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Photo-induced current transient spectroscopy of Al0.48In0.52As semi-insulating layers grown on InP by molecular beam epitaxy
Materials Science and Engineering, B22 (1993) 93-96
93
Photo-induced current transient spectroscopy of Alo.48Ino.52As semi-insulating layers grown on InP by molecular beam epitaxy A. Kalboussi, G. Marrakchi*, A. Tabata and G. Guillot Laboratoire de Physique de la Mati~re (URA CNRS 358), Institut National des Sciences Appliqudes de Lyon, Bdtt 502, 20 avenue Albert Einstein, 69621 Villeurbanne COdex (France)
G. Halkias and K. Zekentes Foundation for Research and Technology, Hellas-IESL, 711I0 Heraklion, Crete (Greece)
A. Georgakilas and A. Cristou University of Maryland, CALCE Electronic Packaging Research Center, College Park, MD 20742 (USA)
Abstract Deep levels in undoped semi-insulating In0.szAIo.48Aslayers grown by molecular beam epitaxy on iron-doped InP have been studied by photoluminescence and photo-induced current transient spectroscopy. The effect of the growth temperature T~ in the range from 300 °C to 530 °C has been investigated. The results show that low T~ causes the material quality to deteriorate and leads to formation of a higher concentration of deep traps. It is shown that optimized material quality can be obtained for lnAIAs layers on InP substrates with Tg around 530 °C with sufficiently high resistivity, reduced trap density and good structural properties which is appropriate for fabrication of high electron mobility transistors.
1. Introduction
InP-based InGaAs/InAIAs high electron mobility transistors (HEMTs) are among the highest speed devices available today [1]. It has been shown that InAIAs alloy grown by molecular beam epitaxy (MBE) becomes semi-insulating in a wide range of growth conditions [2]. Thus the lattice-matched In0.52A10.48As (denoted by InAlAs hereafter) must be used as buffer layer in the InxGa t _xAs/InAIAs HEMT structures on InP. The electrical and structural properties of this alloy material affect strongly the performance of HEMTs. Traps present into the InAIAs buffer layer and/or at the interfaces can be the origin of the kink effect observed on the I - V characteristics of HEMTs [3]. However, if the trap defects are associated with the crystal defect as shown by Claverie et al. [4], these layers cannot be used for the development of high performance devices. Some contradictory results about MBE grown high quality InAlAs layers have been reported by several researchers [2, 5-10]. However, it is clearly established that, on using MBE, the growth temperature Tg deeply affects the material's quality. In
*Author to whom correspondence should be addressed.
this paper, we present the influence of the InP pregrowth thermal desorption temperature Ta- and of the growth temperature Tg on the optical and electrical properties of MBE grown InAIAs. The range of Tg investigated was 300-530 °C. This work is mainly focused on the study of deep levels responsible for the semi-insulating character of the InAlAs material by photo-induced current transient spectroscopy (PICTS) as a function of Tg. On combining photoluminescence (PL) and PICTS results, a strong dependence of the properties of InA1As layers on the growth temperature has been found.
2. Experimental details
The MBE growth was performed on (001) semiinsulating iron-doped InP substrates in the temperature range from 300 °C to 530 °C. Undoped InA1As layers of 1.5-2.0 p m thickness were grown for all the samples. The InAIAs was grown with composition around that required for lattice matching to InP (Xln= 0.52). The growth rate was 0.85 p m h- ~ and the As4/III beam flux equivalent pressure ratio was 26. The growth parameters of the five samples S1-$5 are given in Table 1. Elsevier Sequoia
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A. Kalho,ssi el aL
/
MBt: SI Alo4.Jn,.:As on Inl'
TABLE 1. Molecularbeam epitaxygrowth parameters and photoluminescenceresults at 5 K of semi-insulatinglnAlAs samples Sample S1 $2 $3 $4 $5
7~ (°C)
T-r (°C)
FWHM~ (meV)
lvt.
300 440 480 500 530
500 530 530 500 53()
25 28 23 33 15
1 5 × 10.* 4 × 1().~ 6 × 10-~ 7 x 104
x J,7
0.56 0.56 0.54 0.53 0.56
Pin/1~;~
(/~m)
Resistivity , x 10" f2 cm)
2 1.4 1.65 2 1.65
1~.20 1.30 0.025 4.95
d
2.81 2.76 2.76 2.81 2.95
~'FWHM,full width at half-maximum.
