Pressure as the device for changing electric parameters of Schottky diodes based on Si mono crystal

. The analysis of investigations carried out on studying influence of all-around pressure to Schottky diodes based on n-Si<Bi> construction characteristics has been presented. It has been revealed that before the pressure influence the diodes capacitance is growing monotonically on the increasing temperature, at the same time after depressurization it loses monotony and has the maximal value about 50 pF on 30 o C temperature. It has been shown that the diodes conductivity is growing slowly on temperature both under the pressure and after its removal. When the pressure up to 12 kBar is acting to n-Si<Ni> diodes structures then we deal with monotonic growing the resistivity. Further, when pressure is growing in range of 12 and 60 kBar then changes in n-Si<Ni> samples resistivity on influence pressure has not monotonic character with a maximum value by P ≥ 35 kBar.


Introduction
In the last years great attention for investigation of properties of impurities creating the different defect centers in semiconductors is paid, because of essential role of different impurities in formation Si properties, which is main material for semiconductor microelectronics and optoelectronics.It is known also that the different high temperature processing leads to the changing defect structure of mono crystal Si, in result which takes place creation of the different associated states of technological impurities, for example, oxygen atoms in Si.Although there are many experimental materials, problems deal with the interactions of different impurities with not controlled impurities and structure defects in Si and theirs affect to parameters of semiconductors devices remain is being unexplained still.
For example, in paper [1] the Schottky diodes structure had been investigated, in which the barrier height, ideality coefficient, straight section of the voltage-ampere characteristics and flowing avalanche voltage correspond practically to "ideal" diode in the framework of thermo electron emission theory.In paper [2] the voltage-ampere characteristics of Schottky diode with the different materials (Ti, W, Mo, Ni) of anode based on 4H-SiC polytypic substance had been investigated.
In paper [3] influence of crystal structure and sizes of different metal contacts to electric physical properties of Schottky silicon diodes had been studied.It had been shown that by a definite components combination of the system the main metal alloys have an amorphous structure and other metal films have polycrystalline one.Authors showed that the thermal annealing influences to the crystallization process.The basic parameters and characteristics of the investigated Si Schottky diodes depending on the composition, sizes and crystal structure of the studied films had been revealed too.The voltage-ampere characteristics of some compositions by low straight voltages had been investigated.The causes of excess current had been revealed and height of potential barrier and not ideality coefficient by transforming from amorphous state to poly crystal one had been defined.
In paper [4] the direct and diverse voltage-ampere characteristics of the rectifier Schottky diodes based on 4H-SiC in range of temperature 20-370 o C had been investigated.The maximal current and maximal voltage in these investigations were 10-20 mA and 10-100 Volts, correspondingly.It had been found that diodes close to ideal with Schottky barrier height aprroximately 1.5 eV can be considered.Wherein the direct current in all temperatures range and diverse current on high temperatures are conditioned by thermal electron emission essentially.The upper limit of working temperatures of Schottky diodes based on 4H-SiC by studied currents and voltages correspond approximately to fundamental limit defined by barrier height and in this experiment one reached up to 370 o C.
By authors of paper [5] influence of structure states of Ti membrane for the pressure tenzo transformer to its metrological characteristics had been considered.They showed the differences in characteristics in transformers, which differ on microstructure and method of manufacturing the elastic membrane.In paper [6] influence of hydrostatic pressure to voltage-ampere characteristics of surface-barrier diode structures Sb-p-Si-Au type had been studied, where the height of potential barrier (0.75 eV) and pressure coefficient for its variation (-1.54•10 -11 eV/Pa) had been defined.
Measurements under pressure, in our opinion, can be useful in course of time for construction general theoretical model of impurity states.Investigations on influencing allaround hydrostatical pressure in paper [7] had been detail described.The metalsemiconductor structures (Schottky diodes) in microelectronics both as independent devices and as constituent components of many modern semiconductor devices and integral microcircuits are being used broadly.On the other side the discrete Schottky diodes are convenient object for studying variations of properties of semiconductor materials in the external influences, which is conditioned by simplicity of diodes construction and uniqueness of interpreted results.
One of the main requirements presented to semiconductor devices is the working reliability and sustainability of their characteristic to the different external influences.Due to which we had carried out investigations on influencing all-around hydrostatic pressure to relaxation characteristics of metal-semiconductor structures manufactured based on crystal Si with the different resistance.
In order to measure electric resistance the careful temperature control is required.In order to measure the mobility we need define the carriers concentration variations and find a method for exact amendment to mobility variations.Such amendments to mobility we can realize by extrapolation of pressure coefficient for mobility measured in more high temperatures (when electrons concentration is being constant on pressure because of all impurities are ionized) or calculation of pressure coefficients variations for parameters describing the mobility (in case of they are known).
In the present paper the analysis of results on experimental studying influence of pressure to characteristics of Schottky diodes manufactured based n-Si<Ni> has been presented.
It should be noted the measurements on pressures allow us primarily separate the part connected by temperature coefficients of capacitance and conductivity conditioned by only lattice expansion on the temperature.Such capacitance behavior and existence of maximum on the temperature dependence testify on the overcompensation of base region of studying diodes [7].
Wherein this representation between acceptor deep center concentration, Na, and donor shallow center Nm is fairing am NN  .Using assumptions suggested by author of paper [7] and the equivalent diode circuit from overcompensated semiconductor we can obtain following expression 1.15lg 2.3lg where , C is the measured capacitance, Cb and Cg are the barrier and geometrical capacitances of diode, correspondingly.Ec is the energy corresponding to bottom of the conductivity zone, Ea is the activation energy of compensated impurity,  is the cyclic frequency, ε is the semiconductor dielectric permeability, μn is the main charge carriers mobility, d is the dimensionless parameter.
Neglecting the temperature dependence of ωεd multiplier, which is equivalent to neglecting the temperature dependence of the main charge carriers mobility, on the tangency angle inclination (1) we find the activation energy compensated deep center.In paper [7] the temperature dependences for parameter A in case of two studied diodes had been presented, according to ones the ionization energy of compensated center equals Ес = 0,27  0,03 eV.

