Abstract:
A compound semiconductor workpiece with reduced defects and greater strength that uses Group II-VI semiconductor nanoislands on a substrate. Additional layers of Group II-VI semiconductor are grown on the nanoislands using MBE until the newly formed layers coalesce to form a uniform layer of a desired thickness. In an alternate embodiment, nanoholes are patterned into a silicon nitride layer to expose an elemental silicon surface of a substrate. Group II-VI semiconductor material is grown in the holes until the layers fill the holes and coalesce to form a uniform layer of a desired thickness. Suitable materials for the substrate include silicon and silicon on insulator materials and cadmium telluride may be used as the Group II-VI semiconductor.

Description:
STATEMENT AS TO RIGHTS IN INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
       [0001]    The present invention was made with governmental support under Contract No. W911NF-07-C-0083 awarded by the United States Army. The government has certain rights in the present invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Mercury cadmium telluride (MCT or Hg x Cd 1-x Te, 0&lt;x&lt;1; hereinafter HgCdTe) is the ideal material choice for fabricating infrared photon detectors because of its band gap tunability over a wide range of wavelengths spanning from 1 μm to over 30 μm, covering the short wave infrared (SWIR) to very long wavelength infrared (VLWIR) spectral regions. It is usually grown on Cd x Zn 1-x Te (hereinafter, CdZnTe) or CdTe/Si substrates. Though CdZnTe is better lattice matched to HgCdTe, the size of the CdZnTe substrates is severely constrained by the present growth technology, particularly in the growth of large and uniform bulk boules. The high cost of CdZnTe wafers prevents the large scale commercialization of HgCdTe-based infrared arrays, resulting in their principal use in military and space applications. Moreover, CdZnTe is rather brittle and difficult to handle. 
         [0003]    Silicon-based alternative substrates promise superior performance and cost in comparison with CdZnTe and have several advantages. First, Si substrates are transparent in the infrared except very near the 0.9 μm end of the range, permitting back-illuminated arrays. Second, a direct coupling is possible between the Si readout of arrays and the Si substrate, allowing the production of very large arrays with a long-term thermal cycling reliability. Third, by growing HgCdTe heterostructures directly on Si-based read-out integrated circuits, one could ultimately fabricate truly monolithically integrated arrays. 
         [0004]    There are, however, certain limitations of heteroepitaxy that limit progress in HgCdTe molecular beam epitaxy (MBE) growth on CdTe/Si substrates. To grow high quality HgCdTe, the first step is to grow a high quality CdTe seed layer on silicon. There is a 19.3% lattice mismatch between CdTe and Si that gives rise to a great deal of strain in the heteroepitaxial system. As a result, a large number of threading dislocations form at the heterointerface, which propagate into the CdTe epilayer. These dislocations subsequently propagate into the HgCdTe layer grown on top of the CdTe layer and play a significant role in limiting HgCdTe device performance, especially in the long wavelength infrared (LWIR) and VLWIR regions. Many research and development groups have mounted significant efforts over the past 20 years to reduce the dislocation density in the CdTe epilayers, yet it remains in the low 10 5 /cm 2 . The technique of using a ZnTe buffer layer between the silicon substrate and the CdTe epilayer has resulted in some improvements. However, the defect density in the subsequently grown HgCdTe layer remains higher than that of epilayers grown on CdZnTe substrates. Selective area epitaxy is another technique that has been applied to mitigate strain-related defects, but no significant success has been achieved because of the macro scale patterned features explored in the epitaxial process. 
         [0005]    Thus, because of the high suitability of HgCdTe in infrared detectors and the difficulties in producing high quality, durable semiconductors containing HgCdTe layers, there is a need for a new method of and structures for joining these layers to more robust, less expensive materials. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to workpieces and methods of forming workpieces that join substrates to at least one Group II-VI semiconductor layer, resulting lower defect densities and increased durability of the semiconductor device. 
         [0007]    According to one aspect of the invention, the inventors teach a method of fabricating a compound semiconductor workpiece that comprises providing at least one substrate material from other than a Group II-VI semiconductor, growing a Group II-VI semiconductor seed layer above the substrate, depositing a photoresist layer on the Group II-VI seed layer, and patterning a plurality of islands having a pitch in the range of 100 to 1000 nm into the photoresist layer. The islands are etched into the Group II-VI seed layer to create Group II-VI semiconductor islands. The substrate and islands may then cleaned and at least one additional Group II-VI semiconductor layer is selectively formed on the Group II-VI semiconductor islands. 
         [0008]    Preferably, the islands coalesce to form a uniform layer with reduced dislocations. More preferably, the Group II-VI semiconductor comprises CdTe. 
         [0009]    According to another aspect of the invention, the inventors teach a method of fabricating a compound semiconductor workpiece that comprises providing at least one substrate made from other than Group II-VI material, forming at least one antireflective coating onto the substrate, forming a photoresist layer above the substrate, and patterning the photoresist layer to leave spaced-apart islands of photoresist on the antireflective coating, the islands having a pitch in the range of about 100 nm to about 1000 nm. The antireflective coating and substrate are etched to leave islands or pillars of substrate and antireflective coating, each island having a lower end proximate to the substrate and an upper end remote from the substrate. The substrate may be cleaned and at least one Group II-VI semiconductor layer is selectively deposited on the islands. The growth process may be permitted to continue so the islands coalesce to form a uniform layer. 
