Radiation conversion device and method of manufacturing a radiation conversion device

A radiation conversion device such as a photovoltaic cell, a photodiode or a semiconductor radiation detection device, includes a semiconductor portion with first compensation zones of a first conductivity type and a base portion that separates the first compensation zones from each other. The first compensations zones are arranged in pillar structures. Each pillar structure includes spatially separated first compensation zones and extends in a vertical direction with respect to a main surface of the semiconductor portion. Between neighboring ones of the pillar structures the base portion includes second compensation zones of a second conductivity type, which is complementary to the first conductivity type. The radiation conversion device combines high radiation hardness with cost effective manufacturing.

BACKGROUND

In semiconductor conversion devices incoming radiation generates electron-hole pairs by transferring electrons from a valence band to a conduction band. The generated electron-hole pairs are separated according to their polarity and travel to the respective electrodes where they induce an electric current. The radiation conversion mechanism may be used in semiconductor radiation detectors, photovoltaic cells and photo-detectors. It is desirable to provide improved radiation conversion devices.

SUMMARY

According to an embodiment a radiation conversion device includes a semiconductor portion with first compensation zones of a first conductivity type and a base portion that separates the first compensation zones from each other. The first compensations zones are arranged in pillar structures, wherein each pillar structure includes at least two of the first compensation zones and extends in a vertical direction with respect to a main surface of the semiconductor portion. Between neighboring ones of the pillar structures the base portion includes second compensation zones of a second conductivity type, which is complementary to the first conductivity type.

A radiation conversion device according to another embodiment includes a semiconductor portion with first compensation zones of a first conductivity type arranged in stripe-shaped pillar structures and second compensation zones of a second, complementary conductivity type between neighboring pillar structures. Each pillar structure includes at least two of the first compensation zones and extends in a vertical direction and in a first lateral direction with respect to a main surface of the semiconductor portion. A first electrode structure with a plurality of strips directly adjoins the semiconductor portion. Each strip is assigned to at least two of the pillar structures.

A radiation conversion device according to a further embodiment includes a semiconductor portion with first compensation zones of a first conductivity type and a base portion. The first compensation zones are arranged in pillar structures, wherein each pillar structure extends in a vertical direction with respect to a main surface of the semiconductor portion. The base portion includes a background portion and second compensation zones of a second, complementary conductivity type between neighboring ones of the pillar structures. The second compensation zones form further pillar structures extending in the vertical direction. Vertical impurity profiles of the second compensation zones are Gaussian distributions.

According to a further embodiment, a method of manufacturing a radiation conversion device includes growing by epitaxy a sequence of semiconductor layers on a semiconductor substrate. In at least two of the semiconductor layers impurities of a first conductivity type are introduced into exposed first sections of a process surface of a preceding one of the semiconductor layers before growing a subsequent one of the semiconductor layers on the preceding one. The semiconductor layers are annealed to form first compensation zones from the introduced impurities. The annealing is terminated before the first compensation zones get structurally connected.

DETAILED DESCRIPTION

The term “electrically connected” describes a permanent low-ohmic connection between electrically connected elements, for example a direct contact between the concerned elements or a low-ohmic connection via a metal and/or highly doped semiconductor. The term “electrically coupled” includes that one or more intervening element(s) adapted for signal transmission may be provided between the electrically coupled elements, for example elements that are controllable to temporarily provide a low-ohmic connection in a first state and a high-ohmic electric decoupling in a second state.

FIG. 1Ashows a radiation meter901based on a semiconductor radiation conversion device910configured as a radiation detection device. The radiation conversion device910may be a reverse biased semiconductor diode operated in the non-conducting mode. A DC source940and an amplifier circuit920may be electrically coupled to electrodes of the radiation conversion device910. An evaluation unit930may be electrically coupled to an output terminal of the amplifier circuit920.

