As-grown Defects Research Articles

On the nature of as-grown and irradiation-induced Ga vacancy defects in β-Ga2O3

On the nature of as-grown and irradiation-induced Ga vacancy defects in β-Ga2O3

Secondary defects of as-grown oxygen precipitates in nitrogen doped Czochralski single crystal silicon

In this work, as-grown defects and their secondary defects in 300 mm nitrogen-doped Czochralski-grown single-crystal silicon (NCZ-Si) are investigated. Secondary defects are revealed by homogeneous epitaxial layer growth, and as-grown defects are decorated via gaseous hydrogen chloride (HCl) etching. Localized light scattering (LLS) technique is utilized to inspect defect distribution and provide the latex sphere equivalent (LSE) size to differentiate defect types. With the locations of LLS, the morphology of defect is observed by scanning electron microscopy (SEM). According to the results, it indicated that secondary defects are extrinsic dislocation loops and stacking fault (SF) loops, which are induced by as-grown oxygen precipitates during the growth and cooling process. Furthermore, the correlation between the counts of secondary defects and nitrogen concentration is obtained. It contributes to the optimization of nitrogen doping to avoid the generation of detrimental defects for device operation.

Comprehensive characterization of efficiency limiting defects in the swirl-shaped region of Czochralski silicon

For Czochralski silicon (Cz-Si) solar cells, swirl-shaped regions in silicon wafers could lead to efficiency degradation, usually accompanied by hot spots and thermal breakdown. In this paper, comprehensive characterization methods including electroluminescence (EL), photo-induced current (LBIC), quantum efficiency (QE), cell efficiency, carrier lifetime, scanning electron microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and deep level transient spectroscopy (DLTS) were used to investigate the formation and properties of the Swirl defects. It is found that there exists a strong correlation between the as-grown swirl defects and the degradation in cell performance. The Swirl defects cause an energy level of E v + 0.33 eV in silicon bandgap, with the capture cross section of 2.3 × 10 −15 cm 2 , which is consistent with the feature of silicon/oxide interface states. The swirl defects can further be evolved into stacking faults after annealing at 1050 °C for 16 h, and cause an additional energy level of E v + 0.42 eV with the capture cross section is 1.0 × 10 −14 cm 2 . These results suggest that oxygen precipitates are the vital component or precursor for the formation of swirl-shaped regions in Cz silicon wafers. • A comprehensive study of the nature of Swirl defects in the photovoltaic Czochralski silicon is obtained. • The direct evidence that Swirl defects are related to oxygen precipitates is provided by deep level transient spectroscopy (DLTS) measurements. • The influence of Swirl defects on solar cell performances and material properties is characterized.

Oxygen concentration dependence of as-grown defect formation in nitrogen-doped Czochralski silicon single crystals

The interstitial oxygen concentration ([OI]) dependence of the as-grown defect (voids and dislocation clusters) behavior in nitrogen-doped silicon crystals fabricated by Czochralski technique (Cz-Si) have been investigated. Nitrogen-doped Cz-Si crystals with [OI] values of 7–8 and 1–3 × 1017 atoms cm−3 are evaluated, and it was found that the range of v/G in which as-grown defect-free (AGDF) region can be obtained become wide with decreasing [O I], where v is the crystal growth rate and G is the axial temperature gradient at the solid–liquid interface. This result indicates that as-grown defect suppression effect by nitrogen doping is enhanced at low [OI].By the analysis based on a thermodynamic model, it is shown that nitrogen–vacancy (NV) and nitrogen–oxygen (NO) complexes compete in the nitrogen-doped Cz-Si crystals and their concentration fraction changes with [OI] value. At low [OI], the concentration of the NO complex becomes low; thus, the concentration of nitrogen that can form NV complexes increases, thereby enhancing the as-grown defect suppression effect. Silicon wafers for insulated gate bipolar transistors (IGBTs) are required AGDF and oxide precipitate-free, therefore nitrogen-doped Cz-Si crystals with low [OI] values are suitable for the fabrication of silicon wafers for IGBTs.

