38 Physics and Applications of Dilute Nitrides - conocimientos.com.ve

Physics and Applications of Dilute Nitrides. An Atomistic View of the Electronic Structure of Mixed Anion III–V Nitrides. Band Anticrossing in III-N-V Alloys. Tight-Binding and k·p Theory of Dilute Nitride Alloys. Electronic Properties of (Ga,In)(N,As)-Based Heterostructures. Theory of Defects in Dilute Nitrides. Growth, Characterization, and Band-Gap Engineering of Dilute Nitrides. GaInNAs Long-Wavelength Lasers.

domingo, 25 de julio de 2010

Restoration of the host band-gap by hydrogenation: dilute, amalgamation, and alloy limits

We describe first the effects of hydrogen irradiation on the optical properties of GaAs1−yNy/GaAs epilayers in the very dilute nitrogen limit (y<0.01%). Figure shows the effect of hydrogen irradiation on the sample. Hydrogenation at various H doses, dH, leads to a progressive and finally complete quenching of the Nrelated lines as well as of the broad underlying band. The dH=5×1015 ions/cm2 spectrum closely reproduces that of pure GaAs, where only two bands are observed, namely, the longitudinal optical (LO) phonon replicas of the C-related free-to-bound transition at 1.4934 eV. This H-induced passivation has never been reported before for any isoelectronic impurity, except for a weak reduction in the luminescence intensity of a few N-related lines in GaP:N. Note that a 100% passivation of impurity luminescence bands is hardly attainable even in the common case of H passivation of shallow impurities in GaAs or Si.

We now move to the so-called amalgamation limit, corresponding to the existence of both localized and extended (or Bloch-like) states in the material electronic structure. Figure illustrates such a case for GaAs1−yNy with y=0.1%. The bottom curve shows the PL spectrum of the H-free sample. H irradiation leads first to a passivation of the N cluster states and then to an apparent reopening of the GaAs1−yNy band-gap toward that of the GaAs reference (top curve). As a matter of fact, both the (e,C) and the E− recombination bands converge to those of the GaAs reference. The energy separation between these two transitions decreases with increasing N concentration, most likely dueto the increase of the tensile strain with increasing x. Indeed, for increasing N concentration, the top of thevalence band acquires a more pronounced light-hole character and, in turn, the binding energy of the acceptor impurity decreases. Similar results have been observed in the full alloy limit as shown in the following section.

                                                  ROSSANA C HERNANDEZ C
                                     ELECTRONICA DEL ESTADO SOLIDO
                  http://www.sciencedirect.com/science/book/9780080445021

EFFECTS OF NITROGEN AND HYDROGEN ON THE ELECTRONIC PROPERTIES OF InXGA1−XAs1−YNY

Figure shows the low-temperature (T=10 K) photoluminescence spectra of a representative series of as-grown samples whose N concentration varies over about two orders of magnitude. At the very early stage of N incorporation in GaAs (N concentration less than 0.01%, bottommost spec-trum in Figure ), the low-temperature PL spectra are characterized by a number of sharp lines (line width ≈0.5 meV) between 1.40 and 1.48 eV, which are due to the recombination of excitons localized on N complexes. These lines are attributed to carrier recombination from electronic levels due to N pairs and/or clusters and are superimposed on a broad band also related to N doping. The luminescence intensity associated with these transitions varies from line to line and increases with y (not shown here). An exact assignment of each line to a given N complex is made rather

difficult by the strong dependence of the material optical properties on the growth conditions, as extensively reported in the literature.