The PL spectra have been measured at 5 K using argon laser excitation and classical detection techniques by a germanium liquid nitrogen cooled detector and lock-in treatment. Structural characterizations of the same layers have been made by double-crystal X-ray diffraction (DCXD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and have been reported in detail elsewhere [11, 12]. Details on the PICTS theory and the experimental arrangements for digital PICTS measurements used in this work have reported elsewhere [13, 14]. Only the experimental conditions will be given here. The ohmic contacts have been made by evaporating 1500 A semitransparent indium layers followed by a rapid thermal annealing at 320 °C for 20 s. Light pulses (duration, 100 ms; repetition rate, 1 Hz) provided by an He-Ne laser (1.96 eV) were used as an excitation source and focused on the sample. The photocurrent and photoinduced transient current were measured through a sampling resistor and amplified with a high speed preamplifier. The entire transient was recorded digitally at each temperature and stored on floppy disk. PICTS peaks occur in the spectrum when the emission rate of trapped carriers corresponds to the rate window set by the chosen values of t~ and t2. In the following experiments, the t2/tl ratio is kept at 10 and the sampling time t I was varied between 1 and 20 ms.
3. Results and discussion The growth temperature Tg was found to be a very critical parameter for the structural, optical and electrical properties of InAIAs. This is illustrated by Table 1 and Fig. 1 where the F W H M and the PL near-bandgap edge intensity at 5 K are given as a function of Tg. It is evident that on increasing the InAIAs growth temperature the optical quality of IrtAIAs is improved: the F W H M decreases (15 meV for $5) and the PL intensity increases ( 105 times higher signal intensity for $5 compared with S1). The F W H M value of 15 meV
10' ..., 104.
50 "40
103 102. m 10 ~
"30 20 ~
10° 10 200 300 400 500 600 Tg (°C) Fig. 1. Variations in the PL intensity and FWHM at 5 K for InAlAs samplesgrown at differenttemperatures.
appears to be indicative of a good material quality and defect-free layers [11, 12]. PL deep level bands are present in the energy range 1.2 eV-0.7 eV. The presence of deep level emission appears to be very sensitive to the growth temperature. There is a tendency to reduced deep level PL bands when Tg increases from 300 °C to 530 °C. A deep emission peaking at about 0.85 eV is always detected whatever Tg. In the $5 sample, this band is the only band detected in the deep energy emission range. It seems typical of an intrinsic defect of the InAIAs material situated at about 0.6 eV from the conduction or valence band. In fact PICTS results will reveal the presence of such deep level located at this energy. The influence of the InP substrate thermal cleaning temperature TT is less evident from the optical results but we can qualitatively state that there is a trend to improved material quality when T.r is increased (see Table 1 and variations in the F W H M with Tx ). These results are very well correlated with SEM, TEM and DCXD observations [11, 12]. The InAIAs surface morphology was found to be dramatically dependent on Tg and TT [15]. Very smooth surfaces have been obtained for Tg= TT = 530 °C and significant surface degradation has been observed either for TT= 500 °C or for Tg lower than 500 °C [15]. From all these characterization results, it appears that the PL F W H M values are mainly correlated with
A. Kalboussi et al.
/
the concentration of extended defects in the InA1As layers while the PL intensity and X-ray FWHM are mainly correlated with point defects and alloy clustering (due to the growth temperature). The sample grown with the optimum conditions ( Tg= TT = 530 °C) has a FWHM of 15 meV at 5 K which is very near the stateof-the-art value (10-12 meV from refs. 16 and 17) and presents a very strong signal intensity. Figure 2 shows PICTS spectra obtained from samples $1, $4 and $5. The $2 and $3 samples exhibit the same PICT spectra as sample $5. In sample $1, five levels labelled A, B, C, D and E with different amplitudes are detected at 0.19 eV, 0.36 eV, 0.56 eV, 0.6 eV and 0.82 eV respectively• In samples $2, $3 and $5 two levels labelled P~ and P2 a r e situated at 0.59 eV and 0.66 eV are detected, while $4 exhibits only one level named P and detected at 0.38 eV.
