Al Thickness Research Articles

Effects of Bond Pad Thickness on Shear Strength of Copper Wire Bonds

Abstract Copper (Cu) wire bonding is now widely accepted as a replacement for gold (Au), however, its use in high reliability applications is limited due to early failures in high temperature and humid conditions. The Au to Cu wire transition is mainly driven by cost savings though there are other advantages to Cu such as better electrical and thermal conductivity, slower intermetallic compound (IMC) formation and reduced wire sweep during transfer molding. Some automotive, industrial and aerospace industries are still reluctant to adopt Cu wire bonded products due to perceived risks of wire and bond pad cracks, the potential for corrosion, and some lack of understanding about its reliability in harsh conditions. A wire bond is considered good if destructive sampling qualification tests and periodic monitors pass for the batch. Tests include wire pull strength, wire bond shear, IMC coverage, and thickness of bond pad aluminum (Al) remaining beneath the bond. Nondestructive inspections also verify acceptable ball diameter and Al “splash”. This paper focuses on the bond shear test and its contribution to Cu ball bond reliability assessment, especially when changing Al bond pad thickness. A new revision of the JEDEC Wire Bond Shear Test Method, JESD22-B116B, has just been released, to include Cu wirebonds for the first time. It helps to clarify shear test failure modes for Cu ball bonds. However, there are still questions to be answered through research and experimentation, especially to learn the extent to which one may predict Cu ball bond reliability based on shear test results. Pad Al thickness is not considered in the current industry standards for shear test. Yet bond pad Al thickness varies widely among semiconductor products. This research is intended to contribute toward improved industry standards. In this study, power and time bonding parameters along with bond pad thickness are studied for bond strength. Several wire bonds are created at different conditions, evaluated by optical microscope and SEM, IMC% coverage and bond shear strength. Similar bonding conditions are repeated for bond pads of 4um, 1um and 0.5um thickness.

Fine Control of the Electric Field Distribution in the Heterostructure Multiplication Region of AlGaN Avalanche Photodiodes

To enhance avalanche ionization, we designed a new separate absorption and multiplication AlGaN solar-blind avalanche photodiode (APD) by replacing the conventional AlGaN homogeneous multiplication region with a high/low Al content AlGaN heterostructure layer. The calculated results showed that the improved APD with the Al0.3 Ga0.7N/Al0.45Ga0.55N heterogeneous multiplication region exhibits 51% higher gain than that of the conventional APD, benefiting from the six times higher hole ionization coefficient of Al 0.3Ga0.7N, compared to that of Al0.45Ga0.55N. Furthermore, we inserted an intermediate n-type AlGaN charge layer into the heterogeneous multiplication region to obtain superior performance by finely controlling the electric field distribution. The dependence of breakdown voltage and multiplication gain on the Al content, doping concentration, and thickness of the charge layer is studied in detail to get the optimal structure. Meanwhile, the solar-blind characteristic is also taken into account when optimizing the APD structure.

Dual-channel extraordinary ultraviolet transmission through an aluminum nanohole array

Ultraviolet (UV) surface plasmon (SP) has distinct applications in UV filters, high-density optical storage, spectral enhancement, optical detectors, and nanolithography, which are closely related to plasmon-induced extraordinary optical transmission (EOT). However, such EOT in the UV region has not been the subject of detailed research. We report UV transmission based on theoretical research using the finite-difference time-domain method, by modulating the Al thickness, hole size, array periodicity, and SiO2 overlayer thickness. It is notable that we can obtain dual-channel UV transmission peaks with excellent qualities such as high transmissivity, zero cross-talk, narrow bandwidth, and perfect symmetry, by optimizing the parameters. The UV transmission peaks have been discovered to non-monotonously shift with increasing hole size. Although array periodicity has great influence on the transmission peak position, the peak energy in the UV region is much less than the value predicted by the well-known periodicity-related surface plasmon polariton (SPP) wavelength equation; the energy discrepancy in the UV region can reach above 20%, which is much larger than the value (typically 4%) in the visible-infrared region. Furthermore, the SiO2 overlayer may significantly modify the transmission properties. The Al nanohole arrays have also been found to exhibit distinct multi-band UV electric field enhancement properties with special interface effect and size effect. Such extraordinary dual-channel UV transmission with zero cross-talk, based on a very simple Al nanohole array, has promising application in dual-channel UV filters, high-density optical storage, and plasmon-enhanced fluorescence/Raman spectroscopy, which generally involves two wavebands (writing/reading storage or exciting/emission wavelengths). This study is expected to broaden our fundamental understanding of the UV EOT phenomenon, and provide references for experimental research and application of deep-UV and near-UV-related dual-band plasmonic devices.

