High Density Of Threading Dislocations Research Articles

Germanium-on-insulator virtual substrate for InGaP epitaxy

In this work, we investigate the bonding processes to improve Germanium-on-insulator (GOI) quality for III–V integration, and an InGaP LED has been grown and characterized on this GOI substrate. The threading dislocation density (TDD) of GOI was reduced from 5.1±0.8×108cm−2 to 4.3±0.4×107cm−2 by chemical mechanical polishing (CMP) processes. The thermal and mechanical properties of the CMPed GOI have been characterized. It showed that a 200nm thermal oxide layer effectively absorbed the hydrogen generated from the bonding by-product reactions, which avoided the formation of thermal bubbles and provided sufficient stability during high temperature epitaxy processes. After post-bonding annealing, a high bonding strength of 1720.6mJ/cm2 was achieved. Then, an AlInGaP/InGaP multi-quantum well (MQW) LED was grown on GOI substrate. Compared to LED on bulk Ge substrate, LED on GOI showed high leakage current due to the presence of high TDD in the Ge layer and the LED junctions. The electroluminescence of LED on GOI was approximately 50% of the light output of LED on bulk Ge.

(Invited) InGaN/GaN Nanowire LEDs and Lasers on Nonconventional Substrates

Conventional planar group-III nitride epitaxy for light-emitting diode (LED) is typically grown on sapphire, which results in high threading dislocation densities in the order of 106-108 cm-2 because of thermal- and lattice-mismatch. The challenge in realizing efficient InGaN QWs in the green gap and beyond stems from the spontaneous and piezoelectric polarization fields induced electron-hole wave functions separation as the indium composition increased to 25% and above.1 SiC and GaN bulk substrates, and nonpolar nitride materials have been used to address the lattice-mismatch, efficiency-droop, polarization field, and green gap issues, at the expense of high development and material costs.2 Nanowire LEDs on silicon has numerous advantages over planar epitaxial LEDs, such as a reduced polarization field, wavelength tunability by increasing indium composition and light extraction.3, 4 The blue, green, red and infrared LEDs grown using molecular beam epitaxy (MBE) demonstrated the potential of nanowires emitters for practical applications.5, 6 However, the large specific surface, and therefore a high density of surface states was found to limit the quantum efficiency and output power of nanowire devices.7 The phonon and carrier confinement in nanowires also led to junction heating and reduced heat dissipation. In this talk, we will present our investigations on nanowires LEDs and lasers emitting at green-to-infrared wavelength range. The approach in surface states passivation in InGaN/GaN quantum-disk-in-nanowires LEDs, as well as enhancing heat dissipation by growing nanowires-crystal on metal substrates will also be presented. Ammonium sulfide was used to passivate the nanowires in the laser fabrication process to reduce the p-contact resistance of GaN:Mg.5 We also developed octadecylthiol (ODT) passivation process for LEDs, which led to a ~4 times increase in photoluminescence peak intensity, and 50% increase in relative peak external-quantum-efficiency in passivated LEDs.8 The passivation changed the surface dynamic charge and recovered the band-edge emission. The faster-increasing trend in quantum efficiency is due to the reduced Shockley–Read–Hall (SRH) non-radiative recombination on the nanowires surface, which was confirmed using 4D scanning ultrafast electron microscopy technique, and the associated rate-equations simulations.9 Current high-power planar LEDs have to be transferred to a heat sink, such as Cu or CuMo, via laser liftoff or wafer bonding because of the high junction temperature during operation. It is more severe for nanowire LEDs on Si because of the small diameter, and the formation of SiN layer before nanowires growth. We developed the first high-power red InGaN/GaN Qdisks-in-nanowire LEDs on bulk polycrystalline Mo; the platform simultaneously implements buffer layer, reflector, n-metal contact, and heat-sink, thus greatly simplifies the fabrication process for high-power devices.10 The LEDs showed a low turn-on voltage of ∼2 V without droop, much lower junction temperature compared to LEDs on Si, and longer lifetime, thus is promising for high-power device operation. In conclusion, the InGaN/GaN nanowire edge-emitting lasers at green-to-infrared wavelength range on Si were demonstrated. We showed the first organic chemical passivation treatment on nitride nanowires, to increase external quantum efficiency and reduce SRH non-radiative recombination. We also reported the first nanowires device growth process on metal substrates. The fabricated device from the high-crystal-quality nanowires shows low turn-on voltage, high-power, droop-free, and stable operation.

Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask.

The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

Strong impact of the initial III/V ratio on the crystalline quality of an AlN layer grown by rf-plasma-assisted molecular-beam epitaxy

The initial Al/N ratio for AlN growth of plasma-assisted molecular-beam epitaxy without plasma stabilization is investigated. The in situ growth rate of AlN gradually increased and its temporal variation corresponded to that of nitrogen atoms, which indicated that the initial Al/N ratio was excessively Al-rich. For AlN growth, such a high-Al/N-ratio condition resulted in a three-dimensional growth mode in the initial stage of the growth, and AlN with high threading dislocation density was obtained. By controlling the initial Al/N ratio by introducing a short standby time, the resulting two-dimensional initial growth mode leads to high-quality growth of AlN.

Analysis of EL Emitted by LEDs on Si Substrates Containing GeSn/Ge Multi Quantum Wells as Active Layers

We analyzed multi quantum well light emitting diodes, consisting of ten alternating GeSn/Ge-layers, were grown by molecular beam epitaxy on Si. The Ge barriers were 10 nm thick and the GeSn wells were grown with 7% Sn and thicknesses between 6 and 12 nm. Despite the high threading dislocation density of 109 to 1010 cm−2 the electroluminescence spectra measured at 300 and 80 K yield a broad and intensive luminescence band. Deconvolution revealed three major lines produced by the GeSn wells that can be interpreted in terms of quantum confinement. Biaxial compressive strain causes a splitting of light and heavy holes in the GeSn wells. We interpret the three lines to represent two direct lines, formed by transitions with the light and heavy hole band, respectively, andan indirect line.

Spatial inhomogeneities in AlxGa1−xN quantum wells induced by the surface morphology of AlN/sapphire templates

The effects of the template on the optical and structural properties of Al0.75Ga0.25N/Al0.8Ga0.2N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25° to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/sapphire show optically pumped lasing with a record short wavelength of 237 nm.

Impact of thermal treatment on the optical performance of InGaN/GaN light emitting diodes

This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N2 atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS); (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i)).

Influence of Hydrogen Post-Implantation on Threading Dislocation Density in Strain-Relaxed Sige Layer

According to International Technology Roadmap for Semiconductor (ITRS), the development of MOSFETs based on SiGe or Ge is necessary for the next generation technology beyond 10-nm C-MOSFETs, because electron and hole mobility of Ge substrate is three and four times higher than those of a conventional Si MOSFETs, respectively. However, due to the lattice mismatch of 4.2 % between Si and Ge, many threading dislocations are generated, resulted in increasing the leakage current of the C-MOSFETs. The threading dislocation density of graded SiGe layer utilized currently is higher than 105 cm-2, which is too high to be applied in mass production because of degrading productivity and performance of the C-MOSFETs. To overcome this drawback in realizing sub 10-nm C-MOSFETs, a novel process technology should be developed to reduce the threading dislocation density. In this study, we designed a novel technology to reduce the threading dislocations in strain-relaxed Si1-xGex grown on Si substrate. We applied the H+ post-implantation which forms defect band at the dislocation clusters above the interface between the Si1-xGex layer and the Si substrate, acting as a dislocation sink. We investigated how the hydrogen-ion implantation followed by annealing affected the surface characteristics, such as dislocation pit and cross-hatch pattern density related the threading dislocation. In particular, the cross-hatch pattern density of the Si0.8Ge0.2 layer on the Si substrate was about 4x104 cm-1, as shown in Fig. 1(a). On the other hand, when hydrogen ions were implanted into the relaxed Si0.8Ge0.2 layer with an dose of 5x1015 cm-2, the cross-hatch pattern density was reduced to about 1.18x104 cm-1, as shown in Fig. 1(b). Also, there was a high threading dislocation density in the case of 30-at% Ge concentration, as shown in Fig. 2(a). However, for the case of hydrogen ion implantation, a dose of 5x1015 cm-2, both threading and misfit dislocations near the interface between Si0.7Ge0.3and Si substrate were significantly reduced, as shown in Fig. 2(b). Therefore, we present the mechanism of the strain relaxation resulted in the formation of the threading dislocation in Si1-xGex/Si (100) substrate. In addition, we explain how H+ implantation affects the cross-hatch pattern density related the threading dislocation. Furthermore, we report the process technology and optimized the implanted energy, dose, and the position of dislocation sink in Si1-xGex/Si structure via ion implantation. * This work was financially supported by the Brain Korea 21 Plus Program in 2015 and SiWEDS (Silicon Wafer Engineering and Defect Science). Reference [1] J. G. Park et al., Mat. Sci. Eng. B 134142-153 (2006) [2] S. Takagi, Strained-Si CMOS Technology in Advanced Gate Stacks for High-Mobility Semiconductors, Springer Berlin Heidelberg 1 (2007) Figure 1

