Bound exciton luminescence in GaP:Cu,O

JOURNAL OF LUMINESCENCE 5 (1972) 239-251 © North-Holland Publishing Co.

BOUND EXCITON LUMINESCENCE IN GaP: Cu,O B. MONEMAR Department of Solid State Physics, The Lund Institute of Technology, Box 725, 220 07 Lund 7, Sweden Received 6 March 1972 Luminescence from Cu,O-diffused high-ohmic p-type GaP crystals exhibits hitherto unobserved complexity in the red spectral region (1.6—1.9 eV). The present investigation concerns the features of three different emission series, denoted by C, E 1 and E2, simultaneously present in these crystals. From detailed measurements of emission and excitation spectra at different temperatures the properties of any of these emissions could be studied separately. The low-temperature spectra for the two line emission series E1 (1.78—1.72 eV) and F2 (1.67—1.62 eV) show strong resemblances to well-known properties of isoelectronic complexes such as, for example, Cd,O in GaP. The featureless broad C emission (1.81 eV) is probably of a similar physical origin as Ei and E2, so that all three emissions are believed to originate from excitons bound to different complexes involving Cu and possibly 0. Further work to determine the chemical nature of these defects is suggested.

1. Introduction During the last few years the discovery of efficient luminescence from isoelectronic impurities in several 111—V and 11—VT compound semiconductors has aroused interest in investigating the properties of impurity systems which are more complicated than those that can be described starting from a hydrogen-like level model. Apart point defects, the well-known 2) andfrom ZnTe:03), boundwith states have been examples GaP:N’) GaP:Bi found from compound substitutions in GaP, notably Zn,O and Cd,04’ 5), and recently also Li,06) and Mg,07). In these latter cases, the optical behaviour of the complexes is dominated by the deep donor oxygen, so that the centre acts primarily as a deepbound trap for electrons. Since an electron, bound to a trap, can attract a hole which is, in turn, loosely bound to the centre, bound exciton features can be produced at low temperatures. Recently complicated bound exciton spectra have also been observed in GaP diffused with both Be and 0; these spectra could, however, not be conclusively identified with BeGa—Op isoelectronic substituents and are instead believed to involve interstitial Be, since the diffusion process is mainly an interstitial migration. Similar spectra are also found in Zn,0-doped GaP at energies higher than the well-known red ‘isoelectronic’ emission at 239

240

ft MONEMAR

7). In this paper experimental evidence is presented for new 1.8—1.9 eV luminescent complexes in GaP, formed by the introduction of two impurities, both causing deep levels, namely Cu and 0. These complexes cause strong and complicated emissions in the red part of the spectrum. While the optical properties of oxygen in GaP are quite well-known8 - I this is not the case for Cu. No published work on luminescence in CLI-doped

GaP is known to the author (except for ref. 12. where a broad peak at about 1.67 eV at 90 K is attributed to Cu in GaP). However, several measurements on photoelectrical properties on Cu-doped GaP indicate the presence of one (or more) deep acceptor level with binding energies in the region 0.5 to 0.8 eV’2 ‘c’). Similar conclusions have been drawn from optical absorption measurements’4 7. 18) In an unpublished report on luminescence ii GaP—Cu, Gershenzon et al.’9) foLind broad emission bands in the red and infrared, which were tentatively explained as originating from donor— acceptor pair emissions involving a deep Cu acceptor (E., 0.63 eV), hut no exciton emissions were found, probably since the oxygen concentration was kept low. In our case, oxygen was deliberately introduced in the lattice together with Cu during the doping process, and therefore aggregates involving both impurities can be formed in concentrations high enough to permit easy detection of their optical spectra. In addition to a broad emission band, two series of sharp emission lines together with their phonon replicas are seen at low temperatures in the red spectral region. and arguments are presented below to show that these features are connected with bound exciton recombination. Corresponding line spectra are also seen in absorption, as revealed by luminescence excitation spectra. The exciton spectra reported here are characteristic of a certain type of high-ohmic p-type Cu,0-codoped crystals. In other high-ohmic crystals prepared and doped in a similar way. these spectra are often absent, being replaced by another broad efficient red emission band with entirely different properties. This type of Cu ILiminescence will be discussed elsewhere20). 2. Material and experimental procedure For these investigations solution-grown GaP platelets have been used. Doping with CLI and 0 was carried OLit by a two-step diffusion process2’). in which Cu and 0 were introduced into a surface layer at about 400 C. the diffusion process being subsequently completed in an evacuated quartz vessel at about 900CC for 24 hours to ensure an approximately homogeneous doping. All crystals referred to in this paper were high-ohmic p-type with a resistivity of 106_l08 Q cm. Etching in HCI gave no drastic changes in luminescent properties.

