Metalorganic Chemical Vapor Deposition GaN Research Articles

Trap characterization of high-growth-rate laser-assisted MOCVD GaN

A detailed study comparing defect incorporation between laser-assisted metal-organic chemical vapor deposition (MOCVD)-grown GaN and conventional low- and high-growth-rate MOCVD GaN was conducted. Using deep-level transient and optical spectroscopy, traps throughout the bandgap were characterized where traps were found at EC-0.25 eV, EC-0.57 eV, EC-0.72 eV, EC-0.9 eV, EC-1.35 eV, EC-2.6 eV, and EC-3.28 eV in all three samples. This indicates no new traps were observed in the laser-assisted MOCVD GaN sample. Overall, the trap concentrations in the laser-assisted MOCVD sample were ∼2× higher than the optimal low-growth-rate sample, but this is primarily due to the increase in gallium vacancy EC-2.6 eV and carbon-related EC-3.28 eV trap concentrations. The EC-0.9 eV trap concentration was ∼2× higher in the laser-assisted sample, so proton irradiation experiments were conducted to identify the physical source of this level. The results indicated this was a native point defect likely related to gallium interstitials. Overall, this study shows that the laser-assisted MOCVD growth method is promising for future thick, high-quality GaN epilayers after further growth optimizations.

(Invited) Vertical GaN Diodes: Effect of Growth Parameters, Indium Surfactants, and Device Development for High-Power Electronics

GaN has achieved widespread interest in high-power and high-speed electronics due to its wide-bandgap (WBG), large breakdown field, and high mobility compared to Si and SiC. There is also continued interest in the growth and development of GaN vertical devices owing to their advantageous properties of high current capability from backside ohmic, better field management, scaling feasibility, and enhanced thermal transport, which is not feasible with the lateral device topology. To realize the predicted high breakdown voltage and low on-resistance from vertical devices, high-quality drift layers with low unintentional doping and low compensating defects are fundamentally required. Hence, our research aims to pursue a comprehensive investigation of growth and device development with vertical GaN structures, building from the optimization of thick GaN homoepitaxy with high growth rate but low impurities, improving surface morphology using Indium as a surfactant as well as realizing punch-through field-plate p-n diodes to maximize the average electric field in high-voltage operation.Toward the goal of high-quality GaN epitaxy growth, we performed systematic studies of growth rate effects on the drift layer doping using ammonia molecular beam epitaxy (NH3 MBE) and plasma-assisted molecular beam epitaxy (PA MBE) [1]. The NH3 MBE homoepitaxy GaN demonstrated an unintentional doping (UID) of ~1015 cm-3, significantly lower than that of the conventional metalorganic chemical vapor deposition (MOCVD) reports, which can be potentially contributed by clean growth environment of MBE.We further focused on improving surface morphology using Indium as a surfactant. Through a combination of varying V/III ratios, In flux and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved for fast growth rates (on the order of 1 µm/hr and beam equivalent pressures on the order of 5 × 10-7 Torr). However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant can also reduce silicon impurities as revealed from the Secondary ion mass spectrometry (SIMS) (Fig. 2).Subsequently, we developed vertical p-n GaN power devices using the NH3 MBE-grown high-quality epitaxy [2]. The vertical p-n GaN diode consisted (from bottom up): ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer, ~4 µm UID GaN drift layer for n-type region, ~0.4 µm p+GaN ([Mg]: 3×1019 cm-3) for p-type region, and ~10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact. Circular p-n diodes of 80-100 µm diameter were fabricated with Pd/Pt anode and a backside Ti/ Au ohmic contact. The field management was achieved by field-plate of 15 µm formed with Ti/Au metals on top of a field-plate dielectric of a Al2O3 (26 nm)/ Si3N4 (205 nm) stack (Fig. 3).The p-n GaN diodes exhibited an outstanding forward J-V behavior with a low specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and a minimum ideality factor of 1.36 (Fig. 4), which are among the best achieved in vertical homoepitaxy GaN p-n diodes [2-4]. The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). These reverse-voltage properties imply a punch-through characteristics with peak electric field (EC) >2.6 MV/cm, as revealed by Silvaco simulation (Fig. 5). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly downscaled (~4 um) drift layer compared to the MOCVD GaN p-n diodes of ≥ 8 um (Fig. 6). Work is ongoing to develop NHE MBE p-n diodes with indium surfactant-assisted drift layer growth which will be promising to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45×1018 cm−3 up to 1.10×1020 cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 Ω cm and 8.4×1017 cm−3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