Study of the Effects of GaN Buffer Layer Quality on the dc Characteristics of AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are promising candidates for high power and high current applications with its high mobility and high breakdown field. Epitaxial films for device substrate are typically grown on sapphire or silicon carbide, SiC, with dislocation densities in the order of 108 - 1010cm-2 due to their thermal coefficient and lattice mismatches. The thinner GaN buffer layers on SiC have been reported to have higher threading dislocation density in the GaN, but lead to higher off-state breakdown voltages. However, thick GaN buffer layers on sapphire and their impact on dc performance have not been widely studied. In this study, the impact of GaN buffer layer quality on dc and gate-lag pulse performance of AlGaN/GaN HEMTs was studied. Transmission electron microscopy (TEM) was used to measure the dislocation density in the GaN buffer layer of the HEMT samples. Dc characteristics, including drain I-Vs, transfer characteristics, and gate pulsed drain I-V characteristics of the HEMTs, were measured to establish a correlation between the AlGaN/GaN HEMTs’ dc and gate-pulsed performance and GaN buffer layer quality. AlGaN/GaN HEMT structures grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates were acquired from two different vendors. Both types of wafers had 24% Al concentration in the AlGaN barrier layer. One type of wafer had a 5 µm GaN buffer and the other had a 2 µm GaN buffer layer. HEMT fabrication was started with mesa patterning by conventional optical lithography. The mesa-etching was achieved using a unaxis shuttle-lock reactive ion etcher with inductively coupled plasma module (ICP) for device isolation with a Cl2/Ar plasma. Ohmic metallization of electron-beam evaporated Ti/Al/Ni/Au (25 nm/125 nm/45 nm/100 nm) was patterned by lift-off and subsequently rapid thermally annealed at 850⁰C in a flowing N2 ambient for 45 seconds. Schottky gates of e-beam evaporated Ni/Au (20 nm/80 nm) with 100 μm width and 1 μm length were obtained by standard lift-off patterning. DC current–voltage (I-V) characteristics were measured using an HP4156 parameter analyzer, and gate-lag pulse measurements were measured using an Agilent 8114A pulse generator. Figure 1 shows the bright field cross-sectional transmission electron microscopy (BF-XTEM) pictures of AlGaN/GaN surface for the top layers of (a) HEMT structure with 2 µm GaN buffer layer, (b) HEMT structure with 5 μm GaN buffer layer as well as the GaN/sapphire interface for the (c) HEMT structure with 2 μm GaN buffer layer, and (d) HEMT structure with 5 μm GaN buffer layer. Dislocations are seen to originate near the GaN/sapphire interface and propagate up through the AlGaN/GaN interface to the surface of both HEMT wafers. However, it is clear that the HEMT structure with 2 μm GaN buffer layer exhibited a much higher defect density. The calculated defect density for 2 μm GaN buffer layer substrate had an order of magnitude higher defect density. However, 5μm GaN buffer layer wafer showed that it was under high strain with larger height difference across the 2 inch with lower radius of curvature. There was no major difference in HEMTs’ dc performance was observed. In the other hand, Figure 2 (a) and (b) shows that gate lag measurement on fabricated HEMTs on 5 μm GaN buffer layer wafer showed no degradation and on 2 μm GaN buffer layer wafer showed the otherwise. The effects of defect density on HEMT dc and radio frequency performance were studied. HEMTs were fabricated on 2 and 5 μm of GaN buffer layers with the same composition of active layers. Despite there was no significant difference was seen in dc characteristics, significant reduction in pulse measurements was observed on thinner GaN layer HEMTs. These results suggest that the defects created in GaN buffer layer do not largely influence device dc performance but are closely related to high frequency performance. Figure 1

