David H. Christensen

High-speed 90% Quantum-Efficiency p-i-n Photodiodes with a Resonance Wavelength Adjustable in the 795-835 nm Range

We report GaAs/AlGaAs-based high-speed, high-efficiency, resonant cavity enhanced p–i–n photodiodes. The devices were fabricated by using a microwave-compatible fabrication process. By using a postprocess recess etch, we tuned the resonance wavelength from 835 to 795 nm while keeping the peak efficiencies above 90%. The maximum quantum efficiency was 92% at a resonance wavelength of 823 nm. The photodiode had an experimental setup-limited temporal response of 12 ps. When the system response is deconvolved, the 3 dB bandwidth corresponds to 50 GHz, which is in good agreement with our theoretical calculations.

High bandwidth-efficiency resonant cavity enhanced Schottky photodiodes for 800–850 nm wavelength operation

High-speed resonant cavity enhanced Schottky photodiodes operating in 800–850 nm wavelength region are demonstrated. The devices are fabricated in the AlGaAs/GaAs material system. The Schottky contact is a semitransparent Au film which also serves as the top reflector of the Fabry–Perot cavity. The detectors exhibit a peak quantum efficiency of η=0.5 at λ=827 nm wavelength and a 3 dB bandwidth of more than 50 GHz resulting in a bandwidth-efficiency product of more than 25 GHz.

Optical constants of (Al0.98Ga0.02)xOy native oxides

We report the optical constants of oxidized crystalline and low-temperature-grown (LTG) Al0.98Ga0.02As films, as determined by variable angle spectroscopic ellipsometry. Data were acquired at three angles of incidence over 240–1700 nm and fitted to a Cauchy dispersion function. For oxidized crystalline material, we observe a variation in the real index of ±0.5% for layer thickness variations of ±6%. We show that upon oxidation, LTG material can expand by >25% while crystalline material contracts by <2%. Atomic force microscopy analysis indicates thickness-dependent variations in the oxide microstructure. Additionally, an optical scattering loss of 2.1×10−4%/pass is calculated based on surface roughness measurements for a thin layer of oxidized crystalline material.

High-speed high-efficiency large-area resonant cavity enhanced p-i-n photodiodes for multimode fiber communications

In this letter, we report AlGaAs-GaAs p-i-n photodiodes with a 3-dB bandwidth in excess of 10 GHz for devices as large as 60-/spl mu/m diameter. Resonant cavity enhanced photodetection is employed to improve quantum efficiency, resulting in more than 90% peak quantum efficiency at 850 nm.

Characterization of vertical‐cavity semiconductor structures

Several analytical tools are applied to characterize vertical‐cavity surface‐emitting laser structures grown on GaAs wafers. These epitaxial structures are amenable to x‐ray, electron‐beam, and optical metrologies. Cross‐sectional scanning electron microscopy and transmission electron microscopy were used to measure layer thicknesses and uniformity. Photoluminescence wafer mapping was used to determine alloy composition uniformity across the wafer. Photoreflectance was also used to determine alloy composition. Cross‐sectional microphotoluminescence was used to measure average alloy compositions in the top and bottom mirrors. Reflectance spectroscopy was used to characterize the cavity resonances and mirror layers. Double‐crystal x‐ray diffractometry (DCXRD) was used to characterize mirror layer dimensions, uniformity, and average alloy composition. Excellent agreement was found among these measurement techniques and between simulations and measurements. The results demonstrate the accuracy of the device s...

Spatio-spectral mapping of multimode vertical cavity surface emitting lasers

This paper reports the spatial and spectral characteristics of multimode vertical cavity surface emitting laser (VCSEL) emission using near field scanning optical microscopy (NSOM). We have investigated 15 /spl mu/m diameter proton implanted 850 nm devices used in 2 Gb/s multimode fiber optic links. We have studied their near field spatial distribution of intensity and the lasing wavelengths of their transverse modes. False colored images were created to portray relative intensity, and spatial distribution information for each transverse mode. Correlation with shear force data allowed mapping of the optical distributions to topographical features of the device. Lasing filaments were observed at high drive currents. Spatially overlapping transverse modes were observed to compete for available gain while spatially isolated modes coexisted.

Vertical-cavity surface-emitting lasers with low-ripple optical pumping windows

A general technique for numerically optimizing the optical admittances in GaAs QW vertical-cavity laser structures is used to suppress the interference ripple in the typical reflectance/transmittance spectra. This technique is applicable to any vertical-cavity device whose photonic properties at various wavelengths requires modification for specific applications. In this paper, we report the use of this optimization method to enhance the coupling of pump light into 850-nm vertical-cavity surface-emitting lasers (VCSELs). We have designed and fabricated novel lasers which contain a wideband window of low reflectance amidst the typical interference fringe spectrum. The 750-800-nm region for the low-ripple design has an average reflectance of 5%; the peak-to-peak amplitude of the ripple is 0.25%. The sensitivity of these devices to temperature variations and layer-thickness manufacturing variations is also studied. The low-ripple pump window shifts at a rate of 0.036 nm//spl deg/C, the peak-to-peak ripple of the reflectance varies less than 2%, and the pump bandwidth remains constant, over temperatures ranging from 0/spl deg/C to 100/spl deg/C. The low-ripple structure substantially reduces the temperature and wavelength variation of the pump-field overlap by creating a window of nearly constant reflectance.

Pump intensity profiling of vertical-cavity surface-emitting lasers using near-field scanning optical microscopy

We have mapped the internal pump intensity distribution of an optically pumped vertical-cavity surface-emitting laser. Spontaneous emission from quantum wells placed throughout the distributed Bragg reflectors is correlated to the pump intensity. The emission is monitored along the cleaved edge using the high spatial resolution and shallow depth of field provided by near-field scanning optical microscopy. Our results show a distinct buildup of optical intensity between the mirror stacks. Simulations performed using the transfer matrix method match well with the experimental data.

Narrow photoluminescence linewidths from ensembles of self-assembled InGaAs quantum dots

Self-assembled InGaAs quantum dots have been grown using alternating molecular beams of In, Ga, and As2. The size distribution changes from bimodal to monodisperse as the quantum dots grow larger. Room-temperature photoluminescence experiments on ensembles of these quantum dots show that the emitted intensity remains high as the center wavelength changes from about 1130 to 1345 nm. The linewidths are less than 30 meV for all samples studied, with the narrowest measured linewidth being 18 meV at a peak emission energy of 930.1 meV (1333 nm).

Noise reduction in optical in situ measurements for molecular beam epitaxy by substrate wobble normalization

We demonstrate a normalization method for removing noise introduced into optical in situ measurements by sample rotation wobble during molecular beam epitaxy. The technique consists of measuring the angle of rotation of the sample through optical triggers attached to the sample manipulator rotation drive, acquiring a normalization curve at the various trigger points, then applying the normalization appropriate to each trigger to subsequent data. This cyclic normalization is demonstrated on normal-incidence optical reflection data and atomic absorption measurements in which the flux-monitor light beam is reflected from the sample to allow determination of layer thickness in addition to atomic flux. Noise reductions by factors of 3 to 30 were observed in both systems, with the larger improvements for samples with larger wobble angles, while preserving the original time resolution of the data. We achieve normalized optical reflectance data with a noise standard deviation of 1% over a period of one to two hou...