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New investigation of III–V compounds by the low-angle mid-IR lightscattering technique
MATERIALS SCIENCE & ENGINEERING
g
ELSEVIER
Materials Science and Engineering B33 (1995) 103-114
New investigation of III-V compounds by the low-angle mid-IR lightscattering technique Vladimir A. Yuryev, Victor P. Kalinushkin Laboratory of Laser Defectoscopy of Semiconductors, General Physics Institute of the Russian Academy of Sciences, 38 Vavilov Street, V-333 GSP-1, Moscow 117942, Russia Received 1 August 1994; in revised form 20 January 1995
Abstract
The results of recent investigations of InP and GaAs single crystals by means of the low-angle mid-IR light-scattering technique are presented in this paper. Main attention is paid to the influence of some exposures to wafers, such as ion implantation, electron irradiation, vapor-phase epitaxy and annealing, on the properties of the large-scale accumulations of electrically active defects in crystal volume. The images of lnP an GaAs crystals obtained by scanning low-angle light-scattering microscopy are published for the first time.
Keywords: Gallium arsenide; Stoichiometry and homogeneity; Indium phosphide; Clusters
1. I n t r o d u c t i o n
This paper is devoted to new investigations of indium phosphide and gallium arsenide single crystals by means of the low-angle mid-IR-laser light-scattering technique (LALS) [1-3]. In previous studies [4-13] geometrical and electrophysical parameters of the large-scale accumulations of electrically active defects (LSDAs) in indium phospide [4-8] and gallium arsenide [8-13] were determined, the energy location of some electrically active centers composing LSDAs was estimated, and the model of inner structure of the L S D A was proposed, according to which LSDAs are spherical p-type regions enriched with In or Ga microinclusions and Inp or GaAs antisites in InP and GaAs, respectively [7,8]. Unfortunately, no technique was available that would enable us to visualize LSDAs, and evaluation of their parameters was made on the basis of other methods, for example, laser tomography. Such estimates were not very reliable, as one cannot be absolutely sure that objects registered by L A L S and by other methods were the same ones. Now a new technique of scanning laser microscopy, which is based on L A L S (SLALS), has been developed [14-17]. This new method, being founded on the same physical 0921-5107/95/$9.50 © 1995 -
SSD10921-5107(94)01184-6
Elsevier Science S.A. All rights reserved
principles as the standard LALS, allows examination of the same features as those observed by LALS, i.e. LSDAs. T h e first S L A L S pictures of LSDAs in InP and G a A s are presented in the current paper, and the L S D A parameters estimated on the basis of SLALS patterns are also presented. It is important to know for the practical goals connected with the fabrication of semiconductor devices, how technological processing of original materials affects the properties of LSDAs. In addition, investigation of the physical exposures to LSDAs helps in understanding their properties and nature. T h e major part of this paper is devoted to the investigation of such effects as ion implantation, vapor phase epitaxy (VPE), annealing and electron b o m b a r d m e n t on LSDAs in InP and G a A s single crystals. Let us introduce some designations for measured and estimated parameters of the scattered light and LSDAs, which are used in this paper. 0 is the light scattering angle in the crystal, W is the probe beam power in the crystal per unit beam cross-sectional area, I is the scattered light intensity in the crystal per unit solid angle per unit tested volume, I 0 = I( 0 = 0), C is the concentration of scatterers, and a is a scatterer radius. An m is the maximum effective concentration of free carriers in the accumulations (as in Refs. [7,8]),
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MaterialsScience and Engineering B33 (1995) 103-114
A n m = m c nm/m a -- no~ where mc, a represents respectively the effective masses of current carriers in the crystal bulk outside the scattering LSDAs (c) and inside them (a), n m is the maximum value of free carrier concentration inside the LSDAs (it corresponds to the maximum magnitude of the deviation of the dielectric constant I A e l - f ( ~ ) where ff is the coordinates in the accumulation), and noo is the free carrier concentration outside the LSDAs; the LSDAs and the crystal bulk free of them may have different conductance types, e.g. for InP and GaAs the model of p-type LSDAs in an n-type crystal matrix is under consideration [7,8], and in such a case n m=-pm means the concentration of holes. Pv is the amount of holes within the LSDAs per unit crystal volume, and AE i = E i - Ev is the location of the levels of the point center constituting LSDAs in the band gap with respect to the valence band. The ratio of / / W is measured in sr- 1 cm - 3.
The prototype of the SLALS microscope, which was made in our laboratory, is not perfect and requires engineering improvement. As it is shown in Refs. [15-16], it transfers, almost without distortion and light failure, the images of LSDAs with dimensions of about 20-50/~m. Smaller LSDAs can also be seen but their contrast is lower and their sizes are blurred with diffraction and aberrations. The images of larger LSDAs are suppressed with a "search-light" filter in the SLALS instrument. So we have supposed that the objects seen in the patterns must be attributed to the LSDAs with appropriate sizes determined from LALS diagrams. Further improvement of the instrument will allow one to obtain more precise data, but the principle goal of this work has now been reached: even the instrument in its present state has allowed us to obtain the images of LSDAs in different semiconductors, and particularly in indium-phosphide single crystals.
2. I n d i u m p h o s p h i d e
2.2. Dependence o f the size of L S D A s on their location along the longitudinal axis o f an InP : Fe ingot
2.1. S L A L S images and parameters o f L S D A s
As mentioned above, the technique of scanning LALS (SLALS) has been developed which allows the visualization of LSDAs in semiconductors [14-17]. In this section, the SLALS patterns of indium-phosphide single crystals are presented for the first time. Undoped InP and InP : Fe single crystals, the images of which are depicted in Figs. l(a) and l(b) and the LALS diagrams of which are plotted in Fig. l(c), were grown by the liquid-encapsulated Czochralski (LEC) process in quartz crucibles using B203 as an encapsulant. The free-electron concentration in the sample of undoped InP was 2 x 1016 cm -3, and the specific resistivity of I n P : F e was greater than 107 Q cm. The carrier mobility in both samples at ambient temperature was about 3 x 103 cm 2 V -1 s -1 . The dislocation density in the crystals was around 5 × 104 cm-2. The thicknesses of the samples were 0.5 mm for InP : Fe and 0.6 mm for InP. The bright white spots in the SLALS pictures are associated with LSDAs (2 × 2 mm 2 crystal areas are given). The LSDA concentrations are close to those accepted in Refs. [7,8]. The data on LSDAs in these samples and the estimates of hole concentrations in them made using SLALS data are.gathered in Table 1. It is seen that these data are close to those given in Refs. [7,8], so our former estimates appeared to be correct. We should say a few words about the concentrations of LSDAs given in Table 1. We have attributed all the spots seen in the SLALS patterns to the LSDAs with radii of 1 1 - 1 9 / t m . The reasons for this are as follows.
As it has previously been found in Refs. [7,8], the dominating LSDAs in InP : Fe have characteristic radii of about 9 pro. Fig. 2 presents the dependence of characteristic radii of dominating LSDAs a on the location of the tested wafer in a single crystalline ingot of InP : Fe. The left part of the figure (l = 0) is related to the ingot region which is close to the seed, whereas the right is associated with the near-tail region. The specific resistivity of the material in the seed was 1.02 x 107 Q cm, and that in the tail was 2.9 x 107 if2 cm. It is visible from Fig. 2, part 1, that the size of LSDAs grows with the distance from the seed and becomes constant at a distance of 1/3-1/2 of the ingot length. The size of the LSDAs are constant in the region where the ingot radius R is constant and change in those regions where R changes (Fig. 2, part 2), i.e., they become stable with stabilization of the ingot growth conditions. One can also see the gap in Fig. 2, part 1, (marked with an arrow) which corresponds to the waist in the ingot layout (Fig. 2, part 2). The example given shows clearly that the size of LSDAs in InP : Fe depends strongly on the thermodynamic conditions of crystal growth. 2.3. Influence o f A r + implantation on L S D A s in undoped InP
In the experiments on the influence of Ar + implantation on the light scattering, the samples of undoped indium phosphide crystals grown by the LEC process in the (100) crystallographic direction in quartz crucibles with B203 as an encapsulant under an argon
V.A. Yuryev, V.P. Kalinushkin
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Materials Science and Engineering B33 (1995) 103-114
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Fig. 1. LSDAs in indium phosphide: SIALS patterns for undoped InP (a) and InP : Fe (b) (2 x 2 ram2); and LALS diagrams (c) for InP (curves 1,2) and InP : Fe (curves 3,4,5). R o o m temperature.
Table 1 Parameters of LSDAs in indium phosphide No.
a (~m)
log(lo/W)
CAnrn2 (cm -9)
1
1 1-14 6-7 14-19 6-7
-
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(cm-3)
Pv (cm-3)
(1.5-2) × 10 5
(2.0-2.5) × 1017
(3-6) × 1014
--
--
--
( 2 - 3 ) x 10 5
( 2 - 5 ) × 1016
( 1 - 3 ) × 1014
--
--
--
No. is sample number: 1, undoped InP; 2, InP : Fe. a is the accumulation radius; log(Io/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-1 cm-3)); C A n t o 2 is a concentration parameter; C is the concentration of the LSDAs; A n m is the effective maximum differential carrier concentration in LSDAs; Pm is the maximum hole concentration inside accumulations; Pv is the total content of holes inside LSDAs per crystal unit volume.
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pressure of 60 atm were used as the initial wafers. T h e crystals had n-type conductance, the free carrier concentration in them at 300 K was (1-3) x 1016 c m -3, the carrier mobility was ( 3 - 5 ) x 103 cm 2 V -~ s ~, and their compensation degree was 0.2-0.3. T h e crystals differed sharply in dislocation densities, a part of them had a normal dislocation density (about 5 x 104 c m - 2 ) , the rest had an enhanced o n e ( 1 0 5 - 1 0 6 c m - 2 ) . T h e thickness of the former crystals was 0.5 mm, and that of the latter ones was 1.5 and 3 mm. T h e samples were parallel-sided plates cut from the ingot along the (100) plane. T h e implantation of A r + ions was made using a High Voltage Engineering E u r o p a accelerator. A r + energies were 200 and 250 keV, a dose was 10 ~6 cm -2. T h e implantation was carried out at r o o m temperature, the sample temperature did not exceed 100 °C. T h e studies were carried out in two stages. In the first stage, the as-grown samples were subjected to detailed investigation by LALS. T h e diagrams were measured at different points on each wafer with high density, each point was studied in a different orientation of the sample with respect to the polarization plane of the probe laser beam. T h e n the samples were subjected to ion implantation, and the samples with a thickness of 1.5 m m were half-covered with a mask (so that only a half of the surface of each sample was under the flux, the b o r d e r went through the center of the wafer side). One 3 m m thick sample stayed without exposure. After the implantation, the samples were investigated as thoroughly as before the exposure. T h e characteristic L A L S diagrams are plotted in Figs. 3(a)-3(c), and the parameters of LSDAs before
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Fig. 3. LALS diagrams for undoped indium phosphide before (curves 1,2) and after (curves 3,4,5,6,7) A r ÷ implantation; (curves 6,7) are under a mask. Dislocation densities are about 5 × 104 cm -2 (a) and 10-5-106 cm -2 (b,c); A r ÷ energies are 200 keV (a,b) and 250 keV (c); room temperature.
