Lateral Carrier Diffusion Research Articles

Lateral diffusion of minority carriers in InAsSb-based nBn detectors

We report on the investigation of lateral diffusion of minority carriers in InAsSb based photodetectors with the nBn design. Diffusion lengths (DLs) were extracted from temperature dependent I-V measurements. The behavior of DL as a function of applied bias, temperature, and composition of the barrier layer was investigated. The obtained results suggest that lateral diffusion of minority carriers is not the limiting factor for InAsSb based nBn mid-wave infrared detector performance at high temperatures (>200 K). The detector with an As mole fraction of 10% in the barrier layer has demonstrated values of DL as low as 7 μm (Vb=0.05 V) at 240 K.

Resistance to edge recombination in GaAs-based dots-in-a-well solar cells

Insensitivity to edge recombination is observed in GaAs-based InAs/InGaAs quantum dots-in-a-well (DWELL) solar cells by comparing its current-voltage (IV) plot to GaAs control samples. The edge recombination current component is extracted by analyzing devices of different areas and then compared to DWELL cells of comparable dimensions. The results demonstrate that GaAs-based solar cells incorporating a DWELL design are relatively insensitive to edge recombination by suppressing lateral diffusion of carriers in the intrinsic layer, and thus promising for applications that require small area devices such as concentration or flexible surfaces.

A CMOS STI-Bound Single-Photon Avalanche Diode With 27-ps Timing Resolution and a Reduced Diffusion Tail

Single-photon avalanche diodes (SPADs) measure individual photons' time of arrival. Low detector jitter is required in many SPAD applications. This letter describes a method for significantly reducing SPAD jitter using an area-efficient shallow-trench-isolation guard ring. The structure prevents lateral drift and diffusion of charge carriers, resulting in improved timing resolution. Experimental results of the device, fabricated in a 0.18-mum CMOS technology, are presented. The timing resolution of the SPAD is 27-ps full-width at half-maximum. Importantly, the diffusion tail exhibits only 96-ps full-width at hundredth-maximum, a three times improvement over previously published SPAD results. The reduced jitter can be translated to improved bit error rates in quantum key distribution systems, faster bit rates in pulse position modulation optical links, and greater contrast in high-resolution fluorescence lifetime imaging microscopy.

Direct Observation of Lateral Carrier Diffusion in Ridge Waveguide InGaNAs Lasers

We present results from measurements of the subthreshold lateral spontaneous emission profile in 1.3-mu m wavelength ridge waveguide InGaNAs quantum-well lasers using a scanning near-field optical microscopy technique. The measurements reveal the presence of significant lateral carrier diffusion which has a profound effect on the temperature dependence of the threshold current. This effect is frequently omitted when the characteristic temperature of the threshold current is considered.

Lateral diffusion of minority carriers in nBn based type-II InAs/GaSb strained layer superlattice detectors

We report on the investigation of lateral diffusion of minority carriers in nBn based InAs/GaSb strained layer superlattice photodetectors. Values of diffusion length of minority carriers were extracted from temperature dependent I-V measurements. The behavior of diffusion length as a function of applied bias, temperature, and composition of the barrier layer and the values ranging by two orders of magnitude from 77 to 250 K at the same value of applied bias were investigated. The obtained results suggest that at this point the lateral diffusion current could be the limiting factor of the MWIR nBn detector performance at low (<150 K) temperatures.

Ultrafast decay of femtosecond laser-induced grating in silicon-quantum-dot-based optical waveguides

Femtosecond transient laser-induced grating (LIG) experiments were performed in planar optical waveguides made of luminescent silicon quantum dots. The LIG was created by interference of two pulses from the frequency-doubled output of a Ti–sapphire laser (400 nm, 400 fs, 1 kHz). The LIG exhibits an extremely fast decay in time (several picoseconds) that was found to decrease with decreasing grating period. The standard models based on lateral carrier diffusion cannot explain this observation (this procedure yields an unrealistically high diffusion constant of 420 cm2 s−1). Instead, the results are explained by exciton diffusion and/or enhanced exciton radiative decay rate in a cavity represented by the periodically modulated planar waveguide (Purcell effect).

Scanning Photoinduced Impedance Microscopy Using Amorphous Silicon Photodiode Structures

Scanning photoinduced impedance microscopy (SPIM) is an impedance imaging technique, which is based on photocurrent measurements at electrolyte-insulator-semiconductor (EIS) and metal-insulator-semiconductor (MIS) field-effect structures. The material to be investigated has to be deposited on top of the insulator (E/I or M/I interface). The lateral resolution of SPIM is limited by the lateral diffusion of minority charge carriers. Therefore, it would be advantageous if semiconductors with a short diffusion length of charge carriers such as amorphous silicon could be employed. However, field-effect capacitors fabricated using amorphous silicon suffered from a large number of interface states, high leakage currents through the insulator, and small photocurrents. In this work, field-effect capacitors were replaced by amorphous hydrogenated silicon photodiode structures (a-Si:H p-i-n/SiO2 or n-i-p/SiO2) as this was expected to result in higher photocurrents and eliminate the necessity of a high-quality insulator. The photodiode structures were shown to be suitable for SPIM measurements. The resolution of photocurrent measurements was found to depend strongly on the frequency of the modulated light and the doping concentration of the amorphous silicon layer closest to the insulator. An equivalent circuit model was developed to simulate this behavior.