BOUNL) EXCiTON LUMINESCENCE IN

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For the detection of emission spectra a Zeiss MM-I2 prism double monochromator was used, together with a photomultiplier (EMI 9558) and a recorder (Moseley 7101 BM) for continuous photoelectrical registration with a resolution limit of about 3 A at 7000 A. For excitation, 100 W compact arc Hg lamps or 250 W xenon lamps were used together with suitable glass filters. In cases where excitation within a narrow spectral band was desired, the excitation light passed another Zeiss MM-I2 double monochromator before striking the crystal. With this two-monochromator set-up, both emission and excitation spectra could be recorded alternately at the same temperature, the only change necessary between the two types of measurements being to switch the recorder connection. To achieve the highest resolution (4 A) in excitation spectra, the monochromator on the detection side was exchanged for suitable glass filters. Two different cryostats have been used for these measurements, one for the temperature range 5—300 K employing indirect cooling, and another for measurements at 4.2 K and below, in which the sample was immersed in the liquid He bath. Measurements in this latter cryostat at low temperature were, however, complicated by fluorescence from its sapphire windows. 3. Experimental results 3.1. EMISSION SPECTRA

Gallium phosphide crystals doped with Cu and 0 show bright red luminescence at both intrinsic and extrinsic excitation (see below) even at temperatures above 300 K. In fig. 1 a typical low-temperature emission spectrum of a crystal of the type considered in this report is shown, for moderate excitation with light energies above 2.0 eV. The red emission spectrum with this broad band excitation consists of a broad featureless emission C centred at about 1.81 eV, with an estimated half-width of about 0.09 eV, and a line spectrum E1 originating at about 1.78 eV, with strong phonon replicas extending below 1.72 eV. At still lower energies another similar line spectrum (E2) appears. Starting at about 1.66 eV, it has a similar strong phonon wing extending to below 1.60 eV. More broad emissions connected with Cu apparently exist at lower energies, but these are not discussed in this paper. The broad C emission is very strong at low temperatures, and is excited mainly via a similar broad absorption band at around 2.09 eV (see below). Furthermore, the C emission shows no detectable change in shape or peak position when the excitation intensity is varied by more than two orders of magnitude, indicating that a pair recombination involving charged donors and acceptors is not the process responsible for this emission. The emission intensity depends slightly superlinearly on

242

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Cu 60-4

-

f

I 1.60

20 K

-.

1.65

1.70 PHOTON

-

~ 1.75 ENERGY

I 1.80

1.85

CV)

Fig. 1 . Photoluminesccnce spectrum for a p-type GaP crystal, doped with Cu and 0 at 20 K, excited with photon energies larger than 2.0 eV.

