T. S. Wang

Drift current dominated terahertz radiation from InN at low-density excitation

This letter investigates the polarity of terahertz radiation from indium nitride (InN) excited by femtosecond optical pulses wherein a central wavelength of around 790nm is measured. The InN epilayers are grown by metalorganic chemical vapor deposition on sapphire and silicon substrates. The polarity of the terahertz radiation field from InN is opposite to that from p-InAs whose radiation mechanism is dominated by the photo-Dember effect indicating that the dominant radiation mechanism in InN is the drift current induced by the internal electric field at low-density excitation below 590nJ∕cm2. The internal electric field consists of the surface accumulation field and the spontaneous polarization-induced electric field. In addition, since no azimuthal angle dependence of the terahertz radiation is observed, the optical rectification effect is ruled out. By comparing the wave forms of terahertz radiation from the front and the back of the InN sample grown on sapphire in reflection geometry, the N polarity o...

Effects of weak ordering of InGaPN

The effect of weak ordering on InGaPN∕GaAs heterostructure grown by gas source molecular-beam epitaxy is quantitatively studied by room-temperature Raman, photoluminescence (PL), and photoreflectance spectroscopy in this work. The PL intensity decreases rapidly as the nitrogen concentration increases, implying that more nonradiative centers are generated by the ordering effect and the degradation of the samples. The band gap of InGaPN decreases dramatically as the nitrogen is incorporated. The Raman modes of InGaPN between 130 and 1000cm−1 are analyzed. Polarized Raman spectra reveal that the InGaPN layers become more ordered as more nitrogen is incorporated. A broad Raman structure that appeared around 730cm−1 is attributed to an InGaN-like LO-phonon mode. The transition of the crystal structure from the zinc blende to CuPt structure and the formation of GaN clusters are responsible for the ordering effect in the InGaPN layer.

Valence-band splitting in InGaPN : Effects of epitaxial strain and atomic ordering

Both the epitaxial-strain and atomic-ordering effects in In0.54Ga0.46P1−yNy∕GaAs(y=0%−2.0%) heterostructures are characterized by high-resolution x-ray rocking curve measurements and photoreflectance (PR) spectra at various temperatures. The lattice constant of InGaPN follows a bowing effect instead of Vegard’s law. The valence-band splitting (VBS) and spin-orbit splitting of InGaPN are obtained from PR spectra. As the nitrogen concentration increases, the lattice mismatch (the compressive strain) between the epilayer and GaAs substrate decreases while the VBS increases, which implies an increase in the degree of ordering of InGaPN. In addition, the order parameter η of InGaPN is deduced from the VBS.

Characteristics of InGaPN/GaAs heterostructures investigated by photoreflectance spectroscopy

Photoreflectance (PR) spectra at various temperatures and photoluminescence (PL) spectra and high-resolution x-ray rocking curve (XRC) measurements are used to investigate the band alignment, epitaxial-strain, and atomic-ordering effects in In0.54Ga0.46P1−yNy∕GaAs (y=0%–2.0%) heterostructures. The existence of additional peaks in PR spectra at higher levels of nitrogen (N) incorporation implies that the band alignment switches from type I to type II, due to the lowering of the conduction band. The electric field at the interface is determined and is discussed with the N content. Redshifts of the PR and PL peaks indicate that the band gap of InGaPN is dramatically reduced as N is incorporated. The valence-band splitting (VBS) and the spin-orbit splitting of InGaPN are obtained from PR spectra. High-resolution XRC measurements indicate that as the N content increases, the lattice mismatch and the compressive strain between the epilayer and GaAs substrate decrease while the VBS increases, which implies an in...

