Abstract

We consider and propose a new low temperature technique of p-n junction fabrication in Si. This p-n junction may be produced in p-Si wafer simply by low energy Ar ions irradiation which leads to inversion of conductivity. The observed effect of both formation and propagation of the n-type region in irradiated boron-doped p-Si is evidence in favor of a powerful self-interstitial flux which is directed from the very thin damaged layer into the sample bulk and causes the displacement of boron into interstitial position. Generation of thermal donors also influences this process. Application of the inverse p-n junction fabrication technique is the easiest and the most efficient in some semiconductor technologies, which involve the formation of two-dimensional (flat) p-n junction over a considerable square. This effect is probably the most useful in solar cells production.

Keywords p-n junctions, silicon, inversion of conductivity, ion implantation, doping, Ar ions

Author and Article Information

Author info
1) A.M.Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russia.
2) Lomonosov Moscow State University, Moscow, Russia.
3) MEMS Electronic Materials, Merano, Italy.

RecievedApr 19 2014　　AcceptedJun 9 2014　　PublishedJun 25 2014

CitationBuzynin AN, Osiko VV, Luk'yanov AE, Voronkov VV (2014) New Ecologic Technique of p-n Junctions Fabrication in Si for solar cells. Science Postprint 1(1): e00025. doi: 10.14340/spp.2014.06A0002

Copyright©2014 The Authors. Science Postprint is published by General Healthcare Inc. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs 2.1 Japan (CC BY-NC-ND 2.1 JP) License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

FundingThis case report was not supported by any funds.

Competing interestNo conflict of interest

Corresponding authorAlexander N Buzynin
AddressProkhorov General Physics Institute, Russian Academy of Sciences, 119991, Moscow, Vavilov Str., 38
E-mailabuzynin@yandex.ru

Introduction

The majority of modern technologies of semiconductor devices are based on generation of different conductivity areas in the semiconductor and in particular of p-n junctions. For this purpose the crystals are doped, by means of three basic processes: diffusion, ion implantation, and irradiation 1, 2. The donor and acceptor impurities coexist in real silicon crystals. Therefore there is an alternative possibility: to redistribute available impurities to fabricate the areas of different conductivity type. Traditional doping-based techniques (diffusion, ion implantation, a radiating doping) have the common drawbacks: (1) one needs to use high temperatures. The diffusion doping is usually carried out at the temperature higher than 1,100°C. After ion doping or radiating doping the subsequent high temperature annealing of the radiation defects should be done; (2) unwanted contamination of the crystal by new impurities could occur; (3) nearly all dopants are poisonous, leading to contamination of the environment 1, 2.
In the present work we consider a new technique of p-n junctions fabrication in silicon, which provides simplification and cost reduction of production process of semiconductor devices such as solar cells. The essence of the present technique of p-n junction fabrication in silicon sample is that conductivity is changed in the bulk of the sample occurs as a result of redistribution of the impurity which already exist in the sample before its processing by ions. It differs from techniques of diffusion and ion doping where change of conductivity and p-n junction fabrication in the sample occur as a result of introduction of atoms of other doping impurity from the outside. The offered technique differs also from techniques of radiating doping (when doping impurity is generated in crystal volume.
Presented in this paper the experimental data suggest that the proposed technique is based on essentially new physical effect of fast low temperature impurity redistributions in the semiconductor. It is well known that each real silicon crystal is compensated by donor, and acceptor impurities. Therefore the mine problem is the following one: it is necessary to redistribute available impurity to fabricate the areas of different conductivity type. This redistribution may be provided by some an irradiation of a sample by ions of inert gases. In the paper we consider experimental date, physical mechanism and model of this process.

Materials and Methods

Experimental

The p-Si wafers were cut from boron-doped CZ crystals of various resistivity (with the boron concentration around 10¹⁵ cm^-3). The wafers were irradiated by 1–5 keV Ar ions in a gas discharge plasma (Figure 1, left). Then the wafers were cleaved, and the depth profile of the conductivity type was inspected by SEM–EBIC (scanning electron microscopy-electron beam induced current) technique. For this purpose, a high quality Schottky barrier was made by metal coating. This process resulted in formation of an n–p junction at some depth, which is clearly revealed as a sharp peak of EBIC signal (Figure 1, right).
The formation of an n-type region below the irradiated surface was also confirmed by a conventional thermo-probe technique. Upon increasing the irradiation time, the depth of the n–p junction (denoted by X_d) increases (Figure 2, 3). Some n-type (phosphorus-doped) wafers were irradiated and inspected too; in this case no p–n junction was found at the irradiated surface 3, 4.
The junction depth X_d is a non-linear function of the irradiation time t. No junction was found in reference non-irradiated wafers.
There is some “dead” time (1–15 min) in the junction propagation. After prolonged irradiation, the junction reaches some final position (Figure 3) that can be quite close to the back (non-irradiated) surface of the wafer.
I-V characteristics of inverse p-n junction were similar these for standard diode diffusion p-n junction (Figure 4).
In some range of duration (neither too short nor too long) the depth is roughly proportional to t^1/2 – a typical dependence for a diffusion process.

