Minority Hole Research Articles

Photoconductive study and carrier dynamics of vertically aligned GaAs nanowires

Single wire devices are generally fabricated to study the electrical and photoelectric behaviors of semiconductor nanowires (NWs); however detriment or contamination can hardly be avoided during manipulation of NWs under focused ion and electron beams. This could not be a trivial factor for III-V NWs which are candidates for high efficiency solar energy harvesting and sensitive photodetection. In this study an alternative way to probe the photoconductive property of individual epitaxial GaAs NWs is presented. For the sample preparation, a uniform spin-coated layer of polymer was selected to be the supporting medium for the vertically aligned NWs structure; then the adequate thinning and polishing of the sample exposed the NW tip and also achieved the required height of NW. An external power adjustable laser was introduced as the excitation source, and the dark and photoconductive current-voltage properties of individual NW were measured by the conductive atomic force microscopy. The typical Schottky style photoconductive behavior was observed in the vertically aligned GaAs NW, and its photoresponsivity has been found to be much higher than that of the reported for single NW photodetector. Finally, a numerical model based on the experimental setup was established to simulate the photoelectric behavior of individual NW. The minority hole lifetime has been found to dominate the photoconductive current-voltage properties of NW under the positive sample bias, and can be derived from the quantitative fitting of experimental photo-IV curves.

Distinct Photocurrent Response of Individual GaAs Nanowires Induced by n-Type Doping

The doping-dependent photoconductive properties of individual GaAs nanowires have been studied by conductive atomic force microscopy. Linear responsivity against the bias voltage is observed for moderate n-doped GaAs wires with a Schottky contact under illumination, while that of the undoped ones exhibits a saturated response. The carrier lifetime of a single nanowire can be obtained by simulating the characteristic photoelectric behavior. Consistent with the photoluminescence results, the significant drop of minority hole lifetime, from several hundred to subpicoseconds induced by n-type doping, leads to the distinct photoconductive features. Moreover, by comparing with the photoelectric behavior of AlGaAs shelled nanowires, the equivalent recombination rate of carriers at the surface is assessed to be >1 × 10(12) s(-1) for 2 × 10(17)cm(-3) n-doped bare nanowires, nearly 30 times higher than that of the doping-related bulk effects. This work suggests that intentional doping in nanowires could change the charge status of the surface states and impose significant impact on the electrical and photoelectrical performances of semiconductor nanostructures.

Slow de‐trapping of minority holes in n‐type 4H‐SiC epilayers

AbstractThe carrier lifetime in epilayers of n‐type 4H‐SiC with low concentrations of the Z1/2 lifetime killer has been investigated over a wide range of temperature under low injection conditions. It was found that in addition to the primary initial decay dominated by surface recombination (SR), as determined by previous work, in some samples a slow decay component is observed, exhibiting a thermally activated recombination rate. Such a slow tail on the carrier decay can be of concern in high voltage switching devices. The slow decay was well accounted for by the thermal emission of minority carriers trapped on a defect. The resulting analysis has determined that the responsible trap is located ≈Ev + (0.37–0.58) eV with relatively small capture cross‐sections for both electrons and holes: σp ≈ (0.5–12) × 10−17 cm2; σn < (0.3–8) × 10−18 cm2. Comparing these characteristics with reports in the literature for as‐grown materials, the most likely candidates for the responsible defects are the D‐ and/or i‐centers, which have similar ionization energies and capture cross‐sections.

Nonequilibrium carrier distribution in semiconductor photodetectors: Surface leakage channel under illumination

The nonequilibrium carrier distribution in an InGaAs/InP avalanche photodiode under light illumination is obtained by cross-sectional scanning capacitance microscopy combined with numerical simulation. The sheet density of negative surface charge is determined to be 1.85×1010 cm−2 on the native-oxidized InGaAs (110) face. This surface charge is found responsible for the accumulation of minority holes, which leads to an inversion layer at the sidewall surface of device in the absorption region under illumination exceeding 0.1 mW/cm2. The inversion depth increases up to 200 nm along with the enhancement of excitation intensity. This work suggests that a surface leakage channel may form in semiconductor photodetectors through detection light excitation.

