An avalanche photodetector using a quantum well superlattice in which impact ionization of carriers in the well layers occurs across the band-edge discontinuity is described.

TECHNICAL FIELD 
This invention relates generally to the field of photodetectors and 
particularly to such photodetectors having an avalanche multiplication 
region. 
BACKGROUND OF THE INVENTION 
Many types of photodetector structures have been proposed for diverse 
applications including optical communication systems. For many of these 
applications, it is desirable that the photodetector structure exhibit 
gain without the need for external, electrical amplifiers. Two well known 
types of photodetector structures that exhibit gain are photoconductors 
and avalanche photodetectors. 
In addition to exhibiting gain, an avalanche photodetector should have 
relatively low noise associated with the avalanche multiplication process, 
i.e., the avalanche process should not reduce photodetector sensitivity by 
increasing noise. In silicon avalanche photodetectors, low noise avalanche 
multiplication is relatively easy to achieve as the ratio of the ioization 
coefficients for electrons and holes useful at wavelengths longer than 1.0 
.mu.m and other materials must be used at such wavelengths. However, in 
avalanche photodetectors using Group III-V compound semiconductors and 
structures analogous to those used for silicon, the ratio of the 
ionization coefficients for holes and electrons is typically close to 
unity, and the noise associated with the avalanche multiplication process 
is large. That is, the ratio of the ionization coefficients for bulk Group 
III-V compound semiconductors is typically close to unity. 
Various structures have been disclosed in attempts to reduce the noise 
associated with the avalanche process in avalanche photodetectors using 
compound semiconductors. One such photodetector is described in U.S. Pat. 
No. 4,476,477 issued on Oct. 9, 1984 to Capasso, Tsang and Williams. This 
photodetector, commonly termed a staircase by those skilled in the art, 
uses the energy imparted to carriers as they cross energy band 
discontinuities to impact ionize and produce avalanche multiplication. As 
the discontinuity occurs primarily in one energy band, only one type of 
carrier impact ionizes and, consequently, the noise associated with the 
avalanche process is relatively low. The structures described used a 
graded, i.e., varying, bandgap to obtain the required energy band profile. 
Other types of photodetector structures have been proposed including 
several using superlattices. A superlattice is a structure having a 
periodicity which differs from that of the underlying crystal lattice. 
That is, a superlattice may be viewed as comprising interleaved layers of 
compositions A and B. If one layer, e.g., that with composition A, is 
sufficiently small, quantum effects are significant and the device may be 
called a quantum well structure. An exemplary photodetector using such a 
structure is described in The Journal of Vacuum Science and Technology, 
B1, pp. 376-378, April-June, 1983. The structure described had interleaved 
layers of GaAs and GaAlAs which were uniformly doped n-type. The GaAs 
quantum wells and barrier layers were between n-type Ga.sub.0.6 Al.sub.0.4 
As cladding layers. The device was operated at a sufficiently low 
temperature so that essentially all the electrons were trapped in the 
quantum wells. Application of a bias voltage to an unilluminated structure 
did not result in significant current flow as there were few carriers 
present outside the quantum wells. The bias was applied perpendicular to 
the quantum wells, i.e., to the two cladding layers. However, when the 
device was illuminated and photons absorbed in the quantum wells, 
electrons were excited from the wells and current flowed in the external 
circuit, if the photons had energy sufficient to excite electrons from the 
wells. This device may thus be thought of as a particular type of 
photoconductor. 
The analysis given by the authors is interesting because they found 
substantial photoconductive gain, approximately 10,000, but also an 
exceedingly slow device response time of approximately one second which 
was determined by the electron capture time. There were other effects, 
including a possible avalanche multiplication effect, mentioned, the 
significance, and indeed the presense, of which the authors were not 
certain. For the particular effect just mentioned, it was hyposthesized 
that electrons traveling perpendicular to the quantum wells might scatter 
electrons from within the wells. However, the effects of such an avalanche 
process were neither described nor observed by the authors. 
SUMMARY OF THE INVENTION 
An avalanche photodetector comprising interleaved first and second layers, 
said first and second layers comprising first and second semiconductors 
having first and second bandgaps, said first bandgap being less than said 
second bandgap, the superlattice so formed being disposed between first 
and second cladding layers and a source of carriers to said layers having 
said first bandgap is described. The layers having first and second 
bandgaps may be conveniently termed well and barrier layers respectively. 
The photodetector is operated with a bias voltage small enough so that 
there is no tunneling or thermionic emission from the well layers but 
large enough so that carriers in the barrier layers impact ionize carriers 
in the well layers. The first layers are desirably sufficiently thin so 
that quantum effects are significant although this is not critical. The 
first and second cladding layers may have first and second conductivity 
types, respectively. In another embodiment, the device is a unipolar 
device, and the cladding layers have the same conductivity types but the 
layers having the first bandgap are doped. That is, the well layers are 
doped. In yet another embodiment, there is an electrical contact to the 
layers having the first bandgap to provide the carriers. This decreases 
the device response time.

For reasons of clarity, the elements of the devices depicted are not drawn 
to scale. 
DETAILED DESCRIPTION 
An exemplary embodiment of a photodetector according to our invention is 
schematically depicted in FIG. 1. Depicted are first and second cladding 
layers 1 and 3 and superlattice 5 which is between the first and second 
cladding layers. superlattice 5 comprises interleaved layers 7 and 9, 
respectively, having first and second compositions. For reasons of 
clarity, not all layers are shown. Layers 7 and 9 have first and second 
bandgaps, respectively, with the first bandgap being less that the second 
bandgap. Layers 7 are desirably sufficiently thin so that quantum effects 
are significant although this is not essential for successful device 
operation. 
