N-type doping of zinc telluride

ZnTe is implanted with a first species selected from Group III and a second species selected from Group VII. This may be preformed using sequential implants, implants of the first species and second species that are at least partially simultaneous, or a molecular species comprising an atom selected from Group III and an atom selected from Group VII. The implants may be performed at an elevated temperature in one instance between 70° C. and 800° C.

FIELD

This invention relates to doping zinc telluride (ZnTe) and, more particularly, to n-type doping of ZnTe.

BACKGROUND

Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the lattice of the workpiece material to form an implanted region.

Workpieces or films on workpieces may be composed of many different materials. For example, ZnTe is a wide band gap semiconductor material with a direct band gap of around 2.25 eV. ZnTe may be used in ultra-high efficiency solar cells, pure green light emitting diodes (LEDs), laser diodes, optoelectronic detectors, compound semiconductors, and other applications known to those skilled in the art. However, it is difficult to perform n-type doping of ZnTe or ZnTe workpieces. In-situ doping during ZnTe growth has been performed, such as using molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). Doping during ZnTe growth cannot control the Zn vacancy concentration, which is one mechanism that prevents n-type doping of ZnTe. This is at least partly because in-situ doping during ZnTe growth involves competition between dopants and Zn atoms. This competition results in Zn vacancies. The existence of Zn vacancies is a p-type characteristic and will compensate for n-type doping of ZnTe. What is needed is a new method of doping ZnTe and, more particularly, n-type doping of ZnTe.

SUMMARY

According to a first aspect of the invention, a method of doping is provided. The method comprises implanting a ZnTe layer with a first species selected from Group III. The ZnTe layer also is implanted with a second species selected from Group VII.

According to a second aspect of the invention, a method of doping is provided. The method comprises implanting a ZnTe layer with a first species selected from Group III and a second species selected from Group VII. The ZnTe layer is at a temperature between 70° C. and 800° C. during the implantation of the first species and second species.

According to a third aspect of the invention, a method of doping is provided. The method comprises implanting a ZnTe layer with a molecular species comprising an atom selected from Group III and an atom selected from Group VII.

DETAILED DESCRIPTION

These methods are describe herein in connection with an ion implanter. However, while a beam-line ion implanter is specifically described, other systems and processes involved in semiconductor manufacturing or other systems that use plasma or generate ions also may be used. Some examples include a plasma doping tool, a plasma immersion tool, a flood implanter, an implanter that focuses a plasma or ion beam, or an implanter that modifies the plasma sheath. Thus, the invention is not limited to the specific embodiments described below.

FIG. 1is a view of one embodiment of a ZnTe crystal structure102. ZnTe may have a cubic crystal structure like a diamond. However, ZnTe may have other crystal structures such as hexagonal (wurzite), polycrystalline, or amorphous. The ZnTe crystal structure102illustrated inFIG. 1includes Zn atoms100and Te atoms101(illustrated as black inFIG. 1). The illustration inFIG. 1is a two-dimensional approximation of a three-dimensional structure. Thus, some atoms in the ZnTe crystal structure102would go into or out of the page.

FIG. 2is a cross-sectional view of implanting a workpiece with a first species. A workpiece103, which is ZnTe or has a ZnTe film on at least one surface, is grown. Thus, the workpiece103may be or may contain a ZnTe layer. So while the term “workpiece” is used herein, a ZnTe layer also may be processed using the embodiments disclosed herein. MBE, for example, may be used to grow the workpiece103, though other methods are possible.

The workpiece103is implanted with a first species104. This first species104is selected from Group111. Examples of the first: species104include B, Al, Ga, and In. Of course, other ions may be implanted as the first species104. The first species104implants the entirety of the workpiece103, though implants to particular depths or to particular regions also are possible.

FIG. 3is a cross-sectional view of implanting a workpiece with a second species. The workpiece103is then implanted with a second species105. This second species105is selected from Group VII. Examples of the second species105include F, Cl, Br, and I. Of course, other ions may be implanted as the second species105. The second species105implants the entirety of the workpiece103, though implants to particular depths or to depths different than that of the first species104are possible.