Carrier transport mechanisms of Pnp AlGaN∕GaN heterojunction bipolar transistors

We fabricated Pnp AlGaN∕GaN heterojunction bipolar transistors (HBTs) with various base widths WB and investigated their common-emitter current-voltage characteristics at room temperature to clarify their carrier transport mechanisms. The current gain β increased as WB decreased. The maximum current gain βmax was 40 in a HBT with a WB of 30nm. HBTs with different base widths exhibited almost the same tendency for β to increase with increasing the collector current IC, indicating that the carrier transport mechanism is the same in all the n-GaN base layers. With a low IC, recombination in the emitter-base depletion region is the dominant carrier transport mechanism. β was less affected when IC was high, and the carrier transport was dominated by the minority hole diffusion in the neutral base layer. The minority hole diffusion length obtained from the HBT characteristics agrees well with previous results obtained with electron beam induced current measurements, also indicating that βmax was determined by the minority hole diffusion length in the n-GaN base layer.

Negative and positive persistent photoconductivity effects in Al xGa 1−xAs ySb 1−y/InAs quantum wells

Both negative and positive persistent photoconductivity (NPPC and PPPC) effects on the low-temperature transport properties were studied in the Al x Ga 1− x As y Sb 1− y /InAs quantum wells with a well width of L w=15 and 50 nm. The irradiation of light with λ<1.0 μm causes the NPPC, while the light with λ=1.6 μm causes the PPPC. The Hall coefficient R H( B) in the dark or after illumination considerably increases with increasing B, indicating the coexistence of majority electrons and minority holes and allowing to analyze it by means of the two-carrier model. The PPPC effect leads to an increase in both density (n and p) and mobility of electrons and holes, while the NPPC inversely leads to their decrease, which was confirmed by the PPC effects on the SdH oscillations and the Hall resistance in the regime of quantum Hall effect in high magnetic fields. We have found the experimental relation, p μn 3, so that the minority holes increases (decreases) much more rapidly than the majority electrons increases (decreases) in the PPPC (NPPC).

Kelvin probe force and surface photovoltage microscopy observation of minority holes leaked from active region of working InGaAs∕AlGaAs∕GaAs laser diode

A method for direct observation of carrier leakage from active regions of working semiconductor light-emitting diodes and lasers is suggested. In this method, Kelvin probe force and surface photovoltage microscopies are used to measure local changes in the surface potential of the device mirror on which a high concentration of the leaked carriers is expected. The applicability of the method is demonstrated by studying in detail the leakage current on the mirrors of high-power InGaAs∕AlGaAs∕GaAs laser diodes in action. It is shown that minority holes arrive at the mirror surface from the active zone of the laser and spread over to regions of the n emitter and n substrate. This observation is confirmed by exposing the mirror to external light with photon energy exceeding the band gap of the laser structure and measuring the generated surface photovoltage. Owing to surface channels formed by the surface band bending, the holes can move tens of micrometers from the place of their generation. The leakage currents are evaluated on the basis of the surface potential distributions observed. It is found that as the injection current of the laser increases, the leakage current grows until onset of lasing.

Minority carrier diffusion lengths in MOVPE-grown n- and p-InGaN and performance of AlGaN/InGaN/GaN double heterojunction bipolar transistors

We fabricated InGaN p–n junction diode structures on SiC substrates by metalorganic vapor phase epitaxy, and investigated the minority carrier diffusion length in n- and p-InGaN layers by electron beam induced current measurements. The minority electron diffusion length in p-InGaN was little affected by the In content in InGaN. The diffusion length decreased with increasing Mg-doping concentration. The minority hole diffusion length in n-InGaN was little affected by Si-doping concentration but slightly decreased with increasing In content in InGaN. We also fabricated pnp AlGaN/InGaN/GaN double heterojunction bipolar transistors and investigated their common-emitter current–voltage characteristics. The Si-doping concentration in the base was 4×10 19 cm −3. The maximum current gain was 21 at a collector current of −10 mA for an emitter size of 30 μm×50 μm. This good performance is ascribed to the large conduction band discontinuity between the AlGaN emitter and InGaN base.

Kelvin probe force microscopy of hole leakage from the active region of a working injection-type semiconductor laser diode

A method that enables direct observation and quantitative characterization of carrier leakage from the active region of working semiconductor light-emitting diodes and lasers is developed on the basis of Kelvinprobe microscopy. The method is used to reveal, on the surface of the mirror surfaces of high-power InGaAs/AlGaAs/GaAs laser diodes, minority holes arriving from the active region and spreading to the surface regions over the n-type emitter and n-type substrate. It is shown that holes can move through surface channels formed by regions of surface band bending to distances of tens of micrometers from the place where they originally appear at the surface. It is demonstrated that, as the injection current increases, the amount of leakage gradually grows and stabilizes after the onset of lasing.