Stacking multiple connecting functional materials in tandem organic light-emitting diodes

Tandem device is an important architecture in fabricating high performance organic light-emitting diodes and organic photovoltaic cells. The key element in making a high performance tandem device is the connecting materials stack, which plays an important role in electric field distribution, charge generation and charge injection. For a tandem organic light-emitting diode (OLED) with a simple Liq/Al/MoO3 stack, we discovered that there is a significant current lateral spreading causing light emission over an extremely large area outside the OLED pixel when the Al thickness exceeds 2 nm. This spread light emission, caused by an inductive electric field over one of the device unit, limits one’s ability to fabricate high performance tandem devices. To resolve this issue, a new connecting materials stack with a C60 fullerene buffer layer is reported. This new structure permits optimization of the Al metal layer in the connecting stack and thus enables us to fabricate an efficient tandem OLED having a high 155.6 cd/A current efficiency and a low roll-off (or droop) in current efficiency.

Towards standardization of x-ray beam filters in digital mammography and digital breast tomosynthesis: Monte Carlo simulations and analytical modelling

In digital breast tomosynthesis and digital mammography, the x-ray beam filter material and thickness vary between systems. Replacing K-edge filters with Al was investigated with the intent to reduce exposure duration and to simplify system design. Tungsten target x-ray spectra were simulated with K-edge filters (50 µm Rh; 50 µm Ag) and Al filters of varying thickness. Monte Carlo simulations were conducted to quantify the x-ray scatter from various filters alone, scatter-to-primary ratio (SPR) with compressed breasts, and to determine the radiation dose to the breast. These data were used to analytically compute the signal-difference-to-noise ratio (SDNR) at unit (1 mGy) mean glandular dose (MGD) for W/Rh and W/Ag spectra. At SDNR matched between K-edge and Al filtered spectra, the reductions in exposure duration and MGD were quantified for three strategies: (i) fixed Al thickness and matched tube potential in kilovolts (kV); (ii) fixed Al thickness and varying the kV to match the half-value layer (HVL) between Al and K-edge filtered spectra; and, (iii) matched kV and varying the Al thickness to match the HVL between Al and K-edge filtered spectra. Monte Carlo simulations indicate that the SPR with and without the breast were not different between Al and K-edge filters. Modelling for fixed Al thickness (700 µm) and kV matched to K-edge filtered spectra, identical SDNR was achieved with 37–57% reduction in exposure duration and with 2–20% reduction in MGD, depending on breast thickness. Modelling for fixed Al thickness (700 µm) and HVL matched by increasing the kV over (0,4) range, identical SDNR was achieved with 62–65% decrease in exposure duration and with 2–24% reduction in MGD, depending on breast thickness. For kV and HVL matched to K-edge filtered spectra by varying Al filter thickness over (700, 880) µm range, identical SDNR was achieved with 23–56% reduction in exposure duration and 2–20% reduction in MGD, depending on breast thickness. These simulations indicate that increased fluence with Al filter of fixed or variable thickness substantially decreases exposure duration while providing for similar image quality with moderate reduction in MGD.

Coexistence of tunneling magnetoresistance and Josephson effects in SFIFS junctions

We demonstrate an integration of tunneling magnetoresistance and the Josephson effects within one tunneling junction. Several sets of Nb-Fe-Al-Al2O3-Fe-Nb wafers with varying Al and Fe layers thickness were prepared to systematically explore the competition of TMR and Josephson effects. A coexistence of the critical current IC(dFe) and the tunneling magnetoresistance ratio T M R(dFe) is observed for iron layer dFe thickness range 1.9 and 2.9 nm. Further optimization such as thinner Al2O3 layer leads to an enhancement of the critical current and thus to an extension of the coexistence regime up to dFe≃3.9 nm Fe.

