Ultra-high doped n-type germanium monocrystals and wafers derived therefrom

Abstract

The invention concerns N-type germanium monocrystals comprising phosphorus as a single dopant in an amount of at least 1.0 x 1019 / cm³. Such crystals can be obtained by using the Czochralski pulling technique with GeP as dopant, whereby phosphorus is provided as single dopant in an amount of at least 1.0 x 1019 / cm³. The obtained crystals show that the amount of surface defects is significantly reduced.

Description

[0001] ULTRA-HIGH DOPED N-TYPE GERMANIUM MONOCRYSTALS AND WAFERS DERIVED THEREFROM

[0002] TECHNICAL FIELD

[0003] The present invention concerns a process for the manufacture of dislocation-free monocrystalline germanium, n-type doped with phosphorus having reduced surface imperfections.

[0004] BACKGROUND

[0005] Electronic and opto-electronic device manufacturers require commercially grown, large and uniform germanium single semiconductor crystals which, when sliced and polished, provide substrates for microelectronic device production. The market for n- type Ge wafers is mainly for opto-electronic devices such as vertical-cavity surface emitting lasers (VCSEL) or mid-infrared plasmonic sensors. A VCSEL is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface. VCSELs are used in various laser products, including computer mice, fiber optic communications, laser printers, face recognition, and smart glasses. Plasmonic sensors are based on collective oscillations of free electrons and are suited for various applications such as the detection of molecules.

[0006] N-type doping and a low resistivity are desirable for these applications, as high electron densities achieved in these highly doped wafers enable the plasmonic effect in the germanium films. However, dislocations often degrade their performance.

[0007] Low resistivity dislocation-free semiconductor wafers have been described in US 7,341,787. Very low resistivities are envisaged, such as lower than 2 mQ-cm. To this end, at least two electrically active dopants, which belong to the same group of the periodic system of elements, are simultaneously added. This is said to avoid the problem that, above certain concentrations, some of the dopant atoms incorporated in the semiconductor material may be electrically inactive. This document, although mentioning Si, Ge, and Si-Ge alloys, is clearly dedicated to the doping of Si. No details are disclosed with respect to Ge, and in particular also not for the source of the dopants used. Spitzer et al., J. Appl. Phys. 32, 1822, 1961, describe the preparation of Ge single crystals heavily doped with Sb, As, and P. Resistivities well below 2 m -cm are reported. Herein, the phosphorus-doped samples are all obtained by pulling single crystals with calcium orthophosphate as the source. However, the use of calcium orthophosphate as dopant will necessarily preclude obtaining dislocation-free crystals.

[0008] US 8,574,363 teaches that a high doping level in a silicon-based semiconductor crystal can also be obtained by using a so-called dopant chamber that, for example, includes phosphorus. The chamber is brought into close proximity of the melt. When the temperature is raised to the melting point of phosphorus, the phosphorus is released as a gas, and a part of it will be incorporated into the melt, while the rest is lost to the environment. The gas phase operation thus requires additional attention and calculations to predict the amount of phosphorous released to the melt. In case of doping silicon with phosphorous and germanium, US 8,574,363 further teaches the use of two separate chambers, one for phosphorous, supplied via evaporation, the other for germanium, supplied by liquefication. US 7,132,091 indicates using a dopant chamber where the cover is partially dissolved when lowering it into the melt. The usage of such a chamber can introduce undesired contaminants into the melt.

[0009] WO 2021 / 123461 Al describes a process of growing high-purity germanium crystals using the Czochralski method, with a focus on doping the crystals with GeP to a crystal dopant level between 1 and 6 . 1018atoms / cm3, leading to resistivity variations from 2 to 6 mOhm-cm. N-type doping, achieved through the addition of GeP, introduces high electron densities in the wafers, which is crucial for enabling the plasmonic effect in germanium films. The process described ensures controlled doping and resistivity levels, which are essential for achieving the desired electrical properties. However, the formation of dislocations, which can occur during the growth and cooling process, can degrade the performance of these wafers. The detailed process aims to control these factors, thereby optimizing the crystal's quality for its intended high-tech applications.