(Invited) Ion Implantation into GaN and Implanted GaN Power Transistors

GaN is a promising material for high-power, high-frequency and high-efficiency electron devices owing to their high saturation electron velocity and large break-down electric field (Eb ). The heavily doped layer with a low sheet resistance is strongly desired to reduce the power loss of GaN device and to increase the transconductance and maximum saturation current through the decrease of series resistances in devices. To reduce the series resistances in a device, a heavily doped epitaxial layer is often grown on the active layer, which would be removed by a dry etching technique to fabricate the recess structure [1]. On the other hand ion implantation is also one of very common technology to make lightly and highly selective doping area in Si [2] and SiC [3,4] devices and it is crucial for making high performance devices. We have previously demonstrated Si ion implantation for GaN HEMTs [5] and MESFETs [6] to reduce the contact resistance and fabricate a lightly-doped channel region. In addition, we have recently demonstrated normally-off GaN MISFETs with Si ion implantation into p-type GaN [7] and N ion implantation for the device isolation [8]. Si implanted MISFETs have achieved the high threshold voltage (Vth ) of +3.4 V [9]. Moreover, we have succeeded to fabricate selective p-GaN regions with Mg ion implantation and this result made it possible to control the Vth of GaN MISFETs from -5 V to +1.5 V. These results indicate a definite availability for power switching device applications. As known well, it is very difficult to form a p-type conductive layer with ion implantation on GaN. The GaN-based devices are usually fabricated on either a sapphire substrate or a Si substrate with a large diameter and at low cost. However these substrates cause many defects in GaN due to large lattice mismatch between epitaxial layers and the substrate. Such defects of high threading dislocation density over 109 /cm2 cause substantial current leakage and such layers would be deteriorated by the high temperature annealing for activation of the implanted Mg and create donor-like defects which compensates the Mg acceptors. P-type conversion by Mg ion implantation was attempted using high quality n--GaN epitaxial layer grown on a free-standing GaN substrate. Low temperature PL spectra showed Mg-acceptor related emissions. Vertical diodes showed clear rectifying I-V characteristics. UV and blue-green light emission was observed at forward biased condition. Positive Hall coefficients were observed by van der Pauw measurements. These results indicate the p-type conversion was possible using high quality GaN layers grown on free-standing GaN substrates, which can lead new device structures and processing for future GaN power devices. [1] B.R. Park, et al., Semisonductor Science and technology, Vol. 28, No. 12, (2013) [2] E. Suekawa, et al., Proceeding of ISPSD & ICs, 11.12, 249 – 252 (1998) [3] S.H. Ryu, et al., IEEE Electron Device Letters, Vol. 22, No. 3, 124 – 126, March (2001) [4] K.W. Chu, et al., IEEE Electron Device Letters, Vol. 34, No. 2, 286 – 288, February (2013) [5] K.Nomoto, T. Nakamura, et al., IEEE Electron Device Letters, Vol. 28, No. 11, 939 – 941, November (2007) [6] K.Nomoto, T. Nakamura, et al., Proceeding of MRS Fall Meeting, Vol. 892 (2005) [7] S. Gu, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 10, Issue. 5, 820 – 823, May (2013) [8] H. Kasai, T. Nakamura, et al., physica status solidi (c), Vol. 11, Issue. 3-4, 914 – 917, April (2014) [9] S. Taguchi, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 9, Issue. 3-4, 858 – 860, March (2012)

