R. F. Kopf

Beryllium δ doping of GaAs grown by molecular beam epitaxy

Spatial localization of Be in δ‐doped GaAs within few lattice constants (<20 A) is achieved at low growth temperatures for concentrations N2DBe <1014 cm−2 as indicated by capacitance‐voltage profiles and secondary ion mass spectroscopy. At elevated growth temperatures and at higher Be concentrations, significant spreading of the dopants occurs and is explained by (i) Fermi‐level pinning‐induced segregation, (ii) repulsive Coulomb interaction of dopants, and (iii) diffusion. The highest Be concentration achieved at low growth temperatures exceeds 2×1020 cm−3 and is limited by repulsive dopant interaction. It is shown that the repulsive Coulomb interaction results in a correlated, nonrandom dopant distribution. The diffusion coefficient of Be in GaAs is determined and is found to be much lower than previously reported.

Stability of carbon and beryllium‐doped base GaAs/AlGaAs heterojunction bipolar transistors

GaAs/AlGaAs heterojunction bipolar transitors (HBTs) utilizing highly Be‐doped base layers display a rapid degradation of dc current gain and junction ideality factors during bias application at elevated temperature. For example, the gain of a 2×10 μm2 device with a 4×1019 cm−3 Be‐doped base layer operated at 200 °C with a collector current of 2.5×104 A cm−2 falls from 16 to 1.5 within 2 h. Both the base emitter and base collector junction ideality factors also rise rapidly during device operation, and this current‐induced degradation is consistent with recombination‐enhanced diffusion of Be interstitials producing graded junctions. By sharp contrast, devices with highly C‐doped (p=7×1019 cm−3) base layers operated under the same conditions show no measurable degradation over much longer periods (12 h). This high degree of stability is most likely a result of the fact that C occupies the As sublattice, rather than the Ga sublattice as in the case of Be, and also has a higher solubility than Be. The effect...

Room‐temperature electroabsorption and switching in a GaAs/AlGaAs superlattice

We report room‐temperature observation of Wannier–Stark localization in a GaAs/AlGaAs superlattice. We show that large modulation can be obtained over a wide spectral range and demonstrate the operation of a self‐electro‐optic effect device.

Band offset determination in analog graded parabolic and triangular quantum wells of GaAs/AlGaAs and GaInAs/AlInAs

The band offset parameter Qc = ΔEc/ΔEg for both GaAs/AlGaAs (lattice matched to GaAs), and GaInAs/AlInAs (lattice matched to InP) was extracted from the optical interband transition energies obtained from both triangular and parabolic quantum well shapes of various widths. The wells were grown using continuous analog compositional grading as opposed to the discrete, superlattice (digital) grading used by previous researchers. Electron beam electroreflectance (EBER) was the primary technique used to measure the interband transition energies. By combining the theoretical energies from quantum mechanical potential well calculations with EBER measured energies, it was possible to extract band offset values in a self‐consistent manner. Qc values obtained were 0.658±0.009 and 0.650±0.001 for GaAs/AlGaAs and GaInAs/AlInAs, respectively. Measurements also revealed that Qc was both temperature and concentration independent within the range of composition studied.

Increased fiber communications bandwidth from a resonant cavity light emitting diode emitting at λ=940 nm

Substrate‐emitting InGaAs/AlGaAs resonant cavity light emitting diodes (RCLEDs) emitting at λ=940 nm have been fabricated for use in optical communications. The devices exhibit a high output efficiency, with a far‐field intensity of 85 μW/Steradian from a planar surface at a current of 14 mA. The spontaneous spectrum exhibits a very narrow peak of only 5 nm width, as opposed to the 50‐nm‐wide peak of an 875 nm wavelength reference LED. We show that the narrow spectrum drastically reduces the effects of chromatic dispersion within a 3.37 km length of 62.5 μm core graded index multimode fiber. The resulting −3 dB frequency is 102 MHz for the RCLED and fiber system, as opposed to only 33 MHz for the chromatic dispersion limited reference device.

