Studies of interdiffusion in GemSin strained layer superlattices

We present the results on the characterization and interdiffusion behavior of Ge _m Si _n strained layer superlattices (SLS’s) composed of alternating monolayers of pure Ge and pure Si. Such Ge _m Si _n SLS’s were grown on top of thick relaxed Ge _y Si_1-y buffer layers so as to symmetrize the strain distribution and to maintain the pseudomorphic growth of the superlattices. Samples with different superlattice periodicities (i.e. d = d_Ge + d_Si and different layer thickness ratios (i.e. d_Ge:d_Si were prepared for comparison. Raman scattering spectroscopy and x-ray diffraction were used to characterize these samples. Initial results on thermal stability of these Ge _m Si _n SLS’s are also reported     

Non-Equilibium Al-Ga interdiffusion in MOCVD reactor annealed AIGaAs quantum well heterostructures

AlGaAs-GaAs quantum well heterostructures have been annealed in an atmospheric pressure MOCVD reactor under an AsH₃/H₂ ambient. Photoluminescence spectra show a uniform and reproducible increase in the effective quantum well band-gap. Energy shift data indicate that Al-Ga interdiffusion occurs under “non-equilibrium” conditions resulting in depth-dependent Al-Ga interdiffusion such that quantum wells close to the surface disorder less than those further away. Activation energies vary from approximately 5.1–5.2 eV for the “equilibrium” case to 3.2–3.6 eV for the “non-equilibrium” case. These results suggest that caution must be exercised in using reported activation energies to characterize Al-Ga interdiffusion for photonic device fabrication. IBM Graduate Fellow Howard Hughes Doctoral Fellow     

The study of relaxation in asymmetrically strained Si1−x Ge x Si superlattices

Asymmetrically strained Si/SiGe superlattices consisting of 12 nm Si/4 nm Si_0.65Ge_0.35 have been grown in Si(001) by molecular beam epitaxy (MBE) and studied as a function of thermal treatments. Results indicate that initially, the interdiffusion is very rapid and non-linear, and at later annealing stages a steady-state interdiffusion is attained. Raman spectroscopy has been used to determine the Ge content and the strain independently, and to show that in the very early annealing stages, strain relaxation occurs predominantly by interdiffusion. This is supported by transmission electron microscopy (TEM) which indicates that less than 10% of the initial strain relaxation is caused by dislocation formation. In addition, a low temperature relaxation has been observed which may be related to misoriented SiGe crystallites at the superlattice/substrate interface, and increased in size with annealing at 631‡ C.     

Diffusion induced disorder in AlAs-GaAs superlattice by transition elements

In the present experiments transition elements including Zn, Mn, Cr, Cu, Ag and Au have been diffused into an AlAs-GaAs superlattice at temperatures between 700 and 1000° C. The disordering effects on the superlattice were assessed by shallow-angle polishing technique. Apart from the well-known disordering effect caused by the Zn diffusion, we have found that only Mn induces disordering in the superlattice. However the disordering effect arising from the Mn diffusion may be completely inhibited if the weight of As in the diffusion source is considerably higher than that of Mn. This suppression effect may be related to the formation of MnAs (or MnAs₂), which leaves very little amount of Mn in the vapour phase for diffusion. The diffusions of Cu, Ag and Au showed no disordering effect on the superlattice. These results are explained using the interstitial-substitutional mechanism and the solubility of the dopant in GaAs.     

p型扩散区(SiO2/Si系扩Ga)的俄歇分析

采用Ga在SiO2/Si系扩散途径,对扩Ga硅片进行了俄歇分析,在p型区未发现重金属杂质,表明热氧化SiO2层对外来重金属有良好的屏蔽作用,而且有利于提高硅器件的电参数性能、稳定性和可靠性.     

Enhanced/suppressed interdiffusion of InGaAs-GaAs-AlGaAs strained layers by controlling impurities and gallium vacancies

The interdiffusion of In and Ga at an InGaAs-GaAs interface subjected to different annealing temperatures, times, and environments is demonstrated. The interdiffusion coefficients and activation energies are determined by correlating the shift in the photoluminescence peaks with the calculated quantum well transition energies based on an error function composition profile. The results indicate that a higher In composition In_xGa_1-xGaAs single quantum well (SQW) leads to a higher interdiffusion coefficient of In and Ga in an As overpressure annealing condition. Also, As overpressure increases the interdiffusion, whereas Ga overpressure reduces the interdiffusion. The thermal activation energies for different In composition InGaAs-GaAs SQW’s (x = 0.057, 0.10, 0.15) range from 3.3 to 2.6 eV for an As overpressure environment and from 3 to 2.23 eV for the Ga overpressure situation. With respect to impurity induced disordering by Zn using a Ga or As overpressure significantly effects the depth of the Zn diffusion front but significant mixing does occur in either case when the impurity front reaches the quantum well.     

