Heavy-hole Transitions Research Articles

Systematic study of polarized electron emission from strained GaAs∕GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

Linear and nonlinear pulse propagation in a multiple-quantum-well photonic crystal

We investigate the temporal and spectral properties of subpicosecond pulses transmitted on the heavy-hole exciton transition through a multiple-quantum-well Bragg structure, exhibiting a one-dimensional photonic band gap. At low light intensities, a temporal propagation beating is observed. This beating is strongly dependent on the optical dephasing time ${T}_{2}$ which is dominated by the radiative interwell coupling. In an intermediate intensity regime, the Pauli-blocking nonlinearity leads to gradual suppression of the photonic band gap and vanishing of the linear propagation beating. For highly nonlinear excitation, we find signatures of self-induced transmission due to Rabi flopping and adiabatic following of the carrier density. Numerical simulations using the semiconductor Maxwell-Bloch equations are in excellent agreement with the experimental data up to intensities for which higher many-particle correlations become more important and self-phase modulation occurs in the sample substrate.

Optical properties of GaN/AlN multiple quantum wells

Optical properties of GaN/AlN multiple quantum wells (MQW) have been investigated by Raman scattering, photoluminescence and photoluminescence excitation measurements. A careful examination of the Raman spectrum reveals the fact that the constituent layers of GaN/AlN MQWs are well strained. The experimental results of emission and absorption in MQWs were compared with the calculated solutions of the finite quantum well and the bound states involved in the optical transitions were identified. It is found that the interband transitions up to n=3 bound state can be observed in the strained GaN/AlN MQWs sample. The temperature dependence of the heavy-hole transitions shows an interesting phenomenon, in which the peak energy first increases with increasing temperature and then decreases with the temperature rapidly. The observation can be explained in a consistent way by the strain effects of lattice mismatch due to the interplay between the thermal expansion of GaN and AlN layers. Our results indicate that pseudomorphic GaN/AlN MQWs with good quality can be readily grown, and their applications in optoelectronics can be expected in the near future.

Optimization of InGaAsP–InP tensile strained multiple quantum-well structures emitting at 1.34 µm

A systematic investigation has been undertaken on the growth, characterization and performance of tensile-strained multiple-quantum-well (MQW) lasers emitting at 1.34 µm. Two photoluminescence peaks corresponding to n = 1 electron–light hole (E1–LH1) and n = 1 electron–heavy hole (E1–HH1) transitions are clearly observed in MQW structures. The improvements of tensile strained MQW lasers such as the internal quantum efficiency, internal loss and threshold current are attributed to the increased energy separation between LH1 and HH1 when the tensile strain and well width are larger. The relatively thick barrier layer is required to mitigate the strain relaxation between the quantum wells. The degradation of device performance by the excessive barrier height could be attributed to the exacerbated spatial non-uniformity of the carrier distribution in the valence band. The optimized tensile strained MQW lasers with 300 µm cavity length and ridge waveguide (RWG) structure exhibit a lower threshold current of 13 mA and a higher external differential efficiency per facet of 0.29 mWmA−1 at 25 °C.

TERAHERTZ EMITTERS AND DETECTORS BASED ON SiGe NANOSTRUCTURES

Terahertz (THz) electroluminescence was produced by three different types of sources: intersubband transitions in silicon germanium quantum wells, resonant state transitions in boron-doped strained silicon germanium layers, and hydrogenic transitions from dopant atoms in silicon. The devices were grown by molecular beam epitaxy, fabricated by dry etching, and characterized by infrared spectroscopy. The absorption of THz was observed in silicon germanium quantum wells at energies corresponding to heavy hole and light hole intersubband transitions. These results suggest that SiGe nanotechnology is attractive for THz device applications.

