Minority Carrier Transport Properties Research Articles

Influence of Energetic Particles and Electron Injection on Minority Carrier Transport Properties in Gallium Oxide

The influence of various energetic particles and electron injection on the transport of minority carriers and non-equilibrium carrier recombination in Ga2O3 is summarized in this review. In Ga2O3 semiconductors, if robust p-type material and bipolar structures become available, the diffusion lengths of minority carriers will be of critical significance. The diffusion length of minority carriers dictates the functionality of electronic devices such as diodes, transistors, and detectors. One of the problems in ultrawide-bandgap materials technology is the short carrier diffusion length caused by the scattering on extended defects. Electron injection in n- and p-type gallium oxide results in a significant increase in the diffusion length, even after its deterioration, due to exposure to alpha and proton irradiation. Furthermore, post electron injection, the diffusion length of an irradiated material exceeds that of Ga2O3 prior to irradiation and injection. The root cause of the electron injection-induced effect is attributed to the increase in the minority carrier lifetime in the material due to the trapping of non-equilibrium electrons on native point defects. It is therefore concluded that electron injection is capable of “healing” the adverse impact of radiation in Ga2O3 and can be used for the control of minority carrier transport and, therefore, device performance.

Effect of Electron Injection on Minority Carrier Transport Properties in Unintentionally Doped GaN

GaN is a robust and stable wide bandgap material with a bandgap of 3.4eV. It is extensively studied for applications in light emitting diodes, high-power electronics and optoelectronics. Radiation hardness is another attractive attribute of GaN due to its atomic structure and wide bandgap with possible electronic applications in lower Earth satellite orbits. Like other wide bandgap materials, GaN also suffers from short diffusion length, which has been earlier reported to be ~100-300 nm. Minority carrier transport properties such as carrier lifetime (τ) and diffusion length (L) are of vital importance in bipolar and minority-carrier devices. Previously, it was shown that low energy electron beam irradiation (up to 30 keV) for extended duration (up to 3000 seconds) can be used to enhance L by a factor of 4 at room temperature in other wide bandgap materials like p-ZnO and β-Ga2O3 [1-3] . It was suggested that the increase in L is associated with trapping of non-equilibrium electrons on meta-stable trap levels acting as recombination centers. Dependence of L on τ is given by the relation L=√(Dτ) , where D is the diffusion coefficient. Therefore, removal of recombination centers, due to electron trapping on them would, in turn, increase τ, and hence L. Another effect of low energy electron irradiation, apart from increasing τ, is manifested in decrease in the continuous CL intensity. In this work, unintentionally doped GaN was studied using Electron Beam-Induced Current (EBIC), Continuous (CL) and Time-Resolved Cathodoluminescence (TRCL) techniques to independently measure L and τ, and verify the validity of the proposed model[4]. Measurements were conducted on GaN Schottky rectifiers (~400 µm GaN layer grown by Halide Vapor Phase Epitaxy on sapphire) with electron concentration of ~1x1017 cm-3. L was found to increase from 306 nm to 347 nm for a maximum electron injected charge density of 117.5 nC/µm3 and saturating at 60 nC/µm3. For the same maximum and saturation injected charge densities, τ was found to increase from 77 ps to 101 ps. Fig. 1a shows a plot of TRCL streaks for bare region and the region with maximum injected charge density. Slower decay of streak signal indicates longer lifetime in the electron-injected region. Fig. 1b depicts the plot of L and √τ as a function of injected charge density at room temperature. A clear linear dependence (L=√(Dτ)) is seen till saturation charge density of ~60 nC/µm3. Furthermore, mobility values (from the diffusion coefficient) for various charge injection doses were obtained for the ultrafast regime of TRCL spectroscopy.

Effect of Electron Injection on Minority Carrier Transport in 10 MeV Proton Irradiated β-Ga2O3 Schottky Rectifiers

We report the effect of extended duration electron beam exposure on the minority carrier transport properties of 10 MeV proton irradiated (fluence ∼1014 cm−2) Si-doped β-Ga2O3 Schottky rectifiers. The diffusion length (L) of minority carriers is found to decrease with temperature from 330 nm at 21 °C to 289 nm at 120 °C, with an activation energy of ∼26 meV. This energy corresponds to the presence of shallow Si trap-levels. Extended duration electron beam exposure enhances L from 330 nm to 726 nm at room temperature. The rate of increase for L is lower with increased temperature, with an activation energy of 43 meV. Finally, a brief comparison of the effect of electron injection on proton irradiated, alpha-particle irradiated and a reference Si-doped β-Ga2O3 Schottky rectifiers is presented.