High-temperature annealing of AlN films grown on 4H–SiC

The effect of high-temperature annealing (HTA) at 1700 °C on AlN films grown on 4H–SiC substrates by metalorganic vapor phase epitaxy has been studied. It is shown that the structural quality of the AlN layers improves significantly after HTA similar to what has been demonstrated for AlN grown on sapphire. Dislocation densities reduce by one order of magnitude resulting in 8 × 108 cm−2 for a-type and 1 × 108 cm−2 for c-type dislocations. The high-temperature treatment removes pits from the surface by dissolving nanotubes and dislocations in the material. XRD measurements prove that the residual strain in AlN/4H–SiC is further relaxed after annealing. AlN films grown at higher temperature resulting in a lower as-grown defect density show only a marginal reduction in dislocation density after annealing. Secondary ion mass spectrometry investigation of impurity concentrations reveals an increase of Si after HTA probably due to in-diffusion from the SiC substrate. However, C concentration reduces considerably with HTA that points to an efficient carbon removal process (i.e., CO formation).

Enhanced optical and electrical performance of Ge1−xSnx/Ge/Si(100) (x = 0.062) semiconductor via inductively coupled H2 plasma treatments

The structural, electrical and optical properties of a unintentionally doped Ge1−xSnx (x = 6.2%) alloy semiconductor both as-grown and after a hydrogen plasma treatment or ‘hydrogen passivation’ are investigated by high resolution x-ray diffraction, atomic force microscopy, and temperature-dependent Hall-effect and photoluminescence (PL) measurements. The samples were grown via reactions of SnD4 and Ge3H8 on Ge buffered Si wafers at 305 °C and subsequently subjected to a hydrogen plasma environment generated using an inductively coupled H2 approach (ICP). The plasma treatments showed no appreciable change in the structural, compositional and morphological properties of the samples indicating no measurable degradation of the materials quality has occurred. However, the H passivation significantly alters the electrical activity of as-grown defects and impurities in the epilayer, resulting in electrical conductivity of passivated sample that is more than 2 times higher at 300 K and 30 times higher at 10 K. The as-grown samples showed conduction-type changes with temperature (manifested by singularities in the apparent carrier concentrations around 14 and 255 K), while the background n-type carrier concentration of the hydrogen treated analogs varied smoothly with temperature. In particular at 300 K, the carrier concentration was reduced from the background 4.61 × 1017 cm−3 in the as-grown material to 5.68 × 1016 cm−3 in the H treated counterpart due to potential passivation of deleterious point defects, a highly desirable outcome for effective device performance. The room temperature PL intensity increased ∼5 times (more than 3 times at 5 K) upon hydrogen treatment due to electrical passivation of deep acceptor states. Hydrogen plasma treatments are thus found to enhance the electrical and optical responses of the samples, suggesting that conventional ICP treatments could be used as a processing step to improve the properties of newly developed Sn-based group IV semiconductors using straightforward and relatively low temperature protocols.

Fast-neutron-induced and as-grown structural defects in magnesium aluminate spinel crystals with different stoichiometry

Several hole-type paramagnetic defects (a hole localized at a regular oxygen ion near charged structural defect/defects) have been revealed in fast-neutron-irradiated MgO⋅2.5Al2O3 single crystals using the EPR method. Three of them (a dominant V1 and small amount of V2 and V22) were recently revealed in a neutron-irradiated stoichiometric MgAl2O4, while a novel V4 center, ascribed to a hole in a form of O− ion nearby a complex of a magnesium vacancy and a positively charged antisite defect (V4 ≡ O–‑VMg‑Al|Mg), was created by fast neutrons only in a nonstoichiometric single crystal. The pulse annealing of the EPR signal of these centers was compared to that of radiation induced optical absorption in the same crystals. The EPR and optical characteristics of the holes localized near as-grown structural defects in X-ray irradiated 1:1 and 1:2.5 Mg‑Al spinel crystals were compared as well. The tentative scenario of the thermal annealing process of neutron-induced defects (hole-type and complementary electron F-type ones) in MgO⋅2.5Al2O3 was considered.