Figure 1 Scheme of experiment (left) and SEM microphotograph that made by combination of secondary emission and EBIC modes (right)

Figure 2 SEM microphotograph made both in secondary emission and EBIC modes of inversion p-n junctions on a cleaved Si wafer

The wafers were irradiated at the left side. The dark vertical strip is the image of p-n junction.

Figure 3 The p-n junction depth X_d in dependence of the time of exposure to Ar ions

Figure 4 I-V characteristics of inverse (curves 1, 2) and diffusion (curve 3) p-n junction in Si: 1 - In-Ga contacts; 2 - Ti contacts; 3 - standard diode

Results and Discussion

Peculiarities of n–p junction propagation

The n–p junction propagation was found to be sensitive to the state of the wafer surface. If the irradiated surface is bright polished, the junction moves faster, in comparison to the abrasion-polished surface. The surface defects, like scratches, cause a local distortion of the junction shape. The scratches at the backside ‘attract’ the junction. On the contrary, near the wafer edges, the junction propagation is retarded (Figure 5). Striation non-uniformity of Si affects the shape of inversion p-n junction too (Figure 6).
These results indicate to the role of the irradiation-induced self-interstitials: the local self-interstitial concentration is sensitive to the sinking ability of the sample surface. Particularly, the scratches at the backside may getter the surface impurities from the adjacent regions of the surface, thus improving the sinking ability of those regions.
One can argue that the p–n inversion is caused by some fast-diffusing donor impurity introduced during Ar irradiation. To check this possibility, we used the secondary-ion mass-spectrometry. An irradiated sample with a shallow p–n junction and a reference non-irradiated sample were inspected using layer-by-layer etching. No difference in the impurity content between the two samples was found which proves that the irradiation did not lead to any contamination of the sample near the surface.
It is therefore accepted that the p–n inversion is caused by in-diffusion of intrinsic point defects (self-interstitials) which leads to a loss of boron acceptors by kicking out the boron atoms B_s into the interstitial state B_i. The B_i atoms are known to be donors in p-Si 5. Most likely, B_i will be paired to B_s, into neutral B_iB_s defects. The conductivity will then change to n-type, due to either isolated B_i or due to residual donors (phosphorus and grown-in thermal donors) that are present already in the initial state, before the irradiation.
The thermal donors are well known to be produced by a heat treatment around 450°C, and to be annihilated by annealing at T > 600°C. It was found that after several hours at 450°C, the n–p junction persisted. However, after one hour at 750°C the p–n junction disappeared. This result can be treated as an indication of role of the thermal donors. On the other hand, it can be attributed to conversion of B_i back into B_s by annealing at higher T.
By varying the irradiation conditions (for instance, using a two-side irradiation), multiple junctions can be produced. An example of a double and a triple junction is shown in Figure 7a, b.

Figure 5 Influence of surface damage in Si wafer on the shape of inversion p-n junction (schemes and SEM microphotographs)

Figure 6 Influence of non-uniformity of Si wafer on the form of inversion p-n junction

Figure 7 The depth profile of the SEM–EBIC signal for a sample with two (a) and three (b) irradiation-induced p–n junctions

The horizontal line shows the zero level.

