Vertical-cavity, surface-emitting diode lasers offer many advantages. They may be integrated in a two-dimensional array on an opto-electronic chip. They may have diameters of the order of tens of micrometers or less so that large arrays occupy little space. Their quantum efficiency is fairly high. Nonetheless, the vertical-cavity, surface-emitting diode lasers proposed to date have several disadvantages. A recent review of progress in the field has been summarized by Iga et al in a technical article entitled "Surface emitting semiconductor laser array: Its advantage and future", appearing in Journal Vacuum Science Technology A, volume 7, 1989 at pages 842-846.
Many of the proposals involve growing a vertical laser structure of III-V materials on a GaAs substrate. This structure includes some lateral definition of the active region or current injection means into the active region. Then, the substrate is locally thinned by etching from beneath in the area of the laser structure so as to expose the laser structure in a back-surface via hole bored into the substrate. A partially transmitting metallic or dielectric stack mirror, forming one side of the Fabry-Perot cavity of the laser, is then deposited on the back surface of the laser structure exposed in the hole. The laser light is emitted from the bottom of the chip, through the hole bored into the substrate.
The process of boring a hole through the back of the substrate suffers many disadvantages, such as the critical stopping of the back-side etching at the back mirror surface and the front-to-back alignment. Generally, such back-side processing is difficult and considered undesirable for production lasers.
Jewell et al have disclosed another structure for vertical-cavity, surface-emitting lasers in U.S. patent application, Ser. No. 07/380,996, filed July 17, 1989, now issued as U.S. Pat. No. 4,949,350 and in a technical article entitled "Low-threshold electrically pumped vertical-cavity surface-emitting microlasers", appearing in Electronics Letters, volume 25, 1989 at pages 1123-1124. They grew a laterally undefined, vertical-cavity laser structure on a conducting GaAs substrate. The structure included semiconductor Bragg reflectors on both ends of the cavity. The active layer contained strained quantum wells of InGaAs. Lateral definition was achieved by chemically assisted, ion-beam etching which produced precisely vertical pillars .about.5 .mu.m in height and 2 .mu.m or more in diameter. Each pillar was a laser. Contact was made to the top of each pillar, the conducting substrate serving as the other electrode. Laser light was emitted through the GaAs substrate which passed the 950 nm light emitted by the InGaAs.
The Jewell et al laser structure presents several difficulties. Providing permanent leads to the tops of the pillars is difficult. The current passes through both Bragg reflectors. Therefore, the Bragg reflectors were made of semiconductor materials rather than of a combination of insulators, which is well known to be more efficient. The semiconductor Bragg reflectors were made of alternating layers of GaAs and AlAs, which have a relatively small difference of refractive indices, .DELTA.n.apprxeq.0.62. Thus, many layers were required for high mirror reflectance. Nonetheless, there was a semiconductor heterojunction at each interface. To reduce the junction resistance in order to achieve a high quantum efficiency, a graded superlattice of GaAlAs was provided between the GaAs and AlAs layers of the Bragg reflector. For these reasons, they grew a laser structure including over 500 layers. Clearly, it would be desirable to reduce the number of layers. Further, the Jewell et al laser emitted from the unpatterned back surface of the substrate. It is preferable in many applications that the light be emitted from the patterned front surface. In particular, GaAs has a band-edge at .about.870 nm, near and below which it will not transmit. Thus, GaAs cannot be used as a substrate for a back-surface emitting laser diode emitting at this wavelength or in the visible below .about.700 nm unless a hole is bored in the substrate. Yet further, Jewell et al used a conducting substrate to provide the back contact. If their laser diode is to be integrated with other electrical components on a common substrate, electrical isolation cannot rely simply on lateral displacement on such a conducting substrate.
Orenstein et al have proposed another surface-emitting, vertical-cavity diode laser in U.S. patent application, Ser. No. 07/480,117, filed Feb. 14, 1990 and in a post-deadline technical paper entitled "Vertical cavity laser arrays with planar lateral confinement" appearing in Postdeadline Papers, Optical Society of America, 1989 Annual Meeting, Orlando, Fla., Oct. 15-20, 1989 at page PD22. Their vertical laser structure was the same as for Jewell et al. However, Orenstein et al did not etch pillars but instead implanted resistance-increasing protons into the upper Bragg reflector surrounding the intended lasers. Thereby, they avoided the problem of contacting the high aspect-ratio pillars. Although, the Orenstein et al laser offers some advantages over that of Jewell et al in some applications, it still suffers the disadvantages of back-surface emission and a large number of layers.
Ogura et al have disclosed a front-surface emitting laser in a technical article entitled "Surface-emitting laser diode with distributed Bragg reflector and buried heterostructure" appearing in Electronics Letters, volume 26, 1990 at pages 18-19. They deposited a semiconductor lower Bragg reflector, lower spacer, active region and upper spacer. Then they etched a pillar similarly to Jewell et al. Thereafter, they regrew semiconductor material surrounding the pillar which current isolated the active region and below but which provided a lateral current contact to the upper spacer. Thereafter, they deposited a Si/SiO.sub.2 dielectric stack over the pillar for the upper mirror. Etching the pillar is believed to introduce surface states. The threshold current reported by Ogura et al is considered excessive.