E.H. Bottcher

Ultrafast semiinsulating InP:Fe-InGaAs:Fe-InP:Fe MSM photodetectors: modeling and performance

The response of InGaAs metal-semiconductor-metal (MSM) detectors as a function of all relevant detector parameters is investigated experimentally and theoretically. The calculations of the intrinsic detector response are carried out by taking fully into account the two-dimensional character of the problem. The generalized Ramo theorem is applied to obtain the induced current, which is the only measurable quantity. The calculations clearly demonstrate the distinct contributions of electrons and holes to the response. The extrinsic response is derived taking parasitics, which are obtained in different ways, into account. A full-wave analysis of the detector circuit, solving Maxwell equations, is performed. The resulting S/sub 11/ scattering parameter is measured by means of a network analyzer. Fitting of the calculated and measured S/sub 11/ parameters to an equivalent circuit diagram yields the values of the parasitic elements. Perfect agreement between calculated and measured data is found. >

Measurement and simulation of the turn-on delay time jitter in gain-switched semiconductor lasers

The turn-on delay time jitter of four different unbiased gain-switched laser types was determined by measuring the temporal probability distribution of the leading edge of the emitted optical pulse. One single-mode 1.5- mu m distributed feedback laser and three multimode Fabry-Perot lasers emitting at 750 nm and 1.3 mu m were investigated. The jitter was found to decrease for all lasers with increasing injection current. For multimode lasers it decreases from 8 ps excited slightly above threshold down to below 2 ps at three times the threshold current. The jitter of the distributed feedback (DFB) laser is a factor of 3-5 larger than the jitter of the three multimode lasers. A model for predicting the turn-on delay time jitter is presented and explains the experiments quantitatively. >

Influence of space charges on the impulse response of InGaAs metal-semiconductor-metal photodetectors

The impulse response of interdigitated metal-semiconductor-metal photodetectors fabricated on an Fe-doped InGaAs absorbing layer and an Fe-doped InP barrier enhancement layer is investigated. For ultra-short pulse excitation of 150 fs at lambda =620 nm the photoresponse is found to be less than 13 ps FWHM for detectors with 1 mu m finger spacing. Above a certain level of illumination, the peak amplitude increases sublinearly and the relative contribution of the tail to the detector response is appreciably enhanced. The screening of the electric field by photo-generated space charges is responsible for this nonlinearity. For detectors with 5 mu m finger spacing illuminated with 1.3 mu m light pulses (FWHM=33 ps), space charge perturbation of the impulse response manifests itself by a decrease of the FWHM and an increase of the fall time with increasing illumination level. The practical consequences for the performance of MSM detectors in various applications are discussed. >

Gain modulation of unbiased semiconductor lasers: ultrashort light-pulse generation in the 0.8 μm-1.3 μm wavelength range

Stable production of 15ps pulses having a peak power of 200 mW from fully gain-modulated, unbiased GaAs double-heterostructure and multiple-quantum-well lasers emitting in the 0.8 μm-0.9 μm wavelength range and from InGaAsP double heterostructure lasers emitting in the 1.2 μm-1.3 μm range is reported. The shortest pulses obtained from GaAs DH lasers show a width as small as 2ps. The optical pulses represent the first relaxation oscillation of the laser. Special double avalanche generators are developed to produce large injection-current pulses with widths down to 12Sps at repetition rates up to 10 MHz. Numerical and approximate analytical solutions of a coupled-rate-equation model of a semiconductor laser incorporating bimolecular recombination and realistic injection current pulse shapes are presented. The theoretical predictions are found to be in excellent agreement with the experimental results.