While the second species105is shown being implanted after the first species104, the implantation may be performed in either order. In another particular embodiment, the first species104and second species105are implanted simultaneously or at least partially simultaneously. In one example, a cocktail or plasma containing both the first species104and second species105is formed and implanted into the workpiece103at the same time. In yet another particular embodiment, the first species104and second species105are implanted sequentially without breaking vacuum around the workpiece103. While the entire workpiece103is illustrated as being implanted inFIGS. 2-3, in an alternate embodiment only a portion of the workpiece103or a certain region of the workpiece103is implanted.

In a first instance, the first species104is Ga and the second species105is I. In a second instance, the first species104is Al and the second species105is Cl. The combinations can enhance a doping effect because Ga or Al will replace Zn atoms in the ZnTe and I or Cl will replace Te atoms in the ZnTe. Other combinations of first species104and second species105are possible. These are merely examples. The first species104and second species105may be generated from atomic or molecular feed gases in one embodiment.

In one particular embodiment, the implantation of the first species104or second species105may be followed by an anneal. For example, a laser or flash anneal may be performed. This anneal recrystallizes the workpiece103. Laser annealing, for example, may activate the first species104and second species105without producing additional Zn vacancies. The time duration of the anneal may be configured to reduce the number of Zn vacancies produced. Annealing using a laser anneal or flash anneal may minimize the competition process between the implanted species and Zn vacancies, which may reduce the Zn vacancy concentration. In an alternate embodiment, rapid thermal anneal (RTA) or other annealing methods may be used.

In another embodiment, the implantation of the first species104or second species105may be performed at an elevated temperature. In one instance, the workpiece103is pre-heated prior to the implantation steps to above room temperature. In another instance, the workpiece103is heated during the implantation steps. For example, the workpiece103may be pre-heated or heated to between approximately 70° C. and 800° C. In one particular embodiment, the workpiece103heated to between approximately 300° C. and 800° C. during implantation. Implantation at an elevated temperature may reduce damage to the crystal lattice of the workpiece103or may repair or anneal damage to the crystal lattice of the workpiece103. Reduced damage may enable particular annealing methods that are less effective with more damage to the crystal lattice. The temperature of the workpiece103is configured to reduce or prevent diffusion of the species implanted into the workpiece103. Furthermore, the temperature of the workpiece103is configured to reduce or prevent amorphization of the workpiece103due to implant. Partial amorphization may occur in one instance if this partial amorphization can be removed using, for example, a laser anneal or flash anneal. In one particular embodiment, the workpiece103is heated during implantation to a varying temperature. This temperature may be ramped or otherwise adjusted during the implantation or between the implantation of the first species104and second species105.

FIG. 4is a view of one embodiment of a doped ZnTe crystal structure. The implanted ZnTe crystal structure108includes Zn atoms100, Te atoms101, first species atoms106, and second species atoms107. The first species atoms106are selected to have a size similar to the Zn atoms100in one instance. The second species atoms107are selected to have a size similar to the Te atoms101in a second instance. The first species atoms106, which may be from Group III, replace Zn atoms100in the ZnTe crystal structure108. The second species atoms107, which may be from Group VII, replace Te atoms101in the ZnTe crystal structure108. Similar-sized atoms may reduce stress or strain within the implanted ZnTe crystal structure108because the lattice mismatch between the host material, the first species atoms106, and the second species atoms107is minimized. Dose and energy during implantation of the first species atoms106and second species atoms107are configured to obtain the desired dopant incorporation in the implanted ZnTe crystal structure108.

In an alternate embodiment, the first species atoms106, which may be from Group III, have a size larger than the Zn atoms100. The second species atoms107, which may be from Group VII, may be smaller than the Te atoms101. This combination or other suitable combinations also may reduce stress or strain within the implanted ZnTe crystal structure108.