for broad band excitation hr > 2.0 eV. Since such emission has not previously been reported in the literature, it should be related to the presence of Cu and possibly 0. Our main interest here is the two structural emission bands E~and E, [see figs. 2(a) and 2(b)]. E, can most consistently be described as being composed of two no-phonon lines A~,and B~,at energies of 1.7835 ±0.0003 eV for A~and of 1.7788 ±0.0003 eV for B,~,the uncertainty being determined by spectral resolution and scale inaccuracy. The line-width of these two lines at 10 K appears to be about 1.5 meV at a spectral resolution of 0.8 meV, indicating that the true half-width might be less than kT for both lines (A,~,seems to be somewhat broader). Measurements at the lowest temperatures (below 4 K) were difficult, partly due to the strong C emission which quenches the E, group and partly because the sapphire windows of the cryostat emitted a line spectrum in the same spectral region. Therefore, at the lowest temperatures, the linewidths could not be estimated accurately, but at least the B,~ line seemed to be essentially resolution-limited. The other peaks in the region 1.775—1.74 eV are broader than A,~and B,~and could be classified as phonon replicas of the A,1, and B,1, lines, with a phonon energy of 10.8 ±0.5 meV for both lines. A strong support for this model is the observation that the relative intensity between the lines remains constant between different crystals. If this interpretation is correct, the A’ line replicas are apparently stronger than A,1,, while B’ line replicas are weaker than B,1,. excitation strength

BOUND EXCITON LUMINESCENCE IN

1.72

;

GaP:Cu,O

1.74 1.76 PHOTON ENERGY

243

1.78 cV)

.: 1.62

1.64

1 66

1.68

PHOTON ENERGY (cV)

Fig. 2. A more detailed picture of the line emission spectra E, and E

2 for a p-type Cudoped crystal, coded 62-2, excited with photon energies larger than 2.0 eV. In (a) the two identified zero phonon emission A0’ and ~ are shown, together with at least two phonon replicas of each of the main low-energy phonon of about II meV. (Note that for this crystal Ao’ is much larger compared 1’ and Bo” withis Bo’ believed than to for bethea repetition crystal in of fig.the I.) main The Ao’, new B,,’ series,starting the energy structure with difference lines A0 being an optical phonon. The whole spectrum is here superimposed on the broad A emission (dashed line) and the zero level is depressed for clarity. (b) shows E 2 emission features. The line A02 corresponding to As’ in Et [see (a)] was not detected here: otherwise the main phonon energy seems to be approximately the same as for E1 in (a).

Since the replicas are considerably broadened, the energy of the second replica is poorly defined, and the accuracy in determining phonon energies is not better than 0.5 meV. The existence of less prominent phonon replicas in this region cannot be excluded owing to the limited resolution. [Compare absorption spectra in section 3.2, fig. 4(b).] A repetition of this entire series seems to start at A~’ = 1.7372 ± 0.005 eV, that is, 46.3 ±0.7 meV below the A~1,line. The next line B~’at 1.7326 ±0.0006 eV has the same spacing from the B~line, and the third peak, reasonably well-resolved, has a spacing of 10.5 ± 0.6 meV from the A~’line, being thus a phonon replica of A~’ with the same phonon energy as for the principal A’—B’ wing at 1.78 eV.

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B. MONEMAR

It therefore seems obvious that these peaks involve a 46.3 ± 0.7 meV phonon repetition of the principal E, series, the phonon probably being an

optical lattice phonon. as observed strongly in the phonon wing of GaP: N. A notable fact is that here A~’is stronger than B,1,’ [see fig. 2(a)]. At still lower energies, a new series E, begins with the first peak. detected at 1.6635 ± 0.0005 eV and the second at 1 .6577 ± 0.0005 eV. This series can he accounted for in a similar way as for E, if’ it is assumed that the peak corresponding to A,1, is missing while the second peak might be a phonon replica of this first missing one. The first peak then corresponds to B,1, in E, and will be denoted B~.The third line R~is then a phonon replica of B~. it is interesting to note that, if this interpretation is correct, a common phonon energy of about 10.9 ±0.5 meV is involved. i.e. the same as in the E, series. Note also that the separation between the two series A2 and B2 in E 2 is also approximately the same as for A’ and B’. respectively, in E i.e. 4.4 ±0.5 nieV. The notation used for the E,-series was chosen to point out the close similarity with the E,-series concerning phonon energies and A—B-separation. The fact that the A~-linewas not either seen in absorption (see fig. 4c) points to an alternative description of the E2-series. where B~ in fig. 2b is regarded as the most high-energetic zero-phonon line, thus called A~.and A~above should be called B,~.This gives a slightly different A—B-separation, but has the advantage of relating the most intense zerophonon line to an allowed transition (normally the one of highest energy in emission, here denoted by A). Another bound exciton emission structure was observed in these crystals