MBE SI Alo4flno s2As on InP
95
The apparent activation energies and capture crosssections corresponding to the PICTS peaks observed in all the samples and deduced from an Arrhenius plot of T2/en vs. 1/T are reported in Table 2. In order to have a qualitative interpretation of the PICTS peaks amplitude evolution, we have paid attention to keep the experimental conditions unchanged. In this case, we can consider that the variation in the PICTS peak amplitude is roughly proportional to the corresponding level concentration. Figure 3 shows the evolution of the PICTS peak amplitudes of the Pl and P2 traps detected in $2, $3 and $5 samples as a function of growth temperature Tg. It is clearly evidenced that the concentrations of these levels decrease when Tg increases and reach a minimum at 530 °C. In studied temperature range, we observe a clear correlation between the PL intensity and the concentration of point defects (Figs. 1 and 3), showing that they certainly act as non-radiative centres. The PL emission
SI(Tg = 300"C)
T A B L E 2. Apparent activation energies and capture crosssections of the deep levels detected by photo-induced current transient spectroscopy
E
-1 <
100
200
300
Sample
Tg (°C)
Trap label
Activation energy (eV)
Capture crosssection (cm 2)
S1
300
A B C D E
0.19 0.36 0.56 0.60 0.82
3 x 1 0 -17 1 × 10 -L3 1 x 10-1~ 2 × 10-13 5 x 10 -14
$2
440
P1 P2
0.59 0.65
6 x 10 -14 7 × 1 0 -15
$3
480
P1 P2
0.58 0.67
2 × 10 -13 6 × 10 -14
$4
500
P
0.38
4 x 10-17
$5
530
P1 P2
0.59 0.66
7 x 1 0 -13 2 x l 0 -~5
400
TEMPERATURE (K)
S4(Tg
-g
=
500"C)
P Z
1
100
200
L 300
400
TEMPERATURE (K)
3
--or- P1 (0.59 eV)
SS(Tg = 530"C)
2" ,d Z
P2
o
t 1"
.. i 100
200
300
400
TEMPERATURE (K)
Fig. 2. PICTS spectra of samples $5, $4 and S1 grown respectively at 530 °C, 500 °C and 300 °C.
0 420
460 500 G r o w t h t e m p e r a t u r e (°C)
540
Fig. 3. Evolution of the PICTS signal for P~ and P2 traps as a function of the growth temperature.
96
A. Kalbous~i et al.
/
band detected at 0.85 eV in all the studied samples can be correlated with the deep level observed by PICTS at 0.59 eV in all the samples except for the sample $4. In this case, the amplitude of the 0.38 eV PICTS peak is higher than the total sum of the peak amplitudes in samples $3 and $5. Therefore, the 0.59 eV deep level is probably present in sample $4 with a much lower concentration than the 0.38 eV level and it cannot be clearly observed by PICTS. Several researchers [18, 19] have observed by deep level transient spectroscopy (DLTS) in M B E n-type silicon doped InAlAs a level having the same parameters as the level which we observed by PICTS at 0.59 eV in u n d o p e d InA1As samples. Consequently, this deep level is an electron trap related to a native defect. T h e appearance of the 0.38 eV level and the disappearance of the 0.66 eV level in the sample $4 is correlated with the increase in the F W H M in this sample and can be explained by the decrease in the InP substrate thermal cleaning temperature ( T a-= 500 °C). T h e 0.66 eV level seems to be related to the InP substrate. Indeed, when the substrate temperature Tv decreases from 530 °C to 500 °C, the 0.66 eV level disappears (samples S1 and $4). This indicates that this level probably originates at the substrate-epilayer interface. H o n g et al. [19] have found by DLTS an electron trap at 0.71 eV labelled E A 4 and they suggested that this level is related to the I n A I A s - I n P interface. Taking into account the error estimated by H o n g et al. of 0.07 eV in the determination of activation energies and the deviation in the indium composition xjn, we can assume that the level detected in this work at 0.66 eV corresponds to the 0.71 eV level observed by H o n g et al. T h e C and E levels situated at 0.56 eV and 0.82 eV respectively in sample SI have also been detected in M B E n-type samples by DLTS [18, 19]. Thus, these levels are electron traps which are annihilated when Tg is higher than 300 °C, whereas the A level situated at 0.19 eV in sample S1 could be a hole trap because it is indetectable by DLTS measurements in n-type samples.
