Devices and techniques for coupling radiation to intraband quantum-well semiconductor sensors that are insensitive to the wavelength of the coupled radiation. At least one reflective surface is implemented in the quantum-well region to direct incident radiation towards the quantum-well layers

Bandara, Sumith V.

Gunapala, Sarath D.

Liu, John K.

NASA Technical Reports Server

(12) United States Patent (io) Patent No.: US 7,129,104 B2 
Gunapala et al. (45) Date of Patent: Oct. 31,2006 
(54) 
(75) 
WAVELENGTH-INSENSITIVE RADIATION 
COUPLING FOR MULTI-QUANTUM WELL 
SENSOR BASED ON INTERSUBBAND 
ABSORPTION 
Inventors: Sarath D. Gunapala, Valencia, CA 
(US); Sumith V. Bandara, Valencia, 
CA (US); John K. Liu, Pasadena, CA 
(US) 
Assignee: California Institute of Technology, 
Pasadena, CA (US) 
Subject to any disclaimer, the term of this 
patent is extended or adjusted under 35 
U.S.C. 154(b) by 286 days. 
Notice: 
Appl. No.: 10/409,866 
Filed: Apr. 8, 2003 
Prior Publication Data 
US 200310199113 A1 Oct. 23, 2003 
Related U.S. Application Data 
Division of application No. 091443,925, filed on Nov. 
19, 1999, now Pat. No. 6,545,289. 
Provisional application No. 601109,330, filed on Nov. 
20, 1998. 
Int. C1. 
HOlL 21/00 (2006.01) 
200 
Reflected radiation 
(52) U.S. C1. .............................. 438/29; 43819; 438139; 
438157; 438166 
(58) Field of Classification Search ................ 43817-9, 
438129, 31-40, 57, 63, 66, 73, 700-701, 
4381707-713,726-728,737-739 
See application file for complete search history. 
(56) References Cited 
U.S. PATENT DOCUMENTS 
5,272,356 A 12/1993 Wen et al. 
5,384,469 A 1/1995 Choi 
5,470,761 A 11/1995 McKee et al. 
5,485,015 A 1/1996 Choi 
OTHER PUBLICATIONS 
Song et al , “Magnetically confined plasma reactive ion etching of 
GaAs/AlGaAs/AlAs quantum nanostructures”, J. Vac. Sci. Technol. 
B, vol. 12, No. 6, 1994, pp. 3388-3392.* 
* cited by examiner 
Primary Examiner-Thanhha Pham 
(74) Attorney, Agent, or Firm-Fish & Richardson P.C. 
(57) ABS TRAC ’I 
Devices and techniques for coupling radiation to intraband 
quantum-well semiconductor sensors that are insensitive to 
the wavelength of the coupled radiation. At least one reflec- 
tive surface is implemented in the quantum-well region to 
direct incident radiation towards the quantum-well layers. 
3 Claims, 2 Drawing Sheets 
Incident 
RAdiation ‘ Non-absorbed 
Reflected Radiation 
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U.S. Patent Oct. 31,2006 Sheet 2 of 2 
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US 7,129,104 B2 
US 7,129,104 B2 
1 
WAVELENGTH-INSENSITIVE RADIATION 
COUPLING FOR MULTI-QUANTUM WELL 
SENSOR BASED ON INTERSUBBAND 
ABSORPTION 
CROSS-REFERENCE TO RELATED 
APPLICATIONS 
This application is a divisional application of and claims 
priority to U.S. application Ser. No. 091443,925, filed Nov. 
19, 1999, now U.S. Pat. No. 6,545,289, which claims the 
benefit of U.S. provisional application Ser. No. 601109,330, 
filed Nov. 20, 1998. 
ORIGIN 
The devices and techniques described herein were made 
in the performance of work under a NASA contract, and are 
subject to the provisions of Public Law 96-517 (35 U.S.C. 
g202) in which the Contractor has elected to retain title. 
BACKGROUND 
This specification relates to devices and techniques of 
coupling radiation energy to a quantum-well radiation sen- 
sor. 
An infrared quantum-well semiconductor sensor usually 
includes a quantum-well structure formed of alternating 
active and barrier semiconductor layers. Such a quantum- 
well structure can have different energy bands. Each can 
have multiple quantum states. An intraband transition 
between a ground state and an excited state in the same band 
(i.e., a conduction band or a valance band) can be used to 
detect infrared (“IR’) radiation by absorbing IR radiation at 
or near a selected resonance IR wavelength. The absorption 
of the radiation generates electric charge indicative of the 
amount of received radiation. The radiation-induced charge 
can then be converted into an electrical signal (e.g., a voltage 
or current) to be processed by signal processing circuitry. 
The compositions of lattice-matched semiconductor 
materials of the quantum well layers can be adjusted to cover 
a wide range of wavelengths for infrared detection and 
sensing. In comparison with other radiation detectors, quan- 
tum-well structures can achieve, among other advantages, 
high uniformity, a low noise-equivalent temperature differ- 
ence, large format arrays, high radiation hardness, and low 
