Semiconductor structures including quantum well wires and boxes

Abstract: A method of fabricating semiconductor structures, quantum well wires and boxes is described in which interdiffusion in a semiconductor having a compositional profile is enhanced by the presence of defects created by ion implantation in localized regions. A representative structure is shown in Fig. 5.

Description: Technical Field This invention relates generally to semiconductor structures having compositional variations including potential wells for carrier confinement. Background of the Invention In bulk semiconductors, carriers are free to move in three dimensions as their movement is not restricted by potential wells. However, interesting and useful effects arise if semiconductor structures are fabricated in which carriers have their movement restricted so that they can move in less than three dimensions. For example, heterojunctions have been fabricated with a doped wide bandgap material adjacent to an undoped narrow bandgap material. This type of structure is commonly referred to as modulation doping and has increased carrier mobility, as compared to doped bulk semiconductors, because the carriers are essentially confined to the undoped material where they have high mobility because of the reduced impurity scattering. Modulation doping is useful in field effect transistors and such transistors are commonly referred to as s electively d oped h eterojunction t ransistors (SDHT). Additional and interesting effects may arise if the carrier motion is further confined to dimensions which are sufficiently small so that quantum effects become important. Structures demonstrating such effects are commonly referred to as quantum wells. Two dimensional quantum well structures were first demonstrated in the GaAs/GaAlAs materials system. In these quantum well structures, the carriers are free to move in two dimensions, but quantum effects are significant in the third dimension. That is, the energy levels are quantized in one dimension but are continuous in the other two dimensions. Such quantum well structures are of interest because, for example, the emission frequency of a double heterostructure laser is shifted from that expected for bulk semiconductors due to the change in allowable energy levels caused by the presence of quantum effects. The success of devices using two dimensional carrier confinement and their technological and commercial significance has led to desires for structures in which carrier movement is further restricted, i.e., structures in which carriers can move freely in only one dimension (1 dimensional confinement) or in which their positions are essentially localized. It is hypothesized by those skilled in the art that quantum confinement to one and zero degrees of freedom may lead to new physical phenomena with interesting device applications, and attempts have been made to fabricate structures in which carriers are so confined. However, these proposals and attempts have not yet achieved the success that has been obtained with two dimensional quantum well structures. This relative lack of success undoubtedly has its origins in the technological difficulties associated with the fabrication of structures having, in two or three directions, dimensions which are close to the carrier de Broglie wavelengths. These wavelengths are typically less than approximately 50 nm. Attempts have been made to fabricate small features by implanting impurities to introduce compositional disordering. See, Japanese Journal of Applied Physics , 24, pp L516-L518, July, 1985. Structures in which the carriers are confined so that they have essentially only a single degree of freedom are conveniently termed quantum well wires (QWW). By analogy, structures which restrict the carriers so that they have no degree of freedom can be aptly termed quantum well boxes (QWB). Summary of the Invention According to the invention there is provided a method as set out in the claims. We have found that rapid thermal annealing permits fabrication of quantum well structures such as wires (QWW) and boxes (QWB). The annealing of the implanted semiconductor results in interdiffusion of two species, for example, Al and Ga. Rapid thermal processing, which includes rapid thermal annealing, is described in pages 57-58 of S. Wolf and R.N. Taube r Silicon Processing for the VLSI Era.Volume 1: Process Technology , Lattice Press, Sunset Beach, California, USA, 1986. Brief Description of the Drawings FIG. 1 shows a mask on a semiconductor structure; FIG. 2 is a schematic representation of ion implantation FIG. 3 shows the number of ions implanted versus the distance from the surface; FIG. 4 is a schematic idealized representation of an exemplary embodiment of this invention; FIG. 5 is a schematic representation of another exemplary embodiment of this invention; and FIG. 6 illustrates a mask suitable for fabricating a grating; and FIG. 7 illustrates the bandgap variation in the grating. For reasons of clarity, the elements of the embodiments depicted are not drawn to scale. Detailed Description A structure having an undoped GaAs single quantum well was grown on a (100) GaAs, Cr doped substrate. The structure had, sequentially disposed on the substrate 1, a 50 nm GaAs buffer layer, a 50 nm Ga 0.65 Al 0.35 As first barrier layer 3, a 5 nm GaAs quantum well 5, a 50 nm Ga 0.65 Al 0.35 As second barrier layer 7, and a 5 nm GaAs cap layer. Masks 9 were formed on the surface from evaporated Ti/Au-Pd films which were approximately 150 nm thick. The films were then patterned using conventional electron beam lithography and lift-off processing. Metal wires and dots having sizes between 50 nm and 1000 nm were fabricated. The wires and dots were strips extending into the paper. A sectional view of this structure is shown in FIG. 1. For reasons of clarity, only the substrate and the two barrier layers surrounding the quantum well layer are shown as well as the mask. Ga⁺ ions were then implanted into the quantum well region with an energy of 210 keV and a dose which was typically 5 x 10¹³cm⁻². This step is schematically depicted in FIG. 2 and the number of ions, as a function of depth z, is depicted in FIG. 3. The dose level was typically kept lower than the amorphization threshold. It will be readily appreciated that the implantation step creates a significant number of defects. The mask were then removed using a potassium iodide solution for Au-Pd and dilute HF for Ti. A rapid thermal anneal was performed at a temperature of approximately 900 degrees C with the epitaxial side of the wafer in contact with the GaAs wafer. The rapid thermal anneal of the implanted material results in interdiffusion of Ga and Al across the heterointerface as well as laterally underneath the mask. The anneal is typically between 5 and 15 seconds although somewhat longer times, say approximately 5 minutes, can be used. An idealized representation of the resulting structure is shown in FIG. 4. The anneal has resulted in interdiffusion of Ga and Al to form the wires shown. That is, Al and Ga move into and out of, respectively the quantum well region. Since AlGaAs has a larger bandgap than does GaAs, quantum well wires and boxes are formed underneath the mask. As will be understood by those skilled in the art, the wires and boxes are perpendicular to the plane of the page. An anneal for a shorter period will result in less complete interdiffusion of the Ga and Al. The particular species chosen for the implant step is not critical. Species other than constituents of the semiconductors may be used. However, the semiconductor must have two species which will interdiffuse. It must also have a compositional profile in which the two species have a varying concentration. The rapid thermal annealing step then permits the two species to come to an equilibrium via the mechanism of the defect enhanced interdiffusion. That is, the diffusion coefficient is much larger in the damaged material than in the undamaged material. A more realistic schematic representation of the resulting structure is depicted in FIG. 5. Depicted are GaAs buffer layer 1, barrier layers 3 and 7, quantum well layer 5, and mask 9. L z is the original quantum well layer thickness. Δ i is the interdiffusion length. To facilitate discussion, the z and x directions are explicitly indicated. The variations of the Al concentration along the direction perpendicular, i.e., the z direction, to the interface is given by an error function. Along the x direction, the interdiffusion under the masks decreases smoothly as will be described. The local bandgap is proportional to the aluminum concentration. Thus, as the Al content increases in what was the quantum well layer, GaAs wires are formed under the mask material. The energy for implantation is chosen so that the implanted species is deposited in the desired localized region. The time and temperature should be sufficiently long and high, respectively, that the desired amount of interdiffusion occurs. It will be appreciated that for some applications complete interdiffusion will not be desired. The confinement energy of an electron in a 100 nm quantum well is relatively small, for example, less than 1 meV. The potential well profile perpendicular to the wires is characterized by three parameters: (1) the mask size, (2) the implantation lateral straggling, and (3) the diffusion length of the defect which enhances the interdiffusion. For large masks, that is, over 350 nm in width, the confinement effects are either absent or too small to be resolved. For smaller masks, the profile is not square but rather is rounded. In this potential, the splitting between the first two energy levels is typically larger than in a square well having the same width but it decreases as the quantum number of the energy level increases. However, further decreases in the mask size no longer change the width of the potential which will be defined mainly by the straggling and the defect diffusion length. This is due to the increased interdiffusion of Al into the GaAs QWW, which will reduce the conduction band offset for the confinement in the x direction. It should be noted that there is a significant amount of lateral diffusion, i.e., diffusion under the mask. Some of this is the result of lateral straggling during the implantation step which leads to lateral defect diffusion during the annealing step. A graded profile, i.e., a varying bandgap, under the mask in the x direction is then produced by the annealing step. The structures described are useful in many applications. For example, graded bandgap structures can be easily achieved laterally. This is in contrast to molecular beam epitaxy which typically permits graded bandgap structures to be fabricated only in the vertical, that is, growth, direction. A suitable mask is shown in a top view in Fig. 6. The implanted zones have a width, w, which increases from left to right. Thus, the implanted area, and thus the dose, also increases. However, the dose per unit area is the same. The effective bandgap structure for the thermally annealed AlGaAs is depicted in FIG. 7 for both the conduction and valence bands. As can be seen, the bandgap increases monotonically from left to right as there is more interdiffusion of Al and Ga. These laterally graded devices have two degrees of freedom for introducing built-in fields in a structure without etching. Other embodiments are also contemplated. For example, very small waveguides may be easily fabricated. Additionally, gratings may be fabricated by using a slight modification, of the mask depicted in Fig. 6. These gratings may be used in many applications, such as, e.g., the external grating in a single frequency laser. Additionally, the grating may be optically coupled to the active layer of a double heterostructure to form a distributed feedback laser. As will be readily appreciated by those skilled in the art, no regrowth over the grating is required. It is also contemplated that the quantum well wires and boxes may be part of semiconductor lasers. Chemical dopants may be added to assist carrier relaxation to the ground state.

