FORMATION OF
LOW-DIMENSIONAL IMPURITY
DEFECTIVE COMPOSITIONS IN
DOPED SILICON UNDER THERMAL
AND RADIATION TREATMENTS
Sh. Makhkamov,
N.A. Tursunov, M. Karimov, K.P. Abdurakhmanov, A.R.
Sattiev, A.O. Kurbanov, Sh.A. Mahmudov, M.N. Erdonov
Introduction
Application of semiconductor crystals to microelectronic
manufacture, in particular silicon, doped by fast diffusing impurities,
mainly depends on defectiveness of initial material. Basic defects in
silicon single crystals are dislocations, microdefects, accumulations of
impurity atoms in the form of precipitates and others. Presence of such
defects in the silicon plates can lead to degradation of the device
characteristics and their failure. Therefore detection of these defects in
the crystals and determination of their sizes and concentration have
extremely great practical value. In this aspect despite the
electrophysical and photoelectric properties of silicon doped by
palladium, copper and nickel impurities were examined in detail,
insufficient attention is given to formation of structural defects and their
reconstructions under the external influences on these materials.
Purpose
The current work is dedicated to study regularities of the physical processes of formation
and reorganization of impurity defect composition in silicon doped by fast diffusing
impurities of copper, nickel and palladium at thermoradiation treatment, identification and determination of microstructure defects, homogeneity degree of their distribution and influence of the defect states on the crystal properties.
Samples and experimental methods
:
Silicon single crystals were used with n- and р-types conductivity and resistance from 1 to 40 Ohmcm with dislocation density about 104 cm-2 and with oxygen content about 1017
cm-3. Silicon plates were doped with impurities by thermo diffusion method in the
temperature range of 10501250оС during 0.55 hrs. The irradiation was carried out to
reactor neutron fluences of 51016 51018 cm-2 with the subsequent annealing at 8000С
for 0.5 hrs.
Effectiveness of formation and reconstruction was controlled by the
impurity-defect complexes in the silicon samples before and after radiation exposure by measuring the electrophysical parameters (, n, p) at the standard Hall installation, and recombina-tion parameters () were studied by starecombina-tionary photoconductivity method. Type and structure of defect states were examined on the modernized infrared microscope MIK-1
Methods of study
:
Method of nonstationary capacitive spectroscopy of deep levels (DLTS) with the temporary window of the speeds of emission 50 s (1) and 2 ms (2). It is intended for measuring the concentration, type and ionization energy of level, and also capture cross section of contaminant- defective centers.
Method of Infrared
microscope
Type and structure of defect states were examined on the modernized infrared microscope MIK-1 having B/W CAMERA with data acquisition on the base of the Pentium 4- of 2.0 GHz with 713X TV CARD.
Nuclear reactor VVR-SМ
Low enriched fuel
ТВС 36% (14 pieces)
ТВС 19,8 % (6 pieces)
Maximal fluency of heat
neutrons
8.10
13cм
-2.s
-1Unique Installations
of
Gamma sources-Со
60building
Activity - 20000 Кu
Power – 1500 R/s
Unique Installations of
Microphotographies of investigated silicon samples are shown in fig. 1. As can be seen in (a), two types of microdefects are observed in the undoped samp-les: A (larger - the accumulation of inculcated type of dislocatory loops) and in B (smaller - vacancy type defects) that have oval and roundish form with sizes not exceeding 1015 m. Microdefects are larger in the samples doped by
palladium at 11500C, which is apparently caused by the aggregate of palladium
atoms on these microdefects and by the formation of impurity clouds around them (Fig. 1, b). These defects transform into the needle-shaped defects when the palladium diffusion temperature increases (Fig. 1, c). The transformation of defects in the samples doped by palladium occurs during irradiation to the
neutron fluence 1017 cm-2 can be seen in (Fig. 2, a, b). The bright faultless
regions appear with sizes of 5-7 m around the microdefects as a result of attraction to nucleus of different clusters because of Coulomb interaction.
Experimental results
а b c
Fig. 1. Microphotography of p -type silicon samples: a - undoped sample;
b - doped by palladium at 11500C; c- doped by palladium at 12500C.
