Search

The application of an electric field to a semiconductor can alter its absorption properties. This electroabsorption effect can have a significant impact on the quantum efficiency of detector structures. The photocurrents in bulk InGaAs and GaAsSb p-i-n photodiodes with intrinsic absorber layer thicknesses ranging from 1 to 4.8 μm have been investigated. By using phase-sensitive photocurrent measurements as a function of wavelength, the absorption coefficients as low as 1 cm−1 were extracted for electric fields up to 200 kV/cm. Our findings show that while the absorption coefficients reduce between 1500 and 1650 nm for both materials when subject to an increasing electric field, an absorption coefficient of 100 cm−1 can be obtained at a wavelength of 2 μm, well beyond the bandgap energy when they are subject to a high electric field. The results are shown to be in good agreement with theoretical models that use Airy functions to solve the absorption coefficients in a uniform electric field. [ABSTRACT FROM AUTHOR]

Low noise avalanche photodiodes (APDs) detecting 1550 nm wavelength play a crucial role in optical communication and LiDAR systems. These APDs utilize a separate absorption, charge, and multiplication (SACM) architecture with an absorber for 1400–1650 nm detection and a low noise, high gain multiplier that can be independently optimized for a high signal-to-noise ratio. Recently, GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 SACM APDs have demonstrated ultra-high gain and extremely low noise, possibly improving sensitivity over Si and InGaAs/InP commercial APDs. This accomplishment was achieved using a GaAsSb absorber instead of a conventional InGaAs absorber, mitigating band discontinuities between the absorber and the multiplier. However, further optimization is required to reduce noise due to tunneling and impact ionization from the GaAsSb absorber, which occurs at a high electric field region. This paper focuses on the study of the high-field characteristics of GaAsSb photodiodes (PDs). The tunneling phenomenon is analyzed through current density-voltage measurements, and the impact ionization behavior is evaluated by measuring the multiplication of p-i-n GaAsSb PDs. The result suggests that when designing a SACM APD with a GaAsSb absorber, the electric field in the absorber can be increased to 175 kV/cm without the detrimental effects of ionization occurring in the absorber. The findings from this investigation will assist in optimizing GaAsSb-based SACM APDs and promoting further advancements in the 1550 nm APD technology. [ABSTRACT FROM AUTHOR]

Band gap alignment in GaAs1-xBix/GaAs quantum wells (QWs) was studied experimentally by photoreflectance (PR) and theoretically, ab initio, within the density functional theory in which the supercell based calculations are combined with the alchemical mixing approximation applied to a single atom in a supercell. In PR spectra, the optical transitions related to the excited states in the QW (i.e., the transition between the second heavy-hole and the second electron subband) were clearly observed in addition to the ground state QW transition and the GaAs barrier transition. This observation is clear experimental evidence that this is a type I QW with a deep quantum confinement in the conduction and valence bands. From the comparison of PR data with calculations of optical transitions in GaAs1-xBix/GaAs QW performed for various band gap alignments, the best agreement between experimental data and theoretical calculations has been found for the valence band offset of 5265%. A very similar valence band offset was obtained from ab initio calculations. These calculations show that the incorporation of Bi atoms into GaAs host modifies both the conduction and the valence band. For GaAs1-xBixBix with 01-xBix and GaAs in the range of ~60%-40% (~40%-60%), which is in good agreement with our conclusion derived from PR measurements. [ABSTRACT FROM AUTHOR]

Current-voltage measurements have been performed on InAs/InGaAs/GaAs quantum dot structures with varying growth and design parameters. These measurements show that the forward and reverse bias dark currents decrease with increasing spacer growth temperature, however, they are relatively insensitive to the number of periods of the quantum dot layers. Temperature dependent current-voltage measurements show that the mechanism that governs the reverse bias leakage current is due to generation-recombination via mid-band traps assisted by the Frenkel-Poole emission of carriers from these traps. [ABSTRACT FROM AUTHOR]

We present a study of InAs/GaAs quantum dot-in-well (DWELL) material using transmission electron microscopy and leakage current-voltage measurements. The spacer layers between the DWELL layers have a variety of annealing and growth temperatures. We show that there is a strong correlation between spacer layer, annealing temperature, defect density, and these leakage currents, with the most defective sample having 30 times more defects and a leakage current several orders of magnitude above that of the least defective. Cross section transmission electron microscope (TEM) shows that surface roughness above defective dots is responsible for the high defect densities. However, even in the best sample the reverse bias leakage current is several orders of magnitude above that typically seen in quantum well materials and a measurable density of defective dots are observed in planar view TEM. [ABSTRACT FROM AUTHOR]

