- Email: [email protected]
Oxygen-related deep centres in LEC grown GaAs crystals
0038-1098/9053.00+.00 Pergamon Press plc
Solid S t at e Communications, Vol. 73, No. 7, pp. 495-498, 1990. P ri n t ed in Great B r i t a i n .
OZYGBN-RBLATRD DBBP CBNTRBB I N
LBC GROWN GaAs CRYSTALS
A . V . M a z k o v , V . B . OsvenslLT, A . Y . PolTallcov a n d M.V. T i s l ~ i m Institute of Rare Metals, B. Tolmachevsky, 5, Moscow, 109017, USSR
E.M.OmolJanovsky UN Center for Science and Technology for Development, Plaza, New York, N.Y., 10017, USA (Received 15 December
I UN
1989 by V.M.Agranovich)
Deep levels spectra are measured in low and high-resistivity LEC grown GaAs crystals doped with oxygen. It is shown that the behaviour of midgap centres ELO is consistent with the assumption that they are either simple interstitial oxygen atoms or complexes of these atoms with stoichiometry independent native defects like divacancy V~aVAR. The possibility of growing new type of semi-insul~%fS~ GaAs compensated by ELO centres instead of ELa centres is discussed.
1.INTRODUCTION
works [7,8]. The level position estimate of V.~,_-O complexes in [8] seems rather ~ i g u o s and it is not quite clear which of the variety of traps detected in GaAs could be a proper candidate. We tried to address these issues in the present paper.
OXYGEN is a technologically important impurity in GaAs since it is often present in crystal growth environment (quartz crucible, B203 encapsulant, etc) and besides, because in many cases it is intentlonally added into the melt to reduce crystal contamination with residual impurities (silicon, carbon, etc) [1-3]. Hence a detailed knowledge of oxygen properties is vital to achieve reproducible parameters of GaAs crystals. In the last few years a coherent picture of oxygen behaviour in GaAs gradually started to emerge. An oxygen related midgap centre situated very close to the famous EL2 trap [4] was detected in oxygen implanted [5] or oxygen doped crystals [3] (a so called ELO centre). The model of such centre includlng interstitial oxygen and VGaVAs divacancy was proposed by Wager and Van Vechten [6]. Oxygen lattice position was studied by LVM spectroscopy in a number of works [7,8]. It was shown that oxygen may exist in GaAs in two fo_r~s: interstitial oxygen [7-11] (840 cm -~ LVM band} and V~,-O complexes [8~ with which two LVM ~ n @ s at 715 cm -x [7-10] and 730 cm - ~ [8-10] are associated. A donor trap with a level at around Ec-0.4 eV was attributed to the VA.-O complexes [8]. HoWever, a number of questions still remain. For instance it is not clear where ELO centre fits into the model proposed in recent LVM spectroscopy
2.EXPERIMENTAL Our experiments were carried out on low dislocation density GaAs(In) crystals which are thought to be very promissing as s~bstrates in GaAs LSI applications [12]. The crystals were grown by LEC technique under low (0.7 atm) Ar pressure using quartz crucibles. The inltlal polycrystalllne GaAs was presyntheslzed either in HB process or in cruclble under high Ar pressure. Some details of the crystals history are presented in table 1. The polycrysta11Ine charge composition was close to stoichiometry for all crystals except crystal V (table 1). This latter was grown from the charge enriched by gallium (48.3 at.% As). Indium was introduced via InAs addition so as to obtain In cq~cent~ations in the range (0.8-1.0)'10 ~u cm -= in the seed end of the crystal. Oxygen doping was carried out by adding Ga20 ~ [2,3] into the charge in concen£ra~ions 8.3"10 -3 0.25 % wt. Electrlcal characterization of the samples was performed by conventional conductivity and Hall effect measurements at 300 K. Residual impurities were controlled by spark 495
496
0XYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
Vol. 73, No. 7
Table i Some p a r a m e t e r s of i n v e s t i g a t e d GaAs cr'Tstals !
!