at low temperatures in the yellow region. with a narrow line at 2.177 eV and several different phonon at lower grown energies. have 22’ 23) replicas in GaP crystals by Similar differentspectra techniques, recently been reported and were believed to be connected with 0 V~~Opcentre. Recently this interpretation has been reconsidered, and the 2 rneV spectrum is believed to be due to a bound exciton emission involving Cu (P. J. Dean. private communication). This is supported by the present investigations. 3.2. EXCITATION SPECTRA Characteristic for all the emissions

C. E,

and

E 2

they are excited preferentially

described above is that

in an extrinsic band with a shift towards

higher energy compared to the emission, bands, similar to the case of bound exciton emission in, for example. GaP:Zn.O. Experiments using one Zeiss monochroniator system for excitation and one for detection could unambiguously separate the main absorption bands corresponding to each emission, i.e. E, is excited mainly via a band centred at about 1.95 eV and E2 in a corresponding band centred at about 1.82 eV. as can be seen froni fig. 3. As was

BOUNI) EXCITON LUMINESCENCE IN GaP:Cu,O I

245

I

Cu 62-2 >

3—

-

t) IC)

0

±

-

N U)

z uJ

F o

-

-

E

U)

U)

uJ I I 1.50 2.00 2.20 EXCITATION PHOTON ENERGY (eV)

I 2.40

Fig. 3. Excitation spectrum for the same crystal as in fig. 2 at 10 K. The filter used in front of the detector transmits all radiation below about 1.75 eV. The spectrum is thus mainly characteristic for the Fz emission, but the multiphonon wings of F, (and possibly

also C) contribute, causing the 1.93 eV peak and part of the 2.09 eV peak in absorption. The sharp peak observed below 2.32 eV is believed to be due to the nitrogen exciton absorption line.

mentioned previously, the C-emission is excited via a broad band centred at about 2.09 eV. The excitation spectra taken with bare detector + filters shown in fig. 4 also reveal structure in the low-energy edges in these bands, in good agreement with the phonon structure observed in emission. Thus

for the E2 emission the line B~A corresponding to B~appears at 1.7855 ± 0.0005 eV, while the line A~A (but not its phonon replica) is missing in absorption as well as in emission. The phonon energy in this case seems to be somewhat larger than in emission, about9), 11.6in ±0.5 behaviour which meV. a shiftThis of local mode is similaristoobserved that observed in GaP:Cd,O’ energies from emission to absorption. The separation between B~in emission and B~A in absorption is thus 0.122 ± 0.001 eV. For E 1 emission, the peak corresponding to A~is apparently weak in absorption, and the narrow line at 1.9122 ±0.0004 eV might thus be classified as B,1,A and the other broader peaks at higher energy as phonon replicas of B~1,in absorption. In this case more than one phonon energy must be used (9.0 ± 0.8 meV and 11.6 ±0.8 meV, respectively). From the widths of the peaks in fig. 4 it appears that this identification is not completely certain. However,

246

U. MONEMAR T~

P

T~22.00

2.05

2.10

215

665 13 mm

~ 1.00

192

1.94

1.96

1.98

‘78 L80 1.82 1.84 EXCITATION PHOTON ENERGY leVI

1.86

Fig. 4. Main extrinsic absorption hands at 10 K (detected as excitation spectra) for emissions C, E, and E 2 in crystal 62-2. In (a) the broad absorption peak is selected associaicd with the similarly broad C-emission, detected with a filter transparent for photon energies below approximately 1.87 eV. The superimposed structure between 2.02 and 2.06eV of the low-energy side of this peak is due to a minor contribution of this absorption hand exciting the F, emission structure, whose principal absorption band is shown in (h). This hand is essentially a mirror reflection of the F, emission in fig. 2(a), except for a reduced prominence for the A’ ‘ line series. The optical phonon replica also remains undetected, and all details in the spectrum probably cannot be explained by the introline series dL,ction of just one phonon energy in the II rneV range. In (c) the principal2’absorption seems to be more prominent than using for A’ in (b), hutfig.the hand associated with E2 is shown, the‘ above same filter as in 3. line HereAthe2’ Ais apparently undetected as it was in emission (fig. 2h). 0