Extraction of the minority carrier transport properties of solar cells using the Hovel model and genetic algorithms

In this paper, a quick and accurate method for extraction of the minority carrier transport properties of p-n or n-p junction solar cells, such as diffusion lengths and surface recombination velocities, is presented. The knowledge of these parameters is essential to investigating factors that limit the performance of photovoltaic devices. The proposed method, based on genetic algorithms and the analytical Hovel model, is used to fit the external quantum efficiency (EQE) curves of solar cells with different emitter thicknesses. As a demonstrative example of application of the procedure carried out in this work, theoretical and experimental EQE curves of n-p GaAs solar cells under the standard AM1.5G spectrum have been used in order to extract the desired parameters. Errors less than 2.4% have been obtained, which shows the capability of the developed tool. An analysis of the total number of iterations is presented. The results obtained can be used to improve the design, optimization, and manufacturing process of high-efficiency photovoltaic devices.

Modeling of the tandem III–V dilute nitride bulk or quantum engineered/silicon solar cells

Tandem silicon solar cells are designed in order to lower the levelized cost of solar electricity by increasing the cell’s efficiency to more than 30%. Dual junction thin film III–V/Si cell efficiencies are limited to 20% because of high dislocation densities (>106 cm−2) due to lattice mismatch between them. GaAsyP1−x−yNx has a lattice constant matched with silicon at y = 4.7*x − 0.1 and has an optimal bandgap i.e. between 1.7 and 1.9 eV for tandem III–V’s with Si. This work designs a p-i-n GaAsPN/GaP symmetric or asymmetric (resonant thermos tunneling) multi-quantum well (MQW) device for a top sub-cell of a dual junction silicon tandem to minimize the effect of the degraded minority carrier transport properties of bulk dilute nitride layers and presents the simulated solar cell properties using a realistic drift-diffusion approach. Using extracted experimental spectral responses as well as other solar cell parameters of various silicon solar cell like: HIT, PERC, IBC etc, the tandem efficiencies of designed p-i-n MQWs devices in conjunction with silicon solar cells have been evaluated. The tandem efficiency of an optimized p-i-n MQWs device under AM 1.5 G spectrum can exceed 33% when in conjunction with a 25.6% efficient HIT silicon solar cell.

Quantitative Analysis of the Degradation Behavior of Silicon Solar Cell Irradiated by 1 MeV Electron Beams Using Photocarrier Radiometry Combined with Lock-in Carrierography

Silicon solar cells with cover glass irradiated by 1 MeV electron beams at various fluences were investigated using photocarrier radiometry (PCR) combined with lock-in carrierography (LIC, spectrally gated dynamic photoluminescence). The minority carrier transport properties (i.e., minority carrier lifetime τ, diffusion coefficient D, surface recombination velocities S) and the degradation of these properties were studied using PCR. The relative damage coefficient obtained by LIC was consistent with the PCR measurement. The local series resistance of the solar cell before and after irradiation was characterized by LIC. The results showed that the series resistance increased with electron fluences.

Measurements of ultrafast spin-profiles and spin-diffusion properties in the domain wall area at a metal/ferromagnetic film interface

Exciting a ferromagnetic material with an ultrashort IR laser pulse is known to induce spin dynamics by heating the spin system and by ultrafast spin diffusion processes. Here, we report on measurements of spin-profiles and spin diffusion properties in the vicinity of domain walls in the interface region between a metallic Al layer and a ferromagnetic Co/Pd thin film upon IR excitation. We followed the ultrafast temporal evolution by means of an ultrafast resonant magnetic scattering experiment in surface scattering geometry, which enables us to exploit the evolution of the domain network within a 1/e distance of 3 nm to 5 nm from the Al/FM film interface. We observe a magnetization-reversal close to the domain wall boundaries that becomes more pronounced closer to the Al/FM film interface. This magnetization-reversal is driven by the different transport properties of majority and minority carriers through a magnetically disordered domain network. Its finite lateral extension has allowed us to measure the ultrafast spin-diffusion coefficients and ultrafast spin velocities for majority and minority carriers upon IR excitation.

Heavy and light hole minority carrier transport properties in low-doped n-InGaAs lattice matched to InP

Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due to their higher valence band density of states, but at large diffusion distances, the light holes begin to dominate due to their larger diffusion length. It is found that heavy holes have a diffusion length of 54.5 ± 0.6 μm for an n-InGaAs doping of 8.4 × 1015 cm–3 at room temperature, whereas light holes have a diffusion length in excess of 140 μm. Heavy holes demonstrate a mobility of 692 ± 63 cm2/Vs and a lifetime of 1.7 ± 0.2 μs, whereas light holes demonstrate a mobility of 6200 ± 960 cm–2/Vs and a slightly longer lifetime of 2.6 ± 1.0 μs. The presented method, which is limited to low injection conditions, is capable of accurately resolving minority carrier transport properties.

