E.F. Schubert

The delta-doped field-effect transistor (δFET)

A field-effect transistor (FET) using a two-dimensional electron gas (2DEG) as an electron channel is fabricated from GaAs grown by molecular-beam epitaxy. The doping profile of the field-effect transistor is described by the Dirac delta (δ) function. The subband structure of δ-doped GaAs is calculated. The characteristics of the δFET are a high concentration of the 2DEG, a high breakdown voltage of the Schottky contact, a narrow distance of the 2DEG from the gate, and a high transconductance. These properties are analyzed. Preliminary results for the extrinsic transconductance and for the transit frequency are obtained from δFETs having nonoptimized structures.

GaAs sawtooth superlattice laser emitting at wavelengths λ>0.9 μm

A new type of semiconductor superlattice laser grown by molecular beam epitaxy is realized in GaAs. The active region of the injection laser consists of alternating n and p Dirac-delta doped layers resulting in a sawtooth-shaped conduction-band and valence-band edge. The band gap of this new GaAs superlattice is smaller than the GaAs bulk band gap. Room-temperature operation of broad-area GaAs sawtooth superlattice (STS) injection lasers is demonstrated at a wavelength of 905 nm. The threshold current density of the STS laser is 2.2 kA/sq cm. 10 references.

Electron subband structure in selectively doped n-Al x Ga 1-x As/GaAs heterostructures

The electron subband structure in the triangular potential well of a selectively doped n-Al<inf>x</inf>Ga<inf>1-x</inf>As/GaAs heterostructure is calculated by a new analytical method. The GaAs conduction band edge is approximated by a polygonal curve and the electron de Broglie wavelength is matched to the width of the triangular quantum well. The band bending due to ionized donor impurities, free electrons in the potential well, and residual acceptors in the p^-GaAs buffer layer are taken into account. The electron subband structure is determined for background accepter concentrations of<tex>1 \times 10^{14}</tex>cm^-3<tex>\leq N_{A} \leq 1 \times 10^{16}</tex>cm^-3in the p^-GaAs layer and for two-dimensional carrier concentrations in the potential well ranging from 5 × 10¹⁰cm^-2to 2 × 10¹²cm^-2, and the electron concentrations in the first three subbands are presented. The first excited subband is populated at two-dimensional carrier concentrations of<tex>2.2 \times 10^{11}</tex>to<tex>9.8 \times 10^{11}</tex>cm^-2depending on the background impurity concentration in the p^-GaAs layer. The real-space widths of electron subbands range from 6 to 20 nm. The accuracy of our method for calculation of subband structures is favorable for quantitative device design.

Preparation and properties of a new GaAs sawtooth doping superlattice

Abstract GaAs sawtooth doping superlattices consist of alternating n- and p-delta-doped layers separated by undoped regions. The space-charge-induced sawtooth-shaped periodic modulation of the band edges leads to a confinement of electrons and holes in alternate layers. The energy gap of the superlattice is smaller than that of the GaAs host material, and it remains stable even at high excitation densities due to the short carrier lifetime in the nanosecond range. This feature is in contrast to the tunable electronic properties of conventional doping superlattices. We have fabricated light emitting diodes and injection lasers from the new GaAs sawtooth superlattice which operate at room temperature in the 900 to 1000 nm range.

Photoconductivity in selectively n- and p-doped AlxGa1−xAs/GaAs heterostructures

Abstract The doping characteristics of direct-gap bulk n-AlxGa1−xAs doped with Si and grown by molecular-beam epitaxy are determined by the coexistence of shallow and deep donors, both of which are caused by Si-impurities. The deep donor is ionized thermally at high temperatures and optically at low temperatures, resulting in a persistent increase of the free carrier concentration. The persistent photoconductivity in selectively n-doped n-AlxGa1−xAs/GaAs heterostructures is predominantly caused by the photoionization of the deep Si-donor in the AlxGa1−xAs layer. Electron-hole generation in the GaAs contributes only a minor part to persistent photoconductivity in n-type heterostructures. In p-type heterostructures, however, electron-hole generation is the dominant mechanism responsible for persistent photoconductivity and contributes 5 × 1010 holes per cm2 for a 1 μm thick GaAs buffer layer to the two-dimensional electron- (hole-) gas. The hole concentration in selectively p-type heterostructures is calculated versus (i) Al-mole fraction, (ii) acceptor concentration in the p-AlxGa1−xAs, and (iii) AlxGa1−xAs spacer width. Theoretical hole concentrations agree well with experimental data, if the bandgap difference of GaAs and AlxGa1−xAs is taken to be distributed by a ratio of if 75 25 to the conduction- and valence-band discontinuity. Hall measurements on selectively p-doped AlxGa1−xAs/GaAs heterostructures reveal a freeze-out of carriers and high hole mobilities at low temperatures. The acceptor binding energy in the p = AlxGa1−xAs:Be is determined to be Ea = 26 ± 3 meV for x = 0.45.

GaAs sawtooth superlattice laser emitting at wavelengths greater than 0. 9 micron

A new type of semiconductor superlattice laser grown by molecular beam epitaxy is realized in GaAs. The active region of the injection laser consists of alternating n and p Dirac-delta doped layers resulting in a sawtooth-shaped conduction-band and valence-band edge. The band gap of this new GaAs superlattice is smaller than the GaAs bulk band gap. Room-temperature operation of broad-area GaAs sawtooth superlattice (STS) injection lasers is demonstrated at a wavelength of 905 nm. The threshold current density of the STS laser is 2.2 kA/sq cm. 10 references.

GaAs sawtooth superlattice laser emitting at wavelengths lambda>0. 9. mu. m

A new type of semiconductor superlattice laser grown by molecular beam epitaxy is realized in GaAs. The active region of the injection laser consists of alternating n and p Dirac-delta doped layers resulting in a sawtooth-shaped conduction-band and valence-band edge. The band gap of this new GaAs superlattice is smaller than the GaAs bulk band gap. Room-temperature operation of broad-area GaAs sawtooth superlattice (STS) injection lasers is demonstrated at a wavelength of 905 nm. The threshold current density of the STS laser is 2.2 kA/sq cm. 10 references.