Note: Descriptions are shown in the official language in which they were submitted.
2 ~
.. wos2to82so PCT/USs1/~8~0~
,. .
MINIBAND TRANSPORT QUANTUM WELL INFRARED DETECTOR
BACKGROUND OF THE INVE~TION
; The present invention is directed to a
semiconductor infrared detector and a method of detecting
thermai radiation and more particularly to a semiconductor
infrared detector having a plurality of doped quantum
wells separated by short period superlattice layers which
form a miniband of energy states.
High performance de~ectors and detector arrays
for detecting thermal radiation are u~ed in a wide variety
Of military and commercial electro-optic system such as
night visio~, military surveilance and navigation, the
trackinq o missiles and aircraft and navigationa~ aids
for commercial aircrafts.
Detectors in the medium wavelength infrared
(MWIR) wavelength band (with wavelengths from 3 to 5
microns) have a number of detector materials that are
. . available to be used. However, detectors in the long
., . . . . ~ . i . . . . .... .
,. wavelength infrared (LWIR) wavelength band (from
. wavelengths of 8 to 12 microns) have only been able to be
20- fabricated with a narrow band gap semiconductor of mercury
.. . . . . . ...
- cadmium telluride (MCT). Many problems with respect to
..the uniformity of optical and electrical properties and
; mechanical stability . are associated with ~CT. As a
~ ,result~ routine fabrication of ~WIR dPtec~or arrays is
.. . . ... ., .. ~ .. , . . . . . . , . ,. . . . . : .
, ~;25. prevented. ,,~ , ~ ~
. .. Conventional quantum wéll infrared photo-
. . , detectors ~QWIP~ dètect the presence of thermal
,. . ... ~ , .,, . ~ . .. - .. ,, .. , . . . - : ., .
' " ..... ' ~' ' " ' """ ' ''';, ' " '
' ' '~ ' , ' ' ' " ' ' '
'' ' " '' '. ','''~. ' " ' ' '' ' '
,' " "' ' ,
- 2~Q~
W092/08250 P~TIUS91/0800~ ~.
radiati~n by an internal photoemission of charged carriers
from confined states in a quantum well which changes the
electrical conductivity of the material ~or the QWIP
detectors. The materials used for the conventional QWIP
detectors are III-V compound semiconduct~rs such as
gallium arsenide and aluminum gallium arsenide. Although
processing technology suggests that III-V compound
semiconductors may be used to fabricate detectors with
very uniform detectivities, the performance oÇ such
detectors are an order of magnitude of less than the
performance of the MCT detectors in the ~WIR wavelength
band.
One e~ample of a conventional QWIP detector is
illustrated in ~igure 1. The QWIP detector of Fi~ure 1
includes a stack of quantum wells having widths and depths
chosen to provide two confined states, the ground state
and the e~cited state as illustrated in Figure la. The
energy separation between the sround state and the e~cited
state is equal to the energy of the photon to be
detected. The quantum walls are doped with electron donor
impurities which partially fill the lowest energy state
with electrons. A barrier having a thickness o
appro~imately one hundred angstroms (lOO ~) separates each
of the quantum walls. When a bias is applied to the QWIP
~5 detector `as``illùstrated in Figure lb, electrons are
., i .. ~, . . ~.... . . ............. . . . ....... . .
photoe~cited i`nto the excited state and--tunnel through the
barrier separating the guantum wells to bé -collected as a
.~ photocurrent. However, dark current which fiows in the
.absence of thermal radiation in~the` above described QWIP
detectors is unacceptàbly high~ due to` tunneling of
electrons from the grou~d state through the thin barriers
between ~he guantum wells.
. , : ., : , ., . : . ,
- . .. . ~ ~ .: .
.. ~ , . . : :
.: ' .. .. ,
: ' '
~W092/08250 ~ L ~ ~ ~ PCT/~S91t~004
Another e~ample of a conventional QWIP detector
is illustrated in Figure 2. In this QWIP detector , the
widths of the quantum wells are chosen so that only one
state is bound in the quantum well a~d the virtual ~xcited
state is pushed slightly above the barrier into the
continuum as illustrated in Figure 2a. In these QWIP
deteckors, the barriers between the quantum wells ~re made
to be relatively large, appro~imately 500 A, which reduces
the dark current while still allowing ~he ~arriers to be
collected because photoe~citation pushes the carriers
above the barrier 4Or allowing the carriers to be swept
out by an applied electric field. However, in the above
described QWIP dete~tors, there is only one bound state in
the ~uantum well and the first e~cited state must be close
in energy to the top of the barrier because the absorption
strength drops rapidly as the escited ~tate moves higher
a~ove the barrier which reduces the detectiviky. As a
result o these requirements, the well width and barrier
height are uniquely defined and therefore cannot be varied
.: 20 to optimize detector performance. Additionally, the
~~f ective mass of the carriers must be low in order ~o
: provide one bound staté in the guan~um well. As a result,
i the dopants in the QWIP detector must be n~type (electron
donors) ~ which further limi~s; the de~i~n ~arameters.
