SEMICONDUCTOR FABRICATION UTILIZING LASER RADIATION

Abstract: Process for greatly simplifying the production of certain types of semiconductor devices, e.g., lasers. In one embodiment of the invention, a stripe geometry is fabricated in a laser-diode structure having a plurality of epitaxial layers, including in tandem a p-doped semiconductor layer (4), a moderately n-doped semiconductor layer (5) and a heavily p<s+>s-doped layer (6), by focusing laser radiation on the n-doped semiconductor layer (5). The laser radiation is chosen to have a wavelength which passes through the p<s+>s-doped layer (6) without absorption. When the laser radiation is absorbed in the n-doped layer, heat is generated which causes diffusion of p-dopant from the two adjacent layers to convert the exposed region to p-type. As the laser beam is scanned, a stripe having a forward pn-junction for laser action is formed.

Description: SEMICONDUCTOR FABRICATION UTILIZING LASER RADIATION Background of the Invention b This invention pertains to the field of semiconductor fabrication and more particularly to the application of laser radiation in said fabrication. While . not limited thereto, the invention has particular utility in the fabrication of laser devices. 0 An article entitled "Unstable Horizontal Transverse Modes and Their Stabilization with a New Stripe Structure", IEEE J. Quant. Elec. , Vol. QE-13, No. 8, August 1977, pp. 659-661, by K. Kobayashi, R. Lang, H. Yonezu, Y. Matsumoto, T. Shinohara, I. Sa ' kuma, 5 T. Suzuki, and I. Hayashi discloses that the active region ' of a laser-diode has to form a channel waveguide not more than 5 microns wide for single-mode operation. In fact, as shown in an article entitled "GaAs-Ga- ] __ χ Al χ As Buried- Heterostructure Injection Lasers", _ ; _ Appl. Phys. , Vol. 45, 0 No. 11, November 1974, pp. 4899-4906 by T. Tsukoda, the best single-mode AlGaAs/GaAs laser-diode has a mesa type structure which is only about 1 micron wide. The mesa type of structure shown in the Tsukoda article and known by those skilled in the art as a buried structure requires a 5 two-step growth process and is difficult to fabricate. Summary of the Invention According to the present invention there is provided a method of fabricating a semiconductor device comprising forming first, second, and third semiconductor 0 layers, the second layer being between and contiguous with and having a different conductivity from the first and third layers, and focusing radiation, with respect to which the second layer is absorptive but the third layer is substantially transparent, from a laser source through the 5 third layer onto a selected area of the second layer so as to induce localized diffusion of dopant impurities to change the conductivity of the second layer in the selecte area. The method can be used to produce a stripe geometry in a semiconductor diode. It will be appreciated that no photolithographic processes are necessary to form the stripe geometry and the method can be applied to many laser-diode structures simultaneously as, for example, whe many diodes are to be processed on the same chip in the case of an integrated optic circuit. Brief Description of the Drawing FIG. 1 shows a laser-diode structure before the application of radiation from a laser source; and FIG. 2 shows the application of radiation to the laser-diode structure of FIG. 1 and the results thereof. Detailed Description FIG. 1 shows a specific example of a laser-diode structure before annealing. This structure comprises an n-InP substrate layer 1, n-InP buffer layer 2 (-10 um thick), Ga 3 0 I >70 As Q 26 P . 364 act ive --Y-- - (-0.25 μm thick) designed for emitting 1.3 micron radiation, p-Ga * L7 * L In 829 As 373 P 627 - a Y e τ 4 ( ~ 2 uin thick ) using either Cd or Zn as the dopant, 0.3 micron thick moderately doped n-Ga ^ - - j il g2gAs 373P 527 layer 5, and heavily doped p + -InP layer 6 (-0.5 μm thick). The n-InP buffer layer 2 serves as one of the confinement layers in the double- heterostructure and is grown to provide a smooth surface for the interface between the substrate 1 and the active layer. The p + -InP cap layer 6 may be replaced by a p + quaternary cap layer to obtain a low resistance electrical contact. The structure shown in FIG. 1 may be grown by known liquid phase epitaxy processes. A striped geometry is then formed in the laser structure shown in FIG. 1. FIG. 2 shows the application o laser beam 100 from laser source 10. The laser beam is focused by lens system 15 to a spot size of about 1 micron in diameter. We use the 1.06 micron radiation from a Q- switched NdrYAG laser, which radiation is highly absorbing in the layers 4 and 5 of Ga ^ j ι ~n m 829 As .373 p .627 * We note that caution must be exercised in that the laser beam be applied only in a volume that is typically less than _ Q • } 10 °cm . This is required so that the vapor pressure generated in this volume does not shatter the diode 2 structure. For example, a 60 nanosecond, 2 J/cm laser pulse is used. During the application of the laser radiation, the heat generated in layer 5 is sufficient to cause rapid diffusion of. p-dopant from the two neighboring p-doped layer? -4 and 6 into layer 5 and to convert it into p-type. With the aforementioned laser energy, the p dopants pass entirely through the layer 5 to form a p type channel therethrough at the narrow region heated by the laser beam. If the laser beam is scanned along the structure, an elongated channel or stripe 8 is formed along the length thereof. The resulting structure thus comprises a continuous p type region from the layer 6 through the stripe 8 to the layer 4, the layer 4 forming a p-n junction with the layer 3. On either side of the stripe 8, the original n type layer 5 forms a p-n junction with the layers 4 and 5, thereby restricting current flow from the layer 6 to the layer 4 only through the narrow stripe 8. The narrow current path thus provided by the stripe 8 gives rise to a high current density which, as known, is desirable in laser devices of this type. In order that the optical energy be absorbed by layer 5, the bandgap wavelength of layer 5 must be larger than the wavelength of the laser. The bandgap wavelengths, λ , of In ι_ x Ga x As v p ι _ v alloy compounds which are lattice- matched to InP have been measured. They are: (in micron 0.000 0.000 0.920 0.083 0.155 1.005 0.171 0.373 1.125 C.226 0.472 1.176 0.250 0.531 1.225 0.275 0.585 1.275 0.300 0.636 1.298 0.305 0.658 1.343 0.358 0.790 1.452 0.406 0.880 1.513 0.475 1.000 1.650 The bandgap wavelength may also be computed from the following formula: \„ (in microns) = 1.24/(1.35 - 0.72y + 0.12y^) Thus for y = 0.373, λ_ = 1.125 microns, which wavelength i larger than the laser wavelength 1.06 microns. Therefore compound having this value of y will absorb laser light. On the other hand, InP has λ = 0.92C microns, which y wavelength is smaller than a 1.06 micron laser wavelength and will therefore be transparent to the laser light. The laser light can " only penetrate into the layers 5 and 4 les than 1 micron in combined depth. Thus the laser energy will not reach the active layer 3. This process may be followed, in a second embodiment of the invention, by the step of applying laser radiation from a second laser. This radiation is absorbed in active layer 3 of the device and anneals that layer. apply 1.2 micron radiation from an F-center laser, which radiation is absorbing only in the alloy of layer 3, Ga 0.30 In 0.70 As 0.636 p C364' which alloy has a λ g = 1.3 micron. This step reσrows the active layer to provide a better interface for the lasing pn junction.

