Superlattice Buffer Layer Research Articles

High thermostability in CoFeB/MgO/Ta multilayers by {Pt(t)/Ta(t)}n superlattice buffer layers

Ta/CoFeB/MgO/Ta multilayer known as the fundamental building block for magnetic tunneling junctions, has been studied for decades. However, its thermostability is normally less than 300 °C, which cannot meet the requirements for subsequent semiconductor processes. In this study, buffer layers of {Pt(t)/Ta(t)}n superlattices were introduced into CoFeB/MgO/Ta multilayers grown by magnetron sputtering. The growth period n and the single layer thickness t in the superlattice have been systematically explored. And it is found that the sample can maintain distinct perpendicular magnetic anisotropy (PMA) even after a 440 °C post-annealing treatment at n = 3 and t = 1.5 nm. This has been attributed to that the superlattice buffer layer might serve as a diffusion barrier, reducing Ta atoms from diffusing into the CoFeB/MgO interface. Less Ta at the interface results more FeOx formed and enhances the hybridization of Fe 2p and O 3d orbitals which increases the thermostability of PMA. This suggests that superlattice buffer layer provides a new approach to improve the materials’ thermostability.

High Current Density and Low Ron Quaternary InAlGaN MIS-HEMT on Si for Power Applications

In this work, a quaternary In0.04Al0.63Ga0.33N/GaN metal-insulator-semiconductor (MIS)-high electron mobility transistor (HEMT) on Si substrate using a GaN:C back-barrier (BB) layer and an AlGaN/AlN superlattice (SL) buffer layer to achieve a high breakdown voltage and a high output current density was demonstrated. Compared to the conventional device adopting AlGaN as the barrier layer, the proposed device showed a better 2DEG carrier density up to 1.9 × 1013 cm−3, a higher output current density up to 1,070 mA mm−1 (improved by 47%), an on-resistance (Ron) as low as 7.64 Ω mm (decreased by 26%), an off-state breakdown voltage up to 2,070 V, and an improved dynamic Ron performance (dynamic to static ratio increased to just 1.2 times at an applied drain-to-source stress voltage (VDS,stress) of 400 V). These results indicated the great potential of the InAlGaN/GaN MIS-HEMTs on Si for high-power switching applications.

Effects of AlN/GaN superlattice buffer layer on performances of AlGaN/GaN HEMT grown on silicon for sub-6 GHz applications

In this study, the effects of AlN/GaN superlattice (SL) thickness on performances of AlGaN/GaN high electron mobility transistor (HEMT) heterostructure grown by metal-organic chemical vapor deposition on silicon is investigated. Stress in GaN is controlled by varying the total thickness of the AlN/GaN SL. Improved crystal quality and surface roughness accomplished with 2200 nm-thick AlN/GaN SL, leads to an increase in high electron mobility (1760 cm2 (V s)−1) as well as two-dimensional electron gas concentration (1.04 × 1013 cm−2). AlGaN/GaN metal–insulator-semiconductor HEMT (MIS-HEMT) fabricated on the heterostructure with SL buffer layer exhibits a significant improvement in maximum saturation current of 1100 ± 29 mA mm−1 at V GS = 0 V and a low on-resistance of 4.3 ± 0.15 Ω mm for the optimized AlN/GaN SL. The 2200 nm-thick AlN/GaN SL supports the growth of stress-free GaN heterostructure, which can reduce the insertion loss for sub-6 GHz radio frequency (RF) applications. This GaN HEMT structure based on SL buffer layer is suitable for low-frequency RF power applications.

Fully-relaxed n-AlGaN on AlN / Al2O3 templates using strain-relaxed super-lattice buffer layers

Strain relaxation, surface morphology, and threading dislocations of 2.5–μm-thick n–Al0.6Ga0.4N, grown by metal–organic chemical vapor deposition on AlN/Al2O3 templates using strain relaxed super lattice (SRSL) buffer layers, are investigated. The SRSLs are consisted of 11–115 nm Al0.50Ga0.50N/23 nm Al0.75Ga0.25N multiple layers. The n–Al0.6Ga0.4N layer is fully strain relaxed and smooth surface morphology when grown on the SRSL with Al0.50Ga0.50N thickness over 69 nm. The full width at half maximum values of X–ray rocking curves of n-Al0.6Ga0.4N for (0002) and (101‾2) planes are 237 and 468 arcsec, respectively. The threading screw dislocations and threading edge dislocations are estimated as 1.2 × 108 cm−2 and 1.1 × 109 cm−2, respectively. The SRSL buffer layer enables to achieve 100% strain relaxation together with a smooth surface and a low threading dislocation density.