The DC source940reverse biases the radiation conversion device910and generates a depletion zone in the radiation conversion device910. Incoming radiation990incidents on a radiation-receiving surface of the radiation conversion device910. The incoming radiation990may be ionizing radiation, for example gamma radiation, beta radiation or X-ray radiation, or non-ionizing radiation, e.g. ultra violet light, visible light or infrared light and generates electron-hole pairs. In the depletion zone the generated electrons and holes migrate to the corresponding electrodes and induce current pulses between the electrodes. The amplifier circuit920may amplify the induced current and the evaluation unit930may further process and analyze the amplified current.

The sensitivity of the radiation conversion device910may be increased by a cooling device reducing the intrinsic charge carrier density and/or by applying a comparably high voltage between the electrodes such that the accelerated electrons trigger the generation of further electron-hole pairs. Each of the electrodes may be partitioned and evaluated differently to obtain local information regarding the incoming radiation.

FIG. 1Brefers to a solar generator device902using a radiation conversion device910configured as a photovoltaic cell. The output current of the radiation conversion device910may be fed to a charge controller950controlling the charging of a buffer battery960or supplying current to a load970, e.g. a DC-to-AC converter.

FIG. 1Crefers to a radiation conversion device910which may be configured as a photovoltaic cell, a photodiode or as a radiation detection device. The radiation conversion device910includes a semiconductor portion100provided from a single-crystalline semiconductor material. According to an embodiment, the semiconductor material is a silicon crystal Si, a germanium crystal Ge, or a silicon-germanium crystal SiGe. According to other embodiments, the single-crystalline semiconductor material may be gallium nitride GaN or gallium arsenide GaAs, by way of example.

The semiconductor portion100has a main surface101and a rear side surface102which may be in substance parallel to the main surface101. A thickness between the main surface101and the rear side surface102may be between 50 μm and several millimeters. A silicon-based radiation detection device may have a thickness of at least 150 and at most 400 μm, for example about approximately 250 μm. The shape of the semiconductor portion100may be a rectangle with an edge length in the range of several millimeters or a circle with a diameter of several millimeters. The normal to the main surface101defines a vertical direction and directions orthogonal to the normal direction are lateral directions.

The semiconductor portion100includes first compensation zones111of a first conductivity type which are arranged in pillar structures110. Each pillar structure110extends in the vertical direction and includes at least two first compensation zones111. Some or all of the first compensation zones111of a pillar structure110are spatially separated from each other.

A base portion120separates the first compensation zones111from each other. The base portion120includes second compensation zones122of a second conductivity type, which is complementary to the first conductivity type. The second compensation zones122are formed between neighboring pillar structures110. The base portion120further includes sections121separating the first compensation zones111in the vertical direction.

The sections121may have the second conductivity type which is opposite to the conductivity type of the first compensation zones111. For example, the base portion120including the sections121and the second compensation zones122may have a uniform background impurity concentration. According to other embodiments, the sections121of the base portion120have the same conductivity type as the first compensation zones111, but distinguish from the first compensation zones111significantly as regards the impurity concentration and the vertical impurity profile.

For example, the sections121of the base portion120separating the first compensation zones111in the vertical direction may have a mean net impurity concentration such that they are fully depleted in the operation mode, for example by applying a reverse bias voltage of at least 10 V. The mean net impurity concentration in the first compensation zones111is at least twenty times, or even at least forty times the mean net impurity concentration in the sections121of the base portion120.

The vertical impurity profiles of the sections121of the base portion120may differ significantly from the vertical impurity profile in the first compensation zones111. For example, the vertical impurity profiles of the first compensation zones111may approximate Gaussian distributions, whereas the impurity distribution in the sections121is approximately uniform.

When the radiation conversion device910is reverse-biased, the electric field applied between the first and second compensation zones111,122depletes a predominant part of the semiconductor portion100even at a background impurity concentration in the base portion120which is high compared with an intrinsic layer of a PIN diode. The increased background impurity concentration increases device ruggedness in view of radiation damages gradually shifting the net impurity concentration to higher acceptor concentrations such that only a smaller portion of an original detection volume can be depleted. Where conventional radiation conversion devices based on PIN diodes gradually become less sensitive because the depleted portion shrinks, the long-term influence of the radiation on the sensitivity of radiation detection devices and the efficiency of photovoltaic cells based on the compensation zones111is low.

