Diffusion profile measurements in the base of a microwave transistor

A technique is presented for measuring the impurity atom profile in the base of a microwave transistor. Thin layers of silicon are removed by the growth of an anodic oxide. MOS C(V) measurements are made after each oxidation. The oxide is then removed, a new oxide is grown and another MOS C(V) measurement is made. The process is repeated a number of times. For each MOS C(V) curve the maximum width of the space charge layer, w_m, is determined. An experimental plot is made of w_m vs x_s, the total thickness of silicon removed. The base impurity profile is found by calculating for a series of theoretical profiles the curve w_m(x_s). The profile whose w_m(xs) curve matches the experimental curve is taken as the correct profile. Measurements have also been made of the junction capacitances of both the emitter-base and base-collector junctions. The impurity atom concentrations in the base region near the junctions, determined from the junction capacitance measurements, agree within 10 per cent with the values obtained with the MOS technique.     

Measurement of CdxHgi-xTe composition depth profiles using auger electron spectrometry on bevelled sections

The width of the band gap in the ternary system Cd_xHg_I-xTe (CMT) is a function of the value of x and the assessment of device structures requires reliable techniques for the measurement of x both laterally and with depth. This work describes the development of an Auger electron spectrometry (AES) method for the measurement of CMT composition depth profiles, based on measurement of the Cd/Te ratio along shallow-angle bevelled sections through epitaxial layers. Results are accurate to within 3% relative and the depth resolution (Δz = 2σ) is about 0.04 μm even through layer structures with a total thickness of about 20 μm. Techniques for making the bevel sections are described together with the AES measurement and calibration techniques. A composition depth profile is given for a heterostructure grown by metal organic vapour phase epitaxy (MOVPE), showing the interface width (Δz = 2σ) at the composition change to be 0.3 μm. The CdTe/CMT interface widths in material grown by liquid phase epitaxy (LPE), MOVPE and molecular beam epitaxy (MBE) are shown to be highly dependent on growth temperature, with widths of 1.5, 0.2 and ? 0.04 μm respectively.     

Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallizations

For an alternative front side metallization process without screen printing of metal paste the selective opening of the front surface anti‐reflection coating could be realized by laser ablation. A successful implementation of this scheme requires direct absorption of the laser light within the anti‐reflection coating, since the emitter underneath must not be damaged severely. Additionally, the ablation must be feasible on textured surfaces. In this paper, we show that laser light with a wavelength of 355 nm and a pulse length of approximately 30 ns is absorbed directly by a typical silicon nitride anti‐reflection coating. Based on lifetime measurements on ablated samples it is shown that a damage free laser ablation of SiN_x layers on planar surfaces is possible. The characteristic ablation structure on textured surfaces is explained and quantified by rigorous coupled wave analysis (RCWA) simulations. Finally, high efficiency solar cells with a standard emitter (R_sh approx. 50 Ω/sq) have been processed using laser ablation of the silicon nitride anti‐reflection coating. These cells show efficiencies of up to 19·1%, comparable to the reference solar cells using photolithographically opened contact areas. Copyright © 2008 John Wiley & Sons, Ltd.     

Impurity survey analysis of CdxHgi−xTe by laser scan mass spectrometry

A laser scan mass spectrometry (LSMS) technique has been applied to the analysis of epitaxial layers and bulk Cd_xHg_1?xTe (CMT) materials. The samples are raster scanned under a focused Q-switched Nd–YAG laser beam which ionises all elements with approximately unit sensitivity. The ions are extracted into a high-resolution mass spectrometer and interpretation of the mass spectra gives a complete impurity survey of the material with detection limits down to 1 part in 10⁹ (3 × 10¹³ cm^?1) in bulk materials and 5 parts in 10⁹ in epitaxial layers. Surface impurities are effectively removed in the first scan and subsequent scans over the same area give a true measurement of the impurities present in the material. Successive erosion of the sample surface gives impurity depth profiles with the thickness of material eroded in each scan variable between 1 and 4 μm. Results are given for impurities found in bulk CMT grown by standard Bridgman and ACRT (accelerated crucible rotation technique) processes and in epitaxial CMT produced by liquid phase epitaxy (LPE), metal organic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). Bulk material is shown to be purer in general than epitaxial material. Semiquentitative depth profiles of dopants in CMT, such as iodine, arsenic and phosphorus, can also be obtained and comparative LSMS and secondary ion mass spectrometry (SIMS) depth profiles showing good agreement are given for arsenic in an MOVPE layer.     