V.A. Yuryev, V.P. Kalinushkin
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Materials Science and Engineering B33 (1995) 103-114
107
Table 2 Parameters of LSDAs in undoped InP and their changes after implantation of Ar + ions Wafer parameters Thickness (mm)
Dislocation density (cm -2)
0.5
~- 5 × 104
Ar + energy (keV)
Wafer state
ct (/~m)
log(l0/ W )
200
Initial
13-18 7-10 5-6 20-23 7-9
-
1.6 1.9 2.1 0.4 1.1
36.0-37.2 37.1-37.7 38.2-38.8 36.6-36.8 38.3-38.5
12-14
- 2.1 to - 1.4
36.6-37.2
-~ 4 17-25 7-9 ~<5
- 2.8 to - 2.6 - 1.6 to - 0.2 - 2.4 to - 1.6 ~< - 2 . 5
38.9-39.2 36.4-36.8 37.6-38.0 />38.8
13-14
- 2.8 to - 2.4
35.6-36.2
~<4 15-21 11-12 -- 5 18-20 10-12 5
- 3.3 to - 1.5 to - 1.4 to - 2.6 to = - 2.1 - 2.6 to -~ - 2.8
= 39.3 36.7-37.0 37.5-37.9 38.4-39.0 35.5-35.7 36.5-37.3 = 38.2
After implantation
3
105-106
200
Initial After implantation
1.5
105-106
250
Initial After implantation After implantation under mask
1.7 to 2.7 to 2.9 to 0.8 to 1.5 to
-
-
log( CA n m2) (log(cm-9))
3.0 0.5 1.0 2.2
- 2.2
a is the accumulation radius; log(lo/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-1 cm-3)); CAnto2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs.
and after the i m p l a n t a t i o n are given in Table 2. G e n e r a l g r o w t h of the light scattering intensity is seen in all the samples as well as s o m e c h a n g e in the s h a p e of the scattering diagrams. It has b e e n f o u n d (see Table 2) that the size o f large L S D A s increases, but the effect of ion i m p l a n t a t i o n is manifested m o s t strongly f o r m e d i u m L S D A s with the sizes o f 7 - 1 2 /~m: they b e c o m e visible even in the samples with e n h a n c e d dislocation density, w h e r e they w e r e n o t o b s e r v e d b e f o r e the implantation, a n d their c o n c e n t r a t i o n p a r a m e t e r CA nm 2 g r o w s greatly in all the samples. T h e intensity of light scattering b y small L S D A s also grows (their p a r a m e t e r s c a n n o t be estimated precisely in the samples with n o r m a l dislocation density b e c a u s e o f a h u g e increase in L A L S intensity b y m e d i u m L S D A s ) . T h e p a r a m e t e r s o f L S D A s and the s h a p e o f L A L S diag r a m s for the samples o f I n P with e n h a n c e d dislocation density b e c a m e very close to those f o r n o r m a l InP. M o r e o v e r , the p a r a m e t e r s o f L S D A s c h a n g e d even in the regions of the samples c o v e r e d with masks. A similar b e h a v i o u r of scattering i n h o m o g e n e i t i e s was o b s e r v e d in Si crystals after P+ implantation [18]. T h e changes in L A L S diagrams and, as a rule, a g r o w t h
of L A L S intensity, including the regions c o v e r e d with masks w h e n e x p o s e d was registered in this case as well. C h a n g e s of scattering p r o p e r t i e s of the accumulations, in the o p i n i o n of the a u t h o r s o f Ref. [18], m a y have m a n y causes: n e w point defects m a y have entered the L S D A s ; c o m p e n s a t i n g impurities m a y f o r m electrically active c o m p l e x e s with the intrinsic defects g e n e r a t e d in the process of implantation; dissolution of impurity microclusters and the additional activation of L S D A s m a y be a result of the c a p t u r e of intrinsic defects in the accumulations; and so on. In the case of A r ÷ implantation in InP, the g r o w t h of L A L S intensity (the activation of L S D A s ) m a y m o s t likely o c c u r b e c a u s e of the increase of Inp c o n c e n t r a tion in L S D A s . T h e changes in L A L S diagrams for the samples with e n h a n c e d dislocation density indicate that initially L S D A s in this material are n o t electrically active. M o s t likely, the activating centers (e.g. Vp) migrate to dislocations in the p r o c e s s of p o s t - g r o w t h cooling o f the crystal and the b r e a k d o w n o f n o n stoichiometric solid solutions o f I n - I n P , and the rate of their flux to dislocations, b e c a u s e of very high dislocation density, m a y g r o w in this case in c o m p a r i s o n with
108
V.A. Yu~ev, V.P. Kalinushkin /
MaterialsScienceandEngineeringB33(1995) 103-114
that in normal InP and reach the rate of interstitial In coagulation (for normal dislocation density, the rate of coagulation of In i is much greater than the rate of vacancy flow in both sublattices, and as a result, the concentration of vacancies appears to be greater than the Ini concentration [19]). That, in turn, may result in a drop of Vp concentration with respect to Ing concentration and, as a consequence, in a reduction of Inp concentration, which is formed as a result of the reaction Vp + Ini = Inp. Ion implantation in this case increases sharply the concentration of phosphorus vacancies in material (including LSDAs, e.g. as a result of their capture from intrinsic defect flux), that may result in growth of Inp concentration in LSDAs. In this case the LALS intensity would increase greatly and, the sizes of LSDAs being nearly equal in the materials with different dislocation densities (only the degree of their activation is different), the shapes of LALS diagrams for the material with high dislocation density would coincide with those for ImP with normal dislocation density. Namely this is seen in the experiments. A somewhat lower growth of LALS intensity after ion implantation in the regions of crystals covered with masks is likely to be caused by a lower flux of intrinsic defects in the lateral direction in comparison with their longitudinal flux. Such a "lateral effect", however, was described at ion implantation, for example, in Ref. [18]. It should be noted as a confirmation of the above explanation of the influence of ion implantation on LSDAs, that the PL band with a maximum at 1.392 eV, which is associated with radiation transition of electrons on the center Inp °, was registered in the PL spectra of n-ImP at 4.2 K after bombardment of the crystals with 3.5-4 MeV electrons when the dose was greater than 5 x 1015 cm -2 [20]. When heating, this center switches to the state Inp- as a result of the thermal activation (the energy of the Inp °/- transition is E v+ 30 meV), and the latter determines the light scattering by LSDAs at room temperature in the proposed model.
2. 4. Effect of the probe beam radiation The effect of slow growth of the LALS intensity in InP at liquid nitrogen temperature under CO2-1aser radiation (2 = 10.6/~m) has already been mentioned in Refs. [5,6]. Let us describe it in more detail, as this effect impedes the investigation of LALS temperature dependences in InP. At a sample temperature lower than that of freezing. out the point centers constituting LSDAs in InP [5-8] (it is about 120-140 K, which corresponds to the activation energies of about Ev + (15-50) meV ) growth
of the LALS intensity is observed. When the power density of the probe CO2-1aser is 1-1.5 Wcm -2, the characteristic growth time is 100-120 min (see Figs. 4(a)-4(c), in which the LALS diagrams illustrating this effect are plotted, and Fig. (5), in which the graphs of the scattering intensity growth and relaxation are depicted); the scattering intensity during this time is in excess of the initial one at 300 K, and the concentration An m grows by 2-3 times (this corresponds to growth of the hole concentration Pm by 3 x 1016-3 × 1017 cm-3). After switching off the illumination, a slow decay (for about 80 min) of the LALS intensity is observed (the sample is illuminated with the probe radiation only when LALS is measured). If the cooled sample after freezing out is not illuminated with the probe emission, i.e. it is illuminated only during LALS measurements, the growth of LALS intensity does not occur. The same changes in LALS intensity take place also under CO-laser radiation (2 = 5.4/~m) in the absence of CO2-1aser emission (when the latter is used only as a probe beam for a short time of making measurements). It should be noted that the above effect has been observed in the samples of I n P : F e and InP with normal dislocation densities (see Figs. 4(a) and 4(b)), while it has been registered in the samples of InP with enhanced dislocation density only after Ar + implantation. (The carrier concentration An m grows in this material at 100 K under CO2-1aser emission for 80 min by 1.5-2 times in large LSDAs and by 2.5-3 times in medium ones (see Fig. 4(c)). A 2 h relaxation in darkness returns large LSDAs to the initial state, while the scattering intensity by medium ones in 2 h is just slightly higher than initially. The latter circumstance witnesses in favour of the fact that, as a result of Ar + implantation, LSDAs in this material become analogous in their composition to LSDAs in normal InP. At present, we have no satisfactory explanation for the described phenomenon. Maybe it is conditioned with some effects connected with the spatial separation of charges in the potential relief. We should remark also that the temperature dependences of LALS in InP with enhanced dislocation density unless exposed to Ar + implantation are anomalous (see Refs. [5,6]), the growth of LALS intensity is observed instead of ordinary activation drop. This may also be a result of CO2-1aser action.
3. Gallium arsenide
3.1. S L A L S images and parameters of LSDAs The samples of vertical gradient freeze (VGF) and LEC-grown undoped semi-insulating (SI) GaAs have
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Materials Science and Engineering B33 (1995) 103-114
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Fig. 5. Increase under CO2-1aser radiation (curve 1 ) and decay in darkness (curve 2) of the light-scattering intensity for undoped InP at 100 K.
b e e n studied by b o t h S L A L S and L A L S methods. T h e i r S L A L S images are shown in Figs. 6(a)-6(c) and their L A L S diagrams are depicted in Figs. 6(d) and 6(e). T h e samples of V G F SI G a A s (samples 1 and 2) were 49 m m diameter wafers with a thickness of 0.5 mm. T h e etch pit density (epd) in t h e m was about 1500 c m -2, however this material had relatively high impurity cluster sites (mainly SIO2) f r o m crystal growth process. T h e resistivity was 27.5 x 106 Q cm, and the carrier mobility was 5250 c m 2 V-1 s-1 (both at 300 K). T h e wafers of L E C SI G a A s with a 76 m m diameter had a thickness of 0.58 m m (samples 3 and 4). T h e s e wafers had an e p d of a b o u t 7.5 x 104 cm -2. T h e resistivity was 17 x 106 fl cm, and the carrier mobility was about 6 0 0 0 cm 2 V - 1 s - 1 (at 300 K). Sample 5 was a wafer of L E C SI G a A s grown in a (100) direction in a P B N crucible. A s - g r o w n crystals were annealed u n d e r v a c u u m in a sealed quartz a m p o u l e at 900 °C before cutting. Its resistivity at 300 K was greater than 107 Q cm, its mobility was greater than 4 0 0 0 cm 2 V-1 s - l , and the epd was less than 105 cm -2. T h e wafer diameter was 76 mm, its thickness was 0.625 mm. All the samples were polished on b o t h sides and finished for use in waferprocessing equipment. As in the case of indium phosphide, the white spots in S L A L S patterns are the images of L S D A s ( 1.3 x 1.3 m m 2 areas are presented). L S D A concentrations estim a t e d f r o m the S L A L S patterns are given in Table 3
O (degr.)
