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Photoluminescence spectra of Ga-doped and Al-doped 4H-SiC
1. Phys. Chem. Solids. 1977, Vol. 38, pp. 693-699.. PergamonPress. Printed in Great Britain
PHOTOLUMINESCENCE SPECTRA OF Ga-DOPED AND Al-DOPED 4H-SiC A. SUZUKI, H. MATSUNAMIand T. TANAKA Department of Electronics, Faculty of Engineering, Kyoto University, Kyoto 606, Japan (Received 28 June 1976; accepted in revised four 15 October 1976) Abstract-Blue and violet photoluminescences of Ga-doped and Al-doped 4H-SiC single crystals grown from a Si melt have been studied at 2 - 200K. Luminescence spectra under continuous excitation and their dependences on temperature and excitation intensity as well as temperature dependence of the luminescence intensity are measured. Time-resolved spectra and decay curves after pulsed excitation also are observed at various temperatures. The luminescences at 2 K are found to be due to pair recombination between the N donor introduced unintentionally and the Ga or Al acceptor. The spectra of the two samples resemble each other in shape, and each consists of a zero-phonon peak and its phonon replicas. At higher temperatures, another emission appears due to the recombination of free electrons with bound holes at the acceptors in place of the pair emission. From the energies of the zero-phonon peaks of these two kinds of emissions, the ionization energies of the Ga and the Al acceptors are determined to be 249+ 3 meV + E. and 1682 3 meV + E., respectively, where E. is the exciton binding energy of 4H-Sic, and that of the N donor is estimated to bi 55 27 meV using an appropriate approximation. 1. INTRoDum~
such as Be, SC, B, Al, Ga and In have been known to act as radiative centers of visible luminescence in Sic [l-3]. Properties of Al acceptors as a luminescent center have been investigated in detail for 3C-, 6H- and 4H-SiC doped with N and Al using crystals prepared by vapor-phase growth[4-91. Recently, the luminescence due to B acceptors has been studied for 3C-Sic grown from a silicon melt saturated with carbon[lO, 111. Pair recombination between N donors and Al or B acceptors has accounted for these luminescences at low temperatures. At higher temperatures, the recombination of free electrons with bound holes was found to be dominant. The ionization energies of the N donor, and the Al and the B acceptors have been determined from the luminescence spectra, but they showed somewhat those by Hall-effect different values from measurements[9,12,13]. There have been very few reports about the properties of Be, SC, Ga and In acceptors, and their energy levels in the forbidden gap remain unknown. The authors reported on photoluminescence of 4H-SiC grown from a Si melt[lC16]. Blue and violet luminescences were observed from Ga-doped and Aldoped crystals, respectively. The purpose of the present paper is to find recombination processes of these luminescences and determine the ionization energies of the Ga and the Al acceptors measuring luminescence spectra under continuous excitation and time-resolved spectra after pulsed excitation. The ionization energy of the N donor which seems to participate in the recombination process will also be given. Acceptors
2. FDEmMWrALpIIocEurnms The crystals used in this investigation were grown from a Si melt in a graphite crucible. Usually 3C-SiC grows by this methodll’l], but at higher growth temperature (1750- 18WC) and with less temperature
gradient of the Si melt, 4H-Sic single crystals were obtained[l5]. The typical crystal size was 0.8 x 0.8 x 0.2 mm’ by 8 hr growth. Ga-doped and Al-doped samples were prepared from the Si melt with 1 at.% Ga metal and 0.01-0.05 at.% Al metal in excess. All the samples showed n-type conduction with unknown donors of the order of 10’9cm-3. In the photoluminescence measurements, a 250 W high-pressure mercury lamp was used as a light source for continuous excitation. As an excitation source for time-resolved spectroscopy and luminescence decay measurements, a xenon flash lamp was used (pulse width: about 1 psec, repetition rate: 1 or 10 pulseslsec). The UV light of 365 nm was suitably filtered and focused on the sample. Luminescence from the sample was detected by a grating spectrometer and a photomultiplier. Time-resolved spectra and decay curves were obtained by means of a boxcar integrator.
3.1 Luminescence spectra under continuous excitation Under continuous excitation at low temperatures, blue and violet photoluminescences were observed from the Ga-doped and the Al-doped samples, respectively. Luminescence spectra at 2 K are shown in Fig. 1. The two spectra were very similar in shape, but that of the Ga-doped sample was observed on the lower energy side. Both spectra had several peaks. The highest-energy peak B, (at 2.990eV for the Ga-doped sample and at 3.067eV for the Al-doped sample) was relatively sharp. The photon energies of the other peaks are given in Table 1 (the energy difference between the B0peak and the other peak is given in parentheses). Each peak energy of the Ga-doped sample was smaller than the corresponding one of the Al-doped sample by an almost constant value: 77 2 5 meV. The similarity of the two spectra may suggest similar recombination processes for both luminescences. From the experimental results described hereafter, the lu693
A. SUZUXF et ai
02
B,(Ga)B,(Ai) 2.990
3,067
‘J: EXClTATiON FNTENSITY -u= I --LI= O.f ----- lJ =&Of 2K
?l_ii_ii PHOTON ENERGY
2.7
2.8 PtlOTON
2.9
3.0
ENERGY
3.1 (eV)
3.00 (eV)
Fig. 2. Energy shift of the B, peak of the Ga-doped sample with decreasingexcitationintensity at 2 K.
Fig. 1. Luminescence spectra of Gadoped and Al-doped 4H-SE under continuous excitation at ZK. (The & peak is a zerophonon peak.) Table 1. Emission peaks of the fum~ne~ence spectra at 2 K Peak energy Peak B. BI & B,
4H-SC :Ga 2.9!90ev
2.924(66meV) 2.873(117) 2.824(164) 2.762(228) 2.718(272)
4H-Sic : Al 3.667ev 2.996(71 meV) 2.954(113) 2.904(163) 2.843(224) 2.798(269)
LA LO LA+LO 2LO LA + 2L0
Numbers in parentheses are energy differences between the B, peak and the other peaks.
