Optical Injection Level Research Articles

Investigation of bulk and surface minority carrier lifetimes in metamorphic InAsSb grown on GaAs and Si

Monolithic integration of III–V-based optoelectronic devices onto Si wafers provides enormous benefits to many device manufacturing technologies. Therefore, it is essential to understand the effect of limiting factors such as dislocations on the material properties. In this paper, we study the minority carrier lifetimes in mid-wave infrared InAsSb alloys grown on lattice-matched GaSb and lattice-mismatched semi-insulating GaAs and Ge/Si substrates. Time-resolved microwave reflection measurement has been performed to study the carrier dynamics and different recombination mechanisms over the temperature range of 20–300 K at various optical injection levels. The sample on GaAs is found to have a lower lifetime over the entire temperature region than the sample on the Ge/Si substrate. The threading dislocation density values estimated from the lifetime analysis were found to be 2.9±0.2 times larger in the sample on GaAs when averaged over the temperature range of 70–200 K. Furthermore, we studied the contribution of various recombination mechanisms, and it was shown that the lifetime in the sample on GaAs is dominated by the Shockley–Read–Hall recombination up to 140 K, above which the Auger recombination is the limiting factor. This transition temperature is 80 K and 100 K for the samples on GaSb and Ge/Si, respectively. We have also investigated the effect of the surface recombination on the total lifetime. The extracted bulk lifetime was found to be up to 2× higher when the surface effect was excluded.

Tunable Frequency Comb Generation Using Quantum Cascade Lasers Subject to Optical Injection

We dissect the enabling capabilities of the tunable period-one (P1) limit cycles of optically injected quantum cascade laser (QCL) oscillators for the generation of optical frequency combs. As such, we theoretically investigate the P1 dynamics of a QCL using a single-mode rate equation model. We find that such a P1 limit cycle occupies a rather large and wide region of the optical frequency detuning and injection level ratio map. We have not recorded evidence of chaos in this injected laser system, in marked contrast with quantum well and quantum dot cases. Contrary to interband semiconductor lasers, the QCL's oscillation frequency is generally smaller than the detuning frequency, and is reduced with increasing injection strength, due to the strong injection pulling effect. When the optical injection is operated in the vicinity of the Hopf bifurcation , the P1 oscillation produces dense optical frequency combs, owing to both the frequency pulling effect and the four-wave mixing effect. The comb spacing is continuously tunable from subgigahertz up to a few gigahertz, via fine control of either the detuning frequency and/or the injection ratio. This novel approach of the frequency comb generation is of prime importance for high-resolution detection of narrow absorption lines of gas molecules.

Steady state minority carrier lifetime and defect level occupation in thin film CdTe solar cells

A model consisting of Shockley Read Hall (SRH) recombination under steady state conditions of constant photon injection is proposed in this work to study the steady state minority carrier lifetime in CdS/CdTe thin film solar cells. The SRH recombination rate versus optical injection level is analytically approximated in the junction and neutral regions. In the neutral region, it is found that the recombination rate through certain defect levels has one constant value under lower optical injection conditions and another constant value under higher optical injection conditions with the transition occurring at a critical optical injection level. By simultaneously solving the equations of charge neutrality, charge conservation and SRH recombination in the neutral region, it is found that the compensation of doping and the reduction of minority carrier lifetime by donors in the p-type semiconductor can each be remedied by optical injection. It is also demonstrated that this optical-dependent SRH recombination is significant in large bandgap thin films. The measured minority carrier diffusion length in a CdS/CdTe solar cells, as determined from the steady-state photo-generated carrier collection efficiency, shows the predicted transition of minority carrier lifetime versus optical injection level. A numerical fitting of the indirectly-measured minority carrier lifetime by assuming the minority carrier mobility gives a non-intuitive picture of the p–n junction with a low free hole concentration but a narrow depletion region width.

