value forScN. This value is not yet calculated or measured, therefore,the band bending cannot be estimated for these junctions.

From Arrhenius analysis an electronic trap situated be-

low the conduction band at approximately 0.510.03 eV hasbeen found in the bulk of ScN. A trap concentration profileshows that this defect has a higher concentration near the Siinterface (1.21.61013 cm3). This could be understood ifthe defect is associated with the existence of structural de-fects, such as dislocation lines, a consequence of the mis-match between the two structures of about 10%. If one bearsin mind that the mismatch of about 14% between GaN andsapphire leads to dislocation lines concentration of about1010 cm2 in good films, then the conclusion for ScN/Si isthat the density of structural defects can be very high at theinterface between the two materials as well. Once the filmsbecome thicker, moving away from the Si interface, the con-centration of the defect decreases. This defect might be as-sociated with intrinsic defects, either present in the as-grownmaterial independent of the substrate as is the case of theNGa defect in GaN

14,15 or might be created as a consequenceof structural defects. In either case, it is still possible that theconcentration of the defect would have a dependence uponthe structural environment, as has been found for the EL2defect in GaAs.16

Given the fact that this level has a signature that does notdepend on the duration of the filling pulse implies that thecapture cross section has a temperature independent behaviorand that the electron capture rate is not limited by a time-

dependent Coulomb barrier.16,17

ScN/Si p n heterojunctions have been fabricated. DLTS

FIG. 2. Typical DLTS spectra for ScN/Si heterojunctions. The diode waspulsed from a reverse bias of2 V to a filling pulse of0 V for 1 ms. Forfour different rate windows corresponding to four emission time constantsthe temperature of the diodes was ramped from 280 to 420 K. The concen-tration of the trap is (11.5)1013 cm3.

FIG. 3. Trap concentration profile as a function of distance from the ScN/Siinterface. It can be seen that the concentration of the trap is decreasingfurther from the interface of the two materials, dropping by almost one orderof magnitude.

996 Appl. Phys. Lett., Vol. 80, No. 6, 11 February 2002 Perjeru, Bai, and Kordesch

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analysis shows that an electron trap with activation energy of0.51 eV is present in PAPVD ScN material, and that this traphas a higher concentration closer to the Si interface, due to,probably, existence of a higher number of structural defects.Improvements in the as-grown material as well as theoreticalcalculations are necessary in order to make assumptionsabout the nature of the defect.

This work is supported by the Office of Naval Researchthrough Grant Nos. N00014-96-1782 and N00014-99-1-0975.

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997Appl. Phys. Lett., Vol. 80, No. 6, 11 February 2002 Perjeru, Bai, and Kordesch

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