Growth of InxGa1−xN quantum dots by nitridation of nano-alloyed droplet method using MOCVD

In_xGa_1−xN quantum dots (QDs) were grown on GaN/sapphire (0 0 0 1) substrates by employing nitridation of nano-alloyed droplet (NNAD) method using metal-organic chemical vapor deposition (MOCVD). In+Ga alloy droplets were initially formed by flowing the precursors TMIn and TMGa. Density of the In+Ga alloy droplets was increased with increasing precursors flow rate; however, the droplet size was scarcely changed in the range of about 100–200 nm. Two cases of In_xGa_1−xN QDs growth were investigated by varying the nitridation time and the growth temperature. It was observed that the In_xGa_1−xN QDs size can be easily changed by controlling the nitridation process at the temperature between 680 and 700 °C for the time of 5–30 min. Self-assembled In_xGa_1−xN QDs were successfully grown by employing NNAD method.     

Mg doping behavior of MOVPE InxGa1−xN (x∼0.4)

The Mg doping behavior of MOVPE indium gallium nitride (InGaN), such as secondary ion mass spectrometry (SIMS) Mg profile, crystalline quality and n–p conversion of the films are described and discussed in this paper. The SIMS analysis reveals that the memory effect of Cp₂Mg as a doping source deteriorates the controllability of Mg doping level and profile, especially for thin (−0.4 μm) InGaN. The high residual donors (10¹⁹–10²⁰ cm⁻³) in InGaN with In content from 0.05 to 0.37 can be compensated by Mg doping and p-type conduction is obtained for those with In content up to 0.2. It is found that a higher Cp₂Mg flow rate is needed to get p-type conduction in InGaN with a higher In content x; for example, Cp₂Mg/(TEG+TMI)≈0.5% for x=0 (GaN), ≈2% for x=0.05 and ≈4% for x=0.2. Such a high Cp₂Mg flow rate is needed due to the high residual donor concentration (10¹⁹–10²⁰ cm⁻³) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN is deteriorated with increasing In content as well as Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it is essential to improve the crystalline quality of non-doped InGaN. For this purpose, the use of a thicker GaN interlayer is effective.     

Low-temperature RPCVD of Si,SiGe alloy,and Si1−yCy films on Si substrates using trisilane (Silcore)

Si homo-epitaxial growth by low-temperature reduced pressure chemical vapor deposition (RPCVD) using trisilane (Si₃H₈) has been investigated. The CVD growth of Si films from trisilane and silane on Si substrates are compared at temperatures between 500 and 950 °C. It is demonstrated that trisilane efficiency increases versus silane's one as the surface temperature decreases. Si epilayers from trisilane, with low surface roughness, are achieved at 600 and 550 °C with a growth rate equal to 12.4 and 4.3 nm min⁻¹, respectively. It is also shown that Si_1−xGe_x layers can be deposited using trisilane chemistry.     

Growth kinetics of SiGe/Si superlattices on bulk and silicon-on-insulator substrates for multi-channel devices

We have studied in reduced pressure chemical vapor deposition the growth kinetics of Si and Si_0.8Ge_0.2 on bulk Si(0 0 1) and on silicon-on-insulator (145 nm buried oxide/20 nm Si over-layer) substrates. For this, we have grown at 650 °C, 20 Torr 19 periods (Si_0.8Ge_0.2 19 nm/Si 32 nm) superlattices on both types of substrates that we have studied in secondary ion mass spectrometry, X-ray diffraction and cross-sectional transmission electron microscopy. The Si and SiGe growth rates together with the Ge content are steady on bulk Si(0 0 1), with mean values around 9.5 nm min⁻¹ and 20.2%, respectively. In contrast, growth rates decrease from ∼9.5 nm min⁻¹ down to values around 7.0 nm min⁻¹ (SiGe) and 6.3 nm min⁻¹ (Si), when the deposited thickness on SOI increases from 0 up to slightly more than 100 nm. They then go back up to values around 8.8–9.0 nm min⁻¹ as the thickness increases from 100 up to 400 nm. They then slowly decrease to values around 8.4–8.6 nm min⁻¹ as the thickness increases from 400 up to 800 nm. The Ge concentration follows on SOI exactly the opposite trend: an increase from 19.9% (0 nm) up to 20.6% (∼100 nm) followed by a decrease to values around 20.1% (400 nm) then a slow re-increase up to 20.4% (800 nm). These fluctuations are most likely due to the following SOI surface temperature variations: from 650 °C down to 638 °C (100 nm), back up to 648 °C (400 nm) followed by a slow decrease to 646 °C (800 nm). These data curves will be most useful to grow on conventional SOI substrates large number of periods, regular Si/Si_0.8Ge_0.2 superlattices that will serve as the core of multi-channel or three-dimensional nano-wires field effect transistors.     

