Intersubband transition at 1.55 μm in AlN/GaN multiple quantum wells by metal organic vapor phase epitaxy using the pulse injection method at 770 °C

Intersubband transition (ISBT) at 1.55 μm in AlN/GaN multi quantum wells (MQWs) was realized by metal organic vapor phase epitaxy (MOVPE) using the pulse injection (PI) method to grow GaN well layers at 770 °C. It was shown that a main factor for shifting ISBT wavelength to shorter region to cover 1.55 μm and improving ISBT properties of MQWs is the growth temperature of MQWs. Best structural and ISBT properties are observed at low growth temperature of 770 °C in this study. Carbon incorporation level in GaN layer grown by the PI method (PI-GaN) showed one order smaller value compared with that by the conventional continuous method. Moreover, further decrease in growth temperature to 770 °C did not show significant increase in carbon incorporation in PI-GaN layer. It clearly indicates that the PI method is very effective in reducing carbon concentration in GaN layer, especially at low temperature region. The low carbon concentration of 4×10¹⁸ cm^?3 released by the PI method was indispensable for realizing enough carrier concentration of 1.6×10¹⁹ cm^?3 to achieve strong ISBT at 1.55 μm.     

Growth of thick,continuous GaN layers on 4-in. Si substrates by metalorganic chemical vapor deposition

We report on the growth of thick GaN epilayers on 4-in. Si(1 1 1) substrates by metalorganic chemical vapor deposition. Using intercalated AlN layers that contribute to counterbalance the tensile strain induced by the thermal mismatch between gallium nitride and the silicon substrate, up to 6.7 μm thick crack-free group III-nitride layers have been grown. Root mean-squares surface roughness of 0.5 nm, threading dislocation densities of 1.1×10⁹ cm^?2, as well as X-ray diffraction (XRD) full widths at half-maximum (FWHM) of 406 arcsec for the GaN(0 0 2) and of 1148 arcsec for the GaN(3 0 2) reflection have been measured. The donor bound exciton has a low-temperature photoluminescence line width of 12 meV. The correlation between the threading dislocation density and XRD FWHM, as well as the correlation between the wafer curvature and the GaN in-plane stress is discussed. An increase of the tensile stress is observed upon n-type doping of GaN by silicon.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Fabrication of 3-in GaN substrates by hydride vapor phase epitaxy using void-assisted separation method

By using the hydride vapor phase epitaxy and a void-assisted separation method, freestanding 3-in GaN substrates were successfully fabricated for the first time, and the process showed an excellent reproducibility. A thick GaN layer 3.2 in in diameter was easily separated from the base substrate. No cracks were generated during the separation process. The dislocation density was of the order of 10⁶ cm^–2. The carrier density was approximately 1×10¹⁸ cm^–3 and the mobility was 3.4×10² cm² V^–1 s^–1. The concentrations of impurities, estimated by secondary-ion mass spectrometry, were below the limit of detection, except for Si. The Si concentration was approximately 1×10¹⁸ cm^–3, which is in good agreement with the carrier density.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Characterization of deep levels in a-plane GaN epi-layers grown using various growth techniques

To study the correlation between defects and deep levels in a-plane GaN films grown on r-plane sapphire substrates, transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) have been performed on three types of a-plane GaN samples grown using modified two-step growth (sample I), SiN_x interlayer (sample II), and patterned insulator on sapphire substrate (sample III). From the microstructure evolution in cross-sectional TEM images, it was shown that combination of growth techniques is highly efficient in the reduction of dislocation densities. Average dislocation densities of samples I, II, and III were about 2.2×10⁹ cm^?2, 1.1×10⁹ cm^?2, and 3.4×10⁸ cm^?2, respectively. The trap a_t E_c–E_t～0.13 eV (E1) was observed in only sample I, and three electron traps at 0.28–0.33 eV (E2), 0.52–0.58 eV (E3), and 0.89–0.95 eV (E4) from the conduction band edge were measured common to all the samples. The analysis of trap properties indicated that E2 and E3 trap levels are strongly associated with the partial dislocations in a-plane GaN films.     

