Ground State Subband Research Articles

Topological transitions in size-quantized heterostructures

An exact solution is obtained to the problem of the spectrum of holes, described by the Luttinger Hamiltonian, in a quantum well of finite depth under arbitrary uniaxial stresses in the well and the barrier. Conditions for the topological transitions accompanied by the variation in the connectivity of the isoenergetic surface are found. It is shown that, for certain values of model parameters, the effective mass of holes in the ground-state subband of size quantization becomes negative.

SELF-CONSISTENT CALCULATION OF SEMICONDUCTOR HETEROJUNCTIONS USING QUANTUM GENETIC ALGORITHM

In this study, we have investigated the ground state energy level of electrons in modulation doped GaAs / Al x Ga 1 - x As heterojunctions. For this purpose, Schrödinger and Poisson equations are solved self consistently using quantum genetic algorithm (QGA). In this way, we have found the potential profile, the ground state subband energy and their corresponding envelope functions, Fermi level, and the amount of tunneling charge from barrier to channel region. Their dependence on various device parameters are also examined.

Spin splitting of the excited-state subband inGaAs−Al0.3Ga0.7Asasymmetric two-layer systems

We track the variation of the Landau-level filling factors for the quantization of the Hall resistance in unbalanced double-quantum-well structures when the electron density is a controlled parameter. We observe that the filling factors associated with adjacent quantum Hall states differ by three for two or more consecutive quantum Hall transitions. The anomalous filling-factor increment results from simultaneous depopulations of a spin-degenerate Landau level of the ground-state subband and a spin-polarized Landau level of the excited-state subband. The locking of the topmost Landau levels to the Fermi level gives rise to the phenomenon in which the Zeeman splitting of the excited-state subband can be mistaken to be enormous enough to match the Landau-level separation of the ground-state subband. The temperature dependence of the quantum Hall states evidences, however, that the exchange enhancement of the g factor for the excited-state subband is rather similar to that for the ground-state subband.

SiGe/Si THz laser based on transitions between inverted mass light-hole and heavy-hole subbands

We have investigated a SiGe/Si quantum-well laser based on transitions between the light-hole and heavy-hole subbands. The lasing occurs in the region of k space where the dispersion of ground-state light-hole subband is so nonparabolic that its effective mass is inverted. This kind of lasing mechanism makes total population inversion between the two subbands unnecessary. The laser structure can be electrically pumped through tunneling in a quantum cascade scheme. Optical gain as high as 172/cm at the wavelength of 50 μm can be achieved at the temperature of liquid nitrogen, even when the population of the upper laser subband is 15% less than that of the lower subband.

Intersubband infrared detector with optimized valence band quantum wells for 3–5 μm wavelength region

We present results on a p-type intersubband infrared detector for the 3–5 μm wavelength region with optimal quantum well design. For valence band quantum wells made by the AlGaAs/GaAs heterosystem with negligible strain, the ground state subband is the first heavy hole subband, and a structure with the second light hole subband in resonance with the top of the barrier is optimal for normal incidence. Using this resonance design, the fabricated detector shows good performance with a background limited operating temperature of 100 K. Our experiments also show a clear polarization dependence for different intersubband transitions involving heavy hole to heavy hole and heavy hole to light hole processes.

Valence intersubband lasers with inverted light-hole effective mass

We propose a novel intersubband laser based on transition between the ground-state heavy-hole subband (HH1) and light-hole subband (LH1) in a k-space region where the light-hole effective mass is inverted. The laser structure can be electrically pumped with a simple quantum cascade scheme. Our calculation shows that with only a small fraction of the carrier population in the upper subband (LH1), it is possible to achieve population inversion between the two subbands locally in K space where the light-hole effective mass is inverted. Optical gain in excess of 150/cm can be achieved with a pumping current density on the order to 100 A/cm2 at the temperature of liquid nitrogen.

Doublet structure in the absorption coefficient for tunneling-split intersubband transitions in double quantum wells.

