1.  Field of the Invention
The present invention relates generally to light emitting devices, and more particularly to an improved vertical-cavity light emitting device having an improved intra-cavity lens structure formed by selective oxidation. 
2.  Description of the Prior Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Surface Emitting Lasers (SELs) or Light Emitting Diodes (LEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications.  Vertically emitting devices have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically emitting devices.  The circular nature of the light output beams from these devices also make them ideally suited for coupling into optical fibers as in optical interconnects or other optical systems for integrated circuits and other applications. 
VCSELs or Surface Emitting Lasers SELs whose current flow is controlled by lateral oxidation processes show the best performances of any VCSELs in terms of low threshold current and high efficiency.  In oxidized VCSELs the oxidation occurs in the lateral direction from the sides of etched mesas in the VCSEL wafers, typically under the conditions of 425.degree.  C. temperature with high water-vapor content.  VCSELs or any other vertical light emitting devices employing laterally oxidized layers have been strictly limited only to structures which have been grown upon gallium arsenide (GaAs) substrates.  For further details, see U.S. Pat.  No.  5,493,577, by Choquette et al. 
Generally, oxide apertures are utilized to control current flow and thus function as current apertures.  The appreciation of the use of an oxide aperture for its ability to function as a lens is quite new.  It is now known that a lensing aspect of the oxide structure greatly reduces diffractive losses in VCSEL cavities and that this reduction is largely responsible for the greatly improved efficiencies.  It is also known that the lens formed by a simple oxide aperture is far from ideal.  Light scattered from the abrupt oxide/semiconductor interface is lost from the cavity oscillation.  Similarly, optical aberrations in the lens result in cavity losses.  Thus it follows that an oxidized structure which more closely resembles an ideal aberration-free lens may form the basis for VCSELs with still greater efficiencies and lower thresholds. 
Since VCSELs are presently the subject of intense research and development, a great deal of results and advancements are published monthly.  The following is a summary of the prior art documents which are relevant to the problem of utilizing oxide apertures. 
The lens-like behavior of oxide-defined apertures are described by Coldren et al.  in Appl.  Phys.  Lett.  68, pp.  313-315 (1996) and in Hegblom et al.  in Appl.  Phys.  Lett.  68, pp.  1757-1759 (1996).  Only single layers are used and the publication describes the importance of minimizing the scattering at the inner boundary of the aperture.  Means for minimizing the scattering are the use of a thin oxidized layer, e.g. 200 .ANG.,  and a tapering of the oxidation in the aperture.  Tapering of the aperture has been accomplished by varying the Al concentration within the oxidizable layer, the oxidation proceeding further inward into the aperture for the portion of the layer with higher Al concentration.  The tapering is calculated to reduce scattering significantly. 
The oxidation rate of materials such as AlGaAs is a sensitive function of the Al concentration as described by Choquette et al.  in Electronics Letters 30, pp.  2043-2044 (1994).  It is therefore possible to control the extent of oxidation for multiple layers in a single process by having the layers be of different material compositions.  It has also been found however, that the precise composition of a pre-oxidized layer may have a profound effect on the reliability of the oxidized structure.  For example, oxidized Al.sub.0.98 Ga.sub.0.02 As layers appear to be much more reliable than oxidized AlAs layers.  Thus, it is preferred that all oxidizable layers in the structure have nominally the same material composition. 
Variation of thickness in thin layers of oxidizable material, e.g. AlGaAs, also results in variation of the rates of lateral oxidation.  This variation with thickness is described somewhat inaccurately by Dallesasse et al.,  in Applied Physics Letters 57, 2844-2846 (1990) as a variation with the coarseness of an AlGaAs "alloy."  It was observed that the oxidation process occurred much more slowly in a fine scale alloy represented by a superlattice having 70 .ANG.  thick AlAs and 30 .ANG.  thick GaAs layers as compared to a coarser "alloy" comprising 150 .ANG.  AlAs and 45 .ANG.  GaAs layers.  The variation in oxidation rate with oxidizable layer thickness (when the layers are sufficiently thin, e.g. 300 .ANG.)  may be used for the creation of optical and electronic structures comprising multiple oxidized layers in which the layers oxidize at different rates and to different extents. 
Related to the use of thin oxidizable layers is the control of the oxidation process by layer interdiffusion which is described by the present inventor in U.S. patent application Ser.  No.  08/574165.  The present invention does not require layer interdiffusion but may be combined with it. 
The oxidation rate of AlGaAs is also sensitive to the doping type, e.g. p-type or n-type, as reported by Kish et al.  in Applied Physics Letters, vol.  60, pp.  3165-3167 (1992).  They show that Al.sub.0.6 Ga.sub.0.4 As etches more than 3 times faster when p-doped at a concentration of 9.times.10.sup.18 cm.sup.-3 as compared to Al.sub.0.6 Ga.sub.0.4 As which is n-doped at 4.times.10.sup.17 cm.sup.-3.  For a given dopant type, the variation does not depend specific dopant used.  The variation is explained by a variation in the Fermi level of the semiconductor which varies with the concentration of charged defects from the dopant. 
A thorough discussion on how the oxidation rate varies with temperature is described by Ochiai et al.  in Applied Physics Letters, vol.  68, pp.  1898-1900 (1996).  The authors show that for low oxidation temperatures and small oxidation depths, the oxidation depth varies linearly with time.  For higher oxidation temperatures and/or large oxidation depths, the depth varies as the square root of time. 
MacDougal et al.  in Photonics Technology Letters 8, pp.  310-312 (1996) describe electrically pumped VCSELs in which both mirrors are oxidized throughout their entire lateral extents and which further comprise oxide-defined current apertures above and below the active region.  All the oxidized layers are a quarter-wave thick at the emission wavelength of .about.9940 .ANG.. The refractive index of 1.6 for the oxidized layers implies a physical thickness &gt;1500 .ANG.. To produce oxidized layers which are completely oxidized in the mirrors and similar oxidized layers which form apertures above and below the active region, multiple etches and oxidation processes are performed with a silicon nitride capping between the oxidation processes to prevent further oxidation of the aperture layers.