Junction Electric Field Research Articles

Observation of high junction electric fields in modulation-doped GaAs/AlGaAs heterostructures by room temperature photoreflectance spectroscopy

Room temperature photoreflectance (PR) was utilized to determine the junction electric field in molecular-beam epitaxy (MBE)-grown modulation-doped GaAs/AlGaAs heterostructures (MDH) with varied spacer layer thickness and doping concentration. The PR spectra exhibited Franz–Keldysh oscillations (FKOs) at energies above the GaAs band gap. Linear regression from the energy plots of the FKO extrema allowed for the calculation of the junction electric field strengths. High junction electric fields of magnitude 270–430 kV/cm were obtained from the samples. These measured fields were almost a factor of two higher than what can be accounted for by the measured carrier concentration. The presence of impurities introduced during the growth process, which could account for the added field strength, was checked by photoluminescence (PL) spectroscopy at 10 K. The similar PL spectra of the samples with different electric field strength ruled out carbon as the source of added field observed. Deep level transient spectroscopy (DLTS) measurement performed on MBE-grown Si-doped GaAs revealed electron traps corresponding to M4 and EL2. M4 is an impurity-related trap involving As vacancies while EL2 is an As-antisite trap. Although DLTS was not performed directly on MDH samples, results in the doped GaAs indicate As-related electron traps as the possible cause of the added field observed by PR.

Electric field of a buried interfacial region measured by positron-lifetime spectroscopy

Positrons from a radioactive source are implanted into a reverse-biased metal–semiconductor contact and are drifted back towards the contact by the internal electric field where they trap into voids and annihilate. The electric field dependent interface annihilation fraction is monitored by way of the intensity of the long (∼400–500 ps) void lifetime component using positron-lifetime spectroscopy. Unlike previous analyses of such systems a numerical model involving positron drift, annihilation and trapping into the interfacial state has been constructed to describe the positron dynamics in the presence of the non-uniform junction electric field. The use of the positron-lifetime technique in probing the internal electric field at buried contacts is thus demonstrated. Results obtained using the numerical method for the Au, Al and Ni/Semi-Insulating (SI)-GaAs contact systems are found to be consistent with the findings of previous studies on the Au/SI-GaAs system.

Rhodium-related deep levels in n-type MOCVD GaAs

Preliminary results on the study of deep levels associated with 4d-transition metal, rhodium, in crystalline GaAs grown by metal-organic chemical vapour deposition (MOCVD) technique are reported for the first time. Deep level transient spectroscopy on n-type GaAs doped in situ with Rh during MOCVD growth reveals a broad majority carrier emission peak. The peak corresponds to a band of deep levels extending over the energy range 0.57–0.65 eV below the conduction band edge with lower-energy states having lower electron capture cross-sections. The deep levels show a pronounced dependence of electron emission rate on the junction electric field. Minority carrier (hole) emission spectra at zero bias show a pronounced Rh-related deep-level peak with a low-temperature shoulder. The dominant level in the lower half-gap is found to have a position E v+0.44 eV, with a field-dependent emission rate signature.

Excitonic effects of InAs monolayers in GaAs/AlGaAs-microcavities

We report on optical investigations of differently spaced multiple InAs-monolayers in weak-finesse GaAs/AlGaAs-microcavities. The excitonic properties of the monolayer stacks were studied on non-resonant structures. The energies of the monolayer-induced transitions were determined by photoluminescence. Photocurrent measurements at different p–n junction electric fields show Stark ladder transitions as well as Quantum confined Stark effect. The optical properties of the resonant cavity-structures were investigated by polarized reflection-, photoluminescence-, and photocurrent-measurements and spectroscopic ellipsometry. We have studied the coupling between cavity- and exciton mode dependent on the degree of tuning by varying the angles of incidence and applied electric fields.