(Invited) Vertical GaN Devices for High-Power Electronics

With the demand of high-power and high-speed electronics, GaN has achieved widespread interest owing to its wide-bandgap (WBG), large breakdown field, and high mobility that can provide superior performance compared to the Si and SiC that are limited by inherent material properties. In power electronics, GaN is a potential candidate due to its high-quality p-n junctions and availability of native substrates to prevent dislocation-assisted leakage, particularly for vertical devices. For high-power electronics, these vertical devices are more advantageous compared to their lateral counterparts due to their high current capability from large-area backside ohmic, high breakdown voltage from confined electric field within WBG material, scaling feasibility, and better thermal management. Moreover, the punch-through vertical device can extract the full potential of the WBG semiconductors by maximizing the average electric field over the fully depleted drift layer.For high-performance vertical device, high-quality drift layers with low controlled background doping and low compensating defects are essential to obtain the expected high breakdown voltage and low on-resistance. Hence, we have focused on both materials and device development for GaN power devices. Toward the first goal of high-quality GaN epitaxy, we optimized the growth condition for controlled low unintentionally doped (UID) and low impurity drift layer from ammonia molecular beam epitaxy (NH3 MBE). For this, we performed a systematic study of growth rate effects on the drift layer doping for a series of growth rates from 0.37 µm/hr to 1.68 µm/hr in both sapphire and free-standing GaN substrates (Fig. 1). A low background UID doping of ~1015 cm-3 was achieved with a growth rate 1.4 µm/hr on the free-standing GaN substrate. This UID background doping is one of the lowest reported for GaN homoepitaxy [1-3]. The potential of NH3 MBE-growth for low-doped epitaxy is recognized from our work, contributed from the cleaner growth environment of MBE, compared to the conventional metal-organic chemical vapor deposition (MOCVD).The next step of our work involved developing high-performance power devices with these high-quality epitaxies. Our vertical p-n GaN diode consisted of a ~4 µm UID GaN drift layer on ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer for the n-type region. The p-type region included a 10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact followed by a 0.4 µm p+GaN ([Mg]: 3×1019 cm-3) to form the p-n junction.The sample was fabricated with circular diodes of 80-100 µm diameter Pd/Pt Anode and a backside Ti/ Au Cathode. The field management was achieved by a combination of ~1.4 µm dry etch mesa isolation with a sidewall angle of ~55° and field plate (Fig. 2). For the field-plate dielectric, a stack of atomic layer deposition of Al2O3 (26 nm) and plasma-enhanced chemical vapor deposition of Si3N4 (205 nm) at 300 °C was used. Finally, a Ti/Pt metal stack of 15 µm length was patterned to form the field-plate.Capacitance-Voltage characteristics performed in these p-n diodes showed a doping of ~3×1015 cm-3 in the n- drift layer (Fig. 3a). Secondary ion mass spectrometry (SIMS) detected Si~2×1015 cm-3 and O ~2×1016 cm-3 respectively across the n-layer (Fig. 3b), likely the source of UID doping, accounting for their possible partial ionization. The compensating carbon impurities was less than the SIMS detection limit, certifying the high-quality NH3 MBE-growth. The forward J-V characteristics (Fig. 4) revealed the leakage current below the detection limit (~1.6 pA) for V<2 V. At higher forward biases (V=2.87 V) in the diffusion current region, the ideality factor reaches to a minimum of 1.36. The extracted specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and the minimum ideality factor of 1.36 are among the best achieved in vertical homoepitaxy GaN p-n diodes [1-3]. The highest breakdown voltage of these diodes were > 1 kV (equipment limit 1 kV) whereas some diodes exhibited breakdown at -890 V (Fig. 5). Silvaco simulation showed a punch-through breakdown of the diodes with peak electric field (EC) >2.6 MV/cm. Remarkably, this breakdown performance with NH3 MBE GaN of only ~4 um channel is comparable with the best available MOCVD GaN p-n diodes with a significantly thicker drift layer of ≥ 8 um (Fig. 6). Our work thus demonstrates the potential of high-performance power devices by NH3 MBE, benefitted from high-quality epitaxy, controlled low doping, and fast growth rates, that will be promising for high-voltage devices.