Compositional disordering and the formation of semi-insulating layers in AlAs-GaAs superlattices by MeV oxygen implantation

Data are presented demonstrating the use of MeV oxygen ion implantation and subsequent annealing procedures to induce compositional disordering and to create a semiinsulating region simultaneously within an AlAs-GaAs superlattice. High dose oxygen implantation yields a compositionally disordered region 3500? wide centered 1.25 μm below the surface of the superlattice, as determined by secondary ion mass spectrometry (SIMS) analysis. More extensive disordering of the superlattice occurs at lower implantation temperatures. Current-voltage measurements indicate the formation of a semiinsulating layer which is thermally stable to at least 850° C. The semi-insulating properties of the implanted superlattice are assigned to the disorder-enhanced formation of Al-O pairs and the substitutional introduction of deep level states.     

Layer intermixing and related long-term instability in heavily carbon-doped AlGaAs/GaAs superlattices

Superlattices (SLs) of Al_o.3Ga_o.7As/GaAs grown by metalorganic chemical vapor deposition and heavily doped with carbon using CClp₄ were annealed for 24 h at 825° C under a variety of ambient and surface encapsulation conditions. Photoluminescence atT = 1.7 K has been employed to determine approximate Al-Ga interdiffusion coefficients (D _Ai-Ga) for different annealing conditions. For all encapsulants studiedD _Al-Ga increases with increasing As₄ pressure in the annealing ampoule. This result disagrees with trends reported for Mg-doped crystals, and with predictions of the charged point-defect (Fermilevel) model. The Si₃N₄ cap provides the most effecitve surface sealing against ambientstimulated layer interdiffusion (D _Al-Ga ≈ 1.5-3.9 x 10^-19 cm²/sec). The most extensive layer intermixing has occurred for an uncapped SL annealed under As-rich ambient (D _Al-Ga ≈ 3.3 x 10^-18 cm²/sec). These values are up to ~40 times greater than those previously reported for nominally undoped AIGaAs/GaAs SLs, implying that theC _As doping slightly enhances layer intermixing, but significantly less than predicted by the Fermi-level effect. The discrepancies between the experimental data and the model are discussed. Pronounced changes in the optical properties of the annealed SLs with storage time at room temperature are also reported. These changes may indicate a degraded thermal stability of the annealed crystals due to high-temperature-induced lattice defects. A possibly related effect of the systematic failure to fabricate buried heterostructure quantum well lasers via impurity-induced layer disordering in similarly doped AIGaAs/GaAs crystals is discussed.     

Mg掺杂AlInP 650 nm LD外延片的Zn扩散研究

利用石英闭管法,对Mg掺杂AlInP 650 nm LD外延片进行Zn扩散,分析了扩散温度和时间2个参数对Zn扩散的影响.采用光致发光(PL)谱和电化学蒸涂(ECV)方法研究了Zn扩散产生的影响.PL谱结果表明,Zn扩散引起了AlGaInP/GaInP多量子阱(MQW)有源层的组分无序,使PL谱的峰值蓝移,最大蓝移为54 nm,约175 meV.ECV测量结果显示,Zn已经扩散到MQW有源区,MQW区域的p型载流子浓度为4.4×1017 cm-3.     

Silicide formation and interdiffusion effects in Si-Ta,SiO2-Ta AND Si-PtSi-Ta thin film structures

The formation of TaSi₂ in the Si-PtSi-Ta and Si-Ta systems has been studied using Auger spectroscopy, x-ray diffraction and electron diffraction techniques. The reaction of tantalum with PtSi was observed by Sinha, et al.^l to take place with high temperature (800°-900°c) annealing of thin film systems consisting of Si-PtSi-Ta-W¹. In the present investigation, it is shown that tantalum reacts with PtSi at approximately 600°C to form a mixture of Ta₅Si₃ and TaSi₂ and predominantly TaSi₂ at 785°C. Platinum is displaced at the refractory metal (Ta)-PtSi interface, whereupon the more stable refractory metal-silicide is formed. The displaced platinum reacts further with the excess silicon which diffuses from the Si-PtSi interface. The Si-PtSi-Ta reaction is similar to the Si-PtSi-W reaction. However, unlike tungsten which migrates very little in the Si-PtSi-W system, tantalum appears to interdiffuse with the PtSi at temperatures as low as 600°C. In the case of the Si-Ta couple, TaSi₂ forms at approximately 750°C as determined by transmission electron microscopy (TEM) measurements. The kinetics of TaSi₂ formation at the Si-Ta interface are compared to that which takes place at the PtSi-Ta interface to determine the influence of the PtSi layer. Silicide formation was not observed in SiO₂-Ta specimens. after anneals up to 800°c. At 750°C Ta₂O₅ formed as observed by electron diffraction.     