Short-wave infrared (SWIR) photodetection by InAs/GaAs quantum dot heterostructures grown on Ge (100) substrate without Migration Enhanced Epitaxy layer

Integration of III-V based optoelectronic devices with Silicon (Si) technology is the prime choice of researchers for the futuristic Si-photonics. Profusion research is going on for direct growth of III-V heterostructures on the Germanium (Ge) substrate. Also, monolithic integration of III-V based device structures on Si using Ge layer and Migration Enhanced Epitaxy (MEE) technique have been endeavored. In this study, InAs based quantum dot infrared photodetectors (QDIPs) are grown on Ge substrate without MEE layers. The GaAs/AlAs superlattice buffer (SLB) layers are used to suppress the defects and dislocations. Two different QDIPs with GaAs capped and In 0.15 Ga 0.85 As/GaAs capped InAs QD are grown and characterized using photoluminescence (PL), high-resolution X-Ray Diffraction (HRXRD), and Raman spectroscopy (RS). Red-shifted and narrow PL spectrum of the InGaAs/GaAs capped QDIP with respect to the GaAs capped QDIP attributes to formation of larger dots and homogeneous size distribution. Low strain and high activation energy in the InGaAs capped QDs signify better strain relaxation via the InGaAs strain reducing layer (SRL) and better carrier confinement. Interband-assisted near-infrared (NIR) photoresponse is obtained from both devices till room temperature. However, the InGaAs/GaAs capped QDIP shows shortwave infrared (SWIR) response, which promulgates the formation of QDs with better crystallinity due to the presence of InGaAs SRL. The room temperature (300 K) dark current density of the GaAs and InGaAs capped QDIP is 1.2 × 10 −1 A/cm 2 and 6.6 × 10 −3 A/cm 2 respectively with −2 V bias voltage. This work demonstrates the pathway to uncomplicated growth of high quality QDIP structures on Ge substrate. • A novel technique is used to grow defect free InAs QD structures on Ge substrate. • GaAs/AlAs SLB layers are used to suppress the defects and dislocations. • InAs QDs with GaAs and InGaAs/GaAs capping are investigated and compared. • Both structures show low dark current at room temperature. • Infrared photoresponse till room temperature is obtained for both QDIPs.

Realization of high-quality InGaAs/GaAs quantum dot growth on Ge substrate and improvement of optical property through ex-situ ion implantation

Epitaxial growth of III-V heterostructures on non-native substrates such as Silicon (Si) or Germanium (Ge) is one of the promising research topics for the last two decades. The interface between polar III-V semiconductors and non-polar substrates (Si or Ge) plays a crucial role in monolithic integration. However, there is an anticipation of epitaxial GaAs growth on Ge substrate because of the lower mismatch of lattice constants and thermal expansion coefficients between them. Therefore, the high-quality growth of III-V semiconductor heterostructures on Ge substrates would overcome the impediment to Si-photonics, where the monolithic integration of optoelectronic device structures can be done using a Ge graded layer on Si. Here, we have explored the epitaxial growth of multi-layer InGaAs/GaAs quantum dot heterostructures on Ge substrates and compared the optical and structural properties with the QDs grown on GaAs substrate. The optical properties of all samples are investigated with the help of photoluminescence (PL) and time-resolved photoluminescence (TRPL), whereas the morphology of the QDs is observed through cross-sectional transmission electron microscopy (XTEM) images. An enhancement in the optical characteristics (PL peak wavelength, activation energy, carrier lifetime) is found for the QDs grown on Ge substrate with the super-lattice buffer (SLB) layer. The minimization of defects and dislocations in the heterostructure is also realized for the structure with the SLB layer. Furthermore, a two-fold enhancement in PL intensity and 24 meV increment in activation energy is achieved through ex-situ H− ion-implantation, which approached the values obtained for the QD heterostructure grown on GaAs substrate.