if it is correct, the shift for the B,1, lines between emission and absorption should be 0.133 ±0.001 eV. which is somewhat larger than for the E 2 emission. (The alternative notation for the E2-series mentioned above would, however, give approximately the same shift (0.133 eV) between B~and B~A.)Apparently the F, emission can also to a certain extent be excited via the broad 2.09 absorption hand connected with the 1.81 eV C emission

BOUND EXCITON LUMINESCENCE IN

GaP:Cu,O

247

described above. Weak structure is observed, starting with the first peak (with lowest energy) at 2.0230 ±0.0004 eV, probably corresponding to the A~line, since the second peak is displaced just 4.4 ±0.5 meV towards higher energy, the same A~,—B~ separation as in the E, emission. The phonon energy involved in the higher energy peaks in this absorption spectrum is of the same magnitude as those in emission, i.e. about 10—11 meY; accurate measurements were not possible here. This weak absorption structure corresponds to a shift of about 0.240 eV between emission and absorption of the A~,line. It might also indicate that the origin of the broad C emission at 1.81 eV is intimately connected with (at least) the E1 emission structure. As the excitation spectra for the emissions C, E1 and E2, for experimental reasons, could only be recorded at energies higher than about 0.1 eV from the high energy cut off of the corresponding emission spectra, direct contributions to excitation of these emissions (i.e. without substantial shifts between zero phonon lines in emission resp. absorption) cannot be excluded. Such contributions, however, seem not to be very prominent, since their high energy phonon assisted tails were not observed in excitation spectra. 3.3. TEMPERATURE DEPENDENCE

The temperature dependence of the different emissions described here can only be given qualitatively, since a number of different competing emissions exist at each temperature. Emissions E1 and E2 are strongest below 100 K (the structure is broadened and unresolved above 90 K), E, does not fall off rapidly until above 200 K, at which higher temperatures the excitation comes partly from the broad absorption band centred at about 2.09 eV (fig. 5). (The E2 band falls in a region of lower detector sensitivity and, because selective excitation with a Zeiss spectrometer had to

be used to isolate this emission, its temperature dependence could not be established above 150 K.) As is apparent from fig. 5, the high-temperature red emission spectrum at 270 K is dominated by a strong emission centred at about 1.83 eV, which shifts to higher energy when the temperature is lowered, being centred at about 1.86 eV at 150 K (fig. 5). At lower temperatures, this emission is quenched and replaced by another similar broad emission, referred to as C above, which is centred at about 1.82 eV. This emission persists and increases in intensity down to the lowest temperatures, where it is usually dominant below 15 K [fig. 2(a)]. These broad emissions are excited only extrinsically via the broad 2.09 band (and also to some extent via a band at about 2.28 eV), but not intrinsically except at temperatures below 60 K, where this part also contributes. For the emissions E1 and E2, the intrinsic contribution in excitation starts at higher temperatures

248

B. MONEMAR

i~~\ ~70K

_K~50K~

i~U~

~ 15K

1.70

180

15K

1.90 1.8 2.0 22 PHOTON ENERGY (eVI

~

2.4

Fig. 5. Review of the most prominent emissions at different temperatures for crystal 62-2 (left), together with the excitation spectra for these peaks at each temperature (right). For these measurements spectral detection of the emissions was performed with a Zeiss MM 12 monochromator in combination with another Zeiss monochromator LISed as excitation light source. The proper excitation photon energy for emission spectra was chosen in each case from the corresponding excitation spectrum. The excitation spectrum was taken with the same set-Lip, but with the monochromator on the detection side adjusted to the enlission maximum.

and is important below 120 K for both E, and

~

for E

2. the intrinsic part by far dominates the excitation spectrum at 10 K (see fig. 4). Thus, at higher temperatures (T > 150 K), all the red emissions described in this work seem to originate from transitions within strongly localized centres.

