Multiwavelength infrared focal plane array detector

A multiwavelength focal plane array infrared detector is included on a common substrate having formed on its top face a plurality of In.sub.x Ga.sub.1-x As (x.ltoreq.0.53) absorption layers, between each pair of which a plurality of InAs.sub.y P.sub.1-y (y<1) buffer layers are formed having substantially increasing lattice parameters, respectively, relative to said substrate, for preventing lattice mismatch dislocations from propagating through successive ones of the absorption layers of decreasing bandgap relative to said substrate, whereby a plurality of detectors for detecting different wavelengths of light for a given pixel are provided by removing material above given areas of successive ones of the absorption layers, which areas are doped to form a pn junction with the surrounding unexposed portions of associated absorption layers, respectively, with metal contacts being formed on a portion of each of the exposed areas, and on the bottom of the substrate for facilitating electrical connections thereto.

FIELD OF THE INVENTION 
The present invention relates generally to infrared detectors, and more 
particularly to detectors for detecting light in the infrared of different 
wavelengths. 
BACKGROUND OF THE INVENTION 
Applications involving fiber optic communications systems typically utilize 
light waves having wavelengths in the near infrared (0.8 to 3.0 
micrometers in wavelength). These systems presently represent the greatest 
usage for near infrared detectors. However, other applications such as 
temperature sensing, night vision, eye-safe range finding, process 
control, lidar, and wind-shear detection require detectors with higher 
sensitivity and faster response times in the near IR region. Recently, 
InGaAs detectors have been investigated for light detection at wavelengths 
greater than 1.65 .mu.m because of their potential for high performance 
and reliability. Such detectors have demonstrated high quantum 
efficiencies (&gt;70%), low dark current (&lt;100 mA/cm.sup.2 at -5 V), and rise 
times less than one nanosecond at room temperature. Other materials (Ge, 
PbS, InSb, PtSi, HgCdTe, etc.) have been used for detectors at wavelengths 
greater than 2 .mu.m, but they generally have to be cooled to low 
temperatures, often have very slow response, or have high dark currents. 
Since In.sub.0.53 Ga.sub.0.47 As detects light at wavelengths .ltoreq.1.65 
.mu.m, in order to detect longer wavelengths more indium must be added to 
the ternary compound, thereby decreasing the bandgap. In this case, the 
lattice parameter can no longer match that of the InP substrates. A graded 
layer technique has been developed to accommodate the lattice mismatch 
between the substrate (a.sub.o =5.869 .ANG.) and the In.sub.x Ga.sub.1-x 
As (x&gt;0.53) absorption layer (a.sub.o &gt;5.869 .ANG.). In this technique, 
InAs.sub.y P.sub.1-y (y.ltoreq.1) buffer layers with increasing lattice 
parameters are grown in between the InP substrate and the absorption 
layer. This prevents lattice mismatch dislocations from propagating from 
layer to layer, enabling the growth of absorption layers with good 
optoelectronic properties. It is also possible to use In.sub.x Ga.sub.1-x 
As as grading layers by increasing the indium concentration. However, by 
using InAsP which has a larger bandgap than In.sub.x Ga.sub.1-x As, better 
spectral response is obtained for back illumination (light enters through 
the substrate). Also, using larger bandgap materials as buffer layers 
results in detectors with lower dark current. InGaAs detectors with up to 
2.6 .mu.m cutoff wavelength using InAsP graded layers have been 
successfully fabricated with good opto-electronic properties. 