         [0010]    More preferably, the step of depositing at least one Group II-VI semiconductor layer on the islands comprises forming a Group II-VI nucleation layer at the highest nucleation temperature for the Group II-VI semiconductor on silicon, increasing the first temperature to a second temperature above the highest nucleation temperature for the Group II-VI semiconductor on silicon, thereby desorbing portions of the nucleation layer, decreasing the second temperature to a third temperature below the highest nucleation temperature of the Group II-VI semiconductor on the Group II-VI seed layer, but above the highest nucleation temperature for the Group II-VI semiconductor on silicon, and selectively forming Group II-VI semiconductor on the islands. 
         [0011]    According to another aspect of the invention, the inventors teach a compound semiconductor workpiece that includes at least one substrate material made from a material other than a Group II-VI semiconductor, a plurality of spaced-apart islands having a pitch of from approximately 100 nm to approximately 1000 nm formed from a Group II-VI semiconductor seed layer, each island having a lower end proximate to the substrate and an upper end remote from the substrate, and at least one additional Group II-VI semiconductor layer formed only on the upper end of the islands. Preferably, the substrate is silicon on insulator (SOI). 
         [0012]    In another aspect of the invention, the inventors teach a compound semiconductor workpiece comprising a substrate made of material other than Group II-VI semiconductor material and a monolithic layer of Group II-VI semiconductor material disposed above the substrate where the layer has a predicted dislocation density of less than approximately 5×10 5 /cm 2 . 
         [0013]    According to another aspect of the invention, a semiconductor device comprises a substrate made of material other than Group II-VI semiconductor material and a monolithic layer of Group II-VI semiconductor material disposed above the substrate, the layer having a predicted dislocation density of less than approximately 5×10 5 /cm 2 . 
         [0014]    According to another aspect of the invention, the inventors teach a method of fabricating a compound semiconductor workpiece comprising providing at least one substrate material having an upper face of elemental silicon, forming at least one silicon nitride layer on the upper face of the substrate, depositing at least one photoresist layer above the silicon nitride layer, and patterning a plurality of holes into the photoresist layer having a pitch in the range of 100 nm to 1000 nm. At least the silicon nitride layer is etched and Group II-VI semiconductor material is filled into at least the plurality of holes. 
         [0015]    Preferably, responsive to said step of growing Group II-VI semiconductor material in the holes, the Group II-VI semiconductor material coalesces to form a uniform Group II-VI layer with reduced dislocations. 
         [0016]    According to another aspect of the invention, the inventors teach a compound semiconductor workpiece, including at least one substrate material with an elemental silicon face, at least one silicon nitride layer grown on the substrate, a plurality of holes etched into at least the silicon nitride layer having a pitch in the range of 100 to 1000 nm, and at least one Group II-VI semiconductor layer formed in the plurality of holes. The holes may be etched into one or more moiré patterns. 
         [0017]    Preferably, the semiconductor structure further includes a passivating layer. Even more preferably the structure also includes at least one buffer layer formed to adjoin the Group II-VI semiconductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Further aspects of the invention and their advantages can be discerned in the following detailed description, in which like characters denote like parts and in which: 
           [0019]      FIG. 1  is a first schematic block diagram showing a first method of fabricating a semiconductor workpiece; 
           [0020]      FIG. 2  is a series of successive highly magnified schematic isometric views of a workpiece showing a process for fabrication of nanoislands followed by epitaxial lateral overgrowth; 
           [0021]      FIGS. 3A and 3B  show perspective SEM images of 500 nm pitch grating (left) and 360 nm pitch grating (right); 
           [0022]      FIG. 4  is a photomicrograph showing a highly magnified cross-sectional SEM image of nanoislands fabricated without an anti-reflective coating (ARC); 
           [0023]      FIG. 5  is a highly magnified cross-sectional SEM image showing nanoislands fabricated with an ARC; 
           [0024]      FIG. 6  is a plan view of a workpiece showing developed photoresist after the post exposure bake; 
           [0025]      FIGS. 7A and 7B  are microphotographs showing top and perspective views of CdTe islands on silicon; 
           [0026]      FIG. 8  is a microphotograph taken from the top of a nanoisland array, showing selective layer deposition after etching; 
           [0027]      FIGS. 9A and 9B  are microphotographs showing top views of partially coalesced CdTe; 
           [0028]      FIG. 10  is an AFM image of a nanopatterned CdTe buffer layer substrate with the section analysis; 
