Semiconductor device manufacturing: process – Avalanche diode manufacture
Reexamination Certificate
2003-11-24
2004-09-28
Zarneke, David A. (Department: 2829)
Semiconductor device manufacturing: process
Avalanche diode manufacture
C438S091000
Reexamination Certificate
active
06797581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to avalanche photodiodes and photon counting technology.
2. Description of Related Art
Avalanche Photodiodes (APD) are well known photosensitive devices used to convert optical signals into electrical signals. As such, APD's behaves like standard photodiodes, as both APD's and photodiodes convert optical energy into electrical signal. However, APD's additionally incorporate a gain mechanism internal to the device itself, making it more sensitive. That is, while in a conventional p-i-n photodiode an individual photon is converted into one electron-hole pair, in an APD for each individual photon absorbed multiple electron-hole pairs are generated. This multiplication, however, introduces unwanted noise to the APD's output. Therefore, there's a constant effort by APD researchers and manufacturers to produce a sensitive, but reduced-noise, APD.
FIG. 1
depicts one possible structure of an APD
100
in a somewhat simplified form. While the depicted example APD
100
of
FIG. 1
is in an etched mesa-form, the entire discussion herein is equally applicable to APD of the bulk-planar form. The APD
100
comprises a p-InP substrate
110
; a p-InP buffer layer
120
and an n-InP layer
130
, forming the wide-bandgap multiplication region; an n-InGaAsP grading or bandgap-transition layer
140
of an intermediate bandgap; and an n-InGaAs narrow-bandgap absorption layer
150
. The intermediate-bandgap transition layer
140
is generally provided in order to reduce accumulation of charges at the interface between the multiplication and absorption regions,
130
and
150
, respectively. Layers
105
and
115
are contacts, which can be made of, for example, AuInZn or AuSn. In this example, photons h&ngr; are collected from the substrate side.
The example APD
100
depicted in
FIG. 1
is of the separate absorption and multiplication (SAM) APD type. That is, in order to obtain high sensitivity to infrared light, the APD absorption region is built using narrow-bandgap InGaAs material
150
. Using a wider bandgap material such as InP for the absorption region would not result in the APD having comparable infrared sensitivity. Similarly, in order to obtain adequate gain properties in the APD multiplication region
130
, the multiplication material is optimally a wide-bandgap semiconductor, in this example InP, that is able to support the high electric fields needed to achieve charge multiplication without, at the same time, creating excessive unwanted carriers through an electric field-assisted method known as tunneling. In this manner, the photogeneration of carriers takes place in a material optimized for absorption and not in the multiplication region. Lastly, because SAM APDs comprise two semiconductor materials with distinct bandgaps, one or more grading layers
140
of intermediate bandgap materials are used to prevent trapping of charged carriers that would otherwise occur at the heterointerface between the dissimilar regions
130
and
150
.
The multiplication noise of an APD has been generally shown to be a function of k, the ratio of hole to electron ionization constants within the multiplication medium of the APD, i.e., k=&bgr;/&agr;. Note, however, that in some publications k is provided in terms of electron to holes ionization constants, i.e., k=&agr;/&bgr;. However, unless specifically noted otherwise, in this disclosure the convention k=&bgr;/&agr; applies. In a series of papers, McIntyre et al., demonstrated that to improve the APD's performance, one needs to achieve as low k value as possible. For example, they showed that an APD having k value approaching 1 would have a low gain-bandwidth product, whereas an APD having low values (k much less than 1) would have high gain-bandwidth product.
McIntyre, R. J.,
Multiplication Noise in Uniform Avalanche Diodes
, IEEE Transaction on Electron Devices, ED 13, 164-168 (1966); McIntyre R. J.,
A New Look at Impact ionization—Part I: A Theory of Gain, Noise, Breakdown Probability, and Frequency Response
, IEEE Transaction on Electron Devices, 46, 1623-1631 (1999); Yuan, P., Anselm, K. A., Hu, C., Nie, H., Lenox, C., Holms, A. L., Streetman, B. G., Campbell, J. C., and McIntyre, R. J.,
A New Look at Impact Ionization—Part II: Gain and Noise in Short Avalanche Photodiodes
, IEEE Transactions on Electron Devices, 46, 1632-1639 (1999).