4. Conclusion Semi-insulating behaviour of u n d o p e d M B E InAlAs grown at Tg< 530 °C is investigated by the study of deep levels. We show that the trap concentrations increase when Tg decreases. For T g = 3 0 0 °C five defects are present in the material; however, only two defects remain for T g = 5 3 0 °C. PL results reveal a direct correlation between defect density and optical properties. Very good optical and structural properties are obtained with thermal treatment and a growth temperature both equal to 530 °C.
MBt: SI AI~)4Jn,,:,4s on InP
Acknowledgments This work has been supported by ESPRIT Basic Research 3086 Project. A. Tabata wishes to acknowledge the C A P E S (Brazil) for financial support.
References 1 E I. Ng, D. Pavlidis, Y. Kwon, T. Brock, J. I. Davies, G. Clarke and P. K. Rees, Proc. 4th Int. Conf. on InP and Related Materials, Newport, RL April 1992, IEEE, New York, 1992, p. 18. 2 G. J. Davies, T. Kerr, C. G. Tuppen, B. Wakefield and D. A. Andrews, J. Fac. Sei. Technol. B, 2(1984)219. 3 L. E Palmateer, P. J. Tasker, W. J. Schaff, L. D. Nguyen and L, F. Eastmann, AppL Phys. Lett., 54 ( 1991 ) 2139. 4 A. Claverie, K. M. Yu, W. Swider, Z. Lilienthal-Weber, M. O'Keefe, P. Kilaas, J. Pamulapati and P. K. Bhattacharya, AppL Phys. Lett., 60 (1992)989. 5 J. Massies, J. F. Rochette, P. Etienne, P. Delescluse, A. M. Huber and J. Chevrier, J. Cryst. Growth, 64 (1983) 101. 6 A. Chin, P. Battacharya, W. P. Hong and W. Q. Li, J. Fac. Sei. Technol. B, 6 (1988) 665. 7 A.S. Brown, M. J. Delaney and J. Singh, J. Vac. Sci. TechnoL B, 7(1989)384. 8 J.P. Praseuth, L. Golstein, P. Henoc, J. Primot and G. Danan, J. Appl. Phys., 61 (1987) 215. 9 J. E. Oh, P. K. Bhattacharya, Y. C. Chen, O. Aina and M, Mattingly, J. Electron. Mater., 19 ( 1990 ) 435. 10 E. Tourni6, Y. H. Zhang, N. J. Pulsford and K, Ploog, J. AppL Phys., 70(1991) 7362. 11 A. Georgakilas, A. Cristou, G. Halkias, K. Zekentes, N. Kornilias, C. Papavassiliou, A. Dimoulas, E Peiro, A. Cornet, S. Ababou, A. Tabata and G. Guillot, Proc. SOTAPOCS XVI, 181st Electrochemical Society Meet., St. Louis, MO, May 1992, Electrochemical Society, Pennington, NJ, to
be published. 12 A. Georgakilas, K. Zekentes, K. Tsagaraki, P. Lefebvre, J. Allegre and A. Christou, Proc. 4th Int. Conf. on InP and Related Materials, Newport, RI, April 1992, IEEE, New York, 1992, p. 101. 13 C. Hurtes, M. Boulou, A. Mitonneau and D. Bois, AppL Phys. Lett., 32(1978)821. 14 O. Yoshie and M. Kamihara, Jpn. J. Appl. Phys., 22 (1983) 621. 15 E Peiro, A. Cornet, J. R. Morante, A. Georgakilas and G. Halkias, Proc. SOTAPOCS XV1, 181st Electrochemical Society Meet., St Louis, MO, May 1992, Electrochemical Society, Pennington, NJ, to be published. 16 W. P. Hong, A. Chin, N. Debbar, J. Hinckley, E Bhattacharya, J. Singh and R. Clarke, J. Vac. Sci. Technol. B, 5 (1988) 800. 17 A. S. Brown, U. K. Mishra, J. A. Henige and M. J. Delaney, J. Vac. Sci. Technol. B, 6(1988)678. 18 E S. Whitney, W. Lee and C. G. Fonstad, J. Vac. Sci. TechnoL B, 5(1987)796. 19 W. P. Hong, S. Dhar, E K. Bhattacharya and A. Chin, J. Electron. Mater., 16 (1987)271.
