Nitride-based Heterostructures Research Articles

Mobility-limiting mechanisms in polar semiconductor heterostructures

The mechanisms controlling the carrier mobility of two-dimensional electron gases (2DEGs) in ultrathin polar semiconductor heterostructures, such as III–V nitrides, have been analyzed. InxAl1−xN/AlN/GaN heterostructures with different AlN layer thicknesses have been investigated. These structures can be considered a very good benchmark for the analyses of III–V nitrides, due to the possibility of modulating the strain by varying the In composition. In order to determine an estimate of the mobility, charged dislocation and remote surface roughness scattering lifetimes have been calculated. Atomic force microscopy and scanning tunneling microscopy analyses have been used to measure the parameters required for the lifetime calculation, such as surface roughness, correlation length and dislocation density, and the total mobility has thus been calculated without the need of any a priori assumptions on the values of these parameters. The mobility of InxAl1−xN/AlN/GaN heterostructures has been measured at room temperature and liquid nitrogen temperature by the Hall effect. A comparison between the calculated and the Hall-effect-measured mobilities allowed us to establish, without using any ad hoc assumptions or fitting parameters, that the remote surface roughness is the most effective factor in controlling the transport properties of 2DEGs in nitride-based heterostructures at low temperature.

Intrinsic dynamics of weakly and strongly confined excitons in nonpolar nitride-based heterostructures

Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 10 5 cm −2 . Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K the QW emission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.

Electron microscopy of AlGaN-based multilayers for UV laser devices

UV-Laser technology is one of the most challenging and important areas in nitride semiconductor research. A new growth technology, using a thin GaN interlayer approach, has recently been developed for UV-LEDs in our department. Improved optical quality of so grown LEDs has been demonstrated. A similar growth approach has now been used to grow an UV-laser structure that has shown stimulated emission (lasing) at ~340 nm via optical pumping. We report here a study of the microstructure, the epitaxial quality and the elemental distributions of such an aluminium gallium nitride-based heterostructure using a combination of various electron microscopy techniques.

Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)−1 with corresponding sheet carrier densities of 2.3–1.3 × 1013 cm−2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)−1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm−2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.

The Reduction of Elastic Energy Density in InN Growth on (hkl)-Oriented Planes

Simple calculations performed in the context of linear elasticity theory, indicate that growth of InN on (hkl)-oriented substrates minimizes the density of elastic energy stored in the strained layers and boosts the critical thickness for coherent growth of strained layers to unexpected values. This renders possible to grow two-dimensional InN–GaN structures with InN layers thicknesses larger than one monolayer and, in addition, continuous in the growth plane. How impacting this finding can be for optoelectronic devices using indium nitride-based heterostructures is finally discussed.

Tunable terahertz-frequency resonances and negative dynamic conductivity of two-dimensional electrons in group-III nitrides

We investigated the terahertz (THz)-frequency resonances of two-dimensional electron conductivity under the streaming transport in a GaN quantum well at the nitrogen temperature. The calculation results found that the negative microwave mobility can occur in the narrow windows near the optical-phonon transit-time resonance frequencies, which can be tuned electrically in the 0.2–2.5THz range with the static electric fields of 1–10kV∕cm. The estimated magnitude of the negative mobility reaches hundreds of cm2∕Vs. These effects suggest that the nitride-based heterostructure may enable the development of an electrically pumped, tunable THz source operating at or above 77K.

AlGaN/GaN HEMT based liquid sensors

An AlGaN/GaN high electron mobility transistor structure was used for sensing different liquids present in the gate region. The forward current showed significant decreases upon exposure of the gate area to solvents (water, acetone) or acids (HCl). The pH sensitivity is due to changes in net surface charge that affects the relative depletion in the channel of the transistor. The results indicate that nitride-based heterostructures may have application in integrated chemical, gas and fluid monitoring sensors.