Strong surface Fermi level pinning and surface state density in GaAs0.65Sb0.35 surface intrinsic-n+ structure

Room-temperature photoreflectance is employed to investigate the Fermi level pinning and surface state density of a GaAs0.65Sb0.35 surface intrinsic-n+ (SIN+) structure. Based on the thermionic emission theory and current-transport theory, the surface Fermi level VF and surface state density are determined experimentally from the dependence of the surface barrier height on the pump beam intensity. The surface state density Ds is estimated as approximately 1.91×1013 cm−2, and the Fermi level is located 0.63 eV below the conduction band edge at the surface. By sequential etching of the intrinsic layer, the Fermi level pinning in GaAs0.65Sb0.35 SIN+ structure is further demonstrated.

Temperature-dependent parameters of band anticrossing in InGaPN alloys

Temperature-dependent photoreflectance (PR) measurements are employed to characterize the conduction band structure of In0.54Ga0.46P1−yNy (y=0 and 0.02) grown on GaAs substrates. The band gap and the upper subband E+ transition are observed in InGaPN as predicted by the band anticrossing (BAC) model. To investigate the energetic positions of the features in the PR spectra, a Kramers–Kronig analysis is proposed. Based on the PR data and the BAC model, we find that the energy EN of isolated nitrogen states shifts significantly to higher energies with decreasing temperature. Simultaneously, the interaction potential V between the nitrogen states and the unperturbed conduction band also rises to higher values. At 293 K, EN=2.054 eV and V=1.513 eV are determined. The thermal shifts of EN and V are dEN/dT≈−0.43 meV/K and dV/dT≈−0.67 meV/K, respectively. The temperature-dependent EN level and interaction potential V are attributed to the lattice distortions, which can be affected by temperature-induced changes i...

Studies of terahertz radiation from InAlAs and GaAs surface intrinsic-N/sup +/ structures and the critical electric fields of semiconductors

THz radiation from series of GaAs and InAlAs surface intrinsic-N + (SIN + ) structures with the built-in electric field as bias is studied. When the bias exceeds the so-called critical electric field, the amplitude of THz waves radiated is independent of the built-in electric field. However, the THz amplitude is proportional to the number of photo-excited free charged carriers and thus the thickness of the intrinsic layer of the SIN + structures. The critical electric field determined from the THz amplitude as a function of the electric field may be useful in estimating the Γ to L valley splitting in semiconductors. Lattice-matched GaAs and In0.52Al0.48As surface intrinsic-N + (SIN + ) structures with various thicknesses of intrinsic layers grown by conventional molecular beam epitaxy are used as the THz emitters in this study. The built-in electric field is used as the bias. The as-grown heterosturctures possess a common structure consisting of various thicknesses of undoped layers on top of 1 µm of a Si-doped, n-type buffer layer that have been grown previously on an Fe-doped semi-insulated (100) substrate. The doping concentration in the buffer layer is approximately 8.4×10 17 cm -3 . Samples with various undoped layer thicknesses are also obtained from as-grown samples by subsequent etches. The built-in electric field is determined as a function of the thickness of the intrinsic layers using modulation spectroscopy of photoreflectance (PR). Figure 1a) displays the THz waves radiated from SI (100) GaAs wafer, and GaAs and In0.52Al0.48As SIN + structures. Figure 1b) plots THz waves in the frequency domain obtained by applying the fast Fourier transform to the THz waves in the time domain. The amplitude of the THz wave from the In0.52Al0.48As SIN + structure with an intrinsic layer of 2000A is five times of that from the SI GaAs wafer. Figure 2 plots the amplitude of the THz radiation from In0.52Al0.48As and GaAs SIN + structures as a function of the thickness of the intrinsic layer. Figure 3 depicts the built-in electric field, determined from PR spectra in our previous studies, as a function of the thickness of the intrinsic layer. 5-7 The negative values of thickness represent the thickness of the buffer layer that has been etched away, therefore; the built-in electric field in these samples locates within the charge depletion layer. Figures 2 and 3, however, reveal that the amplitude of the THz radiation is independent of the built-in electric fields in the emitters but is proportional to the thickness of the intrinsic layer of the SIN + structure. This behavior is interpreted