4. Discussion The above description of the spectra from one type of Cu.O-doped GaP crystal indicates the complexity of the luminescent properties of this material.

and this first crude investigation of its properties could not definitely determine the origin of all the emissions observed. Sonic striking featLires, however, point to their physical origin. Since all the emissions in the red spectral

BOUND EXCITON LUMINESCENCE IN

GaP:Cu,O

249

region discussed here are quite efficient, and none of them have been found in GaP crystals doped with other impurities (according to the literature), it is highly probable that they are connected with the diffusion of Cu into the samples, and probably partly also with the presence of oxygen introduced during growth (these emissions were not observed by Gershenzon et a1’9), although they investigated many different crystals, both n-type and p-type, but with low 0-concentration). The two structured red emissions E 1 and E2 show a remarkable resemblance 4’ 5, to, 24):for example, bound exciton spectra from Cd,O complexes in GaP (I) Two series of lines, called A and B above, appear separated by a few

meV, indicating the presence ofan electron—hole pair bound with Jfcoupling. (2) The main phonon replicas involve a local mode of low energy and an optical phonon. (3) Superlinear dependence of the emissions on excitation intensity was also observed. The A 0—B0 ratio is expected to exhibit a strong variation with temperature in the low-temperature region according to this model. This effect was, however, not observed. The reason for this complication is probably to be found in the presence of residual strain in the crystals, which might also explain the difference in the A~—B~1,ratio observed between different crystals [compare. for example, figs. 1 and 2(a)]. It was also found that the A’ line series1 inlines. E1 was stronger excited by intrinsic light than Themuch classification of the phonons observed in the corresponding B emission (and absorption) is also uncertain, since for deep impurities like CLI (and 0) momentum-conserving phonons need not be dominant in optical spectra. This point is, however, not a serious one in the interpretation of E 1 and E2 as bound exciton spectra. From their energy position it is obvious that they could hardly be connected with excitons bound to a single acceptor CU0a, but must instead be bound to a complex involving at least two coupled point defects, for example Cu and 0. The strong phonon wings observed in both emission and absorption47). are typical for emissions due to excitons Assuming that a substitutional CUbound to isoelectronic complexes 0a~Op 9 impurity to constitute complex is involved, the CUGa atom should act as 3d an isoelectronic complex with 0 [it is assumed that Cu (configuration 3d1 O4~t) on the Ga side has to give up three electrons to the valence band of GaP]. Thus one of the emissions E, and E 2 could be8 associated with this isoeleccharge state might also be tronic complex, but hardly both. Cu in a 3d present in these p-type crystals, but does not give an isoelectronic CU~a~Op complex. The crucial point is, of course, the close similarity between the series E, and E 2, in emission as well as in absorption. (The observation that the low energy cut off of the E~spectrum in fig. 4c falls very close to

250

B. MONEMAR

the A,1,-line of the E’-emission (fig. 2a) also points to a possible relation between the F 1- and E2-emissions.) Since the energy difference between F, and F2 is as large as about 115 meV, we consider it improbable that E2 is a phonon replicaT)]. of F, largest phonon found replica in GaPofupthis to and[the a selective multiphonon now is about 70 meV strength would indeed be anomalous. It is interesting to note that two similar emission line series have been observed in diffused GaP: Be,0 as well as in GaP:Zn.07), but their chemical origin is not yet clear.

