- Email: [email protected]
Urbach energy parameter of flash evaporated amorphous gallium arsenide films
Journal of Non-Crystalline Solids 299–302 (2002) 328–332 www.elsevier.com/locate/jnoncrysol
Urbach energy parameter of flash evaporated amorphous gallium arsenide films J.H. Dias da Silva *, R.R. Campomanes Faculdade de Ci^ encias, Departamento de Fısica, Universidade Estadual Paulista, Cep 17033-360, Bauru SP, Brazil
Abstract The dependence of the optical absorption edge on the deposition crucible temperature is used to investigate the electronic states in As-rich a-GaAs flash evaporated films. The Urbach energy parameter, determined from photothermal deflection spectroscopy (PDS), presents large correlated variations with crucible temperature. The optical and electrical results are consistent with the As under coordinated sites being the more important defect in the material. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 73.61.Jc; 78.66.JG; 61.43.Dq
1. Introduction The improvement of the electronic properties of amorphous GaAs films in electronic devices has attracted considerable interest. Amorphous GaAs has already been used as antiguide layers in vertical cavity surface-emitting lasers [1], as buffer layers between Si(1 0 0) and GaAs epitaxial layers [2], and as host material for Er doping [3]. At present, the reduction of deep defect and shallow band-tail state densities is one of the major research goals in amorphous semiconductors with improved transport properties. Such improvements are related to a better understanding of the structure of the material, to the improvement of
*
Corresponding author. Tel.: +55-14 221 6084; fax: +55-14 221 6074. E-mail address: [email protected] (J.H. Dias da Silva).
the preparation methods [4,5], and also from the development of reliable methods for the determination of the density of states. The photothermal deflection spectroscopy (PDS) has attracted considerable attention over the last decades, because it permits the evaluation of small absorption coefficients and is a suitable monitoring tool for process optimization. The Urbach energy parameter (E0 ) can be determined from the exponential absorption edge, and is an indicative of disorder of the material. In the present study we analyze the variation of the Urbach energy parameter and of the dark conductivity with a variation in the evaporation crucible temperature in flash evaporated amorphous As-rich GaAs films. We analyze the existing correlation between the preparation parameter and the optical and electrical data using previous experimental results on the structure of the a-GaAs [6] and on theoretical models [7,8].
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 9 - 9
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
2. Experimental details The samples used in this investigation were deposited by the flash evaporation technique on Corning 7059 glass substrates. The crucible temperature was varied from sample to sample in the range Tcruc 1240–1900 °C. The powder used in the depositions was obtained from high purity (6N) small c-GaAs pieces. For all depositions, the mean powder grain size was 15 lm. The thicknesses of the samples were measured with the help of a profilometer and were in the range 400–1000 nm. The substrate temperature and powder feeding rates were kept constant at 25 °C and 0.65 g/ min for all results presented hereafter. The optical transmission data, obtained from a Lambda-9 Perkin–Elmer spectrophotometer in the vis- and near-infrared range, were processed to obtain the dependence of the index of refraction, the absorption coefficient and the optical gap (Eg Þ with Tan . Also, it is obtained the E04 parameter as being the photon energy corresponding to an absorption coefficient a ¼ 104 cm1 . Low absorption coefficient was obtained by PDS. From the analysis of PDS data we determined the Urbach energy parameter (E0 ), which is related to the disorder of the material. It is well known that the energy distribution of tail stales is associated with the Urbach energy E0 [4], or the characteristic energy of the exponential absorption edge. The optical absorption at the band edges (electronic tail-band transitions and conversely) and at the subgap region were obtained by matching the relative absorption spectra obtained by PDS to the absorption coefficient calculated from absolute optical transmittance measured in the near infrared and visible (NIR– VIS) spectral region. In order to get good confidence on the calculated a values, we have restricted the aðhmÞ values calculated from transmittance data to energies higher than the ones where ad > 0:1. In the region where ad 6 0:1, only the aðhmÞ values determined from PDS measurements were used. The upper limit for aðhmÞ calculated from transmittance values, was established considering absorbances ðlog 1=T Þ smaller than 3.0 even though the nominal limit of the equipment is 3.5.