Analysis of nonradiative recombination mechanisms and their impacts on the device performance of InGaN/GaN light-emitting diodes

We investigate the nonradiative recombination mechanisms of two conventional InGaN/GaN-based blue light-emitting diodes with different threading dislocation densities (TDDs). The current–voltage, the ideality factor, and the slope of the light-versus-current curve on log scales are analyzed to distinguish the dominant nonradiative recombination mechanisms at room temperature. Through the analysis, we infer the dominant nonradiative recombination mechanisms to be the Shockley–Read–Hall process for the sample with a low TDD (∼1 × 108 cm−2) and the defect-assisted tunneling for the sample with a high TDD (∼1 × 109 cm−2). For more detailed analysis of the nonradiative recombination mechanisms and their impacts on the device performance, we execute the temperature-dependent photovoltage and temperature-dependent electroluminescence efficiency experiments. The sample with a low TDD is found to be more prone to the carrier spill-over at cryogenic temperatures due to the deactivation of point defects, while the sample with a high TDD is more robust to the operation at cryogenic temperatures owing to the relative insensitiveness of the defect-assisted tunneling to temperature.

Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Silicon photonics is rapidly growing because of the recent introduction of GeSn alloys as a group IV direct bandgap material. Viability of GeSn alloys for complementary metal-oxide-semiconductor technology, as demonstrated recently, is another step towards commercializing this material for integrated circuits applications [1]. However, ability to grow high quality materials is imperative for device applications. In order to compensate the high lattice mismatch between the Si substrate and the GeSn film different strain relaxed buffer layers, such as Ge, are used. Relaxation of Ge buffer layer introduces a network of misfit dislocations. This can result in high threading dislocation density (TDD) in GeSn degrading the performance of the final device. In this paper, we report on defect formation and propagation mechanism in Ge1-xSnx films.Plastic relaxation of higher lattice constant materials (SiGe, Ge, GeSn) on top of Si forces the layers to form 60o misfit dislocations. These dislocations are caused by slip of (110) and (111) planes in the diamond lattice structure, which have a screw component that threads through the grown film. Fig. 1 (a) schematically shows how defects form and propagate through the film. A network of misfit dislocations results in a cross-hatch pattern on the surface of the film. Fig. 1 (b) shows the plan view transmission electron microscopy (TEM) image of Ge.93Sn.07 on top of Ge buffer layer. The cross-hatch patterns formed at GeSn/Ge interface have overlapped with those of Ge/Si. The denser pattern is attributed to the Ge/Si interface due to higher lattice mismatch. Fig. 1 (c) delineates scanning electron microscopy image of decorated threading dislocations at the surface by using cold Schimmel solution (CrO3+HF) for 10 minutes. Density of pits (1.1×107 cm-2) is in good agreement with TDD measured from TEM micrographs (3×107 cm2)[1]. It also confirms that the quality of GeSn material is higher than Ge buffer layer (TDD: 9.71×108 cm-2) by an order of magnitude.Formation of Lomer misfit dislocaltion prevents propagation of threadig dislocation in the top layer.The main reasons behind this phenomenon is that the Burgers vector in Lomer misfit dislocation, for a (001) substrate, lies parallel to the interface [2]. Therefore, the substrate and film lattices can match while no prependicular component exists. In this case the film is grown relaxed without introducing structural imperfection such as threading dislocations, which degrades the material quality. Different models of Lomer misfit dislocation are proposed including a five-atom ring and seven-atom ring without dangling bonds [3]. Available models show that Lomer dislocations can combine two lattices through symmetric and asymmetric loops [4]. Fig. 1 (d-g) shows formation of the symmetric and asymmetric loops at Ge/GeSn interface films using high resolution TEM images.Our investigations show that formation of symmetric and asymmetric Lomer dislocation at the interface of Ge/GeSn film has reduced the TDD in GeSn film. Results from TEM images and etch pit density measurement verifies this conclusion by showing that 80% of the threading dislocations has not passed through the GeSn film. The current results show achievement of high quality GeSn film as well as indicating that GeSn is a unique candidate to be used as a universal compliant layer.

