Lattice-matched ScAlMgO4 Substrates Research Articles

Ab initio calculations on the initial stages of GaN and ZnO growth on lattice-matched ScAlMgO4 (0001) substrates

The initial stages of GaN and ZnO epitaxial growth on lattice-matched ScAlMgO4 substrates have been investigated by ab initio calculation. The geometrical parameters and electronic structure of ScAlMgO4 bulk and (0001) surface have been investigated by density-functional first-principles study. The effects of different surface terminations have been examined through surface energy and relaxation calculations. The O–Mg–O termination is more favorable than other terminations by comparing the calculated surface energies. It should be accepted as the appropriate surface structure in subsequent calculation. The initial stages of GaN and ZnO epitaxial growths are discussed based on the adsorption and diffusion of the adatoms on reconstructed ScAlMgO4 (0001) surface. According to theoretical characterizations, N adatom on the surface is more stable than Ga. O adatom is more favorable than Zn. These observations lead to the formation of GaN and ZnO epilayer and explain experimentally-confirmed in-plane alignment mechanisms of GaN and ZnO on ScAlMgO4 substrates. Furthermore, the polarity of GaN and ZnO surfaces on ScAlMgO4 (0001) at the initial growth stage have been explored by ab initio calculation. Theoretical studies indicate that the predominant growths of Ga-polar GaN and Zn-polar ZnO are determined by the initial growth stage.

Surface and interface engineering of ZnO based heterostructures fabricated by pulsed-laser deposition

ZnO and related alloys are an important class of materials to realize transparent electronics because of their characteristic wide band-gap and high mobility, and also because of their practical advantages, such as: available n-type materials and bulk single crystals, low-cost production, and absence of toxicity. Our studies have been conducted for more than a decade and they have enabled surface and interface engineering on an atomic scale, presenting a promising technology for developing electrical devices of various kinds. The quality of the epitaxial films was improved drastically when grown on high-temperature annealed buffer layers prepared on lattice-matched ScAlMgO4 substrates using pulsed-laser deposition. We carefully investigated the growth temperature dependence of surface morphology and electrical properties. Electron mobility was recorded as 440 cm2 V−1 s−1 at room temperature and 5500 cm2 V−1 s−1 at 1 K, leading to observation of the integer quantum Hall-effect (QHE) in abrupt ZnO/MgxZn1−xO interfaces. Two-dimensional electron gas (2DEG) was formed spontaneously in the interface because of the polarization mismatch between the layers. The observation of QHE enables us to access the direct determination of the interfacial electronic structure. In addition, the field-effect control of 2DEG has been demonstrated using lattice-matched interfaces as high-mobility channels.

Pulsed laser deposition of thin films and superlattices based on ZnO

The pulsed laser deposition (PLD) technique has been applied for the epitaxial growth of ZnO for more than two decades. The emergence of high-temperature stability of the excitonic lasing was first demonstrated in a microcrystalline ZnO film grown by PLD leading to recent remarkable growth in this field. A number of attempts have been made to improve the crystallinity for realizing p-type materials in the quest for ZnO-based short wavelength light emitting devices (LEDs). In this paper, we describe practical advantages of PLD and currently accomplished intrinsic properties of ZnO films according to the abundant literature. We find that correlation between Hall mobility and lateral grain size captures the effect of grain boundaries for the films grown on sapphire substrates. On the other hand, advantages of the use of lattice-matched ScAlMgO4 substrate are evidenced by the lower residual electron density, higher mobility and sharper exciton peaks in the photoluminescence and absorption spectra. We also focus on the wide-band-gap ternary alloy, MgxZn1−xO, especially in terms of the composition dependence of its lattice parameters and band-gap in two different crystallographic phases, to discuss the stability of this metastable compound. The studies on the PLD growth of multilayer and superlattices are briefly reviewed. We finally present the current capability of electron and hole doping by incorporating Ga and N into films grown on (0001) ScAlMgO4 substrates. We conclude that the PLD technique and related technologies have now mature to meet the requirements for fabricating UV-LEDs.