Fig. 4. LALS diagrams for undoped InP (a) and InP : Fe (b) with a dislocation density of about 5 x 104 cm -2, and those for undoped InP with a dislocation density of 105-106 cm -2 after Ar + implantation (c); curves 1,7, 300 K; curves 2,3,4,5,6, 100 K; curves 1,2, initial sample; curve 3, 1 h under CO2-1aser radiation; curve 4, 1 h under COz-laser radiation and 1 h in darkness; curve 5, 80 min under CO2-1aser radiation; curve 6, 80 min under CO2-1aser radiation and 2 h in darkness; curve 7, after heating.
V.A. Yuryev, V.P. Kalinushkin
1 10
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Materials' Science and Engineering B33 (1995) 103-114
i
-2.0
~A~
~q~
D[3n[3(3
"~ -2.5
--cen~eT -edge
-l.ODDDDD
2
D
~edge
~-1.5-
-eo 3
.'~ - 3. 0
~ -2.5 -3.0
"~ -3.5
~ -3.5 ~ -4.0 -
r~ - 4 . 0
o
-4.5
(d)
iii IIIIIllllllllllllllll
2
3
IIIIlll
I I I [1111 1 1 1 1 1 1 1 1 1 1 1 1 1 1 H I I I f I I I I I I I I I l l
4 5 6 Sco~f;f;er~ng a~gle,
7 8 0 (degT.)
r IIII IIII I
9
--4.5
10 (e)
4 I,lll,lllllllIH~rlllttllrlJll,lrlflllllrlrlll~lllltlr~lll,lllll,111~lllllll,ll
2
3
4 5 6 7 8 Scc~tte~ng a"n.gle, 0 (degr.)
9
I
10
Fig. 6. L S D A s in V G F a n d L E C SI G a A s : S L A L S i m a g e s of s a m p l e 1 (a), s a m p l e 4 (b) a n d s a m p l e 5 (c)(1.3 x 1.3 mm2): a m p l i f i c a t i o n in (a) a n d (b) is 1 0 0 t i m e s h i g h e r t h a n in (c). L A L S d i a g r a m s (d), (e); c u r v e s 1,2, s a m p l e s 1 a n d 2; c u r v e s 3,4, s a m p l e s 3 a n d 4; c u r v e 5, s a m p l e 5. R o o m t e m p e r a t u r e .
Table 3 P a r a m e t e r s o f L S D A s in SI G a A s No.
a (/~m)
log(lo/W )
CAnm ? (c m - 9 )
C(cm-3)
Pm (cm 3)
Pv ( c m - 3 )
1
16-17 8-10
- 3.0 to - 2.9 - 3.5 to - 3.1
= 10 35 ( 1 . 5 - 3 ) × 10 36
-4 × 10 5
-1.5 x 10 ~6
-( 2 - 3 . 5 ) × 10 ~3
2
= 16 8-9
= -2.1 - 2.9 to - 2 . 7
= 10 36 ( 8 - 1 0 ) × 1036
-4 x 105
-3.5 x 1016
-( 4 - 5 . 5 ) x 1013
- 2 . 5 to - 2 . 4
( 3 - 1 0 ) X 1036
( 4 . 5 - 5 ) X 105
( 1 . 5 - 3 . 5 ) × 10 ~6
( 7 - 7 . 5 ) x 1013
. ( 4 - 4 . 5 ) × 105
= 1016
( 1 . 5 - 2 ) x 10 ~3
,~- 1.5 × 1017
= 2.5 x 1014
3
9-12
4
25-30 8-9
- 2.7 to - 2.1 - 3.7 to - 3.5
. = 1036
= 12
= -0.9
= 10 38
5
.
2 × 10 5
.
No. is s a m p l e n u m b e r ; a is t h e a c c u m u l a t i o n radius; l o g ( l 0 / W ) is a l o g a r i t h m o f t h e z e r o - a n g l e s c a t t e r i n g i n t e n s i t y p e r p r o b e - b e a m unit p o w e r (units l o g ( s r - 1 c m - 3 ) ; CAnm 2 is a c o n c e n t r a t i o n p a r a m e t e r ; C is t h e c o n c e n t r a t i o n o f L S D A s ; A n m is t h e effective m a x i m u m differential c a r r i e r c o n c e n t r a t i o n in L S D A s ; Pm is t h e m a x i m u m h o l e c o n c e n t r a t i o n inside a c c u m u l a t i o n s ; Pv is t h e total c o n t e n t o f h o l e s i n s i d e L S D A s p e r crystal unit v o l u m e .
V.A. Yuryev, V.P. Kalinushkin
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Materials Science and Engineering B33 (1995) 103-114
together with other parameters of LSDAs estimated from LALS diagrams. The amplification of the images in Figs. 6(a) and 6(b) is 100 times higher than that in Fig. 6(c), that is fully in agreement with the ratio of the LALS intensities in Figs. 6(d) and 6(e). (It should be noted that the signal-to-noise ratio in Figs. 6(a) and 6(b) is rather close to unity.) As in the case of indium phosphide, the data obtained from SLALS and LALS are in good agreement with those given in Ref. [8].
3.2. I n f l u e n c e o f V P E on L S D A s in G a A s : Cr
To investigate the effect of VPE on light scattering with LSDAs in the bulk substrate, undoped GaAs epitaxial layers were grown using the trichloride vapor phase epitaxy on substrates of chromium-doped LEC gallium arsenide. The dislocation density in the substrates was 104-105 cm -2 and their resistivity was in excess of 108 f2 cm. The substrate surfaces were oriented at angles of 2-6 ° with respect to the (100) plane. The thicknesses of the substrates were 300/xm. The wafers intended for epitaxial growth were cut from the same ingot and were subjected to chemomechanical polishing on one side, which was used to grow epitaxial layers. Before epitaxy, the substrates were washed in solvents, the damaged surface layers were etched away in a liquid etchant, and immediately before growth the substrates were etched again with a vapor etching agent containing arsenic trichloride. The substrates were positioned on a quartz holder during growth. The temperature during vapor etching and growth was 720 °C and the temperature of the source of gallium enriched with arsenic was 810 °C. The film thickness changed from 2 to 15/xm, the film growing rate was 0.1 # m min -1 . The carrier concentration in the epilayers did not exceed 1014 cm-3. After growth of the epilayers, the opposite side of a wafer was polished so as to make the wafer fit for investigation by LALS. Control samples were cut from the same ingot (being usually wafers in the ingot adjacent to the processed ones) and polished on both sides, as they were not subjected to epitaxial growth or any other treatment. Some of the wafers, initially polished on both sides, were also used to grow epilayers being investigated by LALS beforehand. Unfortunatley, the films grown on them were of poor quality, and the epitaxial layers had to be polished off completely before investigating the samples by LALS. In some cases, it was possible to find regions of sufficient size with a perfect enough epilayer. LALS diagrams of these regions were essentially the same before and after polishing, so all the changes in the light scattering during epitaxy may be associated with the substrate bulk, as the scattering of light by the
substrate greatly exceeds in intensity the contribution of the epitaxial layer. Some of the samples were subjected to a thermal treatment at 720 °C in a hydrogen atmosphere, which simulates the VPE procedure. The annealing last 1 h and samples were placed in a quartz holder. One sample was subjected to vapor etching alone and a field effect transistor (FET)-like structure was formed on the other. The parameters of LSDAs estimated from LALS diagrams of these samples (Fig. 7) are listed in Table 4. It is seen that the epitaxy, the complete cycle used in device manufacture and the simulating annealing enhance the intensity of light scattering and slightly increase the size of the LSDAs. The increase in LALS intensity is caused by growth of the free carrier concentration in LSDAs, as it follows from both the LALS diagrams and temperature dependences discussed below. The samples subjected to vapor etching manifested the same trends as the other treated samples. To estimate the activation energies of the centers responsible for higher hole concentration in LSDAs, the LALS temperature dependences (Fig. 8) were measured. The depth of the levels of these centers in the initial material was 6 0 - 9 5 meV, whereas after VPE growth it was 60-110 meV and in the annealed material it was 70-110 meV (the estimates were made as in Refs. [7,8]). It is obviously, in spite of the increase in hole concentration in LSDAs, the energy location of the centers which are responsible for this did not change and was close to the first ionization energy of GaAs antisite. Consequently, it may be concluded that the increase of the carrier concentration in LSDAs,
0.0
00000 -I DOOOD - - 2
-0.5
0
':'
60660 -4 0,0., -5
-I.0 "~ - 1 . 5 •~ - 2 . 0
•~ -2.5 -3.0 -3.5
I'IInI~1~Ir'Iul1'~l~u''~I~I1~rInIql~nII~l~flII~n~q~1Il~I~n~11~I1InIl1nIt11IqIllIIIIl q
2
3
4 5 6 7 8 9 S c a t t e r i n g angle, 0 (degr.)
I0
II
Fig. 7. LALS diagrams for GaAs ; Cr before and after different processings; curves 1,2, initial wafer; curve 3, after simulating thermal treatment; curves 4,5, after VPE; curve 6, after F E T fabrication cycle. R o o m temperature.
11 2
V.A. Yuryev, V.P. Kalinushkin
/
Materials"Science and Engineering B33 (1995) i03-114
Table 4 Parameters of LSDAs in GaAs : Cr and their changes after different processings
GaAs : Cr
a (/tm)
Initial
~
log( CAnto 2) (log(cm-9))
log(lo/W )
17
-
1.9
to
-
1.8
~--36 37.0-37.6 --
Pm (cm - 3)
AEi (meV)
--
--
(3-10) x 1016 --
60-95 --
~<2
- 2.7 to - 1.8 ~ -3
20-25 12-13 ~<2
~0 - 1.2to - 0 . 7 - 2.6 to - 2 . 2
36.9-37.5 37.5-38.1
-(6-20)x 10 ~6
-70-115
--
--
--
VPE
17-25 8-11 ~<2
- 0.8 to 0.0 - 1.4to - 0 . 9 - 2.2 to - 1.8
36.9-37.0 38.2-38.3 --
-( l - 2 ) x 1017 --
-60-110 --
After device fabrication cycle
= 15 =8 2
- 0.7 to - 0.3 - 1.4 to - 1.0 - 2.7 to - 2.2
37.5-37.9 38.5-38.9 --
-~<(2-5) x 1017 --
--
8-10
After annealing at 720 °C After
-
-
--
a is the accumulation radius; log(lo/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr- 1 cm-3); CAnto 2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs; Pm is the maximum hole concentration inside accumulations; AE, is the thermal activation energy of the point centers in LSDAs.
ifll f l l l f l l f l f l r l r frlr~lflf~fl f l l l l l l l l l l l , l I F l l l l l l r , l f l l l l l l l l l l l l f l f , , , , l l l l l l , l ~ l l l l l l l , l l l l l l l
-I.0
1 -
as
2
aft;er
--
grown VPE
3 - after simulating thermal treatment ~-1.5 dt
.I.
