Infrared absorption in indium nitrideT. L. Tansley and C. P. Foley Citation: Journal of Applied Physics 60, 2092 (1986); doi: 10.1063/1.337213 View online: http://dx.doi.org/10.1063/1.337213 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/60/6?ver=pdfcov Published by the AIP Publishing

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Infrared absorption in indium nitride T. L Tansley and C. P. Foley"l School of Math and Physics, Macquarie University, NSW 21 13, Australia

(Received 31 March 1986; accepted for pUblication 13 May 1986)

The room-temperature optical absorption spectra of indium nitride films in the subband-gap energy range 40 me V to 2.0 e V are surveyed. The major features are two broad absorption bands: a band-edge tail with a threshold energy of about 1 e V and a broad peak centered at about 0.3 e V. Both are strongly sample dependent and are identified with crystal defects associated with the observed electrical compensation of the shallow donor impurity. All samples show optical excitation of the 50-60-meV shallow donor seen in thermal excitation measurements, while some exhibit an absorption doublet, whose nature is not understood, around 0.15 eV.

I. INTRODUCTION

It has recentl.y been shown that a modified radio-fre­quency sputtering technique' permits the preparation ofUI­V nitride semiconductors without the deliberate introduc­tion of deep-level compensating impurities. For example, the introduction of Be, Zn, Mg, or Li is apparently necessary to yield gallium nitride of high resistivity. 2

The essence of our method is that group-III metal tar­gets are prenitrided by reaction with a nitrogen plasma prior to growth. The availability of atomic nitrogen as a sputtered species sustains a high degree of stoichiometry in the depos­ited films. 3 One consequence has been the deposition of films of semiconducting indium nitride, in which nitrogen vacan­cies constitute the principal donor, with room-temperature electron densities as low as 2x 10'6 cm- 3

•4

Conductivity measurements indicate the influence of compensating centers on the temperature dependence of mobility below 150 K. Compensation, which is typically 30% for the purest samples (n::::: 1016 cm- 3

), increases fairly linearly to about 80% for the highest doped samples (n:::::5 X 1017 cm- 3

). The mechanism for electron scattering at higher temperatures remains somewhat obscure, showing the qualitative features of space-charge scattering at inter­crystallite boundaries, although the possibility exists that lo­calized phonon modes may also contribute.

The observed high electron mobilities are commensur­ate with a low effective mass, a further consequence of which is the shift of the fundamental optical absorption to higher energies with increasing electron concentration. A Moss­Burstein shift of about 0.2 eV above the low concentration vaJ.ue of 1.89 eV is seen5 for samples with n> 1020 cm- 3

.

These observations concur quantitatively with the calculat­ed band structure,6 the principal features of which include a central f\ conduction-band extremum with a r 3 minimum displaced by about 0.3 eV.

Optical absorption at photon energies below the band gap in indium nitride has seldom been reported,7,8 and only in the photon energy range kv > 0.4 eV. In the present work we extend these measurements down to 40 me V (30 Ji-m wavelength), identify the principal contributions to optical

oj Present address: CSIRO Division of Applied Physics, National Measure. ment Laboratory, P.O. Box 218, Lindfield NSW 2070, Australia.

absorption and, where appropriate, attempt to correlate the data with the general material properties summarized above.

II. EXPERIMENT

Details of growth methods and measurement tech­niques have been given elsewhere.s Briefly, (00.2) oriented polycrystaUine films are prepared by radio-frequency sput­tering of a prenitrided indium target in a reactive nitrogen plasma. Comparison of absorption data taken from films grown on various substrates failed to reveal any substantial dependence of optical properties on choice of substrate. The majority of measurements were made on films grown simul­taneously on glass and polished KBr, a combination giving a good degree of overlap of spectral transparency in the visible and near-infrared regions.

III. RESULTS AND DISCUSSION

Figure 1 shows schematically all of the room-tempera­ture absorption features seen in various samples with elec-

0.01 0.1 1.0 10

photon en_gy(eV)

FIG. I. Schematic infrared absorption spectrum of indium nitride. Princi· pal features are A: direct band-to-band absorption; B and C: deep level exci­tations; D: an infrared doublet of unknown origin; E: excitation of hydro­genic donor levels.