Laser ablation of SiO2 for locally contacted Si solar cells with ultra‐short pulses

We apply ultra‐short pulse laser ablation to create local contact openings in thermally grown passivating SiO₂ layers. This technique can be used for locally contacting oxide passivated Si solar cells. We use an industrially feasible laser with a pulse duration of τ_pulse ∼ 10 ps. The specific contact resistance that we reach with evaporated aluminium on a 100 Ω/sq and P‐diffused emitter is in the range of 0·3–1 mΩ cm². Ultra‐short pulse laser ablation is sufficiently damage free to abandon wet chemical etching after ablation. We measure an emitter saturation current density of J_0e = (6·2 ± 1·6) × 10⁻¹³ A/cm² on the laser‐treated areas after a selective emitter diffusion with R_sheet ∼ 20 Ω/sq into the ablated area; a value that is as low as that of reference samples that have the SiO₂ layer removed by HF‐etching. Thus, laser ablation of dielectrics with pulse durations of about 10 ps is well suited to fabricate high‐efficiency Si solar cells. Copyright © 2007 John Wiley & Sons, Ltd.     

Fabrication of narrow pulse passively Q-switched self- stimulated Raman laser with c-cut Nd:GdVO4

Combining the self-stimulated Raman scattering technology and saturable absorber of Cr4+:YAG, a 1.17 μm c-cut Nd:GdVO4 picosecond Q-switched laser is demonstrated in this paper. With an incident pump power of 10 W, the Q-switched laser with average power of 430 mW for 1.17 μm, pulse width of 270 ps, repetition rate of 13 kHz and the first order Stokes conversion efficiency of 4.3% is obtained. The Q-switched pulse width can be the narrowest in our research. In addition, the yellow laser at 0.58 μm is also achieved by using the LiB3O5 frequency doubling crystal.     

激光脉宽和功率密度对烧蚀性能影响研究

微纳卫星的飞速发展对微推力器的性能提出了更高的的要求。激光推进微推力器因其比冲高、推力控制精确、能耗低等特点,为微纳卫星提供了一种性能优异的微推力器选择方案。文中在透射式烧蚀模式下,研究了半导体激光器的激光功率密度和脉宽对激光烧蚀性能的影响。结果表明,在工质厚度为200 μm的工况下,随着激光功率密度的增加,单脉冲冲量和比冲都逐渐增大,而冲量耦合系数和烧蚀效率都存在一个最优值。随着激光脉宽的增加,单脉冲冲量逐渐增加,比冲呈现出先增大后减小的趋势,在250 μs时,比冲达到最大值,约为221.8 s;冲量耦合系数和烧蚀效率都随着脉宽的增大而减小;脉宽超过一定的临界值时,会对激光烧蚀工质的靶坑产生不良影响,使得激光能量和工质严重浪费。激光参数的优化对于激光推进微推力器的工程化应用提供了参考。     

Formation and crystallization of silicon nanoclusters in SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H films using femtosecond pulsed laser annealings

SiN _x :H films of different compositions grown on glass and silicon substrates using plasma-chemical vapor deposition at a temperature of 380°C have been subjected to pulsed laser annealings. The treatments are performed using titanium-sapphire laser radiation with a wavelength of 800 nm and a pulse duration of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nanoclusters are detected in as-grown films with molar fractions of excess silicon of ∼1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in as-grown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1–2 nm in size in these films.     

