Dislocations in vapor phase epitaxial GaP

Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and <211> directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called “inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 10¹⁹ cm^-3, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 10⁴ cm⁻² to 1 × 10⁶ cm⁻².     

The effect of isolated dislocations on substrate and device properties in low-dislocation czochralski GaAs

Czochralski GaAs grown with In incorporated into the melt has large regions with fewer than 100 cm^-2 dislocations. We have examined the effect of these dislocations on substrate and device properties. Infrared transmission images reveal dark filaments of high EL2 concentration a few tens of microns in diameter surrounding dislocations, Cathodo and photoluminescence images show orders of magnitude contrast in band-edge luminescence intensity near dislocations. Single dislocations appear to be surrounded by bright rings ˜200 μm in diameter in luminescence images, with dark spots 50 to 75 μm across centered on the dislocation. More complex luminescence structures with larger dark regions (˜150 μ across) and central bright spots are centered on small dislocation clusters. Differences in lifetime of photogenerated electrons or holes are the most likely cause of the luminescence contrast. Anneals typical of our post-implant processing substantially lower the luminescence contrast, suggesting the defect lowering the lifetime is removed by annealing. This may partially explain why we do not observe any effect of dislocation proximity on the properties of devices made in the material, in spite of the enormous luminescence contrast observed near dislocations.     

Inhomogeneity of the deep center el2 in GaAs observed by direct infra-red imaging

The distribution of the dominant deep trap EL2 in 7.5cm diameter crystals of semi-insulating GaAs is studied by whole slice infrared imaging. Very significant fluctuations in the neutral EL2 concentration ([EL2]ℴ) are observed, corresponding at most to variations in [EL2]ℴ of up to 80%. The different sorts of fine structure, namely cell structure and bands of high infrared absorption ("sheets" and “streamers”) lying in (110) planes running down the <001> growth directions, are described.     

Strain Mapping at the Atomic Scale in Highly Mismatched Heterointerfaces

A complete characterization of dislocation network in a highly mismatched interface with high spatial resolution has been performed. The interface between InN quantum dots and a (0001) GaN substrate contains three noninteracting sets of regularly‐spaced misfit dislocations lying along <110> directions. The network has a “Star of David” form, with each star bounding a hexagonal region which is pseudomorphic. These misfit dislocations form a threading dislocation network at the island edges due to free surface forces.     

Parallel Dislocation Networks and Cottrell Atmospheres Reduce Thermal Conductivity of PbTe Thermoelectrics

Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>10¹² cm^–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na_0.025Eu_0.03Pb_0.945Te is demonstrated at a moderate dislocation density of 1 × 10¹⁰ cm^–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon-scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge-type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal-insulating applications.     

Identification of prismatic slip bands in 4H SiC boules grown by physical vapor transport

Transmission electron microscopy (TEM) and KOH etching have been used to study the dislocation structure of 4H SiC wafers grown by physical vapor transport. A new type of threading dislocation arrays was observed. Rows of etch pits corresponding to dislocation arrays were observed in vicinity of micropipes, misoriented grains and polytypic inclusions at the periphery of the boules and extended along the directions. Plan view conventional and high resolution TEM showed that the arrays consisted of dislocations threading along the c-axis with Burgers vectors having edge components of the a/3 type. The Burgers vectors were parallel to the corresponding arrays. The dislocation arrays were interpreted as slip bands formed by dislocation glide in the prismatic slip system of hexagonal SiC during post-growth cooling.     

Degradation of ZnSe/ZnTe multiquantum well contacts to p-ZnSe

The work presented in this paper studied the degradation of ZnTe/ZnSe multiquantum well contacts to p-ZnSe under high current loading (1000 to 1500 A/cm²). During degradation, localized heating (up to 200°C > the bulk substrate and heat sink) was observed to occur at the point were electrical power was supplied. Auger data from degraded samples indicated that due to the localized heating, Zn and Te from the ZnTe layers and Zn from the ZnSe layer diffused through the Au metallization to the samples surface. In addition, thermal stress from the localized heating generated micro-cracks in the ZnSe which acted as high diffusivity paths for impurities. Rectangular defects were also found to form in the degraded region. These defects were oriented to the micro-cracks and had similar geometries as dislocation patches (dark line defects) which have been reported to form in the quantum well region of degraded ZnSe based laser devices. The similarities between the rectangular defects and dark line defects suggest the formation of similar dislocation patches in the quantum well region of the multiquantum well contacts.     

