Alternating gradient photodetector

A far infrared (FIR) range responsive photodetector. There is a substrate (28) of degenerate germanium. A a plurality of alternating impurity-band (32) and high resistivity (30) layers of germanium are disposed on the substrate (28). The impurity-band layers (32) have a doping concentration therein sufficiently high to include donor bands which can release electrons upon impingement by FIR photons of energy hv greater than an energy gap .epsilon.. The high resistivity layers (30) have a doping concentration therein sufficiently low as to not include conducting donor bands and are depleted of electrons. Metal contacts (36, 38) are provided for applying an electrical field across the substrate (28) and the plurality of layers (30, 32). In the preferred embodiment as shown, the substrate (28) is degenerate n-type (n.sup.++) germanium; the impurity-band layers (32) are n.sup.+ layers of germanium doped to approximately the low 10.sup.16 cm.sup.-3 range; and, the high resistivity layers (30) are n.sup.- layers of germanium doped to a maximum of approximately 10.sup.15 cm.sup.-3. Additionally, the impurity-band layers (32) have a thickness less than a conduction-electron diffusion length in germanium and likely to bein the range of 0.1-1.0 micron, the plurality of impurity-bands (33) is of a number such that the flux of FIR photons (20) passing therethrough will be substantially totally absorbed therein, the thickness of the high resistivity layers (30) is such compared to the voltage applied that the voltage drop in each the high resistivity layers (30) controls the occurance of impact ionization in the impurity-band layers (32) to a desired level.

1. Technical Field 
The present invention relates to photodetectors for use in infrared 
detection and, more particularly to an infrared photodetector useful in 
the far infrared region comprising, a substrate of a semiconductor 
material; a plurality of alternating impurity-band and high resistivity 
layers of semiconductor material disposed on the substrate, the 
impurity-band layers having a doping concentration therein sufficiently 
high to include donor bands which can release electrons upon impingement 
by infrared photons of energy h.nu. greater than an energy gap .epsilon., 
the high resistivity layers having a doping concentration therein 
sufficiently low as to not include conducting donor bands and being 
depleted of electrons; and, means for applying an electrical field across 
the substrate and the plurality of layers. 
2. Description of the Prior Art 
There is a need in space applications for a far-infrared (FIR) 
photodetector (30-200 .mu.m) having high detectivity as well as the 
potential for being fabricated into focal-plane arrays. The well-known 
technology includes bolometers and extrinsic-semiconductor 
photoconductors. Such devices lack sensitivity and/or response speed, and 
are not readily adaptable to arrays. 
An established approach to long wavelength infrared (LWIR) detection, and 
the most relevant art to the present invention, is the blocked 
impurity-band (BIB) detector as described in great detail in the papers 
"RESPONSIVITY AND NOISE MODELS OF BLOCKED IMPURITY BAND DETECTORS" by M. 
D. Petroff and M. G. Stapelbroek and "ORIGIN OF EXCESS LOW-FREQUENCY NOISE 
AT INTERMEDIATE INFRARED BACKGROUNDS IN BIB DETECTORS" by M. D. Petroff, 
M. G. Staplbroek, J. J. Speer and R. Bharat. The above-referenced papers 
were presented in August of 1984 at the IRIS Specialty Group on IR 
Detectors at Seattle, Wash. Copies of both papers are being filed with 
this application for the convenience of the Patent Office. A basic 
discussion of BIB technology follows hereinafter as well. 
Blocked impurity band (BIB) detectors have been developed with extrinsic 
silicon material for LWIR in the wavelength range of 10-30 .mu.m. Attempts 
to extend this BIB technology to the FIR using extrinsic germanium have 
thus far failed. In the case of germanium, the material becomes degenerate 
(i.e. metal-like), losing its extrinsic photoconducting properties at 
relatively low dopant concentrations (low 10.sup.16 cm.sup.-3) compared 
with silicon (&gt;10.sup.18 cm.sup.-3). This means that the active doped 
region must be much thicker than that of silicon (&gt;&gt;30 .mu.m) to achieve 
good absorption or the quantum efficiency and detectivity will be very 
low. A necessary requirement of the BIB detector, however, is that the 
impurity band of the active (doped) region be depleted of carriers from 
unwanted compensating impurities. This requirement becomes increasingly 
difficult and impractical if the region becomes too thick because of 
severe constraints on the breakdown field due to field emissions of 
electrons (or alternatively, holes) from the impurity band into the 
conduction (valence) band. Improved purity control of the germanium 
material reduces these unwanted carriers; however, there are practical 
limits to what can be achieved and what can be controlled with uniformity 
over a large area such as required for arrays (e.g., 10.sup.11 -10.sup. 12 
cm.sup.-3 compensating impurities). At these impurity limits, the 
thickness of the active region is limited to that used in silicon BIBs 
(e.g. .ltorsim.30 .mu.m). Unfortunately, such thicknesses are inadequate 
for germanium BIB detectors and thus one is limited to much lower quantum 
efficiencies and detectivities. 