Perimeter recombination in thin film solar cells

Surface recombination has a profound effect on the performance of a solar cell, at the illuminated surface reduces its photocurrent and along the cell's perimeter increases its dark current. The perimeter recombination current has two components; the first is due to recombination at the surface that intersects the space-charge layer while the second originates from recombination at the surface of quasi-neutral regions. The current due to recombination at the depleted layer surface is treated in a similar way to that of the bulk, using a simple model. We present an analytical form that produces results that agree well with reported experimental findings. The recombination current outside the space-charge region is of two-dimensional nature, it represents lateral diffusion of minority carriers from the boundary of the depleted layer to the perimeter. This current is calculated by solving numerically a two-dimensional continuity equation. As the ratio of perimeter to area is increased the perimeter current acquires important proportions, consequently the expected bulk recombination current becomes insignificant.

Ambipolar lateral diffusion of photo-induced carriers in a moderate magneticfield

The ambipolar lateral diffusion of photo-induced charge carriers restricted ina plane of a quantum well (QW) under a moderate (non-quantizing) magneticfield is studied theoretically in the framework of the drift-diffusion model. Thecontinuity equation for this case is solved exactly. The analytical expressions for theconcentration of photo-induced electrons and the built-in electric field in the practicallyimportant cases of uniform and bell-shaped light beams are obtained in a closedform. It is shown that the ambipolar lateral diffusion of photo-induced chargecarriers can be suppressed in InGaAs/GaAs QWs by a moderate magnetic field of∼0.5 mT.

Scanning Photo-Induced Impedance Microscopy—Resolution studies and polymer characterization

Scanning Photo-Induced Impedance Microscopy (SPIM) is an impedance imaging technique that is based on photocurrent measurements at field-effect structures. The material under investigation is deposited onto a semiconductor–insulator substrate. A thin metal film or an electrolyte solution with an immersed electrode serves as the gate contact. A modulated light beam focused into the space charge region of the semiconductor produces a photocurrent, which is directly related to the local impedance of the material. The absolute impedance of a polymer film can be measured by calibrating photocurrents using a known impedance in series with the sample. Depending on the wavelength of light used, charge carriers are not only generated in the focus but also throughout the bulk of the semiconductor. This can have adverse effects on the lateral resolution. Two-photon experiments were carried out to confine charge carrier generation to the space charge layer. The lateral resolution of SPIM is also limited by the lateral diffusion of charge carriers in the semiconductor. This problem can be solved by using thin silicon layers as semiconductor substrates. A resolution of better than 1 μm was achieved using silicon on sapphire (SOS) substrates with a 1 μm thick silicon layer.

Pixelless 1.5-/spl mu/m up-conversion imaging device fabricated by wafer fusion

We designed and fabricated a first-ever pixelless optical up-conversion imaging device using wafer-fusion technology. The device consists of an In/sub 0.53/Ga/sub 0.47/As-InP p-i-n detector and a GaAs-AlGaAs light-emitting diode (LED), which were grown on an InP and a GaAs substrate, respectively, and wafer-bonded together. The layer structures and doping profiles of the common region linking the detector and LED were designed such that lateral carrier diffusion was successfully suppressed while effective electrical connection was well preserved. Pixelless up-conversion imaging from 1.5 to 0.87 μm was demonstrated. Moreover, an internal electrical gain of over 100 was observed for the detector part of the integrated device. The internal up-conversion quantum efficiency was measured to be /spl sim/50% at room temperature.

Microsecond time scale lateral-mode dynamics in a narrow stripe InGaN laser

Time-resolved measurements of the spectrum and the far field of InGaN-based laser diodes show lateral-mode changes and gradual tilting of the far field on a microsecond time scale. Numerical simulations based on a microscopic theory are in good agreement with the measurements. The observed effects are attributed to lateral carrier diffusion in combination with thermal lensing.

Numerical stability analysis of a large-scale delay system modeling a lateral semiconductor laser subject to optical feedback.

This paper highlights the use of advanced numerical tools to study the stability of large-scale systems of delay differential equations (DDEs). Specifically, we consider a model describing a semiconductor laser subject to conventional optical feedback and lateral carrier diffusion. The symmetry of the governing rate equations allows external cavity mode solutions (ECMs) to be computed as steady state solutions. Using the software package DDE-BIFTOOL, branches of ECMs are computed as a function of varying feedback strength. The stability along these branches is computed by solving eigenvalue problems, the size of which is governed by a step-length heuristic. In this paper, we employ an improved heuristic which substantially reduces the size of these eigenvalue problems. This approach makes the stability analysis of large-scale systems of DDEs computationally feasible.

