EXPERIMENTAL TECHNIQUE OF CAVITY TUNING

In order to simplify the growth during molecular beam epitaxy (MBE), we designed the minimum number of layers.  The design required a p-i-n diode with MQWs in the intrinsic region plus an etchstop layer for the substrate removal.  The use of the p- contact as a reflective surface avoided a DBR growth.  (The metal stack included Au as the material directly in contact with the GaAs p-cap in order to maximize the reflectivity at that interface.)   The top mirror is formed just from the semiconductor-air interface. Another possibility would have been to deposit a dielectric mirror on the top of the GaAs

buffer, after the substrate removal, to increase the top mirror reflectivity, as in [18]. However, the deposition of a dielectric mirror was undesirable for our design because by keeping the top mirror reflectivity low, the device maintains a wide wavelength range.

If a dielectric mirror was to be avoided, it remained necessary to adjust to growth variations, once the epitaxial growth has been completed.   A post-integration cavity thickness tuning technique was developed to solve this problem [19].   Essentially, the strategy was deliberately to grow the structure a little too thick, to fabricate the device structures, to measure the positions of the Fabry-Perot and exciton peaks, and finally to remove a small amount of semiconductor material in a highly controlled fashion in order to shift the Fabry-Perot resonance to the appropriate wavelength.

Using  the  transfer  matrix  method  simulation  approach  described  earlier,  the device could be designed to take advantage of the resonant effect.  The device processing steps had, by this point, become well-understood.  The position of the Fabry-Perot and exciton peaks could be observed using the tunable Ti:sapphire laser in the probe station in the optics lab before the epoxy removal step (i.e. with no applied bias).   Clearly the critical  step  was  the  final  one  –  removing  a  small  amount  of  semiconductor  in  a controlled manner.  The key was to use a highly selective wet etch that removed only the native oxide layer that formed on the surface of GaAs without etching GaAs [20].  An added benefit of this etching technique is that it has been shown to smooth the surface by a factor of about 5-10 (from rms roughness of 10-20 Å down to ~2 Å).   Because the oxide layer is formed only through the first few monolayers of the surface and can be selectively removed, the cavity thickness can be carefully tuned by increments equal to this oxide thickness (~10-20 Å).

The procedure for cavity tuning was then clear.  Once the devices were fabricated, flip-chip bonded, and exposed via substrate removal, the etchstop was removed by a 90 s dip in HCl:H2O (1:1).   This exposed the GaAs buffer layer, which was intentionally grown a bit too thick, as noted previously.  An oxide was formed by a 30 s dip in H2O2 and selectively removed by a 30 s etch in HCl:H2O (1:1) or NH4OH:H2O (1:9) [20].  We refer to this as one “tuning cycle.”   Fig. 4.11 below shows the result of both 2 tuning cycles (blue curve) and of 10 tuning cycles (green curve) with respect to the original absorption vs. wavelength data (red curve).  The movement of the Fabry-Perot resonance

to shorter wavelengths at a rate of about 0.75 nm per tuning cycle is consistent with a

simple calculation.

This post-integration cavity adjustment enabled the improvement of the contrast ratio and compensated not only for the variations associated with growth thickness from run to run, but also for variations within a single run from wafer edge to wafer center. The device arrays can be integrated and quickly scanned optically to determine the degree of cavity tuning required.   Since cavity tuning was essentially the last step, it could conceivably be done after dicing up the integrated wafer structure.  Alternatively, the wafer could be sectioned into various smaller parts (without complete dicing) and treated with cavity tuning to a varying degree in these pieces.  

Once the Fabry-Perot resonance was properly aligned with respect to the QCSE in wavelength, a significant increase was observed in the contrast ratio for a given voltage drive.  The transfer matrix method simulation and the experimental data matched each other quite closely.