Sample position in the boule (in fraction solidified)
In contents, cm -~
I
0.17
1.2 10 =°
II
0.01
III
0.03
1.0 I0 ~°
8.3"10 -~
0.58
IV
0.10
i.I-10 m°
0.25
0.48
V
0.03
1.0"1020
0.24
5.5
VI-I
0.10
8.8 10 ~9
0.25
1.0"107
VI-2
0.32
1.1"1020
0.25
2.7"108
VII
0.05
8.3"10 ~
0.i0
3.5 I0 v
VIII-i
0.04
1.0-10 ~°
0.10
1.1-106
VIII-2
0.11
1.1"1020
0.10
6.2"107
VIII-3
0.25
1,3"1020
0.I0
~3.1-i07
Sample No
-
Ga203 concentration in the charge % wt
Resistivity at 300 K, Ohm-cm
0.22 8.3
10 -~
1.2
* p - t y p e crystal s o u r c e m a s s s p e c t r o m e t r y SSMS (Si, S, O etc) or by LVM s p e c t r o s c o p y (carbon). Typical Si c o n c e n t r a t i o n s w i t h o u t Ga20, o p i n g ranged from 5.0 to 1 0 . 0 " 1 0 1 5 cm a , typical C c o n c e w ~ r a t i q n s w e r e in the range (0.7-1.4)'I0 x~ cm -J (sample I in table i). Oxygen addition even in c o n c e n t r a t i o n s as low as 8.3"10 -3 % wt. led to v e r y serious r e d u c t i o n of Si and C c o n t a m i n a t i o n in u n d o p e d GaAs ( ~ m p l e ~I, Si c o n c e n t r a t i o n around 5"10 *~ cm~, c a r b o n c o ~ e n t r a t 3 i o n b e l o w d e t e c t i o n limit <7"10 cm- ). However, this p u r i f i c a t i o n effect is less p r o n o u n c e d in the case of In doping and m u c h higher oxygen concentrations are necessary to achieve the same low c o n c e n t r a t i o n s of Si and C. In b o t h cases of u n d o p e d and In doped GaAs a d d i t i o n of oxygen did not s e r i o u s l y i n f l u e n c e the c o n c e n t r a t i o n of sulfur d o n o r s ((0.5-2.0)'10 ~ cm-~). T h e r e f o r e the g r o w n c r y s t a l s w e r e c o n d u c t i v e and had r e s i s t i v i t y in the range 0.I-i0.0 O h m ' c m (crystals I-V in table I). To render the crystals semi-insulating carbon was intentionally introduced into t h e m by i n s e r t i n g a g r a p h i t e ring into the e n c a p s u l a n t [13] (crystals VIVIII in t a b l e I). O t h e r w i s e the g r o w t h conditions for semi-insulating and c o n d u c t i v e samples w e r e quite s i m i l a r and w e h a v e strong reason to b e l i e v e t h a t t h e i r d e e p level spectra w e r e not s e r i o u s l y a l t e r e d by t r a n s i t i o n from low to h l g h - r e s l s t l v i t y state. These spectra in conductive samples were obtained by DLTS technique [14].
~
E x p e r i m e n t a l set-up was similar to that described in [3]. That is, in each p o i n t of the t e m p e r a t u r e scan the w h o l e capacitance relaxation curve was measured and stored into the c o n t r o l l i n g computer. C o n v e n t i o n a l DLTS spectra w i t h d i f f e r e n t t I and t~ w i n d o w p o s i t i o n s w e r e d i s p l a y e d in rial time m o d e d u r i n g the t e m p e r a t u r e scan, w h i l e a f t e r the end of the t e m p e r a t u r e scan individual relaxation curves were analyzed in "suspicious" temperature regions. In our case such analysis was c a r r i e d out for the p e a k around 400 K w h i c h is u s u a l l y a t t r i b u t e d to EL2 trap [4]. In a fair a g r e e m e n t with the results of [3] we o b s e r v e d a s y s t e m a t i c shift of this peak t e m p e r a t u r e to lower values as oxygen contents in the crystal increased. D e c o n v o l u t i o n of the r e l a x a t i o n curves r e v e a l e d the p r e s e n c e of two traps w i t h ionization energy 0 ~ eV and capture cross s~ction 1.0"I0 -±J cm z and 4.0"10 -lj cm z respectively. This p a r a m e t e r s are v e r y close to t ~ p a r a m e t e r s of EL2 (0.81 eV, l.~il0-~ = cm ) and ELO (0.83 eV, 4.8"10- ~ J cm z ) traps q u o t e d in [3].This n o t a t i o n is a d o p t e d in d e s c r i p t i o n of e x p e r i m e n t a l spectra p r e s e n t e d in table 2. O t h e r traps are d e s i g n a t e d a c c o r d i n g to the commonly accepted nomenclature proposed by M a r t i n et al [15]. Deep levels spectra in highr e s i s t i v i t y samples were m e a s u r e d by PERS (other n o t a t i o n s OTCS [16], PITS [17]) w h i c h was d e s c r i b e d in detail in our w o r k [18]. It was shown that u n d e r
73~
Vol.