Studies on dislocation and surface morphology of AlxGa1−xN/GaN heterostructures grown by MOCVD

In this work, AlxGa1−xN epilayers have been grown on sapphire substrate with GaN template by the metal organic chemical vapor deposition (MOCVD) method. The structural and morphological properties of these samples have been investigated and compared. The growth rate of AlGaN has been found to decrease with increasing Al composition. By increasing Al composition of AlGaN epilayer, tilt and twist angle has been found to increase, indicating higher threading dislocation density. Surface morphology and dislocation density (DD) have been investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different types of dislocation etch pits have been observed in AlxGa1−xN/GaN epilayers, using orthophosphoric acid (85% H3PO4) as defect selective etchant. Three types of etch pits like screw type (α), edge type (β) and mixed type (α+β) dislocations have been observed. The mechanism of etch pits formation has been explained using Cabrera’s thermodynamic model. The etch pit density (EPDs) has been correlated with threading dislocation density (TDs) estimated using HRXRD measurements. Photoluminescence (PL) studies have revealed an increase in intensity of near band edge emission due to etching.

Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys.

(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

InGaN/GaN MQWs grown on c-plane sapphire substrate show high internal quantum efficiency, regardless of their high threading dislocation density (108 cm-2~ 1010 cm-2) that is introduced by lattice mismatch. This high-IQE phenomenon is attributed to bandgap energy fluctuation and carrier localization in the QW layer [1].In the present work, temperature-dependent PL, confocal microscopy, and nanometer-scale time-resolved photoluminescence (TRPL) spectroscopy were employed to analyze the carrier localization, carrier transportation and carrier recombination processes in blue-emitting InGaN/GaN MQWs.The temperature-dependent PL spectra were taken from 10 K to 300 K. The decreased spectrum-integrated PL intensity with increasing temperature was observed, which was fitted with second-order Arrhenius equation, as shown in Fig. 1. The fitting indicated two non-radiative channels: the one with high activation energy (dominated at high temperature) corresponded to thermal activation of carriers out of the strongly localized states, while the one with low activation energy (dominated at low temperature) corresponded to thermal activation of carriers out of the weakly localized states. The S-shaped shift of PL peak energy and the inverse S-shaped shift of PL FWHM were also observed, as shown in Fig. 2 and Fig. 3, which were explained with carrier localization and carrier dynamics.The confocal microscope image collecting near bandedge emission showed inhomogeneous PL intensity distribution, as shown in Fig. 4. Previous analysis [2] has shown that the bright region had smaller bandgap energy than dark region did. Nanometer-scale TRPL spectra were taken at different places in the confocal microscope image. As shown in Fig. 5, TRPL spectra taken at the center of bright region and the center of dark region show single exponential decay, which are characterized by a single lifetime and corresponds to carrier localization in strongly localized state and weakly localized state, respectively. Comparing with bright region, dark region has shorter lifetime, indicating a higher probability of nonradiative recombination and carrier transportation from dark region to bright region. TRPL spectrum taken at the boundary of bright region, however, shows double exponential decay, as shown in Fig. 5, which is characterized by two lifetimes. The short lifetime corresponds to carrier transportation from weakly localized state to strongly localized states, while the long lifetime corresponds to carrier recombination at local strongly localized state.