Furthermore, the- wavelength range ~in which the QWIP
detector is sensitive .is;primarily a material parameter
- :; that is determined by the wavelength dependence of the
:process ~ of :absorption from -~a-S discrete .state to the
~ ~- continuum,~ and the wavelen~th.range i can only be changed by
- 30 varying the energy of.the excited state above the:~arrier
~ which grea~ly decreases the:absorption strength~
,: . , ~ . .
. .. . . . ....... . . . . ..... ..
.. ..
, , , , . , , : .
,, ; ~ , ,
. . ~ , . '. ,,, ~
2 ~ 0 ~ 3
WO 92J08250 PCI'/US91~0800~ ¢' . .
SUM~!LARY OF THE INVENTION
One object of the present invention îs to provide
an improved semiconductor deYice for detecting thermal
radiation.
It i~ a further object of the present invention
to provide a miniband transport quantum well infrared
semiconductor detector having a predetermined bandwidth
corresponding to the range of wavelengths to which the
detector is sensitive that is determined by the ~hickness
and comp~sition of the semiconductor layers of the
detector.
Another objec~ of the present inv~ntion is to
obtain an electrically tunable peak absorption and
bandwidth in response to an electric field applied to the
detector.
The objects of the present in~ention are
fulfilled by providing a semiconductor miniband transport
quantum well infrared detector comprising a substrat~, a
- multilay~r structure: disposed on the substrate, and first
` 20 and second contact layers disposed on.the top and bottom
surfaces of the multilayer structure. . The multilayer
~ : stru~ture includes a plurality of doped quan~um wells and
~ superlattice barrier.-layers disposed on each iside of the
;; quantum wells.- The superlattice,barrier lay~rs comprise a
:- -' 2s-`plurality-`of -alternating..~irst and second lay~rs wherein
: the ~irst -layers have a:relatively low,ba~d gap and the
second layers jhave~-a relatively high .band gap. The
: ~ sup~rlattice-barrier -layers form a miniband of energy
istates :which~:transport~ photoe~cited carriers from the
: - . 3~ quantum wells for collectio~ as photocurren~.
:; . .. . .
, : ,: , :
: .: ' ' ' . ,:
,.-;,w092io8250 ~ f~$ ~ PCT/US91/OB004
The semiconduc~or detector of the present
invPntion comprises quantum wells having two or more bound
states so that a wide range of materials and carrier types
may be used for the semiconductor layers. In one
embodiment of the present in~ention, the semiconductor
detector comprises a GaAs/AlGaAs quan~um well system. In
further embodiments, a wide range of semiconductor
materials for the detector may be used such as
InGaAs/InAlAs (lattice-matched to InP substrates) or
InGaAs/InAlGaAs strained layer systems ~or e~ample.
The present invention additionally pro~ides a
method for detecting thermal radiation by a semiconductor
device comprising the s~eps of providing a plurality of
doped ~uan~um well layers, forming a miniband from a
plurality of strongly ~oupled superlattice barrier layers
disposed on each of the quantum well layers, photoe~citing
carriers from the quantum wells to the miniband and
applying an electric ield aCross the superlattice barrier
layers and the quantum -well layers to transport the
? 20 ~ carriers through~: the .- miniband--.for collection as a
photocurrent corresponding ~o the thermal radiation.
Ac~ording to the present invention, a
semiconductor detector and a method is provided which
: allows the quantum wells to be ormed with more than one
bound:state and ~ wide variety of well widths and barrier
heights. The superlattice barrier~layers redu~e the dark
--current~ and the sensitivi~y to proces~ing ~ariations by
- using wider quantum~wells. B~ allowing the quantum wells
~ ' -~to contain two or more1bound states, a larger range of
mat0rials and ~carrier :1.types may be used for the
dëtectors. Furthermore, the bandwidth of the detector may
J
b`é-selected over-a wide range :b~:varying;the coupling of
the layers in the superlattace barrier ,to provide a
,~ , , .
, , :~ ,
,; : ~ : ,
2 ~ ~t~ 3
W092~08250 PCT/US91/~800
broader or narrower miniba~d as desired which broadens or
narrows the absorption range respectively while
maintaining the absorption strength.
Further scope o~ applicability of the present
invention will become apparent from the detailed
description given herein after. However, it should be
understood that the detailed description and specific
e~amples, while indicating preferred embodiments of the
: present invention, are given by way o~ illus~ration only,
since various changes and modi~ications within the spirit
and scope o the present invention will become apparent to
those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE D~AWINGS
The present invention will become more fully
understood from the detailed description given herein
~elow and the accompanying drawings which are given by way
? illustration.-;only:--and thus;are not limitative of the
present i~vention, and wherein:
.