The problems associated with BIB technology and its application to 
germanium and the detection of FIR can best be understood with reference 
to the simplified drawings of FIGS. 1 and 2. The BIB detector, generally 
indicated as 10, comprises a substrate 12 upon which an IR active layer 14 
and blocking layer 16 are "grown" by conventional chemical vapor 
deposition (CVD) technology. The IR active layer 14 is appropriately doped 
while the blocking layer 16 is undoped. The doping can be either n-doping 
or p-doping, as desired, and only affects the polarity of the field 
applied thereto. Electrical contact can be made to the substrate 12 and to 
the blocking layer 16 by means of a transparent contact 18. The IR active 
layer 14, by virtue of the process by which it was created and otherwise, 
contains impurities which cause a background current flow which must be 
"swept" out to allow the signals of interest to be detected. This is 
accomplished by the application of an electrical voltage across the 
detector 10 between the transparent contact 18 and the substrate 12 as 
indicated in FIG. 1. This must be a low voltage (a few volts at most) 
because of breakdown. The blocking layer 16 is necessary to make the 
detector 10 responsive to the incidence of IR photons 20 thereon. Without 
the blocking layer 16, electron flow could take place unhampered because 
of the conductivity of the doped IR active layer 14. With a sufficiently 
high doping level, however, electrons 22 will be excited into the 
conduction (valence) band with low energy photons corresponding to the FIR 
wavelength region. 
As depicted in FIG. 2, the application of the voltage across the IR active 
layer 14 to create the sweeping field causes the electrons 22 to migrate 
towards the anode side of the layer 14 and the "holes" 24 to migrate 
towards the cathode side. Unfortunately, there is a limit to the sweeping 
field which can be applied without exceeding breakdown fields and causing 
the detector 10 itself to cease operation for its intended purpose of 
photodetection. The limiting of the field strength, in turn, limits the 
thickness of the IR active layer 14 which can be effectively depleted of 
carriers. In the silicon BIB detectors employed for LWIR detection, the IR 
active layer 14 has a practical thickness limit of 10-20 microns as 
mentioned in the above-referenced papers filed herewith. Ideally, all the 
energy of the incident IR photons 20 would be absorbed in the IR active 
layer 14 and cause electrons 22 to be excited into the conduction band. 
Just as depicted in the simplified drawing of FIG. 1, however, in the case 
of germanium the thin IR active layer 14 possible within the constraints 
described allows a great portion of the photon energy to pass through the 
detector unused; that is, the quantum efficiency of the detector 10 is 
quite poor. To achieve higher absorption, one would like to increase the 
doping; however, the energy gap .epsilon. goes to zero at high doping and 
the device ceases to work. 
Those skilled in the art should begin to appreciate at this point the 
problems which are encountered when attempting to adopt the 
above-described BIB technology to the detection of FIR radiation. First, a 
material such as germanium must be employed; that is, silicon will not 
work at the energy levels involved. The energy .epsilon. must be kept 
lower than the photon energy of the FIR radiation where .epsilon. is a 
function of the doping level of the IR active layer 14. In attempting to 
adopt BIB technology to FIR detection, the designer falls into a difficult 
situation. Even if one can (through intensive quality control on impurity 
levels, and the like) create an IR active layer 14 of optimum dopant level 
and sufficiently low level of compensating impurities that the sweeping 
field is operative, the process would be virtually non-repeatable from 
detector to detector so that the use of the resultant detectors in a 
multi-detector array might be unachievable. 
