Diffusion procedure for semiconductor compound

A process is described for doping compound semiconductors using a metal fluoride (e.g., ZnF.sub.2) as the source of dopant. The anhydrous metal fluoride is put down on the surface of the compound semiconductor, capped with a suitable encapsulant and heat treated to promote the diffusion. The heat treatment can be carried out in air without danger of surface damage to the compound semiconductor. Also, the diffusion is better controlled as to depth of diffusion and boundary delineation.

TECHNICAL FIELD 
The invention is a process for fabricating semiconductor devices. It is 
most specifically directed to the doping procedure for semiconductor 
compounds. 
BACKGROUND OF THE INVENTION 
Great advances have been made in semiconductor technology in the last few 
years due largely to greater demands for more exotic circuits, denser 
circuit packing, larger memories, higher frequencies and greater speeds. 
These demands have resulted in circuits with elements of smaller size, 
smaller spacing between circuit elements, more precise location of circuit 
features, etc. The trend toward denser packing of circuit elements 
continues and is likely to continue for some time. 
A particular case in point is in the fabrication of various semiconductor 
devices containing one or more compound semiconductors including indium 
phosphide and related compounds such as indium gallium arsenide phosphide 
and indium gallium arsenide. Typical devices are photodetectors, 
light-emitting diodes and semiconductor lasers. 
A particularly important aspect of semiconducting processing is the 
introduction of the dopant to the semiconductor material. Traditional 
techniques involve gaseous diffusion from metal vapor (e.g., Zn, Cd, etc.) 
in sealed ampules. Sometimes other sources are used such as phosphides 
(e.g., ZnP.sub.2, CdP.sub.2 etc.) or arsenides (e.g., ZnAs.sub.2, 
CdAs.sub.2, etc.) in appropriate cases. For many applications, this 
procedure yields excellent results. Conventional doping techniques have 
been described in a number of references including a paper entitled, 
"Zn-Diffused In.sub.0.53 Ga.sub.0.47 As/InP Avalanche Photodetector" by Y. 
Matsushima et al, Applied Physics Letters, 35 (6) (Sept. 15, 1979) and 
"Low Dark Current, High Efficiency Planar In.sub.0.53 Ga.sub.0.47 As/InP 
P-I-N Photodiodes" by S. R. Forrest, IEEE Electron Device Letters, Vol. 
EDL-2, No. 11 (November 1981). Other references include "Planar Type 
Vapor-Phase Epitaxial In.sub.0.53 Ga.sub.0.47 As Photodiode" by N. Susa et 
al, IEEE Electron Device Letters, Vol. EDL-1, No. 4 (April 1980) and 
"Plasma Enhanced CVD Si.sub.3 N.sub.4 Film Applied to InP Avalanche 
Photodiodes", by N. Susa, Japanese Journal of Applied Physics, Vol. 19, 
page L675 (1980). 
SUMMARY OF THE INVENTION 
The invention is a procedure for making semiconductor devices comprising 
III-V semiconductor compound in which the III-V semiconductor compound is 
doped with zinc or cadmium or both by first depositing the fluoride of 
these elements on the surface of the III-V semiconductor compound, 
covering the fluoride with an encapsulant (e.g., Al.sub.2 O.sub.3, 
SiO.sub.2, borosilicate glass, phosphosilicate glass, etc.) and then heat 
treating at elevated temperatures (e.g., 500 degrees C.) to diffuse the 
zinc or cadmium into the semiconductor. The procedure is highly 
advantageous because the diffusion profile can be more closely controlled 
than with conventional procedures and is highly reproducible. Also, the 
time-consuming diffusion process is carried out in air-ambient conditions 
and the semiconductor surface remains protected throughout the heat 
treatment involved in the diffusion process. The concentration of dopant 
(particularly on the surface) can be controlled. The photodiode devices 
formed by this procedure have unusually low dark currents and 
exceptionally good operating characteristics.

DETAILED DESCRIPTION 
The invention is based on the discovery that certain fluorides such as zinc 
fluoride and cadmium fluoride can be used to dope III-V semiconductor 
compounds by diffusing metal from the fluorides into the semiconductor 
compound. Particularly important is that the fluoride be anhydrous so as 
to ensure proper diffusion into the semiconductor compound. Because of the 
requirement that the fluoride be anhydrous, a special preparation 
procedure is used to ensure the essential absence of water, oxyfluoride or 
oxide as well as other impurities. 
The general procedure for carrying out the doping procedure involves 
deposition of the relevant fluoride on the semiconductor surface, 
encapsulation and then heat treatment. A typical procedure is as follows: 
First, the surface of the semiconductor is delineated so that fluoride 
appears only where diffusion is desired. Next, the surface of the 
semiconductor with fluoride is covered with encapsulant, heat treated and 
then further processed to produce a particular device. For example, the 
dielectric and fluoride are often removed and electrical contacts put on 
the areas where diffusion has occurred. 