Claims: Claims 1. A method of fabricating a semiconductor device comprising forming first, second, and third semiconductor layers, the second layer being between, contiguous with and having a different conductivity from the first and third layers, and focusing radiation, with respect to which the second layer is absorptive but the third layer is substantially transparent, from a laser source through the third layer onto a selected area of th second layer so as to induce localized diffusion of dopan impurities to change the conductivity type of the second layer in the selected area. 2. A method as claimed in claim 1 wherein the radiation is scanned so that successive selected areas fo a stripe in the second layer. 3. A method as claimed in claim 1 or claim 2 wherein the radiation has a wavelength of 1.06 micrometers and the laser source is a Nd: YAG laser. 4. A method as claimed in claim 3 wherein the radiation is pulsed, having a pulse width of 60 nanoseconds, and is focused to a spot having a 1 micrometer diameter and an energy density per pulse of 2 Joules per square centimeter. 5. A method as claimed in any of the preceding claims wherein the third layer is of p + -type InP. 6. A method as claimed in any of claims 1 to 4 wherein the third layer is of a p + -type quaternary of th form In ι_ x Ga x As y P ι_y « 7. A method as claimed in claim 5 or claim 6 wherein the first and second layers are of quarternary compounds of the form In j ^Ga^s. ?- ] ^,.. 8. A method as claimed in any of the preceding claims wherein the first layer is formed on and contiguous with a substantially undoped semiconductor layer whereby laser-diode structure is formed. 9. A method as claimed in claim 8 wherein, subsequently to the said reversal of conductivity type, radiation from a second laser source to which the substantially undoped layer is. absorptive but the first, second and third layers are substantially transparent is applied to anneal the substantially undoped layer. 10. A method as claimed in claim 9 wherein the second laser source is an F-center laser which produces radiation having a wavelength of 1.2 micrometers. ' 11. A method as claimed in any of claims 8 to 10 wherein the substantially undoped layer is of In QG- 3o As 636 P 364 fc ^ e f*** rst aΩ( second layers are respectively of p-type and n-type In 829 Ga 171 As 373 P 627 and the third layer is of ρ + -type InP.

Inventors: TIEN P
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC CO INC
Application year: 1981
Main classification: H01S3
IPC codes: H01L21, H01L29, H01S5