Strongly reduced V pit density on InGaNOS substrate by using InGaN/GaN superlattice

Abstract The InGaN pseudo-substrate, namely InGaNOS (InGaN On Sapphire), is used to enhance the In incorporation rate in InyGa1-yN/InxGa1-xN multiple quantum wells (MQWs) to get red emission for micro-display applications. However, the starting material for the InGaNOS fabrication is a non-optimized In0.08Ga0.92N layer grown on GaN on sapphire substrate which exhibits V shaped defects (V pits). Such V pits remain afterwards in the final InGaNOS substrate. We demonstrate here that InxGa1-xN/GaN superlattice has the potential to cover or fill the native V pits while maintaining a pseudomorphic growth. A combination of a thin GaN interlayer and an InGaN layer in a slight tensile strain state for each pair of the superlattice is necessary to achieve this goal. In addition, it is shown that the presence of GaN interlayers improves the material quality and the surface roughness. (0 0 2) X-ray diffraction rocking curve linewidth reduces to 780 arcsec compared to 3000 arcsec for the substrate. Finally, InyGa1-yN/InxGa1-xN multiple quantum wells grown on InxGa1-xN/GaN superlattice buffer layer on InGaNOS 3.205Ȧ substrate shows a central emission wavelength, measured by photoluminescence, of 624 nm at 290 K with an optical internal quantum efficiency value of 6.5%.

Electrical properties of Si and Be doped InSb and InAlSb/InSb superlattice applied to improve the doping efficiency

Abstract 0.8 to 1 μm Si and Be doped InSb were grown by molecular beam epitaxy. X-ray diffraction and atomic force microscope indicate that growth temperature below 340 °C severely results in dislocations and undulating surface. Silicon shows amphoteric doping nature in InSb at higher growth or Si cell temperatures, for the calculation result reveals it needed more energy to take the place of Sb. ECV measurement demonstrates that InSb/In0.9Al0.1Sb superlattice buffer layer can effectively inhibit Be dopant from diffusing to the substrate in the 420 °C InSb, with which both the doping level and crystalline quality are significantly improved. 790–870 °C Be doped InSb transfer from p- to n-type during the Hall measurement temperature increasing from 77 K to 330 K. The transition temperature of each sample is linearly correlated to the Be doping temperature, which is theoretically interpreted by the excitation mechanism and carrier transportation principles inside InSb.

Milliwatt power UV-A LEDs developed by using n-AlGaN superlattice buffer layers grown on AlN templates

Ultraviolet (UV)-A light-emitting diode (LED) light sources are strongly demanded for both medical and photochemical applications. In our previous report, we investigated the conventional n-AlGaN buffer layer (BL)-based UV-A LED devices and a very low output power was achieved. In this work, we aim for the suppression of vertically propagating threading dislocation densities (TDDs) in the n-AlGaN BL including the current spreading layer (CSL) by introducing Si-doped n-Al0.37Ga0.63N/n-Al0.27Ga0.73N superlattices (SLs) between the AlN template and n-AlGaN BL for the demonstration of 341 nm UV-A LEDs. When the conventional n-AlGaN BLs were replaced with n-AlGaN SL-based BLs (with a suitable number of periods up to ~70) in the UV-A multi-quantum wells, then the full width at half maximum of the x-ray rocking curves in the n-AlGaN CSL for the (0 0 0 2) and (10–12) planes, respectively, were reduced to 346 and 431 arcsec and the total TDDs were suppressed to approximately ~1 × 109 cm−2. Finally, when the conventional Ni (20 nm)/Au (150 nm) p-electrodes were replaced with new Ni (1 nm)/Mg (200 nm) p-electrodes in the n-AlGaN SL-based UV-A LEDs, the maximum output power was improved from 2.1 to 2.5 mW.

The Effect of Buffer Types on the In0.82Ga0.18As Epitaxial Layer Grown on an InP (100) Substrate.

