Growth of Polarity-Controlled ZnO Films on (0001) Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The polarity control of ZnO films grown on (0001) Al₂O₃ substrates by plasma-assisted molecular-beam epitaxy (P-MBE) was achieved by using a novel CrN buffer layer. Zn-polar ZnO films were obtained by using a Zn-terminated CrN buffer layer, while O-polar ZnO films were achieved by using a Cr₂O₃ layer formed by O-plasma exposure of a CrN layer. The mechanism of polarity control was proposed. Optical and structural quality of ZnO films was characterized by high-resolution X-ray diffraction and photoluminescence (PL) spectroscopy. Low-temperature PL spectra of Zn-polar and O-polar samples show dominant bound exciton (I₈) and strong free exciton emissions. Finally, one-dimensional periodic structures consisting of Zn-polar and O-polar ZnO films were simultaneously grown on the same substrate. The periodic inversion of polarity was confirmed in terms of growth rate, surface morphology, and piezo response microscopy (PRM) measurement.     

ZnO and related materials: Plasma-Assisted molecular beam epitaxial growth, characterization and application

This paper will address features of plasma-assisted molecular beam epitaxial growth of ZnO and related materials and their characteristics. Two-dimensional, layer-by-layer growth is achieved both on c-plane sampphire by employing MgO buffer layer growth and on (0001) GaN/Al₂O₃ template by predepositing a low-temperature buffer layer followed by high-temperature annealing. Such two-dimensional growth results in the growth of high-quality heteroepitaxial ZnO epilayers. Biexciton emission is obtained from such high quality epilayers The polarity of heteroepitaxial ZnO epilayers is controlled by engineering the heterointerfaces. We achieved selective growth of Zn-polar and O-polar ZnO heteroepitaxial layers. The origin of different polarities can be successfully explained by an interface bonding sequence model. N-type conductivity in Gadoped ZnO epilayers is successfully controlled. High conductivity, enough to be applicable to devices, is achieved. Mg_xZn_1-xO/ZnO heterostructures are grown and emission from a ZnO quantum well is observed. Mg incorporation in a MgZnO alloy is determined by in-situ reflection high-energy electron diffraction intensity oscillations, which enables precise control of the composition. Homoepitaxy on commericial ZnO substrates has been examined. Reflection high-energy electron diffraction intensity oscillations during homoepitaxy growth are observed.     

Polarity Determination of Polarity-Controlled ZnO Films Using Photoresponse Characteristics

We report on the use of the photoresponse characteristics of polarity- controlled ZnO films for determining ZnO polarity. ZnO films were grown on CrN and Cr₂O₃ buffer layers in order to produce Zn- and O-polar films, respectively, and Ti/Au metal contacts were formed. All samples showed Ohmic behavior, but the Zn-polar ZnO film showed lower contact resistance than the O-polar ZnO film. The O-polar ZnO film showed higher photocurrent and longer decay time than the Zn-polar film by photoresponse measurement. These phenomena can be explained by the expansion of the depletion layer into the bulk ZnO surface. Compared with current methods, this method of determination of the polarity of ZnO films through the measurement of photoresponse characteristics is very easy and simple to implement.     

In situ control of gan growth by molecular beam epitaxy

Methods to determine GaN surface temperature, surface composition, and growth rates using in situ desorption mass spectroseopy (DMS) and reflection high energy electron diffraction (RHEED) are demonstrated for molecular beam epitaxial growth of GaN using NH₃. Using these methods, the GaN surface temperature, T_s, and GaN growth rates as a function of T_s, Ga flux, and NH₃ flux were obtained. Surface temperatures were determined from DMS and RHEED measurements of the temperature at which Ga condenses on GaN. NH₃-limited and Ga-limited growth regimes are identified and the transition between these regimes is shown to be abrupt. NH₃-limited samples have a weakly reconstructed (2 × 2) RHEED pattern, while Ga-limited samples reveal a transmission pattern. Atomic force microscopy showed that NH₃-limited samples exhibit atomic steps while Ga-limited samples exhibit faceting.     