Evaluation of low-temperature interdiffusion coefficients in Hg-based superlattices by monitoring the E1 reflectance peak

We show that variations of the E₁ reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E₁ peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100°C for approximately six months, and then at 150°C for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern.     

An internal calibration technique for pseudobinary systems by auger electron spectroscopy

An internal calibration technique has been developed for the quantitative analysis of pseudobinary systems by means of Auger electron spectroscopy without the need for stan-dards of known composition. This technique has been used for analyzing PbO-In₂O₃ two-phase films and A1N-A1₂ O₃ , AlAs-GaAs, and GaP-InP solid solutions. The technique is applicable to systems of the type (AC_m) (BC_n)_1-x if the Auger intensities measured for elemens A, B, and C are given by the linear relationships I = xI_A ^**, I_B = (l-x)I_B ^*, and I_C = xI_C ^** + (l-x)I_{C C} ^* where I_A ^** and I_C ^** are the inten-sities for pure AC_m and I_B ^* and I* are the intensities for pure BC_n . To determine whether this criterion is satisfied, a series of Auger measurements is made on regions of dif-ferent composition within a single specimen (e.g., on speci-mens in which a concentration variation exists across the surface or surfaces exposed by repeated sputter-etching of a sample with an in-depth composition gradient), and plots of I_A vs I_B and I_C/I_B vs I_A/I_B are made. If these plots are linear over tne range of compositions investigated, it is assumed that the above linear relationships are valid over the whole range of the pseudobinary system. It is then possible to construct linear calibration plots of I_A, I_B , and I_C vs x for the whole composition range.     

Reactive interdiffusion between a lead-free solder and Ti/Ni/Ag thin-film metallizations

The reactive interdiffusion between a Sn-3.0wt.%Ag-0.7wt.%Cu solder and thin-film Ti/Ni/Ag metallizations on two semiconductor devices, a diode and a metal-oxide-semiconductor field-effect transistor (MOSFET), and a Au-layer on the substrates are studied. Comprehensive microanalytical techniques, scanning electron microscopy, transmission electron microscopy (TEM), and analytical electron microscopy (AEM) are employed to identify the interdiffusion processes during fabrication and service of the devices. During the reflow process of both diode and MOSFET devices, (1) the Ag layer dissolves in the liquid solder; (2) two intermetallics, (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅, form near the back metal/solder interface; and (3) the Au metallization in the substrate side dissolves in the liquid solder, resulting in precipitation of the (Au,Ni,Cu)Sn₄ intermetallic during solidification. During solid-state aging of both diode and MOSFET solder joints at 125°C and 200°C, the following atomic transport processes occur: (1) interdiffusion of Cu, Ni, and Sn, leading to the growth of a (Ni,Cu)₃Sn₄ layer until the Ni layer is completely consumed; (2) interdiffusion of Au, Cu, Ni, and Sn through the (Ni,Cu)₃Sn₄ layer and unconsumed Ni layer to the Ti layer to form a solid solution; and (3) further interdiffusion of Au, Cu, Ni, and Sn through the (Ni,Cu)₃Sn₄ layer to from an (Au,Ti,Ni,Cu)Sn₄ layer. The growth of the latter layer continues until the entire Ti layer is consumed.     

Anomalous Lateral Zn Surface Diffusion in InP Caused by Zn-Contained Metallization

We report an anomalous Zn surface diffusion in InP during annealing of ohmic contact structures containing Zn. A Pd/Zn/Pd contact was used to demonstrate this phenomenon. Electrical properties of the contact were monitored to corroborate this anomalous surface diffusion. Cross-sectional scanning electron microscopy was also used to delineate the Zn diffusion front lines. It was found that the Zn surface lateral diffusion can extend ≥50 μm for samples annealed at 500°C or higher temperatures. Close attention should be paid to this anomalous lateral surface diffusion during fabrication of devices using Zn-contained ohmic contacts.     

The use of computer controlled group V valved sources for interface type control in InGaSb/InAs strained layer superlattices

The purpose of this research is to demonstrate the necessity of computer controlled valved group V effusion cell sources in the growth of indium gallium antimonide/indium arsenide (InGaSb/InAs). These sources allow enhanced control of the group V flux. This flux control allows the reduction of unwanted cross contamination and complete control of the interface type. For simple structures, this control can be done manually, however, for complicated structures the control must be automated to allow for reproducibility and uniformity. The InGaSb/InAs strained layer superlattice (SLS) is an example of a complicated structure with hundreds of layers that requires interface type control. Arsenic incorporation with typical flux shuttering was found to be a problem in the growth of antimonide layers and limit interface type control. The antimony incorporation was not found to occur for the growth of arsenide layers. In addition, antimony exposure to critical interfaces did not appear to reduce the interface quality. This research demonstrates that the use of computer controlled valve sources is only required for the arsenic source when attempting to create InGaSb/InAs SLS structures.     

