Evidence for the existence of a complex isoelectronic center in Si : In

Journal of Luminescence 21(1980) 329—336 © North-Holland Publishing Company

EVIDENCE FOR THE EXISTENCE OF A COMPLEX ISOELECTRONIC CENTER IN Si : In David H. BROWN and S.R. SMITH University of Dayton Research Institute Dayton, Ohio, 45469, U.S.A. Received 5 March, 1980

The effect of 1 MeV electron irradiation and a 100°Cannealing stage on the intensity of the U 2 line at 1.1182 eV is presented. Evidence is provided from intensity vs. excitation power measurements that the U2 radiative recombination involves only a single exciton. Eight models are discussed, with a complex isoelectronic center being favored.

1. Introduction Attention has recently been focused on a number of photoluminescence lines seen in Si : In that have not yet been identified. These lines were originally labeled U1—U5 by Vouk and Lightowlers [1] who observed them in bulk doped but not in diffusion doped Si In samples. Lyon et al. [2] studied the photoluminescence of Si : Al, Si Ga, and Si In to assess the role of bound excitons (BE) and bound multiple exciton complexes (BMEC) in the radiative recombination of those materials. They observed the satellite line U1 4.0 meV below the In BE(J = 0) line which had a line shape similar to the BE(J = 2) line but had a pump-power dependence typical of BE emission rather than BMEC emission. Subsequently, Sauer et a!. [3] and Elliott et al. [4] provided evidence based on the excitation and temperature dependence of the lines as well as photoluminescence decay data that suggested that the peak labeled U1 was due to recombination of a bound exciton, with the hole left in a split-off ground state, 4 meV above the usual ground state. The line labeled U4 is the TO phonon replica of U1. The lines labeled U2 and U3 by Vouk and Lightowlers [1] and the line B observed by Lyon et a!. [5] remain to be identified. The line-shape and energy position of U5 establishes it as the TO replica of the B line. The line U2 also appears in the data of Dean et al. [6] but they did not discuss that feature of the spectrum. Lyon et al. [5] measured photoluminescence decay times at 15 K for U2 and U3 of 208 ±5 psec and 308 ±21 psec, respectively. More recently, Mitchard et al. [7] measured photoluminescence decay times at 20 K for U2, B, and U3 of 196 ±S ~.zsec,170 ±14 #sec, and 220 ±29 psec, respectively. Ziemelis et al. [8] recently published results of photoluminescece studies on double-doped Si(B, In) in which 329

330

D.H. Brown, S.R. Smith IA complex isoetectronic center in Si: In

a variety of new lines besides those seen before were observed. Interestingly, they observed relative intensities of U2, B, and U3 at 14 K and 20 K that were inverted with respect to the intensities observed by Mitchard et al., and they observed these three lines to decrease in intensity with increasing excitation power relative to the In BE(TA) line. Their preliminary studies of the photoluminescence decay times yielded r >10 ~isecfor the lines as well. Finally, Weber et al. [12] observed lines with similar decay characteristics in Si(P, C), Si(Sb, C), Si(B, C) and Si(Al, C); however, these lines do not match those seen in Si : In—they are higher in energy and the analogue to the Si : In B line appears only upon the application of external magnetic or strain fields. It should be noted from the outset that the problem of identification of the new lines is hampered by the fact that the centers responsible for the U2, B, and U3 luminescence are not electrically active and have not yet been observed in infrared absorption measurements on Si : In. In this paper we report the U2 luminescence intensity dependence on excitation power, electron irradiation, and a 100°Cannealing stage. The current evidence suggests that the U2 line originates from a single bound exciton at a complex isoelectronic center.