Polaron binding energy and effective mass in nitride single semiconductor heterostructures

Polaronic corrections in the conduction band of III–V nitride-based single heterostructures (SHS) are calculated within second-order Rayleigh–Schrödinger perturbation theory, using the dielectric continuum model for the interface and half-space polar optical phonon modes. The particular case of the AlN/GaN system is considered for illustration. The formation of a polarization-induced sheet density charge at the interface between both materials allows to study the single-electron states with the use of an approximate analytical expression for the potential energy function. The band-bending profile is then described in a way that includes many-body effects through a one-dimensional Hartree potential. Comparison of the calculated polaron effective mass in this material with recent experimental results in nitride SHS shows that higher perturbative orders in the interaction should be included in order to obtain better agreement.

Designing strain-balanced GaN/AlGaN quantum well structures: Application to intersubband devices at 1.3 and 1.55 μm wavelengths

A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at λ∼1.3 and 1.55 μm were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55μm and depopulating via resonant longitudinal optical (LO) phonons (ℏωLO≈90 meV).

A study of temperature field in a GaN heterostructure field-effect transistor

This paper presents a three-dimensional finite element based heat transfer model for a Gallium Nitride-based Heterostructure Field-Effect Transistor (henceforth referred to as GaN HFET). Analyses were carried out to study the distribution of temperature in the HFET under steady-state conditions for two different steady-current inputs. Two different substrates for the HFET, sapphire and silicon carbide (SiC), were studied. The paper discusses the effect of using a heat sink and also that of using reasonable contact resistances on the substrate side of the HFET, on the temperature profile. In all cases, the gate region of the HFET was found to attain the highest temperature. Subsequent experiments to validate the results of the computational analysis were carried out at the Oakridge National Laboratories, Knoxville, and are also presented in this paper.

Separation of hot-electron and self-heating effects in two-dimensional AlGaN/GaN-based conducting channels

We address experimental and theoretical study of a two-dimensional electron gas transport at low and moderate electric fields. The devices under study are group-III nitride-based (AlGaN/GaN) gateless heterostructures grown on sapphire. The transmission line model patterns of different channel lengths, L, and of the same channel width are used. A strong dependence of the device I–V characteristics on the channel length has been found. We have developed a simple theoretical model to adequately describe the observed peculiarities in the I–V characteristics measured in steady-state and pulsed (10−6 s) regimes. The effect of the Joule heating of a heterostructure is clearly distinguished. The thermal impedance and the channel temperature rise caused by the Joule self-heating have been extracted for the devices of different L at different values of dissipated power. The current reduction due to both self-heating and hot-electron effects is determined quantitatively as a function of the electric field.

Two‐Dimensional Electron Dynamics in GaN/AlGaN Heterostructures

(a) Institut fu¨r Schichten und Grenzfla¨chen, Forschungszentrum Ju¨lich, 52425 Ju¨lich,Germany(b) Institute of Semiconductor Physics, NASU, Kiev 03028, Ukraine(c) School of Electrical Engineering, Cornell University, Ithaca, New York 14853, USA(Received July 22, 2002; accepted October 1, 2002)PACS: 72.80.Ey; 72.90.+y; 73.40.KpThis report addresses the study of two-dimensional electron gas (2DEG) transport at low andmoderate electric fields. The devices under study are group-III nitride-based (AlGaN/GaN)gateless heterostructures grown on sapphire for HEMT applications. The transmission line model(TLM) patterns of different channel lengths L and the same channel width are used. We havedeveloped a simple theoretical model to adequately describe the observed peculiarities in the I–Vcharacteristics measured in steady-state and pulsed (10

HIGH-FIELD ELECTRON TRANSPORT CONTROLLED BY OPTICAL PHONON EMISSION IN NITRIDES

We have investigated the problem of electron runaway at strong electric fields in polar semiconductors focusing on the nanoscale nitride-based heterostructures. A transport model which takes into account the main features of electrons injected in short devices under high electric fields is developed. The electron distribution as a function of the electron momenta and coordinate is analyzed. We have determined the critical field for the runaway regime and investigated this regime in detail. The electron velocity distribution over the device is studied at different fields. We have applied the model to the group-III nitrides: InN, GaN and AlN. For these materials, the basic parameters and characteristics of the high-field electron transport are obtained. We have found that the transport in the nitrides is always dissipative. However, in the runaway regime, energies and velocities of electrons increase with distance which results in average velocities higher than the peak velocity in bulk-like samples. We demonstrated that the runaway electrons are characterized by the extreme distribution function with the population inversion. A three-terminal heterostructure where the runaway effect can be detected and measured is proposed. We also have considered briefly different nitride-based small-feature-size devices where this effect can have an impact on the device performance.

