Minority Electron Diffusion Length Research Articles

Long‐Range Charge Carrier Transport in Planar Polymer Bulk‐Heterojunction Photovoltaic Cells

One‐dimensional scanning optical beam‐induced current (OBIC) measurements have been carried out on polymer bulk heterojunction (BHJ) photovoltaic cells with a planar, or lateral configuration. The planar P3HT:PCBM cells have parallel aluminum or gold electrodes that are 390 to 560 micrometers apart. When a focused laser beam is scanned across the electrode gap, photocurrent or photovoltage is recorded as a function of beam position along with the transmission of the excitation beam. Despite the large electrode gap size, cells with symmetric Al/Al electrodes exhibit significant photocurrent and photovoltage which are the highest at the electrode interfaces and null at the cell center. The OBIC in these large planar polymer BHJ cells is attributed to the metal/BHJ blend Schottky junction. The larger Schottky barrier of the Al/BHJ junction gives rise to a stronger OBIC response than the Au/BHJ junction. The photocurrent and photovoltage always have opposite signs and are antisymmetric about the cell center. In asymmetric Al/Au cells, the electrode work function difference contributes an additional built‐in field/potential drop and significantly modifies the photocurrent and photovoltage profiles. The depletion width of the Al/BHJ Schottky junction is 110–120 μm, while the minority electron diffusion length is determined to be 43.8 μm.

Impact of radiation and electron trapping on minority carrier transport in p-Ga2O3

Highly resistive undoped p-type gallium oxide samples were subjected to cumulative proton irradiation with energies ranging from 25 to 70 keV and doses in the 1.6 × 1014–3.6 × 1014 cm−2 range. Proton irradiation resulted in up to a factor of 2 reduction of minority electron diffusion length in the samples for temperatures between ∼ 300 and 400 K. Electron injection into the samples under test using a scanning electron microscope beam leads to pronounced elongation of diffusion length beyond the pre-irradiation values, thus demonstrating stable (days after injection) recovery of adverse radiation impact on minority carrier transport. The activation energy of 91 meV estimated from the temperature dependent diffusion length vs electron injection duration experiments is likely related to the local potential barrier height for native defects associated with the phenomenon of interest.

16.8%-Efficient n+/p GaAs Solar Cells on Si With High Short-Circuit Current Density

The highest efficiency heteroepitaxial GaAs solar cells on Si have historically been grown in the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /n polarity, which was preferred due to the decreased sensitivity of open-circuit voltage in such cells to threading dislocations. The n± /p polarity also has potential advantages due to the higher mobility of electrons than holes in GaAs, and most multi-junction solar cells in the literature are grown in this polarity. Here, we demonstrate n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p GaAs solar cells on Si with a certified AM1.5G efficiency of 16.8%, approaching the best certified efficiency of 18.1% for p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /n cells in the literature. The high efficiency of our n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p cells is primarily due to the short-circuit current density of 26.5 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is significantly higher than prior p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /n record cells. The strong carrier collection results from the use of a highly transparent AlInP window layer, thin n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> emitter, and a relatively high minority electron diffusion length in the p-type base. The high quantum efficiency of these n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p cells at wavelengths of 700-880 nm makes them promising for future triple-junction devices on Si, where the GaAs will serve as a middle sub-cell.

Impact of temperature and gamma radiation on electron diffusion length and mobility in p-type InAs/GaSb superlattices

The minority carrier diffusion length was directly measured by the variable-temperature Electron Beam-Induced Current technique in InAs/GaSb type-II strain-layer-superlattice infrared-detector structures. The Molecular Beam Epitaxy-grown midwave infrared superlattices comprised 10 monolayers of InAs and 10 monolayers of GaSb to give a total absorber thickness of 4 μm. The diffusion length of minority electrons in the p-type absorber region of the p-type/barrier/n-type structure was found to increase from 1.08 to 2.24 μm with a thermal activation energy of 13.1 meV for temperatures ranging from 77 to 273 K. These lengths significantly exceed the individual 10-monolayer thicknesses of the InAs and GaSb, possibly indicating a low impact of interface scattering on the minority carrier diffusion length. The corresponding minority electron mobility varied from 48 to 65 cm2/V s. An absorbed gamma irradiation dose of 500 Gy halved the minority carrier diffusion length and increased the thermal activation energy to 18.6 meV, due to creation of radiation-induced defect recombination centers.