           [0029]      FIG. 11  is an AFM image of selectively grown CdTe on the nanopatterned CdTe buffer layer shown in  FIG. 10 , accompanied by a section analysis showing the thickness of the film; 
           [0030]      FIG. 12  is an AFM image of regrown CdTe showing regions of lateral epitaxial overgrowth; 
           [0031]      FIG. 13  is a schematic block diagram showing a second method of fabricating a semiconductor workpiece; 
           [0032]      FIGS. 14C and 14D  show a series of schematic isometric views of a workpiece showing a process for fabrication of nanoislands followed by epitaxial overgrowth; 
           [0033]      FIGS. 15A and 15B  show plan view SEM images of CdTe selectively grown on nanopatterned SOI substrates; 
           [0034]      FIG. 16  is an AFM section analysis of a sample of CdTe selectively grown on SOI having 1 μm pitch nanopillars; 
           [0035]      FIG. 17  is an x-ray diffraction graph (intensity v. 2θ) of selectively CdTe grown on 1 micron pitch nanopatterned SOL 
           [0036]      FIGS. 18A and 18B  are plan view photomicrographs of an array of CdTe nanoislands on SOI showing coalescence of CdTe; 
           [0037]      FIG. 19  is a block diagram showing a method of fabricating a semiconductor workpiece; 
           [0038]      FIG. 20A  (top left) is a schematic diagram of hexagonal moirés; 
           [0039]      FIG. 20B  (top right) is a photomicrograph of hexagonal moirés etched into a substrate; 
           [0040]      FIG. 20C  (bottom left) is a photomicrograph of GaN layers selectively grown in hexagonal moirés; 
           [0041]      FIG. 20D  (bottom right) is a photomicrograph of GaN layers that have coalesced into crack-free layers; and 
           [0042]      FIG. 21  is a schematic of interferometric lithography set up. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    In the following discussion, reference is made to  FIGS. 1 and 2 . In a first aspect of the invention, the inventors disclose a method of fabricating a Group II-VI semiconductor structure with a reduced number of dislocations comprising providing (step  102 ,  FIG. 1 ) at least one substrate material  202  ( FIG. 2 ) made from material other than Group II-VI semiconductor. Suitable substrates include, but are not limited to, silicon, sapphire, and silicon on insulator (SOI) substrates. A monolithic seed layer  204  of Group II-VI semiconductor material is grown above the substrate  202 . 
         [0044]    Prior to growth of the seed layer, silicon (211) wafers are subjected to an ex-situ RCA cleaning procedure to obtain clean atomic surfaces for performing MBE growth. The RCA procedure has three major sequential steps: performing an organic clean by removing insoluble organic contaminants with a 5:1:1 H 2 O:H 2 O 2 :NH 4 OH solution, oxide stripping by removing a thin silicon dioxide layer using a diluted H 2 O:HF solution where metallic contaminants may accumulated as a result of the above step, and ionic cleaning by removing ionic and heavy metal atomic contaminants using a solution of 6:1:1 H 2 O:H 2 O 2 :HCl. 
         [0045]    Following the ex-situ cleaning, each silicon wafer is loaded into an introduction chamber and transferred into a preparation chamber where it is prebaked for several hours. The sample is then transferred into the growth chamber under high vacuum and the temperature is ramped to approximately 1050° C. Each sample is left at that temperature for approximately thirty seconds to desorb any oxide on the silicon surface and the samples may then be subjected (step  104 ) to an arsenic flux while ramping down the temperature to 400° C., leaving a thin passivating layer  203  of arsenic adjacent to the substrate. A thin buffer layer  205 , preferably ZnTe, may be grown (step  106 ) remote from the substrate at 330° C. to preserve the (211) orientation. 
         [0046]    This buffer layer  205  is annealed (step  122 ) at 460° C. for 10 minutes, followed by the growth (step  109 ) of a CdTe epilayer  204  by molecular beam epitaxy (MBE) at a substrate temperature of 390° C. for approximately 10 hrs. The growth rate of CdTe is approximately 1 μm/hr, which results in a CdTe thickness of approximately 10 μm. Double crystal rocking curve (DCRC) measurements performed on samples made according to this procedure indicated high quality CdTe. 
         [0047]    After forming the CdTe epilayer  204 , a photoresist layer  207  is deposited (step  110 ) on the Group II-VI epilayer  204  and baked at 115° C. for 3 minutes. The photoresist is cleaned in acetone for 10 minutes, rinsed in isopropyl alcohol for 5 minutes, rinsed in deionized water for 5 minutes, and blown dry with nitrogen. The substrate is then baked on a contact hot plate at 65° C. for 60 seconds to desorb any moisture. A photoresist adhesion promoter, such as hexamethyl disilane (HMDS), is spun on several samples at 4000 rpm for 30 seconds and the samples are then baked at a temperature of 95° C. for 60 seconds. 
         [0048]    A positive photoresist is spun (step  110 ) onto the samples at 3000 rpm for 30 seconds and had a thickness of 500 nm for these conditions as per the manufacturer&#39;s specifications. The samples are baked at 95° C. for 3 minutes and transferred to the interferometric lithography lab, where the equipment is set up to pattern two-dimensional features of a desired size, shape, and pitch. 