Calculations of the absorption coefficient for tunneling-split intersubband transitions in double quantum wells are presented and the line shape is analyzed as a function of the temperature and dopant concentration. The peaks in the absorption coefficient correspond to tunneling-split intersubband transitions resulting in the resonant absorption of light. The lower-energy absorption peak is induced by electron tunneling. This doublet structure accounts for the transitions between the tunneling-split originally degenerate ground-state subbands in the isolated quantum wells and corresponds to the transition from the Fermi edge of the lower tunneling-split subband and the transition to the Fermi edge of the upper tunneling-split subband. These two transitions are unresolved at low temperatures but become resolved at high temperatures. When the dopant charge density is increased, the absorption peak strength increases. It also brings out another unresolved absorption peak due to the transition between tunneling-split excited states. \textcopyright{} 1996 The American Physical Society.

Voltage-tunable far-infrared photodetector with a very large tuning range

Abstract A highly sensitive tunable far-infrared photodetector using the 1 → 2 intersubband Stark shift in compositionally asymmetric coupled quantum cells (CACQWs) is proposed here. The operation of this device is based on the infrared absorption by electrons in the ground state transited from the ground-state subband E 1 of the CACQW to the first-excited-state subband E 2 . A very large variation of eigenenergy spacing ΔE 21 ( E 2 − E 1 ) under an applied electric field can be achieved. Since the infrared radiation is absorbed via the intersubband resonance absorption ([ hstrok ] ω = E 2 − E 1 ), the detected infrared wavelength can be tuned by the ΔE 21 which can be adjusted by the applied electric field. Based on theoretical calculations, a tuning range from 8 to 16 μm is predicted. This tuning capability is achieved by varying the applied electric field in the 90 to −60 kV/cm range. The absorption coefficient is evaluated by the density-of-state formalism and a very large absorption coefficient is obtained for the proposed CACQW structure.

Magnetotransport, magneto-optical and electronic subband studies in highly strained one-side-modulation-doped double quantum wells

Shubnikov - de Haas (SdH), Van der Pauw Hall effect and cyclotron resonance measurements on asymmetric double quantum wells grown by metalorganic chemical vapour deposition have been carried out to investigate the magnetotransport properties of an electron gas and to determine the effective mass of the electron gas, subband energies and wavefunctions in the quantum well. Transmission electron microscopy measurements show that a 50 Å InAs and a 100 Å quantum well were separated by a 30 Å potential barrier in an active region. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi-two-dimensional electron gas (2DEG) in the quantum wells. The fast Fourier transformation results for the SdH data indicate clearly the electron occupation of two subbands in the double quantum wells. The results of the cyclotron resonance measurements show that one absorption resonance is evidence of electron concentration occupied at the ground state subband in the double quantum well. The electron effective mass determined from the slope of the main peak absorption energies as a function of a magnetic field is . Electronic subband energies and energy wavefunctions in the quantum wells were calculated by a self-consistent method taking into account exchange correlation effects together with strain and non-parabolicity effects. The ground state subband wavefunction is strongly localized in the InAs quantum well side, and the first excited subband wavefunction in the asymmetric quantum well is very weakly coupled over both InAs and wells.

Ultrahigh-magnetic-field cyclotron resonance in a two-dimensional electron gas at the grain boundary of Hg1-x-yCdxMnyTe.

The two-dimensional electron gas adjacent to the grain boundary in a bicrystal of the narrow-gap semiconductor p-type ${\mathrm{Hg}}_{0.79}$${\mathrm{Cd}}_{0.19}$${\mathrm{Mn}}_{0.02}$Te has been studied by means of cyclotron-resonance measurements in the mega-Gauss field range with ${\mathrm{CO}}_{2}$ and CO laser radiation sources. Under these conditions, the resonance energies are larger than both the depth of the confining potential well and the band-gap energy. For a photon energy of \ensuremath{\Elzxh}\ensuremath{\omega}=120 meV a pronounced single resonance line is observed at 23 T. The corresponding cyclotron effective mass exhibits a strong nonparabolicity---it increases about three times between 23 and 100 T, in agreement with the results of our self-consistent band-structure computations. Measurements in tilted magnetic fields reveal a lack of the angular dependence of the resonance field. The resonance linewidth at 23 T is larger by a factor of 7 than that evaluated from low-field transport data. This is interpreted as a result of the degeneracy of the first excited Landau level with the quasicontinuum of higher subband levels. In addition, the linewidth exhibits an abrupt increase in the magnetic-field range between 20 and 30 T. This is discussed in terms of two possible mechanisms: one is the magnetic-field-induced transfer of electrons into the ground-state subband, which is charcterized by efficient scattering of electrons by grain boundary defects; the other is the magnetic-field-induced localization of the two-dimensional electron gas at the grain boundary.

