GB2144151A - Method of selective area epitaxial growth - Google Patents

Abstract

A III-V compound is grown on selected areas of a substrate (1010) by flooding the surface with a thermal beam of group V particles from an effusion oven (1020) and scanning a beam of group III particles from an ion source (1030) over the selected areas. There may be more than one scanning beam and in that case the beams may be of different elements, permitting growth of different compounds in different areas or of tertiary or higher compounds. Also there may be flooding of the surface by a thermal beam of group III particles so that growth outside the selected areas takes place by molecular beam epitaxy. <IMAGE>

Description

SPECIFICATION Method of selective area epitaxial growth This invention relates generally to methods of epitaxial growth and particularly to such methods that grow epitaxial structures on selected areas.

Modern semiconductor technology depends upon methods for growing a multiplicity of high quality semiconductor layers on, for example, a substrate or other epitaxial layers. The layers should have few residual, i.e. undesired, impurities and carrier traps as well as few structural imperfections. Accordingly, several methods have been developed to grow such epitaxial layers. The layers may comprise elemental semiconductors, such as silicon or germanium, or compound semiconductors such as Groups III-V or Group Il-VI binary, ternary, or quaternary semiconductors.

For the growth of Groups Ill-V compounds semiconductor materials, the first method developed and brought to a high degree of perfection was liquid phase epitaxy (LPE). While this method is now well developed and perfectly adequate for the fabrication of many types of epitaxial layers and devices comprising at least one of such layers, it is not without its limitations. For example, LPE requires that a substrate be moved from one melt to another, etc., with epitaxial growth typically occurring in each melt. The melts are typically compositionally varying, and therefore, precautions generally have to be made to prevent undesired transport of melt constituents from one melt to another melt. While this limitation may be overcome by careful apparatus design and operation, other limitations appear more fundamental and difficult to overcome.For example, fabrication of ultra-thin layers, for example, less than 20nm thick, is usually difficult, if not impossible, with this technique.

Even very thin, less than 100nm thick, smooth layers have been difficult to grow reproducibly. Additionally, interfaces between layers having very abrupt, i.e. step function, compositional or doping variations are also difficult to fabricate.

As a result of these and other limitations, as well as for other reasons, additional techniques have been developed for the growth of Group IIl-V compound semiconductor materials. The most promising of these additional techniques at present appears to be molecular beam epitaxy (MBE). This method is described in detail in U.S. Patent No.

3,615,931 issued to John R. Arthur, Jr. on October 26, 1971. In MBE, effusion ovens containing the desired Group Ill and Group V materials are heated to a temperature sufficient to volatilize the materials and the resulting thermally evaporated beams are directed toward the substrate upon which epitaxial growth is desired. The substrate is maintained at a temperature that is high enough for surface diffusion and epitaxial growth to occur but is low enough so that the materials have a reasonable probability of sticking to the surface. The beams have a cosine-square distribution in intensity and, when the basic method is supplemented with other techniques, such as substrate rotation, permit growth of extremely compositionally uniform epitaxial layers over large diameter substrates.Furthermore, molecular beam epitaxy permits fabrication of, for example, extremely thin, in fact, even monolayer, epitaxial layers as well as interfaces having very abrupt compositional and doping variations.

The beams used by molecular beam epitaxy are generally electrically neutral. However, nonthermal charged particle, i.e. ion, beams have been used in at least several semiconductor epitaxial growth techniques. The use of nonthermal ions offers, at least theoretically, the possibility of lower substrate temperatures during growth because the kinetic energy of the deposited particles enhances surface diffusion. The preparation of InSb thin films by an ion beam epitaxy technique is described in Journal of the Vacuum Society of Japan, 20 pp. 241-246, July 1977. The technique used both In and Sb ions.

The ions were accelerated by a high constant voltage, 1000 volts or greater, applied to the substrate.

Uniform area growth, within unspecified tolerances, was apparently demonstrated. However, for purposes of compositional control, flash evaporation was employed, i.e. the composition of the layer deposited was controlled by flash evaporation to completion of preweighed In and Sb charges. The preweighing was required because both the In and Sb particles were charged and thus had high sticking coefficients on the substrate surface. Consequently, the deposited layer might not have perfect stoichiometry as either In or Sb might be incorporated into the layer in excess.

Epitaxial growth of silicon on either germanium, Ge(100), or silicon, Si(100) or Si(111), substrates using ion beam epitaxy is described in Applied Physics Letters, 41, pp. 167-169, July 15, 1982. The method described used silicon ions having energies between 50 and 100 eV and obtained epitaxial silicon growth at substrate temperatures between 300 and 900 degrees K. The silicon beam was formed by thermal evaporation of silicon from an effusion oven, and a discharge voltage of 60 volts accelerated the particles from the plasma. Growth of thin and high-quality silicon layers was reported.