Implanting smaller ions than the examples listed herein into the implanted ZnTe crystal structure108may induce strain in the crystal lattice. For example, B and F may be implanted to induce strain. This occurs because the Zn atoms100, atomic weight 65.39, and Te atoms 127.60, are fairly large compared to smaller n-type dopants. Implantation of smaller ions to cause strain may be beneficial for certain applications.

FIG. 5is a cross-sectional view of implanting a workpiece with a molecule. The workpiece103is implanted with a molecular species109. This molecular species109contains both an atom from Group III and an atom from Group VII. For example, the molecular species109may be BF3ions. The molecular species109also may contain a combination of B, Ga, or Al and F, Cl, or I such as BI3or BCl3. Of course, other examples of the molecular species109are possible. While the entire workpiece103is illustrated as being implanted inFIG. 5, in an alternate embodiment only a portion of the workpiece103or a certain region of the workpiece103is implanted.

In another embodiment, implantation of the first species and second species or the atoms of the molecular species is to approximately the same depth in the workpiece. The implant energies or doses of the first species and second species or the implant energies or doses of the molecular species may be configured to attain a particular depth. Different implant energies or doses may be needed in part due to the size, mass, or charge of the ions or molecule. This may result in approximately matching profiles when implanting two species or two different atoms within a molecular species. Placing the two species or two different atoms at the same depth may provide benefits during the anneal. For example, placing the two species or two different atoms near one another provides local stabilization.

In one instance, B and F were implanted into a ZnTe workpiece. The B was implanted at 8 keV and the F was implanted at 13 keV. This resulted in the B and F both being implanted to the same depth of 20 nm within the ZnTe workpiece.

The dose of the two species or two different atoms may be controlled at a particular implant depth. Particular doses influence the doping process. For example, if more of one atom or species exists at a particular depth than the other atom or species, then the ZnTe workpiece may be counterdoped.

The atomic ratio of the two species or two different atoms also may be controlled at a particular implant depth. This also may provide benefits during the anneal such as better stabilization of the lattice, For example, more F than B may be needed during implantation because some F may be lost in the annealing step. Also, a specific atomic ratio may be used to stabilize the doped structure. This atomic ratio may be 1:1, but may vary based on the particular species or atoms or on the particular depth within the ZnTe workpiece.

In one embodiment, a beam-line ion implanter is used to adjust the implant depth, implant energy, or dose. In another instance, a plasma tool is used. In one example, a plasma is generated from BF3in a plasma tool such as a plasma doping tool. The ZnTe workpiece rests on a platen in a process chamber where the plasma is generated. Such a tool may modify the power that generates the plasma and the bias to the platen on which the ZnTe workpiece rests. The bias to the platen may be equivalent to changing the implant energy.

FIG. 8illustrates one possible embodiment using a plasma doping tool. The line300represents bias voltage to the ZnTe workpiece and the line301(dotted inFIG. 8) represents the power used to generate the plasma. The first voltage302and first power303occur at substantially the same time. The second voltage304and second power305also occur at substantially the same time. The second voltage304is less than the first voltage302. The second power305is less than the first power303. The first power303and second power305may generate plasmas with different ion densities for specific species. For example, with a plasma based on BF3the first power303may predominantly create a higher F ion plasma density and the second power305may predominantly create a higher B ion plasma density. The difference in the first voltage302and second voltage304may ensure that the two species are implanted to a substantially similar depth within the ZnTe workpiece or to form a substantially similar implant profile within the ZnTe workpiece. Of course, other voltage or power combinations may be used. The embodiment ofFIG. 8is merely an example.

In one instance, BF3was implanted into the ZnTe workpiece using an embodiment similar to that illustrated inFIG. 8followed by a flash anneal, photoluminescence, and ellipsometry measurements. The B and F were incorporated into the ZnTe workpiece after the anneal. A new photoluminescence peak around 690 nm (1.8 eV) appeared due to the B and F implantation. Ellipsometry measurements indicated the presence of a doped ZnTe layer at the surface of the ZnTe workpiece.

