JPH059945B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to photoconductive detectors. More particularly, the present invention relates to a device made of photoconductor material having an input bias contact and an output bias contact. Such devices may include one or more additional contacts, such as a voltage contact or multiple voltage contacts, to facilitate radiation detection. Currently, most photoconductive detectors use the material cadmium mercury telluride (CMT) as the photoconductive semiconductor material and have contacts made of gold, indium or aluminum metal. For example, UK patent no.
See US Pat. No. 1,488,258 (US Pat. No. 3,995,159). Conventionally, photoconductive detectors have a rectilinear shape. That is, the semiconductor material is provided in a square or rectangular shape, and contacts are provided at both ends of the semiconductor material, the contacts being formed along a flat edge boundary perpendicular to the longitudinal direction of the semiconductor material. . Some detectors, especially those incorporated into integrated arrays,
Contacts may be provided which are not metallic but are made of heavily doped semiconductor material. That is, the contact is made of a lightly doped photoconductive material or an intrinsic photoconductive material;
It may be formed at the interface between regions of heavily doped conductive material. It may also be a so-called "light-heavy" (l-h) contact. Photoconductive infrared detectors made of CMT materials, particularly currently available high-purity CMT materials, are characterized by long excess carrier (ie, photocarrier) lifetimes. This is thought to be because it takes a long time for the carrier electrons and holes generated by light irradiation to recombine. Typical life time is 1~4μs (8~
14μm band sensitive CMT material) and 10~20μs (3~5μ
m-sensitive CMT material). This means that the detector is susceptible to carrier accumulation effects in the vicinity of the contacts. In the absence of carrier accumulation, the device is normally operated in a sweep-out condition, ie, with carrier removed from the semiconductor material by a bias voltage. In this case, the transit time of the minority carriers throughout the system determines the lifetime of the excess minority carriers. The lifetime of the minority carrier is much shorter than that of the carrier in the CMT material mentioned above. Therefore, it can be seen that the accumulation of carriers causes a delay in the recombination of the minority carriers at the contact point, increasing the life time of the minority carriers.
[The carrier accumulation phenomenon in semiconductors was first suggested by Low (Proc.Phys.Soc.Lond B68, 310
(1955)), the theory of which was developed by Gunn (JnI. Electronics & Control 4, 17 (1958)). Carrier accumulation has two effects on detector characteristics. First, the The detector sensitivity is defined as the voltage corresponding to 1 watt of radiation flux, or the voltage corresponding to 1 watt of radiation flux per area of the square of the detector width in the case of a three-conductor structure, for example, patent No. 1488258. This is because the time that excess carrier is present in the detector is increased, i.e. the sweep-out time is extended.Secondly, the frequency response of the detector is reduced. For the detector disclosed in No. 1488258, this latter effect is expressed as a reduction in resolution produced by the detector. According to the present invention, the aforementioned object is to provide a photoconductive detector for radiation detection, in which elongated light beams in the form of strips are provided. a conductive sensing member; input bias contacts and output bias contacts disposed on the sensing member to define a path for a bias current extending longitudinally along the sensing member; In order to prevent a decrease in resolution, the bias current is converged and the bias electric field is concentrated at the output side bias contact, and the aforementioned current convergence and electric field concentration have a width smaller in the lateral direction than the detection member. Achieved by a photoconductive detector for radiation detection, which is based on at least one of the shape of the output bias contact and the cross-sectional shape of the detection member that is constricted in the vicinity of the output bias contact. According to the photoconductive detector of the present invention, the shape of the output side bias contact is smaller in width in the lateral direction than the photoconductive detection member, and the cross section of the photoconductive detection member is constricted in the vicinity of the output side bias contact. Based on at least one of the shapes, the bias electric field is concentrated at the output bias contact, increasing the strength of the bias electric field near the output bias contact, accelerating the carrier, and increasing the bias electric field near the output bias contact. In the region of increased bias electric field, the carrier modulates the conductivity of the photoconductive sensing member, resulting in a disadvantageous reduction in frequency response or resolution due to carrier accumulation. According to one form of the invention, the detector is characterized by an output bias contact with a distinct longitudinal area. The output bias contact, which extends toward the input bias contact, is shaped to concentrate the bias electric field in the vicinity of the output bias contact. Preferably, the output bias contacts are positioned opposite the ends of the semiconductor material to symmetrically concentrate the bias electric field. This arrangement is provided to minimize the travel time for diffusion of photocarriers incident on the output bias contact. If the detector is made of n-type material, the output bias contact is negatively biased; conversely, if the detector is made of p-type material, the output bias contact is positively biased. It will be done. According to another aspect of the invention, the detector is characterized by an output bias contact area.
