J. Phys. Chem. 1995, 99, 4894-4899
4894
Continuous-Wave, Microwave-Modulated, and Thermal-Modulated Photoluminescence Studies of the Bi13 Layered Semiconductor E. Lifshitz" and L. Bykov Department of Chemistry and Solid State Institute, Technion, Haifa, 32000, Israel Received: March 9, 1994; In Final Form: December 30, 1994@
This paper reports the extensive investigation of the luminescence properties of the bismuth triiodide, BiI3, layered semiconductor. The luminescence spectrum is comprised of relatively narrow stacking fault excitons (R, S, and T) in addition to broader bands at lower energies. The present work emphasizes the investigation of the broader bands. Research of the Bi13 involved utilization of continuous-wave photoluminescence (PL) and PL excitation (PLE), together with two modulated techniques: thermal-modulated PL (TMPL) and microwave-modulated PL (MMPL). The TMPL and the MMPL of the stacking fault excitons served to confirm the previously suggested interaction among the R, S, and T states. Experimental evidence indicates that the broader emission lines may be divided into several subgroups, some of which are associated with band edge to deep state transitions. The deep state within the band gap may be attributed to stoichiometric or strain defects. Results showed that the latter transitions correlated to band edge properties. One subgroup, however, displayed independent behavior, due mainly to an absence of coupling to the band edge. This subgroup is associated with a transition between relatively localized donor and acceptor states.
I. Introduction Bismuth triiodide, Bi13,is an indirect band-gap semiconductor. It crystallizes in a layered structure and exhibits quasi-twodimensional behavior in many of its physical properties. Its layers consist of bismuth atom planes sandwiched between iodine planes to form the sequence I-Bi-I. The intralayer bonding is predominantly ionic, while the layers are held together by weak van der Waals forces. The bismuth ions occupy only two-thirds of the octahedral holes between the adjacent iodine planes. A successive stacking of the three I-Bi-I layers leads to a stable rhombohedral crystal structure with a space symmetry group of Theoretical band structure calculations2 suggest that the topmost valence band comprises an admixture of the I(p) and Bi(s) orbitals, while the bottom of the conduction band is mainly of Bi(p) character. Several previous optical measurements have indicated that the optical processes are associated with the Bi(s) Bi(p) cationic transition. The optical absorption edge of Bi13 consists of an indirect exciton transition assisted by three different momentum conserving phonons, named A, B, and C. The indirect exciton transition (the energy E,, = 2.0081 eV at 2 K, gx = exciton ground state) occurs in close proximity to the direct exciton energy (Edgx = 2.072 eV) and subsequently exhibits an extraordinarily large oscillator strength. In addition, several sharp sample-dependent absorption lines appear below the indirect exciton lines, labeled P, Q, R, S, and T. It has been suggested that the P line is associated with polytypic stacking disorder, while other lines are related to stacking fault (SF) interfaces. 1,3,4 The photoluminescence of Bi13 single crystals consists of the indirect phonon-assisted exciton transition (Lc) and the SF emission lines (P, Q, R, S, and T), with a zero Stokes shift from the corresponding absorption transitions. The luminescence line widths of the latter are very narrow ( < O S meV at 4.2 K).1,3,4Occasionally, an additional series of broader lines
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* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracrs, March 15, 1995.