Free-electron to neutral-carbon acceptor (e,C) and free-exciton (E−) recombinations of GaAs are observed at 1.493 eV and 1.515 eV, respectively. As the nitrogen concentration is increased further (y=0.043 and 0.1%), the energy of the excitonic recombination from the material's band gap, E−, as well as the (e,C) recombination band start redshifting very rapidly, coexisting with and taking in the levels associated with the N complexes, whose energies do not change with N concentration. This highlights the strongly localized character of the N isoelectronic traps, contrary to that of shallow impurities, whose wavefinctions overlap at smaller concentrations (1016–1018 cm−3). Eventually, at higher N concentrations (alloy limit, y>0.1%), the GaAs1−yNy band-gap keeps redshifting along with the C-related states. The dramatic variation of the GaAs host band-gap with y is accompanied by other major effects on the electronic and optical properties of GaAs1−yNy. Indeed, with increasing N concentration, the electron effective mass increases , a sizable Stokes shift between emission and absorption is observed , the band-gap dependence on temperature and hydrostatic pressure decreases, and N resonant states move to higher energy. In the following discussion, we show that most of the above-mentioned changes can be fully and reversibly counteracted by irradiation with atomic hydrogen.

                                        ROSSANA HERNANDEZ
                              ELECTRONICA DEL ESTADO SOLIDO
             http://www.sciencedirect.com/science/book/9780080445021

EXPERIMENTAL: HYDROGENATION AND CHARACTERIZATION TECHNIQUES

The samples considered in this review were grown by different techniques. One set of samples was grown by solid-source molecular-beam epitaxy (MBE) and consists of GaAs1−yNy/GaAs and InxGa1−xAs1−yNy/GaAs single quantum wells (QWs) having In concentration x=25 to 42%, N concentration y=0.7 to 5.2%, and QW thickness L=6.0 to 8.2 nm. MBE-grown GaAs1−y Ny/GaAs epilayers were also considered (layer thickness t=310 nm, with y <0.01 and y=0.81 and 1.3%). Another set of samples consists of four 0.5-μm-thick GaAs1−yNy/GaAs epilayers (y=0.043, 0.1, 0.21, and 0.5%) grown by metalorganic vapor-phase epitaxy [28,29]. Finally, GaP1−yNy epilayers were grown by gas-source MBE on GaP. In this case, nitrogen concentrations are y=0.05, 0.12, 0.6, 0.81, and 1.3%. The GaP1−yNy epilayer thickness is 250 nm for all samples, except for the y=1.3% epilayer, which is 750-nm thick. In all cases, the composition and layer thickness of the samples have been derived by X-ray diffraction measurements. Hydrogenation was obtained by ion-beam irradiation from a Kaufman source with the samples held at 300°C . The ion energy was about 100 eV, and the current density was a few tens of μA/cm2. Several hydrogen doses (dH=1014 to 1020 ions/cm2) were used.

Posthydrogenation thermal annealing was performed at 1.0×10−6 torr at temperatures, Ta, ranging between 220°C and 550°C and for various durations, ta, ranging between 1 and 50 h. The electronic properties of the samples were investigated mainly by means of photoluminescence (PL) spectroscopy. For InxGa1−xAs1−yNy and GaAs1−yNy samples, PL was excited by the 515-nm line of an Ar+ laser or by an Nd-vanadate laser (excitation wavelength equal to 532 nm). For GaP1− yNy, the 458-nm line of an Ar+ laser was used, instead. PL was dispersed then by a 1-m single-grating monochromator or a 0.75-m double monochromator and detected by a liquid-nitrogen-cooled Ge detector or (InGa)As linear array, or by a photomultiplier with a GaAs/Cs cathode.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.sciencedirect.com/science/book/9780080445021

Local N-Environment and Fine Structure of the Band Gap

In the following, we will focus on the rearrangement of the local N environment. In many samples grown by MOVPE or MBE, there are indications of changes of the local N environment from a configuration with four Ga nearest neighbors (nn) to configurations with more In nns . This change of the local N environment can be detected by changes of the local vibrational modes of N in (Ga,In)(N,As) in Raman or infraredabsorption spectra . Very convincing evidence for such changes is obtained by Raman spectroscopy close to the E+ resonance with excitation energies of 2.18 eV and 1.92 eV or by infrared absorption. Recently, these findings were also confirmed by X-ray absorption spectroscopy. It is worth mentioning that under certain growth and annealing conditions, there do not seem to be significant changes of the nn environment of N in (Ga,In)(N,As), i.e., the four-Ga environment dominates even after annealing .

