Photodiodes (10) are fabricated in a single step diffusion process which exploits the characteristic of certain acceptors to form an anomalous diffusion profile (VI) including shallow and deep fronts (VIa and b) joined by an upwardly concave segment (VIc). By performing this type of diffusion into a low-doped n.sup.- -type body (12) with a carrier concentration (VII) below that of the concave segment, a p.sup.+ -p.sup.- junction (15) is formed at the depth of the concave segment and a p.sup.- -n.sup.- junction (17) is formed at a greater depth. The zone (16) between the junctions is at least partially depleted and forms the active region of a p.sup.+ -p.sup.- -n.sup.- photodiode. Specifically described are InP:Cd photodiodes.

BACKGROUND OF THE INVENTION 
This invention relates to detectors of optical radiation (i.e., lightwaves) 
and, more particularly, to semiconductor photodiodes. 
The recent special issue of the Western Electric Engineer, Vol. XXIV, No. 
1, Winter 1980, is a graphic illustration of the burgeoning interest in 
lightwave communications systems, especially fiber optic systems. The 
rapid growth of these systems has engendered commensurate activity in 
optical sources and detectors, primarily GaAs-AlGaAs laser diodes and LEDs 
in conjunction with Si APDs and p-i-n diodes for present applications at 
relatively short wavelengths (e.g., 0.80-0.90 .mu.m), and InP-InGaAsP 
laser diodes and photodiodes for future systems at longer wavelengths 
(e.g., 1.1-1.6 .mu.m). 
In general, a photodiode operates by the absorption of light which 
generates electron-hole pairs in the depletion region of a p-n junction. 
In the photovoltaic mode or under reverse bias, the junction field 
separates the pairs and thereby produces a photocurrent which can be made 
to do useful work in an external load. The optical-to-electrical 
conversion efficiency can be enhanced by employing a p-i-n photodiode 
configuration in which the impurity concentration of the i-layer is low 
enough to produce complete depletion. A depleted i-layer, often called the 
active layer where light is primarily absorbed, means that pairs can be 
readily separated and do not recombine before producing a useful 
photocurrent. Although the i-layer should be made thick enough to absorb a 
substantial fraction of the light incident thereon, it is often made even 
thicker to reduce leakage current, increase the reverse breakdown voltage, 
and lower the capacitance of the photodiode. On the other hand, the 
maximum thickness of the i-layer is limited primarily by the required 
speed of operation. 
Realizing low-doped i-layers can often be a problem depending on the 
materials from which the photodiode is made and the fabrication techniques 
employed. For example, suitable p-i-n photodiodes can be made of silicon 
using, inter alia, high resistivity (&gt;300 .OMEGA.-cm) epitaxial layers and 
ion-implantation (see, U.S. Pat. No. 4,127,932 granted to A. R. Hartman et 
al), yet the ability to controllably fabricate similar devices from Group 
III-V compound semiconductors is complicated by the difficulty of making 
low-doped material (e.g., 10.sup.15 cm.sup.-3). This problem in turn 
limits the maximum depletion width attainable and hence places constraints 
on desired levels of leakage current, breakdown voltage, and capacitance. 
There is a need, therefore, to be able to fabricate relatively wide (e.g., 
10 .mu.m) depletion layers in more highly doped (e.g., mid-10.sup.16 
cm.sup.-3) Group III-V compound semiconductors. 
The fabrication of prior art photodiodes is also disadvantageous because of 
the need to grow a plurality of epitaxial layers of controlled 
composition, conductivity type and thickness often involving sophisticated 
growth procedures (e.g., LPE, MBE, VPE) and a complicated sequence of 
ion-implantation and/or diffusion steps. So, there is also a need to 
simplify the fabrication of photodiodes and, thereby, to increase 
reproducibility and reduce costs. 
SUMMARY OF THE INVENTION 
We have devised a relatively simple, reproducible and inexpensive procedure 
for fabricating a p-i-n photodiode in moderately lowly doped Group III-V 
compound semiconductor n.sup.- -type body in which a single acceptor 
diffusion step produces both the p-i junction and the i-n junction 
separated by a fully depleted, relatively thick p.sup.- active i-region. 