[0010] Germanium wafers according to the state of the art which are devoid of dislocations are frequently characterized by a series of surface imperfections. These imperfections originate from a process known as "vacancy condensation," which predominantly occurs during the crystal's cooling stage post-growth. These surface anomalies primarily manifest as voids on the surface, typically extending several micrometers in diameter. Regrettably, these defects substantially compromise the integrity of subsequent epitaxial material layers deposited on the wafer. The presence of these voids can lead to significant challenges in the uniformity and quality of these layers, impacting their electronic properties. Consequently, it is imperative to mitigate the formation of such defects to enhance the wafer's suitability for high-precision applications.

[0011] SUMMARY

[0012] To this aim, the present invention provides an N-type doped germanium monocrystal according to claim 1. The inventors discovered that high-level phosphorus doping in germanium monocrystals significantly reduces surface voids, contrary to expectations. This reduction enhances the quality and uniformity of epitaxial layers on the wafers, crucial for high-precision semiconductor applications, and improves material and energy efficiency in production processes.

[0013] The invention further relates to germanium wafers comprising said germanium monocrystals and production processes for preparing said germanium monocrystals.

[0014] FIGURES

[0015] By means of further guidance, figures are included to better appreciate the teaching of the present invention. Said figures are intended to assist the description of the invention and are nowhere intended as a limitation of the presently disclosed invention.

[0016] The figures and symbols contained therein have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0017] Figure 1(a) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 4 inch wafer according to Example 2. Figure 1(b) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 4 inch wafer according to Comparative Example 1.

[0018] Figure 2(a) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 8 inch wafer according to Example 4. Figure 2(b) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 8 inch wafer according to Comparative Example 3.

[0019] DETAILED DESCRIPTION OF THE INVENTION

[0020] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0021] As used herein, the following terms have the following meanings:

[0022] "A", "an", and "the" as used herein refers to both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a compartment" refers to one or more than one compartment.

[0023] "About" as used herein referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / - 20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less of and from the specified value, in so far such variations are appropriate to perform in the disclosed invention. However, it is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0024] "Comprise," "comprising," and "comprises" and "comprised of" as used herein are synonymous with "include", "including", "includes" or "contain", "containing", "contains" and are inclusive or open-ended terms that specifies the presence of what follows e.g. component and do not exclude or preclude the presence of additional, non-recited components, features, element, members, steps, known in the art or disclosed therein. The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within that range, as well as the recited endpoints. All percentages are to be understood as percentage by weight, abbreviated as "wt.%", unless otherwise defined or unless a different meaning is obvious to the person skilled in the art from its use and in the context wherein it is used. Concentration of doping elements is expressed as atoms per cm3, and the mentioned concentration refers to the average concentration of the entire ingot or wafer, respectively.

[0025] The growth of a semiconductor crystal according to the present invention involves heating raw material to its melting point to create a crystalline raw material melt, bringing the melt into contact with a high quality seed crystal, and allowing the crystallization of the melt when in contact with the seed crystal. A number of different processes for accomplishing this are known. These include the Czochralski (Cz) process and its variant the Liquid Encapsulated Czochralski (LEC) process, the Horizontal Bridgman and Bridgman-Stockbarger processes (HB) and their vertical variants (VB), and the gradient freeze (GF) and its variant, the vertical gradient freeze (VGF) processes. Such techniques are described in "Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials " P. Clapper, Ed., John Wiley and Sons Ltd, Chichester, England, 2005.

[0026] The crystallization of the melt forms an essentially cylindrical crystal, referred to as an ingot, along a vertical axis with the seed crystal below the crystalline raw material(s). The equipment necessary to form the semiconductor crystal includes a crystal growth furnace, a crucible, and sometimes a crucible support.

[0027] In the context of the present invention, the term 'surface imperfections' is synonymous for the term 'surface voids' and encompasses Light Point Defects (LPDs) and Crystal Originated Particles (COPs) as well as other types of defects such as scratches, pits, or contamination that affect the wafer's surface. Light Point Defects (LPDs) are small, localized crystallographic imperfections in the germanium wafer. They are typically only a few atoms in size and can be either interstitial defects, where extra atoms are inserted into the crystal lattice, or vacancy defects, where atoms are missing from the lattice. Crystal Originated Particles (COPs) are slightly larger defects than LPDs and are often formed during the crystal growth process. COPs are actual particles or clusters of atoms that deviate from the ideal crystal structure. They can originate from various sources, such as impurities in the crystal growth environment, dislocations in the crystal lattice, or irregularities in the melting process of the germanium. COPs are particularly concerning in semiconductor manufacturing as they can lead to non-uniformity in the electrical properties across the wafer.

