P-type Superlattices Research Articles

Design and Growth of Low Resistivity P-Type AlGaN Superlattice Structure.

This work investigated the impact of periodic thickness and doping region on the doping efficiency of the P-type AlGaN superlattice. In this paper, the band structure of the simulated superlattice was analyzed. The superlattice structure of Al0.1Ga0.3N/Al0.4Ga0.6N, and the AlGaN buffer on the sapphire substrate, achieved a resistivity of ~3.3 Ω·cm. The results indicate that barrier doping and low periodic thickness offer significant advantages in introducing a reduction of the resistivity of P-type AlGaN superlattice structures.

Enhanced hole transport of nonpolar InGaN-based light-emitting diodes with lateral p-type superlattice doping structure

In this work, we propose a novel structure for nonpolar (10–10)-plane InGaN-based light-emitting diode (LED) using a lateral p-type Al0.2Ga0.8N/GaN superlattice structure as the hole injection layer. The main objective is to increase the hole concentration and facilitate vertical hole injection. The nonpolar InGaN-based LED lacks polarization along the growth plane (10–10), but the lateral direction along [0001] exhibits strong polarization. Therefore, the Al0.2Ga0.8N/GaN superlattice structure, which is periodic along the [0001] direction, induces net polarization charges at the GaN/Al0.2Ga0.8N interface, resulting in increased ionization rates of the acceptors. Additionally, the high-density two-dimensional hole gases formed at the Al0.2Ga0.8N/GaN interfaces along the [0001] direction can efficiently inject vertically into the quantum wells. Based on the numerical simulation results, the proposed LED structure offers improved electrical characteristics, effective hole injection, and enhanced optical performance compared to the nonpolar LED with conventional p-type doping structure.

Regulation of the Electronic Properties of Graphene via Organic Molecular Intercalation

Stable, efficient, and flexible regulation of the carrier types and concentrations of graphene is important to its multifunctional application. Here, we have synthesized n- and p-type graphene superlattices which are named as Gr/TBA and Gr/Ac via the electrochemical intercalation of organic tetrabutyl ammonium and acetate, respectively. The crystal structure characterization demonstrated that the interlayer spacing of the high-quality Gr/TBA and Gr/Ac was larger than that of pristine graphene. Hall effect measurements indicated that Gr/TBA and Gr/Ac showed n-type and p-type conductive behaviors, respectively. The carrier concentrations and mobility can be effectively tuned by the intercalated organic molecules. The regulation of the electronic properties of graphene via intercalation should result from the electron transfer between graphene and the corresponding organic molecules. Moreover, we further show that the resulting Gr/TBA and Gr/Ac feature excellent stability (chemicals and air), with no apparent change in carrier concentration. Our work provides a new way to regulate the electronic properties of graphene and broaden the application of graphene in multifunctional electrical devices.

Development of type-II superlattice long wavelength infrared focal plane arrays for land imaging

We explore InAs/InAsSb type-II superlattice long wavelength infrared detector based focal plane arrays for potential NASA land imaging applications. We used the complementary barrier infrared detector (CBIRD) architecture with p-type superlattice absorber (p-CBIRD), and with a combination of p-type and n-type superlattice absorbers (pn-CBIRD). CBIRD focal plane arrays fabricated using a conventional process and with an alternative silicon-on-silicon process have both demonstrated good operability and uniformity.

High Thermoelectric Power Factor Realization in Si-Rich SiGe/Si Superlattices by Super-Controlled Interfaces

A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained, not to give strainto the above Si layers. Therein, a drastic thermal conductivity reduction occurs by larger phonon scattering at the interfaces with the large atomic size difference between Si layers and Ge segregation layers, while unstrained Si layers preserve a high conduction band degeneracy leading to a high Seebeck coefficient. As a result, the n-type Si0.7Ge0.3/Si SL with controlled interfaces achieves a higher power factor of ∼25 μW cm-1 K-2 in the in-plane direction at room temperature, which is superior to ever reported SiGe-based films: SiGe-based SLs and SiGe films. The Si0.7Ge0.3/Si SL with controlled interfaces also exhibits a low thermal conductivity of ∼2.5 W m-1 K-1 in the cross-plane direction, which is ∼5 times lower than the reported value in a conventional Si0.7Ge0.3/Si SL. These results demonstrate that strain and atomic differences controlled by ultrathin layers can bring a breakthrough for realizing high-performance light-element-based thermoelectric films.

