ELSEVIER Materials Science and Engineering B46 (1997) 333-335

MATERIALS SCIENCE &

ERGINEERING

B

Deep levels of chromium in 4H-Sic1

Norbert Achtziger *, Wolfgang Witthuhn Institut fiir Festkiirperphysik, UnirersitBt Jma, Max Vi% Plat- 1, D-07743 Jella, German)

Abstract

Deep levels of transition metals in 4H-SiC were investigated. A definite chemical identification is achieved by observing the elemental transmutation of radioactive isotopes. Epitaxial layers of n-type 4H-SiC were doped with the radioactive isotopes 48V and 51Cr by recoil implantation and subsequent furnace annealing at 1600 K. Repeated deep level transient spectroscopy (DLTS) measurements were performed during the elemental transmutation of these isotopes to 4STi and 5’V, respectively. In the case of “Cr three levels at 0.74, 0.18 and 0.15 eV below EC disappear with a time dependence of the nuclear decay, i.e. these levels are due ;o chromium. In the case of the 4sV implantation, there is a comparatively strong tendency to form a compensated layer under identical implantation and annealing conditions. A level at EC - 0.97 eV is identified with vanadium in 4H Sic. 0 1997 Elsevier Science S.A.

Keyoods: Chromium; Deep levels; Silicon carbide; Vanadium

1. Introduction

Transition metals are frequent residual impurities in silicon carbide crystals that strongly influence carrier concentration and lifetime because of their deep level properties. Most of the available data refer to vana- dium in 6H-SiC [1,2], its role as a ‘lifetime-killer’ is discussed controversially [3]. Other transition metals and their properties in the polytype 4H are widely unknown. Data about electron traps are rare and there are no reliable chemical identifications. The present work determines the activation energy and estimates the capture cross section of V and Cr levels in n-type 4H Sic by deep level transient spectroscopy (DLTS) ([4]). In order to achieve a definite chemical identifica- tion of these energy levels, radioactive isotopes of these elements are used. The correlation between their well known elemental transmutation and the concentration changes to be observed by DLTS definitely proves the involvement of a certain element in a defect and di- rectly yields the number of atoms involved. This tech- nique has been used successfully in silicon [5-71 and is now extended to the wide-gap material 4H-SiC where the bandgap states of most elements-except for the usual dopants B, Al and N, are unknown.

* Corresponding author. Tel.: +49 3641 636054; fax: f49 3641 635854; e-mail: achtziger@pinet.nni-jena.de

’ E-MRS 1996 Spring Meeting, session A-1X.4.

0921-5107/97/$17.00 8 1997 Elsevier Science S.A. All rights reserved. PIIs0921-5107(96)02000-4

2. Experiment

All experiments were performed in epitaxial n-type 4H SIC layers on heavily n-doped 4H substrates (grown by Cree Research Inc.). The net donor concentration derived from CV-measurements was 3 x IOr cme3. Doping with the radioactive impurities “*V (decay to 48Ti, half life T1,2 = 16 days) or 51Cr (decay to 51V, r,,, = 27.7 days) was performed by recoil implantation with fluences between 10” and 10” cmP2 using (p, n) reactions (for details see ref. [8]). As a characteristic feature of the recoil implantation technique, the im- plantation of 48V is contaminated with stable Ti and that of ‘rCr with stable V.

Thermal annealing of the implantation damage was done in sealed quartz ampoules at 1600 K for 4 h under an oxygen atmosphere. The oxidizing ambient was cho- sen in order to grow an oxide layer of 100 nm which was removed by hydrofluoric acid (HF) directly before contact evaporation (aluminum, 0.5 mm dia.). Large area, ohmic backside contacts were produced by Ti evaporation and heating at 800 K for 5 min.

3. Results

Some DLTS spectra of a “Cr implanted sample (Cr fluence 10” cm-2) are shown in Fig. l(a). The domi- nating peak 3 vanishes during the observation time.

334 A’. dciriziger, lj'. It’ittimim ; Materials Science and Etzgimcring 846 (1997) 333-335

Two smaller peaks below 100 K decrease as well and the peak 4 increases. The height of these peaks is plotted in Fig. 2. The level parameters are summarized in Table 1. In a reference spectrum measured on an unimplanted reference part of the same sample. none of the previously discussed peaks are present and the peak heights are much smaller (Fig. l(b)).

Except for the absolute concentration (in the order of 10’” cm - 3)p the depth profile shapes of the levels l-4 are identical and quickly decrease with depth (by a factor of two within 300 nm depth). An effect of the electric field strength on the emission time constant (Pool-Frenkel effect) exists for peaks 1 and 2 only. The effect is small compared to the theoretical prediction [9] for a singly charged center.

After ‘*V implantation with the same fluence, the samples had a strongly compensated surface layer. To avoid this effect, the fluence was reduced to 10” cme2. A peak with identical parameters as peak 4 of the Yr implanted samples was found to decrease with a half life identical to the nuclear half life of ‘*V. Details of these experiments will be reported elsewhere [lo].

4. Discussion

The concentration decrease of the peaks 1: 2 and 3 exactly follows an exponential decrease with the nuclear half-life of “Cr as it is demonstrated in Fig, 2. Conse- quently, these defect levels definitely contain one Cr atom.