Luminescence characteristics of magnesium aluminate spinel crystals of different stoichiometry

Magnesium aluminate spinel single crystals with different stoichiometry, MgAl2O4 (1:1 spinel) and MgO 2.5Al2O3 (1:2.5) were investigated using different optical methods (cathode-, photo- and thermally stimulated luminescence (TSL), optical absorption, “creation spectra” of TSL peaks and phosphorescence by VUV radiation). Low-temperature charge carrier traps and the position of intrinsic UV emission bands depend on the degree of stoichiometry. Antisite defects (ADs), Mg2+ or Al3+ located in a “wrong” cation site (Mg|A1 or Al|Mg) are the main as-grown structural defects, which serve also as efficient traps for electrons and holes as well as seeds for bound excitons. AD concentration is especially high in 1:2.5 spinel. There are several manifestations of ADs (electronic excitations near ADs) in the spectral region of 7–7.5 eV, slightly below the energy gap.

Synthesis of In-Plane Artificial Lattices of Monolayer Multijunctions.

Recently, monolayers of van der Waals materials, including transition metal dichalcogenides (TMDs), are considered ideal building blocks for constructing 2D artificial lattices and heterostructures. Heterostructures with multijunctions of more than two monolayer TMDs are intriguing for exploring new physics and materials properties. Obtaining in-plane heterojunctions of monolayer TMDs with atomically sharp interfaces is very significant for fundamental research and applications. Currently, multistep synthesis for more than two monolayer TMDs remains a challenge because decomposition or compositional alloying is thermodynamically favored at the high growth temperature. Here, a multistep chemical vapor deposition (CVD) synthesis of the in-plane multijunctions of monolayer TMDs is presented. A low growth temperature synthesis is developed to avoid compositional fluctuations of as-grown TMDs, defects formations, and interfacial alloying for high heterointerface quality and thermal stability of monolayer TMDs. With optimized parameters, atomically sharp interfaces are successfully achieved in the synthesis of in-plane artificial lattices of the WS2 /WSe2 /MoS2 at reduced growth temperatures. Growth behaviors as well as the heterointerface quality are carefully studied in varying growth parameters. Highly oriented strain patterns are found in the second harmonic generation imaging of the TMD multijunctions, suggesting that the in-plane heteroepitaxial growth may induce distortion for unique material symmetry.

Identification and analysis of partial shading breakdown sites in CuInxGa(1-x)Se2 modules

In this study, CuInxGa(1-x)Se2 (CIGS) mini-modules are stressed under reverse bias, resembling partial shading conditions, to predict and characterize where failures occur. Partial shading can cause permanent damage in the form of ‘wormlike’ defects on thin-film modules due to thermal runaway. This results in module-scale power losses. We have used dark lock-in thermography (DLIT) to spatially observe localized heating when reverse-bias breakdown occurs on various CIGS mini-modules. For better understanding of how and where these defects originated and propagated, we have developed techniques where the current is limited during reverse-bias stressing. This allows for DLIT-based detection and detailed studying of the region where breakdown is initiated before thermal runaway leads to permanent damage. Statistics of breakdown sites using current-limited conditions has allowed for reasonable identification of the as-grown defects where permanent breakdown will likely originate. Scanning electron microscope results and wormlike defect analysis show that breakdown originates in defects such as small pits, craters, or cracks in the CIGS layer, and the wormlike defects propagate near the top CIGS interface.

