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Definitions

• This invention is in the field of materials , and more specifically relates to the conversion of amorphous materials , including amorphous semiconductor films , to crystalline materials .
• the Nd:YAG laser has been found to be particularly desirable with silicon films because of its relatively high overall power efficiency, its high power-output, and because it is well matched to the absorption characteristics of an amorphous silicon film since radiation is absorbed substantially uniformly across a thickness of about 10pm.
• Large silicon crystallites are obtained after scanning, and it is even possible by our method to use a shaped laser spot, such as a slit, to provide a preferred crystalline orientation within the film.
• other semiconductors and other lasers can be used in our method, as long as the laser wavelength and semiconductor are suitably matched and as long as the laser beam provides sufficient energy to improve the crystalline properties of the scanned film.
• our invention comprises the improvement in converting amorphous material to a more crystalline state by moving an energy beam across the material under conditions which provide continuous, controlled motion of the crystallization front.
• One aspect of controlling the crystallization front relates to the scanning rate of the energy beam, which in the case of this invention is typically higher for most materials than scan rates previously employed with the same materials.
• the crystallization efficiency of these increased scan rates is surprising particularly in view of those prior art teachings in which the beam energy was employed to heat local areas of an amorphous or polycrystalline material to a molten state.
• the crystallizations achieved by this invention are done in the solid phase. Such solid phase transformations allow lower temperatures to be employed, are extremely rapid, and result in crystallized materials having exceptionally smooth surfaces.
• Another aspect of controlling the crystallization front relates to modulation of the energy beam, and in particular, spatial and/or temporal modulation of the beam intensity.
• any modulation is sufficient that provides a crystallization front which advances fast enough to avoid quenching and periodic structure while also avoiding crystallization front runaway.
• this invention offers many advantages over prior techniques for crystallizing amorphous materials, and in general, permits the rapid, uniform, solid-phase conversion of a wide variety of amorphous material to a more crystalline state.
• Fig. 1 is a schematic view of an apparatus suitable for scanning amorphous material with a laser beam as described herein;
• Fig. 2 is an optical transmission micrograph of a laser-crystallized germanium film having periodic structural features
• Fig. 3 is an exploded view of a small area of the micrograph of Fig. 2;
• Fig. 4 is a bright-field transmission electron micrograph of a laser-crystallized germanium film;
• Figs . 5 and 6 are transmission electron diffraction patterns for fine-grained and large grained regions of a laser-crystalli zed germanium film;
• Fig. 7 is a schematic diagram illustrating the crystallization of an amorphous semiconductor film by a scanned slit laser image
• Fig. 8 is a plot of normalized position of amorphous-crystalline phase boundary with respect to a laser image as a function of normalized time obtained from the theoretical model for various given parameters
• Fig. 9 is a plot of theoretical and experimental data for spatial periodicity ratio vs . temperature ratio for laser-crystallization of a germanium film
• Fig. 10 is a plot of theoretical and experimental data for the spatial period structural features of laser-crystallized germanium films as a function of background temperature
• Fig. 11 is a block diagram which illustrates one method for fabricating a photovoltaic cell employing laser beam scanning of a semiconductor film as described herein;
• Fig . 12 is a cross-sectional view of a photovoltaic cell which could be prepared by the method illustrated in Fig. 11;
• Fig . 13 is a block diagram which illustrates an alternative method for fabricating a photovoltaic-cell-employing laser beam scanning of the semiconductor film as described herein;
• Fig. 14 is a cross-sectional view of a photovoltaic cell which could be prepared by the method illustrated in Fig. 13;
• Fig. 15 is a block diagram which illustrates still another alternative method for fabricating a photovoltaic cell employing laser beam scanning of a semiconductor film as described herein;
• Fig. 16 is a block diagram of a photovoltaic cell prepared by the method illustrated in Fig. 15;
• Fig. 17 is a schematic diagram illustrating crystallization according to this invention with a spatially and temporally modulated energy beam
• Fig. 18 is a schematic diagram of an apparatus suitable for crystallizing amorphous materials according to this invention by employing an electron beam or an ion beam;
• Fig. 19 is a schematic diagram of an apparatus for crystallizing amorphous materials according to this invention employing a train of electrical pulses simulating a moving energy team.