To reach a better understanding of the effect of the nitrogen nn environment on the band structure of Ga1−yInyNxAs1−x, full sp3s* tight-binding supercell calculations have been performed, in which the central group V site was constrained to have a given number m (=0 to 4) of In nns. In atoms were placed at random on the remaining sites to give the desired overall y. The Hamiltonian was written as H1=H0+ΔH, where H0 is the Hamiltonian for (Ga,In)As, and ΔH is the change due to N incorporation. Pairs of supercells H0 and H1 were defined by placing As and N, respec-tively, onto the central group V site. Calculating separately the wavefunctions ψc0 and ψc1 for H0 and H1, one canderive a nitrogen resonant level wavefunction ψN. This allows one to relate the supercell calculations to the simple level repulsion model by VNc=<ψN |H1|ψc0>, EN=<ψN|H1|ψN>, and Ec=<ψc0|H1|ψc0>. Details of the model are given in Refs. [17,25] and Chapter 3 of this volume. These results are corroborated by other

theoretical models.

For each y, the EN for the five nn configurations are equally spaced with the value for four-Ga nns being always about 220 meV lower than that of four-In nns. Such strong dependence of the energy of isolated, strongly localized impurities on the nn environment is common. Calculations, where the atomic arrangement in the second group III shell and higher shells was altered, only shift EN by ±20 meV. This agrees with experiments on Ga(As,P):N, where a broadening of 30 meV of the localized N state is observed due to disorder on the group V sublattice. The derived matrix element VNc linking the N resonant state and the conduction band edge varies between about 2.00 eV·x1/2 for four- Ga nns to about 1.35 eV·x1/2 for four-In nns, i.e., the strength of the perturbation of the crystal decreases with increasing number of In nns. The large differences of the five nn environments of N are also reflected in the derived conduction-band-edge energies E−. For each y at x=1%, the E− values for the five nn configurations are evenly spread over an energy range of about 80 meV below the corresponding unperturbed Ec of (Ga,In)As, with that for fourGa nns being lowest in energy.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.sciencedirect.com/science/book/9780080445021

TEN-BAND k·p MODEL FOR DILUTE NITRIDE ALLOYS

The k·p and envelope-function methods are widely applied to study III–V semiconductor heterostructures. The strong interaction between the N resonant states and the conduction band edge means that the conventional eight-band k·p method cannot be applied to GaInNAs and related hetero-structures. We must include the interaction between the N resonant states and the conduction band edge to describe the variation of the (zonecenter) conduction-band-edge energy with N. This leads to a modified ten-band k·p Hamiltonian for GaInNAs, with the modified Hamiltonian giving a good description of the conduction-band dispersion over an energy range at least on the order of 200 meV, sufficient for most analyses.

We illustrate this by comparing the band structure of a Ga32As32 and a Ga32N1As31 supercell in Figure a, where the dotted lines show the sp3s* band structure plotted with the spin-orbit interaction Eso set to zero. The GaAs eight-band k·p Hamiltonian reduces to a two-band Hamiltonian for the conduction and light-hole valence bands along the [0,0,1] direction when Eso=0, as illustrated by the thick solid lines, which show the dispersion of these two bands calculated using ψc0 and the light-hole zone-center wavefunction, ψlh0, as the k·p basis states. The k·p matrix elements were found by explicitly evaluating <ψi0|H(kz)|ψj0> using the tight-binding Hamiltonian . We must add the nitrogen resonant state ψN0 to the k·p Hamiltonian for Ga32NAs31. The conduction and light-hole band dispersion are then

found by diagonalizing a 3×3 k·p model. The most general form of this 3×3 Hamiltonian includes k-dependent diagonal and off-diagonal matrix elements linking the ψN0, ψc0.