InAs/GaSb superlattices for advanced infrared focal plane arrays

We report on the development of high performance focal plane arrays for the mid-wavelength infrared spectral range from 3–5 μm (MWIR) on the basis of InAs/GaSb superlattice photodiodes. An investigation on the minority electron diffusion length with a set of six sample ranging from 190 to 1000 superlattice periods confirms that InAs/GaSb superlattice focal plane arrays achieve very high external quantum efficiency. This enabled the fabrication of a range of monospectral MWIR imagers with high spatial and excellent thermal resolution at short integration times. Furthermore, novel dual-color imagers have been developed, which offer advanced functionality due to a simultaneous, pixel-registered detection of two separate spectral channels in the MWIR.

Electron beam induced current profiling of ZnO p-n homojunctions

Variable temperature electron beam induced current technique was employed for the profiling of ZnO p-n homojunctions and the extraction of minority electron diffusion length values in the Sb-doped p-type ZnO region. A thermally induced increase for diffusion length of minority electrons was determined to have an activation energy of ∼145meV. The latter parameter likely represents carrier delocalization energy and determines the increase of the diffusion length due to the reduction in recombination efficiency.

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.

Studies of minority carrier diffusion length increase in p-type ZnO:Sb

Minority electron diffusion length was measured in p-type, Sb-doped ZnO as a function of temperature using the electron beam induced current technique. A thermally induced increase of electron diffusion length was determined to have an activation energy of 184±10meV. Irradiation with a low energy (5kV) electron beam also resulted in an increase of diffusion length with a similar activation energy (219±8meV). Both phenomena are suggested to involve a SbZn–2VZn acceptor complex. Saturation and relaxation dynamics of minority carrier diffusion length are explored. Details of a possible mechanism for diffusion length increase are presented.

Influence of electron injection on the photoresponse of ZnO homojunction diodes

Forward bias electron injection into the p side of a p-n homojunction was shown to result in an improved response of the ZnO photodiodes. Injection of about 25C of charge yielded a nearly 2.5-fold increase of photocurrent at 350nm. This improvement was correlated with the increase of the diffusion length of minority electrons in p-type ZnO:Sb as determined by electron beam induced current measurements. It is suggested that the increase of the diffusion length is related to the carrier trapping on nonionized acceptor levels.

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.

Minority carrier transport in p-type Zn0.9Mg0.1O doped with phosphorus

Minority carrier diffusion length and lifetime in p-Zn0.9Mg0.1O doped with phosphorus were obtained from local electron beam irradiation measurements. The irradiation resulted in an increase of up to 25% in minority electron diffusion length from the initial value of ∼2.12μm and in a simultaneous decrease of the peak near-bandedge cathodoluminescence intensity. The observed phenomena are attributed to charging of phosphorus-related deep acceptor level(s), which is consistent with the activation energy of 256±20meV found for the effect of electron injection in Zn0.9Mg0.1O.

Minority electron transport anisotropy in p-type Al/sub x/Ga/sub 1-x/N/GaN superlattices

The minority electron diffusion length, L, in Mg-doped molecular beam epitaxy (MBE) grown p-type Al/sub x/Ga/sub 1-x/N/GaN superlattices with aluminum content x=0.1 and 0.2 was measured perpendicular and parallel to the superlattice planes by the electron beam induced current technique. A large anisotropy in the transport properties was observed with the effect varying from 1:3 to 1:6. We attribute an experimentally observed diffusion length anisotropy to minority electron scattering during transport across the potential barriers of the superlattice. Reference p-GaN samples were also investigated, and the diffusion length was observed to be isotropic in both MOCVD (L=0.5 /spl mu/m) and MBE (L=0.27 /spl mu/m) grown samples.

Enhancement of minority carrier transport in forward biased GaN p-n junction

Forward bias application to a GaN p-n junction (current density ~12 A/cm2) leads to a 1.6-fold increase in the minority electron diffusion length in the p-layer of an epitaxial p-n structure. The effect persists for several days, and is likely associated with electron injection-induced charging of Mg-related deep levels in p-type GaN.