Model

A proposed mechanism of this process consists mainly in the following 6. The irradiation of the sample by inert ions generates a flux of silicon interstitial atoms Si_I directed from a surface in the bulk of the sample. Due to very high diffusivity of Si_I 7 (even at low temperatures), the steady non-uniform distribution of Si_I in a sample is formed (Figure 8a, b). Equilibrium concentration Si_I at low temperatures is very low, therefore a huge supersaturation of Si_I is created, which results in a sharp increase in boron interstitial component, B_i. Reaction of kicking-out boron and the backward reaction (B_i -> B_s + Si_I) establish dynamically equilibrium ratio between B_i and Si_I. This ratio is proportional to the supersaturation of Si_I. Therefore the loss of boron acceptors will be more pronounced in wafer part with a higher concentration Si_I. As a result local inversion of conductivity occurs in this part and p-n junction are formed (Figure 9).
This model implies that the self-interstitials diffuse very fast at low T (below 100°C), and penetrate to the depth of at least 300 µm within 100 min (Figure 3). Accordingly the self-interstitial diffusivity, at the irradiation temperature, is at least as high as 10 ^-7 cm²/s.
In the subsequent discussion, we concentrate on the boron acceptor loss, assuming that the near-surface region contains some concentration of donors, N_d, which is less than the initial concentration of the boron acceptors, N_o.
A change in the substitutional boron concentration, due to the kick-out reaction (and due to the inverse reaction of kicking out the silicon lattice atoms by B_i), is described by a simple equation,

Figure 8 Schematic profiles of self-interstitials (Si_I), substitutional boron (B_S) and phosphorus (P_S) in the beginning of the process before (a) and at the first moment (b) Ar ion irradiation

Figure 9 Schematic profiles of self-interstitials (Si_I), substitutional boron (B_S) and phosphorus (P_S) at successive time of intermediate stage of junction propagation

where C is the local (depth-dependent) self-interstitial concentration, α is the kinetic constant of the direct kick-out reaction and K is the equilibrium constant in the mass-action law that relates the concentration for the case of equilibrium between the reacting species (N_sC/N_i = K). The highest self-interstitial concentration, C_f, is reached near the front surface; it is defined by the balance of the production rate (proportional to the Ar flux) and the consumption rate by local Ar-produced vacancies and by sinking of self-interstitials at the front surface. With specified C, the concentration ratio of the interstitial and substitutional boron species, N_i/N_s, tends to C/K due to the reaction (1). A strong loss of acceptors occurs if C >> K. It is therefore assumed that this inequality holds at least at the front surface: C_f >> K.

Initial stage of boron acceptor loss

At short irradiation time, the term KN_i in Eq. (1) is negligible, and the boron concentration near the front surface is lost exponentially,

The n–p junction appears when N_s becomes less than N_d. This moment (t_d) lies experimentally, between 1 and 10 min. The product α C_f is estimated, from E_q. (2), to be in the range 0.01–0.001 s^-1

Propagation of the n–p junction

The near-surface region – where a large fraction of boron is already displaced into interstitial state Bi (and then paired into B_iB_s) –expands as more self-interstitials diffuse from the front surface into the bulk. The mass action law, N_sC/N_i = K, is valid at duration longer than the kick-out reaction time (10 min or less). The boron-depleted region corresponds, approximately, to the condition С(x) > K. At not too long duration, the self-interstitials penetrate to some limited depth (Figure 9, t =t₂), and the n–p junction resides at some intermediate position within the sample. Finally, the С(x) profile approaches a steady-state linear shape: the interstitials generated at the front surface are consumed at the back surface (Figure 9, t = t₃). Therefore, the n–p junction does not reach the back surface but stops at some final position, just like observed.
The self-interstitial flux into the sample bulk is, approximately, DC_f/ x_d, where x_d is the size of the boron-depleted region (x_d is almost identical to position X_d of the n–p junction). The total amount of the remaining boron is equal to C_o(L-x_d), where L is the wafer thickness. The boron loss rate, dQ/dt, is twice as large as the above self-interstitial flux (each consumed Si_I leads to a loss of two Bs: one by kick-out, and the other by pairing of B_i to B_s). This loss equation provides a solution for the junction depth x_d(t),

The DC_f product is estimated to be 5x10⁷ cm²с^-1 from Eq. (3). Above, we estimated the product α C_f (where α is the kick-out kinetic coefficient). By these numbers, the α/D ratio is of the order of 10^-10 cm. If the kick-out reaction were limited just by self-interstitial diffusion (which means that any ‘encounter’ of a self-interstitial with the boron atoms immediately leads to the boron displacement into the interstitial state), the coefficient α/D ratio would be equal to 4πr = 4 x 10^-7 cm, where r is of the order of the interatomic distance. The difference between the two numbers indicates some kinetic barrier (roughly, 0.25 eV) for the kick-out reaction.