Laser-Assisted etching and metallisation of via-holes in polyethylene terephthalate

Experimental results are presented on via-hole etching in a thick (100 μm) film of polyethylene terephthalate (PET) with the help of a copper vapour laser (wavelength γ=510 nm, pulse duration 10 ns, repetition rate 8 kHz). The rate of via-hole formation is as high as 100 μms^?1. The effective coupling of the polymer film to the laser radiation is due to the formation of highly absorbing graphitised regions in the bulk of the film, which is initially transparent at the laser wavelength. The side walls of the hole can be metallised by simultaneous decomposition of a Pd(acac)₂ spun-on layer as a precursor for subsequent electroless Ni deposition. The activation of the inner surface of the hole is achieved as a result of the precursor vapour entering the hole during laser etching. The metallised via-hole provides a good electrical interconnection of the two sides of the PET film.     

Laser‐fired contact optimization in c‐Si solar cells

In this work we study the optimization of laser‐fired contact (LFC) processing parameters, namely laser power and number of pulses, based on the electrical resistance measurement of an aluminum single LFC point. LFC process has been made through four passivation layers that are typically used in c‐Si and mc‐Si solar cell fabrication: thermally grown silicon oxide (SiO₂), deposited phosphorus‐doped amorphous silicon carbide (a‐SiC_x/H(n)), aluminum oxide (Al₂O₃) and silicon nitride (SiN_x/H) films. Values for the LFC resistance normalized by the laser spot area in the range of 0.65–3 mΩ cm² have been obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantitative oxygen measurements in OMVPE Al x Ga1−x As grown by methyl precursors

Oxygen has always been considered to be a major contaminant in the organo-metallic vapor phase epitaxy (OMVPE) of Al _x Ga_1−x As. Oxygen incorporation has been invoked as a contributor to low luminescence efficiency, dopant compensation and degradation of surface morphology among other deleterious effects. This study presents quantitative measurements of oxygen concentration in nominally high purity Al _x Ga_1−x As. The oxygen concentration was measured as a function of alloy composition, growth temperature, andV/III ratio. Quantitative secondary ion mass spectroscopy (SIMS) measurements were used to determine the oxygen content as well as the carbon concentration in the film. The oxygen concentration increases with decreased growth temperature and V/III ratio while increasing superlinearly with Al content in the epitaxial layer.     

Controllable Synthesis of Nanosized Amorphous MoSx Using Temporally Shaped Femtosecond Laser for Highly Efficient Electrochemical Hydrogen Production

Amorphous molybdenum sulfide (a‐MoS_x) is regarded as a promising electrocatalyst for hydrogen evolution reaction (HER) due to its disorder structures with a significant number of defect‐rich active sites. Here, a green, one‐step, and controllable method is developed to photoregulate the chemical reactions and synthesize nanosized a‐MoS_x by temporally shaped femtosecond laser ablation of ammonium tetrathiomolybdate aqueous solution. By adjusting the laser energy and pulse delay to control photoinduced and/or photothermal‐induced reduction/oxidation, the S to Mo ratio x can be modulated from 1.53 to 3.07 and the ratio of the Mo^V defect species, bridging S₂^2?, and terminal S₂^2? ligands can be controlled. The optimized a‐MoS_x catalysts (x = 2.73) exhibit high catalytic activity with a low Tafel slope of 40 mV dec^?1, high double‐layer capacitance of 74.47 mF cm^?2, and large current density of 516 mA cm^?2 at an overpotential of 250 mV. The high catalytic activity can be mainly attributed to Mo^V defect species and bridging S₂^2? ligands, or most likely dominated by the Mo^V defect species. This study not only provides an alternatively controllable method to prepare a‐MoS_x as efficient HER catalysts but also contributes to the understanding of the origin of its catalytic activity.     