(Invited) Developments of Wbg GaN-on-GaN Vertical Power Devices & Uwbg Ga2O3, Algao, and Heterostructures

In this talk, metalorganic chemical vapor deposition (MOCVD) development of GaN-on-GaN targeting for vertical power devices will be presented. To achieve vertical GaN PN diodes with high breakdown voltage (Vbr) and low on resistance (Ron), it is critical to develop thick GaN drift layer with low controllable doping (Nd-Na) and high electron mobility. Key impurities in MOCVD GaN serving as charge compensation centers were identified. For example, mechanisms of Fe incorporation in MOCVD GaN were identified and addressed [1]. Source of C incorporation is mainly from metalorganic precursor (TMGa), and the C concentration in GaN is highly dependent on MOCVD growth condition, particularly the growth rate. Typically, as GaN growth rate increases, C incorporation increases undesirably. To address this key challenge for achieving high quality thick GaN drift layer, we developed a laser-assisted MOCVD (LA-MOCVD) growth technique to enable fast GaN epitaxy while suppressing C incorporation. The effects of LA-MOCVD GaN growth parameters on GaN growth rate, impurity incorporation and charge transport properties will be discussed. GaN PN diodes with breakdown voltage ~ 3kV are demonstrated.This talk will also discuss our recent developments of ultrawide bandgap (UWBG) Ga2O3, (AlxGa1-x)2O3, and heterostructures [2-5]. Challenges and opportunities for developing (AlxGa1-x)2O3 on Ga2O3 substrates with different crystal orientations ((010), (100), (-201)) be discussed. Band offsets between AlGaO/GaO heterostructures were determined based on high quality epitaxy of AlGaO/GaO heterostructures. Experimental results agree very well with the values predicted from theoretical density functional theory (DFT). Growth techniques based on MOCVD and low pressure chemical vapor deposition (LPCVD) will be compared and discussed. Acknowledgement: The authors acknowledge the funding support from Advanced Research Projects Agency-Energy (ARPA-E) DE-AR0001036, U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call, the Air Force Office of Scientific Research FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir) and the National Science Foundation (1810041, 1755479, 2019753). [1] Y. Zhang, Z. Chen, W. Li, H.-S. Lee, M. R. Karim, A. R. Arehart, S. A. Ringel, S. Rajan, H. Zhao, J. Appl. Phys., 127, 215707, 2020.[2] Z. Feng, A F M A. U. Bhuiyan, M. R. Karim, H. Zhao, Appl. Phys. Letts., 114, 250601, 2019.[3] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, Z. Chen, H. -L. Huang, J. Hwang, H. Zhao, Appl. Phys. Lett., 115, 120602, 2019.[4] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, H.-L. Huang, J. Sarker, M. Zhu, M. R. Karim, B. Mazumder, J. Hwang, and H. Zhao, APL Materials, 8, 031104, 2020.[5] A F M A. U. Bhuiyan, Z. Feng, J. M. Johnson, H.-L. Huang, J. Hwang, and H. Zhao, Appl. Phys. Letts, 117, 252105, 2020.