329
The compositions of the film were determined by energy dispersive X-ray spectroscopy (EDS). The results indicated As excess in all the samples in the range of crucible temperatures studied. The Ga1x Asx films have x values in the 0.53–0.57 range. However we could obtain no clear relationship between x and Tcruc . This could be due to relatively narrow variation compared to the precision of our measurements, estimated to be around 1%.
3. Results Fig. 1 shows the absorption coefficient as function of the photon energy for samples grown using different crucible temperatures. A wide range of slopes of the absorption edge is observed. In the 0.6–1.1 eV range, it can be noted that with the increase of the crucible temperature there is a shift to lower a values and a gradual decrease of the slope of the spectra until the curve corresponding to Tcruc ¼ 1350 °C is reached. After that a slight
Fig. 1. Absorption coefficient vs photon energy for a-GaAs films prepared at different crucible temperatures. The different line types correspond to different crucible temperatures.
330
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
Fig. 2. Urbach energies and dark conductivity rRT at room temperature as a function of the crucible temperature of flash evaporated a-GaAs. The lines were included just as guide to the eye.
increase of both the a values and of the E0 parameter (Fig. 2) are observed with the increase of Tcruc . For comparison Fig. 2 also displays the dark conductivity at room temperature ðrRT ) as a function of the crucible temperature. The sample prepared at the lowest crucible temperature 1240 °C presents the slowest increase in the absorption edge and consequently is sup-
posed to have the highest degree of disorder. The samples prepared in the 1300–1440 °C have better defined and more abrupt absorption edge and consequent smaller E0 values (Fig. 2). The high Tcruc samples have E0 values slightly bigger, but closer to more resistive and less disordered samples. Analyzing the E0 behavior for the whole set of samples, one can notice that the lower temperature samples show rapidly increasing E0 values as the temperature decreases from 1350 °C values to lower temperatures. For Tcruc higher than 1400 °C we observe only a slight increase of E0 . This behavior is similar to the one presented for the logarithm of the dark conductivity. The optical gap E04 (Table 1) of most samples prepared at intermediate crucible temperature range ð1300 °C 6 Tcruc 6 1770 °CÞ lay in a small range (1.08–1.10), whereas outside this range significant changes occur: 0.72 eV at Tcruc ¼ 1240 °C and 1.17 at Tcruc ¼ 1900 °C. In spite of the relatively unchanged E04 in the intermediate crucible temperature samples we have at least two orders of magnitude change in the room temperature dark conductivity. The combination of these results indicates some significant Fermi energy shift. Determination of the sign of the Seebeck coefficient using hot spot measurements indicated that all samples in the measurable range are p-type at room temperature. The exception are the high resistivity samples in the
Table 1 Main characteristics of the a-GaAs sample series Tcruc (°C) 1240 1270 1300 1322 1350 1375 1380 1440 1500 1600 1690 1770 1900
CAs (%) 54.7 56.8 54.3 55.9 56.7 55.9 54.1 55.1 53.6 53.6 54.0 57.0 56.4
rRT ðX cmÞ1 1
1:9 10 4:4 103 3:4 104 4:3 104 3:2 105 – 5:0 106 1:7 105 1:2 105 2:6 105 9:3 105 5:0 105 –
E04 0:03 (eV)
E0 (meV)
0.72 0.88 1.07 1.09 1.11 1.09 1.10 1.09 1.09 1.06 1.09 1.06 1.17
350 20 242 20 162 15 – 118 10 124 10 120 10 135 15 157 15 147 15 – 165 15 150 15
Tcruc is the temperature of crucible during the deposition, CAs represents the arsenic concentration in the samples as determined from EDS, rRT represents the dc conductivity at room temperature, E04 is the energy corresponding to an absorption coefficient of 104 cm1 and E0 is the characteristic Urbach edge (as determined from PDS measurements).
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
1300–1440 °C Tcruc range, which were below the detection limit of our setup.