Dopant diffusion and segregation in semiconductor heterostructures: Part 1. Zn and Be in III-V compound superlattices

effect on the neutral species; and (ii) a Fermi-level effect on the ionized species, because, in addition to the chemical effect, the solubility of the species also has a dependence on the semiconductor Fermi-level position. For Zn and Be in GaAs and related compounds, their diffusion process is governed by the doubly-positively-charged group III element self-interstitials (I2+ III), whose thermal equilibrium concentration, and hence also the diffusivity of Zn and Be, exhibit also a Fermi-level dependence, i.e., in proportion to p2. A heterojunction consists of a space-charge region with an electric field, in which the hole concentration is different from those in the bulk of either of the two layers forming the junction. This local hole concentration influences the local concentrations of I2+ III and of Zn- or Be-, which in turn influence the distribution of these ionized acceptor atoms. The process involves diffusion and segregation of holes, I2+ III, Zn-, or Be-, and an ionized interstitial acceptor species. The junction electric field also changes with time and position.

Fermi-Level effect and junction carrier concentration effect on p-Type dopant distribution in IlI-V Compound superlattices

AbstractThe pronounced segregation phenomenon in the distribution of p-type dopants Zn and Be in GaAs and related III-V compound heterostructures has been explained quantitatively by treating simultaneously the processes of dopant atom diffusion, segregation, and the effect of heterojunction carrier concentrations on these two aspects. Segregation of a dopant species between two semiconductor heterostructure layers is described by a model incorporating (i) a chemical effect on the neutral species; and (ii) in addition, a Fermi-level effect on the ionized species. The process of Zn and Be diffusion in GaAs and related compounds is governed by the doubly-positively-charged group III element self-interstitials whose thermal equilibrium concentration and hence also the Zn and Be diffusivities exhibit also a Fermi-level dependence, i.e., in proportion to p2.A heterojunction is consisting of a space charge region with an electric field, in which the hole concentration is different from those in the bulk layers. This influences the junction region concentrations of and of Zn− or Be−, which in turn influence the distribution of the ionized acceptor atoms. The overall process involves diffusion and segregation of holes, , Zn− or Be−, and an ionized interstitial acceptor species. The junction electric field also changes with time and position.

Improved modeling of grain boundary recombination in bulk and p-n junction regions of polycrystalline silicon solar cells

This article provides a theoretical investigation of recombination at grain boundaries in both bulk and p-n junction regions of silicon solar cells. Previous models of grain boundaries and grain boundary properties are reviewed. A two dimensional numerical model of grain boundary recombination is presented. This numerical model is compared to existing analytical models of grain boundary recombination within both bulk and p-n junction regions of silicon solar cells. This analysis shows that, under some conditions, existing models poorly predict the recombination current at grain boundaries. Within bulk regions of a device, the effective surface recombination velocity at grain boundaries is overestimated in cases where the region around the grain boundary is not fully depleted of majority carriers. For vertical grain boundaries (columnar grains), existing models are shown to underestimate the recombination current within p-n junction depletion regions. This current has an ideality factor of about 1.8. An improved analytical model for grain boundary recombination within the p-n junction depletion region is presented. This model considers the effect of the grain boundary charge on the electric field within the p-n junction depletion region. The grain boundary charge reduces the p-n junction electric field, at the grain boundary, enhancing recombination in this region. This model is in agreement with the numerical results over a wide range of grain boundary recombination rates. In extreme cases, however, the region of enhanced, high ideality factor recombination can extend well outside the p-n junction depletion region. This leads to a breakdown of analytical models for both bulk and p-n junction recombination, necessitating the use of the numerical model.

Copper as an electron trap in GaAs0.6P0.4

An electron trap having an energy level of 0.14 eV from the conduction band edge was found in the bulk of copper-diffused VPE-grown n-GaAs0.6P0.4 by conventional DLTS measurements and by pulse-duration dependent capacitance amplitude measurements. The capture cross section at room temperature is about 1.0×10−21 cm2 and has a weak temperature dependence. These properties are attributed to a non-repulsive center having a capturing mechanism which involves multiphonon emission processes with hardly any lattice relaxation. Evolution of the spatial distributions of the traps with time under junction electric field were studied. The results suggest that the trap is positively charged and has a high diffusivity under electric field. The center can thus be identified as positively charged interstitial copper ion rather than some form of copper complexes.