Gallium Nitride and Silicon Transistors on 300 mm Silicon Wafers Enabled by 3-D Monolithic Heterogeneous Integration

We demonstrate industry’s first 300 mm GaN transistor technology and 3-D monolithic heterogeneous integration with Si transistors, enabled by 300 mm GaN metal–organic chemical vapor deposition (MOCVD) epitaxy and 300 mm 3-D layer transfer. The 300 mm GaN technology is a high- ${k}$ dielectric enhancement-mode GaN nMOS transistor technology on Si(111) substrate. It is capable of excellent characteristics and figure-of-merits (FOM) for realizing energy-efficient, compact power-delivery and RF front-end components such as power-amplifiers, low-noise amplifiers, and RF-switches. Our GaN nMOS transistors show e-mode operation with: 1) high ${I}_{\text {D,max}} =1.5$ mA/ $\mu \text{m}$ ; 2) low ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of $610\,\,\Omega -\mu \text{m}$ ( ${L}_{\text {G}} =50$ nm); 3) low ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ of 100 pA/ $\mu \text{m}$ ( ${L}_{\text {G}} \,\,=180$ nm), which are significant improvements over GaN HEMT; 4) excellent RF performance: ${f}_{\text {T}} =190$ GHz, ${f}_{\text {MAX}} =300$ GHz, power-added efficiency (PAE) = 56% ( ${L}_{\text {G}} \,\,=50$ nm) at mmwave frequency 28 GHz, and PAE = 77% at 5 GHz ( ${L}_{\text {G}} =180$ nm), significantly better than industry-standard GaAs and Si RF transistors; 5) good RF-switch FOM, ${R}_{ \mathrm{\scriptscriptstyle ON}}{C}_{ \mathrm{\scriptscriptstyle OFF}} =110$ fs; and 6) low noise figure, NF $_{\text {min}} \,\,=1.36$ dB ( ${f} =28$ GHz) and 0.4 dB ( ${f} =5$ GHz), all at SoC-compatible voltages. We further demonstrate GaN transistor innovations all integrated on 300 mm Si(111) wafer, including depletion-mode GaN nMOS transistor with high ${I}_{\text {D}} =1.8$ mA/ $\mu \text{m}$ ; GaN Schottky gate transistor producing high saturated power of 20 dBm ( $80\mu \text{m}$ width) with peak PAE = 57% at 28 GHz; low leakage compact cascode and multigate GaN transistors; and GaN Schottky diodes with ultralow ${C}_{ \mathrm{\scriptscriptstyle OFF}}$ for electrostatic discharge (ESD) protection. The layer-transferred Si transistors, monolithically stacked on top of the GaN transistors by 300 mm 3-D layer transfer, show high drive current performance: 1.0 mA/ $\mu \text{m}$ (Si nMOS) and 0.5 mA/ $\mu \text{m}$ (Si pMOS). Such a monolithic 3-D Monolithic integration of GaN and Si transistors enables full integration of energy-efficient, truly compact power delivery and RF solutions with CMOS digital signal processing, logic computation and control, memory, and analog circuitries.

Probing unintentional Fe impurity incorporation in MOCVD homoepitaxy GaN: Toward GaN vertical power devices

Unintentional impurity incorporation in GaN drift layers represents a challenging issue that can limit their potential performance in vertical power devices. In this paper, we focus on studying the origins of Fe impurity incorporation in metal-organic chemical vapor deposition (MOCVD) grown GaN materials. Acting as a compensator in n-type GaN drift layers, Fe impurities can reduce the electron mobility in GaN and limit the lowest controllable doping level. Two sources, the sample cleaning process and growth susceptor, were identified as the main mechanisms of Fe incorporation in the MOCVD GaN growth process. It was found that solvent cleaning of the wafer can introduce significant Fe contamination at the growth interface, which would slowly be incorporated into the GaN epilayer, thus causing background Fe impurity as high as 1017 cm−3 level. Moreover, the Fe impurity in the coating material on the susceptor can introduce additional Fe impurity during the growth process. Our studies revealed that the Fe impurity level could be significantly suppressed by more than two orders when an alternative cleaning process was used and the susceptor surface was fully covered by substrates. Characterization of the Fe impurity concentrations was performed via secondary ion mass spectrometry. The trap level (EC − 0.57) eV from deep-level transient spectroscopy that had previously been attributed to Fe confirmed the carrier compensation effect from Fe. Room temperature Hall mobility as high as 1007 cm2/V s was achieved on the MOCVD grown low-Fe GaN. Results from this work will provide guidance for achieving high purity GaN toward high performance GaN vertical power devices.