[0028] Surface defect inspection is performed using a KLA-Tencor Surfscan 6000 Series (type 6220; monthly calibrated using PSL sphere drift calibration process with standards: 0.364 micron diameter PSL, Total Deposition 2000 - 5000 spheres; normal incidence laser beam with no polarization) in accordance to the instrument manual specifications according to the following procedure: Position the substrate in the scan plane and move it at a constant velocity through the high-speed telecentric scanning laser beam. Set the laser beam to project a circular spot with 90 pm diameter on the substrate surface. Collect scattered light with the collection optics and direct it onto a low-noise PMT for amplification. Digitize the analogue signal from the PMT with a high-speed analogue to digital converter. Analyse the data to create, display, and manipulate surface measurement data. Implement a two-dimensional signal integration technique to increase signal level by up to 8x compared to the peak amplitude for precise defect sizing. Set detection thresholds to differentiate between actual defects and background noise. For a population of mono-disperse spheres, set a threshold that provides a capture rate of 90% and a false count rate of less than 1%. The procedure monitors the response of a PSL sphere standard calibration wafer(s) and compares it to the most recent calibration response. The instrument calculates a sensitivity drift factor, called the Custom Ratio, which can be used to normalize the data obtained from the system.

[0029] In a first aspect, the present invention provides an N-type germanium monocrystal comprising phosphorus in an amount of at least 1.0 x 1019 / cm3. Also, in a first aspect, the present invention provides an N-type germanium monocrystal having less than 1.50 surface imperfections per cm2, as determined by Surface Defect Inspection. Preferably, phosphorous is present as single dopant. This means that phosphorous is present as the single dopant and that the presence of any further dopants is excluded.

[0030] The inventors have found, quite unexpectedly, that the implementation of an exceptionally high level of phosphorus doping, whereby phosphorus is singularly utilized as a dopant, within the germanium monocrystal structure significantly diminishes the incidence of surface imperfections, which are commonly known as "surface voids". This discovery indicates a notable deviation from anticipated outcomes, as the intricate interplay between high dopant concentrations and defect formation in crystal structures is typically complex and not straightforward. Lower surface voids in germanium wafers have a profound impact on addressing the shortcomings associated with surface imperfections, especially in the context of high- precision applications. Surface voids, resulting from vacancy condensation during the cooling phase of crystal growth, typically manifest as micrometer-scale voids on the wafer surface. These imperfections can significantly compromise the quality and uniformity of epitaxial material layers subsequently deposited on the wafers.

[0031] The finding by the present inventors allows for a new scope of applications, especially high-precision applications. Surface voids create non-uniform regions on the wafer surface, leading to inconsistent growth of epitaxial layers. By reducing the occurrence of these voids, the epitaxial layers can be deposited more uniformly. This uniformity is critical for the consistent electronic properties of the layers, ensuring reliability in their performance. Furthermore, surface voids negatively affect the electronic properties of the germanium wafers, including the subsequently grown epitaxial layers, which is crucial for semiconductor devices where precision and efficiency is demanded. As such, the requirement for high-precision applications can be more easily met. Also, reduction in surface voids leads to fewer rejected wafers, thereby improving the material- and energy-economy of the production processes.

[0032] The number of surface imperfections is determined using Surface Defect Inspection method using KLA Tencor's Surfscan. The number of surface defects can be determined as follows. Before inspection, the semiconductor wafer is thoroughly cleaned to remove any loose particles or contaminants. In a cleanroom environment, the wafer is then loaded into the Surfscan system. The Surfscan system is programmed with specific parameters suitable for the wafer's characteristics. These parameters include sensitivity settings, which determine the size threshold for defect detection, and the scanning resolution. Surface defects with a size from 1 to 10 pm are retained. The location, size, and density of defects is mapped across the wafer surface.