No.
7
Deep
levels d e t e c t e d Concentration
Sample No
ELI4
EL8
1.0-1014
II
III
497
OXYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
_
Table 2 in l o w - r e s i s t i v i t y GaAs
of the centres,
EL2+ELO ~
EL3
EL6
cm -3 ELO
EL2
1.0-i015
1.9-1014
4.0-1015 (381)
3.2-1015
8.0-1014
7.2-1014
1.9-1015
4.0-1014
9.0-1015 (377)
6.3-1015
2.7-10 x5
9.5-1014
4.2-1015
1.8-1014
6.2-1015 (378)
4.4-1015
1.8-1015
IV
7.6-1014
1.2-1015
4.1-1015
2.3-1014
1.0-1016 (369)
5.0-1015
5.0-1015
V
5.8-1013
1.2-1014
3.5-1013
3.5-10 ~3
3.4-1015 (364)
3.0-1014
3.1-1015
*
in
)arenthesis
the DLTS p e a k t e m p e r a t u r e
certain conditions the c o n c e n t r a t i o n s of the t r a p s can be o b t a i n e d and h o l e and e l e c t r o n t r a p s can be d i s c r i m i n a t e d in PERS [18]. However, the c o m p a r i s o n of the m e a s u r e m e n t s by PERS and DLTS is not quite straightforward because capture cross sections and even i o n i z a t i o n e n e r g i e s of the t r a p s m a y r e v e a l any o x y g e n - r e l a t e d LVM b a n d s in our samples. Experiments with higher s e n s i t i v i t y and h i g h e r r e s o l u t i o n are in p r o g r e s s c u r r e n t l y in our l a b o r a t o r y but u nt i l t h e y are f i n i s h e d w e are left with the only possibility to use a l r e a d y p u b l i s h e d d a t a . The a u t h o r s of [7] i d e n t i f i e d 840 cm -~ LVM b a n d w i t h i n t e r s t i t i a l oxygen. T h e y w e r e not able to o b s e r v e ELO t r a p s in t h e i r s a m p l e s which could simply mean that the concen~atio~ of such t r a p s was in the low i0 ~ cm -~ range [7]. This does not altogether contradict the estimated c o n c e n t r a t i o n of i n t e r s t i t i a l oxygen. A question remains whether LVM data c o m p l e t e l y rules out c o m p l e x i n g of this oxygen with VGaVAs divacancy which
Deep
explains fairly well the electronic properties of ELO [6]. M o r e clarity will of c o u r s e a p p e a r if we shall be able to o b s e r v e o x y g e n - r e l a t e d b a n d s in our e l e c t r i c a l l y a s s e s s e d samples. As for VAs-O complexes they were a s s o c i a t e d w i t h E_-0.4 eV e l e c t r o n trap t h a t g i v e s rise t~ 715 cm -I LVM b a n d i~ n e u t r a l c h a r g e state and to 730 cm -~ b a n d w h e n p o s i t i v e l y c h a r g e d [8]. If we look for such a trap in t a b l e 2 we can see t h a t the c o n c e n t r a t i o n of EL6 trap has the "right" i o n i z a t i o n e n e r g y and b e s i d e s seems to d e p e n d on the p r e s e n s e of oxygen. However, the s t o i c h i o m e t r y d e p e n d e n c e of EL6 is o p p o s i t e to w h a t expected for V A s - O complex. It c o u l d rather be VGa-O complex, but the q u e s t i o n r e m a l n s as to w h a t w o u l d be vibrational properties of such a complex. A g a i n ~ e t a i l e d o b s e r v a t i o n s on 715 and 730 cm -± LVM b a n d s i n t e n s i t i e s c o u l d be a final c o n f i r m a t i o n of the p r o p o s e d model. The r e s u l t s for h i g h - r e s i s t i v i t y s a m p l e s are s u m m a r i z e d in t a b l e 3. A
levels d e t e c t e d
Table 3 in h i g h - r e s i s t i v i t y GaAs
Concentration Sample No VI-i
E1 (EL8) -
of the centres,
cm -3
E2 (EL6)
E3 (EL3)
E4 (EL2)
E5 (ELO)
HI
8-1014
8-1014
3-1015
5-1015
-
3-1015
7-1015
-
VI-2 VII
is s h o w n
6-1014
3-1015
6-1015
5-10 I~
8-1014
VIII-i
5-1014
6-1014
2-1015
7-10 Is
5-1015
4-10 ~4
VIII-2
2-10 ~4
3-1014
I-i0 ~4
5-1015
1-1016
5-10 ~4
7-1015
6-10 ~4
VIII-3
-
498
OXYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