In0.82Ga0.18As epitaxial layers were grown on InP (100) substrates at 530 °C by a low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The effects of different buffer structures, such as a single buffer layer, compositionally graded buffer layers, and superlattice buffer layers, on the crystalline quality and property were investigated. Double-crystal X-ray diffraction (DC-XRD) measurement, Raman scattering spectrum, and Hall measurements were used to evaluate the crystalline quality and electrical property. Scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM) were used to characterize the surface morphology and microstructure, respectively. Compared with the In0.82Ga0.18As epitaxial layer directly grown on an InP substrate, the quality of the sample is not obviously improved by using a single In0.82Ga0.18As buffer layer. By introducing the graded InxGa1−xAs buffer layers, it was found that the dislocation density in the epitaxial layer significantly decreased and the surface quality improved remarkably. In addition, the number of dislocations in the epitaxial layer greatly decreased under the combined action of multi-potential wells and potential barriers by the introduction of a In0.82Ga0.18As/In0.82Al0.18As superlattice buffer. However, the surface subsequently roughened, which may be explained by surface undulation.

GaSb and GaSb/AlSb Superlattice Buffer Layers for High-Quality Photodiodes Grown on Commercial GaAs and Si Substrates

The objective of this work is the integration of InGaAs/GaSb/GaAs heterostructures, with high indium content, on GaAs and Si commercial wafers. The design of an interfacial misfit dislocation array, either on GaAs or Si substrates, allowed growth of strain-free devices. The growth of purposely designed superlattices with their active region free of extended defects on both GaAs and Si substrates is demonstrated. Transmission electron microscopy technique is used for the structural characterization and plastic relaxation study. In the first case, on GaAs substrates, the presence of dopants was demonstrated to reduce several times the threading dislocation density through a strain-hardening mechanism avoiding dislocation interactions, while in the second case, on Si substrates, similar reduction of dislocation interactions is obtained using an AlSb/GaSb superlattice. The latter is shown to redistribute spatially the interfacial misfit dislocation array to reduce dislocation interactions.

High quality Ge epilayer on Si (1 0 0) with an ultrathin Si1−xGex/Si buffer layer by RPCVD

The authors report a method to grow high quality strain-relaxed Ge epilayer on a combination of low temperature Ge seed layer and Si1−xGex/Si superlattice buffer layer by reduced pressure chemical vapor deposition system without any subsequent annealing treatment. Prior to the growth of high quality Ge epilayer, an ultrathin Si1−xGex/Si superlattice buffer layer with the thickness of 50 nm and a 460 nm Ge seed layer were deposited successively at low temperature. Then an 840 nm Ge epilayer was grown at high deposition rate with the surface root-mean-square roughness of 0.707 nm and threading dislocation density of 2.5 × 106 cm−2, respectively. Detailed investigations of the influence of ultrathin low-temperature Si1−xGex/Si superlattice buffer layer on the quality of Ge epilayer were performed, which indicates that the crystalline quality of Ge epilayer can be significantly improved by enhancing the Ge concentration of Si1−xGex/Si superlattice buffer layer.

Effect of InSb/In0.9Al0.1Sb superlattice buffer layer on the structural and electronic properties of InSb films

The effect of InSb/In0.9Al0.1Sb buffer layers on InSb thin films grown on GaAs (001) substrate by molecular beam epitaxy (MBE) is investigated. The crystal quality and the surface morphology of InSb are characterized by XRD and AFM. The carrier transport property is researched through variable temperature hall test. The sharp interface between InSb/In0.9Al0.1Sb is demonstrated important for the high quality InSb thin film. We try different superlattice buffer layers by changing ratios, 2–0.5, thickness, 300–450nm, and periods, 20–50. According to the function of the dislocation density to the absolute temperature below 150K with different periods of SL buffers, we can find that the number of periods of superlattice is a major factor to decrease the density of threading dislocations. With the 50 periods SL buffer layer, the electron mobility of InSb at the room temperature and liquid nitrogen cooling temperature is ∼63,000 and ∼4600 cm2/V s, respectively. We deduce that the interface in the SL structure works as a filter layer to prevent the dislocation propagating to the upper InSb thin films.

An Investigation of Carbon-Doping-Induced Current Collapse in GaN-on-Si High Electron Mobility Transistors

This paper reports the successful fabrication of a GaN-on-Si high electron mobility transistor (HEMT) with a 1702 V breakdown voltage (BV) and low current collapse. The strain and threading dislocation density were well-controlled by 100 pairs of AlN/GaN superlattice buffer layers. Relative to the carbon-doped GaN spacer layer, we grew the AlGaN back barrier layer at a high temperature, resulting in a low carbon-doping concentration. The high-bandgap AlGaN provided an effective barrier for blocking leakage from the channel to substrate, leading to a BV comparable to the ordinary carbon-doped GaN HEMTs. In addition, the AlGaN back barrier showed a low dispersion of transiently pulsed ID under substrate bias, implying that the buffer traps were effectively suppressed. Therefore, we obtained a low-dynamic on-resistance with this AlGaN back barrier. These two approaches of high BV with low current collapse improved the device performance, yielding a device that is reliable in power device applications.