(with a full width at half-maximum [fwhm] of -5 meV) is observed at energies below the SF lines. The present work strives to clarify the origin of the broader luminescence lines through the introduction of new experimental evidence. The research utilized conventional photoluminescence (PL) and PL excitation (PLE) in combination with two unique modulation techniques: microwave-modulated photoluminescence (MMPL) and thermal-modulated photoluminescence (TMPL), the latter supplying information non-obtainable through continuous-wave PL spectroscopy. In MMPL measurement one monitors changes in the luminescence intensity ( h l p ~ induced ) by the electric-field component ( E l ) of applied microwave radiation. These changes are associated with free carrier heating induced by E l , which subsequently influences recombination processes. In TMPL measurement one monitors changes in the luminescence intensity ( h l p ~ brought ) about by thermal modulation. This modulation induces a thermal redistribution among the emitting states. The above modulation methods enable the separation of overlapping luminescent events and thus provide for improved determination of recombination processes. 11. Experimental Section
1. Single Crystal Growth. Platelet crystals of Bi13 ( < l o p m thick and lateral dimension of 2 x 2 cm2) were prepared by ~ublimation.~The constituent elements were placed in a Pyrex reactor at the center of a three-temperature-zone horizontal furnace. Growth was performed by passing a flow of argon gas over iodine grains (located at first zone, 298 K), reacting the gas mixture with elemental bismuth (located at the second zone, 583 K), and then forming the platelet crystals at the third furnace zone (473 K). 2. Instrumental. PL, PLE, MMPL, and TMPL spectra were recorded by mounting single crystals of Bi13 on to special sample probes. A detailed description of the MMPL and TMPL probes is given in the following paragraphs. All probes were placed vertically in a 12CNDT Jannis cryogenic Dewar. The samples were cooled either by direct immersion of the entire probe in superfluid helium (1.4 K) or through coupling to liquid helium vapor (4.2 K). The crystals were excited by either a continuous-
0022-3654/95/2099-4894$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 14, 1995 4895
BiI3 Layered Semiconductor wave (CW) 514 nm Ar+ laser (Coherent, Innova 70) with a power output of about 0.1-3 W/cm2 or by a high-pressure xenon arc lamp. The emission (PL and PLE) or modulated emission (MMPL and TMPL) was then dispersed through a holographic grating monochromator (Jobin-Yvon, Model THR1000) and detected by an Hamamatsu R666 photomultiplier tube. An Oriel colored glass filter was used in order to’eliminate scattered excitation light. The MMPL sample probe contained a “home-made’’ TEoll microwave cavity, resonated at 10.8 GHz, with an unfilled Q of approximately 3000. The microwave source was an HP Model 83620A synthesizer sweeper, amplified by an Aventek Model APT-18668 solid state amplifier. The amplifier output was directional-coupled to the cavity via a low-loss, cryogenic coaxial cable with the microwave being loop-coupled at the entrance to the cavity. The microwave power output was amplitude modulated at audio frequencies by a square-wave output from a Wavetek Model 142 waveform generator. The induced changes in luminescence were detected using a conventional lock-in amplifier referenced to the microwave chopping frequency. The MMPL spectra were measured at various microwave power levels, ranging between 6 and 400 mW. The emission was monitored in a direction k, perpendicular to the layers and to the applied microwave electric field (Ell1 layer, ElUc). The TMPL sample probe contained a heater, consisting of a 1-2 ,um thick chrome-nickel alloy film, vapor deposited onto a Si02 substrate. Electrical contact with the heater film was made by indium soldering a no. 40 copper wire to gold contact pads (vapor deposited to partially overlap the chrome-nickel film). The active area of the heater was typically 2 x 5 mm2 with resistance of about 50 52. In the TMPL experiment, the sample was glued to the heater with its c-axis normal to the heater film. The TMPL was recorded for a relatively thin sample having a [sample area]/[sample thickness] ratio of about lo3. This condition is necessary to obtain an efficient transfer of heat across the sample. The power dissipated by the heater (10-300 mW) corresponded approximately to microwave dissipation. Thermal modulation was accomplished by passing a modulated current through the heater. To obtain the TMPL spectra the heater was powered by a square-wave output originating from a pulse generator; the change in luminescence was lock-in detected. The temperature change induced by the heat pulses was estimated theoretically to about 3°.6
111. Results The PL spectra of several samples (i, ii, and iii), recorded at 1.4 K and excited with laser energy > are shown in Figure 1. Each spectrum comprises three main groups of emission lines (I, 11, and m),as labeled. The relative intensities of these groups vary from sample to sample. The individual lines of each group are designated by alphabetic letters for the purpose of the following discussion, and their emission energies are summarized in Table 1. Although all samples were prepared by the same growth procedure (as discussed in section II.l), their corresponding PL spectrum reveals different resolution and relative intensities of the various emission lines. This may be attributed to the varying concentrations of the different defects. Moreover, the PL spectra of samples i and iii were recorded following cleavage of the single crystal. This process exposes fresh layer surfaces, but while inserting additional mechanical stresses. The PL spectra of the various specimens were recorded in the temperature range between 1.4 K and 65 K. Representative spectra of sample ii, recorded at three different temperatures,
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!---a-7--+-11.83
I
J
I
1.85
1.90 1.95 2.00 2.05 ENERGY (eV) Figure 1. PL spectra of three different samples of BiI3 (i, ii, and iii), recorded at 1.4 K with excitation energy > Egx. The three main emission groups (I, 11, and 111) are labeled at the bottom of the figure.