2092 J. Appl. Phys. 60 (6), 15 September 1986 0021-8979/86/182092-04$02.40 @ 1986 American Institute of Physics 2092 [This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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tron concentrations in the range 5 X 1016-2 X 1018 cm -3.

Above the band edge (w > 1.8 eV, region A in Fig. 1) direct valence to conduction-band transitions occur and these have been discussed elsewhere.5 This edge is accompa­nied by a long wavelength tail (1.8 eV > w > 1.2 eV, region B) terminated by the onset of a band of high absorption peaking at about 0.3 e V and tailing into the far infrared (re­gion C). Superimposed on this band is some structure at about 0.15 eV (feature D), whose occasional appearance we were unable to correlate with any other measured param­eters and do not discuss further. Additional structure occurs at about 50-60 me V (feature E). We now proceed to discuss these spectral features in more detail.

A. Deep-level absorption

The photoionization threshold ET at 1 eV corresponds to centers lying approximately at midgap. The associated states are therefore properly described as deep levels. In this case the energy dependence of the absorption cross section varies as E i/2 for Is) like states and E :/2 fori p) like states9 where Ek = w - ET . Recall also that the r l conduction minimum has an Is) like orbital character.

Figure 2 plots a 2 against photon energy between 1.0 and 2.0 eV (region B of Fig. 1). The four samples shown have electron concentrations in the range 1-3 X 1017 em - 3 and the E 112 relationship indicative of j p) -like states is evident.

Extrapolated threshold energies. Fig. 3, decrease linear­ly with increasing carrier concentration. presumably repre­senting the superimposition of random fluctuations in the local potential in less-stoichiometric samples. Also shown is a plot of the absorption coefficient at 1.8 eV against electron concentration from which we deduce an optical absorption cross section of about 10- 14 cm2 given the high degree of compensation, on average about 50%. The combination of

-: 6 d E 2 .. 0 ...

>C

" III 4

2

1.0 1.5 2.0

photon energy(eV)

FIG. 2. Square of absorption coefficient against photon energy (region B of Fig. 1) for four samples with the following electron concentrations: (a) 1.1 X 10" cm-3 ; (b) 1.6 X 10" cm- 3 ; (c) 1.85 X 10'7 cm-'; (d) 2.3 X 1017

cm- 3.

2093 J. Appl. Phys., Vol. 60, No.6, 15 September 1986

8 - /

'e X u

x/ >: .. ; 1.2 II -.. ... IJ.I

>C III

6

X 0

4 o 1.0

2

electron conc.( 1017 cm-~)

FIG. 3. Absorption coefficient at 1.8 eV and excitation threshold energy against electron concentration for the four samples of Fig. 2.

high absorption coefficient, the apparent IP) -like nature of this center. and its midgap location suggest that the defect responsible may be the InN antisite substitution, analogous to the midgap GaAS defect frequently held responsible for the EL2 center in GaAs.

B. IBroad absorption band

Figure 4 shows the absorption coefficient across the broad infrared band centered between 0.35 and 0.4 eV (re­gion C) for three samples from the present study and two examined over a limited range by other workers. 7.8 The prin-

I 1 I I

10 5 - -

';" E ~ III

104 t- -

10~ - 1 I I 1-L-~ __ ~ __ L-~ __ ~ __ ~~ __ -L--J

0.2 0.4 0.6 0.8

photon energy (eVl

FIG. 4. Broad infrared absorption band (region C of Fig. 1) for various samples as follows: (a) Trainor and Rose.' n = 3 X l{)'o cm -3; (b) Hovel and Cuomo,7 n = 5X 10" cm- 3; remainder, this work; (c) n = JOl9 cm-'; (d) n = 10'8 cm-'; (e) n = 5X 10" cm -J. The dotted line is calculated on the basis of transitions between conduction bands and the dashed line is the expected response ofa deep neutral impurity. Both are to be compared with experimental data (d).