However, this method permits the growth only of elemental semiconductors.

As is evident from the preceding discussion, molecular beam epitaxy is not generally a selective area growth technique because the molecular beams impinge upon the entire crystalline substrate, i.e. the beams cover the entire substrate surface. Techniques have been developed which modify the basic molecular beam epitaxy technique described in U.S Patent 3,615,931 to permit selective area growth. However, these techniques require steps, for example, suitable substrate preparation, which necessitate additional processing complexity. See, for example, U.S. Patent No.

3,982,092 issued on December 23, 1975 to William Charles Ballamy and Alfred Yi Cho. The method described in U.S. Patent 3,982,092 prepares planar isolated devices by forming an amorphous insulating layer on a Group Ill-V substrate and removing selected portions of the layer to expose the under lying crystalline material thus yielding a patterned substrate. These steps are performed outside the MBE growth chamber. The now patterned substrate is transported into the MBE growth chamber for deposition of material by MBE. Single crystal material grows only on the exposed portions of the substrate. Other selective area growth techniques, for example, mechanical shadow masking, are known.

Ion beams have been used for other purposes in epitaxial growth processes. For example, Applied Physics Letters, 40, pp.686-688, April 15, 1982, describes the use of a high energy ion beam in silicon fabrication. Amorphous silicon is initially deposited. A high energy, approximately 2.5 MeV, arsenic ion beam is then directed to the amorphous silicon layer. The resulting heating causes the recrystallization of the amorphous silicon with the impurity arsenic atoms from the ion beam located on substitutional sites within the crystal lattice.

According to the present invention there is provided a method of epitaxially growing Group Ill-V semiconductor materal on a selected area within a larger area of a semiconductor substrate comprising scanning a beam of Group Ill particles over the said selected area and flooding the larger area with a thermal beam of Group V particles.

A further beam of Group Ill particles, which may be the same as or different from those in the first said scanning beam, may be scanned over a selected area, which may be the same as or different from the first said selected area, within the said larger area.

The said larger area may also be flooded with a thermal beam of Group Ill particles.

Additionally, ion beam epitaxy according to this invention may be used to grow compound materials in which one or more of the constitutent elements has a significantly lower sticking coefficient at the growth temperature than do the other constituent elements.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of apparatus suitable for the practice of this invention; and Figures 2-8 show exemplary devices which may be expediently fabricated by means of this invention.

For reasons of clarity, the elements in the Figures are not drawn to scale.

Apparatus suitable for the practice of this invention is schematically shown in Figure 1. The apparatus comprises at least one ion beam source for a Group ill element, indicated as 1030, means for scanning 1040 the ion beam, from source 1030, and at least one effusion oven for a Group V element, indicated as 1020. Source 1030 further comprises means for accelerating the particles, for example, an accelerating gun. These elements are enclosed in chamber 1000 which may be evacuated by pump 1050 to a high vacuum, i.e. a vacuum of at most 10-6 Torr and preferably to at most 10-9 Torn The particle beams from the oven and the ion source are indicated schematically by the arrows and are directed to substrate 1010 which is located within chamber 1000.

The ion beam source and the means for scan ning the ion beam that are used are well known to those skilled in the art and can be similar to those described in, for example, Applied Physics Letters, 34, pp. 310-312, 1979. The ion beam source is charged with a Group Ill element. The means for scanning, i.e. deflecting, permits the beam to be swanned to any point on the substrate, that is, it may be deflected to any point on the two-dimensional substrate surface. The points collectively form a selected area within a larger area, e.g. a substrate. The means for deflecting may be, for example, any conventional electrostatic or electro magnetic deflection means. Such means are known to those skilled in the art and need not be described in detail. It will readily be appreciated by those skilled in the art that the ion beam may be focused.Additionally, if desired, the ion beam may be neutralized after it leaves the accelerating gun and the means for scanning has deflected it to the desired direction.

The at least one effusion oven for the Group V element is a conventional effusion oven with means for resistive heating of the oven to evaporative temperatures. More than one effusion oven may be used for Group V elements. The effusion ovens for the Group V elements are charged with the Group V elements and the thermal beams from these ovens flood the substrate. The term "flood" means that the beam strikes the entire substrate or a substantial portion of the substrate. Another effusion oven may be charged with a Group Ill element if molecular and ion beam epita$y are to occur simultaneously. The Group V particles may also be charged if it is desired to improve their sticking coefficient during growth.