SUMMARY OF THE INVENTION 
In one embodiment of the invention, an infrared detector is provided in a 
plural wavelength InGaAs focal plane array pixel element for detecting a 
plurality of wavelengths of light over a predetermined range in the 
infrared or near infrared, where each of the wavelength sensitive 
detectors are formed on a common substrate, and are individually 
addressable. The detector consists of successively smaller bandgap layers 
of In.sub.x Ga.sub.1-x As (x.gtoreq.0.53) formed over a substrate, 
separated by compositionally graded layers of InAs.sub.y P.sub.1-y 
(y.ltoreq.1) to decrease defects induced by lattice mismatch strain with 
the InP substrate. Portions of the various layers are selectively removed 
to form different pn junctions with different wavelength responses, 
respectively

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1A, a prototype detector array pixel sensitive to three different 
selectable infrared wavelengths is shown, for one embodiment of the 
invention as developed for use as a focal plane array imaging device for 
applications such as gas spectroscopy and absolute temperature 
measurements (pyrometry). Since detectors with smaller bandgap which 
detect longer wavelength also have larger dark current, one can use this 
pixel to select the detector with the appropriate absorption layer to 
maximize the quantum efficiency while minimizing the dark current. The 
device 1 includes a plurality of integrated detector pixels, each 
including regions 3, 5, and 7 for detecting different wavelengths of 
light, respectively. In this example, the light is on the near infrared or 
infrared wavelength regions. Optical absorption occurs, with reference to 
FIG. 1B, in 3 .mu.m thick In.sub.0.53 Ga.sub.0.47 As, In.sub.0.7 
Ga.sub.0.3 As, and In.sub.0.85 Ga.sub.0.15 As layers 9, 11, 13, 
respectively, which are grown by vapor phase epitaxy on top of an InP 
substrate 15, InAs.sub.0.3 P.sub.0.7 layer 17, and InAs.sub.0.6 P.sub.0.4 
layer 19, respectively. The bandgaps of the absorption layers 9, 11, 13 
are 0.75, 0.60, and 0.47 eV, respectively, which correspond to cutoff 
wavelengths of 1.65, 2.07, 2.64 .mu.m, respectively. To accommodate the 
lattice mismatch between the absorption layers 9, 11, and 13, step-graded 
1 .mu.m thick InAsP buffer layers 21, 23, 25, 17, 27, 29, and 19 are 
grown. Thus, the respective lattice parameters of the In.sub.x Ga.sub.1-x 
As absorption layers matches the lattice parameter of the InAsP layers 
immediately underneath. The lattice parameter of the InAsP layers 
immediately above the absorption layers should also match, but in this 
structure the lattice parameter of InAs.sub.0.4 P.sub.0.6 layer 27 does 
not exactly match that of In.sub.0.7 Ga.sub.0.3 As absorption layer 11. 
This may be the source of somewhat elevated dark currents in the longer 
wavelength detectors. Accordingly, in a preferred device the lattice 
parameters of layers 27 and 11 should match. All the grown layers were 
undoped (with a background n-type carrier concentration of 
&lt;5.times.10.sup.15 cm.sup.-3), while the (100) InP substrate 15 was doped 
with sulfur to give an n-type doping density of 8.times.10.sup.18 
cm.sup.3. The band diagram in FIG. 1C shows the band gap and the band 
offsets between the different layers 15, 9, 21, 23, 25, 17, 11, 27, 29, 
19, 13, and 31; and FIG. 1B shows the composition, bandgap in electron 
volts (eV) and equivalent wavelength in micrometers (.mu.m), and the 
thickness in micrometers (.mu.m), respectively. 