High Efficient, Intense and Compact Pulsed D2O Terahertz Laser Pumped With a TEA CO2 Laser

A high efficient, intense and compact pulsed D₂O terahertz laser is presented, which is pumped by a multi-transverse mode TEA CO₂ laser. For D₂O gas as the active medium, with the cavity length of 120 cm, pulse energy of the THz laser has been investigated as the variation of pump energy and gas pressure. When the pump energy was 1.41 J, the maximum single pulse energy of 6.2 mJ was achieved at the wavelength of 385 μm. Photon conversion efficiency as high as 36.5% was obtained when laser operated at the maximum output energy. As the pump energy was raised from 0.57 to 1.41 J, the optimum pressure was slightly changed from 400 to 700 Pa. The THz pulse consisted of a spike pulse with pulse width of 120 ns and a tail pulse with pulse width of about 170 ns. The peak power of the spike pulse is about 44.3 kW. Comparing with the occurring time and pulse width of pump pulse, 70 ns delay and 10ns broadening were observed in the THz spike pulse.     

Si1 − x Ge x /Si heterostructures grown by molecular-beam epitaxy on silicon-on-sapphire substrates

The growth of heterostructures with Si_{1 ? x} Ge _x layers on $\left( {1\bar 102} \right)$ sapphire substrates by molecular-beam epitaxy with a silicon sublimation source and a germanium gas source (GeH₄) is reported. The systematic study of the influence of substrate temperature and thickness of the silicon buffer layer shows that the optimal conditions for growing epitaxial Si_{1 ? x} Ge _x layers are provided at a temperature of T _S = 375–400°C. There are significant differences in the orientations of Si_{1 ? x} Ge _x layers, depending on the thickness d of the Si buffer layer: the preferred orientations are (100) at d ≥ 100 nm and (110) for thinner layers. Heterostructures with thick (～1 μm) Si_{1 ? x} Ge _x layers, doped with erbium atoms, exhibit intense photoluminescence at λ = 1.54 μm.     

Curved crystal spectrometer for the measurement of X-ray lines from laser-produced plasmas

In order to diagnose the laser-produced plasmas, a focusing curved crystal spectrometer has been developed for measuring the X-ray lines radiated from a laser-produced plasmas. The design is based on the fact that the ray emitted from a source located at one focus of an ellipse will converge on the other focus by the reflection of the elliptical surface. The focal length and the eccentricity of the ellipse are 1350 mm and 0.9586, respectively. The spectrometer can be used to measure the X- ray lines in the wavelength range of 0.2-0.37 nm, and a LiF crystal （200） （2d = 0.4027 nm） is used as dispersive element covering Bragg angle from 30° to 67.5°. The spectrometer was tested on Shengnang- Ⅱ which can deliver laser energy of 60-80 J/pulse and the laser wavelength is 0.35 μm. Photographs of spectra including the 1 s2p ^1P1-1s^2 ^1S0 resonance line（w）, the 1s2p ^3P2-1s^2 1S0 magnetic quadrupole line（x）, the 1s2p ^3P1-1 s^2 ^1S0 intercombination lines（y）, the 1 s2p ^3S~1-1 s^2 ^1S0 forbidden line（z） in helium-like Ti Ⅹ Ⅺ and the 1 s2s2p ^2P3/2-1 s622s ^2S1/2 line（q） in lithium-like Ti Ⅹ Ⅹhave been recorded with a X-ray CCD camera. The experimental result shows that the wavelength resolution（λ/△ 2） is above 1000 and the elliptical crystal spectrometer is suitable for X-ray spectroscopy.     