Investigation of Al/CuO multilayered thermite ignition

The ignition of the Al/CuO multilayered material is studied experimentally to explore the effects of the heating surface area, layering, and film thickness on the ignition characteristics and reaction performances. After the description of the micro-initiator devices and ignition conditions, we show that the heating surface area must be properly calibrated to optimize the nanothermite ignition performances. We demonstrated experimentally that a heating surface area of 0.25 mm2 is sufficient to ignite a multilayered thermite film of 1.6 mm wide by a few cm long, with a success rate of 100%. A new analytical and phenomenological ignition model based on atomic diffusion across layers and thermal exchange is also proposed. This model considers that CuO first decomposes into Cu2O, and then the oxygen diffuses across the Cu2O and Al2O3 layers before reaching the Al layer, where it reacts to form Al2O3. The theoretical results in terms of ignition response times confirm the experimental observation. The increase of the heating surface area leads to an increase of the ignition response time and ignition power threshold (go/no go condition). We also provide evidence that, for any heating surface area, the ignition time rapidly decreases when the electrical power density increases until an asymptotic value. This time point is referred to as the minimum response ignition time, which is a characteristic of the multilayered thermite itself. At the stoichiometric ratio (Al thickness is half of the CuO thickness), the minimum ignition response time can be easily tuned from 59 μs to 418 ms by tuning the heating surface area. The minimum ignition response time increases when the bilayer thickness increases. This work not only provides a set of micro-initiator design rules to obtain the best ignition conditions and reaction performances but also details a reliable and robust MicroElectroMechanical Systems process to fabricate igniters and brings new understanding of phenomena governing the ignition process of Al/CuO multilayers.

Influence of the metal-induced crystallization on the structural and electrochemical properties of sputtered LiCoO2 thin films

LiCoO2 thin films were fabricated using the metal-induced crystallization (MIC) method. The condition to create the MIC effect was investigated. The location and thickness of the Al coating used to manufacture the LiCoO2 cathode thin film electrodes were varied, and the structural and electrochemical characteristics of the LiCoO2 thin film electrode were investigated by Raman spectroscopy, field emission scanning electron microscopy and charge-discharge tests. The degree of crystallization of LiCoO2 increased in the order of uncoated LiCoO2 (lowest), Al(coating)/LiCoO2/Al foil, and LiCoO2/Al(coating)/Al foil (highest), for identical heat treatment condition (600°C, 1h). The crystallization of LiCoO2 occurred more quickly as the Al thickness decreased, and it is considered that crystallization of LiCoO2 is delayed owing to the formation of an alloy between Li-Al when there is more Al.

Effects of Probe Marks on Shear Test of Copper Ball Bonds in Two Pad Aluminum Thicknesses

Abstract This paper continues the work of reference [1], evaluating shear test results of Cu ball bonds over a variety of probe marks in two different pad aluminum (Al) thicknesses (0.8μm and 3μm). The presence of invasive probe marks on thick Al bond pads lowers certain shear force results.. Lower values of shear force imply reduced Cu bond reliability. Physical factors are investigated relating to poor intermetallic (IMC) formation in the Cu wirebond and bond shear force. Optical microscopic image analysis of Cu bonds, bond contact areas and Al “splash” are studied for correlation with the shear test results. Percent IMC coverage of bond contact areas decrease when invasive probe marks are present beneath the bond, which in turn may reduce the shear force. Probe mark features are studied to discover the characteristics of greatest influence on % IMC coverage and shear test values in each of the pad metal thicknesses.

High Performance OTFTs with CNT/Metal Hybrid Electrodes and Their Application to Amoled Panel

The contact resistance is an important factor to significantly affect performance of organic thin film transistors (OTFTs). In this paper, a new carbon nanotube (CNT) based hybrid electrodes are introduced for a low contact resistance of source and drain electrodes of OTFTs. The hybrid electrodes consist of the solution-processed CNTs and a metal (Al; CNT/Al or Au; CNT/Au) evaporated on the CNTs. The contact resistance of CNT/Al and CNT/Au hybrid electrodes was varied depending upon the thickness of Al and Au, as shown in Fig.1. For CNT/Al hybrid electrodes, the contact resistance exhibited the minimum of 2.9 KW∙cm (VGS = -30 V) at the Al thickness of 5 nm, which was smaller than 13 KW∙cm of the bare CNT electrodes and tremendously smaller than 4 MW∙cm of Au electrode. It was noticeable that the minimum contact resistance of CNT/Au was 0.9 KW∙cm at the Au thickness of 5 nm which was the lowest value ever reported. The mobility of OTFTs, which used pentacene as the semiconductor and polyvinylphenol as the gate dielectric, also followed the same metal thickness dependence as the contact resistance. The maximum mobility of OTFTs using CNT/Al and CNT/Au electrodes was 0.78 cm2/V∙sec and 1.0 cm2/V∙sec, respectively, at the same thickness of 5 nm, which was larger than 0.3 cm2/V∙sec of the bare CNT electrodes. A major origin of such enhancements was identified to be the smallest energy difference, which was obtained at the metal thickness of 5 nm, between the work function of the CNT/metal hybrid electrodes and pentacene HOMO (5.1 eV). The CNT/Au electrodes were applied to OTFTs in AMOLED panel. The AMOLED panel consisted of 64 x 64 pixels where each pixel was composed of 2 OTFTs (switching and driving OTFT), 1 capacitor, and 1 OLED. A shown in Fig.2, the AMOLED panel successfully worked to display some patterns. Figure 1