Gallium concentration dependence of room-temperature near-band-edge luminescence in n-type ZnO:Ga

We investigated the optical properties of epitaxial n-type ZnO films grown on lattice-matched ScAlMgO4 substrates. As the Ga doping concentration increased up to 6×1020cm−3, the absorption edge showed a systematic blueshift, consistent with the Burstein–Moss effect. A bright near-band-edge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.

Emission from the higher-order excitons in ZnO films grown by laser molecular-beam epitaxy

Epitaxial ZnO thin films were grown by laser molecular-beam epitaxy on lattice-matched ScAlMgO4 substrates following the deposition and annealing of suitable buffer layers. The samples were characterized by low-temperature photoluminescence (PL), absorption, and reflectivity measurements. PL from higher order (n=2) excitons (A exciton) was observed at temperatures lower than 40 K. The absorption spectrum contained lines and the reflection spectrum exhibited anomalies that were assigned to the excited-states (n=2,3) of A and B excitons. The optical quality could be improved dramatically by using annealed ZnO or MgZnO buffer layers.

Layer-by-layer growth of high-optical-quality ZnO film on atomically smooth and lattice relaxed ZnO buffer layer

The growth mode of ZnO thin films can be well regulated in a molecular layer-by-layer growth by employing a ZnO buffer layer deposited on a lattice-matched ScAlMgO4 substrate and annealed at high temperature. The annealed buffer layer has atomically flat surface and relaxed (strain-free) crystal structure. The intensity oscillation of reflection high-energy electron diffraction persisted for more than a 100-nm film deposition under optimized conditions on such a buffer layer. Thus prepared thin films show free exciton emissions in a 5 K photoluminescence spectrum and excited-state exciton resonance structures in a reflection spectrum, both indicating very high optical quality.

Photoluminescence properties of ZnO epitaxial layers grown on lattice-matched ScAlMgO4 substrates

Photoluminescence (PL) properties of ZnO films grown epitaxially on lattice-matched ScAlMgO4 substrates are investigated. PL spectra of ZnO layers measured at 5 K are dominated by neutral-acceptor bound exciton emission at 3.359 eV (I6) with a linewidth of 0.8 meV. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 120 K. Temperature dependence of intensity of the free exciton emission is discussed.

Effect of MgZnO-layer capping on optical properties of ZnO epitaxial layers

Photoluminescence (PL) and reflectivity spectra of ZnO epilayers capped with MgxZn1−xO layers (up to x=0.36) are reported. These capped films were epitaxially grown on lattice-matched ScAlMgO4 substrates by laser molecular-beam epitaxy. A photoluminescence spectrum from the ZnO layer taken at 5 K shows emission bands at 3.389, 3.376, and 3.362 eV. The two higher bands are due to A- and B-free exciton emissions and the lowest band is due to emission of a neutral-acceptor bound exciton (I6). The linewidth of the I6 emission in our uncapped sample (0.8 meV) is significantly smaller than that in the capped one (6 meV). This is probably due to strain applied across the ZnO layer because of the difference in lattice constant between the two layers. The spectral assignment of the free exciton emissions is strengthened by a comparison with a temperature-dependent PL study and a reflectivity study. With an increase in temperature, the intensity of the bound exciton emission line decreased drastically and became comparable to that of free exciton lines. Free exciton emissions are already detectable at the liquid He temperature for the capped samples, which is never achieved in the uncapped ones grown under identical conditions. This indicates the high degree of purity of the capped films.