A selective wet etching process was developed in order to access the 
different absorption layers to enable junction diffusion for detection 
regions 3, 5, and 7, respectively (see FIG. 1A). A mixture of 5:1 citric 
acid (50% by weight) : H.sub.2 O.sub.2 was used to etch InGaAs detection 
layers since it is strongly selective of InGaAs (versus InAsP), and leaves 
a good surface morphology. The etch rate of In.sub.x Ga.sub.1-x As 
is.about.1000 .ANG./min at room temperature. To etch InAsP layers, a 
mixture of HCl:H.sub.3 PO.sub.4 :H.sub.2 O.sub.2 in the ratio of 3:1:x was 
used, where x was varied from 0 to 0.3 as the arsenic concentration in 
InAsP was increased. The etch rate of InAsP is .about.200 .ANG./sec at 
room temperature. To fabricate the integrated detector array 1, a area 
square area 33 of 500 .mu.m.sup.2 (see FIG. 4) was first etched above the 
In.sub.0.7 Ga.sub.0.3 As layer 11 and In.sub.0.85 Ga.sub.0.15 As layer 13, 
using a plasma deposited SiN.sub.x film (1000 .ANG.) as an etch mask, for 
initially forming detection region 3. Similarly, a 500 .mu.m.sup.2 area 34 
was etched above layer 11 for initially forming detector region 5. Thin 
layers (1 .mu.m thick) 21 and 27 of InAsP were left on top of the 
absorption layers in regions 3 and 5 as a wider bandgap cap layer in order 
to reduce surface-generated dark current. The pn junctions of all three 
detectors in regions 3, 5, and 7, respectively, were formed in 100 by 150 
.mu.m areas 35, 37, and 39, respectively, using a single sealed ampoule 
diffusion of Zinc Arsenide, with SiN.sub.x used as the diffusion mask. As 
a result, p+ diffusions were formed in regions 35, 37, and 39, thereby 
providing pn junctions with their underlying n doped absorption layers 9, 
11, and 13, respectively. Next, an antireflection coating 41 of SiN.sub.x 
is deposited to a thickness of 2250 .ANG. on the top diode surface (see 
FIG. 1A). Also, 40 .mu.m square Au-Zn alloy contacts 43, 45, and 47 are 
placed on top of the diffused areas 35, 37, 39, respectively, using a 
photoresist lift-off process. Also, overlay metal contacts 40, 44, and 46, 
typically of TiAu material, are formed on top of the antireflective 
coating 41 in association with detectors 3, 5, and 7, for electrically 
contacting contacts 43, 45, and 47, respectively. Contacts 43, 45, and 47 
facilitate making electrical connections either to individual ones of, or 
two or more of detectors 3, 5, and 7, respectively, using integrated 
circuitry techniques, such as flip-chip bonding, or wire bonding. 
In order to maximize performance, the back surface of the detector array 1 
or bottom of substrate 15 should preferably have an antireflection coating 
42 of SiN.sub.x, and an ohmic contact grid 48 (typically GeNiAu alloy), as 
shown in FIG. 1D. More specifically, in the preferred embodiment, the 
contact 48 is formed into a grid pattern with open spaces 42 for allowing 
backlighting or back illumination of the substrate 15 or detector 1. The 
open spaces 42 consist of a transparent antireflective coating of 
SiN.sub.x, in this example. Each space 42 permits back illumination of an 
underlying absorption layer 9, 11, or 13, associated with a given detector 
3, 5, or 7, of a pixel in an array of such pixels, in this example. 
The basic processing steps for the three wavelength infrared focal plane 
array detector element 1 of FIG. 1A are summarized in eight steps as 
follows: 
I. Deposit 1.000 .ANG. SiN.sub.x film by Plasma Enhanced Chemical Vapor 
Deposition (PECVD) 
II. Photolithography to define etch area (500 .mu.m square) 
III. Material selective wet etching 
.cndot.InGaAs: Citric Acid:H.sub.2 O.sub.2 at 5:1 
.cndot.InAsP: HCl:H.sub.3 PO.sub.4 H.sub.2 O.sub.2 at 3:1:x 
IV. Deposit 1000 .ANG. SiN.sub.x diffusion mask 
V. Photolithography to define diffusion area (100 by 150 .mu.m area) 
VI. Sealed ampoule diffusion using Zn.sub.2 As.sub.3 at 500.degree. 
C.-530.degree. C. for 20-40 minutes 
VII. Deposit 2250 .ANG. SiN.sub.x anti-reflective coating on top surface 
VIII. Place Au-Zn alloy contacts (40 .mu.m square) on top of diffused area 
using photoresist lift-off process 
IX. Deposit SiNx AR coating on substrate surface 
X. Deposit GeNiAu on substrate surface over previously exposed and 
developed photoresist layer lift off metal to form grid pattern 
Note that in step II, a chrome or iron oxide photolithography mask can be 
used. In step III, in place of wet etching, reactive ion (dry) etching can 
be used. Also, in step IV, a silicon nitride mask is used. 