Fabrication of Pyramid Structure Substrate Utilized for Epitaxial Growth Free-Standing GaN

Metal–organic chemical vapor deposition (MOCVD)-grown GaN on sapphire substrate was etched by hot phosphoric acids. Pyramid structures were obtained in the N-polar face of the MOCVD–GaN. Details of the formation process and morphology of the structures were discussed. The crystallographic plane index of the pyramid facet was calculated dependent on the symmetry of the wurtzite crystal structure and the tilt angle. The substrates with pyramid structures were utilized in subsequent hydride vapor phase epitaxy (HVPE) growth of GaN. Free-standing crystals were obtained, while HVPE-grown GaN achieved a certain thickness. Raman spectroscopy was employed to obtain the stress conditions of the HVPE–GaN without and with sapphire substrate. The mechanism of the self-separation process was discussed. This facile wet etching method may provide a simple way to acquire free-standing GaN by HVPE growth.

Point defect reduction in MOCVD (Al)GaN by chemical potential control and a comprehensive model of C incorporation in GaN

A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations.

Identification of the primary compensating defect level responsible for determining blocking voltage of vertical GaN power diodes

Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (∼104 - 106 cm−2) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at Ec-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be Nt = 3 × 1012, 2 × 1015, and 5 × 1014 cm−3, respectively. The Ec-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large VBD in the next-generation wide-bandgap power semiconductor devices. Thus, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.

Opportunities and challenges in GaN metal organic chemical vapor deposition for electron devices

The current situation and next challenge in GaN metal organic chemical vapor deposition (MOCVD) for electron devices of both GaN on Si and GaN on GaN are presented. We have examined the possibility of increasing the growth rate of GaN on 200-mm-diameter Si by using a multiwafer production MOCVD machine, in which the vapor phase parasitic reaction is well controlled. The impact of a high-growth-rate strained-layer-superlattice (SLS) buffer layer is presented in terms of material properties. An SLS growth rate of as high as 3.46 µm/h, which was 73% higher than the current optimum, was demonstrated. As a result, comparable material properties were obtained. Next, a typical result of GaN doped with Si of 1 × 1016 cm−3 grown at the growth rate of 3.7 µm/h is shown. For high-voltage application, we need a thick high-purity GaN drift layer with a low carbon concentration, of less than 1016 cm−3. It is shown that achieving a high growth rate by precise control of the vapor phase reaction is still challenge in GaN MOCVD.

DFT study on adduct reaction paths of GaN MOCVD growth

The adduct reaction paths for GaN growth by metal organic chemical vapor deposition (MOCVD) were studied by quantum chemical calculations employing density functional theory (DFT). Five possible adduct reaction paths with or without the excess NH3 were proposed and the corresponding potential energy surfaces were calculated. From the calculation results, it is concluded that after the formation of DMGNH2 from TMG:NH3, the further decomposition paths have very slim probability because of the high energy barriers; whereas the oligomerization pathway to form oligomers [DMGNH2]x (x=2, 3) is probable, because of zero energy barrier. Since the oligomers tend to further polymerize, the nanoparticles are easily formed through this path. When NH3 is in excess, TMG:NH3 tends to combine with the second NH3 to form two new complexes: the coordination-bonded compound H3N:TMG:NH3 and the hydrogen-bonded compound TMG:NH3… NH3. The formation of hydrogen-bonded compound TMG:NH3… NH3 will be more probable because of the lower energy than H3N:TMG:NH3. By comparing the potential energy surfaces in five adduct reaction paths, we postulate that, under the growth conditions of GaN MOCVD, the formation of hydrogen-bonded compound TMG:NH3… NH3 followed by the reversible decomposition may be the main reaction path for GaN thin film growth; while the adduct oligomerization path to generate oligomers [DMGNH2]2 and [DMGNH2]3 might be the main reaction path for nanoparticles formation.