93
Photo-induced current transient spectroscopy of Alo.48Ino.52As semi-insulating layers grown on InP by molecular beam epitaxy A. Kalboussi, G. Marrakchi*, A. Tabata and G. Guillot Laboratoire de Physique de la Mati~re (URA CNRS 358), Institut National des Sciences Appliqudes de Lyon, Bdtt 502, 20 avenue Albert Einstein, 69621 Villeurbanne COdex (France)
G. Halkias and K. Zekentes Foundation for Research and Technology, Hellas-IESL, 711I0 Heraklion, Crete (Greece)
A. Georgakilas and A. Cristou University of Maryland, CALCE Electronic Packaging Research Center, College Park, MD 20742 (USA)
Abstract Deep levels in undoped semi-insulating In0.szAIo.48Aslayers grown by molecular beam epitaxy on iron-doped InP have been studied by photoluminescence and photo-induced current transient spectroscopy. The effect of the growth temperature T~ in the range from 300 °C to 530 °C has been investigated. The results show that low T~ causes the material quality to deteriorate and leads to formation of a higher concentration of deep traps. It is shown that optimized material quality can be obtained for lnAIAs layers on InP substrates with Tg around 530 °C with sufficiently high resistivity, reduced trap density and good structural properties which is appropriate for fabrication of high electron mobility transistors.
1. Introduction
InP-based InGaAs/InAIAs high electron mobility transistors (HEMTs) are among the highest speed devices available today [1]. It has been shown that InAIAs alloy grown by molecular beam epitaxy (MBE) becomes semi-insulating in a wide range of growth conditions [2]. Thus the lattice-matched In0.52A10.48As (denoted by InAlAs hereafter) must be used as buffer layer in the InxGa t _xAs/InAIAs HEMT structures on InP. The electrical and structural properties of this alloy material affect strongly the performance of HEMTs. Traps present into the InAIAs buffer layer and/or at the interfaces can be the origin of the kink effect observed on the I - V characteristics of HEMTs [3]. However, if the trap defects are associated with the crystal defect as shown by Claverie et al. [4], these layers cannot be used for the development of high performance devices. Some contradictory results about MBE grown high quality InAlAs layers have been reported by several researchers [2, 5-10]. However, it is clearly established that, on using MBE, the growth temperature Tg deeply affects the material's quality. In
*Author to whom correspondence should be addressed.
this paper, we present the influence of the InP pregrowth thermal desorption temperature Ta- and of the growth temperature Tg on the optical and electrical properties of MBE grown InAIAs. The range of Tg investigated was 300-530 °C. This work is mainly focused on the study of deep levels responsible for the semi-insulating character of the InAlAs material by photo-induced current transient spectroscopy (PICTS) as a function of Tg. On combining photoluminescence (PL) and PICTS results, a strong dependence of the properties of InA1As layers on the growth temperature has been found.
2. Experimental details
The MBE growth was performed on (001) semiinsulating iron-doped InP substrates in the temperature range from 300 °C to 530 °C. Undoped InA1As layers of 1.5-2.0 p m thickness were grown for all the samples. The InAIAs was grown with composition around that required for lattice matching to InP (Xln= 0.52). The growth rate was 0.85 p m h- ~ and the As4/III beam flux equivalent pressure ratio was 26. The growth parameters of the five samples S1-$5 are given in Table 1. Elsevier Sequoia
94
A. Kalho,ssi el aL
/
MBt: SI Alo4.Jn,.:As on Inl'
TABLE 1. Molecularbeam epitaxygrowth parameters and photoluminescenceresults at 5 K of semi-insulatinglnAlAs samples Sample S1 $2 $3 $4 $5
7~ (°C)
T-r (°C)
FWHM~ (meV)
lvt.
300 440 480 500 530
500 530 530 500 53()
25 28 23 33 15
1 5 × 10.* 4 × 1().~ 6 × 10-~ 7 x 104
x J,7
0.56 0.56 0.54 0.53 0.56
Pin/1~;~
(/~m)
Resistivity , x 10" f2 cm)
2 1.4 1.65 2 1.65
1~.20 1.30 0.025 4.95
d
2.81 2.76 2.76 2.81 2.95
~'FWHM,full width at half-maximum.