minescences are found to be due to pair recom~na~on between an fawn donor and the Ga or the Al acceptor. The I& peak will be explained as a zero-phonon peak and the other peaks as its phonon replicis. The energy difference of 77 + 5 meV between the two spectra is due to the different energy levels of the two acceptors. With decreasing excitation intensity down to l/l00 at 2K, both B, peaks shifted to lower energies by a few meV as shown in Fig. 2 for the Ga-doped sample. This is one of the evidences of pair recombination[l81. At higher temperatures a new peak A, appeared on the higher energy side of the B. peak as shown in Figs. 3 and 4. In the spectrum due to the Ga acceptor (Fig. 31, the new As peak appeared at 3.025 eV at 15OK, though thermal broa~n~ of each peak ~curred, In the WCtrum due to the Al acceptor (Fig. 41, the 1& peak was observed above 118K and located at 3.101eV at 129K. These A0 peaks became dominant in place of the B0 peaks with increasing temperature. The width of the A0 peak was smaller than that of the & peak. The energy differences between the A0 and the Bp peaks of both samples were 30 - 35 meV. Figure 5 shows the spectrai change of the Al-doped 4H-Sic at 118K when the excitation intensity was decreased to If tO0.The B. peak shifted to lower energies
2.96
300 PHOTON
3.04 ENERGY
3.08 feV)
Fii. 3. Spectral change of the A, and the & peaks of the Oaaoped sample with increasing temperature.
as at 2K, while the energy of the A0 peak did not change. The two peaks due to the Ga acceptor also behaved in the same manner. The A0 peaks, therefore, are considered to be due to any other recombination process than pair recombination. The tem~rature dependence of the luminescence intensity is shown in Fig. 6 as a function of reciproc~ temperature. The intensity of the G&doped sample decreased ex~~n~~y with an activation energy of 240 + 20 meV above 175 K, while that of the Al-doped one with 160+ 20 meV above 150K. 3.2 Time-resolved spectra and decay cwves Fiure 7 shows time-resolved spectra of the & peak of the Ga-doped sample observed at different delay times (t) after pulsed excitation. They were measured at 2,77 and 135K. At 2 K, the & peak was very broad at 10 psec delay. With increasing defay time, it shifted to lower energies and the peak width decreased gradually (Fig.
695
Photoluminescence spectra of Ga-doped and AI-doped 4H-Sic TEMPERATURE 400
300
’
1’1
( K
1 100
200
I
I
I
107K
114K
118K
123K
129K
4H:Ga( Eact
2
q
240*20meV
I
I
I
I
I
I
I
3
4
5
6
7
8
9
T
( K-‘1
1000
\ _
I 3.06
I
3.02
I
3.1 0
PHOTON
I
I
I
ENERGY
3.14 (eV)
Fig. 4. Spectral change of the A, and the B, peaks of the Al-doped sample with increasing temperature. 4H:AI Bo
A0
U:EXCITATION
-u= ---
\ \ \ \I ’
INTENSITY
1 U=O.Ol
118K
I
I
I
I
3.06
3.08
3.10
3.12
PHOTON
ENERGY
10
Fig. 6. Temperature dependence of the emission intensities of the Ga-doped and the Al-doped samples.
4H:AI
I
I
(eV)
Fig. 5. Spectral change of the A, and the B, peaks of the Al-doped sample with excitation intensity at 118K.
7a). This feature is another evidence that the B0 peak is due to pair recombination[l8], The origin of a peak appearing at 2.% eV at 7 msec remains unknown. At 77 K, a new emission was observed on the higher energy side of the B0 peak at delay times longer than 500 psec (Fig. 7b). With increasing delay time it became a distinct peak (3.021eV at 7 msec). At 135K, this new peak appeared as a hump even at SO~sec (Fig. 7~). It HIS located at 3.024eV at 7msec, and its intensity
exceeded that of the & peak. From the photon energy, this new peak is considered to be the same A,, peak that was observed at 150K under continuous excitation (in Fig. 3). The feature that each A, peak had much the same energy at any delay time also shows that it is not due to pair recombination. In the spectra at 135K (Fig. 7c), anoth& new peak A, appeared at 2.%1 eV at 7msec. Since the energy difference of 63 meV between this peak and the A,, peak almost agrees with 66meV between the B, and the B. peaks (Table l), it may be the first phonon replica of the A, peak. Time-resolved spectra of the Al-doped sample at 2 and 77 K are shown in Fig. 8. The B, peak showed the features of pair recombination. The same A, peak observed under continuous excitation was found at longer delay times at 77 K as in Fig. 8(b) (3.106eV at 4 msec). It should be noted that at 77 K, the A, peaks of both Gadoped and Al-doped samples were not found in the spectra under continuous excitation, but they appeared in the time-resolved spectra. Decay curves of the total luminescence intensity are shown in Figs. 9 and IO for the Gadoped and the Al-doped samples, respectively. They are normalized at the delay time of 10psec. All the decay curves at 2, 77 and 135K were non-exponential, At longer delays than 5 - 10 msec they varied as t-“. The values of n were 0.90 and 0.84 at 2K for the Ga-doped and the Al-doped samples, respectively. They increased with increasing temperature, and the luminescence decayed more rapidly. 4. DISCUSSION
4.1 Donor-acceptor pair recombination Both B. peaks of the Ga-doped and the Al-doped samples showed the features of pair recombination under
6%
A.
SUZUKI et al. #ELAV TIME lOpsec<
1 50 psec
150
psec
500
psec
50 psec
150 psec 500
psec
1.5ms.w
1.5msec
45msec 4.5 msec 7msec
7 msec
% 4H:Ga lo4
I 2.92
,;:,a,
2K I
I
1 2.96
PHOTON
I 3.00
ENERGY
I
I 3.04
2.92
.135,K
,
,
ENERGY
, \I
3.04
3.00
2.96 PHOTON
(eV)
,
(eV)
7(c)
X4
Fig. 7. Time-resolvedspectra of the B,, peak of the Gadoped sample at (a) 2 K, (b) 77 K and (c) 135 K.
4H
: Ga
I 2.92
77H I
PHOTON
I
I
I
I
ENERGY
I 3.04
3.00
2.96
(eV)
709 continuous excitation and in the time-resolved spectra. The other peaks (B, -B,) on the lower energy side behaved in the same manner as the E0 peak. The nonexponential decay curves at 2 K also give an evidence of pair recombination [ 181.