(Invited) Green Synthesis of Wide Band Gap Single-Crystalline Oxide Semiconductors for Green Innovation

Wide band gap oxide semiconductors are basically “green” materials constituted with abundant and safe elements. Recent evolution has been brought by gallium oxide (Ga2O3) semiconductors, which possess wide band gap of about 5 eV and thus are promising for high-breakdown voltage power devices. Being supported by ideal homoepitaxial growth by MBE or HVPE on thermally-stable orthorhombic β-Ga2O3 substrates, excellent properties of MOSFETs and Schottky barrier diodes (SBDs) with high breakdown voltage as well as high temperature operation have been reported by collaboration of NICT, Tamura Co., Koha Co., and Tokyo University of Agricultural and Technology[1-3]. On the other hand, our interest is to develop safe, inexpensive, and environmental-friendly growth technology. This is the mist CVD technology, where we prepare water and/or alcohol solution of safe chemicals containing the target metal element (for Ga2O3, such as gallium chloride or gallium acetylacetonate) and the mist particles generated by ultrasonic atomization of the source solution are transferred to the growth chamber to cause oxidation reaction forming Ga2O3 on a substrate[4-6]. This technology allowed the growth of highly crystalline-quality corundum-structured Ga2O3 (α-Ga2O3) on sapphire substrates[5,6] which possess corundum structure, followed by the band gap tuning with α-(Al,Ga)2O3 and α-(In,Ga)2O3 alloys from 3.8 to 8.8 eV. In this presentation, the author reports the efforts for the property and conductivity control of Ga2O3-based single crystalline semiconductors by the green synthesis technology of mist CVD as well as the promising potential of these materials for devices leading green innovation. Unintentionally-doped (UID) α-Ga2O3 showed unmeasurable high resistivity. Sn doping is a promising technology for conductivity control. With improvements of the crystal quality, n-type conductivity has been controlled in the range of 1017-1019 cm-3 with the Hall mobility of upto 24 cm2/Vs. Test MOSFETs were fabricated and showed modulation of drain current by the gate voltage, but the mutual conductance was too small to be actually utilized and the off-operation was not performed. This was partly because the activation of donors is significantly affected by dislocation defects originating by heteroepitaxy on sapphire. Our efforts are directing to superlattice buffer layers and epitaxial later overgrowth (ELOG), and will be introduced at the meeting. Recently, SBDs showing very low on-resistance and high breakdown voltage of 0.1 mW×cm2 and 531 V or 0.4 mW×cm2 and 855 V, respectively, were reposted by FLOSFIA Inc. (Kyoto, Japan) using Sn-doped α-Ga2O3 grown by mist CVD[7], and this clearly indicates the potential of α-Ga2O3 for future green innovation. Besides α-Ga2O3, mist CVD has contributed the evolution of various wide band gap oxide semiconductors. One of the examples is corundum-structured α-In2O3, whose band gap is 3.8 eV, showing the x-ray diffraction (0006) rocking curve FWHM of as small as 185 arcsec and the electron Hall mobility of as high as 130 cm2/Vs. An α-In2O3 MOSFET demonstrated the modulation of drain current in terms of the gate voltage from -50 to +5 V with the on-off ratio of >106, while the gate leakage current was as small as <100 pA. Off-state breakdown voltage tends to be 70 V at the present stage. This was the clear demonstration that corundum-structured oxide semiconductors can work as electron devices. Another example is tin oxide, which can be either n-type (SnO2) or p-type (SnO) by controlling the growth conditions of mist CVD so that oxidation reaction is enhanced or reduced, respectively, by adding oxidizing or reducing agent. This is also an attractive advantage of mist CVD, where reaction atmosphere can be chemically controlled. It is the responsibility of us to promote green innovation by green materials synthesized by green technology. The combination of mist CVD and wide band gap oxide semiconductors can do, we hope. [1] M. Higashiwaki et al., Appl. Phys. Lett. 103, 123511 (2013). [2] K. Sasaki et al., IEEE Electron Device Lett. 34, 493 (2013). [3] M. Higashiwaki et al., Conference Digest; 73rd Device Research Conference, 2015, p.29. [4] S. Fujita et al., J. Cryst. Growth 401, 588 (2014). [5] D. Shinohara et al., Jpn. J. Appl. Phys. 47, 7311 (2008). [6] K. Kaneko et al., Jpn. J. Appl. Phys. 51, 020201 (2012). [7] M. Oda et al., presented at Int. Workshop Gallium Oxide and Related Materials, Kyoto, Japan (2015).