Thus, one can properly state that, at present, there is no method and 
apparatus for the detection of FIR radiation which is cost effective and 
sufficiently uniform so as to be incorporatable into arrays. The BIB 
technology which is the most relevant art is limited to low quantum 
efficiencies and requires the most stringent and limited conditions. 
DISCLOSURE OF THE INVENTION 
The above-described shortcomings of the prior art are overcome by the far 
infrared (FIR) photodetector of the present invention comprising, a 
substrate of degenerate germanium; a plurality of alternating 
impurity-band and high resistivity layers of germanium disposed on the 
substrate, the impurity-band layers having a suitable doping concentration 
therein sufficiently high to include donor bands which can release 
electrons upon impingement by FIR photons of energy h.nu. greater than an 
energy gap .epsilon., the high resistivity layers having a doping 
concentration therein sufficiently low as to not include conducting donor 
bands and being depleted of electrons; and, means for applying an 
electrical field across the substrate and the plurality of layers. 
In the preferred embodiment, the substrate is degenerate n-type (n.sup.++) 
germanium; the impurity-band layers are n.sup.+ layers of germanium doped 
to approximately the low 10.sup.16 cm.sup.-3 range; and, the high 
resistivity layers are n.sup.- layers of germanium doped to less than 
approximately 10.sup.15 cm.sup.-3. 
Further in the preferred embodiment, the impurity-band layers have a 
thickness significantly less than a conduction-electron diffusion length 
in germanium, the plurality of impurity-bands is of a number such that the 
flux of FIR photons passing therethrough will be substantially totally 
absorbed therein, the thickness of the high resistivity layers is such 
compared to the voltage applied by the means for applying an electrical 
field across the substrate and the plurality of layers that the voltage 
drop in each the high resistivity layer controls the occurance of impact 
ionization in the impurity-band layers to a desired level, the doping 
level in respective ones of the plurality of impurity-band layers is 
varied with respect to the others of the impurity-band layers to achieve a 
desired spectral response across the entire photodetector, and the doping 
level in each of the plurality of impurity-band layers is varied within 
the layers to increase the breakdown level of the photodetector as well as 
further control the spectral response.

DETAILED DESCRIPTION OF THE INVENTION 
The alternating gradient photodetector (AGP) of the present invention is 
shown in simplified cross section in FIG. 3 wherein it is generally 
indicated as 26. As will be appreciated by those skilled in the art from 
the description which follows hereinafter, the structure is easily adapted 
to fabrication of large focal-plane arrays. In this regard, the 
description hereinafter can be considered to be with respect to one pixel 
of such an array. Fabrication of an AGP structure is readily achieved by 
conventional CVD technology as mentioned above with respect to the prior 
art. The device described hereinafter is for the case of n-doped 
germanium; however, one could employ p-type doping instead. Furthermore, 
other materials, e.g., Si or GaAs, could be chosen to provide alternative 
spectral response. 
The base material or substrate 28 is degenerate n-type (n.sup.++) 
germanium. CVD is employed to grow sequentially the alternating layers 30, 
32 and finally 34 comprising, respectively, n.sup.-, n.sup.+, n.sup.-, 
n.sup.+, n.sup.-, n.sup.+, . . . n.sup.-, n.sup.+, n.sup.-, n.sup.++. The 
last layer 34 (n.sup.++) is transparent contact of thickness "d" 
(typically 1 .mu.m). To the above structure as grown by CVD processes, a 
back metal contact 36 and a top metal contact 38 on the periphery of the 
pixel element are deposited by conventional means. 
FIG. 4 is an expanded doping profile of the several alternating layers 28, 
30, 32, 34 of FIG. 3 with the n.sup.-, n.sup.+ layers 30, 32 having 
thicknesses of "a" and "b", respectively. The total number of n.sup.+ 
layers 32 is "N"; so, the quantity Na is the total absorption thickness 
and ideally should be at least as large as the photon absorption length, 
i.e. greater than 100 .mu.m. Individual layer thickness "a" must be small 
compared to a conduction-electron diffusion length, and is projected to 
have an optimum value in the range 0.1-1.0 .mu.m. The dopant concentration 
n.sup.+ of the impurity-band layers 32 is in the low 10.sup.16 cm.sup.-3 
range. The concentration n.sup.- is significantly lower in order to 
provide the high resistivity layers (of thickness "b") to support the 
applied voltage V, and is likely to be less than 10.sup.15 cm.sup.-3. 