Creation and thermal annealing of structural defects in neutron-irradiated MgAl2O4 single crystals

Abstract Several novel hole-type defects (a hole localized at a regular oxygen ion near a negatively charged structural defect) have been revealed in fast neutron irradiated MgAl2O4 crystals using the EPR method. The pulse annealing of the EPR signal of these centers was compared to that of radiation induced optical absorption in the same crystals. Taking into account the determined models of V1, V2 and V22 paramagnetic centers, the tentative scenario of the thermal annealing process of neutron-induced defects (hole-type and complementary electron F-type ones) is proposed. In addition, one more paramagnetic hole center consisting of an Al|Mg as-grown antisite defect near an aluminum vacancy is created just by X-ray irradiation at room temperature.

Evidence of quantum dot size uniformity in strain-coupled multilayered In(Ga)As/GaAs QDs grown with constant overgrowth percentage

Strain coupled multi-layer quantum dot (QD) structures are limited due to their non-uniform dot size distribution, as-grown defects and dislocations. Even with these limitations, they are subject of interest because of high optical and device efficiency. In this work, we propose a modified growth strategy in which monolayer (ML) deposition of QDs is varied based on reflection high energy electron diffraction (RHEED) pattern to achieve defect free structure with uniform dot size distribution. The overgrowth percentage, i.e. monolayer deposition above critical thickness (minimum thickness required for transition from two-dimensional (2D) to three-dimensional (3D) confined structures) was kept constant in modified strategy. To verify this claim, we have compared it with conventional strain coupled multilayer structure and demonstrated improved optical performance and device characteristics. Photoluminescence spectroscopy exhibited monomodal QD distribution in strain-coupled multi-stacked heterostructure grown by the proposed strategy. Power dependent photoluminescence (PL) measurement confirms existence of single ground state peak. The proposed strategy yielded a defect free structure and better carrier capture rate compared to conventional multilayer QDs. The spectral response observed from proposed quantum dot infrared photodetector (QDIP) exhibited multiple peaks in mid-IR region (3.92µm and 4.54µm), which is one of the essential requirements for fabricating infrared detectors or focal plane arrays. Post-growth thermal annealing treatment at various temperatures (650–800°C) were performed to verify the change in activation energy, ground-state emission peak, and thermal stability. Negligible change in the ground-state PL emission peak (< 2%) and thermal activation energy (< 2%) were observed up to 750°C. The full width at half maximum (FWHM) values of the ground-state peak of the annealed samples exhibited a similar trend. High-temperature thermal stability was achieved probably due to high strain field hindering material interdiffusion and improved quantum confinement realized from uniform QD sizes. The modified growth strategy used in this paper is helpful not only for improved optical characteristics, but also for useful device performances, such as a night vision camera in defense areas, cancer detector in medical applications and fabrication of thermal imagers; focal plane arrays.

Electrical characterization of electron irradiated and annealed lowly-doped 4H-SiC

The effect of high energy electron (HEE) irradiation on nickel Schottky contacts fabricated on lowly-doped n-type 4H-SiC was investigated by deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS. The Schottky contacts were deposited by resistive evaporation of nickel and were observed to be of good rectification quality from current-voltage measurements. DLTS was performed up to 350K to investigate the presence of defects before and after HEE irradiation. HEE irradiation was observed to induce three deep level defects below 350K at 0.42eV, 0.62eV and 0.76eV below the conduction band minimum. These deep level defects are labelled E0.42, E0.62 and E0.76. Defects E0.42 and E0.76 were observed after the same electron fluence and were annealed out at the same temperature, suggesting that the defects could be strongly related. The effect of HEE irradiation and annealing on as-grown defects was also investigated and is reported.