The internal strain in epitaxial layers due to lattice misfit has long been recognized as a limiting factor in the design of semiconductor structures. In strained layer structures above a critical thickness hc, this strain is relaxed by the introduction of misfit dislocations. Here, we show that the interaction between the strain fields of a self-assembled quantum dot and a dislocation can lead to a threading dislocation being trapped, or pinned, by the quantum dot. The strength of this interaction is always larger than the force exerted on the dislocation by a surrounding layer with lower misfit strain. This gives a significant increase in the critical thickness for relaxation hc(QD). In layers between hc and hc(QD), threading dislocations can at best move only small distances, effectively preventing relaxation. Furthermore it is not possible to destabilize such a layer by the deposition of strained layers above it. The classical critical thickness condition thus does not apply to these structures, and they can be produced with essentially no limits to thickness before relaxation occurs. This may be expected to have significant technological consequences and allow a much wider range of structures to be produced than is possible using current strained layer design rules. [ABSTRACT FROM AUTHOR]

A simple Monte Carlo model has been developed to simulate the avalanche multiplication process in In0.52Al0.48As. The model reproduces avalanche multiplication and excess noise factor measured on a wide range of In0.52Al0.48As p+-n-n+, n+-n-p+, and p+-n+ diodes and confirms that very low excess noise factor can be obtained using pure electron injection in very thick diodes with avalanche region greater than 2.21 μm or in very thin diodes with avalanche region lesser than 0.11 μm. In addition we investigated the effect of an electric field gradient in the avalanche region of avalanche photodiodes and found that the excess noise factor can be reduced with electric field gradients. However in thin diodes with avalanche region lesser than 0.20 μm, the onset of tunneling current negates the excess noise reduction achieved using the electric field gradient. Therefore ideal p+-i-n+ diodes still provide the overall preferred structure. [ABSTRACT FROM AUTHOR]

We report bulk GaInNAs p-i-n photodiodes lattice-matched to GaAs substrates, grown by solid source molecular beam epitaxy with photoresponses out to ∼1.3 μm. The as-grown samples were characterized optically, structurally, and electrically. A low background doping concentration in the range of 1014–1015 cm-3 was obtained in the samples. One of the samples with a 0.5 μm thick GaInNAs absorbing layer gave a responsivity of 0.11 A/W for a band edge of 1.28 μm at reverse bias of 2 V. [ABSTRACT FROM AUTHOR]

We demonstrated highly improved optical properties of the AlGaN/GaN multiple quantum well (MQW) structure grown on a GaN layer with significant reduction of dislocations achieved using an AlN buffer directly grown on a sapphire substrate at high temperature, in comparison with the MQW grown using a conventional two-step technique. An enhancement of room-temperature photoluminescence intensity by up to one order of magnitude has been observed, compared with the conventional MQW. Transmission electron microscopy measurements indicated that the dislocation density of the GaN grown using the AlN buffer technique was dramatically reduced. Furthermore, the significantly improved optical properties of this MQW using this AlN buffer technique were also demonstrated by increasing the thickness of the GaN layer underneath, whose crystal quality can be improved by increasing thickness, confirmed by detailed x-ray measurements. Since the technique can significantly improve the optical efficiency in the ultraviolet (UV) spectral region without involvement of any ex situ patterning process, it should be highlighted in the development of high performance UV light-emitting diodes, currently a highly regarded research area. [ABSTRACT FROM AUTHOR]

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350 nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs. [ABSTRACT FROM AUTHOR]

The growth temperature of spacer layers (SPLs) is investigated as a means to obtain identical layers for multilayer quantum dot (QD) structures. A 5-layer 1.3-μm InAs/GaAs QD structure with 50-nm GaAs SPLs served as a model system. It is found that the growth temperature of the GaAs SPLs has pronounced effects on both the structural and optical properties of the InAs QDs. For GaAs SPLs grown at a low temperature of 510 °C, dislocations are observed in the second and subsequent layers, a result of significant surface roughness in the underlying spacer layer. However by increasing the growth temperature to 580 °C for the final 35 nm of the 50-nm GaAs SPLs, a much smoother surface is achieved, allowing the fabrication of essentially identical, defect free QD layers. The suppression of defect formation enhances both the room-temperature photoluminescence efficiency and the performance of 1.3-μm multilayer InAs/GaAs QD lasers. An extremely low continue-wave room-temperature threshold current density of 39 A/cm2 is achieved for an as-cleaved 5-layer device with emission at 1.306 μm and ground state operation up to 100 °C. [ABSTRACT FROM AUTHOR]