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cipal contestants for this contribution are (a) photoexcita­tion of electrons from the conduction band extremum to a higher minimum, in particular the additional minimum at the zone center at a calculated elevation of about 0.3 eV, and (b) photoionization of a deep level for which the threshold energy is about 0.2 eV.

Band-to-band transitions are characterized by an ab­sorption coefficient:

a(IUu)=A(w-Er )2, (1)

where E T is the minimum band separation, assuming para­bolic bands and in the energy-independent matrix element approximation.

Following the arguments of Kaisser and Fan,1O as ap­plied to transitions between valence and partially fined con­duction bands, the dispersion relations for two parabolic conduction bands coincident at the zone center are

E, = fi2k 212mI' E2 = ET + fi2k 212m2' (2)

when the geometrical relationship for an optical transition is

(1Uu - ET ) = fi2k 212m" with

mr = m,m2/(m, - m2)'

The energy-dependent absorption coefficient, including an electron occupancy term for the lower band, thus beComes

a(lUu) = A (fUu - Er )2/{1 + exp[ (fUu - Er )m,1

(3)

Since m rm,==m2/(m, - m 2 ), this absorption band lies in the spectral region 1Uu> E T for m, > m 2 or IUu < E T for m, <m2• The function described by Eq. (3) is sharply peaked at IfUu - ET I -;::;kT for all values of the ratio m,/m, (and is a delta function at fUu = E T for parallel bands, m t = m2)' As an example we have plotted Eq. (3) for room temperature, choosing m I = O.lmo, m 2 = 0.16mo,6 and EF = 0 (corresponding ton = 1Olll cm -3 5) for comparison with relevant experimental data in Fig. 4. This mechanism does not provide a sufficiently broad peak to account for the data. Further, no temperature dependence was seen in the range 290--370 K, over which electron concentration in­creases by a factor of 1.6.

For a deep level which is neutral, the absorption coeffi­cient is ll

a(lUu) =BEr2/fUu[1 + (EkIET )]2, Ek =w-Er . (4)

Equation (4) is modified in the case ofadonor,9 but tends to the above in the limit of sufficient depth. The matrix element between the quantum defect wave function and the final Bloch state is significant over a wide energy range and a broad absorption band ensues. We have included a calculat­ed result in Fig. 4 (curve d) based on a threshold energy of 0.19 eV for comparison with n = lOtS em -2 data. Similar fits were obtained for Er = 0.18 eV (n = 5X lOt8 cm- 3 ) and ET =0.2eV (n = lO'9 cm-3). The generaHorrnofEq. (4) is evident experimentally, reduced absorption at higher en­ergies we attribute to the increasing nonparaboIicity of the conduction band.

2094 J. Appl. Phys., Vol. 50, No.6, 15 September 1986

10·r------r------~-----.------,_----~

_ -0-

, 5 ii

10'· 1017 10'· 10" 101D

electron cone.( cm-*)

FIG. 5. Absorption coefficient at 0.6 eV against electron concentration for the five samples of Fig. 4. The solid line represents Q a: n, deviation from which at n-3 X 10'· cm- 3 coincides with the onset of conduction-band filling.

The extrapolated absorption coefficient at 0.6 eV, is plotted against electron concentration, as an approximation to the density of the compensating center, in Fig. 5. The relationship is again linear for low electron concentrations and with an approximate cross section of 10- 14 cm- 2

• This correlation may imply that the levels of 1.0 and 0.2 eV are excitations of the same defect. We also note that sublinearity above n -;::;3 X 10t8 cm -3 coincides with the onset of degener­ate filling of the conduction band with consequent restric­tion on available final quantum states.s

C. Long wavelength absorption peak

The spectral absorption coefficient for a hydrogenic do­nor is9

a(w) = CE~/2/w[l + (EkIR ~) J4, Ek = IUu - Er . (5)

Here R ~ is the effective Rydberg energy q2/811"ErfJ H' where a H is the Bohr radius of the state involved. Close to thresh­old, Eq. (5) is mUltiplied by the Coulomb factor Co-;::;211"(R '!II Ek ) 112, which shifts the peak response to an

c o :;:: CI. .. o ., Sl 1\1

40 50 60 70 80

photon energy (m·eV)

FIG. 6. Photoionization spectrum of the hydrogenic donor doublet (region E of Fig. I) at 300 K. The deconvolution into two contributions is indicated by the dotted lines.