Compared to approaches using pillar structures110with connected compensation zones, significantly less epitaxial layers and/or shorter anneal times for diffusing implanted impurities are required. The manufacture of the radiation conversion device910is significantly simplified and more cost effective.

InFIGS. 2A to 2Da semiconductor portion100of a radiation conversion device910has a main surface101, which may be the radiation-receiving surface, and a rear side surface102parallel to the main surface101. The semiconductor portion100may have a rectangular or circular shape. Pillar structures110extend in the vertical direction between the main surface101and the rear side surface102. The pillar structures110may be column-like or strip-like. According to an embodiment, a cross-section of the pillar structures110parallel to the main surface101is a circle, an oval, an ellipse or a rectangle, for example a square with rounded corners. According to the illustrated embodiment, the pillar structures110are stripe-shaped and have a length in a first lateral direction parallel to the main surface101that is significantly greater than a width in a second lateral direction orthogonal to the first lateral direction.

Each pillar structure110includes two or more first compensation zones111which are spatially separated from each other. The first compensation zones111have the first conductivity type and are embedded in a base portion120. Sections of the base portion120between neighboring pillar structures110provide second compensation zones122of a second conductivity type, which is the opposite of the first conductivity type. Further sections121of the base portion120separate the first compensation zones111along the vertical direction.

The impurities in the first compensation zone111compensate for the impurities in the second compensation zones122such that by applying a sufficiently high reverse voltage the regions between neighboring pillar structures110can be completely depleted. The resulting depletion zones represent the detection volume where the incoming radiation generates free charge carriers and where the free charge carriers are transported to the respective electrodes.

The required operation reverse voltage (detection voltage) depends on the impurity concentrations in and the dimensions of the first and second compensation zones111,122.

According to an embodiment, the dopant charge of the first conductivity type in the first compensation zones111is higher than the dopant charge of the second conductivity type in the second compensation zones122. In a plane parallel to the first surface101and cutting the first compensation zones111the number of impurity atoms of the first conductivity type exceeds the number of impurity atoms of the second conductivity type.

A typical detector voltage, which is typically between 5% and 95% of the breakdown voltage, fully depletes the second compensation zones122but does not fully deplete the first compensation zones111. The detector voltage and the lateral dimension of the first compensation zones111may be matched such that a lateral width of a remaining non-depleted portion of the first compensation zones111is less than a charge carrier diffusion length prevailing in the first compensation zones111. The width of this non-depleted portion may be, for example, less than 50% of the diffusion length or less than 10% of the diffusion length. In this way a recombination of free charge carriers generated by radiation within the first compensation zones111is minimized and the detection sensitivity maximized.

For example, a ratio between a mean net dopant concentration in the first compensation zones111and a mean net dopant concentration in the second compensation zones122may be between 2 and 105, wherein according to an embodiment the impurity concentration in the base portion120is at least 1.5×1013cm−3or even 5×1013cm−3for Si, at least 2.5×1014cm−3for Ge and at most 1.5×1017cm−3for Si and at most 2.5×1017cm−3for Ge. The pillar structures110may be equally spaced. For a silicon device, a pitch d between the centers of neighboring pillar structures110may be between 10 μm and 200 μm, for example between 20 μm and 160 μm.

A first electrode structure210is arranged in direct contact with the main surface101and directly adjoins the semiconductor portion100. The first electrode structure210may be a contiguous, uniform layer covering a closed section of the main surface101. According to other embodiments, the first electrode structure210includes a plurality of strips, wherein each strip is assigned to one or more pillar structures110such that the respective strip provides to the assigned pillar structures110a potential applied to the first electrode structure210when the detection voltage is applied.

According to the illustrated embodiment each single strip is assigned to one single pillar structure110. According to other embodiments, each single one of the strips is assigned to at least two of the pillar structures110. According to further embodiments, the strips run in a second lateral direction which intersects the first lateral direction. For example, the second lateral direction is orthogonal to the first lateral direction.

