J. Yoshino

Concentration of electrons in selectively doped GaAlAs/GaAs heterojunction and its dependence on spacer‐layer thickness and gate electric field

The concentration Ns of two‐dimensional electrons in N‐GaAlAs/GaAs systems is studied; Ns shows a systematic decrease when the thickness Wsp of an undoped GaAlAs ‘‘spacer layer’’ is increased. Such a dependence is shown to be well explained by the theory in which the size quantization is taken into account. Furthermore, Ns is studied as a function of gate voltage in field‐effect transistor (FET) structures and found to saturate once Ns reaches its limiting value. This phenomenon is strongly dependent on Wsp and is well ascribed to the formation of nondepleted region in the GaAlAs layers. Implications for FET designs are discussed.

Recombination lifetime of carriers in GaAs‐GaAlAs quantum wells near room temperature

The lifetime τ of carriers in undoped GaAs‐GaAlAs quantum well structures was studied at room temperature by using the photoluminescence phase shift method. We have found that τ is inversely proportional to the carrier concentration under the excitation levels of 1016–1018 cm−3. The comparison of these results with both the theory of band‐to‐band recombination and the measured dependence of τ on the carrier concentration at 77 K suggests that the carrier recombination near room temperature is dominated by this band‐to‐band recombination process. We also discussed the dependence of τ on the quantum well thickness.

Effects of electron heating on the two-dimensional magnetotransport in AlGaAs/GaAs heterostructures

Effects of electron heating on Shubnikov-De Haas oscillations are studied at 4.2 K. in n-AlGaAs/GaAs heterostructures. Electron temperatures Te are determined and found to depend mainly on the input power per electron Pe, suggesting the energy relaxation time at low temperatures is independent of sample mobilities and approximately proportional to the inverse of carrier concentrations. When Pe > 10−13 W and Te > 40 K, extra peaks are observed in SdH oscillations, which can be attributed to the resonant emission of optical phonons. The electron heating is found to reduce the plateau width of quantized Hall resistivities, probably because such a heating causes the delocalization of electrons.

Photoluminescence and absorption linewidth of extremely flat GaAs-AlAs quantum wells prepared by molecular beam Epitaxy including interrupted deposition for atomic layer smoothing

Abstract GaAs-AlAs quantum well (QW) structures with extremely flat interfaces are prepared by our new growth procedure and their photoluminescence (PL) and absorption linewidths are studied at various temperatures. The PL linewidths of our QWs measured at 77 and 4.2 K are found to be the narrowest that have ever been published, and indicate the effective interface roughness to be less than 0.19 atomic layer. Excellent correlation was found to exist between the PL and absorption lineshapes, suggesting the validity of estimating interface roughness from the PL linewidths.

Effects of substrate temperatures on the doping profiles of Si in selectively doped AlGaAs/GaAs/AlGaAs double‐heterojunction structures

Electrical properties of selectively doped double heterojunctions with a thin GaAs layer (≤300 A) grown by molecular beam epitaxy are known to depend significantly on the substrate temperatures during growth. Although samples grown at 530u2009°C show high electron mobilities and reasonable electron concentrations, much lower mobilities and rather high electron concentrations have been obtained for those grown at 630u2009°C. Secondary ion mass spectrometry studies have been performed and have revealed that the degraded electrical properties are caused mainly by the surface migration of Si and the subsequent incorporation of Si into the undoped GaAs layer of the quantum well.

Self‐consistent calculation of electronic states in AlGaAs/GaAs/AlGaAs selectively doped double‐heterojunction systems under electric fields

The electronic states in AlGaAs/GaAs/AlGaAs selectively doped double‐heterojunction (SD‐DH) systems or single‐quantum‐well systems have been calculated self‐consistently for the case where an external gate voltage is applied perpendicularly to the surface. The transition from symmetrical to asymmetrical distributions of electrons by the external field is predicted. The calculated variations of electron concentration with the gate voltage are found to be in good agreement with the experiment. The shoulder structures observed in the gate voltage dependence of mobility have been successfully accounted for and ascribed to the onset of electron population in the upper subbands. This analysis is expected to provide powerful means to evaluate the performance of SD‐DH field‐effect transistors and to optimize their design.

Energy levels and electron wave functions in semiconductor quantum wells having superlattice alloylike material (0.9 nm GaAs/0.9 nm AlGaAs) as barrier layers

Energy levels and wave functions of carriers are studied both experimentally and theoretically in 4 nm GaAs quantum wells (QW’s), for the case when barriers are formed with alternating layers of 0.9 nm GaAs/0.9 nm AlxGa1−xAs (x=0.39). The photoluminescence spectra of the QW’s are studied at 77 K and are found nearly equivalent to that of conventional QW’s having alloy barriers with Al content of 0.26, which is much higher than the averaged alloy composition (∼0.2). The modified Kronig–Penney analysis is found effective in predicting the observed energy and has clarified a feature of enhanced penetration of wave function into the novel barrier layer.

Light emission from zero‐dimensional excitons—Photoluminescence from quantum wells in strong magnetic fields

We report, for the first time, the spontaneous light emission from ‘‘zero‐dimensional exciton states’’ in GaAs/GaAlAs quantum wells under a high magnetic field at 80 K. The formation of such fully quantized states is evidenced by observing the wavelength shift Δ λ of the spontaneous emission peak as well as the narrowing of the spectrum width as the magnetic field is raised up to 15 T. The observed shift Δλ is shown to be well explained by the theory in which the high magnetic field effect on two‐dimensional hydrogenic exciton is taken into account. The formation of such a novel state is further evidenced by the strong anisotropy of the photoluminescence spectrum, which depends on the direction of the magnetic field.

Field‐dependent transport of electrons in selectively doped AlGaAs/GaAs/AlGaAs double‐heterojunction systems

The transport properties of high‐mobility electrons in selectively doped AlGaAs/GaAs/AlGaAs double‐heterojunction systems have been investigated for electric fields (E) up to several kV/cm by pulsed Hall and pulsed current‐voltage measurements. It was found that electron mobilities began to decrease drastically in proportion to E−0.8 when the electric fields exceeded (6.2–6.6)×106 (cm/s)/μ0, where μ0 is the low field electron mobility. The saturation of electron velocity was observed at E≥ 2 kV/cm, and its estimated value of (1.5–1.7)×107 cm/s was found to be almost independent both of the temperature in the range below 100 K and the density of electrons in the GaAs well of 3.8×1011 and 1.2×1012/cm2.

Spatially modulated photoconductivity at N‐AlGaAs/GaAs heterojunctions and formation of persistent charge patterns with submicron dimensions

The electron concentration at an N‐AlGaAs/GaAs interface is shown to be spatially modulated at low temperatures by the selective photoionization of deep donors in AlGaAs using two interfering laser beams. The charge pattern thus formed is persistent and detected by measuring the anisotropy of the channel conductivity. Grating patterns with a period as small as 0.6 μm were successfully recorded in accordance with the prediction that spatial resolution comparable with the AlGaAs layer thickness (∼0.1 μm) will be achieved.