[0033] In a preferred embodiment, the present invention provides an N-type germanium monocrystal according to the first aspect of the invention having less than 1.25 surface imperfections per cm2, as determined by Surface Defect Inspection, more preferably less than 1.10 surface imperfections per cm2, and even more preferably less than 1.00 surface imperfections per cm2. Typically, an N-type germanium monocrystal according to the first aspect of the invention has between 0.25 and 0.75 surface imperfections per cm2, more typically between 0.30 and 0.60 surface imperfections per cm2, such as about 0.35, 0.40, 0.45, 0.50 or 0.60 surface imperfections per cm2.

[0034] The dislocation-free nature of the germanium wafer can be validated using X-ray topography (XRT). The presence of streaks, spots, or irregular patterns in the topographic image suggests the presence of dislocations. In contrast, a uniform and regular pattern indicates a dislocation-free wafer. Preferably, said Ge monocrystal is substantially dislocation-free. In the context of the present invention, the term "substantially dislocation-free" is to be interpreted as having less than 0.025 dislocations per cm2, or less than 0.020 dislocations per cm2, or less than 0.015 dislocations per cm2, or less than 0.010 dislocations per cm2, or less than 0.005 dislocations per cm2, and more preferably less than 0.001 dislocations per cm2. Most preferably, said Ge monocrystal has no dislocations, and is thus dislocation-free. The dislocations which occur in crystals grown by the Czochralski method can be determined by etching and / or copper decoration techniques. Sources of dislocations include thermal shock from surface damage or residual dislocations in the seed; poor epitaxy at the seed; plastic generation in the bulk of the crystal; effects of segregation of impurities. The skilled person will appreciate that it is possible to eliminate residual dislocations and thenceforth grow a dislocation-free crystal, e.g. by Dash-necking. In a preferred embodiment, the Ge monocrystal according to the invention is produced in a procedure wherein the crystal growth furnace produces a crystal ingot having no dislocations or lineage. Preferably, said ingots are grown by a vertical growth process.

[0035] In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, comprising phosphorus in an amount of at least 1.1 x 1019 / cm3, or in an amount of at least 1.2 x 1019 / cm3, or in an amount of at least 1.5 x 1019 / cm3, or in an amount of at least 1.8 x 1019 / cm3, or in an amount of at least 2.0 x 1019 / cm3, or in an amount of at least 2.2 x 1019 / cm3, or in an amount of at least 2.4 x 1019 / cm3, or in an amount of at least 2.5 x 1019 / cm3, or in an amount of at least 2.8 x 1019 / cm3, or in an amount of at least 3.0 x 1019 / cm3, or in an amount of at least 3.2 x 1019 / cm3, or in an amount of at least 3.5 x 1019 / cm3, or in an amount of at least 4.0 x 1019 / cm3, or in an amount of at least 4.2 x 1019 / cm3, or in an amount of at least 4.5 x 1019 / cm3, or in an amount of at least 4.7 x 1019 / cm3, or in an amount of at least 5.0 x 1019 / cm3. In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, comprising phosphorus in an amount of at most 5.0 x 102° / cm3, or in an amount of at most 4.0 x 102° / cm3, or in an amount of at most 3.0 x 102° / cm3, or in an amount of at most 2.0 x 102° / cm3, or in an amount of at most 1.0 x 102° / cm3.

[0036] In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, comprising phosphorus in an amount of 2.5 x 1019 / cm3to 5 x 1021 / cm3, more preferably in an amount of 5 x 1019 / cm3to 1.0 x 1021 / cm3, and even more preferably in an amount of 5 x 1019 / cm3to 5 x 102° / cm3, or any value there in between.

[0037] In a preferred embodiment, the present invention provides in an n-type doped Ge monocrystal, wherein said Ge monocrystal has a resistivity of less than 10 mQ-cm, and preferably between 0.1 mQ-cm and 10 mQ-cm, and more preferably between 0.1 mQ-cm and 7 mQ-cm, and more preferably between 0.1 mQ-cm and 5 mQ-cm, and even more preferably between 0.5 mQ-cm and 5 mQ-cm. Most preferably, said resistivity is between 0.5 mQ-cm and 4 mQ-cm, such as 0.5 mQ-cm, 1.0 mQ-cm, 2.0 mQ-cm, 3.0 mQ-cm, 4.0 mQ-cm, or any value there in between.