notable fact is the appearance in oxygen doped samples of the new E5 electron trap which was never observed in low oxygen contents samples (the deep level spectra of such samples have already been described in [20]). Besides, the concentration of this E5 trap does not vary considerably as stoichiometry of the samples is shifted towards Ga along the growing crystal (crystal VIII, samples 1-3; in the last portion of the crystal conductivity becomes p-type because of vanishing of EL2 together with relatlvely high concentration of acceptors). Hence, although the parameters of E5 trap differ from the parameters of ELO we think them to be the same centres and assign the difference to the difference in experimental conditions of DLTS and PERS measurements. 4.CONCLUSIONS Therefore least do not of ELO traps interstitial
we see that our results at contradict the assignment to VGa-VAs-O complexes or oxygen and EL6 traps to
Vol. 73, No. 7
VG--O complexes. Doping with oxygen ma~es it possible to obtain GaAs crystals with concentrati~%s o{6midga ~ ELO centres in the i0 -i0 cmrange. Hence we anticipate the possibility to grow "high-purity" (concentrations of residual impurity ~onors and acceptors less than 10 ~ cm) semi-insulatlng GaAs from Ga-rlch melts. The properties of such materials with low EL2 concentration will be governed by ELO centres compensation by native shallow acceptors GaA-. Finally we would 11ke to mention that we observed no qualitative changes in deep levels spectra that could be related to In doping. Some quantitative changes are explained by the shift of stoichiometry in GaAs(In) to the Gaside. &oknowledgements - It is a pleasure to thank ¥u.N. Bol'sheva for experimental assistance and M.A. If'in and D.V. Kormilitsyn for measurements of residual impurities concentrations. E.F. Astakhova's preparation of Au/GaAs Shottky diodes is gratefully acknowledged.
REFERENCES i. J.F. Woods & N.G. Ainslle, J. Appl. Phys. 43, 1469 (1963). 2. J.R. Oliver, R.D. Fairman, R.T. Chen & P.W. ¥u, Electron. Lett. 17, 839 (1981). 3. J. Lagowsky, D.G. Lin, T. Aoyama & H.C. Gatos, Appl. Phys. Lett. 44, 336 (1984). 4. S. Makram-Ebeid, P. Langlade & G.M. Martin, in Semi-Insulating III-V Materlals (Edited by D.C. Look & J.S. Blakemore) p. 184. Nantwich, Shlva Publishers (1984) 5. M. Taniguchi & T. Ikoma, J. Appl. Phys. 54, 6448 (1983) 6. J.F. Wager & .J.A. Van Vechten, J. Appl. Phys. 62, 4192 (1987) 7. J. Schneider, B. Dishler, H. Seelewlnd et al, Appl. Phys. Lett. 54, 1442 (1989) 8. H.Ch. Aft, Appl. Phys. Lett. 54, 1 4 4 5 (1989) 9. C. Song, W. Ge, D. Jiang & C. Hsu, Appl. Phys. Lett. 50, 1666 (1987) I0. X. Zhong, D. Jiang, W. Ge & C. Song, Appl. Phys. Lett. 52, 628 (1988) 11. Z.L. Akkerman, L.A. Borisova & A.F. Kravchenko, Sov. Phys. Semicond. 10, 590 (1976)
12. J.V. DILorenzo, A.S. Jordan, A.R. Von Nelda & P.O. O~Connor, in SemiInsulating III-V Materials (Edited by D.C. Look & J.S. Blakemore) p. 308. Nantwich, Shiva Publishers (1984) 13. Yu.N. Bol~sheva, M.A. Ii~in, A.V. Markov et al, High-Purlty Substances 2, 210 (1989) 14. D.V. Lang, J. Appl. Phys. 45, 3023 (1974) 15. G.M. Martin, A. Mitonneau & A. Mircea, Electron. Lett. 13, 191 (1977) 16. G.M. Martin & D. Bois, Electrochem. Soc. Proc. 78, 32 (1978) 17. J.K. Rhee, P.K. Bhattacharya & R.J. Koyama, J. Appl. Phys. 53, 3311 (1982) 18. E.M. Omeljanovsky, A.Ya. Polyakov, V.I. Raicbstein et al, in The Physics of semiconductors (Edited by G. Engstrom) p.1007. Singapore, World Scientific (1986) 19. R.G. Kremer, M.C. Arikan, J.C. Abele & J.C. Blakemore, J. Appl. Phys. 62, 2424 (1987) 20. A.V. Govorkov, A.V. Markov, E.M. OmelJanovsky et al, in Semi-Insulating III-V Materials (Edited by L. Ledebo) p.145. Brlstol, Adam Hilger (1988)
Solid S t at e Communications, Vol. 73, No. 7, pp. 495-498, 1990. P ri n t ed in Great B r i t a i n .