Platinum-catalyst-assisted metalorganic vapor phase epitaxy of InN

This paper reports the first attempt of the Pt-catalyst-assisted MOVPE growth of InN. In order to enhance NH₃ decomposition at a relatively low growth temperature (～550 °C), Pt is used as a catalyst. The catalyst is installed in the NH₃ introduction tube in the MOVPE reactor and the tube is located just above the susceptor to be heated. Compared with InN films grown without the catalyst, the samples grown with Pt catalyst show improved electrical properties; a carrier concentration in the order of 10¹⁸ cm^?3 and a Hall mobility as high as 1350 cm²/Vs are obtained. The crystalline quality is also improved by employing the catalyst and a tilt fluctuation as low as 8.6 arcmin is obtained for a sample grown on a GaN/sapphire template. It is confirmed that for InN films grown at 550 °C with Pt catalyst, the electrical and crystallographic properties are also improved with increase in thickness. These results indicate that the growth at around 550 °C with the Pt catalyst is performed under the circumstances where NH₃ is effectively decomposed, whereas the deterioration of InN during growth is significantly suppressed.     

Metalorganic vapor phase epitaxy and characterizations of nearly-lattice-matched AlInN alloys on GaN/sapphire templates and free-standing GaN substrates

The epitaxy optimization studies of high-quality n-type AlInN alloys with different indium contents grown on two types of substrates by metalorganic vapor phase epitaxy (MOVPE) were carried out. The effect of growth pressure and V/III molar ratio on growth rate, indium content, and surface morphology of these MOVPE-grown AlInN thin films were examined. The surface morphologies of the samples were characterized by scanning electron microscopy and atomic force microscopy. By varying the growth temperatures from 860 °C to 750 °C, the indium contents in AlInN alloys were increased from 0.37% up to 21.4% as determined by X-ray diffraction (XRD) measurements. The optimization studies on the growth conditions for achieving nearly-lattice-matched AlInN on GaN templates residing on sapphire and free-standing GaN substrates were performed, and the results were analyzed in a comparative way. Several applications of AlInN alloy for thermoelectric and light-emitting diodes are also discussed.     

InOx semiconductor films deposited on glass substrates for transparent electronics

Transparent undoped semiconductor indium oxide films were deposited by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium at low substrate temperature. It was experimentally verified that the variation of rf power density has a strong influence on the electrical and structural properties of the films. The thickness of the InO_x films is of about 100 nm. Results show that InO_x films show an average visible transmittance of about 85% and energy gap of about 2.6 eV. Structural and electrical conductivity measurements show that films are polycrystalline and there exists a linear variation of conductivity logarithm vs reciprocal of temperature. Electrical conductivity variation of 17.6 to 5.8 × 10⁻³ (Ω cm)⁻¹ for films produced at rf power densities ranging from 3.9 to 78.1 mW cm⁻³ was obtained. This controllable semiconductor behavior can therefore satisfy the requirement of a particular application for these type of films.     

Thermally reversing window in Ge15Te85 − xInx glasses: Nanoindentation and micro-Raman studies

Nanoindentation studies on Ge₁₅Te_85 ? xIn_x glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3 ≤ x ≤ 7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge₁₅Te_85 ? xIn_x glasses in the composition range 3 ≤ x ≤ 7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Further, micro-Raman studies reveal that as-quenched Ge₁₅Te_85 ? xIn_x samples exhibit two prominent peaks, at 123 cm^? 1 and 155 cm^? 1. In thermally annealed samples, the peaks at 120 cm^? 1 and 140 cm^? 1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm^? 1 and 141 cm^? 1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-aging and are more stable.     