The preparation and characterization of strained-layer superlattices in the GaAs + GaP System

The technique of metal organic chemical vapor deposi-tion has been used to prepare strained-layer superlattices in the GaAs + GaP system. The superlattices consist of alternating layers of GaP and GaAs_xP_1?x for x = 0.2 to 1.0, which vary in thickness from 30 to 400 Å. The layers were grown by the decomposition of trlmethylgallium and various mixtures of ASH₃ and PH₃ in H₂ at 800δC. The thickness and uniformity of the layers were determined by optical and transmission electron microscopy and x-ray diffraction. The composition of the layers was determined from x-ray diffraction. A new analysis has been developed to determine the layer strain as well as the composition of thick layers (～ 300 Å). Transmission electron microscopy has been used to yield direct evidence that strained-layer superlattices can be used to remove the misfit dislocations generated during the epitaxial growth of a GaAs_xP_1-x alloy on a lattice mismatched GaP substrate. These results are in agreement with the previous work of Matthews and Blakeslee. Optical absorption, photocurrent spectroscopy and photoluminescence have been used to deter-mine the band gap energy as well as the energies for other optical transitions. The values are in excellent agreement with the values predicted by tight binding and effective mass calculations.     

Study of threading dislocations in the plan-view sample of SiGe/Si(001) superlattices by transmission electron microscopy

Interfacial defects due to a mismatch of 1.378% between substrate and epilayer were examined in a Si_0.67Ge_0.33/Si(001) superlattice by transmission electron microscopy (TEM). Plan-view specimens from the superlattice were prepared to investigate the defects in the structure. It was observed that 60°C-type misfit dislocations associate with point contrast on and at their ends. This point contrast was found to represent threading dislocations by using tilt experiments in the microscope. Consequently, stereo electron microscopy was used to examine the threading dislocations. It was discovered that the threading dislocations are not on the {111} slip planes but can be almost parallel to the [001] zone axis.     

CHF3-O2 reactive ion etching of 4H-SiC and the role of oxygen

Reactive ion etching of 4H-SiC was performed using a CHF₃-O₂ plasma. The etch rate and mean surface roughness were investigated as a function of the ratio of the O₂ flow rate to the total gas flow rate. It was found that oxygen plays an indirect role in contributing to the etching of SiC. An optimum O₂ fraction of 20% was found to give a maximum etch rate of 35 nm/min. On the other hand, the root-mean-square (RMS) surface roughness was found to increase from 1.31 to 2.34 nm when the O₂ fraction increased from 0% to 80%. Auger electron spectroscopy results for the samples etched at the optimum condition of 20% O₂ fraction revealed the presence of oxygen on the etched surface in a form of an oxide-like layer (SiO_x). No carbon residue (carbon rich-layer) and aluminum were found. Based on our results, the role of O₂ in the reactive ion etching of 4H-SiC will be presented.     

The role of oxygen in electron cyclotron resonance etching of silicon carbide

The electron cyclotron resonance (ECR) etching of silicon carbide (SiC) was studied using SF₆ + O₂ based plasma. The role of O₂ was studied by varying the O₂ flow rate while keeping the total gas flow constant. It was found that oxygen enhances the etch rate at low O₂ fraction through releasing more fluorine atoms, while lowers the etch rate at high O₂ fraction by diluting fluorine atoms and forming an oxide-like layer. The etched surface roughness was found to be affected by the surface oxidation and oxygen ion related physical ion bombardment. The role of oxygen in chemical etching of carbon was found to be insignificant. In general, the etched surface is smooth and free of micromasking effect that can arise from Al contamination and C rich layer.     

Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN

Titanium (15 nm)/aluminum (60 nm)/rhenium (20 nm)/gold (50 nm) ohmic contacts to moderately doped n-type GaN (4.07×10¹⁸ cm⁻³) have been investigated as a function of annealing temperature. It is shown that the current-voltage (I–V) characteristics of the contacts are improved upon annealing at temperatures in the range of 550–750°C. Specific contact resistance as low as 1.3 × 10⁻⁶ Ωcm² is obtained after annealing at 750°C for 1 min in a nitrogen ambient. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the sample annealed at 750°C shifts toward the high binding side by 0.71 eV compared with that of the as-deposited one. It is also shown that the contact does not seriously suffer from thermal degradation even when annealed at 750°C for 30 min. Based on Auger electron spectroscopy (AES), glancing angle x-ray diffraction (GXRD), and XPS results, possible explanations for the annealing-induced improvement of the ohmic behavior are described and discussed.