JP3676802B2 - Epitaxial passivation of infrared detectors made of materials from groups (2) to (6) of the periodic table - Google Patents

Description

(Industrial application fields)
The present invention relates to infrared (IR) photodiodes made of materials of the Synchronous Group II-VI, and in particular, HgCdTe having a wide bandgap passivation layer (eg, made of CdTe or CdZnTe) that is epitaxially grown. The present invention relates to an IR photodiode made of a material.
(Prior art and problems to be solved by the invention)
Photodiodes composed of mercury-cadmium-tellurium (Hg _(1-x) Cd _x Te, where x is essentially a value from 0 to 1.0) are usually fabricated as a two-dimensional array, Covered with a passivated layer, the passivation layer consists of low temperature photochemical SiO ₂ , evaporated ZnS, or anodically grown CdS. Suitable for certain imaging applications, but at some subsequent processing stage of the array, a high vacuum at 100 ° C is used to degas the vacuum Dewar containing the photodiode array. A baking cycle is required, which is considered disadvantageous with conventional passivation layers. For example, significant figure of merit degradation has been observed, such as leakage current, diode impedance, quantum efficiency, noise (especially at low frequencies), spectral response, and optical fields. This deterioration is particularly remarkable in the long wave detector. The porosity of the passivation layer and poor adhesion to the underlying HgCdTe surface are also commonly encountered problems with the conventional passivation layers described above.
Furthermore, since these conventional passivation materials do not make more than one coating on the HgCdTe surface, it is difficult or impossible to adjust the band structure and energy level at the boundary between the HgCdTe surface and the passivation layer. It is. Thus, if the array should maintain the desired figure of merit level, especially during and after high temperature processing and storage, creating flatband conditions at the interface between the HgCdTe surface and the passivation layer; It is necessary to maintain this together.
(Means for solving the problem)
The above problems are overcome by the array of IR photodiodes and similar structures according to the method and apparatus of the present invention, and other advantages are realized. That is, according to the present invention, the material has a first region that absorbs radiation and generates charge carriers from the absorbed radiation, and the radiation absorbing region is a material of Group II-VI in the periodic table having the first type conductivity. The second region is also composed of a periodic table group II-VI material having conductivity opposite to that of the first region to form a pn diode junction, and at least the first region A third region overlying the pn diode junction at the boundary between the first and second regions, the third region being composed of an epitaxial layer made of Group II-VI material of the periodic table, It has a method and apparatus for forming heterostructures with materials. The band gaps of the first and second regions may or may not be the same, and the third region has a wider band gap than either of the first and second regions.
According to an embodiment of the present invention, the third region is composed of CdTe, CdZnTe, or an epitaxial passivation layer made of HgCdTe having a wide band gap. It is lattice-matched to Group II-VI semiconductor materials in the periodic table. Optionally, a glass coating layer (overglass layer) may be provided on the passivation layer. This glass coating layer supplements the effect of an epitaxial passivation layer that prevents out-diffusion of Hg during exposure to high temperatures and electrically isolates the contacts of the photodiodes.
The foregoing and other features of the present invention will become more apparent upon reading the following detailed description of the embodiments together with the accompanying drawings.
(Example)
Here, as an example of the present invention, a back-illuminated mesa-type photovoltaic radiation detector will be described. However, the present invention relates to a photoconductive and forward-illuminated radiation detector. Also applies. The invention also applies to both homo and heterojunction devices. The present invention further includes a plurality of regions of opposite type conductivity, i.e., "wells", in which the base layer is formed in the upper surface, and pn at the boundary between the base layer and each region. It also includes a planar type detector in which a diode junction is formed.
Referring first to FIG. 1a, a stylized elevational view of an array 1 of photodiodes 2 is shown, but not to scale. The photodiode is made of a Group II-VI group material such as HgCdTe, and each of them is selectively distinguished by a conductive type to form a plurality of diode junctions. It can be seen that the array 1 is composed of a plurality of photodiodes 2 arranged in a regular two-dimensional array. Incident IR radiation, which can be long, medium, or short wavelength (LWIR, MWIR, or SWIR) radiation, is incident on the bottom surface of the array 1. Array 1 includes a radiation absorbing base layer of Hg _(1-x) Cd _x Te semiconductor material, where the value of x determines whether the array is responsive to LWIR, MWIR, or SWIR. The photodiode 2 is defined by a mesa structure 6 and is typically formed by etching crossed V-shaped grooves that penetrate the base layer through the upper cap layer. Each photodiode is provided with an area of contact metallization 4 on its upper surface, which metallization part is usually connected to an underlying indium bump (not shown). And serves to couple to a readout device (not shown). The upper surface of the array 1 is also provided with a passivation layer 5 made of an epitaxial layer of a periodic table group II-VI material according to the present invention, and an underlying periodic table group II-VI material and heterostructure. Form.