The PL spectra have been measured at 5 K using argon laser excitation and classical detection techniques by a germanium liquid nitrogen cooled detector and lock-in treatment. Structural characterizations of the same layers have been made by double-crystal X-ray diffraction (DCXD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and have been reported in detail elsewhere [11, 12]. Details on the PICTS theory and the experimental arrangements for digital PICTS measurements used in this work have reported elsewhere [13, 14]. Only the experimental conditions will be given here. The ohmic contacts have been made by evaporating 1500 A semitransparent indium layers followed by a rapid thermal annealing at 320 °C for 20 s. Light pulses (duration, 100 ms; repetition rate, 1 Hz) provided by an He-Ne laser (1.96 eV) were used as an excitation source and focused on the sample. The photocurrent and photoinduced transient current were measured through a sampling resistor and amplified with a high speed preamplifier. The entire transient was recorded digitally at each temperature and stored on floppy disk. PICTS peaks occur in the spectrum when the emission rate of trapped carriers corresponds to the rate window set by the chosen values of t~ and t2. In the following experiments, the t2/tl ratio is kept at 10 and the sampling time t I was varied between 1 and 20 ms.
3. Results and discussion The growth temperature Tg was found to be a very critical parameter for the structural, optical and electrical properties of InAIAs. This is illustrated by Table 1 and Fig. 1 where the F W H M and the PL near-bandgap edge intensity at 5 K are given as a function of Tg. It is evident that on increasing the InAIAs growth temperature the optical quality of IrtAIAs is improved: the F W H M decreases (15 meV for $5) and the PL intensity increases ( 105 times higher signal intensity for $5 compared with S1). The F W H M value of 15 meV
10' ..., 104.
50 "40
103 102. m 10 ~
"30 20 ~
10° 10 200 300 400 500 600 Tg (°C) Fig. 1. Variations in the PL intensity and FWHM at 5 K for InAlAs samplesgrown at differenttemperatures.
appears to be indicative of a good material quality and defect-free layers [11, 12]. PL deep level bands are present in the energy range 1.2 eV-0.7 eV. The presence of deep level emission appears to be very sensitive to the growth temperature. There is a tendency to reduced deep level PL bands when Tg increases from 300 °C to 530 °C. A deep emission peaking at about 0.85 eV is always detected whatever Tg. In the $5 sample, this band is the only band detected in the deep energy emission range. It seems typical of an intrinsic defect of the InAIAs material situated at about 0.6 eV from the conduction or valence band. In fact PICTS results will reveal the presence of such deep level located at this energy. The influence of the InP substrate thermal cleaning temperature TT is less evident from the optical results but we can qualitatively state that there is a trend to improved material quality when T.r is increased (see Table 1 and variations in the F W H M with Tx ). These results are very well correlated with SEM, TEM and DCXD observations [11, 12]. The InAIAs surface morphology was found to be dramatically dependent on Tg and TT [15]. Very smooth surfaces have been obtained for Tg= TT = 530 °C and significant surface degradation has been observed either for TT= 500 °C or for Tg lower than 500 °C [15]. From all these characterization results, it appears that the PL F W H M values are mainly correlated with
A. Kalboussi et al.
/
the concentration of extended defects in the InA1As layers while the PL intensity and X-ray FWHM are mainly correlated with point defects and alloy clustering (due to the growth temperature). The sample grown with the optimum conditions ( Tg= TT = 530 °C) has a FWHM of 15 meV at 5 K which is very near the stateof-the-art value (10-12 meV from refs. 16 and 17) and presents a very strong signal intensity. Figure 2 shows PICTS spectra obtained from samples $1, $4 and $5. The $2 and $3 samples exhibit the same PICT spectra as sample $5. In sample $1, five levels labelled A, B, C, D and E with different amplitudes are detected at 0.19 eV, 0.36 eV, 0.56 eV, 0.6 eV and 0.82 eV respectively• In samples $2, $3 and $5 two levels labelled P~ and P2 a r e situated at 0.59 eV and 0.66 eV are detected, while $4 exhibits only one level named P and detected at 0.38 eV.