-2.0 •
3
Loo o< -2.5
II~q'lllll~llVHII qeU~IlIIIUlHIPlIIIIpIIITIIII]IIIllIIIlllIllUIIIIIIIIIIrIIL[IIHIr PII~IIHIII
2
3
4
5
6
7
8
9
fO
! I
12
Reciprocal temperabure, IO00//T ( K - t )
Fig. 8. LALS temperature dependences for GaAs : Cr; curve 1, initial wafer; curve 2, after VPE; curve 3, after simulating thermal treatment.
w h i c h was o b s e r v e d in t h e e x p e r i m e n t , was c a u s e d b y a n n e a l i n g at 7 2 0 °C a n d as f a r as w e c o u l d j u d g e , t h e a t m o s p h e r e o f a n n e a l i n g h a d n o significant i n f l u e n c e o n t h e L S D A s l o c a t e d in t h e b u l k o f t h e s u b s t r a t e . It m a y t h e r e f o r e b e c o n c l u d e d t h a t the p r o c e d u r e o f V P E results in c o n s i d e r a b l e g r o w t h in f r e e c a r r i e r c o n c e n t r a t i o n in L S D A s l o c a t e d in t h e b u l k o f t h e G a A s : C r s u b s t r a t e s . T h i s c h a n g e is likely to b e d u e to an i n c r e a s e in t h e c o n c e n t r a t i o n o f GaAs antisites as a result o f s u b s t r a t e a n n e a l i n g at 7 2 0 °C in t h e p r o c e s s o f V P E . W h e n a F E T - l i k e s t r u c t u r e is f a b r i c a t e d o n
G a A s : C r s u b s t r a t e s , t h e m a i n c h a n g e s in L S D A s in t h e substrfite b u l k o c c u r d u r i n g t h e g r o w t h of t h e e p i t a x i a l layer. It s h o u l d also b e n o t e d t h a t t h e a n n e a l i n g o f t h e wafers of GaAs:Cr f o r 1 h w i t h o u t c o a t i n g at T = 7 5 0 °C in a H2 a t m o s p h e r e r e s u l t e d in a n i n t e n s i v e e s c a p e o f v o l a t i l e A s a t o m s f r o m the s u r f a c e o f t h e s a m p l e s [20] ( a n n e a l i n g at T ~> 8 5 0 °C r e s u l t e d in t h e emergence of gallium droplets on the sample surface [21,22]) a n d p r o d u c e d a p - t y p e l a y e r o f 0.5 ¢tm thick, w h e r e t h e h o l e d e n s i t y was a r o u n d 1016 c m -3 [22,23]. The hole concentration appeared because annealing g e n e r a t e d c e n t e r s with an a c t i v a t i o n e n e r g y o f a b o u t 100 m e V , w h i c h w e r e a t t r i b u t e d in Ref. [23] to GaAs a c c e p t o r s . A t a n n e a l i n g t e m p e r a t u r e s o f 7 0 0 - 7 3 0 °C no conversion producing a p-type layer occurred, p r o b a b l y b e c a u s e of t h e r e d u c t i o n in t h e r a t e o f A s a t o m e v a p o r a t i o n f r o m the surface. I n t e n s i v e f o r m a t i o n o f VAS is n o t e s s e n t i a l f o r t h e f o r m a t i o n o f GaAs c e n t e r s in L S D A s ( a c c o r d i n g to Refs. [7,8,19,24] t h e c o n c e n t r a t i o n o f VAS in L S D A s m a y b e c o n s i d e r a b l e ) , so that the active f o r m a t i o n of t h e GaAs c e n t e r s in L S D A s d o e s t a k e p l a c e at t e m p e r a t u r e s s o m e w h a t b e l o w t h e t h r e s h o l d of f o r m a t i o n of t h e p - t y p e l a y e r o n t h e surface.
3.3. Influence of electron bombardment on L S D A s in undoped GaAs Single c r y s t a l s o f u n d o p e d n - G a A s w e r e g r o w n b y t h e L E C p r o c e s s in t h e (100) d i r e c t i o n . B e f o r e i r r a d i a t i o n t h e e l e c t r o n c o n c e n t r a t i o n in t h e m was 4 × 1015
V.A. Yuryev, V.P. K a l i n u s h k i n
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Materials Science a n d Engineering B33 (1995) 103-114
1.0
Table 5 Parameters of LSDAs in undoped GaAs and their changes after electron irradiation
AAAAA - - I or~nOo - 2
0.5 0.0 .
GaAs
- 0 . 5
-
ct
log(lo/W )
log( CAnm 2) (log(cm-9))
(/~m)
g -t.o .~
113
Initial
23-27 10-12 ~<2
0.3-0.7 - 1.6 to - 1.2 ~- -2.0
37.2-37.4 37.4-37.8 --
Irradiated
21-23 12-13 ~<2.5
0.7-1.1 - 1.2 to -0.6 - 3.3 to - 2.7
38.0-38.2 37.7-38.0 --
1.5
-2.0
-e.5 c~
-3.0 -3.5
Imm~mlll,lllllllIHmqlmllml,l,lmlq,,ImH,IIm,lllqHImllqllmllll
2
3 4 5 6 7 8 S c a t t e r i n g angle, 0 ( d e g r . )
9
I
10
Fig. 9. LALS diagrams for undoped n-GaAs with low resistance before (curve 1) and after (curve 2) electron bombardment. Room temperature.
cm -3 and the mobility was (1-5)x 103 cm 2 V -1 s -1. The dislocation density was around 104 cm-2. Parallelsided GaAs wafers were cut from an ingot along the (100) plane. Samples were irradiated with 3.5 MeV electrons (a dose of 2 x 1015 cm -2) at room temperature. As a result, the carrier concentration in them was reduced to 5 × 1014 cm 3. Fig. 9 shows typical LALS diagrams for GaAs crystals before and after irradiation. The light scattering intensity by large inhomogeneities increased as a result of electron irradiation. No significant influence of sample orientation with respect to probe beam polarization plane upon the diagram shape and scattering intensity was found. So the diagrams may be approximated on the basis of a model of spherically symmetric LSDAs with a Gaussian profile of the radial distribution of the dielectric constant [1,7,8,13]. The parameters of LSDAs estimated from the diagrams are given in Table 5. An increase in the light-scattering intensity could, in our opinion, be attributed to an increase in GaAs concentration in LSDAs [8] (it was reported in Ref. [20] that the electron bombardment of InP results in the formation of the antisite Inp, which is completely analogous to GaAs). The processes, which may enhance the concentration of gallium antisite in LSDAs, are proably associated with the capture of intrinsic defects generated by irradiation outside LSDAs (as in the case of Ar + implantation in InP) or owing to the formation of these defects in LSDAs. The high-level plateau due to the light scattering by particles of radius less than 1-2/~m disappeared from the LALS diagrams indicating that these defects are
a is the accumulation radius; l o g ( l o / W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-L cm-3)); CAnm2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs.
likely to be of different origin to the larger ones. The appearance of intrinsic point defects in these LSDAs could result in their strong compensation (e.g. because of the formation of the complexes of intrinsic defects with defects constituting LSDAs or because of partial dissolution of impurity microprecipitates in LSDAs, etc.).
4. Conclusion
In summary, the main results presented in the paper are as follows. The SLALS images of LSDAs in indium phosphide and gallium arsenide are given. The data obtained from them are all in good agreement with former estimations made in Refs. [7,8]. The size of the LSDAs in InP: Fe is shown to be dependent on ingot growth conditions. The implantation of Ar + ions with energies of 200 and 250 keV with a dose of 1016 cm -2 in the crystals of undoped InP and 3.5 MeV electron bombardment of undoped (low resistance) GaAs with a dose of 2 x 1015 cm- 2 result in an increase of free-carrier concentration in the volume of the LSDAs. Activation of the LSDAs is probably caused by the growth of Ine and GaAs antisite concentrations in LSDAs. When growing the epitaxial layer of undoped GaAs on the substrate of SI GaAs : Cr by the VPE process, the free carrier concentration grows in LSDAs located in the substrate bulk. Analogous growth of the free carrier concentration in LSDAs is registered after wafer thermal treatment, which simulates the VPE process (at 720 °C without coating in an atmosphere of H2). The activation energies of the centers, which determine the LSDAs' conduction type and the carrier concentration in them, do not change after the epitaxy
114
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Materials Science and Engineering B33 (1995) 103-114
and the thermal treatment in c o m p a r i s o n with those in the original material. T h e growth of free carrier concentration in L S D A s is a consequence of wafer annealing and is likely to be connected with the growth of the GaAs concentration in LSDAs. It should also be mentioned that one can find s o m e additional information on the influence of annealing on L S D A s in InP in Ref. [4], on that in G a A s in Ref. [9], and s o m e data on the d e p e n d e n c e of L A L S intensity on G a A s melt stoichiometry in Ref. [11].
Acknowledgments We would like to thank Dr. A.V. Spitsyn for ion implantation, Professor A.K. Abiev for electron b o m b a r d m e n t and Dr. T.V. T h i e m e for growing epitaxial layers. We express our appreciations to Dr. E.V. Russu, Professor I.M. Tiginyanu and Mr. J. Oicles for kindly supplying the experimental samples.
References [1] V.P. Kalinuchkin, Proc. Inst. Gen. Phys. Acad. Sci. USSR, Vol. 4, Laser Methods of Defect Investigations in Semiconductors and Dielectrics, Nova, New York, 1988, p. 1. [2] V.V. Voronkov, S.E. Zabolotskiy, V.P. Kalinushkin, D.I. Murin, M.G. Ploppa and V.A.Yuryev, J. Cryst. Growth, 103 (1990) 126. [3] V.V. Voronkov, G.I. Voronkova, V.P. Kalinushkin, B.V. Zubov, B.B. Krynetsky, T.M. Murina and A.M. Prokhorov, Sov. Phys. Solid State, 23(1981)65. [4] A.N. Georgobiani, V.P. Kalinushkin, A.V. Mikulyonok, D.I. Murin, A.M. Prokhorov, S.I. Radautsan, I.M. Tiginyanu and V.V.Ursaki, Sov. Phys. Semicond., 19 (1985) 810. [5] A.N. Georgobiani, V.P. Kalinushkin, D.I. Murin, T.M. Murina, A.M. Prokhorov, S.I. Radautsan, I.M. Tiginyanu and V.A. Yuryev, Sov. Phys. Semicond., 21 (1987) 2126. [6] V.P. Kalinushkin, D.I. Murin, T.M. Murina, A.M. Prokhorov, S.I. Radautsan, I.M. Tiginyanu and V.A. Yuryev, in G. Grossmann and L. Ledebo (eds.), Proc. 5th Int. Conf. on Semi-insulating llI-V Materials, Malm6, Sweden, June 1-3, 1988, Adam Hilger, Bristol, 1988, p. 153.