Luminescence from Sic crystals containing N donors and Al acceptors has been investigated by several researchers[4-91. They have shown that at low temperatures the luminescence was due to pair recombination and the spectrum consisted of a zero-phonon peak and its phonon replicas. The photon energy of the II0 peak of our Al-doped sample almost agreed with that of a zero-phonon peak of 4H-Sic reported by Gorban’ ef al. [8] and Hagen et aI.[9]. Unknown donors of the order of 10’gcm-3 in our samples are probably nitrogen introduced unintentionally from the ambient during the growth[l9]. Consequently, the B. peak of the Al-doped sample is a zero-phonon peak of pair recombination between the N donors and the Al acceptors, and the other B, - B5 peaks are its phonon replicas. The I&,peak of the Ga-doped sample can be regarded as a zero-phonon peak of pair recombination between the N donors and the Ga acceptors. As compared with the Aldoped sample, the smaller photon energy can be explained by the larger ionization energy of the Ga acceptor. Hagen et al. reported the presence of another different zero-phonon peak at about 50 meV lower energy than the aforementioned one in the spectrum of Al-doped 4HSiG[9]. But Gorban’ et al. did not find such a peak[8]. Hagen et al. explained the two zero-phonon peaks as being due to two kinds of N donors: the “hexagonal” and the “cubic” lattice sites of 4H-Sic. Our Al-doped or Gadoped 4H-Sic showed much the same spectral shape as Al-doped or Ga-doped 3C-Sic prepared in the same growth run [20]. Since 3C-Sic has only one kind of lattice sites, it is
PhotoIu~ne~n~ DELAY
TIME
@O
10‘
I
,(Jyal 105 4H:Al
11
I
I
I
3.04 PHOTON
104
I
I
I
3.08 ENERGY
I
3.12
I
I\
\
-i6
‘1’
, 1 , / , j ‘, j 1i3 DELAY
2K
3.00
697
spectra of Gadoped and Mdoped 4H-Sic
to* TIME
10’
1
t (seconds)
Fig. 9. Normalized decay curves of the luminescence of the Gadoped sample at 2,77 and 135K.
feV)
W ’ DELAY
TIME
80
DELAY TIME t (seconds) Fig. 10. Normalizeddecay curves of the luminescenceof the Al-doped sampleat 2 and 77 K. 4H:AI
ti
??K I
I
3.00
I
3.04 PHOTON
I
I
3.08 ENERGY
3.12 (eV)
8(b) Fig.
8.
Tie-resolved spectra if the BO peak of the Al-doped sample at (a) 2 K and (b) 77 K.
tAs described in Ref. [211,these notations are accurate only for 3C-Sic and somewhat inaccurate for 4H-Sic, but they are used for convenience.
improbable that some of the B, - B5peaksof our Al-doped or Ga-dopled 4H-SiC may be another zero-phonon peak and its phonon replicas. Since the spectra of the Ga-doped and the Al-doped samples resembled each other, the same kind of phonons may contribute to the corresponfling phonon replicas of the two spectra: From the energy difference between the B. peak and eaqh phonon replica (given in Table l), and phonon energies of 4H-SiC reported by other authors[211, the 8, and the Bt peaks are found to be LA? and LO? phonon replicas, respectively. The B,, B, and B5 peaks may be explained as LA + LO, 2L0, and LA + 2L0 phonon replicas, respectively.
698
A. SUZUKI @t d.
4.2 Recombination between free electrons and bound holes Emissions due to the recombination of free electrons with bound holes at acceptors in Sic have been reported by several authors [4,7,8,10,11]. These emissions appeared on the 35 - 40 meV higher energy side than the donor-acceptor pair emissions. The A, peaks of our samples are thought to be due to such recombination by the following reasons: (1) The A, peaks appeared in place of the I&, peaks at higher temperatures. (2) The energy difference between the A0 and the & peaks was 30 - 35 meV. (3) The A. peaks did not show the features of pair recombination. The thermal quenching of the luminescence shown in Fig. 6 is not caused by thermal ionization of electrons at the donors, but by that of holes at the acceptors. The reasons are that (1) even at the the temperature where the A0 peak began to be dominant, the quenching did not occur yet, and (2) the quenching of the Ga-doped sample began at higher temperature and showed larger activation energy than that of the Al-doped sample. The proba~ity of thermal ioni~tion of bound holes at the acceptors may be expressed as s exp (- E&0), where s is a constant and E, is the ionization energy of the acceptor. Then, the quenching activation energy may be regarded as E,, and the quenching begins at higher temperature for larger E, (221.Therefore, the activation energies of 2402 20 meV for the Ga-doped sample and 160+ 20 meV for the Al-doped sample may be around the ionization energies of the respective acceptors. 4.3 Time-resolvedspectra and decay curues The result that the A* peaks appeared at longer delay times in the time-resolved spectra at 77 and 135K is discussed here. Since the numbers of electrons and holes excited by the xenon flash lamp were not so large, all of them were probably bound by the donors and the acceptors. At 2 K, the bound electrons and holes are not ionized thermally before they recombine to generate pair emissions. Since the electrons and holes with larger separations recombine more slowly[l8], long decay curves characteristic of pair emissions were observed at 2 K as shown in Figs. 9 and 10. At higher temperatures, bound electrons and holes with smaller pair separations recombine in shorter delay times, but electrons largely separated from the holes will be ionized thermally before they recombine. (The ionization energy of the donor is much smaller than that of the acceptor as described in the following section.) These free electrons recombine with bound holes and produce the A,, peak at longer delay times in the timeresolved spectra even at such low temperatures as 77 K. With increasing temperature, the electrons begin to be ionized in shorter delay time as in Figs. 7 and 8. The normalized decay curve at hibrher temperature falls more rapidly at long delay times as in Figs. 9 and 10, because electrons largely separated from the holes are ionized much faster. At 135K, only the A0 peak and its phonon replicas may be observed at longer delays than 7msec from the spectra in Fig. 7(c), but the Iuminescence intensity did not decay exponentially during these delay times. This suggests that the thermal ionization and
the recombination processes of the electrons are not simple.
4.4 Zonizationenergies of the N donor and the Ga, Al acceptors From the energy hva of the A, peak, the ionization energy of the acceptor can be obtained by the following relation [23]: E,=E,+E;+kT-hvA,
(1)
where E,, E, and kT are the exciton energy gap, the exciton binding energy, and the thermal energy of a free electron, respectively. The sum of E, and E, is the energy gap Ew Using the values of hu., obtained under continuous excitation and in the time-resolved ‘spectra (E, is given by Ref. [24]), eqn (1) gives E, = 168ir3meVfE,
(2)
for the Al acceptor, and E,,=249&3meV+E,
(3)
for the Ga acceptor. The ionization energy of the Al acceptor given by eqn (2) almost agrees with 180meV + E, by Gorban’ et al.[8] and 150meV + E, by Hagen et a1.[9]. The ionization energy of the Ga acceptor has been determined by the present study for the first time. The exciton binding energy E, of 4H-Sic is unknown. #en the value of I$ is assumed to be of the same order as 13.5meV reported for 3C-Sic [25], both ionization energies of the Al and the Ga acceptors obtained by eqns (2) and (3) are found to be almost in agreement with the activation energies of the thermal quenching of the luminescence in Fig. 6 (1602 20 meV and 240~ 20 meV, respectively), as has been expected in Section 4.2. The photon energy hv(R) of the pair emission with the separition R is expressed by hv(R) = E, t E, -(Ed t E,) t e’/&,
(4)
where Ed is the ionization energy of the donor, and e and 6 are the electron charge and the static dielectric const~t of the material[l8]. Then, from the photon energy hu, due to the pair with extremely large separation, one can obtain Ed t E, by eqn (4): EdtE,,=E;,tE,-hv,.