2. Experimental The samples used in this study were of three types: float zone, Czochralski, or solution grown. In the first two cases the samples were mechanically lapped then polished in a chemical etch to provide a damage-free surface. The solution grown sample was not lapped but merely etched in a solution of S HNO3 : 3 HF. Prior to mounting, each sample was cleaned with warm isopropyl alcohol in an ultrasonic cleaner, rinsed with distilled water, dried, and immediately mounted in a Janis variable temperature dewar. Samples used for the electron irradiation study were irradiated at room temperature with I -MeV electrons inirradiation, a Van de Graaff accelerator 2, After all samples were to fluences ranging from I X 1016 to 1017 e/cm held at liquid-nitrogen temperature until photoluminescence measurements were made. Cleaning and mounting procedures were the same as for the unirradiated samples. In general the solution grown material exhibited rather poor luminescence efficiency, even for the main indium lines. Results for Czochralski and float zoned samples were qualitatively similar except for U 2 intensity which was higher in the float zoned material. The measurements reported here are for float zoned Si : In samples from Dow Corning. Excitation was accomplished by a cw Ar ion laser at 514.5 nm with the power typically stabilized at 0.6 watt. ‘~‘hespot size was approximately 1 mm in diameter. The luminescence was collected from the back side of the sample, focused on the entrance slits of a one meter Jarrell-Ash spectrometer, and measured with conven-

D.H. Brown, S.R. Smith IA complex isoelectronic center in Si In

331

tional lock-in techniques using an RCA 7102 photomultiplier tube cooled to 196 K. The spectra were digitally recorded and corrected for water and air absorption and grating nonlinearities by computer-aided data analysis. Sample temperatures were measured by a GaAs thermometer placed on the sample block. Temperature stabilization via an automatic controller was better than 0.1 K.

3. Results and discussion Figure 1 shows a typical photoluminescence spectrum of Si : In at a temperature of 22 K and excitation power of 0.6 watt. Hall effect measurements determined the indium concentration to be 3.1 XlO’6 cm3. In this figure the lines due to the In BE(NP) and In BE(TO) are clearly visible. In addition the free exciton (FE) TO line is shown at 1098.4 meV and the lines U 2, B, U3, and U5 can be seen at 1118.2 meV, 1116.0 meV, 1108.6 meV, and 1057.8 meV, respectively.

1000.0

10200

l04Q0

1060.0

I

I

I

1060.0

1100.0

1120.0

1140.0

I

I

I

I

3 (FZ) OF Si: In PHOTOLUMINESCENC~STUDY Ni~.3.IXlO~ cm

1

1180.0 I

AE;I6meV

B..~

II~.o PHOTON ENERGY In,.vl

FE(T0,L0)

u 5

1000.0

112

1160.0

I02Q0

10401)

1060.0

InBE(T0,L0)

1080.0

J

-~

~

1100.0

112Q0

In BE(NP)

I

1140.0

1160.0

1180.0

PHOTON ENERGY (meV) Fig. 1. Photoluminescence of Si In at 22 K and excitation power of 0.6 watt. Inset: High resolution spectrum at T= 22 K,P = 0.6W showing the B line at 1116 mey.

D.H. Brown, S.R. Smith IA complex isoelectronic center in Si: In

332

In BE(NP)/U

2 INTENSITY RATIO VS. INCIDENT LASER POWER

0 78

-

3(FZ) PHOTOLUMINESCENCE STUDY OF Si: In ENERGY N1~~3.IXIO~cm OF LINE Uz 1.1182eV

6

-

ENERGY OF In BE(NP): I. 1414eV

-

2

SLO~~

I

0.1

0.2

0.3

0.4

0.5 0.6 0.70.80.91.0

LASER POWER AT 5145 A (WATTS) Fig. 2. Full logarithmic plot of ‘In BE(NP)/1u 2 versus excitation power for T = 5 K

(.) and

T=

10K (.).