Heavily doped first contact zones118of the first conductivity type are provided in sections of the semiconductor portion100directly adjoining the first electrode structure210to provide a low-ohmic contact (Schottky contact) between the first electrode structure210and the semiconductor portion100. For example, for p type silicon (p-Si) the impurity concentration in the first contact zones118may be at least 1016cm−3and for n type silicon (n-Si) at least 3×1019cm−3.

The first contact zones118may be aligned to the pillar structures110. For example, each contact zone118may be completely arranged in the vertical projection of one of the pillar structures110. According to another embodiment, some or all of the contact zones118overlap only partially with the vertical projection of one, two or more pillar structures110. The first contact zones118may be spaced from or may overlap with the closest first compensations zones111.

The semiconductor portion100includes one or more second contact zone(s)128of the second conductivity type directly adjoining the rear side surface102. The second contact zone(s)128provide ohmic contacts to a second electrode structure220. The second electrode structure220may cover a closed area of the rear side surface102. According to other embodiments, the second electrode structure220may include strips, wherein each strip partially or completely overlaps with the vertical projection of at least one second compensation zone122.

Each of the first and second electrode structures210,220may include one or more layers, wherein each layer may include aluminum Al, copper Cu or an aluminum copper alloy, e.g. AlCu or AlSiCu. According to other embodiments, at least one of the first and second electrode structures210,220is provided from a transparent conductive material, for example a tin oxide. At least one of the first and second electrode structures210,220may include one or more layers containing, as main constituent(s), nickel Ni, gold Au, silver Ag, titanium Ti, tantalum Ta or Palladium Pd.

The base portion120, which includes the second compensation zones122and the sections121separating the first compensation zones111in the vertical direction, has an approximately uniform impurity distribution. Vertical impurity profiles of the first compensation zones111are approximately Gaussian distributions. According to the illustrated embodiment, the first conductivity type is p-type and the second conductivity type is n-type.

FIG. 2Bshows the charge carrier distribution nDin the radiation detection device910ofFIG. 2Ain the idle state without a reverse voltage applied.FIG. 2Cshows the charge carrier distribution nDwith a sufficiently high reverse voltage applied between the first and second electrode structures210,220. Despite the fact that the first compensation zones111are not connected to each other, the base portion120is completely depleted between the first compensation zones111and a predominant portion of the semiconductor portion100is effective for converting incoming radiation into electrical current.

Other than in super junction IGFET (insulated gate field effect transistor) devices radiation conversion devices are not operated in the forward or on-state mode such that the fact that the first compensation zones111at least partially float do not affect adversely the performance of the device.

The first compensation zones111may be provided by repeating a sequence including (i) growing semiconductor layers by epitaxy and (ii) implanting impurities in vertically aligned surface sections of the grown semiconductor layers, as well as a final anneal to control the diffusion of the implanted impurities. Where structurally connected first compensation zones111require a sufficiently high number of epitaxial layers and/or a sufficiently long anneal to ensure that the first compensation zones111get connected, the embodiments relying on not-connected first compensation zones111get by with a lower number of epitaxial layers and/or a reduced anneal time such that the manufacture of the radiation conversion device910is simplified and less cost effective.

The radiation conversion device910ofFIG. 2Dcorresponds to the radiation conversion device ofFIG. 2Awith the first impurity type being n-type and the second impurity type being p-type. In addition the base zone120may include a p type buffer layer125for accommodating an applied voltage. An impurity concentration in the buffer layer125may be lower than in portions of the base portion120outside the buffer layer125. The buffer layer125may be grown by epitaxy and in-situ doped during the growth. Since radiation damages gradually increase the acceptor concentration, the embodiment excludes a change of the conductivity type in low-doped sections of the base portion120within the operation life span.