Exfoliation of Threading Dislocation‐Free, Single‐Crystalline, Ultrathin Gallium Nitride Nanomembranes

Despite the recent progress in gallium nitride (GaN) growth technology, the excessively high threading dislocation (TD) density within the GaN crystal, caused by the reliance on heterogeneous substrates, impedes the development of high‐efficiency, low‐cost, GaN‐based heterostructure devices. For the first time, the chemical exfoliation of completely TD‐free, single‐crystalline, ultrathin (tens of nanometers) GaN nanomembranes is demonstrated using UV‐assisted electroless chemical etching. These nanomembranes can act as seeding layers for subsequent overgrowth of high‐quality GaN. A model is proposed, based on scanning and transmission electron microscopy as well as optical measurements to explain the physical processes behind the formation of the GaN nanomembranes. These novel nanomembranes, once transferred to other substrates, present a unique and technologically attractive path towards integrating high‐efficiency GaN optical components along with silicon electronics. Interestingly, due to their nanoscale thickness and macroscopic sizes, these nanomembranes may enable the production of flexible GaN‐based optoelectronics.

Growth and characterization of AlGaN/AlN/GaN/AlGaN double heterojunction structures with AlGaN as buffer layers

High electron mobility transistors (HEMTs) structures with GaN, Al0.025Ga0.975N and Al0.04Ga0.96N high resistivity (HR) buffers were grown on sapphire by metal organic chemical vapor deposition (MOCVD). The structural and electrical properties of these three samples were investigated and compared. By increasing Al composition of AlGaN buffer, full width at half maximum (FWHM) values of (0002) and (10-12) x-ray rocking curves for buffer increase, indicating higher threading dislocation density. Room temperature noncontact Hall measurements were performed, and the measured 2DEG mobility was 1828cm2/Vs for GaN buffer, 1728cm2/Vs for Al0.025Ga0.975N buffer, and 1649cm2/Vs for Al0.04Ga0.96N buffer, respectively. Combining the theoretical calculation with the experiments, it was demonstrated that the decrease of mobility was attributed to higher dislocation density in sample with higher Al composition of AlGaN buffer. Devices were fabricated and it was found that the double heterojunction (DH) HEMT with Al0.025Ga0.975N buffer could effectively reduce the buffer leakage current.

The effect of dislocations on the efficiency of InGaN/GaN solar cells

Two solar cells based on an InGaN/GaN p–i–n hetero-junction, but having different dislocation densities, were fabricated and characterized. The structures were grown on c-plane (0001) GaN-on-sapphire templates with different threading dislocation (TD) densities of 5×108 and 5×109cm−2. Structural characterization revealed the presence of V-defects in the InGaN epilayer. Since each V-defect was associated with a TD, the structural as well as the optical properties worsened with a higher TD density in the GaN/sapphire template. It was also found that additional dislocations were generated in the p-GaN layer over the V-defects in the InGaN layer. Because of its superior structural quality, the peak external quantum efficiency (EQE) of the low TD density sample was three times higher than that of the high TD density sample.

The thermoelectric properties of Ge/SiGe modulation doped superlattices

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1−yGey barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1−yGey, and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques.

Comparative Studies of the Growth and Characterization of Germanium Epitaxial Film on Silicon (001) with 0° and 6° Offcut

The quality of germanium (Ge) epitaxial films grown directly on silicon (Si) (001) with 0° and 6° offcut orientation using a reduced-pressure chemical vapor deposition system is studied and compared. Ge film grown on Si (001) with 6° offcut presents ∼65% higher threading dislocation density and higher root-mean-square (RMS) surface roughness (1.92 nm versus 0.98 nm) than Ge film grown on Si (001) with 0° offcut. Plan-view transmission electron microscopy also reveals that threading dislocations are more severe (in terms of contrast and density) for the 6° offcut. In addition, both high-resolution x-ray diffraction and Raman spectroscopy analyses show that the Ge epilayer on 6° offcut wafer presents higher tensile strain. The poorer quality of the Ge film on Si (001) with 6° offcut is a result of an imbalance in Burgers vectors that favors dislocation nucleation over annihilation.