Low dose 60Co gamma-irradiation effects on electronic carrier transport and DC characteristics of AlGaN/GaN high-electron-mobility transistors

ABSTRACTThe impact of internal irradiation with secondary Compton electrons, generated by gamma-photons, on the characteristics of III-N/GaN-based devices was explored. N-channel AlGaN/GaN high-electron-mobility transistors (HEMTs) were exposed to gamma-radiation from a 60Co source for doses up to 600 Gy. Temperature-dependent electron beam-induced current (EBIC) was employed to measure minority carrier transport properties. For low doses below ∼250 Gy, the minority carrier diffusion length in AlGaN/GaN HEMTs is shown to increase by about 40%. This increase is likely due to longer minority carrier lifetime induced by internal Compton electron irradiation. An associated decrease in activation energy, extracted from temperature-dependent EBIC, was also found. The obtained increase in transconductance and decrease in gate leakage current indicate an improvement in performance of the devices after low doses of irradiation. For high doses of gamma-irradiation, above ∼300 Gy, the performance of HEMTs showed a deterioration. The deterioration results from the onset of increased carrier scattering due to additional radiation-induced defects, as is translated in a decrease of minority carrier diffusion length.

Direct minority carrier transport characterization of InAs/InAsSb superlattice nBn photodetectors

We present an extensive characterization of the minority carrier transport properties in an nBn mid-wave infrared detector incorporating a Ga-free InAs/InAsSb type-II superlattice as the absorbing region. Using a modified electron beam induced current technique in conjunction with time-resolved photoluminescence, we were able to determine several important transport parameters of the absorber region in the device, which uses a barrier layer to reduce dark current. For a device at liquid He temperatures, we report a minority carrier diffusion length of 750 nm and a minority carrier lifetime of 200 ns, with a vertical diffusivity of 3 × 10−2 cm2/s. We also report on the device's optical response characteristics at 78 K.

Investigation of metastability and instability effects on the minority carrier transport properties of microcrystalline silicon thin films by using the steady-state photocarrier grating technique

Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of σDark and σphoto within a factor of two, but they did not affect the minority carrier μτ-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80 °C resulted in more than an order of magnitude increase in both σDark and σphoto and an increase in the diffusion length, LD, by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both σDark and σphoto to their annealed-state values, while LD decreases only slightly. Such increase in the LD values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains.

Plan View and Cross-Sectional View EBIC Measurements: Effect of e-Beam Injection Conditions on Extracted Minority Carrier Transport Properties

The application of low-doped epitaxial layers and the increase of complexity of silicon photodiode design require the knowledge of the basic physical parameters, such as minority carrier lifetime or diffusion length, to improve the photodiode performance simulation. In this paper, electron-beam-induced current technique is used to evaluate minority carrier lifetime and diffusion length on a silicon photodiode. Particular focus is to compare plan view and cross-sectional view testing geometry, and also to evaluate artefacts introduced by high injection conditions unavoidable in lifetime measurement.

Minority carrier lifetime variations associated with misfit dislocation networks in heteroepitaxial GaInP

Variations in minority carrier transport properties associated with networks of misfit dislocations have been measured using a unique optical technique in p-type GaInP grown on Ge. The diffusion length L of minority carriers has been measured with a spatial resolution of 0.4 µm in regions showing alternating light and dark luminescence bands. Periodic variations of ±4% from a mean diffusion length of 3.9 µm were measured and found to be anti-correlated to intensity fluctuations. A model based on the coupling between luminescence intensity and minority carrier lifetime allows for the extraction of spatial variation of both radiative and non-radiative lifetimes. For this high quality material, with relatively low concentrations of non-radiative recombination centers, the results indicate variations in radiative recombination lifetime associated with dopant fluctuations.