         [0049]    Interferometric lithography (step  112 ) is a mask-less and relatively simple method of forming periodic arrays of sub-micron structures on the surface of a substrate that uses interference between coherent laser beams to define a pattern. See  FIG. 21 . There is a large parameter space that needs to be explored when fabricating nanoscale features: the feature density and dimensions, pattern symmetry, the underlying layer structure and also the grouping of the nanoelements. Interferometric lithography is a unique approach that in principle allows all these requirements to be controlled. In one set of experiments, the features were set up on a 500 nm pitch with laser emission at 60 Hz and 50 mJ energy. The samples are exposed in one direction and rotated 90° for a second exposure. After exposure, they are baked at 110° C. for 60 seconds and then developed in a developer to pattern  112  the features  212 . 
         [0050]    We have obtained very uniform 2-dimensional islands  212  of photoresist on a 500 nm pitch on the thin CdTe seed layers, as seen from the perspective view SEM images in  FIGS. 3A and 3B . We further characterized these features with cross-sectional SEM to evaluate the lateral dimensions, sidewall profile and the morphology of the photoresist. A cross-sectional SEM image is shown in  FIG. 4 . 
         [0051]    The cross-sectional SEM image shows approximately 200 nm diameter photoresist islands  212  (approximately 450 nm high) on a 500 nm pitch. Expected pitches may range from approximately 100 nm to approximately 1000 nm and heights of the photoresist islands may range from 15 to approximately 500 nanometers. The sidewalls of the islands are serrated as a consequence of standing wave patterns that are generated with the interference between the incident and the reflected beam from the surface of the sample underneath the photoresist layer. An antireflective coating layer is usually used to absorb the incident laser beam to avoid the standing wave patterns. Although this is desirable to obtain good sidewall profiles, during the pattern transfer stage one must etch the antireflective coating and then etch the underlying layer. We have explored the use of antireflective coating on a few test pieces of CdTe/Si to fabricate two-dimensional islands. Cross-sectional SEM images obtained from the use of antireflective coating underneath the photoresist are shown in  FIG. 5 . 
         [0052]    The SEM image in  FIG. 5  shows that the standing wave patterns are reduced with the use of an antireflective coating on the samples. The property of the antireflective coating that reduces the standing wave patterns is its low refractive index compared to the surface underneath, hence it doesn&#39;t allow the reflected wave from the surface to transmit through. Generally, it is very difficult to fabricate nanoscale patterns in photoresist on a silicon surface without an antireflective coating but CdTe tends to act as an antireflective coating because the refractive index of CdTe is smaller than silicon, thereby reducing the standing wave patterns. 
         [0053]    We have successfully fabricated 150-200 nm diameter islands of photoresist on CdTe seed layers using interferometric lithography. We believe this is the first attempt to fabricate nanoscale features on CdTe epilayers grown on silicon (211) substrates. Islands  212  with diameters from approximately 20 nm to approximately 500 nm may be used. 
         [0054]    Returning to  FIGS. 1 and 2 , to effectively transfer the nanoscale features into the CdTe epilayer  204 , a reliable etch technique with good control over the etch rate, surface morphology and the sidewall profile is required. Wet etching with Br/methanol and Br/lactic acid is a commonly used approach for etching CdTe. However, wet etching is isotropic in nature and limits the feature dimensions that can be etched due to the undercut involved with the process. This requires a dry etching technique that can provide the required resolution of etch to the sub micron dimensions. 
         [0055]    Dry etch technology offers the ability to produce anisotropic etch profiles, resulting in more photodiodes per unit area. In addition to an anisotropic nature that is critical for accurate dimensional control, dry etching provides greater uniformity, smaller amounts of hazardous waste and in situ vacuum processing capability. Dry etching of CdTe with CH 4 /H 2  has been reported and the same chemistry is also being used for the dry etch of HgCdTe. Different dry etch mechanisms have been developed so far for HgCdTe and there is still extensive research going on to develop the best possible technique. Plasma etching with H 2 , Ar and CH 4  has been reported as the extensively used etch chemistry for HgCdTe. Several plasma etch techniques such as reactive ion etching (RIE), electron cyclotron resonance (ECR) and inductively coupled plasma etching (ICP) have been explored for dry etching of II-VI compounds. 
         [0056]    Inductively coupled plasma (ICP) etching is a technique that provides high-density plasma at low pressure. The ICP technique has been reported to result in superior control over etching large area wafers and reproducibility and involves coupling radio frequency (RF) power into a vacuum chamber from an inductive coil while biasing a stage with an independent RF power. This allows separate control over ion energy and ion density during plasma etching or deposition. In the following example, we present the details of the ICP etch process developed for etching CdTe mesas. 
       Example 1 
       [0057]    A 3-inch Si (211) wafer with approximately 5 μm CdTe grown by MBE was spun with photoresist SPR 1818 at 3000 rpm for 30 seconds and baked at 115° C. for 3 minutes. This wafer was then diced into 20 mm×20 mm pieces and the photoresist was stripped away in acetone (this photoresist layer was used to protect the CdTe epilayer from being damaged during the dicing of the wafer). The 20 mm×20 mm samples were then spun with a negative photoresist SU-8 @ 3000 rpm for 30 seconds and were subjected two different baking steps: 60 seconds @ 65° C. and 120 seconds @ 95° C. An iron oxide mask with a 320×256 array format having pixel regions of 5 μm on a 30 μm pitch was used on a MJB-3 mask aligner to perform contact lithography. The exposures were performed for 11 seconds and the samples were then baked in two steps: 60 seconds @ 65° C. and 120 seconds @ 95° C. These were then developed for 12 seconds in the SU-8 developer solution. 