The radiation conversion device910ofFIGS. 3A to 3Cdiffers from the radiation conversion device910ofFIG. 2Ain that the first contact zones118directly adjoin the first conversion zones111closest to the main surface101. In addition the second compensation zones122at least partially emerge from introducing impurities of the second conductivity type in the surface of epitaxial semiconductor layers e.g. by masked or unmasked implants. Vertical impurity profiles of the second compensation zones122approximate the Gaussian distribution. Providing a significant portion, for example at least 50% or more than 90%, of the impurities of the second compensation zones122by an implant process instead of by in-situ doping during the epitaxial growth may increase the precision of the compensation.

If in the course of operation the radiation gradually generates more acceptor atoms and shifts the dopant level in a lightly doped background section126of the base portion120outside the second compensation zones122from n type to p type, the more heavily doped n-doped second compensation zones122still ensure the complete depletion of sections of the base portion120between neighboring pillar structures110.

FIG. 3Bshows the electric field for the radiation conversion device910ofFIG. 3Abefore aging andFIG. 3Cthe electric field for the same device910after a simulated aging, wherein the area concentration of p-type impurities after aging is assumed to be 2×1013cm−2, which is assumed to be significantly higher than p-type area concentrations achieved by regular aging processes.

In addition, the aging results in an electric field gradient that is more favorable in some respects. For example, a maximum field strength occurring at the edge of the pillar structures110is reduced. The effect may improve the radiation hardness of a termination structure in an edge portion of the radiation conversion device910. In case of a p-type base epitaxy aging may result in an even more favorable electric field gradient since the electric field strength gradually decreases along the interfaces of the pillar structures110in the base zone120and the maximum electric field increasingly appears in a lateral layer section of the base zone120parallel and next to the second contact zones128.

According to an embodiment, the n-type first compensation zones111may be provided from fast diffusing donator type impurities, for example sulfur S or selenium Se such that the temperature/time budget of the diffusion process can be significantly reduced. In addition donator materials like sulfur S and selenium Se, which provide an energy level deep in the energy gap of silicon Si, increase the diffusion length in the detector volume as a result of the effectively reduced free charge carrier concentration.

InFIG. 4A, the base zone120of the radiation conversion device910includes second compensation zones122with the vertical impurity profiles being approximately Gaussian distributions. Two or more second compensation zones122are aligned along the vertical direction and form further pillar structures which are spaced from the pillar structures110. A uniformly doped background section126of the base portion120may separate the pillar structures110with the first compensation zones111and the further pillar structures with the second compensation zones122. The pillar structures110may be connected to first contact zones118and the further pillar structures may be connected to one or more second contact zones128.

The net dopant concentration in the second compensation zones122may in substance correspond to the impurity concentration in the corresponding first compensation zones111. The remaining background portion126may be intrinsic, n-type or p-type. The base portion120can be completely depleted even at comparatively high impurity concentrations in the compensation zones111,122.

The first compensation zones111of each pillar structure110may be separated by the uniformly doped background section126of the base portion120, and the second compensation zones122of each further pillar structure may be separated by the uniformly doped background section126.

According to the embodiment ofFIG. 4B, the first compensation zones111of each pillar structure110are structurally connected to each other or overlap with each other and the second compensation zones122of each further pillar structure are structurally connected to each other or overlap with each other.

FIG. 4Crefers to a radiation conversion device910providing both the first and the second electrodes210,220at the main surface101. Non-transparent electrode materials may be used for both the first and second electrodes210,220with the rear side surface102forming the radiation-receiving surface through which the radiation enters the semiconductor portion100.

FIGS. 5A and 5Brefer to a radiation conversion device910with a semiconductor portion100including first compensation zones111of a first conductivity type arranged in stripe-shaped pillar structures110. The first compensation zones111of each pillar structure110may be connected to each other or may be separated from each other. Between neighboring pillar structures110second compensation zones122of a second, complementary conductivity type may form further pillar structures. Each pillar structure110extends in a vertical direction and in a first lateral direction with respect to a main surface101of the semiconductor portion100. A first electrode structure210directly adjoins the semiconductor portion100at the main surface101and includes a plurality of strips. Each strip is assigned to at least two pillar structures110. According to the illustrated embodiment, the strips directly adjoin first contact zones118, wherein each of the first contact zones118is assigned to two parallel pillar structures110.