Catalyst‐Free GaN Nanorods Synthesized by Selective Area Growth

GaN nanorod formation on Ga‐polar GaN by continuous mode metalorganic chemical vapor deposition selective area growth (MOCVD SAG) is achieved under a relatively Ga‐rich condition. The Ga‐rich condition, provided by applying a very low V/III ratio, alters the growth rates of various planes of the defined nanostructure by increasing relative growth rate of the semi‐polar tilted m‐plane {1–101} that usually is the slowest growing plane under continuous growth conditions. This increased growth rate relative to the non‐polar m‐plane {1–100} and even the c‐plane (0001), permits the formation of GaN nanorods with nonpolar sidewalls. In addition, a new growth mode, called the NH₃‐pulsed mode, is introduced, utilizing the advantages of both the continuous mode and the lower growth rate pulsed mode to form nanorods. Finally, nanorods grown under the different growth modes are compared and discussed.     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

In-situ monitoring of surface stoichiometry and growth kinetics study of GaN (0001) in MOVPE by spectroscopic ellipsometry

GaN surface stoichiometry and growth kinetics in MOVPE were studied by in-situ spectroscopic ellipsometry. The effect of MOVPE conditions on both the surface stoichiometry and growth kinetics was investigated. The surface stoichiometry, such as N-rich, Ga-rich and Ga-excess surfaces, was monitored, and was drastically changed by the variation of the NH₃ partial pressure. When the TMG supply was interrupted during the growth, the layer-by-layer decomposition/revaporation was observed in H₂/NH₃ ambient. The decomposition rate was measured as a function of the NH₃ flow rate at the conventional epilayer growth temperatures (1050–1140 C). The decomposition rate was decreased with the increase in the N coverage on the GaN surface. it was found that the surface stoichiometry is a very important parameter for the control of the MOVPE growth kinetics.     

Cathodoluminescence of ZnO/GaN/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> heteroepitaxial structures grown by chemical vapor deposition

The cathodoluminescent properties of ZnO films in ZnO/GaN/α-Al₂O₃ and ZnO/α-Al₂O₃ heteroepitaxial structures grown by chemical vapor deposition in a low-pressure flowing-gas reactor were studied and compared. A superlinear dependence of the excitonic-band intensity in the cathodoluminescence spectrum of the ZnO/GaN/α-Al₂O₃ structures on the electron-beam current is ascertained, which indicates that the emission is stimulated for relatively low thresholds of the excitation intensity. It is shown that the ZnO films grown on the GaN substrates exhibit a much more effective cathodoluminescence compared to the cathodoluminescence in the films grown on α-Al₂O₃. It was observed that the luminescent properties of ZnO layers in the ZnO/GaN/α-Al₂O₃ structures subjected to long-term heat treatment at 750°C in an oxygen atmosphere exhibit a high thermal stability.     

LSAT(111)衬底上ZnO单晶薄膜的分子束外延生长

利用射频等离子体辅助分子束外延技术,在LSAT(111)衬底上制备高质量ZnO单晶薄膜.研究了衬底表面预处理及生长温度对ZnO外延膜的生长过程、外延取向关系以及表面形貌的影响.发现在较低温度下生长ZnO时,薄膜中容易形成30. 旋转畴,而在较高温度下,可完全消除薄膜中的旋转畴,得到具有单一畴的ZnO单晶薄膜，讨论了旋转畴的起源以及生长温度对于消除旋转畴的作用.锐利的3×3 RHEED图像验证了ZnO薄膜具有O极性     

The Effect of Growth Pressure and Growth Rate on the Properties of Mg-Doped GaN

In this work, the effects of growth pressure and growth rate on electrical and structural properties of Mg-doped GaN were investigated. It has been shown that enhanced growth rates induced by higher growth pressures may lead to decreased structural and electrical properties of p-type GaN layers. If the growth rate is kept unchanged, higher growth pressures will be beneficial for the quality of Mg-doped GaN due to the enhanced NH₃ overpressure.     