A possibility of long-range migration of interstitial boron

It was assumed in the above discussion that the boron atoms displaced into interstitial state do not diffuse much from the initial location. The alternative possibility is that the B_i species are of high mobility (comparable to the self-interstitial mobility), and therefore they can migrate to the distance comparable to the sample thickness. In this case, a considerable spatial redistribution of boron impurity would occur. The final profile of B_i would be smoothed by diffusion to some constant (depth-independent) concentration N_i. The mass-action law would then imply that the substitutional boron concentration, N_s = N_iK/C, is inversely proportional to C(x). Therefore, substitutional boron would accumulate near the back surface, where C(x) is at minimum. Such a profile of substitutional impurity (with a well-pronounced accumulation at the back surface) is typical during in-diffusion of Au and Pt impurities 7, 8.
A formation of double and triple junction (Figure. 7a, b) can be accounted for by the long-distance migration of B_i.
The boron profile after the first irradiation is of the type shown in Figure 9, t = t₃, with just one junction. The second (back-side) irradiation creates an n-region near the back surface and formation of n–p–n structure, and also results in the boron acceptor accumulation near the front side (now non-irradiated) (Figure 7a, 10a). Then a region adjacent to the front side becomes again of p-type conductivity. The resulting structure is p–n–p–n (Figure 7b, 10b).

Figure 10 Schematic profiles of self-interstitials (Si_I), substitutional boron (B_S) and phosphorus (P_S) after second (back-side) irradiation at successive time (a,b)

Application of the Technique

Important practical applications of the studied effect is for the new technology of fabrication of p-n junctions in silicon, that provides simplification and cost reduction of production process of semiconductor devices by means of: excluding external doping; switching practically to the room temperature; preventing contamination of crystal and environment by unwanted impurities that would deteriorate crystal properties and pollute the environment 9. Manufacturing of inverse p-n junctions requires also less time and energy compared to traditional diffusion p-n junctions. Some photovoltaic applications of the inversion p-n junctions are also considered in 10.
The problem of the stability of field effect transistor characteristics is very essential in semiconductor technology and IC applications. The p-n junctions (prepared according to any technology) can shift spontaneously in the devices under effect of low-energy irradiation by ions, electrons, or laser radiation that leads to degradation and diminishes service life. A complete investigation of this effect is very important from the point of view its application in microelectronic and optoelectronic technologies. It allows developing anti-degradation methods for different semiconductor and optoelectronic devices and will significantly increase them service.
The irradiation-induced n–p junctions, after some improvements, may provide a basis for a simple low-temperature and environmental-friendly technology of some devices. By varying the irradiation conditions (for instance, using a two-side irradiation), multiple junctions can be produced. An example of a double and a triple junction is shown in Figure 7.
Contrary to the well known ion implantation technique, the ions in the suggested technique penetrate only into sub-surface area of the treated sample. The penetration depth is much less than that of p-n junction required. For the sub-surface penetration we used low energy bombarding ions, 0.01–50 keV; these ions are not additional chemical elements that would change the conductivity type of semiconductor. These ions only cause a redistribution of acceptor and donor impurities, which already exist in the semiconductor.
New technique provides: a) elimination of necessity of external doping; b) reduction of process temperature (from 1,000°C practically down to room temperature); c) prevention of contamination and poisoning both the treated crystal and the environment.

Conclusions

The formation of an n-type region in Ar-irradiated samples is caused, most likely, by the boron acceptor loss due to the self-interstitials produced at the front surface by the Ar flux. The mechanism of acceptor loss is probably the reaction of the boron atoms with self-interstitials (kick-out reaction).
The n–p junctions produced by Ar ion irradiation can be used for device fabrication. Some samples (diodes with irradiation-induced n–p junctions) were already tested as prototypes of solar cells and radiation detectors (for electrons and ions). Encouraging results were obtained even with simple ohmic contacts (Al baking, rubbing in In–Ga alloy). Formation of single and multiple junctions by a low-temperature process (Ar irradiation) is an attractive technological possibility.
Due to physical nature and peculiarities of generation of the inverse p-n junction its application is the easiest and the most efficient in those semiconductor technologies, which involve the formation of two-dimensional (flat) p-n junction over a considerable square: solar cells and similar semiconductor devices.

Author Contributions

Buzynin AN: Wrote the article, fulfilled experiments and analyzed the data.
Osiko VV: Analyzed the data and designed the discussion.
Luk’yanov AE: Fulfilled experiments and designed the discussion.
Voronkov VV: Analyzed the data and developed the model.

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