The influence of lateral carrier diffusion and surface recombination on the behavior of semiconductor optical amplifier (SOA)-based MMIs

The influence of lateral carrier diffusion and surface recombination on the self-imaging properties of semiconductor-optical-amplifier-based multimode interference couplers has been verified by simulations using a beam propagation method. It shows a significant degradation of the self-imaging properties of these devices. Buried heterostructures or deeply etched waveguide structures can decrease the impact when the degree of surface recombination is sufficiently low.

Quasi‐analytic steady‐state solution of VCSEL rate equations including spatial hole burning and carrier diffusion losses

AbstractWe propose a new set of equations describing a cylindrical vertical cavity surface emitting laser (VCSEL) cavity under CW operation, based on the rate equations including lateral carrier diffusion. The only numerical step in the calculation consists in finding the roots of a polynomial expression. This model enables a quasi‐instantaneous calculation of the laser's light‐current characteristic, including such effects as spatial hole burning, current spreading, inhomogeneous optical intensity distribution, and diffusion losses. An analysis of the VCSELs threshold current and differential quantum efficiency is proposed, which illustrates the interplay between injected current profile and diffusion coefficient. Copyright © 2003 John Wiley & Sons, Ltd.

Surface-acoustic-wave-induced carrier transport in quantum wires

The ambipolar transport of photogenerated electron-hole pairs by surface acoustic waves (SAW's) in coupled GaAs quantum wells (QW's) and quantum wires (QWR's) is investigated by spatially and time-resolved photoluminescence. Experimental configurations for SAW propagation direction parallel or perpendicular to the QWR's have been studied. In the first configuration, the QWR confinement potential inhibits lateral carrier diffusion. The carriers are then efficiently transported along the wire as well-defined charge packages with a repetition rate corresponding to the SAW frequency. In the second configuration, we demonstrate that the SAW can be used to transfer electron-hole pairs generated in the QW into the QWR. We also provide clear evidence for the extraction of carriers from the QWR into the QW, when the SAW piezoelectric field is sufficiently strong to overcome the QWR confinement potential.

3D Effects on Minority Carrier Recombination in Homogeneous Silicon Wafers

Calculation of three-dimensional recombination effects in homogeneous silicon wafers is performed. The current continuity equation for minority carriers with surface recombination boundary conditions is solved in cylindrical coordinates. The two most important three-dimensional recombination effects are discussed. Lateral diffusion of minority carriers gives rise to a characteristic decay inversely proportional to time. Shell surface recombination should be taken into account when measuring within the minority carrier diffusion length from the wafer edge. The discrepancy between the one-dimensional and the three-dimensional models is discussed.

Continuous room-temperature operation of electrically pumped quantum-dot microcylinder lasers

We report continuous-wave (cw) room-temperature operation of electrically pumped AlGaAs/GaAs microcylinder lasers using self-assembled InAs/GaAs quantum dots (QDs) as the active material. The threshold currents are lower than those of similar devices with an InGaAs quantum well fabricated for comparison. This is discussed in terms of nonradiative surface recombination at the sidewall of the resonator which has less influence in the case of the QD devices since the lateral carrier confinement suppresses lateral carrier diffusion.

Numerical analysis for static and dynamic characteristics of an optical amplifier-switch integrated device

A physical model is developed for the operation of a functional quantum well optoelectronic integrated devise (QW-OEID). Based on the model, static and dynamic characteristics of the device are analyzed numerically, considering the effects of parameters such as internal optical feedback and lateral diffusion of carriers, for both amplification and switching modes of operation.

Turn‐on transient dynamics of a semiconductor laser with optical feedback

AbstractWe study the effect of lateral carrier diffusion on the transient turn‐on of a semiconductor laser with optical feedback. The numerical simulations are based on the Lang–Kobayashi model, and on an extension of the model that includes lateral profiles for the carrier density and the optical field. For moderately strong feedback, intensity pulses characterize the dynamics of the turn‐on, with a repetition rate at the external cavity round‐trip time. We find that carrier diffusion has an important effect when the laser is biased close to the solitary threshold. For low carrier diffusion the intensity drops to a nearly zero value in between the pulses; for fast carrier diffusion, the spatial holes in the carrier profile burned by the intensity pulses are quickly replenished, and the intensity does not decay to a zero value in between the pulses. The results of our analysis show that the effect of diffusion is qualitatively well modelled by a non‐linear gain saturation coefficient different from zero in the space‐independent Lang–Kobayashi model. Copyright © 2001 John Wiley & Sons, Ltd.