Anti-Fibrotic Roles of Oviductus Ranae in Rats with Hepatic Fibrosis

In this paper, we focus on the role of Oviductus ranae in protecting the liver against fibrogenesis caused by chemical model in rats and to further explore the underlying mechanisms. Rat models of experimental hepatic fibrosis were established by injection with carbon tetrachloride (CCl4). Optical injection, the denaturation and necrosis level of hepatocytes and the distribution of collagen fibrosis increased obviously after hypodermic injection of CCl4 for 9 weeks. Electron microscope, the shape and structure of cell organells were destroyed obviously. Fat storing cells and kuppfer.s cells increased. Collagen fibrosis distributed extensively in the model groups. After 8 weeks treated with Oviductus ranae. The pathological changes and the collagen fibrosis related with hepatic fibrosis decreased significantly. Conclusions: Oviductus ranae can decrease the pathological changes of hepatic fibrosis induced by CCl4, and prevent the development of hepatic fibrosis.

Study on the saturation characteristics of high-speed uni-traveling-carrier photodiodes based on field screening analysis

A back-illuminated mesa-structure InGaAs/InP charge-compensated uni-traveling-carrier (UTC) photodiode (PD) is fabricated, and its saturation characteristics are investigated. The responsivity of the 40-\mu m-diameter PD is as high as 0.83 A/W, and the direct current (DC) saturation current is up to 275 mA. The 1-dB compression point at the 3-dB cutoff frequency of 9 GHz is measured to be 100 mA, corresponding to an output radio frequency (RF) power of up to 20.1 dBm. According to the calculated electric field distributions in the depleted region under both DC and alternating current (AC) conditions, the saturation of the UTC-PD is caused by complete field screening at high optical injection levels.

Modulation response performances of a Fabry-Perot semiconductor laser subjected to light injection from another Fabry-Perot semiconductor laser

The modulation response characteristics of a Fabry-Perot semiconductor laser (slave FP-LD) subjected to light injection from another Fabry-Perot semiconductor laser (master FP-LD) have been investigated theoretically. The results show that the modulation response performances of the slave FP-LD depend on the light injection strength, the central mode frequency detuning Δf and the mode spacing difference Δλ between the slave FP-LD and the master FP-LD. With the increase of the light injection strength from the master FP-LD, the 3 dB modulation bandwidth of the slave FP-LD will be enhanced; however, if the injection strength is increased to certain degree, the front side of the relaxation oscillation peak will be reduced to below 3 dB, which results in the rapid decrease of the 3 dB modulation bandwidth. With the increase of Δf, for a relatively small injection strength level, the 3 dB modulation bandwidth will behave a monotonous enlargement until the slave FP-LD operates in a period-one state (P1). For a relatively strong optical injection level, the slave FP-LD has a broad injection-locked frequency detuning area. Within the injection-locked area, with the increase of frequency detuning Δf, the 3dB modulation bandwidth increases at first and then decreases after experiencing a maximum value. For given light injection strength and frequency detuning, the 3 dB modulation bandwidth of the slave FP-LD has two maximums and behaves a symmetrical distribution with the change of Δλ. By choosing reasonable parameters, the 3 dB modulation bandwidth can be significantly improved. For the parameter values selected in this paper, the 3 dB modulation bandwidth can increase 5.5 times compared with the case of free running.

Characterization and optimization of high-power InGaAs/InP photodiodes

The performance of 34-μm-diameter InGaAs/InP charge compensated modified uni-traveling carrier photodiodes (CC MUTCs) is studied using a commercial device simulator based on a drift-diffusion model. Excellent agreement has been achieved between simulations and experiments. The origin of the degradation of responsivity at high optical injection level is investigated. Parameters of the photodiode are further optimized to maximize the saturation current without sacrificing bandwidth and responsivity.

