n-SiŠB,S‹
M. S. Yunusov, M. Karimov, and P. A. Khalikov
Institute of Nuclear Physics, Uzbek Academy of Sciences, Tashkent
共Submitted April 13, 1999兲
Pis’ma Zh. Tekh. Fiz. 25, 1–4共December 26, 1999兲
Thermal annealing was used to study silicon samples having different sulfur concentrations. It was established that the temperature at which the sulfur centers decay depends on the
concentration of sulfur atoms Ns in the overcompensated silicon. As the distance between the
impurities (Ns⫺1/3) decreases, the decay temperature increases. The effect can be attributed to characteristic features of the distribution of the compensating sulfur impurity atoms inside a region of fluctuation, formed in silicon during doping. © 1999 American Institute of
Physics. 关S1063-7850共99兲01612-2兴
Numerous studies have been made of the thermal stabil- ity of impurity centers in doped silicon having deep energy levels.1–8 However, the physical processes taking place in these materials have not yet been clarified. Lemke1–3 used DLTS to study Au, Pt, Ir, and Rh impurity centers in n- and p-Si before and after repeated heat treatment, and showed that the observed concentrations of impurity centers in sili- con decrease, or are converted into other centers at tempera- tures below 400 °C 共in this case the concentrations of impu- rity centers are Nim⯝1013–1014cm⫺3). In Refs. 4–8 the decay of impurity centers 共Au, Pt, Ir, Rh, and S兲 having concentrations Nim⯝1015–1016cm⫺3 was observed at tem- peratures above 600 °C.
The aim of the present study is to identify the influence of the concentration of sulfur centers on the thermal stability of the conductivity of overcompensated n-Si
具
B,S典
.Sulfur was chosen as the silicon impurity because first, sulfur impurities form donor centers in the upper half of the silicon band gap and second, sulfur has a high solubility (⬃3⫻1016cm⫺3at 1320 °C兲. These two factors allow us to obtain n-type silicon 共overcompensated n-Si
具
B,S典
) from p-type over a wide range of resistivity 共between 1 and 105⍀•cm兲. In this case, sulfur atoms are the principal dop- ant in n-Si具
B,S典
. By establishing the temperature Tp atwhich the transition from n- to p-type conductivity takes place under heat treatment, we can determine the dependence of the concentration of sulfur centers on the stabilization temperature (Ts⫽Tp) for the material conductivity.
The starting material was Czochralski-grown p-type silicon with resistivity ⬃45 ⍀•cm and dislocation density
⬃2⫻104cm⫺2.
The silicon was doped with sulfur impurities by thermal diffusion in the range 1000–1300 °C for ⬃20 h in an open quartz ampoule followed by cooling at a rate of
⬃250 deg/min, when conversion from p- to n-type conduc-
tivity was observed.
We investigated isochronous annealing in n-Si
具
B,S典
in the temperature range 300–900 °C for which the holding time at a particular annealing temperature was ⬃20 min.Ohmic contacts were prepared by diffusion of phosphorus prior to doping the silicon with sulfur. Standard methods of measuring the Hall coefficient were used to determine the concentration and type of silicon conductivity.9
Table I gives results of studying the variation of the concentration of sulfur centers in overcompensated silicon with the temperature Tp for the transition from n- to p-type
conductivity. It can be seen that as the sulfur atomic concen- tration increases, Tp shifts toward higher temperatures. We take the view that this is caused by a nonuniform distribution of sulfur atoms in the bulk of the overcompensated silicon, i.e., these samples have different degrees of impurity fluctuation.
Repeated heat treatment 共resulting in the formation of vacancies V and interstitial sulfur impurity atoms Si)1兲 up to
the total decay temperature Tdec (Tp⯝Tdec) leads to addi- tional lattice strength: after a sulfur atom has migrated from a lattice site to an interstitial site, a previously elongated region 共the radius of the sulfur atom is smaller than the ra- dius of the silicon atom;10in this case the lattice situated in the region of fluctuation is elongated兲 becomes even more elongated. Then, the lattice strength is compensated by the interstitial atom Siand becomes concentrated inside the fluc-
tuation region, i.e., the released V and Si, migrating within
the region have a fairly high probability of encountering each other. As a result, an Sienters a lattice site configuration or a
new 关V⫹Si兴 forms. Thus, in our view, as Ns increases, Tp
shifts toward higher temperatures. The higher the impurity concentration in the region of fluctuation, the shorter the dis- tance between the sulfur atoms (N⫺1/3) and the larger the radius of action of the elastic forces, and as a result the thermal stability of the conductivity of the compensated sili- con is enhanced 关in this case N⫺1/3⯝ 共0.94– 2兲⫻10⫺5cm兴. The proposed mechanism to explain the thermal stability of compensated silicon also confirms the results reported in Refs. 1–3. In these studies the distance between the impurity atoms is N⫺1/3⬍4⫻10⫺4cm.
The condition for efficiency of this mechanism is that the distance between impurities N⫺1/3should be smaller than
TECHNICAL PHYSICS LETTERS VOLUME 25, NUMBER 12 DECEMBER 1999
969
the distance of action of the elastic forces Rel, i.e., N⫺1/3
⬍Rel. As the temperature increases, all the sulfur atoms are simultaneously released from vacancies and the mechanism ceases to operate.
It should be noted that when analyzing the shift of the total decay temperature as a function of the impurity atom concentration, we allowed for dilatational charges, i.e., we assumed that in the bulk of the material the compressed and elongated regions共formed as a result of heat treatment to the total decay temperature兲 have opposite dilatational charges.
To sum up, as a result of this analysis we have proposed a mechanism to explain the dependence of the total decay temperature of sulfur centers on their concentration, which we attribute to the different degrees of fluctuation of the sulfur atoms: the larger the number of sulfur atoms in a re- gion of fluctuation, the higher the probability of the decay product becoming fixed inside this region in the bulk of n-Si
具
B,S典
.It is quite interesting to note that the temperature depen- dence of the annealing of the same center 共in this case an
A-center11兲 depends on the degree of order of the semicon- ductor共i.e., as the radiation dose accumulates兲. However, the radiation conditions in the experiment are such that alterna- tive mechanisms may appear.12
1兲Several types of impurity fluctuations occur in the bulk of the doped ma-
terial. It was assumed that the sulfur centers begin to decay in a fluctuation where the distance between the sulfur atoms is greater than in other fluc- tuations.
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birsk共1981兲, 181 pp. Translated by R. M. Durham TABLE I. Dependence of the conductivity stabilization temperature in
n-Si具B,S典on the concentration of sulfur centers. Concentration of
sulfur centers (Ns), cm⫺3 6⫻1014 1⫻1015 6⫻1015 2⫻1016
Conversion temperature (Tp), °C 680 700 730 780