Epitaxial Growth and Characterization of p-Type ZnO

N-doped p-type ZnO thin films were grown on c-sapphire substrates, semi-insulating GaN templates, and n-type ZnO substrates by metal organic chemical vapor deposition (MOCVD). Diethylzinc and oxygen were used as precursors for Zn and O, respectively, while ammonia (NH₃) and nitrous oxide (N₂O) were employed as the nitrogen dopant sources. X-ray diffraction (XRD) studies depicted highly oriented N-doped ZnO thin films. Photoluminescence (PL) measurements showed a main emission line around 380 nm, corresponding to an energy gap of 3.26 eV. Nitrogen concentration in the grown films was analyzed by secondary ion mass spectrometry (SIMS) and was found to be on the order of 10¹⁸ cm⁻³. Electrical properties of N-doped ZnO epilayers grown on semi-insulating GaN:Mg templates were measured by the Hall effect and the results indicated p-type with carrier concentration on the order of 10¹⁷ cm⁻³.     

Room-Temperature Quantum Cascade Laser: ZnO/Zn1−x Mg x O Versus GaN/Al x Ga1−x N

A ZnO/Zn_1?x Mg _x O-based quantum cascade laser (QCL) is proposed as a candidate for generation of THz radiation at room temperature. The structural and material properties, field dependence of the THz lasing frequency, and generated power are reported for a resonant phonon ZnO/Zn_0.95Mg_0.05O QCL emitting at 5.27 THz. The theoretical results are compared with those from GaN/Al _x Ga_1?x N QCLs of similar geometry. Higher calculated optical output powers [ $ {P}_{\rm{ZnMgO}} $  = 2.89 mW (nonpolar) at 5.27 THz and 2.75 mW (polar) at 4.93 THz] are obtained with the ZnO/Zn_0.95Mg_0.05O structure as compared with GaN/Al_0.05Ga_0.95N QCLs [ $ {P}_{\rm{AlGaN}} $  = 2.37 mW (nonpolar) at 4.67 THz and 2.29 mW (polar) at 4.52 THz]. Furthermore, a higher wall-plug efficiency (WPE) is obtained for ZnO/ZnMgO QCLs [24.61% (nonpolar) and 23.12% (polar)] when compared with GaN/AlGaN structures [14.11% (nonpolar) and 13.87% (polar)]. These results show that ZnO/ZnMgO material is optimally suited for THz QCLs.     

Alternative precursor systems for the MOCVD of aluminium nitride and gallium nitride

Two alternative precursor systems have been investigated for the growth of AlN and GaN by MOCVD. The first involved the reaction between Me₃M (M(DOUBLE BOND)Al, Ga) and tert-butylamine (^tBuNH₂), whilst the second route involved the pyrolysis of single-source precursors such as Me₃M(NH₃) (M(DOUBLE BOND)Al, Ga) and [Me₂Ga(NH₂)]₃. Both routes proved suitable for the deposition of AlN thin films, and epitaxial AlN layers have been deposited on sapphire (0001) from Me₃Al(NH₃) without any added NH₃. Attempts to grow GaN from Me₃Ga/^tBuNH₂ mixtures or Me₃Ga(NH₃) were unsuccessful, leading to the deposition of Ga droplets, although GaN films containing a large excess of Ga were deposited by low-pressure MOCVD from the single-source precursor [Me₂Ga(NH₂)]₃.     

Growth of thick gan films on rf sputtered ain buffer layer by hydride vapor phase epitaxy

High crystalline quality thick GaN films were grown by vapor phase epitaxy using GaCl₃ and NH₃. The growth rate was in the range of 10~15 Μm/h. GaN films grown at higher temperatures (960~ 1020?C) were single crystalline with smooth surface morphologies. No chlorine impurity was incorporated in these films during growth. The best crystalline quality and surface morphology of grown films was achieved by sputtering a thin A1N buffer layer, prior to growth. According to reflection high energy electron diffraction and atomic force microscopy measurements, as-sputtered A1N buffer layer was amorphous with root means square roughness of 0.395 nm and then crystallized during the GaN growth. This improved the GaN growth due to more uniform distribution of GaN nucleation. Rutherford backscattering channeling experiments produced the lowest value from the GaN film grown on a-Al₂O₃ with a 500å A1N buffer layer at 1020?C.     