A
I83
I87
I91 I95 ENERGY (eV)
2 03
I99
Figure 2. PL spectra of BiIs (sample ii) recorded at various temperatures, as indicated in the figure. These spectra were obtained under excitation energy > Egx. TABLE 1: Emission Lines in the PL Spectrum of BiG, Recorded at 1.4 K emission emission emission emission line energy (eV) line energy (eV) group I group II P
Q
R Lc S T LO LO LO
2.010 2.005 1.998 1.995 1.992 1.988 1.970 1.966 1.962
D
E F G
1.932 1.924 1.915 1.908
group I11
H I J K L
1.885 1.880 1.865 1.855 1.840
are shown in Figure 2. The emission lines of group I undergo a blue shift with increasing temperature. The emission lines D, I, J, and L are also blue shifted. Uniquely, the F line was not shifted between 1.4 and 65 K. However, above 65 K the F line intensity rapidly quenched, preventing determination of its
Lifshitz and Bykov
4896 J. Phys. Chem., Vol. 99, No. 14, 1995
A
50
T(K)
1.80
Figure 3. The temperature ( r ) dependence energy shift (AE) of the emission line T (M), D (A), and J (0)with respect to their position at 1.4 K. The solid lines are drawn to guide the eye.
1.85
1.90
1.95
2.00
2.05
E NERGY (eV 1 Figure 5. The TMPL spectrum and the PL derivative [APL/A(hv)]of BiIs (sample ii), recorded at 4.2 K with excitation energy > EgX.
25
4
c
>
-
A
-
E20
r c 0 3
-
C
al .-c c
'5.-c al
A
-I
a
10 I
2
3
5
4 112
6
7
0
I/2
T (K 1 Figure 4. The fwhm dependence on temperature of the D The solid line is a theoretical curve (see text).
l-c (m) line.
central line position. The remaining lines are not resolved above 4.2 K. The energy shift (AE) of lines T, D, and J with respect to their position at 1.4 K are plotted versus temperature ( r ) in Figure 3 (labeled M, A, and 0, respectively, in the figure). It should be noted that the blue shift of the D line cannot be evaluated above 45 K due to the substantial broadening of the adjacent F line. In addition, as seen in Figure 3, A," is larger for the lower energy emission line. The full width at halfmaximum (fwhm)dependence on temperature (T'") of the D (H) line is drawn in Figure 4. The solid line in this figure is a theoretical fit (vide infra). The TMPL spectrum of Bi13 (sample ii), recorded at 4.2 K, is shown in Figure 5 . In this figure the spectrum is compared with the continuous-wave PL derivative [APL/A(hv)]. This comparison indicates that, excepting group I and the F line, all luminescence lines are identical in the TMPL and PL derivative curve. The R, S , T, and F lines do not have appearance of a derivative curve; instead, T and F appear as a negative signal (decrease of luminescence intensity) while the S and R lines appear as positive signals at the corresponding CW-PL energies. The PL spectra of BiI3 (sample ii), observed under various resonance excitation energies, are shown in Figure 6. Excitation energy was approximately 5 meV above the corresponding high energy luminescence line. Spectrum a in the figure was observed under resonance excitation of the P line. This spectrum contains the T, D, F, I, J, and L emission lines as well as the broad shoulder of the LO emission lines, while the weak E, G, and K lines are absent. Spectrum b was observed under resonance excitation of the T line. This spectrum contains a D line of enhanced intensity, and E, F, I, J, and K lines of weaker intensity. Spectrum c was observed under near reso-
1.82
I
1.86
I .94 ENERGY (eV 1 1.90
I .98
Figure 6. PL spectra of BiI3 (sample ii) observed under various resonance conditions: (a) excitation of the P line, (b) excitation of the T line, and (c) excitation of the D line.
