Hole Band State Research Articles

Temperature dependent photoreflectance and photoluminescence characterization of GaInNAs∕GaAs single quantum well structures

Ga 0.69 In 0.31 N x As 1 − x ∕ GaAs single quantum well (SQW) structures with three different nitrogen compositions ( x=0%, 0.6%, and 0.9%) have been characterized, as functions of temperature in the range 10–300K, by the techniques of photoreflectance (PR) and photoluminescence (PL). In PR spectra, clear Franz-Keldysh oscillations (FKOs) above the GaAs band edge and the various excitonic transitions originating from the QW region have been observed. The built-in electric field in the SQW has been determined from FKOs and found to increase with N concentration. The PR signal has been found to decrease for nitrogen incorporated samples when the temperature was lowered due to a weakening of the modulation efficiency induced by carrier localization. A careful analysis of PR and PL spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The anomalous temperature dependent 11H transition energy and linewidth observed in the PL spectra have been explained as originating from the localized states as a result of nitrogen incorporation. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the interband transitions.

Double-exchange mechanisms for Mn-doped III-V ferromagnetic semiconductors

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity d-electrons with the heavy hole band states is largely responsible for the transfer of electrons between the impurities, whereas Hund rule for the electron occupation of the impurity d-shells makes the transfer spin selective. The model is applied to such systems as $n-$type GaN:Mn and $p-$type (Ga,Mn)As, $p-$type (Ga,Mn)P. In $n-$type DMS with Mn$^{2+/3+}$ impurities the exchange mechanisms is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In $p-$type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of heavy hole carriers caused by their hybridization with 3d electrons of Mn$^{2+}$ impurities. Using the molecular field approximation the Curie temperatures $T_C$ are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.

Raman scattering and infrared absorption in multiple boron-doped Ge dots

Multiple boron-doped Ge quantum dots are investigated. The structure, which consists of 20 periods of Ge quantum dots stacked with 6 nm Si spacers, is grown on a Si (100) substrate by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy and atomic force microscopy are used to characterize the structural properties of these Ge dots. Raman spectrum shows a downward shift of the Ge–Ge mode, which is attributed to the phonon confinement in the Ge dots. From polarization dependent Raman spectra, the strong inter-sub-level transition in the Ge quantum dots is observed. The transition is further confirmed by Fourier transform infrared spectroscopy using a waveguide geometry. The observed peak at 5 μm in the infrared absorption spectrum is consistent with that in the Raman spectrum and attributed to the transition between the first two heavy hole band states of the Ge quantum dots. The polarization dependence measurement is used to study the nature of the transitions. This study suggests the possible use of Ge quantum dots for infrared detector application.

Structural and Optical Studies of ZnCdSe/ZnSe/ZnMgSSe Separate Confinement Heterostructures with Different Buffer Layers

Detailed structural and optical studies of ZnCdSe/ZnSe/ZnMgSSe separate confinement heterostructures (SCH) grown on ZnSe, ZnSe/ZnSSe strained-layer superlattices (SLS), and GaAs buffer layers at the II–VI/GaAs interface have been carried out by employing transmission electron microscopy, variable temperature photoluminescence (PL), and contactless electroreflectance (CER) measurements. A significant improvement on the defect reduction and the optical quality has been observed by using either the ZnSe/ZnSSe SLS or GaAs as the buffer layers when compared to that of the sample using only ZnSe as the buffer layer. However, the sample grown with the SLS buffer layers reveals a room temperature PL intensity higher than that of the sample grown with a GaAs buffer layer, which may still suffer from the great ionic differences between the II–V and III–V atoms. Using 15 K CER spectra, we have also studied various excitonic transitions originating from strained Zn0.80Cd0.20Se/ZnSe single quantum well in SCH with different buffer layers. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH (L), between the mth conduction band state and the nth heavy (light)-hole band state. An excellent agreement between experiments and theoretical calculations based on the envelope function approximation model has been achieved.

Contactless electroreflectance study of strained Zn0.79Cd0.21Se/ZnSe double quantum wells

We have studied various excitonic transitions of strained Zn0.79Cd0.21Se/ZnSe double quantum wells, grown by molecular beam epitaxy on (100) GaAs substrates, using contactless electroreflectance (CER) at 15 and 300 K. A number of intersub-band transitions in the CER spectra from the sample have been observed. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The conduction-band offset Qc is used as an adjustable parameter to study the band offset in the strained Zn0.79Cd0.21Se/ZnSe system. The value of Qc is determined to be 0.67±0.03.

Temperature dependence of quantized states in an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of EmnH(L) are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.