         [0058]    Optical microscope images of the photoresist are shown in  FIG. 6 , which shows rounded mesas  212  due to a short over-developing after the post exposure bake. This is desirable in this case because it results in cylindrical photoresist pillars, which will finally lead to circular CdTe features after etching. The samples were rinsed in isopropyl alcohol after developing because the negative photoresist used is sensitive to aqueous solutions. 
         [0059]    ICP etching (step  114 ) of CdTe was performed on a Oxford instruments Plasmalab 100 ICP chamber which has six gas lines CH 4 , H 2 , N 2 , O 2 , Ar and SF 6 , a temperature controller with helium back flow, a custom gas ring to feed gases directly over a 3-inch wafer, and two separate power supplies for controlling plasma density and ion energy. Process development was performed on various samples with various etch process parameters, source gas flow rates, RF power, ICP power and chamber pressure, to optimize the etch conditions for reproducibility. 
         [0060]    Feedback for tuning the etch process (step  114 ) was obtained from profilometry, optical microscopy and SEM. CH 4  and Ar were used as source gases in the plasma chamber with flow rates in the range 0-15 sccm and 0-20 sccm respectively while RF power and ICP power in the range 0-30 Watts and 0-750 Watts, respectively, were used. A chamber pressure of 5 mTorr with a helium back flow of 10 Torr (used to maintain the sample at room temperature in the plasma) was used to maintain an etch rate of approximately 0.22 μm/min. The etch duration was timed to approximately 25 minutes to etch the CdTe layer  204  and layers  207 ,  205 , and  205  down to the silicon substrate  203 . 
         [0061]      FIGS. 7A and 7B  show a plurality of islands  214  in a two dimensional array having lower ends proximate to the substrate and upper ends remote from the substrate. These figures also show a very clean surface  700  after etching, which is indicative of the silicon substrate underneath the CdTe epilayer  204 . The sizes of the mesas  214  obtained after etching are comparable to the features on the mask used for photolithography, which indicates good control of the etch process in retaining the feature dimensions. Cross-sectional SEM was performed to characterize the sidewall profile and the etch depth obtained on the samples. 
         [0062]    These SEM images show a etch depth of between approximately 50 nm and approximately 300 nm, which is within the wider range of approximately 15 nm to approximately 500 nm. The aspect ratio of the features is 1:1, which is a significant result in etching CdTe with a dry etch process. 
         [0063]    We had previously observed that the photoresist had hardened after the etch process and it was difficult to clean from the surface of the CdTe. This could be a potential problem during MBE growth since the carbon associated with the polymer in the photoresist will contaminate the MBE chamber when the sample is loaded into the chamber. Also, surfaces with photoresist residue are not ideal surfaces for the onset of epitaxy. As shown in  FIG. 1 , it is preferred to clean (step  116 ) the surface  700  after the etch process using oxygen plasma to strip the resist from the surface. We found that the surfaces were almost completely cleaned of photoresist after 10 minutes of subjecting the etched surfaces to oxygen plasma. SEM was performed on a sample etched with ICP before the photoresist was cleaned and the same sample cleaned with oxygen plasma for 10 minutes to characterize the surfaces after the etch process. 
         [0064]    Molecular Beam Epitaxy (MBE) is then used to regrow CdTe on nanopatterned CdTe seed islands  214 . In the following example, we present the details of the regrowth of CdTe layers on the islands. 
         [0065]    Prior to growth, the samples were subjected to an ex-situ cleaning process (part of step  116 ) to remove the oxidized CdTe surfaces. We determined that the nanopatterned samples are sensitive to this cleaning process because the usual Br/Methanol etch used for CdTe/Si substrates resulted in nanopillar lift-off. This is due to the high etch rate of the material with this solution. The wet etch process rapidly damaged the substrates through undercutting since the nanopillars are only approximately 200 nm in diameter and of similar thickness. We improved the cleaning process by using a diluted solution of HBr:DI:H 2 O 2  (1:50:10) to treat the nanopillar surfaces. 
         [0066]    Samples of 10 mm×10 mm size were loaded into the MBE chamber and subjected to an tellurium dioxide desorption process at a temperature of between 260° C. and 400° C. Following desorption, CdTe was grown at a temperature of 510° C. 
         [0067]    Reflection high energy electron diffraction (RHEED) analysis from the MBE process shows patterns indicative of single crystal (211) CdTe being present after growth. This shows that single crystal CdTe was regrown  118  on the nanopillars  214 . Moreover, (211) is the preferred orientation of the CdTe layer that will be needed for the subsequent growth of HgCdTe. 
         [0068]    SEM analysis was performed to characterize the regrown layers.  FIG. 8  shows that selective CdTe growth was obtained from the regrowth process on the nanopatterned CdTe seed layer samples. Selectivity of the growth process appears to highly homogeneous on the sample, which is important for subsequent epitaxial lateral overgrowth leading to coalescence, as shown in  FIGS. 9A and 9B . The high degree of uniformity in the periodic nanoscale features on the samples and the optimization of the MBE growth process have led to the successful selective growth  118  of single crystal CdTe (211) on the nanopatterned CdTe substrates. 