Optimization on the luminous efficiency in AlGaN-based ultraviolet light-emitting diodes by amendment of a superlattice hole reservoir layer

The application of a p-type superlattice hole reservoir layer in the traditional ultraviolet light-emitting diodes (UVLED) can obtain better Internal quantum efficiency (IQE) and output power, ease the problem about efficient carrier movement in high Al-content AlGaN material. Through computation and analysis by using the APSYS simulation software, the change of position of the hole reservoir layer can influence the luminous efficiency. The design of a superlattice hole reservoir layer between electron blocking layer (EBL) and p-type AlGaN layer can obviously reduce the hole potential height and increase the electron potential height, produce more hole injection and less electron leak, leading to higher carrier concentration, so as to realize the further increased for carrier recombination rate.

Cluster-packing geometry for Al-based F-type icosahedral alloys

This paper presents a new highly stable periodic approximant to the Al-based F-type icosahedral quasicrystals, i-Al-Pd-TM (TM=transition metals). The structure of this intermetallic Al-Pd-Cr-Fe compound is determined ab initio using single-crystal X-ray diffraction, where the space group is identified to be Pa-3 and the lattice constant 40.5 angstrom. The structure is well described as a dense packing of clusters of two kinds, which are known in the literature as the pseudo-Mackay type and the Bergman type clusters. The clusters are centered at the vertices of a canonical cell tiling, in which the parity of each vertex determines the kind of the associated cluster. Adjacent clusters can be markedly interpenetrated, while the structure requires no glue atoms to fill in the gaps between the clusters. It is shown that the crystal can be designated as a 2x2x2 superstructure of the ordinary cubic 3/2 rational approximant. The superlattice ordering is shown to be of a different kind from the P-type superlattice ordering previously reported in i-Al-Pd-Mn. The present results will greatly improve the understanding of atomic structures of F-type icosahedral quasicrystals and their approximants.

Theoretical luminescence spectra in p-type superlattices based on InGaAsN

In this work, we present a theoretical photoluminescence (PL) for p-doped GaAs/InGaAsN nanostructures arrays. We apply a self-consistent method in the framework of the effective mass theory. Solving a full 8 × 8 Kane's Hamiltonian, generalized to treat different materials in conjunction with the Poisson equation, we calculate the optical properties of these systems. The trends in the calculated PL spectra, due to many-body effects within the quasi-two-dimensional hole gas, are analyzed as a function of the acceptor doping concentration and the well width. Effects of temperature in the PL spectra are also investigated. This is the first attempt to show theoretical luminescence spectra for GaAs/InGaAsN nanostructures and can be used as a guide for the design of nanostructured devices such as optoelectronic devices, solar cells, and others.

Mg dopant distribution in an AlGaN/GaN p-type superlattice assessed using atom probe tomography, TEM and SIMS

P-type conducting layers are critical in GaN-based devices such as LEDs and laser diodes. Such layers are often produced by doping GaN with Mg, but the hole concentration can be enhanced using AlGaN/GaN p-type superlattices by exploiting the built-in polarisation fields. A Mg-doped AlGaN/GaN superlattice was studied using SIMS. Although the AlGaN and GaN were nominally doped to the same level, the SIMS data suggested a difference in doping density between the two materials. Atom probe tomography was then used to investigate the Mg distribution. The superlattice repeats were clearly visible, as expected and, in addition, significant Mg clustering was observed in both the GaN and AlGaN layers. There were many more Mg clusters in the AlGaN layers than the GaN layers, accounting for the difference in doping density suggested by SIMS. To evaluate the structural accuracy of the atom probe reconstruction, layer thicknesses from the atom probe were compared with STEM images. Finally, future work is proposed to investigate the Mg clusters in the TEM.