Temperature dependence of quantized states in an InGaAs/GaAs strained asymmetric triangular quantum well

Photoreflectance (PR), contactless electroreflectance (CER) and piezoreflectance (PzR) measurements of an InGaAs/GaAs strained asymmetric triangular quantum well (ATQW) heterostructure as a function of temperature in the range of 20 to 300 K have been carried out. The structure was fabricated by molecular beam epitaxy using the digital alloy compositional grading method. A careful analysis of the PR, CER and PzR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy(light)-hole band state. Comparison of the observed intersubband transitions with a theoretical calculation based on the envelope function model, including the effects of strain, provide a self-consistent check of the ATQW composition profile. The detailed study of the temperature dependence of the excitonic transition energies indicates that the potential profile of the ATQW varies at different temperatures. The parameters that describe the temperature dependence of are evaluated. The anomalous behaviour of the temperature dependence of the linewidth of 11H, , is compared with recent results for GaAs/AlGaAs and InGaAs/GaAs symmetric rectangular quantum wells of comparable dimensions.

Modulation spectroscopy study of an InGaAs/GaAs-strained asymmetric triangular quantum well heterostructure

We have studied modulation spectra related to the intersubband transitions at 300 K and 20 K from an In GaAs/GaAs-strained asymmetric triangular quantum well (ATQW) heterostructures fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. A careful analysis of the spectra has led to the identification of various excitonic transitions,mnh(1), between them-th conduction band state to then-th, heavy (light)-hole band state. Comparison of the observed intersubband transitions with a theoretical calculation based on the envelope, function model, including the effects of strain provided a self-consistent verification that the DACG method produced the intended ATQW composition profile.

Observation of confinement effects on acceptors in Si/Si 1−xGe x superlattices

We have used photothermal ionization spectroscopy to study boron acceptors in p-Si/Si 1−xGe x, MBE grown on n-Si substrates. The Si 1−xGe x layers were center-doped with boron in concentrations between 10 17 to 10 18 cm -3. For constant Si layer width, at Si 1−xGe x widths of 100 Å and smaller, bands of acceptor transitions appeared between 120 to 400 cm -1 depending on the width. For 100 Å wells, the observations are explained by the simple hydrogenic model of a confined acceptor of Bastard and the assumption that they form states associated to the heavy hole valence band. This assumption is justified by the well known fact that the valence band is splitted from the light hole band by the stress present in these superlattices. For the 30 Å superlattice, appreciable differences with respect to the Bastard model are observed. This indicate the need to include the finite height of the confining barrier and the effect of the light and heavy hole band states for acceptors confined in wells of 30 Å in width and smaller.

Temperature Dependence of Quantized States in Strained-Layer In0.21Ga0.79As/GaAs Single Quantum Well

The piezoreflectance (PzR) and photoreflectance (PR) measurements of a strained-layer (001) In0.21Ga0.79As/GaAs single quantum well as a function of temperature in the range of 20 to 300 K have been carried out. A careful analysis of the PzR and PR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of E mnH(L) are evaluated. The detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of the 11H exciton is evaluated and compared with that of the bulk material.

Infrared absorption in SiGe/Si multiple quantum wells

Infrared absorption has been used to investigate the subband structures in SiGe/Si quantum wells. The quantum wells are prepared using RRH/VLP-CVD and consist of 20 periods of SiGe(6 nm) Si(20 nm) and 60 periods of SiGe(4 nm) Si(40 nm) . The good periodical and interface sharpness of the SiGe/Si quantum wells have been shown by Auger Electron Spectroscopy (AES). The absorption peaks due to transitions between the hole subbands and the conduction band have been observed in infrared absorption spectra. The transverse photocurrent spectrum parallel to the growth plane have also shown absorption peaks due to transitions between the heavy and light hole band states and the conduction band states in quantum wells.

Carrier relaxation and luminescence polarization in quantum wells.

A theory of polarization of luminescence in doped and undoped quantum wells is constructed taking into account the relaxation processes of photoexcited electrons and holes. The hole momentum relaxation by acoustic-phonon interaction yields spin relaxation through the spin mixing of the hole band states, resulting in incomplete spin depolarization in quantum wells. Our theory of hole-phonon interaction in symmetric quantum wells leads to the concept of relaxation through two channels conserving or changing the parity of the hole state. Calculation of the polarization spectra for undoped, n-type-doped, and p-type-doped GaAs/${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum wells produces an explanation of all the observed features in the spectra.

Hole relaxation and luminescence polarization in doped and undoped quantum wells.

Valence-hole relaxation by acoustic-phonon emission yields spin relaxation through the spin mixing of the hole-band states. In quantum wells, the hole-spin relaxation is incomplete. This leads to a unified theory of the polarization spectra for undoped and doped quantum wells, which explains the diverse features in the spectra.

Overlap integrals for Auger recombination in direct gap III-V semiconductors: calculations for conduction and heavy hole band states with wavevectors along the (001) direction in GaAs and InP

Calculations of overlap integrals between conduction band and heavy hole band states with wavevectors in the (001) direction are presented for GaAs and InP. The overlap integrals are calculated using a 15-band full zone empirical k.p method, and are found to be two or three orders of magnitude smaller than conventional estimates based on effective mass sum rules. This discrepancy is resolved by evaluating all the terms that appear in such sum rules. It is found that the usual assumptions as to the dominant terms are incorrect. The small calculated values of the overlap integrals for the conduction and heavy hole bands are to be expected on group theoretical grounds.