Vertically Well‐Aligned ZnO Nanowires on c‐Al2O3 and GaN Substrates by Au Catalyst

In this letter, we report that vertically well‐aligned ZnO nanowires were grown on GaN epilayers and c‐plane sapphire via a vapor‐liquid‐solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au‐coated GaN were vertically well‐aligned, while nanowires normally tilted from the surface when grown on Au‐coated c‐Al₂O₃ substrates. However, pre‐growth annealing of the Au thin layer on c‐Al₂O₃ resulted in the growth of well‐aligned nanowires in a normal surface direction. High‐resolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c‐axis orientation with a single‐crystalline structure.     

White electroluminescence from ZnO/GaN structures

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of ZnO:In onto GaN:Mg/GaN structures MOCVD-grown on Al₂O₃ substrates has been observed. The white light is produced by superposition of the two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al₂O₃ structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN structure is constructed and a qualitative explanation of the EL is suggested.     

A study of parasitic reactions between NH3 and TMGa or TMAI

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH₃ is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH₃. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH₃.     

High-Temperature Growth of GaN and Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N via Ammonia-Based Metalorganic Molecular-Beam Epitaxy

The effect of high-temperature growth on the crystalline quality and surface morphology of GaN and Al _x Ga_1?x N grown by ammonia-based metalorganic molecular-beam epitaxy (NH₃-MOMBE) has been investigated as a means of producing atomically smooth films suitable for device structures. The effects of V/III ratio on the growth rate and surface morphology are described herein. The crystalline quality of both GaN and AlGaN was found to mimic that of the GaN templates, with (002) x-ray diffraction (XRD) full-widths at half- maximum (FWHMs) of ~350 arcsec. Nitrogen-rich growth conditions have been found to provide optimal surface morphologies with a root-mean-square (RMS) roughness of ~0.8 nm, yet excessive N-rich environments have been found to reduce the growth rate and result in the formation of faceted surface pitting. AlGaN exhibits a decreased growth rate, as compared with GaN, due to increased N recombination as a result of the increased pyrolysis of NH₃ in the presence of Al. AlGaN films grown directly on GaN templates exhibited Pendellösung x-ray fringes, indicating an abrupt interface and a planar AlGaN film. AlGaN films grown for this study resulted in an optimal RMS roughness of ~0.85 nm with visible atomic steps.     

Effects of H2/NH3 flow-rate ratio on the luminescent, structural, and electrical properties of GaN epitaxial layers grown by MOCVD

GaN epitaxial layers were grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. The flow-rate ratio of H₂ on the upper stream to NH₃ on the bottom stream is varied from 0.5 to 2. The growth condition and characterization of the GaN epitaxial layers are investigated in detail. The H₂ flow rate of the upper stream strongly affects the reactant gas flow pattern near the substrate surface and thus influences the quality of epitaxial layers. At the optimum H₂/NH₃ flow ratio of 1.0, we can obtain a good quality of GaN epitaxial layers which exhibit a strong near band-edge emis-sion in the 20 K photoluminescence (PL), a full width at half maximum of 66 meV for the 300 K PL, an electron mobility of 266 cm²/V-s and concentration of 1 × 10¹⁸ cm⁻³ at 300 K.     

Growth of zinc- blende GaN on GaAs (100) substrates at high temperature using low-pressure MOVPE with a Low V/lll molar ratio

Zinc-blende GaN films were grown on GaAs (100) substrates by low-pressure metalorganic vapor phase epitaxy using trimethylgallium or triethylgallium and NH₃. Films grown at lower temperatures contained considerable amounts of carbon, but the carbon concentration was reduced in high temperature growth. When the film was grown at 950°C using triethylgallium and NH₃, its carbon concentration was on the order of 10¹⁷ cm⁻³. The crystalline and optical quality of zinc-blende GaN crystal also improved with high-temperature growth at a low V/III ratio using a thin buffer layer. The films exhibited only one sharp photoluminescence peak at 3.20 eV with a full width at half maximum as low as 70 meV at room temperature.