         [0069]    Following the homogeneous selective growth  118  of CdTe on nanopatterned CdTe seed layers, the selective growth of CdTe on nanopatterned CdTe seed layers was accomplished by the MBE growth technique. This is, to our knowledge, the first report of nanoscale selective CdTe growth. Initially, the MBE growth conditions were calibrated to obtain the highest nucleation temperature of CdTe on silicon. Following this, CdTe was regrown on the nanopatterned CdTe seed layers at a temperature slightly higher than the highest nucleation temperature of CdTe. This process enables the nucleation of CdTe only on CdTe. No nucleation was seen on the silicon surface. SEM characterization of the grown samples has shown the selectivity of the growth with an increase in the diameter of the islands as compared to before growth. AFM characterization was performed to characterize the regrown CdTe since it would influence the next stage of the nanopatterned growth, which was the lateral epitaxial overgrowth (LEO)  120 . An AFM image of the substrate and nanoislands  212  before the selective growth of CdTe is shown in  FIG. 10 . The thickness of the CdTe nanopillars  214  before the growth was approximately 53 nm, as indicated by the vertical distance in the line scan analysis of the AFM image shown in  FIG. 10 . 
         [0070]    An AFM image of a selectively grown CdTe sample on the nanopatterned CdTe nanopillars  214  is shown in  FIG. 11 . The thickness of the CdTe after 15 minutes of MBE regrowth was approximately 186 nm. This corresponds to a growth rate of approximately 1.2-1.4 Å/sec. We did not observe any change in the thickness between the CdTe islands, which indicated that the CdTe grown during the regrowth process was indeed selective. 
         [0071]    We observe in  FIG. 12  that a ‘nanobridge’  1200  formed between two CdTe islands  214 . The horizontal distance between the two islands  214  joined by bridge  1200  is approximately 132 nm. It should be noted that the size of the islands  214  before growth was approximately 200 nm and the pitch was approximately 500 nm. This corresponds to a separation of approximately 300 nm between the islands before growth. Based on the size of the nanobridge  1200 , we see that the islands were almost coalescing at the stage shown in  FIG. 12 . The increase in diameter has reduced the separation between the islands and the high degree of uniformity in the growth of the islands indicates a path towards coherent coalescence, which was essential to form a complete film on the substrate. AFM measures a vertical island height of approximately 122 nm, but this was probably an underestimate since it is quite possible that the AFM tip is unable to touch the bottom of the substrate due to the small separation between the islands. 
         [0072]    Lateral epitaxial overgrowth (step  120 ) may be allowed to continue until a completely coalesced layer  220  of CdTe results, with fewer dislocations than have been achieved by prior manufacturing techniques. Additionally, other layers and structures may be formed. Finally, one or more infrared detection layers  222 , preferably formed of HgCdTe, may be grown  122  on the coalesced CdTe layer. The band gap of these layers may be tuned so they detect wavelengths spanning approximately 1 μm to at least approximately 30 μm, effectively covering the short wave infrared to the very long wave infrared spectral regions. 
         [0073]    In another embodiment of the invention, the inventors disclose a method of fabricating a Group II-VI semiconductor structure without the use of the monolithic Group II-VI semiconductor seed layer described above. The steps of this method are illustrated in  FIG. 13 . Schematic sectional views showing different stages in the process are shown in  FIGS. 14A-14D . 
         [0074]    Suitable substrates have an upper surface of elemental Si, and include silicon and silicon on insulator (SOI) substrates. After choosing (step  1302 ) the substrate  1400 , an antireflective coating (ARC)  1402  is deposited (step  1308 ) is to and then to deposit (step  1310 ) a layer of photoresist  1404 . The ARC may have a thickness of between approximately 150-160 nm and the photoresist a thickness of approximately 250 nm. Optionally, a passivating layer  1405 , preferably of arsenic, and buffer layer  1407 , preferably of zinc telluride, may be formed (step  1304  and step  1306 ) before applying the photoresist layer. The following example describes in detail the procedures used. 
       Example 2 
       [0075]    SOI substrates 1400 (20 mm×20 mm pieces) were subjected to a chemical rinse in piranha and then deionized water. A layer  1402  of ARC (XHRI-16) was spun (step  1308 ) on the samples at 4000 rpm for 30 seconds and then hard baked at a temperature of 175° C. for 3 minutes. This process resulted in an ARC thickness of 150 to 160 nm. The bake allowed the ARC  1402  to sustain beam exposure without cracking The ARC  1402  was an organic polymer that serves to avoid standing wave patterns that would otherwise result from the interference between the incident laser beam and the reflected beam from the substrate. 
