R. Kapre

Growth of InxGa1−xAs on patterned GaAs(100) substrates

The influence of finite substrate size on misfit dislocation densities in strained systems is examined through the growth of InxGa1−xAs on nonplanar patterned (100) GaAs substrates consisting of parallel mesas of widths between 6500 A and 1.3 μm and of macroscopic length. Cross‐sectional transmission electron microscopy (XTEM) studies on In0.11Ga0.89As films reveal that while the mean misfit dislocation spacing for the growth in the nonpatterned region was ∼1500 A, for the growth on the mesas no misfit dislocations running parallel to the mesa length were observed. This is likely due to strain relief at the mesa edges, possibly brought about by the ability to transfer strain energy from cluster coalescence boundaries to the mesa edges and/or reduced cluster coalescence boundaries when the mesa size becomes comparable to or less than the effective migration length.

Realization and analysis of GaAs/AlAs/In0.1Ga0.9As based resonant tunneling diodes with high peak-to-valley ratios at room temperature

Investigations of pseudomorphic resonant tunneling diodes based on the GaAs/AlAs/In0.1Ga0.9As material system reveal that the use of undoped In0.1Ga0.9As spacer layers gives rise to a significantly enhanced peak‐to‐valley ratio of 3.2 and 14 at 300 and 77 K, respectively, as opposed to 2 and 5 obtained with the use of conventional GaAs spacers in an otherwise identical structure. This is achieved without any significant degradation of the peak current density but rather through reduction of the undesired nonresonant valley current. Comparison of the experimental results with calculations done using the Airy function transfer matrix approach indicates that the Γ‐X discontinuity at the GaAs/AlAs and In0.1Ga0.9 As/AlAs interface is relevant for electron tunneling, along with the smaller transverse effective mass of 0.19m0 in the AlAs X valley. We also find that In0.1Ga0.9As layers grown under an excess As‐stabilized growth condition give better results than those grown under a barely As‐stabilized condition.

Highly strained GaAs/InGaAs/AlAs resonant tunneling diodes with simultaneously high peak current densities and peak-to-valley ratios at room temperature

Highly strained In0.33Ga0.67As/AlAs‐based resonant tunneling diodes have been fabricated on GaAs(100) substrates without the use of thick strain relieving buffer layers. These structures exhibit a simultaneously high peak current density (Jp) of 125 kA/cm2 and a peak to valley ratio (PVR) of 4.7. A PVR of 5.9 with Jp=73 kA/cm2 is observed on some devices, the highest PVR seen for such devices. The excellent resonant tunneling characteristics of these devices are attributed to accurate device design using a Γ‐X‐Γ‐X‐Γ resonant tunneling path and to high quality interfaces obtained through the use of optimized growth conditions.

Realization of low defect density, ultrathick, strained InGaAs/GaAs multiple quantum well structures via growth on patterned GaAs (100) substrates

Growth of low defect density highly strained InxGa1−xAs/GaAs multiple quantum well (MQW) structures of thicknesses suited for application in spatial light modulator (SLM) devices operating in infrared has been thwarted by the occurrence of strain‐induced defects. Exploiting the notion of strain relief at mesa edges, we report here the first realization of very low defect density MQW structures of thickness as high as 2.38 μm at x=0.20. This has opened up the possibility of realizing a variety of reflective and transmissive SLM structures which also fruitfully exploit the transparent nature of the substrate.

High‐contrast optically bistable optoelectronic switch based on InGaAs/GaAs (100) asymmetric Fabry–Perot modulator, detector, and resonant tunneling diode

The realization at room temperature of a high contrast ratio (20:1) and an on‐state reflectivity of 46.5% in an optically bistable switch involving strained InGaAs/GaAs (100) multiple‐quantum‐well‐based asymmetric Fabry–Perot reflection modulator, detector, and InGaAs/AlAs‐based resonant tunneling diode and an Si field‐effect transistor is reported.