High quality InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

High quality InAlN/GaN heterostructures are successfully grown on the (0 0 0 1) sapphire substrate by pulsed metal organic chemical vapor deposition. The InAlN barrier layer with an indium composition of 17% is observed to be nearly lattice matched to GaN layer, and a smooth surface morphology can be obtained with root mean square roughness of 0.3 nm and without indium droplets and phase separation. The 50 mm InAlN/GaN heterostructure wafer exhibits a mobility of 1402 cm²/V s with a sheet carrier density of 2.01×10¹³  cm^?2, and a low average sheet resistance of 234 Ω/cm² with a sheet resistance nonuniformity of 1.22%. Compared with the conventional continual growth method, PMOCVD reduces the growth temperature of the InAlN layer and the Al related prereaction in the gas phase, and consequently enhances the surface migration, and improves the crystallization quality. Furthermore, indium concentration of InAlN layer can be controlled by adjusting the pulse time ratio of TMIn to TMAl in a unit cycle, the growth temperature and pressure, as well as the InAlN layer thickness by the number of unit cycle repeats.     

Growth and process modeling studies of nickel-catalyzed metalorganic chemical vapor deposition of GaN nanowires

A combination of experimental and computational fluid dynamics-based reactor modeling studies were utilized to study the effects of process conditions on GaN nanowire growth by metalorganic chemical vapor deposition (MOCVD) in an isothermal tube reactor. The GaN nanowires were synthesized on (0 0 0 1) sapphire substrates using nickel thin films as a catalyst. GaN nanowire growth was observed over a furnace temperature range of 800–900 °C at V/III ratios ranging from 33 to 67 and was found to be strongly dependent on the position of the substrate relative to the group III inlet tube. The modeling studies revealed that nanowire growth consistently occurred in a region in the reactor where the GaN thin-film deposition rate was reduced and the gas phase consisted primarily of intermediate species produced by the reaction and decomposition of trimethylgallium–ammonia adduct compounds. The GaN nanowires exhibited a predominant [1 1 2¯ 0] growth direction. Photoluminescence measurements revealed an increase in the GaN near-band edge emission intensity and a reduction in the deep-level yellow luminescence with increasing growth temperature and V/III ratio.     

Formation of GaN nanodots on Si (1 1 1) by droplet nitridation

GaN nanodots (NDs) are obtained by Ga metallic droplet formation on Si (1 1 1) substrates followed by their nitridation. The size and density of Ga droplets and GaN NDs can be controlled by varying the growth temperature within the range 514–640 °C. Atomic force microscopy (AFM) investigation of Ga droplets shows an increase in the average diameter with temperature. The average diameter of GaN NDs increases with growth temperature while their density decreases more than one order of magnitude. In addition, the formation of a GaN crystallite rough layer on Si, in-between NDs, indicates that a spreading mechanism takes place during the nitridation process. High-resolution transmission electron microscopy (HRTEM) is used for the investigation of shape, crystalline quality and surface distribution of GaN dots. X-ray photoelectron spectroscopy (XPS) results confirm that Ga droplets that are transformed into GaN NDs spread over the sample surface during nitridation.     