Growth of compressively‐strained GaN films on Si(111) substrates with thick AlGaN transition and AlGaN superlattice buffer layers

AbstractIn this paper, crack‐free GaN films with step‐graded AlGaN transition layer and AlGaN superlattice layer as buffer layers were grown on Si(111) substrate by metal‐organic chemical vapor deposition(MOCVD). The combination of both buffers effectively improves the properties of GaN layer. With the optimization of the buffer structures, high quality compressively‐strained GaN layers with thickness up to 3.6 μm have been obtained on Si(111) substrates (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

(Invited) Potential and Activities of III-V/Si Tandem Solar Cells

III-V compound/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. However, problems to be solved are high density of dislocations and large residual strain in III-V compound on Si in the case of hetero epitaxy of III-V compound materials on Si substrates. There are other approaches such as epitaxial lift-off and direct bonding technologies in order to prevent such problems. This paper presents efficiency potential of III-V/Si tandem solar cells with efficiencies of more than 35% under 1-sun AM1.5G and effects of dislocation upon solar cell properties of III-V-on-Si single-junction solar cells and III-V/Si tandem solar cells. Because III-V/Si system has large misfit and difference in thermal expansion coefficient, generation of misfit and thermal stress induced dislocations affect on solar cell properties. Effects of dislocations upon minority-carrier lifetime are derived from considering one dimensional transport of minority carriers to dislocations. In order to realize high efficiency solar cells with similar efficiency by homo-epitaxially grown solar cells, low density dislocation of less than 3x105 cm-2 is necessary. The paper also reviews approaches on reduction in dislocation density in III-V compound films on Si and improvements in efficiencies of III-V compound single-junction solar cells on Si substrates and III-V/Si tandem solar cells. Especially, effective dislocation density reduction in III-V compound films on Si substrates by thermal cycle annealing (TCA) and insertion of strained-layer super-lattices (SLS) in our study are demonstrated. As a result of combination of TCA process and SLS insertion, high quality GaAs films with a dislocation density of 1-2x106 cm-2 have been realized. Reduction in dislocation density of GaAs-on-Si layers due to TCA and SLS insertion have been explained by coalescence of dislocations under high temperature and high stress condition and dislocation movement towards edge and interface by mismatch stress, respectively. In addition, effectiveness of thicker-layer growth, selective area growth, low-temperature growth for hetero epitaxy of III-V compound films on Si substrates are also reviewed. We have demonstrated 20% efficiency (under 1-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si. By employing high quality GaAs films on Si, heteroface GaAs thin films solar cells with an Al0.8Gao.2As window layer were fabricated on Si substrates. The thickness of Si substrates was 200mm. The n-GaAs base layer (carrier concentration of 5x1017 cm-3) with a thickness of 1.3-1.5mm was grown on an Al0.6Ga0.4As/GaAs super lattice (SL) layer (20nm/100nm x 5pairs), In0.1Ga0.9 As/GaAs SLS (10nm/10nm x 5 pairs), and thermal cycle annealed GaAs buffer layer (TCA:9000C x 5 times, 2mm thick) which were grown on Si substrates. The thickness and carrier concentration of p-GaAs emitter layer were 0.5 mm and 2x1018 cm-3, respectively. The total epitaxial layer thickness was about 3.5-5 mm. A Si3N4 layer was used as an anti-reflection film. Ti/Ag was used as electrode metal. The cell area was 1 cm2. The total-area conversion efficiency of 18.3% at AM0 and 20.0% at AM1.5G were the highest ever reported for GaAs-on-Si single-junction solar cells. Such high-efficiency GaAs-on-Si cells have been attained by combining TCA, SLS and SL buffer layers. As a result of such a combination, high efficiency GaAs-on-Si solar cells have been realized using high quality GaAs films with a dislocation density of 3.7x106 cm-2. The first space flight of forty-eight 2 cm x 2 cm GaAs-on-Si solar cells with an average AM0 total-area efficiency of 16.9% have been carried out using the Engineering Test Satellite No. 6 (ETS-VI). The GaAs-on-Si solar cells have been demonstrated to be more radiation-resistant on space than GaAs-on-GaAs solar cells and 50, 100 and 200-mm-thick Si space solar cells. These results show that the III-V/Si-based multi-junction solar cells have great potential for space applications. Ohio State University has also demonstrated 17.1% (AM0) efficiency with GaAs single-junction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding. Future prospects are also discussed. Further improvement in efficiency for GaAs-on-Si single-junction solar cells and III-V/Si multi-junction solar cells can be obtained by reducing dislocation densities to less than 3x105 cm-2.