FIG. 1B shows a cross-sectional view of one of the photodiodes in array 1, in particular a double layered HgCdTe heterojunction photodiode 10 having a lower surface that accepts infrared radiation as indicated by arrow hν. If desired, the photodiode 10 can be a homojunction device. The photodiode 10 includes a base layer 12 that absorbs incident radiation and generates charge carriers. The radiation absorbing base layer may be either p-type or n-type semiconductor material and has a cap layer 14 having the opposite type of conductivity for forming the pn junction 15. Therefore, if the radiation absorption base layer 12 is p-type HgCdTe, the cap layer 14 is n-type HgCdTe. Current flows through the pn junction 15 due to charge carriers generated by absorption of IR radiation, which is detected by a readout circuit (not shown) and coupled to the photodiode 10.
For example, the base layer 12 may be p-type and may be doped with arsenic to a concentration of about 5 × 10 ¹⁵ atoms / cm ³ to about 5 × 10 ¹⁶ atoms / cm ³ . Cap layer 14 may be the original n-type or may be doped with indium to a concentration of about 10 ¹⁶ to 10 ¹⁷ atoms / cm ³ .
According to one embodiment of the invention, at least the region of the pn junction 15 of the photodiode 10 is passivated with a relatively thin cadmium telluride (CdTe) epitaxial layer 16. The epitaxial layer 16, which can be called a passivation layer, usually has a thickness of about 5000 angstroms.
According to another embodiment of the present invention, passivation layer 16 comprises an epitaxial layer of cadmium zinc telluride (Cd _(1-y) Zn _y Te), which thickness can also typically be about 5000 angstroms. .
According to yet another embodiment of the present invention, the passivation layer 16 comprises an epitaxial layer of Hg _(1-x) Cd _x Te, the value of x being the base layer 12 and the cap layer 14 overlying the epitaxial layer. Each is chosen to have a wider hand gap.
In the preferred embodiment of the present invention described above, the base layer 12 and the cap layer 14, and also the passivation layer 16, are also Group II-VI semiconductor members of the periodic table, ie Hg _(1-x) Cd _x Te and CdTe, respectively. Or, it is Cd _(1-y) Zn _y Te. As long as the passivation layer 16 is selectively formed by one epitaxial layer growth method, a heterostructure is formed at the boundary between the pn junction 15 and the passivation layer 16. That is, the crystalline structure of the passivation layer 16 is essentially continuous with the crystalline structure of the underlying base layer and cap layer and has a wider band gap than the underlying material. This crystalline continuity favors the continuous widening of the HgCdTe layer, 12 and 14 bandgap structures, typically having an energy gap of 0.1 to 0.3 eV, to the wider bandgap of the passivation layer 16. Work.
For example, CdTe has a band gap of about 1.6 eV. As a result, the conduction band is bent upward, so that electrons are ejected from the boundary between HgCdTe and CdTe. Because of this wider band gap, the valence band will be bent further downward, thereby expelling holes from the boundary. This is illustrated in FIG. 2 and will be described in further detail.
Referring again to FIG. 1B, the diode 10 can be provided with a glass coating layer 18 made of a suitable dielectric material such as Si ₃ N ₄ , SiO ₂ or ZnS. The glass coating layer 18 electrically insulates the metal contacts 20 from similar contacts of adjacent diodes. The metal contact 20 can be made of any suitable material that can be used to form an ohmic contact to the HgCdTe cap layer 14. It is desirable that the metal of the metal contact 20 does not diffuse much into the cap layer 14. Suitable metals for metal contact 20 are palladium and titanium.
The epitaxial CdTe passivation layer 16 has a sufficiently high intrinsic resistance and does not cause lateral shunting between adjacent photodiodes in the array of photodiodes. Is known to be voluntary. However, the presence of the glass coating layer 18 may be useful in that it serves as an additional barrier to the outward diffusion of Hg from the underlying HgCdTe base layer 12 and cap layer 14, respectively. I know. This glass-covered barrier helps to reduce the out-diffusion of Hg, as well as the diffusion hindrance caused by contact metallization itself, so that the resulting photodiode structure can withstand higher temperatures than conventional devices.
In this regard, the crystalline structure of the epitaxial passivation layer 16 at typical processing and storage temperatures is a primary barrier to the out-diffusion of Hg from the HgCdTe layer and the glass coating layer. It should be noted that the additional barrier provided by 18 and the metal contact 20 serves to reinforce the passivation layer 16 barrier. It should also be noted that the problems associated with lack of porosity and adhesion are overcome because the mechanical properties of the crystal structure of the passivation layer 16 are superior to conventional passivation coatings.