Structural,electrical and optical properties of in-situ phosphorous-doped Ge layers

We have studied the impact of temperature and pressure on the structural and electronic properties of Ge:P layers grown with GeH₄+PH₃ on thick Ge buffers, themselves on Si(0 0 1). The maximum phosphorous atomic concentration [P] exponentially decreased as the growth temperature increased, irrespective of pressure (20 Torr, 100 Torr or 250 Torr). The highest values were however achieved at 100 Torr (3.6×10²⁰ cm^?3 at 400 °C, 2.5×10¹⁹ cm^?3 at 600 °C and 10¹⁹ cm^?3 at 750 °C). P atomic depth profiles, “box-like” at 400 °C, became trapezoidal at 600 °C and 750 °C, most likely because of surface segregation. The increase at 100 Torr of [P] with the PH₃ mass-flow, almost linear at 400 °C, saturated quite rapidly at much lower values at 600 °C and 750 °C. Adding PH₃ had however almost no impact on the Ge growth rate (be it at 400 °C or 750 °C). A growth temperature of 400 °C yielded Ge:P layers tensily-strained on the Ge buffers underneath, with a very high concentration of substitutional P atoms (5.4×10²⁰ cm^?3). Such layers were however rough and of rather low crystalline quality in X-ray Diffraction. Ge:P layers grown at 600 °C and 750 °C had the same lattice parameter and smooth surface morphology as the Ge:B buffers underneath, most likely because of lower P atomic concentrations (2.5×10¹⁹ cm^?3 and 10¹⁹ cm^?3, respectively). Four point probe measurements showed that almost all P atoms were electrically active at 600 °C and 750 °C (1/4th at 400 °C). Finally, room temperature photoluminescence measurements confirmed that high temperature Ge:P layers were of high optical quality, with a direct bandgap peak either slightly less intense (750 °C) or more intense (600 °C) than similar thickness intrinsic Ge layers. In contrast, highly phosphorous-doped Ge layers grown at 400 °C were of poor optical quality, in line with structural and electrical results.     

Growth temperature induced effects in non-polar a-plane GaN on r-plane sapphire substrate by RF-MBE

Non-polar a-plane GaN films were grown on an r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The effect of growth temperature on structural, morphological and optical properties has been studied. The growth of non-polar a-plane (1 1 ?2 0) orientation of the GaN epilayers were confirmed by high resolution X-ray diffraction (HRXRD) study. The X-ray rocking curve (XRC) full width at half maximum of the (1 1 ?2 0) reflection shows in-plane anisotropic behavior and found to decrease with increase in growth temperature. The atomic force micrograph (AFM) shows island-like growth for the film grown at a lower temperature. Surface roughness has been decreased with increase in growth temperature. Room temperature photoluminescence shows near band edge emission at 3.434–3.442 eV. The film grown at 800 °C shows emission at 2.2 eV, which is attributed to yellow luminescence along with near band edge emission.     

Growth kinetics and boron doping of very high Ge content SiGe for source/drain engineering

We have studied the in-situ boron doping of high Ge content Si_1?xGe_x layers (x=0.3, 0.4 and 0.5). These layers have been grown at low pressure (20 Torr) and low temperature (600–650 °C) with a heavily chlorinated chemistry on blanket Si(0 0 1) substrates. Such a chemistry yields a full selectivity versus SiO₂ (isolation) and Si₃N₄ (sidewall spacers) on patterned wafers with gate stacks. We have quantified the impact of the diborane flow on the SiGe layer crystalline quality, its resistivity, the SiGe:B growth rate and the apparent Ge concentration. Resistivity values lower than 1 mΩ cm are easily achieved, all the more so for high Ge content layers. The SiGe growth rate increases and the apparent Ge concentration (from X-ray diffraction) decreases as the diborane flow increases. B atoms (much smaller than Si or Ge) indeed partially compensate the compressive strain in the SiGe:B layers. We have also probed the in-situ boron and phosphorus doping of Si at 750 °C, 20 Torr with a heavily chlorinated chemistry. The B ions concentration increases linearly with the diborane flow, then saturates at a value close to 4×10¹⁹ cm^?3. By contrast, the P ions concentration increases sub-linearly with the phosphine flow, with a maximum value close to 9×10¹⁸ cm^?3. Adding diborane (phosphine) to the gaseous mixture leads to a sharp increase (decrease) of the Si:B (the Si:P) growth rates, which has to be taken into account in device layers. All the know-how acquired will be most handy for the formation of in-situ doped recessed or raised sources and drains in metal-oxide semiconductor devices.     