These works led researchers on a quest to discover low-k materials and structures for use in APD multiplication regions. For example, Campbell et al., have demonstrated that noise and gain-bandwidth performance can be significantly improved by utilizing very thin multiplication regions. They noted that InP has approximately equal hole and electron ionization rates (i.e., k≅1) and that, therefore, InP APD's have high multiplication noise. They proposed an APD having a thin (200 nm-400 nm) In
0.52
Al
0.48
As multiplication region; demonstrated to result in k=0.18. They also noted, however, that thinning the multiplication region must be accompanied by an increase in the carrier concentration in the multiplication region. Otherwise, electric field in the narrow-bandgap absorbing layer would be too high and tunneling will ensue, leading to excessive dark current.
Campbell, J. C., Nie H., Lenox, C., Kinsey, g., Yuan, P., Holmes, A. L., Jr. and Streetman, B. G.,
High Speed Resonant
-
Cavity InGaAs/InAlAs Avalanche Photodiodes
, IEEE Journal of High Speed Electronics and Systems 10, 327-337 (2000); Campbell, J. C., Chandrasekhar, S., Tsang, W. T., Qua, G. J., and Johnson, B. C.,
Multiplication Noise of Wide
-
Bandwidth InP/InGaAsP/InGaAs Avalanche Photodiodes
, Journal of Lightwave technology 7, 473-477, (1989); Kinsey, G. S., Hansing, C. C., Holmes, A. L. Jr., Streetman, B. G., Campbell, J. C., and Dentai, A. G.,
Waveguide In
0.53
Ga
0.47
As—In
0.52
Al
0.48
As Avalanche Photodiode
, IEEE Photonics Technology Letters 12, 416-418 (2000); Kinsey, G. S., Campbell, J. C., and Dentai, A. G.,
Waveguide Avalanche Photodiode Operating at
1.55
m with a gain
-
Bandwidth Product of
320
GHz
, IEEE Photonics Tachnology Letters 13, 842-844 (2001).
APDs can be operated in two regimes: the linear regime and the breakdown regime, the latter often referred to as Geiger mode. In the linear regime, the APD is biased below its breakdown voltage, and the output photocurrent of the APD is proportional to the intensity of light striking the absorption region
150
and to the APD gain that occurs in the multiplication region
130
. In the Geiger mode of operation, the APD is biased above its breakdown voltage. In this mode of operation, a single photon can lead to an avalanche breakdown resulting in a detectable current running through the device, which thereafter remains in a conductive state. Consequently, the amplitude of the output signal in Geiger mode is constant and is not proportional to the number of photons absorbed. However, Geiger mode enables using APD's for single-photon detection applications.
Among the various utilities, APD's are used for single photon detection. Various applications require accurate detection of single photons. Among such applications is the detection of photon emission generated by switching semiconductor devices. Detection of such emission can be used to test, debug, and characterize the operation of such devices, especially in integrated circuits (IC's). One system that can be used to detect such emission is described in U.S. patent application Ser. No. 09/995,548, commonly assigned to the assignee of the subject application, and which is hereby incorporated herein by reference in its entirety. Other systems are described in, for example, U.S. Pat. Nos. 4,680,635; 4,811,090; 5,475,316; 5,940,545; 5,208,648; 5,220,403; and Khurana et al.,
Analysis of Product Hot Electron Problem by Gated Emission Microscope
, IEEE/IRPS (1986); all of which are incorporated herein by reference in their entirety.
As can be gathered from the above-cite
Bach Joseph
Credence Systems Corporation
Geyer Scott B.
JB Patents.Com
Zarneke David A.
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