nance excitation of the D line. It exhibits an enhanced intensity of the I line and gradually weaker intensities of the J and K emission lines. The latter set of spectra show that resonance excitation at line P yields a nearly equal intensity of the D and F lines, whereas under any other resonance excitation the F line is substantially weaker than the D line. Figure 7 represents PLE spectra of the D (a) and F (b) emission lines in the energy range between 1.96 eV and 2.03 eV. The PLE of the D line in this energy range shows the indirect edge with re-absorption (negative peaks) at the P, R, S , and T lines. The PLE of the F line is dominated by an enhanced intensity (positive peak) of the P line. Figure 8 shows the MMPL spectrum of BiI3 (sample iii), recorded at 1.4 K, compared to the corresponding PL spectrum (excited with E > E,, = 2.0081 eV). The various emission lines in the MMPL spectrum are negative, indicating that microwave modulation has induced quenching of the luminescing processes. Extended scales of the MMPL and PL spectra of group I1 lines are shown in Figure 9. While the D line intensity is stronger than that of the F line in the CW-PL spectrum, their relative intensities are reversed in the MMPL spectrum. The F line is, therefore, strongly affected by the microwave radiation. The F line emission appears as a doublet in the CW-PL and the MMPL spectra; however, the high energy component of the doublet is more pronounced in the MMPL spectrum. The amplitude of the MMPL signals (AZP~ZPL)shows nearly linear dependence on the microwave (mw) power.
J. Phys. Chem., Vol. 99, No. 14, 1995 4897
BiI3 Layered Semiconductor
1.96
1.97
1.98
1.99
200 2.01
2.02
203
ENERGY (eV) Figure 7. PL excitation (PLE) of the D (curve a) and F (curve b)
emission lines.
I 1.89
?
0
Y
-
I
I
6
1.92 1.93 1.94 I! 5 ENERGY (eV) Figure 9. An extended scale of the MMPL spectrum of BiI3, at the energy region of group II. This region is compared in the figure with the corresponding PL spectrum.
v
- -
I
1.90
1.91
MMPL
1.90 1.95 2.00 2.05 ENERGY (eV) Figure 8. The MMPL spectrum of BiI3 (sample iv), recorded at 1.4 K and excited with energy =- pgx. The latter spectrum is compared with 1.80
1.85
its corresponding PL spectrum. Representative plots of AIPL/IPLversus mw power (in mW) are shown in Figure 10 for the T (H), D (A), and F (0)emission lines. The solid lines in the figure have been drawn as a guide. As seen from the figure, the AIpL/IpdE1) dependence of both T and D are similar while that of F is distinctly stronger.
IV. Discussion A. Group I Transitions. The emission lines of group I in the present work are identical with those reported in the p a ~ t . ' . ~ - ~ According to these previous studies, group I is associated with the indirect edge; i.e., the emission lines are blue-shifted with increasing temperature. This is in agreement with the blue shift of the indirect edge and in contrast to the red shift of the direct band edge. As mentioned in the Introduction, the P line is associated with polytypic disorder, while the R, S, and T are associated with trapped excitons. It has been previously ~ u g g e s t e d ~that . ~ . certain ~ crystals contain a perturbed stacking sequence, that these crystals show reversed stacking of the three I-Bi-I layers within a unit cell, and that a certain type of stacking fault (SF) occurs at the boundary between the normal and reverse stacking. Excitons are bound along these stacking faults, giving rise to the R, S, and T transitions. These excitons are fairly localized in the boundary region, exhibiting a small Bohr radius (-4.7 A) and narrow line width.8 The group I energy region in the PL spectrum (Figure 1) contains additional weak transitions associated with the single- or multiple-phonon
I 0
I
IO0
200
I
300
I 400
MW power (mW) Figure 10. A Z p l j l p ~versus E1 (microwave power in mW) for the T (m), D (A), and F (e)emission lines. The solid lines are drawn to
guide the eye. side bands of the indirect allowed exciton (L,) and the SF excitons (LO). The associated momentum conserving LO phonons, A, B and C, are reported to have the energies 2.7 meV, 7 meV, and 13.7 meV, r e s p e c t i ~ e l y . ~It~has ~ * been ~ suggested that SF defects may occur in 1/10000 of the layers," thus emphasizing their localization. The following section focuses on the influence of thermal and microwave modulations on group I recombination emission. The TMPL spectra of Bi13 single crystals were obtained under heat modulation of 5 Hz. Such a slow response indicates that modulation is associated with the heating of the entire lattice. As indicated in section 111, most lines in the TMPL spectrum of BiI3 resemble the derivative of the PL curve. This is due to the fact that, when heated, the bands undergo an energy shift. The lines of group I, however, deviate from a derivative shape.
