It is now possible to grow high-quality strained-layer superlattices, in which individual layers are composed of semiconductor materials which would normally have significantly different lattice constants. Strained structures open new possibilities for band-structure engineering and applications, but place even higher demands on crystal growth techniques. The authors first describe some of the growth that has been achieved and the theoretical and experimental limits on good quality growth. They then discuss in detail the electronic structure of strained quantum wells, and in particular the possibility of valence band engineering in strained layers. Axial strain splits the degeneracy of the light- and heavy-hole zone centre states, accessing a range of subband structures, including the possibility of the highest band being light-hole-like, and of holes with electron-like zone centre masses. The authors next describe the experimental evidence confirming such subband structures. This strain-based band engineering suggests new device applications, including high hole mobility transistors and low threshold current, long-wavelength lasers. The authors review what has been achieved in respect of the main proposed device applications to date. Strained layers are also of interest for the new materials combinations which they allow. They briefly overview some of the materials-based advantages, and conclude that strained layers will continue to attract significant attention.

https://iopscience.iop.org/article/10.1088/0268-1242/4/3/001/pdf

Valence band engineering in strained-layer structures

A p-n junction diode of cubic boron nitride was made by growing an n-type crystal epitaxially on a p-type seed crystal at a pressure of 55 kilobars and a temperature of about 1700°C. A temperature-difference solvent method was used for the crystal growth, and beryllium and silicon were doped as acceptors and donors, respectively. Formation of the p-n junction was clearly confirmed at 1 bar by rectification characteristics and by existence of a space charge layer of the junction as observed by electron beam induced current measurement. This diode operated at 530°C.

https://science.sciencemag.org/content/sci/238/4824/181.full.pdf

High-temperature cubic boron nitride p-N junction diode made at high pressure.

We present a critical review of the strategies used in the fabrication of mismatched semiconductor heterostructures. By using simple concepts derived from the Matthews model of misfit relief, we show how the relaxation of single layers and complex structures may be analysed and predicted. These techniques allow a broad view of the processes that take place in the relaxation of strained layers. This is followed by a discussion of how the relative misfits and thicknesses of different layers in a heterostructure may influence the behaviour and distribution of dislocations in the structure. Finally, we describe the historical development and status of the experimental work and development that has been carried out in this area.

Plastic relaxation and relaxed buffer layers for semiconductor epitaxy

The materials growth, materials characterization, device fabrication, device results, and modeling of strained-layer InGaAs-GaAs-AlGaAs photopumped and current injection quantum-well lasers are presented. Experimental and theoretical methods for determining the electronic energy states in pseudomorphic quantum wells are presented and discussed, and design curves for the emission energy of biaxially compressed InGaAs and GaAs are presented as a function of indium composition and quantum well width. Photopumped lasers with thresholds comparable to early lattice-matched AlGaAs-GaAs quantum-well lasers, as well as continuous-wave room temperature strained-layer injection lasers, are demonstrated. The temperature dependence of the current injection devices is good (T/sub 0/=147 K) in marked contrast to photopumped samples. Preliminary life test results indicate that long-lived strained-layer injection lasers may be possible. >

Strained-layer InGaAs-GaAs-AlGaAs photopumped and current injection lasers

Semiconductor heterojunctions with ideal lattice matching, well-controlled in fabrication, yield devices that cannot be achieved in any other way. These devices include modulated-doped high-speed field-effect transistors, ultra-high-gain and high-speed bipolar transistors, efficient injection lasers and light-emitting diodes and sensitive photo-detecting structures. Atomic reconstructions take place at heterojunction interfaces and are process-fabrication-dependent and not adequately understood. The barrier discontinuities observed are therefore scattered in value and also somewhat dependent on the determination method. Many papers in this Special Issue contain review aspects of these matters. Others, however, are specific contributions on very particular heterojunction topics. Not all aspects of heterojunctions are dealt with by the papers that follow, and the present article is intended for newcomers to the field as a brief commentary on topics that are not adequately represented.

Semiconductor heterojunction topics: Introduction and overview

Summary Abstract: High quality p–n junctions in InGaAs/GaAs strained‐layer superlattices

Pulsed irradiaion of optimized, MBE grown, AlGaAs/GaAs radiation hardened photodiodes

A set of optoelectronic devices including p-n junction photodetectors, emitters (both an LED and a stripe-geometry, injection LASER), and a gain device have been fabricated from In 0.2 Ga 0.8 As/GaAs strained-layer superlattice (SLS) material. The photodetectors have demonstrated a peak external quantum efficiency at zero volts reverse bias(uncorrected for surface reflection) of 50% at 770nm and an optical absorption edge at 1050nm. Both the LED and injection LASER have peak, room temperature, emission wavelengths near 1020nm. The LASER exhibited cw, 77K operation at 976nm with a threshold current of 95mA, the first reported cw operation of an InGaAs/GaAs SLS LASER. Previously reported studies of prototype, double-gate, modulation-doped FETs (channel length = 2.5 µm) have demonstrated peak intrinsic transconductances of 120mS/mm at room temperature and 190mS/mm at 77K. These encouraging photodetector, LED, LASER, and FET results demonstrate that useful optical and electronic devices can be fabricated from mismatched materials and that the InGaAs/GaAs SLS system is an attractive candidate for integrated optoelectronic applications.

InGaAs/GaAs, strained-layer superlattice (SLS), junction photodetectors, LED's, injection laser's, and FET's for optoelectronic IC applications

Deep level transient spectroscopy measurements have been performed on pn jucntions in a strained‐layer superlattice (SLS) wafer composed of 240‐A‐thick alternating layers of GaP and GaAs0.3P0.7 grown by metalorganic chemical vapor deposition. Comparison of these results with data taken on a standard alloy buffer layer of nearly the same average composition (18% As) reveals that in the SLS there are fewer types of traps, the trap concentrations are usually lower, the sample to sample variations across the wafer are much less, and the trap depth profiles are more uniform. Consequently, this comparison suggests that the SLS material is of better quality than the buffer which is mismatched to the underlying GaP substrate.

Comparison of trapping levels in GaAsP strained‐layer superlattice structures and in their buffer layers

Mesa‐isolated, p+nn+, GaAs/Al0.30Ga0.70 As/GaAs, double heterojunction diodes are reported which have excellent electrical characteristics over the full temperature range from 23 to 400 °C. These devices have diode law ideality factors of approximately 1.1 at any temperature for forward current densities greater than 10−3 A/cm2 and have reverse leakage current densities of 4×10−10 A/cm2 at 23 °C and 2×10−2 A/cm2 at 400 °C. AlxGa1−x As is used only in critical device regions to ensure that GaAs heterojunction contacts are made to the wider band‐gap materials and to allow the chemically reactive AlxGa1−x As to be encapsulated by GaAs. These results demonstrate that the GaAs/AlxGa1−x As chemical system is a good materials candidate in which to base a technology for electronic components operated at high temperatures.

C. E. Barnes

Papers

Summary Abstract: High quality p–n junctions in InGaAs/GaAs strained‐layer superlattices

Pulsed irradiaion of optimized, MBE grown, AlGaAs/GaAs radiation hardened photodiodes

InGaAs/GaAs, strained-layer superlattice (SLS), junction photodetectors, LED's, injection laser's, and FET's for optoelectronic IC applications

Comparison of trapping levels in GaAsP strained‐layer superlattice structures and in their buffer layers

A novel p+nn+ GaAs/Al0.30Ga0.70As/GaAs double heterojunction diode for high‐temperature electronic applications