Using capacitance versus voltage measurements, the inventors obtained the 
carrier concentration in the absorption layers 9, 11, and 13 for each 
detector 3, 5, and 7, respectively. It was determined that the background 
carrier concentration in the absorption layers 11 and 13 is 
&lt;1.times.10.sup.16 cm.sup.3, but higher for the In.sub.0.53 Ga.sub.0.47 As 
layer 9 where the carrier concentration increases near the heavily doped 
substrate 15, due to the diffusion of the sulfur substrate dopant into the 
epitaxially grown layer. Also, at 0 V, the smaller bandgap materials have 
higher capacitance (2.1, 3.0, 7.8 pF for In.sub.0.53 Ga.sub.0.47 As, 
In.sub.0.7 Ga.sub.0.3 As, and In.sub.0.85 Ga.sub.0.15 As layers 9, 11, and 
13, respectively). In the operating range of 5-10 V, all diodes or pn 
junctions exhibit capacitances ranging from 1.2-2.0 pF, again with the 
short wavelength (In.sub.0.53 Ga.sub.0.47 As) detector 3 having the 
smallest capacitance. 
The dark current of each detector 3, 5, and 7 under reverse bias is shown 
as points in FIG. 2, in plots 49, 51, and 53, for absorption layers 9, 11, 
and 13, respectively. The error bars were determined from the sum of 
measurement random error (determined to be five percent of the measured 
value) and small systematic errors. The lines are theoretical fits 
assuming that the total dark current is the sum of the 
generation-recombination current (either in the bulk or at the surface), 
junction shunt current, and diffusion current at low voltages, while 
tunneling dominates at high voltages, as has been shown to be the case in 
previous studies of InGaAs photodiodes. 
The theory fits the measured data, especially for the In.sub.0.53 
Ga.sub.0.47 As layer 9 for detector 3. The main source of low voltage dark 
current for this detector is generation-recombination current which is (at 
V&gt;kT) given by: 
##EQU1## 
where k is the Boltzman constant, T is the absolute temperature, q is the 
electronic charge, .tau..sub.eff is the effective carrier lifetime, 
n.sub.i is the intrinsic carrier concentration, A is the surface or 
cross-sectional area of the depletion region boundary, and W is the 
depletion region width for an abrupt one-sided junction. From the fit, the 
value of .tau..sub.eff is estimated to be 1 .mu.s, indicating that the 
growth and processing of the complex structure shown in FIG. 1 does not 
significantly affect the diode properties. The tunneling current, which 
becomes dominant at V&gt;15 volts for this detector 3, in this example, is 
given by: 
##EQU2## 
where m.sub.o is the free electron mass, .epsilon..sub.g is the energy 
band gap of the absorbing layer material, .eta. is Planck's constant 
divided by 2.pi., .EPSILON..sub.m is the maximum junction electric field 
given by: 
EQU .EPSILON..sub.m =-2(V+V.sub.b1)/W (3) 
and .THETA. depends on the shape of the tunneling barrier. Here, .THETA. 
was estimated to be 0.26 from the fit. The prefactor .gamma. depends on 
the initial and final states of the tunneling carrier. 
The dark currents of In.sub.0.7 Ga.sub.0.3 As and In.sub.0.85 Ga.sub.0.15 
As detectors 5 and 7, respectively, are considerably larger than for the 
In.sub.0.53 Ga.sub.0.47 As detector 3, especially at lower voltages. This 
is due, in part, to the smaller bandgap of the former materials which not 
only leads to an increased intrinsic carrier concentration affecting both 
the diffusion and the generation-recombination currents, but also leads to 
increased tunneling current. Another source of the high dark current is 
the larger concentration of defects in these materials caused by the 
lattice mismatch between the absorption layers 11 and 13, and the InP 
substrate 15. These defects provide midgap generation-recombination 
centers, increasing the generation-recombination current. Indeed, 
.tau..sub.eff for the In.sub.0.7 Ga.sub.0.3 As layer 11 is estimated to be 
110 ns, which is nearly an order of magnitude less than for In.sub.0.53 
Ga.sub.0.47 As layer 9. 