Influence of minority carrier transport in the optical properties of double barrier diodes

The aim of this work is to study the importance of minority carrier transport in double barrier diodes (DBD). We propose a theoretical model capable to describe the photoluminescence properties observed in GaAs-Al0.35Ga0.65As double barrier diodes with the increase of temperature. In this model, we considered that the minority carries (holes) photocreated at the contact of the structure diffuse, drift and then tunnel to the well. To study the influence of previous transport mechanisms, we solved the continuity equation under the influence of an applied bias and an excitation laser light. The theoretical photocurrent and photoluminescence calculations agree quite well with the experimental observations. Within the approaches of our model, we are able to simulate the minority holes mobility from the photocurrent measurements suggesting the use of the experimental technique as a probe of the carrier mobility.

Electron beam-induced current investigation of GaN Schottky diode

In this article, we report the electron beam-induced current (EBIC) measurements in a GaN Schottky diode performed in the line-scan configuration. A theoretical model with an extended generation source was used to accurately extract some minority carrier transport properties of the unintentionally doped n-GaN layer. The minority hole diffusion length is found to increase from ∼0.35 µm near the junction to ∼1.74 µm at the bulk regions. This change is attributed to an increase of the carrier lifetime caused by the polarization effects, which are preponderant in this component. For depth distances exceeding 0.65 µm, it is shown that the measured current is produced by the reabsorption recombination radiation process. This corresponds to an absorption coefficient of 0.178 µm−1, in good agreement with the optical absorption measurement.

Nonequilibrium Carrier Diffusion and Recombination in Heavily-Doped and Semi-Insulating Bulk HTCVD Grown 4H-SiC Crystals

We applied four-wave mixing (FWM) technique for investigation of high temperaturechemical vapour deposition (HTCVD) grown 4H-SiC samples with different doping levels. The determined minority electron and hole mobilities in heavily doped crystals at doping densities of 1019 cm-3 were found to be equal to 116 and 52 cm2/Vs. In semi-insulating (SI) crystals, the ambipolar diffusion coefficient Da = 2.6 − 3.3 cm2/s and carrier lifetimes of 1.5 – 2.5 ns have been measured. Irradiation of SI crystals by 6 MeV electrons resulted in essential decrease of carrier lifetime down to ~ 100 ps and clearly revealed the defect-assisted carrier generation with respect to two-photon interband transitions before irradiation.

Minority carrier diffusion length in GaN: Dislocation density and doping concentration dependence

We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p–n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108cm−2. It increased from 220to950nm with decreasing Mg doping concentration from 3×1019to4×1018cm−3. For relatively high dislocation density above 109cm−2, it was less than 300nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length.

High-voltage operation with high current gain of pnp AlGaN∕GaN heterojunction bipolar transistors with thin n-type GaN base

A pnp AlGaN∕GaN heterojunction bipolar transistor (HBT) with a thin n-GaN base shows high-voltage operation with high current gain in the common-emitter configuration at room temperature. The device structure was grown by metalorganic vapor phase epitaxy on a sapphire substrate. The emitter area is 30μm×50μm. The HBT can operate at high voltage of 70V with the maximum current gain of 40 at the collector current of 10mA. The maximum output power density is 172kW∕cm2. Transport characteristics in the HBT were also investigated. At small collector current, the current gain is dominated by the recombination current at the emitter-base heterojunction. At moderate collector current, the calculated minority hole diffusion length well agreed with that determined from electron beam induced current measurements, indicating the current gain is dominated by the minority carrier diffusion. At large collector current, a high injection effect was observed in the current gain characteristics.