[7] V.P. Kalinushkin, V.A. Yuryev and D.I. Murin, Sov. Phys. Semicond., 25(1991) 1483. [8] V.P. Kalinushkin, V.A. Yuryev, D.I. Murin and M.G. Ploppa, Semicond. Sci. Technol., 7(1992)A255. [9] V.V. Voronkov, G.I. Voronkova, V.P. Kalinushkin, D.I. Murin, E.M. Omeljanovsky, E.A. Petrova, A.M. Prokhorov and V.I. Raihstein, Soy. Phys. Semicond., 18 (1984) 854. [10] V.E Kalinushkin, T.M. Murian, E.M. Omeljanovsky, A.M. Prokhorov and V.I. Raihstein, Proc. 17th Int. Conf. on Physics of Semiconductors, San Francisco, USA, August 6-10, 1984, Springer, Berlin, 1985, p. 769. [11] V.P. Kalinushkin, D.I. Murin, E.M. Omeljanovsky, A.J. Polyakov, A.M. Prohorov and V.I. Raihstein, Semicond. Sci. Technol., 2 (1987) 379. [12] V.P. Kalinushkin, V.A. Yuryev, D.I. Murin, M.G. Ploppa and T.V. Thieme, Semicond., 27(1993) 104. [13] V.P. Kalinushkin and V.A. Yuryev, Semicond., 27 (1993) 103. I14] O.V. Astafiev, V.P. Kalinushkin and V.A. Yuryev, Proc. 2nd Int. Symp. on Advanced Laser Technologies, Prague, Czechia, November 7-9, 1993, SPIE, 2332 (1994) 138. [15] O.V. Astafiev, V.P. Kalinushkin and V.A. Yuryev, Microelectronics, submitted for publication. [16] O.V. Astafiev, V.E Kalinushkin and V.A. Yuryev, Proc. 1st Int. Conf. on Materials for Microelectronics, Barcelona, Spain, October 17-19, 1994, Mater. Sci. Technol., submitted for publication. [17] O.V. Astafiev, V.P. Kalinushkin and V.A. Yuryev, Mater. Sci. Eng. B, in press. [18] V.P. Kalinushkin, A.A. Manenkov, G.N. Mihailova, M.G. Ploppa, A.S. Seferov, Yu. N. Chehonadsky and I.B. Haybulin, Soy. Phys. Microelectron., 15 (6) (1986) 528. [19] A.N. Morozov,V.T. Bublik and V.B. Osvenskii, Sov. Phys. Crystallogr., 28(1983) 458. [20] EP. Korshunov, S.I. Radautsan, N.A. Sobolev, I.M. Tiginyanu and V.V. Ursaky, Sov. Phys. Semicond., 23 (1989) 980. [21] A. Mircea-Roussel, G. Jacob and J.P. Hallais, in G.J. Rees (ed.), Proc. Int. Conf. on Semi-insulating III-V Materials, Nottingham, UK, 1980, Shiva, Orpington, UK, 1980, p. 105. [22] T. Udagawa, M. Higashiura and T. Nakanisi, in G.J. Rees (ed.), Proc. Int. Conf. on Semi-insulating II1-V Materials, Nottingham, England, 1980, Shiva, Orpington, UK, 1980, p. 93. [23] A.N. Georgobiani and I.M. Tiginyanu, Soy. Phys. Semicond., 22(1988) 1. [24] V.T. Bublik, V.V. Karatayev and R.S. Kulagin, Sov. Phys. Crystallogr., 19(1973) 218.
g
ELSEVIER
Materials Science and Engineering B33 (1995) 103-114
New investigation of III-V compounds by the low-angle mid-IR lightscattering technique Vladimir A. Yuryev, Victor P. Kalinushkin Laboratory of Laser Defectoscopy of Semiconductors, General Physics Institute of the Russian Academy of Sciences, 38 Vavilov Street, V-333 GSP-1, Moscow 117942, Russia Received 1 August 1994; in revised form 20 January 1995
Abstract
The results of recent investigations of InP and GaAs single crystals by means of the low-angle mid-IR light-scattering technique are presented in this paper. Main attention is paid to the influence of some exposures to wafers, such as ion implantation, electron irradiation, vapor-phase epitaxy and annealing, on the properties of the large-scale accumulations of electrically active defects in crystal volume. The images of lnP an GaAs crystals obtained by scanning low-angle light-scattering microscopy are published for the first time.
Keywords: Gallium arsenide; Stoichiometry and homogeneity; Indium phosphide; Clusters
1. I n t r o d u c t i o n
This paper is devoted to new investigations of indium phosphide and gallium arsenide single crystals by means of the low-angle mid-IR-laser light-scattering technique (LALS) [1-3]. In previous studies [4-13] geometrical and electrophysical parameters of the large-scale accumulations of electrically active defects (LSDAs) in indium phospide [4-8] and gallium arsenide [8-13] were determined, the energy location of some electrically active centers composing LSDAs was estimated, and the model of inner structure of the L S D A was proposed, according to which LSDAs are spherical p-type regions enriched with In or Ga microinclusions and Inp or GaAs antisites in InP and GaAs, respectively [7,8]. Unfortunately, no technique was available that would enable us to visualize LSDAs, and evaluation of their parameters was made on the basis of other methods, for example, laser tomography. Such estimates were not very reliable, as one cannot be absolutely sure that objects registered by L A L S and by other methods were the same ones. Now a new technique of scanning laser microscopy, which is based on L A L S (SLALS), has been developed [14-17]. This new method, being founded on the same physical 0921-5107/95/$9.50 © 1995 -
SSD10921-5107(94)01184-6
Elsevier Science S.A. All rights reserved
principles as the standard LALS, allows examination of the same features as those observed by LALS, i.e. LSDAs. T h e first S L A L S pictures of LSDAs in InP and G a A s are presented in the current paper, and the L S D A parameters estimated on the basis of SLALS patterns are also presented. It is important to know for the practical goals connected with the fabrication of semiconductor devices, how technological processing of original materials affects the properties of LSDAs. In addition, investigation of the physical exposures to LSDAs helps in understanding their properties and nature. T h e major part of this paper is devoted to the investigation of such effects as ion implantation, vapor phase epitaxy (VPE), annealing and electron b o m b a r d m e n t on LSDAs in InP and G a A s single crystals. Let us introduce some designations for measured and estimated parameters of the scattered light and LSDAs, which are used in this paper. 0 is the light scattering angle in the crystal, W is the probe beam power in the crystal per unit beam cross-sectional area, I is the scattered light intensity in the crystal per unit solid angle per unit tested volume, I 0 = I( 0 = 0), C is the concentration of scatterers, and a is a scatterer radius. An m is the maximum effective concentration of free carriers in the accumulations (as in Refs. [7,8]),
104
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/
MaterialsScience and Engineering B33 (1995) 103-114
A n m = m c nm/m a -- no~ where mc, a represents respectively the effective masses of current carriers in the crystal bulk outside the scattering LSDAs (c) and inside them (a), n m is the maximum value of free carrier concentration inside the LSDAs (it corresponds to the maximum magnitude of the deviation of the dielectric constant I A e l - f ( ~ ) where ff is the coordinates in the accumulation), and noo is the free carrier concentration outside the LSDAs; the LSDAs and the crystal bulk free of them may have different conductance types, e.g. for InP and GaAs the model of p-type LSDAs in an n-type crystal matrix is under consideration [7,8], and in such a case n m=-pm means the concentration of holes. Pv is the amount of holes within the LSDAs per unit crystal volume, and AE i = E i - Ev is the location of the levels of the point center constituting LSDAs in the band gap with respect to the valence band. The ratio of / / W is measured in sr- 1 cm - 3.
The prototype of the SLALS microscope, which was made in our laboratory, is not perfect and requires engineering improvement. As it is shown in Refs. [15-16], it transfers, almost without distortion and light failure, the images of LSDAs with dimensions of about 20-50/~m. Smaller LSDAs can also be seen but their contrast is lower and their sizes are blurred with diffraction and aberrations. The images of larger LSDAs are suppressed with a "search-light" filter in the SLALS instrument. So we have supposed that the objects seen in the patterns must be attributed to the LSDAs with appropriate sizes determined from LALS diagrams. Further improvement of the instrument will allow one to obtain more precise data, but the principle goal of this work has now been reached: even the instrument in its present state has allowed us to obtain the images of LSDAs in different semiconductors, and particularly in indium-phosphide single crystals.
2. I n d i u m p h o s p h i d e
2.2. Dependence o f the size of L S D A s on their location along the longitudinal axis o f an InP : Fe ingot
2.1. S L A L S images and parameters o f L S D A s
As mentioned above, the technique of scanning LALS (SLALS) has been developed which allows the visualization of LSDAs in semiconductors [14-17]. In this section, the SLALS patterns of indium-phosphide single crystals are presented for the first time. Undoped InP and InP : Fe single crystals, the images of which are depicted in Figs. l(a) and l(b) and the LALS diagrams of which are plotted in Fig. l(c), were grown by the liquid-encapsulated Czochralski (LEC) process in quartz crucibles using B203 as an encapsulant. The free-electron concentration in the sample of undoped InP was 2 x 1016 cm -3, and the specific resistivity of I n P : F e was greater than 107 Q cm. The carrier mobility in both samples at ambient temperature was about 3 x 103 cm 2 V -1 s -1 . The dislocation density in the crystals was around 5 × 104 cm-2. The thicknesses of the samples were 0.5 mm for InP : Fe and 0.6 mm for InP. The bright white spots in the SLALS pictures are associated with LSDAs (2 × 2 mm 2 crystal areas are given). The LSDA concentrations are close to those accepted in Refs. [7,8]. The data on LSDAs in these samples and the estimates of hole concentrations in them made using SLALS data are.gathered in Table 1. It is seen that these data are close to those given in Refs. [7,8], so our former estimates appeared to be correct. We should say a few words about the concentrations of LSDAs given in Table 1. We have attributed all the spots seen in the SLALS patterns to the LSDAs with radii of 1 1 - 1 9 / t m . The reasons for this are as follows.
As it has previously been found in Refs. [7,8], the dominating LSDAs in InP : Fe have characteristic radii of about 9 pro. Fig. 2 presents the dependence of characteristic radii of dominating LSDAs a on the location of the tested wafer in a single crystalline ingot of InP : Fe. The left part of the figure (l = 0) is related to the ingot region which is close to the seed, whereas the right is associated with the near-tail region. The specific resistivity of the material in the seed was 1.02 x 107 Q cm, and that in the tail was 2.9 x 107 if2 cm. It is visible from Fig. 2, part 1, that the size of LSDAs grows with the distance from the seed and becomes constant at a distance of 1/3-1/2 of the ingot length. The size of the LSDAs are constant in the region where the ingot radius R is constant and change in those regions where R changes (Fig. 2, part 2), i.e., they become stable with stabilization of the ingot growth conditions. One can also see the gap in Fig. 2, part 1, (marked with an arrow) which corresponds to the waist in the ingot layout (Fig. 2, part 2). The example given shows clearly that the size of LSDAs in InP : Fe depends strongly on the thermodynamic conditions of crystal growth. 2.3. Influence o f A r + implantation on L S D A s in undoped InP
In the experiments on the influence of Ar + implantation on the light scattering, the samples of undoped indium phosphide crystals grown by the LEC process in the (100) crystallographic direction in quartz crucibles with B203 as an encapsulant under an argon
V.A. Yuryev, V.P. Kalinushkin
-0.5
/
Materials Science and Engineering B33 (1995) 103-114
105
-
ooooo -I O000 r7 - - 2
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.-I.0
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7
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Scattering angle, O (degr.)
Fig. 1. LSDAs in indium phosphide: SIALS patterns for undoped InP (a) and InP : Fe (b) (2 x 2 ram2); and LALS diagrams (c) for InP (curves 1,2) and InP : Fe (curves 3,4,5). R o o m temperature.