The a.c. small signal conductance at 0 V was measured at 1 kHz to be 18.2 
nS, 4.54 .mu.S, 9.34 .mu.S which translates to shunt resistances of 55.1 
M.OMEGA., 220 k.OMEGA., and 107 k.OMEGA. for In.sub.0.53 Ga.sub.0.47 As 
layer 9, In.sub.0.7 Ga.sub.0.3 As layer 11, and In.sub.0.85 Ga.sub.0.15 As 
layer 13 for detectors 3, 5, and 7, respectively. Assuming that the 
generation-recombination is the main source of conductance near 0 V, one 
can calculate .tau..sub.eff (using these conductance values) for 
In.sub.0.53 Ga.sub.0.47 As and In.sub.0.7 Ga.sub.0.3 As layers 9, 11, 
respectively, to be 1.1 .mu.s and 61 ns, respectively, which is in good 
agreement with values calculated from the dark current. 
The contribution from shunt current, given by: 
EQU I.sub.ohm =V/R.sub.eff (4) 
where R.sub.eff is the effective resistance, was found to be much greater 
for In.sub.0.7 Ga.sub.0.3 As and In.sub.0.85 Ga.sub.0.15 As detectors, 
where R.sub.eff was approximately 3-4 M.OMEGA., while for In.sub.0.53 
Ga.sub.0.47 As detector, R.sub.eff &gt;5 G.OMEGA.. This may also be due to 
the larger number of defects in the In.sub.0.7 Ga.sub.0.3 As and 
In.sub.0.85 Ga.sub.0.15 As layers, but the physical origin of the shunt 
conduction is not clear. 
The diffusion current is negligible except for the In.sub.0.85 Ga.sub.0.15 
As detector. The diffusion current is given by: 
##EQU3## 
where D.sub.p is hole diffusion constant, .tau..sub.d is the minority 
carrier diffusion lifetime, and N.sub.d is the doping density. The 
diffusion current depends exponentially on the bandgap which is smallest 
for the In.sub.0.85 Ga.sub.0.15 As layer 13, where .tau..sub.d was 
estimated to be 500 ps. 
Note that the tunneling current contribution to the dark current was not 
observed for the In.sub.0.7 Ga.sub.0.3 As and In.sub.0.85 Ga.sub.0.15 As 
detectors 5 and 7 due to the large component of generation, diffusion, and 
shunt currents. It is believed that the integration and processing of the 
three-detector pixel in the above example for detector array 1 does not 
significantly degrade individual device performance. 
The quantum efficiency of each detector 3, 5, and 7 under front (light 
incident from the associated pn junction) and back illumination is shown 
in plots 55, 57, and 59, respectively, of FIG. 3. The measurements were 
made under a reverse bias of 3.5, 6.0, 5.0 volts for In.sub.0.53 
Ga.sub.0.47 As, In.sub.0.7 Ga.sub.0.3 As, and In.sub.0.85 Ga.sub.0.15 As 
detectors 3, 5, and 7, respectively. The measured long wavelength cutoffs 
of 1.7, 2.1, and 2.5 .mu.m correspond to the bandgaps of the absorption 
layer materials. The short wavelength cutoff for the device 1 under back 
illumination is determined by the light absorption properties of layers 
between the substrate 15 and the absorption layers 9, 11, and 13. For 
example, for the In.sub.0.7 Ga.sub.0.3 As layer forming detector 5, light 
absorbed in the underlying In.sub.0.53 Ga.sub.0.47 As layer 9 will not be 
detected, thus the short wavelength cutoff of the In.sub.0.7 Ga.sub.0.3 As 
detector 5 is approximately equal to the cutoff wavelength of In.sub.0.53 
Ga.sub.0.47 As layer 9. The peak quantum efficiency under front 
illumination ranges from 55 to 95%. For the In.sub.0.85 Ga.sub.0.15 As 
detector 7, the peak quantum efficiency was determined to be 55%. This 
lower than expected efficiency is believed due, in part, to the large 
number of heterojunctions and layers underlying the detectors, increasing 
the probability of the carrier being captured and recombining at traps 
prior to being collected. Also, the fact that the diffusion of Zn is 
faster in In.sub.0.85 Ga.sub.0.15 As layer 13 compared with the other 
absorption layers 9 and 11, caused the thickness of the depleted 
absorption region to be less than optimum (&lt;2 .mu.m) in this detector 1, 
affecting its quantum efficiency. The peak quantum efficiency under back 
illumination (between 15% and 60%) is somewhat lower than for front 
illumination since the antireflective coating was deposited only on the 
top surface of device 1, in this particular example. 