Dislocation reduction through nucleation and growth selectivity of metal-organic chemical vapor deposition GaN

A novel serpentine channel structure is used to mask the sapphire substrate for the epitaxial growth of dislocation-free GaN. Compared to the existing epitaxial lateral overgrowth methods, the main advantages of this novel technique are: (a) one-step epitaxial growth; (b) up to 4 times wider defect-free regions; and (c) the as-grown GaN film can be transferred easily to any type of substrate. TEM, etch pits and cathodoluminescence experiments are conducted to characterize the quality of as-grown GaN. The results show that the average etch-pit density in the yet-to-be-optimized GaN epi-layers is about 4 × 105 cm−2. The underlying physics of selective nucleation and growth is investigated using the finite element method (COMSOL). It is concluded that the proximity effect dominates the selective growth of GaN on the serpentine channel structure masked sapphire. This novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors.

The effect of carbon impurities on lightly doped MOCVD GaN Schottky diodes

Abstract

Growth temperature dependence of strain in a GaN epilayer, grown on a c-plane sapphire substrate

Despite the large mismatch in the lattice constants and thermal-expansion coefficients between the epilayer and the sapphire substrate, GaN epilayer is commonly grown on a c-plane sapphire substrate due to the lack of other suitable substrates [1–3]. The GaN epilayer on a c-plane sapphire substrate is widely grown with the use of the twostep metalorganic chemical vapor deposition (MOCVD), employing an AlN [4] or a low-temperature GaN buffer layer [5]. It was reported that a biaxial strain and a hydrostatic strain could coexist in the GaN epilayer [6, 7]. In GaN, the biaxial stress is due to thermal mismatch stress and growth [6–10]. The growth stress can be caused by lattice mismatch, island coalescence, grain growth, or surface stress [8–10]. Some in-situ measurements on the growth stress have been reported during MOCVD of GaN [8–10]. The internal hydrostatic strain was shown to have been introduced by the presence of point defects, which can be compressive or expansive depending on their size [6, 7]. A study was reported on a high-resolution X-ray diffraction (HRXRD) strain–stress analysis of a GaN/sapphire heterostructure grown through molecular beam epitaxy (MBE), particularly in the deformation state, depending on the relative content of N in the Ga1-xNx buffer layer by Harutyunyan et al. [11]. In the authors previous study [12], the strain analysis of a GaN epilayer with different growth times on a c-plane sapphire substrate via a two-step growth, using low-pressure, metalorganic chemical vapor deposition was conducted on the basis of precise measurement of the lattice parameters, using HRXRD. In this continuing study, the growth temperature dependence of the residual strain in a GaN epilayer at room temperature on a c-plane sapphire was studied. The final growth temperature in a fixed two-step MOCVD was varied from 850 to 1,050 C. The cand a-lattice parameters were measured using HRXRD, followed by the out-of-plane and in-plane strains. We have extracted the levels of biaxial and hydrostatic components of strain in the GaN films on sapphire. An undoped GaN epilayer was grown on a c-plane sapphire substrate in a horizontal MOCVD reactor at a low pressure of 300 Torr, as described in the authors’ previous articles [12, 13]. We employed a two-step growth method of GaN—consisting of the deposition of the low-temperature GaN buffer layer and high-temperature GaN epitaxial growth. The growth temperature of the high-temperature GaN was varied from 850 to 1,050 C so that the temperature dependence of the strain of the GaN epilayer induced after the two-step MOCVD growth process could be studied. The V/III ratios were 2,740 and 1,370 for buffer growth and the main high-temperature growth, respectively [12]. The growth rate was fixed at 0.07 lm/min; as such, the thickness of the GaN layer that was grown for 20 min was estimated to be 1.5 lm. The c-lattice and a-lattice parameters of the GaN epilayers were measured with a x-2h scan of the wurtzite GaN epilayer through triple-axis, HRXRD, to obtain precise measurements (PANalytical X’Pert PRO MRD), as described in the authors’ previous article [12]. S. I. Cho K. Chang Department of Chemical Engineering, University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, South Korea

