US2711379A

US2711379A - Method of controlling the concentration of impurities in semi-conducting materials

Info

Publication number: US2711379A
Application number: US302639A
Authority: US
Inventors: Rothstein Jerome
Original assignee: Individual
Current assignee: Individual
Priority date: 1952-08-04
Filing date: 1952-08-04
Publication date: 1955-06-21
Anticipated expiration: 1972-06-21

Description

June 21, 1955 .J. ROTHSTEIN 2,711,379

METHOD OF CONTROLLING THE CONCENTRATION OF IMPURITIES IN SEMI-CONDUCTING MATERIALS Filed Aug. 4, 1952 FIG. 2

INVENTOR.

JEROME ROTHS'TEIN ATTORNEY METHOD OF CONTROLLING THE CONCENTRA TION OF IMPURITIES IN SEMI-CONDUCTING MATERIALS Jerome Rothstein, Belmar, N. J., assignor to the United States of America as represented by the Secretary of the Army Application August 4, 1952, Serial No. 302,639

3 Claims. (Cl. 1481.5) (Granted under Title 35, U. s. Code 1952 see. 266 The invention described herein may bemanutactured and used by or for, the Government for governmental purposes without the paymentof any royalty thereon.

This invention relates to methods of controlling the ice" 4 the electrodes and 5 into the material to be purified. A

potential difference between the electrodes must be main-* tained which is large compared to thermalenergiesat the temperature of operation (measured; in electron volts lflj divided .by the chargeonan electron. lfojur or-five volts I is more than ample. The higher potential diilerence,

the greaterthe speed and efiicacy of electrolysis; The

" electrolysis is continued until the ions ,have been swept concentration of impurities in semi-conducting materials such as germanium, silicon, and other materials.

Purification of germanium, silicon and other high resistivity materials to the degree needed in semi-conductive work is very difiicult to attain. Even traces undetectable chemically may have decisive effects on the electrical properties of semi-conductors. Heretofore the approach has been essentially that of chemical purification followed by crystallization techniques.

The present invention contemplates purification of A semi-conducting material by electrolyzing the materials whereby donor type impurities, which bear an effectively positive charge, will migrate toward the cathode, while acceptor type impurities, which bear an efiectively negative charge will migrate toward the anode. N-type ma terial will thus form around the anode, P-type around A the cathode, with exceptionally pure material in between.

The invention and its objects will be illustrated by the following description of specific embodiments taken in connection with the accompanying drawing wherein Fig.

1 shows a top view of a crucible for-melting the get-' maniumwith the heating coils shown in horizontal sectional view and Fig. 2 a vertical sectional viewv of the assembled apparatus.

Germanium is heated to about its melting point which is 959 C. It is desirable to approach or exceed themelting point so as to get suflicient mobility of the'impurity ions. Exceeding the melting point is advantageous in into the enlarged regions 2'and'3'which contain'the elec- A, trodesleaving a very small concentration of impurities in' the region in between.

The high temperature needed to the materialmayf befobtained by radio frequency heating or bythe'use of conventional heating coils'S and 9 surrounding thecru:

cible 1 why other conventional arrangements.

After finishing :the electrolysis, in the event that material has been melted, cooling is started by first cooling the central region of the melt whichcontainsithe pure material ,beforecooling the partsnear ther electr'odes where theimpurities gather. Otherwise the crystalliza- H tion forces would push the impurities back into the puri= fied region, 1 This selective coolingmaybe accomplished in variousways for instance by making the heating c'o ils 1 8 and 9 slidable inahorizontal direction (not sho'wnzi'n the dr'aWin gyand by pulling these coils with-the aid of,

rods 10 andll'toward the electrodes after the electrolysis V is finished.