         [0076]    The substrates were cooled for 60 seconds and then spun (step  1308 ) with a positive photoresist  1404  at 4000 rpm for 30 seconds. Subsequently, they were soft baked at 95° C. for 3 minutes. The SOI samples were exposed to ultraviolet (UV) radiation for 15 sec and then rotated by 90 degrees for a second exposure to obtain 2-D features. Following the UV exposure, the samples were developed (step  1312 ) in a MF702 developer for 75 seconds. Two-dimensional pillars  1406  ( FIG. 14B ) in photoresist were obtained from this process with feature dimensions between 100 nm to 1000 nm. Three different pitch (center to center spacing) values of 360 nm, 500 nm and 1000 nm were used for fabricating multiple geometry samples. Other pitches ranging from approximately 100 nm to approximately 1000 nm may be used. 
         [0077]    Following the interferometric lithography process (step  1312 ), reactive ion etching (step  1314 ) was used to transfer (step  1314 ) the nanoscale patterns into the substrate  1402  ( FIG. 14C ). Reactive ion etching (step  1314 ) offers precise control over the etch process with good anisotropy and selectivity of etch. The samples were then cleaned (step  1316 ) in piranha solution to strip the ARC  1402  and the photoresist  1404 . 
         [0078]    After etching and cleaning the substrate, a Group II-VI semiconductor layer, preferably CdTe, is formed  1318  on the nanoscale patterns. Example 3 below provides specific details on the procedures and process conditions. 
       Example 3 
       [0079]    A custom holder suitable for holding 6 pieces of 10 mm×10 mm and 2 pieces of 20 mm×20 mm size was used for the CdTe growth on silicon nanopillars  1408 . The samples were subjected to RCA cleaning and blown dry with nitrogen before being loaded into the introduction chamber and transferred into the preparation chamber, where they were prebaked at 500° C. for 8 hours. The samples were then transferred into the growth chamber under high vacuum and the temperature was ramped to 1050° C. The samples were left at that temperature for duration of 30 seconds to desorb the oxide on the silicon surface and subjected to an arsenic flux while ramping down the temperature to approximately 400° C. A CdTe nucleation layer  1410  was deposited for approximately two minutes at a temperature of between approximately 420° C. and approximately 470° C., preferably 440° C. The as-grown layer was then desorbed by ramping the temperature to between approximately 660° C. and 710° C., preferably 680° C., in approximately 20 minutes. An epilayer  1412  was grown (step  1318 ) at approximately a temperature between approximately 480° C. and approximately 540° C., preferably 510° C., for approximately 30 minutes. In the conventional growth process, the as-grown nucleation layer was not desorbed before the subsequent growth of the epilayer. Plan view SEM was used as an initial characterization step to observe the selectivity of CdTe on the islands. SEM images of the samples are shown in  FIGS. 15A and 15B . 
         [0080]    SEM data shows no CdTe growth on the silicon-dioxide growth mask  1414 , which is indicative of the selectivity of CdTe to silicon vs. silicon dioxide. This high temperature process desorbs the CdTe deposited on the silicon islands and the polycrystalline material deposited on the silicon dioxide growth mask. However, since the bond energy between silicon and CdTe was higher than that of silicon dioxide and CdTe, all the polycrystalline material deposited on the silicon dioxide mask was desorbed at this temperature while some CdTe remains bonded to the silicon surfaces on the nanopillars  1416 . When epilayer growth was subsequently performed, CdTe grows on top of the silicon islands, resulting in the selective growth of CdTe. To further characterize selectivity and the thickness of the CdTe obtained on the nanopillars. AFM images of the characterized samples are shown in  FIG. 16 . 
         [0081]    AFM data for the sample ( FIG. 16 ) shows CdTe a thickness of 46 nm. Other pitches showed a thickness of between 34 nm and 46 nm. It can be seen that the CdTe nucleated at the center of each island  1416  while the periphery of the island  1408  is devoid of CdTe, which was probably due to the high desorption rate of CdTe at the edges of the island. 
         [0082]    X-ray diffraction (XRD) measurements were performed on the selective CdTe grown on the nanopatterned SOI substrates to analyze the crystallographic orientation of the CdTe. The ultimate goal of the selective epitaxy process was to obtain single crystalline CdTe on the nanopatterned SOI substrates.  FIG. 17  shows the XRD results of the characterized samples. 
         [0083]    X-ray diffraction data on the substrate shows distinctive peaks corresponding to the silicon (100) orientation and CdTe (111) orientation, which indicated that the selectively grown CdTe is single crystalline and had the orientation that is expected on Si (100) substrates. These results validate that the MBE growth technique can be used to achieve selective epitaxy of CdTe on patterned substrates and obtain single crystalline epitaxial material. 
         [0084]    Following the selective growth of CdTe, we extended the epitaxy process to achieve lateral epitaxial overgrowth (LEO)  1320  of CdTe on the nanopillars. LEO is a technique that has been applied to various heteroepitaxial systems, especially III-V semiconductor materials, to reduce threading dislocation densities. LEO employs the growth of an epilayer from the window regions of growth mask patterned trenches. After the epilayer has reached the top of the trench, it was overgrown  1320  on the growth mask, thereby reducing the dislocation density. Finally, the overgrown regions coalesced  1320  to form a uniform layer  1412  upon which device fabrication can be performed. Example 5 provides more detail on the process conditions of the deposition. 