Stress evolution influenced by oxide charges on GaN metal–organic chemical vapor deposition on silicon-on-insulator substrate

GaN epilayers have been deposited on silicon-on-insulator (SOI) and bulk silicon substrates. The stress transition thickness and the initial compressive stress of a GaN epilayer on the SOI substrate are larger than those on the bulk silicon substrate, as shown in in situ stress measurement results. It is mainly due to the difference of the three-dimensional island density and the threading dislocation density in the GaN layer. It can increase the compressive stress in the initial stage of growth of the GaN layer, and helps to offset the tensile stress generated by the lattice mismatch.

High-Quality GaN Film Grown by HVPE with an Anodized Aluminum Oxide Mask

High-quality GaN films were deposited by hydride vapor phase epitaxy (HVPE) on metalorganic chemical vapor deposition GaN templates using an anodized aluminum oxide mask. High-resolution X-ray diffraction, scanning electron microscopy, and cathod-oluminescence were used to highlight the excellent crystalline quality of the films, and the optical properties at different positions of the as-grown layers. The optical properties of the GaN films improved with increased film thickness. The PL peak position indicates stress relaxation by the AAO mask.

Alumina-rich spinel: A new substrate for the growth of high quality GaN-based light-emitting diodes

A new substrate material, alumina-rich spinel, with better thermal and lattice match to GaN than sapphire, has been developed for growing high-quality GaN-based light-emitting diodes (LEDs). As 1:3 spinel (1 MgO for 3 Al 2O 3), MgAl 6O 10, is chemically close to sapphire, Al 2O 3, we successfully replicated on (1 1 1)-oriented spinel wafers the GaN metal-organic chemical vapor deposition conditions developed for c-plane sapphire. This enabled the simultaneous growth of GaN on both substrates. The resulting GaN layers are structurally, optically and electrically as good on 1:3 spinel as on sapphire. We therefore grew, still simultaneously on both substrates, InGaN LEDs with peak emission wavelengths ranging from 450 nm (blue) to 550 nm (yellow–green). Their electroluminescence (EL) was found to be brighter on 1:3 spinel than on sapphire, with the light output increasing almost linearly prior to breakdown in both cases. The EL full-width at half-maximum was also systematically narrower on 1:3 spinel than on sapphire, again confirming the high quality of the LEDs grown on this new substrate material.

GaN nanowire lasers with low lasing thresholds

We report optically pumped room-temperature lasing in GaN nanowires grown by metalorganic chemical vapor deposition (MOCVD). Electron microscopy images reveal that the nanowires grow along a nonpolar ⟨11-20⟩ direction, have single-crystal structures and triangular cross sections. The nanowires function as free-standing Fabry–Pérot cavities with cavity mode spacings that depend inversely on length. Optical excitation studies demonstrate thresholds for stimulated emission of 22kW∕cm2 that are substantially lower than other previously reported GaN nanowires. Key contributions to low threshold lasing in these MOCVD GaN nanowire cavities and the development of electrically pumped GaN nanowire lasers are discussed.

Ab initio study of the gas phase nucleation mechanism of GaN

The mechanism of formation of gas phase adducts in a Ga(CH3)3–NH3 atmosphere was investigated using density functional theory and fluid dynamic simulations. We found that the GaN gas phase chemistry is dominated by a radical chain mechanism, started by the CH3· radicals originated by the decomposition of Ga(CH3)3, and resulting in the fast formation of linear covalently bonded R(GaN)x adducts. As the number of GaN units in the adduct reaches three, the molecule cyclises. A kinetic mechanism suitable to study the formation of GaN adducts was then proposed and used to simulate the chemistry in a GaN metalorganic chemical vapor deposition reactor. It was found that GaNH2 is among the most important precursors to the film growth.