The height of peak 4 increases as it is expected for the daughter element V. This might be the case for decay induced defects as well. The decrease of such a level during the 48V --f Ti transmutation, however, defin- itely identifies it with vanadium. Obviously the same vanadium level is observed both as parent and as daughter element. Because of the co-implantation of stable V during the 5’Cr implantation, peak 4 of the daughter element V is present already at zero delay.

Only a fraction of 22 ( + 5)% of the transmuting electrically active Cr isotopes that contributed to peak 3 forms electrically active vanadium (peak 4). Obviously there is a branching into different configurations and only one of them (peak 4) has a level in the investigated part of the bandgap. This configuration is identical to the natural site of V in Sic because it is observed also after doping with 48V parent isotopes. The branching is probably caused by the high, decay-induced recoil en- ergy of the daughter isotope 5’V (100 eV) that strongly exceeds the displacement energy (about 20-40 eV [I 1,w.

The concentration of peaks 1 and 2 also exhibits an exponential decrease which identifies them as being chromium related. The apparently remaining offset of peak 1 in the limit of infinite delay probably is an

0 -,- ,, ,11”~1”“1”” 100 200 300 400 500

temperature (K)

Fig. 1. DLTS spectra of n-type 4H-Sic: (a) “Cr recoil implanted, measured after several delay times, (b) unimplanted part of the Same sample (scale enlarged). The measuring conditions were: time window 100 ms, U, = - 2 V, U,, = 0 V. 0, is the 1st Fourier sine-coefficient of the transient. The peak value equals the transient amplitude.

artifact due to the superposition of an additional small peak at 85 K that shows up after the decay of peak 1.

Finally, we discuss the relation between the different Cr levels. When comparing the concentration derived from different peaks, one has to account for the differ- ent level energies and the capacitance change with temperature. that becomes significant below 100 K. Because of the steep depth profiles, the height of a low temperature peak may be reduced compared to the peak height of an equally concentrated deeper level measured at a higher temperature. Therefore, the con- centration of the Cr levels 1, 2 and 3 cannot be deter- mined with an accuracy that would be necessary to derive a detailed model from their relative population. Because of their small energy difference, we propose that the rather shallow levels 1 and 2 at 0.15 and 0.18 eV are due to the occupation of inequivalent lattice sites in the 4H lattice (quasi-cubic or -hexagonal). In the case of level 3 at 0.74 eV, however, there is no

0 0 20 40 60 80 100 120

delay (d)

Fig. 2. DLTS peak heights of peaks l-5 during the elemental transmutation j’Cr + 51V. The solid lines are exponential curves with the half-life of the nuclear dccap (27.7 days).

N. Acitfziger, lV. Wittlutlw / Mutwials Sciertce and Engineering B46 (1997) 333-335 335

Table 1 Thermal activation energy ET (emission to the conduction band), capture cross section 0 (assumed to be constant) and chemical identification of the deep levels l-4

Peak number: 1 2 3 4

ET (eV) 0.15 0.18 0.74 0.97 g (cm2) 2x10-I” 8x10-‘6 2x lo-l5 6x lo-l5 Identification Cr Cr Cr V

indication of a level splitting. We believe that the subtle difference between the inequivalent h- and k-sites is less important for deep levels because of their more local- ized electron states. Within the experimental uncertain- ties, the data are at least consistent with a model, that peak 1 and 2 together reflect one charge state and peak 3 reflects another charge state of the same configura- tion.

A deep level with identical parameters to our deep Cr level 3 has been reported by Uddin et al. [13] from samples containing considerable amounts of impurities, including chromium. Our identifications excludes their careful proposal to identify Cr with two other levels found in that work. The existence of a rather shallow Cr level has been proposed by the same authors to explain the n-type doping effect of Cr observed in their growth experiments [13]. The weak Pool-Frenkel effect observed in the present work, however, does not favor donor-like states.

In the case of vanadium, there is only one level within the investigated part of the bandgap (about 0.15- 1.1 eV below EC). The missing Pool-Frenkel effect hints at an acceptor character. Together with the reported V donor state at midgap [1,14], which is far outside the energy window of DLTS, this assignment gives a consistent level diagram of V in the 4H poly- type: a midgap donor state and an acceptor state at EC - 0.97 eV. This level diagram is completely analogous to V in the polytypes 3C and 6H [15].

Recently, the V acceptor energy was published by Jenny et al. [16] to be 0.80 eV in contrast to our value of 0.97 eV. This difference may have methodical rea- sons: the authors ignore a possible temperature depen- dence of the mobility when analyzing resistivity data and their DLTS measurements were performed in ex- tremely high electric fields and with a relatively high trap concentration. All of these conditions may result in an apparently reduced energy.

The strong compensating effect observed after 48V implantation is probably not due to implantation dam- age or V, but due to the co-implanted Ti. Otherwise, the effect should occur as well during the “‘Cr implan-

tation, because the damage is identical and the V fluence is at least comparable.

5. Conclusion

The deep levels of chromium (0.74, 0.18 and 0.15 eV below EC) and vanadium (E, - 0.97 eV) in the 4H-SiC bandgap were identified by elemental transmutation. There are no further levels of these elements with a comparable concentration in the part of the bandgap investigated.

Acknowledgements

We thank M. Frank and K. Rith for organizing the beamtimes at the tandem accelerator at the university of Erlangen. The work was funded by the German BMBF under the contract number 03WI4JEN9.

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