The process is applicable to a large number of devices where zinc and 
cadmium are suitable dopants. This includes various devices which contain 
III-V compound semiconductors where zinc and cadmium are used as dopants. 
Typical devices are photodetectors (both P-I-N planar diodes and Mesa 
detectors), light-emitting diodes, lasers, field effect transistors, etc. 
A more detailed description of the process is given below. A particularly 
important element of the process is that the fluoride (e.g., ZnF.sub.2, 
CdF.sub.2) be anhydrous (typically containing less than 1.0 mole percent 
oxygen, oxyfluoride or water). Indeed, more often much less water or 
oxygen content is preferred, for example, less than 0.1 or even 0.01 mole 
percent. High purity in the doping fluoride is generally preferred. 
A convenient way of ensuring high purity fluoride is to synthesize the 
fluoride from high purity materials (e.g., metal) under controlled 
conditions. Typical procedures for zinc fluoride and cadmium fluoride are 
given below. 
For zinc fluoride, high purity zinc metal sponge is reacted with 
concentrated aqueous hydrogen fluoride (about 48 weight percent) for 
sufficient time to convert all the zinc into zinc fluoride. The zinc 
fluoride is then dried, sintered and melted in a dry hydrogen fluoride 
atmosphere. The material is then zone refined in an atmosphere of dry 
hydrogen fluoride. 
The preparation procedure for cadmium fluoride is somewhat different. High 
purity cadmium metal (typically 99.999 percent pure) is reacted with 
acetic acid and hydrogen peroxide to yield cadmium acetate. This compound 
is then reacted with hydrogen fluoride to yield cadmium fluoride. The 
cadmium fluoride is slowly heated to the melting point in an atmosphere of 
carbon tetrafluoride to ensure removal of water, oxide and oxyfluoride. 
Where the metal fluoride is already available, various procedures can be 
used to remove water, oxide and oxyfluoride from the metal fluoride. 
Generally, these procedures involve melting, fusing and/or zone refining 
under appropriate atmospheres such as fluorine, hydrogen fluoride or 
organic fluoride such as carbon tetrafluoride. 
The fluoride may be deposited on the surface of the semiconductor by a 
variety of techniques including magnetron sputtering, E-beam (electron 
beam) deposition and evaporation. Introduction of water or oxygen during 
the deposition procedure is to be avoided. Generally, E-beam deposition is 
preferred because of cleanliness of the procedure and the fact that a 
variety of encapsulants can be deposited immediately afterwards in the 
same apparatus without exposing the fluoride to the atmosphere. 
An understanding of the invention is facilitated by a description of E-beam 
technology as a procedure for depositing a material such as zinc fluoride 
or cadmium fluoride on a surface. This technology is discussed in a number 
of references including Physical Vapor Deposition, published by Airco 
Temescal (a division of Airco, Inc.) 2850 Seventh Street, Berkeley, Calif. 
94710. 
In this procedure, an electron beam is used to evaporate a material for 
condensation on a surface. The procedure is highly advantageous for a 
number of reasons. Improved electron guns permit high evaporation rates. 
It is relatively easy to keep the evaporated material free of 
contamination. The process can be precisely controlled and there is an 
excellent economy of material and a high thermal efficiency. 
Generally, the electron beam evaporation process is carried out in a vacuum 
environment. Typically, the chamber pressure is less than about 10.sup.-4 
Torr. Typical chamber pressures are in the range from 10.sup.-5 to 
10.sup.-6 Torr. 
Thickness of the fluoride layer may vary over large limits depending on a 
variety of factors including desired doping profile, temperature and 
duration of the heat treatment, etc. Typical limits are from about 50 to 
5000 Angstroms with about 100 to 1000 Angstroms most useful. For many 
devices, a thickness of 150 to 350 Angstroms is most preferred. 
An encapsulant is used to prevent evaporation of the fluoride during the 
heat treatment and to prevent surface damage to the semiconductor material 
during the heat treatment. Indeed, one of the major advantages of the 
inventive process is the superior operating characteristics of devices 
made in accordance with the invention. This is believed to be due to 
protection of the surface of the semiconductor against damage from the 
heat treatment necessary to diffuse in the dopant. 
Various encapsulants can be used in the practice of the invention. 
Particularly useful are such substances as SiO.sub.2, Al.sub.2 O.sub.3, 
various glasses such as borosilicate glass, phosphosilicate glass, etc. 
Also useful are combinations of these substances such as Al.sub.2 O.sub.3 
over SiO.sub.2. The thickness of the encapsulant may vary somewhat. For 
example, single film encapsulants typically vary from 1000 to 4000 
Angstrom Units in thickness. For composite encapsulants, typical thickness 
for the first film is from 200 to 800 Angstrom Units and the cap film is 
from about 1000 to 3000 Angstrom Units. 