Very large Stark shift in three-coupled-quantum wells and their application to tunable far-infrared photodetectors

The quantum-confined Stark effect in the three-coupled-quantum-well (TCQW) structure is studied theoretically in this paper. The basic TCQW structures are composed of three quantum wells separated by two thin barriers. Coupled one-dimensional Schrödinger and Poisson equations are solved self-consistently to find the sub-band eigenenergies and the envelope wave functions for the TCQW structures. Results indicate that the GaInAs/AlGaAs/GaAs two-depth TCQW structure exhibits both a very large Stark shift and a high absorption coefficient for the 1→3 intersub-band transition. By using a 1→3 intersub-band Stark shift in the two-depth TCQW structure, a highly sensitive tunable far-infrared photodetector is proposed. This photodetector is ideal for device applications in the 8–14 μm atmospheric window region. The operation of this device is based on the infrared absorption by electrons in the ground state transited from the ground-state sub-band E1 of the TCQW to the second-excited-state sub-band E3. A very large variation of eigenenergy spacing ΔE31 between E3 and E1 under an applied electric field can be achieved. Since the infrared radiation is absorbed via the intersub-band resonance absorption (ℏω=E3−E1), the detected infrared wavelength can be tuned by the ΔE31 which can be adjusted by an applied electric field. Based on the theoretical calculations, a tuning range from 7.4 to 14 μm is predicted for the two-depth TCQW structure. This tuning capability is achieved by varying the applied electric field in the 60 to −60 kV/cm range.

Enhancement of the Stark effect in AlInAs/GaInAs coupled quantum wells and their application to tunable midinfrared photodetectors

A tunable midinfrared photodetector using 1→4 intersubband Stark shift in the four-level AlInAs/GaInAs coupled quantum wells (CQWs) is proposed. The operation of this device is based on the infrared absorption by the electrons in the ground state transited from the ground-state subband E1 of the CQWs to the third-excited-state subband E4. A large variation of eigenenergy spacing ΔE41 between E1 and E4 under an applied electric field can be achieved for the proposed AlInAs/GaInAs CQW system. Since the infrared radiation is absorbed via the intersubband resonance absorption (ℏω=E4−E1), the detected infrared wavelength can be tuned by the spacing ΔE41 which can be adjusted by an applied electric field. The tunability of these AlInAs/GaInAs CQWs have been studied theoretically. Based on theoretical calculations, tuning ranges from 3 to 3.9 and 2.9 to 4.2 μm are predicted for the proposed asymmetric CQW structure and compositionally asymmetric CQW structure, respectively. This tuning capability is achieved by varying the applied electric field in the 90 to −90 kV/cm range.

Charge control model of the double delta-doped quantum-well field-effect transistor

The charge control properties of Al/sub 0.3/Ga/sub 0.7/As-GaAs-Al/sub 0.3/Ga/sub 0.7/As quantum-well field-effect transistor (FET) with double /spl delta/-doped planes are studied theoretically. A simple capacitor-like charge control model for the double/spl delta/-doped quantum-well FET's has been proposed and verified through self-consistent calculation. The threshold voltage and the capacitance can be related to the structure through simple analytical equations. The effective separation between capacitor plates is found to be the thickness of AlGaAs layer d+d/sub i/+/spl delta/ plus a correction term to account for the distribution of N/sub 2DEG/ inside the GaAs quantum well. For small well widths, only the ground-state subband is occupied and there is a simple linear relation between N/sub 2DEG/ and the gate bias V/sub G/. For larger well widths, electrons occupied the lowest subband, then the next higher energy subband, and the relation between the N/sub 2DEG/ and the V/sub G/ can be divided into piece-wise linear regions. >