OZYGBN-RBLATRD DBBP CBNTRBB I N
LBC GROWN GaAs CRYSTALS
A . V . M a z k o v , V . B . OsvenslLT, A . Y . PolTallcov a n d M.V. T i s l ~ i m Institute of Rare Metals, B. Tolmachevsky, 5, Moscow, 109017, USSR
E.M.OmolJanovsky UN Center for Science and Technology for Development, Plaza, New York, N.Y., 10017, USA (Received 15 December
I UN
1989 by V.M.Agranovich)
Deep levels spectra are measured in low and high-resistivity LEC grown GaAs crystals doped with oxygen. It is shown that the behaviour of midgap centres ELO is consistent with the assumption that they are either simple interstitial oxygen atoms or complexes of these atoms with stoichiometry independent native defects like divacancy V~aVAR. The possibility of growing new type of semi-insul~%fS~ GaAs compensated by ELO centres instead of ELa centres is discussed.
1.INTRODUCTION
works [7,8]. The level position estimate of V.~,_-O complexes in [8] seems rather ~ i g u o s and it is not quite clear which of the variety of traps detected in GaAs could be a proper candidate. We tried to address these issues in the present paper.
OXYGEN is a technologically important impurity in GaAs since it is often present in crystal growth environment (quartz crucible, B203 encapsulant, etc) and besides, because in many cases it is intentlonally added into the melt to reduce crystal contamination with residual impurities (silicon, carbon, etc) [1-3]. Hence a detailed knowledge of oxygen properties is vital to achieve reproducible parameters of GaAs crystals. In the last few years a coherent picture of oxygen behaviour in GaAs gradually started to emerge. An oxygen related midgap centre situated very close to the famous EL2 trap [4] was detected in oxygen implanted [5] or oxygen doped crystals [3] (a so called ELO centre). The model of such centre includlng interstitial oxygen and VGaVAs divacancy was proposed by Wager and Van Vechten [6]. Oxygen lattice position was studied by LVM spectroscopy in a number of works [7,8]. It was shown that oxygen may exist in GaAs in two fo_r~s: interstitial oxygen [7-11] (840 cm -~ LVM band} and V~,-O complexes [8~ with which two LVM ~ n @ s at 715 cm -x [7-10] and 730 cm - ~ [8-10] are associated. A donor trap with a level at around Ec-0.4 eV was attributed to the VA.-O complexes [8]. HoWever, a number of questions still remain. For instance it is not clear where ELO centre fits into the model proposed in recent LVM spectroscopy
2.EXPERIMENTAL Our experiments were carried out on low dislocation density GaAs(In) crystals which are thought to be very promissing as s~bstrates in GaAs LSI applications [12]. The crystals were grown by LEC technique under low (0.7 atm) Ar pressure using quartz crucibles. The inltlal polycrystalllne GaAs was presyntheslzed either in HB process or in cruclble under high Ar pressure. Some details of the crystals history are presented in table 1. The polycrysta11Ine charge composition was close to stoichiometry for all crystals except crystal V (table 1). This latter was grown from the charge enriched by gallium (48.3 at.% As). Indium was introduced via InAs addition so as to obtain In cq~cent~ations in the range (0.8-1.0)'10 ~u cm -= in the seed end of the crystal. Oxygen doping was carried out by adding Ga20 ~ [2,3] into the charge in concen£ra~ions 8.3"10 -3 0.25 % wt. Electrlcal characterization of the samples was performed by conventional conductivity and Hall effect measurements at 300 K. Residual impurities were controlled by spark 495
496
0XYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
Vol. 73, No. 7
Table i Some p a r a m e t e r s of i n v e s t i g a t e d GaAs cr'Tstals !
