Heterojunctions are ubiquitous to compound semiconductor devices. They are used for both device function and are remedial for problems associated with Fermi level pmining. In the past lattice matching was a critical feature for success. Most devices were made using either the GaAIAs/GaAs or InP/GalnAsP systems. Recently, there have been breakthroughs in the fabrication of devices using non-lattice-matched systems. These include high speed devices made of GaAs grown on Si, InGaAs/GaAs high speed and optoclectronic devices, infra-red photodetectors made of GaInAs grown on GaAs and GeSi/Si strained-layer superlattices. This talk will review this progress and discuss some of the materials and epitaxy issues likely to affect future trends in device research using non-latticematched layers.

The combination of an ion beam assisted etching (IBAE) system and a Molecular Beam Epitaxy (MBE) growth chamber integrally connected in the same set of ultrahigh vacuum (UHV) chambers has allowed us to etch patterns defined by a strontium fluoride mask down into the underlying semiconductor wafer, and regrow monocrystalline material around these patterned structures entirely in situ. Scanning electron microscopy (SEM) analysis of the regrown structures reveals very directional deposition of the overgrown material. Cross-sectional transmission electron microscopy (TEM) reveals a smooth interface and vertical sidewalls at the boundary between the etched surfaces and the overgrowth. Material deposited on the sidewalls is shown to be monocrystalline, while that deposited over the mask is polycrystalline. The SrF2 mask material, previously shown to be an excellent IBAE mask for extremely fine nanostructures, is also shown to be compatible with the high temperatures need for overgrowth, and regrowth around structures as small as ˜200 nm in size has been demonstrated.

Laser-assisted crystal growth experiments have been performed in an upflow vertical metalorganic chemical vapor deposition reactor. An Ar+ laser (514.5 nm) is used to locally heat and photo-excite the surface adlayer during growth. The laser irradiation is observed to enhance the growth rate and Al composition of AlGaAs if the substrate temperature is ˜580 °C. The laser-grown AlGaAs epitaxy is single crystal with good surface morphology and is used to fabricate multiple wavelength light emitting diodes.

The dependence of the density of defects on the pressure ratio PAs/PGa and on the substrate temperature has been studied on a series of GaAs and GaAlAs samples prepared by MBE. Samples have also been desorbed prior to the MBE deposition by UV laser and the density of defects have been compared with the density of defects obtained in samples where thermal desorption occured. Optimum pressure ratios PAs/GPa and growth temperatures have been found for both GaAs and GaAlAs for minimum defect density. It is shown that insitu laser desorption, prior to MBE deposition is a feasible technique for attaining MBE structures with a low defect density, thus simplifying the substrate preparation process.

Selective interdiffusion of Al and Ga at AlxGa1−x As-GaAs heterointerfaces can be carried out by conventional masking procedures and diffusion of acceptor impurities (e.g., Zn), or donor impurities (e.g., Si), or also by ion implantation. This process, impurity-induced layer disordering (IILD), makes it possible to convert quantum well heterostructures (QWHs) such as AlxGa1−xAs-GaAs superlattices (SLs) into bulk homogeneous AlyGa1−yAs where y is the average Al composition of the QWH or SL. Since th IILY process is maskable and thus selective, heterojunctions can be formed in directions perpendicular to the crystal growth direction, i.e., between as-grown “ordered” and IILD “disordered” regions. To date this process has been used most effectively in the fabrication of buriedheterostructure QW lasers, single and multiple stripe, where the disordered regions provide both optical and electrical confinement. The IILD process has also been used to advantage in the fabrication of high power laser diodes with non-absorbing “windows” at the laser facets and thus with better immunity from facet damage. In this paper we present data on the application of the IILD process to the fabrication of buried-heterostructure QW laser diodes. We also describe possible mechanisms by which the impurity-induced layer disordering proceeds based on Column III “Frenkel” defects and the influence of the crystal Fermi level on the defect solubility. These mechanisms are supported by experimental data.

Mixing (or interdiffusion) of AlGaAs superlattice layers is greatly accelerated in the presence of Si doping. We have employed secondary ion mass spectrometry (SIMS) to monitor the depth dependence of the Al diffusion coefficient as well as the Si diffusion profile. Our results reveal unusually complex dependences on implantation and annealing conditions. To isolate the effects of chemistry and lattice damage, several structures were grown by molecular beam epitaxy (MBE) containing plateaus of Si concentration. The doping dependence and activation energy of Al diffusion were then evaluated in as-grown samples and in samples damaged by MeV Ga ion bombardment. To further elucidate the process, samples containing single or multiple implants of various dopants and impurities were examined. Microscopic and electrical characterizations were also performed. Al diffusion was found to be strongly inhibited by lattice damage and by very high Si doping levels. The Al diffusion coefficient has a high power law dependence on Si concentration while its activation energy is relatively unaffected by doping or lattice damage. A wide range of experimental and theoretical mixing studies are surveyed. Mixing models invoking Si pairs, divacancies, Fermi level arguments, and other mechanisms are critically assessed.