Fabrication of freestanding m-plane GaN wafer by using the HVPE technique on an aluminum carbide buffer layer on an m-plane sapphire substrate

A freestanding m-plane GaN wafer is fabricated by using the hydride vapor-phase epitaxy (HVPE) technique on an aluminum carbide buffer layer on an m-plane sapphire substrate. X-ray pole-figure measurements show a clear m-plane orientation of the GaN surface. The full-width at half-maximum (FWHM) of GaN (1 1¯ 0 0) X-ray rocking curve (XRC) with the scattering vector along the [1 1 2¯ 0] direction is approximately 800 arcsec; this indicates good crystallinity. On the other hand, the FWHM for the case in which the scattering vector is oriented along the [0 0 0 1] direction is broad; this suggests the influence of structural defects along this direction. In fact, basal plane stacking faults (BSF) with a density of approximately 3×10⁵ cm⁻¹ is observed by transmission electron microscopy (TEM). The preparation of a 45-mm-diameter m-plane GaN wafer due to spontaneous separation of the GaN layer from the sapphire substrate is demonstrated.     

One-sidewall-seeded epitaxial lateral overgrowth of a-plane GaN by metalorganic vapor-phase epitaxy

The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (1 1 2¯ 0) GaN on r-plane (1 1¯ 0 2) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 10⁶–10⁷ cm⁻² and 10³–10⁴ cm⁻¹, respectively, were successfully realized.     

Structural and electrical properties of Si-doped a-plane GaN grown on r-plane sapphire by MOVPE

Approximately 2-μm-thick Si-doped a-plane GaN films with different doping concentrations were grown on approximately 8-μm-thick undoped a-plane GaN/r-sapphire by metal organic vapor phase epitaxy (MOVPE). The structural and electrical properties of the Si-doped a-plane GaN films were investigated by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and temperature-dependent Hall measurement. The results showed that a small amount of Si doping can improve the surface morphology and decrease the density of pits. Upon increasing the CH₃SiH₃ flow rate, the crystalline quality of the (0 0 0 2) plane was slightly improved. The highest room-temperature mobility of 83.4 cm²/Vs was obtained at a carrier density of 6.2×10¹⁸ with a CH₃SiH₃ flow rate of 10 sccm.     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

Molecular beam epitaxy of crystalline and amorphous GaN layers with high As content

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN_1−xAs_x alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As₂ flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from ∼3.4 to ∼1.35 eV with increasing As content. The energy gap reaches its minimum of ∼1.35 eV at x∼0.6–0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN_1−xAs_x alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN_1−xAs_x alloys have short-range ordering that resembles random crystalline GaN_1−xAs_x alloys.     

Mg segregation in a (1 1¯ 0 1) GaN grown on a 7° off-axis (0 0 1) Si substrate by MOVPE

Redistribution behavior of magnesium (Mg) in the N-terminated (1 1¯ 0 1) gallium nitride (GaN) has been investigated. A nominally undoped GaN layer was grown on a heavily Mg-doped GaN template by metalorganic vapor-phase epitaxy (MOVPE). Mg dopant profiles were measured by secondary ion mass spectrometry (SIMS) analysis. A slow decay of the Mg concentration was observed in the nominally undoped GaN layer due to the surface segregation. The calculated decay lengths of the (1 1¯ 0 1) GaN are ∼75–85 nm/decade. These values are shorter than the decay length determined in the sample grown on the Ga-terminated (0 0 0 1) GaN. This result indicates that Mg exhibited weak surface segregation in the (1 1¯ 0 1) GaN as compared to the (0 0 0 1) GaN. The weak surface segregation is in agreement with the high efficiency of Mg incorporation on the (1 1¯ 0 1) face. The high density of hydrogen was obtained in the (1 1¯ 0 1) GaN, which might enhance the Mg incorporation.     