• Laser 10 which is a high power, high efficiency laser, such as a Nd:YAG laser, emits laser beam 12 which is focused to a spot having the shape and size desired by focusing lens 14.
• a preferred shape is one which has a large aspect ratio, such as an elongated slit, because such shapes can be used to align semiconductor crystallites as well as to enlarge them. This is apparently because the slit produces thermal gradients in the semiconductor film transverse to its long axis.
• the exact size and shape of the focused laser spot will depend upon factors such as laser power, scanning rate, area to be scanned, crystalline properties desired, etc. Various shapes are obtainable by employing beam expanders, cylindrical lenses, mirrors, or other optical or mechanical elements known to those skilled in the art. For reasons of practicality, it is preferred to use a spot size. having an area of at least about 10 -4 cm 2 . It is also particularly preferred, as mentioned above, to employ a laser image having a large aspect ratio.
• Beam splitter 16 is used to divide focused beam 12 so that a first portion is reflected to power detector 18 and a second portion is transmitted to electro-optical scanner 20.
• Power detector 18 serves to measure the exact beam power so that any desired changes in laser power, scanning rate, etc. can be made.
• Electro-optical scanner 20 is one convenient means for scanning focused beam 12. After passing scanner 20, focused beam 12 enters sample chamber 22 through transparent window 24 and strikes the surface of semiconductor 26. Scanning of semiconductor 26 can be acheived by mounting sample chamber 22 upon three translational stages, 28, 30 and 32.
• Translational stages 28, 30 and 32 provide the capability to move chamber 22, and thus semiconductor 26, in the x, y and z directions, respectively.
• Each stage can be independently driven by connecting rotatable arms 34, 36 and 38 to electric motors (not shown).
• Each stage can be driven separately, or any combination can be driven simultaneously.
• the rate at which each stage can be driven is variable. Thus, a great variety of scan patterns and rates is acheivable.
• electro-optic or acousto-optical beam deflectors or other means, to raster the laser beam can also be used, and these are known to those skilled in the art.
• scanning can be achieved by moving either the sample or the beam, or both.
• the scan rate is set, of course, by the dwell time required. As a general rule, however, the scan rates used with this invention are higher than those used with prior methods.
• Radiant heating lamp 40 can be used to help heat the semiconductor 26. Induction heaters, resistance heaters, or other means to heat semicon ductor 26 could, of course, also be employed.
• Fig. 1 The apparatus illustrated in Fig. 1 is typical of that employed in our previous patent, U. S. 4,059,461.
• the teachings of our prior patent are hereby incorporated by reference, particularly in regard to other suitable apparatus for carrying out a laser-crystallization process and for matching the characteristics of the la ⁇ er to the amorphous material to be scanned as well as the determin ation of other appropriate parameters for the method.
• a crystallization system utilizing a cw Nd:YAG laser beam focused to a slit image with power density that is uniform to + 3% over a central area about
• Amorphous germanium (a-Ge) films were deposited by either electron beam evaporation, rf sputtering, or ion-beam sputtering on molybdenum, graphite, or fured -silica substrates.
• the maximum temperature reached by the film during scanning estimated from its color, was about 700°C, far below the melting point of crystalline Ge, which is about 937°C. Because of this, the transformations from amorphous to crystalline described herein are referred to as "solid phase transformations.” This means that the maximum temperature reached by the material as a result of the energy beam is below the melting point of the crystalline phase of the material.
• a series of laser scanning experiments on films about 0.3 urn thick deposited by electron-beam evaporation on 1-mm thick fused silica substrates was then performed. The power density of the central area of the laser image was kept at about 5kW/cm 2 , and the scan rate at 0.5 cm/sec.