Important ion beam parameters include the following: current, beam size, beam energy, scanning rate. ln general, the beam current should be as large as possible as this parameter essentially determines the rate of epitaxial growth and higher current permit faster growth. Typical beam currents are within the range from 5 pamp to approximately 150 lamp. The size of the features, which are essentially one-dimensional Figures, i.e. lines, being written is determined by the ion beam size.

Typical ion beam diameters at the substrate surface are approximately several micrometres for ion beam epitaxy at low ion beam energies. Smaller diameters may be obtained at low ion beam energies but with smaller currents. At higher energies, smaller beam diameters, for example, less than 1 Fm, are easily attained. The features may be formed either by a single scan or by repetitive scans. The beam energy will be typically between approximately 10 eV and approximately 35 KeV per beam particle. Energies greater than 2KeV may result in the removal of some of the underlying material. The upper limit is determined by the desire to have the beam particles cause little damage to the substrate. The lower limit is determined by the operational stability of the ion source.Within this range, higher accelerating voltages, however, permit smaller features to be written as the beam can be more easily focused to a small size. The scanning rate, for a given beam current and beam size, will be determined by the desired thickness in the growth direction of the ion beam initiated epitaxial growth as slower scanning rates will yield layers of greater thickness in the growth direction than will faster scanning rates. Another important growth parameter is the substrate temperature. As will be readily appreciated, the energy of the particles in the ion beam will contribute to the substrate heating and also enhance surface diffusion. Consequently, the substrate generally need not be heated to temperatures as high as those normally used in MBE.

The ovens for both the ordinary effusion cells and the ion source are charged with the desired Group Ill and Group V elements and placed in the growth chamber with the substrate. The substrate comprises a Group III-V compound semiconductor having a lattice constant so that it is at least approximately lattice matched to the desired epitaxial layers. The chamber is evacuated, after appropriate substrate cleaning using well-known techniques, to a pressure of approximately 10-9 Torr and the ovens and substrate heated to their desired temperatures. The temperatures of the thermal effusion ovens are determined, in well-known manner, by the need to obtain desired flux levels of the Group Ill and Group V elements at the substrate surface. Within the ion beam source that contains the Group Ill element, the oven is heated to a temperature sufficient to insure that the charge is molten.The particle flux intensity from the ion source is then controlled by the size of the extracting voltage and the accelerating voltage. The beam of Group Ill particles is scanned, as desired, over the substrate surface while the beam of Group V particles simultaneously floods the surface. The density of Group Ill particles at the surface controls the rate of ion beam initiated epitaxial growth.

Other embodiments are contemplated. For example, a thermal beam comprising Group Ill particles may also flood the substrate. Molecular beam and ion beam epitaxy then proceed simultaneously. If the molecular and ion beam comprise the same Group Ill element, two constituent, i.e. binary, material is grown and if they comprise different elements, three constituent, i.e. ternary, material is grown in those areas they both strike and binary material is grown in those areas that only the molecular beam strikes. It will be readily appreciated that more than one molecular or ion beam may be used thus allowing selective area growth of binary, ternary, and quaternary materials to proceed simultaneously. For example, two or more thermal beams of Group V particles may be used.

Several desirable attributes of the method described will now be apparent. In particular, it will be readily appreciated by those skilled in the art that the highly directional ion beam may be incident on arbitrarily selected substrate areas by appropriately defocusing and/or focusing the beam to obtain the desired beam size and then electrostatically or electromagnetically deflecting the beam to produce the desired pattern.

The growth rate is precisely controlled by both the beam scanning rate and the beam current. By varying these parameters during the ion beam initiated process, the thicknesses of the features in both the growth and lateral directions, as well as the chemical compositions of the epitaxial lines, may be controlled in a very precise manner.

The ion beam, because it is charged, may be abruptly turned on and off by electrostatic or electromagnetic means instead of the mechanical shutters that are conventionally used in molecular beam epitaxy. This permits very abrupt compositional changes to be made.

Several exemplary structures grown with this method are shown in Figures 2 to 8. Figure 2 depicts a buried heterostructure laser comprising a substrate 1, a first epitaxial layer 3 having a first conductivity type, a second epitaxial layer 5 having a second conductivity type, and a plurality of active stripes 7. In one embodiment, the device comprises an n±type GaAs substrate 1, an n-type AIxGa1 xAs first layer 3, a p-type AI1Ga,,As second layer 5, and GaAs active stripes 7. The AI,Ga,,As layers 3 and 5 are grown by conventional MBE techniques. The active stripes 7 are grown by ion beam epitaxy after layer 3 has been grown. These stripes are grown by raster scanning the Ga beam to form the array of GaAs stripes.