[0038] In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, wherein said monocrystal is cylindrical and has a diameter of 1 cm to 50 cm, more preferably a diameter of 5 cm to 35 cm. Most preferably, said diameter is about 100 mm or 4 inch, about 150 mm or 6 inch, about 200 mm or 8 inch, or about 300 mm or 12 inch, or any diameter there in between. Preferably, said Ge monocrystal is in the form of an ingot, more preferably in the form of a Czochralski-grown crystal ingot.

[0039] In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, wherein said monocrystal is cylindrical and has a length of 1 cm to 300 cm, preferably a length of 5 cm to 200 cm, and more preferably a length of 10 cm to 150 cm, such as 30 cm, 45 cm, 60 cm, 75 cm, 90 cm, 105 cm, 120 cm, or 135 cm, or any length there in between. Produced ingots may typically have a length in the range 25 cm to 135 cm.

[0040] In a preferred embodiment, the present invention provides a germanium monocrystal according to the first aspect of the invention, wherein said monocrystal is in the form of an ingot.

[0041] In a second aspect, the present invention provides a wafer comprising an N-type germanium monocrystal comprising phosphorus in an amount of at least 1.0 x 1019 / cm3. Also, in a second aspect, the present invention provides an N-type germanium wafer having less than 1.50 surface imperfections per cm2, as determined by Surface Defect Inspection. Preferably, phosphorous is present as single dopant. Preferably, said germanium monocrystal is dislocation free.

[0042] In a preferred embodiment, the present invention provides an N-type germanium wafer according to the second aspect of the invention having less than 1.25 surface imperfections per cm2, as determined by Surface Defect Inspection, more preferably less than 1.10 surface imperfections per cm2, and even more preferably less than 1.00 surface imperfections per cm2. Typically, an N-type germanium wafer according to the second aspect of the invention has between 0.25 and 0.75 surface imperfections per cm2, more typically between 0.30 and 0.60 surface imperfections per cm2, such as about 0.35, 0.40, 0.45, 0.50 or 0.60 surface imperfections per cm2.

[0043] In a preferred embodiment, the present invention provides a germanium wafer according to the second aspect of the invention, comprising phosphorus in an amount of at least 1.1 x 1019 / cm3, or in an amount of at least 1.2 x 1019 / cm3, or in an amount of at least 1.5 x 1019 / cm3, or in an amount of at least 1.8 x 1019 / cm3, or in an amount of at least 2.0 x 1019 / cm3, or in an amount of at least 2.2 x 1019 / cm3, or in an amount of at least 2.4 x 1019 / cm3, or in an amount of at least 2.5 x 1019 / cm3, or in an amount of at least 2.8 x 1019 / cm3, or in an amount of at least 3.0 x 1019 / cm3, or in an amount of at least 3.2 x 1019 / cm3, or in an amount of at least 3.5 x 1019 / cm3, or in an amount of at least 4.0 x 1019 / cm3, or in an amount of at least 4.2 x 1019 / cm3, or in an amount of at least 4.5 x 1019 / cm3, or in an amount of at least 4.7 x 1019 / cm3, or in an amount of at least 5.0 x 1019 / cm3. In a preferred embodiment, the present invention provides a germanium wafer according to the second aspect of the invention, comprising phosphorus in an amount of at most 5.0 x 102° / cm3, or in an amount of at most 4.0 x 102° / cm3, or in an amount of at most 3.0 x 102° / cm3, or in an amount of at most 2.0 x 102° / cm3, or in an amount of at most 1.0 x 102° / cm3. In a preferred embodiment, the present invention provides a germanium wafer according to the second aspect of the invention, comprising phosphorus in an amount of 2.5 x 1019 / cm3to 5 x 1021 / cm3, more preferably in an amount of 5 x 1019 / cm3to 1.0 x 1021 / cm3, and even more preferably in an amount of 5 x 1019 / cm3to 5 x 102° / cm3, or any value there in between.

[0044] In a preferred embodiment, the present invention provides a germanium wafer having a resistivity of less than 10 mQ-cm, and preferably between 0.1 mQ-cm and 10 mfi'Cm, and more preferably between 0.1 mQ-cm and 5 mQ-cm, and even more preferably between 0.5 mQ-cm and 5 mQ-cm. Most preferably, said resistivity is between 0.5 mQ-cm and 4 mQ-cm, such as 0.5 mQ-cm, 1.0 mQ-cm, 2.0 mQ-cm, 3.0 mQ-cm, 4.0 mQ-cm, or any value there in between.

