"Electric Doping of Hydrogenated Silicon Carbide Amorphous (a-SiC:H)
Obtained by PECVD "

Introduction

The present project negotiates on the doping eletric of films semicondutors of hydrogenated silicon carbide amorphous (a-SiC:H). The interest in this material is due to its likeness with the silicon carbide crystalline (c-SiC) whose notable properties, to know, high mechanical hardness and chemical stability, release band forbidden, high thermal conductivity and high rupture tension, etc., they wake up great interest in as several as the microeletronic areas, metalurgia and medicine (orthopedics). due to its mechanical properties, the silicon carbideo was firstly (and it is even today) very used as abrasive material and in court tools. Of the electric point of view, it is a semiconductor of high gap (2,3 to 3,3 eV) that began to wake up larger interest starting from ends of the decade of 60, with the appearance of techniques for obtaining of crystals of larger purity (method of Lely). in spite of that, the production of crystals of SiC that indeed could be used as substrata in the production of devices semiconductors the base of this material it only happened the ends of the decade of 80. Thus, in the last 10 years it has been growing the interest in understanding its properties better, to dominate its obtaining and in developing technologies that allow to manufacture opt-electronic devices that take sides of its extraordinary properties and that they can be operated in high potencies, high temperatures and in atmospheres chemically aggressive, where the silício cannot be used or it has limited application.

Of the development of bulk methods to obtain wafers of SiC, has also been studied the growth of films epitaxiais of SiC on silícon substract, due to the advantages of simplicity and compatibility with the current technologies of semiconductors that this method back. In a general way even so, a limitation of that technique is the high involved temperatures (above 1000ºC), of there the interest in developing techniques of obtaining of SiC in smaller temperatures, through techniques as the PECVD, that we used in this project. In fact, this technique has been a lot used to produce films of SiC in temperatures between 200 and 400ºC us which initially the largest attractiveness was the possibility to vary the optical gap of the controlled material the content of carbon in the films. The material like this obtained, even so, it is a hydrogenated amorphous league of Silicon-carbon, denominated him ''a-SiC:H '', that it has been used with success in the development of several optical and opt-electronic devices, as cells solares of silicon amorphous with a structure P-I-N, issuing diodes of light, detecting of colors, in applications fotoxerográficas, in transistors of fine film, sensor of image, etc.

Differently of the c-SiC, that is a composition, the to-SiC:H it is essentially a ternary league of carbon, silicon and hydrogen, with variable composition and great concentration of structural defects, what conditions its physical properties strongly. We didn't owe therefore, to wait that has the same properties of the c-SiC. In spite of that, the obtaining in low temperatures has a series of technological advantages (as compatibility with previous processes in the same sheet and/or the growth on sensitive substrata to the temperature) that justify the effort in obtaining to-SiC:H of high quality. Thus, our works with to-SiC:H they largely objectify the obtaining of a material that is the amorphous compensation of crystalline SiC, of which retains, if not whole, some of its properties.

In that context, our researches have been allowing to reach us a high degree of understanding of the processes involved during the growth of the films of to-SiC:H for the technique of PECVD and to obtain a material of high quality, with chemical and structural order similar to the one of crystalline SiC, in which is promising to try to manufacture devices semiconductors.

To manufacture devices nevertheless, it is necessary to control, among other basic processes, the doping electric of N type and P type, that is not still well understood nor in c-SiC nor in to-SiC:H and of the which negotiates to present research proposal. Thus, assisting to this need in the last years has been developing (with support FAPESP and of CNPq) a series of works that have been allowing to demonstrate us the possibility of doping a-SiC:H, obtaining both types of materials, N type and P type. in spite of well happened and of its original character, those works are still partial and they should be continued, objectifying a systematic study. It is that therefore the motivation of this research project.

Objectives

The objective of this project is to study the doping films of a-SiC:H almost estequiometrics obtained by technical PECVD. The proposal is to give continuity to previous works that they demonstrated the viability and pertinence of a study of this type, approaching aspects that should still be studied better and/or otimizados. In our way specific objective it is to systematize the study of the doping N type, for the technique of ionic implantation with nitrogen, and of the doping P type with aluminum for the technique of thermal diffusion of sludges.