cost. Infrared quantum-well sensing arrays may be used for 
various applications, including night vision, navigation, 
flight control, environmental monitoring. 
A quantum-well infrared sensor usually responds to inci- 
dent radiation with a polarization that is perpendicular to the 
quantum well layers, i.e., parallel to the growth direction. 
This is because this polarization can induce an intraband 
transition at a desired infrared wavelength. In applications 
based on imaging at focal plane arrays, the photodetector 
array is often oriented perpendicular to the scene to be 
imaged. Since the electric vector is essentially parallel to the 
quantum well layers in this arrangement, the quantum well 
layers absorb little or no light. 
Hence, a coupling mechanism is often implemented to 
couple incident radiation in a way that at least a portion of 
the incident radiation becomes absorbable by the quantum 
well layers. The coupling efficiency of the coupling mecha- 
nism can be characterized by the percentage of absorbable 
radiation, but not what is actually absorbed, of the total 
incident radiation. One way to provide proper coupling is to 
use random reflectors or corrugated surfaces to scatter a 
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portion of the incident radiation into the correct polarization 
for absorption. Alternatively, one-dimensional or two-di- 
mensional grating couplers can also be used to convert 
normally-incident radiation to waves have components that 
propagate along the quantum well layers. 
However, the coupling efficiencies of these coupling 
schemes are sensitive to the wavelength of the radiation. 
Hence, their applications are limited to detection of radiation 
at a single selected wavelength or a narrow spectral range 
and hence cannot be used for detection of radiation of 
multiple colors. In addition, these coupling schemes direct 
only a portion of the incident beam to be parallel to the 
absorbing quantum well layers and hence limit the coupling 
efficiency to an upper limit that is determined by the 
percentage of the portion of radiation propagating parallel to 
the quantum well layers. 
SUMMARY 
This disclosure includes devices and techniques for cou- 
pling radiation to quantum-well semiconductor sensors that 
are insensitive to the wavelength of the coupled radiation. 
One embodiment of a quantum-well semiconductor device 
includes a radiation-sensing region and non-radiation-sens- 
ing region that are adjacent to each other over a substrate. 
The radiation-sensing region has a plurality of alternating 
quantum-well layers parallel formed over the substrate and 
operating to absorb radiation by at least one intraband 
transition. A reflective surface is formed between the non- 
radiation-sensing region and the radiation-sensing region to 
have an angle with respect to the substrate to reflect incident 
radiation from the substrate towards the radiation-sensing 
region. The angle may be chosen to make the reflected 
radiation to propagate substantially parallel to the quantum- 
well layers. 
These and other features and associated advantages of the 
devices and techniques are described in detail in the follow- 
ing. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows on embodiment of a quantum-well sensor 
that has at least one reflective surface to direct incident 
radiation to the quantum-well layers. 
FIG. 2 shows another embodiment of a quantum-well 
sensor that has pyramid-like quantum-well regions to couple 
radiation to the absorbing quantum layers with a desired 
polarization. 
DETAILED DESCRIPTION 
Present devices and techniques are designed to direct 
incident radiation to propagate within the absorbing quan- 
tum well layers in a direction that is substantially parallel to 
the quantum well layers. Polarization of the radiation is 
generally perpendicular to the direction of propagation. 
Hence, the polarization can be directed to be perpendicular 
to the quantum well layers and therefore be absorbed by the 
quantum well layers to produce charge for detection and 
signal processing. 
In comparison with other coupling techniques which 
direct a beam at an angle with respect to the quantum well 
layers, the present devices and techniques direct the entire 
incident radiation, rather than a portion of it, to be parallel 
to the quantum well layers. When the input radiation is 
linearly polarized, the entirety of the input radiation can be 
so directed to have its polarization perpendicular to the 
US 7,129,104 B2 