[0045] In a preferred embodiment, the present invention provides a germanium wafer according to the first aspect of the invention, said wafer having a thickness of 25 to 1000 pm. More preferably, said wafer has a thickness of 100 pm to 750 pm, and even more preferably, said wafer has a thickness of about 140 pm, 180 pm, 220 pm, 260 pm, 300 pm, 450 pm, 650 pm or any thickness there in between.

[0046] In a third aspect, the present invention provides a process for the manufacture of an N-type doped germanium monocrystal, comprising the steps of:

[0047] - providing a Czochralski pulling furnace;

[0048] - feeding germanium and phosphorus into the furnace in such relative quantities as to obtain a dopant level of at least 1.0 x 1019 / cm3; and,

[0049] - pulling a crystal.

[0050] Phosphorus may be fed to the furnace together with germanium as a single dopant. This means that phosphorous is fed to the furnace together with germanium as the single dopant and that the presence of any further dopants is excluded. Phosphorus may also be fed as a single dopant to achieve a phosphorus concentration in the germanium crystal in an amount of at least 1.1 x 1019 / cm3, or in an amount of at least 1.2 x 1019 / cm3, or in an amount of at least 1.5 x 1019 / cm3, or in an amount of at least 1.8 x 1019 / cm3, or in an amount of at least 2.0 x 1019 / cm3, or in an amount of at least 2.2 x 1019 / cm3, or in an amount of at least 2.4 x 1019 / cm3, or in an amount of at least 2.5 x 1019 / cm3, or in an amount of at least 2.8 x 1019 / cm3, or in an amount of at least 3.0 x 1019 / cm3, or in an amount of at least 3.2 x 1019 / cm3, or in an amount of at least 3.5 x 1019 / cm3, or in an amount of at least 4.0 x 1019 / cm3, or in an amount of at least 4.2 x 1019 / cm3, or in an amount of at least 4.5 x 1019 / cm3, or in an amount of at least 4.7 x 1019 / cm3, or in an amount of at least 5.0 x 1019 / cm3. Phosphorus may also be fed as a single dopant to achieve a phosphorus concentration in the germanium crystal of at most 5.0 x 102° / cm3, or at most 4.0 x 102° / cm3, or at most 3.0 x 102° / cm3, or at most 2.0 x 102° / cm3, or at most 1.0 x 102° / cm3. Phosphorus may also be fed as a single dopant to achieve a phosphorus concentration in the germanium crystal of 2.5 x 1019 / cm3to 5 x 1021 / cm3, or of 5 x 1019 / cm3to 1 x 1021 / cm3, or of 5 x 1019 / cm3to 5 x 102° / cm3, or any value there in between. Said process may provide an N-type doped, dislocation-free Ge monocrystal according to the first aspect of the invention.

[0051] In one embodiment, said N-type doped germanium monocrystal is dislocation-free.

[0052] In a preferred embodiment, the present invention provides a process according to the third aspect of the invention, whereby phosphorus is fed into said furnace in the form of GeP. This ensures that a phosphorus-doped, monocrystalline, and dislocation-free product is obtained. Using GeP as dopant source is highly preferred over, for example, red or white phosphorus because of safety precautions needed when handling the latter two. Moreover, GeP is available in high purity (99.999%), minimizing the addition of undesired impurities to the melt.

[0053] In a preferred embodiment, the present invention provides a process according to the third aspect of the invention, comprising the additional steps of:

[0054] - cutting the crystal into wafers;

[0055] - grinding the wafers with a coarse grit;

[0056] - chemical-mechanical polishing of the wafers;

[0057] - cleaning the wafers' surface. In a fourth aspect, the present invention provides an electronic or opto-electronic device comprising an n-type doped germanium monocrystal according to the first aspect of the invention.

[0058] In a fifth aspect, the present invention provides vertical-cavity surface emitting lasers or infrared plasmonic sensors comprising an n-type doped germanium monocrystal according to the first aspect of the invention.

[0059] In a sixth aspect, the present invention provides the use of a monocrystalline, dislocation-free germanium monocrystal according to the first aspect of the invention for vertical-cavity surface emitting lasers or infrared plasmonic sensors.