Enhancement in optical characteristics of c-axis-oriented radio frequency–sputtered ZnO thin films through growth ambient and annealing temperature optimization

High-quality radio frequency–sputtered ZnO were grown on Si substrates at 400°C at various partial gas pressures (Ar/Ar+O2). Subsequently, to remove as-grown defects, high temperature annealing from 700 to 900°C on as-grown samples in constant oxygen flow for 10s was performed. X-ray diffraction study confirmed the formation of highly crystalline films with a dominant peak at (002). The sample grown in 50% Ar and 50% O2 ambient exhibited the lowest linewidth (2θ=~0.2728°) and highest stoichiometry. Grain size of the as grown samples decreased with increase in the partial pressure of oxygen till a certain ratio (1:1), and photoluminescence (PL) improved with increase in annealing temperature. Low-temperature (18K) PL measurements showed a near-band-edge emission peak at 3.37eV, and the highest peak intensity (more than six orders compared to others with narrow linewidth of ~0.01272eV) was exhibited by the sample annealed at 900°C and was six orders higher than that of the as-grown sample. All as-grown samples exhibited dominant visible-range peaks due to emission from defect states.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106 cm−2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014 cm−3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large VBD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.

(Invited) Simulation of the Effects of As-Grown Defects on GaN-Based Power HEMTs

With the wide band-gap of Gallium Nitride (GaN) and high mobility of 2 dimensional electron gas(2DEG), AlGaN/GaN high electron mobility transistors (HEMTs) are considered ideal candidates for high speed and power applications ranging from hybrid vehicles to space communication. Developments in the fabrication of GaN-based blue LEDs have been incorporated into the processing of AlGaN/GaN HEMT structures leading to improvements in amount of defects and dislocations resulting from heteroepitaxial growth. But evident degradation of dynamic performance of such devices due to the presence of as-grown defects still remains a subject of major concern. Using FLorida Object Oriented Device Simulator (FLOODS) TCAD, this work looks to analyze the impact of variation of defect properties as well as their spatial distribution and concentration on the steady-state and dynamic behavior of AlGaN/GaN HEMTs. FLOODS TCAD’s defect modeling capability will be applied to examine how multiple defect levels, of both donor and acceptor-like nature, can influence device performance. Particular emphasis will be on transient switching, such as gate-lag and drain-lag experiments, in consideration of the slow nature observed frequently in literature. While comprehensive investigation of the role of charged defects at the AlGaN/passivation layer interface on performance degradation has already been performed, contribution by as-grown defects, expected to be in significant quantity in the GaN buffer layer, is yet to be investigated. Building on experimental results, which suggest the presence of multiple acceptor-like defects, impact on steady-state current-voltage characteristics will be observed followed by an examination of how such defects will affect dynamic performance by isolating their contribution to transient and switching behavior. Figure: (a) Drain current against gate voltage at VDS=0.5V for HEMT A with 1 acceptor-like level (nT=1014 cm-3, Et=Ec-0.7eV) and HEMT B with 2 acceptor levels(Level 1: nT=1014 cm-3, Et=Ec-0.7eV; Level 2: nT=1015 cm-3, Et=Ec-0.5eV) (b) Optical pumping like detrapping of electrons, biasing at VDS=1V and VGS=0V. HEMT A(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=4x1013 cm-3, Ec-0.5eV, τ=1ms), HEMT B(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=1013 cm-3, Ec-0.5eV, τ=1ms) Figure 1

Combined Effect of Rapid Thermal Annealing and Crystal Nature on the Gate Oxide Reliability of Czochralski Silicon