!
Sample position in the boule (in fraction solidified)
In contents, cm -~
I
0.17
1.2 10 =°
II
0.01
III
0.03
1.0 I0 ~°
8.3"10 -~
0.58
IV
0.10
i.I-10 m°
0.25
0.48
V
0.03
1.0"1020
0.24
5.5
VI-I
0.10
8.8 10 ~9
0.25
1.0"107
VI-2
0.32
1.1"1020
0.25
2.7"108
VII
0.05
8.3"10 ~
0.i0
3.5 I0 v
VIII-i
0.04
1.0-10 ~°
0.10
1.1-106
VIII-2
0.11
1.1"1020
0.10
6.2"107
VIII-3
0.25
1,3"1020
0.I0
~3.1-i07
Sample No
-
Ga203 concentration in the charge % wt
Resistivity at 300 K, Ohm-cm
0.22 8.3
10 -~
1.2
* p - t y p e crystal s o u r c e m a s s s p e c t r o m e t r y SSMS (Si, S, O etc) or by LVM s p e c t r o s c o p y (carbon). Typical Si c o n c e n t r a t i o n s w i t h o u t Ga20, o p i n g ranged from 5.0 to 1 0 . 0 " 1 0 1 5 cm a , typical C c o n c e w ~ r a t i q n s w e r e in the range (0.7-1.4)'I0 x~ cm -J (sample I in table i). Oxygen addition even in c o n c e n t r a t i o n s as low as 8.3"10 -3 % wt. led to v e r y serious r e d u c t i o n of Si and C c o n t a m i n a t i o n in u n d o p e d GaAs ( ~ m p l e ~I, Si c o n c e n t r a t i o n around 5"10 *~ cm~, c a r b o n c o ~ e n t r a t 3 i o n b e l o w d e t e c t i o n limit <7"10 cm- ). However, this p u r i f i c a t i o n effect is less p r o n o u n c e d in the case of In doping and m u c h higher oxygen concentrations are necessary to achieve the same low c o n c e n t r a t i o n s of Si and C. In b o t h cases of u n d o p e d and In doped GaAs a d d i t i o n of oxygen did not s e r i o u s l y i n f l u e n c e the c o n c e n t r a t i o n of sulfur d o n o r s ((0.5-2.0)'10 ~ cm-~). T h e r e f o r e the g r o w n c r y s t a l s w e r e c o n d u c t i v e and had r e s i s t i v i t y in the range 0.I-i0.0 O h m ' c m (crystals I-V in table I). To render the crystals semi-insulating carbon was intentionally introduced into t h e m by i n s e r t i n g a g r a p h i t e ring into the e n c a p s u l a n t [13] (crystals VIVIII in t a b l e I). O t h e r w i s e the g r o w t h conditions for semi-insulating and c o n d u c t i v e samples w e r e quite s i m i l a r and w e h a v e strong reason to b e l i e v e t h a t t h e i r d e e p level spectra w e r e not s e r i o u s l y a l t e r e d by t r a n s i t i o n from low to h l g h - r e s l s t l v i t y state. These spectra in conductive samples were obtained by DLTS technique [14].
