Semiconductor laser light sources are key components in the rapidly expanding field of optoelectronics. The properties of high packing density, large scale integration into microchips, and low manufacturing cost make Vertical Cavity Surface Emitting Lasers (VCSELs) in particular uniquely attractive for applications, such as massive parallel computing, interconnects capable of up to THz (Tera (1012)) bandwidth, and optical information storage technologies.
A standard mesa type VCSEL, is shown schematically as a cross section in FIG. 1, and includes a group of n-doped semiconductor segments 1, a group of p-doped semiconductor segments 2, and an active region 3 for light emission. The groups of n-doped and p-doped semiconductor segments, 1 and 2, are each commonly referred to as Bragg mirrors. Bragg mirror 1 has, for example, alternating n-doped semiconductor layers of high refraction index N—AlxGa1-xAs (4, 6, 8, 10) and low refraction index N—AlyGa1-yAs (5, 7, 9) formed as a periodic sequence or periodic arrangement of layers. Bragg mirror 2 has, for example, alternating p-doped semiconductor layers of high refraction index P—AlxGa1-xAs (11, 13, 15, 17) and low refraction index P—AlyGa1-yAs (12, 14, 16) and a metal conductor layer 18. The active region 3 includes, for example, a layer of N—AlyGa1-yAs 19, one (or more) active layers (Quantum Wells QW) of low band-gap p-GaAs 20, and a layer of P—AlyGa1-yAs 21 formed sequentially and disposed between Bragg mirror 1 and Bragg mirror 2.
In the typical VCSEL, an electron current Je and a hole current Jp flow in opposite directions through n-doped GaAs semiconductor segments of Bragg mirror 1 and p-doped GaAs semiconductor segments of Bragg mirror 2 until they reach the active region (λ) 3 formed by one or more thin layers of a third semiconductor material sandwiched between the n-doped Bragg mirror 1 and the p-doped Bragg mirror 2. The active region 3 provides light emission via electron-hole pair recombination. The envelope of the radiation profile is characterized by the diameter 2w, w being the radiation waist. The active region 3 material has a smaller energy gap than the semiconductor material of the abutting Bragg mirrors so that (a) the emitted frequency will not be reabsorbed outside the active region and (b) a potential well forms at the p-n junction greatly increasing the carrier density there. The carrier density in the active region, and thus the photon production rate under given external current, increases by orders of magnitude when sub-micron thick active layer structures, known as quantum wells or superlattices are used.
The current-density profile and the light intensity profile in a standard, cylindrical cross-section, single mode VCSEL such as the one shown in FIG. 1, do not match. The light intensity is peaked at the center of the cross-section (axis), as dictated by the cavity fundamental mode profile, while the current intensity is uniform across the cross section because of the uniform resistivity across the cylindrical VCSEL structure of FIG. 1. The rate of electron-hole recombination, being proportional to the emitted laser light intensity, is therefore higher near the cylinder axis than the rate of replenishment by the current, resulting in carrier depletion in the center of the cylinder.
Central carrier depletion causes undesirable mode switching of the VCSEL. As a consequence of carrier depletion (hole-burning) at the center of the cavity cross section, currently manufactured VCSELs have a tendency to switch into higher modes at modest output power levels, when the device current is only a few times above threshold (start-up) current. The resulting change in the radiation profile is highly undesirable for a majority of optoelectronic applications.
To prevent that depletion one needs a non-uniform current profile that peaks at the center so as to provide more carriers where the carriers are consumed faster. Increasing the conductivity near the center of the cavity cross-section has been tried to counteract center cavity carrier depletion. Present methods of achieving increased carrier conductivity at the center of the cavity include ion implantation and oxide aperture techniques. Although these techniques are successful in reducing the lasing threshold they still suffer from multi-mode switching at low currents. Thus, these proposed methods do not improve mode switching.
Moreover, ion implantation and oxide aperture techniques have the disadvantage of requiring time consuming and cost increasing wafer post-processing, whereby grown wafers are removed from the growth chamber and subjected to additional processing (exposure to ion bombardment or oxidizing chemical agent). Therefore, the present VCSELs do not provide a fabrication process that can attain the low cost associated with standard semiconductor integrated circuit VLSI processing approach.
Another important issue, also stemming from the on-axis carrier depletion, is the appearance of an optical tail after the laser current has been turned off. This is important when VCSELs are employed in producing square optical pulses in digitized optical signals or digital communications. Elimination of imperfections in the optical pulse modulation is crucial in achieving better bit-error-rate (BER); the latter sets a limit on the information transmission rate and prevents harvesting the full optical fiber communication bandwidth.