Investigation on the origin of wurtzite domains in thick cubic GaN using reactive ion etching

We measured the depth profiling of photoluminescence (PL) in cubic GaN films. The depth-resolved PL of normal grown GaN layers showed that the near-band-edge luminescence intensities of both cubic and wurtzite domains remained constant only until an etching depth of up to 2.7 mu m, but their ratio remained unchanged at all etching depths. Moreover, when a thin In0.1Ga0.9N layer was sandwiched between two GaN layers, the content of the wurtzite domains increased, and its distribution showed a dependence on thickness. As the reactive ion etching depth increased, the PL intensity ratio of cubic GaN to wurtzite domains increased. Based on the distribution, the strain relaxation, instead of the instability of cubic GaN at high temperature, was attributed to the origin of wurtzite domains. (C) 2000 Elsevier Science S.A. All rights reserved.

Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038 +/- 0.0009 Angstrom, which is in excellent agreement with the theoretical prediction of 4.503 Angstrom. (C) 2000 American Institute of Physics. [S0021-8979(00)07918-4].

Optical transitions in cubic GaN grown by metalorganic chemical vapor deposition on GaAs (100) substrate

Photoluminescence measurements have been performed on cubic GaN films with carrier concentration as low as 3 x 10(13) cm(-3). From the temperature and excitation intensity dependence, the emission lines at 3.268, 3.150 and 3.081 eV were assigned to the excitonic, donor-acceptor pair, and free-to-acceptor transitions, respectively Additionally, we observed two additional emission lines at 2.926 and 2.821 eV, and suggested that they belong to donor-acceptor pair transitions. Furthermore, from the temperature dependence of integral intensities, we confirmed that three donor-acceptor pair transitions (3.150, 2.926, and 2.821 eV) are from a common shallow donor to three different accepters. The excitonic emission at 3.216 eV has a full-width-at-half-maximum value of 41 meV at room temperature, which indicates a good optical quality of our sample.

The content calculation of hexagonal phase inclusions in cubic GaN films on GaAs(001) substrates grown by metalorganic chemical vapor deposition

Cubic GaN(c-GaN) films are grown on GaAs(001) substrates by metalorganic chemical vapor deposition (MOCVD). Two GaN samples were grown with different buffer layer, the deposition time of each was 1 and 3 min, respectively. 4-circle X-ray double crystal diffraction (XRDCD) was used to study the secondary crystallographic phases presented in the c-GaN films. The phase composition of the epilayers was determined by X-ray reciprocal space mapping. The intensities of the c-GaN(002) and h-GaN(10 (1) over bar 1) planes detected in the mapping were investigated by omega scans. The content of the hexagonal phase inclusions in the c-GaN films was calculated to about 1.6 and 7.9%, respectively. The thicker buffer layer is not preferable for growing high quality pure c-GaN films. (C) 2000 Elsevier Science S.A. All rights reserved.

Influences of initial buffer layer deposition on electrical and optical properties in cubic GaN grown on GaAs(100) by metalorganic chemical vapor deposition

We present some results on the effect of initial buffer layer on the crystalline quality of Cubic GaN epitaxial layers grown on GaAs(100) substrates by metalorganic chemical vapor deposition. Photoluminescence and Hall measurements were performed to characterize the electrical and optical properties of cubic GaN. The crystalline quality subsequently grown high-temperature (HT) cubic GaN layers strongly depended on thermal effects during the temperature ramping process after low temperature (LT) growth of the buffer layers. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to investigate this temperature ramping process. Furthermore, the role of thermal treatment during the temperature ramping process was identified. Using the optimum buffer layer, the full width at half maxim (FWHM) at room temperature photoluminescence 5.6 nm was achieved. To our knowledge, this is the best FWHM value for cubic GaN to date. The background carrier concentration was as low as 3 x 10(13) cm(-3). (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Optical characterization of high-purity cubic GaN grown on GaAs (001) substrate by metalorganic chemical vapor deposition

The optical properties of cubic GaN films have been investigated in the temperature range of 10-300 K. Five peaks were observed at 10 K. From the dependence of photoluminescence emissions on the temperature and excitation intensity, we have assigned two of the five peaks (2.926 and 2.821 eV) to donor-acceptor pair (DAP) transitions. Furthermore, these two peaks were found to be related to a common shallow donor involved in the peak position previously reported at 3.150 eV. The intensities of DAP transitions were much weaker than that of excitonic emission even at low temperature, indicating a relatively high purity of our samples. (C) 2000 American Institute of Physics. [S0003-6951(00)00921-9].

Effect of buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(001) substrates

In this letter, we investigated the effect of the buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrate. The reflection high-energy electron diffraction (RHEED) pattern of the low-temperature GaN buffer layers shows that both the deposition temperature and time are important in obtaining a smooth surface. Four-circle X-ray double-crystal diffraction (XRDCD) reciprocal space mapping was used to study the hexagonal phase inclusions in the cubic GaN (c-GaN) films grown on the buffer layers. The calculation of the volume contents of the hexagonal phase shows that higher temperature and longer time deposition of the buffer layer is not preferable for growing pure c-GaN film. Under optimized condition, 47 meV FWHM of near band gap emission of the c-GaN film was achieved. (C) 2000 Elsevier Science B.V. All rights reserved.