~
E x p e r i m e n t a l set-up was similar to that described in [3]. That is, in each p o i n t of the t e m p e r a t u r e scan the w h o l e capacitance relaxation curve was measured and stored into the c o n t r o l l i n g computer. C o n v e n t i o n a l DLTS spectra w i t h d i f f e r e n t t I and t~ w i n d o w p o s i t i o n s w e r e d i s p l a y e d in rial time m o d e d u r i n g the t e m p e r a t u r e scan, w h i l e a f t e r the end of the t e m p e r a t u r e scan individual relaxation curves were analyzed in "suspicious" temperature regions. In our case such analysis was c a r r i e d out for the p e a k around 400 K w h i c h is u s u a l l y a t t r i b u t e d to EL2 trap [4]. In a fair a g r e e m e n t with the results of [3] we o b s e r v e d a s y s t e m a t i c shift of this peak t e m p e r a t u r e to lower values as oxygen contents in the crystal increased. D e c o n v o l u t i o n of the r e l a x a t i o n curves r e v e a l e d the p r e s e n c e of two traps w i t h ionization energy 0 ~ eV and capture cross s~ction 1.0"I0 -±J cm z and 4.0"10 -lj cm z respectively. This p a r a m e t e r s are v e r y close to t ~ p a r a m e t e r s of EL2 (0.81 eV, l.~il0-~ = cm ) and ELO (0.83 eV, 4.8"10- ~ J cm z ) traps q u o t e d in [3].This n o t a t i o n is a d o p t e d in d e s c r i p t i o n of e x p e r i m e n t a l spectra p r e s e n t e d in table 2. O t h e r traps are d e s i g n a t e d a c c o r d i n g to the commonly accepted nomenclature proposed by M a r t i n et al [15]. Deep levels spectra in highr e s i s t i v i t y samples were m e a s u r e d by PERS (other n o t a t i o n s OTCS [16], PITS [17]) w h i c h was d e s c r i b e d in detail in our w o r k [18]. It was shown that u n d e r
73~
Vol.
No.
7
Deep
levels d e t e c t e d Concentration
Sample No
ELI4
EL8
1.0-1014
II
III
497
OXYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
_
Table 2 in l o w - r e s i s t i v i t y GaAs
of the centres,
EL2+ELO ~
EL3
EL6
cm -3 ELO
EL2
1.0-i015
1.9-1014
4.0-1015 (381)
3.2-1015
8.0-1014
7.2-1014
1.9-1015
4.0-1014
9.0-1015 (377)
6.3-1015
2.7-10 x5
9.5-1014
4.2-1015
1.8-1014
6.2-1015 (378)
4.4-1015
1.8-1015
IV
7.6-1014
1.2-1015
4.1-1015
2.3-1014
1.0-1016 (369)
5.0-1015
5.0-1015
V
5.8-1013
1.2-1014
3.5-1013
3.5-10 ~3
3.4-1015 (364)
3.0-1014
3.1-1015
*
in
)arenthesis
the DLTS p e a k t e m p e r a t u r e
certain conditions the c o n c e n t r a t i o n s of the t r a p s can be o b t a i n e d and h o l e and e l e c t r o n t r a p s can be d i s c r i m i n a t e d in PERS [18]. However, the c o m p a r i s o n of the m e a s u r e m e n t s by PERS and DLTS is not quite straightforward because capture cross sections and even i o n i z a t i o n e n e r g i e s of the t r a p s m a y r e v e a l any o x y g e n - r e l a t e d LVM b a n d s in our samples. Experiments with higher s e n s i t i v i t y and h i g h e r r e s o l u t i o n are in p r o g r e s s c u r r e n t l y in our l a b o r a t o r y but u nt i l t h e y are f i n i s h e d w e are left with the only possibility to use a l r e a d y p u b l i s h e d d a t a . The a u t h o r s of [7] i d e n t i f i e d 840 cm -~ LVM b a n d w i t h i n t e r s t i t i a l oxygen. T h e y w e r e not able to o b s e r v e ELO t r a p s in t h e i r s a m p l e s which could simply mean that the concen~atio~ of such t r a p s was in the low i0 ~ cm -~ range [7]. This does not altogether contradict the estimated c o n c e n t r a t i o n of i n t e r s t i t i a l oxygen. A question remains whether LVM data c o m p l e t e l y rules out c o m p l e x i n g of this oxygen with VGaVAs divacancy which
Deep