Depleted active regions thicker than 10 .mu.m can be controllably 
fabricated in semiconductor bodies having a carrier concentration as high 
as the mid-10.sup.16 cm.sup.-3 range. 
More specifically, we exploit the characteristic of certain acceptors under 
particular conditions to deviate from expected error function diffusion 
profiles and to form an anomalous concave section resulting in two 
diffusion fronts. This anomalous profile includes upper and lower 
monotonically decreasing segments joined by the upwardly concave section, 
with the upper segment being closer to the surface and the lower segment 
being deeper. By performing this type of anomalous diffusion into an 
n-type body doped to a carrier concentration below that corresponding 
approximately to the concave section, a p.sup.+ -p.sup.- junction is 
formed at a depth corresponding approximately to that of the concave 
section, and a p.sup.- -n.sup.- junction is formed at a depth 
corresponding to the intersection of the lower segment and the carrier 
concentration of the n.sup.- -body. The p.sup.- -region between the 
junctions is depleted and forms the active i-region where light is 
absorbed and photocarriers are generated. The thickness of the active 
region is controlled by controlling the difference in doping level of the 
body and that of the concave section. 
Although numerous prior art workers have studied these anomalous diffusion 
profiles, none has fabricated photodiodes incorporating the 
characteristic. Representative of the prior art studies are the following: 
______________________________________ 
GaAs:Zn B. Tuck et al, Journal of Materials 
Science, Vol. 7, page 585 (1972) 
InP:Zn B. Tuck et al, J. Phys. D: Appl. Phys., 
Vol. 8, page 1806 (1975) 
GaP:Zn L. L. Chang et al, J. Appl. Phys., Vol. 35, 
page 374 (1964) 
GaAs:Mn M. S. Seltzer, J. Phys. Chem. Solids, 
Vol. 26, page 243 (1965) 
Ge:Cu F. Van der Maesen et al, J. 
Ge:Ni Electrochem. Soc., Vol. 102, 
page 229 (1955) 
CdS:Ag H. H. Woodbury, J. Appl. Phys., 
Vol. 36, page 2287 (1965) 
InP:Cd B. I. Miller and P. K. Tien, 21st EMC, 
Abs. No. G4 (6/1979) 
InP:Cd P. K. Tien and B. I. Miller, Appl. Phys. 
Lett., Vol. 34, page 701 (5/1979) 
______________________________________ 
Indicative of the failure of prior art workers to recognize the utility of 
the anomalous diffusion profile in, for example, InP:Cd or Zn, is the 
report by T. P. Lee et al, Appl. Phys. Lett., Vol. 35, page 511 (10/1979). 
This paper describes InP photodiodes in which Zn or Cd diffusion was 
employed to make simple p-n junctions but, notwithstanding the earlier 
work of Tuck (1975) and the two works of Miller and Tien (1979), did not 
exploit the anomalous diffusion of these acceptors in any way. 
In contrast, we fabricated abrupt p.sup.+ -p.sup.- -n.sup.- junctions in 
nominally undoped InP n.sup.- -substrates under conditions that produced 
the anomalous double diffusion profile. The shallow front (i.e., the 
p.sup.+ -p.sup.- junction) marked the concave section of the profile, and 
the deep front corresponded to the p.sup.- -n.sup.- junction. The 
depletion region extended between the two fronts. To achieve this profile, 
we found the Cd activity should be less than about 0.15 at a substrate 
temperature of 680 degrees C. Unlike previous work which used Cd compounds 
(Tien and Miller, supra), we diffused Cd into n-type InP 
(4.times.10.sup.16 cm.sup.-3) in sealed ampoules using elemental sources. 
The diffusion depths were controlled by varying the diffusion time or the 
concentration of either In or P. 
Mesa photodiodes characterized by large reverse breakdown voltages, low 
leakage current, the wide depletion widths were fabricated using this 
technique. These photodiodes are suitable for detection of light at 
wavelengths .lambda..ltoreq.0.96 .mu.m.