4. Discussion Analyzing the results presented in the previous section one can note that the absorption coefficient at small photon energies (Fig. 1), the Urbach energy parameter, the dark conductivity (Fig. 2) and the optical gap (Table 1) present a consistent variation with the preparation crucible temperature. On the other hand, no correlation is observed when we try to analyze the variation of the same parameters as a function of the determined As relative concentration in the samples. Although care should be taken because the uncertainty of the concentration measurement (1%) is not small compared to the range of variation of the As concentration (53–57%), we have observed that the variation with the crucible temperature is very clear while the correlation with the composition is not noticeable in our data. This evidences the strong influence of the crucible temperature in the optical, electrical and structural properties of the material. But one question arises from this: what could be the mechanism for these influences, considering that the film composition is approximately unchanged, being the film As-rich? We propose that all observed modifications are correlated with the structural arrangement of the constituent elements, and not with the composition variations. Experimental studies using the EXAFS technique [6] have concluded that there is a mixture of triplefold and fourfold co-ordinated As atoms in As-rich a-GaAs. Also, observing the results of the calculations proposed by O’Reilly and Roberston [7] using the tight-binding method, and a more recent quantum molecular dynamics calculation proposed by Fois et al. [8], we have noted that in gross features these studies have shown similar conclusions. Analyzing the As centers in more detail, the mentioned calculations [7,8] place the As-db as acceptor states near to the valence band top, and the As–C3 centers at lower energies, being resonance states inside the valence band. The last ones do not produce gap states. In
331
this way the reduction of the number of As-db, should produce a lowering of the gap state density and a tendency of the Fermi level to move to a position closer to the center of the gap. The proposed model fits in a good way the observed experimental data: we have observed that the samples prepared in both the low and high crucible temperature extreme present p-type conductivity and higher dark conductivity. The mentioned balance between the As related defects also fits well the behavior of the dark conductivity with the behavior of the Urbach parameter: in the more resistive samples, which are the ones that present smaller E0 , the dominating defects could be the As–C3 , placed inside the bands, while in the more conductive samples that present bigger E0 , the dominating center could be the As-db. A relevant point to consider in these As-rich films is the possible influence of As–As wrong bonds. Widely expected in the early papers on amorphous III–V materials [9–11], the existence of these bonds between like atoms has no clear experimental confirmation. In our case, Raman scattering experiments with the as grown samples have shown only the LO and TO GaAs scattering bands (Fig. 3). No band is observed around 200 cm1 where one As related mode is expected to occur. In this way we can discard the existence of pure As clusters in the material, but we cannot rule out the possibility of As–As wrong bonds as isolated defects. Although the a-GaAs produced in this work is As-rich (53–57 at.%), the observed values of E0 and E04 are similar to the values reported previously by other authors, including the ones observed for nearly stoichiometric material [6,12]. This is not in conflict with reported data since studies of highly unbalanced Ga1x Asx ðx > 0:55Þ have shown that the optical gaps do not change drastically. Baker et al. [6] attributed this to the similar optical absorption edge of the amorphous arsenic. The slightly unbalanced material presents small differences in E0 to the ones claimed to be strictly stoichiometric [6,12]. This is probably due to the tact that the defects related to the As atoms have energy inside the bands, or deep in the band tails, not affecting drastically the absorption edge.
332
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
sent a direct correlation with the evaporation crucible temperature of flash evaporated amorphous GaAs. We propose that the mechanism by which E0 and rRT are being influenced is mainly structural, since the composition of the samples is not affected within the experimental uncertainty of the EDS technique. The analysis of the optical and electrical data using the current literature suggest that the main influence of the evaporation crucible temperature is on the formation of under coordinated As sites in the amorphous GaAs. Acknowledgements The authors are grateful to Eng. A.C Costa, Dr F. Alvarez and Dr M. Bica de Moraes to NIR– VIS, PDS, and profilometer facilities, respectively, and Mrs W.B. da Costa for the help in sample depositions. We acknowledgment the financial support of Fapesp (Proc.97/06278-6, 98/12744-2) and ICTP (94-309RG). Fig. 3. The micro-Raman spectra and XRD spectra (inset) of the a-GaAs film grown at crucible temperature of 1370 °C.
