1.- Behavior of Donors and Acceptors in Widegap III-V Semiconductors.
Our studies on donor related deep levels (DX centers) in GaAsP and GaAlAs alloys, for a variety of dopants (Si, Se, Te, Sn) continues, although at a lower pace. Cooperation with former partners in ESPRIT Basic 3168 on that topic is still alive. This effort is also motivated by the growing importance of very wide gap semiconductors, like SiC and GaN, in relation to understand the behavior of dopants in such materials.
The efforts to detect the donor neutral states (either the DX° or
a deep substitutional state) have been very intensive. Based on theoretical
grounds and experimental findings by Hall effect at low temperatures and
high pressures, an effort to detect Sn-related neutral donors is being
pursued. DLTS in the dark, under illumination, and the dependence on hydrostatic
pressure of the persistent photoconductivity regime are being explored
at temperatures in the 4 to 15K range. A level, different from the DX ground
state, with a very small capture barrier, has been univocally detected,
and its fingerprints are being determined.
2.- InGaAs-Based HEMT Devices (InGaAs/GaAs PM HEMT's).
The involvement of HISEL in ESPRIT Giants 2035 and in ESPRIT Basic LDS, led to a significant experimental and modeling efforts in the area of PM HEMT's. One of the main objectives has been to find non-destructive optical procedures to assess the PM HEMT structures concerning In%, quantum well thickness and 2DEG concentration. Photoluminescence has been systematically applied, including room temperature measurements. Self-consistent calculations, Poisson-Schrödinger, have been applied to such PM HEMT structures. The PL line shape has been also modeled. Strain effects were also studied by Raman scattering and PL, and a semi empirical model has been developed.
In cooperation with the IFA-Freiburg, the effects of channel width on
electron mobility have been studied. Structures where the electron supplying
layer is a d-plane embedded into a GaAs QW inside the AlGaAs layer, have
been studied. The channel width varied from 50 to 150A, and the role of
alloy scattering and interface roughness effects is being investigated.
3.- InGaAs/GaAs(111)-based lasers for long wavelength applications
Strained layers of InxGa1-xAs/GaAs Quantum Wells (QWs) on GaAs
substrates offer considerable potential for new optoelectronic devices
in the near infrared wavelength region. Among these, the ones grown on
(001) oriented surfaces have been studied in much greater detail due
to the good epitaxial layer quality that can be achieved using a
relatively wide range of growth conditions. Unfortunately, relaxation
limits the practical applications of these InGaAs/GaAs(001)
heterostructures to wavelengths around 1 mm. A number of groups have
reported a larger critical layer thickness for strain relaxation for
InGaAs layers grown on (111)B orientation as opposed to the
(001). This implies that for a given quantum well width, a higher In
content can be used. InGaAs/GaAs(111) heterostructures grown on
(111)B-oriented substrates therefore offer potential for the
development of optoelectronic devices at wavelengths beyond 1 mm or
for improved reliability devices working around 1 mm. Among possible
applications of high reliability, high efficiency diode lasers working
at 1.06 mm are optical intersatellite links. The scientific
instrumentation and metrology areas would also benefit from these
lasers working in the 1-1.1 mm wavelenth range. The growth and
fabrication of the above lasers is the main subject of the GHISO
Project (GaAs High Index Substrates for Optoelectronics) with funding
from ESPRIT (Project number 35112). In collaboration with the
LAAS-Toulouse, the University of Sheffield and Thomson-CSF, the above
Project has started on 1999.
4.- Piezoelectric optical and electronic devices grown on (111)GaAs substrates
When III-V semiconductor layers are grown on (111) substrates with some degree of lattice mismatch, internal piezoelectric fields are generated. Such fields arise from the polar nature of these semiconductors and have a strong effect on the optical and electrical properties of devices containing them, if properly designed. The ability to control the direction, magnitude and location of internal electric fields in a heterostructure provides a new tool for designing electronic and optoelectronic devices.