Table 1 Parameters of LSDAs in indium phosphide No.
a (~m)
log(lo/W)
CAnrn2 (cm -9)
1
1 1-14 6-7 14-19 6-7
-
(4-8.5) × 10 37 (1.5-3)X 10 39 ( 4 - 1 4 ) x 10 36 (2-1 1 ) × 1 0 39
2
1.2 1.5 1.3 2.0
to to to to
-- 0.8 -- 1.1 - 1.0 - 1.7
C
(cm
-3 )
Pm
(cm-3)
Pv (cm-3)
(1.5-2) × 10 5
(2.0-2.5) × 1017
(3-6) × 1014
--
--
--
( 2 - 3 ) x 10 5
( 2 - 5 ) × 1016
( 1 - 3 ) × 1014
--
--
--
No. is sample number: 1, undoped InP; 2, InP : Fe. a is the accumulation radius; log(Io/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-1 cm-3)); C A n t o 2 is a concentration parameter; C is the concentration of the LSDAs; A n m is the effective maximum differential carrier concentration in LSDAs; Pm is the maximum hole concentration inside accumulations; Pv is the total content of holes inside LSDAs per crystal unit volume.
V.A. Yu~ev,
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Fig. 2. Radii of LSDAs a vs. location l along the longitudinal axis of the InP : Fe ingot (part 1 ) and the ingot layout (part 2). R is the ingot radius. Room temperature.
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pressure of 60 atm were used as the initial wafers. T h e crystals had n-type conductance, the free carrier concentration in them at 300 K was (1-3) x 1016 c m -3, the carrier mobility was ( 3 - 5 ) x 103 cm 2 V -~ s ~, and their compensation degree was 0.2-0.3. T h e crystals differed sharply in dislocation densities, a part of them had a normal dislocation density (about 5 x 104 c m - 2 ) , the rest had an enhanced o n e ( 1 0 5 - 1 0 6 c m - 2 ) . T h e thickness of the former crystals was 0.5 mm, and that of the latter ones was 1.5 and 3 mm. T h e samples were parallel-sided plates cut from the ingot along the (100) plane. T h e implantation of A r + ions was made using a High Voltage Engineering E u r o p a accelerator. A r + energies were 200 and 250 keV, a dose was 10 ~6 cm -2. T h e implantation was carried out at r o o m temperature, the sample temperature did not exceed 100 °C. T h e studies were carried out in two stages. In the first stage, the as-grown samples were subjected to detailed investigation by LALS. T h e diagrams were measured at different points on each wafer with high density, each point was studied in a different orientation of the sample with respect to the polarization plane of the probe laser beam. T h e n the samples were subjected to ion implantation, and the samples with a thickness of 1.5 m m were half-covered with a mask (so that only a half of the surface of each sample was under the flux, the b o r d e r went through the center of the wafer side). One 3 m m thick sample stayed without exposure. After the implantation, the samples were investigated as thoroughly as before the exposure. T h e characteristic L A L S diagrams are plotted in Figs. 3(a)-3(c), and the parameters of LSDAs before
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10
11
Fig. 3. LALS diagrams for undoped indium phosphide before (curves 1,2) and after (curves 3,4,5,6,7) A r ÷ implantation; (curves 6,7) are under a mask. Dislocation densities are about 5 × 104 cm -2 (a) and 10-5-106 cm -2 (b,c); A r ÷ energies are 200 keV (a,b) and 250 keV (c); room temperature.
V.A. Yuryev, V.P. Kalinushkin
/
Materials Science and Engineering B33 (1995) 103-114
107
Table 2 Parameters of LSDAs in undoped InP and their changes after implantation of Ar + ions Wafer parameters Thickness (mm)
Dislocation density (cm -2)
0.5
~- 5 × 104
Ar + energy (keV)
Wafer state
ct (/~m)
log(l0/ W )
200
Initial
13-18 7-10 5-6 20-23 7-9
-
1.6 1.9 2.1 0.4 1.1
36.0-37.2 37.1-37.7 38.2-38.8 36.6-36.8 38.3-38.5
12-14
- 2.1 to - 1.4
36.6-37.2
-~ 4 17-25 7-9 ~<5
- 2.8 to - 2.6 - 1.6 to - 0.2 - 2.4 to - 1.6 ~< - 2 . 5
38.9-39.2 36.4-36.8 37.6-38.0 />38.8
13-14
- 2.8 to - 2.4
35.6-36.2
~<4 15-21 11-12 -- 5 18-20 10-12 5
- 3.3 to - 1.5 to - 1.4 to - 2.6 to = - 2.1 - 2.6 to -~ - 2.8
= 39.3 36.7-37.0 37.5-37.9 38.4-39.0 35.5-35.7 36.5-37.3 = 38.2
After implantation
3
105-106
200
Initial After implantation
1.5
105-106
250
Initial After implantation After implantation under mask
1.7 to 2.7 to 2.9 to 0.8 to 1.5 to
-
-
log( CA n m2) (log(cm-9))
3.0 0.5 1.0 2.2
- 2.2
a is the accumulation radius; log(lo/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-1 cm-3)); CAnto2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs.
and after the i m p l a n t a t i o n are given in Table 2. G e n e r a l g r o w t h of the light scattering intensity is seen in all the samples as well as s o m e c h a n g e in the s h a p e of the scattering diagrams. It has b e e n f o u n d (see Table 2) that the size o f large L S D A s increases, but the effect of ion i m p l a n t a t i o n is manifested m o s t strongly f o r m e d i u m L S D A s with the sizes o f 7 - 1 2 /~m: they b e c o m e visible even in the samples with e n h a n c e d dislocation density, w h e r e they w e r e n o t o b s e r v e d b e f o r e the implantation, a n d their c o n c e n t r a t i o n p a r a m e t e r CA nm 2 g r o w s greatly in all the samples. T h e intensity of light scattering b y small L S D A s also grows (their p a r a m e t e r s c a n n o t be estimated precisely in the samples with n o r m a l dislocation density b e c a u s e o f a h u g e increase in L A L S intensity b y m e d i u m L S D A s ) . T h e p a r a m e t e r s o f L S D A s and the s h a p e o f L A L S diag r a m s for the samples o f I n P with e n h a n c e d dislocation density b e c a m e very close to those f o r n o r m a l InP. M o r e o v e r , the p a r a m e t e r s o f L S D A s c h a n g e d even in the regions of the samples c o v e r e d with masks. A similar b e h a v i o u r of scattering i n h o m o g e n e i t i e s was o b s e r v e d in Si crystals after P+ implantation [18]. T h e changes in L A L S diagrams and, as a rule, a g r o w t h
of L A L S intensity, including the regions c o v e r e d with masks w h e n e x p o s e d was registered in this case as well. C h a n g e s of scattering p r o p e r t i e s of the accumulations, in the o p i n i o n of the a u t h o r s o f Ref. [18], m a y have m a n y causes: n e w point defects m a y have entered the L S D A s ; c o m p e n s a t i n g impurities m a y f o r m electrically active c o m p l e x e s with the intrinsic defects g e n e r a t e d in the process of implantation; dissolution of impurity microclusters and the additional activation of L S D A s m a y be a result of the c a p t u r e of intrinsic defects in the accumulations; and so on. In the case of A r ÷ implantation in InP, the g r o w t h of L A L S intensity (the activation of L S D A s ) m a y m o s t likely o c c u r b e c a u s e of the increase of Inp c o n c e n t r a tion in L S D A s . T h e changes in L A L S diagrams for the samples with e n h a n c e d dislocation density indicate that initially L S D A s in this material are n o t electrically active. M o s t likely, the activating centers (e.g. Vp) migrate to dislocations in the p r o c e s s of p o s t - g r o w t h cooling o f the crystal and the b r e a k d o w n o f n o n stoichiometric solid solutions o f I n - I n P , and the rate of their flux to dislocations, b e c a u s e of very high dislocation density, m a y g r o w in this case in c o m p a r i s o n with
108
V.A. Yu~ev, V.P. Kalinushkin /
MaterialsScienceandEngineeringB33(1995) 103-114
that in normal InP and reach the rate of interstitial In coagulation (for normal dislocation density, the rate of coagulation of In i is much greater than the rate of vacancy flow in both sublattices, and as a result, the concentration of vacancies appears to be greater than the Ini concentration [19]). That, in turn, may result in a drop of Vp concentration with respect to Ing concentration and, as a consequence, in a reduction of Inp concentration, which is formed as a result of the reaction Vp + Ini = Inp. Ion implantation in this case increases sharply the concentration of phosphorus vacancies in material (including LSDAs, e.g. as a result of their capture from intrinsic defect flux), that may result in growth of Inp concentration in LSDAs. In this case the LALS intensity would increase greatly and, the sizes of LSDAs being nearly equal in the materials with different dislocation densities (only the degree of their activation is different), the shapes of LALS diagrams for the material with high dislocation density would coincide with those for ImP with normal dislocation density. Namely this is seen in the experiments. A somewhat lower growth of LALS intensity after ion implantation in the regions of crystals covered with masks is likely to be caused by a lower flux of intrinsic defects in the lateral direction in comparison with their longitudinal flux. Such a "lateral effect", however, was described at ion implantation, for example, in Ref. [18]. It should be noted as a confirmation of the above explanation of the influence of ion implantation on LSDAs, that the PL band with a maximum at 1.392 eV, which is associated with radiation transition of electrons on the center Inp °, was registered in the PL spectra of n-ImP at 4.2 K after bombardment of the crystals with 3.5-4 MeV electrons when the dose was greater than 5 x 1015 cm -2 [20]. When heating, this center switches to the state Inp- as a result of the thermal activation (the energy of the Inp °/- transition is E v+ 30 meV), and the latter determines the light scattering by LSDAs at room temperature in the proposed model.
2. 4. Effect of the probe beam radiation The effect of slow growth of the LALS intensity in InP at liquid nitrogen temperature under CO2-1aser radiation (2 = 10.6/~m) has already been mentioned in Refs. [5,6]. Let us describe it in more detail, as this effect impedes the investigation of LALS temperature dependences in InP. At a sample temperature lower than that of freezing. out the point centers constituting LSDAs in InP [5-8] (it is about 120-140 K, which corresponds to the activation energies of about Ev + (15-50) meV ) growth
of the LALS intensity is observed. When the power density of the probe CO2-1aser is 1-1.5 Wcm -2, the characteristic growth time is 100-120 min (see Figs. 4(a)-4(c), in which the LALS diagrams illustrating this effect are plotted, and Fig. (5), in which the graphs of the scattering intensity growth and relaxation are depicted); the scattering intensity during this time is in excess of the initial one at 300 K, and the concentration An m grows by 2-3 times (this corresponds to growth of the hole concentration Pm by 3 x 1016-3 × 1017 cm-3). After switching off the illumination, a slow decay (for about 80 min) of the LALS intensity is observed (the sample is illuminated with the probe radiation only when LALS is measured). If the cooled sample after freezing out is not illuminated with the probe emission, i.e. it is illuminated only during LALS measurements, the growth of LALS intensity does not occur. The same changes in LALS intensity take place also under CO-laser radiation (2 = 5.4/~m) in the absence of CO2-1aser emission (when the latter is used only as a probe beam for a short time of making measurements). It should be noted that the above effect has been observed in the samples of I n P : F e and InP with normal dislocation densities (see Figs. 4(a) and 4(b)), while it has been registered in the samples of InP with enhanced dislocation density only after Ar + implantation. (The carrier concentration An m grows in this material at 100 K under CO2-1aser emission for 80 min by 1.5-2 times in large LSDAs and by 2.5-3 times in medium ones (see Fig. 4(c)). A 2 h relaxation in darkness returns large LSDAs to the initial state, while the scattering intensity by medium ones in 2 h is just slightly higher than initially. The latter circumstance witnesses in favour of the fact that, as a result of Ar + implantation, LSDAs in this material become analogous in their composition to LSDAs in normal InP. At present, we have no satisfactory explanation for the described phenomenon. Maybe it is conditioned with some effects connected with the spatial separation of charges in the potential relief. We should remark also that the temperature dependences of LALS in InP with enhanced dislocation density unless exposed to Ar + implantation are anomalous (see Refs. [5,6]), the growth of LALS intensity is observed instead of ordinary activation drop. This may also be a result of CO2-1aser action.
