Study of the photodetection behaviour of oxide-confined verticalcavity surface-emitting lasers (VCSELs) C. Bringer*, V. Bardinal, L. Averseng, J. Polesel-Maris, T. Camps, P. Dubreuil, C. Fontaine, E. Bedel-Pereira, C. Vergnenegre LAAS-CNRS, 7, avenue du colonel Roche, F-31077 Toulouse cedex 4, France Elsevier use only: Received date here; revised date here; accepted date here
Abstract We present a theoretical and experimental study of oxide-confined top -emitting VCSELs used as resonant detectors. Oxidemode influence on the detection spectrum is discussed. © 2001 Elsevier Science. All rights reserved. Keywords: semiconductors microcavity; detection quantum efficiency; reflectivity; VCSEL; RCE detector; oxide modes
1. Introduction Vertical semiconductor microcavities are very attractive nanoscale artificial structures due to their high internal optical confinement. The increasing need for high-speed, compact and low cost devices for optoelectronic applications such as bi-directional optical interconnects, has recently led to focus on the ability for the VCSELs (Vertical-Cavity SurfaceEmitting Lasers) to be used as Resonant Cavity Enhanced (RCE) photodetectors for dual purpose operation. Indeed, despite a narrow spectral detection sensitivity due to the high finesse of their cavity, VCSELs diodes are able to efficiently detect lights at their resonance wavelength only by changing their applied voltage polarity. Here we present calculations *
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and measurement of the detection sensitivity of a single-mode oxide-confined top-emitting VCSEL. In this structure, the internal diameter of the top electrodes has been chosen to be much higher than the buried oxide layer diameter to increase detection capability.
2. Quantum efficiency calculation and experiments The resonant-microcavity-enhanced detection quantum efficiency of the VCSEL structure has been calculated using a transfer matrix formalism, including microcavity effects [1] and quantum wells absorption dispersion. The bottom distributed Bragg reflector (DBR) consists of 30,5x (AlAs/Al0.12Ga0.88As) periods, the upper DBR of 25
2
35
0.8
20 1.2 1.0
∆λ ∆λ = 1 7 n m
(b)
(a)
the
0.8
Photocurrent (a.u)
10 0.6 0.4
0.2 0 810
815
820
825
830
835
840
0.0 845
wavelength (nm)
25 20
0.6 15 0.4
10 5
0.2 as-grown VCSEL 0.0 0.2
between
Fig. 2: Photocurrent and reflectiv ity spectra measured in the emission zone (a) (dashed lines) and in the periphery (b) (solids lines) of the VCSEL submitted to a -5V reverse voltage.
30
FWHM
Detection Quantum efficiency
1.0
to the refractive index difference oxidized and non-oxidized zones [2].
Reflectivity
periods, while the cavity contains three GaAs/Al0.3 Ga0.7 As quantum wells. The maximum detection quantum efficiency and the corresponding full width at half maximum (FWHM) are shown in Fig. 1 as a function of the top mirror reflectivity. In structures optimised for laser emission, a maximum detection quantum efficiency of 9% should be obtained, which corresponds to an optical detection sensibility of 0.06 A/W at the Fabry-Pérot wavelength (830 nm). For optimised photodetection, twelve periods of the top mirror have to be removed to increase the FWHM, but in this case laser operation is not achieved. For dual-purpose application, a compromise has then to be found.
0.4
0.6 0.8 Top mirror reflectivity
0 1.0
Fig. 1: Maximum detection quantum efficiency and full width at half maximum (FWHM) of photodetection peak calculated for the initial VCSEL vs. its top mirror reflectivity.
Spatially localised photocurrent and reflectivity spectra measurements have been performed in the 750-920 nm range with a 0.8 nm resolution using a tuneable titanium : sapphire laser with a spot diameter as small as 8 µm on the surface of the VCSEL (Fig. 2). The FWHM value of the detection peak obtained at the centre (a) is 2nm as expected but its maximum sensitivity is higher than the calculated one (0.18 A/W). This quantum efficiency discrepancy (27% instead of 9%) can be attributed to a weaker absorption coefficient and/or to a lower top mirror reflectivity than the calculated values. In the lateral zone (b), a single detection peak is also observed but is found to be 17 nm blue-shifted from the central one, thus leading to a five times lower detection sensitivity. This demonstrates the existence of “oxide modes” originating from the optical path change due
Indeed, this wavelength shift is the same as the shift observed between the dips of reflectivity spectra measured in non-oxidized and oxidized zones (Fig.2). In that case, modelling indicates that this resonance shift originates from the change in refractive index due to AlAs oxidation (Al2 O3: n = 1.6) of lateral zones. In these zones, the decrease of the detection peak can then be explained by a misalignment between the maximum of the quantum well gain and the cavity mode.
3. Conclusions We have theoretically and experimentally studied the optical detection behaviour of oxide-confined VCSELs used as RCE detectors. Simultaneous measurements spatially localised photocurrent and reflectivity spectra have demonstrated the presence of oxide modes in the lateral zones of the device in detection.
References [1] M.S. Ûnlü and S. Strite, J. Appl. Phys. 78, 607 (1995) [2] K.D. Choquette, K.L. Lear, R.P. Schneider, JR, and K.M. Geib, Appl. Phys. Lett. 66 (25) p. 3413 (1995)