Hydrodynamic Simulations of Unitraveling-Carrier Photodiodes

We present simulated results of a unitraveling-carrier photodiode (UTC-PD) using the hydrodynamic carrier transportation model. A maximum responsivity of 0.25 A/W and a small-signal 3-dB bandwidth of 52 GHz were obtained for a 220-nm-thick InGaAs absorption layer. The physical properties of the UTC-PD have been investigated at different optical injection levels. Modulation of the energy-band profile due to the space charge effect has been observed at high injection level, and an electron velocity overshoot of 3 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s has been found to effectively delay the onset of space charge effects. Comparisons with reported simulated results using the drift-diffusion model as well as reported experimental results are presented. The results suggest the necessity of using the hydrodynamic transport equations to accurately model the UTC-PD. In addition, it has been corroborated that the photoresponse of the UTC-PD could be improved by incorporating a graded doping profile in the absorption layer.

Electronic Defect and Contamination Monitoring in Si Wafers Using Spectrally Integrated Photocarrier Radiometry

The ability of spectrally integrated room-temperature photocarrier radiometry to image electronic defects and contamination in silicon wafers is presented. Amplitude and phase imaging contrast is a result of signal sensitivity to local variations in the recombination lifetime of photoexcited carriers. Experimental frequency scans are fitted to carrier density-wave theory to simultaneously obtain the recombination lifetime, diffusivity, and surface recombination velocities (front and back). Lifetime measurements are combined with lateral surface scans to produce quantitative lifetime imaging. Contaminated boron-doped silicon wafers with iron concentration on a baseline of have been successfully imaged and exhibit a much higher spatial resolution than surface photovoltage images. Recombination lifetime measurements before and after photodissociation of FeB pairs have been utilized for quantitative determination of iron concentration; however, lifetime accuracy limitations due to photodissociation by the excitation laser under medium-to-high optical injection levels required for photocarrier radiometric measurements compromise the accuracy of heavy ion (Fe) concentration measurements.

Calculations of band-filling optical nonlinearities in extrinsic semiconductors beyond the low injection limit

Theoretical calculations of room temperature band-filling optical nonlinearities in a bulk semiconductor are reported. The approach, based on self-consistent determination of pseudochemical potentials, improves on other existing calculations as it correctly takes into account an arbitrary optical injection level. The self-consistent approach also enables direct calculation of the nonlinear optical susceptibility without fixing the photogenerated electron-hole density as a parameter, as required in existing phenomenological models. Good agreement with experimental results is obtained, in the case of GaAs, when photoexcited carrier densities above the Mott transition are considered. Calculations of pair density and nonlinear refractive index are shown as a function of the doping level and of the pump intensity for bulk GaAs. Finally, the limits of the model and applications to calculations of optical behavior in more complex structures are discussed.

Numerical modeling study of the InP/InGaAs uni-travelling carrier photodiode

The operation and performance of the InP-based uni-traveling carrier photodiode (UTC-PD) has been studied using a two dimensional, drift–diffusion approach utilizing a commercial numerical device simulator. The UTC-PD has been proposed as a replacement for the InGaAs PIN photodiode for long wavelength optical communications since it retains its performance at high levels of optical injection and exhibits a very high frequency capability. In this work, the results of an investigation of the effects of the device’s structure on the operational performance are reported as well as the effects of device biasing and the optical injection level. In particular, the origins of the degradation in device performance at high optical injection levels have been investigated. The results are discussed in relation to reports of the device’s experimentally observed performance.

Quality evaluation and improvement of iron-doped electromagnetic multycrystalline silicon wafers

The effect of iron doping in electromagnetic cast (EMC) Si wafers on minority carrier recombination lifetime has been extensively investigated by means of microwave detected photo-conductivity decay (μ-PCD) and surface photo-voltage (SPV) to discriminate impurities and defects for the lower crystal quality. Optical and thermal activations of doped iron are investigated to study the difference between the two techniques in obtaining iron concentration. Minority carrier lifetimes increase or decrease after activation due to the different optical injection levels of both the SPV and μ-PCD techniques. No noticeable effect of chemical passivation in obtained iron concentration is observed. Boron and phosphorus gettering and hydrogen passivation techniques are applied to improve minority carrier recombination lifetime. As a result, the crystal quality of the EMC Si wafers depends on defect centers rather than on iron impurities.