Optical study of high density two-dimensional electron gas in GaAs/Al0.3Ga0.7As modulation-doped quantum wells

We have investigated the optical properties of high density electron gas in GaAs/Al0.3Ga0.7As modulation doped quantum wells by using the photoluminescence measurements with the pumping photon energies above and below the band gap of the barrier and the resonant Raman scattering. We point out that the previous assignment of the transition between the second conduction subband and the ground-state heavy-hole subband in the photoluminescence spectra may be flawed. Thus, there is no large breakdown of the parity selection rule of the optical matrix element in quantum wells even if the carrier concentration is as high as 1012 cm−2. Our results are consistent with the theoretical investigation. We have also estimated the band gap shrinkages due to many-body interactions, which are comparable with the previous calculations.

Spectral linewidths of quantum well intersubband transitions

The effect of electron-electron interactions on quantum well intersubband transition spectral linewidths is examined. It is shown that the effect of exchange interactions on absorption linewidths can be comparable in magnitude to observed absorption linewidths, so that previous assumptions about the origin of observed widths may need to be revised. The exchange interaction width mechanism involves differences in the in-plane momentum dependence of electron energies in the ground state subband and the excited state subband. These differences are primarily due to intrasubband exchange interactions in the ground state subband.

Exchange interaction effects in quantum well infrared detectors and absorbers

Infrared excitation energies between the ground-state subband and the first excited-state subband in quantum wells are analyzed including the effect of exchange interactions on the ground-state subband. Analytic and numerical calculations relevant to infrared absorption and infrared detection are performed. Previous work on exchange interaction effects in narrow wells is extended to deal with well widths which can provide absorption peaks and photocurrent peaks throughout the long-wavelength infrared and very-long-wavelength infrared regime. Exchange effects are shown to be substantial.

Temperature dependence of the electron intersubband resonance on (100) Si surfaces

The dependence of the electron intersubband resonance on temperature has been studied between 4.2 and 130 K for photon energies 10.45, 15.81, and 44.32 meV. At the highest energy, distinct transitions from the ground state to each of the next three higher-lying levels (i.e., the 0\ensuremath{\rightarrow}1, 0\ensuremath{\rightarrow}2, 0\ensuremath{\rightarrow}3 resonances) are observed. With rising temperature, the lines originating from the 0 ground-state subband shift and diminish in amplitude. The effect is attributed to thermal population of the higher-lying levels, in particular the 0\ensuremath{'} subband of the fourfold degenerate valleys in the Si (100) surface. Firsthand evidence for the population of this level, and the existence of two distinct types of electrons at finite temperature, comes from the identification of a new resonance (i.e., the 0\ensuremath{'}\ensuremath{\rightarrow}1\ensuremath{'} transition) which grows with increasing temperature.

Evidence for mobility domains in (100) silicon inversion layers

In proof of the existence of domains with different mobilities measurements of Shubnikov-de Haas oscillations, piezoresistance, and transverse “Hall” field due to mobility anisotropy are provided. The energy splitting between the ground state subbands of different valleys has been determined.

Tunneling study of surface quantization inn−PbTe

Using electron tunneling through $n$-type PbTe-oxide-Pb junctions, we have measured some electronic properties of the (100) surface accumulation layer at the PbTe-oxide interface. Three energy levels, resulting from quantization of the electronic motion normal to the surface, are observed in a sample having $n=1.2\ifmmode\times\else\texttimes\fi{}{10}^{18}$ /${\mathrm{cm}}^{2}$. The energies of these three levels are ${E}_{0}=42$ meV, ${E}_{1}=19$ meV, and ${E}_{2}=6$ meV below the conduction band edge. From the magnetotunneling data we obtain ${g}^{*}=29\ifmmode\pm\else\textpm\fi{}3$ and ${m}^{*}=(0.069\ifmmode\pm\else\textpm\fi{}0.008){m}_{0}$ for the ground-state subband. In addition, the Landau levels in the ground-state subband do not cross the Landau levels in the excited-state subband. The interaction between the Landau levels removes the degeneracy at the crossover and a splitting, $\ensuremath{\Delta}E\ensuremath{\approx}4$ meV, has been observed.