The embodiments disclosed herein may introduce fewer Zn vacancies than in-situ doping, such as that performed by MBE or MOCVD, because there is less competition between dopants and the Zn than by in-situ doping during ZnTe growth.FIG. 6is a chart comparing implantation introduced Zn vacancies per intrinsic Zn vacancy to depth. The Zn vacancies inFIG. 6are caused by Al implantation at 5 kV and 1E16 cm−2, In one particular example, 8.4% Zn vacancies were formed. This is approximately ten times less than that caused by MBE or MOCVD in-situ doping.

FIG. 7is a simplified block diagram of a beam-line ion implanter. Those skilled in the art will recognize that the beam-line ion implanter200is only one of many examples of differing beam-line ion implanters. In general, the beam-line ion implanter200includes an ion source201to generate ions that are extracted to form an ion beam202, which may be, for example, a ribbon beam or a spot beam. The ion beam202ofFIG. 7may correspond to the first species104, the second species105, or the molecular species109ofFIG. 2,3, or5.

The ion beam202may be mass analyzed and converted from a diverging ion beam to a ribbon ion beam with substantially parallel ion trajectories in one instance. The ion beam202also may not be mass analyzed prior to implantation. The beam-line ion implanter200may further include an acceleration or deceleration unit203in some embodiments.

An end station204supports one or more workpieces, such as the workpiece103, in the path of the ion beam202such that ions of the desired species are implanted into workpiece103. The end station204may include workpiece holder, such as platen205, to support the workpiece103. The workpiece holder also may be other mechanisms such as a conveyor belt. This particular end station204also may include a scanner (not illustrated) for moving the workpiece103perpendicular to the long dimension of the ion beam202cross-section, thereby distributing ions over the entire surface of workpiece103.

The beam-line ion implanter200may include additional components known to those skilled in the art such as automated workpiece handling equipment, Faraday sensors, or an electron flood gun. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The beam-line ion implanter200may incorporate hot or cold implantation of ions in some embodiments. Hot implantation may use lamps, LEDs, a platen205or other workpiece holder that is heated, or other mechanisms known to those skilled in the art. Pre-heating the workpiece103may be performed on the workpiece holder, a separate area of the end station204, or in a separate chamber of the beam-line ion implanter200.

Turning toFIG. 9, the plasma doping system400includes a process chamber402defining an enclosed volume403. A platen404may be positioned in the process chamber402to support the workpiece103. The workpiece103may be cooled or heated by a temperature regulation system, such as using the platen404or some other method. Thus, the plasma doping system400may incorporate hot or cold implantation of ions in some embodiments. The workpiece103may be clamped to a flat surface of the platen404by electrostatic or mechanical forces. In one embodiment, the platen404may include conductive pins for making connection to the workpiece103.

The plasma doping system400further includes a source401configured to generate a plasma406from an implant gas within the process chamber402. The source401may be an RF source or other sources known to those skilled in the art. The platen404may be biased. This bias may be provided by a DC or RF power supply. The plasma doping system400may further include a shield ring, a Faraday sensor, or other components. In some embodiments, the plasma doping system400is part of a cluster tool, or operatively-linked process chambers402within a single plasma doping system400. Thus, numerous process chambers402may be linked in vacuum.

During operation, the source401is configured to generate the plasma406within the process chamber402. In one embodiment, the source401is an RF source that resonates RF currents in at least one RF antenna to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents in the process chamber402. The RF currents in the process chamber402excite and ionize the implant gas to generate the plasma406. The bias provided to the platen404and, hence, the workpiece103will accelerate ions from the plasma406toward the workpiece103during bias pulse on periods. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate. The amplitude of the pulsed platen signal may be selected to provide a desired energy. With all other parameters being equal, a greater energy will result in a greater implanted depth.