The thick solid lines in Figure b show the band structure of Ga32NAs31 calculated, where we evaluate the matrix elements directly using the tightbinding Hamiltonian. This Hamiltonian gives an excellent fit to the conduction-band dispersion within about 200 meV of the band edge. However, it is notable that the N impurity band in Figure b does not correspond to a specific higher-lying conduction b and in the supercell. This is to be expected from our analysis of resonant states in the previous section.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.sciencedirect.com/science/book/9780080445021

NITROGEN RESONANT STATES IN DISORDERED GaNxAs1−x STRUCTURES

Overall, clearly demonstrate that the conduction band edge in GaNxAs1−x is being perturbed and pushed downward due to its interaction with a higherlying localized resonant state, centered on the nitrogen atoms. Why, then, has this state not been identified in previous calculations? To answer this question, and to investigate the role of disorder, we extend the tight-binding and two-level model to disordered GaNxAs1−x supercells.

We first consider a set of 1000 atom supercells containing up to 15 randomly distributed N atoms. In these supercells we fit the number, but not the distribution, of NN pairs to the number given statistically, so that each cell contains n isolated N sites and p N-N pairs. For each configuration, we used the GULP molecular relaxation package [48] to calculate the equilibrium positions of all the atoms, using a parameterized valence-force-field model, while using Végard's law to vary the unit cell basis vectors as a(x)=x aGaN+(1−x) aGaAs. The calculated relaxed bond lengths are in good agreement with those obtained by other authors [46] who used an ab initio pseudopotential approach.

In a disordered supercell, we can again try to describe the GaNxAs1−x conduction band edge by a Linear Combination of Isolated Nitrogen Resonant States (LCINS) interacting with the unperturbed conduction band edge, ψc0.

H=Ho+ Vn+ Vn-n

where H0 is the Ga500As500 Hamiltonian, ΔVN is the sum of defect potentials associated with the n isolated N atoms, and ΔVNN is the sum of defect Hamiltonians associated with the p N-N pairs. In extension of the approach for ordered structures, we now determine the GaNxAs1−x conduction band edge E− and the N-related conduction-band levels by constructing and solving a (n+2p+1)×(n+2p+1). Hamiltonian matrix involving the GaAs conduction-band-edge wavefunction, and the n+2p N-related states.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.elsevier.com/wps/find/bookdescription.cws_home/703091/description#description

NITROGEN RESONANT STATES IN ORDERED GaNxAs1−x STRUCTURES

The BAC model explains the extreme band-gap bowing observed in InyGa1− yNxAs1−x in terms of an interaction between two levels, one at energy Ec associated with the extended onduction band edge (CBE) states of the InGaAs matrix, and the other at energy EN associated with the localized N impurity states, with the two states linked by a matrix element VNc describing the interaction between them [23]. The CBE energy of Ga(In)NxAs1−x, E−, is then given by the lower eigenvalue of the determinant

FIGURE 3.1

which showed the measured variation in E− and E+ as a function of N composition x in GaNxAs1−x. However, initial pseudopotential calculations found no direct evidence for the upper state, although they do confirm its effect on the conduction band edge, and it has more recently been identified for relatively low N compositions (x<~1%) . To investigate the resonant state, and its behavior, we have developed an accurate sp3s* tight-binding (TB) Hamiltonian to describe the electronic structure of GaInNxAs1−x . This Hamiltonian fully accounts for the observed experimental data, and also gives results in good agreement with pseudopotential calculations. Figure 3.1 shows, for instance, the variation of the band-gap energy across the full alloy range in free-standing GaNxAs1−x, calculated using the sp3s* Hamiltonian: the observed variation matches well that obtained in the literature .

