Determination of the subsUiUutional nitrogen content and the electron effective mass 6 InN>b,-, (001) epitaxial layers

I. Mahboob, T.D. Veal and C.F. McConville

Abstract: The electronic properties of epitaxial layers of InN,Shl-, grown on GaAs (001) substrates havc been investigated using high-resolution electron-energy-loss spectroscopy (HREELS), Hall measurements and band smcture modelled by a modified k.p Hamiltonian. The chemical composition of the epitaxial layers was found to bc InNo ozSb,l.,, from secondary ion mass spectrometry (SIMS). However, electrical measurements and hand structure calculations indicate a band gap of 135 meV. thus entailing an epilayer composition of lnN,,.,,,,Sb,9985, indicating that approximately 7.5% of the nitrogen present is clectrically active, being located substitutionally 011 antimony lattice sites. Hall measurements and simulations of HREEL spectra imply a much larger effective inass at the Fermi level than a conventional Kane hand structure material with an equivalent band gap.

1 Introduction

When a small fraction of anion atoms are replaced with nitrogcn in conventional binary Ill-Valloys the band gap is found to initially decrease rapidly [ 1-31 resulting in a myriad of applications for these materials. E3and gap reduction of 400 meV has been observed in GaAs with the incorporation of just 3% nitrogen on arsettic lattice sites [ I ] . Baud gap reduction is thought to occur BS a result of the formation of a narrow nitrogen level situated above the conduction band the interaction of which, with the conduction band results in band gap reduction [4]. The major attraction of these materials lies in the fact that the band gap can be tuned to previously unavailable regions of the electromagnetic spectrum and the rcsulting material can bc lattice matched to substrate material thus facilitating high-quality growth and incorporation into the existing electronics infrastructure [4-61.

However, little investigation has been undeitaken on antimony-based ternary dilute nitrides [7-10]. This inate- rial holds great promise as it has many applications such as remote detection of pollutant gases, secure co:mniunica- tions and eye safe range finding, to name a few. lnSb has the narrowest band gap of binary Ill-V semiconductors that corresponds to a wavelength of -7pm at room temperature. Extension of this wavelength to -10 pm at room tempcrature is essential for device developmcnt for the above applications. Alloying lnSh with small amounts of nitrogen presents an attractive prospect for the dcvelop- ment of the far infrared wavelength [7, 81

A key issue in dilute Ill-V nitride materials is deternii- nation of the alloy composition, specifically thc amount of electrically active nitrogen. Measurenicnt of the lattice constant by X-ray dimaction (XRD) has been widely used to deduce the substitutional nitrogen content by applying Vegard's law However, deviations from Vegard's have been observed in GaN,As,-, [ I I , 121. In future work we plan to use X-ray photoelectron spectroscopy (XPS). along with XRD and secondary ion mass spectrometry (SIMS) to investigate the variation of lattice parameter with nitrogen content, the relative amounts of substitutional and interstitial nitrogen in InN,Sbl-, and to investigate the nitrogen bonding configurations. Hecht et al. have shown that XPS can distinguish between substitutional and inter- stitial nitrogen in Ill-V semiconductors [13]. It is probable that InN,Shl-, will have a grcater proportion of interstitial nitrogen than othcr dilute Ill-V nitrides because of the large lattice constant of InSb. Additionally, we will employ electrical measurements for this purpose. The results of our initial high-resolution clectron energy loss spcctroscopy (HREELS) and Hall studies are dcscribed in this paper.

Here we report on a sclf-consistent method dcvcloped to derive the substitutional nitrogen content based purely on electrical measurements. HREELS is employed to probe the electronic structure near-surface region of these niate- rials by means of low-energy electrons which couple to the electric fields of surface plasmons arising from the free carriers in the conduction band resulting in electron energy loss spectra. By varying the energy of the probing elec- trons_ the electron plasma at depths of between 100 A and 1500A can be surveyed [14-161.

0 IEE, 2003 IEE Ptuceeding.7 online no. 211030045 Dol: I O . l049lip-opt:2003il045 Paper reccivcd I Ith Novembcr 2002

The authors are with the Department of Physics, University of Warwick. Covently CV4 l A L , UK

I02

2 Experimental

Samples were fabricated by QinetiQ (Malvern, UK) on semi-insulating GaAs (001) substrates with InN,Sbl-, growth via MBE with nitrogen incorporation by means of a plasma source [7, 81. In order to reduce the amount of

IEE Pruc~Opr"rl~~,~.aan. . 1'01. 150. No. I. Febrtriiq 2003

nitrogen incorporated during growth, hydrogen was intro- duced to the plasma. Hydrogen incorporation into the samples has been measured at 10% of the nitrogen level [SI. Nitrogen incorporation was measured with SlMS where data calibration was achieved by means o f nitrogen- implanted standards. These investigations yield a value of s = 0.02 for the chemical incorporation originating from both the substitutional and interstitial latticc sites. Hall mcasuremcnts of the InN,Sb, -,alloy yield a room temperature carrier concentration of n = 9 x 1 0 ’ ~ cm-l and a mobility o f p = 1400cm’ V-‘ s-I. To enable the collection of HREEL spectra, atmospheric contamination of the sample was removed by atomic hydrogen cleaning; a process that does not damagc the electronic propcrties of the material being prepared [ 171.