Comparison of a new optical biometer using swept-source optical coherence tomography and a biometer using optical low-coherence reflectometry

To investigate agreement between the IOLMaster 700 based on swept-source optical coherence tomography (SS-OCT) and the Lenstar LS 900 based on optical low-coherence reflectometry (OLCR). Private practice, Castrop-Rouxelle, Germany. Prospective evaluation of diagnostic test. Axial length (AL), keratometry (K), anterior chamber depth (corneal epithelium to lens) (ACD), lens thickness, and central corneal thickness (CCT) were measured in 183 eyes of 183 patients. Analyses used a paired t test, Pearson correlation coefficient (r), and Bland-Altman plots. Intraocular lens (IOL) power calculations were compared using the Hoffer Q, Holladay 1, and SRK/T formulas. The difference in the mean AL between SS-OCT and OLCR was statistically significant but clinically insignificant (23.61mm±1.27 [SD] and 23.60±1.27mm, respectively; P<.0001); the agreement and correlation were excellent. The mean K with OLCR was flatter by 0.02 diopter (D) than the mean K with SS-OCT, 43.82±1.43 diopters (D) and 43.84±1.43 D, respectively; the difference was not statistically significant. The mean ACD with SS-OCT was deeper by 0.03mm than the mean ACD with OLCR, 3.22±0.44mm and 3.19±0.44mm, respectively; the difference was statistically significant (P<.001) but clinically insignificant. The differences in the mean lens thickness (OLCR 4.63±0.44mm and SS-OCT 4.59±0.43mm) and the mean CCT (OLCR 559±37μm and SS-OCT 554±36μm) were not statistically significant. There was no statistically significant difference between the median absolute errors of the 3 formulas. Agreement between SS-OCT and OLCR was very good. The clinically insignificant but statistically significant differences in AL, ACD, and lens thickness did not reflect a statistically significant difference in IOL power calculation using 3 third-generation formulas. Proprietary or commercial disclosures are listed after the references.

전하생성층 MoOx와 음극 Al의 두께에 따른 양면발광 적층 OLED의 발광 특성

전하생성층 MoO_x와 음극 Al의 두께에 따른 양면발광 적층 OLED의 발광 특성

Performance improvement of organic thin film transistors with carbon nanotube/metal hybrid electrodes for S/D contacts

Electrode contact resistance is an important factor that seriously affects the performance of organic thin film transistors (OTFTs). In this paper, new low contact resistance carbon nanotube (CNT) based hybrid electrodes are introduced for the source and drain electrodes of OTFTs. The hybrid electrodes consist of solution-processed CNTs and a metal (Al; CNT/Al or Au; CNT/Au) layer evaporated on the CNTs. The contact resistance of the CNT/Al and CNT/Au hybrid electrodes was found to vary depending on the thickness of the Al and Au layer. The contact resistance of the CNT/Al hybrid electrodes exhibited a minimum of 2.9 kΩ cm at an Al thickness of 5 nm. It is notable that the minimum contact resistance of the CNT/Au was 0.9 kΩ cm at an Au thickness of 5 nm, and is the lowest value ever reported. It was lower than the 13 kΩ cm of the bare CNT electrodes, and tremendously less than the 4 MΩ cm of the pure Au electrode. The mobility of the OTFTs, which used pentacene as the semiconductor and polyvinylphenol as the gate dielectric, also followed the same dependence on metal thickness as the contact resistance. The maximum mobility of the OTFTs using CNT/Al and CNT/Au electrodes was 0.76 cm2/V sec and 1.0 cm2/V sec, respectively, at the same metal thickness of 5 nm, which was larger than 0.3 cm2/V sec of the bare CNT electrodes. The major origin of these enhancements was found to be the small energy difference between the work function of the CNT/metal hybrid electrodes and pentacene HOMO (5.1 eV), which was obtained at the metal thickness of 5 nm.