To investigate the resonant state and its behavior, we calculated the electronic structure of ordered GaNxAs1−x supercells . By comparing the calculated CBE states ψc1 and ψc0 in large supercells (Ga864N1As863 and Ga864As864, respectively), we canderive the nitrogen resonant state ψN0 associated with an isolated N atom. In the BAC model, ψc1 is a linear combination of ψc0 and ψN0.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.elsevier.com/wps/find/bookdescription.cws_home/703091/description#description

Modulation Spectroscopy and Optical Excitation of Band-to- Band Transitions in Quantum-Well Structures

In QW structures, the electron and hole confinement energies as well as their subband structures are sensitive functions of the electron and hole masses. These physical properties are commonly retrieved from band-to-band optical transitions between the CB and the VB by using photoluminescence excitation (PLE), photovoltaic measurements, ora variant of modulation spectroscopy. In the latter, instead of measuring optical reflectance (or transmittance), the spectral response modified by a repetitive perturbation (e.g., light, an electric field, a heat pulse, or stress) is evaluated. This gives rise to derivativelike spectral features in the photon energy region corresponding to inter-band transitions or critical points of the band structures.

To determine me* from this experimental approach, solid knowledge on some other parameters that are also important for interband transitions is required. For example, values of conduction- and valence-band offsets at the heterointerfaces, the hole effective mass, strain field, and exact QW width should be separately obtained or self-consistently fitted or assumed. Depending on the choices of these parameters, a large uncertainty can arise in some cases.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.elsevier.com/wps/find/bookdescription.cws_home/703091/description#description

FW: THEORY OF BAND ANTICROSSING

The electronic structure of highly mismatched alloys (e.g., GaNxAs1−x) can be described by considering the interaction between the localized states and extended states within the many-impurity Anderson model]. The total Hamiltonian of the system is the sum of three terms.

The first term is the Hamiltonian of the electrons in the band states with energy dispersion The second term corresponds to the electron localized on the jth impurity site with energy The third term describes the change in the single electron energy due to the dynamic mixing between the band states and the localized states. Following Anderson's scheme, the hybridization strength is characterized by the parameter Vkj defined by Anderson.

Where a(r−j) and φd(r−j) are the Wannier function belonging to the band and the localized wavefunction of the impurity on the jth site, respectively. HHF(r) is the singleelectron energy described in the Hartree-Fock approximation.

The hybridization term produces a profound effect on the electronic structure of the system. In general, one shall consider finite but dilute concentrations of impurities, 0<x<<1. In this case, the single-site coherent potential approximation (CPA) is adequate for the manyimpurity system. In the CPA treatment, a configurational averaging is performed, neglecting correlations between positions of the impurities. Consequently, the spacetranslational invariance of the average Green's function is partially restored, and k resumes its well-defined properties as a good quantum number. In momentum space, the diagonal Green's function in CPA can be written as

The integration converges rapidly with in a small range that is proportional to x. The calculated perturbed DOS for GaNxAs1−x with several small values of x is shown in Figure 2.3. Note that the anticrossing interaction leads to a dramatic redistribution of the electronic states in the conduction band. The most striking feature of the DOS curves is the clearly seen gap between E+ and E− that evolves with increasing N content. In the Green's function calculation, the k dependence of Vkj is assumed to be weak on the momentum scale we are interested in.

There is experimental evidence indicating that the values of Vk at the L point in GaNxAs1−x and at the X point in GaNxP1−x are about three to four times smaller than the Vk at the point. This ratio corresponds to a localized wave function decay length (ld) on the order of the lattice constant. This result indicates that the off-zone-center conduction-band minima are affected by the anticrossing interaction only when their energies are close to the localized level. This is consistent with recent measurements of the optical properties of InyGa1−yNxAs1−x alloys, which have shown that alloying with N has only very small effects on the high energy transitions at large k vectors.

ROSSANA HERNANDEZ

ELECTRONICA DEL ESTADO SOLIDO

http://www.elsevier.com/wps/find/bookdescription.cws_home/703091/description#description

THEORY OF BAND ANTICROSSING

Defects in Dilute Nitrides, Gerald Soto, CRF 2010-1, (3er Parcial).

Dilute nitrides, derived from conventional III-V semiconductors such as Ga(In,Al)P and Ga(In)As by insertion of N into the group-V sublattice, have gained increasingly high interest during the last few years. They exhibit unusual and fascinating new physical properties, such as a giant band-gap bowing that allows widely extended band structure engineering. The high interest has also been driven by potential technological advantages provided by the novel dilute nitrides in lattice matching to GaAs and Si substrates. A combination of the remarkable fundamental properties with the technological advantages has provided an unprecedented opportunity to tailor material properties for desired device applications in optoelectronics and photonics, such as improved solid-state lasers for fiber-optic communications, multi-junctional solar cells, integration of efficient III- V optoelectronics with microelectronics based on silicon.