In this case, a semiconductor material is placed near the output bias contact to concentrate the bias electric field toward the output bias contact. In this form of the invention, the semiconductor material may be drilled with one or more pores extending across the length of the semiconductor material. Preferably, a pair of opposing pores are drilled into the semiconductor material extending from either side of the detector to the other side. Preferably, these pores are adjacent to the output bias contacts. Alternatively, a semiconductor material may be formed near the output bias contact to concentrate the electric field toward the output bias contact. The detector may combine both features mentioned above. The present invention will be explained below using preferred embodiments shown in the accompanying drawings. Figure 1 shows an n
A photoconductive detector 101 is shown consisting of a strip-like filament 103 as a photoconductive detection member made of a type cadmium mercury telluride material. The electron density is 5×10 ¹⁴ to 1×10 ¹⁵ cm ^-3 . This material has a Cd _0.2
It has a composition of Hg _0.8 Te and is sensitive to infrared radiation in the 8-14 μm band of the spectrum. At temperatures as low as 80°C, this material is characterized by a photocarrier life time of 1-4 μs when incorporated into a nitrogen temperature or F-3 cryogenic shield. The strip filament 103 is mounted on a sapphire insulating substrate (not shown) and cut and shaped by ion beam milling of larger than normal CMT flakes. Gold metal contacts 105 and 1 are formed as input bias contacts and output bias contacts at both ends of the strip filament 103 by sputtering metal to form a film on the surface of the strip filament 103.
07 is formed. These metal contacts 105,
107 is contoured by photolithographic techniques. At one end of the strip filament 103, the material is bifurcated to provide a voltage contact projection 109. Voltage contact protrusion 109 has gold metal contacts 111 on its upper surface. This contact was also formed during a sputtering or photolithography process. The bifurcated groove or pore 113 between the voltage contact protrusion 109 and the end region of the filament strip 103 is formed by ion beam milling and is approximately 12 μm wide. Metal contact 111
is simply for detecting electrical signals and does not contribute to concentration of the bias electric field. A metal contact 107, an output bias contact, is formed at the bifurcated end of the filament strip 103 to provide local distortion of the bias electric field in the filament strip 103. This bias electric field is
This occurs when a bias is applied to the two metal contacts 105 and 107. As shown, the metal contact 107 has a distinct longitudinal area;
This metal contact 107 is a strip-like filament 10
Metal contact 10 from the bifurcated end of 3
It is sticking out in the direction of 5. The metal contact 107 is in the form of a metal finger with a length of 50 μm and a width (d) of 15 μm and is centered between the ends of the end region of the filament strip 103. This metal contact 10
7 has a smooth outline and no sharp edges.
This ensures an orderly distortion of the bias electric field and, moreover, the finger contacts are even more reproducible of the bias electric field, since sharp-edged metal features are unlikely to occur in any case. For a detector of this geometry, the electric field E at the contacts is E (w/d) E ₀ 120V cm ^-1 (where E ₀ is the bulk electric field of the device and under typical operating conditions approximately is estimated to be 30V cm ^-1 ). This electric field is concentrated approximately four times. The frequency response for this detector is determined by the recombination rate S (typically 1000
cm・S ^-1 ~ 500cm・S ^-1 ). Considering the radiation line incident on the detector, the output pulse produced by this radiation has a time T T0.7×10 ^-7 _sec (S=1000cm・S ^-1 ) 1.2×10 ^-7 _sec (S=500cm・S ^-1 ) during which the output pulse value will decline to 1/e.
From this, it can be assumed that the only source of pulse spread is carrier accumulation. (In fact, in the case of the present invention the pulse broadening would be dominated by thermal diffusion of the carrier.)