According to the embodiment shown inFIG. 6the strips of the first electrode210run in a second lateral direction orthogonally intersecting the first lateral direction defined by the stripe-shaped pillar structures110. Heavily doped first contact zones118run parallel to the strips to provide a Schottky contact between the first electrode structure210and the pillar structures110.

A first pitch d1 of the strips of the first electrodes210is decoupled from a second pitch d2 of the pillar structures110. According to an embodiment, the second pitch d2 of the pillar structures110is significantly smaller than, for example at most half, the first pitch d1 to achieve both a high dopant level in the detector area and a great pitch for the strips of the first electrode structure210simplifying and making more reliable the manufacture of the radiation conversion device910.

FIGS. 7A to 7Crefer to the manufacture of a radiation conversion device. On a semiconductor base substrate100aa semiconductor layer100bis grown by epitaxy. The crystal lattice of the grown semiconductor layer100bgrows in registry with the crystal lattice in the base substrate100a. A mask layer is deposited on the grown semiconductor layer100band patterned by a photolithographic process to obtain an impurity mask310with openings315. The grown semiconductor layer100bmay be an intrinsic layer, lightly p-doped or lightly n-doped. Impurities of a first conductivity type, for example p-type, are introduced through the openings315in the impurity mask310into exposed first surface sections of the grown semiconductor layer100bto form implant zones111a. The implant energy may be selected such that the impurities are implanted in close proximity to the exposed surface of the grown semiconductor layer100b.

FIG. 7Ashows the implanted zones111aclose to the exposed surface of the grown semiconductor layer100bin first sections exposed by the implant mask310. The implant mask310is removed and a cycle including (i) growing by epitaxy a semiconductor layer (ii) providing an implant mask with openings aligned to the openings in the first implant mask310(iii) implanting impurities of the first conductivity type and (iv) removing the implant mask is repeated several times.

FIG. 7Bshows a semiconductor portion100obtained by growing successively five semiconductor layers100b-100fby epitaxy on the base substrate100a. In the first to fourth semiconductor layers100bto100e, implanted zones111a-111dare aligned along the vertical direction orthogonal to a main surface101of the obtained semiconductor portion100. An anneal is performed such that the impurities of the implanted zones111ato111ddiffuse out to form first compensation zones111arranged in pillar structures110. The annealing is terminated before the first compensation zones111overlap each other. Then first contact zones118of the first conductivity type are formed that directly adjoin the main surface101. Second contact zones128of the second conductivity type may be provided at the rear side surface102. Electrode structures210,220are provided that form Schottky contacts with the first and second contact zones118,128.

FIG. 7Cshows the resulting radiation conversion device910similar to the radiation conversion device ofFIG. 2A. According to an other embodiment, each cycle may include an unmasked implant of impurities of the second conductivity type before or after the masked implant of impurities of the first conductivity type to obtain the radiation conversion device ofFIG. 3A.

FIGS. 8A to 8Crefer to a further method providing a second implant mask320covering the implanted zones111aofFIG. 7A. In each cycle impurities of a second conductivity type, which is complementary to the first conductivity type, are implanted into the epitaxial semiconductor layer100bto form further implanted zones122aof the second conductivity type in some or each of the epitaxial semiconductor layers100b-100f. The sequence of the implants of the first and second conductivity type may be inverted for each semiconductor layer100b-100f.

According toFIG. 9a method of manufacturing a radiation conversion device includes growing by epitaxy a sequence of semiconductor layers on a semiconductor substrate (802), wherein impurities of a first conductivity type are introduced into first sections of an exposed surface of each of the grown semiconductor layers before growing a subsequent one of the semiconductor layers (804). The grown semiconductor layers are annealed to form first compensation zones from the introduced impurities (806). The first compensation zones remain separated by a base portion, wherein second compensation zones of a complementary second conductivity type separate the first compensations zones in a lateral direction and further sections of the base portion separate the first compensations zones in a vertical direction.