The effects of Implantation Enhanced Interdiffusion (IEI) in GaAs-GaAlAs quantum well structures are investigated. A Focused Ion Beam (FIB) source is used to implant narrow lines (500Å wide) with Ga+ ions. IEI in these structures is characterized by low temperature Cathodoluminescence. The dose effects and annealing kinetics dependence on IEI are presented. An unusual damage distribution which produces IEI deep below the surface is observed for the case of FIB implant. The possible origins of this effect and the limits of IEI for processing quantum wires and boxes are discussed.

The n+ layers on semi-insulating liquid encapsulated Czochralski GaAs and p+ layers on Si-doped n-type GaAs were formed by rapid thermal diffusion (RTD) from Te- and Zn-doped oxide films, respectively. The Zn diffusion coefficient of the RTD sample at 850°C for 6s with the heating rate of 50°C/s is about two orders of magnitude higher than that of a similar furnacediffused sample at the same temperature. In addition, Zn and Te diffusion are strongly enhanced by the high heating rate of RTD. The shallow and abrupt p+n junction in GaAs is formed by RTD of Zn with the low heating rate. This shallow p+n junction is suitable for the construction of a photodiode. It is observed that the short wavelength spectral response (<800 nm) of the photodiode fabricated by RTD from Zn-doped oxide film decreases as the heating rate of RTD increases. Deep levels in these photodiodes were characterized by deep level transient spectroscopy. A electron trap EZ (Ec -0.57eV) is produced by RTD of Zn in the n-type substrate under p+n junction. The concentration of this trap is independent of the heating rate of RTD.

Te enhanced mixing of AlAs/GaAs superlattice has been observed by secondary ion mass spectrometry. The superlattice sample was grown by organometallic chemical vapor deposition and doped with Te at concentrations of 2×1017 to 5×1018 cm−.3 In the temperature range from 700 to 1000 C, a single activation energy for the Al diffusion of 2.9 eV was observed. Furthermore, it has been found that the relationship between the Al diffusion coefficient and Te concentration is linear. Comparisons have been made between Si and Te induced superlattice mixing.

Recently, a laser-scanning technique for patterning Si-induced layer disordering of GaAs-AlGaAs heterostructures has been reported. This process, called laserassisted disordering (LAD), has been successfully used to fabricate low threshold buried heterostructure lasers. In this report, the LAD process is studied in detail with scanning electron microscopy, transmission electron microscopy and secondary ion mass spectrometry. The results are discussed in the context of device fabrication.

This paper is the first report of the use of Si+ implantation into GaAs/GaAlAs MQW material to form optical waveguides operating at a wavelength of 1.15μm. Lateral confinement is achieved by mixing of the MQW material which is produced by the implantation of Si+ and subsequent annealing at 750°C. The properties of these waveguides are compared with those of chemically etched rib waveguides and are shown to have reasonably low losses.

The nondestructive neutron depth profiling (NDP) technique has been used to measure the boron (10B) distributions in GaAs, CdTe, Hg0.7 Cd0.3 Te, and Hg0.85 Mn0.15Te after multiple energy ion implants. The NDP results are found to be in good agreement with the theoretical ion ranges obtained from Monte Carlo computer simulations. Only minor changes in the boron profiles were seen for the chosen annealing conditions.

The effect of rapid thermal annealing on strain reduction in 1.15 MeV S-implanted GaAs wafers irradiated to a dose of 5 × 1014/cm2 has been studied by double-crystal x-ray diffraction technique. X-ray rocking curves exhibit characteristic thin film fringes between the peak of unstrained GaAs and the major peak of the strained region. The maximum strain, i.e., the separation between the two peaks, as well as the number of minor fringes decreases with increasing RTA temperature, while the relative spacing between the fringes remains constant. At temperatures above 900°C, the main peaks begin to overlap; however, a residual positive strain can be measured for temperatures as high as 1100°C.

The activation kinetics and diffusion behaviour of implanted Be and Si in two different types of MOCVD-grown GaAs-AlGaAs heterostructures on Si substrates were examined by electrochemical C-V profiling, secondary ion mass spectrometry and sheet resistivity measurements. The implanted Be displays a thermal activation energy of 0.70 eV and Si a thermal activation energy of 0.53 eV in heteroepitaxial material, similar to the comparable cases in homoepitaxial GaAs. In addition, there is no evidence for enhanced diffusivity of either species, at least for implants located away from the heterointerface. The remnant lattice disorder in the heterostructures caused by implantation and annealing is negligible compared to the as-grown disorder, as revealed by transmission electron microscopy.