explains fairly well the electronic properties of ELO [6]. M o r e clarity will of c o u r s e a p p e a r if we shall be able to o b s e r v e o x y g e n - r e l a t e d b a n d s in our e l e c t r i c a l l y a s s e s s e d samples. As for VAs-O complexes they were a s s o c i a t e d w i t h E_-0.4 eV e l e c t r o n trap t h a t g i v e s rise t~ 715 cm -I LVM b a n d i~ n e u t r a l c h a r g e state and to 730 cm -~ b a n d w h e n p o s i t i v e l y c h a r g e d [8]. If we look for such a trap in t a b l e 2 we can see t h a t the c o n c e n t r a t i o n of EL6 trap has the "right" i o n i z a t i o n e n e r g y and b e s i d e s seems to d e p e n d on the p r e s e n s e of oxygen. However, the s t o i c h i o m e t r y d e p e n d e n c e of EL6 is o p p o s i t e to w h a t expected for V A s - O complex. It c o u l d rather be VGa-O complex, but the q u e s t i o n r e m a l n s as to w h a t w o u l d be vibrational properties of such a complex. A g a i n ~ e t a i l e d o b s e r v a t i o n s on 715 and 730 cm -± LVM b a n d s i n t e n s i t i e s c o u l d be a final c o n f i r m a t i o n of the p r o p o s e d model. The r e s u l t s for h i g h - r e s i s t i v i t y s a m p l e s are s u m m a r i z e d in t a b l e 3. A
levels d e t e c t e d
Table 3 in h i g h - r e s i s t i v i t y GaAs
Concentration Sample No VI-i
E1 (EL8) -
of the centres,
cm -3
E2 (EL6)
E3 (EL3)
E4 (EL2)
E5 (ELO)
HI
8-1014
8-1014
3-1015
5-1015
-
3-1015
7-1015
-
VI-2 VII
is s h o w n
6-1014
3-1015
6-1015
5-10 I~
8-1014
VIII-i
5-1014
6-1014
2-1015
7-10 Is
5-1015
4-10 ~4
VIII-2
2-10 ~4
3-1014
I-i0 ~4
5-1015
1-1016
5-10 ~4
7-1015
6-10 ~4
VIII-3
-
498
OXYGEN-RELATED DEEP CENTRES IN LEC GROWN GaAs CRYSTALS
notable fact is the appearance in oxygen doped samples of the new E5 electron trap which was never observed in low oxygen contents samples (the deep level spectra of such samples have already been described in [20]). Besides, the concentration of this E5 trap does not vary considerably as stoichiometry of the samples is shifted towards Ga along the growing crystal (crystal VIII, samples 1-3; in the last portion of the crystal conductivity becomes p-type because of vanishing of EL2 together with relatlvely high concentration of acceptors). Hence, although the parameters of E5 trap differ from the parameters of ELO we think them to be the same centres and assign the difference to the difference in experimental conditions of DLTS and PERS measurements. 4.CONCLUSIONS Therefore least do not of ELO traps interstitial
we see that our results at contradict the assignment to VGa-VAs-O complexes or oxygen and EL6 traps to
Vol. 73, No. 7
VG--O complexes. Doping with oxygen ma~es it possible to obtain GaAs crystals with concentrati~%s o{6midga ~ ELO centres in the i0 -i0 cmrange. Hence we anticipate the possibility to grow "high-purity" (concentrations of residual impurity ~onors and acceptors less than 10 ~ cm) semi-insulatlng GaAs from Ga-rlch melts. The properties of such materials with low EL2 concentration will be governed by ELO centres compensation by native shallow acceptors GaA-. Finally we would 11ke to mention that we observed no qualitative changes in deep levels spectra that could be related to In doping. Some quantitative changes are explained by the shift of stoichiometry in GaAs(In) to the Gaside. &oknowledgements - It is a pleasure to thank ¥u.N. Bol'sheva for experimental assistance and M.A. If'in and D.V. Kormilitsyn for measurements of residual impurities concentrations. E.F. Astakhova's preparation of Au/GaAs Shottky diodes is gratefully acknowledged.
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