Phase Analysis of Quantum Oscillations in Graphite

The quantum de Haas-van Alphen (dHvA) and Shubnikov-de Haas oscillations measured in graphite were decomposed by pass-band filtering onto contributions from three different groups of carriers. Generalizing the theory of dHvA oscillations for 2D carriers with an arbitrary spectrum and by detecting the oscillation frequencies using a method of two-dimensional phase-frequency analysis which we developed, we identified these carriers as (i) minority holes having a 2D parabolic massive spectrum p(2)(perpendicular)/2m(perpendicular), (ii) massive majority electrons with a 3D spectrum and (iii) majority holes with a 2D Dirac-like spectrum +/-vp(perpendicular) which seems to be responsible for the unusual strongly-correlated electronic phenomena in graphite.

Impact of dislocations on minority carrier electron and hole lifetimes in GaAs grown on metamorphic SiGe substrates

The minority carrier lifetime of electrons (τn) in p-type GaAs double heterostructures grown on GaAs substrates and compositionally graded Ge/Si1−xGex/Si (SiGe) substrates with varying threading dislocation densities (TDDs) were measured at room temperature using time-resolved photoluminescence. The electron lifetimes for homoepitaxial GaAs and GaAs grown on SiGe (TDD∼1×106 cm−2) with a dopant concentration of 2×1017 cm−3 were ∼21 and ∼1.5 ns, respectively. The electron lifetime measured on SiGe was substantially lower than the previously measured minority carrier hole lifetime (τp) of ∼10 ns, for n-type GaAs grown on SiGe substrates with a similar residual TDD and dopant concentration. The reduced lifetime for electrons is a consequence of their higher mobility, which yields an increased sensitivity to the presence of dislocations in GaAs grown on metamorphic buffers. The disparity in dislocation sensitivity for electron and hole recombination has significant implications for metamorphic III-V devices.

Radiation damages of InGaAs photodiodes by high-temperature electron irradiation

Results are presented of a detailed study of the effects of high-temperature 2-MeV electron irradiation on the performance degradation of InGaAs photodiodes. The macroscopic device performance will be correlated with the radiation-induced defects, observed by DLTS. It was found that the dark current increases after irradiation, while the photo current decreases. After irradiation, one majority electron capture level with ( E c−0.37 eV) was induced in the n-InGaAs layer, while no minority hole traps were found. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decrease with increasing irradiation temperature. For a 300 °C irradiation, the reduction of the photo current is only 40% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.

Induced lattice defects in InGaAs photodiodes by high-temperature electron irradiation

Results of a detailed study of the effects of high-temperature 2-MeV electron irradiation on the performance degradation of InGaAs photodiodes are presented. The macroscopic device performance will be correlated with the radiation-induced defects observed by deep level transient spectroscopy. It was found that the dark current increases after irradiation, while the photocurrent decreases. After irradiation, one majority electron capture level with (Ec−0.37eV) was induced in the n-InGaAs layer, while no minority hole traps were found. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decrease with increasing irradiation temperature. For a 300°C irradiation, the reduction of the photocurrent is only 40% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.

2D device-level simulation study of strained-Si pnp heterojunction bipolar transistors on virtual substrates

A novel strained-Si pnp heterojunction bipolar transistor (HBT) design, suitable for virtual substrate technology, is proposed that is inherently free from the detrimental valence band barrier effects usually encountered in conventional SiGe pnp HBTs on silicon. It takes advantage of the heterojunction formed between a strained-Si layer and a relaxed SiGe buffer (virtual substrate), whose associated valence band offset appears favorable for minority hole transport at the base/collector junction. From two-dimensional (2D) numerical simulation, it is found that the newly proposed strained-Si pnp HBT substantially outperforms the equivalent BJT on a silicon substrate in terms of DC and high-frequency characteristics. A threefold increase in maximum current gain β, a fourfold improvement in peak f t and a 2.5 times increase in peak f max are predicted for strained-Si pnp HBTs on a 50% Ge virtual substrate in comparison with identical conventional silicon pnp BJTs.

Radiation damage induced in Si photodiodes by high-temperature neutron irradiation

Results are presented of a detailed study of the effects of high-temperature 4-MeV neutron irradiation on the performance degradation of Si pin photodiodes together with the radiation-induced defects, observed by deep level transient spectroscopy (DLTS). It was found that the dark current increases after irradiation, while the photocurrent decreases. After irradiation, two majority electron capture levels with (E c -0.22 eV) and (E c -0.40 eV) were induced in the n-Si substrate, while one minority hole capture level with (E v + 0.37 eV) was found. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decreases with increasing irradiation temperature. For a 250 °C irradiation, the reduction of the reverse current is only 20% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.