DETAILED DESCRIPTION 
With reference now to FIG. 1, there is shown a p-i-n photodiode, e.g., 
p.sup.+ -p.sup.- -n.sup.- photodiode 10 which, in accordance with one 
aspect of our invention, is fabricated by means of a single diffusion step 
performed under conditions which result in an anomalous double diffusion 
profile. The photodiode 10 includes a relatively low-doped n.sup.- -type 
semiconductor body 12, which may be a substrate of single crystal 
semiconductor or such a substrate with one or more epitaxial layers (not 
shown) grown thereon. By suitable single step acceptor diffusion described 
hereinafter, a surface layer 14 of body 12 is converted to p.sup.+ -type 
and an underlying layer 16 is converted to p.sup.- -type. The latter layer 
of thickness t is the active region of the photodiode where light 18 is 
primarily absorbed and generates photocarriers. These carriers are 
collected by suitable electrical contacts, illustratively a broad area 
contact 20 on the bottom of body 12 and an annular contact 22 on the top 
of layer 14. Light 18 passes through the annulus of contact 22 to the 
active region. Typically, for high speed detection a reverse bias is 
applied across contacts 20 and 22. The photocurrent, whether generated in 
the photovoltaic or reverse bias mode, then can do work in an external 
load (not shown) connected across these contacts. Illustratively, 
photodiode 10 is formed in the shape of a mesa, typically to reduce device 
capacitance. 
In order to realize an anomalous diffusion profile, such as shown by curve 
VI of FIG. 3, three basic conditions should be observed: (1) the diffusing 
acceptor should have a high diffusivity, e.g., a fast diffusing impurity 
such as Cu or Ni in Ge and Zn or Cd in Group III-V compounds; (2) the 
surface concentration of the impurity should be relatively high (e.g., in 
the upper 10.sup.18 cm.sup.-3 range), and (3) the diffusion time should be 
relatively long; i.e., more than a few minutes to insure that acceptors 
have time to penetrate deep enough into the body to form the deeper p-n 
junction, but less than many hours to prevent equilibration; 1 to 4 hours 
is suitable for InP:Cd. Furthermore, in order to exploit the anomalous 
diffusion profile to make a p.sup.+ -p.sup.- -n.sup.- photodiode, the 
carrier concentration (line VII, FIG. 3) of the low-doped body 12 must be 
less than net impurity concentration corresponding to the concave section 
VIc. Under these circumstances, the shallow diffusion profile VIa produces 
a p.sup.+ -p.sup.- junction at depth d.sub.2 corresponding approximately 
to the concave section VIc, whereas the deeper diffusion profile VIb forms 
a p.sup.- -n.sup.- junction at depth d.sub.3 corresponding to its 
intersection with line VII. The width t=d.sub.3 -d.sub.2 of the active 
region between these junctions is controlled by the difference in carrier 
concentrations between concave section (VIc) and body 12 (line VII). 
Advantageously, this active region is at least partially depleted (because 
it is compensated) and can be made relatively wide (e.g., 10 .mu.m), 
thereby decreasing leakage current, increasing reverse breakdown voltage, 
and lowering capacitance. 
EXAMPLE 
The following example describes the fabrication of a p.sup.+ -p.sup.- 
-n.sup.- InP photodiode by a single step diffusion of Cd. 
In this example, Cd was diffused into n-type InP using elemental (Cd and P) 
or (Cd and In) as sources. p.sup.+ -p.sup.- -n.sup.- junctions were 
formed using either Cd source. Higher reverse breakdown voltages and lower 
reverse leakage currents, exceeding those of state-of-the-art, abrupt 
p.sup.+ -n.sup.- InP avalanche photodiodes (APDs), were obtained from our 
p.sup.+ -p.sup.- -n.sup.- diodes. 
Unintentionally doped (4.times.10.sup.16 cm.sup.-3) InP substrates (e.g., 
body 12) were cut along the &lt;100&gt; orientation from twin free, liquid 
encapsulated (LEC) grown crystals. One surface of the substrates was 
polished using Br-methanol in order to accurately determine the diffusion 
depth and to qualitatively assess the degree of thermally induced 
decomposition of the surface. The diffusion anneals were carried out in 
sealed quartz ampoules for four hours at 680 degrees C. in a vertical 
furnace with the diffusion source at 675 degrees C. to prevent 
condensation of droplets onto the InP wafer. The ampoule volume and amount 
of Cd were held constant from run to run at 5 cm.sup.3 and 3.5 mg, 
respectively. The diffusion depth was controlled by varying the diffusion 
time or the concentration of either In or P. The diffusion front was 
revealed by staining a cleaved edge with a well-known AB etch at room 
temperature for five minutes. Examination of the diffused samples using 
Nomarski interference microscopy showed no significant change in surface 
quality. Similarly, photoluminescence measurements of the diffused samples 
were consistent with the absence of surface damage. 