Internal gettering (IG) of metallic impurities (MI) in Czochralski silicon (CZ-Si) is one of the key techniques in modern microelectronic engineering to manufacture high-performance devices with high process yield. Various silicon substrates such as EPI wafers with heavily boron-doped substrates, ion-implanted wafers, and rapid-thermal annealing (RTA) treated wafers have been used to enhance IG ability [1-2] and retain denuded zone (DZ) for the active area of devices. Especially, RTA-treatment is an easy way to form IG and DZ simultaneously with a rapid heat process that injects the intrinsic point defects, mainly vacancies, into the bulk. However, it was reported that vacancy clusters, or ‘nano voids’ whose size was less than 50nm [2-3] could be formed in the near-surface region after RTA process via vacancy self-trapping [4], which can possibly degrade the performance and yield of device. In this study, we investigated the effect of RTA treatment on the gate oxide reliability and their relation to the crystal nature, i.e., as-grown point defect distribution in silicon wafers.As shown in Fig. 1 a), we performed the Cu-contamination method [5] on the prepared samples to verify the crystal nature of each sample. After that, we separated the samples into two groups whether they treated by RTA or not, and carried out time-zero dielectric breakdown (TZDB) reliability tests on the patterned poly-Si/oxide/Semiconductor capacitors with the gate oxide thickness of 10 nm. As shown in Fig. 1 b), distribution of TZDB failed cell on the wafer was closely related to the crystal nature. Moreover, it was revealed that a vacancy-dominant region tends to have lower breakdown field than an interstitial-dominant region. These phenomena were only found in RTA-treated wafers and entire fail rate was drastically increases when RTA effect was combined with vacancy-dominated region, which was shown in Fig. 1 b). We assumed that as-grown vacancies contained in the vacancy-dominated region tend to enhance the supersaturation of vacancy beneath the oxide-semiconductor interface, so the formation of nano voids was accelerated and the breakdown field was lowered due to the presence of such defects. A detailed mechanism of the experimental result will be discussed. Figure 1. a) Crystal nature measured by Cu contamination method and b) TZDB frequency with or without RTP

Combined Effect of Rapid Thermal Annealing and Crystal Nature on the Gate Oxide Reliability of Czochralski Silicon

The effect of RTA treatment on the gate oxide reliability in the manner of as-grown and RTA-induced defect state was investigated. The RTA pre-treatment brings significant degradation of GOI yield, and the effect becomes severe as the v/G of a sample is increased. On the other hand, GOI yield was recovered when successive CMP processes was applied on the wafer after finishing the RTA process. In summary, void-rich layer with a thickness of few micrometers was formed in the wafer surface after the RTA treatment, and the density of void is clearly affected by the as-grown vacancy concentration.

(Invited) Simulation of the Effects of As-Grown Defects on GaN-Based Power HEMTs

Recent developments in identification of as-grown defects and impurities in the GaN buffer region of AlGaN/GaN High Electron Mobility Transistors (HEMTs) provide an opportunity to identify their role in degradation of dynamic switching behavior. Using TCAD simulation, effect of capture-emission by electron and hole traps on drain current transients during drain lag switching experiments is presented which is otherwise difficult to predict analytically.

Temperature-dependent Hall effect measurements on Cz-grown silicon pulled from compensated and recycled feedstock materials

In this work, temperature-dependent Hall effect measurements in the temperature range 88–350K were carried out to investigate the electrical properties of three solar grade p-type Czochralski (Cz) silicon ingots, pulled from recycled p-type multi-crystalline silicon top cuts and compensated solar grade (SoG) feedstock. Material bulk properties including Hall mobility, carrier density and resistivity as functions of temperature were studied to evaluate the influence of compensation and impurities.Recycled top cut replacing poly-silicon as feedstock leads to a more uniform resistivity. In addition, higher concentrations of O and C, give rise to oxygen related defects, which act as neutral scattering centers displaying only a slight influence on the electrical properties at low temperature compared to the dominant compensation effect.The electrical performances of all samples are shown to be strongly dependent on compensation level, especially at the lowest temperature (~88K). A significant presence of incompletely ionized phosphorus was deduced through the measured carrier density. The temperature-dependent Hall effect measurements fit Klaassen׳s mobility model very well at low temperatures (<150K), showing consistency with the explanation of a reduced screening effect on ionized dopants for lightly doped silicon, while the deviation at the high temperature probably may be accounted for by the presence of as-grown defects, such as oxygen related defects and phosphorus clusters, which are usually neglected in most mobility models.