Ellipsometric and ion backscattering techniques were used to investigate the changes in refractive index and thicknest of a surface layer of high purity silica modified by transition metal ion implantation. Samples were implanted with Cr+, Mn+, and Fe+ ions to doses ranging from 0.5 × 1016 to 6.0 × 1016 ions/cm2. The refractive index of the ion implanted region increased with increasing implantation dose. A single layer model for the implanted region was determined to be consistent with the ion backscattering profiles and ellipsometric data.

This paper describes preliminary device work on GexSi1−x, based devices and examines progress towards the realization of a silicon based heterostructure technology. Target areas for further work are identified including: An enhanced understanding of metastable-growth processes; The need for defect confinement in discommensurate epitaxy; Elimination of manufacturing barriers caused by particulate induced defects in MBE and low growth rates in low temperature CVD.

Over the last five years, studies of GexSi1−x have yielded an understanding of crystal growth mechanisms and information on electrical and optical properties such as bandstructure and heterojunction band alignment. These in turn lead to a number of preliminary device applications including optical detectors, modulation doped transistors and heterojunction bipolar transistors. In this paper, I will try to answer the question posed by the symposium organizers: In the germanium silicide system, has our understanding and experience reached the point that we have the basis for a technology, and if not, where must further work be done?

The recent demonstrations of room-temperature cw operation of diode lasers fabricated in GaAs/AlGaAs layers grown on Si wafers have encouraged efforts to develop monolithic GaAs/Si integration technology for applications such as optical interconnects between VLSI subsystems. This paper summarizes our current work in this area, which is focused on the development of a highspeed, MSI-scale monolithic GaAs/Si test chip that integrates Si MOSFET circuits with diode lasers, LEDs, photoconductive detectors, and MESFET logic circuits fabricated in GaAs/AlGaAs layers grown by molecular beam epitaxy. Growth issues and processing considerations that affect device and circuit performance are addressed, and the characteristics of LEDs monolithically integrated with Si driver circuits and of GaAs microwave MESFETs fabricated on high-resistivity Si substrates are reported.

We present DC characteristics of all-OMCVD grown GaAs MESFET structures on Si substrates with unintentionally doped GaAs or AlGaAs buffer layers. MESFETs fabricated in two and three inch GaAs on Si wafers pinch off well and exhibit reasonably high transconductances up to 110 mS/mm for 1 μm gate devices. The reverse Schottky breakdown voltage of the MESFET gate is as high as 15 V and the forward turn on voltage is ∼0.65 V. The ohmic isolation is comparable to the typical homoepitaxial layer with a leakage current of 100 nA at a 1 V bias. The low background doping levels of unintentionally doped GaAs buffer layers is the key factor for this successful MESFET operation.

High quality GaAs and InP have been grown on silicon substrates, using low pressure metalorganic chemical vapor deposition technique. The growth temperature is 550°C and the growth rate 100 A/min.

Photoluminescence, X-ray diffraction and electrochemical profiling verified the high quality of these layers. The use of superlattices as buffer layers, (GaAs/GaInP) in the case of GaAs/Si and (GaInAsP/InP) in the case of InP/Si, decreased the amount of misfit dislocations in the epitaxial layer. Carrier concentrations as low as 5.1015 cm−3 have been measured by electrochemical profiling.

GaAs layers free of antiphase domains (APD's) have been grown by molecular beam epitaxy (MBE) on nominally (001)-oriented Si substrates. This is achieved by preheating the substrates at 950°C over 60 min or at 1000°C over 5 min in an ultrahigh vacuum. The maximum Hall mobility at 293 K is 5300 cm2/Vs for the APD-free GaAs layer doped with Si at a concentration of 2×1016 cm−3. Selective epitaxial growth of GaAs has been carried out on a Si substrate pattrened with SiO2, which was formed by wet O2 oxidation. By choosing an appropriate thickness of the SiO2 layer, thzxcSe warpage of wafers can be reduced to zero. While single-crystalline GaAs is grown on Si-exposed areas, highly-resistive (p ≧ 105 Ωcm) poly-crystalline GaAs is deposited on SiO2. This technique has been successfully applied for the device isolation of modulation-doped FET's (MODFET's, HEMT's, etc.) on Si without mesa-etching. The transconductance of the MODFET with a 3 μm-long gate reaches 88 mS/mm at 293 K.