Transparent conductive Ta/Al/Ta-grid electrode for optoelectronic and electromagnetic interference shielding applications

Ta/Al/Ta multilayer grid electrodes were fabricated by combining DC magnetron sputtering with a photolithography process. The optical transparency and sheet resistance were investigated for the electrode when the thickness of Al was changed from 20 to 150nm. Predicted and measured sheet resistance and transmittance are in good agreement. The electrode shows insignificant increase in sheet resistance after annealing at 250°C for 2h in air. It also exhibits excellent adhesion, showing no increase in sheet resistance after repeated scotch tape tests and ultrasonic treatment in acetone for 30min. With 150nm Al thickness, the electrode has sheet resistance of 10.2Ω/sq and transmittance of 86% in the whole investigated Vis-NIR spectral range, and the figure of merit is 21.7×10−3Ω−1 which is comparable to ITO. Furthermore, the EMI shielding effectiveness simulations for all the samples were performed over the frequency range of 1 to 12GHz. It was observed that the shielding effectiveness was enhanced with increased thickness of Al and at 6GHz shielding effectiveness was 48dB with 150nm Al thickness.

WE‐DE‐207B‐08: Towards Standardization of X‐Ray Filters in Digital Mammography‐Enabled Breast Tomosynthesis Systems

Purpose:In digital breast tomosynthesis (DBT) systems capable of digital mammography (DM), Al filters are used during DBT and K‐edge filters during DM. The potential for standardizing the x‐ray filters with Al, instead of K‐edge filters, was investigated with intent to reduce exposure duration and to promote a simpler system design.Methods:Analytical computations of the half‐value thickness (HVT) and the photon fluence per mAs (photons/mm2/mAs) for K‐edge filters (50µm Rh; 50µm Ag) were compared with Al filters of varying thickness. Two strategies for matching the HVT from K‐edge and Al filtered spectra were investigated: varying the kVp for fixed Al thickness, or varying the Al thickness at matched kVp. For both strategies, Al filters were an order of magnitude thicker than K‐edge filters. Hence, Monte Carlo simulations were conducted with the GEANT4 toolkit to determine if the scatter‐to‐primary ratio (SPR) and the point spread function of scatter (scatter PSF) differed between Al and K‐edge filters.Results:Results show the potential for replacing currently used Kedge filters with Al. For fixed Al thickness (700µm), ±1 kVp and +(1–3) kVp change, matched HVT of Rh and Ag filtered spectra. At matched kVp, Al thickness range (650,750)µm and (750,860)µm matched the HVT from Rh and Ag filtered spectra. Photon fluence/mAs with Al filters were 1.5–2.5 times higher, depending on kVp and Al thickness, compared to K‐edge filters. Although Al thickness was an order higher than K‐edge filters, neither the SPR nor the scatter PSF differed from K‐edge filters.Conclusion:The use of Al filters for digital mammography is potentially feasible. The increased fluence/mAs with Al could decrease exposure duration for the combined DBT+DM exam and simplify system design. Effect of x‐ray spectrum change due to Al filtration on radiation dose, signal, noise, contrast and related metrics are being investigated.Funding support: Supported in part by NIH R21CA176470 and R01CA195512. The contents are solely the responsibility of the authors and do not reflect the official views of the NIH or NCI.

MO-FG-CAMPUS-IeP2-01: Characterization of Beam Shaping Filters and Photon Spectra From HVL Profiles in CT