A 2 kbit array of symmetric self-electrooptic effect devices

A 64*32 array of symmetric self-electrooptic effect devices, each of which can be operated as a memory element or logic gate, is discussed. The required optical switching energies of the devices were approximately 800 fJ and approximately 2.5 pJ at 6 and 15 V bias, respectively, and the fastest switching time measured was approximately 1 ns. Either state of the devices could be held with continuous or pulsed incident optical signals with an average optical incident power per input beam of approximately 200 nW or less than 1 mW for the entire array. Photocurrent and reflectivity were measured for all 2048 devices. Only one device failed to have the negative resistance required for bistability, and only nine of the devices fell outside a band of +or-20% of the mean. Additionally, over 200 devices in the array were operated in parallel using low-power semiconductor laser diodes.<<ETX>>

Elimination of heterojunction band discontinuities by modulation doping

Heterojunction band discontinuities have been an active field of research during the last decade’ and made possible the realization of new device concepts such as modulation-doped transistors, heterobipolar transistors, and quantum-well lasers. The physical principles of these devices are based on heterojunction band discontinuities. In other device structures, however, heterojunction band discontinuities impede the flow of charge carriers across the junction. These structures include the optical distributed Bragg reflector which consists of alternating layers of two semiconductors with different refractive index, each having a thickness of a quarter wavelength. If distributed Bragg reflectors are used for current conduction, the constituent heterojunction band discontinuities impede the current flow, which is a highly undesired concomitant effect. It is the purpose of this publication to demonstrate that unipolar heterojunction band discontinuities can be eliminated by modulation doping and compositional grading of heterojunctions. The charge carrier transport across a heterojunction is illustrated in Fig. 1, which shows the band diagram of two semiconductors “A” and “B.” Band discontinuities occur in the conduction and valence band since the fundamental gap of semiconductor B is larger than the gap of A. Such discontinuities are usually referred to as type-1 heterojunctions, which contrast to type-11 (staggered) and type-III (broken gap) heterostructures. Transport across the heterojunction barrier can occur via thermal emission or via tunneling as schematically illustrated in Fig. 1. For sufficiently thick and high barriers, tunneling and thermal emission of carriers are not efficient transport mechanisms across the barrier. It is therefore desirable to eliminate such heterojunction band discontinuities in the conduction or valence band. Modulation doping of a parabolically graded heterojunction will next be shown to result in a flat-band-edge potential. The band diagram of a parabolically graded conduction-band edge is shown in Fig. 2 (a). The energy of the band edge increases parabolically with a positive second derivative between the points z, and z,. The band edge further increases parabolically with a negative second derivative between z2 and zs. The energy of the band edge can be expressed as / -&(z,) + 2(zf~z,)‘iz - zd’

Operation of a fully integrated GaAs-Al/sub x/Ga/sub 1-x/As FET-SEED: a basic optically addressed integrated circuit

The authors experimentally demonstrate the operation of a fully integrated optoelectronic circuit with optical input and output consisting of a p-i-n photodetector and load resistor, a depletion-mode GaAs-Al/sub x/Ga/sub 1-x/As heterostructure field-effect transistor (HFET) and self-biased HFET load, together with an output GaAs-Al/sub x/Ga/sub 1-x/As multiple quantum-well optical modulator. All elements have been monolithically integrated within a 50- mu m*50- mu m area. A low optical power input causes a modulation of a higher-power output, demonstrating optical signal amplification.<<ETX>>

Photoluminescence studies of the effects of interruption during the growth of single GaAs/Al0.37Ga0.63As quantum wells

Low‐temperature excitation and photoluminescence spectra are described for single GaAs/Al0.37Ga0.63As quantum wells grown by molecular beam epitaxy with and without a 2‐min interruption of growth at the heterointerfaces. The spectra from samples grown with interruption include well‐resolved multiple sharp peaks which are due to changes in well thickness of one monolayer and to bound excitons. These peaks are as narrow as 1.0, 1.7, and 6.0 meV for single wells of width 57, 28, and 17 A, respectively.

Shallow donors and D-X centers neutralization by atomic hydrogen in GaAlAs doped with silicon

Hydrogen plasma exposure of silicon‐doped Ga1−xAlxAs epilayers with x<0.37 causes a strong reduction of the free‐electron concentration in the layers. For x<0.29, this effect is accompanied by a simultaneous increase of the electron mobility. This is interpreted, as in GaAs, in terms of a neutralization of the active silicon donors by atomic hydrogen. The neutralization efficiency of the shallow donors increases as x increases. For x≂0.25, the D‐X centers are very efficiently neutralized by hydrogen and, as a consequence, the conductivity mechanisms after exposure are only governed by the remaining shallow donors. For 0.29<x<0.37, most of the D‐X centers are neutralized, but the electron mobility after hydrogenation is reduced.