Through-thickness characterization of copper electrodeposit

Through-thickness crystallographic texture, defect structure, and tensile embrittlement of 35 μm thick electrodeposit are characterized by successive thinning. An initially random grain structure, inherited from the substrate, evolves into a strong <220> fiber texture. The random to oriented grain transformation begins at the inception of thickening and is complete after about 15 μm deposit thickness, where about 0.9 volume fraction of grains become oriented near <220>. Further thickening of the deposit sharpens the texture, reducing the scatter around the <220> ideal orientation. A duplex coarse/fine particle (coherent domain) structure is obtained. Coarse particles along <220> are less defective and have smaller lattice strains; fine particles along <200>, presumably associated with the random grains, are defect-saturated with finely spaced twins, high dislocation density and enhanced lattice strains. With increasing distance from the shiny surface (of initial film formation), especially following the initial 10 μm deposit thickness, (a) along <220>: particle size and twin spacing increase whereas dislocation density and root mean square (rms) strains decrease, (b) along <200>: particle size increases gradually, dislocation density and rms strains increase sharply and the already fine twin spacing remains unchanged, and (c) the effective particle size ratio D_eff<220>:D_eff<200> exceeds 1.4, suggesting a twinning-induced z-direction particle shape anisotropy. A substantial decrease in tensile elongation is observed at 180°C. The embrittlement increases with the deposit thickness, attributed to the development of low density regions in the morphological boundaries. High elongation and embrittlement directional anisotropies are observed near the shiny surface, perhaps due to preferred nucleation on the substrate asperities.     

The microstructure of laterally seeded silicon-on-oxide

The production of large scale integrated circuits in thin silicon films on insulating substrates is currently of much interest in the electronics industry. One of the most promising techniques of forming this composite structure is by lateral seeding. We have used optical microscopy and transmission electron microscopy to characterize the microstructure of silicon-on-oxide formed by scanning CW laser induced lateral epitaxy. The primary defects are dislocations. Dislocation rearrangement leads to the formation of both small angle boundaries (stable, regular dislocation arrays) and grain boundaries. The grains were found to be misoriented to the <100> direction perpendicular to the film plane by ≤ 4° and to the <100> directions in the plane of the film by ≤ 2°. Internal reflection twins are a common defect. Microtwinning was found to occur at the vertical step caused by the substrate-oxide interface if the substrate to oxide step height was > 120 nm. The microstructure is continuous across successive scan lines. Microstructural defects are found to initiate at the same topographical location in different oxide pads. We propose that this is due to the meeting of two crystallization growth fronts. The liquid silicon between the fronts causes large stresses in this area because of the 9% volume increase during solidification. The defects observed in the bulk may form by a similar mechanism or by dislocation generation at substrate-oxide interface irregularities. The models predict that slower growth leads to improved material quality. This has been observed experimentally.     

Transmission electron microscopy and x-ray topography study of mercury-cadmium-telluride

The fault structure of MCT crystals was studied by means of X-ray topography and transmission electron microscopy. Subgrain boundaries were revealed and identified by both techniques, and their tilt angle was calculated to be of the order of 10^-3 rad. The dislocations which form the subgrain boundaries were identified by TEM to have Burger‘s vectors parallel to the <110> directions. Twins in large numbers were observed by TEM. The twins were identified to have a {111} habit plane.     

Temperature-dependent light-emitting characteristics of InGaN/GaN diodes

Temperature-dependent light-emitting and current-voltage characteristics of multiple-quantum well (MQW) InGaN/GaN blue LEDs were measured for temperature ranging from 100 to 500 K. The measurement results revealed two kinds of defects that have pronounced impact on the electroluminescent (EL) intensity and device reliability of the LEDs. At low-temperature (<150 K), in addition to the carrier freezing effect, shallow defects such as nitrogen vacancies or oxygen in nitrogen sites can trap the injected carriers and reduces the EL intensity. At high temperature (>300 K), deep traps due to the structure dislocations at the interfaces significantly reduce the efficiency for radiative recombination though they can enhance both forward and reverse currents significantly. In addition, the significant enhancement of trap-assisted tunneling current causes a large heat dissipation and results in a large redshift of the emission peak at high temperature.     