For the hydrogenated material (a-GaAs:H) prepared by the rf sputtering technique [11,13] the observed values of E04 are generally higher (1.45 and 1.33 eV) and the values of E0 are generally lower (108–125 meV) than the ones observed here. These differences between the hydrogenated material and our data are not large. The relatively good structural characteristic of the material studied here is reinforced by the Raman spectra, where a well defined the separation of the TO and LO modes as is observed (Fig. 3). In the previous reports found in the literature for amorphous GaAs [14], a broad band is observed in the region of the two peaks. The separation of the modes is probably due to local ordering of the material, since it is not related to the presence of crystallites in the film, as confirmed by the difractogram in the inset of Fig. 3. 5. Conclusions We have observed that the Urbach energy parameter, and the dark electrical conductivity pre-
References [1] B.S. Yoo, H.Y. Chu, H.H. Park, H.G. Lee, J. Lee, IEEE J. Quantum Electron. 33 (10) (1997) 1794. [2] K.I. Choo, W.K. Choo, J.Y. Lee, S.C. Park, T. Nishinaga, J. Appl. Phys. 69 (1991) 237. [3] A.R. Zanatta, Appl. Phys. Lett. 75 (1999) 3279. [4] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University, Cambridge, 1991. [5] A. Gheorghiu, M.L. Theye, Philos. Mag. B 44 (2) (1981) 285. [6] S.H. Baker, M.I. Manssor, S.J. Gurman, S.C. Baliss, E.A. Davis, J. Non-Cryst. Solids 144 (1992) 63. [7] E.P. O’Reilly, J. Robertson, Phys. Rev. B 34 (1986) 8684. [8] E. Fois, A. Selloni, G. Pastore, Q.M. Zhang, R. Car, Phys. Rev. B 45 (1992) 13378. [9] J.D. Joannopoulos, M.L. Cohen, Phys. Rev. B 10 (1974) 1545. [10] N.J. Shevchik, J. Tejada, M. Cardona, Phys. Rev. B 9 (1974) 2727. [11] W. Paul, T.D. Moustakas, D.A. Anderson, E. Freeman, Proceedings of the 7th International Conference on Amorphous and Liquid Semiconductors, Edinburgh, 1977, p. 467. [12] M.L. Theye, A. Gheorghiu, K. Driss-Khodja, C. Boccara, J. Non-Cryst. Solids 77&78 (1985) 1293. [13] M.I. Manssor, E.A. Davis, J. Phys.: Condens. Mater. 2 (40) (1990) 8063. [14] R. Zallen, M. Holtz, A.E. Geissberger, R.A. Sadler, W. Paul, M.L. Theye, J. Non-Cryst. Solids 114 (1989) 795.
Urbach energy parameter of flash evaporated amorphous gallium arsenide films J.H. Dias da Silva *, R.R. Campomanes Faculdade de Ci^ encias, Departamento de Fısica, Universidade Estadual Paulista, Cep 17033-360, Bauru SP, Brazil
Abstract The dependence of the optical absorption edge on the deposition crucible temperature is used to investigate the electronic states in As-rich a-GaAs flash evaporated films. The Urbach energy parameter, determined from photothermal deflection spectroscopy (PDS), presents large correlated variations with crucible temperature. The optical and electrical results are consistent with the As under coordinated sites being the more important defect in the material. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 73.61.Jc; 78.66.JG; 61.43.Dq
1. Introduction The improvement of the electronic properties of amorphous GaAs films in electronic devices has attracted considerable interest. Amorphous GaAs has already been used as antiguide layers in vertical cavity surface-emitting lasers [1], as buffer layers between Si(1 0 0) and GaAs epitaxial layers [2], and as host material for Er doping [3]. At present, the reduction of deep defect and shallow band-tail state densities is one of the major research goals in amorphous semiconductors with improved transport properties. Such improvements are related to a better understanding of the structure of the material, to the improvement of
*
Corresponding author. Tel.: +55-14 221 6084; fax: +55-14 221 6074. E-mail address: [email protected] (J.H. Dias da Silva).
the preparation methods [4,5], and also from the development of reliable methods for the determination of the density of states. The photothermal deflection spectroscopy (PDS) has attracted considerable attention over the last decades, because it permits the evaluation of small absorption coefficients and is a suitable monitoring tool for process optimization. The Urbach energy parameter (E0 ) can be determined from the exponential absorption edge, and is an indicative of disorder of the material. In the present study we analyze the variation of the Urbach energy parameter and of the dark conductivity with a variation in the evaporation crucible temperature in flash evaporated amorphous As-rich GaAs films. We analyze the existing correlation between the preparation parameter and the optical and electrical data using previous experimental results on the structure of the a-GaAs [6] and on theoretical models [7,8].