Laterally patterned band structure in micromachined semiconductors

We demonstrate that micromachining lattice-matched InGaAs quantum wells grown on (001) InP with strained barriers produces precise laterally patterned modifications to the semiconductor band structure. The light-hole and heavy-hole excitonic transitions are mixed and differentially shifted by the micromachining, inducing a surface-normal optical anisotropy characterized by a peak birefringence of Δn=0.028. The measured optical properties agree with calculations based on finite-element models of the strain combined with an eight-band k⋅p model that includes deformation potentials. This technique may find applications in fields such as surface-normal polarization modulators, quasi-phase matching, and optically-acitve piezoelectric materials.

Electroreflectance of surface-intrinsic-n+-type-doped GaAs by using a large modulating field

It is known that electroreflectance of surface-intrinsic-n+-type-doped GaAs has exhibited many Franz–Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy- and light-hole transitions. However, each peak still contains two components, which belong to F+δF/2 and F−δF/2, respectively, where F is the electric-field strength in the undoped layer and δF is the modulating field of applied ac voltage (Vac). In order to resolve the heavy- and light-hole transitions, δF was kept much smaller than F in the previous works. In this work, we have used a larger Vac and, hence, a larger δF, to further separate the peaks. The peaks can be divided into two groups which belong to F+δF/2 and F−δF/2, respectively. The peak belonging to the heavy-hole transition and F−δF/2 can be singled out to compare with the Airy function theory.

The effects of light–heavy hole transitions on the cutoff wavelengths of far infrared detectors

Results are presented on the effects of doping variation on the cutoff wavelength ( λ c) of homojunction interfacial workfunction internal photoemission far infrared detectors. The behavior at low doping (<10 19 cm −3) is well predicted by the free carrier absorption model used previously. However at high doping the observed λ c is much shorter than the values predicted by the workfunction obtained from Arrhenius plots. An explanation for the reduced λ c in the high doping region is presented using a model for depletion of the heavy hole band due to direct transitions from the heavy hole to light hole band.

Theoretical and experimental studies of cyclotron resonance in p-type InAs and InMnAs at ultrahigh magnetic fields

We report theoretical and experimental ultrahigh magnetic field cyclotron resonance (CR) studies of paramagnetic p-type InAs and InMnAs. Experimental results are compared with an 8 band Pidgeon–Brown model which includes (i) the wave vector dependence of the electronic states along the magnetic field, and (ii) s–d and p–d exchange interactions with Mn ions. CR spectra are computed using Fermi’s golden rule. Results show two strong peaks associated with heavy and light hole transitions. Line shapes of the transitions provide information on the carrier densities.

Spin-polarizing properties of the InAs/(AlSb)/GaMnSb/(AlSb)/InAs ferromagnetic resonant interband tunneling diode

We theoretically investigate the low-temperature spin-dependent current-voltage characteristics of the ferromagnetic resonant interband tunneling diode (FRITD). The simulations are based on a transmission formulation of the effective bond-orbital method (EBOM), which self-consistently includes both the electrostatic potential and the magnetic exchange interaction. Despite negligible spin splitting in the InAs emitter and collector regions, this type-II InAs/(AlSb)/GaMnSb/(AlSb)/InAs structure can function either as a source of spin-polarized electrons, whose polarity depends on the applied bias, or as a spin valve. For unpolarized emitter electrons, the spin polarization of carriers transmitted to the collector can be as high as \ensuremath{\approx}90%. We consider both thicker-well structures, in which the interband tunneling occurs primarily via light-hole-like states, and thinner-well devices whose transport is dominated by heavy-hole transitions. Since the maximum FRITD operating temperature is limited primarily by the Curie temperature in the GaMnSb hole quantum well, the same favorable device performance may be achievable at 77 K by employing the digital incorporation of Mn into the center of a GaSb hole well.