Lifshitz and Bykov
4898 J. Phys. Chem., Vol. 99, No. 14, 1995
The R and S lines appear as a positive signal in the TMPL spectrum, while the T line appears as a strong negative signal (Figure 5). The lack of a derivative-like curve in group I is due to the relatively weaker blue shift observed upon heating (Figure 3). Instead, the thermal modulation results in redistribution of the recombining carriers among the emitting states, transferring them from the T state into the R and S states. This process is in agreement with the previously observed efficient energy transfer between the S and R and the T emitting states.' The LO duplicates of group I appear as negative signals in the MMPL spectrum. These LO duplicates follow the behavior of the T transition, indicating their coupling to the latter exciton line. The MMPL spectra of the group I transitions were observed under modulation in the kHz range (Figure 8). This emphasizes the relatively short sample response time to microwave modulation as compared to the response to thermal effect. In addition, it may indicate that the microwave absorption results in the heating of free carriers, without raising the temperature of the whole lattice (nonequilibrium heating). As shown in Figure 8, the luminescence processes of the R, S, and T transitions were quenched upon microwave application. A representative dependence of this quenching on microwave power is shown in Figure 10 for the T line; identical relations have also been observed for the S and R transitions (not shown). Different microwave-induced mechanisms could in principle be considered to explain the observed PL quenching: (1) impact ionization; (2) heating effect; (3) Stark effect; (4) a change in the radiative lifetime; and (5) a change in cross section of localization of the exciton. Arguments 2-4 can be disregarded for the following reasons. The heating effect had been eliminated by exciting the samples at 1.4 K with relatively low laser intensity. In addition, thermal modulation showed an influence differing substantially from the microwave effect. A Stark field is able to affect the PL only by exceeding a few kV/cm, while in the MMPL experiments, the highest El field was about 100 V/cm. Karasawa et aL9 have shown that the lifetime of the SF excitons is '10 ns and does not vary in the temperature range between 1.4 and 20 K. If joule heating of the crystal had taken place upon microwave radiation, it could not have exceeded a temperature change of more than 4-5", therefore, eliminating the possibility of changes in the lifetime of recombination. In the traditional model of impact ionization, lo photoexcited free carriers gain sufficient energy from the microwave electric field to impact ionize localized excitons. Luminescence processes related to these localized centers are then quenched in the presence of competing recombination processes. The latter may include radiative band-to-impurity recombination, bandto-band recombination, or nonradiative processes. Occasionally the presence of impact ionization process is recognizable by the existence of a strongly superlinear dependence of AZPJZPL versus mw power. For example, the ALlpJlp~ avalanches as a function of applied mw power as a result of the impact ionization of excitons or shallow donors. It is important to note that a failure to observe an avalanche point does not preclude the existence of impact ionization,ll particularly when the ionization centers have low concentration. On the other hand, Ashkinadze et al.,'* Booth et al.,13Viohl,14 and Lax15have suggested that the microwave induced changes in the PL are due to the induced changes in the cross section of formation of an exciton. This cross section depends continuously on the inverse of the free electron temperature ( UTe), the latter being proportional to the microwave power (T, = P). Thus, the latter mechanism leads to linear decrease of AIPJIPLwith increasing microwave power.