On the degradation of InGaAsP/InP-based bulk lasers

Degradation of InGaAsP-InP-based buried-heterostructure bulk (BH-bulk) lasers has been studied by means of electroluminescence (EL), photoluminescence (PL), electron-beam-induced current (EBIC) and transmission electron microscopy (TEM). Lasers with p/n as well as semi-insulating (SI) current-blocking layers were studied. The results show that moderate increases in the threshold current correlate well with formation of dark defects (DD's) (i.e., dark-line defects (DLD's) or dark-spot defects (DSD's), which cannot be distinguished in our case due to the narrowness of the laser stripe.) The DD's were found to be caused by dislocation loops. The dependence of threshold current increase on the number of DD's is explained in terms of a model which includes effects due to the DD's, as well as changes in the regions outside the DD's. The latter is found to be responsible for the major part of the threshold current increase. Values for the ratio between the carrier lifetimes inside and outside the DD's are presented, for the first time. In our lasers, strong degradation differs from moderate degradation in that DD's do not form during aging. The presence of dislocation loops only at the sidewalls of the active stripe in lasers with p/n current-blocking layers points to the sidewalls as being critical. The near absence of dislocation loops and the smaller increase in threshold current in SI lasers which have degraded strongly, compared to the strongly degrading p/n lasers, suggest that strong degradation is a synergistic combination of damage in the sidewalls and Zn indiffusion from the current-blocking layers     

The dissolution mechanism of oxide precipitates in czochralski silicon degenerately doped with boron during high temperature annealing

The morphology of oxide precipitation induced defects in Czochralski silicon degenerately doped with boron and annealed at 800° and 1050°C, respectively, was examined using a transmission electron microscope. After an extended annealing at 800°C, the predominantly observed defects were the oxide precipitate platelets having the {001}-type habit planes and sides parallel to <110> and <112> crystallographic directions. The morphology of the oxide precipitates as derived from the residual oxygen calculation is suggested to be that of a thin octahedral shape. During a subsequent high temperature annealing, the octahedral precipitate platelets became thermodynamically unstable and dissolved. Based upon the defect morphology observed after a 1050°C anneal, it is suggested that the dissolving precipitate introduces a tensile strain into the surrounding silicon lattice. Contrary to precipitate growth, the lattice strain introduced by precipitate dissolution is relieved primarily through mechanisms involving vacancy injection from the precipitate interface and a condensation of excess silicon interstitials via a formation of an interstitial-type dislocation loop.     

Positive feedback model of defect formation in gradually degraded GaAlAs light emitting devices

Defects in gradually degraded GaAlAs (0.8 µm wavelength) light emitting diodes have been evaluated by transmission electron microscope (TEM) and deep-level transient spectroscopy (DLTS). Two stages increase of deep-level defects has been observed by DLTS during device operation (reported in a previous paper). On the other hand, very small dislocation loops (15-50 nm in diameter) were observed in the active region by TEM at the end of the device operation. The nature of the dislocation loops was determined to be interstitial Frank loops with Burgers vector of (a/3)     

金属间化合物Ni3Al中超点阵位错的原位加热观察

利用透射电子显微镜弱束成像技术对金属间化合物Ｎｉ＿３Ａｌ单晶中的超点阵位错进行原位动态观察。结果发现，在（００１）面上分解的位错，其分解宽度在室温至７７３Ｋ之间没有明显变化，在７７３Ｋ恒温３０分钟后两分解位错之间出现连结点，在８９８Ｋ恒温２０分钟后两分解位错完全束集。说明（００１）面上反相畴界的能量随着温度升高而增加。本文讨论了这一现象对材料力学性能的影响。     

Traces of HgCdTe Defects as Revealed by Etch Pits

The characteristics of defects in HgCdTe liquid-phase epitaxy (LPE) epilayers were investigated by using Schaake’s and Chen’s etches. By tracking the etch pits (EP), two kinds of threading dislocations with <110> and <211> orientations were observed for the first time in HgCdTe LPE epilayers. They are ascribed to perfect 60 deg dislocation and Shockley partial screw dislocations. The kinds of dislocation etch pits revealed by Schaake’s and Chen’s etches were experimentally confirmed to be correlated one-to-one. In addition to the threading dislocation etch pits, another kind of etch pits without the threading feature was also observed using both etch methods. The density of the nonthreading etch pits increases in the regions close to epilayer-substrate interfaces, scratched areas, and melt droplets. The etch pit density (EPD) varies from 10⁴ cm⁻² to 10⁷ cm⁻² from sample to sample or at different places on the same sample, indicating that they are correlated to stresses and should be considered in the assessment of HgCdTe epilayers.     