Claims: A method of fabricating a semiconductor structure comprising the steps of implanting a species into localized regions of a crystalline semiconductor body including at least two elements and having two heterojunctions therein, said implanting having a dose level less than the amorphization threshold, and creating defects, and heating said crystalline semiconductor structure CHARACTERIZED IN THAT said heating comprises a rapid thermal annealing process to cause interdiffusion of atoms of said at least two elements of said semiconductor via defect-enhanced mechanism, said interdiffusion producing regions of increased bandgap and thereby producing regions in which carriers are confined with zero or one degree of freedom. A method as recited in claim 1 in which said heterojunctions, form a potential well region, said species being implanted into said regions. A method as recited in claim 2 in which said localized regions define one or more regions confined in two dimensions and extending in one dimension. A method as recited in claim 3 in which said regions extending in one dimension have varying widths. A method as recited in claim 4 in which said annealing produces a laterally varying bandgap of said semiconductor body.

Inventors: CIBERT JOEL BERNARD, GOSSARD ARTHUR CHARLES, PEARTON STEPHEN JOHN, PETROFF PIERRE MARC, CIBERT, JOEL BERNARD, GOSSARD, ARTHUR CHARLES, PEARTON, STEPHEN JOHN, PETROFF, PIERRE MARC
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1987
Main classification: H01L21
IPC codes: H01L21, H01L29, H01S5