In summary of one embodiment of the invention, as described above, a novel 
three wavelength InGaAs focal plane array pixel element 1 for detection at 
wavelengths from 0.9-2.6 .mu.m is shown, where each of three 
wavelength-sensitive detectors 3, 5, and 7 are individually addressable. 
This device 1 consists of successively smaller bandgap layers of In.sub.x 
Ga.sub.1-x As (x.gtoreq.0.53) 9, 11, and 13, grown on an InP substrate 15, 
separated by layers of InAs.sub.y P.sub.1-y to decrease defects induced by 
lattice mismatch strain with the substrate 15. The various layers were 
selectively removed so that pn junctions with different wavelength 
response can be separately contacted. All three detectors 3, 5, and 7 have 
quantum efficiencies between 15 and 95% (depending on wavelength and 
illumination direction) and dark currents from 0.01 to 10 mA/cm.sup.2 
--values comparable to discrete photodiodes with similar wavelength 
responses. 
To improve the performance of the three wavelength infrared focal plane 
array detector 1 of FIGS. 1A, and 1B, the present inventors believe that 
the modified device as shown in FIG. 5A is preferred. As shown, relative 
to the prototype detector element 1 of FIG. 1B, the preferred embodiment 
thereof of FIG. 5A includes an additional transparent strain relief layer 
18 between absorption layer 11 and strain relief layer 27, as shown. 
Improved performance is expected to be obtained in that the lattice 
parameter of the InA.sub.0.3 P.sub.0.7 layers 17 and 18, immediately below 
and above the absorption layer 11, are matched. In this manner, the 
magnitude of the dark current associated with absorption layer 11 is 
expected to be reduced, as previously indicated above. Also, the improved 
performance can be observed by comparing the band diagram shown in FIG. 1C 
for the prototype device 1, relative to the band diagram of FIG. 5B for 
the preferred configuration of FIG. 5A, whereby as shown the bandgap is 
extended for strain relief layers 19, 29 and 27, by the addition of strain 
relief layer 18. 
The present invention, within practical limits, can be extended to provide 
a focal plane array detector element capable of detecting "N" different 
wavelengths, where N is any integer number 1, 2, 3, 4, 5, . . . . N. As 
shown in FIG. 6A, such a device includes a cap layer 62, analogous to 
layer 31 of FIG. 5A, a substrate 50, a first absorption layer 52, under 
strain relief layers 54 (analogous to layers 17, 25, 23, and 21 of the 
device of FIG. 5A), a second absorption layer 56, followed by alternating 
buffer absorption layers 58 to the Nth order or degree, followed by an Nth 
absorption layer, followed by the previously mentioned cap layer 62. The 
material for each of these layers is generally indicated in Table 1 as 
shown below, as is the doping for each of these layers. Also, in FIG. 6B a 
band diagram is included showing the bandgaps of the various layers 
associated with the N absorption layer device of FIG. 6A. 
TABLE 1 
______________________________________ 
Lay- 
er Name Material Doping 
______________________________________ 
50 Substrate InP n+ 
52 First Absorption 
InGaAs Undoped (n-) 
Layer 
54 Buffer Layers InAsP, GaAlAsSb, 
Undoped (n-) 
InGaPSb, 
InGaAsSb, 
or InAlAsSb 
56 Second Absorption 
InGaAs Undoped (n-) 
Layer 
58 Alternating Buffer Undoped (n-) 
and 
Absorption Layers 
60 Nth Absorption 
InGaAs Undoped (n-) 
Layer 
62 Cap Layer Same as Layer C 
Undoped (n-) 
______________________________________ 
Although various embodiments of the invention are described herein for 
purposes of illustration, they are not meant to be limiting. Those of 
skill in the art may recognize modifications that can be made in the 
illustrated embodiments. Such modifications are meant to be covered by the 
spirit and scope of the appended claims. For example, a plurality of 
pixels each providing the capability of detecting up to (N+1) different 
wavelengths of light can be provided on a common substrate, with each 
pixel including (N+1) detectors, thereby providing an array of such pixels 
through use of the present invention.