Strain and photoluminescence characterization of cubic (In,Ga)N films grown on GaAs(001) substrates

Cubic InxGa1-xN films were successfully grown on GaAs(001) substrates by metalorganic chemical-vapor deposition. The values of x content ranging from 0.10 to 0.24 obtained at different growth conditions were measured by double-crystal x-ray diffraction (XRD). The perpendicular and parallel elastic strain of the In0.2Ga0.8N layer, epsilon(perpendicular to)=0.4% and epsilon(parallel to)=-0.4% for GaN and epsilon(perpendicular to)=0.37% and epsilon(parallel to)=-0.37% for InGaN, respectively, were derived using the XRD measurements. The inhomogeneous strain and the average grain size of the In0.2Ga0.8N/GaN films were also studied by XRD. Photoluminescence spectra were used to measure the optical characterization of the InxGa1-xN thin films with different In composition, and the near-band-edge emission dependence of cubic InxGa1-xN on the x value is nearly linear with In content x less than or equal to 0.24. (C) 2000 American Institute of Physics. [S0021-8979(00)03908-6].

Steady and transient optical properties of cubic InGaN epilayers

Photoluminescence (PL) and time-resolved PI, were employed to study the steady and transient optical properties of cubic InxGa1-xN epilayers grown by MBE. The results suggest that the PL transitions in InGaN epilayers are mainly from localized exciton states. The localization energies are estimated to be 60 meV, independent of In composition. The PL decay is characterized by a hi-exponential dependence. The fast process (50 ps at 12K) is related to the fast relaxation of excitons, while a slower contribution (200-270 ps at 12K) is attributed to the decay process of localized excitons.

Room-temperature optical transitions in Mg-doped cubic GaN/GaAs(100) grown by metalorganic chemical vapor deposition

The mechanism of room-temperature optical transitions in a Mg-doped cubic GaN epilayer grown on GaAs(100) by metalorganic chemical vapor deposition has been investigated. By examining the dependence of photoluminescence on the excitation intensity (which varied over four orders) at room temperature, four different emissions with different origins were identified. A blue emission at similar to 3.037 eV was associated with a shallow Mg acceptor, while three different lower-energy emissions at similar to 2.895, similar to 2.716, and similar to 2.639 eV were associated with a deep Mg complex. In addition to a shallow acceptor at E congruent to 0.213 eV, three Mg-related deep defect levels were also found at around 215, 374, and 570 meV (from the conduction band). (C) 2000 American Institute of Physics. [S0021-8979(00)01904-6].

Pulsed excimer laser annealing of Mg-doped cubic GaN

A KrF (248 nm) excimer laser with a 38 ns pulse width was used to study pulsed laser annealing (PLA) on Mg-doped cubic GaN alms. The laser-induced changes were monitored by photoluminescence (PL) measurement. It indicated that deep levels in as-grown cubic GaN : Mg films were neutralized by H and PLA treatment could break Mg-H-N complex. The evolution of emissions around 426 and 468 nm with different PLA conditions reflected the different activation of the involved deep levels. Rapid thermal annealing (RTA) in N-2 atmosphere reverts the luminescence of laser annealed samples to that of the pre-annealing state. The reason is that most H atoms still remained in the epilayers after PLA due to the short duration of the pulses and reoccupied the original locations during RTA. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Vv; 61.72.Cc; 18.55. -m.

Photovoltaic effect of cubic GaN/GaAs(100)

We have studied the photovoltaic effect in cubic GaN on GaAs at room temperature. The photovoltaic spectra of cubic GaN epitaxial film were concealed by the photovoltaic effect from the GaAs substrate unless additional illumination of a 632.8 nm He-Ne laser beam was used to remove the interference of the GaAs absorption in the measurement. On the basis of the near-band-edge photovoltaic spectra of cubic GaN, we obtained the minority carrier diffusion lengths of about 0.32 and 0.14 mu m for two undoped n-type cubic GaN samples with background concentrations of 10(14) and 10(18) cm(-3), respectively. (C) 1999 American Institute of Physics. [S0003-6951(99)00450-7].

Correlation of calibration equations for test fixtures

This paper begins from the thru-short-open (TSO) and thru-line-match (TLM) methods to investigate the correlation of the calibration equations of these two methods, The relations among the measurements with the corresponding standards are obtained. It is found that the line standard with zero length can be used instead of ideal open and short, in case that two test fixtures are symmetrical. For asymmetrical fixtures, the measurements with the standards line, open and short are related at certain frequencies, and the matched load can be replaced by the line standards. The relations established are used to test short and match standards and analyze the freqPuency limits of the TSO method, Good agreement between theory and experiment is obtained, It is found that the TSO method becomes very poor when the insertion phase of the thru standard is near n pi/4, and this method has a lower frequency limit. The TLM method is found unsuitable for calibrating asymmetrical fixtures.

Effect of Si doping on cubic GaN films grown on GaAs(100)

Epitaxial layers of cubic GaN have been grown by metalorganic vapor-phase epitaxy (MOVPE) with Si-doping carrier concentration ranging from 3 x 10(18) to 2.4 x 10(20)/cm(3). Si-doping decreased the yellow emission of GaN. However, the heavily doped n-type material has been found to induce phase transformation. As the Si-doping concentration increases, the hexagonal GaN nanoparticles increase. Judged from the linewidth of X-ray rocking curve, Si-doping increases the density of dislocations and stacking faults. Based on these observations, a model is proposed to interpret the phase transformation induced by the generated microdefects, such as dislocations and precipitates, and induced stacking faults under heavy Si-doping. (C) 1999 Elsevier Science B.V. All rights reserved.