A simple Monte Carlo model is used to simulate the avalanche process in a multiplication region which incorporates a heterojunction, intended to introduce localization into the ionization process and reduce excess avalanche noise. The results are compared with those of models where the ionization path length distribution is represented by an exponential decay, displaced from the origin by a ballistic dead space. While the latter results depend sensitively on the arbitrary choice of scheme used to evaluate the model parameters, they agree remarkably well with Monte Carlo, considering the simplicity of the model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Impact ionization in 1-2.5 μm thick Al[SUBx]Ga[SUB1-x]As/GaAs multiple quantum well (MQW) avalanche photodiodes with various barrier and well widths has been investigated in detail. The measured avalanche multiplication suggests that in all cases no enhancement of the ionization coefficients over that of equivalent GaAs is possible. Excess noise measurements on these structures corroborate the multiplication results and show that no enhancement of the ionization coefficient ratio can be obtained in Al[SUBx]Ga[SUB1-x]As/GaAs MQWs by virtue of the heterojunction interfaces, irrespective of the number of repeating units. These results suggest that the excess energy gained by carriers crossing the Al[SUBx]Ga[SUB1-x]As/GaAs heterojunction interface is negligible and that the lower ionization coefficients of the Al[SUBx]Ga[SUB1-x]As results in behavior that is similar to that of a pseudoalloy of the wells and barriers. [ABSTRACT FROM AUTHOR]

The duration of the avalanche multiplication process in thin GaAs avalanche photodiodes is investigated using a full band Monte Carlo (FBMC) model. The results are compared with those of a simple random path length (RPL) model which makes the conventional assumptions of a displaced exponential for the ionization path length probability distribution function and that carriers always travel at their saturated drift velocities. We find that the avalanche duration calculated by the RPL model is almost twice of that predicted by the FBMC model, although the constant drift velocities used in the former model are estimated using the latter. The faster response predicted by FBMC model arises partly from the reduced dead space but mainly from the velocity overshoot of ionizing carriers. While the feedback multiplication processes forced by the effects of dead space extend the avalanche duration in short structures, the effects of velocity overshoot in the realistic model more than compensate, significantly improving multiplication bandwidth. [ABSTRACT FROM AUTHOR]

A Fokker–Planck equation for carrier transport in semiconductors is derived from the Boltzmann transport equation by expanding in Legendre polynomials and assuming the phonon energy exchanged at momentum randomizing collisions is small compared with the mean carrier energy. The method is used to compute impact ionization probability distributions in space and time and the results agree well with those generated by an equivalent Monte Carlo model over a wide range of electric fields from 300 kV/cm to 1 MV/cm. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The time dependent current response to an impulse of injected carriers is calculated for an avalanche photodiode using Monte Carlo simulation. For low electric fields and long avalanche regions the results agree with the conventional model, which assumes that carriers travel always with their saturated drift velocities. However, while diffusion remains unimportant, for high fields and short avalanche regions, the conventional model underestimates the device speed. Monte Carlo simulations show that the mean downstream average velocity of ionizing carriers is significantly enhanced at high electric fields and agreement is restored if we allow for this effect in the conventional model. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The electron saturation velocity in Ga[sub 0.52]In[sub 0.48]P has been measured as a function of temperature, utilizing the Kirk effect in double heterojunction bipolar transistors. An AlGaAs base was used to eliminate the conduction band spike, and measurements were carefully performed using pulse biasing to minimize self-heating. Voltage drops across the base and collector series resistances were also taken into account. The measured room temperature saturation velocity of 5.0×10[sup 6] cm/s decreased rapidly to 2.9×10[sup 6] cm/s at 200 °C. These results provide useful data for modeling and design, and are particularly important for the prediction of frequency performance of Ga[sub 0.52]In[sub 0.48]P/(Al)GaAs/Ga[sub 0.52]In[sub 0.48]P double heterojunction bipolar transistors when operating at typical elevated temperatures in power applications. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The impact ionization probability functions, P(x), are effectively histograms of the positions carriers ionize along the field direction, x, which determine the avalanche multiplication properties of a semiconductor device. In this work, we use a Monte Carlo model to investigate the form of these as multidimensional functions; in not just x, but also of the elapsed time, t, and the distance traveled perpendicular to the electric field direction, y. Despite most previous temporal calculations of the avalanche process assuming that all carriers travel at the drift velocity, v[sub d], it is shown that electrons which ionize at the shortest distances travel several times faster than v[sub d]. There is also a significant spread in possible velocities with which ionizing carriers travel along the x direction due to diffusion. Diffusive spreading of the ionization probability in both x and y is also described. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photo multiplication initiated by electrons and holes measured in sub micron Si p (sub +)-i-n (sub +) and n (sub +)-i-p (sub +) diodes are described. The effect of ionization dead space (DS) on avalanche multiplication and breakdown is highlighted.