Clearly there is no general agreement in the literature about the experimental distinction between impact ionization and cross section of formation processes. However, it is more likely that there is a minor contribution of the impact ionization process in the present case. This is due to the existence of a relatively large Rydberg energy of the stacking fault excitons (-180 meV, ref 16). Thus, ionization of these excitons requires that the temperature of the hot carriers be substantially higher than is viable under the experimental conditions. On the other hand, the linear variation of AZPJZPL versus mw power in the present case (Figure 10) suggests that the reduction of the cross section of formation induced by microwave irradiation is the major mechanism. Thus, the microwave radiation reduces the population of localized SF excitons, converting them into delocalized states. B. Group I1 Transitions. Group Il transitions may be divided into two subgroups. The D, E, and G lines are obviously related to each other (named below the D subgroup). This statement is based upon three factors: the resonance excitation of the PL (Figure 6, spectrum b), the PL excitation spectra (Figure 7b), and the identical blue shift observed upon heating (Figure 2). These observations indicate that the D subgroup is associated with the indirect band edge. The D line is the zerophonon (ZF) transition of this subgroup. E and G are coupled to D by B and A B C phonons, respectively. Thus, the D emission is associated with a state within the band gap and is coupled to the indirect edge by the LO phonons. Phonon coupling is also pronounced in the change of the full width at half-maximum (fwhm) of line D with temperature as shown in Figure 4 (W, experimental). The solid line is a theoretical curve generated from the following relation:17
+ +
This expression predicts the broadening of the line associated with a transition from a relatively deep level (according to the configuration coordinate model) due to coupling with a phonon mode (hv). Our simulation utilized a phonon mode of 7.8 meV, which is in close agreement with the B momentum conserving mode in Bi13 crystals. The F line constitutes the second subgroup of group 11, with a temperature dependence substantially different to the previous subgroup. The latter showed no shift of the peak position in the temperature range 1.4-65 K. This deviates both from the indirect and direct band edge temperature dependences, thus indicating strong localization of the F state within the band gap. The TMPL of the D and F subgroups also serves to emphasize their differences. The D subgroup appears as a derivative curve while the F line appears as a negative signal (Figure 5) without any energy shift from the corresponding PL peak position. The latter effect may be attributed to thermal detachment of carriers from the F state and their migration into nonradiative states (thermal quenching). The responses of the D and F lines under microwave radiation in the MMPL experiment differ in several ways. The F line appeared as a doublet with a stronger intensity quenching than that of the D line. In the case of both the D and F lines, quenching exhibited a linear dependence (Figure 10) on microwave power, suggesting a decrease of the cross section of localization, similar in manner to the linear dependence observed for the lines in group I (section 1V.A). The above paragraph cites several of the differences existing between the D and F subgroups. Surprisingly, though, as seen from Figure 1, their appearance is always correlated. The observations discussed above suggest that the D and F transitions
Bi13 Layered Semiconductor are both associated with at least one common localized state within the band gap. The D line may be associated with the band edge to the localized state transition, thus explaining its strong dependence on the indirect edge. In general, a blue shift of an indirect edge upon heating is due to a change in the Bi-I bond length, while a red shift can be observed due to phonon coupling. In BiI3 crystals a blue shift is observed with an increase in temperature, suggesting a change in bond length as the dominant effect. Moreover, the conduction band edge is affected more strongly by the latter change, as it originates from the Bi(p) orbital. On the basis of this argument, we suggest that the D transition is associated with a conduction band to acceptor (relatively deep state) transition. The F line may be associated with a donor-acceptor recombination transition between relatively localized states, weakly coupled to the band edge. Thus, the D and F transitions may have a common acceptor state. The participating donor and acceptor states may be stoichiometric or strain defects. It was shown in section I11 that the F line transition is enhanced by excitation of the P line. The latter is associated with a polytypic disorder which, being a long range defect, enables coupling with defect sites (Le., donor and acceptors) distributed uniformly within the lattice. The R, S, and T lines, on the other hand, originate from a stacking fault defect appearing at specific localized areas (about every 1/10 000 layer), and thus avoid interaction with long range defects, such as the F line’s donor and acceptor states. C. Group III. The temperature shift of this line also follows the indirect edge; it does, however, differ slightly from the D state. Therefore, we suggest that this group of lines is not associated with group 11. Nonetheless, this group may be excited the T energy excitation, thus indicating its correlation to the indirect band edge. In this group the zero phonon line represents the I transition, while the other lines (J, K, L) are the multiphonon duplicates. This relation is made apparent mainly by the resonance excitation presented in Figure 5 (spectrum c). Under nonresonance conditions, the intensity of the J replica is dominant due to a shift of the excited state potential minimum from its ground state equilibrium position.13 The group 111 transitions can be associated with transitions between the band edge and localized states within the band gap (i.e., impurity or defect state) or between shallow donor (acceptor) to deep acceptor (donor) states. In any event, one is a relatively deep state within the band gap and is independent from those states participating in the electronic transitions of group 11.