Doping profiles can be varied in a number of ways including varying the 
temperature and time of the heat treatment, varying the composition and 
thickness of the doping compound and varying the composition and thickness 
of the encapsulating substance. For example, the encapsulating compound 
can be arranged so that part of the doping compound diffuses into the 
encapsulating compound and part into the III-V semiconductor compound. 
This procedure reduces the concentration of dopant (Zn or Cd) on the 
surface and generally reduces the concentration in the depth profile. For 
example, ZnF.sub.2 covered with relatively nonporous encapsulant (e.g., 
Al.sub.2 O.sub.3, borosilicate glass) yields surface doping levels of 
about 10.sup.18 zinc acceptors per cubic centimeter (this number is 
considerably smaller than the number of Zn atoms diffused per cubic 
centimeter) whereas with a more porous encapsulant such as SiO.sub.2 
(typically with thicknesses of 600 to 2000 Angstroms) surface doping 
levels are about 10.sup.17 zinc acceptors per cubic centimeters. Further 
reduction in the surface doping levels can be made by making the layer of 
fluoride unusually thin (50-150 Angstroms) and to some extent by making 
the thickness of porous encapsulant unusually thick. 
Various III-V semiconductor compounds may be used in the practice of the 
invention. The two most important III-V semiconductor compounds 
commercially are InP and GaAs but other compounds are also of interest 
such as InSb, GaSb, AlSb, InAs, etc. Also of interest are the ternary and 
quaternary III-V semiconductor compounds particularly those that are 
lattice matched to these binary compounds. Typical examples are compounds 
such as InGaAs (In.sub.0.53 Ga.sub.0.47 As), AlGaPAs, GaInPAs, GaInAsSb, 
etc. A list of such compounds is contained in a book by H. C. Casey, Jr. 
and M. B. Panish entitled, Heterostructure Lasers, Part 13, Academic 
Press, New York, page 33. 
The time and temperature of the heat treatment are of particular importance 
in the practice of the invention. These variables determine the depth 
profile of the doping atoms. Generally, the dependence of depth and 
concentration profile on time and temperature of the heat treatment is 
different for different fluorides (i.e., ZnF.sub.2 and CdF.sub.2) and 
semiconductor compounds. 
In general, the temperature varies from about 300 to about 650 degrees C. 
Below 300 degrees C., diffusion is generally too slow to be of commercial 
interest. Above 650 degrees C., there is often substantial danger of 
damage to the semiconductor although for short times higher temperatures 
might be permitted. 
Preferred temperatures often depend on the fluoride used and the 
semiconductor involved. Preferred temperature ranges often are a balance 
between short heat treatment times and the danger of causing surface 
damage to the semiconductor involved. 
For ZnF.sub.2 and InP, the preferred temperature range is from 300 to 550 
degrees C. Higher temperatures are generally preferred because the 
diffusion times are less. Generally, the limitation on high temperature is 
the danger of damage to the semiconductor and control of the diffusion 
process. Diffusion times vary from about 1 minute to about 100 hours. The 
depth of diffusion increases as the square root of the diffusion time at a 
particular temperature as expected from the diffusion equation. 
For ZnF.sub.2 on InGaAs, the preferred temperature range for the heat 
treatment is 450 to 600 degrees C. For CdF.sub.2 on InP, the preferred 
temperature range is 450 to 650 degrees C. and for CdF.sub.2 on InGaAs, 
the preferred temperature range is 550 to 650 degrees C. 
Diffusion depths for several diffusion times and temperatures in InP with 
ZnF.sub.2 were measured with the following results: 15.0 .mu.m at 550 
degrees C. for 2 hours; 3.0 .mu.m at 500 degrees C. for 0.5 hours; 1.7 
.mu.m at 400 degrees C. for 2 hours and 0.13 .mu.m at 350 degrees C. for 
72 hours. These results are consistent with an activation energy of 1.4 
e.v. 
For ZnF.sub.2 on InGaAs (unintentionally doped n-type), the preferred 
temperature range is 450 to 600 degrees C. Times may vary over the same 
ranges as for InP. Typical results are 1.0 .mu.m diffusion depth at a 
diffusion temperature of 550 degrees C. for a diffusion time of 2 hours. 
For CdF.sub.2 on InP (unintentionally doped n-type), a 2 hour diffusion at 
550 degrees C. produced a diffusion depth of 4 .mu.m and on epitaxially 
grown InGaAs (again unintentionally n-type), a 0.5 .mu.m depth was 
produced in about 15 minutes at 600 degrees C. 