FM–AM conversion in injection-locked semiconductor lasers

The possibility of FM–AM conversion in an injection-locked FP semiconductor laser has been investigated analytically in this paper. It is found that the efficiency of FM–AM conversion depends upon the value of some key parameters of the device such as nonlinearity parameter, optical injection level and linewidth enhancement factor. Index of converted AM has been numerically estimated. The injection-locked laser of the proposed FM–AM converter reduces the Relative Intensity Noise (RIN) of the input signal drastically.

Influence of adjacent channel interference on the frequency-modulated WDM optical communication system

The effect of adjacent channel interference on the frequency modulated wavelength division multiplexing (FM-WDM) optical communication system is analyzed in this paper. This system uses injection-locked optical discriminator at the output for the demodulation of FM channels. Adjacent channel interference results in crosstalk in the FM-WDM communication system which has been calculated. Dependence of crosstalk on the channel spacing, received signal power in a particular channel, and linewidth enhancement factor of the slave laser incorporated in the optical discriminator has been numerically investigated. The injection-locked discriminator has a typical output crosstalk level of -50 dB for a channel spacing of 100 GHz with optical injection level of -30 dB into the slave laser and is a suitable subsystem to be used in FM-WDM communication system.

Optical frequency bistability and power bistability in semiconductor lasers

Optical frequency bistability and power bistability in semiconductor lasers biased from below to above threshold are presented using the unified description of semiconductor lasers with external light injection. A new method is given to calculate the bistable frequency and power loop widths. When the bias current of a laser is fixed, there is a lowest power for the external light to realize optical frequency bistability. And this lowest power reaches its minimum just at threshold. The optical injection level (defined as the ratio of the injected power to the power emitted by the free-running laser) must be larger than a certain value to obtain the bistability. This level may be larger than unity for a laser biased below threshold. On the other hand, for a given bias current, there is a lowest frequency detuning between the electric field of the laser and the injected field to achieve the power bistability. And this detuning must be negative for any bias current (below or above threshold) of the laser. The results are in good agreement with experiments. >

Simple method for measuring the linewidth enhancement factor of semiconductor lasers by optical injection locking

A simple method for measuring the linewidth enhancement factor alpha of a semiconductor laser by optical injection locking is described. Without knowledge of the absolute values of detuning and the optical injection level, the value of alpha is evaluated from only changes in optical power in the stable optical injection-locking state. The value of alpha of a 0.83-microm channeled substrate planar laser is evaluated to be 2.65 +/- 0.2. The measurement error of this method is also discussed.

Injection locking in distributed feedback semiconductor lasers

Injection locking properties of distributed feedback semiconductor lasers are studied systematically. Due to the high side mode suppression, these devices show different locking properties when compared to lasers with Fabry-Perot structures. The main result is the identification of four regimes for different injection levels. In particular, a symmetrical locking band at low optical injection level is confirmed. The presence of this symmetrical band can be exploited in some applications. As examples, the measurement of the linewidth enhancement factor and the phase-shift-keying modulation capability are reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of direct-discrimination of FSK modulated optical signal using injection-locked DFB lasers

Direct discrimination of FM and FSK optical signals using injection-locked DFB semiconductor lasers is analysed. The demodulation efficiency is found to be independent of the optical injection level in the steady state. However, the optical injection level is important in determining the dynamic response. There exists an optimum optical injection rate to achieve the highest demodulation speed. The noise analysis revealed that the signal to noise ratio of this method is of the same order as that of the passive interferometer demodulation method.

Novel measurement technique of α factor in DFB semiconductor lasers by injection locking

A simple and accurate method to measure the linewidth enhancement factor a in DFB semiconductor lasers is proposed. This method, based on the principle of external optical injection locking, does not require the knowledge of the absolute value of optical injection level.