Reports on the growth of high quality pseudomorphic indium-arsenic-phosphorus/indium-phosphorus single and multi-quantum well (MQW) structures by solid molecular beam epitaxy. Use of valved cracker sources to control the group ratio; Photoluminescence of a series of MQW grown in a configuration; Characteristics of devices fabricated from the structures.

Presents a study which evaluated the effect of strain relaxation on forward bias dark currents in gallium arsenide and indium gallium arsenide multi-quantum well p-i-n diodes. Discussion on the equilibrium theory on strain and misfit dislocation; Measurement of dark line densities; Correlation between the dark line density and the dark current density.

Reports the use of room- and low-temperature photoluminescence spectroscopy for the assessment of n-type pseudomorphic AlGaAs/InGaAs gallium arsenide high-electron mobility transistor structures. Characteristics of the samples used in the study; Theoretical method used to calculate the energy transitions in the quantum wells; Procedure used to assess the structural parameters of the structure.

Presents a study on excitation-power-dependent blue shifts in photoluminescence and electroluminiscence in piezoelectric strained layer multiple quantum wells incorporated in p-i-n diodes. Introduction strained quantum wells; Experimental setup; Results.

Presents a study that examined the growth and characterization of high quality epitaxial layers of gallium compounds and gallium compounds-containing quantum wells grown by solid-source molecular beam epitaxy (MBE). Evidence for the presence of long-range ordering in MBE-grown gallium-compounds; Comparison of measured transition energies; Information on the cross-sectional transmission electron micrograph of samples.

Presents a study that extended photoluminescence excitation (PLE) investigations to samples with varying degrees and details of ordering growth by both organometallic vapor phase epitaxy and solid source molecular beam epitaxy. Function of PLE; Scope of the expression degrees and details of ordering; Contrast of the PLE properties seen in the samples of GaInP with varying degrees of ordering with those seen in high quality AlGaAs in which very little ordering was present.

It is demonstrated that the emission of InAs quantum dots (QDs) capped with GaAsSb can be extended from 1.28 to 1.6 μm by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55 μm injection lasers are discussed. [ABSTRACT FROM AUTHOR]

Avalanche breakdown voltage has been measured in a series of AlxGa1-xAs (0+in+ and n+ip+ diodes at temperatures between 20 and 500 K. The temperature sensitivity falls with decreasing avalanche region width for all alloy compositions, with a minimum sensitivity at x=0.6, for which we offer a physical explanation. While Monte Carlo simulations show that alloy scattering reduces the temperature dependence of breakdown voltage, the effect is too small to account for the observations. [ABSTRACT FROM AUTHOR]

Photomultiplication measured in Al[sub 0.6]Ga[sub 0.4]As p[sup +]-i-n[sup +] diodes falls with temperature in a manner that becomes less marked with decreasing i-region width and increasing electric field. Simple arguments relate this result to the effects of the ionization dead space, which is expected to depend weakly on temperature, and to the reduced scattering in the high electric fields present in thin avalanching structures. The temperature dependence of the "enabled" ionization coefficient can be approximately accounted for by simply subtracting the ballistic dead space from the width of the avalanche region. The simple arguments suggest the effect is independent of materials system. [ABSTRACT FROM AUTHOR]

Avalanche breakdown voltages were determined for a series of In[sub 0.53]Ga[sub 0.47]As p-i-n diodes with i-region thicknesses ranging from 0.4 to 4.79 μm using measurements of reverse dark current and phase-sensitive photomultiplication. Despite its narrower bandgap In[sub 0.53]Ga[sub 0.47]As is found to have a very similar breakdown voltage to GaAs. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]