[0060] EXAMPLES

[0061] This process is illustrated in the following examples. The following example is intended to further clarify the present invention, and is nowhere intended to limit the scope of the present invention.

[0062] EXAMPLE 1

[0063] A load of 100 kg of high purity germanium is introduced into a Czochralski furnace, in a graphite crucible, to which 623 g of GeP is added as a dopant.

[0064] This amount of dopant corresponds to a dopant level in the crystal, which varies between 1 and 6 x 1019 / cm3, corresponding to a variation in the resistivity of about 0.8 to 3.7 mfi'Cm. The variation is due to the well-known fact of impurity segregation at the solid interface during crystal growth. As a result, the concentration of phosphorus increases towards the tail of the crystal, causing a lower resistivity there.

[0065] The furnace is first heated to 1000 °C in a nitrogen environment, so as to melt the germanium. Around 725 °C, germanium phosphide dissociates into solid Ge and liquid phosphorus. At 1000 °C, the germanium is also molten, and phosphorus is incorporated into the melt. When the germanium is fully molten, the temperature of the furnace is lowered to 950 °C. After 6 hours, the temperature is stabilized, whereupon a germanium seed crystal is lowered through the shaft of the furnace until it contacts the molten bath. A procedure called "Dash necking", well known to the skilled person, is then executed in order to initiate the growth. More specifically, a thin 5 mm diameter crystal is grown for a length of 150 mm. The crystal is then slowly pulled up at a controlled rate of 10 mm / h so as to increase its diameter to 150 mm. The diameter of the crystal is then stabilized by controlling the heater temperature and / or the pulling rate. This phase is commonly called body growth. A pulling rate of about 10 mm / h is achieved.

[0066] At the end of the body growth, a tail is formed on the crystal by steadily reducing its diameter, whereupon the crystal is detached from the melt. This procedure is useful to minimize the thermal shock as an aid to avoid dislocations. The crystal is pulled up into the shaft of the furnace and slowly cooled to below 50 °C for 28 h. It is then unloaded from the furnace. The crystal is sawn perpendicular to the crystal axis into cylindrical pieces.

[0067] EXAMPLES 2-4

[0068] Wafers were prepared according to the above procedure with different wafer sizes, 4 inch, 6 inch and 8 inch, respectively, as Examples 2-4. This corresponds to wafers having a diameter of about 102 mm, about 152 mm and about 203 mm, respectively. Resistivity was measured on 6 different locations along the length of the crystal using a 4-point probe. The results are reported in Table 1. That the crystal is dislocation- free was verified by X-ray topography (XRT).

[0069] Figure 1(a) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 4 inch wafer according to Example 2. Figure 1(b) shows the result of Surface Defect Inspection using a KLA Tencor's Surfscan of the 4 inch wafer according to Comparative Example 1. The scans shows a comparatively lower amount of surface imperfections for the germanium wafer according to the present invention. Figures 2(a) and 2(b) show the results of Surface Defect Inspection using a KLA Tencor's Surfscan of 8 inch wafers according to Example 4 and Comparative Example 3, respectively. Also here, the germanium wafer according to the invention shows a markedly reduced amount of surface imperfections. Table 1. Ultra-high phosphorus doped germanium wafers.

[0070] Examples Wafer size Resistivity Surface imperfections

[0071] (inch) (mQ-cm) (cm2)

[0072] 2 4 0.87 - 3.02 0.44

[0073] 3 6 1.63 - 3.7 < 0.35

[0074] 4 8 1.75 - 2.97 1.01

[0075] Comparative Wafer size Resistivity Surface imperfections examples (inch) (mQ-cm) (cm2)

[0076] 1 4 n.d. 3.74

[0077] 2 6 n.d. 3.03

[0078] 3 8 n.d. 1.57 n.d. = not determined.

[0079] COMPARATIVE EXAMPLES 1-3

[0080] Ga-doped wafers were provided with different wafer sizes, 4 inch, 6 inch and 8 inch, respectively, as Comparative Examples 1-3. This corresponds to wafers having a diameter of about 102 mm, about 152 mm and about 203 mm, respectively. Resistivity was measured on 6 different locations along the length of the crystal using a 4-point probe. The results are reported in Table 1.