Structure and Strain Properties of GaN Films Grown on Si (111) Substrates with AlxGa1−xN/AlyGa1−yN Superlattices**Supported by the National Natural Science Foundation of China under Grant Nos 61076120 and 61106130, and the Natural Science Foundation and Scientific Support Plan of Jiangsu Province under Grant Nos BK2012516, BK20131072, and BE2012007.

GaN films with an AlxGa1−xN/AlyGa1−yN superlattice (SL) buffer layer are grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD). The structure and strain properties of the samples are studied by optical microscopy, Raman spectroscopy, x-ray diffractometry and atomic force microscopy. The results show that the strain status and crystalline quality of the GaN layers are strongly dependent on the difference of the Al composition between AlxGa1−xN barriers and AlyGa1−yN wells in the SLs. With a large Al composition difference, the GaN film tends to generate cracks on the surface due to the severe relaxation of the SLs. Otherwise, when using a small Al composition difference, the crystalline quality of the GaN layer degrades due to the poor function of the SLs in filtering dislocations. Under an optimized condition that the Al composition difference equals 0.1, the crack-free and compressive strained GaN film with an improved crystalline quality is achieved. Therefore, the AlxGa1−xN/AlyGa1−yN SL buffer layer is a promising buffer structure for growing thick GaN films on Si substrates without crack generation.

Ultrathin low temperature Si0.75Ge0.25/Si buffer layer for the growth of high quality Ge epilayer on Si (100) by RPCVD

An approach to grow high quality strain-relaxed Ge by reduced pressure chemical vapor deposition system has been proposed in this paper. Prior to epitaxial growth high quality Ge layer, an ultrathin Si0.75Ge0.25/Si superlattice buffer layer with thickness of 54nm and a 460nm Ge seed layer were deposited successively at low temperature. Then an 840nm thickness Ge layer was grown at high temperature for faster deposition rate. After achieving high-temperature annealing at hydrogen atmosphere, the root-mean-square roughness of the epitaxial Ge lowers to 0.61nm at 10×10μm2 scan area, and the threading dislocation density is 1×106cm−2. Micro-Raman spectra manifests the extremely uniform distribution of tensile strain in Ge layer at room temperature, which can be contributed to the higher thermal expansion coefficient of Ge than that of Si. In contrast to the epitaxial Ge sample without buffer layer, the crystal quality and Hall hole mobility of the Ge epilayer incorporated ultrathin Si0.75Ge0.25/Si buffer improve significantly, indicating that the ultrathin low temperature Si0.75Ge0.25/Si superlattice buffer layer plays an important role on fabricating high quality Ge epilayer.

Optimizations of ZnO/Si(100) with ZnO/ZnMgO Super Lattice Buffer Layers Grown by Molecular Beam Epitaxy

ZnO with ZnO/ZnMgO super lattice buffer layers grown by molecular beam epitaxy on Si (100) substrates at room temperature were studied by reversing the epitaxial sequence of ZnO or ZnMgO in the super lattice buffer layers, tuning the oxygen power and the vacuum pressure. The crystal quality was improved by supper lattice buffer layers started with ZnO, using higher oxygen power, and proper vacuum pressure in the growth chamber.