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 9 - 9
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
2. Experimental details The samples used in this investigation were deposited by the flash evaporation technique on Corning 7059 glass substrates. The crucible temperature was varied from sample to sample in the range Tcruc 1240–1900 °C. The powder used in the depositions was obtained from high purity (6N) small c-GaAs pieces. For all depositions, the mean powder grain size was 15 lm. The thicknesses of the samples were measured with the help of a profilometer and were in the range 400–1000 nm. The substrate temperature and powder feeding rates were kept constant at 25 °C and 0.65 g/ min for all results presented hereafter. The optical transmission data, obtained from a Lambda-9 Perkin–Elmer spectrophotometer in the vis- and near-infrared range, were processed to obtain the dependence of the index of refraction, the absorption coefficient and the optical gap (Eg Þ with Tan . Also, it is obtained the E04 parameter as being the photon energy corresponding to an absorption coefficient a ¼ 104 cm1 . Low absorption coefficient was obtained by PDS. From the analysis of PDS data we determined the Urbach energy parameter (E0 ), which is related to the disorder of the material. It is well known that the energy distribution of tail stales is associated with the Urbach energy E0 [4], or the characteristic energy of the exponential absorption edge. The optical absorption at the band edges (electronic tail-band transitions and conversely) and at the subgap region were obtained by matching the relative absorption spectra obtained by PDS to the absorption coefficient calculated from absolute optical transmittance measured in the near infrared and visible (NIR– VIS) spectral region. In order to get good confidence on the calculated a values, we have restricted the aðhmÞ values calculated from transmittance data to energies higher than the ones where ad > 0:1. In the region where ad 6 0:1, only the aðhmÞ values determined from PDS measurements were used. The upper limit for aðhmÞ calculated from transmittance values, was established considering absorbances ðlog 1=T Þ smaller than 3.0 even though the nominal limit of the equipment is 3.5.
329
The compositions of the film were determined by energy dispersive X-ray spectroscopy (EDS). The results indicated As excess in all the samples in the range of crucible temperatures studied. The Ga1x Asx films have x values in the 0.53–0.57 range. However we could obtain no clear relationship between x and Tcruc . This could be due to relatively narrow variation compared to the precision of our measurements, estimated to be around 1%.
3. Results Fig. 1 shows the absorption coefficient as function of the photon energy for samples grown using different crucible temperatures. A wide range of slopes of the absorption edge is observed. In the 0.6–1.1 eV range, it can be noted that with the increase of the crucible temperature there is a shift to lower a values and a gradual decrease of the slope of the spectra until the curve corresponding to Tcruc ¼ 1350 °C is reached. After that a slight
Fig. 1. Absorption coefficient vs photon energy for a-GaAs films prepared at different crucible temperatures. The different line types correspond to different crucible temperatures.
330
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
Fig. 2. Urbach energies and dark conductivity rRT at room temperature as a function of the crucible temperature of flash evaporated a-GaAs. The lines were included just as guide to the eye.
increase of both the a values and of the E0 parameter (Fig. 2) are observed with the increase of Tcruc . For comparison Fig. 2 also displays the dark conductivity at room temperature ðrRT ) as a function of the crucible temperature. The sample prepared at the lowest crucible temperature 1240 °C presents the slowest increase in the absorption edge and consequently is sup-
posed to have the highest degree of disorder. The samples prepared in the 1300–1440 °C have better defined and more abrupt absorption edge and consequent smaller E0 values (Fig. 2). The high Tcruc samples have E0 values slightly bigger, but closer to more resistive and less disordered samples. Analyzing the E0 behavior for the whole set of samples, one can notice that the lower temperature samples show rapidly increasing E0 values as the temperature decreases from 1350 °C values to lower temperatures. For Tcruc higher than 1400 °C we observe only a slight increase of E0 . This behavior is similar to the one presented for the logarithm of the dark conductivity. The optical gap E04 (Table 1) of most samples prepared at intermediate crucible temperature range ð1300 °C 6 Tcruc 6 1770 °CÞ lay in a small range (1.08–1.10), whereas outside this range significant changes occur: 0.72 eV at Tcruc ¼ 1240 °C and 1.17 at Tcruc ¼ 1900 °C. In spite of the relatively unchanged E04 in the intermediate crucible temperature samples we have at least two orders of magnitude change in the room temperature dark conductivity. The combination of these results indicates some significant Fermi energy shift. Determination of the sign of the Seebeck coefficient using hot spot measurements indicated that all samples in the measurable range are p-type at room temperature. The exception are the high resistivity samples in the