Systematic examination of carrier polarity in composition spread ZnO thin films codoped with Ga and N

We have grown high-crystallinity ZnO thin films on lattice-matched ScAlMgO4 substrates by pulsed-laser deposition with doping donor (Ga) and acceptor (N) simultaneously. Alternating ablation of ceramics with concentrated Ga addition and highly pure single crystal targets yielded in a controlled Ga concentration (CGa) in a wide range of 1018–1020 cm−3 with minimal contamination of undesired impurities such as Al and Si. The use of the originally developed temperature-gradient method, where controlled and continuous gradient of the growth temperature is given to the single substrate with a range of about 50–200 °C, results in a continuous spread of N concentration (CN) in a controlled fashion. Therefore, the ratio of CN/CGa can be varied continuously in a wide range for each film, assuring that a region satisfying p-type codoping condition predicted by T. Yamamoto and H. K. Yoshida [Jpn. J. Appl. Phys., Part 2 38, L166 (1999)] is included in the sample. The electrical properties were measured for over thousand specimens of lithographically patterned Hall bars without observing any sign of p-type conduction.

Observation of Biexciton Emission in ZnO/ZnMgO Multi-Quantum Wells

We report on the experimental observation of the radiative recombination of localized biexcitons at low temperature (5 K) in ZnO/ZnMgO multi-quantum wells grown by laser-molecular-beam epitaxy on a lattice-matched ScAlMgO4 substrate (0001). The emission components due to the recombinations of localized excitons and biexcitons and due to the exciton–exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton–exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced compared with the corresponding value of bulk ZnO.

Biexciton emission from ZnO/Zn0.74Mg0.26O multiquantum wells

Luminescence due to the radiative recombination of localized biexcitons has been observed at low temperature (5 K) in ZnO/Zn0.74Mg0.26O multiquantum wells grown by laser-molecular-beam epilaxy on a lattice-matched ScAlMgO4 substrate (0001). The emission components due to the recombination of localized excitons and biexcitons and due to the exciton–exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton–exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced by quantum confinement effect.

Band gap engineering based on MgxZn1−xO and CdyZn1−yO ternary alloy films

We describe the structural and optical properties of II–VI oxide alloys, MgxZn1−xO and CdyZn1−yO, grown by pulsed-laser deposition. Single-phase alloyed films of (Mg,Zn)O and (Cd,Zn)O with c-axis orientations were epitaxially grown on sapphire (0001) substrates. The maximum magnesium and cadmium concentrations (x=0.33 and y=0.07, respectively) were significantly larger than the thermodynamic solubility limits. The band gap energies systematically changed from 3.0 (y=0.07) to 4.0 eV (x=0.33) at room temperature. The photoluminescence peak energy deduced at 4.2 K could be tuned from 3.19 to 3.87 eV by using Cd0.07Zn0.93O and Mg0.33Zn0.67O at both ends, respectively. The lattice constants of the a axis were monotonically increasing functions of the concentrations of both alloys. The exciton–phonon coupling strength was determined in Cd0.01Zn0.99O grown on a lattice-matched ScAlMgO4 substrate.

Room-temperature luminescence of excitons in ZnO/(Mg, Zn)O multiple quantum wells on lattice-matched substrates

We report on the optical properties of ZnO/(Mg, Zn)O multiple quantum wells (MQWs) on lattice-matched ScAlMgO4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence (PL) and absorption peaks showed a systematic blueshift, consistent with the quantum-size effect. Moreover, a bright PL of free excitons could be observed even at room temperature. As a result, the PL could be tuned in the energy range of 3.3–3.6 eV by choosing the appropriate barrier height and well layer thickness. The widest tunability on the room-temperature luminescence of the excitons could be attained on the basis of the ZnO quantum structure. These favorable properties could not be attained in the MQWs on lattice-mismatched sapphire substrates.

Exciton spectra of ZnO epitaxial layers on lattice-matched substrates grown with laser-molecular-beam epitaxy

Optical properties in undoped-ZnO epilayers grown by the laser-molecular-beam epitaxy method on lattice-matched ScAlMgO4 substrates were investigated. The absorption spectrum at 5 K has two sharp peaks, both of which are attributed to resonances of A and B excitons, which reflect a small nonradiative damping constant of excitons as well as high film crystallinity accomplished by the virtue of lattice matching. The coupling strengths of exciton-acoustic phonon and of exciton–longitudinal-optical phonon were directly determined from the temperature dependence of exciton absorption spectra independently for A and B excitons, which are close in energy and obey the same selection rule for each other.