       Example 5 
       [0085]    CdTe was initially nucleated on nanopatterned compliant SOI substrates at a temperature of 440° C. It was then desorbed at a temperature of 680° C. for 5 minutes. After CdTe desorption producing CdTe-topped nanopillars  1416  as seen in  FIG. 14C , an epilayer  1412  was grown at the higher temperature of 510° C. for 60 minutes. A schematic sectional view is shown in  FIG. 14D . We observed that the CdTe grew selectively on the silicon nanoislands  1408  and then lateral overgrowth was obtained (even with a symmetric growth ratio) followed by coalescence of CdTe. The pitch between the nanoislands  1408  is of the order 500 nm. When the thickness of the grown CdTe  1412  reaches a scale comparable to the spacing between the nanoislands, it coalesced. Previous work on LEO of CdTe was performed on trenches 5 μm wide, which required significant lateral growth to achieve coalescence. In the case of nanopatterned substrates, we showed that it was possible to achieve coalescence even with the growth of a thin CdTe epilayer. The achieved coalescence is non-homogeneous in the images shown in  FIGS. 17A and 17B  at this time, as can be seen from the SEM images. This was probably due to the non-uniformity of temperature on different regions of the sample. 
         [0086]    As with the first method, once the Group II-VI semiconductor layer  1420  has formed, the method may further comprise depositing  1322  at least one HgCdTe layer  1430  above the substrate for use in infrared photon detectors. 
         [0087]    In another embodiment of the invention, the inventors teach a method of fabricating a Group II-VI semiconductor structure on a substrate that reduces stress within the semiconductor by patterning holes into the substrate. Thermal mismatch arises due to different coefficients of expansion of the various materials involved and increases the internal stress within the semiconductor. If the internal stresses become too high, the layers separate or crack. 
         [0088]    In the following discussion, reference is made to FIGS.  19  and  20 A-D. Suitable substrates for the structure include silicon, SOI, and sapphire. After selecting (step  1902 ) a substrate, at least one silicon nitride layer is formed (step  1910 ) above the substrate. Several optional layers may be deposited as well. First, at least one passivation layer, preferably arsenic, may be deposited (step  1904 ) adjacent to the substrate. Second, at least one buffer layer, preferably ZnTe, may be deposited (step  1906 ) as well. Third, at least one Group II-VI semiconductor layer, preferably CdTe, may be deposited (step  1908 ) as well. 
         [0089]    After deposition (step  1910 ) of the silicon nitride layer, at least one photoresist is deposited (step  1912 ) above the silicon nitride layer and a plurality of holes having at least one diameter is patterned (step  1914 ) into at least the photoresist layer. The pitches of the holes are at least approximately 100 nm and the diameters of the holes are diameters greater than 50 nm. 
         [0090]    Patterning (step  1914 ) of the holes into the photoresist is done by interferometric lithography. The pattern is then etched into at least the silicon nitride layer  1916 , preferably with a reactive ion etch or an inductively coupled plasma etch. The substrate may be cleaned (step  1918 ), preferably with oxygen plasma as discussed above to remove any remaining photoresist. At least one Group II-VI semiconductor layer, preferably CdTe, is then filled (step  1920 ), preferably by molecular beam epitaxy, into at least the plurality of holes. Preferably, the at least one Group II-VI semiconductor material coalesces (step  1922 ) to form at least one uniform layer. Finally, at least one additional layer of HgCdTe may be formed (step  1924 ) above the uniform layer. 
         [0091]    As above, the step of depositing (step  1920 ) the Group II-VI semiconductor material may be a several step process. In the first step, a Group II-VI nucleation layer is deposited (part of step  1920 ) at a first temperature. Second, the temperature is ramped, or increased, (part of step  1920 ) to a second temperature, thereby desorbing the nucleation layer. Third, the temperature is decreased (part of step  1920 ) to a third temperature and at least one additional layer of Group II-VI semiconductor is then deposited (part of step  1920 ). 
         [0092]    As shown in  FIGS. 20A-20D , the plurality of holes may be arranged into one or more geometric shapes, or moirés, which may be spaced apart from each other as shown in  FIGS. 20A and 20B . The shapes may be octagonal or hexagonal as shown in  FIG. 20A  and the distances between the shapes include distances of between approximately 2 μm to approximately 20 μm. The shapes may have a width of greater than approximately 3 μm across. Similar work has been done in Group II-V semiconductors, the results of which are shown in  FIGS. 20C and 20D . 
         [0093]    In another aspect of the invention, the process described above for the formation of uniform Group II-VI semiconductor layers may be used to form arrays of cells that act as pixels for infrared detectors. The nanoscale features in each pixel is a significant improvement over the prior art, since it provides greater detail and a large amount of redundancy in the system in the event of failure of a number of the pixels. As with the previous embodiments, at least one layer of infrared detection material, preferably HgCdTe, may be deposited above the uniform layer of Group II-VI semiconductor and may cover wavelengths from approximately 1 μm to approximately 30 μm. 
         [0094]    While illustrated embodiments of the present invention have been described and illustrated in the appended drawings, the present invention is not limited thereto but only by the scope and spirit of the appended claims.