AlGaN/GaN HEMTs regrown by MBE on epi-ready semi-insulating GaN-on-sapphire with inhibited interface contamination

In this work, we show that, by carefully designing the subsurface Fe doping profile in SI-GaN templates grown by MOVPE and by optimizing the MBE regrowth conditions, a highly resistive GaN buffer can be achieved on these epi-ready GaN-on-sapphire templates without any addition of acceptors during the regrowth. As a result, high-quality high electron mobility transistors can be fabricated. Furthermore, we report on the excellent properties of two-dimensional electron gas and device performances with electron mobility greater than 2000 cm²/V s at room temperature and off-state buffer leakage currents as low as 5 μA/mm at 100 V.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

Hydrogenated amorphous and nanocrystalline silicon solar cells deposited by HWCVD and RF-PECVD on plastic substrates at 150 °C

In this work we present a study of the structural, optoelectronic and transport properties of a series of Si films deposited in a parameter region (namely hydrogen dilution) corresponding to a transition from amorphous-to-nanocrystalline silicon by hot-wire (HW) and radio-frequency plasma enhanced chemical vapor deposition (RF) on plastic substrates at 150 °C. To achieve a higher deposition rate of Si films by RF we used a relatively high power density (350 mW/cm²) and deposition pressure (1.5 Torr). For certain hydrogen dilution values, these deposition conditions can lead to the formation of Si crystals in the silane plasma and to a growth of polymorphous silicon film. This material has improved carrier transport properties (ambipolar diffusion length = 220 nm) and very high photosensitivity (>5 × 10⁶). The best HW amorphous silicon films exhibited lower photosensitivity (7 × 10⁴) and an ambipolar diffusion length of only 100 nm. For solar cell fabrication, we optimized the RF deposition conditions to produce very thin amorphous and nanocrystalline phosphorous and boron doped silicon layers. Our best n–i–p solar cell, with a polymorphous Si intrinsic layer deposited on plastic, has an efficiency of 5.5%, FF = 52.5%, V_OC = 920 mV, J_SC = 11.6 mA/cm². For solar cells with a nanocrystalline Si active layer deposited on glass the following results were achieved: efficiency = 3.4%, FF = 43.5%, V_OC = 460 mV, J_SC = 17.2 mA/cm²; and on plastic substrate: efficiency = 2.2%, FF = 32.7%, V_OC = 397 mV, J_SC = 17.2 mA/cm².     

Growth and characterization of InxBi2−xTe3 single crystals

The V–VI group narrow band gap compounds are known to have important photoconductivity and thermoelectric properties. Among these, Bi₂Te₃ is the most potential material for thermoelectric devices having a direct band gap of 0.16 eV. There has been ample study reported on crystal growth and polycrystalline thin films of both pure and indium doped Bi₂Te₃ pertaining to its basic semiconducting, optoelectronic and thermoelectric properties. It has been shown that on exceeding certain limiting concentration of indium in Bi₂Te₃, the conductivity changes from p-type to n-type. However, there is hardly any work reported in literature on crystal growth, dislocation etching and optical band gap of In_xBi_2?xTe₃ (x=0.1, 0.2, 0.5) single crystals. The authors have grown their single crystals using the zone melting method. The freezing interface temperature gradient of 70 °C/ cm^?1 has been found to yield the best quality crystals obtainable at the growth rate of 0.4 cm/h. The as-grown crystals have been observed to exhibit certain typical features on their top free surfaces. The crystals have been characterized using XRD technique. A chemical dislocation etchant has been used for estimating perfection in terms of dislocation density in the crystals. The optical absorption was measured in the wave number range 500 to 4000 cm^?1. The transitions in all the cases were observed to be allowed direct type. The detailed results are reported in the paper.     

Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces

Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O₂ system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO_x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O₂ fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.