(micrometrical scale with 10 m step is given the on the inserts). А
The most complete qualitative explanation of the observed processes of forming the microdefects in initial and doped silicon can be done on the basis of recombination- diffusion mechanism. Three diagrams are possible for formation of microdefects in the undoped samples according to this mechanism: sequential (primary clusters and disordered regions are converted into B - the defects, large ones of which are transformed in A defects), parallel (simultaneous transformation of clusters and disordered regions in A and B - defects) and independent (A and B - defects which are formed by the independent mechanisms).
The analysis of the obtained results shows that the presence of the palladium impurity in the doped samples, whose covalent atomic radius differs from that of silicon, leads to change of the crystal elastic energy and impurity accumulations around the microdefects, followed by their coarsening. An increase of the palladium concentration or irradiation fluency results in the decomposition of impurity accumulations, decrease of the nuclear sizes of microdefects and reconstruction of palladium atoms with their subsequent incorporation into the chain of needles.
Основные результаты
а b
Experimental results
Fig. 2. Microphotography of the silicon samples doped by Pd at 11500C
Several different pictures were observed in the silicon samples
doped by copper and nickel impurities. Doping with copper impuritiy
the star-shaped impurity- defective low-dimensional compositions
are formed in the silicon volume, with laminar distribution in plane,
perpendicular to the direction of the copper diffusion (Fig. 3, a).
It was discovered in this case, that the boron concentration
decrease at the constant copper concentration or at the copper
concentration growth at the constant boron concentration leads to
the confluence of defects from several planes at each other and the
growth of the sizes of impurity - defective low-dimensional
compositions from 10 -15
m to 50 -70 m (Fig. 3, b, c). This effect
was observed at the irradiation. Analogous results were obtained
also in silicon with nickel impurity.
Основные результаты
а b c
Fig. 3. Microphotography of the silicon samples doped by copper at 12200C
with different initial specific resistances: a - 4; b - 10; c- 20 Ohmcm.
Parameters of doped and control Si samples.
No Samples
init. , Оhmсm , OhmcmElectrophysical parametersp, cm-3 , cm2V-1s-1 charge carriers, , sLife time of
1 р-Si<B> р-Si<B,Cu> p-Si<В,Ni > p-Si<В,Pd > 4 4 4.2 4.1 4 5.401015 5.471015 5.421015 5.381015 290 280 300 270 1.010-6 2.210-6 2.110-6 8.910-7 2 р-Si<B> р-Si<B,Cu > p-Si<В,Ni > p-Si<В,Pd > .10 10.1 9.5 9.1 10.2 1.801015 1.901015 1.931015 1.781015 343 323 330 319 9.010-7 2.010-6 1.7∙10-6 6.510-7 3 р-Si<B> р-Si<B,Cu > 17.722 8.510 14 1.11015 334 345 7.010 -7 2.410-6
The electrophysical and recombination parameters of the doped
and reference silicon samples are given in the table.
As can be seen from table, in the silicon samples doped by
copper and by nickel the values of the life time of the minority
charge carriers are 2- 3 times higher, and in p -Si<B, Pd > samples
are less than in the references. The discovered anomalous effect is
explained by formation of different low-dimensional defect structures
in the silicon volume, caused by “impurity-oxygen” type
nanocomposition.
1. It is shows that the presence of the palladium impurity in the doped samples, whose covalent atomic radius differs from that of silicon, leads to change of the crystal elastic energy and impurity accumulations around the microdefects, followed by their coarsening. An increase of the palladium concentration or irradiation fluency results in the decomposition of impurity accumulations, decrease of the nuclear sizes of microdefects and reconstruction of palladium atoms with their subsequent incorporation into the chain of needles.
2. It was discovered that the boron concentration decrease at the constant
copper concentration or at the copper concentration growth at the constant boron concentration leads to the confluence of defects from several planes at each other and the growth of the sizes of impurity - defective
low-dimensional compositions from 10 -15 m to 50 -70 m
3. It is established that in the silicon samples doped by copper and by nickel
the values of the life time of the minority charge carriers are 2- 3 times higher, and in p -Si<B, Pd > samples are less than in the references. The