MBE SI Alo4flno s2As on InP
95
The apparent activation energies and capture crosssections corresponding to the PICTS peaks observed in all the samples and deduced from an Arrhenius plot of T2/en vs. 1/T are reported in Table 2. In order to have a qualitative interpretation of the PICTS peaks amplitude evolution, we have paid attention to keep the experimental conditions unchanged. In this case, we can consider that the variation in the PICTS peak amplitude is roughly proportional to the corresponding level concentration. Figure 3 shows the evolution of the PICTS peak amplitudes of the Pl and P2 traps detected in $2, $3 and $5 samples as a function of growth temperature Tg. It is clearly evidenced that the concentrations of these levels decrease when Tg increases and reach a minimum at 530 °C. In studied temperature range, we observe a clear correlation between the PL intensity and the concentration of point defects (Figs. 1 and 3), showing that they certainly act as non-radiative centres. The PL emission
SI(Tg = 300"C)
T A B L E 2. Apparent activation energies and capture crosssections of the deep levels detected by photo-induced current transient spectroscopy
E
-1 <
100
200
300
Sample
Tg (°C)
Trap label
Activation energy (eV)
Capture crosssection (cm 2)
S1
300
A B C D E
0.19 0.36 0.56 0.60 0.82
3 x 1 0 -17 1 × 10 -L3 1 x 10-1~ 2 × 10-13 5 x 10 -14
$2
440
P1 P2
0.59 0.65
6 x 10 -14 7 × 1 0 -15
$3
480
P1 P2
0.58 0.67
2 × 10 -13 6 × 10 -14
$4
500
P
0.38
4 x 10-17
$5
530
P1 P2
0.59 0.66
7 x 1 0 -13 2 x l 0 -~5
400
TEMPERATURE (K)
S4(Tg
-g
=
500"C)
P Z
1
100
200
L 300
400
TEMPERATURE (K)
3
--or- P1 (0.59 eV)
SS(Tg = 530"C)
2" ,d Z
P2
o
t 1"
.. i 100
200
300
400
TEMPERATURE (K)
Fig. 2. PICTS spectra of samples $5, $4 and S1 grown respectively at 530 °C, 500 °C and 300 °C.
0 420
460 500 G r o w t h t e m p e r a t u r e (°C)
540
Fig. 3. Evolution of the PICTS signal for P~ and P2 traps as a function of the growth temperature.
96
A. Kalbous~i et al.
/
band detected at 0.85 eV in all the studied samples can be correlated with the deep level observed by PICTS at 0.59 eV in all the samples except for the sample $4. In this case, the amplitude of the 0.38 eV PICTS peak is higher than the total sum of the peak amplitudes in samples $3 and $5. Therefore, the 0.59 eV deep level is probably present in sample $4 with a much lower concentration than the 0.38 eV level and it cannot be clearly observed by PICTS. Several researchers [18, 19] have observed by deep level transient spectroscopy (DLTS) in M B E n-type silicon doped InAlAs a level having the same parameters as the level which we observed by PICTS at 0.59 eV in u n d o p e d InA1As samples. Consequently, this deep level is an electron trap related to a native defect. T h e appearance of the 0.38 eV level and the disappearance of the 0.66 eV level in the sample $4 is correlated with the increase in the F W H M in this sample and can be explained by the decrease in the InP substrate thermal cleaning temperature ( T a-= 500 °C). T h e 0.66 eV level seems to be related to the InP substrate. Indeed, when the substrate temperature Tv decreases from 530 °C to 500 °C, the 0.66 eV level disappears (samples S1 and $4). This indicates that this level probably originates at the substrate-epilayer interface. H o n g et al. [19] have found by DLTS an electron trap at 0.71 eV labelled E A 4 and they suggested that this level is related to the I n A I A s - I n P interface. Taking into account the error estimated by H o n g et al. of 0.07 eV in the determination of activation energies and the deviation in the indium composition xjn, we can assume that the level detected in this work at 0.66 eV corresponds to the 0.71 eV level observed by H o n g et al. T h e C and E levels situated at 0.56 eV and 0.82 eV respectively in sample SI have also been detected in M B E n-type samples by DLTS [18, 19]. Thus, these levels are electron traps which are annihilated when Tg is higher than 300 °C, whereas the A level situated at 0.19 eV in sample S1 could be a hole trap because it is indetectable by DLTS measurements in n-type samples.
4. Conclusion Semi-insulating behaviour of u n d o p e d M B E InAlAs grown at Tg< 530 °C is investigated by the study of deep levels. We show that the trap concentrations increase when Tg decreases. For T g = 3 0 0 °C five defects are present in the material; however, only two defects remain for T g = 5 3 0 °C. PL results reveal a direct correlation between defect density and optical properties. Very good optical and structural properties are obtained with thermal treatment and a growth temperature both equal to 530 °C.
MBt: SI AI~)4Jn,,:,4s on InP
Acknowledgments This work has been supported by ESPRIT Basic Research 3086 Project. A. Tabata wishes to acknowledge the C A P E S (Brazil) for financial support.
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