Miniband Structure Formation of p-type delta-doped Superlattices in GaAs

We present the electronic structure of flnite p-type delta-doped superlattices in GaAs. We use the flrst neighbors sp 3 s ⁄ tight-binding approximation including spin for the miniband structure analysis. The calculation is based on analytical expression for the Hartree potential of the inhomogeneous part, previously obtained within the Thomas-Fermi (TF) ap- proximation. This potential is considered as an external potential in the computations, so, it is added to the diagonal terms of the tight-binding Hamiltonian. We give a detail description of the delta-doped superlattices, this is, we study the miniband formation as a function of the impurity density (n2D) and the superlattice period (d), obtaining the regions of superlattice, multiple well and isolate well behavior. We also compare our results with the theoretical and experimental data available, obtaining a reasonable agreement.

Characteristics of Green Light-Emitting Diodes Using an InGaN:Mg/GaN:Mg Superlattice as p-Type Hole Injection and Contact Layers

Mg-doped InGaN/GaN p-type short-period superlattices (SPSLs) are developed for hole injection and contact layers of green light-emitting diodes (LEDs). V-defect-related pits, which are commonly found in an InGaN bulk layer, can be eliminated in an InGaN/GaN superlattice with thickness and average composition comparable to those of the bulk InGaN layer. Mg-doped InGaN/GaN SPSLs show significantly improved electrical properties with resistivity as low as ∼0.35 ohm-cm, which is lower than that of GaN:Mg and InGaN:Mg bulk layers grown under optimized growth conditions. Green LEDs employing Mg-doped InGaN/GaN SPSLs for hole injection and contact layers have significantly lower reverse leakage current, which is considered to be attributed to improved surface morphology. The peak electroluminescence intensity of LEDs with a SPSL is compared to that with InGaN:Mg bulk hole injection and contact layers.

Surface channel current in InAs∕GaSb type-II superlattice photodiodes

We observed experimental evidences of surface channel current on the sidewall of InAs∕GaSb superlattice photodiodes. We investigated the surface channel current by measuring the current-voltage (I-V) characteristics of the diodes. The experimental data compare very well with our theoretical model before and after ammonium sulfide passivation. By using the passivation, we reduced the surface channel current by five times, which supports that the surface channel current is induced by surface carriers. We believe that the surface channel current results from the inversion layer of a p-type superlattice with surface Fermi levels pinned above the conduction-band minimum in InAs∕GaSb superlattices.

Electrical, photoelectrical, and luminescent properties of doped p-type GaN superlattices

Electrical photoelectrical, and microcathodoluminescence properties were measured on doped p-GaN superlattices prepared by molecular beam epitaxy on c-plane sapphire substrates. The photosensitivity of such superlattices is at least two orders of magnitude higher and the microcathodoluminescence intensity is about five times lower than for similarly grown uniformly doped p-GaN films. The difference is explained by the spatial separation of the photoexcited electrons and holes by the superlattice potential. The sheet carrier concentration in the superlattice is shown to be several times (6.6×1012cm−2 vs 2×1013cm−2) lower and the effective hole mobility almost two times higher (20cm2∕Vs vs 12cm2∕Vs) than for bulk p-GaN samples with similar doping.

Electrical and optical properties of doped p-type GaN superlattices

A comparison is reported of the electrical, photoelectrical, and microcathodoluminescence properties of bulk p-GaN films and p-type p-GaN(Mg)∕GaN superlattices grown by molecular beam epitaxy. It is shown that the microcathodoluminescence intensity is much lower and the photocurrent (both on samples with indium Ohmic contacts and on Au Schottky diodes) much higher for the doped superlattices due to the spatial separation of photoexcited electrons and holes in the doped and undoped layers and corresponding increase of the lifetime in comparison to the bulk samples. The in-plane conductivity in the doped superlattices is not enhanced significantly relative to conventional p-GaN, but the hole mobilities are increased (from 12to20cm2∕Vs).

Theoretical study of current overflow in GaN based light emitters with superlattice cladding layers

We investigate the effect of the short-period superlattice cladding layer on electron current overflow in nitride light emitters. The classical drift-diffusion current flow and quantum tunneling transport through the miniband are considered. We show that the drift-diffusion electron current in the p-type superlattice cladding layer is drastically reduced by the presence of the intrinsic built-in electric fields. Based on this finding, we propose a design of the electron blocking layer which should considerably lower the electron current overflow in nitride light emitters.