Description of Project

As mentioned previously, our objective is to deepen the study of the electric doping of fine films of a-SiC:H obtained by PECVD. With this end, samples of a-SiC:H grown in different conditions they will be dopadas N type and P type for techniques as ionic implantation and the thermal diffusion. Before doing a detailed description of the activities they be she accomplished, it will be instructive we make a small introduction on our works with to-SiC:H already obtained by PECVD and a brief description of the results on doping obtained.

Silicon Carbide obtained by PECVD (a-SiC:H)

Of the point of view of the obtaining technique, the main appeals of the a-SiC:H obtained by PECVD they are the low temperatures in that the material can be obtained. Thus for example, our samples are grown starting from a silano (SiH4) and matano (CH4) plasm produced by an electric field of radio frequency (RF) in temperatures of ~ 300ºC.

Of our part, we are one of the groups that work with films of to-SiC:H in Brazil and our researches with this material has been allowing to reach us a reasonable understanding of its properties and of the necessary conditions for obtaining of a material of good quality. To this respect we can say that our main contribution has been to identify a condition peculiar of growth (condition of " starving plasma'') in which the obtained material presents smaller microporos concentrations, larger optical gaps (of up to ~4eV, in rich films in carbon), larger contents of carbon and, in films almost estequiometrics, a similar chemical and structural order the one of crystalline SiC. In other words, a material with structure similar to the one of crystalline SiC, with low concentrations of pores, low incorporation of hydrogen and with the atoms of Itself linking to atoms of C and vice and it turns.

Those properties the most important and promising for our objective in the moment is the chemical and structural order of the material, since we are speaking about an amorphous league, in the ones which in general, the structural disorder is a serious obstacle to the obtaining of efficient electric doping. In fact, it is a characteristic one almost universal of the amorphous semiconductors the difficulty of if they get high doping efficiencies. The amorphous silicon for example, became only useful as semiconductor after the discovery that atoms of hydrogen can saturate the incomplete connections in its structure, reducing like this the density of states located in the forbidden band. In that way, the order properties in our films went the main incentive to study the electric doping of the material and in fact, as we will see to proceed, our expectations have been confirmed by the success in doping plenty those films. At the present time our researches have been allowing to demonstrate us that the chemical and structural order of the material can still be improved more growing the films in larger potencies of R.F. and diluting in :2 the precursory mixture of SiH4 + CH4. In fact, the dilution in hydrogen is a well-known plenty technique to obtain films I sleep crystalline of silicon and we have been using it to try to obtain I sleep crystals of SiC:H, in which we have not still been totally well happened. In spite of that, the results indicate that our material in those conditions is indeed more orderly, so that it is our objective in this work to also study the doping of this material.

Previous results of doping of a-SiC:H

In function of the properties presented by our films of to-SiC:H we began a series of studies recently about the possibility of eletric doping the material. We have been using the technique of ionic implantation to introduce sludges as nitrogen, match and boro, and the thermal diffusion to introduce the aluminum. To proceed we summarized the most important conclusions:

- The samples of a-SiC:H chemically ordered in fact they present a larger dopagem efficiency. Its activation energy is among 2 to 3 times minor than those obtained in a-SiC: H grown in conditions non ideal;

- In the obtaining of doping N type the nitrogen is a more efficient dopante than the match. In the best results, samples doping with this element exhibits an activation energy Ea = 0,12 eV and a conductivity of the order of 10E-3 (Ohm.cm)-1;

- In the dopagem P type the boro implantation drove the due not very reliable and not repetitive results the thermal instabilish observed in the electric conductivity of the samples;

- The best results with dopagem P type was obtained in samples dopings with aluminum, which was introduced by thermal diffusion in temperatures of 450ºC. In the best results the activation energy was of Ea = 0,2 eV and the conductivity the ambient temperature of approximately of 10E-6 (Ohm. cm)-1. These results, although they have to be improved, healthy very good if we consider the doping P type is still a problem in having opened even in the technology of crystalline SiC;

- The technique of thermal diffusion was adopted because up to now Al's ionic implantation has been showing unviable due the difficulties related to the obtaining of a stable bunch of Al. The advantages of this technique even so, and Al's superior acting as element doping indicates that would be of great interest to get the doping P type through the implantação of aluminum.