As observed by Vouk and Lightowlers [1], the species responsible for the photoluminescence line U2 seemed to vary in concentration down the boule and across each wafer and precise rastering of the excitation beam across the sample was frequently necessary to obtain sufficient intensity. In an attempt to ascertain the nature of the exciton binding at the U2 site, a study was made of the ratio of the intensity of the In BE(NP) to the U2 emission as a function of excitation intensity. The results are summarized in fig. 2 which shows a plot of log [‘In/lu2] versus log F, for two temperatures, 5 K and 10K. Least squares fits to the data yield a slope of 1.1 and 1.2 for the 5 K and 10 K data, respectively. We conclude that in this temperature range only one exciton per site is involved in the U2 luminescence. Because of the fact that acceptor-acceptor molecular-type interactions have recently been implicated in the In : X level [9] and the fact that electron irradiation tends to increase In : X concentration [10], we decided to investigate the change in U2 intensity with electron beam irradiation and subsequent annealing. 2) are Preliminary frompeak that heights study ofinaeach high.fluence sample (~ 1017 e/cm presented inresults fig. 3. The trace have been normalized to the In BE(NP) line and a decrease in U~intensity upon irradiation is readily apparent.

D.H. Brown, S.R. Smith IA complex isoelectronic centerin Si: In

333

U~INTENSITY AFTER ELECTRON IRRADIATION AND ANNEALING SAMPLE 011-226 0107 (HIGHEST FLUENCE)

AFTER IOOC ANNEAL

mIiIIIIiII I0~O.01040.0 050.0 1060.0 1070.0 10800 1090.0 1100.0 1110.0

1120.0

1130.0 1140.0 1150.0 1160.0

PHOTON ENERGY(MEV)

Fig. 3. Photoluminescence spectra of Si : In in the as grown condition (bottom trace), after electron irradiation (middle trace) and following a 100°Cannealing stage (top trace). Spectra were taken at T = 22 K and P = 0.6 W.

Table 1 Hall data and photoluminescence integrated intensities for as-grown (AG), irradiated (IRR), and annealed samples (100°C) 3)

Condition

ND (cm

AG IRR 100°C

3.10 x 1012 1.26 X 1016

N

3)

NA (cm3)

1~(cm 3.21 2.53 b)

x x

1016 1016

1u

I 1~(NP)

6.79 6.79

x x

1012 1012 a)

247 204 167

2 18.5 4.48 0

Iin(TO) 56.5 42.9 37.4

a) Concentration and energy position of second acceptor, believed to be aluminum, held constant during three-level fit to Hall data. b) Hall samples were taken to 100°C during measurements, therefore they were not taken through another 100°Cannealing stage.

334

D.H. Brown, S.R. Smith IA complex isoelectronic center in Si: In

Table 2

Proposed models for U

2 center in Si: In

Model

Remarks

References

Sinple acceptor or donor

Unlikely — not seen in Hall Effect or IR absorption. Lifetime too long

[5,7]

Csj

Unlikely Si: In

[121

Cs~-Ins~ pair

Ziemelis’ fmdings tend to eliminate this possibility

[8]

Ins~-O~ pair

Improbable as U intensity greater in FZ material

This work

Isolated, simple Group IV on Si site

Unlikely, U2 seems to require presence of In; also Baldereschi’s theoretical results

[111

Donot-Acceptor pairs

Improbable, line positions do not scale with discrete lattice positions

[13]

In : X center

Unlikely — U2 intensity in opposite direction to N~upon irradiation

[10] and

Most likely due to decrease in intensity after irradiation (follows decrease with N10 so far). More low temp. annealing studies needed

[10] and this work

Complex center requiring presence of Inst

—

Weber’s data do not match

this work

After a 15 minute anneal at 100°C,the U2 line has disappeared completely. The reason for the slight shift in energy position of the three lines after irradiation is not understood at this point and is being investigated. Hall data and photoluminescence integrated intensities for the high fluence samples are presented in table 1. The decrease in electrically active indium following irradiation is to be expected as some In atoms are moved from substitutional to interstitial positions by the l-MeV electrons. The increase in donors is largely due to the production of indium interstitials (In1), interstitial-substitutional indium (In1—Ins~)pairs, and divacancies [10]. Possible models for the U2 center are presented in table 2. The evidence suggests that the center responsible for the U2 photoluminescence is complex in nature and that migration of the entities involved via damage-created vacancies may be occurring. The disappearance of U2 upon a 100°Canneal suggests

a loosely bound complex center with an extremely small energy of dissociation. This could explain the vast differences in U2 intensity seen by different workers. The extremely long lifetime and high radiative efficiency of the U2 line suggests that an isoelectronic center is responsible for this transition. Because there are no free particles attached to the center, there exists no mechanism for a nonradiative Auger process; therefore, the radiative efficiency should be quite high compared

