Y. C. Kao

Nine-state resonant tunneling diode memory

The authors demonstrate an epitaxial series combination of eight pseudomorphic AlAs/In/sub 0.53/Ga/sub 0.47/As/InAs resonant tunneling diodes (RTDs) grown by molecular beam epitaxy on InP. This series RTD produces an eight-peak multiple negative differential resistance characteristic with a peak-to-valley current ratio (PVR) exceeding 2 per peak at a peak current density of approximately 6 kA/cm/sup 2/. Hysteresis in the current-voltage characteristic is reduced by uniformly Si doping the double-barrier resonant tunneling region at a density of 5*10/sup 16/ cm/sup -3/. Using this multiple-peak RTD in series with a field-effect transistor load, a nine-state multivalued memory circuit is demonstrated.<<ETX>>

Phonon mode study of near-lattice-matched InxGa1-xAs using micro-Raman spectroscopy

We identify the four allowable phonon modes in InxGa1−xAs on InP:InAs‐like transverse optical (TO) (225±2 cm−1), InAs‐like longitudinal optical (LO) (233±1 cm−1), GaAs‐like TO (255±2 cm−1), and GaAs‐like LO (269±1 cm−1), using the selectivity of first‐order Raman scattering off the (100) normal surface and the (011) cleaved plane and detailed line‐shape analysis employing a sequential simplex optimization procedure. Raman scattering off the (011) cleaved plane was achieved for the first time in thin‐film InGaAs using microprobing capabilities (∼1 μm). We also identify another phonon mode R* at 244 cm−1 which is attributed to an alloy disorder mode in these films. For the five identified phonon modes, a linear relationship between the Raman frequencies and composition determined from x‐ray diffraction was determined for near‐lattice‐matched conditions (0.42<1−x<0.52).

Generation and propagation of threading dislocations in GaAs grown on Si

The generation and propagation of threading dislocations in GaAs grown on Si was studied by transmission electron microscopy. This study was conducted by analyzing dislocation structures in GaAs at various growth stages. The dislocation structure changes as the GaAs grows from initial islands to a thick film. These changes are explained by the following physical processes: (1) nucleation of dislocations in GaAs islands, (2) generation and propagation of threading dislocations during coalescence of initial GaAs islands and their subsequent growth, and (3) bending of threading dislocations under misfit‐strain force and their interactions. The second process results in threading dislocations extending from the interface to the surface in thin films. The third process causes an improvement in near‐surface quality in thick films. The iplication of this study in confining threading dislocations to near the interface is discussed.

Detection and reduction of indium segregation during molecular-beam epitaxial growth of InGaAs/GaAs using in situ reflection mass spectrometry

The use of reflection mass spectrometry (REMS) as an in situ diagnostic for surface segregation of indium during InGaAs layer growth is reported. InGaAs growth at substrate temperatures above 500 °C yields a REMS signature which indicates a thickness‐dependent surface In content. Adapting a simple segregation model [K. Muraki, S. Fukatsu, Y. Shiraki, and R. Ito, Appl. Phys. Lett. 61, 557 (1992)], the segregation ratio R was extracted under various deposition conditions in real time. The segregation ratio obtained during InGaAs/GaAs multiple quantum well growth at 500–530 °C suggests the presence of InGaAs composition grading near interfaces, and agrees qualitatively with ex situ characterization by x‐ray diffraction. The high values of R (0.7–0.8) observed under normal device‐layer growth produces a surface layer with high In content (i.e., InAs). The segregation ratio was not sensitive to unrelaxed layer strain, but showed a large increase under conditions which probably produce island (three‐dimensional...

Experimental sensitivity analysis of pseudomorphic InGaAs/AlAs resonant‐tunneling diodes

Through the use of a novel vertically integrated resonant‐tunneling diode (RTD) heterostructure we have established experimentally the relationship between intentional variations in the structural parameters of the pseudomorphic In0.53Ga0.47As/AlAs resonant tunneling diode (i.e., barrier thickness, quantum‐well thickness, quantum‐well composition, and doping density) and the measured current–voltage characteristics of the device. Based upon the results of these experiments, we have determined that a 1 monolayer increase in AlAs barrier width, InGaAs quantum‐well width, or InAs subwell width results in a peak current reduction of 56%±7%, 19%±2%, and 18%±3%, respectively. Further, a 1% decrease in indium mole fraction of the InGaAs quantum well has been found to increase the peak current by 10%±1%. Sensitivity parameters have been tabulated for both the peak current and the peak voltage of the RTD. Through the use of these parameters, the maximum allowed fluctuation in the RTDs structural parameters has bee...