(5)
The energy hv, can be estimated from the peak energy hv, and the half-width SE of the B, peak (the zerophonon peak) under continuous excitation as described in Ref.[26]. Since the excitation intensity was not high enough, our experimental condition corresponds to the unsaturated limit in Ref. [2&l.Thus, hv, = hv, - (1~0.76)~~.
(6)
Using the values of hv, and SE given in Table 2 and
P~tolu~eenee
spectraof Ga-doped and ~-do~
T&de 2. & + E, bv, and &, calculated from the peak energy hv. and the half-width SE of the B. peak at 2 K
Sample 4H”SiCIt% 4HSiC : Al
hva (eV)
SE (eV)
0
(“a
:::
2.990 3,067
0.019 0.022
2.965 3.038
58 5a
300+ E, 227+ Ex
4H-Sii
699
determined for the N donor, the Ga acceptor, and the Al acceptor, respectively. The ionization energy of the Ga acceptor was determined by the present study for the first time.
Ed+& =227mev+E3, for the Al-doped SampIe, and E,+E,
=3OOmeV+E,
(8)
for the Gadoped sample. Using the values previously determined for & @pus 2 and 3), one obtains &=59‘9-+3meV for the Al-doped sample, and &=51-+3meV
ilO
for the Ca-doped sample. Since the donors in both samples are probably the same nitrogen, the ionization energy of the N donor is estimated to be 55 &7 meV from eqns (9) and (10). This value may be compared with 65 meV from the luminescence data by Gorban’ et aL(8] and 33 meY by Hall-eiTect measurementsIl3]. The pair separation &. makin the most ~on~~utiou to the B. peak is derived from eqns (4) and (5) by &v(R) = hFg: R$ = e*~~~~~~ - irv,).
f. Addmnimm A, J, ~~t~~kern. SOGIB, Z34@6?$. 2. Blank J. M., In Si&xm Cwkide-1973 (Edited by R. C. Marsh& J. W. Faust, Jr. and C. E. Ryan), p. 550. University of South Carolina Press, Columbia (1974). 3. Kholuyanov G. F, and Vodakov Yu. A., In Silicon CarNde1973(Edited by R, C. Marshall, 1. W. Faust, Jr. and C. E. Ryan), p, 574. University of South Carolina Press, Columbia (1974). 4. Zanmarchi G., J. P&s. C&n. Soiids 29, 1727{l%Q. 5. Choykg:W. J. and Patrick L. Pkys. Reu. BZ, 4959 flW0). 6. Lann N. N., Nedzvetskii D. S.. Prokofeva N. K. and Reifman-M. &, bpf.specfrosc 29;388[197@. 7. Long N. N., N&vet&ii I). S., Prokofeva N. K. end Reifman-M. B,, opt.speefrase. 3% 165(1971). 8. &&an’ f. 9.. ~~nov V. A. and Btimov V. M., Soo. ~ky~.-~l~ S&e 84,2(X0 fl973). 9. Hagen S. H., van Kemenade A. W. C., van der Does de Rye J. A. W., 1. ~~rnine~~ence8, 18 (1973). 10. Yamada S. and Kuwabara H., In Silicon C&ride-1973 (Edited by R, C. Marshall, J. W. Faust, Jr. and C. E. Ryan), p. 305. University of South Carolina Press, Columbia (1974). 11. Kuwabara H. and Yamada S., Phys. Stat. Soi. 30(a), 739 (1975). 12. Lomskina G. A., Vodakov Yu. A., Mokhov E. N., Odin&V. G,, aad K~luya~v G. F., Sov. Pkys.-Solid State 12, 2356 f1971). 13. LMi&ina G. I#,, in Sibm Cadge-1~3 IEdited by R. c. Marshail,5. W. Faust, Jr. and C. B. Ryan), p. 520. ~~iv~s~y of South Carolina Press. cohlntbii (19741.
00
Using 10 as c[27& 50 and %A are obtained for the Al-doped and the Ga-doped samples, respectively. 5. 8uMMABY In order to find the recombination processes of the
blue and the violet photoluminescences from the Gadoped and the Al-doped 4&SiC, luminescence spectra under con~no~s excrtation, ~reso~v~ spectra and decay curves were measured at va&us temperatures. As a result, the huninescences at 2 K were found to be pair emissions between the N donor and the Ga or Al acceptor. The spectra of the two sampies ~se~~d each other in shape, and consisted of a zero-shouts peak and its phouon replicas. Two kinds of phonons and their combinations contributed to the replicas. At bigher temperatnres, another emission due to the recombination of free electrons with bound holes at the Go or Al acceptors was observed. From the energies of the zero-phonon peaks of these two kinds of emissions, the ionization energies of 554 7u~V, 249+3meV+& and 168c3meV+& were
15. Matsunami H., Suzuki A. and Tanaka T., In Silicon Curkide1973(Edited by IL C, Marshall, J. W. Faust, Jr. and C. E. Ryan), p. 618. University of South Carolina Press, Columbia (1974). 16. Suzuki A., Matsunami H. and Tanaka T., Japan. S, A&. Pkys. 14, 891 (1975)” 17. Nelson y. E., Maiden F. A. and Rosengreen A., L Appl. Pkyt. 37,333 (1966), 18. Thomas D. O., HopReId J. J. and Augustyniak W. M., P&V. Be& 14& A2%?(I!%% 19. SuzuukiA., Ikeda hi., Nagao N., Ma~onam~ II. and Tanaka T., J AppL P&y**47,4546 (1976). 20. Suzuki A,, ~a~un~i H. and Tanaka T., .I. ~ec~~~. Sot. 124% 241 (1977). 21. Patrick L,, Choyke W, J. and Han&on D. R., Pkys. Rev, t3?, A151.5(1965). 22. Tajima M. and Aoki Id., Japan.i.Appt. Pkys. I&819 (1974). 23. Colbow K., Pkvs. Rev. 141.742 (1966). 24. Choyke W. J., Patrick L. aad H~mil& D. R., In Prac. 71 Conf. on Physics of Semiconductors Paris, p. 7.51.Dnnod, Paris (1964) 25. Nedzvetskli D. S., Novikov B. V., Prokof’eva N. K. and R&man M. B,, Sov. Pkw-Semiconductors 2.914 11969). 26. Morgan T. N., Plaskett f S. and Pettit G. L?.,ihys. keu. i80, 845 ff%9).