Experimental method
As the initial material we used n-type Si crystal.Wherein plates on 1250 o C in duration 2 hours have been heat treated and fast cooling (>200 o C/minute) up to home temperatures.The Schottky diodes by vacuum spraying if Ni to Si plate's surface are manufactured.Area of the metal contacts equals 7.1 .10 -2 centimeter 2 .Part of the manufactured diodes to allaround compression up to 70 kBar have been subjected.

Experimental results
Results for ionization energies and pressure coefficients obtained on the charge carrier's concentration variations have been presented on Table 1.Values for these coefficients coincide with ones calculated theoretically and pressure coefficients for effective mass and dielectric constant found in other experiments [9].Variations of the diodes parameters by dark voltage-farad characteristics have been controlled, which using bridge compensation method on frequency 150 kHz have been measured.The diodes temperature in measuring process has been stabilized with accuracy 0.5 o C.
The obtained results for temperature dependences of capacitance and conductivity measured on the parallel equivalent circuit for one of the studied diodes by reverse voltage 4 V have been presented on Figure 1.It is seen from one that before the pressure influence the diodes capacitance on the temperature is growing monotonically (curve 1) up to value approximately 80 pF, at the same time after depressurization it loses monotony and has the maximal value about 50 50 pF on temperature approximately -30 o C (curve 3).As to the conductivity, it is increasing on temperature slowly both as pressure influence (curve 2) and as after depressurization (curve 4).Wherein values for the conductivity were smaller approximately 40 milliSiemens in comparison of the depressurization variant.The temperature dependence of relaxation time constant for two Schottky diodes by ionization energies ЕС=0.27  0.03 eV and ЕС=0.54  0.03 eV subjected to pressure 4 kBar has been presented on Figure 2. Comparison of obtained dependences with data of papers [5,6] showed very good match, that is both as in these papers and as in our case, recharge of the identical centers has been observed.
Taking into account, the diodes described in paper [5] had been created using boron diffusion and in our case Si plates were subjected only analogical thermal effects, we can conclude that the centers observed in diodes studied ourselves and described in papers [5,6] have the similar nature.The pressure impact leads only to decreasing its concentration without changing the center structure.It should be noted that the capture cross section of major carriers by pressure influence is not changing, which follows from constant of dependences τ(T) in diodes both in compensation presence and when it is absent.
Measurements of the current density in normal conditions and by 12 kbar pressure in more general point which deal with precipitation of oxygen atoms showed that here the free interstitial oxygen enters to second phase with formation of SiO2 particles [8] (see, Figure 3).When the strong electric field influences to n-type semiconductors many electrons gain energy quickly which is enough for exciting in 0.2 eV or higher over the edge conductivity zone.
Application of 12 kBar pressure changes situation.Then oxygen atoms settle in SiO2 clumps, in result they, probably, lose the electric activity.We can catch sight of the Figure that in case influence 12 kBar pressure the current density saturation happens on its smaller value in of the neglecting pressure case.The results for resistivity of n-Si<Ni> samples by consequent increasing (curve 1) and decreasing (curve 2) all-around compression pressure have been presented on Figure 4.It is seen from Figure that, unlike from results of paper [7], under pressure 12 kBar in studied ourselves diode n-Si<Ni> structures consisting impurity centers in the semiconductor volume we deal with monotonic increasing resistivity.Hence, it follows that observed ourselves centers are structure defects of semiconductor conditioned by its fast cooling.
Further when pressure is increasing in range of 12-60 kBar then changing resistivity of n-Si<Ni> samples under pressure (see curves 1 and 2 in Figure 4) has not monotonic character with appearance maximum by P ≥ 35 kBar.Formation of this maximum, probably, it is conditioned by unlike of great solubility of Ni atoms in Si (7 .10 17 centimeter -3 ), their main part (99 %) located in the volume as electric neutral atoms [9] and form impurity precipitates, only the insignificant part (1 %, about of 5 .10 14 centimeter -3 ) are being electric active state.Namely, these atoms in Si protected zone will form two acceptor levels (Еv= 0.2 eV and Ec = 0.4 eV).We can also catch sight from Figure that by further increasing the pressure up to 60 kBar the resistivity is decreasing essentially.This situation, probably, conditioned by the experimental observed nonmonotonicity in ρ=f(Р) dependence is the result of two counter processes.The first of that corresponds to decreasing band gap and changing ionization energy of deep Ni levels, which leads to increasing resistivity.The second it is conditioned by decay of impurity precipitates with increasing concentration of electric active centers which by pressure influence displace from the semiconductor volume and change the spectrum of distribution of surface charge on the limit between metal-semiconductor phases.