Purpose: Advanced dosimetry in CT (e.g. the Monte Carlo method) requires an accurate characterization of the shaped filter and radiation quality used during a scan. The purpose of this work was to develop a method where half value layer (HVL) profiles along shaped filters could be made. From the HVL profiles the beam shaping properties and effective photon spectrum for a particular scan can be inferred. Methods: A measurement rig was developed to allow determinations of the HVL under a scatter-free narrow-beam geometry and constant focal spot to ionization chamber distance for different fan angles. For each fan angle the HVL is obtained by fitting the transmission of radiation through different thicknesses of an Al absorber (type 1100) using an appropriate model. The effective Al thickness of shaped filters and effective photon spectra are estimated using a model of photon emission from a Tungsten anode. This method is used to obtain the effective photon spectra and effective Al thickness of shaped filters for a CT scanner recently introduced to the market. Results: This study resulted in a set of effective photon spectra (central ray) for each kVp along with effective Al thicknesses of the different shaped filters. The effective photon spectra and effective Al thicknesses of shaped filters were used to obtain numerically approximated HVL profiles and compared to measured HVL profiles (mean absolute percentage error = 0.02). The central axis HVL found in the vendor's technical documentation were compared to approximated HVL values (mean absolute percentage error = 0.03). Conclusion: This work has resulted in a unique method of measuring HVL profiles along shaped filters in CT. Further the effective photon spectra and the effective Al thicknesses of shaped filters that were obtained can be incorporated into Monte Carlo simulations.

Asymmetric quantum-well structures for AlGaN/GaN/AlGaN resonant tunneling diodes

Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39 A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates that RTD with a 2.8-nm-thick In0.03Ga0.97N sub-well in front of a 2.0-nm-thick GaN main-well can exhibit the I-V characteristic having Ip and PVCR of 0.07 A and 11.6, about 7 times and double the value of RTD without sub-QW, respectively. The purpose of improving the structure of GaN-based QW is to solve apparent contradiction between the device structure and the device manufacturability of new generation RTDs for sub-millimeter and terahertz applications.

Interconnection reliability and interfacial structure between Au alloy bump and Al pad using ultrasonic bonding

Flip chip technology with Au bumps on a substrace has been widely applied to electronic equipment such as smart phones. The purposes of this study are to examine the effect of Al pad thickness on the bondability of flip chip using ultrasonic bonding and to clarify interfacial structures between Au alloy bumps and Al pads by ultrasonic bonding compared before and after a thermal cycle test. Suitable Al thickness for excellent initial Au/Al bonding without chip cracking are 0.8 μm because a thin Al layer could not reduce stress to a chip under an Al pad during the ultrasonic bonding process. Intermetallic compounds between the Au alloy bump and chip after reflows consisted of five Au-Al layers, and a pure Al layer remained. On the other hand, after the temperature cycle test at 218/423 K, intermetallic compounds between the Au alloy bump and chip were changed into two kinds of Au-Al layers, so a pure Al layer did not exist. In addition, if thick intermetallic compound layers existed around the bonding region, bondability deteriorated easily by thermal stress due to a thermal cycle test, therefore the open failure rate was rising when the Au thickness was 1.2 μm.

Al doping of 4H-SiC by laser irradiation to coated Al film and its application to junction barrier Schottky diode

Aluminum (Al) is doped into 4H-SiC by excimer laser irradiation to a coated Al film on the 4H-SiC surface. Deep (∼200 nm in the depth) and high-concentration (over 1 × 1021/cm3 on the surface) Al doping is realized by adjusting the Al film thickness for the laser fluence. Optical emission measurements clearly show that Al plasma is generated on and above the sample surface by laser irradiation. Surface morphology observation suggests that, under an optimum combination between Al thickness and laser fluence, high-temperature molten Al is formed on the 4H-SiC surface and the molten Al serves as the dopant source. We adopt this laser Al doping to fabricate a junction barrier Schottky (JBS) diode. Selective doping is performed by using SiO2 as the doping mask. The fabrication is thoroughly carried out without heating the substrate except for film deposition and metal sintering processes. The fabricated JBS diode shows satisfactory operation as compared with a pn junction diode.

Quantum efficiency of transmission-mode AlxGa1−xAs/GaAs photocathodes with graded-composition and exponential-doping structure

A transmission-mode AlxGa1−xAs/GaAs photocathode with the combination of composition-graded AlxGa1−xAs window layer and exponential-doping GaAs emission layer is developed to maximize the cathode performance. The theoretical quantum efficiency model with this complex structure containing twofold built-in electric fields is deduced by solving the one dimensional continuity equations combined with the three-step model. By comparison of spectral characteristics of photocathodes with different composition and doping structures, and through analysis of cathode structure parameters, it is found that the twofold built-in electric fields can effectively improve photoemission performance of AlxGa1−xAs/GaAs photocathode, which is related to Al proportion variation range and thicknesses of window layer and emission layer. The quantum efficiency model would provide theoretical guidance for better design of transmission-mode graded bandgap photocathodes.