Table 1 Main characteristics of the a-GaAs sample series Tcruc (°C) 1240 1270 1300 1322 1350 1375 1380 1440 1500 1600 1690 1770 1900
CAs (%) 54.7 56.8 54.3 55.9 56.7 55.9 54.1 55.1 53.6 53.6 54.0 57.0 56.4
rRT ðX cmÞ1 1
1:9 10 4:4 103 3:4 104 4:3 104 3:2 105 – 5:0 106 1:7 105 1:2 105 2:6 105 9:3 105 5:0 105 –
E04 0:03 (eV)
E0 (meV)
0.72 0.88 1.07 1.09 1.11 1.09 1.10 1.09 1.09 1.06 1.09 1.06 1.17
350 20 242 20 162 15 – 118 10 124 10 120 10 135 15 157 15 147 15 – 165 15 150 15
Tcruc is the temperature of crucible during the deposition, CAs represents the arsenic concentration in the samples as determined from EDS, rRT represents the dc conductivity at room temperature, E04 is the energy corresponding to an absorption coefficient of 104 cm1 and E0 is the characteristic Urbach edge (as determined from PDS measurements).
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
1300–1440 °C Tcruc range, which were below the detection limit of our setup.
4. Discussion Analyzing the results presented in the previous section one can note that the absorption coefficient at small photon energies (Fig. 1), the Urbach energy parameter, the dark conductivity (Fig. 2) and the optical gap (Table 1) present a consistent variation with the preparation crucible temperature. On the other hand, no correlation is observed when we try to analyze the variation of the same parameters as a function of the determined As relative concentration in the samples. Although care should be taken because the uncertainty of the concentration measurement (1%) is not small compared to the range of variation of the As concentration (53–57%), we have observed that the variation with the crucible temperature is very clear while the correlation with the composition is not noticeable in our data. This evidences the strong influence of the crucible temperature in the optical, electrical and structural properties of the material. But one question arises from this: what could be the mechanism for these influences, considering that the film composition is approximately unchanged, being the film As-rich? We propose that all observed modifications are correlated with the structural arrangement of the constituent elements, and not with the composition variations. Experimental studies using the EXAFS technique [6] have concluded that there is a mixture of triplefold and fourfold co-ordinated As atoms in As-rich a-GaAs. Also, observing the results of the calculations proposed by O’Reilly and Roberston [7] using the tight-binding method, and a more recent quantum molecular dynamics calculation proposed by Fois et al. [8], we have noted that in gross features these studies have shown similar conclusions. Analyzing the As centers in more detail, the mentioned calculations [7,8] place the As-db as acceptor states near to the valence band top, and the As–C3 centers at lower energies, being resonance states inside the valence band. The last ones do not produce gap states. In
331
this way the reduction of the number of As-db, should produce a lowering of the gap state density and a tendency of the Fermi level to move to a position closer to the center of the gap. The proposed model fits in a good way the observed experimental data: we have observed that the samples prepared in both the low and high crucible temperature extreme present p-type conductivity and higher dark conductivity. The mentioned balance between the As related defects also fits well the behavior of the dark conductivity with the behavior of the Urbach parameter: in the more resistive samples, which are the ones that present smaller E0 , the dominating defects could be the As–C3 , placed inside the bands, while in the more conductive samples that present bigger E0 , the dominating center could be the As-db. A relevant point to consider in these As-rich films is the possible influence of As–As wrong bonds. Widely expected in the early papers on amorphous III–V materials [9–11], the existence of these bonds between like atoms has no clear experimental confirmation. In our case, Raman scattering experiments with the as grown samples have shown only the LO and TO GaAs scattering bands (Fig. 3). No band is observed around 200 cm1 where one As related mode is expected to occur. In this way we can discard the existence of pure As clusters in the material, but we cannot rule out the possibility of As–As wrong bonds as isolated defects. Although the a-GaAs produced in this work is As-rich (53–57 at.%), the observed values of E0 and E04 are similar to the values reported previously by other authors, including the ones observed for nearly stoichiometric material [6,12]. This is not in conflict with reported data since studies of highly unbalanced Ga1x Asx ðx > 0:55Þ have shown that the optical gaps do not change drastically. Baker et al. [6] attributed this to the similar optical absorption edge of the amorphous arsenic. The slightly unbalanced material presents small differences in E0 to the ones claimed to be strictly stoichiometric [6,12]. This is probably due to the tact that the defects related to the As atoms have energy inside the bands, or deep in the band tails, not affecting drastically the absorption edge.