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temperature(K)

300 250 200 101.r-~------~-----------r----,

~ u ..... U c: o u c o .. .. u • i 1017

101.~--~----~-----L--__ -L __ ~

3 4 5

FIG. 7. Hall measurements of electron concentration for four samples at several temperatures (data points). The solid lines represent thermal acti­vation energies of 45 meV (two higher-doped samples) and 50 meV (two lower-doped samples).

energy only O.llR 1:. above threshold. Thermal broadening reduces the sharpness of this threshold, however. Experi­mentally, the absorption spectrum, Fig. 6, can be resolved into two contributions with maxima at 55 and 70 meV, cor­responding to thresholds at 50 and 65 meV, respectively. The excitation energy of the larger peak is in excellent agree­ment with the termal excitation energy, 45-55 meV, of the principal donor obtained from measurements of tempera­ture dependence of electron concentration as shown in Fig. 7. The Boltzmann factor, exp liE IkT, for levels -20 meV apart is about two at room temperature, hence the smaller peak corresponds to either a lower density of more populat­ed levels, or a cross section about five times smaller than that of the shallower level.

Further measurements at 320 and 350 K, showed ab­sorption decreases of 10% and 25%, respectively, corre­sponding to increased thermal depopulation of the donor levels with the appropriate activation energy. The optical

2095 J. Appl. Phys., Vol. 60, No.6, 15 September 1986

cross section for this transition is of the order of 10- 18 cm2,

taking into account the relative occupancy of conduction band and donor levels.

IV. SUMMARY

Room-temperature infrared absorption measurements on sputtered indium nitride films exhibit three principal fea­tures in the energy range 40 meV to 1.9 eV. Each of these features varies systematically with electron concentration and hence with the level of compensation present.

An absorption tail appended to the principal valence to conduction-band transition apparently corresponds to a mid gap (Ec - 1 eV) level of jp> like character with an opti­cal cross section of 10- 14 cm2

• A similar cross section is presented by a further level with a threshold excitation of 0.2 eV, for which the possibility of transitions between conduc­tion band minima can be dismissed. In highly doped samples there is evidence that the transition is inhibited by degener­ate filling of the conduction band. The similarity of behavior of these two levels suggests that they might be related, possi­bly as excitations of an InN antisite defect.

Independently of the presence of deep levels, a shallow hydrogenic donor doublet at 50 and 65 meV is present and has properties fully compatible with thermally excited elec­tron concentration data.

ACKNOWLEDGMENTS

We wish to thank B. Noroozi-Homsey, I. Guy, and 1. Patterson for their contribution to this work.

'c. P. Foley and T. L. Tansley, Appl. Surf. Sci. 22,663 (1985). 2J, 1. Pankove, J. E, Berkeyheiser, and E. A. Miller, J. App!. Phys. 45, 1280 ( 1974),

'T. L. Tansley and C. P. Foley, in Semi-insulating III- V Compounds, edited by D. C. Look and J. S. Blakemore (Shiva, London, (984), p.497.

·T. L. Tansley and C. P. Foley, Electron. Lett. 20, 1066 (1984). 5T. L. Tansley and C. P. Foley, J. App!. Phys. 59, 3241 (1986). 6c. P. Foley and T. L. Tansley, Phys Rev. B 33, 1430 (1986). 'H. J. Hovel and J. J. Cuomo, App!. Phys. Lett. 20, 71 (1972). "J. W. Trainor and K. Rose, J. Electron. Mater. 3, 821 (1974). 9B. K. Ridley, Quantum Processes in Semiconductors (Clarendon, Oxford, 1982), Chap. 5.

'OW. Kaisser and H. Y. Fan, Phys. Rev. 98, 966 (1955). "G. Lucovsky, Solid State Commun. 3, 299 (1965).

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