[0081] It is shown that doping a germanium melt with GeP effectively results in low-resistive, n-type germanium crystals with a resistivity ranging between 0.5 and 4.0 mQ-cm. This variation in resistivity is perfectly acceptable for the intended applications. Theoretical calculations further indicate that with these quantities of added dopant, close to all phosphorus atoms are electrically active inside of the germanium lattice.

[0082] Moreover, the results of Table 1 clearly depict a lower appearance of surface defects in the phosphorous doped germanium wafers compared to the gallium doped germanium wafers, as determined Surface Defect Inspection using KLA Tencor's Surfscan. Voids detected typically have a size in the range of about 1 to 10 pm.

[0083] For the obtention of wafers, the crystal can be further processed as follows. The crown and tail of the crystal are cropped and subsequently the crystal is ground to the desired diameter using cylindrical grinding. A flat or notch is then machined on the rounded crystal to mark the crystal orientation. Afterwards, a wire-saw cuts the crystal into individual wafers. These wafers are laser-marked with a unique identification code for tracking purposes. The edge of the wafer is rounded to prevent wafer breakage in further processing. In order to remove thickness variations that are present after sawing, the wafer surface is ground with a course grit. Any subsurface damage induced by this process is removed by chemical etching. Then, chemical-mechanical polishing is applied until a mirror-like surface is obtained. A final clean ensures an "epi-ready" surface for the growth of various epi-layers.

Claims

CLAIMS1. N-type, dislocation-free germanium monocrystal comprising phosphorus as single dopant in an amount of at least 1.0 x 1019 / cm3.

2. Germanium monocrystal according to claim 1 comprising phosphorus in an amount of at least 5.0 x 1019 / cm3.

3. Germanium monocrystal according to claim 1 or 2 comprising phosphorus in an amount of between 5.0 x 1019 / cm3and 5.0 x 102° / cm3.

4. Germanium monocrystal according to any of claims 1 to 3, wherein said monocrystal is cylindrical and has a diameter of 1 cm to 50 cm.

5. Germanium monocrystal according to any of claims 1 to 4, wherein said monocrystal is cylindrical and has a length of 1 cm to 300 cm.

6. Germanium monocrystal according to any of claims 1 to 5, wherein said monocrystal is in the form of an ingot.

7. Germanium monocrystal according to any of claims 1 to 6, having less than 1.50 surface imperfections per cm2, as determined by Surface Defect Inspection.

8. Wafer comprising an N-type, dislocation-free germanium monocrystal comprising phosphorus as a single dopant in an amount of at least 1.0 x 1019 / cm3.

9. Wafer according to claim 8, comprising phosphorus in an amount of at least 2.5 x 1019 / cm3.

10. Wafer according to claim 8 or 9, comprising phosphorus in an amount of between 2.6 x 1019 / cm3and 5.0 x 102° / cm3.

11. Wafer according to any of claims 8 to 10, whereby said wafer has a thickness of 25 to 1000 pm.

12. Wafer according to any of claims 8 to 11 having less than 1.50 surface imperfections per cm2, as determined by Surface Defect Inspection.

13. Process for manufacturing an N-type, dislocation-free germanium monocrystal, comprising the steps of:- providing a Czochralski pulling furnace;- feeding germanium and phosphorus into the furnace in such relative quantities as to obtain a dopant level of at least 1.0 x 1019 / cm3, whereby phosphorus is the single dopant; and,- pulling a crystal.

14. Process according to claim 13, whereby phosphorus is fed into said furnace in the form of GeP.

15. Process according to claim 13 or 14, comprising the additional steps of:- cutting the crystal into wafers;- grinding the wafers with a coarse grit;- chemical-mechanical polishing of the wafers;- cleaning the wafers' surface.

16. Electronic or opto-electronic device comprising an N-type doped, dislocation-free germanium monocrystal according to any of claims 1 to 3.

17. Vertical-cavity surface emitting lasers or infrared plasmonic sensors comprising an N-type doped, disclocation-free germanium monocrystal according to any of claims 1 to 3.

18. Use of a monocrystalline, dislocation-free germanium monocrystal according to any one of claims 1 to 3 for vertical-cavity surface emitting lasers or infrared plasmonic sensors.

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