3 Results

A series of normalised HREEL spectra, recorded with a range of incident electron energies from the clean InN,Sb, _ x surface, are shown in Fig. I along with dielectric theory simulations. The main feature of the spectra is the conduction band electron plasmon peak at - 65 meV At low probing electron energies, a surface phonon feature at -23 meV can be observed as a shoulder on the elastic peak. The plasmon peak does not dispersc significantly with respect to electron loss energy because of two conflicting effects. Downward dispersion of the plas- mon feature with decreased probing electron energy, Ei, arises bccause of a depletion of free carriers in the near- surface region [IS]. This is compensated for by upward dispersion due to the wavevector dependence of the plasmon excitations (spatial dispersion). Simulation of the HREEL spectra is essential in order to extract quanti- tative infomiation about the system under investigation [16, 191. The HREEL spectra were simulated using a two- layer model consisting of a 60 A depletion layer and a bulk layer with a plasma frequency of 70ineV. This plasma

-50 0 50 100 150

frequency (U,’) and the carrier concentration (n) from the Hall measurements are related by

where i:(oo) is the high-frequency dielectric constant, giving an effective mass at the Fermi level me of 0.160. This particular combination of cup, n and m,? occurring at the particular Fermi level is determined by the details of the conduction hand dispersion relation, which, in turn, is defined by the band structure and therefore the electrically active nitrogen content in the alloy.

The band structure of InN,Sbl - ~ for sinall nitrogen values is predicted using a modified k.p Hamiltonian previously dcveloped for GaN,As, - . ~ and modified for InN,Sb, _ , ~ [9, IO , 20, 211. The modified k.p model is used, rather than the hand auticrossing model (BAC), to include the interaction of the valence bands with the conduction band due to the very narrow hand gaps of InSb-based dilute nitride alloys. Fig. 2 shows the band structure of InN,Sbl ~ for x=0 and 0.0015, incorporat- ing, the nitrogen level, conduction band. hcavy, light and split off hole bands. Details of the band structure model employed can be found in [9] and references therein. Thesc results are then uscd to compute the plasma frequency U,>

and the effective mass at the Fermi level mp as a function of the carrier concentration n. Thc results of these calcu- lations are shown in Fig. 3 for x=0.0015, the alloy composition for which the experimentally determined (up, n and niF values coincide. Comparing this x value with the nominal alloy composition determined using SlMS suggests that approximately one thirteenth of the nitrogen present occupies substitutional lattice sites. This figure compares well with the figure for the substitutional nitro- gen content deduced from the spectral output of LEDs fabricated from this material [8].

The effective mass at the Fermi level for IuN,,~,,,,,sSbo.,,8, of 0. I 6 is significantly larger than the value for an equivalent band gap Kane band material of 0.072. This largc effective mass at the Fermi level of luN,.o,,lsSb0.998, can be understood when one examines the E- band (Fig. 2) at the Fermi level (-275 meV) for the observed carrier concentration. The E- hand is highly

800 1

k . A-1

Fig. 2 Dolled line: x = O , Es= 18OmeV Solid linc: .x=O.OO15. Ex= 135 ineV

Bond sfriicture calcdated for InNJb, - .~

ID3

I ' " ' I I I 0.30 5 Acknowledgments

The authors would like to thank T. Burke and T. Ashley of QinetiQ, Malvern, UK for MBE growth of the InN,Sbl-, samples.

6 References

' 0 1018 1019

carrier concentration, c m 3

Fig. 3 Plasma fiequencj! U,,, mid efecrive niass at the Fermi l e v d m;., colcrrlared us a,function ofcarrier cuncentrarion. n. jrum dispev.sion of E- batid f i r x=0.0015 The dotted line denotes the experimentally determined w,,. n and m; values

non-parabolic and flattens distinctly at high wavevectors. The effective mass at the Fermi level is determinsd by the curvature of the dispersion relation, according to

Obviously the lack of curvature at this particu'lar Fermi level implies a large effective mass. Murdin er al. also observed large effective masses in InN,Sbl, with regards to suppression of Anger recombination losses tc' improve the operating tcmperatnre in emitter and detector devices [9, IO]. Further, large effective masses have also been observed in other dilute nitrides and are of critical impor- tance for carrier confinement in the superlattice structure for emitter devices.

4 Conclusions

HREELS and Hall measurements were used to d'Educe the substitutiond~ nitrogen content in InN,Sbl_, using dielec- tric theory simulations, carrier statistics and calculations of the band structure modelled by a modified k.p Hamiltonian. Our data show that 7.5% of the total nitrogen content occupies substitutional lattices sites. The remaining nitro- gen either occupies interstitial lattice sites, indium lattice sites or is passivated by the atomic hydrogen incorporated during the sample growth. A large enhancement of the effective mass when compared with a conventional Kane band material has been obscrved and is a consequence of the flattening of the E_ conduction band resulting; from the interaction with the resonant nitrogen level.

1 energy loss spectroscnpy and . . . . .. - -.

IRE, R., LOWE, M.J.. and McCONVILLE, C.F.:. to he published.

18 JONES,T.S., SCHWEITZER, M.O., RICHARDSON, N.V,B,ELL,G.R.. and McCONVILLE, C.F.: 'Depletion layers, plasmon dispersion, and the effccts of temperature in dcgenerate l n S h ( 1 0 0 t a study by electron- cnergy-losripecfroscopy'. Phys. Rev. E, 1995,51,(24).pp. 17615-17680

19 LAMBIN, Ph., VIFNERON, J.P., and LUCAS, A.A.: 'Computation of the surface electron-energy-loss spectrum in specular geometry for an arhitrarv olane-stntificd medium'. Cornmi. Plivs. Co,nntan.. 1'990. 60. (3), pp.~3Sl-364

20 LINDSAY. A.. and O'REILLY, E.P: 'Theory ofenhanced handgap nan- paraholicity in GaN,As,-, and related alloys', Solid Sfale Cwnm~m., 1999, 112, (8). pp. 443447

21 O'REILLY, E.P., and LINDSAY, A.: 'k.p model of ordcrcd GaN,As, -L', Plzys. Sfolio Solidi E, 1999. 216. ( I ) , pp. 131-134