PHOTOLUMINESCENCE SPECTRA OF Ga-DOPED AND Al-DOPED 4H-SiC A. SUZUKI, H. MATSUNAMIand T. TANAKA Department of Electronics, Faculty of Engineering, Kyoto University, Kyoto 606, Japan (Received 28 June 1976; accepted in revised four 15 October 1976) Abstract-Blue and violet photoluminescences of Ga-doped and Al-doped 4H-SiC single crystals grown from a Si melt have been studied at 2 - 200K. Luminescence spectra under continuous excitation and their dependences on temperature and excitation intensity as well as temperature dependence of the luminescence intensity are measured. Time-resolved spectra and decay curves after pulsed excitation also are observed at various temperatures. The luminescences at 2 K are found to be due to pair recombination between the N donor introduced unintentionally and the Ga or Al acceptor. The spectra of the two samples resemble each other in shape, and each consists of a zero-phonon peak and its phonon replicas. At higher temperatures, another emission appears due to the recombination of free electrons with bound holes at the acceptors in place of the pair emission. From the energies of the zero-phonon peaks of these two kinds of emissions, the ionization energies of the Ga and the Al acceptors are determined to be 249+ 3 meV + E. and 1682 3 meV + E., respectively, where E. is the exciton binding energy of 4H-Sic, and that of the N donor is estimated to bi 55 27 meV using an appropriate approximation. 1. INTRoDum~
such as Be, SC, B, Al, Ga and In have been known to act as radiative centers of visible luminescence in Sic [l-3]. Properties of Al acceptors as a luminescent center have been investigated in detail for 3C-, 6H- and 4H-SiC doped with N and Al using crystals prepared by vapor-phase growth[4-91. Recently, the luminescence due to B acceptors has been studied for 3C-Sic grown from a silicon melt saturated with carbon[lO, 111. Pair recombination between N donors and Al or B acceptors has accounted for these luminescences at low temperatures. At higher temperatures, the recombination of free electrons with bound holes was found to be dominant. The ionization energies of the N donor, and the Al and the B acceptors have been determined from the luminescence spectra, but they showed somewhat those by Hall-effect different values from measurements[9,12,13]. There have been very few reports about the properties of Be, SC, Ga and In acceptors, and their energy levels in the forbidden gap remain unknown. The authors reported on photoluminescence of 4H-SiC grown from a Si melt[lC16]. Blue and violet luminescences were observed from Ga-doped and Aldoped crystals, respectively. The purpose of the present paper is to find recombination processes of these luminescences and determine the ionization energies of the Ga and the Al acceptors measuring luminescence spectra under continuous excitation and time-resolved spectra after pulsed excitation. The ionization energy of the N donor which seems to participate in the recombination process will also be given. Acceptors
2. FDEmMWrALpIIocEurnms The crystals used in this investigation were grown from a Si melt in a graphite crucible. Usually 3C-SiC grows by this methodll’l], but at higher growth temperature (1750- 18WC) and with less temperature
gradient of the Si melt, 4H-Sic single crystals were obtained[l5]. The typical crystal size was 0.8 x 0.8 x 0.2 mm’ by 8 hr growth. Ga-doped and Al-doped samples were prepared from the Si melt with 1 at.% Ga metal and 0.01-0.05 at.% Al metal in excess. All the samples showed n-type conduction with unknown donors of the order of 10’9cm-3. In the photoluminescence measurements, a 250 W high-pressure mercury lamp was used as a light source for continuous excitation. As an excitation source for time-resolved spectroscopy and luminescence decay measurements, a xenon flash lamp was used (pulse width: about 1 psec, repetition rate: 1 or 10 pulseslsec). The UV light of 365 nm was suitably filtered and focused on the sample. Luminescence from the sample was detected by a grating spectrometer and a photomultiplier. Time-resolved spectra and decay curves were obtained by means of a boxcar integrator.
3.1 Luminescence spectra under continuous excitation Under continuous excitation at low temperatures, blue and violet photoluminescences were observed from the Ga-doped and the Al-doped samples, respectively. Luminescence spectra at 2 K are shown in Fig. 1. The two spectra were very similar in shape, but that of the Ga-doped sample was observed on the lower energy side. Both spectra had several peaks. The highest-energy peak B, (at 2.990eV for the Ga-doped sample and at 3.067eV for the Al-doped sample) was relatively sharp. The photon energies of the other peaks are given in Table 1 (the energy difference between the B0peak and the other peak is given in parentheses). Each peak energy of the Ga-doped sample was smaller than the corresponding one of the Al-doped sample by an almost constant value: 77 2 5 meV. The similarity of the two spectra may suggest similar recombination processes for both luminescences. From the experimental results described hereafter, the lu693
A. SUZUXF et ai
02
B,(Ga)B,(Ai) 2.990
3,067
‘J: EXClTATiON FNTENSITY -u= I --LI= O.f ----- lJ =&Of 2K
?l_ii_ii PHOTON ENERGY
2.7
2.8 PtlOTON
2.9
3.0
ENERGY
3.1 (eV)
3.00 (eV)
Fig. 2. Energy shift of the B, peak of the Ga-doped sample with decreasingexcitationintensity at 2 K.
Fig. 1. Luminescence spectra of Gadoped and Al-doped 4H-SE under continuous excitation at ZK. (The & peak is a zerophonon peak.) Table 1. Emission peaks of the fum~ne~ence spectra at 2 K Peak energy Peak B. BI & B,
4H-SC :Ga 2.9!90ev
2.924(66meV) 2.873(117) 2.824(164) 2.762(228) 2.718(272)
4H-Sic : Al 3.667ev 2.996(71 meV) 2.954(113) 2.904(163) 2.843(224) 2.798(269)
LA LO LA+LO 2LO LA + 2L0
Numbers in parentheses are energy differences between the B, peak and the other peaks.