Unfortunately, epitaxial growth of dilute nitrides remains as a great chal-lenge. The required non-equilibrium growth conditions together with the disparity between N and the replaced group-V atoms are known to favor formation of various defects. As the optical quality of dilute nitrides has been shown to deteriorate with increasing N composition, even in the best available materials that are free of structural defects, there is currently a great need in identifying growin point defects and in assessing their role in non-radiative carrier recombination. In fact, Below we shall provide a brief review of our recent results from optically detected magnetic resonance (ODMR) studies of grownin non-radiative defects in Ga(In)NAs and Ga(Al,In)NP, in an effort to provide chemical identi¯cation and experimental signatures of defects and to evaluate their role in carrier recombination. Among them, defect complexes involving intrinsic defects such as antisites and self-interstitials have been positively identifyed and effects of growth conditions, chemical compositions and post-growth processing on formation of the defects have been studied. Non-radiative defects can be monitored using the ODMR technique because a magnetic resonance induced increase in efficiency of dominant non-radiative recombination channels can lead to a corresponding decrease in free carrier concentration available for radiative recombination and thus to a decrease in photoluminescence (PL) intensity.

the issue of defects is one of main problems we are currently facing that hinders a rapid progress of dilute nitrides for various device applications in optoelectronics and photonics. For example, it has been shown that about 50% of threshold current in the best available 1.3 um GaInNAs lasers has been found to be due to defect-related non-radiative recombination channels. Many key parameters of GaInNAs for solar-cell applications have also been found to be limited by defects. Identifying and understanding of the relevant defects in dilute nitrides and designing strategies to eliminate them are therefore crucial to the success of these materials for optoelectronic device applications.

Experimental evidence for the existence of AsGa antisites in Ga(In)NAs alloys grown by gas source molecular beam epitaxy (GS-MBE) has been provided by the ODMR measurements. The participation of an As atom in the defect was concluded from the experimentally resolved hyperfine (hf) structure, i.e. a group of four ODMR lines, characteristic of the hf interaction between an unpaired electron spin S = 1/2 and the nuclear spin I = 3/2 of the 75 As atom (100% natural abundance) | Fig. 1. The hf splitting parameter, A = 737 x 10^4 cm^1, and the g-value of the unpaired electron localized at the defect, g = 2, were determined by fitting experimental data with the effective spin Hamiltonian

The first and second terms in Eq. (1) are the electron Zeeman and hyperfine interaction terms, respectively; ¹B denotes the Bohr magneton. The obtained A value is about 20% smaller than that known for the isolated AsGa in GaAs, suggesting that the revealed defect is a complex involving AsGa. The microscopic structure of the complex does not depend on the N composition in the GaNxAs1¡x layers for x = 1-3%, as the defect parameters do not change with x.

Fuente:

http://przyrbwn.icm.edu.pl/APP/PDF/108/a108z404.pdf

Gerald Soto, CRF 2010-1.

http://gerald-ees.blogspot.com/

Reducing GaN HEMT degradation with InAlN barrier, Gerald Soto, CRF 2010-1, (3er Parcial).

By using indium aluminum nitride (InAlN) rather than the more usual aluminum gallium nitride (AlGaN) as the barrier layer, one can lattice match (In0.17Al0.83N) with the underlying gallium nitride (GaN), removing strain effects. It is found that spontaneous polarization in the nitride semiconductor materials is sufficient to create the two-dimensional electron gas (2DEG) channel at the barrier/substrate interface that is necessary for HEMT operation. In other words, one does not need strain-induced (piezoelectric) polarization from a mismatched barrier to produce GaN HEMTs.