The gross features of the broad C emission are very similar to the E, and E 2 emissions, and the low quenching temperature might indicate a bound exciton recombination (with one particle bound more loosely than for the E1 series) as the physical origin also for this emission. The mixing observed in excitation spectra [fig. 4(a)] also suggests sonic connection with the E, emission. The large width of the peak should then be due to strong unresolved phonon interactions, such as are found example. the red 3’ 4), It is interesting to note thatin,in for the excitation spectra GaP: Zn,0 emission of all three emissions a peak is observed just below the band gap at 2.25—

2.3 eV (fig. 5) [besides that due to the nitrogen line (fig. 4)]. which niight be interpreted as a contribution from the tunnelling of shallow bound excitons into these Cu complexes before recombination. In conclusion, it might be stated that this first investigation of luminescence in GaP:Cu,0 crystals reveals interesting new emission features, which could be most naturally ascribed to excitons bound to complexes involving Cu, and, probably in some cases, oxygen. The rather simple phonon structure indicates that at least the E, and F 2 complexes are of low order, such as, for example, diatomic pairs. The participation of 0 in complex might 7).a However, more be settled by looking for isotope shifts in emission lines detailed information about the chemical nature of the radiating defects in

GaP:Cu,0 will only he gained from substantial experimental work, including lifetime measurements and Zeeman spectroscopy at a higher spectral resolution than was available in this work. Acknowledgement

The author is much indebted to Professor H. G. Grimmeiss f’or fruitful discussions.

References I) D. G. Thomas and J. J. Hopfield, Phys. Rev. 150 (1966) 680. 2) F. A. Trumbore, M. Gershenzon and D. G. Thomas, AppI. Phys. Lett. 9 (1966) 4. 3) J. J. Hopfield, D. G. Thomas and R. T. Lynch, Phys. Rev. Lctt. 17(1966) 312.

BOUND EXCITON LUMINESCENCE IN

4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) IS) 16) 17) 18) 19) 20) 21) 22) 23) 24)

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T. N. Morgan, B. Welber and R. N. Bhargava, Phys. Rev. 166 (1968) 751. C. El. Henry, P. J. Dean and J. D. Cuthbert, Phys. Rev. 166 (1968) 754. P. J. Dean, Phys. Rev. B 4(1971) 2596. P. J. Dean and M. Ilegems, J. Luminescence 4(1971) 201. P. J. Dean, C. H. Henry and C. J. Frosch, Phys. Rev. 168 (1968) 812. P. J. Dean and C. H. Henry, Phys. Rev. 176 (1968) 928. J. M. Dishman, Phys. Rev. B 3 (1971) 2588. R. N. Bhargava, Phys. Rev. B 2(1971) 387. H. G. Grimmeiss and H. Scholz, Philips Res. Rep. 20(1965)107. J. W. Allen and R. J. Cherry, J. Phys. Cheni. Solids 23 (1962) 509. R. J. Cherry and J. W. Allen, in: Proc. Intern. Conf. on the Physics of Semiconductors Exeter (The Institute of Physics and the Physical Society, London, 1962) p. 384. I—I. G. Grimmeiss and M. 0. Ottosson, Phys. status solidi (a) 5 (1971) 481. M. 0. Ottosson, Solid State Electron. 14 (1971) 305. R. Olsson, Phys. Status solidi (b) 46 (1971) 299. J. M. Dishman and M. Di Domenico, Jr., Phys. Rev. B 4(1971) 2621. M. Gershenzon, F. A. Trumbore, R. M. Mikulyak and M. Kowaichik, unpublished, 1966. B. Monemar, to be published. H. G. Grimmeiss, W. Kischio and I-I. Scholz, Philips Tech. Rdsch. 10/11 (1963/64) 386. R. N. Bhargava, S. K. K. Kurtz, A. T. Vink and R. C. Peters, Phys. Rev. Lett. 27 (1971) 183. P. J. Dean, Solid State Commun. 9 (1971) 2211. C. H. Henry, P. J. Dean, D. G. Thomas and J. J. Hopfield, Localized Excitations in Solid, Ed. R. F. Wallis (Plenum Press, New York 1968) pp. 267—275.