Conclusion
Thus, based on analysis of investigations carried out studying influence all-around pressure to Schottky diodes characteristics manufactured based n-Si<Ni> we can conclude the following conclusions.
We have revealed that if before pressure when the temperature is growing the diodes capacitance is increasing monotonic up to value about 80 pF, at the same time after pressure is maintain in duration of 2 hours and depressurization then it loses monotony and has maximal value approximately 50 pF on the temperature about -30 o C. Measurements of diodes conductivity showed that it is growing slowly on the temperature both as pressure compression and as after its depressurization.Wherein the conductivity values were being smaller approximately to 40 milliCimens in comparison of the depressurization variant.
It has been shown that in case of pressure compression, 12 kBar, saturation of the current density occurs at its lower and by higher voltage values then in case of the depressurization variant.When we grow the pressure up to 12 kBar in the studied n-Si<Ni> diode structures consisting impurity centers in the semiconductor volume then one deals with the monotonic increasing resistivity.This situation, probably, is conditioned by the observed ourselves centers are the semiconductor structure defects dealing with theirs rapid cooling process.
Further, when we grow the pressure in range of 12 up to 60 kBar then changing the resistivity of n-Si<Ni> samples has nonmonotonic character forming maximum value at P ≥ 35 kBar.Formation of this maximum is conditioned by that unlike to big solubility of Ni atoms in Si (7 .10 17 centimeter -3 ) their essential part (99 %) occurs in the crystal volume in electric neutral atoms [8] forming impurity precipitates and only insignificant part (1 %, about 5 .10 14 cantimeter -3 ) are being electric active state.Namely, these atoms will form two acceptor levels (Еv= 0.2 eV and Ec = 0.4 eV) in the protected zone.
We showed that when pressure is growing up to 60 kBar then resistivity decreases noticeable.This situation, probably, conditioned by that the experimental observed nonmonotonic character in ρ=f(Р) dependence is results of two contrary processes.
The first of these processes corresponds to decreasing band gap and changing ionization energy of deep Ni levels, which leads to increasing resistivity.The second process is conditioned by decay of impurity precipitates with increasing concentration of electric active centers, which by pressure influence displace from the semiconductor volume and change the spectrum of distribution of surface charge on the limit between metal-semiconductor phases.

Fig. 2 .
Fig.2.The temperature dependence of the relaxation time constant for two Schottky diodes (1, 3 and 2, 4), subjected to pressure 4 kBar.Here solid lines were plotted based data of paper[5]

Fig. 4 .
Fig. 4. Resistivity of n-Si<Ni> samples under all-round hydrostatic impact (here lines 1 and 2 correspond to pressure increasing and decreasing, correspondingly)

Table 1 .
The ionization energies and pressure coefficients of impurities for Si[9]