• the background temperature T b which was found to be critical in crystallization behavior, was varied from room temperature to 500°C.
• T b equal to room temperature, after laser scanning, the films exhibited periodic structural features. Within each film the spatial period initially increased, but after several periods, it reached a steady-state value of about 50 pm.
• Fig. 2 shows an optical transmission micrograph of one such film, obtained with visible radiation from a xenon lamp. The dark areas in the micrograph are regions of untransformed a-Ge, which for the film thickness used was almost opaque to visible and near-infrared radiation, while the bright areas are regions of crystalline Ge, which had significant transmission in the red and near-infrared. Similar contrast was also obtained using infrared microscopy.
• Each periodic feature consisted of four different regions, as shown in Fig. 3. These were a narrow amorphous region, followed by another region containing a mixture of amorphous material and fine grains, and then a broad region of fine crystallites, and finally another broad region of much larger, elongated crystallites having dimensions of about 1-2 x 20 micrometers, generally aligned parallel to each other but at an angle of about 55o to the laser scan direction.
• the large crystallites within each periodic feature formed a roughly chevronlike pattern, with the two halves of the pattern symmetrical about an axis parallel to the scan direction and located near the center of the laser slit image.
• the angles between the long axes of the crystallites and the scan direction were similar in magnitude but of opposite sense for the two halves of the pattern.
• the symmetry axis lies close to or just below the bottom of the micrograph, so that only half the chevron pattern can be seen.
• Fig. 4 is a TEM bright-field micrograph of a portion of a film that was laser-scanned with T b equal to room temperature. The micrograph illustrates the same sequence of regions found by optical microscopy and shown in Fig. 2: amorphous (the dark area), amorphous-plus-crystalline, fine-grain and large grain. The fine-grain region yields a transmission electron diffraction pattern like the one shown in Fig. 5, with the rings typical of polycrystalline material.
• TEM transmission electron microscopy
• the grain size gradually increases until it reaches about 0.3 ⁇ m, the thickness of the film, then increases abruptly to give the large, well-aligned crystallites of the final region, which are clearly visible as ribbonlike structures in the lower left corner of the micrograph of Fig. 4. This is the end portion of the preceeding periodic feature. It has been demonstrated by bright-field and dark-field TEM that each of these structures is a single grain, and that they yield characteristic single-crystal transmission electron diffraction patterns. Fig. 6 is an example of such a diffraction pattern.
• Fig. 7 is a schematic diagram illustrating the geometry considered.
• the film is deposited on a thick substrate that is heated to T b , and the laser image is scanned from left to right at velocity v.
• the approximation was made that the amorphous-crystalline (a-c) transformation occurs instantaneously when the film reaches a critical temperature T c .
• the basis for this approximation is that the rate of transformation increases exponentially with temperature. See Blum, N. A. and Feldman, C, J. Non-Cryst. Solids, 22, 29 (1976). This means that over a narrow temperature interval the ratio of the time required for transformation to the time required for laser scan changes from>>1 to ⁇ 1.
• the a-c boundary therefore coincides with a T c isotherm, and the motion of the boundary can be found by calculating the motion of the isotherm.
• Crystallization of the amorphous film accompanied by liberation of latent heat, begins when the film temperature reaches T c along a line at the left edge of the film that is parallel to the long axis of the laser image. Because the film temperature ahead of the image is then increased by conduction of the latent heat as well as by laser heating, the T c isotherm, and therefore the a-c boundary, move to the right at a velocity v ac that is initially much higher than v. As the boundary moves away from the laser image, the contribution of laser heating to the temperature at the boundary decreases rapidly.
• T b With increasing T b , this distance increases , since less heat is required to increase the film temperature to T c , so that the a-c boundary moves farther beyond the laser image before it comes to rest. If T b exceeds T r , the continuing release of latent heat is sufficient to maintain the propagation of the T c isotherm indefinitely, resulting in self-sustaining crystallization. Such self-sustaining crystallization does not result in large, aligned crystallites.