As we can see, the possibility of dopar eficientemente the a-SiC: H is evident. That is clear to the we remind qua the activation energy and the electric conductivity of the films of a-SiC: H non dopings (intrinsic) they are of Ea = 1,2 eV and the electric conductivity of the order of 10E-15 (Ohm.cm)-1. We see therefore that is possible to promote a quite significant variation in the relative position of the level of Fermi and an increase of the conductivity of a lot of orders of greatness.

Description of Project and Methodology

In function of the methodology exposed to be adopted pursues the following points previously:

1. To do a systematic study of the doping N type and P type for I/I of nitrogen with the use of different doses and/or energy and of new treatments thermal powders implantation. The objectives are to extend and improvement previous results through the study of implantation profiles specifically projected for silício carbeto and of thermal treatments that improvement electric activation of the implanted sludges;

2. To do a systematic study of the doping P type for thermal diffusion of aluminum. The objective is improvement the diffusion process studying different temperatures and times of recozimento that drive to a larger electric conductivity. This study also includes the determination of the concentration of sludges introduced in the head office of to-SiC: H for the diffusion processes, thing that it was not still done;

3. To study the dopagem P type for I/I of Boro. Although the acting of B as element doping belongs inferior to the to Al and have driven the instabilidades, the origin of these it was not illuminated so that it is also of our interest to try a stable doping with that element and/or indentify the origin of the observed instabilidades;

4. To study the doping P type for I/I of Aluminum. In spite of the previous insucessos in getting to implant aluminum, the doping P type using this technique would have several advantages. Like this being, in case the technique comes becoming viable this study it will also be accomplished .

The nitrogen ionic implantation (in the doping N type), it will be executed in the Center of Component Semiconductors, of Unicamp, and the processes of thermal diffusion of aluminum (in the dopagem P type) they will be accomplished in our laboratory. Thus, the experimental activities will be initiate with those two studies being developed in parallel, for improviment the available time.

As the objective is to evaluate the performance of materials obtained in different deposition conditions, the doping processes will be applied in two groups of samples:

1. In the first group the samples of a-SiC: H will already be grown in the same conditions of the samples studied previously, that is to say, they will almost be samples estequiometrics, with a gap of energy of ~ 2,5 eV and that exhibit a chemical and structural order similar to the one of crystalline SiC. The objective here is to extend and improviment the previous results seeking, at the same time, to deepen our understanding of the doping process;

2. In the second group the samples will also be almost estequiométricas, but in this in case the films of a-SiC:H will be grown in dilution in H2, what eats we already say it still promotes the growth of a material more orderly. The objective here is to study the dopagem in a material with better properties, in which the electric doping was not still studied.

The doping efficiency will be evaluated essentially starting from measures of electric conductivity of the samples in function of the temperature, from where it is obtained not only the electric conductivity, but also the activation energy.

Of the point of view of the infrastructure, our laboratory counts with all the necessary ways to take to good term the project. For growth of the samples we totally have 2 deposition systems for PECVD operational. For the electric characterization, thermal treatments and diffusion processes, we have the whole necessary aparelhagem, set up recently with the aid of FAPESP. In the same way, we counted with the metalização processes (Evaporation, Sputtering, etc. ) and necessary fotolitografia to define the electric contacts. The optical structural characterization will also be accomplished in our laboratory, since we have ''Spectrometer of Infravermelho for Transformed of Fourier" (FTIR), besides Perfilometer for measures of thickness. The processes of ionic implantação will be accomplished in the implantator of the Center of Component Semiconductors of Unicamp, where it will also be made the determination of the concentration of sludges using the technical "Second Íon Mass Spectrocopy" (SIMS), close to the Laboratory of Research in Devices of the Institute of Physical Gleb Wataghin/Unicamp. Alternatively the concentration of sludges and composition of the samples will be made by " Rutherford Back Scattering " (RBS) close to the Laboratory of Analysis (LAMFI), of the Institute of Physics of University of São Paulo.