Polarization-resolved electro-absorption in InAs/GaAs quantum dots in waveguide structures—modeling of size, shape and In-content

Integrating InAs self-assembled quantum dots into p–i–n (and n–i–p) waveguide structures allows for polarization resolved electro-absorption measurements of quantum dot single excitonic transitions. In this geometry, the field dependence of three electron heavy-hole and three electron light-hole transitions has been investigated. The light-hole ground state exciton is separated from the heavy-hole ground state transition by 160 meV . A zero field polarization of the heavy-hole excitons is found, which compensates at −75 kV/cm . The light hole excitons, however, do not indicate any dipole moment. In contrast to the heavy-hole transitions they show a strongly pronounced field dependence. Modeling these extensive results, the parameter space with respect to shape, size and In-content of the quantum dot turns out to be very narrow. As a key issue, the light-/heavy-hole splitting is strongly influenced by the biaxial strain, which in turn depends on the width/height ratio of the dot. The best agreement with the experimental data has been achieved by assuming a lens-shaped dot with a trumpet-shaped In-distribution.

Differential surface photovoltage spectroscopy characterization of a 1.3 μm InGaAlAs/InP vertical-cavity surface-emitting laser structure

We have investigated a 1.3 μm InGaAlAs/InP vertical-cavity surface-emitting laser (VCSEL) structure using angle- and temperature-dependent wavelength-modulated differential surface photovoltage spectroscopy (DSPS). The DSPS measurements as functions of incident angle and temperature have been carried out in the ranges 0°⩽θ⩽60° and 300 K⩽T⩽420 K, respectively. Angle-dependent reflectance (R) and surface photovoltage spectroscopy (SPS) measurements have also been performed to illustrate the superior features of the DSPS technique. The differential surface photovoltage (DSPV) and SPV spectra exhibit both the fundamental conduction to heavy-hole excitonic transition of quantum well and cavity mode (CM) plus a rich interference pattern related to the mirror stacks, whereas in the R spectra only the CM and interference features are clearly visible. The energies of the excitonic transition and CM are accurately determined from the DSPV spectra. By changing the angle of incidence in the DSPS measurements the energy positions of the CM and distributed Bragg reflector features show a blueshift while the excitonic transition remains unchanged. At a fixed incident angle, the energy positions of the excitonic feature and CM show a different rate of redshift with increasing temperature, with the latter at a much slower pace. The results demonstrate considerable potential of DSPS for the nondestructive characterization of the VCSEL structures.

Magneto-Optical Study of Diluted Magnetic Semiconductor Nanostructures Prepared by Pulsed Laser Deposition

The CdTe/Cd1−xMn x Te quantum well structures have been grown by pulsed laser deposition. X-ray diffraction analysis demonstrates high structural quality of the deposited layers and nanostructures. In the magnetoabsorption spectra the large Zeeman splitting of exciton peak which is corresponded to the heavy-hole exciton transition in the CdTe quantum well layers is revealed. The Faraday rotation spectra of the superlattices is interpreted in framework of the exciton transition for the Cd1−xMn x Te layers at a higher photon energy. Influence of different factors on behavior of the Zeeman splitting and the Faraday rotation in the studied nanostructures is discussed.

Si/SiGe quantum-cascade emitters for terahertz applications

The quantum cascade laser provides one possible method of realizing high efficiency light emission from indirect band gap materials such as silicon. Strain-symmetrized Si/SiGe samples designed to investigate the intersubband properties of quantum wells are examined. Electroluminescence data from Si/SiGe quantum-cascade staircases demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions are presented. In surface-normal emission only light-hole to heavy-hole electroluminescence is observed at 2.9 THz ( 103 μm wavelength) as predicted by theory. Modulation-doped SiGe quantum well samples are also investigated to determine the underlying physics in the system. Results of picosecond time resolved studies of the dynamics of the intersubband transitions using a free electron laser are presented which show approximately constant relaxation times of 10 ps below 100 K .