• the semiconductor film which is deposited on a thick substrate of poor thermal conductivity, is of infinite extent in the y and z directions and so thin that its temperature is constant in the x direction.
• the laser slit image is of infinite length in the z direction and moves at a velocity v from left to right in the y direction.
• the phase boundary is located at y , with the crystalline phase to the left (y ⁇ y o ) and the untransformed amorphous phase to the right (y>y o ).
• the laser image carries with it a steady temperature profile ⁇ 1 (y-vt).
• the temperature at the phase boundary reaches T c and the boundary begins to move toward the right, with heat being liberated at a rate per boundary unit cross-sectional area of fLpY(t), where L is the latent heat of a-c trans formation of the semiconductor, is the semiconductor density, Y(t) is the position of the boundary at time t, and f is a factor less than 1 that accounts in an approximate way for the loss of latent heat to the substrate. Then the temperature T(y,t) at any point y along the film at time t is given by the one dimensional integral relation
• T b is a uniform, time independent background temperature and the temperature contribution due to the laser is described by a Gaussian of width a and magnitude
• ⁇ ⁇ T 1 /(T c - T b ) (4a)
• ⁇ fL/C ⁇ 1/2 (T c -T b ) (4b)
• U i the position of the phase boundary measured from.the center of the laser Image.
• Equation (3) is not in itself sufficient to give physically acceptable solutions for the motion of the phase boundary, since it allows negative values of +V], which imply a motion of the phase boundary back toward the laser image, with the reconversion of crystalline material to the amorphous state, accompanied by the reabsorption of latent heat.
• +V negative values
• Equation (3) is required that, when the numerical solution of Eq. (3) yields + V] ⁇ O, this quantity is to be set equal to zero, with the phase boundary remaining stationary.
• the spatial period of the structural features of laser-crystallized Ge films was determined as a function of T b .
• T b and T c are expressed in oC, room temperature is much less than T c , so that t room temperature , and ⁇ Y o corresponds to .
• the value obtained for the front velocity, v ac is dependent upon the time interval for numerical integration, and represents a lower limit, since smaller values of St lead to higher values of v ac .
• the values obtained from the calculation for the spatial period of the oscillations are, however, independent of Results of preliminary measurements of v ac for a 0.3 ⁇ m film of amorphous germanium indicate that the value for v ac is between 150 and 400 cm/sec.
• the exact v ac value will depend upon such variables as film thickness, the particular semiconductor, and the substrate material and temperature.
• the front velocity, v ac is much higher than the scan velocity.
• Fig. 10 presents the experimental and theoretical data obtained in a slightly different manner. The points shown are the experimental data whereas the computer solution of Eq. 3 is plotted as a continuous curve. From these data, it can be appreciated that larger, aligned crystallites are produced by longer spatial periods.
• the scan rate of the laser beam or other energy beam can be used itself or in conjunction with the background temperature to achieve continuous, controlled motion of the crystallization front.
• the scan rate can be much more rapid than has traditionally been employed which is possible because of the solid phase nature of the amorphous-crystalline transformation.
• the scan rate of the laser beam or other energy beam need not be continuous, but may itself be pulsed in a manner which applies a pulse of energy to the crystallization front prior to the time that the front would quench to a temperature which would result in periodic structure.
• Fig. 11 illustrates diagrammatically the fabrication of a photovoltaic cell employing the laser scanning procedure described herein
• Fig. 12 illustrates a cell produced thereby.
• the first step in this fabrication is the formation of a conductive layer 52 on a substrate 50. This could be acheived, for example, by vacuum depositing a metal layer such as copper, silver, tin, gold or other metal onto substrate 50.
• Substrate 50 need not be transparent, but certainly could be. Also, substrate 50 itself could be conducting, which satisfies the first step in this fabrication.
• a film of amorphous semiconductor 54 is deposited onto conducting layer 52 or directly onto substrate 50 if it is conducting.