Growth of GaN based structures on Si(1 1 0) by molecular beam epitaxy

The growth of GaN based structures on Si(1 1 0) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical and electrical properties of such structures are assessed and are quite similar to the ones obtained on Si(1 1 1) in-spite of the very different substrate surface symmetry. A threading dislocation density of 3.7×10⁹ cm⁻² is evaluated by transmission electron microscopy, which is in the low range of typical densities obtained on up to 2 μm thick GaN structures grown on Si(1 1 1). To assess the potential of such structure for device realization, AlGaN/GaN high electron mobility transistor and InGaN/GaN light emitting diode heterostructures were grown and their properties are compared with the ones obtained on Si(1 1 1).     

Effects of morphologies on the field emission characteristics of GaN nanorods grown on Si (0 0 1) by MBE

GaN nanorods were grown on Si (0 0 1) substrates with a native oxide layer by molecular beam epitaxy. The changes in the morphologies and their effects on the field emission characteristics of GaN nanorods were investigated by varying growth conditions, namely, growth time of low-temperature GaN buffer layer, growth time of GaN nanorods, Ga flux during growth of GaN nanorods, and growth temperature of GaN nanorods. GaN nanorods with a low aspect ratio measured by diode configuration showed better field emission characteristics than those with a high aspect ratio, which may be due to the effects of screening and the surface depletion layer. In addition, the distance between the GaN nanorods and the anode played an important role in the field emission characteristics such as turn-on field, field enhancement factor, and field distribution on the emitter surface.     

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10⁵ cm⁻², revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.     

InN excitonic deformation potentials determined experimentally

We report the experimental determination of the interband deformation potentials of indium nitride by combining both optical spectroscopy investigations and high-resolution X-ray measurements performed on a series of InN films grown by metal organic vapour-phase epitaxy. Our approach, which follows the one used for GaN by Shan et al. [Phys. Rev. B. 54 (1996) 13460], gives here for InN a₁=−7.66 eV, a₂=−2.59 eV, b₁=5.06 eV, and b₂=−2.53 eV.     

AlN buffer layer growth for GaN epitaxy on (1 1 1) Si: Al or N first?

AlN is generally used as buffer layer for the epitaxial growth of GaN on Si(1 1 1) substrate. In this work, we specifically address the relationship between the way the AlN growth is initiated on the Si(1 1 1) surface and the overall properties of the final GaN epitaxial layer. The growth is performed by molecular beam epitaxy with ammonia (NH₃) as nitrogen source. Two procedures have been compared: exposing the Si surface first to NH₃ or Al. The AlN nucleation is followed in real-time by reflection high-energy electron diffraction and critical stages are also investigated in real space using scanning tunnelling microscopy and transmission electron microscopy. Atomic force microscopy, X-ray diffraction and photoluminescence are also used to assess the properties of the final GaN epitaxial layer. It is shown that best results in terms of GaN overall properties are obtained when the growth is initiated by exposing the Si(1 1 1) surface to NH₃ first. This is mainly due to the fact that almost an order of magnitude decrease of the dislocation density is obtained.     

Ammonothermal epitaxy of wurtzite GaN using an NH4I mineralizer

Purely wurtzite phase needle crystals and epitaxial layers of GaN were grown by the ammonothermal method using an NH₄I mineralizer. The inclusion of zincblende phase GaN was effectively eliminated by increasing the growth temperature higher than 500 °C. Accordingly, an approximately 20-μm-thick GaN epitaxial layer was achieved on the Ga-polar face of a c-plane GaN seed wafer at 520 °C. Although the characteristic deep state emission band dominated the room temperature photoluminescence spectrum, the near-band-edge emission of GaN was observed for both the needle crystals and the epitaxial layers. These results encourage one to grow better quality GaN crystals at a high growth rate under high-temperature growth conditions.     

AlGaN epitaxial lateral overgrowth on Ti-evaporated GaN/sapphire substrate

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.