Analysis of contacts and V-defects in GaN device structures by transmission electron microscopy

This paper describes the microstructure of ohmic contacts to an AlGaN/GaN heterostructure, of interest for high power transistors, and an analysis of V-defects in an InGaN/GaN multi-quantum well (MQW) light-emitting structure. A combination of different transmission electron microscopy (TEM) techniques has been employed, as they provide complementary information. These include bright field and dark field TEM, high-resolution electron microscopy, X-ray mapping, energy filtered TEM and high angle annular dark field. A full determination of the phase distribution in the ohmic contacts was achieved. The onset of low contact resistance was found to correspond with the formation of an interfacial layer containing both TiN and AlN, and of an intermetallic layer containing Al, Ti and Au in contact with it. The MQW structures were capped with a p-type GaN layer, and TEM and ADF studies of the samples show a number of V-defects 100-200 nm apart along the MQW. Each V-defect incorporates a pure edge (b = 1/3 <11-20>) dislocation, which runs through its apex up to the free surface. The defects contain GaN with no InGaN layers, suggesting the V-pits have been filled in by the capping layer.     

Growth and characterization of iron aluminide films on compound semiconductors

Transmission electron microscopy (TEM), selected area electron channeling patterns (SACP), and reflection high-energy electron diffraction (RHEED) are used to investigate the growth and quality of epitaxial iron aluminide films on GaAs(100) substrates. These films are observed to grow layer-by-layer even when the constituents are codeposited. During growth the lattice relaxation is determined fromin situ RHEED measurements of the separation of two diffracted beams. After growth TEM measurements of Moiré fringes are used to determine the local residual strain and the Burger's vector of the iron aluminide film. Selected area electron channeling is used to determine the residual strain in the films without thin specimen artifacts. The results indicate that strain in the FeA1 films relaxes by the formation of misfit dislocations with anα<100> Burger's vector. The relaxation does not occur as quickly as the Matthews equilibrium model predicts. Since the FeAl and GaAs have different slip systems, the dislocations inducing the relaxation must nucleate in the epilayer. The defects present suggest growth by nucleation and subsequent climb of dislocation half-loops.     

Light Amplification and Efficient Electroluminescence from a Solution-Processable Diketopyrrolopyrrole Derivative via Triplet-to-Singlet Upconversion

Over the years, achieving efficient electroluminescence (EL) while simultaneously having low light amplification thresholds under optical excitation has been the key to progression toward the long-thought objective of electrically pumped organic lasers. While significant progress in this regard has been made for organic semiconductors emitting in the blue–green region of the visible spectrum, organic laser dyes with low-energy emission (>600 nm) still suffer from high amplified spontaneous emission (ASE) thresholds and low external quantum efficiencies (EQEs) in devices. Herein, low ASE thresholds and efficient EL are reported from a solution-processable organic laser dye dithiophenyl diketopyrrolopyrrole (DT-DPP). The ASE threshold of 4 µJ cm⁻² at the wavelength of 620 nm is obtained while making constructive use of triplet excitons by doping DT-DPP in a green-emitting host matrix, which exhibits thermally activated delayed fluorescence (TADF). The organic light-emitting diode fabricated from this system gives a high EQE of 7.9% due to the efficient utilization of triplet excitons. Transient EL studies further show that a high reverse intersystem crossing rate is crucial in achieving lasing under electrical pumping from such TADF-assisted fluorescent systems.     

Unusual temperature dependence of electroluminescence intensity in blue InGaN single quantum well diodes

Temperature dependence of electroluminescence (EL) spectral intensity of the super-bright blue InGaN single quantum well (SQW) light emitting diodes (LEDs) has been carefully investigated over a wide temperature range (T=15-300 K) and as a function of injection current level (0.1-10 mA) in comparison with high quality GaAs SQW-LEDs. When T is slightly decreased to 180 K, the EL intensity efficiently increases in both cases due to the reduced non-radiative recombination processes. However, further decreasing T below 100 K, striking differences exist in EL intensity as well as injection current dependences between the two types of diodes. That is, the EL efficiency at lower T is found to be quite low for the blue diode in strong contrast to that of red GaAs SQW-LED where significant enhancement of the EL efficiency persists down to 15 K. These results indicate that the carrier capture efficiency of the blue SQW diode is unusually worse at lower T than at T=180-300 K, reflecting the unique radiative recombination processes under the presence of high-density dislocation (10¹⁰ cm⁻²).