-' Figures la and lb illustrate the energy states of
20 -a conventional quantum well i~frared photodetector where
' ' ''' J'; no ~ias;is applied~to the detector.in Figur~ la and a bias
'is applied to:the-detector in Figure lb; ~.
r ~Figures 2a andr2b illustrate the~energy state~ of
another~conventional- quantum well inrared photodetector
~ 25 :whére no bi~as is applied to the detector in Figure 2a and
. a bias is applied.to;the detector in Fi~ure 2b;.
- -:- : ~ Figurer?~.;3 ~illustrates -the structure for the
- --~miniband ~transport-.quantum well .in rared detector for an
embodiment of ~he present invention; . .
- :, " , ,,,~, " . :: ..
, ; . , ~. -
~q~ J~ ~ .
wos2~o~2so PcT/US91/OgO~q
Figure 4 illustrates a miniband transport quantum
well infrared detector for an embodiment of the present
invention which utilizes the semiconductor structure of
Figure 3;
Figures 5a and 5b illustrate the energy states
for the miniband ~ransport detec~or ~or the present
invention where no bias is applied in Figure 5a and a bias
is applied in Figure 5b;
Figure 6 illustrates the wavelength dependence of
photocurrent obtained from a miniband transport detector
of the present invention; and
Figure 7 illustrates the bias dependence of the
detectivity of the mi~iband transport detector of the
present inYention.
DET~ILED DESCRIPTION OF THE PREFERRED EM~ODI~ENTS
Figure 3 is first referred to and illustrates an
embodiment of the structure for a miniband transport
quantùm well infrared~d~tector (her2inafter referred to as
MBT detector). The MBT detector is formed of a multilayer
structure:~ 40 which has alternating quantum. wells lol,
- 102, . . . 10n and superlattice barrier layers 201,
202,~ . . . 20n. A - capping layer 60 and a buried
- contact 70 are disposed on each.end of thP multilayer
-'- structure 4~. : The-^above described structure is formed on
`25 a sùbstrate 30.
Figure 4 illustrates.:an-embo~iment .of the present
`- invention ~which includes an array -of d~tector elements
pi~els~. Each pisel is delineated.and.the buried.`contact
is; exposed by etching::away the rmaterial surrounding the
pixels. Each pîxel i~cludes a layer 80 having an optical
diffraction grating etched into the capping layer 60 and a
- . .. . . . . .
. :. .....
.. .
: : . ~, , ~ .,
. - . : ~ ; , , . .;
. . . .. .
2 ~ ~ G3 3
WO 92/082~0 PCr/US91/08004 (
metal contact deposited on the ~urface of the optical
diffraction grating. Radiation is incident through the
substrate 30 and is coupled into the detector layers by
the optical diffraction grating.
The quantum wells o~ the MBT detector contain two
or more bound states which include at least a ground state
- and an ea:cited stat~. A miniband o energy states is
formed by strongly coupling the superlattice layers of the
structure. The superlattice barrier layers include a
plurality of thin quantum wells ~eparated by thin harrier
layers. These plurality of layers correspond to
alternating low band gap layers and high band gap layers
which are chosen so that the lowest energy miniband is in
resonance with the e~cited state o~ the quantum wells.
An esample o~ this structure is illustrated in
Figure ~a where the quantum well includes a ground state
and an e~cited state which fall~ between the energy l~vels
of the miniband 100. Fi~ure 5b illustrates the
application of an electric field to the structure wherein
.1the minibahd is tilted -and-breaks up into a series of
- : quasi-discrete states (not shown in the figuses)
~ e~tending through several periods of the superlattice
:. :layers. The quantum.wells-are doped so that the ground
states are partially-filled with carriers. The carriers
25 n in:.the ground :states-o the quantum wells may thereby be
~ v~- e~cited::into~.the -ne~t,-higher energy -~state~(or a~y odd
: order energy ~tate as dictated by ~selection rules for
`optical ~ransition) by infrarea photons~ ~s a result, the
carriers -are-~placed in~!the,miniband which allows the
: 30~i:carrier~ to.:~move ~:through ; the barrier layers with
J relatively~;~ease foricollection as photscurrent in the
, 3 !; coIltact ' layerS'~
'' ' , ', ' "'' '' . ' '~:' ' ` .