Excitonic and electron‐hole contributions to the spontaneous recombination rate of injected charge carriers in GaAs‐GaAlAs multiple quantum well lasers at room temperature

The spontaneous recombination rate at room temperature of GaAs‐GaAlAs multiple quantum well lasers is investigated at carrier densities of 2×1017–1.7×1018 cm−3 using the small‐signal electroluminescence technique. A monomolecular, partly excitonic, and a bimolecular term contribute to the recombination rate R(n)=An+Bn2, where A and B are 2.08×108 s−1 and 1.5×10−10 cm3 s−1, respectively, for a well width Lz =7.5 mm. The radiative and nonradiative contributions to the linear recombination term, A=1/τr+1/τnr, are determined as τr =8 ns and τnr =12 ns from the dependence of the spontaneously emitted light power on the injection current density. In addition the carrier density dependence of the internal quantum efficiency is reported and ηi is found to be close to unity. The results demonstrate that even at fairly high excitation levels, excitonic enhancement of radiative recombination in high quality multiple quantum well structures is an important factor.

Turn‐on delay time fluctuations in gain‐switched AlGaAs/GaAs multiple‐quantum‐well lasers

The turn‐on delay time jitter in unbiased gain‐switched AlGaAs/GaAs multiple‐quantum‐well lasers is determined by measuring the transient fluctuations of the total emitted power with the use of a fixed‐time sampling technique with a picosecond temporal resolution. The turn‐on jitter is found to decrease significantly with increasing pumping rate, particularly when the lasers are operated at an excitation level at which only the first relaxation oscillation is emitted. The root‐mean‐square jitter of the emitted optical pulses decreases from about 20 ps just above the laser threshold to a value of 14 ps at the threshold for the appearance of the second relaxation oscillation. These results demonstrate that an accurate adjustment of the pumping rate is essential for a low‐jitter single‐pulse operation of the gain‐switched lasers.

Improved performance of large‐area InP/InGaAs metal‐semiconductor‐metal photodetectors by sulfur passivation

We report on sulfur based surface passivation technique for InGaAs metal‐semiconductor‐metal (MSM) photodetectors with an InP barrier enhancement layer. We show that excessive leakage current and photocurrent gain, which are the two major performance‐limiting factors in MSM detectors, can be largely suppressed by a treatment of the InP surface with ammonium polysulfide. The dark current and photocurrent characteristics of such passivated devices were monitored over a period of half a year and were found to be stable. The improved performance of the device characteristics is explained in terms of a passivation‐induced reduction of surface charging effects.

Ultra-wide-band (>40 GHz) submicron InGaAs metal-semiconductor-metal photodetectors

Submicron feature size InGaAs metal-semiconductor-metal (MSM) photodetectors with 3-dB bandwidths in excess of 40 GHz are demonstrated. Devices with a 0.3-/spl mu/m-thick active layer and an interelectrode spacing of <0.5 /spl mu/m show a roll-off of the frequency response of /spl les/2 dB up to 40 GHz when operated at 5-V bias under front illumination with 1.3-/spl mu/m light.

Picosecond spectra of gain‐switched AlGaAs/GaAs multiple quantum well lasers

The temporal waveform of the single longitudinal modes emitted by gain‐switched AlGaAs/GaAs multiple quantum well lasers is analyzed by means of a synchroscan streak camera system coupled to a spectrograph. It is found that only 50% of the number of the longitudinal modes contributing to the time‐averaged spectrum are present during the first relaxation oscillation. The time‐dependent shift of the envelope of the mode spectrum is quantitatively explained by a spectral shift of the gain maximum as a function of the time‐varying charge carrier density. The results demonstrate that, for the analysis of transient multimode spectra of gain‐switched injection lasers, inclusion of the energy dependence of the gain and gain compression is mandatory.

65 GHz InGaAs/InAlGaAs/InP waveguide-integrated photodetectors for the 1.3–1.55 μm wavelength regime

We report on ultrafast waveguide-integrated metal-semiconductor-metal photodetectors based on low pressure metal organic chemical vapor deposition grown semiinsulating InP/InGaAs/InAlGaAs/InP layers. The vertically coupled detectors have an internal coupling efficiency of >90% at 1.3 and 1.55 μm wavelength for detector lengths of 30 μm. A 3 dB bandwidth of 65 GHz at 1.55 μm wavelength is achieved by employing 0.3 μm feature-size finger electrodes and an active layer thickness of 150 nm. Furthermore, we present results on high-performance devices with a buried waveguide structure fabricated by regrowth of InP:Fe.