Such is the case of PIN diodes containing a multiquantum well structure (MQW) in their intrinsic region. These diodes exhibit a blue-shift for increasing electric applied field instead of the redshift one displayed by (100) grown PIN-MQW due to the Quantum Confined Stark Effect (QCSE). A blue-shifting MQW diode has important consequences for self-electro-optic effect devices (SEEDs), which achieve bistability with external light intensity because the PIN diode has higher absorption at lower biases. Thus, a blue-shifting SEED operates at reduced voltages and has much lower absorption in its transmitting state than (100) devices.
On the other hand, other electronic devices can also take advantage
of internal piezoelectric fields. First, carriers can be better confined
in High Electron Mobility Transistors (HEMT's) by means of these internal
fields which allow a higher carrier concentration with a reduction of hetero-interface
scattering. Other high frequency devices, as Double Barrier Resonant Tunneling
Diodes (DBRTD's), can be better tailored to their requirements using these
piezoelectric effects. Besides the band engineering obtained by varying
composition, strain and doping levels, new possibilities appear when piezoelectric
fields are brought into the structure. It is important to point out an
important feature of the piezoelectric field: it is generated at an atomic
level (very fine grain), as opposite to that of the electric fields due
to unscreened ionized donors or acceptors that have a coarser grain structure
(mean distance between donors or acceptors). This is important for high
mobility structures and for quantum devices, as DBRTD's, where the main
dimension is comparable to the above mean distance. An otherwise "rough"
DBRTD collector (unscreened donors) can be made more continuous or smoother
by means of an internal piezoelectric field, producing an accumulation
region in the collector electrode
5. Technology for blue and ultraviolet detectors and light emitting devices: GaN and AlN alloys
Wide gap semiconductors have always attracted a great interest because the need for electronic devices operating at high temperatures, high power levels, and for the potential applications of optical materials active in the blue and ultraviolet wavelengths. The III-V nitrides are a good candidate for optoelectronic applications, with better electronic properties than SiC. Both SiC and GaN have small lattice constants. We have started a program on the growth of GaN and related compounds by MBE on Si substrates, in which the reactive nitrogen is supplied by microwave plasma excitation and aiming to obtain UV photodetectors, blue LED's, and lasers. In cooperation with CNRS-Valbonne, Univ. Nottingham, Padderborn and Laussane, and with funding from ESPRIT (project 20968, LAQUANI), the growth of laser quality structures has started. The problem of intrinsic levels, yellow luminescence ohmic contacts and Schottky barrier formation are being addressed. From our experience with the behavior of donors in widegap semiconductors, we aim to determine the nature of the donor levels and acceptor in GaN, its dependence on the chemical species, and the optimum selection of dopants.
The application of GaN and AlGaN for solar UV detectors, to be
visible blind, is being studied. P-n junction, photoconductive (non-intentionally
doped) and Schottky barrier photodetectors have been fabricated and evaluated
for solar UV detection. The gain mechanism in photodetectors, and the responsivity
of the various photodetectors are being studied. A full system, based on
GaN p-n junctions and Schottky barrier detectors, is being used as a monitor
system for the solar UV insolation reaching the UPM.
6. Multicolor Quantum Well Infrared Photodetectors (QWIP)
One of the advantages of QWIP technology is that both wavelenght and spectral bandwidth can be selected while maintaining a good lattice matching with the substrate. Hence, it is possible to monolitically stack several lattice-matched QWIP during crystal growth in order to achieve the desired multispectral response.
In collaboration with the Spanish Navy Research Lab (CIDA), the design of multicolor QWIP based on n-doped QW's has been made. Normal incidence detection in these doped QWIP's, and its dependence on crystal orientation are being investigated.
Investigations are also focused on voltage controlled tunable two-color IR detectors, with photovoltaic and phtoconductive dual-mode operation at 3-5 (m and 8-12 (m, using GaAs/AlAs/AlGaAs double barrier quantum wells and bound-to-continuum GaAs/AlGaAs quantum wells.
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