With respect to FIG. 2, the wider bandgap passivation layer 16 is made of CdTe, and the narrower bandgap material is the idealized energy of the photodiode 10 of FIG. 1 constituting the HgCdTe base layer 12 or the HgCdTe cap layer 14. • A band diagram is shown. As can be seen, the change in potential energy in the conduction band and valence band is continuous, the conduction band is bent upward and the valence band is bent downward. Therefore, both electrons and holes are ejected from the boundary between HgCdTe and CdTe, respectively. Otherwise, due to the repulsion from the boundary between both electrons and holes, the surface state generation current becomes excessive due to the relatively high density of dislocations and impurities and the service life of the charge carriers is shortened. Such photodiodes perform better than conventional SiO ₂ passivated photodiodes.
Further, if desired, the upper surface of the CdTe passivation layer 16 may be doped to insulate the charge on the CdTe surface from the underlying HgCdTe surface. In the diagram of FIG. 2, the upper surface of the CdTe passivation layer 16 is doped with n-type impurities. If desired, p-type impurities can be used instead. A typical doping concentration value on the top surface of the passivation layer 16 is about 10 ¹⁷ atoms / cm ³ .
For example, the use of Cd _(1-y) Zn _y Te as an epitaxial surface passivation material minimizes dislocation at the boundary between HgCdTe and CdZnTe by lattice matching the passivation layer and the underlying HgCdTe layer. There is an additional advantage of being able to. For example, y can be selected to have a value of about 0.4. It is optional to provide a glass coating layer 18 for the resistivity of CdZnTe passivation, as already described for photodiodes passivated with CdTe.
The passivation layer 16 can be epitaxially grown to a desired thickness on the photodiode by a vapor phase epitaxy (VPE) method. Of course, a metal organic compound having a high vapor pressure is heated on a substrate heated in a vacuum chamber. Some other suitable epitaxial layer formation method may be used, such as a method of sending a vapor of the metal to grow the metal film on the substrate (metal organic chemical vapor deposition (MOCVD)). In general, any epitaxial growth method may be used as long as the crystalline order of the passivation layer 16 results in repeating the same order as the crystalline order of the HgCdTe layer underneath. After the passivation layer 16 is grown, the device can be processed in a conventional manner to deposit the glass coating layer 18 and the metal contact 20. Using an appropriate mask during the growth of the epitaxial passivation layer 16, to prevent the growth of the epilayer on the surface portion of the cap layer 14 to be contact metallized Can do. Desirably, a passivation layer is grown on the entire surface of the cap layer 14, and a part of the obtained passivation layer is selectively removed from the metallized portion in the next processing step.
It consists of a photodiode, a gated and ungated variable area photodiode, and a 5-by-5 array of MIS capacitors, which are composed of HgCdTe radiation absorbing material for both LWIR and MWIR. A test structure was produced. Some of these test structures were provided with conventional SiO ₂ passivation layers, while others were provided with epitaxially grown CdTe passivation layers. In addition, several other test structures were provided with a passivation layer using both CdTe and conventional SiO ₂ on the same HgCdTe wafer.
FIGS. 4, 5, 6, and 7 show graphs and performance curves that support the performance improvement of photodiode devices using an epitaxial passivation layer 16 comprised of elements of Groups II-VI of the periodic table according to the present invention.
As described above, a number of preferred embodiments according to the present invention have been described. Those skilled in the art can make some modifications to each of the above preferred embodiments based on the foregoing disclosure. For example, although the embodiments shown herein have been described with reference to back-illuminated mesa photovoltaic radiation detectors, the teachings of the present invention also apply to front-illuminated radiation and planar type photodiodes, Applies to all types of photovoltaic and photoconductive devices consisting of Group II-VI elements of the Periodic Table. Accordingly, it is to be understood that the invention is not limited to the embodiments shown, but is limited to the scope described in the claims.
[Brief description of the drawings]
FIG. 1A shows, on a reduced scale, a portion of an array 1 of photodiodes 2 made of Group II-VI elements of the Periodic Table having an epitaxial passivation layer 5 made of Group II-VI materials of the Periodic Table according to the present invention. It is not a stylized perspective view.
FIG. 1B is a cross-sectional view of a photodiode 10 having a HgCdTe radiation absorbing base layer 12, a HgCdTe cap layer 14, and a CdTe, CdZnTe or HgCdTe passivation layer 16. FIG.
FIG. 2 is a representative energy bandgap diagram of the epitaxial passivated photodiode of FIG. 1B.