3 
quantum well layers and hence be absorbed. Thus, the 
coupling efficiency can essentially be up to 100%. When the 
input radiation is randomly polarized, the coupling effi- 
ciency can be up to 50%. Hence, with other device param- 
eters being equal, the coupling efficiencies of the present 
devices and techniques are significantly higher than many 
conventional coupling schemes. 
At least one reflective surface is implemented to intercept 
the quantum-well layers at a desired angle so that incident 
radiation at or around a known incident angle can be 
reflected to propagate in a direction substantially parallel to 
the quantum-well layers. For example, when the device is 
designed to detect incident radiation that is nominally per- 
pendicular to the quantum-well layers, the angle of the 
reflective surface may be set at or around 45". 
The present devices and techniques are insensitive to the 
wavelength of the radiation under detection. The reflection 
of the reflective surface can be configured to be insensitive 
to the wavelength of the incident radiation. For example, the 
reflective surface may be coated with a reflective layer 
having one or more layers of appropriate materials to 
produce a substantially constant reflectivity for a range of 
wavelengths to which the quantum-well layers are respon- 
sive Also, the reflective surface may be simply a boundary 
surface between the quantum-well layers and another dif- 
ferent material. This difference in the materials on the two 
sides of the boundary can make the surface reflective to 
radiation of different wavelengths. 
FIG. 1 shows one embodiment of an intraband quantum- 
well sensing device 100 that implements a reflective surface 
for radiation coupling. The device 100 includes a substrate 
110 on which structures of the sensing device are formed. 
The substrate 110 is semi-insulating and may be formed of 
a semiconductor material (e.g., GaAs). In addition, the 
substrate 110 is substantially transparent to radiation at one 
or more wavelengths to be detected and functions to receive 
incident radiation. A conductive contact layer 120 is formed 
over the substrate 110 to provide an electrical bias to the 
device 100. The contact layer 120 may be made of a heavily 
doped semiconductor material such as doped GaAs. 
A quantum-well radiation-sensing region 130 and a non- 
radiation-sensing region 140 are formed adjacent to each 
other above the contact layer 120. The quantum-well region 
130 includes multiple layers of alternating active and barrier 
layers of two different semiconductor materials and absorb 
radiation at one or more wavelengths. The quantum-well 
layers are parallel to the surface of the substrate 120. The 
quantum-well layers may include two or more different 
stacks of different quantum well structures that have intra- 
band transitions at different wavelengths to allow the device 
100 to simultaneously detect radiation of different colors. 
The region 140 is formed of a material different from the 
quantum-well region 130. An insulator material, e.g., a glass 
material or polymide, may be used to construct the region 
140. The regions 130 and 140 are divided by a slanted 
surface 150 that forms an angle with respect to the substrate 
120. The surface 150 is designed to be reflective to the 
radiation to be detected. The angle of the surface 150 is set 
to reflect radiation incident from the side of the substrate 110 
at one or a range of incident angles to propagate in the 
quantum-well region 130 in a direction that is substantially 
parallel to the quantum-well layers. 
The device 100 further includes another conductive con- 
tact layer 160 over the quantum-well region 130 which is 
used to provide a potential different from that on the contact 
layer 120 to bias the quantum-well region 140. A metallic 
ohmic layer 170 is formed atop the contact layer 160 to 
4 
supply the electrical potential to the contact layer 160. The 
metallic layer 170 covers at least the contact layer 160 and 
may also cover the surface of the non-radiation-sensing 
region 140 as shown in FIG. 1. Above the metallic layer 170 
5 is one or more circuit layers 190 that are formed on a silicon 
substrate to receive and process the radiation-induced output 
signal generated in the quantum-well region 130. A layer of 
indium bumps 180 may be formed over the metallic layer to 
provide an electrical interface between the circuit layer 190 
lo and the quantum-well region 130. Eachpixel has one indium 
bump 180. 