There are also electrical contacts 11 and 13 to cladding layers 1 and 3, 
respectively. These are connected to a bias source. The narrow bandgap, 
i.e., first, layers may be doped or undoped and the cladding layers may 
have the same or opposite conductivity types. However, there must be 
carriers present in the narrow bandgap layers for the device to operate 
successfully. If the superlattice layers are undoped, thermal generation 
must be adequate to yield the requisite number of carriers in the narrow 
bandgap layers. Thermal generation thus acts as a source of carriers in 
the narrow bandgap layers. The bias applied to contacts 11 and 13 must be 
within a critical range to yield the desired avalanche characteristics. In 
particular, the bias must be large enough so that the carriers in the 
barrier layers acquire sufficient energy to impart ionize carriers out of 
the well layers. However, it must also be small enough so that there is 
not a significant amount of either tunneling or thremionic emission from 
the well layers. 
In an exemplary embodiment, the first and second cladding layers had 
opposite conductivity types and the superlattice comprised Al.sub.0.48 
In.sub.0.52 As layers interleaved with Ga.sub.0.47 In.sub.0.53 As layers. 
The cladding layers were Ga.sub.0.47 In.sub.0.53 As and were lattice 
matched to an InP substrate. Another embodiment comprised an AlSb/GaSb 
superlattice. Well widths varied from 100 to 400 Angstroms, and the widths 
of the barrier layers varied between 100 and 400 Angstroms also. Other 
Group III-V compound semiconductors may also be used. 
FIG. 2 shows the energy band diagram for the superlattice region of an 
exemplary embodiment in which the wells layers are doped n-type. As 
depicted, a single hot electron gaining energy in the first barrier layer 
impact ionizes a bound electron out of the first well yielding two 
carriers in the second barrier layer. Each of these carriers impact 
ionizes in the second well to yield four carriers in the next barrier 
layers, etc. 
FIG. 3 shows the energy band diagram for a portion of the superlattice 
region in which the wells are undoped and ionization across the band-edge 
discontinuity of carriers dynamically stored in the wells occurs. The 
carriers orginate from a thermal generation process via mid gap centers. 
Alternatively, the well layers maybe contacted directly to yield a three 
terminal device. The third electrical contact provides a source of 
carriers to the well layers. This embodiment is depicted in FIG. 4. 
Depicted are n+ substrate 41, n- layers 43, n-type absorbing layer 47, n+ 
contact layer 49, n+ contacts 51 and 53, and a superlattice comprising 
alternating layers 451 and 453. The bandgap structure of the superlattice 
has already been described. The contacts may be formed by, for example, 
diffusion, ion implantation or epitaxial regrowth. The contact should be 
ohmic to the low bandgap layers. This type of contact is relatively easy 
to obtain because of the carrier concentration in the low bandgap layers. 
Of course, layers having the opposite conductivity types may be used. The 
diodes were cooled and illuminated with either chopped light from a 
monochrometer or a helium neon laser. As the reverse bias was increased to 
20 V, a substantial increase in the photocurrent response was observed. 
This increase was shown to be the result of a multiplication process. It 
was also observed that as the bias increased, the spectral response was 
enhanced more at longer wavelengths. This indicates that holes are 
multiplied significantly more than are electrons. It was also noted that 
multiplication began to occur at an electric field which is a factor of 
approximately two smaller than the threshold field for impact ionization 
across the bandgap in bulk Ga.sub.0.47 In.sub.0.53 As at the same 
temperature. This clearly indicates that multiplication is caused by a 
type of impact ionization collision which differs from that of the bulk 
material. 
As the sample temperature was increased from 70 to 300 degrees K, the onset 
of avalanche gain shifted to lower voltages. This shift precludes the 
possiblity of band-to-band impact ionization. However, the frequency 
dependence of the gain indicates the presence of a relatively long time 
constant and strongly suggests that the multiplication is due to presence 
of the well layers. In fact, deep level ionization can be ruled out as 
being unlikely. 
Thus, impact ionization across the band-edge discontinuity of carriers 
dynamically stored in the quantum wells is the source of the observed 
phenomena. In the steady state, the flux of carriers entering any well 
must balance the flux of carriers leaving the well due to the thermionic 
emission. When this is achieved, it is apparent that if the band offset is 
sufficiently larger than the carrier temperature, considerd in terms of 
energy units, the carrier densities in the well layers will be greater by 
orders of magnitude that the values that are normally obtained in the 
absence of well layers for the same dark current level. This leads to 
carrier concentrations in the wells in excess of 10.sup.15 /cm.sup.3 for 
microampere dark currents. If a hole is now photo-injected into the 
structure and gains energy in the barrier layers, it impact ionizes one of 
the holes stored in the low bandgap layers out of the well and across the 
valence band discontinuity. The ionization threshold energy for this 
process is greater than 2.DELTA.E.sub.v. The ionization rate is 
proportional to the hole density in the wells which in turn is 
proportional to the dark current and increases strongly with temperature. 
Since the conduction band discontinuity is larger than the valence band 
discontinuity the electron ionization rate is smaller than the hole 
ionization rate. 
The reservoir of carriers must be continuously replenished to sustain the 
avalanche process. This may occur via thermal generation of electron-hole 
pairs in the wells. The time constant can be very long at low temperatures 
and determines the role of the multiplication curve as a function of 
chopping frequency.