It is worthy of note at this point that, while not yet investigated in 
depth at this time, the voltage drop V/N in each n.sup.- layer can be 
used to control the desired level (or absence) of impact ionization. This 
potentially vital aspect will be developed as further work is undertaken; 
however, it is to be considered as part of the scope and spirit of the 
present invention as disclosed herein. 
The principle of the detector of the present invention can best be 
understood with reference to FIGS. 5 and 8. The n.sup.+ layers 32 contain 
donor bands 33 which can release electrons 22 upon impingement by IR 
photons 20 of energy h.nu. greater than the energy gap .epsilon.. The 
n.sup.- layers 30, on the other hand, cannot release electrons since they 
do not contain donor bands and are depleted of electrons due to the donor 
concentration n.sup.-. In the presence of a voltage "V" being connected 
across the structure 26 in the manner of FIG. 1, local fields are produced 
across the layers 30, 32 as depicted FIG. 8. Upon impingement by an IR 
photon of energy h.nu. greater than .epsilon.,each of the n.sup.+ layers 
32 has electrons 22 which can be excited into the conduction band where 
they drift to the adjacent n.sup.- layer 30. 
The above-described structure of the present invention and its method of 
operation provide several beneficial effects. For one, because there are 
multiple layers 32 sufficient in total thickness to absorb virtually all 
of the impinging IR photons, the quantum efficiency can be high. This high 
quantum efficiency is achieved with only moderate requirements on doping 
control and background impurities. These relaxed material requirements 
over the prior BIB technology makes the AGP detector of the present 
invention much more suitable for array technology. These relaxed 
requirements also make it a more robust detector and less susceptable to 
radiation damage in space applications. It should be specifically noted 
that these advantages may also be achieved in Si and GaAs AGP 
architectures in other spectral regions. When employing Si in an AGP 
structure for LWIR detection, for example, the n+ layers would be doped in 
the 10.sup.17 cm.sup.-3 range with the n.sup.- at 10.sup.15 cm.sup.-3 
maximum. 
The AGP detector 26 of the present invention also has the potential for 
"gain" provided from two sources. For one, photo electrons 22 released 
from a particular layer 32 move through the remaining layers 32 of the 
detector 26. In the process, they may impact ionize other electrons 22 in 
their path causing an increase in the number of electrons 22 flowing as a 
result of the initiating impingement at the first layer 32, e.g. a gain in 
the signal produced by the structure. For another, as each electron 22 is 
released, a "hole" 24 is left behind. The electrical imbalance caused by 
the presence of the hole 24 causes electrons 22 from adjacent layers to be 
attracted during the life of the hole 24. This increase in the number of 
electrons 22 flowing is a gain in the signal produced and is given by the 
formula--GAIN =LIFE OF HOLE/TRANSIT TIME OF ELECTRON. 
As an additional consideration, because of its multilayered construction as 
opposed to the single active layer construction of BIB detectors as 
described above, the AGP detector of the present invention inherently 
provides certain desired attributes not possible with BIB detectors. 
Moreover, the construction invites customization to achieve specific 
characteristics which may be desirable in a particular application. For 
example, the energy gap .epsilon. and thus the spectral response of an AGP 
detector depends on the concentration of n.sup.+ and, in the structure 
according to the present invention, can be tailored to specific 
applications. n.sup.+ can be varied within each layer 32' as depicted in 
FIG. 9 as compared with FIG. 6, or can be varied from layer 32 to layer 32 
as shown in the photodetector 26' of FIG. 11 to achieve, for example, 
flat, broad-band response. 
It is also worthy of special note with respect to the structure of the 
present invention that if the dopant is homogeneous throughout the n.sup.+ 
layers 32 as depicted in FIG. 6, the donor band/conduction band interface 
will have a sharp break as depicted in FIG. 7. If, on the other hand, the 
dopant is graded in concentration from the center of the n.sup.+ layers 
32 towards the edges as depicted in FIG. 9 the donor band/conduction band 
interface will taper-off as depicted in FIG. 10, thus increasing the 
breakdown level. The ability to manipulate the performance and 
characteristics of the detector of the present invention is a major point 
of novelty thereof over the prior art.