FIGS. 3A through 3D are performance curves for CdTe passivated LWIR HgCdTe photodiodes, with typical curves from I to V representing Ro A vs. temperature, spectral response, and noise spectrum at 80 K, respectively. Indicates.
FIGS. 4A to 4D are performance curves of LWIR HgCdTe photodiodes that are SiO passivated according to the prior art.
FIGS. 5A and 5B are for MWIR photodiodes fabricated from different parts of the same HgCdTe wafer and CdTe passivated on each wafer and MWIR photodiodes passivated by conventional SiO methods. A comparison of current-voltage curves is shown.
FIGS. 6A to 6C show the LWIR large area, small area, and respective RA for 5 × 5 arrays of HgCdTe photodiodes for CdTe and conventional SiO ₂ respectively at 100 ° C. as a function of temperature. Shows product comparison.
FIGS. 7A and 7B show CdTe passivated photodiodes as a function of baking time at 100 ° C. for surface recombination velocity (SRV) and photodiodes passivated by the conventional SiO method, respectively. The comparison about is shown.

Claims (14)

In IR radiation sensitive photodiodes,
A radiation-absorbing first layer having a surface for receiving IR radiation for generating charge carriers from the absorbed IR radiation, the radiation-absorbing layer having a first type of conductivity in the periodic table; Composed of II-VI group materials,
A plurality of mesa regions overlying the upper surface of the first layer, and each mesa region is also made of a material of Group II-VI of the periodic table having conductivity opposite to that of the first layer. This forms a pn diode junction in the boundary region between each of the plurality of mesa regions and the first layer,
Having a passivation layer overlying at least the boundary region, the passivation layer comprising an epitaxial layer of a material of Group II-VI of the periodic table that can be used to form a heterostructure with the boundary region;
The first layer and the plurality of mesa regions have first and second energy band gaps, respectively. In this case, the passivation layer is more than one of the first and second energy band gaps. Having a wide third energy band gap,
An array of IR radiation sensitive photodiodes. 2. The photodiode array according to claim 1, wherein the first layer and the plurality of mesa regions are made of HgCdTe, and the passivation layer is an epitaxial layer made of CdTe. The photodiode according to claim 1, wherein the first layer and the plurality of mesa regions are made of HgCdTe, and the passivation layer is an epitaxial layer lattice-matched to the underlying mesa region and the first layer. Array. 4. The photodiode array of claim 3, wherein the epitaxial layer is made of CdZnTe. 4. An array of photodiodes as claimed in claim 3, wherein the epitaxial layer comprises HgCdTe having a wider band gap than the mesa region or the first layer. 2. The array of photodiodes of claim 1, further comprising a third layer overlying the passivation layer, the third layer comprising a dielectric material. In a method of passivating a photodiode made of a material of Group II-VI of the periodic table, the method comprises:
A first region made of Group II-VI material of the periodic table having a first type conductivity and a first energy band gap, and a plurality of mesa regions overlying the top surface of the first region . The second region is composed of a Group II-VI material of the periodic table having two types of conductivity and a second energy band gap, and the second region is in contact with the first region and is in contact with the first region. And providing a photodiode forming a pn diode junction at the boundary between the second regions,
It consists of a material of Group II-VI of the periodic table having a third energy band gap wider than either one of the first and second energy band gaps over at least the interface between the first and second regions Growing an epitaxial layer, thereby positioning at least its pn diode junction below the epitaxial layer having a wider band gap;
A method for passivation of a photodiode made of a material of Group II-VI of the periodic table. 8. The method of claim 7, wherein the first and second regions are each composed of Hg _(1-x) Cd _x Te, and the step of growing the epitaxial layer is performed by growing an epitaxial layer of CdTe. The first and second regions are each composed of Hg _(1-x) Cd _x Te, and the step of growing the epitaxial layer is performed by growing an epitaxial layer of Cd _(1-y) Zn _y Te. 7. The method according to 7. The first and second regions are each made of Hg _(1-x) Cd _x Te, and the step of growing the epitaxial layer is performed by growing an epitaxial layer made of Hg _(1-x) Cd _x Te. 8. The method of claim 7, wherein x in the layer material is greater than x in the underlying first and second materials. The method of claim 9, wherein the value of y is about 0.4. The method of claim 7, wherein the step of growing the epitaxial layer is performed such that the thickness of the epitaxial layer is about 5000 Angstroms. 8. The method of claim 7, further comprising doping the upper surface of the epitaxial layer with a desired doping agent to a predetermined concentration. The method of claim 13, wherein the predetermined concentration is about 10 ¹⁷ atoms / cm ³ .

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