minescences are found to be due to pair recom~na~on between an fawn donor and the Ga or the Al acceptor. The I& peak will be explained as a zero-phonon peak and the other peaks as its phonon replicis. The energy difference of 77 + 5 meV between the two spectra is due to the different energy levels of the two acceptors. With decreasing excitation intensity down to l/l00 at 2K, both B, peaks shifted to lower energies by a few meV as shown in Fig. 2 for the Ga-doped sample. This is one of the evidences of pair recombination[l81. At higher temperatures a new peak A, appeared on the higher energy side of the B. peak as shown in Figs. 3 and 4. In the spectrum due to the Ga acceptor (Fig. 31, the new As peak appeared at 3.025 eV at 15OK, though thermal broa~n~ of each peak ~curred, In the WCtrum due to the Al acceptor (Fig. 41, the 1& peak was observed above 118K and located at 3.101eV at 129K. These A0 peaks became dominant in place of the B0 peaks with increasing temperature. The width of the A0 peak was smaller than that of the & peak. The energy differences between the A0 and the Bp peaks of both samples were 30 - 35 meV. Figure 5 shows the spectrai change of the Al-doped 4H-Sic at 118K when the excitation intensity was decreased to If tO0.The B. peak shifted to lower energies
2.96
300 PHOTON
3.04 ENERGY
3.08 feV)
Fii. 3. Spectral change of the A, and the & peaks of the Oaaoped sample with increasing temperature.
as at 2K, while the energy of the A0 peak did not change. The two peaks due to the Ga acceptor also behaved in the same manner. The A0 peaks, therefore, are considered to be due to any other recombination process than pair recombination. The tem~rature dependence of the luminescence intensity is shown in Fig. 6 as a function of reciproc~ temperature. The intensity of the G&doped sample decreased ex~~n~~y with an activation energy of 240 + 20 meV above 175 K, while that of the Al-doped one with 160+ 20 meV above 150K. 3.2 Time-resolved spectra and decay cwves Fiure 7 shows time-resolved spectra of the & peak of the Ga-doped sample observed at different delay times (t) after pulsed excitation. They were measured at 2,77 and 135K. At 2 K, the & peak was very broad at 10 psec delay. With increasing defay time, it shifted to lower energies and the peak width decreased gradually (Fig.
695
Photoluminescence spectra of Ga-doped and AI-doped 4H-Sic TEMPERATURE 400
300
’
1’1
( K
1 100
200
I
I
I
107K
114K
118K
123K
129K
4H:Ga( Eact
2
q
240*20meV
I
I
I
I
I
I
I
3
4
5
6
7
8
9
T
( K-‘1
1000
\ _
I 3.06
I
3.02
I
3.1 0
PHOTON
I
I
I
ENERGY
3.14 (eV)
Fig. 4. Spectral change of the A, and the B, peaks of the Al-doped sample with increasing temperature. 4H:AI Bo
A0
U:EXCITATION
-u= ---
\ \ \ \I ’
INTENSITY
1 U=O.Ol
118K
I
I
I
I
3.06
3.08
3.10
3.12
PHOTON
ENERGY
10
Fig. 6. Temperature dependence of the emission intensities of the Ga-doped and the Al-doped samples.
4H:AI
I
I
(eV)
Fig. 5. Spectral change of the A, and the B, peaks of the Al-doped sample with excitation intensity at 118K.
7a). This feature is another evidence that the B0 peak is due to pair recombination[l8], The origin of a peak appearing at 2.% eV at 7 msec remains unknown. At 77 K, a new emission was observed on the higher energy side of the B0 peak at delay times longer than 500 psec (Fig. 7b). With increasing delay time it became a distinct peak (3.021eV at 7 msec). At 135K, this new peak appeared as a hump even at SO~sec (Fig. 7~). It HIS located at 3.024eV at 7msec, and its intensity
exceeded that of the & peak. From the photon energy, this new peak is considered to be the same A,, peak that was observed at 150K under continuous excitation (in Fig. 3). The feature that each A, peak had much the same energy at any delay time also shows that it is not due to pair recombination. In the spectra at 135K (Fig. 7c), anoth& new peak A, appeared at 2.%1 eV at 7msec. Since the energy difference of 63 meV between this peak and the A,, peak almost agrees with 66meV between the B, and the B. peaks (Table l), it may be the first phonon replica of the A, peak. Time-resolved spectra of the Al-doped sample at 2 and 77 K are shown in Fig. 8. The B, peak showed the features of pair recombination. The same A, peak observed under continuous excitation was found at longer delay times at 77 K as in Fig. 8(b) (3.106eV at 4 msec). It should be noted that at 77 K, the A, peaks of both Gadoped and Al-doped samples were not found in the spectra under continuous excitation, but they appeared in the time-resolved spectra. Decay curves of the total luminescence intensity are shown in Figs. 9 and IO for the Gadoped and the Al-doped samples, respectively. They are normalized at the delay time of 10psec. All the decay curves at 2, 77 and 135K were non-exponential, At longer delays than 5 - 10 msec they varied as t-“. The values of n were 0.90 and 0.84 at 2K for the Ga-doped and the Al-doped samples, respectively. They increased with increasing temperature, and the luminescence decayed more rapidly. 4. DISCUSSION
4.1 Donor-acceptor pair recombination Both B. peaks of the Ga-doped and the Al-doped samples showed the features of pair recombination under
6%
A.
SUZUKI et al. #ELAV TIME lOpsec<
1 50 psec
150
psec
500
psec
50 psec
150 psec 500
psec
1.5ms.w
1.5msec
45msec 4.5 msec 7msec
7 msec
% 4H:Ga lo4
I 2.92
,;:,a,
2K I
I
1 2.96
PHOTON
I 3.00
ENERGY
I
I 3.04
2.92
.135,K
,
,
ENERGY
, \I
3.04
3.00
2.96 PHOTON
(eV)
,
(eV)
7(c)
X4
Fig. 7. Time-resolvedspectra of the B,, peak of the Gadoped sample at (a) 2 K, (b) 77 K and (c) 135 K.
4H
: Ga
I 2.92
77H I
PHOTON
I
I
I
I
ENERGY
I 3.04
3.00
2.96
(eV)
709 continuous excitation and in the time-resolved spectra. The other peaks (B, -B,) on the lower energy side behaved in the same manner as the E0 peak. The nonexponential decay curves at 2 K also give an evidence of pair recombination [ 181.