1488258)),
Pulse broadening corresponds to limiting resolution. Therefore, for a typical image sweep speed of 130 m/s, the typical or purely theoretical resolutions are 8 μm for S = 1000 cm S ^-1 and 8 μm for S = 500 cm S ^-1 Calculated as 14μm. This limit in resolution is less of a problem than the typical limit on thermal diffusion. A typical limit for thermal diffusion in the device described is about 50 μm. The detector sensitivity (R) is increased by the carrier accumulation effect. The contribution of the carrier accumulation effect is RACCDηψ _s τE/SN _t (where T is the carrier diffusion coefficient and "E"
is the electric field at the metal contact 107, and “N”
is the parallel electron density and ψ _s is estimated as the photon flux corresponding to Watts/(width of signal emission) ² (11 μm wavelength). Typical value of life is τ
2μs, detector thickness t=8μm, quantum efficiency η=
1. Recombination speed S = 1000cm・S ^-1 , N = 1×
10 ¹⁵ cm ^-3 , E = 120 V cm -1 , R _ACC (11 μm) 1.3 × 10 ⁶ Vw ^-1 This detector sensitivity is ~6 times that of the factor obtained using conventional output contacts. be. Although the detector end is shown in FIG.
region). As in the embodiment of FIG. 1, photoconductive strip 203 has an elongated contact finger 207 as an output bias contact with an output bias contact. However,
A voltage contact 211 is provided adjacent to this finger-like protrusion 207 . In the region closest to the tip of the finger 207, the voltage contact 211 has the same shape as the contour of an "undisturbed" equipotential surface (ie, an equipotential surface calculated for a detector without voltage contacts). Therefore, in this case the shape of the bias electric field in the vicinity of finger 207 is relatively undisturbed. Detector 301 shown in FIG. 3 has an alternative configuration. A photoconductive sensing strip 303 is placed near the output bias contact 307. A portion of the photoconductive semiconductor material is removed by ion beam milling to create two opposing pores 315 and 317. In this way, the width of the strip 303 is reduced. When a bias is applied to this output bias contact 307, the electric field in the immediate vicinity of the output bias contact 307 is distorted. The narrowing width is 10 μm.
Pores 315 and 317 have similar lengths and widths and act to symmetrically distort the bias electric field.
strip 30 when the bias electric field is suitably uniform.
The carrier moving toward the output bias contact 307 from the region 3, for example, line X--X shown in FIG. 3, reaches the output bias contact 307 with a spread of arrival time. The introduced symmetry ensures that this spread is minimized. Pore 3
15 and 317 are adjacent to the output side bias contact 307. However, these pores 315, 31
7 can be moved from the output bias contact 307, but the space should be small compared to the width "d" of the recess, otherwise the performance will be poor. Another alternative is shown in FIG. The detector 401 includes a strip 403 as a photoconductive detection member.
FIG. Output side bias contact 40
7, the strip 403 width gradually changes. The profile is symmetrical to minimize carrier travel time spread. FIG. 5 shows a high speed, high sensitivity separate array detector. The detector 501 consists of a photoconductive sensing element 503 made of CMT material or other n-type photoconductive material. A metal coating 505 is formed at the end of this thin piece 503 as a common input bias contact. Several output bias contacts 507 (three shown) are attached to the lamina 50.
It is provided at the other end of 3. Each of the output bias contacts 507 is formed as a metal finger approximately 10 μm wide (d) with a center-to-center pitch of approximately 50 μm from adjacent neighbor. This distance corresponds to approximately twice the lifetime of the carrier, which limits the spread of carrier diffusion in CMT (8-14 μm material). In this way, detector 501 is effectively divided into sections each approximately 50 μm wide (w). However, if desired, aperture 513 between output bias contacts 507 can be used.
These compartments may be delineated by introducing . Regarding this detector 501, the sensitivity due to accumulation is R _ACC 4D/πCSR ₀ [However, R ₀ is the sweep-out ultimate sensitivity of the conventional detector R ₀ (2・E〓μ _p N _t ) ^-1 (However, E〓 is the photon energy and μ _p is the Hall mobility). Table 1 below shows these values:

[Table] Figure 6 shows an alternative high-speed, high-sensitivity detector.