3. Gallium arsenide
3.1. S L A L S images and parameters of LSDAs The samples of vertical gradient freeze (VGF) and LEC-grown undoped semi-insulating (SI) GaAs have
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109
Materials Science and Engineering B33 (1995) 103-114
9
Fig. 5. Increase under CO2-1aser radiation (curve 1 ) and decay in darkness (curve 2) of the light-scattering intensity for undoped InP at 100 K.
b e e n studied by b o t h S L A L S and L A L S methods. T h e i r S L A L S images are shown in Figs. 6(a)-6(c) and their L A L S diagrams are depicted in Figs. 6(d) and 6(e). T h e samples of V G F SI G a A s (samples 1 and 2) were 49 m m diameter wafers with a thickness of 0.5 mm. T h e etch pit density (epd) in t h e m was about 1500 c m -2, however this material had relatively high impurity cluster sites (mainly SIO2) f r o m crystal growth process. T h e resistivity was 27.5 x 106 Q cm, and the carrier mobility was 5250 c m 2 V-1 s-1 (both at 300 K). T h e wafers of L E C SI G a A s with a 76 m m diameter had a thickness of 0.58 m m (samples 3 and 4). T h e s e wafers had an e p d of a b o u t 7.5 x 104 cm -2. T h e resistivity was 17 x 106 fl cm, and the carrier mobility was about 6 0 0 0 cm 2 V - 1 s - 1 (at 300 K). Sample 5 was a wafer of L E C SI G a A s grown in a (100) direction in a P B N crucible. A s - g r o w n crystals were annealed u n d e r v a c u u m in a sealed quartz a m p o u l e at 900 °C before cutting. Its resistivity at 300 K was greater than 107 Q cm, its mobility was greater than 4 0 0 0 cm 2 V-1 s - l , and the epd was less than 105 cm -2. T h e wafer diameter was 76 mm, its thickness was 0.625 mm. All the samples were polished on b o t h sides and finished for use in waferprocessing equipment. As in the case of indium phosphide, the white spots in S L A L S patterns are the images of L S D A s ( 1.3 x 1.3 m m 2 areas are presented). L S D A concentrations estim a t e d f r o m the S L A L S patterns are given in Table 3
O (degr.)
Fig. 4. LALS diagrams for undoped InP (a) and InP : Fe (b) with a dislocation density of about 5 x 104 cm -2, and those for undoped InP with a dislocation density of 105-106 cm -2 after Ar + implantation (c); curves 1,7, 300 K; curves 2,3,4,5,6, 100 K; curves 1,2, initial sample; curve 3, 1 h under CO2-1aser radiation; curve 4, 1 h under COz-laser radiation and 1 h in darkness; curve 5, 80 min under CO2-1aser radiation; curve 6, 80 min under CO2-1aser radiation and 2 h in darkness; curve 7, after heating.
V.A. Yuryev, V.P. Kalinushkin
1 10
/
Materials' Science and Engineering B33 (1995) 103-114
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--4.5
10 (e)
4 I,lll,lllllllIH~rlllttllrlJll,lrlflllllrlrlll~lllltlr~lll,lllll,111~lllllll,ll
2
3
4 5 6 7 8 Scc~tte~ng a"n.gle, 0 (degr.)
9
I
10
Fig. 6. L S D A s in V G F a n d L E C SI G a A s : S L A L S i m a g e s of s a m p l e 1 (a), s a m p l e 4 (b) a n d s a m p l e 5 (c)(1.3 x 1.3 mm2): a m p l i f i c a t i o n in (a) a n d (b) is 1 0 0 t i m e s h i g h e r t h a n in (c). L A L S d i a g r a m s (d), (e); c u r v e s 1,2, s a m p l e s 1 a n d 2; c u r v e s 3,4, s a m p l e s 3 a n d 4; c u r v e 5, s a m p l e 5. R o o m t e m p e r a t u r e .
Table 3 P a r a m e t e r s o f L S D A s in SI G a A s No.
a (/~m)
log(lo/W )
CAnm ? (c m - 9 )
C(cm-3)
Pm (cm 3)
Pv ( c m - 3 )
1
16-17 8-10
- 3.0 to - 2.9 - 3.5 to - 3.1
= 10 35 ( 1 . 5 - 3 ) × 10 36
-4 × 10 5
-1.5 x 10 ~6
-( 2 - 3 . 5 ) × 10 ~3
2
= 16 8-9
= -2.1 - 2.9 to - 2 . 7
= 10 36 ( 8 - 1 0 ) × 1036
-4 x 105
-3.5 x 1016
-( 4 - 5 . 5 ) x 1013
- 2 . 5 to - 2 . 4
( 3 - 1 0 ) X 1036
( 4 . 5 - 5 ) X 105
( 1 . 5 - 3 . 5 ) × 10 ~6
( 7 - 7 . 5 ) x 1013
. ( 4 - 4 . 5 ) × 105
= 1016
( 1 . 5 - 2 ) x 10 ~3
,~- 1.5 × 1017
= 2.5 x 1014
3
9-12
4
25-30 8-9
- 2.7 to - 2.1 - 3.7 to - 3.5
. = 1036
= 12
= -0.9
= 10 38
5
.
2 × 10 5
.
No. is s a m p l e n u m b e r ; a is t h e a c c u m u l a t i o n radius; l o g ( l 0 / W ) is a l o g a r i t h m o f t h e z e r o - a n g l e s c a t t e r i n g i n t e n s i t y p e r p r o b e - b e a m unit p o w e r (units l o g ( s r - 1 c m - 3 ) ; CAnm 2 is a c o n c e n t r a t i o n p a r a m e t e r ; C is t h e c o n c e n t r a t i o n o f L S D A s ; A n m is t h e effective m a x i m u m differential c a r r i e r c o n c e n t r a t i o n in L S D A s ; Pm is t h e m a x i m u m h o l e c o n c e n t r a t i o n inside a c c u m u l a t i o n s ; Pv is t h e total c o n t e n t o f h o l e s i n s i d e L S D A s p e r crystal unit v o l u m e .
V.A. Yuryev, V.P. Kalinushkin
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Materials Science and Engineering B33 (1995) 103-114
together with other parameters of LSDAs estimated from LALS diagrams. The amplification of the images in Figs. 6(a) and 6(b) is 100 times higher than that in Fig. 6(c), that is fully in agreement with the ratio of the LALS intensities in Figs. 6(d) and 6(e). (It should be noted that the signal-to-noise ratio in Figs. 6(a) and 6(b) is rather close to unity.) As in the case of indium phosphide, the data obtained from SLALS and LALS are in good agreement with those given in Ref. [8].
3.2. I n f l u e n c e o f V P E on L S D A s in G a A s : Cr
To investigate the effect of VPE on light scattering with LSDAs in the bulk substrate, undoped GaAs epitaxial layers were grown using the trichloride vapor phase epitaxy on substrates of chromium-doped LEC gallium arsenide. The dislocation density in the substrates was 104-105 cm -2 and their resistivity was in excess of 108 f2 cm. The substrate surfaces were oriented at angles of 2-6 ° with respect to the (100) plane. The thicknesses of the substrates were 300/xm. The wafers intended for epitaxial growth were cut from the same ingot and were subjected to chemomechanical polishing on one side, which was used to grow epitaxial layers. Before epitaxy, the substrates were washed in solvents, the damaged surface layers were etched away in a liquid etchant, and immediately before growth the substrates were etched again with a vapor etching agent containing arsenic trichloride. The substrates were positioned on a quartz holder during growth. The temperature during vapor etching and growth was 720 °C and the temperature of the source of gallium enriched with arsenic was 810 °C. The film thickness changed from 2 to 15/xm, the film growing rate was 0.1 # m min -1 . The carrier concentration in the epilayers did not exceed 1014 cm-3. After growth of the epilayers, the opposite side of a wafer was polished so as to make the wafer fit for investigation by LALS. Control samples were cut from the same ingot (being usually wafers in the ingot adjacent to the processed ones) and polished on both sides, as they were not subjected to epitaxial growth or any other treatment. Some of the wafers, initially polished on both sides, were also used to grow epilayers being investigated by LALS beforehand. Unfortunatley, the films grown on them were of poor quality, and the epitaxial layers had to be polished off completely before investigating the samples by LALS. In some cases, it was possible to find regions of sufficient size with a perfect enough epilayer. LALS diagrams of these regions were essentially the same before and after polishing, so all the changes in the light scattering during epitaxy may be associated with the substrate bulk, as the scattering of light by the
substrate greatly exceeds in intensity the contribution of the epitaxial layer. Some of the samples were subjected to a thermal treatment at 720 °C in a hydrogen atmosphere, which simulates the VPE procedure. The annealing last 1 h and samples were placed in a quartz holder. One sample was subjected to vapor etching alone and a field effect transistor (FET)-like structure was formed on the other. The parameters of LSDAs estimated from LALS diagrams of these samples (Fig. 7) are listed in Table 4. It is seen that the epitaxy, the complete cycle used in device manufacture and the simulating annealing enhance the intensity of light scattering and slightly increase the size of the LSDAs. The increase in LALS intensity is caused by growth of the free carrier concentration in LSDAs, as it follows from both the LALS diagrams and temperature dependences discussed below. The samples subjected to vapor etching manifested the same trends as the other treated samples. To estimate the activation energies of the centers responsible for higher hole concentration in LSDAs, the LALS temperature dependences (Fig. 8) were measured. The depth of the levels of these centers in the initial material was 6 0 - 9 5 meV, whereas after VPE growth it was 60-110 meV and in the annealed material it was 70-110 meV (the estimates were made as in Refs. [7,8]). It is obviously, in spite of the increase in hole concentration in LSDAs, the energy location of the centers which are responsible for this did not change and was close to the first ionization energy of GaAs antisite. Consequently, it may be concluded that the increase of the carrier concentration in LSDAs,
0.0
00000 -I DOOOD - - 2
-0.5
0
':'
60660 -4 0,0., -5
-I.0 "~ - 1 . 5 •~ - 2 . 0
•~ -2.5 -3.0 -3.5
I'IInI~1~Ir'Iul1'~l~u''~I~I1~rInIql~nII~l~flII~n~q~1Il~I~n~11~I1InIl1nIt11IqIllIIIIl q
2
3
4 5 6 7 8 9 S c a t t e r i n g angle, 0 (degr.)
I0
II
Fig. 7. LALS diagrams for GaAs ; Cr before and after different processings; curves 1,2, initial wafer; curve 3, after simulating thermal treatment; curves 4,5, after VPE; curve 6, after F E T fabrication cycle. R o o m temperature.