Photoluminescence of deep levels in (CdZn) Te-correlation with diffusion length measurement

Photoluminescence of deep levels near mid-gap in P-(Cd1−xZnx)Te (x≈0.05–0.07) grown by the vertical gradient freezing method was investigated. Correlation with recombination properties manifested by temperature dependence of diffusion length of minority electrons and the corresponding mobility-lifetime product was studied. Low-temperature photoluminescence (PL) in the spectral range 0.68–1.3 eV and a temperature dependence of diffusion length of minority electrons (DL) measured by EBIC in the temperature range 60–300 K on both as-grown and annealed samples (800°C and 900°C) were investigated. We observed, that while as-grown samples exhibited a steep increase in DL with decreasing temperature in the temperature range 60–140 K, annealing and subsequent quenching resulted in a significant decrease of DL at these temperatures. Photoluminescence band with a peak at approx. 0.84 eV in annealed samples with increased recombination and therefore low DL was observed, while no such peak was detected in the as-grown samples with high DL. Luminescence in the ≈0.8 eV band is usually attributed to a VCd related defect which is supposed to act as a recombination centre. It can be concluded, that a correlation between effects of increased recombination manifested by decreased values of DL at temperatures 60–140 K and detection of the ≈0.8 eV PL band was observed. Reduction of the mobility-lifetime product by increased recombination in (CdZn)Te single crystals is therefore probably connected with a presence of a VCd-related defect.

Heterojunction bipolar transistors in AlGaInP/GaAs grown by metalorganic vapor phase epitaxy

A series of low-pressure metalorganic vapor phase epitaxy grown AlxGa0.52−xIn0.48P/GaAs single heterojunction bipolar transistors with x=0, 0.18, 0.30, 0.40, and 0.52 have been studied. These devices consistently exhibit near-ideal characteristics with base and collector current ideality factors close to unity except in the ungraded AlInP case. A low conduction band spike height which is believed to be due to unintentional grading at the emitter-base junction of the partially graded devices (intentionally graded down close to the Ga0.52In0.48P lattice-matched composition) provides effective hole current suppression over the whole range of compositions. Base bulk recombination current was determined to be the current-gain limiting mechanism for x≤0.3 in this heterojunction bipolar transistors (HBT) system. The common-emitter dc current gain showed a maximum value of 380 at JC=4×103 A/cm2 for x=0.18 which translates to a minority electron diffusion length of 1.1 μm. These are the highest values reported for a base sheet resistance of 440 Ω/⧠.

Luminescence in scanning tunneling microscopy on III–V nanostructures

Using electroluminescence associated with scanning tunneling microscope in AlxGa1−xAs heterostructures, we show that: (a) luminescence due to recombination can be induced within single quantum wells of dimensions down to a few nm and can also be used to image them, (b) the energy of bulk bands can be determined, and (c) transport parameters can be measured, e.g., the thermalization length and the diffusion length of minority electrons. This technique opens up new possibilities for the study and characterization of semiconductors and devices—including the possibility of studying surface states and single trapping centers associated with lattice defects, impurities, chemisorbed species, etc.

HIGH RESOLUTION ELECTRON BEAM INJECTION IN SEMICONDUCTORS USING A SCANNING TUNNELING MICROSCOPE

The tip of a scanning tunneling microscope is used as a source of ultralow-energy electrons to excite luminescence in model AlGaAs/GaAs(001) heterostructures. This beam injection technique has the advantage of providing a nanometer-sized charge generation volume. We demonstrate the application of this technique to determine minority charge carrier transport parameters such as the thermalization length of hot electrons and the diffusion length of minority electrons.

Time and injection rate dependence of minority-carrier diffusion length in solar cells irradiated with x rays

The minority-electron diffusion length in commercial n+p silicon solar cells was determined from measurements of transient photocurrent generated by low-energy x rays. The results show an order of magnitude increase in diffusion length over a time interval inversely proportional to the square of the injection rate. This accounts for the occurrence of dose rate sensitivity of silicon detectors of x rays at very low injection rates. The strong rate dependence observed suggests a space-charge trapping mechanism.

Variation of diffusion length with processing in dielectrically isolated π-silicon tubs

The variation of minority-electron diffusion length with processing in dielectrically isolated π-silicon tubs is investigated using electron-beam-induced current measurements. The analysis of these measurements is complicated by the six boundary conditions on the π-silicon. The diffusion lengths measured in virgin π-tub silicon are ≤ 250 µm while those in π-silicon undergoing a phosphorus getter are >> 250 µm. The determination of exact values for the diffusion lengths as a function of processing are not possible without a more complete understanding of the parasitic effects of the inversion region at the tub walls. Trends in the diffusion lengths are indicative of trends in the carrier lifetime, which are critical in the high-voltage applications for silicon tubs.