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

Angle-dependent differential-photovoltage spectroscopy for the characterization of a GaAs/GaAlAs based vertical-cavity surface-emitting laser structure

An angle-dependent wavelength-modulated differential-photovoltage spectroscopy (DPVS) investigation has been performed at room temperature on a bare as-grown wafer of GaAs/GaAlAs-based vertical-cavity surface-emitting laser (VCSEL) structure, designed for emitting at a wavelength near 850 nm. The differential-photovoltage (DPV) spectra exhibit both the fundamental conduction to heavy-hole excitonic transition and cavity mode plus an interference pattern related to the mirror stacks. By changing the angle of incidence in the DPV measurements the energy positions of the cavity mode and distributed Bragg reflector features show a blueshift while the excitonic transition remains unchanged. The energies of the excitonic transition and cavity mode are accurately determined from the DPV spectra. The advantages of DPVS in relation to other methods of characterizing VCSEL structures, such as surface photovoltage, photoreflectance, photocurrent, and differential photocurrent spectroscopy, are discussed. The results demonstrate considerable potential of DPVS for the nondestructive characterization of these structures at room temperature.

Surface photovoltage spectroscopy and normal-incidence reflectivity characterization of a 1.3 μm InGaAlAs/InP vertical-cavity surface-emitting laser structure

We have investigated a 1.3 μm InGaAlAs/InP (001) vertical-cavity surface-emitting laser (VCSEL) structure using surface photovoltage spectroscopy (SPS) and normal-incidence reflectivity. The SPS measurements were performed as a function of angle of incidence relative to the normal (0°–55°) and temperature (300 K&amp;lt;T&amp;lt;420 K). The SPS spectra exhibit both the fundamental conduction to heavy-hole excitonic transition and cavity mode plus a rich interference pattern related to the mirror stack. The advantages of SPS in relation to other methods to study VCSELs, such as photoreflectance and photocurrent spectroscopy, are discussed. This experiment demonstrates the considerable potential of SPS for the characterization of these device structures.

Inter- and Intraband Transitions in Cubic Nitride Quantum Wells

In this work we analyze the luminescence emissions from selected isolated GaN/InGaN quantum wells comparing measured and theoretical photoluminescence (PL) spectra. The calculations are performed within the k · p method by means of an 8 × 8 Kane Hamiltonian, generalized to treat different materials. Strain effects due to the large lattice mismatch between InN and GaN are taken into account. From the direct comparison with experimental results, we found evidence for transitions involving confined levels which, besides those related to quantum dots, may be ascribed to the first electron–heavy-hole transition in the quantum wells. Since the studies of optical properties of quantum wells based on cubic nitrides are at an early stage, the results reported here will provide guidelines for the interpretation of forthcoming experiments.

Optical properties of wetting layers in stacked InAs/GaAs quantum dot structures

The optical transitions of the wetting layers in two-fold self-assembled InAs/GaAs quantum dot samples are studied as a function of GaAs spacer thickness by various methods. The absorption related studies by photoreflectance and selective photoluminescence excitation spectroscopy reveal already for thick barriers, for which coupling effects can be excluded, two energetically separated heavy-hole transitions. This splitting indicates the formation of two wetting layers during growth with a 10% difference in width and reflects strain field interaction between the island layers. Thin spacer layer samples show in addition the expected wetting layer coupling as confirmed by subband calculations.

N-Composition and Pressure Dependence of the Inter Band Transitions of Ga(N,As)/GaAs Quantum Wells

A series of GaN x As 1 m x /GaAs quantum well structures with well widths of about 20 nm and x varying between 1% and 3.5% has been grown by metal-organic vapour phase epitaxy. We have studied the evolution of the quantum well states under hydrostatic pressure up to 20 kbar at 300 K by photomodulated reflectance (PR) spectroscopy. The energy positions of the quantum well transitions have been obtained by fitting the PR spectra. The pressure dependence of the allowed heavy-hole transitions e n hh n decreases with increasing n . This directly reflects the strong non-parabolic dispersion of the conduction band originating from the interaction of the N-impurity level with the bands of the GaAs host. The fitted energy positions and their pressure dependence can be well described by a 10 band k.p model. The observed splitting between the lowest light-hole and heavy-hole transitions are in agreement with a type I band alignment.