This detector 601 consists of a thin piece of photoconductive material 603 as a photoconductive detection element. An annular metal conductor 605 provides the input bias contact. A disk-shaped metal conductor 607 is provided at the center of the annular contact as an output side bias contact. The bias electric field created when a bias is applied to metal conductors 605 and 607 has cylindrical symmetry and converges in the direction of metal conductor 607. An insulating layer may be provided across the detector 601 to facilitate access to the metal conductor 607. Access may then be provided through a window in the insulating layer. The improvement in detector sensitivity may be evaluated by comparing the sensitivity of this annular structure with the sensitivity of a conventional rectilinear shaped detector of width w: R _ACC (annular)/R _ACC (Rectangle) W/2πr1 = 1.6 (however, W = 50 μm, and radius r1 of output bias contraction part = 5 μm). This assumes the same materials, the same doping and the same thickness, and perfect sweep-out. The detectors described in the above embodiments can be used for imaging applications. As is conventional, the detectors may be cooled and may be placed individually or in multiples in the image plane of a shielded optical assembly. The image thickened by this assembly may be stationary or scanned. The assembly in the latter case is
To scan the image across each detector,
Rotating mirror or flapping mirror
mirrors) or both. A two-contact detector is used to derive signal information by measuring the bias current (at constant bias voltage) or by measuring the voltage across the bias contacts (at constant bias current). It's okay. A detector containing one or more voltage contacts may be used, and the voltage between the voltage contacts or between a voltage contact and an output bias contact (at a constant bias current) is measured.

[Brief explanation of the drawing]

FIG. 1 is a plan view of an embodiment of a photoconductive detector with an output bias contact, and FIG. 2 is a partial plan view of another embodiment of a photoconductive detector with an output bias contact. , FIG. 3 is a plan view showing only a portion of still another embodiment of a photoconductive detector having a pair of pores near the output bias contact, and FIG. 4 shows a photoconductive detector in which the output bias contact is formed. A plan view showing only a portion of still another embodiment of the photoconductive detector;
FIG. 5 is a plan view of an embodiment of a photoconductive detector array with a plurality of output bias contacts, and FIG. 6 is a plan view of an embodiment of a photoconductive detector arrangement in an annular arrangement. 101, 201, 301, 401, 501, 6
01...Photoconductive detector, 103,203,30
3,403,503,603...Photoconductive detection member, 105,505,605...Input side bias contact, 107,207,307,407,50
7,607...Output side bias contact, 315,3
17...Pore.

Claims (1)

[Scope of Claims] 1. A photoconductive detector for detecting radiation, comprising a strip-like elongated photoconductive detection member, and a photoconductive detector provided on the detection member and extending in the longitudinal direction along the detection member. The input side bias contact and the output side bias contact are arranged to define the path of the bias current, and in order to prevent deterioration of frequency response and resolution, the bias current converges on the output side bias contact and the bias electric field is The convergence of the current and the concentration of the electric field are caused by the shape of the output bias contact having a width smaller in the lateral direction than the detection member, and the detection member constricting near the output bias contact. A photoconductive detector for radiation detection, the photoconductive detector being based on at least one of the cross-sectional shapes of. 2. The detector of claim 1, wherein the output bias contact has a protrusion extending toward the input bias contact. 3. Detection according to claim 2, wherein the input side bias contact is provided at one end of the detection member, and the output side bias contact is provided at the other end of the detection member and faces the input side bias contact. vessel. 4. The detector according to claim 1, wherein the detection member has an aperture near the output side bias contact. 5. The detector according to claim 4, wherein the detection member has a pair of opposing pores to form the aperture near the output bias contact. 6. The detector of claim 5, wherein the pore is adjacent to the output bias contact. 7. The detector of claim 4, wherein the sensing member tapers toward the output bias contact to form the aperture in the vicinity of the output bias contact. 8. The detector according to claim 1, wherein the output side bias contact and the input side bias contact are arranged concentrically with each other, and the output side bias contact is inside the input side bias contact.