While strain can produce desirable effects in some semiconductors, such as increased mobility in certain directions, it can also lead to unstable structures that can fail under electrical and thermal stress. Among the achievements for InAlN/GaN HEMTs have been drain current densities exceeding 3A/mm and HEMT operation at 1000°C without permanent damage.

Figure 1: Comparisons of normalized drain current (a), intrinsic channel resistance (b), and threshold voltage (c) after negative gate bias (NGB), off, and semi-on stresses at the same drain-gate potential VDG. VGS=–3V during semi-on stresses. Corresponding bias conditions are VDG =–VG=VDS – VGS.

The researchers from the Technische Universität Wien (TU Vienna), Institute of Electrical Engineering Slovak Academy of Sciences and École Polytechnique Fédérale de Lausanne (EPFL) aimed to fill the gap in the analysis of possible degradation mechanisms in InAlN/GaN HEMTs at various device working points and electrical stressing conditions in a similar way to other groups studying AlGaN/GaN devices. The team also hoped to confirm its expectation that the absence of strain in InAlN can reduce some device degradation processes.

The epitaxial layers (10nm InAlN/1nm AlN/1μm GaN/150nm AlN) for the tested devices were constructed using metal-organic chemical vapor deposition (MOCVD). Source/drain contacts consisted of titanium, aluminum, nickel and gold, while the Schottky barrier for the gate consisted of nickel and gold. No passivation procedure was used.

Stressing experiments were carried out under negative gate bias (NGB), off and semi-on conditions. NGB stresses are found to damage AlGaN/GaN devices because inverse piezoelectric effects (strain produced from electric fields) generate defects. The InAlN/GaN devices were also subjected to testing under different temperature conditions up to 250°C. Five or six devices were subjected to each test and it was found that there were no qualitative differences in behavior.

NGB tests revealed less variability in parameters when compared with published results for AlGaN/GaN HEMTs. Although gate leakage is initially a little higher for InAlN/GaN with gate-source voltage (VGS) less than –26V, it increases by less than 80% in going to –50V (catastrophic breakdown), in contrast to the four-order-of-magnitude increase for AlGaN/GaN. Further, when degradation does occur, it is reversible, unlike the traditional AlGaN/GaN HEMT set-up. While some parameters need several hours to recover their initial values, the drain and gate leakage currents need only about 100 minutes.

Irreversible damage was seen in off and semi-on tests when the drain-gate voltage (VDG) exceeds 38V. In the off-state the intrinsic channel resistance (Rch) and drain current are most affected. The resistance increased by one order of magnitude and the drain current decreased by 70% after off-state bias tests to a VDG of 50V. The researchers believe that these degradation effects are related to hot-carrier injection into the GaN buffer layer, creating defects and ionizing existing states.

Improvements may be sought using double-heterostructure channels, field plates or recessed gates, with the aim of reducing the hot-carrier injection. Surface passivation is another possible route to more reliable InAlN/GaN HEMTs.

Fuente:

http://www.semiconductor-today.com/news_items/2010/JAN/GANHEMT_080110.htm

Gerald Soto, CRF 2010-1.

http://gerald-ees.blogspot.com/

InSbN delivers infrared detection, Gerald Soto, CRF 2010-1, (3er Parcial).

InSbN photovoltaic infrared detectors offer a promising alternative to the HgCdTe incumbent by combining superior material quality with lower Auger recombination and a range of fabrication techniques.

A team of researchers in Singapore claims to have built the first InSbN-based photodiodes for mid- and long-wavelength infrared detection.

These photovoltaic devices incorporate tiny amounts of nitrogen and produce their strongest photocurrent peak at 5.3 microns, which originates from the binary InSb. Subsidiary peaks in three different samples occurred at 6.30, 6.33 and 6.47 microns. According to team member Dao Hua Zhang from Nanyang Technology University, one application for these InSbN devices is night vision. In addition, they could be used to detect gases such as sulphur dioxide, ammonia and chlorofluorocarbon refrigerant compounds. If InSbN detectors are to kick-on and enjoy commercial success, then they needs to take market share from HgCdTe devices that

provide detection across the 1 to 25 micron spectral range.