Strain-relaxed, high-speed In 0.2Ga 0.8As MQW p-i-n photodetectors grown by MBE

We have fabricated p-i-n photodetectors with 10, 15 and 20 well In 0.2Ga 0.8As/GaAs multiple quantum wells (MQWs) in the intrinsic region, whose bandwidths exceed 23 GHz. Cathodoluminescence (CL) images are characterized by rectangular grids, indicating strain relaxation through the formation of misfit dislocations. The MQW photoluminescence (PL) is strongly affected by the junction electric field, but PL from the p ++ GaAs cap shows that increasing the number of QWs enhances the non-radiative recombination associated with the misfit dislocation network. Infrared absorption measurements reveal sharp, room temperature excitonic absorption at 980 nm, which shifts to 1000 nm with a 6 V reverse bias. The peak excitonic absorption intensity and linewidth improve as the number of wells increases. Room temperature dark current measurements on 250 μm x 250 μm p-i-n photodetectors yield leakage currents as low as 2.5 nA for a reverse bias of 4.4 V. These results demonstrate the usefulness the usefulness of strain-relaxed InGaAs/GaAs MQW photodetectors for high speed applications.

Evidence for an electric-field-induced phase transition in a spin-polarized two-dimensional electron gas.

A junction between two coplanar two-dimensional electron gases in a ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs heterostructure at which the Fermi level crosses between adjacent spin-split Landau levels has remarkable current-voltage characteristics: current is time dependent, a consequence of dynamic nuclear polarization at the junction. Also, when biased so as to increase the junction electric field, the junction exhibits a series of abrupt transitions between distinct states, an effect that we attribute to an electric-field-induced phase transition of the spin-polarized electrons at the junction.

Negative base current and impact ionization phenomena in AlGaAs/GaAs HBTs

Impact ionization phenomena in the collector region of AlGaAs/GaAs heterojunction bipolar transistors give rise to base current reduction and reversal. These phenomena can be characterized by extracting the M-1 coefficient, which can be evaluated by measuring base current changes. Measurements of M-1 are affected at low current densities by the presence of the collector-base junction reverse current I/sub CBO/. At high current densities, three effects contribute to lower the measured M-1 value: voltage drops due to collector (R/sub C/) and base (R/sub B/) parasitic resistances, device self-heating, and lowering of the base-collector junction electric field due to mobile carriers. By appropriately choosing the emitter current value, parasitic phenomena are avoided and the behavior of M-1 as a function of the collector-base voltage V/sub CB/ in AlGaAs/GaAs HBTs is accurately characterized.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effects of X-rays on p-n junction leakage currents

The effects of X-ray exposure on two types of p-n junction leakage current are presented. X-rays generate surface states at the oxide-silicon interface and lower the junction electric field at the surface. As a result, the usual field-insensitive generation-recombination current (type 1) increases and the field-sensitive leakage current (type 2) decreases. The type 1 current increases linearly with the incident energy density. From the increment of the type 1 leakage, the surface recombination velocity increases by 1*10/sup 3/ cm/s for every 120 mJ/cm/sup 2/ of incident X-ray exposure. Some surface states are responsible for the reduction of the surface electric field and thus the surface component of the type 2 current. The results resemble that of the late stage of hot-carrier stress. The surface states affecting these two types of leakage have different annealing properties. The ones that increase the type 1 current can be annealed out with a short heat cycle, while the ones that lower the type 2 current require a very long heat cycle to remove. >

Interface properties of (Cd,Zn)S/CuInSe2 single-crystal solar cells

Metal contacts to p-type CuInSe2 and heterojunctions of the form n-(CdZn)S/p-CuInSe2 have been prepared, to investigate the electrical behavior and chemical composition of the resulting interfaces and to correlate the results with the composition and corresponding electrical properties of CuInSe2 . The results indicate that the observed variability of the back contacts and front junctions of different CuInSe2 samples can be attributed to the variable initial equilibrium concentration of the intrinsic defect states which dominate the crystals. The type and concentration of the dominant defects can be changed by thermal treatments and by interface induced effects due to the junction electric field and the chemical potential gradients. Diffusion into or out of the CuInSe2 crystals was not detected. More over, no composition or other chemical variation due to the deposition of the sulfide layer was observed.