A Nomarski optical micrograph was made to observe the typical stained edge 
of a nominally undoped, n-type InP (4.times.10.sup.16 cm.sup.-3) wafer 
after Cd diffusion. Two diffusion fronts 15 and 17 (FIG. 1) were formed, 
one at a depth d.sub.2 of 11.4 .mu.m and the other at a depth d.sub.3 of 
24.6 .mu.m for an amount of P in the diffusion source equal to about 3.4 
mg. The two fronts were either changes in doping density or dopant type 
since etching revealed both types of variations. Examination of the 
stained edge with a scanning electron microscope (SEM) showed that a step, 
.about.0.2 .mu.m deep, was delineated at the shallow diffusion front 15 
for five minutes etch time; no step was observed at the deep front 17 even 
after thirty minutes etch time. 
A high magnification electron beam induced current (EBIC) image of the 
sample was also made, and the positions of the shallow and deep diffusion 
fronts obtained from the optical micrograph were compared. This EBIC 
profile demonstrated the nature of the junction. The EBIC signal reached a 
peak at the shallow front 15, remained at the peak value for .about.3 
.mu.m, and decreased to a second inflection point at the position of the 
deep front 17. The EBIC measurement thus revealed the presence of a 
depletion region (active region 16) bounded by the two fronts. The EBIC 
profile was characteristic of a p.sup.+ -p.sup.- -n.sup.- junction. 
Further investigation of the active region 16 bounded by the two diffusion 
fronts was made using thermal probing and photoluminescence. Using a 2 
percent Brmethanol solution, a surface layer, approximately 20 .mu.m 
thick, was removed from a top portion of the wafer. A thermal probe 
measurement of the original surface of the sample showed it to be strongly 
p-type. Thermal probing of the etched surface at 20 .mu.m depth showed it 
to be low doped; the conductivity type could not be determined. Low 
temperature (4.2 degrees K.) photoluminescence measurements also indicated 
a high Cd concentration at the original surface. Similar photoluminescence 
measurements of the etched surface indicated that it was p-type as shown 
by the width and strength of the Cd impurity line at 1.365 eV. An amount 
of Cd, lower than the surface concentration by about an order of magnitude 
was found. The formation of a p.sup.+ -p.sup.- -n.sup.- junction was thus 
confirmed by EBIC, thermal probing, and photoluminescence measurements. 
FIG. 2 summarizes the results of diffusing Cd into undoped 
(4.times.10.sup.16 cm.sup.-3) n.sup.- -type InP. The diffusion depths for 
both shallow and deep fronts versus the amount of either P or In in the 
source are plotted; i.e., curves I and II correspond to the shallow 
p.sup.+ -p.sup.- junction 15 and the deeper p.sup.- -n.sup.- junction 
17, respectively, for an In-Cd source, whereas curves III and IV represent 
the same junctions for a P-Cd source. For example, at 20 mg of In, the 
distance between curves I and II gives a p.sup.- -active region 16 which 
is about t.sub.1 =8 .mu.m thick. Similarly, at 20 mg of P, the active 
region thickness is about t.sub.2 =10 .mu.m. The data show that the 
diffusion depths decrease for increasing amounts of either P or In and 
that the dependence of diffusion depth on In concentration in the ampoule 
is greater than for P. Although the dependence on P has been previously 
demonstrated by Tien and Miller, supra, the dependence on In is presented 
for the first time. 
We also found that at a substrate temperature of 680 degrees C., and for an 
In-Cd diffusion source, the anomalous profile was produced only for Cd 
activities in excess of about 0.15. No similar limitation was observed, 
however, for a P-Cd source. We believe that Cd activities less than 0.15 
at this temperature effectively reduce the surface concentration of 
acceptors below the desired upper 10.sup.18 cm.sup.-3 range and thus 
prevent the formation of two junctions. 