V. Summary The current project utilized continuous-wave PL (both at resonance and above band-gap excitation), PLE, TMPL, and
J. Phys. Chem., Vol. 99, No. 14, 1995 4899 MMPL in order to clarify the origins of the recombination processes in Bi13 single crystals. The different forms of recombination can be divided into three main groups. Group I is associated with the stacking fault excitons. The intensities of group 11 and group 111 are dependent on the sample quality, suggesting that they are associated with sample imperfections. Group 11contains two subgroup recombination emissions, more likely associated with band edge-acceptor and donor-acceptor transitions. The coexistence of these two subgroups may indicate a common deep state. Group 111 is associated either with a band edge to deep state transition or with shallow to deep state (donor-acceptor) emission. The deep and shallow states may be linked with stoichiometric or strain defects. Results indicate that groups I, 111, and D-subgroup-11 are associated with the indirect edge. Conversely, the F-subgroupI1 states are fairly localized and does not interact with the band edge.
Acknowledgment. This project was supported by the Israel Ministry of Science, Contract No. 4178-1-93, and by the Israel Science Foundation, Contract No. 394194-1. References and Notes (1) Watanabe, K.; Karasawa, T.; Komatsu, T.; Kaifu, Y. J . Phys. SOC. Jpn. 1986, 55, 897. (2) Schlutter, M.; Kohn, S.; Cohen, M.; Fong, C. Phys. Starus Solidi 1976, B78. 137. (3) Kaifu, Y.; Komatsu, T.; Aikami,T. Nuovo Cimento 1977, 38B, 449. (4) Tatsumi, S.; Karasawa, T.; Komatsu, T.; Kaifu, Y. Solid State Commun. 1985, 54, 587. ( 5 ) Curtis, B. J.; Brunner, H. R. Mat. Res. Bull. 1974, 9, 715. (6) Parker, H.; Hipps, K. W.; Francis, A. H. Chem. Phys. 1977, 23, 117. (7) Petroff, Y.; Yu, P. Y.; Shen, Y. R. Phys. Status Solidi 1974, 61, 419. (8) Kaifu, Y.; Komatsu, T.; Aikami,T. Nuovo Cimento 1977,38,449. (9) Tatsumi, S.; Karasawa, T.; Komatsu, T.; Kaifu, Y. Solid State Commun. 1985, 54, 587. (10) Wang, F. P.; Monemar, B.; Ahlstrom, M. Phys. Rev. B 1986, 39, 11195. (11) Delong, D. C.; Ohlsen, W. D.; Viohl, I.; Yin, X.; Taylor, P. C.; Sengupta, D.; Stillman, G. E.; Olson, J. M.; Harrison, W. A. Phys. Rev. B 1993,48, 5157. (12) Ashkinadze, B. M.; Bel’kov, V. V.; Krasinskaya, A. G. Sov. Phys. Semicond. 1990, 24, 555. (13) Booth, I. J.; Schwedtfeger, C. F. Solid State Commun. 1985, 55, 817. (14) Voihl, I. Ph.D. Thesis, University of Utah, 1991. (15) Lax,M. Phys. JETP 1976, 43, 359. (16) Komatsu, T.; Kaifu, Y. J. Phys. SOC.Jpn. 1976, 40, N 4. (17) Nick, C. C.; Schulman, J. H. In Solid State Physics; Seitz, F., Trunbull, D., Eds.; Academic Press: New York, 1957; Vol. 5 . JP940605F