Surface Plasmon-mediated Luminescence in Organic Field-effect Transistors

AbstractConsiderable attention has recently been paid to light-emitting organic filed-effect transistors (LEOFETs) because of their potential applications to novel opto-electronic devices. In most LEOFETs, light emission occurs through the recombination of carriers injected from electrodes. Various efforts have been, thus, made to improve injection and transport properties of minority carriers in organic materials. On the other hand, light emission was frequently observed when we applied reverse bias voltages, that is, positive bias voltages, to the drain electrode for p-type FETs with grounded source and negatively biased gate electrodes. In this circuit, minority carriers were not injected into organic semiconductors. Nevertheless, the luminous intensity increased with drain voltage.In the present work, we have prepared such novel type LEOFETs with various device structures. We have inserted thin metal wires between the source and drain electrodes in the channel region of pentacene FETs. It was found that the light emission was observed around the metal wires inserted and the luminous efficiency was improved with the number of wires. The luminescence spectrum was identical to that of pentacene films. The light emission was thought to be generated by via energy transfer to the pentacene films from plasmon excited on metal surfaces by current injection.

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.

Carrier-density-wave transport and local internal electric field measurements in biased metal-oxide-semiconductor n-Si devices using contactless laser photo-carrier radiometry

Laser infrared photo-carrier radiometry was used with an n-type Si metal-oxide-semiconductor (MOS) diode and with a Si–SiO2 structure with a transparent electrode and under external bias. Application of three-dimensional PCR theory yielded values of the minority carrier (hole) transport properties in the presence of the thus created local internal electric field at fixed frequencies. Furthermore, the internal electric field at fixed applied voltage was calculated. Under the combination of increased temperature and voltage, the sub-interface position of the carrier-density-wave centroid was found to depend on a trade-off between increased recombination lifetime and decreased ambipolar (conductivity) mobility. The ability of PCR to measure local internal electric fields by combining applied bias sweeps and frequency scans appears to pave the way towards the contactless reconstruction of depth profiles of these fields in active devices.

Minority carrier transport properties of GaInNAs heterojunction bipolar transistors with 2% nitrogen

GaAs-based heterojunction bipolar transistors (HBTs) have a relatively large turn-on voltage of approximately 1.4 V that can only be decreased by reducing the band-gap energy of the base material. For a variety of applications, particularly operation with low power supply voltage and reduced power dissipation, it would be desirable to have a smaller value of turn-on voltage. We report the performance of NpN double heterojunction bipolar transistors fabricated on a GaAs substrate with a Ga0.89In0.11N0.02As0.98 base that has a band-gap energy (Eg) of 0.98 eV; this is achieved with a nitrogen composition of 2%. These devices have a turn-on voltage VBE that is 0.4 V lower than that of their GaAs-base counterparts. The peak incremental current gain HFE is 8. The current gain of nitrogen containing HBTs is degraded due to the complex change in transport properties of carriers through the GaInNAs base region. In this article, the transport properties of GaInNAs-base HBTs are investigated by temperature-dependent dc current–voltage characteristics, optical and high frequency small-signal electrical measurements. The characterization of these devices is important to understand the effect of nitrogen on the device performance.

Electron transport in the barriers of AlGaAs/GaAs quantum well structures observed by scanning-tunneling-microscope light-emission spectroscopy

Using scanning-tunneling-microscope light-emission (STM-LE) spectroscopy, we have investigated the transport properties of minority carriers in p-Al0.3Ga0.7As/p-GaAs quantum well (QW) structures. The optical measurements were performed on a cleaved (110) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the STM tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants.

Microcrystalline silicon deposited by the hot-wire CVD technique

Hot-wire chemical vapour deposition (HW-CVD) is a well-known technique to deposit amorphous silicon with high deposition rates from the decomposition of silane and hydrogen gases. By changing the hydrogen and silane flow rates, it is possible to observe a transition from amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). In this study, structural and electrical properties of layers deposited as a function of silane concentration in the gas are presented. Ellipsometry and X-ray diffractometry have been used to assess the structure of the films. Steady-state photoconductivity, steady-state photocarrier grating, and modulated photocurrent experiments have been carried out to characterise both majority and minority carrier transport properties. Finally, the addition of phosphine and diborane in the reactor allows the deposition of n- and p-type layers with conductivities up to 10 and 1 s cm −1, respectively, making possible the realisation of HW-CVD solar cells.

Correlation between the surface defect distribution and minority carrier transport properties in GaN

We have studied linear dislocations and surface defects in p- and n-type metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy grown GaN films on sapphire with atomic force microscopy. The surface pits due to threading dislocations were found not to be distributed randomly but on the boundaries of growth columns. The dislocations are thought to be electrically active since the average distance between them (average column size) is comparable to minority carrier diffusion lengths as measured by electron beam induced current experiments on Schottky diodes fabricated with the same material. Diffusion lengths found for holes and electrons are on the order of Lp=0.28 μm and Le=0.16 μm which corresponded to the sizes of regions free from surface dislocations in both cases and can be described by a simple model of recombination on grain boundaries.