Luminescence from Sic crystals containing N donors and Al acceptors has been investigated by several researchers[4-91. They have shown that at low temperatures the luminescence was due to pair recombination and the spectrum consisted of a zero-phonon peak and its phonon replicas. The photon energy of the II0 peak of our Al-doped sample almost agreed with that of a zero-phonon peak of 4H-Sic reported by Gorban’ ef al. [8] and Hagen et aI.[9]. Unknown donors of the order of 10’gcm-3 in our samples are probably nitrogen introduced unintentionally from the ambient during the growth[l9]. Consequently, the B. peak of the Al-doped sample is a zero-phonon peak of pair recombination between the N donors and the Al acceptors, and the other B, - B5 peaks are its phonon replicas. The I&,peak of the Ga-doped sample can be regarded as a zero-phonon peak of pair recombination between the N donors and the Ga acceptors. As compared with the Aldoped sample, the smaller photon energy can be explained by the larger ionization energy of the Ga acceptor. Hagen et al. reported the presence of another different zero-phonon peak at about 50 meV lower energy than the aforementioned one in the spectrum of Al-doped 4HSiG[9]. But Gorban’ et al. did not find such a peak[8]. Hagen et al. explained the two zero-phonon peaks as being due to two kinds of N donors: the “hexagonal” and the “cubic” lattice sites of 4H-Sic. Our Al-doped or Gadoped 4H-Sic showed much the same spectral shape as Al-doped or Ga-doped 3C-Sic prepared in the same growth run [20]. Since 3C-Sic has only one kind of lattice sites, it is
PhotoIu~ne~n~ DELAY
TIME
@O
10‘
I
,(Jyal 105 4H:Al
11
I
I
I
3.04 PHOTON
104
I
I
I
3.08 ENERGY
I
3.12
I
I\
\
-i6
‘1’
, 1 , / , j ‘, j 1i3 DELAY
2K
3.00
697
spectra of Gadoped and Mdoped 4H-Sic
to* TIME
10’
1
t (seconds)
Fig. 9. Normalized decay curves of the luminescence of the Gadoped sample at 2,77 and 135K.
feV)
W ’ DELAY
TIME
80
DELAY TIME t (seconds) Fig. 10. Normalizeddecay curves of the luminescenceof the Al-doped sampleat 2 and 77 K. 4H:AI
ti
??K I
I
3.00
I
3.04 PHOTON
I
I
3.08 ENERGY
3.12 (eV)
8(b) Fig.
8.
Tie-resolved spectra if the BO peak of the Al-doped sample at (a) 2 K and (b) 77 K.
tAs described in Ref. [211,these notations are accurate only for 3C-Sic and somewhat inaccurate for 4H-Sic, but they are used for convenience.
improbable that some of the B, - B5peaksof our Al-doped or Ga-dopled 4H-SiC may be another zero-phonon peak and its phonon replicas. Since the spectra of the Ga-doped and the Al-doped samples resembled each other, the same kind of phonons may contribute to the corresponfling phonon replicas of the two spectra: From the energy difference between the B. peak and eaqh phonon replica (given in Table l), and phonon energies of 4H-SiC reported by other authors[211, the 8, and the Bt peaks are found to be LA? and LO? phonon replicas, respectively. The B,, B, and B5 peaks may be explained as LA + LO, 2L0, and LA + 2L0 phonon replicas, respectively.
698
A. SUZUKI @t d.
4.2 Recombination between free electrons and bound holes Emissions due to the recombination of free electrons with bound holes at acceptors in Sic have been reported by several authors [4,7,8,10,11]. These emissions appeared on the 35 - 40 meV higher energy side than the donor-acceptor pair emissions. The A, peaks of our samples are thought to be due to such recombination by the following reasons: (1) The A, peaks appeared in place of the I&, peaks at higher temperatures. (2) The energy difference between the A0 and the & peaks was 30 - 35 meV. (3) The A. peaks did not show the features of pair recombination. The thermal quenching of the luminescence shown in Fig. 6 is not caused by thermal ionization of electrons at the donors, but by that of holes at the acceptors. The reasons are that (1) even at the the temperature where the A0 peak began to be dominant, the quenching did not occur yet, and (2) the quenching of the Ga-doped sample began at higher temperature and showed larger activation energy than that of the Al-doped sample. The proba~ity of thermal ioni~tion of bound holes at the acceptors may be expressed as s exp (- E&0), where s is a constant and E, is the ionization energy of the acceptor. Then, the quenching activation energy may be regarded as E,, and the quenching begins at higher temperature for larger E, (221.Therefore, the activation energies of 2402 20 meV for the Ga-doped sample and 160+ 20 meV for the Al-doped sample may be around the ionization energies of the respective acceptors. 4.3 Time-resolvedspectra and decay curues The result that the A* peaks appeared at longer delay times in the time-resolved spectra at 77 and 135K is discussed here. Since the numbers of electrons and holes excited by the xenon flash lamp were not so large, all of them were probably bound by the donors and the acceptors. At 2 K, the bound electrons and holes are not ionized thermally before they recombine to generate pair emissions. Since the electrons and holes with larger separations recombine more slowly[l8], long decay curves characteristic of pair emissions were observed at 2 K as shown in Figs. 9 and 10. At higher temperatures, bound electrons and holes with smaller pair separations recombine in shorter delay times, but electrons largely separated from the holes will be ionized thermally before they recombine. (The ionization energy of the donor is much smaller than that of the acceptor as described in the following section.) These free electrons recombine with bound holes and produce the A,, peak at longer delay times in the timeresolved spectra even at such low temperatures as 77 K. With increasing temperature, the electrons begin to be ionized in shorter delay time as in Figs. 7 and 8. The normalized decay curve at hibrher temperature falls more rapidly at long delay times as in Figs. 9 and 10, because electrons largely separated from the holes are ionized much faster. At 135K, only the A0 peak and its phonon replicas may be observed at longer delays than 7msec from the spectra in Fig. 7(c), but the Iuminescence intensity did not decay exponentially during these delay times. This suggests that the thermal ionization and
the recombination processes of the electrons are not simple.
4.4 Zonizationenergies of the N donor and the Ga, Al acceptors From the energy hva of the A, peak, the ionization energy of the acceptor can be obtained by the following relation [23]: E,=E,+E;+kT-hvA,
(1)
where E,, E, and kT are the exciton energy gap, the exciton binding energy, and the thermal energy of a free electron, respectively. The sum of E, and E, is the energy gap Ew Using the values of hu., obtained under continuous excitation and in the time-resolved ‘spectra (E, is given by Ref. [24]), eqn (1) gives E, = 168ir3meVfE,
(2)
for the Al acceptor, and E,,=249&3meV+E,
(3)
for the Ga acceptor. The ionization energy of the Al acceptor given by eqn (2) almost agrees with 180meV + E, by Gorban’ et al.[8] and 150meV + E, by Hagen et a1.[9]. The ionization energy of the Ga acceptor has been determined by the present study for the first time. The exciton binding energy E, of 4H-Sic is unknown. #en the value of I$ is assumed to be of the same order as 13.5meV reported for 3C-Sic [25], both ionization energies of the Al and the Ga acceptors obtained by eqns (2) and (3) are found to be almost in agreement with the activation energies of the thermal quenching of the luminescence in Fig. 6 (1602 20 meV and 240~ 20 meV, respectively), as has been expected in Section 4.2. The photon energy hv(R) of the pair emission with the separition R is expressed by hv(R) = E, t E, -(Ed t E,) t e’/&,
(4)
where Ed is the ionization energy of the donor, and e and 6 are the electron charge and the static dielectric const~t of the material[l8]. Then, from the photon energy hu, due to the pair with extremely large separation, one can obtain Ed t E, by eqn (4): EdtE,,=E;,tE,-hv,.