~ ,WO92/082so P~T/US9~/08004
, .,
The superlattice barrier layers may be made of a
thickness so that the probability of carriers tu~neling
directly from the ground sta~e into ~he ne~t quantum well
or into the continuum is substantially zero e~cept when
very hi~h biases are applied to the structure. In other
words, the tunneling componen~s of the dark current is
appro~imately zero. Also, the energy of the superlattice
barrier layers may be chosen to be sufficiently large for
limiting the thermionic emission o~ carriers into the
continuum at the desired operating temperature of 77
Kelvin. The precise quantum well and superlattice barrier
layer geometry is determined by solving ~he Schroedinger
equatio~ ~or the system in order to obtain the desired
wavelength pea~ and range in the responsitivity of the
detector. The number of layers in the superlattice
barrier layers are chosen based on the crystal growth time
~; or other similar considerations. The quantum well layers
are doped so that the ground states are partially filled
with carriers where the layers are usually doped with
electron donors. ` ; ~
- ,
The energy a~d the spatial extents of the
- ~ quasi-discrete states in the miniband are strong functions
of the applied électric field. As a result, properties of
. the MBT detector such as the peak absorption and the
responsitivity bandwidth may be electrically-tuned. :
.. .. . . ... . . ..
One e~amplé of an MBT detector that was formed by
molecular béam epitaxy includes forty GaAs quan~um wells,
. which have a thickness of àpproximately 78 ~ and are doped
~ - -- -17 ;-~
w~th silicon to- a level of 4xl~ Ocm~, s~parated by
. 30 ~uperlattice barrier layérs having -nine Ga~s wells,
.. appro~mately 20 ~ thick, and ten AlGaAs barriers,
.; . ajprosimatëiy 40 ~ thick.` Doped contact layers having a
thicknéss of 1 ~ m were formed above and below the active
: ~ :, ", .. ..
: . . , . : . -.. .,; ~;
.. .,, ,.. ,. - . ,
., .... , . . : .. ::: . . ,
: ;
Wo92/08250 2 ~ b ~ ~ Pcr/us9l/080O4 ~
quantum well regions. Detector elements having various
areas were thereaf~er defined hy c~emical etching in order
to form ohmic metalization areas. A conventional 4 ~ m
pitch triangular grating is etched into the back of the
wafer in order to collect light with a polarization
component perpendicular to the quantum well layers.
The ~3T detector is not restricted to this
specific Ga~s-AlGaAs quan~um well system. A wide varie~y
of layers, layer thicknesses and materials may be used
for the M~T detector. For e~ample, InGaAs/In~lAs (la~tice
matched to InP substrates) or InGaAs~InAlGaAs strained
layer systems may be used. The widths of the quantum
wells and the thicknesses of the superlattice barrier
layers may range from one monolayer to se~eral hundred
Angstroms. The wavelength sensitivity is determined by
material parameters such as the effecti~e mass, band
offsets and the well depths. The strueture of the present
invention allows the llse of p-type acceptor doping in the
quantum wells. As a result, the higher effective mass of
the p-type carriers reducss the dark curren~ of the device
by reducing the Fermi level and the corresponding
. ~ thermionic emission.
Figures 6 and 7 illustrate the improved
.. characteristics of the ~BT detector of the present
invention. . Figure 6 illustrates the wavelength depende~ce
. ~ . . . , , , , . . ," ...
. . ~ of.the photocurrent which has a péak response of about
.. . ~ . . . , .., . , ~; . .
- .. 10.5 ~ m and.a long wavelength cutoff (a half power point)
.. of 11.1 ~ m. Figure 7 illustrates the bias~dependence of
; th~ peak detectivity at a temperature of 77 Kelvin which
obtains a value of lxlOlOcm ~ /W.
- The embodiments of the present invention provide
.. . ..
- an...infrared detector for .collecting photocurrent by
- quantum wells having a wide range of depths and widths.
.
. ..
,"
: , , ,, ,, : .
- - . .
r
~092/08250 ~ PCr/US91/08004
Because the MBT detectors allow materials having more than
one bound state for the guantum wells to be used, the
range of materials and carrier types are greater.
Furthermore, the bandwidth of the MBT detector may be
selected o~er a wide range by strongly coupling the
superlattice barriers layers to provide a broader miniband
which increases the absorption range while maintaining the
strength of absorption in th~ quasi-bound state.
Accordingly, an e~hanced semiconductor device and a method
for detecting thermal radiation is provided i~ the present
invention.
The invention being thus described, it will be
obvious that the same may be varied in many ways. Such
variations are not to be regarded as a departufe from he
lS spirit and scope of the invention, and all such
modifications as would be obvious to one skilled in the
art are intended to be included within the scope of the
.-follo~ing claims~
., . . ~ , , .
.
, . , .. . ~ , . . . . . ... ... . . . .... . .
' I' ! ~ , ,
. , . . ~ . . .
.. ..
~. , .
" . `, ::,. ` ' . . ~ '. '
, '." : , .,
;. ' ` " " :' ' ' . , ' ', ' ' : , ' '.
. ~ - ~, ' : " ", ": . ', ' ' ' , ' . :'.' " ' ' ' :
, '