High contrast ratio self-electro-optic effect devices based on inverted InGaAs/GaAs asymmetric Fabry-Perot modulator

We demonstrate a new class of ‘‘normally off’’ high contrast ratio asymmetric Fabry–Perot (ASFP) reflection modulators based on a blue‐shift of the Fabry–Perot mode under bias. The negative differential resistance (NDR) exhibited by the photocurrent‐bias behavior was exploited to implement self‐electro‐optic devices (SEED) using as the load (a) a photodiode (D‐SEED), and (b) a phototransistor (T‐SEED). In the D‐SEED scheme, a contrast ratio of ∼50:1 with ∼20% throughput was realized. The T‐SEED configuration was found to offer nearly as high contrast ratio but with a ∼200:1 gain from the control light beam. Moreover, the ASFP modulators used belong to the inverted configuration which offers considerable convenience in device integration and optical interconnections.

Interfacet migration and defect formation in heteroepitaxy on patterned substrates: AlGaAs and InGaAs on GaAs (100) in MBE

We present results for lattice matched (AlxGai..xAs, x=0.5 and x=0.0) and lattice mismatched (InxGai..xAs, x□0.25) growth on patterned GaAs (100) substrates. For the AlGai..xAs structures, the GaAs substrates were patterned in the form of elongated mesas parallel to [011 II with widths of approximately 3 tim. Interfacet migration effects observed on the nesas via cross-section transmission electron microscope studies are explained in terms of a ledge-ledge interaction on the vicinal surfaces formed due to growth on the mesas. InxGai..xAs (x□0.25) structures were grown on GaAs (100) substrates patterned in the form of elongated mesas parallel to [01 11 with widths of approximately 1 tim. This patterning direction was chosen since under cutting in the [0 1 1] direction eliminates inter-facet migration effects so that compositional change induced strain effects can be minimised. For x □0.15, we find a reduction in misfit dislocation densities in films upto five times the nominal critical thickness for growths on the patterned mesas as compared to the growths on the corresponding non patterned regions. For x=0.25 no such difference is observed and a large number ofthreading dislocations ( around iO cm2) are found in both the patterned and the non patterned regions. This is believed to be a consequence of the onset ofa 3-D island growth mode. Finally we present some results for the growth of InØ5Gaj•75As I AlAs resonant tunneling diode (RTD) structures and a 100 period InjØGaØ8As (80 A) IGaAs (160 A) Multiple Quantum Well (MQW) such as suited for spatial light modulator (SLM) structures on GaAs (100) substrates patterned in both <01 1> directions on a length scale of 12 to 20 tm. For the RTD structures we conclude that benefits from patterning are expected for x□0.25 provided the growth kinetics are appropriately adjusted to prevent 3D island growth mode. For the MQW -SLMstructure we demonstrate superior optical properties for the growth in the patterned region and a corresponding absence of threading dislocations in the central region of the mesas.

In/sub 0.25/Ga/sub 0.75/As/AlAs-based resonant tunneling diodes grown on prepatterned and nonpatterned GaAs

The DC current-voltage characteristics of strained In/sub 0.25/Ga/sub 0.75/As/AlAs resonant tunneling diode (RTD) structures grown on GaAs

Highly strained pseudomorphic InxGa1−xAs/AlAs based resonant tunneling diodes grown on patterned and non-patterned GaAs(100) substrates

Resonant tunneling diodes (RTDs) with strained well and spacer regions made of InxGa1-xAs alloys or (InAs)M/ (GaAs)N short period multiple quantum well (SPMQW) have been fabricated on GaAs(100) substrates. With increasing x, the presence of strain results in generation of misfit dislocations and/or change to three-dimensional island mode of growth, both detrimental to the performance of the RTDs. Consequently, a systematic study of the electrical and structural properties of RTDs with 0.10 ≤ x ≤ 0.33 was carried out with emphasis on control of the growth kinetics. This has led to RTDs with (InAs)1/(GaAs)2 wells having room temperature peak to valley ratios (PVR) of 4.7 with peak current densities (Jp) of 125 kA/cm2. This is the first time that devices grown on GaAs substrate without any thick strain relieving intermediate layers have shown simultaneously high PVRs and Jp.

High contrast optically bistable optoelectronic switches based on InGaAs/GaAs(100) conventional and inverted asymmetric Fabry–Pérot modulators grown via molecular‐beam epitaxy

High contrast ratio InGaAs based asymmetric Fabry–Perot reflection modulators, realized in both conventional and inverted geometry, were used to obtain high contrast optically bistable switches in various schemes utilizing both ‘‘normally on’’ and ‘‘normally off’’ types of modulators. The flexibility provided by the inverted structure in realizing novel optical and electrical interconnections is demonstrated.