• An example is a depositon of a thin film of silicon during the final step in the preparation of pure silicon by a method such as the chlorosilane process.
• the cost of the amorphous silicon film could be covered in the silicon purification step.
• Any suitable semiconductor deposition process which provided the purity required could be used, of course, since the usual crystalline perfection requirements are not present because the semiconductor film will undergo laser beam or other, energy beam scanning.
• Amorphous semiconductor film 54 is then scanned with a focused laser beam from a high power laser to improve its crystalline properties. This might be acheived, for example, in the case of an amorphous silicon film by the mechanical linear sweep of the silicon sheet under a slit image from a Nd:YAG laser and/or with the slit image scanned laterally back and forth by an acousto-optic or electro-optic scanner.
• a transparent rectifying contact 56 is formed with a laser-scanned semiconductor film 54. This could be done, for example, by depositing a transparent, highly conducting tin-doped indium oxide film over a thin transparent metallic layer to form a Schottky barrier. Suitable tin doped indium oxide films are described in Fan, J.C.C, and Bachner, F. J., J. Electro Chem. Soc, 122, 1719 (1975), the teachings of which are hereby incorporated by reference.
• a rectifying contact would be formed by laser scanning in a doping atmosphere to form a p-n junction at the surface of the laser-scanned film. As illustrated in Fig. 12, sunlight enters the photovoltaic cell through the transparent, rectifying contact 56 and passes to the laser scanned semiconductor film 54 where a photovoltaic current is generated.
• Fig. 13 illustrates diagrammatically an alterna tive procedure for fabricating a photovoltaic cell employing the laser scanning procedures described herein
• Fig. 14 illustrates the cell itself.
• a transparent, conductive substrate 60 is used which can be formed by depositing on transparent support 62 a conductive layer 64.
• Transparent support 62 can be glass or transparent plastic and conductive layer 64 can be a thin layer of a conductive metal.
• a conductive layer 64 is a material which will also form a recitfying contact with a semiconductor layer subsequently deposited thereon.
• a layer of tin-doped indium oxide, as previously mentioned, would serve as a conductor and would also form a Schottky barrier with some semiconductors such as silicon.
• conductive layer 64 another layer which is capable of forming a rectifying contact with the semiconductor could be deposited on to conductive layer 64.
• one material which is transparent, conductive and capable of forming a rectifying contact with the semiconductor layer could also be used.
• the rectifying contact required can be formed upon deposition-of the semiconductor film, or alterna tively, in a post treatment of the film. In fact, one suitable post treatment is the laser scanning of the semiconductor film.
• a semiconductor film 66 such as a thin film of amorphous silicon, is then deposited on top of transparent, conductive substrate 60.
• many of the known processes for depositing semi conductor films can be used since the usual limitations on crystal perfection are obviated with sub sequent laser scanning of the film.
• the semiconductor film 66 is then scanned with a focused laser beam from a high power laser to improve the film's crystalline properties.
• a conductive layer 68 such as aluminum or other metal, is formed on top of laser-scanned semiconductor film 66. As can be seen in Fig. 14, sunlight enters this photovoltaic cell through the transparent, conductive substrate 60.
• Fig. 15 illustrates diagrammatically still another alternative procedure for fabricating a photovoltaic cell employing the laser scanning procedures described herein, and Fig. 16 illus trates the cell produced thereby.
• an amorphous semiconductor layer 70 is deposited on a substrate 72 which could be a metal or heavily-doped semiconductor.
• This deposited layer 70 might comprise, for example, a p + germanium layer having a thickness between 0.1 and 500 ⁇ m and being formed from amorphous germanium doped with p-dopants to a carrier level of about 10 carriers/cm 3 or greater.
• the amorphous germanium layer 70 is then scanned with a high power laser beam as described herein to provide a crystalline semiconductor layer 70.
• a gallium arsenide layer 74 is applied to the germanium substrate by chemical vapor deposition or other deposition technique.