11 2
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Materials"Science and Engineering B33 (1995) i03-114
Table 4 Parameters of LSDAs in GaAs : Cr and their changes after different processings
GaAs : Cr
a (/tm)
Initial
~
log( CAnto 2) (log(cm-9))
log(lo/W )
17
-
1.9
to
-
1.8
~--36 37.0-37.6 --
Pm (cm - 3)
AEi (meV)
--
--
(3-10) x 1016 --
60-95 --
~<2
- 2.7 to - 1.8 ~ -3
20-25 12-13 ~<2
~0 - 1.2to - 0 . 7 - 2.6 to - 2 . 2
36.9-37.5 37.5-38.1
-(6-20)x 10 ~6
-70-115
--
--
--
VPE
17-25 8-11 ~<2
- 0.8 to 0.0 - 1.4to - 0 . 9 - 2.2 to - 1.8
36.9-37.0 38.2-38.3 --
-( l - 2 ) x 1017 --
-60-110 --
After device fabrication cycle
= 15 =8 2
- 0.7 to - 0.3 - 1.4 to - 1.0 - 2.7 to - 2.2
37.5-37.9 38.5-38.9 --
-~<(2-5) x 1017 --
--
8-10
After annealing at 720 °C After
-
-
--
a is the accumulation radius; log(lo/W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr- 1 cm-3); CAnto 2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs; Pm is the maximum hole concentration inside accumulations; AE, is the thermal activation energy of the point centers in LSDAs.
ifll f l l l f l l f l f l r l r frlr~lflf~fl f l l l l l l l l l l l , l I F l l l l l l r , l f l l l l l l l l l l l l f l f , , , , l l l l l l , l ~ l l l l l l l , l l l l l l l
-I.0
1 -
as
2
aft;er
--
grown VPE
3 - after simulating thermal treatment ~-1.5 dt
.I.
-2.0 •
3
Loo o< -2.5
II~q'lllll~llVHII qeU~IlIIIUlHIPlIIIIpIIITIIII]IIIllIIIlllIllUIIIIIIIIIIrIIL[IIHIr PII~IIHIII
2
3
4
5
6
7
8
9
fO
! I
12
Reciprocal temperabure, IO00//T ( K - t )
Fig. 8. LALS temperature dependences for GaAs : Cr; curve 1, initial wafer; curve 2, after VPE; curve 3, after simulating thermal treatment.
w h i c h was o b s e r v e d in t h e e x p e r i m e n t , was c a u s e d b y a n n e a l i n g at 7 2 0 °C a n d as f a r as w e c o u l d j u d g e , t h e a t m o s p h e r e o f a n n e a l i n g h a d n o significant i n f l u e n c e o n t h e L S D A s l o c a t e d in t h e b u l k o f t h e s u b s t r a t e . It m a y t h e r e f o r e b e c o n c l u d e d t h a t the p r o c e d u r e o f V P E results in c o n s i d e r a b l e g r o w t h in f r e e c a r r i e r c o n c e n t r a t i o n in L S D A s l o c a t e d in t h e b u l k o f t h e G a A s : C r s u b s t r a t e s . T h i s c h a n g e is likely to b e d u e to an i n c r e a s e in t h e c o n c e n t r a t i o n o f GaAs antisites as a result o f s u b s t r a t e a n n e a l i n g at 7 2 0 °C in t h e p r o c e s s o f V P E . W h e n a F E T - l i k e s t r u c t u r e is f a b r i c a t e d o n
G a A s : C r s u b s t r a t e s , t h e m a i n c h a n g e s in L S D A s in t h e substrfite b u l k o c c u r d u r i n g t h e g r o w t h of t h e e p i t a x i a l layer. It s h o u l d also b e n o t e d t h a t t h e a n n e a l i n g o f t h e wafers of GaAs:Cr f o r 1 h w i t h o u t c o a t i n g at T = 7 5 0 °C in a H2 a t m o s p h e r e r e s u l t e d in a n i n t e n s i v e e s c a p e o f v o l a t i l e A s a t o m s f r o m the s u r f a c e o f t h e s a m p l e s [20] ( a n n e a l i n g at T ~> 8 5 0 °C r e s u l t e d in t h e emergence of gallium droplets on the sample surface [21,22]) a n d p r o d u c e d a p - t y p e l a y e r o f 0.5 ¢tm thick, w h e r e t h e h o l e d e n s i t y was a r o u n d 1016 c m -3 [22,23]. The hole concentration appeared because annealing g e n e r a t e d c e n t e r s with an a c t i v a t i o n e n e r g y o f a b o u t 100 m e V , w h i c h w e r e a t t r i b u t e d in Ref. [23] to GaAs a c c e p t o r s . A t a n n e a l i n g t e m p e r a t u r e s o f 7 0 0 - 7 3 0 °C no conversion producing a p-type layer occurred, p r o b a b l y b e c a u s e of t h e r e d u c t i o n in t h e r a t e o f A s a t o m e v a p o r a t i o n f r o m the surface. I n t e n s i v e f o r m a t i o n o f VAS is n o t e s s e n t i a l f o r t h e f o r m a t i o n o f GaAs c e n t e r s in L S D A s ( a c c o r d i n g to Refs. [7,8,19,24] t h e c o n c e n t r a t i o n o f VAS in L S D A s m a y b e c o n s i d e r a b l e ) , so that the active f o r m a t i o n of t h e GaAs c e n t e r s in L S D A s d o e s t a k e p l a c e at t e m p e r a t u r e s s o m e w h a t b e l o w t h e t h r e s h o l d of f o r m a t i o n of t h e p - t y p e l a y e r o n t h e surface.
3.3. Influence of electron bombardment on L S D A s in undoped GaAs Single c r y s t a l s o f u n d o p e d n - G a A s w e r e g r o w n b y t h e L E C p r o c e s s in t h e (100) d i r e c t i o n . B e f o r e i r r a d i a t i o n t h e e l e c t r o n c o n c e n t r a t i o n in t h e m was 4 × 1015
V.A. Yuryev, V.P. K a l i n u s h k i n
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Materials Science a n d Engineering B33 (1995) 103-114
1.0
Table 5 Parameters of LSDAs in undoped GaAs and their changes after electron irradiation
AAAAA - - I or~nOo - 2
0.5 0.0 .
GaAs
- 0 . 5
-
ct
log(lo/W )
log( CAnm 2) (log(cm-9))
(/~m)
g -t.o .~
113
Initial
23-27 10-12 ~<2
0.3-0.7 - 1.6 to - 1.2 ~- -2.0
37.2-37.4 37.4-37.8 --
Irradiated
21-23 12-13 ~<2.5
0.7-1.1 - 1.2 to -0.6 - 3.3 to - 2.7
38.0-38.2 37.7-38.0 --
1.5
-2.0
-e.5 c~
-3.0 -3.5
Imm~mlll,lllllllIHmqlmllml,l,lmlq,,ImH,IIm,lllqHImllqllmllll
2
3 4 5 6 7 8 S c a t t e r i n g angle, 0 ( d e g r . )
9
I
10
Fig. 9. LALS diagrams for undoped n-GaAs with low resistance before (curve 1) and after (curve 2) electron bombardment. Room temperature.
cm -3 and the mobility was (1-5)x 103 cm 2 V -1 s -1. The dislocation density was around 104 cm-2. Parallelsided GaAs wafers were cut from an ingot along the (100) plane. Samples were irradiated with 3.5 MeV electrons (a dose of 2 x 1015 cm -2) at room temperature. As a result, the carrier concentration in them was reduced to 5 × 1014 cm 3. Fig. 9 shows typical LALS diagrams for GaAs crystals before and after irradiation. The light scattering intensity by large inhomogeneities increased as a result of electron irradiation. No significant influence of sample orientation with respect to probe beam polarization plane upon the diagram shape and scattering intensity was found. So the diagrams may be approximated on the basis of a model of spherically symmetric LSDAs with a Gaussian profile of the radial distribution of the dielectric constant [1,7,8,13]. The parameters of LSDAs estimated from the diagrams are given in Table 5. An increase in the light-scattering intensity could, in our opinion, be attributed to an increase in GaAs concentration in LSDAs [8] (it was reported in Ref. [20] that the electron bombardment of InP results in the formation of the antisite Inp, which is completely analogous to GaAs). The processes, which may enhance the concentration of gallium antisite in LSDAs, are proably associated with the capture of intrinsic defects generated by irradiation outside LSDAs (as in the case of Ar + implantation in InP) or owing to the formation of these defects in LSDAs. The high-level plateau due to the light scattering by particles of radius less than 1-2/~m disappeared from the LALS diagrams indicating that these defects are
a is the accumulation radius; l o g ( l o / W ) is a logarithm of the zero-angle scattering intensity per probe-beam unit power (units log(sr-L cm-3)); CAnm2 is a concentration parameter; C is the concentration of LSDAs; An m is the effective maximum differential carrier concentration in LSDAs.
likely to be of different origin to the larger ones. The appearance of intrinsic point defects in these LSDAs could result in their strong compensation (e.g. because of the formation of the complexes of intrinsic defects with defects constituting LSDAs or because of partial dissolution of impurity microprecipitates in LSDAs, etc.).
4. Conclusion
In summary, the main results presented in the paper are as follows. The SLALS images of LSDAs in indium phosphide and gallium arsenide are given. The data obtained from them are all in good agreement with former estimations made in Refs. [7,8]. The size of the LSDAs in InP: Fe is shown to be dependent on ingot growth conditions. The implantation of Ar + ions with energies of 200 and 250 keV with a dose of 1016 cm -2 in the crystals of undoped InP and 3.5 MeV electron bombardment of undoped (low resistance) GaAs with a dose of 2 x 1015 cm- 2 result in an increase of free-carrier concentration in the volume of the LSDAs. Activation of the LSDAs is probably caused by the growth of Ine and GaAs antisite concentrations in LSDAs. When growing the epitaxial layer of undoped GaAs on the substrate of SI GaAs : Cr by the VPE process, the free carrier concentration grows in LSDAs located in the substrate bulk. Analogous growth of the free carrier concentration in LSDAs is registered after wafer thermal treatment, which simulates the VPE process (at 720 °C without coating in an atmosphere of H2). The activation energies of the centers, which determine the LSDAs' conduction type and the carrier concentration in them, do not change after the epitaxy
114
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Materials Science and Engineering B33 (1995) 103-114
and the thermal treatment in c o m p a r i s o n with those in the original material. T h e growth of free carrier concentration in L S D A s is a consequence of wafer annealing and is likely to be connected with the growth of the GaAs concentration in LSDAs. It should also be mentioned that one can find s o m e additional information on the influence of annealing on L S D A s in InP in Ref. [4], on that in G a A s in Ref. [9], and s o m e data on the d e p e n d e n c e of L A L S intensity on G a A s melt stoichiometry in Ref. [11].
Acknowledgments We would like to thank Dr. A.V. Spitsyn for ion implantation, Professor A.K. Abiev for electron b o m b a r d m e n t and Dr. T.V. T h i e m e for growing epitaxial layers. We express our appreciations to Dr. E.V. Russu, Professor I.M. Tiginyanu and Mr. J. Oicles for kindly supplying the experimental samples.
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