Figure 3 depicts a structure comprising one-dimensional conducting channels for electronic devices. The term "one dimensional" means that the dimensions in two directions of the features formed are sufficiently small that the carrier energy levels are quantized. The structure comprises a substrate 31, a plurality of epitaxial layers 33, 35, 37 and 39 having a first conductivity type, and a plurality of one-dimensional undoped channel regions 301. The one-dimensional channel regions are grown by ion beam epitaxy and the layers enclosing them are grown by MBE as described above. The channels become highly conducting as carriers (electrons or holes) move into the undoped high purity semiconductor channel regions from the surrounding n-type or p-type semiconductor layers. As will be appreciated by those skilled in the art, layers 33, 35, 37 and 39 may be either ntype or p-type and the channel regions will thus contain one-dimensional electron or hole gases within the same matrix.

Figure 4 shows a high mobility field effect transistor having such one-dimensional channel regions. The device has source, drain, and gate electrodes indicated as 40, 47 and 48, respectively.

Regions 41 and 45 are semi-insulating semiconductors while regions 42, 43 and 44 comprise metals for the source, gate, and drain region, respectively. The one-dimensional electron or hole gases formed in the channel regions indicated as 401 provide high mobility conduction between the source and drain regions.

The one-dimensional channel regions need not be straight. Another field effect transistor having a one-dimensional channel is depicted in Figure 5.

The transistor comprises substrate 51; first epitaxial layer 53 having a first conductivity type; source, gate and drain electrodes 55, 56 and 57, respectively. The source and drain regions are connected by the undoped one-dimensional channel region 59.

Figure 6 depicts a structure comprising a body 61 and a three-dimensional array of electron or hole gas pockets 63. The pockets form a three-dimensional structure having a periodicity that is easily varied. The pockets are formed by modifying the growth technique used for the one-dimensional tubes so that the ion beam strikes the substrate periodically for a time sufficient to form the desired pockets.

Ion beam epitaxy is also useful in growing integrated optics devices. For example, Figure 7 shows a laser structure having multiple active areas feeding a single waveguide that may be fabricated expediently with ion beam epitaxy. The structure comprises substrate 71, first epitaxial layer 73 having a first conductivity type, second epitaxial layer 75 having a second conductivity type; and, at the interface of layers 73 and 75, a plurality of active stripes 701 which are optically coupled to waveguide 710. The active stripes are grown by ion beam epitaxy which proceeds simultaneously with the molecular beam epitaxy.Because both the ion beam epitaxy and molecular beam epitaxy are carried on simultaneously, the composition of the waveguide section may vary from that of the active stripes if the growth rate of the ion beam epitaxy during the growth of the waveguide section differs from that of the active stripes.

Figure 8 illustrates yet another application of ion beam epitaxy as described to the fabrication of integrated optics devices. The structure comprises substrate 81, waveguide 83, and devices 85 and 87.

Curved waveguide 83 optically couples devices 85 and 87. As is shown, waveguides having essentially arbitrary configurations may be fabricated for the purpose of optically connecting various optical components.

If the ion beam has accelerated to a coefficiently high energy, removal of material may occur. If the ion beam is now scanned while MBE occurs, different growth rates, and perhaps compositions, will occur in the areas scanned and not scanned by the ion beam. Additionally, the scanning and flooding steps may occur sequentially rather than simultaneously.

As will be readily appreciated, other materials systems can also be used. They can be, for example, other Group IIl-V compound semiconductors, metals, semimetals, or superconductors. Additionally, all of the devices described as having one-dimensional channels, may be formed with larger dimensions if it is not desired that the carrier energy level be quantized with low quantum numbers. Devices other than those described may also be fabricated. For example, one-dimensional quantum well lasers may be fabricated.

Claims (8)

1. A method of epitaxially growing Group Ill-V semiconductor material on a selected area within a larger area of a semiconductor substrate comprising scanning a beam, of Group Ill particles over the said selected area and flooding the larger area with a thermal beam of Group V particles.

2. A method as claimed in claim 1 including scanning a further beam of Group Ill particles over a selected area within the said larger area.

3. A method as claimed in claim 1 including flooding the said larger area with a thermal beam of Group III particles.

4. A method as claimed in any of the preceding claims wherein the or each scanning beam is formed by ionizing the Group ill particles.

5. A method as claimed in claim 4 in which the Group Ill particles in the or each scanning beam are accelerated by a voltage greater than 10 volts and less than 35000 volts.

6. A method as claimed in claim 5, wherein the voltage is greater than 2000 volts and the scanning and flooding occur sequentially.

7. A method as claimed in claim 5 wherein the voltage is less than 2000 volts and the flooding and scanning steps occur simultaneously.

8. A method as claimed in any of claims 4 to 7 wherein the scanning is by electromagnetic means.