To prevent oxidation of the heated materialthe elec-, trolysis may be carried out in vacuo or in an inert atmosphere. For this purpose the crucible 1 is inserted into 1/ f tube 14 of an appropriate material, such asquart; glass. The water-cooled" holders 6 and 7 and the wiring @12 and V t 13 for the heatingcoiIs-S and 9 are hermeticallysealed I t into fthewallsof thetube '14. ,The ends otjtube 14 (not i shown in the drawings) .iare closed and" connected in well} known rnanner with thenecessary implements forsupply- I ing either high vacnumor an inert atmosphere;- Y Away of circumventing contamination of moltenlsemi conductingmaterial by the containe'r'lis tor-the molten 1 I portion to be surrounded by a solid cocled outer porltion whereby the outer cooler and solid portion of the ma terial (such as silicon or germanium) act as the container that themobility is greater in the liquid state than in the A solid. However, the disadvantage of going to very high temperatures is that impurities begin to diffuse into the melt from the container. I

The use of electrolysis for the purpose of controlling the concentration of impurities is not obvious because semi-conductors conduct electronically, rather than ionia cally. Even under the conditions that exist in the above mentioned melt most of the current may be carried by electrons or holes (or both). At the temperatures con templated, however, the lattice structure of the solid is broken down either by melting or by generation of lattice defects below the melting point to such a high degree that atoms and ions become quite mobile whereby the charged impurities can drift under the influence of an i applied electric field. i

The apparatus shown in Figs, 1 and 2 of the drawing uses an elongated shallow quartz crucible 1' having enlarged and deepened ends 2 and 3 into which pure carbon electrodes 4 and 5 are dipped. The germanium in the immediate vicinity of the electrodes is cool and solid because of thermal loss thru the inserted carbon electrodes.

The carbon electrodes 4 and 5 are preferably held by water-cooled holders 6 and 7 to insure cooling of the parts of the metal in the neighborhood of these carbon electrodes. This prevents diffusion of impurities from untilino further changeoccursone can obtain regions'of N-type and P-type material around the electrodes and a pure material in thefcentral region)": If," however, electrolysis is stopped at an intermediate point rnaterials with, variousdistributionsof donor and acceptor type impurities h and materials. with .various' degrees of purity will :bef obtained; 7 7; y Aparticular advantage of the present 'invention overthe known purification by crystal growing methods cone i- .sists in the fact that very large batches of semi-conducting materialmay be purified. The amountsto be purified with the aid of electrolysis are limited only by the power available.

What isclaimed-is:

'1; Method of controlling the concentrationof impuri ties insemi-conducting materials comprising submitting a material selected from the group consisting 'of'germariitim and silicon to the action of an electric field in" the presence of spaced anode and cathodeata temperature that brings; 7

about sufiicient "mobility ,offthe' impurity ions whereby I donor type. impurities migrateioward thecathcde, While acceptor type impurities migrate toward the anode, coir tinuing' the treatment until material of the desired degrees I of purity and impurity is obtained, and selectively cooling the-material byfirst coolingthe central-region that con- Patented .lune .21, 1955:

3 tains the desired degree of purity before cooling the parts near the electrodes.

2. Method of controlling the concentration of impurities in semi-conducting materials according to claim 1 in which the material is heated in an inert atmosphere to about its melting point.

3. Method of controlling the concentration of impurities in semi-conducting materials according to claim I in which an inner portion of the material is heated whereby the outer cooler portion of said material acts as the container 'for the inner molten portion.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. METHOD OF CONTROLLING THE CONCENTRATION OF IMPURITIES IN SEMI-CONDUCING MATERIALS COMPRISING SUBMITTING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF GERMANIUM AND SILICON TO THE ACTION OF AN ELECTRIC FIELD IN THE PRESENCE OF SPACED ANODE AND CATHODE AT A TEMPERATURE THAT BRINGS ABOUT SUFFICIENT MOBILITY OF THE IMPURITY IONS WHEREBY DONOR TYPE IMPURITIES MIGRATE TOWARD THE CATHODE, WHILE ACCEPTOR TYPE IMPURITIES MIGRATE TOWARD THE ANODE, CONTINUING THE TREATMENT UNTIL MATERIAL OF THE DESIRED DEGREES OF PURITY AND IMPURITY IS OBTAINED, AND SELECTIVELY COOLING THE MATERIAL BY FIRST COOLING THE CENTRAL REGION THAT CONTAINS THE DESIRED DEGREE OF PURITY BEFORE COOLING THE PARTS NEAR THE ELECTRODES.