(5)
The energy hv, can be estimated from the peak energy hv, and the half-width SE of the B, peak (the zerophonon peak) under continuous excitation as described in Ref.[26]. Since the excitation intensity was not high enough, our experimental condition corresponds to the unsaturated limit in Ref. [2&l.Thus, hv, = hv, - (1~0.76)~~.
(6)
Using the values of hv, and SE given in Table 2 and
P~tolu~eenee
spectraof Ga-doped and ~-do~
T&de 2. & + E, bv, and &, calculated from the peak energy hv. and the half-width SE of the B. peak at 2 K
Sample 4H”SiCIt% 4HSiC : Al
hva (eV)
SE (eV)
0
(“a
:::
2.990 3,067
0.019 0.022
2.965 3.038
58 5a
300+ E, 227+ Ex
4H-Sii
699
determined for the N donor, the Ga acceptor, and the Al acceptor, respectively. The ionization energy of the Ga acceptor was determined by the present study for the first time.
Ed+& =227mev+E3, for the Al-doped SampIe, and E,+E,
=3OOmeV+E,
(8)
for the Gadoped sample. Using the values previously determined for & @pus 2 and 3), one obtains &=59‘9-+3meV for the Al-doped sample, and &=51-+3meV
ilO
for the Ca-doped sample. Since the donors in both samples are probably the same nitrogen, the ionization energy of the N donor is estimated to be 55 &7 meV from eqns (9) and (10). This value may be compared with 65 meV from the luminescence data by Gorban’ et aL(8] and 33 meY by Hall-eiTect measurementsIl3]. The pair separation &. makin the most ~on~~utiou to the B. peak is derived from eqns (4) and (5) by &v(R) = hFg: R$ = e*~~~~~~ - irv,).
f. Addmnimm A, J, ~~t~~kern. SOGIB, Z34@6?$. 2. Blank J. M., In Si&xm Cwkide-1973 (Edited by R. C. Marsh& J. W. Faust, Jr. and C. E. Ryan), p. 550. University of South Carolina Press, Columbia (1974). 3. Kholuyanov G. F, and Vodakov Yu. A., In Silicon CarNde1973(Edited by R, C. Marshall, 1. W. Faust, Jr. and C. E. Ryan), p, 574. University of South Carolina Press, Columbia (1974). 4. Zanmarchi G., J. P&s. C&n. Soiids 29, 1727{l%Q. 5. Choykg:W. J. and Patrick L. Pkys. Reu. BZ, 4959 flW0). 6. Lann N. N., Nedzvetskii D. S.. Prokofeva N. K. and Reifman-M. &, bpf.specfrosc 29;388[197@. 7. Long N. N., N&vet&ii I). S., Prokofeva N. K. end Reifman-M. B,, opt.speefrase. 3% 165(1971). 8. &&an’ f. 9.. ~~nov V. A. and Btimov V. M., Soo. ~ky~.-~l~ S&e 84,2(X0 fl973). 9. Hagen S. H., van Kemenade A. W. C., van der Does de Rye J. A. W., 1. ~~rnine~~ence8, 18 (1973). 10. Yamada S. and Kuwabara H., In Silicon C&ride-1973 (Edited by R, C. Marshall, J. W. Faust, Jr. and C. E. Ryan), p. 305. University of South Carolina Press, Columbia (1974). 11. Kuwabara H. and Yamada S., Phys. Stat. Soi. 30(a), 739 (1975). 12. Lomskina G. A., Vodakov Yu. A., Mokhov E. N., Odin&V. G,, aad K~luya~v G. F., Sov. Pkys.-Solid State 12, 2356 f1971). 13. LMi&ina G. I#,, in Sibm Cadge-1~3 IEdited by R. c. Marshail,5. W. Faust, Jr. and C. B. Ryan), p. 520. ~~iv~s~y of South Carolina Press. cohlntbii (19741.
00
Using 10 as c[27& 50 and %A are obtained for the Al-doped and the Ga-doped samples, respectively. 5. 8uMMABY In order to find the recombination processes of the
blue and the violet photoluminescences from the Gadoped and the Al-doped 4&SiC, luminescence spectra under con~no~s excrtation, ~reso~v~ spectra and decay curves were measured at va&us temperatures. As a result, the huninescences at 2 K were found to be pair emissions between the N donor and the Ga or Al acceptor. The spectra of the two sampies ~se~~d each other in shape, and consisted of a zero-shouts peak and its phouon replicas. Two kinds of phonons and their combinations contributed to the replicas. At bigher temperatnres, another emission due to the recombination of free electrons with bound holes at the Go or Al acceptors was observed. From the energies of the zero-phonon peaks of these two kinds of emissions, the ionization energies of 554 7u~V, 249+3meV+& and 168c3meV+& were
15. Matsunami H., Suzuki A. and Tanaka T., In Silicon Curkide1973(Edited by IL C, Marshall, J. W. Faust, Jr. and C. E. Ryan), p. 618. University of South Carolina Press, Columbia (1974). 16. Suzuki A., Matsunami H. and Tanaka T., Japan. S, A&. Pkys. 14, 891 (1975)” 17. Nelson y. E., Maiden F. A. and Rosengreen A., L Appl. Pkyt. 37,333 (1966), 18. Thomas D. O., HopReId J. J. and Augustyniak W. M., P&V. Be& 14& A2%?(I!%% 19. SuzuukiA., Ikeda hi., Nagao N., Ma~onam~ II. and Tanaka T., J AppL P&y**47,4546 (1976). 20. Suzuki A,, ~a~un~i H. and Tanaka T., .I. ~ec~~~. Sot. 124% 241 (1977). 21. Patrick L,, Choyke W, J. and Han&on D. R., Pkys. Rev, t3?, A151.5(1965). 22. Tajima M. and Aoki Id., Japan.i.Appt. Pkys. I&819 (1974). 23. Colbow K., Pkvs. Rev. 141.742 (1966). 24. Choyke W. J., Patrick L. aad H~mil& D. R., In Prac. 71 Conf. on Physics of Semiconductors Paris, p. 7.51.Dnnod, Paris (1964) 25. Nedzvetskli D. S., Novikov B. V., Prokof’eva N. K. and R&man M. B,, Sov. Pkw-Semiconductors 2.914 11969). 26. Morgan T. N., Plaskett f S. and Pettit G. L?.,ihys. keu. i80, 845 ff%9).