Co-integration of resonant tunneling and double heterojunction bipolar transistors on InP

The authors report the first co-integration of resonant tunneling and heterojunction bipolar transistors. Both transistors are produced from a single epitaxial growth by metalorganic molecular beam epitaxy, on InP substrates. The fabrication process yields 9- mu m/sup 2/-emitter resonant tunneling bipolar transistors (RTBTs) operating at room temperature with peak-to-valley current ratios (PVRs) in the common-emitter transistor configuration, exceeding 70, at a resonant peak current density of 10 kA/cm/sup 2/, and a differential current gain at resonance of 19. The breakdown voltage of the In/sub 0.53/Ga/sub 0.47/As-InP base/collector junction, V/sub CBO/, is 4.2 V, which is sufficient for logic function demonstrations. Co-integrated 9- mu m/sup 2/-emitter double heterojunction bipolar transistors (DHBTs) with low collector/emitter offset voltage, 200 mV, and DC current gain as high as 32 are also obtained. On-wafer S-parameter measurements of the current gain cutoff frequency (f/sub T/) and the maximum frequency of oscillation (f/sub max/) yielded f/sub T/ and f/sub max/ values of 11 and 21 GHz for the RTBT and 59 and 43 GHz for the HBT, respectively.<<ETX>>

In situ detection of relaxation in InGaAs/GaAs strained layer superlattices using laser light scattering

We report the use of laser light scattering (LLS) for the in situ detection of strained epitaxial layer relaxation. Strained layer superlattices (SLSs) of InGaAs/GaAs were prepared by molecular beam epitaxy. The rapid increase in the LLS signal was interpreted as increased surface roughness due to surface steps generated during InGaAs relaxation. The LLS signal was sharply peaked with respect to the azimuthal angle (the rotation angle between crystal axes and the detection axis), indicating the scattering comes primarily from α misfit dislocations which run parallel to the (011) direction. The growth time at which the LLS signal onset occurred, together with the InGaAs growth rate, yielded the critical layer thickness, hc. The hc value for SLSs of In0.17Ga0.83As/GaAs with thicknesses of 4.6/17 and 4.6/7.8 nm were 25 and 23 nm, respectively, and almost identical to values obtained for single InGaAs layers. The observed values of hc are greater than those calculated using the standard force‐balance model. ...

Multibit resonant tunneling diode SRAM cell based on slew-rate addressing

We propose and demonstrate a resonant-tunneling diode (RTD) based memory cell in which N bits are stored in a series combination of N RTDs without internal node contacts. The slew rate of an applied voltage signal determines the circuit switching dynamics and allows addressing of the bits. We verify slew rate dependent switching order of up to four series RTDs experimentally and through SPICE simulation incorporating a physics-based RTD model. The new addressing scheme allows N bits to be stored in a stack of N vertically integrated RTDs compared to log/sub 2/ (N) bits in previous demonstrations. We demonstrate a two-bit two-RTD static memory cell based on the new method.

A study of the coupling efficiency versus grating periodicity in a normal incident GaAs/AlGaAs multiquantum well infrared detector

A detailed study of the dependence of coupling quantum efficiency on the grating periodicity of planar metal grating coupled GaAs/AlGaAs quantum well infrared photodetectors has been made in this work. Five different detector samples with grating periodicities of Λ=1.1, 3.2, 5, 7, and 10 μm have been fabricated for the present study. The results showed that the device with a 5 μm grating periodicity gave the best front‐side coupling efficiency, which was in good agreement with our theoretical prediction. A single pass quantum efficiency (η) of 11% was achieved for the front‐side illumination at λp=9.8 μm and T=77 K.

New field‐effect resonant tunneling transistor: Observation of oscillatory transconductance

We present device characteristics of a field‐effect, unipolar, resonant tunneling transistor. An oscillatory tunneling current in the transfer characteristics is observed for the first time. Our observation confirms a recent hypothesis that a mere three‐to‐two dimensional resonant tunneling can occur when scattering rate is less than the attempt frequency of tunneling electrons in the quantum well.