D.H. Brown, S.R. Smith IA complex isoelectronic center in Si: In

335

to bound exciton recombination processes at acceptors in silicon, even deep acceptors such as indium. But the conditions for bound-states to exist for isoelectronic

centers are quite rigid as the size and electronegativity differences between the isoelectronic and host atom must be great enough to deepen the potential well relative to the kinetic energy of the particle to be captured. Theoretical calculations by Baldereschi [11] indicate that no elements from Group IV should produce hole traps in silicon. Inversion of the relative intensities of the U2, B, and U3 lines (U3 strongest, U2 weakest) as reported by Ziemelis is contrary to results presented here and elsewhere [1,7], and cannot be explained by differences in detector response. Our measurements on an RCA 7102 phototube with S-l response yield a decrease of only 5% from 1118 meV to 1109 meV, not enough to explain such a large difference in relative intensity. It seems rather more likely that U2, B, and U3 originate from differ-

ent defect centers involving indium and that either different growth parameters are responsible for the effect or the large concentration of boron renders the U3 center preferentially able to bind an exciton. Clearly the role of molecular associates and clusters in Si photoluminescence must be critically reexamined; to that end more extensive studies are proceeding on the annealing behavior of the U2, B, and U3 lines after electron and y irradiation.

4. Summary We have presented evidence that the U2 line in Si : In photoluminescence originates from a single bound exciton. Moreover, the behavior of U2 intensity after

irradiation and annealing suggests that the isoelectronic center is complex in nature with a low energy of dissociation.

Acknowledgments The authors would like to acknowledge valuable discussions with Dr. F. Szmulowicz and Mr. J.A. Detrio; they would also like to thank Mr. P. VonRichter for making Hall measurements, Mr. E. Soltis for performing the photoluminescence measurements, and J. Baker of Dow Corning for providing the float-zoned material used in this work. This research was sponsored by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio under Contract F33615-78-C-5064. References [1] M.A. Vouk and E.C. Lightowlers, J. Lum. 15(1977)357. [2] S.A. Lyon, D.L. Smith, and T.C. McGill, Phys. Rev. B17 (1978) 2620. [31R. Sauer, W. Schmid, and J. Weber, Sol. State Comm. 27 (1978) 705.

336

D.H. Brown, S.R. Smith IA complex isoelectronic center in Si: In

[4] K.R. Elliott, S.A. Lyon, D.L. Smith, and T.C. McGill, Phys. Lett. 70A (1979) 52. [5] S.A. Lyon, KR. Elliott, G. Mitchard, DL. Smith, T.C. McGill, S.P. Baukus, R. Baron, M.H. Young and O.J. Marsh, IRIS Meeting, June 1978. [6] P.J. Dean, J.R. Haynes, and W.F. Flood, Phys. Rev. 161 (1967) 711. [7] G.S. Mitchard, S.A. Lyon, K.R. Elliott, and T.C. McGffl, Sol. State Comm. 29 (1979) 425. [8] U.O. Ziemelis, R.R. Parsons, and M. Voos, Sol. State Comm. 32 (1979) 445. [9] M.C. Ohmer and J.E. Lang, Appl. Phys. Lett. 34 (1979) 750. [101 V. Swaminathan, J.E. Lang, P.M. Hemenger, and SR. Smith, Appl. Phys. Lett. 35 (1979) 184. [11] A. Baldereschi, J. Lum. 7 (1973) 79. [12] J. Weber, W. Schmid, and R. Sauer, J. Lum. 18/19 (1979) 93. [13] F.E. Williams, J. Phys. Chem. Solids 12(1960) 265.