In order to obtain a clearer understanding of the invention, a particular 
example will be described. In particular, a description of the fabrication 
of a P-I-N planar photodiode is described. A wafer of InP is first 
obtained. This wafer serves as the substrate for all the devices made on 
the wafer. Typical substrate material is doped n-type with tin or sulfur 
for convenience in growing. The wafer is covered with a buffer layer of 
n-type InP grown generally by liquid phase epitaxial growth and thereby a 
layer of n-type InGaAs (In.sub.0.53 Ga.sub.0.47 As) also grown by the 
liquid phase epitaxial method. 
Prior to deposition of the films, the surface is polished mechanically with 
a chemical polish such as one percent bromine in methanol. Generally, this 
is done until a mirror-like finish is produced. The surface is then 
degreased in chloroform, acetone and methanol and then etched for 1 minute 
in a 1:1 solution of hydrofluoric acid in deionized water. The surface is 
then thoroughly rinsed in deionized water and blown dry with nitrogen gas. 
The sample was then immediately transferred to an E-beam evaporator for 
sequential deposition of 150 Angstroms of ZnF.sub.2 and 1000 Angstroms of 
SiO.sub.2. The surface is then patterned by conventional photolithographic 
techniques. For example, a protective coating is left on small islands 
(typically about 75 .mu.m in diameter) and the remainder of the ZnF.sub.2 
and encapsulant removed by conventional means (generally CF.sub.4 plasma 
and buffered HF etch). The entire surface is then covered with a thick 
layer (e.g., 1200 Angstroms) of SiO.sub.2, borosilicate glass or 
phosphosilicate glass using E-beam evaporation. 
The sample is then heat treated in an air-ambient, open tube for a 
prescribed time and temperature to yield a desired doping level and 
junction depth. Typical diffusion temperatures and times are about 550 
degrees C. and 2.0 hours for In.sub.0.53 Ga.sub.0.47 As and 500 degrees C. 
and 0.5 hours for InP. 
Upon completion of the heat treatment, the sample is quenched to room 
temperature by simply removing it from the furnace. The samples are then 
etched in concentrated HF to remove the films. The samples are then 
metallized and passivated by conventional means which results in a 
finished planar P-I-N photodiode. 
FIG. 1 shows a photodiode 10 profitably made in accordance with the 
invention. The device is made up of several layers including a substrate 
layer 11 of n-type InP (generally doped with tin or sulfur), a buffer 
layer 12 of n-type InP (often doped with tellurium) typically grown with 
liquid phase epitaxial procedure and a layer of n-type indium gallium 
arsenide 13 with composition so that it is lattice matched to InP. The 
indium gallium arsenide is usually unintentionally doped n-type and is 
usually deposited by a liquid phase epitaxial procedure. A portion of the 
n-type indium gallium arsenide is doped p-type by the diffusion of zinc in 
accordance with the invention. A metal contact 15 (generally gold-zinc 
alloy) is used for electrical contact and the area surrounding the metal 
contact is covered with encapsulant 16, usually silicon nitride. The metal 
contact 17 on the opposite surface is usually gold-tin alloy. 
A similar procedure may be used to make planar P-I-N photodiodes where 
cadmium is the dopant. Cadmium fluoride is used as the dopant fluoride and 
the heat treatment is usually at a higher temperature (typically 550 
degrees C. for InP and 600 degrees C. for InGaAs). Because of the higher 
temperature required for cadmium diffusion, it is likely to be more stable 
thermally than the zinc dopant. 
Mesa type devices were also made by a procedure similar to that described 
above, at least as far as the doping procedure is concerned. A typical 
Mesa photodiode 20 is shown in FIG. 2. The Mesa structure is made up of a 
substrate 21 of tin doped InP with a buffer layer 22 of tellurium doped 
indium phosphide grown by a liquid phase epitaxial procedure. On top of 
this layer is an epitaxial layer of indium gallium arsenide grown n-type 
originally and then partially doped p-type with zinc in accordance with 
the invention. This leaves two layers, one n-type 23 and one p-type 24. A 
bottom electrical contact is made of gold-tin alloy and a top contact 26 
is made of gold-zinc alloy. 
A variety of photodiodes made in accordance with the invention were tested 
for dark current and capacitance as a function of voltage. At room 
temperature, the planar diodes typically exhibit a dark current ranging 
from about 3.times.10.sup.-11 to about 10.sup.-8 amperes at -10 volts with 
very sharp breakdown voltages varying from 70 volts to 150 volts. These 
measurements were repeated after a 30 second etch in 0.05 weight percent 
bromine in methanol solution. The dark currents and breakdown voltages 
were slightly improved, yielding a range from 5.times.10.sup.-12 to 
10.sup.-8 amperes for the dark current and 100 to 200 volts for breakdown 
voltage. Capacitance was also very low, often less than the lowest 
capacitance measurable with the experimental apparatus (0.1 pF at 0 
volts). Mesa type devices exhibited comparable electrical characteristics.