Effects of emitter-base junction gradation on the minority-carrier transport in the base region of bipolar junction transistors

The effects of emitter-base junction gradation, mobility gradients, and electric field gradients on the injected minority-carrier density distributions in the base region of narrow-base transistors are analyzed. It is shown that the collector current of transistors having a graded emitter-base junction deviates from the ideal characteristics. The factors which contribute to this departure are analyzed in this paper. It is also shown that the emitter-base forward bias voltage affects the transit time significantly in narrow-base transistors, even when high injection level conditions are absent.

Using silicon dioxide as device engineering parameter in the fabrication of photovoltaic cells through masked ion implantation

The main goal of the research reported in this work was to demonstrate the use of silicon dioxide (SiO 2) as an effective device engineering parameter shaping the main electro-optical characteristics of p + −n single-crystalline silicon solar cells fabricated through low-energy masked ion implantation of boron ( 11B +) into n-wafers. It is shown experimentally that by variation of the SiO 2 mask thickness in the vicinity of the projected range R p (1) the peak responsivity wavelength λ s of the silicon photonic devices could be shifted into the visible portion of the light spectrum and (2) the junction electric field, aiding the collection of carriers in the illuminated mode, could be conveniently adjusted. The physical mechanisms underlying the peculiar behavior of photocells fabricated by 11B + implant through SiO 2 layers of variable thickness are explained in detail. The AM1 measurements performed on the test p + −n devices had shown that high-efficiency, “preferentially current-generating” solar cells could be fabricated by means of the technological approach developed, even without back contact and surface passivation.

On the carrier emission from donor-related centres in GaAs 1- xP x and Al xGa 1- xAs

From an experimental study of the electron thermal emission and capture rates of Te-related centres, donor-related (DX) traps have been identified in GaAsP and GaAlAs compounds. It is shown that under thermal excitation nonexponential transient waveforms markedly deviate from theory. The relation of the nonexponential degree with alloy composition and junction electric field has been studied. In addition, optical activated transients, a model based on trap charge dependent emission and capture coefficients, and the drawbacks of Deep Level Transient Spectroscopy (DLTS) techniques to study DX centres are presented.

Alteration of diffusion profiles in semiconductors due to p− n junctions

Concentration profiles of diffusing species in semiconductors are calculated including the effects of the electric fields at p− junctions. The junction electric field can significantly alter diffusion behavior near the junction at the growth temperature, and thus affect dopant uniformity and junction placement for some commonly occurring epitaxial growth conditions. The junction electric field affects diffusion in the same manner as the internal electric field that results from a dopant concentration gradient. The field can produce diffusion profiles with either enhanced or retarded diffusion rates at the junction and pile-up or depletion of the diffusing species near the junction. Several experimental examples for diffusion of Mg across a p− n junction in (Al,Ga)As during growth by liquid phase epitaxy are presented.

Alteration of diffusion profiles in semiconductors due to p- n junctions

Concentration profiles of diffusing species in semiconductors are calculated including the effects of the electric fields at p- n junctions. The junction electric field can significantly alter diffusion behavior near the junction at the growth temperature, and thus affect dopant uniformity and junction placement for some commonly occurring epitaxial growth conditions. The junction electric field affects diffusion in the same manner as the internal electric field that results from a dopant concentration gradient. The field can produce diffusion profiles with either enhanced or retarded diffusion rates at the junction and pile-up or depletion of the diffusing species near the junction. Several experimental examples for diffusion of Mg across a p- n junction in (Al, Ga). As during growth by liquid phase epitaxy are presented.

Capacitance method applied in studying defects in heavily doped semiconductors

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Evidence for tunneling in reverse-biased III-V photodetector diodes

Photodiodes made from III-V group semiconductor alloys have been found to exhibit anomalously high dark currents. We present evidence that tunneling is the dominant source of dark current in many cases. The tunneling current becomes substantial at peak junction electric fields as low as 105 V/cm due to the small direct energy gaps and small effective masses of the materials tested. Tunneling sets limits on the magnitude of the electric field attainable in these materials, and therefore has serious implications on photodetector design and performance.