When the incident radiation is polarized, either the device 
100 or the incident polarization may be rotated to ensure that 
15 the polarization of the reflected radiation in the quantum- 
well region 130 is substantially perpendicular to the quan- 
tum-well layers. In the latter, a polarization rotating element 
102 may be placed in front of the substrate 110 to control the 
incident polarization. 
An additional reflective surface 152 may be desirable to 
efficiently use the quantum-well region 130 in the device 
100 of FIG. 1. The additional reflective surface 152 is also 
slanted to form an angle with respect to the substrate 110. 
Incident radiation impinging the additional reflective surface 
25 152 is reflected to travel along the quantum-well layers in 
the region 130 towards the reflective surface 150. This 
configuration of two opposing reflective surfaces increases 
the active area for receiving and converting the radiation 
into a state with a detectable polarization. 
FIG. 2 shows another embodiment of an intraband quan- 
tum-well sensing device 200 that has an array of pyramid- 
like quantum-well regions 230 based on the above 2 reflec- 
tive surface configuration. Each quantum-well region 230 
has two slanted reflective surfaces 232 and 234 to form a 
35 pyramid-like structure. The fabrication of the quantum well 
structure may use the PMMA (polymethylmethacrylate) 
pattern transfer technology. Pyramid regions 230 can be 
fabricated by first forming a PMMA mask layer over GaAs 
epitaxial materials by using e-beam lithograph and PMMA 
40 development. The pattern on the PMMA mask layer is then 
transferred to epitaxially grown GaAs by using ECR (elec- 
tron cyclotron resonance) plasma reactive ion etching. Non- 
radiation-sensing regions 240 fill in the voids between the 
quantum-well regions 230 with a different material such as 
On top of each quantum region 230, a small flat surface 
236 is formed between the surfaces 232 and 234 to support 
a conductive contact layer 260 (e.g., heavily-doped semi- 
5o conductor like GaAs). Ametallic contact layer 270 is formed 
to cover all contact layers 260 and the top surface of the 
non-radiation-sensing regions 240. The angle of slanted 
surfaces 232 and 234 may be around 45" to receive radiation 
incident at a direction substantially perpendicular to the 
In operation, the two reflective surfaces 232 and 234 in 
each region 230 reflect all received radiation to propagate 
either substantially parallel to the quantum-well layers or at 
angles with respect to the quantum-well layers. Such 
A quantum sensing array may be constructed based on the 
configuration of the device 200. Each pixel may include at 
one or more sensing regions 230. The slanted reflective 
surfaces 230 and 232 also confine radiation in pixel from 
65 entering other pixels to blur the image. The non-absorbed 
reflected radiation is reflected towards the substrate 110 
rather than being received by other pixels. 
20 
30 
45 an insulator. 
55 substrate 110. 
60 reflected radiation becomes absorbable. 
US 7,129,104 B2 
5 6 
Although only a few embodiments are disclosed, other 
embodiments, variations, and modifications are intended to 
be encompassed by the following claims. 
substrate to a direction parallel to the quantum-well 
layers thereby increasing coupling efficiency of the 
quantum-well sensing array; and 
filling the pyramid-shaped voids with a non-radiation- 
sensing insulator material, 
2. The method as in claim 1, further comprising making 
each side surface of each pyramid-shaped void to be about 
45 degrees with respect to the quantum-well layers. 
3. The method as in claim 1, further comprising making 
the quantum-well layers to have two different quantum-well 
structures stacking together to absorb light of at least two 
different wavelengths. 
What is claimed is: 
1. A method for fabricating a quantum-well sensing array 5 
fabricating quantum-well layers over a substrate; 
forming a patterned PMMA (polymethylmethacrylate~ 
layer over the quantum-well layers; 
performing reactive ion etching to the quantum-well 10 
layers though the PMMA layer to form an 
array of pyramid-shaped voids in the quantum-well 
layers with boundary planes each at an angle of about 
45 degrees with respect to said substrate wherein said 
comprising: 
boundary planes reflecting incident radiation form the * * * * *  


Wavelength-insensitive radiation coupling for multi-quantum well sensor based on intersubband absorption

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