• the p layer of gallium arsenide 74 can then be diffused with n-type dopant to form a shallow homojunction therein, or a separate n GaAs layer 76 can be deposited.
• An ohmic contact 78 is then attached, and an anti-reflection coating 80 can be deposited upon the light receiving surface of the cell.
• Shallow.homojunction devices of this type are described in detail in copending application Serial Number 22,405, filed March 21, 1979, and the teachings of this application are hereby incorporated by reference.
• amorphous germanium films Although much of the actual experimental work described herein was performed employing amorphous germanium films, similar effects were observed in gallium arsenide and silicon films. It is also believed that other semiconductors could be crystallized by the method of this invention, including, but not limited to, indium phosphide, cadmium sulfide, cadmium telluride, gallium phosphide, germanium/silicon alloy, etc. Moreover, the invention described herein can also be employed in the crystallization of amorphous materials which are not semiconductor materials. These include: inorganic oxides or nitrides such as aluminum oxide or silicon dioxide; amorphous carbon; and even amorphous metals.
• the invention is suitable with materials which are not 100 percent amorphous, including mixtures of amorphous and crystalline materials. In certain cases, the latent heat liberated upon crystallization might even cause further reaction of materials.
• the work described above was performed with laser beams from a Nd:YAG laser, other lasers could be employed, as well as other energy beams. Examples of other suitable energy beams include ion beams, electron beams, or trains of electrical pulses which simulate moving energy beams.
• one possi ble method is to scan the energy beam at a velocity as fast as the crystallization front velocity v in order to maintain the control of the front propagation.
• the energy beam initially has a high intensity causing initiation of crystallization and advancement of the crystallization front to a position in front of the beam.
• the beam intensity is lowered but the crystallization front advances further ahead of the beam, as in Fig. 17(b).
• the beam is advanced and almost catches the crystallization front, as shown in Fig. 17(c).
• Fig. 17(a) the energy beam initially has a high intensity causing initiation of crystallization and advancement of the crystallization front to a position in front of the beam.
• the beam intensity is lowered but the crystallization front advances further ahead of the beam, as in Fig. 17(b).
• the beam is advanced and almost catches the crystallization front, as shown in Fig. 17(c).
• Fig. 18 illustrates an apparatus suitable for crystallizing amorphous materials by an electron or ion beam.
• This apparatus is contained within a vacuum chamber 80.
• Gun 82 can be an electron or ion beam gun, which produces energy beam 84.
• Beam 84 is focused by focusing means 86 and caused to scan by scanning means 88.
• Sample 90 is the amorphous material to be crystallized and is supported on sample holder 92 which has resistance heater 94 therein to heat sample 90.
• gun 82 is an ion beam gum, simultaneous doping of a semiconductor is possible.
• Fig. 19 illustrates still another embodiment of apparatus suitable for carrying out the invention.
• a series of voltage sources 100, 100', 100", 100''' are employed to consecutively apply electrical pulses to conducting pad sets 102, 102', 102'', 102'''. This simulates a moving beam of energy thereby causing a crystallization front to move along amorphous material 104, if the appropriate conditions are chosen.
• the invention described herein has industrial applicability in the conversion of amorphous materials, such as semiconductor films, to crystalline materials. As described above, such crystalline semiconductor films are employed in photovoltaic devices. It also has industrial applicability in the very rapid conversion of amorphous materials other than semiconductor materials to a crystalline state.

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• 1980
  • 1980-02-22 US US06/123,745 patent/US4309225A/en not_active Expired - Lifetime
  • 1980-09-08 JP JP50226780A patent/JPS56501508A/ja active Pending
  • 1980-09-08 WO PCT/US1980/001151 patent/WO1981000789A1/en not_active Ceased
  • 1980-09-08 DE DE8080901915T patent/DE3072022D1/de not_active Expired
• 1981
  • 1981-03-23 EP EP80901915A patent/EP0035561B1/en not_active Expired

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