Improvement on Thermoelectric Characteristics of Layered Nanostructure by Ion Beam Bombardment

ABSTRACTWe made p-type nanoscale super lattice thermoelectric cooling devices which consist of multiple periodic layers of Si1−x Gex / Si, The thickness of each layer ranges between 10 and 50 nm. The super lattice was bombarded by 5 MeV Si ion with different fluencies aiming to form nano-cluster quantum dot structures. We estimated the thermo-electric efficiency of the so fabricated devices, measuring the thin film cross plane thermal conductivity by the 3rd harmonic method, measuring the cross plane Seebeck coefficient, and finally measuring the cross plane electric conductivity before and after ion bombardment. As predicted, the thermo-electric Figure of Merit of the films increases with increasing Si ion fluencies. In addition to the effect of quantum well confinement of the phonon transmission, the nano-scale crystal quantum dots produced by the incident Si beam further adversely affects the thermal conductivity by absorbing and dissipating phonon along the lattice, and therefore further reduces the cross plane thermal conductivity, This process increases the electron density of state therefore increasing Seebeck coefficient, and the electric conductivity.

AlN/AlGaInN superlattice light-emitting diodes at 280 nm

Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN(1.2 nm thick)/AlGaInN(0.5 nm thick) doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7–1.1)×1018 cm−3, with the mobility of 3–4 cm2/V s and electron concentrations of 3×1019 cm−3, with the mobility of 10–20 cm2/V s, at room temperature. These carrier concentrations are sufficient to form effective p–n junctions needed in UV light sources.

Mechanism for thermoelectric figure-of-merit enhancement in regimented quantum dot superlattices

We propose a mechanism for enhancement of the thermoelectric figure-of-merit in regimented quantum dot superlattices. A proof-of-concept calculation has been carried out for p-type regimented superlattice of Ge dots on Si. It is shown that when conditions for miniband formations are satisfied, carrier transport in such structures can be tuned in a favorable way leading to large carrier mobility, Seebeck coefficient, and, as a result, to the thermoelectric figure-of-merit enhancement. To maximize the improvement, one has to tune the parameters of quantum dot superlattice in such a way that electrical current is mostly through the well-separated minibands of relatively large width (at least several kBT, where kB is Boltzmann’s constant and T is temperature).

Thermal Stability of p-type Bi2Te3/Sb2Te3 and n-type Bi2Te3/Bi2Te2-xSex Thermoelectric Superlattice Thin Film Devices

ABSTRACTThermolectric devices have been constructed using Bi2Te3/Sb2Te3 and Bi2Te3/Bi2Te2-xSex superlattice thin films. Since these devices are intended for use in systems that will operate at elevated temperatures over their lifetime as in many power conversion applications, thermal stability of the thermoelectric figure of merit is an important consideration. The ZTe of p-type and n-type superlattice thin film elements was evaluated at specific intervals during exposure to elevated temperatures of 150°C for up to 60 hrs. Results indicate that the figure of merit for p- and n-type superlattice films is not compromised over time when exposed to these operating temperatures. In fact, both p- and n-type superlattice thin film ZTe remains very constant or improves slightly when subjected to continuous exposure to elevated temperatures. Evaluation of these thin film thermoelements is reported and implications of these results are considered for thin film thermoelectric modules.

Plasma etching of AlN/AlGaInN superlattices for device fabrication

We report a study of plasma etching of GaN, AlN, and AlN/AlGaN superlattices for the processing of deep ultraviolet light emitting diodes. Etching was carried out using inductively coupled plasma of chlorine diluted with argon under reactive ion etching conditions. Using parameters selected for etch rate, anisotropy, and surface smoothness, we study etching of n- and p-type superlattices. The former etches at a rate of 250 nm/min, which is intermediate to that of AlN and GaN, while the latter exhibits a slower etch rate of 60 nm/min. Based on these studies, we prepare low-leakage p–n junctions and mesa light emitting diodes with peak emission at 280 nm.