Changes in properties of a 〈porous silicon〉/silicon system during gradual etching off of the porous silicon layer

Scanning electron microscopy has shown etching of a porous silicon layer in an HF solution to be irregular. The intensity of porous silicon photoluminescence significantly decreases during gradual etching off, and its peak initially shifts to shorter and then to longer wavelengths. Under red-light pulse excitation, photo-voltage measurements have shown that the boundary potential ?_s of the p-Si substrate is positive, and ?_s grows with the etching time and as the temperature decreases from 300 to 200 K. At T<230 K, the photomemory of ?_s caused by nonequilibrium electron capture by p-Si boundary traps is observed. The concentration of shallow traps and boundary electron states in p-Si increases as porous silicon is etched. At T<180 K, the system of boundary electron states is rearranged. Photovoltage measurements with white-light pulses have revealed electron capture at oxide traps of aged porous silicon.     

Laser induced defects in laser doped solar cells

Laser doping offers a promising method to define selective emitters for solar cells. Its main advantage is the localised nature of the laser beam, which allows melting of the surface area without heating the bulk. The ability to perform this process over a dielectric film offers further benefits, such as the possibility of creating self‐aligned metallisation patterns simultaneously with the selective emitter formation. However, laser induced defects, contaminations and discontinuities in the selective emitter can reduce solar cell performance. In this work the influence of different dielectric films on defect formation is investigated. It was found that a thin oxide beneath the SiN_x improves the implied open circuit voltage of the solar cells for a wide range of laser output powers. Fewer defects were observed when using this SiO₂/SiN_x stack compared to the standard single SiN_x anti‐reflection coating layer. It was also found that the recrystallised silicon layer grows epitaxially according the substrate orientation. No dislocation or stacking faults were observed in deeper areas using transmission electron microscopy, although some defects were observed near the surface. Electron beam induced current images revealed discontinuities in junctions formed with high laser powers. We conclude that micro‐cracks create these discontinuities, which can potentially induce shunts. Finally, laser doped solar cells with a standard SiN_x and with a double SiO₂/SiN_x stack layer as anti‐reflection coating were compared. An efficiency of 18.4% on a large area commercial grade p‐type CZ substrate was achieved. Copyright © 2010 John Wiley & Sons, Ltd.     

Realizing high breakdown voltage for a novel interface charges islands structure based on partial-SOI substrate

A novel interface charge islands partial-SOI (ICI PSOI) high voltage device with a silicon window under the source and its mechanism are studied in this paper. ICI PSOI is characterized by a series of equidistant high concentration n⁺-regions on the bottom interface of top silicon layer. On the condition of high-voltage blocking state, inversion holes located in the spacing of two n⁺-regions effectively enhance the electric field of the buried oxide layer (E_I) and reduce the electric field of the silicon layer (E_S), resulting in a high breakdown voltage (V_B). It is shown by the simulations that the enhanced field ΔE_I and reduced field ΔE_S by the accumulated holes reach to 449 V/μm and 24 V/μm, respectively, which makes V_B of ICI PSOI increase to 663 V from 266 V of the conventional PSOI on 5 μm silicon layer and 1 μm buried oxide layer with the same silicon window length. On-resistance of ICI PSOI is lower than that of the conventional PSOI. Moreover, self-heating-effect is alleviated by the silicon window in comparison with the conventional SOI at the same power of 1 mW/μm.     

SiC via holes by laser drilling

Through-wafer vias were formed in 400-μm-thick bulk 4H-SiC substrates by CO₂ laser drilling (1.06-μm λ) with a ?-switched pulse width of ∼30 nsec and a pulse frequency of 8 Hz. The resultant pulse energy delivered to the SiC surface was on the order of 60 mJ/pulse. Laser drilling produces much higher etch rates (229–870 μm/min) than conventional dry etching (0.2–1.3 μm/min) and the via entry can be tapered to facilitate subsequent metallization. Laser drilling combines optical and mass spectroscopic methods to in-situ monitor and control the laser ablation plume and ionized debris, reducing the total residual surface contamination.     

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

In this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN_x, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO₂/SiN_x passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO₂/SiN_x passivation layers, without inducing appreciable damage or defects on the surface of the n⁺ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.