It should be noted that the Cd activity limit is also a function of 
substrate temperature. At higher temperatures, as the vacancy 
concentration increases, a higher Cd activity than 0.15 is required. At 
present, however, we do not understand why there is no similar limit for a 
P-Cd source. In any event, we expect that the anomalous profile can be 
attained in bulk InP using the above-described procedure with diffusion 
temperatures in the range of about 630 to 850 degrees C. We have 
successfully obtained such profiles for diffusion times in the range of 1 
to 4 hours at 680 degrees C. 
The formation of a p.sup.+ -p.sup.- -n.sup.- junction in InP which is 
delineated as two diffusion fronts by etching can be understood from FIG. 
3. Curves V and VI represent the normal error function profile and the 
anomalous double diffusion profile of Cd in InP, respectively. Curve VI 
deviates from curve V for depths greater than the concave section VIc of 
curve VI. For the normal diffusion profile, only the position where the 
diffusing species is equal to the substrate doping level, i.e., the p-n 
junction at d.sub.1, is revealed by etching. If the diffusion profile is 
that of curve VI, two possibilities arise. For substrates doped above the 
level of the concave section, one front is present as discussed for curve 
V. However, for substrates doped (line VII) below concave section VIc, 
etching reveals both the p.sup.- -n.sup.- junction at d.sub.3 and the 
position of the concave section in the anomalous diffusion profile, i.e., 
the p.sup.+ -p.sup.31 junction at d.sub.2. 
To demonstrate the quality of the p.sup.+ -p.sup.- -n.sup.- junctions, 
mesa diodes were fabricated from a Cd diffused wafer with the p.sup.+ 
-p.sup.- and p.sup.- -n.sup.- junctions at 3.1 .mu.m and 20.0 .mu.m, 
respectively. The diffusion source comprised 3.5 mg Cd and 3.4 mg In. 
Mesas were defined by standard photolithography techniques and etched 
using 2 percent Brmethanol. Be/Au and Sn/Au were used for the p-contact 22 
and n-contact metallization 20, respectively. The I-V characteristics of 
these mesa diodes, 150 .mu.m in diameter, showed a reverse breakdown 
voltage between 90 V and 220 V and a reverse leakage current from .about.5 
pA to .about.10 pA at half breakdown. The reverse I-V characteristics of 
the diode with the highest breakdown voltage exhibited double breakdown, 
at 160 V and 220 V and was typical of the devices. The two breakdowns are 
probably due to the variation in dopant concentration across the p.sup.- 
-region but are not an intrinsic property of the device. 
For a donor carrier concentration of 4.times.10.sup.16 cm.sup.-3, the 
avalanche breakdown voltage of state-of-the-art, abrupt p.sup.+ -n.sup.- 
InP APDs is .about.30 V (see T. P. Lee et al, supra). The p.sup.+ -p.sup.- 
-n.sup.- diodes fabricated in this study from n.sup.- =4.times.10.sup.16 
cm.sup.-3 InP crystals had breakdown voltages exceeding that of p.sup.+ 
-n.sup.- diodes by a factor of .about.3 to .about.6. The higher breakdown 
voltage of the p.sup.+ -p.sup.- -n.sup.- diodes is due to the reduction 
of the junction electric field by the formation of the p.sup.- -region. 
It is to be understood that the above-described arrangements are merely 
illustrative of the many possible specific embodiments which can be 
devised to represent application of the principles of the invention. 
Numerous and varied other arrangements can be devised in accordance with 
these principles by those skilled in the art without departing from the 
spirit and scope of the invention. In particular, while only InP:Cd 
photodiodes have been shown by example, it is apparent that the anomalous 
diffusion profile in other materials using other acceptors can be 
exploited in a similar fashion; e.g., Group III-V compounds such as 
GaAs:Zn, InP:Zn, GaP:Zn, GaAs:Mn, InGaAsP:Cd or Zn; and other 
semiconductors such as Ge:Cu or Ni and CdS:Ag.