Enhancement of hole injection for nitride-based light-emitting devices

A novel device design is proposed for a strong enhancement of hole injection current in nitride-based light-emitting heterostructures. Preliminary calculations show orders of magnitude increase in injected hole current when using the proposed superlattice hole injector device based on the real-space transfer concept.

Relaxation of misfit-induced stress in nitride-based heterostructures

Relaxation processes of misfit-induced stress in GaN/Al y Ga 1− y N and Al x Ga 1− x N/Al y Ga 1− y N structures ( x> y) were studied. In case of GaN on relaxed Al y Ga 1− y N, relaxation of compressive-stress occurs in a very early stage of the GaN growth, while in case of Al y Ga 1− y N on Al x Ga 1− x N ( x> y), tensile-stress relaxation does not occur until the catastrophic relaxation by fracture formation. Surface roughness of the GaN grown on Al y Ga 1− y N can be suppressed by doping with Mg.

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Enhancement of base conductivity via the piezoelectric effect in AlGaN/GaN HBTs

Large polarization effects in nitride-based heterostructures provide opportunities for controllably introducing negative charge, equivalent to acceptor-doping, in AlGaN/GaN HBT structures. This paper reviews evidence for polarization-based doping effects in nitride-based materials, and discusses potential applications of these effects for the improvement of p-contacts and p-type base conductivity. It is shown that in conventional HBTs grown with Ga-terminated faces in the emitter-up configuration, the polarization-induced charges detract from the acceptor doping of the base. In contrast, the polarization effects add to effective doping in the base in structures that are grown and processed as collector-up devices, or in devices that are grown emitter-up and utilize the transferred substrate approach.

Carrier screening and polarization fields in nitride-based heterostructure devices

We have shown that free carriers and fixed doping charges can effectively screen macroscopic spontaneous and piezoelectric polarization fields in nitride quantum wells. A rather high sheet density is needed to achieve these conditions. We also explain the red shifts versus well width and blue shifts versus sheet density observed in the experiments as resulting from the interplay of carrier screening and polarization fields.

Zeeman Effect in Wurtzite-Based and Strained Cubic Heterostructures

We investigate theoretically the Zeeman effect on the lowest confined subbands in wurtzite heterostructures. The powerful approach suggested recently for zincblende-based nanostructures is extended to a new symmetry class and analytical expressions are derived for the first time for the g-factors in the technologically important nitride-based quantum wells and AIIBVI heterostructures, in the framework of the quasi-cubical Kane model. In our calculation we make allowance for the strain in heterolayers and huge internal electric fields due to spontaneous polarization and lattice-constant mismatch between the compositional materials.

Plastic versus elastic misfit relaxation in III-nitrides grown by molecular beam epitaxy

With a view to understand the misfit relaxation mechanisms for binary III-nitride semiconductors, a combined RHEED and TEM study of strain accommodation for AlN/GaN, GaN/AlN and InN/GaN MBE heteroepitaxial combinations is presented. It is shown that AlN grows on GaN (2.7% tensile strain) in a two-dimensional way in accordance with a Frank-van der Merwe mode with plastic relaxation occurring through the formation of non-interacting edge-type dislocations. On the other hand, GaN epitaxy on AlN (2.7% compressive strain) proceeds in different modes according to the growth temperature: in particular, a strained Stranski–Krastanow mode can be evidenced at intermediate temperatures. For the case of InN on GaN (10% compressive strain), strain relaxation occurs through the formation of dislocated islands on the surface, i.e. through a relaxed Stranski–Krastanow mode. The implications of these different growth processes to the development of nitride-based quantum dots heterostructures are presented.