The incumbent technology has many strengths: a tunable bandgap governed by alloy composition; a high optical absorption coefficient; high electron mobility; and readily available doping techniques. These benefits have to be weighed against several disadvantages, including lattice, surface and interface instabilities of HgCdTe, which can lead to large variations in stoichiometry and transport properties.

"Mass production [of HgCdTe detectors] has a very low uniformity and low yield, and HgCdTe is a highly toxic material," adds Zhang. In comparison, InSbN features better material quality and uniformity, thanks to the very small amounts of nitrogen needed to red-shift the bandgap. There are also many options for fabrication, because this ternary can be fabricated by MOCVD, MBE or multi-step ion implantation.

"More importantly, the Auger recombination rate of the InSbN alloy is only one-third of HgCdTe with an equivalent bandgap, which makes InSbN the best candidate for making mid- and long-wavelength infrared photodetectors," says Wang.

Fabrication of InSbN devices began by depositing a 100 nm-thick, SiN film onto n-type InSb substrates by PECVD. InSbN layers were formed by nitrogen ion implantation, using energies of 90, 180 and 530 keV to ensure a uniform nitrogen profile. The top p-region was then created by magnesium ion implantation.

Annealing the wafers at 550K for 4 hours removed damage caused by implantation, and activated the incorporated nitrogen. Standard photolithography then created mesa-like structures with a 250-micron diameter. Detectors produced a range of photocurrent spectra, and the longest wavelength device had a photocurrent peak at 6.47 microns and a cut-off wavelength of 9.4 microns. This device has a nitrogen composition of 0.43 percent, according to theoretical band structure calculations with a 10-band k.p model. Zhang and his colleagues are now planning to build devices spanning the mid and longwavelength infrared.

Fuente:

http://compoundsemiconductor.net/csc/adminpanel/uploads/magazine_images/11748_Compound%20Semi_July2010.pdf

Gerald Soto, CRF 2010-1.

http://gerald-ees.blogspot.com/

sábado, 24 de julio de 2010

GaAs-based detectors extend to the far infrared, Gerald Soto, CRF 2010-1, (3er Parcial).

A team of French researchers claims that it has fabricated the first GaAs/AlGaAs quantum cascade detector (QCD) capable of operating at very long infrared wavelengths. Development of this 15 μm detector could provide a stepping stone towards the manufacture of focal plane arrays operating in this spectral range that could be used for meteorology, atmospheric chemistry studies, and Earth observation missions. Corresponding author Amandine Buffaz from the University of Paris, Diderot-Paris 7, says that the performance of the team’s detectors are comparable to those of the incumbent technology, quantum well infrared photodetectors.

However, the cascading detectors have one distinct advantage – very low dark currents that enable long integration times. The team, which also includes researchers from the Alcatel-Thales 3-5 lab, produced their detectors via MBE growth on a semiinsulating GaAs (001) substrate.

The detector’s epitaxial layers consist of 30 identical periods of 4 coupled quantum wells that feature AlGaAs barriers with a 232 meV conduction band offset.

Square shaped mesas with 50 μm and 100 μm sides were created with dry-etching techniques, and Au/Ge/Ni ohmic contacts were deposited onto these pixels.

The detector has a responsitivity peak of 14.3 μm, and its detectivity at 25 K and an applied bias of –0.6V is 1 x 1012 Jones. The detector’s performance can be taken to a new level by cutting the tunneling current. “To reach that aim we will use two theoretical models of electronic transport in QCDs: a ‘thermalized subbands’ approach that models transport based on diffusion mechanisms; and a resonant tunnelling model.”

Comparing the results of each of these calculations should uncover a structure that has carrier transport dominated by diffusion rather than tunneling. Another of the team’s goals is to develop detectors operating in other regions of the infrared spectrum.

“The first QCD detecting in the terahertz is under study, and in the immediate future the first thermal imager based on QCD detectors should be fabricated.”