332
J.H. Dias da Silva, R.R. Campomanes / Journal of Non-Crystalline Solids 299–302 (2002) 328–332
sent a direct correlation with the evaporation crucible temperature of flash evaporated amorphous GaAs. We propose that the mechanism by which E0 and rRT are being influenced is mainly structural, since the composition of the samples is not affected within the experimental uncertainty of the EDS technique. The analysis of the optical and electrical data using the current literature suggest that the main influence of the evaporation crucible temperature is on the formation of under coordinated As sites in the amorphous GaAs. Acknowledgements The authors are grateful to Eng. A.C Costa, Dr F. Alvarez and Dr M. Bica de Moraes to NIR– VIS, PDS, and profilometer facilities, respectively, and Mrs W.B. da Costa for the help in sample depositions. We acknowledgment the financial support of Fapesp (Proc.97/06278-6, 98/12744-2) and ICTP (94-309RG). Fig. 3. The micro-Raman spectra and XRD spectra (inset) of the a-GaAs film grown at crucible temperature of 1370 °C.
For the hydrogenated material (a-GaAs:H) prepared by the rf sputtering technique [11,13] the observed values of E04 are generally higher (1.45 and 1.33 eV) and the values of E0 are generally lower (108–125 meV) than the ones observed here. These differences between the hydrogenated material and our data are not large. The relatively good structural characteristic of the material studied here is reinforced by the Raman spectra, where a well defined the separation of the TO and LO modes as is observed (Fig. 3). In the previous reports found in the literature for amorphous GaAs [14], a broad band is observed in the region of the two peaks. The separation of the modes is probably due to local ordering of the material, since it is not related to the presence of crystallites in the film, as confirmed by the difractogram in the inset of Fig. 3. 5. Conclusions We have observed that the Urbach energy parameter, and the dark electrical conductivity pre-
References [1] B.S. Yoo, H.Y. Chu, H.H. Park, H.G. Lee, J. Lee, IEEE J. Quantum Electron. 33 (10) (1997) 1794. [2] K.I. Choo, W.K. Choo, J.Y. Lee, S.C. Park, T. Nishinaga, J. Appl. Phys. 69 (1991) 237. [3] A.R. Zanatta, Appl. Phys. Lett. 75 (1999) 3279. [4] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University, Cambridge, 1991. [5] A. Gheorghiu, M.L. Theye, Philos. Mag. B 44 (2) (1981) 285. [6] S.H. Baker, M.I. Manssor, S.J. Gurman, S.C. Baliss, E.A. Davis, J. Non-Cryst. Solids 144 (1992) 63. [7] E.P. O’Reilly, J. Robertson, Phys. Rev. B 34 (1986) 8684. [8] E. Fois, A. Selloni, G. Pastore, Q.M. Zhang, R. Car, Phys. Rev. B 45 (1992) 13378. [9] J.D. Joannopoulos, M.L. Cohen, Phys. Rev. B 10 (1974) 1545. [10] N.J. Shevchik, J. Tejada, M. Cardona, Phys. Rev. B 9 (1974) 2727. [11] W. Paul, T.D. Moustakas, D.A. Anderson, E. Freeman, Proceedings of the 7th International Conference on Amorphous and Liquid Semiconductors, Edinburgh, 1977, p. 467. [12] M.L. Theye, A. Gheorghiu, K. Driss-Khodja, C. Boccara, J. Non-Cryst. Solids 77&78 (1985) 1293. [13] M.I. Manssor, E.A. Davis, J. Phys.: Condens. Mater. 2 (40) (1990) 8063. [14] R. Zallen, M. Holtz, A.E. Geissberger, R.A. Sadler, W. Paul, M.L. Theye, J. Non-Cryst. Solids 114 (1989) 795.